Hello experts! I am trying to figure out how the circuit would like in a 'short circuit' event. I am reading the following text book which refers to current and voltage relays connected to secondary sides of current transformers; how is it that 1600A (current due to short circuit) flows through both the CT and the relay? Our relay coil is connected to the secondary of the CT; therefore, primary current is one value and secondary current is another value. In case of a short circuit event, what happens to this model? If someone can model this circuit with the return path I would very appreciate your help. Please refer to image attached below.

 

You may want to give Transmission line protectiona read and watch the little videos which are quite good and informative. The video does the modeling for you in a nice and concise as well as easy to understand way. The current transformer secondary may or may not use the relay coil as a burden resistance. Many involve more sophisticated electronics but the end result is the same as in the transmission like is broken (switched open) when the current is excessive. Some systems will "try again" after a delay and if a short condition still exist then they open until service. The CT itself can be small or rated for thousands of amps.

Ron

 

You may want to give Transmission line protectiona read and watch the little videos which are quite good and informative. The video does the modeling for you in a nice and concise as well as easy to understand way. The current transformer secondary may or may not use the relay coil as a burden resistance. Many involve more sophisticated electronics but the end result is the same as in the transmission like is broken (switched open) when the current is excessive. Some systems will "try again" after a delay and if a short condition still exist then they open until service. The CT itself can be small or rated for thousands of amps.

Ron

Hi Ron,
Thanks, I will take a look. However, it would be interesting to see how the circuit looks like in this particular case, from the book example. If anyone could draw it out I would very appreciate it.

 

It would look a little like the below which is not to scale. The idea being as we travel further down a transmission line the line resistance increases. So if a transmission line carries for example 500 Amps any short to ground along the line will cause an increase in monitored current on the current transformer primary resulting in an increased current in the secondary. The series resistors just represent increasing line resistance. There are switches to ground which are switched at points in time.



V2, V3 and V4 are are just signals to turn the shorts to ground on and off.The normal example current is a little over 2 Amps but turning on SW1 Note the current increase. SW2 has less errect further down the line and finally SW3 has even less effect. The cirsuit approximates what you posted and what is covered in the link I provided.

Ron

 

It would look a little like the below which is not to scale. The idea being as we travel further down a transmission line the line resistance increases. So if a transmission line carries for example 500 Amps any short to ground along the line will cause an increase in monitored current on the current transformer primary resulting in an increased current in the secondary. The series resistors just represent increasing line resistance. There are switches to ground which are switched at points in time.

View attachment 157883

V2, V3 and V4 are are just signals to turn the shorts to ground on and off.The normal example current is a little over 2 Amps but turning on SW1 Note the current increase. SW2 has less errect further down the line and finally SW3 has even less effect. The cirsuit approximates what you posted and what is covered in the link I provided.

Ron

Ron,
Thank you very much, with this explanation the concept is clearer to me now.
If I will have further questions, I will contact you.