Timothy A. Haskew

Optimal and Direct-Current Vector Control of Direct-Driven PMSG Wind Turbines

With the advances of power electronic technology, direct-driven permanent magnet synchronous generators (PMSGs) have increasingly drawn the interest of wind turbine manufacturers. At the present time, a commercial PMSG wind turbine primarily makes use of a passive rectifier followed by an insulated gate bipolar transistor (IGBT) inverter. Although a PMSG wind turbine with two back-to-back voltage source IGBT converters is considered more efficient, it has not been widely adopted by the wind power industry. This paper investigates both the conventional and a novel vector control mechanism for a PMSG wind turbine that has two side-by-side voltage source pulsewidth modulation converters. The proposed approach is based on a direct-current vector control mechanism for control of both machine- and grid-side converters of a PMSG wind turbine. Then, an optimal control strategy is developed for integrated control of PMSG maximum power extraction, reactive power, and grid voltage support controls. A transient system simulation using SimPowerSystem is built to investigate the performance of the conventional and proposed control techniques for the PMSG wind turbine under steady and gusty wind conditions. This paper shows that when using the direct-current vector control structure, a PMSG system has excellent performance in various aspects.

Control of HVDC Light System Using Conventional and Direct Current Vector Control Approaches

With the advance of power electronic technology, the application of HVdc light systems based on voltage-source insulated gate bipolar transistor (IGBT) converters has increased rapidly in renewable, microgrid, and electric power systems. This paper proposes an optimal control strategy for an HVdc light system using a direct current vector control mechanism. The proposed approach is compared with the traditional vector control method for different HVdc control requirements, such as active power, reactive power, and grid voltage support control. A limitation of the conventional control mechanism is analyzed through a theoretical study and computer simulation. Closed-loop control evaluation demonstrates that the proposed approach works well for HVdc light system control both within and beyond the physical constraints of the system, such as rated power and saturation of pulse width modulation (PWM). The evaluation also shows that the conventional control technique could result in over voltage and system oscillation, especially when the controller operates beyond the PWM saturation limit.

Adaptive Step Size With Adaptive-Perturbation-Frequency Digital MPPT Controller for a Single-Sensor Photovoltaic Solar System

This paper presents a load-current-based maximum power point tracking (MPPT) digital controller with an adaptive-step-size and adaptive-perturbation-frequency algorithm. Only one sensor is needed in the controller circuitry since the MPPT controller is only utilizing the load current information. By utilizing a variable step-size algorithm, the speed, accuracy, and efficiency of the PV system MPPT are improved when compared to the fixed step-size load-current-based algorithm. Furthermore, the proposed adaptive algorithm utilizes a novel variable perturbation frequency scheme which further improves the controller speed. The concept and operation of the load-current adaptive-step-size and adaptive-perturbation-frequency MPPT controller are presented, analyzed, and verified by results obtained from a proof-of-concept experimental prototype.

Control of DFIG Wind Turbine With Direct-Current Vector Control Configuration

The doubly-fed induction generator (DFIG) wind turbine is a variable speed wind turbine widely used in the modern wind power industry. At present, commercial DFIG wind turbines primarily make use of the technology that was developed a decade ago. But, it is found in this paper that there is a limitation in the conventional vector control technique. This paper presents a direct-current vector control method in a DFIG wind turbine, based on which an integrated control strategy is developed for wind energy extraction, reactive power, and grid voltage support controls of the wind turbine. A transient simulation system using SimPowerSystem is built to validate the effectiveness of the proposed control method. The conventional control approach is compared with the proposed control technique for DFIG wind turbine control under both steady and gust wind conditions. The paper shows that under the dc vector control configuration, a DFIG system has a superior performance in various aspects.

A Time-Domain AC Electric Arc Furnace Model for Flicker Planning Studies

A time-domain model of an AC electric arc furnace (EAF) was developed for power system (flicker) planning studies. The proposed model was implemented in the Electromagnetic Transient Program (EMTP), and it focuses on the behavior of the EAF during the early stages of the melt cycle, thus providing an accurate prediction of the short term flicker created by the EAF, specifically Pst99%.ldr The primary advantages of the proposed model over existing models are: 1) it uses system data that is readily available to the planning engineer; 2) it is a three phase model and can accurately model imbalance and predict flicker at the point of common coupling (PCC) as well as remote buses in the power system; and 3) its accuracy has been verified using synchronized flicker measurements of an actual EAF. Existing time-domain EAF models that are used in flicker planning studies require measurement or statistical data that is difficult to obtain during the planning stages of a project. Frequency domain methods are a popular means of estimating the flicker created by an EAF; however, when these methods are used in flicker planning studies, the operational uncertainty of the EAF introduces error into the calculations. Also, in many cases, frequency domain methods struggle to accurately predict the flicker level at buses remote from the PCC. Thus, a time-domain EAF model which can accurately predict Pst99% at points of interest and uses readily available system information is needed. The following paper describes such an EAF model. Validation of the proposed model is performed by comparing simulation results with flicker measurements of an actual EAF that were time synchronized using Global Positioning Systems (GPS).

Characteristic Study of Vector-controlled Direct-driven Permanent Magnet Synchronous Generator in Wind Power Generation

Abstract With the advance of power electronics, direct-driven permanent magnet synchronous generators have drawn increased interest to wind turbine manufacturers due to its advantages over other variable-speed wind turbines. This article studies permanent magnet synchronous generator characteristics under the general d-q control strategy in the rotor-flux-oriented frame so as to benefit the development of advanced permanent magnet synchronous generator control technology. Compared to traditional approaches, the specific features of the article are (1) a steady-state permanent magnet synchronous generator model in a d-q reference frame, (2) a simulation mechanism that reflects the general permanent magnet synchronous generator d-q control strategy, (3) an integrative study that combines generator-converted power with extracted wind power characteristics, and (4) a joint investigation incorporating both steady-state and transient evaluations. Extensive simulation-based analysis is conducted to study how permanent magnet synchronous generator characteristics are affected by different d-q control conditions and are interacted with wind power drive characteristics in power generation and speed regulation of a permanent magnet synchronous generator wind turbine.

Transient and Steady-State Simulation Study of Decoupled d-q Vector Control in PWM Converter of Variable Speed Wind Turbines

Variable-speed wind turbines are attractive to the high performance and are commonly used by the wind turbine industry today. They are based on variable-speed operation with pitch control using either a direct driven synchronous generator (without gearbox) or a doubly-fed induction generator. For both, there is an AC/DC/AC PWM converter that is used for wind turbine control and grid interface. The AC/DC/AC converter usually consists of a machine-side converter and a grid-side converter. In order for effective control and integration of variable-speed wind turbines with the electrical grid, it is important to understand the power control characteristics of the two PWM converters. This paper focuses on the analysis of decoupled d-q vector control approaches applied to the grid-side converter control in variable-speed wind turbines and studies the power control characteristics of the PWM converters through both steady-state and transient simulation techniques. A typical decoupled d-q control concept that has been widely used in the grid-side converter control is reviewed in the paper. Deficiencies of conventional d-q control mechanisms are discovered and analyzed both analytically and through computer simulation. An extensive simulation-based study, in both steady-state and transient, is performed to examine the characteristics of the grid- side converter under different d-q control conditions in variable- speed wind turbines.

An algorithm for steady-state thermal analysis of electrical cables with radiation by reduced Newton-Raphson techniques

Thermal analysis of electrical cables and cable systems is a topic that has received considerable attention by many researchers. In typical analyses, nonlinear boundary conditions resulting from convection and radiation have been addressed. A finite-difference heat transfer model is employed, with nonlinearities treated via the Newton-Raphson technique with symbolic reduction. This reduces the dimension of the system of equations requiring iteration as well as the number of iterations required by offering quadratic convergence. The procedure for implementation of this reduced iterative algorithm is the major emphasis of this paper. In order to illustrate the procedure for implementation, only a single cable with radiation at the boundary is treated. Appropriate considerations for the extension of the method for more complex systems are discussed in a general sense. The overall scope of this paper is to illustrate the procedure for application of the algorithm to nonlinear thermal analyses. The finite-difference thermal model is obtained from power balance equations at each node of a solution grid imposed on the cable cross-section. All calculations are based on a per-unit length section with constant RMS conductor currents. Conductor resistance variations with temperature are considered, and no conductors are assumed isothermal. The convergence of the presented algorithm has proven to provide substantial speed-up over standard and accelerated Gauss-Seidel methods. >

Characteristic Study of Vector-controlled Doubly-fed Induction Generator in Stator-flux-oriented Frame

Abstract The doubly-fed induction generator (DFIG) is a “special” variable-speed induction machine that is widely utilized in modern large wind turbine generators. This article focuses on the DFIG characteristic study in the stator-flux-oriented frame. Compared to traditional approaches, the specific features of this article are (1) a DFIG steady-state model in the d-q reference frame, (2) simulation-based characteristic study under general d-q control strategies in the stator-flux-oriented frame, (3) a numerical iteration technique, (4) power simulation from both the stator and rotor paths, and (5) an integrated study that enables assessment of various DFIG parametric data simultaneously in an integrative environment. In addition, transient simulation using the MatLab SimPowerSystem is developed to validate the effectiveness of steady-state results and to further investigate a DFIG in a transient environment. Extensive simulation-based analysis is performed to inspect how DFIG characteristics are affected by different d-q control conditions in the stator-flux-oriented frame, such as torque-speed and real and reactive power versus speed characteristics.

Impact of uneven shading and bypass diodes on energy extraction characteristics of solar photovoltaic modules and arrays

Solar photovoltaic (PV) energy is becoming an increasingly important part of the worlds renewable energy. In order for effective energy extraction from a solar PV system, this paper investigates I–V and P–V characteristics of solar PV modules and arrays. The paper particularly focuses on I–V and P–V characteristics of PV modules and arrays under uneven shading conditions, and considers both the physics and electrical characteristics of a solar PV system in the model development. The article examines how different bypass diode arrangements could affect maximum power extraction characteristics of a solar PV module or array. It is found in this article that under uneven shading conditions, solar PV cells may perform in very different ways and a solar PV system may exhibit multiple peaks in its P–V characteristics. The study of this article also shows that the arrangement of largely distributed bypass diodes within a PV module could effectively improve efficiency and maximum power point tracking strategies for energy conversion of solar PV systems.