GB2536896B

GB2536896B - Repair of Conductors

Info

Publication number: GB2536896B
Application number: GB1505419.0A
Authority: GB
Inventors: Mirafsari Hodi
Original assignee: Houghton International Electrical Services Ltd
Current assignee: Houghton International Electrical Services Ltd
Priority date: 2015-03-28
Filing date: 2015-03-28
Publication date: 2019-07-24
Anticipated expiration: 2035-03-28
Also published as: GB201505419D0; GB2536896A

Description

Repair of Conductors

The present invention relates to a method of repairing a conductor.

Electric railway systems are known to include trains which utilise direct current (DC) line inductors. Inductors of the type are configured to withstand high frequency currents in use. However, in cold weather or winter periods, inductors of this type can be exposed to moisture from, in particular, snow and ice which leads to condensation on the coil windings of the inductors. Such condensation can result in internal electrical arcing and damage to the coils due to the overheating and shorting out of the circuit which results.

In order to overcome this, manufacturers add protection to the inductors in the form of guards or similar. This process is known as winterisation. Unfortunately, winterisation does not solve the problem adequately, and significant damage can occur to inductors. These inductors are considered unable to function to the standards required and are therefore scrapped and/or disposed of.

According to an aspect of the present invention, there is provided a method of repairing a conductor in accordance with claim 1.

The invention will now be described by way of example only, with reference to the accompanying drawings of which:

Figure 1 shows a carriage of an electric train;

Figure 2 shows a damaged line inductor awaiting repair;

Figure 3 shows a step of repairing a conductor of a line inductor;

Figure 4 shows a qualified operative repairing the conductor by means of welding;

Figure 5 shows a diagrammatic representation of the magnified cross section of a repaired conductor; and

Figure 6 shows a diagrammatic illustration of the path of electricity through a conductor in accordance with the present invention.

Figure 1 A carriage 101 of an electric train is illustrated in Figure 1. In this example, drive power is received from a third rail 102 and used to drive traction motors which power the vehicle. A direct current (DC) line inductor 103 is attached to the electric train and acts as a choke so as to block higher frequency alternating current (AC) while passing lower frequency or direct current (DC). The line inductors therefore comprise conductor coils or winding which are typically manufactured from aluminium alloys. In alternative embodiments, the conductor coils are manufactured from other suitable conductive materials and it is appreciated that the invention is not limited to aluminium coils.

During operation, approximately four hundred (400) amperes of direct current (DC) can pass through the line inductor 103. Taking into account additional currents, the conductor of the line inductor 103 can rise to a temperature of around ninety (90) degrees Celsius (°C) while the train is running.

In order to achieve optimum performance, line inductors used on such trains are serviced at regular intervals for damage. Due to the high temperatures and currents the line inductors are subjected to in use, it is not uncommon for the inductors to be provided for servicing showing severe damage as a result of electrical flashover, shorting and/or overheating.

When such damage occurs to the line inductors, they are typically disposed of as it is not considered possible to repair a line inductor or the conductive windings of the line inductor to a standard which enables the repaired line inductor to withstand the currents and temperatures described.

Figure 2 A line inductor 103 is shown in Figure 2 prior to servicing and winterisation. Line inductor 103 comprises a plurality of aluminium conductors 201 which are wound to form inductor windings or coils 202. The inductor coils 202 are attached to a frame 203 which, in use, is mounted to an electrical train, such as the vehicle shown in Figure 1. Inductor 103 further includes a pair of output terminals 204.

In this illustrated embodiment, aluminium conductor 201D has suffered overheating in use, such that a portion of the aluminium conductor has melted and eroded away. A gap 205 is now present in part of the aluminium winding, and, conventionally, the inductor would be disposed of and replaced. Gap 205 clearly indicates a piece of the inductor is now missing, and, in alternative embodiments, this problem can include a complete break across the cross section of the conductor 201. This is particularly a problem with aluminium conductors which have a relatively low melting point of around six hundred and sixty (660) degrees Celsius (°C).

Once damaged, the current flow through the conductor is forced to bottleneck through the now smaller cross sectional area of the conductor which arises from the break or gap. The smaller cross section leads to higher resistivity (and lower conductivity) which in turn leads to a further increase in temperature through the conductor. Thus, the inductor as a whole is therefore disposed of to avoid the risk of fire.

Figure 3

Damage incurred to line inductors of the type described in Figure 2 can be repaired by the method which will now be described with reference to Figures 3 and 4. A first step of a method of repairing a conductor of a line inductor, such as line inductor 103 described in Figure 2, is illustrated in Figure 3.

As illustrated in Figure 3, a portion of line inductor 103 is shown having an aluminium conductor 301 which has suffered damage in the form of gap 302. Aluminium conductor 301 is shown being prepared in anticipation of repair. In this illustrated embodiment, gap 302 presents a damaged surface 303 which is initially cleaned and smoothed using file 304. Minor cleaning can also be achieved by use of a wire brush. Alternative grinding tools are used to prepare and clean the damaged surface of the conductor in other embodiments, and it is appreciated that any other suitable alternatives can be used in further embodiments.

Figure 4

Once the damaged surface 303 of the conductor 301 has been cleaned appropriately in line with usual welding practices and standards, surface 303 is welded to effect a repair such that conductor 301 comprises an original portion 401 and a repaired portion 402.

In this embodiment, operative 403 uses a standard high quality tungsten inert gas (TIG) welding procedure to create a filler weld which utilises an inert gas with a tungsten tip to create the repaired portion 402. The TIG process maintains a constant temperature to create plasma which allows for transfer from a metal stick to the repaired portion or weld.

The welding procedure is conducted in line with a particular welding procedure specification (WPS) which is authorised and which specifies the welding stick and filler material used, along with gas composition used and flow rates to be used. The WPS advantageously allows the process to be completed repeatedly and to a consistent standard.

In the example described herein, for an aluminium conductor repair, the filler material is an aluminium 5356 wire provided EN ISO18273:2004 standard and available under the trade name Nexus (RTM) 5356. The gas used comprises seventy-five percent (75%) helium and twenty-five percent (25%) argon which are required to be delivered at a flow rate of between ten (10) and twelve (12) litres per minute (l/min). The tungsten tip is three point two (3.2) millimetres in diameter and is zirconiated. The welding should also be performed at a temperature of between one hundred (100) and two hundred (200) degrees Celsius (°C). Once the welding procedure is completed, the weld is allowed to cool at ambient temperature.

In the embodiment illustrated, a suitably qualified operative fills a gap by means of a filler weld. However, in alternative embodiments, depending on the damage to the conductor in question, the conductor is repaired using a butt welding technique. This involves cutting the conductor piece through the entire cross section and welding two pieces of the conductor together. A substantially similar specification from the WPS can be used for the same aluminium alloy to achieve this.

Whether by filler welding or butt welding, once repaired, the conductor comprises a repaired portion in which the average grain size is smaller than the average grain size of the original portion of the conductor that was undamaged.

Figure 5

Repair of the conductor is performed by the method described in Figures 3 and 4 to result in a conductor forming part of a line inductor in which the average grain size of the repaired portion (the welded portion) is less than or equal to the average grain size of the original portion. A diagrammatic representation of the magnified cross section of a repaired conductor in accordance with an aspect of the present invention is shown in Figure 5.

Conductor 501 is substantially similar to previous conductors herein described. In an embodiment, conductor 501 comprises an aluminium alloy and is used as conductive windings on a DC line inductor on electric trains. Conductor 501 comprises an original portion 502. Original portion 502 is so called as, prior to damage caused to conductor 501, the conductor as a whole consisted of the original portion only. Original portion 502 comprises a plurality of grains, such as grains 503 and 504 which are of an average grain size which is determined by the processes by which the original metal has been manufactured.

When manufacturing the conductor of aluminium alloy, the rate of cooling of the hot metal determines the size of the grains in the microstructure of the metal. In order to control the grain size, processes such as annealing are performed. Annealing, for example, allows the grains to grow over time. In contrast, quenching can be utilised to allow the metal to cool quickly resulting in smaller grain sizes. It is appreciated throughout that the term average grain size is known in the art of metallurgy to relate to the average diameter across each crystal, or grain of the metal in question.

Conductor 501 also includes a repaired portion 505 which includes a further plurality of grains such as grains 506 and 507. As repaired portion 505 has been formed through the process of welding as previously described in Figures 3 to 4, processes such as annealing or quenching are not performed to affect the grain size, and the weld cools at ambient temperature. Thus, the grain size is determined by the specific process of welding performed to the specification of the WPS described previously, for example.

Significantly, as is show in Figure 5, the average grain size of the repaired portion 505 is smaller than the average grain size of the original portion 502.

In the illustrated embodiment, both the repaired portion 505 and the original portion 502 comprise aluminium alloys of substantially similar compositions. Once repaired, the inductor comprising conductor 501 can be put back into service instead of being disposed of.

Repaired conductor 501 advantageously does not suffer in terms of resistivity or conductivity as would be expected so long as the grain size of the repaired portion is less than or equal to the grain size of the original portion. This is because, in practice, electrons (and thereby electricity) pass through the metal through the grain boundaries between each of the grains, for example, grain boundary 508. Thus, the smaller the grain size, the larger the number of grain boundaries, giving the free electrons an easier passage through the smaller grains than the larger ones. Thus, it is appreciated that, in the embodiment, electricity is free to pass through both the original portion and the repaired portion. In contrast, if the average grain size in the repaired portion was larger than the original portion, then the electricity would bottleneck through the original portion, and the conductor would overheat from the excessive current. Thus, in the embodiment, the temperature in the repaired portion will also be less than or equal to the temperature of the original portion, thus preventing overheating and bottlenecking simultaneously.

Figure 6

The path of electricity through a repaired conductor 601 is illustrated diagrammatically in Figure 6. Conductor 601 comprises an original portion 602 and a repaired portion 603. In use, for example when connected as part of a DC line inductor for electric trains, electrons providing electricity enter the conductor 601 as indicated by arrow 604. Given repaired portion 603 has an average grain size which is smaller than the average grain size of the original portion 602, the electrons follow the grain boundaries as previously described and are transmitted through both the repaired portion 603 and the original portion 602, as illustrated by arrows 605 and 606 respectively.

The applicant has performed testing of conductors of the type described herein. The testing included samples of aluminium conductors having repaired portions of three (3) millimetre (mm), five (5) millimetre (mm) and ten (10) millimetre (mm) diameters. Current was applied through the tested aluminium conductors to achieve temperature of between eighty (80) and one hundred (100) degrees Celsius (°C), maintaining the required maximum variation of between zero (0) and twenty (20) degrees Celsius (°C) between the original portions and the repaired portions of the conductors.

While the application described herein specifically relates to the repair of conductors used in reference to DC line inductors, it is appreciated that the same method of repair and the repaired conductors themselves are applicable in alternative applications, such as relating to the repair of transformer windings or other similar equipment. It is further appreciated that the method of repair and the repaired conductors described herein can be utilised in any other application which requires the use of conductors subjected to high current and high temperatures and which are susceptible to damage from such currents and temperatures.

Claims

1. A method of repairing a conductor for use in direct current line inductor coils, comprising the steps of: preparing a damaged surface of a conductor for repair; welding said surface so as to effect a repair such that said conductor comprises an original portion and a repaired portion; cooling said repaired portion at ambient temperature such that the average grain size of the repaired portion is less than or equal to the average grain size of the original portion, thereby providing an electrical path through said original portion and said repaired portion, such that when a current is applied through the repaired conductor to achieve a temperature of between 80 and 100 degrees Celsius through the electrical path, the temperature through the electrical path of said repaired portion is less than or equal to the temperature through the electrical path of said original portion within a maximum temperature variation of between 0 and 20 degrees Celsius between said original portion and said repaired portion; wherein said step of welding said surface involves tungsten inert gas welding and the gas used comprises seventy-five percent helium and twenty-five percent argon and said step of welding includes a flow rate of said gas of between 10 and 12 litres per minute.

2. A method according to claim 1, wherein said step of preparing said conductor for repair comprises cleaning said damaged surface using a grinding tool.

3. A method according to claim 1 or claim 2, wherein said conductor comprises aluminium alloy.

4. A method according to any one of claims 1 to 3, wherein said repaired portion is repaired by means of a butt weld.

5. A method according to any one of claims 1 to 3, wherein said repaired portion is repaired by means of a filler weld.

6. A method according to any one of claims 1 to 5, wherein said repaired portion is repaired with a filler metal comprising an aluminium alloy.