Impact of distributed photovoltaic power access on the system

In recent years, China's photovoltaic industry has developed rapidly and will play an important role in the future power supply. As more and more distributed photovoltaic power sources are connected to the distribution network system, new challenges are posed to the traditional distribution network. The interaction between distributed photovoltaic power sources and distribution networks includes the impact of photovoltaic power sources on distribution networks and the impact of distribution networks on photovoltaic power sources.

1. Impact on voltage

The distribution network of centralized power supply is generally radial. Under steady-state operation, the voltage gradually decreases along the feeder flow direction. After the photovoltaic power source is connected, the voltage at each load node along the feeder is raised due to the reduction of transmission power on the feeder, which may cause the voltage deviation of some load nodes to exceed the standard. The amount of voltage increase is closely related to the location and total capacity of the photovoltaic power source. Under normal circumstances, the voltage deviation of the load node can be controlled within the specified range by setting up voltage regulating equipment such as on-load tap-changing transformers and voltage regulators in the medium and low voltage distribution network. For the voltage adjustment of the distribution network, it is important to reasonably set the operation mode of the photovoltaic power source. When the sun is sufficient at noon, the output of the photovoltaic power source is usually large. If the line is lightly loaded, the photovoltaic power source will significantly raise the voltage of the access point. If the access point is at the end of the feeder line, the voltage at the access point is likely to exceed the upper limit. At this time, the operation mode of the photovoltaic power source must be reasonably set, such as stipulating that the photovoltaic power source must participate in voltage regulation to absorb excess reactive power in the line. During the heavy load period at night, the photovoltaic power source usually has no output, but it can still provide reactive output to improve the voltage quality of the line. The impact of photovoltaic power on voltage is also reflected in the possible voltage fluctuation and flicker. Since the output of photovoltaic power varies with the incident solar irradiance, it may cause voltage fluctuation and flicker in local distribution lines. If combined with load changes, it will cause greater voltage fluctuation and flicker. Although the photovoltaic power in actual operation does not cause significant voltage fluctuation and flicker, it is still important to reasonably plan the access location and capacity when a large number of grid-connected photovoltaic power sources are connected.

2. Contribution to short-circuit current

It is generally believed that when a short circuit occurs on the distribution network side, the photovoltaic power source connected to the distribution network does not contribute much to the short-circuit current. The steady-state short-circuit current is generally only 10%~20% larger than the rated output current of the photovoltaic power source. The peak current at the moment of short circuit is related to the energy storage components and output control performance of the photovoltaic power inverter itself. In the distribution network, short-circuit protection generally adopts overcurrent protection plus fuse protection. For photovoltaic power sources with high penetration, when a short-circuit fault occurs on the feeder line, the feeder line may not be able to detect the short-circuit fault because the photovoltaic power source provides most of the short-circuit current. In 1999, IEA-PVPS-Task-5 (Photovoltaic Technology Working Group in the International Energy Agency) used inverters with controlled current injection from four different manufacturers in Japan to connect to a column transformer on a distribution network, and then conducted a short-circuit test on the other side of the transformer. The test showed that the short-circuit current did not rise more than twice before the fault, and the fault was isolated in 1~2 cycles. In addition, Japan also conducted a short-circuit test on a 200kWp photovoltaic power system. The study found that after the short-circuit current passes through the transformer, the current becomes smaller and the transformer overcurrent protection does not work. In 2003, the NERL (National Renewable Energy Laboratory) of the United States conducted a study on the interaction between distributed generation and distribution networks. Using a distributed power source connected in the form of an inverter, the simulation prototype was established on a 13.2kV ​​medium-voltage distribution network. The capacity of the distributed power source is 5MW, and the research focus is on the fuse protection characteristics. The results show that when single-phase and three-phase faults occur, the distributed power source connected in the form of an inverter contributes little to the short-circuit current. The short-circuit current mainly comes from the main grid, which is even much smaller than the short-circuit current provided by the 5MW induction motor. Therefore, it can be concluded that the photovoltaic power inverter with controlled current injection does not contribute much to the short-circuit current.