First, the role of reactive power compensation:
With the continuous expansion of the power distribution network, the capacity of the power grid continues to increase, and the requirements for the reactive power supply of the power grid are increasing. Reactive power is the same as active power, which is indispensable for ensuring power quality, voltage quality, network loss and safe operation of power systems. In addition to the active load of the user, the power supply and power supply departments also supply the user with a certain amount of reactive load. The active load is the user's measurement and the electrical energy that must be consumed by the normal operation of the electrical equipment. The reactive load is for maintenance. The energy required to oscillate between the power source and the user's inductive and capacitive devices, and in the power system network, this energy is unavoidable. At the same time, it also shows that in the power network, the balance of active power and reactive power should be maintained. Otherwise, the system voltage will be reduced, the output of the electrical equipment will be insufficient, the network transmission capacity will be reduced, the loss will increase, and the equipment will be damaged in serious cases, and the system will be disassembled. Solving the problem of network reactive power compensation has important significance for power grid loss reduction and energy conservation.
Second, the way of reactive power compensation:
The capacitive element is installed proportionally in the electromagnetic component circuit, so that the currents of the two cancel each other, and the angle between the vector of the current and the voltage vector is reduced, thereby improving the ability of the electric energy to work, which is the principle of reactive power compensation. At present, reactive power compensation technology is widely used in China, and there are generally three types of compensation methods: 1. Centralized compensation. The compensation capacitor assembly is located in the busbar of the substation station and can be automatically compensated manually or in groups. In combination with the actual situation of the power grid, the compensation is selected according to different methods such as power factor, voltage reactive power, voltage reactive power integrated control or voltage reactive power integrated control and filtering. 2. Dynamic reactive compensation. The switching capacitor is controlled by the thyristor. The reaction speed of this control mode is generally 20ms. There is no charging current and overvoltage when switching. However, since the thyristor has the characteristic of natural conduction voltage, the capacitor switching will generate harmonics. . 3. Automatic reactive power compensation for power supply line grouping. It is generally used for on-site compensation for low-pressure parts of factories and mines and distribution transformers. When considering the choice of reactive power compensation mode, in order to give full play to the potential of the power supply and power supply equipment, try to make the generator less reactive, and give priority to the reactive power required by the user and the reactive power loss in the power grid. Compensation to increase the allowable temperature rise of each component of the power supply network and the transmission capacity of the allowable voltage drop, reducing power loss in the network.
Third, the reactive power compensation capacitor installation and operation safety issues:
According to SD325-89 "Technical Guidelines for Voltage and Reactive Power of Standard Power System of the Ministry of Energy of the People's Republic of China": When the maximum load of the main transformer is used, the power factor of the high voltage side is not less than 0.95, and the power factor of the medium and low voltage side is not less than 0.9. The high-voltage power supply industrial users and high-voltage power supply are equipped with power users with load-adjusting voltage devices, and the power factor is above 0.9. According to the specifications, we have calculated the power factor values. Within the jurisdiction of the company, there are four substations and 159 line distribution transformers. The main urban area is powered by the 110kv hub station; in cooperation with the construction of the Danjiangkou dam heightening project in the South-to-North Water Diversion Project, a 35kv distribution substation was built in 2005; and a 110kv distribution substation was built to meet the production needs of large and medium-sized factories and mines in the suburbs. . In the system operation, due to a large number of inductive loads such as line distribution transformers, electric motors and other power equipment, these devices need to absorb reactive power during no-load operation, and the absorbed reactive power accounts for more than 60% of the full load, resulting in the company's grid power. The factor reduces the voltage drop. After monitoring, the line power factor is between 0.72 and 0.78, which has an impact on the production and life of enterprises and residents. According to the improvement of the power factor, the compensation capacity mode needs to be determined. After the analysis and calculation, two sets of reactive power compensation devices with capacity of 900kva and 1000kva are installed in the 35kv and 110kv substations respectively, and the power factor-voltage integrated control method is adopted to improve. System voltage level. After the operation, the power factor of both stations reached 0.91 or more, and the compensation effect was obvious. Explain our installation of reactive power compensation device capacitors and safety issues in actual operation to raise concerns about the safe use of capacitors.
1. Installation wiring should meet technical requirements . For capacitors with centralized compensation, a separate capacitor chamber should be built, and the indoors should be well ventilated to avoid sunlight. The capacitor bank is connected to the busbar via a circuit breaker and cable. At the same time, for the medium-point non-direct grounding system, when the capacitor is star-connected, in order to prevent the single-phase grounding voltage of the system from rising, the outer casing should be insulated from the ground and the insulation level should be consistent with the rated voltage of the power grid.
2. The influence of ambient temperature. The operating temperature of the capacitor is an important monitoring item for the operator. The temperature does not exceed the allowable range is an important condition to ensure the safe operation of the capacitor and the expected service life. Excessive operating temperatures result in a decrease in dielectric breakdown strength or a rapid increase in dielectric loss. If the temperature rise continues, it will destroy the heat balance and cause thermal breakdown. Under low temperature conditions, the inside of the capacitor will be negative and the free voltage will drop. Therefore, the power departments in different regions select the corresponding types according to the local climatic conditions and refer to the capacitor technical regulations to ensure the safety of operation.
3. The overcurrent hazard of the capacitor . 1 When the capacitor bank is put into operation, the voltage of the capacitor terminal cannot be changed, and the inrush current will be generated. The general inrush current is 6-8 times of the normal current amplitude. The arcing chamber of the circuit breaker generates a large mechanical stress, which causes the arc extinguishing chamber to be damaged. To prevent damage caused by the inrush current, the number of switching should be minimized, or a small reactor should be inserted in the capacitor bank. 2 The operating voltage rises to over-current the capacitor. Since the reactive power of the capacitor is proportional to the square of the operating voltage, the operating voltage of the system increases and the reactive current also increases, causing the capacitor temperature to rise, which is not conducive to safe operation. In the operation, the system bus voltage should be monitored, and the abnormality should be adjusted in time to keep the operating voltage in the equilibrium position. 3 The power supply voltage waveform is distorted and the capacitor is overcurrent. In the power system, factories and mines use high-power thyristor rectifiers as widely used DC power supplies, electrolytic processes and saturation of transformer cores, etc., which will distort the voltage waveform and generate higher harmonics. In severe cases, the compensation capacitors will The inductive load in the line forms an oscillating circuit, and the outgoing current resonates, causing a serious overload of the capacitor, causing the fuse to break repeatedly, and the capacitor bank cannot be put into operation. The industrial users of our 110kv substation are mostly of such nature. The technicians conducted continuous harmonic tests on the running lines in the station and found that the higher harmonics in the system were more serious in 5, 7, and 11 times. For this reason, we are not In the power compensation device, a reactor with a series resistance of 12% in the capacitor circuit is suppressed, and harmonics of three or more times are suppressed.
4. Overvoltage damage of capacitors . The regulations stipulate that the capacitor can operate for a long time at 1.1 times the rated voltage, but in many cases, the operating voltage at night is often higher than the specified voltage allowable value. The running overvoltage has an effect on the life of the capacitor. The test found that the life of the capacitor is inversely proportional to the 7 to 8 power of the voltage. If the voltage increases by 15%, its life may be shortened to 32.7% to 37.6% when operating at rated voltage. Therefore, in order to avoid overvoltage in actual operation, capacitors with higher rated voltages are preferentially selected in the device selection, and series connected in the same phase requiring little difference in capacitance values, and the three-phase difference cannot be excessively large. The circuit breakers specially designed for capacitor banks should be properly selected, generally vacuum, SF6 circuit breakers, etc. are used. In the operation, the number of switching and cutting times of the capacitor group is minimized to avoid frequent occurrence of operating overvoltage surge.
5. It is forbidden to charge the capacitor group with charge . If the polarity of the voltage at the moment of closing is exactly opposite to the polarity of the residual charge on the capacitor, the addition of the two voltages will generate a large inrush current on the loop, causing an overheating explosion. Therefore, each time the capacitor bank is reclosed, it must be performed after the circuit breaker is disconnected from the capacitor bank for 5 minutes. The data shows that a substation had coincident due to the tripping of the 110kv power line, causing the capacitor group on the 10kv busbar to explode and damaging the 19 capacitors. To prevent such accidents, first install the protection device that automatically disconnects the circuit breaker when the voltage is lost. Second, the operating personnel should strengthen their safety awareness and avoid such operations. Third, the capacitor end should be connected in parallel with the discharge resistor, so that it can be automatically discharged after power failure. At present, most manufacturers have connected the discharge resistor into the capacitor bank before the equipment leaves the factory.
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