High Stability Margin Research Articles

A Root-Locus Design Methodology Derived From the Impedance/Admittance Stability Formulation and Its Application for LCL Grid-Connected Converters in Wind Turbines

This paper presents a systematic methodology for the design and the tuning of the current controller in LCL grid-connected converters for wind turbine applications. The design target is formulated as a minimization of the current loop dominant time constant, which is in accordance with standard design guidelines for wind turbine controllers (fast time response and high stability margins). The proposed approach is derived from the impedance/admittance stability formulation, which, on one hand, has been proved to be suitable for the controller design when the active damping is implemented and, on the other hand, has also been proved to be very suitable for system-level studies in applications with a high penetration of renewable energy resources. The tuning methodology is as follows: first, the physical system is modeled in terms of the converter admittance and its equivalent grid impedance; then, a sensitivity transfer function is derived, from which the closed-loop eigenvalues can be calculated; finally, the set of control gains that minimize the dominant time constant are obtained by direct search optimization. A case study that models the target system in a low-power scale is provided, and experimental verification validates the theoretical analysis. More specifically, it has been found that the solution that solves the minimization of the current controller time constant (wind turbine controller target) also corresponds to a highly damped electrical response (robustness provided by the active damping).

Dynamic margin of stability during gait is altered in persons with multiple sclerosis

Persons with multiple sclerosis (PwMS) have high fall risk due to altered balance. To measure dynamic balance during walking, margin of stability (MoS) examines how the extrapolated center of mass moves relative to the base of support. This study investigates how MoS is affected in PwMS during walking at preferred, slow, and fast speeds, as well as the relationship between MoS and the Expanded Disability Severity Score (EDSS), fall history, and self-report balance confidence questionnaire. MoS was evaluated in PwMS without clinical gait impairment (MS1; n=20), PwMS with clinical gait impairment (MS2; n=20), and age-matched healthy controls (HC) (n=20), in the anterior/posterior (AP) and medial/lateral (ML) direction at heel strike and midstance. In the AP direction, MS2 had a higher MoS than HC (p<0.001) and MS1 (p<0.001) at heel strike and midstance. In the ML direction, MS2 had a higher MoS than HC (p<0.001) at heel strike only. At midstance, slow pace had a lower MoS than preferred pace (p<0.001) and fast pace (p=0.007). Compared to HC, PwMS walk slower which increases their AP MoS. In the ML direction, slow walking causes lower MoS at midstance, so PwMS increase their MoS by taking wider steps. AP MoS correlated with EDSS (p=0.008) and number of falls (p=0.001), and ML MoS correlated with number of falls (p=0.027). Walking slower, with shorter step length, and with wider step widths increases MoS for PwMS but may be a poor adaptive gait strategy since falls still occur.

Modified PID control with H∞ loop shaping synthesis for RO desalination plants

This paper deals with the PID control synthesis with robust (H∞) loop shaping framework to manage MIMO reverse osmosis (RO) desalination plants. This new method takes advantage of the robust controller which has a fixed low order of conventional PID scheme guaranteeing robust stability and performance under large parametric uncertainties, external disturbances, and sensor noises. The coprime factor uncertainty description in H∞ loop shaping methodology can cover a wide variety of uncertainties over all frequencies for the RO plants. The test results demonstrate that the achieved controller has high stability margin, showing disturbance and noise attenuation abilities. Furthermore, most interactions between control channels have been decoupled in the complete control system. The presented control approach can help reduce membrane cleaning, save energy, and reduce product water costs for the RO plants. Note that weather changes increase unstableness in RO plants but the water treatment facilities are only dimensioned for PID control. In addition, this study addresses control robustness as well as performance of a RO system against uncertainties, disturbances, and noises. Finally, the present control method can be utilized for efficiently controlling water quality (including quantity) and managing water facility of the RO desalination plants.

A Novel Type-1 Frequency-Locked Loop for Fast Detection of Frequency and Phase With Improved Stability Margins

The synchronous reference frame phase-locked loop (SRF-PLL) is a widely used synchronization technique in power electronics and power systems applications due to its ease of implementation and robust performance. The conventional SRF-PLL is a type-2 control system due to the use of proportional-integral controller as loop filter. With higher bandwidth design, it can achieve fast detection of frequency and phase under ideal grid conditions. However, its bandwidth should be sufficiently lowered to obtain proper disturbance rejection under unbalanced and distorted grid conditions. This results in a slower detection speed. Recently, several advanced PLLs with pre/in-loop filtering stage have been proposed to improve the detection speed. A major challenge with the PLLs is how to further improve their dynamic performance without compromising the disturbance rejection capability and stability. To resolve this issue, in this paper, a novel type-1 frequency-locked loop (FLL) is proposed. The disturbance rejection capability of the proposed FLL is improved using a modified structure low-pass filter with selective harmonics filtering ability. As the proposed FLL is type-1 control system, it achieves better dynamic performance with higher stability margins. The effectiveness of the proposed FLL is confirmed through experimental results and comparison with advanced type-2 PLLs.

Maximum constant boost approach for controlling quasi‐Z‐source‐based interlinking converters in hybrid AC–DC microgrids

This study presents the mathematical modelling and control design methodology of bidirectional quasi-Z-source inverters (qZSIs) for interlinking converter application in hybrid AC–DC microgrids. The proposed control scheme provides the maximum constant boost (MCB) control capability for the qZSI in both grid-connected and islanded modes of operation even in the presence of unbalanced AC loads. The proposed approach maintains the MCB without any additional control loop for regulating the voltage of the DC-side capacitor of the qZSI. This is done by determining the shoot through duty cycle of the qZSI based on the inverter modulation index. Moreover, maintaining the MCB condition, the proposed control scheme provides a high stability margin for the qZSI network. The effectiveness of the proposed method is demonstrated through simulation studies conducted on a typical AC–DC hybrid microgrid in MATLAB/Simulink environment for both islanded and grid-connected modes of operation.

Correlations between measures of dynamic balance in individuals with post-stroke hemiparesis

Mediolateral balance control during walking is a challenging task in post-stroke hemiparetic individuals. To detect and treat dynamic balance disorders, it is important to assess balance using reliable methods. The Berg Balance Scale (BBS), Dynamic Gait Index (DGI), margin-of-stability (MoS), and peak-to-peak range of angular-momentum (H) are some of the most commonly used measures to assess dynamic balance and fall risk in clinical and laboratory settings. However, it is not clear if these measures lead to similar conclusions. Thus, the purpose of this study was to assess dynamic balance in post-stroke hemiparetic individuals using BBS, DGI, MoS and the range of H and determine if these measure are correlated. BBS and DGI were collected from 19 individuals post-stroke. Additionally, kinematic and kinetic data were collected while the same individuals walked at their self-selected speed. MoS and the range of H were calculated in the mediolateral direction for each participant. Correlation analyses revealed moderate associations between all measures. Overall, a higher range of angular-momentum was associated with a higher MoS, wider step width and lower BBS and DGI scores, indicating poor balance control. Further, only the MoS from the paretic foot placement, but not the nonparetic foot, correlated with the other balance measures. Although moderate correlations existed between all the balance measures, these findings do not necessarily advocate the use of a single measure as each test may assess different constructs of dynamic balance. These findings have important implications for the use and interpretation of dynamic balance assessments.

Performance Improvement of Quasi-Type-1 PLL by using a Complex Notch Filter

The synchronous reference frame phase-locked loop (SRF-PLL) is widely used for synchronization applications. However, it suffers from a poor performance under unbalanced and distorted grid conditions. To improve the filtering capability of SRF-PLL, moving average filter (MAF) is incorporated into its control loop at the cost of a slow transient response of PLL in recently published literature. To further improve their dynamic performance without compromising the disturbance rejection capability and stability, a novel PLL based on quasi-type-1 PLL structure is proposed in this paper. A complex notch filter is incorporated into the QT1-PLL to eliminate the fundamental frequency negative sequence voltage component. The window length of MAF in QT1-PLL is reduced. And MAF is responsible for rejecting the rest of harmonics. Parameter design guidelines are suggested to obtain the minimum settling time for both phase jump and frequency jump. The proposed PLL provides a faster transient response with higher stability margins. The effectiveness of the proposed PLL is confirmed through simulation and experimental results and comparison with advanced PLLs.

A Quasi-Type-1 Phase-Locked Loop Structure

The grid voltage phase and frequency are crucial information in control of most grid-connected power electronic-based equipment. Most often, a phase-locked loop (PLL) is employed for this purpose. A PLL is a closed-loop feedback control system that the phase of its output signal is related to the phase of its input signal. Arguably, the simplest PLL is a type-1 PLL. The type-1 PLLs are characterized by having only one integrator in their control loop and, therefore, having a high stability margin. However, they suffer from a serious drawback: They cannot achieve zero average steady-state phase error in the presence of frequency drifts. To overcome this drawback of type-1 PLLs, and at the same time, to achieve a fast dynamic response and high filtering capability, a modified PLL structure is proposed in this letter. The proposed PLL has a similar structure to a type-1 PLL, but from the control point of view is a type-2 control system. For this reason, it is called the quasi-type-1 PLL. The effectiveness of the proposed PLL is confirmed through simulation and experimental results and comparison with standard PLLs.

Novel Comprehensive Control Framework for Incorporating VSCs to Smart Power Grids Using Bidirectional Synchronous-VSC

This paper presents a new control strategy for voltage-source converters (VSCs) in the frequency-angle domain which enables dc-link voltage regulation via frequency and load angle adjustment. A major advantage of the proposed controller is emulating the behavior of synchronous machines (SMs) with proper regulation of dc-link voltage which eases integration of VSCs interfacing distributed and renewable generation units into ac systems in the presence of conventional SMs. A cascaded frequency, angle and virtual torque control topology is developed to emulate the mechanical behavior of an SM which offers synchronization power to eliminate the need for a phase-locked-loop after initial converter synchronization, and damping power dynamics to damp power oscillations; and presents frequency dynamics similar to SMs, thus it introduces some inertia to the grid. The controller presents high stability margin and fast dc-link voltage regulation, whereas it can provide frequency support in the ac-side during contingencies. Frequency and voltage amplitude are adjusted by two separate loops. Two different variants are proposed for dc-link voltage control; namely direct dc-link voltage control and indirect dc-link voltage control via a dc-link voltage controller. Small-signal dynamics, analysis, and design process are presented. Both simulation and experimental results are provided to validate the controller effectiveness.

The Effect of Transformer Leakage Inductance on the Steady State Performance of Push-pull based Converter with Continuous Current

As a result of the advantages such as high efficiency, continuous current and high stability margin, push-pull converter with continuous current (PPCWCC) is competitive for battery discharge regulator (BDR) which plays an important role in power conditioning unit (PCU). Leakage inductance yields current spike in low-ripple current of PPCWCCs. The operating modes are added due to leakage inductance. Therefore the steady state performance is affected, which is embodied in the spike of low-ripple current. PPCWCCs which are suitable for BDR can be separated into three types by current spike characteristics. Three representative topologies IIs1, IIcb2 and Is3 are analyzed in order to investigate the factors on the magnitude and duration of spike. Equivalent current sampling method (ECSM) which eliminates the sampling time delay and achieves excellent dynamic performance is adopted to prevent the spike disturbance on current sampling. However, ECSM reduces the sampling accuracy and telemetry accuracy due to neglecting the spike. In this paper, ECSM used in PPCWCCs is summarized. The current sampling error is analyzed in quality and quantity, which provides the foundation for offsetting and enhancing the telemetry accuracy. Finally, current sampling error rate of three topologies is compared by experiment results, which verify the theoretical analysis.

Novel uniplanar flexible Artificial Magnetic Conductor

A novel flexible uniplanar Artificial Magnetic Conductor (AMC) design is presented. A prototype is manufactured and characterized under flat and bent conditions in an anechoic chamber. The designed prototype shows broad AMC operation bandwidth, polarization angle independency, and high angular stability margin when operating under oblique incidence.

Auto-adaptive and string stable strategy for intelligent cruise control

This study completes the study of a longitudinal control strategy based on a non-linear acceleration profile model. The model is thoroughly defined by a set of two design parameters that make the control policy auto-adaptive. The control strategy is safe and Stop&amp;Go consistent. The string stability property of the control policy is also analysed. Simulation results show that the control policy has a good string stability property with high string stability margin with respect to Pipes and Chandler&apos;s control strategy.

Integral resonant control scheme for cancelling human-induced vibrations in light-weight pedestrian structures

SUMMARY Integral resonant control (IRC) has been recently introduced as a simple, robust and high-performance technique for vibration control of smart structures instrumented with collocated piezoelectric actuator–sensor pairs. This work deals with the design and implementation of an active vibration control (AVC) system based on an IRC strategy for the mitigation of human-induced vibrations in light-weight civil engineering structures, such as floors and footbridges, via proof-mass actuators. This work presents a new AVC strategy that combines an approximate inversion of the proof-mass actuator dynamics with an IRC-based strategy. The result is a control scheme with the following desirable characteristics: (i) the closed-loop system exhibits very high stability margins, (ii) the risk of stroke saturation at low frequencies is significantly reduced so that the saturation nonlinearity, which has to be included to keep the system hardware safe, can be designed to account only for force saturation (i.e. the actuator performance is enhanced), (iii) rigorous stability analysis and systematic design can be proposed and (iv) it is not necessary to measure the actuator force. The stability analysis is carried out using the recently developed stability theorem based on the positive feedback interconnection of systems with negative imaginary frequency response. The control scheme is validated on a full-scale prestressed concrete laboratory structure. Excellent vibration reduction performance is reported for frequency-response-function-based tests and for walking excitations. Copyright © 2010 John Wiley & Sons, Ltd.

Enhancing Stability of an Autonomous Microgrid using a Gain Scheduled Angle Droop Controller with Derivative Feedback

This paper discusses the stability of an autonomous microgrid in which, load sharing by a gain scheduled angle droop controller with derivative feedback is proposed. It is assumed that all the DGs are connected through Voltage Source Converter (VSC). The VSCs are controlled by state feedback controller to achieve desired voltage and current outputs that are decided by a droop controller. First a load sharing strategy is derived in terms of droop controller gain and converter output inductance, combining angle droop controller and DC load flow analysis. Then state space models of converters with its associated feedback controller are derived to investigate system stability as a function of the droop controller gain and converter output inductance through eigen value analysis. It is shown that with proposed angle droop controllers, where the droop gains are scheduled based on the output power along with derivative feedback it is possible to achieve a higher stability margin with better load sharing. These observations are then verified through simulation studies using PSCAD/EMTDC. It will be shown that the simulation results closely agree with stability behavior predicted by the eigenvalue analysis.

Effects of Compressor Tip Injection on Aircraft Engine Performance and Stability

This analytical study discusses the system aspects of active stability enhancement using mass flow injection in front of the rotor blade tip of a high pressure compressor. Tip injection is modeled as a recirculating bleed in a performance simulation of a commercial turbofan engine. A map correction procedure accounts for the changes in compressor characteristics caused by injection. The correction factors are derived from stage stacking calculations, which include a simple correlation for stability enhancement. The operational characteristic of the actively controlled engine is simulated in steady and transient states. The basic steady-state effect consists of a local change in mass flow and a local increase in gas temperature. This alters the component matching in the engine. The mechanism can be described by the compressor-to-turbine flow ratio and the injection temperature ratio. Both effects reduce the cycle efficiency resulting in an increased turbine temperature and fuel consumption at constant thrust. The negative performance impact becomes negligible if compressor recirculation is only employed at the transient part power and if valves remain closed at the steady-state operation. Detailed calculations show that engine handling requirements and temperature limits will still be met. Tip injection increases the high pressure compressor stability margin substantially during critical maneuvers. The proposed concept in combination with an adequate control logic offers promising benefits at transient operation, leading to an improvement potential for the overall engine performance.

Dynamic stability control in forward falls: postural corrections after muscle fatigue in young and older adults

Many studies report that muscle strength loss may alter the human system's capacity to generate rapid force for balance corrections after perturbations, leading to deficient recovery behaviours. Yet little is known regarding the effect of modifications in the neuromuscular system induced by fatigue on dynamic stability control during postural perturbations. This study investigates the effect of muscle strength decline induced by fatiguing contractions on the dynamic stability control of young and older adults during forward falls. Eleven young and eleven older male adults had to regain balance after sudden falls before and after submaximal fatiguing knee extension-flexion contractions. Young subjects had a higher margin of stability than older ones before and after the fatiguing task. This reflects their enhanced ability in using mechanisms for maintaining dynamic stability (i.e. a greater base of support). The margin of stability, the boundary of the base of support and the position of the extrapolated centre of mass, remained unaffected by the reduction in muscle strength induced by the fatiguing contractions, indicating an appropriate adjustment of the motor commands to compensate the deficit in muscle strength. Both young and older adults were able to counteract the decreased horizontal ground reaction forces after the fatiguing task by flexing their knee to a greater extent, leading to similar decreases in the horizontal velocity of centre of mass as in the pre fatigue condition. The results demonstrate the ability of the central nervous system to rapidly modify the execution of postural corrections including mechanisms for maintaining dynamic stability.

Stability analysis of high temperature superconducting coil in liquid hydrogen

Recently, it is expected that hydrogen plays an important role in energy source including electric power in near future. Liquid hydrogen has high potential for cooling down superconducting coil wound with high temperature superconductors (HTS), such as BSCCO, YBCO. In this paper, we study stabilities of the coils wound with BSCCO tapes, which are immersed in the liquid hydrogen, and compare stability results with those cooled by liquid helium. We treat a minimum propagation zone (MPZ) theory to evaluate the coil stability considering boiling heat flux of the liquid hydrogen, and specific heat, heat conduction and resistivity of HTS materials as a function of temperature. It is found that the coil cooled by the liquid hydrogen has higher stability margin than that cooled by the liquid helium. We compare the stability margins of both coils wound with Bi-2223/Ag tape and Bi-2212/Ag tape in liquid hydrogen. As a result, it is found that the stability of Bi-2212 coil is equivalent to that of Bi-2223 coil in low and high magnetic field, while the maximum current of Bi-2212 coil exceeds a little bit that of Bi-2223 coil in both magnetic fields.

Cascaded integrator comb filters with smoothly varying coefficients for reduced delay in synchrotron feedback loops

The Rapid Cycling Synchrotron (RCS) of the J-PARC complex in Tokai, Japan, is designed to accelerate a high intensity proton beam from 181 MeV, and later 400 MeV to 3 GeV in 20 ms within the 40 ms machine cycle. The beam power up to 1 MW demands a stable beam control to avoid excessive losses and activation of the accelerator chain. The fully digital control system is based on quadrature modulation and demodulation. In the amplitude control loops standard FIR filters separate the harmonics (h � 2) and (h � 4) after down conversion. For the phase loops at (h � 2) and (h � 4), intended to damp synchrotron oscillations, the delay in a FIR filter would limit the loop stability. Cascaded integrator comb filters, also called CIC filters, provide a shorter delay because they filter the longitudinal beam signal only where it is necessary. The notches are located at multiples of the revolution frequency of the proton beam. For fixed frequency accelerator applications, digital comb filters with fixed clock frequency are widely used to improve loop stability. For variable frequency accelerator applications, as in a proton synchrotron, where the frequency swing is larger than the notch width, usually the clock frequency of the comb filter is variable and chosen to be an integer multiple of the particle revolution frequency. At J-PARC RCS, the clock frequency has to be fixed. Tracking the frequency would require a variable noninteger number of filter taps. Here we present a filter, based on the weighted output of 2 CIC filters with variable length, and one tap difference. The filter function looks like a CIC with smoothly varying coefficients, where the notches follow the revolution frequency of the proton beam. The delay of this filter is approximately half of the corresponding FIR filter, so that the phase loops have a higher stability margin.

17.13 Quantification of the level of effort at the plantarflexors and hip extensors and flexor muscles in persons with hemiparesis walking at self-selected and maximal paces

Mediolateral balance control during walking is a challenging task in post-stroke hemiparetic individuals. To detect and treat dynamic balance disorders, it is important to assess balance using reliable methods. The Berg Balance Scale (BBS), Dynamic Gait Index (DGI), margin-of-stability (MoS), and peak-to-peak range of angular-momentum (H) are some of the most commonly used measures to assess dynamic balance and fall risk in clinical and laboratory settings. However, it is not clear if these measures lead to similar conclusions. Thus, the purpose of this study was to assess dynamic balance in post-stroke hemiparetic individuals using BBS, DGI, MoS and the range of H and determine if these measure are correlated. BBS and DGI were collected from 19 individuals post-stroke. Additionally, kinematic and kinetic data were collected while the same individuals walked at their self-selected speed. MoS and the range of H were calculated in the mediolateral direction for each participant. Correlation analyses revealed moderate associations between all measures. Overall, a higher range of angular-momentum was associated with a higher MoS, wider step width and lower BBS and DGI scores, indicating poor balance control. Further, only the MoS from the paretic foot placement, but not the nonparetic foot, correlated with the other balance measures. Although moderate correlations existed between all the balance measures, these findings do not necessarily advocate the use of a single measure as each test may assess different constructs of dynamic balance. These findings have important implications for the use and interpretation of dynamic balance assessments.

Muscle Activity, Internal Loads, and Stability of the Human Spine in Standing Postures: Combined Model and In Vivo Studies

The load in active and passive spinal components as well as the stability margin in standing postures +/- load in hands are studied using both computational model and in vivo studies. To investigate muscle activity, spinal loads, and system stability in standing postures. Study of the human trunk yields a redundant system, the satisfactory solution of which remains yet to be done. Existing biomechanical models are often oversimplified or attempt to solve the problem by equilibrium of loads at only one cross section along the spine. In vivo measurements are performed to obtain kinematics (by skin markers) as input data into model and EMG activity (by surface electrodes) for validation of predictions. A thoracolumbar model, while accounting for nonlinear ligamentous properties and trunk musculature, solved the redundant active-passive system by a novel kinematics-based approach that used both the posture and gravity/external loads as input data. In both studies, neutral standing posture was considered with weights up to 380 N held in hands with arms extended close to the body either in front or on sides. Predicted muscle forces were in satisfactory agreement with measured EMG activities. The activity in extensor muscles significantly increased with the load magnitude when held in front, a trend that disappeared as loads were held on sides. Abdominal muscles remained relatively silent. Large compression forces of approximately 2000 N were computed in lower lumbar levels when 380 N was held in front. Coactivity in abdominal muscles markedly increased internal loads and stability margin. A tradeoff exists between lower loads in passive tissues (i.e., tissue risk of failure) and higher stability margins as both increase with greater muscle coactivation. Greater muscle activity observed under load held in front did not necessarily yield larger stability margin as the position of load appeared to play an important role as well. The strength of the proposed model is in realistic consideration of both passive-active structures under postures and gravity/external loads, yielding results that satisfy kinematics, equilibrium, and stability requirements in all directions along the spine.