Principles for Controlling Harmonics

Harmonic distortion is present to some degree on all power systems. Fundamentally, one needs to control harmonics only when they become a problem. There are three common causes of harmonic problems:

1.

The source of harmonic currents is too great.2.

The path in which the currents flow is too long (electrically), resulting in either high voltage distortion or telephone interference.

3.

The response of the system magnifies one or more harmonics to a greater degree than can be tolerated.

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Principles for Controlling Harmonics►

When a problem occurs, the basic options for controlling harmonics are:

1.

Reduce the harmonic currents produced by the load.2.

Add filters to either bypass the harmonic currents off the system, block the currents from entering the system, or supply the harmonic currents locally.

3.

Modify the frequency response of the system by filters, inductors, or capacitors.

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Modifying the System Frequency Response

There are a number of methods to modify adverse system responses to harmonics:

1.

Add a shunt filter. Not only does this shunt a troublesome harmonic current off the system, but it completely changes the system response, most often, but not always, for the better.

2.

Add a reactor to detune the system. Harmful resonances generally occur between the system inductance and shunt power factor correction capacitors. The reactor must be added between the capacitor and the supply system source.

One method is to simply put a reactor in series with the capacitor to move the system resonance without actually tuning the capacitor to create a filter. Another is to add reactance in the line

4

Modifying the System Frequency Response3.

Change the capacitor size. This is often one of the least expensive options for both utilities and industrial customers.

4.

Move a capacitor to a point on the system with a different short-circuit impedance or higher losses. This is also an option for utilities when a new bank causes telephone interference—

moving the bank to another branch of the feeder may very well resolve the problem. This is frequently not an option for industrial users because the capacitor cannot be moved far enough to make a difference.

5.

Remove the capacitor and simply accept the higher losses, lower voltage, and power factor penalty. If technically feasible, this is occasionally the best economic choice.

5

Modifying the System Frequency Response EndEnd--User FacilitiesUser Facilities

When harmonic problems arise in an end-user facility, the first step is to determine if the main cause is resonance with power factor

capacitors in the facility. When it is, first attempt a simple solution by using a different capacitor size. With automatic power factor

controllers, it may be possible to select a control scheme that avoids the configuration that causes problems. In other cases, there will be so many capacitors switched at random with loads that it will be

impossible to avoid resonant conditions. Filtering will be necessary.

When the magnitude of harmonic currents injected by loads is excessive, industrial users should also investigate means of reducing harmonics by using different transformer connections and line chokes. In office buildings, zigzag transformers and triplen

harmonic

filters can reduce the impact of triplen

harmonic currents on neutral circuits

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7

Harmonic Mitigation Filters

Harmonic mitigation is achieved by installing harmonic filters. Those filters are divided into THREE categories.

Passive Filters Active Filters

Harmonic mitigation is achieved by installing harmonic filters. Those filters are divided into THREE categories.

Passive Filters Active Filters

Passive Filters

Passive filters are inductance, capacitance, and resistance elements configured and tuned to control harmonics.

They are commonly used and are relatively inexpensive compared with other means for eliminating harmonic distortion.

However, they have the disadvantage of potentially interacting adversely with the power system, and it is important to check all possible system interactions when they are designed.

They are employed either to shunt the harmonic currents off the line or to block their flow between parts of the system by tuning the elements to create a resonance at a selected frequency.

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Passive Filters

Shunt passive filters. The most common type of passive filter is the single-tuned “notch”

filter. This is the most economical

type and is frequently sufficient for the application. The notch filter is series-tuned to present a low impedance to a particular

harmonic current and is connected in shunt with the power system. Thus, harmonic currents are diverted from their normal flow path on the line through the filter.

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The filter designed for medium-voltage applications has a dry-type iron-core reactor positioned atop the capacitors, which are connected in Y configuration with the other phases. Each capacitor can is fused with a current-limiting fuse to minimize damage in case of a can failure.

In outdoor installations it is often more economical to use air-core reactors. Iron-core reactors may also be oil-insulated

Passive Filters

10

0 100 200 300 400 5000

0.2

0.4

0.6

0.8

1

|Vo|

= (

L-1/

(C

)|I|,

V

1 o 2 , R/s

.7071

Notch filters can provide power factor correction in addition to harmonic suppression. In fact, power factor correction capacitors may be used to make notch filters.

SeriesSeries--SingleSingle--Tuned Notch Filter Tuned Notch Filter & Filter Frequency Response.& Filter Frequency Response.

Passive Filters

An example of a common 480-V filter arrangement is illustrated below. The figure shows a delta-connected low-voltage capacitor bank converted into a filter by adding an inductance in series with the phases. In this case, the notch harmonic hnotch

is related to the fundamental frequency reactances

by.

F

cnotch X

Xh 3

11

Low-voltage filter configuration

Note that Xc in this case is the reactance of one leg of the delta rather than the equivalent line-to-

neutral capacitive reactance. If Line-to-Line voltage and three-

phase kVAR

are used to compute Xc ,

the factor 3 would be omitted.

cap

LLc Q

VX2

12

Creating a fifth-harmonic notch filter and its effect on system response

Passive Filters

One important side effect of this type of filter is that it creates a sharp parallel resonance point at a frequency below the notch frequency.

This resonant frequency must be safely away from any significant harmonic or other frequency component that may be produced by

the load.

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Passive FiltersIn many systems with potential series resonance problems,

parallel resonance also arises due to the circuit topology.Parallel resonance is formed by the parallel combination

between Xsource and a series between XT and XC . The resulting parallel resonant frequency is always smaller than its series resonant frequency due to the source inductance contribution.

The parallel resonant frequency can be represented by the following equation:

sourceT

cr XX

Xh

T

cs X

Xh

Series resonance:

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Passive FiltersFilters are commonly tuned slightly lower than the harmonic to be

filtered to provide a margin of safety in case there is some change in system parameters that would raise the notch frequency.

If they were tuned exactly to the harmonic, changes in either capacitance or inductance with temperature or failure might shift the parallel resonance higher into the harmonic being filtered. This could present a situation worse than one without a filter because the resonance is generally very sharp.

To avoid problems with this resonance, filters are added to the system starting with the lowest significant harmonic found in the system. For example, installing a seventh-harmonic filter usually requires that a fifth-harmonic filter also be installed. The new parallel resonance with a seventh-harmonic filter alone is often very near the fifth, which is generally disastrous

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Passive Filters►The filter configuration (-connected Cap.)

does not admit zero-

sequence currents because the capacitor is delta-connected, which makes it ineffective for filtering zero-sequence triplen

harmonics.

► Because 480-V capacitors are usually delta-configured, other solutions must be employed when it becomes necessary to control zero-sequence third-

harmonic currents in many industrial and

commercial building facilities. ►In contrast, capacitors on utility distribution systems are more

commonly Y-connected. This gives the option of controlling the zero-

sequence triplen

harmonics simply by changing the neutral

connection.►Placing a tapped reactor in the neutral of a capacitor is a common

way to force the bank to filter only zero-sequence harmonics. This technique is often employed to eliminate telephone. interference.

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Passive FiltersSeries passive filters.

Unlike a notch filter which is connected in

shunt with the power system, a series passive filter is connected in series with the load.

The inductance and capacitance are connected in parallel and are tuned to provide a high impedance at a selected harmonic

frequency. The high impedance then blocks the flow of harmonic currents at the tuned frequency only.

At fundamental frequency, the filter would be designed to yield a low impedance, thereby allowing the fundamental current to follow with only minor additional impedance and losses.

A series passive filter

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Passive FiltersSeries filters are used to block a single harmonic current (such

as

the third harmonic) and are especially useful in a single-phase circuit where it is not possible to take advantage of zero-sequence characteristics.

The use of the series filters is limited in blocking multiple harmonic currents. Each harmonic current requires a series filter tuned to that harmonic. This arrangement can create significant losses at the fundamental frequency.

Furthermore, like other series components in power systems, a series filter must be designed to carry a full rated load current and must have an overcurrent

protection scheme. Thus, series filters

are much less commonly applied than shunt filters.

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Passive FiltersC filters.

C filters are used for reducing multiple harmonic

frequencies simultaneously in industrial and utility systems. They can attenuate a wide range of steady-

state and time-varying

harmonic and interharmonic

frequencies generated by electronic converters, induction furnaces, cycloconverters.

The configuration of a C filter is nearly identical to that of the second-order high-pass filter.

The auxiliary capacitor Ca is sized in such a way that its capacitive reactance cancels out Lm

at the fundamental frequency, bypassing the damping resistance R.

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Active FiltersActive filters are sophisticated power electronics-based devices

for eliminating harmonics. They are much more expensive than passive filters. They have the distinct advantage that they do not resonate with

the system. The basic idea is to replace the portion of the sine wave that is

missing in the current in a nonlinear load.An electronic control monitors the line voltage and/or current,

switching the power electronics very precisely to track the load current or voltage and force it to be sinusoidal

Application of an active filter at a load.

2

Equipment withstand capabilities

According to existing equipment standards, harmonics should be limited to the equipment withstand capabilities.

When transformers are operating at rated load, the total harmonic current distortion should be limited to 5% as defined in IEEE Std C57-2000. IEEE Std C57-1998 defines the method for derating

transformers when supplying

nonsinusoidal

loads.

Harmonic Filter Design- Limitations

3

IEEE Std 1036-1992 and IEEE Std 18-2002 state that capacitors are intended to be operated at or below their

rated

voltage. Capacitors shall be capable of continuous operation under contingency system and bank conditions provided that none of the following limitations are exceeded:

1.

135% of nameplate kVAR2.

110% of rated rms

voltage (including harmonics but excluding transients)

3.

135% of rated rms

current (including fundamental and harmonic current)4.

120% of peak voltage (including harmonics).

Harmonic Filter Design- Limitations

IEEE Standard for Shunt Power Capacitors (IEEE Standard 18- 2000) specifies the following continuous capacitor ratings:

4

Filter design procedures are detailed in the steps shown below. The best way to illustrate the design procedures is through an example.

A single-tuned notch filter will be designed for an industrial facility and applied at a 480-V bus. The load where the filter will be installed is approximately 1200 kVA

with a relatively

poor displacement power factor of 0.75 lagging. The total harmonic current produced by this load is approximately 30 percent of the fundamental current, with a maximum of 25 percent fifth harmonic.

Harmonic Filter Design- A Case Study

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The facility is supplied by a 1500-kVA transformer with 6.0 percent of impedance. The fifth-harmonic background voltage distortion on the utility side of the transformer is 1.0

percent of the fundamental when there is no load. The harmonic design procedures are provided in the following steps.

Harmonic Filter Design- A Case Study

Industrial facility where the filter will be applied

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Harmonics Filter Design ExampleThis example deals with the designing consideration for a

single-tuned filter for a 480 V bus as shown in the figure.

The system parameters are as follows:The load where the filter to be installed is 1.2 MVA.

Load power factor is 0.75 lagging.Total harmonic current produced by the load is about 30 % of the

fundamental current with a maximum of 25 % 5th harmonic component. The facility is supplied via a 1.5 MVA transformer with 6 %

impedance.At no-load, the 5th

harmonic voltage distortion on the utility side of the

transformer is 1 % of the fundamental.