Selection of Reactive Power Compensation Capacitor Operating Characteristics (2)

3. Selection of low voltage capacitor rated voltage and steady state overvoltage range:

From the above analysis, it can be seen that the rated voltage of the capacitor higher than the operating voltage may cause the compensation capacity to be insufficient or increase the investment of the equipment; and if the rated voltage of the capacitor is too low, the value of the capacitance C may decrease due to the partial breakdown of the dielectric and thus affect the compensation effect. The correct selection principle should be based on the premise of guaranteeing the compensation effect, not only satisfying less equipment investment in the early stage of construction, but also requiring that the dielectric breakdown rate during the operation of the capacitor is low, that is, the capacitor operating voltage should be maintained as much as possible. The allowable steady-state over-voltage range of the capacitor. GB1274721991 "Self-healing low-voltage shunt capacitors" Section 4.1.5.1 of the capacitor is clearly defined as the steady-state over-voltage capacitor "The capacitor should be able to run at 1.1 times the rated voltage for a long period of time, and can withstand 200 times within the entire service life 1.15 times the rated voltage value of the voltage impact.” The power supply radius of the low-voltage transformer station in the public transformers in Zhongshan City has basically reached 250m within the township and within 500m in the rural areas since 1998. The transformer is installed on the principle of deepening the load center, and more than 75% of the low-voltage load distribution. Within 150m in the vicinity of the transformer, the voltage deviation at the power receiving end of the three-phase power supply users of 10kV and below is 7% according to the National "Power Supply Business Rules," and the voltage deviation at the power receiving end of 220V single-phase power supply users is +7% to -10%. In line with the principle of taking into account both the voltage of the transformer's near-end user's high-end voltage and the voltage of the remote user's low-end voltage, the average voltage of the low-voltage output of the common 100.4kV transformer remains at 400-440V, but there is still a high 440V. Voltage.

Recently, 1300 sets of sampling data of 550 public distribution transformers have been statistically analyzed, and 11.81 million sets of sampling data with voltages of 400 to 440 have been seen, accounting for about 90.85%, and 1,084,400 sets with voltages of 440 to 460V. Accounted for 8.34%; voltage ≥ 460V 106,000 groups, accounting for about 0.82%. According to the above transformer low-voltage operating conditions, if the rated voltage of the capacitor is selected to be 400V, the capacitor configured in each transformer may experience approximately 193 times of 1.15 times the rated voltage per year. Obviously, it is very unfavorable for the partial breakdown of the capacitor plates and even the operating life. If the rated voltage of the capacitor is increased, it will greatly increase the investment in equipment. In this regard, the approach adopted is to select the rated voltage of the capacitor to be 400V according to the national standard's priority recommended value, and at the same time, the principle of parallel control is adopted for capacitor switching.

4. Parallel control switching capacitors:

The integrated control of the power factor and voltage parameters of the capacitor installation is taken as the basis for the switching control to ensure that the capacitor operation area of the reactive power compensation device is always maintained at a stable overvoltage range of 1.1 times.

The condition that the capacitor is put into operation is that the power factor is lower than the design requirement and the voltage is lower than the upper limit value, or the voltage is lower than the lower limit value and the power factor is less than 1; the condition for removing the capacitor from operation is that the power factor is higher than 1 or the voltage is higher than Limit value. The upper limit of the general voltage is 1.05 times the rated voltage, and the lower limit of the voltage is 0.9 times the rated voltage.

Recently, the author conducted a test test on 320 sets of utility transformer low-voltage side reactive compensation capacitors installed in the region in 2002. The obtained data show that the power factor at the installation is between 0.91 and 0.99, and the high zui operation voltage is 419V. The average decline was 1.2%. Facts have proved that adopting the power factor and voltage in parallel to control the switching mode, the effect of the compensation of reactive power and the operating voltage of the stabilized capacitor are all effective.