Redundant Vectors Research Articles

Fault Tolerant Control of Grid Connected Inverters Based on Low-Voltage Ride Through

To enhance the reliability of new energy grid connected systems, a fault-tolerant model predictive control strategy with low-voltage ride-through capability for grid connected inverter bridge arms has been proposed. Firstly, the operational principles following a single-phase bridge arm fault in the grid-tied inverter are analyzed. Then, a fault-tolerant structure for the grid connected inverter is established by creating a virtual bridge arm using direct current-side capacitors. Subsequently, a current prediction model is established, and control is implemented using redundant vectors. Additionally, by dynamically adjusting reactive current compensation commands based on the relationship between grid voltage drops and reactive current commands, the system is ensured to maintain continuous and stable operation even during grid voltage fluctuations. Finally, experimental results demonstrate that this fault-tolerant control strategy can still maintain the continuous and stable operation of the system when bridge arm faults occur due to grid voltage drops at the inverter grid connection point.

Model predictive control for Vienna rectifier with constant frequency based on inductance parameters online identification

When the finite control set model predictive control (FCS-MPC) is applied to the three-level converter, there are problems such as large current harmonics, high requirements for the computing efficiency of the microcontroller, complex multi-objective optimization and limited output vector switching. In addition,the mismatch of inductance parameter may directly affect the observation accuracy of FCS-MPC. To solve the above problem,a double vector model predictive control strategy with constant frequency strategy based on the inductance parameters on-line identification is presented.First, a single objective cost function based on the direct power is constructed by optimizing the redundant vector which is selected to balance the neutral-point potential, the design of weighting factor is avoided.At the same time, in order to effectively reduce the grid current ripple under single vector modulation, space vector pulse width modulation (SVPWM) is realized by combining with zero vector control mode. And, a parameter identification method based on model reference adaptive system(MARS) is proposed to overcome the effect of model parameter mismatch.Finally, the results show that the proposed F-MPCCF has good steady-state and dynamic performance from the static, transient and neutral-point potential control.

Decoupled Modeling and Wide-Range Power Distribution Strategy for the Multisource Inverter in Microgrids

Multi-source inverter (MSI) provides a low-cost and high-power-density solution for microgrids (MGs) applications due to the removal of the dc/dc converter, which offers direct power flow between dc-side and ac-side. The existing modulation strategies for MSI transplanted from those of multilevel inverters experience a limited power distribution range due to the finite control freedom of degree provided by redundant vectors. Moreover, the power distribution between the dc ports is highly coupled with the vector synthesis, this is especially true when the voltages of the dc-port are unbalanced. To address the above issue, this article proposes a decoupled modeling method and a wide-range power distribution strategy for the MSI. The decoupled model is proposed to simplify the modulation implementation process and power distribution analysis. In the proposed model, the reference voltage vector of MSI is proportionally decomposed into two decoupled parts which can be generated independently. As a result, it can not only avoid complex modulation calculations but also ensure decoupled power distribution. Moreover, detailed theoretical analysis indicates that the proposed solution can offer an expanded power distribution range by increasing the switching actions in one switching cycle. Finally, the effectiveness of the proposed multiple-carrier modulation strategy is verified by islanded MGs experimental tests.

Fault-Tolerant Hardware Acceleration for High-Performance Edge-Computing Nodes

High-performance embedded systems with powerful processors, specialized hardware accelerators, and advanced software techniques are all key technologies driving the growth of the IoT. By combining hardware and software techniques, it is possible to increase the overall reliability and safety of these systems by designing embedded architectures that can continue to function correctly in the event of a failure or malfunction. In this work, we fully investigate the integration of a configurable hardware vector acceleration unit in the fault-tolerant RISC-V Klessydra-fT03 soft core, introducing two different redundant vector co-processors coupled with the Interleaved-Multi-Threading paradigm on which the microprocessor is based. We then illustrate the pros and cons of both approaches, comparing their impacts on performance and hardware utilization with their vulnerability, presenting a quantitative large-fault-injection simulation analysis on typical vector computing benchmarks, and comparing and classifying the obtained results. The results demonstrate, under specific conditions, that it is possible to add a hardware co-processor to a fault-tolerant microprocessor, improving performance without degrading safety and reliability.

Optimal Switching Sequence Model Predictive Control for Modular Multilevel Converter

This paper proposes a new optimal switching sequence model predictive control method for modular multilevel converter (MMC). In this method, the common mode voltage (CMV) is cosidered, which makes the model more accurate. The proposed method realizes the optimization of multiple control objectives of output current tracking, circulating current suppression (CCS), and submodule voltage balancing. The sequence that allows the output current to track its reference value is calculated in the <inline-formula><tex-math notation="LaTeX">$\alpha -\beta$</tex-math></inline-formula> coordinate system. And the virtual vectors aim to suppress the circulating current without influencing the output current are inserted. Capacitor voltage balancing is achieved by redundant vectors selection and tranditional selection algorithm. These control objectives are calculated independently, thus weighting factor is not required. At the same time, the computation burden is reduced by restricting the voltage mutation. This method has a satisfactory dynamic response and steady-state performance, the simulation and experimental results are given to verify the effectiveness of the method.

An SVPWM Algorithm for a Novel Multilevel Rectifier with DC-Side Capacitor Voltage Balance

The recently proposed novel unidirectional multilevel rectifier, a three single-phase star-connected multilevel rectifier, has the characteristic of having a large number of DC-side capacitors and a complex capacitor voltage balancing control structure under conventional carrier-based phase-shift sine wave pulse-width modulation (SPWM). Hence, a space vector pulse-width modulation (SVPWM) algorithm for the novel multilevel rectifier is proposed in this paper, which can quickly balance the capacitor voltage without an additional voltage balancing control structure. Firstly, it divides the space vectors of the rectifier, then it determines the two basis voltage vectors that synthesize the output reference voltage. After that, based on the analysis of the relationship between switching states and the charge–discharge of capacitors, the final action sequences of redundant vectors are determined according to the principle of keeping the capacitor charge–discharge time consistent. Thus, the capacitor voltages can be automatically balanced without an additional voltage balancing control structure. Finally, simulation and experimental results validated the feasibility and effectiveness of the proposed SVPWM algorithm. The results also show improvements in current quality, capacitor voltage balance and the fluctuation of the neutral point voltage on the DC-link, allowing for further reduction in the overall volume and cost of the rectifier.

A simplified space vector pulse width modulation method for a three‐level four‐leg inverter considering the neutral point potential balance

AbstractThis paper presents a simple three‐dimensional space vector modulation method in ghγ coordinates for a three‐level four‐leg neutral‐point‐clamped inverter. The idea uses redundant vectors as centre points to establish two‐level space vector diagrams simplifying the original space, the first sector of which is designated as subspaces, where a modulation process is implemented. The duty cycle and all possible switching sequences in a tetrahedron can be calculated automatically without manual storage and can avoid a full‐scale search process. Additionally, the neutral point potential can be balanced by three types of charge through cancellation. Such features enable the inverter to demonstrate good performance under closed‐loop control. Moreover, the proposed method can be applied to a multilevel inverter. Simulation and experimental results demonstrate the effectiveness of the proposed concept.

A Computationally Efficient Finite Control Set Model Predictive Control for Multiphase PMSM Drives

Finite control set model predictive control (FCS-MPC) has been widely studied and applied in power converters and machine drives. It takes advantage of the nonlinear finite switching states of the power converters, and a discrete model is used to predict the behavior of the system. The main advantages of FCS-MPC lie in easy inclusion of nonlinear constraints, simple structure and intuitive concept. However, FCS-MPC applied in multiphase machine drives will face with the challenge of heavy computation cost. In this article, a computationally efficient FCS-MPC method is proposed for multiphase drives. The concept of virtual voltage vector is introduced to eliminate low-order harmonic currents in open-loop mode, resulting in simplified prediction model and cost function. To further simplify the enumerative optimization process, the redundant vectors are eliminated, and only five evaluations are required. By this way, the computation complexity is considerably reduced and is approximately irrespective of phase number. Thus, it can be easily generalized to multiphase drives. Moreover, the merit of simple structure is kept, which is the most important characteristic of FCS-MPC. The proposed method is compared with the existing FCS-MPC methods to illustrate its effectiveness. The experimental results have verified that the proposed method can reduce computation complexity, achieve superior steady-state and dynamic performance.

A Hybrid Modulation Strategy With Neutral Point Voltage Balance Capability for Electrolytic Capacitorless Vienna Rectifiers

Electrolytic capacitorless Vienna rectifiers have many advantages, such as longer lifetime and higher power density. However, the reduced dc-link capacitance aggravates the voltage stress due to the neutral point (NP) fluctuation. In this article, a hybrid modulation strategy combining the modulations of redundant vector and compression vector is proposed, which can realize the NP voltage balance for electrolytic capacitorless Vienna rectifiers. On the basis of analyzing the limitation of NP-balance capability of conventional space vector pulsewidth modulation (PWM) strategy in the region of high modulation index, the negative influence of reduced capacitance on the NP fluctuation and the input current harmonics is revealed. To suppress the NP fluctuation, the compression coefficient is introduced to redistribute the duty cycles of voltage vector, which can reduce the negative effect of medium vector on the NP voltage. In addition, the phase voltage error is modeled to reduce the input current harmonics with the appropriate adjustment factor. The implementation of the proposed strategy is realized by the equivalent carrier-based PWM, which can avoid the sector judgment and angle calculation. By using the proposed strategy, the NP voltage balance and the input current harmonics reduction can be realized simultaneously. The effectiveness of the proposed strategy is verified on an electrolytic capacitorless Vienna rectifier platform.

Model Predictive Duty Cycle Control for Three-Phase Vienna Rectifiers With Reduced Neutral-Point Voltage Ripple Under Unbalanced DC Links

Under unbalanced dc links, the control of a three-phase Vienna rectifier is challenging. To deal with the unbalanced dc links, the optimal switching sequence model predictive control (MPC) regulates the neutral-point (NP) voltage through preselecting a redundant vector and grid current by reconstructing duty cycles of the optimal switching sequence. However, its one-redundant-vector-per-control-interval characteristic leads to large NP voltage ripple. To solve this problem, in this article, a model predictive duty cycle control method is proposed for three-phase Vienna rectifiers under unbalanced dc links. First, we study the impacts of duty cycles on the variations of instantaneous current, and then, the three-phase duty cycles are derived by predefined objective function minimization. Second, considering both the operational characteristics and unbalanced dc links of the Vienna rectifier, a new modification method for duty cycles is presented. This method ensures the grid current quality and decouples the control of grid current and NP voltage. Finally, an MPC-based NP voltage control strategy is proposed to reduce the NP voltage ripple. Experimental results demonstrate that the presented method can effectively reduce the NP voltage ripple.

Neutral Point Voltage Balancing Control Based on Adjusting Application Times of Redundant Vectors for Three-Level NPC Inverter

Three-level neutral-point-clamped (NPC) inverter is used in many industrial applications due to its attractive advantages in terms of harmonics content, achieved power level, and electromagnetic interference reduction. However, the main concern in this topology is the imbalance of the neutral point (NP) voltage in the dc side of the inverter. Indeed, the NP voltage distorts the output voltage of the inverter and increases voltage stress on its switching devices. In this article, a new NP voltage balancing control based on modified three-level space vector pulse width modulation (SVPWM) is proposed. The core idea of this method consists in adjusting the application times of redundant vectors in such a way that the NP voltage is kept balanced. In this method, the appropriate adjustment direction is determined by measuring only the capacitor voltages and the output currents of the inverter. The performances of the proposed method are validated and compared through various experimental tests, and the obtained results show an excellent NP voltage balancing regardless of the modulation index values.

Simplified model predictive current control based on fast vector selection method in a VIENNA rectifier

This paper presents a simplified finite-control-set model predictive control (S-FCS-MPC) based on fast vector selection method in a three-level VIENNA rectifier. This method features a high-power factor, a low input current THD as a well-controlled DC-link voltage with fewer redundant vectors and lower computational load. Moreover, the converter with the proposed control technique exhibits a faster dynamic response in terms of input current and DC-link voltage compared with conventional finite-control-set model predictive control (C-FCS-MPC). In addition, the average switching frequency can be effectively reduced due to fewer switching times in a subset sector using the proposed method, which means fewer switching losses. Finally, the operation principle of the proposed algorithm has been analysed and an execution time comparison between S-FCS-MPC and C-FCS-M has been undertaken. The effectiveness of the proposed control technique has been validated using both simulation and experimental results.

A Novel Low-Frequency Virtual Space Vector Modulation With the Improved Continuity of Output Average-Voltage and Bus Voltage Utilization for Single-Phase Neutral Point Clamped Three-Level Inverter

In the high-power superconducting fusion converter, the single-phase neutral point clamped (NPC) three-level converter is applied to replace the traditional thyristor converter using an integrated gate-commutated thyristor (IGCT) as a power device. The operating frequency of IGCT is usually lower, only a few hundred Hz. An optimized hybrid space vector pulsewidth modulation (SVPWM)-8H is proposed to solve the problems of discontinuous output average voltage, low dc bus voltage utilization rate, dead time compensation, and neutral point potential balance. The new sectors are inserted to make the range of output average voltage of adjacent sectors intersect, which improves the continuity of output average voltage. Furthermore, some sectors are composed of four vectors to reduce the minimum working time of redundant small vectors, improving the dc bus voltage utilization ratio and the converter’s performance. Moreover, by analyzing the influence of dead time on redundant small vectors, redundant vectors are rearranged to make redundant small vectors’ working time synchronously in sector and sector switching, which reduces the influence of dead time on neutral point potential balance. Finally, by analyzing the influence of dead time on the output average voltage, dead time compensation is applied, which reduces the difficulty of compensation implementation. Simulations and experiments verify the rationality of the optimized hybrid SVPWM-8H.

Model Predictive Control for Three-Phase Three-Level NPC Inverter Based APF Interfacing Single Stage Photovoltaic System to the Grid

A finite control set model predictive control (FCS-MPC) based controller has a fast dynamic response and robustness. furthermore, the presence of a cost function gives designers a degree of freedom to include system control targets, constraints and system non-linearities. On the other hand, Multilevel inverter (MI) topologies are becoming a strong alternative in distributed power generation system (DPGS), among these topologies is the three-phase three-level NPC (TTLNPC) inverter. Generally, to properly operate this topology, the applied current control ensures the achievement of two main objectives. First, the output current must be controlled to track its reference. Second, the two dc-link capacitor voltages have to be equal and balanced. In this paper, FCS-MPC is proposed to control the TTLNPC inverter based parallel active power filter (APF) adopted to connect a photovoltaic system (PVS) to the grid and perform a harmonic mitigation. The proposed FCS-MPC exploit the model of the system to predict the future values of the inverter currents by selecting the best voltage vector that aims to minimize a predefined cost function. Instead of using the popular redundant vectors algorithms to balance the two-split dc-link capacitor voltages, another term will be added to the expression of the cost function to achieve this goal. The PV panel is coupled directly to the inverter without DC/DC converter, the P&amp;O MPPT algorithm is responsible to generate the capacitor reference voltage whatever the climatic conditions are. Simulations using Matlab/Simulink were performed to prove the efficiency of the proposed technique to mitigate the grid current harmonics, and to ensure a continuous power injection and perform a load power sharing.

Performance Investigation of Deadbeat Predictive Controllers for Three-Level Neutral Point Clamped Inverter

This article aims to improve the performance of the deadbeat (DB) predictive controller for a three-level neutral point clamped (NPC) inverter. The effect of the number of effective vectors considered for the cost function evaluation on steady-state and dynamic performance is investigated. To do that, three DB predictive controllers with different numbers of effective vectors, namely, 19 vectors-based, six vectors-based, and three vectors-based DB, are compared beside the conventional current-based model predictive control (MPC). The neutral-point (NP) voltage is balanced using the redundant vectors. Simulations and experimental tests are performed to evaluate the performance of the competing MPC algorithms in terms of four main criteria, namely: NP voltage balancing error, total harmonic distortion (THD), the computational effort required, average switching frequency, power loss, and sensitivity to parameters mismatch. Compared to conventional MPC, the experimental results show that the three vectors-based DB predictive controller has the best steady-state and dynamic performance with a reduction of computational burden up to 60% and a reduction of the current THD up to 72%.

Model predictive duty cycle control for three‐phase Vienna rectifiers

The Vienna rectifiers are widely used in industrial applications. The computationally efficient optimal switching sequence model predictive control directly optimizes the switching sequences. Thus, it requires to store the switching states of 25 voltage vectors, which increases the memory usage. Moreover, the neutral-point (NP) voltage is regulated by redundant vector preselection. As only one redundant vector is applied during the whole sampling period, the NP voltage ripple is relatively large, and the grid current performance is still not satisfactory. To overcome these drawbacks, this paper proposes a model predictive duty cycle control strategy for three-phase Vienna rectifiers, which directly optimizes three-phase duty cycles. The three-phase duty cycles are first obtained by predicting the effects of three-phase duty cycles on instantaneous current variations and minimizing a cost function. To guarantee the grid current performance, a novel three-phase duty cycles modification method is proposed by considering the structural characteristics of the Vienna rectifier. To suppress the NP voltage ripple, the obtained three-phase duty cycles are further optimized. With the proposed method, low memory usage, low harmonic distortion in the grid current, and low NP voltage ripple are achieved simultaneously. Experiments are conducted to verify the effectiveness of the proposed method.

New Optimized Control of Cascaded Multilevel Converters for Grid Tied Photovoltaic Power Generation

This paper proposes a new optimized control of photovoltaic two stages conversion cascade composed by Three Levels Boost (TLB) and Three Levels Neutral Point Clamped (TLNPC) inverter. In order to extract the maximum power from photovoltaic generator and get a balanced DC bus voltage, the duty cycles of the two TLB switches are determinate from a Fuzzy Logic Controller (FLC) for the first switch and by adding to the first duty cycle an additional duty cycle obtained by integration of the error between the two capacitors voltages of DC bus. Balancing the bus voltages by the TLB using a single regulator avoid us to use a complex balancing algorithm by the redundant vectors of TLNPC inverter. For the control of the inverter, we used a Proportional Integral (PI) regulator optimized by PSO. This command allows us to have on one side a constant DC bus voltage and a current injection in phase with the grid voltage. To have an efficient follow-up of the TLNPC inverter reference voltages, the Space Vector Pulse Width Modulation (SVPWM) is applied. The simulation is carried out in MATLAB/SIMULINK platform. The results obtained from the application of the FLC command associated with PI PSO are better compared to the simulation without optimization in terms of sum of the absolute values of the errors at the inputs of the three PI regulators.

Computationally Efficient Optimal Switching Sequence Model Predictive Control for Three-Phase Vienna Rectifier Under Balanced and Unbalanced DC Links

Unbalanced dc links usually occur when a Vienna rectifier is used as a charger for series-connected battery packs. The redundant vector preselection optimal switching sequence model predictive control (OSS-MPC) scheme does not require weighting factor tuning and is applicable to unbalanced dc links. However, its use is complicated for switching sequence selections and duty cycle calculations. Therefore, this article proposes a computationally efficient OSS-MPC scheme for three-phase Vienna rectifier operating under balanced and unbalanced dc links. The scheme achieves the same performance as that achieved by the redundant vector preselection OSS-MPC scheme, however, its execution is significantly more efficient. In the proposed method, an unbalanced factor is proposed by considering unbalanced dc links, and a look-up table is derived to describe the equivalence relation among the six feasible switching sequences. Then, duty cycles are calculated for only one switching sequence, and the OSS and its duty cycles are reconstructed based on the predefined table. Unlike the redundant vector preselection OSS-MPC scheme, the enumeration process is eliminated in the proposed method, thus, the computational burden is significantly reduced. Experimental results are provided to validate the effectiveness of the proposed method. This article is accompanied by a PDF demonstrating the look-up tables.

Circulating Current Mitigation and Harmonic Current Compensation for Multifunction Parallel Three-Level Four-Leg Converters

The multifunction parallel three-level four-leg converters are proposed for high-power applications, which combine grid-connected renewable generation and active power filter (APF). However, the challenge of zero-sequence circulating current (ZSCC) is inevitable which degrades the output currents’ quality and reduces the stability of the system. To overcome this limitation, a proportional integral (PI) + feedforward control strategy and space vector modulation based on the nonaxial redundant vectors (NARVs) are proposed in this article. First, the model of ZSCC is established and analyzed in detail. The analysis reveals that ZSCC is determined by zero-sequence duty-cycle difference and zero-sequence benchmark function. Then, by analyzing the space vector of the three-level four-leg converter, it is concluded that ZSCC is affected by different NARVs. Consequently, a PI + feedforward control strategy is proposed to mitigate ZSCC by adjusting the duty cycle of NARVs in each pulse width modulation (PWM) sampling period. The simulation and experimental results verify the effectiveness of the proposed control strategy.

Optimal Vector Sequences for Simultaneous Reduction of the Switching Loss, Zero-Sequence Circulating Current, and Torque Ripple in Two Parallel Interleaved Inverter-Fed PMSM Drives

The two-parallel interleaved converters system is essentially a multilevel converter and introduces more vector redundancies than a single converter, leaving a large space for vector sequences optimization for the simultaneous optimization of crucial indicators. However, the vector redundancies have not been fully explored by previous studies to achieve multiple crucial indicators optimization. This article aims to obtain high efficiency and low torque ripples for the two-parallel interleaved converters-fed permanent-magnet synchronous motor (PMSM), which are dominantly affected by the switching loss, zero-sequence circulating current (ZSCC), and torque ripples. First, given the vector redundancies, this article explores the available vector sequences according to switching times, the minimal torque ripple, and ZSCC with a priority of the switching times. The resultant vector sequences split each 60° sector into eight subsectors, but each subsector has redundant vector sequences. Then, this article further quantitatively analyzes the torque ripple and ZSCC of the redundant vector sequences in the subsectors, based on which each subsector applies an optimal vector sequence. Despite the complexity of the divisions of the subsectors, this article proposes a simple carrier-based modulation scheme and the implementation of the proposed optimal vector sequences amounts to corresponding common-mode voltage (CMV) injections into the references, which only require a simple logic judgment. The theoretical analysis confirms that the proposed method yields smaller torque ripples, ZSCCs, and switching losses than existing pulsewidth modulation (PWM) schemes over the whole modulation indices. Finally, the experimental results further verify the theoretical analysis and the effectiveness of the proposed strategy.