Model and analysis of a broken conductor (source isolated) earth fault on radial 11kV distribution feeders

Geary, A. (2013) Model and analysis of a broken conductor (source isolated) earth fault on radial 11kV distribution feeders. [USQ Project]

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Abstract

Protection schemes of electricity distribution networks are designed to limit the damage to the network in the event of a fault, and to provide some security and safety to the network. This thesis examines the electrical characteristics of the Source Isolated Earth
(SIE) Fault.

The Source Isolated Earth Fault is a type of high impedance earth fault that can occur on overhead electricity networks. SIE faults are caused by a broken overhead conductor falling to ground on the load side of the span with the source end of the span isolated from the ground.

A simplified model for calculation of the earth fault levels in SIE faults was developed by circuit reduction of the fault schematic. The SIE fault was reduced to the equivalent of a Phase to Phase to Ground fault.

Results obtained from the simplified model were compared to two peer reviewed models for SIE fault calculations in order to validate the simplified method. The comparison was undertaken in two stages, by first varying one factor at a time to determine the most significant factors and then by carrying out designed experiments on the significant factors and analysing the interactions between these factors.

The results of the one factor at a time analysis identi�ed which of the factors had the largest effect on the earth fault current. The most significant factor in determining the earth fault level in SIE faults is the pre-fault load beyond the fault location. This knowledge can be used to identify areas where SIE fault levels may be low.

Computational effciency of the three models was compared using MATLAB profiling.
The simplified model was found to be significantly faster than the other methods.

Confidence in the theory was bolstered by the calculation of fault levels for a case study. The results were compared between all three models and data captured during
an actual SIE fault event.

A process was developed that allowed existing 11 kV network feeder models to be analysed using the SIE fault models. Sections of feeder where SIE fault levels may be
below conventional Sensitive Earth Fault (SEF) protection pickup levels were identifed. Attempts to optimise the feeder analysis led to methods of reducing the number of network nodes to be tested to find the limit of the protection zone.

The extreme case analysis led to the discovery of the circuit conditions that must exist for these types of faults to be undetectable by conventional SEF protection schemes.It was discovered that the maximum possible SIE fault current can be easily estimated by applying a factor to the pre-fault load downstream of the fault location.


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Item Type: USQ Project
Item Status: Live Archive
Faculty / Department / School: Current - Faculty of Health, Engineering and Sciences - School of Mechanical and Electrical Engineering
Supervisors: Ahfock, Tony
Date Deposited: 24 Jun 2014 00:10
Last Modified: 24 Jun 2014 00:10
Uncontrolled Keywords: model and analysis, broken conductor earth fault; radial 11kv; distribution feeders
Fields of Research : 09 Engineering > 0906 Electrical and Electronic Engineering > 090607 Power and Energy Systems Engineering (excl. Renewable Power)
URI: http://eprints.usq.edu.au/id/eprint/24630

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