Making Short-Circuit Calculations Easy

Jun 1, 2000 12:00 PM, By John Paschal, P.E.

Using the new kVA method, you only need one calculation to determine the short-circuit values at every point within an entire electrical power system.

In this day of high fault currents, it's more important than ever to protect electrical equipment from extremely high current levels. Otherwise, the equipment will explode as it attempts to interrupt the fault. But fault current calculations have always been difficult to get a handle on, until now.

The new "Easy Way" kVA approach is taking the place of the abstract "Per Unit" method of short-circuit calculations from the past. With the kVA method, you can easily visualize what currents will flow where. And you can calculate them using an inexpensive handheld calculator in moments, regardless of the complexity of the electrical power system.

This method is simple because there are no awkward "base" changes to make, since kVAs are the same on both the primary and secondary sides of every transformer. Perhaps best of all, you only need one calculation to determine the short-circuit values at every point within the entire electrical power system. With the old Per Unit method, you needed a separate calculation for each point in the system.

You can obtain short-circuit kVA values from the electrical utility company, but short circuit power is also protected by generators and motors. The kVA produced by motors equals the motor starting inrush current, and the kVA produced by generators equals the kVA nameplate rating divided by its nameplate subtransient reactance rating "Xd."

For example, a 1000kVA generator with a subtransient rating of 0.15 instantaneously produces 1000/0.15, or 6666kVA. A 100hp motor instantaneously produces 100,000/.17kVA, or 588kVA. If this motor and generator connect to the same bus, then the short-circuit power available at that bus is the sum (6666 + 588), or 7254kVA. If the electrical utility is rated to deliver 100kVA to this same bus, then the total short-circuit power available at that bus is 107,254kVA.

Using the kVA method also greatly simplifies the short-circuit power attenuation (or holdback) provided by reactors, transformers, and conductors. For example, a 2000kVA 7% impedance transformer will pass through its windings a maximum of 2000/.07, or 28571kVA of power, if infinite power flows to one side of its windings. If instead of an infinite current source, the above bus connects to this transformer, then the amount of power that will be "let through" the transformer is the reciprocal of the sum of the reciprocals of the two, or 1/(1/107254 + 1/28571), or 22561kVA. You can determine transformer impedance, reactor impedance, or cable size with the kVA method quickly enough to make "what if" calculations.

Comparisons over several years have found results of the kVA method to be accurate within 3% of computer calculations using expensive software, so you can even use the kVA method as a "check" on the input and output of a computer calculation. This is an excellent benefit because standard engineering procedure requires you to check calculations using a different method from the one originally used.

Editors Note: EC&M's book, "Short-Circuit Calculations The Easy Way," explains the entire "Easy Way" kVA method in a step-by-step format. Available from EC&M Books, call (800) 543-7771 to order.

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