1. Overview With the development of generating units towards larger capacity and higher parameters, the automation level of thermal power plants has received increasing attention. Since the late 1980s, distributed control systems (DCS) based on computer microprocessors have been widely used in newly built large generating units in China. To improve the automation level of units and meet the needs of power generation enterprises for competitive bidding for grid connection, reducing manpower and increasing efficiency, and participating in market competition, some older 200MW and 125MW units in China are also actively undergoing DCS retrofitting. Due to conservative traditional concepts and the high requirements for safety and reliability of electrical equipment control, most domestic power plants, when carrying out DCS retrofitting, only modify the thermal dynamic components, including DAS, MCS, SCS, FSSS, and DEH, while the monitoring of electrical equipment (ECS) is rarely modified. Some systems simply delegate a portion of the auxiliary equipment switch control functions to the DCS, participating in the sequential control of the thermal system; others send some electrical analog and switching quantities into the DAS system to meet some data acquisition requirements; more thorough electrical modifications only involve having the tripping and closing control of some switches in the main units such as generators and transformers handled manually within the DCS. The electrical feedback panels and control consoles still retain a large number of signals, meters, manual controls, relays, etc. The Huaibei Power Plant's Unit #6 is a domestically produced 200MW unit. During the unit's overhaul, a distributed control system (DCS) was modified. As part of the DCS, the electrical equipment monitoring (ECS) was also modified simultaneously. The DCS modification of the electrical equipment in Huaibei Power Plant's Unit #6 is one of the most thorough modifications currently being made in China. Not only are the control, interlocking, metering, and signaling functions of the high and low voltage auxiliary equipment switches and main unit switches handled by the DCS, but the automatic transfer logic of the high and low voltage plant power supplies is also implemented by the DCS. We also fully utilized the advantages of computer control to design and implement some electrical sequential control logic. Examples include: automatic generator voltage boosting and automatic synchronizing grid connection logic, automatic generator disconnection logic, automatic switching logic for plant power, and manual/automatic switching logic for the excitation system. The DCS retrofit of Unit #6 at Huaibei Power Plant was jointly designed and completed by Shanghai Automation Instrumentation Co., Ltd. and Huaibei Power Plant, using a complete set of the newly imported Westinghouse MAX-1000 system from the United States. This article will introduce the ECS retrofit of Unit #6 at Huaibei Power Plant and discuss several issues that should be noted after the ECS retrofit. 2. Implementing Basic Functions of Electrical Equipment Monitoring with DCS After the DCS retrofit, the control, interlocking, metering, and signaling functions of the original high and low voltage auxiliary equipment switches and the main unit switches are all completed by the DCS. Automatic synchronizing switches, manual voltage boosting and deboosting switches, etc., are also controlled via the software manual operation within the DCS. 2.1 Switch Control 1) Through the software manual operation within the DCS, point D0 (AC220, 10A) is sent out to connect to the switch's trip and close circuits to achieve trip and close control. Most switches are only equipped with soft manual operation, but for safety reasons, for some important auxiliary machines, such as the main power supply for the powder feeder, cooling water pump, fuel pump, and high-pressure AC oil pump, in addition to the soft manual operation in the DCS, hard manual operation is still retained on the panel. 2) Based on the switch return status value DI point, the switch status is displayed by different colors on the CRT. This includes six states: closed, open, abnormal, etc. 3) The switch status values ​​are indicated by trip and close relays, rather than by the switch's auxiliary contacts. This not only indicates the switch status but also monitors whether the trip and close circuits are intact and whether the operating fuse is intact. An alarm signal is issued when there is a fault in the trip or close circuit or when the operating fuse blows. 4) The oil pressure, water pressure, and temperature contacts that were originally supplied to the electrical circuit by the thermal components for interlocking start, interlocking closing, and tripping are directly fed into the DCS, and the corresponding functions are realized through the logic within the DCS. The original electrical interlocks between auxiliary machines and the boiler main interlocks are also implemented by the DCS based on the status variables sent to the DCS. 2.2 Meters, Sounds, and Signals 1) Most of the original panel meters are eliminated. Parameters such as current, voltage, frequency, active power, and reactive power are sent to the DCS in the form of 4-20mA AI points through instrument transmitters for data acquisition, meter indication, participation in sequential control, automatic transfer logic, overcurrent alarms, etc. All meters are displayed in appropriate positions on the main wiring operation screen and monitoring screen within the CRT for easy operation and monitoring, based on the overall coordination and aesthetics of the screen. Only a very few meters for generator active power, reactive power, frequency, etc., are retained on the panel. 2) The fault signals and warning signals on the original panel are sent to the DCS in the form of DI points. A dedicated fault signal and warning signal screen is designed within the DCS. Fault signals are red, and warning signals are yellow. Different sounds are emitted when fault signals and warning signals are issued, and the signal screen can be quickly switched to view the issued signals. Some important signals, after entering the DCS, are output through the DO point and connected to the flashing alarm on the screen. 2.3 The original automatic synchronizing circuit electrical automatic synchronizing device, synchronizing meter, and synchronizing interlocking relay are retained. The synchronizing switches such as TK, STK, and DTK in the original synchronizing circuit are implemented by manual operation within the DCS. The additional contacts required for TK, STK, and DTK are extended and sent out by the DCS. The synchronizing device, synchronizing meter, and synchronizing interlocking relay are retained to maintain the last line of defense for synchronous grid connection. 2.4 The excitation increase and decrease operation switches of the voltage regulation and speed regulation circuit are also implemented by manual operation within the DCS and output in pulse form. The original generator speed regulation was performed by the synchronous motor. After the DEH modification, the increase and decrease operation of the electrical part was cancelled. Only the turbine professional has the right to increase and decrease the generator, and it is operated through the CRT of the DEH. 3. Automatic Transfer Logic for High and Low Voltage Plant Auxiliary Power The automatic transfer logic for high and low voltage plant auxiliary power is a broad concept, encompassing three aspects: low-voltage protection logic for the working power supply, automatic transfer closing logic for the standby power supply, and accelerated tripping logic after a fault. Traditionally, automatic transfer logic for high and low voltage plant auxiliary power is implemented using relays. After DCS (Digital Control System) upgrades, the necessary external conditions for the automatic transfer logic are fed into the DCS, which performs logical judgments to achieve functions such as low-voltage protection tripping for the working power supply, automatic transfer closing for the standby power supply, and accelerated tripping after a fault. The automatic transfer logic implemented by the DCS is functionally the same as the original relay-based logic. It is worth noting that in the automatic transfer logic for high and low voltage plant auxiliary power, some power plants still use relays to determine undervoltage (low-voltage protection) and loss of voltage for the working power supply, as well as the presence of voltage for the standby power supply. When the relay reaches its set value, it activates, and its contacts are sent to the DCS as logic conditions. Our plant utilizes voltage transmitters on the working and standby busbars to determine undervoltage and loss of voltage in the working power supply and voltage presence in the standby power supply. The setpoints are determined internally by the DCS (Digital Control System). After the DCS upgrade, to achieve data display and acquisition, the voltages of the working and standby busbars must be fed into the DCS; therefore, additional voltage relays for automatic transfer logic are no longer needed. 4. Generator P/Q Curves The DCS upgrade of Unit #6 at Huaibei Power Plant integrated the P/Q curves provided by the generator manufacturer into the DCS. The dynamic P/Q values ​​during generator operation are displayed on the horizontal and vertical axes of the P/Q curves, thus determining the generator's dynamic operating point. Due to software and design limitations, the P/Q curves for Unit #6 at Huaibei Power Plant are relatively simple. With more powerful computer software, the P/Q curves could be used more effectively. For example, different operating areas of the generator could be displayed in different colors; alarms or tripping could be triggered if the generator exceeds operating limits; and historical generator operating data could be recorded for easy access. 5. Sequential Control Logic of Electrical Equipment The sequential control logic of electrical equipment mainly includes the automatic quasi-synchronous grid connection logic for generator step-up, the automatic generator disconnection logic, the automatic switching logic for plant auxiliary power, the manual/automatic switching logic for the excitation system, and the automatic switching logic for main and standby excitation. 5.1 Automatic Quasi-synchronous Grid Connection Logic for Generator Step-up This section includes automatic excitation step-up automatic quasi-synchronous grid connection logic, manual excitation step-up automatic quasi-synchronous grid connection logic, and standby excitation step-up automatic quasi-synchronous grid connection logic. The generator can achieve automatic step-up automatic quasi-synchronous grid connection under automatic excitation, manual excitation, or standby excitation operation. When the generator main switch is in hot standby mode, after the generator reaches full speed, regardless of whether it is in automatic excitation, manual excitation, or standby excitation operation, once an automatic grid connection command is issued, the generator can automatically execute the remaining operations according to its current state until it is connected to the grid. 5.2 Automatic Disconnection Logic for Generator During normal operation, the generator's active power is reduced to approximately 4MW. At this time, once the generator receives the automatic disconnection command, it can automatically reduce active and reactive power to 0 and realize the automatic switching of plant auxiliary power until the generator is disconnected. 5.3 Automatic switching logic of plant auxiliary power High and low voltage plant auxiliary power can realize the automatic switching from working power supply to standby power supply or standby power supply to working power supply according to the command. 5.4 Automatic switching logic of generator automatic excitation and manual excitation The generator can realize the automatic switching from automatic excitation to manual excitation or from manual excitation to automatic excitation according to the command. The guiding idea of ​​the generator sequential control logic design is to send some voltage, current, power, speed and switch status that need to be observed during manual operation into DCS, and add some interlocking conditions. DCS makes logical judgments to realize the self-regulation of voltage and power and the automatic operation of switches. 6 Several issues that should be noted in the application of DCS control technology in electrical monitoring 6.1 After the DCS is upgraded, some electrical analog quantities are no longer just data displays as before, but are used as logical conditions to participate in some logical control through transmitters. Examples include voltage in the automatic transfer logic of plant power, reactive power and reactive power in the DEH, and voltage and current in plant power switching. Therefore, instrumentation professionals must undergo a shift in understanding and fully consider the impact on relevant logic when operating transmitters. 6.2 All electrical analog quantities are sent to the DCS through transmitters. Most transmitters require an external auxiliary AC power supply, which must be sufficiently reliable and connected via a UPS. If the auxiliary power supply is not connected via a UPS and is simply selected from a general power cabinet, even if the backup power supply automatically transfers quickly, the transmitter will not function properly due to the momentary loss of auxiliary power, and the automatic transfer cannot be guaranteed to succeed. 6.3 After the DEH modification, once the generator main switch is closed, the DEH device will determine that the unit is connected to the grid based on the switch auxiliary contacts and automatically add a 5000kW load. Therefore, when conducting unit sham connection tests, zero-start voltage boost tests with main switch, or other main switch closing tests under DEH operating conditions, the auxiliary contacts of the switch supplied to DEH should be removed to prevent unit overspeed. 6.4 After DCS modification, generator voltage increase/decrease operations are performed via pulse output through soft manual operation within the DCS. It should be noted that soft manual operation differs from traditional hard manual operation. The soft manual operation within the DCS will remember the number of mouse clicks; the generator voltage will increase or decrease by the same number of clicks. Since there is a certain lag time in the actual voltage increase/decrease of the generator, rapid and repeated mouse clicks may cause generator overvoltage during generator voltage boost operations. Therefore, generator overvoltage limits should be set along with the soft manual operation for voltage increase/decrease. Generator overvoltage limits should also be set in the generator sequential voltage boost logic. 6.5 After DCS retrofitting, the DCS controls some equipment and switches via pulse-based manual operation, such as generator sequential control, manual increase/decrease operation, increase/decrease operation, and switch tripping/closing. Besides noting the potential for operation memory issues mentioned in 6.4, we must also pay attention to the pulse length settings. Improper pulse length settings can cause problems. For example, in generator voltage regulation, if the system still uses an electromagnetic excitation regulator or a microcomputer excitation regulator without a step size limit, a too-wide voltage regulation pulse in the DCS may result in a large voltage adjustment range, affecting the unit's operational stability. However, this problem does not exist with the SAVR-2000 excitation regulator produced by NARI Excitation Co., Ltd., because this regulator itself has a step size limit. As long as the regulator's step size is set appropriately, the DCS pulse length will not affect the generator voltage adjustment range. Furthermore, the pulse length settings for all switch tripping and closing controls should also be carefully considered. High-voltage switches such as 220kV and 110kV typically have tripping and closing speeds of only tens of milliseconds or around 100 milliseconds. Even some slow-closing switches, such as some SN2 type oil switches, have closing times of only around 600 milliseconds. Therefore, the tripping and closing pulses should not be set too long; just ensure that the switch has sufficient tripping and closing time. Otherwise, phenomena such as switch tripping, burnt-out tripping and closing coils may occur. Special attention should be paid to some low-voltage switches such as DW15. Although they are all DW15 switches, their closing procedures may differ, so the impact of the closing pulse length on the switch should be carefully considered. Some switches begin storing energy and closing as soon as the pulse appears; others begin storing energy when the pulse appears and only begin closing when the pulse disappears. Especially for the latter, if the DCS closing pulse is too wide, the switch will remain in the energy-storing state, and the closing coil will remain energized, which will burn out, and the switch will be unable to close. This situation has occurred in our factory. 6.6 The relevant regulations regarding DCS applications are all aimed at the decentralized control of thermal systems, but DCS technology is increasingly being applied to the monitoring of electrical equipment. Due to the lack of relevant regulations and standards, the design of circuits or logic during the DCS retrofitting of electrical systems varies from unit to unit, and some problems may arise. For example, in the control of high-voltage switches, some units only use the auxiliary contacts of the switch to reflect the switch status, thus eliminating the original functions of the pushbuttons in monitoring trip/close circuits and controlling circuit disconnection. Our factory added trip/close position relays, using the normally open contacts of the trip/close relays to reflect the switch status, and simultaneously using the normally closed contacts of the trip/close relays in series to reflect whether the control circuit is disconnected, effectively solving these problems. Furthermore, the regulations for DCS control in thermal systems stipulate that the shielding layer of shielded cables should be grounded at one end, while the regulations for electrical secondary circuits stipulate that the shielding layer of shielded cables should be grounded at both ends, creating a contradiction. Therefore, relevant institutions or departments should formulate relevant regulations and standards for the application of DCS control technology in electrical monitoring as soon as possible. 6.7 Some power plants and DCS control developers have now incorporated standby power supply fault switching and synchronizing devices into their DCS control systems. Based on information from relevant manufacturers, and considering the importance and complexity of standby power supply switching and synchronizing devices, I believe it's best to avoid implementing these functions through the DCS and instead use dedicated devices. This is primarily based on the following considerations: 1) Most DCS developers are not specialists in these areas. They haven't conducted extensive research on the changes in electrical quantities in the plant's power supply and synchronizing systems under fault, abnormal, and other conditions. They simply implement these functions through software based on the logical relationships and electrical quantity relationships provided by power plant professionals. This often only achieves some basic functions of these devices, and in some very special cases, it's often difficult to meet the requirements. For example, the switching device for the plant's power supply only implements the original automatic standby power transfer function, and its reliability is incomparable to the fast-switching devices widely used in the system. Fast-switching devices should be used for plant power supply. 2) Given the importance and complexity of synchronizing and fast-switching devices, these devices should generally undergo dynamic model testing, expert evaluation, grid connection trial operation, and grid access permitting before being widely applied to the system. Products promoted and applied within the grid should be highly mature, reliable, and proven. However, some backup power switching and synchronizing grid connection cards developed by DCS developers generally have not undergone these procedures. If even slight negligence occurs, the resulting losses will be incalculable. 3) Once synchronizing and fast-switching devices are finalized, their internal circuits or programs will be relatively fixed and generally unlikely to change after successful commissioning. However, the logic configurations and related circuits of some backup power switching and synchronizing grid connection cards developed by DCS developers are open to some DCS engineering station management professionals. Since power plant DCS management is generally handled by thermal engineers, who may lack sufficient understanding of the importance or characteristics of these devices, errors could lead to serious problems. In conclusion, I believe that fast-switching devices should be used for plant power supply. A dedicated synchronizing device should be selected to ensure the final safety and prevent major accidents.