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We present a method for designing a stabilizing reference circulating current for modular multilevel converters. To this end, an optimization problem is formulated. This optimization problem takes into account conflicting control requirements such as reducing the oscillating component of the circulating current as well as the oscillating component of the arm voltages. Tracking of the obtained reference...
This paper focuses on the experimental implementation of local and primary controls for MT-HVDC in a real test bench (Pnom = 6 kW). A local control with two different time scales, one for the current and the other for the voltage, and primary control (droop control), which is responsible to vary the reference voltage when a disturbance appears in the network, are addressed in this paper.
With the rapid development of Voltage Source Converters and the recent progresses for DC fault management through DC breakers or inherent fault limiting converters, HVDC has become more and more attractive to grid operators and the possibility of a DC grid layer superimposed to the AC grid is gaining ground. Still, a lot of questions remain concerning the appropriate operation of such a grid. One...
This work presents a general approach for calculating the circulating current references of Modular Multilevel Converters (MMC) based on mathematical optimization by means of Lagrange Multipliers in the ABC frame. The resulting control method is especially suitable for operation of MMCs in multi-terminal HVDC systems, under both balanced and unbalanced AC grid voltages, since it is designed for eliminating...
The Multi-terminal HVdc systems is a key technology for bulk power transmission. The stability analysis of such power electronics-dominated power systems is a new challenge. This paper focuses on the analysis based on impedance representation of a modular multilevel converter. Therefore, a small signal input impedance modeling method is described for high and low frequencies. A comparative evaluation...
This work presents a generalized and versatile control approach for Modular Multilevel Converters using Lagrange Multipliers in the ABC frame. The methodology is capable of analytically obtaining desired operative conditions by calculating the differential current references previously established by the constraints in the optimization formulation, whilst obtaining the result with minimum I) differential...
This work presents a generalized and versatile approach for controlling Modular Multilevel Converters (MMC), with three or more phases, by means of Lagrange Multipliers in the ABC frame. The resulting control method is suitable for operation of HVDC converters under both balanced and unbalanced AC grid voltages, since it is designed for eliminating oscillations in the DC power flow independently of...
The Multi-terminal HVdc systems is a key technology for bulk power transmission. The stability analysis of such power electronics-dominated power systems is a new challenge. This paper focuses on the analysis based on impedance representation of a modular multilevel converter. Therefore, a small signal input impedance modeling method is described for high and low frequencies. A comparative evaluation...
This work presents a simple approach to eliminate DC power ripple in a three-phase Modular Multilevel Converter (MMC) while operating with balanced (load) currents during unbalanced grid faults. In this study, the MMC is connected to an AC grid on one of its terminals, and to a constant ideal DC source on the other, while a single phase fault takes place. The presented “first” approach is based on...
This paper presents a control strategy capable of eliminating the mean value of the energy differences between the upper and lower arms in a three-phase Modular Multilevel Converter (MMC) in Park Coordinates for HVDC applications. This is achieved by controlling the active and reactive differential currents using a rotating reference frame at fundamental frequency in order to eliminate the zero sequence...
In this work, a generalized optimization methodology based on Lagrange Multipliers was used to propose circulating current references in order to properly control the Modular Multilevel Converter (MMC) under different conditions. The optimization strategy finds the local minima of the objective function, which is typically the power losses of the MMC system, subject to equality constraints, which...
This work consists of the presentation of a regulation strategy capable of controlling the energy stored in the DC capacitors of the upper and lower arms of a Modular Multilevel Converter (MMC) for each phase. This is achieved by regulating the positive, negative and zero sequences in dqo coordinates of the differential current using a rotating reference frame at twice the fundamental value. Active...
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