Reza Iravani

Dynamic Performance of a Modular Multilevel Back-to-Back HVDC System

The modular multilevel converter (MMC) is a newly introduced switch-mode converter topology with the potential for high-voltage direct current (HVDC) transmission applications. This paper focuses on the dynamic performance of an MMC-based, back-to-back HVDC system. A phase-disposition (PD) sinusoidal pulsewidth modulation (SPWM) strategy, including a voltage balancing method, for the operation of an MMC is presented in this paper. Based on the proposed PD-SPWM switching strategy, a mathematical model for the MMC-HVDC system, under both balanced and unbalanced grid operation modes, is developed. Dynamic performance of the MMC-based back-to-back HVDC converter system, based on time-domain simulation studies in the PSCAD/EMTDC environment, is then evaluated. The reported time-domain simulation results show that based on the adopted PD-SPWM switching strategy, the MMC-HVDC station can respond satisfactorily to the system dynamics and control commands under balanced and unbalanced conditions while maintaining voltage balance of the dc capacitors.

Trends in Microgrid Control

The increasing interest in integrating intermittent renewable energy sources into microgrids presents major challenges from the viewpoints of reliable operation and control. In this paper, the major issues and challenges in microgrid control are discussed, and a review of state-of-the-art control strategies and trends is presented; a general overview of the main control principles (e.g., droop control, model predictive control, multi-agent systems) is also included. The paper classifies microgrid control strategies into three levels: primary, secondary, and tertiary, where primary and secondary levels are associated with the operation of the microgrid itself, and tertiary level pertains to the coordinated operation of the microgrid and the host grid. Each control level is discussed in detail in view of the relevant existing technical literature.

Potential-Function Based Control of a Microgrid in Islanded and Grid-Connected Modes

This paper introduces the potential-function based method for secondary (as well as tertiary) control of a microgrid, in both islanded and grid-connected modes. A potential function is defined for each controllable unit of the microgrid such that the minimum of the potential function corresponds to the control goal. The dynamic set points are updated, using communication within the microgrid. The proposed potential function method is applied for the secondary voltage control of two microgrids with single and multiple feeders. Both islanded and grid-connected modes are investigated. The studies are conducted in the time-domain, using the PSCAD/EMTDC software environment. The study results demonstrate feasibility of the proposed potential function method and viability of the secondary voltage control method for a microgrid.

Negative-Sequence Current Injection for Fast Islanding Detection of a Distributed Resource Unit

This paper presents an active islanding detection method for a distributed resource (DR) unit which is coupled to a utility grid through a three-phase voltage-sourced converter (VSC). The method is based on injecting a negative-sequence current through the VSC controller and detecting and quantifying the corresponding negative-sequence voltage at the point of common coupling of the VSC by means of a unified three-phase signal processor (UTSP). UTSP is an enhanced phase-locked loop system which provides high degree of immunity to noise, and thus enable islanding detection based on injecting a small (3%) negative-sequence current. The negative-sequence current is injected by a negative-sequence controller which is adopted as the complementary of the conventional VSC current controller. Based on simulation studies in the PSCAD/EMTDC environment, performance of the islanding detection method under UL1741 anti-islanding test is evaluated, and its sensitivity to noise, grid short-circuit ratio, grid voltage imbalance, and deviations in the UL1741 test parameters are presented. The studies show that based on negative-sequence current injection of about 2% to 3%, islanding can be detected within 60 ms even for the worst case scenario.

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.

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 neutral-point clamped converter system for direct-drive variable-speed wind power unit

Recent and ongoing developments in wind turbine technology indicate a trend towards utilization of high capacity (e.g., up to 5 MW) wind power units in large wind farms. Higher capacity of the wind turbine necessitates operation of the corresponding electric machine and the static converter system at higher voltages. This paper presents a neutral point diode clamped (NPC) converter system that inherently accommodates higher voltage and power ratings of a high capacity wind power unit. The overall control strategy of an NPC-based wind power unit and the details of the ac side and the dc side controls of the NPC converter system are also described. The generator-side NPC converter provides torque-speed control of the turbine-generator unit. The network-side NPC converter controls real and reactive power flow to the network and thus regulates the dc bus voltage and the ac side power-factor (or voltage) respectively. The paper also presents a new control approach to balance the dc capacitor voltages. The NPC converter system is augmented with a dc chopper that controls the synchronous generator field current. The NPC-based converter system is used to interface a 3 MW, direct-drive (gearless), synchronous machine based wind power unit to the utility grid. Performance of the overall NPC-based wind power unit, under the proposed controls, is evaluated based on time domain simulations in the power systems computer aided design (PSCAD) electromagnetic transient for DC (EMTDC) environment.

A Decentralized Robust Control Strategy for Multi-DER Microgrids—Part I: Fundamental Concepts

This paper presents fundamental concepts of a central power-management system (PMS) and a decentralized, robust control strategy for autonomous mode of operation of a microgrid that includes multiple distributed energy resource (DER) units. The DER units are interfaced to the utility grid through voltage-sourced converters (VSCs). The frequency of each DER unit is specified by its independent internal oscillator and all oscillators are synchronized by a common time-reference signal received from a global positioning system. The PMS specifies the voltage set points for the local controllers. A linear, time-invariant, multivariable, robust, decentralized, servomechanism control system is designed to track the set points. Each control agent guarantees fast tracking, zero steady-state error, and robust performance despite uncertainties of the microgrid parameter, topology, and the operating point. The theoretical concept of the proposed control strategy, including the existence conditions, design of the controller, robust stability analysis of the closed-loop system, time-delay tolerance, tolerance to high-frequency effects and its gain-margins, are presented in this Part I paper. Part II reports on the performance of the control strategy based on digital time-domain simulation and hardware-in-the-loop case studies.

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.

Unbalanced Model and Power-Flow Analysis of Microgrids and Active Distribution Systems

This paper presents a three-phase power-flow algorithm, in the sequence-component frame, for the microgrid (μgrid) and active distribution system (ADS) applications. The developed algorithm accommodates single-phase laterals, unbalanced loads and lines, and three/four-wire distribution lines. This paper also presents steady-state sequence-component frame models of distributed energy resource (DER) units for the developed power-flow approach under balanced/unbalanced conditions. The DER models represent the synchronous-generator based and the electronically-coupled DER units. Both constant power (PQ) and regulated-voltage (PV) modes of operation of DER units are considered. The application of the developed power-flow method for two study systems is presented. The study results are validated based on comparison with the detailed solution of the system differential equations in time domain, using the PSCAD/EMTDC software tool.