Showing papers on "Voltage droop published in 2012"

Power generator having a power selector and organic light emitting display device using the same

Abstract: A power generator includes a booster that boosts an input voltage supplied from a power supply unit and that supplies a boosted input voltage to an output terminal, a selector that selects one of the input voltage and a voltage at the output terminal as a selected voltage and supplies the selected voltage as an output voltage, a reference voltage generator that generates a reference voltage based on the output voltage, a comparator that compares a feedback voltage supplied from the booster and the reference voltage with each other, and a controller that controls the booster to output a chosen voltage from the output terminal according to a comparison result of the comparator.

Secondary Control Scheme for Voltage Unbalance Compensation in an Islanded Droop-Controlled Microgrid

Abstract: The concept of microgrid hierarchical control is presented recently. In this paper, a hierarchical scheme is proposed which includes primary and secondary control levels. The primary level comprises distributed generators (DGs) local controllers. The local controllers mainly consist of power, voltage and current controllers, and virtual impedance control loop. The central secondary controller is designed to manage the compensation of voltage unbalance at the point of common coupling (PCC) in an islanded microgrid. Unbalance compensation is achieved by sending proper control signals to the DGs local controllers. The design procedure of the control system is discussed in detail and the simulation results are presented. The results show the effectiveness of the proposed control structure in compensating the voltage unbalance.

An Enhanced Microgrid Load Demand Sharing Strategy

Abstract: For the operation of autonomous microgrids, an important task is to share the load demand using multiple distributed generation (DG) units. In order to realize satisfied power sharing without the communication between DG units, the voltage droop control and its different variations have been reported in the literature. However, in a low-voltage microgrid, due to the effects of nontrivial feeder impedance, the conventional droop control is subject to the real and reactive power coupling and steady-state reactive power sharing errors. Furthermore, complex microgrid configurations (looped or mesh networks) often make the reactive power sharing more challenging. To improve the reactive power sharing accuracy, this paper proposes an enhanced control strategy that estimates the reactive power control error through injecting small real power disturbances, which is activated by the low-bandwidth synchronization signals from the central controller. At the same time, a slow integration term for reactive power sharing error elimination is added to the conventional reactive power droop control. The proposed compensation method achieves accurate reactive power sharing at the steady state, just like the performance of real power sharing through frequency droop control. Simulation and experimental results validate the feasibility of the proposed method.

Frequency Control in Autonomous Power Systems With High Wind Power Penetration

Abstract: This paper presents an investigation on wind turbine (WT) contribution to the frequency control of noninterconnected island systems The capability of WTs to participate in the primary frequency control and offer primary reserve is discussed The investigation includes both transient frequency support (inertial response) and permanent frequency response (droop characteristic), as well as the combined application of these concepts A quantitative analysis is presented for the expected benefits and drawbacks of each method, including the appropriate selection of their parameters The power system of Rhodes Island has been selected as a study case, which includes different types of conventional generation and the three basic WT types, based on Active-Stall Induction Generator (ASIG), Doubly Fed Induction Generator (DFIG), and Permanent Magnet Synchronous Generator (PMSG)

Impact of DC Line Voltage Drops on Power Flow of MTDC Using Droop Control

Abstract: This paper discusses the impact of dc transmission voltage drops on the distribution of dc grid balancing power when dc voltage droop control is applied. DC line voltage drops in a multiterminal VSC-HVDC (MTDC) system result in nonuniform variations of dc bus voltages when changes in dc grid power flow occur. This in turn affects the distribution of instantaneous balancing power in a MTDC that uses dc voltage droop control. The values of dc voltage droop constants determine the degree of impact that dc voltage drops will have on the sharing of balancing power in the dc grid. In this paper, an analytical expression for estimating the distribution of balancing power which accounts for dc line voltage drops is derived. A five-terminal MTDC was modelled in PSCAD for demonstrating the effects of dc line voltage drops as well as for validating the proposed analytical expression which estimates balancing power distribution.

Autonomous DC Voltage Control of a DC Microgrid With Multiple Slack Terminals

Abstract: Autonomous DC voltage control for a DC microgrid with multiple power and slack terminals is studied in this paper. Slack terminals respond to the generation variation and load step within a DC microgrid to maintain the DC voltage. The slack terminals considered here are grid connected VSC and energy storage systems. A voltage droop based power sharing and coordination strategy among the slack terminals is proposed for power smoothing during grid-connected condition and normal operation during islanding condition. A prototype microgrid with two power and two slack terminals is established to demonstrate the excellent operation performance of the proposed control system during various operating conditions such as power variation, islanding, and grid reconnection.

Size-dependent efficiency and efficiency droop of blue InGaN micro-light emitting diodes

Abstract: The mechanisms of size-dependent efficiency and efficiency droop of blue InGaN micro-pixel light emitting diodes (μLEDs) have been investigated experimentally and by simulation. Electrical characterisation confirms the improvement of current spreading for smaller μLEDs, which enables the achievement of the higher efficiency at high injection current densities. Owing to the higher ratio of sidewall perimeter to mesa area of smaller μLEDs, a lower efficiency was observed at a low injection current density, resulting from defect-related Shockley-Read-Hall non-radiative recombination. We demonstrate that such sidewall etch defects can be partially recovered by increased thermal annealing time, consequently improving the efficiency at low current densities.

Analogy Between Conventional Grid Control and Islanded Microgrid Control Based on a Global DC-Link Voltage Droop

Abstract: For islanded microgrids, droop-based control methods are often used to achieve a reliable energy supply. However, in case of resistive microgrids, these control strategies can be rather different to what conventional grid control is accustomed to. Therefore, in this paper, the theoretical analogy between conventional grid control by means of synchronous generators (SGs) and the control of converter-interfaced distributed generation (CIDG) units in microgrids is studied. The conventional grid control is based on the frequency as a global parameter showing differences between mechanical power and ac power. The SGs act on changes of frequency through their P/f droop controller, without interunit communication. For CIDG units, a difference between dc-side power and ac-side power is visible in the dc-link voltage of each unit. Opposed to grid frequency, this is not a global parameter. Thus, in order to make a theoretical analogy, a global measure of the dc-link voltages is required. A control strategy based on this global voltage is presented and the analogy with the conventional grid control is studied, with the emphasis on the need for interunit communication to achieve this analogy. A known control strategy in resistive microgrids, called the voltage-based droop control for CIDG units, approximates this analogy closely, but avoids interunit communication. Therefore, this control strategy is straightforward for implementation since it is close to what control engineers are used to. Also, it has some specific advantages for the integration of renewables in the network.

Active Power Management of Multihybrid Fuel Cell/Supercapacitor Power Conversion System in a Medium Voltage Microgrid

Abstract: This paper proposes a hierarchical active power management strategy for a medium voltage (MV) islanded microgrid including a multihybrid power conversion system (MHPCS). To guarantee excellent power management, a modular power conversion system is realized by parallel connection of small MHPCS units. The hybrid system includes fuel cells (FC) as main and supercapacitors (SC) as complementary power sources. The SC energy storage compensates the slow transient response of the FC stack and supports the FC to meet the grid power demand. The proposed control strategy of the MHPCS comprises three control loops; dc-link voltage controller, power management controller, and load current sharing controller. Each distributed generation (DG) unit uses an adaptive proportional resonance (PR) controller for regulating the load voltage, and a droop control strategy for average power sharing among the DG units. The performance of the proposed control strategy is verified by using digital time-domain simulation studies in the PSCAD/EMTDC software environment.

Decentralized Cooperative Control Strategy of Microsources for Stabilizing Autonomous VSC-Based Microgrids

Abstract: In designing procedure of a power sharing controller for a voltage source converter (VSC)-based microgrid with no communication link, three issues should be considered. Firstly, in VSC-based microgrids, which use droop controller method, the desired frequency of VSCs is altering regarding the output active power. Consequently, the conventional load frequency control techniques are inappropriate since their operation is based on a fixed pre-specified desired frequency. Secondly, to prevent circulating current and thermally overstressing, all DGs should participate in active power supply. In addition, since there is no communication link, the steady state value of each micro-source active power is unknown. Therefore, the conventional fixed active power control method for DGs is not appropriate. Thirdly, when the microgrid loads are increased, the output power of VSCs is increased rapidly; however, the output power of each VSC's primary source could not change in the same rate to respond. It causes the DC voltage of VSCs to decrease, which could affect the appropriate performance of VSCs. In this paper, a novel control strategy for VSCs and an energy storage system in a VSC-based microgrid without communication link accompanied with a novel hybrid model of VSC-based DGs, which considers primary source effect, is proposed.

Analytic model for the efficiency droop in semiconductors with asymmetric carrier-transport properties based on drift-induced reduction of injection efficiency

Abstract: An analytic model is developed for the droop in the efficiency-versus-current curve for light-emitting diodes (LEDs) made from semiconductors having strong asymmetry in carrier concentration and mobility. For pn-junction diodes made of such semiconductors, the high-injection condition is generalized to include mobilities. Under high-injection conditions, electron drift in the p-type layer causes a reduction in injection efficiency. The drift-induced leakage term is shown to have a 3rd and 4th power dependence on the carrier concentration in the active region; the values of the 3rd- and 4th-order coefficients are derived. The model is suited to explain experimental efficiency-versus-current curves of LEDs. V C 2012 American Institute of Physics .[ http://dx.doi.org/10.1063/1.4704366]

Suppression of electron overflow and efficiency droop in N-polar GaN green light emitting diodes

Abstract: In this letter, we experimentally demonstrate direct correlation between efficiency droop and carrier overflow in InGaN/GaN green light emitting diodes (LEDs). Further, we demonstrate flat external quantum efficiency curve up to 400 A/cm2 in a plasma assisted molecular beam epitaxy grown N-polar double quantum well LED without electron blocking layers. This is achieved by exploring the superior properties of reverse polarization field of N-face polarity, such as effective carrier injection and higher potential barriers against carrier overflow mechanism. The LEDs were found to operate with a low (∼2.3 V) turn-on voltage.

Autonomous control of inverter-interfaced Distributed Generation units for harmonic current filtering and resonance damping in an islanded microgrid

Abstract: Harmonic current filtering and resonance damping have become important concerns on the control of an islanded microgrids. To address these challenges, this paper proposes a control method of inverter-interfaced Distributed Generation (DG) units, which can autonomously share harmonic currents and resonance damping burdens. The approach employs a load compensator based on the decomposition of output current, in addition to the outer droop-based power controller, as well as inner voltage and current controllers. The load compensator consists of a virtual fundamental impedance loop for enhanced sharing of reactive power, and a variable harmonic impedance loop which allows to counteract harmonic voltage drops across the grid-side inductance of the DG inverter, and to damp the harmonic resonance propagation throughout a distribution feeder. Experiments on a three-phase microgrid are performed to validate the performance of the proposed control scheme.

A New Control Strategy for a Multi-Bus MV Microgrid Under Unbalanced Conditions

Abstract: This paper proposes a new control strategy for the islanded operation of a multi-bus medium voltage (MV) microgrid. The microgrid consists of several dispatchable electronically-coupled distributed generation (DG) units. Each DG unit supplies a local load which can be unbalanced due to the inclusion of single-phase loads. The proposed control strategy of each DG comprises a proportional resonance (PR) controller with an adjustable resonance frequency, a droop control strategy, and a negative-sequence impedance controller (NSIC). The PR and droop controllers are, respectively, used to regulate the load voltage and share the average power components among the DG units. The NSIC is used to effectively compensate the negative-sequence currents of the unbalanced loads and to improve the performance of the overall microgrid system. Moreover, the NSIC minimizes the negative-sequence currents in the MV lines and thus, improving the power quality of the microgrid. The performance of the proposed control strategy is verified by using digital time-domain simulation studies in the PSCAD/EMTDC software environment.

Automatic Power-Sharing Modification of $P$ / $V$ Droop Controllers in Low-Voltage Resistive Microgrids

Abstract: Microgrids are receiving an increasing interest to integrate the growing share of distributed-generation (DG) units in the electrical network. For the islanded operation of the microgrid, several control strategies for the primary control have been developed to ensure stable microgrid operation. In low-voltage (LV) microgrids, active power/voltage (P/V) droop controllers are gaining attention as they take the resistive nature of the network lines and the lack of directly coupled rotating inertia into account. However, a problem often cited with these droop controllers is that the grid voltage is not a global parameter. This can influence the power sharing between different units. In this paper, it is investigated whether this is actually a disadvantage of the control strategy. It is shown that with P/V droop control, the DG units that are located electrically far from the load centers automatically deliver a lower share of the power. This automatic power-sharing modification can lead to decreased line losses; therefore, there is overall better efficiency compared to the methods that focus on perfect power sharing. In this paper, the P/V and P/f droop control strategies are compared with respect to this power-sharing modification and the line losses.

Study on the Current Spreading Effect and Light Extraction Enhancement of Vertical GaN/InGaN LEDs

Abstract: This study analyzes the current spreading effect and light extraction efficiency (LEE) of lateral and vertical light-emitting diodes (LEDs). Specifically, this study uses a fully 2-D model that solves drift-diffusion and Poisson equations to investigate current flow paths and radiative recombination regions. The ray-tracing technique was used to calculate the LEE of the top surface. First, this study discusses the current spreading effect of the lateral and conventional vertical LED and determines the efficiency droop even with a transparent conducting layer. Different electrode configurations in the vertical LED were tested to optimize the current spreading effect, which, in turn, suppresses the carrier leakage and mitigates the efficiency droop under high injection conditions. This study also discusses the wall-plug efficiency in overall cases to identify the design rules for higher power conversion efficiency.

Voltage Control Technique for the Extension of DC-Link Voltage Utilization of Finite-Speed SPMSM Drives

Abstract: This paper presents a new voltage control technique which extends dc-link voltage utilization for finite-speed surface-mounted permanent magnet synchronous motor drives. Therefore, significantly increasing the output power and torque under flux-weakening operation can be achieved. The switching period and summation of active switching times for inverter pulsewidth modulation control are used to modify the -axis current reference under the flux-weakening region such that the voltage trajectory of inverter output can be extended to move to the hexagon of the space vector modulation, thereby extending the utilization of dc-link voltage. Moreover, it will be shown that the proposed voltage control technique has some additional special features, such as not sensitive to motor parameter, no need of voltage and current feedback, and no need of lookup table. The presented method is realized by software and does not require additional hardware. Comparisons of experimental results will be presented for confirming the presented technique.

The consequences of high injected carrier densities on carrier localization and efficiency droop in InGaN/GaN quantum well structures

Abstract: There is a great deal of interest in the underlying causes of efficiency droop in InGaN/GaN quantum welllight emitting diodes, with several physical mechanisms being put forward to explain the phenomenon. In this paper we report on the observation of a reduction in the localization induced S-shape temperature dependence of the peak photoluminescence energy with increasing excitation power density. This S-shape dependence is a key fingerprint of carrier localization. Over the range of excitation power density where the depth of the S shape is reduced, we also observe a reduction in the integrated photoluminescence intensity per unit excitation power, i.e., efficiency droop. Hence, the onset of efficiency droop occurs at the same carrier density as the onset of carrier delocalization. We correlate these experimental results with the predictions of a theoretical model of the effects of carrier localization due to local variations in the concentration of the randomly distributed In atoms on the optical properties of InGaN/GaN quantum wells. On the basis of this comparison of theory with experiment we attribute the reduction in the S-shape temperature dependence to the saturation of the available localized states. We propose that this saturation of the localized states is a contributory factor to efficiency droop whereby nonlocalized carriers recombine non-radiatively.

Temperature dependent efficiency droop in GaInN light-emitting diodes with different current densities

Abstract: The effect of chip area on the temperature-dependent light-output power (LOP) in GaInN-based light-emitting diodes (LEDs) is investigated. The larger the chip size, the faster the reduction in LOP with increasing temperature becomes, indicating that increasing the size of LED chips, a technology trend for reducing the efficiency droop at high currents, is detrimental for high temperature-tolerant LEDs. In addition, it is found that regardless of chip size, the temperature-dependent LOP is identical for the LEDs operating at the same current density.

Analysis of efficiency droop in nitride light-emitting diodes by the reduced effective volume of InGaN active material

Abstract: In InGaN quantum wells (QWs), effective active volume can be greatly reduced due to carrier localization in In-rich region and inhomogeneous carrier distribution. The authors investigate the efficiency droop of InGaN-based light-emitting diodes (LEDs) based on the carrier rate equation including the influence of the reduced effective active volume. It is found that efficiency droop characteristics can be modeled well without employing a large Auger recombination coefficient by assuming that only a small portion of the QWs is effectively used as active region. The presented model is expected to provide insight into the realization of droop-free operation in nitride LEDs.

Study of droop phenomena in InGaN-based blue and green light-emitting diodes by temperature-dependent electroluminescence

Abstract: InGaN-based blue and green light-emitting diodes are studied by temperature-dependent electroluminescence (EL) from 300 to 50 K to elucidate the effects of carrier overflow and the saturation in radiative recombination rate on the efficiency droop. Severe efficiency droop at cryogenic temperatures is attributed to the carrier overflow, which is confirmed by the EL spectra. The degree of overflow is thought to be related to the reduced effective active volume and the subsequent saturation in radiative recombination rate. Carrier transport and indium clustering in the active region are discussed in relation to the reduced effective active volume.

Distributed secondary control for islanded MicroGrids - A networked control systems approach

Abstract: This paper presents a novel approach to conceive the secondary control in droop-controlled MicroGrids. The conventional approach is based on restoring the frequency and amplitude deviations produced by the local droop controllers by using a MicroGrid Central Controller. A distributed networked control system is used in order to implement a distributed secondary control thus avoiding the use of a MicroGrid Central Control. The proposed approach is not only able to restore frequency and voltage of the MicroGrid but also ensures reactive power sharing. The distributed secondary control do not relies on a central control, so that the failure of a single unit will not produce the fail down of the whole system. Experimental results are presented to show the feasibility of the distributed control. The time latency limits of the communication systems are studied as well.

Modified droop controller for paralleling of dc-dc converters in standalone dc system

Abstract: Dc systems are gaining popularity because of its high efficiency, high reliability and easy interconnection of renewable sources as compared to ac systems. In standalone dc system, parallel dc-dc converters are used to interconnect storage element and loads. Use of master-slave controller for paralleling is limited because of its high cost, low reliability and complexity. Although conventional droop controllers overcome these limitations, they cannot simultaneously ensure equal current sharing (in per unit) and low-voltage regulation. This is due to the error in measurement of voltage feedback signal. To address this limitation, modified droop controller is proposed in this study. Circulating current between converters is used to modify nominal voltages such that error between them is reduced. This improves current sharing among converters. The advantage of the proposed method is that, equal current sharing is achieved along with low-voltage regulation. The effectiveness of the proposed scheme is verified through detailed simulation study. To confirm the viability of the scheme, experimental studies are carried out on a scaled-down laboratory prototype developed for the purpose and results are included in this study.

A State Equation Model of a Single-Phase Grid-Connected Inverter Using a Droop Control Scheme With Extra Phase Shift Control Action

Abstract: This paper presents a state equation model of a single-phase pulsewidth modulation inverter connected to the grid, using frequency-active power and voltage-reactive power droops, including an extra phase shift loop. The influence of the control parameters on the system's behavior can be studied using the proposed state equation model, which was obtained from the small-signal analysis. The model's results are compared with numerical simulations of the nonlinear system to verify their accuracy. Comparisons are also made with experimental data reported in the literature.

Multilayer Control for Inverters in Parallel Operation Without Intercommunications

Abstract: In this paper, a multilayer control is proposed for inverters that are able to operate in parallel without intercommunications. The first control layer is an improved droop method that introduces power proportional terms into the conventional droop scheme, letting both active and reactive power to be shared among the inverters. The second layer is designed to compensate the voltage deviations caused by the aforementioned droop control, thus improving the load-voltage regulation of the system. The third layer is a quasi-synchronization control that roughly adjusts the angle of the inverter to be close to the common ac bus. This layer ensures that the phase difference of each inverter is inside a limited margin with the help of the phase signal sensed from the common ac bus. The principle of operation of the control scheme has been analyzed in detail. A small-signal model has been developed in order to study the system dynamics, which can be used for adjusting the main control parameters. A prototype consisting of a two 35-kVA-inverter system has been built and tested in order to verify the feasibility of the proposed approach.

Controllable Harmonic Current Sharing in Islanded Microgrids: DG Units With Programmable Resistive Behavior Toward Harmonics

Abstract: In order to obtain a coordinated approach to integrate the increasing presence of distributed generation (DG) units in the electric power system, the microgrid concept has been introduced. Since most DG units use a converter as an interface with the grid, new control strategies for these converters are being developed. For the islanded mode of the microgrid, with the voltage-based droop control strategy, active and reactive power balancing and sharing between multiple DG units are achieved. This method also makes it easy to delay changing the output power of the renewable DG units compared to that of the dispatchable DG units without communication. However, to deal with harmonic and nonlinear loads, the power control strategies in general, with the voltage-based control strategy as a specific example, need to be modified. Otherwise, the grid-forming DG units form short circuits for harmonic currents. Therefore, in this paper, the shunt harmonic impedance method for harmonic damping in the grid-connected mode is adapted for application in islanded microgrids. The voltage-based droop control strategy of the DG units is extended with programmable resistive behavior toward harmonics. In this way, harmonic current sharing between DG units can be achieved in a controllable manner (e.g., according to the ratings of the units).

SoC-based droop method for distributed energy storage in DC microgrid applications

Abstract: With the progress of distributed generation nowadays, microgrid is employed to integrate different renewable energy sources into a certain area. For several kinds of renewable sources have DC outputs, DC microgrid has drawn more attention recently. Meanwhile, to deal with the uncertainty in the output of microgrid system, distributed energy storage is usually adopted. Considering that the state-of-charge (SoC) of each battery may not be the same, decentralized droop control method based on SoC is shown in this paper to reach proportional load power sharing. With this method, the battery with higher SoC supplies more load power, while the one with lower SoC supplies less load power. Different kinds of relationship between SoC and droop coefficient are selected and their performance of power sharing speed is evaluated. Small signal model of SoC-based droop method is reached to test the system stability. Theoretical analysis is validated by both simulation and experimental results.