Abstract: This paper details the purpose, wiring method and setting principle of each protection configuration in the typical design of dual configuration of microprocessor transformer protection, and further discusses the transformer failure protection, non-electrical quantity protection countermeasures and the simplification of transformer protection pressure plates. Keywords: Transformer protection; Dual configuration; Protection configuration 1 Introduction According to the spirit of the State Power Corporation's "Twenty-Five Key Requirements for Preventing Major Accidents in Power Production" that "the microprocessor protection of 220kV main transformers must adopt dual configuration", and in combination with the implementation of countermeasures, design standardization and on-site operation safety, a seminar was organized with the participation of production, design and operation departments. The seminar fully discussed how to make the protection configuration and setting operation more unified, reasonable and practical on the basis of safety and reliability, and formulated the "Typical Design of Dual Configuration of Microprocessor Transformer Protection" suitable for step-down transformers in the 220kV system of Anhui Province. The basic principles are briefly introduced as follows. 2 Protection Panel Method [1] The basic principle of the panel method is: mutual independence, safety and reliability, and taking into account the flexibility and convenience of starting, stopping and maintenance. The primary consideration is that even if one protection system fails, the other can still effectively protect the transformer. Secondly, the design minimizes the wiring between the two panels to reduce errors or hidden dangers caused by complex secondary circuit wiring, which could lead to malfunctions. Statistical analysis of main transformer protection actions nationwide over the past two years shows that the complexity of the secondary circuit is one of the main causes of incorrect main transformer protection operation. Therefore, the design adopts a dual-main-dual-backup configuration, meaning it is designed with two panels, each equipped with one main protection system and one identical backup protection system. One panel, in addition to the main and backup protections, also includes non-electrical quantity, failure, and incomplete phase protection, a 110kV side operating box (voltage switching box), and a 35kV (or 6kV) side operating box. The other panel, in addition to the main and backup protections, also includes a 220kV side operating box (voltage switching box). 3. Main Protection Configuration The design selects differential protection with second harmonic restraint and transformer differential protection based on waveform symmetry as the main protections, leveraging their respective advantages for complementarity. Most mature transformer differential protection systems utilize the second harmonic braking principle to avoid inrush current. However, with this principle, when a single-phase or two-phase internal fault occurs during no-load energization of the transformer, the differential protection fails to operate due to inrush current braking. Large transformers have long time constants, with inrush current processes typically exceeding 5 seconds. In the event of such faults, the main protection must wait for the inrush current to disappear before issuing an alert, delaying its operation. In contrast, transformer differential protection based on waveform symmetry uses a waveform symmetry algorithm to distinguish between the inrush current and fault current generated during no-load energization of the transformer. Specifically, the differential current flowing into the relay is differentiated, and the first and second half-waves of the differentiated differential current are compared symmetrically. When a fault occurs during transformer energization, the waveform symmetry principle is used to calculate that the protection is unaffected by the healthy phase, enabling rapid alerts and reliable operation. Dynamic model tests conducted in East and North China also illustrate this point. In tests involving transformer no-load closing and inter-turn faults of 5%, the differential protection based on the second harmonic braking principle typically had an output time of 100ms, while the output time of the transformer differential protection based on the waveform symmetry principle was around 25ms. Furthermore, zero-differential protection has high sensitivity to transformer faults, especially internal faults in autotransformers, and is unaffected by inrush current. However, due to the difficulty of conducting polarity tests in the field, and the numerous instances of zero-differential protection malfunctions in Anhui Province, the principle for setting up zero-differential protection is: if the device has an automatic polarity verification function, it can be used; otherwise, it is recommended not to use it. 4. Configuration of Backup Protection The principle for configuring backup protection is to ensure safe and reliable operation in situations where faults occur on the medium and low voltage busbars of the transformer, and the protection or circuit breaker fails to operate, thus preventing fault isolation. It also aims to minimize the probability of maloperation when only one protection system is operational due to reasons such as a faulty protection system or bypass circuit breaker failure. Therefore, we require that the configuration of the main transformer backup protection ensures that there are backup isolation methods for faults occurring at various electrical locations from the independent current transformer (CT) on the high voltage side of the main transformer to the medium and low voltage busbars. This configuration should meet the grid stability requirements under various operating and maintenance modes and possess remote backup functionality for adjacent electrical equipment. The specific configuration is as follows: 4.1 220kV Side Protection Configuration 4.1.1 Overcurrent and Zero-Sequence Overcurrent Protection with Overvoltage Interlock: This is a two-time-limited protection section (Section I). The first time-limited protection trips the circuit breaker on this side, and the second time-limited protection trips all three circuit breakers. The protection settings should have sufficient sensitivity for the medium and low voltage busbars, thus ensuring that the main power supply side has a protection section with sufficient sensitivity for all three sides. To prevent the high-voltage busbar voltage from failing to drop due to a fault on the low-voltage side, the interlocking function is implemented using a three-side voltage parallel method. The voltage on each side can be flexibly switched on and off via pressure plates according to changes in the operating mode. The main function of this protection is to act as backup when the medium and low voltage side protection or switch fails to operate. 4.1.2 Directional Overcurrent and Zero-Sequence Directional Overcurrent Protection with Overvoltage Interlock: These are two-time-limited protection sections (Section I). The first time-limited protection trips the circuit breaker on this side, and the second time-limited protection trips all three circuit breakers. For transformers operating under pure load or with power supply on the medium-voltage side, the direction of the power supply is towards the transformer. The setting can be coordinated only with the medium-voltage side overcurrent protection (interlocking directional overcurrent and zero-sequence directional overcurrent protection, Stage I). Therefore, for severe faults occurring on the leads between the transformer switch's independent current transformer (CT) and the transformer, or on a portion of the transformer's high-voltage winding, the clearing time can be shorter. 4.1.3 When bypassing: Only one set of main and downstream protections is switched to the bypass circuit breaker. This is mainly due to the following reasons: First, the probability of bypassing is low, and the duration is short; second, switching too many circuit breakers is unsafe. According to years of differential protection malfunction analysis in Anhui Province, improper operation by operators accounts for one-quarter of all incorrect differential protection actions; third, previously operating transformers were only equipped with one set of protection, and taking the line protection with relatively mature operating experience as an example, dual sets of protection were configured during operation on this line, and only one set of protection was switched during bypassing. Based on past operating experience, there have been no cases of protection malfunctions during bypassing due to a single-set configuration. 4.1.4 Neutral Point Gap Zero-Sequence Overcurrent and Zero-Sequence Overvoltage Protection (Three-Winding Transformer): One-stage, one-time-limit protection; the protection trips all three circuit breakers after a delay. 4.2 110kV Side Protection Configuration 4.2.1 Overcurrent and Zero-Sequence Overcurrent Protection with Overvoltage Interlock: One-stage, two-time-limit protection; the first-time-limit protection trips the local switch, and the second-time-limit protection trips all three circuit breakers. The protection settings coordinate with the 110kV outgoing line backup protection and ensure sensitivity on the busbar opposite the 110kV outgoing line. Its purpose is to clear the fault when the 110kV outgoing line protection or switch fails to operate. Because the 110kV system does not have failure protection, a switch failure could lead to an escalation of the accident. 4.2.2 Directional Overcurrent and Zero-Sequence Directional Overcurrent Protection with Overvoltage Interlock: One-stage, two-time-limit protection; the first time-limit trips the bus tie or sectionalizing circuit breaker, and the second time-limit trips the local circuit breaker. For a 110kV side pure load transformer or one with a small power source, its direction should be towards the 110kV busbar. The protection settings should coordinate with the 110kV outgoing line protection (Stage I) to ensure sensitivity to 110kV busbar faults and serve as backup for the 110kV busbar when the 110kV bus differential protection is out of service or fails to operate. For a 110kV side with a stronger power source, the overcurrent protection with overvoltage blocking and zero-sequence directional overcurrent protection on one of the two panels can be directed towards the transformer. The protection settings should ensure sensitivity to the 220kV busbar, ensuring rapid power disconnection when the 220kV bus differential protection is out of service or fails to operate. The overcurrent protection with overvoltage blocking and zero-sequence directional overcurrent protection on the other panel should still be directed towards the 110kV busbar. (Source: Power Transmission and Distribution Equipment Network) 4.2.3 When bypassing: Only one set of main and downstream protections should be switched to the bypass. 4.2.4 Neutral Point Gap Zero-Sequence Overcurrent and Zero-Sequence Overvoltage Protection (Three-Wave Transformer): One-stage, one-time-delay protection; the protection trips all three circuit breakers after a delay. 4.3 35kV Side Protection Configuration 4.3.1 Low-Voltage Side Overcurrent Protection with Interlocking: Two identical sets of low-voltage interlocking overcurrent protection are configured, each with two time delays. The first set trips the circuit breaker on its own side with the first time delay protection, and trips all three circuit breakers on the transformer with the second time delay protection. The setting coordinates with the outgoing line I stage, effectively serving as low-voltage side busbar protection. The second set trips the circuit breaker on its own side with the first time delay protection, and trips all three circuit breakers on the transformer with the second time delay protection. The setting coordinates with the outgoing line backup protection, serving as the overall backup for low-voltage side outgoing line protection. This configuration satisfies the system stability requirements and avoids damage to the main equipment caused by fault-side protection failure and circuit breaker failure. This is also a summary of lessons learned from accidents. 5. Protection Enabling and Disabling Methods [2] Conventional protection is generally enabled and disabled by pressure plates. After the pressure plate is disconnected, a clear disconnection point is created in the circuit connection. In addition to enabling and disabling by pressure plates, microprocessor-based protection can also be enabled and disabled by function control words, but this must be performed by relay protection professionals. Transformer protection needs to trip three-sided switches, and each side has several sets of protection, and each set of protection is divided into several segments. If each time-limited segment is enabled and disabled by pressure plates, the number of pressure plates will be very large, which will cause great inconvenience to operation and easily lead to misoperation. According to the analysis of transformer protection operation over the years, operator misoperation accounts for nearly one-third of the total number of transformer protection misoperations. Therefore, the design prioritizes simplification and safety in setting up the pressure plates. Specifically: 1) The zero-sequence overcurrent and zero-sequence directional overcurrent protections of the backup protection are combined into one pressure plate; overcurrent protection and directional overcurrent protection are combined into one; 2) When bypassing, only one set of main and backup protections is switched to the bypass, further reducing the number of pressure plates; 3) Each time-limit segment is enabled/disabled by control words, without going through pressure plates. Source: http://tede.cn 6. Malfunction Protection Considering the frequent malfunctions of malfunction protection, such as both the medium and low voltage side protections of the main transformer malfunctioning, the wiring would become complex, increasing the probability of malfunction. Therefore, the design only requires that the electrical quantity protection with fast return on the 220kV side can activate malfunction protection; non-electrical quantity protections do not activate malfunction protection. Malfunction protection activation adopts a method of protection action + current discrimination + series connection of switch trip position and closing position to ensure that malfunction protection is activated when a switch malfunction actually occurs. After the protection system is activated, a voltage lockout release signal is first sent to resolve the issue of the 220kV bus voltage not dropping sufficiently during a low-voltage side fault in the transformer. Then, a time-delay trip is initiated. The failure protection current discrimination element takes the phase current or zero-sequence/negative-sequence current from the independent current transformer (CT) on the high-voltage side. During bypass operation, the transformer protection action contacts are switched to bypass mode, and the failure current of the bypass switch is used to start the circuit. 7. Improvement of Non-Electrical Quantity Protection In microprocessor-based transformer protection, the implementation of non-electrical quantity protection involves connecting the contacts from the non-electrical quantity protection to the transformer protection panel. The output relay is activated via the re-closing relay on the transformer protection panel. Simultaneously, the action behavior of the non-electrical quantity protection is recorded in the microprocessor device via the re-closing relay for analysis. Because non-electrical quantity protection is mostly installed outdoors, rainy weather can easily cause cables to become damp and insulation to deteriorate, leading to maloperation of the protection. Anhui Province has experienced several instances of transformer cooler complete shutdown and maloperation of the protection system. The time relay for the transformer cooler full shutdown protection is moved from the outdoor area to the microcomputer protection panel, which can effectively prevent protection maloperation caused by cable dampness and reduced insulation. 8 AC and DC power distribution 8.1 AC configuration (1) Two sets of differential main protection are connected to two independent CTs on each side of the switch, so that the main protection is independent of each other and the protection range is maximized. (2) The backup protection current loop on each side is the same as the differential main protection, and the independent CTs on each side of the transformer are taken respectively. The protection overvoltage blocking is introduced into the three-side voltage through the pressure plate. (3) The current of the neutral point gap zero-sequence overcurrent and zero-sequence overvoltage protection is taken from the transformer neutral point discharge gap CT, and the voltage is taken from the open delta voltage of the high-voltage side bus PT. 8.2 DC Configuration: Each panel's main protection and backup protection devices are equipped with one set of fuses; non-full-phase and failure current starting are equipped with one set of fuses; non-electrical quantity protection is equipped with one set of fuses; the 220kV operating circuit is equipped with two sets of fuses; the 110kV side switch and low-voltage side switch operating circuits are each equipped with one set of fuses. 9 Conclusion From the development trend of microprocessor-based transformer protection, the choice of a dual-main-dual-backup, main-backup integrated configuration, where protection functions are implemented by independent CPU modules and the output tripping circuits are separate, highlights the characteristics and advantages of microprocessor-based protection. It features multi-CPU parallel processing, a compact overall structure, data sharing, relatively simple panel setup, clear circuits, simple external wiring, convenient commissioning and decommissioning, and strong independence. This represents the future direction of microprocessor-based transformer protection development. References [1] Wang Weijian, Hou Bingyun. Theoretical Basis of Relay Protection for Large Generating Units (Second Edition) [M]. Beijing: China Electric Power Press, 1997. [2] Mao Jinqing, Zhao Zigang, Ma Jie. Key Points of Accident Prevention Technology for Relay Protection and Automatic Safety Devices in Power Systems [S]. Compilation of Power Dispatch Technical Standards [M]. Beijing: China Electric Power Press, 1999.