Analysis of the influence of long cables on transformer impulse closing overvoltage

作者： GOZ Electric 时间：2024-10-05 09:35:50 阅读：12

The scenario of switching on and off no-load transformers in power systems is quite common. For example, there is a phenomenon of reverse power supply during the grid connection process of power plants. The second most common is that the newly put into operation transformer needs to undergo no-load full voltage closing impact test. This is because when the no-load transformer is put into operation under power, an excitation surge current will be generated, and its value can reach 6 to 8 times the rated current. In addition, due to the saturation characteristics of the excitation inductance, a large resonant overvoltage and excitation surge current may be generated under certain parameters of the external circuit.

During the closing process of the no-load transformer, the nonlinear magnetic saturation characteristics of the transformer winding will cause the system to generate a non-sinusoidal excitation surge current, and the excitation surge current will contain multiple harmonic components. According to the Thevenin theorem, from the switch of the operating bus to the system side, there will be a parallel resonant frequency close to a certain harmonic frequency in the system impedance due to the harmonic components in the excitation surge current, so high-frequency resonant overvoltage may be generated. Secondly, if a branch of the operating bus has a large capacitive characteristic, the impedance of the system seen from the switch of the operating bus to the system side will have a series resonant frequency close to a certain harmonic frequency, and then the components of this branch will be subjected to a large resonant overvoltage and harmonic current. Finally, since the resistance of the power system is generally small, the resonance phenomenon generated in the system will have the characteristics of long duration and slow decay, which will seriously cause insulation damage to system equipment and the operation of relay protection equipment.

Related literature quantifies the closing overvoltage of the main transformer of the GIS booster station, studies the influence of factors such as equipment commissioning, number of outgoing lines, and the length of the pipeline connecting the transformer and the GIS bus on the closing overvoltage of the transformer, and concludes that the refraction and reflection of the wave during GIS operation is the main reason for the increase in closing overvoltage, but the related literature simplifies and ignores the magnetic saturation characteristics of the transformer itself, and does not consider the nonlinear characteristics of the transformer during simulation modeling, so the conclusion is relatively simple. The related literature builds a simulation model of no-load closing of 220kV transformer, and studies the three-phase oscillation overvoltage, the oscillation overvoltage of closing in different periods, and the three-phase resonance overvoltage. However, the research focus is limited to whether the insulation withstand voltage levels on each side of the transformer match, and no further research has been conducted on the factors affecting the closing overvoltage of the transformer. The related literature builds a simulation model of no-load closing of transformer based on the transformer excitation characteristic data measured in the test, and studies the influence of closing resistance and residual magnetism on closing overvoltage, but does not conduct a separate study and analysis on the large-scale investment of long cables.

Based on the transformer excitation curve data of a 500kV transformer factory test, this paper uses MATLAB programming to convert it into ψ-i parameters, and establishes a 500kV transformer opening and closing simulation model with long cable incoming lines in the EMTP simulation software with the background of the newly commissioned main transformer impulse power transmission. The focus is on the influence of the XPLE cable length on the high-voltage side of the transformer and the closing angle on the closing overvoltage, and suppression suggestions are given accordingly. Based on the transformer excitation curve data of a 500kV transformer factory test, MATLAB programming is used to convert it into l-i parameters, and a 500kV transformer split-and-close simulation model with long cable incoming lines is established in the EMTP simulation software with the impact power transmission of the newly commissioned main transformer as the background. The focus is on the influence of the XPLE cable length on the high-voltage side of the transformer and the closing angle on the closing overvoltage, and the following conclusions are drawn:

(1) The impact closing process of the transformer with long cable power transmission will produce obvious closing no-load transformer overvoltage and excitation inrush current. Under the closing angle of 0°, the maximum closing overvoltage can reach 742.61kV, which is 1.733 times the rated voltage, and the overvoltage duration is about 80ms.

(2) The length of the transmission cable has an impact on the amplitude of the overvoltage and excitation surge current of the no-load transformer when it is closed, which has a great impact on the vibration attenuation of the waveform, and has a significant impact on the charging current of the long cable at the moment of closing. Compared with the case of 100m, the surge current on the high-voltage side of the main transformer increased by 3.75% in the case of 2100m, and the instantaneous charging current of the long cable increased by more than 20 times.

(3) The most effective method to suppress the closing overvoltage and excitation surge current during the impact closing of the transformer with long cable transmission is to connect the long cable series reactor. Under the premise of configuring the reactor parameters well, it can effectively suppress the overvoltage and excitation surge current of the no-load transformer when it is closed.