Low-Frequency Mathematical Model For Microgrid And Stability Study

Acting as the energy conversion interface in the microgrid which is composed of new energy, energy storage units and energy conversion devices, the inverter has the characteristics of small inertia. This feature makes the microgrid stability becomes the research emphasis. And the premise to study stability is to establish the mathematical model of the system.Common modeling methods for microgrid are analysed and compared at first. Aiming at the impedance-based modeling method, the voltage-source inverter, current source inverter, grid connected inverter and paralleled multi-inverter system are derived which ignores the power loop. This is mainly applicable to analysing the high-frequency resonance. For system state-space modeling method, the complete model is built whose research emphasis is low-frequency stability. However, this method is very complicated. At last, derive the reduced order modeling method ignoring the voltage and current loops and calculate the particular solution of the angle. This method achieves the purpose of simplification, however, it is just applied to the single inverter.To overcome the shortcomings of the above modeling method, a novel reduced order low-frequency small signal model for microgrid system is proposed in this paper. It is built including the dynamic characteristics of the droop controllers, the power measurement and the network. By this method, the low-frequency stability of the whole system is studied and the purpose of order reduction is achieved. The basic principle is as follows: the eigenvalues are derived around an operating poing at first. The trend of eigenvalues with the variation of droop coefficient indicates the sability of the microgrid system. Then comparing the proposed model with the benchmark, the results prove the feasibility of this modeling method in the research of low-frequency stability problem. And this model can be used for islanded and grid-connected modes. At the same time, another purpose of this paper is to model for the microgrid with IM(Induction Motor) load and CP(Constant Power) load based on proposed modeling method and research the stabilities of them.At last, this paper builds up the simulation system of microgrid which proves the theory above. An experimental platform of two voltage source inverters(VSIs) in parallel is built based on TMS320F2812 DSP, and the simulationg and experimental results prove the feasibility of proposed modeling method.