Oil filled cables at 132kV are paper insulated. During routine factory testing, following manufacture, Low Pressure Oil Filled (LPOF) cables are tested using ac. This requires some sizeable transformers. The ac test is used both as a withstand test and to establish the dielectric loss angle. This latter test gives an indication as to the presence of contaminants in the insulation, most notably moisture.
For site tests after laying, the standards dictate that an HV dc test is performed. There are two reasons for this, the first is that the dc test set is portable (i.e. you can get a set in a good sized car and assemble it on site) and secondly that DC voltage on paper cables has a reasonable chance of finding a serious defect (but it has to be serious). Paper cables use paper taped insulation, the papers are laid in such a way that the tapes have a gap between the edges of the same layer. This gap allows the cable to bend as the gap opens and closes; without the tape egdes butting into each other. These 'butt gaps', though small, are filled with oil which has a lower impulse strength than impregated paper. Paper cable insulation thicknesses are thus designed on impulse voltage strength rather than ac voltage strength. A paper cable will have more insulation than it needs for ac voltage strength alone and this has much to do with the longevity of paper cable designs.
The real question is why do you need ac site testing for polymeric cables? This is because faulty/damaged polymeric cables are remarkably resistant to high voltage dc withstand tests. There have been technical papers issued on this subject and ac testing is now the preferred standard method.
A dc oversheath (jacket) test is still the norm however.
