According to the characteristics of power load in coal industry, dynamic reactive power compensation technology has been studied at home and abroad, and the main types are as follows:
1, grouping capacitor switching mode. In the switching mode of vacuum contactor (or circuit breaker), because the capacitor circuit will produce high operating overvoltage and surge current during the switching process, an arc will be generated between the switch contacts during the switching process. Therefore, reactors are usually connected in series in the loop to suppress inrush current and control corresponding harmonics. Its characteristics are simple principle and low cost.
2. Static and dynamic reactive power compensation device (SVC). The device is thyristor controlled reactor+filter device (TCR+FC) or thyristor switching capacitor (TSC). It has the functions of smoothly adjusting reactive power compensation capacity, fast system response and comprehensive harmonic control, and is widely used in coal mine system, metallurgical industry, power system and electrified railway.
3. Solenoid valve compensation mode. The device consists of a compensation capacitor and a parallel adjustable reactor. By adjusting the magnetic flux of the high impedance reactor, it is balanced with the redundant capacitive reactive capacity in the shunt capacitor. This is a variant product of self-saturation reactor compensation mode. Because of its large loss, high operating cost, slow adjustment speed and limited compensation range, it belongs to obsolete technology.
These compensation methods have some disadvantages. Combined with the distribution form of coal mine, it is urgent to develop reactive power compensation equipment suitable for coal mine application, which is also in response to national policies.
At present, there are three commonly used reactive power compensation methods in coal mine distribution network: centralized compensation, decentralized compensation and local compensation.
1, centralized compensation
Centralized compensation is to install the capacitor on the low-voltage bus of the user-specific substation or distribution room to uniformly compensate reactive power. This compensation method is more suitable for situations with concentrated load, close to substations and large reactive power compensation capacity.
Xuzhou Xinji Coal Mine adopted centralized compensation. This mine analyzes the grouping switching capacitor banks, voltage regulating reactive power capacity compensation and dynamic reactive power compensation (SVC). Based on the economy of the three schemes, it is determined to replace the original fixed switching capacitor with the grouped switching capacitor, which ensures the power supply quality and power factor of the mine.
The advantage of centralized compensation is that reactive power loss of transformer can be compensated locally. As the reactive current of the transformer decreases, the capacity of the transformer decreases accordingly, that is to say, the active load carried by the transformer can increase; It can compensate the power loss of substation bus, transformer and power receiving line, and save energy; When the load changes, the bus voltage can be adjusted to some extent, thus improving the voltage quality; Convenient for management, maintenance, operation and centralized control.
The disadvantage is that it can only reduce the loss caused by the transmission of reactive power to lines and transformers above the installation point, but can not reduce the loss caused by the transmission of reactive power to electrical equipment through low-voltage lines within users.
2. Decentralized compensation
Decentralized compensation means that according to the reactive load distribution of low-voltage distribution network, capacitor banks are grouped and installed on corresponding buses, or directly connected with low-voltage trunk lines, forming a multi-group decentralized compensation method for low-voltage distribution network, which is suitable for compensation occasions with relatively dispersed loads.
Jixi Mining Group adopted the decentralized compensation method. The specific method is to install the mine flameproof reactive power automatic compensation device on the low voltage side of the mobile substation in the fully mechanized mining face.
Advantages of decentralized compensation: for power users with relatively dispersed loads, it is beneficial to reactive power partition control, realize local balance of reactive load, reduce reactive current loss and voltage loss of distribution network and distribution transformer, and significantly reduce line loss; Under the condition of constant load, the output capacity of the network can be increased; The compensation method is flexible and easy to control.
The disadvantages of decentralized compensation are: if the installed capacitors cannot be grouped, the compensation capacity cannot be adjusted, and there may be overcompensation or undercompensation in operation; The utilization rate of compensation equipment is lower than that of centralized compensation; The installation is scattered and the maintenance and management are inconvenient.
3. On-site compensation
This method is to compensate the reactive power consumed by on-site electrical equipment (mainly motors), and install the capacitor bank directly next to the electrical equipment in parallel with the power supply circuit of the electrical equipment, thus improving the power factor of the electrical system and obtaining obvious loss reduction benefits.
The advantage of local compensation is that reactive current only exchanges with nearby electrical equipment and does not flow to other points in the network. The reactive power loss and voltage loss of reactive current in the network are small, which not only compensates the reactive power loss of the system but also compensates the internal reactive power loss of users, greatly reducing the power loss, and the compensated network operation is the most economical; Under the condition of unchanged distribution equipment, the power supply capacity of the network can be increased and the conductor section can be reduced accordingly; Good adaptability, can be used for three-phase compensation, large-capacity motor compensation alone, two-phase and single-phase compensation. Moreover, the capacity of a single compensation device is small, and the impulse current of capacitor switching is small, which is especially suitable for reactive power compensation in hotels, buildings and other occasions.
The disadvantage of local compensation is that compared with centralized compensation and decentralized compensation, the total capacity of compensation capacitors and the total number of compensation devices needed to compensate reactive loads with the same capacity increase, resulting in large investment and low utilization rate of compensation devices. At the same time, due to the harsh underground environment, maintenance can not keep up, which is easy to cause equipment damage. [26]
Due to the above shortcomings, many coal mines in China adopt a combination of centralized compensation and decentralized compensation for capacitor reactive power according to the principle of economic operation. Capacitors are installed on the main transformer bus of mine ground substation to compensate the reactive power of the whole mine. Capacitors are set in groups, and automatic tracking compensation devices are set when necessary to adjust the power factor of the whole mine; Install capacitors on the secondary side of underground central substation or mining face as decentralized compensation; For motors with large capacity and long-term stable operation without reverse rotation or reverse braking, local independent compensation is adopted.
Jiaozuo Coal Industry Co., Ltd. adopts the compensation mode of combining centralized compensation with local compensation. The specific method is to take centralized compensation for loads close to the mine ground substation and with small electricity consumption, such as the ground domestic electricity consumption and the load of compressor room. For places where the load is relatively concentrated, such as machine maintenance, air shaft, etc., decentralized compensation is adopted. For motors with long distance and heavy load (such as shearer and water pump), separate compensation is adopted.
Through the analysis of compensation methods commonly used in domestic coal mines, we can see that although various compensation methods have been adopted to minimize the network loss, there are still many problems in reactive power compensation of many coal mine distribution networks, which are as follows:
1, single compensation mode
Through the research on the compensation methods of main coal mines in Chongzhou mining area, it is found that the compensation methods are single, and most of them only adopt one compensation method. Some coal mines adopt high-voltage centralized compensation, and it is found that the reactive power compensator in coal mines can only compensate the 6kV bus, which has little effect on the internal compensation of coal mines.
2. The compensation position is not ideal.
Because there are many large-scale power electronic equipment in coal mines, such as hoists with a power of 680kw, many coal mines are directly compensated on the spot. However, this kind of compensation for heavy load supplied by large-capacity thyristor power supply will lead to waveform distortion, increase of harmonic components and decrease of power factor. Moreover, because this kind of load often changes rapidly, the harmonic frequency increases, which endangers the quality of power supply and has a great impact on communication equipment, it is unsafe and inappropriate to use local compensation for this kind of load.
3. Insufficient compensation capacity
Generally, coal mines are far away from power supply substations, and the power supply voltage fluctuates greatly. When the voltage of the normally designed capacitor rises beyond the set range of the automatic device, it will quit operation and lose its compensation function.
Through the above analysis, we can find that there are many problems in the calculation of reactive power compensation capacity and the selection of compensation location in coal mine distribution network. In order to effectively determine the compensation capacity and compensation mode, reactive power optimization must be adopted to calculate the power flow of the whole mine distribution network.
Second, the development trend of reactive power compensation equipment in coal industry-static reactive power generator
The main body of static var generator is voltage source inverter. By properly turning on and off the thyristor, the DC voltage on the capacitor is converted into a three-phase AC voltage synchronized with the power system voltage, and then connected to the power grid in parallel through the reactor and transformer. By properly controlling the output voltage of the inverter, its working state can be flexibly changed to make it in capacitive, inductive or no-load state. Compared with the static var compensator, the static var generator has faster response speed and smaller harmonic current, and can still inject more reactive power into the system when the system voltage is low.
With the rapid development of power electronics 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 urban power network, operating units often need to install reactive power compensation controller and distribution comprehensive tester at the same time on the low voltage side of distribution transformer, so the integration of reactive power compensation controller and distribution comprehensive tester is proposed.
2. Quickly and accurately detect the reactive power parameters of the system, improve the dynamic response time, and quickly switch the capacitor to meet the harsh working conditions (such as heavy impact load or frequent load fluctuation). With the development of computer digital control technology and intelligent control theory, some advanced control methods, such as fuzzy 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 thyristor withstand voltage level, high voltage system is connected by transformer, such as used in electrified railway traction substation. It is of great significance to develop a high voltage dynamic reactive power compensation device. The key problem is to solve the withstand voltage of thyristor and diode of compensation device, that is, the series connection and voltage sharing of multiple thyristor elements, the synchronization of trigger control and so on.
4. From 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, there are a lot of harmonics in power supply system or load. Research and development of thyristor switching filter with dual advantages of reactive power compensation and power filtering will become an effective means to improve system power factor, suppress harmonics, stabilize system voltage and improve power quality.