Transformer Ground Fault Question

[djb79]

Hi All,
I'm trying to get it right in my head as to why when a single phase on the secondary side of the transformer goes to ground (earth fault), the remaining 2 phases increase in voltage by a factor of sqrt3?

I know it's a relatively elementary question, but I'm in the process of a career switch from embedded comm's engineering to EIC commissioning. I've got the instrumentation stuff sussed, and a basic grasp for distributed control systems, just in the process of teaching myself transformer protection.

Cheers

 

[7anoter4]

If the neutral is grounded then it does not occur, but between this phase and Ground a short-circuit [a phase-to-ground short-circuit -directly or through a resistance] will occur. If it is no return from ground to neutral then the Ground potential rise up to grounded phase potential keeping the potential difference to the other phase [VL-L=sqrt(3)*VL-N]

 

[dpc]

The voltage between the phases stays the same, but the voltage from phase to ground increases in the unfaulted phases.

 

[sibeen]

Draw a Y (star).

Under normal conditions the voltages are from the tip of each leg to the centre point (neutral). Now make one of the tips the ground point - draw the vector from the other two legs to the earth (short) point.!

Of course, as 7anoter4 mentions, if the centre point is already grounded the voltage raise will not occur.

 

[djb79]

Hi All,
Thanks for the posts, makes a lot more sense when you draw out the vectors.

I realise now that my issue was that I was assuming that the Neutral point of the star transformer was bonded to ground as this is the way I've always seen them. In Australia (were I'm from, I assume this is a mainly UK based board from what I've read) just after the transformer there is an MEN link between the neutral point of the transformer and the ground. My understanding for this is so that there is only a single earth return point which means one can perform earth leakage detection much easier (in a single place).

The above being the case, in what instances would the neutral point NOT be connected to ground.

Regards
Darryl

 

[ScottyUK]

In the UK a system where the neutral is used to supply a load will almost certainly have the neutral solidly earthed. Utility and most private LV final distribution systems are solidly earthed, in either a TNC-S or TNS configuration. Impedance earthed systems are typically found in the distribution and sub-transmission voltage range from 3.3kV up to 132kV. At 275kV and 400kV the systems are solidly earthed. Truly non-earthed systems are rare in the UK although they are found occasionally in small private installations for special purposes.

 

[sibeen]

You see it on MV site backup generators, where a Neutral - Earth resistor is sometimes used at the star point of the generator.

Keeps the single phase fault current down.

The only time I've run across LV systems, in Australia, where the neutral is not solidly earthed was in petrochemical plants and on shore to ship systems.

 

[djb79]

Hi All,
Thanks for all the sweet info and helping re-skill and embedded designer. Whilst working as a grad embedded designer I loved the fact that I never had to deal with anything over 12V.

Through working as an EIC commissioning engineer, as heap of instruments, PLC chains as well as 415V, 3k3V and 6kV VSDs as well as an 11kV substation. 18months ago, I had never even thought about working in this area and now I'm into it in a deep way. The learning curve over the last 14 months has been steep to say the least and it's been awesome to find a site where I can pose questions that I've faced and have them answered sensibly and accurately.

Cheers
Darryl