I. Introduction Circuit breaker failure protection refers to the protection system that detects circuit breaker failure when the relay protection of a faulty electrical device issues a trip command but the circuit breaker fails to operate. It utilizes the protection action information of the faulty device and the current information of the refusing circuit breaker to determine the circuit breaker failure. This allows for the rapid disconnection of other related circuit breakers within the same substation, minimizing the power outage area and ensuring the stable operation of the entire power grid. It also prevents severe burnout of faulty components such as generators and transformers, and avoids grid collapse. Circuit breaker failure to operate represents a double fault—a power grid fault compounded by circuit breaker malfunction. While it's permissible to appropriately reduce protection requirements, the principle must be that the fault can ultimately be cleared. In modern high-voltage and ultra-high-voltage power grids, circuit breaker failure protection is widely used as a near-backup protection method. II. Basic Components and Functions of Failure Protection Failure protection consists of a relay interlocking element, a starting circuit composed of protection action and current discrimination, a timing element, and a trip output circuit. The starting circuit is crucial for ensuring the correct operation of the entire protection system. It must be safe and reliable, and should implement dual discrimination to prevent malfunctions caused by a single condition indicating circuit breaker failure, as well as false starts due to jammed or unreturned protection contacts, accidental contact, or accidental energization. The starting circuit includes a starting element and a discrimination element; the two elements form an AND logic, as shown in Figure 1. The starting element typically utilizes the circuit breaker's automatic trip output circuit itself. It can be directly used with the instantaneously returning output trip relay contact, or with the instantaneously returning auxiliary intermediate relay contact connected in parallel with the output trip relay. Failure of the contact to return indicates circuit breaker failure. The discrimination element identifies whether the fault has not been eliminated in different ways. Existing operating equipment uses a "current present" discrimination method based on phase current (line) and zero-sequence current (transformer). After the protection operates, the presence of current in the circuit indicates that the fault has not been eliminated. The time element is an intermediate link in circuit breaker failure protection. To prevent false trips caused by a single time element failure, the time element should form an AND logic with the starting circuit before activating the output relay. Voltage blocking for line failure protection generally consists of busbar undervoltage, negative sequence voltage, and zero sequence low voltage relays. When the failure protection and bus differential protection share the same tripping circuit, they also share the same voltage blocking element. III. Main Problems and Improvement Measures (I) Problems with Line Failure Protection Conventional circuit breaker failure protection uses phase current elements that can quickly reset as the discrimination element for circuit breaker failure. The contacts of this discrimination relay cooperate with the protection contacts to form single-phase tripping and three-phase tripping failure initiation circuits respectively. The purpose of adding discrimination elements is to prevent the protection output contacts from getting stuck and not returning, or from accidental contact or energization, which would cause the switch failure protection to be falsely activated, thus making the failure protection work safer and more reliable. However, in the actual setting process, because the system operation mode and the fact that the phase current element should still have sufficient sensitivity when the line end fault occurs after the bus tie switch trips, it is difficult for its setting value to avoid the load current during normal operation. This results in the current discrimination element being in an active state during normal line operation, and therefore, it does not play a role in preventing false tripping. In fact, before the addition of composite voltage blocking, the system may experience maloperation of the failure protection due to reasons such as forgetting to disconnect the start failure connection (the switch failure current discrimination element is in the activated state) during transmission protection. If the phase current discrimination element of the failure protection does not operate during normal operation, these maloperations can be completely avoided. In addition, for electromagnetic relays, when the load current is close to the set value, it will cause the relay tongue and contacts to jitter. After a long period of operation, the relay shaft will fall off, causing the failure protection to fail to operate. (II) Problems and solutions of failure protection for generator-transformer units and transformers When an internal fault occurs on the low-voltage side of the transformer (or the high-voltage switch of the generator-transformer unit experiences a phase loss operation), the failure protection voltage blocking element installed in the bus differential protection, which only reflects the composite voltage on the 220 kV side, often cannot be opened. Therefore, in addition to paying attention to separating the gas protection (or other protection with delayed return contacts) output and electrical quantity output, attention should also be paid to the unlocking of the composite voltage blocking element for transformer and generator-transformer unit start failure protection. The following measures can be taken. 1. For a 220 kV generator-transformer unit, unlocking can be achieved by connecting a current discrimination element, a protection output, and a normally open contact of a closing position relay in series to form an AND gate. The current discrimination element can be constructed by connecting zero-sequence current and phase current in parallel (or a gate); the protection output is the output of the high-voltage side switch. Additionally, a pressure plate can be added to the unlocking circuit to disconnect the unlocking circuit during high-voltage switch maintenance in certain special circumstances. 2. For transformer failure protection, unlocking can be achieved by connecting a current discrimination element, a protection output, and a composite voltage blocking contact in series to form an AND gate. The current discrimination element can be constructed by connecting zero-sequence current and phase current in parallel (or a gate); the protection output is the output of the high-voltage side switch; the composite voltage blocking contact should be a composite voltage contact on the low-voltage side, and the voltage contact should return after a delay. The voltage blocking contact includes the low-voltage side voltage, mainly to prevent the high-voltage side composite voltage element from lacking sensitivity and failing to open the failure protection when there is a low-voltage side fault; the delayed return is mainly... This is because if the transformer differential protection trips the low-voltage switch, the voltage on the low-voltage bus may immediately return to normal (e.g., there is a small power supply on the low-voltage side of the transformer or the low-voltage side of the transformer is operating in parallel), thus failing to achieve the purpose of opening and blocking. The delay time should ensure that even if a fault occurs within the low-voltage side zone, the differential protection or low-voltage side backup protection has sufficient time to initiate the failure protection to trip all components on the bus where the faulty transformer is located. In other words, the delay time should be greater than the difference between the setting time for tripping the low-voltage switch and the tripping time of the three-sided switch after the low-voltage side protection outputs (generally 0.3 seconds). (s～0.5s), plus the time for all components on the faulty transformer bus to trip after the failure protection starts (generally 0.5s), considering a certain margin, 3s is generally sufficient. The above method ensures that there is voltage control when there is a mis-transmission, and "current discrimination" and "protection output" control when the external fault voltage is opened. The advantage of this method is that it can also unlock when the three phases of the high-voltage switch fail. In addition, when the low-voltage switch of the transformer is under maintenance, the low-voltage bus may lose voltage. At this time, the voltage blocking in the unlocking circuit will be opened. Therefore, a pressure plate can also be connected in series in the unlocking circuit to disconnect the unlocking circuit. 3. Problem of low sensitivity of current discrimination element The current discrimination element of the circuit breaker failure protection should meet the requirement of not operating when the system is running normally and after the faulty line switch is opened, and at the same time, it should have sufficient sensitivity when various faults occur at the end of the line. This is so that the current discrimination element can play the role of output control. The following two methods can be adopted: 1) Use the current change in quantity to start the element to logically block the three phase current elements; 2) The positive power supply for controlling the failure-starting current relay is controlled by a current surge initiation element. In this way, during normal system operation, the failure-starting current discrimination element will not operate because the current surge initiation element does not operate; when a system fault occurs, the current surge initiation element operates and then extends the current discrimination circuit for a period of time (greater than the backup protection time, e.g., 7 seconds). The current surge initiation element (composed of positive and negative sequence currents) should ensure sufficient sensitivity and reliable starting when a fault occurs at the end of the line. The failure protection current discrimination circuit constructed using the above method ensures that it will not operate during normal operation due to the current surge initiation element, and that it will not operate after the switch is opened due to the phase current element, thereby improving the safety of failure protection during normal system operation. When a circuit breaker fails, the current discrimination element used to determine the failure must operate reliably to ensure the failure protection trips. For generators and transformers, when an internal inter-turn short-circuit fault occurs, although the differential protection can trip, the fault current sensed by the current measuring element at the high-voltage side circuit breaker is not large enough to reach the tripping value of the "current present" current discrimination element for circuit breaker failure. Thus, the failure protection cannot be guaranteed to operate correctly when the high-voltage side circuit breaker fails. Because the magnitude of the fault current sensed by the current measuring element at the high-voltage side circuit breaker during an internal inter-turn short-circuit fault in a generator or transformer is highly uncertain and greatly dependent on the number of short-circuited turns, it is unlikely that the "current present" discrimination element can sensitively detect this fault and distinguish between a faulty and non-faulty circuit. Therefore, the author believes that the setting value of this current discrimination element should be set very small, so that it operates as soon as current flows through the circuit breaker and reliably returns after the circuit breaker trips. With the continuous expansion of the application scope of microprocessor-based protection, the setting accuracy of the circuit breaker failure current discrimination element will be very high. Moreover, since this current discrimination element is implemented in software, it can effectively avoid incorrect operation states of the current element contacts caused by contact sticking, similar to current relay contacts. The setting value of the failure current discrimination element should have the same setting value as the "no-current discrimination element" commonly referred to in line microprocessor-based protection, or even higher. It may not require user setting. IV. Several issues to note when applying circuit breaker failure protection: 1. Using non-electrical quantity protection as the starting quantity for circuit breaker failure protection is inappropriate. Failure protection should not be activated at the main transformer heavy gas, pressure relief, or generator water shortage protection outputs. This is because the contact action and return time of non-electrical quantity protection are slow, resulting in poor reliability of initiating failure protection; when non-electrical quantity protection operates, sometimes the current does not increase rapidly enough to reach the failure initiation current value, and the failure protection will not activate in this case. It is recommended to cancel the design of the generator water shortage protection output as a failure protection initiation setting. 2. Backup protection cannot directly initiate failure protection. Designing generator inverse-time symmetrical overload protection, inverse-time asymmetrical overload protection, and overexcitation protection to initiate failure protection at the output is inappropriate and fundamentally flawed. The concept of "programmed tripping" is that when the protection trips, the turbine main steam valve is closed first. Once the generator experiences reverse power and reaches the reverse power setpoint, and the main steam valve closing contact closes, the programmed reverse power protection completes the demagnetization and disconnection. The closing of the turbine main steam valve and the generation of reverse power by the generator is a complex physical process, generally exceeding 1 second. The failure protection's operating time is typically set to trip the bus tie at 0.3 seconds and the main circuit breaker at 0.5 seconds. Therefore, simultaneously initiating failure protection with the protection's programmed tripping will inevitably cause the failure protection to trip erroneously in the event of a generator-transformer unit fault, expanding the scope of the accident. 3. Auxiliary protection should not initiate failure protection. For example, if the main transformer cooler complete shutdown protection is used as an auxiliary protection for the main transformer, once this protection trips and demagnetizes, the protection contacts will not return for a short time. The cooler operation or backup power must be manually restored before the protection contacts can return, which can easily cause malfunctions. Therefore, this type of protection should not initiate failure protection. Some power plants are designed to initiate failure protection; it is recommended to correct this. 4. Differentiate between generator-transformer unit failure protection and line failure protection. First, because there are many types of failure protection that initiate protection in large generator-transformer units, and the principles of each protection are different, the action and return times of various protections are also different, some faster and some slower. Second, generator-transformer units generally use three-phase linkage switches, which have a longer action time than line phase-by-phase operating switches. For example, the opening time of the LW6-220 type SF6 three-phase linkage switch is no more than 38 ms, while the opening time of the LW6-220 type SF6 phase-by-phase operating switch is no more than 28 ms. Therefore, in practical applications, it is necessary to distinguish between component-based failure protection and line-based failure protection to improve the reliability of the protection. V. Conclusion The low commissioning rate of circuit breaker failure protection in large generating units is due to various factors, among which non-standard design of circuit breaker failure protection leading to low reliability of its operation is the main reason why failure protection cannot be properly commissioned. The requirements for failure protection are: 1. The protection of a certain circuit breaker has indeed started and does not reset; 2. It is determined that the circuit breaker has indeed not been disconnected; 3. Add fault discrimination elements. At the same time, to improve reliability, the contacts of the discrimination elements should be connected in the output tripping circuit and a "one-to-one" wiring method should be adopted, that is, each tripping circuit has a pair of discrimination element contacts in series, avoiding one pair of discrimination element contacts controlling several tripping circuits; 4. Reclosing should be blocked after the failure protection operates to prevent reclosing to the fault.