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

Online Synchronous Machine Parameter Extraction from Small-Signal Injection Techniques

This paper proposes using a novel line-to-line voltage perturbation as a technique for online measurement of synchronous machine parameters. The perturbation is created by a chopper circuit connected between two phases of the machine. Using this method, it is possible to obtain the full set of four complex small-signal impedances of the synchronous machine d-q model over a wide frequency range. Typically, two chopper switching frequencies are needed to obtain one data point. However, it is shown herein that, due to the symmetry of the machine equations, only one chopper switching frequency is needed to obtain the information. A 3.7-kW machine system is simulated, and then constructed for validation of the impedance measurement technique. A genetic algorithm is then used to obtain IEEE standard model parameters from the d -q impedances. The resulting parameters are shown to be similar to those obtained by a series of tests involving synchronous reactance measurements and a standstill frequency response

Overdistention Operation of Cascaded Multilevel Inverters

In the past decade, the multilevel power converter has transitioned from an experimental concept to a standard product of many medium-voltage drive manufacturers. By utilizing small voltage steps, the multilevel topology offers higher power quality, higher voltage capability, lower switching losses, and improved electromagnetic compatibility over standard topologies. Recently, several researchers have focused on the cascaded multilevel inverter whereby two multilevel inverters are series connected to a motor load by splitting the neutral connection. The resulting performance is exceptional in terms of power quality since the overall number of voltage levels is effectively the product of the two cascaded inverters. This paper demonstrates that it is possible to extend this performance to an even higher number of voltage levels referred to as overdistended operation. This further improves the power quality that is significant in applications that have stringent total harmonic distorsion requirements, such as naval ship propulsion. A new control is introduced for overdistention operation and is validated with computer simulation and laboratory measurements

Over-Distention Operation of Cascaded Multilevel Inverters

Established research has shown that cascaded multilevel inverters can provide more voltage vectors per number of active semiconductors compared to typical multilevel converters. This feature is significant for increasing the drive performance as well as reducing the drive complexity and losses. When two inverters are cascaded, the maximum number of effective levels (or maximal distention operation) is the product of the number of levels of the individual inverters. It is possible to operate the cascaded inverter beyond maximum distention. The over-distention operation is desirable since it effectively increases the number of voltage levels in spite of some missing switching levels. This paper studies over-distention operation based on an inverter system where two three-level inverters are cascaded, which can generate eleven equivalent converting levels instead of nine levels under maximal distention condition. An advanced modulation technique is introduced to handle both the missing line-to-ground voltage levels and the balance of DC link capacitor voltages in over-distention operation. Computer simulation and experimental validation are presented to verify the proposed methods

Optimal SVM Switching for a Multilevel Multi-Phase Machine using Modified Discrete PSO

This paper searches for the best possible switching sequence in a multilevel multi-phase inverter that gives the lowest amount of voltage harmonics. A modified discrete particle swarm (MDPSO) algorithm is used in an attempt to find the optimal space vector modulation switching sequence that results in the lowest voltage THD. As with typical PSO cognitive and social parameters are used to guide the search, but an additional mutation term is added to broaden the amount of area searched. The search space is the feasible solutions for the predetermined vectors at a given modulation index. Comparison of the MDPSO algorithm to an integer particle swarm optimization (IPSO) is presented for all three modulation indices tested. The resulting switching sequences found show that the MDPSO algorithm is capable of finding a minimal THD solution for all modulations indices tested. The MDPSO algorithm performed better overall than the IPSO in terms of converging to the best solution with significantly lower iterations

AC Impedance Measurement Techniques

Naval ship as well as aerospace power systems are incorporating a greater degree of power electronic switching sources and loads. Although these power electronics based components provide exceptional performance, they are prone to instability due to their high efficiency and constant power characteristics which lead to negative impedance. When designing these systems, integrators must consider the impedance versus frequency at a system interface (which designates source and load). Stability criterions have been developed in terms of source and load impedance 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 suggests several methods for measuring ac impedance including utilization of power converters, induction machines and chopper circuits. Simulation results on an example ac system demonstrate the effectiveness of the proposed method

Extending Voltage Range and Reducing Torque Ripple of Five-Phase Motor Drives with Added Voltage Harmonics

As multi-phase (defined as greater than three-phase) drives become more popular and practical, new research in this area investigates potential advantages including lower torque ripple and better power density. The added dimensions of a multi-phase machine leads to a completely different operating nature than standard three-phase machines. It can be shown physically and mathematically that certain harmonics do not contribute to torque production and therefore the torque is not directly tied to the current wave-shape. This paper utilizes this property to demonstrate a substantial increase in voltage range and a reduction in torque ripple through the use of added voltage harmonics. An analysis of a five-phase motor is presented followed by a range of modulation techniques. It is shown that by proper selection of third, fifth, and seventh harmonics, the required dc voltage can be reduced by eighteen percent and the torque ripple can be reduced by nearly sixty percent over traditional methods at the expense of higher current THD; which may not be a disadvantage in certain applications. Further investigation is then carried out in applying a unique space-vector modulation patter to the five-phase motor drive. This further reduces the torque ripple. Detailed simulation and laboratory tests are used to demonstrate this concept

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)

Control of Cascaded Multilevel Inverters

A new type of multilevel inverter is introduced which is created by cascading two three-phase three-level inverters using the load connection, but requires only one dc voltage source. This new inverter can operate as a seven-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)

Voltage Balancing Control of Diode-Clamped Multilevel Rectifier/Inverter Systems

This paper presents a new voltage balancing control for the diode-clamped multilevel rectifier/inverter system. A complete analysis of the voltage balance theory for a 5-level back-to-back system is given. The analysis is based on fundamental frequency switching control and then extended to pulse-width modulation. The method involves obtaining optimal switching angles; a process which is described in detail in this paper. The proposed control strategy regulates the DC bus voltage, balances the capacitors, and decreases the harmonic components of the voltage and current. Simulation and experimental results demonstrate the validity of the optimizing method and control theory