Load Compensation Research Articles

Design and Decoupled Compensation Methods of a PM Motor Capable of 6-D Force/Torque Actuation for Minimum Bearing Reaction

This paper presents a methodology to design and control a permanent magnet (PM) motor capable of 6-D force/torque actuation for real-time compensation of external loads to achieve minimum bearing reaction (MBR). Unlike conventional multiphase designs, the current inputs to the stator-electromagnets (EMs) can be flexibly configured to enable 6-D force and/or torque actuation in one motor; two common motor structures (radial and axial types) are illustrated. Both the forward and inverse force/torque models are presented in terms of coordinate-independent kernel functions that characterize the force between an EM and a PM pole pair. Two closed-form solutions to the inverse model that solves for the current-input vector minimizing the total input energy to generate a desired force/torque vector, which can be computed within 1 ms, are derived and verified numerically. A feedforward MBR compensator designed to argument the proportional-integral-derivative (PID) speed regulator has been experimentally evaluated on a structurally smart spindle system to minimize bearing reactions. Experiments show that the MBR compensation effectively reduces vibrations and improves cutting quality.

Two alternative robust optimization models for flexible power management of electric vehicles in distribution networks

This paper presents a robust optimization problem of the flexible bidirectional power management of a smart distribution network and harmonic compensation of nonlinear loads using electric vehicles (EVs) equipped with bidirectional chargers. The base deterministic model of the proposed problem is as mixed-integer nonlinear programming (MINLP), having the objective function to minimize the economic and technical indices subject to harmonic load flow equations, EVs constraints, system operation and harmonic indices limits. This model is converted to a mixed-integer linear programming (MILP) model in the next step. In the proposed MILP model, the active, reactive and apparent loads, electrical energy, reactive power and harmonic current prices, as well as EVs characteristics, are considered uncertain parameters. Accordingly, two alternative robust optimization approaches have been implemented for the conditions of having both the probability distribution function or the bounded uncertainty in the proposed MILP problem model. The proposed model is tested on distribution networks to demonstrate its efficiency and performance. The results show that the MINLP model can be substituted by the proposed high-speed MILP model. In addition, the capacity of the injecting power of EVs is reduced in the worst-case scenario with respect to the scenario that is used in the deterministic model, while the consumed power of loads and EVs and energy price increases in this scenario. Finally, the payment of EV owners is reduced by considering EVs power and harmonic control.

Schedule of air-conditioning systems with thermal energy storage considering wind power forecast errors

As the penetration of uncontrollable renewable energy sources (RESs) increases, energy storage and flexible demand will play a more important role in future power systems. In this paper, air-conditioning systems with thermal energy storage (A/C storage systems) are studied as a way of compensating uncertainties from wind power. Wind power forecast errors are analyzed from different perspectives in order to better assist the schedule of storage devices. An operation scheme is proposed for A/C storage systems for both day-ahead scheduling and real-time operation, based on the features of wind power forecast errors. The targets include load management and compensation of wind power forecast errors. Simulations illustrate the effectiveness of the proposed scheme to support power systems with high wind penetration.

Analysis of star and delta connected modular multilevel cascaded converter-based STATCOM for load unbalanced compensation

This paper compares the operating capabilities of STATCOMs based on Modular Multilevel Cascaded Converters (MMCC) using star and delta connections, with special attention to unbalanced load compensation. Zero sequence voltage for star connection, and zero sequence current for delta, need to be applied to overcome the phase cluster DC-voltage unbalance. Expressions are derived for both zero sequence elements as functions of the degree of load unbalance defined as the ratio of negative to positive sequence load current. They show that the zero sequence voltage in star connection reaches a very high level as the degree of load unbalance increases, making the MMCC DC-link voltage too high for correct functioning. However the delta connected MMCC can cope with the high level of load current unbalance. Experimental results are presented to validate this analysis.

Modeling and loading compensation of a rotary valve-controlled pitch system for wind turbines

A rotary valve-controlled pitch system is proposed to regulate the generator power and smooth power fluctuations for a wind turbine. Design details and dynamic modeling of this pitch system are presented and analyzed. A practical loading compensation approach is synthesized and involved in the pitch system to compensate for the external uncertain pitch loads. The proposed pitch system and loading compensation approach have been experimentally evaluated in terms of generator power smoothing and control accuracy. As demonstrated by the comparative experimental results, the proposed pitch system can be used to significantly smooth the generator power fluctuations and hence to improve the power quality as compared with a servo valve-controlled pitch system under the same operating conditions. The loading compensation approach can also be used to significantly attenuate the effects of external pitch loads and improve the robustness and reliability of the pitch system. The proposed pitch system features good control accuracy and cost-efficiency and hence is attractive for applications in modern largescale wind turbines.

Power Quality Improvement by Solar Photo-voltaic / Wind Energy Integrated System Using Unified Power Quality Conditioner

The real problems in diminution of power quality occurs due to the rapid growth of nonlinear load are leads to sudden decrease of source voltage for a few seconds i.e sag, swell, harmonics in source and load current, voltage unbalance etc. All these problems can be compensated by using Unified Power Quality Controller (UPQC) and the operation of UPQC depends upon the available voltage across capacitor present in dc link. If the capacitor voltage is maintained constant then it gives satisfactory performance. The proposed research is basically on designing of Photo Voltaic (PV) /Wind energy fed to the dc link capacitor of UPQC so as to maintain proper voltage across it and operate the UPQC for power quality analysis. The said model is simulated in Matlab and results are verified by using FFT analysis.The proposed PV/ Wind energy-UPQC is design in Matlab simulation for reduction of voltage sag, swell, interruption of voltage, harmonics in load current and compensation of active and reactive power.

The Role of Visual Noise in Influencing Mental Load and Fatigue in a Steady-State Motion Visual Evoked Potential-Based Brain-Computer Interface.

As a spatial selective attention-based brain-computer interface (BCI) paradigm, steady-state visual evoked potential (SSVEP) BCI has the advantages of high information transfer rate, high tolerance to artifacts, and robust performance across users. However, its benefits come at the cost of mental load and fatigue occurring in the concentration on the visual stimuli. Noise, as a ubiquitous random perturbation with the power of randomness, may be exploited by the human visual system to enhance higher-level brain functions. In this study, a novel steady-state motion visual evoked potential (SSMVEP, i.e., one kind of SSVEP)-based BCI paradigm with spatiotemporal visual noise was used to investigate the influence of noise on the compensation of mental load and fatigue deterioration during prolonged attention tasks. Changes in α, θ, θ + α powers, θ/α ratio, and electroencephalography (EEG) properties of amplitude, signal-to-noise ratio (SNR), and online accuracy, were used to evaluate mental load and fatigue. We showed that presenting a moderate visual noise to participants could reliably alleviate the mental load and fatigue during online operation of visual BCI that places demands on the attentional processes. This demonstrated that noise could provide a superior solution to the implementation of visual attention controlling-based BCI applications.

Comparing the Effectiveness of Feedback and Load Compensation Techniques in Controlling the Disturbed State of Nominal-T Configuration

Stability of Power system is the ability of a system, for a given initial operating condition, to regain a state of operating equilibrium after being subjected to a physical disturbance, with most system variables bounded so that practically the entire system remains intact. This research work stated clearly the effectiveness of the feedback and load compensation techniques in stabilizing a disturbed state of a medium transmission line using a Nominal-T configuration network. In order to achieve the set objectives, Osogbo - Akure transmission line data was obtained from Akure 132kV transmission substation. This configuration was modeled into transfer function and state space models, the compensator circuit which happens to be the phase lag circuit was also modeled. The transfer function model contained the line parameters extracted from this transmission substation logbook. A state space model was obtained from the transfer function model with a code written in MATLAB environment. The effectiveness of these compensation techniques were compared. The result revealed that load compensation technique offered a perfect compensation to unit step disturbance and unit impulse disturbance. While feedback compensation technique provides perfect compensation to unit impulse disturbance only.

Comparing the Effectiveness of Feedback and Load Compensation Techniques in Controlling the Disturbed State of Nominal-T Configuration

Stability of Power system is the ability of a system, for a given initial operating condition, to regain a state of operating equilibrium after being subjected to a physical disturbance, with most system variables bounded so that practically the entire system remains intact. This research work stated clearly the effectiveness of the feedback and load compensation techniques in stabilizing a disturbed state of a medium transmission line using a Nominal-T configuration network. In order to achieve the set objectives, Osogbo - Akure transmission line data was obtained from Akure 132kV transmission substation. This configuration was modeled into transfer function and state space models, the compensator circuit which happens to be the phase lag circuit was also modeled. The transfer function model contained the line parameters extracted from this transmission substation logbook. A state space model was obtained from the transfer function model with a code written in MATLAB environment. The effectiveness of these compensation techniques were compared. The result revealed that load compensation technique offered a perfect compensation to unit step disturbance and unit impulse disturbance. While feedback compensation technique provides perfect compensation to unit impulse disturbance only.

Distributed Wind Energy Conversion System Interface to the Grid Using Cascaded Multi-Level Converter

ABSTRACTIn this paper, a distributed wind energy conversion system based on the cascaded H-bridge multi-level converter topology has been proposed to be used for the medium-voltage grid interface application. This includes load compensation and injection of real power requirement of the load. The load compensation includes reactive power compensation, load harmonic mitigation, and correction for the unbalancing problem. An algorithm has been proposed for generating references for the voltage source converter in current control mode in order to inject real power together with the reactive and harmonic components of the load. Modular structure of the wind turbine-alternator sets and cascaded multi-level inverter using H-bridges suits the distributed wind energy conversion system interface to the medium-voltage grid. The factors like equal distribution of switching stress and power losses, reduced power ratings of the inverter, redundancy, and high quality of inverter output voltage make the proposed arrangement efficient for high-power applications. The cascaded topology with phase-shifted multi-carrier pulse width modulation (PWM) has the features of increasing the effective switching frequency and reducing the ripple magnitude. The proposed scheme has been verified through the simulation study using power system transient analysis tool (PSCAD/EMTDC).

Design and Implementation of a STATCOM Based on a Multilevel FHB Converter with Delta-Connected Configuration for Unbalanced Load Compensation

A delta-connected STATCOM with cascade multilevel full H-bridge (FHB) converter has been applied for phase balancing and power factor correction of unbalanced load in a three-phase, three-wire power distribution system. In this paper, a feedforward compensation formula is presented for the STATCOM by using the symmetrical components method to compensate unbalanced load. Computer simulation is performed for preliminary verification. Accordingly, a hardware prototype employing a TMS320F2812 DSP-based system is built for final verification test. Experimental results show that the proposed STATCOM is very effective for real-time unbalanced load compensation.

Implementation of Echo-State Network-Based Control for Power Quality Improvement

This paper proposes a new control algorithm for mitigation of power quality problems at the distribution level. The control algorithm is designed on the basis of echo-state network, which is a type of recurrent neural network. This technique is used to extract fundamental weight components from nonsinusoidal load currents. These weight components are used to evaluate reference grid currents and consequently generate switching logic for the voltage source converter used in the active filter. The shunt-connected active filter with the proposed control algorithm is used to overcome several current-related power quality problems and provides the load compensation. Problems such as the presence of lower order harmonics, insufficient reactive power, and load unbalancing are corrected using a shunt filter in both the power factor correction and voltage regulation modes. The proposed control algorithm is validated on a real time system which is a prototype of a shunt filter. The algorithm is tested in MATLAB using Sim Power Systems and SIMULINK and implemented on a digital signal processor.

A dual EHA system for the improvement of position control performance via active load compensation

The dynamic response of a single-rod type EHA (electro-hydrostatic actuator) in the closed-loop control of piston position is inherently dependent on load directions due to the unequal piston areas of the rod-side and head-side. In particular, the response time of the extension and retraction of the piston rod changes significantly when the direction of the external load is changed. In this study, a dual EHA system is proposed, where a force-controlled double-rod type EHA was added to a single-rod type EHA to isolate it from external loads. Force signals for the force control were obtained by measuring the cylinder chamber pressures. The dual EHA system exhibited significantly improved position control performance, even when using basic proportional controllers. An expected collateral benefit is that the system may be reconfigured to a redundant system for fail-safe operation in the emergency case where the main position controlled EHA is malfunctioning.

Grid synchronization and control of distributed generation unit with flexible load compensation capabilities using multi-output LMS-filter

This paper deals with grid synchronization and control of single-phase voltage source inverter in distributed generation (DG) systems using a multi-output adaptive filter. Besides the active power injection, the proposed control enables the inverter to compensate load harmonic and reactive currents based on the capacity of inverter. The currents to be compensated are obtained using a multi-output adaptive filter structure that operates on the principle of variable step size (VSS) least mean squares (LMS) algorithm. The main objective of VSS-LMS filter is to decompose the load current into harmonic and reactive current components and compute their RMS values. The currents extracted by the VSS-LMS filter are used in developing a flexible compensation based on load compensation factors. The load compensation factors are calculated based on the remaining capacity of the inverter and priority. Harmonic currents compensation is given first priority over the reactive currents. In addition to harmonic currents detection, the same VSS-LMS filter is used in developing a phase locked loop unit for synchronizing the inverter with fundamental point of common coupling (PCC) voltage. Synchronizing the DG inverter with fundamental PCC voltage would make the proposed control algorithm insensitive to grid voltage harmonics and frequency fluctuations. The feasibility and the efficacy of the proposed control algorithm are demonstrated using hardware-in-the-loop (HIL) based experiments.

A Sag Compensator That Eliminates the Possibility of Inrush Current While Powering Transformer-Coupled Loads

Sags are the foremost power quality problem encountered by industrial consumers because they frequently cause the interruption of manufacturing processes, which can result in a significant economic loss. To resolve these problems, industrial consumers often install voltage sag compensators in critical industrial applications. However, these compensating topologies prove to be costly, and some take a few milliseconds to being operating when a sag occurs. The transformer, which is usually mounted in front of the critical load, is likely to be exposed to sag voltages during this time interval. As a result, when the compensator reinstates the load voltage, a serious magnitude of transient current begins to flow toward the sensitive load. This inrush current can either damage the equipment or activate the protection devices. In either case, the compensation will eventually fail. To eliminate the inrush current and to achieve seamless compensation for sensitive loads, we propose in this paper a sag compensator established on a customary current-regulated inverter. This compensator employs a current-regulating scheme using a stationary frame of reference to regulate the current for the load by connecting it to the secondary winding of the load transformer. We verify the operation of the proposed system through laboratory test results obtained under different possible sag types.

Damped-SOGI-Based Control Algorithm for Solar PV Power Generating System

This paper deals with two-stage solar photovoltaic (PV) system with a damped-second-order generalized integrator (SOGI) algorithm. Proposed topology not only integrates the PV energy to the grid, in addition, it provides the load compensation, power factor correction, and harmonics elimination. Therefore, a double stage system is proposed where the first stage is a dc–dc boost converter, which performs the maximum power point tracking by regulating its duty ratio. For extracting maximum power from the PV array, an incremental conductance-based approach is utilized. Moreover, in the next stage, a voltage source converter (VSC) is utilized. To control the VSC, a damped-SOGI algorithm is used. By using a damped-SOGI-based control algorithm, fundamental active components of load currents are extracted for evaluating the reference grid currents. After comparing reference grid currents with sensed grid currents, switching pulses for the grid tied VSC are produced. A prototype of the proposed system is developed in the laboratory and test results are presented for verification of control of solar photovoltaic system.

Effect of crop load on the phenological, vegetative and reproductive behavior of the ‘Frantoio’ olive tree (Olea europaea L.)

The aim of this study was to characterize the phenological, vegetative and reproductive behavior variables of ‘Frantoio’ olive trees in an “off” year (with a low crop load) and an “on” year (with a high crop load). To do this, during the 2011–2012 and 2012–2013 seasons, “off” and “on”, respectively, phenological variables, plant growth (shoots and roots) and reproductive growth (type of flower, fruit set and growth), were monitored biweekly. In addition, the yield per tree was assessed, and the crop load was estimated. The main results showed that crop load strongly influenced these variables. The distribution and intensity of vegetative growth, both in the roots and aboveground, decreased more and had lower intensity in the high fruiting season, with a large part of this growth occurring prior to flowering. With respect to reproductive variables, a higher percentage of perfect flowers and fruit set in the low flowering season was observed compared to the high load season. These results suggest a partial crop load compensation mechanism under low flowering conditions. This mechanism, however, does not compensate for lower production in the “off” season. In terms of volume, fruit growth was similar between seasons until pit hardening, then the fruit growth rate dropped dramatically in the “on” season, indicating that fruit growth, until pit hardening, would not be affected by the presence of a higher number of fruits per tree.

The Role and Load of the Athletic Training Clinical Education Coordinator

Context: The position of clinical education coordinator has been identified as a required one in athletic training education. However, the literature has yet to address the job responsibilities of clinical education coordinators and the commensurate work load/release time needed to accomplish these responsibilities in athletic training education. Objective: To determine the current practices of clinical education coordinators in athletic training program, their current load compensation, whether or not they feel their load compensation is appropriate, and what their ideal load compensation should be. Design: Mixed methods. Setting: Commission on Accreditation of Athletic Training Education–accredited education programs. Patients or Other Participants: A total of 120 clinical education coordinators. Main Outcome Measure(s): A survey was administered including both quantitative and qualitative questions. Quantitative data were analyzed statistically and qualitative data were analyzed using an inductive approach, revealing themes. Results: Statistically significant differences were found when comparing tenure with clinical track in the areas of scholarship and athletic training room coverage. Common job responsibilities for the clinical education coordinator were identified. The study also identified the current and perceived ideal load/release credit for clinical education coordinators based on their program size. Conclusions: Clinical education coordinators are incredibly valuable to all athletic training education programs. This study attempted to solidify their value through identifying job responsibilities and load credit criteria to ensure quality clinical education experiences for students.

Power Quality Enhancement in Grid Connected PV Systems using High Step Up DC-DC Converter

Renewable energy sources (RES) are gaining more importance in the present scenario due to the depletion of fossil fuels and increasing power demand. Solar energy is the one of the most promising as it is clean and easily available source. The voltage obtained from the PV system is low. This voltage is increased by high step up dc-dc converter which uses only one switch leads to low switching losses and hence the efficiency of this converter is high. To get the good response this converter is operated in closed loop manner. Integration of PV system with existing grid has so many issues like distorted voltage, current and reactive power control etc. This paper presents a four leg inverter which works on hysteresis current control technique to address the power quality issues like reactive power compensation, balanced load currents and compensation of neutral current. The switching to the inverter is designed in such a way that it supplies the extra current to stabilise the current of the grid that is being supplied to the loads. Finally, the proposed technique is validated by using mat lab/Simulink software and corresponding results are presented in this paper.

Spectral dependence of aerosol light absorption at an urban and a remote site over the Tibetan Plateau

We present a study of aerosol light absorption by using a 7-wavelength Aethalometer model AE33 at an urban site (Lhasa) and a remote site (Lulang) in the Tibetan Plateau. Approximately 5 times greater aerosol absorption values were observed at Lhasa (53±46Mm−1 at 370nm and 20±18Mm−1 at 950nm, respectively) in comparison to Lulang (15±19Mm−1 at 370nm and 4±5Mm−1 at 950nm, respectively). Black carbon (BC) was the dominant light absorbing aerosol component at all wavelengths. The brown carbon (BrC) absorption at 370nm is 32±15% of the total aerosol absorption at Lulang, whereas it is 8±6% at Lhasa. Higher value of absorption Ångström exponent (AAE, 370–950nm) was obtained for Lulang (1.18) than that for Lhasa (1.04) due to the presence of BrC. The AAEs (370–950nm) of BrC were directly extracted at Lulang (3.8) and Lhasa (3.3). The loading compensation parameters (k) increased with wavelengths for both sites, and lower values were obtained at Lulang than those observed at Lhasa for all wavelengths. This study underlines the relatively high percentage of BrC absorption contribution in remote area compared to urban site over the Tibetan Plateau.