This was the first of the two variable-speed units installed in the plant. It is also the first unit of this type, starting operation in Europe. Up to date, only few Japanese pumped-storage units provide the feature of variable-speed.

The pump-turbine was supplied by Voith Siemens Hydro Power Generation in a consortium and is equipped with an asynchronous motor-generator. The unit’s rated speed of 333 rpm over an operating range of -10% to +4% is automatically controlled by the turbine governor. Both, electricity generation during turbine mode as well as operation during pumping mode, can be adapted to individual load conditions in the grid. Pumping mode within a range of 170 to 300 MW, for example, can be regulated with very little changes in speed.

An intensive phase of commissioning works, started after first grid synchronization on December 17, saw a series of testing and optimization activities for stationary operation during pump and turbine mode as well as for transient operation modes. Above this, parallel operation of one variable-speed and one fixed-speed unit, both connected to one common draft tube, was tested. Operation of the variable-speed pump-turbine after 30 days of trial operation proved extremely smooth and partially even exceeded expectations.

By means of the variable-speed control, power input during pumping can be controlled continuously. Also, efficiency especially in turbine part load ranges is improved remarkably. Start-up of the turbine is only considerably faster and provides much smoother operation for the complete unit compared to fixed-speed units. Variable-speed is, therefore, an advantage for overall unit performance, and also for flexibility of operation within grid regulation.

Voith Siemens Hydro is convinced that this will be of increasing importance to operators of pumped storage plants within public grids.

The Goldisthal pumped-storage plant is expected to reduce peak load deficits and to provide a minute reserve for losses of larger power station blocks in the Vattenfall grid. These features make a significant contribution to the safe operation of the grid. The maximum output of each pump-turbine at a maximum head is 325 Megawatt of clean, renewable energy.

The pump turbines, each rated at 265 Megawatt, were supplied in a consortium with VA Tech Hydro and CKD Blansko Engineering, in which Voith Siemens Hydro Power Generation was primarily responsible for the hydraulic design and the entire control system, including the technology of variable-speed turbine control in two of four machines.

Voith Siemens Hydro Power Generation is a Group Division of Voith and - with a workforce of around 2,200 employees and an order intake of around 600 Mio. € per business year - Voith Siemens Hydro Power Generation belongs to the worldwide leading companies for hydro power equipment.

Voith sets standards worldwide for papermaking technology, power transmission, energy technology, and industrial services. Voith was founded on January 1st 1867. With annual sales of approximately. € 3.1 billion, 24,000 employees and 180 locations worldwide, Voith is one of the large family-owned companies in Europe.