Improvement of ultrasonic method for testing of power transformers

Ultrasonic method of detecting PD is based on the fact that the electrical energy of the PD transforms in a mechanical energy, an ultrasonic acoustic wave that spreads through the transformer to the tank wall. These mechanical waves are detected with piezzoelectric sensors and transformed into electrical signals which are processed with a special instrument called Acoustic Emission System (AES). Since not only defects inside the transformer can produce acoustic waves, during analysis there is a problem distinguishing the actual origin of the detected acoustic waves. Therefore, additional sensors can be added to the system which monitor some parameters of the transformer such as temperature of the tank, load, OLTC operations, fan and pump operations, PD level during laboratory testing (in accordance with [1]) etc. These parameters can simplify analysis using a time correlation between detected hits (hit is a designation for event when sensor detects an acoustic wave) and in-service parameters of the transformer. Another problem with location of the source in 3D system is the lack of knowledge of the zero time of the hit. Using a high frequency current transformer (Rogowski coil) and the fact that the electric signal has a high propagation velocity, zero time can be added to the system providing higher accuracy.