Autonomous and accurately sized solution
Synchronous generator for hybrid systems
- Objective: Efficient energy conversion within a compact design solution by intelligently combining a diesel generator and solar energy to create an autonomous power supply for off-grid mobile phone transmitters
- Implementation: A 6 kW synchronous generator from Johannes Huebner Giessen combined with a diesel engine and individually adapted to suit the special characteristics of the battery
- Result: Solution reduces operating costs by achieving savings on diesel fuel and avoids high transport costs to remote destinations
A reliable power supply is not something that can be taken for granted in some regions of the world, such as in parts of Africa and Asia. Consequently, the business division Energy and Drive Systems of the leading specialist Johannes Hübner – Fabrik elektrischer Maschinen GmbH – concentrates on hybrid solutions that make it possible to provide an autonomous power supply including in remote off-grid areas. The efforts of the company based in Gießen, Germany, resulted in the development of a permanent magnet three-phase synchronous generator. Together with a diesel generator this can be deployed to power mobile phone transmitters, for example, and also combined with solar and wind energy as well as other sources of energy. The primary objective is to make the best-possible use of regenerative energies in combination with diesel engines while at the same time keeping the need for diesel fuel as low as possible.
When running within its optimum operating range of 1900–2100 rpm the diesel generator produces 6 kW of power. Powered by the diesel generator the three-phase synchronous generator from the DSG-P series charges a 48 V / 320 Ah lithium-ion battery via an electronic charge controller and cell protection circuitry. At the same time optimum start-up characteristics guarantee a continual power supply at low speeds.
And because the technical characteristics of the lithium-ion battery requires a constant input of 100 A direct current, the system is equipped with both an electronic charge controller and cell monitoring circuitry. The battery voltage increases gradually during the charging process. As a consequence, the synchronous generator progressively needs to generate more generator voltage. Successively increasing the generator voltage requires a faster speed or rather faster rotation, because both characteristic quantities are directly proportional to one another. A control system ensures that a constant 100 A direct current is fed to charge the battery.
At the same time the six-pole synchronous generator from Huebner Giessen acts as the link between the characteristics of the diesel engine and those of the battery. So it is absolutely essential to take into consideration the specifications to be able to guarantee both the best-possible speed range of the diesel engine and supply the battery with the correct voltage and current.
However, due to the different characteristics of individual batteries it is extremely difficult to calculate the right design concept to feed the energy from the generator directly to a battery. That is why the experts and Huebner Giessen recommend turning to empirical values. For example, these could be the measurement results gained from similar machines. And a trial run in conjunction with the original application is highly recommended.
Fundamental test results
Since the prototype of the synchronous generator was built in 2010, the company has supplied customers with types DSG P 112.17-10 and DSG P 112.17-6 generating sets – and test results are now available. For example, one customer has tested the interaction between the generator DSG P 112.17-6 and the original battery from his application. This saw the original 48 V battery being charged over a time frame of 1 hour and 15 minutes. In this test configuration the speed of the synchronous generator increased within the first 45 minutes of the warm-up phase from 2000 rpm to 2212 rpm. During this phase the controls maintained a constant current. The battery voltage increased slightly from 55 V to 58 V over the same period. The generator speed remained constant after the first 45 minutes elapsed. As a consequence, it was possible to avoid too fast a speed and to maintain the optimum operating point. Current and output fell slightly following the 45 minute charging process.
In the operating range between 2000-2100 rpm the generator charged the battery with an output of 5 kW. Exact winding as well as extremely precise sizing are necessary to avoid the synchronous generator overheating and failing to charge the battery to its full capacity. That means it is also necessary to accurately adapt the operating range of the synchronous generator to match the optimum speed range of the diesel engine.
Sizing the synchronous generator incorporating its own cooling system at the limit of its power range required compact and space saving packaging. The result: the entire system can be accommodated in a container measuring approx. 120 cm in width, 80 cm in depth and 200 cm in height. Furthermore, the thermal class F type DSG P 112.17-6 generator is designed for a maximum temperature T max. of 155 °C at a cooled temperature of 40 °C. The synchronous generator also fulfils protection class IP55 requirements; as a consequence it is protected against the ingress of dust deposits and hosing water jets from all directions and fully screened to prevent accidental contact.
In the operating mode S1 the generator is designed for continual operation under constant load. It boasts a nominal power output of 5 kW as well as a nominal speed of 2000 rpm. Further performance parameters specified by the manufacturer include a no-load voltage of 3 x 48 V AC / 65 V DC, nominal voltage of 3 x 43 V AC / 58 V DC and nominal current of 67 A AC / 86 A DC. An optional winding shield is available.
Other advantageous special features are: in addition to the low voltage winding the generator is also equipped with a large terminal box. That makes it possible to connect cable cross-sections designed for 100 A. Finally, as there are no components that require intensive maintenance such as slip rings and carbon brushes the synchronous generator is as good as maintenance-free. All that is necessary is to replace the ball bearings approximately every five years.
There is no rectifier integrated in the three-phase synchronous generator. However, Huebner Giessen does offer a three-phase bridge rectifier with an attached heat sink. This weighs 2.3 kg and can be installed separately at the customer's application switchboard.