Significance of Frequency Domain Dielectric Spectrum Measurement of Impulse Voltage Transformer Bushing

Author： GOZ Electric Time：2024-08-14 09:29:01 Read：11

    As an indispensable supporting component of power transformers, oil-paper insulated high-voltage bushings play the role of connecting the winding lead wires to the external power grid and fixing the lead wires, and are widely used in power systems. During operation, the bushings are subjected to high voltage, high current, strong mechanical loads and complex external environments for a long time, and their insulation performance is severely tested. Once a bushing fails, it will directly cause the transformer to stop operating. In severe cases, it will cause transformer failures and even cause fires and explosions, expanding the scope of the accident. In recent years, power accidents caused by transformer bushing failures have occurred frequently. Statistics from the 2015 CIGREA2.37 Working Group's report on the causes of transformer failures show that as the voltage level increases, the proportion of transformer failures caused by bushings gradually increases. When the voltage level is 500kV and above, the proportion reaches 27.8%. Therefore, the safe operation of the bushing directly affects the stability of the entire power grid. Timely and effective evaluation of the operating status of the bushing and accurate judgment of its defect type have become key issues to ensure equipment safety and power grid operation reliability.

    Professor K.Jonscher proposed the dielectric response theory in the 1990s and gradually began to apply it to the detection of oil-paper insulation status. The oil-paper insulation diagnosis methods based on the dielectric response theory mainly include polarization and depolarization current method (PDC), return voltage meter (RVM) and frequency domain spectroscopy (FDS). Among them, the frequency domain dielectric spectroscopy has the advantages of rich insulation information, low measurement voltage and strong anti-interference ability, and is widely used in the diagnosis and evaluation of insulation status of on-site power equipment. WsZaeng et al. proposed an empirical formula for moisture evaluation based on nonlinear fitting of the minimum value of the dielectric loss curve to evaluate the moisture content of oil-paper insulation. Liao Ruijin et al. designed an experiment to study the influence of moisture on the characteristics of the FDS curve, and proposed an empirical formula for evaluating the degree of moisture according to the dielectric loss value at different frequencies of the curve. Xu Xiaowei et al. studied that moisture content has a significant effect on the relaxation time constant, relaxation strength and shape parameter of the H-N dielectric relaxation model, and proposed a moisture content evaluation method. It can be seen that domestic and foreign scholars have conducted in-depth research on the frequency domain dielectric spectroscopy method and used it in the diagnosis of aging and moisture insulation status of oil-paper insulation and transformer bushings. However, the traditional frequency domain dielectric spectroscopy method is carried out by sweeping frequency point by point testing. If the dielectric response information of 1 mHz~1kHz is obtained, the on-site test time is as long as 1h, and the on-site transformer power outage detection time window is limited, and the change of transformer and bushing insulation temperature during the test will also affect the measurement and evaluation effect. Therefore, it is of great engineering significance to study the fast measurement method of transformer bushing frequency domain dielectric spectrum.

Fast measurement method of transformer bushing dielectric response under impulse voltage excitation. This method applies transient impulse voltage to the central guide rod of the transformer bushing, uses the rich frequency components contained in the transient impulse voltage as excitation, obtains the response current of the corresponding frequency component of the bushing end screen, and processes the voltage and current signals, so as to quickly obtain the frequency domain dielectric spectrum curve of the transformer bushing under transient voltage excitation. Based on the extended Debye model, a simulation platform for the dielectric response of bushings under impulse voltage excitation is built in this paper. An arbitrary wave generator and a high-voltage power generator are used to build an experimental platform. The dielectric response of bushings under impulse voltage excitation is simulated and tested, and the feasibility and accuracy of the fast dielectric response measurement method proposed in this paper are verified.

    As one of the most important power equipment in the substation, the insulation state of oil-paper insulated bushing is crucial to the safe and stable operation of the power system. The frequency domain dielectric spectroscopy method can realize non-destructive detection of the insulation state of the bushing, but the traditional sweep frequency test takes a long time, which brings inconvenience to the field application of frequency domain dielectric spectroscopy. This paper proposes a fast measurement method for the dielectric response of transformer bushings under impulse voltage excitation. The rich frequency components contained in the impulse voltage signal are used as the excitation signal to obtain the bushing end screen response current and perform data processing and calculation to obtain the frequency domain dielectric response information. Based on the extended Debye model, a simulation platform for dielectric response under impulse voltage excitation was built in this paper, and an experimental test platform was built using arbitrary waveform generators, high-voltage power amplifiers and other equipment. The feasibility and accuracy of the rapid measurement method of dielectric response under impulse voltage excitation were verified, which is of great significance for shortening the transformer power outage maintenance time window and improving the efficiency of on-site bushing insulation status diagnosis.