Independent control of real and reactive power of superconductive magnetic energy storage systems

The real power and reactive power (P-Q) characteristic of a superconductive magnetic energy storage (SMES) system is very important to its applications in power systems. The P-Q characteristic of a SMES system solely depends on the power conditioning circuit consisting of AC/DC reversible converters. This dissertation studies some AD/DC current-sourced converters commonly used in the motor drive industry and the high voltage direct current (HVDC) transmission.; After studying AC/DC converters and P-Q control algorithms, this dissertation proposes a new method using parallel hybrid GTO/SCR converters to independently control the real and reactive power of a SMES unit. The GTO converter and the SCR converter of a hybrid converter are controlled with leading and lagging firing angles, respectively. The dc current distribution between the GTO converter and SCR converter is controlled by a proportional-plus-integral (P-I) controller to meet the desired real and reactive power input or output of the SMES unit. With hybrid GTO/SCR converters as the power conditioning circuit for the SMES unit, four quadrant P-Q operation can be realized.; A GTO converter with a special energy recovery circuit was designed and built for the hybrid GTO/SCR converter. The energy recovery circuit recovers the energy trapped in the ac line inductances during commutations between the GTOs, and hence increases the efficiency of the GTO converter.; The proposed independent P-Q control with hybrid GTO/SCR converters was tested with the UW-Madison laboratory model SMES unit. The experimental results show that the real and reactive power of the model SMES unit are independently controlled.; This dissertation also analyzes the losses and the round-trip energy conversion efficiency of the power conditioning circuit of the UW-Madison model SMES unit. The circuit has an efficiency over 70% for a simulated daily power conversion cycle. The efficiency of a practical full sized SMES unit is also studied. For a 5000 MWh, 1000 MW SMES unit, the analysis shows that an efficiency over 96% is achievable.