MVA Method Short Circuit Calculation

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 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.

sld2_a

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

  • Sieg Monjardin Sanchez

    H sir thank you for this very informative article. May I ask, How do you assign a value for the line impedance of lumped loads. Many thanks in advance.

  • Teto

    Thanks Sir! here is additional information. distribution feeder is designed from AAAC with: positive impedace=0.598 + j0.39483ohm/km, negative impedance = 0.598+j0.39483 , zero sequence impedance 0.77218 + j2.03059. it is uniform for total of 110km long distribution feeder. here I am not considering for line down stream to each transformers or there is no line at low voltage side of transformer. I.e each transformer supply load directly. thanks!

    • With the positive & negative sequence impedance the same and with the zero sequence impedance much higher, I do not see any reason why LLG fault is much higher than 3L fault.

  • Teto

    Thanks Ing.Ver. while I simulate short circuit analysis using ETAP for my power distribution system, I get result of fault current report of each fault type. In this result double line to ground (L-L-G) fault current is higher than three phase fault current. what is the reason for this? thanks!

    • It is due to the transmission line design. It may be that the negative sequence impedance of your transmission line is lower than the positive sequence impedance.

  • Teto

    Sir! l have no word to say thank you! here is my last question! don’t hesitate me! does100000MVA simply inserted to MVASc given on ETAP? how to calculate MVASc value for single phase of this power grid point? l can’t get X/R from utility. so, what will be X/R for this power grid point! thanks!

    • As per ANSI Std C37.010, typical X/R ratio is the range of 5-12.

  • Teto

    Thank you very much! your great Engineer! Sir don’t hesitate me! l have my last questions! what does 100000MVA mean? can l simply insert 100000 value to MVASc place on ETAP for three phase(my feeder is 3phase)? If so, how I calculate for single phase? I can’t get X/R data for power grid. from I can get this data also? Thanks!

  • Teto

    Thank you sir for soon response!
    Have 15KV radial power distribution feeder for certain rural area with 50 distribution transformers(15KV/400V) of different rating. most of the loads are small rating and residential loads. I assume there is no any fault currents from generator or motors.so, the only fault source will be feeder supply point or grid point according to ETAP circuit model. From our utility I can’t get MVASc value that is grid MVASc is unknown. but, to run this short circuit analysis ETAP require MVASc value. so, can you help me sir! Thank you in advance!

    • If you don’t know the MVAsc, do the followling

      1. Ask your utility for the MVAsc at the point of coupling
      2. if the utility can not provide MVAsc, assume the MVAsc at the point of coupling to be infinite bus. In ETAP this is 100000 MVA
  • Teto

    how to guess MVASc for feeder line analysis. if there is no MVASc data at utility! how to guess MVASc?

    • If you do not know the MVAsc of the utility, then assume that it is an infinite bus.

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