Cables and their accessories are an integral part of power networks and, whenever there is a transition to them from either overhead lines or busbars, the ends are sealed using terminations. Terminations are of different construction depending on whether the application is medium or high voltage. In both cases, however, care must be taken in regard to pollution withstand since they are an interface with the service environment. While a pollution test has already been standardized for MV terminations, no similar standard yet exists for HV terminations. This INMR article from 2013, contributed by Heiko Jahn and Wolfgang Manzke of CESI (FGH Engineering & Test) in Germany, reviewed existing standards as well as some of the important factors when it comes to testing terminations for pollution performance.
Fig. 1 provides an overview of the various IEC and CENELEC standards used for testing medium and high voltage cable accessories. It is worthwhile noting that this assortment of standards only defines minimum requirements to satisfy the quality demands of power utilities. Some users have chosen to issue their own test specifications, based on these standards but with increased stresses placed on the accessories.
For MV terminations, a test with respect to pollution behaviour has been standardized (Clause 13 of IEC 61442). Depending on application, this test is either:
• a humidity test for indoor terminations; or
• a salt fog test in the case of outdoor terminations.
MV terminations are directly installed onto cable ends, either with all components integrated into a single piece or with the individual elements (i.e. field grading, heat-shrinkable tube, weathersheds) installed separately. Due to their relatively limited size, the weight of medium voltage terminations is typically low enough that no additional mechanical support is required.
With increasing nominal voltage, terminations become larger and therefore installation is often made in discrete steps with the various elements installed individually. Housings can be either hollow porcelains or, as increasingly being used, hollow composite insulators with different silicone rubber material.
For glass and porcelain insulators, pollution tests have been standardized in IEC 60507 for AC and IEC/TS 61245 for DC. In both cases, however, the scope of the standards does not include terminations as the test object. While a cable termination can basically be regarded as a bushing, meaning that both applicable standards can apply, care has to be taken to avoid internal breakdown.
Composite insulators are excluded from the scope of the standards referred to above since the test procedures were developed for hydrophilic surfaces and hydrophobic silicone results in certain unwanted effects. As a result, the only standardized test procedure for hollow composite insulators is the tracking and erosion test according to IEC 62217, which is really a test of the housing material and not of pollution performance. Therefore, when it comes to HV terminations equipped with composite insulators, either a test procedure has to be agreed upon between manufacturer and user or some specification from the purchaser must be made available. Tests that have been carried out in this regard are described below:
Humidity & Salt Fog Tests on MV Terminations
To ensure appropriate behavior, a humidity test was introduced for indoor cable terminations and a salt fog test for outdoor terminations. The main differences lie in test duration and conductivity of the saline water. The relevant humidity and salt fog test conditions are defined in IEC 61442 (see Table 1).
A corrosion-proof test chamber with inclined roof is essential to avoid any influence from water that might drip down onto the test object. Moreover, the circuit feeding the test has to provide low impedance to ensure only a small voltage drop at high test currents. When testing 3-phase cable accessories with spreader caps, the voltage must also be supplied 3-phase so as to correctly test the whole termination system.
Medium voltage terminations are often installed with significant mechanical strain on the cable. As such, in cases where tracking affects the edge of the termination or erosion becomes too deep, the material may rupture and there will be moisture ingress. Eventually, the termination may fail.
The various conditions specified can sometimes require significant time and effort to build up the test circuit. Moreover, since such a test lasts up to 1000 hours, most customers require that it yield extended information beyond simply ‘pass or fail’. For example, while only over current detection is required by the standard, today’s test systems also provide for long-term measurement of leakage current. From such a curve, it is possible to establish if there has been a loss of hydrophobicity; the stress on the cable termination can also be estimated. With today’s systems, an oscillogram is available should any over current situation occur (see Fig. 2). This allows the possibility to carry out additional investigations should there be failure.