A VERSATILE CONTROL SCHEME FOR A DYNAMIC

VOLTAGE RESTORER FOR POWER-QUALITY

IMPROVEMENT

ABSTRACT

This paper presents a control system based on a repetitive controller to compensate for

key power-quality disturbances, namely voltage sag, harmonic voltages, and voltage imbalances,

using a dynamic voltage restorer (DVR). The control scheme deals with all three disturbances

simultaneously within a bandwidth. The control structure is quite simple and yet very robust; it

contains a feed forward term to improve the transient response and a feedback term to enable

zero error in steady state. Simulation results show that the control approach performs very

effectively and yields excellent voltage regulation.

This paper focuses on the design of a closed-loop control law for a two-level DVR, based

on the so-called repetitive control, aiming at compensating key voltage-quality disturbances,

namely, voltage sags ,harmonic voltages, and voltage imbalances. A detailed analysis of various

repetitive control configurations is reported. The repetitive control was originally applied to

eliminate speed fluctuations in electric motors but it has since been adopted in a wide range of

power-electronics applications. A repetitive controller is applied to obtain an output voltage with

low distortion in a constant voltage, constant frequency three-phase PWM inverter. A repetitive

controller is used to achieve zero tracking error in the output current of a three-phase rectifier in

order to improve its power factor. The repetitive controller presented in this paper has a wider

range of applicability; it is used in a DVR system to ameliorate voltage sags, harmonic voltages,

and voltage imbalances within a bandwidth. Unlike other schemes, which also have a

comparable range of applicability, only one controller is needed to cancel all three disturbances

simultaneously. The control structure contains a grid voltage feed forward term to improve the

system transient response, and a closed-loop control which comprises a feedback of the load

voltage with the repetitive controller in order to warrant zero tracking error in steady state.

A case in point is the so called repetitive controller proposed in this paper, which has a

fast transient response and ensures zero error in steady state for any sinusoidal reference input

and for any sinusoidal disturbance whose frequencies are an integer multiple of the fundamental

frequency. To achieve this, the controller has been provided with a feed forward term and

feedback term. The design has been carried out by studying the stability of the closed-loop

system including possible modeling errors, resulting in a controller which possesses very good

transient and steady-state performances for various kinds of disturbances. Only one controller is

required to eliminate three PQ disturbances, namely, voltage sags, harmonic voltages, and

voltage imbalances.

I. INTRODUCTION

THE importance of power quality (PQ) has risen very considerably over the last two

decades due to a marked increase in the number of equipment which is sensitive to adverse PQ

environments, the disturbances introduced by nonlinear loads, and the proliferation of renewable

energy sources, among others. At least 50% of all PQ disturbances are of the voltage quality

type, where the interest is the study of any deviation of the voltage waveform from its ideal form

[1]. The best well-known disturbances are voltage sags and swells, harmonic and inter harmonic

voltages, and, for three-phase systems, voltage imbalances. A voltage sag is normally caused by

short-circuit faults in the power network [2], [3] or by the starting up of induction motors of large

rating [4]. The ensuing adverse consequences are a reduction in the energy transfers of electric

motors and the disconnection of sensitive equipment and industrial processes brought to a

standstill. A comprehensive description of voltage sags can be found in [5].

Harmonics are produced by nonlinear equipment, such as electric arc furnaces, variable

speed drives, large concentrations of arc discharge lamps, and loads which use power electronics.

Harmonic currents generated by a nonlinear device or created as a result of existing harmonic

voltages will exacerbate copper and iron losses in electrical equipment. In rotating machinery,

they will produce pulsating torques and overheating [6]. Voltage imbalances are normally

brought about by unbalanced loads or unbalanced short-circuit faults, thus producing overheating

in synchronous machines and, in some extreme cases, leading to load shutdowns and equipment

failure. The DVR is essentially a voltage-source converter connected in series with the

acnetwork via aninterfacing transformer, which was originallyconceivedtoamelioratevoltage sags

[7].However, as shown in this paper, its range of applicability can be extended very considerably

when provided with a suitable control scheme. The basic operating principle behind the DVR is

the injection of an inphase series voltage with the incoming supply to the load, sufficient enough

to reestablish the voltage to its presag state. Its rate of success in combating voltage sags in

actual installations is well documented [8], this being one of the reasons why it continues to

attract a great deal of interest in industry and in academic circles. Research work has been

reported on DVR two-level [9] and multilevel [10] topologies as well as on control and

operation. The latter may be divided into several topics. 1) The configuration, whether two-level

or multilevel, relates to the availability, or otherwise, of energy storage [2], the output filter [11],

and the capacity to cancel out unbalanced voltages in three-phase four-wire systems [12]. 2) The

voltage-sag detection. Several techniques have been used to detect the instant of sag appearance,

such as measurement of the peak value of the grid voltage. A comprehensive analysis of these

techniques can be found in [13]. 3) The control strategy. The DVR may be operated to inject the

series voltage according to several criteria, such as minimum energy exchange with the grid. The

three most popular strategies to compensate voltage sags are [14]: 1) presag compensation. The

injected DVR voltage is calculated to simply compensate the load voltage to its presag

condition; 2) inphase compensation. The DVR voltage is always in phase with the grid voltage;

and 3) optimal energy compensation. This strategy minimizes the energy transfer between the

energy storage and the grid during steady-state operation. Although these are the best well-

known control strategies, many efforts are being made to develop new ones to enable better DVR

utilization, as amply discussed in [15]–[18]. 4) The design of the control law. The controller is

normally designed with some specific aims firmly in mind, such as the kind of disturbances it

should ameliorate, the velocity of time response, error in steady-state, etc. Most of the published

work on DVR uses a simple proportional-integral (PI) control law implemented in a frame of

reference which rotates with the frequency of the grid voltage. This basic approach is sufficient

to enable voltage sag compensation, to warrant zero tracking error for the fundamental

component, and to compensate certain kinds of unbalanced conditions. However, this simple

control law is insufficient when dealing with high-performance applications and more complex

controllers are required [19], [20]. The former reference adds resonant control filters to the

existing PI control scheme in order to eliminate harmonic voltages [21]. The main drawback of

this structure is that one filter is required for each harmonic to be eliminated if the system is

unbalanced and only half that number if the system is balanced. The latter reference takes the

approach of adding a feed forward loop to the feedback PI controller in order to improve the

control overall performance, taking into account the time delay of the sampled system and the

DVR output filter constraints. This paper focuses on the design of a closed-loop control law for a

two-level DVR, based on the so-called repetitive control, aiming at compensating key voltage-

quality disturbances, namely, voltage sags, harmonic voltages, and voltage imbalances.

Repetitive control was first introduced in [22] to eliminate periodic disturbances and to track

periodic reference signals with zero tracking error. A detailed analysis of various repetitive

control Configurations is reported in [23]. The repetitive control was originally applied to

eliminate speed fluctuations in electric motors but it has since been adopted in a wide range of

power-electronics applications. In [24], a repetitive controller is applied to obtain an output

voltage with low distortion in a constant voltage, constant frequency three-phase PWM inverter.

In [25], a repetitive controller is used to achieve zero tracking error in the output current of a

three-phase rectifier in order to improve its power factor. A more recent example is found in

[26], where a repetitive controller is used in a parallel active filter to cancel out harmonic

currents produced by a nonlinear load. The repetitive controller presented in this paper has a

wider range of applicability; it is used in a DVR system to ameliorate voltage sags, harmonic

voltages, and voltage imbalances within a bandwidth. Unlike other schemes, which also have a

comparable range of applicability, only one controller is needed to cancel all three disturbances

simultaneously. The control structure contains a grid voltage feed forward term to improve the

system transient response, and a closed-loop control which comprises a feedback of the load

voltage with the repetitive controller in order to warrant zero tracking error in steady state. This

paper is organized as follows. The DVR model is presented in Section II. The fundamentals of

the control system and the proposed control scheme are studied in Section III. The modeling of

the repetitive controller using the well-developed graphical facilities available in

PSCAD/EMTDC and simulation results is presented in Section IV. The main conclusions of the

current investigation are drawn in Section V.

DVR

The major objectives are to increase the capacity utilization of distribution feeders (by

minimizing the rms values of the line currents for a specified power demand), reduce the losses

and improve power quality at the load bus. The major assumption was to neglect the variations

In the source voltages. This essentially implies that the dynamics of the source voltage is much

slower than the load dynamics.

When the fast variations in the source voltage cannot be ignored, these can a®ect the

performance of critical loads such as (a) semiconductor fabrication plants (b) paper mills (c) food

processing plants and (d) automotive assembly plants. The most common disturbances in the

source voltages are the voltage sags or swells that can be due to (i) disturbances arising in the

transmission system, (ii) adjacent feeder faults and (iii) fuse or breaker operation. Voltage sags

of even 10% lasting for 5-10 cycles can result in costly damage in critical loads. The voltage sags

can arise due to symmetrical or unsymmetrical faults. In the latter case, negative and zero

sequence components are also present. Uncompensated nonlinear loads in the distribution system

can cause harmonic components in the supply voltages. To mitigate the problems caused by poor

quality of power supply, series connected compensators are used.

These are called as Dynamic Voltage Restorer (DVR) in the literature as their primary

application is to compensate for voltage sags and swells. Their configuration is similar to that of

SSSC, discussed in chapter 7. However, the control techniques are di®erent. Also, a DVR is

expected to respond fast (less than 1/4 cycle) and thus employs PWM converters using IGBT or

IGCT devices. The first DVR entered commercial service on the Duke Power System in U.S.A.

in August 1996. It has a rating of 2 MVA with 660 kJ of energy storage and is capable of

compensating 50% voltage sag for a period of 0.5 second (30 cycles). It was installed to protect a

highly automated yarn manufacturing and rug weaving facility.

Since then, several DVRs have been installed to protect microprocessor fabrication

plants, paper mills etc. Typically, DVRs are made of modular design with a module rating of 2

MVA or 5 MVA. They have been installed in substations of voltage rating from 11 kV to 69 kV.

A DVR has to supply energy to the load during the voltage sags. If a DVR has to supply active

power over longer periods, it is convenient to provide a shunt converter that is connected to the

DVR on the DC side. As a matter of fact one could envisage a combination of DSTATCOM and

DVR connected on the DC side to compensate for both load and supply voltage variations. In

this section, we discuss the application of DVR for fundamental frequency voltage...

The voltage source converter is typically one or more converters connected in series to

provide the required voltage rating. The DVR can inject a (fundamental frequency) voltage in

each phase of required magnitude and phase. The DVR has two operating modes

1. Standby (also termed as short circuit operation (SCO) mode) where the voltage injected has

zero magnitude.

2. Boost (when the DVR injects a required voltage of appropriate magnitude and phase to restore

the prefault load bus voltage).

The power circuit of DVR shown in Fig. 14.1 has four components listed below.

1. Voltage Source Converter (VSC)

This could be a 3 phase - 3 wire VSC or 3 phase - 4 wire VSC. The latter permits the injection of

zero-sequence voltages. Either a conventional two level converter (Graetz bridge) or a three level

converter is used.

2. Boost or Injection Transformers

Three single phase transformers are connected in series with the distribution feeder to

couple the VSC (at the lower voltage level) to the higher distribution voltage level. The three

single transformers can be connected with star/open star winding or delta/open star winding. The

latter does not permit the injection of the zero sequence voltage. The choice of the injection

transformer winding depends on the connections of the step down trans- former that feeds the

load. If a ¢ ¡ Y connected transformer (as shown in Fig. 14.1) is used, there is no need to

compensate the zero sequence volt- ages.

However if Y ¡ Y connection with neutral grounding is used, the zero sequence voltage

may have to be compensated. It is essential to avoid the saturation in the injection transformers.

3. Passive Filters

The passive filters can be placed either on the high voltage side or the converter side of

the boost transformers. The advantages of the converter side filters are (a) the components are

rated at lower voltage and (b) higher order harmonic currents (due to the VSC) do not °own

through the transformer windings. The disadvantages are that the filter inductor causes voltage

drop and phase (angle) shift in the (fundamental component of) voltage injected. This can a®ect

the control scheme of DVR. The location of the filter o the high voltage side overcomes the

drawbacks (the leakage reactance of the transformer can be used as a filter inductor), but results

in higher ratings of the transformers as high frequency currents can °ow through the windings.

4. Energy Storage

This is required to provide active power to the load during deep voltage sags. Lead-acid

batteries, °ywheel or SMES can be used for energy storage. It is also possible to provide the

required power on the DC side of the VSC by an auxiliary bridge converter that is fed from an

auxiliary AC supply

CONTROL STRATEGY :

There are three basic control strategies as follows.

1. Pre-Sag Compensation

The supply voltage is continuously tracked and the load voltage is compensated to the

pre-sag condition. This method results in (nearly) undisturbed load voltage, but generally

requires higher rating of the DVR. Before a sag occur, VS = VL = Vo. The voltage sag results in

drop in the magnitude of the supply voltage to VS1. The phase angle of the supply also may shift

see Fig. 14.2). The DVR injects a voltage VC1 such that the load voltage (VL = VS1 + VC1)

remains at Vo (both in magnitude and phase). It is claimed that some loads are sensitive to phase

jumps and it is necessary to compensate for both the phase jumps and the voltage sags.

2. In-phase Compensation

The voltage injected by the DVR is always in phase with the supply voltage regardless of

the load current and the pre-sag voltage (Vo). This control strategy results in the minimum value

of the injected voltage (magnitude). However, the phase of the load voltage is disturbed. For

loads which are not sensitive to the phase jumps, this control strategy results in optimum

utilization of the voltage rating of the DVR. The power requirements for the DVR are not zero

for these strategies

3. Minimum Energy Compensation

Neglecting losses, the power requirements of the DVR are zero if the injected voltage

(VC) is in quadrature with the load current. To raise the voltage at the load bus, the voltage

injected by the DVR is capacitive and VL leads VS1 (see Fig. 14.3). Fig. 14.3 also shows the in-

phase compensation for comparison. It is to be noted that the current phasor is determined by the

load bus voltage phasor and the power factor of the load.

Implementation of the minimum energy compensation requires the measurement of the

load current phasor in addition to the supply voltage. When VC is in quadrature with the load

current, DVR supplies only reactive power. However, full load voltage compensation is not

possible Unless the supply voltage is above a minimum value that depends on the load power

factor.

When the magnitude of VC is not constrained, the minimum value of VS that still allows

full compensation is where Á is the power factor angle and Vo is the required magnitude of the

Load bus voltage. If the magnitude of the injected voltage is limited (V max C ), the mini- mum

supply voltage that allows full compensation is given by The expressions (14.1) and (14.2)

follow from the phasor diagrams shown in Fig. 14.4. Note that at the minimum source voltage,

the current is in phase with VS for the case (a).

CONTROL AND PROTECTION

The control and protection of a DVR designed to compensate voltage sags must consider

the following functional requirements.

1. When the supply voltage is normal, the DVR operates in a standby mode with zero voltage

injection. However if the energy storage device (say batteries) is to be charged, then the DVR

can operate in a self- charging control mode.

2. When a voltage sag/swell occurs, the DVR needs to inject three single phase voltages in

synchronism with the supply in a very short time. Each phase of the injected voltage can be

controlled independently in magnitude and phase. However, zero sequence voltage can be

eliminated in situations where it has no e®ect. The DVR draws active power from the energy

source and supplies this along with the reactive power (required) to the load.

3. If there is a fault on the downstream of the DVR, the converter is by- passed temporarily using

thyristor switches to protect the DVR against over currents. The threshold is determined by the

current ratings of the DVR.

The overall design of DVR must consider the following parameters:

1. Ratings of the load and power factor

2. Voltage rating of the distribution line

3. Maximum single phase sag (in percentage)

4. Maximum three phase sag (in percentage)

5. Duration of the voltage sag (in milliseconds)

6. The voltage time area (this is an indication of the energy requirements)

7. Recovery time for the DC link voltage to 100%

8. Over current capability without going into bypass mode.

Typically, a DVR may be designed to protect a sensitive load against 35% of three phase voltage

sags or 50% of the single phase sag. The duration of the sag could be 200 ms. The DVR can

compensate higher voltage sags lasting for shorter durations or allow longer durations (up to 500

ms) for smaller voltage sags. The response time could be as small as 1 ms.

POWER QUALITYThe contemporary container crane industry, like many other industry segments, is often enamored by the bells and whistles, colorful diagnostic displays, high speed performance, and levels of automation that can be achieved. Although these features and their indirectly related computer based enhancements are key issues to an efficient terminal operation, we must not forget the foundation upon which we are building. Power quality is the mortar which bonds thefoundation blocks. Power quality also affects terminal operating economics, crane reliability, our environment, and initial investment in power distribution systems to support new crane installations. To quote the utility company newsletter which accompanied the last monthly issue of my home utility billing: ‘Using electricity wisely is a good environmental and business practice which saves you money, reduces emissions from generating plants, and conserves ournatural resources.’ As we are all aware, container crane performance requirements continue to increase at an astounding rate. Next generation container cranes, already in the bidding process, will require average power demands of 1500 to 2000 kW – almost double the total averagedemand three years ago. The rapid increase in power demand levels, an increase in container crane population, SCR converter crane drive retrofits and the large AC and DC drives needed to power and control these cranes will increase awareness of the power quality issue in the very near future.

POWER QUALITY PROBLEMSFor the purpose of this article, we shall define power quality problems as:‘Any power problem that results in failure or misoperation of customer equipment,manifests itself as an economic burden to the user, or produces negative impacts onthe environment.’

When applied to the container crane industry, the power issues which degrade power quality include:• Power Factor• Harmonic Distortion• Voltage Transients• Voltage Sags or Dips• Voltage SwellsThe AC and DC variable speed drives utilized on board container cranes are significant contributors to total harmonic current and voltage distortion. Whereas SCR phase control creates the desirable average power factor, DC SCR drives operate at less than this. In addition, line notching occurs when SCR’s commutate, creating transient peak recovery voltages that can be 3 to 4 times the nominal line voltage depending upon the system impedance and the size of the drives. The frequency and severity of these power system disturbances varies with the speed of

the drive. Harmonic current injection by AC and DC drives will be highest when the drives are operating at slow speeds. Power factor will be lowest when DC drives are operating at slow speeds or during initial acceleration and deceleration periods, increasing to its maximum value when the SCR’s are phased on to produce rated or base speed. Above base speed, the power factor essentially remains constant. Unfortunately, container cranes can spend considerable time at low speeds as the operator attempts to spot and land containers. Poor power factor places a greater kVA demand burden on the utility or engine-alternator power source. Low power factor loads can also affect the voltage stability which can ultimately result in detrimental effects on thelife of sensitive electronic equipment or even intermittent malfunction. Voltage transients created by DC drive SCR line notching, AC drive voltage chopping, and high frequency harmonic voltages and currents are all significant sources of noise and disturbance to sensitive electronic equipment

It has been our experience that end users often do not associate power quality problems withContainer cranes, either because they are totally unaware of such issues or there was no economic Consequence if power quality was not addressed. Before the advent of solid-state power supplies, Power factor was reasonable, and harmonic current injection was minimal. Not until the crane Population multiplied, power demands per crane increased, and static power conversion became the way of life, did power quality issues begin to emerge. Even as harmonic distortion and power Factor issues surfaced, no one was really prepared. Even today, crane builders and electrical drive System vendors avoid the issue during competitive bidding for new cranes. Rather than focus on Awareness and understanding of the potential issues, the power quality issue is intentionally or Unintentionally ignored. Power quality problem solutions are available. Although the solutions are not free, in most cases, they do represent a good return on investment. However, if power quality is not specified, it most likely will not be delivered.

Power quality can be improved through:• Power factor correction,• Harmonic filtering,• Special line notch filtering,• Transient voltage surge suppression,• Proper earthing systems.In most cases, the person specifying and/or buying a container crane may not be fully aware of the potential power quality issues. If this article accomplishes nothing else, we would hope toprovide that awareness.

In many cases, those involved with specification and procurement of container cranes may not be cognizant of such issues, do not pay the utility billings, or consider it someone else’s concern. As a result, container crane specifications may not include definitive power quality criteria such as power factor correction and/or harmonic filtering. Also, many of those specifications which do

require power quality equipment do not properly define the criteria. Early in the process of preparing the crane specification:• Consult with the utility company to determine regulatory or contract requirements that must besatisfied, if any.• Consult with the electrical drive suppliers and determine the power quality profiles that can beexpected based on the drive sizes and technologies proposed for the specific project.• Evaluate the economics of power quality correction not only on the present situation, but consider the impact of future utility deregulation and the future development plans for the terminal

THE BENEFITS OF POWER QUALITYPower quality in the container terminal environment impacts the economics of the terminal operation, affects reliability of the terminal equipment, and affects other consumers served by the same utility service. Each of these concerns is explored in the following paragraphs.1. Economic ImpactThe economic impact of power quality is the foremost incentive to container terminal operators. Economic impact can be significant and manifest itself in several ways:

a. Power Factor PenaltiesMany utility companies invoke penalties for low power factor on monthly billings. There is no industry standard followed by utility companies. Methods of metering and calculating power factor penalties vary from one utility company to the next. Some utility companies actually meter kVAR usage and establish a fixed rate times the number of kVAR-hours consumed. Other utility companies monitor kVAR demands and calculate power factor. If the power factor falls below a fixed limit value over a demand period, a penalty is billed in the form of an adjustment to the peak demand charges. A number of utility companies servicing container terminal equipment do not yet invoke power factor penalties. However, their service contract with the Port may still require that a minimum power factor over a defined demand period be met. The utility company may not continuously monitor power factor or kVAR usage and reflect them in the monthly utility billings; however, they do reserve the right to monitor the Port service at any time. Ifthe power factor criteria set forth in the service contract are not met, the user may be penalized, or required to take corrective actions at the user’s expense. One utility company, which supplies power service to several east coast container terminals in the USA, does not reflect powerfactor penalties in their monthly billings, however, their service contract with the terminal reads as follows:

‘The average power factor under operating conditions of customer’s load at the point where service is metered shall be not less than 85%. If below 85%, the customer may be required to furnish, install and maintain at its expense corrective apparatus which will increase thePower factor of the entire installation to not less than 85%. The customer shall ensure that no excessive harmonics or transients are introduced on to the [utility] system. This may require special power conditioning equipment or filters. The IEEE Std. 519-1992 is used as a guide inDetermining appropriate design requirements.’

The Port or terminal operations personnel, who are responsible for maintaining container cranes, or specifying new container crane equipment, should be aware of these requirements. Utility deregulation will most likely force utilities to enforce requirements such as the example above.Terminal operators who do not deal with penalty issues today may be faced with some rather severe penalties in the future. A sound, future terminal growth plan should include contingencies for addressing the possible economic impact of utility deregulation.

b. System LossesHarmonic currents and low power factor created by nonlinear loads, not only result in possible power factor penalties, but also increase the power losses in the distribution system. These losses are not visible as a separate item on your monthly utility billing, but you pay for them each month. Container cranes are significant contributors to harmonic currents and low power factor. Based on the typical demands of today’s high speed container cranes, correction of power factoralone on a typical state of the art quay crane can result in a reduction of system losses that converts to a 6 to 10% reduction in the monthly utility billing. For most of the larger terminals, this is a significant annual saving in the cost of operation.

c. Power Service Initial Capital InvestmentsThe power distribution system design and installation for new terminals, as well as modification of systems for terminal capacity upgrades, involves high cost, specialized, high and medium voltage equipment. Transformers, switchgear, feeder cables, cable reel trailing cables, collector bars, etc. must be sized based on the kVA demand. Thus cost of the equipment is directly related to the total kVA demand. As the relationship above indicates, kVA demand is inversely proportional to the overall power factor, i.e. a lower power factor demands higher kVA for the same kW load. Container cranes are one of the most significant users of power in the terminal. Since container cranes with DC, 6 pulse, SCR drives operate at relatively low power factor, the total kVA demand is significantly larger than would be the case if power factor correction equipment were supplied on board each crane or at some common bus location in the terminal. In the absence of power quality corrective equipment, transformers are larger, switchgear current ratings must be higher, feeder cable copper sizes are larger, collector system and cable reel cables must be larger, etc. Consequently, the cost of the initial power distribution system

equipment for a system which does not address power quality will most likely be higher than the same system which includes power quality equipment.

2. Equipment ReliabilityPoor power quality can affect machine or equipment reliability and reduce the life of components. Harmonics, voltage transients, and voltage system sags and swells are all power quality problems and are all interdependent. Harmonics affect power factor, voltage transients can induce harmonics, the same phenomena which create harmonic current injection in DC SCRvariable speed drives are responsible for poor power factor, and dynamically varying power factor of the same drives can create voltage sags and swells. The effects of harmonic distortion, harmonic currents, and line notch ringing can be mitigated using specially designed filters.

3. Power System AdequacyWhen considering the installation of additional cranes to an existing power distribution system, a power system analysis should be completed to determine the adequacy of the system to support additional crane loads. Power quality corrective actions may be dictated due to inadequacy of existing power distribution systems to which new or relocated cranes are to be connected. In other words, addition of power quality equipment may render a workable scenario on an existing power distribution system, which would otherwise be inadequate to support additional cranes without high risk of problems.4. EnvironmentNo issue might be as important as the effect of power quality on our environment. Reduction in system losses and lower demands equate to a reduction in the consumption of our natural nm resources and reduction in power plant emissions. It is our responsibility as occupants of this planet to encourage conservation of our natural resources and support measures which improve our air quality

VOLTAGE SAG

Voltage sags and momentary power interruptions are probably the most important

PQ problem affecting industrial and large commercial customers. These events are usually

associated with a fault at some location in the supplying power system. Interruptions occur

when the fault is on the circuit supplying the customer. But voltage sags occur even if the

faults happen to be far away from the customer's site. Voltage sags lasting only 4-5 cycles can

cause a wide range of sensitive customer equipment to drop out. To industrial customers,

voltage sag and a momentary interruption are equivalent if both shut their process down. A

typical example of voltage sag is shown in fig 1. The susceptibility of utilization equipment to

voltage sag is dependent upon duration and magnitude of voltage sags and can be define

Characteristics of Voltage Sags:

Voltage sags which can cause equipment impacts are caused by faults on the power system.

Motor starting also results in voltage sags but the magnitudes are usually not severe enough to

cause equipment mis operation

How a fault results in voltage sag at a customer facility?

The one line diagram given below in fig. 3 can be used to explain this phenomenon.

Consider a customer on the feeder controlled by breaker 1. In the case of a fault on

this feeder, the customer will experience voltage sag during the fault and an interruption when

the breaker opens to clear the fault. For temporary fault, enclosure may be successful.

Anyway, sensitive equipment will almost surely trip during this interruption. Another

kind of likely event would be a fault on one of the feeders from the substation or a

fault somewhere on the transmission system, In either of these cases, the customer will

experience a voltage sag during the actual period of fault. As soon as breakers open to clear the

fault, normal voltage will be restarted at the customer's end. Fig 4 is a plot of rms voltage versus

time and the waveform characteristics at the customer's location for one of these fault conditions.

This waveform is typical of the customer voltage during a fault on a parallel feeder

circuit that is cleared quickly by the substation breaker. The total duration of fault is 150m sec.

The voltage during a fault on a parallel feeder will depend on the distance from the substation to

fault point. A fault close to substation will result in much more significant sag than a fault near

the end of feeder. Fig 5 shows the voltage sag magnitude at

the plant bus as a function of fault location for an example system.

A single line to ground fault condition results in a much less severe voltage sag than 3-

phase fault Condition due to a delta--star transformer connection at the plant. Transmission

related voltage sags are normally much more consistent than those related to distribution.

Because of large amounts of energy associated with transmission faults, they are cleared as soon

as possible.

This normally corresponds to 3-6 cycles, which is the total time for fault detection and

breaker operation Normally customers do not experience an interruption for transmission

fault. Transmission systems are looped or networked, as distinct from radial distribution

systems. If a fault occurs as shown on the 115KV system, the protective relaying will sense

the fault and breakers A and B will open to clear the fault. While the fault is on the transmission

system, the entire power system, including the distribution system will experience

Voltage sag. Fig 6 shown the magnitude of measured voltage sags at an industrial plant supplied

from a 115 kV system. Most of the voltages were 10-30% below nominal voltage, and no

momentary interrupts were measured at the plant during the monitoring period (about a year).

Fig7 given a three-dimensional plot illustrating the number of sags experienced as a function of

both the voltage sag magnitude and the duration.

This is a convenient way to completely characterize the actual or expected voltage sag

conditions at a site. Evaluating the impact of voltage sags at a customer plant involves

estimating the member of voltage sags that can be expected as a function of the voltage sag

magnitude and then comparing this with equipment sensitivity.

The estimate of voltage sag performance are developed by performing short-circuit

simulations to determine the plant voltage as a function of fault location throughout the power

system. Total circuit miles of line exposure that can affect the plant (area of vulnerability) are

determined for a particular sag level.

Historical fault performance (fault per year per 100 miles) can, then be used to

estimate the number of sags per year that can be expected below the magnitude. A chart such as

the one in fig 8. Can be drawn in splitting the expected number of voltage sags by magnitude.

This information can be used directly by the customers to determine the need for power

conditioning equipment at sensitive loads in the plant.

Voltage-Sag Analysis- Methodology

The methodology is outlined in chapter9 (proposed) of IEEE Gold book (IEEE standard

493, Recommended practice for the design of reliable industrial and commercial power system)

The methodology basically consists of the following four steps:

Load Flow:

A load flow representing the existing or modified system is required with an accurate zero-

sequence representation. The machine reactance Xd" or Xd ' is also required. The reactance

used is dependent upon the post fault time frame of interest. The machine and zero-sequence

reactance are not required to calculate the voltage sag magnitude.

Voltage Sag Calculation:

Sliding faults which include line-line, line to ground, line to line- to ground and three phase are

applied to all the lines in the load flow. Each line is divided into equal sections and each section

is faulted as shown in fig 9.

Voltage Sag Occurrence Calculation:

Based upon the utilities reliability data (the number of times each line section will

experience a fault) and the results of load flow and voltage sag calculations, the number of

voltage sags at the customer site due to remote faults can be calculated. Depending upon the

equipment connection, the voltage sag occurrence rate may be calculated in terms of either phase

or line voltages dependent upon the load connection. For some facilities, both line and phase

voltages may be required. The data thus obtained from load flow, Voltage sag calculation,

and voltage sag occurrence calculation can be sorted and tabulated by sag magnitude,

fault type, location of fault and nominal system voltage at the fault location

Study of Results of Sag- Analysis:

The results can be tabulated and displayed in many different ways to recognize difficult aspects.

Area of vulnerability can be plotted on a geographical map or one - line diagram (fig 9). These

plots can be used to target transmission and distribution lines for enhancements in reliability.

Further bar charts, and pie-charts showing the total number of voltage sags with reference to

voltage level at fault point, area/zone of fault, or the fault type can be developed to help

utilities focus on their system improvements (figs. 10 and 11) To examining the existing

system, system modifications aimed at mitigating or reducing voltage sags can also be identified,

thus enabling cost benefits analysis. Possible such system structural changes that can be

identified include.

Reconnection of a customer from one voltage level to another, Installation of Ferro-resonant

transformers or time delayed under voltage, drop out relay to facilitate easy ride - through the sag

Application of static transfer switch and energy storage system., Application of fast acting

synchronous condensers, Neighborhood generation capacity addition , Increase service voltage

addition through transformer tap changing, By enhancement of system reliability

Equipment Sensitivity Studies:

Process controllers can be very sensitive to voltage sags. An electronic component

manufacturer was experiencing problems with large chiller motors tripping off-line during

voltage sag conditions. A 15VA process controller which regulates water temperature was

thought to be causing individual chillers to trip. This controller was tested using a voltage

sag simulator for voltage sags from 0.5-1000 cycles in duration. The controller was found

to be very sensitive to voltage sags tripping at around 80% of voltage regardless of duration.

B Chip Testers:

Electronic chip testers are very sensitive to voltage variations, and because of the complexity

involved, often require 30 minutes or more to restart. In addition, the chips involved in the

testing process can

be damaged and several days' later internal electronic circuit boards in the testers may fail. A

chip tester consists of a collection of electronic loads, printers, computers, monitors etc. If any

one component of the total package goes down, the entire testing process is disrupted. The chip

testers can be 50KVA or larger in size.

C.DC Drives:

DC drives are used in many industrial proc esses, including printing presses and plastics

manufacturing. The plastic extrusion process is one of the common applications where

voltage sag can be particularly important. The extruders melt and grind plastic pellets into liquid

plastic. The liquid plastic may then be blowup into a bag or processed in some other way before

winder winds the plastic into spools. During voltage sag, the controls to the D.C. drives and

winders may trip. These operations are typically completely automated and an interruption

can cause very expensive cleanup and restarting requirements. Losses may be of the order of

Rs. 15 lakhs / event and a plant fed from a distribution system is likely to experience at least one

event per month. Extra ders begin to have problems when the voltage sags to only 88% of

normal, which indicates a very high level of sensitivity. Faults May miles away from the plant

will cause voltage sags down to 88% level. Even protecting only the winders and controls does

not serve the purpose always. When they are protected and voltage sag occurs, the controls and

winders continue to work properly. However, the dc drives slow down. For severe voltage dips,

the slowing down is so much that the process is interrupted. Therefore D.C. drives also need to

be helped to ride through all voltage sags.

D .Programmable Logic Controllers.

Their overall sensitivity to voltage sags varies greatly by portions of an overall PLC system

have been found to be very sensitive The remote I/O units have been found to trip for

voltages as high as 90% for a few cycles.

E. Machine Tools: Robots or complicated machines used in cutting, drilling and metal

processing can be very sensitive to voltage variation. Any variation in voltage can affect the

quality of the part that is being machined. Robots generally need very constant voltage to

operate properly and safely. Any voltage fluctuations especially sags. May cause unsafe

operation of robot. Therefore these types of machines re often set to trip at voltage levels of only

90%

Solutions to Voltage Sag Problems:

Efforts by utilities and customers can reduce the number and severity of sags.

A. Utility solutions: Utilities can take two main steps to reduce the detrimental effects of sags –

(1) Prevent fault

(2) Improve fault clearing methods

Fault prevention methods include activities like tree trimming, adding line arrests, washing

insulators and installing animal guards. Improved fault clearing practices include activities like

adding line recloses, eliminating fast tripping, adding loop schemes and modifying feeder design.

These may reduce the number and /or duration of momentary interruptions and voltage sags but

faults cannot be eliminated completely.

B. Customer solutions: Power conditioning is the general concept behind these methods. Fig 12

is a schematic f the general approach used.

Power conditioning helps to

1. Isolate equipment from high frequency noise and transients.

2. Provide voltage sag ride through capability

The following are some of the solutions available to provide ride - through capability to

critical loads.

Motor generator sets (M-G sets)

Uninterruptible Power supply (UPS's)

Ferro resonant, constant voltage transformers (CVT's)

Magnetic synthesizers

Super conducting storage devices (SSD's)

MG sets usually utilize flying wheels for energy storage. They completely decouple the loads

from electric power system Relational energy in the flywheel provides voltage regulation and

voltage support during under voltage conditions. MG sets have relatively high efficiency and

low initial cost. UPS's (Fig.13): utilize batteries to store energy which is converted to usable

form during an outage or voltage sag UPS technology is well established and there are many

UPS configurations to choose

From.

CTS can be used to enhance voltage sag ride through capability. CVT's are basically 1;

transformers which are excited high on their saturation curves, thereby supplying output voltage

which is fairly independent of input voltage variations. Magnetic synthesizers are generally used

for larger loads. A load of at least several KVA is needed to make these units cost effective.

They are often used to protect large computers and other sensitive electronic equipment, This is

an electromagnetic device which generates a clean three phase ac output way form regardless of

input power quality (Fig . 14) SSD's utilize a super conducting magnet (Fig.15) store energy in

the same way a UPS uses batteries to store energy. SSD's occupy less space and use fewer

electrical connections as compared to UPS's thus promising better reliability. They are also

expected to become economically competitive.

Economic Evaluation

If the less-expensive solutions mentioned in this brief are not effective, the next step is to evaluate the life-cycle costs and effectiveness of voltage sag mitigation technologies. This task can be very challenging and tends to be beyond the expertise of most industrial facility managers. This type of evaluation requires an analysis of the costs of your voltage sag problems in terms of downtime and lost production, the costs of the devices, and an Understanding of how the mitigation devices work, including partial solutions. A good place to start in performing this type of analysis is to ask your utility or a power quality consultant for assistance. Many utilities offer power quality mitigation services or can refer you to outside specialists

What is a Harmonic?

The typical definition for a harmonic is “a sinusoidal component of a periodic wave or\

quantity having a frequency that is an integral multiple of the fundamental frequency.” [1]. Some

references refer to “clean” or “pure” power as those without any harmonics. But such clean

waveforms typically only exist in a laboratory. Harmonics have been around for a long time and

will continue to do so. In fact, musicians have been aware of such since the invention of the first

string or woodwind instrument. Harmonics (called “overtones” in music) are responsible for

what makes a trumpet sound like a trumpet, and a clarinet like a clarinet.

Electrical generators try to produce electric power where the voltage waveform has only

one frequency associated with it, the fundamental frequency. In the North America, this

frequency is 60 Hz, or cycles per second. In European countries and other parts of the world, this

frequency is usually 50 Hz. Aircraft often uses 400 Hz as the fundamental frequency. At 60 Hz,

this means that sixty times a second, the voltage waveform increases to a maximum positive

value, then decreases to zero, further decreasing to a maximum negative value, and then back to

zero. The rate at which these changes occur is the trigometric function called a sine wave, as

shown in figure 1. This function occurs in many natural phenomena, such as the speed of a

pendulum as it swings back and forth, or the way a string on a voilin vibrates when plucked.

Fig1. Sine wave

The frequency of the harmonics is different, depending on the fundamental frequency.

For example, the 2nd harmonic on a 60 Hz system is 2*60 or 120 Hz. At 50Hz, the second

harmonic is 2* 50 or 100Hz.

300Hz is the 5th harmonic in a 60 Hz system, or the 6th harmonic in a 50 Hz system.

Figure 2 shows how a signal with two harmonics would appear on an oscilloscope-type display,

which some power quality analyzers provide.

Figure2. Fundamental with two harmonics

In order to be able to analyze complex signals that have many different frequencies

present, a number of mathematical methods were developed. One of the more popular is called

the Fourier Transform. However, duplicating the mathematical steps required in a

microprocessor or computer-based instrument is quite difficult. So more compatible processes,

called the FFT for Fast Fourier transform, or DFT for Discrete Fourier Transform, are used.

These methods only work properly if the signal is composed of only the fundamental and

harmonic frequencies in a certain frequency range (called the Nyquist frequency, which is one-

half of the sampling frequency). The frequency values must not change during the measurement

period. Failure of these rules to be maintained can result in mis-information. For example, if a

voltage waveform is comprised of 60 Hz and 200 Hz signals, the FFT cannot directly see the 200

Hz. It only knows 60, 120, 180, 240,..., which are often called “bins”. The result would be that

the energy of the 200 Hz signal would appear partially in the 180Hz bin, and partially in the 240

Hz bin. An FFT-based processer could show a voltage value of 115V at 60 Hz, 18 V at the 3rd

harmonic, and 12 V at the 4th harmonic, when it really should have been 30 V at 200 Hz.

These in-between frequencies are called “inter harmonics”. There is also a special

category of inter harmonics, which are frequency values less than the fundamental frequency

value, called sub-harmonics. For example, the process of melting metal in an electric arc furnace

can result large currents that are comprised of the fundamental , inter harmonic, and sub

harmonic frequencies being drawn from the electric power grid. These levels can be quite high

during the melt-down phase, and usually effect the voltage waveform.

Why Worry About Them

The presence of harmonics does not mean that the factory or office cannot run properly.

Like other power quality phenomena, it depends on the “stiffness” of the power distribution

system and the susceptibility of the equipment. As shown below, there are a number of different

types of equipment that can have mis operations or failures due to high harmonic voltage and/or

current levels. In addition, one factory may be the source of high harmonics but able to run

properly. This harmonic pollution is often carried back onto the electric utility distribution

system, and may effect facilities on the same system which are more susceptible.

Some typical types of equipment susceptible to harmonic pollution include: - Excessive

neutral current, resulting in overheated neutrals. The odd triplen harmonics in three phase wye

circuits are actually additive in the neutral. This is because the harmonic number multiplied by

the 120 degree phase shift between phases is a integer multiple of 360 degrees. This puts the

harmonics from each of the three phase legs “in-phase” with each other in the neutral, as shown

in Figure 3.

Figure3. Additive Third Harmonics

- Incorrect reading meters, including induction disc W-hr meters and averaging type current

meters.

- Reduced true PF, where PF= Watts/VA.

- Overheated transformers, especially delta windings where triplen harmonics generated on the

load side of a delta-wye transformer will circulate in the primary side. Some type of losses go up

as the square of harmonic value (such as skin effect and eddy current losses). This is also true for

solenoid coils and lighting ballasts.

- Zero, negative sequence voltages on motors and generators. In a balanced system, voltage

harmonics can either be positive (fundamental, 4th, 7th,...), negative (2nd, 5th, 8th...) or zero

(3rd, 6th, 9th,...) sequencing values. This means that the voltage at that particular frequency tries

to rotate the motor forward, backward, or neither (just heats up the motor), respectively. There is

also heating from increased losses as in a transformer.

Table3. Harmonic Sequencing Values in Balanced Systems

- Nuisance operation of protective devices, including false tripping of relays and failure of a UPS

to transfer properly, especially if controls incorporate zero-crossing sensing circuits.

- Bearing failure from shaft currents through un insulated bearings of electric motors.

- Blown-fuses on PF correction caps, due to high voltage and currents from resonance with line

impedance.

- Mis-operation or failure of electronic equipment

- If there are voltage sub harmonics in the range of 1-30Hz, the effect on lighting is called

flicker. This is especially true at 8.8Hz, where the human eye is most sensitive, and just 0.5%

variation in the voltage is noticeable with some types of lighting. [2]

Where They Come From

How this electricity is used by the different type of loads can have an effect on “purity”

of the voltage waveform. Some loads cause the voltage and current waveforms to lose this pure

sine wave appearance and become distorted. This distortion may consist of predominately

harmonics, depending on the type of load and system impedances.

Since this article is about harmonics, we will concentrate on those types of sources.

“The main sources of harmonic current are at present the phase angle controlled rectifiers and

inverters.” [3] These are often called static power converters. These devices take AC power and

convert it to another form, sometimes back to AC power at the same or different frequency,

based on the firing scheme. The firing scheme refers to the controlling mechanism that

determines how and when current is conducted. One major variation is the phase angle at which

conduction begins and ends.

A typical such converter is the switching-type power supplies found in most personal

computers and peripheral equipment, such as printers. While they offer many benefits in size,

weight and cost, the large increase of this type of equipment over the past fifteen years is largely

responsible for the increased attention to harmonics.

Figure shows below how a switching-type power supply works. The AC voltage is

converted into a DC voltage, which is further converted into other voltages that the equipment

needs to run. The rectifier consists of semi-conductor devices (such as diodes) that only conduct

current in one direction. In order to do so, the voltage on the one end must be greater than the

other end. These devices feed current into a capacitor, where the voltage value on the cap at any

time depends on how much energy is being taken out by the rest of the power supply.

When the input voltage value is higher than voltage on the capacitor, the diode will conduct

current through it. This results in a current waveform as shown in Figure 5, and harmonic

spectrum in Figure 6. Obviously, this is not a pure sinusoidal waveform with only a 60 Hz

frequency component.

Figure5. Current Waveform

Figure6. Harmonic Spectrum of Current Waveform Shown in Figure 5

If the rectifier had only been a half wave rectifier, the waveform would only have every

other current pulse, and the harmonic spectrum would be different, as shown in Figure 7.

Fluorescent lights can be the source of harmonics, as the ballasts are non-linear inductors.

The third harmonic is the predominate harmonic in this case. (See Table 3) As previously

mentioned, the third harmonic current from each phase in a four-wire wye or star system will be

additive in the neutral, instead of cancelling out Some of the newer electronic ballasts have very

significant harmonic problems, as they operate somewhat like a switching power supply, but can

result in current harmonic distortion levels over 30%.

Table3. Sample of Harmonic Values for Fluorescent lighting [4]

Low power, AC voltage regulators for light dimmers and small induction motors adjust

the phase angle or point on the wave where conduction occurs. Medium power converters are

used for motor control in manufacturing and railroad applications, and include such equipment as

ASDs (adjustable speed drives) and VFDs (variable frequency drives). Metal reduction

operations, like electric arc furnaces, and high voltage DC transmission employ large power

converters, in the 2-20MVA rating.

This type of 3-phase equipment may also cause other types of power quality problems.

When the semiconductor device is suppose to turn-off, it does not do so abruptly. This happens

under “naturally” commutated conditions, where the voltage that was larger on the anode side

compared to the cathode is now the opposite. This occurs each cycle as the voltage waveform

goes through the sine waveform. It also happens under “forced” commutation conditions, where

the semi-conductor device has a “gate”-type control mechanism built in to it. This commutation

period is a time when two semiconductor devices are both conducting current at the same time,

effectively shorting one phase to the other and resulting in large current transients.

When transformers are first energized, the current drawn is different from the steady state

condition. This is caused by the inrush of the magnetizing current. The harmonics during this

period varies over time. Some harmonics have zero value for part of the time, and then increase

for a while before returning to zero. An unbalanced transformer (where either the output current,

winding impedance or input voltage on each leg are not equal) will cause harmonics, as will

overvoltage saturation of a transformer.

Where to look for them

Wherever the aforementioned equipment is used, one can suspect that harmonics are

present. The amount of voltage harmonics will often depend on the amount of harmonic currents

being drawn by the load, and the source impedance, which includes all of the wiring and

transformers back to the source of the electricity. Ohm’s Law says that Voltage equals Current

multipled by Impedance. This is true for harmonic values as well. If the source harmonic

impedance is very low (often referred to as a “stiff” system) then the harmonic currents will

result in lower harmonic voltages than if the source impedance were high (such as found with

some types of isolation transformers).

Like any power quality investigation, the search can begin at the equipment effected by

the problem or at the point-of-common-coupling (PCC), where the utility service meets the

building distribution system. If only one piece of equipment is effected (or suspected), it is often

easier to start the monitoring process there. If the source is suspected to be from the utility

service side (such is the case when there is a neighboring factory that is known to generate high

harmonics), then monitoring usually begins at the PCC.

The phase voltages and currents, as well as the neutral-to-ground voltage and neutral

current should be monitored, where possible. This will aid in pinpointing problems, or detecting

marginal systems. Monitoring the neutral will often show a high 3rd harmonic value, indicating

the presence of non-linear loads in the facility.

How do you find them

Hand-held harmonic meters can be useful tools for making spot checks for known

harmonic problems. However, harmonic values will often change during the day, as different

loads are turned on and off within the facility or in other facilities on the same electric utility

distribution system. This requires the use of a harmonic monitor or power quality monitor with

harmonic capabilities (such as shown in Figure 8), which can record the harmonic values over a

period of time.

Figure8. Power Quality Monitor with Harmonic Analysis

Typically, monitoring will last for one business cycle. A business cycle is how long it

takes for the normal operation of the plant to repeat itself. For example, if a plant runs three

identical shifts, seven days a week, then a business cycle would be eight hours. More typically, a

business cycle is one week, as different operations take place on a Monday, when the plant

equipment is restarted after being off over the weekend, then on a Wednesday, or a Saturday,

when only a Skelton crew may be working.

Certain types of loads also generate typical harmonic spectrum signatures that can point

the investigator towards the source. This is related to the number of pulses, or paths of

conduction. The general equation is h = ( n * p ) +/- 1, where h is the harmonic number, n is any

integer (1,2,3,..) and p is the number of pulses in the circuit, and the magnitude decreases as the

ration of 1/h (1/3, 1/5, 1/7, 1/9,...). Table 4 shows examples of such.

Table4. Typical Harmonics Found for Different Converters.

When are they a problem?

Most electrical loads (except half-wave rectifiers) produce symmetrical current

waveforms, which mean that the positive half of the waveform looks like a mirror image of the

negative half. This results in only odd harmonic values being present. Even harmonics will

disrupt this half-wave symmetry. The presence of these even harmonics should cause the

investigator to suspect there is a half-wave rectifier on the circuit. This also results from a full

wave rectifier when one side of the rectifier has blown or damaged components. Early detection

of this condition in a UPS system can prevent a complete failure when the load is switched onto

back-up power.

To determine what is normal or acceptable levels, a number of standards have been

developed by various organizations. ANSI/IEEE C57.110 Recommended Practice for

Establishing Transformer Compatibility When Supplying No sinusoidal Load Currents is a

useful document for determining how much a transformer should be derated from its nameplate

rating when operating in the presence of harmonics. There are two parameters typically used,

called K-factor and TDF (transformer dereading factor). Some power quality harmonic monitors

will automatically calculate these values.

IEEE 519-1992 Recommended Practices and Requirements for Harmonic Control in

Electrical Power Systems provides guidelines from determining what acceptable limits are. The

harmonic limits for current depend on the ratio of Short Circuit Current (SCC) at PCC (or how

stiff it is) to average Load Current of maximum demand over 1 year, as illustrated in Table 5.

Note how the limit decreases at the higher harmonic values, and increases with larger ratios.

Table5. Current Harmonic Limits as per IEEE 519-1992

For voltage harmonics, the voltage level of the system is used to determine the limits, as

shown in Table 6. At the higher voltages, more customers will be effective, hence, the lower

limits.

Table6. Voltage Harmonic Limits as per IEEE 519-1992

The European Community has also developed susceptibility and emission limits for\

harmonics. Formerly known as the 555-2 standard for appliances of less than 16 A, a more

encompassing set of standards under IEC 1000-4-7 are now in effect.

How do you get rid of them?

Care should be undertaken to make sure that the corrective action taken to minimize the

harmonic problems don’t actually make the system worse. This can be the result of resonance

between harmonic filters, PF correcting capacitors and the system impedance.

Isolating harmonic pollution devices on separate circuits with or without the use of

harmonic filters are typical ways of mitigating the effects of such. Loads can be relocated to try

to balance the system better. Neutral conductors should be properly sized according to the latest

NEC-1996 requirements covering such. Whereas the neutral may have been undersized in the

past, it may now be necessary to run a second neutral wire that is the same size as the phase

conductors. This is particularly important with some modular office partition-type walls, which

can exhibit high impedance values. The operating limits of transformers and motors should be

derated, in accordance with industry standards from IEEE, ANSI and NEMA on such. Use of

higher pulse converters, such as 24-pulse rectifiers, can eliminate lower harmonic values, but at

the expense of creating higher harmonic values.

TOTAL HARMONIC DISTORTION

Harmonic problems are almost always introduced by the consumers’ equipment and installation

practices. Harmonic distortion is caused by the high use of non-linear load equipment such as

computer power supplies, electronic ballasts, compact fluorescent lamps and variable speed

drives etc, which create high current flow with harmonic frequency components. The limiting

rating for most electrical circuit elements is determined by the amount of heat that can be

dissipated to avoid overheating of bus bars, circuit breakers, neutral conductors, transformer

windings or generator alternators.

Definition

THD is defined as the RMS value of the waveform remaining when the fundamental is removed.

A perfect sine wave is 100%, the fundamental is the system frequency of 50 or 60Hz. Harmonic

distortion is caused by the introduction of waveforms at frequencies in multiplies of the

fundamental ie: 3rd harmonic is 3x the fundamental frequency / 150Hz. Total harmonic

distortion is a easurement of the sum value of the waveform that is distorted.

Power Measurement

Despite the use of good quality test meter instrumentation, high current flow can often remain

undetected or under estimated by as much 40%. This severe underestimation causes overly high

running temperatures of equipment and nuisance tripping. This is simply because the average

reading test meters commonly used by maintenance technicians, are not designed to accurately

measure distorted currents, and can only provide indication of the condition of the supply at the

time of checking. Power quality conditions change continuously, and only instruments offering

true RMS measurement of distorted waveforms and neutral currents can provide the correct

measurements to accurately determine the ratings of cables, bus bars and circuit breakers.

Neutral Currents

High harmonic environments can produce unexpected and dangerous neutral currents. In a

balanced system, the fundamental currents will cancel out, but, triple- N’s will add, so harmonic

currents at the 3rd, 9th, 15th etc. will flow in the neutral. Traditional 3 phase system meters are

only able to calculate the vector of line to neutral current measurements, which may not register

the true reading. Integra 1530, 1560 and 1580 offer a 3 phase 4 wire version with a neutral 4th

CT allowing true neutral current measurement and protection in high harmonic environments.

Harmonic Profiles

There is much discussion over the practical harmonic range of a measurement instrument,

however study of the harmonic profiles of typically installed equipment can guide the system

designer to the practical solution. A typical harmonic profile graph will show a logarithmic

decay as the harmonic frequency increases. It is necessary to establish the upper level at which

the harmonic content is negligible.

For Example:

A laptop switch mode power supply causes approximately 25% of 3rd harmonic, 19% of 5th

harmonic, 10% of 7th harmonic and 5% of 9th harmonic etc. Therefore it can be seen that almost

all the harmonic content in an IT dominated load will be below the 15th harmonic. In a 3 phase

load incorporating 6 pulse bridge technology as is common in many variable speed drives, UPS

systems and DC converters, similar profiles will be observed but extending to the 25th and 27th

harmonic. It can therefore be deduced

that in the majority of industrial and commercial applications an instrument measuring up to the

31st harmonic is ideal.

Pulse width Modulation

What is PWM?

Pulse Width Modulation (PWM) is the most effective means to achieve constant

voltage battery charging by switching the solar system controller’s power devices. When in

PWM regulation, the current from the solar array tapers according to the battery’s condition and

recharging needs Consider a waveform such as this: it is a voltage switching between 0v and

12v. It is fairly obvious that, since the voltage is at 12v for exactly as long as it is at 0v, then a

'suitable device' connected to its output will see the average voltage and think it is being fed 6v -

exactly half of 12v. So by varying the width of the positive pulse - we can vary the 'average'

voltage.

Similarly, if the switches keep the voltage at 12 for 3 times as long as at 0v, the average

will be 3/4 of 12v - or 9v, as shown below.

and if the output pulse of 12v lasts only 25% of the overall time, then the average is

By varying - or 'modulating' - the time that the output is at 12v (i.e. the width of the

positive pulse) we can alter the average voltage. So we are doing 'pulse width modulation'. I said

earlier that the output had to feed 'a suitable device'. A radio would not work from this: the radio

would see 12v then 0v, and would probably not work properly. However a device such as a

motor will respond to the average, so PWM is a natural for motor control.

Pulse Width modulator

So, how do we generate a PWM waveform? It's actually very easy, there are circuits

available in the TEC site. First you generate a triangle waveform as shown in the diagram below.

You compare this with a d.c voltage, which you adjust to control the ratio of on to off time that

you require. When the triangle is above the 'demand' voltage, the output goes high. When the

triangle is below the demand voltage, the

When the demand speed it in the middle (A) you get a 50:50 output, as in black. Half the

time the output is high and half the time it is low. Fortunately, there is an IC (Integrated circuit)

called a comparator: these come usually 4 sections in a single package. One can be used as the

oscillator to produce the triangular waveform and another to do the comparing, so a complete

oscillator and modulator can be done with half an IC and maybe 7 other bits.

The triangle waveform, which has approximately equal rise and fall slopes, is one of the

commonest used, but you can use a saw tooth (where the voltage falls quickly and rinses slowly).

You could use other waveforms and the exact linearity (how good the rise and fall are) is not too

important.

Traditional solenoid driver electronics rely on linear control, which is the application of a

constant voltage across a resistance to produce an output current that is directly proportional to

the voltage. Feedback can be used to achieve an output that matches exactly the control signal.

However, this scheme dissipates a lot of power as heat, and it is therefore very inefficient.

A more efficient technique employs pulse width modulation (PWM) to produce the

constant current through the coil. A PWM signal is not constant. Rather, the signal is on for part

of its period, and off for the rest. The duty cycle, D, refers to the percentage of the period for

which the signal is on. The duty cycle can be anywhere

from 0, the signal is always off, to 1, where the signal is constantly on. A 50% D results in a

perfect square wave. (Figure 1)

A solenoid is a length of wire wound in a coil. Because of this configuration, the solenoid

has, in addition to its resistance, R, a certain inductance, L. When a voltage, V, is applied across

an inductive element, the current, I, produced in that element does not jump up to its constant

value, but gradually rises to its maximum over a period of time called the rise time (Figure 2).

Conversely, I does not disappear instantaneously, even if V is removed abruptly, but decreases

back to zero in the same amount of time as

the rise time.

Therefore, when a low frequency PWM voltage is applied across a solenoid, the current

through it will be increasing and decreasing as V turns on and off. If D is shorter than the rise

time, I will never achieve its maximum value, and will be discontinuous since it will go back to

zero during V’s off period (Figure 3).* In contrast, if D is larger than the rise time, I will never

fall back to zero, so it will be continuous, and have a DC average value. The current will not be

constant, however, but will have a ripple (Figure 4).

At high frequencies, V turns on and off very quickly, regardless of D, such that the

current does not have time to decrease very far before the voltage is turned back on. The

resulting current through the solenoid is therefore considered to be constant. By adjusting the D,

the amount of output current can be controlled. With a small D, the current will not have much

time to rise before the high frequency PWM voltage takes effect and the current stays constant.

With a large D, the current will be able to rise higher before it becomes constant. (Figure 5)

Dither

Static friction, stiction, and hysteresis can cause the control of a hydraulic valve to be

erratic and unpredictable. Stiction can prevent the valve spool from moving with small input

changes, and hysteresis can cause the shift to be different for the same input signal. In order to

counteract the effects of stiction and hysteresis, small vibrations about the desired position are

created in the spool. This constantly breaks the static friction ensuring that it will move even

with small input changes, and the effects of hysteresis are average out.

Dither is a small ripple in the solenoid current that causes the desired vibration and there

by increases the linearity of the valve. The amplitude and frequency of the dither must be

carefully chosen. The amplitude must be large enough and the frequency slow enough that the

spool will respond, yet they must also be small and fast enough not to result in a pulsating

output.

The optimum dither must be chosen such that the problems of stiction and hysteresis are

overcome without new problems being created. Dither in the output current is a byproduct of low

frequency PWM, as seen above. However, the frequency and amplitude of the dither will be a

function of the duty cycle, which is also used to set the output current level. This means that low

frequency dither is not independent of current magnitude. The advantage of using high frequency

PWM is that dither can be generated separately, and then superimposed on top of the output

current.

This allows the user to independently set the current magnitude (by adjusting the D), as

well as the dither frequency and amplitude. The optimum dither, as set by the user, will therefore

be constant at all current levels.

Why the PWM frequency is important:

The PWM is a large amplitude digital signal that swings from one voltage extreme to the

other. And, this wide voltage swing takes a lot of filtering to smooth out. When the PWM

frequency is close to the frequency of the waveform that you are generating, then any PWM

filter will also smooth out your generated waveform and drastically reduce its amplitude. So, a

good rule of thumb is to keep the PWM frequency much higher than the frequency of any

waveform you generate.

Finally, filtering pulses is not just about the pulse frequency but about the duty cycle and

how much energy is in the pulse. The same filter will do better on a low or high duty cycle pulse

compared to a 50% duty cycle pulse. Because the wider pulse has more time to integrate to a

stable filter voltage and the smaller pulse has less time to disturb it the inspiration was a request

to control the speed of a large positive displacement fuel pump. The pump was sized to allow full

power of a boosted engine in excess of 600 Hp.

At idle or highway cruise, this same engine needs far less fuel yet the pump still normally

supplies the same amount of fuel. As a result the fuel gets recycled back to the fuel tank,

unnecessarily heating the fuel. This PWM controller circuit is intended to run the pump at a low

speed setting during low power and allow full pump speed when needed at high engine power

levels.

Motor Speed Control (Power Control)

Typically when most of us think about controlling the speed of a DC motor we think of

varying the voltage to the motor. This is normally done with a variable resistor and provides a

limited useful range of operation. The operational range is limited for most applications

primarily because torque drops off faster than the voltage drops.

Most DC motors cannot effectively operate with a very low voltage. This method also

causes overheating of the coils and eventual failure of the motor if operated too slowly. Of

course, DC motors have had speed controllers based on varying voltage for years, but the range

of low speed operation had to stay above the failure zone described above.

Additionally, the controlling resistors are large and dissipate a large percentage of energy

in the form of heat. With the advent of solid state electronics in the 1950’s and 1960’s and this

technology becoming very affordable in the 1970’s & 80’s the use of pulse width modulation

(PWM) became much more practical. The basic concept is to keep the voltage at the full value

and simply vary the amount of time the voltage is applied to the motor windings. Most PWM

circuits use large transistors to simply allow power On & Off, like a very fast switch.

This sends a steady frequency of pulses into the motor windings. When full power is

needed one pulse ends just as the next pulse begins, 100% modulation. At lower power settings

the pulses are of shorter duration. When the pulse is On as long as it is Off, the motor is

operating at 50% modulation. Several advantages of PWM are efficiency, wider operational

range and longer lived motors. All of these advantages result from keeping the voltage at full

scale resulting in current being limited to a safe limit for the windings.

PWM allows a very linear response in motor torque even down to low PWM% without

causing damage to the motor. Most motor manufacturers recommend PWM control rather than

the older voltage control method. PWM controllers can be operated at a wide range of

frequencies. In theory very high frequencies (greater than 20 kHz) will be less efficient than

lower frequencies (as low as 100 Hz) because of switching losses.

The large transistors used for this On/Off activity have resistance when flowing current, a

loss that exists at any frequency. These transistors also have a loss every time they “turn on” and

every time they “turn off”. So at very high frequencies, the “turn on/off” losses become much

more significant. For our purposes the circuit as designed is running at 526 Hz. Somewhat of an

arbitrary frequency, it works fine.

Depending on the motor used, there can be a hum from the motor at lower PWM%. If

objectionable the frequency can be changed to a much higher frequency above our normal

hearing level (>20,000Hz) .

PWM Controller Features:

This controller offers a basic “Hi Speed” and “Low Speed” setting and has the option to

use a “Progressive” increase between Low and Hi speed. Low Speed is set with a trim pot inside

the controller box. Normally when installing the controller, this speed will be set depending on

the minimum speed/load needed for the motor. Normally the controller keeps the motor at this

Lo Speed except when Progressive is used and when Hi Speed is commanded (see below). Low

Speed can vary anywhere from 0% PWM to 100%.

Progressive control is commanded by a 0-5 volt input signal. This starts to increase PWM

% from the low speed setting as the 0-5 volt signal climbs. This signal can be generated from a

throttle position sensor, a Mass Air Flow sensor, a Manifold Absolute Pressure sensor or any

other way the user wants to create a 0-5 volt signal. This function could be set to increase fuel

pump power as turbo boost starts to climb (MAP sensor). Or, if controlling a water injection

pump, Low Speed could be set at zero PWM% and as the TPS signal climbs it could increase

PWM%, effectively increasing water flow to the engine as engine load increases. This controller

could even be used as a secondary injector driver (several injectors could be driven in a batch

mode, hi impedance only), with Progressive control (0-100%) you could control their output for

fuel or water with the 0-5 volt signal.

Progressive control adds enormous flexibility to the use of this controller. Hi Speed is

that same as hard wiring the motor to a steady 12 volt DC source. The controller is providing

100% PWM, steady 12 volt DC power. Hi Speed is selected three different ways on this

controller: 1) Hi Speed is automatically selected for about one second when power goes on. This

gives the motor full torque at the start. If needed this time can be increased ( the value of C1

would need to be increased). 2) High Speed can also be selected by applying 12 volts to the High

Speed signal wire. This gives Hi Speed regardless of the Progressive signal.

When the Progressive signal gets to approximately 4.5 volts, the circuit achieves 100%

PWM – Hi Speed.

How does this technology help ?:

The benefits noted above are technology driven. The more important question is how the PWM

technology Jumping from a 1970’s technology into the new millennium offers:

• Longer battery life:

– reducing the costs of the solar system

– reducing battery disposal problems

• More battery reserve capacity:

– increasing the reliability of the solar system

– reducing load disconnects

– opportunity to reduce battery size to lower

the system cost

• Greater user satisfaction:

– get more power when you need it for less

money!!

Space Vector PWM

The Space Vector PWM generation module accepts modulation index commands and

generates the appropriate gate drive waveforms for each PWM cycle. This section describes the

operation and configuration of the SVPWM module.

A three-phase 2-level inverter with dc link configuration can have eight possible

switching states, which generates output voltage of the inverter. Each inverter switching state

generates a voltage Space Vector (V1 to V6 active vectors, V7 and V8 zero voltage vectors) in

the Space Vector plane (Figure: space vector diagram). The magnitude of each active vector

(V1to V6) is 2/3 Vdc (dc bus voltage).

The Space Vector PWM (SVPWM) module inputs modulation index commands

(U_Alpha and U_Beta) which are orthogonal signals (Alpha and Beta) as shown in Figure. The

gain characteristic of the SVPWM module is given in Figure . The vertical axis of Figure

represents the normalized peak motor phase voltage (V/Vdc) and the horizontal axis represents

the normalized modulation index (M).

The inverter fundamental line-to-line Rms output voltage (Vline) can be approximated (linear

range) by the following equation:

………….. (1)

Where dc bus voltage (Vdc) is in volts

Space Vector Diagram

This document is the property of International Rectifier and may not be copied or

distributed without expressed consent

Transfer Characteristics

The maximum achievable modulation (Umag_L) in the linear operating range is given

by:

………….. (2)

Over modulation occurs when modulation Umag > Umag_L. This corresponds to the

condition where the voltage vector in (Figure: voltage vector rescaling)increases beyond the

hexagon boundary. Under such circumstance, the Space Vector PWM algorithm will rescale the

magnitude of the voltage vector to fit within the Hexagon limit. The magnitude of the voltage

vector is restricted within the Hexagon; however, the phase angle (θ) is always preserved. The

transfer gain (Figure :transfer characteristics) of the PWM modulator reduces and becomes non-

linear in the over modulation region.

Voltage Vector Rescaling

This document is the property of International Rectifier and may not be copied or

distributed without expressed consent.

PWM Operation

Upon receiving the modulation index commands (UAlpha and UBeta) the sub-module

SVPW M_Tm starts its calculations at the rising edge of the PWM Load signal. The SVPWM

_Tm module implements an algorithm that selects (based on sector determination) the active

space vectors (V1 to V6) being used and calculates the appropriate time duration (w.r.t. one

PWM cycle) for each active vector. The appropriated zero vectors are also being selected. The

SVPWM _Tm module consumes 11 clock cycles typically and 35 clock cycles (worst case Tr) in

over modulation cases. At the falling edge of nSYNC, a new set of Space Vector times and

vectors are readily available for actual PWM generation (PhaseU, PhaseV, PhaseW) by sub

module Pwm Generation. It is crucial to trigger PwmLoad at least 35 clock cycles prior to the

falling edge of nSYNC signal; otherwise new modulation commands will not be implemented at

the earliest PWM cycle.

The above Figures voltage vector rescaling illustrates the PWM waveforms for a voltage

vector locates in sector I of the Space Vector plane (shown in Figure). The gating pattern outputs

(PWMUH … PWMWL) include dead time insertion

3-phase Space Vector PWM

2-phase (6-step PWM) Space Vector PWM

PWM Carrier Period:

Input variable PwmCval controls the duration of a PWM cycle. It should be populated

by the system clock frequency (Clk) and Pwm frequency (PwmFreq) selection. The variable

should be calculated as:

……….. (3)

The input resolution of the Space Vector PWM modulator signals U_Alpha and U_Beta

is 16-bit signed integer. However, the actual PWM resolution (PwmCval) is limited by the

system clock frequency.

Dead time Insertion Logic Dead time is inserted at the output of the PWM Generation

Module. The resolution is 1 clock cycle or 30nsec at a 33.3 MHz clock and is the same as those

of the voltage command registers and the PWM carrier frequency register.

The dead time insertion logic chops off the high side commanded volt*seconds by the

amount of dead time and adds the same amount of volt*seconds to the low side signal. Thus, it

eliminates the complete high side turn on pulse if the commanded volt*seconds is less than the

programmed dead time.

Dead time Insertion

The dead time insertion logic inserts the programmed dead time between two high and

low side of the gate signals within a phase. The dead time register is also double buffered to

allow “on the fly” dead time change and control while PWM logic is inactive.

Symmetrical and Asymmetrical Mode Operation

There are two modes of operation available for PWM waveform generation, namely the

Center Aligned Symmetrical PWM (Figure) and the Center Aligned Asymmetrical PWM

(Figure)The volt-sec can be changed every half a PWM cycle (Tpwm) since Pwm Load occurs

every half a PWM cycle (compare Figure :symmetrical pwm and Figure :asymmetrical PWM).

With Symmetrical PWM mode, the inverter voltage Config = 0), the inverter voltage can be

changed at two times the rate of the switching frequency. This will provide an increase in voltage

control bandwidth, however, at the expense of increased current harmonic

Asymmetrical PWM Mode

Three-Phase and Two-Phase Modulation

Three-phase and two-phase Space Vector PWM modulation options are provided for the

IRMCx203. The Volt-sec generated by the two PWM strategies are identical; however with 2-

phase modulation the switching losses can be reduced significantly, especially when high

switching frequency (>10Khz) is employed. Figure: three-phase and two phase modulation

shows the switching pattern for one PWM cycle when the voltage vector is inside sector 1

Three Phase and Two Phase Modulation

The field Two Phase PWM of the PWM Config write register group provides selection of

three-phase or two-phase modulation. The default setting is three-phase modulation. Successful

operation of two-phase modulation in the entire speed operating range will depend on hardware

configuration. If the gate driver employs a bootstrap power supply strategy, disoperation will

occur at low motor fundamental frequencies (< 2Hz) under two-phase modulation control.

Sinusoidal Pulse Width Modulation

In many industrial applications, Sinusoidal Pulse Width Modulation (SPWM), also called

Sine coded Pulse Width Modulation, is used to control the inverter output voltage. SPWM

maintains good performance of the drive in the entire range of operation between zero and 78

percent of the value that would be reached by square-wave operation. If the modulation index

exceeds this value, linear relationship between modulation index and output voltage is not

maintained and the over-modulation methods are required

Space Vector Pulse Width Modulation

A different approach to SPWM is based on the space vector representation of voltages in

the d, q plane. The d, q components are found by Park transform, where the total power, as well

as the impedance, remains unchanged.

Fig: space vector shows 8 space vectors in according to 8 switching positions of inverter,

V* is the phase-to-center voltage which is obtained by proper selection of adjacent vectors V1

and V2.

Inverter output voltage space vector

Determination of Switching times

The reference space vector V* is given by Equation (1), where T1, T2 are the intervals of

application of vector V1 and V2 respectively, and zero vectors V0 and V7 are selected for T0.

V* Tz = V1 *T1 + V2 *T2 + V0 *(T0/2) + V7 *(T0/2)……….(4)

Space Vector Pulse Width Modulation (continued)

Fig. below shows that the inverter switching state for the period T1 for vector V1 and for

vector V2, resulting switching patterns of each phase of inverter are shown in Fig. pulse pattern

of space vector PWM.

Inverter switching state for (a)V1, (b) V2

Pulse pattern of Space vector PWM

Comparison

In Fig:- comparison, U is the phase to- center voltage containing the triple order

harmonics that are generated by space vector PWM, and U1 is the sinusoidal reference voltage.

But the triple order harmonics are not appeared in the phase-to-phase voltage as well. This leads

to the higher modulation index compared to the SPWM.

Comparison of SPWM and Space Vector PWM

As mentioned above, SPWM only reaches to 78 percent of square wave operation, but the

amplitude of maximum possible voltage is 90 percent of square-wave in the case of space vector

PWM. The maximum phase-to-center voltage by sinusoidal and space vector

PWM are respectively

Vmax = Vdc/2 : Sinusoidal PWM

Vmax = Vdc/√3 : Space Vector PWM

Where, Vdc is DC-Link voltage.

This means that Space Vector PWM can produce about 15 percent higher than

Sinusoidal PWM in output voltage.

SVM PWM Technique

The Pulse Width modulation technique permits to obtain three phase system voltages,

which can be applied to the controlled output. Space Vector Modulation (SVM) principle differs

from other PWM processes in the fact that all three drive signals for the inverter will be created

simultaneously. The implementation of SVM process in digital systems necessitates less

operation time and also less program memory.

The SVM algorithm is based on the principle of the space vector u*, which describes all

three output voltages ua, ub and uc :

u* = 2/3 . ( ua + a . ub + a2 . uc ) ………(5)

Where a = -1/2 + j . v3/2 We can distinguish six sectors limited by eight discrete vectors

u0…u7 (fig:- inverter output voltage space vector), which correspond to the 23 = 8 possible

switching states of the power switches of the inverter.

Space vector Modulation

The amplitude of u0 and u7 equals 0. The other vectors u1…u6 have the same amplitude

and are 60 degrees shifted.

By varying the relative on-switching time Tc of the different vectors, the space vector u*

and also the output voltages ua, ub and uc can be varied and is defined as:

ua = Re ( u* )

ub = Re ( u* . a-1)

uc = Re ( u* . a-2) …………(6)

During a switching period Tc and considering for example the first sector, the vectors u0,

u1 and u2 will be switched on alternatively.

Definition of the Space vector

Depending on the switching times t0, t1 and t2 the space vector u* is defined as:

u* = 1/Tc . ( t0 . u0 + t1 . u1 + t2 . u2 )

u* = t0 . u0 + t1 . u1 + t2 . u2

u* = t1 . u1 + t2 . u2 ………….. (7)

Where

t0 + t1 + t2 = Tc and

t0 + t1 + t2 = 1

t0, t1 and t2 are the relative values of the on switching times.

They are defined as: t1 = m . cos ( a + p/6)

t2 = m . sin a

t0 = 1 - t1 - t2

Their values are implemented in a table for a modulation factor m = 1. Then it will be

easy to calculate the space vector u* and the output voltages ua, ub and uc. The voltage vector

u* can be provided directly by the optimal vector control laws w1, vsa and vsb. In order to

generate the phase voltages ua, ub and uc corresponding to the desired voltage vector u* the

following SVM strategy is proposed.

The Costs

Harmonic currents add to the fundamental load current and can affect revenue billing by

introducing errors into kilowatt hour metering systems, which will directly increase the net

billable kilowatt demand and kilowatt hour consumption charges.

The commercial effects of harmonic distortion to power quality are dramatically shorter

equipment lifetimes, reduced energy efficiency and a susceptibility to nuisance tripping. The

costs of supply interruption are high, however caused, resulting in data corruption, disruption of

process manufacturing and failure of telecommunications facilities etc.

II. MODEL OF THE DVR-CONNECTION SYSTEMAtypical test system, incorporating aDVR, is depicted in Fig. 1. Various kinds of loads are

connected at the point of common coupling (PCC), including a linear load, a nonlinear load, and

a sensitive load. The series connection of the voltage-source converter (VSC) making up the

DVR with the ac system is achieved by means of a coupling transformer whose primary is

connected in series between the mains and the load. Although a passive LC filter is normally

used to obtain a switching-ripple-free DVR voltage, in this paper, this filter is not considered in

order to fully assess the harmonic cancelling properties of the repetitive controller.

Fig. 1. System configuration with a DVR.

Fig. 2. Single-phase equivalent circuit for the DVR

Fig. 2 shows the equivalent circuit for the DVR, where Vs is the supply voltage Zs, Is is the line

impedance, is the current supplied by the source, which splits at the PCC into a current injected

into the sensitive load and a current injected into other loads . The voltage Upcc is the measured

voltage at the PCC, U is the voltage representing the DVR, which is modeled as an ideal voltage

source. Also R,L and are the resistance and inductance of the coupling transformer, respectively,

and is the measured voltage across the sensitive load. The sensitive-load voltage can be obtained

as

III. DESIGN OF THE CONTROL SYSTEM

The aim of the control system is to regulate the load voltage in the presence of various kinds of

disturbances. The control structure proposed in this paper is based on the use of a feedforward

term of the voltage at the PCC to obtain a fast transient response, and a feedback term of the load

voltage to ensure zero error in steady state. The continuous time of the whole control system is

depicted in Fig. 3 where C(S) represents the controller. If the switching frequency is high

enough, the DVR can be modeled as a linear amplifier with a pure delay [20].

This delay is the sum of one-sample-period plus the time delay of the inverter due to PWM

switching. The former applies in cases of microprocessor-based implementations [27] and the

latter can be taken to be half the switching period [20]. The transfer function is equal to

is the reference voltage for the load, is the control output, whereas is

the output voltage of the DVR and is the load voltage. The inputs stand

for the grid voltage and the current through the load, respectively. Both inputs are assumed to be

measurable. The model may be extended with ease to three-phase applications. The load voltage

is

-----------------2

Repetitive control is a contemporary control technique that may be used to cancel out,

simultaneously, voltage sags, voltage harmonics, and voltage imbalances, characteristics rarely

achieved with other control techniques, such as PI controllers. As a first approximation, as

described in conventional repetitive-control theory [23], C(s) the controller can be written as

----------------6

The substitution of (6) into (3)–(5) yields

In order to calculate the frequency response of (7)–(9), the variable is substituted by . It

should be noticed that the term is always zero whenever is an integer

multiple of the frequency (e.g., , then ). Hence, the frequency response

shows that and for frequencies

therefore, if the closed-loop system is stable, the error in

steady state is zero for sinusoidal reference inputs or sinusoidal disturbance inputs of frequency.

Since the delay is smaller than the grid-voltage period

, the transfer function can be chosen as

------------------------10

With the substitution of (7)–(9) and (10) into the load voltage, (2) yields

Unfortunately, the delay t0 is not exactly known and the closed loop system will not be stable if

a controller is used with (6) and (10) designed for an estimated . To tackle this problem,

a modified controller is proposed

as

----------------------12

Where Q(s) is the transfer function of a low-pass filter [23], is the estimated value for the DVR

delay, with , and is a design parameter which is smaller than the period of the grid

voltage

.

The characteristic equation of the resulting closed-loop system is

----------------------16

In order to guarantee stability, the term G(s) in (16) must comply with the Nyquist criterion: if

the number of unstable poles of the open-loop G(s) system is equal to zero(P=0) , then the

number of counterclockwise encirclements of the point(-1 0) of the term G(jw) must be

zero(N=0) with .

Since all of the poles of are stable, which implies that , then must be zero to guarantee stability,

and a sufficient condition for can be obtained by making

----------------17

which is fulfilled if

----------18

Note that condition (18) is independent of the delay value of the controller C(s) in (12). A low-

pass filter, which is approximated by a constant time Delay within its pass

band, can be designed With being the time delay of the filter. For continuous systems,

Bessel filters can be used because they can be approximated by a constant time delay [28], while

for discrete time systems, finite-impulse-response (FIR) filters with a linear phase in their

passband can be used [29]. Therefore, the design parameter can be chosen to cancel out the filter

time delay ; and under such conditions, the closed-loop-system frequency response

will satisfy while the approximation of a constant time

delay is valid. Obviously, the bandwidth of the controller will be limited because the magnitude

characteristic of the filter will decrease as frequency increases.

V. CONCLUSION

The use of dynamic voltage restorers in PQ-related applications is increasing. The most

popular application has been on voltage sags amelioration but other voltage-squality phenomena

may also benefit from its use, provided that more robust control schemes than the basic PI

controller become available. A case in point is the so called repetitive controller proposed in this

paper, which has a fast transient response and ensures zero error in steady state for any

sinusoidal reference input and for any sinusoidal disturbance whose frequencies are an integer

multiple of the fundamental frequency. To achieve this, the controller has been provided with a

feedforward term and feedback term. The design has been carried out by studying the stability of

the closed-loop system including possible modelling errors, resulting in a controller which

possesses very good transient and steady-state performances for various kinds of disturbances. A

key feature of this control scheme is its simplicity; only one controller is required to eliminate

three PQ disturbances, namely, voltage sags, harmonic voltages, and voltage imbalances. The

controller can be implemented by using either a stationary reference frame or a rotating reference

frame. Comprehensive simulation results using a simple but realistic test system show that the

repetitive controller and the DVR yield excellent voltage regulation, thus screening a sensitive

load point from upstream PQ disturbances.