Since humidity and salt fog testing lasts for up to 1000 hours, most customers seek additional information on the accessory, apart from simply monitoring its surface condition. Therefore, while only detection of over current (flashover) is requested in the standard, today’s test systems also provide for long-term measurement of leakage current. From such data, any loss of hydrophobicity can be monitored and the stress on the cable termination estimated. With the latest test systems, it is also possible to save all relevant test data in the event of flashover, thereby providing the opportunity to carry out additional investigation into its cause (see example in Fig. 3). Moreover, using currently available technology, test bays can even be observed by remote access to the measuring system, allowing the test engineer to be kept fully informed by phone or e-mail.
While the pollution behaviour of MV terminations is tested as described above, no similar test procedure is as yet requested in the case of HV terminations equipped with non-ceramic housings. This stands in contrast to cable terminations using porcelain, which are required to be tested according to either IEC 60507 for AC or IEC 61245 for DC. Still, given the fact that non-ceramic housings are now increasingly common in HV terminations, there is clearly the need to test their surface behaviour as well. At the moment, the only applicable IEC test for the hollow core composite insulator is the tracking and erosion test according to IEC 62217/IEC 61462. Unfortunately, this is not a full-scale test and therefore does not account for the influence of length and field grading of the termination (as is the case of bushings). As such, the only information arising from this test is the quality of the cover material and design. Given this, some power utilities have started to issue their own test specifications to verify the performance of HV cable accessories under pollution.
Electrical Type Tests
A thermal cycling test under applied voltage is requested for MV and HV accessories with load current up to the maximum conductor temperature. Since conductor temperature must then be measured, a so-called dummy loop has to be used. The heating current of the test loop is regulated such that sheath temperature of the test loop is the same as for the dummy loop. As such, it can be concluded that conductor temperature has this same value. Fig. 4 shows a typical such test circuit for the electrical type test of HV cable systems as well as their accessories.
Proper set-up requires the possibility to adjust and measure voltage and current in the test loop as well as in the dummy loop. A typical thermal cycle then consists of:
1. A heating interval where the test loop conductor is heated to the maximum temperature;
2. A stabilization interval during which temperature is held close to this maximum; and
3. A cooling part with the heating current switched off.
A specific number of such cycles have to be carried out followed by whatever high voltage withstand tests and measurements are specified. Since the heating current is mostly inductive, the feeding current can be reduced by the use of capacitive compensation circuits. This way, the requested power is influenced mostly by losses in the cable and in the heating transformers.
Pre-qualification testing is similar to the electrical type test described above except that its duration is 8760 hours (equivalent to one year) and that it is performed under realistic installation conditions for the complete cable system, e.g. in natural soil, a concrete tunnel or a PE duct.
The relevant standard for the prequalification test requires a test loop length of at least 100 m. Because of this large size and the installation conditions to be simulated, these tests must be carried out in an outdoor test field. As defined in IEC 62067 for cable systems with rated voltage of 170 kV and higher, a total of 180 thermal load cycles at 1.7 times the rated conductor-to-earth voltage have to be applied over the course of the one year test period. Moreover, with edition 2.0 of IEC 60840, this test is now also recommended for systems from 36 kV to 170 kV in cases where the dielectric field strength is similar to that for cables with higher rated voltages, i.e. 8 kV/mm at the inner conductive layer. This means that manufacturers of such products will now also be required to provide evidence of performance. By contrast, this test is not required for complete systems (including accessories) in the MV range and only a long duration test of the cable itself is needed.