Showing papers on "Voltage sag published in 2016"

Control Strategy to Maximize the Power Capability of PV Three-Phase Inverters During Voltage Sags

Abstract: Under voltage sags, grid-tied photovoltaic inverters should remain connected to the grid according to low-voltage ride-through requirements. During such perturbations, it is interesting to exploit completely the distributed power provisions to contribute to the stability and reliability of the grid. In this sense, this paper proposes a low-voltage ride-through control strategy that maximizes the inverter power capability by injecting the maximum-rated current during the sag. To achieve this objective, two possible active power situations have been considered, i.e., high- and low-power production scenarios. In the first case, if the source is unable to deliver the whole generated power to the grid, the controller applies active power curtailment to guarantee that the maximum rated current is not surpassed. In the second case, the maximum allowed current is not reached, thus, the control strategy determined the amount of reactive power that can be injected up to reach it. The control objective can be fulfilled by means of a flexible current injection strategy that combines a proper balance between positive- and negative-current sequences, which limits the inverter output current to the maximum rated value and avoid active power oscillations. Selected experimental and simulation results are reported in order to validate the effectiveness of the proposed control strategy.

Small Signal Instability of PLL-Synchronized Type-4 Wind Turbines Connected to High-Impedance AC Grid During LVRT

Abstract: Instability phenomenon of type-4 wind turbine during grid faults is increasingly concerned by researchers and may result in failure of low-voltage ride through (LVRT), especially when connected to a high-impedance ac grid In order to carry out the stability analysis for type-4 wind turbines during deep voltage sag, dynamic model, which reflects the interaction between phase locked loop (PLL) and alternating current control (ACC), is first developed Eigenvalues and modal analysis show that there is a risk that a pair of poorly damped poles may become unstable, which is mainly dominated by PLL as well as the interaction between PLL and ACC The mechanism of small-signal instability during LVRT is identified as the insufficiency of damping Complex torque coefficient approach is further proposed to study the characteristic of the damping component, which is composed of the inherent one determined by PLL and the additional influenced by ACC Finally, simulated results are presented to verify the analytical results

Power Quality Improvement of Grid Connected Wind Farms through Voltage Restoration Using Dynamic Voltage Restorer

Abstract: Increasing sensitivity of the loads with respect to power quality has gained the interest of power system analysis and power quality improvement techniques. The voltage sags or swells which are characterized by rms voltage variations outside the normal operating range of voltages due to faults may lead to improper disconnection of wind turbines. This paper deals with the effective voltage sag/swell mitigation using Dynamic Voltage Restorer (DVR), to regulate the terminal voltage of the wind farm. The DVR utilizes a feed forward vector control based algorithm to generate the PWM based firing signals for injecting appropriate compensation voltages. The actual wind farm field data of the voltage sag and swell events during fault conditions are re- created using MATLAB/Simulink and restored by employing DVR. The simulation results are shown to verify the operation of DVR during balanced voltage sag and swell conditions.

Modular battery powered handheld surgical instrument with voltage sag resistant battery pack

Abstract: A system including a battery assembly for use in a modular handheld surgical instrument, wherein the battery assembly includes a primary battery pack, a secondary battery pack, and a circuit configured to supply a first output voltage to a first load of the modular handheld surgical instrument via the primary battery pack, supply a second output voltage to a second load of the modular handheld surgical instrument via the secondary battery pack, and upon sensing a sagging voltage from the primary battery pack, supply a supplemental voltage to the first load, via the secondary battery pack, to prevent the battery assembly from supplying an output voltage below a predetermined threshold output voltage under the first load.

Impact of dynamic behavior of photovoltaic power generation systems on short-term voltage stability

Abstract: In this study, we investigate the impact of the dynamic behavior of photovoltaic (PV) power generation systems on short-term voltage stability of the transmission system. First, the impact of the fault ride-through capability of a PV model is studied by setting several recovery speeds of the active current output when the operation of the PV system is interrupted because of a voltage sag. The results are analyzed by using transient P-V curves and a stability boundary, which has been proposed in our previous research. Further, we show that the installation of PVs severely impairs the short-term voltage stability if the PVs shut off after a voltage sag, and its recovery speed is low. Next, two countermeasures to control short-term voltage instability phenomena are tested. One is the operation of the PV system at a leading power factor in the normal state, and the other is the dynamic reactive power control by the inverters of the PV system after a voltage sag. Numerical examples are carried out for a one-load infinite-bus power system and a five-machine five-load power system. The results show that these countermeasures can play a substantial role in preventing the voltage instability phenomena caused when a PV system is suddenly interrupted because of a fault.

PLL-Less Nonlinear Current-Limiting Controller for Single-Phase Grid-Tied Inverters: Design, Stability Analysis, and Operation Under Grid Faults

Abstract: A nonlinear controller for single-phase grid-tied inverters, that can operate under both a normal and a faulty grid with guaranteed closed-loop stability, is proposed. The proposed controller acts independently from the system parameters, does not require a phase-locked loop, and can achieve the desired real power regulation and unity power factor operation. Based on nonlinear input-to-state stability theory, it is analytically proven that the inverter current always remains below a given value, even during transients, independently from grid variations or faults (short circuit or voltage sag). The desired performance and stability of the closed-loop system are rigorously proven since the controller has a structure that does not require any switches, additional limiters, or monitoring devices for its implementation. Therefore, nonlinear stability of a grid-tied inverter with a given current-limiting property is proven for both normal and faulty grid conditions. The effectiveness of the proposed approach is experimentally verified under different operating conditions of the grid.

Compensation of voltage sag using DVR with PI controller

Abstract: A power quality problem is an occurrence which depicted as a nonstandard voltage, current or frequency that results in a failure or a dis-functioning of end use equipments causing a havoc to system performance. The aim therefore, in this work to maintain a proper voltage profile by mitigation of sag due to faults in power system occurred due to several reasons like energizing of heavy loads, transformers, induction motors, bad weather, insulation breakdown, closing & reclosing of circuit breakers. In this paper compensation of voltage sag is studied in various faults and minimize the total harmonic distortion in the transmission line.

Design and performance of a 1 MW-5 s high temperature superconductor magnetic energy storage system

Abstract: The feasibility of a 1 MW-5 s superconducting magnetic energy storage (SMES) system based on state-of-the-art high-temperature superconductor (HTS) materials is investigated in detail. Both YBCO coated conductors and MgB2 are considered. A procedure for the electromagnetic design of the coil is introduced and the final layout is arrived at and compared for the two materials. The choice of the inductance of the coil is carried out as part of the design procedure. Both low-field (3 T) and high-field (8 T) designs are considered for the YBCO. AC losses during a complete charge/discharge cycle at full power are estimated and the cooling power needed for continuous operation is derived. The power conditioning system and control algorithms needed to carry out various operations are discussed in detail. Performances of the SMES system during voltage sag compensation, load leveling and power factor correction are investigated by means of numerical simulation.

Impact of Voltage Sags on Electric-Vehicle Charger and Critical Voltage Sag Determination

Abstract: This letter investigates the impact of voltage sags on electric-vehicle chargers and presents a method to determine the minimum symmetrical voltage sag during and after which chargers can both be kept stable. The proposed method is based on the constraints of pulse-width modulation (PWM), electric circuits, and small-signal stability.

Analysis and Design of a DSTATCOM Based on Sliding Mode Control Strategy for Improvement of Voltage Sag in Distribution Systems

Abstract: Voltage sag is considered to be the most serious problem of power quality. It is caused by faults in the power system or by the starting of large induction motors. Voltage sag causes about 80% of the power quality problems in power systems. One of the main reasons for voltage sag is short circuit fault, which can be compensated for by a distribution static compensator (DSTATCOM) as an efficient and economical flexible AC transmission system (FACTS) device. In this paper, compensation of this voltage sag using DSTATCOM is reviewed, in which a sliding mode control (SMC) technique is employed. The results of this control system are compared with a P+Resonant control system. It will be shown that this control system is able to compensate the voltage sag over a broader range compared to other common control systems. Simulation results are obtained using PSCAD/EMTDC software and compared to that of a similar method.

An Integrated High Side Var-Voltage Control Strategy to Improve Short-Term Voltage Stability of Receiving-End Power Systems

Abstract: To make better use of local generators' dynamic var reserve to improve short-term voltage stability (STVS), this paper proposes an integrated high side var-voltage control (IHSV $^{2}$ C) for power plants in receiving-end power systems. The IHSV $^{2}$ C consists of a plant-level multi-machine var coordinator (MMVC) and several unit-level high side voltage controllers (HSVCs). MMVC coordinates the reactive power output among generators and provides control parameters for HSVCs, while HSVCs can maintain the voltage of a pre-defined voltage control point (VCP) by regulating the voltage reference of the excitation control of each generator. Therefore, when the system suffers a serious fault, IHSV $^{2}$ C can drive the generators to provide stronger var support. Consequently, the voltage stability can be improved. A conceptual model of the receiving-end power system in China Southern Power Grid (CSG) is established, and the IHSV $^{2}$ C has been validated through a simulation analysis on the system. Both time-domain simulation results and the voltage sag severity index (VSSI) have fully demonstrated its performance. Therefore IHSV $^{2}$ C offers a new and effective approach to improve STVS of receiving-end power systems.

Compensation of voltage sags and swells by using Dynamic Voltage Restorer (DVR)

Abstract: The most important aspect for electrical engineering is power quality in recent year. Power quality problem is an event of abnormal voltage, current or frequency. Utility distribution networks experience from various types of disturbances. The major problems dealt here are the voltage sags/swells. Now the fast developments in power electronics technology have made it possible to mitigate power quality problems. Such as, dynamic voltage restorer can provide the most mercantile solution to compensate voltage sag/swell by injecting voltage in to the sensitive system. Dynamic Voltage Restorer is a series connected power electronics based device that can mitigate the power quality issues in the system and restore the load voltage to the pre-fault value. This review paper first gives an introduction to related power quality problems for a DVR and power electronics controllers for voltage sag/swell compensation. Afterwards the operation and elements in DVR is described by using new control strategy.

Voltage quality assessment considering low voltage ride-through requirement for wind turbines

Abstract: This study proposes an effective method for voltage quality assessment considering low voltage ride-through (LVRT) requirement for wind turbines. With increasing wind power generation, there is a strong need for system connection requirements of wind turbines, for stable system operation even under fault conditions. The LVRT requirements specify acceptable voltage magnitude and duration for maintaining the connection to power systems under voltage sags. In general, different grid connection points of wind farms have different voltage quality performance. Therefore, LVRT requirements can be severe for certain connecting points and wind turbines. The proposed method can efficiently assess voltage sag performance at grid connection points, and vulnerable connection points that can be expected to incur more severe voltage sags causing wind turbine disconnections can be identified.

An optimized compensation strategy of DVR for micro-grid voltage sag

Abstract: This paper uses a dynamic voltage restorer (DVR) to improve the voltage quality from voltage sags. It is difficult to satisfy various of compensation quality and time of the voltage sag by using single compensation method. Furthermore, high-power consumption of the phase jump compensation increases the size and cost of a dynamic voltage restorer (DVR). In order to improve the compensating efficiency of DVR, an optimized compensation strategy is proposed for voltage sag of micro-grid caused by interconnection and sensitive loads. The proposed compensation strategy increases the supporting time for long voltage sags. Firstly, the power flow and the maximum compensation time of DVR are analyzed using three basic compensation strategies. Then, the phase jump is corrected by pre-sag compensation. And a quadratic transition curve, which involves the injected voltage phases of pre-sag strategy and minimum energy strategy, is used to transform pre-sag compensation to minimum energy compensation of DVR. The transition utilizes the storage system to reduce the rate of discharge. As a result, the proposed strategy increases the supporting time for long voltage sags. The analytical study shows that the presented method significantly increases compensation time of DVR. The simulation results performed by MATLAB/SIMULINK also confirm the effectiveness of the proposed method.

Smart Grid Connection of an Induction Motor Using a Three-Phase Floating H-bridge System as a Series Compensator

Abstract: Electrical grid voltage sags are a significant industrial power quality concern. According to a survey result across the US, voltage sags and short-duration power outages are responsible for 92% of power quality problems faced by industrial customers. These power interruptions often impose severe cost penalties in plant shutdowns for many industries. A series compensation scheme for an induction motor is presented with inherent voltage sag ride through capability. The system utilizes a system of three-phase floating capacitor H-bridge converters located in each phase between the utility grid and a squirrel-cage induction motor. By injecting a series voltage in each phase, the proposed system can manipulate the voltage supplied to a motor, increasing its tolerance of grid voltage sags. The voltage injection scheme has an inherently leading grid power factor under steady state and, hence, generates VARs into the grid, over a wide range of load conditions. The paper develops mathematical analysis of the proposed system to quantify both the voltage sag ride-through and the reactive power generation that results. The analysis shows that voltage sag tolerance of the proposed system is closely related to the motor fundamental power factor. Moreover, it is found that unity power factor operation of the system, as seen from the grid, is possible and that the reactive power generation capability can also be accurately quantified. A 5-hp experimental testbed is used to validate both the grid voltage ride-through capability feature and the reactive power generation characteristics.

Reactive power compensation of grid-connected inverter in vehicle-to-grid application to mitigate balanced grid voltage sag

Abstract: Modernized grid needs ancillary service as a variety of distributed energy resources are connected. Vehicle-to-grid (V2G) application is one of promising distributed energy resources, which enables to transfer bidirectional power between batteries of plug-in electric vehicle and the grid as well as offer ancillary service to the grid. Benefits of V2G application are voltage and frequency regulation, and spinning reserves. To achieve benefits of V2G application, grid-connected inverter (GCI) and its control method are significant. This paper presents control methods of GCI in V2G application to mitigate balanced voltage sag. Voltage sag detector and reactive power compensation are implemented. A 10 kW GCI with output LCL filter and closed-loop control is simulated for V2G application.