The notion of power quality may be familiar to many in the electricity sector, but it is gaining in importance and needs to be tackled.
The reasons include an increase in apparent power demand and the growth in distributed generation.
The latter is not given much weight at present because its influence percentage-wise is low. Yet there are already examples, even at low-voltage (LV) distribution levels, in which the high penetration of rooftop solar photo-voltaic (PV) generation causes harmonics and elevated voltage effects.
In addition, the power factor is lowered on days of high insolation.
The growth of solar PV and wind farms will accentuate the importance of power quality. These are examples of distributed generation (DG), adding to the familiar mix of combined cycle, diesel and gas generation.
Although the effect of DG is receiving attention from power generation and distribution authorities, the overall control of power quality will pose challenges as the growth of DG continues.
The electrical contracting industry is familiar with certain constraints placed on electrical installations at the point of common coupling (PCC).
The Australian Standard AS/NZS 61000-3-4 limits harmonic contribution at the PCC for LV distribution.
A similar Standard, AS/NZS 61000.3.6, pertains to medium-voltage (MV) distribution and high-voltage transmission. Larger installations are often connected to the MV network. These are likely to be 11kV installations, or lower voltages.
The growth in solar PV farms and wind farms is mainly at distribution voltage levels, which can be as high as 132kV. However, connection is very much dictated by geographical constraints, so lower voltages will also be encountered.
These renewable energy sources affect power quality because there is limited control over frequency, power factor and reactive power.
Until now, the concept of power quality has been understood as controlling harmonic emission from installations. The future will be quite different because of power quality problems on the supply side.
Low inertia sources
Although many issues affecting power quality from wind and solar PV sources are beyond the scope of this article, some aspects are highlighted because of their importance with higher levels of DG penetration.
Both types of DG are low-inertia forms and therefore dependent on the grid stability provided by high rotational inertia synchronous generators in base-load power stations.
The latter generators damp power surges by virtue of this high inertia. Wind and solar generation can result in large phase angle differences across distribution lines and consequent power surges.
In the case of stiff networks, those with a high reactance-to-resistance ratio (X/R), power flow can take place without much change to voltage. For slack networks, with an X/R less than 4, appreciable voltage effects are noticed.
Whose problem is it?
The foregoing considerations highlight the need for forensic power quality analysis.
For mission critical installations, and those in which electrical power plant is a large part of the productive assets, power quality monitoring will be of increasing importance.
Take harmonics – analysis of dominant harmonics on the incomers may be required because of their potential to stress the power factor correction equipment.
The harmonic aspect is already a vexing one. Is harmonic voltage distortion the power supplier’s problem, or is it brought on by the installation?
To gain forensic information, synchronous timing with the supplier is an essential feature. This article provides basic information on power quality analysers as furnished by participating companies.
As will be evident from a reading 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.
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 Pn of an individual harmonic of order n is given by the familiar formula:
This formula looks the same as that used for determining kilowatts given rms voltage, current and displacement power factor (cos ) for an installation.
The elements that have changed are the subscripts. Thus Vn and In are the rms voltage and current of the nth harmonic. The phase angle n 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 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 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.
To download the comparison guide, click here.