Characteristics of transformerless MMC-based HVDC systems and the principle of decentralized controlled potential definition

Inhalt:
Up till now converters of HVDC systems are connected to the electricity grid by transformers. This limit short-circuits currents, mitigates zero-sequence components caused by the switching of the converter valves and offers a potential separation. Especially with view on offshore applications, transformerless concepts were considered early to reduce costs and weight. Modern self-commutated modular multilevel converters allow now the realization of such concepts effectively. This paper analyses the transformerless construction of a HVDC system with special regard to the binding of electric potentials. It demonstrates that under fault conditions effective current limitation and mastering of overvoltages is viable. Compared to transformer-based solutions, many advantages result especially if DC cables have to be protected from overvoltages.