Power Flow Electronics Help Solve Transmission Line Load Problems
By Teresa Hansen, Managing Editor
Much of North America`s current interconnected power grid was designed and built in the 1950s and 1960s. While these transmission systems were more than adequate at that time, today`s demands and requirements are much larger and more complicated. As a result, the nation`s power grid is now expected to perform functions that were not anticipated when it was designed.
In order to accommodate these changes, advanced power flow technologies are being implemented. Power electronic devices are being developed that can provide control over electricity flow in transmission systems, while simultaneously improving system stability and helping prevent cascading outages. Many believe this power electronic technology holds the key to providing customers with reliable, quality electricity over the nation`s existing power grid, at an affordable cost.
AEP`s Investment in Power Electronics
One utility that has invested heavily in power electronics is Ohio-based American Electric Power (AEP). AEP is one of the United States` largest investor-owned utilities, providing energy to 2.9 million customers in Ohio, Virginia, West Virginia, Indiana, Michigan, Tennessee and Kentucky. Due to recent growth in Kentucky, some of the utility`s long 138 kV transmission lines were becoming fully loaded and the likelihood for service problems to some key industrial areas was apparent. Bruce Renz, AEP`s energy delivery support vice president spoke about the project at Electric Power Research Institute`s (EPRI`s) Transmission and Distribution Conference recently held in New Orleans. According to Renz, these lines had a high concentration of switched shunt capacitors and were cause for significant power concerns in the region. They were operating with high losses, both real and reactive, he said. In addition, the voltage was low and there was poor voltage control. In order to avoid service problems, AEP had to reinforce its transmission system serving this area--the Inez area. "The Inez area is a large, important area on our power system, representing over 2,000 MW of load," said Renz. "It is a major coal mining area, and electricity sales have steadily increased over the years."
The utility decided to build an additional high capacity 138 kV transmission line, which would serve the Inez Station. However, analysis showed that the new line would not naturally carry its share of regional loading because of its relatively high impedance--power would seek out and overload other transmission lines.
AEP decided to use flexible AC transmission system (FACTS) technology-based unified power flow controllers (UPFCs) to remedy this problem. "The UPFC will be a great benefit as it will help us maximize utilization of our lines. It will enable control of power flow into and out of the area, effectively maintain voltage throughout the area and improve stability," said Renz.
Without the UPFC control, the new 138 kV transmission line would have been able to carry about 670 MW during peak load into the Inez Station at a voltage close to acceptable levels. With the UPFC control, however, the line will be capable of transferring about 770 MW with well-controlled voltage, solving a significant transmission grid problem for AEP.
The project was implemented in two phases. Installation of the shunt inverter was completed during Phase I. Connection of the series inverter to the new high capacity 138 kV transmission line was completed in Phase II. The new system was dedicated on June 26, 1998.
Increasing Demands and Limited Capacity
The successful implementation of UPFC at AEP not only solves a problem for the utility, but it also represents a major milestone in the transmission industry. It brings the power industry into a new era of electronic control, and is expected to play a major role in reducing capacity problems in much of the nation`s transmission lines.
Limited construction of new transmission lines in North America has resulted in transmission systems that are quickly reaching their capacity. According to EPRI, over the past decade, electrical loads have grown at an average annual rate of 2 percent. However, in this same period, little new transmission capacity has been installed, mainly because of the high cost of such lines (EPRI estimates the cost to be about $1 million per mile for a 500 kV line).
Besides the steady load growth and increased power transfers that have occurred in the last 30 years due to increases in electricity use by utility customers, competition in the utility industry is also straining the nation`s power grid. The volume of bulk power transactions carried by U.S. transmission systems is expected to grow substantially over the next several years, as competitive electricity buying and selling becomes a common occurrence.
Not only are the demands on the power grid increasing due to increases in loads, but also the loads themselves are changing. Microprocessor-based technologies are now common in manufacturing, agriculture, entertainment, home automation and many other areas, making reliable, quality power even more important. Making the transmission system reliable enough to meet not only today`s demands, but also future demands, is a big challenge. This is especially true in today`s environment where utilities have little incentive to invest their money in transmission system upgrades since they may be required to turnover transmission system operation to an independent organization in the near future.
And, even when utilities do want to build new transmission lines, many are running into public opposition. For example, AEP has been unable to get permission from the Virginia Corporation Commission to build a 132-mile, 765,000 V transmission system. The new transmission project was first proposed in 1990 to be completed this year. Now, however, the earliest it can be completed is 2002.
Utility officials say the new system is needed to carry increasing electricity demand. However, convincing the regulators of this need has not been easy. The utility is facing strong opposition from an organization called Friends of Regional Culture & Environment and other critics, accusing AEP of wanting the new lines mainly to sell excess power to other areas. Unfortunately, AEP`s situation is not unusual. Many utilities are faced with similar challenges. Obtaining new rights-of-way is becoming quite difficult, making it even more important for utilities to find ways to make the most of their existing transmission systems.
FACTS--The Technology Solution
The current business and political environment facing utilities makes it easy to see why they are looking for technologies that will allow them to continue to provide reliable power and stable transmission using existing lines.
FACTS technology, which EPRI has championed for years, is now available to help maximize the capacity of existing transmission systems. Basically, FACTS technology creates utility networks that operate like giant, high-tech integrated circuits. When combined with conventional equipment, the technology can substantially increase transmission capacity.
According to EPRI, controlling power flow along specific "contract paths" is one of the fundamental problems of AC transmission system control. The lines` electrical characteristics determine how electricity will flow along each network path. Each transmission line`s characteristics and electricity flow are determined by three key parameters: terminal bus voltages, line impedance and the relative phase angle between the sending and receiving end.
Traditionally, transmission controls, mostly electromechanical devices, have been used to change these parameters. However, these devices are typically not able to respond fast enough to changing conditions to provide real-time flow control. FACTS devices, also called controllers, use a thyristor--a silicon "megachip" that is the high voltage equivalent of a transistor. By combining this technology with powerful computers, communications technology and power system analysis software, utilities can increase individual line capacity by up to 50 percent.
The first generation FACTS devices have actually been around for several years. Although not called a FACTS device, the static var compensator, introduced about 20 years ago, helped provide voltage support. It consisted of a fast thyristor switch controlling a shunt capacitor bank and/or reactor. Later, another first generation device--the thyristor-controlled series capacitor (TCSC)--was introduced. This device uses silicon-controlled rectifiers to control a capacitor bank connected in series with a line, allowing increased power transfer on particular lines over longer distances by controlling impedance.
The second generation of FACTS controllers was recently demonstrated at Tennessee Valley Authority`s Sullivan substation. This controller can perform the voltage support and power control functions of the first generation controllers, but does not require large external circuit elements, such as capacitor banks, shunt reactors or phase-shifting transformers. Instead, an advanced configuration of gate turn-off thyristors is used. These devices can mimic reactors and capacitors electronically, thus potentially reducing the applications` cost while improving grid performance.
In November 1995, the first static synchronous compensator (STATCOM) began operation at the Sullivan substation. This controller provides voltage support to the transmission line by generating or absorbing reactive power through an all-electronic shunt connection. A complementary second-generation FACTS controller, the static synchronous series compensator (SSSC), is currently being designed. It is expected to perform the functions of the TCSC. According to EPRI, the SSSC is expected to be used in new installations.
The UPFC installed at AEP is the newest and most advanced FACTS device. This third generation FACTS device combines the STATCOM and SSSC into a single device with a common control system (see figure). By combining these functions, the unique capability to control simultaneously both real and reactive power flows on a transmission corridor is offered. In addition, according to EPRI, the design permits a much smaller installation "footprint." For example, the Inez UPFC occupies a standard substation building approximately 100 feet by 200 feet, while a typical TCSC requires an area the size of a football field. The UPFC was developed through a joint effort between EPRI, Westinghouse and AEP.
EPRI estimates that new transmission lines in North America cost $1 million per mile for a 500-kV line, thus, utilities have little incentive to build new transmission capacity. Photo courtesy of ABB Power Lines.
UPFCs, like those recently implemented at AEP, will give utilities full control of power flows on transmission lines. Photo courtesy of EPRI.