Merging Unit (MU)

The MU interfaces directly with conventional or non-conventional instrument transformers to capture high-fidelity analog waveforms. It typically digitizes multiple current and voltage channels simultaneously:

• 4 Voltage Channels: Three phase-to-ground voltages plus a neutral voltage from a voltage transformer (VT).
• 4 Current Channels: Three phase currents plus a neutral current from a current transformer (CT).
• High Dynamic Range: Accurately captures both steady-state load currents and high-magnitude fault transients without saturation clipping.

Precise time alignment is the defining characteristic of a Merging Unit. It uses the Precision Time Protocol (PTP) to synchronize its internal clock to a grandmaster clock with sub-microsecond accuracy.

• Timestamping: Each sampled value packet is tagged with a precise timestamp based on the synchronized clock.
• Multi-IED Alignment: This ensures that Sampled Values from different MUs across the substation are temporally coherent when they arrive at a protection relay, enabling accurate differential protection calculations.

The core output of a Merging Unit is a continuous, unidirectional stream of Sampled Values (SV) as defined by IEC 61850-9-2. The MU acts as a publisher on the process bus:

• High Sampling Rate: Typically publishes 80 samples per cycle (4 kHz) for protection applications and 256 samples per cycle (12.8 kHz) for power quality analysis.
• Multicast Ethernet Frames: Each frame contains the digitized instantaneous values for all current and voltage channels, allowing multiple subscribing Intelligent Electronic Devices (IEDs) to receive the data simultaneously.

Beyond analog digitization, the Merging Unit often integrates binary hardwired signals from primary switchgear, converting them to digital GOOSE messages:

• Circuit Breaker Status: Digitizes the open/closed status of the circuit breaker auxiliary contacts.
• Gas Pressure Alarms: Converts SF6 gas density monitor alarm contacts into digital status points.
• GOOSE Publication: Publishes these binary states as Generic Object Oriented Substation Event (GOOSE) messages, eliminating dedicated copper wires for status indication.

To ensure zero recovery time for critical protection signals, MUs support network redundancy protocols directly at the hardware interface:

• Parallel Redundancy Protocol (PRP): The MU duplicates every SV frame and sends it simultaneously over two independent, parallel Ethernet networks (LAN A and LAN B).
• High-availability Seamless Redundancy (HSR): The MU sends frames in both directions around a ring topology.
• Seamless Failover: If one network path fails, the receiving IED discards the duplicate and processes the valid frame without any delay or data loss.

Merging Units include operational modes that facilitate commissioning and troubleshooting without tripping live equipment:

• Test Mode: The MU sets a 'test' quality bit in the SV frame. Subscribing IEDs can be configured to process these test values for secondary injection testing while ignoring them for operational tripping.
• Simulation: Allows the MU to inject pre-recorded or generated waveforms onto the process bus, enabling end-to-end testing of protection schemes using COMTRADE files without physical signal injection.

The primary output protocol of a merging unit, defined in IEC 61850-9-2. The MU packages digitized current and voltage measurements into Ethernet frames and publishes them at a fixed rate—typically 4,000 Hz for protection and 80 Hz for metering. Each frame includes a sample counter synchronized to a common time source, allowing subscribing IEDs to align data from multiple MUs for differential protection schemes.

Merging units rely on IEEE 1588 PTP to achieve sub-microsecond clock synchronization across the substation network. This precision is non-negotiable for protection applications where phase angle accuracy between multiple feeders is critical. A PTP grandmaster clock distributes time to the MU, which timestamps each Sampled Value frame, enabling downstream relays to correctly align and compare measurements from different bays.

The subscribing device that consumes Sampled Values published by the MU. Modern protection IEDs replace discrete copper wiring with a process bus interface, receiving digitized current and voltage streams directly from the MU. This architecture eliminates complex analog wiring, reduces copper costs, and allows for virtualized protection schemes where a single IED can subscribe to multiple MU data streams for busbar or transformer differential protection.

The MU serves as the bridge between conventional or non-conventional instrument transformers and the digital world. It accepts inputs from:

• Conventional CTs/VTs: 1A/5A current and 110V/220V voltage signals
• Non-Conventional Instrument Transformers (NCITs): Direct digital outputs via fiber
• Rogowski Coils: Air-core current sensors requiring integrator circuits The MU performs anti-aliasing filtering and analog-to-digital conversion with high resolution—typically 16-bit or greater—to preserve waveform fidelity for protection algorithms.

The UCA International Users Group published the 9-2LE (Lite Edition) implementation guideline to ensure multi-vendor interoperability between merging units and subscribing IEDs. This profile standardizes:

• Sampling rate: 80 samples/cycle for protection (4,000 Hz at 50 Hz)
• Dataset structure: Fixed arrangement of phase currents, voltages, and neutral values
• Ethernet configuration: VLAN tagging and priority settings Adherence to 9-2LE ensures that an MU from one manufacturer can seamlessly communicate with a protection relay from another.