Assignment 2 Power System Protection

Name: Wan Mohd Shahrulfarh bin Wan Shamsuddin

ID: EP078782Lecturer : Prof. Madya Dr. Izham

Current Transformer (CT)

a) Operating Principles and types• CT is defined as instrument transformer which the

secondary current is substantially proportional to the

primary current .• Work on principle of variable flux. • In ideal CT, secondary current would be exactly equal (when

multiplied by the turns ration) and opposite to the primary current.

• Types – Donut /window CT, Bar CT

Donut or Window-Type CT

Bar-type CT

3 Primary Loops + 2 secondary loops added

b) Characteristic under steady-state and transient

• Transient – The core of a CT mainly determines the performance of the CT under fault conditions, since it causes the non-linear behaviour of the transformer. It is well known that d.c. offset in the fault current following into a protective current transformer can cause steel to saturate and produce a distorted secondary current. Core hysteresis results in a remnant or residual flux which also adversely affects CT performance.

Zb represents burden to the CT = meters, cabling, CB and anything connected to the CT

c) Configurations/connections• Ring Core CT - for measuring currents from 50 to 5000 amps,

with windows (power conductor opening size) from 1" to 8“ diameter.

• Split Cote CT - for measuring currents from 100 to 5000 amps, with windows in varying sizes from 1" by 2" to 13" by 30". Split core CT's have one end removable so that the load conductor or bus bar does not have to be disconnected to install the CT.

• Wound Primary CT - designed to measure currents from 1 amp to 100 amps. Since the load current passes through primary windings in the CT, screw terminals are provided for the load and secondary conductors. Wound primary CT's are available in ratios from 2.5:5 to 100:5

Voltage Transformer (VT)

a) Operating principle and types• VT is defined as the secondary voltage is substantially

proportional to the primary voltage and differs in phase from it by an angle which approximately zero for an appropriate direction of the connections

• In an "ideal" transformer, the secondary voltage vector is exactly opposite and equal to the primary voltage vector, when multiplied by the turns ratio.

• n a "practical" transformer, errors are introduced because some current is drawn for the magnetization of the core and because of drops in the primary and secondary windings due to leakage reactance and winding resistance.

b) Performance under steady-state and transient• Steady-state

• Under steady state operating conditions, the load current drawn by the parallel combination of Zf and Zb is relatively small.• There may be a phase shift between the primary voltage and the voltage appearing at the load.

• Transient – VT produces secondary voltages during transient conditions which may be significantly different from the primary system voltage• power frequency voltages are and even low a small transient component may cause problems for relaying operations.

c) Configurations/connections

References• http://www.kappaelectricals.com/technical.html#current• S.H. Horowitz, A.G. Phadke Text Book, 1992• http://www.metersusa.com/Transformers/Data/CTTheory.pdf• http://www.kilowattclassroom.com/Archive/CurrentTransf.pdf• Mehmet Tu, R. R. S. Simpson and H. El-Khatroushi, “Dynamic model of a

current transformer”.• Pradeep Chawande, S.A. Soman, P. R. Apte, and Shubha Pandit,

“Experimental Evaluation of Current Transformer Performance under Saturation”.