Impact of high-frequency harmonics (0–9 kHz) generated by grid-connected inverters on distribution transformers

The level of current harmonics circulating in a transformer winding can affect its operating temperature and lifetime. Although the existing standards mainly consider the impact of harmonics up to 2 kHz, higher frequency harmonics generated by high power converters utilized in renewable energy sources can also contribute to the temperature rise of a transformer. Pulse Width Modulated (PWM) voltage generated by power converters can generate significant high-frequency harmonics at its switching frequency. The switching frequency of converters in high-power applications is mainly between 2 and 9 kHz. Consequently, in this article, the importance of also using higher frequency harmonics in calculating transformer core loss and temperature rise is investigated as per IEEE standard conclusions. For that purpose, transformer K-factor values have been calculated in simulated and practical distribution systems, in both long-term and transient conditions. The results confirm the importance of considering higher-order current harmonics (more than 2 kHz) when calculating K-factor and estimating transformer temperature rise and lifetime