Merging Unit
Process-bus device that samples analogue voltage and current from instrument transformers and publishes them as IEC 61850-9-2 Sampled Values frames. The enabling technology for retrofit-in-place modernisation.
Also: MU, merging unit, SAMU, stand-alone merging unit, LPIT
A merging unit (MU) is a device that samples analogue voltage and current signals from instrument transformers (CTs and VTs) at the process level and publishes them as digital Sampled Values frames on the process bus.
Two flavours
- Stand-Alone Merging Unit (SAMU) — terminates on the secondaries of conventional analogue CTs and VTs out in the substation yard, digitises their output. Specified in IEC 61869-13:2021. The retrofit-friendly variant.
- Low-Power Instrument Transformer (LPIT) — sensing and digitisation integrated in a single primary-yard device. The new-build variant.
Either form delivers IEC 61850-9-2 Sampled Values to subscribing IEDs.
Why SAMUs are the retrofit hinge
The traditional protection wiring is a copper trunk from each CT/VT secondary back to a central control room — kilometres of cable per site, expensive to renew, slow to engineer. A SAMU replaces all of that with one fibre-optic process-bus connection per bay. The CTs and VTs themselves don’t need replacing. The legacy copper doesn’t need digging up in one hit. Only the secondary systems get refreshed.
That’s the engineering pattern that lets the protection layer cycle every 15-20 years on top of primary plant that cycles every 30-40. The lifecycle mismatch becomes manageable.
Time synchronisation
Merging units publish to a multicast Ethernet group that multiple IEDs subscribe to. The IEDs reconstruct phasors from the relative timing of multiple sample streams, so every MU on the bus must share a common time reference — typically PTP under the IEC/IEEE 61850-9-3 Power Utility Profile.