DC-presaturated fault current limiter for high voltage direct current transmission systems

The authors present a 500 kV real-dimension DC-presaturated fault current limiter (PFCL) with a steady-state rated current of 2 kA for limiting large fault current of 8 kA with high rate of rise. These characteristics of the fault current can be a threat to high voltage direct current (HVDC) transmission systems, and hence PFCL design and performance improvement are investigated through three-dimensional, time-domain, magnetic-field and electric-circuit coupled model using finite element simulation of COMSOL Multiphysics package. The nonlinear magnetic characteristics of the soft magnet ensure variable inductance depending on demagnetisation magnetomotive force generated by the line current. This DC-biased PFCL can replace the traditional smoothing reactor during the normal operation of the system by controlling the level of presaturation. In addition, PFCL is a self-triggered device and therefore it can automatically limit the fault current and reduce its value below the interruption rating of the used HVDC circuit breakers (CBs). Moreover, the rate of rise of the fault current can be controlled to ensure the compatibility with the available type of HVDC CBs. The dynamic performance of PFCL is investigated during fault condition through the fault current clipping ratio and rate of rise of the fault current, and during steady-state operation through the voltage drop across PFCL and its power losses. It is found that the proposed PFCL presents adequate capability in limiting large fault currents with extremely low values of voltage drop and power losses during the steady-state condition. However, the switching transient overvoltages that appear at terminals of PFCL coils during the fault duration have been successfully suppressed by carefully selected ratings of zinc-oxide surge arresters.