DC partial discharge testing on transformers and bushings, OMICRON Energy Solutions
For at least the past two decades, high-voltage HVAC partial discharge (PD) measurements have typically been performed on power transformers and bushings to assess performance and detect potential insulation defects in the factory and in the field. However, the shift towards decentralizing energy supplies and the growing number of solar power generation stations, remote offshore wind farms, electric vehicle charging stations, and HVDC power transmission systems, have dramatically increased the development of DC power grids. This in turn has intensified the need for measuring PD under DC voltage on related electrical assets.
Measurement differences and challenges
As PD pulses are highly complex, the gathered PD data needs to be interpreted. When it comes to AC PD measurements performed simultaneously with three, six or more OMICRON MPD data acquisition devices connected to each phase (depending on the ratings of the transformer), the common method for PD evaluation is using phase-resolved PD (PRPD) patterns that show PD magnitude and quantity over the phase angle of the test voltage (φ-q-n). The PRPD diagram can be used to show filtered out pulses in real time while greying out the residual pulses in the background. PD analysis is further simplified by filtering tools, such as channel gating or the 3-Phase Amplitude Relation Diagram (3PARD).
However, no such phase information exists for DC PD measurements, which are primarily performed on a single phase, and the total number of PD events is significantly lower than AC PD measurements. Therefore, it becomes even more important to measure every pulse and assign the pulses to their sources. In principle, there are two essential parameters for measuring and detecting PD under DC voltage. These are the time difference (Δti) between two PD pulses and the apparent charge (qi) with its polarity information.