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Test winding in zero sequence current transformers
Red_Micro:
Sometimes it is common to see a zero sequence current transformer with four output wires, i.e., the secondary winding and the test winding as shown in the attachment. What kind of signal is the test signal? Is it like injecting a current signal that it is induced in the actual secondary? I want to perform this test with a microcontroller, so what comes to my mind is kind of a voltage to current converter for the test signal?
Source of the picture: T. Novak, L. A. Morley and F. C. Trutt, "Sensitive ground-fault relaying," in IEEE Transactions on Industry Applications, vol. 24, no. 5, pp. 853-861, Sept.-Oct. 1988.
duak:
I'm not in the energy industry and don't have any hands on experience but was interested in your question as I sometimes diagnose industrial electrical problems. I understand the test winding is there for testing and calibration when an external source injects a test current. I expect for accuracy's sake, the CT is disconnected from the measurement system and the primary conductors. Various companies make equipment to do this: http://www.powermetrix.com/wp-content/uploads/2015/11/CT-Testing-with-the-PowerMaster.pdf
Here's a few other links to look at:
https://www.eng-tips.com/viewthread.cfm?qid=285242
https://www.eng-tips.com/viewthread.cfm?qid=331624
By the way, CTs are designed to work with a particular load resistance called the Burden. The amount of current in the secondary is a function of the primary current, the turns ratio between the primary & the secondary and the Burden resistance. The test winding will develop a voltage that is a function of the secondary voltage and the turns ratio between the two windings. I have a couple of industrial CTs and a 270 degree dial type meter. If memory serves, the secondary current is 5 Amps AC. Note also, that if the burden is disconnected, hazardous voltages can be developed on an open circuited CT secondary.
Red_Micro:
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?
duak:
Red Micro,
Perhaps you could give us more info on what you're trying to do here. I don't have access to the IEEE paper and can only conjecture on what you are trying to do.
Is this circuit supposed to be connected while the CT is part of the electrical measurement system? ie., are you trying to test the CT in situ? The voltage developed across the test winding is related to the Burden voltage and the turns ratio between the secondary and test winding. During typical operation there should be minimal voltage on the test winding but during a fault condition, there could be a large voltage developed there. This could damage the driver because it is likely a high voltage from a low impedance source. I would consider a relay to isolate the driver from the test coil except during test mode.
The driver will have a DC bias. At such currents it's unlikely to cause the CT core to saturate but I think you will find it difficult to drive enough AC signal into the test winding to get a sensible signal from the secondary. Here's a link to an article on typical industrial strength CT burden calculations: https://sdvelectrical.blogspot.com/2019/05/how-to-calculate-burden-of-ct.html If you're injecting 20 mA into the test winding and the turns ratio is 1:10 you will get 2 mA in the secondary. If the burden is one ohm you should see 2 mV. This should be recoverable if you're using synchronous detection ie., a Lock-in Amplifier. I would consider a more powerful driver, perhaps an H-bridge driven from the PWM port.
Vovk_Z:
--- 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 ---
There will be a large DC component so it must be AC current source only.
If you need 20 mA there is no need in PWM with MOSFET. Many opamps can do 20-50 mA, even up to 150 mA.
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