Page 10 - Industrial Technology EXTRA - 6th April 2020

POWER RESISTORS... Cressall Resistors

miles. While they may carry almost all of our braking voltage of the unit cannot be connected to the
communications, many of us are barely aware of their vessel, it is not possible to use any of the re-generative
existence. capacities of a motor to power the ship. This means
A cable layer is a deep-sea vessel used to lay that a large resistor is required to dissipate all of the
underwater cables on ocean floors for braking energy, as none of it can be fed back into the
telecommunications and electric power transmission. system.
Cables are stowed on the vessels in a spiral cable To combat these challenges and manage excess
carousel and unreeled according to the speed at braking energy onboard cable layers, engineers need
which the cable laying takes place. Today, the largest to turn to a rugged solution. Most ships have a chilled
cable layer vessels can store as much as 9,000 water system, making it straightforward to
tonnes of cables on board. incorporate resistors into the cable laying
The ships themselves are also of an immense infrastructure.
weight, with a tonnage of up to 11,000 tons and the Cressall’s EV2 water-cooled resistor is a 25kW
ability to lay several cable lines at once. With anything unit available as a single unit or as a block of multiple
up to a mile of cable being laid out over the side units up to MW with a common cable box attached to
during passage, the weight of the cable is so massive braking power input, all using a common fresh water
that the drive motor has to reverse its function from supply. Cooling is achieved in the EV2 by pumping
motor to generator in order to brake the cable reel. cold water, which comes into one end of the system
Braking needs to occur regularly while the cable is and then absorbs the heat. It can be pumped through
lowered onto the sea floor to control the laying speed. a radiator, which can be located some way from the
This frequent braking, along with atmospheric heat generating equipment.
problems such as salt, water ingress and extreme However, if the end users chooses to cool the
temperatures creates a pretty tough working resistor using sea water, they would require a different
environment. solution. In this case, titanium-sheathed elements in
To safely dissipate the excess energy created by high-grade stainless-steel vessels are preferred as they
braking, a bank of braking resistors is required. These are able to withstand constant harsh environ-ments.
resistors must be able to withstand harsh, corrosive This means that the resistor’s voltage capabilities and
environments so that the components do not degrade its durable materials can safely dissipate all of the
over time. An additional challenge that these braking excess heat created during cable laying braking to
resistors face is insulation resistance. As the DC make sure the operation is carried out without causing
harm to other components on the ship.
Underwater cables are the invisible force driving
the internet. These cables are a critical part of modern
infrastructure and making sure they reach the ocean
floor without any problems is an important task. To
successfully manage the frequent braking that is
required during this process, a durable and
capable resistor should always be found onboard.
www.cressall.com

6 April 2020 • INDUSTRIAL TECHNOLOGY EXTRA! • p10