Power Converter DC Link Ripple and Network Unbalance as Active Constraints in Distribution System Optimal Power Flow

TL;DR

This paper introduces a novel optimal power flow method that incorporates dc link ripple and network unbalance as active constraints, improving system design and stability analysis for multi-terminal converters.

Contribution

It derives a bilinear OPF constraint for dc link ripple in multi-terminal converters and demonstrates its importance through case studies and simulations.

Findings

Modeling dc link ripple affects capacitor and leg sizing decisions.

Tradeoffs exist between capacitor size and phase current unbalance mitigation.

Simulation results validate the algebraic formulation's accuracy.

Abstract

The mitigation of unbalanced grid voltages or currents by voltage source converters results in power ripple on the dc link, and is a key converter design parameter due to hardware or stability considerations. Despite the importance of this issue for system design and operation, the use of Optimal Power Flow (OPF)-based methods capturing the interaction between dc link ripple and converter unbalanced operation has been largely unexplored. In this work, the magnitude of the power ripple is derived for generic multi-terminal converters, then introduced as a bilinear OPF constraint for two-level converter topologies. OPF case studies demonstrate the necessity to model both neutral current and dc link ripple, with tradeoffs between capacitor sizing and leg sizing highlighted for phase current unbalance mitigation applications. Time domain simulations of a grid-connected four-wire voltage source converter verify the accuracy and validity of the algebraic formulation. It is concluded that awareness of dc link ripple impacts and constraints will be of growing importance for distribution system operators.