Analysis and mitigation of excessive zero-sequence harmonic currents in distribution systems

The increased penetration of electronic loads in distribution systems results in more frequently occurring problems for the utilities due to triplen harmonics - a trend that has been confirmed in a recent utility survey conducted as part of DSTAR project 13–5. In this paper, we investigate an inexpensive mitigation option that has been recently applied successfully on an actual distribution feeder resulting in a substantial reduction of third harmonics in the neutral conductor. The mitigation option was converting the capacitor banks on the distribution feeder from ‘grounded wye’ to ‘floating wye’. To verify and explain the field results, the authors built a detailed model of the distribution feeder with the harmonic loads using a transient simulation software application. We successfully recreated the problem system in the simulation and also reproduced the effect of the applied mitigation option. Our simulation shows a resonance point near the third harmonic frequency, which caused the third harmonic problem. Converting the capacitor banks from ‘grounded wye’ to ‘floating wye’ causes a shift of this resonance point away from the third harmonic frequency towards a higher frequency, which explains why this mitigation option is successful in reducing third harmonic current in the system. We conclude that the ‘floating wye’ mitigation option is a viable option for mitigating third harmonic problems that are caused by resonance conditions.