TVA Locates Faulty Equipment

By Ralph McKosky, Mark Goff and Joseph Graziano, Tennessee Valley Authority
Antenna-array technology is used to locate partial-discharge signature of equipment that goes bad
Tennessee Valley Authority has had to Deal with Pollution Problems on Insulators and lightning arresters at its Paradise Fossil Plant Switchyard. Particulates from the combustion process have caused insulator pollution. The worst case is when this pollution leads to an insulator flashover, resulting in a protection relay and circuit breaker operating to mitigate the problem. TVA (Knoxville, Tennessee, U.S.) has changed its plant blow-down operations and now periodically washes the insulators. These steps have minimized but not solved the problem.
Working with the Electric Power Research Institute's (Palo Alto, California, U.S.) Substation Task Force and the University of Strathclyde (Glasgow, Scotland), TVA and other utilities are developing an antenna-array system to continuously monitor partial-discharge (PD) activity in a substation as an effective way to locate possible faults. TVA is using a PD system that was developed by the Institute for Energy and Environment at the University of Strathclyde. The antenna-array system constantly listens for all discharge activity within its range. The antenna-array PD system was demonstrated successfully by National Grid in the United Kingdom.

The antenna-array PD system measures radio-frequency (RF) signals to detect and locate PD. To accomplish this, the antenna-array PD system uses four specially designed omni-directional disk-cone antennas, a high-speed wide-band digitizing oscilloscope, custom software and algorithms to record and analyze the RF sources. The technology detects and captures the discharge and then computes the time-of-flight data to locate the source of PD. This is done by finding the solution of nonlinear equations.
The goal of the project is for the system to screen and identify suspected problems such as incipient faults and insulator pollution issues. With this knowledge, TVA could schedule insulator washing before a flashover risk occurs or take action to replace or repair high-voltage equipment showing internal PD.
In December 2005, TVA installed an antenna array in a wireless sensor lab of a Paradise mobile field trailer. Since that time, the utility has been tracking two different sources of PD. On average, 2000 PD impulses have been recorded a day. One source had a more energetic level of PD activity; the other source just seemed less energetic.
On May 18, 2006, TVA took a 69-kV bus off-line to install sensors for another research project (on wireless back-scatter sensors). During the outage, power-system operations, transmission operations and maintenance personnel performed a follow-up power-factor retest on the B-phase voltage transformer (VT).
The retest showed a sharp increase of the power factor; hence, it was decided to replace the VT during this outage. Once the 69-kV bus was back in service, the less energetic of the previously tracked discharges disappeared. The post-processed data after analysis was then plotted on a substation overview. A closer look at the data revealed that the source that seemed to have disappeared coincided with the replaced VT.

To read more, click here.