The application of heating cycles to a cable system installed in an outdoor test field is far more difficult than performing a type test in an indoor HV test laboratory. For example, the ambient temperatures of systems can differ by more than 40°K over the course of the one year test period and this has to be taken into account in terms of feeding power of the heating circuits. SCR-controlled heating circuits are therefore necessary to ensure the heating cycle will follow the defined temperature curve exactly. This type of system takes into account the maximum heating current allowed as well as the conductor and sheath temperatures of the test and dummy loops and applies precisely the current needed to follow a pre-defined temperature curve. Even higher dielectric losses of the test loop on which voltage is applied have to be taken into consideration using an offset and any deviation between target and real conductor temperature must be kept to less than 0.2°K. The resulting curves are then shaped perfectly and the number of invalid cycles significantly reduced. Fig. 5 shows an example of such a heating cycle.
As shown, there is an offset between the heating currents of the test loop and of the dummy loop and this offset is necessary to get the sheath temperatures of both loops as close together as possible. Even though the dummy loop conductor temperature may appear artificial, it is in fact a real measured temperature curve. This curve consists of several basic points connected by straight lines. The position of these points are stored as constants within the program for the SCR controller but can also be changed manually. The controller regulates the heating currents so as to follow the straight lines precisely. This, of course, is not possible manually if an operator is not constantly observing measured values during the heating period. Manual adjustment of the curve parameters can be used to minimize energy consumption of the heating system.
Introduction of pre-qualification tests to lower voltage levels has required additional testing capacity and today large laboratories offer test bays for several test loops, in some cases up to a test voltage of as high as 800 kV. A growing request in recent years when it comes to testing is the possibility to observe test loop behaviour on a real time basis as the test progresses. Upon request, some facilities offer remote viewing access to the measuring system via the internet. This allows not only the client but also test engineers to access data from the systems, in the latter case with writing capabilities to the equipment as well so as to permit process parameters to be adjusted if necessary without the need to be physically present. Further evidence of progress in this field is the possibility to test the whole test loop with lightning impulse upon conclusion of the cycles. In the past, the standard was met simply by choosing the most stressed part of the cable and cutting out a section of 30 meters to test. With mobile impulse generators, however, it is now possible to place the generator close to the test loop and carry out the lightning impulse test on the whole loop, including accessories, without any need to cut or move the cable. While this type of test procedure is not required by the standards, it increases confidence in test results.
On Site Tests
The performance of any cable system depends greatly on the quality of installation of its various accessories. Since the cable itself is routine-tested at the factory, the probability of it failing is very low. Accessories, by contrast, are assembled on site, often employing a range of different materials. Final system quality, therefore, can be assured only by a clean installation with no damage to any of its components. To ensure that no serious defect has occurred during installation, on site commissioning tests can identify any possible weaknesses. For example, modern equipment now allows measuring partial discharges in the field and this means that the likelihood of detecting installation problems has increased significantly. Since the capacity of an installed cable system is relatively high, resonant test transformers are the preferred equipment for AC tests. Test frequencies vary between 20 and 300 Hz while test current is in the range from 80 to 240 A. The photo below illustrates an example of such on-site measurement that involves mobile test platforms that guarantee high availability and reduced transport costs.