Test Methods & Acceptance Criteria for On & Offshore Power Cables

Cables & Accessories, HV/HP Testing

High voltage power cable circuits are strategic elements of on- and offshore power grids. Capital Expenditures (CapEx), as made by investors and stakeholders, are directly related to Operational Expenditures (OpEx), that in turn are fundamental to keeping reliability of supply at some agreed level (see Fig. 1).

Fig. 1: Balance of financial values and costs versus quality assurance of newly installed and maintenance of service aged cable circuits is important for reliable operation of on- and offshore power cable systems.

Given the above, from a technical point of view:

1. Overall technical quality of newly installed cable circuits is important to provide investors with so-called ‘value for money’ for their investments;

2. During the service life, the main tasks of the system operators and/or the asset managers are to have the reliability costs at an agreed level and of course without a risk of long-duration outages at least until the end of design lifetimes.

The challenge when managing power cable systems as strategic assets is to keep their CapEx value at the expected level and also to maintain the reliability of these assets based on the agreed OpEx. Considering the normal lengths of XLPE insulated transmission cable systems, it is typically the cable accessories that contribute most to failure statistics and most cable breakdowns occur within the first 3 years of operation. By contrast, for service-aged cable circuits that have been in service 30 years, the majority of failures occur within the cable insulation.

Statistics show that there is a chance of a failure every 42 km of cable circuit length. This requires dedicated condition-based maintenance for service-aged cables. However, there are still no widely-accepted methods and criteria to perform this. For example, in the case of offshore wind generation where long cable circuits are frequently used, severity and frequency of failures of export and inter-array cables is among the highest. The explanation for this probably lies in the fact that guidelines in regard to technical quality control of power cable circuits are based mainly on rules provided by cable manufacturers.

The key goal of an after-laying test is to control, on-site, the quality of the complete cable installation, which might consist of different cable sections, cable terminations and different types of cable accessories. Yet the standards dedicate less than a page of more than 100 in the complete document for after-laying testing of cable circuits after final installation. Moreover, despite the fact that new more sensitive and less destructive developments have come into use worldwide for many years now, proposed destructive test parameters by many cable manufacturers remain the same as 30 years ago.

Table 1: Example of Discrepancy in Recommendations as Given for After Laying Testing of 220 kV Cable

Looking at the example in Table 1, it is clear that from a technical point of view e.g. electrical stresses and over-voltage durations, all options are not consistent. Rather, they create a gap between after-laying test expectations and exclude the possibility to detect weak spots in a cable system (e.g. terminations, joints, cable sections) that might not experience breakdown during the test times given. As such, present international recommendations for on-site testing of power cables do not necessarily provide uniform and sufficient quality assurance for detection of insulation defects in power cables as necessary from the viewpoint of demand for power network reliability.

Fig. 1: Example overview of DAC field test possibilities for different testing goals of cable systems.

A typical after-laying test is based mostly on an overvoltage test (Pass or Fail the voltage withstand test) e.g. at 1.7 U0 for 1 hour. Still, it cannot be excluded that after e.g. 1 hour and 10 minutes, a failure will occur. Morover, in about 80% of cases, partial discharges are observed prior to insulation breakdowns. Application of the withstand test without monitoring partial discharges is therefore not sufficient to identify all relevant installation problems, particularly if there are a variety of test possibilities, as per Table 1.

Sensitive, calibrated partial discharge detection during on-site acceptance testing is needed. Moreover PD detection and localization in all types of accessories and in the cable insulation is also important. Indeed, documents describe the importance of selecting adequate methods for on-site testing and diagnostics (see Fig. 2).

Attend the 2022 INMR WORLD CONGRESS in Berlin where cable-testing expert, Dr. Edward Gulski will present the case that current international regulations for on-site quality control of power cables are deficient. He will then explain how best to achieve higher reliability and consequently lower downtime costs by utilizing more dedicated test methods that will lead to better quality control of newly installed onshore and offshore power cables.