Hossein Sepahvand

Active Capacitor Voltage Balancing in Single-Phase Flying-Capacitor Multilevel Power Converters

Two active capacitor voltage balancing schemes are proposed for single-phase (H-bridge) flying-capacitor multilevel converters. They are based on the circuit equations of flying-capacitor converters. Consequently, they can be implemented using straightforward control rules. In particular, the first technique is based on an algorithm which follows the standard multilevel modulation. Then, it utilizes a redundant state selection table for capacitor voltage balancing. In the second method, multiple duty cycles are defined and modulated in direct response to the capacitor voltages. The most important advantage of these two proposed methods is that they can be utilized to converters with any desired number of levels in their output voltage. Moreover, the analysis and implementation of both methods are straightforward. Through simulation and experimental implementation, these methods are shown to be effective on capacitor voltage regulation in flying-capacitor multilevel converters.

Fault Detection and Mitigation in Multilevel Converter STATCOMs

Many static synchronous compensators (STATCOMs) utilize multilevel converters due to the following: 1) lower harmonic injection into the power system; 2) decreased stress on the electronic components due to decreased voltages; and 3) lower switching losses. One disadvantage, however, is the increased likelihood of a switch failure due to the increased number of switches in a multilevel converter. A single switch failure, however, does not necessarily force an (2n + 1)-level STATCOM offline. Even with a reduced number of switches, a STATCOM can still provide a significant range of control by removing the module of the faulted switch and continuing with (2n - 1) levels. This paper introduces an approach to detect the existence of the faulted switch, identify which switch is faulty, and reconfigure the STATCOM. This approach is illustrated on an eleven-level STATCOM and the effect on the dynamic performance and the total harmonic distortion (THD) is analyzed.

Capacitor Voltage Regulation in Single-DC-Source Cascaded H-Bridge Multilevel Converters Using Phase-Shift Modulation

Cascaded H-bridge multilevel power electronic converters generally require several dc sources. An alternative option is to replace all the separate dc sources feeding the H-bridge cells with capacitors, leaving only one H-bridge cell with a real dc voltage source. This will yield a cost-effective converter. However, the required capacitor voltage balancing is challenging. In this paper, using the phase-shift modulation approach, a new control method for cascaded H-bridge multilevel converters fed with only one independent dc source is presented. The proposed method has a wide voltage regulation range for the replacement capacitors in the H-bridge cells. Experimental and simulation results support the proposed control method.

Investigation on Capacitor Voltage Regulation in Cascaded H-Bridge Multilevel Converters With Fundamental Frequency Switching

Multilevel power electronic converters have gained popularity in high-power applications due to their lower switch voltage stress and modularity. Cascaded H-bridge converters are a promising breed of multilevel converters which generally require several separate dc voltage sources. By utilizing the redundant switching states, it is possible to replace the separate dc voltage sources with capacitors and keep only the one with the highest voltage level. Redundancy in the charge and discharge modes of the capacitors is assumed to be adequate for their voltage regulation. However, the effects of the output current of the converter as well as the time duration of the redundant switching states have been neglected. In this paper, the impacts of the connected load to the cascaded H-bridge converter as well as the switching angles on the voltage regulation of the capacitors are studied. This paper proves that voltage regulation is only attainable in a much limited operating conditions that it was originally reported. In addition, based on the analysis of the converter, a simplified formula is found which can be used to find those modulation indices that regulate the voltage of the capacitor. This formula can be used in harmonic minimization problems while capacitor voltage regulation is ensured. Simulation and laboratory results are provided to confirm the analysis.

Harmonic Distortion Optimization of Cascaded H-Bridge Inverters Considering Device Voltage Drops and Noninteger DC Voltage Ratios

This paper considers achieving the minimum total harmonic distortion (THD) or frequency-weighted THD (WTHD) of the staircase-modulated output voltage of single-phase multilevel inverters, with or without elimination of the lowest order harmonics. The minimal THD values, together with the corresponding step angles and dc voltage source ratios, have been obtained for the 5-, 7-, 9-, 11-, and 13-level cases; accounting for the device voltage drops when the load is resistive or is moderately inductive is described. Similarly, the minimal WTHD values, together with the corresponding step angles and dc source voltage ratios, have been obtained for the five-, seven-, and nine-level waveform cases. The results show that requiring harmonic elimination leads to larger W/THD than the minimum W/THD that can be achieved without this requirement. Furthermore, a 13-level waveform is needed to attain a voltage THD less than 5%, and a nine-level waveform is needed to attain a WTHD less than 0.5%. Experimental measurements are presented as verification of the analytical results.

A generalized capacitor voltage balancing scheme for flying capacitor multilevel converters

Multilevel power electronic converters are the converter of choice in medium-voltage applications due to their reduced switch voltage stress, better harmonic performance, and lower switching losses. Although it has received little attention, the flying-capacitor multilevel converter has a distinct advantage in terms of its ease of capacitor voltage balancing. A number of techniques have been presented in the literature for capacitor voltage balancing, some relying on “self-balancing” properties. However, self balancing cannot guarantee balancing of capacitor voltages in practical applications. Other researchers present closed-loop control schemes which force voltage balancing of capacitors. In this paper, a new closed loop control scheme is proposed which regulates the capacitor voltages for a multilevel flying capacitor converter. The proposed scheme is based on the converter equations and involves implementing simple rules. In particular, multiple duty cycles are defined and modulated in direct response to the capacitor voltages. Through simulation, the method is shown to work on four, eight and nine-level flying capacitor inverters.

Hysteresis-Based Control of a Single-Phase Multilevel Flying Capacitor Active Rectifier

The deployment of a single-phase, multilevel flying capacitor converter as a 1.5-kW active rectifier is discussed in this paper. The challenges associated with this approach include providing unity power factor, regulating the dc output voltage, and maintaining balanced voltages across the flying capacitors. In such applications, the input current is usually controlled via a reference-frame-based control approach. Here, a different approach, which is based on the hysteresis current-mode control scheme, is applied. The simulation and experimental results for both reference-frame-based and hysteresis-based controllers are presented and compared. The results show that the hysteresis-based method exhibits a tighter regulation of the input current and offers easier hardware implementation.

Start-up Procedure and Switching Loss Reduction for a Single-Phase Flying Capacitor Active Rectifier

This paper describes a research conducted on a single-phase flying capacitor active rectifier. An online process that reduces the total number of switching events, which leads to a reduction of switching losses in the converter, is introduced. This method is based on redundant state selection, which is used to regulate the voltage of flying capacitors. The proposed approach is general and can be applied to flying capacitor converters with any number of levels. Furthermore, in order to control the inrush current at the start of operation and avoid extra voltage and current stress for active switches and capacitors, a start-up procedure that precharges the flying capacitors is proposed. With this precharge procedure, no additional hardware is needed. Simulation and laboratory results demonstrate these new concepts.

Fault recovery strategy for hybrid cascaded H-bridge multi-level inverters

In the configuration of the multilevel inverter which is used in this study, two cascaded H-bridge cells are connected in series with each phase of a three-phase three-level neutralpoint clamped (NPC) inverter. The NPC inverter is fed by a single DC source; whereas, all of the cascaded H-bridge cells are supplied by capacitors. In this paper, in order to regulate the voltage across the H-bridge capacitors, a small fundamental harmonic is added to or subtracted from the original PWM reference of each H-bridge cell. In addition, the operation of the inverter during a fault in the cascaded H-bridge cells is studied and a method for recovery from fault and compensation of fault using the remaining H-bridge in the corresponding phase is proposed. This method allows acceptable operation of the inverter even when one of H-bridge cells is not functioning. Verification of the method using simulation shows the proper operation of the voltage capacitor regulation and strategy of fault recovery.

A hybrid multilevel inverter with both staircase and PWM switching schemes

In this paper, a new method to control multilevel converters is proposed. The considered multilevel converter consists of a three-phase three-level diode-clamped converter (main converter). Two H-bridge cells are then connected in series with each output phase of the main converter. The operation of the main converter and one of the cascading Hbridge cells is based on the staircase switching method. The firing angles of these converters are selected in a way that the dc voltage required for the last H-bridge cell is minimized. The switching pattern of the second H-bridge cell is based on the pulse-width modulation method. This last cell generates the remaining parts of the desired sinusoidal output voltage. The combination of these converters and their switching methods result in an output waveform with low harmonics. Here, a fifteen-level converter is designed based on this approach. Simulation results and laboratory measurements verify the effectiveness of the proposed topology and modulation method.