Multi-machine System Research Articles

Control Algorithm for UPFC Based on Non-linear Model of Power System

This article proposes a state-variable stabilizing control algorithm for unified power flow controller in a multi-machine power system. This stabilizing control is activated in the transient state of the power system and is complementary to the main steady-state control designed for power flow regulation. The stabilizing control algorithm proposed in this article is derived for a non-linear multi-machine system model in order to maximize the damping of power swings in a power system. This article also shows that the effects of multiple control devices on power swing damping are additive, i.e. each new device added to power system network increases damping.

Development of generalized unified power flow controller (GUPFC) detailed model with a comparison with multi-UPFC

Unified power flow controller (UPFC) is a unique type FACTS device which is able to regulate power system parameters such as active and reactive power along with voltage magnitude and phase angle. In restructured power system, use of multi-machine systems increasing exponentially. In such systems, there is a need of control of parameters of more than one line. In such situation, multiple Unified power flow controllers (M-UPFCs) is used to achieve these power system requirements. A modified form of UPFC which is known as generalized unified power flow controller (GUPFC) is also used to control the power system parameters for more than one bus or even a subsystem. This paper envelops a innovative approach based on voltage source converter is used for the designing of detailed model of GUPFC in MATLAB/Simulink. A detailed comparison is done between a detailed GUPFC model and M-UPFC for power flow control (active power, reactive power and voltage). The simulation results the superiority of GUPFC over M-UPFC. This paper states first time designing of the detailed model of GUPFC in MATLAB/Simulink.

Eigenvalues Optimization Method of Low Frequency Oscillation for Multi-machine System based on Genetic Algorithm

This paper presents a method that simultaneously tunes the control parameters using genetic algorithms (GAs) to optimize the eigenvalues of multi-machine system, thereby achieving the effect of suppressing low frequency oscillation of the system. Due to the complexity and scale of the multi-machine system, it is necessary to rationally and efficiently optimize the algorithm for the parameter setting of this grid. In this article, the method of adjusting the population size and mutation rate and adopting several improved eigenvalues optimization objective functions is proposed to improve the performance of the genetic algorithm. These improvement measures are applied for control parameters tuning to the modified IEEE-39 system that contains DFIG-based wind farms and energy storage devices. The optimization results show that the efficiency and convergence of the genetic algorithm have indeed improved.

Sizing of Modular Cascade Machines System for Electric Vehicles

Modular cascade machines (MCM), as a new multi-machine system, are expected to be applied for electric vehicles (EVs). This paper focuses on the sizing of MCM systems. Expanding MCM high-efficiency area and reducing MCM mass are selected as optimization targets for sizing. The proposed machine efficiency map and MCM mass evaluation methods are the means to achieve this goal without machine pre-design. The efficiency evaluation method enables to evaluate machines efficiency maps only with four basic design specifications (rated speed, rated torque, overload factor, and speed expansion ratio). Neither finite element (FE) nor experimental data (inductance and resistance) are used. Its calculation is fast and can be finished within several milliseconds. The method is validated by comparisons with the experimental efficiency maps of two prototypes. The proposed methods are proven having an accuracy that is acceptable for the MCM definition and pre-design. The exemption of machine pre-design saves a lot of time during sizing stage. The sizing optimization is realized by genetic algorithm. And the comparisons of different sizing are conducted with the same vehicle model. Moreover, general sizing rules for the similar EVs are proposed. For the studied EV, the number of machines is proposed as a function of traction power. The sized specifications could be used for more accurate machine design by the FE method in future. The sizing method could be extended for the other kind of multi-machine systems.

Comparative analysis for optimum position of A PWM based UPFC and ASVC applied to multi-machine systems with non-linear load model

Comparative analysis for optimum position of A PWM based UPFC and ASVC applied to multi-machine systems with non-linear load model

Optimization of damping controller for PSS and SSSC to improve stability of interconnected system with DFIG based wind farm

This paper studies the stability issues of the Double Fed Induction Generator (DFIG) based wind farm in the power system with variable wind speed in strong and weak grid. With the intermittent generation of the wind power, it experiences instability and malfunction. The nature of Electromechanical Modes of Oscillations (EMOs) for DFIG incorporated into the multi-machine system under various conditions is explored considering system uncertainties. In this paper, the impact of Power System Stabilizer (PSS) and Static Series Synchronous Compensator (SSSC) on the stability of wind power system is analyzed by utilizing modified IEEE 14-bus test system. The rotor speed deviation is the objective function for particle swarm optimization and is formulated based on the transient and small signal stability. The transient and small signal stability investigations are performed by using time domain simulation and eigenvalue analysis respectively. The sensitivity analysis is carried out with degree of compensation of SSSC and coefficient of damping gain of PSS as sensitivity parameter. The performance of the modified test system is examined for transient and small signal stability for DFIG based wind farm for variable wind speed and grid strength considering different combinations of PSS and SSSC. From the results, it is noticed that oscillations in the system are minimized, damping, transient and small signal stability are enhanced and also the capability of synchronous generator and DFIG under extreme disturbance is intensified by optimization the coordinated controllers’ parameters. The analyses also show that damping ratio is improved for most of the EMOs. Sensitivity analysis identifies EMOs that have detrimental and beneficial impacts on the system.

Research on Simulated Inertia Coordination Control Strategy of VSG Based on Energy Storage

With the development of new energy and micro grid, inverter plays an important role as the interface of distributed new energy to realize power conversion. However, the introduction of various power electronic components not only enhances the response speed and flexibility of the micro grid, but also greatly reduces the overall inertia of the system. VSG control technology is introduced in this paper. At the same time, through the in-depth analysis of energy storage system, the inertial parameters of energy storage system are deduced. The VSG control and the energy storage system inertial parameters are matched to increase the system inertia and damping. Aiming at the power response part of multi-machine system, the inertial parameters of multi-machine system are controlled uniformly, which makes the whole system achieve good power response and distribution. At last, a simulation model was set up by MATLAB for verification and analysis.

Influence of Adaptive Power System Stabilizer on the Transient and Dynamic Performance of a Multimachine System

This paper presents a power system stabilizer (PSS) for improving the performance of a multimachine system when the system is subjected to different types of disturbances. An adaptive proportional-plus-derivative-plus-integral (PID) PSS is proposed in this paper. In this technique, the controller parameters (proportional, integral, and derivative gains) are not fixed but they are varying according to the system operating conditions. The derivative of power angle, error in output power, terminal voltage, and error in power angle are selected to be the input signals to the proposed stabilizer. A weighting factors are assigned to these signals to obtain the best performance characteristics of each machine. The obtained computer results from simulation study are compared with those of a conventional lead-lag stabilizer.

An energy-based analysis of reduced-order models of (networked) synchronous machines

ABSTRACT Stability of power networks is an increasingly important topic because of the high penetration of renewable distributed generation units. This requires the development of advanced techniques for the analysis and controller design of power networks. Although there are widely accepted reduced-order models to describe the power network dynamics, they are commonly presented without details about the reduction procedure. The present article aims to provide a modular model derivation of multi-machine power networks. Starting from first-principle fundamental physics, we present detailed dynamical models of synchronous machines and clearly state the underlying assumptions which lead to some of the standard reduced-order multi-machine models. In addition, the energy functions for these models are derived, which allows to represent the multi-machine systems as port-Hamiltonian systems. Moreover, the systems are proven to be shifted passive, which permits for a power-preserving interconnection with other passive components.

Reduced Order Observer for Two-Connected Machine Drive

The paper presents a study of a reduced order observer with the vector control of a series-connected two-motor drive supplied by a single inverter. The multimachine system consists of a six-phase induction machine and a three-phase induction machine. The stators windings of both machines are connected in series in appropriate manner. The dynamic decoupling of each machine from the group is obtained using the indirect vector control algorithm. Modeling of the multi-machine system showed the possibility of independent control of each machine in the group. The independent control is demonstrated by analyzing the characteristics of the torque and speed of each machine obtained via simulation of multi-machine system under indirect vector control scheme. A reduced-order observer is applied to sensor-less control of multimachine. The simulation results show that the closed-loop system performances are satisfactory using the proposed algorithm.

Lyapunov Stability Based Wide Area Control Systems for Excitation Boosters in Synchronous Generators

Excitation boosters (EBs) are aimed to improve fault ride through capability of synchronous generators equipped with bus fed static excitation systems by means of adding a supplementary voltage or current source to the rectifier. However, this paper shows that EBs governed by local on–off controls may result in adverse effects in case of remote faults from generators that may occur in multi-machine systems. To sort this problem out, a novel EB Wide Area Control System (WACS) that modulates the EB voltage using the generator speed deviation with respect to the speed of the center of inertia (COI) is proposed. The speed of the COI is obtained using a Wide Area Measurement System (WAMS), whereas the control laws are deduced from the Lyapunov stability theory. Results show that transient stability of multi-machine systems can be enhanced using EB WACS.

Analysis of harmonic resonance mechanism of PV power plant

Great amount of PV inverters connected with the PV power plant causes the power system to become electronically power and affects the operation mode of the traditional power system. High-frequency switching characteristics of PV inverters carry a large number of harmonic components, even lead to the harmonic resonance of a multi-machine parallel system. In the paper, based on physical mechanism of power electronics, a high-order model of harmonic impedance of a typical PV power plant is established, and harmonic resonance mechanism in parallel operation of PV power plant is analyzed, as well as key factors.

Transient Stability Assessment Using Individual Machine Equal Area Criterion PART III: Reference Machine

The analysis of the transient stability of a multimachine system via the use of a few machines is a promising technique for transient security assessment. This paper proposes a genuine nonglobal direct-time-domain method that is developed from the previous COI-reference-based IMEAC method. In this paper, the reference machine (RM) is chosen as a real machine that does not separate from the system after fault clearing. The key concept of this paper is to select the reference machine (RM) first and then replace the COI with the RM during the post-fault period in the transient security assessment. In this manner, the motions of all machines in the system are redefined in RM reference. Furthermore, it is found that the Kimbark curve of the RM-reference-based critical machine has a distinctive quasi-sinusoidal characteristic, and thus the curve is predictable after fault clearing. Following the individual-machine thinking, the stability analysis of the multimachine system is transformed into monitoring the stability state of a few RM-reference-based critical machines, which completely avoids the usage of COI. The simulation results show the effectiveness of the proposed method in the transient stability analysis of multimachine systems.

Large-Scale Power Base’s Impact on Low Frequency Oscillation Characteristic in UHVAC Power Transmission System

The dynamic stability of ultra-high voltage alternating current (UHVAC) transmission system with large-scale power bases has not been clearly discussed before. This paper studies the low frequency oscillation (LFO) characteristic of UHVAC system through complex torque coefficient method (CTCM) and finds out the reasons why UHVAC system is more vulnerable to LFO problem. It is pointed out that the bulk capacity and long distance make UHVAC system’s damping torque coefficient much smaller than conventional system’s, which lets UHVAC system’s operation point closer to the safety edge and decreases system’s dynamic stability. The comparisons between UHVAC system and conventional system in practical multi-machine systems further validate the theoretical analysis. Finally, the suggestions for UHVAC systems are given based on the analysis in the view of dynamic stability.

Исследование позиционной модели энергетической системы

The position model specific features are investigated; the eigenvalues and eigenvectors of the small perturbation equation matrix are analyzed, and a criterion of limiting operation modes in terms of aperiodic steady-state stability is obtained, which generalizes the classic Wagner - Evans criterion for complex multimachine systems. It has been found that the position model, i.e., a model in which the powers and torques of generators depend only on the relative angles of machine rotors, implicitly uses the distributed slack bus philosophy, and its limiting modes in terms of aperiodic steady-state stability are exactly identical with the limiting modes of a load flow model with a distributed slack bus in which the coefficients characterizing the participation of nodes in active power balancing are assigned to be directly proportional of the synchronous machines’ inertia constants. It has been found from a comparative analysis of the limiting mode parameters in the position model and the model with an infinite bus that the position model yields a conservative (underestimated) assessment of the electric power system steady-state stability margin. It is shown that the steady-state stability margins are estimated in the most adequate manner by means of the load flow model with an infinite bus, in the limiting modes of which the relative power loss increments do not exceed unity for all nodes.

Power System Stabilization Using Energy-Dissipating Hybrid Control

A new energy-dissipating hybrid control framework is proposed in this paper. The proposed framework incorporates a plant with continuous dynamics connected in parallel with a hybrid controller that combines logical switches with continuous dynamics. The overall closed-loop system can be described as a hybrid impulsive dynamical system in which the redundant energy preserved in the plant can be damped quickly and continuously due to the enhanced dissipativity of the closed-loop system. The proposed framework is applied to design decentralized nonlinear excitation controllers for synchronous generators in a multimachine system to improve its transient stability, in which the synchronous generators are modeled as port-controlled Hamiltonian systems. Simulation studies on the IEEE 10-generator, 39-bus New England power system are carried out to verify the effectiveness of the proposed energy-dissipating hybrid excitation control to improve the transient stability and dissipativity of the interconnected power system under various fault conditions.

Investigation and damping of low‐frequency oscillations of stochastic solar penetrated power system by optimal dual UPFC

Solar power being stochastic and asynchronous in nature, its integration with conventional power generation has so many challenges in damping of low-frequency oscillations. This study presents an investigation of low-frequency oscillation and damping of stochastic solar power integrated power system by a unified power flow controller (UPFC)-based dual optimal controller, which gains are optimised by a novel hybrid particle swarm optimisation and improved grey wolf optimiser. The dual controller simultaneously controls the modulation index of series and phase angle of shunt converters of UPFC, which co-operate with each other, optimising its efficacy and thereby implementing the advantages of both static synchronous series compensator and static synchronous compensator. A detailed Eigen value analysis has been performed with time domain simulations to study the damped oscillatory response of variable and random solar penetration with a power system along with the interaction of solar power with variable synchronous power generations. For a multi-machine system, a multi-input single output dual controller is proposed. A maximum sensor and time delay have been considered to design the controller providing a realistic approach. It is observed that random and heavy solar penetration has a more detrimental effect on system oscillations, which can be damped heavily with the proposed controller, in contrast to particle swarm optimisation, differential evolution, grey wolf optimiser, differential evolution particle swarm optimisation optimised lead-lag and dual controllers.

A practical method of transient stability analysis of stochastic power systems based on EEAC

With the diversification of power systems and the application of power electronic technology, the uncertainty of power systems is becoming more and more serious, and the traditional deterministic transient stability analysis methods encounter severe challenges. In this paper, a practical method is proposed for transient stability analysis of stochastic multi-machine systems based on EEAC. Firstly, the stochastic differential equation model of multi-machine systems is established. Secondly, the stochastic multi-machine system is equivalent to a stochastic single-machine infinite bus system according to coherency. Finally, the acceleration and deceleration area is constructed, and Heun’s method is used to seek the critical clearing time of the system. The method is applied to analyse transient stability of four machine system. Compared with the numerical simulation based on Monte Carlo method, the simplicity and effectiveness of the method are verified. The impact of stochastic disturbance intensity on stability is also presented.

Oscillatory Stability Prediction Using PSO Based Synchronizing and Damping Torque Coefficients

This paper presents the assessment of stability domains for the angle stability condition of the power system using Particle Swarm Optimization (PSO) technique. An efficient optimization method using PSO for synchronizing torque coefficients Ks and damping torque coefficients Kd to identify the angle stability condition on multi-machine system. In order to accelerate the determination of angle stability, PSO is proposed to be implemented in this study. The application of the proposed algorithm has been justified as the most accurate with lower computation time as compared to other optimization techniques such as Evolutionary Programming (EP) and Artificial Immune System (AIS). Validation with respect to eigenvalues determination, Least Square (LS) method and minimum damping ratio ξmin confirmed that the proposed technique is feasible to solve the angle stability problems.

PMU-based model-free method for transient instability prediction and emergency generator-shedding control

Using PMU measurements, this paper proposes a model-free method to predict post-fault transient instability and develop emergency generator-shedding control. First, the multi-machine system is converted to an equivalent one-machine-infinite-bus (OMIB) system based on online generator clustering, and then the stability criteria are derived to judge the transient stability using the OMIB rotor speed (OMIB-ω) trajectory. Next, a new trajectory prediction algorithm based on ensemble online sequential learning machine (E-OS-ELM) is proposed to predict the OMIB-ω trajectory with an adaptive prediction window. The post-fault transient instability status can be detected in advance on basis of the predicted ω trajectory and the derived stability criteria. Lastly, when the system is foreseen to lose stability, an analytical generator-shedding control algorithm is presented, and the relationship between the generator-shedding amount and the time delay is illustrated. Case studies on the New-England 39-bus system, the NPCC 140-bus system and a realistic province power system in China are presented to show the proposed methodology can detect the instability status early, and help the system maintain synchronism.