Design of a flywheel energy storage system for high current pulsating loads

In today's industrial production (e.g. cars), resistance spot welding with dynamic current control is essential. Besides the ability to control the welding current, the peak power demand at the point of common coupling of these welding systems should be as low and steady as possible, making an energy storage mandatory. Previous investigations showed, that a flywheel energy storage offers significant advantages in terms of cycle stability, volume and efficiency compared to capacitor-based solutions. This contribution describes the topology of a complete welding system including a flywheel storage and its design process. Unlike state-of-the-art systems, the proposed topology incorporates a storage and the ability of dynamic current control. This paper addresses the specific challenges of this application. On the one hand, system dynamics have to be superior to typical flywheel energy storages; on the other hand, the topology has to be compact and efficient. Finally, robustness and simplicity of the system are of great importance to be industry-tailored. Because of the systems complexity, computer based modeling is used to simulate system performance and to optimize relevant parameters such as flywheel size and speed, motor parameters, transformers leakage-inductance, capacity of DC-link capacitors, etc. with respect to the goals stated before. Using different computer-based modeling tools, a simulation of the overall system shows that the proposed topology is practical and meets the specifications regarding size, efficiency and system dynamics. Based on these results, suitable components are selected. A prototype of the last converter stage is built which confirms expectations regarding leg current symmetry, dynamics and switching performance.