Abstract: This paper analyzes the characteristics of electrical system control, introduces the scope and control requirements of integrating electrical systems into DCS monitoring based on practical engineering applications, and discusses relevant issues such as DCS hardware configuration, software functions, and public system control, providing a reference for similar projects in the future. Keywords: Electrical system monitoring, DCS application. The thermal control of 300MW and above capacity units has fully adopted DCS control systems, gradually forming four major systems: Data Acquisition System (DAS), Analog Control System (MCS), Sequential Control System (SCS), and Burner Management System (BMS). From actual operation, DCS has effectively realized its control functions and demonstrated its advantages of safety, economy, and reliability, achieving good results. However, electrical control still uses relatively conventional control methods, employing a one-to-one manual operation on the control panel. With the rapid development of computers and the continuous improvement of control technology, the coordination between boiler and turbine control and electrical control has become increasingly apparent, and the control level gap has gradually widened. To resolve this contradiction, an effective solution is to integrate electrical control into DCS control, thus utilizing the mature distributed control technology of DCS while improving the level of electrical control. After electrical control is integrated into a DCS (Distributed Control System), the capabilities of the DCS can be fully utilized to make functions such as preventing electrical misoperation more convenient and complete. Furthermore, the display and alarm of relevant quantities can be organically combined with the control and regulation of electrical equipment, effectively improving the safety and reliability of the entire electrical control system. In July 1997, the former Ministry of Electric Power convened the sixth expanded meeting of the Leading Group for Thermal Automation, which put forward specific opinions on how electrical design should be coordinated with DCS: "It is necessary and feasible under the current technology and equipment conditions to achieve centralized management of boilers, turbines, and electrical systems, and to coordinate the development of boiler, turbine, and electrical control levels in the power plant's main control room. This can save manpower and main control room space, facilitate centralized control operation, and does not increase costs. The Leading Group requires a positive attitude to participate in pilot projects for integrating electrical systems into DCS." In accordance with this spirit, the electrical systems of the Jiujiang Power Plant Phase III (2×350MW), Xinyang Power Plant Phase I (2×300MW), and Yiyang Power Plant Phase I (2×300MW) projects were all integrated into DCS monitoring. Based on the specific implementation process and requirements of these projects, this paper analyzes and discusses some relevant issues concerning the integration of electrical systems into DCS monitoring. 1. Characteristics of Electrical System Control Compared with thermal automation, electrical systems have many differences in control requirements and operation. The main characteristics of electrical systems are: (1) Electrical equipment has fewer control objects and lower operating frequency compared to thermal equipment. Some systems or equipment may only be operated once every few months or longer when they are running normally. (2) Electrical equipment protection automatic devices require high reliability and fast action speed. The action speed of generator-transformer group protection is required to be within 40ms; automatic quasi-synchronization adopts synchronous voltage method, and speed, voltage adjustment and sliding voltage control are required to be within 5ms; the voltage automatic adjustment device requires a very short excitation time; the fast switching time of the plant power supply fast switching device is generally less than 60-80ms, and the phase difference of synchronization identification is 5°-20°. (3) For 300MW and above units, generally two units share one starter/standby transformer. The maintenance of any unit should not affect the normal operation of the other unit. Therefore, the DCS control should consider its control mode to ensure that only one unit's DCS can control the shared part, while the other unit's DCS can monitor it in real time, and this operation control authority can be switched. (4) The interlocking logic of the electrical system of electrical equipment is relatively simple, but the operating mechanism of the electrical equipment itself is complex. Therefore, the inclusion of the unit's electrical system in DCS control requires the control system to have high reliability. In addition to being able to realize normal start-up, shutdown and operation, it is especially required to be able to realize the real-time display of various data and statuses under abnormal operation and accident conditions, and provide corresponding operation guidance and emergency handling measures to ensure that the automatic control of the electrical system works under the safest and most reasonable operating conditions. 2 Scope of inclusion of electrical system in DCS monitoring According to the characteristics of unit operation and electrical control, in engineering applications, the control of electrical systems such as generator-transformer groups and plant power supply is included in DCS monitoring. All electrical systems within the main power plant buildings from the substation (excluding the substation itself) to the power plant side are integrated into the DCS monitoring system. The main systems are summarized as follows: Generator-transformer unit system (excluding the transformer's own ventilation system); Generator excitation system; High-voltage plant auxiliary power system (including normal switching of auxiliary power); Low-voltage plant auxiliary power system and 400VPC; High-voltage start-up/standby transformer power system (shared by two units); Diesel generator set and emergency power supply (monitoring only); DC system and UPS system (monitoring only); Automatic synchronizing system; Plant auxiliary power fast switching system. Currently, implementing the functions of the main electrical protection and safety automatic devices through the DCS is quite difficult and would incur considerable costs. In principle, their functions are not required to be implemented through the DCS; these devices are retained, but they must have an interface with the DCS, typically using a hard interface. The retained dedicated automatic protection devices mainly include: Generator-transformer unit protection devices; Automatic quasi-synchronizing device (ASS); Automatic voltage regulator (AVR); Plant auxiliary power fast switching device; Fault recording device. Electrical systems of auxiliary systems outside the main plant, such as coal conveying, water treatment, hydrogen production, and electrostatic precipitators, are not currently included in the DCS system. 3. DCS Control Level and Technical Requirements for Electrical Systems During the overall startup of the unit, from boiler ignition to generator grid connection with initial load, automatic program control can be achieved throughout the entire process. (Of course, a limited number of breakpoints can be set for manual analysis and judgment before intervention), or the DCS can safely shut down the unit when it stops. The requirements for unit automatic start-up and shutdown control are: During normal unit startup, when the generator reaches its rated speed, the DCS will engage the AVR. When the generator voltage reaches its rated value, the DCS will engage the synchronizing device. Synchronization between the generator and the grid is automatically achieved by the synchronizing device. During the synchronization process, the DCS controls the AVR and DEH, and when the synchronization conditions are met, a closing command is sent to the generator circuit breaker. After successful synchronization and the generator electrical load reaches a certain value, the DCS quickly switches the high-voltage plant power system from the standby transformer to the high-voltage plant transformer. When the unit is shut down normally, the DCS controls the reduction of the unit load. When the unit load drops to a certain value, the DCS quickly switches the high-voltage plant power system to the start/standby transformer system for power supply. When the unit load continues to drop to zero, the main switch is tripped, the turbine is tripped (the main steam valve is closed), and the generator is demagnetized. The requirements for the plant power system control are: when the unit is started, power is supplied to the plant load through the start/standby transformer; when the unit is in normal power use, power is supplied by the plant transformer and supplied to the 400VMCC low-voltage load through the low-voltage plant transformer to start the necessary auxiliary equipment of the unit; when the plant power is lost, in order to protect the safety of the equipment and system, the plant power fast switching device should quickly switch the plant working load to the start/standby transformer; when the safety section busbar is confirmed to be de-energized, the emergency standby diesel engine should be started to supply power to ensure the safety of the equipment. Based on the above control level requirements, the requirements for electrical monitoring to be incorporated into DCS technology are: (1) The generator system can realize program control and soft manual control to make the generator start from zero speed up, increase voltage until it is connected to the grid with the initial load. (2) The plant power system can be controlled and manually operated according to the requirements of the start/stop phase and normal operation phase. (3) It can display and record various data and statuses of the above-mentioned generator-transformer group system and plant power system under normal operation, abnormal operation and accident status in real time, and provide operation guidance and emergency handling measures. (4) The unit unit (boiler electromechanical) realizes full CRT monitoring. 4 Requirements for DCS software and hardware In order to better illustrate the requirements of electrical system to DCS for software and hardware, especially to provide a more detailed description of the software, the proposed project scale is 2 unit units. The main electrical wiring adopts generator-transformer group unit wiring and is connected to the 220kV/500kV power distribution system. The 6kV high-voltage plant power bus of the main plant adopts a wiring method with two working sections and a dedicated common section. 4.1 Hardware Configuration Requirements Following traditional functional divisions, after electrical systems are integrated into the DCS monitoring system, their functions can be divided into two parts: Data Acquisition (DAS) and Sequential Control System (SCS). Since electrical quantities do not have special requirements compared to thermal quantities, and relatively fast-acting and highly reliable control functions are implemented through retained dedicated electrical devices, there are, in principle, no special requirements for DCS hardware. The hardware configuration requirements are consistent with those for thermal systems, with their functions integrated into the DCS functions. However, due to historical reasons, the DCS is managed by thermal engineers, who often lack knowledge of electrical control principles. Therefore, this work still needs to be completed by electrical personnel in various stages, including design, commissioning, production, and maintenance. Furthermore, since the electrical power supply components for plant auxiliary power need to be commissioned and put into operation in advance during project construction, it is still recommended to set up a separate controller for the electrical control function, i.e., Sequential Control System (SCS), to facilitate smooth operation. Its data acquisition section should be integrated with the thermal system. 4.2 Description of Software Functions 4.2.1 Data Acquisition System (DAS) Functions The Data Acquisition System (DAS) should continuously acquire and process all important measurement point signals and equipment status signals related to the electrical system, so as to provide relevant operating information to operators in a timely manner and realize the safe operation of the unit. Once any abnormal operating condition occurs in the unit, an alarm should be triggered in time to improve the availability of the unit. Generally, it has the following functions: (1) Display: including operation display, group display, bar chart display, trend display, alarm display, etc. (2) Tabulation and Recording: including periodic records, accident recollection records, sequence of events (SOE) records, trip overview, etc. (3) Historical data storage and retrieval. (4) Secondary calculation and unit performance. 4.2.2 Sequential Control System SCS (G/A) Functional Electrical Part Sequential control mainly consists of 3 functional groups: (1) Generator-Transformer Functional Group, which includes the following sequential control subgroups: Generator-Transformer High Voltage Side Circuit Breaker and Knife Switch Subgroup: This subgroup includes the generator-transformer high voltage side circuit breaker and high voltage side I bus and II bus disconnect switches, etc.; Synchronization Subgroup: This subgroup includes the generator synchronization system. The automatic synchronization control of the generator is completed by the electrical special device (ASS). The DCS should have an operation window; Excitation Subgroup: This subgroup includes the generator excitation system and AVR, etc. (2) Plant power supply function group, which includes the following sequential control subgroup items: High voltage plant power supply subgroup item: This subgroup item includes high voltage plant working and standby branches, etc.; Low voltage plant power supply subgroup item: This subgroup item includes 6 kV side circuit breaker, 380V side circuit breaker and 380V side sectional circuit breaker of the plant power transformer connected to 6 kV, etc.; Security power supply subgroup item: This subgroup item includes circuit breakers of working and standby incoming lines of security section, circuit breakers of instrument control transformer incoming lines and instrument control sectional circuit breakers, etc.; Plant power fast switching subgroup item: This subgroup item includes the function of plant power normal fast switching device. (3) Plant utility and backup power supply function group, which includes the following sequential control subgroup items: High voltage start/backup power supply subgroup item: This subgroup item includes the high voltage side circuit breaker, disconnecting switch and 6kV side circuit breaker of the start/backup transformer, on-load tap changer of the start/backup transformer, etc.; Low voltage plant utility section subgroup item: This subgroup item includes the 6kV side circuit breaker, 380V side circuit breaker and 380V side circuit breaker of the transformer connected to 6kV, etc. 5 Configuration of backup monitoring equipment Based on the operating experience of the 300MW units that have been built, after adopting DCS with CRT and keyboard monitoring as the center in the boiler and turbine control of Jiujiang, Xinyang and Yiyang power plants, the display instruments, operators/switches, alarm windows, etc. on the control panel should be reduced as much as possible. Only the backup monitoring equipment is retained as follows: (1) All analog signals are displayed in DCS, and only a few display instruments are retained on the panel. (2) Only a few important electrical switches are retained and the electrical control panel/panel is cancelled. The control function is realized in DCS. Regarding the retention of manual synchronizing switches, since the reliability of automatic synchronizing devices has become increasingly higher, it is recommended to cancel manual synchronizing switches. The backup manual switches to be retained are: emergency trip button for generator-transformer circuit breaker; trip button for generator demagnetizing switch; emergency button for diesel generator. (3) The number of alarm light signs should be retained at around 20, which will be combined with thermal alarms, reducing the types of equipment and facilitating the layout. 6 Common System Control Methods For the common systems of the two units, such as plant utility and backup power systems, the DCS configuration should be the same as the thermal control system, so that when the unit stops, the operators of the other unit can monitor the common system. Reliable measures should be taken to ensure the uniqueness of its control commands, that is, only one DCS system is allowed to control the common equipment at the same time. The existence of the common system should not cause the DCS of the two units to be coupled together. In the actual application of the project, two typical methods were adopted according to the characteristics of DCS to realize the control of the common system. The DCS for Jiujiang Phase III is the HIACS7000 system manufactured by HITACHI. In the DCS configuration, identical hardware and software are configured for both generating units, with operation control switching achieved via a transfer switch. Signals from external devices are sent to the two DCS systems separately through redundant configuration (analog signals) or extended conversion (digital signals). To prevent operational errors, a significant number of decision criteria and I/O measurement points have been added. The advantage of this approach is that it allows for convenient inspection of the DCS when one unit is shut down or under maintenance, and it also increases control reliability. The disadvantages are increased number of external devices and increased costs. Adding a step also increases the possibility of failure. The DCS for Xinyang Phase I and Yiyang Phase I is the INFI-90 system provided by Bailey. Its system architecture utilizes a common loop, and all control of the common system is implemented through the common loop configuration. The two DCS systems are controlled via Tagnumbers, thus effectively solving the coupling problem. The advantages of this control method are fewer configurations and no need to add external equipment, effectively utilizing information resources. The disadvantage is that equipment in the common loop cannot be maintained during power outages unless both units are shut down for maintenance, increasing the requirements for equipment operational safety. 7. Coordination of DCS Commissioning with Plant Auxiliary Power Supply Schedule After the plant auxiliary power supply control is integrated into the DCS, since there is no independent manual operation system separate from the DCS, and based on past power plant construction experience, the DCS is generally not yet operational when backfeeding plant auxiliary power. At this time, high-voltage start-up/standby transformers and high/low-voltage plant auxiliary power supply operations cannot be realized. Therefore, the installation, commissioning, and operation of the DCS equipment must be coordinated in terms of schedule. The installation and commissioning of the DCS were carried out simultaneously with the installation and commissioning of the plant auxiliary power equipment, especially the commissioning of subsystems related to plant auxiliary power, so that it could be put into operation before backfeeding plant auxiliary power to meet the requirements of electrical backfeeding of plant auxiliary power and the partial trial operation of various process systems within the plant.