The principle of local balance should be adopted to balance the total output of reactive power (including reactive power compensation) and total reactive power (including total reactive power loss) at any time. The Wangkui Bureau has realized the centralized compensation of the substation, which is not covered in this paper. It only discusses the 10kV line, the distribution transformer and the compensation of the motor.
(1) Reactive power compensation for 10kV distribution lines:
Wangkui Bureau installed 1 or 2 high-voltage reactive power compensation devices on each 10kV distribution line, and the compensation capacity is controlled at 10% of the total capacity of the line distribution transformer. Wangkui Bureau's common distribution capacity is 40,500kVA, and the compensation for reactive capacity is about 4000kvar, which requires about 550,000 yuan. It is calculated that when installing a place, the reactive power automatic compensation device should be installed at the length of 2/3 line from the head end of the line. When two installations are installed, the first is installed at the length of the 2/5 line from the head end of the line, and the other is installed at the length of the 4/5 line from the head end of the line. The capacity is half of the total compensation capacity of the line. For specific installations, consideration should also be given to ease of operation, maintenance and overhaul work.
(2) Reactive power compensation of distribution transformer:
Most of the distribution transformers in the rural power grid have large changes in day and night load. Many villages use electricity for residential electricity. Most transformers are in light or no load during the day and night. We know that the loss of the transformer includes active loss and reactive loss, and the reactive loss includes no-load excitation loss and magnetic flux leakage loss. From the theoretical calculation of the distribution line loss, the reactive loss of the distribution transformer accounts for about 60% of the total loss of the distribution network. In order to effectively compensate the reactive power of the distribution transformer itself, avoid the power factor leading, light voltage and saving money when the light load is light, and implement the static reactive power compensation for the distribution transformer with the capacity below 200kVA according to about 5% of the distribution capacity. The compensation device is installed at the distribution low-pressure outlet, and is cut at the same time as the distribution. Automatic tracking compensation device for distribution transformers of 200kVA and above.
(3) Reactive power compensation of the motor:
For motors with a high commissioning rate of 7.5 kW and above, it is best to perform reactive power compensation. To prevent the resonant overvoltage generated by over-compensation and burn the motor, the power factor of the motor at no-load should be compensated to close to 1. Because the reactive load of the motor at no load is the smallest, the motor power factor of the fully loaded motor after compensation is still lagging, thus avoiding the occurrence of overcompensation. The low voltage capacitor is switched together with the device to directly compensate the reactive loss of the device itself.
1 Motor with less mechanical inertia (such as fan):
Q c ≈0.9Q o (1)
Where Q c -- compensation capacity, kvar
Q o -- motor no-load reactive power, kvar
The no-load current of the motor can be provided by the manufacturer. If not, it can be determined by referring to (2):
I o =2I e (1-cosφ), A(2)
Where I o -- motor no-load current, A
I e -- motor rated current, A
Cosφ--power factor of rated load of electric motor
2 Motors with large mechanical load inertia (such as pumps):
Q c = (1.3 ~ 1.5) Q o (3)
3 The centralized compensation capacity of the workshop and factory can be determined according to formula (4):
Q c =P m (tgφ 1 -tgφ 2 ) (4)
Where P m is the average active power at the highest load
Tgφ 1 -- the tangent of the power factor angle before compensation
Tgφ 2 -- the tangent of the power factor angle after compensation
The reactive power compensation of the motor, because the beneficiary is mainly the customer itself, the investment should be borne by the customer.
2 Economic Benefit Analysis(1) Analysis of economic benefits of distribution transformer reactive power compensation:
After the power grid realizes reactive power compensation, it not only reduces the loss of the power distribution equipment, but also reduces the high and low voltage distribution currents, resulting in a decrease in the line loss rate, and at the same time, the main transformer copper loss and the wire loss of the upper transmission line are reduced. All considerations will complicate the calculation. In order to simplify the calculation procedure, the economic equivalent of reactive power compensation can be used to calculate the economic benefit after reactive power compensation. Its physical meaning is that each 1kvar of compensation capacitors is installed, which is equivalent to how many kilowatts of active loss is reduced. The compensation device is on the low-voltage bus side of the distribution transformer, and the relevant manual for the reactive economic equivalent value can be taken as 0.15. In order to make the calculation more scientific, according to the actual situation of Wangkui County, take 0.1 when calculating. The power supply area of ​​Wangkui County needs to be installed with a reactive power compensation capacity of 2500kvar. After calculation, the annual power loss can be reduced by 1.7 million kW·h. The unit price per kW·h is calculated at 0.3 yuan, and the annual income can be 500,000 yuan.
(2) Analysis of economic benefits of reactive power compensation for 10kV distribution lines:
The total reactive power compensation capacity of the 10kV distribution line is about 4000kvar, the manual of the reactive economy equivalent is about 0.06, and the compensation equipment is put into operation every day for about 6 hours. After calculation, the power loss can be reduced by 500,000 kW·h per year. kW·h is calculated based on the unit price of 0.3 yuan, and there is a profit of 150,000 yuan per year.
(3) Analysis of the economic benefits of the reactive power compensation equipment itself:
After installing the reactive power compensation device, the power lost by the device itself can be calculated as follows:
A=Q c ·tgφ (5)
Where Q c -- put capacitor capacity, kvar
Tgφ--the tangent of the dielectric loss angle of the capacitor
T-capacitor commissioning time
After calculation, the annual power consumption of reactive power compensation equipment is 160,000 kW·h, and there is a negative return of 50,000 yuan per year.
Through the above analysis, the total investment of reactive power compensation is about 1 million yuan. After the equipment is put into operation, it can have a return of 600,000 yuan per year, and the entire investment can be recovered in two years.
3 ConclusionIt is indeed an effective way to reduce the line loss and improve the quality of power by making a reasonable reactive power compensation with small investment, quick effect, high profit and practicality.
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