Control System

The development of I.C. technology has evolved a new field of miniaturizing the earlier

giant looking electronic devices. Further advancement of I.C. came forward with the

concept of scale of circuit integration. Thus today we have circuits of VLSI scale(very

large scale integration) which carry more than one lach of transistors on a single wafer of

silicon of millimeter size. This advancement of I.C. gave birth to the microprocessor, use

of which amazingly has reduced the size of control equipment. Thus today a desk size

controller is capable to control a huge power plant more reliably and efficiently than the

old one which needed a very large room for its accommodation.

The control system of this power plant categorically relates to the microprocessor family.

In consistence with the variety of machines installed at this power station, the control, too,

is quite versatile. Thus at unit No.1,2,13,14 and 15 SIEMENS control of TELEPERM ME

and ISKAMATIC family is employed. At unit No. 9 & 10 ABB control of

PROCONTROL K and SIEMENS TELEPERM ME is in service. For unit 5-8 GE control

under the trade name of MARK IV SPEEDTRONIC is in action. For the FIAT machines

unit 3&4 the control equipment is the conventional relay type. Unit no 11&12 are being

controlled through ALSTHOM control CEG LEC.

SIEMENS control TELEPERM ME provides all the requirement of an automation

process, e.g. the open and closed loop operation, data acquisition, processing,

calculations, optimization, monitoring, signaling, operation and visualization of the

process in the interactive mode on a monitor via a control desk. From design point of

view the whole system is divided into the following subsystems.

As automation subsystem OS operation, monitoring and configuring subsystem IS

process information system CS 275 coupling sub system

The AS 220 EA automation subsystem forms the heart of TELEPERM ME. It is

selectively redundant automation system where the function modules and the input/output

bus control can be configured in 1 out of 2 redundancy or nonredundantly. The AS 220

EHF is the highly reliable and fail-safe design of the AS 220 E. The central unit of AS

220 EHF consists of three independent central processing units with synchronous

commands whose output signals are linked in a 2 out of 3 configuration. The central unit

consists of function blocks and organizational functions and thus forms the unit control,

group level and sub group level controls. The central processing unit then process these

blocks according to the plant configuration. The function block contain the basic function

like integrator, differentiator, adder, maximum/minimum value selector, close loop

controller, command blocks and step blocks. Since the function blocks are processed

sequentially, they can be used several times without having to extend the hardware.

The OS 265 operation and monitoring system is used for the monitor based operation and

supervision of power plant. It provides detailed plant graphic displays, function graphic

displays and individual displays for measured values, open and close loop controls. In the

event of fault the operator is directly guided into the relevant plant or detailed graphic

displays by flashing indicators on the screen.Thus all faults that occur including their

causes are recognized at an early stage and the process is protected from disturbance.

Process operations are carried out by touching the screen at the requisite point of display

block using a light pen.

The information subsystem MADAM S is a modular arranged data acquisition and

monitoring system, devised by SIEMENS. The system provides alarm annunciation,

analog value display, digital status display, plant mimic display, curve display and the

operating point display. The incident review log provides 10 minutes pre and post history

alarm sequence logs. Each page of the display accommodates 8 alarms. Likewise 8 analog

value and binary values are displayed on a page with a value updating cycle of 5 and 1

second respectively.

The CS 275 bus subsystem is used to transfer data between the components of the system

i.e. AS220, OS 265, MADAM S, WS30 and PG 750. The bus system is divided into a

LOCAL (upto 20 meters distance) and REMOTE (upto 4 km. distance) area. The system

employs distributed transmission control for transfer of data among the system

components. It uses "the flying master principle" i.e. each participant takes over the

transmission control. The rest of the participants become slave till they get their turn to

become master. This change of master status is carried out through bus protocol which

assigns the master function to any participant via REQUEST, COMMAND and TIME-

OUT CONTROL.

The ISKAMATIK system, on the other hand is an integration of different dedicated

modules which are hardwired to obtain a particular job. Three different categories of

modules are in use. Iskamatic module A is used for analog, Module B for binary and

Module C for protection circuits. Each module has a specified hardware and by

hardwiring the outputs of different modules, an automation process can be tailored to suit

any specific task.

SPEEDTRONIC MARK IV.

General Electric Control: MARK IV SPEEDTRONIC, controls the operation and

monitoring of the gas turbine unit 5-8. It employs three identical computers, identified as

controllers R, S, and T which perform all calculations necessary to keep the gas turbine

running after it has reached complete sequence and also for the normal shut down of the

unit. Each computer is designed such that it drives its output in a defined direction on loss

of power or failure of any computer but for reliability the two-thirds voting concept is

provided in the system configuration. A fourth computer called as "The communicator" C

supervises the three controllers and initiates an audible alarm when there is disagreement

between any control parameter or logic signal in the three controllers. The discrepancy is

then displayed on the CRT for maintenance personnel to evaluate the problem. The

turbine however continues to run because the control is responding to the median value.

The field trip contacts are wired to the two contact input modules, named as (CIM1),

(CIM2) from here the signals are paralleled to the three optical couplers which feed them

to the separate digital inputs cards in R,S and T. The communicator C monitors the input

seen by R,S & T and performs the majority vote and feeds the voted and individual values

to the CRT for display.

Field contacts relevant to operation of the gas turbine are connected to the Contact Input

Module 3-6 and via optocouplers go the communicator C. Power for each computer is

supplied by an independent power supply which is separately fused from the 125 v dc

battery system. Voting of the logic outputs is performed by the logic driver card in the

relay drive module. Power for this cards is supplied from the two redundant 28 vdc power

sources under the supervision of computer C. Relays used for the trip circuits such as fuel

solenoids, have additional protection which removes power from the coil when a trip

condition occurs. This protects against the relay driver failing in the turned on mode.

For analog control system, the temperature input signals are connected to the I/O Modules

(ICM) and all others to the Modules (AI01-4) where they are filtered prior to being fed to

the controllers R,S & T or the communicator C. These incoming signals are then

multiplexed on the computer card dedicated to specific functions and then wired to an

other analog input card for a second stage of multiplexing and the final analog to digital

conversion. Median selection of key parameters such as fuel stroke reference, FSR is

calculated in C and fed back to R, S and T to bias their individual calculation and in case

of computer failure to up date the computer prior to reinitializing it for an automatic

restart. All analog control loops are calculated in the software except for the fuel servos

which are regulated in hardware. In this case the controllers R, S and T calculate the

required fuel which is converted from digital to analog, compared with the LVDT

position, and/or flow divider feedback and then used to drive the three coil servos. The

application of three coil servos extend the voting principal to the gas turbine.

The thermocouple inputs are processed similar to other analog inputs. Here the trip and

essential control thermocouples are connected to separate input modules, (TCM.R),

(TCM.S) and (TCM.T), for filtering, cold junction reference and offset measurement prior

to being fed to the controllers ®, (S) and (T). The monitoring thermocouples are wired to

one of the input module (TCM1-3) which feed to the communicator, "C".

The monitoring system provides a lot of information for the operator and the maintenance

personnel. Some of the features are:

 

  1

The "normal display" shows all the key gas turbine parameters relevant to a

particular operating mode, i.e. startup, running and shutdown and automatically

changes displays when the operating mode changes. Any display can be copied

through the panel mounted printer.

  2

Two (2) field selectable "demand Displays" can be provided with thirty two

(32) points each including any control or logic signal displayed in engineering

unit. It can be logged at specified time intervals or printed manually on

demand.

  3256 alarm messages ae available, and additional alarms are dedicated to

internal diagnostics.

  4

Gains, time constants and setpoints can be displayed and changed from the

operator interface module, (OPM) and the changes will be retained in BRAM,

BATTERY-BACKED RAM, for up to ten (10) days at 25 degree C if DC

power is removed from the panel.

  5The status of all contact inputs can be displayed simultaneously to assist in

troubleshooting.

  6 Logic signals can be forced from the Orator Interface Module (OPM), if the

correct security code is entered in the computer by qualified maintenance

personnel.

  7Two (2) levels of security codes prevent unauthorized adjustment of guarded

signals.

  8Key parameters are recorded time/date wise over a period of 72 hours by an

historical log monitor.