High Voltage Cable Tunnel Cleats, 275-400kV

Electrical Review  - March 2011
Ellis Patents High Voltage Cable Supports for Cable Tunnels

Centaur Cable Saddles
 

                     Also - High Voltage Substation Cable Duct Sealing Systems

As a result of the increasingly congested and over populated nature of our major cities, high voltage cableEllis Patents Centaur Saddles - Cable Cleats For 132kV 275kV 400kV tunnels are fast becoming the preferred option for delivering electrical power – most notably because they provide a means of carrying large HV cables in a non-intrusive manner.

Andy Booth, development manager of leading cable cleat manufacturer, Ellis Patents talks to Electrical Review about a paper the company co-presented at the recent CIGRE Session in Paris, which focused on the issues concerning the clamping and supporting of HV cable systems in underground cable tunnels.

Many of the leading utilities companies are currently investing heavily in HV cable tunnels, and to give some idea of the scale of this investment it’s worth highlighting some of the proposed cable tunnels in London are 3m in diameter and up to 25km long. Furthermore, the level of investment in new HV cable installations is likely to grow substantially over the next few years due to the fact many of the UK’s existing fluid filled 275kV and 400kV HV cable circuits are reaching the end of their original design life and will need to be replaced with new XLPE circuits.

As a rule of thumb, these HV single core cable systems are installed in a vertical flexible arrangement. This means the cables are supported, generally by cable cleats, at intervals of between 5 and 8.5 metres and allowed to sag in between.

This cable sagging allows the cables to take up thermal expansion and contraction in steady state conditions without the exertion of large thermo-mechanical forces. During load cycles the thermo-mechanical thrust developed by the cable conductor and sheath in an axial direction needs to be constrained by the cable cleats without damaging the cable oversheath.

In the case of short circuit faults and lightning strikes, the resulting high fault currents flowing through the cable will result in large lateral electro-mechanical forces between cores, which cause the cables to shake violently. During these conditions accelerated sidewall pressures are experienced on the cable at the fixing points, which can compromise the integrity of not just the cleats, supports, clamps and the cable, but the entire HV cable network.

Amazingly, and despite the huge amount of theories, standards and literature regarding fault protection, very little attention is given to arguably the single most important piece of equipment in any fault protection system – the cable cleat.

Yes, IEC61914:2009 describes the appropriate requirements for cable cleats for electrical installations, but this only allows the use of 600V – 1kV cables in a series of tests to confirm the resistance to electro-magnetic forces.

Until now, methods for supporting large HV cables at fixed points have been designed on a project by project basis, but there have been no tests or related publications to determine how these HV cable fixings should perform in the event of a fault. Therefore, our intention when starting work on this project was simple – we wanted to develop a standard product that would provide adequate fault protection for all HV cable installations.

 

Rationale for appropriate fixings for power cable systems.

The operating time for a typical breaker is generally between 3 and 5 cycles, which is equivalent to 0.06 - 0.1 seconds on a 50Hz system. Exceptionally quick relays may operate at 1.5 cycles. However, when considering three-phase faults and the instance of the peak forces, the time frame will be a quarter of a cycle or 0.005 seconds.

On the occurrence of a fault the highest repulsive force is proportional to the square of the peak short circuit current. This is then followed by a residual, pulsating, oscillating stress at a frequency of twice the operating frequency, known as the fault RMS. However, it is accepted that the forces at the peak of the fault are the highest, the most instantaneous, and in turn the most destructive, when considering system protection.

Recommendations for the calculation of short circuit currents are given in the IEC 60909:2001 series. For three phase short circuit faults the most severe repulsive force for flat spaced (horizontally or vertically) cables is experienced in the central phase due to the oscillating effects of mutually induced forces by the outside phases. For trefoil installed cables an equal force (at peak) is experienced in all three phases due to the symmetry.

Further consideration should also be given to the linear stresses along the actual conductor. It is common with cable installation assessments to use the calculation method simulating a bar fixed at both ends, thus determining the transverse deflection rate due to electro-magnetic forces during a fault. Further consideration, as a result of the instantaneous forces during a fault, is the effect of the surface pressure from the moving cables and its effect on the inner loop of the cleat itself.

The time duration for short circuit faults, such as 1 or 3 seconds, which is often specified by clients or in installation specifications, is often misinterpreted with respect to the duration of an actual short circuit fault. The 1s or 3s requirement quoted is the thermal withstand characteristic of the cable and considers conductor cross section and its ability to carry a level of current and therefore heat.


Design Criteria for HV Cable Saddles and Cable Straps
A longitudinal ‘saddle’ type of design, rather than the traditional cable clamp design tends to be best suited to this type of installations. Firstly, the cable saddle should be able to support the weight of the cable in its final installed position – and remember a 2500mm² copper conductor, lead sheathed cable can weigh almost 50kg per metre.

If we assume an 8m fixing distance, then the cable saddle must be able to support 4m (200kg) on one side and 4m on the other side, without deflecting or changing its original profile. Furthermore, the longitudinal saddle must also be radiused along its length to ensure the cable is adequately and safely supported. Various cable construction types affect the radius of the installed cables. For example a lead sheathed, copper conductor cable will have considerably different characteristics to an aluminium sheathed, aluminium conductor cable. The saddle manufacturer must ensure their product design matches this specific cable sag radius on any particular project.

It is undesirable for a cable to be in contact with any sharp edge of a cable cleat. To alleviate the problems various steps can be taken: All sharp edges must be removed as a matter of course from any face which may come into contact with the cable, either during installation, or when the cable is in its final, fixed position.

Generally, the base portion of the cable saddle for this type of installation is a minimum of 600mm long. As the cable is installed over the top of this 600mm long ‘beam’ it becomes curved when sagged. It is essential that the 600mm long ‘base section’ is also curved along its entire length, to ensure support is given to the cable over an area which is as large as possible. On each end of this curved ‘saddle’ section, as the cable leaves the saddle, an additional ‘flare’ should be added to further reduce the possibility of the cable being in contact with a defined edge, and therefore becoming damaged. Once these general rules have been applied to an initial cleat concept, the actual cleat spacing and installation sag can be calculated.

 

Short Circuit Testing of a HV Cable Saddle Installation
There is very little empirical research, or cited publications with regard to short circuit testing cable fixings for HV cables. That said, major utility groups around the world use National Grid in the UK for technical expertise and knowledge and so it seemed sensible to use the technical specifications of National Grid as the basis for a series of live short circuit tests.

These tests were carried out at KEMA, an internationally recognised testing station in The Netherlands. The design of the test rig corresponded to the worst case scenario for the peak forces, and 8.4m fixing centres and a phase to phase spacing of 500mm was selected. If the calculation methods from IEC 61914:2009 are employed the maximum theoretical forces between each cleat can be shown as follows:

For a 2 phase fault:
F = 0.2 x 157.5² = 9922.5N/m or 9.923kN/m
          0.5

The figure of 157.5 was obtained by using a multiplication factor of 2.5 on a 63kA RMS. This was the theoretical calculation used to obtain the appropriate peak force levels of the fault.


Post Test Cable Examinations
Immediately after the tests were completed and the cables were still in position, an electrical test was performed on each individual cable. Each cable sample satisfactorily withstood a 5kV direct voltage, applied between the lead sheath and the earthed conductive screen for one minute without breakdown or incident. This procedure follows the requirements of ENA C55/4.

Upon dismantling the test rig a 1m section of cable was identified adjacent to each saddle and each intermediate strap (500mm each side of the saddle or clamp), and cut away for later examination. For each 1m length the following aspects were examined in great detail: Outer jacket over sheath, lead sheath, copper wire screen, lead sheath, and the interior surface core screen.

There were no features or defects attributable to the cleats or intermediate straps. Some features attributable to manufacturing and/or handling of the cable were seen, but as they were independent of the position of the cable cleats, it was evident that they were not due to the presence of the cleats. In any case these features were not of such a severity to compromise the performance of the cable.

Conclusion
With HV power cable installations becoming ever more commonplace, it was absolutely imperative that a tried, tested and trusted means of ensuring these cables remain intact and working during a short circuit situation was available to the industry. The Centaur saddle cleat that we developed as a result of this research has certainly been enthusiastically welcomed and is currently being installed in a major HV cable installation in the UK. That said, from an industry perspective, there is still a long way to go. It seems that every new type of HV cable and accessory seems to be tested to a known standard with the exception of the cable cleat. However, now that a precedence has been set by our research and development it should follow that every cable saddle, cleat, strap or clamp that is to be used on a flexibly installed, HV, underground system should be fully and independently tested to meet, or exceed, the requirements of the specific project. Furthermore, all engineers in the field need to become ever more aware of the importance of cable fixings.

A full copy of the CIGRE session paper is available upon request.

Video : Cable Tunnel Project From nkt Cables. 400kV Extra High Voltage Cable Installation. 30km 400kV 2,500sqmm copper XLPE cable with an APL fire retardant outer sheath. nkt project scope :
• Supply of 400kV straight joints and dry terminations
• Installation of the EHV cables by nkt cables fitters and jointers
• Tests on site using nkt cables test trailers
• Installation of a monitoring system




Adobe Cable Tunnel Design Standard - UKPN - March 2013

Adobe Centaur Cable Saddles for Cleating EHV Cables 132kV, 275kV, 400kV