Analysis of reactive power compensation scheme

1 How to perform reactive power compensation

The principle of local balance should be adopted to keep the total reactive power output (including reactive compensation) and total reactive load (including total reactive loss) of the power grid balanced at any time. Wangkui Substation has realized centralized compensation of substations, which will not be covered in this article. Only the compensation of 10kV lines, distribution transformers and motors will be discussed.

(1) Reactive power compensation of 10kV distribution lines:

Wangkui Bureau installs 1-2 high-voltage reactive power automatic compensation devices on each 10kV distribution line, and the compensation capacity is controlled at 10% of the total capacity of the line distribution transformer. The public distribution transformer capacity of Wangkui Bureau is 40500kVA, and the reactive power capacity to be compensated is about 4000kvar, which requires about 550,000 yuan. According to calculations, when installing one, the reactive power automatic compensation device should be installed at 2/3 of the line length from the head end of the line. When installing two, the first one is installed at 2/5 of the line length from the head end of the line, and the other one is installed at 4/5 of the line length from the head end of the line. The capacity of each is half of the total compensation capacity of the line. When installing specifically, it should also be considered to facilitate operation, maintenance and overhaul work.

(2) Reactive power compensation of distribution transformer:

Most distribution transformers in rural power grids have large load changes during the day and night. Many villages and towns use electricity for residents' daily life. Most transformers are lightly loaded or no-loaded during the day and late at night. We know that transformer losses include active power loss and reactive power loss. Reactive power loss includes no-load excitation loss and leakage reactive power loss. From the theoretical calculation of distribution network line loss, it can be seen that the reactive power loss of distribution transformers accounts for about 60% of the total loss of distribution network. In order to effectively compensate for the reactive power of the distribution transformer itself, avoid the leading power factor and voltage increase when lightly loaded, and save money, static reactive power compensation is implemented for distribution transformers with a capacity of less than 200kVA at about 5% of the distribution transformer capacity. The compensation device is installed at the low-voltage outlet of the distribution transformer and switched on and off at the same time as the distribution transformer. Automatic tracking compensation devices are installed for distribution transformers of 200kVA and above.

(3) Reactive power compensation of motor:

It is best to perform reactive power compensation on motors with a high commissioning rate of 7.5kW and above. To prevent the occurrence of resonant overvoltage caused by overcompensation and burn the motor, the power factor of the motor when it is not loaded should be compensated to close to 1. Because the reactive load of the motor is the smallest when it is not loaded, the power factor of the motor with full load after compensation is still lagging, thus avoiding the occurrence of overcompensation. Switch on and off the low-voltage capacitor together with the equipment to directly compensate for the reactive power loss of the equipment itself.

① Motors with small mechanical load inertia (such as fans, etc.):

Q _c ≈0.9Q _o (1)

Where Qc _-- 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 formula (2):

I _o =2I _e (1-cosφ), A(2)

Where I _o -- no-load current of the motor, A

I _e -- rated current of the motor, A

cosφ--power factor of motor at rated load

② Motors with large mechanical load inertia (such as water pumps, etc.):

Q _c =(1.3～1.5)Q _o (3)

③ The centralized compensation capacity of workshops and factories can be determined according to formula (4):

Q _c =P _m (tgφ ₁ -tgφ ₂ ) (4)

Where Pm _-- average active power at maximum load

tgφ ₁ -- tangent value of power factor angle before compensation

tgφ ₂ -- Tangent value of the power factor angle after compensation

As the beneficiaries of reactive power compensation for motors are mainly customers themselves, the investment should be borne by the customers themselves.

2 Economic Benefit Analysis

(1) Economic benefit analysis of reactive power compensation for distribution transformers:

After the power grid realizes reactive power compensation, it not only reduces the loss of distribution and transformation equipment, but also reduces the high and low voltage distribution current, resulting in a reduction in line loss rate, while reducing the copper loss of the main transformer and the conductor loss of the upper level transmission line. All considerations will make the calculation complicated. In order to simplify the calculation procedure, the economic equivalent of reactive power compensation can be used to calculate the economic benefits after reactive power compensation. Its physical meaning is that every 1kvar of compensation capacitor installed is equivalent to how many kilowatts of active power loss is reduced. The compensation device is installed on the low-voltage bus side of the distribution transformer. The reactive economic equivalent value can be taken as 0.15 according to the relevant manual. In order to make the calculation more scientific, according to the actual situation in Wangkui County, 0.1 is taken when calculating. The power supply area of ​​Wangkui County needs to install a reactive power compensation capacity of 2500kvar. According to calculations, the power loss can be reduced by 1.7 million kW·h each year. The unit price of electricity purchase per kW·h is calculated at 0.3 yuan, and there can be a profit of 500,000 yuan per year.

(2) Economic Benefit Analysis of Reactive Power Compensation for 10kV Distribution Lines:

The total reactive power compensation capacity required for 10kV distribution lines is about 4000kvar. The reactive economic equivalent can be taken as 0.06 according to the relevant manual. The compensation equipment is put into operation for about 6 hours a day. It is calculated that the power loss can be reduced by 500,000 kW·h each year. Calculated at a unit price of 0.3 yuan per kW·h, there can be an annual profit of 150,000 yuan.

(3) Economic benefit analysis of reactive power compensation equipment itself:

After installing the reactive power compensation equipment, the power consumption of the equipment itself can be calculated as follows:

A=Q _c ·tgφ (5)

Where Qc _-- Capacity of capacitor in operation, kvar

tgφ--the tangent value of the capacitor dielectric loss angle

T--Capacitor commissioning time

According to calculations, the reactive power compensation equipment consumes 160,000 kW·h of electricity per year, resulting in a negative return of 50,000 yuan per year.

The above analysis shows that the total investment in reactive power compensation is about 1 million yuan. After the equipment is put into operation, it can generate an annual profit of 600,000 yuan, and the entire investment can be recovered in two years.

3 Conclusion

Reasonable reactive power compensation is indeed an effective way with small investment, quick results, high returns, feasibility, and the ability to significantly reduce line losses and improve power quality.