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POWER SYSTEM 2016

Category Archives

Distributed Noise-resilient Networked Synchrony of Active Distribution Systems

Abstract

This paper proposes a distributed noise resilient control technique for voltage and frequency synchronization in inverter-based AC microgrids. Existing cooperative control techniques assume ideal communication among inverters. The effect of additive noise in communication links among inverters, and between the reference signal and inverters, on the synchronization process, is studied. Distributed least mean-square solutions estimate the reference set points, and a local least meansquare algorithm estimates neighboring inverter frequencies and voltages. The efficacy of the proposed solution, for an islanded microgrid test system under the additive noise in reference communication links and links connecting neighboring inverters, is evaluated for a modified IEEE 34-bus feeder system. An upper bound for the noise-induced deviation in the consensus parameter is analytically derived and verified by the simulated testbed.


Offset-free Direct Power Control of DFIG Under Continuous-Time Model Predictive Control

Abstract

This paper presents a robust continuoustime model predictive direct power control for doubly fed induction generator (DFIG). The proposed approach uses Taylor series expansion to predict the stator current in the synchronous reference frame over a finite time horizon. The predicted stator current is directly used to compute the required rotor voltage in order to minimize the difference between the actual stator currents and their references over the predictive time. However, as the proposed strategy is sensitive to parameter variations and external disturbances, a disturbance observer is embedded into the control loop to remove the steady-state error of the stator current. It turns out that the steady-state and the transient performances can be identified by simple design parameters. In this work, the reference of the stator current is directly calculated from the desired stator active and reactive powerswithout encompassing the parameters of the machine itself. Hence, no extra power control loop is required in the control structure to ensure smooth operation of the DFIG. The feasibility of the proposed strategy is verified by experimental results of grid-connected DFIG and satisfactory performances are obtained.


Control of DFIG Wind Power Generators in Unbalanced Microgrids Based on Instantaneous Power Theory

Abstract

This paper presents a model and a control strategy for a doubly-fed induction generator (DFIG) windenergy system in an unbalanced microgrid based on instantaneous power theory. The proposed model uses instantaneous real/reactive power components as the system state variables. In addition to thecontrol of real/reactive powers, the controllers use the rotor-side converter for mitigating the torque and reactive power pulsations. The control scheme also uses the grid-side converter for partial compensation of unbalanced stator voltage. The main features of the proposed control method are its feedback variables are independent of reference frame transformations and it does not require sequential decomposition of current components. These features simplify the structure of required controllers under an unbalanced voltage condition and inherently improve the robustness of the controllers. A power limiting algorithm is also introduced to protect power converters against over rating and define the priority of real/reactive power references within the control scheme. The performance of the proposed strategy in reducing torque ripples and unbalanced stator voltage is investigated based on the time-domain simulation of a DFIG study system under unbalanced grid voltage.


Active Power Control Strategies for Transient Stability Enhancement of AC/DC Grids With VSC-HVDC Multi-Terminal Systems

Abstract

Multiterminal High Voltage Direct Current (HVDC) using Voltage Source Converters (VSCHVDC) is a promising technology which provides flexible control of active and reactive power and facilitates remote renewable energy integration, above all using long cables. This paper analyses an active power controlstrategy for multiterminal VSCHVDC systems tailored to enhance transient stability of hybrid AC/DCgrids. The proposed strategy controls each VSC using frequency measurements of all terminals. Its performance is compared to a strategy in which each VSC is controlled using only local frequency measurements of the AC side, proving that the proposed strategy shows better performance, even taking into account reasonable communication delays. The paper also shows that the proposedstrategy generally gives similar results to those obtained when each VSC is controlled using the speed of the centre of inertia (COI). The speed of the COI is a more comprehensive and richer figure than the one proposed in this paper but it is also much more complex to obtain. Simulation results with PSS/E of a test system have been used to illustrate the comparisons and the main contributions of the proposal.


High Frequency Resonance Damping of DFIG based wind power system under weak network

Abstract

When operating in a micro or weak grid which has a relatively large network impedance, the Doubly Fed Induction Generator (DFIG) based wind power generation system is prone to suffer high frequencyresonance due to the impedance interaction between DFIG system and the parallel compensatednetwork (series RL + shunt C). In order to improve the performance of the DFIG system as well as other units and loads connected to the weak grid, the high frequency resonance needs to be effectivelydamped. In this paper, the proposed active damping control strategy is able to implement effectivedamping either in the Rotor Side Converter (RSC) or in the Grid Side Converter (GSC), through the introduction of virtual positive capacitor or virtual negative inductor to reshape the DFIG systemimpedance and mitigate the high frequency resonance. A detailed theoretical explanation on the virtual positive capacitor or virtual negative inductor has been given, and their parameters are also optimally designed. The proposed DFIG system damping control strategy has been validated by experimental results


An Enhanced DC Voltage Droop-Control for the VSC-HVDC Grid

Abstract

This paper introduces an enhanced droop-based DC voltage control method, including dead-band, for applications to the high-voltage direct-current (HVDC) grid that utilizes the voltage-sourced converter (VSC) technology. The proposed droopcontrol structure also autonomously imposes energy balance between the HVDC grid and its host AC system. The droopcontrol method (i) divides the VSC stations into four groups, (ii) activates the droopcontrol of each group based on a pre-specified voltage margin and (iii) introduces an improved power-voltage characteristic for desirable VSC station dynamic performance. Feasibility and performance of the proposed control method are evaluated based on time-domain simulation studies in the PSCAD platform, using the IEEE-39-Bus system which imbeds a five-terminal VSCHVDC grid. Each VSC station is a monopolar modular multilevel converter (MMC). The study results show that the proposed droopcontrol method enables the HVDC-AC system to reach a new steady-state after transient events.


Analysis and Performance Enhancement of Vector-Controlled VSC in HVDC Links Connected to Very Weak Grids

Abstract

Voltage source converter (VSC)-based high-voltage direct current (HVDC) transmission systems have been employed widely in recent years. However, connecting a VSCHVDC link to a very weak grid (a high-impedance grid) is challenging. A vectorcontrolled VSC is incapable of injecting/absorbing its maximum theoretical active power in such grids. A simple yet effective control system for a standardvectorcontrolled VSC in a very weak grid condition has not been reported in the literature. This paper, benefiting from a comprehensive small-signal model, presents a detailed analysis of the VSC dynamics and shows how the assumptions made for designing VSC regulators in strong grids are no longer valid in very weak grids. The paper then proposes and compares two straightforward solutions: retuning the control parameters and using an artificial bus for converter-grid synchronization. Both methods enable the VSC to operate at the maximum theoretical active power at a very weak grid condition (i.e., at unity short-circuit ratio) by minimal modification in the widely accepted vector control method. The advantages and disadvantages of each method are discussed. The analytical results are verified by detailed nonlinear time-domain simulation results.


Impact of Switching Harmonics on Capacitor Cells Balancing in Phase-Shifted PWM Based Cascaded H-Bridge STATCOM

Abstract

The purpose of this paper is to investigate the impact of switching harmonics on the instantaneous power distribution in the cells of a cascaded HBridge based STATCOM when using PhaseShiftedPWM. The case of high and low switching frequency for the converter cells are investigated and the interaction between voltage and current harmonics is analyzed. It is shown that in both cases, this interaction results in an uneven power distribution among the cells in the same phase leg, leading to drifting of the dc-capacitor voltages and thereby the need for proper stabilization control loops. It is also shown that the selected frequency modulation ratio affects the active power distribution among thecells. In particular, the selection of a non-integer frequency modulation ratio helps in providing a more uniform power distribution among cells of the same phase leg, thus contributing to the capacitorsbalancing. A methodology for optimal selection of the frequency modulation ratio is given. Theoretical conclusions are validated through simulation and experimental results.


Review of Active and Reactive Power Sharing Strategiesin Hierarchical Controlled Micro grids

Abstract

Microgrids consist of multiple parallel-connected distributed generation (DG) units with coordinatedcontrol strategies, which are able to operate in both grid-connected and islanded mode. Microgrids are attracting more and more attention since they can alleviate the stress of main transmission systems, reduce feeder losses, and improve system power quality. When the islanded microgrids are concerned, it is important to maintain system stability and achieve load power sharing among the multiple parallel-connected DG units. However, the poor active and reactive power sharing problems due to the influence of impedance mismatch of the DG feeders and the different ratings of the DG units are inevitable when the conventional droop control scheme is adopted. Therefore, the adaptive/improved droop control, network-based control methods and cost-based droop schemes are compared and summarized in this paper for active power sharing. Moreover, nonlinear and unbalanced loads could further affect the reactive power sharing when regulating the active power, and it is difficult to share thereactive power accurately only by using the enhanced virtual impedance method. Therefore, thehierarchical control strategies are utilized as supplements of the conventional droop controls and virtual impedance methods. The improved hierarchical control approaches such as the algorithms based on graph theory, multi-agent system, the gain scheduling method and predictive control have been proposed to achieve proper reactive power sharing for islanded microgrids and eliminate the effect of the communication delays on hierarchical control. Finally, the future research trends on islanded microgrids are also discussed in this paper.


Fast Model Predictive Control for Multilevel Cascaded H-Bridge STATCOM with Polynomial Computation Time

Abstract

To solve the issue of exponentially increasing computational burden of finite control set model predictivecontrol (FCS-MPC) for multilevel converters, a fast MPC scheme for multilevel cascaded HbridgeSTATCOM is presented. The proposed approach consists of three steps. First, with the partially stratified optimization approach, the multiobjective programming of MPC is divided into two suboptimization problems, i.e., current-control MPC and voltage-balancing MPC. Second, a dynamic programming algorithm is proposed for the optimization of current-control MPC. Third, a mixed algorithm, which combines dynamic programming and greedy algorithm (0-1 programing), is proposed to select the optimal switching combination from all the redundant switching combinations for the voltage balancing MPC achieving a global optimization. Through the analysis of the time complexity, with the proposed scheme, the total computation of FCS-MPC can be reduced to polynomial time from exponential time. The proposed approaches will not deteriorate the control performance. The controlperformance is validated by simulation results and the effectiveness is further demonstrated by implementing the algorithm on a low-cost DSP (TMS320F28335) in real time.


Resolving Power Quality Issues Raised by Aerodynamic Aspects of Wind Turbine in Isolated Microgrids Using Fuel Cell/Electrolyzer System

Abstract

This paper shows how the power quality issues can be resolved in wind-diesel microgrids by means of a fuel cell/electrolyzer (FC/ELZ) system. In this regard, an autonomous hybrid power system, including a diesel generator and a fixed-speed wind turbine (FSWT) equipped with a FC/ELZ system, has been investigated. The main aim of employing the FC/ELZ system is to reduce fuel consumption and mitigate the aerodynamic effects of wind turbine (i.e., tower shadow, wind shears, yaw error, and turbulence) onpower quality in the microgrid. To conduct a comprehensive study, the detailed models of the devices are used. The aerodynamic and mechanical aspects of WT are simulated using AeroDyn and FAST, and the thermodynamic and electrochemical aspects of FC/ELZ are simulated using models validatedby experimental data. Furthermore, control of power electronic interfaces for the FC/ELZ system, including a bidirectional dc/ac voltage source converter (VSC), a dc/dc converter to boost the FC output voltage, and a dc/dc converter to reduce input voltage to the ELZ, is presented. The studied system was implemented in a MATLAB/Simulink software environment. The simulation results demonstrate the efficacy of the FC/ELZ system in reducing the flicker level and suppressing the voltage fluctuations induced by yaw error and turbulence.


Droop-Based Distributed Cooperative Control for Micro grids with Time-varying Delays

Abstract

This paper develops a droopbased distributed cooperative control scheme for microgrids under a switching communication network with non-uniform timevarying delays. We first design a pinning-based frequency/voltage controller containing a distributed voltage observer and then design a consensus-based active/reactive power controller, which are employed into the secondary controlstage to generate the nominal set points used in the primary control stage for different distributedgenerators (DGs). By this approach, the frequencies and the weighted average value of all DGs’ voltages can be pinned to the desired values while maintaining the precise active and reactive power sharing. With the proposed scheme, each DG only needs to communicate with its neighbors intermittently, even if their communication networks are local and timevarying, and their variant delaysmay be non-uniform. Sufficient conditions on the requirements for the network connectivity and thedelay upper bound that guarantee the stability and reliability of the microgrid are presented. The effectiveness of the proposed control scheme is verified by the simulation of a microgrid test system.


Harmonic Profile Injection-Based Hybrid Active Islanding Detection Technique for PV-VSC-Based Microgrid System

Abstract

In this paper, a new investigation on islanding detection method is presented for PVbased VSC system. The proposed PVVSC is represented as one of the distributed energy resource (DER) for a microgridenvironment. A new multivariable dynamic model is presented in terms of two stage conversion with a dc–dc converter for the proposed PVVSC system. The proposed VSC system is controlled primarily by a positive sequence P–Q loop, at the PCC. A new backstepping fast sliding mode control (BFSMC) is proposed for primary PWM control of VSC system. During sudden grid disconnection an islandingprotection loop based on rate of change in voltage harmonic profile injection is proposed. A binary tree (BT) threshold filter is designed for the proposed hybrid islanding protection loop, to avoid falsedetection during other grid operational issues. UL 1741 (section 46) standard is followed for the PVVSCsystem to consider the worst-case scenario. Performance of the proposed hybrid islanding protection scheme is investigated for IEEE 9 bus system with multiple DERs, implemented in MATLAB/Simulink environment. The effectiveness of the proposed scheme is determined by result analysis, where no significant effect on power quality is recorded during nominal grid operation.


Optimal Placement of Energy Storage Devices in Microgrids via Structure Preserving Energy Function

Abstract

As system transient stability is one of the most important criterions of microgrid (MG) security operation, and the performance of an MG strongly depends on the placement of its energy storage devices(ESDs); optimal placement of ESDs for improving system transient stability is required for MGs. An MGstructure preserving energy function is first developed for voltage source inverter-based MGs since the existing energy functions, based on synchronous generators and the conventional power system, are not applicable for MGs. The concept of internal potential energy of distributed energy resource is presented instead of the kinetic energy term in traditional energy function. Then, a novel approach for the optimal placement of ESDs is proposed based on MG structure preserving energy function for improving MG transient stability. Simulation and experimental results show that the proposed method can be used to find the optimal placement of ESDs and improve the system stability effectively.


Cooperative Planning of Renewable Generations for Interconnected Micro grids

Abstract

We study the renewable energy generations in Hong Kong based on realistic meteorological data, and find that different renewable sources exhibit diverse time-varying and location-dependent profiles. To efficiently explore and utilize the diverse renewable energy generations, we propose a theoretical framework for the cooperative planning of renewable generations in a system of interconnectedmicrogrids. The cooperative framework considers the self-interested behaviors of microgrids, and incorporates both their long-term investment costs and shortterm operational costs over the planninghorizon. Specifically, interconnected microgrids jointly decide where and how much to deploy renewableenergy generations, and how to split the associated investment cost. We show that the cooperativeframework minimizes the overall system cost. We also design a fair cost sharing method based on Nash bargaining to incentivize cooperative planning, such that all microgrids will benefit fromcooperative planning. Using realistic data obtained from the Hong Kong observatory, we validate thecooperative planning framework, and demonstrate that all microgrids benefit through the cooperation, and the overall system cost is reduced by 35.9% compared to the noncooperative planning benchmark.


Efficient Single Phase Transformer less Inverter for Grid-Tied PVG System With Reactive Power Control

Abstract

There has been an increasing interest in transformerless inverter for grid-tied photovoltaic (PV) system due to low cost, high efficiency, light weight, etc. Therefore, many transformerless topologies have been proposed and verified with real power injection only. Recently, almost every international regulation has imposed that a definite amount of reactive power should be handled by the grid-tied PV inverter. According to the standard VDEAR- N 4105, grid-tied PV inverter of power rating below 3.68KVA, should attain power factor (PF) from 0.95 leading to 0.95 lagging. In this paper, a new high efficiency transformerless topology is proposed for grid-tied PV system with reactive power control. The new topology structure and detail operation principle with reactive power flow is described. The high frequency commonmode (CM) model and the control of the proposed topology are analyzed. The inherent circuit structure of the proposed topology does not lead itself to the reverse recovery issues even when inject reactive power which allow utilizing MOSFET switches to boost the overall efficiency. The CM voltage is kept constant at midpoint of dc input voltage, results low leakage current. Finally, to validate the proposed topology, a 1 kW laboratory prototype is built and tested. The experimental results show that the proposed topology can inject reactive power into the utility grid without any additional current distortion and leakage current. The maximum efficiency and European efficiency of the proposed topology are measured and found to be 98.54% and 98.29%, respectively.


Adaptive DC Stabilizer with Reduced DC Fault Current for Active Distribution Power System Application

Abstract

This paper takes a systematic view on the control and protection of medium power DC networks in anactive distribution power system considering fault current limiting, system control and converter design.Reduced terminal capacitance and extra DC impedance are used to limit DC fault current and reducethe required converter current rating for medium power DC networks. An adaptive DC power stabilizeris proposed to alleviate possible system instability brought by the fault current limiting settings in the presence of constant power load. The effect of the current limiting method and the proposed stabilizeron DC fault current and stability enhancement are validated by simulation studies using a simple two-converter DC network and a multi-terminal DC network in an active distribution power system.


Synchronous Power Controller with Flexible Droop Characteristics for Renewable Power Generation Systems

Abstract

The increasing amount of renewable power gene-ration systems is a challenging issue for the control and operation of the electrical networks. One of the main issues is their lack of inertia, which is becoming a greater problem as much as the share of the power plants based on traditionalsynchronous generators gets reduced. In this regard the new grid codes ask these plants to provide new functionalities such as the frequency support and inertia emulation. In this paper, a synchronouspower controller for grid- connected converters is proposed as a good solution for the renewablegeneration systems with energy storage. It provides inertia, damping and flexible droop characteristics. Different from the faithful replication of the swing equation of synchronous machines, an alternative control structure is proposed, by which the damping and inherent droop slope can be configured independently to meet the requirements in both dynamics and frequency regulations. Analysis and experimental results are both shown to validate the proposed controller.


Combined LMS-LMF Based Control Algorithm of DSTATCOM for Power Quality Enhancement in Distribution System

Abstract

This paper deals with the combined least mean square-least mean fourth (LMSLMF)-based controlalgorithm for distribution static compensator (DSTATCOM) in three-phase distribution system to alleviate the power quality problems caused by solid-state equipments and devices. The combinedLMSLMFbased algorithm is simulated using Sim Power System (SPS) toolbox in MATLAB for obtaining the corresponding active and reactive weights and supply reference currents. The proposedcontrol algorithm has advantages of both LMF– and LMSbased control algorithms, which helps in fast and accurate response with a robust design. Depending on the value of error signal obtained in any of the phases either of LMS– or LMFbased control is used to minimize the error. The developed combinedLMSLMFbased algorithm is implemented on the prototype of the proposed system and responses obtained are found satisfactory with harmonic spectra of the supply currents meeting the power qualitystandards.


A Hybrid-STATCOM with Wide Compensation Range and Low DC-Link Voltage

Abstract

This paper proposes a hybrid static synchronous compensator (hybridSTATCOM) in a three-phase power transmission system that has a wide compensation range and low dclink voltage. Because of these prominent characteristics, the system costs can be greatly reduced. In this paper, the circuit configuration of hybridSTATCOM is introduced first. Its V-I characteristic is then analyzed, discussed, and compared with traditional STATCOM and capacitive-coupled STATCOM (C-STATCOM). The system parameter design is then proposed on the basis of consideration of the reactive powercompensation range and avoidance of the potential resonance problem. After that, a control strategy forhybridSTATCOM is proposed to allow operation under different voltage and current conditions, such as unbalanced current, voltage dip, and voltage fault. Finally, simulation and experimental results are provided to verify the wide compensation range and low dclink voltage characteristics and the good dynamic performance of the proposed hybridSTATCOM.


Experimental Investigation on a Hybrid Series Active Power Compensator to Improve Power Quality of Typical Households

Abstract

 In this paper a Transformerless Hybrid Series Active Filter (THSeAF) using a sliding mode control algorithm and a Notch harmonic detection technique are implemented on a Single-phase distribution feeder. This method provides compensation for source current harmonics coming from a voltage fed type of nonlinear load (VSC) and reactive power regulation of a residential consumer. The realized active power filter enhances the power quality while cleaning the point of common coupling (PCC) from possible voltage distortions, sags, and swells initiated through the grid. Furthermore, to overcome drawbacks of real-time control delay, a computational delay compensation method, which accurately generates reference voltages, is proposed. Based on an improved compensation strategy, while the grid current remains clean even with a small compensation gain, voltage disturbances initiated by the power system are obstructed by the compensator, and the PCC became free of voltage harmonics and protected from sag and swell. Simulation and experimental results carried on a 1.6-kVA prototype are presented and discussed.


Control and Analysis of the Modular Multilevel DC De-Icer with STATCOM Functionality

Abstract

Deicing of overhead transmission lines are necessary to prevent damage to the transmission system and outages. In this paper, a modular multilevel DC de-icer (M2D2) is introduced, based on the structure of modular multilevel converter (MMC) with full-bridge submodules. The M2D2 exhibits a lot of salient features: high modularity, scalability, compactness, high efficiency, wide DC voltage range, and no harmonic filter requirements, etc. Moreover, during good weather, the M2D2 can be configured as a static synchronous compensator (STATCOM), providing reactive power support to the grid and alleviating power quality problems. Operating principle of the M2D2 is explained, harmonic features under phase-shifted carrier (PSC) modulation are analyzed, and selections of the phase-shifted angle are derived to achieve best harmonic performance. Control schemes for M2D2 are also developed, including control of de-icing current, capacitor voltages, and reactive power. Finally, experimental results, utilizing a downscaled prototype, confirm the validity of this topology and effectiveness of the proposed control strategies.


Design of External Inductor for Improving Performance of Voltage Controlled DSTATCOM

Abstract

A distribution static compensator (DSTATCOM) is used for load voltage regulation and its performance mainly depends upon the feeder impedance and its nature (resistive, inductive, stiff, non-stiff). However, a study for analyzing voltage regulation performance of DSTATCOM depending upon network parameters is not well defined. This paper aims to provide a comprehensive study of design, operation, and flexible control of a DSTATCOM operating in voltage control mode. A detailed analysis of the voltage regulation capability of DSTATCOM under various feeder impedances is presented. Then, a benchmark design procedure to compute the value of external inductor is presented. A dynamic reference load voltage generation scheme is also developed which allows DSTATCOM to compensate load reactive power during normal operation, in addition to providing voltage support during disturbances. Simulation and experimental results validate the effectiveness of the proposed scheme.


A Low Capacitance Cascaded H-Bridge Multi-Level StatCom

Abstract

This paper introduces a cascaded H-bridge multilevel converter (CHB-MC) based StatCom system that is able to operate with extremely low dc capacitance values. The theoretical limit is calculated for the maximum capacitor voltage ripple, and hence minimum dc capacitance values that can be used in the converter. The proposed low-capacitance StatCom (LC-StatCom) is able to operate with large capacitor voltage ripples, which are very close to the calculated theoretical maximum voltage ripple. The maximum voltage stress on the semiconductors in the LC-StatCom is lower than in a conventional StatCom system. The variable cluster voltage magnitude in the LC-StatCom system drops well below the maximum grid voltage, which allows a fixed maximum voltage on the individual capacitors. It is demonstrated that the proposed LC-StatCom has an asymmetric V-I characteristic, which is especially suited for operation as a reactive power source within the capacitive region. A high-bandwidth control system is designed for the proposed StatCom to provide control of the capacitor voltages during highly dynamic transient events. The proposed LC-StatCom system is experimentally verified on a low-voltage 7-level CHB-MC prototype. The experimental results show successful operation of the system with ripples as high as 90% of the nominal dc voltage. The required energy storage for the LC-StatCom system shows significant reduction compared to a conventional StatCom design.


Adaptive Neuro Fuzzy Inference System Least Mean Square Based Control Algorithm for DSTATCOM

Abstract

This paper proposes the real time implementation of a three phase distribution static compensator (DSTATCOM) using adaptive neuro fuzzy inference system least mean square (ANFIS-LMS) based control algorithm for compensation of current related power quality problems. This algorithm is verified for various functions of DSTATCOM such as harmonics compensation, unity power factor, load balancing and voltage regulation. The ANFIS-LMS based control algorithm is used for the extraction of fundamental active and reactive power components from non-sinusoidal load currents to estimate reference supply currents. Real time validation of the proposed control algorithm is performed on a developed laboratory prototype of a shunt compensator. The real time performance of shunt compensator with ANFIS-LMS based control algorithm is found satisfactory under steady state and dynamic load conditions. The performance of proposed control algorithm is also compared with fixed step LMS and variable step LMS (VSLMS) to demonstrate its improved performance.


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