Maryam Saeedifard

Analysis and Control of DC-Capacitor-Voltage-Drift Phenomenon of a Passive Front-End Five-Level Converter

The phenomenon of dc-capacitor-voltage drift is the main technical drawback of a passive front-end multilevel diode-clamped converter (DCC). This paper formulates and analyzes the dc-capacitor-voltage-drift phenomenon of a passive front-end five-level DCC, which operates based on a sinusoidal pulsewidth-modulation (SPWM) switching strategy. The analysis shows dependence of the voltage drift on the modulation index and the ac-side power factor of the DCC. The analysis concludes that an SPWM strategy, without the use of auxiliary power circuitry, is not able to prevent the voltage-drift phenomenon of a five-level DCC. This paper also proposes a space-vector-modulation (SVM)-based switching strategy that takes advantage of redundant switching vectors of the SVM method to counteract the voltage-drift phenomenon. The limit to the range of operation of a five-level DCC, which is based on the proposed SVM strategy, is also presented. The salient feature of the proposed strategy is that it enables voltage balancing of the dc capacitors with no requirements for additional controls or auxiliary-power circuitry, within the specified range of operation. The performance of a DCC under various operating conditions, based on time-domain simulation studies in the MATLAB/SIMULINK environment, is evaluated. This paper demonstrates capability of the proposed SVM strategy to control and maintain voltage balance of dc capacitors.

A Space Vector Modulation Strategy for a Back-to-Back Five-Level HVDC Converter System

The DC-capacitor voltage drift is the main technical drawback of a multilevel diode-clamped converter (DCC) system. This paper proposes a space vector modulation (SVM)-based switching strategy that takes advantage of the redundant switching states of the SVM to counteract the voltage drift phenomenon of a five-level DCC-based back-to-back high-voltage direct-current (HVDC) converter system. The proposed strategy is based on online minimization of a quadratic cost function, associated with the voltage deviations of the dc capacitors. The salient feature of the proposed strategy is that it enables voltage balancing of the DC capacitors with no requirements for offline calculations, additional controls, or auxiliary power circuitry. Performance of the proposed SVM-based balancing strategy for a back-to-back HVDC converter system, based on time-domain simulation studies in the PSCAD/EMTDC environment, is evaluated and experimentally verified. The studies demonstrate capability of the proposed SVM strategy to control and maintain voltage balance of DC capacitors.

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

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.

Low switching frequency space vector modulators for high power multimodule converters

Force-commutated multiconverter systems have been proposed and recently implemented for high power applications, particularly high-voltage direct current (HVDC) transmission and flexible ac transmission systems (FACTS) devices, including STAtic var COMpensators (STATCOM). This paper demonstrates that pulse-width modulation (PWM) based on space vector modulation (SVM), and using low switching frequencies (a few hundred Hertz) can be implemented in multimodule power converter systems. The proposed scheme is based on a delayed sampling technique and allows output voltage control and minimization of harmonic components. Pattern generation options are analyzed and system waveforms are presented for different switching frequencies and number of modules. Results are validated by simulation and confirmed by experiments on a 5-kVA prototype unit.

A Space Vector Modulation Approach for a Back-to-Back Connected Four-Level Converter

The phenomenon of DC-capacitor voltages drift is the main technical challenge of a back-to-back connected diode-clamped multi-level converter (DCMC). This paper (i) develops a mathematical model for a back-to-back connected four-level DCMC and formulates the capacitor voltages drift phenomenon, and (ii) based on the developed model proposes a space vector modulation (SVM) approach to control the capacitor voltages drift. The online voltage balancing task is achieved based on a coordination between the two SVM modulators that minimize a cost function related to voltage unbalance of DC-capacitors. The main feature of the proposed SVM-based balancing strategy is that it enables online DC-capacitor voltages balancing with no requirement for lookup tables, additional controls or auxiliary devices. Performance of the overall back-to-back DCMC system and effectiveness of the proposed SVM-based balancing strategy are evaluated based on time-domain simulations in the PSCAD/EMTDC software environment.

Neuro-Computing Vector Classification SVM Schemes to Integrate the Overmodulation Region in Neutral Point Clamped (NPC) Converters

Three-level neutral point clamped (NPC) voltage source converters have recently emerged as important alternatives to conventional two-level converter topologies in high-power medium-voltage energy conversion applications, particularly in high performance ac motor drive systems. An all-inclusive modulation strategy for NPC converters should have the capability of extending the operating range of the converter into the overmodulation region with a smooth and linear transition characteristic. An overmodulation switching strategy based on the space vector classification technique for three-level NPC converters is introduced in this paper. The proposed overmodulation modes, make possible continuous control of the output voltage up to the maximum possible with a smooth linear transition characteristic, and minimum distortions. A theoretical basis for the vector classification space vector modulation technique in the overmodulation region is presented, and the proposed overmodulation schemes are validated by analysis, simulation and experimentation on a 2-KVA three-level NPC converter laboratory prototype

A three-level converter based micro-turbine distributed generation system

This paper proposes a novel converter configuration as a power electronic interface between a high-speed micro-turbine generator and a utility distribution system. The converter system includes a three-level voltage-sourced converter that is connected in a back-to-back configuration to a two-level converter. A space vector modulation based switching strategy is employed to control the converter system. A comprehensive mathematical model is developed for the three- to two-level converter based micro-turbine generator system. The developed model is used to decouple the multi-input multi-output converter controller to multiple single-input single-output subcontrollers. Performance of the overall micro-turbine based generation system is evaluated based on time-domain simulation studies in the PSCAD/EMTDC software environment

A new adaptive harmonic extraction scheme for single-phase active power filters

This paper introduces a new adaptive filter for harmonic extraction in Active Power Filters (APF). The performance of this adaptive filtering algorithm is explained on a shunt active power filter connected in parallel with a typical uncontrolled bridge rectifier feeding a variable load. Finally, simulation results obtained confirm the feasibility and the features of the proposed APF system.

Extending the operating range of the neuro-computing vector classification space vector modulation algorithm of three-level inverters into overmodulation region

Three-level voltage source pulse width-modulated (PWM) inverters are used in high power industrial drive applications. Operating the converter in the undermodulation range of PWM technique restricts the drive operation in constant torque region. Therefore, an overmodulation strategy to extend the modulation region is essential. This paper is an extension to the operating range of the classification algorithm for the implementation of overmodulation schemes for SVM in three-level converters. Two schemes, depending upon the output voltage level, are developed to continuously increase the output voltage, in a controllable manner, up to the maximum attainable value. The technique requires less computational time, when compared with conventional SVM methods. The mathematical analysis is verified by means of simulation results.