Restraining EHV cables under fault conditions
06 March 2013
Thanks to a ready range of cable saddles designed specifically to secure extra high voltage (EHV) cables, a Yorkshire based cable cleat manufacturer managed to beat off competition to supply a new Hydro Accumulative power plant in Russia’s Moscow Region.
The cable saddles (from Ellis Patents' Centaur range) were specified by Russian cable engineering and cable supply company, Systec LLC for use in the second phase development of the Zagorskaya Pumped Storage Plant, which is located in the administrative district’s Sergiev Posad area. The plant uses two lakes at different altitudes to generate sufficient power to cope with Moscow's peak electricity demands, and when complete, will deliver 1,200MW of generated power and 1,320MW of consumable power.
Centaur is a heavy-duty extruded aluminium saddle cleat that was designed and developed in-house by Ellis in response to a serious safety issue surrounding the restraint of high voltage and EHV cables up to 160mm in diameter. Ellis’ export sales director, Tony Conroy provides some background to the Systec order:
“When we launched Centaur in 2008 neither the British nor European Standards took into account cleats on cables of this size. This meant those specifying such projects were very much in the hands of the manufacturers, who in most cases simply provided warranties for their products.
“The problem with this was that none of the products available had been short-circuit tested, and so the warranties were based purely on calculations and mechanical tests. As such there was no proof the saddle cleats being used would withstand the most testing elements of the job they’d been bought for.”
Prior to launch, Ellis put its Centaur cable saddles through the most rigorous of testing procedures. The company shipped the them to the Netherlands where they were tested on EHV cable to 163kA peak and 63kA rms for one second, in both three-phase and phase-to-phase fault scenarios.
Systec senior project manager, Pavel Udovitsky summarises: “We knew that Ellis had developed a cable cleat for EHV cables and when we spoke to them about the benefits it would bring to this project, the answers they provided left us in absolutely no doubt that its Centaur cable saddle was the ideal specification option.”
Centaur cable saddles and accessories were also recently used to secure high voltage cables in the National Grid’s London Power Tunnels project.
Reclassify cleats as protective equipment
Richard Shaw has called on the IET to address a longstanding installation issue that has the potential to cause serious health, safety and system integrity problems, by reclassifying cable cleats as protective equipment.
“For many years we have banged the drum about the sheer importance of correctly cleating cables and the dangers that arise if short cuts are taken, yet still we see far too many installations where this advice has been ignored and corners cut simply to save money,” he says.
“Where to point the finger of blame is a complex and difficult problem, but how to resolve the situation is straightforward,” he continued. By reclassifying cable cleats as protective equipment you would immediately see them being correctly specified and installed in every new electrical installation.” Ellis’ argument for this reclassification is based on three key points:
- In the event of a short circuit fault the maximum electromechanical stress between the conductors occurs during the first quarter cycle – i.e. at or before 0.005 seconds
- Typical circuit breakers and other protection devices don’t trip and interrupt a fault until between three and five cycles (0.06 to 0.1 seconds)
- In contrast, correctly specified cable cleats earn their crust during the all important first quarter cycle, ensuring the cables remain intact and operational
“What this means in practice is that without properly specified cable cleats, the time, effort and expense spent assembling a circuit breaking system will all go to waste as any electrical installation will be irrevocably damaged by electromechanical stresses long before the short circuit protection devices are put to the test.”
Cable cleats restrain electrical cables in a manner that can withstand the forces they generate, including those generated during a short circuit. Without them, the dangers are obvious – costly damage to cables and cable management systems, plus the risk to life posed by incorrectly or poorly restrained live cables.
“What always need to be remembered when considering cable cleats is that in a short circuit situation all an under or incorrectly specified product will do is add to the shrapnel,” adds Mr Shaw.
Contact Details and Archive...