Research On Key Technologies Of Primary Modulation And Inertia Control Of Distributed Generation System

With the increasing proportion of renewable energy in the energy composition of the grid,power system is gradually changing from centralized generation to distributed generation.A large number of distributed power sources are connected to the power grid through power electronic device,which poses an enormous challenge to the safe and stable operation of power systems because the device lacks inertia and damping as the traditional synchronous machines.The virtual synchronous machine technology enables the distributed power sources or controllable load to participate in the primary frequency modulation and voltage regulation of the system while mimicing the rotational inertia of the synchronous machine.Thus,it realizes the independent maintenance of the stable operation of the system,which is significant for solving the problems of frequency and voltage stability of distributed generation systems.With the support of National High-tech R&D Program(863 Program)"Development and Demonstration Application of Photovoltaic Microgrid Core Equipment and Control System"(2015AA050603)and Hebei Province Natural Science Fund Project "Research on Parallel Technology of Self-Synchronous Voltage Source Inverter in Microgrid"(E2015502046),the key technologies for the primary regulation and inertial control of distributed generation systems are focused on in this work.The virtual synchronous machine control strategy with considering the dynamic characteristics of primary source,and the synchronous regulation characteristics analysis method and inertia matching principle of distributed power sources with different control forms have been further studied.And then a high-precision small-signal model of multi-VSGs parallel system was established.Moreover,the stability analysis of the core control parameters of VSG have been performed.An semi-physical experimental setup based on dSPACE has been designed to demonstrate the excellent performance of the proposed control strategies and analyzes methods.The main research contents and achievements of this work are presented as follows:(1)The stability problem of distributed power generation system with high permeability of renewable power sources is studied.Meanwhile,the influence of primary regulation and inertial support ability to system frequency and voltage stable operation is analyzed.Then the general idea,evolution of technology,typical applications and development directions of the virtual synchronous generators are combed and summarized.The key issues of synchronous generator that need to be further studied and the related technology development trends in the future are summarized in this work,respectively.(2)A variable power point tracking(VPPT)control method with a variable tracking direction of the PV operation point is proposed,and then the specific implementation process of the method is described.The method can follow the MPPT control to realize the maximum power support for the grid when the output power of the PV array is insufficient,meanwhile,it also can adaptively adjust the PV output power to keep the grid power stability when the output power of the PV array is adequate.In addition,a photovoltaic virtual synchronous generator control strategy based on VPPT control is proposed,with consider of the influence of randomness and volatility of the renewable energy.The strategy overcomes the technical defects that the VSG based on the traditional MPPT control can easily cause system power imbalance and DC bus voltage collapse when the photovoltaic power is sufficient.(3)A unified mathematical model of virtual synchronous machine is established by the analogous way of traditional droop control DG inverter,with which the unity in the physical sense is realized between droop control equation and swing equation,and between the droop coefficient and the mechanical time constant.The physical nature of the frequency and power dynamic regulation process of the distributed generation system suffering from load disturbance or source output disturbance is studied.It is pointed out that the system can operate stably after receiving small disturbances essentially depends on whether the angular acceleration and the relative angular velocity of the rotor can be zero at the same time or not.The disturbance power distribution rules and the core control parameter configuration principles of three typical parallel structures of distributed generation such as two SGs parallel system,two VSGs parallel systems,and droop controlled inverter and VSG parallel system are proposed.Aiming at the problems of frequency and power transient synchronism and consistency in multi-generators parallel systems,an inertia matching method for multi-generators parallel operation in distributed power generation systems is proposed.(4)A method for establishing a precision small-signal model is proposed for VSG stand-alone system and VSG multi-machine parallel system,which includes all state variables in the control system.This model takes the VSG control strategy that based on virtual impedance control and voltage-current double-loop control as the object,through which accurate system characteristic root locus analysis and sensitivity analysis can be performed for VSG main circuit parameters such as droop control parameter,virtual inertia parameter,double closed loop PI regulator parameter,line impedance parameter,virtual impedance parameter etc.It lays a theoretical foundation for the stability analysis of VSG stand-alone and multi-VSGs parallel system,and provides basic design principles and stability criteria for the VSG parameter optimization design.(5)A semi-physical experimental platform of VSG based on dSPACE real-time simulation system is built in laboratory.The VSG characteristics such as primary frequency modulation,primary voltage regulation,rotation inertia,power decoupling and load power distribution are analyzed under the conditions such as different load disturbance and in grid-connected or island mode,for single VSG and two parallel VSGs respectively.Furthermore,the correctness of the analysis method and control strategy presented above is verified.