What is a Short Circuit Study?
A short circuit study calculates the short circuit capacity at designated locations within a power system or power delivery infrastructure.
Why Do We Make Short Circuit Study?
The resulting data from the Short Circuit Study may also be utilized to select electrical equipment ratings, may serve as the basis for protective device coordination study and Arc-Flash hazard analysis.
Assumptions for MVA Method Short Circuit Calculations
For Short Circuit calculations
- The Power Utility, generators and all motors are sources of electrical energy or short circuit currents.
- Transformers, reactors and cables limit short circuit currents.
- Capacitors and static loads such as heaters and lighting do not contribute to short circuit current.
- If actual motor impedance are not known
- All motors 37kW or less are lumped and assigned an impedance Z = 25%.
- All motors abover 37kW are lumped and assigned an impedance Z = 17%.
- For motors, 1 HP = 0.75 kW = 1 KVA
Short Circuit KVA of Circuit Elements
Utility: KVASC = Utility FAULT DUTY (KVA)
Fault Duty = 0.04 pu @ 100MVA
KVASC = (100/0.04) x 1000 = 2,500,000 kVA
Generator: KVASC = (100 x KVAG) / %Z = KVAG / X"d
Generator 50 MVA, 11 000 V, X"d = 0.113
KVASC = (50 x 1000) / 0.113 = 442,478 kVA
Motor: KVASC = (100 x KVAM) / %Z = KVAM / X"d
Motor 1500 HP, 4000V, FLA = 193, X"d = 0.167.
KVASC = 1500 / 0.167 = 9000 kVA
Transformer: KVASC = (100 x KVAT) / %Z = KVAT / Zpu
Transformer 132kV / 11kV, 3PH, 50/56MVA @ 55OC, 66.5 / 74.5 MVA @ 65OC, OA/FA, Z = 9% @ 50MVA
KVASC = 50 x 1000 / 0.09 = 555,555 kVA
Reactor: KVASC = (1000 x KVA2) / Z (ohms)
Reactor 11 kV, 0.125 ohms
KVASC = 112 x 1000 / 0.125 = 1,523,520 kVA
Cable: KVASC = (1000 x KVA2) / Z (ohms)
Cable : 3/C - 185 mm2, 400V, 150 m, R = 0.0258 / km, X = 0.027 / km
Z = (0.02582 + 0.0272)0.5 x 150 / 1000 = 0.0056 ohms
KVASC = 0.402 x 1000 / 0.0056 = 28,571 kVA
In the next part of this tutorial, we shall be having examples.