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.