Single phase to ground fault calculation, confusion! 1

[Power0020]

Math wise the ground fault current is If= 3E/(Z1+Z2+Z0)

where E is the phase voltage, Z1 is positive seq. impedance, Z2 is negative sequence impedance and Z0 is zero sequence impedance. with transmission lines and cables it is common to assume that Z1=Z2=Z then If = 3E/(2Z+Z0).

The zero sequence impedance included 3 times any ground connected resistance (e.g. NER, tower footing resistance,...etc)

However, I have seen some literature calculate the fault current with a simple KVL loop including phase voltage as a source, earth loop resistance/impedance including grid resistance and tower footing, no consideration for any sequence impedance.

If=E/ZE

I am confused!

any clues?

 

[WHiPCPL]

I'm currently writing a thesis on the application of symmetrical components for line and motor protection.

The only time I've ever seen If=E/ZE, was as an equation in a instructional manual for an installation tester.

In that one ZE is the measured loop through a conductor and the PE (protective earthing) conductor. In this ZE it measures the resistance by sending a current impulse through the phase, with return through the PE. Here it measures the voltage drop and calculates the impedance and from the impedance the short circuit values.

I don't think it's very helpful to the reader to write ZE as it may be misintepreted as the earth impedance. I think what the equation does is equate Z1=Z2=Z0=ZE, by which you have If=3E/(3ZE). Obviously this reduces to If=E/ZE. Alternatively it may assume that ZE is a fault impedance by which you get: If=3E/(Z1+Z2+Z0+3ZE), and then assuming that 3ZE >> Z1+Z2+Z0 thereby neglecting the sequence impedancens. Once again this results in If=3E/(3ZE) which is once again If=E/ZE.

As for whether or not that's the case I can't say.

For your own sake stick to If=3E/(Z1+Z2+Z0).

Hope it was at least somewhat helpful.

 

[Power0020]

@WHiPCPL Exactly the same as I thought.

Good luck with your thesis.

 

[prc]

Power0020-You have brought up an interesting issue that had puzzled me also for sometime back. Calculation of grounding transformer (zig-zag winding) with a Neutral resistor. When NGR is used, Z of winding is ignored for If calculation. Without NGR, only winding Z is used for If calculation. So second alternative seems the real reason.

 

[HamburgerHelper]

If you have high impedance grounding, you'll get what they got. The grounding impedance swamps out everything else. ZE (earth impedance) = 3 *Z0. A quirk of most high impedance grounding systems is that the ground fault current doesn't change with location.

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If you can't explain it to a six year old, you don't understand it yourself.

 

[7anoter4]

See also:

http://www.studiecd.dk/cahiers_techniques/Calculation_of_short_circuit_currents.pdf

page no.12

 

[veritas]

Power0020

One can either use sequence imepdances or the KVL method. However, with the KVL method it is critical to factor in the source impedance, i.e. Zs (and Zs is defined in terms of its sequence components) should be included in the loop. Also, it can be shown that Z0 (cable) = Z1 + 3*Ze., where Ze is the impedance of the ground return path (typically the ground cable).

I wrote a spreadsheet detailing exactly the above as I found often engineers (or others) were calculating the groundfault loop impedance for LV systems incorrectly. Calculation was required to ensure protection had sufficient sensitivity for faults at end of feeder. A good place to start is using symmetrical components and then to check the KVL answers against those obtained using symmetrical components.

I'll try and find the spreadsheet and post it here.