How to understand apparent power, active power and reactive power?

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1. Several common questions

Is reactive power useless power?

What is Power Factor?

Does the higher the power factor, the more power it saves?

Why do power supply companies have requirements for the power factor of factory electrical equipment?

2. Concept Introduction

The components in the circuit can be roughly divided into three categories, resistors, inductors and capacitors.

The voltage and current across the resistor are equal in phase;

The voltage phase of the inductor leads the current phase by 90°;

The capacitor voltage phase lags the current phase by 90°.

1. Active power

Official definition: In an AC circuit, active power refers to the average value of the integration of the instantaneous power emitted or consumed by the load in one cycle (or the power consumed by the load resistance), so it is also called average power.

illustrate:

Resistors are energy-consuming components . When current passes through a resistor, heat is generated. Electrical energy is converted into thermal energy, which means that electrical power is consumed. It can only absorb power but cannot release it. Its characteristic is that it is irreversible .

2. Reactive power

Official definition: Reactive power is generated by the reactor (inductor or capacitor) in the AC circuit. Since the voltage across it is 90 degrees out of phase with the current flowing through it, it cannot do work and does not consume active power, but it participates in the energy exchange with the power supply, which generates reactive power and reduces the power supply efficiency of the generator and the power grid.

illustrate:

Inductors and capacitors are energy storage components . Although they also absorb power from the external circuit, they only store the absorbed power and release it at the appropriate time, exchanging energy with the external circuit (power source), which is characterized by reversibility .

3. Apparent power

Apparent power indicates the capacity of AC electrical equipment. It is the total power that the AC power supply can provide, and its value is equal to the product of the effective value of voltage and the effective value of current in the AC circuit.

Apparent power is neither equal to active power nor reactive power, but it includes both active power and reactive power.

4. Power factor

The power factor reflects the extent to which the apparent power output of the power supply is effectively utilized.

In the power triangle, the ratio of active power P to apparent power S is called the power factor cosφ, and its calculation is: P=U×Icosφ, where φ refers to the phase difference between voltage and current.

illustrate:

The power factor is an indicator used to describe the amount of active power and reactive power. The higher the power factor, the greater the proportion of active power.

We hope that the power factor is as large as possible. In this way, the reactive power in the circuit can be reduced to a minimum, and most of the apparent power will be used to supply active power, thereby increasing the efficiency of electric energy transmission.

3. Focus on understanding reactive power

In a factory environment, there are a large number of inductive loads. The power consumed by them to establish a magnetic field during operation is called reactive power. Without reactive power, motors and transformers cannot establish a working magnetic field. Therefore, reactive power is also useful in some occasions.

For a certain factory, capacitive or inductive loads have little impact on power consumption, that is, the power factor has little impact. However, if hundreds of factories are like this, a large amount of reactive power will cause low power efficiency in the power grid.

The capacity of the power supply equipment cannot be fully utilized. When a certain active power is transmitted to the load at a certain voltage, the current through the transmission line increases, and the energy loss of the wire resistance and the wire impedance will cause voltage drop. Therefore, the power supply company has requirements for the power factor of electrical equipment.

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