When I watch videos of high speed trains I always see explosions of electricity near the top, or arcing. Why does that happen? I know that the Acela does it a lot but other high speed trains have it, too.
Several factors influence this:
- at high speed, there's a higher chance of the pantograph losing contact with the catenary wire: at higher speed, bumps in the wire cause a more violent excursion that can exceed the capability of the suspension of the pantograph.
- Low-speed trains can show arcing too.
- high-speed trains often use high voltage (15 or 25 kV), which is capable of arcing across longer distances than the lower voltage (e.g. 1500 V) used in older trains.
The point where the pantograph of an electric train makes contact with the trolley wire creates one of the most complex and challenging environments for rail component manufacturers and test engineers to understand, let alone predict and improve.
For trains to operate efficiently, the pantograph must maintain constant contact with trolley wires suspended from catenary systems. Yet these wires and their support structures exhibit different vertical stiffnesses along any given section. The catenary system zigzags in 30 to 100 meter intervals to prevent grooving. The force the pantograph applies to the wire must stay within a well-defined range (70N to 120N). If it is too low, loss of contact results in arcing, which not only causes the train to lose power but damages the trolley wire and the contact bar through etching and overheating. If the force is too high, the resulting friction wears down the wire and contact bar prematurely.
Delivering the right amount of force requires variable vertical motion. But when trains move at higher speeds, pantographs lose their ability to react appropriately. Even when the trolley wire is as flat as possible, it is only flat when it hangs undisturbed. When the pantograph lifts the wire, the resulting deformation creates a wave. If there is too much uplift, the pantograph creates a much larger waveform that causes contact problems for the next pantograph coming down the line.
The catenary wire isn't stationary: it gets moved around by trains and by the wind.
In general, when a pantograph runs underneath the catenary, it sets up a wave-like disturbance which travels down the wire with a speed determined by the tension in the wire and its mass per unit length. When a train approaches this critical speed, the pantograph catches up with the disturbance, resulting in dangerously large vertical displacements of the wire as well as contact interruptions. The top speed of the train is then limited by the critical speed of the catenary. This problem was central to the test runs, since it was desired to test set 325 at speeds well above the critical speed of standard TGV catenary.
As others have posted, a temporary gap between pantograph and overhead conductor is part of the answer, however that's not the complete story. The other big factor is that the train's motors are an inductive load, which seriously complicates what happens when the circuit is interrupted.
When there is an interruption of a circuit with an inductive load, the current through the load cannot go to zero instantly. Instead, current continues to flow through the load, generating an voltage spike at the point of interruption. (The extra energy to do this is actually come from the inductive load.) The voltage rises suddenly until a breakdown (e.g. arc) occurs. One an arc has formed, the voltage drops, but less voltage is needed to sustain an arc because plasma is more conductive than air at typical temperatures.
The currents flowing for a high-speed train will typically be much higher than their low-speed counterparts, so the voltage developed when the circuit is interrupted will be higher.
The upforce on a pantograph is 15-40 pounds, 60 pounds at the outside. (7-18kg, max 30 or so).
The trolley (contact) wire is made of solid bronze or copper, typically 4/0 to 400kcmil (107-200mm^2), with a stranded steel messenger (catenary) wire of 3/8-1/2" (10-13mm) diameter. The messenger wire is supported every 100-200 feet (30-60m) and it supports the contact wire every 6-10' (2-3m). So the contact wire is free to rise up even a foot (0.3m) as the train passes. It often has a stabilizer bar to keep it from moving laterally but is free to move vertically.
As discussed, any irregularity in the contact wire, or in how it's hung, can cause pantograph and wire to separate for a moment.
Wave action in the wire can also cause a momentary separation. Sufficient wire or train movement can cause the wire to move out onto the curved "horn" of the pantograph.
Irregularities in the pantograph's running surface can also cause arcing. There are typically inset copper or bronze slides; physical damage to a slide or simply a burned spot from arcing may cause the wire to lose contact.
Also a pantograph typically has two slides, fore and aft, and the pantograph has either linkage or strong springs to keep it level. If there is any binding or broken linkage or a fatigued or broken spring, it may not be level and may ride on its heel or toe, causing poor contact.
The arcing of course is caused by current. Current might remain continuous through the arc (that tendency being proportional to voltage, more likely on the high voltage systems used in high-speed rail) -- however high speed air movement is likely to snuff out the arc, severing power to the train momentarily. Talk about voltage spikes!
It's not about voltage*, it's about current.
When a high-current circuit is interrupted (especially an inductive one), an arc is formed between the contacts breaking away. The high current then sustains the arc: ohmic heating turns air into plasma, while the plasma conducts the current. It's the basis of arc welding, which uses hundreds of amps at voltage as low as 20V.
Even low-voltage (usually 600-800V) trams moving at walking pace do produce arcing and sparks at breakpoints in catenary, while subway does same at the level of the power rail.
Because of the high-current requirement, sparks happen mostly when the train is accelerating (eg from a standstill), or when it draws lots of power to sustain high speed, but they never happen when it's coasting at idle power, despite voltage being the same.
In low-speed operation, this happens mostly when a break of contact is introduced to the wire-shoe system externally, eg by a physical gap separating different circuits or by contamination from ice, snow or leaves.
In high-speed operation, in addition to all the low-speed ones, extra breaks are created by the pantograph jumping on catenary irregularities, just like an off-road truck which gets it's wheel momentarily airborne when it goes too fast on bumps. Some of those irregularities are introduced by the pantograph itself: one can imagine pantograph on a catenary as an upside-down acrobat on a tightrope. Instead of gravity acting on the acrobat downwards, the pantograph pushes the catenary upwards via a spring, so the whole system jumps up and down when passing under suspension points.
When I watch videos of high speed trains I always see explosions of electricity near the top, or arcing. Why does that happen?
There is a gap in the contact, the electrons shooting through the gap turn the air into a plasma and breakdown the air. Because the air is a plasma, it can conduct current, this happens at around 3kV/mm so you know there is some voltage involved.
Another factor is that overhead line profile changes much more rapidly at high speed. The contact wire isn't at exactly the same distance from the rail all the time.
The pantograph is constantly readjusted to apply a constant pressure on the contact wire, but at high speed that doesn't happen fast enough. When the pressure on the contact wire is insufficient, a small bump is all it takes to send the pantograph down a few mm, creating a visible arc.
Just for reference, low-voltage trains are capable to create quite visible arcs just as well (the lower voltage is usually compensated by the fact that it's DC), if they go fast enough or the contact wire is in a bad shape.