Power supply to Rappenseehütte (Rappensee Alpine Hut) modernized: synchronous generator helps to avoid downstream electronics
Water turbine and an oversized generator from Johannes Huebner Giessen
- Objective: Remove the need for a diesel generator and reduce fuel consumption by combining a solar energy system and water power
- Implementation: Modernize a hydroelectric power plant with Pelton turbine and permanent magnet synchronous generator
- Result: Constant mains quality power supply powered by water and without an inverter.
Many mountain huts belonging to members of the German Alpine Association have turned to renewable energy to generate power autonomously and protect the pristine nature of their local environment from exhaust gases. Since its power supply was upgraded at the beginning of the year, the Rappenseehütte, the largest German Alpine Hut, has been generating its own power from a combination of water power and solar energy. Prior to that its power was generated by means of a diesel generator and a photovoltaic system. Electricity from the PV system was stored in batteries and the generating set was brought online when there was little sunshine; the generating set was replaced by a hydroelectric power plant when the new energy concept was implemented. Located some 2047 meters above sea level the Rappensee mountain lake is the source of the water drawn via a pipeline to power a Pelton turbine. The effective output is delivered to a coupled generator supplied by Johannes Huebner Giessen.
The specialist for encoders and drive technology designed the permanent magnet three-phase synchronous generator especially to meet the specific needs of this application. The DSG P 200.25-10 generator is oversized and, as a consequence, it is able to maintain the nominal voltage tolerances to within mains quality requirements and improve efficiency at rated load. "The concept behind the generator design meant there was no need for downstream electronics and direct drive is possible without an inverter", Maik Will, graduate engineer (University of Applied Sciences) from the department of energy and drive systems at Johannes Huebner Giessen.
Constant source of energy without an inverter
Originally, the power supply concept envisaged a smaller generator in conjunction with an inverter to feed the power generated by the hydroelectric power plant to the central off-grid inverter of the photovoltaic system. The oversized Huebner generator ensures it is still possible to generate electrical power autonomously even if the battery or the off-grid inverter of the photovoltaic system should fail. "The generator voltage would drop too much under load making autonomous power generation impossible with a smaller generator" explains Maik Will. Eliminating the need for an inverter provided other advantages: the added workload configuring generator and inverter parameters was no longer required, installation was easier and planning no longer needed to include the possibility of operational downtimes resulting from malfunctions and failures to which inverters are susceptible.
Turning special requests into reality
To maintain a constant voltage and frequency for the off-grid power supply it was necessary to control the generator speed at 600 rpm. At 600 rpm the generator frequency has the 50 Hz value required by the control frequency of the off-grid inverter. The generator of the hydroelectric power turbine was synchronized to this value and designed to produce precisely 3x 420 V AC in no-load operation and 3x 400 VAC under rated load. If the inverter of the photovoltaic system should malfunction external controls automatically regulate the generator to produce a constant 50 Hz – which is the only way to generate electricity autonomously. Further application-specific adaptations were undertaken to stabilize the speed in the face of rapid changes or surges: The bearing assembly was strengthened and a flywheel weighing some 130 kg mounted to the second shaft end to compensate for irregular rotational movements. Maik Will summarizes: "Taken together the individual adaptations make it possible for the generator to provide full output at all times and ensure the Rappensee Alpine Hut is able to rely on a constant source of power."