Electronics > Projects, Designs, and Technical Stuff
Test winding in zero sequence current transformers
floobydust:
That circuit would not work, it always presents a load to the test winding as C2 & R2.
You want to open-circuit the test winding drive otherwise you wreck the CT's sensitivity because it always sees R2 loading.
I use square-wave DC drive excitation with tri-state tiny logic 74LVC2G125.
Normally your main winding has an RC filter to deal with EMI so the (test) square-wave fast edges are not a big problem. Although some CT's have a ton of inter-winding capacitance.
I also add transient protection as the winding will generate high voltage under fault conditions, and when the load contactor turns on/off as all 3-phases are never simultaneously switched. I use no burden resistor across the test winding.
Red_Micro:
--- Quote from: floobydust on March 16, 2020, 12:04:12 am ---That circuit would not work, it always presents a load to the test winding as C2 & R2.
You want to open-circuit the test winding drive otherwise you wreck the CT's sensitivity because it always sees R2 loading.
I use square-wave DC drive excitation with tri-state tiny logic 74LVC2G125.
Normally your main winding has an RC filter to deal with EMI so the (test) square-wave fast edges are not a big problem. Although some CT's have a ton of inter-winding capacitance.
I also add transient protection as the winding will generate high voltage under fault conditions, and when the load contactor turns on/off as all 3-phases are never simultaneously switched. I use no burden resistor across the test winding.
--- End quote ---
Ok, Thanks. So to fix the last circuit, I have to open-circuit the test winding drive, right? Would it work to set the micro port at high impedance? I forgot to mention that the test winding has a dc resistance of 80 ohms. I have to be careful on the max current capability of the micro or opamp.
Circlotron:
--- Quote from: Red_Micro on March 15, 2020, 01:50:10 am ---What do you think of the idea below? A PWM to a low pass filter to get a sinusoidal wave and the amplifier with the FET to inject a 20 mA 50Hz to the test winding?
--- End quote ---
A better method might be to use an audio power IC instead of the opamp + mosfet combination, but same general principle, i.e. feedback from that current sense resistor.
Edit -> If you only need 20uA test signal then an opamp all by itself would be plenty. No mosfet needed.
Edit 2 -> oops! 20mA not 20uA. Anyway, still achievable with the right opamp.
floobydust:
I would say you can't drive the test winding directly with an MCU unless its substrate diodes can withstand overload with CT saturation under an unbalanced fault, like a line-ground short at the load. The CT can then see >100's amps or more depending on the size/voltage of the system, until a breaker trips. So the test winding circuit can see a surprise. The two drive resistors do lessen the surge current but I'll add clamp diodes. Tri-stating the MCU outputs would work.
The design I did used H-bridge '125 from 3.3V with 1k ohm drive to a 100:1 test winding around 4 ohms, I think giving around 3mApk drive but it was a high current trip test.
Red_Micro:
One additional question... The resistance of the winding (measured with a meter) is about 80 ohms. As the turns ratio between the test and secondary winding is 1:1, would the burden resistor across the secondary represent a load for the test winding?
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