Current Transformer danger when secondary is left open

Discussion in 'General Electronics Chat' started by MrBuggy, Sep 21, 2014.

  1. MrBuggy

    Thread Starter New Member

    Nov 7, 2013
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    Hi guys,

    I've learned from my seniors that when the secondary of CTs it poses a huge hazard (CT secondary voltage will shoot up and possibly destroy the device). I do not understand this concept as I know that its okay to leave the secondary of normal power transformers. Can anyone shed light into this issue?

    Sincerely,
    MrBuggy
     
  2. MaxHeadRoom

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    Jul 18, 2013
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  3. t_n_k

    AAC Fanatic!

    Mar 6, 2009
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    I've always assumed the danger is to people who may be (inadvertently) shocked or electrocuted by the open CT secondary. Measuring CT's are considered as isolated from the active voltage line which the CT is monitoring. The temptation may be to think they are therefore intrinsically safe on the secondary side.
    One wouldn't intentionally touch the open or unloaded secondary of a live power transformer. One may however think the CT is safe because it is not directly connected to the active conductor. Beware of unforseen hazards.
     
    Last edited: Sep 21, 2014
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  4. MrBuggy

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    Nov 7, 2013
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    For Transformers in general I thought voltage comes first and then the current (the same for current transformers, the magnetic flux created by the AC current should induce a voltage in the secondary winding, only then when there is a load a current will flow and be provided). So in the case of the Secondary winding of the current transformer there is already a high voltage across the terminating leads whether there is a low impedance ammeter or not.
     
  5. MrBuggy

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    Nov 7, 2013
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    Here attached is a snippet from the book Power Systems Analysis and Design. The steps outline how to determine the missing parameters. In a way I disagree to this as Step 1 starts with a known current rather than a known secondary voltage. In electromagnetics theory we have Lenz law that states there will be voltage when there is a change in magnetic flux instead of a current.
     
  6. #12

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    Nov 30, 2010
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    The whole point of a current transformer is that it transforms current, not voltage. That's why it has a different name. If 500 amps flows through the core of a 500:1 current transformer, one amp is going to try to come out the secondary, and it will develop as many volts as necessary to do that.

    It's like trying to turn off a relay coil. The magnetic field has to collapse, and that energy has to go somewhere, even if it has to arc through the transistor that tried to shut it off.

    You just have to accept that this one works, "backwards". Do the math for the current and the voltage will fall into place.
     
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  7. Brownout

    Well-Known Member

    Jan 10, 2012
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    There isn't any such thing as voltage first or current first. Voltage and current exists concurrently in any circuit. As a thought experienme, consider the following circuit: A current source of xA connects to a resistor of 1ohm. A voltage of 1V is measured across the resistor. Can you determine the value of the current source from the voltage measurement?

    Now consider this: a wire loop is connected to a 1 ohm resistor, and the magnetic flux and number of turns are such that 1V is produced at the ends of the coil (per Lenz) Is there current flowing in the resistor currently with the voltage produced? Can you determine the voltage by measuring the current?
     
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  8. subtech

    Senior Member

    Nov 21, 2006
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    There is no magic here.

    The CT transforms current, according to its ratio, primary to secondary, and the CT secondary voltage varies in direct proportion to the magnitude of the connected impedance. (up to it's design limit)

    The PT transforms voltage, according to its ratio, primary to secondary, and the PT secondary
    current varies in inverse proportion to the magnitude of the connected impedance.

    A careful and detailed study of the physical relationship of windings to the core of each device
    may be helpful.
     
    Last edited: Sep 22, 2014
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  9. MrBuggy

    Thread Starter New Member

    Nov 7, 2013
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    There's a lot of things out there that is just the way it is, including CTs I guess. In my head, it doesn't matter where the flux comes from when it goes through the coils, whether the flux comes from the primary of a PT or the primary of the CT. A potential must always be induced in order to balance the flux inside the coils as much as possible, ie if the external flux is increasing inside the coils there will be induced flux such to decrease this phenomena. The concept of current transformation is still difficult for me to understand :(.

    I get what you are saying. But what then would happen to a secondary of a CT? If there is infinity impedance (open circuit), there should be a finite amount of voltage across the secondary.

    I have done this through Lenz Law. Voltage is produced across terminals when there is a changing flux inside a coil. So shouldn't it be safe to have the secondary open? It will be a high finite voltage, yes, but it shouldn;t destroy the CT...
     
  10. #12

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    Well...infinity is the theoretical limit if there was no impedance in the windings and there was no insulation failure inside the current transformer and there was no stray capacitance. Let's just say it tries to get to infinity. ;)

    With a common inductor, energy = 1/2 LI^2. If you try to stop the current instantly, the induced voltage would approach infinity except it finds stray capacitance to charge up to the tune of energy = 1/2 CV^2 and some of the energy is destroyed as heat in the resistance of the winding, and if that isn't enough it commits suicide by arcing in the coil. Much like a bullet, you can give it somewhere safe to go or get knocked on your @ss. :D
     
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  11. crutschow

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    There really is no difference between an ideal voltage and and ideal current transformer in theory. In each one the secondary voltage is equal to the primary voltage times the turns ratio and the secondary current is equal to the primary current divided by the turns ratio.

    So lets take an ideal 1:100 transformer (it's immaterial whether it's called a current or voltage type). Then if you have 1A going through the primary, there will be 10mA going through the secondary. If the secondary has a 1k load then the secondary voltage drop will be 10V and the primary voltage drop will be 100mV. Now suppose you open circuit the secondary. The secondary voltage will rise as determined by the available primary source voltage and the turns ratio. If the primary source generating the current is 110V then the secondary peak voltage will rise to 110V x 100 x 1.4 = 15,400V :eek: which is obviously a very dangerous voltage and will likely arc over and break down the secondary insulation. That's why you don't ever want to open circuit the secondary of a transformer used to measure current.
     
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  12. HighVoltage!

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    Apr 28, 2014
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  13. HighVoltage!

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    Nice explanation! However, where did the 100 & 1.4 from (110 X 100 X 1.4) come from? Is 100 the turns ratio? How about the 1.4?
     
  14. crutschow

    Expert

    Mar 14, 2008
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    Yes, 100 is the turns ratio and 1.4 is simply the peak voltage of a sinewave whose EMF is given in RMS volts.
     
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