A portable acoustic emission monitoring system has been developed and used
to test both laboratory samples and practical equipment. A fibre optic cabl
e provides electrical isolation and protection for the operator and the mon
itor. This has been tested on high voltage equipment up to 400kV. In labora
tory samples it has been possible to distinguish between discharges arising
from different sources by converting the acoustic emission sinal from the
time domain to the frequency domain by fast Fourier transformation and reco
gnition of the frequency spectra ('signatures') thus produced. By a similar
procedure it has been possible to discriminate between signals generated b
y discharge activity in oil in a ring main unit, in the oil alone and in oi
l containing dry and wet fibres. The optically isolated acoustic emission d
etector has been successfully used to locate and identify a number of pract
ical sources of discharge activity, for example in transformer tanks and ca
ble terminations, A portable system, including the detection and signal pro
cessing capability, has been developed and installed for the on-line monito
ring of substation equipment.