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Analysis of the Advantages of Installing Generator Outlet Circuit Breakers 1. Usage and Development of GCBs at Home and Abroad In developed countries such as the United States, the United Kingdom, and France, GCBs are considered for use at the outlets of large-capacity generators in power plant design. Currently, domestic power plants use GCBs or generator load switches, and such plants can be found in Jixian, Tianjin; Suizhong, Liaoning; Yimin Power Plant; Shajiao C Power Plant (3×600 MW); Waigaoqiao Power Plant in Shanghai (2×900 MW); Panshan in Tianjin (2×600 MW), as well as the Gezhouba Hydropower Plant, Ertan Hydropower Plant, Lijiaxia, Tianshengqiao, and other facilities. In the past, GCBs were widely used in hydropower and nuclear power projects. In recent years, with the development of large-scale power grids, large-sized generating units, and ultra-high voltage systems in China, there has been an increasing emphasis on simplifying the operation of power plants, improving the availability of these generating units, as well as enhancing the safety and stability of the systems. The excellent properties of GCBs can fully meet these requirements. At present, domestic manufacturers do not yet have the capability to produce GCBs suitable for 600 MW class large-capacity units; abroad, only a few well-known large companies such as ABB, GEC-ALSTHOM, and MITSUBISHI are capable of producing them (the main technical parameters are detailed in Table 1). The GCB types mainly include the low-oil type, air type, SF6 gas type, and vacuum type. Oil-reduced GCBs such as the SN3 and SN4 models produced by Shenyang High Voltage Switch Factory in the 1960s have a rated current of 5000–8000 A, and a rated breaking current of 58 kA. Air-type GCBs, such as the PKG2 model produced by the French company A-A, have a rated voltage of 36 kV, a rated current of 11,000 A, and a rated breaking current of 58 kA. The main disadvantages of these circuit breakers are their large size, high noise level, and the lack of circuit breakers with medium capacity. They are used in the Gezhouba Hydropower Plant in China, where they perform well. With the advancement of power equipment manufacturing technology, in the 1980s companies such as ABB introduced GCBs that use SF6 gas as the arc-quenching medium. These devices take advantage of the self-arc-quenching property of SF6: when the moving contact is separated, an arc is generated which heats the SF6 gas, causing it to expand and create the gas necessary for arc extinguishment. At the same time, current flowing through the coil inside the fixed contact generates a magnetic field that causes the arc to rotate and separate, ensuring proper operation of both the load contact and the arc-quenching contact. SF6-type GCBs are widely used in power plants both domestically and internationally. They have a rated current of up to 24,000 A and a breaking capacity of 160 kA. Moreover, they feature a compact design, a lower failure rate (<0.3%), and can integrate devices such as CTs, PTs, and grounding switches, thus becoming multi-functional switchgear. In summary, the technological development of GCBs abroad is progressing very rapidly at present. Various companies are competing to develop innovative technologies, moving from oil-minimized types to SF6 and vacuum circuit breakers. These circuit breakers are becoming smaller in size, while their rated current and breaking capacity are increasing. Their mechanical lifespan exceeds 10,000 operations. As research and development capabilities as well as manufacturing techniques improve, the protection features integrated into GCBs will become more sophisticated, resulting in higher reliability and lower failure rates. 2 Technical Analysis of GCB Installation The low-voltage, high-current circuit breaker installed at the generator outlet plays a crucial role. In the past, the high rated current and short-circuit current of generators, along with the large DC component in their breaking current, made it difficult to manufacture GCBs and increased their cost significantly. Taking technical and economic factors into account, aside from small-capacity units (with a single-unit capacity of 200 MW or less) that are equipped with oil-free circuit breakers at the generator outlet, large-scale units (with a single-unit capacity of 200 MW or more) generally adopt a generator-transformer unit connection scheme, making use of phase-separated enclosed busbars without outlet circuit breakers or isolating switches wherever possible. In recent years, with improvements in the manufacturing quality and technology of GCBs, their prices have continued to drop, and greater emphasis is being placed on enhancing the safety and stability of these systems. The following is an analysis of the advantages of installing circuit breakers at the generator outlet. 2.1 Improving system safety and stability The advantage of the unit connection scheme for generators and transformer sets used in units of 200 MW and above is that it eliminates the need for GCBs, as well as the corresponding relay protection systems. However, this simplified wiring method means that the stable operation of the generator, transformer, and the entire system depends to a large extent on the reliability of the high-voltage circuit breakers on the high-voltage side of the main transformer. When a high-voltage circuit breaker is in normal operation, during operations to disconnect or connect circuits, or while operating under fault conditions, if one or two of the circuit breakers fail to operate properly, operate incorrectly, or experience insulation breakdown at their contacts, resulting in a non-full-phase closing or opening condition, the safe and stable operation of the power grid will be severely threatened. Non-full-phase operation can lead to damage to the insulation of transformers, even causing fires; it can also cause damage to the insulation of generator rotors due to negative-sequence currents, leading to fires as well. Furthermore, it can disrupt system stability and result in disconnections that cause widespread power outages and other serious accidents. Many similar accidents have occurred in domestic power plants; for example, at one plant, Unit 2 had to be shut down for inspection due to a malfunction. When the operators tried to trip the main transformer circuit breaker of Unit 2, they found that phase A of the circuit breaker would not open. After unsuccessful attempts to trip it manually at the step-up substation, they tripped the busbar circuit breaker to disconnect the main transformer of Unit 2 from the system, resulting in a condition of partial operation that lasted for 8 minutes, which led to the burnout of the rotor of Unit 2. For example, during a reverse power test on Unit 2 at the Second Plant in Shidongkou, the reverse power protection of Unit 2 activated, which in turn caused the circuit breakers connected in parallel on the high-voltage side of the main transformer to trip in all three phases. As one circuit breaker failed to open completely, it operated in a non-full-phase condition, which caused another 600 MW unit in the power plant that was operating, four 500 kV lines in the power grid, three 220 kV lines, as well as one 500 kV transformer and one 220 kV transformer at the Huangdu substation to trip one after another. From these cases, it can be seen that the causes of the accidents were serious incidents resulting from faults in the high-voltage circuit breaker itself, the operating mechanism, the control circuit, as well as improper handling by the operators. Damage to generators and transformers not only severely affects the safety and stability of the entire system but also results in substantial economic losses. If the installation of GCBs can truly reduce the occurrence of accidents, they are able to disconnect the power unit from the point of failure within 50–60 ms** – this shortens the duration of accidents and thus effectively protects the power unit, ensuring the long-term stable operation of the power system. Therefore, the use of GCBs enhances the safety and stability of the system’s operation. 2.2 Protection of the generator and main transformer When the generator operates under an unbalanced load or experiences an asymmetric short circuit, either internally or externally, severe mechanical and thermal stresses are generated in the generator. The fault current, along with the negative sequence components resulting from partial-phase operation, causes thermal stress that is applied to the damping windings of the generator rotor; this leads to abnormally high temperatures and serious damage to the generator rotor. In addition, asynchronous opening and closing of high-voltage circuit breakers, damage to arresters, and ground faults caused by upward wave reflections in overhead lines or GIS connection bushings can all affect the generator. GCBs can quickly isolate such faults, preventing the generator from being damaged. But if a GCB is not installed, the generator will continue to supply an unbalanced load to the fault point until the demagnetizing device takes effect. Since the demagnetization process often lasts for several seconds, or even more than 10 seconds, it can cause severe damage to the generator. Although it is impossible for a GCB to prevent the occurrence of a fault within the system, as such a fault may stem from an inherent weakness in a device or from external factors, it can reduce the various stresses exerted on the devices and the extent of damage caused by faults. As an example, assume that there is a fault between the high-voltage side bushing of the transformer and ground; in this case, the system fault current can be interrupted by the high-voltage circuit breaker. Without a GCB, the generator would continue to supply current to the fault site until the demagnetization device takes effect. The typical de-excitation time ranges from 5 to 20 seconds; especially in the case of faults on the high-voltage side of the main transformer, within the first 40 ms, the arcing current comes from both the system side and the generator side, causing the pressure inside the transformer tank to rise extremely rapidly. At 40 ms, the high-voltage circuit breaker separates the system from the fault point; the arcing current is then supplied solely by the generator. In the absence of a GCB, the generator would continue to supply a current that decreases due to demagnetization to the arcing site, and this current would be maintained for several seconds. Eventually, the pressure inside the transformer tank rises to the explosive limit, resulting in an explosion of the transformer tank. If GCB is applied, it will operate at 60 ms to cut off the fault current in the generator, allowing the pressure to be kept below the level at which an explosion would occur; thus, the transformer can be prevented from exploding. It can be seen that the use of GCB can protect the main transformer. 2.3 Improving protection selectivity When a fault occurs on the generator side, the operation of the GCB isolates the faulty point from the system, preventing accidents related to the auxiliary power supply. This simplifies the control and protection wiring for the auxiliary power source and reduces the complexity of the interlocking mechanisms associated with protection actions. In the event of a fault on the main transformer side, the GCB can be quickly disconnected, allowing the generator, the main transformer, and the station service high-voltage transformer to be under their respective independent protection systems. 2.4 Facilitates debugging and improvement of synchronization conditions. The GCB is able to carry out all the operational tasks required by the unit because it is located in the most appropriate position within the circuit; it allows the generator to be disconnected without interrupting the auxiliary power supply. This reduces the number of operations needed by the operators and helps to avoid the possibility of errors. When conducting short-circuit tests with the unit in operation, it is convenient to use a grounding switch; otherwise, wiring changes are required for the tests, which entails additional costs and time, as well as the potential for unnecessary risks. When the connection between the power plant and the power grid is made via a high-voltage circuit breaker and through the main transformer to receive power, synchronization can be achieved using a GCB. For synchronous operations, what is the difference between using the high-voltage side circuit breaker of the main transformer and a GCB for such operations? Recent foreign research shows that the delayed zero-crossing currents resulting from synchronous and asynchronous operations, achieved respectively by high-voltage circuit breakers and GCBs, have different effects on the system. During reverse synchronous operation, the rapid rotation of the generator rotor generates delayed zero-crossing currents; high-voltage circuit breakers have very limited capacity to interrupt such currents, whereas GCBs possess sufficient capability to do so. When operations are performed on the high-voltage side during the same period, the high-voltage circuit breaker may be subjected to overvoltage. In cases of severe pollution, it may cause flashover in the external insulating medium of high-voltage circuit breakers. Furthermore, high-voltage circuit breakers are generally not mechanically interlocked in three phases; as a result, significant phase discrepancies can occur during simultaneous operation. This leads to an unbalanced load, which imposes severe mechanical and thermal stresses on the generator, potentially damaging it. When operations are carried out at the generator voltage level during the same period, reducing the circuit breaker voltage level helps prevent external insulation flashover. Implementing simultaneous operations using GCB remains entirely within the control scope of the power plant; the substation control does not need to get involved, thereby avoiding any overlap in control responsibilities. 3 Economic comparison of installing GCBs With the improvement in the manufacturing quality of main transformers and advancements in GCB manufacturing technology, the principles governing the selection of starting (standby) power supplies for large-capacity units are changing. When the price of the GCB is close to that of equipment such as start/standby transformers and high/low voltage side switches, it is possible to consider omitting a dedicated start/standby transformer and instead using the main transformer, powered by an auxiliary service transformer, to provide the starting power source, thereby minimizing the initial investment. Even when taking into account the investment in starting/standby transformer GCBs, there is still a considerable increase in economic benefits while improving the plant’s availability. The following is an economic comparison of the two commonly used electrical wiring schemes for 600 MW units: Scheme 1: Uses a generator-transformer unit configuration; no GCB is installed at the generator outlet. Two starting/standby transformers are provided, with power to the transformers supplied from a 200 kV substation located about 10 km away. The two starting/standby transformers are connected via two circuits using overhead lines, and the substation is equipped with either one and a half circuit breakers or a double-busbar wiring arrangement. When the high-voltage service transformer fails or is under maintenance, the service power is supplied by the start-up/standby transformer. The schematic diagram of its main wiring is shown in Figure 1. Option 2: A generator-transformer unit wiring scheme is adopted, with a GCB installed at the generator outlet; during unit startup and normal shutdown, the auxiliary power supply is provided by the system via the main transformer. A standby transformer is provided for emergency shutdown purposes; the power supply for this standby transformer comes from a 220 kV substation located about 10 km away. Overhead lines are used for the connection, and the substation is equipped with either one and a half circuit breakers or a double-bus configuration. The schematic diagram of its main wiring is shown in Figure 2. (1) Utilization rate comparison: The reliability data for various components in the main wiring schemes 1 and 2 are based on the data published by the International Conference on Large Power Systems; the calculation results are shown in Table 2. As can be seen from the table above, compared to Plan 1, Plan 2 results in an average utilization rate that is 0.69% higher, and an average annual downtime of 60.4 hours less; therefore, installing GCBs will yield significant economic benefits. (2) Comparison of initial investments Based on the calculations of the initial investments for Options 1 and 2 (see Table 3), Option 2 requires an additional investment of approximately 6.3 million yuan compared to Option 1. (3) Analysis of operational benefits According to the results regarding availability, the average downtime for Option 2 is 60.4 hours less than that for Option 1. Assuming an annual operating time of 6,000 hours per unit, this means the unit can generate 49,640 kW more electricity per year. After deducting 6% for plant internal consumption, the amount of electricity supplied to the grid increases by 46.6616 million kWh per year. At a grid selling price of 0.34 yuan/kW·h, the plant’s annual income can increase by 15.865 million yuan. Therefore, Option 2 offers significant operational benefits, enabling the initial investment to be recovered more quickly. (4) Analysis of losses due to failures and power outages According to statistics from relevant literature, the failure rate of 500 kV main transformers is 2 times per 100 units per year. Assuming an operating hours of 6,000 hours per year, a generation profit of 0.14 yuan per kW·h, and a GCB service life of 20 years, adopting Plan B would enable the power plant to reduce its annual losses due to power outages by: 0.02×2×6000/8760×20 × (177×24 – 1187) × 60 × 0.14 × 0.8 = 112.714 million yuan. Obviously, this benefit far exceeds the difference in the initial investment. 4 Conclusion As can be seen from the above, the use of GCBs not only helps to protect generators and transformers, reduces the average maintenance time for these devices, and improves their operational conditions, thereby enhancing the safety and reliability of the entire power plant; it also facilitates the management, operation, and maintenance of the plant. Additionally, it brings significant economic benefits by reducing maintenance costs throughout the plant’s lifecycle and accelerating the return on investment. Therefore, installing GCBs on the motors of large-capacity units is one option worth considering. References: Northwest Design Institute of the Ministry of Water Resources and Electric Power. Handbook for Electrical Design in Power Engineering. Beijing: China Electric Power Press, 1999. Ye Delong, et al. Domestic and International Levels of High-Voltage Switchgear Products in 1995. Liu Guanghua. Discussion on the Use of GCBs in 00MW Generating Units. Beijing: Power Design Electrical Professional Technology Information Network, 2001. Jiang Baiqing. Selection of Generator Outlet Switches for Units 1 and 2 at Taishan Power Plant [J]. Guangdong Electric Power, 2001, (12). Wu Zhicheng, Cheng Zhaohui. Discussion on the Application of Generator Circuit Breakers in Large-Scale Generating Units [C]. Beijing: Power Design Electrical Professional Technology Information Network, 2001