On-line PD Testing for Cables & HV Plant

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by:
Ross Mackinlay, High Voltage Solutions UK
Arthur Bryan, M&B Systems Power UK.

The new generation of PD testers is geared to give the best range of measurement methods ever offered.

Twenty years ago the main method of testing the insulation of both switchgear and cables was to arrange an outage, apply a high voltage and then measure the tan delta or measure the partial discharge (PD) activity.

Technology-wise, things have come a long way since then. Now, however, a huge driver for industry and utilities is to keep the system live with as small an outage requirement as possible; hence the boom in on-line methods for testing high voltage plant.

For this reason M&B Systems Power Instrumentation Ltd., along with its consultant Dr Ross Mackinlay, has developed a new generation of digital PD detector. This instrument provides engineers with a single piece of equipment capable of testing motors, generators, switchgear and cables. The system includes Rogowski type current transducers, capacitive probes and acoustic transducers.

The main reasons for testing cables and switchgear on-line are…

• Prediction of service performance and likely failure risk
• Safety to operational staff working with the plant
• Planning maintenance and other scheduled outages
• Commissioning tests on new plant
• Monitoring quality of repairs and installations (whether with contractors or in house)

Actually, it makes a great deal of sense to assure that critical plant is not likely to fail or give service difficulties. However, the methods must be cost effective and accepted. Both of these criteria are starting to be met.

Spot testing using PD as the basis of insulation testing is becoming much more reliable, and certainly user friendly, with on-line methods giving more secure data. Only in rarer cases when the PD activity is highly non predictable, is continuous monitoring needed. In most cases spot testing for 10 to 15 minutes provides a good indication of system PD performance. It also shows when continuous monitoring would be an advantage.

With the new 'longshot' technology (see figure 1 below), the latest Partial Discharge testing instruments use all of the methods of peak and count for Partial Discharge detection across the phase of the power cycle.

Figure 1: Extracting PD waveforms from one cycle of PD information

In addition, they use the waveforms of the PD pulses themselves to carry out simple locations on cables, measure the PD pulses on cables in pC (Pico Coulombs) with no calibration required and, for switchgear, reducing noise by eliminating pulse shapes which are not PD based.

The integration of all the PD methods yields the following measurement advantages:-

• Correction of PD pulses for attenuation in on-line monitoring
• Mapping of the PD signals in the normal way
• Simultaneous representation of the mapping data on a magnitude/phase plot
• Noise reduction algorithms using active filters or wavelet transforms to reduce the effect of radio interference
• Capturing of sequences of ‘longshot’ waveforms of the power cycle can be used to do pulsed noise reduction by gating and other similar methods
• Use of transponders for PD location on cables can still be used for accurate location

The instruments to carry out these new 'longshot' methods must have long memory digitizers with sampling rates between 100 and 500 Msamples/Sec and this has been achieved.

The new generation of PD testers, seen at right, is geared to give the best range of measurement methods ever offered in a PD measurement instrument. As the power of the testing for either PD location in cables, or routine scanning of switchgear, lies in the software, this means that investment is preserved as new facilities arrive in the form of software updates.