IEEE C37.118 and the Synchrophasor Standards
The IEEE C37.118 family — and its IEC/IEEE 60255-118-1 successor — that defines synchrophasor measurements and data transfer for Phasor Measurement Units. The standard that turns GPS-time-stamped voltage and current phasors into the substrate for wide-area monitoring (WAMS) and the Dynamic Line Rating systems modern grid operators depend on.
Also: C37.118, synchrophasor, PMU, phasor measurement unit, 60255-118-1, WAMS
The C37.118 family defines the Phasor Measurement Unit (PMU) — a device that produces GPS-time-stamped voltage and current phasor measurements, typically at 30, 50, 60, or 120 frames per second, and streams them to a Phasor Data Concentrator (PDC) for wide-area use. PMUs are the substrate for Wide-Area Monitoring Systems (WAMS), dynamic stability detection, and the inter-area oscillation analysis that grid operators with high renewables penetration increasingly depend on.
The standard’s name has shifted: the measurement specification originally published as IEEE C37.118.1-2011 was promoted to a joint IEC/IEEE standard as IEC/IEEE 60255-118-1:2018. The data-transfer half remains as IEEE C37.118.2. Vendors and engineers still typically say “C37.118” for both halves of the family.
What a synchrophasor actually is
A synchrophasor is the magnitude and phase angle of an AC voltage or current waveform, computed at a known UTC time stamp accurate to ±1 µs. Two PMUs hundreds of kilometres apart, each producing a 50 Hz frame, produce phasor pairs whose relative phase angle directly indicates the power flowing between their measurement points. Aggregated at a PDC, the phasor estimates from many PMUs across a region produce a real-time picture of grid state at sub-second resolution.
This is the data that the older state-estimator-based EMS view of the grid (SCADA polling at 30-second cycles, see SCADA) is structurally incapable of producing.
Why this is not the same as Sampled Values
Sampled Values and synchrophasors are both timestamped electrical measurements; they operate at different scales for different purposes:
| Sampled Values | Synchrophasors | |
|---|---|---|
| Scope | Single bay, single substation | Wide area, grid-region |
| Rate | 4,800-15,360 samples per second | 30-120 frames per second |
| What’s transmitted | Raw waveform samples | Computed phasor estimates |
| Latency target | <3 ms (per-frame) | <100 ms (end-to-end) |
| Consumer | Protection IED in the same substation | PDC at control centre / operator |
| Standard | IEC 61850-9-2 | IEEE C37.118 / IEC/IEEE 60255-118-1 |
| Time source | PTP on the process bus | GPS at the PMU itself, traceable to UTC |
A PMU and an IED can be the same physical device — many modern protection relays embed a PMU function — but the data path and the data semantics are different.
Where the data goes
The architecture is hierarchical:
- PMU in the substation produces phasor frames over Ethernet using the C37.118.2 transport (typically TCP, sometimes UDP for unreliable lossy links).
- Local PDC in the substation or regional centre aggregates frames from multiple PMUs, time-aligns them (because each PMU’s frame may arrive with different network latency), and forwards the aligned set onwards.
- Super PDC / Wide-Area PDC at the system operator combines regional streams into the grid-wide view.
- WAMS applications consume the super-PDC stream — oscillation detection, voltage stability margin estimation, post-disturbance forensics, dynamic line rating.
In GB, NESO operates a national WAMS — historically known as PhasorPoint — that consumes PMU streams from the transmission network and feeds operator displays and analytics.
What it adds to the operator’s toolkit
WAMS does not replace SCADA; it sits beside it, answering questions SCADA cannot:
- Inter-area oscillations in the 0.1-1 Hz range — invisible at 30-second SCADA polling, obvious at 50 Hz PMU frame rate.
- Dynamic Line Rating — combining PMU-derived line state with weather data to lift seasonal capacity ratings beyond the static “worst case” values.
- Islanding detection — when a section of the grid disconnects, the phase-angle drift across the boundary becomes large within cycles.
- Post-event forensics — high-resolution playback of grid state in the seconds before and after a disturbance, far beyond what fault recorders capture per substation.
The frequency excursion that triggered the 9 August 2019 GB blackout was reconstructed in detail from PMU data; the SCADA telemetry was useful but coarse by comparison.
Security and the wide-area path
The C37.118.2 transport itself has no built-in security — the same plain-text problem that DNP3 had until 62351-5 addressed it. The conventional protection is to wrap the C37.118.2 stream in TLS, which the 62351-3 profile supports because it is TCP-borne — not because C37.118.2 specifies it. Most operational deployments still rely on private MPLS isolation as the compensating control, which is the same posture as the rest of the SCADA estate.
Where it fits in the modernisation story
PMUs are not new — they have been deployed in transmission substations since the late 1990s — but their density and operational role have shifted in the renewables era. Where a 2010 transmission substation might have had one or two PMUs as a research curiosity, a 2026 substation supporting high renewable infeed has PMUs as core protection-and-control infrastructure, with the PDC sitting on the same hypervisor cluster as the ADMS and EMS.
The PMU’s GPS dependence is also one of the operator’s larger concerns under GPS-denial scenarios, and is part of what is driving the substation-grandmaster-with-rubidium-holdover specification described under PTP.