High-voltage cable testing: type test experiences and new insights into pre-qualification.
During testing, highly stressed cable designs and their accessories are subjected to heating cycles under voltage. The cable is usually installed under various laying conditions, resulting in different conductor temperatures. The prequalification test requires voltage to be applied for the duration of one year and at least 180 heating cycles, each for a minimum of 24 hours. This method of testing leaves room for interpretation, and consequently, various ways to execute this test resulting in different outcomes depending on the choices made at the start.
KEMA Laboratories has performed this test at their test facility in Arnhem, the Netherlands, since the first edition of IEC 62067, and has witnessed this test at various manufacturers and test facilities world-wide. Based on our experience, a guideline - in line with the background for this test – has been developed on how to properly execute this prequalification test in accordance with the requirements in the standard.
Type test statistics from 1993-2013 on MV and HV cable systems (cables and accessories) show that a fraction of a few tens of percent are not passing certification tests; a number that is not decreasing over time.
Transformer short-circuit withstand testing and classification: Experience and developments
The role and rationale of short-circuit testing of power transformers in the quality assurance process has been well documented, as well as a range of utility considerations that rely on short-circuit testing to verify withstand against the mechanical forces of short-circuit current. Experience gathered during the past 20 years from short-circuit testing of large power transformers (≥ 25 MVA) has been examined thoroughly by KEMA Laboratories. In total, 297 test series involving 258 different power transformers have been analyzed.
Statistics show that at first access to standard IEC 60076-5, 23% of the short circuit tests performed resulted in a failure initially. Failure to pass short-circuit tests is caused mainly by an increase of short-circuit reactance beyond what is accepted in the IEC standard. However, several other directly apparent failure modes are frequently observed, such as oil spill, damage to bushings and leads, etc., and are demonstrated in the paper submitted at CEPSI written as a result of the analysis.
A similar analysis describing test methods and test circuit setups, was carried out on five years-worth of short-circuit testing data, gathered from 250 distribution transformers (≤ 2.5 MVA). It was observed that around 20% failed to pass the test initially.
Test-methods and test-circuit setups are described in the CEPSI paper. KEMA laboratory’s expansion plan is illustrated, in which two new, additional generators and four new additional test transformers are in operation to enable the short-circuit testing of transformers up to the 800 kV class.
Developments in EHV/UHV circuit breaker testing
Nowadays, a global trend of increased system voltage demand up to 800 kV AC and above can be observed in the market. This calls for the need of having test facilities to verify the current interruption performance of circuit breakers for such “super grids”.
A recently commissioned test- circuit consisting of a two-stage synthetic installation, will be able to demonstrate what is required, using actual test examples of 800 kV and 1100 kV circuit breakers, for which TRVs are in excess of 2000 kV. The CEPSI paper highlights the test-circuits and test-methods that have been developed to carry out full short-circuit test programs on circuit breakers up to 1200 kV. Examples of short-circuit current making- and breaking tests are given, including the full-power testing of a switch, designed to activate closing resistors in an 800 kV GIS circuit breaker.
The expansion enables full three-phase synthetic testing, even under effectively earthed conditions, examples of which will be shown.