Control System Of Turbine | Civil Engineering Optional Notes for UPSC PDF Download

Control System

The turbine control system manages the opening of control valves in response to demand signals, allowing steam to flow into the turbine with the aid of a governing system. This system ensures the turbo set operates effectively within an interconnected grid. The governing system performs several crucial functions:

• Speed and load control
• Start-up and shut-down control
• Over-speed control
• Turbine stress calculation
• Initial run-up and synchronization of the unit
• Matching power generation to demand by responding to system frequency changes
• Regulating steam control valve positions to manage load generation in response to operator signals or load dispatch center instructions
• Restricting speed rise within acceptable limits if the unit gets disconnected from the grid (islanding)
• Providing some protective trip functions

Continuous research and advancements in modern turbine technology have led to the development of three main types of governing systems used in power plants today:

Hydraulic Governing System

• Function: Hydraulic governors use a speed controller loop to measure and indicate machine speed through a primary oil measure.
• Speed Transducer: A centrifugal pump that discharges pressure based on machine speed, sending signals to a hydraulic converter to generate a high-power hydraulic signal for operating control valves.
• Speed Control Range: 2790-3210 rpm
• Speed Regulation (Droop): 7%
• Isolation: Typically, hydraulic governors cannot be mechanically isolated like electrohydraulic governors.

Electrohydraulic Governing System

• Control Loops: Consists of speed, load, and pressure control loops.
• Speed Transducer: Uses an electronic transducer to measure machine speed, processes the signal electronically, and sends it to an electrohydraulic converter (E-H) to convert electronic signals into proportional hydraulic signals (I-P converter) for operating control valves.
• Advantages: Provides faster response and precise frequency control.
• Speed Control Range: 0-3210 rpm
• Speed Regulation (Droop): 5%
• Isolation: Electrohydraulic governors can be mechanically isolated by closing the secondary oil line locally.

Major Devices/Components of Governing Systems

1. Remote trip solenoid
2. Turbine trip gear (main trip valve)
3. Starting and load limiting devices
4. Speeder gear
5. Auxiliary follow-up piston
6. Follow-up piston
7. Hydraulic amplifier
8. Electro-hydraulic converter
9. Sequence trimming device
10. Solenoid for load shedding relay

Block Diagram of EH Governing system

Oil Lube Flow in the Circuit

• Control Oil: Oil is sourced from the AOP/MOP discharge header for the governing system.
• Trip/Auxiliary Trip Oil: These circuits are established through the main trip valves under reset conditions.
• Start-Up/Auxiliary Start-Up Oil: These circuits are pressurized through the starting device at the 0% position, with pressure reducing as the starting device position is raised.
• Auxiliary Secondary Oil: This circuit is the output of the hydraulic governor and the input to the hydraulic converter.
• Secondary Oil: This oil is the output to the hydraulic and electrohydraulic converter.
• Test Oil: This circuit is used to test overspeed trip devices when the turbine is running at rated speed.
• Remote Trip Solenoid Valves: There are two solenoid valves. When a turbine trip is initiated, the solenoids are energized, draining the control oil through the valves, which trips the MTV.
• Main Trip Valve: These two main trip gears facilitate all turbine tripping and, under non-trip conditions, establish trip and auxiliary trip oil circuits. They are used for resetting/opening stop valves and producing HP/IP secondary oil.
• Starting Device: This device resets and opens the stop valves, main trip valves, and hydraulic trip devices. It can be operated manually by a handwheel or remotely (MCR) through a motorized actuator.
• Speeder Gear: This device, combined with the starting device, forms a hydraulic governor that provides an input hydraulic signal to the hydraulic converter. It can be operated manually by a handwheel or remotely (MCR) through a motorized actuator.
• Hydraulic/Electro-Hydraulic Converter: This device amplifies and converts hydraulic/electrical signals into hydraulic signals (HP/IP secondary oil) for operating the HP/IP control valve.
• Hydraulic Trip Devices: These devices provide mandatory turbine protection by draining auxiliary trip oil and are reset by auxiliary start-up oil.

Digital Electro-Hydraulic (DEH) Governing System

The DEH control system manages the turbine's operations, including start-up (auto-judging the thermal state), speed and load control, protection, supervision, and communication. It consists of electronic components and an electrohydraulic system that controls speed/load by adjusting valve openness. Key features include:

• Exact load frequency droop with high sensitivity
• Reliable operation during isolation from power grids
• Dependable control during load rejection
• Low transient and steady-state speed deviations under all conditions
• Excellent operational reliability and dependability
• Safe operation with the Turbine Stress Evaluator (TSE) / Controller (TSC)

Operating Method of DEH Governing

1. The HP/IP stop valve opens using HP fire-resistant oil when the dump valve is closed.
2. The dump valve closes when the trip oil is pressurized.
3. Trip oil is pressurized when the trip solenoid valves (5,6,7,8 YV on the trip block) are de-energized, pressurizing the HP trip oil header.
4. Oil enters the bottom of the piston through the isolating valve, causing the stop valve to open slowly against the spring force.
5. In case of a trip, the trip solenoid valves energize, de-energizing AST valves and depressurizing the HP trip oil header. The dump valve opens, draining the oil to LP accumulators, causing the valve to close quickly due to spring force.
6. After reset, the HP trip oil is formed. The de-energized trip solenoid valve pressurizes oil to the dump valve to keep it closed.
7. Pressurized oil reaches the EH servo valve.
8. The EH servo valve, based on DEH command and valve position feedback (from LVDT), connects HP fire-resistant oil to the lower piston port or drains it until matching is achieved.
9. In case of a trip, the dump valve opens due to trip solenoid energization or HP trip fluid header depressurization, leading to the control valve closing quickly due to spring force.

Type of Control System Loops

• Speed Control Loop: The speed control loop compares the speed reference generated by the speed reference limiter circuit with the actual speed of the turbine. Based on this comparison, it provides an output to the valve lift controller. This loop determines the control valve position to adjust the turbine speed to the set value before the generator is synchronized with the power system.
• Power Control Loop: After synchronization with the power system, the task performed by the speed control loop is transferred to the power control loop (or MSP control loop). In this mode, the steam turbine is controlled based on MW output. If there is a drop in grid frequency, the governor valves open instantaneously to provide additional MW for frequency support. When the main steam pressure (MSP) control mode is in operation, the MW-controller tracks the MSP controller.
• Turbine Inlet Steam Pressure (MSP) Control Loop: In MSP mode (Turbine Follow mode), the governing system modulates steam flow to maintain steam pressure at a fixed value. The control system automatically switches to this mode when faults occur that prevent normal boiler control. MSP mode takes over after a 10% drop or a 3% increase in steam pressure. It can also be manually switched to this mode. When the power control mode is active, the steam pressure controller tracks the power controller.
• Steam Pressure Limiter Control Loop: The pressure limiter overrides the governor to progressively reduce steam flow to the turbine if the steam pressure before the HP steam turbine governing valves drops below a predetermined value, limiting severe drops in steam temperature.
• Condenser Vacuum Limiter Control Loop: This limiter controller overrides the governor to progressively reduce steam flow to the turbine if the condenser vacuum falls over a predetermined range, aiming to maintain the condenser vacuum at a set value. The setting is adjustable, and it can be overridden during vacuum raising. It does not operate below 1000 rpm.
• Control Valve (CV) Position Control Loop: Outputs from the above loops are fed into the CV position control loop as its setting. Consequently, it adjusts the CV position according to its characteristic curves. This part outputs the opening command to the Electro-hydraulic (E/H) converter mounted on each CV separately. Each E/H converter has two magnet coils, and two separate signal (±10V) lines are connected to them.

Valves Comprised in the EHG Circuit

Turbine speed control, load control, load shedding relay control, and emergency control are carried out by the open and close actions of the following valves:

• Main Steam Stop Valve (MSSV)
• Main Steam Control Valve (MSCV)
• Reheat Steam Stop Valve (RSSV)
• Reheat Steam Control Valve (RSCV)

Common Failure-to-Trip Conditions

Most failure-to-trip conditions can be attributed to five basic problems:

1. Steam Deposits on the Valve Stem(s)
2. Lubrication Deposits: Accumulation of soaps, dirt, detergents, etc., in the top works of the valve exposed to the elements.
3. Mechanical Failures: Issues such as bent stems within the valve or upper works, damaged split couplings, etc., typically occurring near the center of the valve mechanism.
4. Galling of the Piston in the Hydraulic Latch Cylinder
5. Jamming of the Screw Spindle: Occurs in larger cylinder-type valve designs due to forcing by operations personnel.

Control Oil - Fire-Resistant Fluid (FRF)

A fire-resistant control fluid (FRF) is essential for control and governing systems to reduce fire risks due to its higher ignition temperature compared to mineral oil. Here are the critical requirements and properties for FRF:

• Corrosion Resistance: The fluid should not cause corrosion to steel, copper, copper alloys, zinc, tin, or aluminum. Compatibility must be verified.
• Regeneration: The fluid should be capable of continuous regeneration using Fuller’s earth, ICB resins, or an equivalent regeneration agent.
• Non-Erosive and Non-Corrosive: It must not cause any erosion or corrosion on the edges of control elements.
• Shear Stability: The fluid must be shear-stable and should not contain any viscosity index improvers.
• High Ignition Temperature: FRF leaking from the system should not ignite or burn when in contact with hot surfaces up to 550°C.
• Thermal Stability: It must withstand continuous operating temperatures of 75°C without physical or chemical degradation.
• Miscibility: The fluid should be miscible with up to 3% by volume of TXP from another brand without deterioration.
• Air Release: The air release properties of FRF should not deteriorate in the presence of fluoroelastomer seals and packing used in the system.
• Ortho-Cresol Compounds: The fluid must be free of ortho-cresol compounds.
• Safety and Health: FRF must not pose a safety or health hazard to personnel, provided that the requisite hygiene regulations are followed.

These properties ensure that FRF is reliable and safe for use in turbine control and protection systems, minimizing the risk of fire and ensuring long-term stability and compatibility with system materials.

Contamination in FRF/ Degradation of Phosphate Ester

Phosphate ester fluid, when maintained properly, can offer long service life of around 20 years. However, contamination or improper lubrication practices can degrade the oil and impact the reliability of the machinery. The major contaminants and factors contributing to oil degradation are:

Water Contamination

• Hydrolysis: Phosphate ester has a tendency to hydrolyze, breaking down into acid and alcohol. This process accelerates with increasing temperature and is catalyzed by strong acids and some metals.
• Effect on Fluid: Water contamination can lead to acid formation, affecting the performance and life of phosphate esters. Acidic products can react with metals, forming soaps or salts that can cause stickiness in servo valves.

Fluid Degradation Cycle

• Acid Formation: Uncontrolled generation of acidic products can harm the life and performance of phosphate esters. These acids can react with metals, forming soaps or salts that affect foaming and volume resistivity.
• Lowering of Resistivity: Resistivity is a crucial performance indicator for phosphate ester fluid quality. Low resistivity values are linked to electrokinetic wear, a common failure mechanism of servo valves.

Impact on Components

• Sticking Valves: Fluid degradation can lead to sticking valves, impacting the operation of the system.
• Eroded Servo Valves: The presence of acids and soaps/salts can erode servo valves, reducing their effectiveness.
• Plugged Filters and Blocked Strainers: Contaminants can lead to clogged filters and strainers, affecting the flow of fluid through the system.

Prevention and Maintenance

• Regular monitoring of fluid quality and moisture levels.
• Use of proper dehydration units to remove moisture from the fluid.
• Implementing good lubrication practices and avoiding contamination sources.
• Regular fluid analysis and maintenance to ensure optimal performance and longevity of the phosphate ester fluid.

Solid Contamination

• Particulate contamination in phosphate ester fluid can cause fluid darkening and is typically produced from micro-dieseling, a high-temperature breakdown caused by air release issues.
• Solid contaminants larger than 4 microns are measured by ISO4460, but those smaller than 4 microns are not detected. 
• Investigations have shown that 90% of total solid contamination in EHC fluids is below 4 microns, leading to issues like plugged filters and blocked servo valve strainers.

Oxidation Degradation

• Oxidation is a critical process that occurs due to contamination, resulting in heat generation. 
• This oxidative degradation produces a range of acids and low molecular weight hydrocarbons, which can plate out as varnish or form sludge. These byproducts can increase fluid viscosity, reduce resistivity, and block filters, leading to valve sticking. 
• External heat sources, such as steam lines or welding torches near hydraulic lines, can contribute to oxidation. 
• Unlike internal "hot spots" caused by fluid circulation, external heat sources affect the fluid when it is static.

Aeration (Thermal Stability and Pyrolysis)

• A small amount of dissolved oxygen in the fluid can lead to degradation at lower temperatures, especially in the presence of metals. 
• Changes in the physical and chemical properties of the fluid may be attributed to oxidation rather than pure thermal breakdown. 
• Proper maintenance and monitoring are crucial to prevent these issues and ensure the long-term performance of phosphate ester fluid in hydraulic systems.

Maintenance of FRF Fluid

The maintenance of fire-resistant control fluid (FRF) is crucial for ensuring the proper operation and longevity of hydraulic systems. Here are key maintenance practices:

1. Keep the Fluid Dry, Clean, and Purified: Preventing water, dirt, and air contamination is essential. Use effective filtration systems and ensure proper sealing of the hydraulic system.

2. Check Material Compatibility: Ensure that the FRF is compatible with all system materials. Avoid using materials that may react with the fluid and cause degradation.

3. Regular Fluid Condition Monitoring: Implement regular monitoring of the fluid's condition. This includes checking for contaminants, acidity levels, and overall fluid health.

4. Follow Guidelines for Handling Control Fluid: Adhere to manufacturer guidelines for handling and storing FRF. This includes proper disposal procedures for used fluid.

5. Treatment of Control Fluid Systems: Use proper treatment methods to maintain fluid integrity. This may include filtration, additive replenishment, and moisture removal.

6. Preservative Agents in Control Fluid System: Consider using preservative agents to extend the life of the FRF and protect it from degradation.

7. Precautions for Compatibility: Take precautions to ensure that the control fluid is compatible with other materials in the system. Avoid mixing incompatible fluids or materials.

8. Sampling and Analysis Programs: Implement a regular sampling and analysis program to monitor the condition of the FRF. This can help identify issues early and prevent major problems.

By following these maintenance practices, you can ensure the optimal performance and longevity of your FRF fluid and hydraulic systems.

Purification of FRF Fluid

Ion exchange systems using resins are commonly used for purifying fire-resistant fluids (FRF) in EHC systems. These systems are effective in controlling acidity generation and reducing metal soap content in the fluid. Weak Base Anionic (WBA) resin in hydroxide form can reduce acidity, while a strong acid cationic resin (SAC) can reduce metal soap content. Here are key points regarding FRF oil sampling:

1. Sampling Location: Always sample from the same location directly from the system.
2. Recirculation: Recirculate the FRF long enough before sampling to avoid settling and ensure a homogeneous sample.
3. Sampling During Operation: Perform sampling while the FRF system is in operation.
4. Flush Sampling Point: Prior to sampling, flush the sampling point by draining some FRF into a clean receptacle and then return it to the system.
5. Flushing: Allow some FRF to pass through the sampling point before filling the sample flask.
6. Avoid Aids: Do not use aids like syringes or beakers for sampling.
7. Identification: Mark the specimens clearly and durably for identification.
8. Sampling Records: Complete sampling records and send them to the analyzing laboratory along with the FRF sample.

By following these guidelines, you can ensure accurate and reliable sampling of FRF for purification and analysis.