A brief discussion on the experience of reactive power compensation in low-voltage networks

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Pingshun County Hydropower Group Company of Shanxi Province is responsible for comprehensively transforming the rural power grid with a population of 35,000 in 2 townships and 1 town in the county's self-supply water and electricity area, and is required to complete it before the end of June 2002. In the past, due to the uncertainty of the load, the rural power grid was heavily loaded during the spring and autumn harvest seasons and during the irrigating period, but there was basically no load from 0 to 8 o'clock and during the day. The power supply quality was extremely abnormal, and the user-side voltage was very unstable, even making some low-voltage power users unable to work. People complained quite a lot. Therefore, improving the quality of power supply and improving the voltage and power factor COSφ at the user end during the transformation and construction of rural power grids are important issues to be solved, and it was decided to carry out low-voltage reactive power compensation. 　　 　　1. The concept of low-voltage reactive power compensation Low-voltage reactive power compensation refers to the installation of compensation devices in the low-voltage 400 (380) volt network of distribution transformers, including random compensation, follower compensation, and tracking compensation. 　　 　　　　random compensation: Random compensation is to connect the low-voltage capacitor in parallel with the motor through the fuse, and through control, the protection device and the motor are switched on and off at the same time. 　　 　　Follower compensation: Follower compensation is to fix the low-voltage capacitor to the low-voltage side of the distribution transformer through a fuse to compensate for the excitation and magnetic leakage reactive power losses of the transformer. 　　 　　　　tracking compensation: Tracking compensation refers to using the reactive power compensation switching device as a control and protection device, and connecting the low-voltage capacitor bank to the 400-volt bus of the large user. This compensation method is equivalent to the function of follower compensation. Several sets of low-voltage capacitors are also selected for manual or automatic switching to compensate for changing reactive loads in the 400-volt network at any time. 　　 　　2. Brief analysis of reactive load of rural power grid In our existing 10 kV power transmission system, one line is often connected to several, dozens or even twenty or thirty distribution transformers with different capacities. Due to scattered users and small transformer capacity, more than 70% of transformers are below 75 kVA, and most transformers operate close to no-load for nearly 15 hours a day, with a few operating between 20% and 40% of the rated capacity. During planting or harvesting seasons, reactive power imbalance will occur. 　　 　　　　Under the above network conditions, among the original rural power grid users, none of the 10 kV line sending end or power distribution transformer users are equipped with compensation devices, resulting in a very low power factor COSφ value at the 10 kV sending end. The main reason is that many small-capacity distribution transformers often operate under low loads. The no-load and leakage losses of many distribution transformers and the reactive power consumption of household appliances add up to occupy a large amount of reactive power sent out by the 10 kV line, causing the COSφ value to fail to meet the specified requirements and the line loss to decrease. * * Increase. 　　 　　3. Requirements for the power factor COSφ value. According to the provisions of the "Technical Guidelines for Power System Voltage and Reactive Power" of the Ministry of Water and Power and the technical requirements for rural power grid transformation, the power factor of power users should meet the following requirements: 　　 　　1． The power factor of industrial users with high-voltage power supply and power users equipped with load-adjusting voltage devices must be above 0.90. ； 　　 　　2． Other power users of 100 kVA (kilowatt) and above and large and medium-sized power drainage and irrigation stations, the power factor is above 0.85 ； 　　 　　3． For bulk sales and agricultural electricity, the power factor is above 0.80. 　　 　　　　Those who fail to meet the above requirements after efforts should install necessary compensation devices. 　　 　　　　According to the provisions of the "Electricity Supply and Consumption Rules" of the former Ministry of Water and Power: Users of high-voltage power supply must ensure that the power factor is above 0.9, and other users should keep it above 0.85. 　　 　　4. Comparison of the advantages and disadvantages of several reactive power compensation methods For the 10 kV power supply system, installing a parallel capacitor bank with centralized compensation method on the 10 kV busbar of the substation can only increase the power between the transformer and the 10 kV busbar and the upper level. The power factor of the voltage level line cannot be changed for the power factor COSφ value at the head end of the 10 kV bus line. The reactive power provided by each distribution transformer on the line still needs to be sent out here, and the line loss on each sending line cannot be reduced. Therefore, for the reactive power compensation of the 10 kV power supply system, it is best to install a low-voltage reactive power compensation device along with the distribution transformer on the line for decentralized compensation. This method makes it easy to select the compensation capacity according to the needs of the reactive load. It has the characteristics of "making up where there is a shortage and making up as much as there is a shortage". It can partially reduce the line losses of 10 kV and its upper voltage level, and has good compensation effect and high economic benefits. 　　 　　　　On the 10 kV lines of rural power grids, there are generally many "T" connected transformers, and the phenomenon of "big horses and small carts" of transformers is very common. Most of the time, they are close to no-load operation. The power factor COSφ value at the head end of the 10 kV line is generally only 0.5 to 0.75. The power supply range of distribution transformers is mostly based on natural villages. Each village has one or two power processing workshops and electric irrigation stations. Under the above circumstances, it is appropriate to adopt the random and random compensation methods, that is, the parallel capacitor compensation is selected based on the no-load current calculation on the low-voltage side of the transformer, and the parallel capacitor compensation is selected based on the motor capacity calculation in the processing workshop. The compensation capacitor adopts manual switching method, which can * * Reduce investment (about 30 yuan per thousand dollars). Only large-scale reactive power dispersion compensation can reduce the line loss of rural power grid lines and reduce rural electricity prices. 　　 　　　　Regarding the electricity consumption characteristics of rural drainage and irrigation stations, one is "strong seasonality", with the cumulative time of electricity consumption being 1 to 2 months per year. The second is "the single motor capacity is large", generally 30 to 80 kVA. Third, due to the short power consumption time, no attention is paid to reactive power demand, and no compensation device is installed. In addition, many drainage and irrigation stations in rural areas are put into operation at the same time, resulting in an imbalance of reactive power and electricity in the system. These drainage and irrigation stations should select parallel capacitors for random compensation based on motor capacity calculations to promote local reactive power balancing. 　　 　　5. The wiring of the compensation device. The wiring of the measuring point of the compensation device is mainly the capacitor bank of the compensation device and the introduction point of the current. Especially the introduction point of the current is often ignored in actual wiring. The introduction point of the capacitor bank refers to the "T" contact point of the total incoming line of the capacitor bank in the system being compensated. ； The introduction point of the current refers to the installation point of the current transformer used by the compensation device in the system being compensated. The correct way is: Taking the power supply of the load as the reference point, the installation point of the current transformer must be between the "T" contact power supply of the main incoming line of the capacitor bank, that is, the current measured by the current transformer must include the current flowing through the capacitor bank. Otherwise, when the capacitor group is switched on and off, the active power, reactive power and COSφ values ​​measured and displayed by the reactive power compensation device will not change, making it impossible to judge the switching effect of the reactive power compensation device. 　　 　　6. Benefit evaluation of reactive power compensation According to the relevant regulations of Guodian Corporation, if the power factor COSφ value does not meet the standard, the installation of compensation devices should be urged to reduce line losses. When installing compensation devices, we must proceed from long-term interests and overcome the current lack of funds. From the installation of low-voltage reactive power compensation devices, users will benefit the most, followed by the power supply system. We have installed low-voltage reactive power compensation capacitors on a voluntary basis for rural power grid users affiliated to our company through careful analysis and calculation, careful design, and on a voluntary basis. This has completely solved the problem of unstable power quality for users, achieved good social benefits, and won praise from users.