Around the World, the Demand for Electricity is Steadily Increasing, and the delivery system is being severely challenged. In industrialized countries, the bulk-power transmission infrastructure has not kept up with growth, and it has aged beyond all normal expectations. In developing and emerging countries, such infrastructure is nonexistent or extremely limited. The answer for both problems is adequate bulk power transmission interconnections implemented by advanced technologies.

Interconnections or grids make it possible for areas with excess-generation resources to serve load centers in areas without generation resources. A grid is composed of several transmission lines in proximity to each other, commonly referred to as paths or corridors. Grids are a blessing and a curse, because these paths can be parallel to each other. Multiple paths do not share well. These lines have a resistance to the flow of electricity, which is called impedance. In a grid configuration, transmission lines with lower resistance will carry more electricity than those with higher resistance, which is called interaction.

GRID ISSUES

Interaction between transmission lines can be significant enough to require the limiting of the path's rating. The limiting factors are thermal overload, instability and poor voltage performance. To make matters more complex, the multiple paths the grid provides can cause bottlenecks, or congestion, and loop flows.

A loop flow is a phenomenon also called unscheduled flow. Power flows through all the parallel paths of the grid but not equally. A power transaction may call for power to go directly from Arizona for use in Utah, but because of loop flow, it goes to California, Oregon and Idaho before reaching Utah. This is unscheduled flow, and it can cause utilities to curtail power transactions to avoid overloading the system; thus, path rating gets to be a sticky issue.

This rating is determined by a regional transmission organization, which is made up of the owners of the interconnected systems and who reach an agreement for each path by negotiating. Sometimes the operating conditions keep the path's actual power transfer below the path rating, which is based on the maximum power transfer that can be reliably managed under all conditions.

TURNING THE CORNER

Without additional transmission system investment, grid congestion will increase, making it more difficult for available supply to meet demands and facilitate full use of capacity/demand diversity. In some situations, this can lead to supply shortages and involuntary customer interruptions. But, building lines takes time. Mark Lauby, manager of reliability assessments for North American Electric Reliability Corp. (NERC), points out that it takes seven years or longer for engineering design, licensing and construction of the typical transmission line. Lauby says the real challenge is the ability to site new generation and lines where they are needed, not where they can be permitted. The hurdles to greenfield transmission are too onerous; the fallback is to use advanced technologies to improve the existing facilities. NERC is technology neutral, as long as the steps taken improve reliability, says Lauby.

Edison Electric Institute (EEI) reports in its 2007 publication Transmission Projects at a Glance that our industry appears to have turned a corner. The lethargic investment in the transmission system so prevalent from the early 1970s through 1999 has ended. EEI-member companies report spending more than US$28 billion from 2000 to 2005 on transmission projects and plan to increase spending to approximately US$31.5 billion for the period from 2006 to 2009. EEI members understand that, with a transmission system made up of 20^th century technology, it is almost impossible to direct electricity along a specific path. That is why we see several nontransmission-line projects listed in the report that do not add to the overall line mileage, but enable us to improve and effectively use existing facilities. Advanced technologies of the intelligent grid provide tangible benefits to the transmission system including increased transfer capability, increased controllability and strengthening of the overall transmission system.

SIMPLIFYING THE COMPLEX

Southern California Edison (SCE; Rosemead, California, U.S.) has more than 6600 miles (10,622 km) of transmission and 17 local remedial action schemes (RAS) placed on its transmission corridors. RAS is an automated process of detecting a system problem and automatically executing steps to mitigate any adverse effects on the transmission system. It is faster at detection and execution than human intervention would be. It also is able to do operations that would not be permitted without a lot of time spent getting approvals.

SCE proposes taking this to the next logical step and replacing the many local RAS with a centralized system, or C-RAS. SCE will use off-the-shelf advanced technology to simplify the entire process. The system will use phasor monitoring units, coupled with IEC 61850-compatible protection and control equipment, and high-speed generic object-oriented system event Ethernet communications (see “The Digital Utility” supplement, Transmission & Distribution World December 2006). SCE expects to increase transmission capacity ratings, make additions to the system more easily and reduce system-wide stability problems without adding more transmission.

JUST THE FACTS

Real power is measured in watts and does the work of turning motors, lighting and heating/cooling. Reactive power is measured in volt-amperes reactive (VARs), and supports the magnetic and electric fields required for ac to function. In its most basic mode, flexible ac transmission systems (FACTS) devices, also referred to as controllers, are power electronics-based systems that control VARs. In a more complex configuration, FACTS provides control for one or more ac transmission system parameters, which include the dynamic control of voltage, current, impedance and phase angles.

Dr. Narain Hingorani, considered by many to be the father of FACTS, says FACTS is an enabling technology. It allows a utility to overcome any constraints or limitations on its system brought about by poor voltage performance, transients or stability with continuous control of active and reactive power flows. Hingorani believes that applications of FACTS technology have not taken place as quickly as expected due to market uncertainties created by the regulatory environment.

Mechanically switched shunt capacitors can be used to provide VARs, and mechanically switched shunt reactors can be used to consume VARs, but they can't handle dynamic switching. This is where FACTS devices play a strong role. Combining the two shunt devices and switching them with power electronics results in a FACTS device known as a static VAR compensator (SVC), which provides a variable source of VARs.

Siemens Power Transmission & Distribution (Erlangen, Germany) recently commissioned the largest FACTS installation in North America, which is used to strengthen the transmission path from Arizona to California. It is a 525-kV -110/+440-MVAR SVC located in SCE's Devers substation.

HVDC TO THE RESCUE

High-voltage direct current (HVDC) also can be used to combat loop flows by actually directing the flow of power exactly where the marketer says the power transaction is going. Siemens' Neptune Interconnection in the United States is an example of this concept. The interconnection is a 65-mile (105-km) undersea and underground HVDC system that was commissioned at the end of June 2007.

Neptune provides a directional transmission link from Sayreville, New Jersey, into Long Island, New York. Siemens reports that the HVDC system is rated for 500 kV and carries 660 MW. It is owned and operated by the Neptune Regional Transmission System, LLC (Fairfield, Connecticut, U.S.). The Long Island Power Authority (LIPA) will receive power from the PJM Interconnection, a regional transmission owner.

The Neptune Interconnection was built by a consortium led by Siemens, who managed the project and supplied all the HVDC equipment. The submarine and subterranean cable was supplied and installed by Prysmian (Milan, Italy). Siemens states that the project enabled the utility to meet load growth without the need for another power plant located on Long Island. Siemens also says the Neptune Interconnection provides power without increasing the short-circuit current in the network, which means counter measures are not needed, saving time and money. HVDC offers another benefit in that the control functions are extremely fast acting, which helps to stabilize the connected networks.

HVDC technology has taken giant steps forward. We've seen increases in the voltage rating from 600 kV, first introduced in the late 1970s, to 800 kV. Dr. Ram Adapa, technical executive of power delivery and markets for Electric Power Research Institute (EPRI; Palo Alto, California), is excited about recent developments for 800-kV ultra-HVDC (UHVDC) technology. He points out that it will be possible to transfer larger blocks of power greater distances and with reduced losses. It should be possible to increase power-transfer ratings from 4000 MW to approximately 6000 MW.

EPRI, ABB (Zurich, Switzerland), Siemens and AREVA (Paris, France) have been developing HVDC at this voltage level for several years. Adapa reports that the EPRI high-voltage laboratory in Lenox, Massachusetts, U.S., is testing a section of the UHVDC transmission line. ABB unveiled its UHVDC test facility in Ludvika, Sweden, last year. The company reports this is the first major increase in HVDC in more than 25 years. And Siemens recently announced it has won an order from China Southern Power Grid Co. (Guangzhou, China) to build the first 800-kV UHVDC system. It is scheduled to commence commercial service by mid-2010. This is but another example that indicates advanced technologies of the intelligent grid are developing exponentially.

THE ULTIMATE UHVDC

The Chinese UHVDC transmission system is part of a massive transmission infrastructure-building project. China has three power grids taking power from the energy-rich western regions to the load centers on the east coast. The southern corridor brings hydroelectric and thermal power from southwest Yunnan, Guizhou and Guangxi to southeast Guangdong. The central corridor transports hydroelectric power from the Yangtze River (the Three Gorges Project being the most famous) to eastern China. The northern corridor connects the hydroelectric plants on the Yellow River and the thermal plants in the northern area to the Beijing-Tianjin-Tanshan power grid.

The UHVDC system will be located in the southern corridor and connect the provinces of Yunnan and Guangdong. This will be the world's first 800-kV dc transmission system. It will be able to transfer 5000 MW of power to load centers on the southeast coast.

To put that in perspective, transferring the same amount of power using the 1960s 500-kV ac technology would require approximately four transmission lines. If the 1970s 500-kV HVDC transmission technology were used, it would require two HVDC systems. The new system will be a big step, but China has a lot of experience with HVDC. The 800-kV Siemens system will be the fifth HVDC system in China.

ABB commissioned the first interconnection, Gezhouba-Shanghai (1200 MW at 500 kV), in 1989. Siemens commissioned the second HVDC interconnection, Tianshengqiao-Guangzhou (1800 MW at ±500 kV), in 2001. The third interconnection, Guizhou-Guangdoung I (3000 MW at 500 kV), was commissioned by ABB in 2004. ABB is also doing the fourth interconnection, Guizhou-Guangdoung II (3000 MW at 500 kV), which is scheduled to be energized by the end of 2007. China has definitely become a leader in the installation of HVDC interconnections.

A GLOBAL INITIATIVE

UHVDC is generating a lot of interest not only in China, but also in other developing and emerging economies around the world. India and Africa are studying UHVDC bulk-power systems. Pat Naidoo, senior general manager of special projects for Eskom Holdings Ltd., leads an effort to electrify southern Africa. According to Naidoo, less than 20% of Africa's population has access to electricity. Eskom is taking part in a massive 800-kV UHVDC undertaking known as the Western Power Corridor (WESCOR). It will interconnect the countries of South Africa, the Democratic Republic of Congo (DRC), Botswana, Angola and Namibia.

The Congo River is a huge renewable resource for Africa. Naidoo describes the Inga hydroelectric facility operated by the Societe Nationale d' Electricite, the state-owned utility of DRC, as having two power plants — Inga 1 and 2 — with a combined output of 1770 MW. Inga 3 will be a 3500-MW expansion to this facility. The Grand Inga would follow this expansion. It is estimated that the potential output would be approximately 44 GW.

Naidoo acknowledged that the nearest major load centers are 1864 miles to 3728 miles (3000 km to 6000 km) from the hydroelectric facilities. The WESCOR project calls for innovative thinking, and that's what the UHVDC voltage level is. UHVDC is ideal for traversing the distances involved and the extremely large amounts of power needed to electrify southern Africa.

WE CAN REBUILD IT

Our bulk transmission systems are under attack. Our customers need electricity in amounts never imagined by past generations. This demand keeps growing faster than new facilities can be installed. There is also a push for renewable energy, which translates into more generation spread on the transmission system located at some of its weakest points. With proper use of conventional and advanced technologies, we are now moving to increase the robustness and the integrity of the bulk transmission grid.