Before high-voltage (hv) equipment is placed into service, it must be checked for its operational safety and functional ability. This takes place by means of tests which are laid down in regulations and recommendations. The tests which are laid down in regulations and recommendations.

The tests should anticipate the service conditions in abbreviated form and to reveal weak points. However, they should not overstrain the equipment and shorten the life of the apparatus.

Equipment, which in spite of having passed tests, fails during operation and hence an additional check on its quality without doing any harm would be appreciable. Weakness of the insulation, whether they were there originally, or resulted from gradual ageing cause ionisation or partial discharges (PD) at a certain voltage. These occur as electrical oscillations in the measuring circuit in the form of ultra-sonics and light emission.

If these symptoms are recognised early enough, the danger of a total break down (BD) can be promptly foreseen. Therefore, with increasing electrical stress and further tightening of the construction of HV apparatus, non-destructive test like the PD measurement (PDM) is an important aid to quality control (QC) of electrical insulation.

2. PDM AS A SUBSTITUTE FOR USUAL TESTS

The test voltages are generally, high compared with the operating voltages, and of long duration and may create weak points. In order to reduce the danger of creating weak points - with the simultaneous measurement of PDs - the test voltage can be reduced in amount and duration can be increased, whereby the requirements to be met during operation later, still must be reproduced or substituted accordingly.

3. PREMISES FOR QC BY MEANS OF PDM AND DISTINCTION OF THE ORIGIN OF PDs

PDs in the hv insulation cause a change of voltage charge and energy of the sample-object as a whole. As a measuring quantity the IEC recommends preferably the apparent charge in pico coulombs (pc). The measured quantity is much smaller than the actual discharge quantity in most cases and may be different for different terminals of the sample. This latter property is used for locating PDs in hv equipments. If PDs occur, which appear dangerous, because they exceed the prescribed values during the test, ensuring that these pulses are due to discharge occurring in the specimen and not in terminations or other parts of the test circuit. Finally an estimation must be made of the type- internal, surface or corona discharges, and the danger posed by detected discharge. Internal discharges, in many cases are crucial for the life of the insulation, the surface discharge stands the next and so on. This task requires much expertise, which is frequently not available since experts are few and also takes a long time to consolidate.

4. STATISTICAL APPROACH

Individual discharge magnitude could indicate the relative size of degradation sites. The magnitude/phase relationship of these pulse on the power cycle and the way these vary with applied voltage and time, could provide valuable data on the nature and form of degradation prevalent. Shapes of the PD distributions also are considered for evaluation.

Being highly random, inconsistent and subjective, PDM are deciphered into unambiguous mathematical descriptors and discharge parameters; these signatures are characteristic for different types of discharges and offer good discrimination between different defects.

5. SOFTWARE TOOL (DPDSTOP) AS A QC TOOL AND CASE STUDY

A highly user friendly software using Turbo C compiler was developed and tested using a standard set of distribution patterns. The results obtained were encouraging. Batch processing of Metal-oxide Surge arresters (SA) was taken as a case study to test the tool DPDSTOP (Detection of PD Sources based on STatistical OPerators). Input to the program is PD distributions and output is a set of values characterising the discharge sources or defect. This classification allows the QC crew to decide whether the PDs are non-dangerous or dangerous (>50 pc) whereby mere tightening of the SA-assembly or change of one or more internal blocks of the assembly and so on. With this tool any powerapparatus could be the subject and QC is thus accomplished. Further support of the results can be done by application of FFT analysis to the data of industrial objects acquired.