Analysis of the superiority of installing generator outlet circuit breaker

Fan Jiangdong (Zhejiang Electric Power Company, Zhejiang Hangzhou) compares two common electrical wiring schemes for 600MW units. It is believed that the generator outlet circuit breaker can improve the safety of the system, simplify the operation mode, facilitate the commissioning and maintenance, and can produce obvious Economic benefits.

Ground/fracture 75/10060/9075/1060/8060/80<38 equipped with mechanism pneumatic pneumatic pneumatic pneumatic pneumatic pneumatic circuit breaker arc extinguishing medium compressed air SF6 compressed air SF6SF6 vacuum 1 domestic and international GCB use and development status United States, In the design of power plants in the UK, France and other developed countries, the large-capacity generator exports are considered to be equipped with GCB. At present, domestic power plants use GCB or generator load switch power plants, mainly Tianjin Jixian, Liaoning Yuzhong, Yimin Power Plant, Shajiao C. Power plant (3X00MW) Shanghai Waigaoqiao Power Plant (2X90MW), Tianjin Panshan (2K600MW), Gezhouba Hydropower Plant, Ertan Hydropower Plant, Lijiaxia, Tianshengqiao and other projects. In the past, GCB was widely used in hydropower projects and nuclear power projects. In recent years, with the development of China's power system large power grid, large units, and ultra-high voltage, how to simplify the operation of power plants, improve the availability of units, and improve system safety and stability. Issues such as sex are getting more and more attention, and GCB's superior features can fully meet these requirements.

At present, domestic manufacturers have not been able to produce GCBs that are compatible with 600MW high-capacity units. Only a few well-known large companies such as ABB, GEC-ALSTHOM, and MITSUBISHI have the ability to produce them. (The main technical parameters are shown in the table.) GCB types mainly include Less oil type, air type, SF6 gas type and vacuum type. Less oil type GCB, such as SN3SN4 produced by Shenyang High Voltage Switch Factory in the 1960s, rated current is 5000~8000A, rated breaking current is 58kA. Air type GCB, such as PKG2 type rated voltage produced by French A-A company is 36kV The rated current is 11000A and the rated breaking current is 58kA. This type of circuit breaker mainly has the disadvantages of large volume, noise, lack of medium capacity circuit breaker, etc. It is used in Gezhouba Hydropower Plant in China and its operation is good. With the development of power equipment manufacturing technology, ABB and other companies introduced GCB with SF6 gas as the arc extinguishing medium in the 1980s. It uses the SF6 self-extinguishing principle to generate an arc to separate the moving contacts to heat the SF6 gas and expand it. The gas required for extinction is formed, and the current flows through the coil in the fixed contact to generate a magnetic field, which causes the arc to rotate and separate, and ensures that the load contact and the arc-extinguishing contact work normally. The SF6 GCB is currently used in a large number of domestic and foreign power plants. It has a rated current of 2000A, a breaking capacity of 16kA and a compact structure with a low failure rate (<03%). It can also integrate CT, PT, grounding switches and other equipment. Become a versatile combination.

In summary, the current technology development of foreign GCB is very rapid, major companies compete to develop innovative technologies, from the original oil-less type to SF6 and vacuum circuit breakers, the volume is getting smaller, the rated current and breaking current Increasingly large, the mechanical life is up to 1000 times. With the improvement of R&D capability and manufacturing technology, GCB configuration protection will be more perfect, and the reliability will be higher and the failure rate will be lower.

2 Install GCB technology to analyze the low-voltage, high-current circuit breaker installed at the generator outlet, its role can be described as very important. In the past, due to the large rated current and short-circuit current of the generator and the large DC component of the breaking current, the GCB was difficult to manufacture and the cost was also high. Considering technical and economic factors, except for the small-capacity unit's generator outlet with a small oil circuit breaker (single unit capacity of 200MW or less), the general large unit (single unit capacity of 20MW and above) mostly uses generator-transformer unit wiring, try to use The phase-closed busbar does not have an outlet circuit breaker and an isolating switch. In recent years, with the advancement of GCB manufacturing quality and technology, prices have been decreasing, and how to improve the safety and stability of the system will be paid more and more attention. The following is an analysis of the superiority of setting the circuit breaker at the generator outlet.

21 Improve system safety and stability The advantages of the unit wiring method of the generator and transformer group used by the 2MW and above units are that the GCB is omitted and the corresponding relay protection is also omitted. However, this simplified wiring method makes the stable operation of generators, transformers and systems largely dependent on the operational reliability of the high voltage circuit breaker on the high voltage side of the main transformer. When the high-voltage circuit breaker is in normal operation, during the operation of the disengagement or parallel operation, in the event of an accident, if the one-phase or two-phase circuit breaker occurs due to refusal, misoperation or breakage insulation breakdown In the case of non-full phase separation and closing, the safe and stable operation of the power grid will be seriously threatened. It is very likely that the transformer insulation damage or even the fire will be destroyed due to non-full phase operation. The generator rotor is caused by the negative sequence current. Insulation damage or even fire and destruction, system stability suffered damage and the breakdown caused major accidents such as large-scale power outages. There have been many similar accidents in domestic power plants. For example, a power plant was shut down due to the failure of No. 2 furnace. When the operating personnel operated the No. 2 main transformer circuit breaker, it was found that the circuit breaker A phase refused, and the booster station did not jump. After the success, the jumper busbar breaker disconnected the main transformer of the No. 2 machine from the system, resulting in a non-full phase operation time of up to 8 minutes, causing the No. 2 rotor to burn. Another example is the reverse power test of No. 2 of Shidongkou No. 2 Plant. The No. 2 machine reverses the power protection action, and at the same time causes the three-phase opening of the circuit breaker connected in parallel with the high voltage side of the main transformer. Because one circuit breaker failed to open to the end, the circuit breaker was not fully phase-operated, resulting in another 600MW unit of the power plant, four 50kV lines, three 22kV lines, and one 500kV transformer of Huangduchang. The 20kV transformer has tripped.

It can be seen from these cases that the cause of the accident is a major accident caused by the high-voltage circuit breaker body, the operating mechanism, the control circuit failure, and the improper handling of the operating personnel. Damage to generators and transformers will not only seriously affect the safety and stability of the entire system, but also cause huge economic losses. If the installation of GCB can completely reduce the accident, GCB can greatly reduce the accident time by separating the unit from the fault point, thus effectively protecting the unit and ensuring long-term stable operation of the power system. Therefore, using GCB will improve the safety of the system. Sex and stability.

22 Protection of generators and main transformers When the generator is operated with unbalanced load and an asymmetrical short circuit occurs internally or externally, it will cause severe mechanical and thermal stress on the generator. This fault current and its non-full phase operation are negative. The thermal stress caused by the sequence component is applied to the damper winding of the generator rotor, which will cause abnormally high temperatures and severely damage the generator rotor. In addition, the combination of high-voltage circuit breakers in different periods, the damage of the arrester, the ground fault caused by the upward wave reflection of the overhead line or GIS connection casing will have an impact on the power generation. The GCB can quickly remove these faults and make the generator Free of damage. However, if the GCB generator is not installed, the unbalanced load will continue to be supplied to the fault point until the de-excitation device functions. Since the de-excitation process often lasts for a few seconds, it can even exceed 1 s, causing serious damage to the generator.

Although it is impossible for GCB to avoid a certain fault in the system, because the fault may be caused by the inherent weakness or external cause of a certain equipment, GCB can reduce the damage caused by various stresses and faults added to the equipment. degree. As an example, assuming that the transformer high-voltage side casing fails to the ground, the system fault current can be cut off by the high-voltage circuit breaker. If there is no GCB generator, the current will be continuously sent to the fault point until the de-excitation device functions. Generally, the demagnetization time needs to be 5~2s, especially for those faults on the high voltage side of the main transformer. In the first 4ms, the arcing current comes from the system side and the generator side, and the internal pressure of the transformer tank rises extremely rapidly. At 4ms, the high voltage circuit breaker separates the system from the fault point. The arcing current is only supplied by the generator. If there is no GCB generator, a current source that is attenuated due to demagnetization will be continuously sent to the arcing point and maintained for several s. At the time, the internal pressure of the transformer tank eventually rises to the limit pressure at which the explosion occurs, causing the transformer tank to explode. If the GCB action cuts the generator fault current at 6ms when the GCB is applied, the pressure can be limited below the explosion pressure and the transformer can avoid an explosion. It can be seen that the main transformer can be protected by GCB.

23 Improve protection selectivity When the generator side fails, the GCB action isolates the fault point from the system, avoiding the switching of the plant power accident, simplifying the control and protection wiring of the plant power supply, and reducing the interlocking complexity of the protection action. When the main transformer side fails, the GCB can be quickly removed, so that the generator, main transformer and factory high voltage transformer are in separate protection ranges.

24 Facilitating Commissioning and Improvement of Concurrent Conditions GCB is able to perform all the operational tasks required by the unit because its position is in the most appropriate place in the circuit and the generator can be disconnected without interrupting the power supply to the plant. This way the operator also reduces the operation and avoids the possibility of error. When the unit is put into operation for short-circuit test, it is convenient to use the grounding switch. Otherwise, it is necessary to invest additional funds and time to carry out trial and change wiring, and it may take unnecessary risks.

When the connection between the power plant and the grid is received by the main transformer via the high voltage circuit breaker, the coincidence point can be realized by the GCB. For the synchronous operation, what is the difference between the main transformer high-voltage side circuit breaker and the GCB for the synchronous operation? The latest foreign research shows that the high-voltage circuit breaker and GCB respectively realize the delay caused by the synchronous operation and the different operation. Zero current has different effects on the system. During the reverse synchronous operation, due to the delayed zero-crossing current generated by the rapid rotation of the generator rotor, the high-voltage circuit breaker has very limited ability to cut off the reverse-phase synchronous current, and the GCB is sufficient. The ability to cut the current.

The high voltage circuit breaker may be subjected to an overvoltage when operating on the high voltage side during the same period. In the case of heavy pollution, the external insulation medium of the high voltage circuit breaker may be flashed. Moreover, the high-voltage circuit breakers are generally not three-phase mechanical linkage, so there may be a large difference in the same period of operation, which will produce an unbalanced load, which will bring serious mechanical and thermal stress to the generator. , thus damaging the generator.

When operating at the generator voltage level for the same period, the reduction in the voltage level of the circuit breaker helps prevent external insulation flashover. The simultaneous operation with GCB is completely within the control range of the power plant, and the substation control can be carried out without intervening, so that there is no overlap in any control responsibility.

3 Installation of GCB economic comparison With the improvement of the manufacturing quality of the main transformer and the advancement of GCB manufacturing technology, the setting principle of the startup (standby) power supply of large-capacity units is changing. When the price of the GCB is close to the price of the start/standby change, high and low voltage side switches, etc., it can be considered that there is no special start/standby change, and the main change is provided by the main work to provide the start power supply. Investment is reduced to a minimum. Even if the start/standby change is taken into account, the GCB investment is taken into consideration, and while the plant availability is increased, there is still considerable economic benefit. The following is an economic comparison between the two electrical wiring schemes commonly used in 600MW units: Option 1: Use generator-transformer wiring, generator outlet without GCB, set two start/standby changes, transformer power supply from 200km near 1km The substation is connected. The two start/standby switches use two-circuit lines, the connecting lines use overhead lines, and the substation uses one-half circuit breaker or double busbar wiring. When the high-voltage plant transformer is faulty or overhauled, the factory power supply is provided by the start/standby change. The main wiring diagram is shown in Scheme 2: the generator-transformer wiring is used, and the generator outlet is equipped with GCB. When the unit starts and stops normally, the factory power supply is supplied by the system through the main transformer. Set up an accident shutdown standby change, the standby transformer power supply is led from the 22kV substation near lkm, the connecting line uses overhead lines, and the substation uses one-half circuit breaker or double busbar wiring. The main wiring diagram is shown in the first and second main wiring reliability data of each component using the data published by the International Large Power Grid Conference. The calculation results are shown in Table 2, Table 2, and the annual average utilization rate under the two schemes. The failure time (inside)%2 is known from the above table. The average annual availability of the scheme is increased by 069% compared with the scheme, and the annual average failure time is reduced by 604h. The installation of GCB will produce obvious economic benefits.

According to the initial investment calculation of schemes I and II (see table), scheme 2 is about 6.3 million yuan more than the initial investment of scheme 1. According to the calculation result of availability, the average failure time scheme 2 is 604h less than scheme one, such as the unit. The annual operating hours are assumed to be 6h, so the unit can generate 4,960 kW of electricity per year. After deducting 6% of the plant's electricity consumption, the annual on-grid electricity can be increased by 466.616 million degrees, the on-grid price is 0 34 yuan/kWh, and the annual revenue of the power plant can be 15.765 million. Yuan, so the operating income of the second plan is significant, and the initial investment can be recovered quickly.

According to relevant statistics, the failure rate of the 500kV main transformer is 2 times/10 units. In the year, if the operating hours are 60001/year, the power generation profit is calculated according to 0 4 yuan/cWh, and the GCB life time is 20 years. If the scheme 2 power plant is used, the power loss will be reduced every year: 0 Table 3 Yuan name - scheme one scheme two (not installed GCB) start / standby power transformer circuit breaker isolation switch current transformer voltage transformer lightning arrester 220kV overhead line 6kV switchgear 6kV closed bus main transformer pressure switch other total Note: (1) Only the difference between the main equipments of the two schemes is compared, and the same part is not listed; (2GCB is the price of domestic equipment according to the price of imported equipment.

08 failure rate X 2 main changes X years of operation hours X years of use X (no GCB failure repair days X 4h * GCB failure recovery time) X profit X (GCB function) such failure rate) = 1171 4 million . Obviously, this benefit is far greater than the difference in initial investment.

4 Conclusion As can be seen from the above, the use of GCB not only can protect generators and transformers, reduce the average maintenance time of equipment, improve the conditions of the same period, greatly improve the safety and reliability of the entire power plant, facilitate power plant management, operation and maintenance, but also bring The obvious economic benefits reduce the maintenance cost of the entire power plant life cycle and accelerate the return on investment of the power plant. Therefore, installing the GCB of the generator outlet of the large-capacity unit is one of the solutions worth considering.

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