Investigation into open phase faults on transmission circuits.
Date
2017
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Abstract
Open phase faults are not commonly encountered on the South African power transmission
systems. However, when these faults do occur it results in major power system interruptions and
spurious tripping of healthy (circuits without faults) circuits to trip without giving any indication of
the type or location of the fault. This results in lengthy restoration times to find the open phase fault
without any fault detection devices. The intention of this study was to investigate open phase faults
and assess the impact thereof. An investigation was conducted on an open phase fault on the bussection
of the transmission high voltage yard at Koeberg Power Station. The investigation utilised
the Koeberg network configuration, power system data, and protection settings in order to simulate
the fault and validate the results with the simulation model. The simulation model was tested by
simulating short circuit faults with the current feeder circuit protection scheme characteristics and
settings. The results of the investigation confirmed that the current feeder protection schemes do
not take open phase fault detection into account. The back-up earth-fault protection which is
normally utilised to detect and trip for high resistance faults, did indeed detect and trip for open
phase faults where the unbalance currents summation was above the minimum setting threshold of
300 A although the fault clearing times was extremely long. However, this was not the case for all
instances. The feeder tripped due to zero sequence currents instead of negative sequence currents.
In addition the impact of open phase faults was investigated on the Koeberg generator circuit to
confirm that the generator would be protected against negative sequence currents and trip based on
the generator protection philosophy, the coordinated and configure generator protection settings.
The literature research comprised of the present feeder protection philosophies, a review of
currently used feeder protection schemes, available new feeder protection schemes, technologies
available or technologies that have the potential to detect an open phase fault. An evaluation of the
currently used protection schemes and new protection schemes available was conducted.
Considerations with respect to the protection scheme flexibility, adaptability with regards to coordination
and configuration of the protection scheme in conjunction with the feeder protection
philosophies, modifications and additions to the current feeder protection schemes were
considered.
Description
Masters Degree. University of KwaZulu-Natal Natal, Durban.