Unbalanced Voltage Sags Research Articles

New Control Technique Applied in Dynamic Voltage Restorer for balanced and unbalanced Voltage Sag /swell

New Control Technique Applied in Dynamic Voltage Restorer for balanced and unbalanced Voltage Sag /swell

Current Control Reference Calculation Issues for the Operation of Renewable Source Grid Interface VSCs Under Unbalanced Voltage Sags

This paper analyzes the current reference calculation for the control of grid-connected voltage source converters meant to operate under voltage unbalanced sags produced by grid faults. The well-known reference calculation method that allows to control the active power ripple produced by the existence of negative-sequence components in the grid voltage is extensively analyzed. A type of voltage sag that produces unfeasible current reference values is identified and a possible workaround is proposed. Also, the need to compensate the power ripple produced by the filter inductance of the converter is demonstrated and an extension of the calculation method to compensate for this is introduced. The theoretical results are confirmed using simulation tools and experimental tests.

Modeling and Validation of DFIG 3-MW Wind Turbine Using Field Test Data of Balanced and Unbalanced Voltage Sags

A complete model of a doubly fed induction generator-based wind turbine generation system and its control scheme are developed. All components such as converters, dc-link, protections, and grid are modeled. Control techniques for torque, reactive power, and dc-link voltage are analyzed for balanced and unbalanced faults ride-through performance. Simulation results are validated and compared with field test measurements of an operational 3-MW wind turbine. Balanced and unbalanced voltage sags have been provoked and the data required by the grid codes to perform a validation process have been sampled by means of a data acquisition system. A comparison between field test data and simulation results is done and the variables used to validate the model are plotted showing their evolution along the voltage dip and after the fault clearance. The results are validated according to Spanish and German grid codes.

Grid-Fault Tolerant Operation of a DFIG Wind Turbine Generator Using a Passive Resistance Network

With the increasing penetration of wind turbines in the utility grid, new regulation codes have been issued that require them to have low-voltage ride-through capability. In this paper, a passive resistive network consisting of shunt and series elements that are applied at the stator side of a doubly fed induction generation wind turbine is presented. The network is inactive during steady-state operation and enabled for short intervals of time during the initiation of voltage sag and recovery events. Computer simulation and experimental results confirming the validity of this operation during balanced and unbalanced voltage sags are shown in this paper.

Mitigation of Voltage Sags With Phase Jumps by UPQC With PSO-Based ANFIS

The unified power-quality conditioner (UPQC) is used to mitigate the current and voltage-related power-quality (PQ) problems simultaneously in power distribution systems. Among all of the PQ problems, voltage sag is a crucial problem in distribution systems. In this paper, a new methodology is proposed to mitigate the unbalanced voltage sag with phase jumps by UPQC with minimum real power injection. To obtain the minimum real power injection by UPQC, an objective function is derived along with practical constraints, such as the injected voltage limit on the series active filter, phase jump mitigation, and angle of voltage injection. Particle swarm optimization (PSO) has been used to find the solution of the objective function derived for minimizing real power injection of UPQC along with the constraints. Adaptive neuro-fuzzy inference systems have been used to make the proposed methodology online for minimum real power injection with UPQC by using the PSO-based data for different voltage sag conditions. The proposed method has been validated through detailed simulation and experimental studies.

Grid-Fault Control Scheme for Three-Phase Photovoltaic Inverters With Adjustable Power Quality Characteristics

The power quality of a three-phase photovoltaic (PV) inverter drastically deteriorates in the presence of grid faults with unbalanced voltages. A ripple in the injected power and an increase in the current harmonic distortion are the main noticeable adverse effects produced by this abnormal grid situation. Several grid-fault control schemes are nowadays available for operating under unbalanced grid voltage. These control schemes usually have extreme power quality characteristics. Some of them have been conceived to completely avoid power ripple during unbalanced voltage sags, but at an expense of high current harmonic distortion. With other schemes, the harmonic distortion is totally eliminated but at an expense of high ripple in the injected power. This paper further explores the performance of PV inverters under unbalanced voltage sags. It has three theoretical contributions: 1) a generalized control scheme, which includes the aforementioned grid-fault controllers as particular cases; 2) a control strategy based on the use of continuous values for the control parameters. This original approach gives adjustable power quality characteristics that cannot be achieved with the previous control schemes; 3) three different control algorithms for calculating the continuous values of the control parameters. These contributions are experimentally validated with a digital signal processor-based laboratory prototype.

Voltage Control and Active Power Management of Hybrid Fuel-Cell/Energy-Storage Power Conversion System Under Unbalanced Voltage Sag Conditions

This paper concentrates on the control of hybrid fuel-cell (FC)/energy-storage distributed generation (DG) systems under voltage sag in distribution systems. The proposed control strategy makes hybrid DG system work properly when a voltage disturbance occurs in distribution system, and it stays connected to the main grid. To distribute the power between dc power sources and stabilize the dc-link power, a Lyapunov-based neuro-fuzzy control strategy has been developed. This controller determines the supercapacitor power that should be generated according to the amount of available energy in dc-link. Also, current control strategies for the FC converter (boost) and supercapacitor converter (buck-boost converter) are designed by proportional-integral and sliding-mode control consequently. Moreover, a complementary control strategy for voltage source converter based on positive and negative symmetrical components is presented to investigate the voltage sag ride-through and voltage control capability. The hybrid system is studied under unbalance voltage sag condition. Simulation results are given to show the overall system performance including active power control and voltage sag ride-through capability of the hybrid DG system.

Robust control of hybrid fuel cell/energy storage distributed power generation system in weak grid under balanced and unbalanced voltage sag

SUMMARY The operation and control of fuel cell (FC) distributed generation (DG) systems combined with energy storage are studied in this paper. For this purpose, modeling of hybrid FC/energy storage DG system is developed. Dynamic models for proton exchange membrane fuel cell (PEMFC), supercapacitor, DC–DC converters, and grid connected inverter are described. Then a PI controller for the FC converter (boost) and a sliding mode controller for supercapacitor converter (buck-boost converter) are designed to regulate their duty cycles based on the requested current. Moreover, a robust current control strategy for voltage source converter based on positive and negative symmetrical components is presented for treating the unbalance and balance voltage sag conditions. In order to control the power between dc power sources and stabilize the dc-link power during normal and fault conditions, a self-tuning fuzzy control strategy has been designed. This controller determines the supercapacitor power that should be generated according to the amount of available energy in dc-link. The hybrid system is studied under unbalance and balance voltage sag condition. Simulation results are given to show the response of whole system under voltage sag and illustrate the performance including active power control and voltage sag ride-through capability of the proposed control strategy. Copyright# 2010 John Wiley & Sons, Ltd.

Time–optimal current control with constant switching frequency for STATCOM

In this paper a time–optimal current control algorithm with constant switching frequency for a STATCOM is presented. The method enables a fast transient response of the device limited only by its output voltage rating. The main idea behind the time–optimal current control is to find such control voltage that enables us to reach current reference values in a pre-determined minimal length of time. This control approach utilizes the mathematical model of the STATCOM and computes the exact required output to move the system state to the reference waveform. The algorithm is tested by means of simulations using the STATCOM mathematical model and a detailed model. The performance of the control algorithm is validated for reference current tracking and balanced and unbalanced network voltage sags. Simulations were carried out in PSCAD.

Unbalanced Voltage Sag Ride-Through of a Doubly Fed Induction Generator Wind Turbine With Series Grid-Side Converter

Regulatory standards for grid interconnection require wind generators to ride through disturbances such as faults and support the grid during such events. Conventional accommodations for providing voltage sag ride-through for doubly fed induction generator (DFIG) wind turbines result in compromised control of the turbine shaft and grid current during unbalanced faults. This paper presents the analysis and control design of a DFIG wind turbine with a series grid-side converter for ride-through during unbalanced voltage sag events. A dynamic model and control structure is developed for unbalanced operating conditions. Experimental results from 2-kW laboratory hardware are used to verify the proposed concepts. Hardware results illustrate an excellent ride-through response of the DFIG system under various sag conditions.

Ride-Through Control of a Doubly Fed Induction Generator Under Unbalanced Voltage Sags

This paper deals with the control of doubly fed induction generators to ride through unbalanced voltage sags. A control strategy is proposed by choosing certain current reference values in the positive and negative sequences so that the torque and the DC voltage are kept stable during such unbalanced sags. Both rotor- and grid-side converters are considered, detailing the control scheme of each converter while considering the effect of the crowbar protection. The control strategy is validated by means of simulations.

Effects of balanced and unbalanced voltage sags on DC adjustable-speed drives

This paper analyzes the sensitivity of DC adjustable-speed drives to balanced and unbalanced voltage sags. The influence of sag type, depth, duration and phase-angle jump on DC drives is studied. The control of the DC drive has been taken into account to understand drive behavior in the presence of voltage sags. Two working modes of the DC motor are considered in the study: as a consumer load and as a regenerative load. When the DC motor works as a consumer load, the study shows that sag type and depth have a significant influence on drive behavior. However, the voltage sag can be ridden through if the rectifier firing angle is set correctly by the control. When the DC motor works as a regenerative load, the study shows the consequences of the three-phase rectifier commutation failure due to the voltage sag.

An Analytical Approach for Stochastic Assessment of Balanced and Unbalanced Voltage Sags in Large Systems

In this paper, voltage sags produced by balanced and unbalanced short circuits are analyzed by means of a new analytical method. The main feature of this approach is that contrary to the fault positions method, it does not require a preassignation of discrete fault positions along the lines. The developed algorithm has been applied to the 24-buses IEEE Reliability Test System (RTS) and to a real transmission system to illustrate its application. Results are analyzed and compared with the fault positions method. The mathematical formulation of the proposed approach is straightforward and different fault distributions along the lines can be effortlessly considered. The proposed method is adequate for the analysis of any size networks.

SSC Compensation Capability of Unbalanced Voltage Sags

In this paper, the capacity of a static series compensator (SSC) to compensate for voltage sags is discussed, and a complete analysis of the effects of unbalanced sags on the SSC compensation capability is carried out. Expressions are obtained for the missing voltage during unbalanced voltage sags. These expressions are used to estimate how often a voltage sag occurs where the missing voltage exceeds the injection capability of the SSC. The method is applied to a typical Italian network configuration.

Unified Approach for Mitigating Voltage Sag and Voltage Flicker Using The DSTATCOM

This paper introduces a novel method for the mitigation of the voltage sag and voltage flicker by using Kalman filter and its derivatives (adaptive, and extended). The Kalman filter is used as a tool to extract both the instantaneous envelope of the voltage sags, and to extract the Instantaneous Flicker Level (IFL) of the voltage flicker. Also, this paper demonstrates the advantages of using the Kalman filter instead of the existing tools for tracking and extracting voltage disturbances. Digital simulation results are presented to illustrate the mitigation of unbalanced voltage sags, and the compensation of the cyclic and noncyclic voltage flicker by employing the proposed algorithm.

An Optimum UPQC with Minimum VA Requirement and Mitigation of Unbalanced Voltage Sag

The paper deals with an optimized Unified Power Quality Conditioner (UPQC), which aims at the integration of series active and shunt active power filters with minimum VA loading of the UPQC. The series active filter is a Dynamic Voltage Restorer (DVR), which is operated in twofold functioning mode. During the unbalanced voltage sag, the DVR compensates the unbalance in the voltage sag. It also regulates the voltage at the load end with minimum VA loading of the overall UPQC by optimum angle voltage injection. Validity of the proposed scheme is proved through extensive simulation results.

Mitigation of Voltage Disturbances Using Adaptive Perceptron-Based Control Algorithm

This paper presents a generalized control algorithm for voltage disturbance extraction and mitigation. The proposed mitigating device is the dynamic voltage restorer (DVR). A DVR is commonly used to mitigate the voltage sags. In this paper the proposed DVR can compensate the voltage unbalance and mitigate voltage harmonics in the time of normal operation as well as performs its basic function during the fault condition. The suggested control algorithm employs an adaptive perceptron to effectively and adaptively track and extract the most common voltage harmonics, voltage unbalance (which include negative and zero sequence voltage drops), and different types of voltage sags, which include balanced and unbalanced voltage sags. Digital simulation results are obtained using PSCAD/EMTDC to verify the effectiveness of the proposed control algorithm. Experimental results are demonstrated to prove the practicality of the mitigating device.

Increasing Usability of Dynamic Voltage Restorer in Distribution Systems

This article presents novel functionalities of the Dynamic Voltage Restorer (DVR). A DVR is commonly used to mitigate voltage sags. This research increases the usability index of the DVR. The proposed DVR can compensate the voltage unbalance and mitigate voltage harmonics in the time of normal operation in addition to its basic function in fault time. The suggested control algorithm employs an adaptive perceptron to effectively and adaptively track and extract the most common voltage harmonics, voltage unbalance (which includes negative and zero-sequence voltage drops), and different types of voltage sags (which include balanced and unbalanced voltage sags). Digital simulation results are obtained using PSCAD/EMTDC to verify the effectiveness of the proposed functionalities and the generalized control algorithm.

Effects of Balanced and Unbalanced Voltage Sags on VSI-Fed Adjustable-Speed Drives

This paper analyzes the effects of voltage sags on the three-phase rectifier of a voltage-source-inverter (VSI)-fed adjustable-speed drive. These effects are dc voltage drop and ac current peaks. As rectifier behavior is univocally characterized by two parameters (which depend on the ac reactance and the dc capacitor), their influence on the dc voltage drop and the ac current peaks is studied. Diagrams in function of these two parameters are presented in order to easily predetermine the worst effects of voltage sags originated by single-phase faults. The results of the study suggest that if the trip levels are correctly adjusted, the drive can ride through the voltage sags produced by single-phase faults (assuming the drive has typical values of L and C).