Medium-voltage Dc Application Research Articles

Reliability Analysis of Modular Multilevel Converters in MVDC Applications

In modular multilevel converters (MMCs) designed for medium-voltage DC (MVDC) systems, since the required sub-modules (SMs) in one arm are fewer, the converters cannot afford the same level of redundancy as observed in high-voltage DC (HVDC) applications. In order to ensure high availability of converters in MVDC systems, reliability becomes a challenge and requires different design approaches. Considering the redundancy design, operating modes, and correlation across failures of SMs in one arm, this paper analyses and compares reliability in MMCs for MVDC systems with multiple SM topologies. Under different scenarios in MVDC systems, SM topologies with better performance of reliability have been identified in this research. It is demonstrated that, for a given output voltage in converters, three-level SMs generally provide higher reliability than two-level SMs. By implementing passive and load-sharing operation schemes in converters, the MMC reliability can be improved compared with the conventional active mode. In terms of the correlation between lifetime of SMs in the same arm, the smaller disturbance introduced by one faulty SM promotes the improvement of converter reliability. Also, the correlation between failures of SMs in one arm has greater effects on the reliability of an MMC with higher level of redundancy. The outcomes of this work contribute to the optimisation of converter design and operation with high reliability and low investment for an MMC in MVDC systems.

An Improved AC-Link Voltage Matching Control for the Multiport Modular Multilevel DC Transformer in MVDC Applications

In this paper, an improved AC-link voltage matching control (IVM) strategy is proposed for the multiport modular multilevel DC transformer (M3DCT), which comprises a single-phase modular multilevel converter (MMC) and a series of cascaded H-bridge units. The objective of the proposed IVM strategy is to address the AC-link voltage mismatch phenomenon. Distinct from existing control methods, such as various phase-shifting control methods and the conventional AC-link voltage matching control strategy, the proposed IVM strategy orchestrates the operation of the M3DCT in an innovative fashion. It allows the sum of inserted submodule (SM) numbers in the upper and lower arms to be flexible, no longer confined to a specific SM number per arm. Consequently, the AC-link voltage of the M3DCT is maintained proximate to the matched operating condition, regardless of the degree of mismatch in the DC side voltage of the M3DCT. This enables the enhancement of the M3DCT’s overall operational performance, particularly under conditions of light loads within medium-voltage DC (MVDC) distribution systems. The correctness and effectiveness of the proposed control strategy and the corresponding analysis are substantiated through simulation results.

Coprime Polynomial Based Dual-Circulant Modulation for Inherent Submodule Voltage Balancing in MMDC

The modular multilevel dc-dc converters (MMDCs) have attracted much interest in medium voltage dc applications, but have to face the main issue of balancing submodule (SM) capacitor voltages. This paper proposes a dual-circulant modulation and proves it possessing the inherent balancing capability for arbitrary operation cases. High-speed communication for real-time sensor data transfer could be avoided, which reduces implementation costs and also enhances the system reliability. Two sets of circular switching patterns are preset and combined to complete the full constrains on SM voltages. The associated polynomial of circulant matrix is introduced and the coprime of polynomials demonstrates the full-rank feature of the extended switching matrix that promises the inherent balancing for any operation cases. Then the switching patterns are reallocated and optimized to keep the SM uniformity and reduce the capacitor voltage ripple at the same time. Full-scale simulations on MMDC models and down-scaled experiments on prototypes are both presented, which validates the inherent voltage balancing capability and the optimization of SM capacitor voltage ripple with the proposed dual-circulant modulation.

Dual-Circulant Modulation for Modular Multilevel DC/DC Converters With Inherent Balance Capability

Modular multilevel DC/DC converters (MMDC) have emerged recently for medium-voltage dc applications. Under single circulant modulation, the sub-module (SM) capacitor voltages in MMDC are found to be evenly divided only at some specific modulation coefficients. To obtain the inherent balance capability under all modulation coefficients and expand the application range of circulant modulation, a simple dual circulant modulation is proposed in this letter. On the basis of single circulant modulation, the self-balancing of the SM capacitor voltages is realized by using the additional switching patterns generated by a simple circulant permutation method. The theoretical analysis of the proposed method is presented, and the inherent balance capability of MMDC under the dual circulant modulation method is verified by experiments.

Design and prototyping of transformerless DC-DC converter with high voltage ratio for MVDC applications

Power boost DC-DC voltage converters have become a key element for medium-voltage DC (MVDC) applications as a link between generation and distribution grids, it is possible to classify the DC-DC converters into four families, depending on their characteristics, as well, one of the actual principal needs is to reduce the number of passive and active components needed for voltage conversions. This article presents a novel transformerles DC-DC boost-resonant converter, which is destined for the interconnection of medium and high voltage DC smart grids. The proposed DC-DC converter is composed of two inductors, a capacitor, and five IGBTs interrupters, which are responsible for the interaction between resonant and boost converters, in the same way, the resonant converter is composed of one resonant inductor and one resonant capacitor while the boost converter is composed by one boost inductor. This paper also presents the operation principle of the proposed converter, as well as its control, modular connection, and simulation to probe the behavior of the converter and an experimental 100 W prototype with 9.5 gain.

Bridge-to-Bridge Independent Control Method for Dual-Active-Bridge Interlinking Converter

In dual-active-bridge (DAB) converters, the driving-synchronization signal is supposed to be required to be transmitted across the primary bridge and the secondary bridge to realize a phase shift for power flow. In this letter, a novel bridge-to-bridge independent (B <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> I) control method is proposed. In this control method, cross-bridge transmission of sampling/driving signals is not required. Thus, full modularity of each side in the DAB converter was realized. B <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> I control is suitable for high-voltage series low-voltage parallel (HSLP) modular DAB interlinking converter in medium-voltage dc applications because: first, the bridge-level modularity of B <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> I control benefits electromagnetic compatibility and insulation design with the HSLP connection; second, B <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> I control features series-side voltage sharing; and third, B <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> I control can adapt for various operation modes of microgrids.

Comparative Assessment of Multi-Port MMCs for High-Power Applications

Multi-port converters can interconnect different dc and ac systems while reducing semiconductor requirements and losses by eliminating redundant power conversion stages. Modular multilevel converter (MMC)-based multiport systems are well suited for application in mixed ac-dc grids containing high-voltage dc (HVDC) and medium-voltage dc (MVDC) systems. This paper conducts a detailed comparative assessment of multi-port dc-dc-ac MMCs for high power applications. Four representative topologies are chosen for study due to their contrasting internal power processing characteristics. Three different network scenarios are investigated that include HVDC and MVDC applications, covering several different power flow cases. The multi-port MMCs are compared in terms of losses, semiconductor effort, internal energy storage and magnetics requirements. The results are extensively discussed and general conclusions are summarized.

Performance Evaluation and Limitations of Overvoltage Suppression Circuits for Low- and Medium-Voltage DC Solid-State Breakers

This paper presents a design and performance evaluation of three different overvoltage suppression circuits employed in a solid-state circuit breaker for medium- to high-power and low- to medium-voltage DC grids. The evaluated criteria are the requirements for passive components, as well as the breaker performance in terms of peak switch overvoltage, peak short-circuit current, fault clearance time and rate of voltage rise during the fault clearing process. Additionally, the impact of the stray inductance in the solid-state breaker design on the anticipated overvoltage is also investigated. Design and operating limitations of each configuration from an application perspective are also discussed. From simulations of a 1.8 kV and 500 A breaker for high-power medium-voltage DC applications it is shown that the most effective overvoltage suppression is achieved by means of using a metal-oxide varistor and a snubber capacitor connected in parallel with the main switch at a cost of high fault current. A medium-power solid-state DC breaker prototype rated at 1.5 kV and 50 A was also designed and constructed. The experimental results reveal that using only a metal-oxide varistor can be sufficient when the breaker employs high-voltage discrete semiconductors with long current falling time and is used in medium-power medium-voltage grids.

FB-ZCS DC–DC Converter With Dual-Capacitor Resonant Circuit for Renewable Energy Integration With MVDC Grids

Full-bridge zero-current-switching (FB-ZCS) dc–dc converters realize quasi-resonant soft-switching and smooth current commutation by utilizing the leakage inductance of high-frequency transformer and a resonant capacitor. The following two types of resonant capacitor configurations are proposed in the literature: First, shunt-connected across the transformer-secondary and, second, series-connected with the transformer-primary. Shunt-connected capacitor has the drawbacks of storing full-load-rated resonant energy irrespective of converter loading. This results in large duty-cycle loss and consequently regulation loss at light-loads. Series-connected capacitor addresses this issue but suffers from higher peak-voltage stress across components and insufficient voltage for resonance at light-loads. To address these limitations, a dual-capacitor resonant circuit is proposed in this article. Series-capacitor facilitates current-adaptive resonant energy storage and shunt-capacitor is designed only for a fraction of full-load-rated resonant energy, which lowers the duty-cycle loss and supports soft-switching at light-loads. This combination also reduces the series-capacitor voltage required for resonance, thereby reducing the peak-voltage stress. Steady-state analysis of converter operation is presented and the advantages over existing converters are demonstrated through simulations for medium-voltage dc application. Experimental results from a scaled-down prototype are presented to validate the proposed converter.

Comparative Analysis of an MV Neutral Point Clamped AC-CHB Converter With DC Fault Ride-Through Capability

The ac-side cascaded H-bridge converter with a two-level (2L) main bridge has previously been proposed as a fault-tolerant converter for high-voltage dc. This paper explores the benefits of replacing the 2L bridge with a neutral point clamped (NPC) three-level bridge for medium voltage dc applications and defines the optimum operating conditions for this case. By modifying the topology to include an NPC main bridge, the peak stack voltage during normal operation is decreased considerably, which results in a 58% reduction in the required number of submodules (SM), thereby significantly increasing efficiency. However, this sacrifices the fault ride-through capability as the stacks are no longer able to support the ac voltage and thus two new SM topologies are proposed. The proposed topologies function as single full-bridges with two capacitors in parallel during normal operation. Under fault conditions, the SMs divide into two series-connected full-bridges to enable dc fault ride-through. Reverse-blocking integrated gate-commutated thyristors or ultra-fast mechanical switches are used to bypass the insulated gate bipolar transistors that are unused in normal operation and therefore the topology maintains a high efficiency. Simulation results are shown, and the proposed topologies are compared with the more conventional designs in terms of efficiency, energy storage requirement, and device count.

A Hybrid Resonant ZVZCS Three-Level Converter for MVDC-Connected Offshore Wind Power Collection Systems

A novel hybrid resonant zero-voltage zero-current switching three-level converter with capacitive output filter is proposed in this paper, which undergoes half of the input voltage for switches and is suitable for offshore wind farms in medium-voltage dc (MVDC) collection systems. The proposed converter, utilizing two transformers with very different power ratings, adopts insulated-gate bipolar transistors for all six power switches and operates in discontinuous current mode and can achieve zero-current switching for the four main switches and rectifier diodes over the whole load range. Meanwhile, the auxiliary switches with small current rating can realize zero-voltage switching naturally. Hence, the switching loss is reduced, which is vital in MVDC applications. By the trajectory formulas of the resonant current and voltage, the trajectory paths can be achieved, which is helpful to compare the peak and turn- off currents of switches. The design rules of main parameters can be deduced with the trajectory formulas and paths. The influence of the turns ratio of auxiliary transformer and the resonant capacitance on the peak and turn- off currents is discussed in detail. The operation principles of the proposed converter are analyzed and verified by the simulation and experimental results.

Study of Liquid Metal Fault Current Limiter for Medium-Voltage DC Power Systems

Fault current limitation is an urgent but challenging technology for medium-voltage dc (MVDC) power systems. For a liquid metal current limiter (LMCL), the arc discharge characteristics often determine its current-limiting properties. The evolving arc in liquid metal is measured by contacting voltage probes, and the properties of the formed dumbbell-like arc column are studied and discussed in detail. It indicates that the total arc voltage almost behaves as a linear function of arc current for an LMCL. Besides, a simplified numerical arc model based on magnetohydrodynamics theory is built and solved by the software FLUENT. It is concluded that the temperature rather than the slightly fluctuant pressure dominates the arc properties, which coincides well with test results to some extent. Finally, a fault current limiter topology including serial LMCLs and parallel resistors is put forward, whose feasibility for MVDC application is discussed on the basis of its current-limiting behaviors.

Isolated Modular Multilevel DC–DC Converter With DC Fault Current Control Capability Based on Current-Fed Dual Active Bridge for MVDC Application

In this paper, a current-fed modular multilevel dual-active-bridge (CF-MDAB) dc–dc converter is proposed for medium-voltage dc (MVDC) application. The proposed converter inherits favorite characteristics of DAB circuits including soft switching and small passive components. Thereby, high efficiency and high power density can be achieved. Moreover, with direct input and output dc current control, the CF-MDAB is suitable for a breakerless MVDC system since it can realize dc fault ride-though operation. In addition, the dv / dt in the converter is mitigated with the quasi-three-level modulation. In this paper, the proposed converter is applied to integrate the battery energy storage to a MVDC grid as an example to illustrate its operation principles and fault current control capability. The operation principles are presented for both normal and dc fault conditions; the dynamic models are also derived not only under normal operation mode but under dc fault operation mode as well. The control systems under different operation modes are designed, respectively, based on the developed mathematical models. A downscaled 40-kHz 3-kW CF-MDAB prototype was built in the laboratory. The experimental results under both normal condition and dc fault condition verified the analysis as well as the control performance of the proposed converter.

Comparative Analysis of Multilevel-High-Frequency-Link and Multilevel-DC-Link DC–DC Transformers Based on MMC and Dual-Active Bridge for MVDC Application

This paper gives a comprehensive comparison of multilevel high-frequency-link (HFL) dc transformer (abbreviated MDCT) based on modular multilevel converter (MMC) and multilevel-dc-link dc transformer (abbreviated ADCT) based on dual-active-bridge (DAB) for medium-voltage dc (MVDC) application. The topology, operation, HFL voltage and current, active and circulating power, characteristic currents, switching behaviors, and power loss are analyzed in detail. Both MDCT and ADCT have fault treatment ability for MVDC application. However, the installation and commissioning of ADCT are more flexible and simpler. In addition, the MDCT needs more switches and high-frequency (HF) inductors than ADCT in the MVDC side with the same voltage level, but the number of HF transformer can be reduced. Compared with MDCT, the ADCT has higher power transfer ability and lower circulating power, lower HFL voltage, and RMS current and peak values with the same transmission power in the MVDC side. However, the arm RMS and average currents of MDCT are lower than those of ADCT in the MVDC side. The switching performance of MDCT deteriorates when the MVDC voltage fluctuates, but the switching behaviors of half bridges are added to ADCT. The loss of ADCT almost keep the same when the MVDC voltage fluctuates, but the loss of MDCT changes a lot. Finally, a small-scale prototype platform with 1 kW/450 V/150 V is built and experimental results verify the correctness and effectiveness of the theoretical analysis.

Lyapunov‐based high‐performance controller for modular resonant DC/DC converters for medium‐voltage DC grids

This study presents a high-performance controller based on the Lyapunov stability criterion that enhances the dynamic performance and disturbance rejection capability of resonant DC/DC converters when compared with classical PI control. The series–parallel resonant converter (SPRC) is used as the candidate converter to which this controller design is applied but the design can be generalised to other types of resonant DC/DC converters. By using a multiple module approach, low-power modules of this resonant converter are stacked to enable operation at medium-voltage DC (MVDC). The proposed controller design is applied to modular structure of the SPRC to verify its high-performance output in conjunction with active sharing control loops that ensure uniform current/voltage distribution across the multiple interconnected modules. Detailed controller design, closed-loop stability criteria, robustness and parameter sensitivity are investigated and controller performance is compared and verified against the classical PI control in simulation and low-scaled experimental prototype. Operations in single-module and two-module input-series output-parallel modes are both studied. The study affirms the selection of the modular DC/DC converter architecture and its associated proposed controls for high-performance MVDC applications.

Hybrid Resonant ZVZCS PWM Full-Bridge Converter for Large Photovoltaic Parks Connecting to MVDC Grids

A novel hybrid resonant zero-voltage zero-current switching pulsewidth modulation full-bridge converter with capacitive output filter is proposed in this paper, which is suitable for large photovoltaic parks connecting to medium-voltage dc (MVDC) grids. The hybrid resonant converter, with two full-bridge cells sharing a bridge leg and connecting the secondary windings of two transformers in series, operates in discontinuous current mode and can achieve zero-current switching for main power switches and rectifier diodes over the whole load range. Meanwhile, the auxiliary switches with small current rating can realize zero-voltage switching naturally. Hence, the switching loss is reduced, which is vital in MVDC applications. By the trajectory formulas of the resonant current and voltage, the trajectory paths can be achieved, which is helpful to compare the peak and turn-off currents of switches. The design rules of main parameters can be deduced with the trajectory formulas and paths. The influence of the turns ratio of auxiliary transformer and the resonant capacitance on the peak and turn-off currents is discussed in detail. A simulation model is established in the software PLECS and the operation principles of the converter are verified. A 150 V–1500 V/2 kW prototype was built to validate the operation principles and parameters optimization.

A Hybrid ZVZCS Dual-Transformer-Based Full-Bridge Converter Operating in DCM for MVDC Grids

High-power dc–dc converter is the essential component for connecting the renewable energy sources to medium-voltage dc (MVDC) grids. In this paper, a novel zero-voltage zero-current switching converter with two full-bridge cells sharing a bridge leg and connecting the secondary windings of two transformers in series is proposed for MVDC applications. One big feature of the proposed converter is that the required inductance used for energy transmission is reduced remarkably and even can be embedded in the transformer in some cases. Special but simple control strategy adopted by the converter makes it work in discontinuous current mode, which can realize zero-current switching for the main switches and rectifier diodes in the whole load range. Meanwhile, the auxiliary switches with small current rating can realize zero-voltage switching naturally. Hence, the switching loss is reduced, which is very important for high-power applications. The effects of the turns ratio of auxiliary transformer on the total loss and needed inductance are comprehensively analyzed and proved. With a detailed parameters design procedure, a simulation model is established in the software PLECS and the operation principle of the converter is verified. A 120 V–1200 V/1 kW prototype was built to validate the operation principle of the proposed converter.