Analysis of protection coordination in MV distribution system of Sri Lanka

Introduction to MV distribution system

The main purpose of an electrical utility in a country is to supply an un-interrupted power to the end customers. Hence, transmission and distribution network ensure the transferring of the generated electrical power to end users. Power transmission is done in High Voltage (HV) while power distribution is done in Medium Voltage (MV) and Low Voltage (LV) levels.

In various countries, these HV, MV and LV levels are defined in various limits but these are approximately same. In Sri Lanka, MV level is defined as 33 kV to 11 kV.

Overhead MV distribution system is subjected to various electrical faults. These faults are mainly categorized in to transient (temporary) faults and permanent faults, depending on the nature of the fault.

Transient faults are faults which do not damage insulation permanently while allowing the circuit to safely re-energize after a short period. More than 80% of faults are transient and usually these faults occur when phase conductors are electrically in contact with each other or ground momentary owing to lightning strikes, insulator flashovers, high winds, trees, birds or other animals and so on.

On the other hand, permanent faults cause permanent damage to the insulation while damaging equipments which have to be repaired before drestoration / re-energize.

The protective device co-ordination is the process of determining most appropriate timing of power system interruption during abnormal conditions in the power system. Hence, most appropriate protection scheme is required for the power system mainly to minimize the fault duration and to minimize the number of customers affected.

Identification of the Problem

There are about 60 numbers of Grid Substations (GSSs) in power system network in Sri Lanka to step down the transmitted electrical power from HV to MV.

Hence, proper selection of protective devices with proper protection coordination is required to maintain the power system reliability as well as to avoid damages to very costly equipment such as power transformers.

The correct operation of protective relays and auto-reclosers (ARs) during transient faults will minimize permanent trippings of the distribution network. Therefore, co-ordination of protective relays at GSS and downstream ARs are very much essential to maintain the high reliability in MV distribution network.

Objective of the Research

The objective of the study is to determine the optimum protection co-ordination of MV system of Sri Lankan power system by analyzing the existing protection settings and behavior of MV network. Protection settings of relays at GSS and downstream ARs of selected GSS will be analyzed with the standards to determine the optimum co-ordination in MV level.

This study will present the optimum protection settings for the MV system of Sri Lankan power system.