Page 60 - Energize October 2021

TECHNICAL

Figure 8: Typical trace of a PD measurement (Megger) Figure 10: CRW waveform for PD testing (Megger)

and level of discharges. A typical PD test will yield the result shown in The cosine rectangular waveform
Figure 8, which records each PV event, the magnitude and the location. A CRW of the form shown in Figure 10 can also be used for PD testing.
In some cases it is useful to record the voltage phase at which PD The voltage of the waveform needs to be increased to √2 x RMS to
occurs and this requires synchronisation of the recording device with activate the void PD.
the input waveform.
PD location
PD Test waveforms The location of the partial discharge can also be determined by
Accurate evaluation of PD activity requires a repetitive varying incorporating time domain reflectometry into the test. A partial
waveform, and two methods are used primarily, the damped oscillatory discharge signal will travel in two directions, firstly back to the test end,
waveform (DAC) and the cosine rectangular wave, described in the and secondly to the far end where it will be reflected back to the test
previous section. end. The time difference between the two signals can be used to give an
indication of the location of the PD (Figure 11).
Damped oscillatory waveform (DAC)
DAC is the most well established waveform in the MV sector and is
recognised in international standards such as IEC 60270. DAC testing,
which has been in worldwide use for many years, and has become
the proven standard method for carrying out non-destructive PD field
diagnostics on MV and HV cables, since it exposes the main insulation
of the cable under test to the minimum possible stress (the voltage
exposure time is only around 10 cycles). 2
The wave has both the peak voltage required for cable voids and
the rate of change required for interfaces and has been proven to have
comparable results to 50 Hz testing. The DAC is illustrated in Figure 9. Figure 11: TDR location of PD activity [Megger]
The DAC is created by using resonance between the cable
capacitance and an inductance in the test set. A DC source charges the Online PD monitoring
cable, and a discharge switch is closed to create a series resonant circuit. Online PD monitoring can be used to monitor the occurrence of PD activity
The resonant frequency is a function of the inductance of the of a cable under operating conditions. The system makes use of the fact
inductive reactor in the test set, the capacitance of the auxiliary that PD activity generates HF signals, and uses HF current transformers
capacitor in the test set and the capacitance of the cable under test. attached to the cable to detect the presence of PD activity. Typical systems
It is typically in the range 30 to 500 Hz, and each oscillation lasts will record system information such as cable loading and voltage levels that
about 200 ms. can be correlated to PD activity. Advanced systems include the facility to
PD results obtained with DAC have been proved to show very good determine the location of the PD activity in the cable length. n
correlation with results at 50/60 Hz power frequency, and thus are fully
comparable. DAC is the only excitation technology capable of properly References
measuring PDIV and PDEV in the same test cycle. 1. Megger:“Fault finding solutions”, MEG-231/MIL/3M/11.2003,
https://megger.com/applications/cables/cable-fault-location
2. R Probst: “Partial discharge testing in a cable’s life management”,
Megger online presentation.
3. J Perkel and J Hernandez: “Medium voltage cable system partial
discharge”, Georgia Tech Research Corporation, 2016.
4. W Higginbotham: “Online vs. offline partial discharge testing for cable
assessment”, EAtechnology.com, 2017
5. R Youngblood: “VLF tan delta cable testing: Is it right for your
company?”, Netaworldjournal.org, April 2020.

Figure 9: Damped oscillatory wave with partial discharge activity (Megger) Send your comments to rogerl@nowmedia.co.za

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