Contd.

ACOUSTIC PD INSTRUMENTATION

Acoustic partial discharge detection set up is very simple, consisting of a sensor, filter, preamplifier, some type of data acquisition system (e.g., storage oscilloscope) and some output device.

The detected AE signals from the sensors are preamplified and filtered for spurious signals. These filtered signals are further amplified and are passed to a digital storage oscilloscope where the transients are captured and stored digitally. The stored transient can be transferred to an X-Y recorder via a digital to analog converter, or passed in the digital form to a spectrum analyzer.

Alternatively, the amplified analog signal can bypass the digital storage oscilloscope and be fed to a data acquisition system and a PC, were the signals can be analyzed.

The system frequency response (time constant) determines most of the system detection characteristics. The sensitivity and signal-to-noise ratio are determined primarily by the amplitude and frequency characteristics of the signal that arrives at the sensor and the ambient mechanical back-ground noise.

The coupling from the apparatus under test to the sensor should be considered as an integral part of the system, as it strongly influences system characteristics.

LOCATION OF DISCHARGES

In order to determine the severity of a PD problem, it is necessary to locate its geometric location as well as its magnitude. The possibility of PD location is one of the major features of acoustic discharge determination. Location can be based on either measurement of time of signal arrival at a sensor or of signal level.

An arrangement for on-line measurement instrumentation for PD location in case of transformer is shown in fig. 4. Two AE detection channels are used in this set up. From the analysis of the difference in signal magnitude and the time delay of the two signals the location of the PD source is determined.

A test chamber (transformer tank model) as shown in fig. 5 was designed and fabricated for the experiment purpose. Various types of partial discharges can be simulated inside the chamber by using the electrode system and the PD measurements by both the conventional electrical methods and acoustic emission methods can be performed at laboratory. Then the correlation between the test results of the two methods shall be performed.

CONCLUSION