HVDC – Securing supply

When the ‘battle of the currents’ ended in the late 19th century with Westinghouse and the AC lobby victorious over Edison’s DC lobby, AC became the dominant means for transmission of electrical power; a method which was to remain dominant for over a century. AC, with the ability for stepping the voltage up or down for high voltage transmission with transformers, was the easiest solution. However, AC transmission has significant drawbacks which become increasingly serious for long distances, particularly when sea cables are used. For this reason. High Voltage Direct Current (HVDC) is playing an increasingly important role in offshore wind, as wind resources are exploited at greater distances from the shore. 

Why HVDC?

HVDC first appeared in a commercially viable form in the 1950s but remained largely a niche technology until the early part of the 21st century. Today HVDC is becoming increasingly important in a range of different circumstances.

DC transmission has several advantages over AC: it is cheaper over long distances, it allows asynchronous AC systems to be connected together and – most importantly in the context of offshore wind – it greatly increases the distance over which power can be transmitted by under-sea cable.

In high voltage cables, the distance from the high voltage conductor to the earthed sheath is only a few centimetres and thus a cable has quite a high capacitance, typically 200-250nF per km. When energised by AC voltage, the cable capacitance results in an alternating current flow, creating power losses and using up part of the current rating of the cable. Consequently, with long sections of AC cable, the entire current rating of the cable is used up by the charging current and no useful power is delivered to the other end.

DC avoids this problem because the cable capacitance only needs to charge up once, when the system is energised. It is for this reason that HVDC is increasingly becoming the preferred option for transmitting power under the sea where the distance exceeds approximately 50km.

Types of HVDC technology

Two quite different types of HVDC technology are available on the market today. The more traditional of the two is the ‘Line-Commutated Converter’ (LCC) HVDC technology, using thyristors. LCC-HVDC is still the most efficient option for very long distance, high power transmission but it has certain operational limitations and the converter stations occupy large amounts of land, making them an uneconomic option for an offshore platform installation.

The second type of HVDC technology in use today is the ‘Voltage-Sourced Converter’ (VSC) type. VSC-HVDC uses ‘insulated-gate bipolar transistors’ (IGBTs) instead of thyristors. VSC-HVDC can be used in situations where LCC-HVDC cannot, for example where one of the interconnected AC systems has no directly connected generators of its own. VSC-HVDC also occupies about half as much space as LCC-HVDC, and as a result, has become the preferred HVDC technology for offshore wind applications.

VSC technology, in turn, can be subdivided into two sub-categories depending on how the IGBTs are switched. Most VSC-HVDC schemes built until 2010 used ‘two-level converter’ or ‘three-level converter’ technologies, scaled up from variable-speed motor drives. Such converters use hundreds of IGBTs in series, all switched simultaneously at a relatively high switching frequency (around 1kHz). This leads to significant levels of harmonic distortion and much poorer efficiency than LCC-HVDC.

In recent years, ‘two-level’ and ‘three-level’ converters have been displaced by the ‘Modular Multi-Level Converter’ (MMC). Here, again, hundreds of IGBTs are used but they are arranged differently, being grouped into submodules containing two IGBTs and a very large capacitor (several millifarads). Depending on which of the two IGBTs is turned on, the capacitor is either inserted into the circuit or is bypassed, contributing no voltage. Instead of switching all the IGBTs simultaneously, the submodules are switched at different times in order to synthesise a ‘staircase’ of different voltage levels. With enough steps in the staircase (typically, around 400 are used) the converter can produce an exceptionally high quality sinewave at the AC terminals of the converter, with negligible harmonic distortion. The efficiency is also much better than for the two level converter because of the low frequency at which the IGBTs can be switched.

HVDC in offshore wind

Although plans exist in many countries, Germany is leading the way for HVDC-connected offshore wind and already has eight separate schemes either completed or under construction. The first of these, Borwin 1, used a two-level converter but all of the subsequent connections have used variants of the Modular Multi-level Converter. The most recently ordered is the 900MW, ±320kV, Dolwin 3 project, awarded to Alstom Grid in February 2013.

Offshore wind uses the largest turbines available: 5MW and above, which are generally of the ‘full converter’ type. This means that the generator, instead of being connected directly to the grid, is instead connected via a back to back power electronic converter which allows the rotor and grid frequencies to be decoupled and hence permit efficient operation for a wide range of wind speeds. The converter in the turbine normally uses IGBTs in a two-level or three-level converter configuration and in fact is similar in concept to an HVDC link, albeit on a smaller scale. Consequently, although wind turbine generators are normally regarded as AC machines, this description is over-simplified.

The turbine cluster (typically 100-200 turbines) is connected to the HVDC converter in a radial arrangement. The turbine output voltage of 690V AC is stepped up to a 33kV collector array, several of which are brought together onto an AC substation platform where the AC voltage is stepped up again to an intermediate voltage such as 155kV.

The power is then fed to the HVDC converter platform where it is converted to DC, typically with a voltage of ±320kV, transmitted to shore via two insulated DC cables, before being converted back to AC in a second (onshore) converter station and fed into the onshore grid.

An important point to note here is that when an HVDC connection is used, the wind turbines are completely isolated from the onshore grid and thus form an electrical ‘island’. This can be an advantage because it means the offshore grid does not necessarily have to run at the same frequency as the onshore grid, but care needs to be taken in the design of the islanded offshore AC grid, a field in which there is little experience in the offshore wind industry.

The interaction between the offshore AC grid and HVDC converter is complex and important; requiring detailed modelling of the complete system, including the control behaviour of both the HVDC converter and wind turbine converters.

Most HVDC schemes are designed to be connected into relatively ‘strong’ AC power systems, where the HVDC converter plays only a subordinate role, tracking and ‘locking on to’ the AC system. However, in the case of an islanded AC system connected to an offshore wind farm, the roles of AC system and HVDC converter are effectively reversed.

The offshore AC grid consists of one very large power electronic converter (the HVDC converter), several hundred much smaller converters (in the turbines), and no other loads or generators. Consequently the role of the HVDC converter is to create the AC voltage reference for the turbine converters to lock on to, playing the part that would normally be played by the AC system in a conventional land based HVDC system.

The latest generation of MMC HVDC converters produce almost no harmonic distortion, but the earlier generation of ‘two level’ HVDC converters, along with the converters in the turbines themselves, produce significant harmonic distortion which needs to be filtered out if it is not to create potential harmonic distortion problems on the offshore AC grid.

The offshore grid does not behave in the same way as a conventional onshore grid. It contains no directly connected synchronous machines and has no loads, which means that it is much more lightly damped at harmonic frequencies than would be the case for a similar onshore grid. As a result, any harmonic distortion produced either by the HVDC converter itself or the converters within each turbine can be amplified, producing a waveform that becomes too distorted for the turbine converters to lock on to.

The future of offshore HVDC

To date, virtually all HVDC systems built, either onshore or offshore, have been simple, two-terminal, point to point schemes because multi-terminal schemes were difficult to engineer with LCC HVDC. However, with VSC-HVDC technology, multi-terminal schemes are easier to realise and so to allow possible expansion in future, many of the VSC-HVDC schemes being built today are designed to be ‘multi-terminal ready’.

For offshore wind, multi-terminal HVDC offers some interesting possibilities to improve the security of supply. With simple point to point HVDC links which only import power from a single wind farm, the HVDC link constitutes a single point of failure, with a repair time that could easily be measured in months. Connecting together two separate offshore HVDC-connected wind farms between their offshore DC terminals (or one wind power plant to two different countries or landmasses) would provide increased security.

However, the large-scale adoption of multi-terminal HVDC systems will require new technologies and products that are not yet commercially available on the market. Most discussed of these is the DC circuit breaker. DC circuit breakers are likely to be required to segregate the DC network into zones so that faults affect only part of the network instead of the entire network. DC circuit breakers are more complex to design than AC circuit breakers because there are no naturally-occurring current zeroes at which the current can be interrupted. Instead, the DC circuit breaker has to force the current to zero by inserting a voltage source in the opposite direction to the supply voltage. The key technological challenge is to achieve this fast enough (a few milliseconds) without incurring excessive power losses. Until recently this challenge had remained unsolved, but now Alstom is one of two HVDC manufacturers to have demonstrated prototype DC circuit breakers of this type.

Not every multi-terminal HVDC system will require HVDC circuit breakers: for moderate-sized systems with three or four converters, it is possible to use a modified design of converter which allows faults to be cleared electronically, but for more complex ‘DC grids’, DC breakers will become essential.

More fundamentally, protection and control strategies need to be worked out for a DC Grid so that the products and technologies necessary to realise such a grid can be defined.

Another challenge posed by multi-terminal HVDC systems is that such systems are quite likely to emerge ‘organically’, progressively linking together several existing point to point systems. For this to be possible, both the nominal DC voltage and its permitted operating range must be the same on all systems, but today there is no universally agreed DC operating voltage. The 8 offshore HVDC schemes already ordered in Germany use four different DC voltages: ±150kV, ±250kV, ±300kV and ±320kV. Even ±300kV and ±320kV cannot be directly connected together without some form of ‘DC transformer’, or DC-DC converter.

DC-DC converters are well known at low voltage, for example in computer power supplies, but extending their use to the very high voltages and powers needed for HVDC transmission, while retaining the very high efficiency of today’s HVDC schemes, is a major challenge which is now stimulating considerable R&D effort. Once DC-DC converters become commercially available for HVDC with suitably low power losses, a considerable simplification to the structure of the wind farm becomes possible. Present-day HVDC-connected wind farms involve two complete conversions from AC to DC and back to AC: one inside the turbine nacelles and the second in the HVDC link. Clearly this approach is wasteful.

A technically preferable option would be to use DC for all of the collection and transmission, leaving only the wind generator and onshore power system as AC. In doing so, not only would the overall efficiency improve but the synchronisation problems between the turbines and the HVDC converter would disappear at a stroke.

Conclusions

After 60 years as a commercial technology for transmitting electrical power, HVDC is finding important new application fields for offshore wind, using Voltage-Sourced Converters. HVDC is essential for connecting the far offshore wind farms that are becoming increasingly necessary. However, future HVDC schemes will not just be point to point: HVDC can create entire ‘DC grids’ linking multiple offshore wind farms together and allowing their generated power to be shared between several different countries. Hence HVDC is vital in order for the full potential of offshore wind to be exploited.

Thanks to Colin Davidson, Chief Technology Officer – HVDC at Alstom Grid