Filter compensation design

Reactive compensation should be based on a comprehensive planning, reasonable layout, classification compensation, the principle of local balance to determine the optimal compensation capacity and distribution, the specific content is as follows:

■ The overall reactive balance is combined with local reactive power balance. It is necessary to meet the total demand for power supply network, but also to meet the sub-line, sub-station substation and the user reactive power balance

■ Combination of centralized compensation and decentralized compensation. To decentralized compensation, which requires the focus of the load to compensate, it is necessary in the substation for large-capacity centralized compensation, but also in the distribution lines, distribution transformers and electrical equipment at the decentralized compensation, so that reactive To balance, reduce the loss of transformers and lines.

■ High-voltage compensation combined with low-voltage compensation. The high-voltage reactive power compensation device should be installed on the main load side of the transformer. When the conditions are not available, it can be installed on the third winding side of the transformer and no pressure on the high pressure side. Compensation device.

■ combination of damage reduction and pressure regulation. To reduce the loss of the main, taking into account the pressure regulator. This is for the power supply radius is longer, more branches, the load is scattered, the natural power factor low line. This line load is low, the line of the power supply transformer work in the no-load or light load conditions, the line loss is large, if the line compensation, can significantly improve the line power supply capacity.

〓The general structure of reactive power compensation

The device is mainly composed of parallel capacitor, capacitor dedicated fuse, series reactor, compensation controller, protection device (high voltage), instrument, discharge coil, zinc oxide arrester, isolation grounding switch (high voltage), pillar insulator, connecting bus and capacitor frame Equipment composition. If the use of dual star wiring neutral point unbalanced current protection or single star wiring bridge difference protection, there should be current transformer.

■ Series reactors are connected in series in the circuit of the capacitor bank to resist high harmonics and limit the closing inrush.

     Used to resist 5 times with the above harmonics, the reactor can be Xl / Xc = 7% configuration.

     When used to resist more than three harmonics, the reactor can be configured according to Xl / Xc = 14%.

     Only for limiting inrush current, the reactor can be configured according to Xl / Xc = 0.5% -1%.

■ Zinc oxide arrester is connected to the capacitor bank line to limit the overvoltage caused by the switching capacitor.

■ The discharge coil is connected to both ends of the capacitor bank. When the capacitor group is switched on, the voltage remaining at both ends of the capacitor can be reduced from 0.1 to 50 times of the rated voltage or 50V in 5 seconds to 20 seconds.

〓Determination of compensation capacity

■ High pressure compensation capacity:

   Substation in accordance with the main transformer capacity of 10-15% configuration, line compensation in accordance with 15% configuration

■ Low-pressure centralized compensation capacity:

   According to the transformer capacity of 20-40% configuration

■ In situ compensation capacity:

   General motor, the compensation capacity is not greater than no load reactive power, generally in accordance with the motor power of 25-40%; irrigation and drainage motor in accordance with 50-60% configuration; metallurgical motor, for the 8-pole motor in accordance with the power of 60% configuration, 10-pole motor According to 75% configuration

〓Number of reactive power compensation circuits

In the case of compensation capacity, the more the number of compensated circuits is divided, the smaller the compensation capacity of each circuit, the better the compensation effect, but the greater the investment, the higher the cost of the equipment, the worse the product cost. While the more the number of capacitors in the group, the greater the probability of resonance with the system.

     The capacity of the single branch compensation is usually determined according to the minimum load fluctuation characteristics to determine the number of compensated circuits. 0.4kV system of outdoor compensation box generally choose 2 to 4, indoor compensation cabinet generally choose 4 to 10 Road, high pressure compensation generally 1 to 4, the more the number of compensated circuits, in the presence of harmonics with the system The greater the probability of harmonic resonance. Therefore, in the case of guaranteed switching accuracy, the less the cost of the selected circuit.

〓Determination of reactance rate of reactor

■ For resisting 5 times with the above harmonics, the reactor can be configured according to Xl / Xc = 4.5% -7%. For the six-pulse rectifier equipment, frequency converter, DC arc furnace, DC mill and IF furnace and other non-linear load is usually generated by the five harmonics, seven harmonics, in order to suppress the capacitor harmonic amplification, select the series reactance (4.5 to 6)% of the reactor.

■ For resisting 3 times with the above harmonics, the reactor can be configured according to Xl / Xc = 12% -14%. For the welding machine, AC arc furnace, ferroalloy furnace, calcium carbide furnace and other loads usually produce the characteristic harmonics containing the third harmonic, should choose the series reactance rate (12 ~ 13)% of the reactor

■ Only when the inrush current is limited, the reactor can be configured according to Xl / Xc = 0.5% -1%

〓Switch to determine the switch

Low-voltage reactive power compensation device in three switches (contactor, non-contact switch, composite intelligent switch) switching capacitor. High pressure in accordance with the breaking current capacity of two (contactor, circuit breaker)

■ Contactor

     Switching capacitor when the impact of large current, with arcing, switch life is short, but the flow of low power consumption, more suitable for the basic load fluctuations or fluctuations are not frequent occasions.

■ SCR

     Cross-zero switching capacitor, no inrush current, capacitor cut off no overvoltage, long switching life, no need to discharge the capacitor can be re-input, but the flow of a certain power consumption, more suitable for particularly frequent fluctuations in the occasion.

■ Composite switch

Zero-crossing switching capacitor, capacitor input without inrush current, no over-voltage cut off, switch longer life (more than 500,000 times), power consumption, switching speed between the contactor and non-contact switch. Composite switch is a two-way thyristor and contactor in parallel to run the form of the switch, which is equipped with control circuit, so that when the switch into the thyristor before the first switch, cut off the thyristor after resection, to achieve no arc switching; normal operation, by the contactor Bearing current.

〓Capacitor determination and switching based on

The usual control objectives are: power factor, reactive power, reactive current, voltage. According to the specific circumstances, in order to maximize the transformer and distribution capacity of the main purpose of the transformer and power grid to minimize the loss of the main target, so the voltage should not be the control target. The power factor is the detection amount, the disadvantage is that the light load is easy to produce switching oscillation, heavy load compensation is not sufficient; reactive current for the detection, reactive power for the control target, reactive power upper and lower limits according to the system load characteristics set Set the system to improve the power factor to the best.

■ When the current distortion rate <10%, the use of standard capacitors;

■ When the current distortion rate> 10% and <50%, the use of tuning filter capacitor;

■ When the current distortion rate> 50%, the use of harmonic control and reactive power compensation comprehensive measures.

〓Reactive power compensation capacity estimation

Qc=P×F          Qc: Capacitor reactive power required for correction P: Capacitor reactive power to be corrected F: Conversion factor for target power factor