A case study of fault level calculations for a MV/LV network using direct method

Step-by-step fault level calculations

This technical article represents the case study of fault level calculations for an MV/LV network using the direct method. There are many advantages of the direct approach, and the main of them is that it directly uses the system single line diagram, equipment data, and basic electrical equations.

It’s worth mentioning that the direct method of fault level calculations is more straightforward to comprehend than the per-unit method.

For a start, it’s important to prepare a single line diagram of the electrical power supply and distribution network, clearly indicating all the significant network elements (power supply sources, transformers, electric motors, etc.), fault current contributors, short circuit protective devices, etc.

Let’s now describe the network components for which we will calculate the fault levels:

1. The 11kV incoming terminals (source fault current)
2. The auxiliary transformer’s 11kV terminals
3. The 6.6kV switchgear bus
4. The service transformer’s 6.6kV terminals
5. The 415V PCC panel bus
6. The 415V MCC panel bus
7. Motor contribution (510kW motor, PF=0.8; eff=85%) at the 6.6kV switchgear bus, assuming that:
   1. Only one motor in operation
   2. Both motors in operation
8. Motor contribution (110kW motors; I_rM = 193.23A) at the 415V PCC panel, assuming five such motors are in operation.