High-voltage direct current (HVDC) technology offers several advantages compared to alternating current transmission systems. For example, it allows more efficient bulk power transfer over long distances. However, cost is an important variable in the equation. Once installed, HVDC transmission systems are an integral part of the electrical power system, improving stability, reliability, and transmission capacity.

Typical utility-scale power plants generate alternating current (AC) electricity, and most electrical loads run on AC power. Thus, the majority of transmission lines carrying power around the world are of the AC type. However, there are instances when high-voltage direct current (HVDC) transmission systems offer significant benefits.

“One big advantage to HVDC is the efficiency of power transmission over long distances,” George Culbertson, vice president of power delivery markets for HDR, told POWER. “If the transmission line route is longer than about 300 miles, DC is a better option because AC lines have more line losses than DC for bulk power transfer.”

Converting from AC to DC

The challenge, however, is that to transmit via HVDC, two converter stations are needed. First, the AC power must be converted to DC to begin the transmission process, and then when it gets to the desired tie-in destination, the DC power must be converted back to AC to be utilized on the grid.

Conversion technology is well-established. Electrical pioneers were working on the building blocks for HVDC-links back in the late 1800s. Conventional HVDC converter technology is based on the use of line-commutated or phase-commutated converters. In 1954, ASEA, the predecessor of ABB, used this classic technology utilizing mercury arc valves to construct the world’s first commercial HVDC link between Västervik, on the east coast of Sweden, and Ygne, on the island of Gotland in the Baltic Sea. The original Gotland link could transfer 20 MW over a 98-kilometer (km)-long submarine cable with a voltage of 100 kV. The service was re-engineered in 1970, increasing capacity to 30 MW at a voltage of 150 kV through the addition of a thyristor valve bridge.

ASEA continued to push boundaries, developing new HVDC systems during the decades that followed. In 1997, ABB commissioned the world’s first HVDC demonstration project using voltage source converters (VSCs). VSC technology uses gate turn-off switching devices, such as insulated-gate bipolar transistors (IGBTs), to perform the conversion. An IGBTs high switching frequency capability allows more-precise VSC control and less-complex circuit configuration through the use of pulse width modulation techniques. ABB named its new VSC-based product HVDC Light.

VSC technology was further improved when Siemens introduced a modular multilevel converter (MMC). The Trans Bay Cable project, which runs between San Francisco and Pittsburg, California, was completed in 2010, using Siemens’ HVDC Plus system. MMC technology offers excellent harmonic performance and reduced power losses compared to previous VSCs. All HVDC manufacturers are applying MMC technology in VSCs today.

Platform to Shore

Vince Curci, underground transmission project manager with HDR, said one of the advantages of VSC technology is that it is very compact. “They require maybe 30% of the area of a conventional converter and about 50% of the weight,” Curci said. That makes them a good choice for offshore wind farms. “A 600-MW VSC requires less than one acre of land, whereas a conventional converter requires three or four acres. So, the advantage of this new technology is that you can put them at sea on a small footprint and transfer power to land via submarine cables.”

One example of that is the DolWin2 project (Figure 1). TenneT, a European transmission system operator with operations in the Netherlands and Germany, required a 916-MW HVDC link to connect the Nordsee One, Gode Wind I, and Gode Wind II wind farms to the onshore transmission grid. ABB designed, supplied, installed, and commissioned the compact offshore and onshore converter stations, as well as the subsea and underground cable systems.

1. Offshore link. DolWin2—completed in 2017—ties three North Sea wind farms to the German power grid via a 916-MW high-voltage direct current (HVDC) transmission link. Courtesy: ABB

 

The wind farms are connected with AC cables to an HVDC converter station installed on an offshore platform in the North Sea. The DC power is then transmitted through a 45-km-long sea-cable system (Figure 2) and further 90-km-long land cable to an onshore HVDC station at the grid connection point of Dörpen West. The project was completed by ABB and handed over to TenneT in June 2017.

2. Subsea cables. Power generated by the Nordsee One, Gode Wind I, and Gode Wind II offshore wind farms is transmitted to land via HVDC subsea cables, shown here during installation. Courtesy: TenneT

 

“HVDC is the technology of choice for reliably and efficiently transmitting large amounts of power over long distances with minimal losses. It is ideal for integrating remote renewable energy into the power grid,” Claudio Facchin, president of ABB’s Power Grids division, said in a press release announcing the project’s completion. Siemens has also completed projects of this nature.

Analyzing Options

One thing that often puts the kibosh on an HVDC project is cost. Converter stations are expensive. “VSCs for a large HVDC transmission project could cost in excess of $100 million and depends on voltage and power rating,” Curci said. Therefore, it’s prudent to complete a study of the available alternatives. Three main factors need to be considered.

“It depends on the distance, it depends on the voltage, and it depends on the power transfer,” Curci said. “There are typically breakeven studies done, which include lifecycle cost, and then you reach a point where the HVDC system becomes more economical based on these factors.

“AC systems have lower capital costs, but a much steeper line slope as you increase the distance. Along the length, they need compensation, especially at high voltages, because they require what we call VAR [volt-ampere reactive] support,” continued Curci. “HVDC systems have a much higher capital cost, but as the distance increases the slope of the line is flatter. So, there is a point where these two lines intersect, and that’s your breakeven point—that’s a function of distance, voltage, and power transfer.”

Culbertson recalled a study he had been involved with early in his career. It was completed for a gas company that was trying to determine if it was more cost-effective to build a natural gas pipeline or an HVDC transmission line from Turkmenistan, where gas was plentiful, to Pakistan, where power was needed, by way of Afghanistan. Both options were very expensive. Ultimately, the project never got off the ground due in large part to political unrest in the region.

But there are plenty of projects that are moving forward. In March 2017, a consortium between Siemens and Sumitomo Electric Industries Ltd. was awarded an HVDC order from Indian transmission operator Power Grid Corp. of India. The team will construct a 200-km-long HVDC connection, using both underground cable and overhead lines, between Pugalur, Tamil Nadu, and Trichur, Kerala. It will be India’s first HVDC link featuring VSC technology. Siemens is supplying two converter stations with two 1,000-MW parallel converters, while Sumitomo Electric is responsible for the cross-linked polyethylene HVDC cable system in the DC circuit. The combined order volume for the two companies is about $520 million. Grid connection is scheduled in the first half of 2020.

Siemens is also involved in a couple of UK projects. The Nemo Link will interconnect the British and Belgian national grids via subsea cable. Siemens is responsible for the turnkey installation of a converter station on an 8-hectare site in southeast England, formerly occupied by the Richborough Power Station, and a similar converter station in the Herdersbrug industry zone in Bruges, Belgium. The 140-km-long link, with a 1,000-MW capacity and 400-kV operating voltage, is expected to enter commercial operation in 2019. Additionally, the ElecLink will connect the British and French power grids. HVDC cables will be routed through the Channel Tunnel as part of that project. The 51-km-long link will have a 1,000-MW capacity and 320-kV operating voltage (Figure 3).

3. HVDC converter station. The converter hall shown here is part of an HVDC transmission link between France and Spain. It utilizes Siemens’ HVDC Plus insulated-gate bipolar transistor converter modules to supply capacity of 1,000 MW with voltage of 320 kV, which is currently the most powerful link in the world, using voltage source converter technology. Courtesy: Siemens

 

ABB is also working on a project that will connect the English and French markets. With a capacity of 1,000 MW, the link will run from Chilling, Hampshire, on the southern coast of England, to Tourbe in northern France—a distance of 240 km across the English Channel. Furthermore, ABB received an order in early July to upgrade the HVDC link that interconnects New Zealand’s north and south islands.

Permitting and Cost

“From my perspective, one of the biggest challenges to any project is permitting, especially when you’re talking about a 500- or 1,000-mile line,” said Culbertson. “You’re going to be crossing different jurisdictions—cities, counties, states, or even countries.”

However, that challenge isn’t isolated to HVDC projects. Any transmission project can face difficulty obtaining the necessary permits. There is often a negative public reaction from impacted residents who don’t want to see towers running through their neighborhoods or across their land. In the western U.S., there is a lot of federal land that may need to be crossed, which adds complexity with respect to obtaining permits from agencies such as the Bureau of Land Management.

Almost all projects require some form of environmental impact study to address temporary and permanent impacts, and the process can be time-consuming, sometimes requiring years to complete. Furthermore, there are right-of-way requirements that must be adhered to in terms of width for installation, operation, and maintenance, depending on voltage and number of lines. There are also horizontal and vertical clearance obligations—really nothing is left to chance.

Although converter stations are expensive, HVDC projects do have some cost advantages over AC systems. “DC lines can actually be cheaper per mile because of the way the conductors are configured,” said Culbertson. “You have to have three separate phases for AC, so for a large line you have three sets of conductors, usually they’re multiple bundles of conductor—very heavy—and the towers have to be pretty massive to hold all that weight. That additional steel and aluminum also adds to the visual impact.

“A DC line can deliver comparable amounts—or even higher amounts—of power using only two sets of conductors as opposed to three, so the towers don’t have to be quite as large resulting in much less installed cost on the transmission part of it. You can also run longer DC lines underground. So, there can be a big advantage to DC where permitting and visual impact is a concern,” Culbertson said. ■

—Aaron Larson is POWER’s executive editor.

The world is in desperate need of an electrical revolution. Now, more than ever.

High-Voltage Direct Current (HVDC) can be transmitted over long-distances with minimal power losses, unlike Alternating Current (AC) electricity. This means that, once cost-effective infrastructure for HVDC is developed, it will be very beneficial to primarily transmit DC power because transmitting it will cut down on both energy waste and copper use. Not to mention, when power can more efficiently be distributed to buildings, less electricity needs to be generated to satisfy electrical demands. When less electricity needs to be generated, less carbon emissions are produced, making DC electricity a significant piece of the puzzle when it comes to meeting global emission reduction targets.

In this article we'll cover the scientific reasons why HVDC has less power losses than AC electricity, discuss the benefits of this, and discuss why these things make it worth it to work towards a world powered by DC electricity.

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The 3 Main Benefits of HVDC Transmission Systems: 

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1. HVDC is more efficient to transmit over long distances

According to George Culbertson, VP of Power Delivery Markets for HDR, “One big advantage to HVDC is the efficiency of power transmission over long distances. If the transmission line route is longer than the break-even distance, DC is a better option because AC lines have more line losses than DC for bulk power transfer." According to the scientific journal, PNAS, the conventionally cited break-even distance for new DC over AC overhead lines is ∼600 km to 800 km. The reason why high-voltage DC has less energy losses over transmission lines is because high-voltage AC has much more "capacitive" losses than DC power, especially when conductors are closer to the ground. Therefore, DC power is inherently more efficient to transmit, especially underground or underwater, than AC electricity. In fact, the break-even distance is much shorter for cables travelling underground or underwater; for underground cables the break-even distance is 50 - 95km, and underwater it's about 24 - 50km.

Break-Even Distance Definition According to Electrical Technology: In order for high-voltage DC to be worth the initial investment of infrastructure, the transmission distance must be a certain length (the Break-Even Distance). This distance depends on the type of transmission. This Break-Even Distance exists because, after that point, the energy and cost savings of transmission make up for the cost of the initial investment.

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What are the scientific reasons why DC electricity is more efficient to transmit? 

There are 3 main reasons: 

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1. HVDC is purely active power, so there are no reactive power losses 

In AC circuits or systems there are reactive components due to the alternating behaviour of AC electricity.

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AC Electricity Sine Graph demonstrating the alternating behaviour of AC electricity

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Reactive power is the quantity of unusable power that is developed by these reactive components. In order to calculate the amount of reactive power, or unused power, consumed, one would calculate power factor. Power factor is an expression of energy efficiency that is influenced by the amount of reactive power (or unused power) in an electrical load (like your lights).

One analogy you can use to understand reactive power and power factor is a simple glass of beer: 

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Image Source: Power factor visualized

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So AC circuits have reactive power because they have these reactive components. Because DC power contains only active power, the power factor doesn’t need to be taken into consideration in DC systems. Not only is this considered when designing energy efficient electrical systems, but the power factor is also taken into consideration by electrical companies when they are determining how much electricity to generate. In AC systems, power companies must make up for reactive power losses due to poor power factor by sending more energy than will be actively used by the consumer. This is one factor that makes AC systems more inefficient than DC systems (which contain only active power).

Additionally, power with reactive components doesn’t travel as far, in electrical systems, as power that is only made up of active power. Because it doesn't travel as far, the lengths of High-Voltage Alternating Current (HVAC) transmission lines are maintained below a specific point.

The Corona Discharge

So now you know that, because AC power alternates voltage levels, it contains reactive power. But that's not all that's caused by current that alternates: alternating currents also inherently produce electrical frequencies. Frequencies are also important to consider because they cause AC electricity to lose almost 3 times more energy than high-voltage DC electricity because of a phenomenon called the corona discharge. The corona discharge doesn’t affect DC electricity as much because DC electricity has no frequency. But what is the corona discharge? It is a term used to describe when a voltage increases above a certain threshold, and the air surrounding the conductor starts ionizing and generates sparks. These sparks waste some energy, and that waste is called the corona discharge. 

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2. Less copper or conductive materials are necessary for HVDC cables

To reduce the corona effect, bundled conductors are used in AC power lines. The International Journal of Engineering and Advanced Technology states that: 

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Conductor bundling increases the effective radius of the lines conductor and also reduces the electric field strength near the conductors. Therefore increasing the number of conductors in a bundle reduces the effects of corona discharge.

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In other words, the larger the diameter of the conductor, the less corona discharge effects line losses. Since bundling conductors together increases the diameter of the conductor, this is an effective method to reduce the effect of corona discharge. Energy losses due to the corona discharge are dependent on the frequency of the system. According to the mathematical expression of how to calculate these losses, the higher the frequency of the transmission system, the higher the losses to the corona discharge will be. Because DC electricity has no frequency, it's not as affected by the corona discharge, and it's therefore not necessary to bundle conductors together in high-voltage DC transmission systems. When it's not necessary to bundle conductors together, less conductor material is necessary, which results in a less expensive infrastructure for HVDC systems with respect to cable costs. Additionally, less cables are needed in HVDC transmission systems; DC transmission requires only 1 - 2 conductors per circuit, whereas AC transmission systems require a 3 phase circuit. This further brings down the cost of cabling HVDC systems significantly.

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Image Source: Bundled Power Line Cables

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The Skin Effect

Additionally, AC power lines are affected by a phenomenon called the Skin Effect. Essentially the skin effect exists in AC circuits, especially those with higher frequencies. The simple reason that the skin effect exists is that alternating current induces circulating eddy currents in the conductor, which oppose current flow deeper within the conductor. The higher the frequency, the shallower the region (and closer to the surface) in which the current is conducted. Thus, the higher the frequency, the thicker the cable has to be in order to transmit the same amount of power. This is why, not only is cable bundling necessary, but each bundled cable must also be thick enough to carry an effective amount of power with respect to the skin effect. The skin effect does not exist in DC electrical cables because DC electricity does not oscillate, thus can travel directly through a cable. This means that the conductor within DC power lines can be thinner, and use less material. Based on this information, DC power lines are cheaper to produce because they require less conductive material (like copper).  

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Image Source: The Skin Effect

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3. Certain infrastructure for HVDC power lines is cheaper than infrastructure for HVAC power lines

High-voltage DC power lines can transport significantly more power over greater distances than high-voltage AC lines. The two primary reasons for this are: HVDC lines can carry a higher voltage than HVAC lines with the same wire thickness due to the corona discharge and the skin effect. It's also possible to transmit about twice the amount of voltage in an HVDC power system in comparison to a high-voltage AC power system, which explains most of the advantages of overhead HVDC lines compared to overhead HVAC lines.

The ability for HVDC lines to transmit more power over greater distances gives it two main advantages over HVAC:

1. HVAC transmission systems are limited to shorter power lines, meaning that more power towers must be created, which boosts the costs high-voltage AC transmission systems.
2. Not only are there more power towers in HVAC systems, but the power towers are also bulkier and more expensive to construct than power towers in HVDC systems. The support needs to be stronger, wider and taller than HVDC transmission towers in order to accommodate for the heavy mechanical load on AC transmission towers, which makes them more expensive. 

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In total, the cable cost for high-voltage AC lines is more than three times the cost of HVDC cables.

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If HVDC is that much more efficient to transmit, then why are high-voltage AC transmission systems considered cheaper?

The fact is, even with all of these infrastructure advantages that HVDC has over HVAC, HVAC is still cheaper and more efficient to deliver power under certain transmission distances. The break-even distance for overhead lines is around 600 km, for example (as mentioned previously). HVAC is less expensive to deliver power under the break-even distance because HVDC transmission systems require terminal converter stations (which are very expensive), whereas HVAC does not.  

See the image below from Electricaleasy.com to get a better understanding of the break even distance that depicts when HVDC becomes more efficient than HVAC transmission. If you're wondering why HVDC transmission systems require converter stations, and HVAC transmission systems do not, you can learn more about this in our article comparing DC and AC electricity in general. To simplify the answer, DC power does not work with transformers, so the converter stations act as a work around to this. Let's move onto the second benefit of HVDC transmission systems over HVAC transmission systems.

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2. It's more environmentally friendly to transmit HVDC

This point may seem obvious since we’ve already discussed the lighter and cheaper cables and power towers involved in HVDC transmission systems, as well as the fact that HVDC doesn’t waste as much energy in transmission. However, this point bears noting simply to give a value to how much energy and carbon can be conserved with HVDC transmission systems. EE Power points out that the increased efficiency of HVDC over HVAC reduces losses from 5 - 10% in an AC transmission system to around 2 - 3% for the same application in HVDC. 

Additionally, because HVDC has lower capacitive losses than HVAC, it can travel underground, underwater, and through the air with significantly less losses in energy. This makes it ideal for integration with renewable energy sources, such as wind, hydro and solar. The European Commission says Europe needs between 230 and 450 GW of offshore wind by 2050, as 450 GW would meet 30% of Europe’s electricity demand in 2050. WindEurope emphasizes that the longer links required for connecting renewable energy sources to cities and distribution networks, mean that high-voltage direct current (HVDC) grid infrastructure will play an increasingly important role. This is to reduce energy losses, and reduce our global carbon emissions by implementing more sources of renewable energy. 

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‍3. HVDC Supports the Globalization of Power Grids

As we discussed earlier, DC electricity has no frequency, whereas AC electricity does. An additional problem is that different countries often distribute electricity at different frequencies. For example, Peru uses the electrical frequency of 60Hz, whereas Bolivia (directly below Peru), uses 50Hz. This can cause issues if the countries wanted to connect their power grids because, for AC grid connection, the rated voltage and frequency must be the same. 

On the other hand, because DC electricity has no frequency, a system involving it can be used to interconnect two substations with different frequencies. Thus the transmission of power is independent of the frequencies on the sending and receiving ends, which allows each country or power grid receiving DC electricity to choose to convert the DC power they receive into the frequency of AC electricity that works for their infrastructure. This is called Asynchronous Interconnection.

Conclusion

In order to proliferate the market with DC power transmission and distribution, and reap the benefits of HVDC transmission, it’s essential to consider the gaps in technology that exist. As technology in the electrical engineering space continues to develop, we’ll see the infrastructure costs associated with DC transmission systems go down. For now, AC transmission systems are generally less expensive (up to the break-even distance) because they don’t require terminal converter stations.

It’s still important to note that, because of the many benefits of DC transmission systems, they are still currently the most cost effective option under these conditions: 

• The transmission distance is longer than the break-even distance (as discussed)
• Electrical cables are travelling underground or underwater 
• Transmission cables cross country borders 
  

If the cost of transmission infrastructure is not taken into consideration, DC electricity is the superior option. It’s more energy efficient mainly for these reasons: 

• It has low capacitance losses and can travel longer distances 
• The corona discharge and the skin effect do not apply to DC power 
• It's an active power so the power factor doesn't apply to it
  

As we strive to meet our climate targets for 2050, it’s important to consider where we’re wasting energy, and what technological advancements can be made to reduce this waste. We know that switching to DC electricity will reduce energy consumption in power grids and buildings. The question is, how quickly will it be adopted into our infrastructure? We actually don’t have to wait until HVDC transmission technology advances to do so. There are many systems out there, including , that allow commercial buildings to easily switch to DC power distribution, even if power grids still supply AC electricity. If building managers chose to prioritize this change at the local level, they wouldn’t only be reducing the carbon footprint of their buildings, they would also be saving on energy costs. Building managers don’t need to wait for advancements to make this change, this is something they can do right now to make the world a better place and bring their building into the future. 

Learn more about how distributing DC power at the local level would save your building energy by checking out this article: 5 Reasons DC Electricity Should Replace AC in Buildings