Reasonable Way of Reactive Power Compensation in Distribution Network

The reasonable selection of reactive compensation points and the determination of compensation capacity can effectively maintain the system voltage level, improve the system's voltage stability, avoid large amounts of reactive long-distance transmission, and thus reduce active power loss. Moreover, due to the long-term lack of reactive power in China's power distribution network, the resulting network losses are quite large. Therefore, reactive power compensation is an effective scheme for reducing investment and recovering losses. Distribution network reactive power compensation methods are commonly used: substation centralized compensation method, low-voltage centralized compensation method, pole reactive power compensation method and user terminal dispersion compensation method.

Distribution network reactive power compensation scheme

1 Substation centralized compensation method

For the reactive power balance of the transmission grid, centralized compensation is performed at the substation (as shown in Figure 1, Mode 1). The compensation device includes a shunt capacitor, a synchrotron camera, a static compensator, etc. The main purpose is to improve the power factor of the transmission network and improve the terminal change. The voltage of the power station and the reactive power loss of the main transformer. These compensation devices are generally connected to the 10kV bus in the substation, which has the advantages of easy management and convenient maintenance.

In order to achieve voltage control in substations, reactive power compensation devices (generally shunt capacitor banks) are commonly used in conjunction with on-load voltage regulation of transformers. Through the coordination of the two for voltage/var control has accumulated a wealth of experience in the country, the nine-zone map is an effective method of substation voltage / reactive power control. However, the operation is still more troublesome, because the limit value needs to be adjusted with different operating modes, and even in some areas, oscillation may occur; and due to the actual operation of the transformer on-load tap adjustment and capacitor bank projection The number of cuts is limited, and there is no corresponding judgment in the nine districts. Therefore, the effectiveness of the current nine-zone map adjustment needs further improvement.

2 Low-pressure centralized compensation

In the distribution network, the most commonly used reactive power compensation method in China is to perform centralized compensation on the 380V side of the distribution transformer (in the manner shown in Figure 1). Usually, a low voltage shunt capacitor cabinet controlled by a microcomputer is used, with a capacity of several tens to A few hundred kilograms or so, according to the fluctuation of the user load level, put a corresponding number of capacitors for tracking compensation. Its main purpose is to increase the power factor of the dedicated variable users, achieve local balance of reactive power compensation, and have a positive effect on the loss reduction of the distribution network and distribution, and also help to ensure the user's voltage level. The investment and maintenance of this compensation method are all undertaken by dedicated users. At present, the automatic compensation devices produced by domestic manufacturers are usually based on the power factor to automatically switch capacitors. In this case, although it helps to ensure the power quality of the user, it is not desirable for the power system. Although the fluctuation of the line voltage is mainly caused by the change of reactive power, the voltage level of the line is often determined by the system conditions. When the line voltage reference value is high or low, the reactive power input and output may be far away from the actual demand, and reactive over-compensation or under-compensation may easily occur.

For the distribution system, there are many public changes in addition to the special changes. However, public utilities for the majority of home users and other small users are unrealistic because they are usually installed on outdoor rod racks, and it is unrealistic to realize low-voltage reactive power compensation: it is difficult to maintain, control, and manage, and production safety hazards are easily caused. In this way, the reactive power compensation of the distribution network is greatly limited.

3-pole compensation method

Since the large number of common transformers in the distribution network are not compensated by low voltage, the degree of compensation is limited. The resulting large reactive power gap needs to be filled by substations or power plants. A large number of reactive power transmissions along the line make the distribution network loss still high. Therefore, 10kV outdoor shunt capacitors can be installed on the poles of overhead lines (or another pole) to perform reactive power compensation (as shown in Figure 3, Mode 3) to increase the power factor of the distribution network and achieve the purpose of reducing losses and boosting voltage. However, due to the fact that the shunt capacitors installed on the poles are far from the substations, problems such as the difficulty in the configuration of the protection, the high control cost, the large maintenance workload, and the limitations of the installation environment and space and other objective conditions are likely to occur. Therefore, the reactive power optimization compensation on the pole must be combined with the following actual engineering requirements:

(1) There should be less compensation points. It is recommended that a single point of compensation should be used on one distribution line, and it is not advisable to use multiple points of compensation;

(2) The control method is simple. It is suggested that there should be no group switching for compensation on the pole;

(3) The proposed compensation capacity should not be too large. Too large compensation capacity will lead to overvoltage and over-compensation of the distribution line at light loads. In addition, the space on the rod is limited. Too many capacitors are installed on the same rod, which is neither safe nor conducive to heat dissipation of the capacitor.

(4) The proposed method of protection should be simplified. The main use of fuses and zinc oxide arresters for simple protection.

Obviously, the reactive power compensation on the pole is mainly to compensate for the reactive power needed for the utility change of the 10kV feeder. Because it has the advantages of small investment, fast recovery, high compensation efficiency, and easy management and maintenance, it is suitable for low power factor. The long load distribution lines with heavy loads, but because the load often fluctuates and the compensation method is a long-term fixed compensation, so its poor adaptability, should actively develop and apply the capacitor on the pole can be automatically switched to the reactive power compensation technology.

4User terminal dispersion compensation

Currently in China's urban and rural areas, the power consumption of low-voltage users grows rapidly. The reactive power demand of enterprises, factories, mines, and communities is very large. Reactive power compensation directly to the user's terminal (mode 4 in Figure 1) will be most appropriately reduced. The loss of the distribution network and the effective measures to maintain the voltage level of the distribution network. For enterprises and factories and mines in the motor, local reactive power compensation should be performed, that is, random compensation; for the user terminal of the cell, due to small user load, large fluctuations, scattered locations, no management, it should actively develop and apply a new type of low pressure. Terminal reactive power compensation device and meet the following requirements: 1 intelligent control, maintenance-free; 2 small size, easy to install; 3 complete function, lower cost.

Compared with the above three compensation methods, this compensation method can better reflect the following advantages: 1 The line loss rate can be reduced by about 20%; 2 Reduce the voltage loss, improve the voltage quality, and then improve the starting and operating conditions of the electrical equipment; 3 release System energy, improve the line power supply capacity. The disadvantage is that the low-voltage reactive power compensation usually determines the installation capacity according to the maximum reactive power demand of the low-voltage side of the distribution transformer, and the different timeliness of the low-voltage load fluctuations of the distribution transformers causes a large number of capacitors to be idle when the load is light, and the utilization rate of the equipment is not high.

The above four reactive power compensation methods are combined and the technical performance is compared as shown in Table 1.

Problems encountered in reactive power compensation of distribution network

With people's attention to distribution network construction and the development of reactive power compensation technology, low-voltage reactive power compensation technology has begun to gain popularity in distribution systems. From static compensation to dynamic compensation, from contact compensation to contactless compensation, we have gained rich experience. However, some problems have also been exposed in practice and must be taken seriously.

(1) The starting point of reactive power compensation is usually on the user side, and only pay attention to the compensation of the user's power factor. However, in order to achieve effective loss reduction, we must start from the perspective of the power system to determine the compensation method, the optimal compensation capacity, and the compensation location of the distribution network by calculating the reactive power flow of the entire network, so as to maximize the benefits of limited funds.

(2) Currently, the load point on the 10kV distribution network is generally not metered, and the accuracy and simultaneity of the recorded data cannot be guaranteed. This brings great difficulties to power flow calculation and reactive power optimization calculation of distribution networks.

(3) The capacitor itself has a certain anti-harmonic ability, but it also has the side effect of amplifying harmonics. When the harmonic content is too large, it will affect the life of the capacitor, and even cause premature destruction of the capacitor, and at the same time make the harmonic disturbance of the distribution network more serious.