Online short circuit calculations example

Develop a comprehensive one-line radial diagram of the power system being analized. It is recommended that you use a copy of the same one-line diagram which is applied when laying out the distribution system. This approach provides the most complete system.

Each basic component within the industrial electrical distribution system is pre-assigned a single short circuit MVA factor (SC MVA) based on the impedance it adds to the system. Generators, motors, transformers are normally given their own rated MVA, X/R ratios and impedance ratings [typical X/R and impedance ratings].The short circuit MVA of each is equal to its MVA rating divided by its own per unit impedance. For instance, a 750KVA, 5.75%Z transformer has a SC MVA = Rated MVA / Z = 13MVA, where Z is the tranformer impedance in p.u. For a feeder where the voltage is given and its impedance or reactance is known, its short circuit MVA is equal to (kV)² divided by its impedance in ohms. Loads not contributing to system's short circuit current (lighting, heating) are assigned a SC MVA value of 0. Unloaded transformers should also be terminated with the SC MVA = 0 node - this allows to calculate short circuits on the unloaded transformer secondary. Incoming line short circuit duty in MVA is normally given by power companies. Therefore, with these values no conversion is required. However, if impedance at the terminal is given, find it's short circuit MVA by dividing (kV)² by its ohms.

The calculator performs vector operations on the short circuit MVA values from power corporation, generators, high voltage motors and low voltage motors through transformers. Since reactance of all circuit components usually far exceeds resistance producing a consistently high X/R ratio through out system, direct arithmetic addition of the short circuit MVA and their reciprocal values is an easy safe close approximation recognized and widely acceptable by industry in calculating power system short circuits. This approach results in higher (typicaly by 5 to 10 percent) short circuit values. Accurate equipment X/R ratios should be used wherever available.

Develop a hierarchical tree in terms of database records

+--+----------+-------+---+------+---+--------------------------------+----+
|id| label    | SC MVA| % |  x/r | % | Description                    |p_id|
+--+----------+-------+---+------+---+--------------------------------+----+
| 1| Power Co.| 500.00| 1 | 12.0 | 5 | 500MVA 34.4kV Service Entrance |  0 |
| 2| TRSFRM 1 |  83.30| 1 | 12.0 | 5 | 5MVA 6%Z, 4.16kV Secondary     |  1 |
| 3| TRSFRM 2 |  13.00| 1 | 12.0 | 5 | 750kVA, 5.75%Z, 208V Secondary |  2 |
| 4| MOTORS 2 |   1.70| 1 | 12.0 | 5 | 350kVA Motor Load, 20%Z, 208V  |  3 |
| 5| TRSFRM 3 |  17.40| 1 | 12.0 | 5 | 1000kVA, 5.75%Z, 480V Secondary|  2 |
| 6| MOTORS 3 |   2.50| 1 | 12.0 | 5 | 500kVA Motor Load, 20%Z, 480V  |  5 |
| 7| GENerator|   1.80| 1 | 12.0 | 5 | 300HP Synch. Generator         |  2 |
| 8| TRSFRM 4 |  17.40| 1 | 12.0 | 5 | 1000kVA, 5.75%Z, 480V Secondary|  2 |
| 9| MOTORS 4 |   4.40| 1 | 12.0 | 5 | 1000kVA Motor Load, 23%Z, 480V |  8 |
|10| 75ft.Cabl|  102.0| 1 | 12.0 | 5 | 75 ft. Cable 2-300 MCM / Mag. C|  5 |
|11| MOTORS3_1|   1.00| 1 | 12.0 | 5 | 250kVA Motor Load, 25%Z, 480V  | 10 |
|12| 50ft.Cabl|  38.80| 1 | 12.0 | 5 | 50 ft. Cable 1-1/0 / � Mag. Con| 10 |
|13| Trsf3_1  |   4.30| 1 | 12.0 | 5 | 4.3 MVA Transformer            | 12 |
|14| Void Node|   0.00| 1 | 12.0 | 5 | Lighting Load                  | 13 |
+--+----------+-------+---+------+---+--------------------------------+----+

The system equipment tree should be broken into levels, with each level being more focused than the last. The tree consists of nodes connected to each other by branches. Please note that a node may have one or more children, but can have only one parent. Equipment parent id equals the id of the upstream device feeding the equipment. In the above example, TRSFRM1 is fed from Power Co. The TRSFRM1 is assigned parent id of "1" which equals the Power Co id value. Power Co is a root feeding the system, its parent id is assigned "0" by default. You will need the reference table such as the one above to input your system information required for short circuit calculations.

Run the program and observe the results

• Power Co.[ 500(12X/R) +  8.71(12X/R) =  51 X 10¹  (12X/R) ]
  • TRSFRM 1[ 71.1(12X/R) +  9.73(12X/R) =  81 (12X/R) ]
    • TRSFRM 2[ 11.1(12X/R) +  1.70(12X/R) =  13 (12X/R) ]
      • MOTORS 2[1.70]
    • TRSFRM 3[ 14.2(12X/R) +  3.49(12X/R) =  18 (12X/R) ]
      • MOTORS 3[2.50]
      • 75ft.Cable[ 14.2(12X/R) +  1.00(12X/R) =  15 (12X/R) ]
        • MOTORS3_1[1.00]
        • 50ft.Cable[ 10.6(12X/R) +  0(12.0X/R) =  11 (12X/R) ]
          • Trsf3_1[ 3.05(12X/R) +  0.00(12X/R) =  3.1 (12X/R) ]
            • Lighting[0.00]
    • GENerator[1.80]
    • TRSFRM 4[ 14.2(12X/R) +  4.4(12X/R) =  19 (12X/R) ]
      • MOTORS 4[4.40]

The program will output the hierarchical system equipment tree with short circuit MVA calculated at each node. Short circuits contributed by upstream (red) and downstream (blue) equipment are listed across each node. Upstream and downstream values above (blue and red color respectively) contain one more significant digit than required by error analysis rules. This digit is dropped off the final result (in green color). In this manner, phenomenon known as "round-off error" is effectively avoided.

Use the accompanying SC MVA to kA Converter or divide total SC MVA values by 1.73 * kV_LL to get 3 phase short circuit current values in kA. Since short circuit MVA values are added vectorially, ariphmetic addition of upstream and downstream SC MVA does not generally equals the total SC MVA. By performing short circuit MVA analysis for positive, negative and zero sequences, unsymmetrical phase to ground, phase to phase double phase to ground fault currents can be resolved.

Suggested report format

You may transfer the calculated short circuit MVA onto the original one-line diagram as displayed below:

Your account gives you the advantage of saving the entire system diagram. This is practical for systems with multiple scenarios of interconnections where the system goes through ongoing changes over time. You may come back to continue your analysis by means of modifying equipment data and / or extending the distribution diagram.