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Home » News » industry information » Selection of Reactive Power Compensation Capacitor Operating Characteristics (1)

Selection of Reactive Power Compensation Capacitor Operating Characteristics (1)

Views: 1     Author: Site Editor     Publish Time: 2018-06-21      Origin: Site

1. Power capacitor and its main characteristic parameters:

The power capacitor is the main component of the reactive power compensation device. With technological progress and process renewal, paper dielectric capacitors have been replaced by self-healing capacitors that use electrodes on both sides of the metal in the dielectric to vaporize the metal body. The big improvement is the fact that the capacitor partially penetrates the dielectric. Insulation has a natural recovery performance, that is, when the dielectric is partially broken down, the metal coating near the breakdown will melt and vaporize and form a cavity, which will cause the reduction of the area of the plate to reduce the capacitance C and the corresponding reactive power. But does not affect the normal operation of the capacitor. The main characteristic parameters of self-healing capacitors are rated voltage, capacitance, and reactive power.

 

1.1 Rated voltage:

GB1274721991 "self-healing low-voltage shunt capacitors" section 3.2 provides that "the capacitor rated voltage priority values are as follows 0.23,0.4,0.525 and 0.69kV." The rated voltage of the capacitor is generally higher than the rated operating voltage of the electrical equipment by 5%.

 

1.2 Capacitance:

The capacitance of the capacitor is the ratio of the charge on the polar plate to the voltage between the plates. This value is related to the area of the plate, the insulation thickness between the plates, and the dielectric coefficient of the insulating medium. The formula is C=14πε×SD. ε is the dielectric coefficient of the insulating medium between the plates; S is the area of the capacitor plate; D is the thickness of the insulating layer of the capacitor. In the above equation, the value of capacitor C is not directly related to the voltage. The value of C seems to depend only on the capacitor plate area and the insulating medium, but this is only the static condition when the capacitor is not connected to the network. After the network is connected, the capacitor is used as the dielectric. Partial breakdown causes a reduction in the area of the plate and thus affects the decrease in the value of the capacitance C. Therefore, during operation, the capacitance C is a variable that decays year by year, and its decay speed depends on the operating voltage condition and its own steady state overvoltage capability. The capacitor value of the factory capacitor is defined as the static capacitance. Generally, the rate of decrease of the capacitance value should be within 2% after the first year of operation, and the rate of decrease of the capacitance value between 1% and 2% should be between 1% and 2% from the second year to the fifth year. After the fifth year, the dielectric value will decrease due to dielectric aging. When the capacitance value drops below 85% of the factory, the lifetime of the capacitor is considered to end.

 

1.3 Reactive power:

In the AC circuit, the reactive power QC=UIsinφ is very small due to the capacitor dielectric loss angle, φ=90°, so sinφ=1, then the reactive power QC=UI=ωCU2×103=2πfCU2×103 (μF From this formula, it can be seen that the reactive power of the capacitor depends not only on the capacitance C, but also directly on the power supply frequency f and the terminal voltage U. The exact definition of the rated reactive power of the capacitor should be the static capacitance when the rated voltage is externally connected at the standard frequency. Corresponding no power. If the actual reactive power output by the capacitor after the network is connected and put into operation can reach the nominal capacity, it depends on the operating voltage condition. When the grid voltage is lower than the rated voltage of the capacitor, the reactive power output by the capacitor will be less than the calibrated value. Therefore, if the capacitor rated voltage is selected to be high, the actual operating voltage of the capacitor is lower than the rated value for a long period of time, it is likely that the reactive power output of the capacitor is lower than the design value and the reactive power shortage of the grid is caused.

 

2. Reactive power compensation Selection error of capacitor device parameters:

When the reactive power compensation device is used for design selection and equipment ordering, the parameters provided to the manufacturer are often just capacitance compensation cabinet types and reactive power values. The two important parameters of capacitor rated voltage and static capacitance are often ignored. Because the capacitor manufacturers do not understand the voltage conditions at the product installation site, they often focus on reducing the production cost and reducing the dielectric breakdown when the product is designed. The rated voltage of the selected capacitor is often higher than the national standard recommended value. In progress. What effect does the reactive power compensation effect have on the power grid construction? What changes will result? The following cases can be used for analysis.

 

For example, for a 100.4kV transformer, according to the power factor of 0.9 operating requirements, a centralized reactive power compensation must be performed on the low voltage side of the transformer. After calculation, the reactive power 100kvar should be compensated. If the capacitor is configured according to the rated voltage U=450V, according to QC=ωCU2× Calculated from 10-3, the static capacitor value C of the capacitor bank is 1572μF. After the power grid is connected to the power grid and the operating voltage U is 400V, the actual reactive power QC provided by the capacitor to the grid is 79kvar, and the compensation effect is only as expected as 79. %. On the contrary, under the above conditions, to ensure that the actual compensation effect is 100 kvar, at least the capacitor reactive power must be configured to be 127 kvar, which means that the equipment investment needs to increase by 27%. In 2004, the number of transformers in Zhongshan City increased by 898 units, with a total capacity of nearly 600,000 kvar. According to the 30% compensation rate, the reactive power to be compensated was nearly 180,000 kvar. According to the above analysis, if the rated voltage is selected to be 450V, the amount of reactive power compensation must be increased by 48,600 kvar over the original figure. Even if the AC contactor is used to switch the static compensation method, the investment in equipment purchase also needs to increase by 10.8 million yuan. If thyristor switching dynamic compensation is adopted, the increase in equipment purchase investment will reach 19.8 million yuan. Therefore, in order to ensure that the actual compensation effect is consistent with the design, ordering of reactive power compensation equipment should not only propose the type of the capacitance compensation cabinet and the amount of reactive power, but also specify the rated voltage of the capacitor. At the same time, the manufacturer of the equipment should be marked on the nameplate of the capacitor. Power power quantity, rated voltage value and static capacitance C value.


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