A switchboard in commercial building is fed from the utility through a 1500 kVA transformer during normal operating condition. If the utility power is not available, a 650 kVA standby generator provides the power to the switchboard through an Automatic Transfer Switch (ATS) that has a published short circuit interrupting rating of 8-kAIC at 480V. The following data is given below:
Utility data
Transformer data
Generator data
Capacitor Bank
Feeders data
Panels data
Motors data
- Use the provided data and draw the one-line diagram using the SKM program
- Hand calculate transformer T1 full load ampere at the secondary 480V side. Show your work
- With the 150 kVAR capacitor bank connected to the switchboard bus, and the Cap. Switch is Normally Open (N.O), perform short circuit “Balanced System Studies” and provide the following:
a) 3-phase fault magnitude on transformer T-1 secondary side in KA.
b) With the switch to motor M-7 is N.O, what is single-line-to ground fault on Panel I in KA.
c) What is the 3-phase fault contribution from motor M-7 to the Swbd bus?
- Perform “Load flow” analysis when the switchboard is fed from the transformer and the Cap. Switch is N.C, and provide the following results:
a. Real power (KW) and reactive power (kVAR) flowing in the feeder from the ATS to the switchboard.
b. Is reactive power in 4a being absorbed or delivered?
c. What is the real power (kW) absorbed by motor M-2?
d. Hand calculate the power factor at motor M-2. Show your work
e. Are the feeders to motors M-2 and M-4 overloaded? If yes, by what percentage?
5.With the switch to the capacitor bank is N.C and the switch to motor M-7 is N.O:
a. What is the reactive power on the switchboard bus
b. Is reactive power being absorbed by the switchboard or delivered from the switchboard back to the utility and by how many kVAR; Explain why.
- Hand calculate the power factor on feeder C-4
- When the switchboard is fed from the generator, and the switch to the Cap Bank is N.C, perform short circuit “Balanced System Studies”, and provide the following:
a) 3-pahse fault contribution from the generator in KA
b) 3-phase fault magnitude on the ATS emergency-side blade in KA.
c) What is the 3-phase fault magnitude at the switchboard bus in KA.
d) Comparing the ATS published AIC interrupting rating to the 3-phase bolted fault on the ATS, is the ATS overdutied, and if yes by what percentage?
e) Is it safe to use the ATS based on findings in 7.d) above? What do you recommend?
Sample Solution