Catenary wire is a wire which runs directly above the railroad tracks and provides the overhead power to the locomotive or electrified multiple units (EMUs) which connect to it via the pantograph. Pantograph transmits the energy from catenary wire to the rest of the traction power system. In the United States, catenary systems are usually powered by 12kV power feed (from the local traction power substation). Because of the high voltage involved and its function, it is critical that the catenary system is properly maintained and any issues are addressed prior to the disruption of service or even more serious accidents.
What is the basic structure of the catenary system?
As mentioned above catenary system is a system which feeds power to the train in contact with it (via pantograph). Some of the key components of the system (exact terminology can vary a bit) are: contact wire, messenger wire, droppers (or hangers), insulators, catenary poles, registration arm and bracket. Most modern catenary systems are constant tension, meaning that the constant tension is maintained on the “wire assembly” irrelevant of the temperature outside (higher temperatures can cause wire sagging). The wire assembly is typically a contact wire (which touches pantographs) and a messenger wire (which supports the contact wire via the vertical droppers or hangers, as well as shares electrical feed load). The brackets are components which stick out from the poles and keep the entire wire assembly fixed above the track via catenary poles (or catenary bridge structure in case of multi-track territory). This is by no means an exhaustive description of the system, but covers the parts relevant to the current discussion (a deeper dive will be subject of the future articles).
What is a pantograph?
It is basically a mechanical means to make a physical connection with contact wire. It touches the contact wire using the carbon strips located on its “head”. It then feeds the current from the contact wire to the primary transformer of the locomotive’s/car’s traction system. It usually consists of two arms allowing it to articulate up or down. Note that it rests on the insulators to insulate it from the rest of the car body.
What can possibly go wrong?
Well quite a bit can go wrong. If any parts of the catenary system gets loose the wire can get caught by the pantograph and “dragged” along with the train. Alternatively a broken pantograph can “catch” pantograph wire and “rip out” the section of the wire (as well as cause electrical shorts). Alternatively a foreign object (such as a tree branch) can hang on the wires creating an electrical path where there shouldn’t be one. In a rare instances, due to interactions between pantograph and catenary system, pantograph can brake off and swing into passenger section. Hence it is critical to maintain both systems are in a good working conditions despite a typically heavy usage.
Proactive monitoring
In order to avoid serious accidents and train delays one of the best approaches is proactive inspection of the catenary system. This can be done in one of two ways: either through the use of specialized catenary inspection vehicle (a train car instrumented to scan and check various details of the wire) or through the use of “passive systems” placed on or around the regular equipment (already equipped with a pantograph). Catenary inspection vehicle is a specialized equipment which travels throughout electrified territory (with catenary wire) and provides a detailed inspection of the catenary system at fairly low speed. It usually requires dedicated track time to perform its work and can thus interfere with the normal train traffic. As a result it is typically scheduled to be performed at set intervals.
The “passive systems” are usually those which are mounted on the regular equipment with a pantograph. They usually contain a camera (for visual inspection with video recording) and sensors to provide accelerometer readings (to measure impacts “sensed” by the pantograph). This information in combination with GPS coordinates can provide a good overview of the current state of the system. Another form of this “passive system” is pantograph cameras located at various locations throughout the railroad. The objective here is to take pictures of the pantographs and utilize machine learning to spot any pantograph which might have missing or broken parts (which may damage the catenary wire).
Reactive monitoring
The reactive monitoring stems from the running daily operation of the catenary system. Like every complex electrical system, catenary system has its own computer based supervision system (aka SCADA) as well as various forms of fault detection and control. Entire system is also sectionalized into individual parts which can be configured as needed by the system operator. So for example, if a section of catenary is grounded due to a tree falling on it, it can be isolated from the rest of the system through the use of sectionalizing switches, enabling continuation of operation while crew are dispatched to fix it. Furthermore, the utilization of sectionalizing enables easier fault location by knowing approximate area of the fault (between which switches).
Relevant railroad accidents
This incident involved a pantograph falling from the roof of the car and into the passenger portion of the car. It resulted from the pantograph hitting the bracket holding catenary wire and being “ripped off” from the roof. This is a great example why both parts of the system must be maintained with utmost care.