Ashish R. Bendre

Modeling and design of a neutral-point voltage regulator for a three-level diode-clamped inverter using multiple-carrier modulation

The three-level diode-clamped multilevel converter commonly called the neutral-point-clamped converter has become established to be a preferred topology for high-power motor drive applications operating at several kilovolts. Although solutions to the problem of maintaining a stable neutral-point voltage in the converter continue to be the topic of research, a simple solution based on a design-oriented dynamic model of the system is not widely known. This paper presents the design, analysis, and implementation of a simple neutral-point voltage regulator for a three-level diode-clamped multilevel inverter, which uses a multiple-carrier sine-triangle modulator in conjunction with a closed-loop controller for neutral-point regulation. Redundant state choices are controlled via a continuous offset voltage that regulates the dc injection into the midpoint of the dc bus. A small-signal transfer function is developed in closed form, for neutral-point regulation, with the voltage offset as the control variable. Besides maintaining dc-bus voltage balance, the use of the approach leads to a significant reduction in the voltage distortion at the neutral point, allowing a definitive reduction in the required dc bus capacitance. Analytical, computer simulation, and experimental results verifying the approach are presented in this paper.

Circuit Breaker Technologies for Advanced Ship Power Systems

The AC power systems on navy ships use circuit breakers based on traditional commercial technology. New naval power systems employ higher voltage DC distribution and use solid state power converters that actively (and instantaneously) limit the available fault current. When conventional circuit breakers are used in these systems, they have a relatively long clearing time, causing the voltage to collapse for a significant time. The application of fast-acting solid state circuit breakers, based on IGBTs and IGCTs will eliminate such voltage dips and result in a superior power system. Solid state circuit breakers also offer advantages when used in traditional ship power systems by limiting fault currents to levels far below those dictated by the supplying generators. This leads to lower magnetic and thermal stresses on distribution components, lower energy delivered to faults, faster fault isolation and minimal voltage disturbance.

Unity Power Factor Control for Three-Phase Three-Level Rectifiers Without Current Sensors

Three-level rectifiers with reduced number of switches (such as the Vienna rectifier) to improve the input power quality of rectifier systems have been receiving wide interest in the past years. In this paper, a new carrier-based pulsewidth-modulation control algorithm is proposed for such converters to eliminate the low-frequency harmonics in the line current while achieving unity power factor at the rectifier input terminals. The operating constraints of the Vienna rectifier with the carrier-based modulation strategy are examined carefully, and the proposed control algorithm ensures that appropriate voltage/current directional constraints are met. A promising cost-reduction opportunity can be seen with the elimination of input current sensing to operate the Vienna rectifier. The control algorithm is verified via Saber simulation and experimental results.

Mitigating Circulating Common-Mode Currents Between Parallel Soft-Switched Drive Systems

A mathematical model that Is developed for a generalized drive system, including common-mode passive and active elements, is used to explore the issues of paralleling soft-switched resonant dc-link drive systems. Differences between the modulation pattern for each drive system cause common voltage disturbances, which lead to significant circulating currents between the drive systems. Control methods for actively compensating for common-mode circulating currents or reducing the common- mode voltage disturbances are investigated. Practical modifications to the drive system controls are implemented to reduce the circulating currents between paralleled systems.

Equipment failures caused by power quality disturbances

This paper provides insight into a failure mechanism that impacts a broad range of industrial equipment. Voltage surges have often been blamed for unexplained equipment failure in the field. Extensive voltage monitoring data suggests that voltage sags occur much more frequently than voltage surges, and that current surges that accompany voltage sag recovery may be the actual culprit causing equipment damage. A serious limitation in equipment specification is highlighted, pointing to what is possibly the root-cause for a large percentage of unexplained equipment field failures. This paper also outlines the need for a standard governing the behavior of equipment under voltage sags, and suggests solutions to protect existing equipment in the field.

Implementation of a Non-Isolated Three Level DC/DC Converter Suitable for High Power Systems

This paper describes the implementation of a non-isolated three-level DC/DC buck converter that is suitable for high power systems. This topology has the advantage of achieving all of the design objectives for this classification of power converters. The authors have developed, built and tested a system which utilizes a general purpose power electronic module, which can be configured via software/firmware programming and modifications to its input/output power connections to implement various types of power systems. The work behind this paper has proven the usefulness of this topology and has addressed many of the issues associated with successful implementation and commercialization. The paper describes the control implementations and simulation results compared to measured results used in tuning and commissioning of the system. The final 500 V, 100 kW system is implemented in hardware and experimental results are provided showing its performance under step load and step short circuit conditions.

Neutral current ripple minimization in a three-level rectifier

This paper proposes a novel neutral-point voltage regulator for a three-level diode clamped rectifier that uses a fast space vector modulator in conjunction with a sharing function controller for neutral-point regulation. Redundant state choices are controlled via a continuous sharing function, and small signal models are developed for closed-loop regulators with this sharing function as the control variable. A significant improvement is seen in the voltage distortion at the neutral point when the regulator is used, and this leads to a definitive reduction in the required dc bus capacitance.

Implementation of a Four-Pole Dead-Time-Compensated Neutral-Point-Clamped Three-Phase Inverter With Low Common-Mode Voltage Output

This paper describes the implementation of a four-pole neutral-point-clamped (NPC) three-phase inverter that produces virtually no common-mode voltage. The low common-mode voltage output is achieved by constraining the switch states of the NPC inverter to only those states that produce zero common-mode voltage in dead-time compensation, which enhances the capability of the circuit to produce low common-mode voltage by compensating common-mode voltages produced by reverse diode commutations. The low common-mode voltage performance is achieved at the expense of reduced voltage utilization and loss of DC-link voltage balance control. In order to overcome this limitation, a fourth pole and associated control are added to balance the upper and lower dc-link voltages. This paper describes the control implementations and simulation results compared to measured results used in tuning and commissioning of the system. A 450-V 78-kW system is implemented in hardware, and experimental results are provided, showing its differential mode transient and steady-state harmonic performance as well as its common-mode output voltage.

Power-Dense Shipboard-Compatible Low-Horsepower Variable-Frequency Drives

A variable-frequency drive (VFD) having a 440-V front-end current source rectifier (CSR) interface to a voltage source inverter (VSI) feeding a Permanent-Magnet Axial-flux Air Core motor combination is a solution for low-horsepower pump and fan control that is both power dense and compatible with a shipboard environment. This paper describes the control and power platforms for the CSR/VSI and provides experimental results for a 250-V 3.3-kW system. Power density and efficiency comparisons are made between equivalent CSR/VSI- and voltage-source-conversion-based VFDs to demonstrate that the CSR/VSI-based VFD is more power dense.

Floating capacitor voltage regulation in diode clamped hybrid multilevel converters

The family of hybrid topologies consisting primarily of a main neutral point clamped (NPC) 3-phase inverter followed by one or more series connected H-bridges in each output phase show promise in medium voltage applications requiring high power quality and minimal size and weight. Another distinct advantage of this topology is the ability to supply the main NPC converter with a single independent fixed or variable DC source if floating capacitors are used in the series connected H-bridge inverters. A new method is proposed for regulating floating capacitor voltages in the series connected H-bridge inverters. The proposed method provides independent control of each capacitor by decoupling floating capacitor voltage regulation of a particular capacitor from all other floating capacitors within the converter.