GESPEEDTRONIC™ MARK V GAS TURBINE CONTROL SYSTEM

INTRODUCTION

The SPEEDTRONIC™ Mark V Gas Turbine

Control System is the latest derivative in the

highly successful SPEEDTRONIC™ series.

Preceding systems were based on automated turbine control, protection and sequencing techniques dating back to the late 1940s, and have

grown and developed with the available technology. Implementation of electronic turbine control, protection and sequencing originated with

the Mark I system in 1968. The Mark V system is

a digital implementation of the turbine automation techniques learned and refined in more

than 40 years of successful experience, over 80%

of which has been through the use of electronic

control technology.

The SPEEDTRONIC™ Mark V Gas Turbine

Control System employs current state-of-the-art

technology, including triple-redundant 16-bit

microprocessor controllers, two-out-of-three voting redundancy on critical control and protection parameters and Software-Implemented

Fault Tolerance (SIFT). Critical control and protection sensors are triple redundant and voted

by all three control processors. System output

signals are voted at the contact level for critical

solenoids, at the logic level for the remaining

contact outputs and at three coil servo valves for

analog control signals, thus maximizing both

protective and running reliability. An independent protective module provides triple redundant hardwired detection and shutdown on

overspeed along with detecting flame. This module also synchronizes the turbine generator to

the power system. Synchronization is backed up

by a check function in the three control processors.

The Mark V Control System is designed to fulfill all gas turbine control requirements. These

include control of liquid, gas or both fuels in

accordance with the requirements of the speed,

load control under part-load conditions, temperature control under maximum capability

conditions or during startup conditions. In addition, inlet guide vanes and water or steam injection are controlled to meet emissions and operating requirements. If emissions control uses

Dry Low NOx techniques, fuel staging and combustion mode are controlled by the Mark V system, which also monitors the process.

Sequencing of the auxiliaries to allow fully automated startup, shutdown and cooldown are also

handled by the Mark V Control System. Turbine

protection against adverse operating situations

and annunciation of abnormal conditions are

incorporated into the basic system. The operator interface consists of a color

graphic monitor and keyboard to provide feedback regarding current operating conditions.

Input commands from the operator are entered

using a cursor positioning device. An arm/execute sequence is used to prevent inadvertent turbine operation. Communication between the

operator interface and the turbine control is

through the Common Data Processor, or <C>, to

the three control processors called <R>, <S> and

<T>. The operator interface also handles communication functions with remote and external

devices. An optional arrangement, using a

redundant operator interface, is available for

those applications where integrity of the external data link is considered essential to continued plant operations. SIFT technology protects

against module failure and propagation of data

errors. A panel mounted back-up operator display, directly connected to the control processors, allows continued gas turbine operation in

the unlikely event of a failure of the primary

operator interface or the <C> module.

Built-in diagnostics for troubleshooting purposes are extensive and include “power-up,”

background and manually initiated diagnostic

routines capable of identifying both control

panel and sensor faults. These faults are identified down to the board level for the panel and

to the circuit level for the sensor or actuator

components. The ability for on-line replacement

of boards is built into the panel design and is

available for those turbine sensors where physical access and system isolation are feasible. Set

points, tuning parameters and control constants

are adjustable during operation using a security

password system to prevent unauthorized access.

Minor modifications to sequencing and the

addition of relatively simple algorithms can be

accomplished when the turbine is not operating.

They are also protected by a security password.

A printer is included in the control system

and is connected via the operator interface. The

printer is capable of copying any alpha-numeric

display shown on the monitor. One of these displays is an operator configurable demand display that can be automatically printed at a

selectable interval. It provides an easy means to

obtain periodic and shift logs. The printer automatically logs time-tagged alarms, as well as the

clearance of alarms. In addition, the printer will

print the historical trip log that is frozen in

memory in the unlikely event of a protective

trip. The log assists in identifying the cause of a

trip for trouble shooting purposes.

The statistical measures of reliability and availability for SPEEDTRONIC™ Mark V systems have

quickly established the effectiveness of the new

control because it builds on the highly successful SPEEDTRONIC™ Mark IV system.

Improvements in the new design have been

made in microprocessors, I/O capacity, SIFT

technology, diagnostics, standardization and

operator information, along with continued

application flexibility and careful design for

maintainability. SPEEDTRONIC™ Mark V control is achieving greater reliability, faster meantime-to repair and improved control system

availability than the SPEEDTRONIC™ Mark IV

applications.

As of May 1994, almost 264 Mark V systems

had entered commercial service and system

operation has exceeded 1.4 million hours. The

established Mark V level of system reliability,

including sensors and actuators, exceeds 99.9

percent, and the fleet mean-time-betweenforced-outages (MTBFO) stands at 28,000

hours. As of May 1994, there were 424 gas turbine Mark V systems and 106 steam turbine

Mark V systems shipped or on order.

CONTROL SYSTEM HISTORY

The gas turbine was introduced as an industrial and utility prime mover in the late 1940s with

initial applications in gas pipeline pumping and

utility peaking. The early control systems were

based on hydro-mechanical steam turbine governing practice, supplemented by a pneumatic

temperature control, preset startup fuel limiting

and manual sequencing. Independent devices

provided protection against overspeed, overtemperature, fire, loss of flame, loss of lube oil and

high vibration.

Through the early years of the industry, gas

turbine control designs benefited from the

rapid growth in the field of control technology.

The hydro-mechanical design culminated in the

“fuel regulator” and automatic relay sequencing

for automatic startup, shutdown and cooldown

where appropriate for unattended installations.

The automatic relay sequencing, in combination

with rudimentary annunciator monitoring, also

allowed interfacing with SCADA (Supervisory

Control and Data Acquisition) systems for true

continuous remote control operation.

This was the basis for introduction of the first

electronic gas turbine control in 1968. This system, ultimately known as the SPEEDTRONIC™

Mark I Control, replaced the fuel regulator,

pneumatic temperature control and electromechanical starting fuel control with an electronic equivalent. The automatic relay sequencing was retained and the independent protective

functions were upgraded with electronic equivalents where appropriate. Because of its electrically dependent nature, emphasis was placed on

integrity of the power supply system, leading to a

DC-based system with AC- and shaft-powered

back-ups. These early electronic systems provided an order of magnitude increase in running

reliability and maintainability.

Once the changeover to electronics was

achieved, the rapid advances in electronic system technology resulted in similar advances in

gas turbine control technology (Table 1). Note

that more than 40 years of gas turbine control

experience has involved more than 5,400 units,

while the 26 years of electronic control experience has been centered on more than 4,400 turbine installations. Throughout this time period,

the control philosophy shown in Table 2 has

developed and matured to match the capabilities of the existing technology. This philosophy

emphasizes safety of operation, reliability, flexibility, maintainability and ease of use, in that

order.

CONTROL SYSTEM 

FUNCTIONS

The SPEEDTRONIC™ Gas Turbine Control

System performs many functions including fuel,

air and emissions control; sequencing of turbine

fuel and auxiliaries for startup, shutdown and

cooldown; synchronization and voltage matching of the generator and system; monitoring of

all turbine, control and auxiliary functions; and

protection against unsafe and adverse operating

conditions. All of these functions are performed

in an integrated manner that is tailored to

achieve the previously described philosophy in

the stated priority.

The speed and load control function acts to

control the fuel flow under part-load conditions

to satisfy the needs of the governor.

Temperature control limits fuel flow to a maximum consistent with achieving rated firing temperatures and controls air flow via the inlet

guide vanes to optimize part-load heat rates on

heat recovery applications. The operating limits

of the fuel control are shown in Figure 1. A

block diagram of the fuel, air and emissions control systems is shown in Figure 2. The input to

the system is the operator command for speed

(when separated from the grid) or load (when

connected). The outputs are the commands to

the gas and liquid fuel control systems, the inlet

guide vane positioning system and the emissions

control system. A more detailed discussion of

the control functionality required by the gas turbine may be found in Reference 1.

The fuel command signal is passed to the gas

and liquid fuel systems via the fuel signal divider

in accordance with the operator’s fuel selection.

Startup can be on either fuel and transfers

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GER-3658D

Table 2

GAS TURBINE CONTROL PHILOSOPHY

• Single control failure alarms when running or during

startup

• Protection backs up control, thus independent

• Two independent means of shutdown will be available

• Double failure may cause shutdown, but will always

result in safe shutdown

• Generator-drive turbines will tolerate full-load rejection

without overspeeding

• Critical sensors are redundant

• Control is redundant

• Alarm any control system problems

• Standardize hardware and software to enhance reliability while maintaining flexibility

Figure 2. Gas turbine fuel control

under load are accomplished by transitioning

from one system to the other after an appropriate fill time to minimize load excursions. System

characteristics during a transfer from gas to liquid fuel are illustrated in Figure 3. Purging of

the idle fuel system is automatic and continuously monitored to ensure proper operation.

Transfer can be automatically initiated on loss of

supply of the running fuel, which will be

alarmed, and will proceed to completion without operator intervention. Return to the original fuel is manually initiated.

The gas fuel control system is shown schematically in Figure 4. It is a two-stage system, incorporating a pressure control proportional to

speed and a flow control proportional to fuel

command. Two stages provide a stable turndown ratio in excess of 100:1, which is more

than adequate for control under starting and

warm-up conditions, as well as maximum flow

for peak output at minimum ambient temperature. The stop/speed ratio valve also acts as an

independent stop valve. It is equipped with an

interposed, hydraulically-actuated trip relay that

can trip the valve closed independent of control

signals to the servo valve. Both the stop ratio

and control valves are hydraulically actuated,

single-acting valves that will fail to the closed

position on loss of either signal or hydraulic

pressure. Fuel distribution to the gas fuel nozzles in the multiple combustors is accomplished

by a ring manifold in conjunction with careful

control of fuel nozzle flow areas.

The liquid fuel control system is shown

schematically in Figure 5. Since the fuel pump is

a positive displacement pump, the system

achieves flow control by recirculating excess fuel