In an AC circuit, there are two types of electrical power supplied to the load by the power supply: one is active power and the other is reactive power. The voltage and current are in the same phase, the power supply supplies power to the load, and the load converts the electrical energy into other energy, which is called active power. The part of the voltage and current in different phases, the power supply and the load exchange electric energy. This part (except line loss) of the electric energy is not converted into other energy (other than electromagnetic), which is called reactive power.
Active power
Active power is the electrical power required to maintain the normal operation of electrical equipment, that is, the electrical power that converts electrical energy into other forms of energy (mechanical energy, light energy, thermal energy). For example: a 5.5 kilowatt motor converts 5.5 kilowatts of electrical energy into mechanical energy to drive a water pump to pump water or a thresher to thresh grains; various lighting equipment converts electrical energy into light energy for lighting people's lives and work. The symbol of active power is represented by P, and the units include watts (W), kilowatts (kW), and megawatts (MW).
Active power: In an AC circuit, the average value of the instantaneous power emitted by the power supply within a cycle (or the power consumed by the load resistor) is called "active power". Excessive low active power leads to increased line losses, reduced capacity, and reduced equipment utilization, which leads to increased waste of electrical energy.
Reactive power
Inductive loads in the power grid (such as motors, chokes, transformers, induction heaters and welding machines, etc.) will produce varying degrees of electrical hysteresis, which is the so-called inductance.
Inductive loads have such a characteristic that even if the applied voltage changes direction, the hysteresis of the inductive load can still maintain the direction of the current (such as forward) for a period of time. Once this phase difference between current and voltage exists, negative power is generated and fed back into the grid. When the current and voltage are in the same phase again, the same amount of electrical energy is needed to establish a magnetic field in the inductive load. This magnetic field reverse electrical energy is called reactive power.
Definition: In a circuit with an inductor or capacitor, in each half cycle, the power supply energy is converted into magnetic field (or electric field) energy and stored, and then released, and the stored magnetic field (or electric field) energy is returned to the circuit. The power supply only performs this energy exchange and does not actually consume energy. We call the power value of this exchange "reactive power".
Reactive power is relatively abstract. It is the electrical power used to exchange electric fields and magnetic fields within a circuit and to establish and maintain magnetic fields in electrical equipment. It does not do work externally, but is converted into other forms of energy. Any electrical equipment with electromagnetic coils consumes reactive power to establish a magnetic field. For example, a 40-watt fluorescent lamp requires more than 40 watts of active power (the ballast also consumes part of the active power) to emit light, and also requires about 80 watts of reactive power for the ballast coil to create an alternating magnetic field. Because it does no external work, it is called "reactive". The symbol of reactive power is represented by Q, and the unit is Var (Var) or kVar (kVar).
Disadvantages of too high reactive power:
1) Reactive power will lead to an increase in current and apparent power, resulting in a decrease in system capacity;
2) The increase in reactive power will increase the total current, thereby increasing the losses of equipment and lines;
3) The voltage drop of the line increases, and the impact of reactive load will also cause the voltage to fluctuate violently.
After the inductive electrical equipment in the distribution network, such as transformers, motors, welders, air conditioners, washing machines, refrigerators, sodium lamps, fluorescent lamps, etc., are put into operation, they must not only absorb active power from the power grid for work, but also absorb inert power. Work power creates a magnetic field, which results in a generally low natural power factor for electricity customers. Our country stipulates power factor standards that must be met for electricity consumption by electricity customers.
Reactive power is by no means useless power, it has great uses. The motor needs to establish and maintain a rotating magnetic field to rotate the rotor, thus driving mechanical movement. The rotor magnetic field of the motor is established by obtaining reactive power from the power source. Transformers also require reactive power to generate a magnetic field in the primary coil of the transformer and induce voltage in the secondary coil. Therefore, without reactive power, the motor will not rotate, the transformer will not change voltage, and the AC contactor will not close. In order to vividly illustrate this problem, here is an example: rural water conservancy construction requires excavation and soil transportation. When transporting soil, bamboo baskets are filled with soil. The soil picked up is like active power, and the empty bamboo basket is like reactive power. , Bamboo baskets are not useless. How can soil be transported to the embankment without bamboo baskets?
Under normal circumstances, electrical equipment not only needs to obtain active power from the power source, but also needs to obtain reactive power from the power source. If the reactive power in the power grid is in short supply, the electrical equipment will not have enough reactive power to establish a normal electromagnetic field. Then, these electrical equipment will not be able to maintain operation under rated conditions, and the terminal voltage of the electrical equipment will drop. This affects the normal operation of electrical equipment.
Reactive power has certain adverse effects on electricity supply and consumption, mainly in:
(1) Reduce the active power output of the generator.
(2) Reduce the power supply capacity of power transmission and transformation equipment.
(3) Causes increased line voltage loss and increased power loss.
(4) Causes low power factor operation and voltage drop, so that the capacity of electrical equipment cannot be fully utilized.
The reactive power supplied from generators and high-voltage transmission lines is far from meeting the needs of the load. Therefore, some reactive power compensation devices must be set up in the power grid to supplement the reactive power to ensure users' needs for reactive power. In this way, Electrical equipment can only work at rated voltage. This is why the power grid needs to install reactive power compensation devices.
