Chopped Lightning Impulse Research Articles

Precise Modeling of Chopped Lightning Impulse Test of Power Transformers

Performing insulation tests on power transformers is one of the most essential topics in high-voltage laboratories. According to international standards like IEC 60,076, full and chopped wave Lightning Impulse (LI) voltages must be applied to high voltage transformers. Since the wave shape parameters of the applied impulses must comply with the constraints that are specified in these standards, the setting of the impulse generator and test circuit must be modified correctly by test engineers. An accurate computerized simulation method is very desirable to help them accomplish this duty quickly and enhance the outcome results. Therefore, this paper proposes a method that is capable of simulating both Full Wave (FW) and Chopped Wave (CW) impulses in the time domain accurately. In this method, the frequency response of the transformer is measured and then the Vector Fitting (VF) technique is used to obtain its admittance model. Additionally, the transformer's impulse test circuit elements are precisely modelled. Afterwards, by solving the equations of the equivalent circuit in the state-space system, the time domain signals of all state variables are calculated. Finally, the output impulse voltages are computed. Several practical tests are accomplished on two power transformers to ensure the correctness of the proposed method.

Investigation on the Resonant Oscillations in an 11 kV Distribution Transformer under Standard and Chopped Lightning Impulse Overvoltages with Different Shield Placement Configurations

This paper presents an investigation on the resonant oscillations of an 11 kV layer-type winding transformer under standard and chopped lightning impulse overvoltage conditions based on calculated parameters. The resistances, inductances and capacitances were calculated in order to develop the transformer winding equivalent circuit. The impulse overvoltages were applied to the high voltage (HV) winding and the resonant oscillations were simulated for each of the layers based on different electrostatic shield placement configurations. It is found that the placement of grounded shields between layer 13 and layer 14 results in the highest resonant oscillation and non-linear initial voltage distribution. The oscillation and linear stress distributions are at the lowest for shield placement between the HV and low voltage (LV) windings.

Effect of the overshoot level in an alternative method for processing of tail chopped lightning impulses

The construction of new Ultra High Voltage laboratories requires more studies about standards for equipment as well as for testing and measurement techniques. Currently, the analysis of tail chopped lightning impulse waveforms is possible only by prior application of a full reference lightning impulse, creating difficulties for testing and processing of waveforms. A new method for evaluating the relevant parameters of tail chopped waveforms has been previously developed and this paper studies the effect of the overshoot in this alternative method.

High-Frequency Model of the Power Transformer Based on Frequency-Response Measurements

In this paper, a new high-frequency black-box model of the power transformer is presented for the calculation of transferred lightning overvoltages. The model is based on frequency-response analysis measurements and the application of four-terminal network theory. The computational verification of the presented model was successfully performed on a white-box transformer model in Electromagnetic Transients Program-Restructured Version software. The experimental verification was carried out on 130-kVA and 36-MVA three-phase power transformers. The presented calculation and measurement results confirm the validity of the proposed model for full and chopped lightning impulse voltages.

Resonant behavior of EHV transformer windings under system originated oscillatory transient over voltages

Grid connected EHV transformers experience various terminal disturbances when in service. The present work attempts to investigate the voltage stresses that may develop on the transformer insulations under a variety of terminal disturbances. A number of standard and non-standard wave shapes like lightning impulse, chopped lightning impulse, steep-front long tail switching surge and oscillatory transient over voltages have been simulated and impressed on the terminals of a 400 kV EHV power transformer operating in the Indian power grid to ascertain how the winding insulations are stressed under these disturbances. Relevant section of the Indian power grid and the transformer has been modeled using Alternative Transient Program (ATP). It has been established that oscillatory system transients can trigger natural resonate frequencies of the transformers causing high voltage stresses on the insulations. Short Time Fourier Transform (STFT) analysis of the oscillatory voltage response of the windings confirmed the presence of resonant frequencies indicating forced resonance. Some remedial measures involving winding design modifications have been suggested in the paper to overcome the problem.

Evaluation of parameters of lightning impulse voltages from transformer tests using the new k-factor approach

Present procedures for evaluating full and chopped lightning impulse voltages in accordance with IEC 60060-1:1989 [1] are only an approximation of the physical behavior of the insulation and do not always give reproducible results for impulse waveforms with superimposed oscillation and overshoot. The results of a recent study have provided technical basis for developing new procedures with improved accuracy. This paper evaluates a number of procedures being considered by the relevant CIGRE, IEC and IEEE working groups. In particular, practical waveforms recorded during testing power transformers are evaluated in this study. The evaluation of these waveforms is useful and important because they cover a wide range of practical impulse shapes, overshoot amplitude and frequencies. In addition, the results of the new procedures are compared with those of the procedure specified in IEC 60060-1:1989.

Multi-resolution wavelet analysis for chopped impulse voltage measurements and feature extraction

In this study, an application of the wavelet transform based on the multi-resolution analysis (MRA) is aimed for evaluation of amplitude and time parameters of the chopped impulse voltage. In terms of getting data set, three types of the chopped lightning impulse voltages with different chopping times are considered and MRA is applied to these data as different case studies. Hence, some characteristic properties are extracted from the impulse waveforms at some special frequency values. In this sense, frequency component of 33 MHz is found with the maximum peak value for each case before the chopping time and their appearing times for three cases are determined using the time-frequency analysis. Hence the difference between the chopping time and occurring time of the maximum peak values at 33 MHz is calculated easily. Also, some physical interpretations of the frequency component of 33 MHz are defined through the circuit parameters of the measurement system. In usual application, the MRA can be presented as a powerful technique to extract the noise effects, which come from different sources, in the high voltage measurements.

FDTD Analysis of the Unit Step Response of a Circuit Measuring Chopped Lightning Impulse Voltages

Unit step response (USR) parameters of a circuit for measuring chopped lightning impulse voltages are calculated using the finite-difference time-domain method for solving Maxwell's equations, and compared with those of a USR-measuring circuit recommended by the International Electrotechnical Commission (IEC). Although the circuit for measuring chopped lightning impulse voltages is quite different from the IEC-recommended USR measuring circuit, USR parameters calculated for the former circuit are not much different from those calculated for the latter circuit except the case when the former circuit includes both a low-impedance impulse high voltage generator and a damping resistor.

Digital measurement system for high-voltage impulse tests on transformers

More information from test data can be obtained when digital measuring techniques are used. A description of a tailor made measurement system in use by a Dutch high-voltage laboratory is presented. Schematic diagrams of the test setup and the measurement system are shown and the precautions taken to avoid electromagnetic compatibility (EMC) problems are described. Software facilities and their realization are described together with algorithms for waveshape determination. The use of a digital signal processor board for transfer function calculations is explained. Examples of tests with full and chopped lightning impulses are presented both in time and frequency domain.

An impulse voltage calibrator of low output impedance

A calibrator that produces full lightning impulses and chopped lightning impulses with variable chopping time is described. The calibrator uses series-connected power metal-oxide field-effect transistors as the main impulse switch and insulated gate bipolar transistors (IGBTs) as the chopping switch, and its output impedance is low to drive a termination impedance of 50 /spl Omega/. The performance of the calibrator is verified by comparing the simulated calibrator output parameters based on the calibrator component values and output parameters calculated from measured output waveforms using a calibrated digitizer.

数値電磁界解析による裁断波雷インパルス電圧試験回路の直角波応答評価

数値電磁界解析による裁断波雷インパルス電圧試験回路の直角波応答評価

A transfer function is a reliable tool for comparison of full- and chopped lightning impulse tests

A new diagnostic tool available with digitally recorded waveforms for lightning impulse tests on a transformer is the so called transfer function. For the calculation of the transfer function, two waveforms are recorded. The first waveform is the impulse voltage and the second waveform is the impulse current. Both waveforms recorded in the time-domain are transformed to the frequency-domain and the transfer function is calculated. This paper proves that the transfer functions of a sequence of impulse tests with the same test circuit can be compared with each other. In the author's opinion, the differences found between transfer functions are mostly due to hardware and software which is not suitable for the determination of the transfer function. A statement about reliability is only possible if the technology used for determination of the transfer function is beyond any doubt. This paper compares the transfer functions of the full impulse waveform and the chopped impulse waveform on the same terminal of the transformer and test circuit, which is the worst case condition for the comparison, to prove that comparison of transfer functions is possible.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Function representation of lightning impulses produced by a testing circuit with continuity of voltages and currents at a tested object

Mathematical formula describing full and chopped lightning impulses occurring when a tested device is represented by a complex impedance, and for nonzero initial conditions at a chopping moment, have been derived. The derived method of parameter calculation of the formula fitting chopping of the impulse is presented. Interpolation formulas for computation of the full impulse formula coefficients are proposed. An application example of the method is also given.

Interactions between two dividers used in simultaneous comparison measurements (of power system impulses)

A revised international standard for the measurement of lightning and front-chopped lightning impulses is presently under consideration. This standard states that the accuracy of these measuring systems is to be determined by comparison to reference systems maintained by appropriate national laboratories. Investigations have been made of the interactions between two systems configured for simultaneous measurements and of methods for minimizing these interactions. Step responses were measured for different configurations and a model developed to predict divider response. Simultaneous measurements were made of full and chopped lightning impulses using different divider systems to determine the effects of divider interactions on such measurements. >