Assignment no 2

Assignment no 2Power system Analysis

Symmetrical & unsymmetrical faults

Q1A small generating station has a bus bar divided into three stations. Each section is connected to a tie bar with reactors, each rated 5 MVA 0.1 pu reactance. A generator of 8 MVA rating and 0.15 pu reactance is connected to each section of the bus bar. Determine short circuit capacity of the bus bar if a three phase fault takes place on one section of the bus bar. Ans: 78,73MVA

Q2A generating station A has a short circuit capacity of 1000 MVA. Another station B has a short circuit capacity of 650 MVA. They are operating at 11 KV. Find the short circuit MVA if

(a) they are connected in parallel.

(b) If they are connected by a cable of 0.5 ohm reactance per phase and three phase fault takes place at generating station A.

Ans: (a) 1650 MVA (b) 1176.47 MVA

Q3A single line to ground fault occurs on phase a at a point P in the circuit whose single line diagram is shown below.

Determine the fault current. The reactances of the transmission lines are X1=X2=12% and X0=40% on a base of 1.25 MVA and 4160 Volts.

(Ans: 2765.6A)

Q4A 30 MVA, 13.8 KV, 3-phase alternator has a sub transient reactance of 15% and negative and zero sequence reactances of 15% and 5% respectively. The alternator supplies two motors over a transmission line having transformers at both ends as shown on one line diagram. The motors have rated input of 20MVA and 10 MVA both 12.5 KV, with 20% sub transient recatances and negative & zero sequence reactances are 20% and 5% respectively. Current limiting reactors are 2.0 ohm each in neutral of alternator and large motor. The rating of both transformers are 35 MVA, 13.2/115KV /Y connected with leakage reactance of 10%. Series reactance of the line is 80 ohms. The zero sequence reactance of the line is 200 ohms. Determine the fault current when

(a) L-G (b) L-L and L-L-G fault takes place at point P.

Q5Three 6.6 KV, 3 phase, 10 MVA alternators are connected to a common bus. Each alternator has a positive reactance of 0.15 pu. The negative and zero sequence impedances are 75% and 30% of positive sequence reactance. A single line to ground fault occurs on the bus. Find the fault current if

(a) All alternator neutrals are solidly grounded.

(b) One alternator neutrals is solidly grounded and other two neutrals are isolated.

(c) One alternator neutral is solidly grounded through 0.3 ohm resistance and other two neutrals are isolated.

(Ans: (a) 25603.6A (b) 19805.5A (c) 10677.8A) Q6Explain the term short circuit capacity.

A power station has two 30 MVA generators each with 0.15 pu reactance and one 10MVA generator with 0.125 pu reactance. It is supplying to 3.3 kV switchgear through a 5 MVA transformer. If the short circuit capacity of switch gear is 100MVA, find the safe minimum reactance of transformer.Ans: 0.40pu (0.087 ohm). Q7Show that a line to ground fault at the terminals of the synchronous generator with solidly grounded neutral is more severe than a three phase fault. Q8Figure below shows single line diagram of a three phase power system. ( HC Rai P 345)

Both generator and motor are rated 2500kVA, 600volts with X1=X2= 10% and X0=5%. T1 and T2 are transformers each rated 2500kVA, 600V- delta and 4160V star with X1=X2=Xo= 5%. The transmission line reactances are X1=X2=15% and Xo= 50% on a base of 2500kVA, 4.16kV. A double line to ground fault occurs on line b & c at point P in the system. Assuming no load condition and neglecting pre-fault current determine fault current.Q9A 50 MVA 11 KV three phase synchronous generator was subjected to different types of fault. The faults currents are as follows:

LG fault - 4200A; LL fault - 2600A; LLL (three phase fault) - 2000A

The generator neutral is solidly grounded. Find the per unit value of positive, negative and zero sequence reactances of the generator.

(Ans: X0=0.126pu, X1= 1.312pu, X2= 0.436pu) Q10A three phase 10 MVA 11 KV synchronous generator with a solidly grounded neutral point, supplies a feeder. The relevant impedances of the generator and the feeder are as follows:

GeneratorFeeder

Positive sequencej 1.2

j 1.0

Negative sequencej 0.9

j 1.0

Zero sequence j 0.4

j 3.0

A fault from one phase to ground occurs at the far end of the feeder. Determine the voltage to neutral of the faulty phase at the terminals of the generator.

(Ans: 4233.9V)

Q11Draw the sequence networks for the system shown in the figure. Determine the fault current when LL fault occurs at point F. the per unit reactances all referred to the same base are as follows:

X0X1X2

Generator G1

0.050.30.2

Generator G2

0.030.250.15

Line 1

0.700.30.3

Line 2

0.700.30.3

Transformer T10.120.120.12

Transformer T20.10.10.1

Both the generators are generating 1 pu voltage. If rating of generators and transformers are 100 MVA, 11KV and 100 MVA11/132KV. Find the actual fault currents of all the three lines.

(Ans: If = Ib = Ic = 4.266pu, Ia=0)