Winter Worries

February 11th 2005

At this time of year most people will have encountered the seasonal change in climate, which occurs in winter. Those cold and draughty station platforms, that late train which is frosty or the outside, and hopefully warm and welcoming on the inside.

The rail industry has in place a winter preparedness plan that will have swung into action in the autumn. The plan includes arrangements to cope with the effects of snow and ice, both on the platforms and the track. So what is likely to be happening on the electrification front and how is this part of the industry likely to be affected?

Change in Temperature
Temperature has the greatest effect on the overhead line conductors, which are metallic, and as a consequence begin to contract. Some of the conductors extend to 900 metres in length, consequently changes become noticeable to the trained observer. At the tensioning structures the weight stacks will migrate towards the upper limit of their free range on auto tensioned systems. Contact wire registration arms and support bridles, as a consequence, will tend towards the mid point anchor as the system begins to contract. All of these characteristics are normal and are accommodated within the system design.

The effects of thermal contraction are also seen elsewhere. Fixed conductors, such as earth wires and return conductors together with fixed catenary systems, will also be responding in a similar way. The tension will be increasing and their sag profile will diminish.

What about the consequences?
This all sounds quite logical, but what are the potential consequences? The tension changes to the fixed conductors, particularly under bridges and at close obstructions, may cause a reduction in clearance, which may often be electrical in nature. Although a normal function of the system it does equate to an increase in risk of flashover.

Thermal cycling, with resultant tension variations, is a form of repetitive stress. This situation, in extreme cases and particularly where surface imperfections exist on the conductor strands, could result in the development of fatigue failures over a period of time.

Conductors are not alone in being subjected to the effects of thermal cyclic stress. Ceramic insulators can also be affected, although designed for such environments the effects of time and subsequent ageing under certain conditions results in crazing of the surface glaze. This can lead to failure due to fracture, or as the results of the ingress of water. Failures of this type often present themselves at the onset of early hard frosts.

Cold Weather & Snow
When the cold weather really bites and the snow begins to fall, the main will be the formation of icicles on the underside of structures that over the railway. Of most concern will be bridges where the waterproofing is poor allowing water seepage. Icicles quickly grow towards the live overhead line equipment, presenting a risk of flashover. These are removed by routine inspection patrols equipped with insulated poles and hooks, which are used to knock them off.

When snow falls, the overhead line equipment is generally unaffected. Radial icing of the conductors poses the greatest threat, but is a condition that is seldom seen in this country. The formation of radial ice occurs when either a build up of wet snow, or severe frost, or frozen fog particles, become attached to the conductor by compaction and subsequent re-freezing. The result is a radial coating of ice, which causes increased weight, and resistance to wind due to increased surface area.

System design caters for this, particularly where the weight of ice on the conductors causes an increase in conductor sag. This can cause a reduction in contact wire height at low bridges and level crossings, where clearances are frequently at the extremes of the operating envelope.

Electric Train Pantographs
When most people think of electrification systems, they forget the pantograph on the train. The pantograph is a mechanical unit mounted on the roof of the train. The device collects electrical current from the overhead line equipment, and delivers it to the train’s traction and ancillary equipment. This key component of the system can be affected by winter weather conditions and special action is required to ensure that it continues to function correctly.

Current collection by the pantograph is achieved by means of specialised carbon strips mounted on a frame, or head as it is called, which makes direct contact with the underside of the contact wire. The mounting arrangement is such that the strips form part of an enclosure, or air gallery, which when ruptured due to wear or damage, causes the pantograph to be automatically lowered, thereby limiting the potential damage to the overhead line equipment.

During the winter months ice regularly forms on the underside of the contact wire, particularly at night or early morning. This presents a rough surface, which causes irregular transition at the interface between pantograph and contact wire. This results in a display of arcing as the train draws current whilst moving. This situation leads directly to excessive carbon erosion, necessitating additional maintenance and more frequent carbon changes during the winter months.

The air gallery itself, which forms part of a pneumatic circuit of the pantograph protection system, can also be affected by freezing conditions. The compression of air leads to moisture condensation, which collects within the air circuit. In the past pantograph performance has been affected by poor efficiency caused by frozen moisture within the system. This has required the introduction of counter measures’ such as the introduction of anti-freezing agents on some units to ensure that operation is maintained under all conditions.


Wind & Gales
One of the most disruptive influences of winter weather is gale force wind. On a number of days every year electric train services are disrupted as a result. The mitigation measures used are the application of speed restrictions. Although sensibly applied, they are never well received by either the industry or the general public.

A recent claim by a Secretary of State during an interview with a rail publication* was that the problem was a result of the masts being too far apart. Whilst true in some instances, the statement represents an oversimplification. So what are the issues during high winds and is it just an overhead line problem?

Not entirely, there are several factors, which are overlooked for a number of reasons, a typical human reaction you might say.

Conductor displacement
The process of conductor displacement with respect to catenary engineering is a simple one. Conductors are of a fixed shape, size and tension. When the wind acts against the conductor it is displaced in proportion its span length [or distance between supports], its height above ground level and its exposure.

The wind acting on the masts supporting the conductor are also a factor, as is the amount of conductor offset at the structure or stagger, which allows the conductor to sweep across the pantograph to give an even wear pattern.

These factors are considered at the design stage, together with the expected mean hourly wind speed for a particular location, based upon historical meteorological data. This information is obtained from an isopleth, or wind chart. The analysis also considers the probability of occurrence, based on the anticipated train availability and the fact that wind forces act as gusts.

Track & Train Interfaces
So that is the overhead line side, what about other influences! Well there are two other factors that are a key part of the equation, the track and the train. The overhead line equipment may be considered in this context as static, however the track and train are not.

The track is a small but constantly moving interface both horizontally and vertically as a result of the passage of trains. The track must be maintained within the defined parameters, particularly its lateral dimension. This is most important on curves and transitions, where the maximum deflection of the wire under wind loading may not occur at mid point in the span.

The mechanism for controlling this is the track datum management system. This is designed to ensure that the track is maintained to a fixed dimension both horizontally and vertically from a point on the overhead line mast using x and y co-ordinates in conjunction with a set tolerance. The information is recorded on each mast for each track.

The Train, the Final Frontier
The train itself is the final piece in the equation [other than perhaps the human one, which in itself could be an interesting subject in its own right!] The train is a moving and swaying unit operating at speed, with its pantograph traversing the wire.

The overhead line engineer makes an allowance for vehicle sway, together with pantograph height, at the design stage. But is it that simple? Well the variables include the wheel track interface, the spring stiffness, damping characteristics of the train and the alignment of the pantograph, not to mention the degree of play in the pantograph itself, which consists of several linked and moving parts.

The pantograph is also subjected to the effects of high wind, and the effects depend on whether the device is leading or trailing. High winds cause a significant increase in uplift forces, increasing the risk of damage and ultimately dewirement.

The overhead line equipment is a fixed parameter. Some spans may be too great, but there are many factors that can contribute to a failure. Perhaps the greatest factor is the human one and the discipline that is required by different groups to maintain satisfactory operation. So the next time there are delays caused by high winds associated with the overhead line equipment, remember it’s not quite as simple as it sounds!

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