How to solve the power factor and reactive power compensation problems in industrial and commercial power plants

At the end of the day, a photovoltaic system is a power generation system that generates direct current without any reactive power or power factor issues. However, after being converted into an AC power grid by an inverter, photovoltaic power generation will encounter a series of matching problems with the grid, with reactive power being one of the main issues.

In fact, the issue of reactive power varies greatly depending on the connection method of photovoltaic power plants. 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 differences in access methods and load conditions within the power grid, the issue of reactive power reflected and the requirements for reactive power compensation are also completely different, and should be treated differently and measures should be taken according to the actual situation.

A large-scale photovoltaic power station connected to the high-voltage power grid. Usually, this type of power station adopts a dedicated line design, which directly connects the power of the power station to the transformer of the higher-level substation. Therefore, the reactive power problem of such power stations is relatively small and simple. During the day, when a photovoltaic power station generates electricity, there is no other load between the photovoltaic power station and the substation. The inductance of the upper stage step-up transformer is the main factor affecting the reactive power in the grid. However, due to the large amount of electricity generated during the day, the transformer is operating at almost full load, so the relative reactive power ratio is relatively small. Most photovoltaic power stations can meet the requirements of the grid. However, at night, the inverter of the photovoltaic power station is in a stopped state. As the transformer in the station is always connected to the high-voltage grid, it 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 bus to absorb the reactive power generated by the night line, so that the power factor of the photovoltaic power station meets the requirements of the power grid at night.

Schematic diagram of adding reactive power compensation to the high-voltage busbar of a large photovoltaic power station

Distributed photovoltaic power stations connected to the user side. The issue of reactive power factor has been a prominent problem 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 inductive loads such as motors. 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 of the grid and the reactive power in the grid. Therefore, the reactive power problem in the grid can be automatically compensated through the reactive power compensation equipment set in the grid, so that the reactive power problem in the grid meets the requirements of the grid.

But when the 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 the sun is abundant at noon, the photovoltaic power generation system will generate strong electricity, greatly reducing the power supply load of the grid. The active power displayed on the electricity meter will decrease as the power of the photovoltaic system increases, and there may even be situations where surplus electricity from photovoltaic power generation is sent out. At this time, if the load in the grid remains unchanged, 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 supply load and consumption load of the large power grid, resulting in an increase in the proportion of reactive power in the grid and a decrease in power factor.

In recent years, many companies that install photovoltaics have been fined by power grid companies due to the power factor of the user side power grid not meeting the requirements of the power grid company. The specific form of the fine is that the power grid company will charge high electricity 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 network, special attention should be paid to the selection of photovoltaic grid connection points. If conditions permit, the photovoltaic power generation access point should be set at the front A point of the original reactive power compensation equipment. This connection method can fully utilize the original reactive power compensation system and reduce investment.

Schematic diagram of photovoltaic power generation access point set at the front end of the original reactive power compensation equipment

However, if the construction site of the photovoltaic power station is far away from point A, or for some other reason, it cannot be connected at point A, and it has to be connected at point B or other locations, then although the sampling point of the original reactive power compensation equipment can sense the reactive power situation in the grid, it refers to the power supply of the grid and cannot perceive the output power of photovoltaic power generation, which may not fully meet the function of correctly adjusting reactive power. To solve this problem, it is necessary to install specialized 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 station to supplement the lack of high-capacity reactive power compensation capability in the photovoltaic power generation system and meet the needs of the power grid.

Schematic diagram of installing reactive power compensation device on the output side of photovoltaic power generation system

Most industrial and commercial photovoltaic inverters have a power factor regulation capability of around 20%. For systems with small installed capacity of photovoltaic power generation in the grid and less inductive load, the issue of reactive power compensation equipment can be ignored, and a portion of reactive power compensation can be provided through inverters. But when the reactive power in the grid is too high, exceeding the capacity of the inverter to withstand and regulate, 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 the photovoltaic power station.

Reactive power compensation capability curve of industrial and commercial inverters

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

Summary

The power factor issue is one of the important problems in many photovoltaic power plants, especially in industrial and commercial photovoltaic power plants. It is necessary to fully consider the existing transformers, reactive power compensation devices, transmission cables, load types, and power in the design stage. For industrial and commercial photovoltaic power stations connected to the grid on the user side, attention should be paid to the location of the access point to ensure that the reactive power compensation device can function normally, the power quality meets the requirements of the grid, and the photovoltaic power station can operate efficiently and stably, achieving maximum economic benefits.