Validity of circuit‐based models for air‐cored helical windings at high‐frequency regimes

Helical coils in different forms find applications in various areas of electrical engineering. While power engineers use close‐pitched coils in inductors, reactors and transformer windings, communication engineers use large‐pitched helical coils as antennas. In classical power engineering, coils are represented as lumped inductors for power frequency applications and switch‐over to distributed circuit models while dealing with switching and lightning transients. Even for very fast rising excitations such as very fast transient overvoltages, chopped lightning waves and propagation of partial discharge pulses, multi‐conductor transmission‐line‐based models have been employed, which appears to be an over‐simplification. It would be very useful if an upper‐frequency limit for the circuit‐based models is quantified, which requires an extensive solution of the electromagnetic fields. After realising the inherent late‐time instability of marching‐on‐in‐time methods, a relatively new method called the marching‐on‐in‐degree‐based scheme is adopted for the solution of the associated electric field integral equation. A deeper insight is first obtained by analysing the helical antennas. Subsequently, based on the extensive simulation results for the single‐layer helical coils, an empirical relation relating the upper‐frequency bound for the circuit‐based modelling has been deduced.