Performing the Calculation (Simplified Approach)
3. The Magic Formula (Sort Of)
Alright, let’s get down to the nitty-gritty (just kidding, we’re avoiding that term!). While a full-blown short circuit study can involve complex software and detailed modeling, we can use a simplified approach to get a good estimate of the 3 phase short circuit current. This approach is suitable for many practical applications and provides a solid understanding of the underlying principles. Essentially, we’ll be calculating the fault current at various points in the electrical system using a per-unit or impedance method.
The basic formula you’ll be using, in a simplified form, is: Ifault = Ibase / Ztotal, where Ifault is the fault current, Ibase is the base current, and Ztotal is the total impedance to the point of the fault. Base current needs to be calculated from system voltage and MVA base. Ztotal includes the impedance of the source, transformer, and any cables in between. You’ll need to convert all impedances to a common base (usually the transformer’s kVA or MVA rating) to ensure consistency.
Let’s break it down even further. Imagine a simple system with a utility source, a transformer, and a cable feeding a load. You’d first calculate the base current for the system. Then, convert the utility source impedance, transformer impedance, and cable impedance to per-unit values based on the same kVA or MVA. Add these per-unit impedances together to get the total impedance (Ztotal). Finally, divide the base current by the total per-unit impedance to get the estimated fault current. Remember to multiply the per-unit fault current by the base current to obtain the actual fault current in Amperes or kA. Sounds easy right?
This simplified calculation gives you a good starting point for understanding the magnitude of the fault current. However, it’s important to remember that this is an approximation. For more complex systems or critical applications, a professional short circuit study using specialized software is highly recommended. It’s like using a map versus a GPS. The map gets you in the general area, but the GPS provides precise directions.