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Thailand is located in the tropical zone dominated by monsoons for more than six months in a year. These climatic conditions illustrate elevated relative humidity and temperature that contributes to Potential Induced Degradation (PID). Humidity has been reported to be potential cause of degradation in the systems with high voltages. Moisture helps leakage currents in passing through laminate permeation and back sheet from cell to ground frame. The PID occure in PV systems when connected in series using DC voltages of high orders. This research shows comparisons of both PV systems, with negative grounding and floating grounding with a system voltage about 800 Vdc. Investigations of insulation resistance (MΩ) in PV strings and modules after two years operation has been done. It was found that the decrement in average insulation resistance (MΩ) of PV string using negative grounding, floating grounding was 14.5 MΩ, 11.0 MΩ and average leakage current per string was 68.9 µA, 90.9 µA respectively. Moreover, the wet leakage current per module showed the decrement in average insulation resistance (MΩ.m2) was 174 MΩ.m2, 133 MΩ.m2 and average leakage current per module was 5.75 µA, 7.51 µA respectively.

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