Ryszard Malewski

Impulse testing of power transformers using the transfer function method

The transfer function of a transformer winding is deconvoluted in the frequency domain from the digitally recorded neutral current and high voltage applied during impulse tests. The integrity of the winding insulation is determined by comparing the transfer function obtained at full and reduced test voltage. Differences between the transfer function plots reveal local breakdowns in the winding that can be dissociated from partial discharges. Thus the method permits unambiguous acceptance or rejection if the transformer and, since the transfer function is theoretically immune to changes in the applied impulse, also allows evaluation of the chopped-impulse test. Some 100 windings of large HV power transformers have been tested using the transfer function method, which on several occasions has revealed transformer faults as well a test setup problems that would have been missed or misinterpreted by conventional techniques. >

Measurement of switching transients in 735 kV substations and assessment of their severity for transformer insulation

An automatic measuring system that digitally records transients generated by EHV-transmission-system switching operations and lightning strokes has been developed. The severity of these transients was assessed by comparing them with test voltages applied to the transformers during acceptance tests. In order to quantify in-service transients with respect to the impulse-test voltages, the frequency spectra of the time-domain records were calculated and expressed as a percentage of the test level. The presence of high-frequency winding oscillations was revealed by simultaneous measurements of the bushing voltage and neutral current. The results of these measurements are presented in the form of a transfer function, which can also be used to detect winding displacements in service. >

Comparative evaluation of computer methods for calculating the best-fit sinusoid to the digital record of a high-purity sine wave

A comparative evaluation of programs used in the “equivalent bit” test is described. Four methods of determining the best-fit sine wave were tested using computer generated data contaminated to varying degrees with white noise. These data were supplemented with actual records of sine waves obtained using fast digitizers with 6- to 12-bit resolution and record lengths of between 512 to 16 000 words. The programs were evaluated for relative accuracy, absolute accuracy, and efficiency, both in terms of how often each method converged and also of how much computer time each method took to converge.

Modeling of the dynamic performance of transient recorders used for high voltage impulse tests

Digital recorders are becoming more commonly used in the recording of steep-front HV impulses. Although many digitizers exhibit a nonlinear deterioration in dynamic performance with increasing input signal steepness, there is no accepted method for predicting the magnitude of the resulting errors. The authors describe the development and implementation of a computer modeling technique for establishing the magnitude of these errors. The model is based on time-domain test data combined with a basic understanding of the digitizers operating principles. The model is general in nature and can be used for the prediction of errors generated by any recorder that has systematic errors. The model was used to predict the maximum error which can occur when a particular EBS (electron-bombarded semiconductor) recorder is used to monitor HV impulse tests on power apparatus. Its use allowed for the determination of error limits for records of steep-front impulse tests which will be standardized in the near future. >

Comparison of Three Techniques of Partial Discharge Measurements in Power Transformers

The paper presents a comparison of partial discharge (PD) measurements taken on 140 large power transformers using radio-influence voltage (RIV) meters calibrated in pV according to NEMA 107 Standard and also with the aid of wide-band and narrow-band PD detectors calibrated in pC according to IEC Publication 270. Results are presented in the form of pC/¿V ratios and compared to data from the literature. In conclusion the RIV measuring technique is considered inappropriate for PD measurements on power transformers, although it can be employed to detect the presence of the discharges.

Elimination of the Skin Effect Error in Heavy-Current Shunts

A mathematical analysis of the transient skin effect in tubular and flat conductors has led to the development of a theory of ideal heavy-current shunts immune to the measuring error caused by this effect. A general expression was derived for calculating a profile of the measuring circuit laid inside the shunt wall that would give a step response undistorted by the skin effect. A model of the new shunt ws examined with a current step; the recorded response followed the applied pulse form.

Digital Techniques in High-Voltage Measurements

A review of digital techniques and instruments developed and applied to high-voltage measurements is presented. Advantages of digital instruments over their analog homologues are exposed together with some examples of processing of digitized signals recorded in HV laboratories.

Micro-ohm shunts for precise recording of short-circuit currents

A new design of the tubular shunt is presented which offers a reduction of the response time up to 10 times without changing the volume of the resistive material and preserving a regularly shaped response. A new formula is derived which gives the necessary data for calculating the improved shunt. Another possible design option is also presented which gives a shunt with zero response time but then the response curve indicates a larger overshoot. A simple mechanical design is presented, particularly suitable for heavy current shunts in the microohm range. Two such shunts were built for precise recording of steep front, short-circuit currents with a large asymmetry. They were designed according to the new formula and experimentally examined with a step impulse current. The measured response time of these shunts are in agreement with the calculated data.

A Comparison of Instrumentation for Measuring the Losses of Large Power Transformers

The results of a comparison of several different types of equipment and techniques for measuring the short circuit (copper) and open circuit (iron) losses of a large power transformer are presented. The measurements were made on a 233-MVA, 735-kV, single phase, 60-Hz power transformer with a short circuit power factor of 1.3%, at currents up to rated, and open circuit voltages to 115 percent of the rating. The short circuit tests were performed using two current comparator type high-voltage capacitance bridges, a thermal wattmeter, an electro-dynamic wattmeter system with conventional instrument transformers and a new semi-automated digital wattmeter system. For the open- circuit tests only the three wattmeters were employed. The average discrepancy between the five measuring systems used in the short circuit tests was less than 1% and between the three systems in the open circuit tests less than 0.5 %.

Digital Impulse Recorder for High-Voltage Laboratories

A digital recorder has been developed for use in HV laboratories as a prospective replacement for conventional impulse oscilloscopes. The new apparatus has a sufficiently high time resolution to record the fastest microsecond impulses used for HV testing and also allows slower switching transients to be monitored.