How to solve the power factor and reactive power compensation problems of industrial and commercial power stations

In the final analysis, the photovoltaic system is a power generation system. Photovoltaic power generation emits direct current, which does not have any reactive power or power factor problems. However, after the photovoltaic power generation is converted into an AC power grid through an inverter, it will cause a series of problems in matching with the power grid. , the problem of reactive power is one of the main problems.

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In fact, the connection methods of photovoltaic power stations are different, and the reactive power problems reflected are also very different. 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 problems and reactive power compensation requirements reflected are completely different. They should be treated differently and measures should be taken according to the actual situation.

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Large-scale photovoltaic power stations connected to the high-voltage grid side. Usually, such power stations adopt a dedicated line design to directly connect the power of the power station to the transformer of the upper-level substation. Therefore, the reactive power problem of this type of power station is smaller and relatively simple. During the day, when the photovoltaic power station generates electricity, since there are no other loads between the photovoltaic power station and the substation, the inductance of the upper-stage step-up transformer is the main factor of the reactive power in the network. However, due to the large power generation during the day, the transformer is at almost full load. operation, so the relative reactive power ratio is relatively small, and most photovoltaic power stations can meet the requirements of the power grid. However, at night, the inverter of the photovoltaic power station is stopped. However, since the transformer in the station is always connected to the high-voltage power grid, at this time, the transformer in the station becomes an inductive load, which not only consumes part of the active power without load, but also Reactive power losses occur in the high-voltage circuits of power stations and upper-level substations. The light-load operation of the external cable lines and main transformers of large photovoltaic power stations will generate reactive power. Usually, a set of reactive power compensators can be added to the high-voltage bus to absorb the reactive power generated by the lines at night, so that the nighttime power factor of the photovoltaic power station can meet the requirements of the power grid.

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

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Distributed photovoltaic power stations connected to the user side. The problem of reactive power factor has been a prominent problem encountered by distributed photovoltaic power stations on the Internet in recent years, especially industrial and commercial photovoltaic power stations built in factories and enterprises. Since such power stations are usually connected to the enterprise’s internal low-voltage or medium-voltage power grid, There are many electrical equipment connected to the network, especially many inductive loads such as motors. When the photovoltaic power generation system was not originally installed, the reactive power compensation system in the network automatically adjusted the compensation based on the active power supply of the grid and the reactive power in the network. Therefore, the reactive power problem in the network could be solved through the settings in the network. Reactive power compensation equipment achieves the purpose of automatic compensation, making the reactive power problem in the network meet the requirements of the power grid.

However, when the photovoltaic power generation system is connected to the network, it means that there are second or even multiple power sources in the network. When the sun is sufficient at noon, the photovoltaic power generation system will emit powerful power, greatly reducing the power supply of the grid. The active power displayed by the load and electricity consumption meter will decrease as the power of the photovoltaic system increases, and it may even occur that the remaining power from photovoltaic power generation is sent out. At this time, if the load in the network remains unchanged, especially when the inductive load is relatively large, the original reactive power compensation system in the network will make wrong instructions and actions based on the power supply load and power load of the large power grid, resulting in The proportion of reactive power increases, causing the power factor to decrease.

In recent years, many companies that install photovoltaics have been fined by the power grid company because the power factor of the user-side power grid cannot meet the requirements of the power grid company. The specific form of fine is that the power grid company will charge high power adjustment fees.

In order 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 coupling capacity greater than 50% of the transformer capacity and 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 point A at the front end of the original reactive power compensation equipment, this connection method can make full use of the original reactive power compensation system and reduce investment.

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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 from point A, or due to some other reasons, it cannot be connected at point A and has to be connected at point B or other locations, the original reactive power compensation equipment will Although the sampling point can sense the reactive power in the network, it refers to the power supply of the grid and cannot sense the output power of photovoltaic power generation. It may not or cannot fully meet the function of correctly adjusting reactive power. To solve this problem, it is necessary to install special 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 large-capacity reactive power compensation capabilities of the photovoltaic power generation system and meet the needs of the power grid.

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

Most industrial and commercial photovoltaic inverters have a power factor adjustment capability of about 20%. For systems with a small installed capacity of photovoltaic power generation in the network and less inductive loads in the network, there is no need to consider the issue of reactive power compensation equipment. Part of the reactive power compensation is provided through the inverter. However, when the reactive power in the grid is too large and exceeds the tolerance and adjustment capabilities 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 the photovoltaic power station.

From the perspective of photovoltaic system operation stability, try to choose the grid connection point of photovoltaic power generation as close to the transformer as possible. The closer to the transformer, the more stable the voltage will be. At the end of the grid far away from the transformer, the voltage fluctuation will be greater. Frequent voltage and power fluctuations will cause the reactive power compensation device to fail to operate normally or malfunction.

Summarize

Power factor problem is one of the important problems in many photovoltaic power stations, especially industrial and commercial photovoltaic power stations. Issues such as original transformers, reactive power compensation devices, transmission cables, load types and power must be fully considered during the design process. 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 and the power quality meets the requirements of the power grid, so that the photovoltaic power station can operate efficiently and stably to achieve maximum economic benefits.

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