Search

Premium Membership

Save 15% on Pro Premium Plan with discount code: UX778 (expires on September 31st). Study specialized LV/MV/HV technical articles and papers.

Home / Technical Articles / 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.

A case study of fault level calculations for a MV/LV network using direct method
A case study of fault level calculations for a MV/LV network using direct method (photo credit: Portastor via Flickr)

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; IrM = 193.23A) at the 415V PCC panel, assuming five such motors are in operation.
Single line diagram of a network for which faults are calculated
Single line diagram of a network for which faults are calculated

Since an article is quite long, it is divided into four parts. The first part will deal with the calculation of resistances, reactances, and impedances on various network points through fifteen steps. The second part is a table where all these values are nicely summarized. The third part presents a summary of fault current calculation formulae and its value for each of the six network points explained above.

Finally, the fourth part of this article investigates the motor contribution to the overall fault at the 6.6kV switchgear bus and the LV PCC bus.

Table of contents:

  1. Calculation steps:
    1. 11kV source
    2. 11kV cable from 11kV source to the auxiliary transformer (C1)
    3. 11kV source + 11kV Cable
    4. Conversion of 11kV impedances to the 6.9kV side
    5. 11kV/6.9kV 7.5MVA transformer (T1) impedance
    6. Sum of 11kV reflected impedance and T1 impedance
    7. The connection from the 11kV/6.9kV transformer’s LV terminals to the 6.9kV
    8. 6.6kV Cable from 6.6kV switchgear to the 6.6kV/433V, 2.5MVA transformer (C2)
    9. Sum of 11kV reflected impedance, T1 impedance and Cable C2 impedance
    10. 6.6kV side impedances reflected to the 433V side
    11. 6.6kV/433V 2.5MVA transformer (T2) impedance
    12. Sum of 6.6kV side reflected impedance and T2 impedance
    13. The connection from the 6.9kV/433V transformer’s LV terminals to the 433V PCC
    14. 1.1kV cable from PCC outgoing to the MCC (C3)
    15. Sum of 6.6kV side reflected impedance, T2 impedance and cable C3 impedance
  2. Table of resistances, reactances and impedances
  3. Summary of fault current calculations
  4. Motor contribution:
    1. At the 6.6kV switchgear bus
    2. At the LV PCC bus

Premium Membership Required

This technical article/guide requires a Premium Membership. You can choose an annually based Plus, Pro, or Enterprise membership plan. Subscribe and enjoy studying specialized technical articles, online video courses, electrical engineering guides, and papers. With EEP’s premium membership, you get additional essence that enhances your knowledge and experience in low- medium- and high-voltage engineering fields.

Check out each plan’s benefits and choose the membership plan that works best for you or your organization.

Save 15% – Get 15% discount on Pro Plan, apply discount code: UX778

Log In »Learn More »

Premium Membership

Get access to premium HV/MV/LV technical articles, electrical engineering guides, research studies and much more! It helps you to shape up your technical skills in your everyday life as an electrical engineer.
More Information
author-pic

Sivakumar K

A professional electrical engineer, having more than 37 years of 'hands-on' experience in the Design, Specification, Selection, Procurement, Inspection, Installation, Testing, Commissioning, Operation, Trouble-shooting & Maintenance of a wide variety of EHV/HV/MV/LV Electrical Equipment in various large industrial projects & process industries. Also, worked/working as a professional electrical trainer, conducting high-end training programmes for industrial electrical professionals & faculty/students of electrical engineering. Published more than 50 technical articles and presented more than 15 technical papers

Leave a Comment

Tell us what you're thinking... we care about your opinion! Please keep in mind that comments are moderated and rel="nofollow" is in use. So, please do not use a spammy keyword or a domain as your name, or it will be deleted. Let's have a professional and meaningful conversation instead. Thanks for dropping by!

sixty five  −    =  sixty

EEP Academy Video Courses

Learn to design LV/MV/HV power systems through professional video courses. Lifetime access. Enjoy learning and don't forget to apply a 20% discount code SEP21