Power Electronics Question

[Circlenaut]

Hey guys,

I got this question in an exam and I'm having a hard time finding the reasoning behind it. Do any of you know?

How do I determine the primary and secondary side of an MV 130kV to LV 2kV transformer if I had no markings and no measurement apparatus?

 

[Mr Evil]

I'm guessing they're talking about huge transformers where you can't see the windings, so you can't just look at the thickness of the wire. Maybe the size of the insulators at the output terminals? Or you could try connecting it up - if you get it the wrong way around it will explode.

 

[Pick2]

... Maybe the size of the insulators ...

Yes , I think you nailed it , Mr Evil

 

[Red Squirrel]

The question is kinda vague, if they just want you to know which side is lower voltage and which side his high, then you can probably go by the leading cable size and the isolator size. The high voltage side will have much larger isolators and thinner wire. The low voltage side will have thicker wires.

For pole transformers the low voltage side will often have insulated wires too as those are also going into the homes. You can also go by the height of the wires on the pole. The high voltage ones will be much higher (usually the very top), either single, or in groups of 3 (3 phase). The pole transformer will tap a single phase then have 3 conductors out to a lower cable span, and then taps to the houses.

 

[MrDudeMan]

The question is kinda vague, if they just want you to know which side is lower voltage and which side his high, then you can probably go by the leading cable size and the isolator size. The high voltage side will have much larger isolators and thinner wire. The low voltage side will have thicker wires.

For pole transformers the low voltage side will often have insulated wires too as those are also going into the homes. You can also go by the height of the wires on the pole. The high voltage ones will be much higher (usually the very top), either single, or in groups of 3 (3 phase). The pole transformer will tap a single phase then have 3 conductors out to a lower cable span, and then taps to the houses.

It's probably about current, not voltage. High voltage doesn't dictate a conductor size, but high current does. The higher voltage side of the transformer will have thinner wires because there will be less current. The total power through the transformer (ignoring losses, which could be significant) should stay the same, which means high V * low I = low V * high I. Low current = thinner wire. High current = thicker wire.

 

[MrDudeMan]

Yes , I think you nailed it , Mr Evil

Insulator size is another possible answer on top of conductor size. Either would be correct imo with insulator size potentially being more obvious depending on the material.

 

[Red Squirrel]

It's probably about current, not voltage. High voltage doesn't dictate a conductor size, but high current does. The higher voltage side of the transformer will have thinner wires because there will be less current. The total power through the transformer (ignoring losses, which could be significant) should stay the same, which means high V * low I = low V * high I. Low current = thinner wire. High current = thicker wire.

It was implied but I should have mentioned that. Given any transformer the current on the high voltage side will be lower than the low voltage side as the amount of watts is the same. (roughly)

The lower the current, the less conductor losses you get too, which is why they boost the voltage up for long distances.

You should see the size of wires and bus bars in a 48vdc telcom setup. You won't get a shock touching the bare bus bars because the voltage is too low for any current to be delivered to you, but the currents going through are in the thousands of amps.

 

[who?]

oops.

 

[MrDudeMan]

It was implied but I should have mentioned that. Given any transformer the current on the high voltage side will be lower than the low voltage side as the amount of watts is the same. (roughly)

The lower the current, the less conductor losses you get too, which is why they boost the voltage up for long distances.

You should see the size of wires and bus bars in a 48vdc telcom setup. You won't get a shock touching the bare bus bars because the voltage is too low for any current to be delivered to you, but the currents going through are in the thousands of amps.

Heat loss becomes trivial once the voltage is above a certain threshold. The voltage level for carrying bulk power is decided by transmission distance and the surge impedance load. Lower heat loss is a bonus, but it's not the primary motivation.

 

[lakedude]

Red Squirrel's conductor size method would work every time for any large commercial transformer (for ampacity reasons as MrDudeMan pointed out). The only time this method might fail is if someone made a homemade transformer and used the same size wire on both sides, perhaps out of wire they already had laying around (this would most likely be a much smaller and lower voltage transformer).

I don't know if this was mentioned but the high voltage side would also have many more "turns" of wire than the low voltage side. The exact ratio should be easy to determine from the voltages. Yes the high voltage side will have 65 times as many turns as the low voltage side. Even if you could count the exact number of turns, a ratio of 65 to 1 should be pretty easy to spot.