Current status and development trend of flexible AC transmission technology

Introduction

Flexible AC transmission (FACTS) technology is the product of the combination of modern power electronics technology and traditional power flow control. It uses high-reliability high-power thyristor components instead of mechanical high-voltage switches, so that the three main electrical parameters (voltage, line impedance and power angle) that affect the power flow distribution in the power system can be quickly adjusted according to the needs of the system, in order to achieve the reasonable distribution of transmission power, reasonable control of voltage, reduce power loss and power generation costs, and greatly improve system stability and reliability. This technology is an important means to achieve safe, economical and comprehensive control of power systems.
Once proposed, FACTS technology immediately received high attention from power workers in various countries. Some authoritative people at home and abroad have predicted that flexible AC transmission, integrated automation and EMS technology will be identified as "three supporting technologies for the new era of future power transmission systems". Developed countries such as the United States and Japan, as well as China, have invested a lot of manpower and material resources in development and research. Many devices have been put into actual operation and play an important role in the power system.

Controllers in FACTS

1. Static VAR Compensator (SVC)
Typical representatives of static VAR compensators are thyristor-switched capacitors (TSC) and thyristor-controlled reactors (TCR). In practical applications, TCR is used in conjunction with parallel capacitors. According to the different components of the switched capacitors, it can be divided into static VAR compensators using TCR and fixed capacitors, compensators using TCR and circuit breaker switched capacitors, and reactive compensators using TCR and TSC. The important feature of these combined SVCs is that it can continuously adjust the reactive power of the compensation device and perform dynamic compensation so that the voltage at the compensation point remains nearly unchanged. However, SVC can only compensate the voltage of the system, and its reactive output is proportional to the square of the voltage at the compensation point node. When the voltage decreases, its compensation effect will weaken. The main function of SVC is voltage control. After adopting appropriate control methods, SVC can also have the functions of damping system power oscillations and increasing stability. At present, SVC technology is relatively mature. Foreign countries have begun to apply SVC since the 1960s, and began to use it for voltage control of power transmission systems in the late 1970s. After decades of development, static VAR compensators are not only used for voltage control of power transmission systems, but also for compensation and control of distribution systems, and can also be used for reactive power compensation and voltage control of power terminal users.

2. Static synchronous compensator STATCOM
static synchronous compensator can also be called ASVG-active static VAR generator. Its basic principle is to connect the self-commutated bridge circuit directly or through a reactor to the power grid in parallel, and appropriately adjust the amplitude and phase of the output voltage on the AC side of the bridge circuit, so that the circuit can absorb or emit reactive current that meets the requirements and realize dynamic reactive power compensation. ASVG can be divided into voltage type and current type according to the two different energy storage elements of capacitors and inductors used on the DC side. It can exchange reactive power with the system by controlling its capacitive or inductive current, and can output rated reactive power under any system voltage. Compared with SVC, static synchronous compensator can maintain system voltage, improve system transient stability and suppress system oscillation in the case of system failure. In the past two decades, static synchronous compensator has been widely valued by experts and scholars at home and abroad. Japan developed the first 20Mvar forced self-commutation bridge ASVG in 1980, and put into operation a ±80Mvar ASVG in 1991, which was successfully operated on 154kV transmission line, while the United States put into operation a ±100Mvar ASVG in 1995. Tsinghua University and Henan Electric Power Bureau jointly developed a ±20Mvar static VAR compensator in my country, and put it into operation in Chaoyang Substation in Luoyang, Henan in 1999.

3. Parallel energy storage system
Parallel energy storage devices include battery energy storage systems (BESS) and superconducting magnetic energy storage devices (SMES). They are energy storage systems that use parallel voltage source converters. Their converters can quickly adjust to supply or absorb electrical energy to the AC system. Using SMES for frequency control of the two-machine system can effectively suppress the frequency offset between the two systems. SMES can also be combined with static phase shifters for load frequency control of interconnected systems. However, this type of superconducting energy storage device not only has high technical requirements, but also has relatively expensive investment costs under current conditions. There are still certain difficulties in large-scale system operation.

4. Thyristor-controlled series capacitor TCSC
The module of thyristor-controlled series capacitor is mainly composed of series capacitor and parallel circuit containing reactance and thyristor switch. Through thyristor control, the compensation capacity can be flexibly and continuously changed to achieve a fast response effect. TCSC has many advantages in improving the performance of power system. The use of TCSC in high-voltage transmission system can give full play to the potential of existing system, improve the power transmission limit, flexibly adjust the system flow, increase the system damping effect, and is an important measure to ensure the safe and stable operation of ultra-high voltage power grid.
Compared with other FACTS devices, TCSC has a relatively simple flow control function and has received attention and attention from large companies such as GE, ABB and Siemens. TCSC has been installed in three places in the United States and is operating well. Sweden, Brazil and other countries have also put TCSC into actual operation. China uses series compensation technology on the double-circuit transmission line from Yimin Power Plant to Fengtun Substation in Qiqihar area.

5. Static Synchronous Series Compensator SSSC
Static Synchronous Series Compensator is based on DC/AC inverter. Its basic principle is to inject a controllable voltage with a 90° difference from the voltage into the line to quickly control the effective impedance of the line and thus effectively control the system. Its role in the system is somewhat similar to TCSC, but its ability to control power flow is far greater than the TCSC controller with a single-direction line impedance reduction function, and its harmonic content is small.

6. Thyristor-controlled phase-shifting transformer TCPST
Thyristor-controlled phase-shifting transformer uses thyristor switches to control the phase shift angle to change the phase shift angle on both sides of the line to control the size or direction of the power flow. The development of phase shifters is relatively early. As early as the 1930s, the first phase shifter was put into operation in the United States. With the development of power electronics technology, power experts from various countries began to combine thyristors with phase shifters in the 1970s and began to study thyristor-controlled phase shifters TCPST. Research has shown that TCPST has the functions of improving the transmission flow of the interconnection line, suppressing small interference, improving system stability, damping power oscillation, bus voltage control, and regulating the interconnection line flow. The control speed of the thyristor-controlled phase shifter is fast, and the phase angle step can be very small, or even stepless regulation. However, the thyristor-controlled phase shifter has a disadvantage. It consumes reactive power itself and generally needs to be used in conjunction with a reactive compensation device during operation. In addition, the harmonic content is high, so it has a certain impact on the power quality.

7. Convertible Static Compensator CSC
Convertible Static Compensator is a new product of FACTS controller launched in the past two years. It actually uses the series-parallel compensator technology based on synchronous converters to achieve flexibility in structure, so that it can more flexibly respond to the changing requirements of the power system. CSC consists of two voltage source converters, a transformer connected in parallel with the transmission line, and two transformers connected in series. The different operating states of the compensator are realized by switching the switch. According to different control targets, CSC can provide four basic control modes: static synchronous VAR compensator, static synchronous series VAR compensator, unified power flow controller and line power flow controller.

8. Unified power flow controller UPFC
The concept of UPFC was first introduced by L. Gyugyi of Westinghouse Technology Center in the United States in 1992. Unified power flow controller is a new type of power flow control device developed on the basis of the original power flow control device. It consists of a parallel converter and a series converter coupled by a capacitor on the common side. Only by changing the control quantity can the functions of parallel compensation, series compensation or phase shifter be realized respectively, or the functions of the three can be combined. Through the design of different control strategies, UPFC can not only be used to control bus voltage, line power flow, improve system dynamic and transient stability, and suppress system oscillation, but also can quickly switch working state to adapt to the needs of emergency state of the system. It is considered to be the most representative, most powerful and most technically complex member of the FACTS family.

Conclusion

With the development of power electronics technology, various switching devices are applied to FACTS controllers as switches to extend the service life of the devices and improve economic benefits. The application of FACTS technology can make interconnected power grids serve as backup for each other, reduce the cold and hot backup capacity, flexibly control the system parameters that affect the power flow distribution, control the transmission power on the tie line, make the power flow to the designated line, improve the power flow distribution in the system, reduce the circulation in the large power grid, improve the dynamic stability of the system, and ensure that the power transmitted on the transmission line can be close to the thermal stability limit, but can meet the requirements of safe and economic operation.