A comprehensive summary of DEH system knowledge for steam turbines!

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I. Operating Principle of the DEH System The main purpose of the DEH control system is to regulate the speed and power of the steam turbine generator set, thereby meeting the power supply requirements of the power plant. For the DEH control of heating units, it also controls the heating pressure and flow rate. The DEH system is equipped with basic control circuits such as a speed control circuit, a power control circuit, a main steam pressure control circuit, and an overspeed protection circuit, as well as logical circuits for synchronization, frequency regulation limitations, and signal selection and judgment. The DEH system controls the high-pressure valves through electro-hydraulic servovalves, thereby achieving control over the unit’s speed and power. During startup and normal operation of the unit, the DEH receives commands from the CCS or adjustment commands sent by operators via the human-machine interface. It collects signals such as the speed and power of the turbine unit as well as feedback on the position of the control valves, analyzes and processes these signals, performs comprehensive calculations, and outputs control signals to the electro-hydraulic servovalves in order to adjust the opening degree of these valves and thus control the operation of the unit. During the speed-up process of the unit (that is, when the unit is not connected to the grid), the DEH control system controls the unit’s speed through the speed regulation circuit; the power control circuit does not function at this time. In this loop, the DEH control system receives the speed signal from the on-site turbine; after being processed by the DEH optimization logic, it is used as a feedback signal for the speed. This signal is compared with the DEH speed setpoint, sent to the speed loop regulator for error calculation, adjusted via PID, and then the actuator opening setpoint is output to the servo card. This given signal is compared with the position feedback signal from the field LVDT actuator within the servo card, and a control signal is generated to control the opening degree of the electro-hydraulic servo valve, thereby controlling the opening degree of the control valve and hence the speed of the unit. During acceleration, the operator sets the target speed and acceleration rate. After the unit is connected to the grid, the DEH control system switches to the power control mode, with the turbine speed serving as a primary frequency regulation signal for control purposes. Under this circuit, there are two control methods: (1) valve position control, in which the load setting is controlled by the operator. After setting the desired opening degree, the DEH outputs a valve opening setpoint signal to the servo card; this signal is compared with the feedback signal from the valve position, and a control signal is then sent to the electro-hydraulic servo valve in order to control the valve opening and achieve the desired degree of opening. In this approach, power is controlled using the valve opening as internal feedback; errors may occur during actual operation, but this method does not require high specifications for the valve characteristics. (2) Power control mode: In this case, the load circuit regulator comes into play. The DEH receives the on-site power signal, compares it with the set power value, and sends the difference to the load circuit regulator for differential amplification and comprehensive processing. Through PID control, it generates a signal indicating the valve opening degree, which is sent to the servo card. After comparison with the valve position feedback signal, a control signal is output to the electro-hydraulic servo valve, thereby controlling the valve opening degree to achieve the desired power level. Power input control requires good flow characteristics of the valve; otherwise, it will cause load fluctuations. For steam turbine generator sets, there is a nonlinear relationship between the opening degree of the control valve and the steam flow rate; therefore, linear correction of the valve is necessary. The DEH control system incorporates a valve correction function F(X) to carry out this linear correction. When the unit trips, the valve opening setpoint signal is set to 0, closing all valves. The DEH control system is equipped with various protections such as OPC protection, valve position limitation, and quick load shedding. A single-frequency modulation dead zone can also be set. The DEH control system has three operating modes: turbine remote control, turbine automatic mode, and turbine manual mode. (3) The main steam pressure control system serves as an auxiliary control loop for the DEH; it uses the operator-set value as the setpoint and the actual main steam pressure as feedback, thereby performing closed-loop control on the main steam pressure on the turbine side through a PID regulator. II. Working principle of the ETS protection system: ETS stands for the Turbine Emergency Trip Protection System. It is used to monitor certain parameters that have a significant impact on the safety of the unit; when these parameters exceed safe levels, this system activates to close all the steam inlet valves of the turbine, thereby achieving an emergency shutdown. The ETS system features various protection switching functions, automatic trip protection, and first-cause memory capabilities. When either of the conditions in the following example occurs, ETS can send a turbine trip signal, which activates the AST solenoid valve to achieve an emergency shutdown. 1) Turbine overspeed at 110% (from DEH) 2) Turbine overspeed at 110% (from overspeed protection device) 3) Large axial displacement threshold (+1.3mm, -0.7mm from TSI) 4) Excessive expansion difference threshold (+4.5mm, -3.5mm from TSI) 5) High vibration level threshold (greater than or equal to 80um) 6) Radial bearing temperatures of #1~~~#4 exceeding 110 degrees Celsius (4 points, logical OR) 7) Return oil temperature of radial bearings of #1~~~#4 exceeding 75 degrees Celsius (4 points, logical OR) 8) Temperature of positive thrust bearings exceeding 110 degrees Celsius (10 points, logical OR) 9) Return oil temperature of positive and negative thrust bearings exceeding 75 degrees Celsius (2 points, logic) 10) Low lubricating oil pressure threshold of 0.02 (from local sensor) 11) Activation of the generator’s main protection system (from electrical system) 12) Activation of the DEH shutdown protection system (from DEH) 13) Manual shutdown (via two buttons located on the control panel; no switching function)

III. Main functions of the DEH control system:
1) Ability to remotely activate the brake before startup.
2) Setting of the static parameters of the servo system.
3) Control prior to startup.
4) Speed control.
5) Load control.
6) Applying initial load during grid connection.
7) Load feedback control.
8) Primary frequency regulation.
9) CCS control.
10) Load limitation.
11) Rapid load reduction.
12) Valve position limitation.
13) Main steam pressure control.
14) Protection against low main steam pressure.
15) Overspeed control.
16) Online testing.
17) Parameter modification and configuration possible from the engineer station.
18) Complete data recording, display, and printing functions.

IV. Turbine startup and shutdown procedures:
1. Automatic braking activation: The conditions for activating automatic braking must be met simultaneously: (1) “Tripped” status is true; (2) The “Brake” button is pressed.
2. Resetting of automatic braking: Automatic braking can be reset if either of the following conditions is met: (1) “Brake” status is true; (2) 30 seconds have passed since automatic braking was activated. 3. Manual locking of the interface allows for remote locking; this locking action is achieved by energizing the locking electromagnet, which in turn creates a reset oil flow. After the switch is turned on, manual locking allows the locking electromagnet to be powered in order to create a reset oil pressure; therefore, the manual locking switch should be set to the off position when the unit is operating normally. The purpose of setting the static relationship of the servo system is to enable the hydraulic actuator to be controlled by the servo valve throughout its entire stroke. The relationship between the reset signal and the travel of the hydraulic actuator is as follows: a given value of 0–100% corresponds to a lift of 0–100%. To maintain good linearity in this correspondence, it is required that the LVDT used for feedback on the hydraulic actuator be installed so that its core moves within the intermediate linear range. The allowable setting conditions are: both of the following must be met: (1) the rotation speed is below 500 revolutions, and (2) the unit is not connected to the grid ； (3) The “Valve Position Calibration Test Start” button is pressed. After the tuning is complete, set all valve control commands to 0 and then click the “Valve Position Disconnection” button. Note: When enabling valve position calibration, make sure that the steam supply has been disconnected. 4. The control of the steam turbine during startup is a heating process for the cylinder, rotor, and other components. To reduce thermal stress during the starting process, different starting curves should be employed for different initial fruit setting temperatures. When shutting down the turbine daily, DEH can select the appropriate thermal state based on the temperature of the turbine walls. The following are the reference ranges: T < 150 degrees Celsius – cold state; 150 degrees Celsius ≤ T < 300 degrees Celsius – warm state; 150 degrees Celsius ≤ T < 400 degrees Celsius – hot state; T = 400 degrees Celsius – extremely hot state. 5. Speed control: Before the turbine generator set is connected to the grid, DEH functions as a closed-loop speed control system. Its set point is the given speed. The difference between the desired speed and the actual speed. After PID control processing, the servo system is used to regulate the opening of the hydraulic actuator, so that the actual rotational speed adjusts in accordance with the set rotational speed. Once the target speed is set, it automatically approaches that speed at a predetermined acceleration rate. When entering the critical speed range, the acceleration rate is automatically changed to 600 RPM (adjustable) to quickly pass through the critical zone. During the speed-up process, it is usually necessary to warm up the turbine at medium and high speeds in order to reduce thermal stress. (1) In addition to the operator being able to set the target speed via the panel, the DEH automatically sets the target speed under the following conditions: when the turbine just starts operating, the target speed is the current speed; when the oil switch is disconnected, the target speed is 3000 RPM; and when the turbine has tripped, the target speed is zero. (2) The rising rate is set by the operator; the rate lies within the critical speed range of (0, 500) R/mim/mim, and in such cases it is forced to 3000 r/mim/min. (3) A critical speed range is defined by the DEH in order to prevent the turbine from operating within that critical speed range. When the turbine speed enters this critical range, the DEH automatically adjusts at the highest rate. 6. The warming-up speed of the turbine for warming up is 500, 1200, 2500, and 3000 rpm; therefore, the target values are usually set at 500, 1200, 2500, and 3000 rpm. Once the target speed is reached, the speed increase will stop automatically to allow for warming up. If it is necessary to pause the speed increase during this process, the following action can be taken: click the “Hold” button using the mouse on the control screen. When within the critical speed range, the hold command is invalid; only the target speed can be modified. 7. At a constant speed of 3000 RPM, once the turbine speed is stable around 3000 RPM, checks are carried out on all systems prior to grid connection. 8. Once the DEH automatically enters synchronization mode during the same period, its target speed adjusts, based on the value it had when synchronization mode was first entered, in accordance with the speed increase commands sent by the synchronization device as well as the change rate of 100 RPM/min, so that the frequency and phase of the generator meet the requirements for grid connection. 9. Conduct fake grid connection tests on the generator in order to check the reliability of the automatic synchronization system and the accuracy of its adjustments. During the test, the isolating switch on the generator side generated a fake grid-connected test signal. As in normal conditions, during the same period the system uses the DEH and the generator excitation system to adjust the generator’s frequency and voltage. When the simultaneous conditions are met, the oil switch closes. The oil switch closes. Since the disconnector is open, in reality the generator is not connected to the grid. 10. Load-controlled grid connection: When the conditions for synchronizing are met, the synchronization device commands the oil circuit breaker to close; the DEH immediately increases the set value so that the generator can take on an initial load, thereby preventing reverse power from occurring. The synchronous operation mode will be automatically terminated under one of the following conditions: (1) the speed is less than 2950 RPM; (2) the unit is already connected to the grid; (3) the turbine has tripped due to an increase in load. After the turbo-generator set is connected to the grid, load feedback can also be enabled during testing or when operating at basic load. When load feedback is enabled, both the target and setpoint are expressed in MW. When load feedback is not active, the target and set values are expressed as a percentage of the rated load at the rated pressure. After setting the target, the given value automatically approaches the target value at the set load rate, causing the generator load to gradually increase. In addition to the operator being able to set the target load via the panel, the DEH automatically sets the target load under the following conditions: 1) When load feedback is first activated, the target is the current load value (MW); 2) When the generator is first connected to the grid, the target is the initial load setting (%); 3) When feedback is disabled, the target is the reference value (%); 4) In the event of a trip, the target is zero. 5) Under the CCS control mode, the target load rate is set by the operator as per the CCS setpoint. When the load rate changes within the range of 0–50% of the rated power, in MW/min under CCS control mode, for each pulse of load change, the oil actuator causes a change in valve opening equivalent to 0.5% of that in pure condensing mode, in accordance with international practice. The load control method employs a PI controller, which compares the set value with the actual power; after calculation, it outputs signals to control the throttle valve. The controller can be activated by the operator once all of the following conditions are met: 1) The switch is in the closed position; 2) No ETS action is occurring; 3) It is in the “Operator Automatic Mode”; 4) It is connected to the grid; 5) There is no activation of the main steam pressure protection; 6) There are no faults in the power channel; 7) It is not in “Remote Control Mode”. The load controller is deactivated when any of the following conditions occur: 1) The switch is not in the closed position; 2) An ETS action occurs; 3) It is in the “Manual Mode”; 4) It is not connected to the grid; 5) The main steam pressure protection is activated; 6) There are complete faults in the power channel; 7) It is in “Remote Control Mode”; 8) The difference between the set value and the measured value of the power PID is large (greater than 10% of the rated power); 9) The power circuit is disconnected. When the primary frequency regulation steam turbine generator set is operating connected to the grid, the primary frequency regulation function can be activated to meet the requirements regarding grid frequency for ensuring power supply quality. When the unit’s speed is within the deadband range, the frequency adjustment output is zero and primary frequency control does not operate; when the speed is outside this range, primary frequency control activates, and the frequency adjustment setting changes proportionally with the speed (with a default speed ratio of 6%). To ensure that the unit assumes a reasonable amount of primary frequency control load, the DEH’s ratio and deadband are set to match those of the hydraulic control system. 1) The non-uniformity can be adjusted within 3–6%. 2) The dead zone is adjustable within 0–30 RPM. 3) The deadband range is 3000+, with a deadband value of 12. CCS control can be activated by the operator under the following conditions: 1) The operator initiates it automatically; 2) There are no faults in the remote control setpoint channel; 3) A DCS remote control request is received; 4) The system is connected to the grid; 5) There is no ETS active. In CCS mode, the DEH accepts the CCS setpoints and disables load feedback. The CCS shutdown method is activated only when any of the following conditions are met: 1) “Manual” mode; 2) failure in the remote control setpoint channel; 3) a DCS remote control request in the future; 4) not connected to the grid; 5) ETS activation; 6) the “Remote Shutdown” button being pressed. 13. Rapid load reduction: When a fault occurs in the turbine generator set, the valve opening is reduced rapidly to reduce the load and prevent the fault from worsening. During the operation of the rapid load reduction function, when the DEH receives a rapid load reduction input signal, it immediately reduces the load to the corresponding value using a pre-set target value and load reduction rate. DEH—NTK has a rapid load reduction function, which is divided into automatic rapid reduction and manual rapid reduction. Among them, the automatic fast reduction has one setting; manual fast reduction has two settings. For Fast Reduction 1, the rate is a 50% reduction in rated power per minute, with a target load of 20% of the rated power. For Fast Reduction 2, the rate is also a 50% reduction in rated power per minute, with a target load of 50% of the rated power. Activation of automatic rapid load reduction 1: All of the following must be satisfied: 1) The switch for “automatic rapid reduction allowed” is in the on position; 2) RUNBACK is set to true; 3) The load setting is greater than 20% of the rated power. Deactivation of automatic rapid load reduction 1: Only one of the following conditions needs to be met: 1) The switch for “automatic rapid reduction allowed” is in the off position; 2) RUNBACK is set to false; 3) The load setting is less than 20% of the rated power. Activation of manual rapid load reduction 1: All of the following must be satisfied: 1) The “Manual Rapid Reduction 1” button is pressed; 2) The load setting is greater than 20% of the rated power; 3) The system is connected to the grid. Deactivation of manual rapid load reduction 1: Only one of the following conditions needs to be met: 1) The “Manual Rapid Reduction Reset” button is pressed; 2) The system is disconnected from the grid. Activation of manual rapid load reduction 2: All of the following must be satisfied: 1) The “Manual Rapid Reduction 2” button is pressed; 2) The load setting is greater than 50% of the rated power; 3) The system is connected to the grid. Deactivation of manual rapid load reduction 2: Only one of the following conditions needs to be met. Load limitation: When it is not desired for the load on a steam turbine generator set to be too high for a certain period of time, the operator can set a high load limit value, so that the target value set by the DEH is always higher than this limit value. The low-load limit should generally be set at 0 MW. In cases where, for some reason, it is not desired for the valve to be opened too wide for a certain period of time, the operator can set a valve position limit. 16. Main steam pressure control: The main steam pressure is maintained at a normal level by adjusting the opening degree of the high-pressure throttle valve. For the main steam pressure control to be enabled, the following conditions must all be met: 1) The turbine has been synchronized; 2) No ETS actions have occurred; 3) The system is in the “Operator Automatic Mode”; 4) The turbine is connected to the grid; 5) No “Main Steam Pressure Protection Actions” have taken place; 6) There are no faults in the main steam pressure channel; 7) The system is not in “Remote Control Mode”; 8) The “Enable Main Steam Pressure Control” button has been pressed. For the main steam pressure control to be disabled, any one of the following conditions is sufficient: 1) The turbine has not been synchronized; 2) An ETS action has occurred; 3) The system is in the “Manual Mode”; 4) The turbine is not connected to the grid; 5) A Main Steam Pressure Protection Action has occurred; 6) There is a fault in the main steam pressure channel; 7) The system is in “Remote Control Mode”; 8) The “Disable Main Steam Pressure Control” button has been pressed; 9) The deviation between the setpoint and measured value of the main steam pressure PID controller is significant. 17. Main steam pressure protection: When the main steam pressure reaches the set upper limit, load increase is blocked. When the main steam pressure is below the set lower limit, the valve will be gradually closed until the main steam pressure returns to the normal range. Conditions allowing the main steam pressure protection to be activated: All of the following conditions must be met simultaneously: 1) The turbine has been synchronized; 2) No ETS actions have occurred; 3) The system is in “Operator Automatic Mode”; 4) The turbine is connected to the grid; 5) There is no “Main Steam Pressure Protection Activation” signal; 6) There are no faults in the main steam pressure monitoring channel; 7) The system is not in “Remote Control Mode”; 8) The load is greater than 10% of the rated power; 9) The main steam pressure falls within the preset upper and lower limits; 10) The “Activate Main Steam Pressure Protection” button has been pressed.

Conditions for deactivating the main steam pressure protection: Any one of the following conditions is sufficient: 1) The turbine has not been synchronized; 2) An ETS action has occurred; 3) The system is in “Manual Mode”; 4) The turbine is connected to the grid; 5) There is a fault in the main steam pressure monitoring channel; 6) The system is in “Remote Control Mode”; 7) The load is less than 10% of the rated power; 8) The “Deactivate Main Steam Pressure Protection” button has been pressed.

For the high-main steam pressure protection to be triggered, all of the following conditions must be met: 1) The “Main Steam Pressure Protection” has been activated; 2) The main steam pressure exceeds the preset upper limit value. After this protection is activated, any adjustments to the load are prohibited; that is, it becomes impossible to change the target load value. “For the “low main steam pressure protection” to activate, the following conditions must be met simultaneously: 1) The “main steam pressure protection” function is enabled; 2) The main steam pressure is below the “specified main steam pressure value”. Once this protection activates, control will be switched to valve position control, and the load command will be reduced at a rate of 0.5%/S ； When the actual load is less than 10% of the rated load, the low main steam pressure protection activates and automatically resets. 18. Overspeed protection: If the rotational speed of the turbine becomes too high, centrifugal forces may damage the turbine. Although an overspeed limitation function is incorporated into the DEH system to prevent turbine overspeeding, when the rotational speed exceeds the preset value (3300 RPM), the turbine is immediately shut down and all main steam valves are swiftly closed. For safety and reliability, multiple overspeed protection mechanisms are incorporated into the system: 1) DEH overspeed protection at 103%; 2) DEH electrical overspeed protection at 110%; 3) Mechanical overspeed protection via emergency trip rings at 110%–112%. The DEH also features the following shutdown functions: 1) Manual shutdown via the operator interface; 2) Operator-initiated manual shutdown. 19. Online test – Main steam valve tightness test: 1) The main steam valve must undergo a tightness test during the initial installation or major overhaul of the steam turbine. 2) Conditions for initiating the main steam valve tightness test: The following conditions must be met simultaneously: 1) The “Initiate Main Steam Valve Tightness Test” button is pressed and the system is in automatic mode; 2) There are no trip pulses; 3) The turbine is not yet connected to the grid. Test procedure: On the overspeed test screen, click the “Initiate Main Steam Valve Tightness Test” button. Upon initiation of this test, the solenoid valves controlling the main steam valve are energized, causing the main steam valve to close slowly. After the main steam valve has closed completely and its opening degree falls below 5%, all control valves open fully. The turbine then coasts to a stop; observe whether its speed drops below 1,000 RPM. After the test is completed, click the “Main Steam Valve Leakage Test Reset” button to exit the main steam valve leakage test. High-pressure valve tightness test: 1) The high-pressure valve must be subjected to a tightness test during its first installation or after major maintenance. 2) The test conditions require that both of the following be met: 1) The button for initiating the valve tightness test is pressed, and the system is in automatic mode; 2) The turbine is not connected to the grid. Test procedure: On the overspeed test screen, click the “Initiate Valve Tightness Test” button. After the valve is fully opened, close all valves, allow the turbine to run down at its own speed, and check whether the rotational speed drops below 1000 RPM. Once the test is complete, click the “Reset Valve Tightness Test” button to end the test. The oil injection test is conducted to ensure that the emergency trip flyweight can promptly fly out to shut off the steam turbine in the event of an overspeed condition of the unit; therefore, periodic mobility tests on the flyweight are necessary. This test involves spraying oil into the flying ring to increase the centrifugal force and cause it to fly out; however, since the flying ring flies out as a result of the oil spraying test, the brake should not be engaged. Throttle movement test: To ensure the throttle operates smoothly, a movement test can be conducted on it to prevent sticking. Test conditions: All of the following must be met simultaneously – 1) the “Start Throttle Movement Test” button is pressed, 2) the throttle is fully open, 3) the button for selecting the valve to be tested is pressed. Test procedure: On the test screen, click the button corresponding to the valve to be tested, then press the “Start Test” button; once the throttle has moved, click the “Cancel Test” button. After the experiment is completed, click the “Reset Experiment” button to exit the throttle activity test. During the initial installation or major overhaul of a turbine, an overspeed protection test is necessary to verify the accuracy of the overspeed protection’s operation; each type of overspeed protection must be tested and verified. Since the overspeed protection in the system employs both software and hardware safeguards, during overspeed tests, in addition to toggling the buttons on the operating interface, it is also necessary to operate the overspeed test toggle switch in the control cabinet accordingly, so as to disable the output of the hardware circuit. When the toggle switch is set to the left, the setting value for the hardware overspeed protection component is 3605 RPM; when it is set to the right, the setting value is 3365 RPM. When it is set to the middle, the setting value returns to its original value. OPC solenoid valve test: On the overspeed test screen, in the disconnection mode, click the “103% Overspeed” button; the target speed will then automatically increase to 3095 RPM. When the actual speed exceeds 3090 RPM, the 103% overspeed protection activates, and the target speed is set to 2950 RPM automatically, until the actual speed drops to that target value. 110% overspeed test: On the ETS protection switching screen, set the turbine overspeed protection switch to the on position, and also move the overspeed protection switch located in the DEH-NTK cabinet to the left side in order to increase the setting value for the hardware overspeed protection component to 3302 RPM. Click the “OPC disable button” and then the “110% overspeed” button; this will set the target speed to 3305 RPM. When the actual speed exceeds 3090 RPM, the 103% protection should not activate. However, when the actual speed exceeds 3300 RPM, a signal for 110% overspeed protection is sent to the ETS system to initiate shutdown. To prevent failures in the ETS system, an OPC activation signal from the hardware overspeed protection component is also sent out when the speed exceeds 3302 RPM. The ETS turbine emergency shutdown system continuously receives alarm and shutdown signals from the TSI system of the turbogenerator set, the boiler FSSS system, the DEH system, and other equipment, and performs comprehensive logical processing on these signals. When situations arise that prevent the proper operation of thermal systems and equipment or pose a threat to their safety, it is important to issue various shutdown/alarm signals, as well as signals to shut down the units and close the extraction check valves, in order to ensure the safe shutdown of the steam turbine units. Additionally, it provides shutdown diagnostic information for DCS and SOE systems; this makes it one of the essential protection systems in a power plant’s thermal control equipment. This system is centered around a programmable controller (PLC), and it uses redundant CPUs, power supplies, and I/Os to enhance the reliability and flexibility of the protection system.