Increased Operating Flexibility for Superconducting Magnetic Energy Storage Systems Through the Use of Self-Commutation

In a superconducting magnetic energy storage (SMES) system, it is convenient to use a twelve pulse converter as the electrical interface between the high voltage transmission system and the superconducting coil. A demonstration 30 MJ (8.333 kWh) SMES unit with a 10 MW converter installed and commissioned in the USA in 1983 is a good example of a practical solution. Line-commutation, however, limits the phase delay angle of the bridges to α_max = 140 °; this limits the operating range in the inverter mode. The paper presents a technique for the construction of a circular PQ diagram to analyze power flow for a converter and SMES. The method indicates that if self-commutation is used, any operating state in the PQ plane may be attained. This flexibility in operating state implies that all operating conditions, from unity power factor, to zero power factor lagging, are attainable in the rectifier as well as the inverter modes. The implications of this flexibility, including harmonic impact, are discussed.