Transformer Voltage in case of Short-Circuiting

[QwertyXP]

Consider a distribution transformer: The primary winding (HV) has a nominal voltage of 14.4 KV, whereas secondary (LV) has a nominal voltage of 120 V. One terminal from each of the primary and secondary windings is solidly grounded. A HV line (14.4 KV) is connected to the primary.

Now, if the HV line is short-circuited with the LV line, what will the voltage be across the secondary windings?

According to the book I have, the voltage will not be much larger than 120V. I find this very surprising. After the HV and LV lines are short-circuited, the potential difference across secondary should also have been 14.4KV, not a bit larger or smaller!

 

[QwertyXP]

I think I have the answer already. The book was probably only referring to normal situations. The voltage between a terminal and ground can be slightly higher than 120V because of the resistance of ground wire.

 

[Baluncore]

When the primary to secondary circuit short is made, the transformer will have what is effectively several turns shorted that will saturate the core. This will remove voltage control on the secondary so the voltage on the secondary circuit will rise close to the HV and destroy LV equipment connected to the secondary circuit. The HV fuse will then open, but only after LV equipment has been damaged.

This happens when LV cables are on the same poles below the HV distribution cables. When a cable breaks and drops onto the LV wires you need a good insurance policy. You can tell when this has happened with a 3 phase MEN system because one third of the houses in the street have damaged equipment. Those were the houses that were powered by the LV phase that shorted.

 

[QwertyXP]

Thank you Baluncore!

 

[alphy]

I would like to clarify that the voltage across the secondary windings in this scenario will not be exactly 14.4 KV, but it will also not be significantly larger than 120V. This is because a short circuit in the transformer causes a significant increase in current flow, leading to a drop in the voltage across the transformer.

The voltage across the secondary windings will depend on the impedance of the transformer, which is the ratio of the voltage to the current in the windings. In the case of a short circuit, the impedance decreases due to the increased current flow, resulting in a decrease in voltage across the transformer.

Therefore, it is expected that the voltage across the secondary windings will be slightly higher than 120V, but not as high as 14.4 KV. This is a normal behavior of a transformer during a short circuit and is a result of the transformer's design and construction.