The static reactive power capacitor compensation cabinet is composed of the switching electric reactors and the capacitors controlled by the thyristor. Because the thyristor responds very quickly to the control signals, and it is possible that the number of switching on/off is not to be limited. When there is a change in the voltage, the static reactive power capacitor compensation cabinet can make adjustments quickly and smoothly so as to meet the requests of the dynamic reactive power capacitor compensation cabinet; it can also achieve compensation for a single phase. As to three-phase unbalanced load and impact load, it has a strong adaptability; however, because high order harmonics may be produce while the thyristor is performing control on the switching process of the electric reactor, it is necessary to install a special filter.
In addition, the static reactive power capacitor compensation cabinet technology has great objective demand in the wind farms, the metallurgy industry, the electrified railways, the coal industry and other industrial fields. At present, the SVC (static var compensation) is very effective in solving the reactive power shock caused by various loads. The fluctuations in the voltage have been significantly improved and the power factor of the power grid has been significantly increased. It is a new type of energy saving device with high technical content as well as remarkable economic benefit. The thyristor converter and the control system can achieve this function. The values of reactive current and the values of voltage of the bus bar are collected to make the synthetic value of the reactive power, then the result is compared with the constant value set of the reactive power, and the calculation of the trigger angle is performed. The required current can flow through the thyristor by means of the thyristor activator device.
1. The dynamic reactive power capacitor compensation cabinet
In the static synchronous compensator (Static Synchronous Compensator, STATCOM) a new generation of power electronic devices (such as the gate turn-off thyristor (GTO), the insulated gate bipolar transistor (IGBT) and the integrated gate commutation thyristor are used.
The integrated gate commutated thyristor (IGCT) and the modern control technology (such as the inverse system, the direct feedback linearization, etc.) have many functions, such as making compensation for the system’s inductive reactive power and the capacitive reactive power, increasing the power factor of the system , improving the power quality, improving the stability of the power system and so on. Since its appearance, STATCOM has attracted extensive attention from the electric power scientific researches institutes as well as the electric power industry in various countries. It has undergone rapid development and applications. Up to now, it is the best reactive power capacitor compensation cabinet used in the power system and is the core of the flexible alternating current transmission system.
The function of STATCOM in the power system is to perform compensation for the reactive power, keep the voltage of the connection point at a specified value, improve the stability of the system voltage, improve the steady-state performance and the dynamic performance of the system. STATCOM is based on the concept of instantaneous reactive power and the principle of compensation. It uses the fully controlled switching devices to form the self-commutation inverter, and use additional small capacity energy storage elements to form the reactive power compensation device. Compared with the existing SVC (static var compensation) type reactive power capacitor compensation cabinet, it has the advantages such as faster adjustment speed, wider operation range, continuous absorption of the reactive power, low harmonic current, low loss, the capacity and installation area of the reactors and capacitors having been greatly reduced, and so on.
Another traditional method to for the compensation of harmonics and reactive current is to install the passive filter, which is usually composed of the power capacitors, the electric reactors and the resistors which are mounted in series or in parallel. With this method, it is not possible to perform compensation of the harmonics, but also perform compensation for the reactive power. At present, the commonly used reactive power regulation equipment in China is still the mechanical shunt reactors and the switching capacitors. It is hard to meet the needs of the rapid change of the operation mode of the system by means of these static voltage regulating methods, because the regulations are discontinued and the response speed is slow.
The response speed of another voltage regulating device SVC (static var compensation) is very fast. However, because of its constant impedance characteristic, it can not provide the required support for the reactive power in the case that the voltage is low, so its ability for preventing against emergencies is weak; in order to suppress the harmonics, it is necessary to install the filter and thus a large area is to be occupied. In addition, if there are too many SVC (static var compensation) devices, it will be easy to cause system oscillations. In contrast, the new STATCOM type reactive power capacitor compensation cabinet is an effective method for voltage regulation. Its reactive current output can remain constant in a wide variation range of the voltage; it can still provide strong reactive power support even when the voltage is low, and it is continuously adjustable in the whole range from the inductive current to the capacitive current.
2. The principle of reactive power capacitor compensation cabinet:
The output power of the power grid consists of two parts: one is the part of active power, the other is the part of reactive power. The part of the power which directly consumes the electric energy, converts the electric energy into the mechanical energy, the thermal energy, the chemical energy or the sound energy, and uses these energy to do work, is called the active power. The part of the power which does not consume the electric energy, but converts the electric energy into another form of energy, and which can be used as a necessary condition for enabling the electrical equipment to do work, and which is periodically converted to the electric energy in the power grid, is called the reactive power, for example, the the electric energy used by the electromagnetic element in establishes the magnetic field, the electric energy used by the capacitor in establishing the electric field. When the current works in the inductor, the current lags behind the voltage by 90 °. . When the current works in the capacitor element, the current surpasses the voltage by 90 °. In the same circuit, the inductive currentand the capacitive current are in the opposite directions, and the difference between them is 180 °. If the capacitor element is installed in the circuit of the electromagnetic element in a proportionate manner, so that the currents of the two elements can offset each other and the angle between the current vector and the voltage vector is reduced, the purpose of the reactive power capacitor compensation cabinet can be achieved.
The research and development of the reactive power capacitor compensation cabinet and the harmonic control device for the distribution network has been successful, and these products have been put into mass production. This can bring great social benefits. Generally speaking, at present, there are not sufficient reactive power capacitor compensation cabinets in the distribution network, and the existing reactive power capacitor compensation cabinets are not reasonably arranged. There is the phenomenon that the capacitor capacity in the urban and rural area and that in the regional power grids need to be relocated between them. There is a great demand for the reactive power from the users of the distribution network above 10KV voltage grade. The effective and reasonable use of the reactive power capacitor compensation cabinets and the harmonic control devices to make compensation for the reactive power and the harmonics in the distribution network not only can achieve the purpose of saving the energy and reducing the consumption, but also can reduce the damages to the electric devices as well as accidents caused by the harmonics.