Topic: Power System Protection and Controls Problems

Problem 1 - The 12.5 kV distribution feeder (Figure P12.1) has two taps. One is protected by three oil circuit reclosers with 70/140A coils set as in Table P12.1. The other tap is a single-phase circuit protected by one 30A fuse operating as shown in Table P12.2. The data for the 46 kV fuse is in Table P12.3. The phase and ground relays are very inverse time overcurrent with instantaneous units. Their time overcurrent characteristics are shown in the typical curve s of P12.11. Fault currents are in amperes at 12.5 kV.

Notes:

1. Assume the transformer is a delta-wye bank.

2. Refer to Blackburn Figure 9.20 for shifting the 65A fuse characteristic to the 12.5kV side of the transformer for comparison with low side relay operating times. Recall that the shifting factor depends on fault type.

a. Determine the 46 kV fuse time-current characteristics in terms of 12.5 kVA for 12.5 kV three-phase, phase-to-phase and phase-to-ground faults. Draw these high-side fuse curves along with the recloser and 30 A fuse curves on time-current log paper, such as K & E 48 5257, with 12.5 kVA as the abscissa and time in seconds as the ordinate.

b. Select a suitable ratio for the current transformers to the phase and ground relays.

c. Set and coordinate the phase and ground relays. Provide a minimum 0.2sec coordination interval between the reclosers and the relays, and a minimum 0.5sec between the 46kV fuse and the relays. Specify the time-overcurrent relay tap selected (available taps are 1, 1.2, 1.5, 2, 2.5, 3, 3.5, 4, 5, 6, 7, 8, 10), the time dial, and the instantaneous current pickup for both phase and ground relays. Plot the coordination on the curve of part 1.

Problem 2 -

a. Apply and set distance-type relays at Stations H and R for the protection of line HR in the system in Figure P12.5. Set zone 1 units for 90% of the protected line, zone 2 to reach 50% into the next line section beyond the protected line, and zone 3 for 120% of the next line section.

b. Plot this system on an R-X diagram with the origin at bus H. Plot the relay settings of part 1 using mho-type characteristics. The mathematical formula for a circle through the origin or relay location is where Z_s, is the relay setting at 75^o: Z = ½(Z_S - Z_S∠φ).

The first term is the offset from the origin at 75^o and the second term is the radius. This when φ is 75^o, Z=0, the relay location; when φ is 255^o, Z = Z_S the forward reach.

Notes:  3. It may be easier to plot Buses H and R relays on separate graphs instead on combining them on a single graph as the problem text indicates.

c. What is the maximum load in MVA at 87% pf. that can be carried over line HR without the distance relays operating? Assume that the voltage transformer ratio R_V = 1000 and the current transformer ratio R_e = 80.

Problem 3 - The 60 mile, 115 kV line GH (Figure P12.8) is operating with the voltages at each end 30^o out of phase when a three-phase fault occurs at 80% of the distance from bus G. This fault has 12 Ω are resistance. The currents flowing to the fault are as shown and are in per unit at 100 MVA, 115 kV.

a. Determine the apparent impedance seen by the distance relays at G for this fault.

b. Determine if the zone 1 mho unit at G set for 90% of the line GH can operate on this fault. Assume that the angle of the mho characteristic (Figure 6.13b) is 75^o.

c. Determine the apparent impedance seen by the distance relays at H for this fault.

d. Determine if the zone 1 mho unit at H set for 90% of the line GH can operate for this fault. Assume that the angle of the mho characteristic is 75^o.

All figures, tables, graphs needed given in attached file.

Attachment:- Assignment File.rar