sui@qdczpower.com

2. 30MW Condensing Steam Turbine Electrical Heating System Retrofit

Overview of Original System Operation

The power plant utilizes fully hydraulic or semi-hydraulic turbine regulation systems. During operation, various regulation deficiencies are frequently observed: sluggish speed (load) response, excessive oil motor oscillation, unstable speed control, load fluctuations, slow response to changing conditions, or overshoot. These phenomena are directly related to the response characteristics of the regulation system itself.

The turbine regulation system employs either a fully hydraulic or an electro-hydraulic hybrid system, reflecting a transitional phase in market control systems. Although an electro-hydraulic controller is used for signal processing, the servo amplifier and actuator system still rely on relatively traditional mechanical components. The precision and sensitivity of these mechanical elements fail to harmonize effectively with the controller, preventing the overall system from achieving precise control.

Considering modern grid requirements for automated and intelligent power plant control, existing systems exhibit the following shortcomings:

1) Speed measurement relies on a pulse pump, generating a pulse oil pressure signal proportional to the square of rotational speed. At low speeds, pulse oil pressure becomes negligible, making closed-loop speed control technically challenging;

2) In flow-balancing systems, decreasing pulse oil pressure Px opens the steam valve, while increasing it closes the valve. This means that if an oil pipe ruptures or a gasket leaks, causing Px to drop, the valve moves toward further opening—contrary to safety design principles.

3) The low mechanical spring stiffness in crude oil motors results in significant valve response delay, failing to meet rapid adjustment requirements.

4) The system exhibits slippage under certain load conditions, indicating poor output stiffness of the hydraulic motor.

5) Steam extraction and power (backpressure) cannot achieve self-regulating control.

Summary of the original system's shortcomings:

First, the regulation safety system and lubrication system share a common oil tank, failing to ensure the cleanliness of the regulating oil. This significantly increases the likelihood of sticking and sluggishness in the speed regulation system.

Second, the entire hydraulic speed control system comprises transmission amplification components such as electro-hydraulic converters, regulating spool valves, differential valves, hydraulic motors, mechanical feedback levers, and start valves. The system's control circuitry is lengthy and interlinked, requiring stability at every stage. Its inherent self-regulating capability is insufficient, creating multiple potential failure points and complicating maintenance.

Equipment Modification Plan

With the continuous advancement of automation and intelligent technology in the industry, high-voltage cut-off electric feedback servo systems have become the most automated and precise control systems for small and medium-sized turbomachinery.

DEH is built upon the coordinated operation of both electrical and hydraulic systems. This necessitates not only the modification of the electronic control system but also the critical overhaul of the hydraulic control system, specifically:

DEH Electronic Control System: Utilizes the latest DEH electronic controller.

DEH Hydraulic Control System: Modifies the high-pressure regulating valve hydraulic motor and extraction steam hydraulic motor into an electro-hydraulic servo system with an independent high-pressure oil source.

The upgraded self-contained hydraulic motor system (Figure 1) employs an independent control oil station. This direct-acting hydraulic motor directly drives the regulating steam valve to control steam intake. It utilizes electrical signals for position feedback, eliminating intermediate amplification stages (such as pressure transducers and differential oil valves) to reduce response delays and cumulative errors. This system significantly outperforms traditional hydraulic and transitional regulation systems in terms of regulation accuracy, linearity, and operational convenience.

The self-contained oil-powered servo system features:

(1) High precision and speed, with high hydraulic motor stiffness and redundant pure electrical signal feedback for stroke feedback, ensuring exceptional control accuracy and quality.

(2) The regulating system's oil supply is completely separate and independent from the original steam turbine oil supply system, guaranteeing the cleanliness of regulating oil while remaining unaffected by fluctuations in other oil systems.

(3) Convenient on-site commissioning with easy fault detection and troubleshooting.

Tailored optimization and upgrade solutions are proposed for each unit based on its unique characteristics and extensive field application experience. These solutions effectively address control deficiencies in fully or semi-hydraulic speed regulation systems, enhancing operational stability, improving efficiency, reducing system failure rates, and lowering specialized maintenance requirements.

Implementation of the Plan:

DEH Control System Solution:

The DEH system is built upon the coordinated operation of both electronic and hydraulic control systems. It requires not only modifications to the electronic control system but also critical upgrades to the hydraulic control system, specifically.

DEH Electronic Control System:

Adopt the latest DEH electronic controller.

DEH Hydraulic Control System:

Modify the high-pressure regulating valve hydraulic motor and extraction steam hydraulic motor into an electro-hydraulic servo system with an independent high-pressure oil supply.

Retrofit Installation:

Highly adjustable performance enables comprehensive modification of the original unit's regulation characteristics, resolving numerous issues within the hydraulic system such as load slippage and sticking. Ensures rapid responsiveness and control precision in regulation quality.

Simple retrofit installation: On-site modification requires only removing the original hydraulic motor and control system components. The self-contained hydraulic motor is installed in the original motor's position and connected to the independent oil supply line, minimizing field work. High control precision: Utilizing a complete self-contained hydraulic motor system enhances output stiffness, ensuring stable operation of the DEH control system.

Features of the New System

● Hydraulic motors deliver high output stiffness, ensuring exceptional control precision and quality:

The unit is equipped with servo hydraulic motors to replace the original governor hydraulic motors and extraction hydraulic motors. The hydraulic motor system operates at pressures up to 14 MPa, completely eliminating constraints from the original hydraulic system. By directly connecting valves through the steam distribution mechanism, it achieves high control precision and stability. Full closed-loop positioning control delivers positioning accuracy of 0.01 mm. The hydraulic motor's dynamic response and closing speed reach 0.2 seconds, matching the control performance of high-pressure fire-resistant hydraulic systems.

● The oil supply for the regulating system is separate from the original turbine oil supply system.

The control system requires an electro-hydraulic servo system with high precision, demanding exceptionally clean oil. Utilizing an independent oil supply system, coupled with multiple precision filters, fully ensures the required filtration accuracy.

● Independent gas station

The oil supply system has been upgraded to a modular configuration with a single oil supply unit. The primary advantage of adopting this hydraulic actuator system lies in its high working oil pressure of 14 MPa, which enables a reduction in the size of the servo hydraulic actuator controlling the valve while maintaining equivalent valve lifting force.

● Redundant configuration of the oil supply system

The oil supply system employs a redundant configuration with two sets of oil pumps serving as mutual backups to ensure reliable oil delivery. The oil source utilizes one active and one standby pump, enabling online switching, and features outputs such as low-pressure interlock and high-pressure alarm.

● Simple on-site debugging with easy fault inspection and resolution

All hydraulic equipment undergoes factory testing and commissioning to ensure that after on-site installation, only simple static adjustments are required before startup conditions are met. Multiple monitoring points are added to the hydraulic motors, enabling real-time on-site data monitoring for each unit and significantly simplifying fault diagnosis.

● Reduce maintenance requirements

Compared to low-pressure turbine oil systems, annual minor overhauls require no disassembly of the hydraulic motor; only the seals of hydraulic components in the pump station and hydraulic motor need to be replaced.

Features of the DEH Electronic Control System:

The upgraded DEH controller features a Chinese human-machine interface to enhance system operability. It supports standalone operation, thermal-electric coupling regulation, steam injection regulation, extraction steam regulation, sliding pressure operation, automatic speed ramping and speed control, power control, main steam pressure control, primary frequency regulation, turbine online testing, simulation, historical trend queries and printing, and other functions. This improves the unit's automation level and protective measures while extending its service life.

(1) Automatic adjustment control function

● Speed Ramp-up

After the operator sets the target speed, the unit automatically controls the regulating valve along the empirical curve corresponding to the current thermal state. This completes the speed ramp-up, warm-up, and critical speed transition until reaching steady-state control at 3000 r/min. (The ramp rate is automatically adjusted when crossing the critical speed.) During the ramp-up process, the operator can also control the unit's speed ramp-up by modifying the target speed, ramp rate, speed hold time, and other parameters.

● 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

Upon generator grid connection, the DEH automatically increases the setpoint to apply an initial load (6 MW, adjustable) to the generator, preventing reverse power flow.

● Load Ramp

After grid connection, the operator can control the unit using valve control mode, power control mode, pressure control mode, or CCS mode as needed.

● Valve Control Mode

The operator directly controls the valve opening by setting the target valve position, and the DEH maintains this valve position. At this time, the unit load and steam pressure automatically balance.

● Power Control Mode

The operator controls unit load by setting target power. DEH performs power closed-loop control using actual turbine output as feedback, maintaining constant unit load.

● Pressure Control Mode

The operator controls pre-turbine pressure by setting target pressure. DEH controls valve opening to maintain constant main steam pressure.

(2) Restriction Control Function

● Load and Valve Position Limitation

Limit values are manually set. DEH automatically restricts load within upper and lower limits and limits valve position below the specified value.

● Power Feedback Limitation

When the difference between actual power and power setpoint becomes excessive, the power feedback loop is automatically disconnected, switching to open-loop control while simultaneously reducing the power setpoint.

● Low Main Steam Pressure Limitation

When main steam pressure falls below the limit value, the DEH automatically reduces valve opening to limit load, allowing main steam pressure to recover.

● Rapid Load Shedding

The DEH features three rapid load shedding rates (fast, medium, slow) to address 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 (Over-Speed Protection)

The speed measurement module receives high acceleration signals to activate the OPC solenoid valve and acceleration correction voltage signals. It rapidly closes the regulating steam valve to reduce overshoot during transition. After a delay period or when speed drops below 103% n (or 105% n0, adjustable), it automatically reopens and maintains the unit speed at 3000 r/min.

(3) Test Control Function

● Mock Grid Connection Test

Upon receiving the signal that the mock grid connection test isolating switch is open, the DEH can automatically coordinate with the electrical system to complete the mock grid connection test.

● Overspeed Test

The driver can operate via the LCD screen to increase the rotational speed, triggering the overspeed protection to verify the impact pin and the electrical overspeed protection's activation speed. During mechanical overspeed testing, the DEH's electrical overspeed protection setpoint automatically adjusts from 3300 r/min to 3390 r/min, functioning as a backup overspeed safeguard.

● Valve Tightness Test

Operators can use the LCD interface to conduct tightness tests on the throttle valve and flow valve, with automatic recording of coasting time.

(4) Protection and Control Functions

● System Status Monitoring

Through system network data communication, all parameters and statuses of the steam turbine are intuitively displayed on the LCD. The system features over-limit alarms, fault alarms, and convenient access to alarm items. Critical signals such as tripping, quick shutdown, and others are equipped with SOE functionality, with an event resolution of 1ms.

The historical database holds over 500 entries, storing data for more than one year. Historical trend screens can be accessed at any time.

● Overspeed Protection

Upon unit uncoupling, when rotational speed exceeds 110% of rated speed, the DEH issues a signal to interrupt system operations, rapidly closing the main steam valve and regulating steam valve.

(5) Enhanced Automation Capabilities

● Automatic Logbook Generation

Drivers can set schedules or event-triggered daily/hourly logs for automated recording.

● Historical Data Logging and SOE Recording

● Simulation Functionality

(6) Key Technical Specifications

● Speed control range: 200~3600 r/min

● Speed control accuracy: ≤+1 r/min

● Speed deviation rate: Online adjustable within 3%~6% range

● Load control range: 0~120%

● Load control accuracy: ≤+0.2% of rated value

● Main steam pressure control accuracy: +0.1MPa