Independent-phase current control of a three-phase VSC under unbalanced operating conditions

Abstract

This study presents an independent phase-current-control architecture for a three-phase voltage source converter. It comprises three independent single-phase phase locked loops (PLLs), and a dual-loop scheme including an outer loop (responsible for active power and ac voltage control) and three similar inner current control loops. To obtain reliable phase-tracking, an enhanced second-order generalised integrator-based PLL is developed. The phase-tracking angle is strategically selected from the three PLLs to minimise the impact of imbalance-induced distortions. To validate the proposed control scheme, extensive simulations are conducted for a 101-level, 500-MW, 500-kV (dc) half-bridge modular multi-level converter in PSCAD/EMTDC. The simulation results prove that the proposed scheme enables the system to ride through unbalanced conditions effectively and reliably.

Keywords

• phase locked loops
• power convertors
• voltage control
• electric current control
• power control
• unbalanced operating conditions
• independent phase-current-control architecture
• three-phase voltage source converter
• PLLs
• dual-loop scheme
• ac voltage control
• second-order generalised integrator-based PLL
• phase-tracking angle
• active power control
• inner current control loops
• three-phase VSC
• single-phase phase locked loops
• imbalance-induced distortions
• half-bridge modular multilevel converter
• PSCAD/EMTDC
• power 500 MW
• voltage 500 kV