Question

PROBLEM 5 a Discuss the consequences of short circuit faults on electric power systems [5 Points Protective schemes are routinely used for electric power transformers. Name at least three different types of transformer protective schemes (by function) and explain briefly their principles of operation. [5 Points] Consider the system shown in the single-line diagram of Figure (4 per unit to the same base. Assume that the voltage at both sources is 1 p.u .) All reactances are shown in ind the fault current due to a bolted- three-phase short circuit in the middle of line B. [5 Points] d Find the voltages at buses 4 and 5 under the fault conditions of part c above [10 Points] 3 0.2 p.u ( 1 0.2 p.u 0.2. Pu. Llne A 0.1 p.u 4 0.2. P.u. Line B 0.1 P.u. - circuit breaker a.c. generator 38 0.1 p.u transformer 0.2 p.u Figure (4) Single-line diagram for Problem (5)

Answer 1

Solutio for Question A

Short Circuit Occur as a result of breakdown in the insulation of current Carrying Conductors relative to earth or insulation between phases

Different types of short circuit fault occur in the power systam are

1 Single phase to Ground

2 Phase to phase

3 Two phase to ground

4 phase to phase and third phase to ground

5 All Three phases to ground

6 All 3 phases short circuited

Faults 1 to 4 are classified as unsymmetrical faults

Faults 5 to 6 are classified as symmetrical faults

Consequence of Short circuit Faults

1 Heavy currents due to short circuits cause excessive heating which results in fire or explosion

2 Stability of power system may be adversely affected even leading to shutdown of entire plant

3 Damage will be caused to other apparatus in the system due to overheating

4 Reduction in voltage will be so large that relays having high pressure coils tend to fails

5 Interruption in power supply to consumers when power circuits or generating units are switched off

6 Considerable reduction in voltage on healthy feeders connected to the system having faults which lead to abnormally high current drawn by the motors

B Transformer Protctive Schemes

1 Buchloz Protection

When Internal faults occur in an oil immersed power transformer gas is usually generated slowly for an incipient fault and violently for heavy faults.most short circuit caused either by impulse breakdown between adjacent turns at the end turns of the winding or as a poor initial point contact which will immediately heat to arcing temperature.The heat generated by large local currents causes the transformer oil to decompose and generate gases which may be used in detection of winding faults.The relays based on this principle are gas accumulated relays known as Buchhloz relay This type of protection is mainly used for oil immersed transformer

2 Core Balance leakage protection

Due to earth faults there will be partial break down of winding insulation to ground.this leakage current will be very small compred to short circuit currents.If this earth faults persisits for a long time and causes considerable damage before it ultimately devlops in to short circuit faults.An earth faults relay used for this is an overcurrent relay of lower setting and operates when earth faults develops.

3 Combined leakage and overeload protection

Used for protection against phase to phase and earth faults.The overload relays are used to provide protection against phase to phase faults while earth faults relay has low current setting and operates under leakage or earth faults only

4 Differential Protection

The Differential protection used in power transformers provied protection against internal phase to phase and phase to earth faults and is generally used for transformers of rating exceeding 2 MVA.This protection is based on Merz-price circulating current principle.