4. Effectiveness of Steam Turbine Speed Control System Retrofit

Introduction to the DEH Electric Control Independent Oil System Retrofit

I. Pre-Retrofit Overview:

The site utilizes a 7.5MW extraction-condensing steam turbine generator unit. Its configuration includes: one high-pressure main steam valve, one high-pressure regulating valve, and one low-pressure extraction steam regulating valve. The high-pressure main steam valve is manually operated via a handwheel and functions as an on/off control valve; The high-pressure regulating valve and extraction regulating valve are each driven by a hydraulic motor via a lever mechanism, i.e., the governor hydraulic motor and extraction hydraulic motor are continuous control types. The original turbine regulation system was a 505E control system, with the hydraulic control system utilizing a low-pressure turbine oil system employing CPC electro-hydraulic converters. The unit system exhibited load fluctuation phenomena, poor output stiffness of the hydraulic motors, and severe valve position deviation, failing to meet operational requirements.

II. DEH Electric Control Independent Oil System Retrofit Plan

1. Considering economic viability and stable operation, retrofits will primarily target the following two components:

(1) Electronic Control System:

The MACS-DCS K series products will be employed, integrated with DEH-specific speed measurement modules, servo modules, high-speed CAN communication hardware, and software products. This configuration ensures speed feedback and acceleration feedback cycles are less than 50ms to form the DEH control system.

(2) Hydraulic Control System:

Convert the high/low-pressure regulating valve hydraulic motors into an electro-hydraulic servo system with an independent oil supply.

a. Advantages of Modification:

High regulation performance comprehensively alters the original unit's regulation characteristics, eliminating numerous hydraulic system issues such as slippage and sticking. Ensures rapid response and control precision for regulation quality. Achieves regulation performance comparable to high-pressure fire-resistant oil DEH systems. Simple retrofit installation: on-site modification only requires removing the original hydraulic motor, installing the self-contained hydraulic motor in its position, and connecting the oil supply lines, minimizing field work. High control accuracy: the complete self-contained hydraulic motor system enhances output stiffness, guaranteeing stable operation of the DEH control system.

b. Modification Features:

● High output stiffness of hydraulic motors delivers exceptional control precision and quality.

● Separate oil supply system for the control system from the original turbine oil supply.

● Independent oil station hydraulic motor retrofit approach.

The added oil supply system is a modular unit. The primary advantage of this hydraulic motor system is its high working oil pressure of 14 MPa, enabling reduced servo hydraulic motor size for control valves while maintaining equivalent valve lifting force.

● Redundant oil supply system configuration

The oil supply pumps are redundantly configured, with two sets serving as mutual backups to ensure reliable oil delivery. The two pumps operate in a hot-standby configuration with online switching capability, featuring low-pressure interlock and high-pressure alarm outputs.

● Simplified On-Site Commissioning and Troubleshooting

All hydraulic equipment undergoes factory testing and commissioning. Upon installation at the site, only basic static adjustments are required before startup readiness. Multiple monitoring points are added to the hydraulic motors, enabling real-time on-site data monitoring for each unit and significantly simplifying fault diagnosis.

● Reduced maintenance requirements

Compared to low-pressure turbine oil systems, annual minor maintenance requires no disassembly of hydraulic motors—only replacement of seals in hydraulic components at the pump station and motors. Compared to high-pressure fire-resistant oil systems, no maintenance of regeneration units is required.

1. Fluctuations in High- and Low-Voltage Speed Regulators Before Modification

2. DEH Electronic Control Unit Functions and Technical Specifications

2.1 Primary Functions:

Control circuits individually or jointly implement functions including programmable turbine start-up, automatic regulation, parameter limiting, protection, monitoring, and testing.

2.1.1 Automatic Regulation Control Functions:

● Speed Ramp-up: After setting the target speed, the unit automatically controls the regulating valve along the empirical curve corresponding to the current thermal state, completing the ramp-up, warm-up, and critical speed transition until reaching steady-state speed control at 3000 r/min. During ramp-up, the process can also be controlled by modifying the target speed, ramp rate, speed hold time, etc.

● Automatic Synchronization: After the turbine reaches steady speed, the DEH accepts commands from the automatic synchronizing device to automatically control the unit to synchronous speed.

● Grid Connection with Initial Load: After generator grid connection, the DEH automatically increases the setpoint to enable the generator to automatically assume initial load, preventing reverse power.

● Load Ramp: After grid connection, the unit can be controlled via valve-controlled mode, power-controlled mode, pressure-controlled mode, or CCS mode as needed. This coordinates with the boiler control system to complete the steady-state-slip-to-steady load ramp process.

● Valve-Controlled Mode: Directly controls the valve opening by setting the target valve position. The DEH maintains the valve position constant, automatically balancing unit load with steam pressure.

● Power Control Mode: Unit load is controlled by setting a target power value. DEH performs power closed-loop control using actual turbine output power as feedback to maintain constant unit load. If generator active power signal is used as the power signal, certain logic processing is required. Note: Avoid using power control mode if boiler steam pressure is not in automatic regulation.

● Pressure Control Mode: Controls pre-turbine pressure by setting a target pressure. The DEH adjusts the valve opening to maintain constant main steam pressure.

● CCS Mode: In CCS mode, the DEH receives valve position setpoint signals from the CCS master controller and directly controls the valve opening. The DEH and CCS master controller work together to achieve various control functions, including turbine-boiler synchronization, boiler-turbine synchronization, and coordinated turbine-boiler operation.

● Primary Frequency Regulation: Digital signals are transmitted via CAN bus. The deviation between the set speed and actual speed undergoes unequal rate processing to generate a primary frequency regulation setpoint. This setpoint can be applied to either the power setpoint or the valve control setpoint. When the deviation between actual speed and 3000 r/min exceeds ±15 r/min, the system exits power control mode, voltage control mode, and CCS mode, switching to valve control mode.

● Automatic Secondary Frequency Regulation: Automatically performs secondary frequency regulation to restore frequency to 50 Hz.

2.1.2. Limiting Control Function:

● Load and Valve Position Limiting: Manual setting of limit values. DEH automatically restricts load within upper/lower limits and limits valve position below the set value.

● Low Main Steam Pressure Limiting: When main steam pressure falls below the limit value, DEH automatically reduces valve opening to limit load, allowing main steam pressure to recover.

● Low Vacuum Limit: When vacuum drops to the limit value, the DEH automatically reduces valve opening to decrease load.

● Rapid Load Shedding: The DEH features three rapid load shedding rates (fast, medium, slow) corresponding to different auxiliary equipment failures. When the CCS issues a rapid load shedding signal, the DEH reduces load to the corresponding value at the specified rate.

● OPC Control: During unit load shedding, the DEH receives signals from oil switch tripping or overspeed (103% n). It rapidly closes the regulating steam valve to reduce overshoot during transition. After a delay or when speed drops below 103% n, it automatically reopens and maintains unit speed at 3000 r/min, awaiting re-synchronization.

2.1.3 Test Control Functions:

● Mock Synchronization Test: Upon receiving the mock synchronization test isolation switch open signal, the DEH automatically coordinates with electrical systems to complete the mock synchronization test.

● Friction Inspection: Equipped with friction inspection capability, it issues alarm prompts and automatically determines the next turbine operating mode based on unit conditions.

● Overspeed Test:

● Operable via LCD interface, this function increases rotational speed to verify the activation of the overspeed protection system, including the release of the overspeed hammer and the electrical overspeed protection's trip speed. During mechanical overspeed testing, the DEH system automatically adjusts the electrical overspeed protection trip value from 3300 r/min to 3390 r/min, functioning as a backup overspeed protection measure.

● Valve Operation Test: To ensure reliable valve closure during accidents, the DEH system enables online operation testing of high-pressure main steam valves. Turbine operation and load remain unaffected during these online valve tests.

● Valve Tightness Test: Operable via the LCD screen, this function performs tightness tests on both regulating valves and main steam valves, automatically recording deadband time.

2.1.4 Protection Control Functions:

● System Status Monitoring: Through system network data communication, the DEH shares DCS data to monitor real-time changes in key parameters during startup or operation. Historical data can be recorded using the DCS system's configuration history database.

● Fault alarm indicators are set on the LCD for alarm item identification. Critical signals such as tripping, quick shutdown, etc., feature SOE functionality with 1ms event resolution.

● Overspeed Protection: When unit is uncoupled and rotational speed exceeds 110% of rated speed, the DEH issues a signal to interrupt system actions, rapidly closing the main steam valve and regulating steam valve.

2.1.5 Automation Level Functions:

● Automatic Meter Reading: Supports scheduled or event-triggered daily/hourly meter readings for automated recording.

● Simulation Capability.

2. Fluctuations in the high-pressure governor prior to modification

2.2 Key Technical Specifications:

● Speed Control Range: Handwheel speed ~ 3600 r/min

● Speed Control Accuracy: <±1 r/min

● Speed Unevenness: 3-6% continuously adjustable

● Exhaust Pressure Unevenness: 0~20% continuously adjustable

● Load Control Range: 0-120%

● Load control accuracy: ≤0.2%

● Main steam pressure control accuracy: +0.1 MPa

● Speed-up rate control accuracy: +0.1%

● Control system insensitivity: <0.06%

● Speed overshoot during load shedding: <7% of rated speed, maintaining 3000 r/min

● Mean Time Between Failures (MTBF) for DEH device: ≥25000 hours

● System availability: ≥99.9%

● Maximum overshoot speed during full load shedding: <7% n

● Control system cycle time: <50 ms

3. Fluctuations in the low-voltage governor prior to modification

3. Retrofit Plan for Hydraulic Servo Mechanism

The adoption of a complete self-contained hydraulic motor system addresses the issue of “insufficient output stiffness in conventional hydraulic motors failing to overcome sliding loads generated by the steam distribution mechanism,” thereby ensuring stable operation of the DEH control system. This represents the optimal solution for retrofitting traditional low-pressure turbine DEH systems.

The self-contained actuator control system constitutes a component of the turbine digital electro-hydraulic (DEH) control system. It primarily comprises the oil supply system (oil station, accumulator, anti-wear hydraulic oil, etc.), actuators (oil motor, servo valve, OPC solenoid directional valve, etc.), and oil piping system (oil lines and various valves). This system enables precise control of turbine speed ramping and load regulation.

This solution retains the safety control components of the original system (including emergency shut-off valves, gate valves, magnetic circuit breaker throttles, etc.) and the lift plate-type steam distribution mechanism, while replacing all regulating components (original hydraulic motors and all other regulating assemblies). Self-contained hydraulic motors are employed.

(1) Implementation Plan:

Retain: Steam distribution mechanism, components of the original hydraulic safety system (main steam valve automatic shut-off device, emergency shut-off valve, magnetic circuit breaker throttle, etc.), and certain operational controls—primarily the manual emergency stop device at the turbine head and the emergency shut-off valve oil injection test device.

Remove: Pressure transducers, differential oil valves, hydraulic motors, throttle orifice regulating devices, and associated oil pipelines (dismantle and seal).

Add: Self-contained hydraulic motor compatible with electro-hydraulic converter (for continuous servo regulation), OPC solenoid valve assembly, low-pressure pressure switch assembly, etc.

Increase: One high-pressure independent anti-wear hydraulic oil supply system; add speed sensor and other hydraulic auxiliary equipment.

Principle: Replaces the pulse pump with a magnetic resistance encoder to achieve wide-range speed measurement. Replaces the synchronizer and pressure transducer with an electro-hydraulic converter. It receives control commands from the DEH to directly generate pulsed hydraulic pressure, controlling the hydraulic motor. Through the servo board, it receives real-time displacement feedback signals from the LVDT mounted on the hydraulic motor piston, thereby forming an electro-hydraulic servo system to achieve continuous control of the high-pressure regulating valve.

(2) Hydraulic Servo System and Working Principle of the DEH Device:

The hydraulic servo system of this DEH device comprises a self-contained actuator, oil supply system, and control system. The working principle diagram is as follows:

1. Self-Contained Actuators

(1) High- and Low-Pressure Hydraulic Motors

High- and low-pressure hydraulic motors employ closed-loop control, utilizing dual LVDTs for position feedback. They receive safety oil cutoff signals from the unit to rapidly close their respective steam valves, achieving the quick-close function. Quick-close is initiated by energizing the OPC solenoid valve.

(2) Hydraulic Motor Operating Principle

Hydraulic actuators serve two functions: continuous control of valve opening and rapid closure of the valve system, i.e., the valve's quick-closing capability.

(3) Oil Pipeline System

The oil pipeline system primarily consists of a set of oil pipes and a high-pressure accumulator. The oil pipes connect the oil supply system to the actuator, forming a working circuit and conveying the working medium.

(4) Primary Structure and Components of the Hydraulic Actuator

The hydraulic actuator comprises a cylinder, displacement sensor, electro-hydraulic converter, and control block. The control block houses components such as a servo valve, unloading valve, OPC solenoid valve, check valve, and pressure monitoring port for the control valve actuator. The actuator piston is driven by anti-wear oil. For double-acting pistons, both opening and closing are driven by pressurized oil. To ensure the impact force of the steam valve disc against the seat remains within permissible limits during rapid cylinder closure, a buffer device is installed at the rear end of the cylinder piston. This device decelerates the piston to zero speed upon reaching the end of its stroke.

(5) Hydraulic Cylinder

The hydraulic cylinder comprises a piston rod, piston, front end cap, rear end cap, cylinder barrel, buffer device, dust guide ring, piston rod tandem seal, piston seal, and corresponding connecting components. All seals exhibit excellent physical and chemical compatibility with high-pressure anti-wear fuel oil.

(6) Electro-Hydraulic Servo Valve

An imported electro-hydraulic servo valve serves as the electro-hydraulic converter.

(7) Unloader Valve

The unloader valve is mounted on the hydraulic manifold of the hydraulic motor. Its primary function is to release pressure oil from the lower chamber of the cylinder through the unloader valve after the safety oil pressure drops, when the steam turbine requires emergency shutdown or a single valve needs rapid closure testing. At this point, regardless of the magnitude of the signal output by the servo valve amplifier, the valve will close.

(8) Redundant Displacement Sensor

The displacement sensor is connected to the hydraulic actuator's piston rod. When the actuator moves, the sensor outputs an electrical signal measuring its displacement, thereby adjusting the steam valve opening. The displacement sensor is redundantly configured.

4. Fluctuations in Load Data on the DCS Interface

2. Independent Oil Supply System

The anti-wear oil supply system (hereinafter referred to as the supply unit) is a combined integrated assembly providing qualified high-pressure working fluid for the entire hydraulic system's normal operation. It utilizes high-pressure anti-wear hydraulic oil as the working medium.

The supply unit's pump station employs high-pressure pumps. Each oil supply unit features two independent pump sets. These can operate simultaneously to double oil flow or function independently as mutual backups during normal operation. When steam turbine startup requires higher flow rates or system pressure drops below normal levels, electrical interlocks automatically activate the backup pump set to meet system demands. The unit's design allows for pump set switching and repairs during single-line failures.

The oil supply system incorporates a pilot-operated relief valve as a safety device. Should system pressure exceed the set value for any reason, the relief valve activates to prevent the system from sustaining excessive pressure surges. Additionally, several pressure switches are installed to provide alarms for both low and high system pressure conditions. When system pressure falls below the set value, these switches activate the standby pump set.

● The oil supply system features a self-contained oil filtration and oil cooling system.

(1) Composition and Main Components Overview

The oil supply unit adopts a modular design, with primary equipment including oil pumps, oil filters, relief valves, oil coolers, accumulators, air filters, liquid level indicators, and essential monitoring instruments.

① Oil Tank

Constructed from stainless steel welded sealing structure, featuring an access hatch for future maintenance and tank cleaning. An air filter is mounted on the tank to provide sufficient air filtration precision during system ventilation, ensuring oil system cleanliness.

② Pump Assembly

This assembly employs a high-pressure pump with a flexible sleeve pin coupling. The pump and motor connection uses a flange sleeve coupling for easy maintenance access.

③ Oil Filter and Return Oil Filter

The oil filter is designed as an integrated plate type as required, while the return oil filter is a cartridge type.

④ Oil Cooler

The oil cooler employs a shell-and-tube design, installed independently for easy maintenance and replacement.

⑤ Accumulator Assembly

The oil supply unit incorporates a bladder-type accumulator. The accumulator assembly includes a shut-off valve, safety valve, pressure gauge, etc., ensuring convenient and reliable operation.

⑥ Essential Monitoring Instruments

The oil supply unit also includes essential monitoring instruments such as pump outlet pressure gauge, system pressure gauge, return oil pressure gauge, pressure sensors, liquid level thermometer, and thermocouples. These instruments interface with the central control room instrument panel, computer control system, and safety systems to monitor and control the operation of the oil supply unit and hydraulic system.

5. Test Procedure for Self-Contained Hydraulic Motor Pull-Valve After Modification

3. Control System

(1) Normal Control

The valve position control signal from the DEH controller operates the servo valve to drive the hydraulic cylinder, with position feedback provided by the LVDT. The hydraulic motor has a safety factor of 2x, ensuring that fluctuations in oil supply pressure during pressure regulation do not affect its control capability.

(2) Quick-Close Control

The actuator incorporates a quick-close solenoid valve enabling “shut-off” and “quick-close” functions. Designed as an “energized shut-off” valve, the quick-close solenoid activates the hydraulic motor to enter the ‘disengaged’ state when energized. Upon engagement of the safety system, the safety hydraulic pressure builds. This pressure activates the pressure switch, establishing the safety voltage and transitioning the actuator to the “engaged” state. Upon “trip” or ‘close’ of the safety system, safety hydraulic pressure is lost, safety voltage is de-energized, and the actuator enters the “shutdown” state, stopping the unit.

(3) Performance:

● 100% continuously adjustable operation with a closed-loop servo control system, ensuring high reliability.

● Positioning accuracy < 0.1% of full stroke.

● Repeatability < 0.1% of full stroke.

● Control deadband < 0.05% of full stroke.

● Dynamic response time < 20 milliseconds; quick-close time 0.2 seconds.

● Mechatronic design with two series configurations: simple external hydraulic piping or no external piping. Long service life, maintenance-free.

● Compact size, easy installation, and highly convenient for on-site maintenance.

● Integrated control system accepts any standard control signal. Equipped with digital communication interface for seamless integration with other control methods.

● Intelligent design allows on-site adjustment of actuator displacement, stroke speed, valve preload, and more. Supports multiple functions including controlled object characteristic correction (e.g., valve curve adjustment), water hammer protection, and minimum control point.

● Exceptionally high output stiffness. Hydraulic system operating pressure range: 10–15 MPa. Supports online adjustment based on site conditions, completely unrestricted by load type.

● Intermittent reliable oil supply control mode with extremely low motor input power, representing a low-energy consumption, energy-efficient design approach.

● Environmentally friendly fluid working medium, independent sealed oil supply system, reliable cleanliness assurance, and zero environmental pollution, resolving the environmental contamination issues associated with fire-resistant oil.

● When integrated with Hollysys' MACS(DEH) electronic control system, it comprehensively resolves performance limitations of DEH low-pressure turbine oil systems (such as low control accuracy and low output stiffness). It achieves the same control precision as high-pressure fire-resistant oil systems without requiring the extensive maintenance associated with fire-resistant oil media.