CIRCUIT PROTECTION FOR DISTRIBUTION SYSTEM FEEDER LINE

CIRCUIT PROTECTION FOR DISTRIBUTION SYSTEM FEEDER LINE

How an electric utility protects its distribution system line?

An electric utility’s role in an electric power system does not only focus on the delivery of power to its customers. They also need to concentrate in the conservation of their assets in the network. Assets in the system like the transformers, wires, structures, etc., have an equivalent monetary value where the utility must always take care of.

The ability of the electric distributor to minimize the damage during failure at the fastest time possible is very important since this means property loss can also be in the minimum. Relaying fundamentals is an important tool that protection engineers must possess in order to carefully analyse their own system.

The most common form of distribution system feeder line protection which is also the most important is the overcurrent protection. Its basic application is mostly on substation protection and distribution feeder line protection.

Distribution lines are always subjected to different kinds of threats, the most common would be by faults and man-caused accidents. Faults can be classified into single-line to ground, 3-phase, double-line to ground, and line to line. Other events that need utility’s action through protection are the likes of open circuits, phase discontinuity, unbalance, high impedance, differential, undercurrent, over/under voltage, load shedding, etc.

Utilities monitors their feeder lines by means of carefully recording electrical parameters associated with their operation. Line amperes, voltage, frequency, impedances, power are some of these parameters that are considered usually using control automation and scada system. System configuration like delta, wye, delta-wye, open delta, open wye, laterals are also included in the analysis since different configuration means different treatments.

The required characteristics in effectively protecting the distribution system must possess the following considerations;

1. Sensitivity – applies to the ability of the relay to operate reliably under the actual condition that produces the least operating tendency.

2. Selectivity - Is the ability of the relay to differentiate between those conditions for which immediate action is required and those for which no action or a time-delayed operation is required. The relay must be able to recognize faults on their own protected equipment and ignore, in certain cases, all faults outside their protective area.

3. Speed - Is the ability of the relay to operate in the required time period. Speed is important in clearing a fault since it has a direct bearing on the damage done by the short-circuit current or system condition; thus, the ultimate goal of the protective equipment is to disconnect the faulty equipment as quickly as possible.

4. Reliability - This is the basic requirement of protective relaying, it must be reliable. Refers to the ability of the relay system to perform correctly. It denotes the certainty of correct operation from all extraneous causes. The proper application of protective relaying equipment involves the correct choice not only of relaying equipment but also of the associated apparatus.

5. Economic - Continuity of service and costly equipment failures is protected to operate.