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For power current transformer, the secondary is not allowed to open when the primary is conducting current of several KAs. So I want to design a over voltage protection circuit for this situation, when there is over voltage (several KVs), then short out the secondary of the CT.

I want to use GDT or MOV for this purpose, but all they labeled as "transient". Is it suitable to use them under power frequency?

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    \$\begingroup\$ Check the long term power dissipation. Generally we protect the output of a current transformer with antiparallel silicon diodes, as very little voltage is needed, so the power dissipation is low, even at high current. If you really want to clamp your output at high voltage, then you will have to deal with a lot of power. \$\endgroup\$ – Neil_UK May 26 '17 at 7:44
  • \$\begingroup\$ Yes, for some reason, I must leave the secondary of the CT open, until the voltage beyond some value. I want to check the voltage on MOV, if it beyond some value, then close a relay to short out the secondary of the CT. But, apparently, there will a delay from the over voltage to the shorting out action. \$\endgroup\$ – diverger May 26 '17 at 8:56
  • \$\begingroup\$ Why must you leave the secondary unconnected to a burden. It fails to be a CT when the secondary is open circuit and the core can saturate and burn. \$\endgroup\$ – Andy aka May 26 '17 at 9:43
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A current transformer has to be connected to burden resistor or short circuit. Your problem can't have a solution, since if you cannot ensure a burden resistor to be connected all the time, how can you ensure that MOV will be? Connect the burden resistor as you plan to connect the MOV and the problem is solved.

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  • \$\begingroup\$ Yes, for some reason, I must leave the secondary of the CT open, until the voltage beyond some value. I want to check the voltage on MOV, if it beyond some value, then close a relay to short out the secondary of the CT. But, apparently, there will a delay from the over voltage to the shorting out action. \$\endgroup\$ – diverger May 26 '17 at 8:54
  • \$\begingroup\$ Then use a two zener diodes mounted on heatsink. But it is not clear if this circuit will have a burden resistor as well or it is just some test equipment with voltage limiting device and short circuit relay? Anyway, what's the purpose of this circuit. \$\endgroup\$ – Marko Buršič May 26 '17 at 9:35

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