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High power DC transmission lines are coming back

Wednesday, November 30, 2022

Many things that are basic knowledge to us today were not fully understood in the early days of electricity and power (in the late 1800s/early 1900s), but one thing was clear even then: Ohmic (resistive) losses in power-transmission lines were unavoidable. The only solution was to increase the line voltage used for power transmission, thus decreasing the line current by the same factor.

Because ohmic power losses are proportional to the square of the current (P = I2R), the savings are exponential and significant. Novice electrical engineers today quickly learn this basic lesson about the relative efficiency of higher voltages for power transmission early and often, starting with small-scale projects like PC boards and chassis ranging up the power scale.

The big issue in the early days was whether to use AC or DC power for generators and transmission lines. At that time, there was a highly publicized battle between Edison on one side, as a staunch DC proponent, and Tesla, Westinghouse, and others, who favored AC. Power transmission using AC could be easily stepped up/down via transformers as needed and thus support longer transmission-line distances, while DC was limited to shorter runs and needed many relatively local generators to serve neighborhoods (electricity was first adopted for dense urban areas, of course).

Fast-forward about 100 years to the recent past, and the situation has changed dramatically. Many transmission-line designs are using HVDC, with many new HVDC installations being planned. There are many reasons why, but chief among them is the familiar, mutually beneficial, positive-feedback push-pull of applications and technology advances: New components make new approaches increasingly viable, while that increased viability increases design-ins and the demand for these components, which in turn increases demand for new components, and the cycle begins again.

In the case of HVDC, advances in technologies like high-voltage/-power IGBTs and thyristors have been among the many new components needed (it’s interesting that many of these same devices and technologies were initially developed for all-electric and diesel-electric locomotives). For HVDC, the most common types of converter stations are line-commutated converters (LCCs) and voltage-source converters (VSCs). There’s a lot of interesting design work being done for LCCs and VSCs, as well as related HVDC subsystems that use new components and unique design rules.

The market changes

How is HVDC doing? The short answer is “very well,” with many numbers, depending on who you ask. Market-research organizations each have their own high-precision numbers looking out to five years and more (how they can predict the future to three and even four significant digits always fascinates me). It varies depending on what they include in their HVDC market assessment and the methodology they use, but the rough consensus is that the HVDC market range was about $10 billion each for 2020 and 2021 and is expected to grow to between $17 and $18 billion by 2026, for a compound annual growth rate (CAGR) of between 6% and 8%. In contrast, the all-important numbers for HVAC growth are several points and billions of dollars lower.

With respect to HVAC versus HVDC costs, a rough guide is that if you exclude the cost of conversion circuitry at each end — and that’s a very big “if,” as it is admittedly a large cost—HVDC lines make sense for distances greater than about 500mi/1,000km for overhead lines, 15–30mi/30–60km for submarine cables, and 30–60 miles/60–120km for underground cables. While conversion circuitry for HVDC is more expensive, that is balanced out somewhat, as its transmission lines require smaller towers and in their simplest arrangement need only two conductors rather than three. Determining that total-cost breakeven point takes fairly sophisticated analysis (see Figure 2).

While cost is certainly a major consideration, the HVAC-versus-HVDC case also has important technical aspects. First, HVDC is asynchronous and does not require synchronization among the many sources to maintain stability, so these can be added/dropped as needed or available; in contrast, AC sources require careful initial synchronization procedures and must not go out of sync. Furthermore, the power factor of the DC line is always unity, so no reactive compensation is needed. Also, there is no “skin effect” as there is with AC lines, a detrimental aspect that reduces the effective current-carrying cross-section of the line and increases resistive loss.

In short, there’s a good case for using HVDC, and the case gets stronger as the grid increasingly counts on a multiplicity of distant sources like wind power, solar farms, and even battery-based energy storage systems rather than generation plants. We’re even seeing some commercial power systems use 350VDC as the primary power line for office buildings, factories, and large apartment complexes, with conversion to 120/240VAC for the end applications.

Even if HVDC takes on a larger role in power-line transmission, Edison won’t get a full victory, as HVAC and HVDC each play a legitimate role and will be used where they make the most technical and economic sense. Perhaps the most we can expect is that in their afterlife, Edison and Tesla will recognize that reality and at least become “frenemies.”

By: DocMemory
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