Research On Loop Closing Between Multiple Grounding Systems In City Distribution Network

The loop closing operation is one of the very important means in providing a high level power supply reliability in city distribution network, while the chosen of the neutral grounding methods will directly affect the over voltage and short-circuit current as well as the chosen of the relay protection in the short circuit fault. In china, low-resistance grounding and resonance grounding systems exist in some large and medium-sized cities and they are in the mixed operation state. This article provides a new way of analyzing the transient state of the single-phase fault in both low-resistance and resonance grounding on the basis of the previous studies. Upon this theory, the process of the loop closing operation and a single-phase fault after this have been discussed, and a comprehensive evaluation of several loop closing methods is conducted, so lead to the chosen of the best strategy.The article firstly analyzed the transient and steady state of the single-phase fault in the resonance and low-resistance grounding systems. A new type of analysis is conducted to complement the deficiency of the previous one. PSCAD/EMTDC program is used in modeling and simulating a lOkV distribution network, and the result is compared with the calculations of the conventional method and the new one. The waveform and FFT analysis comparisons proved that the new method is much more accurate than the conventional one. Some important complements are made in the analysis of steady-state during the fault, thus the theory of transient and steady state in the single-phase fault is complete.According to the actual test conditions which has two transformer substations respectively using resonance grounding and low-resistance grounding system, a model inside of PSCAD program is built. Based on the loop-closing analysis, a simuliation was made to testify the calculation of the loop-closing current, while the necessary condition for the loop-closing and the calculation of the best action time is acquired at the same time.The actual arc characteristics in a lOkV grid single-phase fault test are shown, which provides the data to model an improved Mayr-based arc model in PSCAD. The arc model parameters for both transient and steady state are derived using the Multiple Run function in PSCAD. According to the presented three kinds of loop-closing methods and using each of the arc model built before, the single-phase to ground fault is simulated. Accoding to the over voltage, the zero sequence cuurent and the influence of the relay, the best loop-closing pattern is choosed:two kinds of grounding systems should accomplish the loop-closing withing their own grounding methods, while a zero sequence cureent protection should be added in the closing point and coopertate with the protection of the original one in the low-resistance grounding side. The protection should still be useful after the loop is cut open.