Aiming for flexibility and economy9 July 1998
Joachim Klein* discusses aspects of the the variable speed pump turbine that will be used at Goldisthal
Radial single-stage pump-turbines represent the most economical equipment for modern pumped storage schemes. With a wide range of specific speeds, this type of machine can be installed at sites with available geodetic heads from 60m up to 800m and more, and with unit capacities ranging from 50MW to 500MW. Using variable speed machines provides even greater operating flexibility and improved plant efficiency by covering a wider operating range.
The new Goldisthal pumped storage plant in Thüringen, Germany will be equipped with four radial single-stage pump-turbines. Each of the two penstocks will be connected to one machine operating at a constant synchronous speed of 333.33rpm and one machine operating at variable speed. The contract for the pump-turbines, including governors, was awarded in October 1997 to a consortium known as Goldisthal Hydro. Voith Hydro’s share in the scope of supply is about 45%, and includes hydraulic development and model testing. Commissioning of the units is scheduled for 2002.
For both the synchronous and the variable speed turbines the main machine components and the setting of the machine are identical. There are various reasons for this, which are largely based on reducing cost. With this configuration, for example, civil construction costs are reduced, and spare parts can be interchanged. Even runners will be interchangeable although the runner blade profiles will be optimised individually for the synchronous machines and the variable speed machines.
Developing the variable speed turbines has mainly been driven by the task of regulating power in the pumping mode. However, variable speed also offers the additional advantage of improving part load efficiency and operating behaviour in the turbine mode.
Besides the challenges on the electrical side, the major target for hydraulic development is to ‘widen’ the cavitation characteristic in pumping mode from a conventional pump-turbine runner design to a customised design for variable speed application and to achieve a stable head-Q-curve within this range. Depending on the desired range for power variation, the setting of the machine has to be lowered. A power variation range of about 30% (±15%) at each individual head is quite reasonable and does not affect the setting drastically.
A wide head-range plant (maximum to minimum delivery head) provides less flexibility for power variation than a limited head-range plant. This is because of the instability phenomenon in pumping characteristics and the suction side leading edge cavitation at high delivery heads, as well as the pressure side leading edge cavitation at low delivery heads. In such a case, at high head power variation means mainly power increase while at low head it means power decrease.
In turbine mode a machine with constant synchronous speed is usually operating far outside the peak efficiency range because of the application requirements driven by the pumping mode. The red lines in the diagram show the head range in turbine mode with synchronous speed. The pump characteristic marked by the black line shows sufficient safety margin to the instability region; even the head losses, which will increase the delivery head in pumping mode, are not reflected by these red lines. Variable speed makes it possible to optimise the operating behaviour by reducing the speed.
Using variable speeds improves the efficiency, especially at turbine part load where the gain more than off-sets the additional losses of the frequency converter of about 2%. It also reduces the cavitation and pressure fluctuation behaviour. The resulting possibilities for part-load operation are of great benefit to the owner and substantially improve the flexibility of such units.
Projects using low heads are often set up to exploit special topographical conditions in fast growing industrialised areas, and may require special cost-reduction efforts to optimise the economical benefits. In such plants relative head variation is often much larger than for medium or high head plants, and variable speed machines may offer advantages in these applications because they can cover a wider head range. Civil construction cost can be decreased by reducing headwater and tailwater pond surface area, while still having the same active volume. This results in larger level fluctuations. In such a case, a variable speed machine would offer little opportunity for power regulation in the pumping mode because the head and tailwater level already fluctuate, but does have reasonably high machine settings and no risk of cavitation damage.
The pure hydraulic efficiency of pump turbines has undergone continued development, and it has reached a level today where further big improvements cannot be expected.
Nevertheless, development in the field of equipment for pump-turbines is still brisk. Now the aim is to enhance the operating flexibility and to improve the overall economics of pumped storage plants.
From now on, development will be driven by the requirements of the grid: to operate pump-turbines more and more in pump mode to control the frequency, and to use pump-turbines as standby capacity, or as emergency power supply if one of the thermal or nuclear power plants trips off the system or has to be shut down. A variable speed machine allows these tasks to be performed successfully.