Transmission Technologies: HVDC
High-voltage direct current (HVDC) systems offer an alternative solution to the problem of electrical transmission, one with strengths not offered by the traditional AC grid system. While DC-based systems are not a new technology—at the advent of the electrical era, Thomas Edison envisioned that they would become the predominant choice—the ability to incorporate them with the existing AC grid is a relatively new development.
In general, transmitting electricity from the generator to the consumer at a higher voltage reduces the amount of power lost in the process. However, there are functional limitations that make transmitting across high-voltage AC lines an ineffective option. HVAC lines lose large amounts of power due to induction, capacitance, the “skin effect” (wherein the current moves to the outside of the cable, forming a “skin” and thus failing to utilize the cable in its entirety), and the ionization of the air around the cable, which draws electrons away from the path of transmission. HVDC, by eliminating the alternating current, eliminates these problems, but introduces a new challenge: switching to an HVDC grid necessitates either the construction of DC generators to replace the AC generators currently in use, or the systematic conversion of AC to DC at the point of transmission. As AC generators are generally understood to be less expensive and easier to maintain than DC generators, the second option is more desirable.
While crude technologies for this sort of conversion existed as early as the 1930s, it was not until the 1970s that conversion between the AC power emitted from generators and the DC power suitable for high-voltage (and thus low-loss) transmission could be achieved with a great degree of efficacy. The converter station—the keystone of HVDC technology— is generally designed with dual-conversion capability; each can convert AC power to DC (a process called rectification) and DC power to AC (inversion). The device operating at the core of the conversion process is the thyristor, a semiconductor which can only carry current in one direction. The thyristor acts as a sort of “gateway” in the converter, allowing the voltage and the direction of power flow to be controlled remotely.
Most often, thyristors are arranged in a series to form thyristor valves, in a type of system known as natural commutated conversion. This basic system can be improved with the addition of commutation capacitors, which, inserted between the converter transformers and the thyristor valves, increase the accuracy with which the valves “fire” in synchronicity to control the direction and magnitude of power. This kind of system is known as a capacitor commutated converter. A third kind of converter station, the voltage source converter, is built with semiconductors which can turn off or on via remote control almost instantaneously, allowing for an even greater degree of load control, as well as the ability to control active and reactive power independently of one another. This allows the converter station to act as a mechanism to control reactive power, further regulating the voltage of the system.
Thyristor valves are arrayed in groups of six, performing six commutations per period. The number of thyristors that comprises each valve can be varied to produce the desired voltage output. As they are the most fundamental element of the converter station and therefore of HVDC technology, development of new models of thyristor have driven improvements in HVDC technology overall. For instance, photons can be used to trigger their valve action, allowing the development of light-operated thyristors with an 80% reduction in necessary components.
HVDC systems are not flawless; they have important drawbacks to consider. The synchronous pulses that allow the converter station to operate create harmonics which can cause interference with neighboring telecommunications systems, as well as mechanical damage over the long term. Usually, harmonic filtering systems must be installed to counteract the potential for damage. The station itself is susceptible to a high level of load stress, requiring observation and maintenance as it is subjected to wear. However, the most significant disadvantage of an HVDC system is the cost of the converter station, which must be designed and constructed to work in conjunction with the existing generator at a site.
Even with this cost in mind, HVDC can still be a more economical option overall in situations where AC systems are inappropriate or fall short of meeting a demonstrated need. In areas where adjacent regions wishing to share power have networks with different nominal frequencies, HVDC cables are sometimes used to allow power exchange. They can also be used to transmit power underwater, or overhead in cases where AC lines would be too expensive or unsightly—DC lines require only two main conductors, whereas AC lines require three and thus consume more space and materials.
Whether the benefits and drawbacks of an HVDC system make it an efficient option or a wasteful is widely variable based on individual situations; the site of generation and consumption, the path of transmission, the type of power, and the nature of the existing grid can all be factors. However, over long distances, HVDC has proven to be an excellent choice. In addition to stability it adds to the grid, an HVDC system succeeds where as an AC cable would fail, because the latter suffers from greater line losses and the need for intermittent substations to regulate current. Therefore, AC systems can only effectively transmit power a certain distance before becoming ineffective. (See Figure 74.)
Even after accounting for the costs associated with converter stations, HVDC becomes a more efficient option than AC systems over long distances. The break-even point is generally assumed to be between 400 and 500 miles (Rudervall)
Is it Competitive? A Cost Analysis of Wind Energy
Overview of the section:
In this section we will discuss the feasibility of wind-based electricity generation in America. We will first address the two primary disadvantages of wind-based electricity generation: availability and variability of resources. We then present two scenarios of the potential market for wind-generated resources in America. The first assumes that wind-generated electricity will compete in the market of the nearest major metropolitan area. The second assumes that wind-generated electricity will take advantage of variation in the price of electricity across states. Specifically, we consider the proposed Frontier Line, an HVDC line that would transmit large amounts of electricity to Western cities from remote generation sites. We then analyze the cost of wind-generated electricity under both scenarios. We find that wind-generated electricity is not competitive in nearby energy markets, but is competitive to service Californian demand. Finally, we consider the impact of two polices would have on the economic viability of wind energy: the Renewable Electricity Producer Tax Credit and a cap-and-trade.
The Availability of Wind Resources
One legitimate drawback to wind-based electricity generation is that the inputs to production are geographically fixed. Unlike fossil fuels or uranium, wind resources cannot be extracted from the earth and transported to demand sites. Instead, they must be converted to electricity at the site where they are found, and the electricity must be transmitted to demand sites.
This limitation is not inherently problematic, because in many scenarios wind resources are very close to demand sites. Denmark, for example, receives a large proportion of their energy from offshore winds that are quite close to demand sites. The U.S. also has abundant offshore wind resources, and with 53% of the US Population living in a coastal county (Crosset 2004), many electricity demand sites have a large potential supply of wind-generated electricity nearby.
The map below is published by the National Renewable Energy Laboratory (NREL), a facility operated by the U.S. Department of Energy. It illustrates the availability of wind resources at different wind power classes (WPCs). Current technology limits wind-based electricity generation to WPCs of 4 and above, which roughly corresponds to winds average speeds greater than 12.5 mph. These correspond to the pink, purple, red, and blue areas on the map below.