Transient Stability And Its Operation Characteristics Analysis For The Rural Isolated Microgrid

The shortage of power supply is a key problem limiting Pakistan’s energy development.Nearly 40% of Pakistan’s population lives in remote areas,and some areas are not connected to the main power grid.For the residents in these areas,the most feasible way to get rid of no power is to integrate various renewable energy generations to form a community-based independent microgrid with distributed generation and load balancing on the spot.In the independent microgrid,the inverter type power supply replaces the conventional generator set,and its instability form is no longer the power angle instability in the large power grid;due to the weak anti disturbance ability of the independent microgrid,the voltage and frequency instability occur simultaneously under large disturbance,that is,its ability to maintain frequency and voltage transient stability is weak.For Pakistan community-based independent microgrid,due to the lack of support of diesel generator or energy storage,the stable operation of microgrid after large disturbance must be maintained through the optimization and coordination of load side and power side control strategy.This paper focuses on the transient operation characteristics of Pakistan community independent microgrid under large disturbance.Firstly,combined with the distribution characteristics and power consumption characteristics of primary energy in Pakistan,the dynamic models of photovoltaic(PV arrays),doubly fed induction machine(DFIG)wind sources and micro gas turbine representing biogas power station,as well as the Vdc/Q and P/Q control of grid connected converter of PV system and DFIG wind power generation system are established;the dynamic model of induction machine(IM)is established to simulate the local plant load,and the transient stability simulation model of community independent microgrid is built in PSCAD/EMTDC.Secondly,the microgrid system with an IM is analyzed by using Thevenin equivalent method.According to the mechanical characteristics of IM and the critical fault clearing time of large disturbance stability,the influence of IM parameters and system parameters on large disturbance stability is obtained.The influence of multi-IM starting process on transient stability of independent microgrid is studied by simulation.The strategy of step-down start-up and superposition start-up of IM is proposed,which can effectively improve the dynamic characteristics of the start-up process and ensure the stable operation of the microgrid.Then,the influence of short-circuit fault on IM and distributed generation in microgrid is studied,and the control strategy of distributed generation is improved.The coordinated control strategy of PV system is proposed.On the premise that the gridconnected inverter does not exceed the current limit,the dynamic reactive current is injected according to different voltage sags,which plays a supporting role in the voltage of grid connection point.When the voltage drop is deep,the MPPT control in chopper circuit is switched to Vdc control,which can control the DC side voltage without adding hardware equipment.The Vrms-Q control strategy of DFIG is proposed,which can provide reactive power support during fault by controlling DFIG rotor side injection of reactive current.In the phase of voltage recovery after fault,Vrms-Q control is used to control the voltage at the junction point without difference.Finally,the simulation results show that the control strategy has more advantages in improving the voltage recovery after fault,and has better dynamic characteristics than the traditional low voltage ride through control.