Showing papers on "Transient (oscillation) published in 2015"

A General Method for Analyzing Resonant and Soft-Charging Operation of Switched-Capacitor Converters

Abstract: Traditionally, switched-capacitor (SC) converters have suffered from high transient currents, which limit both the efficiency and power density of such converters. Soft-charging operation can be employed to eliminate the current transients and greatly improve the power density of SC converters. In this approach, a second-stage magnetic converter is cascaded with the SC stage to act as a controlled current load. Another approach is to use resonant SC converters with zero-current switching. This paper shows that resonant and soft-charging operations of SC converters are closely related, and a technique will be proposed, which achieves either operation by adding a single inductor to existing SC topologies. In addition, since most preexisting resonant or soft-charging SC converters were devised in an ad-hoc manner, this paper formulates an analytical method that can determine whether an existing conventional SC converter topology is compatible with the proposed approach. A number of common SC topologies are analyzed, including Dickson, series-parallel, ladder, Fibonacci, and doubler configurations. Through comparison to simulated results, as well as experimental work, the proposed method is validated and a family of high-performance SC converters is obtained.

Model Predictive Direct Current Control of Modular Multilevel Converters: Modeling, Analysis, and Experimental Evaluation

Abstract: Modular multilevel converters (M2LCs) are typically controlled by a hierarchical control scheme, which essentially requires at least two control loops: one to control the load current and another to control circulating currents. This paper presents an M2LC with a single controller, which is based on model predictive direct current control (MPDCC) with long prediction horizons. The proposed MPDCC scheme maintains the load current within tight bounds around sinusoidal references and minimizes capacitor voltage variations and circulating currents. An internal prediction model of the M2LC is used to minimize the number of switching transitions for a given current ripple at steady state while providing a fast current response during transient conditions. A state-space model, which is generalized for an N number of modules per each arm of the M2LC, is also presented to investigate the dynamic behavior of arm currents and capacitor voltages. Simulated performance of the converter, under various operating conditions, is presented in comparison to measured performance of a single-phase, three-level 860-VA M2LC prototype to demonstrate the proposed MPDCC philosophy.

Mitigating Voltage and Frequency Fluctuation in Microgrids Using Electric Springs

Abstract: Voltage and frequency fluctuation associated with renewable integration have been well identified by power system operators and planners. At the microgrid level, a novel device for the implementation of dynamic load response, which is known as the electric springs (ES), has been developed for mitigating both active and reactive power imbalances. In this paper, a comprehensive control strategy is proposed for ES to participate in both voltage and frequency response control. It adopts the phase angle and amplitude control which respectively adjust the active power and the reactive power of the system. The proposed control strategy is validated using a model established with power system computer aided design/electro-magnetic transient in dc system. Results from the case studies show that with appropriate setting and operating strategy, ES can mitigate the voltage and frequency fluctuation caused by wind speed fluctuation, load fluctuation, and generator tripping wherever it is installed in the microgrid.

Proportional-Resonant Current Controllers Design Based on Desired Transient Performance

Abstract: Stationary reference frame proportional-resonant (PR) controllers have recently became a viable alternative to rotation reference frame proportional-integrative compensators in ac applications because of their ability of achieving zero steady-state error without the need for computational-intensive reference frame transformations. While extensive efforts have been put into performance comparison of the two control schemes, design of PR controllers according to desired closed-loop time-domain transient performance was barely investigated. This letter proposes a method for deriving PR controller structure and coefficients according to desired transient behavior of ac signal amplitude, applied to typical power converter current loop. The method is based on the fact that if ac signal envelope is perceived as dc signal, its transient behavior may be easily shaped utilizing well-known approaches employed in dc systems loop shaping while keeping zero phase tracking error at all times. On the other hand, while desired transient performance is easily achieved, the relation between crossover frequency and transient processes time constant is not as straightforward as in dc systems. The validity of the presented theoretical analysis is evaluated by simulations.

Dynamical transition in the temporal relaxation of stochastic processes under resetting.

Abstract: A stochastic process, when subject to resetting to its initial condition at a constant rate, generically reaches a nonequilibrium steady state. We study analytically how the steady state is approached in time and find an unusual relaxation mechanism in these systems. We show that as time progresses an inner core region around the resetting point reaches the steady state, while the region outside the core is still transient. The boundaries of the core region grow with time as power laws at late times with new exponents. Alternatively, at a fixed spatial point, the system undergoes a dynamical transition from the transient to the steady state at a characteristic space-dependent timescale ${t}^{*}(x)$. We calculate analytically in several examples the large deviation function associated with this spatiotemporal fluctuation and show that, generically, it has a second-order discontinuity at a pair of critical points characterizing the edges of the inner core. These singularities act as separatrices between typical and atypical trajectories. Our results are verified in the numerical simulations of several models, such as simple diffusion and fluctuating one-dimensional interfaces.

A Fully-Integrated Low-Dropout Regulator With Full-Spectrum Power Supply Rejection

Abstract: A fully-integrated low-dropout regulator (LDO) with fast transient response and full spectrum power supply rejection (PSR) is proposed to provide a clean supply for noise-sensitive building blocks in wideband communication systems. With the proposed point-of-load LDO, chip-level high-frequency glitches are well attenuated, consequently the system performance is improved. A tri-loop LDO architecture is proposed and verified in a 65 nm CMOS process. In comparison to other fully-integrated designs, the output pole is set to be the dominant pole, and the internal poles are pushed to higher frequencies with only 50 μA of total quiescent current. For a 1.2 V input voltage and 1 V output voltage, the measured undershoot and overshoot is only 43 mV and 82 mV, respectively, for load transient of 0 μA to 10 mA within edge times of 200 ps. It achieves a transient response time of 1.15 ns and the figure-of-merit (FOM) of 5.74 ps. PSR is measured to be better than -12 dB over the whole spectrum (DC to 20 GHz tested). The prototype chip measures 260×90 μm 2 , including 140 pF of stacked on-chip capacitors.

The Impact of Nonlinear Junction Capacitance on Switching Transient and Its Modeling for SiC MOSFET

Abstract: The nonlinear junction capacitances of power devices are critical for the switching transient, which should be fully considered in the modeling and transient analysis, especially for high-frequency applications. The silicon carbide (SiC) MOSFET combined with SiC Schottky Barrier Diode (SBD) is recognized as the proposed choice for high-power and high-frequency converters. However, in the existing SiC MOSFET models only the nonlinearity of gate-drain capacitance is considered meticulously, but the drain–source capacitance, which affects the switching commutation process significantly, is generally regarded as constant. In addition, the nonlinearity of diode junction capacitance is neglected in some simplified analysis. Experiments show that without full consideration of nonlinear junction capacitances, some significant deviations between simulated and measured results will emerge in the switching waveforms. In this paper, the nonlinear characteristics of drain–source capacitance in SiC MOSFET are studied in detail, and the simplified modeling methods for engineering applications are presented. On this basis, the SiC MOSFET model is improved and the simulation results with improved model correspond with the measured results much better than before, which verify the analysis and modeling.

Transient Stability and Voltage Regulation in Multimachine Power Systems Vis-à-Vis STATCOM and Battery Energy Storage

Abstract: This paper examines the application of STATCOM and battery energy storage to enhance the transient stability of large-scale multimachine power systems with synchronous and doubly-fed induction generators (DFIGs). A passivity-based control design method [interconnection and damping assignment passivity-based control (IDA-PBC)] is developed for multimachine power systems and its performance is evaluated on a two-area system consisting of two synchronous generators (SGs) and two DFIG along with STATCOM/battery energy storage system. The main contributions of this paper are threefold: 1) use of a STATCOM and battery energy storage system to enhance transient stability and provide voltage regulation with SG and DFIG; 2) demonstrating the application of nonlinear control theory (specifically the IDA-PBC methodology) for the design of a stabilizing feedback controller in large-scale power systems to improve transient system performance; and 3) developing a methodology that can use the additional degrees of freedom in large-scale power systems in order to further improve system performance, in particular the transient stability margin [measured through critical clearing time (CCT)] and the dynamic transient performance of the system. In order to achieve power angle stability along with the simultaneous regulation of frequency and voltage, the performance of the proposed control scheme after the occurrence of large disturbances is evaluated and compared with a conventional power system stabilizer and a feedback linearizing controller.

Fault Detection for IGBT Using Adaptive Thresholds During the Turn-on Transient

Abstract: This paper presents the analysis and design of an electronic failure detection system applied to the insulated gate bipolar transistor (IGBT), this proposal is based on the direct measurement of behavior of the gate signal during the turn-on transient. The failures by short-circuit and open-circuit devices only are considered in this paper. To achieve early detection, the IGBT gate signal behavior during turn-on transient is used and to increase the effectiveness of the detection and to tolerate the variations of input to system, adaptable thresholds have been added to the analog electronics circuit implemented. The experimental tests are presented in order to validate the proposed fault-detection technique.

Transient Performance Improvement of Microgrid by a Resistive Superconducting Fault Current Limiter

Abstract: In this paper, a resistive-type superconducting fault current limiter (SFCL) is suggested to improve the transient performance of a microgrid system during a fault. The microgrid is connected to the main network at the point of common coupling, where the resistive-type SFCL is applied. When a short-circuit fault happens at the connecting line, the SFCL can mitigate the fault current, and its action signal will be sent to the master distributed generation (DG) included in the microgrid. Accordingly, the switching between the master DG's two control patterns can be flexibly performed; furthermore the microgrid system is expected to achieve a smooth transition between its grid-connected and islanded modes. Theoretical analysis and a technical discussion are conducted, and the simulation model of a typical microgrid with the SFCL is built in MATLAB. From the demonstrated results, employing the resistive-type SFCL can effectively limit the transient fault current to a lower level, help guarantee the microgrid system's power balance, and enhance its voltage and frequency stability.

Fast response integrated MEMS microheaters for ultra low power gas detection

Abstract: Semiconducting metal oxide (SMO) gas sensors typically operate at a few hundred degrees Celsius and consume hundreds of milliwatts of power, limiting their application in battery-powered devices. An analytical model is presented for the optimization of the heater dimensions, which suggests the minimal power consumption is achieved when heat loss through air conduction and supporting beam conduction are equal. We demonstrate micromachined SMO sensors with optimized microheaters, which consume only ∼2 mW of power when operated continuously at 300 °C. We also measure an ultra-fast thermal response time of 33 μs via a transient temperature–resistivity response method. The short response time allows the heaters to be operated in ultra-short pulsing mode decreasing the average power consumption to the μW level. These micromachined SMO sensors are used in proof-of-principle experiments as ultralow power hydrogen sulfide SMO gas sensors.

On the Reduction of Second Harmonic Current and Improvement of Dynamic Response for Two-Stage Single-Phase Inverter

Abstract: Two-stage single-phase inverters have been widely used as they can achieve voltage matching and galvanic isolation between the input and output. Due to the pulsating output power of the downstream inverter, an ac current at twice the output frequency, which is called second harmonic current (SHC), arises in the input side of the downstream inverter. This SHC will penetrate to the front-end dc-dc converter, leading to reduced conversion efficiency. This paper first analyzes the propagation mechanism of the SHC and load transient response of two-stage single-phase inverters from the viewpoint of output impedance. Then, based on the current mode control and load current feed forward, two control methods to achieve low SHC in the front-end dc-dc converter and fast dynamic performance during load transient are proposed in this paper. Finally, a 2-kW two-stage single-phase inverter prototype has been constructed and tested, and the experimental results are provided to verify the effectiveness of the proposed control methods.

Advanced Hybrid Transient Stability and EMT Simulation for VSC-HVDC Systems

Abstract: This paper deals with advanced hybrid transient stability and electromagnetic-transient (EMT) simulation of combined ac/dc power systems containing large amounts of renewable energy sources interfaced through voltage-source converter–high-voltage direct current (VSC-HVDC). The concerning transient stability studies require the dynamic phenomena of interest to be included with adequate detail and reasonable simulation speed. Hybrid simulation offers this functionality, and this contribution focuses on its application to (multiterminal) VSC-HVDC systems. Existing numerical interfacing methods have been evaluated and improved for averaged VSC modeling. These innovations include: 1) ac system equivalent impedance refactorization after faults; 2) amended interaction protocols for improved Thevenin equivalent source updating inside the EMT-type simulation; and 3) a special new interaction protocol for improved phasor determination during faults. The improvements introduced in this contribution lead to more accurate ac/VSC-HVDC transient stability assessment compared to conventional interfacing techniques.

Small quantum absorption refrigerator in the transient regime: Time scales, enhanced cooling, and entanglement.

Abstract: A small quantum absorption refrigerator, consisting of three qubits, is discussed in the transient regime. We discuss time scales for coherent dynamics, damping, and approach to the steady state, and we study cooling and entanglement. We observe that cooling can be enhanced in the transient regime, in the sense that lower temperatures can be achieved compared to the steady-state regime. This is a consequence of coherent dynamics but can occur even when this dynamics is strongly damped by the dissipative thermal environment, and we note that precise control over couplings or timing is not needed to achieve enhanced cooling. We also show that the amount of entanglement present in the refrigerator can be much larger in the transient regime compared to the steady state. These results are of relevance to future implementations of quantum thermal machines.

Real-Time Testing of a Fuzzy-Logic-Controller-Based Grid-Connected Photovoltaic Inverter System

Abstract: This paper presents a novel fuzzy-logic-based high-performance control of a three-phase photovoltaic grid-connected inverter. With the aid of the inverter model and fuzzy-logic-based voltage and current-control schemes, a digital signal processor controller board DS1104 generates the sinusoidal pulsewidth modulated signals for the inverter operation in both stand-alone and grid-connected modes. An inverter prototype was built to verify the effectiveness of the control algorithm. The system demonstrates stable ac output voltage satisfactorily during both transient and steady state with grid and load disturbances. The control system generates 2.48% and 4.64% voltage and current total harmonic distortions, respectively. The output waveforms such as output voltage, injected current, and the system power flow are presented to validate the effectiveness of the control strategy.

Transient Stability Enhancement of Doubly Fed Induction Machine-Based Wind Generator by Bridge-Type Fault Current Limiter

Abstract: Transient stability is a major concern for doubly fed induction machine (DFIM). A DFIM-based wind generator is readily affected by faults at the grid side as its stator windings are interfaced to grid. However, the wind generators need to remain connected and continue operation during faults at the grid side according to the grid code requirements. Therefore, it is important to enhance the transient stability of the DFIM-based wind generators. To achieve enhanced transient stability of the DFIM, a bridge-type fault current limiter (BFCL) is proposed in this study. Symmetrical as well as unsymmetrical faults were applied to the test system to check the efficacy of the BFCL in transient stability enhancement. Simulations were carried out in Matlab/Simulink environment. To demonstrate the effectiveness of the proposed BFCL, its performance is compared with that of the series dynamic braking resistor (SDBR). Simulation results show that the BFCL is a very effective device to attain better stabilization of the DFIM and outperforms the SDBR in all aspects.

A New Reactive Current Reference Algorithm for the STATCOM System Based on Cascaded Multilevel Inverters

Abstract: This paper presents a simple controller integrating a new reactive current reference algorithm for enhancing the transient performance of static synchronous compensator (STATCOM). A multilevel cascaded inverter with separated dc capacitors which is driven by carrier-based pulse width modulation is used to implement the STATCOM. The voltage across each dc-link capacitor is regulated by the rotated switching swapping scheme. In this paper, the STATCOM is controlled to provide both reactive power (VAR) compensation and grid power factor correction at the point of common coupling with a dynamically varying reactive load system. The proposed algorithm enhances the transient performance of the closed-loop system with only proportional controller and minimizes the STATCOM reactive current ripples. STATCOM based on a five-level cascaded inverter is presented in this paper and the performance of the proposed controller is investigated through various simulation studies using MATLAB/Simulink software for both steady state and transient conditions. A laboratory prototype is also developed to verify the simulation results where a good match between simulation and experimental results is achieved.

Three-Phase Fault Detection During Power Swing by Transient Monitor

Abstract: Distance relays are immune to inadvertent operation during power swings by a block known as “power swing blocking (PSB)”. Its main function is discriminating faults from power swings. However, if a fault occurs during a power swing, PSB should be de-blocked and let the distance relay to operate normally and clear the fault accordingly. Meanwhile, discrimination of a three-phase fault from a power swing is the most difficult task due to their likelihood. This paper proposes a new method to overcome this issue by monitoring the transient period established in the process of current dynamic phasor estimation. In this regard, transient monitor (TM) index, which is the difference between the estimated current samples regenerated from the calculated dynamic phasors and the actual sample values, is used to distinguish the fault from the power swing. Fourier-Taylor transformation is applied to estimate the dynamic phasor of the current signal. Simulation results verify the inherent potential of the proposed method to overcome the issue.

Transient modeling of a flash tank vapor injection heat pump system – Part I: Model development

Abstract: This two-part article explores the dynamic behavior of a flash tank vapor injection heat pump system from a numerical simulation perspective. Part I provides a first-principles model describing the transient heat transfer and flow phenomena of the system with detailed modeling techniques for each component. The vapor injection scroll compressor is analyzed with the internal heat transfer between the refrigerant and metallic parts taken into account. Lumped-parameter models are developed for the flash tank and expansion devices. Heat exchangers are modeled using a finite volume approach and accounting for the complex tube circuitry. The separated flow model without interfacial exchange is utilized for two-phase flows in order to incorporate an appropriate void fraction model so that a more accurate prediction for refrigerant mass distribution can be achieved. The modular nature of the component models allows flexibility in the system configuration. Transient simulations are carried out for start-up and shut-down operations. A detailed comparison of model predictions against experimental data is presented in the companion paper.

Coupled Electromagnetic–Thermal Analysis of Electric Machines Including Transient Operation Based on Finite-Element Techniques

Abstract: Since the mass and volume of electric machines are heavily dependent on their thermal constraints, it is important to find ways to analyze and simultaneously optimize their electromagnetic (EM) and thermal performances. This paper presents an approach for coupling the finite-element EM and thermal analyses of electrical machines using temperature-dependent material properties, so that temperatures inside a candidate machine can be predicted simultaneously with its EM performance. In addition to steady-state conditions, the coupled analysis has been extended in this paper to transient operation for machines that are required to deliver high torque/power for short intervals. Three 30-kW 10-pole 12-slot surface permanent magnet machines optimized for maximum torque density, minimum cost, and maximum efficiency, respectively, have been investigated. This coupled EM–thermal analysis makes it easier for designers to maximize the winding current density to achieve the highest possible torque/power ratings within thermal limits set by the winding insulation or demagnetization limits.

A Parallel Capacitor Control Strategy for Enhanced FRT Capability of DFIG

Abstract: This paper presents a novel dc-link scheme for enhancing the fault ride-through (FRT) capability of doubly fed induction generator-based wind turbine (DFIG-WT). The proposed system consists of parallel capacitors with a dedicated control strategy designed to provide means for power evacuation during grid fault conditions. This technically simple and cost-effective scheme was developed considering transmission line autoreclosing which may cause multiple fault inceptions. Simulation studies were carried out to compare the performance of the introduced solution with a DFIG-WT, equipped with the dc chopper and crowbar. The simulation results demonstrate the enhanced performance of the proposed approach in maintaining the dc-link voltage, transient rotor voltages, and currents within the permissible operating range during a bolted three-phase-to-ground fault. The proposed schemes were also tested in response to asymmetrical grid faults, and the enhancement in transient response has been verified. An experimental setup was developed to emulate the behavior of the dc-link circuit during fault conditions. These experimental results demonstrate the effectiveness of the switching parallel capacitors in preventing dc-link overvoltage during imbalance power operation. The discharging capacitor circuit highlighted the capability of tackling the multiple fault inception problems while adhering to grid code requirements.

Transient Reconfiguration and Coordinated Control for Power Converters to Enhance the LVRT of a DFIG Wind Turbine With an Energy Storage Device

Abstract: This paper proposes a novel transient reconfiguration solution and coordinating control strategy for power converters to enhance the fault ride through and transient voltage support capabilities of a doubly-fed induction generator with an energy storage device (DFIG-ESD). During a grid fault, the connection of the grid-side converter is reconfigured such that it is connected to the rotor circuit in parallel with the rotor-side converter to provide an additional route for the rotor current, while the ESD is responsible for dc-link voltage regulation. A coordinated demagnetizing and reactive current control strategy is designed for the reconfigured DFIG during transient conditions. Specifically, the demagnetizing current is used to counteract the dc and negative-sequence stator flux components so that the transient electromotive force will be reduced. Simultaneously, the reactive current is added to meet the reactive power support requirement. The enhanced low-voltage ride through (LVRT) and transient voltage support capabilities obtained from the proposed design are demonstrated on the DFIG-ESD wind conversion system under different severe fault scenarios (asymmetrical and symmetrical fault). Additionally, The enhanced transient voltage support capability of the proposed design is further demonstrated by comparing with different control strategies.

Switched Model Predictive Control for Improved Transient and Steady-State Performance

Abstract: This work presents a novel switched model predictive control (MPC) formulation for power converters During transients, the proposed method uses horizon-one nonlinear finite control set (FCS) MPC to drive the system toward the desired reference When the converter state is close to the reference, the controller switches to linear operation using an approximate converter model and a pulse-width modulation modulator As an illustrative example, the proposed switched MPC is applied to a flying capacitor converter As evidenced by experimental results, the proposed control strategy provides quick disturbance compensation, while giving excellent steady-state performance

An Adaptive Voltage-Sensor-Based MPPT for Photovoltaic Systems With SEPIC Converter Including Steady-State and Drift Analysis

Abstract: An adaptive voltage-sensor-based maximum power point tracking algorithm employing a variable scaling factor for a single-ended primary-inductance converter is presented. In this method, only a voltage divider circuit is used to sense the photovoltaic (PV) panel voltage. This method can effectively improve both transient and steady-state performance by varying the scaling factor as compared with the fixed step size and adaptive step size with fixed scaling factor. For sudden change in solar insolation or in start-up, this method leads to faster tracking, whereas in steady state, it leads to lower oscillations around maximum power point. The steady-state behavior and drift phenomena are also addressed in this paper to determine the tracking efficiency. The duty cycle is generated directly without using any proportional–integral control loop to simplify the control circuit. MATLAB/Simulink is used for simulation studies, and a microcontroller is used as a digital platform to implement the proposed algorithm for experimental validation. The proposed system is implemented and tested successfully on a PV panel in the laboratory.

Improved Ride-Through Control of DFIG During Grid Voltage Swell

Abstract: Grid voltage swell causes a transient dc flux component on doubly fed induction generator (DFIG) stator winding even stronger than grid voltage dip, resulting in a much more serious stator, rotor current, and torque oscillation. This paper analyzes the dynamic behavior of DFIG during grid voltage swell. Based on the analysis results, the virtual resistance control strategy manages best to suppress the rotor current and torque oscillation but prolongs the transient duration, resulting in a higher rotor voltage. Thus, this paper proposed a virtual impedance control strategy to enhance the high-voltage ride-through capability of DFIG. In order to improve the dynamic performance, the optimization algorithm of virtual impedance is proposed in the paper. The effectiveness of the proposed control strategy was verified by simulation and experimental results.

Protection scheme for high-voltage direct-current transmission lines based on transient AC current

Abstract: This study presents a new protection scheme for high-voltage direct-current (HVDC) transmission lines which only uses a specific frequency AC current (SFAC) at one of the line terminals during fault transients. The fluctuation characteristic of the root mean square (RMS) of SFAC is analysed under different fault conditions. The fluctuation in the RMS of SFAC during the transient period of internal and external faults is quite different. A fluctuation coefficient (FC) is proposed to measure the fluctuation. Fault types and the faulted line can be distinguished by calculating FC. A HVDC test system modelled in PSCAD/EMTDC is used to validate the proposed scheme considering different fault types, operation modes, sampling frequency and interference. Comprehensive test studies show that the performance of the proposed protection scheme is inspiring, and it is simpler and is of higher reliability compared with some traditional protections based on harmonic currents.