Calculation Of Stability Margin Research Articles

Bus-impedance-based stability improvement of DC power distribution system considering dynamic performance

A direct-current (DC) power distribution system (PDS) enjoys the benefits of flexibility and efficiency. However, increasing stability issues occur due to the interaction among feedback-controlled converters in PDS. For ensuring stability, various impedance-based method is discussed by scholars. Unlike the traditional analysis method based on minor-loop-gain (MLG), Passivity-Based Stability Criteria (PBSC) is a novel bus-impedance-based method. Although PBSC is a promising measure of stability analysis for the distribution system, the difficulty of stability margin calculation and dynamic characteristic analysis limits its application. Therefore, the relative stability margin (RSM) is proposed to assess the damping levels. Based on RSM, the dynamic performance optimization algorithm (DPOA) is presented to optimize the proposed MMC impedance reshaping (MIR) control. Compared with the past stability analysis, dynamic behaviour is considered in this paper when stability improvement is implemented. By means of the MIR and DPOA, the stability and dynamic performance of the distribution system are enhanced. Simulation results confirm the validity of the proposed viewpoints and method.

Voltage Stability Analysis in Medium-Voltage Distribution Networks Using a Second-Order Cone Approximation

This paper addresses the voltage stability margin calculation in medium-voltage distribution networks in the context of exact mathematical modeling. This margin calculation is performed with a second-order cone (SOCP) reformulation of the classical nonlinear non-convex optimal power flow problems. The main idea around the SOCP approximation is to guarantee the global optimal solution via convex optimization, considering as the objective function the λ-coefficient associated with the maximum possible increment of the load consumption at all the nodes. Different simulation cases are considered in one test feeder, described as follows: (i) the distribution network without penetration of distributed generation; (ii) the distribution network with penetration of distributed generation; and (iii) the distribution grid with capacitive compensation. Numerical results in the test system demonstrated the effectiveness of the proposed SOCP approximation to determine the λ-coefficient. In addition, the proposed approximation is compared with nonlinear tools available in the literature. All the simulations are carried out in the MATLAB software with the CVX package and the Gurobi solver.

Static voltage stability margin calculation of power systems with high wind power penetration based on the interval optimisation method

Calculation of the static voltage stability margin (SVSM) of a power system with high wind-power penetration requires consideration of the uncertain fluctuation of wind farms' (WFs') output. Using interval numbers to describe the uncertain WF output fluctuation and based on the interval optimisation theory, a new SVSM interval calculation method is proposed. The method establishes two bi-level optimal power flow (OPF) models to calculate the SVSM interval, including a min–min model for the upper bound and a max–min model for the lower bound. Through the dual theory of convex programming, the max–min model is transformed into a max–max model, which is solved by transforming it into a one-layer optimisation model. Using correlation angles to describe the correlation of different WF output intervals and using linear transformation to decorrelate them into independent intervals, two bi-level OPF models of calculating the SVSM interval considering different WFs' correlation are established. Take the IEEE-39 bus system and an actual 964-bus provincial power grid as examples, compared with the Monte-Carlo method and the continuous power flow and affine interval integrated method, the results indicate the proposed method obtains more accurate SVSM interval and has better calculation efficiency.

Computation of Time Delay Stability Margin for the Automated Vehicular Platoon

Due to the limited band width and congestion of communication channels in the wireless vehicle-to-vehicle (V2V) communication, time delay inevitably arises and dramatically leads to the disturbances for the automated vehicular platoon. This paper focuses on computing the exact time delay stability margin. In this study, we treat this problem as a stability issue of a consensus system with time delay, where each vehicle in the platoon is recognized as a node, and the interconnected information flow is represented as a graph. Then, the distributed controller is designed by combining the states of the vehicle itself and its neighbouring vehicles. Furthermore, the stability of the entire platoon is analysed according to the characteristic equation of the closed-loop system, and a necessary and sufficient condition for the exact time delay stability margin is obtained. Especially, for the automated vehicular platoon with undirected topology, it is revealed that exact time delay stability margin is determined by the largest eigenvalue of the augmented Laplacian matrix. Furthermore, a rapid method for finding exact time delay stability margin is proposed. Finally, numerical simulations demonstrate that this work generates exact and satisfactory time delay stability margin for the automated vehicular platoon.

Robust and Adaptive Anticoagulant Control

Summary We consider a control theory approach to adaptive dose allocation of anticoagulants, based on an analysis of records of 152 patients on long-term warfarin treatment. We consider a selection of statistical models for the relationship between the dose of drug and subsequent blood clotting speed, measured through the international normalized ratio. Our main focus is on subsequent use of the model in guiding the choice of the next dose adaptively as patient-specific information accrues. We compare a naive long-term approach with a proportional-integral-plus method, with parameters estimated by either linear quadratic optimization or by stochastic resource allocation. We demonstrate advantages of the control approaches in comparison with a naive approach in simulations and through calculation of robust stability margins for the observed data.

Automatic Identification and Importance Evaluation Technology of Power Transmission Section

In theory, the cross section is the minimum cut set in the network. For the actual power grid, the transmission cross section is a collection of transmission lines between different regions, which is mainly used for power transmission to achieve the purpose of generating load balance. In this paper, a power grid zoning algorithm is proposed based on the topological structure characteristics and current operation state of the power grid, and the key transmission sections can be directly obtained without depending on the calculation of safety and stability margin. A practical system state partition model and key section sorting method are introduced, and a set of automatic identification method flow of key transmission sections of the system is established by searching for electrical betweenness. The importance of the transmission section is quantitatively evaluated from three angles: the margin from the stability limit, the hub position in the current power flow state, and the impact on the power grid after the power cut-off. The evaluation results can be used as a Reference for decision-making in the planning and reconstruction of the transmission section, safety monitoring, protection configuration, etc.

Radial basis function for fast voltage stability assessment using Phasor Measurement Units

A simple method, based on Machine Learning Radial Basis Functions, RBF, is developed for estimating voltage stability margins in power systems. A reduced set of magnitude and angles of bus voltage phasors is used as input. Observability optimization technique for locating Phasor Measurement Units, PMUs, is applied. A RBF is designed and used for fast calculation of voltage stability margins for online applications with PMUs. The method allows estimating active local and global power margins in normal operation and under contingencies. Optimized placement of PMUs leads to a minimum number of these devices to estimate the margins, but is shown that it is not a matter of PMUs quantity but of PMUs location for decreasing training time or having success in estimation convergence. Compared with previous work, the most significant enhancement is that our RBF learns from PMU data. To test the proposed method, validations in the IEEE 14-bus system and in a real electrical network are done.

Real-Time Voltage Stability Assessment Method for the Korean Power System Based on Estimation of Thévenin Equivalent Impedance

This study aims to develop a real-time assessing methodology for a power systems voltage stability. The proposed algorithm is based on the Thévenin equivalent (TE) impedance estimation method, which applies the phasor measurement unit technology practically. To present an accurate analysis of the real-time situations of a power system, the developed voltage stability index can be used as useful information for system operators to establish appropriate countermeasures. Moreover, by considering the results of voltage stability margin calculation within the Korean power system, the effect of voltage stability on the dynamic behavior of the system is presented. Furthermore, to increase the accuracy, load model parameter estimation is introduced in this algorithm. The load model might be used for calculating the stability margin more accurately. The power-voltage curve is drawn in theory using the TE voltages and impedances. To validate the case study of the proposed method, simulations were executed using the Matlab software. The simulations demonstrated the effectiveness of the proposed method and detected voltage stability/instability under severe contingency scenarios.

Robust Design for the Roll-Channel Autopilot of a Canard-Guided Dual-Spin Projectile

Abstract This article addresses the design of a robust feedback controller for the roll dynamics of a dual-spin guided projectile equipped with low performance actuators. Exploiting characteristics of realistic flight scenarios and the corresponding parameter variations, a simple yet efficient gain-scheduling strategy tailored for this application is devised. Controller synthesis is based on robust control techniques, and takes into account potential hardware limitations to safeguard against performance loss upon digital implementation. Robust stability analysis with respect to aerodynamic and actuator uncertainties is also investigated using µ-analysis techniques and includes computation of worst-case stability margins. The designed autopilot is then tested on a complete nonlinear simulator of the projectile to validate the proposed solution.

Управление созданием бизнес-ценности в организационных ИТ-проектах

The paper is devoted to the new approach to value creation management in IT-based organizational change projects that utilizes the product service model, management by stages and value control. The approach solves the problem that a business-value created in a project is out of control because it is realized in the post-project stage. The paper studies value control problems that are conditioned by the utilization of the traditional IT-project lifecycle. The method of value metrics definition that is based on the product service model is presented together with the released value method a new method of value control that extends the earned value method with a vector of value creation performance indices. Those are to be considered in conjunction with the schedule performance index. Scope changes are used as an instrument to control the value creation process. The scope creep risk control is performed by calculation of stability margin based on previously done changes data.

Explicit Phase Lead Filter Design in Repetitive Control for Voltage Harmonic Mitigation of VSI-Based Islanded Microgrids

Repetitive control strategies have been commonly applied in pulse-width-modulated (PWM) voltage source inverters (VSIs) for many industrial applications. This paper presents a repetitive controller for voltage harmonic mitigation of VSI-based islanded microgrids. The phase delay in the overall control system, e.g., the delay caused by the digital duty cycle calculation, PWM generation, and repetitive controller, has to be compensated by elaborately designed phase lead filters in order to prevent control performance deterioration and system instability. Nevertheless, quantificational analysis and practical design of the time advance unit in such filters are hardly found in existing literature works. In view of this, this paper proposes an explicit analysis of the phase lead filters and a novel design method of the time advance unit in repetitive controllers to ensure system stability. Moreover, with the help of the proposed method, the overall system stability margin is predictable and improved controller performance is achieved as well. The proposed method is implemented experimentally to show the accurate stability margin calculation as well as the excellent steady state and dynamic performances of the repetitive control scheme.

An Efficient Approach for Fast and Accurate Voltage Stability Margin Computation in Large Power Grids

This paper proposes an efficient approach for the computation of voltage stability margin (VSM) in a large-scale power grid. The objective is to accurately and rapidly determine the load power margin which corresponds to voltage collapse phenomena. The proposed approach is based on the impedance match-based technique and the model-based technique. It combines the Thevenin equivalent (TE) network method with cubic spline extrapolation technique and the continuation technique to achieve fast and accurate VSM computation for a bulk power grid. Moreover, the generator Q limits are taken into account for practical applications. Extensive case studies carried out on Institute of Electrical and Electronics Engineers (IEEE) benchmark systems and the Taiwan Power Company (Taipower, Taipei, Taiwan) system are used to demonstrate the effectiveness of the proposed approach.

A novel recursive approach for real-time transient stability assessment based on corrected kinetic energy

Real-time assessment of transient stability is one of the main issues of power system operators in online applications. This paper proposes a novel recursive approach based on corrected kinetic energy which has the capability of real-time assessment and real time computation of transient stability margin in the power system. This approach considers all details of power system by using network preserving model to simulate transient stability. This paper uses a hybrid method based on the new concept of equal area criterion to estimate initial value of critical point of the system and corrected Kinetic Energy Function to estimate high precision value of the critical point. Also, this paper proposes a recursive method which uses large change sensitively (LCS) analysis to correct initial condition point of the system when the topology of system is changed by a disturbance. In order to validate the proposed method, comprehensive case studies have been conducted on IEEE39-bus test system. Comprehensiveness in considering the details, simplicity in implementation and low computational cost are the outstanding features of the proposed approach. Also, simulation results approve that the proposed approach can be used in real-time application without loss of any detail in the transient stability assessment.

Estimating the Voltage Stability Margin Using PMU Measurements

This paper presents a new method based on phasor measurement units (PMUs) for the estimation of voltage stability margin in a power system to increase operator's situational awareness. The method assumes a PMU measurement preprocessing technique as a priori in order to eliminate data inconsistency and uncertainty caused by random load disturbances. The measurements are then used in the computation of voltage stability margin based on the coupled single-port Thevenin equivalent model and the cubic spline extrapolation technique. Moreover, some practical operating constraints such as the generator reactive power limits are taken into account for practical assessment of the method's performance. Extensive case studies conducted on several standard IEEE test systems are used to demonstrate the effectiveness of the proposed method.

Voltage collapse detection based on local measurements

This paper is concerned with voltage collapse detection in power systems. The concepts of voltage support surface and local balancing equation are used. Likewise, the use of the impedance matching concept, requiring only measurements taken at the corresponding bus without needing a reference signal, is proposed. The impedance matching equation is solved by direct and a recursive least squares algorithm. The estimated equivalent impedance becomes a multiple of the load impedance. Thus, proximity indices to the point of collapse (PoC) that requires just voltage magnitudes are presented. Voltage instability is evaluated through direct detection of PoC in loads and transit (no-load) buses. Voltage stability margin calculation without requiring the load flow solution at the bifurcation point is proposed. Taking constant P–Q load characteristics into account, the method is tested on the IEEE 14-bus test system by the continuation power flow (CPF) to estimate the path that the impedance and the proposed indices follow through stable and unstable operation regions. A dynamic evaluation of the method is performed when exponential recovery and motor loads are embedded.

Static Voltage Stability Margin Calculation for the Microgrid Based on Immune Algorithm

To the questions of approaching degree between operating point and collapse point, and operating mechanism in the instability, the main method to analyze power system is often the calculation of static voltage stability index. In this paper, a method of microgrid static voltage stability margin based on immune algorithm is proposed. Compared with traditional method, the method not only can search for the global optimal solution effectively, but also deal with discrete variables in the system; meanwhile, complex node of the microgrid is considered entirely .The simulation results confirm that the method is more reasonable and effective for microgrid static voltage stability margin.

Lead-Lag Damper Robustness Analysis for Helicopter Ground Resonance

Aeroelastic stability is a key issue that drives the design of modern rotorcraft. Rotary wing vehicles may present significant parameter variability and uncertainty related to environment, operating conditions, and wear. The capability for analyzing the robustness of the stability is fundamental to determine the amount of freedom a designer has in defining the key properties in specific rotorcraft problems dominated by stability. This paper discusses robust stability analysis, focusing on the generalization of the computation of frequency-dependent stability margins for multi-input/multi-output systems. The proposed method is applied to the ground resonance problem, cast in a form that isolates a closed loop containing the lead-lag damper, which is considered the uncertain component. The problem is solved for linear and nonlinear models of the damper, illustrating how stability requirements can be used to determine uncertainty bounds on the stiffness and damping properties of the damper.

An Adaptive Power System Equivalent for Real-Time Estimation of Stability Margin Using Phase-Plane Trajectories

This paper studies real-time estimation of stability margin of a power system under disturbances by continuous measurement data. The paper presents a ball-on-concave-surface (BOCS) mechanics system as a power system's adaptive equivalent representing its real-time status and the stability region about a monitored variable. The parameters of the equivalent are adaptive to the operating condition and can online be identified from the phase-plane trajectories of the monitored variable. Accordingly, the stability margin and risk of instability can be estimated. Case studies on a two-generator system and a 179-bus system show that the BOCS system can be applied either locally or for wide-area stability monitoring in real-time calculation of stability margin. This paper proposes a new idea for using real-time measurements to develop and identify a power system's adaptive equivalent as a basis for online prediction of instability.

Application of Two Polynomial Optimization Methods for Computing Robust Stability and Robust Performance Margins

AbstractIn this paper, the computation of robust stability and robust performance margins for linear time invariant systems under polynomial parameter uncertainty is considered. The characteristic equations of such systems appear as polynomial functions of uncertain parameters. The problem is formulated as a polynomial optimization problem using the zero-exclusion theorem. Then, two global optimization techniques are used to solve the optimization problems. One of the algorithms is based on the Reformulation-Linearization Technique (RLT), and the other uses the Linear Matrix Inequalities (LMI) scheme. The two methods are applied to two case studies and the results are compared. The outcomes show the superiority of the RLT methodology in converging to a global solution, and in computational time.

Computation of Stability Margins for Uncertain Linear Fractional-Order Systems

The present paper proposes an algorithm for finding the stability margins and cross over frequencies for an uncertain fractional-order system using the interval constraint propagation technique. It is first shown that the problem of finding the stability margins and crossover frequencies can be formulated as an interval constraint satisfaction problem and then solved using the branch and prune algorithm. The algorithm guarantees that the stability margins and the crossover frequencies are computed to the prescribed accuracy. The proposed algorithm is demonstrated on a noninductive cable system and also on a practical application of a gas turbine plant.