Study Of AC Network Response Behaviors Under Voltage Source Converter Excitation

With trend of power-electronized reform about power system,voltage source converter replaces synchronous generator,becoming the main power generation equipment of power system.Compared with synchronous generator,the dynamic characteristics of VSC under large disturbances are determined by the physical circuit,auxiliary control and protection circuit.Different from the internal voltage of the synchronous generator,the instantaneous amplitude and instantaneous frequency of the internal voltage of VSC are time varying under a large disturbance.When the internal voltage of VSC,which is more complex,is used as an excitation to the AC network,there are some new challenges in analyzing AC network response behaviors and modeling equivalent circuit of AC network under a large disturbance.The analysis of AC network response behaviors plays an important role in making the principle of relay protection of power-electronized power system and modeling equivalent circuit.The internal voltage of VSC was compared with the internal voltage of synchronous generator by a single machine infinite bus system and it is shown that the instantaneous amplitude and instantaneous frequency of the internal voltage of VSC are time varying under a large disturbance.By simulation of a typical grid-connected VSC system in MATLAB/SIMULINK,it is found that the instantaneous amplitude and frequency of each node voltage are both determined by time variable and position variable;π-equivalent circuit cannot be applied in electromagnetic transient program of power-electronized power system.Furthermore,the determinants of AC network response behaviors were analyzed,such as controller parameter and strength of the grid.The results of the instantaneous amplitude/frequency of the internal voltage of VSC were processed by Hilbert transform and it is shown that the modulation of the instantaneous amplitude/frequency are not ideal periodic modulation.As the modulation of the instantaneous amplitude/frequency of VSC are complicated under a large disturbance,internal voltage of amplitude-frequence periodic modulation,a special type of excitation,was introduced to figure out the laws of AC network node voltage/current response.And the laws are concluded below:(1)The instantaneous amplitude of node voltage/current under the internal voltage of amplitude-frequency periodic modulation consists of two parts.One part is the instantaneous amplitude of node voltage/current under the internal voltage of pure frequency periodic modulation.The other is the instantaneous amplitude fluctuation of node voltage/current under the internal voltage of pure amplitude periodic modulation.(2)The instantaneous frequency of node voltage/current under the internal voltage of amplitude-frequency periodic modulation mainly consists of two parts.One part is the instantaneous frequency of node voltage/current under the internal voltage of pure frequency periodic modulation.The other is the instantaneous frequency fluctuation of node voltage/current under the internal voltage of pure amplitude periodic modulation.