Power utility companies have to protect high voltage lines monitoring them constantly. This supervision requires the transmission of information between the power substations in order to ensure correct operation while controlling every single alarm and failure. Legacy telecom networks where interconnected with metallic wires, the problem is that this environment is characterized by a high level of electromagnetic fields that may disturb BNC and any copper wires. To avoid this perturbations it is recommended the use of optical links at the physical layer -instead of the traditional metallic cables- to solve the above mentioned issues following the IEEE C37.94 standard that defines the rules to interconnect tele-protection and multiplexer devices of different manufacturers using optical fiber.
Teleprotection of High Voltage lines
Authorities use a tele-protection scheme to enable substations to communicate with one another to selectively isolate faults on high voltage lines, transformers, reactors and other important elements of the electrical plants. This functionality requires the continuos exchange of critical data in order to assure correct operation. In order to warranty the operation the telecom network should always be in perfect conditions in terms of availability, performance, quality and delays. Initially these networks were electrical, then the 56–64 kbps channels became vulnerable to electromagnetic and radio interferences (EMI/RFI), signal ground loops, and ground potential rise because were based on metallic conductive media. Obviously this is not good for the reliability of the protections protocols. The substation environment is usually characterized by a high level of electromagnetic fields caused by high voltages and currents in power lines. Moreover, during fault conditions electromagnetic perturbations may rise significantly and disturb those communications channels based on copper wires. The reliability of the communications link interconnecting the protection relays is critical and must be resilient to the effects encountered in high voltage areas such as high frequency induction and ground potential rise.
Fig 1. Teleprotection architecture using C37.94 in normal operation.
This is the reason why the industry moved to optical fibers to connect the different items of the installed in the substations. Fiber optics do not need ground and are immune to the interferences caused by electrical noise therefore eliminates a lot of the errors that electrical connections do suffer. In other words, it is convenient the use of fully optical links from power relays to Multiplexers as the IEEE C37.94 standard defines. The protection scheme is able to be upgraded to a more sophisticated architecture using fault tolerant networks. Then instead of using direct relay connection and dedicated fibres redundant network are able to make the protection procedures process more reliable increasing the availability critical data interchanges.
The IEEE C37.94
Teleprotection systems must isolate faults very quickly to preventing damage to the network and power outages. Then the IEEE committee defined the C37.94 as a programmable n x 64 kbps (n=1…12) multimode optical fibre interface to provide transparent communications between teleprotection relays and multiplexers for distances of up to 2 km. Later on the industry adopted monomode optical fibre too in order to reach longer distances. The standard defines the protection and communications equipment inside a substation using optical fibers, the method for clock recovery, the jitter tolerances allowed in the signals, the physical connection method, and the actions the protection equipment must follow when any kind of network anomalies and faults occur. C37.94 has already implemented by many protection relay manufacturers including ABB, SEL, RFL, RAD and others. Teleprotection equipment use to offer a choice of transmission interfaces including the IEEE C37.94 compliant optical fiber interface for transmission over fiber pairs, and G.703, 64Kbps co-directional and E1 interfaces.