Analysis and Control of an MMC-HVDC System Under an Unbalanced Grid Condition
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ORIGINAL ARTICLE
Analysis and Control of an MMC‑HVDC System Under an Unbalanced Grid Condition Kim Tuan Luu1 · Minxiao Han1 · Senior Member, IEEE · Xiaojun Tang2 Received: 9 January 2020 / Revised: 11 June 2020 / Accepted: 17 September 2020 © The Korean Institute of Electrical Engineers 2020
Abstract Modular multilevel converter (MMC)-based high voltage direct current (HVDC) systems have been increasingly researched and installed in recent years. During operation, systems may be subjected to various unbalanced conditions, the single-line to ground (SLG) fault is one of the most common unbalanced faults. When a SLG fault occurs in a three-phase alternating current (AC) system, the double-line frequency positive- and zero-sequence harmonic components will be produced besides the double-line frequency negative-sequence harmonic component. And if the control system cannot suppress these harmonic components, they can propagate into the direct current (DC) side of the HVDC system and cause ripples in DC-line voltage and current as well as other unwanted consequences. Among the proposed controllers in the published papers on this topic, rarely controllers can both effectively suppress the circulating current and completely eliminate ripples in the DC-line voltage and current while still keeping the AC-side current balanced under unbalanced grid conditions. Therefore, this paper first analyses the propagation of harmonic components in a two-terminal MMC-HVDC system under a SLG fault. Then, based on the mathematical analysis, a proposed controller including an inner current controller for controlling the negativeand positive-sequence components and a controller for suppressing the zero-sequence component is designed to eliminate harmonic propagation from the faulty three-phase AC system to the DC side of the HVDC system, resulting in suppressing ripples in DC-line voltage and current. The theoretical analysis and proposed controller are verified by a simulation model in PSCAD platform. Keywords Harmonic propagation · Harmonic suppression · Modular multilevel converter (MMC) · MMC-HVDC system · Unbalanced grid conditions List of Symbols Idc DC-line current Udc DC-line voltage PCC Point of common coupling AC Alternating current DC Direct current ua,ub,uc AC input voltages ia, ib, ic AC input currents L0 Arm inductor R0 Arm resistor vua, vub, vuc Upper arm voltages vla, vlb, vlc Lower arm voltages
* Kim Tuan Luu [email protected] 1
School of Electrical and Electronic Engineering, North China Electric Power University, Beijing, China
China Electric Power Research Institute, Beijing, China
2
iua, iub, iuc Upper arm currents ila, ilb, ilc Lower arm currents idiffa, idiffb, idiffc Inner difference currents iza,izb, izc Circulating currents ea, eb, ec Inner electromotive forces (emf) ea_ref, eb_ref, ec_ref References of inner electromotive forces udiffa_ref, udiffb_ref, udiffc_ref References of inner difference voltages u+, u− Positive- and negative-sequence voltages of AC grid i+, i− Posit
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