Steam Turbine Control

Steam Turbine Control System

Basic of Steam Turbine

Turbine • The turbine converts the high-pressure driving force created by steam, water, combustion gas into rotation energy (mechanical energy) that turns the generator rotor. • The generator rotor is connected to the turbine via a shaft.

Fossil Fired Steam Unit

Turbine Stage – The moving blade Æ “Buckets” – A set of stationary blade Æ “Nozzle Partition”

HP/IP turbine blading

HP & IP Rotor

LP Rotor

TYPE OF STEAM TURBINE

Straight Condensing Turbine CONTROL VALVE INITIAL PRESSURE

TO CONDENSER EXHAUST PRESSURE LESS THAN ATMOSPHERIC

Straight Non-condensing Turbine CONTROL VALVE INITIAL PRESSURE

EXHAUST PRESSURE GREATER THAN ATMOSPHERIC

Non automatic Extraction Turbine (Condensing or Non condensing) CONTROL VALVE INITIAL PRESSURE

EXCTRACTION ONE OR MORE

EXHAUST PRESSURE

Automatic Extraction Turbine (Condensing or Non condensing) CONTROL VALVE INITIAL PRESSURE

CONTROLLED EXTRACTION PRESSURE

EXUAST PRESSURE

Mixed Press Turbine (Condensing or Non- condensing) P2 = INITIAL PRESSURE P1 = INITIAL PRESSURE

P1 ≠ P2

EXHAUST PRESSURE

Reheat Turbine (Condensing or Non-condensing) FROM REHEAT CONTROL VALVE

CONTROL VALVE

INITIAL PRESSURE

REHEATER

CONDENSER

Turbine Sections (Cylinders) – Single cylinder – Multi-cylinder • Tandem-compound • Cross-compound

• Tandem compound turbine-generator – High press (HP), intermediate pressure

(IP), Low pressure (LP) and generator on a common shaft

From boiler

Cross over

MSV CV

HP

MSV Æ Main Stop Valve CV

Æ Control Valve

LP

LP

shaft

GEN

Condenser

Non-reheat tandem compound steam turbine

From boiler

Cross over

MSV CV

HP

IP

LP

LP

shaft

GEN

RSV IV

Condenser

RH RH

Æ Reheater

RSV

Æ Reheat Stop Valve

IV

Æ Intercept Valve

Single reheat tandem compound steam turbine

From boiler

Cross over

MSV CV

HP

IP

IP RSV IV

LP

LP

LP

LP

GEN

Condenser

RH RH

Æ Reheater

RSV

Æ Reheat Stop Valve

IV

Æ Intercept Valve

Single reheat tandem compound steam turbine

Steam Steam turbine turbine rotor rotor

• Cross compound turbine- generator – Commonly with HP, IP and one generator on one shaft , LP and another generator on a second shaft

• Greater capacity • Improved efficiency • More expensive

100 MW – 700 MW HE series

• H turbine - Single flow HP turbine module - one stop and control valve • E turbine - Single flow IP / LP section combined in one cylinder

Modular Design of the E-Turbine

250 MW – 700 MW KN series

• K turbine - combined HP/IP • N turbine - Double flow LP section

Modular Design of the K-Turbine

Stop and Control valve

LEFT Generator

sv cv HP stage LP stage

RIGHT

sv cv LP HEADER

HP HEADER

Process parameter control • • • •

MW output. Main steam Fuel / Air Feed water

Steam Turbine Controller

The tasks of steam turbine controller • • • • • • • • •

Starting up and shutting down the turbine Synchronizing with grid* Loading the turbine Influencing the grid regulator Pressure stabilization Limitation of load Control of Isolated operation Turbine interception during load rejection Monitoring function

Steam turbine controller ( Conventional Hydraulic Governor)

Steam turbine control (DEH Governor)

Comparison between DEH governor and conventional hydraulic governor

Speed and Load Control Speed signal

+

TNH 2 out of 3 Voting Logic

-

1/S

+

Speed summer Speed setpoint

Load Setpoint

+ TNR

Hold

TNE

Intgrl Summer

Speed Gain

+ +

Speed /Load summer

PWR

Hard ware of turbine controller • Very fast response digital system - Microprocessor-based controller • Redundant structure – Dual, TMR

• Man machine interface with CRT • Standardize interface technique with other systems • Control panel* – Hard wired push buttons comply with minimum requirements for backup operation

SPEEDTRONIC TM Mark V Steam Turbine Controls

Mark V Control Simplex Configuration

Mark V Control TMR Configuration

Mark VI

Siemens AG, Power Generation Modern Automation Concepts

Alstom P400 Turbine Control

GE Fanuc PACSystem RX7i

TMR SYSTEM

Mark V TMR Control Sequence

Hardware voting for analog Outputs

Hardware voting of logic outputs

Servo Position Controller (SPC)

Block diagram of SPC

Single coil wiring - Integrating Servo Application

Over Speed Protection Device

Over views diagram of TurboSentry Application

Steam Turbine Operation

STARTING AND LOADING • Successful operation • Maintenance • Long life a turbine-generator

Main factors involved during starting and loading • • • •

1) Thermal Stress 2) Vibration 3) Shell and Rotor differential expansion 4) HP exhaust temperature

Thermal Stress • Operating Transients can be produce high thermal stresses in stem turbine rotor and shell – Start-ups – Load changes – Shutdowns

Turbine Stress Evaluation (TSE) • Limiting the acceleration during start-up • Load ramp gradients under normal loading operation

Differential expansion • Rotor temperature change faster than outer shells. • Lead to differential expansion large enough to cause internal rubbing

Oil and Hydraulic Fluid System Check lube oil temp. > 50 F°/ 70 F°

Starting AC lube oil pump

Check lube oil pressure & flow to bearing

A Check lube oil protection sequence

Return to normal configuration

Recheck lube oil pressure

Lube oil ready

Oil and Hydraulic Fluid System Check EH fluid reservoir level

Check EH fluid temp.

Open pump Start number 1 suction valve fluid pump (if used)

B Adjust cooling water flow

Place backup fluid Check auto. EH fluid pump on auto. Start of ready backup pump

Oil and Hydraulic Fluid System Lube oil and hydraulic systems ready

A AND B

Turbine rotor prewarming • rotor warming (prewarm) prior to roll-off after the vacuum system is in operation. • rotor warming (heatsoak) be performed during turbine acceleration and involves a “part speed hold” • In either case the manufacturer ‘s specific recommendations should be followed

Valve trip testing • the main turbine valves should be tested to verify that the testable functions of the trip system are working properly. - verifying that all valves involved are in the proper position. - manually tripping the turbine from the control-room panel and observing that all valves return to the tripped status.

Turbine preroll • Energize the supervisory instruments. • Energize the EH (electro-hydraulic) electronic governor at least 2 h before admitting steam into the turbine. • If an electrical-trip system shall be energized prior to turbine roll-off. • Control & Stop valves are closed • resetting the turbine trip system and the status of the various valves when reset vary with manufacturers.

Energize supervisory instrument Energize electronic governor Energize electric trip system Check main stop valve closed Check control valve closed Check intercept valve closed Check generator ok Lube oil system ok

Unit on turning gear AND

Unit on turning gear Hydraulic fluid system ok

Turbine rotor prewarmed (if required)

Vacuum ok AND Steam seal ok Drain valve open Eccentricity ok Eccentricity ok

1

1

Reset turbine

All trip lockouts reset Ready to roll Deactivate initial pressure reg. Check main stream press. & Temp ok Water induction preroll checks

AND

Digital Electro-Hydraulic Control System (DEHC)

RATCHABURI THERMAL POWER PLANT

Turbine Specification • Rated output • Revolution • Steam condition

735,000 KW 3000 r.p.m.

– Main steam pressure 24.22 MPa – Main steam temp. 538 °C – Reheat steam temp. 566 °C – Condenser vacuum 700 mm Hg – Number of Extraction 8

Unit Capability and Minimum Load • Unit capability refers to the maximum possible megawatt output that the generating unit can safely produce. • Minimum load is the smallest amount of generation that a unit can sustain for an extended period.

Boiler MSV

GV

HP MSV

IP

LP LP LP LP

GV

DEHC

Reheater RSV

ICV

RSV

ICV

GEN

• • • •

MSV : Main Stop Valve GV : Governing Valve ICV : Interceptor Valve RSV : Reheat Stop Valve

Servo drive

Solenoid valve

Stop Valves / Throttle Valves - Normally provide fast interruption of the main energy input to the turbine. ‰ A stop valve is defined as an open or closed valve ‰ A throttle valve have some portion of its opening through which it can modulate flow ( used for turbine control during startup)

DEHC Function • Control function – – – – – – –

Speed up control Valve transfer control Close all valves Load / Frequency control Load limiter IMP control (Impulse chamber pressure) Valve management

DEHC Function • Protection function – EOST (Electrical Over Speed Trip) – OPC (Over Speed Protection Control) – IPR (Initial Pressure Regulator)

DEHC Function • Test function – Valve closing test – OPC test – MOST test (Mechanic Over Speed Trip)

Start-up curve rpm 3000 2200

400 T/B reset Rub check Startup

Heat soak

Valve transfer

41E Synchronizing on

Speed control • Turbine speed is controlled by MSV from starting to rated speed. • GV full open

Speed control position

RSV GV (full open)

MSV

T/B reset

EH AUTO

Target speed Program select 400 rpm GO

Speed Control Speed reference

+

3000 Hz Speed A Speed B

>H

-

K

MSV control

• What is 3000 r.p.m. ? r.p.m. Æ round / minute ( r/min ) • Frequency of network = 50 Hz (Thailand) Hz

Æ round / second (r/sec)

50 HZ = 50 round / second In one minute = 60 Second Rotation in one minute = 50 x 60 = 3,000 round or 3000 round / minute = 3000 r.p.m.

• • • •

Turbine speed = 3000 rpm. No. of gear teeth = 60 Pulse / min = 60 x 3000 Pulse / sec. = 60 x 3000 = 3000 Hz 60

Taget speed & Speed change rate • Taget Speed – 400 rpm – 2,200 rpm – 3,000 rpm

• Speed Change Rate – 75 rpm/min – 150 rpm/min – 300 rpm/min

Valve transfer • After turbine has reached the rated speed. • Speed control change from MSV to GV

Valve transfer GV

Full open Bias 0%

GV position demand

(No load valve position) Full open

MSV Valve transfer

Valve transfer complete

41E permit

GV

MSV

Start-up

Full open

Speed control

Transfer

Speed Control

Full open

Synchronize

Load Control

Full open

Close all valves • For turbine stop and rub check • MSV, GV and ICV are fully Closed by the sevo valve. • RSV still open.

Load control • Load frequency control • Load limiter – limit steam flow

Load control DCS

Load setter

GV control

+

111%

speed

>111%

speed

>111%

2/3

Turbine Trip

MOST (Mechanical Overspeed Trip) test • To confirm mechanical overspeed protection device (before synchronization) • EOST setting 111% Æ 115% • OPC is blocked

MOST setting

+ 15 rpm - 15 rpm

MOST 110% (+ 0.5%)

OPC (Overspeed Protection Control) • Power-load imbalance • To avoid overspeed trip when load rejection • Speed up to 107.5% OPC valve solenoid are energized.

• Power output Æ IP Turbine inlet pressure (Mechanical) • Power output Æ CT (current transformer) (Electrical)

OPC (Overspeed Protection Control) Boiler MSV

GV

HP MSV

IP turbine inlet pressure

Px

IP

LP LP LP LP

GV

Gen.Current

GEN

IPR (Initial Pressure Regulator) • Protect against excessive decrease of the initial (main) steam pressure • Main steam deviate from main steam pressure reference > 25 bar • Turbine water damage prevention • Load reference runback rate 200%/min.

IPR (Initial Pressure Regulator) Load setter 100%

200% / min

GV = 25%

Time (sec)

Valve Closing Test • Confirm the safety function – GV & MSV valve test (RH, LH) – RSV & ICV valve test (RH, LH)

GV – MSV Valve test MSV IMP IN GV

Test

ICV – RSV Valve test RSV

energized

ICV

De-energized

Close

OPC Test energized GV De-energized ICV

1 Sec.

Overall unit control • Boiler following mode (Turbine leading) • Turbine following mode (Boiler leading) • The integrated or coordinated boiler turbine control • Sliding pressure mode

Boiler following mode (Turbine leading) • Change in generation are initiated by turbine control valves. • The boiler controls respond to the resulting changes in steam flow and pressure by changing steam productions.

Turbine following mode (Boiler leading) • Change in generation are initiated by change input to the boiler. • The MW demand signal is applied to the combustion control. • The turbine control valves regulate the boiler pressure.

The integrated or coordinated boiler- turbine control • Provides an adjustable blend of both boilerfollowing and turbine-following mode of control. • The improvement in unit response achieved through integrated control. • The integrated control strikes a compromise between fast response a boiler safety.

The sliding pressure mode • The control valves are left wide open. • The turbine power output is controlled by controlling the throttling pressure through manipulation of the boiler control.