92 / 2021-06-05 13:36:57

SIMULATION OF DIELECTRIC RESPONSE OF TRANSFORMER BUSHING UNDER DIFFERENT VOLTAGE EXCITATION

 The oil-paper bushing is the key equipment for connecting bus and transformer in power system, and its insulation status is of great significance to the safety and stability of the power grid. Statistics show that the unplanned shutdown of transformers caused by bushing faults accounts for a high proportion in the total number of accidents, even causing catastrophic accidents such as fire or explosion ^[1]. Frequency domain spectroscopy (FDS) has the advantages of abundant insulation information and strong anti-interference ability, which can realize nondestructive diagnosis of oil-paper insulation state ^[2]. However, this method needs to perform outage test on the bushing with a long detection time and is difficult to find the potential insulation fault in time, and the field application is difficult to conduct. Therefore, it is urgent to research on the on-line measurement method of FDS of transformer bushing and the on-line diagnosis and evaluation technology of bushing insulation state.

In addition to the valid power-frequency component in power grid, transient overvoltage with different waveform is generated due to various operations, which contains ample frequency components. Under the excitation of different voltage, response current of corresponding frequency components will be generated at the end screen through the main insulation. In this paper, an online measurement method of frequency domain dielectric response of transformer bushing is proposed which utilizes the different voltage waveform during the transient process in power grid as an excitation source. The transient overvoltage caused by common operation in the power grid, as well as the response current at the end screen of the bushing are simulated and the frequency domain characteristics are studied. Simulation results show that the transient voltage of the power grid contains ample frequency components and has high signal strength, which can be used as the excitation source for on-line measurement of the dielectric response of the transformer bushing. The dielectric response of bushing under different voltage excitation was simulated based on the extended Debye model. The voltage and current signals in the simulation are converted to the frequency domain by Fourier transform for FDS computation. The simulation results were consistent with the experimental test curves.

In conclusion, different voltage waveforms can be generated during operation or failure of the operating power grid. These different voltages contain a wide range of frequency components. Under the excitation of different voltages, the current signal with the same frequency component will also be generated at the end screen of the transformer bushing. Through the proposed dielectric response algorithm of bushing under different voltage excitation, the FDS curves of bushing can be accurately obtained, which is expected to realize the on-line diagnosis of bushing insulation status.