Continuing to share my learning from attending CIGRE 2024 B5 discussions and reading the latest papers on digital substation…

A Process Interface Unit (PIU) and a Merging Unit (MU) are components used in digital substations based on the IEC 61850 standard. They have different roles and levels of complexity, primarily focusing on interfacing with the substation’s primary equipment and converting data into digital formats for communication over the process bus.

Schematic of a digital substation architecture showing MU v. PIU devices

The PIU is an evolution of the MU to cover all types of data coming from the field, not just the analog voltage and current measurements. As digital substations become more complex and data-rich, a standalone MU is no longer sufficient to realize the full benefit of IEC 61850 digital substations. This is why PIUs are now being specified as multi-function smart devices capable of processing data out in the switchyard and communicating both with the process bus, providing sample values to protection relays, and the station bus receiving and sending GOOSE messages to all subscribed devices within the substation.

Merging Unit (MU): The primary function of a Merging Unit is to digitize analog signals from current transformers (CTs) and voltage transformers (VTs) and convert them into time-synchronized streams of sampled values. This digitized information is sent to protection and control devices over the IEC 61850 process bus in compliance with the IEC 61850-9-2 or the more recent IEC 61869-9 standard. The standalone MU is generally limited to these specific data types and does not have protection or control functions built into them. Thus, a MU remains simple and lower cost.

Process Interface Unit (PIU): Besides the analog current and voltage data from the primary equipment, other critical measurements from multiple sources could benefit the grid operator. We can group them into three additional categories:

1. Switchgear Control Unit: receiving and sending binary status
2. Condition Monitoring Data: non-electrical data such as temperature, pressure, etc.
3. Other data from the switchyard (for example, data from an energy storage system)

A PIU is a more complex device that integrates multiple functions beyond digitizing analog signals. It handles electrical and non-electrical measurements and combines different process-level data in one device. A PIU can handle data conversion for non-conventional instrument transformers (NCITs) and even perform some control functions.

While MUs will remain helpful in future digital substations, there is a trend to either augment their functionality beyond their initial data conversion function or merge multiple functionalities into a more complex software-defined PIU that is ruggedized to be located close to the equipment in the switchyard to 1) reduce the cost of cabling to send all relevant data to the station secondary devices and 2) perform more efficient local data processing and control actions.