Frequency Control Strategy Of Isolated Microgrid With Multi-source Energy

Microgrid system has many advantages,such as high energy efficiency and environment friendly.In recent years,microgrid technology has also been developed rapidly.As an important part of smart distribution grid,microgrid in isolated operation can provide better power supply solutions for islands,frontier defense,rural areas and regions where requiring high reliability of power supply.Because isolated microgrid contains a large number of power electronic devices and intermittent renewable energy sources,which are different from the characteristics of large power grid,how to ensure the rapid realization of active power regulation to ensure the frequency stability of the system has become a hot research topic.At the same time,the isolated microgrid contains heterogeneous power sources composed of multiple energy generation modes,and the cost characteristics of each power source are different.Therefore,how to ensure both safety and economy in frequency control and operation is also an urgent issue to be investigated.In this paper,the active power and frequency regulation of isolated microgrid composed of multi-energy generation is studied.A distributed active power control scheme considering both the frequency regulation performance and the economic cost of regulation is proposed.Firstly,this paper designs a local primary frequency modulation control system for different heterogeneous power sources in microgrid.Based on the research of the characteristics of sources,it is proposed that the traditional droop control strategy is adopted in the gas turbine power generation system;the control strategy based on virtual synchronous generator is adopted in the battery energy storage system;the frequency control strategy based on disturbance observation method is adopted in the photovoltaic power generation system;the doubly-fed wind generator system uses the combination of rotor overspeed and variable blade to participate in frequency modulation and the virtual inertia control unit is added.The designed control system meets the requirement of “plug and play” for distributed generators and the requirement of frequency control under normal operation conditions.Secondly,a distributed bisection method based on consistency algorithm is proposed to optimize the distributed operation of heterogeneous power sources which operate at system level in isolated microgrids.The objective of optimal operation is to minimize the overall operating cost of the system on the premise of meeting the power supply and demand balance and also the respective constraints of sources.The designed distributed algorithm does not need a central controller,and the N-1 rule is used to design the communication system structure to improve the reliability of the whole system.Aiming at the implementation of the algorithm,this paper proposes a scheme of using S-function in Matlab/Simulink to realize the communication and algorithm of the controller.Thirdly,according to the operation characteristics of isolated microgrid,a second frequency modulation control scheme is designed,which is different from that of large grid.By analyzing the principle of primary frequency modulation and using the steadystate deviation of the system to estimate the active power gap of each micro-source,thus the generation demand of the system can be obtained.In order to compensate for the frequency deviations,several rapid power reference value adjustments are utilized during the implementation of secondary frequency modulation.The accuracy of the proposed method is verified by theoretical derivation and simulation analysis.Finally,a two-level control scheme of the isolated microgrid system is established,which can be called "upper coordination optimization-lower self-control".On the basis of satisfying the frequency stability in the isolated microgrid,the optimal economic operation aiming at the lowest total operating cost of the system is realized.