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.