UNIVERSITY OF MORATUWATUTORIAL 2

OC, EF, DOC and DEF PROTECTION

1. Determine the CT ratios, plug settings and TMS to provide discriminative over current protection using standard IDMT relays for the power system shown in Figure 2-1.

Location A B C D ELoad (A) 50 75 80 70 50Min FL (A) 2000 1500 1300 800 600Max FL (A) 5000 4000 3000 2000 1500

Figure 2-1

2. Power system shown in the Figure Q2 is required to be provided with over-current protection. Design currents of the 33kV double circuit line and the 11kV outgoing feeders are 400Amps and 200Amps respectively.

a) Determine the CT ratios at the locations (A1, A2), (B1 ,B2 ), (C1 C2), (D1,D2) and (E1 E2)

b) Mark the CT locations c) Calculate

1. 11kV feeder (D1,D2 ,E1 ,E2) over current relay settings 2. Transformer 33kV over current, , High set Instantaneous over current

relay settings 3. 33kV incomer, directional over current relay settings 4. 33kV incomer over current relay settings

3. Power system shown in the Figure Q3 is required to be protected with discriminative over-current and earth fault protection. Determine

a) Plug setting and time settings of the OC, EF relays of the 11kV feeders A. A’ and A”. b) Plug setting and time settings of the OC, EF, HSIOC, HSIEF relays of at the

location B c) Plug setting and time settings of the DOC, DEF relays at D and D’. d) Plug setting and time settings of the OC, EF relays E and E’

Figure Q 3

4. Select the appropriate relays and determine the settings to provide discriminative over current protection for the power system shown in Figure Q4.

Figure Q-4

Current (A) 430 650 1150 2000 3500Op time (Secs) 100 10 1 0.1 0.01

150 Amp fuse characteristicsCurrent (PSM)

2 3 4 5 6 7

Cold curve 245 90 50 31 22 16.5Hot curve 85 31 16 10.5 7.4 5.7

Thermal Characteristics of the motor protecting relay. PS -100% and Inst - 700%