Variable-Speed Drives
for Combined Cycle Power Plants
Voith Turbo’s proven technology
in Combined Cycle Power Plants
Speed variation produces significant energy savings and efficiency
improvement in Combined Cycle Power Plants, in cogeneration
plants as well as in repowering projects.
160
Fuel
H.Pr. l.Pr.
2 x 50% Pump
L.Pr.
140
1 x 50% Pump
Air
Stack
TC
Gas Turbine
GT Generator
Steam Turbine
M
Generator
Fuel
H.Pr. l.Pr.
Cooling
Water
L.Pr.
Test Point
100% MCR
100
BFP
80
Boiler Characteristic
60
40
20
Air
Stack
TC
Gas Turbine
Pressure [bar]
120
GT Generator
M
Condensate
Pump
HP/IP BFP variable speed coupling
Simplified flow schematic of a multi shaft,
triple pressure Combined Cycle Power Plant.
0
0
10 20 30 40 50 60 70 80 90 100 110 120 130
Flow [%]
Boiler and pump characteristic curves.
Voith variable-speed and geared
variable-speed turbo couplings
enable the reliable and economic
speed control of HP/IP boiler feed
pumps. In addition, Voith torque
converters are very reliably and
efficiently used as gas turbine
starting devices in advanced Combined Cycle Power Plants.
Boiler feed pump drive with Voith
variable-speed coupling.
TC
Gas turbine Start-up Device
with Voith torque converter.
2
Energy reduction and performance improvement
of boiler feed pumps
In Combined Cycle Power Plants,
experience shows that every boiler
– including heat recovery boilers –
has to fulfill a so called test point,
thus ensuring the safety of the
equipment at overload conditions.
A more economical way to control
the pump flow is to change its
speed, resulting in different pump
characteristic curves, that match
the system’s unthrottled resistance
curve.
When using speed regulation,
additional cost savings can be
achieved by selection of a less expensive recirculation control flow
valve and a 100% control valve
(small differential pressure).
The operating point of a pump
results from the intersection of
pump characteristic curve with the
system’s resistance curve. Other
operating points may be obtained
by increasing the system’s head
loss by throttling. In addition to the
head loss, the pump efficiency decreases significantly when operating at lower flow rates.
The use of a variable-speed coupling allows reducing the speed of
the boiler feed pump for overload,
as well as for the speed of base
load conditions and also for partial
load. This results in significant
savings of auxiliary energy. The
pump can be operated close to its
optimum efficiency over the whole
speed range.
Service life of a power plant is
about 30 years. Nobody can predict
fuel and gas prices as well as load
patterns over this period of time.
Flexibility is a key issue for today’s
advanced power station technology. The operation of boiler feed
pumps should be flexible, too. This
can only be achieved by variablespeed operation of pumps.
Cost comparison of throttle control versus variable-speed turbo coupling
3 x 50% Boiler Feed Pumps
120
Turbo Couplings
425 MW Test Block
425 MW Test Block
Operating hours
[h/yr]
7,990
10
7,990
10
Flow
[kg/s]
62.70
78.38
62.70
78.38
Head
[bar]
131.99
115.21
93.50
115.21
Feed water density
[kg/m3]
925.87
925.87
925.87
925.87
Pump efficiency
[%]
78.96
80.00
80.00
80.00
Pump speed
[rpm]
2,980
2,980
2,487
2,872
40
Absorbed power
[kW]
1,132
1,219
791
1,219
Slip losses
[kW]
–
–
157
46
20
Mechanical losses
[kW]
–
–
12
12
Total power
[kW]
2 x 1,132
2 x 1,219
2 x 960
2 x 1,277
Energy per year
[kWh]
24,380 15,340,800
25,540
Total energy per year
[kWh]
Energy cost 1 kWh: 0.0407 $
[$]
100
Motor power [%]
Throttle Control
Operating points
Throttle control
80
Power Savings
60
Variable-speed
turbo coupling
0
0
10
20
30
40
50 60
Flow [%]
70
80
90
100 110
18,089,360
18,113,740
15,366,340
737,229
625,410
Savings: 111,819 $ per year
Example of power comparison between throttle control and variablespeed coupling.
3
Example: Cost return studies have shown that
amortization time by speed variation is usually between 6 and
20 months.
Variable-speed and geared variable-speed
turbo couplings
Variable-speed turbo couplings
for HP/IP boiler feed pumps
Variable-speed turbo couplings are
fluid couplings, connecting the
motor with the boiler feed pump.
They enable stepless speed control
of the pump. Hydrodynamic couplings provide wear-free power transmission and vibration damping
between input and output. Other
benefits include exceptional reliability and availability, smooth acceleration of pump and long service
life. All these features result in a
more efficient and reliable operation
of the pump.
Variable-speed turbo coupling type SVTL,
self-supported, with antifriction bearings.
Geared variable-speed couplings
PDS
for HP/IP boiler feed pumps with
high power and speed
PI
PS
PS
For higher power ranges, geared
variable-speed turbo couplings are
used. Input powers of more than
10,000 kW and speeds exceeding
6,000 rpm are possible.
4...20 mA
Geared variable-speed turbo coupling type
R..K..M. Combination of high-performance speed
increasing gearbox and variable-speed coupling.
Based on their robust design, a reliability of 99.8% can be achieved in
combination with a squirrel cage
motor. Low operating and maintenance costs, high control accuracy
and low energy consumption are
additional benefits for operators.
4
Torque converters for starting and turning
of gas turbines
Due to its torque/speed characteristic, the Voith torque converter
is an excellent device for starting gas turbines. A high breakaway
and acceleration torque is available for start-up of the gas turbine
into the self-sustaining speed range.
Complete Start-up Device with electric motor,
Voith torque converter and turning gear.
Simplified sectional view of a Voith torque
converter with integrated turning gear.
Turning gear
M
TC
Starter System
Generator
Gas Turbine
Schematic view of a gas turbine train. Starting package with motor, torque converter
and turning gear.
Hydrodynamic torque converters
adapt themselves easily and precisely to the gas turbine torque and
speed. Optimal start-up is guaranteed by adjustable guide vanes and
customized hydraulic circuits.
High start-up reliability is secured
with Voith. The system reliability is
over 99.9%.
The torque converter is drainable,
eliminating the need of a clutch for
disconnection of the starter from
the turbine. Torsional vibration is
no subject in hydrodynamic power
transmissions.
For slow roll requirements of the
turbines, a turning gear can be
attached to the torque converter
output, achieving a space saving
economical unit.
Complete Start-up Devices
Torque converter, motor, oil supply
and base frame are assembled into
a complete drive unit and therefore
offer the advantages of a complete
package from one single source.
This means a reduced number of
interfaces, minimized space requirements and project responsibility in
one hand.
5
cr303en, aik / WA, 01.2009, 2 000. Dimensions and illustrations without obligation. Subject to modifications.
Voith Turbo GmbH & Co. KG
Variable-speed Drives
Voithstr. 1
74564 Crailsheim, Germany
Tel. +49 7951 32-261
Fax +49 7951 32-650
[email protected]
www.voithturbo.com/variable-speed