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| A Method for Stray Capacitance Measuring and Refined Modeling of Potential Transformer Based on Low Frequency and Narrow Band Scanning |
| Shen Zeliang1, Wang Jingang1, Wang Qian2, Zhao Pengcheng1, Yan Xiaojun1 |
1. State Key Laboratory of Power Transmission Equipment & System Security and New Technology Chongqing University Chongqing 400044 China;
2. Chongqing Electric Power Research Institute Chongqing 401123 China |
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Abstract The low-frequency responses of potential transformers (PT) are essential in power system protection. Due to the complex winding structure and large transformation ratio, the stray capacitance will significantly impact the dynamic performance of the PT. Many studies have analyzed the distribution of the electric field and magnetic field energy of the PT at low frequencies to determine its modeling method of it. However, it is difficult to obtain the stray capacitance value of a PT without its detailed internal structure parameters, which makes it challenging to build an accurate low-frequency simulation model.
Firstly, the electric field energy distribution for two typical winding structures was analyzed, and the corresponding stray capacitance values could be calculated. The capacitance calculation method explained the generation mechanism of the PT and could provide references for the design progress. However, most users of the PT are not involved in the design progress and need help understanding its detailed internal structure parameters. Therefore, a non-destructive experiment method is necessary for the stray capacitance evaluation. Secondly, the equivalent model of PT stray capacitance distribution at low frequency was built, and the low-frequency dynamic responses under electromagnetic field coupling of the core and coil were analyzed. The obtained model can provide support for the experimental designs. Finally, a series of tests were carried out to obtain the specific value of PT stray capacitance. By connecting the inductance in series between the PT and the frequency conversion power supply port, the resonance point of the sweep frequency test was reduced to the low-frequency range, and the value of the stray capacitance was accurately calculated. Since this method requires a narrow scanning frequency band and a low scanning frequency, the hardware requirements for power supply and measuring equipment are reduced, and the loss problem caused by high-frequency testing is avoided. Furthermore, the capacitive and excitation current components in the PT port current could be separated, and the hysteresis curve could be accurately drawn.
Based on the problem that it is difficult to measure the PT stray capacitance and hysteresis curve in engineering, this paper reduced the resonant frequency point of the sweep frequency test using series inductance and calculated the stray capacitance. Furthermore, the capacitive component in the current of the PT port could be excluded, and the accurate drawing of the hysteresis curve of the iron core could be realized. Combined with the short-circuit test and DC resistance measurement results, the π-type equivalent circuit of PT was constructed. The simulation results had good frequency response characteristics. The Pearson correlation coefficient was introduced to study the accuracy of the simulation model, and the correlation coefficient with the test results remained above 95 %. Due to the accurate description of the inductive and capacitive coupling, the model can be applied to the ferromagnetic resonance simulation of a power system to obtain more accurate simulation results and provide a reference for the design of the anti-resonance scheme.
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Received: 24 January 2022
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2023, Vol. 38 
