Synchrongenerator for a Historic Water Plant

Targets of the Project

The aim of the project was to improve the drive and control technology of a listed hydropower plant and to achieve a significant increase in the efficiency of energy generation.

The "Steinerne Renne" hydropower plant is located in the Hasserode district of Wernigerode below the eponymous railway station on the Harzquerbahn. This technical monument from 1899 has been operated by Stadtwerke Wernigerode since 2002.

The power plant was once built to supply power to the gravel and granite plant at the time. This was opposite, at the location of today's company WERBAT. The "Steinerne Renne" hydropower plant supplied the lighting for the quarry, the factory railway and the quarry itself with electrical energy. In the left extension there was a steam engine for times when the water was weak. Today you can visit an interesting exhibition of historical household electrical appliances.

At a weir below the "Steinerne Renne" inn, part of the river water from the Steinerne Renne is branched off into a covered moat. It flows around 1,700 meters through the Harz forest to the computing house, where coarse dirt such as branches and leaves are retained. The water flows to the turbines of the hydropower plant via an underground cast iron pressure line with a height difference of around 160 m.

The hydropower plant has two machine sets with Pelton turbines. The older one has an output of approx. 90 kilowatts (generator output) and dates from the year 1899. In the 1950s, a second turbine with an output of 265 kilowatts was installed. Both turbines drive asynchronous generators.

According to Stadtwerke Wernigerode, the electrical work generated is up to one million kilowatt hours per year. Around 900 m³ / h of water are required for the maximum output - an amount that is naturally not continuously available.

The conversion of the historic 240kW asynchronous generator to a PM generator is described below. The outer historical machine image was left unchanged with a clear improvement in the energy yield.

State before modernization of the generator

Large machine set before modernization (green paint)

Old generator: MWS, asynchronous

• Year of construction: 1954, last general repairs 1985 (winding) and 1995 (bearing) 380V / 525A / 240kW (shaft power as motor)
• Turbine: MMW (VEB Maschinen- und Mühlenbau Wittenberg) Year of construction: 1954 Power: 265 kW at 154 m water column and 214 ltr / sec absorption volume

Small machine set (black paint)

• Generator: AEG, asynchronous Year of construction: 1899 380V / 171A / 90kW
• Turbine: manufacturer unknown Year of construction: 1899 Power: approx. 75 kW at 154 m water column and approx. 90 ltr / sec swallowing volume

The two machine sets are controlled depending on the water level behind the rake. The operating and fault data of the power plant are transferred to the control system of the SWW via a secure VPN connection. The graphic still shows the old visualization 20 years ago.

One of the pictures still shows the old visualization 20 years ago.

Synchronous generator to increase the yield

The conversion of the existing asynchronous generator to the permanently excited synchronous generator was carried out by the company Krebs & Aulich. The rotor was exchanged for a permanently excited synchronous rotor with a damper cage. The stand was re-tinned and received a new winding. The plain bearings have been renewed. Vibration sensors were arranged on the bearing housings. The engine housing was used again during the renovation, the old appearance remained unchanged. One picture shows the permanently excited rotor before installation.
 

Efficiency of the old generator

The old generator was an asynchronous motor of historical design, which was mainly operated at part load. The generator delivered approximately 225 kW of power to the network when the Pelton nozzles were fully open. The Pelton turbine generates 265 kW of turbine power on the shaft, see nameplate.

The efficiency of the old generator was calculated as:
Eta = 225/265 = 0.849

The generator had an efficiency of around 85% at nominal power.
 

Efficiency of the old generator in the partial load range

The following diagram shows the course of the efficiency versus the power that is fed into the network. The red line is based on the test report of the new generator from Krebs & Aulich.
As expected, the blue characteristic of the old generator deteriorated sharply in the partial load range.

Efficiency of the new synchronous generator

The red curve in the  diagram shows the course of the efficiency.

The efficiency remains almost constant across the entire performance range. It was based on the data from the test protocol dated 02.12.2016 from the company Krebs & Aulich.

The conversion to a permanently excited synchronous generator resulted in a significantly improved efficiency. This can be seen from the fact that the same opening of the Pelton nozzles generates more electrical power. The improvements achieved are shown in a picture below.

Explanation of the technical causes

The improved efficiency can also be explained by the fact that with permanent excitation no reactive current is drawn from the network for field construction. In the old 240 kW generator, this reactive current was approx. 40% of the nominal current and caused approx. 5.2 kW heat loss at the ohmic resistance of the stator winding (0.2 ohm) at a synchronous speed of 600 revolutions per minute. (So ​​without real power being delivered by the generator). The reactive power compensation (and the associated heat losses) can now be omitted, which results in further savings. It is also assumed from experience that the large machine set is mainly operated under partial load, in a range between 110 and 135 kW. With predominantly part-load operation of the generator from 50% to 60%, corresponding to 110 to 135 kW feed-in power, the efficiency improves from old 0.81 to 0.83 to 0.97. There was an improvement in the generator efficiency (and thus the energy yield) of 14% to 16%.

Power factor and synchronization

The following diagram shows the measured power factor of the new generator as a function of the generated power. The curve reaches 0.8 from just 10% of the nominal output. The synchronization takes place via a Woodward controller without noticeable transients. As a reminder: With the old generator there were clear transient currents.