Design and Application of Low-voltage Reactive Power Compensation Device

Xu Shuang

Ankerui Electric Co., Ltd.

Abstract: With the rapid development of China's economy, enterprises have put forward higher requirements for the demand for power supply and the quality of power equipment. The economics of power system operation are closely related to power quality and reactive power. The design of the power compensation device was described and studied, and the application of the reactive power compensation device in the field was introduced.

Keywords: power quality reactive power reactive power compensation design application

0 Preface

In recent years, with the rapid development of the economy, the concept of power quality management has received widespread attention in major companies. The concept of “safety of electricity, quality of electricity, and economy of electricity” is deeply rooted in the hearts of people, and reactive power compensation is used as power quality management. The important part of it has also become the direction of the majority of electric power research. The state has adopted an iron fist policy for enterprises that do not meet the target of reactive power, and the power rate has also become a burden on some power companies. In this paper, the design and selection of low-voltage reactive power compensation devices are described and analyzed. The operation and application of low-voltage reactive power devices in the field are illustrated by examples.

1 Active power and reactive power

In an AC circuit, there are two kinds of electric power supplied from a power source to a load; one is active power and the other is reactive power.

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, optical, thermal).

Reactive power is relatively abstract. It is used for the exchange of electric and magnetic fields within a circuit and is used to establish and maintain the electric power of magnetic fields in electrical equipment.

The relationship between apparent power, active power, and reactive power can be derived from the power triangle. The formula is as follows:

For example, the electric motor needs to establish and maintain a rotating magnetic field, which causes the rotor to rotate, thereby driving the mechanical movement. The rotor magnetic field of the motor is established by obtaining reactive power from the power supply. The transformer also requires reactive power to generate a magnetic field in the primary coil of the transformer and induce a voltage in the secondary coil.

Therefore, reactive power is not useless power and it occupies an important position in the power system. Without reactive power, the motor will not rotate, the transformer cannot be transformed, and the AC contactor will not pick up.

2 Introduction of Low Voltage Reactive Power Compensation Device

The low-voltage reactive power compensation device plays a role in the power supply system to increase the power factor of the power grid, reduce the loss of the power transformer and the transmission line, improve the power supply efficiency, and improve the power supply environment. Therefore, the reactive power compensation device is in an indispensable position in the power supply system. Reasonable choice of low-voltage reactive power compensation device can minimize the loss of the network, so that users improve the quality of electricity. Conversely, unreasonable choices or improper methods of use may also cause uncontrollable factors such as voltage fluctuations, flicker, and harmonic amplification in the power supply system.

3 Principle of reactive power compensation device

As can be seen from the power triangle, the user's transformer capacity (S) is fixed, if the user's equipment requires reactive power (Q) is too large, it will make the active power (P) output reduced. This will not only make the use of electrical equipment not work properly, it may also result in overload of the equipment resulting in damage to the equipment. In this case, reactive power compensation of the power system is required.

As shown in Fig. 1, before the reactive power compensation is performed, the reactive power and active power of the load all come from the power grid through the transformer, which leads to a reduction of the power factor of the power grid, a drop in the power quality, and a poor power consumption efficiency.

Fig. 2 compensates the reactive power of the equipment. At this time, the active power of the equipment comes from the power grid through the transformer, and the reactive power is taken from our reactive power compensation equipment. It is assumed that the reactive power of the device is extracted from the reactive power compensation device. At this time, from the grid side, the reactive power (Q) is 0, and the apparent power (S) at this time is equal to the active power (P). Is 1. At this time, the power supply efficiency of the power grid is the highest. Although this condition is ideal, in the actual operation, as long as the compensation is done properly, it is no problem that the monthly average power factor is above 0.95.

Figure 1 Reactive compensation before the legend Figure 2 Reactive compensation after the legend

4 The importance of reactive compensation

In general, the significance of using reactive power compensation devices to increase power factor is reflected in two aspects: First, it can reduce power loss on transmission lines; second, it can fully exploit the potential of power equipment (such as motors, transformers, etc.). Because electrical appliances always operate at a certain voltage and a certain active power, if the power factor is low, larger currents are needed to ensure normal operation of electrical appliances, and the transmission current becomes larger, resulting in increased line losses. In addition, any electrical equipment always works within a certain rated voltage and rated current. If it exceeds the rated voltage, it will threaten the insulation performance of the equipment; if the operating current exceeds the rated value, the internal temperature rise will be too high, thus reducing the equipment. The service life. For some power generation equipment, the increase of power factor can greatly increase the efficiency, for example: a generator capacity of 1500kW, when the power system power factor increases from 0.6 to 0.8, the actual power generation capacity can be increased to 3000kW.

5 Economic Benefits of Reactive Compensation

The economic benefits of reactive power compensation are mainly reflected in the rate of electricity, rate of electricity = active energy × electricity price × power rate adjustment factor. The power rate adjustment factor is obtained by looking up the table according to the power factor value. The benchmark tariff rate adjustment coefficients for different benchmarks are shown in Tables 1 and 2.

Table 1 uses 0.90 as a standard worth rate adjustment schedule

Table 2 uses 0.85 as a standard worth rate adjustment schedule

The state has also made provisions for the value of the standard, where users who require transformers with a capacity of 100 kVA and over 10 kV should perform a power factor assessment. The specific standards are as follows:

♦ For users below 100kVA 160kVA (excluding 160kVA), the power factor must reach 0.85;

♦ The user power factor above 160kVA should reach above 0.90.

♦ The user's power factor is greater than the standard to reduce its monthly electricity bill, and below the standard to increase its monthly electricity bill.

For example: a transformer user's transformer capacity of 160kVA, using low-voltage metering, the user's power consumption to change the user's power consumption is 25000kWH, reactive power is 27000kWH, assuming the power fee is 0.68 yuan / degree. According to research data, the transformer's active loss is 1036 kWH, reactive power loss is 3790 kWH, so the user consumes a total of 26,036 kW of active power, and the amount of reactive power consumed is 30,790 kWH. Through the power triangle or lookup table, the user's power factor is 0.65. . Through the look-up table, we can see that the user's power rate adjustment factor is +15%, a total of 25000×0.68 is actually charged for 17,000 yuan, and the required power rate is 25,000×0.68×0.15 is 2550 yuan, for a total of 19,550 yuan. .

Now through the reactive power compensation for the user to increase the power factor from 0.65 to 0.95, through the lookup table that the user power rate tariff adjustment factor is -0.75%, a total of 25000*0.68 to be charged for the actual consumption of electricity is 17,000 yuan, need to add The power rate for the power rate is 25000 x 0.68 x (-0.0075) -127.5 yuan, for a total of 1,6872.5 yuan.

From the examples, we can see that the use of reactive compensation can generate great economic benefits for the company. Generally, equipment costs can be recovered in 10 months to a year, creating revenue for the company.

6 Reactive power compensation selection

6.1 Capacity Selection

6.1 Capacity Selection

6.1.1 New Project

Because the on-site parameters cannot be determined for the newly-built project, in order to facilitate rapid selection, the industry is first classified, and the categories of each industry belong to the following table ( , Refers to the reactive compensation capacity, Refers to transformer capacity, Refers to the coefficient of reactive compensation industry):

Industry, Occasion

category

Industry, Occasion

category

Industry, Occasion

category

LED screen

II

Coal mine

III

Hospital office

I

Office building

I

Car charging station

III

Bank Securities

I

Glass factory

III

Car manufacturer

III

print

III

Supermarket

II

Commercial Plaza

II

cinema

II

Electroplating factory

III

Petrochemical

III

Entertainment venue

II

TV studio

II

food processing

III

papermaking

III

Electronic processing

III

Plastic processing

III

Exhibition Center

II

power plant

III

Tunnel lighting

II

Residential building

I

Textile

III

stadium

II

shop

I

Wind power plant

III

library

II

Radar base station

III

Steel

III

Sewage treatment

III

metallurgy

III

port

III

Stage Plaza

II

Medical Technology Building

II

Rail

III

Office building

I

tobacco

III

welding

III

SCHOOL

I

Machining

III

Traffic lighting

II

Pharmaceutical industry

III

I: coefficient About 0.3, suitable for offices, residential and other harmonics, reactive power demand is relatively small occasions;

II: coefficient About 0.4, suitable for commercial, rail transit and other harmonics, reactive power demand is relatively moderate occasions;

III: Coefficient About 0.55, suitable for heavy industries, manufacturing and other harmonics, relatively large demand for reactive power;

6.1.2 Retrofit Project

The reconstruction project generally needs to go to the site for field measurement. According to the actual situation on the site, the corresponding reactive power compensation scheme is configured. The points to be measured include: active power, reactive power, apparent power, actual power factor, target power factor, Load conditions, grid background, etc. The capacity of the reactive power compensation device can be calculated through actual measurement. For the above example, assume that the user transformer is fully loaded before compensation. The user's transformer capacity is 160kVA, the power factor is 0.65, and the target power factor is 0.95. as follows:

The compensation front view power is , power factor The active power before compensation is , Reactive power before compensation is .

The active power before compensation and after compensation does not change to , target power factor The compensated rear view power is The compensated reactive power is .

Therefore, the reactive power that needs to be compensated is .

6.2 Model Description

6.3 Technical parameters

Electrical parameters

Rated voltage

AC400V

Compensation capacity

60～630kvar

working frequency

50Hz

Response time

≤20ms

Overload capacity

1.1 times voltage overload, 1.3 times current overload

Target power factor

0.8 to 1.0 (can be set)

Control characteristics

Compensation method

Total compensation, sub-compensation, and mixed compensation

Control the number of ways

2 to 16 loops

control method

With manual, automatic switching modes

Switching algorithm

Cycle switching

Protective function

Overvoltage, undervoltage, phase loss, short circuit protection

Structure

Protection level

IP30

colour

RAL7035 (can be customized according to customer requirements)

Installation method

Indoor installation, fixing method and inlet method optional

Environmental conditions

Ambient temperature

-25°C to +50°C

Relative humidity

40°C, ≤50%, 20°C, ≤90%

altitude

2000 meters above sea level

7 Reactive power compensation modular solution

At present, the low-voltage reactive power compensation devices on the market basically take the form of a complete cabinet. Basically, they all belong to the “one-stroke deal”. Although they can meet the user's on-site compensation requirements, they are very important in terms of maintenance and capacity expansion after compensation. The inconvenience, it is very difficult to replace the entire cabinet when the user needs to carry out the transformation. Fig. 3 is a reactive compensation module that incorporates a set of reactive power compensation circuits including capacitors, reactances, thyristor switching switches, fuses, and secondary circuits into one module. In actual use, the modules and modules are They are independent of each other and do not interfere with each other. In the course of maintenance and use, the modules can be increased or decreased in capacity and maintained. Fig. 3 Reactive power compensation module

8 Application Cases of ANSVC in Jiangsu Hongliu Sheet Co., Ltd. Project

8.1 Project Introduction

Project Address: Jiangsu Hongliu Sheet Co., Ltd. The project address is located in Jiangyin City, Wuxi City, Jiangsu Province, which is beautiful and richly endowed. The project is located in the Yangtze River in the north, near the Taihu Lake in the south, Changshu, Zhangjiagang in the east and Changzhou in the west. It lies in the geometric center of the “Golden Triangle” in Suzhou, Wuxi and Changzhou, and the transportation hub is convenient. . The average elevation of 6 meters, the temperature conditions throughout the year better, four distinct seasons. Jiangsu Hongliu Sheet Co., Ltd. covers an area of ​​150,000 square meters and a building area of ​​120,000 square meters. The project has purchased 2 sets of reactive power compensation cabinets and 1 active filter cabinet from our company. For reactive power compensation, the 1# transformer outlet side was measured. The measurement results are shown in Figure 4 and the target power factor is 0.98. The on-site power distribution system harmonic sources are mainly frequency converters and switching power supplies. Harmonic frequencies are 3, 5, and 7 The second and eleventh times are typical 6n±1 characteristic harmonics, which are controlled by the ANAPF active filter to protect the capacitors. The measurement is schematically shown in Figure 5: (black circles are measurement points).

Fig. 4 Reactive power compensation field measurement results

Figure 5 on-site measurement

8.2 Reactive Power Compensation Capacity Calculation

The compensation front view power is , power factor The active power before compensation is , Reactive power before compensation is .

The active power before compensation and after compensation does not change to , target power factor The compensated rear view power is The compensated reactive power is .

Therefore, the reactive power that needs to be compensated is .

Considering the difference between the actual compensation capacity and the nominal compensation capacity, considering that only 800mm depth cabinets can be placed in the field, a 350kvar main cabinet and a 350kvar auxiliary cabinet are selected for a total of 700kvar.

8.3 Field Usage

Figure 5 site installation diagram

Figure 6 site completion

9 Conclusion

This paper describes the design and selection of low-voltage reactive power compensation devices from the aspects of basic knowledge, selection parameters, economic benefits, and example design of low-voltage reactive power compensation devices. A new idea of ​​modular low-voltage reactive power compensation device was proposed. The rational allocation of low-voltage reactive power compensation devices not only can reduce line losses, improve equipment life and operating efficiency, but also can reduce the rate of fines for enterprises, obtain electricity rewards, and create economic benefits intuitively.

【references】

GB15576-2008-T Low Voltage Complete Reactive Power Compensation Device Ankerui Electric Co., Ltd. Product Manual. 2013.01.

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