1. What is reactive power
Most of the power equipment in the power grid work according to the principle of electromagnetic induction. They establish an alternating magnetic field in the process of energy conversion, and the absorbed power is equal to the released power in a cycle. The power supply energy has no energy loss when passing through the pure inductance or pure capacitance circuit. It is only exchanged back and forth between the load and the power supply and flows between three phases. Because this exchange power does not do external work, it is called reactive power.
Reactive power is divided into:
Inductive reactive power: the current vector lags behind the voltage vector by 90 degrees, such as motors, transformers, thyristor converter equipment, etc;
Capacitive reactive power: the current vector is 90 degrees ahead of the voltage vector, such as capacitors, cables, transmission and distribution lines, etc;
Fundamental reactive power: reactive power equal to the power frequency (50Hz);
Harmonic reactive power: reactive power that is not equal to the power frequency.
Explain inductive reactive power from the physical concept: because the inductive coil is an element that stores magnetic field energy, when the coil is applied with AC voltage and the voltage changes, the corresponding magnetic field energy also changes with it. When the voltage increases, the current and magnetic field energy will be strengthened accordingly. At this time, the magnetic field energy of the coil will store the energy supplied by the external power supply in the form of magnetic field energy; When the current decreases and the magnetic field energy decreases, the coil releases the magnetic field energy and transmits it back to the external circuit. AC inductance circuit does not consume power, and the circuit is only the reciprocating conversion between power supply energy and magnetic field energy.
Explain capacitive reactive power from the physical concept: since the capacitor is an element that stores electric field energy, when the capacitor is applied with AC voltage and the voltage changes, the corresponding electric field energy also changes. When the voltage increases, the current and electric field energy will be strengthened accordingly. At this time, the electric field energy of the capacitor will store the energy supplied by the external power supply in the form of electric field energy; When the voltage decreases and the electric field energy decreases, the capacitor releases the electric field energy and transmits it back to the external circuit. The AC capacitor circuit does not consume power, and the circuit is only the reciprocating conversion between power supply energy and electric field energy.
2. Power factor
The power load in the actual power supply and consumption system is not purely inductive or capacitive, but a load with inductance, capacitance and resistance. There is a certain phase difference between the voltage and current phasor of this load. The cosine of the phase angle is called power factor, also known as force rate, which is the ratio of active power to apparent power.
The calculation formula of three-phase power factor is:
Where: cos Φ—— Power factor; P - active power, kW; Q - reactive power, kvar; S - apparent power, KVA
Power factor is usually divided into natural power factor, instantaneous power factor and weighted average power factor. In a three-phase symmetrical circuit, the voltage and current of each phase are symmetrical and the power factor is the same. Then the total power factor of the three-phase circuit is equal to the power factor of each phase.
China has certain requirements for power factor, and the requirements for power supply companies are 110kV stations with power factor between 0.95 and 0.98; 220kV station with power factor above 0.95. The requirements for users are transformers above 100kVA, and the power factor is greater than 0.9. The requirements for agricultural irrigation are transformers above 100kVA, and the power factor is greater than 0.8.
3. Reactive power compensation
In power system, not only active power but also reactive power should be balanced. The phasor relationship among active power, reactive power and apparent power is shown in the figure.
By formula cos Φ= P / s shows that under a certain active power, the power factor cos Φ The smaller, the greater the reactive power required. In order to meet the power demand, the capacity of power supply lines and transformers needs to be increased, which will not only increase the power supply investment and reduce the equipment utilization, but also increase the line loss. In order to improve the economic operation efficiency of the power grid, the reactive power of the line is offset by artificially compensating capacitive reactive power or inductive reactive power according to the reactive power type in the power grid.
The main function of reactive power compensation is to improve power factor to reduce equipment capacity and power loss, stabilize voltage and improve power supply quality, improve transmission stability and transmission capacity in long-distance transmission, and balance the active and reactive power of three-phase load. Installing shunt capacitors for reactive power compensation can limit the transmission of reactive power in the power grid, reduce the voltage loss of the line and improve the voltage quality of the distribution network. Reactive power compensation mainly has the following functions.
(1) Improve voltage quality
Divide the current in the line into active current IA and reactive current IR, then the voltage loss in the line:
Where: P - active power, kW; Q - reactive power, kvar; U - rated voltage, kV; R - total line resistance, Ω; Xi - line inductive reactance, Ω
Therefore, increasing the power factor can reduce the reactive power Q transmitted on the line. If the active power remains unchanged and R and Xi are fixed values, the smaller the reactive power q is, the smaller the voltage loss is, so as to improve the voltage quality.
(2) Improve the utilization rate of transformer and reduce investment
The power factor is determined by cos Φ 1 to Cos Φ 2. Improve transformer utilization:
It can be seen that the utilization rate of transformer after compensation is higher than that before compensation Δ S%, which can carry more load and reduce the investment in power transmission and transformation equipment.
(3) Reduce user electricity expenditure
a. It can avoid being punished because the power factor is lower than the specified value;
b. It can reduce the active power loss caused by the transmission and distribution of reactive power within the user, and the electricity charge can be reduced accordingly.
(4) Improve the transmission capacity of power network
The relationship between active power and apparent power is: P = SCOs Φ， It can be seen that under the condition of transmitting a certain active power, the higher the power factor, the smaller the power that needs to be transmitted by the power grid.
4. Arrangement of reactive power compensation
Centralized compensation: it is installed on the 6 ~ 35kV bus of the enterprise or local general substation, which can reduce the reactive power loss of the high-voltage line and improve the power supply voltage quality of the substation.
Decentralized compensation: installed on the high-voltage or low-voltage bus of workshop or village terminal substation and distribution substation with low power factor. This method has the same advantages as centralized compensation, but the reactive capacity is small and the effect is obvious.
Local compensation: it is installed near asynchronous motor or inductive electrical equipment for local compensation. This method can not only improve the power factor of the power supply circuit of electric equipment, but also change the voltage quality of electric equipment.
The energy saving of reactive power compensation only reduces the power supply loss from the compensation point to the generator, so the reactive power compensation on the high-voltage side can not reduce the loss on the low-voltage network side, nor improve the utilization of the low-voltage power supply transformer. According to the optimal compensation theory, the energy-saving effect of local compensation is the most significant. Centralized compensation is combined with decentralized compensation, mainly decentralized compensation; Adjustment compensation is combined with fixed compensation, mainly fixed compensation; High voltage compensation is combined with low voltage compensation, mainly low voltage compensation.
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