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
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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 stateof-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:-
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Start up and shut down sequence performed in a reliable way.
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Protect drives and related auxiliaries.
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Uniform and sequential information- to operator about the process.
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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 167
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
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.
These controls obtain:
Group controls
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group controls
(GC)
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sub group controls (SGC)
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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 169
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 c ertain 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 a nd load set print devices of 172
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. 173
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 174
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 instru ment 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 175
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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