Cable condition assessment in a 40 MW Solar power plant!!

Cable condition assessment in a 40 MW Solar power plant for One of the most renowned power utilities in Gujarat, India with the help of UHF Partial Discharge Testing.

Problem Overview –

• They experienced a major outage caused by a simple cable failure and hence decided to go for a partial discharge analysis. The cable failure caused them a long outage time resulting in huge economic losses.
• Because of the unavailability of the required instrument and the competency required to operate the same, it was hard to conduct it internally also the availability of a very less number of competent service providers in the area made it more difficult to find a service provider to quickly locate the cable fault, repair the cable and commission the circuit.
• As Elegrow Technology had already conducted a Safety Audit for them and because of that trust they offered us the opportunity.

Cable details:

• Number of circuits: 18 circuits, 4 years old
• of Feeders: 18
• Type: XLPE
• Type of conductor: Aluminium
• Cable rating: 33kV
• Instrument used: Megger UHF PD, VLF Sine 62-TD, PDS 62-SIN, S1-568

Sequence of testing:

• Scanning with TEV – PD Scanner
• Scanning with HFCT UHF PD
• Tan delta measurement @ 0.1Hz
• IR and DAR measurement
• Offline PD measurement @ 0.1Hz

Testing and Observations

 

• 17 From the 18 tested cable circuits were found in good condition and a periodic monitoring recommendation was to monitor its ageing over time.
• One circuit indicated PD signs; TD & PD results were not within the tolerance level. For this specific cable direct action was recommended.
• The identified cable had a length of 1,002 m.
• The IR and DAR values indicated differences between the phases when looking to the phase to ground resistance.
• The phase-to-phase values were stable. But the TD diagnostics showed a large tip-up and absolute value in the Y-phase.
• The other two phases had normal levels. Different TD values between phases often indicate a local problem, such as issues with a joint.

Results – 

The above graphs show the TD trend for each individual phase. It can be clearly recognized that the Y phase has far higher values compared to the R and B phase, the dielectric losses in the Y phase at 1.5 Uo are in an order of 20 to 40 times higher. The results were compared with the evaluation criteria set out by the IEEE 400.2-2013.

The recommendations suggested ‘No action required’ for the R and B phase but ‘Action required’ for the Y-phase. As it is a local problem, it is recommended to either perform a withstand test or PD diagnostics. As with a withstand test it cannot be guaranteed that the local problem will be brought to controlled breakdown, the PD diagnostics is the best option of the two.

Test Voltage	0.5 Uo	1 Uo	1.5 Uo
Phase R
Tan d (10^3)	1.07	1.39	2.56
Tan d Deviation(10^3)	0.00	0.01	0.06
Phase Y
Tan d (10^3)	0.95	5.27	49.78
Tan d Deviation (10^3)	0.03	0.85	16.70
Phase B
Tan d (10^3)	0.47	0.52	1.30
Tan d Deviation (10^3)	0.00	0.00	0.00

 

Megger make PD detector was used to carry out partial discharge measurements along-with the VLF Sine 62. Following test voltages were applied to the cable, 0.2Uo, 0.5Uo, 0.8Uo, 1Uo, 1.2Uo, 1.5Uo and 1.7Uo. Specifically for these local problems one should do the PD diagnostics longer than normal, this to give discharges a chance to be visible also in wet environments. Up to Uo (phase to earth voltage) no discharges have been recognized as shown in the graph.

With increasing voltage over Uo, PD’s were localized at the terminations. After analyzing the PRPD pattern this turned out to be corona discharges. Additionally in the Y-phase, which had high Tan delta value, another PD location became visible, discharges have been located at about 800 m away from the near end.

Based on the first incident, which caused revenue losses, Partial Discharge Testing was performed on all the MV cables within the solar power plant. From the 18 tested cables 17 were found to be in good condition. Only one cable showed defects. Especially the TD diagnosis properly indicated this. The subsequent PD diagnostics located the local problem. With help from the condition assessment the utility now has a good fingerprint of the complete MV network, one highly degraded joint has been identified and replaced. This prevented the power utility from another outage which could have caused additional high revenue losses. As the complete cable diagnostics work was carried out during night, when the solar plant does not generate any power, the condition assessment program itself did not result in loss of income.