Underground high-pressure fluid-filled (HPFF) pipe-type cables are widely used for high-voltage transmission
networks in densely populated urban areas. The protection setting development of these HPFF pipe-type
cables is challenging, particularly because of its non-linear zero-sequence impedance. The zero-sequence
impedance is dependent on the permeability of the steel pipe, which, in turn, is dependent on the zero-
sequence current. As a result, the zero-sequence impedance becomes a non-linear function of fault current.
There are, in general, two methods to calculate the zero-sequence impedance of an HPFF pipe-type cable:
(1) empirical formula published by J. H. Neher in a 1960 paper, and (2) finite-element method considering
detailed electromechanical dynamics. It is worth noting that currently, many commonly used short circuit
programs (ASPEN and CAPE) and electromechanical transient (EMT) simulation programs (PSCAD and
Simulink) do not support simulating this non-linear current-dependence effect out of the box.
This paper introduces a novel method to accurately model the current-dependent HPFF pipe-type zero-
sequence impedance in the Real-Time Digital Simulator (RTDS). This method not only accounts for current-
dependent zero-sequence impedance, pipe cross-bounding, and system grounding but also enables
hardware-in-the-loop simulation with protective relays to validate developed protection settings.