Search

To cope with the weak grid stability issue of grid-tied voltage source converters (VSCs), this article proposes an artificial neural network (ANN) based approach for online stabilization control of the grid-tied VSC with the pole-tracking feature. First, an ANN is adopted to establish the mapping between the control parameters and the closed-loop poles of the grid-VSC system, serving as a computationally light model surrogate that is favorable for real-time control applications. Then, an online parameter search algorithm enabling simultaneous tuning of multiple controllers and parameters is developed, by which the system's poles under a new grid condition can be pulled to the reference ones, i.e., achieving the pole-tracking-based stabilization control of this article. Finally, the efficacy of the proposed method along with its stabilization effect is verified by MATLAB simulations and experimental results. [ABSTRACT FROM AUTHOR]

Mission profile emulation (MPE) for submodules (SMs) in the modular multilevel converter (MMC) is a crucial step before a full-scale system is put into field operation. Existing methods suffer from limitations of the testing accuracy due to the staircase-like arm voltages that result in distorted loading current and require large filter inductance. This is especially the case when a limited number of SMs are under test. In this article, an efficient MPE method is proposed to emulate the loading behaviors of multiple SMs in MMC. The testing circuit allows two arms of MMC with flexible SM number, operating in inverting and rectifying mode under nearest level modulation, to be tested simultaneously with reduced dc supply voltage. A common H-bridge circuit is introduced in the testing circuit, with each arm of the H-bridge circuit functioning as the voltage compensator by using feedforward control. In this way, the current distortion can be significantly suppressed, and testing accuracy can be guaranteed in a cost-efficient manner even though no large inductor is included in the testing circuit. The proposed method allows SMs under test to be loaded as in the actual back-to-back MMC system. Simulations and experimental measurements are given to validate the proposed method. [ABSTRACT FROM AUTHOR]

The high penetration of renewables has sparked new oscillatory concerns in hybrid AC/DC grids. Knowledge of the oscillation regarding its source and propagation mechanism, is crucial to system operation. In this respect, the frequency-domain modal analysis (FMA) provides a way (e.g., participation factor) to extract these oscillation properties, and has been widely applied in converter-based AC grids. Compared to AC grids, oscillation properties of hybrid AC/DC grids are much more complex. Such complexity mainly originates from the diversity of oscillation propagation behaviors in hybrid AC/DC grids, where a distinctive local-area oscillation will emerge. Therefore, a dedicated oscillation propagation analysis for hybrid AC/DC grids is desired, which is the main focus of this paper. First, the mechanism of the distinctive local-area oscillation is analyzed considering AC/DC coupled dynamics of converters. Then, a systematic method for the oscillation propagation analysis is presented, where indices for characterizing the local-area oscillation and its impact area are established. Finally, some propagation properties are discussed, by which a strategy for the online oscillation source locating is designed. Case studies are carried out in a typical hybrid AC/DC grid to validate the propagation analysis in this paper. [ABSTRACT FROM AUTHOR]

Insulated gate bipolar transistor (IGBT) modules with multiple chips have wide range of applications, and the correct estimation for the thermal behaviors inside IGBT modules is becoming crucial. Thermal impedance matrix is one of the most adopted approaches to describe the thermal-coupling effect of IGBT module. For simplicity of analysis, the heatsink or ambient temperature is typically chosen as the reference node for the thermal-coupling impedance term, while the case temperature is simplified or ignored. This letter provides a thermal coupling model enabling case temperature as reference node. This proposed model decouples the thermal coupling impedances of IGBT module itself and external cooling condition, so that the modeling of cooling conditions outside device and inside the power module, can be separately considered. The characterization method and advantages of the proposed model are verified by an experimental demonstration, and the potential applications are further discussed. [ABSTRACT FROM AUTHOR]

Conventional half bridge modular multilevel converter (MMC) system will suffer great over-current when there is a dc side fault. Therefore, topology study in sub-modules with dc side fault blocking capability have become one of the hotspots in MMC field. However, fault blocking topologies using conventional IGBT devices must employ more semiconductors than half bridge topology. The extra semiconductor devices employed in the topology will lead to the increase of cost and conduction loss. To reduce the conduction loss of the fault blocking topology, a novel sub-module topology is proposed in this paper. The proposed topology employs reverse blocking devices to form the low conduction loss path. Conduction loss analysis is conducted in this paper to show the priority of the proposed topology. Simulation and experiment results verified the dc fault blocking capability of the proposed topology. [ABSTRACT FROM AUTHOR]

In this article, a modified LLC resonant converter with LC antiresonant circuit in parallel branch is proposed. The proposed converter is derived by placing LC antiresonant circuit on parallel branch. In comparison with LLC resonant converter, the equivalent inductance of parallel branch of the proposed converter decreases along with switching frequency within regulating frequency range. This advantageous property contributes for that the turn-off current and circulating conduction loss in the primary side can be reduced at unity voltage conversion ratio due to large impedance of LC antiresonant circuit. Moreover, by optimizing resonant parameters design of parallel branch based on time-domain analysis, the magnetizing current is in close proximity to a sine wave and the conduction loss in the secondary side will be reduced, especially in the light load. A higher voltage conversion ratio can be achieved at low switching frequency due to decreased impedance in parallel branch. In addition, a design flowchart and a design practical case are given to help to implement an efficiency optimization algorithm to decide five resonant design parameters. Finally, a 2 kW 250 V—400 V-input and 48 V-output prototype is built, and the main experimental results are provided to verify the feasibility of the proposed converter and compare with LLC resonant converter. Comparative state-plane trajectories are depicted to compare their characteristics and to verify the model-domain analysis. [ABSTRACT FROM AUTHOR]

Grid-synchronization stability (GSS) is an emerging stability issue of grid-tied voltage source converters (VSCs), which can be provoked by severe grid voltage sags. Although a qualitative understanding of the mechanism behind the loss of synchronization has been acquired in recent studies, an analytical method for quantitative assessment of GSS of grid-tied VSCs is still missing. To bridge this gap, a dedicated Lyapunov function is analytically proposed, and its corresponding stability criterion for GSS analysis of grid-tied VSCs is rigorously constructed. Both theoretical analysis and simulation result demonstrate that the proposed method can provide a credible GSS evaluation compared to the previous EAC/EF-based method. Therefore, it can be applied for fast GSS evaluation of the grid-tied VSCs as is exemplified in this letter, as well as an analytical tool for some GSS-related issues, e.g., GSS-oriented parameter design and stabilization control. [ABSTRACT FROM AUTHOR]

Modular multilevel converter (MMC) is one of the most promising converter topologies for medium/high-voltage applications, where the nearest level modulation (NLM) is widely applied. However, the capacitor voltage balancing methods under this modulation mode exist some inefficient actions, which are useless for capacitor voltage balancing but lead to unnecessary switching of power semiconductor devices. In this article, the disturbance and unbalance mechanisms of capacitor voltages in MMC under NLM are analytically revealed and solved. The disturbing metrics for capacitor voltage are digitally extracted to identify the accurate time interval in which the switching actions have immediate effects on capacitor voltage balancing. Then, a demand-oriented scaling factor is introduced to adjust the balancing strength by changing the range of the time intervals which are highly efficient with respect to capacitor voltage balancing, thus flexible tradeoff between the switching frequency of power semiconductor devices and the balancing level of capacitor voltages can be achieved. Finally, a dedicated mission-profile-emulator for the testing of MMC is designed and built with comprehensive validations to verify the effectiveness of the proposed method. [ABSTRACT FROM AUTHOR]

Modular multilevel converter (MMC)-based solid-state transformers (M-SSTs) show promise in hybrid ac/dc distribution grids where there is need for a medium-voltage dc (MVdc) interface. It can be used to interconnect several distribution networks with different voltage forms and levels and enable flexible power flow among them. In an M-SST, the MVdc short-circuit fault is a critical issue due to the rapidity of the fault current, which can cause device damage and power failure of the low-voltage (LV) grids. Aiming at addressing this issue, this article proposes a fault-mode control strategy applied to a hybrid M-SST topology for uninterrupted power supply of its LV ports. The topology consists of a hybrid MMC with both half- and full-bridge submodules and isolated bidirectional dc–dc converters. Through the proposed control, the fault current is eliminated when an MVdc short-circuit fault occurs. The hybrid M-SST can keep uninterrupted power interaction between the MVac port and the LVdc port during the fault, thereby improving power supply reliability. Moreover, an improved capacitor voltage balance control and an optimized modulation scheme are developed and included in the uninterrupted operation scheme to maintain the harmonic performance of the ac port. The feasibility and effectiveness of the proposed topology and control are verified by simulation and experimental results. [ABSTRACT FROM AUTHOR]

Recent studies have shown that the grid connection of offshore wind farms through modular multilevel converter–high voltage direct current (MMC-HVdc) transmission system has brought low inertia issue. It is an urgent requirement for offshore wind farms to compensate for the inertia. After analysis and evaluation in this article, it is found that integrating an ultracapacitor energy storage system (UESS) into an MMC has an advantage comprehensively considering cost, size, and dynamic. Based on the evaluation, this article proposes an approach to inertial compensation of HVdc offshore wind farms by MMC-UESS integration. An ultracapacitor is selected as an energy storage element (ESE) for its high power density since the inertial response is a short-term response and it is hoped that added ESE does not significantly increase the size of an MMC submodule. Batteries with high energy density but low power density are not suitable for such applications. The optimal operating voltage of a UESS and the control scheme between MMC loops and inertial emulation are proposed in this article. Experimental results from real-time simulation and a downscaled prototype are presented to verify the effectiveness of the proposed method. [ABSTRACT FROM AUTHOR]

Ring and meshed dc distribution grids have attracted increasing interest lately. Their multipoint and multiterminal structure facilitates flexible operational control and reliable power supply across the network. However, the dc grids implemented with existing power converters lack sufficient control freedoms for proper power flow adjustment; therefore, it is necessary to develop additional dc power flow controllers (DCPFCs) to be added to these grids for improved power flow control capability. Specific to multiterminal dc networks built with modular multilevel converters (MMCs), this article proposes an embedded DCPFC (e-DCPFC) directly integrated in the MMC topology. Unlike previously proposed solutions, the e-DCPFC configuration is derived from the conventional MMC and does not need any isolation transformers or external power supply. Its other main features include modular construction, convenience for multiport extension and bidirectional power flow adjustment capability. In this article, the topology, working principle, and the coordinated control strategy for e-DCPFC and its host MMC are elaborated. Experimental results from a lab prototype verify the proposed topology and its performance under typical working conditions. [ABSTRACT FROM AUTHOR]

The grid-connected converters are becoming a fundamental building block in the modern power grid, and the stability of grid-connected converters, especially when they are connected to weak grid, has drawn increasing attention. In order to test the interaction with various grid impedances and validate the stability behaviors, more accurate and advanced emulation of grid impedance is becoming an emerging need. However, the existing approaches are still difficult to flexibly shape the grid impedance under medium-to-high frequency range. In this article, a novel configuration for programable impedance emulation of power grid is proposed. The proposed grid emulator is composed of two power electronics converters with special control functions, i.e., the impedance forming converter (IFC) and power supporting converter (PSC). The IFC is designed with a higher switching frequency to virtually emulate different grid impedances under medium-to-high frequency band. On the other hand, the PSC is designed with a lower switching frequency to generate/absorb the power of the converter under test. Besides, a Norton-circuit-based virtual impedance control strategy is introduced to eliminate the derivation term in the control structure of virtual impedance. Finally, the performance of the proposed emulator is verified by various simulations and experimental prototype. [ABSTRACT FROM AUTHOR]

This article proposed the concept of monopolar fault reconfiguration for bipolar dc distribution system realized with Bipolar Half Bridge (BiHB) converter. Monopolar fault reconfiguration oriented from bipolar high voltage direct current (HVdc) transmission grid. For dc distribution, the operation aim is to provide uninterrupted power supply to critical load under monopolar fault. It requires interfacing circuit with fault-tolerant capability as well as control design considering both bipolar and monopolar working mode. The proposed BiHB converter achieves mode switching by reconfiguring the converter switching pattern. Considering both monopolar and bipolar working mode, a bipolar modulation method is proposed for coordinate circuit design. Key influence factors relating to the transient voltage spikes have been identified, including the fault detection delay and the modulation method. Besides, to suppress the voltage spikes caused by monopolar fault, feed-forward compensation is adopted for smooth mode transition. The proposed BiHB converter and control strategy have been comparatively evaluated under fault protection and restoration. Uninterrupted power supply of load has been attained, in validation of the proposed monopolar fault reconfiguration concept. [ABSTRACT FROM AUTHOR]

A bleeding resistor is usually adopted to be parallel with the capacitor in the submodule (SM) to avoid static voltage imbalance caused by auxiliary power supply during start-up process of modular multilevel converters (MMCs). However, the bleeding resistor causes unnecessary power loss during normal operation. In this article, a resistor controllable SM based on a normally-on silicon carbide (SiC) junction field-effect transistor is proposed to reduce the power loss on bleeding resistor at normal operation of MMCs. The proposed control can not only ensure the static voltage balance during start-up of MMCs, but can also completely eliminate the power loss on bleeding resistor during normal operation of MMCs for high efficiency. In addition, the proposed control can shorten the SM capacitor discharging time at shut-down process. Experimental studies are conducted and the results confirm the effectiveness of the proposed control. [ABSTRACT FROM AUTHOR]

Modular multilevel converter (MMC) based solid state transformer (M-SST) shows promise in hybrid ac/dc distribution grids. To further meet low voltage dc (LVdc) users’ requirements, a bipolar-type M-SST (BM-SST) is developed in this article, which not only has medium voltage ac and dc ports but also directly provides a bipolar low voltage dc (BLVdc) port for various LVdc sources and loads such that an extra SST or voltage balancer for achieving similar outcomes could be saved. Featuring various ac and dc ports, easy voltage conversion, and flexible power control capability, BM-SST is most suitable for multiple ac and dc grids interconnection as an energy hub. The most crucial issue of BM-SST is how to deal with the unbalanced power caused by the bipolar LVdc port which leads to submodule capacitor voltage deviation and uncontrollable circulating current between upper and lower arms of MMC. An arm voltage regulation (AVR) combing with active circulating current injection (ACCI) method is proposed in this article to eliminate the unbalanced power and its negative effects. Based on detailed circuit analysis of the BM-SST, the coordinated control of the BM-SST with optimum AVR&ACCI is designed. Moreover, the effects of AVR&ACCI method on current stress, voltage stress, and power loss of BM-SST are evaluated. The feasibility of the proposed BM-SST has been verified by experimental results. [ABSTRACT FROM AUTHOR]

Impedance modeling of a permanent magnet synchronous generator (PMSG)-based wind energy conversion system (WECS) usually ignores the machine-side dynamics, instead, the machine-side system is simplified as a constant power source, while merely keeping the detailed grid-side converter model. This simplification is mainly based upon the intuition that the dc-bus capacitor is large enough so that the machine- and grid-side dynamics can be decoupled. However, the simplification probably does not hold in practice since there is always a tendency to minimize the capacitor size to achieve a smaller package for installation. Therefore, the machine-side system should not be ignored, otherwise maybe leading to error for stability analysis. To address this issue, this paper developed a complete ac-side impedance model of the PMSG-based WECS. Then, impact factors about the coupling characteristics between machine- and grid-side systems of the PMSG-based WECS are discussed. Finally, a case study of sub/super synchronous oscillation is carried out on a practical offshore wind farm integrated with a modular multilevel converter based high-voltage dc (MMC-HVDC) transmission system in China. The results show that ignoring the machine-side system would lead to an optimistic stability judgement, which confirms the necessity of the complete model of PMSG-based WECS. Furthermore, the system can be stabilized by either changing the couplings between machine- and grid-side systems of PMSG-based WECS or implementing active damping in the wind farm side MMC. [ABSTRACT FROM AUTHOR]

The mission profile emulator (MPE) proves to be an effective and cost-efficient approach to validate the design and reliability of the submodule (SM) in a modular multilevel converter (MMC). This article established the detailed models for the capacitor voltages of SM in the MMC system. The factors that can determine the dc component of capacitor voltages in SM of MMC are analytically solved. The difference between the capacitor voltage of SM in a complete MMC system and that of SM in MPE is discovered, explained, and verified. In order to improve the emulating accuracy for the capacitor voltage in SMs of the MPE, a novel voltage control method, which is more adaptive to the operating conditions of MMC, is proposed. The simulation models and experimental setup are built to verify the validity of the proposed methods. [ABSTRACT FROM AUTHOR]

The modular multilevel converter (MMC) is attractive for high-voltage direct current (HVdc) applications. The dc line short-circuit fault is one of the key challenges for the HVdc system. In this article, a crossing thyristor branches (CTB)-based hybrid MMC (HMMC) with current interruption capability is proposed to protect the HVdc system under dc line short-circuit faults. In the CTB-HMMC, the thyristor branches are proposed to crossing-connect different arm inductors and each arm consists of unipolar full-bridge (UFB) submodules (SMs) and half-bridge SMs, where the CTB effectively reduces the UFB-SMs number in the arm of the HMMC with current interruption capability. The current interruption operations, including dc-side and ac-side current interruption, are also proposed for the CTB-HMMC in case of dc line short-circuit faults, where the proposed CTB-HMMC can interrupt the fault current with the advantages of fewer UFB-SMs in the arm, short current interruption time, low semiconductor cost, and low power losses. The simulation studies are conducted and a down-scale prototype is built, and the study results verify the effectiveness of the proposed CTB-HMMC and operation principles. [ABSTRACT FROM AUTHOR]

Capacitance monitoring is one of the important issues for a modular multilevel converter (MMC) to obtain high reliability. Since pulsewidth modulation is usually implemented in the field-programmable gate array (FPGA), produced precise switching states in the FPGA cannot be directly accessed by the top digital signal processor (DSP) controller, which poses challenges to the existing capacitance monitoring methods based on precise switching states of the MMC. This article presents a submodule (SM) capacitance monitoring strategy for the MMC with a simple algorithm, where the fundamental frequency components of the SM capacitor voltage and current are extracted to estimate the SM capacitance based on reference, but not the precise switching states. The proposed scheme not only simplifies the implementation and calculation, but also avoids control limitations and heavy communication burden between the DSP and the FPGA. Besides, the proposed strategy effectively eliminates noise impact from sensors and increases accuracy. Simulation and experimental studies are implemented, and the results confirm the effectiveness of the proposed strategy. [ABSTRACT FROM AUTHOR]

Frequency-domain model reduction is a crucial concern in applying the prevailing impedance method for the stability analysis of complex systems, e.g., the modular multilevel converter (MMC). Recently, it has been shown that under symmetric conditions, a 2 × 2 matrix-based impedance model characterizing the two coupled frequencies of MMC are sufficient for its stability analysis. However, when the asymmetry occurs, principally, a much higher number of frequency couplings will appear in the MMC and thereby leads to a significant rise in the model dimension. Enlighted by this issue, there is an urgent need of finding a suitable frequency-domain method that can serve as a general criterion for model reduction. To this end, this article proposes a block diagonal dominance (BDD)-based model reduction method and applied it to the asymmetric MMC. Basically, the BDD can decompose an N-dimensional task to N one-dimensional tasks, via which a significant reduction in model dimension can be realized. It is shown that by properly shifting the impedance model from one domain to another (e.g., α-β domain to d-q domain), the BDD property can be achieved for most asymmetric scenarios. Finally, various case studies considering different asymmetry degrees are conducted to validate the effectiveness of the proposed method. [ABSTRACT FROM AUTHOR]