Solutions for reactive power compensation and voltage regulation of power systems

In the power system, the power quality is an important index to evaluate the performance of the power system, and the voltage is an important index to measure the power quality. Therefore, the stability of the voltage is particularly important for the performance of the power system.

1.introduction

In the power system, the power quality is an important index to evaluate the performance of the power system, and the voltage is an important index to measure the power quality. Therefore, the stability of the voltage is particularly important for the performance of the power system. Whether the voltage is stable or not mainly depends on the balance of reactive power in the system. If the reactive power demand of the electrical load fluctuates greatly, and the reactive power source and distribution of the grid cannot be regulated in time, the line voltage will exceed the allowable limit; In addition, for the load side, the power system is mostly composed of transmission and distribution lines, transformers, generators, etc., and its internal impedance is mainly inductive, which makes the change of the load reactive power extremely adverse to the stability of the grid voltage.

Reactive power compensation is a major subject involving power electronics technology, power system, electrical automation technology, theoretical electrical engineering and other fields. As the application of power Electronic technology devices is increasingly popular in various fields of production and life, the problem of reactive power compensation has attracted more and more attention. According to relevant scientific statistics, if the reactive power compensation devices are installed nationwide by optimizing the configuration calculation, it is estimated that about 300 million kilowatt-hours of electricity can be saved every year under the condition that the total investment remains unchanged. Therefore, the reactive power compensation and voltage adjustment of the power system are important measures to ensure the safe, high-quality and economical operation of the power grid. At present, due to the rapid progress of power electronics technology, breakthroughs have been made in reactive power compensation.

2.Development history, current situation and development prospect of continuous reactive power compensation device

The reactive power compensators used in engineering mainly include rotating reactive power compensators and static reactive power compensators, and their specific classifications are shown in Figure 1.

Solutions for reactive power compensation and voltage regulation of power systems

Solutions for reactive power compensation and voltage regulation of power systems

2.1 Development history of continuous reactive power compensation device

The rotary reactive power compensator is represented by the synchronous camera. The synchronous camera is actually a synchronous motor running in the state of over-excitation or under-excitation. It can emit both capacitive reactive power and inductive reactive power. The camera can dynamically compensate for changing reactive power. Due to its many shortcomings (see Table 1), it has been gradually replaced by static var compensators since the 1970s.

Static reactive power compensation technology has experienced 3 generations of development as shown in Figure 1:

The first generation belongs to the slow reactive power compensation device, which was applied earlier in the power system and is still in use at present;

The second generation is a passive, fast dynamic reactive power compensation device, which appeared in the 1970s and is widely used in foreign countries. At present, there are certain applications in my country. It is mainly used in power distribution systems and rarely used in transmission networks. SVC can be regarded as a It is a reactive element whose susceptance value can be adjusted. It relies on the switch of power electronic devices to realize reactive power adjustment. When SVC is used as system compensation, it can continuously adjust and exchange reactive power with the system;

The third generation is a fast dynamic reactive power compensation device. It has been studied abroad since the 1980s and has been widely used in the late 1990s. With the emergence of high-power fully-controlled power electronic devices GTO, IGBT, and IGCT, especially the introduction of phase control technology, pulse width modulation technology (PWM), and four-quadrant converter technology, power electronic inverter technology has developed rapidly , the reactive power compensation technology based on this has also developed rapidly. As the most important member of the FACTS family, the static synchronous compensator has been successfully applied in the United States, Germany, Japan and China. In addition, compared with SVC, SVG also has the advantages of faster adjustment speed, wider adjustment range, stronger reactive power adjustment ability under undervoltage conditions, etc. At the same time, the harmonic content and occupied space are greatly reduced. The advantages and disadvantages of the 3rd generation reactive power compensation device are shown in Table 1.

Solutions for reactive power compensation and voltage regulation of power systems

2.2 Current status of dynamic reactive power compensation in domestic and foreign power grids

The most widely used compensation device in my country’s power grid is the parallel capacitor bank with mechanical switching. In order to meet the voltage regulation requirements, a large number of parallel capacitor banks are installed in the low-voltage power supply network, and a small number of parallel capacitor banks are installed in the medium-voltage distribution network. The G(X) JF1 capacitor tracking switching cabinet (box) designed and produced by Mudanjiang Kehai Electric Equipment Co., Ltd. adopts the KH-ZK capacitor intelligent switching switch; G(X) JK1 type contactor capacitor tracking switching cabinet ( In the process of switching capacitors, the inrush current is small, the service life of the whole machine is long, the maintenance amount is small, the reactive power compensation response is fast, frequent switching, and multi-level compensation is in place at one time. Including G(X)D1 type capacitor switching products are the progress and development of this compensation device.

At present, a total of 6 sets of large-capacity SVCs in 5 places in my country’s power transmission system have been put into use, and they have been installed in 500kV substations in Jiangmen, Guangdong, Yuntian, Hunan, Fenghuangshan, Hubei (2 sets), Xiaoliu, Henan, and Shaling, Liaoning. . Most of these SVCs are imported, and 3 sets of them are ABB products.

SVC is widely used in large industrial and mining enterprises, especially in iron and steel enterprises. Wuhan Iron and Steel Company, Baotou Iron and Steel Company, Baoshan Iron and Steel Company, Jinan Iron and Steel Company, Zhangjiagang Sha Iron and Steel Company, Tianjin Steel Pipe Company, etc. are equipped with this compensation. For example, a set of SVCs with a capacity of 25Mvar designed and manufactured by Siemens was installed on the 35kV busbar in the second-phase project of Jinan Iron and Steel Company’s medium and heavy plate plant.

From an international perspective, both SVC and SVG have been widely used. SVC appeared early and has been used for a long time. ABB alone has more than 370 SVCs in operation around the world. The total installed SVC capacity of ABB and Siemens is about 90,000 Mvar (including decommissioned devices). . SVG devices were mainly demonstration projects in the 20th century. From the end of the 1990s to the beginning of this century, SVG has been widely used in Japan, Europe and the United States, especially in metallurgy, railways and other occasions that require fast dynamic reactive power compensation. In March 1999, my country’s first industrialized STATCON was successfully connected to the grid at Chaoyang Substation, Luoyang City, Henan Province, indicating that my country has mastered the design and manufacturing technology of high-voltage and large-capacity FACTS equipment.

2.3 Development prospect of static reactive power compensation device

With the rapid development of power electronic technology and the cross influence of various disciplines, the development trend of static reactive power compensation mainly includes the following points:

(1) In the transformation of the urban network, the operating unit often needs to install a reactive power compensation controller and a power distribution comprehensive tester on the low-voltage side of the distribution transformer. problem.

(2) Quickly and accurately detect the reactive power parameters of the system, improve the dynamic response time, and quickly switch capacitors to meet the harsh working conditions (such as large impact loads or occasions with frequent load fluctuations). With the development of computer digital control technology and intelligent control theory, some advanced control methods, such as fuzzy control and microcomputer control, can be introduced into reactive power compensation.

(3) At present, reactive power compensation technology is mainly used in low-voltage systems. Due to the limitation of the thyristor level, the high-voltage system is connected through a transformer to reduce the voltage, such as for electrified railway traction substations. It is of great significance to develop a high-voltage dynamic reactive power compensation device. The key is to solve the problems of the withstand voltage of the thyristor and diode of the compensation device, that is, the series connection of multiple thyristor elements, the voltage equalization, and the synchronization of trigger control.

(4) From a single reactive power compensation to the function of filtering and suppressing harmonics. With the development of power electronic technology and the popularization and application of power electronic products, the power supply system or load contains a large number of harmonics. The research and development of a thyristor switching filter with the dual advantages of reactive power compensation and power filter will become an effective means to improve system power factor, suppress harmonics, stabilize system voltage and improve power quality. Active power filter (APF) and unified power flow controller (UPFC) are devices that can compensate both harmonics and reactive power. Although there are high-order harmonics in the current, a single unit has low capacity and relatively low cost. Advanced problems, but its development prospects are still promising.

(5) An AC voltage generated by a thyristor converter is connected in series and superimposed on the phase voltage of the transmission line, so that its amplitude and phase angle can be continuously changed, so as to achieve accurate adjustment of the active and reactive power of the line, and Improves conveying capacity and dampens system oscillations. At present, the comprehensive power flow controller (UPFC) is developing rapidly. The United States Westinghouse Electric Company has developed a series power flow controller (SPFC), whose cost is significantly lower than that of UPFC, and its function is comparable to and better than SVG.

3.The fundamental purpose of reactive power compensation

In engineering applications, in order to improve the power factor of the grid and stabilize the grid voltage, reactive power compensation devices are usually introduced. The reactive power compensation of the system can change the power factor, reduce the system loss, and greatly improve the operating efficiency of the grid power. In addition, reactive power compensation can also reduce voltage flicker, reduce overvoltage, and improve the static and dynamic stability of the power system, which is of great significance in terms of its economic value.

3.1 Reduce line voltage drop and improve voltage stability

The introduction of the reactive power compensation device balances the reactive power in the system and improves the stability of the voltage. Since the transmission current of the line is small, the line voltage loss of the system is also reduced accordingly, which is conducive to the stability of the system voltage (prevent the voltage from rising too high due to the leading current during light load), and is conducive to the starting of large motor devices.

3.2 Reduce system energy consumption and improve resource utilization

The improvement of the power factor can reduce the line loss and the copper loss of the transformer to a certain extent.

Solutions for reactive power compensation and voltage regulation of power systems

It can be seen from equation (5) that when the power factor is increased from 0.8 to 0.9, the copper consumption is equivalent to 79% of the original.

3.3 Improve the power factor and reduce the corresponding electricity bill

According to the “Measures for Adjusting Electricity Charges for Power Factor” promulgated by the Ministry of Water and Electricity and the Price Bureau, three standard values ​​of power factor are stipulated, and the electric charge is reduced accordingly:

(1) The power consumption unit of high-voltage power supply has a power factor of 0.9 or more.

(2) The power consumption unit of low-voltage power supply has a power factor of more than 0.85.

(3) For agricultural users with low-voltage power supply, the power factor is above 0.8.
According to the “Measures”, the power factor after compensation should not exceed 0.95, 0.94, and 0.92 respectively, because beyond this value, the electricity bill will not decrease. On the contrary, it is uneconomical to increase the equipment for the first time.

3.4 Increase power supply and reduce investment in electricity consumption

For the original power supply equipment, under the same active power, the power factor increases and the load current decreases. Therefore, the transformers, switches, wires and other power distribution equipment through which the power is transmitted to the load have increased power reserves, giving full play to the power of the equipment. potential. For new projects, the transformer capacity is reduced, the investment cost is reduced, and the basic electricity cost after operation is also reduced.

4.General method of reactive power compensation

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

4.1 Low voltage individual compensation

Low-voltage individual compensation is to connect single or multiple low-voltage capacitor banks in parallel with the electrical equipment in a decentralized manner according to the reactive power requirements of individual electrical equipment.It is related to electrical equipment
The equipment shares a set of circuit breakers, and the control and protection devices are switched on and off with the motor at the same time. The random compensation is suitable for compensating the reactive power consumption of individual large-capacity and continuous operation (such as large and medium-sized asynchronous motors), mainly to supplement the excitation reactive power. The advantages of low-voltage individual compensation: according to the operation or shutdown of the electrical equipment, the reactive power compensation is input or withdrawn, and the reactive power will not be reversed. It has the advantages of less investment, small size, easy installation, convenient configuration, flexible operation, simple maintenance and low accident rate.

4.2 Low-voltage centralized compensation

Low-voltage centralized compensation refers to connecting low-voltage capacitors to the low-voltage bus side of distribution transformers through low-voltage switches, and using reactive power compensation switching devices as control and protection devices to directly control the switching of capacitors according to the reactive load on the low-voltage bus. The switching of capacitors is carried out in the whole group, and smooth adjustment cannot be achieved. The advantages of low-voltage centralized compensation: easy configuration, simple maintenance, and quick balance, thereby improving the utilization rate of distribution transformers, reducing network losses, and having high economic value. It is one of the commonly used methods for reactive power compensation.

4.3 High-voltage centralized compensation

High-voltage centralized compensation refers to the compensation method in which the parallel capacitor bank is directly connected to the 6-10kV high-voltage bus of the substation. It is suitable for users who are far away from the substation or at the end of the power supply line. When the user has a certain high-voltage load, it can reduce the consumption of reactive power in the power system and play a certain compensation role; the compensation device automatically switches according to the size of the load. , so as to reasonably improve the power factor of the user and avoid the increase of the electricity bill caused by the decrease of the power factor. Advantages of high-voltage centralized compensation: flexible configuration, simple maintenance, and high compensation benefits.

5.Basic Principles of Reactive Power Compensation

In the power system, the dynamic compensation of reactive power can realize the following functions, such as:

① Power factor correction for dynamic reactive load; ② Adjust voltage; ③ Improve dynamic and static stability of power system; ④ Reduce overvoltage; ⑤ Reduce voltage flicker; ⑥ Damping power oscillation; ⑦ Damping subsynchronous oscillation; and current imbalance.

Although the above eight functions are related to each other, the actual static reactive power compensation device often can only take one or several of them as the direct control target, and take into account other functions as much as possible. the focus. This article only introduces the basic functions of improving voltage regulation.
Compensation principle:

The circuit is divided into an equivalent circuit of three parts: system, load and compensator. The principle of dynamic compensation is shown in Figure 2.

Solutions for reactive power compensation and voltage regulation of power systems

The characteristic curve of the system can be approximated by the following formula:

Solutions for reactive power compensation and voltage regulation of power systems

Solutions for reactive power compensation and voltage regulation of power systems

It can be seen from equation (7) that the change of reactive power causes a proportional change of the system voltage, and the reactive power supplied by the system is the sum of the reactive power of the load and the compensator, namely:
Q=QL+QY
In the process of power engineering operation, when the load reactive power QL changes, the reactive power QY of the compensator can always compensate for the change of the load reactive power QL, so that △Q=Q1-Q2, and the reactive power Q remains unchanged. It can be seen from (7) that △U=0, the system voltage U remains constant, which is the basic principle of dynamic compensation for reactive power.

Figure 2b plots the dynamic reactive power compensation, the operating point of the system is kept at the point of Q=QA, that is, U=UA; when the operating point of the system is kept at the point C of Q=0, that is, U=U0 , the system realizes the complete compensation of power factor.

In the actual application process of the project, the load is generally included in the system, and the overall equivalence is carried out. The system and the load part in Figure 2a are equivalent to the part in the virtual box of the system in Figure 3a. Neglect resistance in internal impedance, reactance XS. Since the compensator has the function of maintaining a constant voltage at the continuous point, it can be regarded as a constant voltage source, and the voltage value is taken as the supply voltage Urd when the compensator is not connected at the connection point before the equivalent and the reactive power of the load remains unchanged.

When Xr is zero, the compensator has the horizontal ideal compensator characteristic shown in Fig. 3b, while the actual static var compensation device is not designed to have the horizontal voltage-current characteristic, but the slope shown in this figure Characteristics, the direction of the slope is that the voltage increases with the increase of the absorbed inductive current. This slope characteristic can also take into account the requirements of compensator capacity and voltage stability, can improve the current distribution between parallel compensators, and is conducive to reserve Reactive power reserve for stability requirements.
After the compensator is put in, the reactive power absorbed by the compensator is:

Because Xr is not zero in the actual compensator, the reactive power absorbed by the compensator is reduced relative to the ideal compensation situation. The connection point voltage does not keep the original normal value like the ideal compensator, but changes:

Therefore, in the reactive power characteristics with slope characteristics, the required capacity of the actual compensator is greatly reduced compared to that of the ideal compensator. When Xr=Xs, a compensator whose connection point voltage change is half of the system power supply voltage change can be maintained, and the required capacity is half that of an ideal compensator. This is the so-called compromise between compensator capacity and voltage adjustment.

6.Talking about the Function of Reactive Power Compensation Combined with Examples

Taking a large-scale project energy center as an example, the voltage level of the power supply of this project is 10kV, and the installed capacity of the equipment is about more than 21,000 kilowatts. After economic analysis, 10kV is used as the power supply voltage level of the high-voltage motor, which saves the investment, reduces the substation links, and also reduces the failure points. According to the load calculation, a total of six 10kV power supplies are used, which are directly matched to the high-voltage motors.
In this project, high-voltage motors are mainly used in central air-conditioning units, chilled water circulating pumps and cooling water circulating pumps. The single-unit capacity of these equipment is very large. The maximum single-unit centrifugal unit is 2810kW (5 units in total), the small unit is 870kW (4 units in total), the single unit for chilled water circulation pump is 560kW (9 units in total), and the single unit for cooling water circulation pump is 380kW (3 units in total). The natural power factor is around 0.8. If the compensation capacitor is concentrated in the 10kV power distribution room and the high-voltage reactive power automatic compensation is not used, the start and stop of such a large-capacity motor will make the power factor of the 10kV side unstable, which may cause overcompensation and cause the system voltage to rise. At the same time, the line from the power distribution room to the high-voltage motor in the refrigeration room is 50m short and 140m farthest, and the line loss is considerable. Considering the high requirements of high-voltage automatic compensation components, technology and price, high-voltage capacitors are used for local compensation. The motor switches at the same time. The high-voltage capacitor bank is placed near the motor. These motors adopt the auto-voltage step-down starting method. The high-voltage local compensation device is mainly composed of shunt capacitors. The fuse is used for protection. harmonic. Doing so not only improves the power factor of the motor and reduces the line loss, but also releases the system capacity, reduces the section of the feeder cable, and saves investment.

For low-voltage equipment, it is distributed by two 1000kVA and two 1600kVA transformers, and the low-voltage motor configuration is relatively scattered. Therefore, the capacitor bank is used for centralized automatic compensation on the low-voltage side of the transformer in the substation. Although the capacity of some low-voltage motors is not small, these devices are mainly used in boiler rooms and water supply and drainage equipment. The equipment in the boiler room is not as concentrated as that in the refrigeration room, the environment is poor, and the management is inconvenient. Therefore, in the low-voltage power distribution room, the power factor Size self-compensation is more appropriate.

7.Epilogue

With the development of power electronic technology and the continuous research and innovation of power electronic devices, reactive power compensation is also in continuous development. At present, the research results at home and abroad are developing rapidly, and many types of SVC and SVG compensation devices have appeared. For example: G(X)JF1 type, G(X)JK1 type (contactor type), G(X)D1 type capacitor tracking switching cabinet (box) and VQCL-D12/ J12 reactive power compensation controller; HVC high-voltage automatic reactive power and voltage comprehensive adjustment device and TSC series thyristor dynamic reactive power compensator developed by Harbin Gongda Weihan Technology Development Co., Ltd.; Shenzhen Saiyuan Electric Technology Co., Ltd. research and development Developed JKWA-15A and JKWA-12J low-voltage reactive power compensation controllers and so on. Although the compensation device with the optimal configuration taking into account a series of factors such as price, quality, volume and operation has not yet been launched, the development prospect is relatively broad.

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