You're seeing line traps (also sometimes called wave traps) on those two conductors. Here's a breakdown of what they are and why you're only seeing them on two of the three phases

What are Line Traps?
Line traps are tuned circuits installed on high-voltage transmission lines, designed to block or attenuate high-frequency signals. Think of them as a kind of "filter" for the power line carrier (PLC) signals. They typically consist of a large air-core inductor (that's what you're seeing as the "coil") in parallel with a capacitor. This forms a parallel resonant circuit. When the frequency of the PLC signal matches the resonant frequency of the trap, it presents a very high impedance, effectively blocking the signal from passing beyond the trap. For frequencies away from the resonant frequency, the trap has a low impedance and allows the normal 50/60Hz power to pass freely.

Why Use Line Traps? Power Line Carrier (PLC) Communication

Utilities use PLC to send communication signals along the power lines. This includes things like: Teleprotection signals (e.g., relay commands to trip circuit breakers in case of faults). SCADA (Supervisory Control and Data Acquisition) information (e.g., voltage, current, power flow readings). Voice communication.

Preventing Interference

Line traps prevent these high-frequency communication signals from propagating to other parts of the grid where they could cause interference or be intercepted. They confine the PLC signal to a specific section of the transmission line.

Directing Signals

By strategically placing line traps, utilities can direct the PLC signals to intended destinations, ensuring reliable communication.

Why Only Two Phases?

This is the key question, and there are a few potential reasons, it boils down to the configuration of the communication system used by the utility: Carrier Configuration

The PLC communication system might be designed to only use two of the three phases for its signals. There are different PLC coupling methods:

Phase-to-Phase

The communication signal is injected between two of the phases. In this case, you would need line traps on those two phases to prevent the signal from leaking out.

Phase-to-Ground

The communication signal is injected between one phase and ground. You would only need a line trap on that one phase.

Cost

Line traps are not cheap. It may be acceptable to place them on two phases to allow adequate communication, and still be cost effective.

In summary: You're seeing line traps used to confine the high-frequency PLC communication signals to specific sections of the transmission line. The placement of the traps on only two phases suggests that the utility is using a two-phase PLC system, or is only attempting to filter the communications in a way that best fits its needs. Flag for review