The role of HVDC in energy transition

THE ROLE OF HVDC IN ENERGY TRANSITION

power transmission

With the rise of offshore windfarms and international grid links, effectively and efficiently transmitting electricity over long distances is more crucial than ever before.

Simone Bruckner, Managing Director of Cressall, explains the role of high voltage direct current (HVDC) and filter resistors in making long-distance energy transition possible.

The UK has four times more offshore windfarms in operation than in 2012, with the number set to rise significantly as the government looks to reach its goal of generating 50 gigawatts (GW) of offshore wind by 2030.

Along with this rise, international and intercontinental grid links have increased as the UK trades excess power with other countries, much of which is generated by renewable means. Trading the surplus not only saves energy, but also prevents Brits paying to turn off turbines when more energy is generated than the grid can take.

As the UK currently has 13.9GW offshore wind capacity compared to its 50GW goal, it is important that this output is used efficiently and energy loss is kept at a minimum. Although alternating current (AC) is standard in electrical power transmission, the current often concentrates near the conductor’s surface – known as the skin effect – which causes energy loss.

HOW HVDC HELPS

HVDC is a transmission system that uses direct current (DC) for the transfer of power over long distances. As remote offshore windfarms and the grid are often far apart, HVDC enables effective transmission due to its uniform current density throughout the line.

Additionally, HVDC supports the trading of excess power between unsynchronised AC distribution systems, which run at a set frequency and cannot be connected to those with a different frequency. As HVDC does not have a frequency, multiple circuits can be interconnected and converted to both system voltage and frequency levels of the system at point of use.

While HVDC is used for international grid links, it must be converted back to AC at the local grid level. However, converters create harmonic distortion, which in turn can cause lower efficiency, overheating and increased chance of equipment failure.

Therefore, harmonic filter resistors are a vital part of HVDC and SVC converter stations, helping to remove harmonics by dissipating them as heat. This ensures a safe and assured power supply for the UK and countries across the continent.

The UK’s rollout of offshore windfarms currently puts it among world leaders, and with the pipeline of projects close to 100GW, Britain could soon supply many countries with surplus energy. With the ever-increasing need for sustainable energy, HVDC ensures that countries across the world can safely and securely benefit from wind power.

Why are wind turbines being switched off?

WHY ARE WIND TURBINES BEING SWITCHED OFF?

POWER TRANSMISSION IS JUST AS IMPORTANT AS GENERATION

UK windfarms hit an all-time high in wind power last year, generating more than 80 thousand gigawatt hours (GWh) and enough power for over 22 million homes. Yet, reports also came out of wind turbines being switched off due to overcapacity — at the expense of customers.

Despite reaching impressive milestones in recent years, there’s a massive problem with the renewable — and particularly wind sector — power wastage. In 2022, it was reported that Brits paid millions to switch off wind turbines as networks were unable to deal with the levels of power generated.

The UK has set ambitious goals for renewable energy sources for the next few years, aiming for a more sustainable approach while reducing dependency on both fossil fuels and external suppliers. As the past 18 months or so have highlighted, the volatility of global markets means it’s essential that the country is able to secure its own energy supply.

Fortunately, the UK does have the natural resources to do so. With the greatest wind energy potential in Europe, it’s clear why wind power has been a preferred route for planners and developers to take. So why are wind turbines still being switched off, and why is this energy being wasted?

DISTANCE FROM THE GRID

Offshore wind farms are often a significant distance from the Grid. Typically, these farms are connected to the Grid with a specialist, individual cable connection through a converter and into the transmission network, allowing the farm to distribute power.

The issue with this setup is that the offshore system will typically have fewer connections readily available than an equivalent farm on land. Because of this, there are less options available when it comes to distributing power during surges or when there are problems with the on-land network.

DISTANCE FROM DEMAND

Furthermore, many of these wind farm installations are being built in remote areas of Scotland or in the North Sea, where winds are stronger. Though this is certainly positive when it comes to power generation, the issue is that the local area isn’t where the demand is.

More power is needed in the south of the country, far from where the electricity is being generated. And while the transmission networks can transport electricity great distances, without efficient connections and cable routes a lot of power can be lost before it reaches crucial areas.

A FOCUS ON INFRASTRUCTURE

It’s clear from these issues that improving power infrastructure is just as vital as delivering new power generation projects. Reassuringly, there are developments underway to address these issues. One such example is the ‘Eastern Green Link 2’ (EGL2), which involves the manufacture and installation of a high voltage direct current (HVDC) subsea cable from Peterhead in the North of Scotland down to Drax in Yorkshire.

A crucial element of these power transmission systems is the host of resistors within that help to facilitate the safe movement of electricity. Pre-insertion resistors, for example, can absorb and control transient magnetising currents within transformers throughout the network. This control helps keep voltages consistent with minimal dips, reducing potential disturbances for users of the power network. They can also help mitigate against temporary overvoltages, such as those caused by exceptionally strong winds.

Discharge resistors are another vital component, particularly in terms of safety. These can reduce the risk of sudden overvoltages from capacitors and inductors that have become isolated from their networks or in situations where an emergency shutdown is required. In offshore farms that are far from other connections, the inclusion of discharge resistors is essential in having a sufficient ability to remove excess electricity when required.

Implementing resistor technologies as new projects are built helps both to ensure safety from dangerous overvoltages, as well as safeguard electricity on the Grid from fluctuations and dips.

So, as the UK continues to invest heavily in the renewable energy sector, considering how we’ll transport this energy will be just as important as thinking about how we will generate it in the first place. With projects like EGL2 on the horizon, it’s clear that the industry is taking the right steps to secure a reliable network from the turbine all the way to our homes.

CR590

Power Control Chopper

POWER PROVE LAUNCHES POWER CONTROL CHOPPER FOR CRITICAL TESTING

In response to growing demand for more precise power dissipation, load bank manufacturer Power Prove has launched a dedicated IGBT-based electronic power control chopper, for continuous regulation of its load bank product offering. The power control chopper can be easily integrated into load banks to achieve high power dissipation and a degree of precision superior to that offered by any competitor.

Power Prove, the load bank division Cressall Resistors, commissioned the design of the power control chopper to Italian Internet of Things (IoT) solution developer Techmakers. The combination of Power Prove’s in-depth knowledge of load banks and Techmakers’ expertise in electronic and software-controlled devices has resulted in a powerful, yet cost-effective, solution that meets the growing demands of the market.

A power control chopper is an electrically controlled solid state switch that is used to control the amount of current permitted to flow through a circuit. Normally, a high-power variable load requires multiple fixed value load sections ranging in values for power dissipation with contactors and a logic controller. However, by integrating the power control chopper into the system, a near-infinite set of values for power dissipation can be achieved using just a single resistor.

Power Prove’s chopper also has a closed-loop regulation circuit, which is capable of adapting to fluctuations in voltage and cold resistance variation without any input. Multiple units can be combined to reach high-power dissipation, enabling the load bank to withstand even the greatest of power values with high precision.

Anywhere that requires constant power, whether that’s a healthcare facility, manufacturing plant, or IT data centre, simply cannot afford a complete loss of power. These layers of infrastructure are often secured by an uninterruptible power supply (UPS) that provides power for critical operations if supply from the grid fails.

“The challenge for the managers these systems, which are often deployed as sources of back-up power in a black-out situation is how to determine whether the system is operational and will not fail on the relatively infrequent occasions when their use is required at a critical moment. Regular testing of emergency systems using load banks is therefore essential,” explained Andrew Keith, division director of Power Prove.

“Since these systems provide such a critical safety mechanism, a high level of precision is vital,” continued Keith. “The new power control chopper allows us to provide our load bank customers with a customisable load bank that can be easily integrated into an existing system to provide infinite levels of power adjustment at a degree of precision that is simply not available elsewhere on the market.”

An example of the power control chopper’s application is with battery discharge testing. The chopper can be used with a current feedback loop to provide a genuine constant current load on battery systems up to 1000 V DC. Multiple chopper units can be fitted inside the same load bank, or a combination of traditional fixed loads and chopper modules can be used to create a load bank with the current discharge capacity to suit its application.

In addition, the increasing adoption of electrical vehicles powered by batteries and fuel cells is generating a wide range of operating scenarios that need to be simulated. The development of the power electronic control module allows Power Prove to produce load banks that simulate a much more diverse range of operating conditions for research and development (R&D) testing, system commissioning tests and regular planned maintenance load testing.

The power control chopper is available globally from Power Prove, for more information, visit the website.

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A transcontinental renewable network?

A TRANSCONTINENTAL RENEWABLE NETWORK?

HOW HIGH VOLTAGE DIRECT CURRENT TRANSMISSION COULD TRANSFORM POWER SUPPLY

The SuperSmart Grid (SSG) is a theoretical concept that involves the creation of a transcontinental electricity network connecting Europe, the Middle East and North Africa to deliver low cost, high capacity, low loss electricity. To support global efforts to decarbonise power generation, could the SSG become a reality?

The SSG is a fusion of a super grid, a wide-area, often transcontinental transmission network and a smart grid, which uses digital technology, such as smart meters, to react to fluctuations in energy demand.

By implementing this system across Europe, the Middle East and Africa, this geographical area could benefit from an entirely renewable energy supply, which in turn supports the United Nations’ Sustainable Development Goal Seven: ensure access to affordable, reliable, sustainable and modern energy for all.

Offshore wind farms and solar power are the two resources that offer great potential, given the large number of suitable sites for both systems throughout the region. Having identified potential energy resources, how can this energy be transmitted to meet demand over such a vast area?

DEVIATING FROM THE AC NORM

High voltage direct current (HVDC) uses direct current (DC) for most electrical power transmission. Although DC is less common than standard alternating current (AC) systems, it meets the demands of the SSG for a variety of reasons.

HVDC transmission is a proven method of achieving power transmission over very long distances. It would play a vital part of the SSG, since it allows power to be transmitted from areas where it is in abundance to areas experiencing a shortage, which would secure the energy supply across the entire region. It would also facilitate the use of offshore wind farms — whose natural location is so distant from areas of electricity demand that HVDC is essential to ensuring efficient transmission.

HVDC also allows power transmission between unsynchronised AC distribution systems. AC systems operate at a set frequency and if these frequencies are different, the systems cannot be connected. HVDC circuits do not have a frequency, eliminating this problem and allowing multiple circuits to be interconnected.

Most significantly, HVDC suffers lower electrical losses than AC transmission. It has a uniform current density throughout the line, so there is no skin effect as there is in AC circuits. Although the corona effect, which is an electrical discharge that appears around a charged conductor, is still generated in a HVDC system, it is considerably lower than in AC systems, facilitating more efficient electricity transmission across the vast area encompassed by the SuperSmart Grid.

CONVERTING BACK TO AC

HVDC is ideal for transmitting over long distances, but when transmitting electricity into the local AC transmission grid, the direct current must be switched back to alternating current using a converter system. All converters, including HVDC converters, generate harmonic distortion to some degree.

If harmonics are not controlled, they can wreak havoc with the transmission system, jeopardising power quality and increasing the chances of equipment malfunction and electrical losses on the line. Therefore, it is important to integrate harmonic filters into the HVDC converter stations to block these unwanted currents.

Harmonic filters allow current at the frequency of the AC network to pass through, while redirecting distorted harmonic currents into a harmonic filter resistor, where they are dissipated as heat. This ensures that the unwanted currents are safely removed from the transmission network in a controlled way, which helps to secure the power supply when converting from DC to AC.

Although the SuperSmart Grid is purely theoretical, it’s clear that the technology necessary to realise this concept already exists. With countries all over the region setting ambitious renewable energy targets, perhaps this could be the solution to providing a secure, sustainable power source across all three continents.

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