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The Benefits of HVDC Transmission Systems (High-Voltage Direct Current)

March 18, 2022

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.

The 3 Main Benefits of HVDC Transmission Systems: 

  1. HVDC is more efficient to transmit over long distances
  2. HVDC is more environmentally friendly
  3. HVDC can support the globalization of power grids

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.

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

There are 3 main reasons: 

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.

AC Electricity Sine Graph demonstrating the alternating behaviour of AC electricity

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: 

Image Source: Power factor visualized

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. 


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: 

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.

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.

bundled power line cables for an ac system
Image Source: Bundled Power Line Cables
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).  

Image Source: The Skin Effect

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

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.

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. 

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 the Argentum system, 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


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Erin Kelly

Erin is the Creative Director at Argentum Electronics. She has a New Media degree from the University of Toronto and 5 years of experience in the communications field. From 2017 - 2019 she worked with a manufacturing company on their YouTube content and strategy, and has done digital content creation for dozens of clients through her own business called Story Unlocked. Erin has built two computers, and loves technology, especially when it makes the world a better place.

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