The IEC 60270 states that a coupling capacitor must be used when performing a PD measurement.
Such an approach provides information about the voltage which is needed for a phase-synchronized PD measurement. OMICRON offers standard coupling capacitors from 12 kV up to 100 kV with and without external measuring impedance.
The more energy that is saved in the capacitor, the higher the recharging current is in case of a PD event, and the sensitivity is higher. Therefore a capacitor value should be as high as possible.
When using a coupling capacitor without an integrated measuring impedance, the low side of the coupling capacitor has to be connected to the input of the CPL measuring impedance (basic test setup with measurement on ground potential). Connect the PD output of the CPL measuring impedance to the PD input of the MPD (Data acquisition unit) and do the same for the Testing voltage.
Please note that the CPL impedance box and acquisition unit can be placed on various positions, such as on HV potential or within the test object path due to the fiber optic approach. Therefore, different setups have their benefits.
If more than one grounding wire is used, it should be considered to extend the length of one of these wires to feed both grounding wires through the HFCT. Otherwise, the HFCT will measure only parts of the high-frequency signals. The percentage of the signals measured is defined by the high-frequency impedance of the wires. If the impedance of both wires would be similar, a maximum of 50% of the signal will be detected.
Several advantages come up when tapped bushings are available:
- NO external coupling capacitor is required
- Less background noise in the measuring system
- The coupling device is directly connected to the measuring tap
- On line measurements are possible in case of permanent installed measuring units
- Nevertheless, the installation of the measuring set up must be done when the transformer is disconnected.
There are several connectors available depending on the bushing tap structure. It is strongly recommended to avoid any kind of clamps, because of the damages that can appear as a result of accidental disconnections. Here, a tap adapter was used to prevent accidental disconnection during the measurement
Ultra-High Frequency PD measurement
The frequency range of UHF measurement is 300MHz to 3GHz and typically used range is the range from 200MHz to 1.5GHz depending on device under test. Interferences can often be avoided through changing the center frequency. This unconventional UHF measuring method can be used for commissioning tests as well as on-site and online diagnostics.
A brief description of the two methods (IEC and UHF) is provided here. Until now there is no standard procedure to calibrate UHF measurements.