What is a COMTRADE file?
Protection relays are continuously monitoring voltage and current waveforms, frequency and power for the purpose of detecting faults and clearing them to protect the infrastructure. This is usually done by taking oscillography samples at a given sampling frequency and performing mathematical operations in the digital domain, for example the Clarke Transform to detect homopolar components in three-phase systems.
When a fault is detected these oscillography samples taken before and after the fault can also be used to analyze the behavior of the system in order to improve it. Common analysis are, for example;
- Evaluating the degree of success of the protection system in clearing the fault, for example how long it has taken the protection relays to detect and clear the fault
- Gaining understanding into the behavior of the different elements; for example, knowing the fault current flowing through a transformer and the duration of the fault may help analyze its possible degradation, estimate its lifetime and trigger a decision on preventive maintenance
- Collecting statistical data about the most common faults in a given line, for example phase-to-earth faults because of vegetation or phase-to-phase faults because of the wind
- In large blackouts affecting large geographical areas forensic analysis may help understand how the fault was originated and how it propagated through the grid
These samples are stored in files often created by the protection relays or other IEDs and analyzed with the aid of computer tools. To facilitate the analysis and ensure compatibility between protection relay manufacturers and software tools it was clear from the beginning that these samples should be stored in a standardized format. For this purpose, the Institute for Electrical and Electronics Engineers approved IEEE Standard C37.111 in 1991, defining the so-called COMTRADE file format (Common Format for Transient Data Exchange). Two revisions of the standard were made in 1999 and 2013 to keep pace with the evolution of the technology and improve the accuracy of the results; for example, 16-bit, 32-bit or floating-point formats were introduced.
IEEE C37.111‑1999 is the most widely used version of the COMTRADE format and was later adopted by the International Electrotechnical Commission as IEC 60255–24. As a standard to normalize the format of the samples, IEEE Std C37.111‑1999 defines the following files associated with the COMTRADE recording;
- *.DAT file: data file containing numerical values of all samples together with a time stamp
- *.CFG file: a configuration file that states how to interpret the *.dat file (samplig rate, frequency of power system, number of channels in the recording, binary format of the information, …)
- *.HDR file; an optional header file with additional information, for example test conditions if the recording has been taken in a laboratory test
In a real substation the COMTRADE file can be generated by different devices, often depending on the type of IED and the communications protocol. It can be generated, for example, by the protection relays or the RTUs or gateways. The File Transfer capability, included for example in IEC 60870–5‑104 and DNP3.0 and present in iGrid’s portfolio, becomes so a useful tool to handle COMTRADE files.
The attached diagram shows a typical substation with protection relays and an iRTU (RTU device) or an iGW protocol converter communicating to an iControl SCADA system. The following different scenarios are possible;
- When the protection relays communicate via IEC 60870–5‑103 protocol, they send the samples together with their respective time stamps to the iRTU using a single packet. The iRTU/iGW will create the COMTRADE file with the information given by the relays, and store it internally. This file may be later collected by the control center, like iControl SCADA, using SFTP, DNP3.0 or IEC 60870–5‑104
- In IEC 61850, IEDs create the COMTRADE file, which can be collected later by the iRTU/iGW or the iControl SCADA using IEC 61850 file transfer or SFTP. The files are stored and made available to the control center
The File Transfer capability allows the iGW/iRTU to play an active role and transfer the COMTRADE and other IEC 61850 files to the Control Center, via protocols (IEC 61850, IEC 60870–5‑104 or DNP3.0) or SFTP without waiting for the SCADA to retrieve them. Of course, the File Transfer capability can also be used to transfer any other type of files, such as IEC 61850 CID configuration files or log files.