Advancements in Tech Needed to Standardize HVDC Transmission
June 20, 2022
As more of our modern, digital devices require DC (direct current) power, a stronger need for transmission and distribution systems is developing. For example, data centers represent 2% of the electrical use in the U.S. right now, and are powered almost entirely by direct current (DC) power. In general, by powering a data center with DC electricity directly (and therefore cutting out AC to DC conversions), a typical 800kW data center could save about 4.2% per year in energy costs, which could account for about US$ 33,215 per year savings.
This is only one application of DC electricity that demonstrates just how much energy DC transmission and distribution would save. We know that high-voltage DC (HVDC) has many benefits, but the majority of power grids still supply AC power. So, how can we solve this? What technology specifically needs to be developed to power a world of DC devices with DC power?
Even with the many benefits that DC power provides, there are technological reasons why we still distribute AC power. In a previous article we discussed these reasons, and explained why AC power is often the cheaper option for power distribution. We also discussed what gaps in technology exist that make it more difficult to distribute high-voltage DC power. For example, the fact that transformers isolate power, and have the ability to change voltage levels, but are only compatible with AC power, is a major reason why power grids distribute AC power.
If you're interested in learning about what technological advancements need to be made in order to make DC power transmission more affordable and efficient, you've come to the right place. Perhaps you'll even be part of the solution one day and help to power the world with DC power.
Technological Solutions to Develop
It may seem overwhelming to consider the challenges associated with using DC electricity to power the world, but remember the main benefits of doing so.
The Main Benefits of DC Electricity and the Transmission of DC Electricity:
HVDC inherently has less power losses when travelling than AC electricity
Cables and power towers are lighter and more environmentally friendly in HVDC systems
HVDC can support the globalization of power grids by making them asynchronous
Less copper and cables are involved in HVDC transmission than high-voltage AC transmission
DC electricity is more compatible with smart building systems
DC power distribution to buildings eliminates the need for inefficient power converters in building systems (including servers in data centers)
DC electricity is safer to handle
Many DC powered devices are intrinsically efficient (such as LED fixtures and HVAC systems with DC motors)
If you get nothing else from this article remember this: It’s worth it to work towards a world powered by DC electricity.
Solutions to the Four Problems
In a previous article, we covered the 4 challenges that we face in order to power the world with DC electricity. For reference, we'll list those challenges here, and then dive into the technological advancements that could be made to overcome these hurdles. If you haven't read this article, check it out here for further information.
Now let's dive into the solutions for these problems. Here we'll break down what advancements in technology already exist, and could exist in the future, to power the world with DC electricity.
Solution to Problem 1: Making It Safe to Switch Off DC Electricity
When using a mechanical switch, disconnecting a circuit with AC electricity flowing through it is more safe than disconnecting a circuit with DC electricity flowing through it. As a brief review, it’s not safe to disconnect a circuit with DC electricity flowing through it because there’s the high possibility of the phenomenon of “arcing” to occur.
What is Arcing?
Arcing occurs when electricity makes contact with a gas (such as air) and then discharges because of it. This discharge causes an electrical spark and, quite often, arcing is the cause of electrical fires.
Arcing can occur in mechanical switches because when a mechanical switch opens, it creates an air gap. When electricity reacts with air (in an air gap like this, for example), electrical sparks, or arcs, could occur. AC electricity reacts with air in the same way that DC electricity does, but the difference is that AC electricity alternates, thus passing the 0 volts threshold 50 - 60x per a second. Because of this, even if AC electricity reacts with air present in open mechanical switches, its voltage will lower so quickly to 0 volts that it's very unlikely that a dangerous spark will occur. On the other hand, DC electricity is constantly positively charged, so when it reacts with air, the effect is much more dangerous.
To solve this problem, it’s necessary to eliminate the air gap caused by mechanical switches. That’s where solid state switches come in. Solid state switches, such as transistors and solid state relay (SSR) switches, have no moving parts and use semiconductors to switch between an on and off state. If you’d like to learn more about how semiconductors work within a solid state switch, we recommend checking out the video Transistors Explained - How Transistors Work by the Engineering Mindset on YouTube.
Solid state switches (or relays) have been around for about 40 years; they were invented by Crydom engineers in 1972. Aside from the fact that solid state switches can be used to safely connect and disconnect DC powered circuits, they also have many benefits that have made them increasingly popular over the past few decades. Their main benefits over mechanical switches include:
Low power consumption
Ideal for harsh environments as well as shock and vibration resistant
The market for solid state switches is also growing more quickly than the market for mechanical switches. The market for solid state switches is growing at a CAGR rate of 6.38% from 2021 to 2028, whereas the market for electromechanical relay switches is only growing at a CAGR rate of 3.09% from 2021 to 2026.
This demonstrates that electrical systems will be more compatible with DC electricity as the market size for solid state switches continues to expand.
Solution to Problem 2: Making it Easier to Increase or Decrease Voltage Levels for DC Electricity
As mentioned above, DC power isn’t inherently compatible with transformers. This is a problem because transformers are used at almost every stage in an electrical transmission and distribution system. They are used to change voltage levels as well as isolate the electrical supply from an electrical load. It's essential to be able to do these two things in transmission systems in order to improve safety, step up voltages for when electricity is to travel further, and step down voltages for when electricity is to be distributed to electrical loads with various voltage requirements. Despite the fact that DC electricity is incompatible with transformers, high-voltage DC transmission systems do exist. In fact, the first one was built by ABB in the late 1950s when mercury arc valves were developed.
So how were we able to overcome the challenge of transformers being incompatible with DC power?
Engineers developed a work-around that successfully enables safe DC power transmission, but also adds an extra step to the transmission process. Additionally, even though DC power can be transmitted at long distances, it cannot yet be distributed throughout communities or directly to buildings unless a DC microgrid is being used to do so.
This is the typical transmission process from power generator to electrical load in an AC transmission system:
As you can see, power is generated, then a transformer is used to step up the voltage levels for transmission, then another transformer is used to step down voltages for power distribution to electrical loads with various voltage requirements. There are additionally smaller transformers that you could find in your subdivision; these are used to step voltages down even further to supply power to your house. Then, as electricity is distributed to devices in your home or building, each device has its own, local transformer, which is used to provide each individual device with the specific voltage it requires (like in your laptop charger).
The difference between an HVAC transmission system and a HVDC transmission system is that an extra step is included. Here's how it works: AC power is generated (because most electrical generators produce AC power), the voltages are stepped up for transmission, then AC electricity is converted to DC electricity (this is the extra step). This DC electricity is transmitted at a high voltage to another converter station, where it is converted back into AC electricity so it can be stepped down by a transformer, and distributed to towns, subdivisions, buildings, and individual electrical loads in the same way as in AC electrical distribution systems.
We have the technology right now to increase or decrease the voltage levels of DC electricity without this work-around system; devices that do this are called buck and boost converters. The trouble is, these devices do not isolate electricity, and so they don't provide the important safety feature that transformers inherently provide. Remember, transformers not only change voltage levels, but also isolate electricity to prevent over-current faults. Additionally, buck and boost converters can only be used for lower voltage applications; they haven't been optimized for high-voltage electrical distribution quite yet.
As you can imagine, it's great that we can benefit from the energy savings that DC electricity inherently provides in HVDC transmission systems. However, because of this extra step involving converter stations, HVDC transmission systems are more expensive to construct. Additionally, they still typically provide AC electricity to towns and buildings because power must be stepped down by a transformer before it's distributed at that level. In this way, DC powered devices are still obtaining AC electricity either way. In sum, the technology we need in order to distribute DC power to buildings and devices without a DC microgrid, is a system that can step up or down DC voltages, while isolating power at the same time so that the system is safe. This new system must be transformer-less.
As mentioned above, our current hope for technological development to solve this problem potentially rests in the arms of buck and boost converters. If we can find a way to isolate high-voltage power without a transformer, and use these converters as well, we could have a safe, transformer-less DC power transmission and distribution system. This would save significant amounts of energy in the transmission process, as well as for building systems and devices powered by DC electricity (which is about 80% of the electrical loads in a building).
Let's take a moment to clear up the terminology a little.
Transformers are used to change voltage levels and isolate AC power. They are also considered AC-DC converters. On the other hand, we have DC-DC converters, which step up or down DC voltages. There are two types of DC-DC converters:
1. Isolated: An isolated DC-DC converter uses a transformer because it first uses a "chopper" to convert DC into AC, then a transformer is used to step the voltage up or down, then power is converted back from AC to DC at the desired voltage using a rectifier. The downside to this method is that it involves transformers, which increase the cost and decrease the efficiency of the system. On the positive side, this method is the safest because, since the transformer also provides isolation, it drastically reduces the possibility of over-current faults. For now, this is the only method that works for HVDC distribution, as we discussed above.
There are a few types of non-isolated DC-DC converters, here are the most popular two.
Buck Converters: These use capacitors, inductors, transistors, diodes, and a solid state semiconductor switch to decrease the voltage of DC electricity.
Boost Converters: These increase the voltage of DC electricity using the same parts. The main physical difference between these converters is that the placement of the inductor and transistor is switched depending on if it’s a boost or buck converter.
The downfall of non-isolated DC-DC converters is of course that they’re non-isolated. This makes them more dangerous, and it wouldn't be possible at the moment to include them in high-voltage applications. Doing so would drastically increase the possibility of electrical fires in transmission systems.
So what’s the solution? If the goal is to reap the benefits of HVDC power distribution, there are a few potential solutions:
Supply the power grid with entirely DC electricity. The problem with this is that, for now, our only sources of DC power are solar and wind. Additionally, solar energy only provided about 2.3% of total U.S. electricity in 2020.
Develop a way to achieve electrical isolation alongside the buck and boost converter methods, and scale this up so that isolation can be achieved even with HVDC.
Make transformers more efficient so they can be better utilized with a HVDC power distribution method.
Solution to Problem 3: Make it Easier to Isolate an Electrical Load from a DC Power Supply
We discussed above how AC electricity is compatible with transformers, which makes it easier to change voltage levels in AC transmission system. But, as you'll recall, transformers serve two purposes: they increase or decrease voltage levels, but they also isolate the power supply from a load. Essentially, isolated power supplies provide a clear barrier across which dangerous voltages won’t pass. Isolation can also be explained as the ability to couple one circuit to another without the use of direct wire connections.
In the last section we discussed alternatives to transformers in a DC power system, but only provided one option that isolates power (and it involves transformers). Unfortunately, this option is less than ideal; it is inefficient and expensive because of the extra steps and costly equipment. As of right now, this is the main technology gap that prevents the widespread proliferation of DC power distribution systems in the market.
So what technology are we in need of to replace transformers in high-voltage DC transmission? Transformers employ a method called galvanic isolation in order to isolate the supply from an electrical load. In its most basic sense, galvanic isolation is when there is no physical electrical connection between two circuits. However, energy or information can still be transferred across circuits through other means, such as with electric fields, capacitance, or even via optical or acoustical signals. In fact, a device called an optoisolator (among other names) is a very popular method of galvanic isolation; it is a semiconductor device that transfers an electrical signal between isolated circuits using light.
There is space in the market for methods of transformer-less DC-DC isolation, as there aren’t very many solutions readily available at the time of writing this article, for high power applications. In fact, even with the few solutions we have to achieve isolation in a DC-DC system without transformers, these only work in low - medium voltage situations. In order for HVDC to be distributed safely, the power supply and power loads should be isolated, so the best solution would be to develop a technology that could do so without a transformer for high-voltage applications. If a technology is devised that utilizes galvanic isolation that works in high-voltage applications, the market could potentially be proliferated with DC power transmission systems.
Solution to Problem 4: How to Generate More DC Power
As discussed in our previous article, typically only solar panels and windmills produce DC electricity. Therefore, if we wanted to implement more DC transmission systems, it would make sense that we would want to make it easier and more economical to proliferate the power generation industry with solar and wind. According to our last article:
Implementing more solar panels would help to push the advancement of DC electrical systems because, if more DC electricity is being produced, there's less reason to convert that DC electricity into AC electricity for distribution considering the benefits of distributing DC electricity. Additionally, it's a simpler process to convert AC into DC than DC into AC, so there would be even more resistance towards the idea of converting the DC power generated by solar or wind into AC power.
Currently solar panels only produce about 4.4% of the energy needed to power earth, and wind accounts for just 5% to 6% of global electricity production. The reason we don't produce more of the earth's energy with solar is because better systems for storing solar energy need to be developed. It's necessary to have an efficient and economical solution for storing solar energy because, depending on weather, solar panels could go through long phases of not producing any new energy. For example, if it's cloudy or rains for a few days in a row, energy generated from previous sunny days will need to be stored in order to power buildings during this time. The problem is, the batteries for storing solar energy are very large and expensive. In fact, the more power you need, the larger your battery needs to be.
Generating more DC power is not the only reason we have for developing the technology behind solar and wind power generation; a major benefit of generating power via solar and wind is that they are renewable sources of energy and produce less carbon emissions than their alternatives. Solar specifically produces 5x - 10x less carbon emissions per unit of energy relative to coal or natural gas, and you might be surprised to learn that wind is even more environmental friendly than solar with respect to carbon emissions. In fact:
Bernstein Research "determined that wind power has a carbon footprint 99% less than coal-fired power plants, 98% less than natural gas, and a surprising 75% less than solar".
What can be done?
So now that we've brushed up on the reason why we don't produce more solar power, and the environmental benefits of using both wind and solar for power generation, let's dive into what can be done to power more of the world with these sources. Early adopters of solar panels used a chain of lead acid batteries to store solar energy. This system did its job, but poses its own disadvantages, including that it could be risky to work with these batteries. Since the development of Tesla's Power Wall and some of the other new storage options. Large Lithium Ion batteries or the low maintenance equivalent of lead-acid batteries (VRLA batteries), are often the choice when replacing more outdated systems.
On the other hand, wind energy generation is one of the fastest growing methods of electrical generation in the world. Some of the main challenges facing the proliferation of wind farms include:
It's more practical to build wind farms in rural locations, this makes it necessary to build more transmission lines from wind farms to urban areas. These transmission lines would allow for more cities and urban areas to be powered by wind, increasing the total energy produced by renewable sources worldwide.
Wind projects can alter animal habitats that they're built on, which can be dangerous for certain species. For example, birds or bats have been killed by flying into the blades of windmills. However, research is ongoing to solve these problems, and many of them have been resolved or greatly reduced already with technological development, or by properly siting wind plants.
The number one technology necessary to standardize high-voltage DC (HVDC) transmission and distribution systems is a transformer-less, isolated, DC-DC converter for high-voltage applications. Including a transformer in our current HVDC systems is inefficient and costly because it adds an extra step to the process of transmitting DC electricity over long distances. This extra step involves converter stations, and these are so expensive that they negate the energy savings inherently provided by DC electricity up until a certain distance (usually about 600 km). Additionally, developing this technology would also allow DC electricity to be distributed directly to buildings and our many DC powered devices.
But there is hope; there is a way to power building systems and devices with DC electricity even when power grids are not distributing it.
Building owners, designers and engineers could decide to implement a DC distribution system locally in their building. These systems make one highly efficient conversion at the electrical panel, and then DC electricity is distributed throughout the building to power building systems and devices. Cence provides an easy-to-install system just like this, and additionally allows building systems (BACnet and non-BACnet) to be controlled via the cloud based on sensor data. In this way, building managers using this system not only benefit from DC power distribution, but also from the energy optimization benefits that come from smart building automation systems. Additionally, by implementing a DC power distribution system, buildings can gain 18 points towards a LEED certification through the DC LEED Pilot Credit.
Erin is the Creative Director at Cence Power. She has a New Media degree from the University of Toronto and 5 years of experience in the communications field. She has also done digital content creation for dozens of clients through her own business called Story Unlocked. Erin loves technology, especially when it makes the world a better place.
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