A short insight to governing of steam turbines.This will be more useful and mean more to Power Plant professionals and for those who are beginners or ...
PRESENTATION BY M DEENADAYALAN C M NIKHIL B VASANTH KUMAR
TOPICS OF DISCUSSION
INTRODUCTION
CONCEPT OF GOVERNING SYSTEM
FEATURES OF KWU GOVERNING SYSTEM
GOVERNING SYSTEM DEVICES
WORKING OF GOVERNING SYSTEM
HYDRAULIC AND ELECTROHYDRAULIC SYSTEM
PROTECTIVE DEVICES IN GOVERNING SYSTEM
QUERIES
WHAT IS GOVERNING SYSTEM ?
Turbine Governing system is meant for regulation of turbine speed under no load and varying load condition.
To regulate the steam control valve position and hence load generated.
To control the initial run up and synchronization of the machine
To assist in matching the power generated to that demanded by responding to the network frequency changes.
SALENT FEATURES OF 200 MW KWU TURBINE GOVERNING SYSTEM
KWU 200 MW turbine is a throttle governed machine. Two governors, electro-hydraulic and hydraulic are incorporated. Both the governors are connected in parallel. Only one governor can be selected at a time through the process of hydraulic minimum selection. The control valve opening depends on secondary oil pressure and there is a definite relationship between the opening of HP control valve and IP control valve. Between 40 to 200MW the turbine load is varied by changing the HP control valve throttling alone.
HYDRAULIC GOVERNOR SALIENT FEATURES
Hydraulic governor is having only one control loop called speed controller. Machine speed is measured and indicated as primary oil pressure. Range of speed control is 2800-3210 rpm. Speed regulation range (droop) of HG is 7%. HG acts as a back-up to the EHG. HG can not be isolated mechanically like EHG and HG is considered to be the primary governor of the KWU turbine
ELECTRO-HYDRAULIC GOVERNOR SALIENT FEATURES
EHG consists of three control loops for speed, load and pressure. EHG optimizes the turbine life by conservative operation with the aid of TSE. Faster response and precise frequency control. Actual speed of the turbine is measured by using hall probes Range of speed control is wider (0-3210) rpm. Speed regulation (Droop) can be changed from 2.5% to 8% in a step of 0.5% even when the machine runs on load. The usual setting of droop in EHG is 5%. EHG can be mechanically isolated by closing the secondary oil line from the local.
GOVERNING SYSTEM DEVICES
Remote trip solenoids
Follow-up piston
Turbine trip gear
Hydraulic amplifier
(Main trip valve)
Electro-hydraulic
convertor
Starting and load limiting device
Sequence trimming device
Speeder gear
Solenoid for load shedding
Auxiliary follow-up piston
relay
DIFFERENT OILS USED IN KWU TURBINE
Control oil (source oil)
Auxiliary secondary oil
Auxiliary start-up oil
secondary oil
Start-up oil
Test oil
Auxiliary trip oil
Primary oil
Lube oil
Jacking oil
Trip oil
REMOTE TRIP SOLENOIDS
TURBINE TRIP GEAR
Trip gear is the main protection mechanism of the turbine. It diverts
the control oil coming from the RTS in to TRIP OIL and AUX.TRIP OIL. Aux.trip oil keeps the trip gear spool valve in open condition so that uninterrupted trip oil pressure is ensured. Aux.trip oil is also connected to three hydraulic safety devices namely Over speed , Low
vacuum &Thrust bearing Trip Devices.
STARTING AND LOAD LIMITING DEVICE
USED FOR RESETTING THE TURBINE AFTER TRIPPING FOR OPENING THE STOP VALVES AND RELEASING THE CONTROL VALVES FOR OPENING STARTING DEVICE AT 0% POSITION
BELLOWS COMPRESSED (CONTROL VALVE CLOSED POSITION) AUX STARTUP OIL RESETS TRIP DEVICES STARTUP OIL RESETS STOP VALVES (ESVs AND Ivs)
PILOT VALVE(STARTING DEVICE) POSITION RAISED
START-UP OIL AND AUXILIARY START-UP OIL CIRCUITS ARE DRAINED ESVS OPEN AT 42% AND IVS OPEN AT 56% POSITIONS OF THE STARTING DEVICE
SPEEDER GEAR
ASSEMBLY OF A BELLOW AND A SPRING BELLOW COMPRESSION CONTROLLED BY
STARTING DEVICE POSITOIN SPEEDER GEAR POSITION (ALTERS SPRING TENSION) PRIMARY OIL PRESSURE
OPERATED FROM UCB OR LOCAL PRIMARY OIL PRESSURE (SPEED FEEDBACK)
AUXILIARY FOLLOW UP-PISTONS
TEST VALVES
HP/IP STOP VALVES RECEIVE TRIP OIL THROUGH THEIR RESPECTIVE TEST VALVES PORT OPENINGS FOR BOTH TRIP OIL AND START-UP OIL USED IN RESETTING OF STOP VALVES AFTER TURBINE TRIP VALVE CAN BE OPERATED WITH HANDWHEEL
WORKING OF GOVERNING SYSTEM CONTROL OIL
SPEEDER GEAR TRIP OIL
AUX START UP OIL TO RESET TRIP V/V
STARTING DEVICE
TRIP VALVE TRIP OIL
AFUP EHC INPUT
CONTROL OIL
TRIP OIL
CONTROL OIL FROM RTS
HYRAULIC CONVERTER
EHC
HP/IP STOP VALVE RESET
AUX TRIP OIL AUX TRIP OIL
LOW VACUUM TRIP DEVICE OVER SPEED DEVICE
FUP
FUP IP SECONDARY OIL
HP SECONDAY OIL
THRUST BEARING DEVICE
SELECTION BETWEEN HYDRAULIC AND ELECTRO HYDRAULIC GOVERNING MINIMUM PRESSURE BETWEEN HP AND IP SECONDARY OIL TURBINE STARTUP USING EHC WITH HC SYSTEM FOR BACK UP SECONDARY OIL PRESSURE SET HIGHER THAN THE EHC SYSTEM AND MAINTAINED USING A TRACKING SYSTEM
ESV OPENING
PROTECTIVE DEVICES IN GOVERNING SYSTEM
TURBINE PROTECTON Two type of protection a)Electrical protection b)Mechanical tripping
Electrical protection(through RTS)
Turbine UCB manual trip Stator water conductivity >21 mho/sec Stator water flow <13 cubic m/s Boiler to turbine through UTR Gen to turbine through UTR Lube oil pressure low -low <1.2 ksc(2/3) Fire protection(channel 1&2) UTR manual trip ATRS shut down programme
Mechanical devices
TG over speed trip (bolt-1 –3300 rpm,2-3330 rpm) Condenser vacuum low Axial shift high
GOVERNING CONTROL SYSTEM Governor control systems consist of three basic elements: These elements are Sensing elements – fly ball weight etc. Transmitting elements - mechanical linkage, hydraulic or pneumatic pressure, electrical signals or, as is most common, a combination. and Correcting elements - the valve or valves that control the flow of steam to the turbine.
MECHANICAL PROTECTION DEVICESOVER SPEED TRIP DEVICE OR EMERGENCY GOVERNORS •
The function is to stop turbine when permissible speed is exceeded.
•
Two over speed bolts, one acts when speed reaches 110% (3300rpm) of the rated speed and others comes in action at 111% of the rated speed (3330 rpm)
•
It is situated in the front end bearing pedestal & consists of eccentric bolt, adjusting screw, spring, pawl and screw plug. .
•
The speed is measured with the help of hall probe.
•
When the over speed occurs & goes beyond the preset value(s) the bolts fly out due to centrifugal force & strike against the respective pawls. This will lead to the opening of the Drain port through which Auxiliary trip oil will be drained, resulting in the draining of the trip oil & thereby tripping the turbine.
•
Below the tripping speed the bolt is held in position by spring and bears against the plug.
•
At speed higher than over speed limit, the centrifugal force overcomes the spring force and forces bolt out of turbine rotor. This activates the turbine trip mechanism.
•
Before over speed device comes into action, an another term called DROOP should come in action to reduce the speed of the turbine.
•
Speed regulation of the turbine is called Droop
DROOP OR PROPORTIONAL BAND
Droop= amount of speed change from no load to full load divided by rated speed In KWU turbines Droop is in the range from 2.5 to 8%. Usually it is taken as 5% of the rated speed. This maximum speed is called ‘Dead band’ beyond which Droop comes in action to increase or decrease the speed of the turbine by opening or closing the control valve . It is necessary in the control system to sense a change in speed & thus to reposition the valves
If droop =5%, it means +2.5% from 3000 rpm.
LOW VACUUM TRIP DEVICE
The function of the device is to trip the turbine on dropping of condenser vacuum below a limit. Condenser vacuum permissible is -0.9 ksc. It consists of twin chamber arrangement with a separating diaphragm. Upper side is connected to the condenser vacuum and lower side to atmosphere. Upper side is also subjected to the primary oil pressure through the piston. Under poor vacuum with sufficiently high machine speed(>600rpm), the net force acting on the upper side is more than the lower side . This causes the opening of the drain port through which the Aux.trip oil will drain. This protection is not effective during low speed because of the non availability of the primary oil pressure. For resetting it doesn’t require Auxiliary start up oil.
THRUST BEARING DEVICE
It is used to protect the turbine against high axial shift and high thrust pad wear. It consists of a two pair of small cams, one set before and another after a pawl are fitted on the turbine shaft 180⁰ opposite. The pawl is so arranged that whenever the shaft moves horizontally too much (>0.6mm)either in left side or right side, one sets of cam comes in contact with the pawl and thereby thrust bearing trip device’s latch will be released. This will cause the opening of the drain port through which auxiliary trip will be drained.
