DEH (Digital Electro-Hydraulic Control System) is the "brain and nerve center" of a turbine. It uses a computer as the core and high-pressure fire-resistant oil as power to achieve precise control of the turbine's speed, load, and valves, as well as comprehensive protection. It is the core system for the safe and stable operation of modern thermal power units.
I. Basic and Core Control Functions
1. Q: What stages are included in the DEH system's core speed control function? How does each stage operate?
A: Speed control is the fundamental function of DEH, covering the entire process of the unit from cranking to grid connection, with a control accuracy of ±1 rpm. It is key to the safe startup of the unit.
- Cranking Control: After the unit shuts down, the cranking motor drives the main shaft to rotate at a low speed of 2~3 rpm to prevent bending caused by temperature differences. DEH is responsible for cranking engagement and disengagement, speed monitoring, and protection. If the cranking current is too high or the speed is abnormal, it will automatically trip.
- Acceleration Control: Operators set the target speed and acceleration rate (usually 100~300 rpm/min), and DEH automatically adjusts the main steam valves/governing valves to control steam flow, ensuring the turbine speed rises smoothly. In the critical speed range (e.g., 1200~1800 rpm), DEH will automatically increase the acceleration rate to quickly pass through the resonance zone and avoid severe unit vibrations.
- Warm-up Control: When the speed reaches the warm-up speed (usually 2040 rpm), DEH maintains speed stability for low-speed and high-speed warm-up, ensuring uniform heating of the turbine cylinders and rotor, reducing thermal stress. The warm-up duration is automatically calculated based on cylinder temperatures, and once conditions are met, acceleration continues automatically.
- Constant Speed Control: Once the speed reaches the rated speed of 3000 rpm, DEH enters speed closed-loop control, automatically stabilizing the speed at 3000±1 rpm, preparing for grid connection. At this point, even if grid frequency fluctuates, the unit speed can remain stable.
- Load Rejection Speed Control: When the unit experiences load rejection, DEH quickly closes the regulating valves to suppress overspeed, then automatically adjusts valve openings to stabilize the speed at 3000 rpm, creating conditions for rapid reconnection to the grid.
2. Question: What are the modes of the DEH load control function? What operating conditions are they suitable for?
Answer: After the unit is connected to the grid, the DEH automatically switches to load control mode, mainly including the following four types:
- Valve Position Control Mode (Open-loop Control): The DEH directly controls the governor according to the valve opening command set by the operator, and the load fluctuates with changes in steam parameters. This mode is suitable for conditions where the unit has just been connected to the grid, steam parameters are unstable, or the CCS system fails.
- Power Closed-loop Control Mode: The DEH uses the generator's active power as a feedback signal and automatically adjusts the governor opening to maintain the load at the set value. The control accuracy can reach ±1% of the rated load, and this is the main mode for normal unit operation.
- Pressure Control Mode (Boiler-following Turbine Mode): The DEH uses main steam pressure as a feedback signal and automatically adjusts the governor opening to maintain stable main steam pressure. This mode is suitable for boiler-side faults or load-restricted conditions, where the boiler is responsible for adjusting the load and the turbine is responsible for regulating pressure.
- Coordinated Control Mode (CCS): The DEH works in coordination with the boiler control system to jointly adjust the unit load and main steam pressure. The DEH is responsible for the fast response to load commands, while the boiler slowly adjusts the fuel amount to ensure stable steam parameters. This is the basic mode for AGC (Automatic Generation Control) operation.
3. Question: What is primary frequency regulation? How does the DEH achieve the primary frequency regulation function?
Answer: Primary frequency regulation is the first line of defense for grid frequency stability. It refers to the automatic adjustment of the turbine load based on frequency deviations when the grid frequency changes, compensating for power deficits or surpluses in the grid.
- Implementation principle: DEH has a built-in speed-load static characteristic curve (i.e., droop characteristic, usually 4%-5%). When the grid frequency drops (speed decreases), DEH automatically opens the turbine valve further to increase the unit output; when the grid frequency rises (speed increases), it automatically closes the turbine valve to reduce the unit output.
- Key parameters:
- Dead band: generally ±2 rpm (equivalent to ±0.033 Hz), to avoid frequent load adjustments due to minor frequency fluctuations
- Limit: the maximum adjustment range of primary frequency regulation is generally ±10% of the rated load to prevent unit overload
- Response time: ≤3 seconds, providing fast response to grid frequency changes
- Operational requirements: The primary frequency regulation function must be fully engaged at all times and must not be withdrawn arbitrarily, otherwise it will affect grid frequency stability.
II. Valve Management and Optimization Functions
4. Question: What are the core components of DEH's valve management function?
Answer: Valve management is an important feature that distinguishes DEH from traditional hydraulic control systems. It achieves efficient and economical operation of the unit by optimizing the opening sequence and opening degree of valves.
- Valve Flow Characteristic Correction: DEH has built-in valve flow characteristic curves, which are corrected based on actual operating data to ensure a linear relationship between valve opening and steam flow, improving control accuracy.
- Single Valve / Sequential Valve Switching: Automatically or manually switches valve control modes based on unit load, balancing stability at low load and economy at high load.
- Valve Tightness Control: Verifies the tightness of the main and regulating valves through tightness tests to prevent internal leakage that could cause overspeed or reduced efficiency of the unit.
- Valve Operation Test: Regularly operates the valves to prevent them from sticking in one position over a long period, ensuring reliable closure of valves during protection actions.
5. Question: What is the difference between single-valve control and sequence valve control? How should they be switched correctly?
Answer: These are the two core modes of valve management, directly affecting the unit's thermal efficiency and safety.
Comparison Items | Single-Valve Control (Throttling Regulation) | Sequence Valve Control (Nozzle Regulation)
--- | --- | ---
Working Principle | All regulating valves open simultaneously, adjusting the steam flow by throttling | Regulators open sequentially, with only the last valve providing throttling
Thermal Efficiency | Large throttling loss at low load, low thermal efficiency | Small throttling loss at low load, high thermal efficiency (1%~2% higher than single-valve)
Thermal Stress | Steam enters the cylinder evenly, rotor thermal stress is low, unit stability is good | Steam partially enters the cylinder, rotor heats unevenly, thermal stress is high, vibration prone
Applicable Conditions | Unit startup, low-load operation, frequent load changes | Stable high-load operation
Switching Requirements:
- Switching Conditions: Unit load stable at 50%~70% of rated load, main steam parameters stable, no major operations
- Switching Process: DEH automatically adjusts the valve openings step by step; switching time is about 10~15 minutes
- Precautions: During switching, closely monitor unit vibration, axial displacement, and cylinder temperature changes. If any abnormalities occur, stop switching immediately.
III. Protection and Interlock Functions
6. Q: How is the overspeed protection system of the DEH system structured? What is the action logic of each protection?
A: Overspeed is the most dangerous fault for a turbine. The DEH system builds a "three-line defense" overspeed protection system, with multiple layers of protection to ensure the safety of the unit.
- First line of defense: OPC overspeed protection (103% of rated speed, 3090 rpm)
- Action logic: When the speed exceeds 3090 rpm, the DEH quickly closes all governing valves and extraction stop valves, while keeping the main steam valves open.
- Action result: After the speed drops below 3000 rpm, the DEH automatically reopens the regulating valves to maintain stable speed.
- Feature: Fast action (≤0.1 seconds), does not trip the unit, effectively suppresses speed rise during load rejection.
- Second line of defense: 110% electric overspeed protection (3300 rpm)
- Action logic: When the speed exceeds 3300 rpm, the DEH sends a trip command to the ETS (Emergency Trip System).
- Action result: Closes all main steam valves, regulating valves, and extraction stop valves; the unit performs emergency shutdown.
- Third line of defense: 112% mechanical overspeed protection (3360 rpm)
- Action logic: When the speed exceeds 3360 rpm, the fly-hammer type emergency trip device acts, mechanically shutting down the unit through linkages.
- Feature: Fully mechanical structure, unaffected by electrical system failures, providing the last line of safety defense.
7. Question: Besides overspeed protection, what other important interlock protection functions does DEH have?
Answer: DEH works in coordination with the ETS and TSI (turbine supervisory instrumentation) systems to provide comprehensive protection:
- Load rejection protection: When the generator output breaker trips, DEH immediately triggers OPC protection, quickly closing the governor valves to prevent overspeed.
- Low vacuum protection: When the condenser vacuum drops below the limit, DEH triggers a shutdown through ETS to prevent high exhaust temperature in the low-pressure cylinder from damaging equipment.
- Low lubricating oil pressure protection: When the lubricating oil pressure drops below the limit, DEH triggers a shutdown through ETS to prevent the bearings from being burned.
- Low EH oil pressure protection: When the EH oil pressure drops below the limit, DEH triggers a shutdown through ETS to prevent the valves from failing to close.
- Large axial displacement protection: When the rotor's axial displacement exceeds the limit, DEH triggers a shutdown through ETS to prevent friction between moving and stationary parts.
- Manual shutdown protection: Operators can manually trigger a shutdown via the emergency stop button on the DEH control panel.
