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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.

As wind penetration increases in power systems around the world, new challenges to the controllability and operation of a power system are encountered. In particular, frequency response is impacted when a considerable amount of power-electronics interfaced generation, such as wind, is connected to the system. This paper uses small-signal analysis and dynamic simulation to study frequency response in power systems and investigate how Type-3 DFAG wind turbines can impact this response on a test power system, whose frequency response is determined mainly by a frequency-regulation mode. By operating the wind turbines in a deloaded mode, a proposed pitch-angle controller is designed using a root-locus analysis. Time simulations are used to demonstrate the transient and steady-state performance of the proposed controller in the test system with 25% and 50% wind penetration.

This paper proposes a solution for dynamic model validation and calibration leveraging measurement data. The proposed solution is designed to meet key engineering challenges associated with implementing a generic and scalable solution, and can work across multiple dynamic simulation engines with minimal exchange of information. A nonlinear least squares based parameter estimation formulation is considered and its benefits over some of the variants of Kalman filters are discussed. The efficacy of the proposed algorithm is demonstrated on realistic dynamic models of a variety of power plants and measurement data provided by NERC. Finally, an envisioned product implementation accounting for practical implementation challenges is presented.

With the continued development and proliferation of renewable energy systems worldwide, particularly wind and photovoltaic (PV) generation, computer simulation models for these technologies to be used in large interconnected power-system stability analyses have been a key focus over the past several years. Such computer simulation models are used by power-system planners and operators to simulate various scenarios of large interconnected systems?e.g., continental Europe and the western or eastern North American grid?to assess the stability and reliability of the bulk electric power system. In Europe, much of this work has been led by efforts within the International Electrotechnical Commission, while in North America, most of the major efforts in modeling has gone through the Renewable Energy Modeling Task Force (REMTF) of the Western Electricity Coordinating Council (WECC). The total nameplate capacity of wind generation installed worldwide has surpassed 360,000 MW, and PV generation has surpassed 130,000 MW, with the top three countries in terms of cumulative total capacity being China, the United States, and Germany.

The dynamics governing power systems are being modified by the continued increase in the installation of wind turbine-generators (WTGs). This paper presents a small-signal analysis on the impact of the power electronic interface of large quantities of wind generation have on the inter-area oscillation of a two-area system. In particular the sensitivity of this inter-area mode with respect to: (i) increased wind penetration, (ii) loading conditions in the face of wind integration, and (iii) the active and reactive power control parameters of a WTG are presented in this paper. The results show that wind integration increases the damping of this oscillation for most of the cases except when implemented in the area that is importing power.

The uncontrolled intermittent availability of renewable energy sources makes integration of such devices into today's grid a challenge. Thus, it is imperative that dynamic simulation tools used to analyze power system performance are able to support systems with high amounts of photovoltaic (PV) generation. Additionally, simulation durations expanding beyond minutes into hours must be supported. This paper aims to identify the path forward for dynamic simulation tools to accommodate these needs by characterizing the properties of power systems (with high PV penetration), analyzing how these properties affect dynamic simulation software, and offering solutions for potential problems. In particular, the system eigenvalue configuration of representative power system models is examined and how this configuration influences numerical integration scheme selection is discussed.

The power electronic grid interface of converter-based energy systems such as wind turbine-generators (WTGs) is modifying the dynamics governing power systems. For a fundamental understanding of the impact of power electronic interface on the electromechanical modes, that is, the synchronizing and damping torque on synchronous machines, this paper provides a small-signal analysis about the impact of WTGs on the local mode of a single-machine infinite-bus (SMIB) system. The main contribution is the incorporation of the WTG voltage control into the modal decomposition diagram of Heffron-Phillips and deMello-Concordia. Sensitivity coefficients are developed for the WTG voltage control loop and their impact on the real and imaginary parts of the local mode is illustrated with an example.

This paper describes three models developed through the Western Electricity Coordinating Council (WECC) SVC Task Force, to represent static Var systems (SVS) for power-flow and time-domain stability simulations. The goal was to develop a set of model structures that are generic, and can be easily parameterized to represent a variety of SVS systems. The term generic is used to imply a model structure that is not specific to a given vendor or equipment, and that is non-proprietary and public. These models have been implemented by several software vendors, and may soon be adopted by others. These models offer: 1) a suitable non-proprietary, not vendor specific set of models that can be used to evaluate SVS solution options for planning studies, and 2) the means to move away from the proliferation of user-written models that are becoming hard to manage in large interconnected power system models such as the WECC. The dissemination of the models and modeling documentation helps provide guidance to power system planners and operators about the latest SVS technologies and their application.

This paper describes generic models of photovoltaic (PV) systems developed for implementation in Western Electric Coordinating Council (WECC) base cases. The scope encompasses both transmission-connected, central station PV plants and distributed PV systems. These models were added to the WECC Approved Dynamic Model Library in March of 2014.

On August 14, 2003, a cascading outage of transmission and generation facilities in the North American Eastern Interconnection resulted in a blackout of most of New York state as well as parts of Pennsylvania, Ohio, Michigan, and Ontario, Canada. On September 23, 2003, nearly four million customers lost power in eastern Denmark and southern Sweden following a cascading outage that struck Scandinavia. Days later, a cascading outage between Italy and the rest of central Europe left most of Italy in darkness on September 28. These major blackouts are among the worst power system failures in the last few decades. The Power System Stability and Power System Stability Controls Subcommittees of the IEEE PES Power System Dynamic Performance Committee sponsored an all day panel session with experts from around the world. The experts described their recent work on the investigation of grid blackouts. The session offered a unique forum for discussion of possible root causes and necessary steps to reduce the risk of blackouts. This white paper presents the major conclusions drawn from the presentations and ensuing discussions during the all day session, focusing on the root causes of grid blackouts. This paper presents general conclusions drawn by this Committee together with recommendations based on lessons learned.

This paper summarizes the work done by the Task Force on Assessing the Need to Include Higher Order Terms for Small-Signal (Modal) Analysis. This Task Force was created by the Power System Dynamic Performance Committee to investigate the need to include higher order terms for small signal (modal) analysis. The focus of the work reported here is on establishing and documenting the practical significance of these terms in stability analysis using the method of Normal Forms. Special emphasis was placed on determining and describing conditions when higher order terms need to be included to accurately describe modal interactions. Test cases were developed on a standard test system to demonstrate the application of appropriate indices to detect the occurrence of nonlinear interaction and hence the need for higher order terms in stability analyzes. The use of the higher order terms in the site selection for a damping controller is also documented.

Enhancing predictability with “faster than real-time” dynamic simulation will enable the dynamic stability margin, proactive real-time control and will improve grid resiliency to fast time-scale phenomena. The existing tools used for dynamic simulation are optimized for single core of a PC. Therefore, they do not take advantage of multiple computational cores and do not meet the real-time need of the industry. The recent advancement in High Performance Computing (HPC) techniques and parallel numerical libraries presents an opportunity to improve the computational speed of dynamic simulation engines. In this context, this paper presents two approaches for parallelization. In the first approach the time domain simulation engine of GE Concorda PSLF has been parallelized using OpenMP under the existing architecture. Improvements in speed have been observed in cases that are large enough to slow down simulation time, such as the US Eastern Interconnect. Further speed gains are dependent on the acceleration of network solution, which is being researched. In the second approach an implicit integration technique with simultaneous solution of multiple time-steps is presented. An initial implementation of this technique on a small test system shows promising results.

This paper is based on material presented at the Panel Session: Recent Applications of Linear Analysis Techniques conducted during the 1998 IEEE PES Summer Meeting. The topics discussed include: control system design; system identification; large scale system applications; simultaneous coordination of power system stabilisers and FACTS controllers; robust power system stabilisation; small signal stability analysis based on time-domain identified models; EM transient studies modal analysis; and power system dynamics nonlinearities, normal forms method.

We have investigated the use of multiple input signals for the design of power system stabilizers and thyristor-controlled series-compensation controllers. These examples show the benefits of combining input signals to move the system zeros to more desirable locations, allowing the controller to be more effective in providing additional damping to the interarea modes. The multiple-input-signal controller can be used as the primary control system or as a backup system when some of the controllers are temporarily unavailable. In some cases, a multiple-input-signal controller structure may be more cost effective than installing a new control device. The choices of the input signals used for the controller designs in the example systems in the article have been based on good knowledge of the systems.

This paper describes the results of a study to evaluate the performance of three identification methods for the study of low frequency electromechanical oscillations. The three identification methods considered are: the Steiglitz-McBride algorithm; the eigensystem realization algorithm; and the Prony method. The identification methods are used to identify low order linear systems of power systems modeled in standard transient stability programs. This is accomplished by processing the system response to a simple probing pulse. The frequency domain characteristics of several identified systems are compared using three power systems with lightly damped electromechanical modes.

This paper presents an integrated approach for designing damping controllers for large power systems. First, a low-order model is identified from a time simulation of the nonlinear power system model. Then an exact multi-modal decomposition is used to develop design indices for selecting on appropriate input signal to the controller. The method is illustrated with the design of thyristor-controlled series compensator damping controllers in two test systems.

This paper presents the design and application of a simple controller to damp interarea oscillations. The controller coordinates the control action of a thyristor controlled series capacitor (TCSC) and a thyristor controlled phase angle regulator (TCPAR) to allow for increased power transfers with improved system damping. The controller is based on the application of projective controls and coordinates readily available measurements with control signals applied to the FACTS devices.

This paper presents the implementation of an efficient method for computing low order linear system models of power systems from time domain simulations. The method is the identification algorithm initially introduced by K. Steiglitz and L.E. McBride for calculating the transfer function of a linear system from samples of its input and output. The paper describes the original algorithm, added extensions, and issues addressed for its practical implementation. The applicability of the method is demonstrated by computing a low-order equivalent linear system of a detailed three-phase power system, modeled on an Electromagnetic Transients Program (EMTP). The paper discusses practical implementation issues.

Traditionally, the linear analysis of a power system is performed by first computing a state space model, followed by the application of a suitable analysis method. However, the applicability of this approach is restricted by several practical limitations: typical power systems are represented by very large state matrices that require specialized large-scale eigen analysis programs; the program where the power system models reside does not include linearization capabilities; only time domain data is available. This article illustrates the applicability of a new system identification software program, SYSID, developed to circumvent these limitations. The program incorporates several identification techniques that allow for the computation of equivalent linear models, suitable for control design applications, from time domain simulation data. The program has been used to calculate equivalent models from simulations performed with standard stability programs, as well as from the Electromagnetic Transient Program (EMTP) simulations of detailed three-phase power systems. The equivalent systems are small and retain the modal characteristics of the original system. The identification techniques are based on various algorithms including Prony's method, transfer function fitting, and Hankel matrices. In addition to the identification algorithms, the program also includes signal processing, plotting, modal analysis functions, and basic control design capabilities to assess closed loop system performance.

This paper describes the main results and conclusions from the application of two different long-term stability programs to the analysis of a system islanding scenario for a study case developed by Red Electrica de Espana (REE), based on the Spanish system. Two main goals were to evaluate the performance of both simulation programs in the long-term stability area, and to analyze the influence of some important control and protection elements (tie-line loss-of-synchronism relays, underfrequency load-shedding, load-frequency control, and power plant dynamics). Conclusions about modeling and computational requirements for system islanding (frequency) scenarios and use of long-term stability programs are presented.

This paper describes an algorithm to calculate low-order linear models of power systems from small-signal time-domain simulation data. The linear system models preserve the modal characteristics of the underlying power system. The method is based on the computation of a real symmetric matrix and is well suited for the analysis of large data sets. The results presented in the paper include the computation of a transfer function suitable for power system stabilizer design and the synthesis of an equivalent linear system from a three-phase power system model.

This paper describes the application of a reduction method to power systems to obtain simplified models that facilitate the design of damping controllers. The reduced system retains the lightly damped swing of interest and exhibits modal characteristics similar to the unreduced system in the frequency range associated with swing modes. A power system damping controller is designed using a reduced system. The modal characteristics of the reduced and unreduced system with the damping controller included, are compared. The validity of the control design is assessed via nonlinear simulations. The reduction method is simple to implement and is based on the computation of the observability and controllability Gramians.

The design of controllers sited in the transmission network for damping interarea power oscillations requires several types of analytical tools and field verification methods. Probably the most important aspect of such control design is the selection of proper feedback measurements from the network. This paper describes concepts which provide design engineers with the insight to control performance and the understanding needed to ensure the secure operation of the bulk transmission system. Specific attention is directed to procedures for selecting feedback signals. >

GE Energy has an ongoing effort dedicated to the development of models of GE wind turbine generators (WTG) suitable for representing wind plants in system impact studies. This paper documents the present recommendations for dynamic modeling of GE wind plants using either Doubly Fed Asynchronous Generators (DFAG) or Full Conversion (FC) technology. This includes GE's 1.5 and 1.6 MW, as well as 2.5 and 2.75 MW WTGs. The paper presents both the overall model structure and validation results for the non-generic GE equipment models. The assumptions, capabilities and limitations of the plant model will also be discussed.

The need for generic, standard, non-proprietary models for wind power plants continues to be the subject of much discussion and debate. From a technical point of view, the representation of the often complex dynamic behavior of modern wind power plants is not trivial. However, system planners and compliance organizations continue to struggle with the process deficiencies associated with the black-box and proprietary nature of manufacturer-specific models. For several years, the Western Electricity Coordinating Council (WECC) has championed the development of generic models for wind power plant models, and the progress to date is reported in this document. Recently, other organizations including the International Electromechanical Commission (IEC), manufacturers, software developers, and even utilities have been pursuing similar technical goals. It is anticipated that, through the collective efforts of these stakeholders, generic models will fulfill a much needed gap. This paper reports on the progress made to-date within the Western Electricity Coordinating Council (WECC) regarding the development of generic models suitable for representing wind power plants in typical transmission planning studies. The manuscript address technical issues associated with the representation of wind turbine generators for load flow and transient stability analyses. Current capabilities and envisioned enhancements to existing models are also discussed.

This paper summarizes work performed by the WECC Wind Generation Modeling Group and the IEEE Working Group on Dynamic Performance of Wind Power Generation regarding generic Wind Turbine Generator models, their development, and specifications.

The application of the Extended Time Scale Stability Analysis program (EXSTAB) is illustrated. The program was developed for the numerical simulation of power system phenomena spanning time periods from several seconds to several hours and involving slow events, such as load cycles, as well as fast transients, such as line trips. The integration algorithm in this problem automatically reduces the time step of integration to capture fast transients and gradually increases it as the transients decay. The algorithm is based on the simultaneous solution of the differential and algebraic equations describing a power system, using a numerically stable, implicit, low-order integrator similar to the trapezoidal rule. Recognizing that the simulation of severe transients over periods of time of a few seconds is handled more efficiently using explicit methods with fixed time steps, the program automatically switches to an explicit integrator following the occurrence of a severe disturbance. The program automatically returns to an implicit mode once the severe transient is over. >

A new program, EXSTAB (extended time-scale stability) has been developed for representing a wide variety of power system performance problems, from transient stability through long-term dynamics and voltage instability. The capability of the program includes multiple execution modes and automatic step size selection to address conflicting goals of accuracy and efficiency. The modeling includes a broad range of apparatus to provide the needed time-scale representation (four orders of magnitude). Models for automatic generation control, plant characteristics and control, voltage and reactive power control, static and dynamic loads, and protective relaying for apparatus and system connections are provided. Technologies were developed to perform analysis of voltage stability and prediction of peak power transfer to avoid voltage collapse. >

The feasibility of automating the identification of electromechanical models based on well-established procedures is investigated. The software environment to perform the modal identification is described and representative results are discussed.

This paper describes the current status of the work being conducted by the Western Electricity Coordinating Council (WECC) SVC Task Force to develop generic power flow and time-domain stability models to represent all existing and future (modern) static Var systems (SVS) in the Western Interconnection. The intent is to have these models implemented by software vendors such as GE, Siemens PTI, Powertech Labs and others so that these models are then available to user's in WECC (and worldwide) for their use in system planning studies. The value of this exercise is three fold: (i) it provides suitable non-proprietary, non-vendor specific models that can be used to evaluate SVS solution options for planning studies, (ii) it helps to move away from the proliferation of user-written models that are becoming hard to manage in large interconnected power system models such as the WECC, and (iii) the dissemination of the models and modeling documentation helps to educate power system planners and operators about the latest SVS technologies and their application.

A system-wide dynamic controller is designed to improve power system damping. Multivariable control techniques, involving many machines and associated measurements and controls, are applied on a large-scale system with realistic dynamics. An iterative design method with multiple tasks is presented. It uses eigenvector-based dynamic reduction, suboptimal state space control design, reconstitution of design to full system representation, and verification. The resulting hierarchical controller is shown, with time simulations and eigenvalues, to stabilize the system for heavy power transfer without adverse side effects. >

This paper presents the results of an investigation to establish the feasibility of using a simple aerodynamic model for wind turbine generators, suitable for application in transient stability studies

The work reported in this paper highlights differences and similarities between conventional linear analysis and the method of normal forms. The paper is focused on the effect that second order modal interactions have in the dynamic performance of a power system and identifies dynamic characteristics not captured by conventional linear analysis.

This paper presents the results of an investigation aimed at ascertaining the effect that a sizeable wind farm exerts on the oscillation modes of a two-area, four-generator power system. The modal characteristics of the system, when a wind farm replaces one of the generators, are analyzed.

This paper describes the identification of low-order linear models for two power systems modeled in a standard transient stability program. The approach used in the paper is the eigensystem realization algorithm/observer (ERA/OBS) identification method. The identified models retain the modal characteristics associated with lightly damped electromechanical modes.

Interconnection of wind generation with power distribution systems may result in rapid voltage variations, or flicker, much more readily than for HV transmission interconnections. Wind turbine generators employing doubly-fed asynchronous generators provide a means to rapidly control reactive power, as well as to smooth variations in real power. This capability is exploited to minimize flicker, despite rapid variations of wind velocity. The principles of distribution voltage control using WTGs with doubly-fed technology are explained, and practical examples of performance are illustrated. Comparisons between DFAG and pitch-regulated induction generators are provided.

GE power systems has an ongoing effort dedicated to development of models of GE wind turbine generators (WTG) suitable for use in system impact studies. This paper documents the present recommendations for dynamic modeling of the GE 1.5 and 3.6 MW WTG for use in system impact studies. The paper includes recommended model structure and data, as well the assumptions, capabilities and limitations of the resulting model.

Feedback control of FACTS (flexible AC transmission system) devices typically relies on network variables because generator signals are not usually available locally. Based on the multi-modal decomposition approach, a crucial factor in selecting a feedback variable is the impact of the variable on the inner-loop gain, which consists of mostly the sensitivity of the network variable with respect to the change in the control input variable. Recent investigations have shown that flow variables are more suitable for shunt control devices and nodal variables are more suitable for series control devices. In this paper, the authors use a simple system to demonstrate this concept. In addition, the impact of the interarea modes on the inner-loop gain is studied with a system with two interarea modes.

The basic function of the PSS is to apply a signal to the excitation system, creating electrical torques that damp out power oscillations. There is a significant body of literature about the design, tuning and performance of PSS controls. In some cases this previous work has not fully considered some important practical aspects that can affect the way the PSS controls influence the unit response. Although the primary function of the PSS is to supply positive damping torque contributions, experience indicates that it can impact the generator-power system transient performance under certain conditions. The various aspects of the PSS performance considered in this paper can be summarized in four areas: (1) the effect of PSS on system volt/VAr swings; (2) the interaction of the PSS with governor action during slow frequency regulation, or unit loading/unloading; (3) the interaction of the PSS with other controls such as UEL, VAr controllers, and HVDC, SVC, or TCSC devices; and (4) the possible interaction of the PSS with the turbine generator shaft torsional dynamics. These issues are the focus of this paper. Test results and computer simulations are shown to illustrate the concepts, and guidelines are presented to provide a unified tuning.

This paper describes the application of a Prony-based method for the identification of low-order linear models of detailed three-phase power systems modeled in electromagnetic transient programs (EMTP). A description of the method and discussion of practical issues are included. It is shown that the identified model approximates the response of the three-phase system and preserves interarea and torsional modes.

The backpropagation algorithm for neural network training is formulated as the solution of a set of sparse differential algebraic equations (DAE). These equations are then solved as a function of time. The solution of the differential equations is performed using an implicit integrator with adjustable time step. The topology of the Jacobian matrix associated with the DAE's is illustrated. A training example is included. >

This paper describes the development of a variable time step algorithm for the numerical simulation of power system phenomena spanning time periods from several seconds to several hours. The algorithm automatically reduces the time step of integration to capture fast transients and increases it when the system variables are varying slowly. The solution algorithm is based on a numerically stable, implicit, low order integration similar to the trapezoidal rule. Computer simulation results for two scenarios are presented. These scenarios are: a severe power system load ramp leading to voltage collapse; and a six-hour operational cycle.

This paper describes the application of a model reduction method to obtain linear power system models for control system design. The reduction method is based on the computation of an eigensubspace of the product of the observability and controllability Gramians. The technique avoids illconditioned balancing transformations. The method is applied to coherency reduced power systems to generate smaller systems that are suitable for control design applications. These reduced systems retain the interarea modes and their input/output characteristics are similar to those of the coherent models. A 376-state and a 184-state power system models are used to illustrate the method. For the 184-state model, a control design verifies that a 45-state reduced model is a valid design model. For systems with very large sets of state variables, a coherency input-output approach is proposed. In this approach, the reduction method is applied to individual coherent areas.

This paper describes the potential performance benefits that can be realized by the application of a STATCOM in conjunction with a conventional generator, by allowing the generator to increase its power output beyond its rated power.

This paper demonstrates the feasibility of using the system identification method known as the eigensystem realization algorithm (ERA) as a means for computing the subsynchronous torsional modes of turbine-generator sets. Measured data obtained during an out-of-phase synchronization test of a large turbine generator are used to perform the modal computation. Both front and rear standard speeds are used. The results are compared with those obtained using more conventional methods.

The maximum continuous power output of a gas turbine depends on the system frequency and the ambient temperature. The temperature control of a gas turbine limits the exhaust temperature by reducing the fuel flow as the air flow decreases with the shaft speed. This paper addresses the frequency dependency of the gas turbine power output and presents a physically-based model of a gas turbine that may enable parameter estimation of the frequency dependency from the ambient temperature dependency. The model is suitable for long-term simulation of power system dynamic performance involving abnormal frequency conditions.

Power system events have shown that voltage collapse can be a root cause of some widespread outages. The thrust of this paper is to discuss some means of mitigating system voltage depressions that are available through excitation control of high side voltage. While such control is usually available for individual units through reactive current control, it was difficult to arrange for this control in multi-unit plants where units share a common step-up transformer. Advances in the control architecture allow for such control to be easily implemented. The advantages of this control are outlined in this paper, together with some typical results to show expected benefits. Also, we briefly review some related VAr control strategies for station and system voltage controls and describe their application.

This paper demonstrates the applicability of the identification method of Steiglitz and McBride (1965) to the identification of low-order linear models using time domain measured data from a power system simulator. It is shown that the identified linear models retain the essential dynamic characteristics of the actual system and are suitable for modal analysis and control design for damping low frequency electromechanical oscillations.

The 11 papers in this special section focus on wind energy. The objective of the section is to provide a wide perspective on different aspects associated with wind power as it relates to power system engineering, with an emphasis on dynamic modeling and simulation.