Transformer testing

[Rocboni]

Hi All,

So i was asked to look at at large 3 phase to 120V transformer around 85kVA - it was tripping the MCB instantly (it never used to)

I insulation resistance tested the primary windings to earth - >2000ohms

I continuity tested the windings and got 0.46 ohms per winding, they were all the same.

Thinking back it must have been internally star connected as only 3 connections out which explains the consistent reading.

What other tests should have been done and is the winding resistance low ?

The theory behind inductors is briefly coming back to me regarding chokes ect. that they would appear a dead short but due to inductance when energised so does that explain the low resistance?

my conclusion was the transformer is fine a but the MCB is at fault but they have a spare and it is fine with that plugged in.

Any advice regarding testing would be greatly appreciated to be honest I never really thought about it until i was asked to test one......

 

[Mikegh]

I did a bit of that before forget the details

IR test between primary and secondary ?

 

[Lucien Nunes]

I insulation resistance tested the primary windings to earth - >2000ohms

That is probably true but I don't think it's what you meant. >2000MΩ perhaps?

Thinking back it must have been internally star connected as only 3 connections out

Commonly delta-star for various reasons, but on a transformer of this size there might be winding data on the plate.

Yes, winding resistance should be low, manufacturers make it as low as economically practical, because resistance only creates losses. Yes, it is the inductive reactance that governs the magnetising current (the component of the supply current that sets up the magnetic flux, and lags the supply by 90°).

Faults you are looking for include shorts to earth, to other windings and shorted turns within each winding, the latter being the most common. Shorted turns create a heavy load on the transformer as though there was a secondary feeding an output that is shorted. The fact that the winding resistances are equal on all phases is a good start, but a small fraction of the winding shorted might not be revealed in the two significant figures of the resistance reading. The way to spot that would be to energise the transformer through a resistance, or from a low voltage, and measure the line currents and phase angles. A much greater discrepancy is likely to show up here than in the resistances, with a shorted winding showing a much larger in-phase component of current due to what is effectively a resistive load. The short might be in the secondary of course.

I am not sure whether you mean a spare transformer works on that breaker or a spare breaker works with that transformer? Presumably not actually an MCB as it must be >125A rating? What is it and what is it set to?

 

[pc1966]

As above 85kVA would be around 123A load current so probably fed from something like a 200A MCCB.

8.5kVA on the other hand could well be from something like a 40A C-curve or 25A D-curve MCB.