Variable-Speed Drives for Combined Cycle Power Plants

Boiler and pump characteristic curves.

Speed variation produces significant energy savings and efficiency

improvement in Combined Cycle Power Plants, in cogeneration

plants as well as in repowering projects.

Voith variable-speed and geared

vari able-speed turbo couplings

enable the reliable and economic

speed control of HP/IP boiler feed

pumps. In addition, Voith torque

converters are very reli ably and

efficiently used as gas turbine

starting devices in advanced Com-

bined Cycle Power Plants.

Gas turbine Start-up Device with Voith torque converter.

Boiler feed pump drive with Voith variable-speed coupling.

Simplified flow schematic of a multi shaft, triple pressure Combined Cycle Power Plant.

Boiler Characteristic

100% MCRPr

essu

re [b

ar]

Flow [%]

120

100

80

60

40

20

00 10 20 30 40 50 60 70 80 90 100 110 120 130

140

160

1 x 50% Pump Test Point

2 x 50% Pump

BFP

Fuel

Air

Gas Turbine GT Generator

Fuel

Air

Gas Turbine GT Generator

Stack

Stack

Steam Turbine

Generator

Cooling Water

H.Pr. l.Pr. L.Pr.

H.Pr. l.Pr. L.Pr.

Condensate Pump

HP/IP BFP variable speed coupling

M

M

TC

TC

TC

2

Voith Turbos proven technology in Combined Cycle Power Plants

3 x 50% Boiler Feed Pumps Throttle Control Turbo Couplings

Operating points 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

Absorbed power [kW] 1,132 1,219 791 1,219

Slip losses [kW] 157 46

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] 18,089,360 24,380 15,340,800 25,540

Total energy per year [kWh] 18,113,740 15,366,340

Energy cost 1 kWh: 0.0407 $ [$] 737,229 625,410

Savings: 111,819 $ per year

Example: Cost return studies have shown that amortization time by speed variation is usually between 6 and 20 months.

Example of power comparison between throttle control and variable-speed coupling.

A more economical way to control

the pump flow is to change its

speed, resulting in different pump

characteristic curves, that match

the systems unthrottled resistance

curve.

The use of a variable-speed coup-

ling 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.

When using speed regulation,

additional cost savings can be

achieved by selection of a less ex -

pensive re circulation control flow

valve and a 100% control valve

(small differen tial pressure).

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 todays

advanced power station techno l-

ogy. The operation of boiler feed

pumps should be flexible, too. This

can only be achieved by variable-

speed operation of 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

equip ment at overload conditions.

The operating point of a pump

re sults from the intersection of

pump characteristic curve with the

systems resistance curve. Other

operating points may be obtained

by increasing the systems head

loss by throttling. In addition to the

head loss, the pump efficiency de -

creases significantly when oper-

ating at lower flow rates.

Cost comparison of throttle control versus variable-speed turbo coupling

Throttle control

Variable-speed turbo coupling

Power Savings

Mot

or p

ower

[%]

Flow [%]

120

100

80

60

40

20

00 10 20 30 40 50 60 70 80 90 100 110

3

Energy reduction and performance improvement of boiler feed pumps

4...20 mA

PI PS PS

PDS

Variable-speed turbo coupling type SVTL, self-supported, with antifriction bearings.

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 coup-

lings provide wear-free power trans-

mission and vibration damping

between input and output. Other

benefits include exceptional reli -

ability and availability, smooth ac -

celeration of pump and long service

life. All these features result in a

more efficient and reliable operation

of the pump.

Geared variable-speed couplings

for HP/IP boiler feed pumps with

high power and speed

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.

Based on their robust design, a reli-

ability of 99.8% can be achieved in

combination with a squirrel cage

motor. Low operating and mainten-

ance costs, high control accuracy

and low energy consumption are

additional benefits for operators.

Geared variable-speed turbo coupling type R..K..M. Combination of high-performance speed increasing gearbox and variable-speed coupling.

4

Variable-speed and geared variable-speed turbo couplings

Simplified sectional view of a Voith torque converter with integrated turning gear.

Complete Start-up Device with electric motor, Voith torque converter and turning gear.

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.

Hydrodynamic torque converters

adapt themselves easily and pre-

cisely to the gas turbine torque and

speed. Optimal start-up is guaran-

teed by adjustable guide vanes and

customized hydraulic circuits.

High start-up reliability is secured

with Voith. The system reliability is

over 99.9%.

Turning gear

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 require-

ments and project responsibility in

one hand.

Schematic view of a gas turbine train. Starting package with motor, torque converter and turning gear.

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.

Generator Gas TurbineStarter System

M TC

5

Torque converters for starting and turning of gas turbines

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Voith Turbo GmbH & Co. KG

Variable-speed Drives

Voithstr. 1

74564 Crailsheim, Germany

Tel. +49 7951 32-261

Fax +49 7951 32-650

vs.drives@voith.com

www.voithturbo.com/variable-speed