Showing papers on "Voltage sag published in 2013"

Flexible Voltage Support Control for Three-Phase Distributed Generation Inverters Under Grid Fault

Abstract: Ancillary services for distributed generation (DG) systems become a challenging issue to smartly integrate renewable-energy sources into the grid. Voltage control is one of these ancillary services which can ride through and support the voltage under grid faults. Grid codes from the transmission system operators describe the behavior of the energy source, regulating voltage limits and reactive power injection to remain connected and support the grid under fault. On the basis that different kinds of voltage sags require different voltage support strategies, a flexible control scheme for three-phase grid-connected inverters is proposed. In three-phase balanced voltage sags, the inverter should inject reactive power in order to raise the voltage in all phases. In one- or two-phase faults, the main concern of the DG inverter is to equalize voltages by reducing the negative symmetric sequence and clear the phase jump. Due to system limitations, a balance between these two extreme policies is mandatory. Thus, over- and undervoltage can be avoided, and the proposed control scheme prevents disconnection while achieving the desired voltage support service. The main contribution of this work is the introduction of a control algorithm for reference current generation that provides flexible voltage support under grid faults. Two different voltage sags have been experimentally tested to illustrate the behavior of the proposed voltage support control scheme.

Assessment and Optimization of the Transient Response of Proportional-Resonant Current Controllers for Distributed Power Generation Systems

Abstract: The increasing number of distributed power generation systems (DPGSs) is changing the traditional organization of the electrical network. An important part of these DPGSs is based on renewable energy sources. In order to guarantee an efficient integration of renewable-based generation units, grid codes must be fulfilled. Their most demanding requirements, such as low-voltage ride-through and grid support, need a really fast transient response of the power electronics devices. In this manner, the current controller speed is a key point. This paper proposes a methodology to assess and optimize the transient response of proportional-resonant current controllers. The proposed methodology is based on the study of the error signal transfer function roots by means of pole-zero plots. Optimal gains are set to achieve fast and nonoscillating transient responses, i.e., to optimize the settling time. It is proved that optimal gain selection results from a tradeoff between transients caused by reference changes and transients caused by changes at the point of common coupling. Experimental results obtained by means of a three-phase voltage source converter prototype validate the approach. Short transient times are achieved even when tests emulate very demanding realistic conditions: a +90° phase-angle jump in the current reference and a “type C” voltage sag at the point of common coupling.

Voltage Control of Multiterminal VSC-HVDC Transmission Systems for Offshore Wind Power Plants: Design and Implementation in a Scaled Platform

Abstract: This paper deals with multiterminal voltage source converter (VSC) HVDC transmission systems for the connection of offshore wind power plants to the main land ac grid. A droop-based control scheme is considered. The droop controllers have been designed base on a mixed sensitivity criterion by solving a convex optimization problem with linear matrix inequalities. The system is analyzed by means of simulations and experimentally in a scaled platform. Simulations show the control performance during a wind speed change and a voltage sag in the main ac grid. Experimental results include wind power changes (increase and decrease) and an eventual VSC loss (both considering grid-side and wind farm VSC loss). In all the cases, the simulation and the experimental results have shown a good system performance.

Reactive Power Compensation in Single-Phase Operation of Microgrid

Abstract: A coordinated control of distributed generators (DG) and distribution static compensator (DSTATCOM) in a microgrid is proposed in this paper. With high penetration of distributed sources and single-phase operation of the system, voltage unbalance can often go beyond the acceptable limit. With the feeders geographically spread out, it is not always possible to achieve reactive compensation at optimum location with the three-phase devices. In this paper, a simple control strategy for DSTATCOM with communication in loop is proposed. The proposed reactive compensation technique is based on the voltage sag and the power flow in the line. The power flow and the voltage at different locations of the feeders are communicated to the DSTATCOM to modulate the reactive compensation. The single-phase DSTATCOM compensates for the reactive power deficiency in the phase while the DGs supply “maximum available active power.” During reactive power limit of the DG, the “maximum available active power” is fixed to a value lower than maximum active power to increase reactive power injection capability of the DGs. A primary control loop based on local measurement in the DSTATCOM always ensures a part of reactive compensation in case of communication failure. It is shown that the proposed method can always ensure to achieve acceptable voltage regulation. The data traffic analysis of the communication scheme and closed-loop simulation of power network and communication network are presented to validate the proposed method.

Assessment of Voltage Sag Indices Based on Scaling and Wavelet Coefficient Energy Analysis

Abstract: Summary form only given. The two main voltage sag indices are magnitude and duration, defined in terms of the well-known rms (root mean square) voltages. The spectral energy of the voltages provides the same voltage sag indices of the rms voltage analysis and less computational effort is required. However, neither of them provide point-on-wave of sag initiation and recovery. This paper presents a wavelet-based methodology for characterization of voltage sags, in which the spectral energy of a voltage is decomposed in terms of the scaling and wavelet coefficient energies. The scaling coefficient energies of the phase voltages are used for voltage sag characterization, providing sag indices (magnitude and duration) in agreement with the definition. However, the analysis of the wavelet coefficient energies of such voltages provides additional information for identification of the point-on-wave of voltage sag initiation and recovery, important parameters for power system protection and voltage sag mitigation devices. The performance of the proposed wavelet-based methodology was assessed with actual data and it was scarcely affected by the choice of the mother wavelet. Therefore, a compact mother wavelet can be used for voltage sag analysis with computational effort equivalent to the rms method and in agreement with practical applications. The maximal overlap discrete wavelet transform (MODWT) presented better performance than the discrete wavelet transform (DWT). All the equations provided in this paper were developed for real-time analysis.

Low-voltage ride-through techniques for DFIG-based wind turbines: state-of-the-art review and future trends

Abstract: This paper deals with low-voltage ride-through (LVRT) capability of wind turbines (WTs) and in particular those driven by a doubly-fed induction generator (DFIG). This is one of the biggest challenges facing massive deployment of wind farms. With increasing penetration of WTs in the grid, grid connection codes in most countries require that WTs should remain connected to the grid to maintain the reliability during and after a short-term fault. This results in LVRT with only 15% remaining voltage at the point of common coupling (PCC), possibly even less. In addition, it is required for WTs to contribute to system stability during and after fault clearance. To fulfill the LVRT requirement for DFIG-based WTs, there are two problems to be addressed, namely, rotor inrush current that may exceed the converter limit and the dc-link overvoltage. Further, it is required to limit the DFIG transient response oscillations during the voltage sag to increase the gear lifetime and generator reliability. There is a rich literature addressing countermeasures for LVRT capability enhancement in DFIGs; this paper is therefore intended as a comprehensive state-of-the-art review of solutions to the LVRT issue. Moreover, attempts are made to highlight future issues so as to index some emerging solutions.

Optimum D-STATCOM placement using firefly algorithm for power quality enhancement

Abstract: This paper presents a novel method to optimally place the Distribution Static Synchronous Compensator (DSTATCOM) in a distribution system using the firefly algorithm (FA) for enhancing power quality. In the proposed method, the average voltage total harmonic distortion (THDV), average voltage deviation and total investment cost are considered as the objective functions, where voltage and power limits for individual buses are chosen as the optimization constraints. The performance of the proposed FA is investigated using the Matlab software on the radial IEEE 16bus test system. The obtained results are also compared with the particle swarm optimization and genetic algorithm. The simulation results verify the ability of FA in accurately determining the optimal location and size of the D-STATCOM in radial distribution systems.

Implementation of discrete PWM control scheme on Dynamic Voltage Restorer for the mitigation of voltage sag /swell

Abstract: A Dynamic Voltage Restorer (DVR) is one of the most common custom power devices to compensate for the voltage sag and swell. The main functions of the DVR are the injection of voltage to the power line and maintain the pre-sag voltage condition in the load side. Different control strategies are available depending upon the compensation technique. In this paper, a simple method for the generation of reference voltage for a DVR is presented. This control scheme provides superior performance compared to conventional control methods because it directly measurers the rms voltage at the load point without involving any transformation process. Even under system disturbances, this control scheme maintains a constant voltage at the load point. The control scheme is rather simple, flexible in cost and has an excellent voltage regulation capability. The simulation was carried using MATLAB/ SIMULINK and the results of the simulation show that this proposed method is able to provide the desirable power quality in the presence of a wide range of disturbances.

Investigation on Voltage Sags Caused by DG Anti-Islanding Protection

Abstract: This paper investigates the voltage sag caused by distributed generation anti-islanding protection. This is a new power-quality concern associated with distributed generation expansion. Anti-islanding protection aims to disconnect distributed generators immediately after the opening of a recloser. As a consequence, after the reclosing operation, voltage sag will occur in some parts of the distribution system, since the loads are energized immediately but the generators are no longer present. In this context, this work presents the findings on our investigations about this problem. The results show that the resulting voltage sag level can violate power-quality limits. Thus, a set of indices has been introduced to characterize the severity of the voltage sag, and a load flow-based method has been proposed to predict the phenomenon. Sensitivity studies are also conducted in order to identify some key factors that influence this type of voltage depression.

Unbalance Voltage Sag Fault-Type Characterization Algorithm for Recorded Waveform

Abstract: This paper presents a new algorithm, namely, a three-phase-angle algorithm, for accurate characterization of three-phase unbalanced voltage sags, associated with line-to-line and single-line-to-ground faults, from recorded voltage waveforms. This new algorithm uses the relation between the angles of three-phase voltages and the angle of characteristic voltage. The performance of the proposed algorithm is evaluated by considering numerical examples and recorded voltage waveforms available with the IEEE database. A thorough comparison of the results, obtained using the proposed algorithm and other existing algorithms, is made to demonstrate its efficiency. The results suggest that the proposed algorithm is advantageous in accurate characterization of voltage sags in two ways. First, it is proficient under large/small phase-angle jump as well as for solidly and resistance-grounded systems. Second, it is capable of extracting the information about faults, associated with voltage sags.

Fixed and variable power angle control methods for unified power quality conditioner: operation, control and impact assessment on shunt and series inverter kVA loadings

Abstract: The concept of power angle control (PAC) of unified power quality conditioner (UPQC) provides an effective way to utilise the full capacity of series inverter of UPQC. This study deals with the operation and control of UPQC with PAC under voltage sag and swell conditions. The power angle, a resultant angle between actual source voltage and resultant load voltage, is used as a control variable. Two new control approaches, namely, fixed power angle and variable power angle, are proposed. It is shown that these approaches can affect the overall kVA loadings of shunt and series inverters. A thorough investigation on these fixed and variable power angle methods is presented in this study. A Matlab/Simulink-based detailed simulation and digital signal processor-based experimental studies are discussed to evaluate the performance of proposed two methods.

Novel way for classification and type detection of voltage sag

Abstract: This study presents a new classification of voltage sags which is based on its types, characteristic voltage and zero-sequence component of voltage. It is extremely difficult to distinguish between the voltage sags which have same type, same characteristic voltage and same zero-sequence component of voltage due to load effects and hence such classification is significant. The proposed classification is mathematically justified by introducing two new indices, namely, phase-to-neutral and phase-to-phase voltage indices. Using the theoretical relation between these two indices, it has been revealed that the voltage sags which have the same type, same characteristic voltage and same zero-sequence component of voltages, have same mathematical relations. Further, to reveal the accuracy of the proposed classification and type detection, it is validated through recorded waveforms available in IEEE database, Scottish Power, data obtained from Matlab simulation under different conditions of voltage sags and data of real power station. In addition, the proposed classification and type detection are compared with other established algorithms for type detection of voltage sags. The comparative results show that the proposed classification not only removes the existing anomalies in the earlier proposals but also shows its superiority by presenting more accurate and less confusing results.

OPEN unified power quality conditioner with control based on enhanced phase locked loop

Abstract: OPEN unified power quality conditioner (UPQC) offers different power quality levels with different electricity bill to their users. OPEN UPQC has more flexible modularity than traditional UPQC for field applications. OPEN UPQC consists of two controlled pulse-width modulated shunt and series inverters that have no common DC link. Shunt inverter eliminates current harmonics originating from non-linear load side and series inverter mitigates voltage sag/swell at the point of common coupling caused from faults on transmission system. In this study, controllers for both shunt and series inverters are based on enhanced phase locked loop (E-PLL) and non-linear adaptive filter. For the controller of shunt inverter, proportional integrator controller-based DC-link control strategy is adapted to E-PLL. A fast sag/swell detection method is also presented. Functionalities of OPEN-UPQC system is deeply analysed and tested through simulation studies with power system computer aided design/electromagnetic transients DC analysis program. The test results verify that OPEN UPQC achieves good performance for mitigating the effects of voltage sag/swell and suppressing the load current harmonics.

Voltage Sag Analysis in Loop Power Distribution System With SFCL

Abstract: In this paper, the effects of a superconducting fault current limiter (SFCL) installed in loop power distribution systems on voltage sags are assessed and analyzed. The power distribution system will be operated to a type of loop. In this case, voltage drops (sags) are severe because of the increased fault current when a fault occurs. If SFCL is installed in the loop power distribution system, the fault current decreases based on the location and resistance value of the SFCL, and voltage sags are improved. In this paper, the improvement of the voltage sag is analyzed according to the fault location, resistance value of SFCL, and the length of the loop power distribution system. First, a resistor-type SFCL model is used using the PSCAD/EMTDC. Next, the loop power distribution system is modeled. Finally, when the SFCL is installed in the radial or loop power distribution system with various lengths, voltage sags are evaluated according to various fault locations. The results of voltage sag analysis in the loop system are compared with the voltage sags in radial power distribution system.

Enhancing Accuracy While Reducing Computation Complexity for Voltage-Sag-Based Distribution Fault Location

Abstract: A fault-location method for radial distribution systems is proposed in this paper. The proposed method uses voltage and current phasors from feeder root and voltage sags measured at sparse nodes along the feeder, and pinpoints faults to the nearest node. Decision-tree (DT)-based fault segment identification is introduced before the process of node selection to reduce the computational complexity and improve fault-location accuracy. The method has been implemented on a practical distribution system and tested under a large number of fault scenarios. Test results are compared with those from the traditional voltage-sag-based fault-location algorithm using the same inputs, and the conclusion is that the proposed method can achieve more reliable results while maintaining computational simplicity. A quantitative method to suggest the optimal placement of measurement units based on the DT variable importance is proposed at the end.

Safe-Commutation Strategy for the Novel Family of Quasi-Z-Source AC–AC Converter

Abstract: A novel quasi-Z-source ac-ac converter for voltage sag/swell compensation in smart grids is proposed in this paper. Derived from the basic ac-ac safe-commutation mechanism, a new voltage-based commutation strategy is developed, which is achieved without snub circuit and only by sampling the output voltage. Therefore, the control circuit is simplified greatly. Two switches in a one-phase loop are always turned on for current freewheel path in dead time, and current spike and voltage spike are solved well. Compared with conventional ac-ac converters, fewer switches conduct with soft switching during operation, resulting in high efficiency. Then, the proposed converter is analyzed, including the operating modes and voltage ration based on a state-space average. The experimental results validate the theoretical predication well. The safe-commutation strategy can also be easily applied to other converters of the proposed family.

Low voltage ride-through of single-phase transformerless photovoltaic inverters

Abstract: Transformerless photovoltaic (PV) inverters are going to be more widely adopted in order to achieve high efficiency, as the penetration level of PV systems is continuously booming. However, problems may arise in highly PV-integrated distribution systems. For example, a sudden stoppage of all PV systems due to anti-islanding protection may trigger grid disturbances. Thus, standards featuring with ancillary services for the next generation PV systems are under a revision in some countries. The future PV systems have to provide a full range of services as what the conventional power plants do, e.g. Low Voltage Ride-Through (LVRT) under grid faults and grid support service. In order to map future challenges, the LVRT capability of three mainstream single-phase transformerless PV inverters under grid faults are explored in this paper. Control strategies with reactive power injections are also discussed. The selected inverters are the full-bridge inverter with bipolar modulation, full-bridge inverter with DC bypass and the Highly Efficient and Reliable Inverter Concept (HERIC). A 1 kW single-phase grid-connected PV system is analyzed to verify the discussions. The tests confirmed that, although the HERIC inverter is the best candidate in terms of efficiency, it is not very feasible in case of a voltage sag. The other topologies are capable of providing reactive current during LVRT. A benchmarking of those inverters is also provided, which offers the possibility to select appropriate devices and to further optimize the system.

Simulation of power quality events using simulink model

Abstract: This paper presents a comprehensive set of Simulink models, utilizing SimPowerSystems blockset to simulate various power quality events in distribution system. These models include distribution line fault, induction motor starting and transformer energizing models which are used to simulate various types of voltage sag. Capacitor bank switching model is also presented to simulate voltage transient at the distribution network. There are also three phase non linear load models used to simulate voltage notches and harmonic power quality events at the load side. For simplicity, these models are developed with minimum blocks and setting without scarifying the essence of power quality event. These models are also useful to simulate various power quality waveforms for power quality analysis algorithm research. These models serve as a fundamental model for analysis of complex power quality events as well as contribute to the power engineering education curriculum.

Sliding-mode variable structure controller for cascade STATic var COMpensator

Abstract: The authors propose a novel sliding-mode variable structure controller for cascade STATic var COMpensator (STATCOM), which integrates the direct feedback linearisation control for the balanced system and the individual phase instantaneous current tracking control for the unbalanced system, to achieve a versatile STATCOM controller. It can adapt automatically to various distribution system conditions. Based on the degree of system unbalance, the authors design a switching function to make the STATCOM work reliably with satisfactory performance, when the serious voltage sag occurs or the distribution system enters serious asymmetrical condition. The authors also propose a hierarchical DC-link voltages balance control strategy. The authors build a detailed ± 10 MVar STATCOM model with power system CAD and electro magnetic transient in DC system (PSCAD/EMTDC) and validate the effectiveness and advantage of the proposed control strategy with the simulation study.

Performance investigation of dynamic voltage restorer using PI and fuzzy controller

Abstract: This paper investigates the performance of Dynamic Voltage Restorer for compensating different voltage sag levels with various faults and to reduce the Total Harmonic Distortion during the mitigation process. The DVR is implemented with three phase voltage source inverter and is connected at the point of common coupling in order to regulate the load side voltage. The compensation is based on PI and Mamdani Fuzzy Controller. Extensive simulation studies under different magnitude of sag for faults on load side for balanced and unbalanced conditions are conducted using fault generator. Simulation result analysis reveals that DVR performs perfectly with PI and Fuzzy control approach. In addition, capability and performance of DVR for various energy storage capacities and injection transformer rating are also analyzed. The performance of these controllers are validated with simulation results using Matlab/Simulink.

SVD Applied to Voltage Sag State Estimation

Abstract: The method presented in this paper addresses the problem of voltage sag state estimation. The problem consists in estimating the voltage sags frequency at nonmonitored buses from the number of sags measured at monitored sites. Usually, due to limitations on the number of available voltage sag monitors, this is an underdetermined problem. In this approach, the mathematical formulation presented is based on the fault positions concept and is solved by means of the singular value decomposition technique. The proposed estimation method has been validated by using the IEEE 118 test system and the results obtained have been very satisfactory.

Realization of cascaded H-bridge 5-Level multilevel inverter as Dynamic Voltage Restorer

Abstract: This paper presents a modern industrial devices are mostly based on electronic devices such as programmable logic controllers and electronic drives. The electronic devices are very sensitive to disturbances and become less tolerant to power quality problems such as voltage sags, swells and harmonics. In this multi level inverter is used because to improve output voltage like Voltage Profile & Reduce Total Harmonic Distortion compared to voltage source inverter, Due to the power quality issues like voltage sag, voltage swell, unbalanced voltage, voltage flickering, Interruptions etc. load side voltage is not constant. The main requirement of any system is to maintain load side voltage constant. Among the entire power quality issues, voltage sag and voltage swell occupy a major role. So my paper deals with compensating different power quality issues at load side using 5-level multi level inverter as a Dynamic Voltage Restorer.

Positive- and negative-sequence control of grid-connected photovoltaic systems under unbalanced voltage conditions

Abstract: This paper discusses the control of the positive- and negative-sequence components of a large-scale grid-connected photovoltaic system (GCPS) under unbalanced voltage sag conditions in the grid. Some issues regarding stability and dynamic performance of the system occur when applying PI controllers in the current control loops. The reason is the delay that the filtering method imposes when extracting the current/voltage sequences. Because of such a delay, the dynamic response of the system becomes slower compared with the case when no filtering technique is needed. Furthermore, there is a strong restriction on choosing suitable parameters for the current/voltage loop controllers without compromising system stability. All these issues are discussed in this paper on a 1-MVA GCPV system using MATLAB/Simulink software.