4.3.4 Economic comparison of variable-speed motor (VSM) drive with induction motor plus fluid-coupling drive
At the presently preferred auxiliary electrical system voltage of 11 kV, the direct on-line starting of squirrel-cage induction motors for 50% duty feed pumps on large modern units (900 MW and over) would involve the development of quick-switching techniques not currently available.
The squirrel-cage motor and geared hydraulic coupling gives the minimum first cost, but the direct drive high speed synchronous motor (about 6000 r/min), fed from a converter to give variable speed, has a soft-start capability (only 1.5 times against 4.5 times full-load current) which solves the station electrical system problems.
In addition to the flow and head margins applied to the feed pump (and hence to the drive) that have already been described, electric motor drives are currently required to be able to give their full rated output at a minimum electrical system frequency of 49.5 Hz so, at the normal operating condition of 50 Hz, there is again a margin to be compensated for, in this case by VSM converter or by fluid coupling slip. Hence, the normal duty point for the feed pump drive has significantly less power output than its 100% rated output. Figure 1.74 compares the overall efficiencies of VSM and motors with fluid couplings, for decreasing pump outputs. Even at the normal duty point, the advantage of VSM drive can clearly be seen, and the difference in efficiency affects station lifetime costs signficantly. This difference at the duty point is mainly due to two factors:
- Fluid coupling slip — it is an inherent feature of fluid-drive designs that there is still some slip between the input impeller and output turbine, even at their rated output, which creates losses in the form of heat in the working fluid (removed
by oil coolers).
- Gearbox losses — VSMs have the same advantage as steam turbine drives in that they can drive the high speed (high power) pressure stage pump directly, so that the gearbox (reduction) only has to transmit the suction stage power, which is about 20% or less of the total feed pump power. For the same gearbox efficiency the actual power lost in the gearbox will therefore be up to four times as much for an induction motor/fluid coupling pump, where the slow-speed motor has to transmit over 80% of its power to the pressure stage pump through a step-up gearbox (usually incorporated in the fluid coupling).
Rapidly increasing slip losses in the fluid coupling lead to a much sharper decrease in drive efficiency than the VSM, for decreasing pump speed (output). Lifetime running costs will increase even further, relative to the VSM, if the fixed-speed motor and fluid-coupling feed pump has to be run at part-loads for any significant periods of time.