Fast, In Situ Demagnetization Method for Protection Current Transformers

This paper proposes a fast,
in situ
demagnetization method for protection current transformers (CTs). Simulations show that the method can automatically demagnetize a CT within 300 ms
in situ
, i.e., while the CT is deenergized but still connected to the substation. Therefore, unlike existing methods that require the CT to be disconnected from the substation, this new method can be used to demagnetize a CT during a short interruption, e.g., the time it takes an autorecloser to operate, rather than requiring an extended outage of the substation. This will dramatically increase the number of opportunities for the demagnetization of protection CTs, which will alleviate the threat of remanent flux in the CT from causing saturation and compromising the performance of power system protection. Furthermore, by demagnetizing the CT
in situ
, the method avoids the risk of the improper reconnection of the CT, creating a source of hidden failure. The new method uses the existing dc voltage source in the substation to charge the CT and a parallel-connected inductor. An automatic control scheme is presented that manages the charge/discharge of the inductor, and thereby the demagnetization, by controlling three switches that reconfigure an
$RLC$
circuit. The additional hardware required by the method is the three switches, the switch controller, and the
$RLC$
circuit. The new method is tested for different hysteresis characteristics, levels of remanent flux, and errors in the assumed element parameters and measured hysteresis characteristics using EMTP-RV simulations. The results indicate that the proposed method can quickly and effectively demagnetize a protection CT
in situ
.