b001-c0ntrol system intr0ducti0n

8/2/2019 b001-c0ntrol System Intr0ducti0n

1/4

SYSTEM INSTRUCTION HPGC: 2X300MWDeenbandhu Chhotu Ram

TPP, Yamunanagar

B-OO1 C0NTROL SYSTEM INTR0DUCTI0N

The Digital E lectr0-Hydraulic or"DEH" ControlSystem provided for this turbo-generator

unit combines the advantages of solid state

electronics with those of high pressure

hydraulics to control the steam flow through

the turbine.

This control system consists of five major

components which are listed below.

1. A solid state electronic controller cabinet.

2. Operator' keyboard, CRT, typewriter.

3. Steam valve servo-actuators.

4. An EH fluid supply system.

5. Emergency trip system components.

The following information is offered as a

general description and an introduction to

the "DEH" system apparatus.

ELECTR0NIC C0NTROLLER

The electronic controller performs basic

computations on reference and turbine

feedback signals and generates an output

signal to the steam valve actuators. The

controller hardware consists of a digital

computer and a hybrid section with printed

circuit cards that are cage mounted to form

rack units. The DPU and circuitry, which

includes the logic, reference, signal input

channels, amplifiers, automatic controllers,

power suppl ies, and terminal str ips for

interconnecting wiring, are contained in the

controller cabinets.

0PERATOR' keyboard, CRT, TYPEWRITER

The operator 's keyboard and CRT which areusually located in the control room, are the

unit 's control center. The operator canchange the reference input to the electronic

controller to vary the speed or load at

different rates, and also provides access to

the turbine for an automatic synchronizer or

coordinated control.

Operator setting made at the keyboard and

CRT are used by the electronic controller,

which compares the turbine speed ,first

stage steam pressure, and megawatt signals

to the reference settings selected by the

operator, to position the steam valves.

The Cathode Ray Tube (CRT) is also a means

by which alarms ,messages ,and various

turbine measurements such as temperatures

and pressures can be viewed by the

operator.

The typewriter, wh ich is usually located inthe control room, is a second means by

which alarms and messages may be viewed

by the operators.

MAIN STEAM VALVE ACTUAT0RS

The position of each steam valve is

controlled by an actuator which consists of a

hydraulic cylinder using fluid pressure to

open and spring action to close. The cylinder

is connected to a control block upon which is

mounted isolation, dump, and check valves.

Additional components are added to form

two basic types of actuator assemblies.

The reheat stop valve actuators posit ion

these steam valves only in the fully open or

full closed position. High-pressure fluid is

supplied through an orifice to the area belowthe hydraulic cylinder piston. Fluid pressure

DOCUMENT NO.:71.H156-7E04 Page 1 of 4


2/4


TPP, Yamunanagar

in this area is controlled by a pilot-operated

dump valve. With the turbine control system

latched, the pilot-operated dump valves

close to build up fluid pressure under thecylinder piston thus opening the reheat stop

valves. A solenoid valve, provided for test

purposes, opens the dump valve and

releases the fluid to drain through an orificed

line which ,for the test ,allows the valves to

close slowly.

The throttle valve, governor valve and

interceptor valve actuators position these

steam valves in any intermediate position to

proportionately regula1 the steam flow to

the required amount. The actuators are

provided with a servo-valve and an LVDT

(Linear Variable Differential Transformer).

High pressure fluid is supplied through a 3

micron filter to the servo-valve which

controls the actuator position in response to

a position signal from the servo amplifiers.

The LVDT develops an analog signa ls that isproportional to the valve position and feeds

it back to the contro1ler to complete the

control loop.

The iso lation valves permit on-line Main-

tenance o f the actuator components

including the hydraulic cylinder. The check

valves prevent fluid back flow from the drain

or emergency trip circuits.

EH FLUID SUPPLY SYSTEM

The function of the EH fluid supply system is

to provide high pressure fluid and thereby a

motive force to the servo-actuators which

position the turb ine steam va lves in

response to electrical commands from the

electronic controller. The control fluid

medium is a triaryl phosphate ester, whichpossesses qualities for fire resistance and

fluid stability.

The system consists of a stainless steel

reservoir assembly mounted on a steelframe, the associated tubing, controls,

pumps, motors, filters, and heat exchangers.

The components are arranged in two

duplicate sets. While one set is in

0peration,the other set is on a stand-by

status and will automatically function if the

need arises. During normal turbine operation

with the control system latched, one pump-

motor combination is sufficient to supply the

fluid requirements. Occasionally, during

operation with the control system unlatched,

the second pum p-motor set may cut in. This

can be expected and does not indicate a

system malfunction. However, if the second

pump-motor set cuts in when the control

system is latched, it indicates excessive fluid

leakage, and the system should be checked

to correct the problem.

Fluid discharged by the pump enters the

high pressure fluid header via EH control

block, filters, check and relief valves, and

accumulators. The normal pressure of fluid

header maintained at 140kg/cm2. The fluid

returning to the reservoir passes through a

directional control valve that directs the fluid

through one of the redundant heat

exchanger sets. The relief valve, which is

common to both the primary and stand-by

systems, protects the EH system against

excess1ve pressure by discharging fluid into

the reservoir when the pressure increases to

170kg/cm2.

NOTE

The pressure specified in theproceeding paragraph are nominal and



3/4


TPP, Yamunanagar

may vary somewhat on each unit. For

the actual design values for this turbine

refer to the Control Setting

Instructions.

EMERGENCY TRIP SYSTEM

The interface diaphragm valve, wh ich is

mounted on the governor pedestal, provides

an interface between the Auto Stop

Emergency Trip portion of the high pressure

fluid system and Mechanical Overspeed and

Manual Trip portion of the lubrication oil

system. Lubrication oil from the Mechanical

overspeed and Manual Trip Header supplied

to the top of the diaphragm valve acts to

overcome a spring force to hold the valve

closed and thereby block a path to drain of

the fluid in the Auto Stop Emergency Trip

Header. Any decay in the Mechanical

Overspeed and Manual Trip Header pressure

such as could be caused by a tripping action,

allows the spring to open the valve releasingthe emergency trip fluid to drain and tripping

the turbine. Lubrication oil and EH fluid are

never in contact with each other.

Six solenoid valves are contained in the

Emergency Trip Control Block mounted on

the governor pedestal.

Four of the solenoid valves are Auto Stop Trip

(2O/AST) solenoids, which are energized

closed under normal operating conditions.

When the solenoid valves are closed, they

block a path to drain of the Auto Stop

Emergency Trip Header fluid and pressure

can be established under the steam inlet

valve actuators. When the solenoid valves

open, the header f luid goes to drain and

causes the steam inlet valves to close and

trip the turbine. The 20/AST solenoid valvesare arranged into a series-parallel

con figuration to provide redundant

protection. At least one solenoid valve from

each channel must open to cause a trip.

A turb ine tr ip is in itiated by pressure

switches and electronic sensors which

monitor the turbine for conditions of low

bearing oil pressure, low EH fluid pressure,

high thrust wear, low condenser vacuum,

over-speed, and purchasers trip.

The remaining two-solenoid valves are

Overspeed Protection Control (2O/OPC)

solenoids. The OPC portion of the DEH

controller controls them. They are arranged

in parallel and are automatic closed under

normal operating conditions. In the closed

position, they block a path to drain of the

OPC Trip Header fluid, and pressure can be

established under the interceptor valve and

governor valve servo actuators. In the event

of an OPC action, such as occurs if the unit

reaches 1O3% of rated speed, the solenoidvalves energized open releasing the OPC trip

Header fluid to drain thereby unseating the

dump valves and causing the immediate

closing of the interceptor valves and

governor valves.

Check va lves between the auto stop

emergency trip fluid circuit and the OPC fluid

circuit retain the pressure in the auto stop

emergency trip line, and the reheat stop and

throttle valves remain open. With a reduction

in speed to rated speed, the solenoid valves

will close, the interceptor and governor

valves will reopen, and the governor valves

will take over control of the turbine and keep

the unit at rated speed.

A stainless steel test block, mounted on the

side of the governor pedestal, provides ameans of monitoring the EH fluid pressure



4/4


TPP, Yamunanagar

and testing the operation of the pressure

switches. The test block is orificed off from

the system supply on 0ne side and

connected to a drain on the other side. Thetest block is arranged into two identical

channels, and either the entire block or

individual components can be isolated for

maintenance by means of isolation valves.

The test block for the other monitoredmediums are similar in construction.


b001-c0ntrol system intr0ducti0n

Documents