CT Saturation Error Correction Within Merging Units Using Dynamic State Estimation

Current transformer (CT) saturation errors can pose serious concerns for metering and protective relaying functions. Protective relays depend on the fidelity of Protection-Grade CT instrumentation channel outputs to perform reliably, and meters similarly rely on Metering-Grade CTs for accurate data. The paper presents a new online method for estimating the CT primary current using the Unconstrained Weighted-LeastSquares (UWLS) Dynamic State Estimation (DSE) algorithm. The method can potentially be implemented in real-time in the merging unit (MU). The MU can then broadcast the estimated primary current in a sample-by-sample format to networked devices. The CT primary current is among multiple state variables dynamically estimated using this method. The paper also presents a new and simple algorithm for near-instantaneous CT saturation detection, based on using the dynamically estimated flux linkage.