Offline PD Cable Mapping
Partial Discharge Mapping is an offline test where the cable system is isolated from the electrical network and energised with a discharge free test supply. This allows the measurement of the magnitude and origin of partial discharge activity within high voltage cable insulation with minimal background noise. By capturing many partial discharge events, a graphical representation of the discharge activity along a length of cable may be generated.
In order to measure partial discharge activity, the cable must be energised at a voltage level of at least the normal working voltage (U0). Supplying this test voltage for long cables is difficult due to the high-power requirements necessary to provide the charging current to energise the cable capacitance. To reduce the power requirements, a Very Low Frequency (VLF) high voltage test supply is used. This allows long lengths of cable to be energised at the required test voltage from a 230V 13A outlet. The frequency of the test voltage is dependent on the cable length but is typically 0.1Hz.
Each Partial discharge event is detected via a high voltage coupling filter connected at the end of the cable under test. The signals are captured by a digital oscilloscope and transferred to a computer for storage and analysis.
The recorded partial discharge traces are subsequently processed using a software package to determine the origin and the magnitude of each individual partial discharge event. Finally, the processed data is used to produce a graphical representation of the discharge activity along the route length in the form of a partial discharge map.