| With the development of the society, energy and environmental issues have become a key factor restricting social development, so the higher requirements for the development of electric power system are put forward.On the basis of research of distributed generation,micro-grid system was proposed.Micro-grid is a kind of tiny power system,which connects different distributed generations and energy storage devices,nears to the customer and can operate independently and be connected to the grid. Researching the strategy of controlling micro-grid system has great significance to the stability and flexibility of the running system,the reliability of supplying power and the improvement of power quality.The photovoltaic is one of the most important micro-power in microgrid.But it’s easy to be influenced by the external environment conditions, the output power of the photovoltaic has obvious randomness and intermittent.The system needs to be equipped with energy storage system to suppress the power fluctuation.So the microgrid control strategy research is necessary.The research object of this paper is the photovoltaic and energy-storage hybrid microgrid which composed of the photovoltaic power generation system, energy-storage system,the controllable load and so on.This paper introduces and analyzes each module’s the mathematical model of the microgrid,the output characteristics and control strategies. Firstly,the mathematical models of photovoltaic,battery,inverter and the two-way DC/DC converter of the micro-power grid are separately set up,and their output characteristics will be analyzed.After it mainly analyses the control strategy of the system and all of parts.The PV system is controlled by two classes.The first stage DC/DC achieves the maximum power tracking control to stabilize the voltage of DC bus,and the voltage can be increased to meet the need of post-stage inverter.The post-stage DC/AC inverter adapts the voltage and current dual loop control strategy.Voltage outer loop control is to achieve unsteady error control, the voltage across the capacitor of DC side is equal to the command value,maintain the DC bus voltage stability,and the output current delivers as the given input current of inner loop. The control of inner current loop is mainly to improve the dynamic response of the system, controlling the current to track instruction value and the grid voltage are the same phase, to control the phrase and waveform of gird-connected current. The energy storage system is connected to the DC bus through the two-way DC/DC converter. According to the relationship between the output power of the photovoltaic power generation system and the load power, the energy-storage system will be work in different modes.There are grid-connected operation and island operation of microgrid.This paper puts forward a method of combination of energy flow direction,comprehensively analyzing the mode of microgrid in different operating conditions.Besides,on the basis of master-slave control and peer-to-peer control,the coordination control strategy between photovoltaic and battery is proposed.In the grid-connected mode,the PV array and the battery adopt PQ control strategy.But in island mode, the system mainly uses the master-slave control strategy.Battery as the master unit with V/f control, the PV array is still using PQ control strategy.This paper also analyzes the impact of microgrid in grid-connected condition to the power grid and introduces different methods of testing the island condition.At last,the simulation model of photovoltaic and energy-storage hybrid microgrid is built under Matlab/Simulink software. Through the different examples of microgrid’s operation in simulation software,the control strategy is tested to be correct. |
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