For a technology many considered obsolete, dc continues to grow, expand and improve the grid.
Sometimes technological advancement is not about a new discovery, it is about steady progress, hard work and improvement of existing technologies. Although direct current (dc) and alternating current (ac) started out as competing technologies, they are really complementary technologies; unfortunately, many still do not recognize this fact.
Strangely, early accounts of these technologies sounded more like something from a Harry Potter movie than the start-up of the electric industry. It was an epic battle between two wizards for dominance — the Wizard of Menlo Park (Thomas Edison's dc) and the Wizard of the West (Nikola Tesla's ac). The clash even had Edison staging the electrocution of an elephant to show how dangerous ac was compared with dc, which brought about the electric chair as a new form of capital punishment.
Timing is Everything
The first commercial dynamo, the dc generator became available about the same time Edison invented the lightbulb, which proved to be fortunate since the newfangled lightbulb worked fine with electricity of the dc variety. As a result, Edison invented or improved a lot of devices needed for the early dc electric system, earning him many patents on dc equipment. Within a short time, there were more than 200 electric utilities in North America with dc systems, and they were all paying patent royalties to Edison.
With the growth of his electrical empire, Edison hired a young engineer from Europe, Nikola Tesla, to improve the equipment used in the dc distribution systems. Tesla improved the dynamo, but he also presented Edison with some innovative ideas based on the new ac technology. Needless to say, Edison was less than enthusiastic about ac and Tesla; he was too vested in dc technology.
The wizards parted company less than amicably and became pitted against each other for dominance. Once Tesla was on his own, he designed a complete ac system. He was awarded seven U.S. patents for polyphase ac motors and power transmission equipment. It was around the same time that George Westinghouse entered the battle. He believed in the new ac technology and struck a deal with Tesla to purchase his patents. The “War of the Currents,” as historians call it, was under way.
It is Economics
Calling it a “war” may be a little fanciful but representative of the idea. A great deal of turmoil existed until the Westinghouse-Tesla ac system was selected to illuminate the 1893 Chicago World's Fair. Tesla's polyphase system of ac power generation and transmission system was about half the price of the dc system and required far less infrastructure.
From that point forward, more than 80% of the electrical devices ordered in the United States were for ac voltages. While ac technology became the preferred method of power transmission throughout the world, dc technology was never entirely defeated in this battle of currents. From the start, engineers recognized ac and dc were complementary rather than competing technologies, but a great deal of work was needed.
Research and Development
Before dc could be a truly complementary technology, it had to be compatible with ac. The advocates of dc technology knew the best way to make dc successful was to make it an integral part of the ac grid. A great deal of research took place, and probably the biggest breakthrough came when Dr. Uno Lamm and a group of ASEA (now ABB) engineers developed the low-pressure mercury arc valve (valve referring to early-day vacuum tubes).
The mercury arc valve brought about high-voltage direct-current (HVDC) applications. It was used for the efficient conversion of ac to dc and then back to ac again. Power could be transmitted at high dc voltages and then converted to ac voltages, allowing utilities to meet customers' lower-voltage needs.
ASEA combined the mercury arc valve and submarine cables to connect the Swedish mainland with Gotland Island, making it the first comprehensive application of this technology. The distance was about 60 miles (97 km), which was too far for an ac cable transmission but perfect for HVDC. The interconnection had an initial capacity of 20 MW. From that point, the mercury arc valve became the workhorse of HVDC transmission (from the mid-1950s until the late 1970s).
Utilities saw the advantages this new type of transmission offered the industry, but they wanted a competitive marketplace. ASEA saw the wisdom of competition and developed licensing agreements with other manufacturers of dc equipment. From this start, a new technology became a business.
In the mid-1950s, German scientists at Siemens developed the monocrystalline silicon semiconductor. A short time later, a sample of this material made its way across the Atlantic to GE's research laboratories where engineers began tinkering with it. The result of that tinkering was the thyristor, a bipolar semiconductor switching device that only conducts current from anode to cathode. In effect, a thyristor valve is a controllable unidirectional (diode-like) switch. This made it possible to build the silicon-controlled rectifier, and thus, solid-state power electronics was born.
Just as the semiconductor impacted electronics, the thyristor did the same for HVDC. Mercury arc valves were finicky and massively complicated pieces of machinery. They required a great deal of maintenance to keep them operational. With the advent of the thyristor valve, things became simpler and HVDC schemes increased significantly throughout the world. But, once again, utilities wanted competition, so GE licensed the thyristor technology to others.
The first application of thyristor valves took place at Gotland with ASEA adding a 10-MW thyristor-based converter group to the Gotland link. This was followed by GE's 320-MW Eel River Converter Station, which was the first all solid-state converter system in operation.
Before going any further, it is important to review HVDC technology. Simply put, ac power is fed into what is referred to as a converter. It is a rectifier that changes the ac to dc, hence the name converter.
The dc power is transmitted through a conductor, cable or busbar to a second converter. This converter is operating as an inverter with an output of ac power. The ac power matches the frequency and phasing of the receiving system.
Without going into a great deal of detail, there are three basic configurations, or schemes, for HVDC converters:
- Back to back.
The monopolar HVDC system generally consists of one or more three-phase, full-wave bridges, known as six-pulse or Graetz bridges, at each end. Power is transferred by one conductor and returns through the earth.
The bipolar configuration is a combination of two monopolar systems. The poles are made up of one or more 12-pulse bridges in series or parallel. This scheme can be found with a ground return, a dedicated metallic return or without a dedicated return path used for monopolar operation.
The back-to-back converter is a special adaptation of monopolar interconnections without the dc transmission. Both the rectifier and the inverter are located in the same station and are connected with busbar. These schemes are mainly used for power transmission between two asynchronous ac grids.
Advantages vs. Disadvantages
Because of the availability of the ac power transformer, ac quickly became the preferred method of power transmission. The transformer permitted power to be generated at low voltages, stepped up to higher voltages for transmission and stepped down to lower voltages again for the customer's use, but that dominance carried a price.
Unfortunately, ac has some inherent challenges that are substantial issues for today's grid. The reactive elements (inductance and capacitance) found in overhead lines and cables place limitations on transmission capacity and distance. They also may require additional equipment on the line for compensation such as series capacitors or shunt reactors.
All too often, connecting several ac systems can be problematic. There can be frequency differences between ac systems along with phasing concerns. Even linking two ac systems with the same frequency can bring about problems due to system instability, increasing short-circuit levels and undesirable power-flow scenarios.
HVDC addresses these issues and offers several additional advantages. Power transfers can be controlled and measured precisely. Being able to control the power flow is a huge advantage. Transmission congestion can be ignored by being able to direct exactly where the power is injected into the ac grid.
Another consideration is the effect of the HVDC link on power quality. Since HVDC links have the ability to control the ac output voltage and frequency, they can improve the power quality of the ac grid to which they are connected. This also reduces the phenomena known as flicker, which can impact lighting systems and cause thermal losses in electronic and electrical equipment.
Another evolving HVDC application is the multiterminal system. They are more complex than the point-to-point system, but they are gaining a lot of interest as dc schemes become more numerous. The world's first multiterminal system was built between 1987 and 1992 with line-commutated converter (LCC) technology for Hydro-Québec and National Grid USA (formerly New England Electric Systems). It is a 2,000-MW transmission system that originally had five terminals connecting load centers in Canada and the United States (New England).
Environmentally, HVDC offers a much smaller footprint for transmission rights-of-way, which means less land is required, less disturbance and less right-of-way maintenance. Of course, the converters require land, but an HVDC transmission line carries a great deal more power than an ac line.
A typical 6,000-MW transmission link would need seven power lines to carry the power if the ac voltage level was 500-kV. That same amount of power would require two 600-kV dc transmission lines or one 800-kV dc transmission line, which is a substantial savings of right-of-way. In addition, magnetic fields from HVDC transmission lines are negligible in comparison to corresponding magnetic fields for ac lines.
HVDC Station Designs
Several topologies are available for HVDC links today, but the two most prevalent are the LCC and the self-commutated voltage source converter (VSC). The LCC is considered the classic design, using thyristors for its valve design.
The valves are the 12-pulse design with voltage being controlled from maximum positive to maximum negative and unidirectional current (current flows in one direction no matter the direction of the power flow). Unfortunately, LCCs consume but cannot supply reactive power.
This design also produces harmonic distortions and requires harmonic filters. In addition, the LCC requires special converter transformers with more robust insulation to handle the dc currents found in the scheme. The LCC's major advantage is its ability to operate at extremely high power levels for efficient cross-country transmission.
On the other hand, VSCs use insulated-gate bipolar transistor technology for valve designs. These two technologies are very similar, but the VSC is considered to be more flexible than the LCC. One major advantage of the VSC technology is the fact that, with gate turn-off properties, the VSC schemes can supply reactive power and it is much simpler to change power flow direction. Since they do not require any driving system voltage, they have black-start capability.
ABB developed the VSC technology in the 1990s. The first VSC transmission system was commissioned in 1999 connecting a wind farm on the south end of the Gotland Island to the city of Visby. This was followed by the 36-MW Eagle Pass VSC back-to-back scheme connecting Mexico and the United States (Electric Reliability Council of Texas).
The Expanding Presence of HVDC
From its earliest days, HVDC technology has stirred debates as to its role in the grid. From the historical perspective, dc has had an interesting function. It went from dominance to dormancy to ascendancy.
HVDC may have been the first type of transmission system, but it was replaced by ac technology and interest in it waned. In recent years, however, there have been great advancements in the technology, sparking more interest and awareness. The technology is now at a point where it is capable of making significant contributions as understanding of its potential increases.
Moreover, several new market developments, like integration of renewables, interconnectors and long-distance transmission, lend themselves to HVDC technology.
ABB | www.abb.com
GE | www.ge.com
Hydro-Québec | www.hydroquebec.com
National Grid US | www.nationalgridus.com
Siemens | www.siemens.com