PFC (Part 2): How Current Harmonics Cause Distortion And The Role of the Delta-Wye Transformer

Focus:

The first part of this two-part article reviewed the evolution of power factor correction (PFC) requirements and discussed the IEC 61000-3-2 PFC standard. Here in part 2, the relationship between line frequency harmonics and distortion is analyzed, and the authors explain how the delta-wye transformer corrects this distortion on the power grid. The article begins by expressing power in terms of a dc current term plus a Fourier series of ac current terms, representing the current fundamental and its harmonics. From this wattage expression, various expressions for THD are derived and terms such as displacement factor, distortion factor, and power factor are defined. Then a graphical analysis is used to explain how a delta-to-wye transformer removes current harmonics from the power grid. In the last section, the operation of a delta-wye transformer connected to a motor drive is simulated in LT Spice.

What you’ll learn:

• How to understand the relationship between line frequency harmonics and THD as these terms apply to power factor correction
• How to understand the operation of a delta-wye transformer in canceling line frequency harmonics

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Author & Publication:

Kevin Parmenter, Chair, and James Spangler, Co-chair, PSMA Safety and Compliance Committee , How2Power Today, Mar 13 2020

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