1.3 Speed of rotation
In the normal case, steam turbines are directly coupled to the electrical generator, no gearbox being necessary. The speed of rotation is thus given by:
f = pn
where f = frequency of the electrical grid system
p = number of generator pole pairs
n = the rotational speed
Although there have been exceptions in the past, effectively the only two grid frequencies in use worldwide are 50 Hz and 60 Hz, and generators are usually either of two pole or four pole design. Large turbine-generators therefore fall into one of the following four categories:
Machine type Rotational speed (r/min)
System frequency 50 Hz 60 Hz
Two-pole (full-speed) 3000 (50 Hz) 3600 (60 Hz)
Four-pole (half-speed) 1500 (25 Hz) 1800 (30 Hz)
It is, however, not uncommon for smaller turbines — particularly those associated with special applications such as Combined Heat and Power, or Combined Cycle — to operate at a higher speed, but driving the generator through a speed-reducing gearbox. This results in a smaller and less costly turbine, although the gearbox may introduce losses and affect availability. Mechanically-driven turbines are required to operate at variable speed to cover the operational range of the driven machine. In the power generation field, the largest such turbines are used to drive the boiler feed pumps for large units (see Section 4.3 of this chapter). The maximum operational speed range may be as high as 8500 r/min. Variable-speed turbines have also been used to drive the gas circulators in some gas-cooled reactors, and small single-stage turbines may also be used to drive emergency feed pumps in nuclear stations.
The selection of the rotational speed of a turbine-generator depends on a number of factors, as follows:
- Unit size, initial steam conditions and availability of designs.
- Standardisation, affecting spares holdings.
- Relative size and weights, affecting cost and transport.
- Relative expectations of reliability, operational flexibility and ease of maintenance.
- Relative thermal efficiency and economic comparison of alternatives.
- Available LP turbine modules, with choice of exhaust area to suit output and exhaust pressure.
For superheat plant, a full-speed machine is normally preferred. However, when a plant is proposed of higher output than a current range which would have led to very high stresses or very high LP turbine exhaust loadings, this favours a half-speed machine. Half-speed tandem-compound machines are almost unknown for superheat plant, but cross-compound machines with a full-speed HP/IP line and a half-speed LP line have been used, particularly in 60 Hz systems where the exhaust loading is exacerbated.
For saturated-steam plant, the balance is much more even. The higher steam volumetric flow rate for a given output makes the exhaust loading more critical so, for 60 Hz systems, the rotational speed is almost universally 1800 r/min. In 50 Hz systems, this exhaust loading is less critical, so there are a number of full-speed machines. However, at the time the choice was made of standard reactor sizes, 1500 r/min machines were often chosen as stress levels on 3000 r/min machines would have been beyond the experience at that time. In some cases, including Sizewell В (the first PWR in the UK), two half-size, but full-speed, machines were selected, to take advantage of modules such as the LP turbine and the generator developed and proven for the full-speed superheat machines then in service. A comprehensive review of the choice of speed for saturated-steam machines in 50 Hz systems has been made by Harris and Kalderon . They indicate that 1500 r/min machines may be more economical for the lowest optimum exhaust pressures, i.e., in those countries with the lowest cooling water temperatures. By contrast, 3600 r/min machines could only become the preferred solution in 60 Hz systems where the optimum exhaust pressure is above 90 mbar; such high values are rarely the optimum, even in the warmest climates.