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This article proposes an adaptive dynamic reference (ADR) control for power converters in a microgrid. Conventionally, two separate controllers are required to control the input current and the dc bus voltage. The proposed ADR control eliminates the need for an additional controller by utilizing a simple discrete-time model to generate appropriate input current reference for the dc bus voltage regulation. The proposed technique is easy to design and guarantees a dynamic convergence of the dc bus voltage to its nominal reference even during nonidealities in the system, such as model parameter uncertainties, sensor imperfections, and unmodeled dynamics. A simple design procedure of the control parameters for the desired converter response is also provided based on a theoretical analysis. Unlike traditional linear controllers, the control parameter design of the proposed model is independent of the converter’s operating point. Finally, the performance of the proposed technique is validated experimentally by using a finite control set model predictive control of a typical grid-connected application and is compared with both conventional dynamic reference control and traditional linear controller. [ABSTRACT FROM AUTHOR]

This paper presents a single-objective model predictive control for a bidirectional DC-DC flying capacitor (FC) converter, which integrates a battery to the microgrid. The presence of multiple FCs facilitates the converter to interface a low-voltage battery to a high-voltage DC bus at reduced voltage stress on its power switches. However, such converter requires a controller that fulfils multiple objectives, namely DC bus and FC voltage regulations, and bidirectional power flow. The key feature of the proposed controller is that it utilizes a model predictive control with a single-objective cost function based on battery current and a redundant state selection scheme to attain these multiple objectives. Adopting such a strategy not only reduces computational burden but also eliminates the need for weighting factors and requirement of complex numerical models for its estimation. Another notable aspect is that the controller employs an improved dynamic reference model to generate appropriate battery current reference for the DC bus voltage regulation, without necessitating a secondary control loop and additional current sensors. Finally, experimental results of the proposed system for a step-response of the DC bus voltage, varying PV power, and loads are validated and compared against finite control set model predictive control. [ABSTRACT FROM AUTHOR]

This article presents the modulation and operating limits of the modified cascaded multioutput multilevel converter. A phase-disposition pulsewidth modulation strategy is developed to apply the optimal state that leads to a minimized device switching. A Hamming distance-based cost function is adopted to evaluate the number of device transitions. The cost function is further modified to identify the optimal state from a number of feasible states. Moreover, the operating limits of the converter are analyzed for the safe and valid operation of the converter. Simulation and experiment results validate the theoretical analysis, and the converter is operational under a wide range of operation points. The proposed optimal state modulation strategy is shown to be effective in reducing the average device switching frequency. [ABSTRACT FROM AUTHOR]

This paper introduces a modulation method for the stacked multicell converters (SMCs). The proposed method is implemented using phase-disposition pulse width modulation and is capable of balancing and regulating the voltages of all the capacitors in the topology, i.e., the flying capacitors and the dc-link capacitors. The proposed method is also able to eliminate the low-frequency voltage ripples that may appear in the neutral point (NP) of SMCs. In SMCs with two stacks, the NP voltage level can be generated by direct connection of the output to the dc-link NP, but also using two extra states available. This redundancy is used to regulate the NP voltage. Furthermore, since the proposed method can eliminate the low-frequency voltage ripples in the capacitors, the capacitances of all the capacitors of the SMCs can be reduced, while maintaining high quality of the output voltages and currents. This enables the use of film capacitors. Simulation and experimental results are presented from a three-phase five-level $2\times 2$ SMC to verify the effectiveness of the proposed method. [ABSTRACT FROM AUTHOR]

This letter proposes a phase-shifted pulse-width modulation (PS-PWM) technique for the flying capacitor (FC) multilevel converter that improves the quality of the line-to-line voltages. In traditional PS-PWM, the line-to-line voltages include intervals that switch between nonadjacent voltage levels, which deteriorates the harmonic performance. The proposed PS-PWM constantly achieves line-to-line voltages with switching transitions between two adjacent levels. The proposed modulation technique is implemented using two sets of $n$ evenly phase-shifted carriers that are alternatively applied depending on the instantaneous value of the reference signal of each phase-leg. Furthermore, it can be implemented in FC multilevel converters with any number of levels while maintaining natural capacitor voltage balance. Experimental results from a four-level FC converter prototype are presented. [ABSTRACT FROM AUTHOR]

This paper proposes and evaluates an active voltage-balancing method for stacked multicell converters (SMCs) using phase-shifted pulse width modulation. The method is easy to implement and can be applied to SMCs with any number of levels. The proposed method balances the voltages of the capacitors by modifying the duty cycle of each switch of the SMC using a proportional controller. The direction of the output current and the crossed effect among the duty cycles of the switches is considered and used to improve capacitor voltage balance. The performance of the proposed voltage-balancing method is verified experimentally for different loads, such as linear and nonlinear loads, and under transient conditions. [ABSTRACT FROM PUBLISHER]

This letter proposes a new implementation of phase-disposition pulse-width modulation (PD-PWM) for multilevel flying capacitor (FC) converters using a single triangular carrier. The proposed implementation is much simpler than conventional PD-PWM techniques based on multiple trapezoidal-shaped carriers, generates the same results as far as natural capacitor voltage balance is concerned and offers better quality line-to-line voltages when compared to phase-shifted PWM. The proposed algorithm is based on reshaping the reference signal to fit within the range of a single carrier and assigning each crossing of the reference signal with the carrier to a particular pair of switches at any time. The proposed algorithm is suitable for digital implementation taking maximum benefit from the PWM units available in the processor. Simulation and experimental results are presented from the five-level FC converter to verify the proposed PD-PWM implementation. [ABSTRACT FROM AUTHOR]

This paper proposes a voltage balancing method for stacked multicell converters (SMCs) based on phase-disposition pulse-width modulation (PD-PWM). In order to reduce the switching transitions of the power devices, only optimal transitions between consecutive voltage levels are used. Selection of the optimal transition sequence is performed by evaluating a cost function. Significant reductions in the switching transitions and power losses of the power devices are achieved as compared to the optimal-state voltage balancing method where nonoptimal transitions are allowed. Simulation and experimental results from a seven-level $3\times 2$ SMC verifies that the proposed PD-PWM voltage balancing method is robust under linear/nonlinear loads and transients. [ABSTRACT FROM AUTHOR]

This paper presents a voltage-balancing method for stacked multicell converters (SMCs) based on phase disposition pulsewidth modulation. A cost function is evaluated to select the optimum state from a pool of redundant possible ones at each voltage level. It is demonstrated that, if sawtooth carriers are used for modulation instead of standard triangular ones, significant reduction in the switching frequency and the associated power losses of the power devices can be achieved without affecting the voltage-balancing capability. Experimental results from a seven-level SMC laboratory prototype are provided to verify the proposed voltage-balancing method against linear and nonlinear loads, as well as under transient conditions. [ABSTRACT FROM PUBLISHER]

This paper proposes a voltage balancing method for flying capacitor multilevel converters based on phase-disposition pulse width modulation (PD-PWM) using optimal transitions between converter switching states. The optimal transitions between converter switching states are possible due to the converter states' redundancy and are decided with the continuous evaluation and minimization of a cost function. Simulation and experimental results verify the robustness of the proposed voltage balancing method against different load conditions. A significant reduction of the average switching frequency is achieved, when compared to the use of the optimal state voltage balancing method, where the optimal states are decided independently for each voltage level ignoring how transitions impact on the converter switching frequency. Moreover, the line-to-line output voltages are compared with the ones obtained with other modulation techniques in terms of total harmonic distortion values, and PD-PWM with the proposed voltage balancing method produces the best results. [ABSTRACT FROM AUTHOR]

This paper proposes a voltage balancing method for a flying capacitor multilevel converter based on the phase-disposition sinusoidal pulsewidth modulation (PD-SPWM). This voltage balancing method uses redundant switching states, which are determined by the minimization of a cost function. The implementation of the PD-SPWM using standard triangular carriers leads to additional switching events that are due to the transitions within the same voltage level, thus increasing the switching frequency of the power devices. Such drawback can be avoided by using sawtooth carrier waveforms. Another characteristic of the proposed voltage balancing method is that it does not require the tuning of parameters to achieve optimal performance. Simulation and experimental results confirm the robustness of the proposed method operating with different loads and transient conditions. [ABSTRACT FROM PUBLISHER]

Multilevel flying capacitor (FC) converters provide natural capacitor voltage balance under phase-shifted pulse width modulation (PS-PWM). However, natural balancing may not be robust enough to maintain the voltages at the reference values, especially under certain transient conditions. Furthermore, natural balancing dynamics depend on the load and it may be very slow in some practical applications. Therefore, a more robust balancing mechanism of maintaining the FC voltages at the desired values is required. This paper proposes a new closed-loop voltage-balancing method for the multilevel FC converters using PS-PWM. The proposed method balances the voltages of the FCs by modifying the duty cycle of each switch of the FC converter using a proportional controller. The crossed effect between FC currents and duty cycles is considered and is used for optimal FC voltage balancing. Simulation and experimental results verify that the proposed voltage-balancing method is very robust to different operating conditions, such as load transients, linear/nonlinear and unbalanced loads. [ABSTRACT FROM AUTHOR]

This letter reports a method for the initial charging of capacitors in grid-connected flying capacitor (FC) multilevel converters. A resistor is inserted between each phase of the FC converter and the grid. A voltage balancing algorithm is activated from the beginning of the process and the FC converter generates proper output voltages to achieve balanced charging of both the dc-bus capacitor and the FCs. The proposed initial charging method achieves low voltage and current stress on the power devices and the passive components. The method is simple to implement and can be applied to an FC with any number of levels. Experimental results demonstrate the effectiveness of the proposed method. [ABSTRACT FROM AUTHOR]

In a conventional finite control set model predictive control (FCS-MPC) formulation, active and reactive power control loops rely on the predictive controller, whereas the dc-bus voltage is usually governed by a PI-based control loop. This originates from fact that the dynamic equations for describing the predictions of these variables are heavily coupled. In this paper, a cascade-free FCS-MPC for single-phase grid-connected power converters is presented. The proposed control algorithm is formulated in terms of established dynamic references design, which was originally proposed to directly govern active and reactive power, and dc-voltage in three-phase power converters. In this paper, the dynamic reference design concept is extended to control single-phase grid-connected power converters. The proposed control algorithm does not use instantaneous ac-power calculations; instead, it directly formulates the optimal control problem on the grid-current in the original stationary reference frame. The experimental results obtained with a single-phase grid-connected neutral point clamped (NPC) converter confirm a successful design, where system constraints, e.g., maximum power and weighted switching frequency, are easily taken into account. [ABSTRACT FROM PUBLISHER]

This paper proposes a technique for accurate phase estimation of distorted three-phase grid voltages including unbalanced amplitudes and/or phase angles. An adaptive cascaded delayed signal cancellation (CDSC) strategy is used for the generation of the amplitude balanced three-phase voltages. The CDSC strategy also eliminates both odd and even harmonics from the input voltage. An algorithm is also reported for removing the phase angle deviations from the three-phase voltages including unbalanced phase angles. Finally, a phase-locked loop (PLL) is applied to estimate the phase angle of the reference phase voltage. It requires only one PLL to estimate three phase angles of the thee-phase voltages, respectively, suffering from the unbalanced amplitudes and phase angles. It is also immune to harmonic distortions at changeable frequency environment. When compared to the CDSC-PLL and dqCDSC-PLL techniques reported in the technical literature, it can provide improved phase estimation under amplitude and phase unbalanced condition. Simulated and experimental results of the technique are acquired from the MATLAB/Simulink and dSPACE1104 control board platform, respectively, for a number of case studies observed in the grid voltage. [ABSTRACT FROM AUTHOR]

In this paper, a regeneration protection solution is proposed to address the dc-link overvoltage issue and the unequal power sharing in the parallel uninterruptible power supply (UPS) systems. First, a dc-link voltage protection control strategy is proposed to protect the inverter against the excessive dc-link voltage that may trigger the protection mechanism of the UPS system. In addition, an active power-sharing control strategy by regulating the virtual resistance is proposed to solve the circulating current caused by UPS regeneration issue. Finally, the feasibility of the proposed regeneration protection solution is verified by experimental results from the parallel UPS system prototype. [ABSTRACT FROM AUTHOR]

Reliability worth analysis is of great importance in the area of distribution network planning and operation. The reliability worth's precision can be affected greatly by the customer interruption cost model used. The choice of the cost models can change system and load point reliability indices. In this study, a cascade correlation neural network is adopted to further develop two cost models comprising a probabilistic distribution model and an average or aggregate model. A contingency-based analytical technique is adopted to conduct the reliability worth analysis. Furthermore, the possible effects of adding distributed generation units into the network are evaluated. The proposed approach has been tested on a radial distribution test network evaluating the reliability worth. The results show that the probabilistic distribution model provides a more realistic model for the reliability analysis. [ABSTRACT FROM PUBLISHER]