Over the last ten years shaft-driven shipboard mechanical power transmission has been increasingly replaced by electrical power. For example, both the Royal Navy Type 45 destroyer and the Queen Elizabeth carrier are electrically driven. The prime mover is used solely for generating electricity, which is then distributed to various motors, including those driving the propeller shaft(s), and other consumer loads.
The logic behind all-electric ship design that is two-fold. Firstly, ship layout – a large motor does not need to be in the middle of the ship, which is better used as prime passenger and cargo space. Secondly – energy efficiency. Traditional ships could have up to four diesel engines located mid-ship running at low power under normal operating circumstances. This often made them inefficient. By using smaller diesel engines together with a couple of gas turbines, the right number of prime movers can be fired up to suit power demand, whether it is a fully-laden oil tanker afloat at sea or stationary at port.
In drilling or cable-laying vessels, where anything up to a mile of cable has to be paid out over the side during passage, the weight of the cable is so massive, that the drive motor has to reverse its function from motor to generator, in order to brake the cable reel. Air cooled resistors have heating elements enclosed within a fan cooled cabinet. This makes them suitable for deck mounting, often on anti-vibration mounts.
An important benefit from using an electric drive is that reliable systems of regenerative and dynamic braking are available to complement or replace traditional mechanical braking systems. The advantages of electric braking include control, reliability, mechanical simplicity, weight saving and, in certain cases, the opportunity to make use of the regenerated braking energy.
In such applications, as well as in cranes, lifts, hoists and conveyors, air cooled resistors are common, but in medium voltage, high power applications liquid cooling wins out, according to David Atkins, projects director at Cressall Resistors. “A medium voltage drive running off a 3.3kV supply to turn a 500kW motor will put a severe heating load on the application’s power transistors – hence the need for water cooling,” Atkins says. “As a rule of thumb, if 1MW or more of heat needs to be dissipated, direct water cooling makes more sense.”
Cressall’s The new EV2 advanced water cooled modular resistor range for low and medium voltage applications is especially designed to function in severe environments, like the ones marine systems have to function in. This patented design encapsulates and completely separates the resistor elements from the coolant.
A typical 1MW fresh water-cooled brake resistor for marine use is basically a 10 foot tube with large heating elements similar to those found in common household kettles. The braking electricity that is being regenerated powers these elements. Most ships have a chilled water system, which circulates cool water throughout the vessel, used for both air conditioning and equipment cooling.
There is also a sea water variant with construction and ratings similar to the fresh water designs, but with titanium-sheathed elements in higher-grade stainless steel vessels, suitable for continuous duty in hot sea water. The problem is that heating elements cannot be used here at an operating voltage of much above 1.5kV, whereas medium voltage drives will typically use a 3.3kV supply.
Cressall’s EV2 resistor is not limited by this restriction. This is a 25kW unit available as a single unit or as a block with a common cable box attached to 250kW braking power input using a common water supply. Cold water comes in one end and hot water comes out the other. The EV2 has the advantage of higher operating voltage, lower weight and compactness, and is therefore ideal for marine applications.