Rotor Speed Deviation Research Articles

Development and Field Experiments of a Generator Terminal Subsynchronous Damper

This paper presents a novel controlling device, named generator terminal subsynchronous damper (GTSSD), to mitigate subsynchronous resonance (SSR) in the power system. The proposed GTSSD consists of a multimodal complementary current calculator (MCCC) and a complementary current generator (CCG). The CCG is composed of a current tracking controller and a power-electronic converter. The rotor speed deviation of the generator is fed back to the MCCC to calculate the current references of the CCG. The outputs of CCG consist of both sub- and supersynchronous complementary current, part of which flows into the generator and creates damping torque on the rotor. The relationship between the control parameters of the GTSSD and the resultant damping improvement is derived. A 10-MVA GTSSD prototype has been developed and tested in an actual series-compensated power system. The experimental results fully demonstrate its ability to enhance torsional damping and to depress subsynchronous oscillation, proving that the GTSSD provides a new and effective approach to solve SSR problems.

A multi-objective gravitational search algorithm based approach of power system stability enhancement with UPFC

On the basis of the theoretical analysis of a single-machine infinite-bus (SMIB), using the modified linearized Phillips-Heffron model installed with unified power flow controller (UPFC), the potential of the UPFC supplementary controller to enhance the dynamic stability of a power system is evaluated by measuring the electromechanical controllability through singular value decomposition (SVD) analysis. This controller is tuned to simultaneously shift the undamped electromechanical modes to a prescribed zone in the s-plane. The problem of robust UPFC based damping controller is formulated as an optimization problem according to the eigenvalue-based multi-objective function comprising the damping factor, and the damping ratio of the undamped electromechanical modes to be solved using gravitational search algorithm (GSA) that has a strong ability to find the most optimistic results. The different loading conditions are simulated on a SMIB system and the rotor speed deviation, internal voltage deviation, DC voltage deviation and electrical power deviation responses are studied with the effect of this flexible AC transmission systems (FACTS) controller. The results reveal that the tuned GSA based UPFC controller using the proposed multi-objective function has an excellent capability in damping power system with low frequency oscillations and greatly enhances the dynamic stability of the power systems.

A comparative study in power oscillation damping by STATCOM and SSSC based on the multiobjective PSO algorithm

To improve the damping of power system oscillations by supplementary controller design for the static synchronous series compensator (SSSC) and static synchronous compensator (STATCOM), a multiobjective function based on the particle swarm optimization (PSO) algorithm for solving this optimization problem is introduced. The presented objective function includes the damping factor and the damping ratio of the lightly damped and undamped electromechanical modes. These controllers are adjusted to concurrently transfer the lightly damped and undamped electromechanical modes to a recommended region in the s-plane. For this purpose, the reduced linearized Phillips-Heffron model of the power system with a single machine and infinite bus, integrated with a STATCOM and a SSSC, is used. In this paper, to solve the mentioned optimization problem, the PSO technique is used. It is a robust stochastic optimization technique and has a high capability for discovering the most optimal results. The different loading conditions are simulated and the effects of these flexible alternating current transmission system controllers over the rotor angle and rotor speed deviations are studied. Simulation results reveal that the SSSC's performance is better than that of the STATCOM and it provides higher damping than the STATCOM. MATLAB/Simulink software is used for running the dynamic simulations.

Application of subsynchronous damping controller (SSDC) to STATCOM

In this paper a novel supplementary subsynchronous damping controller (SSDC) is proposed for the STATic synchronous COMpensator (STATCOM) which is capable of damping out subsynchronous oscillations in power system with series compensated transmission lines. An auxiliary subsynchronous damping controller (SSDC) for a STATCOM using the generator rotor speed deviation signal as the stabilizing signal has been designed to damp subsynchronous oscillations. Eigenvalue analysis and transient simulations of detailed nonlinear system are considered to investigate the performance of the controller. Robustness of the controller has been analyzed by facing the system with disturbances leading to significant changes in generator operating point. This paper deals with a cascaded multilevel converter model, which is a 48-pulse (three levels) source converter. The IEEE Second Benchmark (SBM) model is considered for the analysis and the complete digital simulation of the STATCOM within the power system is performed in the MATLAB/Simulink environment.

Novel Scheme for Improving the Performance of a Wind Driven Doubly Fed Induction Generator during Grid Fault

Enhancement of fault ride-through (FRT) capability and subsequent improvement of rotor speed stability of wind farms equipped with doubly fed induction generator (DFIG) is the objective of this paper. The objective is achieved by employing a novel FRT scheme with suitable control strategy. The proposed FRT scheme, which is connected between the rotor circuit and dc link capacitor in parallel with Rotor Side Converter, consists of an uncontrolled rectifier, two sets of IGBT switches, a diode and an inductor. In this scheme, the input mechanical energy of the wind turbine during grid fault is stored and utilized at the moment of fault clearance, instead of being dissipated in the resistors of the crowbar circuit as in the existing FRT schemes. Consequently, torque balance between the electrical and mechanical quantities is achieved and hence the rotor speed deviation and electromagnetic torque fluctuations are reduced. This results in reduced reactive power requirement and rapid reestablishment of terminal voltage on fault clearance. Furthermore, the stored electromagnetic energy in the inductor is transferred into the dc link capacitor on fault clearance and hence the grid side converter is relieved from charging the dc link capacitor, which is very crucial at this moment, and this converter can be utilized to its full capacity for rapid restoration of terminal voltage and normal operation of DFIG. Extensive simulation study carried out employing MATLAB/SIMULINK software vividly demonstrates the potential capabilities of the proposed scheme in enhancing the performance of DFIG based wind farms to fault ride-through.

Damping of subsynchronous oscillations in power system using static synchronous series compensator

In this study, a static synchronous series compensator (SSSC) is used to damp the subsynchronous oscillation in a power system compensated by the series capacitor. In order to achieve an effective damping, a supplementary subsynchronous damping controller (SSDC) is added to the SSSC. The only input signal for the SSDC is the rotor speed deviation to generate the modulation index for controlling the injected voltage of the voltage-sourced converter (VSC). Also, the chaotic optimisation algorithm is employed to tune the parameter of SSDC. The design objective is to suppress the subsynchronous resonance (SSR) caused by the series capacitor. By using the SSDC, the SSSC connected at the transmission line is able to damp the SSR. The first system of IEEE second benchmark model is used to evaluate the effective of SSDC on the torsional oscillations. The several simulations are used to demonstrate the ability of SSDC in damping the SSR.

ENHANCEMENT OF FAULT RIDE THROUGH CAPABILITY OF A WIND DRIVEN DFIG CONNECTED TO THE GRID

Enhancement of fault ride-through (FRT) capability and subsequent improvement of rotor speed stability of wind farms equipped with doubly fed induction generator (DFIG) is the objective of this paper. The objective is achieved by employing a novel FRT scheme with suitable control strategy. The proposed FRT scheme, which is connected between the rotor circuit and dc link capacitor in parallel with Rotor Side Converter, consists of an uncontrolled rectifier, two sets of IGBT switches, a diode and an inductor. In this scheme, the input mechanical energy of the wind turbine during grid fault is stored and utilized at the moment of fault clearance, instead of being dissipated in the resistors of the crowbar circuit as in the existing FRT schemes. Consequently, torque balance between the electrical and mechanical quantities is achieved and hence the rotor speed deviation and electromagnetic torque fluctuations are reduced. This results in reduced reactive power requirement and rapid reestablishment of terminal voltage on fault clearance. Furthermore, the stored electromagnetic energy in the inductor is transferred into the dc link capacitor on fault clearance and hence the grid side converter is relieved from charging the dc link capacitor, which is very crucial at this moment. The converter in this case can be utilized to its full capacity for rapid restoration of terminal voltage and normal operation of DFIG. Extensive simulation study carried out employing MATLAB/SIMULINK software demonstrates the potential capabilities of the proposed scheme in enhancing the performance of wind farms DFIG to fault ride-through.

NEURAL NETWORK PREDICTIVE CONTROL BASED POWER SYSTEM STABILIZER

The present study investigates the power system stabilizer based on neural predictive control for improving power system dynamic performance over a wide range of operating conditions. In this study a design and application of the Neural Network Model Predictive Controller (NN-MPC) on a simple power system composed of a synchronous generator connected to an infinite bus through a transmission line is proposed. The synchronous machine is represented in detail, taking into account the effect of the machine saliency and the damper winding. Neural network model predictive control combines reliable prediction of neural network model with excellent performance of model predictive control using nonlinear Levenberg-Marquardt optimization. This control system is used the rotor speed deviation as a feedback signal. Furthermore, the using performance system of the proposed controller is compared with the system performance using conventional one (PID controller) through simulation studies. Digital simulation has been carried out in order to validate the effectiveness proposed NN-MPC power system stabilizer for achieving excellent performance. The results demonstrate that the effectiveness and superiority of the proposed controller in terms of fast response and small settling time.

Novel scheme for enhancement of fault ride-through capability of doubly fed induction generator based wind farms

Enhancement of fault ride-through (FRT) capability and subsequent improvement of rotor speed stability of wind farms equipped with doubly fed induction generator (DFIG) is the objective of this paper. The objective is achieved by employing a novel FRT scheme with suitable control strategy. The proposed FRT scheme, which is connected between the rotor circuit and dc link capacitor in parallel with Rotor Side Converter, consists of an uncontrolled rectifier, two sets of IGBT switches, a diode and an inductor. In this scheme, the input mechanical energy of the wind turbine during grid fault is stored and utilized at the moment of fault clearance, instead of being dissipated in the resistors of the crowbar circuit as in the existing FRT schemes. Consequently, torque balance between the electrical and mechanical quantities is achieved and hence the rotor speed deviation and electromagnetic torque fluctuations are reduced. This results in reduced reactive power requirement and rapid reestablishment of terminal voltage on fault clearance. Furthermore, the stored electromagnetic energy in the inductor is transferred into the dc link capacitor on fault clearance and hence the grid side converter is relieved from charging the dc link capacitor, which is very crucial at this moment, and this converter can be utilized to its full capacity for rapid restoration of terminal voltage and normal operation of DFIG. Extensive simulation study carried out employing PSCAD/EMTDC software vividly demonstrates the potential capabilities of the proposed scheme in enhancing the performance of DFIG based wind farms to fault ride-through.

Enhanced Fault Ride-through Scheme for Mitigating Rotor Speed Instability of Doubly Fed Induction Generator Based Wind Farms

This paper analyzes and proposes a novel scheme with suitable control strategy to increase the probability of successful fault ride-through (FRT) as well as to enhance the rotor speed stability of doubly fed induction generator (DFIG) based wind farms. The proposed FRT scheme consists of an uncontrolled rectifier, two sets of IGBT switches, a diode and an inductor connected between the rotor circuit and dc link capacitor, in parallel with the rotor side converter of DFIG. In the proposed scheme, during a fault event, the input mechanical energy of the wind turbine is stored as electromagnetic energy in the inductor. Consequently, torque balance between the electrical and mechanical quantities is maintained and the rotor speed deviation is thereby reduced. This results in reduced reactive power requirement and rapid reestablishment of terminal voltage on fault clearance. Moreover, the stored electromagnetic energy in the inductor is transferred into the dc link capacitor on fault clearance; thereby relieving the grid side converter from charging the dc link capacitor. The proposed FRT scheme is validated through an extensive time domain simulation study employing PSCAD/EMTDC software. Simulation results demonstrate the effectiveness of the proposed FRT scheme in achieving successful fault ride-through and rotor speed stability of DFIG based wind farms.

Three-dimensional power system stabilizer

Power system stabilizers (PSSs) are used to enhance damping of power system oscillations through excitation control of synchronous generator. The objective of the PSS is to generate a stabilizing signal, which produces a damping torque component on the generator shaft. Conventional PSSs are designed with the phase compensation technique in the frequency domain and include the lead-lag blocks whose parameters are determined according to a linearized power system model. The performance of conventional PSSs (CPSSs) depends upon the generator operating point and the system parameters, but a reasonable level of robustness can be achieved depending on the tuning method. This paper presents a new three-dimensional PSS (3D PSS), which uses rotor speed deviation, rotor acceleration and load angle deviation as input signals. The 3D PSS attempts to return the generator to the state-space origin, based on the generator’s trajectory in state-space and the achievement of torque equilibrium. The 3D PSS is robust to system parameters changes. The proposed algorithm was implemented in a digital control system, tested in a laboratory environment on a synchronous generator connected to the power system, and then compared with CPSS. Experimental results show that the proposed PSS achieves better performance than the CPSS in damping oscillations.

A Multi-step Q(Λ) learning Approach to Power System Stabilizer

Power system stabilizers (PSS) are used to generate supplementary control signals for excitation system in order to damp the low frequency power system oscillation. To overcome the drawbacks of conventional PSS (CPSS), numerous techniques have been proposed in the literature. This paper proposes an optimal control methodology based on reinforcement learning (RL) for damping the low frequency power system oscillations. Q(Λ) learning is a relative new model-free RL algorithm that extends the one-step Q-learning by combining it with TD(Λ) returns for general Λ in a incremental way for the delayed RL problem. The Q(Λ) learning makes use of TD(Λ) returns as the value function estimator through a trace mechanism, and additional traces appear to help bridge the gap between frequency and recency information for heuristic events. In this paper, rotor speed deviation is adopted as the state feedback input, and the voltage deviation, power deviation and rotor speed deviation are formulated as reward function. The case study shows that both of them are very helpful to enhance the small-disturbance dynamics of power system. The multi-step Q(Λ) learning can enhance convergence rate of reinforcement learning algorithm.

Implementation of Bio Inspired Genetic Optimizer in enhancing Power System Stability

The phenomenon of Stability of Modern Power Systems has received a great deal of attention in recent years. With the increasing complexity of the Modern Power Systems, the system stability is affected by Low frequency oscillations produced in the system. This paper provides a systematic approach to design a damping controller to damp the oscillations based on Bio inspired Genetic Algorithm(GA).The design problem is formulated as an Multiobjective Optimization criterion comprising of Eigen value based and Time domain based objective functions to compute the optimal controller parameters. The main objective here is to shift the closed loop eigen values to better positions in s plane and also to minimize the integral squared error involving rotor speed and power angle deviations so that the system stability will be enhanced to a greater extent. To validate the effectiveness and robustness of the proposed controller, Non linear simulations has been implemented under various system operating conditions and also introduction of disturbances in the form of outage of Transmission line in the system considered. The proposed controller design is also compared with the conventional Lead lag controller design to show the importance of the proposed Bio inspired controller design.

Fully Evolvable Optimal Neurofuzzy Controller Using Adaptive Critic Designs

A near-optimal neurofuzzy external controller is designed in this paper for a static compensator (STATCOM) in a multimachine power system. The controller provides an auxiliary reference signal for the STATCOM in such a way that it improves the damping of the rotor speed deviations of its neighboring generators. A zero-order Takagi-Sugeno fuzzy rule base constitutes the core of the controller. A heuristic dynamic programming (HDP) based approach is used to further train the controller and enable it to provide nonlinear near-optimal control at different operating conditions of the power system. Based on the connectionist systems theory, the parameters of the neurofuzzy controller, including the membership functions, undergo training. Simulation results are provided that compare the performance of the neurofuzzy controller with and without updating the fuzzy set parameters. Simulation results indicate that updating the membership functions can noticeably improve the performance of the controller and reduce the size of the STATCOM, which leads to lower capital investment.

Optimal Neuro-Fuzzy External Controller for a STATCOM in the 12-Bus Benchmark Power System

An optimal neuro-fuzzy external controller is designed in this paper for a static compensator (STATCOM) in the 12-bus benchmark power system. The controller provides an auxiliary reference signal for the STATCOM in such a way that it improves the damping of the rotor speed deviations of its neighboring generators. A Mamdani fuzzy rule base constitutes the core of the controller. A heuristic dynamic programming-based approach is used to further train the controller and enable it to provide nonlinear optimal control at different operating conditions of the power system. Simulation results are provided that indicate the proposed neuro-fuzzy external controller is more effective than a linear external controller for damping out the speed deviations of the generators. In addition, the two controllers are compared in terms of the control effort generated by each one during various disturbances and the proposed neuro-fuzzy controller proves to be more effective with smaller control effort.

APPLICATIONS OF POLE SHIFTING SELF TUNING SPEED GOVERNOR CONTROLLERS FOR MULTIMACHINE POWER SYSTEMS

Examination of the performance of fixed parameter controllers has resulted in the development of pole shifting strategies for speed governor control of multimachine power system. For a pole shifting controller design an input signal is incorporated in the self tuner. This input signal is of a composite output term of the terminal voltage and the rotor speed deviation of each generating unit. The control is based on a criterion which automatically generates a pole-shifting factor to shift the closed loop poles of the system towards the origin in the z domain without violating the control constraints. The pole-shifting factor is depicted on line at each sampling instant depending on the error measurements of the terminal voltage of each generating unit. The output of the pole shifting controller is fed in the summing point of the speed governor of each unit of the multimachine power system. Digital simulations of a 9 bus, 3 machine of a multimachine power system are performed and the comparisons of conventional governor control and self tuning control performances are presented. Nonlinear digital simulation results show the effectiveness of the proposed technique to damp out the oscillation and work effectively over a wide range of loading conditions.

Optimal Wide Area Controller and State Predictor for a Power System

An optimal wide area controller is designed in this paper for a 12-bus power system together with a Static Compensator (STATCOM). The controller provides auxiliary reference signals for the automatic voltage regulators (AVR) of the generators as well as the line voltage controller of the STATCOM in such a way that it improves the damping of the rotor speed deviations of the synchronous machines. Adaptive critic designs theory is used to implement the controller and enable it to provide nonlinear optimal control over the infinite horizon time of the problem and at different operating conditions of the power system. Simulation results are provided to indicate that the proposed wide area controller improves the damping of the rotor speed deviations of the generators during large scale disturbances. Moreover, a robust radial basis function network based identifier is presented in this paper to predict the states of a multimachine power system in real-time. This wide area state predictor (WASP) compensates for transport lags associated with the present communication technology for wide area monitoring of the electric power grid. The WASP is also robust to partial loss of information caused by larger than expected transport lags or even failed sensors throughout the network

Modelling, simulation and optimal tuning of TCSC controller

A systematic procedure for modeling, simulation and optimal tuning the parameters of a Thyristor Controlled Series Compensator (TCSC) controller, for the power system stability enhancement is presented in this paper. For the simulation purpose, the model of singlemachine infinite bus (SMIB) power system with TCSC controller is developed in MATLAB/SIMULINK. The design problem of TCSC controller is formulated as an optimisation problem and genetic algorithm (GA) is employed to search for the optimal TCSC controller parameters. By minimizing a time-domain based objective function, in which the oscillatory rotor speed deviation of the generator is involved; stability performance of the power system is improved. The results obtained from simulations validate the effectiveness of proposed modelling and tuning approach for power system stability improvement. The simulation results also show that the proposed TCSC controller is effective in damping a range of small disturbance conditions in the power system. (Received in October 2006, accepted in February 2007. This paper was with the authors 1 month for 1 revision.)

State-variable control of shunt FACTS devices using phasor measurements

This paper addresses the problem of state-variable stabilizing control of power system using shunt FACTS devices. This stabilizing control is activated in the transient state of a power system and is supplementary with respect to the main steady-state control of a FACTS device. Stabilizing control laws have been derived for a non-linear multi-machine system model using direct Lyapunov method with the aim to maximize the rate of energy dissipation during power swings and therefore maximization of damping. The proposed control strategy is executed by a non-linear multi-loop controller with rotor angles and speed deviations of synchronous generators used as the input signals. The input signals, obtained from a phasor measurement system, are necessary only from a small area around the controlled shunt FACTS device. Validity of the proposed state-variable control has been confirmed by computer simulation for a small multi-machine test system.

ANALYTICAL INVESTIGATION OF THE EFFECT OF GENERATOR MODELLING ON ELECTROMECHANICAL MODE DAMPING

Power system analytical tools differ in their components modelling. The differences affect electromechanical modes damping. This paper investigates the effect of including rotor speed deviation in stator voltage calculation–with the stator transients neglected- and the modelling of turbine output, on electromechanical mode damping of a single machine infinite bus system. We use a sixth order generator model with different excitation control configurations. We analyse results obtained with EUROSTAG and compare these with results obtained with three other industrial-grade tools. Our results show that: (i) if rotor speed deviation is included in the stator voltage calculation, the results are more conservative than those obtained if speed deviation is neglected. (ii) if the turbine model output is torque, the results are more conservative than those obtained if the output model is power.