Abstract

The Long Stator Winding (LSW) propulsion of the high-speed magnetic levitation (MAGLEV) Transrapid Shanghai project represents the most innovative technology application for a special MV rubber power cable with formerly unequalled mechanical and electrical requirements. For the first time, this article provides details about this special MV rubber power cable solution, using high-quality rubber compounds for conductor insulation and semi-conducting outer sheath with gliding coating for the long stator winding of the Transrapid Shanghai propulsion, and to report how the high-demands and specifications of the whole system were fulfilled.

Introduction

The Transrapid magnetic levitation and propulsion is the first fundamental innovation in railway technology since the first railway was built.

Figure 1: Long stator winding motor principle[1]

The MAGLEV propulsion technology is based on the principle of a long stator linear synchronous motor. Contrary to the conventional propulsion of trains running on tracks, the propulsion of the Transrapid is divided into in the long stator being integrated in the guide-way (Figure 2) and the armatures being integrated in the train, thus significantly reducing the propulsion’s contribution to the total weight of the train.

Figure 2: Guide-way girder with stator packs, grounding sleeves and cable on both sides

The long stator of the Transrapid’s linear motor comprising stator packs (Figure 3), stainless steel grounding sleeves, grounding cable and a 3-phase motor winding is integrated on both sides of the guide-way. The train is driven by the levitation magnets acting as linear propulsion armatures (Figure 4) in the electromagnetic travelling field of the long stator.

Figure 3: The long stator motor’s travelling field

Figure 4: Powering principle of the Transrapid propulsion

The electric system of the guide-way is subdivided into motor sections having an average length of 1,000m. One of the main economical benefits of this propulsion system is, that only the motor section is being energised where the Transrapid is just passing.

The Shanghai Transrapid project is the first commercial application of the Transrapid high-speed MAGLEV technology with the basic technical concept being transferred from the Transrapid MAGLEV test track in Lathen / Emsland, Germany. A 30km double track line connects Shanghai’s Pudong International Airport with the Long Young Road Station of downtown Shanghai. Approximately 1,000km of the special MV rubber power cable, the so called LSW cable, were used in individual lengths up to 3,000m. Nexans produced these cables from December 2001 to September 2002, installed and surveyed the laying of the LSW in Shanghai, which was finished in November 2002. After the successful VIP run on December 31st, 2002, the Transrapid Shanghai started its regular service in January 2004, running at a speed up to 430km/h.

Figure 5: The Transrapid Shanghai on the track

The Transrapid Shanghai LSW

1. The Shanghai LSW Concept
The LSW is a three-phase motor winding with a 86cm longitudinal spacing between the different phases and a width of 400mm with about 55 per cent of the winding expanding outside the stator packs. The LSW comprises different shape patterns for the upper layer (blue), middle layer (green) and lower layer (red). A U-bend is the common bend pattern for all layers. The meander then received crimping. A marginal upwards crimp of the lower layer pattern for stability reasons and a sharper upwards bend for the upper layer. The middle layer received the is double crimped in terms

Figure 6: Three Phase Motor Winding

The Shanghai LSW was designed self-supporting in two ways:

• Once being shaped in the appropriate pattern, the LSW cable has to maintain this pattern (Figure 6)
• After the cable was pressed in the stator grooves, it had to stay there without any additional support of clamps (Figure 7).

Figure 7: LSW cable in the stator groove

As the Transrapid demands always 10mm of free space underneath the long stator during operation under all circumstances, the requirements above represent a very strong mechanical system requirement. Electrical-wise, the LSW cable is not energised such as a regular power cable. The LSW is driven by switched rotary current with a peak current load of 1,500A at the maximum acceleration points, a peak voltage of 20kV at the steepest sections of the guide-way with a frequency ranging from 50 up to 250Hz.

2. The LSW Production and Laying Technology
The 1m long stator packs representing the mounting media for the LSW cable and the iron core of the long stator were pre-mounted to the girders before incorporating them to the guide-way on site. Different to the test track, where pre-shaped LSW cables were press in the stator packs at moderate height, the Transrapid Shanghai project introduced a completely new and unique laying concept due to the specific route of the guide-way passing rivers and motorways in an altitude up to 13m. Shaping and laying of the layers should be done continuously in one sequential process by an automated robot operating just from the guide-way. The only bottom activity necessary should be loading and unloading of the drums by cranes. Development and production of these automated laying robots, done in approximately one and a half year by Ferrostaal Maintenance, Germany, represented another outstanding engineering achievement of the ambitious Transrapid Shanghai project. The robots consisted of two vehicles, a propulsion vehicle carrying the power supply etc. and a laying vehicle containing the bending and crimping unit.

Figure 8: Automated robot laying vehicle on the track in Shanghai

The LSW cable drum, weighing 6 tonnes, was loaded perpendicular to the longitudinal axis of the track axis in the pay-off of the vehicle. The cable was fed non-continuously to the bending and crimping unit giving enough time for the bending and crimping process. The shaped meander was then forwarded to the pressing unit with a camera supported self-alignment system for the longitudinal and transversal axis. The first being necessary to prevent serious failure of the pressing unit, and the second being an obligatory requirement for a symmetric travelling field. After successful alignment, the meander was pressed in the stator grove. The laying robots were fully computer controlled, i. e. once set-up for the installation of a specific layer pattern, the machine operated completely self-controlled. The laying robots were operated by a Chinese crews each headed by a Nexans supervisor, who had to take care for the training status of the Chinese operators too. Up to six automated laying robots operated on the track in Shanghai at the same time, enabling the installation of the three phase motor winding on both track which equals approximately 1,000km of cable in a remarkable time period of six months from May to November 2002.

The LSW Cable

1. Product Specification Profile
The above outlined LSW concept incorporating permanent shaping, self-supporting concept, robot processing, pulsed power operation, and grounding through the outer sheath generated a very specific product specification profile with formerly unknown strong mechanical aspects. The specification profile included outer diameter with narrow two sided tolerances, high-radial elastic properties, smooth surface for automatic processing, homogeneous mechanical properties, extreme bending radius up to 1.5D, long-term stability of shaping, long-term electrical stability after shaping, as well as unique screen and sheathing design

2. Cable Design
The LSW cable is basically designed as a 10/17.5 (20)kV MV power cable interpolated in size from IEC 60502 according international standards, as showed in Figure 9. The permanent shaping of the layers required an aluminium conductor and paying tribute to the self-supporting concept of the LSW, insulation and sheathing had to have superior elastic properties resulting in the only possible choice of rubber for the design.

Figure 9: LSW cable design

3. Materials
The LSW cable uses a special designed 300mm2 aluminium conductor, where the mechanical functionality probably superseded the electrical functionality. The conductor was to deliver superior homogeneous bending properties and had to maintain the shape given by the bending and crimping unit of the laying robot without degradation of the electrical performance. It took several trials to work out the best suitable pattern of lay lengths, layer orientation, compacting ratio and in particular the appropriate annealing process in the Nexans Lens plant, France (Figure 10).

Figure 10: Simulation of middle layer shaping[4]

In addition, analytic approaches were made by the Frauenhofer Institute Produktionstechnik using finite element analysis to investigate the interaction of the shaping tools and the conductor’s behaviour under the stress of the bending and crimping process to optimise tools and conductor make-up. The final conductor design used for the Transrapid Shanghai project had an outer diameter of 20.3mm, leaving additional space for the insulation thickness, which enables to give the cable even more electrical headroom than required by the product specification.

Similar to the conductor, there was at least two major mechanical aspect of the insulation system, supporting the general choice of a rubber insulation:

• Representing almost 35 per cent of the total cross section and 73 per cent of the elastic volume of the cable, the insulation had to deliver a high-compression ratio at comparatively low radial forces, due to the self-supporting concept described above;
• The insulation had to act like an elastic corset for the stranded aluminium conductor during the bending and crimping process with some kind of self healing properties resulting from the elastic back-spring forces of rubber.

To support the economical power feeding concept of the Transrapid, a special low-loss EPR insulation compound was developed for the LSW cable with a dissipation factor of < 0.003 at the the Nexans plant Mönchengladbach, Germany. EPR again is the basic material of the compounds used for inner and outer conducting layer of the MV power cable triple insulation system. The compounds used for the inner and outer conducting layers were produced according Nexans recipes. Different from the common MV power cable design, no regular copper screen could be applied to the LSW cable, as it was expected that the mechanical stress of the cable during the bending and crimping process would seriously damage the geometry and thereby the electrical function of stranded or braided metallic screens or a wrapping using solid metallic tapes. In addition the untypical system requirement of a resistance window of (1 - 10)mΩ/cm for the screen resistance comprising the metallic screen, semi conducting outer sheath and the semi conducting gliding coating, asked for such a low copper content, that this could not properly achieved with standard metallic screen designs. The solution was a special designed copper woven tape wrapped helical over the outer conducting layer of the insulation system. This unique design showed nearly impact on the screen resistance after the bending and crimping process. The x-ray in Figure 11 proved this stability of copper screen tape and aluminium conductor of the LSW cable at the most extreme bent section of phase 2.

Figure 11: X-ray analysis of bent LSW cable

The outer sheath is again a multifunctional component of the LSW cable matching high electrical, mechanical and chemical requirements:

• High-elastic properties finally lead to the basic choice of a rubber based material again;
• Grounding of the LSW cable should be done by outer sheath;
• Exposition to plain air and implied resistance against several environmental and chemical media;
• The application of the gliding coating asked for a sufficient adhesive surface;
• Flame-retardant behaviour.

Nexans succeeded in developing a flame retardant semi-conducting PCP sheathing compound being flame-retardant acc. VDE 0472 part 804; ozone-resistant according to VDE 0472 part 805; oil-resistant according to VDE 0472 part 803; and passing a custom defined climate simulation in combination with UV-light exposition[2]. The gliding coating was a short-term component required by the automatic laying process. The only long-term perspective was, that no impact on the electrical and environmental properties of the LSW cable was permitted. The solution was a Teflon-based semi-conducting coating with an average thickness of (20 ±10)µm and minimum spot thickness of 5µm cable on a CV-line. A laser-driven diameter control took care of the outer diameter tolerance. Supporting manual measurements were continuously performed during the production, to assure critical outer diameter tolerance of 38.7 (±0.2)mm for the average and 38.7 (±0.4)mm for the individual diameter.

4. LSW Cable Qualification
The basic requirements of the Table I were adopted from VDE 0276 respectively IEC 60502-2 for a maximum compliance with the other components of the Transrapid propulsion system and power supply.

Table I: Basic requirements according to IEC 60502-3[2]

Paying tribute to the specific powering conditions of the LSW under operation, the investigation of the breakdown voltage (Figure 12) with continuous and impulse feeding and at minimum and maximum conductor temperature delivered a head-room of a factor 10 above the maximum utilised voltage. The full LSW cable qualification program listed a total number of forty tests. Two outstanding system qualification tests will be described to demonstrate the extraordinary demands on the long term electrical stability of the ready installed cable. Among others, these two system test were performed during the VDE qualification program at the Nexans Energy Networks HV laboratory in Hannover.

Figure 12: Breakdown voltage of LSW cable

Five stator pack were mounted on a support to form a 5m long motor section. A three-phase motor winding was installed according the system specification for the Shanghai project. Then a two step test procedure was applied with a three section long-term test spreading over almost a three months period. Six thermal sensor were installed to one phase of the motor section at defined positions, to determine a conductor temperature of 90°C from measuring the sheath temperature. The electrical loading conditions to achieve this temperature were empirically determined on a different motor section before.

Figure 13: Test setup for serviceability of three phase long stator winding[3]

The basic electrical parameters of the motor section were tested first:

• DC resistance, capacitance and dissipation factor of the LSW;
• Screen voltage between grounding sleeve and middle of the meander of the long stator winding when operated at 25kV.

As an initial test for the long-term part of the test, all phases of the motor section received earth resistance of the grounding system, voltage test at 30kV, and partial discharge test at 24kV. The phases of the motor section equipped with the thermal sensors then received 500 temperature cycles, of an one hour heat up to 90°C conductor temperature followed by a three hours cool down time summing up to a 2,000h-test duration. After that, the measurements of initial test were repeated verifying that the electrical characteristics of the motor section still meet the system specification. In particular the sensible partial discharge measurement showed no significant degradation of the cable even when compared to the final cable test, predicting a high long-term stability of the insulation system of the installed cable.

A 12m-cable sample is mounted on a 2m-frame with double bends having a 1.5D bending radius and holding in place by metal clamps. The mounted cable then receives thermal aging at 90°C for seven days. The following initial tests were applied:

• Voltage test at 30kV;
• Partial discharge test at 24kV;
• Impulse test at 125kV.

The cable was then connected to a 24kV high-voltage supply and receives 63 heating cycles from a 1,000Hz high-voltage generator with each cycle having a five hours heating time to a conductor temperature of 90°C and followed by an at least three hours cooling down time. After this long-term test the initial test sequence was repeated verifying that the LSW cable still meets the system requirements (Figure 14).

Figure 14: Test set-up for bent and thermally treated LSW cable[3]

5. Specific Regular Sample Test
The unique installation conditions generated specific mechanical requirements with some special designed made test equipment. In addition, the control of the automated laying robot, based on empirical, sets of parameters with a limited tolerance range. For example, low variations of the bending properties were required to minimise the risk of a failure on site. Therefore a regular sample test was introduced, to determine the bending force ≤ 100N and the back-spring angle ≤ 20° of the cable for one 90° bend.

Figure 15: Bending/crimping unit producing a phase 2 shape

To further assure a failure-free operation of the laying robots on site, a stationary bending and crimping unit, similar to the equipment used on site in Shanghai, was installed to determine the stability of the shaping of the LSW cable. The height of the upper bow was measured immediately after the bending and crimping process and re-measured after 30 minutes and 10 hours. The LSW cable proved to be remarkably stable after the bending and crimping process with typical deviations of < 0.1 per cent. In addition to the in-line control of the coating thickness, a regular sample test was established to determine the effective gliding properties of the coating around the circumference of the cable where a cable sample was mounted upright in a reception channel and a stainless steel slider was pulled by a universal testing machine to measure the pulling force.

As previously stated, the long stator cable was self-supporting, asking for minimum extraction forces of 800N. The automatic pressing of the cable required a maximum pressing force of 3.5kN to enable the system to distinguish a non-properly aligned position. Therefore, a pressing and extraction test was introduced as a regular sample test using a universal testing machine.

Outlook

Apart from the Bavaria Transrapid project in Germany, which is already scheduled for 2007/2008, representing another airport to city line application and further Chinese projects under consideration, the successful operation of the Shanghai Transrapid might break way for the use of this fascinating technology for the California-Nevada Super Speed Train Commission (2011) too.

Conclusions

This article showed how the very specific system requirements for the LSW cable of the Transrapid Shanghai propulsion were successfully transformed in a unique MV rubber cable design of formerly unknown mechanical and electrical performance. It shows that a well-elaborated production control was able to deliver a low-tolerance product for automated robot processing. This is in particular remarkable, as we presented a “big size” cable featuring compounds based on “natural” rubber raw materials, being considered as not comparable controllable like the common VPE.

References

[1] www.transrapid.de
[2] Nexans product specification PS.TR.WFL.02
[3] VDE test report 21330.2-5920-0090/4007G, 2003-04-30
[4] FEM-Simulation des Kabelbiegeprozesses für die WFL zur Herstellung von Statoren für den Antrieb des Transrapid,

This paper was delivered at the 52nd seminar IWCS
Philadelphia, Pennsylvania, USA - November 2003
Printed by courtesy of IWCS - © IWCS 2003

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