By John Thompson, Westek Electronics Pty Ltd

Power quality testing requires instrumentation capable of measuring harmonics, inter-harmonics, flicker as well as spikes, and voltage sags and swells. The need for power quality testing is not confined to supply authorities and companies. It is equally applicable to internal reticulation for commercial and industrial installations. Although specific instruments are available for flicker testing, the combination of this test with that of harmonics, etc is a far more cost-effective since flicker is affected by changes in harmonic spectra, inter-harmonics, and impedance changes. Thus with an instrument such as the Voltech PM6000, which combines these measurement capabilities, analysis and finding solutions to improving power quality is greatly facilitated.
 
Each measurement channel on the PM6000 is a fully floating wattmeter with 10 MHz effective bandwidth and rejection of common mode signals (CMRR). Up to 6 channels may be fitted into a PM6000 chassis, which has an embedded PC running the Linux operating system. The VGA display provides numeric and graphic measurements and shows the intuitive measurement menu. Connectivity includes RS232, printer and Ethernet. The display may be split to show numeric and graphical information at various zoom levels. Colour is used to identify measurements from one particular phase or group of phases. Communications options include RS232, IEEE488 and printer.
 
High sampling speed is necessary in order to capture dynamic effects such as inrush current, changes in harmonic spectra because of load variations, inter-harmonics through leakage of variable speed drive inverter signals via the DC link to the input converter stages, sub harmonics caused by rotor slot deformation and rotor vibration, etc. The advent of DSP chips with high-speed sampling has made dynamic analysis a reality. The Voltech PM6000 uses 40 MHz DSPs and with minimum sampling frequency of 5 MHz. Unlike many other analysers, the Voltech PM6000 uses the correlation (comparison with a standard signal) technique to compute the Fourier series. This is a highly accurate technique for discrete Fourier transformation (DFT) and in many ways superior to FFT (the Fast Fourier Transform as originated by Cooley and Tukey in 1965). Because of the high floating-point operations necessary per unit time, a high speed central processor capable of utilising the 10-cycle sample width (50 Hz) or 12 cycle (60 Hz) complements the instrument.
 
One of the frequently ignored harmonic effects is the straining of transformer capacity. High levels of harmonic current cause high heat loads in transformers by virtue of the increased eddy current losses in the windings over and above the level due to the fundamental (50 Hz) current as well as increased iron losses. The additional losses are proportional to the square of the frequency and to the square of the harmonic current. The ends of transformer windings are particularly susceptible to overheating because of a concentration of leakage magnetic fields. In practice the presence of high levels of harmonic current requires the derating of the transformer. There are however problems associated with derating transformers. These can include primary over-current protection levels, which when set lower to take account of the derating factor, may cause tripping of protection gear during inrush current conditions and also higher than necessary core losses through the use of oversize cores. The use of so-called K-factor transformers, on the other hand takes into account the harmonic levels and therefore is a much better solution. This is particularly so when growth of an electrical installation is taken into account; a derated transformer is far more likely to become overloaded as more and more harmonic loads are connected.
 
The Voltech PM6000 can make K-factor calculations using the formula as shown below:
 

 
Flicker measurement is a more comprehensive indicator of power quality than individual measurements as listed above. Digital measurement is the most repeatable process. Flicker measurement is based on modelling the behaviour of a 60-watt, 230 volt Tungsten lamp. The voltage is digitised, passed through a quadratic converter on to a high pass (0.53 Hz) filter, then on to a low pass (35 Hz) filter. Thereafter a weighting filter with peak pass at 10 Hz provides the signal to second quadratic converter and finally to a low pass (0.53 Hz) filter modelling the human eye. The short-term measurement of flicker, PST is the basic measurement from which the long-term flicker PLT is derived. The principle of measurement is the calculation of time percentages that the voltage is between and outside limits. The flicker tests are part of the Voltech PM6000 repertoire of tests.
 
The ability to measure harmonics and flicker accurately translates into product testing and is provided for in the Voltech PM6000. The relevant standards are IEC61000-3-2 and IEC61000-4-7 thus including inter-harmonics groupings and IEC61000-3-3 and IEC61000-4-15 for flicker testing. To complete product testing pure sine wave power supplies with definable output impedances are required.


<< Click here to contact us regarding this article >>