Novel notions of zero injection property of buses in optimal PMU location with efficient observability enhancement focusing on security concepts

•Novel concepts in zero-injection buses (ZIBs) are introduced.•The reliability of fragile areas monitoring from overvoltage viewpoint is enhanced.•PMUs/redundant measurements are optimally located based on overvoltage risk sensitivity of buses.•New security concepts are introduced to model channel failure, PMU loss, and branch outage.•In the case of branch outage, selective security criteria are considered so as to enhance security of network observability against cascading failures.