Closed-Form Screening of Phase-Overcurrent Relay Blinding and Sympathetic Tripping for Balanced Faults in PV-Rich Radial MV Networks

Preprint notice: This manuscript was submitted to IEEE Transactions on Power Delivery on August 15, 2026 (manuscript under review), and has not yet been peer reviewed. The reproduction package (code and data) is openly archived on Zenodo at https://doi.org/10.5281/zenodo.21366816. Abstract: Current-limited photovoltaic (PV) inverters change the fault currents on which overcurrent protection of radial MV networks was coordinated, degrading relay sensitivity, backup speed, and selectivity. This paper derives an exact closed-form expression for the fault-period current of any series branch of a radial feeder under multiple current-limited PV injections with arbitrary phase angles, using the bus impedance matrix and the protected-branch series impedance. A nonnegative screening index derived from the real component of the PV-induced current perturbation is proven to upper-bound relay-current reduction, yielding a sufficient relay-pickup guarantee and a worst-case coordination-margin bound built from two-sided branch-current intervals, false-negative-free by design. Cross-platform OpenDSS studies over three disjoint sets — calibration, validation with pre-fault PV representation and load-model sensitivity, and a third set with all three protected branches instrumented — selected an asymmetric allowance of −4/+6% of the no-PV branch current that enclosed every observed ratio error. On the IEEE 33-bus feeder with power-flow-based settings, the screen applied to 4,608 pair configurations produced 34 conservative flags (0.75% false-positive rate); 62 configurations lost backup pickup, none violated selectivity, and the worst backup slowdown was 1.64 s. A sympathetic-tripping bound with a tiered uncertainty envelope completes the screen.