Signal Processing for High Impedance Differential Protection Schemes

High Impedance differential protections (87Z) schemes for bus and transformer Restricted Earth Fault (REF) applications are common in the industry because of their security, dependability, and simplicity. 87Z schemes consist of a dedicated set of CTs, one per terminal, with all the secondary windings connected in parallel and terminated at a resistor, referred to as a stabilizing resistor, and MOVs. The interaction of these components results in voltage and current signals that deviate from typical power systems signals. In this paper, we present data from testing at a 2 MVA test facility and draw conclusions from the effect of these distortions on the signal processing of protective relays. We show that these effects are limited to a narrow range of fault currents. We also present signal processing and logic for the 87Z scheme that is secure and dependable.