14. Automatic Turbine Run Up System INTRODUCTION To have the higher unit availability and the optimum utilization of the fuel and capacity of

the power generating units, keeping in view the high capital involved, high-fuel cost and

ever increasing demand in the grid, it is always been a point to have minimum outage of

the power generating unit reduce the extent of damage in case of faults and the facility

to detect the faults and their causes, facility to have and overview of the system and its

performance and the facility for start up and shut down of the power generating unit in

the minimum operating time. it is always desirable to have proven and reliable system to

perform all the above mentioned functions and have high availability of the system. In

view of the above, the turbine is equipped with Automatic Turbine Run Up System

(ATRS), Turbine Supervisory Instruments (TSI), Turbine stress evaluator (TSE), Electro

hydraulic Governing system (EHG) etc.

The ATRS works in conjunction with other turbine related controls like TSE, TSI, EHG etc

to have a centralised control of turbine and related auxiliaries. The signals,

commands and feedback status of various above mentioned turbine control system are to

be acquired, validated and executed so as to achieve proper operation and control of

Turbine generator set.

For start up, acquisition and analysis of a wide variety of information pertaining to

various parameters of steam turbine, demand quick decisions and numerous operations

from the operating personnel. In order to reduce the arduous task of monitoring various

parameters and effect sequential start up, minimise the possible human errors and to

achieve start up in minimum time in optimum way, Automatic Run Up System (ATRS) is

introduced.

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For all our previous projects like Singrauli, Korba, Ramagundam, Farakka Stage I & II

etc. We have ATRS implemented in solid state hardware i.e. electronic part of the ATRS

wore realised in printed circuit boards using transistors, resistors etc. and for operator

intervention and observation and plant control & monitoring, conventional pushbutton,

hand/auto stations indicating lights, lamp indications etc. were being provided on unit

control Desk, (UCD) and Unit Control Panel (UCP). However for NTPC's future projects 165

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due to the advent of microprocessor based technology and CRT/KBD based plant

control and monitoring philosophy, it has been decided to specify ATRS based on state-

of-the-art micro-processor technology having CRT/KBD operational features, so as to

have uniform and operating hardware philosophy for plant C & I as well as for turbine C

and I. Accordingly from Farakka Stage-Ill onwards micro-processor based ATRS with

CRT/KBD control and monitoring facility is being specified for turbine operation and

control.

CONTROL AND MONITORING PHILOSOPHY The ATRS is based on functional group philosophy i.e. the main plant is divided into

clearly defined sections called functional groups such as oil system, vacuum system,

turbine system (Fig No. 30 Fig No. 31). Each functional group is organised and

arranged in sub group control (SGC), sub loop control (SLC) and control interface (Cl).

Each functional group continues to function automatically all the time demanding enable

criteria based on process requirements and from neighboring functional groups if

required. In the absence of desired criteria. the system will act in such a manner as to

ensure the safety of the main equipment.

The task of the ATRS control system is to control, monitor and protect all devices/drives

for:-

- Start up and shut down sequence performed in a reliable way.

- Protect drives and related auxiliaries.

- Uniform and sequential information- to operator about the process.

- Distinct information about the nature and location of faults.

The ATRS is based on functionally decentralised Hierarchical structure. Functionally

decentralised means that each and every drive/actuator has its own dedicated set of

hardware including controllers, interface modules etc. The "Hierarchical structure" refers

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to the control system divided into three control levels to achieve higher degree of

automation. Each control level has its own specific task and depends on the

subordinate lower control levels. If the higher control level fails, the next control level is

not affected and allows the plant to run safely.

Levels of control available in ATRS are as follows: Functional group control

These controls obtain: Group controls

The functional group control is the automatic control

of a part of the system, this part being mostly

independent (functional group) or of a part process.

- group controls (GC)

- sub group controls (SGC)

- sub loop controls (SLC)

Group controls contain the operational logic circuits of

the underlying sub group controls. They are designed

with logic technique and not with sequential

technique.

The group control has the task of deciding when. how

many and which of the underlying sub-group controls

shall be operating or stopped.

In all cases when a functional group is only composed

of one single subgroup, the group control only

decides when the functional group shall be operating

or stopped. For a unified concept and a unified

operational technique for all functional group controls

the group control shall also be used for functional 168

groups with one subgroup control only.

Subgroup controls The subgroup controls contain the sequential logic for

switching machines on and off, including all auxiliary

equipment (subgroups). They are preferentially

designed in sequential technique, except for cases

when technical and economic reasons permit a

renouncement on the sequential technique. In these

cases a pure logic circuit is used.

Sub loop Control Sub loop controls are different from other group controls, as they can be switched on and off

manually, but they can only receive directional control

commands (servo HIGHER/LOWER, motor ON/OFF,

etc.) from criteria (such as auxiliary oil pump

automatic controls, pressure and level monitoring

controls).

System Operation Modes The following three operating modes are possible under sequence start up modes :

Automatic Mode In automatic mode the sub-group control co-ordinator is first switched to "Automatic

ON". The automatic control becomes effective and induces the desired operating status

only when a program has been selected ("Startup or Shutdown").

The step is set if the criteria for it are fulfilled and a command is sent to the sub-ordinate

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control interface. Preparation are simultaneously made for setting the next step. The

program continues in this manner from step to step. The step which outputs the control

and the criteria for the next step can be displayed in the CRT' and control station.

Manual interventions are not necessary when the program runs withouts faults.

Semi-Automatic Mode In the semi-automatic mode enables continuation of the program step by step despite the

fact that the step criteria are missing. The presence of all criteria necessary for a step

are simulated by manual operation of a keyswitch; this enables continuation of the

program. This is useful if the program stops because e.g.. fulfilled plant criteria cannot be

detected because of a defective transmitter. Exact knowledge of the process events is a

prerequisite (or this, however.

Operator Guide Mode The automatic control only processes information (criteria from the plant but does not

output any commands. All commands must be input to the control interface level

manually the selected step sequence now continues with the process independent of

the fulfillment of criteria.

Control Interface (Cl) The control interface module forms the link between the individual commends and the

power plant. Each remote controlled drive has a control interface module. The module

consists of command section monitoring section, power supply and alarm section. The

command section proves the control commands according to their priority and validity

and passes actuation signals to the interposing relays in the switch gear. Solenoid

valves can be actuated directly up to certain capacity (36 W). The monitoring section 170

normally checks the command functions. the position of the drive and the check back

signals and transmitters to the desk, protection logic and FGC. System Description Automatic turbine run up system consists of three sub group controls viz. Oil supply

system, condensate and evaluation system and turbine system. These groups in

conjunction with Electrohydraulic speed control and Turbine stress evaluator achieves

the task of synchronising and block loading of the machine or orderly shut down as

required. SGC Oil Supply and Fire Protection System The sub-group control for oil system performs its tasks comprising starting relevant oil

pumps, ensuring Turning of the turbine during start up and hot rolling to ensure even

distribution o? heat, this task is accomplished through jacking oil pumps & shaft turning

gear pump and ensuring the Lubricating oil at pre-defined temperature under various

circumstances. The programme is accomplished through a number of steps and seven

SLCs, these include, controls of turning gear system, AC/DC Lub oil pumps, jacking oil

pumps, oil temperature controller etc.

Shut Down Programme The shut down programme essentially switches OFF all the equipments in the oil

system. The shut down programme would take OFF if the SGC is "ON" and operator

presses the button for shut down programme and the release criterion for shut down

programme is available. The release criterion would be available only if the HP turbine

casing has cooled down and the metal temperature is below 100°C at the top and

bottom of the casing. This interlock is necessary as an inadvertant initiating of shut 171

down programme, could lead to starving of the oil supply to the bearings as well as

depriving the turbine of turning operation. SGC Condensate and Evaluation System The Sub-group control for condensate and evacuation system accomplishes its task

which comprises of keeping at least one of the two condensate pumps in operation,

evacuating the non-condensate gases from the system, maintaining the desired level of

condenser pressure when turbine is in operation and breaking the vacuum as and when

required by the mechanical process. The SGC acts directly on condensate pumps,

discharge valves, air isolation valves, vacuum pumps, air ejectors and air ejector

bypasses etc. Shut Down Programme The shut down programme can also be initiated by the operator provided the following

release criteria are available:

i. The speed of turbine is less than 200 rpm and

ii. LP bypass system has closed, SGC Turbine The sub-group control turbine, during start-up executes the tasks which comprise of

"Warming-up the Turbine" "Speed increase" synchronization and subsequent block

loading. This sub-group also shuts down the turbine which comprises unloading closure

of steam supply to turbine, isolation from grid and bringing the turbine drains to desired

positions. This SGC acts directly on the following systems drains, warm up controller,

starting device of Turbine Governing System, Speed and load set print devices of

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Turbine Governing System, Auto Synchroniser etc.

Shut Down Programme If the operator switches ON SGC and press the manual push button for shut down

programme, the shut down programme would commence. No release criteria is defined

for initiating the shut down programme manually. In other words, the shut down

programme can be initiated by the operator as per the convenience of the power

system. Unloading and Shut Down During the planned shutdown, prior to beginning of unloading, if possible, the main

steam temperature is to be reduced, in order to keep the metal Temperature at a lower

value when the turbine is re-started after short shut down. The load and main steam

temperature are not to be reduced simultaneously. The rate of unloading is governed by the

margins shown on turbine stress evaluator.

The unloading of the set is accomplished with the help of electro-hydraulic governing

system. The generator gets isolated from the grid as a result of the action of low forward

power relay.

In the event of emergency, the turbine can be shut down under any load condition, by

operating the automatic trip gear, The turbine can be shut down either by remote

tripping via the solenoid valve or by manual tripping.

However, the shut down programme commences automatically under protection

channel if the "Condition 1 AND condition 3 "exist simultaneously" "OR condition 2 AND

condition 3 exist simultaneously". The condition "1" and "2" and "3" are described

below.

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Condition - 1 Emergency stop valve 1 and "OR" Interceptor valve 1 AND 2 are

closed and the pressure of the fluid in the governing system is less

than 5Kg/cm2.

Condition - 2 "HP control valve 1 or 2 not closed "AND" the start-up programme

is in "AND" starting device is at a position less than 56%. Condition - 3 Generator breaker is "OFF".

Auto synchroniser (type-siemens 7 VE 2): This device precisely adjusts the generator

voltage and frequency with the grid voltage and frequency and closing command is

given before 'the phase coincidence point taking into account circuit breaker closing

time.

TURBINE STRESS EVALUATOR Function The recent advances in steam turbine design caused power outputs and main steam

conditions to climb steadily higher. This has also involved a higher degree of material

utilization and as a result it has become necessary to pay special attention to the

additional thermal stresses which result from temperature changes. Each time a turbine

is started m from the hot or cold .condition,, each change in load and each time it is shut

down involves free thermal expansion and restricted thermal expansion which produces

the extra stress.

For economic reasons the question most important to the operator is how quickly the

turbine can be started up and loaded without causing damage or premature ageing of

the components. Under certain conditions a rapid application of load may even be

necessary in order to safeguard the turbine. Furthermore, the permissible rate of toad

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change is of importance for the performance of the turbine generator in the power

system.

Whereas temperature differences within the individual turbine components are

responsible for thermal stresses, it is the mean temperature of the components which

determines the free thermal expansion. Free thermal expansion is monitored by the

turbine monitoring system.

To prevent damage due to excessive thermal stresses, recommended values for

permissible speed an bad changes are quoted by the turbine builder. Naturally, these

estimated values cannot be comprehensive enough to execute all operational changes

utilizing the permissible margins for the particular turbine to the fullest extent and taking

into account the instantaneous thermal condition of the machine.

An instrument which, from the turbine wall temperatures, determines the permissible

operational changes under all operating conditions, also taking into account the recent

operating history. The measuring points are located in the body of the first mainstream,

combined emergency stop valve and control valve in the line and the H.P. and 1.P.

turbine cylinders. the shaft of the H.P. turbine is partly responsible, and the shaft of the

1.P. turbine primarily responsible for the performance restrictions.

This instruments, the turbine stress evaluator, allows turbines to be driven in an

optimum manner, i.e. effecting changes as rapidly as possible while incurring minimum

stresses. It comprises three principal parts : a measuring section, electrical computing

circuits and a display instrument. LATEST TREND IN TURBINE STRESS EVAIUATORS With the advent of state-of-the-art microprocessor based technology having CRT/KBD

control and monitoring facilities, the TSE has also got a face change. The improved

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version of TSE now available is known as Turbine Stress Control System (TSCS). This

TSCS works in conjunction with ATRS and EHG to achieve all the functional

requirements

This new TSCS not only performs the general functions of TSE like computation and

display of stress margins available, continuous on-line monitoring of thermal stress

levels, limits of speed and load changes allowable, but also carry out the fatigue

analyses and provide at least three modes of turbine operation i.e. slow, normal and

fast. That is to say, depending upon the urgency of unit start up, the operator shall be

able to select any of the three modes of turbine run-up and loading.

The TSCS has its own dedicated CRT/KBD and one suitable conventional display

instrument for indication to operator. The stress margin and speed/load gradient

displays changes colour so as to attract the operators attention whenever the

permissible limits are reached/exceeded. ADDITIONAL ADVANTAGES OF TSCS 1. Computation of residual life of Turbine.

2. Three rates of Turbine run up and loading.

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