Open Circuit Protection - 3 phase AC

kappa_am said:

Suppose that the time one phase is crossing zero, the combination will be active active deactive--->fault. while the system is ok and just current of one of the phases is at zero crossing situation.

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You have to qualify the "un-active" phase.
The simplest is if the "zero crossing" lasts longer than say half a cycle (8.3333 ms) then it is a legitimate fault.
To prevent nuisance tripping, you can requalify for several consecutive cycles. If it remains at zero for 10 consecutive cycles, then it isn't a glitch but a real fault.

kappa_am said:

Suppose that the time one phase is crossing zero, the combination will be active active deactive--->fault. while the system is ok and just current of one of the phases is at zero crossing situation.

Click to expand...

What you need to do is to process the secondary voltage induced- just like a clamp on meter does. You rectify the voltage, filter it, and clamp it to a logic level. If the phase is carrying current, the logic will be one else it will be zero. When you filter it, you are averaging it over several cycles, certainly more than one. The load you put after the capacitor filter tells the time constant that will determine the response time of the system. It will be good to keep the response time two or more periods for reliable operation.

c_mitra said:

What you need to do is to process the secondary voltage induced- just like a clamp on meter does. You rectify the voltage, filter it, and clamp it to a logic level. If the phase is carrying current, the logic will be one else it will be zero. When you filter it, you are averaging it over several cycles, certainly more than one. The load you put after the capacitor filter tells the time constant that will determine the response time of the system. It will be good to keep the response time two or more periods for reliable operation.

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That is an analog, very valid way of achieving the same result as I described in the previous thread.

When you filter it, you are averaging it over several cycles, certainly more than one. The load you put after the capacitor filter tells the time constant that will determine the response time of the system. It will be good to keep the response time two or more periods for reliable operation.

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Same as proposed method by FvM.
How about this method:
We measure all three phases (instantaneous values), when a phase is crossing zero, if one of the other phases is zero, we have a fault. Seems fairly rapid and cheap compared with averaging...
What's your opinion?

Hi;
I thought this method through. there are some question:
1- in rectifying stage, isn't voltage drop of the diodes problematic in low currents?
2- how we determine the resistor paralleled with the capacitor when our AC waveform frequency varies from 1Hz to over 80 Hz?
Thank you.

1- in rectifying stage, isn't voltage drop of the diodes problematic in low currents?

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You really do not need power diodes for this purpose; you can use any general purpose small signal diodes that can rectify currents in the range of uA and have reverse currents of nA. Voltage drop of the diodes will not matter for the current; it will simply reduce the output voltage. You cannot use these diodes to rectify voltages that are much less than 1V.

2- how we determine the resistor paralleled with the capacitor when our AC waveform frequency varies from 1Hz to over 80 Hz?

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There is no need to change the resistor load on the current transformer as the frequency change is rather small. However, please ensure that both the CT and VT will work reliably at 1Hz. If they are specified to work at 50Hz, they will work fine in the 30-70Hz range but I am not at all sure about 1 Hz.

Common, of the shelf transformers won't go down to 1Hz.
They won't even go down to 20Hz.

At 1 Hz the magnetizing current will be 50 or 60 times the nominal.
If you wan't to go down to 1 Hz, you'll have to use hall sensors or resistors.