Research Of Equivalence Methods And Related Relay Protection For Ultra-high Voltage AC/DC Parallel Transmission Network

With the extensive application of HVDC transmission and UHV transmission，ultra-high voltage（UHV） AC/DC parallel transmission network is becoming the newcharacteristics of China’s power grid. The safe and stable operation of UHV AC/DCparallel transmission network is related to the safety of the power grid. Equivalentcalculation method for UHV AC/DC parallel transmission network is the base of theresearch on it. In eastern China power grid as an example in this paper, according to thecharacteristics of UHV AC/DC parallel transmission network, the steps of equivalentcalculation method are detailed. Keeping the dc circuit and uhv network is the basicprinciple. Set the DC circuit and UHV network as the initial network, expand outsideaccording tree-like divergence structure.The result of expansion is the internal network.Devide the internal and external network.Do the equivalent to the external network throughmultiple network equivalent module in BPA. The equivalent network model in PSCAD isbuilt. The special attention to the equivalent problem is emphasized. Single-phasegrounding fault and three-phase grounding fault are set on each boundary node. Positivesequence short-circuit current and zero sequence short-circuit current on each boundarynode are compared between the equivalent model and the model in BPA. The comparisonshows that the equivalent method is much easier with less complicated calculation. At thesame time it can meet the requirement of equivalent.Relay protection of UHV power grid has its own particularity. The application ofcapacitive voltage transformer (CVT) and the influence of distributed capacitance causetransient overreach in distance protection of UHV network. The new algorithm of distanceprotection based on the theory of virtual digital transfer can solve this problem. To verifythe validity of the new algorithm, dynamic test on the new algorithm is needed. At first,dynamic model of CVT is needed. The scheme of CVT is designed according to the studyof CVT. According to the equivalent circuit of CVT, modification can be made on PT tosimulate CVT。It is verified that the model can accurately simulate the transient process ofCVT, and the model was applied to the distance protection based on the the newalgorithm.Through the dynamic model experiments, it is verified that the convergence time is shorter by the new algorithm and the new algorithm is better at fault location comparedwith algorithm of differential equation...