What is a COMTRADE File? Format Guide (IEEE C37.111)

If you work in power system protection, you've almost certainly encountered COMTRADE files. Every time a protective relay operates, a digital fault recorder captures an event, or a power quality monitor triggers, the data is almost always stored in COMTRADE format. This guide explains what COMTRADE is, how the files are structured, and how to open and analyze them.

The Standard: IEEE C37.111

COMTRADE stands for Common Format for Transient Data Exchange. It is defined by the IEEE standard C37.111, first published in 1991. The standard was created to give relay manufacturers, utilities, and analysis software a single, vendor-neutral format for exchanging oscillographic recordings of power system transient events — faults, switching operations, and disturbances.

Before COMTRADE, every relay manufacturer used a proprietary binary format. Sharing data between utilities or analysing it with third-party software required custom converters for each device. COMTRADE solved this by defining a human-readable configuration file paired with a compact data file, with a published spec that any software vendor could implement.

Today, IEEE C37.111 is the universal lingua franca of power system recordings. Siemens, ABB, GE, SEL, Schneider Electric, and virtually every other relay manufacturer exports fault data in this format. You can open them right now in Detego's free COMTRADE viewer.

File Structure

A COMTRADE recording is not a single file — it is a set of files that share a common base filename. The core components are:

.cfg — Configuration File

The configuration file is a plain-text ASCII file that describes everything about the recording: the station name, device ID, number of analog channels, number of digital channels, channel names, units, scaling factors, sample rates, start timestamp, trigger timestamp, and the revision year of the standard being used. It is always human-readable and can be opened in any text editor.

Each analog channel entry in the .cfg file specifies a multiplier (a) and offset (b) so that raw integer sample values in the .dat file can be converted to engineering units: value = a × raw + b.

.dat — Data File

The data file contains the actual time-series samples. Its format (ASCII or binary) is specified in the .cfg file. In ASCII format, each row contains the sample index, timestamp (in microseconds), and the raw integer values for all analog and digital channels. In binary format, the same data is packed into fixed-width binary records for compactness and faster parsing.

.cff — Combined File Format

Introduced in the 2013 revision of IEEE C37.111, the CFF format merges the .cfg and .dat files — along with optional .hdr and .inf files — into a single file with clearly delimited sections. This simplifies file management: instead of tracking multiple files per recording, you have one .cff file per event. The .dat section within a .cff can still use any of the standard encoding formats.

Optional Files (.hdr, .inf)

The .hdr (header) file is a free-form text file for human-readable comments about the event. The .inf (information) file contains device-specific configuration data. Both are optional and rarely needed for analysis, but both are included in the .cff combined format when present.

IEEE Versions

There have been three published revisions of IEEE C37.111, each adding capabilities while remaining broadly backward-compatible:

• 1991 — The original standard. Defined ASCII and Binary (16-bit integer) data formats. Timestamps are stored as date and time strings with microsecond resolution.
• 1999 — Added the timemult field to the .cfg file, allowing sample timestamps to be scaled for very high-speed recordings. Also clarified channel naming and scaling conventions.
• 2013 — The most significant update. Added two new data formats: Binary32 (32-bit integer) and Float32 (32-bit IEEE 754 floating point), enabling higher precision. Introduced the CFF combined file format. Added support for GPS-synchronized timestamps.

The revision year is specified on the first line of the .cfg file (or the CFG section of a .cff). A value of 1999 or 2013 tells the parser which features to expect.

Data Encoding Formats

The .cfg file specifies the data format using the ft field. Four encoding formats are defined:

• ASCII — Human-readable comma-separated text. The largest files, but trivial to inspect manually. Common in older relays and simulation exports.
• Binary — 16-bit signed integers per analog sample, 1 bit per digital channel. The most common format in modern relays. Typically 5–10x smaller than ASCII.
• Binary32 — 32-bit signed integers per analog sample. Provides higher dynamic range for high-precision recordings. Introduced in the 2013 revision.
• Float32 — 32-bit IEEE 754 floating point per analog sample. Used when the recording software stores pre-scaled engineering-unit values directly, avoiding the need for the a/b multiplier/offset.

Where Do COMTRADE Files Come From?

Any device that records power system transient data can produce COMTRADE files. The most common sources are:

• Protective relays — Modern numerical relays from Siemens (SIPROTEC), ABB (REL/RET/REF series), GE (L90, D60, T60), SEL (300/400/500 series), and others store oscillographic records triggered by relay operation or manual command.
• Digital fault recorders (DFRs) — Dedicated standalone recorders installed at substations capture all channels simultaneously at high sample rates, independent of relay operation.
• Power quality monitors — PQ instruments triggered by voltage sags, swells, harmonics, or transients also export in COMTRADE format.
• Simulation software — PSCAD, EMTP, and similar transient simulation tools can export synthetic COMTRADE files, useful for relay testing and model validation.
• Hardware-in-the-loop (HIL) test systems — Relay test sets (OMICRON, Doble, RTDS) both play back COMTRADE files and record test results in the same format.

How to Open and Analyze COMTRADE Files

The simplest way to open a COMTRADE file is Detego's free online viewer. Drop your .cfg + .dat pair (or a single .cff file) into the browser — no install, no account required. You get interactive waveforms, phasor diagrams, symmetrical components, impedance trajectories, harmonics analysis, and distance/overcurrent protection assessment. All parsing and computation runs in your browser; your files never leave your machine.

If you're evaluating other options — desktop tools, legacy viewers, or paid software — see the COMTRADE viewers compared guide for an honest breakdown of what each tool does and doesn't do.