Analysis On The Resonance Characteristic In DC Distribution Network

With the development of the smart grid technologies,various inverter based distributed energies and novel DC loads are connected with power system increasingly.Power-electronized distribution network becomes more common.DC distribution network is taken more attention because of its high controllability,consumption capacity and power quality.In addition,there are no frequency stability and reactive power compensation issues in DC distribution network.However,due to the interconnections with many power electronic devices,the harmonics are diverse and the frequency characteristics of the system are complicated.The risk of resonance in the DC distribution network increases greatly which affects the safe and stable operation of the system seriously.Therefore,this thesis focuses on the resonance characteristics of DC distribution network.The main work is as follows.(1)Based on the dynamic phasor,the mathematical models of both the elements and DC distribution network were established.The models included the dynamic phasor sequence component models of three-phase grid-connected voltage source converter(G-VSC),threephase AC load interfaced voltage source converter(L-VSC),isolated bidirectional DC/DC converter and the dynamic phasor models of single-phase voltage source converter(SPVSC),DC lines and DC loads.By correlating the equations of devices,the mathematical model of DC distribution network described with state space equations was achieved.(2)A method to analyze the resonance characteristics of DC distribution network based on node impedance matrix was proposed.By the simplification of the mathematical models,the steady-state equivalent circuit model of each element was established.Then the equivalent circuit model of the DC distribution network was constructed.The relationships between the injection currents and the voltages on the nodes were described by the node impedance equations.By analyzing the frequency characteristics of self impedance and mutual impedance in the node impedance matrix,the resonance characteristics and its impact factors of the DC distribution networked were obtained.The impact factors included the DC line parameters and converter support capacitance.(3)A method to analyze the resonance characteristics of DC distribution network based on small singal model was proposed.Based on the state-space equations of DC distribution network,a small signal model of DC distribution network was established.Eigenvalues were calculated and modal analysis was performed to obtain the resonance modes of DC distribution network.The main issues causing resonance were identified by sensitivity analysis and the study of the variation of dominant eigenvalues with the control parameters,power and capacitors.(4)The resonance characteristics of DC distribution network with single-phase gridconnected converter was analyzed by the small signal model.The differences of resonance characteristics of DC distribution network with single-phase and three-phase grid-connected coverter were obtained.The impact of second order filters on resonance characteristics of DC distribution network with single-phase grid-connected converter was analyzed especially.The proposed methods can provide theoretical base and tools to analyze the resonance characteristics of DC distribution network with different topologies and instruct the decision of the mitigation strategy for the resonance.