By René Smeets and Bas Verhoeven
Cables and accessories
A cable system consists of a cable and its accessories: joints and terminations. The system’s insulation is designed to control the high-voltage stress which is verified by a series of dielectric tests under various operational conditions.
Laboratory test results are collected from over 1000 AC cable systems having (almost all) XLPE insulation. The test-objects are classified as “MV” (53% of components), when tested against IEC 60502 and CENELEC HD 6210 and HD 629.1 (including indoor and outdoor terminations), and “HV” (47%) when tested against IEC 60840 and IEC 62067 (including GIS and outdoor terminations).
Figure 1 gives the observed initial test failure rates of cable and accessories over time (1993 - 2019). Trendlines are added, showing a slight net increase over the years (updated from ).
This survey shows that 10 - 50% of all components fail to pass the standardized type-test.
It is interesting to compare these data with failure data from service. CIGRE WG B1.57 (2020)  concludes a service failure rate due to internal causes of 0.069 /100 km.yr for cables (60 – 500 kV, XLPE AC land cable), which is 30% higher than reported in the earlier CIGRE survey from 2009 . For joints this number is 0.002 / 100 units.yr (40% below the 2009 survey data) and for terminations a failure rate of 0.051 /100 units.yr is reported (300% increase from 2009).
The most severe stress to transformers is passage of short-circuit current. In that event, the windings are subjected to very large electro-dynamic forces that shall not lead to deformation or other damage . Tests to verify withstand of short-circuit are formulated in IEC 60076-5. Fig. 2 displays the experience with short-circuit testing of 251 distribution transformers (315 - 2000 kVA) in a six-year period and 423 large power transformers (LPT) (25 – 600 MVA, 33 - 800 kV) in 29 years of testing . In both cases the test-failure rate is in the order of 25%. The most prominent failure mode is deformation of windings, but also external abnormalities such a as break of bushing, oil spill, tank rupture have been observed. The trend is almost constant, though the last six years the LPT test-failure rate heads upward.
Short-circuit testing is not a type-test but is performed on individual LPTs prior to installation. The test is non-destructive and is acknowledged by CIGRE SC A2 as “the most comprehensive solution because all parts are verified”.
Transformer service failure data are collected by CIGRE in 2015 . In this survey a (major) failure rate  of 0.53% is found (in the voltage range ≥ 69 kV). Winding failures were identified as the largest contributor (37%) to the in-service major failures, and external short-circuit is the second largest known cause (after ageing) of major failures.
Circuit breakers are the key protection hardware in the system: they must interrupt very high current in case of a fault in the grid. CIGRE has conducted various TSO surveys, from which a fault incidence of a few faults per 100 km overhead line per year emerges .
Test statistics of HV SF6 circuit breakers rated 72.5 - 800 kV, being subjected to 1241 standardized IEC short-circuit test-series between 2010 – 2016 are shown in fig. 3. The overall failure rate (to pass a test-duty) is 32%, and roughly constant over the years. Details can be found in  where it is demonstrated that the “short-line fault” test-duty is the most critical one to pass.
In-service field data are also available for circuit breakers and summarized in . The latest information (2004-2007) shows an overall failure rate of 0.30% , down from 0.67% in the period 1988-1991. The period 2014-2017 is now under evaluation by CIGRE WG A3.48.
Improvement of materials, calculation methods and production technologies are ongoing, but these do not result in a decrease of failure rate in type-testing, which, over decades, remains in the 25% range. The in-service failure rate of power system components is much lower. This is because type-testing aims to verify the components compliance to the standards, which cover about 90% of practical applications.
In order to maintain the low service failure rate, type-testing and independent certification remains a key de-risking instrument to distinguish the well-designed, well-manufactured products from the inferior ones.
Thumbnail credit: Omid Armin on Unsplash
-  E. Pultrum, W. Sloot, J. Fernandez, R.P.P. Smeets, “High-voltage Cable Testing: Type Test Experiences and New Insights into Pre-Qualification”, 21st Conference on the Electric Power Supply Industry, Bangkok, 2016
-  CIGRE TB 815: “Update of Service Experience of HV Underground and Submarine Cable Systems”, CIGRE WG B1.57, 2020 - e-cigre.org/publication/815-update-of-service-experience-of-hv-underground-and-submarine-cable-systems
-  CIGRE TB 379: “Update of Service Experience of HV Underground and Submarine Cable Systems”, CIGRE WG B1.10, 2009 - e-cigre.org/publication/379-update-of-service-experience-of-hv-underground-and-submarine-cable-systems
-  R.P.P. Smeets, A. Derviskadic, S. Subramany, “Verification of the Short-Circuit Withstand Capability of Transformers”, CIGRE Sci. and Eng., No. 10, Feb. 2018 - e-cigre.org/publication/CSE010-cse-010
-  R.P.P. Smeets, R. Bruil, ”Experiences and innovations in power transformer short-circuit current withstand testing”, CIGRE Conference, report A2-304, 2016 - e-cigre.org/publication/A2-304_2016
-  CIGRE TB 642, ”Transformer Reliability Survey”, CIGRE WG A2.37, 2015 - e-cigre.org/publication/642-transformer-reliability-survey
-  Failure rate = (number of units failed/operating unit years) x 100 [%/yr]
-  CIGRE TF A3.01: “Statistical Analysis of Electrical Stresses on High-Voltage Circuit Breakers in Service”, Electra 24, No. 220, 2005 - e-cigre.org/publication/ELT_220_2-statistical-analysis-of-electrical-stresses-on-high-voltage-circuit-breakers-in-service
-  R.P.P. Smeets, B. Baum, R. Nijman, D. Petropoulos, T. Ohtaka, “High-Voltage Circuit Breaker Test Statistics 2011-2016 and Test Analysis Tools”, CIGRE Conference, report A3-102, 2018 - e-cigre.org/publication/SESSION2018_A3-102
-  CIGRE AG A3.01, ”CIGRE Reliability Survey on Equipment”, CIGRE Conference, report A3-201, 2020 - e-cigre.org/publication/session2021a3-sessions-2020--2021-sc-a3-package
-  CIGRE TB 510, “Final Report of the 2004-2007 International Enquiry on Reliability of High-Voltage Equipment – part 2: Reliability of High-Voltage SF6 Circuit Breakers”, CIGRE WG A3.06, 2012 - e-cigre.org/publication/510-final-report-of-the-2004---2007-international-enquiry-on-reliability-of-high-voltage-equipment---part-2-sf6-circuit-breakers