Diving into marine resistor design

DIVING INTO MARINE RESISTOR DESIGN

DESIGN CONSIDERATIONS FOR OFFSHORE ELECTRICAL COMPONENTS

Covering over 70 per cent of the Earth’s surface, the oceans are a vital element of our planet’s ecosystem. However, for the millions of vessels that cross them, the aquatic environment can present a problem. Vessels are increasingly using electrical systems to power across oceans, but a component’s design must account for these extreme conditions.

Whether for main propulsion propellors, crane or lifting systems, or cable laying, electrical drives can be found at the heart of many marine operations, offering increased control, reliability and mechanical simplicity. Dynamic braking resistors (DBRs) are an essential part of an electric drive system that remove excess energy from the system when braking to either dissipate as heat if system is not receptive to regeneration or if system is receptive, but energy level goes beyond the system limits, so needs to be removed.

When designing electrical components for offshore applications, material selection is key from the start of the process to guarantee that equipment will perform under harsh conditions, including saline atmosphere, high wind loadings and corrosive sea water.

Engineers tasked with designing resistors for marine applications must consider material choice, structural stability and cooling method.

CORROSION-RESISTANT MATERIALS

Sea water and the saline atmosphere is corrosive, which could leave equipment inoperable. Due to this, stainless steel, combined with special paint systems, is typically used for the enclosure metalwork for resistor elements. With materials containing at least 10.5 per cent chromium, stainless steel reacts with oxygen in the air to produce a protective layer on its surface to prevent corrosion if not painted.

There are many grades of stainless steel that can offer high corrosion resistance, which can be further enhanced by the addition of extra elements. For below-deck applications, 316 and 304 stainless steel contain nickel to broaden the protective layer created by the chromium, and can be used in unpainted condition.

However, for above-deck components, 316 stainless steel has a higher nickel quantity and added molybdenum, so the resistor unit’s metalwork receives optimum protection against the marine atmosphere, but in some conditions, painting will also be required. Cressall’s resistor enclosures for the EV2 resistor terminal cover boast at least an IP56 ingress protection rating, certifying that sea water cannot enter the unit to cause harm.

In addition to the exterior, it is important that the resistor’s element can withstand the harsh conditions. For these applications, Alloy 825 sheathed mineral-insulated elements are less vulnerable to atmospheric corrosion. As the element in encased within the mineral insulated sheathing, the sheath is at earth potential, so if water or high humidity is present this will prevent accidental contact with the live element, making them a much safer choice for marine applications.

STRUCTURAL STABILITY

Weather at sea is unpredictable, so vessels must be able to withstand the large variance in wind and harsh sea conditions found worldwide. Many offshore structures such as wind turbines are located in areas with high winds, so if the system requires resistors to help provide stability to their electrical components these considerations must be considered within a resistor’s design.

Considering the impact of a vessel’s rotational motions — its side-to-side motion, or pitch, and its front-to-back motion, or roll, is crucial. Design engineers need to ensure that there is enough mechanical support in the structure to stabilise the resistors for safe operation when it is subjected to these forces.

Cressall can conduct finite element analysis (FEA) to help ensure structural stability. FEA allows design engineers to predict a product’s performance in the real world, then see the impact of forces and make changes accordingly. This ensures the resistor performs well in the potentially extreme weather conditions.

It’s also important to consider the size constraints of marine applications. In contrast to onshore units, offshore electrical components must fit into a compact area, so the size of the unit’s support structures must be minimised without compromising durability.

COOLING METHOD

An essential part of a resistor is its cooling system. Since the resistor dissipates excess energy as heat, the cooling system is responsible for cooling the resistor element to ensure continued operation. Depending on the layout and resources of the system, resistors can be naturally or forced air or water-cooled.

Air-cooled resistors come in two types — forced and naturally cooled systems. Forced cooling systems use a fan to dissipate heat in a compact space. These units are suitable for deck mounting and can be secured using anti-vibration mounts. Natural cooling is the most common in marine applications, offering a higher power rating and can be mounted in machinery spaces, protected environments or on deck. For machinery spaces or protected areas, consideration should be given to how the hot air released from the resistors should be evacuated to ensure other equipment mounted locally does not overheat.

Alternatively, the cooling system can use the vessel’s chilled water system, which circulates cool water for air conditioning and equipment cooling. If the chilled system uses sea water, titanium-sheathed elements with super duplex steel metalwork can be incorporated, for continuous use in acidic, tropical sea water and downgraded to 316 stainless steel for freshwater systems.

The ocean is a valuable asset for energy, transport and trade. Ongoing development of electric drives for marine applications can be challenging, but taking these conditions and energy savings into account makes them a viable and advantageous option for powering vessel and for use in offshore structures.

When required Cressall can design the resistors to help with your application. Contact us here.

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Industry the planet’s close friend

INDUSTRY: THE PLANET’S CLOSEST FRIEND OR MOST SWORN ENEMY?

industrialpollution and global warming

REALISING THE GOVERNMENT’S BUILD BACK BETTER PLAN

Britain — the birthplace of the Industrial Revolution, the golden age of innovation that transformed society. However, the fossil fuels that powered the revolution have left a detrimental mark on our world, which we are fighting to change with Net Zero. Is the industry that triggered the climate crisis in the first place part of its solution?

The Industrial Revolution transformed the world, igniting technological development that continues to this day. But it has also had disastrous consequences for the planet, with carbon emissions from fossil fuel use triggering the climate crisis. 

However, the necessity of industry is well recognised. The UK’s manufacturing and refining sectors contribute £180 billion to the economy and provide millions of jobs, both directly and indirectly across the entire manufacturing value chain, presenting a dilemma — is industry a help or a hindrance to the planet’s future?

INDUSTRY’S CARBON CONTRIBUTIONS

The Industrial Revolution triggered a rise in the Earth’s core temperature that is yet to stabilise. Since 2018, the Intergovernmental Panel on Climate Change (IPCC) has been warning us that a temperature increase of more than 1.5 degrees Celsius (°C) above pre-industrial levels will result in irreparable damage from extreme weather, failed harvests and species extinction.

The Government’s Net Zero strategy provides a roadmap to successfully combatting the climate crisis. Published in October 2021, Build Back Better gives details on how the UK will achieve Net Zero carbon emissions by 2050. Industry is at the heart of this challenge, both as a carbon contributor and emission eliminator.

Industry is a major source of carbon emissions, producing 15 per cent of the UK’s total. The Government estimates that emissions associated with industry need to drop by as much as 96 per cent by 2050 to achieve Net Zero status — demonstrating the magnitude of its current contribution to the climate crisis.

RESOLVING INDUSTRY’S PROBLEM

Industry’s damage to the planet has incrementally decreased over the last couple of decades. However, to keep momentum, further innovation is necessary to reach Net Zero in this huge carbon-emitting sector, both directly and indirectly. 

According to the International Energy Agency (IEA), industry’s indirect carbon contribution through its colossal energy consumption accounts for 40 per cent of the globe’s total. The move to a decarbonised renewable power supply will help eliminate this. 

However, the situation is more severe with direct CO2 industrial emissions. Since some crucial processes don’t currently have a carbon-free alternative, emission elimination is not always possible — reduction is as far as it can go. CCS is key to aligning industry with Net Zero, ensuring essential carbon-emitting processes continue without the climate consequences.

INDUSTRY’S INNOVATIVE INPUT

Despite being responsible for a large proportion of emissions and acting as a catalyst for the birth of the climate crisis, industry is also the planet’s saving grace. 

The Government’s Net Zero strategy is striving for a fully decarbonised, reliable power supply that integrates both renewable sources, like solar and wind, and dispatchable net-zero sources like natural gas with carbon capture and storage (CCS). In transportation, the goal is to ensure all cars are zero-emission capable by 2035, end the sale of petrol and diesel heavy goods vehicles (HGVs) by 2040 and achieve a net-zero rail network by 2050. 

Reaching these challenging targets involves key manufacturers developing innovative products and services to enable Net Zero. For example, at Cressall Resistors, we manufacture a range of resistors crucial to reaching Net Zero. For the automotive market, the EV2 dynamic braking resistor facilitates regenerative braking in electric vehicles, helping to increase vehicle range and improve the viability of a fully electric national fleet at an unrivalled weight and size to power ratio.

When it comes to decarbonising the nation’s power supply, pre-insertion resistors are used to prevent overvoltages caused by renewable energy’s variable input, while load banks safeguard all power systems by proving their power generation capability. Resistors are necessary to protect every electrical system and make Net Zero a realistic goal.

The Industrial Revolution is by and large to blame for the catastrophic levels of CO2 that have been emitted into our atmosphere since the eighteenth century. But it’s also a crucial part of the solution. Not only through eliminating its own carbon footprint, but also by developing the components to decarbonise other sectors. 

With the full Net Zero strategy revealed, now’s the time for industry to step up and take responsibility for preventing more damage to the planet and shift its position from the planet’s most sworn enemy to its closest friend. 

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Futureproofing tidal power

FURURE PROOFING TIDAL POWER

HOW TECHNOLOGY CAN HELP TIDAL POWER TO REALISE ITS POTENTIAL

The UK Government estimates that tidal energy could meet around 20 per cent of the country’s electricity demands. Considering the UK is an island and entirely surrounded by water, this comes as no surprise. Despite this fortunate position, uptake of tidal power has been slow. How should we encourage the development of this promising resource?

Tidal power functions in a similar way to wind power. Tidal turbines are placed underwater where the change in tide from high to low and low to high turns the blades to produce electricity. Tidal power is more reliable than solar or wind because we can easily predict the movement of the tides, which is determined by the Moon.

However, tidal power comes with extremely high upfront costs. To make the resource more feasible, its technology needs to deliver a high performance, allowing this cost to be recovered more quickly and making tidal power more appealing.

BIOFOULING PROTECTION

Biofouling occurs when plants and animals attach themselves to underwater constructions as often seen on the hulls of ships. However, biofouling also alters the hydrodynamics of submerged tidal turbines, presenting a productivity problem.

The biofouling organisms attach themselves to the surface of turbine blades making them rougher, which increases losses due to friction and therefore reduces the efficiency of the turbine. This, in turn, will lower tidal power’s performance and make it less cost-efficient.

Antifouling methods, such as a non-toxic coating with a low friction, can prevent organisms from attaching to surfaces whilst avoiding damage to surrounding marine life. These coatings are currently used in the shipping industry, but we must explore their applications in tidal power to reduce maintenance costs and improve efficiency.

CALMING THE STORM

Protecting submerged turbines from their marine co-habitants isn’t the only step tidal power plants should take. Sudden changes in water flow can be equally challenging for tidal turbines. Although the time between high and low tide is consistent, the distance between them, known as tidal range, is not. The tides are determined by the Moon and the Sun, and in some circumstances, extreme tidal forces such as spring tides can occur.

Tidal turbines need to be able to cope with these forces, as well as any unexpected and extreme weather conditions. By placing a dynamic braking resistor (DBR) in the generation and control circuit, can protect against any excess power generated by strong currents can be safely dissipated. The turbine system will therefore be less prone to damage, increasing its performance capacity and decreasing the chance of regular repairs.

The use of Cressall’s EV2 advanced, water-cooled resistor, which is suitable for low and medium voltage applications. The range is modular, so multiple resistors can be combined to handle power outputs up to one Megawatt. The EV2 also boasts an IP56 ingress protection rating, making it able to withstand harsh marine environments and suitable for the tidal turbine application.

BLADE DEVELOPMENT

Location also plays a major role in tidal electricity generation, with generator requirements including the need for a flow speed greater than two metres per second. Locations that can offer this are limited, which is one of the reasons for tidal power’s slow uptake. In the UK, only the north coast consistently meets this requirement.

Turbine blades with a high tip-speed ratio are slimmer and produce less drag. With less drag, the turbines can achieve a larger number of rotations at a lower speed. Through the development of blades that can operate at lower flow speeds, the number of sites at which tidal power can operate can increase, making it a more viable option.

Expensive installation costs cannot be avoided when increasing tidal power. However, by investing in technological developments that ensure less maintenance, higher efficiency and increased site suitability, tidal power can realise its potential and increase the prevalence of renewables globally.

For more information on Cressall’s tidal resistor technologies click here

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Beyond the factory floor

BEYOND THE FACTORY FLOOR

POWER RESISTORS AREN’T ONLY FOR INDUSTRIAL PLANTS

While many of us probably have a vague idea of a resistor’s function, you most likely think of them as part of an industrial plant or large-scale operation. In reality, you’re never too far away from this essential power component. Mark Barfield, engineering and R&D manager, explores the range of applications for industrial resistors.

In electronic circuits, resistors are used to reduce current flow, adjust signal levels, divide voltages and handle unnecessary influxes of power. High-power resistors that can dissipate large quantities of electrical power as heat have uses as part of motor controls, in power distribution systems or as test loads for generators.

To anybody that doesn’t possess an in-depth and technical knowledge of a resistor’s function, it may be difficult to understand how these applications are important to everyday functions.

UP AND AWAY

While a DBR may seem like a standard piece of elevator equipment, its design demands a number of variables in order to keep the lift safe and functioning. Key considerations include calculating the energy per stop, the duty cycle and the ohmic value. Once these factors have been determined, the resistor manufacturer can determine the required DBR peak and average power in order to produce the right DBR for the job.

Dynamic braking resistors (DBRs) are an essential component in elevator operations, where speed control is essential. Without them, the elevator mechanism wouldn’t slow down in the time determined by the drive, risking the lives of its passengers. When elevators and lifts descend, there is excess potential energy that usually drives the lift’s motor in reverse, making it operate like an alternator. But an alternator is responsible for charging and powering electrics, such as in an automotive charging system. This is far from what we want an elevator’s motor to do — we definitely don’t want the carriage to speed up during its descent — so this excess energy must be dissipated safely so that the elevator doesn’t descend too quickly and cause harm.

ALL ABOARD

Stopping a train also requires the dissipation of a vast amount of energy. Conventional disc brakes alone suffer a lot of wear, so dynamic braking is often used as an additional braking system to absorb the high amounts of energy generated by stopping electric trains.

Railway braking resistors operate in the same way as those on elevators. However, electrified railways also benefit from regenerative braking, where the power produced during braking is either immediately reused by other locomotives or is stored for later use. This method is particularly beneficial for intensively used underground rail services, as the generated power can be immediately fed back into the next approaching train.

Crowbar resistors, such as those supplied by Cressall, are another resistor type commonly found track side. These resistors are used in traction power supply circuits to deal with the effects of transient or longer lasting over-voltage conditions. A soft crowbar pulses to dissipate transient over-voltages, then if these persist or worsen the main breakers are opened and the system is short circuited using a hard crowbar to absorb the stored energy.

POWER PROTECTION

Power cuts are an inconvenience to anyone, at almost any time of day. But there are some buildings that cannot afford even a couple of minutes of blackout time. Take the care industry, for example. If a hospital was to plunge into darkness, surgery would be suspended, life-sustaining equipment would cut off and vulnerable patients would be placed at risk.

As a result, every hospital has a standby power supply plan in place in case of a power cut, so that the building never has to go a second without. A battery-powered uninterruptible power supply (UPS) can instantaneously take over if the regular power supply fails. In addition, most hospitals also have a diesel generator that kicks in when there isn’t a power supply from the grid. However, our fortune that power cuts are a rarity in the Western world can also be the generator’s downfall — it never has the opportunity to prove its power.

To make sure hospital generators are able to operate during power cuts, their efficiency must be tested using a fixed load bank. The load bank allows the building manager to verify the performance of emergency backup generators without interrupting ordinary power operations by regularly running on sets of at least 25 per cent of the generator’s rated power for 10-20 minutes. Running on load uses up expensive fuel, so the appropriate load for routine testing is the lowest one for the shortest time that will ensure the diesel and its ancillaries are brought up to their full working temperature.

Cressall’s load banks for fixed installations are designed as a stage bolt-on addition to the generator set, requiring a space of only 40–800 millimeters (mm) between the radiator and the acoustic splitters, making them an easy addition during any initial generator set up. Load banks are now easier than ever to operate thanks to features such as touch screen controllers and ethernet connectivity.

While power resistors may seem as though they belong in large, industrial operations, it’s never too difficult to identify where they are required in everyday life. Without this important piece of electrical equipment, many of our services that require power in order to function simply wouldn’t be safe and usable.

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