When using optical protection ground wires (OPGWs) for line protection, the maximum distance is effectively unlimited for practical power system applications, unlike traditional pilot wires which are limited to a few tens of kilometers.

Optical Cable in Line Protection

Optical fibers are increasingly used in longitudinal differential protection of transmission lines. Unlike pilot wire relays, which are limited to short lines (typically a few tens of kilometers), OPGWs allow communication over very long distances without being a limiting factor because optical signals can travel tens to hundreds of kilometers with proper design and amplification . OPGWs serve a dual purpose: they act as a ground wire and provide a communication channel for protection signals.

Factors Affecting Maximum Distance

1. Fiber Type:
   • Single-mode fibers (SMF) are preferred for long-distance protection links because they support distances of tens of kilometers to over 100 km without repeaters, depending on transmitter power and receiver sensitivity .
   • Multimode fibers (MMF) are limited to short distances, typically up to 2 km at 100 Mbps, due to modal dispersion .
2. Optical Power and Receiver Sensitivity:
   • Higher transmitter power and more sensitive receivers extend the maximum distance. Proper design ensures the signal remains above the receiver's minimum detection threshold over the entire line .
3. Dispersion and Bandwidth:
   • Chromatic and modal dispersion reduce the effective distance for high-speed signals. For protection relays, which typically use low-bandwidth signals, this is less restrictive than for data networks .
4. Repeaters or Amplifiers:
   • For extremely long lines, optical amplifiers or regenerators can be used to maintain signal integrity, though in most transmission line protection applications, distances are within the range of standard single-mode fiber without amplification.

Practical Implementation

• OPGW Installation: The cable is installed along the top of transmission towers, providing both grounding and communication.
• Relay Communication: Differential relays at both ends of the line compare currents and trip the line in case of faults. Optical fiber ensures fast, reliable, and secure communication over long distances .
• Distance Considerations: With single-mode OPGW, distances of 50–100 km or more are routinely achievable without repeaters, making optical fiber suitable for almost all transmission line lengths.

Summary

For line protection using optical cables, the maximum distance is no longer constrained by the cable itself when using single-mode OPGWs. Multimode fibers are limited to short distances (≤2 km), but single-mode fibers can support tens of kilometers, and with proper design, distances exceeding 100 km are feasible. This makes optical fiber the preferred choice for modern long-distance differential protection in power systems .