Merging unit – an essential building block of the future digital medium-voltage substation

Merging unit – an essential building block of the future digital medium-voltage substation

Mega trends, such as digitalization and integration of renewables, are driving power systems to the next level. Is the next innovation a digital substation completely without wires? Or is it a substation with only basic measurement products, while the protection and control algorithms are computed through the cloud? At the very least, the first step is a merging unit.

In high-voltage transmission protection systems, the merging unit concept is an ideal solution, as there are large distances between the protection relays and the instrument transformers located in the outdoor switchyards. Typically, there are strict requirements to build protection redundancy and the merging unit concept is no longer a new solution but one that has been proven to work in practice.

So, why don’t we have merging units in the medium-voltage distribution systems? Here the protection devices are typically installed into compact switchgear bays together with the instrument transformers, so the reason to use a merging unit would not be as obvious as with high-voltage.

A unique way of looking at a merging unit is to view it simply as a measuring interface between the primary process and the protection functionality. It is the interface between the physical and the digital world in a substation. The merging unit is implemented in a physical device.

Today a single device can operate at various functional levels. You can choose the most suitable integration level from merging and supervision to control and protection functionalities.

How does a merging unit bring value to a medium-voltage substation?

A merging unit alone doesn’t accomplish anything, as it is only one part of a complete measurement chain. The advantage here is that by incorporating the merging unit, the chain becomes fully digital enabling much more powerful information processing.

Digital measurements from the merging unit can easily be connected to several protection devices over the IEC 61850-based substation communication. Digital commands originating from the protection and control devices can be connected back to the physical world via an intelligent merging unit. IEC 61850 enables device independence and great flexibility for configuration modifications in a software environment.

Sharing voltage information without hardwiring inside a substation is one of the examples that is enabled by the merging unit functionality. The merging unit functionality is available in today’s modern IEC 61850-based protection relays. Future centralized protection and control systems will also be enabled by merging units. Out of the box thinking will likely discover many more applications on top of these.

The merging unit will be a part of the centralized protection and control solutions in medium-voltage substations. The substation merging unit SMU615 and ABB AbilityTM smart substation control and protection SSC600 are the building blocks of the future digital substation.

Ultimately, the benefits of substation digitalization are flexibility, increased supervision, interoperability and scalability. With end goals of future-proofing by making upgrades and the maintenance of assets easier, a complete system can gain large cost efficiencies over its lifetime.

If you would like to hear more, please meet ABB at Cigré Session 2018 where one of our conference papers entitled ‘Levels of functionality for power distribution systems employing intelligent merging unit and low power instrument transformer technologies’, will be available.

To see how centralized protection concept works in practice visit the Caruna’s substation in Noormarkku, Finland.


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