Amer M. Y. M. Ghias

Energy Management and Control System for Laboratory Scale Microgrid Based Wind-PV-Battery

This paper proposes an energy management and control system for laboratory scale microgrid based on hybrid energy resources such as wind, solar, and battery. Power converters and control algorithms have been used along with dedicated energy resources for the efficient operation of the microgrid. The control algorithms are developed to provide power compatibility and energy management between different resources in the microgrid. It provides stable operation of the control in all microgrid subsystems under various power generation and load conditions. The proposed microgrid, based on hybrid energy resources, operates in autonomous mode and has an open architecture platform for testing multiple different control configurations. A real-time control system has been used to operate and validate the hybrid resources in the microgrid experimentally. The proposed laboratory scale microgrid can be used as a benchmark for future research in smart grid applications.

Voltage-Balancing Method Using Phase-Shifted PWM for the Flying Capacitor Multilevel Converter

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.

Voltage Balancing Method for a Flying Capacitor Multilevel Converter Using Phase Disposition PWM

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.

Initial Capacitor Charging in Grid-Connected Flying Capacitor Multilevel Converters

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.

Single-Carrier Phase-Disposition PWM Implementation for Multilevel Flying Capacitor Converters

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.

Voltage-Balancing Method for Stacked Multicell Converters Using Phase-Disposition PWM

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.

Performance evaluation of a five-level flying capacitor converter with reduced DC bus capacitance under two different modulation schemes

A back-to-back HVDC converter system is typically used to connect two separate electrical grids or a remote large-scale renewable energy system with the electrical grid through a DC bus. The DC bus typically contains a large electrolytic capacitor giving a robust performance to the converter system. However, it is well known that the electrolytic DC bus capacitor is bulky and has a low reliability. Therefore, a possible solution to these drawbacks is to use a reduced DC bus capacitance, which will allow film capacitors to be used instead of electrolytic capacitors. This paper presents the performance evaluation of a five-level flying capacitor (FC) back-to-back converter system with reduced DC bus capacitance under different modulation schemes. Two carrier-based modulation strategies are studied, namely the phase shifted pulse-width modulation (PS-PWM) and the phase disposition pulse-width modulation (PD-PWM), along with their appropriate flying capacitor voltage balancing techniques. The classical synchronous reference frame control strategy has been implemented in order to provide DC bus voltage regulation and active/reactive power control. The performance of the two different modulation schemes is tested against the DC bus and FCs voltage ripples using the MATLAB/Simulink-PLECS software platform. The analyzed case studies demonstrate that the PS-PWM strategy is more suitable to operate the proposed FC converter system using reduced values for the DC bus and flying capacitors.

On Reducing Power Losses in Stack Multicell Converters with Optimal Voltage Balancing Method

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 3×2 SMC verifies that the proposed PD-PWM voltage balancing method is robust under linear/nonlinear loads and transients.

An Active Voltage-Balancing Method Based on Phase-Shifted PWM for Stacked Multicell Converters

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.

Voltage balancing of a five-level flying capacitor converter using optimum switching transitions

The flying capacitor (FC) multilevel converter requires a capacitor voltage balancing mechanism for proper operation. This paper proposes a voltage balancing scheme for a five-level FC converter based on phase disposition pulse-width modulation (PD-PWM) using transitions that produce the minimum number of switching events. This strategy will be called optimal-transition voltage balancing (OTVB) scheme. Since multiple optimum switching transitions between two consecutive voltage levels may be available, such a redundancy is used to regulate the FC voltages at their desired levels. Those transitions that produce more switching events are avoided. The rest of transitions are evaluated by means of a cost function and the one that produces the minimum value is selected. Simulation results show a significant reduction of the average switching frequency as compared to the use of the optimal-state voltage balancing (OSVB) scheme, while maintaining the balance of the FC voltages. Moreover, the proposed PD-PWM voltage balancing strategy is robust to static and dynamic load conditions.