Dynamic modelling of saturable reactor for HVDC applications

Experimental development of dynamic models for the saturable reactor for HVDC applications was made by taking account of the effects of the eddy-current, the air gap, and the nonlinear magnetisation of the cores of the 3% grain-oriented silicon steels used. The experimentally developed models were confirmed by comparing them to measurements with regard to step current, rate of change of the reactor current, and the current reversal as a result of the oscillations between the storage capacitance and the equivalent inductance of the saturable reactor. The magnetising inductances and the rules for approaching saturation of the cores were determined from actual static magnetisation curves, corrected according to the core geometry and the size of the air gaps used. The linear relationships between the mean magnetic polarisations and the eddy-current resistance factors, which correspond to the eddy-current resistance for a reactor with a 1-turn exciting coil and a unit core cross-sectional area-to-magnetic path length ratio, were established using a circuit representing the capacitance discharging in typical HVDC applications. The work presented simplifies the optimisation process, which aims to minimise the reactor cost and losses, but without compromising the safe protection of the semiconductors for HVDC applications