There has been continued growth of insulated cables as an alternative to overhead lines in spite of their much higher investment and life-cycle costs. An underground system provides greater safety and higher reliability to a network due to lower probability of faults and also allows extending the grid into areas where new overhead lines are not permitted for technical or environmental reasons.
International standards determine the test requirements to validate the ratings of a cable and its compliance with operating conditions based on network parameters. Whereas the type test sequence serves to validate the design of a cable as representative of a whole batch or production order, routine and sample tests apply to every length of cable produced in the factory. The need for accessories to lay extended lengths of cables and connect them to the grid is another key factor in reliability of a power system. These accessories are therefore also subject to a factory type and routine test program.
But even when compliance of cables and their accessories with standard requirements has been proven at the end of production, there is still a long way from factory to installation and final operation. This process includes critical phases such as storage, transport, handling, laying and assembly. These activities are mostly performed on-site and sometimes under adverse ambient conditions and are therefore susceptible to causing failures, which must be identified and solved before commissioning of a circuit. As such, so-called after-installation tests are critical to guarantee the safe and smooth operation of cables under the rated operating conditions to which they have been designed.
International standards for high voltage cables –IEC 60840, IEC 62067 provide an extensive portfolio of factory tests to validate the design of a cable and cable system as well as every single length of cable produced. However, the after-installation tests considered in the standards are limited to performing voltage withstand tests on the main insulation of the cable and sheath. Partial discharge measurements are recommended, particularly for the higher voltages of a transmission system, and can be carried out as a complementary technique by agreement between purchaser and manufacturer.
Departing from IEC standards, various country legislation and larger utilities might go further in required test sequence and include an extended portfolio of tests as part of their own standards and specifications. Off-line tests and verifications can include:
• Verification of the phase sequence;
• Measurement of resistance of the conductor;
• Measurement of resistance of the screen;
• DC withstand test of the sheath
• Measurement of capacitance of the main insulation;
• Measurement of power factor – tan delta – of the cable insulation;
• AC withstand test of the main insulation;
• Partial discharge measurements on accessories: joints and terminations;
• Verification of the ground system at link boxes: SVLs, contact resistance, etc.;
• Measurement of line impedance.
On-line tests can include:
• Partial discharge measurements on accessories: joints and terminations.
Attend the 2022 INMR WORLD CONGRESS in Berlin where cable testing expert, Aitor Kortajarena of Tecnalia Labs in Spain will offer recommendations for commissioning testing of high voltage insulated cables based on lessons learned during performance and interpretation of numerous such tests over 20 years.