Why current transformer secondary side short circuit?

Answer 1

A current transformer (or CT) is constructed in a specific way so as to step down the current in a high power circuit for measuring or protective relaying purposes. Typically, it will have a toroidal-shaped iron core with the secondary windings wrapped around it. the primary "winding" is usually the conductor of the main power system passing directly through the hole in the center of the CT. As a result of this construction, if the secondary windings are left open, a very large flux can develop, resulting in damage to the CT and possibly even the other equipment it is attached to. As such, if the CT is not going to be used, its secondary windings need to be shorted.

Answer 2

The voltage in the secondary of a CT is directly dependent on the amount of current flowing through the CT and the load (resistance) on the secondary side. If the load is infinite (open circuitted), the CT will attempt to build voltage up to infinite. This will cause arcing arcoss the open circuit at some point, and likely damage the CT.

Answer 3

This depends on source impedance, voltage level, and transformer impedance.

If you wish to assume zero source impedance (maximum short circuit currents), you can calculate a three phase fault by:

I = (V) / (1.732 x Zt x Zb)

I = the individual phase current

V = phase to phase voltage

Zt = the transformer percent impedance (usually between 4-20%)

Zb = scalar of base MVA used to calculate the percent impedance

Zb = V^2 / (BASE MVA)

So the (maximum) secondary current for a 115k/34.5kV 30MVA transformer, with a percent impedance of 5% at 30MVA is:

I = (34.5k)/(1.732x .05 x 39.675) = 10,041 A.

The above is just assuming no source impedance, and following ohms law, I = V/R. If values are not provided in percent impedance, this should be more straight forward (although primary and secondary impedance will then be different and must be provided separately).

Answer 4

A current transformer scales primary current to a lower value current at the secondary winding. It ideally will provide an exact secondary current reflection, such as 1/120th of the primary current. In reality, the transformer core can become saturated when the secondary is overvoltaged, which results in nonlinear representation of primary current. This over voltaging results from too high a resistance connected to the secondary of the transformer.

So you can infer from this that if you open circuit the secondary, and you have current flowing in the primary, the transformer will attempt to push current through the (open circuited) secondary, which will cause the transformer to saturate. This will also cause a large voltage build up on the secondary until it gets to the point it can arc across whatever is open circuited. If you talk with any sub techs that have been working for any serious amount of time, they will tell you "...you only open circuit a CT once."

This can also damage the CT. It's a good idea to test any CT that has been open circuited while primary current is flowing. The higher the primary current, the more likely it is the CT was damaged.

Answer 5

A: i am afraid of a voltage transformer is not a good description of a power converter That is what a transformer does it convert power [minus effefiency ] from one source to another. A shorted primary or secondary will eventually destroy the transformer if exceed the transformer power dissipation

Some transformer are used for specific purposes. Current Transformers are designed so they attempt to push a ratio of the current in the primary through the secondary. For this reason the secondary is typically loaded with a small resistance; an open circuitted secondary will cause the transformer to attempt to push the secondary voltage up to primary voltage potential (often thousands of volts); Never open the secondary of an active CT. A potential transformer, on the other hand, should not have the secondary shorted. A short will overload the transformer; they are designed to provide a reflection of the primary voltage, and so the secondary is usually loaded with a large resistance (so low current flow); this is why the secondary is typically fused with relatively small fuses.

Answer 6

If you open circuit a current transformer that has primary current flowing, what happens is the transformer attempt to push a percentage of the primary current through the now open circuited secondary; it won't be able to until the voltage on the secondary becomes large enough to allow current to jump across the short circuit. The voltage will continue to increase until it is large enough to jump across the short, or is near primary voltage, whichever occurs first.

Answer 7

if we open the CT secondary then there will be produced huge voltage to circulate the secondary current through air from secondary first terminal to 2nd terminal.it produce high voltage untill breakdown the insulated air. and when breakdown the insulated air then flashing will occure and that is harmful to us and device.

Answer 8

Depending on the application of the transformer used, the worst case can be described below:

Input voltage to transformer will go to zero. The primary input will immediately have high inrush current that will increase to maximum until the transformer overheats and the short burns open or until source is shut off.

The secondary output voltage will collapse to zero. Because the input voltage is at zero and in rush current is being applied only to the short in the primary winding of the transformer and there is no magnetic field to induce voltage or current to the secondary winding.

Another Answer

If only a few turns are short circuited, then the turns ratio will change and the secondary voltage will change too.

Answer 9

Hopefully a fuse blows, otherwise you will lose the PT (it will overheat, catastrophically fail, potential explode, etc.).

Answer 10

The transformer will saturate, and the secondary voltage will climb until it is high enough to allow arcing, or the CT fails, whichever comes first.