Dynamic Equivalence Research Of AC/DC Hybrid Large Scale Power Grid For Real Time Simulation

With the development of HVDC transmission technology, the AC/DChybrid power grid has been established in many places. In order to study theinteraction of AC and DC system, the simulation platform of AC/DC hybridpower grid based on RTDS needs to be established. Because of thelimitation of simulation size, the AC/DC hybrid large scale power grid mustbe simplified. The equivalent power grid instead of original power grid isused to carry out the simulation research.The equivalence of large scale power grid preserved research systemwith detailed model, the rest part is substituted by equivalent system with itsstatic power flow and dynamic response to research system unchanged. Theequivalence of large scale power grid can be divided into static equivalenceand dynamic equivalence. Static equivalence includes WARD equivalencemethods and REI equivalence methods. The essences of these two methodsare network simplifications. They are based on fundamental frequencyelectric quantities and can’t reflect the high frequency electromagneticcharacteristics of original network. Frequency dependent networkequivalence method is related to frequency. The equivalent network canreflect the characteristics of original network accurately in a wide frequencyrange. It generally needs three steps to get the frequency dependentequivalent network. First, we sample the admittance matrix of originalnetwork in different frequency. Second, rational function fitting offrequency characteristic of original network is made and next we obtainequivalent circuit in time domain.Dynamic equivalence requires that dynamic characteristics of original network are not distortion after the equivalence. The most important part ofdynamic equivalence of power grid is the equivalence of generator units.Coherency based dynamic equivalent method aggregates generators that arecoherent into an equivalent generator. The parameters of equivalentgenerator can be obtained through the iterative process of parameteroptimization.This paper put forward the basic idea and method of AC/DC hybridlarge scale power grid equivalence. According to the purpose of equivalenceresearch, we choose DC inverter station and nearby AC grid as internalsystem and the rest part is external system. The external system has a largescale and has a long electrical distance with study area. The coherency baseddynamic equivalent method is adopted for external system equivalence. Theexternal system is equivalent to the combination of some equivalentgenerators and equivalent loads. This paper summed up the steps ofdynamic equivalence of external system. Data processing is made to fulfillthe requirement of BPA equivalence software. There are many lowervoltage sub-networks in internal system. It will occupy the simulationresource greatly to establish detailed models of these lower voltagesub-networks on RTDS. A wide band equivalence method is adopted andwe get equivalent module of every lower voltage sub-network respectively.The simulation model of AC/DC hybrid large scale power grid isestablished after the equivalence. The data conversion from BPA format toRTDS format is studied in this paper. The division of simulation subsystemsof RTDS is discussed too.Finally, the equivalence of a practical AC/DC hybrid power grid usingthe proposed method is carried out. According to the comparisons of staticpower flow and dynamic response between original grid and equivalent grid,we proved the effectiveness and practicality of the proposed method.