Alternate Arm Converter Research Articles

Optimization of Switching Control and Microgrid Energy Management System with Alternate Arm Converter Based on Bacterial Foraging Algorithm

The global demand of electrical power has increased enormously due to various reasons such as fast-changing and challenging technologies, climatic change, economic growth, and lifestyle of mankind. Due to global warming issues and the depletion of conventional sources of power generation, the utilization of renewable energy sources has drastically increased. So, many challenges exist in integrating the microgrid with AC grid and load. The alternative arm converter (AAC) is among the most innovative converter topologies used in high voltage direct current (HVDC) applications. This research work presents a new control strategy to generate pulses to trigger the switches in the AAC in a proper sequence to obtain a smooth output waveform. The AAC output is controlled by implementing various controllers such as the proportional-integral-derivative (PID) controller, fractional order PID (FOPID) controller, and FOPID controller tuned by metaheuristic algorithm-bacterial foraging optimization technique (BFOT). Also, a comparative analysis is performed based on the spectral analysis of the output voltage obtained. In comparison to other controllers, the FOPID controller optimized by the bacterial foraging optimization technique (BFOT) produced the least total harmonic distortion (THD) of the AAC output voltage. In addition, this paper also discusses about the performance and analysis on the design of an energy management system (EMS) to optimally utilize the energy sources such as PV system, wind system, and battery based on their availability which feed the AAC. The energy management system controls the entire integrated system in association with an integrated CUK-SEPIC converter to fulfil the load demand at the point of common coupling, either from the microgrid or the AC grid. A nine-level AAC model is designed integrating the microgrid, grid, and industrial loads with an energy management system using MATLAB/Simulink. The performance parameters of the entire model are analysed at every stage to provide stabilized output to meet the load demand.

Capacitor Voltage Balancing for Alternate Arm Converter Based on Conduction Angle and Zero-Sequence Voltage

The alternate arm converter (AAC) owns additional advantages over the traditional modular multilevel converter, such as reduced number and capacitance of submodules (SMs), and dc fault ride-through capability. The capacitor voltage balance of the AAC has always been a hot and difficult research topic. The balance performance of the conventional short-overlap control is limited by the maximum circulating currents, which may also lead to additional power losses. In this article, the conduction angle of the direction switches is employed to balance the SM capacitor voltage and a modified SM energy balance condition is developed. As the upper and lower arms are strictly alternatively conducted, the circulating currents no longer exist, leading to an improvement in system efficiency. The operation range of the AAC is also expanded under this method, so the AAC can operate far away from the sweet spot, such as grid voltage sags. A three-layer closed-loop control method based on conduction angle, zero-sequence voltage, and nearest-level modulation is further proposed, which enables the voltage balance of all SMs even under unbalanced conditions. Finally, the effectiveness of the proposed methods is verified by the simulation and hardware-in-the-loop tests.

A Cost-Effective and DC-Fault Tolerant Alternate Arm Converter With Wide Range Voltage Adaptability

In this article, an enhanced three-arm alternate arm multilevel converter (AAMC), called T-type AAMC (T-AAMC), is proposed for achieving dc fault ride-through capability with reduced semiconductor power devices compared to the hybrid modular multilevel converter (MMC). The upper and lower dc arms of T-AAMC are formed by half-bridge submodules (SMs) and series devices for lower construction costs, while the ac arm uses full-bridge SMs to realize dc fault tolerance. First, the arm phase-shift conducting modulation is applied for energy balance of converter stacks under wide-range operation by adjusting the arm phase-shift angle to match ac grid voltage and power factor. Then, the SM number, power device number, and SM capacitance are evaluated for T-AAMC under optimized modulation design. Analyses illustrate that 43% SMs, 17% power devices, and 44% energy storage requirement are reduced with the proposed topology than that with hybrid MMC. Furthermore, the closed-loop control system is designed to dynamically regulate the energy sharing between dc and ac stacks. Finally, full-scale simulations and down-scaled experiments are performed to verify the feasibility of the proposed converter and the theoretical analysis of its modulation and implementation.

AC/DC fault handling and expanded DC power flow expression in hybrid multi-converter DC grids

Hybrid multi-converter dc grids are extended dc grids formed by combinations of different converter topologies such as the line-commutated converter (LCC), modular multilevel converter (MMC), alternate arm converter (AAC) and other various topologies. The stable and secure operation of such systems in steady-stage and transient conditions is critical. This paper investigates the ac and dc fault-ride-through (FRT) capability and the expanded expression of dc power flow in a hybrid multi-converter dc grid consisting of LCCs, MMCs and AACs. Fault handling schemes of single converters are combined, extended and coordinated to provide satisfactory transient response of the hybrid dc grid under both ac and dc faults. This paper proposes an expanded expression of dc power flow considering the initial power flow determination and the power flow after converter outages under mixed P/V and I/V droop control. The expanded dc power flow expression can be used in LCC and modular VSC-based hybrid multi-converter dc grids to derive the initial dc power flow and assess the system static security after converter outages. Simulation results for steady-state and transient operation based on a detailed equivalent model verify the accuracy of the proposed dc power flow expression, and validate the fault handling capability of the hybrid dc grid.

The Impact of Overlap Period on the Stability of Current-Controlled Alternate Arm Converter Based on dq Frame Impedance Analysis

As a variant modular converter configuration, the alternate arm converter (AAC) is well-suited for high-voltage power transmission and large-scale integration of renewables. In contrast to conventional multilevel converters, the director switches in the arms of AAC lead to the introduction of an overlap period, during which circuiting current can flow through the two arms in the same phase. Thus, fixed or variable overlap period control can be implemented in AAC systems so as to dynamically balance stored arm energy. However, the control of overlap period is linked to instability issues that might impede the safe operation of AAC systems, which are yet to be reported. In this paper, the stability of an AAC system is demonstrated based on measured grid and converter impedance, in conjunction with impedance-based stability criterion in the dq frame. The interaction between harmonic sources at AC and DC sides of the AAC system is analyzed to determine resonant frequencies in the AC current when any potential resonance is identified in the dq frame. Novel results with respect to the impact of overlap period on the system stability are obtained by depicting and comparing the Eigenloci in the polar plot, which are validated by real-time simulations.

T-shaped hybrid alternate arm converter with arm energy balancing control for battery energy storage systems

Abstract Multilevel voltage source converters (VSCs), such as modular multilevel converter (MMC), cascaded H-Bridge (CHB) and alternate arm converter (AAC), are competent topologies for battery energy storage systems (BESSs) due to modularity, scalability and low harmonic distortion. However, there is a lack of studies about interfacing AAC with a BESS due to the arm energy balancing issue. Redundant sub-modules (SMs) are inserted passively into MMC, CHB and AAC to achieve high reliability; consequently, some of them are constantly idling, resulting in low SM utilization. We propose a novel topology -T-shaped hybrid alternate arm converter (TSHAAC) for BESS applications. In addition to the aforementioned features, the proposed TSHAAC requires lower number of SMs than MMC and AAC, along with lower number of switches than CHB. Moreover, an adapted arm energy balancing control is proposed to take advantage of the redundant SMs that are idling to achieve faster balancing than in conventional AAC configuration. The simulation results validate the integration of TSHAAC configuration in a BESS; the adapted arm energy balancing control is able to improve the balancing duration by 27 %.

Interoperability of modular VSC topologies in Multi-converter multiterminal DC systems

The ongoing transformation of power systems driven by the need for more interconnected networks will eventually lead multiterminal dc (MTDC) grids to become more commonplace in future electricity networks. Addressing interoperability challenges associated with multiple converter topologies for future MTDC implementations is a paramount requirement. This paper introduces a multi-converter MTDC (MC-MTDC) system combining both state-of-the-art and emerging modular voltage source converter (VSC) topologies. The developed MC-MTDC system, (i) demonstrates operation of the modular multilevel converter (MMC) and the alternate arm converter (AAC) in different MTDC configurations, and (ii) sets the basis for further development of MTDC configurations with diverse modular VSC topologies. The developed MC-MTDC system will also facilitate accurate testing and validation of multi-converter interoperability using real-time electromagnetic transient (EMT) models. The design parameters of the AAC-based terminals are determined ensuring compliance with existing MMC-based dc power systems. Detailed analysis and a complete set of results based on real-time implementation for different operating scenarios including steady-state, transient, and contingencies, verify multi-converter interoperability and associated control functions in different MTDC configurations.

Interoperability of Modular Multilevel and Alternate Arm Converters in Hybrid HVDC Systems

This paper demonstrates the interoperability of an emerging alternate arm converter (AAC) with the state-of-the-art modular multilevel converter (MMC) in high-voltage direct current (HVDC) systems based on a hybrid VSC-HVDC system. The paper also showcases the parameter derivation of the hybrid HVDC system and its detailed control structure. The study provides preliminary steps towards detailed analysis of AAC interoperability in complex hybrid dc grid configurations. A detailed set of results based on the 800 MVA hybrid voltage source converter (VSC)-HVDC system showcases the interoperability performance of the AAC under different operating scenarios and verifies its associated control functions.

Hybrid LCC-AAC HVDC transmission system

A hybrid HVDC transmission system based on a line-commutated converter (LCC) and an alternate arm converter (AAC) is developed in this article. The basic operation principles and control methods of the proposed LCC-AAC hybrid HVDC system are demonstrated following with appropriate system design and parameter selection. In addition, this paper proposes a set of control schemes for riding through ac faults and handling dc fault. The operation and performance of both converters in such a hybrid HVDC configuration are verified through a detailed set of results based on a 3000 MVA LCC-AAC hybrid HVDC system including steady state operation, ac-grid harmonics & dc-side ripple analysis, reference tracking (active power change at LCC station, reactive power change at AAC station) and system performance under ac-grid faults without blocking converter at both terminals of the HVDC system. Also, the response of the hybrid system under dc fault presents potential dc fault handling capability compared with the hybrid system based on LCC and modular multilevel converter (MMC) with half-bridge submodules (HBSMs). The hybrid LCC-AAC HVDC transmission system lays preliminary steps for research on complex mixed converter multiterminal dc configurations in future dc super grids.

The Alternate Arm Converter “Extended-Overlap” Mode: AC Faults

This article presents ac fault ride-through strategies for the “extended-overlap” operating mode of the alternate arm converter (AAC), which is a type of modular multilevel voltage source converter that has been proposed for HVdc transmission applications. The AAC offers several benefits over the half-bridge modular multilevel converter, such as requiring fewer submodules with a smaller capacitance and providing dc fault ride-through capability. Novel symmetrical and asymmetrical ac fault ride-through strategies are described and these strategies are experimentally validated by using a small scale prototype.

Extended ZCS Region of AAC-HVDC Systems Through On-Load Tap Changer Coordination

Arm inductances and the operation of two arms of a phase leg in an alternate arm converter (AAC) can lead to hard-switching of the director switches (DSs) during steady-state operation, increasing voltage stresses across the DSs. The DS voltage stress also depends on the magnitude of the interrupted arm current, which, in turn, depends on the output active/reactive power. Hard-switching of the DSs limits the operating region of the AAC. This article characterizes the zero-current switching (ZCS) capability of the AAC and proposes extended ZCS operation in steady state for HVDC applications. This is achieved by coordinating the AAC operating point and the transformer on-load tap changer (OLTC). The coordinated operation utilizes the tap positions of the HVDC transformer effectively to extend the ZCS operating region with no added effort on circulating current control. Results based on an AAC-HVDC transmission system demonstrate the effective coordination of the transformer OLTC and the converter operating point to achieve extended ZCS performance.

Arm energy balancing control in AACs: a comparative analysis

The alternate arm converter (AAC) is an emerging state-of-the-art topology for voltage-source converter (VSC) by combining two-level converter and modular multilevel converter (MMC). The AAC performs better than the MMC in terms of the ability to block DC-side faults and ride through AC-side faults. Moreover, the number of sub-modules (SMs) per arm of the AAC is reduced by about 50% compared with the MMC. The key issue with AAC is the energy deviation during the alternate operation between the upper and lower arms. Presently, several AAC balancing techniques have been proposed, but no comparative analysis has been performed to provide insight on the effect of each balancing technique on AAC performance. In this study, five proposed balancing techniques are reviewed accordingly, followed by comparing them in terms of four parameters – the presence of DC filter, balancing capability, the presence of redundant SMs and zero-current switching control. Furthermore, a simulation work has been performed to study and compare the effects of each of the five balancing techniques. Both literature and simulation-based comparative analysis are significant to help select appropriate balancing technique as well as to provide helpful insights to improve the existing balancing techniques.

Balancing Algorithm for a Self-Powered High-Voltage Switch Using Series-Connected IGBTs for HVDC Applications

This paper presents a balancing algorithm for the so-called Director Switch, a controlled, reverse conducting, self-powered, high-voltage switch, using series-connected insulated gate bipolar transistors (IGBTs). This is a key component on two new voltage-source, high-voltage direct current (HVDC) converter topologies. These, which promise efficiency, cost, and size reductions, are an alternate arm converter and a series bridge converter. One of the main design challenges of the director switch is how to supply the IGBT gate drivers, and other auxiliary electronics while overcoming the high-voltage insulation barrier to ground level. In this paper, this is achieved by using the energy stored in a capacitive clamp snubber circuit, which also helps with the overvoltage mitigation during rare hard-switching events. However, the clamp capacitor and by proxy the IGBT voltages become highly unbalanced during operation, compromising the integrity of the switch. This paper presents a balancing strategy that consists in applying a controlled delay in the switching of the IGBT levels, in a way that forces a voltage balance on all the switch levels. The paper explains the unbalance. Then, the balancing strategy is explained and validated on simulation and on a high-voltage experimental rig, on switches comprising two and three series-connected IGBTs, respectively.

Alternate Arm Converter Energy Balancing Under Parameter Variation

Energy balancing of the alternate arm converter (AAC) is a major challenge and overlap period-based circulating current control methods are commonly used. Component parameter variation occurs due to tolerances, component aging, and internal faults leading to stored energy imbalances of modular voltage source converter (VSC) topologies. If not controlled, such variations can severely impact the operation of the AAC. Hence, component parameter variation is a key consideration for energy balancing of all sorts of modular VSC topologies, but has not been addressed in the existing literature for the AAC. This paper investigates the energy balancing capability of the AAC in high-voltage direct current (HVDC) applications under component parameter variations and proposes a compensation method to further improve its energy balancing performance. The effectiveness of the proposed method is verified in a real-time model of an AAC-HVDC transmission system from widely accepted existing CIGRE benchmark models.

Zero-Current Switching Control of the Alternate Arm HVdc Converter Station With an Extended Overlap Period

The alternate arm converter (AAC) is a promising candidate for HVdc converter stations due to its reduced number of devices and capability of blocking the dc fault current. This paper is focused on modeling and control of the AAC with an extended overlap period. First, a generic mathematical model considering all the switching state combinations of the AAC is developed, which governs the characteristic of the time-varying, three-phase imbalanced circuit parameters of the AAC. Based on the developed model, an ac-side current control strategy with a feed-forward compensation method to overcome the time-varying nature of the circuit parameters is proposed. Then, by introducing the so-called “double-overlap period,” a synthetic circulating current strategy is proposed, which enables both dc current active filtering and zero-current switching of the director switches. The validity of the model and effectiveness of the proposed strategies are confirmed by simulation studies on an HVdc system.

Alternate Arm Converters-Based HVDC Model Compatible With the CIGRE B4 DC Grid Test System

This paper develops an alternate arm converter (AAC)-based point-to-point high-voltage direct current (HVDC) transmission system compatible with the existing benchmark models of modular multilevel converter (MMC)-based HVDC and multiterminal dc (MTDC) grids. The developed AAC-based HVDC system expedites the future development of multi-converter, multiterminal benchmark models that enable the system level studies with detailed models of multiple topologies. The configuration and design parameters of the topology, are derived in alignment with the existing MMC-based benchmark systems. Detailed analysis and simulation studies based on real-time digital simulations verify the operation and performance of the developed AAC-based HVDC system and the associated control functions. The model developed in RTDS is also provided as supplementary material in order to enable further research and development on multiple topologies-based dc grids.

Hybrid converter topologies for dc transmission systems

This study presents types 1 and 2 hybrid converters with reduced power circuit complexity compared with the mixed cell modular multilevel converter (MC-MMC). The type 1 converter is formed by replacing the director switches of the alternate arm converter by high-voltage (HV) half-bridge (HB) cells rated for half of the dc-link voltage. Also, it resembles special case of MC-MMC, where the entire HB cells of each arm are lumped into a single HV HB cell, with both capacitors of the half- and full-bridge cells are exposed to fundamental current as in the conventional MMC. The upper and lower arms of the type 2 converter resemble a front-to-front connection of two three-phase hybrid cascaded two-level converters, where the cell capacitors of the three-phase two-level converters that act as director switches do not experience fundamental currents. Therefore, the type 2 converter offers compact design compared with type 1 converter and the MC-MMC. The technical viabilities of the proposed hybrid converters are assessed using simulations, with both converters modelled in MATLAB-Simulink using electromagnetic transient simulation approach, considering normal and transient conditions. Experimental results obtained from single-phase type 1 converter confirm the practical viability of the proposed converters.

Energy and director switches commutation controls for the alternate arm converter

The Alternate Arm Converter (AAC) is a promising multilevel Voltage Source Converter (VSC) suitable for High Voltage Direct Current (HVDC) transmission systems. This converter exhibits interesting features such as a DC Fault Ride Through capability thanks to the use of Full-Bridge Sub-Modules (SM) and a smaller footprint than an equivalent Modular Multilevel Converter (MMC).After an analysis of the converter operating modes called Non-overlap and Overlap mode, a sequential representation of the AAC operation is proposed. The main originality of this paper is the use of the Petri Net to describe all the phases and to highlight their sequencing. According to the phases identified thanks to the sequential approach, models and control structures for the grid currents, the internal energy and the Zero Current Switching (ZCS) are detailed. Furthermore, the step-by-step approach proposed in this paper allows a clear and rigorous modelling of this complex converter.

The Asymmetric Alternate Arm Converter: A Compact Voltage Source Converter Design for HVDC

The Modular Multilevel Converter (MMC) has become one of the most practical topologies for integration of renewable energy power plants into the grid, through submarine HVDC cables and HVDC lines. Nonetheless, the MMC requires significant large DC storage capacitors to mitigate for large power, low frequency pulsations due to the single-phase like AC to DC conversion process in their arms. This work addresses the recently introduced Asymmetric Alternate Arm Converter, a hybride MMC topology which uses modular converter arms only on the lower side of the main converter whereas the upper side uses only standard ON-OFF valves topology. This converter is similar to the Alternate Arm Converter (AAC) but dispenses of the modules on the upper side and of the director switch on the lower side. Like the AAC, the Asymmetric AAC offers a significant reduction in DC energy storage requirements and in the number of modules. In addition, the Asymmetric AAC offers a less restricted operation than the AAC, in terms of operating over a wide range of AC to DC voltage ratio, and the possibility of achieving both distortion-free AC current and ripple-free DC current. The principle of operation of the Asymmetric AAC is revisited here and a suitable control strategy to demonstrate its operation is developed. In addition, requirements for energy storage, voltage and current rating of converter modules and converter losses are examined by means of simplified analyses. Proposals are verified through simulation studies and experiments conducted on a low power prototype having three H-bridge modules per arm. Results confirm good operation of the proposed converter and control scheme.

The Alternate Arm Converter (AAC)—“Short-Overlap” Mode Operation—Analysis and Design Parameter Selection

This paper presents converter operation principles and theoretical analyses for “short-overlap” mode operation of the alternate arm converter (AAC), which is a type of modular multilevel voltage source converter that has been proposed for HVdc transmission applications. Fourier series expressions for the ideal arm current and reference voltage are derived, for the first time, in order to develop an expression for the submodule capacitance required to give a selected peak–peak voltage ripple of the summed submodule capacitor voltages in an arm. The dc converter current contains nonnegligible low-order even harmonics; this is verified by deriving, for the first time, a Fourier series expression for this current. As the dc converter current needs to be filtered to form a smooth dc grid current, a novel dc filter arrangement is proposed, which uses the characteristics of a simplified dc cable model, as well as the capacitance of the dc link and additional dc-link damping resistance, in order to form a passive low-pass filter. Results obtained from a simulation model, which is based on an industrial HVdc demonstrator, are used in order to verify the presented converter operation principles and theoretical analyses.