Marko Jaksic

Three-phase AC system impedance measurement unit (IMU) using chirp signal injection

Impedance defined in synchronous coordinates is a useful tool to predict and evaluate stability of three-phase AC systems. For online impedance measurement in AC systems, the widely used frequency sweep method takes a long time and may not be practical in systems where the operating point cannot be maintained for a long time. A wide bandwidth signal could be used to significantly reduce the measurement time. This paper suggests a chirp signal and describes its advantages over other signals. Experimental results demonstrate the effectiveness of this approach.

Design and implementation of three-phase AC impedance measurement unit (IMU) with series and shunt injection

Electric power systems based on power electronics technology are prone to instability due to the constant power nature of power electronics converters. The impedances defined in synchronous coordinates are useful tools to predict and evaluate three-phase small-signal system stability. This paper presents the design and implementation of an impedance measurement unit used in three-phase systems up to 100kW. It extracts the system impedance online at any specific operating point. The unit utilizes both series and shunt injection modes to perform impedance identification for any load and source conditions, even if the source is very stiff. Experimental results demonstrate the effectiveness of the designed unit with both linear and nonlinear loads.

Small-signal model of a voltage source inverter (VSI) considering the dead-time effect and space vector modulation types

This paper presents a modified small-signal model of a voltage source inverter (VSI) that captures model nonlinearities such as dead time and modulation effects that were not presented in the literature. Previous research has concentrated on developing compensation methods for these types of phenomena in a switching model, but none actually derived an analytical model that can predict them. In this paper, the small-signal model of the VSI in literature is used, and the different phenomena are then incorporated into the model to get a complete one. In addition, the output impedance of the VSI is derived with the modified small-signal model, compared to the conventional one and validated with switching model and experimental results.

Modular scalable medium-voltage impedance measurement unit using 10 kV SiC MOSFET PEBBs

This paper describes the design and implementation of the first functional medium-voltage impedance measurement unit capable of characterizing in-situ source and load impedances of dc- and ac-networks (4160 V ac, 6000 V dc, 300 A, 2.2 MVA) in the frequency range from 0.1 Hz-1 kHz. It comprises three power electronics building blocks, each built using SiC MOSFET H-bridges, features great reconfigurability, and allows both series and shunt perturbation injection in order to achieve accurate impedance characterization of the Navys shipboard power systems. With extraordinary advantages featured by the power electronics building block modular concept, and unconventional power processing benefits offered by SiC semiconductors, development of the unit shown in this paper unquestionably enables both, improvement of the existing, and design of the future, stable and reliable electrical Navy shipboard platforms with advanced electrical energy generation and modern distribution architecture.

Dynamic interactions in hybrid ac/dc electronic power distribution systems

Hybrid electronic power distribution systems have majority of loads interfaced to energy sources through power electronics converters. Furthermore, all alternative, sustainable, and distributed energy sources can only be connected to electric grid through power electronics equipment. However, one of the main challenges in designing and developing of these hybrid ac/dc systems has been modeling and analysis of dynamic interactions between converters at the synchronous and higher frequencies. In order to address these problems, this paper employs terminal-behavioral modeling of power converters as possible methodology for analysis, system-level design, stability, and subsystem interactions study. Some simulation and experimental results are both shown for model verification and validation purposes.

Modeling of a virtual synchronous machine-based grid-interface converter for renewable energy systems integration

The common, and with some exceptions, the only method to interconnect high power renewable energy sources and energy storage systems to the power system has been using power electronics converters that operate as current sources to the grid, for the purpose of achieving maximum primary-source power tracking. When grid is not available, such sources are not allowed to continue operating and are shut-down after anti-islanded algorithms recognize the loss of the grid. Although existing standards and requirements still limit grid-interface converters to regulate voltage in the grid, this functionality will most probably be the dominant mode of their operation in the future grid; hence the growing trend in industry and academia to concentrate research towards control of the grid-interface converters that resemble behavior of the classic synchronous generators. This paper addresses modeling of the virtual synchronous machine-based grid-interface converters for renewable energy systems integration. Being completely equivalent to the synchronous generator model, an average model of the grid-interface converter features an inherent frequency-locked loop, in which the virtual rotor angle is obtained by integrating dc-link voltage.

Un-terminated, low-frequency terminal-behavioral d-q model of three-phase converters

Frequency-domain terminal-behavioral modeling of ac systems is unquestionably attracting more and more interest in engineering practice. New electronic power distribution systems built for airplanes, ships, electric vehicles, data-centers and even homes, dominantly comprise a variety of power electronics converters with very different dynamic characteristics. If their behavior is not examined carefully before system is integrated, instability can become one of the major concerns. This paper addresses low-frequency terminal-behavioral modeling of three-phase converters which dynamics can be captured on-line, in a non-intrusive way, and later decoupled from the source and load in order to get un-terminated model of a particular converter (or a system). A few examples of different converters are given at the end to verify modeling and decoupling procedure with the average and switching models.

Modular interleaved single-phase series voltage injection converter used in small-signal dq impedance identification

Stability analysis of modern three-phase ac power systems is performed using small-signal impedances identified in synchronous reference dq frame. A single-phase series voltage injection is used to perturb ac and dc systems in order to perform small-signal impedance identification. This paper proposes to use interleaved cascaded H-bridge converter to generate a voltage perturbation in series with power system. Special attention is focused on the design procedure, including the selection of inductors and capacitors values, trying to optimize converters performance and maximize injection range. The decoupling control is proposed to regulate ac injection voltage, providing robust and reliable strategy to regulate series voltage injection. Furthermore, low bandwidth dc voltage control is included into the control, assuring dc voltage control for each H-bridge module. Analytical expressions, which describe the control procedure, are derived and presented in the analysis. The proposed converter is extensively simulated using switching simulation model and excellent agreement with the developed design procedure is presented. Simulation model and hardware prototype results verify effectiveness of the proposed series voltage injection solution.

Test environment for a novel medium voltage impedance measurement unit

Small signal stability of converter-based power distribution systems can be evaluated with the help of impedance characterization, which provides a means to predict the effect of interactions. These techniques have great Navy relevance as future ships may be based on medium voltage distribution networks of interconnected feedback-controlled switching power converters. These networks will be subject to converter interactions and potential instability. This paper summarizes work done to prepare full scale testing of the first medium voltage, MW scale impedance measurement unit which was recently developed using SiC technology.

Small-signal impedance identification of three-phase diode rectifier with multi-tone injection

AC and DC impedances of power converters are used for stability analysis of modern power systems at AC and DC interfaces. The basic idea for impedance extraction is AC sweep approach with sinusoidal signal injection, here denoted as single-tone. However, the approach is time consuming because the time-domain measurement will run many times. Therefore the approach of multi-tone signal injection is implemented in which multiple frequency responses could be measured in only one time-domain measurement. In linear loads, the multi-tone approach could give the same results with single-tone approach because superposition holds for linear system. For three-phase nonlinear loads, different results are observed from multi-tone and single-tone approach. In this paper, 6-pulse Diode Bridge Rectifier is studied to analyze the reason of result difference. An algorithm is proposed to improve the multi-tone approach and avoid result differences.