Keith A. Corzine

Control of cascaded multi-level inverters

A new type of multi-level inverter is introduced which is created by cascading two three-phase three-level inverters using the load connection. This new inverter can operate as a nine-level inverter and naturally splits the power conversion into a higher voltage lower-frequency inverter and a lower-voltage higher frequency inverter. This type of system presents particular advantages to naval ship propulsion systems which rely on high power quality, survivable drives. New control methods are described involving both joint and separate control of the individual three-level inverters. Simulation results demonstrate the effectiveness of both controls. A laboratory set-up at the Naval Surface Warfare Center power electronics laboratory was used to validate the proposed joint-inverter control. Due to the effect of compounding levels in the cascaded inverter, a high number of levels are available resulting in a voltage THD of 9 % (without filtering).

Performance characteristics of a cascaded two-level converter

A cascaded two-level power converter is proposed which utilizes two six-transistor inverters and is capable of producing voltages which are identical to those of three-level and four-level converters. Since the machine voltages are the same, the converter performance is the same as is verified through laboratory tests. The advantages and disadvantages of the proposed cascaded converter are explored. The proposed converter is simpler to construct and offers more nonredundant switching states per number of active semiconductors than standard multi-level converters.

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.

Multiple Reference Frame-Based Control of Three-Phase PWM Boost Rectifiers under Unbalanced and Distorted Input Conditions

Many control algorithms and circuits for three-phase pulse width modulation active rectifiers have been proposed in the past decades. In most of the research, it is often assumed that the input voltages are balanced or contain only fundamental frequency components. In this paper, a selective harmonic compensation method is proposed based on an improved multiple reference frame algorithm, which decouples signals of different frequencies before reference frame transformation. This technique eliminates interactions between the fundamental-frequency positive-sequence components and harmonic and/or negative-sequence components in the input currents, so that fast and accurate regulation of harmonic and unbalanced currents can be achieved. A decoupled phase-locked loop algorithm is used for proper synchronization with the utility voltage, which also benefits from the multiple reference frame technique. The proposed control method leads to considerable reduction in low-order harmonic contents in the rectifier input current and achieves almost zero steady-state error through feedback loops. Extensive experimental tests based on a fixed-point digital signal processor controlled 2 kW prototype are used to verify the effectiveness of the proposed ideas.

Transient and dynamic average-value modeling of synchronous machine fed load-commutated converters

A new average-value model of a synchronous machine fed load-commutated converter is set forth in which the stator dynamics are combined with the DC link dynamics. This model is shown to he extremely accurate in predicting system transients and in predicting frequency-domain characteristics such as the impedance looking into the synchronous machine fed load-commutated converter. The model is verified against a detailed computer simulation and against a hardware test system, thus providing a three-way comparison. The proposed model is shown to be much more accurate than models in which the stator dynamics are neglected.

Small-Signal Impedance Measurement of Power-Electronics-Based AC Power Systems Using Line-to-Line Current Injection

Naval ships as well as aerospace power systems are incorporating a greater degree of power electronic switching sources and loads. Although these components provide exceptional performance, they are prone to instability due to their high efficiency and constant power characteristics that can exhibit negative impedance nature at certain frequencies. When designing these systems, integrators must consider the impedance versus frequency at an interface (which designates source and load). Stability criteria have been developed in terms of source and load impedances for both dc and ac systems, and it is often helpful to have techniques for impedance measurement. For dc systems, the measurement techniques have been well established. This paper introduces a new method of impedance measurement for three-phase ac systems. By injecting an unbalanced line-to-line current between two lines of the ac system, all impedance information in the traditional synchronous reference frame d-q model can be determined. For medium-voltage systems, the proposed technique is simpler and less costly than having an injection circuit for each phase. Since the current injection is between only two phase lines, the proposed measurement device can be used for both ac and dc interfaces. Simulation and laboratory measurements demonstrate the effectiveness of this new technique.

Multilevel voltage-source duty-cycle modulation: analysis and implementation

Multilevel converters have become increasingly popular due to high power quality, high-voltage capability, low switching losses, and low electromagnetic compatibility concerns. Considering these advantages, the multilevel converter is a suitable candidate for implementation of future naval ship propulsion systems. This paper focuses on modulation techniques for the multilevel converter. In particular, a novel voltage-source method of multilevel modulation is introduced and compared to existing methods. The proposed method is discrete in nature and can therefore be readily implemented on a digital signal processor. The method is also readily extendable to any number of voltage levels. Results of experimental implementation are demonstrated using a four-level rectifier/inverter system, which incorporates diode-clamped multilevel converters and an 11-level cascaded multilevel H-bridge inverter.

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.

Reduced-parts-count multilevel rectifiers

Multi-level power converters have gained much attention due to their high power quality, low switching losses, and high-voltage capability. These advantages make the multi-level converter a candidate topology for the next generation of naval ship propulsion systems. The primary disadvantage of these systems is the large number of semiconductors involved. This paper presents a reduced parts-count rectifier which is well suited for naval rectifier applications where bi-directional power flow is not required. The proposed converter is analyzed and experimentally verified on an 18 kW four-level rectifier/inverter system.

New Techniques for Measuring Impedance Characteristics of Three-Phase AC Power Systems

Naval ships as well as aerospace power systems are incorporating an increasing amount of power electronic switching sources and loads. Although these power-electronics-based components provide exceptional performance, they are prone to instability due to their constant power characteristics that lead to negative impedance. When designing these systems, integrators must consider the impedance versus frequency at a power system interface (which designates source and load). Stability criteria have been developed in terms of source and load impedance for both dc and ac power systems, and it is often helpful to have techniques for impedance measurement. For dc power systems, the measurement techniques have been well established. This paper suggests several methods for measuring ac impedance including utilization of power converters, wound-rotor induction machines, and chopper circuits. Simulation and laboratory results on an example ac power system demonstrate the effectiveness of the proposed methods.