December 20, 2022
Not all power transmission systems are created equal. Despite alternating current (AC) power having won the War of the Currents, direct current (DC) power suffers from far less line losses along electrical cables. In fact, about 8 - 15% of power is lost between power plants, and consumers in alternating current (AC) transmission and distribution systems.
In this article, we’ll break down what the main three types of line losses in transmission systems are, why they occur, and how they affect high-voltage AC and high-voltage DC (HVDC) transmission systems differently.
In a nutshell, AC power won the War of the Currents because it’s compatible with transformers and the alternative, DC power, is not. Because HVDC isn’t compatible with transformers, it couldn’t be sent over long distances at the time. In the 1950s, however, mercury arc valves were developed, and transmission systems began using them to enable HVDC power transmission. Mercury arc valves are an additional piece of infrastructure required, so they make HVDC transmission systems generally more costly than high-voltage AC transmission systems (in terms of capital expenses). However, because HVDC suffers from far less line losses than high-voltage AC transmission (and is therefore much more efficient to transmit), the cost of HVDC transmission systems can be justified at a break-even distance of about 600 km. To demonstrate just how efficient HVDC systems are, 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. The closer power lines carrying AC power are to the ground, the more line losses they suffer because their electric field will more strongly react to the electromagnetic field of the earth. Essentially, HVDC systems are always more efficient when it comes to power transmission because they only suffer from one of the three main types of line losses (resistive power losses), while HVAC systems suffer from all three types of line losses. Check out our video on this, or read on, for a full explanation of why this is the case.
Transmission line losses make a significant impact on our carbon footprint as a whole, so reducing them would make a big difference. The first step to reducing them is to understand where they come from. Let’s get into it.
Most conductors, like power lines, are not perfect. This means that when power travels down cables, some of it is lost. Typically, electrical cables carrying higher voltages incur less line losses. This can be understood better by considering the formula for power: Power = current x voltage.
As a general rule, longer power lines carry higher voltages in order to reduce line losses. This reduces line losses because (according to the formula for power) the higher the voltage, the lower the current, for the same amount of power. Since a higher current means that more heat is generated (and heat is lost energy), we can conclude that transmitting higher voltages reduces the current, thereby also reducing heat/energy lost along power lines.
Additionally, power lines that carry lower voltages (and higher currents) must also be a heavier gauge. You can think of this like water flowing through a pipe. Electrical cables carrying higher voltages can be thought of as pipes with a higher water pressure; the higher the water pressure, the faster water will flow through pipes. When pressure is reduced (or voltage is lowered), a wider pipe is needed for the same amount of water to flow through. This is why electrical cables with lower voltages (and therefore higher currents) need wider gauges.
Resistive, capacitive and inductive line losses do not only occur in high-voltage transmission, but also in low and medium voltage scenarios. All three of these types of line losses are caused, in part, by heat loss from power being impeded along power lines. Alternating current (AC) power suffers from all three of these line losses, regardless of voltage, and direct current (DC) power only suffers from some types of resistive power losses (yes, there are subtypes to each of these losses). For example, even though losses from the skin effect are considered resistive losses, DC systems are not affected by them.
Does this come as a surprise to you? It’s an extremely common misconception that transmitting DC power is less efficient than transmitting AC power. After all, if DC power has less line losses, then why do most power grids transmit high-voltage AC? The simple answer to this is that AC power won the war of the currents back in the late 19th century (as we mentioned before) because it’s compatible with transformers, and DC power is not. Because AC power is compatible with transformers, it’s more easy and cost effective to step AC voltages up or down, as well as isolate power for safety. This makes the infrastructure to transmit AC power generally cheaper, and that’s why transmission systems are usually high-voltage AC. Anyway, back to where that misconception came from. Because high-voltage AC is usually what power grids are transmitting, DC power is more often associated with low-voltage distribution. As we covered earlier when we discussed the formula for power, a lower voltage also implies a higher current, and a higher current produces more heat and energy losses. So, if we compare a high-voltage AC transmission system to a low-voltage DC distribution system, the high-voltage AC transmission is more efficient. This is essentially where the misconception that DC power is less efficient came from: because people associate DC power with lower voltages, they might assume DC power is just less efficient. However, if we were to compare AC and DC distribution at the same voltage level (high or low), AC power would suffer from all three types of line losses, while DC power would only suffer from certain types of resistive power losses, making AC power ultimately less efficient at all voltage levels. Let’s dive deeper into what resistive line losses are exactly, and why DC power is affected by them, but not the other types of line losses along electrical cables.
Resistive power loss is when electrical power is lost due to the resistance of a conductor, like a power line. There are no perfect conductors, except for superconductors. Aside from those, all conductors have a bit of electrical resistance. This is why, when either type of electricity meets any resistance, electrical power is converted into thermal power (or heat). Thus, energy is lost in the form of heat, and this causes the voltage on conductors, like power lines, to drop. Especially over long distances. Resistive line losses are also called by a couple other names, including conductor losses, because they are more often the fault of the conductor than anything else. There are also a handful of subtypes of resistive power losses, including the skin effect, and dielectric loss.
The formula to calculate resistive power loss is as follows:
Capacitive losses only occur in AC circuits, not DC circuits, and are a type of reactive power loss. Like resistive power losses, reactive power losses come in the form of heat. If you watched our video about power factor, you might remember what reactive power is. Check it out below.
When two conductors run parallel to each other, they create a “capacitance” with each other, especially if they’re close together. So, with this in mind, consider these two conductors: power lines and the earth. Every electrical cable has a “parasitic capacitance” with the earth. “Parasitic”, in this context, means something unwanted, like a parasite. As for capacitance, capacitance occurs when two conductors are close enough together that they can store energy in an electric field. In the case of power transmission, capacitance occurs between the earth and power lines (our two conductors). When energy is stored in an electric field, there is some loss of power, which is known as capacitive line loss.
It’s also interesting to note that this is why power lines need to be so high above the ground! The higher they are, the less capacitive losses they incur because their electric fields cannot react with the earth as much.
Capacitive effects don’t occur in DC circuits because the voltage of a DC circuit is steady (it doesn’t alternate). This means that it doesn’t create an electric field, and so capacitance doesn’t occur between the electric fields of a power line and the earth in this case.
Inductive line losses are the third type of line loss experienced by AC circuits, including power lines. Inductive losses are essentially power losses that occur when a magnetic field is built and collapsed repeatedly, on a wire.
To explain this further, an inductor is an electrical component that stores energy in a magnetic field. When AC power alternates, it charges up a parasitic inductor (created by the wire), creating a magnetic field that collapses and changes direction repeatedly. When power is stored in these magnetic fields, it doesn’t make it to power loads, which makes it a type of line loss. Additionally, each time these parasitic inductors charge up and down, power is lost in the form of heat. The lost power that’s stored up in these magnetic fields and lost in the form of heat, is considered an inductive line loss.
Direct current (DC) power doesn’t have an alternating frequency, so its voltage is steady. This means it doesn’t charge a parasitic inductor up and down repeatedly, like AC power, and so it doesn’t incur inductive power losses.
When discussing lost power in the power generation, transmission and distribution processes, it’s easy to get hung up on all the different types and subtypes of line losses and how they occur. But, when it comes down to it, most line losses occur due to the frequency created by the oscillating voltage of AC power, and the consequential heat loss. AC power was chosen over DC power a century ago because it could easily be transmitted over long distances due to its compatibility with transformers. Additionally, many electrical loads at that time were compatible with AC electricity anyway (such as incandescent light bulbs). Today, things are very different. First of all, power has to travel further, so there are an increasing number of projects over 600 km (the break even distance to justify HVDC transmission). These power grids sometimes also have to traverse country borders, and some bordering countries don’t distribute power at the same frequency. AC power is transmitted globally at a frequency of either 50 or 60 Hz. So if one country distributing 50 Hz (for example), wants to transmit power to a country distributing electricity at 60 Hz, power must be converted to DC power first because DC power doesn’t have a frequency. That way, it can be converted back to AC power at the correct frequency. When DC power grids, on the other hand, traverse borders, these are known as asynchronous grids because there is no need to consider frequencies in this context, so less equipment is required.
Another reason DC power grids are proliferating the market is because, as a society, we’re transitioning over to greener energy solutions for power generation. Power generation for DC power usually involves cleaner technologies like solar panels and windmills, for example. Thirdly, an increasing number of our devices require DC power to operate. In fact, DC consumption currently makes up about 74% of total electrical loads in homes that use electric vehicles and HVAC equipment with DC motors, and that number is on the rise as an increasing proportion of power loads are digital.
As you can see, high-voltage DC power transmission, or HVDC transmission, is not only becoming increasingly popular because of its efficiency along electrical cables. It’s also proliferating the power grid market because it’s better suited for buildings of the future, and to reduce our collective carbon footprint.
If you’d like to learn about how DC power can benefit buildings (including yours), contact Cence to talk to a specialist. We help building owners make their electrical systems and power loads more efficient, saving buildings up to 40% on operating costs for lighting and HVAC.