Design of Protection Coordination of a Distribution Network in ETAP

Objectives

This tutorial teaches the design of a protection coordination scheme for a power distribution network using ETAP. To test the protection and coordination settings of this system by generating TCCs. ETAP STAR TCC Features are also demonstrated in it.

Introduction

Radial network

The Radial distribution system is the cheapest to build and is widely used in sparsely populated areas. A radial system has only one power source for a group of customers. A power failure, short-circuit, or a downed power line would interrupt power in the entire line which must be fixed before power can be restored.

Protection

Protective devices are used in distribution systems to minimize the duration of a fault and isolate the affected areas of a fault. Commonly used protective devices in distribution systems are fuses, reclosers, and circuit breakers, which are usually controlled by relays. Their function will be discussed separately

Fuses

The fuse is a reliable overcurrent protective device, primarily used as a circuit protection device for overcurrents, overloads and short-circuits. A “fusible” link or links encapsulated in a tube and connected to contact terminals comprise the fundamental elements of the basic fuse. Electrical resistance of the link is so low that it simply acts as a conductor. However, when destructive currents occur, the link very quickly melts and opens the circuit to protect conductors and other circuit components and loads. Fuse characteristics are stable. A fuse does not require periodic maintenance or testing. The fuse has three unique performance characteristics

1. They are safe. The modern fuse has an extremely “high interrupting” rating-can withstand very high fault currents without rupturing.
2. Properly applied, a fuse will prevent “blackouts.” Only a fuse nearest a fault opens without upstream fuses (feeders or mains) being affected, thus, they provide “selective coordination.” (These terms are precisely defined in subsequent pages.)
3. A fuse provides optimum component protection by keeping fault currents to a low value… They are said to be “current limiting.”

Circuit breakers

Electrical circuit breaker is a switching device which can be operated manually and automatically for controlling and protection of electrical power system respectively. As the modern power system deals with huge currents, the special attention should be given during designing of circuit breaker for safe interruption of arc produced during the operation of circuit breaker. This was the basic definition of circuit breaker.

Reclosers

In electric power distribution, a recloser, or autorecloser, is a circuit breaker equipped with a mechanism that can automatically close the breaker after it has been opened due to a fault. Reclosers are used on overhead distribution systems to detect and interrupt momentary faults.

Coordination

• Fuses protecting reclosers
• Fuses protecting fuses
• Reclosers protecting fuses
• Reclosers protecting reclosers
• Coordination between fuses and CBs

Setup

Draw following one-line diagram in etap

Procedure

Go to Star – Protection and Coordination Study toolbox and Create TCC (Time current characteristics curve) of CB4, CB5 and Fuse1.

Put a 3 phase fault on Bus 3. Run short circuit analysis.

View Sequence of Operation of tripping devices and View tripping times of protective devices.

View protection zones by extending to source

Generate Short Circuit, Devise Settings, SQOP reports.

Conclusion

• TCC curves for fuse and circuit breakers were observed
• Tripping sequence and tripping time of devices were observed. It can be seen the coordination is done in such a way that circuit breaker 4 has to operate first followed by fuse and finally relay.
• Zones of operation for these devices were also observed.