MV Feeder Earth-fault Protection

Introduction

This chapter contains a description of the earth-fault protection of MV feeders. A feeder may consist of:

1. An overhead line with bare conductors,
2. An overhead line with covered conductors,
3. An aerial cable,
4. An underground cable or
5. A combination of such line and cable sections.

The feeder is a part of a distribution system that may be:

1. Effectively earthed or
2. Non-effectively earthed.

The relays may have independent time or dependent time characteristics and may or may not be combined with a directional element. Neutral displacement voltage and neutral point current are the most common polarizing quantity for directional elements. Backup protection is often provided by a neutral displacement overvoltage relays.

Faults and Abnormal Conditions

Various forms of earth-faults are common on distribution systems. The most common fault is the single phase-to-earth fault but other types of earth fault also occur. They are:

• Phase-to-phase-to-earth faults
• Cross-country faults
• Back-fed earth faults

A long discussion on phase-to-phase-to-earth faults is not necessary. This fault may be detected by either the short-circuit protection or the earth-fault protection or by both.