Single-Shaft Combined Cycle Power Plant: a Great Invention or an Elaborate Joke?

Introduction

A combined cycle power plant (CCPP) uses both steam and gas turbines which increases the efficiency up to 50 percent compared to a simple-cycle plant. Conventional CCPP applications use separate gas and steam turbines and route the waste heat from the gas turbine to the nearby steam turbine to generate extra power. In recent years, an alternative design for a CCPP has been developed with single-shaft rotors.

So, what are the drawbacks and advantages of single-shaft CCPP design? Is it both possible and (more importantly) a good idea to have a single-shaft CCPP? To answer that we need to look at how one would work.

The typical steam and gas turbine rotors for a conventional CCPP application (high power ~200MW) are presented in Figure 1. The first power train (gas turbine) consists of a generator, compressor, and gas turbine parts. The second power train (steam turbine) contains high-intermediate and low-pressure turbine rotors and another generator.

Separate Gas Turbine and Steam Turbine Rotors
Fig. 1 – Separate gas turbine and steam turbine rotors (AxSTREAM RotorDynamics models)

In a single-shaft application, only one generator would be driven by the gas-steam-turbine power train. An optimal variant would be to have the generator between the gas turbine and a steam turbine as shown in Figure 2.

 

Fig. 2 – Single shaft CCPP design configuration
Fig. 2 – Single shaft CCPP design configuration

Figure 2 shows it is conceptually possible.  This single-shaft configuration could provide a more compact design with fewer rotors and bearings which would reduce installation and maintenance costs. These are important considerations. Despite a more complicated and heavier generator design for a single-shaft CCPP, it is still cost-effective to have one generator instead of two because of the slightly higher efficiency of a single generator due to fewer mechanical connections and lower losses. Additional savings for single-shaft use come from having fewer auxiliary turbine components and only one generator connected to the grid.  The lower cost of the auxiliary electrical components could increase the overall reliability of the generator unit, which is also an important consideration for modern power plants.

A potential advantage of the multi-shaft CCPP design is the capability of independently operating the gas turbines. A multi-shaft CCPP design with several gas/steam turbines has more flexibility for maintenance intervals, unit repair, and upgrade options than a single-shaft CCPP.

Reliability of a Single-Shaft Combined Cycle Power Plant

When comparing single and multi-shaft combined cycle power plants, the reliability question becomes the most critical for consideration. At first sight, the multi-shaft CCPP designs with separated gas and steam turbines seem to be more reliable and are proven designs. However, most of the challenging structural and lifetime tasks can be resolved for single-shaft configuration as well.

One of the crucial considerations for reliability is thermal management: thermal stresses, rotor-to-casings differential expansions, etc. In single-shaft CCPP the problem is solved by integration of two thrust bearings (one for a steam turbine, another for a gas turbine) and a flexible coupling between the steam turbine and generator rotor parts, which accommodates the relative movement of the rotors. This flexible coupling allows us to consider lateral rotor dynamics for the rotor parts separately, however, we need to be very careful with torsional analysis, where the whole single-shaft train should be considered as well as all torsional excitation sources coming from the generator or steam/gas turbines. The higher torque required for a large generator is distributed between both ends – steam turbine from one side and a gas turbine from another, thus the required shaft strength safety factors can also be achieved.

The undeniable advantage of a single-shaft configuration is control simplicity of the machine. Operating flexibility may be considered as the key features of single-shaft combined-cycle systems. Ramping up speed in the case of the single-shaft design is the same for gas and steam turbines. Because of this, the steam turbine with its thick casings would be designed to start up faster in comparison to conventional applications to avoid premature LCF damage. This problem can be solved by optimizing the design and incorporating additional preheating systems.

Closing

So, as we see, it is not only technically feasible to have a single-shaft for combined-cycle power plant, but this configuration may also bring economic benefits. Let’s also note this comparatively new design requires very careful consideration of reliability questions as well as more reports from the field operating experience and statistics. Nevertheless, an idea which may have seemed like a silly design joke for conservative engineers has become a reality and a great reminder to us all that concepts which seem unorthodox can go on to be incredible inventions.

Regardless of the combined-cycle power plant configuration that you have (either single or multi-shaft), AxSTREAM RotorDynamics allows you to perform lateral and torsional analysis with the highest accuracy. To learn more about AxSTREAM RotorDynamics, request a trial.

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