Gralla Run-of-River Power Plant

VERBUND’s Gralla power plant is a run-of-river power plant on the Mur, situated in the municipality of Gralla in Styria.

Technical Description 

The Gralla power plant was constructed from 1962 to 1964. After more than 40 years of operation, the turbines and generators in the power plant were dismounted and modernised. By doing this, the effectiveness of the power plant's machinery was improved and the electricity generation increased by around 5 percent. With the modernised turbines and generators, the Gralla power plant on the Mur River can cover the annual electricity needs of around 21,000 households. Two vertical Kaplan turbines with DC coupled three phase current synchronous generators generate an average yearly output of around 71 GWh. The Gralla power plant is also equipped with a fish ladder. 

Technical data:

Owner: VERBUND Hydro Power GmbH
Operating company: VERBUND Hydro Power GmbH
Beginning of operation: 1964/2012
Type:  Run-of-river
Country, State: Austria, Styria
Wates: Mur
Capacity: 15,1 MW
Annual generation: 71.884 MWh
Turbines: Kaplan (2)
Fish bypass: yes

Machine Renovation after more than 40 Years in Operation 

Initial situation

Electrical wear

  • Deteriorated condition of the generator stator windings (insulation) and pole coils 
    Mechanical wear
  • Cast steel impeller blades, in some cases with anti-rust armour, cracks in the transition area between disk and blade 
  • Rotor discs: Low remaining wall strength and rusted Niro plating 

Cooling system

  • Partial obstruction of the cooling water pipers due to manganese deposits (increased pumping demands) 
  • Highly contaminated coolers and therefore poor heat transfer 

Project goals

As a result of the major overhaul and production of new electromechanical components of the two machine sets, the efficiency level of the plant was increased and the annual output increased by around 5 %. The renovated machines increase the performance reliability and reduce servicing time and costs. Not least thanks to the installation of a low-maintenance, closed cooling system. 

Renovation measures (2009-2012)

  • Dismantling of the two machine sets (approx. 88 to rotating mass) 
  • Production of new turbine impeller blades with optimised contours (verification with model test). 
  • Renewal of the turbine rotor discs including connection to the structure 
  • Supply of new generator stators and coolers. 
  • Replacement of pole coils and generator ventilators 
  • Renovation of the braking system 
  • Inspection and repair of portions remaining in use 
  • Complete renewal of corrosion protection 
  • Installation of a closed cooling system with a pipe coil cooler in the turbine inlets
Machine renovation at the Gralla run-of-river power plant