Hassan Nikkhajoei

Control of an Electronically-Coupled Distributed Resource Unit Subsequent to an Islanding Event

This paper presents a new control strategy for islanded (autonomous) operation of an electronically coupled distributed generation (DG) unit and its local load. The DG unit utilizes a voltage-sourced converter (VSC) as the coupling medium. In a grid-connected mode, based on the conventional dq-current control strategy, the VSC controls real- and reactive-power components of the DG unit. Subsequent to an islanding detection and confirmation, the dq-current controller is disabled and the proposed controller is activated. The proposed controller utilizes (1) an internal oscillator for frequency control and (2) a voltage feedback signal to regulate the island voltage. Despite uncertainty of load parameters, the proposed controller guarantees robust stability and prespecified performance criteria (e.g., fast transient response and zero steady-state error). The performance of the proposed controller, based on time-domain simulation studies in the PSCAD/EMTDC software environment, is also presented.

A matrix converter based micro-turbine distributed generation system

This paper investigates the use of an AC-AC matrix converter, as an alternative to AC-DC-AC converter system, to interface a high-speed micro-turbine generator (MTG) to a utility grid as a distributed generation unit. A new switching strategy and a control mechanism for the converter are presented. Based on time-domain simulations in the PSCAD/EMTDC environment, the dynamic behavior of a micro-turbine generation system, including the dynamic models of the micro-turbine (thermodynamics), generator, matrix converter, converter control and micro-turbine control, is studied. The studies conclude that a matrix converter is technically a viable option to interface a MTG to a utility grid.

Steady-State Model and Power Flow Analysis of Electronically-Coupled Distributed Resource Units

This paper presents closed-form steady-state, fundamental-frequency models of power electronic converter systems for coupling distributed resource (DR) units to the utility power grid. Based on the developed models, the paper introduces a two-step power flow analysis approach that accurately and efficiently represents electronically-coupled DR units. A feature of the proposed approach is that it solves for the internal variables of each DR unit in a non-iterative fashion based on the presented closed-form DR models, and thus, enhances efficiency and accuracy of the solution. This papers provides the details for steady-state closed-form formulation of matrix converter and voltage-sourced ac-dc-ac converter for coupling DR units. However, the presented methodology is conceptually applicable to other types of converter systems. An application of the proposed power flow approach to a micro-grid that includes two electronically-coupled DR units is also presented

A Space Vector Modulated STATCOM Based on a Three-Level Neutral Point Clamped Converter

This paper presents a space vector modulation (SVM)-based switching strategy for a three-level neutral point clamped (NPC) converter that is adapted as a STATCOM. The main feature of the proposed SVM switching strategy is that it enables balancing voltages of the dc capacitors, with no requirement of additional controls or auxiliary devices. The proposed SVM strategy also improves the STATCOM ac-side harmonic spectrum and reduces voltage variations of the converter neutral point. Based on a dynamic model of the NPC-based STATCOM, two controllers to regulate the dc-side voltage and control the injected reactive power are designed. Performance of the STATCOM and the corresponding controllers are evaluated based on digital time-domain simulation studies in the PSCAD/EMTDC environment. The studies demonstrate the capabilities of the proposed SVM technique to maintain voltage balance of the dc capacitors and control the STATCOM reactive power exchange for various operating conditions

Dynamic Model and Control of AC–DC–AC Voltage-Sourced Converter System for Distributed Resources

This paper introduces a dynamic model of the AC-DC-AC voltage-sourced converter (VSC) system which provides a linear relationship between the input and output variables. The model is developed in two steps. First, the exact abc frame switched model of a VSC is transformed to a Switching Reference Frame (SRF). Then, the SRF model is transformed to an appropriate dqO reference frame. The fundamental-frequency dynamic model of the VSC is deduced from the dqO model and provides a set of single-input single-output controls for the VSC system. The fundamental-frequency steady-state model of the VSC system is deduced from the dynamic model and provides a closed-form solution to the VSC system. The developed models are used to design controllers and define steady-state settings, e.g. modulation indices, for electronically-coupled distributed resources. The developed models are applied to a test system and the results are validated based on time-domain simulation studies in the PSCAD/EMTDC environment.

A Space Vector Modulation Approach for a Multimodule HVDC Converter System

This paper presents the application of a space vector modulation (SVM) strategy for a multimodule converter system that 1) enables low switching frequency, 2) eliminates/minimizes ac-side voltage harmonics, particularly low-order harmonics, and 3) provides maximum ac-side fundamental voltage component. The SVM strategy is based on a sequential sampling technique. The SVM provides harmonic cancellation/minimization by introducing appropriate phase shift for the corresponding voltage harmonics of the converter modules, while maintaining the fundamental voltage components of modules in-phase to obtain maximum ac-side voltage. The SVM eliminates the need for complicated transformer arrangement for harmonic reduction, and thus provides high degree of modularity by utilization of identical transformers for converter modules. The proposed SVM strategy is developed for a back-to-back HVDC converter station in which each converter system is composed of four identical modules. Based on a dynamic model of the system, converter controls are designed and performance of the SVM strategy in terms of converter harmonics and dynamic performance are presented. The studies are performed in time-domain, using the PSCAD/EMTDC software tool.

A Linear Quadratic Gaussian Controller for a Stand-alone Distributed Resource Unit-Simulation Case Studies

This paper develops a dynamic model and designs a control system for autonomous operation of a stand-alone distributed resource (DR). A stand-alone DR includes a distributed resource and a local load which are interfaced electronically. The DR is represented by a DC voltage source in series with a three-phase voltage-sourced inverter and an R filter, and the local load is modeled by a parallel (S) network. A state-space dynamic model is developed for the DR including the (S) network. Based on dynamic model of the DR, a controller is designed to maintain stability and control voltage and frequency of the stand-alone DR. Performance of the stand-alone DR for various scenarios of operation is evaluated based on digital time-domain simulation studies in the PSCAD/EMTDC software environment. The simulation results verify effectiveness of the designed control system in terms of maintaining voltage, frequency and stability of the stand-alone DR.

Maximum power extraction for a wind-turbine generator with no wind speed sensor

This paper presents an algorithm for maximum power extraction and reactive power control of an inverter based variable-speed wind-turbine generator with no wind speed sensor. The algorithm does not require information about the wind and generator speeds or the inverter DC-link voltage and thus, is dependent of specifications of the wind-turbine generation system. Detail of the algorithm for maximum power extraction and an approach for reactive power control are presented. Dynamic performance of the wind-turbine generation system is evaluated based on digital time-domain simulation studies.

Modeling, control and islanding detection of microgrids with passive loads

This paper presents a control scheme for microgrids with passive loads. The existing control techniques for distributed generation systems are designed to operate either in the grid-connected or in the islanded mode. In the grid-connected mode of operation, a current-controlled scheme based on d-q variables is used for the regulation of real and reactive powers of the converter. In the islanded mode of operation, the converter is controlled in the voltage-controlled mode. This paper deals with the modeling of both control schemes. Furthermore, reconnection from the islanded mode to the grid-connected mode based on the use of adequate Phase-Locked Loops (PLLs) has been demonstrated.

A Current Source Matrix Converter for High-Power Applications

This paper introduces a direct AC-AC current source converter based on the structure of conventional matrix converter. A novel switching strategy is developed to operate the matrix converter in a current source based configuration. The switching strategy is based on minimizing low-order harmonics of the converter input-output currents, which reduces the size of converter filtering components. A control system is developed to regulate magnitude and angle of the converter output current and to control the load real and reactive power components. Performance of the proposed current source converter is evaluated in terms of effectiveness of the developed switching strategy and control system for harmonic cancelation and load power control.