Antennas in partial discharge sensing system

Abstract

The exponential growing demand of electricity has stimulated the manufacture of electric equipment with high rated powers withstanding tens and hundreds of kilovolts. These devices have to be insulated to ensure a safe and reliable service while their size and cost are reduced. Unfortunately, insulations deteriorate over time by being in operation under load and exposed to harsh environments that can degrade their behavior and lead to unexpected equipment outages and failures. The continuous monitoring of these assets is paramount in the operation of electric power systems, and one of the most popular methods to evaluate the ageing is the detection of partial discharges. Partial discharges are ionization processes that take place in voids filled with gas or oil inside the insulation, in dielectric surfaces, and in the proximity of sharp metallic objects. The chemical and physical structure of the insulation is changed, and eventually weakened, by the continuous action of the discharges. Then, their apparition can be directly a signal of problems in the insulation, but they can also be the consequence of other degradation processes. Partial discharges can be measured with a wide range of detectors including inductive, capacitive, acoustic, and light sensors. Because partial discharges occur in extremely short times, well below 1 ns, the radiofrequency measurement of the phenomenon in the HF, VHF, and UHF bands is also part of the unconventional methods used for their detection. EM sensors or antennas have the ability of performing a complete study on the measurement of partial discharges. They can detect pulses, localize the partial discharge site, and, to some extent, classify the type of partial discharge online. However, one of the most important challenges when using antennas in the diagnostic of insulations is the difficulty of relating the RF emissions to the severity of the PD. Another determent in the wide application of antennas as partial discharge detectors is the sample rate needed to obtain information from the signals in the time and frequency domains. The chapter also explores the most common configurations of antennas used in the detection of partial discharges as well as how they are installed and used in different electrical machines.