Electrical Connection Winter 2016

www. e l e c t r i c a l c o n n e c t i o n . c om . a u

of the technical literature on power

quality analysis, specific measurement

tasks require careful checking of an

instrument’s capabilities. Always

discuss your requirements with

suppliers of power analysis

equipment. In general, compliance to

IEC61000-4-30 part A is necessary for

authoritative measurements.

DISTURBANCES

Harmonic analysis is a basic

requirement and there are several

aspects to be considered.

The principal matters are speed of

analysis, data gathering and statistics.

Speed is obviously important

when current and voltage distortion

fluctuate rapidly.

Data gathering (aggregation in 10

cycle groups for example) is of value

for comparing local plant conditions

with incoming power line observations.

In order to be complete, harmonic

analysis requires the ability to measure

inter-harmonics and sub-harmonics.

The former are high-frequency

components but not integers of the

fundamental frequency; the latter

are integer fractions of the

fundamental frequency.

The ability to determine power flow

is important. In principle, considering

power frequency harmonics, the power

of an individual harmonic of order is

given by the familiar formula:

P =V

cos

This formula looks the same as that

used for determining kilowatts given

rms voltage, current and displacement

power factor (cosФ) for an installation.

The elements that have changed

are the subscripts. Thus

and

are

the rms voltage and current of the nth

harmonic. The phase angle

is the

one between voltage and current of

the same harmonic order.

When it comes to harmonics,

we tend to be on the defensive.

It’s assumed that voltage to the

installation is harmonics free, and

we are the ones pumping out current

harmonics.

On that basis an installation is

exporting harmonic power, and

the phase angles for the harmonics

are somewhere between +90

and

180

. However, it is just as likely that

harmonics are being imported, that

is, with phase angles between +90

and 0

The phase angle ranges for export

and import allow for capacitive and

inductive reactive current components.

Long-term observation and

switching in and out of loads suspected

of being harmonic current contributors

can help with confirming the export-

import question.

A better method is to use a

power quality analyser that is GPS

synchronised so that observations

at an installation can be correlated

with data gathered by the

power supplier.

UNBALANCED VOLTAGES

Unbalanced voltages and

currents can be analysed in

symmetrical components.

Readers familiar with protective

relays for transmission and

distribution will know all this.

Symmetrical components analysis

resolves voltage and current phasors

into positive (a-b-c sequence),

negative (a-c-b sequence) and zero

sequence components.

This may seem academic but there

are very practical consequences.

Induction motors, for example, do

not respond well to unbalanced

voltages. Rotating machinery and

transformers in general have different

impedances for negative and zero

sequence components.

Therefore the ability of a power

quality analyser to resolve unbalance

by means of symmetrical components

is an advantage.

There are other methods for

calculating imbalance – for example,

taking the max difference to the

average three-phase quantities, as a

percentage or ratio.

Symmetrical components analysis is

superior in that it quantifies the value

of the negative sequence component,

as that is the one with a deleterious

effect on rotating machinery.

FLICKER

Flicker annoyance stems from

fluctuation in luminance to the

human eye.

It is caused by voltage variations,

and measurement of something that is

essentially subjective is not easy.

The eye has a frequency selective

response, being most sensitive to a

frequency of 8.8Hz, and falling to zero

at 30Hz and above.

Notwithstanding basic subjectivity,

once a standard for measurement has

been established, the advantage of

uniformity of analysis prevails.

The basic IEC method of flicker

measurement uses a squaring

demodulator to extract the

‘modulation’ superimposed on the

voltage (flicker). This is followed by

a low-pass filter to model the human

eye, and a squaring amplifier to

provide the lamp response.

There are other methods,

including analysis based on fast

Fourier transform, providing flicker

measurements in accordance with

the IEC Standard (IEC61000-4-15).

BUILD IT INTO INSTALLATIONS

There will be more instances in

which power quality analysis needs

to be conducted at or near a PCC for

a planned installation.

This will properly provide for

mitigation of harmonics and flicker,

and will ensure that realistic demands

are being made by the supplier once

connection is made.

n n

PHIL

KREVELD