371 / 2019-03-14 15:36:09

The Study of A Secondary Ferroresonance Elimination Method Based on Combined Detection of the Zero-sequence Voltage and Potential Transformer Current

electromagnetic potential transformers (PT),ferroresonance,secondary resonance elimination,PT current,PSCAD/EMTDC

The electromagnetic potential transformers (PTs) are widely used in 10-35kV distribution networks. However, some transient electromagnetic disturbances, such as lightning strike, single phase grounding or disconnection faults, may cause the magnetic saturation of PT, which will often result in the ferroresonance issues in the distribution networks, breaking PT fuses and even exploding PT. Therefore it is of great significance to carry out the researches of the ferroresonance characteristics and their elimination measures. In this paper, a secondary resonance elimination method was mainly studied. Based on the previous research works, it was focused that how to improve the accuracy of ferroresonance detection from a few conventional methods of secondary resonance eliminations, especially the detection of fundamental frequency resonance, or the response speed of the secondary resonance elimination device. Firstly, two testing platforms for simulating ferroresonance of 10 and 35 kV distribution networks were built respectively in the laboratory, and the corresponding simulation circuit models based on PSCAD/EMTDC software were founded in this paper. The excitation characteristics of PTs from different manufactures were simulated by applying the saturation characteristics of the UMEC(unified magnetic equivalent circuit) transformer model based on the measured voltage-current characteristic curves of PTs. Secondly, under different line to ground capacitances and PTs, the ferroresonance overvoltage and current characteristics of the PTs were calculated, respectively. The calculated results show that the current waveform of PT under the fundamental resonance behaves the distinct characteristic with respect to the normal current waveform, which can be used to recognize the fundamental resonance, especially suitable for the operation condition with low capacitance current. And then, under different damping resistances in a secondary resonance elimination device, the influences of damping resistances on the effect of resonance elimination were studied, and an appropriate range of damping resistance values was gotten also. Finally, a secondary elimination control method based on the combined detection of the zero-sequence voltage and PT current was established, specifically with which it could be controlled to close the switch device by detecting the zero-sequence voltage combined with PT current, and to open the switch device by detecting the PT open-delta current combined with the Joule heat of the damping resistor. The validity effectiveness of this secondary resonance elimination method have been verified by the simulating experiments in two ferroresonance testing platforms and field operating experiment in a distribution substation, respectively. The studied results show that this control method can effectively detect the ferroresonances with various resonant frequencies, and zero-sequence voltage and the PT current reduced to zero as the operation of the damping resistor. Furthermore, the threshold value of PT open-delta current has a great influence on working time of secondary resonance elimination, and then the damping resistance has a great influence on the effect of resonance elimination. From the simulated and tested results, it is suggested that the threshold value of PT open-delta current and the damping resistance should be taken as 30 ~ 80 mA and 2 ~ 10 Ω, respectively, in a 10 kV network system, while 50 ~ 100 mA and 5 ~ 20 Ω in a 35 kV network system.