Power factor & reactive power compensation in power plants

A photovoltaic system is a power generation system that emits direct current and does not have any issues with reactive power or power factor. However, after photovoltaic power generation is converted into an AC power grid by an inverter, it will generate a series of problems in matching with the power grid, and the issue of reactive power is one of the main issues.

The connection methods of photovoltaic power stations vary greatly, resulting in significant differences in reactive power issues. At present, photovoltaic power generation systems can be divided into large-scale photovoltaic power stations connected to the high-voltage grid side and distributed photovoltaic power stations connected to the user side. Due to the different access methods and load conditions within the power grid, the reactive power issues reflected and the requirements for reactive power compensation are also completely different, and measures should be taken according to the actual situation.

Large photovoltaic power plants connected to the high-voltage grid side. Usually, this type of power station adopts a dedicated line design, which directly connects the power of the station to the transformer of the upper level substation. Therefore, the reactive power problem of such power plants is relatively small and simple. During the day, when photovoltaic power plants generate electricity, due to the absence of other loads between the photovoltaic power plant and the substation, the inductance of the upper level step-up transformer is the main factor in reactive power within the grid. However, due to the large amount of electricity generated during the day, the transformer operates at almost full load, resulting in a relatively small proportion of reactive power. Most photovoltaic power plants can meet the requirements of the grid. However, at night, the inverter of the photovoltaic power station is in a stopped state. However, due to the constant connection of the transformer to the high-voltage power grid, the transformer in the station becomes an inductive load, which not only consumes some active power without load, but also generates reactive power loss in the high-voltage circuit of the power station and the upper level substation. The external cable lines and main transformers of large photovoltaic power plants will generate reactive power during light load operation. Usually, a set of reactive power compensators can be added to the high-voltage busbar to absorb the reactive power generated by the nighttime line, so that the nighttime power factor of the photovoltaic power plant meets the requirements of the grid.

Distributed photovoltaic power plants connected to the user side. The issue of reactive power factor has been a prominent issue encountered by distributed photovoltaic power plants in recent years, especially for industrial and commercial photovoltaic power plants built in factories and enterprises. Due to the fact that such power plants are usually connected to the internal low-voltage or medium voltage power grid of the enterprise, there are many electrical equipment connected to the grid, especially motors and other inductive loads. When the photovoltaic power generation system was not originally installed, the reactive power compensation system in the grid automatically adjusted and compensated based on the active power supply and reactive power in the grid. Therefore, the reactive power problem in the grid can be automatically compensated through the reactive power compensation equipment set up in the grid, so that the reactive power problem in the grid meets the requirements of the grid.

However, when a photovoltaic power generation system is connected to the grid, it means that there is a second or even multiple power sources in the grid. When there is sufficient sunlight at noon, the photovoltaic power generation system will emit strong electricity, greatly reducing the power supply load of the grid. The active power rate displayed by the electricity meter will decrease with the increase of the photovoltaic system power, and even generate the situation of photovoltaic power generation surplus being transmitted. At this point, if the load in the grid remains constant, especially when the inductive load is relatively large, the original reactive power compensation system in the grid will make incorrect instructions and actions based on the power supply load and consumption load of the large grid, resulting in an increase in the proportion of reactive power in the grid and a decrease in power factor.

In recent years, many photovoltaic installation companies have been fined by the grid company due to the power factor of the user side grid not meeting the requirements of the grid company. The specific method of the fine is that the grid company will charge high power regulation fees.

To solve this problem, in addition to ensuring that the original reactive power compensation device of the power grid is configured correctly, installed correctly, and operates well. For photovoltaic power generation systems with a matching capacity greater than 50% of the transformer capacity, and with a large number of inductive loads such as motors in the grid, special attention should be paid to the selection of photovoltaic connection points. If conditions permit, the connection point for photovoltaic power generation should be set at the front end of the original reactive power compensation equipment. This connection method can fully utilize the original reactive power compensation system and reduce investment.

However, if the construction site of the photovoltaic power station is far from the front-end, or due to some other reasons, it cannot be connected at the front-end and has to be connected at other parts, the sampling point of the original reactive power compensation equipment can perceive the reactive power situation in the grid, but it refers to the power supply of the grid and cannot perceive the output power of the photovoltaic power generation, It may not be possible or fully meet the function of correctly adjusting reactive power. To solve this problem, it is necessary to install dedicated reactive power compensation equipment for this photovoltaic power generation system. Usually, this set of reactive power compensation equipment should be installed on the output side of the photovoltaic power plant to supplement the lack of capacity for reactive power compensation in the photovoltaic power generation system and meet the needs of the power grid.

Most industrial and commercial photovoltaic inverters have a power factor regulation capability of around 20%. For systems with small installed capacity for grid photovoltaic power generation and fewer inductive loads in the grid, the issue of reactive power compensation equipment can be avoided, and a portion of reactive power compensation can be provided through inverters. However, when the reactive power in the grid is too large to exceed the withstand and regulating capacity of the inverter, configuring an appropriate amount of reactive power compensation equipment is the main means to improve the efficiency of the grid and the effective power generation of photovoltaic power plants.

From the perspective of the stability of photovoltaic system operation, it is also recommended to choose the grid connection point for photovoltaic power generation as close to the transformer as possible. The closer the transformer is, the more stable the voltage is. The farther away the transformer is from the end of the power grid, the greater the voltage fluctuation. Frequent voltage and power fluctuations can cause reactive power compensation devices to malfunction or malfunction.

The power factor issue is one of the important issues in many photovoltaic power plants, especially in industrial and commercial photovoltaic power plants. It is necessary to fully consider the existing transformer, reactive power compensation device, transmission cable, load type, and power in the design process. For industrial and commercial photovoltaic power plants connected to the grid on the user side, attention should be paid to the location of access points to ensure that reactive power compensation devices can function normally, and the power quality meets the requirements of the power grid, so as to ensure efficient and stable operation of photovoltaic power plants and achieve maximum economic benefits.