High Stability Margin Research Articles

Identification of novel CA IX inhibitor: Pharmacophore modeling, docking, DFT, and dynamic simulation

Human Carbonic anhydrase IX (hCA IX) is found to be an essential biomarker for the treatment of hypoxic tumors in both the early and metastatic stages of cancer. Due to its active function in maintaining pH levels and overexpression in hypoxic conditions, hCA IX inhibitors can be a potential candidate specifically designed to target cancer development at various stages. In search of selective hCA IX inhibitors, we developed a pharmacophore model from the existing natural product inhibitors with IC50 values less than 50 nm. The identified hit molecules were then investigated on protein-ligand interactions using molecular docking experiments followed by molecular dynamics simulations. Among the zinc database 186 hits with an RMSD value less than 1 were obtained, indicating good contact with key residues HIS94, HIS96, HIS119, THR199, and ZN301 required for optimum activity. The top three compounds were subjected to molecular dynamics simulations for 100 ns to know the protein-ligand complex stability. Based on the obtained MD simulation results, binding free energies are calculated. Density Functional Theory (DFT) studies confirmed the energy variation between the Highest Occupied Molecular Orbital (HOMO) and Lowest Unoccupied Molecular Orbital (LUMO). The current study has led to the discovery of lead compounds that show considerable promise as hCA IX inhibitors and suggests that three compounds with special molecular features are more likely to be better-inhibiting hCA IX. Compound S35, characterized by a higher stability margin and a smaller energy gap in quantum studies, is an ideal candidate for selective inhibition of CA IX.

Ligand based pharmacophore modelling and integrated computational approaches in the quest for small molecule inhibitors against hCA IX.

Carbonic anhydrase IX is an important biomarker to fight hypoxic tumours in both initial and metastatic stages of many forms of cancer. Overexpression of hCA IX in the hypoxic environment, has an active role in pH maintenance and makes the hCA IX a better target for the inhibitors targeting specific types of cancer stages. Being a member of the carbonic anhydrase family and having sixteen isoforms, it is important to have a selective inhibition of hCA IX to limit the disruption in the biological and metabolic pathways where other isoforms of hCA are localised and to avoid the other toxicity and adverse effects we try to find selective hCA IX inhibitors from a natural derivative. In the process of finding selective hCA inhibitors we developed a pharmacophore model based on existing inhibitors with IC50 values of less than 50 nm, which is then validated with the external decoy set and used for database searching followed by virtual screening to identify the hits based on the pharmacophore fit score and RMSD. Molecular docking studies were performed to identify protein ligand interaction and molecular dynamics simulation studies to analyse the stability of the complex and DFT studies were carried out. The initial screening yielded 43 hits with the RMSD value less than 1, which when subjected to docking exhibited very good interaction with key residues ZN301, HIS94, HIS96 and HIS119. The top 4 compounds in the molecular dynamics simulation studies for 100 ns provided useful insights on the stability of the complex and the DFT studies confirmed the energy variation between HOMO and LUMO is within an acceptable range. An average binding score of -7.8 Kcal mol-1 for the lead compounds and high stability margin in the dynamics study concludes that these lead compounds demonstrated outstanding potential for hCA IX inhibitory action theoretically and that further experimental studies for selective inhibition are inevitable.

Torsional active-passive dampers on rotational machinery

Recent hybrid control concepts, mixing active control with a fail-safe passive device, have drawn attractivity as they benefit from a theoretical hyperstability, ensuring very high stability margins. However, they have been experimentally tested until now only for the control of bending vibrations. The main purpose of this paper is to prove the performance of such systems also on torsional vibrations, with a specifically designed electromagnetic device, in order to contemplate the possibility to use them also for rotating machinery. As these control laws are model-free and require the adjustment of a few parameters related to the passive damper only, we demonstrate here their ease of integration for an application in rotating machines and we exhibit a direct experimental validation on an academic test bench. The latter which enables this experience despite the peculiarities of the rotation is therefore described, and the torsion reduction permitted by the hybrid control laws is assessed. A comparison between two of the newest hybrid control laws is also provided, proving the robustness and performance of these two hyperstable control laws. The main control parameters have been investigated and the performances are given through classical indicators but also in terms of energy consumption.

An Improved Frequency-Adaptive Virtual Variable Sampling-Based Repetitive Control for an Active Power Filter

To eliminate the harmonics caused by nonlinear loads, repetitive controllers are widely applied as current controllers for active power filters (APF). In practice, a variation in grid frequency leads to the appearance of a fractional-order delay filter. As a result, the resonant frequency of the repetitive controller will deviate from the fundamental frequency and the controller cannot compensate for harmonics accurately. To solve this problem, an improved frequency-adaptive repetitive controller based on virtual variable sampling (IMFA-VVS-RC) for APF is proposed in this paper. To enhance the system stability margin, the proposed RC introduces an infinite impulse response (IIR) low-pass filter. The proposed RC has a high stability margin at high frequencies due to the low gain of the IIR low-pass filter in the region above the cutoff frequency. In this way, the influence of model uncertainty and parameter uncertainty on system stability are reduced at high frequencies. At the same time, compared with the conventional repetitive controller (CRC), the proposed RC for APF has a better harmonic suppression ability when the frequency varies. Experiments have verified the effectiveness of the scheme adopted for APF.

A Zero-Tracking SOGI-Based Frequency-Locked Loop

This article proposes a zero-tracking second-order generalized integrator-based frequency-locked loop (ZT-SOGI-FLL) with better dynamics and stability than the conventional SOGI-FLL. Unlike the internal model principle, the ZT-SOGI-FLL reconstructs the input signal under frequency deviation by tracking the zeros. Also, it has the same compact structure as the conventional SOGI-FLL. A linearized model is then derived to show that the ZT-SOGI-FLL is always an over-damped or a critical damper system. More importantly, the derived model proves that ZT-SOGI-FLL has a larger damping factor and higher bandwidth than SOGI-FLL when the same SOGI gain is tuned. Finally, the input-to-state stability of ZT-SOGI-FLL is analyzed to support the high robustness to severe grid faults. Comparative experimental results show that ZT-SOGI-FLL has a faster and smoother dynamic response with a higher stability margin than some commonly used SOGI-based methods under grid faults.

Transient stability enhancement control strategy for droop-controlled voltage source converter

The droop-controlled voltage source converter (VSC) can support the voltage at the point of common coupling (PCC) and realize frequency adjustment by simulating the swing equation of the synchronous generator (SG) and has a higher stability margin in the extremely weak grid. Under large-signal disturbances such as short-circuit faults in the power grid, the active power output of the VSC decreases, and the continuous increase of the power angle causes the VSC to have a power angle instability similar to that of the SG. In addition, in order to ensure the safety of the equipment, the VSC control system contains a current-limiting strategy, which will cause a large change in the transient external characteristics of the VSC under large-signal disturbances. This paper firstly studies the transient synchronization mechanism between VSC and the power grid under grid faults and expounds on the reasons for the transient instability of VSC with the current limiting strategy. Based on the above analysis, a transient enhancement strategy is proposed to reduce the imbalance of active power and reduce the risk of VSC transient instability. It is worth mentioning that the proposed transient control strategy will not affect the steady-state output characteristics of the VSC. Finally, simulation and experiment verify the effectiveness of the proposed control strategy.

Accurate Sensorless Multistable Microsystem With a Single Actuator

A compact and monolithic multistable microsystem was developed to realize the accurate positioning on the microscale by using a single actuator, without any positioning sensors. The mobile part can be maintained in several stable positions at rest with accurate intermediate steps and a high stability margin at each position. Switching between stable positions can be performed in both directions on one axis by using a single actuator. Using the mechanical intelligence encoded in the internal mechanisms, the mobile part position can be switched one step forward or back to the initial position through a single activation of the actuator. Multistable devices were designed and fabricated based on the working principle, and their satisfactory operation, with effective holding at stable positions and switching in two directions, was experimentally demonstrated.

RESEARCH STABILITY OF MAST ROPE-PIN DRIVE OF DOWNHOLE SUCKER-ROD PUMP

This scientific article is about the research stability of mast rope-pin drive of downhole sucker-rod pump. The design of downhole sucker-rod pump drive in the form of a mast, which is mounted directly on the column of the wellhead has been proposed. Calculations of the frequency of natural vibrations of the system from the rotation of the cranks during the breakage of the rods are performed, which completely eliminates the possibility of resonant phenomena that destroy the mast. This proves that the mast drive has a high stability margin. Key words: downhole sucker-rod pump drive, mast, long stroke drive.

Theoretical Disquisition on the Static and Dynamic Characteristics of an Adaptive Stepped Hydrostatic Thrust Bearing with a Displacement Compensator

Stepped hydrostatic thrust bearings used in metal-cutting machines are characterized by high load capacity and damping, which ensure the stable operation of structures. However, in comparison with throttle thrust bearings, they have a high compliance. It is preferable that, in addition to the main bearing function, a modern hydrostatic bearing has the ability to provide low (including negative) compliance for the implementation of an adaptive function in order to actively compensate for the deformation of the machine resilient system, thereby increasing the accuracy of metalworking. This paper considers the design of a stepped hydrostatic thrust bearing, which, in order to reduce the compliance to negative values, features a technical improvement consisting of the use of an active displacement compensator on an elastic suspension. In this paper, the results of mathematical modeling and theoretical research of stationary and non-stationary modes of operation of the adaptive thrust bearing are presented. The possibility of a significant reduction in the static compliance of the structure, including the negative compliance values, is shown. It was found that negative compliance is provided in a wide range of loads, which can be up to 80% of the range of permissible bearing loads. The study of the dynamic characteristics showed that with a targeted selection of parameters that ensure optimal performance, the adaptive thrust bearing is able to operate stably in the entire range of permissible loads. It has been established that an adaptive stepped hydrostatic thrust bearing with a displacement compensator has a high stability margin, sufficient to ensure its operability when implementing the adaptive function.

Power Stability Analysis and Evaluation Criteria of Dual-Infeed HVDC with LCC-HVDC and VSC-HVDC

This paper analyzes the power stability of the hybrid dual-infeed high-voltage direct-current (HVDC) system containing a line commutated converter-based HVDC (LCC-HVDC) and a voltage source converter-based HVDC (VSC-HVDC). First, the concept and the calculation method of power stability for the hybrid dual-infeed HVDC system are introduced. Second, the influence of VSC-HVDC on the power stability of the system is investigated. Third, the relationship between the power stability and the effective short circuit ratio (ESCR) is discussed under different system parameters. Then, the value range of the critical effective short circuit ratio is determined. Finally, the evaluation criteria of power stability are proposed. The results show that the evaluation criteria of the single-infeed LCC-HVDC system can still be used, if the VSC-HVDC is in constant AC voltage control mode; if the VSC-HVDC is in constant reactive power control mode, the hybrid dual-infeed HVDC system cannot operate stably when the ESCR is less than 2.0 and can operate stably with high power stability margin when the ESCR is greater than 3.0. The ESCR index can still be used to measure the power stability of the hybrid dual-infeed HVDC system.

Fast Current Control Without Computational Delay by Minimizing Update Latency

This letter proposes a fast current control scheme for ac drives by minimizing the latency from current sampling to the updation of a pulsewidth-modulation (PWM) command. By reconstructing the architecture of a linear controller, the minimal necessary steps to obtain the output voltage are deduced as the <i>primary calculation</i>, and the other computation work of the current loop is done in the <i>postcalculation</i> after updating the PWM command. With the proposed method, the computational delay is reduced with negligible loss in the available modulation index, and a higher stability margin or system bandwidth is realized. Also, the developed controller structure exhibits inherited antiwindup characteristics. The technique has been validated by experimental investigations on a permanent magnet synchronous motor.

Theoretical Investigation on Performance Characteristics of Aerostatic Journal Bearings with Active Displacement Compensator

Active aerostatic bearings are capable of providing negative compliance, which can be successfully used to automatically compensate for deformation of the machine tool system in order to reduce the time and improve the quality of metalworking. The article considers an aerostatic radial bearing with external combined throttling systems and an elastic displacement compensator, which is an alternative to aerostatic bearings with air flow rate compensators. The results of the mathematical modeling and theoretical research of stationary and nonstationary modes of operation of bearings with slotted and diaphragm throttling systems are presented. A counter-matrix sweep method has been developed for solving linear and nonlinear boundary value problems in partial derivatives with respect to the function of the square of the pressure in the bearing gap and inter-throttling bearing cavities for any values of the relative shaft eccentricity. A numerical method is proposed for calculating the dynamic quality criteria, and the transfer function of the dynamic compliance of a bearing with small displacements is considered as a linear automatic control system with distributed parameters. An experimental verification of the theoretical characteristics of the bearing was carried out, which showed a satisfactory correspondence among the compared data. It is shown that bearings with a throttle system have the best quantitative and qualitative load characteristics. The possibility of optimal determination of the values of a number of important parameters that provide the bearing with optimal performance and a high stability margin is established. It is shown that bearings with an elastic suspension of the movable sleeve allow one to compensate for significant movements, which can be larger than the size of the air gap by an order of magnitude or more. In these conditions, similar bearings with air flow compensators would be obviously inoperative.

A fast and robust reference current generation algorithm for three-phase shunt active power filter

The identification of the reference currents constitutes an important part of the control of the active power filter. This part requires an accurate estimation of the frequency, phase, and proper extraction of the load current harmonics. This makes the modeling more difficult and requests a rigorous selection of techniques to be used. For the sake of simplicity, the direct method is motivated by the need for the simplicity and flexibility than the existing techniques such as the instantaneous power theory and diphase currents method. However, this method requires a robust phase-locked loop to extract the unity voltages and a robust controller to estimate the magnitude of the source current. To this end, this paper proposes the hybrid phase-locked loop (HPLL) as a good option mainly because 1) it achieves zero phase error under frequency drifts, 2) Fast dynamic response, 3) totally block the DC offset, 4) From the control point of view, it is a type 1 control system which results in high stability margin. To the best of authors’ knowledge, the HPLL has not been used in active power filter yet. Furthermore, a neural PI regulator is used to estimate the magnitude of the source current. Simulation results show the efficiency of the proposed technique and illustrate all its interesting features. For the sake of comparison, the proposed method is compared to other advanced techniques.

Mitigation and analysis of sub synchronous resonance for DFIG based wind farm using STATCOM controller

Large-scale integration in transmission networks has led to several challenges; one of which is the need for increased transmission capacity to transport a bulk amount of power without affecting the voltage. In case of transmitting power to large distances series compensation is used along with Doubly-Fed Induction Generator (DFIG) wind turbine and it is simulated using MATLAB Simulink and the results obtained are validated. DFIG wind turbine is operated at normal operating state. Due to series compensation Sub Synchronous Resonance (SSR) occurs which is quite harmful to the wind generator because of the frequency variation, the gear mechanics of the generator gets damaged. When a larger disturbance occurred in any part of the study system at any bus bar, the effect of SSR would be more harmful. The effect of SSR depends on the compensation percentage we used for series compensation. SSR can be mitigated by using Flexible AC transmission System (FACTS) devices such as STATCOM, SVC, and SSSC. We use STATCOM because of its high stability margin, low switching loss and variable voltage rating. Whenever disturbance occurs in the transmission line then STATCOM comes into action by supplying reactive power and absorbing real power thus maintaining the system stability. The STATCOM controller gives pulse to the STATCOM for respective action such as real power absorption or reactive power injection. Thus the Wind turbine is protected and system stability is maintained.

Structured H∞ Control of an Electric Power Steering System

Electric power steering (EPS) systems are prone to oscillations because of a very small phase angle margin, so a stable controller is required to increase the stability margin. In addition, the EPS system has parameter disturbances in the gain of the torque map under different conditions, which requires a certain degree of robustness in the control design. This paper synthesizes the multidimensional performance requirements considering the stability margin, robustness, and bandwidth of the system to form an H∞ optimization matrix with multidimensional performance output in using the structured H∞ control design. The structured H∞ controller not only retains the characteristics of traditional H∞ controllers with excellent robust performance and high stability margin but also has a lower order, which can be better applied in practice. Based on the performance requirements of the system and practical implementation, the structured H∞ controllers with different orders were designed, and the feasibility of the structured controller was confirmed through comparison and theoretical analysis.

Harmonic Instability and Amplification for Grid-Connected Inverter With Voltage Harmonics Compensation Considering Phase-Locked Loop

Grid-connected inverters (GCIs) with a voltage harmonics compensation function (VHCF) have been widely applied to ensure the quality of bus voltages in weak grids, while in the analysis and design process of VHCF-GCIs, the influence of the phase-locked loop (PLL) is usually ignored. To cover this gap, the harmonic instability and the harmonic amplification of the VHCF-GCI considering the influence of the PLL are explored. With the small-signal model of the PLL, the mathematical descriptions of the influence of PLL dynamics on the harmonic impedance control loop and the current control loop are derived, and the small-signal output impedance of the VHCF-GCI including the PLL dynamics is presented. Based on the impedance-based analysis, the stability and the harmonic suppression performance of the VHCF-GCI with different parameters of stability margin and the bandwidth for the PLL are studied. The study results reveal that differing from the conventional conclusion that the PLL with a high bandwidth and a high stability margin ensures stability for the GCI in a low-frequency domain, the PLL with a high bandwidth and a high stability margin brings about the instability and the harmonic amplification for the VHCF-GCI in the harmonic-frequency domain. Thus, to attain stability and harmonic suppression performance of the VHCF-GCI, the design approach for PLL parameters in a full-frequency range is presented. Lastly, the theoretical analysis is verified by simulation and experimental results.

A comparative study of hybrid simulation and dynamical substructuring system schemes for shake table tests

In shaking table experiments, the dynamics of the shake table is strongly affected by a specimen to be tested, particularly when the specimen is very heavy. This is also the case when substructuring experiments are implemented by the shaking tables. This study numerically examines two substructuring schemes applied in shaking table experiments. Hybrid simulation scheme, which is commonly used in substructuring experiments, directly uses the output of the numerical substructure as the input to the physical substructure. Dynamical substructuring system (DSS) scheme, which requires more complicated control design, determines the control input to minimize control error obtained by outputs of numerical and physical substructures. This study has found that, in shaking experiments, the stability of the hybrid simulation scheme is very sensitive to a pure time delay. It is also found that DSS scheme can provide much higher stability margin against the pure time delay than the hybrid simulation scheme. The effort to rather complicated controller design of DSS scheme will be rewarded at actual control practices.

A modernistic PLL based on feed forward frequency estimator with selective harmonic pre filter for grid imperfection

This paper describes a synchronisation technique to track phase and frequency of input signal under abnormal grid condition like frequency variation. Proposed control topology consist of a conventional synchronous reference frame phase locked loop (SRF PLL) type-2 along with a feed forward frequency estimator loop with selective harmonic filtering technique. It eliminates phase and frequency error under wide frequency deviation. Phase error is fed into proportional integral regulator which reduces it to zero. It is observed that faster and better dynamic performance is achieved by frequency estimator with higher stability margin. The stability is evaluated by bode plot. An estimated frequency is smooth and close to fundamental frequency as it uses the information from all three phase to estimate a single value for frequency. The feed forward action is implemented with pre filtering stage to improve dynamic performance of system. Simulation results are presented to validate effectiveness of proposed technique.

Study on high robustness and fast dynamic response synchronous rectifier buck converter

Here, a novel control strategy combined by constant-frequency V 2 control and H ∞ robust control is proposed and tested in a buck converter. The control strategy not only demonstrates the fast transient response of ordinary V 2 control, but also shows strong robustness with different input conditions. The main drawbacks of V 2 control, such as high interference susceptibility and low input perturbation stability margin, can be improved by introducing H ∞ robust strategy into the controller design. During the analysis and design process, the original controlled system based on the V 2 control is modelled at first, then the H ∞ weighting functions are calculated according to the small-signal analysis of V 2 controlled buck converter. After that, the H ∞ robust compensator parameters based on constant-frequency V 2 control are calculated. Finally, a buck converter controlled by the proposed control strategy is built and tested, with 48 V input voltage, 12 V output voltage, and 120 W output power. The experimental results demonstrate the effectiveness of the converter with the proposed control, which also proves the proposed method has great potential for high robustness and fast dynamic response applications.

Small-signal stability improvement of an islanded microgrid with electronically-interfaced distributed energy resources in the presence of parametric uncertainties

This paper deals with the concept of small signal stability (SSS) issue in an electronically-interfaced microgrid with large penetration of distributed energy resources (DERs) subjected to the parametric uncertainties. Utilizing a method to enhance SSS of the system stemmed from industrial experiences is the first achievement of this study, which is realized by employing two extra feed-forward loops in the power controller section of DER unit. This control structure damps the active and reactive power oscillations by shifting the dominant eigenvalues to the left half-plane (LHP) in case of any changes in the operating point or droop coefficients. For investigating the impacts of parametric uncertainties on the overall stability, the basic theorem of the proposed robust control strategy, frequency-domain modeling of the microgrid, robustness and close-loop stability analyses are outlined. This method shows that improving the power control section topology results in higher stability margins in case of uncertain load perturbations. Furthermore, it demonstrates how much load variations are allowed to preserve SSS of the system in the frequency-domain which is a more convenient approach and takes less time than time-domain simulations. Finally, time-domain simulations conducted in MATLAB/Simulink on the sample test system will show proper results of the frequency-domain findings.