Abstract : This paper introduces the retrofitting of the steam-driven feedwater pump thermal control system of Unit #4 at Fengzhen Power Plant. The MEH system adopts the same hardware as the main DCS system, sharing operator and engineer workstations. The control system features high automation, accurate measurement, comprehensive protection configuration, reliable operation, and high stability. This control technology has been successfully applied to the retrofitting of the steam-driven feedwater pump of Unit #4, providing a reference for the application of similar technologies in power plants.
Keywords: MEH system automated protection configuration, stability
I. Introduction:
With the rapid development of my country's power market, competition among power generation companies is becoming increasingly fierce. The safe, stable, and efficient operation of generator units is a reliable guarantee for improving the market competitiveness of power generation companies. Furthermore, the long-term safe and stable operation of generator units requires a stable and reliable control system. The design of the control system, the selection of equipment, the level of automation, and the protection configuration will determine the safety and stability of the equipment operation. The steam-driven feedwater pump control system (MEH) for Unit #4 of Fengzhen Power Plant is designed and applied based on this control concept.
II. Upgrade of the feedwater pump turbine control system
1. Introduction to the Control System
MEH (Feedwater Pump Turbine Electro-hydraulic Control System) refers to the electro-hydraulic control system for small steam turbines. Its main functions include speed control, load control, valve management, automatic control of the small steam turbine, and overspeed protection. The MEH's electronic equipment uses the same hardware as the DCS, including a microprocessor-based control cabinet, operator control board, and I/O modules. The MEH's hydraulic system uses imported MOOG electro-hydraulic converters from the USA, and the actuators are imported intelligent integrated ROTORK (UK) products. The MEH does not have separate operator and engineer stations; it shares these stations with the DCS (ETS is included in the MEH system). The TSI system includes monitoring items and devices necessary for the safe operation of the steam turbine. To improve system reliability, the main components are redundantly designed, resulting in high reliability and stability. The electro-hydraulic regulation uses pure electric control, and the oil supply system uses turbine oil with good stability as the working medium to manage and control the regulating valves and main steam valves.
2. MEH Functional Characteristics
2.1 The MEH controller will be connected to the DCS high-speed data highway as a node. Its controller (DPU) module is configured with 1:1 redundancy and has SOE function. The system is configured with a sufficient number of controllers, and the load rate is <40%. Each I/O analog channel has an individual A/D and D/A converter and single-channel isolation; speed cards, servo cards and bit inversion cards are configured individually on a one-to-one basis; each type of I/O point and slot should have a 15% spare capacity.
2.2 The digital electro-hydraulic control system (MEH) changes the steam flow rate of the driving turbine according to the feedwater control signal of the coordinated control system (MCS), and controls the speed of the feedwater pump turbine to match the output feedwater flow rate of the driven feedwater pump with the boiler load.
2.3 The MEH can start the steam-driven feedwater pump automatically or manually, increasing the speed from 0 to 3000 rpm. Above 3000 rpm, the feedwater pump control can be switched to the DCS feedwater control system. The feedwater pump turbine speed is controlled based on the control signal (4-20mA) from the boiler feedwater control system. The control accuracy is ±1‰.
2.4 Failure of individual components within the MEH system will not affect the operation of the entire system. A single fault will not cause the protection system to malfunction or fail to operate, nor will it cause the operating feedwater pump to stop.
2.5 The MEH system performs well under the following operating modes of the unit:
• Unit start-up/shutdown
• The unit is operating normally
• Sliding operation
• Constant pressure operation
• Rapid load reduction (RB)
2.6 Control system performance requirements:
• Closed-loop speed control range: not less than 10%NH to 120%NH (NH is the maximum operating speed of the feed water pump)
• Speed control accuracy: <0.1%NH
• Rotational speed setting accuracy: <0.1%NH
• Static characteristics: Dead zone < 0.1% NH
• Dynamic characteristics: Turbine speed tracking lag <0.1%NH
• Control system execution speed: The full travel time of the main steam valve trip should not exceed 0.5 seconds.
2.7 The MEH system has three control modes: manual, automatic, and remote control. The control system is designed with "fail-safety protection" and "safety self-locking" functions, and the system has the following functions:
(1) Automatic speed increase control: The MEH system automatically increases the turbine speed from the lowest speed to the target speed at the rate of increase set by the operator in advance.
(2) Feedwater pump speed control: The MEH system receives the feedwater flow demand signal from the boiler closed-loop control system and realizes automatic control of the feedwater pump turbine speed.
(3) Interlock protection: The MEH system is equipped with oil pressure interlock, overspeed protection and temperature protection for the feedwater pump turbine.
(4) Valve test: In order to ensure that the valve can be reliably closed in the event of an accident, the MEH system is designed with an online test function for the main steam valve. During the test, the operation of the feedwater pump turbine should not be affected.
(5) Trip test: The MEH system has the function of performing online electrical overspeed trip test and mechanical overspeed trip test respectively to determine whether the overspeed protection system is functioning normally. Moreover, the two functions are mutually interlocked during the test. When the electrical overspeed trip test is performed, the mechanical overspeed protection is isolated; when the mechanical overspeed trip test is performed, the electrical overspeed protection does not cause a trip.
(6) Self-diagnostic function: The MEH system has a self-diagnostic function to detect internal system faults that may cause unexpected actions (such as power supply, communication, controller, module faults, etc.).
(7) In the remote control speed automatic control mode (receiving instructions from the water supply control system), the MEH system receives instructions from the water supply automatic control system to perform automatic speed control.
(8) When the system switches to manual operation, both speed channels are faulty, the turbine trips, the regulating valve communication fails, or the main steam valve is not open, it will enter the manual speed control mode (soft manual operation). In the manual speed control mode, the operator controls the opening and closing position of the speed regulating valve by increasing or decreasing the speed through the operator station.
(9) The MEH system is equipped with emergency shutdown functions including soft manual shutdown, hard manual shutdown and local shutdown.
2.8 The hydraulic system adopts an independent oil source. The oil supply system should use turbine oil with good stability as the working medium. When the system power supply fails or the oil source loses pressure, it can ensure that the feedwater pump turbine can be safely shut down.
2.9 The failure of any individual component of the MEH system should not affect the operation of the entire system. A single failure should not cause the protection system to malfunction or fail to operate. A single failure should not cause the operation of the feedwater pump to stop, and should not hinder the activation of the standby feedwater pump.
3. Functional characteristics of the small steam turbine safety monitoring system (TSI) and the steam turbine emergency protection system (ETS) :
3.1 The TSI system should include the monitoring items and devices necessary for the safe operation of the steam turbine. Each channel should be equipped with a buffered output terminal and communication interface for connection to the TDM fault diagnosis system, a 4-20mA DC standard signal for the DCS, and two pairs of switch alarm signals. The monitoring items include steam turbine shaft vibration, speed, and axial displacement. The TSI measuring device should be an MMS6000 series product.
3.2 The sensor monitoring systems used for local speed display, control room speed display, and turbine overspeed protection of small steam turbines are set up separately and independently, with the overspeed protection sensors set up with triple redundancy.
3.3 Two sets of axial displacement sensor systems for small steam turbines are set up to meet the selection principle of choosing one of two important parameters.
3.4 The small turbine protection and tripping system (ETS) is equipped with tripping functions for electrical overspeed, mechanical overspeed, low lubricating oil pressure, low vacuum, large axial displacement, large vibration, high bearing temperature, manual tripping, and pump group protection. Among them, the important shutdown protection parameters such as electrical overspeed, low lubricating oil pressure, and low vacuum adopt the principle of selecting two out of three.
3.5 An interlock function is set to interlock the start of the AC/DC lubricating oil pump when the lubricating oil pressure is low.
III. Functional Characteristics of the Steam-Driven Feedwater Pump Set Control System
The steam-driven feedwater pump set includes a main pump, a booster pump, and a monitoring and control system for the pump set. It is equipped with measuring points for pressure, temperature, vibration, etc. The start and stop of the pump and the opening and closing of the electric door can be controlled remotely or locally.
2. Protection configuration of steam-driven feedwater pump set: The pump set protection is set with protection against tripping of the booster pump, bearing temperature and return oil temperature, mechanical seal water temperature, and high temperature of booster pump motor stator, which can cause the turbine to trip.
3. Main unit tripping protection settings: The main unit's main protection action, AST action, and OPC action are all set to trip the auxiliary unit after a 5-second delay.
4. Pump Interlock : An interlock function is set between the steam-driven pump and the electric pump. When the steam pump trips, it will connect to the #1 electric pump (the #1 and #2 electric pumps are interconnected). When the small turbine trips, it will connect to close the steam pump outlet electric valve and the four intake steam valves.
IV. System Operation and Functions
1. Switch
During normal start-up of the small turbine, click the "Small Turbine Braking" button on the MEH screen, and then click the "Break-on Confirmation" button in the pop-up window. The "Small Turbine Braking" indicator will light up, the brake coil will be energized, the brake-off solenoid valve drain port will close, and safety oil pressure will be established. Simultaneously, the main steam valve will open. When the safety oil pressure reaches 1.2 MPa, or 5 seconds after the small turbine brake-on command is issued, or when the operator presses the cancel brake-on button, the brake coil will de-energize, and the "Small Turbine Braking" indicator will turn off, indicating that brake-on is complete. After de-energization, the brake coil will remain in its original position. When the main steam valve is fully open, the "Start-up Permitted" indicator will light up.
If the "Start-Allow" indicator light is not illuminated, the next acceleration operation will be invalid. Conditions for the "Start-Allow" indicator light to illuminate: main steam valve fully open; no tripping conditions; normal safety oil pressure (greater than 1.2 MPa).
Figure 1. Schematic diagram of the switch operation.
2. Cancel the gate operation.
During the gate-hanging process, the gate-hanging command is automatically reset 5 seconds after it is issued. Alternatively, the gate-hanging coil can be de-energized by clicking the "Cancel Gate-Hanging" button to prevent it from burning out due to prolonged energization.
3. Manual/Automatic
Click the "Manual/Automatic" button, and the pop-up window will offer three modes to choose from: manual, automatic, and remote control.
a. Manual mode: Click the "Manual mode" button, the "Manual mode" light will illuminate. In this mode, only valve position operation can be performed. Then click the "Manual valve position setting" button. In the pop-up window, click the valve position increase/decrease buttons to adjust the speed valve opening or closing, thereby controlling the turbine speed.
b. Automatic Mode: Click the "Automatic Mode" button. The "Automatic Mode" indicator will light up. In this mode, you can only set the target speed and acceleration rate. Then click the "Speed Target Setting" button. In the pop-up window, you can select the target speed and acceleration rate by clicking the target speed and acceleration rate increase/decrease buttons. Then click the corresponding "Confirm" button in the window. At this time, MEH will automatically accelerate to the given target speed according to the given acceleration rate. When the small machine speed reaches 1800 rpm, it enters the critical zone, and the "Critical Breakthrough" indicator light will light up. MEH will automatically set the acceleration rate to 1200 rpm. When the small machine speed reaches 3000 rpm, the "Critical Breakthrough" indicator light will go out, the critical breakthrough ends, and MEH will automatically set the acceleration rate back to the originally given acceleration rate.
C. Remote Control Mode: When the speed of the small machine reaches 3000 rpm or higher, the "Remote Control Request" indicator light will illuminate, indicating that this mode can be selected; otherwise, the selection will be invalid. After clicking the "Remote Control Allow" button on the furnace side, the "Remote Control Satisfaction" indicator light will illuminate. Clicking the "Remote Control Mode" button will illuminate the "Remote Control Mode" indicator light, indicating that remote control mode is activated. In this mode, only the furnace can operate the machine to set the target speed. The speed of the small machine will automatically increase or decrease at a rate of 300 rpm until the target speed is reached.
When the deviation between the remote target speed and the actual speed is greater than 500 rpm, it will automatically switch to "automatic mode". In "remote control mode", you cannot directly switch to "manual mode". You must first switch to "automatic mode" and then switch to "manual mode".
After the switch is engaged, if no "manual/automatic" mode selection is made, MEH will default to "manual mode".
Figure 2 Control Method
4. Speed Hold/Cancel Hold
During the speed increase of the small machine, clicking the "Speed Hold" button will immediately hold the speed of the small machine at the current speed. Clicking the "Cancel Hold" button will automatically start increasing the speed of the small machine.
5. Overspeed test
The acceleration rate must be automatically set to 100 rpm in "Automatic mode".
Click the "Overspeed Test" button. In the pop-up window, you will see three options: Electrical Overspeed, Mechanical Overspeed, and Cancel.
5.1. Click the “Electrical Overspeed” button. The MEH will automatically set the target speed to 5460 rpm and automatically increase the speed at a rate of 100 rpm. When the turbine speed reaches 5450 rpm, the electrical overspeed protection will activate, the main steam valve and the speed regulating valve will close, and the turbine will trip.
5.2. Click the “Mechanical Overspeed” button and simultaneously lock the “Electrical Overspeed” test. The MEH will automatically set the target speed to 5600 rpm and automatically increase the speed at a rate of 100 rpm. When the turbine speed reaches 5450-5550 rpm, the mechanical overspeed protection will activate, the main steam valve and speed regulating valve will close, and the turbine will trip.
During the overspeed test, manual intervention can be made according to the operation of the small machine. Click the "Cancel" button and MEH will automatically set the target speed to the current speed, which is the speed before the test.
Figure 3 Control Principle
6. Main steam valve movement test
Click the "Main Steam Valve Activity Test" button, and then click the "Main Steam Valve Half-Closed Test" button in the pop-up window. The main steam valve will start to close slightly. When the main steam valve is closed halfway, it will automatically return to full opening.
During the test, manual intervention is possible. Clicking the "Cancel" button will end the "Main Steam Valve Half-Closed Test" and automatically restore the valve to its open position.
7. Simulation Experiment
Click the "Simulation Test" button, and then click the "Enter" button in the pop-up window. The "Simulation Test" indicator will light up, and the MEH minicomputer will enter the simulation function, which can simulate the startup, speed-up, and various tests of the minicomputer.
Simulation Test Activation Conditions: When the simulation test is activated, the operator clicks the "Turbine Braking" button on the MEH screen, and then clicks the "Confirm Braking" button in the pop-up window. The "Turbine Braking" indicator light illuminates, the brake coil is energized, and safety oil pressure is established. Simultaneously, after the main steam valve opens, the "Start-Allowed" indicator light illuminates. If the "Start-Allowed" indicator light does not illuminate, the next speed-up operation will be invalid. When the main steam valve is fully closed, or if any of the following conditions are met (turbine tripping, low safety oil pressure): the "Start-Allowed" indicator light dims, meaning the turbine cannot be started.
Figure 4 Simulation Input Conditions
V. Effects after renovation
1 MEH system
When the steam-driven feedwater pump starts, the MEH system realizes the automatic speed increase control function. When the speed exceeds 3000 rpm, the MEH system receives instructions from the boiler feedwater automatic control system to realize automatic control of the feedwater pump turbine speed.
2. Small Steam Turbine Safety Monitoring System (TSI)
The small steam turbine safety monitoring system provides accurate and reliable monitoring data for important parameters such as speed, vibration, and shaft misalignment during the safe start-up, shutdown, and stable operation of the small steam turbine.
3. Steam Turbine Emergency Protection System (ETS)
(1) It can realize all the protection functions set for small steam turbines.
(2) The interlocking mechanism for restarting the electric pump after the steam-driven feedwater pump trips has been implemented.
(3) The protection function of the host computer and the small computer is realized.
VI. Benefit Analysis
Since the commissioning of the #4 unit's steam-driven feedwater pump, the automatic start-up rate and protection start-up rate have been 100%, improving the automation level of the steam-driven feedwater pump and ensuring its long-term stable operation. Comparative analysis of indicators before and after commissioning at loads of 140MW and 180MW showed that the unit's power consumption was reduced by 2.45%-2.07%, the overall coal consumption decreased by 1.06-1.56 g/kWh, saving approximately 1226-1577 tons of coal annually, saving approximately 29.687-32.206 million kWh of electricity annually, and reducing costs by approximately 9.54625-10.4198 million yuan. (Coal price calculated at 280 yuan/ton, electricity price at 0.31 yuan/kWh)
VII. Recommendations
(1) In case of an accident, it is recommended that boiler operators switch MEH to manual mode and directly increase or decrease the valve position command of the regulating valve to achieve the purpose of quickly increasing or decreasing the boiler feedwater.
(2) Our plant's steam pumps are supplied with steam from the fourth extraction unit. During operation, it was found that when the valve opening is around 70%, further opening the valve does not change the speed. This suggests that the valve is no longer throttling and is in a full-flow state. Recommendation
Replacing the steam pump supply source with a busbar will meet the system output requirements and improve the stability and reliability of the regulating system.
About the author:
Zhang Xuejun: Male, from Inner Mongolia, engineer, graduated from Inner Mongolia University of Technology. Specializes in thermal control management and automated control.
Mailing Address: Thermal Maintenance Team, Fengzhen Power Plant, Inner Mongolia
Contact Person: Zhang Xuejun
Date of birth : June 1970
Education : Bachelor's Degree
Graduation School: School of Electrical Engineering, Inner Mongolia University of Technology
Major: Power System Automation
Research areas: Thermal engineering control, automation, and PLC control of power plants.
Telephone : 13500648069 Work Address: Thermal Maintenance Team, Fengzhen Power Plant, Fengzhen City, Inner Mongolia
