The grid-connected photovoltaic power generation system is directly connected to the distribution network, and the electric energy is directly input into the power grid. At present, the energy storage system is generally not arranged, and the photovoltaic and wind power generation “disposal of light and power limitation” is serious, and the photovoltaic and wind power generation system power output After factors such as large fluctuations and the increasing use and promotion of renewable energy, the allocation of energy storage in grid-connected photovoltaic systems has become one of the research directions of large-scale energy storage systems.
The configuration of energy storage in the grid-connected photovoltaic power generation system is determined by the energy storage target. The energy storage targets can be divided into: smooth output, economic dispatch, and microgrid composition.
1) Smooth output
Photovoltaic power generation is a process in which solar energy is converted into electric energy. Its output power is drastically changed by environmental factors such as solar radiation intensity and temperature. In addition, since the photovoltaic power output is a direct current, it needs to be converted into alternating current by the inverter and then connected to the power grid. Harmonics are generated during the inverter process. The instability of photovoltaic power and the presence of harmonics make the access of photovoltaic power impact on the grid. Therefore, an important purpose of energy storage in grid-connected photovoltaic power generation systems is to smooth photovoltaic power output and improve photovoltaic power quality. The energy storage system configuration aiming at the smooth photovoltaic power generation output is generally configured with a centralized energy storage system on the photovoltaic power generation side and a photovoltaic energy storage system structure with a smooth output.
The capacity of the energy storage system is determined by the grid-connected smoothing strategy, and the energy storage power is generally determined by the smoothing target. The photovoltaic grid-connected smoothing strategy based on energy storage system currently has a fixed time constant low-pass filtering smoothing strategy, fuzzy control/SOC (storage battery state of charge) smoothing strategy, and photovoltaic power forecasting smoothing strategy. The low-pass filtering smoothing strategy has a general smoothing effect, but the control is simple and the cost is low, which is a control strategy with a broad application prospect.
2) Power peaking
After photovoltaic power is connected to the grid, it needs to accept grid dispatching, but its peak peak of power output is inconsistent with the peak stage of grid load. Combined with the influence of peak and valley electricity price factors in the power market, the energy storage system is used to realize the translation of photovoltaic power generation in time coordinates. The participation of power in power grid peaking is also one of the research hotspots of photovoltaic energy storage systems. Through power peaking, the accessibility of photovoltaic power in the power grid and the economics of photovoltaic power can be improved.
The energy storage system capacity of this type of configuration is generally large, and the cost of the energy storage system is high and the unreasonable charging and discharging control will seriously damage the life of the energy storage system. Therefore, the capacity of the centralized energy storage system disposed on the grid side is currently The power configuration is determined by factors such as peak shaving requirements, energy storage and discharge control strategies, and energy storage costs. The algorithms for solving the peak-filling strategy of battery energy storage system mainly include gradient class algorithm, intelligent algorithm and dynamic programming algorithm. Different power grid peaking requirements and energy storage control strategies have different requirements on power and capacity. In practical applications, the configuration of energy storage systems needs to be carried out under various practical conditions. At present, large-scale energy storage power stations in China are still in their infancy, and only experimental or demonstration energy storage power stations are operating, and they have not been put into commercial use on a large scale.
3) Microgrid application
The microgrid is a new type of power grid structure proposed to promote the utilization of renewable energy. It is a regional power grid consisting of renewable energy, energy storage systems and loads. As an independent whole, it can be operated on the grid or Running in an off-grid state. The energy storage system as a component of the microgrid is an energy buffering link in the microgrid, which plays an important role in improving the control stability, improving the power quality of the microgrid, maintaining the power balance of the microgrid, and improving the anti-interference ability of the microgrid. . In addition, the energy storage system in the microgrid can also be used for emergency backup in case of power supply interruption.
The energy storage system deployed in the microgrid is generally configured in parallel with the renewable energy generation system, and has an independent energy storage management system (such as battery control system, BESS), and its operation mode follows the micro network operation mode (off-network/grid )Variety. The capacity and power configuration of the energy storage battery depends on the different microgrid composition and operation mode, and is also restricted by the operation mode of the energy storage system. The configuration and control strategy of the energy storage system in the microgrid is a hot topic in the current research of microgrid.