How to refer fault levels across a transformer 

By on

Over the past year or so I have been involved in on going discussions related to referring fault levels from the secondary of a transformer to the primary side. While this is easy for some people to grasp, for others it's not so straight forward. Here is a brief explanation of how to do this.

ReferTransformerFaultThe image shows a simple electrical system around a transformer (primary voltage VP, secondary VS).  A fault, IFS (shown on the secondary), will also been seen on the primary side of the transformer IFP.  Because, the voltage level on the primary of the transformer is different than the secondary, the two values of current will be different. 

The value of the primary side fault current is determined by the ratio of the transformer voltages:

image002

That really all there is to it.  If you wanted to refer from the primary to the secondary, just reverse the ratio of the voltages.

Proof of correctness

To verify that the above is true, consider the power (S) transfer from the primary to the secondary of the transformer; shown by the green line.  The power is the flow of energy into the fault and the 'law of conservation of energy' states that the energy entering the system must be the same as that exiting the system.

Ignoring losses (which are small compared to the fault), the power (energy) into the fault must be supplied by via the primary of the transformer and is the same on both sides of the transformer. 

In a single phase system, the electrical power is given by:

image005

applying to both the primary and secondary circuits:

image007

which can be rearranged to give:

image003

For a three phase system (with a symmetrical three phase fault), we have the same result.  The power would be three times that of a single phase system, with voltages as line to neutral.  The three on both sides of the equation cancel, leaving the same answer. 

For three phase systems, unbalanced or earth faults are more problematic.  In these instance it is better to use techniques such as symmetrical components to calculate what is happening.

If you need a little more information on working three phase power from single phase power, you can see:

Any other tips or insights, please add to the comments below.



Steven McFadyen's avatar Steven McFadyen

Steven has over twenty five years experience working on some of the largest construction projects. He has a deep technical understanding of electrical engineering and is keen to share this knowledge. About the author

myElectrical Engineering

comments powered by Disqus

  1. Ella 's avatar Ella says:
    3/6/2013 1:35 PM

    Please assist,

    If I have a Ynd1 transformer and a phase to phase fault exists on the secondary side am I correct to assume that the fault current at the primary side can be represented as a 3 phase fault with a ratio of Vs/2Vp ?

    • Steven's avatar Steven says:
      3/10/2013 9:50 AM

      The above applies to balanced faults. With an unbalanced fault such as yours, the primary currents will also be unbalanced.

  2. Ella's avatar Ella says:
    3/11/2013 12:20 PM

    Thanks for the feedback. Regarding primary currents in unbalanced situations I am trying to understand distribution of fault currents through transformers. If I have a wye grounded - delta transformer with a phase to earth fault on the secondary side, am I correct to assume that the primary fault currents on the wye side will be 0 through each of the lines?

    Furthermore, if the delta side (of the wye groudned delta transformer) is grounded through a zig-zag transformer will the primary currents still be 0?

    Your assistance is much appreciated.

    Thanks
    Ella






Comments are closed for this post:
  • have a question or need help, please use our Questions Section
  • spotted an error or have additional info that you think should be in this post, feel free to Contact Us



Material Properties

Everything physical in electrical engineering from insulations to conductors revolves around materials. Here we are listing common materials along with...

Differential protection, the good old days

This morning I was explaining how differential protection works to a junior engineer. To give him something to read I opened up the NPAG (Network Protection...

Fire Resistant and Fire Retardant Cables

Fire resistant and fire retardant cable sheaths are design to resist combustion and limit the propagation of flames. Low smokes cables have a sheath designed...

Lighting Design - An Introduction

From the earliest times, humans have found ways to create light. Pre-historic peoples used natural materials (moss, grass, etc.) soaked in animal fat and...

Smarter Electrical Distribution

The other day I came across an article in Technology Review on the development of a smart transformer. A professor at North Carolina State University is...

Aluminium Windings - Dry Type Transformers

The other day I was talking to a colleague who is a building services consultant.  Despite regularly specifying dry-type/cast resin transformers he was...

Induction Motor Calculator

Just added a page to the tools, which will allow you to calculate the synchronous speed, slip and rated torque for an induction motor. Not a particularly...

Nikola Tesla

Nikola Tesla was born exactly at midnight on July 10, 1856 in the tiny village of Smiljan, Lika in Croatia. In his late teens, Tesla left the village to...

Battery Cars A to Z

Battery powered cars are a hot topic and widely debated. The pros, cons, issues and time frames can be talked about endlessly. An article by the Telegraph...

Motor Efficiency Classification

Electric motors are one of the most widely used items of electrical equipment. Improving motor efficiency benefits include, reduced power demand, lower...

Have some knowledge to share

If you have some expert knowledge or experience, why not consider sharing this with our community.  

By writing an electrical note, you will be educating our users and at the same time promoting your expertise within the engineering community.

To get started and understand our policy, you can read our How to Write an Electrical Note