Abstract The steam turbine system of the second phase of the Jinan Iron and Steel Waste Heat Power Generation Project is controlled by a DCS system, which can effectively protect the turbine and recover the working fluid when the unit sheds load. The operation results show that the BPC system plays a role in increasing the steam temperature and pressure during the process from boiler ignition to turbine tripping, and plays an important role in improving the unit's start-up performance and shortening the start-up time. Keywords BPS system, Losing load, Overspeed [align=center]The Practice of Steam Turbine Side Loop Control Wei Hong Liu Yanbo[/align] Abstract The waste heat generation project second phase adopts the DCS control system. The system can effectively protect the turbine and recover from loss of load. Practice shows that the BPS system can raise steam temperature, increase pressure when the boiler is lit up, and when the turbine is held. It acts on the starting properties of the system and reduces the starting time. Keywords BPS system, Losing load, Overspeed 1 System Overview The Jinan Iron and Steel Group waste heat power generation phase II project consists of three combined cycle generator units with a "2+2+2+1" configuration, meaning each unit has two coal presses, two gas turbines, two boilers, and one steam turbine. The steam turbine is one of the main power generation devices in the gas-steam combined cycle power generation project, utilizing the thermal energy of steam to convert it into rotational mechanical energy. Because the steam turbine generator unit operates at high speed and under high temperature and pressure, the interlocking control and protection of the steam turbine control system is of great significance. The steam turbine system is controlled by a DCS system, mainly including the detection and control of steam system pressure and temperature, bypass system, EH oil system, lubricating oil system, condensate system, and vacuum system. The steam turbine bypass system is a high-low pressure series system, which accelerates the working fluid circulation process during unit startup, increases the unit's temperature and pressure rise rate, and controls the opening of the pressure reducing valve according to the pressure setpoint to prevent overpressure. It effectively protects the turbine and recovers the working fluid during load shedding. 2. Control System Composition The control system adopts a DCS distributed control system with reliable redundancy technology. It consists of a supervisory control station, field control stations, and a communication network (Ethernet). It includes a central I/O unit, extended I/O units, a CPU module, connection modules, and various I/O modules to realize data acquisition, loop control, and sequential control functions. A DP network connects to remote stations, and the Ethernet communication network is used to connect to the monitoring station and transmit data. The operating platform is Windows 2000 Professional + SP4, and the development software includes Control Builder F professional software. Used for hardware configuration, process-level and operator station configuration, establishing a unified database across the system. Digivis Chinese version. Used for process display, alarm information management, trend archiving, various records, system diagnostics, operation control, etc. Control IT Basic (xxx points). I/O point limit. Trend Server acts as a trend server, collecting historical records. During the implementation of the control system's lower-level programming software, based on the technical characteristics of the process flow and field equipment, functions such as signal processing, electric door control, GPS clock synchronization, and system communication are developed using configuration software, while the regulation loop configuration uses a function chart development method. The monitoring interface completes data acquisition, calculation, discrimination, alarm, and protection of analog quantities, switch quantities, pulse quantities, temperature quantities, and protection information of the system, as well as event sequence recording (SOE), report statistics, curve analysis, and, as needed, issues commands to the field protection and control unit layer to achieve control and regulation of electrical equipment. The interface is user-friendly, convenient, and the database is secure and reliable. Operation is carried out in the centralized control room. The console is the center of unit operation monitoring and control, and a CRT operator station is arranged on the console. Except for a few emergency buttons, all operations are controlled via a host computer, and the unit can also be monitored and operated via a large screen. 3. Main Functions and Control Strategies Turbine start-up has three modes: automatic, manual, and semi-automatic. After start-up, the turbine speed is controlled at the minimum speed. In idle/rated conditions, the minimum speed is the idle value; in sequential start-up, the minimum speed is the low idle value. Automatic sequential start-up automatically completes the entire process from low idle to warm-up, to high idle, to the set ratio speed. Warm-up time and acceleration rate depend on whether the turbine is cold-started or hot-started. To ensure safe operation of the unit, various automatic protection devices are generally installed on the turbine. When operating parameters exceed the allowable range for safe operation, they will activate promptly, automatically shutting down the turbine to prevent further escalation of the accident. The bypass system is a crucial piece of equipment in the generator set. The reliability of the bypass system significantly impacts the safe and economical operation of the power plant. It ensures the safe start-up and operation of thermal equipment; extends the unit's service life as much as possible; and improves the unit's overall economic efficiency. The bypass system (BPC) control strategy enables the automatic or manual (remote control) rapid start-up of valves during unit startup, normal operation, and load shedding. It ensures that when the operating pressure or temperature of the main steam or reheat steam exceeds the set range, the bypass device automatically opens or closes, and automatically adjusts the pressure and temperature according to the unit's operating conditions until normal values ​​are restored. The bypass system ensures that when the pressure or temperature of the new steam exceeds the set operating pressure, it opens promptly to release a portion of the steam, thus ensuring that the pressure entering the main steam valve remains stable within a certain range. The steam entering the bypass ultimately enters the condenser. Due to the high steam temperature, the bypass system has a two-stage desuperheating mechanism. Additionally, the bypass system is equipped with interlocks to trigger corresponding fast-opening or fast-closing actions when certain process parameters in the system are not met. The bypass flow diagram is shown in Figure 1. [align=center] Figure 1 Bypass Flow Diagram[/align] 3.1 Functional Settings The turbine bypass is a two-stage series bypass system with high and low pressure. The high-pressure bypass controls pressure reducing valves, water spray isolation valves, and water spray regulating valves; the low-pressure bypass controls pressure reducing valves, primary water spray regulating valves, and secondary water spray valves. All valves are driven by single-speed electric actuators. The bypass control system has three main functions: start-up, overflow, and safety (i.e., three-way valve function), as well as functions such as working fluid recovery, pipe warming, cleaning, and reducing valve and blade erosion. ● Start-up function: Its purpose is to improve the unit's start-up characteristics. It can increase the boiler's combustion rate during start-up; achieve optimal matching between steam temperature and turbine cylinder temperature; thereby shortening the unit's start-up time and reducing lifespan loss. ● Overflow function: Its purpose is to absorb unbalanced loads between the turbine and boiler. It can discharge residual steam during rapid load changes; adjust and stabilize the steam pressure; and maintain the boiler's minimum stable combustion load without oil injection. ● Safety function: Replaces the function of the boiler safety valve. After the unit bypass system is put into standby mode, when the difference between the actual pressure at the turbine inlet and the setpoint of the high-pressure bypass pressure exceeds the overpressure offset setpoint of the bypass, the bypass system will automatically participate in pressure regulation to maintain the main steam pressure equal to the setpoint. 3.2 Regulation, Control, and Protection Functions of the Bypass System: Before boiler ignition, the high and low pressure bypasses are closed. If the bypass system is put into automatic mode, the system opens the valves according to a certain curve. When the turbine's start-up parameters are reached, the DEH sends a closing signal to the BPC, and the high and low pressure bypasses close according to a certain logic. Under normal operating conditions, it will not open again. The bypass system can increase steam temperature and pressure during startup. Putting the high and low pressure bypass systems (also called high bypass and low bypass) into operation between boiler ignition and turbine start-up can accelerate the steam temperature and pressure rise, shortening the unit startup time. It also has functions for stabilizing steam pressure and protection under emergency conditions. It can adapt to the start-up requirements of the unit under various conditions such as cold and hot states; pressure regulation during load changes; protection against overheating, reduction of safety valve action, and recovery of working fluid. It also has an overflow function. It can also adapt to turbine load shedding to maintain no-load operation and turbine tripping to achieve shutdown without shutting down the boiler. 3.2.1 Pressure Control: Controls the main steam pressure, with constant valve position control and constant pressure control. The low-pressure bypass valve has low pressure limits and high pressure limits to ensure the low-pressure bypass valve is fully closed after the unit load increases. Temperature Control: Controls the temperature after the high and low pressure bypass valves, maintaining the cold section steam temperature and preventing the condenser temperature from becoming too high; it can achieve variable parameter adjustment under different steam flow conditions. 3.2.2 Protection Functions: ● Bypass Valve Quick Opening and Quick Closing Function: For the safety of the unit and equipment, quick closing takes precedence over quick opening; high-pressure bypass valve open, low-pressure bypass valve open; low-pressure bypass valve closed, high-pressure bypass valve closed. ● Quick Opening: In case of unit accident conditions, the bypass valve opens quickly to provide overpressure protection. ● Quick Closure: High bypass outlet temperature too high or low bypass closure will cause high bypass to close quickly; low condenser vacuum, high condenser temperature, high condenser water level, low low bypass desuperheating water pressure, etc. will also cause low bypass to close quickly. 3.3 Bypass Control Strategy 3.3.1 High Pressure Bypass Under high bypass valve control mode, the target valve position and valve position change rate can be set through the bypass main control screen to adjust the main steam pressure. The high pressure bypass pressure regulating valve uses the steam pressure in front of the electric isolation valve as the process value. When it is higher than the set value, the high pressure bypass pressure regulating valve opens; when it is lower than the set value, the regulating valve closes. The high bypass control principle is shown in Figure 2. [align=center] Figure 2 High Pressure Bypass Control Principle[/align] Y max —BP Valve position maximum value setting; BP—High bypass valve; P sactual —Final main steam pressure setting value; dp—Pressure deviation setting; Y s —Valve position command; BPE—High bypass spray valve; P steam —Main steam pressure; BD—High bypass isolation valve; PT—Proportional integrator. 3.3.2 Low-Pressure Bypass System The function of the low-pressure bypass system is to bypass reheat steam to the condenser during startup or load shedding, thereby protecting the preheater and turbine. The low-pressure bypass system includes two systems: low-pressure bypass pressure control and low-pressure bypass temperature control. During unit startup, operators can set a minimum pressure Pmin (externally given 1×10⁵ Pa) to control the reheater outlet pressure, maintaining a certain steam flow through the preheater. After the turbine starts running, the pressure setpoint Ps changes with the turbine regulating stage pressure (representing the turbine load), operating under low-pressure sliding pressure. The pressure setpoint Ps = PRH + P, ensuring the LBP valve is closed. To protect the condenser, the LBP valve is equipped with a quick-closing device (SSB). This SSB will activate and close the LBP valve within 2 seconds if any of the following conditions occur: ● Condenser pressure > -5.066×10⁴ Pa; ● Condenser temperature > 80℃; ● Spray water pressure < 5×10⁵ Pa; ● Condenser water level is high. For the low-temperature bypass temperature control system, the controlled variable is not the low-temperature bypass temperature, but rather the opening degree of the low-temperature bypass desuperheating regulating valve. That is, the low-temperature bypass temperature control is a follow-up regulation. The opening degree of the low-temperature bypass desuperheating regulating valve is jointly determined by the reheat steam pressure, the opening degree of the low-temperature bypass pressure reducing valve, and the reheat steam temperature, as expressed below. Let Ta = MAX (T - 250, 0) A = 1 + 0.0043Ta ×0.14 (0.5P + 1) B = f (P) ×20 + 0.2C = 1.18f (L) L Then K = KP + A •B•C Where T is the reheat steam temperature; P is the reheat steam pressure; L is the opening degree of the low-side pressure reducing valve; KP is the minimum opening degree of the low-side desuperheating water regulating valve; K is the opening degree of the low-side desuperheating water regulating valve. 3.4 Bypass Safety Control and Protection The high-pressure bypass pressure reducing valve will be forcibly closed when any of the following conditions are met: ● Turbine overspeed 110% (bypass engaged); ● High-pressure bypass temperature exceeds 390℃ (delay 10s); ● High-pressure bypass spray pressure is low; ● DEH requires bypass disconnection. Without a high-pressure bypass forced closure signal, and with the turbine inlet pressure greater than 6 MPa, the high-pressure bypass pressure reducing valve will be forcibly opened under any of the following conditions: ● Turbine trip; ● Generator trip; ● Turbine inlet pressure rises too quickly; ● Low-pressure bypass protection. The low-pressure bypass pressure reducing valve will be forcibly closed under any of the following conditions: ● Condenser vacuum is below (out of 3) 85 kPa; ● Low-pressure bypass spray pressure is low; ● Low-pressure bypass downstream temperature is above 190 ℃; ● Condenser water level is high; DEH requires bypass disconnection (low-pressure bypass inlet pressure below 0.11 MPa). Without a low-pressure bypass forced closure signal, and with the low-pressure bypass primary spray valve exceeding 10%, the low-pressure bypass pressure reducing valve will be forcibly opened under any of the following conditions: ● High-pressure bypass forced open; ● Turbine overspeed 110%. 4. Conclusion The unit's operation results show that the BPC system plays a role in increasing steam temperature and pressure during the process from boiler ignition to turbine tripping, thus improving the unit's start-up performance and shortening start-up time. The control system configuration software is transparent and offers rich visuals. Compared with the hydraulic bypass system, the electric bypass system has the advantages of being easier to maintain and operate. It ensures the reliable, safe, and stable operation of the power plant, playing a significant role in developing a circular economy, achieving optimal benefits, and creating a clean factory. Postcode 250101, No. 21, Industrial North Road, Jinan, Shandong Province, Automation Department, Jigang Group. Tel: 0531-88866561. Email: [email protected]