Reactive Power

Definition of Reactive Power

The transformer needs to establish an alternating magnetic field before energy conversion and transmission can be carried out.

 In order to establish an alternating magnetic field and induced magnetic flux, electric power is required.

This electric power is called the reactive power of the transformer.

The maximum speed of energy exchange between the inductor and the power source is called reactive power. It is represented by the symbol Q, and the unit of calculation is Var or KVar.

Reactive power is by no means useless power. In an AC power supply system, both inductors and capacitors are indispensable loads. Ferromagnetic loads such as motors and transformers.

The transformer needs reactive power to make the primary coil of the transformer generate a magnetic field and induce a voltage in the secondary coil.

Therefore, without reactive power, the motor will not rotate, the transformer will not transform, and the AC contactor will not pull in.

Table of Contents
Transformer reactive power compensation box
Transformer reactive power compensation box

Reactive power calculation formula

Q-reactive power (var); U-phase voltage (V); I-phase current (A).

1. Single phase: Q=UIsin

2. Three-phase: Q= UIsin

When the angle is between 0 and 90, Sin(Θ) is a positive value;
When the angle is between 270 – 360, Sin(Θ) is a negative value;

When it is between 0 and 90, it means that the characteristic of the load is inductive, and the voltage signal on the actual load leads the signal.

When it is between 270 and 360, it means that the characteristic of the load is partial capacitive, and the current signal on the actual load leads the voltage signal.

The positive/negative reactive power factor reflects the characteristics of the load.

When the reactive power decreases or increases, the active power does not change. But when the reactive power decreases, the current will decrease and the line loss will decrease. On the contrary, the line loss will increase.

What is the no load losses of transformer?What is the short-circuit loss of the transformer

Main factors affecting reactive power factor

When the supply voltage is higher than 10% of the rated value, the reactive power will increase very quickly due to the saturation of the magnetic circuit. When the supply voltage is lower than the rated value, the reactive power will also respond to the reduction so that their power factor has Improved.

Undesirable effects caused by reactive power

  • Reduce the output of the generator’s active power.
  • Increasing reactive power will reduce the power supply capacity of power transmission and transformation equipment.
  • The flow of reactive power in the power grid will increase the line voltage loss and increase the power loss.
  • When the system lacks reactive power, it will cause low power factor operation and voltage drop, so that the capacity of electrical equipment cannot be fully utilized.

What kind of influence does reactive power have on the transformer?

The reactive power consumed by the transformer is generally about 10% to 15% of its rated capacity, and its no-load reactive power is about 1/3 of the full load. Therefore, in order to improve the power factor of the power system and the enterprise, the transformer should not operate at no-load or in a long-term low-load operation state.

  • The power factor of the load wasting power resources is not equal to 1, then its reactive power is not equal to zero. This means that part of the energy it receives from the power supply is used for reactive power exchange rather than active power consumption.
  • Affecting the function of power generation equipment For power generation equipment in operation, reactive power should be supplied by generators or other reactive power sources.

The power factor of transformers in more developed countries is relatively high.

Vigorously improving the power factor of transformers, reducing line losses, and saving energy are the inevitable directions for the development and progress of the current power grid.

Parallel installation of a certain capacity capacitor or other reactive power compensation device at the appropriate position of the transmission line can play a role in improving the power factor and reactive power compensation.

Therefore, the transformer has a great demand for a control system that can intelligently switch the compensation capacitor according to the change of the reactive power in the line and realize the reactive power compensation. Solving the problem of reactive power compensation is extremely important for transformer loss reduction and energy saving.



How to calculate the reactive power of a transformer?

Assume that the rated capacity of a transformer is S=400KVA and the power factor is 0.9.

Then the active power is P=S*power factor=400*0.9=360KW

Reactive power is Q=(S^2-P^2)^0.5

The general way of reactive power compensation

There are three main methods of reactive power compensation: low-voltage individual compensation, centralized compensation, and decentralized compensation.

Low-voltage individual compensation

Low-voltage individual compensation is to randomly connect single or multiple low-voltage capacitor banks with the electrical equipment according to the reactive power requirements of the individual electrical equipment. It shares a set of circuit breakers with the electrical equipment. Through process control, shelter assembly and electromechanical switching at the same time.

Centralized compensation

Centralized compensation means that all capacitor banks are installed on the bus on the primary or secondary side of the substation. This compensation method is simple to install and reliable in operation, but the compensation effect is worse than the previous two compensation methods, and the cost is relatively high.

Decentralized compensation

Decentralized compensation is to install capacitors in groups on the outlet of each branch of the workshop power distribution room or substation. It can put in or remove the capacitor bank according to the change of the system load, and the compensation effect is better. But the cost is relatively high.

The benefit of reactive power compensation

Compensate reactive power and improve power factor;

Reduce the loss of transmission lines and transformers;

Increase the transmission capacity of the power grid and improve equipment utilization;

Improve voltage quality;

Save electricity bills.

9 Responses

  1. It’s interesting to know that transformers would be establishing alternating magnetic fields for converting energy and transmission services. I can imagine how these pieces of equipment would really be vital for communities and companies. It would probably be the best thing to do to have refurbished electrical transformers services when you have that kind of unit for a long time to ensure that it can still run.

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  8. I like what you said the reactive power, current, and line loss will all decrease as the reactive power falls. This is a good article, my uncle is looking for info and this should help him. I wanted to thank you for your explanation about power transformers.

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