1. Overview of Generator Circuit Breaker Development Since the early 1940s, with the improvement of industrial and manufacturing levels and the continuous increase in electricity demand, the production and manufacturing of large-capacity generator sets not only became possible but also a requirement to meet the needs of the electricity market. The continuous increase in generator unit capacity led to a rapid increase in short-circuit current, making ordinary medium-voltage switches unable to meet the breaking capacity requirements. Simultaneously, to improve safety and reliability, the rapid development of phase-separated enclosed busbars and the widespread adoption of generator and transformer unit wiring were observed. By the mid-1960s, to simplify power plant operation, improve unit availability, and meet the needs of nuclear power technology, more and more experts believed that the use of generator circuit breakers was essential. It was this need that led BBC to develop the first DR-type air circuit breaker, which could be directly operated at the generator terminals of large generators, in 1969. This circuit breaker is a phase-separated, fully enclosed structure, using compressed air as the arc-extinguishing medium. The operating mechanism also uses compressed air. It has a rated current of 50,000A and a breaking capacity of 250kA. Circuit breakers with a rated current below 11,000A use natural air cooling, those between 11,000 and 20,000A use forced water circulation air cooling, and those between 20,000 and 50,000A use water cooling. Since the first DR-type circuit breaker was put into operation, BBC/ABB has produced approximately 600 units and launched them on the market. The failure rate has gradually decreased to around 0.5% and is becoming increasingly stable. The operational failure rate is 20 × 10⁻⁶, equivalent to one failure per 50,000 operations, which is about 20 times lower than that of high-voltage circuit breakers. In 1984, ABB launched the first HE-type SF6 generator circuit breaker, using SF6 gas as the arc-extinguishing medium, while the operating mechanism remained pneumatic. It utilizes the self-extinguishing principle of SF6 gas. The electric arc generated when the contacts separate heats the SF6 gas, causing it to expand and form the gas required for arc extinguishing. Simultaneously, the current flowing through the coil in the fixed contacts generates a magnetic field, causing the arc to rotate, thus minimizing burns to the contacts. Furthermore, the relatively independent load contacts and arc-extinguishing contacts ensure continuous current carrying capacity. In March 1996, ABB's HEC-8 generator circuit breaker passed the KEMA test in the Netherlands. This circuit breaker has a rated current of up to 24,000A and a breaking capacity of 160kA, making it possible to use SF6 circuit breakers at the outlet of a 600MW generator. The use of SF6 makes the overall circuit breaker structure more compact and reduces the failure rate. ABB's 1200 HE-type (HEC and HEK) generator circuit breakers have a failure rate of less than 0.3%. Modern generator circuit breakers are no longer just circuit breakers; they integrate voltage transformers, current transformers, disconnect switches, grounding switches, and other equipment between the generator and the main transformer, becoming multi-functional combined electrical appliances. Besides ABB, companies like GEC-ALSTOM (France) and Mitsubishi (Japan) have also developed SF6 generator circuit breakers. GEC-ALSTOM has a long history of generator manufacturing, and while it also produces SF6 generator circuit breakers, it specializes in air-cooled generator circuit breakers. Its SF6 generator circuit breaker development has been slower, and the capacity is also smaller. The maximum parameters of an air-cooled generator circuit breaker are: rated voltage 36kV, rated current 37500A, and breaking current 275kA. Air-cooled generator circuit breakers utilize compressed air as the insulating and arc-extinguishing medium, as well as the operating and cooling air for the circuit breaker. Although air-cooled generator circuit breakers are a highly versatile and comprehensive product, they are large in size, occupy a lot of space, are expensive, and require a separate compressed air system. Mitsubishi Electric Corporation of Japan began producing generator circuit breakers in 1977 using SF6 gas as the insulating medium. The maximum breaking current is 110kA, with a breaking capacity of 10 times; the breaking capacity is 50 times when the rated breaking current is 42000A. Currently, generator circuit breakers with a breaking current of 125kA are under construction. In addition to SF6 generator circuit breakers and air-type generator circuit breakers using SF6 gas and air as the insulating medium, vacuum-type generator circuit breakers were developed in the late 1980s and early 1990s using vacuum technology. These generator circuit breakers were developed and manufactured by Siemens of Germany. The 8BK40 and 8BK41 series are vacuum-type generator circuit breakers manufactured by Siemens, with rated voltages of 7.2–17.5kV. The 8BK40 has a rated current >5000A and a maximum breaking current of 63kA; the 8BK41 has a rated current of 4000–12000A and a maximum breaking current of 80kA. Vacuum generator circuit breakers are limited in their breaking capacity; manufacturing vacuum circuit breakers with breaking currents greater than 63kA is extremely difficult and expensive. Therefore, they are currently used more in small-capacity units, with limited application in large units. The development level of generator circuit breakers in China is relatively low. Currently, only Shenyang High Voltage Switchgear Factory produces generator circuit breakers. In the 1980s, it introduced manufacturing technology from GEC-ALSTOM of France and began developing the PKG2 type generator circuit breaker. From 1986 to 1988, 14 of these circuit breakers were put into operation at the Gezhouba Hydropower Plant. Overall, their operation was good, but the product was large, noisy, and required significant maintenance. It is understood that the Gezhouba Hydropower Plant has replaced all 14 circuit breakers with ABB's SF6 circuit breakers. Currently, Shenyang High Voltage Switchgear Factory is actively developing and manufacturing SF6 generator circuit breakers. Due to the unique nature of generator circuit breakers, in addition to meeting existing switch manufacturing standards such as IEC56-1987 (High-voltage altemating-current circuit breakers), IEEE/ANSI-C37.08-1987, IEC694-1980 (Common clauses for high-voltage switch-gear and control-gear standards), and IEC298-1990 (AC metal-enclosed switch-gear and control gear for rated voltages above 1kV and up to and including 52kV), IEEE also issued a specific manufacturing standard for generator circuit breakers in 1993: IEEE C37.013-1993 (ANSI/IEEE Standard for AC high-voltage generator circuit breakers rated on a symmetrical current basis), thus standardizing the manufacturing, testing, and installation of modern generator circuit breakers. China has successively promulgated corresponding industry standards and national standards: DL427-92 "Technical Conditions for Ordering Indoor AC Generator Circuit Breakers" and GB/T14824 "General Technical Standards for Generator Circuit Breakers". 2. Application of Generator Circuit Breakers in Power Plants Since the invention of the first generator circuit breaker in 1969, generator circuit breakers have been widely used in countries around the world. According to a survey conducted by CIGRE, more than 50% of nuclear power plants and more than 10% of thermal power plants worldwide currently use generator circuit breakers. The use of generator circuit breakers has become a trend, especially in nuclear power plants and large-capacity thermal power plants with high requirements for the reliability of their power systems. In developed countries such as the United States, the United Kingdom, and France, where electricity is mainly generated by nuclear power and pumped storage power plants with large unit capacities, circuit breakers are installed at the generator outlets. In Germany, newly constructed nuclear power plants with a capacity greater than 259 MVA and thermal power plants with a capacity greater than 588 MVA also have generator circuit breakers installed in their generators. Design codes in countries like Russia and Finland explicitly mandate the installation of outlet circuit breakers in thermal power plants. Article 8.9 of Russia's "Technical Code for Thermal Power Plant Design" stipulates that "for each generator with a single unit capacity of 300MW or more…in all cases of unit connection, a circuit breaker shall be installed between the generator and the transformer…". Therefore, in Russia, all units of 300MW and above are equipped with circuit breakers or load switches. For this reason, projects imported from Russia in China, such as the Tianjin Panshan Power Plant (2×500MW), Yimin Power Plant (2×500MW), Liaoning Suizhong Power Plant (2×800MW), and Liaoning Yingkou Power Plant, all have installed load switches. The reason for using load switches instead of outlet circuit breakers is that Russia only produces open-type air circuit breakers, which cannot completely seal the main circuit, reducing the reliability of the unit's operation. Load switches, on the other hand, are enclosed and can be connected to the enclosed busbar at both ends of the casing, completely sealing the generator's main circuit, preventing three-phase short circuits at the generator outlet, and improving the unit's reliability. Relevant design codes in my country also stipulate the principles for using generator circuit breakers. Article 12.1.6 of the "Design Code for Small Thermal Power Plants" (GB50049-94) states, "When a generator and a two-winding transformer are connected as a unit, for heating units, a circuit breaker may be installed between the generator and the transformer; for condensing units, a circuit breaker should not be installed. When a generator and a three-winding transformer are connected as a unit, a circuit breaker and a disconnecting switch should preferably be installed between the generator and the transformer. Plant service branch lines should be connected between the transformer and the circuit breaker." Article 11.2.6 of the "Technical Code for Design of Thermal Power Plants" (DL500-94) states, "When a generator with a capacity of 125MW or less is connected as a unit with a two-winding transformer, a circuit breaker should not be installed between the generator and the transformer; when a generator and a three-winding transformer or autotransformer are connected as a unit..." When connecting generators and transformers, circuit breakers and disconnect switches should be installed between them, and plant service branch lines should be connected between the transformer and the circuit breaker. When a generator with a capacity of 200-300MW is connected to a double-winding transformer as a unit, a circuit breaker, load switch, or disconnect switch should not be installed between the generator and the transformer. For a 600MW generator, if the voltage increase is only 330kV or higher and is technically and economically reasonable, a generator outlet circuit breaker or load switch may be installed. When two generators are connected to one transformer or two sets of generator double-winding transformers as an expanded unit, a circuit breaker and disconnect switch should be installed between the generator and the transformer. In recent years, generator circuit breakers have been widely installed in domestic nuclear power plants, hydropower plants, and newly constructed thermal power plants. Due to their importance, nuclear power units all have circuit breakers installed at the generator outlet, such as the Qinshan and Daya Bay nuclear power plants. Hydropower units, due to their need for peak shaving and frequent starts, are all equipped with generator circuit breakers, such as the recently constructed Ertan Hydropower Station in Sichuan, Yantan Hydropower Station in Guangxi, Lijiaxia Hydropower Station in Qinghai, Xiaolangdi Hydropower Station on the Yellow River, and Conghua Hydropower Station in Guangdong. Among thermal power plants, of the 14 600MW units put into operation by the end of March 1998, only the three units at the Shajiao C Hydropower Plant in Guangdong were equipped with generator circuit breakers; the other units were not. However, most thermal power plants currently in the early design phase have generator circuit breaker plans, such as the Qinbei Hydropower Plant in Henan and the Huanggang Hydropower Plant in Hubei. Therefore, the use of generator circuit breakers in power plants in China, especially in large-capacity units with high safety requirements, is gradually becoming a trend. 3. Main Functions of Generator Circuit Breakers The main functions of installing generator circuit breakers in power plants are to simplify operating procedures, reduce the scope of generator and transformer accidents, simplify synchronization operations, improve reliability, and facilitate commissioning and maintenance. 3.1 Simplifying Plant Auxiliary Power Switching/Operating Procedures Currently, in China's 300MW and below units and some 600MW thermal power plants, dedicated starting/standby transformers are installed. Whether it's normal unit startup or shutdown, or due to a fault or maintenance of the auxiliary power transformer, a switch in auxiliary power supply is required. During normal generator startup, starting power is first obtained through the starting/standby transformer. Once the generator establishes normal voltage and carries a certain load, power is switched to the auxiliary power transformer via the auxiliary power switching device. The shutdown process is the reverse. Therefore, in power plants without generator circuit breakers, normal startup and shutdown inevitably require a connection switch between the auxiliary power transformer and the starting/standby transformer. Since the auxiliary power transformer and the starting/standby transformer draw power from different systems, and their impedance values ​​are different, an initial phase difference exists between the low-voltage side busbars of the two transformers. Due to this initial phase difference, a significant circulating current will be generated between the two transformers during normal parallel switching. In severe cases, circulating currents can reach thousands of amperes. Such large circulating currents, even if they do not damage the transformer during parallel switching, will have a cumulative impact on its lifespan. This poses a significant threat to the safe operation of transformers. (Please visit: Power Transmission and Distribution Equipment Network for more information.) During emergency switching of power plant auxiliary power, there is also a phase difference between the working bus voltage and the starting/standby bus voltage during normal auxiliary power switching. Excessive phase difference makes it difficult to guarantee successful emergency switching and can directly harm equipment. For example, during rapid emergency switching, if the allowed phase difference setting is too large (exceeding 40°), the transient inrush current to the high-voltage motor can reach 18 times the rated value, potentially causing damage to the high-voltage motor, which is unacceptable for safe operation. Even if the phase difference is set within the allowable range, the overvoltage, overcurrent, and overload caused by frequent auxiliary power switching will still adversely affect the service life and safe operation of equipment. Therefore, reducing or avoiding auxiliary power switching will improve the safety and reliability of power plant operation. After adopting a generator circuit breaker, the generator set's start-up and shutdown power supply is obtained by backfeeding power from the main transformer to the station service transformer. From unit startup to generator grid connection, no station service power switching is required. Station service power switching is only necessary when the station service transformer or main transformer fails. Analysis results show that using a generator circuit breaker reduces station service power switching to approximately 1/348, significantly improving the power plant's safety and reliability. Simultaneously, it greatly reduces the difficulty of operating and maintaining station service power. 3.2 Improving the Protection Level of Generators and Transformers Using a generator circuit breaker improves the selectivity of protection in the event of operational faults, system oscillations, or short-circuit faults in the generator or transformer, thereby enhancing the safety and reliability of unit operation. Operational faults or system oscillations cause power fluctuations between the generator and the grid, and unbalanced currents can cause overheating of the generator rotor windings. After a fault occurs, simply disconnecting the generator circuit breaker eliminates the need for station service power switching. After the fault disappears, the generator and the grid can be quickly reconnected via the generator circuit breaker, avoiding a plant-wide blackout caused by a power supply switching failure. Similarly, when an internal generator fault occurs, the generator circuit breaker can disconnect the fault without switching the power supply, ensuring a safe shutdown. The use of the generator circuit breaker not only allows for separate and selective protection tripping of the generator and transformer, simplifying protection wiring, but also eliminates the need to trip the high-voltage circuit breaker for internal unit faults, thus avoiding power supply switching. This is highly beneficial for eliminating transient faults, especially those originating from boilers and turbines, quickly restoring unit operation, and preventing losses due to misoperation. According to the experience of Shajiao C Power Plant, the three units tripped more than 800 times during commissioning, and in most cases, unit operation could be restored within tens of minutes. Another significant benefit of selectively tripping the generator circuit breaker after a fault occurs, reducing the probability of tripping the high-voltage circuit breaker, is that it avoids or reduces damage to the generator caused by incomplete phase operation of the high-voltage circuit breaker. In reality, for generator-transformer unit wiring, the high-voltage circuit breakers, due to their high rated voltage (220-500kV) and large phase-to-phase distances of open-type circuit breakers, cannot be configured for three-phase mechanical linkage. Furthermore, each phase circuit breaker often has multiple breaks, and non-full-phase operation of the high-voltage circuit breaker frequently occurs even during normal operation. Undoubtedly, non-full-phase operation (running) of the high-voltage circuit breaker will generate negative sequence current on the generator stator, and the generator rotor's ability to withstand negative sequence magnetic fields is very limited (the negative sequence operation limit (I2/TN) 2t under generator fault conditions is approximately 8s), which can lead to rotor damage in severe cases. Such accidents have occurred repeatedly both domestically and internationally. In my country, 50 such accidents occurred between 1980 and the end of 1992, with 12 occurring in 1991 alone. Each such accident resulted in severe rotor damage. For example, on June 12, 1990, the high-voltage side circuit breaker B phase of the No. 1 generator-transformer unit at Shouyangshan Power Plant failed to open, causing the generator rotor to overheat and a 9.1mm wide crack to appear in the rotor retaining ring. The direct cause of this serious consequence is that the high-voltage circuit breaker is not three-phase mechanically linked, making it prone to incomplete phase operation. Currently, generator circuit breakers are designed and manufactured with three-phase mechanical linkage in mind, preventing incomplete phase operation. Furthermore, the fast-acting characteristics of generator circuit breakers are also crucial for ensuring generator set safety. The inherent operating time of a generator circuit breaker, including the protection operating time, is approximately 75ms. When a fault occurs (such as a single-phase or two-phase fault), the generator circuit breaker will quickly operate and disconnect the fault, effectively preventing damage to the generator set. Conversely, without a generator circuit breaker, the generator will continue to supply unbalanced current until the demagnetization process is complete. The demagnetization process can last for several seconds (5-20s), during which time the generator will suffer severe damage. This is because research shows that, according to national standards, the critical time limits for single-phase operation in generator sets are: approximately 70s under no-load conditions; approximately 6s under full load; and approximately 4.5s when the main transformer and common high-voltage side are short-circuited (two-phase/three-phase). Therefore, without a generator circuit breaker, the duration of the demagnetization process (especially in brushless excitation systems) (5-20 seconds) significantly exceeds the generator set's tolerance time. Similarly, due to the rapid-action characteristics of the generator circuit breaker, the short-circuit current supplied by the generator is limited. Therefore, for transformer high-voltage side bushing grounding faults (30% probability) or internal transformer faults, the generator circuit breaker will reduce the hazard and effectively improve safety and reliability. 3.3 Simplified Synchronization Operation, Facilitating Maintenance and Commissioning When using a high-voltage circuit breaker for synchronization, the circuit breaker will be subjected to voltage stress. Under contamination conditions, this voltage stress can cause flashover of the external insulation medium of the circuit breaker. When synchronization is performed at the generator voltage level, the voltage stress on the high-voltage circuit breaker disappears. Using a generator circuit breaker for synchronization compares the voltages of the same level on both sides of the generator circuit breaker, thus making the synchronization system simpler and more reliable. Furthermore, since the generator circuit breaker is installed in an indoor enclosed metal casing, the environmental conditions are good, and its sufficient insulation safety margin ensures more reliable synchronization operation. The generator circuit breaker separates the generator-transformer unit into two parts: the generator section and the transformer section. This electrical separation is achieved by combining the generator circuit breaker/disconnector, allowing different circuit breakers to be tested in groups and at each stage. Furthermore, when the plant's auxiliary power is supplied by the main transformer, the generator can be tested under under-excitation conditions. This physical separation achieved by the generator circuit breaker provides greater convenience for the commissioning and maintenance of the generator and transformer. The generator circuit breaker also facilitates generator short-circuit testing. 3.4 Adapting to the Need for Separation of Power Plant and Grid The reform of power industry management advocates the separation of power plant and grid, and competitive bidding for grid connection. This requires power plants to pay a basic electricity fee in addition to electricity consumption fees for their starting/standby transformers. This basic electricity fee is related to the capacity of the starting/standby transformer, and its amount is 8-12 yuan per kVA per month. For example, a 40MVA transformer would have to pay 3.84-5.76 million yuan in basic electricity fees annually. In today's investment environment where investors increasingly focus on investment efficiency, this issue cannot be ignored. The preferred solution is to reduce the capacity of the starting/standby transformer. One method to reduce the capacity of starting/standby transformers is to install generator circuit breakers. Power is fed back from the main transformer to the high-voltage station service transformer to provide start-up/shutdown power. Only a standby transformer is installed for emergency shutdowns, or no standby transformer is required at all. This reduces the number and capacity of transformers, lowers operating costs, and improves economic efficiency. 4. Application Prospects of Generator Circuit Breakers In summary, the development and application of generator circuit breakers both domestically and internationally are very rapid. They have evolved from the original compressed air type to SF6 type and are now more widely used in thermal power plants, hydropower plants, nuclear power plants, pumped storage power plants, and other power plants. The technical level of generator circuit breakers is constantly improving; their size is decreasing, noise is decreasing, while rated current and breaking current are increasing. Furthermore, the mechanical life of generator circuit breakers is also increasing, exceeding 10,000 cycles for high-voltage circuit breakers, far exceeding the 3,000-5,000 cycles of ordinary high-voltage circuit breakers. Due to the competitive development efforts of various companies, the structural design of generator circuit breakers has undergone significant innovation, evolving from compressed air type to SF6 gas type and vacuum type. The SF6 gas type has further developed from dual-pressure type to single-pressure type, self-extinguishing arc type, and compressed air plus self-extinguishing arc type generator circuit breakers. With continuous improvements in development and manufacturing capabilities, the configuration and protection performance of generator circuit breakers have become more sophisticated, and their reliability has greatly improved. These advancements in development and manufacturing capabilities have led to the wider application of generator circuit breakers, and the broad market prospects have in turn driven continuous improvements in generator circuit breaker technology. The widespread use of generator circuit breakers has gradually become a trend.