Research On Stable Control Strategies For DC Microgrid Under AC Weak Grid Fault

DC microgrid can accept the distributed DC renewable generators(like photovoltaic)and the increasing DC loads,as well as effectively reducing the power loss of frequent conversion of converters.Thus,it is a desirable solution to improve the local consumption capacity of renewable energy.However,for the DC microgrid which connected to the terminal of AC distribution network,the line impedance can’t be ignored.Thus,the AC grid can be regarded as a weak grid for the DC microgrid.Moreover,the DC microgrid has low inertia and weak ability of over-current.Once a fault occurs in the AC weak grid,the stable operation and high quality and reliable power supply of the DC microgrid will face challenges.This paper concentrates on the stable coordinated control for the DC microgrid under AC weak grid fault.According to the clues from transient stability analysis,to instantaneous fault ride-through control,and finally to the islanded operation control,the main work is summarized as follows.Firstly,to solve the stability problem caused by the interaction of AC weak grid and grid following bidirectional power converter under instantaneous fault,the transient stable control strategy based on virtual synchronous damping is proposed,which can enhance the transient stability.Considering the AC weak grid voltage drop,phase shift,line impedance and the control structure change of the BPC,a nonlinear reduced-order model of grid-connected DC microgrid under complex interaction is constructed.A modified equal area method is utilized to analyze the transient stability and the instability mechanism is revealed.Based on the analysis,a virtual synchronous damping control strategy is proposed to improve the transient stability of DC microgrid during fault process.The control strategy increases the equivalent inertia and equivalent damping of the phase locked loop.Secondly,considering that the converter output current limiting,DC bus voltage supporting,and maximum reactive power supporting should be dealt with at the same time during the fault ride-through process,a multi-objective fault ride-through control strategy which can be applied to both symmetric and asymmetric instantaneous faults is proposed.Focusing on asymmetric instantaneous fault,the synchronous reference frame-phase locked loop based on virtual synchronous damping control and the unified expression model of different positive and negative sequence current inner loop instructions are analyzed.Based on the analysis,the differences of output power reference between the symmetrical fault and asymmetric fault are proposed when the BPC is set to output maximum reactive power to support the weak grid.With the BPC maximum reactive power output as the objective function and the maximum three phase current limitation of BPC as constraints,the multi-objective optimization model of fault ride-through control is established.A heuristic algorithm is introduced to optimize the active and reactive power reference.Moreover,in order to ensure that the dc bus voltage does not reach high voltage or low voltage due to the BPC interactive power mutation in the fault process,an emergency power control strategy of distributed generator is proposed.Through the power coordination among distributed generators,the bus voltage can be stabilized without chopper circuit.Finally,when the persistent fault occurs in AC weak grid,the DC microgrid should switch to islanded mode.Considering that the fluctuation of voltage and the rate of change voltage in the low inertia DC microgrid are large and the DC bus needs interactive support under disturbance of such as the power fluctuation of distributed generators and loads,a decentralized inertia supporting control strategy for the distributed generators and adjustable load is proposed,which can reduce the voltage fluctuation and improve the power quality.The dynamic response of distributed PV under DC voltage fluctuation is analyzed,which can describe the output change of PV controlled by traditional MPPT.Thus,an active inertia supporting control based on feedforward control is proposed for the distributed generators,which can actively withdraw MPPT in exchange for power margin in the islanded mode,and the power margin can realize the fast power response and reduce the short-term fluctuation of bus voltage together with the port capacitor of PV.Moreover,an adaptive inertia implementing control strategy based on an arctan function is presented for the adjustable load with two-stage converter.The built-in capacitor is controlled to supply part of power buffer during the dynamic process so that the rate of change of voltage can be reduced and the power quality can be improved.