Larger Transformers having smaller impedance

[mull982]

I was looking at some one-lines lately and noticed that most large utility transformers have a smaller impedance compared to smaller LV distribution transformers. It appears that most 13.2kV - 480V transformers have an impedance of 2.5% or so while most 480V-240/120V units have an impedance of about 4-5%. Does anyone know the reason for these impeance differences?

Also I have read somewhere that a 480V-480V isolation transformer will have a higher impedance than a 480V-120V transformer of the same size. Is there any truth to this?

 

[jim dungar]

There are several inter-related reasons, so these are very, very simplistic answers.

Impedance is, effectively, losses in the transformer. Because the utility owns the transformer, they want as low of losses as possible, so they pay for a low impedance up front in order to have fewer losses over the units life.

Impedance can also affect the voltage regulation of the transformers. again, the utility is required to provide customers with a very tight voltage (usually +/- <5%), but inside the customer's facility equipment can actually tolerate +/-10%

Different construction methods, like being oil-filled, can impact the size and shape of the transformer and therefore its impedance.

 

[skeshesh]

To add to Jim's comments (again in simple terms), the customer side is generally less concerned with voltage drop and more interested in limiting the available fault current. With a higher imedapnce transformer on the load side, the customer is able to have a lower the available fault current which allows using equipment with lower short circuit withstand and OCPDs with lower interrupting capacity.

 

[G._S._Ohm]

most large utility transformers have a smaller impedance compared to smaller LV distribution transformers.

"Economy of Scale"? Larger "things" are more efficient than smaller "things".

These tiny wall transformers to charge your cell phone are about 25% efficient, if you compare the DC output with AC input.

And, the hp per pound for lawnmower engines is probably less than the hp/pound for a car engine.
E.g., per ton of payload, for a
car it's 100 hp
semi truck, it's 10 hp
locomotive, it's 1 hp.

That's why the car industry wants to, or wanted to, do away with commercial transportation and streetcars. . .?