Variable speed drives: the dark side

VARIABLE SPEED DRIVES: THE DARK SIDE

While labelling variable speed drives (VSDs) as the Jekyll and Hyde of industry would be extreme, it is accurate to say that they possess a, widely unknown, darker side. VSDs are rightly hailed as effective energy savers and help industrial applications to reduce their power outputs, but their impact on power quality is less often discussed. Here’s how electrical engineers can combat VSDs’ darker side.

According to ABB, the power and automation company, the addition of a VSD can reduce energy consumption by as much as 60 per cent. This means that, if a 90-kilowatt (kW) motor in continuous operation is combined with a VSD, financial savings can amass to over £9,000 per year.

A VSD can help achieve these savings by better catering for the needs of a specific application — we could refer to this as the device’s positive Dr. Jekyll side. Traditionally, induction motors run at fixed speeds and are suited to applications that require a constant motor output speed, such as in pumps or fans. Yet, sometimes, varying motor output speeds are preferable to meet the changing requirements of the load, such as in fans, pumps and precision tools.

Also known as a frequency converter or adjustable speed drive, a VSD is able to control the speed and torque of the motor to better match the process requirements of the machine it is driving. It is the slowing down, when necessary, that helps recoup energy and costs that would otherwise go to waste.

RIDING THE WAVE

Of course, the bottom line of any plant manager’s ambitions is to reduce costs and improve operational efficiency, and a VSD helps to achieve just that. While a manufacturer should not be dissuaded from purchasing VSDs for use with electrical equipment, they must pay attention to an “unwanted ingredient” that the device might add to the power mix.

When existing equipment has to share its power network with connected add-ons, harmonics can become a problem. These harmonics are voltage or current waveforms that have a different frequency to that of the network, and may cause devices to behave erratically.

The undesirable Mr. Hyde aspect of a VSD is that it can create these harmonic currents due to the conversion of an incoming alternative current (AC) waveform to a direct current (DC) source, in order to create modulated pulses that control the AC motor. This back and forth, from AC to DC, results in current waveforms that are greater than the network frequency can handle.

As a result of the unwanted currents, cables may overheat which damages their insulation. Other unwanted consequences include that motors can be at risk of overheating and becoming noisy; circuit breakers may trip; meters can give false readings; or equipment might fail altogether.

CUT THE CURRENTS

To prevent these unwanted effects from occurring, manufacturers can implement a number of techniques. Reduction is one obvious remedy, which involves the use of AC line reactors, known as chokes. These chokes are fitted either inside or outside the drive, to reduce the harmonics to a level where they no longer cause serious issues.

However, the use of a large choke can have major size and cost drawbacks, which makes the solution unsuitable for some applications. An AC choke also has a voltage drop that impacts the system.

FILTER THEM OUT

Harmonics caused by VSDs can be reduced to acceptable levels by using passive filter circuits that consist of inductors, capacitators and resistors. The filter circuit allows the fundamental frequency to pass through while diverting any harmonic frequencies to the resistor bank. Here, the frequencies are dissipated as heat and are removed from the system.

The introduction of a dampening resistor can also offer a number of benefits to the system. They include better filtering characteristics for higher frequencies, reduced amplification at parallel resonance frequency, as well as higher filter losses at the fundamental frequency.

Cressall builds discharge resistors that meet the stringent operating conditions of customers such as Siemens, Areva and also the National Grid Company, both in the UK and its counterparts overseas. Cressall’s design expertise in the field is well-known, as a result.

Based on Cressall’s experiences within the industry, perhaps the most commonly used material in the design of harmonic filter resistors is expanded mesh. This material has a high surface area, which gives it excellent heat dissipation and makes it ideal for continuous filtering duties.

The active material, insulators and mountings on expanded mesh resistor elements maximise the use of convection to avoid hot spots and local overheating. However, as the elements are thin, expanded mesh can bow when exposed to high levels of heat, and this uncontrollable bowing can cause sparks.

To remedy this, Cressall has developed a technique that allows bowing to take place in the same direction. By improving the shape of expanded mesh, the company has been able to prevent this fault from occurring so that dampening resistors made from expanded mesh can filter VSD harmonics, without the risk of sparking.

Given their many advantages, it wouldn’t be right to label VSDs as being solely a Mr. Hyde “electrical circuit villain”. After all, the additional levels of performance flexibility that the devices give to motors are essential — as are the resulting cost savings. However, to stop VSDs from drifting to the dark side, unwanted levels of harmonics must be tackled to allow for optimal performance.

To learn more about Cressall’s harmonic filtering technologies, click here

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ELECTRIFYING THE UK HEAVY VEHICLE MARKET

ELECTRIFYING THE UK HEAVY VEHICLE MARKET

The Department for Transport Statistics reports that there were 485,900 heavy goods vehicles (HGVs) licenced in the UK in 2020, but only 400 of these were battery electric powered. With HGVs being a significant contributor of carbon emissions, will we see an increase in electric power?

HGVs account for around 17 per cent of greenhouse gas emissions while contributing to just five per cent of vehicle miles. Switching from diesel or petrol to electric power reduces the tailpipe emissions of vehicles, while also providing performance benefits. However, electric HGVs remain in the early stages. For electric heavy vehicles to become commonplace, there is a need for further development of the technology.

BATTERY ELECTRIC VERSUS HYDROGEN FUEL CELL

A challenge of electrifying heavy vehicles is finding an energy storage solution that doesn’t add too much weight, which would increase energy consumption. Batteries must also possess a long range, allowing long distance freight. The main contenders for reducing vehicle emissions are battery electric and hydrogen fuel cell electric. Battery Electric Vehicles (BEVs) use chemical energy that is stored in rechargeable battery packs and use electric motors for propulsion.

However, the range between charges is limited, making it not so suitable for HGVs travelling a few hundred miles a day. This is exacerbated by the lengthy charge time of BEVs, extending to many hours for heavy vehicles depending on the charger.

Fuel Cell Electric Vehicles (FCEVs) also use an electric motor for propulsion but with a much smaller battery pack, with the fuel cell constantly converting the hydrogen to electricity, which only emits water from the tailpipe. FCEVs typically have a longer range and shorter fill time than BEVs, making them a stronger candidate for long-distance vehicles. Furthermore, the fuel cells can be stacked together to scale up power for a heavy vehicle. Fuel cells are more compact and lightweight than electric batteries, and most of the fuel cell can be recycled at end of life.

However, the majority of hydrogen currently being produced is made using fossil fuels through steam reforming, meaning hydrogen power is not emission free when its whole lifecycle is considered. If developments are made that allow more hydrogen to be produced from renewable resources, then FCEVs can become a more environmentally friendly option.

PERFORMANCE, RELIABILITY AND SAFETY

Electric vehicles (EVs) are generally more reliable than Internal Combustion Engine (ICE) vehicles as they consist of fewer moving parts, reducing the risk of breakdowns and the need for frequent servicing. Electric motors can deliver torque quickly with almost instant acceleration, making vehicles quicker to start. This is particularly beneficial for heavy vehicles that are carrying large loads on fast motorways or on an inclined gradient.

Heavy vehicles brake differently to cars, as they do not purely rely on their service brakes to slow down. Instead, they also use auxiliary and endurance braking systems, which don’t overheat as quickly on long declines and reduce the risk of brake fade or failure of the service brakes. In electric heavy vehicles, this braking is regenerative, which minimises wear on the service brakes and adds charge and range to the battery packs.

However, if there is a failure in the system, or the battery pack’s state of charge is unable to accept the charge, this could become dangerous. Using a dynamic braking resistor will dissipate the excess energy as heat to improve the safety of the braking system. Regenerative braking aided by braking resistors can also boost heating efficiency by feeding the dissipated energy back into the vehicle to heat the internal cabin. The resistor needs to be compact and meet the current ECE R13 Type –IIA endurance braking performance test. To pass this test, the resistor must allow the heavy vehicle to travel 6km at 30kph on a seven per cent decline with the endurance braking system active and without the service brakes overheating and failing.

FUTURE UPTAKE

Currently, the UK has banned the sale of petrol, diesel and hybrid cars from 2035 onwards. However, there have been talks on proposing a ban on diesel heavy goods vehicles by 2040 in order to remove all carbon emissions from freight transportation by 2050. The race for electrifying heavy vehicles is on, and there could be penalties in the future for those who do not use electric.

With only 400 battery electric heavy vehicles in the UK in 2020, electrifying the heavy vehicle market is in its early stages. However, with potential diesel bans looming, we must power ahead into an electric HGV future.

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Reforming the railways

IS THE END OF RAIL FRANCHISING ENOUGH?

In September 2020, after 24 years, the UK Government announced the end of rail franchising. The goal is to move to a simpler and more effective operating model that improves the transport experience for passengers. In parallel with the transition to the new rail system, what more can be done to reform our railways? David Atkins, projects director of Cressall, looks at the system change and some of the technologies that are improving rail transport.

It’s no question that the UK’s railway system has caused and will continue to cause heated debate in recent years. Poor reliability and rising ticket prices have been large problems for travellers. In fact, independent consumer body Which? found that passengers lost almost four million hours to significantly delayed trains in 2018 — equivalent to 448 years.

Many regard rail franchising as a factor in the widespread dissatisfaction with rail transport. The implementation of rail franchising in the 1990s involved awarding contracts to private train companies for a limited time through a bidding and competition process. The aim was to benefit the industry for passengers through strong competition between operators, and to increase passenger numbers.

FRANCHISE FAULTS

However, franchising hasn’t lived up to its high hopes, causing a complicated system for all. With different train operators dominating different routes, passengers face a complex ticket system that can see high price jumps when their route uses two or more operators. This disconnected ticket system can also cause a lack of coordination on the tracks.

Train operators are performing to profit margins, so if a route yields a low profit, its service will be reduced. This may help the operator’s finances, but does not aid the commuter who relies on that route for work.

The franchise system doesn’t only negatively affect passengers. Operators can overbid for services and be left unable to keep up payments due to overestimated passenger predictions. While a train operator can attempt to draw in more custom, there are many external factors that affect passenger numbers that are beyond their control, such as the general state of the economy.

The Government’s announcement to end rail franchising is the first step towards creating a simpler and more coordinated rail system. Operators are being moved onto transitional contracts called Emergency Recovery Measures Agreements (ERMAs), which will help address the continuing impact of COVID-19 while beginning the replacement of the current franchising system.

The new change is expected to create a more effective rail structure that is built around passengers. The agreements focus on high performance targets and simpler journeys, requiring rail operators to coordinate better with each other.

A SUPPORTING ROLE

A change in the rail management structure is a large step towards improving the UK’s railways, which can be further enhanced by technology. For example, introducing more trains onto routes that travel faster and arrive on time will require fine speed control using advanced braking techniques.

As trains become faster, braking powers will increase. Traditional disc brakes can become unsuitable because of their high wear rates and resulting maintenance costs. Instead, both regenerative and dynamic braking should be favoured, which uses the electric traction motor as a generator to produce the braking torque, converting excess kinetic energy into electrical energy.

The generated electrical energy can be fed back into the line as part of regenerative braking systems to power other trains on the line, a process that’s already used extensively on underground lines. However, when there are no other trains on the line, or the distance between trains is too great, the excess energy can be safely dissipated as heat by a resistor.

Cressall has supplied resistors to the transport sector for over 60 years, and remains at the forefront of technology. Our EV2 advanced water cooled resistor can withstand severe conditions in traction, and is proven to meet all major shock and vibration standards for traction use.

Franchising’s end has been regarded as the biggest change to the railways in 25 years. The move to a simpler system brings hope that trains will become more reliable and fares made simpler. However, reforming the railways will require policy and technology to go hand in hand in order to create a more effective rail transport system for all.

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Offshore wind

ADVANCING OFFSHORE WIND

HOW CAN WE EXPAND OFFSHORE WIND TO REACH 2030’S 40 GW TARGET?

The UK’s history is enriched with maritime activity. Surrounded by water from John o’ Groats to Lands’ End, the surrounding waters have played a pivotal role in trade, travel, and most recently, electricity production. Achieving the Government’s target of generating 40 gigawatts (GW) of offshore wind power every year by 2030 will require continued investment and development in power equipment.

Offshore wind power plays to the nation’s geographical strengths while also providing a clean energy source to fuel the country’s path to net zero. The North Sea’s high quality wind resources and relatively shallow water make it an ideal location for offshore wind farms. According to the International Renewable Energy Agency (IRENA), around 90 per cent of global offshore wind capacity is located in the North Sea, which is why the UK is already a world leader in this renewable power source.

However, to reach the Government’s 2030 production goal, energy suppliers must make advancements in wind turbine technology, while simultaneously considering how their generated power will be safely transferred to the grid.

IMPROVED TURBINE TECHNOLOGY

Turbines capable of producing more power per rotation are essential for the development of efficient offshore wind farms. One way of improving turbine efficiency is to increase the blade length.

An increased blade length means that stronger forces will act on the turbine, so the blade material needs to be appropriately chosen. To achieve an adequate stiffness-to-weight ratio to avoid deflection, carbon fibre or fibreglass blades are typically favoured. However, there is an expanding market for hybrid reinforcements, which combine the two materials together for optimum sturdiness.

Improvements in wind turbine technologies have already triggered a move into deeper waters to use sites with better wind resources. Static wind turbines are still restricted to waters at a maximum depth of 60 metres, so to upscale the UK’s wind power output, floating wind turbines will be essential.

MORE SUITABLE SITES

Once all viable sites within 60 metres of shore have been constructed, floating wind projects will become vital to offshore’s growth. Floating offshore wind farms, which can be located up to 80 kilometres (km) from land, could play a key role in the long-term decarbonisation of the power sector.


Floating wind turbines sit on a steel and concrete floating system instead of a fixed base, meaning they can be placed in a larger number of sites up to 200 metres deep. They can also be towed, allowing them to be relocated without much additional cost. This broadens the potential output that offshore wind could provide and brings it one step closer to the 40 GW target.

SECURED POWER SUPPLY

Like all renewable energy, offshore wind can be unpredictable and inconsistent, which can make grid connection challenging. In periods of high wind, large inrush currents occur, which can lead to overvoltages on the grid and subsequent equipment malfunctioning.

It’s important to prepare for these inevitable inrush currents by integrating technologies such as pre-insertion resistors (PIRs). Already in use across many of the UK’s windfarms, Cressall’s PIRs have a high thermal mass, which allows them to absorb excess energy produced by the inrush current and safely dissipate it as heat. This prevents damage to the grid and improves the reliability of offshore wind’s power supply.

Offshore wind holds great potential in the shift towards renewable energy and could be the key to decarbonising electricity generation. However, we must continue to advance critical power protection technologies to prevent any obstacles in its upscaling and to enable this powerful resource to flourish.

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