Five-level Flying Capacitor Research Articles

A Novel Unidirectional Three-Phase Multilevel Rectifier Composed of Star-Connected Three Single-Phase Topology Based on Five-Level Flying Capacitor DC–DC Converter

A novel unidirectional three-phase multilevel rectifier composed of star-connected three single-phase topology based on five-level flying capacitor DC-DC converter is proposed, which can be widely applied to many industrial applications, especially for the front-end of multilevel converters for AC adjustable motor drives with open-end winding connection. In addition to not requiring transformers (either line-frequency or high-frequency), compared to other high voltage rectifiers, this proposed topology features fewer active switches with lower voltage stress, a high voltage common DC bus, high input power factor and low input current harmonics. In this paper, a comprehensive analysis to the basic operating principles including the topology configuration, the steady-state modeling and phase-shifted SPWM modulation are presented. A hybrid control strategy is studied, including the input power factor control, the dc-link voltage balancing control and the flying capacitor voltage balancing control with balanced and unbalanced loads. Finally, the experimental results are presented to validate the feasibility and correctness of the proposed solution.

A Second-Order Voltage Ripple Suppression Strategy of Five-Level Flying Capacitor Rectifiers Under Unbalanced AC Voltages

Unbalanced ac voltages drastically deteriorate the performance of five-level flying capacitor (FC) rectifiers. Without introducing an extra buffering circuit to absorb the second-order oscillating power, it is an incompatible contradiction for realizing both negative-sequence current reduction and second-order voltage ripple suppression. In this article, based on the idea of using FCs in the rectifier to absorb the oscillating power, a second-order voltage ripple suppression strategy for unbalanced ac voltage conditions is proposed. Reducing the negative-sequence currents and suppressing the dc voltage ripples can be simultaneously realized. Simulations and experimental results verified the effectiveness and performance of the proposed scheme well.

A Carrier-Based Discontinuous PWM Method for a Five-Level Flying Capacitor Rectifier With Unbalanced DC-Link Voltages

Unbalanced dc-link voltages occur when five-level flying capacitor (5L-FC) rectifier has dual dc loads of different rated voltages. Compared with traditional continuous pulsewidth modulation (CPWM) strategies, discontinuous PWM (DPWM) methods have more advantages in high-frequency and high-power applications for less commutation and higher efficiency. Therefore, this article proposes a carrier-based DPWM method for 5L-FC rectifiers with unbalanced dc-link voltages. The proposed method is first analyzed in the revised space vector diagram considering unbalanced dc voltages to divide subsectors and find available clamping modes, then it is implemented by applying carrier-based PWM with injecting clamping compensation component, which greatly reduces the implementation complexity. There is a pair of redundant clamping modes with opposite effects on the neutral-point (NP) in each subsector. Thus, the NP voltage can be actively controlled by selecting suitable clamping mode in each subsector. In addition, the allowable ranges of unbalanced voltage degree and unbalanced power degree are investigated in detail. Simulations and experiments verified the effectiveness and performance of the proposed method.

Static Synchronous Series Compensator (SSSC) for Series Compensation of Transmission Line using Flying Capacitor Multilevel Inverter (FCMLI)

This paper deals with Five-Level Flying Capacitor Multilevel Inverter (FCMLI) based Static Synchronous Series Compensator (SSSC) as used for the series compensation of transmission line. Here, voltage is injected in series with the transmission line. It is preferred to other compensating devices because apart from controlling the active and reactive power flow, it helps to damp out unwanted oscillations, such as reducing the Sub-Synchronous Resonance (SSR), Ferro-resonance and thus helps in improving the Power Quality (PQ). Hence, FCMLI is used which helps to ameliorate the output waveform and reduces the auxiliary filtering requirements. Moreover, the control logic for the inverter that has been shown in such a way that voltage stress across each capacitor connected to DC-link voltage is equal. The control logic has been implemented in MATLAB/Simulink environment and the result is shown as there are no SSR phenomena and considerable less line loss.

Capacitor Voltages Measurement and Balancing in Flying Capacitor Multilevel Converters Utilizing a Single Voltage Sensor

This paper proposes a new method for measuring capacitor voltages in multilevel flying capacitor (FC) converters that requires only one voltage sensor per phase leg. Multiple dc voltage sensors traditionally used to measure the capacitor voltages are replaced with a single voltage sensor at the ac side of the phase leg. The proposed method is subsequently used to balance the capacitor voltages using only the measured ac voltage. The operation of the proposed measurement and balancing method is independent of the number of the converter levels. Experimental results presented for a five-level FC converter verify effective operation of the proposed method.

A Novel Five-Level Flying-Capacitor Dual Buck Inverter

This paper focuses on the development of a Five-Level Flying-Capacitor Dual Buck Inverter (FLFCDBI) based on the main circuit of dual buck inverters. This topology has been described as not having any shoot-through problems, no body-diode reverse recovery problems and the half-cycle work mode found in the traditional Multi-Level Flying-Capacitor Inverter (MLFCI). It has been shown that the flying-capacitor voltages of this inverter can be regulated by the redundant state selection within one pole. The voltage balance of the flying-capacitors can be achieved by charging or discharging in the positive (negative) half cycles by choosing the proper logical algorithms. This system has a simple structure but demonstrates improved performance and reliability. The validity of this inverter is conformed through computer-aided simulation and experimental investigations.

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.

A Novel Modulation Technique and a New Balancing Control Strategy for a Single-Phase Five-Level ANPC Converter

This paper proposes a novel modulation technique and a new balancing control strategy for a single-phase five-level flying-capacitor (FC)-based active-neutral-point-clamped (ANPC) converter. The proposed modulator can control the FC voltage to follow the requested reference value and simultaneously generate the required ac output voltage regardless of the values of the dc capacitor voltages of the converter. By implementing this method, smaller values of the dc-link capacitor and FC can be used even in applications that could experience ripple or transient in the capacitor voltage. In a single-phase five-level ANPC converter applications, where the capacitors can experience pulsation power and dc-link balancing issues, such as grid-connected photovoltaic system, the selection of the reference voltage value for the FC can play an important role to balance the average values of the dc-link capacitor voltage. The proposed new control strategy uses a new reference voltage for the FC to be applied by the new modulator to have an average balanced dc-link voltages as well as an ac output voltage with good power quality. Simulation studies and experimental results demonstrate the effectiveness of the proposed modulation technique and control strategy even with relatively small dc capacitors to produce high-quality output voltage and current waveforms while maintaining an average balanced dc-link voltages.

Single-Phase Semi-Bridge Five-Level Flying-Capacitor Rectifier

For unity power factor applications such as grid-connected rectifiers, semi-bridge converters offer significant advantages over their full-bridge counterparts because of their reduced active switch count and shoot-through-free phase leg structure. However, semi-bridge rectifiers have intrinsic operating limits that require a tradeoff between current distortion and switching ripple. This paper presents a new single-phase semi-bridge flying-capacitor (FC) rectifier that significantly improves this tradeoff, with effective doubling in switching frequency because of the multilevel topology, and the capability to rectify higher output voltages with lower voltage rated, more efficient, devices. For rectifier applications, the new topology offers a better balance between cost and performance than either a diode rectifier followed by a single-phase-leg three-level boost power factor corrector (PFC) or a full-bridge single-phase five-level FC rectifier. Matching simulation and experimental results are presented to fully validate the new converter structure.

Hybrid Flying-Capacitor-Based Active-Neutral-Point-Clamped Five-Level Converter Operated With SHE-PWM

A five-level flying-capacitor (FC)-based active-neutral-point-clamped (ANPC) converter is an arrangement of a three-level ANPC converter and a two-level cell. In this paper, a control strategy is proposed to regulate the voltage across the FCs at their respective reference voltage levels by swapping the switching patterns of the switches based on the polarity of the output current, the polarity of the FC voltage, and the polarity of the fundamental line-to-neutral voltage under selective harmonic elimination pulsewidth modulation. The voltage across the FCs and the dc-link capacitors are simultaneously controlled at their reference voltage levels. The proposed control strategy is applied using power system computer aided design/electromagnetic transients including dc on a static synchronous compensator operating under a power-factor-correction mode to verify its performance. Experimental results are also presented for low and high number of angles per quarter period using a low-power laboratory prototype.

Practical Results of a Five-level Flying Capacitor Inverter

This paper investigates the realization of a five-level Flying Capacitor Inverter. After a brief description of general Power Electronic Converters and an introduction to the advantages of Multilevel Inverters over conventional two-level Inverters the main focus is on the five-level Flying Capacitor Inverter. The Flying Capacitor Multilevel Inverter (FCMI) is a Multilevel Inverter (MI) where the capacitor voltage can be balanced using only a control strategy for any number of levels. After a general description of five-level FCMI topology, the simulation and experimental results are presented. The capacitor voltage is stabilized here with various output voltage amplitude values. The simulation and experimental results of five-level FCMI show that the voltage is stabilized on capacitors using the control strategy. A single-phase five-level FCMI model is currently being developed and constructed in the laboratory. Some of the experimental results are available.

Dynamic Voltage Restorer Based on Flying Capacitor Multilevel Converters Operated by Repetitive Control

This paper presents the control system based on the so-called repetitive control for a five-level flying-capacitor dynamic voltage restorer (DVR). This DVR multilevel topology is suitable for medium-voltage applications and operated by the control scheme developed in this paper. It is able to mitigate power-quality disturbances, such as voltage sags, harmonic voltages, and voltage imbalances simultaneously within a bandwidth. The control structure has been divided into three subsystems; the first one improves the transient response of the filter used to eliminate the modulation high-frequency harmonics, the second one deals with the load voltage; and the third is charged with maintaining balanced voltages in the flying capacitors. The well-developed graphical facilities available in PSCAD/EMTDC are used to carry out all modelling aspects of the repetitive controller and test system. Simulation results show that the control approach performs very effectively and yields excellent voltage regulation.