As a technologist, it is hard to watch as underinvestment in America's integrated transmission grid leaves the nation unable to provide secure, reliable power. The move to deregulate generation has had unintended consequences. Today, most independent generators are rewarded only for selling watts. To support the voltage for the transmission system, we still need access to reactive power (VARs). The lack of fast voltage support has placed the U.S. grid at risk. The scarcity of VARs, especially dynamic VARs, was a contributor to the spread and cascade of the North American blackout of Aug. 14, 2003.

Paying the Price

In the last decade, voltage collapse of the transmission system has caused more than 90% of the blackouts. The fast, dynamic nature of reactive loads is further aggravated by nonlinear loads of the growing digital economy. These include dc power supplies of millions of computers and power-electronic-based adjustable speed drive motors. This potentially can cause cascading voltage collapse or frequency instability. EPRI (Palo Alto, California, U.S.) estimates the cost to the U.S. economy for the loss of power (primarily due to insufficient supplies of fast, dynamic sources of reactive power) to be in the range of US$100 to $188 billion per year. This is almost equal to the total revenue received by the electric utility industry.

VAR Compensation is Critical

The shortage of VARs is greatest during the summer, when the demand for local sources of reactive compensation is heightened by increased use of large air-conditioning units, irrigation motor loads and industrial motor loads. When local sources do not meet reactive power demands, the voltage on the transmission line drops or sags. A decrease in voltage can lead to a blackout unless reserve capacitors are quickly added or generators are able to quickly increase their supply of reactive power. Of course, for-profit generators are less than enthusiastic about overexciting generators to produce VARs, because it results in the reduction of real power output and a lost opportunity to sell incremental power at high prices.

Since the mid 1970s, the trend in the transmission business has been to maximize the use of existing transmission assets. Unfortunately, increases in line current increases line sag, which can result in lines tripping out when lines contact under-builds or trees. This can trigger a dynamic cascade reaction with the resultant voltage and power swings, causing sequential tripping of adjacent lines and generators. Also, as thermal limits are reached, transmission lines and interfaces become congested with costs exceeding billions of dollars annually. Because of the rising demand for electricity, loading on existing lines is expected to increase at 2% to 3% per year.

As lines approach thermal limits, several options can be considered. Lines can be re-tensioned to operate at higher temperatures. Existing conductors can be replaced with high-temperature, low-sag conductors — especially in areas of low clearance. Replacement options include Aluminum Conductor Steel Supported Trapezoidal wire or more exotic conductors, including Aluminum Conductor Composite Core (ACCC), Aluminum Conductor Composite Reinforced (ACCR) GAP conductors or InVAR conductors.

These advanced conductors allow a 60% to 100% increase in the current-carrying capability of the transmission line in the existing right-of-way, although cost premiums over standard conductors can be 50% to 500% or more. However, the higher currents available in these exotic conductors greatly increase the need for reactive power compensation.

VAR Control

The industry needs fast-acting dynamic sources of reactive power to counter fast voltage-collapse events. On the generation side, fossil, nuclear or combustion turbine generators typically provide dynamic VARs. System operators have learned that the actual MVAR ratings for generators are often 60% to 70% of their design ratings because of generator hydrogen cooling leaks, increased use of auxiliary power and aging. Nuclear units, in particular, can increase their real power output by more than 10% by operating at 0.95 power factor versus 0.9 power factor; but in exchange, the reactive power output from the units drops by 50% or more.

The same is true of combustion turbines, combined cycle units, and any fossil or thermal unit that has the capability to operate at 5% over pressure for emergencies or can provide additional steam to upgraded turbines. Calculations show that lost opportunity costs can range from $500/kW to $1000/kW, depending on the region of the country, the availability of capacity and market prices. Note that MVARs do not travel long distances. For example, studies show that only 10% of the reactive compensation from condensing hydroelectric units (operating as a synchronous condenser) can be delivered just 10 miles (16 km) on a 161-kV line to a T&D interface. As a rule of thumb, a MVAR at a T&D interface close to a customer is worth two to three times an MVAR produced on the transmission system a distance from a T&D interface or substation.

Available Solutions

Transmission companies can use slightly higher-cost series capacitors, whose capacitance increases as the current squared. However, this raises concerns of sub-synchronous resonance. Other options are to use low-cost shunt capacitors or moderately priced Static VAR Compensators (SVCs). But the capacitance of SVCs decreases as the voltage squared, and SVCs are dependent on off-site power for auxiliary power and thus sensitive to close-in momentary faults. Another option is to use power electronic devices, including current-generation Gate Turn Off (GTO) or Gate Commutated Thyristor (GCT)-based Static Shunt Compensators (STATCOMs), which are relatively expensive, costing more than $100/kVAR.

Vendors and universities have begun to explore advanced power electronic silicon-based switches to operate at higher voltages (reduced cost for transformers), higher current (reduced number of switches) and higher switching frequency (lower cost for snubbers to form square pulses). These switches are less complex and have improved power quality. Advanced power electronic switches — such as the Integrated Gate Bipolar Transistor (IGBT), Integrated Gate Commutated Thyristors (IGCT), and Super Gate Turn-Off Thyristor (SGTO) — are being developed to reduce the cost, and improve the reliability and robustness of future power electronic devices.

In the past, a third-generation Emitter-Turn-Off-Thyristor (ETO) had been developed as a relatively low cost, easy-to-manufacture device assembled on a printed circuit board. Virginia Tech and EPRI Power Electronics Applications Center (PEAC) are testing a three-phase, third-generation ETO-based STATCOM at 2 kV. The STATCOM will be integrated with ultracapacitors (hybrid battery capacitor devices that deliver high levels of energy in under a second), which can be discharged into a relatively low-cost four-quadrant controlled Flexible AC Transmission System (FACTS) device.

Today, TVA, with Virginia Tech and the Department of Energy (DOE), is investigating FACTS devices using an evolutionary, advanced fourth-generation power electronics switch, the ETO thyristor. The fourth-generation ETO has the potential to produce new FACTS devices that could cost the same or less than SVCs, be mobile and relocatable, and have internal fault-detecting capabilities, making them more robust and reliable. The fourth-generation ETO, which should be available this year, eventually could result in a STATCOM device that will fit in a mobile trailer, with output voltages of 13.8 kV (adequate for connection at a typical T&D interface without a transformer) with total harmonic distortion less than 2%. The potential cost reduction could be 30% to 50% of current generation FACTS devices.

Future Solutions

TVA is working with EPRI, other collaborating utilities and the DOE to integrate the ultracapacitors with third-generation ETOs to develop and test a FACTS device (the Transmission-level Ultra CAPacitor or TUCAP) that can operate at 2 kV, provide 10 MVARs of reactive compensation plus 3 MW of real power for one second, provide four-quadrant control of real and reactive power, and act like a unified power flow control device.

TVA also is working with Vanderbilt University to revolutionize the power electronics business in the next five years by “going back to the future” using vacuum tubes made out of chemical vapor-deposited diamond tips that will operate like today's diodes, triodes, thyristor fast electronic switches.

Advances in power electronics will be able to provide value to transmission companies as they look to provide low-cost reliable power to end-use customers. Initially, the improved reliability and reduced cost will be from evolutionary changes such as the ETO. In the long term, there will be a revolution in improvement from diamond-based devices yielding much lower cost FACTS devices, HVDC terminals, electronic circuit breakers and transfer switches.

Acknowledgments

The author gratefully acknowledges the efforts of Mike Ingram, Terry Boston, Terry Johnson, Aty Edris, Rich Lordan, Arshad Mansoor, Tom Geist, Dr. Alex Huang, Haresh Kamath, Dr. Imre Gyuk and Dr. Jim Davidson for their contribution to the original version of this document.

Dale T. Bradshaw is a senior manager at the Tennessee Valley Authority's (TVA) Energy Research & Technology Applications organization. He manages power-delivery technologies specializing in research, development, demonstration and deployment (RDD&D) of new or first-of-a-kind technologies to increase revenues, reduce operating and capital costs, improve reliability and increase the system security for TVA's transmission system.

Prior to TVA, Bradshaw spent two years at Public Service of New Mexico. Bradshaw is a member of the IEEE PES and holds a BS degree in engineering physics from the University of Oklahoma, an MS degree in mechanical engineering from the University of Oklahoma, an ABD in nuclear engineering from the University of New Mexico and an MBA in finance from the University of Tennessee at Chattanooga, Tennessee.