ON MODELING OF AIR-CORE INDUCTORS FOR DC PROTECTION STUDIES

Abstract

Most presently considered HVDC circuit breakers make use of series air-core inductors to limit the rate-of-rise of the DC-side fault current. The inductors are also useful for identifying whether a fault is on the line to be protected or not, as they form a filter for wave-fronts originating from external faults. In DC-side fault studies, they are predominantly represented by their (lumped) inductance, thereby neglecting resonance effects due to stray capacitances, whose effect is within this context as yet not investigated. To fill this gap, this paper documents the resonance behavior of air-core inductors and the effect of including this behavior in DC-side fault studies. The paper furthermore compares results as obtained with EMT-type simulation software with those obtained with the Numerical Inverse Laplace Transform (NILT). Including the resonance effect is found to be more important for overhead line than in cable systems, and attention needs to be paid to the choice of simulation time step in EMT-type software.