IEEE C37.118

Defines the synchrophasor estimation algorithm performance under steady-state and dynamic conditions. It specifies Total Vector Error (TVE) limits, requiring <1% error during nominal operation. The standard introduces two performance classes: P-class (Protection) for fast response with minimal filtering, and M-class (Measurement) for higher precision under harmonic distortion and out-of-band interference. Compliance testing mandates specific frequency ramp, magnitude modulation, and phase modulation tests.

Specifies the real-time data framing for streaming synchrophasors over TCP/IP or serial links. It defines four message types: Data Frames (streaming measurements), Configuration Frames (machine-readable PMU metadata), Header Frames (human-readable station info), and Command Frames (remote control). The protocol uses a binary, fixed-length format for deterministic latency, with a Common Header block containing a SOC (Second of Century) timestamp and a FRACSEC (Fraction of Second) for sub-microsecond time alignment.

The standard mandates specific nominal reporting rates (Fs) for phasor output. For 60 Hz systems, standard rates are 10, 12, 15, 20, 30, and 60 frames per second. For 50 Hz systems, rates are 10, 25, and 50 fps. The integer number of phasor estimates per second directly dictates the Nyquist frequency for observable grid dynamics. A PMU must state its reporting rate in the Configuration Frame to allow the Phasor Data Concentrator (PDC) to correctly align streams.

IEEE C37.118 mandates that all synchrophasor timestamps be traceable to Coordinated Universal Time (UTC) with a stated accuracy. The standard relies on an external time source, typically a GPS Disciplined Oscillator (GPSDO) providing a 1 Pulse Per Second (1PPS) signal and timecode. The IRIG-B timecode format is commonly used to distribute this time within the substation. Loss of time sync must be flagged immediately in the PMU's data stream via a dedicated status bit to prevent corrupted state estimation.

The standard defines the phasor representation as a cosine reference where the magnitude is the RMS value of the waveform. The phase angle is the angular offset from a cosine at nominal system frequency that peaks at the top of the UTC second. This convention is critical: a 0-degree angle indicates the waveform peak aligns exactly with the 1PPS signal. All PMUs must adhere to this identical convention to ensure that angle difference monitoring between two buses yields a physically meaningful measure of power transfer stress.

Recognizing the global convergence of substation automation, IEEE C37.118.1 has been harmonized with the IEC 61850 framework and is being superseded by IEC/IEEE 60255-118-1. This new joint standard preserves the measurement performance classes (P and M) but integrates the data model into IEC 61850's object-oriented structure. The communication protocol (C37.118.2) is being transitioned to IEC 61850-90-5, which encapsulates synchrophasor data in routable IP multicast packets for true wide-area distribution.

The intelligent electronic device that implements the C37.118 standard at the source. A PMU samples AC waveforms at 30-120 samples per second, computes synchrophasors, and time-stamps each frame using a GPS-disciplined oscillator. C37.118 defines the accuracy classes—P-class for protection (fast response, lower accuracy) and M-class for measurement (higher precision, slower response).

The primary accuracy metric defined in C37.118.1. TVE quantifies the vector difference between the measured and theoretical phasor value, combining both magnitude and phase angle errors into a single percentage:

• Steady-state TVE: Must not exceed 1% for M-class PMUs
• Dynamic TVE: Tested under modulation, frequency ramps, and step changes
• Compliance testing verifies TVE remains within limits across specified operating ranges

A data aggregation node that receives streaming C37.118 frames from multiple PMUs, time-aligns them by GPS timestamp, and outputs a coherent system-wide dataset. PDCs handle:

• Data alignment: Correlating frames with identical timestamps
• Latency management: Waiting for delayed streams before outputting
• Output formatting: Retransmitting aligned data via C37.118 or IEEE C37.118.2 to higher-level applications

The complementary international standard that specifies routable synchrophasor communication over IP networks. While C37.118 defines the measurement and frame format, IEC 61850-90-5 addresses:

• IP multicast transport for wide-area distribution
• Security via IEC 62351 encryption and authentication
• Integration with substation automation systems using IEC 61850 object models Many modern PMUs support both C37.118 and IEC 61850-90-5 output protocols.

The precision timing source that makes C37.118 synchrophasors possible. A GPSDO combines a stable local oscillator (typically an oven-controlled crystal or rubidium atomic clock) with GPS satellite signals to provide:

• 1 microsecond absolute accuracy for phase angle synchronization
• Holdover capability: Maintaining accuracy during GPS outages
• 1 PPS (pulse-per-second) output that triggers PMU sampling Without a GPSDO, phase angle measurements across distant buses would be meaningless.

A critical derived measurement specified in C37.118. ROCOF quantifies how quickly system frequency is falling during a generation-loss event, measured in Hz/s. Key applications:

• Fast-frequency response: Triggering battery storage injection when ROCOF exceeds thresholds
• Under-frequency load shedding: Determining the urgency of load tripping
• Inertia estimation: Calculating system inertia constant from ROCOF during disturbances C37.118 defines ROCOF accuracy requirements and testing procedures.