Improvement Of Stability Margin Research Articles

Spindle speed variation technique in turning operations: Modeling and real implementation

Chatter is still one of the most challenging problems in machining vibrations. Researchers have focused their efforts to prevent, avoid or reduce chatter vibrations by introducing more accurate predictive physical methods. Among them, the techniques based on varying the rotational speed of the spindle (or SSV, Spindle Speed ​​Variation) have gained great relevance. However, several problems need to be addressed due to technical and practical reasons. On one hand, they can generate harmful overheating of the spindle especially at high speeds. On the other hand, the machine may be unable to perform the interpolation properly. Moreover, it is not trivial to select the most appropriate tuning parameters. This paper conducts a study of the real implementation of the SSV technique in turning systems. First, a stability model based on perturbation theory was developed for simulation purposes. Secondly, the procedure to realistically implement the technique in a conventional turning center was tested and developed. The balance between the improved stability margins and acceptable behavior of the spindle is ensured by energy consumption measurements. Mathematical model shows good agreement with experimental cutting tests.

Stability Margin Improvement of Vehicular Platoon Considering Undirected Topology and Asymmetric Control

The platooning of autonomous vehicles has the potential to significantly improve traffic capacity, enhance highway safety, and reduce fuel consumption. This paper studies the scalability limitations of large-scale vehicular platoons moving in rigid formation, and proposes two basic ways to improve stability margins, i.e., enlarging information topology and employing asymmetric control. A vehicular platoon is considered as a combination of four components: 1) node dynamics; 2) decentralized controller; 3) information flow topology; and 4) formation geometry. Tools, such as the algebraic graph theory and matrix factorization technique, are employed to model and analyze scalability limitations. The major findings include: 1) under linear identical decentralized controllers, the stability thresholds of control gains are explicitly established for platoons under undirected topologies. It is proved that the stability margins decay to zero as the platoon size increases unless there is a large number of following vehicles pinned to the leader and 2) the stability margins of vehicular platoons under bidirectional topologies using asymmetric controllers are always bounded away from zero and independent of the platoon size. Simulations with a platoon of passenger cars are used to demonstrate the findings.

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.

A Synchrophasor Based Optimal Voltage Control Scheme with Successive Voltage Stability Margin Improvement

This paper proposes an optimal control scheme based on a synchronized phasor (synchrophasor) for power system secondary voltage control. The framework covers voltage stability monitoring and control. Specifically, a voltage stability margin estimation algorithm is developed and built in the newly designed adaptive secondary voltage control (ASVC) method to achieve more reliable and efficient voltage regulation in power systems. This new approach is applied to improve voltage profile across the entire power grid by an optimized plan for VAR (reactive power) sources allocation; therefore, voltage stability margin of a power system can be increased to reduce the risk of voltage collapse. An extensive simulation study on the IEEE 30-bus test system is carried out to demonstrate the feasibility and effectiveness of the proposed scheme.

Assessing the Effective Parameters on Operation Improvement of SCIG based Wind Farms Connected to Network

As the penetration level of wind energy in power system increases, stable operation of power system would get impacted by wind turbine's characteristics. The stability issue derived from this fact that the squirrel cage induction generators (SCIG) used in these turbines potentially would cause voltage drop and voltage stability problems in network, regarding to its reactive power absorption tendency. Thus, detection of effective parameters in increment and reduction of unstability occurrence probability could make SCIG based wind farm predictable and eventually cause improvement in voltage stability margin of whole of power system. In this paper, effective factors of wind farm operation connected to the network are assessed and studied in two sections. The first section includes parameters which depend upon substantive and functional characteristics of induction generator and could threat the stability of power system. The second section containing such parameters and characteristics of power system as which are able to affect the stable operation of SCIGs and can be lead to instability in whole of the power system. Eventually reactive power has been identified as more effective option in system designing. The studies are concentrated on voltage stability and small signal stability. In order to make the results more practical, the 660KW inductive generator which is widely used in Iran is studied in this paper.

An optimised FOPID controller for dynamic voltage stability and reactive power management in a stand-alone micro grid

This paper proposes the application of fractional order PID controller (FOPID) for reactive power compensation and stability analysis in a stand-alone micro grid. For enhancement of voltage stability and reactive compensation of the isolated system, a SVC based controller has been incorporated. This paper emphasizes the role of fractional PID based SVC controller for reactive power management and improved stability in the stand alone micro grid, as it provides a special advantage of having two more degree of freedom for accurate tuning in comparison with the conventional controller The system performance, particularly the variations in different parameters values are studied properly with different input parameters and loading conditions. Further improvement of stability margin and optimisation of the system parameters have been achieved by the controller, based on Imperialist competitive algorithm.

Improvement of small signal stability margin and transient response in inverter-dominated microgrids

Abstract Inverter-based distributed generators (DGs) based on renewable sources are widely used in microgrids. Most of these sources operate in droop control mode for effective load sharing. Higher droop is desired in these systems to improve dynamic power sharing, however, such systems suffer from stability issues. Stability margin of a system having different combinations of inverter and synchronous generator-based sources is compared in this work. A microgrid is modeled here with three DGs on a modified IEEE 13-bus system using state space approach. The study showed that lack of stability of inverters restricts the use of higher droop gains, resulting in poor power-sharing dynamics. A modified droop control is proposed to improve transient response and stability margin in such cases. The results are validated with time domain simulations using Simulink/MATLAB.

Modelling and attitude control of a miniature ducted fan UAV

Ducted fan UAV (Unmanned Aerial Vehicle) has broad application prospective. To study flight dynamics and flight control problems of ducted fan UAV, this paper developed a refined aerodynamic model for the ducted fan UAV by wind tunnel test. Based on that, the flight dynamic model of ducted fan UAV was built, which showed features as being nonlinear and unstable and of strong coupling and, therefore, inspired this paper to adopt a strong adaptive control approach based on decoupling to perform attitude control on the ducted fan vehicle. The approach combined model tracking control and reference model adaptive control together, which highly reduced attitude tracking error and improved stability margin of the closed-loop system, and was successfully applied in flight control simulation and flight tests of the vehicle.

Time-Delayed Control of SISO Systems for Improved Stability Margins

While time delays typically lead to poor control performance, and even instability, previous research shows that time delays can, in some cases, be beneficial. This paper presents a new benefit of time-delayed control (TDC) for single-input single-output (SISO) linear time invariant (LTI) systems: it can be used to improve robustness. Time delays can be used to approximate state derivative feedback (SSD), which together with state feedback (SF) can reduce sensitivity and improve stability margins. Additional sensors are not required since the state derivatives are approximated using available measurements and time delays. A systematic design approach, based on solution of delay differential equations (DDEs) using the Lambert W method, is presented using a scalar example. The method is then applied to both single- and two-degree of freedom (DOF) mechanical systems. The simulation results demonstrate excellent performance with improved stability margins.

Optimal placement of DG in radial distribution systems based on new voltage stability index under load growth

Abstract This paper presents a comparison of Novel Power Loss Sensitivity, Power Stability Index (PSI), and proposed voltage stability index (VSI) methods for optimal location and sizing of distributed generation (DG) in radial distribution network. The main contribution of the paper is: (i) optimal placement of DGs based on Novel Power Loss Sensitivity and PSI methods, (ii) proposed voltage stability index method for optimal DG placement, (iii) comparison of sensitivity methods for DG location and their size calculations, (iv) optimal placement of DG in the presence of load growth, (v) impact of DG placement at combined load power factor, (vii) impact of DG on voltage stability margin improvement. Voltage profile, the real and reactive powers intake by the grid, real and reactive power flow patterns, cost of energy losses, savings in cost of energy loss and cost of power obtained from DGs are determined. The results show the importance of installing the suitable size of DG at the suitable location. The results are obtained with all sensitivity based methods on the IEEE 12-bus, modified 12-bus, 69-bus and 85-bus test systems.

Time-Delayed Vibration Control Of Two Degree-Of-Freedom Mechanical System For Improved Stability Margins

Time-delayed vibration control is used for a two degree-of-freedom mechanical system to approximate state-derivative feedback and to reduce sensitivity and improve stability margins. Additional sensors are not required since state-derivatives are approximated using available measurements and time delays. A systematic design approach, based on solution of delay differential equations using the Lambert W method, is presented. The simulation results demonstrate excellent performance with improved stability margins over state feedback control only.

Preventive Control Stability Via Neural Network Sensitivity

This paper discusses the power systems stability margin improvement by means of preventive control based on generation re-dispatch using a neural sensitivity model. This model uses multilayer perceptron networks with memory structure in the input layer. The training of this model is made with temporal data samples from time domain simulations, incorporating information about the dynamic behavior of the system, unlike the methods proposed in the literature in which the pre-fault system data are used instead. The sensitivity is used as a guideline in selecting the most effective set of generators in the reallocation of the amount of active power capable of increasing system security. The effectiveness of the proposed methodology has been demonstrated through the application to a large system.

Nonlinear Controller With the Dead-Zone and Saturation for Optical Disk Drive Systems in the Presence of External Shocks

In this paper, a nonlinear controller with dead-zone and saturation is proposed in order to improve the antishock performance. Since using dead-zone and saturation improves the stability margin, we can use a higher gain for the dead-zone than in the previous methods. In addition, we find that the overall controller with a linear lead-lag controller has an improved stability margin over controllers with the PID linear control in this application. The experimental results confirmed that the proposed method provides improved protection against external shock.

Voltage Stability and Loading Margin Improvement in Power System by Optimal Placement of SVC

Proper installation of flexible ac transmission systems (FACTS) in existing transmission networks can improve transmission system loading margin (LM) to a certain degree and reduce network expansion cost. In the paper, under each contingency with high risk index (RI) value, the modal analysis (MA) technique is used to determine which buses need static var compensator (SVC) installation, and with maximum LM and minimum SVC installation cost composed into the multiobjective function the optimal LM enhancement problem is formulated as a multi-objective optimization problem (MOP) and solved by using the fitness sharing multi-objective particle swarm optimization (MOPSO) algorithm for a Pareto front set. In the Pareto front set for each considered contingency, the solution with the biggest performance index value is determined for SVC installation. Finally, an SVC installation scheme derived from the union of the SVC installations for all considered contingencies is recommended for LM enhancement. The proposed method is validated on the IEEE 30-bus reliability test system (RTS) and a practical power system.

STABILITY OF A MULTI-MACHINE POWER SYSTEM EQUIPPED WITH LQG CONTROLLERS

This paper presents the design of a novel control strategy of a Power System Stabilizer (PSS) for generators in multimachine electric power systems using the Linear Quadratic Gawssian (LQG) control technique. The controller combines both the excitation and governor control loops. The technique is applied using a sample power system which includes a number of generators connected to a large power system. An optimal full state controller has been designed and a state observer is included using Kalman Filter theory. A comprehensive stability analysis and time domain performance analysis of the system with the new controller have been presented. The results prove the robustness and powerful of proposed LQG controller in improve stability margins and adding positive with a noticeable reduction in settling time.

Improving Stability Margin in Electric Power Systems by Linear Quadratic Regulator and Disturbance Model

This article deals with the design of a voltage controller using state feedback with integral action for an electric power system. The control system is tailored to achieve zero error of the voltage magnitude. First the transient response and the attraction region of the non-linear model is improved by using a state-feedback linear quadratic regulator controller. On the basis of this design, the integral action is designed as a second loop according to the internal model controller. Both feedback control systems are compared in terms of the robustness of the closed-loop dynamic response and the existing coupling between active and reactive powers when the load reactive power is changed. Simulation results show that the integral controller achieves zero voltage error even for the non-linear system.

IMPROVEMENT OF TRANSIENT STABILITY OF POWER SYSTEM USING SOLID STATE CIRCUIT BREAKER

The aim of this study is to use a solid state circuit breaker for the improvement of transient stability margin. Long distance AC transmission is often subject to stability problems, which limits the transmission capability. Interconnected power systems suffer from weakly damped swings between synchronous generator and sub systems. The development of modern power system has lead to an increasing complexity in the study of power system and also presents new challenges to power system stability and in particular to the aspects of transient stability and small signal stability. Transient stability control plays a significant role in ensuring the stable operation of power systems in the event of large disturbances and faults and is thus a significant area of research. This study investigates the improvement of transient stability of a two area system, using Solid State Circuit Breaker which is capable of fast switching in case of faults and thus controlling the real and reactive power flows in a faulted transmission line. Simulations are carried out in Matlab/Simulink environment for two area system model with Solid State Circuit Breaker to analyze the effects of SSCB on transient stability performance of the system. From the simulation results, swing curves of the three phase faulted power system without SSCB gets increases monotonically and thus the system can be considered as unstable whereas the swing curves of the system with an SSCB reaches to a steady state.

Analysis and improvement of performance in LCL filter‐based PWM rectifier/inverter application using hybrid damping approach

A hybrid passive-active damping solution with improved system stability margin and enhanced dynamic performance is proposed for high power grid interactive converters. In grid connected active rectifier/inverter application, line side LCL filter improves the high frequency attenuation and makes the converter compatible with the stringent grid power quality regulations. Passive damping though offers a simple and reliable solution but it reduces overall converter efficiency. Active damping solutions do not increase the system losses but can guarantee the stable operation up to a certain speed of dynamic response which is limited by the maximum bandwidth of the current controller. This paper examines this limit and introduces a concept of hybrid passive-active damping solution with improved stability margin and high dynamic performance for line side LCL filter based active rectifier/inverter applications. A detailed design, analysis of the hybrid approach and trade-off between system losses and dynamic performance in grid connected applications are reported. Simulation and experimental results from a 10 kVA prototype demonstrate the effectiveness of the proposed solution. An analytical study on system stability and dynamic response with the variations of various controller and passive filter parameters is presented.

Relaxed Ziegler-Nichols Closed Loop Tuning of PI Controllers

A modication of the PI setting of the Ziegler-Nichols closed loop tuning method is proposed. The modication is based on a combination of the Skogestad SIMC tuning formulas for \integrator plus time-delay processes with the Ziegler-Nichols tuning formulas assuming that the process is modeled as an \integrator plus time-delay process. The resulting PI settings provide improved stability margins compared with those obtained with the original Ziegler-Nichols PI settings. Compared with the well-known TyreusLuyben PI settings, the proposed PI settings give improved disturbance compensation. For processes with zero or a negligible time-delay, but with some lags in the form of time-constants, tuning based on ultimate gain and ultimate period may give poor results. Successful PI settings for such processes are proposed.

Optimization of Journal Bearing Profile for Higher Dynamic Stability Limits

This work presents an optimization procedure to find bearing profiles that improve stability margins of rotor-bearing systems. The profile is defined by control points and cubic splines. Stability margins are estimated using bearing dynamic coefficients, and obtained solutions are analyzed as a function of the number of control points and of the Sommerfeld number at optimization. Results show the feasibility of finding shapes for the bearing that significantly improve the stability margins. Some of the obtained solutions overcome the stability margins of conventional bearings, such as the journal bearing and preloaded bearings with 0.5 and 0.67 preload. A time domain simulation of a flexible shaft rotating system supported by such bearings corroborates the results.