Circulating Current Suppression for MMC-HVDC under Unbalanced Grid Conditions

Wang, Jinyu; Liang, Jun; Wang, Chengfu; Dong, Xiaoming

doi:10.1109/tia.2017.2675992

Cited by 90 publications

(34 citation statements)

References 30 publications

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“…Since the analysis and control of MMCs is currently being widely studied in the scientific community, several different methods have been proposed for controlling the circulating currents when operating under unbalanced grid voltage conditions [14]- [16], [18]- [20], [22]- [25]. However, many of the these proposed control strategies rely on a Circulating Current Suppression Controller (CCSC) and/or other control loops implemented in a synchronously rotating reference frame.…”

Section: Context and Contributionsmentioning

confidence: 99%

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Optimal Shaping of the MMC Circulating Currents for Preventing AC-Side Power Oscillations From Propagating Into HVdc Grids

Bergna-Diaz

Suul

Berne

et al. 2019

IEEE J. Emerg. Sel. Topics Power Electron.

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A constrained optimization problem based on the Lagrange multipliers method is formulated to derive the circulating current references of Modular Multilevel Converters (MMC) directly in abc coordinates. The resulting analytic expressions for calculating the circulating current reference signals are designed to eliminate oscillations in the dc-side power flow, independently from the ac-side operation of the MMC. As a result of the constrained optimization, the circulating currents are shaped to optimally utilize the degrees of freedom provided by the internal energy buffering capacity of the MMC, to effectively decouple the ac grid conditions from the dc bus. This property of the proposed control method makes it especially suitable for preventing oscillations due to unbalanced ac grid voltage conditions from propagating into multi-terminal HVDC systems. It is shown that the power flow at the dc-side of the MMC will be most effectively decoupled from ac-side transients if the desired steady-state power flow is imposed by acting directly on the circulating current references instead of by acting on the acside current references. The operation of an MMC controlled by the proposed approach is demonstrated by simulation studies, verifying the ability of keeping the dc power flow free of second harmonic oscillations, independently of the power control objectives applied for calculating the ac-side current references of the converter.

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Section: Context and Contributionsmentioning

confidence: 99%

“…Thus, the MMC can be controlled to act as a "power oscillation firewall" or an "energy buffer" during unbalanced faults, preventing power oscillations on the ac side from propagating into the dc system. This can be achieved even if the ac-side currents are kept balanced during unbalanced conditions [14]- [20].…”

Section: Introductionmentioning

confidence: 99%

Optimal Shaping of the MMC Circulating Currents for Preventing AC-Side Power Oscillations From Propagating Into HVdc Grids

Bergna-Diaz

Suul

Berne

et al. 2019

IEEE J. Emerg. Sel. Topics Power Electron.

View full text Add to dashboard Cite

show abstract

“…Some publications have adopted the use of a dedicated controller to ensure horizontal balancing by forcing the dc link current to be equally split among the three legs of the three-phase MMC. Such enforced equalization of the common-mode dc current of the three legs can lead to unnecessary curtailment of the dc or active power exchanged between the dc and ac sides during unbalanced and asymmetric ac faults [4], [32], [38]. Although this approach performs well during normal operation, it exhibits unsatisfactory performance during operation with unbalanced ac voltages and asymmetric ac faults (as equalization of dc current per leg could be an unachievable control objective in some situations).…”

Section: Comprehensive Analysis Of Internal Arm-level Characterismentioning

confidence: 99%

“…Also, MMC capacitor voltages affect the synthesis of common and differential mode arm voltages that create common and differential mode currents respectively. Normally, the common-mode current within the MMC leg consists of dc and even harmonic currents, and its dc component helps transfer power between the dc side and the MMC arms, while the ac component (usually referred to as circulating current) increases capacitor voltage ripple and semiconductor losses [4]- [7]. Many control strategies have been developed to suppress the circulating current and capacitor voltage ripple, and these strategies broadly exploit injection of appropriate harmonics into the common-mode voltage or arm currents [8]- [12].…”

Section: Introductionmentioning

confidence: 99%

Analysis and Assessment of Modular Multilevel Converter Internal Control Schemes

Wang

Adam

Massoud

et al. 2020

IEEE J. Emerg. Sel. Topics Power Electron.

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Adoption of distributed submodule (SM) capacitors in a modular multilevel converter (MMC) necessitates complex controllers to ensure the stability of its internal dynamics. This paper presents comprehensive analysis and assessment of different proportional resonant (PR)-based control schemes proposed to stabilize the internal dynamics and ensure ac and dc sides power quality of the MMC within a dc transmission system. With the consideration of passive component tolerances, different energy and voltage based control schemes under various conditions are analyzed. It has been established that without vertical voltage balance control, unequal passive component values in the upper and lower arms of the same phase-leg may cause: unbalanced fundamental currents in the arms, unequal dc voltage across the arms, and fundamental oscillations in the common-mode currents that lead to fundamental frequency ripple in the dc link current. The theoretical analysis that explains this mechanism is presented, and is used to show that vertical voltage balancing is necessary for the nullification of arm voltage difference and suppression of odd oscillations caused by capacitive/inductive asymmetry between arms of the same phase-leg. Simulations support the theoretical analysis and the effectiveness of voltage balancing in ensuring correct operation, independent of tolerances of the MMC passive elements and operating conditions. A new direct method for elimination of fundamental oscillations in the common-mode and dc link current is proposed. Experimental results from a singlephase MMC prototype validate the presented theoretical discussions and simulations.

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“…This has led to the MMCC [5,6], which provides superior features of modularity, scalability, low harmonic contents in voltage waveforms at low switching frequencies. The MMCC has gained huge attention because of its vast industrial application in grid-connected converters [7], static synchronous compensators [8,9,10], unified power flow controllers [11,12,13] and HVDC transmission system [14,15,16].…”

Section: Introductionmentioning

confidence: 99%

Carrier-based sinusoidal pulse-width modulation techniques for flying capacitor modular multi-level cascaded converter

Oghorada

Zhang

Esan

et al. 2019

Heliyon

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Carrier-based sinusoidal pulse-width modulation (PWM) techniques, such as phase disposoed PWM(PD-PWM) and phase shifted PWM (PS-PWM), are widely applied to control the modular multilevel cascaded converters (MMCC) having full H-bridge as sub-modules. This paper evaluates these PWM techniques when controlling a variant of the H-bridge MMCC, i.e. the MMCC five-level flying capacitor converter as sub-modules. This MMCC poses an extra challenge to PWM schemes; namely maintaining two inner floating capacitor voltage balancing. Two novel PWM techniques known as the swapped carrier PWM techniques are introduced for the control of this converter. The paper compares them with the two conventional ones using a performance metrics composed of voltage waveform performance, capability in natural flying capacitor voltage balancing, converter power loss, and switch utilisation. The results show that the proposed new PWM schemes outperform both conventional methods in both switching and conduction power losses and achieve similar performance like the PS-PWM under the three other metrics.

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Circulating Current Suppression for MMC-HVDC under Unbalanced Grid Conditions

Cited by 90 publications

References 30 publications

Optimal Shaping of the MMC Circulating Currents for Preventing AC-Side Power Oscillations From Propagating Into HVdc Grids

Optimal Shaping of the MMC Circulating Currents for Preventing AC-Side Power Oscillations From Propagating Into HVdc Grids

Analysis and Assessment of Modular Multilevel Converter Internal Control Schemes

Carrier-based sinusoidal pulse-width modulation techniques for flying capacitor modular multi-level cascaded converter

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