Hi folks.
Here we are going to talk about a very important practical aspect of power system engineering.
Q. Why secondary of the Current transformer is never kept open or should not be opened?
Let us first see what is current transformer. It has same working principle (self and mutual induction) as that of voltage transformer. I want to measure the AC current flowing through a conductor. For that purpose, I put a circular magnetic core around it. I wrap a wire on that core. Now the rate of change of flux in the core in responsible for the induced EMF in the wrapped wire. That flux will be produced by the straight conductor carrying current. So the current flowing through the wrapped wire is giving me the reflection of the current flowing through the straight conductor. This is nothing but current transformer.
To have the transformer action, the flux produced by primary (straight conductor) should be opposed by the flux produced by the secondary (wrapped conductor). If we open the secondary of the Voltage transformer (traditional transformer, you know from 12th), the primary current is very small (no load current). So the flux produced in the core is very small and will not cause any undesirable effects.
For a current transformer, The current in primary is not depending on the current in secondary. It is depending on the load connected across it. So when secondary is opened, The opposing flux produced by secondary is vanished. So the only flux present in core is flux due to primary and it is very high. This large flux induces extremely high voltages across open secondary. This voltages may rise to several KV that is enough to damage any equipment, or the insulation or in worst cases to kill the operator.
You can also understand this as CT always tries to maintain constant secondary current. When there is some resistance across its secondary, (typical relay coil resistance is 0.5 Ohm) Small voltage is enough to make flow rated current. When the secondary is opened, To make flow the same current through high air resistance, the CT develops high voltages across it.
So there is spacial care taken provide a shorting switches across the secondary of the transformer. Many relay constructions have a spring operated shorting switch that keeps secondary of CT shorted when the body of relay is removed form its plastic casing. Please keep this thing in mind. Just for information, The voltage developed by a protective CT should be greater then the voltage developed by metering CT. We will soon talk about Difference between these two as it is interesting to see how hardware construction changes to satisfy peculiar purpose.
Thank you for your time and please feel free to leave comments about your views. You can also mail me at dnachiketa1010@gmail.com
Here we are going to talk about a very important practical aspect of power system engineering.
Q. Why secondary of the Current transformer is never kept open or should not be opened?
Let us first see what is current transformer. It has same working principle (self and mutual induction) as that of voltage transformer. I want to measure the AC current flowing through a conductor. For that purpose, I put a circular magnetic core around it. I wrap a wire on that core. Now the rate of change of flux in the core in responsible for the induced EMF in the wrapped wire. That flux will be produced by the straight conductor carrying current. So the current flowing through the wrapped wire is giving me the reflection of the current flowing through the straight conductor. This is nothing but current transformer.
To have the transformer action, the flux produced by primary (straight conductor) should be opposed by the flux produced by the secondary (wrapped conductor). If we open the secondary of the Voltage transformer (traditional transformer, you know from 12th), the primary current is very small (no load current). So the flux produced in the core is very small and will not cause any undesirable effects.
For a current transformer, The current in primary is not depending on the current in secondary. It is depending on the load connected across it. So when secondary is opened, The opposing flux produced by secondary is vanished. So the only flux present in core is flux due to primary and it is very high. This large flux induces extremely high voltages across open secondary. This voltages may rise to several KV that is enough to damage any equipment, or the insulation or in worst cases to kill the operator.
You can also understand this as CT always tries to maintain constant secondary current. When there is some resistance across its secondary, (typical relay coil resistance is 0.5 Ohm) Small voltage is enough to make flow rated current. When the secondary is opened, To make flow the same current through high air resistance, the CT develops high voltages across it.
So there is spacial care taken provide a shorting switches across the secondary of the transformer. Many relay constructions have a spring operated shorting switch that keeps secondary of CT shorted when the body of relay is removed form its plastic casing. Please keep this thing in mind. Just for information, The voltage developed by a protective CT should be greater then the voltage developed by metering CT. We will soon talk about Difference between these two as it is interesting to see how hardware construction changes to satisfy peculiar purpose.
Thank you for your time and please feel free to leave comments about your views. You can also mail me at dnachiketa1010@gmail.com
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