Introduction Flexible AC Transmission (FACTS) technology is the result of a combination of modern power electronics technology and traditional power flow control. It replaces the mechanical high-voltage switch with high-reliability high-power thyristor components, 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 Reasonable distribution of transmission power, reasonable control of voltage, reduction of power loss and power generation cost, and greatly improve system stability and reliability. This technology is an important means to realize the safety, economy and comprehensive control of the power system.
Once FACTS technology was proposed, it was immediately highly valued by power workers in various countries. Some authoritative figures at home and abroad have already predicted flexible AC transmission, integrated automation and EMS technology to define it as “three supporting technologies for the new era of transmission systems in the futureâ€. Developed countries such as the United States and Japan, as well as China, have invested a lot of manpower and material resources to develop and research this. Many devices have been put into practical operation and play an important role in the power system.
Controller 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, the TCR is used in conjunction with a shunt capacitor. According to the components of the switching capacitor, it can be divided into a static var compensator for use with TCR and a fixed capacitor, and a compensator for use with a TCR and a circuit breaker switching capacitor. And the reactive power compensator used in conjunction with TCR and TSC. The important characteristic of these combined SVCs is that it can continuously adjust the reactive power of the compensation device and perform dynamic compensation to make the voltage of the compensation point stay close to the same, but the SVC can only compensate the voltage of the system, and its reactive output and compensation The square of the point node voltage is proportional, and its compensation is weakened when the voltage is lowered. The main function of SVC is voltage control. After adopting appropriate control mode, SVC can also have the function of damping system power oscillation and increasing stability. At present, SVC technology has been relatively mature. SVC has been used in foreign countries since the 1960s. It has been used for voltage control of transmission systems in the late 1970s. After decades of development, not only static var compensators but also transmission systems are used. The voltage control is also used for compensation and control of the power distribution system, and can also be used for reactive power compensation and voltage control of power terminal users.
2, static synchronous compensator STATCOM
The static sync compensator can also be called ASVG - an active static var generator. Its basic principle is to connect the self-commutated bridge circuit directly or through the reactor to the grid, and properly adjust the amplitude and phase of the output voltage of the AC side of the bridge circuit, so that the circuit can absorb or emit the reactive power that meets the requirements. Current, dynamic reactive compensation. ASVG can be divided into voltage type and current type according to two different energy storage components, namely capacitor and inductor used on the DC side. It can exchange reactive power with the system by controlling its capacitive or inductive current. It can output rated reactive power in any system voltage. Compared with SVC, the static synchronous compensator maintains in the case of system failure. The system voltage, the system transient stability and the suppression of system oscillation are more obvious; in the past twenty years, the static synchronous compensator has received widespread attention from experts and scholars at home and abroad. Japan developed the first 20Mvar forcing from 1980. The self-transforming bridge ASVG, in 1991, invested another ±80Mvar ASVG successfully on the 154kV transmission line, while the United States invested a ±100Mvar ASVG in 1995. China's Tsinghua University and Henan Electric Power Bureau jointly developed a ±20Mvar static var compensator, which was put into operation in 1999 at the Chaoyang Substation in Luoyang, Henan.
3. Parallel energy storage system Parallel energy storage device, including battery energy storage system (BESS) and superconducting magnetic energy storage (SMES), is an energy storage system using a parallel voltage source converter, and its inverter can be quickly adjusted. Supply or absorb electrical energy to the AC system. Using SMES for the frequency control of the two-machine system can effectively suppress the frequency offset between the two systems. SMES can also be combined with a static phase shifter for interconnect system load frequency control. However, this superconducting energy storage device not only has high technical requirements, but also has relatively high investment costs under current conditions, and there are still some difficulties in investing in a large number of systems.
4. Thyristor controlled series capacitor TCSC
The thyristor-controlled series capacitor module is mainly composed of a series capacitor and a parallel circuit containing a reactance and a thyristor switch. The thyristor control can flexibly and continuously change the compensation capacity to achieve a fast response effect. TCSC has many advantages in improving the performance of power system. The TCSC is used in high-voltage transmission systems, which can exploit the potential of existing systems, improve power transmission limits, flexibly adjust system currents, and increase system damping. It is to ensure the safety and stability of ultra-high voltage power grids. Important measures for operation.
Compared with other FACTS devices, TCSC has a simple trend control function and has received attention and attention from major companies such as GE, ABB and Siemens. In the United States, TCSC has been installed in three locations and is operating well. Sweden, Brazil and other countries have also put TCSC into actual operation. China uses series compensation technology on the double-return transmission line of Fengjing Substation from Yimin Power Plant to Qiqihar Area.
5, static synchronous series compensator SSSC
The static synchronous series compensator is based on a DC/AC inverter. Its basic principle is to inject a controllable voltage with a voltage difference of 90 into the line to quickly control the effective impedance of the line for effective system control. Its role in the system is somewhat similar to TCSC, but its ability to control power flow is much greater than that of a TCSC controller that reduces line impedance in a single direction and has a low harmonic content.
6. Thyristor controlled phase shifting transformer TCPST
The thyristor-controlled phase shifting transformer uses a thyristor switch to control the phase shifting angle to change the phase shift angle on both sides of the line to control the magnitude or direction of the current. The development of phase shifters was relatively early. As early as the first phase shifters in the 1930s, they were put into operation in the United States. With the development of power electronics technology, power experts in various countries began to combine thyristors and phase shifters to start thyristors in the 1970s. Control of the phase shifter TCPST. The research shows that TCPST has the functions of improving the transmission line of the tie line, suppressing small interference, improving system stability, damping power oscillation, bus voltage control, and regulation tie line current. The control speed of the thyristor controlled phase shifter is fast, and the phase angle step can be Very small, even reaching stepless regulation, but the thyristor controlled phase shifter has a disadvantage, it itself needs to consume reactive power, it generally needs to be used in combination with the reactive power compensation device, and the harmonic content is high, so Power quality has a certain impact.
7, convertible static compensator CSC
The convertible static compensator is a new product of the FACTS controller introduced in the past two years. It is actually a series-parallel compensator technology based on synchronous converter, which can be more flexible by structurally making it more flexible. Flexibility to respond to changing power system requirements. The CSC consists of two voltage source converters, one transformer in parallel with the transmission line and two transformers connected in series. Through the conversion of the switch, the different operating states of the compensator can be realized. According to different control objectives, the CSC can provide four basic types: static synchronous var compensator, static synchronous series var compensator, unified power flow controller and inter-line power flow controller. control method.
8, unified power flow controller UPFC
The concept of UPFC was first introduced by L.Gyugyi of Westinghouse Technology Center in 1992. The unified power flow controller is a new type of power flow control device developed from the original power flow control device. It consists of a parallel connection. The inverter and a series inverter are coupled by the capacitive coupling on the common side. The function of parallel compensation, series compensation or phase shifter can be realized separately by the change of the control amount, and the functions of the three can also be combined. . Through the design of different control strategies, UPFC can be used not only to control the bus voltage. Line currents, improved system dynamics and transient stability, suppress system oscillations, and can quickly switch operating conditions to meet the needs of the system's emergency. It is considered to be the most representative, most powerful and technically most complex member of the FACTS family.
Conclusion With the development of power electronics technology, the application of various switching devices to the FACTS controller as a switch can extend the service life of the device and improve economic efficiency. The application of FACTS technology can make the interconnected grids alternate with each other, reduce the hot and cold spare capacity, flexibly control the system parameters affecting the power flow distribution, control the transmission power on the contact line, and make the current flow to the designated line, improving the system. The tidal current distribution reduces the circulation in the large power grid, improves the dynamic stability of the system, and ensures that the power transmitted on the transmission line can approach the thermal stability limit, but it can meet the requirements of safe and economic operation.
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