1.8 MW, 6.6 KV motor starts. BUS, consists of 2X6.5 MVA Trafo,Buscoupler Closed. Under voltage 5-6 Sec < 90 % in 6.6 KV bus.U/V limit ? Fault level ?

The Under voltage limits are defined by the Utilities. UV limit will also depend on the other loads on the bus, UV limits must be established such that other loads on the bus continue to operate undisturbed. At the time of Motor Starting a drop of 15% is considered allowable.

SC level on the bus would depend on the sources and we should look at the complete SLD for more accurate values. If assuming that the bus has only this motor and transformers connected then the SC level would be - (assuming transformer %Z of 5%)

through Fault current of each trafo = Ir/Uk ; 6500/1.732*6.6*0.05 = 11.37kA;

For Two transformers = 11.37*2 = 22.74kA

Adding another 700A (Motor Contribution Thumb rule 4 times Ir@0.8pf as per ANSI Std.)

Short Circuit Current Isc = 22.7kA + 0.7kA = 23.4kA.

Other SSC sources on the bus should also be taken into consideration for concluding Short Circuit Current at the bus.

A method is a modification of the Ohmic method where the impedance of a circuit equals the sum of the impedances of components constituting the circuit. Using the admittances, it follows that the reciprocal of the system impedance is the sum of the reciprocals of the admittances of the components. By very definition, the circuit component admittance is the maximum current or KVA at unit voltage which would flow through the circuit or component to a short circuit or fault when supplied from a source of infinite capacity. 

The 1500 MVA power supply is merely given a short circuit MVA rating. Sometimes, if the system MVA is not available, but its voltage and impedance are given, the short circuit MVA can be calculated by MVASC = KV2 * Y formula. The very same formula is used to calculate the short circuit MVA rating of the 69 kV X=3.87 Ohm cable. Next, for the 69/12kV X=0.076 p.u. 15 MVA transformer use MVASC = MVA / Zp.u. formula. The short circuit MVA contribution of the 15 MVA Xd=0.2 motor is equal to its own MVA base divided its own per unit impedance. As a conversion is being made, an MVA diagram is being developed. One line diagram a) is replaced with MVA diagram b). If a short circuit is taken at the 12 kV bus, there will be a series flow of MVA=1500, MVA=1230 and MVA=198, and their combination will be in parallel with the motor SC MVA=75. Combined MVA of components connected in series and parallel are calculated using following equations: