Operating area for modular multilevel converter based high‐voltage direct current systems

Abstract

This study presents a 'seek' and 'check' approach to determine the operating area of the modular multilevel converter based high-voltage direct current (MMC-HVDC) transmission systems. First, the equivalent circuit for analysing the operating area of MMC-HVDC systems is derived and the operating area is represented by discrete operating points. Then, three operating constraints are proposed according to the MMC operation principles, i.e. the voltage, current and voltage stability constraints. Therefore, the determination of the operating area is transformed into checking whether the operating point meets the proposed constraints. The case studies are performed on a oneterminal MMC-HVDC system under six typical short circuit ratios. The results show that the top boundary of the operating area is limited by the voltage constraint, and the bottom boundary of the operating area is limited by the current constraint. The influence of the third harmonic injection and the ac impedance characteristic on the operating area is also studied. Lastly, the validity of the 'seek' and 'check' approach is proved by time-domain simulation results.

Minimum Short Circuit Ratio Requirement for MMC-HVDC Systems Based on Small-Signal Stability Analysis

“…where M, ω g and ϕ j are the modulation index [13], the fundamental angle frequency and the initial phase of arm average switching function. Note that, in normal operation conditions u nj ≈ u pj , u Ceq in (5) could be simplified as (8) by substituting (7) into it:…”

“…where M, ωg and φj are the modulation index [13], the fundamental angle frequency and the initial phase of arm average switching function.…”

“…When SCR is less than 1.95, the AC system equivalent voltage source is limited to the voltage safety limit (1.2 pu), and the decrease would cause the voltage drop at PCC. In consideration of the current constraint [13], the active power of rectifier would inevitably decrease. The relevant MMC-HVDC could operate stably under this circumstance.…”

Recent contributions and future prospects of the modular multilevel converters: A comprehensive review

“…MMCs produce an output voltage whose shape is close to sinusoidal voltage. As a result, MMCs need lower output filter requirement, particularly at high levels, in MMC‐based HVDC application . Typical network models of VSC‐ and MMC‐based HVDC system are illustrated in Figure A and B, respectively.…”

“…As a result, MMCs need lower output filter requirement, particularly at high levels, in MMC-based HVDC application. [156][157][158][159] Typical network models of VSC-and MMC-based HVDC system are illustrated in Figure 11A and 11B, respectively.…”