Control methods for fault current limiting using hybrid HV DC Circuit Breakers

This paper studies the integration of fault current limiting mode of hybrid HVDC breakers (HHB) with dc grid protections and HHB internal controls. Fault current limiting is particularly beneficial in case of delayed protection operation in outer zones. This might happen for a range of reasons like: slow HVDC breakers, breakers (temporary) not ready, or (temporary) protection failure.

It is proposed to use two signals to communicate between HHB and relay: 1) Kinner denoting request for tripping because of fault in inner zone, and Kouter, request for fault current limiting because of protection system delays/problems. The paper discusses the options for enabling and disabling fault current limiting mode. HHB may enter current limiting mode either by grid order or based on its own measurements. It may exit current limiting mode either in open status or in closed status depending whether external fault has been cleared or not. The study shows that HHB has inherent property of exiting in closed status on fault clearing even if there is no external signal, which improves system reliability.

A 3-line, single node, 320kV DC system is used for PSCAD simulations. Simulation results illustrate several typical cases of fault current limiting with exiting in closed and in open status. It is concluded that fault current limiting is beneficial since current magnitudes are substantially reduced for delayed protection operation in outer zones.

An analytical model for energy dissipation in surge arresters is also presented for both normal breaker opening and fault current limiting mode. This model is used to analyse current limiting mode depending on the size of energy absorbers.