THE TENNESSEE VALLEY AUTHORITY POWER SYSTEM OPERATIONS (PSO) ORGANIZATION is focused on ensuring safe, reliable and efficient power-system operation. To do this, TVA (Chattanooga, Tennessee, U.S.) needs to monitor and maintain its transmission system to avoid unnecessary outages. Along with other utilities, TVA has conducted collaborative research with the Electric Power Research Institute (EPRI; Palo Alto, California, U.S.) to provide a monitoring platform to accomplish this.

REQUIREMENTS

Power lines in the utility industry are critical components to the reliability of the electric system. The infrastructure of the power system is aging and being loaded closer to its limits now more than ever before. Working with EPRI's Inspection and Assessment Overhead Transmission Task Force, TVA identified a need for low-cost sensors. The project funders developed eight basic requirements:

1. Low cost
2. Remote inspection of live-lines
3. Low maintenance
4. Useful in inaccessible locations
5. Multiple applications
6. Feasible (small) dimensions
7. No electrical magnetic compatibility (EMC) issues
8. Secure.

When EPRI sent out a request for proposals, several technologies were proposed but only one seemed to fit the bill. Radio backscatter technology was chosen over more conventional wireless transceivers because the sensor electronics are inherently less complex. This most promising proposal came from Southwest Research Institute (SwRI; San Antonio, Texas, U.S.). Thus, the project funders, working with EPRI and SwRI, conducted a feasibility study that led to a series of development efforts for wireless/remote sensors using radio backscatter technology.

Backscatter technology has been around since World War II. SwRI has had experience with radio backscatter technology since the early 1980s. In recent years, it has really started to take off in items such as radio frequency identification (RFID) tags. Wal-Mart is a big player in pushing the development of RFID tags.

TAG WITH SENSOR

A basic RFID system consists of a tag, a reader and a processor. A tag contains data that a reader can download, and the processor turns tag data into useful information. There are many types of tags available now and in development, but the tags for this project have sensor technology.

The EPRI/SwRI team developed a flexible, low-cost electronic design for the core of backscatter sensor design. The tag sensor (package) has basically three parts: antenna, microcontroller/processor and sensor interface. The sensor can be part of the tag package or external to the package. In basic terms, the sensor information is processed to generate a frequency shift-keying (FSK) signal onto the antenna at sensor frequency F2. This FSK signal then gets amplitude modulated (AM) onto the illuminating frequency F1 as AM sidebands to form a backscatter return signal (F1 + F2) to the reader.

The EPRI sensor development was a planned incremental development: First develop the basic tag and reader, then develop the sensors for specific applications.

SPLICE SENSOR

The integrity of the conductor is critical to the reliability of the transmission system. Conductor splices are arguably the weak link in this integrity chain. Presently, there are few sensors to monitor the critical parameters of these components. The first sensor developed was the splice sensor. The approach of the splice sensor is twofold: Directly measure the temperature of the splice and measure the current flowing through the splice. Both pieces of data are important because the temperature rise depends on the amount of on-line current. But there can be high temperature without high current, if the splice is bad.

The sensor coil uses a high-permeability core to sense the magnetic-field strength for current measurement. It also can be used for power harvesting to supply power to the sensor/tag. When the line current is above 80 A (typical for an energized transmission line), the coil can provide enough energy to self-power the sensor, eliminating the need for a separate battery. This reduces ownership cost and improves sensor reliability.

This sensor has six values it outputs along with its identification number (ID): the present temperature, the present line current, the peak temperature seen, the line current at the time of peak temperature and the sensor-board temperature. Prototype backscatter splice sensors were successfully tested with high current, voltage and line temperature at EPRI's Lenox Center, a high-voltage lab in Lenox, Massachusetts, U.S. Other mechanical tests were performed at SwRi's laboratories. The device is also being adapted for conductor thermal rating use.

CONTAMINATION SENSOR

Another sensor EPRI developed was the insulator contamination sensor or leakage current sensor. The power line insulator's main function is to support and insulate the line from the tower. Over time, pollution buildup on the insulators' surface or material breakdown can cause the onset of leakage currents. Eventually, this can lead to insulator flashover and line outages. Ideally, utilities would like to wash or replace insulators before they fail, but this is expensive and difficult to schedule.

EPRI and other researchers have shown that insulator leakage currents correlate with the level of contamination, but it is affected by environmental factors such as condensation, wind and humidity. Typically, it takes expensive laboratory-grade instruments and large amounts of data to measure this leakage current.

The sensor uses low-power, low-cost circuitry and lithium batteries. It also uses a ferrite core to measure the leakage current on the ground side of the insulator string. While power harvesting cannot be used in this application, battery life should be 10 to 15 years. Solar will be investigated as a power source in the future.

The sensor has eight values that it outputs: its ID, present temperature, five count registers (or bins of the leakage current) and sensor-board temperature. The sensor will continuously measure leakage current and, by accumulating bin counts, it develops a histogram of leakage current recordings. Based on the trending of this histogram, action such as scheduling washing may be taken.

Typical values of leakage current of these bins are:

• Bin 1 > 10 mA
• Bin 2 10 mA > 50 mA
• Bin 3 50 mA > 100 mA
• Bin 4 100 mA > 200 mA
• Bin 5 200 mA > 500 mA.

TESTING AND FIELD TRIALS

Reader/processor is the data collector of the system. The reader's basic components are transmit-and-receive antennas, a RF head (transceiver), an analog-to-digital (A/D) card and a computer. The computer controls the transceiver; the A/D card converts the data; and the computer processes and stores the data.

The field trials and testing of these new sensors are taking place at TVA's Paradise Fossil Plant in Drakesboro, Kentucky, U.S. TVA and EPRI have set up a wireless sensor lab there in a mobile field trailer. In the environment of a substation, TVA, EPRI and SwRI have adapted the splice sensors to be reading the current and temperature of high-voltage disconnect switches. The interest here, as with splices, is temperature and current. Typically, a switch has low resistance to power flow, but occasionally this changes. Increased resistance will generate more heat with increasing power flow; typically, maintenance is required to avoid a sudden failure and possible unplanned outage.

TVA has installed five modified splice sensors on the top and bottom of each phase on a 69-kV disconnect switch. Two sensors are power harvesting and three are using batteries. This setup is operating as a backscatter sensor system demonstration. The reader and antennas are set up in the mobile sensor trailer adjacent to the substation. Every 15 minutes, the reader turns on and reads the sensors. The reader computer is connected to the TVA network, so EPRI, SwRI and TVA can collect the data and monitor the system remotely.

TVA has also installed six insulator contamination sensors on lightning arrester grounds. By measuring the leakage current through the arrester or insulator, TVA expects to judge the health or condition of this equipment. The same reader for disconnect switch sensors is also reading the insulator contamination sensors. An RF switch had to be installed on the reader hardware to share the reader with two sets of antennas. Two antennas were necessary due to the sensors being in opposite directions from the trailer.

FUTURE DIRECTION

The sensors have been operating for almost a year with mixed results. They are performing as expected, but power harvesting could not be demonstrated due to insufficient current (less than 80 A). The antennas proved to be more directional than expected and than previous tests showed. An omnidirectional antenna is being developed and tested. Vibration testing showed a need for different packaging (slip and vibration) of the sensor. Therefore, the original packaging has been redesigned before deployment of the splice sensor on the transmission line.

In November 2006, three newly designed splice sensors were installed on a transmission line near Nashville, Tennessee. Also during this installation, custom-designed hotstick tools were tested. The sensors were installed during an outage, but installation was performed with custom-designed hotstick tools as if the line were energized (hot and in-service). This exercise demonstrated the possibility of installing the sensors on a line without removing the line from service. A prototype portable reader was also successfully tested.

A handheld field reader is under development and a commercial manufacturer is being sought. This future reader will be the size of a laptop computer with external antennas. The reader signal processing is being improved to enable high-speed drive-by testing. The leakage current sensors are seeing further accelerated aging and environmental tests by the EPRI Lenox Center.

Wireless sensor technology promises a good fit for substation and transmission line monitoring. User applications are being developed. The need for data management and data format will be addressed. An out-of-the-box application needs to be developed to show the true potential of these low-cost sensors. The world is going wireless and the utility industry is, too.

ACKNOWLEDGEMENTS

The authors would like to acknowledge Dr. Andrew Phillips of EPRI, Mark Majors of SwRI and 11 project member utilities for their technical expertise, leadership, insight and support.

Ralph H. McKosky joined Tennessee Valley Authority after earning his BSEE degree from Mississippi State University in 1987. He has worked in the Nuclear Power department and is presently a project engineer in the Research and Technology Applications department, Transmission Technologies Group. As a project engineer, McKosky is responsible for research, development and demonstration of new technologies that improve TVA's transmission system. rhmckosky@tva.gov

Mark B. Goff is a staff system engineer in the Transmission Operations and Maintenance department, Substation/Power Equipment group, for Tennessee Valley Authority. He received a BSEE degree from the University of Kentucky in 1983 and went to work for TVA as a field test engineer. He joined TVA's transmission staff in 1990 as a lead engineer for substation large power equipment. Goff developed a predictive maintenance program for TVA's substation and is currently developing an on-line transformer-monitoring program for TVA's 500-kV grid. He is a registered professional engineer in Kentucky. mbgoff@tva.gov

Joseph A. Graziano is a senior manager in the Research and Technology Applications department, Transmission Technologies Group, at the Tennessee Valley Authority. Graziano started his career with Westinghouse Electric, General Electric, Teledyne Continental Motors, Babcock and Wilcox before joining TVA's Nuclear Power department. He received his BSME degree from West Virginia Institute of Technology, his MSME dgree from the University of Cincinnati and his PhD in civil engineering from the University of Tennessee, and is a registered professional engineer in Tennessee. jagraziano@tva.gov

WHAT IS BACKSCATTER?

The precursor to today's backscatter technology is the British invention used to identify airplanes as friend or foe, which was used by the allies in World War II. Transponders such as this are still being used in military and commercial aircraft today. An early work exploring RFID is “Communications by Means of Reflected Power” by Harry Stockman in the Proceedings of the Institute of Radio Engineers in October 1948. The first wide-scale commercial deployment of RFID technology was for electronic toll collection in the United States in 1992. Today, a lot of U.S. metropolitan areas use this technology in what is now commonly called E-Z Pass or Fast Pass for electronic toll collection. The 21st century has brought a renewed interest in RFID, which has caused an explosion in the technology.

Backscatter is simply the reflection of signals back to their source. Sophisticated instances of backscatter would include all radar devices. RFID relies on local storage and remote retrieval of data. An RFID tag is a small object containing silicon chips and an antenna that can be attached to or incorporated into a product or other object. RFID tags are then able to receive and respond to radio-frequency queries, thus fitting into the genre backscatter devices. Passive tags require no internal power source, whereas active tags do require a power source. This technology also enables high-speed inspection (i.e., from a helicopter) as the sensor is always available.