Basslink: The reflection shows the 500 kV AC switchgear bay arrester at the Loy Yang converter station on the Australian mainland<br>Credit: Siemens
Basslink: The reflection shows the 500 kV AC switchgear bay arrester at the Loy Yang converter station on the Australian mainland
Credit: Siemens

With current HVDC systems being point to point, the great challenge is to put in place solutions that allow for meshed HVDC networks. Robert Stokes finds out how close the industry is to this breakthrough.

Recent technical advances and headline projects show how companies with market leadership in high-voltage direct current (HVDC) and ultra HVDC are pushing the envelope of what these technologies can do.

Since Sweden’s ASEA – now part of Swiss multinational ABB – installed the world’s first commercial HVDC link under the Baltic Sea to the Swedish island of Gotland in 1954, it has become the technology of choice for transmitting current over very long distances on land or subsea. A little history also helps to understand the story. Early HVDC systems had fairly high losses in converting between AC and DC and vice versa. Research and development over decades focused on reducing those losses, and these are now well below 1 per cent in converter stations.

Much effort also goes into raising power ratings in response to demand from locations such as China, India and Brazil where huge power generation schemes can be far distant from where the electricity is needed.

Take China. “It has more than 1000 GW of power installed,” says Dr Yanny Fu, senior leading consultant at leading global energy consulting and testing and certification company, DNV KEMA.

“Seventy per cent is generated from coal, more than 20 per cent from large hydro schemes, 7 per cent from renewables and 1 per cent from nuclear. Large hydro stations along rivers in west and south China are more than 1500 km away from large load centres on the coast.”

HVDC is a “very suitable and competitive” solution for these transmission needs, notes Claes Rytoft, chief technology officer of ABB’s power systems division.

Industrialisation and rising wealth has seen demand for power in China rise remorselessly for more than a decade and there is no end in sight to this trend as it continues towards becoming the world’s largest economy.

Since the late 1980s, HVDC overhead transmission lines in China have scaled up progressively from 500 kV/3 GW through 660 kV/4 GW, 800 kV/5 GW and now 800 kV/ 6-7 GW, according to Dr Fu. “The next project is 800 kV/8 GW, next year they will have 800 kV/10 GW, and I hear they are planning to go to 1100 kV and at least 10 GW in a few years.” The rating for each of those large Chinese hydropower stations is 10 GW.

HVDC Classic, first deployed by ABB, involves AC/DC conversion using line-commutated converters based on thyristors and has been used from the 1970s until now. It can handle very high power ratings with minimal losses, but is not the easiest type of HVDC to control. It is the dominant HVDC technology for China’s long distance lines, explains Dr Fu.

ABB’s HVDC Light, the voltage source converter (VSC) technology introduced in the mid-1990s, is based on transistors, chiefly insulated gate bi-polar transistors (IGBT), which allow faster switching, many times more per cycle than HVDC Classic, thus giving better control over current. Crucially, IBGTs can switch current both on and off: thyristors can only be turned on.

In April, the world's most powerful HVDC Light, rated at 320 kV, was unveiled<br>Credit: ABB
In April, the world’s most powerful HVDC Light, rated at 320 kV, was unveiled
Credit: ABB

VSC HVDC is more suitable for connecting to the grid: weak AC networks; offshore wind farms; and ‘future-proofing’ DC systems that could conceivably become part of meshed, multi-terminal grids.

ABB had the VSC field to itself for around a decade before rivals switched on to the potential that it had for the energy futures being promoted by governments, supra-national entities, such as the European Commission, and environmental lobbyists.

“Competition began around five years ago and has now led to an explosion of VSC technologies in the market,” explains Rytoft.

Realising VSC tech’s full potential

In April, ABB unveiled the world’s most powerful HVDC Light, rated at 320 kV, more than 50 per cent higher than the previous record for VSC HVDC applications.

While, the converter stations being supplied by Germany’s Siemens Energy for the planned 420-km Western HVDC Link between Hunterston in Scotland and Connah’s Quay in Wales – part of a consortium with Italy’s Prysmian – will help to bypass limitations in the UK electricity grid and to balance supply and demand in the national grid, as well as enabling more renewables come on stream. The interconnector will run at a world record 600 kV DC for a submarine cable and will be rated at 2200 MW when it becomes operational in late-2015.

Second generation VSC HVDC technology branded as MaxSine by France’s Alstom can handle multiple connections such as offshore wind farms, while also supporting traditional large grids. The company says it offers added functionality for use in multi-terminal DC grids, where HVDC MaxSine can isolate a DC fault and support the AC grid if a major fault occurs.

In 2010, Siemens Energy installed the first ever commercial-use VSC multi-modular converter (VSC-MMC), for the 200 kV/ 400 MW Trans Bay HVDC submarine cable in San Francisco, CA, US.

The German OEM is also now in the late stages of installing two 320 kV/1000 MW underground transmission links in parallel over 65 km between France and Spain for the Interconnexion electrique France-Espagne (Inelfe) project, part of the trans-European electricity network that has been built across the European Union (EU) in the past 20 years, which addresses the notorious France-Spain power bottleneck.

The converter stations for Inelfe are VSC-MMC and allow: independent exchange of reactive power for each network, a technology which reduces power reductions in the lines; and black-start capability which can restart a collapsed network.

“It shows the great strides that there have been in increasing the capability of VSC systems in a short time,” says Peter Kohnstam, HVDC business development manager at Siemens Energy.

He adds: “I believe VSC will play a large part in integrated networks. VSC is not brand new but is gaining a great deal of traction. People are starting to understand the benefits of independent power control at either end, as well as the reactive compensation that VSC brings. They are now saying, ‘oh, you can do that’. So while it may not be dramatic on the technical side, the application is now becoming recognised.”

This rising awareness will in itself drive further innovation, he believes. “As we open people’s minds to the applications that are possible, they will start challenging us to do more, more, more.”

New technology drivers

There is always market pressure for stepwise incremental reduction in the cost, size, weight and power losses of HVDC systems, and for higher and higher voltages and ratings. However, new drivers are catalysing greater innovation and this is expected to accelerate in the mid to longer term.

Chief among these is the rise of modern renewable energies such as offshore wind located at great distances from load centres. In Europe at least, some of the largest offshore wind projects will be getting underway in the next two to three years.

“We are seeing a lot of demand from government and the public for greater integration of renewable resources into the world’s electricity networks,” says Kohnstam.

A key policy here is the development of the so-called Supergrid across Europe, which has been backed by EU leaders and companies, and envisages HVDC backbones connecting a wide range of traditional and renewable generation with national and more local distribution systems, some DC, some AC, to balance supply and demand more effectively, increase energy security and create more efficient markets.

With current HVDC systems being point to point, the great challenge is to put in place solutions that allow for meshed HVDC networks

“That will be the real breakthrough,” says Claes Scheibe, vice-president of power electronics activities with Alstom Grid. “The four key elements for a meshed DC system are control systems, DC breakers, protection, and transformers to step from one DC voltage to another, so systems can be connected.”

There are no breakers for HVDC in current commercial use. For example, ABB is building a three terminal, 800 kV UVHDC Northeast Agra system in India, from Agra, near Delhi, stretching east to Assam, but without circuit breakers: if one line goes down, all go down.

An advanced DC breaker in tests interrupted currents >3000 A in less than 2.5 ms<br>Credit: Alstom Grid
An advanced DC breaker in tests interrupted currents >3000 A in less than 2.5 ms
Credit: Alstom Grid

In November 2012 though, ABB again broke new ground by announcing the development of a DC breaker for HVDC. It combines very fast mechanics with power electronics, and will be capable of ‘interrupting’ power flows equivalent to the output of a large power station within 5 milliseconds (ms). Tests performed during 2012 passed 9000 Amperes (A) in short-circuit current.

The breakthrough removes a 100-year old barrier to the development of DC transmission grids, which will enable the efficient integration and exchange of renewable energy. DC grids will also improve grid reliability and enhance the capability of existing AC (alternating current) networks.

“We have eliminated a major technology stumbling block for building DC grids and the hybrid solution not only ensure speed but keeps the losses to a minimum,” explains ABB’s Rytoft. “With this breaker you can isolate faults i.e. if something happens in one line you have to be able to switch it off. It is only for one voltage level at the moment, but it is still a big step forward.”

Where is this leading and how soon? “I think we could see something embryonic within the next five years that could develop into a DC grid,” predicts Rytoft. “Germany is going to build three HVDC lines north to south to connect northern offshore wind and southern solar power to the grid. It is all point to point, but they have decided that they should think about the next step [meshed grids] as well.”

Alstom’s Scheibe also sees demand for meshed HVDC appearing within a five-year timeframe. “Meanwhile, we need to further develop DC breakers to meet future demand,” he agrees. “This will mean breaking DC current in a bigger system and that will need to be ten to 20 times faster than a breaker for AC because the fall in a DC system is very much faster than in AC.”

Initial tests earlier this year of an advanced DC breaker in a prototype system at Alstom’s Villeurbanne plant in France generated record results: in less than 2.5 ms, it interrupted currents greater than 3000 A.

“So we can now break at the speed we need to achieve and are ramping up current and voltage,” says Scheibe. “Our target is to establish a breaker for a DC voltage level of 320 kV and to industrialise the technology by making it smaller and more efficient. We are planning to do 170 kV this autumn and we also want to be sure we do not introduce new losses that would reduce DC’s advantage over AC.”

Circuit breakers will undoubtedly be part of the DC grids solution, agrees Kohnstam at Siemens Energy. “But it is not necessarily the ultimate panacea, to be blunt. We believe that a full-bridge VSC offers huge advantages in certain scenarios. The important thing is that every situation needs to be analysed and the optimised solution chosen. That will depend on where it is now and where it wants to go. So there are many elements that need to be brought together to implement DC grids successfully.”

Step-change in transformer capacity

Some of the most striking examples of moving huge amounts of power over very long distances are the transmission systems for hydropower in China, Brazil and India, which generates issues in insulation and insulation distances and, hence, on transformers.

ABB’s 1100 kV converter transformer has been one of the keys to enabling development of 1100 kV UHVDC transmission equipment which is in the late stages of preparation for potential sale to China for a point-to-point system. This would be the largest capacity and efficiency leap in HVDC for two decades, according to ABB, which says the system could deliver some 10,000 MW over thousands of kilometres with minimal transmission losses. “Technically, we believe it’s doable and now it’s up to China to take the next step,” says Rytoft.

Typically though, UHVDC systems currently on order around the world involve 800 kV. Alstom says it has tested this satisfactorily and is working towards full operational deployment in two years’ time of 800 kV line commutated converter (LCC) transformers for the Power Grid Corporation of India.

Solving insulation challenges requires a balance between the cost, size, weight and transportability of transformers, so all the manufacturers keep a close eye on materials research and development. “And to go from 500 kV to 800 kV is a big step requiring a lot of trying by doing, not only for the transformer itself but of the bushings (the transformer’s connection to the system) that we make in-house in Italy,” says Alstom’s Scheibe.

Similarly, Siemens is working on 800 kV systems in China with power ratings a fraction under 8000 MW on a single line. “For me, as a poor engineer, the thought of getting that much down a single line is just mind blowing,” declares Kohnstam.

“It’s pushing everything – technology, materials science – and we believe it will have a fundamental role in the backbone of electricity networks.”

Cables & controllability

Two other drivers of the HVDC technology story tend to be less discussed, stresses Scheibe, who believes that onshore and offshore cables, and controllability, will assume increasing importance.

“Utilities don’t talk about cables much because they are quite expensive compared with overhead lines, but it is totally forbidden in some countries, such as Sweden and Denmark, to install new overhead lines,” he says. “For cables, DC is the key.”

Traditional mass impregnated non-draining (MIND) HVDC cables for use up to at least 500 kV are oil-impregnated and very reliable for a wide range of voltages, but their heaviness weighs against them for onshore use as single cables over long distances. “You would need a lot of joints and it becomes very expensive and complex,” says Scheibe.

The desire to move large quantities of power over huge distances is driving innovation in transformer technology<br>Credit: Siemens Energy
The desire to move large quantities of power over huge distances is driving innovation in transformer technology
Credit: Siemens Energy

“With VSC HVDC technology a lighter cable called XLPE [cross-linked polyethylene] can be used and this will facilitate implementation of cables onshore,” he adds.

For example, ABB’s HVDC extruded cables are dry, lighter than MIND cables, need fewer joints, so are easier to install over long distances on land and are used subsea too. With VSC converters at both ends, they are good for connecting weak AC networks and when good voltage, power and short-circuit control is needed. Furthermore, Kohnstam points out that renewables locations are often in more sparsely populated areas lacking robust AC infrastructure that was once considered vital for the effective operation of AC/DC converter stations for Classic HVDC.

“But we’ve pushed the envelope there too in being able to connect to weak AC networks with classic technology. For example, we have a back-to-back connection between Georgia and Turkey where we needed to artificially reinforce the AC network to allow us to operate the system there properly,” he says.

This was achieved by using an ostensibly old technology, synchronous condensers, to provide inertia and active compensation to the system but combined with more modern technology, static var compensators, to allow emulation of a stronger AC network so that DC could be applied. “It’s involved some sophisticated modelling because we don’t want to over-engineer it, and we can see opportunities around the world – it will be good for renewables,” adds Kohnstam.

On controllability, Scheibe explains: “With DC, it is easier to control the power flow from one point to another, which means you can trade power in a very predictable way. This is being discussed in many countries where they want to set up reliable trading.”

He continues: “It is well-known that we are seeing continuous and fast development of control systems for HVDC, but we also want to add new functions. This is important for cyber security and is critical for the supergrid. To know how to integrate an HVDC link within a supergrid, the more information you have the better. So what we do on the network management side is very important for what we do on the HVDC side.”

Alstom Grid announced recently that it had struck an agreement with US computer chipmaker Intel to work together on embedding intelligence and IT security into networks to speed evolution of smart grids, smart cities and the integration of power from renewables. For one of the burning issues in security is how to prevent activists and terrorists gaining control of power networks by hacking into IT control systems.

“One big question mark about DC systems is how to control and protect them,” says Scheibe. “Someone has to tell the breaker when it should operate and how. We need more information than today, and at great speed. So there is a lot of research going on into how to control and protect HVDC meshed grids, the breaker being one part of it.”

Similarly, Kohnstam identifies control algorithms as the key challenge and innovation associated with the UK Western HVDC Link. “It’s allowing us to be more flexible and therefore to support the reliability that everyone wants from such large power links.”

He adds: “Changes in our control algorithms mean DC systems are able to respond much better to system parameters and that means we can apply them in different places that we would have ruled out as impractical before.”

Scheibe concludes: “Somewhere in the long-term plans for most big utilities today there is a DC link, and that maturity is a big step forward. When we see meshed systems, that will be the real breakthrough and then we’ll see supergrids. Still, we should not underestimate the value and need for continued incremental steps on costs, power losses, equipment size, and on standardisation. I see AC as a competitor and to fight them we have to be faster, lower cost, all these things.”

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