The Hidden Regression: Risks of Embedding Protection and Control Functions in IEC 61850 Merging Units

The introduction of IEC 61850 marked a turning point in substation automation, offering a vendor-neutral, interoperable, and modular framework that facilitates better system transparency, knowledge retention, and long-term sustainability. Central to this architecture is the process bus, where Merging Units (MUs) digitise analogue signals from field equipment and publish them in IEC 61850 format to subscribing Intelligent Electronic Devices (IEDs) over Ethernet. This clear functional separation promotes knowledge management at utilities’ engineers and personnels, scalable system design, streamlined asset management, and crossvendor knowledge transfer across teams and technology generations. A practical and proven example of this advantage is found in modern IEC 61850 process bus based busbar protection systems, where a central unit can seamlessly integrate with peripheral units (merging units) from different vendors. The success of such systems is not limited to fully digital substations, many IEC 61850 process bus based busbar protection systems have also been successfully implemented in conventional substations. Recognising these benefits, multiple utilities have adopted and mandated the use of open, non-proprietary communication protocols as a requirement for their busbar protection systems.

However, a concerning trend has recently emerged: embedding protection and control (P&C) functions directly into MUs/PIUs. While this may provide short-term benefits such as simplified engineering or backup resilience on the case of failure of both redundant ethernet network, it reintroduces the very problems IEC 61850 sought to eliminate.

This paper critically evaluates the operation and knowledge management implications of embedding P&C functions in MUs/PIUs. It explores how this design shift impacts utilities’ ability to sustain technical know-how, support multi-vendor interoperability, and maintain costeffective operations. Deployment challenges including field device function segregation, environmental resilience, vendor neutral and standardized configuration management, maintainability, and testing procedures are analysed from a knowledge governance perspective.

Through comparative architecture studies, the paper advocates for preserving the original role of MUs/PIUs: as vendor-agnostic, hardware-neutral devices focused solely on standardised signal acquisition and publishing.

The paper concludes with practical guidance for utilities, including procurement strategies, architectural best practices, IEDs configuration management and knowledge-centric governance models that align with IEC 61850’s foundational principles. By resisting the regression toward embedded P&C functions in MU/PIU, and recommitting to open, modular system design, utilities can protect the long-term integrity, adaptability, and knowledge transparency of their digital substations and power network long term development.