High voltage transmission system availability and system security are key
performance criteria for electricity utilities worldwide. System disturbances need to be
cleared quickly and accurately in order to minimise the impact of faults and to
facilitate speedy system restoration. In this context, the South African utility, Eskom
has maintained a process of refreshing protective relaying technology as older
equipment becomes obsolete and is no longer capable of meeting the utility’s
requirements.
The difficulties which a process of equipment renewal presents the organisation with
include the risk of incorrectly applying the newer technologies within the complex
electrical network. The application of new technology is affected by the complexities
of the newer technology with respect to the older, more familiar technologies. Some
of the difficulties can be addressed with revised commissioning procedures or the
use of modern test equipment. Enhanced relay algorithms and settings calculation
methodologies can however not be simplified.
Protective relay maloperations cannot always be completely avoided and when they
do occur, these must be investigated and addressed to prevent future recurrences.
The research covered by this dissertation focuses on a number of protective relay
maloperations on transmission lines using impedance protection algorithms. The
research undertaken identifies the previously unidentified causes of the
maloperations and describes a relay settings solution for improving the accuracy of
the protective relays.
The methodology that was followed in the research covers the following aspects:
• Identification and highlighting of some of the protection relay maloperations
that occurred during system faults,
• Review of the fundamental principles involved in system fault analysis, • Comprehensive study of the theory involved in the calculation of an overhead
line conductor self and mutual-inductance, as well as the calculation of the
positive, negative and zero sequence impedances of an overhead line,
• Brief evaluation of the effect of load impedance on relay measurements and
the impact on fault clearing operation,
• Analysis of the theoretical operation of various numerical relays during singlephase-
to-earth faults in radial and meshed (complex) network conditions,
• Mathematical calculations using typical Newton-Raphson methods to study
the impact of resistive single-phase-to-earth faults on the voltage and current
measurements at the relaying position with the exclusion of the capacitive
components between conductors and conductors and earth,
• Comparison and evaluation of mathematical calculations and system studies
using network simulation software which included all steady state network
parameters,
• Review and analysis of actual system faults that had been previously analysed
without definitive conclusion. The faults were re-analysed in an attempt to
correlate findings with the hypothesis of the research,
• Comparison of the performance of protective relay impedance charactersitics
using positive sequence domain versus loop domain analysis techniques.
This study concluded that significant benefits can be achieved by analysing system
faults and relay operation using loop quantities in primary impedance values as
opposed to positive sequence or apparent impedance quantities in secondary values.
The inherent differences between the positive or apparent impedance characteristics
of the relays are nullified when considered in the loop impedance domain, provided
that the relays reach settings were calculated correctly.
The study also showed that load current cannot be ignored when calculating settings
as it has significant impact on the actual impedance measured during fault
conditions. It is therefore crucial that when relays from different manufacturers are
being used to protect the same circuit that the differences between the relays and the
subsequent measurements are clearly understood and compensated for. Finally relay setting changes have been proposed for implementation based on the
findings of this research. The combination of the theory, network simulations and
secondary injections performed on the relays all correlate and therefore validate the
research. It is left for the utility and or users of these relays to evaluate the results of
this research and implement the necessary changes as applicable. / Thesis (MSc (Electrical Engineering))--North-West University, Potchefstroom Campus, 2012
| Identifer | oai:union.ndltd.org:NWUBOLOKA1/oai:dspace.nwu.ac.za:10394/8454 |
| Date | January 2012 |
| Creators | Troskie, Hercules Johannes |
| Publisher | North-West University |
| Source Sets | North-West University |
| Language | English |
| Detected Language | English |
| Type | Thesis |
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