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What Is Grid-connected PV Energy Storage System?

Ⅰ. What is grid-connected PV energy storage system?


Grid-connected PV power generation system is directly connected to the distribution grid, and the electric energy is directly input to the grid. Currently, the energy storage system is generally not configured. The "light abandonment and power limit" phenomenon is serious in photovoltaic and wind power generation, and the power output of photovoltaic and wind power generation system fluctuates greatly, so the utilization and promotion of renewable energy are increasingly restricted. Therefore, configuring energy storage in grid-connected photovoltaic systems has become one of the research directions of large-scale energy storage systems.


Ⅱ. Energy storage objectives of grid-connected PV energy storage system


The configuration of energy storage in grid-connected PV power generation system is determined by the energy storage objective. The energy storage objective can be divided into smooth output, economic scheduling and microgrid.


1. Smooth power output:


Photovoltaic power generation is a process of converting solar energy into electric energy. Its output power is greatly affected by environmental factors such as solar radiation intensity and temperature. In addition, since photovoltaic power output is DC current, it needs to be converted into AC power by an inverter and then connected to the power grid, and harmonics will be generated in the inverter process.


Due to the instability of PV power and the existence of harmonics, PV power access will impact the power grid. Therefore, an important purpose of energy storage in grid-connected PV power generation system is to smooth photovoltaic power output and improve the quality of photovoltaic power.


In order to realize the smooth photovoltaic power generation output, the centralized energy storage system is generally configured on the photovoltaic power generation side.


The capacity of the energy storage system is determined by the smoothing strategy, while the energy storage power is generally determined by the smoothing objective.


At present, the grid-connected PV smoothing strategies based on energy storage systems mainly include low-pass filtering smoothing strategy with fixed time constant, fuzzy control /SOC smoothing strategy, PV power prediction smoothing strategy, etc. The low-pass filtering smoothing strategy has a general smoothing effect, but its control is simple and the cost is low. It is a control strategy with broad application prospect at present.


2. Peak load regulation of electric power


After the photovoltaic power is connected to the grid, it needs to accept the grid dispatching, but the peak stage of the power output is not consistent with the peak stage of the grid load. In addition, with the influence of peak-valley electricity price factors in the power market, photovoltaic energy storage system is used to realize the translation of photovoltaic power generation in the time coordinate, so that the participation of photovoltaic power in peak load regulation is also one of the research hotspots of photovoltaic energy storage system. The peak load regulation of electric power can improve the access capacity of photovoltaic power in the power grid and the economy of photovoltaic power.


The PV energy storage system based on the purpose of power peak regulation is generally equipped with centralized energy storage system on the power grid side.


The capacity of the energy storage system in this configuration is generally large, but the cost of the energy storage system is high and the unreasonable charge and discharge control will seriously damage the life of the energy storage system. Therefore, the capacity and power configuration of the centralized energy storage system on the grid side are jointly determined by the peak load regulation requirements of the grid, the charge and discharge control strategy of the energy storage, and the energy storage cost.


The algorithms for solving peak load - cutting and valley - filling strategy of battery energy storage system mainly include gradient algorithm, intelligent algorithm and dynamic programming algorithm. Different peak load regulation requirements and energy storage control strategies have different requirements on power and capacity. Therefore, in practical applications, the PV energy storage system should be configured under the restriction of various practical conditions.


At present, large-scale energy storage power stations in China are still in the initial stage. Only experimental or demonstration energy storage power stations are in operation, but they have not been put into commercial use on a large scale.


3. Microgrid application


Microgrid is a new power grid structure proposed to promote the utilization of renewable energy. Specifically, it is a regional power grid form composed of renewable energy, energy storage system and load. As an independent whole, it can be connected to the grid or isolated in the off-grid state.


As the component unit of microgrid, photovoltaic energy storage system is the energy buffer link in microgrid, which plays an important role in improving the control stability, improving the power quality of microgrid, maintaining the power balance of microgrid and improving the anti-interference ability of microgrid. In addition, the energy storage system in microgrid can be used as emergency backup in case of power supply interruption.


The energy storage systems deployed in microgrids are typically configured in parallel with renewable energy generation systems, and they have independent energy storage management systems (such as battery control system, BESS), whose operation mode varies with the operation mode of microgrid (off-grid/grid-connected). The capacity and power configuration of energy storage battery depends on different microgrid structures and operation modes, and is also restricted by the operation mode of energy storage systems. The configuration and control strategy of energy storage system in microgrid is the focus of microgrid research.

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