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GE turbines Combined Cycle
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ABSTRACTCombined-cycle power plants have demonstrat-
ed high thermal efficiency, high reliability/avail-ability and economic power generation for appli-cation in base load and cyclic duty utility service.The single-shaft combined-cycle power generationsystem; in which the gas turbine, steam turbineand generator (STAG™) are installed in a tandemarrangement on a single shaft; has emerged as apreferred configuration resulting from simplicityof controls and operation and demonstrated highreliability.
This paper describes the design features, expe-rience and operating characteristics of theGeneral Electric pre-engineered product line ofsingle-shaft STAG combined-cycle systems forpower generation applications.
INTRODUCTIONHeat recovery type steam and gas turbine com-
bined-cycle systems are the economic choice forgas- or oil-fired power generation. Integrationwith environmentally clean gasification systems isextending their economic application to low costsolid fuel utilization. The features contributing totheir outstanding generation economics are:
• High thermal efficiency• Low installed cost• Fuel flexibility – wide range of gas and
liquid fuels• Low operation and maintenance cost• Operating flexibility – base, mid-range,
daily start• High reliability• High availability• Short installation time• High efficiency in small capacity increments• Minimum environmental impact – low
stack gas emissions and heat rejectionCombined-cycle power generation equipment
is manufactured by GE in two basic configura-tions, single-shaft and multi-shaft. The single-shaftcombined cycle system consists of one gas turbine,one steam turbine, one generator and one heatrecovery steam generator (HRSG), with the gasturbine and steam turbine coupled to a singlegenerator in a tandem arrangement. Multi-shaftcombined-cycle systems have one or more gas tur-
bine generators and HRSGs that supply steamthrough a common header to a separate singlesteam turbine generator unit. Both configurationsperform their specific functions, but the single-shaft configuration excels in the base load andmid-range power generation applications.
The multi-shaft combined-cycle system configu-ration is most frequently applied in phased instal-lations in which the gas turbines are installed andoperated prior to the steam cycle installation andwhere it is desired to operate the gas turbinesindependent of the steam system. The multi-shaftconfiguration was applied most widely in the earlyhistory of heat recovery combined-cycles primarilybecause it was the least departure from the famil-iar conventional steam power plants. The single-shaft combined-cycle system has emerged as thepreferred configuration for single phase applica-tions in which the gas turbine and steam turbineinstallation and commercial operation are concur-rent.
The first GE single-shaft combined-cycle gener-ation units entered service in 1968. The earlyunits demonstrated the excellent generation eco-nomics that could be achieved by these simple,efficient, reliable and compact systems. The sin-gle-shaft combined-cycle applications are increas-ing as a result of: the power generation industryacceptance of GE’s demonstrated outstandingoperating characteristics; the development of larg-er gas turbines making possible large single com-bined-cycle units; the application of efficient butcomplex steam cycles (multiple pressure, reheatand steam cooling of the gas turbine) that are sim-plified by a unitized system. Also, the single-shaftsystem complements the unitized plant configura-tion that is most advantageous for solid fuel firedintegrated gasification combined-cycle (IGCC)generation systems. Table 1, which shows theexperience of GE single-shaft heat recovery com-bined-cycle generation systems, illustrates theincreasing application of this type of equipment.
The H Technology combined cycles, Figure 1,are the most efficient power generation equip-ment available today. High efficiency is achievedby the high gas turbine firing temperature withminimum use of cycle air for cooling that isenabled by closed circuit steam cooling of the tur-bine. The single shaft configuration is ideally suit-
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GER-3767C
SINGLE-SHAFT COMBINED-CYCLE POWERGENERATION SYSTEM
L.O. Tomlinson and S. McCulloughGE Power SystemsSchenectady, NY
ed to this large, highly integrated combined cyclesystem.
STEAM CYCLE ANDPERFORMANCE
A single-shaft STAG combined-cycle system hasbeen optimized for each of the heavy duty gas tur-bines in the GE gas turbine product line. The per-formance ratings for the base configurationsburning natural gas fuel are presented in Table 2.In the STAG system designation, the first digit des-ignates a single gas turbine and the third digitwith the following letters designates the gas tur-bine frame size.
The base configuration for the GE single-shaftproduct line is summarized in Table 3. The threepressure non-reheat steam cycle with natural cir-culation HRSG for the gas turbines with exhaustgas temperature of 1000 F/538 C is shown inFigure 2. The three-pressure reheat cycle that isapplied with gas turbines with exhaust gas temper-
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GER-3767C
Table 1SINGLE-SHAFT COMBINED-CYCLE EXPERIENCE
GT23825D
GT18516
Figure 1. H technology single-shaftcombined-cycle
ature higher than 1000 F/538 C is shown inFigure 3. Figure 4 shows the three pressure reheatsteam cycle that is integrated with the steam cool-ing system for the H technology gas turbine. Thenominal steam conditions at rated output are pre-sented in Table 4 and the major equipment forthe single-shaft product line is presented in Table5.
The single-shaft combined-cycle units followthe typical ambient temperature effects for GESTAG combined-cycle generation systems asshown in Figure 5. The heat rate does not vary sig-nificantly with ambient air temperature but thepower output increases significantly at low ambi-
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GER-3767C
Table 2SINGLE-SHAFT STAG PRODUCT LINE RATINGS
GT23826B
GT23102A
Figure 2. Single-shaft, combined-cycle, three-pressure non-reheat steam cycle
Table 3SINGLE-SHAFT STAG COMBINED-CYCLE
BASE CONFIGURATION
UNIT NET PLANT NET PLANT HEAT RATE (LHV) THERMALDESIGNATION STEAM CYCLE POWER Btu/kWhr KJkWhr EFFICIENCY
60HzS106B Non-Reheat, 3-Pressure 59.8 7005 7390 48.7S106FA Reheat, 3-Pressure 107.1 6440 6795 53.0S107EA Non-Reheat, 3-Pressure 130.2 6800 7175 50.2S107FA Reheat , 3-Pressure 258.8 6090 6425 56.1S107G Reheat, 3-Pressure 350.0 5885 6210 58.0S107H Reheat, 3-Pressure 400.0 5690 6000 60.0
50HzS106B Non-Reheat, 3-Pressure 59.8 7005 7390 48.7S106FA Reheat, 3-Pressure 107.4 6420 6775 53.2S109E Non-Reheat, 3-Pressure 189.2 6570 6935 52.0S109EC Reheat, 3-Pressure 259.3 6315 6660 54.0S109FA Reheat, 3-Pressure 376.2 6060 6395 56.3S109H Reheat, 3-Pressure 480.0 5690 6000 60.0
Notes: 1. Site Conditions-59 F, 14.7 psia, 60% RH (15 C, 1.013 bar, 60%)
4
GER-3767C
GT23103
Figure 3. Single-shaft, combined-cycle, three-pressure reheat steam cycle
Figure 4. STAG 107H/109H cycle diagramGT25020C
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Table 4PRODUCT LINE NOMINAL STEAM CONDITIONS
GT23833AA
Table 5SINGLE-SHAFT STAG PRODUCT LINE EQUIPMENT
GT23828A
ent air temperature. Figure 6 illustrates typicalpart load performance for a single-shaft com-bined-cycle unit. The heat rate is near the ratedvalue from approximately 80% to 100% load,while the gas turbine inlet guide vanes are modu-lated to maintain high cycle temperature. At lowerload, the output is varied by variation in fuel flowwith varying cycle temperature so the heat rateincreases as load is reduced.
ENVIRONMENTAL IMPACTMinimum environmental impact is a hallmark
of all combined-cycle systems. Exhaust emissionsare low as a result of high quality combustion inthe gas turbines, and heat rejected to coolingwater is low since the power produced by thesteam cycle is one-third of the unit power output.The Dry Low NOx combustion systems achieveNOx emissions of 25 ppmvd at 15% oxygen
(43 g/GJ) or less. Water or steam injection abatesNOx emissions to 25 ppmvd at 15% oxygen (43 g/GJ) when burning distillate oil fuel. IGCCsystems can achieve NOx emissions as low as 9ppmvd at 15% oxygen (16 g/GJ) with nitrogeninjection or fuel moisturization by saturation.
For environmentally sensitive applicationswhere extremely low NOx emissions are required,selective catalytic reduction (SCR) can be readilyadapted to single-shaft combined-cycle systems.SCRs require a gas temperature range lower thanthe gas turbine exhaust gas temperature so theyare installed in the HRSG in the appropriate zoneto suit their operating temperature range. The sin-gle-shaft combined-cycle does not have an exhaustgas bypass stack, so the exhaust gas passes throughthe SCR for reducing NOx emissions at all times.
The single-shaft combined-cycle is also ideallysuited for incorporation into IGCC systems whichcan burn coal in an environmentally acceptable
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GER-3767C
GT23822
Figure 6. Typical single-shaft combined-cycle unit heat rate variation with power output
GT23821
Figure 5. Ambient temperature effect on performance
manner. The unitized system enables convenientintegration with a gasifier to form a flexible operat-ing unit. Unitizing the IGCC system is highly desir-able because there are a large number of flow inter-connections between components. The unit systemdoes not require the control complexity to splitflows between components as would be required fornon-unitized systems. Figure 7 presents a unitizedIGCC system with air, coal gas, nitrogen, water andsteam interchange between components.
EQUIPMENTCONFIGURATIONS
The single-shaft combined-cycle units with non-reheat steam turbine capability less than approxi-mately 40 MW for 60 Hz units and 60 MW for 50Hz units are configured with the generatorbetween the gas turbine and the steam turbine, as
shown in Figure 8. This configuration is appliedfor the STAG 107EA and STAG 109E systems. Thisenables the equipment to be installed on a slabtype foundation. The gas turbine and steam tur-bine both have thrust bearings for individuallycontrolling their shaft position. A flexible cou-pling between the steam turbine and generatoraccommodates the relative movement of the rotorbetween the generator and the steam turbine.
A cross section of the steam turbine applied ina typical STAG 107EA system is shown in Figure 9.This is a simple, single casing unit with full arcsteam admission and off-chest stop/control valves.This steam turbine with axial exhaust drives fromthe inlet end through a diaphragm type flexiblecoupling. All connections are flanged since thesteam turbine is moved to remove the generatorrotor. A similar steam turbine configuration isapplied on the STAG 109E, STAG 106FA andSTAG 106B single-shaft units.
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GT23824
Figure 8. STAG 107EA, 109E configuration
GT23823
Figure 7. Unitized IGCC system
Single-shaft combined-cycle units with larger,reheat steam turbines are configured with thesteam turbine between the gas turbine and thegenerator as shown in Figure 10. The steam tur-bine is not required to drive through the genera-tor collector and the more complex steam turbineis not required to be moved for generator rotorremoval. A single thrust bearing in the gas turbinecontrols the rotor position. Solid couplings areused throughout the equipment train.
The reheat steam turbine with double flowexhaust for application on the STAG 107FA withlow exhaust pressure and the 50 Hz STAG 109FAcombined-cycle is shown in Figure 11. Theopposed flow HP-IP section is keyed at the frontstandard which is near the unit thrust bearing inthe gas turbine. Tie rods from the front standardalso fix the position of the gas turbine thrust bear-ing in relation to the steam turbine. Figure 12shows a cross-section of the MS7001FA gas turbineand single flow steam turbine in a STAG 107FA
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GER-3767C
GT21950
Figure 12. STAG 107FA combined-cycle unit with single-flow steam turbine and gas turbine
GT00437-1B
Figure 9. STAG 107EA steam turbineGT21939
Figure 10. Large single- shaft combined-cycleequipment configuration
B. Downward Exhaust, Double Flow
GT21524
Figure 11. Two-flow reheat steam turbine
combined-cycle unit. Figure 13 shows a perspec-tive view of the rotating equipment for a STAG109FA combined-cycle unit.
The simplicity of the GE single shaft combinedcycle equipment provides easy starting and opera-tion and minimum maintenance which results inhigh reliability and availability. A clutch in theshaft line to enable independent operation of thegas turbine and generator is not included becauseit requires either an HRSG gas bypass or largesteam by pass to a dump condenser or to the mainsteam turbine exhaust condenser to enable gasturbine operation. These either complicate thecombined cycle system or they prevent perfor-mance of maintenance work on the steam turbineduring independent operation of the gas turbine.
A comprehensive study of equipment configu-rations was conducted during the development ofthe H technology single shaft combined-cycles.The configuration with the generator between thegas turbine and steam turbine with a clutchbetween the steam turbine and generator wasincluded in this study. While this configurationenables a low, slab type foundation for applica-tions with single flow, axial exhaust steam tur-bines, the installed cost advantage achieved bylower foundation pedestal cost is offset by othercosts resulting from the increased train lengthand the requirement for special features to enableremoval of the generator rotor for inspection andmaintenance. A summary of the comprehensiveinstalled cost analysis is presented on Table 6.Since there is not a distinct cost or operationaladvantage, the GE large single shaft combined-cycles will continue to be configured with the gasturbine exhausting axially to the HRSG and driv-ing through the steam turbine to the generatorbecause of the following:
Compactness - less land space - higher power density
Simplicity - no clutch and auxiliaries, no gas bypass damper, no dump condenser, etc.
Maintainability - easy access for convenient maintenance
Operability - Simplest equipment and control configurations
AUXILIARIESThe unit mechanical auxiliaries include the
condenser and auxiliaries, circulation waterpumps, condensate pump, feedwater pump, con-denser cooling water pump and HRSG evaporatorcirculation pumps, if required. Deaeration offeedwater in the condenser has been applied onmost GE single-shaft combined-cycle units.Alternately, positive pressure deaeration by adeaerator integral with the low-pressure evapora-tor in the HRSG or a separate deaerator withsteam supplied from the low-pressure evaporatorcan be applied. Single train mechanical auxiliarieshave been applied on all single-shaft units. Thesystem simplicity resulting from application of sin-gle train auxiliaries has contributed to the out-standing reliability and availability achieved bythese systems.
Common lubrication system and hydraulicpower systems are employed for the major equip-ment train. These are critical systems and areequipped with redundant pumps, including a d-cpowered lubrication pump for emergency shut-down and cooldown in event of a failure of the a-cauxiliary power supply. The unit accessories for theSTAG 106FA, 107FA and 109FA are motor driven.These accessories are packaged on an integratedskid installed beside the unit in the basement. Theunit accessories for the STAG 106B, 107EA, STAG109E and 109EC are shaft driven from an accessorygear unit mounted on the lubricating oil reservoirwhich is located at the compressor end of the gasturbine.
Two methods of starting the large STAG 107FA,STAG 109EC and STAG 109FA combined-cycleunits are available. These are the steam turbinewith steam supplied from an auxiliary steam sup-ply or boiler, or the generator may be used withpower furnished from the auxiliary electrical sys-tem through a variable frequency load commutat-ed inverter (LCI) system. The smaller STAG 106B,STAG 106FA, STAG 107EA and STAG 109E single-shaft units are started with an electric motor andtorque convertor.
A small auxiliary steam supply is required forthe steam turbine seals for starting. The auxiliarysteam requirement varies from approximately2000 lb/hr/907 kg/hr for the STAG 106B unit to4000 lb/hr/1814 kg/hr for the STAG 109FA unit.Auxiliary steam conditions are 600 F/316 C and100 psig/6.9 atg. An auxiliary steam header with
9
GER-3767C
GT20907C
Figure 13. STAG 109FA perspective view
10
GER-3767C
Tab
le 6
ALT
ERN
ATE
SIN
GLE
-SH
AFT
PO
WER
PLA
NT
DES
IGN
INST
ALL
ED C
OST
EVA
LUAT
ION
PLA
NT
CO
ST
CO
MP
AR
ISO
NS
ING
LE-S
HA
FT C
OM
BIN
ED
-CY
CLE
ALT
ER
NA
TE P
OW
ER
P
LAN
T A
NA
LYS
IS
STA
G 1
07H
OR
109
H
PR
OV
EN
GE
PO
WE
R T
RA
INA
LTE
RN
ATE
PO
WE
R T
RA
INE
VA
LUA
TIO
N E
XA
MP
LES
YM
BO
L LE
GE
ND
AN
D E
VA
LUA
TIO
N N
OTE
S O
NA
LTE
RN
ATE
PO
WE
R P
LAN
T D
ES
IGN
CLU
TCH
, CO
UP
LIN
G A
ND
AX
IAL-
FLO
W C
ON
DE
NS
ER
WID
TH A
DD
ED
TO
MA
IN P
OW
ER
TR
AIN
(MP
T)
LEN
GTH
AX
IAL-
FLO
W C
ON
DE
NS
ER
AD
DE
D T
O M
PT
LEN
GTH
, PLU
S E
XTR
A L
EN
GTH
OF
CLU
TCH
AS
SE
MB
LY
STE
AM
PP
G A
ND
VA
LVE
S M
OV
ED
UP
TO
OP
ER
ATI
NG
FLO
OR
SE
CO
ND
TU
RN
ING
GE
AR
RE
QU
IRE
D
AX
IAL-
FLO
W V
S. D
OW
N F
LOW
EX
HA
US
T D
ES
IGN
; AD
DIT
ION
AL
THR
US
T B
RG
.
DO
UB
LE-E
ND
DR
IVE
RO
TOR
DE
SIG
N C
OS
T
INC
RE
AS
ED
- O
IL C
ON
SU
MP
TIO
N O
F C
LUTC
H A
ND
TH
RU
ST
BE
AR
ING
, IN
CR
EA
SE
D C
OM
PLE
XIT
Y O
F
RE
HE
AT
VA
LVE
AN
D O
VE
R S
PE
ED
PR
OTE
CTI
ON
LOW
ER
SU
PP
OR
T E
LEV
ATI
ON
TYP
ICA
L A
XIA
L-FL
OW
CO
ND
EN
SE
R, S
IMP
LE-C
YC
LE O
PE
RA
TIO
N “D
UM
PIN
G”
DE
CR
EA
SE
D S
TAR
TIN
G P
OW
ER
- G
T A
ND
GE
NE
RA
TOR
ON
LY
CLU
TCH
, CO
UP
LIN
G, L
UB
RIC
ATI
ON
ELE
CTR
IC P
OW
ER
AN
D E
NC
LOS
UR
E
INC
RE
AS
ED
LE
NG
TH O
F S
UP
PO
RT
SLA
BS
/ S
HO
RTE
R C
OLU
MN
S
INC
RE
AS
E A
RE
A D
UE
TO
INC
RE
AS
ED
MP
T LE
NG
TH; M
AIN
T. C
RA
NE
SIZ
ED
TO
LIF
T A
SS
EM
BLE
D
GE
NE
RA
TOR
? / D
EC
RE
AS
ED
VO
LUM
E D
UE
TO
LO
WE
R B
LDG
. ELE
V
PO
SS
IBLE
AD
DIT
ION
AL
EN
CLO
SU
RE
FO
R C
LUTC
H E
QU
IPM
EN
T
LON
GE
R P
PG
RU
NS
FR
OM
HR
SG
TO
STE
AM
TU
RB
INE
LOW
ER
ELE
VA
TIO
N /
LON
GE
R D
ISTA
NC
E B
ETW
EE
N M
AJO
R L
OA
DS
, e.g
. TH
RU
ST
BE
AR
ING
S
HR
SG
US
ED
FO
R S
EA
LIN
G A
ND
CO
OLI
NG
STE
AM
/ A
DD
ITIO
N O
F S
UP
ER
HE
ATE
R A
TTE
MP
ER
ATO
R
LON
GE
R O
UTP
UT
PO
WE
R A
ND
STA
RT-
UP
TR
AN
SFO
RM
ER
CIR
CU
ITS
RU
N N
OR
MA
L TO
T-G
AX
IS
LOW
ER
ELE
VA
TIO
N, S
OM
E S
HO
RTE
R C
IRC
UIT
S
PO
SS
IBLE
DE
CR
EA
SE
D B
AC
K P
RE
SS
UR
E F
OR
AX
IAL
EX
HA
US
T
FAS
TER
STA
RT-
UP
S, I
.E. F
IRS
T G
T-G
FO
LLO
WE
D B
Y S
T (N
ON
“H” T
EC
HN
OLO
GY
)
DIF
FIC
ULT
GE
NE
RA
TOR
RO
TOR
RE
MO
VA
L; A
DD
I’L C
LUTC
H M
AIN
TEN
AN
CE
; IN
DE
PE
ND
EN
T G
T &
ST
MA
INT.
FLE
XIB
LE C
ON
DE
NS
ER
& S
T IN
STA
LLA
TIO
N S
CH
ED
ULE
AD
DIT
ION
AL
MP
T E
QU
IPM
EN
T
TO IN
STA
LL, A
LIG
N, T
ES
T, E
TC.
TUR
BIN
E-G
EN
ER
ATO
R-C
ON
DE
NS
ER
LE
NG
TH
TUR
BIN
E-G
EN
ER
ATO
R B
UIL
DIN
G A
RE
A C
OS
T
ST-
G S
HA
FT C
EN
TER
LIN
E E
LEV
ATI
ON
CO
ST
GA
S T
UR
BIN
E C
OS
T
STE
AM
TU
RB
INE
CO
ST
GE
NE
RA
TOR
CO
ST
T-G
AU
XIL
IAR
IES
CO
ST
HR
SG
CO
ST
CO
ND
EN
SE
R C
OS
T
STA
TIC
STA
RT
CO
ST
AD
DIT
ION
AL
MA
IN P
OW
ER
TR
AIN
(MP
T) E
QU
IP.
PE
DE
STA
L / F
OU
ND
ATI
ON
CO
ST
BU
ILD
ING
/ S
TRU
CTU
RE
CO
ST
AC
OU
STI
C T
RE
ATM
EN
T C
OS
T
STE
AM
LIN
ES
, VA
LVE
S, S
UP
PO
RTS
, IN
SU
LATI
ON
CO
ST
AU
XIL
IAR
IES
PP
G IN
TER
CO
NN
EC
TIO
NS
CO
ST
AU
XIL
IAR
Y B
OIL
ER
CO
ST
HIG
H V
OLT
AG
E C
IRC
UIT
S C
OS
T
ME
DIU
M /
LOW
VO
LTA
GE
CIR
CU
ITS
CO
ST
EFF
ICIE
NC
Y
OP
ER
ATI
ON
AL
CO
ST
MA
INTE
NA
NC
E C
OS
T
CO
NS
TRU
CTI
ON
CO
ST
/ SC
HE
DU
LE
BA
SE
LE
NG
TH /
CO
ST
BA
SE
AR
EA
/ C
OS
T
BA
SE
ELE
VA
TIO
N /
CO
ST
BA
SE
CO
ST
BA
SE
CO
ST
BA
SE
CO
ST
BA
SE
CO
ST
BA
SE
CO
ST
BA
SE
CO
ST
BA
SE
CO
ST
BA
SE
CO
ST
BA
SE
CO
ST
BA
SE
CO
ST
BA
SE
CO
ST
BA
SE
LE
NG
TH /
CO
ST
BA
SE
LE
NG
TH /
CO
ST
BA
SE
CO
ST
BA
SE
CO
ST
BA
SE
CO
ST
BA
SE
EFF
ICIE
NC
Y
BA
SE
CO
ST
BA
SE
CO
ST
BA
SE
CO
ST
/ SC
HE
DU
LE
INC
RE
AS
E -
12M
(39
FT)
INC
RE
AS
E -
420M
2 (
4,50
0 S
Q. F
T)
DE
CR
EA
SE
- 3M
(10
FT)
INC
RE
AS
E
CO
MP
AR
AB
LE/IN
CR
EA
SE
INC
RE
AS
E
INC
RE
AS
E
DE
CR
EA
SE
INC
RE
AS
E
DE
CR
EA
SE
INC
RE
AS
E
INC
RE
AS
E /
DE
CR
EA
SE
INC
RE
AS
E /
DE
CR
EA
SE
INC
RE
AS
E
INC
RE
AS
E
DE
CR
EA
SE
/ IN
CR
EA
SE
DE
CR
EA
SE
/ IN
CR
EA
SE
INC
RE
AS
E
DE
CR
EA
SE
CO
MP
AR
AB
LE
CO
MP
AR
AB
LE
INC
RE
AS
E
DE
CR
EA
SE
/ IN
CR
EA
SE
CO
ND
EN
SE
R
ELE
CTR
ICA
L G
EN
ER
ATO
R
HE
AT
RE
CO
V. S
TEA
M G
EN
.
ST-
HP
& L
P S
EC
TIO
NS
GT-
TUR
BIN
E &
CO
MP
RE
SS
OR
CLU
TCH
steam supplied from an operating unit is typicallyapplied on installations with multiple units.
Exhaust gas bypass stacks and dampers are notrequired on single-shaft combined-cycle units. Asimple steam bypass from the superheaters directto the condenser is applied on units either withreheat or non-reheat steam systems.
PLANT ARRANGEMENTSThe single-shaft STAG combined-cycle equip-
ment is configured for low cost installation withconvenient maintenance. The STAG 106B, 107EAand 109E units are arranged for installation on aslab-type foundation with minimum basement.The modern, high technology units with multiplepressure, reheat cycles are typically installed on apedestal to allow space for the piping. Those sys-tems with the steam turbine between the gas tur-bine and generator are usually arranged with thesteam turbine exhaust condenser below the steamturbine requiring a pedestal type foundation.
The STAG 109E rotating equipment, shown inFigure 14 during erection at the TEPCO FuttsuStation, is designed for inexpensive installation on a
slab-type foundation. While it is not required, this tur-bine building has a basement for convenient accom-modation of the condenser, the auxiliaries, andsteam and water piping. An aerial view of the 2310MW TEPCO Futtsu Station is shown in Figure 15.Seven units are installed in each of the two turbinebuildings. Figure 16 shows the Western FarmersElectric Cooperative Anadarko plant which has threeSTAG 107E units. These units are installed on slabtype foundations with piping installed above the floorand convenient maintenance access and laydownspace provided by an open aisle between the units.
A typical plant arrangement for the STAG 107FAsingle-shaft combined-cycle is shown in Figures 17,18 and 19 for an indoor installation. The turbinebuilding includes a basement for the condenser,auxiliaries and piping. This arrangement includesHRSGs with horizontal gas flow and natural circula-tion evaporators.
The Tokyo Electric Power Co. STAG 109FAinstallation at the Yokohama Station has beendesigned with the air intake duct routed throughthe foundation under the gas turbine. Thisarrangement minimizes building plan space. Arendering of this 2800 MW plant with eight STAG109FA units is shown in Figure 20.
CONTROLS AND OPERATIONSimplicity of controls and operating flexibility
are key features of single-shaft combined-cycle sys-tems. Easy starting and excellent load followingcapability suit these systems for daily start, mid-range peaking service, and their high reliabilityand availability suits them for base load, continu-ous operation.
Each single-shaft unit operates independently,so a problem on one unit does not affect others.Differentiating from the multi-shaft combined-cycles, where the steam turbine generator has dif-ferent operating characteristics than the gas tur-
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Figure 16. Western Farmers Electrical Cooperative Plant
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Figure 15. 2310 MW TEPCO Futtsu station – 14STAG units
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Figure 14. STAG 109E equipment
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Figure 19. STAG 107FA elevation sectionthrough gas turbine
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Figure 17. STAG 107FA equipment arrangement plan
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Figure 18. STAG 107FA equipment arrangement elevation
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Figure 20. 2800-MW TEPCO Yokohama Station
bine generator, multiple single-shaft units in a sta-tion have identical operating requirements.
A block diagram of a typical distributed controlsystem for a plant with multiple single-shaft units isshown in Figure 21. A unit control system, which isusually installed near the major equipment, con-trols and protects the single-shaft unit. The unitcontrol system communicates with the central plantcontrol through a data link. The central control isusually in a remote central control room. Figure 22shows a typical central control room layout. Thissystem enables one central room operator to coor-dinate the operation of several single-shaft com-bined-cycle units.
Complex steam systems such as the three-pres-sure reheat system are simplified in the single-shaftcombined cycle. The diagram of the steam cycle inFigure 23 illustrates some key areas of simplifica-tion. Valves which isolate high pressure steam lines,reheaters and low pressure steam lines are notrequired as they are for multi-shaft reheat com-bined cycles.
When the steam turbine shell and rotor are hot,the starting and loading are fast, typically one hourfollowing a 12-hour overnight shutdown. The start-ing and loading time after a weekend shutdown isapproximately two hours, and three hours if thesteam turbine is cold as shown in Figure 24. Figure
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Figure 23. Reheat single-shaft combined-cycle diagram
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Figure 22. Typical control room layoutGT21526B
Figure 21. Control configuration
25 shows the variation of several key parametersduring a normal shutdown and hot start for aSTAG 109E unit at the TEPCO Futtsu Station. Thisunit completes a hot start within a one hour periodfollowing an overnight shutdown period.
RELIABILITY ANDAVAILABILITY
Combined-cycle plant operating informationgathered through the GE user weekly log systemfor the Operation Reliability Analysis Program(ORAP) and information reported by the NorthAmerican Reliability Council (NERC) depict anoperating history characterized by high reliabilityand availability with a trend for continued reliabil-
ity improvement. Operating statistics from 1989through 1995 for several utility power generationcombined-cycle installations are summarized inTable 7. This data shows single-shaft combined-cycle systems achieve generally higher plant relia-bility and availability than multi-shaft combined-cycle systems. These excellent operatingcharacteristics are the result of simplicity of con-trol and operation and proper maintenance prac-tices.
Operating data for the 14 STAG 109E units atthe TEPCO Futtsu Station are shown on Table 8.This is an outstanding installation with excellentoperation and maintenance practices, whichenables the equipment to achieve outstandingoperating results. The Japanese governmentrequires a thorough inspection that is the equiva-lent of a major inspection at calendar intervals oftwo years. This impairs the availability, but enablesreliability near 100%.
MAINTENANCEMaintenance of single-shaft combined-cycles is
performed on units individually, which does notimpair the operation of other units in the station.This contrasts with multi-shaft combined-cycleunits where steam turbine maintenance impairsthe operation of several gas turbines. Operatingintervals between maintenance on the compo-nents in single-shaft units are similar so that manyplanned maintenance activities can be performed
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Figure 25. TEPCO Futtsu STAG 109E normal shutdown and hot start
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Figure 24. Starting and loading characteristics
concurrently. The gas turbine combustion inspec-tion interval is 8,000 hours which coincides withthe annual HRSG inspection at one year intervalsif base loaded or at two year intervals for mid-range peaking service (4,000 hrs/yr.). The hot gaspath inspection on the gas turbine at 24,000hours, which also includes a combustion inspec-tion, coincides with an HRSG inspection. Themajor inspection of the gas turbine at 48,000hours occurs at the recommended steam turbinemajor inspection, generator rotor removal andinspection, and HRSG annual inspection.
An example of the low maintenance cost
required by single-shaft STAG combined-cyclegeneration units is that of the three STAG 107Eunits at the Western Farmers Electric CoopAnadarko Station. These units have operated inmid-range peaking service and all have completedtwo maintenance cycles including two majorinspections with the intermediate hot-gas-path,combustion and HRSG inspections. The averagemaintenance cost is 0.74 mills/kWh (Table 9).
Single-shaft STAG units with the generatorbetween the gas turbine and steam turbinerequire removal of the steam turbine for genera-tor rotor removal. The steam turbine has been
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Table 9WESTERN FARMERS – 3 STAG 107E
OPERATION AND MAINTENANCE DATA
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Table 8TEPCO FUTTSU STATION OPERATION
SUMMARY – MARCH 31, 1996
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Table 7STAG COMBINED-CYCLE PLANT SEVEN-YEAR OPERATING STATISTICS
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packaged and skid mounted to accommodate thisrequirement as shown in Figure 26. The exhaust isaxial, all connections are flanged and the electri-cal connections are grouped at junction boxes.Appropriate jacking, pull points and rails in theinstallation enables the steam turbine to be easilymoved without lifting.
PRODUCT STRUCTUREThe basic structure for the single-shaft STAG
combined-cycle product line is the GE furnishedEngineering Equipment Package (EEP). The EEPincludes a unique combination of equipment andservices such that the owner receives a plant per-formance guarantee and warranty of operationand serviceability of the combined-cycle systemwhile retaining the capability to customize theplant design, auxiliaries, structures, etc. This isachieved by including in the GE scope the majorcombined cycle equipment which requires closecoordination for assurance of meeting the perfor-mance objectives.
The equipment scope split between GE and theowner is shown in Figure 27 and the correspond-ing services and software split is presented inFigure 28. Key elements in the GE EEP scope arethe combined-cycle equipment and system designand interface definition that enable the owner, hisengineer, or engineer constructor to design theplant to satisfy his specific requirements.
COMPARISON WITHMULTI-SHAFT
The key advantage achieved by the single-shaftcombined-cycle is its operating simplicity and
independence of unit operation and maintenancewhich results in high reliability and availabilityand low maintenance cost. Other differences,such as installed cost, are not as distinct and resultin trade-offs that may be influenced by siterequirements and owner operating requirementsor preferences.
Installed CostThe installed cost is approximately equal for
multi-shaft and single-shaft systems; however, thereare trade-offs that may influence a specific applica-tion. Also, there are differences between the non-reheat and reheat systems that result in differentconsiderations regarding a comparison of installed
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Figure 27. Equipment scope split with Engi-neered Equipment Package
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Figure 28. Services and software scope splitwith Engineered Equipment Package
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Figure 26. Packaged base-mounted steam tur-bine
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Figure 29. STAG 207FA multi-shaft combined-cycle equipment arrangement plan
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Figure 31. STAG 207FA multi-shaft combined-cycle gas turbine and HRSG elevation
Figure 30. STAG 207FA multi-shaft combined-cycle steam turbine elevation
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Figure 33. Combined-cycle system part load performanceGT23112
Figure 32. Reheat multi-shaft combined-cycle steam cycle control diagramGT23834A
cost differences between multi-shaft and single-shaftcombined cycles. The smaller non-reheat single-shaft systems can be installed on a slab type founda-tion at grade level so that their foundation cost islower than the multi-shaft which would have a largepedestal mounted steam turbine. The larger reheatsystems require a basement for the increased quan-tity of piping. The steam turbine is installedbetween the gas turbine and generator whichrequires a down exhaust to a condenser under theturbine. This requires the gas turbine, generatorand steam turbine to be mounted on a pedestalwhereas the gas turbine foundations can be lower inmulti-shaft systems. Figures 29, 30 and 31 show theequipment arrangement drawings for a STAG207FA system with a reheat steam cycle. The steamturbine pedestal is shown in Figure 30 while Figure31 shows the lower foundation for the gas turbines.
Steam piping and valves are a major cost adderfor the multi-shaft system. Figure 32 is a diagramof the major steam cycle piping showing the isola-tion valves, that are not required by the single-shaft system shown in Figure 23. Also, the lengthof steam piping is increased since the steam tur-bine is remote from the HRSGs for the multi-shaftsystems. Figure 29 shows the arrangement withlowest cost steam piping. Longer steam pipingruns result if both gas turbines and HRSG arelocated on the same side of the steam turbine.
The larger number of generators and mainelectrical connections also increase the cost of thesystem. Table 10 presents a comparison configura-tion summary for combined-cycles with three-pressure, reheat steam cycles. This chart illustratesqualitatively that the installed cost of the multi-shaft system with one gas turbine and steam tur-bine has the highest installed cost while the twogas turbine per steam turbine multi-shaft hasapproximately equal installed cost trade-offs. Theoperating simplicity favors the single-shaft.
Independent Operation of GasTurbines
Operation of gas turbines independent of thesteam system is a potential advantage that can berealized by the multi-shaft combined-cycle.However, this requires either exhaust gas bypassstacks and dampers, full steam bypass to the con-denser, or full steam bypass to a separate dumpcondenser. All of these systems increase installedcost and operating complexity and reduce avail-ability and reliability.
PerformanceThe heat rate of a multiple-unit, multi-shaft
combined-cycle system will usually be slightly bet-
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Table 10THREE-PRESSURE REHEAT COMBINED-CYCLE CONFIGURATION COMPARISON SUMMARY
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ter than that of a comparable multiple-unit single-shaft system. This small difference in heat rate isapproximately 10 Btu/kWhr (10.5 kJ/kWhr) andresults from higher efficiency of the larger highpressure section of the steam turbine. The partload heat rate of multiple single-shaft units is bet-ter than that of a multi-shaft system because of thereduced efficiency of the steam turbine atreduced steam flow. Figure 33 illustrates this dif-ference.
CONCLUSIONCombined-cycle systems provide reliable and
economic service in electric utility power genera-tion applications. Flexibility in equipment selec-tion and arrangement, thermal cycle, type of fuel,emission control and duty cycle enable optimiza-tion to meet a wide variety of owner requirements.Single-shaft combined-cycle systems offer furtheradvantages as follows:
• Simplicity of single-unit control• Low plant cost• Minimum land area use• Simplified control and operation with
multiple pressure and reheat steam cycle• High reliability and availabilityTherefore, consideration of the single-shaft unit is
recommended for all power generation combined-cycles that are installed in a single phase.
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© 1996 GE Company
LIST OF FIGURES
Figure 1. H technology single shaft combined-cycle unitFigure 2. Single-shaft, combined-cycle, three-pressure non-reheat steam cycleFigure 3. Single-shaft, combined-cycle, three-pressure reheat steam cycleFigure 4. Stag 107H/109H cycle diagramFigure 5. Ambient temperature effect on performanceFigure 6. Typical single-shaft combined-cycle unit heat rate variation with power outputFigure 7. Unitized IGCC systemFigure 8. STAG 107EA, 109E configurationFigure 9. STAG 107EA steam turbineFigure 10. Large single shaft combined-cycle equipment configurationFigure 11. Two-flow reheat steam turbineFigure 12. STAG 107FA combined-cycle unit with single-flow steam turbine and gas turbineFigure 13. STAG 109FA perspective viewFigure 14. STAG 109E equipmentFigure 15. 2310 MW TEPCO Futtsu station – 14 STAG unitsFigure 16. Western Farmers Electrical Cooperative PlantFigure 17. STAG 107FA equipment arrangement planFigure 18. STAG 107FA equipment arrangement elevationFigure 19. STAG 107FA elevation section through gas turbineFigure 20. 2800-MW TEPCO Yokohama StationFigure 21. Control configurationFigure 22. Typical control room layoutFigure 23. Reheat single-shaft combined-cycle diagramFigure 24. Starting and loading characteristicsFigure 25. TEPCO Futtsu STAG 109E normal shutdown and hot startFigure 26. Packaged base-mounted steam turbineFigure 27. Equipment scope split with Engineered Equipment PackageFigure 28. Services and software scope split with Engineered Equipment PackageFigure 29. STAG 207FA multi-shaft combined-cycle equipment arrangement planFigure 30. STAG 207FA multi-shaft combined-cycle steam turbine elevationFigure 31. STAG 207FA multi-shaft combined-cycle gas turbine and HRSG elevationFigure 32. Reheat multi-shaft combined-cycle steam cycle control diagramFigure 33. Combined-cycle system part load performance
LIST OF TABLES
Table 1 Single-shaft combined-cycle experienceTable 2 Single-shaft STAG product line ratingsTable 3 Single-shaft STAG combined-cycle base configurationTable 4 Product line nominal steam conditionsTable 5 Single-shaft STAG product line equipmentTable 6 Alternate single-shaft power plant design installed cost evaluationTable 7 STAG combined-cycle plant seven-year operating statistics.Table 8. TEPCO Futtsu station operation summary - March 31, 1996Table 9 Western Farmers – 3 STAG 107E operation and maintenance dataTable 10 Three-pressure reheat combined-cycle configuration comparison summary
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Leroy O. TomlinsonLeroy O. Tomlinson is Chief Engineer of the Controls, Accessories and
System Engineering Department in GE Power Generation Engineering.Responsibilities of this department include design and development of gasturbine and steam turbine controls, electrical systems, accessories and fuildsystems and gas turbine and combined cycle system engineering.
Leroy has held engineering management assignments in GE for 23 years,including Gas Turbine and Combined-Cycle Application Engineering andPower Plant Systems Engineering and he has 42 years of engineering experi-ence in gas turbine and combined-cycle power generation equipment andplant design, test, application and operation.
Mr. Tomlinson is active on several ASME Gas Turbine and Power TestCode committees. He is a registered engineer in Pennsylvania, Florida andTexas.
Lists of figures and tables appear at the end of this paper.
Sean McCulloughSean McCullough is Manager of Reference Plant and Standards
Development in the Global Power Plant Systems Department. He is currentlyresponsible for leading efforts to reduce proposal, engineering, and construc-tion cycle and cost, through the development of Reference Plant Designs andConstructability Initatives.
Sean holds a degree in mechanical engineering, is a licensed professionalengineer, and recipient of the Power System’s Engineering Awards in 1985and 1994. He also holds two patent awards. Since joining GE in 1972, he hasheld a variety of positions including Start-up Engineer, Design Engineer,Service Manager, and Project Manager.
