It is very easy for people to understand effective power, but it is not easy to deeply understand ineffective power. In a sinusoidal circuit, the concept of reactive power is clear, but in the presence of harmonics, the definition of reactive power is not clear. However, the concept of reactive power and the importance of reactive power compensation are consistent. Reactive power should include the compensation of basic reactive power and harmonic reactive power.
Reactive power is of great significance to power supply system and load operation. The impedance of power system network components is primarily inductive. Therefore, in order to transmit the active force, a phase difference is required between the transmitter and receiver, which can be achieved over a fairly wide range. In order to transmit reactive power, there is a numerical difference between the voltages at both ends, which can only be realized within a narrow range. In addition to many network components consuming reactive loads, many loads also need to consume reactive loads. The reactive power required by network components and loads must be available somewhere in the network. Obviously, these reactive powers are all supplied by generators, and long-distance transportation is unreasonable and usually impossible. A reasonable way is to generate reactive power where reactive power needs to be consumed, which is reactive power compensation.
1. The meaning of reactive power compensation
In the power distribution system, in order to evaluate the quality of power supply, the significance of reactive power compensation has the following three elements:
1. In order to reduce the capacity of grid equipment and increase equipment output
Under the condition that the effective power does not change, the power factor of the power grid increases and the reactive power also decreases. It can be seen from the formula S-√P2+Q2 that the power will inevitably decrease. For example, if a power consumption unit requires a 200kW electric load, and the power factor is 0.4, it can be obtained from COSφ=P/S, S=P/cosφ=500kV.A, that is, the power factor of a transformer that requires 500kV.A is 0.8, only need to install a 250kV.A transformer. It can be seen that as the power coefficient increases, the required equipment capacity can be reduced accordingly.
2. Whether the voltage and frequency of the power point are close to constant.
(A) Whether the power factor is close to 1.
(b) In a three-phase system, whether the phase currents and phase voltages are balanced.
The use of reactive power compensation to improve the power factor can not only reduce the power loss caused by reactive current transmission, but also effectively improve and increase the voltage of end users, and improve the economical operation level of electrical equipment. Therefore, reactive power compensation has always been an important part of the power supply and distribution system.
3. In order to save electricity costs
According to the current electricity tariff policy in our country, customers whose electrical equipment volume exceeds 100kV.A (kW) shall adjust the electricity bill, and fine when the electricity bill is less than the standard value. Reactive power compensation has improved the power factor, reduced or avoided the increase in electricity bills due to low power factor, and saved electricity bills.
4. In order to reduce fines of power companies
With the increasing emphasis on environmental protection, power companies gradually strictly control the power waste of enterprises, so power companies have imposed more and more fines in some companies. In order to reduce the fines of power companies, companies began to transfer capacitors to compensate for reactive power. , Reduce power consumption.
5. Extend the service life of equipment
In terms of production cost, the company needs to calculate the depreciation rate of the equipment to calculate the production cost and finally determine the company’s annual net profit. However, many equipment have to be abandoned due to serious equipment wear and tear and often use for 3-5 years, a large part of which is due to reactive power. High, leading to equipment aging, so more and more companies begin to pay for compensation capacitors to prolong the service life of equipment.
Second, the role of reactive power compensation
The function of the reactive power compensation cabinet is to provide the necessary reactive power according to the reactive power compensation equipment through reactive power compensation. Power supply environment, improve grid quality.
Reactive power compensation cabinet plays an important role in power supply. Using a reasonable compensation device can reduce the loss of the power grid. On the contrary, selection and improper use may cause various factors such as power supply system, voltage fluctuation, and harmonic increase.
Reactive power compensation is to use an external current source to compensate the reactive power consumed by the load during operation. The device that provides this current source becomes a reactive power compensation device. The common compensation device is a parallel power capacitor.
1. Improve power supply system and load power factor, reduce equipment capacity, and reduce power consumption
2. Improving the quality of power supply and equipment operating conditions can ensure that the equipment operates under normal working conditions, which is conducive to safe production.
3. Save electricity, reduce production costs, and reduce enterprise electricity bills.
4. It can reduce line power consumption and improve power grid transmission efficiency.
5. Stabilize the voltage of the receiving end and the power grid, and improve the quality of power supply. Dynamic reactive power compensation The dynamic reactive power at the appropriate position of the long-distance transmission line can improve the stability of the transmission system and increase the transmission capacity.
6. In the case of unbalanced three-phase loads such as electrified railways, the effective and ineffective loads of the three phases can be balanced by appropriate ineffective compensation.
3. The principle of reactive power compensation
Connect a device with a capacitive electrical load and an inductive electrical load on the same circuit, the inductive load absorbs energy when the capacitive load releases energy, and the capacitive load absorbs energy when the inductive load releases energy, and the energy is shared between the two loads exchange between. In this way, the principle of reactive compensation is that the reactive power absorbed by the inductive load is compensated by the reactive power output by the capacitive load.
In the actual power system, most of the loads are asynchronous motors, and the equivalent circuit of most electrical equipment including asynchronous motors can be regarded as a circuit in which resistance r and inductance l are connected in series, and its power factor is
In the formula
After connecting the R and L circuits in parallel and then connecting them to capacitor C, the circuit is shown in Figure (a) below. The current equation of this circuit is:
It can be seen from the phasor diagram in the figure below that the phase difference between the voltage U and the current I becomes smaller after the capacitor is connected in parallel, that is, the power factor of the power supply circuit increases. At this time, the phase of the supply current I lags behind the voltage U, which is called undercompensation.
The circuit and phasor diagram of parallel capacitance compensation reactive power in the figure
(a) circuits;
(b) Phasor diagram (undercompensated);
(c) Phasor diagram (overcompensation)
The capacitance of the capacitor c is too large, and the phase of the feed current I exceeds the voltage u, which is called overcompensation, and its phasor diagram is shown in Figure (c). Usually, the condition of undesirable overcompensation will cause the secondary voltage of the transformer to rise, and the capacitive reactive power will increase the power loss just like the transmission power line. When the voltage of the power line rises, the power loss of the capacitor itself will also increase, and the temperature rise will increase. , will affect the life of the capacitor.
4. Why do we need to increase reactive power compensation, and what effect does it bring?
The amount of reactive power compensation increases at a certain point in the power grid, and the reactive power flow of all connecting lines and transformers from this point to the power supply decreases, and the power loss connected to this point decreases, realizing power saving and power quality improvement.
Reactive power compensation requires centralized compensation for invalid economic equivalents. Select the compensation point and compensation capacity. Using electric power, customers can perform reactive power compensation in accordance with the principle of improving power factor. Compensation distribution first considers the requirements of voltage regulation to make invalid long-distance transmission invalid. Compensation The configuration of equipment is planned according to the principle of “level compensation, local balance” to realize that there are invalid loads.
Reactive power compensation usually does not want to overcompensate, because it will increase the secondary voltage of the transformer, and the capacity of reactive power transmission on the power line will also increase the power loss, that is, the power supply equipment reverses the reactive power grid. This situation is mainly caused by the reactive power of the power grid. The overvoltage caused by the excess may cause overvoltage damage to the grid, so it is necessary to install a reactor to absorb reactive power. In the power system, if it is unbalanced, the voltage of the system will drop, and in serious cases, the equipment will be damaged and the system will be disarmed. At the same time, the decline of network power factor and voltage leads to the inability of electrical equipment to be fully utilized, the decrease of network transmission capacity, and the increase of loss. Therefore, it is of great practical significance to improve the quality of the working voltage, improve the power factor, reduce system loss, and improve the efficiency of the power supply system.
Post time: Apr-13-2023