Testing of Insulation Liquids for Subsea Power Converters - Partial Discharge and Breakdown Testing
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- Institutt for elkraftteknikk 
To realise the future subsea power systems, pressure compensated, liquid insulated power converters are subject to development. The liquid insulation provides a higher breakdown strength than air-insulated systems and works like a natural cooling agent. However, liquid contaminations can cause high local electric fields that could deteriorate the insulation. In order to avoid the influence of contaminations, electrode surface covering like Parylene can be applied. Sharp edges in electrode trenches at the electrodes in power electronic equipment give rise to high local electric fields. Electrical field values in the range of 〖10〗^8 V/m may initiate partial discharges (PD) or ultimately breakdown. PD activity and breakdown behaviour in high electric fields in such electrode trenches have been investigated by progressive breakdown testing of Printed Circuit Boards (PCBs). Measurements with both sinusoidal and fast turn-on switching voltage (100 ns rise time) have been performed with three different dielectric liquids. A mineral oil (Nytro 10XN), a synthetic oil (Midel 7131) and one fluorinated liquid (Galden HT230) have been tested to get a variety of liquid properties. The PCBs submerged in liquid are tested both with and without Parylene surface cover. The PD activity has been recorded in an attempt to determine the PD inception voltage (PDIV). PDs are detected by the use of electrical, optical and acoustic methods in a low-noise test setup. The setup had a peak-to-peak noise less than 2 pC. Test results show that PDs were detectable for sinusoidal voltage testing. Optical and acoustic PD detection methods should be immune to switching noise, however, results indicate that the transients at fast switching will influence them. When performing tests with positive polarity switching voltage, the PDs were not measurable in most cases because of the noise. In the commonly used transformer oil, Nytro 10XN, some optical and acoustic PD activity was recorded at approximately 20 kV during the fast switching testing before breakdown occurred. At negative switching polarity, PDs were more frequently observed, due to the effect of space charges. Comparison of sinusoidal and fast turn-on switching at low power-frequency of 38.5 Hz, was performed with Weibull statistical analysis. The fast switching showed at slightly higher breakdown voltage (BDV) for Nytro and Midel, with an increase from 21.8 to 23.1 kV (5 %) and 20.6 to 23.7 kV (15 %) respectively. Fast turn-on resulted in significantly higher BDV for Galden at 34.2 kV, which corresponds to an increase of 58 %. The reason for this increase can be that only homo charges can be generated at unipolar switching. Parylene cover increased the breakdown strength by 65 % compared to at same conditions without cover. The test voltage across the object was limited to 40 kV peak voltage that was not sufficient to break down all covered PCBs at fast switching. This high breakdown strength is a promising sign for the objective of insulating subsea converters, which will require high reliability.