Search

The parallel-hybrid converter (PHC) based high-voltage dc (HVdc) plant (PHC-HVdc) mandates the injection of additional harmonic-voltage components into its chain-links (CLs) to regulate its output voltage/modulation index (MI). However, these harmonic voltages are restricted to have limited magnitude and their zero crossings must coincide with the respective output voltages of converter. Hence, the operating range of MI for the PHC-HVdc has narrow limits. This article explores the reactive power compensating capabilities of the PHC based static compensator (PHC-STATCOM), which is relieved from the fixed dc source and is free to operate at any MI without requiring any additional efforts. Two degrees of freedom inherent with PHC-STATCOM are discovered, which proves that PHC is utilized to its maximum when applied for reactive compensation. The capacitor voltages of the PHC-STATCOM belonging to different CLs divert from desired value under unbalanced grid/load conditions. By the virtue of degrees of freedom associated with PHC-STATCOM, a capacitor voltage control technique is proposed using fundamental-frequency zero-sequence voltage (FFZSV). The generalized expression of the reference FFZSV is derived, which regulates the capacitor voltages. The loci planes indicating the maximum range of load unbalance that the PHC-STATCOM can compensate with specific grid voltage conditions is obtained and plotted. The working principles and proposed capacitor control are verified using experimental studies. A detailed comparative study is presented to substantiate the advantages of the proposed topology. [ABSTRACT FROM AUTHOR]

Recently, the idea of combining both thyristors and IGBTs in a voltage-source converter (VSC) structure has gained significant research interests for HVDC applications. The main objectives of deriving these topologies are to achieve forced-commutation of thyristors, increased efficiency, reduced energy-storage requirement and dc-fault tolerance besides retaining the VSC control flexibility. A novel topology named ‘thyristor-augmented modular bridge converter’ (TAMBC) is proposed in this paper. Each TAMBC phase contains four chain-links comprising of series-connected stacks. Each stack contains a parallel connection of an IGBT-based submodule-branch and a thyrsitor-branch. A control technique is developed that allows the main current to flow primarily through the thyrsitor-branch to achieve higher efficiency. In each phase of TAMBC, two chain-links generate the required ac voltage in the first half-cycle and are bypassed through thyrsitor-branch during the next half-cycle. Consequently, it enables current distribution among the chain-links resulting in reduced capacitors size and enhanced efficiency. The ability of TAMBC-HVDC systems to function over a wide operating range, to ride-through ac faults and to provide independent active and reactive power control are verified using PSCAD. Capacitors design, loss analysis and comparative evaluations are carried out to highlight the key features of the TAMBC over the existing ones. [ABSTRACT FROM AUTHOR]

The modular multilevel converter (MMC) is the most preferred choice for high-voltage direct current (HVdc) applications. In this article, a new hybrid-legs bridge converter (HLBC) is presented as an alternative to the MMC. This converter is a hybrid of a two-level converter and an MMC. For a three-phase system, three single-phase HLBCs are connected either in series [series-connected HLBC (SHLBC)] or in parallel [parallel-connected HLBC (PHLBC)] across the dc-link. Compared to the MMC and the series-connected MMC, the SHLBC requires only one-third and half the submodules (SMs), respectively. Besides, the SHLBC eliminates the requirement of any arm inductor or dc-side filter. The structure and operating principle of the HLBC are described. Moreover, the minimum SM capacitance required to limit its voltage fluctuation is calculated. The dc fault-tolerant capability of the SHLBC and PHLBC is achieved by replacing some of the unipolar SMs with bipolar SMs. The operation, applicability for HVdc systems, ac fault ride through and dc fault-tolerant capability of the proposed converter are verified using PSCAD simulations. Furthermore, the feasibility of the proposed converter under normal and dc fault conditions is revalidated experimentally by using a three-phase grid-connected HLBC laboratory prototype. A comparative analysis is carried out to highlight the benefits of the proposed HLBC over the existing converter topologies. [ABSTRACT FROM AUTHOR]

The modular multilevel converter (MMC) is the most preferred voltage source converter high voltage direct current (VSC-HVdc) topology. However, its limitations such as, large energy storage requirements and a large number of devices and capacitors required in its chain-links (CLs), have led to the development of some hybrid multilevel converter topologies. These topologies aim to address the major technical constraints of MMC. The parallel hybrid converter (PHC) is such a hybrid topology, which has evolved as a promising alternative for high power applications because its CLs are not subjected to the fundamental component of ac current and hence the size of its submodule capacitors is substantially lower. Furthermore, for the same power rating, the device count in PHC is only a fraction of that in MMC. However, the PHC suffers from some operational constraints. The dc and ac sides of PHC are strongly coupled, which leads to a single stable operating point resulting in a fixed output voltage if a dedicated controller is not used to tackle this issue. Moreover, the PHC loses control during dc-side faults and it also necessitates a bulky dc filter inductor to filter the dc-side harmonics. Various advanced control methods and derived PHC topologies have been proposed in literature to eradicate these shortcomings of the PHC. This article presents a detailed analysis and review of these state-of-art control strategies and derived PHC topologies. The capability of the PHC and the derived topologies to ride through ac and dc faults are also discussed. This article contributes in summarizing key benefits and limitations of various existing control methods through extensive analyses, simulation, and experimental studies. [ABSTRACT FROM AUTHOR]

Hybrid multilevel converters using bidirectional thyristors and IGBTs have gained significant interest in recent times, as they possess important features of both voltage-source and line-commutated converter technologies. This article presents a classification of hybrid multilevel converters for HVdc applications. The methodologies to derive, evaluate, and select these converter topologies are proposed as well. The hybrid converters are classified from the power device to the system configuration based on the analogy of the voltage-source converter and the current-source converter. Derivation methods of hybrid cell, cluster, arm, phase, and converter, based on the topology tree presented in this article, are applied to derive both existing and new hybrid converters. Comparisons show that hybrid converters have functionality similar to either the modular multilevel converter or the line-commutated converter, depending on the ratio between thyristor and active switch IGBT counts employed. Several hybrid-station configurations are discussed in order to suggest the most suitable converter topology for each configuration. The presented analysis can provide a point of reference and a useful framework for the future developments of hybrid multilevel converters for HVdc applications. [ABSTRACT FROM AUTHOR]

The parallel hybrid converter (PHC) offers compact and highly efficient converter solution for high-power applications. One of the major drawbacks of PHC is the requirement of a bulky dc-side inductor to minimize the dc ripples. Modified third-harmonic injection method and zero-sequence voltage injection method can minimize these ripples. However, their complete mitigation is not possible with these existing methods. In this letter, a new control scheme is proposed, which eliminates the dc-side voltage/current ripple by injecting a suitable common-mode voltage into the chain-link of PHC. The presented method also increases the ac output voltage for a given dc-link voltage and effective utilization of the available dc voltage is achieved. Hence, an enhanced-PHC (EPHC) topology is achieved. The analyses presented are verified using PSCAD/EMTDC. A 25-level EPHC–HVdc system is also developed to substantiate the applicability of the proposed method for HVdc system. Experimental studies are performed using a three-phase 13-level laboratory prototype in order to verify the simulation outcomes and the findings acquired are observed to be in agreement with the simulation studies. [ABSTRACT FROM AUTHOR]

Modular multilevel converter containing half-bridge submodules (SMs) is the most preferred topology for high voltage dc (HVDC) applications. However, the inability to block/limit dc fault current and large number of capacitors requirement are some of its limitations. Recently, hybrid topologies utilizing thyristors for such applications have gained significant interest as they offer lower on-state voltage drop, higher reliability, and power handling capability. In this article, the modular embedded thyristor directed converter (METDC) is proposed, which uses series-connected thyristors as the high voltage switches and chain-links comprising of IGBT based half and full-bridge SMs. A control technique is proposed for force-commutation of thyristors by using full-bridge SMs for lagging loads, while they are naturally commutated for leading loads. The METDC offers wide operating range besides being fault tolerant. The results obtained from the METDC-HVDC model developed in PSCAD substantiate its ability to provide independent active and reactive power control required for HVDC application. The fault studies carried out validate the dc fault tolerant capability of METDC. The results obtained from the various ac fault/unbalanced case studies substantiate the efficacy of proposed controller to force-commutate the thyristors even during contingencies. Loss computation validates the ability of converter to provide higher efficiency. [ABSTRACT FROM AUTHOR]

An H-bridge hybrid modular converter (HBHMC) is proposed for high-voltage direct current (HVDC) applications. It uses a wave-shaping circuit consisting of series-connected full-bridge submodules (FBSMs) at the output of the main H-bridge converter. For a three-phase system, three HBHMCs are connected either in series (series-HBHMC) or in parallel (parallel-HBHMC) across the dc-link. The operating modes of HBHMC, novel modulation strategies for voltage balancing of FBSMs, and control of HBHMC-based HVDC system are presented in this paper. A detailed comparison between HBHMC and other hybrid topologies is performed on the basis of required number of switches and capacitors. The HBHMC has the features of dc fault blocking capability, lower footprint structure, and extra degree of freedom for submodules capacitor voltage balancing. The efficacy of the HBHMC-based HVDC system for three-phase balanced and unbalanced grid conditions and its fault-tolerant capability are validated using PSCAD simulation studies. Furthermore, the feasibility of the proposed converter under normal and dc fault conditions and of the proposed capacitor voltage control scheme is validated experimentally by using a three-phase grid-connected HBHMC laboratory prototype. The results demonstrate the effectiveness of the proposed HBHMC topology, control techniques, and satisfactory responses of the HBHMC-based HVDC system. [ABSTRACT FROM AUTHOR]

Multilevel current source converters are a smart choice for the high-power applications with low-voltage and high-current requirements. This paper presents the detailed analysis of a current source modular multilevel converter (CSMMC) and its application as a STATCOM. Various properties of CSMMC are compared with the voltage-source modular multilevel converter (VSMMC). The submodule inductance selection method and a sorting-based algorithm for inductor current balancing are proposed for CSMMC. The validity of the inductance selection method and the inductor current balancing algorithm are confirmed by the simulation of a three-phase CSMMC using PSCAD/EMTDC. The carrier phase-shifted sinusoidal pulsewidth modulation is used as the switching technique. The performance of a converter is analyzed with interleaved and noninterleaved carrier signals for the upper and lower arms. Results show that CSMMCs and corresponding VSMMCs have analogous features. Hence, the development on VSMMCs can be used for reference to CSMMCs and vice-versa. Furthermore, the control designs for CSMMC-based STATCOM are presented and verified using simulations. The obtained results of the proposed STATCOM show that the steady-state and transient performance of the system are quite satisfactory. This makes the CSMMC suitable for HVDC and flexible ac transmission systems applications. [ABSTRACT FROM PUBLISHER]