Eff. of Harmonics

8/3/2019 Eff. of Harmonics

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Harmonics

The Effect of Harmonics

ORIGINS OF HARMONIC DISTORTION

The ever increasing demand of industry and commercefor stability, adjustability and accuracy of control of

electrical equipment led to the development of relatively

low cost power diodes and thyristors.Now used widely for rectifier circuits for U.P.S. systems,

static converters, and D.C. motor control, these moderndevices replace the Mercury Arc Rectifiers of earlier

years and in consequence create new and challengingconditions for the power engineer of today.

Although solid state devices such as the thyristor havebrought significant improvements in control techniques,

they have the disadvantage that they produce harmonic

currents.

Harmonic currents can cause an unacceptabledisturbance on the supply network and adversely affectthe operation of other electrical equipment including

power factor correction capacitors.

WAVEFORM

All complex waveforms can be resolved into a series of

sinusoidal waves of various frequencies, hence anycomplex waveform is the sum of a number of odd or

even harmonics of lesser or greater value.Thyristor convertors or rectifiers are usually referred to

by the number of DC current pulses they produce eachcycle, the most commonly used being 6 pulse and 12

pulse.

There are many factors that can influence the harmonic

content but typical harmonic currents, shown as a
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percentage of the fundamental current are given in the

following table. Some content of the harmonics not

listed will always be present to some degree but forpractical reasons they have been ignored.

Order ofHarmonic

Percentage of HarmonicContent

6 Pulse 12 Pulse

1 100 100

5 20 --

7 14 --

11 9 9

13 8 8

17 6 --

19 5 --

23 4 4

25 4 4

HARMONIC OVERLOADING OF CAPACITORS

The impedance of a circuit dictates the current flow in

that circuit.

As the supply impedance is generally considered to be

inductive, the network impedance increases withfrequency while the impedance of a capacitor decreases.

This encourages a greater proportion of the currents

circulating at frequencies above the fundamental supply

frequency to be absorbed by the capacitor, and allequipment associated with the capacitor.

In certain circumstances such currents can exceed the

value of the fundamental (50Hz) capacitor current.These currents in turn cause increased voltage to be

applied across the dielectric of the capacitor. The

harmonic voltage due to each harmonic current added


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arithmetically to the fundamental voltage dictates the

voltage stress to be sustained by the capacitor dielectric

and for which the capacitor must be designed.

Capacitors of the correct dielectric voltage stress must

always be used in conditions of harmonic distortion toavoid premature failure.

HARMONIC RESONANCE

As frequency varies, so reactance varies and a point can

be reached when the capacitor reactance and the supply

reactance are equal. This point is known as the circuit orselective resonant frequency.

Whenever power factor correction is applied to adistribution network, bringing together capacitance and

inductance, there will always be a frequency at whichthe capacitors are in parallel resonance with the supply.

If this condition occurs at, or close to, one of theharmonics generated by any solid state control

equipment, then large harmonic currents can circulate

between the supply network and the capacitorequipment, limited only by the damping resistance in

the circuit. Such currents will add to the harmonicvoltage disturbance in the network causing an increased

voltage distortion.

These results in an unacceptably high voltage across the

capacitor dielectric coupled with an excessive current

through all the capacitor ancillary components. Themost common order of harmonics are 5th, 7th, 11th and

13th but resonance can occur at any frequency.

AVOIDING RESONANCE

There are a number of ways to avoid resonance wheninstalling capacitors. On larger systems it may be

possible to re-position the proposed capacitorinstallation onto another part of the system.

The same value of kvar installed at high voltage rather

than at low voltage may eliminate a resonant difficulty,or there may be other low voltage busbars where there

is no harmonic generating load. Varying the outputrating of the capacitor bank will alter the resonant


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frequency.

With multi stage capacitor switching there will be a

different resonant frequency for each stage. Changingthe number of switching stages may avoid resonance at

each stage of switching.

OVERCOMING RESONANCE

If resonance cannot be avoided an alternative solution isrequired.

A reactor must be connected in series with eachcapacitor switching section such that the

capacitor/reactor combination is inductive at the

dangerous frequencies but capacitive at fundamentalfrequency. To achieve this capacitor and series

connected reactor must have a tuning frequency belowthe lowest order of harmonic to be experienced, which

is usually the 5th.

This means the tuning frequency is usually in the range

of 175Hz to 230Hz, although the actual frequency will

depend upon the magnitude of the harmonic currentspresent. The actual tuning frequency will be varied to

suit the specific needs of each case.

The inclusion of a reactor in the capacitor circuit

increases the fundamental voltage across the capacitorin the order of 5 to 9% in addition to the harmonic

voltages previously mentioned.

COST EFFECTIVENESS

Due to varying site conditions, it is not always possible

to determine with certainty that resonance will occur.

Adding series reactors to power factor correction

equipment is expensive and can increase the cost touneconomic levels. If later found not to be required,

then unnecessary expenditure is incurred.

An intermediate step is to install appropriate capacitors

with facilities for the addition of reactors if found to benecessary at a later date, thus lowering considerably the

initial capital cost.

When capacitors are used in series with reactors they


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are rated at higher than system voltage, so when used

without reactors they have the ability to withstand

higher levels of harmonic overload, which alone mayresolve the situation.

If resonance does actually occur reactors can be addedto the existing power factor correction equipment at

minimum extra cost

LIMITS OF HARMONIC DISTORTION

Harmonic distortion can cause severe disturbance to

certain electrical equipment and as it is the duty of theelectric utility to provide a clean supply, many

countries now set limits to the harmonic distortionallowed on the distribution networks.

In the U.K. the Electricity Council EngineeringRecommendation G5/3 provides for three levels of

acceptance for the connection of harmonic generating

equipment, defined as stages.

STAGE 1 permits the connection of individual loads up to

14kVA at 415 volt and 25OkVA at 11 kV without special

consideration.

STAGE 2 limits the total harmonic current which anyinstallation may produce at the point of connection with

the supply authority, as follows:

HARMONI

C2 4 5 7 11 13

415V 48A 22 56 40 19 16A

6.6 - 11kV 13A 6 10 8 7 6A

STAGE 3. Individual analysis of systems is required to

ensure total harmonic distortion does not exceed 5% at415 volts and 4% at 11 kV

Before accepting harmonic generating loads, the

existing harmonic voltage distortion on the supplynetwork is taken into consideration in setting the

individual limits of Stage 3, and may also restrict themaximum limits as tabulated for Stage 2.


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Where these limits are exceeded, it may be necessary

to reduce or eliminate the harmonics produced.

REDUCTION OF HARMONIC DISTORTION

Harmonic currents can never be totally eliminated froman electrical system. They can, however, be verysignificantly reduced by using a harmonic filter.

In its basic form a filter comprises a capacitor connectedin series with a reactor tuned to the frequency to be

eliminated. In theory the impedance of the filter is zero

at the tuning frequency and therefore all of theparticular harmonic current is absorbed by the filter.

In practice, however, the capacitor and reactor areusually tuned slightly below the harmonic frequency.

This together with the natural resistance of the circuitmeans that only a small acceptable level of harmonic

current will flow in the network.

When it is necessary to reduce more than one harmonic,

a multi arm filter may be required.

TYPES OF FILTER

The effectiveness of any filter scheme depends on the

net reactive output of the filter, filter tuning accuracy

and the impedance of the network at the point ofconnection.

Harmonics below the filter tuning frequency will be

amplified. The experience of the filter designer istherefore important to ensure that insignificant

distortion is not amplified to unacceptable levels.

Where there are several harmonics present, a single

arm filter may reduce some harmonics whilst increasing

others, e.g. a filter for 11th harmonic may createresonance in the vicinity of 7th harmonic and high

magnification of any 5th harmonic already on thenetwork

In these cases it may be necessary to use a multi-armfilter where each arm is tuned to a different frequency

Experience is paramount in the design of such filters to

ensure:


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the most efficient and cost-effective solution is chosen

there is no adverse interaction either between the system

and the filter or between branches within the filter.

LOAD ALTERATION

Whenever load expansion is considered, with or without

additional power factor correction equipment, thenetwork impedance is likely to change and existing filter

equipment must be re-appraised in conjunction with thenew load condition and be suitably up rated.

It is not recommended to have two or more filters finetuned to the same frequency connected on the same

busbar system. Slight tuning differences may cause one

filter to take a much larger share of the harmonicdistortion, or even cause a harmonic resonance

condition leading to amplification of the very harmonicorder for which the equipment has been designed to

reduce.

When there is a need to vary the power factor

correction component of a harmonic filter, careful

consideration of all parameters is necessary.

HARMONIC ANALYSIS

To determine capacitor and filter requirements to meetspecific harmonic conditions, it is necessary to establish

with accuracy the impedance of the supply network andthe value of each harmonic current experienced at the

point of intended connection of any filter or power factor

correction capacitor.

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For further details contact

[email protected]: +44 (0)28 44 821197 Mobile: +44 (0)7860 638065

Fax: +44 (0)28 44 821360Copyright 2000 Philip J. Alcorn & Co.

While every care is taken to ensure that the information contained in this publication is correct, no legal responsibility canbe accepted for any inaccuracy. The Company reserves the right to alter or modify the information contained herein at any

time in the light of technical or other developments.
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