active power filter review

Active power filters: A review

M.El-Habrouk, M.K.Darwish and PMehta

Abstract: In recent years there has been considerable interest in the development and applications of active fdters because of the increasing concern over power quality, at both distribution and consumer levels, and the need to control reactive power and voltage stability at transmission levels. The existing approaches are classified and assessed to provide a framework of references for both researchers in this field and for generators, suppliers and consumers of electrical power who are, or may be, concerned about the problems associated with power quality and are considering installing active filters for their particular sets of problems.

1 Introduction

The term active filter is a generic one and is applied to a group of power-electronic circuits incorporating power- switchmg devices and passive energy-storage-circuit elements, such as inductors and capacitors. The functions of these circuits vary depending on the applications. They are generally used for controlling current harmonics in supply networks at the low- to medium-voltage distribution level or for reactive power and/or voltage control at high-voltage-distribution level [l]. These functions may be combined in a single circuit or in separate active filters.

In the past, there has been a number of surveys on various aspects of this topic. Several publications [2-91 have described the development of active-filtering techniques. Some of them focus mainly on circuit configurations and their possible interconnections [2-6]. Others review the control techniques associated with some of these circuits [&9]. However, there is a lack of published material whch provides an overview such that designers and users of active filters can evaluate various techniques in a subjective fash- ion. This contribution is aimed at filling this gap.

To identify the criteria for classifying active filters, a generalised block diagram of an active power filter is presented in Section 2. The subsequent Sections deal with the classification of active filters based on five identifiable criteria.

2 Classification of active filters

Fig. 1 shows the components of a typical active-power-filter system and their interconnections. The information regard- ing the harmonic current, generated by a nonlinear load, for example, is supplied to the reference-currentlvoltage estimator together with information about other system variables. The reference signal from the current estimator, as well as other signals, drives the overall system controller. This in turn provides the control for the PWM switching- pattern generator. The output of the PWM pattern generator controls the power circuit via a suitable interface. The

0 IEE, 2000 IEE Proceedings online no. 2oooO522 DOL lO.l049/ipepa:2oooO522 Paper fmt received 7th December 1999 and in revised form 16th March 2000 The authors are with the Department of Electronic and Computer Engineering, Brunel University, Uxbridge, UK

power circuit in the generalised block diagram can be connected in parallel, series or parallelkeries configurations, depending on the connection transformer used.

siinnlv rr., _ _ connection nonlinear transformer load

variables pattern

switching

effort signal

Fig. 1 Geeerdived block dkgrm, for active power j l ters

On the basis of the above, the published work in this field can be classified using the following criteria. (U) power rating and speed of response required in compensated systems; (b) power-circuit configuration and connections; (c) system parameters to be compensated (e.g. current harmonics, power factor, unbalanced three-phase system etc.); (cl) control techniques employed; and (e) technique used for estimating the reference currentholtage. The following Sections classify the systems according to the above criteria. This will provide a better understanding in dealing with these systems, as it shows the merits and draw- backs of each type.

3 speed of response required in the compensated system

Classification according to power rating and

Fig. 2 shows the classification based on this criterion. The power rating of the compensated system and its speed of response play a major role in deciding the control philosophy to implement the required filter. These two factors follow a reciprocal relationship [10-121. In general, the cost of any particular system is proportional to the required speed of response [lo].

403 IEE Proc.-Elertr. Power Appl. , Vol. 147, No. 5, September 2000

active power-system filters 1

\- low-power

applications (4 00 kVA)

medium-power high-power applications applications

( > I O MVA) (100 kVA to 10 MVA) 1 I

response, 1 -phase

compensator

response, 3 x 1 -phase

compensators

100 vs-IO ms response, 3-phase

compensator

100 ms-1 s response, 3-phase

compensator

response, 3-phase

compensator

Fig. 2 Suhdiviswn of paws system jiliers crccording to power ratkg and speed of sespome

3.1 Low-power applications This type of application is mainly concerned with systems of power ratings below 100kVA. It is mainly associated with residential areas, commercial buildings, hospitals and for a wide range of small to medium-sized factory loads and motor-drive systems. This range of applications employs sophisticated techniques of dynamic active filters, especially those with high-pulse-number PWM voltage- or current-source inverters. Their response time is relatively much faster than other techniques, ranging from tens of microseconds to milliseconds, and resulting in a considerable reduction in the power compensation range as stated above. This type comprises the following two categories.

3.7. I Single-phase systems: Single-phase active filters [9, 13-30] are generally available in low power ratings. They are suitable for retrofit applications, such as in commercial or educational buildings with computer loads [3 I], small factories etc., where the current harmonics can be dealt with at the point of common coupling. Thus several lower-power filters can be connected on a given distribution site rather than using one large filter on the incoming supply. This is due to the large number of the single-phase loads withn one building and the harmful consequences the presence of large amounts of harmonic in the neutral line. This allows for more selective compensation as the operating conditions vary. On the other hand, residential loads do not generate such large values of concentrated harmonics, [I , 321 and hence the effects on the neutral are not significant. However, in the absence of compulsory harmonic regulations [33-351, residential customers are not likely to invest in single-phase active power filters.

The main advantage of single-phase filters is that they have to deal with low powers and hence can be operated at relatively hgher frequencies leading to improved performance.

3.1.2 Three-phase systems: For three-phase applications, the choice of filterskonfigurations depends on whether the three-phase loads are balanced or not. At relatively low power levels (IOOkVA), a three-phase system can use either three single-phase or one three-phase compensator. For balanced loads, a single three-phase-inverter configuration is employed [2-5, 7, 8, 10-12, 36581. This is acceptable if there is no requirement to balance currents or voltages in each phase and the aim is simply to eliminate as many current harmonics as possible, assuming that the magnitudes and respective phase angles in each phase are the same [2-5, 7, 8, 10-12, 3&58]. For unbalanced load currents or unsymmetrical supply voltages, especially in

404

three-phase four-wire distribution systems, three single- phase inverter circuits [58] or alternative configurations [2&30, 551 may provide acceptable solutions. These alternative configurations are discussed later on in this paper. The connection of three single-phase filters is recommended by some designers [2, 5, 581, especially those who do not rely upon standard inverter configurations such as lattice structures, switched-capacitor techniques and power-regulator configuration [20-301. It can incorporate three independent currentlvoltage-feedback signals that will balance the supply currents or voltages.

3.2 Medium-power applications In this category, three-phase systems ranging from lOOkVA to lOMVA [2, 5, 591 are mainly considered. Medium- to hgh-voltage distribution systems [l, 351 and high-power high-voltage drive systems [2, 51, where the effect of phase unbalance is more or less negligible, fall within this classification. Here the main aim is to eliminate or reduce the current harmonics. Because of economic considerations, reactive-power compensation using active filters at the high-voltage distribution level is not generally regarded as viable [l 11 because of the high voltage and its accompanying problems of isolation and serieslparallel connections of switches. Even if step-down transformers are used, the resulting high currents are difficult to handle. Alternative approaches, including capacitive and inductive static compensators as well as quasidynamic systems (such as relay- controlled LC circuits), tuneable harmonic filters, line-com- mutated thyristor converters [60], synchronous condensers [ 11 and cascaded multilevel-inverter VAR compensators [6145], are considered more suitable. The speed of response expected in this range is of the order of tens of dliseconds.

3.3 High-power applications The implementation of very high-power dynamic filters is extremely cost ineffective, because of the lack of hgh- switching-frequency power devices that can control the current flow at such power ratings, is a major limitation for such systems [2, 5, 111. As with medium-power applications, extra high voltages of a few hundred kilovolts cannot be tolerated, even by state-of-the-art semiconductor devices whch can withstand only a few kilovolts. The series-paral- le1 combinations of these switches is possible, but difficult to implement and cost-ineffective. Fortunately, the harmonic pollution in high-power ranges, which include systems with ratings above IOMVA, is not such a major problem as in lower-power systems. These high-power systems include power-transmission grids and ultrahigh-

JEE Proc -Elecrr Powei Appl 1'01 147 No 5 September 2000

power DC drives as well as DC transmission systems. The effect of harmonics generated at the low-power side would be minimised, either naturally or by the installation of several medium- and low-power active filters downstream to contribute to the compensation in such cases. The static- VAR compensation is then the major concern and is usually compensated for by using traditional static power conditionersifilters [I, 2, 51 as well as several sets of synchronous condensers connected in parallel [ 11 and cascaded multilevel-inverter VAR compensators [61-65]. The required response time for such cases is in the range of tens of seconds, which is sufficient for contactors and circuit breakers to operate after taking the optimal-switching decision [l, 111. Power fluctuations in the range of a few seconds are, on the other hand, treated by the generat- ing stations' anciliary devices.

One of the few applications of active filters in high-power systems is the Japanese bullet train (Sinkansen) [2, 5, 661, which uses a parallel combination of several active filters. The control and co-ordination requirements of these filters are, however, complicated [2].

standard lattice voltage switched inverter capacitor structure regulator

4 configuration and connections

Classification according to the power-circuit

parallel AF series AF parallel AF + + +

series AF parallel PF parallel PF

Power-circuit configurations play an important role in the selection of the applications, as some circuits are suitable only for certain aspects of control and power ranges, as discussed in this Section and illustrated in the block diagram shown in Fig. 3.

4.7 Parallel active filters This class of filter configurations constitutes those most important and most widely used in industrial processes [2-91. It is connected to the main power circuit, as shown in the single-line diagram of Fig. 4. The purpose is to cancel the load current harmonics fed to the supply. It can also contribute to reactive-power compensation and balancing of three-phase currents, as mentioned above. Parallel fdters have the advantage of carrying only the compensation current plus a small amount of active fundamental current supplied to compensate for system losses. It is also possible to connect several filters in parallel to cater for higher currents, which makes this type of circuit suitable for a wide range of power ratings. This configuration consists of four distinct categories of circuit, namely inverter configurations, switched-capacitor circuits, lattice-structured filters and voltage-regulator-type filters. Information on these circuits is given in Table 1.

supply impedance

filter

Fig. 4 paralkd active filter configuration

nonlinear load

DC-link 7 1 DC-link

inductor smi% and , jcapacitor active filter

a Fig. 5 Invcrter b u d activejilter,s LI Current fed inverter h Voltage fed inverter

coupling active inductors filter

b

/'$7 a b

Fig. 6 Switched ccpacitor filters

supply nonlinear

m b

nonlinear

C

Fig. 7 Lattice structure configurations

Fig. 3 A F Active filter PF: Passive filter

Subdiviion of power systtmjZters accordmg to power cirmit configurations and connections

IEE Proc-Electr Power Appl.. Vol. 147, No. 5, September. 2000

in series with parallel PF

405

Table 1: Comparison of different active-power-filter configurations

Switched-capacitor Lattice structures Voltage-regulator filters active filters

Inverter/converter configurations

CSI VSI Comparison criteria

l-phase and 3-phase

Acts with a superimposed current-control loop to generate desired waveform

Fig. 5b

Low/medium-power applications

Simple (keep capacitor voltage constant)

Fast (-0.1)

l-phase

Controls filter voltage continuously to control current indirectly

l-phase l-phase

Controls filter voltage continuously to control current' indirectly

No of phases Normally 3-phase

Function Injects currents at PCC to eliminate load-current harmonics

Controls filter current/ voltage directly

Circuits Fig. 5a

Power rating Medium-power

Control complexity Complex (keep

applications

inductor current constant)

Speed of response, Medium (-1) ms

No of active devices 6 switches, 6 diodes

Fig. 6


Complex (optimisation technique)

Slow (-100)

Fig. 7

Low-power applications

Complex (optimisatioi technique)

Slow (-100)

Fig. 8


Simple (track capacitor-voltage reference)

Fast (-0.1)

1

l-phase: 4 switches14 diodes 3-phase: 6 switched6 diodes

-20-30

l-phase: 2 bidirectional switches

l-phase: 4 bidirectional switches

l-phase: 4switches/4 diodes

Switching frequency, Hz

DC energy storage

-2-5 -2-4 -2-4 -4-6

Large DC inductor (-100 mH)

(1.3-1.5) x rated supply current

N/A

Large DC capacitor

(1.3-1.5) x rated supply voltage

N/A

(-4700-9OOOvF) NIA 2 small DC capacitors

(200 pF)

Rated supply voltage DC-link voltage or current

AC components

N/A N/A

1 or 2 AC capacitors (4WOpF)

1 or 2 AC capacitors (80wF) 1 or 2 inductors (1-3 mH)

1.5 x rated-voltage AC capacitors

Optimised PWM of voltage

1 AC capacitor (4GIOOpF) 1 small inductor (< ImH)

1.2-1.5 x rated- voltage AC capacitors

Continuous PAM of voltage

AC voltage ratings N/A NIA 1.5 x rated-voltage AC capacitors

Optimised PWM of voltage

Control method PWM of DC-link current

Current-controlled PWM of DC-link voltage

Very high rates of voltage and change (+Vdcand-Vdc)

[8, 12-16, 40, 47, 48,50,51, 54,55,59, 67-7 1 1

Voltage or current discontinuities

Very high rates of current and change (+Idc and -Idc)

12, 3, 5, 11, 18, 3639, 49,52,53,571

Smooth voltage variations



References 124-261

rating considerably compared with parallel filters, especially in the secondary side of the coupling transformer (increasing the 12R losses and the physical size of the filter). The main advantage of series filters over parallel ones is that they are ideal for eliminating voltage-waveform harmonics, and for balancing three-phase voltages [17, 19, 4143, 581. This, in fact, means that this category of filter is used to improve the quality of the system voltage for the benefit of the load. It provides the load with a pure sinusoidal waveform, which is important for voltage-sensitive devices (such as superconductive magnetic-energy storage and power- system-protection devices). Note that most of the circuit

supply connection impedance transformer (-1

Fig. 8 Kdtuge rrguhtor uct ivejhx

4.2 Series active filters The active fdter in this configuration produces a PWM voltage waveform which is addeasubtracted, on an instantaneous basis, to/from the supply voltage to maintain a pure sinusoidal voltage waveform across the load [2, 581. The main power-circuit configuration is shown in Fig. 9. The inverter configuration accompanying such a system is a voltage-fed inverter without any current-control loops. Series active filters are less common industrially than their rivals, parallel active filters. This is because of the main drawback of series circuits, namely that they have to handle high load currents, which increases their current

406

active

Fig. 9 Series active filter configuration

IEE Proc.-Electr. Power Appl.. Vol. 147, No. 5. September 2000

configurations of parallel filters can be used in series configurations, but only the inverter configuration is reported in the literature.

4.3 Other filter combinations Combinations of several types of filter can achieve greater benefits for some applications.

4.3.1 Combination of both parallel and series active filters: To gain the advantages of both series and parallel inverter-type configurations, a combination of both types of fdter, shown in Fig. 10, can be used to achieve the demanding power-system requirements. The demand for combined filters is limited because of their control complexity and higher cost. The control complexity is due to the dependency of the switching pattern of both parallel and series circuits. Consequently, these types of filter have received less attention than other configurations [2, 5, 72, 731. The arrangement is, however, frequently used for other purposes in power systems for flexible AC transmission systems (FACTS) [74].

load

combination U

Fig. 1 0 Conlhination oypurullel und .series uctivefilters

connection

Fig. 1 1 Series uctive and purallel pu.ssive$lrer combination

4.3.2 Combination of series active and parallel passive filters: To reduce the complexity of the previous filter combination, the inverter-type series active fdter, which constitutes a high impedance for high-frequency harmonics, is accompanied by a parallel passive filter to provide a path for the harmonic currents of the load [44, 731. This combination, represented by Fig. 11, permits an improvement over the characteristics of plain series active filters and the extension of their capabilities to include current-harmonic reduction and voltage-harmonic elimination [44, 461. The configuration, however appealing, has not yet been studied thoroughly owing to lack of interest in series active filters.

4.3.3 Combination of parallel active and passive filters: This combinatiqn, shown in Fig. 12, represents a very important mixture of passive and active inverter-type filters. The active filter is designed to eliminate only part of the low-order current harmonics while the passive filter is designed to eliminate the bulk of the load-current harmonics. In such combinations, the system can be designed for higher powers without excessive costs for high-power switching [45, 731. The main drawback of thls technique is that it contains too many power components, especially for the passive filter. Since passive filters are permanently con-

IEE Proc.-Ele[ectr. Power Appl. , Vol. 147, N o 5, September 2000

nected to the system, this approach is only suitable for single load with a predefined harmonic source.

a

PCC nonlinear load < passive I filter

parallel active filter

nonlinear load

active filter =i Fig. 13 Activejefilter in series with parullel pcwsivefilter cornhinution

4.3.4 Active filter in series with parallel passive filters: This configuration, shown in Fig. 13, is a subject of several publications [46, 57, 73, 75, 761 and is fairly important, especially for medium- and high-voltage applications where the passive filter reduces the voltage stress applied to the switches in the active filter. For this reason, this concept shows promise for higher-voltage applications; however, further research is still needed to assess the effectiveness of the configuration.

5 variable

Classification according to the compensated

Active filters are built to improve some of the characteristics of power systems under question. These characteristics are signified by the system parameters to be controlled, leading to the subdivisions presented in Fig. 14 and discussed in the following Sections.

5. I Reactive-power compensation ('VA R' correction) Most researchers normally regard reactive-power compensation [30, 3941, 43, 481 as not requiring active filters. However, compensation of power factor in conjunction with current harmonics is fairly popular and is addressed by many publications. On the other hand, active-filter configurations rarely treat the problem of power-factor correction on its own owing to the fact that other quasidynamic, cheaper and slower-in-response reactive-power compensators are available in the market. This technique (in this case called active-power filter for reactive-power compensation), if applied, would normally be suited for low-power applications, since the currents needed for reactive-power compensation are of the same order of magnitude as the rated current of the load. It would be a waste of sophsticated equipment to tackle them without the use of other power- factor-correction devices, such as thyristor-controlled reactors and capacitors; especially in single-phase systems, where in certain specific applications the requirement is for accurate compensation without harmonics generation.

5.2 Harmonic compensation This is the most important system parameter requiring compensation in power systems and it is subdivided into voltage- and current-harmonic compensation as follows.

407

active power-system filters 1

compensation

t voltage current

Fig. 14 Subdivision according to cot?lpcxwied variables

5.2. I Compensation of voltage harmonics: The subject of compensating voltage harmonics [2, 19, 42, 671 is not widely addressed because power supplies usually have low impedance. The terminal voltage at the consumer point of common coupling (PCC) is normally maintained withn the standard limits for voltage sag and total harmonic distortion and does not normally vary much with loading. This problem is usually important for harmonic-voltage- sensitive devices [l], which require the supply to be purely sinusoidal, such as power-system-protection devices and superconducting magnetic-energy storage [77]. Note that the compensation of voltage and current harmonics is interrelated. The reduction of voltage harmonics at the PCC helps a great deal to reduce current harmonics, especially for the particular cases of nonlinear loads with reso- nance at the harmonic frequencies. However, the compensation of the voltage harmonics at the PCC does not eliminate the need for current-harmonic compensation for nonlinear loads.

5.2.2 Compensation of current harmonics: Com- pensation of current harmonics is very important in low- and medium-power applications and is covered by many publications [2-5, 10-12, 16, 18, 2&30, 3638, 4446, 521. As mentioned above, the compensation of current harmonics reduces to a great extent the amount of distortion in the voltage at the point of common coupling. The magnitude of the current and its waveform determine many of the power-system-design criteria. It is always recommended that the RMS value of the total current be reduced as much as possible (to reduce cable and feeder losses), which implies the reduction in current harmonics. This is because the total RMS value of the nonlinear load current is equal to the sum of the squares of the RMS values of each of the individual harmonics. The imposition of harmonics stand- ards [33-35] will soon oblige factories and establishments to control the amount of harmonics they inject into the power system.

5.3 Balancing of three-phase systems This problem exists mainly in low- and medium-voltage distribution systems where the currents, and consequently the voltages, in the three phases are not balanced and are not spaced in time 120" apart.

5.3.1 Balancing of mains voltages in three-phase systems: The degree of system imbalance depends on the amount of current imbalance and the magnitude of the supply impedance. These can cause the three-phase voltages to be unequal in magnitude and unequally spaced in time. T h s is due to the presence of a significant amount of supply impedance. The remedy to this problem is to add to

408

t voltage current

+ current harmonics + VAR compensation voltage harmonics + VAR compensation current harmonics +

* voltage harmonics

current harmonics +voltage harmonics +VAR compensation

-*

6ach phase the corresponding amount of instantaneous voltage to force it to follow the reference sinusoidal waveform. The system, in such cases, is normally of the low- power category because in medium- and high-power systems the supply impedance does not have any significant effect on system performance [2, 751.

5.3.2 Balancing of mains currents in three-phase systems: As with balancing voltages, this compensation is mainly concerned with three-phase systems for low-power applications. The reason is that the magnitudes of currents to be supplied to the grid depend entirely on the amount of imbalance in the system, which mostly occurs in low-voltage distribution systems for residential loads. The compensator under consideration [2, 49, 75, 781 would sometimes be forced to supply the rated value of current, which limits its power-handling capabilities. The power circuit of this system normally consists of three single-phase type (H- bridge inverters) having the same energy-storage element.

5.4 Multiple compensation Different combinations of the above systems can be used to improve the effectiveness of filters. The following are the most frequently used combinations.

5.4. I Harmonic currents with reactive power compensation: The most common and popular filters are those which compensate for both the reactive power and the harmonic currents in order to maintain the supply current completely free of harmonics and in phase with the supply voltage [4, G9, 13-15, 50, 511. These techniques have several advantages over other alternatives, as only one filter is needed to compensate for everything, which is much more attractive than using many different types of compensators. However, because of the limits imposed by the ratings of power switches, one can only use this application for low powers. The resulting switching frequency would need to be lower for higher-power applications which restricts the filter under consideration to small powers.

5.4.2 Harmonic voltages with reactive-power compensation: This combination [2], however rare, takes place in certain configurations for controlling the voltage harmonics, which would normally affect indirectly (using suitable feedback) the reactive-power compensation. This compensation system is only suitable for low-power applications.

5.4.3 Harmonic currents and voltages: The problem of addressing harmonic currents and voltages simulta- neously can only be treated by using the seriedparallel

IEE Proc.-Electr. Power Appl. , Vol. 147, No. 5 . September 2000

combination of active-filter configurations. This, of course, is very important and very beneficial in malung both the supply and the load free from harmonic effects [2, 5, 491. However, this complex type is normally used only for very sensitive devices such as power-system-protection equipment and superconducting magnetic-energy storage systems.

5.4.4 Harmonic currents and voltages with reactive-power compensation: This scheme is the ultimate in sophistication since it controls harmonics and reactive power [2, 5, 491. This technique requires the use of the par- alleVseries active-filter combination. It is not employed very often because its control is rather difficult and the information available on it in the literature is very limited.

6 Classification based on the control technique

Fig. 15 shows a convenient method of classifying the presently available control techniques. These techniques fall into the main categories of open- and closed-loop controls. The closed-loop controls can be further subdivided into other techniques as shown in Fig. 15.

6. I Open-loop control systems Open-loop systems sense the load current and the harmonics it contains. They simply inject a fixed amount of power in the form of current (mainly reactive) into the system, which may compensate for most of the harmonics and/or reactive power available. These systems do not check how successful the compensation has been. T h s has been the case with some of the traditional filtering techniques. Such techniques inay include switched passive-filter banks, which are not discussed in this paper, in addition to the following techniques: (U) harmonic elimination by tlurd-harmonic injection [23, 791, (b) harmonic-cancellation devices [23], and (c ) systems with known constant load-harmonic pattern 1261.

6.2 Closed-loop control systems As opposed to open-loop systems, the closed-loop techniques incorporate a feedback loop wluch senses the required variables under consideration. These systems are more accurate from the point of view of the amount of harmonic and reactive-power reduction they can achieve. Almost all new techniques in use are of this type. These controllers usually employ digital signal processors (DSPs). Control loops, considered in this Section, should not be confused with the frequently used inner-hysteresis control loops which are now used as a standard block in most current-controlled voltage-source inverters.

6.2. I Constant-capacitor-voltage technique: This technique, which is suitable for single- and three-phase inverter configurations with a capacitor in the DC link, relies on the fact that the capacitor voltage is the voltage source which controls the current waveform simply by con- necting the capacitor to the mains supply through the smoothing inductor. The resulting current is then controlled by ordinary PWM techniques. Because energy is supplied to or taken from the DC capacitor, the voltage across its terminals fluctuates. To keep this voltage within limits, a reference voltage is chosen. The error difference between the actual capacitor voltage and its reference value deter- mines the active component of power necessary to compensate for losses in the filter. This error difference is added to the current-controller error signal to determine the overall system error to be processed by the current controller [2-8, 12-17, 19, 40, 43, 4 U 8 , 50, 51, 67, 751. This technique is very popular, as is clear from the large number of references provided.

6.2.2 Constant-inductor-current technique: This control technique, on the other hand, is suitable for standard converters with an inductor in the DC link. The operation of the system is then very similar to that of the constant-capacitor-voltage system when the capacitor voltage is replaced with the inductor current. Two main methods are used to implement this technique: (i) Current pulse-width modulation: As in the constant- capacitor-voltage case, the PWM control is used to provide the appropriate pulses to represent the average value of the current signal in a specific time interval [3, 10, 11, 18, 36-39, 41, 44, 45, 491. (ii) Current pulse-amplitude modulation: This new control method [52] provides active filters with a basis for amplitude modulation of the required current waveform. Although the concept is well established, it is not possible to implement it with present power-electronics technology ~521.

6.2.3 Optimisation techniques: The optimisation procedure for switched-capacitor and lattice-filter circuits is the same [9, 20-271. The rate of rise of the current and the amplitude depend mainly on the size of the capacitors and the initial voltages on them. These factors are functions of the switching patterns, and they provide considerable flexi- bility in shaping the waveform of the current drawn by the filter. The key to controlling these filter configurations is to determine the appropriate switching function for the switches. The main task of the system controller is to minimise a predetermined number of individual load-current harmonics, in addition minimising either the THD or the fundamental component of the filter current. However, this is not performed instantaneously. A time delay exists

active power-system filters

THD minimisation minimum filter current

Fig. 15 ,%bdivirioM according to control techniylres

IEE ProcElec tr . Power Appl. . Vol. 147. No. 5, September 2000 409

between the detection of a change in the harmonic current and the application of the new set of switching angles obtained from the optimisation procedure. This system is mainly suitable for constant or slowly varying loads.

6.2.4 Linear-voltage-control technique: This method is only suitable for the voltage-regulator type of active filter [29, 301. The voltage across the output capacitor is linearly controlled through the continuous charging and discharging. The capacitor-voltage reference is calculated from the harmonic reference and includes the rate of change of the load-current harmonics [30]. The continuous and smooth variation of the capacitor voltage, in contrast to the sudden changes of the inverter-voltage waveforms, ensures that the current change in the supply/filter loop is controlled and hence the switching frequency is further reduced. The main advantage of this technique lies in the fact that no sudden variation of voltage is caused on the supply side. This, in fact, reduces the amount of high-frequency harmonics injected into the supply due to the presence of the PWM inverter.

time domain '+

7

current/voltage- current/voltage- reference synthesis reference calculation

frequency

6.2.5 Other techniques: Other control techniques exist [53, 54, SO]. They simply provide small changes to the aforementioned techniques, providing simply newer or better performance over their predecessors. These techniques may include the use of state-of-the-art adaptive, predictive and sliding-mode controllers, which are normally dificult to implement without the use of DSPs. These techniques can be implemented in either the time domain or the frequency domain.

+ synchronous detection + constant active power

--* synchronous frame + synchronous flux detection

constant (unity) power factor fictitious power compensation

7 Classification according to current/voltage- reference-estimation technique

As shown in Fig. 1, the reference currentholtage to be processed by the control loops constitutes an important and crucial measure for subdividing active-filtering techniques. Fig. 16 illustrates these estimation techniques, which cannot be considered to belong to the control loop since they perform an independent task by providing it with the required reference for further processing. Despite the fact that some references do not mention their source of compensating-current reference, these estimation techniques can be classified as follows.

highpass-filter method lowpass-filter method

7.1 CurreniYvoltage-reference synthesis (continuous time-domain control) T h s technique uses an analogue signal filter to determine the harmonics contained in the main supply current. This technique is preferred because of the simplicity of its implementation, using analogue devices, in the time domain. However, it suffers from a serious drawback, in that the phase and magnitude errors introduced by the signal active filter employed are considerable. Two main categories emerge.

domain

other algorithms

- conventional Fourier and FFT sine multiplication modified Fourier series

(a) High-pass-filter method: The use of a high-pass filter is straightforward for removing low-order frequencies in the load-current signal. The resulting high-frequency components constitute the desired reference [3]. T h s filtering technique is, considered to be equivalent to differentiation, which makes this technique vulnerable to noise. (6) Low-pass-filter method: However indirect, this method is preferred over the high-pass-filter method because it reduces the effect of differentiation in the resulting filtered component. Filtering the fundamental component and then subtracting it from the total load current yields the desired reference [3, 51, 671. As mentioned above, the system suffers from large magnitude and phase errors.

7.2 CurrenVvoltage-reference calculation (discrete-time or frequency-domain control) The calculation of harmonics is usually adopted because of the main drawback of the technique described in Section 7.1 which incorporates phase-angle and magnitude errors. Most conventional methods of calculation can be classified either as time-domain or frequency-domain. Other modern techniques exist.

7.2. I TTme-domain approaches: The following seven subdivisions of time-domain approaches are mainly used for three-phase systems except for the fictitious-power-compensation technique which can be adopted for single- or three-phase systems. (i) Instcmtaneous-rec~~tive-power algorithm: In this technique, suitable only for three-phase systems, the instantaneous power of the load is calculated. It consists of a DC component and an oscillating component. The oscillating component is separated over a certain interval of time (an integral number of cycles). The reference signals are then calculated by distributing the total current equally to each of the three phases. This operation takes place only under the assump- tion that the three-phase system is balanced and that the voltage waveforms are purely sinusoidal [S, 40, 41, 501. If, on the other hand, this technique is applied to contaminated supplies, the resulting performance is proven to be poor [SI. (ii) Synchronous-detection ulgorith: This technique [8], which is very similar to the previous one, relies in the fact that the three phase currents are balanced. The average power is calculated and divided equally between the three phases. The signal is then synchronised relative to the mains voltage for each phase. This technique, however easy to implement, suffers from the fact that it depends to a great extent on the harmonics in the voltage signal. (iii) Constant-active-power algorithm: The instantaneous and average powers of the load are calculated. The active- power component of the system is controlled to keep the instantaneous real power constant, while maintaining the imaginary power to zero. This technique performs fairly

410 IEE Proc.-Elecir Power Appl., Vol. 147, No. 5, September 2000

well under ordinary conditions. However, the performance deteriorates when the supply is contaminated [7]. (iv) Constant- (unity-) power-fuctor algorithm: This is another technique, which is very similar to that in (iii) above, except the fact that it forces the instantaneous current signal to track the voltage-reference waveform. This implies that the power factor would be fured to unity and the system would only be suitable for the combined system of VAR and current-hamionic compensation [7, 81. (v) Fictitious-power-compensation algorithm: This technique relies on the principle of fictitious power compensation developed in [1&12]. Despite the opposition to the theory by [8 1-83], this principle was proven to operate satisfacto- rily. The system controller is designed to minimise the undesired component of power. In this aspect, it is similar to the instantaneous-reactive-power algorithm but with a different definition of power. This approach is suitable for both single- and three-phase systems. However it involves a large amount of computation. (vi) Synchronous~~unze-based algorithm: This algorithm relies on the Park transformations to transform the three- phase system from a stationary reference frame into synchronously rotating direct, quadrature and zero-sequence components. These can easily be analysed since the fundamental-frequency component is transformed into DC quantities [84]. The active and reactive components of the system are represented by the direct and quadrature components, respectively. The high-order harmonics still remain in the signal; however they are modulated at different frequencies. These are the undesired components to be elimi- nated from the system and they represent the reference harmonic current. The system is very stable since the controller deals mainly with DC quantities. The computation is instantaneous but incurs time delays in filtering the DC quantities. This method is applicable only to three-phase systems. (vii) Synchronous-Jux-detection algorithm: This technique is similar to that in (vi) above, in applying Park transformations to transfer the system into synchronously rotating direct, quadrature and zero-sequence frames of reference. However, it applies the transformation on the flux linkage of the filter inductance, which is then controlled using the output voltages and currents in separate integral loops [U]. The presence of these integral loops incorporates time delays, which depend on the frequency response of the spe- cial feedfonvard and feedback integrators.

7.2.2 Frequency-domain approaches: Frequency- domain approaches are suitable for both single- and three- phase systems. They mainly derived from the conventional Fourier analysis and include the following three subdivisions. (i) Conventional Fourier and FFT algorithms: Using fast Fourier transforms, the harmonic current can be recon- structed by eliminating the fundamental component from the transformed current signal and then the inverse transform is applied to obtain a time-domain signal [3, 18, 37, 391. The main disadvantage of this system is the accompanying time delay. This technique needs to take samples of one complete cycle (or an integral number of cycles) to generate the Fourier coefficients and it is therefore suitable for slowly varying load conditions. (ii) Sine-multiplication technique: T h s method relies on the process of multiplying the current signal by a sine wave of the fundamental frequency and integrating the result. Th~s results in a loss of all the high-order harmonics using a

IEE PIOC -Electr Power A p p l , Vol 147, No 5, September 2000

simple low-pass filter [13, 28, 461. The performance is still slow (more than one complete mains cycle). This technique is similar to the Fourier techniques presented above; it is, however, differently implemented. (iii) ModiJed-Fourier-series techniques: One of these techniques was developed in [30]. The principle behind it is that only the fundamental component of current is calculated and th s is used to separate the total harmonic signal from the sampled load-current waveform. The practical iniple- mentation of this technique relies on modifying the main Fourier series equations to generate a recursive formula with a sliding window. This technique is adapted to use two different circular arrays to store the components of the sine and cosine coefficients computed every sampling sub- cycle. The newly computed values of the desired coefficient are stored in place of the old ones and the overall sums of the sine and cosine coefficients are updated continuously. The computation time is much less than that of other techniques used for single-phase applications. This technique is equally suitable for single- or three-phase systems.

Another modified Fourier-series technique was developed in [55]. It relies on the decomposition of the three- phase signals into synchronously rotating direct and quadrature axes. The technique is used to compensate for all ‘nonactive’ components of load-current signal. The nonactive current definition in the dq reference frame is used to generate the desired supply currents. Sliding-window computation techniques are used (similar to that above) to calculate the reference value of the filter current. This technique is suitable only for three-phase systems.

7.2.3 Other algorithms: There are numerous optimisation and estimation techniques, and all the utilities and libraries for estimation can be used to perform ths task. However some new methods arise, such as the neural- network and adaptive-estimation techniques which are fairly accurate and have, of course, much better response [9, 53, 54, 8&88]. Unfortunately, presently available control hardware is not suitable for implementation of these techniques.

8 Conclusions

The review and classification of published work in this field shows that there has been a significant increase in interest in active filters and associated control methods. This is due to increasing concern about power quality and the availa- bility of suitable power-switching devices at affordable prices. To facilitate understanding and selection of particular configuration and control techniques for a given application, the classification is based on five main criteria. The power-circuit configurations of active filters and the ratings of the compensated systems define the two broad categories. The other three classification criteria are based on the control strategies, control techniques and reference-estimation methods generally employed. The review also takes into account the criteria for selecting passive components, and the switching frequencies and losses for the various configurations are also discussed.

Control circuits constitute a minor portion of the total cost of active filters. This is because the new generation of microcontrollers and digital signal processors (DSPs) can operate at extremely high frequencies and at very low cost. The number of instructions and operations performed per second is phenomenal. Thus the most complex control requirements can be incorporated without a great deal of concern about this part of the cost for any system.

41 1

The manner in which the paper has classified the different aspects of active filters, although not providing a detailed analysis, should help research workers, users and suppliers of electrical power to gain an overview and inspi- ration for further research on this subject. It is obvious from the survey that a great deal more work still needs to be done; particularly as the problems associated with generation, transmission, distribution and consumption of power become more serious.

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