Western System Coordinating Council Research Articles

Non-Stationary Stabilized Fast Transversal RLS Filter for Online Power System Modal Estimation

Online monitoring of inter-area oscillations is a vital need for secure operation of large-scale power systems. This paper aims at online estimation of inter-area modes by a novel stabilized fast transversal recursive least squares (SFTRLS) filter running on ambient power system data received from phasor measurement units. The SFTRLS filter is a fast finite-impulse response adaptive filter that is widely used in communication applications, such as noise and echo cancelation. It has the lowest computational complexity among least-squares filters and directly calculates coefficients of the parametric model selected for identifying a system under study. In the proposed method, the SFTRLS filter along with digital band-pass Butterworth filter and down-sampling blocks are implemented in online manner for identification of an autoregressive model that describes the measured power system ambient signal. Applicability and accuracy of the proposed method are evaluated using real measurement data of western systems coordinating council (WSCC) breakup on August 10, 1996, as well as simulated ambient data of the IEEE 16-machine, 5-area test system excited by white noise random load modulations. Its performance is investigated under different conditions of load modulations, measurement noises, sudden change in damping ratio, and occurrence of three-phase faults. The simulation results verify efficient and robust performance of the proposed method for modal estimation of real large-scale power systems.

Passivity Sliding Mode Control of Large-Scale Power Systems

In this brief, a sliding mode (SM) decentralized excitation controller for multimachine power systems is designed to stabilize the speed and regulate the terminal voltage of each generator in the system. To achieve the speed stabilization, a sliding manifold is designed using the passivity technique. An integral SM controller is proposed to reject internal and external perturbations, taking advantage of the energy properties of the power systems, in the SM motion on the designed manifold. Then, an SM voltage regulator is included in the control scheme, and a logic control is formulated to manage two proposed control laws for the speed and terminal voltage. Finally, to estimate the unmeasured rotor fluxes a nonlinear observer is designed. The proposed control scheme was tested through simulation on an equivalent of the Western System Coordinating Council under small and large perturbations.

A new approach for df/dt and active power imbalance in power system estimation using Huang’s Empirical Mode decomposition

In this paper, Huang’s Empirical Mode Decomposition approach is proposed for df/dt and active power imbalance in power system estimation. Applied approach implies availability of synchronized phasor measurement units. In addition to the successful applications in the analysis of nonstationary dynamic behavior of power system, identifications and analyses of low-frequency electromechanical oscillations and signals denoising, this approach also enables direct estimation of rate of change of a weighted average frequency (frequency of the center of inertia), as well as assessment of the overall imbalance in the power system. This demonstration is performed using computer simulation testing on the 39 Bus New England System and Western System Coordinating Council 118 bus test systems in the DigSILENT PowerFactory power system analysis software package. To validate the proposed approach the actual frequency information are used. Empirical Mode Decomposition approach is compared with Discrete Wavelet Transform, Method of Least Squares and the results from the DigSILENT PowerFactory. Also, performance of the empirical mode decomposition are compared with performances of the multivariate empirical mode decomposition and noise assisted multivariate empirical mode decomposition on both, simulated signals and field measurements. Applied approach is implemented in the MATLAB environment and results show very high accuracy.

DC offset estimation‐based fault detection in transmission line during power swing using ensemble of decision tree

The nuisance tripping of distance relay during power swing has been identified as one of the major causes for power system blackouts. Avoiding the risk of maloperation of distance relay and hence improving resilience during power swing, demands the development of a scheme capable of generating blocking signal during power swing and detecting a fault occurring with power swing from the measurement of current signal. In this regard, a protection scheme based on the hybrid framework of an ensemble of the tree, least square, and Adaline algorithm to perform the task of fault detection/classification, zone identification, location estimation, and power swing detection is proposed. The proposed scheme is based on the online estimation of DC offset information along with the fundamental component of the current signal. The performance of the proposed scheme has been analysed for diverse power swing and fault scenarios for two-machine system and Western System Coordinating Council 9-bus system. The simulation results reflect the effectiveness of the developed scheme in terms of sensitivity and reliability. Furthermore, to authenticate the effectiveness of the proposed scheme for practical scenarios, implementation, and validation of the proposed scheme has been carried out in a real-time environment using the OPAL-RT digital simulator.

Mean Square Stability Analysis of Stochastic Continuous-Time Linear Networked Systems

In this technical note, we study the mean square stability based analysis of stochastic continuous-time linear networked systems. The stochastic uncertainty is assumed to enter multiplicatively in system dynamics through input and output channels of the plant. Necessary and sufficient conditions for mean square exponential stability are expressed in terms of the input–output property of deterministic or nominal system dynamics captured by the mean square system norm and variance of channel uncertainty. The stability results can also be interpreted as a small-gain theorem for continuous-time stochastic systems. Linear matrix inequalities based optimization formulation is provided for the computation of mean square system norm for the stability analysis. For a special case of single-input channel uncertainty, we also prove a fundamental limitation result that arises in the mean square exponential stabilization of the continuous-time linear system. Overall, the contributions in this technical note generalize the existing results on stability analysis from discrete-time linear systems to continuous-time linear systems with multiplicative uncertainty. Simulation results are presented for Western System Coordinating Council 9-bus power system to demonstrate the application of the developed framework.

Decentralized nonlinear model predictive control of a multimachine power system

This paper proposes a novel decentralized nonlinear model predictive control approach for excitation control in multimachine power systems. A key feature of the proposed approach is the reduction of the multimachine control problem to multiple single-machine control problems. These reduced size control problems only require direct measurements from individual generators, thereby making implementation simple in practice. The effectiveness of the proposed approach is demonstrated through computer simulations on a one small-size Western System Coordinating Council (WSCC) and one large-size New England power system under a wide range of operating conditions. The performance of the proposed approach is evaluated in terms of damping power system oscillations, critical time enhancement, and control signal energy consumption. The results are compared with those obtained using power system stabilizer and feedback linearization based control schemes and found to be impressive.

Supervising zone‐3 operation of the distance relay using synchronised phasor measurements

The study presents a new protection scheme to enhance the zone-3 operation of the distance relay. The proposed scheme develops an index based on the rate of change of active power and angle derived from synchronised phasor measurement units (PMUs). It is found that it clearly distinguishes faults from several stressed conditions those are likely to occur, and takes necessary action. The proposed scheme is developed and validated for a modified Western System Coordinating Council 9-bus, 3-machine system and New England 39-bus system with different fault scenarios on a MATLAB/Simulink platform. Further to test the robustness, the proposed method is tested on a real-time digital simulator platform using phasors received from gigabit transceiver network PMUs. Extensive performance testing for faulted as well as stressed conditions such as stable power swing, load encroachment and voltage stressed conditions indicate that the proposed scheme is highly reliable in enhancing the zone-3 operation of distance relay.

Robust protection scheme against cyber‐physical attacks in power systems

This study presents a robust defence strategy in reaction to destabilising cyber-physical attacks launched against linear time invariant systems and its application to power systems. The proposed protection scheme aims at making the dynamics of a selected subsystem decoupled from the dynamics of the subsystem targeted by the attack. The standard decoupling methods are made robust, in spite of poor information about plant parameters and lack of state measurement, with the aid of an extended observer. In this way, it is possible to keep the protected dynamics arbitrarily close to the one of a suitably chosen stable system, so long as the dynamics being targeted by the attack remain within the prescribed bounds. The proposed defence strategy is presented in the context of modern power systems, wherein generators and transmission networks are operated by different players and shown to be effective using the Western System Coordinating Council 9-bus test power network.

Networked control of a power system: A non-uniform sampling approach

This article deals with the damping of low-frequency power systems oscillations using a networked controller on a smart grid. We assume that the network channel is band-limited and has random communication delay whose probability distribution can be found. To mitigate the effect of delay on control system performance, the measurements from the system are sampled at a random and non-uniform period but the inputs are updated at a faster and constant rate. This makes the closed-loop control system a stochastic switching system. A framework to analyze the mean square stability of the closed-loop system is developed and a switching observer-based controller is designed to ensure the stability of the system at random sampling instants. A case study has been performed using the Western-System-Coordinating-Council (WSCC) 3-machines, 9-bus system for verification of the technique. The contribution of the paper is a bandwidth efficient delay compensation technique and its application to power systems stability enhancement in a smart grid.

Calibrating Parameters of Power System Stability Models Using Advanced Ensemble Kalman Filter

With the ever increasing penetration of renewable energy, smart loads, energy storage, and new market behavior, today's power grid becomes more dynamic and stochastic, which may invalidate traditional study assumptions and pose great operational challenges. Thus, it is of critical importance to maintain good-quality models for secure and economic planning and real-time operation. Following the 1996 Western Systems Coordinating Council system blackout, North American Electric Reliability Corporation (NERC) and Western Electricity Coordinating Council (WECC) in North America enforced a number of policies and standards to guide the power industry to periodically validate power grid models and calibrate poor parameters with the goal of building sufficient confidence in model quality. The PMU-based approach using online measurements without interfering with the operation of generators provides a low-cost alternative to meet NERC standards. This paper presents an innovative procedure and tool suites to validate and calibrate models based on a trajectory sensitivity analysis method and an advanced ensemble Kalman filter algorithm. The developed prototype demonstrates excellent performance in identifying and calibrating bad parameters of a realistic hydro power plant against multiple system events.

Robust fault analysis in transmission lines using Synchrophasor measurements

As more electric utilities and transmission system operators move toward the smart grid concept, robust fault analysis has become increasingly complex. This paper proposes a methodology for the detection, classification, and localization of transmission line faults using Synchrophasor measurements. The technique involves the extraction of phasors from the instantaneous three-phase voltages and currents at each bus in the system which are then decomposed into their symmetrical components. These components are sent to the phasor data concentrator (PDC) for real-time fault analysis, which is completed within 2–3 cycles after fault inception. The advantages of this technique are its accuracy and speed, so that fault information may be appropriately communicated to facilitate system restoration. The proposed algorithm is independent of the transmission system topology and displays high accuracy in its results, even with varying parameters such as fault distance, fault inception angle and fault impedance. The proposed algorithm is validated using a three-bus system as well as the Western System Coordinating Council (WSCC) nine bus system. The proposed algorithm is shown to accurately detect the faulted line and classify the fault in all the test cases presented.

Power system frequency stabiliser for modern power systems

Modern power system involves growing use of open channel communication in secondary frequency regulation which introduces time delay in the automatic generation control loop which may lead to system instability. This study proposes a novel power system frequency stabiliser (PSFS) which is a modified secondary controller in order to improve the system stability in the presence of large communication delays. An auxiliary signal is added to the secondary controller to provide additional damping. The gain of the controller is obtained by meta-heuristic methods. The proposed controller is first developed on a single area hydropower system and the same is then applied on the Western System Coordinating Council 3-machine, 9-bus power system. Furthermore, to show the effectiveness of the proposed PSFS in a large practical multi-area system; it is also applied to the IEEE 39-bus system. Results show that the proposed controller significantly improves the stability of a power system operating with large communication delay which is first considered as constant and then the stability improvement with the proposed PSFS in the presence of random communication delays is also shown. The PSFS performance is tested by the results obtained with MATLAB/Simulink simulations and further verified on a real-time simulator.

New Performance Indices for Voltage Stability Analysis in a Power System

The frequent occurrence of voltage instability in a modern power system is alarming and thus, has been of great concern to power system utilities. In this paper, a new performance index based on the power flow solutions for voltage stability assessment of a power system is presented. First, the voltage deviation with respect to reactive power load variation at each load bus is found. Thereafter, the performance voltage bus index for each load bus is computed. An improved modal analysis technique (IMAT) is used to identify weak nodes that are liable to voltage instability in a power system. This technique uses a submatrix of the full Jacobian matrix for voltage stability analysis. Comparison of the proposed method is done with existing voltage stability indices and the conventional modal analysis technique (CMAT). The effectiveness of all the approaches presented are tested on both Western system coordinating Council (WSCC) 9-bus, IEEE 30 bus and IEEE 57 bus test systems. Results obtained show that the proposed techniques can serve as an alternative tool to other conventional techniques for voltage stability assessment in a power system and can be of tremendous benefits in the planning and operation of a power system by system operators.

Prediction Model of the Power System Frequency Using a Cross-Entropy Ensemble Algorithm

Frequency prediction after a disturbance has received increasing research attention given its substantial value in providing a decision-making foundation in power system emergency control. With the advancing development of machine learning, analysis power systems with machine-learning methods has become completely different from traditional approaches. In this paper, an ensemble algorithm using cross-entropy as a combination strategy is presented to address the trade-off between prediction accuracy and calculation speed. The prediction difficulty caused by inadequate numbers of severe disturbance samples is also overcome by the ensemble model. In the proposed ensemble algorithm, base learners are selected following the principle of diversity, which guarantees the ensemble algorithm’s accuracy. Cross-entropy is applied to evaluate the fitting performance of the base learners and to set the weight coefficient in the ensemble algorithm. Subsequently, an online prediction model based on the algorithm is established that integrates training, prediction and updating. In the Western System Coordinating Council 9-bus (WSCC 9) system and the Institute of Electrical and Electronics Engineers 39-bus (IEEE 39) system, the algorithm is shown to significantly improve the prediction accuracy in both sample-rich and sample-poor situations, verifying the effectiveness and superiority of the proposed ensemble algorithm.

Small Signal Stability Analysis of Grid Connected Renewable Energy Resources with the Effect of Uncertain Wind Power Penetration

Small Signal Stability Analysis (SSSA) for a hybrid renewable energy system including uncertainties requires an extensive attention to make a system more stable. This paper analyses the method involved in the stability studies of integrated hybrid power system with the help of Lyapunov's stability criteria. The stability studies can be performed for different renewable energy sources like wind generation system, solar PV system and a micro hydro system. In this paper, a study is carried out on a wind generation system which is integrated with the Western System Coordinating Council (WSCC) four machine system. The stability analysis concentrates on the different states of operation of WSCC four machine system with random values of generation and demand. The simulation studies on WSCC four machine system are carried out with the help of MATLAB/Simulink to evaluate stability. This study is validated with the aid of Monte Carlo Approach for evaluations based on iterations. This investigation highlights the significant advantage of conducting stability studies with uncertainty constraints involved, over the deterministic stability studies like probable limitations for transfer capability to cancel out poorly damped oscillations from power system operation.

A backup wide‐area protection technique for power transmission network

This paper proposes a wide‐area backup protection technique for transmission lines using a limited number of synchronized phasor measurement units. A binary integral linear programming is used for optimal placement of phasor measurement units to achieve fault observability. The positive‐sequence voltage and current components are utilized for the detection and location of transmission lines faults. The proposed technique considers the randomness of faults on the transmission grid, different fault types, frequency fluctuations, synchronization and measurement errors, different fault inception angles, and different fault resistances, as well as various loading conditions. The proposed scheme is applied to the Western System Coordinating Council (WSCC) 9‐bus test system and the New England 39‐bus test system. Simulation results prove that the proposed protection scheme can detect and locate all transmission line faults correctly. © 2017 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc.

Analogous elastic network for the analysis of power system energy structure

A new approach to the analysis of the transient energy interaction in power systems is presented in this work on the basis of an analogous elastic network. A power system is first rigorously mapped to an elastic network consisting of elastic branches with different elasticity coefficients. On the basis of elasticity mechanics and the analogous elastic network, the transient energy of the different parts, such as the synchronous generators, branches, and loads, is defined and has been proved to agree with the law of energy conservation. Subsequently, based on the law of energy conservation and energy feature analysis, the concept of the power system energy structure (PSES) is built to exhibit the distribution and interaction of transient energy in the power system. Wide area and local area transient energy interaction can be analysed using the measurement-based energy structure, which shows obvious potential in on-line monitoring and analysis of power system oscillation. Finally, the simulations of Western Systems Coordinating Council (WSCC) three-generator test system and a practical power system are performed and analysed to verify the rationality of the analogous elastic network and the validity of the proposed PSES.

Optimizing PSS parameters for a multi-machine power system using genetic algorithm and neural network techniques

Power system stabilizers (PSSs) are generally used to solve the low-frequency oscillation problem. To overcome this type of oscillation, this paper presented a new method by using the genetic algorithm (GA) to identify PSS settings. The tuning of PSS parameters was formulated relying on an eigenvalue-based objective function aiming at maximizing the stability margin. This was achieved by increasing the sum of the squares of negative real parts of the system eigenvalues. The small disruptions in the form of load change take place routinely; the controller parameters should be adjusted to the changing conditions. To remedy this defect, an adjustment of parameters as a function of the load was required. Thus, a neuronal model by using a historical database determined by the GA solutions for various load levels can approximate the simulation studies, since it could estimate the stabilizer parameters in real time after the learning phase. The validity of the proposed technique was checked through the evolution simulation of the regulator parameters for a load forecast curve. These concepts were discussed in details on a multi-machine network Western System Coordinating Council (WSCC) comprising three generators and nine nodes. The eigenvalue analysis and time domain simulation results were presented at different operating conditions and under various disturbances in order to show the effectiveness of this study.

A Non-Stationary Analysis of Low-Frequency Electromechanical Oscillations Based on a Refined Margenau-Hill Distribution

This paper focuses on the use of quadratic time-frequency distributions (QTFDs) to characterize the temporal variation of low-frequency electromechanical oscillations (LFEOs) using ringdown data. A refined Margenau-Hill distribution (MHD) and a procedure to extract the instantaneous frequencies and damping factors as well as initial phase angles of system low-frequency modes when the analyzed signal is multi-component are proposed. The refinement process enjoys a fast-converging iterative algorithm and without loss of generality it can be applied to other QTFDs for using in other applications. A non-stationary synthetic signal as well as simulated ringdown data of a 5-area, 16-machine power system and real measured data of Western Systems Coordinating Council (WSCC) system are used to demonstrate the efficiency of the proposed method in the presence of measurement noise.

Real-time simulation and performance analysis of multimachine power systems using dSPACE simulator

The analysis of the characteristics of multiple machines constituting a power system in real time is of paramount importance. The reported work uses dSPACE, a dynamic simulator, in investigating the characteristics of the said multimachine system. The transient model of the Western System Coordinating Council (WSCC) nine-bus, three-unit system is developed using MATLAB/Simulink. Using real-time interface, the model is executed in the digital signal processor (DSP) of dSPACE hardware. The model is analyzed for its performance against multiple changes in mechanical torque and load variation. The transient behavior of the model upon incidence of a significant network fault and its robustness towards to perturbations in network and system parameters are thoroughly analyzed. The conditions are emulated by usage of the digital signal processor and DS1104 Controller Board.