**0 Introduction** The rapid and accurate operation of busbar protection is crucial for the stable operation of ultra-high voltage (UHV) power grids. It is also a vital link in ensuring the selectivity of UHV power grid relay protection, and its role in protecting the safety of power equipment cannot be ignored. Due to the special and important position of busbar protection in the power grid, people hold a cautious attitude towards the promotion and application of microprocessor-based busbar protection. Whether microprocessor-based busbar protection will be rapidly promoted, and in which direction it should develop, are worthy of in-depth discussion. **1 Brief Analysis of Busbar Protection Operation** Currently, the main busbar protection in power grid operation is current differential protection, with a small number of comparison-type busbar protection systems also in operation. The main type uses a combination of current differential protection and busbar protection to compare the phase of the bus tie current as the selection element for dual busbar protection. Statistics from the Northeast Power Grid over the past 10 years show that busbar protection operated 70 times, with 12 incorrect operations, resulting in a correct operation rate of only 82.85%. Of these 12 incorrect actions, 2 were caused by poor manufacturing quality (one each of abnormal outlet reed relay and AC current relay contact jamming); 5 were due to direct human error such as improper debugging and accidental contact; and 5 were related to bus protection performance, 4 of which were also related to improper use. It is evident that human factors are the main cause of incorrect bus protection actions. Therefore, in addition to meeting the automation requirements of the plant, the developed microcomputer-based bus protection system should fully utilize self-checking functions to reduce the workload of operation and maintenance personnel. [b]2 Centralized Microcomputer Bus Protection[/b] With the successful application of microcomputer-based line protection, many research and manufacturing departments have focused their attention on microcomputer-based bus protection. Microcomputer-based bus protection systems applied in the field can be divided into two categories: one is fast bus differential protection using the ratio braking principle; the other uses the current difference action as the starting element to first trip the bus coupler circuit breaker, and then uses the difference in voltage between the two busbars after the bus coupler trips to select the faulty busbar. Microprocessor-based bus differential protection generally lacks a common differential current loop. Instead, it calculates the differential and braking quantities by performing analog-to-digital conversion on the currents of each component. Microprocessor-based bus differential protection utilizes software compensation to address the issue of inconsistent current transformer (CT) ratios, and its digital calculations facilitate the application of more complex and reliable criteria. Some microprocessor-based bus differential protection systems utilize the sampling value differential principle to accelerate the operation speed of fault protection within the zone, thereby reducing the impact of CT saturation on the bus differential protection. Microprocessor-based bus differential protection can also analyze and determine CT saturation through algorithms, avoiding maloperation of the bus differential protection and thus reducing the requirements of the CT. The successful application of microprocessor-based bus differential protection has provided opportunities for the development of bus differential protection. However, it should also be noted that current microprocessor-based bus differential protection requires the connection of current quantities and disconnector switch positions for all bus components, making its secondary circuit more complex than that of fixed-connection low-impedance bus differential protection. For applications where the operating time requirement is not stringent, the older low-impedance bus differential protection still holds certain advantages due to the use of more secondary cables compared to traditional low-impedance bus differential protection. For situations with strict requirements on bus fault clearing time, microprocessor-based bus differential protection can rival medium-impedance bus differential protection in terms of operating speed, and it does not require auxiliary current transformers (CTs) or switching relays, simplifying the secondary circuit structure and reducing on-site maintenance. However, further operational experience is needed regarding the impact of CT saturation during external faults. The simple operating signals of bus differential protection and the low probability of enabling/disabling operations, coupled with the fact that some conventional bus differential protection systems also monitor CT disconnection and other aspects, mean that the impact on plant automation is not significant, which is also a reason for the limited adoption of microprocessor-based bus differential protection. With the rapid advancement of computer and communication technologies, the decentralization of relay protection to switchyards has been implemented and is gradually gaining acceptance. The higher the voltage level of a plant, the larger its footprint and the more cables it consumes, making the economic benefits of decentralizing relay protection more pronounced. This decentralization of relay protection places new demands on microprocessor-based bus differential protection. 3. Distributed Layout and Implementation of Bus Protection: Conventional bus differential protection requires connecting the current loops of all components connected to the busbar together, making distributed layout difficult. Microcomputer-based bus differential protection does not have a common differential current loop. The differential and braking quantities are obtained through calculation of digital quantities. With the help of communication networks, it is easy to achieve the requirement of decentralized layout. There is already successful operation experience of microcomputer-based decentralized bus differential protection abroad, such as ABB's REB500 microcomputer-based bus differential protection and Tokyo Electric Power Company's manned substation system. Compared with centralized bus differential protection, the decentralized layout scheme has more advantages [8]: a. It can reduce the probability of bus differential protection malfunction and multiple circuit breakers tripping simultaneously due to misoperation or accidental contact. b. It is not necessary to concentrate all TA secondary cables on the bus differential protection panel, reducing TA secondary circuit cables, simplifying the TA secondary circuit, and making it possible for bus differential protection to share a set of TA with other protections. c. It is easy to combine with the main protection of the line or transformer and to mutually back up with the backup protection. Realizing the decentralized layout of bus protection can completely eliminate the electrical connection between different equipment units in the plant, and has significant economic benefits in reducing cable investment and reducing the central control room and plant footprint. Faced with the major trend of decentralizing relay protection, accelerating the development of distributed microprocessor-based busbar protection should be the main direction of microprocessor-based busbar protection development. 3.1 Directional Distributed Busbar Protection In centralized busbar protection applications, current differential protection is simple in principle and easy to implement, and has been the mainstream of busbar protection for many years. For distributed schemes, current differential circuits are difficult to form, and other principles of busbar protection should be actively explored to adapt to the needs of distributed arrangements. For interlocked longitudinal line protection, there are usually starting signaling elements for faults outside the reaction zone and stopping signaling elements for faults inside the reaction zone. In the event of a fault outside the zone, the starting element should be more sensitive than the stopping signaling element of the opposite protection to interlock the opposite protection and prevent false tripping. Excluding the influence of factors such as the path, the reliability of interlocked protection itself is widely recognized. As the main protection, microprocessor-based interlocked longitudinal line protection can fully reflect various types of faults on the line. The busbar can be treated as an ultra-short line, and the various electrical components connected to the busbar can be regarded as one end of a multi-terminal line. Since busbar protection does not require phase-selective tripping, if the busbar is located within a single switching station (i.e., only lines are connected to it), a corresponding busbar protection system can be constructed using the principle of interlocked line protection. This system utilizes the sensitive Stage II protection (sensitive to the entire line) of each line protection as the starting element to respond to faults outside the busbar protection zone. A reverse-direction element, i.e., an element pointing towards the busbar, with the same function as Stage I protection in the line protection, is used to respond to busbar faults. This element, while ensuring sensitivity to busbar faults, works in conjunction with Stage II protection of each outgoing line. Combining the elements that respond to faults outside and within the protection zone for each line constitutes busbar protection. When a line fault occurs, the instantaneous contact of Stage II on the faulty line sends a blocking signal, and the busbar protection cannot trip. When a busbar fault occurs, the element responding to faults within the protection zone operates, and there is no blocking signal, so the busbar protection operates correctly. Because it is based on a sophisticated microcomputer-based line protection system, these forward and reverse-direction elements should be able to instantaneously respond to various types of faults. The use of the instantaneous contact of the second stage of line protection as a component for detecting faults outside the protection zone is not unique; its setting range can be determined entirely based on the needs of the power grid. The component for detecting busbar faults should coordinate with the component for detecting faults outside the protection zone, while ensuring sensitivity during busbar faults. For ordinary substations, microprocessor-based line protection devices can be added to the main transformer, generator-transformer unit, and bus tie (section) unit as a component of directional busbar protection. The microprocessor-based protection of the bus tie unit ensures the selectivity of busbar protection; components for detecting faults inside and outside the protection zone should be provided in directions pointing towards different busbars. This directional, distributed busbar protection requires minimal information exchange between bay units and can be implemented through a simple and reliable communication network. It fully utilizes existing microprocessor-based line protection, enabling dual protection as the line protection becomes dualized, further reducing project costs. The busbar protection is built on a mature microprocessor-based line protection foundation, and the voltage blocking quantity can be distributed, resulting in high reliability. The busbar protection will also no longer be affected by current transformer (CT) saturation. Combined with microprocessor-based line protection, directional bus protection has a simple structure and can adapt to centralized bus protection situations, which should be given sufficient attention. Directional bus protection can also provide a dual protection option for important substations in ultra-high voltage power grids. 3.2 Microprocessor-based Bus Protection without a Master Station Compared with centralized microprocessor-based bus differential protection, distributed microprocessor-based bus protection using the current differential principle requires solving two technical challenges: first, real-time transmission of large amounts of data; and second, high-precision synchronous sampling technology. The large amount of data communication leads to a complex communication network structure, and the complexity of the communication network equipment may even exceed that of the relay protection equipment itself. This not only increases equipment investment but also reduces the reliability of the bus differential protection to some extent. Microprocessor-based current differential bus protection obtains differential and braking quantities through digital calculations, requiring extremely high sampling synchronization between different bay units, especially for rapid actions such as those using the sampling point differential principle. This is one of the reasons why currently deployed distributed microprocessor-based bus differential protection systems all have a master station. Through theoretical analysis and laboratory verification, we have found a solution to the above problems and have prototyped a distributed bus differential protection system without a master station. For the dual-bus configuration, each bay unit of the bus differential protection system consists of five modules: a large differential module, a selective differential module (including voltage blocking), a communication module, a signal and output module, and a power supply module. Different distributed units are connected via a series-connected ring communication network. The communication network uses fiber optic media and has four 384 kbit/s serial communication ports. Two are used for the large differential module to achieve data transmission redundancy and improve reliability; the other two are used for the two selective elements of the dual bus. The large differential module is connected to a dedicated bus differential protection current transformer (CT), while the selective elements share a CT with other protection systems. The bus differential protection system does not have a master unit; it is composed of bay units connected in series via a ring communication network, and the software and hardware configurations of each bay unit are identical. The bus differential protection system uses a ratio braking principle. Considering that the protection has been distributed and the amount of cable used is very small, the operating mode is still determined by directly judging the auxiliary contacts of the disconnecting switch. The protection system includes corresponding software to monitor and determine whether the status of disconnecting switches and circuit breaker auxiliary contacts is abnormal, and whether the current transformer (CT) has broken. Using this ring communication network, based on existing hardware, when sampling 36 points per cycle, the ring network can connect up to 18 units, and the sampling synchronization problem is also properly solved. Laboratory tests show that, without connecting to external clocks such as GPS, after technical processing, the maximum sampling asynchrony time between different interval units is less than 1 μs. The microcomputer-based bus differential protection without a master station consists of interval units with identical software and hardware to the primary equipment, requiring less stringent communication network requirements and facilitating protection decentralization. Fewer secondary cables and a simpler communication network allow the microcomputer-based bus differential protection without a master station to fully utilize the advantages of low CT characteristic requirements, increase hardware redundancy, and significantly improve the reliability of the bus differential protection by connecting different CT windings through large differential and selection elements. [b]4 Conclusion[/b] a. The main problems with conventional bus differential protection are usage and operation/maintenance issues; there is no need to be overly pessimistic about the reliability of the device itself. b. It will take time to fully replace conventional bus differential protection with existing centralized microprocessor-based bus differential protection. c. Microprocessor-based distributed bus protection offers excellent performance and represents the main development direction for the microprocessorization of bus protection. d. Bus protection should not be limited to the current differential principle. Especially considering the new situation of protection decentralization, existing microprocessor-based protection resources should be fully utilized, and the use of bus protection based on other principles should be actively explored. e. Directional bus protection based on microprocessor-based line protection has a simple structure and is suitable for protection decentralization schemes and the need for dual bus protection; it should be given full attention. f. Distributed bus differential protection without a master station has low hardware requirements and high redundancy, providing a new option for protection decentralization. **References** 1 Tang Ping, Chen Yaqiang, Xia Jun, et al. Development of Microcomputer-based Busbar Protection Device. Electric Power Automation Equipment, 1996(4) 2 Cheng Lijun, Feng Guodong, Liu Yong. Research on Adaptive Microcomputer Busbar Protection Device. See: Proceedings of the 6th National Symposium on Relay Protection. 1996 3 Li Kang, Liu Jiping, Liu Zhigang. Principle Analysis and Practical Application of BP-type Microcomputer Busbar Protection. Electric Power System Automation, 1998, 22(5) 4 Li Dong, Mao Yasheng, Lu Zhengjun. BP-2A Microcomputer Busbar Differential Protection. Electric Power System Automation, 1998, 22(6) 5 Chen Deshu, Ma Tianhao, Liu Pei, et al. Some Problems of Sample Value Current Differential Microcomputer Protection. Electric Power Automation Equipment, 1996(4) 6 Lü Hang, Li Li. Research on Anti-CT Saturation Measures for Busbar Protection Based on Waveform Characteristics. See: Proceedings of the 7th National Symposium on Relay Protection. 1998 7 He Benteng, Ma Yongsheng. The Influence of Current Transformer Saturation on Bus Protection. Relay, 1998(4) 8 He Jiali, Luo Shanshan, Wang Gang. Implementation of a Distributed Digital Bus Protection System. IEEE Trans on Power Delivery, 1997,12(4)