Consider the following loads, a DG set is required to run these loads.
- 2 nos 150 KW, 415 volt motors with start/delta starter
- 2 nos 150 KW 415 volt motors with Variable frequency drive(VFD) start
- 300 KVA Lighting and other small motor loads
As we all know , that a motor draws more current during starting than running. so , power requirement from the DG set will be more at starting than running.
Assumption: we Consider that all the motors are working one , no standby . suppose one 150 KW motor fed by start delta starter is running and all the three other motors are started simultaneously. So, we will have to find out the power drawn by the running 150 KW motor and power drawn when all the three motors are started.
once we have these data, we will have no problem in reaching to DG sizing.
150 KW Motor KVA input = 150/(0.95*0.95)= 166 KVA (Consider 0.95 power factor and 95 percent motor efficiency)
Starting KVA of a motor = 2*166 =332 KVA
For VFD controlled motor, starting KVA of a motor =1.5 X166= 249 KVA
The starting KVA = 300 +(2*249) +166+332 = 1296 KVA
Therefore, the DG set capacity required will be = 1.25 X 1296 =1620 KVA(Take 25 Percent extra, as DG will not give its 100 percent . it has a load factor)
Nearest standard size available = 1800 KVA