Power system II

2.  A 100 MVA, 10kv equipment has an impedance of 1 ohm. Find it's per unit and percentage impedance. It now the base values are chosen as 20 MVA, kV, find the new percentage impedance.

3. A single-phase 8MVA, 33/11kV transformer has equivalent impedance referred to LV side as (1+j9) ohms. choosing the transformer rating as the base values, the per-unit equivalent impedance referred to the LV side and HV side respectively are:

4. A three-phase 10MVA, 11KV alternator has 10% sub transient reactance. If a symmetrical fault occurs at its terminals, the short circuit MVA and the current respectively are:

5.Two 11Kv, three-phase, % MVA generators having sub transient reactance of 3% and 2% respectively operate in parallel. Suppose the power loaded through an 11/220 kv, 10MVA transformer has a percentage equivalent reactance of 4%. Calculate the fault current and fault MVA for three phase occurring on the Ht side of the transformer as shown in the figure. Also calculate the fault current supplied by each generator.

6. Two generators rated 11kv, 5MVA  having 5% reactance are interconnected by a transmission line of length 50km. The reactance of the line is 0.4 ohm/phase/km. generators are connected to the line through a set up transformer rated 11/33kV , 8MVA and having 4% reactance. A three phase fault occurs at the middle of the transmission line. Calculate the fault MVA and current.  Neglect the load current.

7. A power system has four generators is shown in the figure below. Two generators rated 15 MVA have a reactance of 15% and the other two rated 20MVA have 20% reactance. The generators are connected to station bus bars. Power is drawn by the load centers through two power transformers rated 50MVA each having percentage reactance of 10%. It is required to design circuit breakers with rating that are to be provided at the LT and HT side of the transformer. The CB rating at the HV side ad LV side of the transformer respectively are:

8. A generating station has four generators; each rated 11 kV, 100 MVA and each having a sub transient reactance of 10%. A sectionalized bus reactor is placed as shown in the figure. A three phase fault occurred at a point near the bus as shown in the figure. a) Determine fault level %X=0 ohms. b) Determine the X value in ohms to limit the fault level to 3000 MVA *

9. Particulars of three generators A, B and C are as follows:GA: 11 kV, 40 MVA, X=10%GB: 11 kV, 60 MVA, X=12%GB: 11 kV, 25 MVA,

X=10%These generators are

interconnected as shown in the figure. The generators are provided

with bus bar reactor, each having Xre=10% reactance based on their ratings (Generator A). A feeder directly connected to generator A as shown in the figure has an impedance of Z=0.05+j0.1

ohms/phase. Estimate the short circuit MVA if a symmetrical fault occurs at the far end of the feeder.

10. A 50 Hz, 30 MVA, 11 kV reactance of 10% supplies power to three identical motors through a transformer as shown in the figure. Each motor is rated for 8 MVA, 6.6 kV with subtransient and transient reactance of jo.15 and jo.25 pu respectively. The transformer is rated for 30 MVA, 11kV/6.6 kV and leakage reactance 8%. The motor busbar voltage is 6.6 kV when a three phase fault takes place at F. Determine the subtransient current

generator has subtransient