Enhancing the Ride-Through Capability of DC-Link Voltage in NPC Multilevel Unified Power-Flow Controllers

Multilevel converters are attractive for unified power-flow controller (UPFC) applications, due to their high-voltage and power capability. Among multilevel topologies, the three-level neutral-point-clamped (NPC) converter allows back-to-back connection as the UPFC shunt and series converters. Besides the pulsewidth-modulated (PWM) multilevel control schemes, UPFCs require constant dc-link voltage and balanced voltages in the NPC multilevel dc capacitors. This paper proposes three main contributions to increase the dc-link voltage steadiness of multilevel UPFCs under line faults: 1) decoupled active and reactive linear power controllers; 2) real-time PWM generation; and 3) double balancing of dc capacitor voltages. A case study using part of the Portuguese transmission network is presented. The results show the effectiveness of the real-time PWM generation and dc-link capacitor voltages balancing included in NPC series and shunt converters to keep the dc-link voltage steadiness under line faults, overall enhancing the UPFC ride-through capability.

I. Introduction

Environmental and economical constraints affect the building of electric power stations and transmission lines, while power demand continues to grow. Thus, increasing power controllability and optimize existing power system capacities by suitable control of the power flow (PF) during steady state as well as in transient conditions is needed. The flexible ac transmission systems (FACTS) concept [1], using advanced solid-state converters, namely, the unified power-flow controller (UPFC), can provide the necessary PF optimization in transmission lines while improving power oscillation damping [2]–[4].