Power factor compensation

Most of the power loads in the power grid, such as motors, transformers, fluorescent lamps and arc furnaces, are inductive loads. These inductive devices need to absorb not only active power from the power system during operation, but also reactive power. Therefore, after installing parallel capacitor reactive power compensation equipment in the power grid, it can provide compensation for the reactive power consumed by the inductive load, reducing the reactive power provided by the power grid power supply side to the inductive load and transmitted by the line.

1 Overview

In an AC circuit, the cosine of the phase difference (Φ) between voltage and current is called the power factor , represented by the symbol cosΦ. Numerically, the power factor is the ratio of active power to apparent power, that is, cosΦ=P/S. [1]

Most of the power loads in the power grid, such as motors, transformers, fluorescent lamps and arc furnaces, are inductive loads. These inductive devices need to absorb not only active power from the power system during operation, but also reactive power. Therefore, after installing parallel capacitor reactive power compensation equipment in the power grid, it will be able to provide compensation for the reactive power consumed by the inductive load, reducing the reactive power provided by the power supply side of the power grid to the inductive load and transmitted by the line. Reducing the flow of reactive power in the power grid can reduce the power loss caused by the transmission of reactive power by transformers and buses in the transmission and distribution lines. This measure is called power factor compensation.

Since the fundamental reason for improving power factor is the reduction of reactive power, power factor compensation is usually called reactive power compensation.

In large systems, reactive power compensation is also used to adjust the voltage of the power grid and improve the stability of the power grid.

2 Theoretical analysis

The size of the power factor is related to the load properties of the circuit. For example, the power factor of a resistive load such as an incandescent bulb or a resistance furnace is 1. Generally, the power factor of a circuit with an inductive or capacitive load is less than 1. The power factor is an important technical data of the power system. The power factor is a coefficient that measures the efficiency of electrical equipment. A low power factor indicates that the reactive power used by the circuit for alternating magnetic field conversion is large, thereby reducing the utilization rate of the equipment and increasing the line power supply loss. Therefore, the power supply department has certain standard requirements for the power factor of power users.

(1) The most basic analysis: Take the equipment as an example. For example: the equipment power is 100 units, that is, 100 units of power are delivered to the equipment. However, due to the inherent reactive power loss in most electrical systems, only 70 units of power can be used. Unfortunately, although only 70 units are used, 100 units of fees must be paid. In this example, the power factor is 0.7 (if the power factor of most equipment is less than 0.9, a fine will be imposed). This reactive power loss mainly exists in motor equipment (such as blowers, pumps, compressors, etc.), also known as inductive loads. Power factor is a measure of motor efficiency.

(2) Basic analysis: Every motor system consumes two types of power, namely, real useful power (called kilowatts) and reactive useless power. The power factor is the ratio between useful power and total power. The higher the power factor, the higher the ratio between useful power and total power, and the more efficient the system operation.

(3) Advanced analysis: In an inductive load circuit, the current waveform peak occurs after the voltage waveform peak. The separation of the two waveform peaks can be expressed by the power factor. The lower the power factor, the greater the separation between the two waveform peaks. Paulkin can bring the two peaks closer together, thereby improving system operating efficiency.

3 Methods

The main purpose of reactive power compensation is to improve the power factor of the compensation system. Because the electricity generated by the power supply bureau is calculated in KVA or MVA, but the charges are in KW, that is, the actual useful work done. There is a difference in reactive power between the two, which is generally the reactive power in KVAR. Most of the reactive work is inductive, which is generally the so-called electric motors, transformers, fluorescent lamps... Almost all reactive work is inductive, and capacitive is very rare. It is because of the existence of this inductance that a KVAR value is created in the system. The relationship between the three is a trigonometric function.

KVA square = KW square + KVAR square

Simply put, in the above formula, if today's KVAR value is zero, KVA will be equal to KW, then 1KVA of electricity generated by the power supply bureau is equal to 1KW of user consumption, and the cost-effectiveness is the highest at this time, so the power factor is a coefficient that the power supply bureau cares about very much. If the user does not achieve the ideal power factor, it is relatively consuming the resources of the power supply bureau, so this is why the power factor is a regulatory restriction. At present, the power factor regulations in China must be between 0.9 and 1 of the inductance, and penalties will be imposed if it is lower than 0.9. This is why we must control the power factor within a very precise range.

In order to improve their cost-effectiveness, the power supply bureau requires users to improve the power factor. What are the benefits of improving the power factor for our users?

① By improving the power factor, the total current in the line and the capacity of electrical components in the power supply system, such as transformers, electrical equipment, and wires, are reduced. This not only reduces investment costs, but also reduces the loss of electrical energy itself.

② By ensuring a good power factor value, the voltage loss in the power supply system can be reduced, the load voltage can be made more stable, and the quality of electric energy can be improved.

③ It can increase the system margin and tap the potential of power generation and supply equipment. If the power factor of the system is low, then installing capacitors can improve the power factor and increase the capacity of the load while keeping the capacity of the existing equipment unchanged.

For example, when the power factor of a 1000KVA transformer is increased from 0.8 to 0.98:

Before compensation: 1000×0.8=800KW After compensation: 1000×0.98=980KW

The same 1000KVA transformer can bear an additional 180KW load after the power factor changes.

④ Reduce the user's electricity bill expenditure; through the reduction of losses in the above-mentioned components and the improvement of power factor, electricity bill discounts are achieved.

In addition, some power electronic equipment such as rectifiers, inverters, switching power supplies, etc.; saturable equipment such as transformers, motors, generators, etc.; arc equipment and electric light source equipment such as arc furnaces, fluorescent lamps, etc., are the main sources of harmonics and will generate a large amount of harmonics during operation. Harmonics have varying degrees of harm to all electrical equipment connected to the power grid, such as engines, transformers, motors, capacitors, etc., mainly manifested in the generation of additional harmonic losses, causing equipment overload and overheating, and harmonic overvoltage accelerating the insulation aging of equipment.

The capacitors connected in parallel to the line for reactive power compensation will amplify the harmonics, making the system voltage and current distortion more serious. In addition, the harmonic current superimposed on the fundamental current of the capacitor will increase the effective value of the capacitor current, causing the temperature to rise and shortening the service life of the capacitor.

Harmonic current increases the copper loss of the transformer, causing local overheating, vibration, increased noise, additional heating of the windings, etc.

Harmonic pollution will also increase the loss of transmission lines such as cables. Harmonic pollution also affects the quality of communication. When the current harmonic component is high, it may cause the overvoltage protection and overcurrent protection of the relay protection to malfunction.

Therefore, if the system measures that the harmonic content is too high, in addition to connecting an appropriate detuned reactor in series with the capacitor, a harmonic improvement device should be installed based on the load characteristics.

4 Significance

Power factor is one of the important technical data of AC circuit and has great significance.

The power factor is of great significance to the utilization rate of electrical equipment and the analysis and research of power consumption.

The so-called power factor refers to the cosine of the phase difference between the voltage U at both ends of any two-terminal network (a circuit with two contacts to the outside world) and the current I therein. The power consumed in a two-terminal network refers to the average power, also known as active power, which is equal to voltage × current × cosine of the phase difference between voltage and current.

From this, we can see that the power P consumed in the circuit depends not only on the voltage V and the current I, but also on the power factor. The size of the power factor depends on the nature of the load in the circuit. For resistive loads, the phase difference between the voltage and current is 0, so the power factor of the circuit is the largest (); while for pure inductive circuits, the phase difference between the voltage and current is π/2, and the voltage leads the current; in pure capacitive circuits, the phase difference between the voltage and current is -(π/2), that is, the current leads the voltage. In the latter two circuits, the power factor is 0. For circuits with general loads, the power factor is between 0 and 1.

Generally speaking, in a two-terminal network, improving the power factor of electrical appliances has two meanings: one is to reduce the power loss on the transmission line; the other is to give full play to the potential of power equipment (such as generators, transformers, etc.). Because electrical appliances always work under a certain voltage U and a certain active power P, according to the formula P = UIcosΦ

It can be seen that if the power factor is too low, a larger current must be used to ensure the normal operation of the electrical appliances. At the same time, the transmission current on the transmission line increases, which leads to an increase in Joule heat loss on the line. In addition, the voltage drop on the resistance of the transmission line and the internal group of the power supply are proportional to the current in the electrical appliance. Increasing the current will inevitably increase the voltage loss in the transmission line and the power supply. Therefore, improving the power factor of the electrical appliance can reduce the transmission current, thereby reducing the power loss on the transmission line.

Improving the power factor can give full play to the potential of power equipment, which is not difficult to understand. Because any power equipment always works within a certain rated voltage and rated current limit. If the working voltage exceeds the rated value, it will threaten the insulation performance of the equipment; if the working current exceeds the rated value, the internal temperature of the equipment will rise too high, thereby reducing the service life of the equipment. For power equipment, the product of the voltage and current ratings is called the rated apparent power S of this equipment.

S amount = U amount I amount

It is also called the capacity of the equipment. For a generator, this capacity is the maximum power that the generator can output. It indicates the power generation potential of the generator. As for how much power the generator actually outputs, it is related to the power factor of the electrical appliances. The power consumed by the electrical appliances is

A high power factor indicates that the proportion of active power to rated apparent power is large, and the electric energy output by the generator is fully utilized. For example, if the capacity of the generator is 15,000 kVA, when the power factor of the power system is increased from 0.6 to 0.8, the actual power generation capacity of the generator can be increased by 3,000 kW. Isn't this the potential of the generator? The utilization of the equipment is also more reasonable. From this perspective, the power factor can be expressed as the ratio of active power to machine power, that is,

How to improve the power factor is an important and practical issue that needs to be seriously considered in the power industry. In the circuits with inductive loads that we usually encounter, such as fluorescent lamp circuits, suitable capacitors are generally connected in parallel to improve the power factor of the entire circuit.

In small systems, the three-phase unbalanced current can also be adjusted through appropriate reactive power compensation methods. According to Wang's theorem: the inductor or capacitor connected between phases can transfer active current between phases. Therefore, for a system with unbalanced three-phase current, as long as capacitors of different capacities are properly connected between each phase and between each phase and the neutral line, not only can the power factor of each phase be compensated to 1, but also the active current of each phase can be balanced.