A Short Circuit Study is an important tool in determining the ratings of electrical equipment to be installed in a project. It is also used as a basis in setting protection devices. Computer software simplifies this process however, in cases where it is not available, alternative methods should be used. The per-unit and ohmic method are very tedious manual calculation. These hand calculations are very prone to errors due to so many conversion required. In per unit, base conversion is a normal part of the calculation method while in ohmic method, complex entities conversion.The easy way to do hand calculation is the MVA method.
In this example, we shall be presenting a short circuit study of a power system. Motors are are already lumped with ratings 37kW and below assigned an impedance value of 25% while larger motors are 17%. A 4MVA generator is also included into the system to augment the utility.
Figure 1
Utility: 33KV, 250 MVAsc
Transformer 1: 10 MVA, 33/11KV, 9% Z
11KV Bus
Generator: 3MVA, X"d = 0.113
Transformer 2: 5 MVA, 11/6.6KV, 7% Z
Motor 1: 5MVA (Lumped), 17% Z
6.6KV Bus
Transformer 3: 2 MVA, 6.6KV/400V, 6% Z
Motor 3: 6.8 MVA (Lumped), 17% Z
400V Bus
Motor 4: 300 KVA (Lumped), 17% Z
Motor 5: 596 KVA (Lumped), 25% Z
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this is very good example of short circuit calc. by MVA method.i like this.thanks a lot!
Very good article!……I presume that the single phase fault mentioned in page 5 is a line to ground fault. How about a line to line fault? a line to line to ground fault ? Mabuhay ka kabayan!…
In the MVA method of short circuit calculation
The generator MVAsc = 35.4 + 133.39 = 168.79 MVA
From the calculation it is not clear that where from the value 133.39 is derived. Is it possible to explain that where from the value derived.
The motor 1 MVAsc = 139.38 + 29.41 = 168.79 MVA
From the calculation it is not clear that where from the value 139.38 is derived. Is it possible to explain that where from the value derived.
Both values are calculated:
133.39 = 76.87 + 27.11 + 29.41
139.38 = 76.87 + 27.11 + 35.4