Optimized use of an internal-combustion engine with a variable-speed synchronous generator and rectifier

When making an energy system a common DC bus can be an appropriate choice. On a DC bus many energy sources and storage systems can be easily coupled in parallel and an electronic DC/AC converter can feed the AC grid in optimum conditions. In such a system an internal-combustion engine (ICE) can be used as an energy source and the ICE does not have to run at a speed synchronized with the AC grid frequency. At low load it will be better to run the ICE at low speed. For building a cost-effective and energy-efficient system it is important to make use of off the shelf components with minor modifications and to know their energy efficiency. These efficiencies are difficult to calculate and hard to find. Therefore a test set up was built for accurate efficiency measurements. This paper includes a trade off and research results for different topologies. For the experiments a three phase synchronous generator with brushless excitation system was used, only the field excitation control system was replaced and modified. Different types of rectifiers are evaluated: a diode rectifier with DC voltage feed back control to the field excitation, a constant AC voltage regulator with thyristor rectifier and finally an active boost rectifier with IGBTs. The efficiency of a diesel engine was measured in the torque versus speed plane. The results show that with a variable load a generation system with common DC bus can increase system efficiency. The results are useful for evaluating new concepts for renewable energy systems, mobile generating sets, cogeneration systems, hybrid vehicle traction and variable-speed windmills.