Abstract: This paper systematically introduces the current status and development trend of the electrical secondary systems of thermal power plants, providing a reference for engineering design and technological transformation. 1 Introduction Thermal power plants are highly technology-intensive enterprises, where a large number of advanced technologies are applied, providing strong technical support for modern power generation technology. In just fifty years, the electrical control system of thermal power plants has undergone a complete process from simple distributed control systems to microcomputer control. To this end, the China Electric Power Planning & Engineering Institute (CEPI) conducted research on the design model of the 2000 demonstration power plant, assigning six institutes to carry out the design research work for the 2000 demonstration power plant, laying a solid foundation for reducing engineering costs and improving the management and automation levels of power plants. This paper, combined with the development of the Inner Mongolia Western Power Grid, analyzes the current status of the electrical control system of thermal power plants in China, and studies the development trend of the electrical control system of thermal power plants in China based on the design model of the 2000 demonstration power plant. 2. Control, Signaling, and Measurement Systems 2.1 Unit Control Rooms and Main Control Rooms The control methods of thermal power plants can be divided into two categories according to the control location: main control rooms and unit control rooms. The latter includes unit control rooms with network control, as well as those with independent unit control rooms and network control rooms. Generally speaking, power plants with a single unit capacity of less than 100MW should use a main control room, while units with a single unit capacity of 200MW and above should use unit control rooms. Among the existing power plants in the Inner Mongolia Western Power Grid, the Fengzhen Power Plant's 6x200MW units and the Dalate Power Plant's 4x330MW units both adopted a scheme where each unit has its own unit control room and a separate network control room. The Hohhot Power Plant's 2x200MW units and the Inner Mongolia Guohua Zhungeer Power Plant's second-phase expansion 2x330MW units, both under construction, adopted a scheme where two units share one unit control room, without a separate network control room, and the network control is integrated into the unit control room. From an electrical engineering perspective, both single-unit one-control and two-unit one-control schemes have their advantages and disadvantages. Single-unit one-control offers advantages such as strong unit integration in installation, operation, monitoring, measurement, commissioning, and protection; interference-free fault handling; spacious control room; and good operating conditions. However, it also has disadvantages such as two units sharing equipment requiring control from two different locations, decentralized maintenance and management, inconvenient communication, and a large number of operating personnel. Two-unit one-control offers advantages such as strong unit integration in operation, monitoring, measurement, commissioning, and protection; relatively centralized arrangement of shared equipment between the two units; no two-location control issues; simplified wiring; convenient operation; more compact control room layout; fewer on-duty personnel; and centralized operation and maintenance management. However, it has the disadvantage that the installation, commissioning, maintenance, and fault handling of one unit can interfere with the other. A comparison of the single-unit one-control and two-unit one-control schemes shows that the two-unit one-control scheme is significantly superior to the single-unit one-control scheme. Furthermore, where conditions permit, eliminating the network control room and incorporating network control into the unit control room would have significant practical implications for reducing control room area, reducing on-duty personnel, and lowering project costs. 2.2 Control Methods Power plants employ three control methods: one-to-one high-voltage control, low-voltage line selection control, and microcomputer monitoring. The control circuit directly affects the tripping and closing of circuit breakers, requiring high reliability. Currently, most circuit breaker operating mechanisms only have high-voltage parameters. Using low-voltage control would require a high-voltage/low-voltage conversion link, resulting in complex wiring and low reliability. Therefore, low-voltage control is not used in thermal power plants to ensure operational reliability. High-voltage control offers advantages such as simple and clear wiring, convenient operation, easy debugging, and high safety and reliability. Therefore, high-voltage control is adopted in large and medium-sized thermal power plants. With the development of the times and the advancement of science and technology, microcomputer-based integrated automation technology has matured and is widely used in substations, accumulating rich engineering and operational experience. Using microcomputer monitoring in power plants, integrating electrical control into the DCS system, can greatly improve the automation level of the units and create favorable conditions for unified operation of boiler and electromechanical units. It also aligns with the principles and objectives of the 2000 demonstration power plant electrical automation design. The adoption of microcomputer monitoring in the electrical systems of thermal power plants is a requirement of the times and an inevitable result of scientific and technological development. In the western Inner Mongolia power grid, the Fengzhen Power Plant's 6x200MW units and the Dalate Power Plant's 4x300MW units both employ a one-to-one high-voltage control method. Meanwhile, the Hohhot Power Plant's 2x200MW units (under construction) and the Inner Mongolia Guohua Zhungeer Power Plant's second-phase expansion (2x330MW units) respectively employ one-to-one high-voltage control and microcomputer monitoring methods, with the latter designed entirely according to the standards of the 2000 demonstration power plant. 2.3 Signal and Measurement Systems The central signaling system of a power plant is the core of the plant's signaling. Current engineering designs include traditional central signaling systems composed of impulse relays and indicator lights, capable of repeated operation and manual/automatic reset, as well as central signaling systems composed of microcomputer-based flashing alarms. Central signaling devices composed of impulse relays were widely used in past engineering designs. However, with advancements in science and technology, their shortcomings have become increasingly apparent, such as a single alarm signal, inability to memorize instantaneous signals, poor reliability, short lifespan of indicator lights, high power consumption, severe heat generation, complex circuitry, heavy maintenance workload, incompatibility with thermal control systems in large and medium-sized thermal power plants, and unattractive appearance. Consequently, they are gradually being phased out in current engineering designs. In contrast, microcomputer-based flashing alarms offer instantaneous signal retention, flashing indicator lights, signal circuit design that eliminates the need to consider repetitive actions, virtually unlimited signal quantity in principle, simple wiring, high technical content, aesthetically pleasing appearance, and comprehensive functionality, making them widely adopted in current engineering designs. When the electrical components of a thermal power plant are monitored by microcomputers, the central signal can be integrated into a DCS system, where the computer handles the central signal tasks. This reduces the number of devices required for the central signaling system, simplifies the system, enhances functionality, and represents the most advanced and best-performing solution. Among the existing power plants in the western Inner Mongolia power grid, the Fengzhen Power Plant's 6x200MW units all use impulse relays to form a central signaling system, while the Dalate Power Plant's 4x330MW units all use microcomputer-based flashing alarms. The Hohhot Power Plant's 2x200MW units, currently under construction, also use microcomputer-based flashing alarms. The Inner Mongolia Guohua Zhungeer Power Plant's second-phase expansion project, with its 2x330MW units, uses microcomputer monitoring. Following the 2000 demonstration power plant design, it has eliminated the central signaling system and integrated into a DCS system, representing the most advanced approach. In conventional thermal power plants, electrical measurements are performed using standard measuring instruments, typically trough-type or square-type meters, installed on the control panel, while electricity meters are conventional. More advanced power plants use digital display instruments, install intelligent electricity meters, and set up electricity billing systems. In the western Inner Mongolia power grid, the Fengzhen Power Plant's 6x200MW units all use conventional measuring instruments, while the Dalate Power Plant's 4x330MW units use conventional measuring instruments, but the electricity meters have been changed to pulse-type. The Hohhot Power Plant, currently under construction, uses digital display instruments and smart electricity meters. The second phase expansion of the Inner Mongolia Guohua Zhungeer Power Plant, designed according to the 2000 demonstration power plant plan, has adopted a microcomputer monitoring system, eliminating conventional measuring instruments and integrating measurements into the DCS system. This saves on control panels and related cables, and also simplifies operation and maintenance. 3 Principles and Objectives of Automation Design for the 2000 Demonstration Power Plant In view of the problems existing in China's thermal power plants and the gap between them and the international advanced level, in August 1997, the China Electric Power Planning & Design Institute proposed the "Suggestions on Accelerating Two Fundamental Transformations, Reforming Design Ideas and Methods, and Building a 2000 Demonstration Coal-fired Power Plant". The "Suggestions" analyzed the seven gaps between the current design of coal-fired power plants and the international advanced level, the five reasons for the gaps, and proposed ten improvement objectives, eight specific ideas, and five implementation suggestions. For the electrical control system, the main problems are: (1) The control levels of boilers, turbines, generators/transformers, and plant power in the unit are not coordinated. Most power plants have not achieved unified duty of unit units. The automation level of auxiliary workshops and auxiliary systems is not high, and there are too many duty personnel. (2) The unit control room is large. The structure of the control system has not kept up with the latest technological development in the world. The amount of cables and their engineering work is getting larger and larger. (3) The automation function level of the unit is still far from the international advanced level. (4) The unit control center is further intelligentized, automated, and the control room is miniaturized. The sequential control of the generator set and plant power is put into the distributed control system (DCS), so that the electrical control is coordinated with the thermal control of boilers, steam turbines and other thermal systems, creating good conditions for the unified duty of boiler and electromechanical units. The boiler, turbine and electrical systems are fully monitored by CRT, and the backup monitoring equipment (including traditional control panels) is eliminated. The network control is computerized, and the full CRT monitoring is also realized. It is incorporated into the first unit control room, and the network control room is eliminated. The control room area of ​​the two unit units is reduced from the current 350-400m2 to less than 150m2. (5) The unit monitoring system is designed according to the principle of functional and physical decentralization. The electronic equipment is distributed and a small central control room is set up. The cable interlayer is eliminated, which saves a lot of expensive control cables and computer cables. It is estimated that 30% to 40% of the cables can be saved. Due to the reduction in the amount of cable laying work, it is conducive to shortening the construction period, and at the same time, it is conducive to fire prevention and safe operation of the power plant. (6) Improve the overall automation level of the plant and realize the networking of the plant monitoring and management information system. Auxiliary workshops and systems such as pump houses, water treatment, coal conveying, and ash (slag) removal, as well as remote and network control systems, should adhere to unified design principles, unified monitoring equipment selection, and unified design standards. Automation levels should be improved to achieve full CRT monitoring without backup, enhancing the operational safety and economy of these systems and reducing on-duty personnel. The design principles and objectives for electrical control systems in the 2000 demonstration power plant serve as the direction and principles for future engineering design. New construction projects should follow these standards, and the renovation of large and medium-sized thermal power plants should also adhere to this approach. 4. Plant Auxiliary Motor Control In thermal power plants, in addition to the steam turbines and boiler motors in the main plant building, there are numerous auxiliary systems. The more concentrated systems include coal conveying systems, ash removal systems, chemical water systems, and hydraulic systems. For steam turbines and boiler motors, previous engineering designs have adopted a one-to-one high-voltage control method. The 6x200MW units of the Zhongfengzhen Power Plant in the Inner Mongolia Western Power Grid and the 1 and 2x330MW units of the Dalate Power Plant both employ a centralized high-voltage control method. Based on the design, installation, commissioning, and operation of Units 1 and 2 at the Dalate Power Plant, when high-voltage control is used for steam turbines and boiler motors, the interlocking of the motors employs relay-based logic circuits. These interlocking circuits are complex and have low reliability. In the Dalate Power Plant's Units 3 and 4 (2x330MW), the boiler motors, including induced draft fans, forced draft fans, and primary air fans, were integrated into the thermal power DCS system, simplifying wiring and achieving good results (but there is still some distance to go before all steam turbines and boiler motors are fully integrated into the DCS system). The Hohhot Power Plant's 2x200MW units, currently under construction, have integrated all steam turbines and boiler motors into the thermal power DCS system. Meanwhile, the Inner Mongolia Guohua Zhungeer Power Plant's expansion of 2x330MW units, also under construction, has integrated the entire electrical system, including generator-transformer units, high and low voltage plant auxiliary power, public and backup power supplies, steam turbines, and boiler motors, into the thermal power DCS system. This represents the most complete and modern design scheme and is the path that future large and medium-sized thermal power plant designs should follow. Coal conveying system control methods can be divided into three types: local manual control, centralized control, and program control. Centralized control is used in all large and medium-sized thermal power plants already built in the western Inner Mongolia power grid, while local control is used in some small units. Practice has shown that manual local operation and monitoring not only involves a large number of personnel, high labor intensity, and a high incidence of occupational diseases, but also makes it difficult for equipment to achieve reasonable and safe operation due to the lack of close coordination between different positions. For coal conveying systems in large and medium-sized thermal power plants, when centralized control is used, the logic interlocking circuits formed by relays are complex, with numerous control components and a large amount of control energy consumed. The automation level of coal conveying systems is relatively low. With the advancement of science and technology, achieving automation of coal conveying systems has become an urgent need for the modern and civilized production of thermal power plants. In recent years, with the rapid development of microprocessor technology, coal conveying program control has also greatly developed, and its technology has matured. It has been widely applied in large and medium-sized thermal power plants in China, accumulating rich experience, and also conforming to the design concept of the 2000 demonstration power plant. Therefore, in the ongoing construction projects of the Inner Mongolia Western Power Grid, the coal conveying systems of the Hohhot Power Plant's 2x200MW units and the second phase expansion of the Inner Mongolia Guohua Zhungeer Power Plant's 2x330MW units have all adopted program control. Similarly, the ash removal system and chemical water system should also adopt program control as much as possible to improve the overall plant automation level. In future engineering designs, efforts should be made to improve the comprehensive automation level in accordance with the design principles and objectives of the 2000 demonstration power plant. For the control of auxiliary systems, the entire plant's monitoring and information system should be networked, and design principles, monitoring equipment, and selection should be unified as much as possible. 5. Component Relay Protection With the development of the times and the continuous improvement of science and technology, the development of component-based relay protection devices has gone through a process from electromagnetic type, rectifier type, transistor type, integrated circuit type to microcomputer type. In the existing power plants of the Inner Mongolia Western Power Grid, units below 50MW have adopted electromagnetic relay protection devices, 50MW units have adopted rectifier type relay protection devices, 100MW units have adopted transistor type relay protection devices, the existing 200MW units have adopted transistor type relay protection devices, and the 330MW units have adopted integrated circuit type relay protection devices. Basically, It can meet the requirements for safe operation. With the advancement of science and technology, the development of microprocessor-based relay protection devices has also entered a new era. Microprocessor-based transformer protection devices have been widely used in substations, and microprocessor-based line protection devices have completely replaced other types of protection devices. Furthermore, large and medium-sized thermal power generating units put into operation in recent years have also gradually adopted microprocessor-based component protection devices. In view of this, the 2x200MW units of the Hohhot Power Plant under construction and the 2x330MW units of the second phase expansion of the Inner Mongolia Guohua Zhungeer Power Plant have both selected microprocessor-based component relay protection devices. Electromagnetic protective relays are simple in principle and wiring, easy to maintain and master, and have extensive operating experience. They were widely used in small and medium-capacity generating units designed in the 1980s. However, some protective devices do not meet the protection requirements of large-capacity generating units in terms of principle criteria and sensitivity coefficients. Therefore, electromagnetic products are less commonly used in large and medium-sized thermal power generating units. Rectifier-type and transistor-type protective devices are superior in terms of sensitivity coefficients, speed, and selectivity, and have the advantages of small size, low power consumption, and good shock resistance. They were the main protective devices for large and medium-sized thermal power generating units from the mid-1980s to the early 1990s. Integrated circuit protection devices, intended as a transitional product to microprocessor-based protection devices, were quickly superseded by the superior performance of microprocessor-based devices before they could be widely adopted. The rapid development of microprocessor technology has permeated almost every aspect of component protection, from integrated protection devices for motors, capacitors, lines, and transformers to relay protection devices for components in large and medium-sized thermal power plants and substations. Their superior performance, advanced technology, and convenient data processing methods have made them popular among technical professionals, leading to their widespread adoption and eventual replacement of other types of relay protection devices. In future engineering designs, microprocessor-based relay protection devices should be used for newly built large and medium-sized thermal power plants. For renovated medium and small-capacity units, appropriate protection devices should be selected based on a comprehensive analysis and comparison of the specific project conditions, the opinions of the construction party, and the investment situation. However, when selecting electromagnetic, rectifier, or transistor-type relay protection devices, the availability of manufacturers and spare parts should be considered. 6. DC Operating Power Supply System In thermal power plants, a reliable DC power supply is required to supply power to control, signaling, protection, automatic devices, emergency lighting, DC oil pumps, and uninterruptible power supplies. DC systems composed of batteries fall into three categories: stationary acid-proof explosion-proof batteries, valve-regulated lead-acid batteries, and alkaline nickel-phosphate batteries. Ordinary lead-acid batteries are widely used in current engineering designs, accumulating rich experience in manufacturing, installation, maintenance, and operation management. However, they also have many drawbacks in practical applications, such as: large battery size, occupying a significant amount of battery room space; acid mist emission during operation polluting the environment; the need for acid adjustment rooms and acid replenishment equipment; and complex and cumbersome operation and maintenance. Alkaline nickel-phosphate batteries include medium-rate and high-rate types. They offer advantages such as simple installation and maintenance, and reliable operation. However, due to slow improvements in manufacturing processes, problems such as alkali creep and leakage have not been effectively resolved, and they require regular replenishment. Furthermore, their price is significantly higher than ordinary lead-acid batteries. Therefore, they are rarely used in large and medium-sized thermal power plants in China, and are only used in auxiliary workshops or systems far from the main plant that require DC power, typically using 10-20AH high-rate alkaline batteries. Valve-regulated lead-acid (VRA) batteries have experienced rapid development in recent years. Because they maintain a valve-regulated seal during use, they require no acid or water addition for maintenance, produce no acid mist, do not pollute the environment, and do not corrode equipment. The batteries can be installed upright, horizontally, or in a modular fashion without needing to consider acid protection. For smaller capacities, they can even be directly mounted on control panels in control rooms. Due to their superior performance, although their adoption started relatively late, VRA batteries are used in large and medium-sized thermal power plants and substations, and are exclusively used in telecommunications projects. VRA batteries are an ideal replacement for stationary lead-acid batteries and nickel-phosphate batteries. In the western Inner Mongolia power grid, the Fengzhen and Dalate power plants use stationary lead-acid batteries for their main power plant DC systems, while nickel-phosphate batteries are used in auxiliary workshops. The Hohhot power plant, currently under construction, has chosen VRA batteries. For lead-acid batteries, the battery charging equipment selected in current engineering designs generally falls into three categories: silicon rectifier charging devices, microcomputer-based charging devices, and high-frequency switching power supplies. The difference between the first two is only whether the controller is a general type or a microcomputer type, both of which belong to phase-controlled power supplies, while the latter belongs to switching power supplies. Silicon rectifier charging devices and microcomputer-based charging devices are widely used in current engineering designs along with stationary lead-acid batteries, while the development of high-frequency switching power supplies is synchronized with valve-regulated batteries. High-frequency switching power supplies are a product of modern scientific and technological development. Compared with traditional phase-controlled power supplies, they have great advantages, which are: (1) High reliability. Because the rectifier modules are connected in parallel for power supply, and the N+1 backup mode is used, if one module fails, the other modules can still work normally, while the phase-controlled power supply is a master-slave backup mode. (2) Lightweight, small in size, and easy to maintain. (3) High efficiency and high power factor. The power conversion efficiency of switching power supplies is 65%-95%, while that of phase-controlled power supplies is only 20-30%. (4) High voltage regulation accuracy and low output ripple coefficient are beneficial to the reliable operation of control and protection equipment and can also improve battery life. In view of the excellent performance of high frequency switching power supply, the Hohhot Power Plant 2X200MW unit selected high frequency switching power supply as charging device. 7 Automatic Devices In order to ensure the reliable and economical operation of the power system and reduce the labor intensity of operators, power plants are equipped with various automatic devices, such as automatic backup power supply switching devices, automatic reclosing devices, automatic synchronizing devices, and automatic generator excitation adjustment devices. In the current engineering design, the automatic reclosing device and the automatic generator excitation adjustment device are both microcomputer type. As for the automatic synchronizing device, in the past engineering designs, the ZZQ-3A, ZZQ-3B and ZZQ-5 integrated circuit type devices were generally used, which can meet the requirements of safe operation of large and medium-sized thermal power plants. In recent years, microcomputer type automatic synchronizing devices have also begun to appear. Microcomputer type automatic synchronizing devices are an inevitable trend of technological development. In future engineering designs, they should be used purposefully and step by step. There are three types of automatic transfer switches for backup power in power plants: electromagnetic, integrated circuit, and microcomputer. All of these have been used in previous engineering designs. With the increase in unit capacity, issues such as the switching time of auxiliary power, the impact on equipment during the switching process, and the stability of boiler system operation have received increasing attention. In the western Inner Mongolia power grid, the 6x200MW units of Fengzhen Power Plant and the 1 and 2x330MW units of Dalate Power Plant, due to the use of low-oil switches, cannot meet the conditions for rapid switching and both adopt the traditional slow switching method, where one power source on the auxiliary bus is disconnected before another is connected. This switching method results in a long auxiliary power switching time, and the impact on the stable operation of the boiler and auxiliary motors during the switching process is significant, making it difficult to meet the requirements for safe and reliable switching of auxiliary power for large units. Therefore, Fengzhen Power Plant and Dalate Power Plant Units 1 and 2 have made certain improvements during commissioning and operation, but the results have not been ideal. To address the problems existing in auxiliary power switching, in the engineering design of the 3 and 4x330MW units of Dalate Power Plant… Vacuum circuit breakers were selected for the 6kV switches, and fast switching devices were used for the plant auxiliary power supply switching, effectively solving the problems caused by slow switching. Given the successful application of fast switching devices for plant auxiliary power supply in Units 3 and 4 of the Dalate Power Plant, the plant subsequently upgraded the plant auxiliary power supply switching system of Units 1 and 2, replacing the 6kV vacuum circuit breakers with fast switching devices. This successfully resolved the problems encountered in previous projects regarding plant auxiliary power supply switching, meeting the requirements for safe and stable operation of large and medium-sized thermal power plants. Future engineering design and upgrades should strive to create conditions to promote the application of fast switching devices for plant auxiliary power supply. In summary, this paper discusses the current status and existing problems of the electrical secondary systems in large and medium-sized thermal power plants in detail, based on the design model of the 2000 demonstration power plant, and proposes design ideas for the electrical secondary systems of newly built and renovated large and medium-sized thermal power plants for reference in engineering design and upgrades.