After-installation testing of very long HVAC submarine cables
According to IEC 62067, after-installation testing of HVAC land cables requires sinusoidal AC test voltages in the frequency range 20 – 300 Hz. Mobile resonant HVAC voltage test system proved as suited equipment to carry out after-installation tests of HVAC extruded cable systems. Due to weight limits in street transport, the test power of a single mobile resonant test system is limited, e.g. to 260kV, 80A. Depending on the actual AC test voltage level and on the capacitance of one phase of a three-phase HVAC cable system, the maximum single-phase cable length that can be tested with one mobile resonant test system is consequently limited as well. To overcome this limitation for ever increasing length of cable, resonant test system are put in parallel or – for very high test voltage levels – in mixed parallel/serial connection.
With large offshore wind farm projects, the demand for suited submarine export cables grew significantly. In case of very long length to shore, HVDC is the only possible solution. But high costs of HVDC converter stations (especially offshore) led in some projects to very long HVAC export cable connections as an alternative. Current projects in the baltic sea include HVAC cables 220kV 100km long. With 1.7 Uo test voltage (220kV line to ground), the capacitive current at 20Hz could exceed the current carrying capability of the lead sheath. With mobile resonant testing, the capacitive current can only be reduced by either lowering the test voltage or by lowering the test frequency. 10Hz instead of 20Hz (according to IEC 60060-3) would reduce the current by factor of 2 but requires special (new) resonant reactors suited for such low resonant frequency at the desired voltage. HV VLF test voltages could serve as another alternative.
This paper deals with investigations on HV VLF testing to check its suitability as an alternative after-installation test voltage for very long length of extruded HVAC cables. Since the segments of these cable systems are tested at the factory, the biggest threat arises from poor workmanship during the jointing process of the segments. Therefore XLPE-insulated MV cables with artificial defects in joints were used as downscaled test objects for 220kV submarine cable systems. VLF voltages with different waveforms (sinusoidal and cos-rectangular) were compared with AC 50 Hz (sinusoidal) as the reference. Accompanying measurements of partial discharges (PD) were carried out to improve the informational value of HV VLF tests. Currently, subsea PD measurements on submarine cable joints appear to be very difficult, especially for very long cables. Some ideas to overcome these difficulties will be investigated in the future of this research.