Discussion on setting up generator circuit breakers for hydropower stations Li Dingzhong (Hydroelectric Water Conservancy Planning and Design Institute, Beijing 100011) Large unit circuit, three-line transformer or self-depleted transformer circuit, circuit breakers should be installed at the generator outlet. The advantages are: adapting peak shaving; improving the reliability and flexibility of the power supply of the plant; reducing the fault range and increasing the selectivity of the protection; suitable for synchronous operation; with the integration of the generator circuit breaker, the design can be simplified and facilitated Arrangement. When comparing the installation scheme of the circuit breaker, factors such as price, mechanical life (no fault operation), failure rate, power supply reliability and power station return rate should be considered. The dynamic evaluation of the economics of the power station should be emphasized. 1 Relevant regulations for installing generator circuit breakers for hydropower stations The original water conservancy and electric power ministry department's standard SD173-85 (Technical Specifications for Electromechanical Design of Hydropower Plants) (Trial) states: When the generator-transformer group is connected by units, the generator A disconnector should be installed at the exit. The following circuits should be equipped with circuit breakers at the generator outlet: 1 need to reverse the power consumption of the plant, and connect the unit circuit of the common factory transformer; 2 the peaking hydropower plant with frequent startup and shutdown, need to reduce the high-voltage side circuit breaker Unit loop of the number of operations; 3 joint unit loop. The following circuits must be equipped with circuit breakers at the generator outlet: 1 expansion unit circuit; 2 three-line transformer or self-turning transformer circuit. "2 main advantages of installing generator circuit breakers 21 Adapting to the needs of some types of electrical main wiring of hydropower stations. The wiring mode of some main electrical mains wirings must be equipped with generator circuit breakers. For example, for expansion unit loops, three-wire transformer circuits or The generator circuit breaker shall be installed in the self-tapping transformer circuit. In addition, the generator circuit breaker shall be installed when the pumped storage power station is commutating on the voltage side of the generator motor. For example, Guangzhou, Tianhuangping, Yanghu, Xi The pumping storage power station is equipped with a generator breaker with commutation function. 22 Adapting to the peaking requirements of hydropower stations For hydropower stations with 3/2, 4/3 circuit breaker wiring or polygonal ring connections on the high voltage side of the main transformer, it is advisable to install generator breakers. Hydropower stations often serve as peaking and frequency-modulating tasks in the system. They start and stop frequently, and they have to start and stop 2~3 times a day, up to 700~1000 times a year. If the high voltage side of the main transformer (hereinafter referred to as the main transformer) is a 3/2, 4/3 circuit breaker wiring or a polygonal ring connection, and the main transformer low voltage side is not equipped with a generator circuit breaker, each time the shutdown is started It affects the short-time open-loop operation of the high-voltage side, and doubles the number of breaking times due to the joint of two high-voltage side circuit breakers. Because the general high-voltage side circuit breaker is more advantageous. The Ertan Hydropower Station is equipped with six units with a capacity of 550 MW. It is the backbone power station of the Sichuan Power System. The high-voltage side of the main transformer is a hybrid connection of 3/2 and 4/3 circuit breakers, and the energy is selected in the generator-main transformer unit circuit. A generator breaker that satisfies the parameters of the plant. Guangxi Yantan and Yunnan Manwan Hydropower Stations are all hydropower stations with a total installed capacity of 1200MW (4X300MWO). The original generator-main transformer circuit is the unit wiring of the generatorless circuit breaker, and the high voltage side of the main transformer is the 3/2 circuit breaker wiring. After the power station is put into production, it will become the peaking and frequency-modulated backbone power plant of the system. Frequent start-stop will affect the open-loop operation of the high-voltage side wiring, and the number of high-voltage side circuit breaker operations will be very unfavorable for the reliable and flexible operation of the power plant, according to the operation requirements. And the expert group's recommendations have added generator breakers, and the current operation is well reflected. 23 Improving the Reliability and Flexibility of Power Supply for Plants 85 (Technical Specifications for Electromechanical Design of Hydropower Plants (Trial) stipulates that: The power supply of large-scale hydropower plants should be no less than three independent power sources, and should be maintained during shutdown of the whole plant. There is a working power supply and a reliable backup power supply. In the recently reviewed Guangxi Pingban (3X 165MWO, Hunan Fengtan expansion (2X170MW) and other hydropower stations, because the external power supply is only one time, it is recommended to install a generator circuit breaker to use the system power supply of the main transformer as the power supply. One of the reliable independent power supplies. Xiaolangdi Water Conservancy Project (6X300MW installed in power station) is mainly based on flood control. The operation of dam gates and hoists, especially flood discharge equipment, requires high reliability and flexibility. Although the high-voltage side of the main wiring of the power station is a 220kV double-bus double-segment wiring, a generator breaker is also installed in the generator-main transformer unit circuit, which is necessary to ensure that the plant power is reversed from the system. 24 Reducing the fault range and increasing the protection of the selective generator circuit breaker can separate the generator from the main transformer area, which is beneficial to separately setting the protection system and increasing the selectivity of protection. In the case of a generator accident, the generator can be cut off in the shortest time due to the improved resolution of the fault, and the scope of the shutdown is reduced to avoid accident expansion. When the generator fails, the generator breaker can quickly remove the faulty generator while continuing to maintain power to the plant and power plant auxiliary equipment. Single-phase or two-phase short-circuit faults on the high-voltage side of the main transformer (such as single-phase grounding, non-synchronous operation of the circuit breaker, etc.) will cause the unit to be subjected to excessive mechanical and thermal stresses, causing the unit to vibrate, causing metal fatigue and mechanical damage, especially The negative sequence component of the short circuit current will cause the generator rotor damper winding to overheat in a short time and burn out the rotor. The unbalanced fault current still exists after the high voltage circuit breaker is disconnected. Generator breakers typically cut and protect the unit within 60 to 80 ms. If there is no generator breaker, the generator will continue to provide unbalanced fault current until the demagnetization device disappears after about 5~20s of demagnetization. This duration has greatly exceeded the tolerance time of the generator set. damaged severely. Such accidents have occurred many times at home and abroad. From 1980 to the end of 1992, there were 50 cases in China. Only in 1991, there were 12 cases, all of which were based on thermal power. Because there are many generator breakers installed in thermal power plants. Each such accident can cause serious damage to the rotor. When the main transformer itself fails, such as insulation damage, casing flashover, carbonization of the pressure regulating switch, deterioration of oil moisture, etc., the evaporation of oil and gas generated by the fault arc current will cause the pressure in the transformer tank to rise. The increase of the pressure is proportional to the arcing time. In a short period of 130~150ms, the transformer tank may be caused to rupture or explode due to excessive pressure. The generator breaker helps to disconnect the generator to limit the fault current in time. Avoid accidents from expanding. 25 It is suitable for synchronous operation of generator breakers for three-phase mechanical linkage. The phase-to-phase switching is extremely small in different periods, and there is no refusal. In addition, it is installed in a closed metal shell, with good environmental conditions and high insulation margin. Ideal for reliable, safe operation over the same period of time and with sufficient ability to shut off the reverse current. High-voltage circuit breakers are generally not three-phase mechanical linkage, and each phase will be multi-fractured. During the synchronous operation, single-phase or two-phase rejection may occur, thereby jeopardizing the operational safety of the generator and transformer. At the same time, the ability of the high voltage circuit breaker to cut off the reverse phase current is also limited. When the high voltage side of the main transformer is being carried out at the same time, it is also possible to be affected by external environmental pollution under high voltage stress to cause flashover of the outer insulation of the circuit breaker. 6 The integration of generator breakers will simplify the design and facilitate the layout of generator breakers. The technology innovation is very fast. It has developed a combination of multiple functions, integrating voltage transformers, current transformers, isolating switches, grounding switches. , short circuit connection, protection capacitor, lightning arrester, monitoring protection, operating mechanism and other equipment, the combination method can be flexible and changeable, installed in a shell, small size, high reliability, easy to repair and maintain, conducive to layout. Although the current price of such products is relatively high, and there are not many foreign manufacturers, integrated products are definitely the trend of the development of generator voltage distribution devices in the future. 3 The problem that should be paid attention to in the comparison of the circuit breaker installation scheme 1 The price is connected to the joint unit. If the power plant main transformer high-voltage side power distribution device adopts 5(1) kVGLS, the price of the GIS breaker is often 25~3 of the SF6 generator breaker. If the main transformer high-voltage side equipment uses 500 kV conventional open-type SF6 equipment, the price will be lower than that of the SF6 generator circuit breaker. It is necessary to comprehensively compare other factors. 32 Mechanical life and failure rate Hydropower stations are frequently shut down due to peak shaving and frequency modulation, and can reach 700~1000 times a year. The mechanical life of the high-voltage side circuit breaker is short, the number of trouble-free operation is 3000~5000 times, and the long-term operation period without fault is short, only 4~7a; if it is considered to avoid the high-voltage side wiring open loop operation, it will increase by 1 times. The number of breaks, its fault-free continuous operation will be reduced to 3~4a. The generator breaker is designed and manufactured for the frequent operation of the unit, and its mechanical life is long, and the number of trouble-free operations can reach 5000~10000 times. Therefore, hydropower stations with ring connections of 3/2, 4/3 circuit breakers or polygons on the high voltage side of the main transformer should fully consider the doubling of the number of high-voltage side circuit breaker operations and their failure due to their short mechanical life. The impact of increased rate, fault repair time and increased power outage costs. Dynamic evaluation of economic efficiency of 33 power station Although the initial investment is large, it can improve the availability and power supply reliability of the power station, reduce downtime, increase power generation efficiency, reduce the degree of equipment failure and reduce the average maintenance time. Improve equipment reliability and power plant return rate; easy to operate remotely, maintain convenient, and save indirect costs. Therefore, in the technical and economic comparison of the program, it is necessary to comprehensively analyze the calculation of the main wiring reliability, accident power failure loss, maintenance and repair costs, annual operating costs and so on. Attention should be paid to the dynamic assessment of the economics of the power station, and the static investment of the equipment of each scheme should not be simply compared. The State Planning Commission announced the re-measurement of the on-grid tariffs issued by the State Planning Commission: "The State Planning Commission's notice on regulating electricity price management issues pointed out that the goal of China's electricity price reform is to further implement the separation of plant and network on the basis of deepening the reform of the power system. Bidding online. Before the implementation of the bidding online reform, in order to constrain the increase in electricity costs and reduce the price of electricity, it is decided to make appropriate adjustments to the specific measures for verifying the on-grid tariff. The main items related to the verification of the on-grid tariff are: 1 The average on-grid tariff is approved according to the operating period of the power generation project. The operating period of power generation projects is 20 years for thermal power. Hydropower is calculated on a 30-year basis. 2 The on-grid tariff is verified according to the average social cost of the advanced enterprise. 3 The power generation enterprises directly invested by foreign investors shall, in principle, verify the electricity price according to the above measures. 4 Establish a reasonable grid transmission and distribution price mechanism. The grid transmission and distribution price should be based on reasonable compensation for cost. The principle of determining the income reasonably and calculating the personal tax according to law shall be approved. 5 Strictly control the old power plants to increase electricity prices due to restructuring and selling property rights to domestic and foreign enterprises. The notice requires that all localities should re-calculate the electricity price of each power generation project according to the above regulations and the changes in market coal prices and bank loan interest rates. 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