Universal fractional-order design of linear phase lead compensation multirate repetitive control for PWM inverters

Repetitive control (RC) with linear phase lead compensation provides a simple but very effective control solution for any periodic signal with a known period. Multirate repetitive control (MRC) with a downsampling rate can reduce the need of memory size and computational cost, and then leads to a more feasible design of the plug-in repetitive control systems in practical applications. However, with fixed sampling rate, both MRC and its linear phase lead compensator are sensitive to the ratio of the sampling frequency to the frequency of interested periodic signals: (1) MRC might fails to exactly compensate the periodic signal in the case of a fractional ratio; (2) linear phase lead compensation might fail to enable MRC to achieve satisfactory performance in the case of a low ratio. In this paper, a universal fractional-order design of linear phase lead compensation MRC is proposed to tackle periodic signals with high accuracy, fast dynamic response, good robustness, and cost-effective implementation regardless of the frequency ratio, which offers a unified framework for housing various RC schemes in extensive engineering application. An application example of programmable AC power supply is explored to comprehensively testify the effectiveness of the proposed control scheme

A multifunctional dynamic voltage restorer for power quality improvement

Power quality is a major concern in electrical power systems. The power quality disturbances such as sags, swells, harmonic distortion and other interruptions have an impact on the electrical devices and machines and in severe cases can cause serious damages. Therefore it is necessary to recognize and compensate all types of disturbances at an earliest time to ensure normal and efficient operation of the power system. To solve these problems, many types of power devices are used. At the present time, one of those devices, Dynamic Voltage Restorer (DVR) is the most efficient and effective device used in power distribution systems. In this paper, design and modeling of a new structure and a new control method of multifunctional DVRs for voltage quality correction are presented. The new control method was built in the stationary frame by combining Proportional Resonant controllers and Sequence-Decouple Resonant controllers. The performance of the device and this method under different conditions such as voltage swell, voltage sag due to symmetrical and unsymmetrical short circuit, starting of motors, and voltage distortion are described. Simulation result show the superior capability of the proposed DVR to improve power quality under different operating conditions and the effectiveness of the proposed method. The proposed new DVR controller is able to detect the voltage disturbances and control the converter to inject appropriate voltages independently for each phase and compensate to load voltage through three single-phase transformers.Web of Science116art. no. 135

Unified Equivalent-circuit Models for Voltage-source Inverters that Capture Averaged Dynamics and Power-flow Solutions in Distribution Networks

This paper demonstrates how three-phase distribution networks composed of voltage-source inverters can be modeled as a single unified equivalent-circuit network realized with familiar circuit elements. Such a model is derived by representing all physical- and control-subsystem dynamics as equivalent circuits. Two versions are put forth: the first captures averaged dynamics; while the second is a steady-state version of the first and it captures the power-flow solution in sinusoidal steady state. The main challenge in undertaking such an effort is presented by the fact that inverters are composed of subsystems (filters, pulse width modulators, phase-locked loops, controllers, direct-quadrature reference-frame transformations) that belong to multiple domains (physical and control). We demonstrate how all these constituent subsystems can be transcribed as equivalent circuits which then promote a single and unified circuit model that captures network physical- and control-layer dynamics. Numerical simulations for a representative distribution network compare results from the averaged model and the steady-state model with high-fidelity switch-level simulations. The results establish the validity of the circuit-based models and the computational benefits of the proposed approach

Unified Power Quality Conditioner for Grid Integration of Wind Generators

A Unified Power Quality Conditioner (UPQC) is relatively a new member of the custom power device family. It is a comprehensive custom power device, with integrated shunt and series active filters. The cost of the device, which is higher than other custom power/FACTS devices, because of twin inverter structure and control complexity, will have to be justified by exploring new areas of application where the cost of saving power quality events outweighs the initial cost of installation. Distributed generation (such as wind generation) is one field where the UPQC can find its potential application. There has been a considerable increase in the power generation from wind farms. This has created the necessity for wind farms connectivity with the grid during power system faults, voltage sags and frequency variations. The application of active filters/custom power devices in the field of wind generation to provide reactive power compensation, additional fault ride through capability and to maintain Power Quality (PQ) at the point of common coupling is gaining popularity. Wind generation like other forms of distributed generation often relies on power electronics technology for flexible interconnection to the power grid. The application of power electronics in wind generation has resulted in improved power quality and increased energy capture. The rapid development in power electronics, which has resulted in high kVA rating of the devices and low price per kVA, encourages the application of such devices at distribution level. This work focuses on development of a laboratory prototype of a UPQC, and investigation of its application for the flexible grid integration of fixed and variable speed wind generators through dynamic simulation studies. A DSP based fully digital controller and interfacing hardware has been developed for a 24 kVA (12 kVA-shunt compensator and 12 kVA-series compensator) laboratory prototype of UPQC. The modular control approach facilitates the operation of the device either as individual series or shunt compensator or as a UPQC. Different laboratory tests have been carried out to demonstrate the effectiveness of developed control schemes.A simulation-based analysis is carried out to investigate the suitability of application of a UPQC to achieve Irish grid code compliance of a 2 MW Fixed Speed Induction Generator (FSIG). The rating requirement of the UPQC for the wind generation application has been investigated. A general principle is proposed to choose the practical and economical rating of the UPQC for this type of application. A concept of UPQC integrated Wind Generator (UPQC-WG) has been proposed. The UPQC-WG is a doubly fed induction machine with converters integrated in the stator and rotor circuits and is capable of adjustable speed operation. The operation of UPQC-WG under sub and super-synchronous speed range has been demonstrated. The Irish grid code compliance of the same has been demonstrated with a detailed dynamic simulation

Balance control of grid currents for UPQC under unbalanced loads based on matching-ratio compensation algorithm

Abstract In three-phase four-wire systems, unbalanced loads can cause grid currents to be unbalanced, and this may cause the neutral point potential on the grid side to shift. The neutral point potential shift will worsen the control precision as well as the performance of the three-phase four-wire unified power quality conditioner (UPQC), and it also leads to unbalanced three-phase output voltage, even causing damage to electric equipment. To deal with unbalanced loads, this paper proposes a matching-ratio compensation algorithm (MCA) for the fundamental active component of load currents, and by employing this MCA, balanced three-phase grid currents can be realized under 100% unbalanced loads. The steady-state fluctuation and the transient drop of the DC bus voltage can also be restrained. This paper establishes the mathematical model of the UPQC, analyzes the mechanism of the DC bus voltage fluctuations, and elaborates the interaction between unbalanced grid currents and DC bus voltage fluctuations; two control strategies of UPQC under three-phase stationary coordinate based on the MCA are given, and finally, the feasibility and effectiveness of the proposed control strategy are verified by experiment results