In the AC circuit, there are two kinds of electric power supplied by the power supply to the load; One is active power and the other is reactive power.

有功功率是保持用电设备正常运行所需的电功率，也就是将电能转换为其他形式能量(机械能、光能、热能)的电功率。各种照明设备将电能转换为光能，供人们生活和工作照明。

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, optical energy, thermal energy). Various lighting devices convert electric energy into light energy for people's living and working lighting.

有功功率的符号用P表示，单位有瓦(W)、千瓦(kW)、兆瓦(MW)。

The symbol of active power is represented by P, and the units are watt (W), kilowatt (kw) and megawatt (MW).

无功功率比较抽象，它是用于电路内电场与磁场的交换，并用来在电气设备中建立和维持磁场的电功率。它不对外作功，而是转变为其他形式的能量。凡是有电磁线圈的电气设备，要建立磁场，就要消耗无功功率。由于它不对外做功，才被称之为“无功”。无功功率的符号用Q表示，单位为乏(Var)或千乏(kVar)。

Reactive power is more abstract. It is used for the exchange of electric field and magnetic field in the circuit, and is used to establish and maintain the electric power of magnetic field in electrical equipment. It does not work externally, but is transformed into other forms of energy. All electrical equipment with electromagnetic coils must consume reactive power in order to establish a magnetic field. Because it does not work externally, it is called "reactive power". The symbol of reactive power is represented by Q, and the unit is var or kvar.

无功功率决不是无用功率，它的用处很大。电动机需要建立和维持旋转磁场，使转子转动，从而带动机械运动，电动机的转子磁场就是靠从电源取得无功功率建立的。变压器也同样需要无功功率，才能使变压器的一次线圈产生磁场，在二次线圈感应出电压。因此，没有无功功率，电动机就不会转动，变压器也不能变压，交流接触器不会吸合。

Reactive power is by no means useless power, it is very useful. The motor needs to establish and maintain the rotating magnetic field to make the rotor rotate, so as to drive the mechanical movement. The rotor magnetic field of the motor is established by obtaining reactive power from the power supply. The transformer also needs reactive power to generate 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 pull in.

在正常情况下，用电设备不但要从电源取得有功功率，同时还需要从电源取得无功功率。如果电网中的无功功率供不应求，用电设备就没有足够的无功功率来建立正常的电磁场，那么，这些用电设备就不能维持在额定情况下工作，用电设备的端电压就要下降，从而影响用电设备的正常运行。无功功率对供、用电产生一定的不良影响，主要表现在：

Under normal circumstances, electrical equipment should not only obtain active power from the power supply, but also obtain reactive power from the power supply. If the reactive power in the power grid is in short supply, the electrical equipment does not have enough reactive power to establish a normal electromagnetic field, then these electrical equipment can not work under the rated condition, and the terminal voltage of the electrical equipment will drop, thus affecting the normal operation of the electrical equipment. Reactive power has certain adverse effects on power supply and consumption, mainly in the following aspects:

1、降低发电机有功功率的输出。

1. Reduce the output of generator active power.

2、降低输、变电设备的供电能力。

2. Reduce the power supply capacity of power transmission and transformation equipment.

3、造成线路电压损失增大和电能损耗的增加。

3. It causes the increase of line voltage loss and electric energy loss.

4、造成低功率因数运行和电压下降，使电气设备容量得不到充分发挥。

4. It causes low power factor operation and voltage drop, so that the capacity of electrical equipment can not be brought into full play.

从发电机和高压输电线供给的无功功率，远远满足不了负荷的需要，所以在电网中要设置一些无功补偿装置来补充无功功率，以保证用户对无功功率的需要，这样用电设备才能在额定电压下工作。这就是电网需要装设无功补偿装置的道理。

The reactive power supplied from the generator and high-voltage transmission line is far from meeting the needs of the load. Therefore, some reactive power compensation devices should be set in the power grid to supplement the reactive power, so as to ensure the needs of users for reactive power, so that the electrical equipment can work under the rated voltage. This is why the power grid needs to be equipped with reactive power compensation devices.

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