V. Ajjarapu

Iowa State University, Ames, IA, United States

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Publications (106)114 Total impact

  • [Show abstract] [Hide abstract]
    ABSTRACT: Short term voltage stability poses a significant threat to system stability and reliability. This paper applies dynamic VAr injection to ensure short term voltage stability following a large disturbance in a power system with high concentration of induction motor loads. Decelerating and stalling of induction motor loads is considered to be the major cause of fault induced delayed voltage recovery (FIDVR) and short term voltage stability. If system dynamics are not taken into account properly, the proposed control solution may be an expensive over design or an under design that is not capable of eliminating FIDVR problems completely. In this work, the optimal amount and locations for installing dynamic reactive resources are found by control vector parameterization (CVP), a dynamic optimization approach. The efficiency and effectiveness of this approach is improved by utilizing results from trajectory sensitivity analysis, singular value decomposition and linear programming optimization. Dynamic optimization based on CVP approach is tested in an IEEE 162-bus system and a realistic large scale utility power system.
    IEEE Transactions on Power Systems 01/2013; 28(4):3865-3873. · 2.92 Impact Factor
  • B. Leonardi, V. Ajjarapu
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    ABSTRACT: This study proposes a man-in-loop control method to boost reactive power reserves (RPRs) while maintaining a minimum amount of voltage stability margin (VSM) bus voltage limits. The objective is to determine the most effective control actions in order to reestablish critical RPRs across the system. Initially, the concept of reactive power reserve sensitivity with respect to control actions is introduced. In the sequel, a control approach based on convex quadratic optimization is used to find the minimal amount of control necessary to increase RPRs above their pre-specified (offline) levels. Voltage stability margin constraints are incorporated using a linear approximation of critical RPRs.
    IEEE Transactions on Power Systems 01/2013; 28(2):615-625. · 2.92 Impact Factor
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    ABSTRACT: We develop a model-free approach for the short-term voltage stability monitoring of a power system. Finite time Lyapunov exponents are used as the certificate of stability. The time-series voltage data from phasor measurement units (PMU) are used to compute the Lyapunov exponent to predict voltage stability in real time. Issues related to practical implementation of the proposed method, such as phasor measurement noise, communication delay, and the finite window size for prediction, are also discussed. Furthermore, the stability certificate in the form of Lyapunov exponents is also used to determine the stability/instability contributions of the individual buses to the overall system stability and for computation of critical clearing time. Simulation results are provided for the IEEE 162-bus system to demonstrate the application of the developed method.
    IEEE Transactions on Power Systems 01/2013; 28(4):3702-3711. · 2.92 Impact Factor
  • M.H. Fan, C.S. Wang, V. Ajjarapu
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    ABSTRACT: This study presents the continuation scheme of the recursive projection method (RPM) for power system equilibrium tracing and bifurcation detection using time-domain simulation code (TDSC). For a given operational condition, a basic RPM algorithm has been demonstrated to be very effective to extract power system steady-state and small-signal stability information from TDSC. It is now combined with arc-length continuation technique to mitigate singularity problems when the operational parameter variations are considered. Along the equilibrium tracing path, saddle node bifurcations (SNBs) and Hopf bifurcations (HBs) can be detected conveniently from byproducts of the RPM procedure. The proposed method takes advantage of available resources and avoids full state-space linearisation and large-scale eigenspectrum computation. The significance of the proposed approach is validated by a test example on New England 39-bus system.
    IET Generation Transmission & Distribution 01/2012; 6(3):199-206. · 1.41 Impact Factor
  • K.K. Yagnik, V. Ajjarapu
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    ABSTRACT: The purpose of this paper is to examine the effects of replacing conventional generation by wind and solar generation on the grid voltage performance. California is one of the states which will be facing adverse effects of such generation shift. California has also very high renewable portfolio standards (RPS) target. Therefore, a western electricity coordination council (WECC) equivalent system is examined to identify the issues with voltage performance after such generation alteration. Two approaches are examined to mitigate the negative impacts on voltage performance by adding synchronous condenser at the retiring conventional units and by operating wind generation with voltage control mode of operation.
    Power and Energy Society General Meeting, 2012 IEEE; 01/2012
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    ABSTRACT: In this paper, we propose a model-free approach for short term voltage stability monitoring of power system. Our data-driven approach makes use of Phasor Measurement Units (PMU) data to generate the stability certificate for verifying the voltage stability. We employ Lyapunov exponent, a stability tool adapted from ergodic theory of dynamical system, to generate the stability certificate. The time-series voltage data from PMU is used for the online computation of Lyapunov exponent. The proposed method can not only be used to determine the voltage stability of the entire system but can also be used to determine stability/instability contribution of individual buses to the overall system stability. Simulation results are presented on WSCC nine bus system using different load models.
    Power and Energy Society General Meeting, 2012 IEEE; 01/2012
  • [Show abstract] [Hide abstract]
    ABSTRACT: Recursive projection method (RPM) is investigated to extract power system steady state and small signal stability information from time-domain simulation code (TDSC). RPM takes outputs from TDSC to identify an unstable/slow invariant subspace of the full state space. On this subspace, a Newton type of method is applied to enhance convergence for unstable/slowly-converging modes. While on the complement, fixed point iteration of TDSC is kept to evolve stable/fast-decaying modes. Dominant eigenspectrum of the integration scheme is also obtained as byproduct, from which system small signal stability features can be reconstructed. Thus, RPM can enhance convergence of fixed point iteration and enable TDSC to perform small signal stability analysis. At the same time, modeling and numerical advantages of original TDSC are preserved and extra programming costs are saved. The approach is validated by examples on WECC nine-bus, New England 39-bus, and 230-bus systems.
    IEEE Transactions on Power Systems 03/2011; · 2.92 Impact Factor
  • Hua Bai, V. Ajjarapu
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    ABSTRACT: This paper develops a novel online fast load shedding strategy aimed at shedding the most effective load to mitigate fault-induced delayed voltage recovery (FIDVR). Induction motor kinetic energy deviation has been shown to be identical to the integral of power imbalance. Instead of using voltage as an indicator, the proposed strategy applies a centralized scheme making use of equivalent motor kinetic energy to identify the most effective loads to shed. In order to derive kinetic energy, the equivalent inertia and the speed of the induction motor are needed. A methodology is proposed to obtain these variables through online measurements. Simulations on a sample power system illustrate the novel load shedding strategy and its effectiveness. The proposed strategy is compared with conventional four-stage under-voltage load shedding (UVLS) scheme. The results have validated that the proposed strategy can effectively mitigate fault-induced delayed voltage recovery and require much less load to be shed.
    IEEE Transactions on Power Systems 03/2011; · 2.92 Impact Factor
  • B. Leonardi, V. Ajjarapu
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    ABSTRACT: This paper investigates the use of reactive power reserves (RPR) as an indicator to estimate voltage stability margin (VSM) in an online environment. The methodology relies upon the relationship between system-wide RPRs and VSM. Statistical multilinear regression models (MLRM) are utilized in order to express how variations in RPRs can be transformed into direct information about VSM. Data regarding RPRs and system VSM are obtained through an offline voltage stability assessment (VSA) and stored in a database for further MLRM development. Different load increase directions and a comprehensive list of contingencies are considered to account for uncertainty present in real-time operations. Once properly designed and validated, the MLRMs are ready to be used in the online environment. The methodology is tested on the IEEE 30-bus system and a real size test system containing 1648 buses. Preliminary results show that MLRMs can be successfully employed in online VSM estimation.
    IEEE Transactions on Power Systems 03/2011; · 2.92 Impact Factor
  • Cheng Luo, V. Ajjarapu
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    ABSTRACT: This letter proposes a new method to calculate eigenvalue sensitivities using continuation of invariant subspaces (CIS). Mathematical proof is demonstrated for the successive eigenvalue sensitivities extracted from CIS. The numerical results on New England 39-bus system are also described.
    IEEE Transactions on Power Systems 03/2011; · 2.92 Impact Factor
  • A. Tiwari, V. Ajjarapu
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    ABSTRACT: Dynamic security of power systems is a major concern during contingencies, especially in present environment when there is inadequate transmission line capacity and dynamic reactive power support. This paper addresses dynamic VAR allocation to ensure acceptable transient voltage performance and short-term stability against severe contingency. To filter out severe contingency, severity indices are proposed. An algorithm based upon trajectory sensitivity analysis is used to select candidate VAR locations. The overall problem is formulated (in dynamic framework) as mixed integer dynamic optimization (MIDO). To solve MIDO problem, it is converted into mixed integer nonlinear problem (MINLP) by complete discretization using simultaneous discretization approach. The effectiveness of proposed approach is illustrated using New England 39-bus system.
    IEEE Transactions on Power Systems 03/2011; · 2.92 Impact Factor
  • Cheng Luo, Venkataramana Ajjarapu
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    ABSTRACT: Successiveeigenvaluesensitivitiesextractedfromcon- tinuation of invariant subspaces (CIS) are proposed for efficient identification of oscillatory stability margin and damping margin in power systems. Spectrum transformation-based methods are in- troduced to calculate the critical eigenvalues of interest for CIS initialization. The predictor-corrector method is applied to trace the movements of eigenvalues as power system parameter changes. Theeigenvaluesensitivitiesare by-productsofthealgorithm.From thisinformation,astepsizecontrolstrategyisproposedtospeedup the oscillatorystabilitymarginanddampingmarginidentification. The proposed method is numerically stable, robust, and converges rapidly. The simulation results and computation performance on New England 39-bus system and IEEE 145-bus system are demon- strated in details. Index Terms—Cayley transform, continuation of invariant subspaces, critical eigenvalues, damping margin, eigenvalue sensi- tivity, eigenvalue trajectory, Hopf bifurcation, oscillatory stability margin.
    IEEE Transactions on Power Systems 01/2011; 26(3):1484-1492. · 2.92 Impact Factor
  • Bruno Leonardi, Venkataramana Ajjarapu, Miodrag Djukanovic, Pei Zhang
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    ABSTRACT: This work proposes a practical two-stage architecture for online voltage stability margin estimation using statistical models and classification techniques. The approach models the relationship between reactive power reserves and voltage stability margin using multi-linear regression models. In order to handle uncertainty related to variable loading conditions and network topology, a few regression models are required. A classification tool is then developed in order to discern which regression model should be employed at any given operating condition. Both the regression models and the classification tool are developed offline form a database generated through a comprehensive VSA. The methodology is implemented on a reduced case of the U.S. eastern interconnection, which contains around 21k buses. The studied area represents a large part of the state of Iowa and small portions of neighboring states. NERC category B, C and D contingencies have been considered in the study. Several load increase directions are used in order to account for uncertainty in load variation. Results have shown that the methodology can successfully estimate voltage stability margin in the presence of uncertainty related to variable loading condition and network topology.
    01/2011;
  • Subhadarshi Sarkar, Venkataramana Ajjarapu
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    ABSTRACT: The combined utilization of renewables such as solar and wind energy is becoming increasingly attractive. Proper methods need to be employed that consider the inherent variability of these two technologies while determining the performance of a wind-solar hybrid energy conversion system (ECS). In this paper, a stochastic approach has been utilized to develop the megawatt resource assessment model (MWRAM) of a wind-solar hybrid ECS at any selected location. The parameters required to define the probabilistic models have been computed from site-specific data using the maximum likelihood estimation method. The wind portion consists of several interconnected wind turbines while the solar component is a parabolic trough solar thermal electric generating system. Different applications of the model to assess resource benefits including capacity factors and reserve require- ments from effective utilization of both wind and solar energy have been explored at different levels with varying wind-solar proportions.
    IEEE Transactions on Sustainable Energy 01/2011; 2(4):383-391. · 3.84 Impact Factor
  • Sheng Yang, V. Ajjarapu
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    ABSTRACT: In this paper, a speed-adaptive reduced-order observer for sensorless vector control of doubly fed induction generators (DFIGs) is proposed. The observer is a simulation of the rotor current dynamic model with feedback of the estimation error and a speed-adaptation loop. Feedback and adaptation gains are designed based on the closed-loop observer model. A parameter sensitivity analysis reveals that this observer is robust against machine parameter variations in the normal operating regions. Simulation results demonstrate desired steady-state and dynamic performance of this sensorless control approach for DFIG-based variable-speed wind turbines.
    IEEE Transactions on Energy Conversion 10/2010; · 2.43 Impact Factor
  • B. Lee, S.-G. Kang, S. Seo, V. Ajjarapu, H. Song
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    ABSTRACT: A new algorithm for the determination of adequate generation re-dispatch as a protective measure against voltage instability is presented in this study. The study applies the method to the Jeju island system in Korea with a frequency control by the current sourced converter (CSC) high-voltage direct current (HVDC) system, which regulates the local system frequency by the active power injection from the mainland system. In order to transfer the active power using the CSC HVDC system, reactive power consumption is essential, and the frequency control of the HVDC is very fast. Therefore voltage instability may occur if the dynamic reactive support is not enough in the local power system at severe contingencies such as generator¿s outages. The authors suggest generation against voltage curves (g¿V curves), constructed by generation continuation power flow, and propose the procedure to determine voltage stability limit of active power generation at each generator using g¿V curves. An illustrative example of the proposed algorithm is shown as applied to the Jeju island system.
    IET Generation Transmission & Distribution 06/2010; · 1.41 Impact Factor
  • Bruno Leonardi, Venkataramana Ajjarapu, Miodrag Djukanovic, Pei Zhang
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    ABSTRACT: This paper investigates the use of multi-linear regression models (MLRMs) and machine learning techniques for online voltage stability margin prediction. The methodology relies upon the relationship between system wide reactive power reserves and voltage stability margin. A comprehensive voltage stability assessment considering an extensive contingency list and several load increase directions is performed. Data regarding reactive power reserves and voltage stability margin are stored for further MLRM development. Once properly designed and validated, the MLRMs are ready to be used in the online environment. As a few models are necessary to represent all contingencies in the list, an identification tool named MLRM-IDtool is necessary to identify what model to use based on current system conditions. Decision trees and neural networks are tested as classification tools to identify which multi-linear regression model to use. The methodology is tested in the IEEE 30 bus system with promising results. It will be shown that the two-stage proposed approach can successfully estimate voltage stability margin in the online environment and also handle uncertainty related to load behavior.
    01/2010;
  • Source
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    ABSTRACT: Hybrid wind systems couple wind turbines with storage and/or other generation technologies to maximize wind energy production while meeting operational requirements in terms of frequency control, regulation, load following, and scheduling. To increase fuel savings and emission reductions, a hierarchical control structure is proposed in this paper that combines supervisory control and component local control. In supervisory control, unit commitment and power dispatch are optimized and communicated to component local controllers. Frequency/voltage droop (P/Q droop) functions are developed for battery banks and diesel units to regulate system frequency and voltage. PSCAD simulation results indicate that the proposed hierarchical control strategy enables high wind penetration without compromising system performance.
    North American Power Symposium (NAPS), 2009; 11/2009
  • R.J. Konopinski, P. Vijayan, V. Ajjarapu
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    ABSTRACT: This paper discusses the impact of utilizing the capability curve of a doubly fed induction generator (DFIG) based wind park on steady state and dynamic power system operation. The interconnection requirements set forth by FERC in order 661-A mandate the operation of wind parks within a power factor range of 0.95 leading and lagging. This operation drastically underutilized the reactive output of the machine. The results presented demonstrate that committing the full reactive capability of a DFIG park for generation dispatch produce a significant reduction in system losses. This additional reactive support was also found to improve post-fault voltage profiles by damping oscillations and preventing overshoots immediately after being subjected to a disturbance. This utilization of extended reactive limits in voltage control may prevent system collapse.
    IEEE Transactions on Power Systems 09/2009; · 2.92 Impact Factor
  • Hua Bai, Pei Zhang, V. Ajjarapu
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    ABSTRACT: Parameter identification is the key technology in measurement-based load modeling. A hybrid learning algorithm is proposed to identify parameters for the aggregate load model (ZIP augmented with induction motor). The hybrid learning algorithm combines the genetic algorithm (GA) and the nonlinear Levenberg-Marquardt (L-M) algorithm. It takes advantages of the global search ability of GA and the local search ability of L-M algorithm, which is a more powerful search technique. The proposed algorithm is tested for load parameter identifications using both simulation data and field measurement data. Numerical results illustrate that the hybrid learning algorithm can improve the accuracy and reduce the computation time for load model parameter identifications.
    IEEE Transactions on Power Systems 09/2009; · 2.92 Impact Factor

Publication Stats

1k Citations
114.00 Total Impact Points

Institutions

  • 1989–2011
    • Iowa State University
      • Department of Electrical and Computer Engineering
      Ames, IA, United States
  • 2009
    • Seoul National University of Science and Technology
      Sŏul, Seoul, South Korea
  • 2008
    • Annamalai University
      • Department of Electrical Engineering
      Chidambaram, State of Tamil Nadu, India
  • 2005–2006
    • EMC Corporation
      Hopkinton, Massachusetts, United States
  • 2000–2003
    • Korea University
      • Department of Electrical Engineering
      Seoul, Seoul, South Korea
  • 2002
    • Kepco
      Sŏul, Seoul, South Korea