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1 Turbine Governing Introduction Concept of Governing System Features of KWU Governing System Governing System Devices Working of Governing Systems Hydraulic and Electro Hydraulic System Protective Devices in Governing System What is Governing System? Turbine governing system is meant for regulation of turbine speed under no load and varying load conditions. To regulate the steam control valve position and hence the 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. Salient 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 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 the HP control valve and IP control valve. Between 40 and 200 MW the turbine load is varied by changing the HP control valve throttling alone. Hydraulic governor salient features Hydraulic governor has only one control loop called the speed controller. Machine speed is measured and indicated as primary oil pressure. Range of speed control is 2800 rpm – 3210 rpm. Speed regulation range (Droop) of hydraulic governor is 7%. Hydraulic governor acts as a backup to the electro hydraulic governor. Hydraulic governor cannot be isolated mechanically like the electro hydraulic governor and is considered to be the primary governor of the KWU turbine. Electro hydraulic governor salient features Electro Hydraulic Governor consists of three control loops for speed, load and pressure. Electro hydraulic governor optimizes the turbine life by conservative operation with the aid of the Turbine Stress Evaluator. 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.

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Turbine Governing Introduction

� Concept of Governing System � Features of KWU Governing System � Governing System Devices � Working of Governing Systems � Hydraulic and Electro Hydraulic System � Protective Devices in Governing System

What is Governing System?

� Turbine governing system is meant for regulation of turbine speed under no load and varying load conditions.

� To regulate the steam control valve position and hence the 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. Salient 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 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 the HP control valve and IP control valve.

� Between 40 and 200 MW the turbine load is varied by changing the HP control valve throttling alone.

Hydraulic governor salient features

� Hydraulic governor has only one control loop called the speed controller. � Machine speed is measured and indicated as primary oil pressure. � Range of speed control is 2800 rpm – 3210 rpm. � Speed regulation range (Droop) of hydraulic governor is 7%. � Hydraulic governor acts as a backup to the electro hydraulic governor. � Hydraulic governor cannot be isolated mechanically like the electro hydraulic

governor and is considered to be the primary governor of the KWU turbine. Electro hydraulic governor salient features

� Electro Hydraulic Governor consists of three control loops for speed, load and pressure.

� Electro hydraulic governor optimizes the turbine life by conservative operation with the aid of the Turbine Stress Evaluator.

� 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.

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� Speed regulation (Droop) can be changed from 2.5% to 8% in steps of 0.5% even when the machine is running on load. The usual setting of droop in electro hydraulic governor is 5%.

� Electro hydraulic governor can be mechanically isolated by closing the secondary oil line.

Governing system devices

� Remote trip solenoids � Turbine trip gear (main trip valve) � Starting and load limiting device � Speeder gear � Auxiliary follow-up piston � Follow-up piston � Hydraulic amplifiers � Electro hydraulic converter � Sequence trimming device � Solenoid for load shedding relay

Different nomenclature for oil systems used in KWU turbine

� Control oil � Auxiliary startup oil � Startup oil � Auxiliary trip oil � Trip oil � Auxiliary secondary oil � Secondary oil � Test oil � Primary oil � Lub oil � Jacking oil

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Trip gear is the main protection mechanism of the turbine. It diverts the control oil coming from the RTS into trip oil and auxiliary trip oil. Auxiliary trip oil keeps the trip gear spool valve in open condition so that uninterrupted trip oil pressure is ensured. Auxiliary trip oil is also connected to three hydraulic safety devices namely overspeed, low vacuum and 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) � Auxiliary startup oil resets trip devices � Startup oil resets stop valves (ESV’s and IV’s)

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Pilot valve (starting device) position raised

� Startup oil and auxiliary startup oil circuits are drained � ESV’s open at 42% and IV’s open at 56% positions of starting device

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Speeder gear

� Assembly of a bellow and spring � Bellow compression controlled by

1. Starting device position 2. Speeder gear position (alters spring tension) 3. Primary oil pressure

� Operated from UCR/CCR or local � Primary oil pressure (speed feedback)

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Test valves

� HP/IP stop valves receive trip oil through their respective test valves � Port openings for both trip oil and startup oil � Used in resetting of stop valves after turbine trip � Valve can be operated with hand wheel

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Selection between hydraulic and electro hydraulic governing

� Minimum pressure between HP and IP secondary oil � Turbine startup using EHC with hydraulic system for backup � Secondary oil pressure set higher than the EHC system and maintained using a

tracking system

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Protective devices in governing system

� Two type of protection 1. Electrical protection 2. Mechanical tripping

Electrical protection (through RTS)

� Turbine UCB manual trip � Stator water conductivity > 21 mho/sec � Stator water flow < 13m3/sec � Boiler to turbine through UTR � Generator to turbine through UTR � Lub oil pressure low < 1.2 ksc (2/3) � Fire protection (channel 1 and 2) � UTR manual trip � ATRS shutdown program

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Mechanical devices

� T/G overspeed trip (bolt 1 – 3300 rpm, bolt 2 – 3330 rpm) � Condenser vacuum low � Axial shift high

Governing control system

� Governing control system consist of three basic elements, which are: 1. Sensing elements – flyball weights etc. 2. Transmitting elements – mechanical linkage, hydraulic or pneumatic pressure,

electrical signals, or as is most common a combination of these. 3. Correcting elements – the valve or valves that control the flow of steam to the

turbine. Mechanical protection devices – overspeed trip device or emergency governors

� The function is to stop the turbine when permissible speed is exceeded � Two overspeed bolts, one acts when speed reaches 110% (3300 rpm) of the rated

speed and the other comes into action at 111% of the rated speed (3330 rpm) � It is situated in the front end bearing pedestal and consists of an eccentric bolt,

adjusting screw, spring, pawl and screw plug � The speed is measured with the help of a Hall probe � When the overspeed occurs and goes beyond the preset value (s) the bolts fly out

due to centrifugal force and 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 and 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 overspeed limit, the centrifugal force overcomes the spring force and forces the bolt out of the turbine rotor. This activates the turbine trip mechanism

� Before overspeed device comes into action, 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 and thus to reposition the valves accordingly

� If droop = 5%, it means ± 2.5% from 3000 rpm

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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 a twin chamber arrangement with a separating diaphragm. The upper

side is connected to the condenser vacuum and the 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 (> 600 rpm), 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 auxiliary 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 does not require auxiliary startup oil

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Thrust bearing device

� It is used to protect the turbine against high axial shift and high thrust pad wear � It consists of two diametrically opposed pairs of small cams on the turbine shaft,

positioned one set before and another set after a pawl � The pawl is so arranged so that whenever the shaft moves axially too much (> 0.6

mm) in either direction one set of cams comes in contact with the pawl and thereby thrust bearing trip device latch will be released. This will cause the opening of the drain port through which auxiliary trip oil will be drained

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