Singleshaft Combined Cycle (CCGT)

Combined Cycle Gas Turbines (CCGT), where the energy in the gas turbine (GT) exhaust is recovered by producing steam and using that to drive a steam turbine (ST) make exceptional power plants due their high efficiency, quick installation and relatively low capital cost.

‘Multishaft’ machines, where the GT’s and ST drive separate generators, used to be the basic configuration but the need to reduce cost, block size and footprint has made Singleshaft CCGT where the GT and ST drive into the same generator the standardised arrangement. Singleshafts were originally solidly coupled but the advantages of adding an SSS Clutch on the ST side to allow the GT to be started independently quickly became clear to turbo machinery designers and end users alike.

In today’s market solidly coupled singleshaft CCGT’s can no longer be considered for most sites, due to their lack of flexibility and slow starting whilst multi-shaft solutions are often too large and unwieldy particularly with the increasing size of the most efficient GT’s.

A site with multiple Singleshaft CCGT’s with SSS Clutches has inherent redundancy, for increased availability and ease of maintainability, whilst maintaining the operational flexibility which means that they often operate as Daily Start Stop (DSS) and in the future, with the increasing renewables such as solar generation, can be used for 2xDSS or even more.

Key features

• Faster Starts - the GT start time is only limited by the GT itself and the Heat Recovery Steam Generator (HRSG)
• Improved shutdown - steam to the ST can be shut-off before the GT starts any cool down operation allowing the ST to be stopped in a hotter condition ready for a faster restart.
• Reduced emissions - The improved start and stop procedures reduce the time at low power when the machine is less efficient with greater emissions.
• Maintainability - GT inspection can be carried out whilst the ST is still on turning gear, speeding up the maintenance time and increasing availability.

For the turbo machinery designer/power plant supplier

• Flexibility and standardisation.

For the owner/operator

• Reliability and low cost

Many Singleshaft CCGT’s are for electricity generation only but the same machines can be used for…

• Co-generation or Combined Heat and Power (CHP) where the steam can also be used where there is a thermal demand.
• Grid support even when not producing active power. In many areas of the world combustion turbines will only run where there is not enough renewable or stored energy operation like a traditional peaker. By including an SSS Clutch in the drive the turbine can be disconnected leaving the generator spinning, synchronised and adding reactive power, inertia and fault security to the grid even when no active power is needed. See Synchronous Condensing.