Abstract:This article mainly introduces the role of capacitor reactive power compensation, and briefly analyzes the types of reactive power compensation for low-voltage parallel capacitors, the determination of capacitor compensation capacity, and the selection of compensation control.

KeywordsLow voltage parallel capacitors; Reactive power compensation; Technology; economy

0 Introduction

Reactive power compensation is currently the main factor in maintaining the balance of the power system. In the transmission and distribution system, reactive power compensation is a condition for the normal operation of inductive equipment and a common means to ensure voltage quality and the normal operation of power supply operations. By optimizing the reactive power balance of the power system and improving the power factor of its load, the active power loss in transformers and lines, as well as the energy loss in other parts, can be effectively reduced, improving the quality of electricity and ensuring the normal operation of the power system and the production development of various industries.

The role of reactive power compensation

1.1 Beneficial for improving the utilization rate of transformer and distribution equipment and reducing investment expenses

Reactive power compensation is applied to loads with low power factor, connected to parallel capacitors. When the reactive current is compensated, the load current is correspondingly reduced.

In power distribution equipment, the reduced capacity (kVA) caused by the increase of power factor can be expressed by formula (1):

In the formula, cos φ 1 is the power factor of the load before compensation; Cos φ 1 is the power factor of the compensated load; △ S is the reduced device capacity; P is the active power of the load.

Before compensation, the power factor of the 1000kW load capacity is 0.7. Using formula (1), it can be calculated that when the compensation power factor value reaches 0.95, the corresponding reduction in the capacity of the load transmission and distribution equipment reaches 376kVA. That is to say, in new construction projects, the capacity of the transformer and distribution equipment can be reduced by 376 kVA compared to the original rated capacity, thereby reducing basic electricity expenses and overall investment costs of the new project, resulting in significant economic benefits.

1.2 Reduce power loss in the power grid

When the power factor of the load decreases from 1 to cos φ, the percentage increase in power loss in the power grid is approximately:

1.3 Beneficial for reducing line voltage drop

The current transmission in the line decreases with the increase of power factor, thereby reducing the loss of line voltage in the system and improving the power quality at the end of the line.

1.4 Improve power factor and reduce power consumption costs

In 1983, the Ministry of Water Resources and Electric Power and the State Administration for Market Regulation issued the "Electricity Fee Method for Power Factor Adjustment", which clearly stipulated the standard values of three power factors. Based on this, the cost of electricity was correspondingly reduced.

Classification of reactive power compensation for low-voltage parallel capacitors

The reactive power compensation of low-voltage parallel capacitors can be divided into three methods: centralized compensation, individual compensation, and group compensation: (1) centralized compensation. Connect the phase-shifting transformer directly to the busbar of the step-down substation or local substation. This compensation method has a high utilization rate of capacitors, and the reactive load of the main transformers in the power system and substations is also reduced, as well as the reactive load of the power supply lines. However, for low-voltage power grids, centralized compensation methods cannot reduce their reactive load. (2) Individual compensation. Individual compensation methods are common in voltage networks, where capacitors are directly connected to electrical equipment. Some compensation methods provide better reactive power compensation. Through individual compensation, the reactive current of high-voltage lines and transformers can be reduced, and the reactive current of low-voltage main lines and their branch lines can also be reduced. This is a significant improvement in active loss control of lines and transformers. However, when using individual compensation methods, the disadvantage is that the investment for capacitors is too large and the utilization rate of capacitors is not high. Therefore, some compensation methods have certain limitations. They often have high practical value for large capacity electrical equipment or loads that require significant reactive power compensation, and are also suitable for electrical equipment on long-distance power supply lines. (3) Group compensation. The group compensation method is to directly connect the phase-shifting capacitor to the busbar in the workshop distribution room. Compared with individual compensation methods, group compensation has a higher utilization rate for capacitors. For high-voltage power supply lines and transformers, group compensation can effectively reduce their reactive load, and can implement input or disconnection of capacitor banks based on load changes. However, the installation of group compensation is relatively cumbersome and cannot reduce the reactive current in branch lines.

3 Technical issues to be noted in reactive power compensation

3.1 Preventing Surge Current

During the process of inserting capacitors, there is often a significant surge current. The calculation formula (2) for capacitor inrush current in IEC publication 831 capacitor section is as follows:

In the formula, Is is the surge current (A) when the capacitor is put into operation; In is the rated current of the capacitor (A); S is the short-circuit power (MVA) at the location where the capacitor is installed; Q is the capacitance of the capacitor (Mvar).

In response to this situation, the following three methods can be adopted to control the low-voltage capacitor circuit: (1) connecting the reactor in series; (2) Improve the capacity of switching capacitors; (3) Use contactors specifically designed for capacitor switching.

3.2 Preventing the Effects of System Harmonic Generation

As an LC circuit, the capacitor circuit is prone to resonance with certain specific harmonics, which amplifies the harmonics and increases the current and voltage accordingly. To solve this phenomenon, it is possible to consider connecting reactors with a certain inductance value in series to avoid resonance. Here we take the percentage of reactors as the letter K. In the power grid, when the 5th harmonic is high and the 3rd harmonic is slightly low, the K value should be taken as 4.5%; If the 3rd harmonic is too high, the K value is taken as 12%; When the harmonic is not high, a K value of 0.5% is optimal.

3.3 Preventing the occurrence of self excitation situations

In the process of using capacitors for on-site compensation of reactive power in electric motors, the capacitors are directly connected in parallel with the motor. When the power supply is cut off, due to inertia, the motor will continue to operate for a period of time, and the discharge current of the capacitors becomes the excitation current. When the capacity of the capacitor used for reactive power compensation is too large, the magnetic field of the motor will be self excited, thereby generating voltage, that is, the motor is still in the working state of generating electricity. Therefore, in the process of reactive power compensation, the compensation capacity of the capacitor should be lower than the no-load capacity of the motor, usually taken as 0.9 times the no-load capacity. The calculation formula (3) is as follows:

In the formula, Qc is the compensation capacitor capacity; U is the system voltage; Io is the engine no-load current.

4. Selection of capacitor compensation control and determination of compensation capacity

In the power system, some end substations have excessively long transmission lines, high loads, and significantly low voltages, which have a significant impact on the normal operation of electrical equipment. In order to improve the quality of power supply, the power department usually adds capacitor banks in certain lines or substations to increase the terminal voltage and achieve the goal of balanced voltage.

4.1 Switching method of capacitor bank

There are two ways to switch capacitors: manual switching and automatic switching. Manual switching is suitable for compensating low-voltage reactive power and long-term stable high-voltage capacitor banks; Automatic switching is suitable for avoiding excessive compensation, high voltage under light load, and equipment damage. If the high voltage compensation effect is the same as the low voltage compensation effect, priority should be given to the low voltage automatic compensation device.

4.2 Determine the compensation capacity of capacitors

Before determining the compensation capacity of the capacitor, load calculation should be carried out to determine the reactive power Q and active power P. The power factor to be compensated is cos φ 2, and the natural power factor before compensation is cos φ 1. Therefore, the compensation capacitor capacity is:

When determining the reactive power compensation capacity of capacitors, the following three points should be ensured: (1) Under light load conditions, excessive compensation should be avoided to prevent unnecessary work from increasing power loss and causing resource waste. (2) The change in power factor is inversely proportional to the reduction in compensating capacity loss per kilowatt. If the power factor is high, the effect of reducing losses will be less significant. We generally believe that the power factor value for reasonable compensation is 0.95. (3) The excitation current is the main parameter for selecting the capacity of the on-site compensation capacitor. The necessary condition for selecting the capacitor capacity is whether the capacitor causes self excitation phenomenon. Here, formula (3) can be used for calculation.

Introduction to 5 Ankerui AZC/AZCL Intelligent Power Capacitor Products

5.1 Overview

The AZC series intelligent capacitors are a new generation of reactive power compensation equipment for 0.4KV, 50Hz low-voltage distribution, which can save energy, reduce line loss, improve power factor and power quality. It consists of an intelligent measurement and control unit, a thyristor composite switch circuit, a line protection unit, and two low-voltage power capacitors that are jointly or separately compensated. Replace the conventional automatic reactive power compensation device composed of loose components such as fuses, composite switches or mechanical contactors, thermal relays, low-voltage power capacitors, indicator lights, etc. connected by wires inside and on the cabinet surface. By changing the bulky and cumbersome structural mode of traditional reactive power compensation devices, the new generation of low-voltage reactive power compensation equipment has the characteristics of better compensation effect, smaller size, lower power consumption, cheaper price, more cost savings, more flexible use, more convenient maintenance, longer service life, and higher reliability, adapting to the higher requirements of modern power grids for reactive power compensation.

The AZC series intelligent capacitors adopt a customized segmented LCD display, which can display real-time three-phase bus voltage, three-phase bus current, three-phase power factor, frequency, capacitor circuit number and switching status, active power, reactive power, total harmonic voltage distortion rate, and capacitor temperature.

On the basis of AZC, the AZCL series intelligent integrated power capacitor compensation device is connected in series with a suitable reactance rate (7% suitable for harmonic environments above 5/7, 14&suitable for harmonic environments above 3/5/7), which can effectively resist harmonics, avoid resonance amplification of harmonics, and protect the life of the capacitor cabinet itself.

5.2 Application scenarios

Hospitals, commercial centers, data centers, inverter industry, photovoltaic industry, ports/oil fields, chemical/smelting industry ..

5.3 Selection of Ankerui AZC/AZCL series intelligent capacitors

AZC intelligent power capacitor compensation device

AZCL intelligent integrated power capacitor compensation device

6 Conclusion

Reactive power compensation is an energy-saving measure that can effectively improve power factor, with the characteristics of quick effectiveness and low investment. The working principle of parallel compensation capacitors is simple, easy to operate, and has high economic benefits. They can also be grouped and switched to ensure the rationality of voltage qualification rate and power factor. At present, the average factor of distribution and rural power grids in a large part of China is too low. It is believed that by using compensation capacitors for reasonable compensation, the power supply quality can be effectively improved and more significant economic benefits can be achieved.