78 66th AMEU Convention 2017

66thAMEUConvent ion2017

Innovative technology permits predictive maintenance on an HV power networkby Patrick O’Halloran, City Power

This paper covers innovative technologies that permits predictive maintenance to be done on a high voltage power network. Results obtained from performing condition monitoring testing on high voltage equipment have prevented numerous failures. Many of these failures would have occurred and caused extensive and extended outages in Johannesburg.

City Power and Singapore Power Global Solutions entered into a strategic partnership that is now bearing fruit. Singapore Power Global Solutions is the consulting division of Singapore Power responsible for ensuring that electrical utilities can implement condition monitoring to improve the reliability and quality of supply to their customers.

S i ngapo re Powe r has succe s s f u l l y implemented condition monitoring over the past 20 years. Condition monitoring is one of the key contributing factors for Singapore Power becoming one of the most reliable utilities in the world. Based on the latest performance indices, a customer in Singapore would experience a power failure only once in every 142 years (System Average Interruption Frequency Index – SAIFI) and the longest fault any customer would experience is roughly 30 seconds (System Average Interruption Duration Index – SAIDI). These figures are absolutely amazing, but possible (see Figs. 1 and 2).

Benefits of performing condition monitoring

When condition monitoring testing is implemented, the following benefits shall be achieved:

l Improved reliability and quality of supply due to reduced faults and forced outages (improved SAIDI, CAIFI and NRS 048, etc.).

l The root cause of the partial discharge is not destroyed and preventive maintenance can be done before failures occur. Rectification costs prior to a failure are much cheaper than repair cost after a failure.

l Extend intrusive maintenance intervals and perform condition based maintenance only

l Extended equipment life

Road map to make condition monitoring a success

Due to the nature of electrical networks, faults will still occur and because of this a business needs to contain the fault and prevent any unnecessary upstream trips. Once a fault has been cleared it is essential to get to the

root cause of the failure. Once this is known, condition monitoring programs may need to be changed to detect such faults in the future before they occur. Quality assurance is critical to eliminate installation errors. Lastly, when a certain product has reached its end of life, it should be replaced before it fails.

Fig. 3 shows Singapore’s recommended condition monitoring road map.

Root cause of equipment failures

Partial discharge (PD) is the root cause of most HV and MV network related failures experienced by City Power, excluding

Fig. 1: Singapore Power System Average Interruption Frequency Index (SAIFI).

Fig. 2: Singapore Power System Average Interruption Duration Index (SAIDI).

66th AMEU Convention 2017 79

66thAMEUConvent ion2017

vandalism and theft. Once PD has begun it will always worsen, leading to insulation breakdown equipment failure. It is therefore imperative that once PD is detected, it is rectified before the failure occurs.

PD is defined as a localised discharge process in which the distance between two electrodes is partially bridged. PDs may originate directly at one of the electrodes or occur in a void or cavity inside the dielectric. (See Fig. 4a, b and c.)

The various types of partial discharge are outlined below:

l Corona discharges: occur as a result of a non uniform field on sharp edges of a conductor subjected to high voltages.

l Surface discharges: occur on the surface of the different dielectric material.

l Cavity discharges: occur when cavities are formed in solid or liquid insulating materials where the gas in the cavity is overstressed and discharges occur.

PD is a result of many contributing factors including:

l Poor workmanship (clearances, installation errors and lack of skills)

l Incorrect application of products (technology changes)

l Overload leading to insulation breakdown (heat causes insulation to breakdown)

l Manufacturing defects

l Equipment designs

Once the PD causes the insulation medium to breakdown, a power flashover will occur. This PD can be detected and assessed, but the exact failure time can’t be predicted.

The recommended Singapore condition monitoring test equipment and techniques are able to detect potential faults on equipment before a failure occurs. As these potential faults are detected before a failure occurs, their location and root cause can be identified and rectified before they fail. This then ensures safety of staff and communities by preventing potentially dangerous failures. The reliability and quality of supply is improved every time we prevent a failure which would normally result in customer outages and voltage dips. City Power is able to carry out pro-active condition-based maintenance on the network and is moving away from current time-based maintenance practices, which have in the past missed many potential faults. City Power in the future would only shutdown the power plant if it is absolutely necessary to perform preventative maintenance. City Power has already saved lots of money on maintenance and post failure repair costs. One day power interruptions should be reduced to hopefully

Condition monitoring system What they detect Applied to

Thermal scanning Overheating Equipment

Dissolve gas analyst Abnormal oil contents Equipment and cables

Oil pressure monitoring Low pressure Cables

Distributed temperature sensing Hot spots Cables

Very low frequency test Low insulation Cables

Partial discharge monitoring Minute current leakage Equipment and cables

Operating mechanism monitoring Abnormal operation Equipment

Table 1: Summary of possible condition monitoring systems that can be applied.

Fig. 4: Various partial discharges: (a) corona discharges, (b) and (c) surface discharges, (d) and (e) cavity discharges.

Fig. 3: Singapore’s condition monitoring road map.

Fig. 5: CBI’s EHV/HV test trailer testing 275 kV and 132 kV cable terminations at Sebenza SS.

80 66th AMEU Convention 2017

66thAMEUConvent ion2017

Fig. 6: MR’s EHV/HV test trailer testing a 250 MVA 275/88 kV power transformer at Delta SS.

Fig. 7: Singapore technical staff performing PD testing on a transformer cable box.

Fig. 8: Infrared thermal image of GIS switchgear clearly shows a difference in temperature.

only vandalism and theft related failures.

The following non-intrusive online condition monitoring testing techniques should be implemented to best determine the operating condition of assets:

l Ultrasonic

l Transient earth voltages (TEV)

l Radio frequency (RF)

l Inductive coupling frequency response via high frequency current transformers (HFCT)

l Capacitive coupling frequency response

l Dissolved gas analysis (DGA)

l Infrared

l Leakage current measurements

It is now possible to detect insulation problems in cables, overhead transmission lines, power transformers, switchgears, and other electrical equipment in our power electrical networks. Condition monitoring testing shall become part of everyday business within City Power and not a special once off project (see Table 1).

Due to the nature of insulation systems, online PD testing will give you a good indication that there is a problem, but it will not always give you the exact location of the PD source, especially in cable networks. Online PD measurements are also at Uo system voltage. The best way to test the actual condition of any insulation system is with offline testing, where an overvoltage is applied, for example 3 x Uo.

Until recently no portable on site HV and EHV testing equipment was available in South Africa. This has now changed and both HV and EHV cables and power transformers can be tested on site at suitable high over voltages (see Figs. 5 and 6).

Simple way to perform condition monitoring testing

Fig. 7 outlines the simple steps to test a transformer cable box, but the same test will be done on all MV switchgear.

Infrared thermal scanning of all equipment has evolved and can even be done with certain smart phones. This condition monitoring technique is cheap and very effective for detecting differences in temperatures during operating conditions (see Fig. 8).

PD in medium voltage switchgear is measurable in two different ways with EA Technology UltraTEV Plus2 handheld instruments:

l Capacitive probe for the detection of TEVs in the VHF electromagnetic spectrum (3 – 80 MHz).

- Internal discharge - High level surface discharge to earth Fig. 9: EA Technologies UltraTEV Plus handheld PD tester unit.

66th AMEU Convention 2017 81

66thAMEUConvent ion2017

Fig. 10: SEBA OWTS PD test on a PILC MV cable – PD scattered and difficult to analyse.

Fig. 11: SEBA OWTS PD test on a XLPE MV cable – PD scattered and simple to analyse.

Fig. 12: SEBA OWTS PD test on a XLPE MV cable after first joint was removed out of the circuit.

Fig. 13: DGA interpretation to identify potential internal fault.

Fig. 14: Typical on-line DGA monitor installed on a power transformer.

l Ultrasonic airborne microphone or contact probe generally centred at 40 kHz.

- Surface discharge.

The latest UltraTEV Plus2 unit offers so much more than a measurement value which can be confused with background noise or corona. The new unit automatically diagnoses the measurements and confirms if it is PD or noise from a loose plate, etc. (see Fig. 9).

The highest failure rates in City Power are from cable termination and joint failures. The reason for these failures has a lot to do with the jointer’s skills. In City Power we subcontract lots of work to contractors and to ensure they perform the joint or termination you need to perform the correct test which will confirm the quality of the workmanship.

In the past, City Power has done typical SANS 10198-13 recommended commissioning tests which include over voltage pressure testing which are “go” or “no go” tests. This has proven not to be adequate as a poorly installed joint may withstand the applied voltage pressure test but if PD is present a fault occurs after a certain time period because the PD eventually weakens

the insulation which leads to a failure. It must be remembered that all insulation ages, and it is important for us to monitor and know the condition of the insulation to prevent failures.

Cable testing with DC voltage has also proven to be unable to detect potential faults in joints, terminations and cables, unless they are bad faults. The use of AC voltage test equipment is a must to ensure we test the permittivity of insulation materials which is what all equipment will experience when energised.

Singapore Power now PD tests all new XLPE cable systems to detect any unacceptable PD activity in joints and terminations. This detected PD over time breaks down the insulation if not corrected immediately. The days of only performing a “go” or “no go” pressure test to prove the reliability of the cable are over. It is now time to record finger prints of MV cable networks and then monitor them over their life cycle to ensure reliability of supply.

If PD is detected during testing, the system cable system should not be energised, but investigated.

City Power have now changed from paper

82 66th AMEU Convention 2017

66thAMEUConvent ion2017

Fig. 15: Typical 88 kV cable termination (sealing end) and failure point.

Fig. 16: 88 kV cable termination international failure point after PD has occurred.

Fig. 17: Heat shrink sheath does not seal correctly and moisture enters the termination at the critical main earthing point.

Fig. 18: Epoxy resin that is damaged due to the moisture ingress. The epoxy resin protects the main earth from moisture.

Fig. 19: Single point bonded HV cable system.

Fig. 20: XLPE insulation eroded away from the partial discharge activity. HV cable insulation will breakdown under these high electrical stresses.

insulated MV cable (PILC) to crosslinked polyethylene insulated MV cables (XLPE) so that PD testing cable be performed to ensure the joints are PD-free to acceptable predefined limits. Remember that the new generation these days is water resistant, so the same bad failures experienced in the 1970s will not be experienced.

By design PILC cables PD is present and this makes PD location very difficult (see Figs. 10, 11 and 12).

Dissolved gas analysis (DGA) is a very important condition monitoring test for power transformers. City Power has been doing DGA tests for 15 years already and we are able to analyse the results in our own laboratory. When analysing the GDA results for a specific transformer the data needs to be trended to ensure correct analysis is done and correct decisions are made with regards to these critical assets (see Fig. 13).

City Power has installed on-line transformers 9 Gas DGA monitors. On-line transformers 9 Gas DGA monitors are a must for all of City Power’s new power transformers (see Fig. 14).

88 kV termination failure case study

Over the last few years City Power has experienced numerous 88 kV cable termination failures at two specific substations. After detailed investigations and field testing with the OEM who designed, manufactured, supplied and installed the 88 kV terminations the root cause was identified (see Fig. 15).

The failure mode identified was moisture ingress into the critical earthing connection point of the termination. At this point the CSA and tin copper earth braids are connected via a wiped connection. Due to the design and the ingress of moisture, the aluminium sheath corroded away and the main earth was no longer effectively earthed (see Figs. 16, 17 and 18).

Due to the single earthing system applied, the sheath of the 88 kV was now not clamped to 0 V (ground potential). All four inspected terminations failed at the same location way below the stress control cone. The OEM recommended simple offline ductor testing (see Figs. 19 and 20).

66th AMEU Convention 2017 83

66thAMEUConvent ion2017

Fig. 21: Online testing of 88 kV cable termination and cable.

Fig. 22: Online testing results.

City Power have roughly 350 88 kV terminations of the same design. Although this simple offline ductor test is extremely reliable and repeatable, outages are not always possible. City Power worked with MARTEC who have specialised online testing techniques to test and establish the unique PD pattern for the failure mode that had been identified. The PD failure mode was previously eliminated as corona when in fact it was partial arcing at the main earth contact (see Figs. 21 and 22).

Conclusion

The vision of City Power’s management is commendable to strategically partner with Singapore Power who is one of the world’s most reliable electricity utilities to make a step difference with regards to preventing potential failures and also improving the reliability of supply to end customers.

The results obtained from performing condition monitoring testing have prevented many failures. Many of these failures would have caused extended outages across the city of Johannesburg.

Regional condition monitoring interest groups need to be established to share all PD findings and to teach interested engineers how to test and what to look for during inspections. Many failures can be prevented once such interest groups are proactively interacting and producing national recommendations.

Contact Patrick O’Halloran, City Power Johannesburg, Tel 011 490-7485, pohalloran@citypower.co.za