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For the allocation of unbalanced power when the loads change in the Multi-terminal HVDC transmission system, a DC voltage slope control strategy considering the Optimal Unbalanced Power Allocation (OUPA) is proposed. The control strategy using hierarchical control includes processing layer and implementation layer. In the processing layer, a mathematical model of unbalanced power allocation is established,...
This paper proposes a transformer-less modular multilevel DC/DC converter. The proposed converter can be used to interconnect two HVDC systems with different dc voltage levels and realize bidirectional power flow. Based on the arm energy balance condition, the optimal design arming at minimum inner circulating current is presented. A power close-loop control method is adopted to regulate the converter...
This study deals with impacts of the shunt conductances associated with HVDC cables and VSC-HVDC stations on optimal operation of Multi-Terminal HVDC (MT-HVDC). In this study, for the first time, shunt conductances are integrated to HVDC Optimal Power-Flow (OPF) program that is executed at the Power Dispatch Center (PDC) of the MT-HVDC grid. With the objective of losses minimization, optimal reference...
The modular multilevel converter (MMC) has become the most promising converter technology for high power applications. However, to get more number of levels in the output voltage, a conventional MMC requires large number of submodules. The main technical challenges of an MMC, with a large number of submodules, are the requirement of large number of semiconductor switches, submodule capacitors and...
DC grid technology is regarded as a promising solution for future electric networks integrating a great amount of renewable energies. It calls for high-efficiency bidirectional dc-dc converters with a high step-up ratio to interconnect mediumvoltage dc (MVDC) distribution grids and high-voltage dc (HVDC) transmission grids. This paper focuses on dynamic performance and fault-tolerant capability of...
In this paper, an asymmetric mixed MMC topology of multilevel converter for bipolar HVDC transmission systems is introduced. Among the existing MMC topologies for the bipolar HVDC systems, the asymmetric mixed MMC topology provides the benefits of: a lower number of submodule cells, fewer switching devices and lower conduction losses. Furthermore, the HVDC based on asymmetric mixed MMC can be flexibly...
Frequency and voltage regulation are required to support the grid under different scenarios. In order to handle these scenarios, classical control strategies demand complex systems to fulfill these requirements. However, synchronous controllers are trending strategies to contribute with frequency and voltage regulation functionalities without the requirement of external elements. Among these strategies,...
This paper presents the Enhanced Series Bridge Converter (ESBC), a hybrid modular multilevel converter with H-bridge front end suitable for high power grid applications. It retains the advantages of other modular multilevel topologies while offering compact structure, making it attractive for offshore stations, back-back HVDC stations, and city centre infeeds. The structure, operating principles and...
Nowadays, the Modular Multilevel Converter (MMC) based HVDC has been considered as a promising technology to meet the increasing electric power demand. However, constructing new transmission lines becomes more and more difficult because of environmental issues. Consequently, the newly constructed DC lines have to share the right of way with existing AC transmission in close proximity. Meanwhile, as...
This paper develops an alternate arm converter (AAC) high-voltage direct current (HVDC) terminal model that is compatible with the existing benchmark models of modular multilevel converter (MMC) based HVDC and dc-grid benchmark models. The developed model is a first step towards an AAC-based point-to-point transmission system and the future development of multi-converter, multiterminal benchmark models...
The negative- and zero-sequence currents of a modular multilevel converter (MMC) under imbalanced ac-grid conditions increase the maximum voltage of the submodule (SM) capacitors, which should be kept below a maximum voltage limit. This paper investigates the impact of offset pulse-width modulation (OPWM) on the capacitor voltage of the MMC under imbalanced ac-grid conditions. It is demonstrated that...
Three-channel controller of FB-MMC HVDC that enables four-quadrant operation on DC side and fault ride through on AC and DC sides without blocking IGBTs is proposed. Phasor modeling of FB MMC is studied to provide insight into independent AC and DC control. Three modulation indices are used: the conventional d-axis, q-axis AC-side modulation indices Md, Mq and a newly proposed DC modulation index...
A transformer-less Modular Multilevel DC-DC converter for High Voltage Direct Current (HVDC) applications is proposed in this paper. This topology is suitable for interfacing two HVDC links at different voltage levels using only conventional half bridge submodules. The uniqueness of the proposed converter is in its symmetry in performance in the event of reversal of polarity of one of the DC links...
This study proposed a novel protective action for converter side phase-to-ground fault of FB-MMC-based bipolar HVDC. The faulty converter is kept in operation during fault, and its dc-link voltage reference is set to zero in order to avoid serious overvoltage of the sub-module capacitors. This protective action was verified by simulation. The overcurrent and overvoltage of converter arms are moderate...
The hybrid high voltage direct current (Hybrid HVDC) system based on line commutated converter (LCC) and modular multilevel converter (MMC) is a feasible solution for long distance bulk power transmission. In order to handle the dc-side short-circuit fault, two kinds of methods can be adopted. The first one is to replace half-bridge sub-modules (HBSMs) with full-bridge SMs or clamp-double SMs which...
Hybrid modular multi-level converter (Hybrid MMC) topology, consisting of both half-bridge (HB) and full-bridge (FB) submodules, is suitable for HVDC application as it combines the advantages of both HB-MMC and FB-MMC. This paper focuses on the control aspects of hybrid MMC with the objective of limiting DC fault current. It is also controlled to exchange reactive power with the AC grid during DC...
High voltage direct current (HVDC) transmission systems play an increasingly important role in offshore wind energy and long distance energy transmission systems. Modular multilevel converter (MMC) is an attractive topology for HVDC systems, due to its good modularity, scalability and inherent fault tolerant capabilities. For such topology, model predictive control (MPC) is a promising alternative...
Distributed diode rectifier units have been proposed as a robust and cost efficient solution for the connection of large off-shore wind farms to HVDC links. However, since the diode rectifier platforms are connected in series with cables of varying distances, their dc-side voltage might not be the same in all diode rectifier platforms. This paper introduces a diode rectifier platform dc-voltage balancing...
In order to study the application of VSC-MTDC in hydropower transmission, make full use of the advantages of VSC-HVDC, established VSC-MTDC hydropower in transportion system model, the analysis of the mathematical model and control strategy. Finally, the three terminal system is simulated by PSCAD, verified the conservation of power system, and analyzes the influence of system of three-phase short...
The control of multiterminal HVDC systems based on local variables has shown a better performance, in terms of dynamical response and implementation simplicity, than constant DC voltage and droop control. However, power oscillations can be produced due to the interaction among different HVDC stations. In this paper, a modified control logic for this type of control is proposed in order to reduce the...
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