Haifeng Sun

Modeling of Transformer Windings Under Very Fast Transient Overvoltages

To calculate the overvoltages in transformer windings under very fast transient overvoltages (VFTOs) generated by switching operations in gas-insulated substation (GIS), a single-input and multiple-output circuit model, which is based on a multiconductor transmission line (MTL) model, is presented in this paper. The procedure to determine the transfer function is deduced. Vector fitting and recursive convolution are used to calculate the time domain response. The resonant frequencies and the location of the resonance are determined by an analysis of the amplitude-frequency response characteristics. For validation, the calculated results are compared with the measurements on a transformer winding model, and the results are found to be satisfactory

Analysis of resonance in transformer windings under very fast transient overvoltages

To study the resonance in transformer windings, under very fast transient overvoltages (VFTOs) generated by switching operations in gas insulated substation (GIS), a single input and multiple output circuit model is presented in this paper which is based on multi-conductor transmission line (MTL) model of transformer windings. The procedure to determine the transfer function is deduced. The resonant frequencies and the location of resonance are determined by the analysis of amplitude frequency response characteristics. For validation, the calculated results are compared with the measurements on a transformer winding model and the result is satisfactory

High-frequency EMTP model of transformer windings

An EMTP model is presented for calculating overvoltage in transformer windings under VFTO in this paper. Firstly, the voltage transfer functions are calculated from the S parameters which are measured by network analyzer or calculated from the distributed parameters of transformer windings. Secondly, the voltage transfer functions are fitted with rational functions by vector fitting codes and then the rational transfer functions are order-reduced by Fade approximation. Lastly, the resultant voltage transfer functions are synthesized by the elements in EMTP. The calculated voltages from the circuit by EMTP are in good agreement with the measured voltages. This shows that the method is simple and robust

Calculation of overvoltage distribution in HVDC thyristor valves

Thyristor valves are the core components in HVDC transmission system. These valves in the system are exposed to various overvoltages both external and internal. So it is important to analyze the characteristic of the converter valves under different overvoltages. Wide-band equivalent circuits of the valve components such as thyristor, saturable reactor are obtained based on impedance measurement. And stray capacitances of valve sections in valve tower are included in these wideband models. Based on these models, overvoltage distributions in the valve tower are calculated and analyzed under different kinds of surges, including switching, lightning and fast front. To illustrate the effects of shielding plate, three different cases are calculated and analyzed. And some useful conclusions are presented in this paper.

Study on parameters calculation and order reduction of transformer model

Accurate elementary parameters play a very important role in building the high-frequency model of a transformer. A very efficient method, the simplified finite element method, for computing elementary parameters (turn to turn capacitances and inductances) is presented in this paper, which greatly promotes the calculation efficiency. The multi-conductor transmission line model is one of most accurate models, which can be used to analyze the transient response of the transformer windings in detail. This model is based on the representation of the winding by its individual turns. Each turn is simulated as one transmission line, so the intercoupling transmission lines can represent the whole winding. This means we have to deal with the equations with large scale. With the technology of order reduction, we can smoothly settle this problem. For validation, the calculated results are compared with the measurements on a real transformer winding and the result is satisfactory.

A New Substitution Waveform of Very Fast Transient Overvoltage in GIS

So far there is no standard waveform of very fast transient overvoltage. Considering the amplitude, frequency and frequency weighted from a great deal of data and according to IEC 61321-1, an accurate substitution waveform of very fast transient overvoltage is presented by means of probability statistical and time–frequency analysis technique. Simulation validation is carried out while the simulated waveforms and substitution waveforms excite the transformer winding and potential transformer, respectively. Test results indicate that the proposed method is highly reliable.

Power electronics devices modeling by traditional equivalent circuit and black-box theory

With the application of semiconductor switch in power systems, wideband modeling of related devices is necessary during the recent years. In most cases, simple equivalent circuit models are available to the designers, which correlate well with the measured parameters at lower frequencies, however, deviate at higher frequencies. Black-box modeling can obtain precise results, but the elements in these models have no physical meaning. In order to address these difficulties, this paper presents an efficient method to model the passive devices with traditional equivalent circuit combined with black-box theory. The new method enables the designers to retain their existing physical models while providing a means to capture the high frequency effects accurately.

Modeling of The Transformer Windings under VFTO based on Transfer Function

A systematic methodology is presented for establishing the high frequency circuit model of transformer windings under VFTO based on transfer functions in this paper. Firstly, the voltage transfer functions are deduced from the S parameters which are measured by network analyzer. Secondly, the voltage transfer functions are fitted with rational functions by vector fitting and then order-reduced by modified Routh approximation method. Lastly, the resultant voltage transfer functions are synthesized by lumped elements. The simulation results from the circuit are in good agreement with the measured. This shows that the method is correct.