Merging Recordings

Overview

The merge feature combines channels from two separate COMTRADE recordings into a single file with a unified timeline. This is essential when analysing a power system event captured by devices at different locations -- for example, relays at both ends of a transmission line during the same fault.

Merging enables several analyses that are impossible with a single recording:

• Differential protection analysis -- compare operate and restraint quantities using currents from both ends of the protected zone
• Two-ended fault location -- use voltage and current measurements from both line terminals for more accurate fault distance estimation
• Event correlation -- see relay operations, breaker status, and waveforms from multiple devices on the same time axis

The key challenge is that relay clocks are often not synchronized. Detego automatically detects the same fault event in both files and aligns them so the physical event appears at the same point on the timeline.

How to Merge

Follow these steps to merge two recordings:

1. Go to the Recordings page -- navigate to the recordings library where your saved recordings are listed.
2. Select exactly 2 recordings -- use the checkboxes to select two recordings that capture the same event. This works in both the ungrouped list view and inside folders.
3. Click the Merge button -- when exactly 2 recordings are selected, the Merge button appears in the action bar at the top of the list.
4. Review the alignment quality -- the merge dialog shows how the two recordings will be aligned, including the detected offset, alignment method used, and an accuracy estimate.
5. Optionally expand Advanced settings -- to change the alignment method or select specific channels to include in the merged output.
6. Click Merge -- a new recording appears in your library with a purple "Merged" badge indicating it was created from a merge operation.

Alignment Methods

Detego offers three alignment methods to synchronize the two recordings. The best method depends on the type of recording and whether the relay clocks are GPS-synced.

Cross-Validation

When both fault detection and timestamp alignment are available, Detego cross-validates them by comparing the two independently computed offsets:

• If they agree -- a "Clocks synced" badge is shown along with an estimated accuracy (e.g., "< 1 ms"). The accuracy estimate is the absolute difference between the two methods.
• If they disagree -- a "Clocks not synced" badge appears, indicating the relay clocks have a significant offset. Fault detection alignment is recommended in this case.

Cross-validation provides confidence that the alignment is correct -- when two independent methods agree, the result is much more trustworthy than either method alone.

If the two recordings come from relays configured with different timezones -- for example, one set to UTC and the other to UTC+1 -- Detego removes the whole-hour offset automatically and shows it in the alignment status (e.g., "+1h timezone offset corrected"). The remaining sub-second residual is what's used for cross-validation.

Channel Selection

By default, all channels from both files are included in the merged output. Channels are prefixed with the source station or device name to avoid naming conflicts -- for example, "Station1_IA" and "Station2_IA".

To customise which channels are included:

• Expand the Advanced section in the merge dialog
• Open the Channels panel to see all available channels from both files
• Deselect any channels you do not need -- both analog and digital channels can be toggled independently

Output

The merged recording has the following characteristics:

• Time window: Covers only the overlapping portion of the two recordings. Samples outside the overlap are discarded.
• File format: Saved as standard IEEE C37.111 CFF format. The merged file can be downloaded and opened in any COMTRADE-compatible tool.
• Sample rate: Uses the higher of the two input sample rates. The lower-rate file is resampled to match.
• Analog channels: Resampled using linear interpolation to produce smooth waveforms at the target rate.
• Digital channels: Resampled using nearest-neighbor interpolation to preserve binary values (no fractional states).
• Badge: Merged recordings show a "Merged" badge in the recordings list and in the viewer's bottom status bar (first element on the left), making merged files recognisable at a glance everywhere they appear.
• AI context: When you run AI analysis on a merged file, the AI engine automatically receives context about the merge operation, including which channels came from which source. The analysis dashboard also splits the channel breakdown into per-source sections (Source A / Source B) so you can see at a glance which channels came from each end.