Aluminum replaced copper as the material of choice for the transmission line conductor at the turn of the 20th century. The transmission line's physical characteristics are continuing to evolve through more capacity, more strength and more intelligence. Growth and reliability will force the industry to add new transmission lines and to upgrade, refurbish or replace the existing transmission system. Some experts predict this is a crisis of huge proportions, but the intelligent technology available should mitigate many problems and break them down into manageable tasks.

This next generation of intelligent power equipment, which includes smart monitoring technology using sophisticated sensing and improved information technology with high-speed communications systems, currently has about US$15 billion in annual global sales, according to Climate Solutions' 2005 study titled “Powering Up the Smart Grid.”

THE CHALLENGE OF POWER TRANSFER

The transmission line designer's challenge has always been to achieve the transfer of power up to the thermal limitation of the transmission line's design. If that could be done, more power could reach the end user over the existing infrastructure. Only a relatively small percentage of transmission circuits are thermally limited under non-contingency conditions. These circuits limit the useful transfer capabilities of the network by 10% to 20%. In other words, we could increase the overall efficiency of the system by identifying and overcoming the thermal limitations of that portion of the total grid.

Transmission lines are located in interacting networks, which cause other problems. Stability, voltage limits and loop flows must be dealt with long before reaching the thermal limits of most transmission lines. Intelligent technology seeks to address these issues and will lead us on our way to a more reliable and secure grid.

KEEPING IT SIMPLE

Remember the old saying about plucking the low-hanging fruit first? Some utilities have taken that direction with fairly straightforward approaches. Transmission line conductor ratings are conservatively calculated using wind speeds of 2 ft/sec (0.609 m/sec) in the sun with a 25°C (77°F) ambient temperature rise and a conductor temperature of 75°C (167°F). Some utilities have investigated the interaction of wind and conductor temperature rise. Public Service Company of New Mexico (PNM; Albuquerque, New Mexico, U.S.) gathered weather data from the NOAA National Weather Service for the Albuquerque metropolitan area and determined the wind speeds to be higher than the 2-ft/sec general assumption.

PNM's engineers installed weather stations along strategic transmission corridors with research-grade sonic anemometers and gathered real-time data. This allowed them to increase the rating of critical existing lines. Increasing the wind speed for thermal rating to 4 ft/sec (1.219 m/sec) allowed the line rating to be increased by 18.2%, and 5 ft/sec (1.524 m/sec) increases the rating by 25.1%.

Blake Forbes, PNM's principal engineer of lines, says, “There is no free lunch in this scenario.” The utility has to do a lot of weather-related research first and know each line's ground clearance and mechanical condition before it can apply higher criteria and increase the line's rating. San Diego Gas & Electric, Long Island Power Authority and Tennessee Valley Authority are also investigating wind speeds and line ratings.

Another simple technological approach aimed at increasing transmission line capacities is PhaseRaiser structure lifting system from Laminated Wood Structures Inc. (LWS; Seward, Nebraska, U.S.). This system allows a transmission line's ground clearance to be increased while the line is energized. The pole is cut, raised hydraulically and stabilized by bolting steel members to the ends of the cut pole. Increasing the transmission line's ground-line clearance while keeping the line energized is a tremendous benefit to a utility. According to LWS, Xcel Energy, PacifiCorp, Central Maine Power, PNM, Omaha Public Power District and Minnesota Power have raised more than 2100 structures using the system.

INCREASED CAPACITY WITHOUT ADDITIONAL LINES

For many years, the transmission engineer could choose from a variety of conductor sizes but had a limited choice in the material or composition of the conductor. That has all changed with the introduction of new materials and alloys. These high-temperature, low-sag advanced-technology conductors are able to carry much higher currents continuously without exceeding sag clearances.

The U.S. Department of Energy (DOE) opened an accelerated testing facility at the Oak Ridge National Laboratory to demonstrate these advanced materials in a controlled environment. The Tennessee Valley Authority is also taking part in the testing program.

3M introduced the first metal matrix conductor in the early 2000s. Doug Johnson, product development specialist for 3M, describes it as a multi-stranded core of heat-resistant aluminum matrix composite wire. The conductor's core of aluminum oxide fibers mixed with ceramic can withstand high temperatures while resisting corrosion and sag. It also can carry up to three times the current of an ACSR conductor at the same diameter without reaching sag limits. Xcel Energy, the DOE, Western Area Power Authority (WAPA), Salt River Project (SRP) and Hawaiian Electric Co. (HECO) conducted early testing. Xcel Energy installed 477 kcmil at 115 kV in Minneapolis. DOE and WAPA installed 1 mile (1.6 km) of 795-kcmil aluminum conductor composite reinforced (ACCR) conductor on a 230-kV transmission line in Fargo, North Dakota. HECO installed 477 kcmil on a 46-kV line on the North Shore of Oahu and is subject to harsh coastal environmental conditions. SRP installed a test section of 795 kcmil of 69-kV line subject to extremely high temperatures of the desert in the Phoenix area. Recently, Arizona Public Service (APS) installed 6 miles (9.7 km) of the ACCR conductor into downtown Phoenix.

Composite Technology Corp. (CTC; Irvine, California, U.S.) introduced a composite core conductor using carbon fiber, glass and epoxy for the core instead of steel. Composite core conductor sags about 10% as much as the same size aluminum core steel reinforced conductor (ACSR). In fact, it gets stronger when heat is applied to it allowing for 28% more aluminum conductor to be wrapped around the core for greater current carrying capability. Tests are underway with 21 miles of this conductor being installed and tested in Kansas. Utah Power has reconductored and placed in service 6.7 miles (10.8 km) of transmission line in Salt Lake City, and CTC recently announced the installation of 37.3 miles (60-km) in Fujian Province, China.

Aluminum Conductor Steel Supported (ACSS) is a composite conductor that can operate at very high temperatures (over 200°C) without increasing sag. Arizona Public Service, Kansas City Power & Light, Pacific Gas and Electric, Reliant Energy, and Xcel are a few of the utilities using ACSS. Reliant Energy reconductored 7 miles (11.3 km) of ACSR transmission line with Southwire's ACSS/TW (trapezoidal configuration) conductor. Reliant Energy estimates cost savings of more than $5 million. Reliant went on to double the capacity of two 345-kV transmission lines by using ACSS/TW instead of ACSR, and they estimate a cost savings of $25 million on the project. The newer HS-285 high-strength-core version of the ACSS conductor is also seeing use in utilities seeking to take further advantage of this technology.

INTELLIGENT SENSORS

Imagine being able to increase a transmission line's power transfer rating by 18% or more just by being able to monitor the sag of the line in real time. Manufacturers have developed sensors and computer programs to provide intelligence to the transmission lines. EPRI Solutions Inc.'s Engineering & Test Center (Haslet, Texas, U.S.) and EDM International Inc. (Fort Collins, Colorado, U.S.) have created the Sagometer, a smart camera with a target, data logger and communications system. It calculates the ground clearance in all light conditions and sends this information to the system operator. It also gives the actual condition of the line as more power is transferred across it.

The Valley Group Inc. (Ridgefield, Connecticut, U.S.) has developed the CAT-1 transmission line monitoring system. Its load cells, radiation sensors, anemometers and communications capability to provide real time dynamic thermal line ratings. The Valley Group estimates the CAT-1 can increase the transfer capacity by more than 30% for 95% of the time. Shaw Energy Power Technologies Inc. (Charlotte, North Carolina, U.S.) developed its ThermalRate Monitoring system, which models the transmission lines from monitors placed along the transmission line corridor. USi's Power Donut2 installs directly on the transmission conductor and sends transmission line data back to the utility. These technologies are installed on existing transmission lines around the world.

THERE IS NO SILVER BULLET

To meet the industry's demands and challenges, the grid must be improved through the upgrade of old facilities and the addition of new facilities. Using the transmission system to its most efficient levels will go a long way solving the capacity problems that plague today's utility grids. The intelligent transmission system's new advanced conductors, monitoring systems and data-gathering software are a step in that direction.

Comparison of Available Conductors

Homogeneous conductor	Non-homogeneous conductor	Advanced materials conductor	Advanced alloys
Copper	ACSR (aluminum conductor steel reinforced)	ACCR (aluminum conductor composite reinforced)	ACFR (aluminum conductor fiber reinforced)
AAC (all-aluminum conductor)	AACSR (aluminum alloy conductor steel reinforced)	ACCR/TW (aluminum conductor composite reinforced trapezoidal wire)	TACFR (thermal-resistant aluminum alloy conductor fiber reinforced)
AAAC (all-aluminum alloy conductor)	ACSS (aluminum conductor steel supported)	ACCC (aluminum conductor composite core)	ZTACIR (Zirconium high-temperature aluminum alloy conductor Invar steel Reinforced)
	ACAR (aluminum conductor alloy reinforced)	ACCC/TW (aluminum conductor composite core trapezoidal wire)	GZTACSR (gap-type ultra-thermal-resistant aluminum alloy steel reinforced)

SAGGING LINE MITIGATOR

PECO's outage management process begins with either a customer call, a last-gasp from a meter or a message from the Interactive Voice Response system. An Outage record is created and is sent to the OMS system. SCADA Events are sent directly to the OMS system. A dispatcher reviews the outage record and assigns it to an appropriate crew. When power is restored, the event is closed and validated with the power-up message from the meter.

Line-to-ground clearances dictate transmission line loading levels in many key situations. Excessive sag from high-temperature conductor operation impacts the ability to transmit more power and can pose safety, reliability and liability issues.

The Sagging Line Mitigator (SLiM) is a new class of line hardware that reduces the effective length of the conductor during high-temperature conductor conditions. This then eliminates the excess sag in the transmission line, allowing higher line transfer capability just when it is needed most. The passive design of the SLiM device and its ruggedness allow utilities to treat SLiM like typical transmission line hardware, such as insulator strings, and treat the mitigation as a permanent solution.

The functionality and reliability of SLiM has been studied, tested and demonstrated at Pacific Gas and Electric (PG&E), Hydro-Quebéc Research Institute, the Kinectrics Labs, as well as at the manufacturer's testing facilities. SLiM's functionality and reliability were demonstrated on an operating transmission line in the San Diego Gas & Electric (SDG&E) system under an EPRI tailored-collaboration research project, sponsored by SDG&E, Public Service of New Mexico, Consolidated Edison, British Columbia Hydro, National Grid Transco, Northeast Utilities, PG&E, Southern California Edison and the California Energy Commission.

The demonstration results indicated that the device performs exactly as designed and intended, and reduced line sag during high-temperature conductor conditions.

The SLiM device has been purchased by utilities in South Africa and China for additional evaluation and commercial operation.