Why Is Current Transformer Secondary Short Circuited?

>>Colin et al,

I have a similar question but with a little twist...

What happens to the inductance on the primary side when one of the turns is shorted on the secondary side...ie lets say you have 10 turns on primary and 100 turns on secondary and you solder turn 50 and 51 together (not to ground or anything else just each other). What happens?
 
Ali Ali... very good question. Essentially there are two ‘negative’ results for the example you cited:

1) The turns-ratio Nsec/Npri will be reduced by one turn, that is, from the normal 10:1 to 9:1. This of course will affect measurement accuracy!

2) More importantly, transformer-action will magnetically couple the 'shorted-turn' to the other 99 turns having and effective turns-ratio of 99:1. The resultant induced current in the single loop will produce localized heating that will cause additional damage to insulation, winding, dand core!

The above discussion ignores other exacerbating factors such as frequency, current wave-shape (if not sinusoidal) and capacitance, if present.

Regards, Phil Corso
 
P

Patric Bernard

> CT secondary will develop a very high voltage when secondary is open circuited with the primary energised. But if the
> CT secondary is kept open initially and the primary is energised, will the CT get damaged?

Depending on how long the secondary is kept open, not necessarily. This is exactly how Spark Plugs work in a car, and have worked for... well almost forever. The Secondary coil of the ignition coil, or the distributor, opens, causing a high voltage build up in the Primary, When the Secondary closes again,the extremely high voltage transfers to the secondary, and finally across the spark plug gap to ground, causing the spark in the spark plug, and that happens to the spark plugs/coil thousands of times a minute in some cases.
 
M

M.jagan kumar

> The current transformer secondary is short circuited during the operation.

ANS:. If the transformer secondary is not short circuited, during the operation - it will opened means, it damage the coil and burnt out. It will affect the surrounding peoples also. It is very dangerous.
 
All comments where very helpful. In addition, i think one can also use the transformation formula V1/V2=I2/I1 to quickly asses the secondary voltage V2 which is equal to V1xI1/I2. The answer is very obvious: if I2 is zero, the result is infinity. thank you.
 
With this, how do you explain the fact that the secondary of a PT should always be kept open? :)

> All comments where very helpful. In addition, i think one can also use the
> transformation formula V1/V2=I2/I1 to quickly asses the secondary voltage V2
> which is equal to V1xI1/I2. The answer is very obvious: if I2 is zero, the result is infinity.
 
Though the secondary voltage is very very high, it is not infinity and there is a correct way to calculate it. Thanks.

> All comments where very helpful. In addition, i think one can also use the transformation formula V1/V2=I2/I1 to
> quickly asses the secondary voltage V2 which is equal to V1xI1/I2. The answer is very obvious: if I2 is zero, the
> result is infinity. thank you.
 
Well said Colin!

I must make one correction though, you stated "Therefore, from V=IR, the voltage developed across the primary winding is approximately zero when the secondary is shorted." which is a bit off. There will be no voltage developed "across" the primary winding of the CT whether the secondary is shorted or not. There will be voltage measured to ground of course, but since the primary "winding" of a CT is simply the current carrying conductor that passes ONCE through the window of a CT you will measure ZERO voltage from any place on this conductor to another on the same conductor, energized or not. I dislike the term "winding" for the primary of a CT because it is not wound at all, (unlike power transformers) and is therefore confusing, I prefer primary current to be measured and secondary winding.

Other than that small detail, nice work!
 
J
> The current transformer secondary is short circuited during
> the operation. What is the reason, and explain with details.

look at it like this:
a VT is a constant voltage source
open circuit- normal voltage (voltage across terminals) normal current when load applied

Short circuit - unsafe "huge currents across short"
loads must be connected in parallel not series

A CT is a constant current source
Open circuit - unsafe - huge voltages
Short circuit - safe (small volts constant current
Loads must be connected in series
 
Basic formula, V=IR.

Voltage is directly proportional to resistance with constant I. Remember that voltage also known as potential energy (voltage drop only happen when there is difference in voltage between two point!). So when we short the wires, there is no resistance exist (or negligible value resistance of wire). Meaning no voltage occur between the joined wire.

> why voltage not high when we short the C.T secondary side?
> Pls explain with principle.
 
I was once given the task of testing some new MCC feed breakers, the owner wanted to see them trip by injecting a high current directly across the busbars, this we did using a Variac and a 1KVA transformer rewound with just one turn on the secondary.

It all went very well until we came to one breaker where we couldn't get enough current through the busbars to exceed the breaker rating. It turned out one of the metering CTs was wired wrong and the secondary was open. There was enough impedance in the busbar primary of the CT that we couldn't get enough current through it.

If you think about the CT ratio e.g. 500:5, if you put 100 Volts across the primary you should be able to get 10,000 Volts on the secondary. I know that's pretty simplistic and completely disregarding losses but you get the idea.
 
You are missing a very important fact. In ignition coil the voltage raises tremendously additionally because the speed of contracting magnetic field due the presence of condenser connected in parallel to the contacts (old systems) or solid state switches (modern system) when they open.
 
How about the case, if CT has a test conductor (current injection through CT window) and shorted on terminals, turn ratio and accuracy will change on the secondary? and is that also necessary to short the terminals of such test conductor or leave open?
 
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