Merits of Application-Focused Merging Units

The implementation of process bus enabled protection systems has been met with increasing acceptance in the US Electric Utility market. This paper addresses inclusion of hardwired backup protection (BUP) in a Merging Unit (MU). While this can offer limited fallback if redundant Digital Substation (DSS) systems fail (relays, networks, MUs), it reintroduces the very problems IEC 61850 set out to solve. Impacts from MU embedded BUP can include: - Increased CAPEX for BUP functionality, increased size of MU enclosures and additional hardwiring - Increased OPEX for additional testing and maintenance considerations - Increased engineering and documentation for BUP and associated wiring - Possibility of vendor lock for BUP equipped MUs, present and future - Possibility of vendor specific software tools for BUP equipped MUs, which impacts overall IEC 61850 top-down engineering processes - Vendor neutrality issues for upgrades to IEC 61850 based system - Increased vendor dependence and firmware coupling - Increasingly difficult configuration control and traceability of protection intent across evolving platforms - Impacts of substation expansion that include BUP in MUs and inclusion of schemes and wiring, in particular hardwired breaker failure BUP - Coordination between Digital Substation protections using process/station bus and the hardwired MU embedded BUP - Selectivity of the MU embedded BUP

As usual in system protection, there are conflicts among the many goals of system design. The paper will discuss the above mentioned impacts of MU embedded BUP, discuss reliability of fully redundant IEC 61850-based DSS systems and juxtaposition it to the issues that the addition of MU embedded BUP contributes to the protection system.