V. Ajjarapu

Iowa State University, Ames, Iowa, United States

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Publications (117)143.97 Total impact

  • IEEE Transactions on Power Systems 01/2015; DOI:10.1109/TPWRS.2014.2385031 · 3.53 Impact Factor
  • Guangchao Geng, Venkataramana Ajjarapu, Quanyuan Jiang
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    ABSTRACT: Stability-constrained optimal power flow (SOPF) is an effective and economic tool to enhance stability performance by adjusting initial steady-state operating conditions, with the consideration of rotor angle and short-term voltage performance criteria. SOPF belongs to the category of dynamic optimization problems which are computationally expensive. In order to reduce its computational complexity, a hybrid dynamic optimization approach is proposed for efficient and robust solving SOPF problems. Based on the direct multiple shooting method, this approach combines the algorithmic advantages from existing direct sequential and simultaneous approaches. Coarse-grained parallelism among multiple shooting intervals is explored. A modular-based implementation architecture is designed to take advantage of the loose coupling between time-domain simulation and optimization. Case studies on various test systems indicate that the proposed approach is able to reduce computation time compared with other direct approaches for dynamic optimization. Also, the investigated parallelizations are effective to achieve acceleration on a symmetric multiprocessing platform.
    IEEE Transactions on Power Systems 09/2014; 29(5):2138-2149. DOI:10.1109/TPWRS.2014.2306431 · 3.53 Impact Factor
  • Nicholas A. Brown, Venkataramana Ajjarapu, Nivad Navid
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    ABSTRACT: Ramping products are planned or in effect in multiple power markets overseen by independent system operators (ISOs) in the United States (US). They are intended to provide flexibility in security-constrained economic dispatch, given higher uncertainty in net load (load minus non-dispatchable renewable power) due to higher wind penetration. When conventional generators respond to unforeseen wind ramps, transmission congestion can arise. In such situations, reserves may not be deliverable as power across the network. This paper presents an extension of the original Midcontinent Independent System Operator (MISO) ramping capability formulation. The key characteristic of this proposed deliverable ramping capability formulation is that it recognizes the magnitude of net load uncertainty by location. An example is given in which the economic dispatch with deliverable ramping capability avoids congestion.
    IEEE PES General Meeting, National Harbor, MD; 07/2014
  • Guangchao Geng, Venkataramana Ajjarapu, Quanyuan Jiang
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    ABSTRACT: Trajectory sensitivity is one of the most important analysis tools for power system dynamic performance assessment and control. Calculating Jacobian matrix has been identified as one of the most computational-intensive and error-prone algorithmic procedures. Automatic differentiation (AD) is able to efficiently generate derivatives of a specified function without any additional hand coding work. Application and improvement of AD in trajectory sensitivity is investigated in this paper. A novel partial-decoupled AD strategy is developed. The proposed approach enables a module-based design separating dynamic component models from time-domain simulator. Maintainability and flexibility in software architecture can be achieved. Numerical results demonstrate that the application of AD retains numerical accuracy and achieves high computational performance compared with numerical finite differentiation.
    2014 IEEE/PES Transmission & Distribution Conference & Exposition (T&D); 04/2014
  • Juan Yu, Wenyuan Li, Venkataramana Ajjarapu, Wei Yan, Xia Zhao
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    ABSTRACT: A new branch equivalent is proposed to identify and locate long-term voltage instability in both distribution and transmission network. In the proposed equivalent, not only the power flow and the sensitivity information remain consistent before and after the equivalence, but also the equivalent voltages and admittances are completely independent of loads, which ensure the equivalent accuracy in voltage stability analysis. Based on the proposed equivalent, an approach is presented to identify system voltage stability. The proposed approach also locates weak branches and buses, where an enhancement or operational measure can be used to improve system voltage stability. The effectiveness of the proposed equivalent and approach is demonstrated using two radial systems, five IEEE systems and four actual utility systems with a system size from 5-buses to 3120-buses.
    IET Generation Transmission & Distribution 01/2014; 8(1):46-54. DOI:10.1049/iet-gtd.2012.0498 · 1.31 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 11/2013; 28(4):3702-3711. DOI:10.1109/TPWRS.2013.2258946 · 3.53 Impact Factor
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    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 11/2013; 28(4):3865-3873. DOI:10.1109/TPWRS.2013.2271260 · 3.53 Impact Factor
  • Juan Yu, Wenyuan Li, Venkataramana Ajjarapu, Wei Yan, Xia Zhao
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    ABSTRACT: With the integration of a quadratic optimisation method into one single Monte Carlo simulation, the system-wide and local risk index functions are developed to quantify the relations between the system-wide and local voltage stability risk and load levels. Each defined risk index function is a function of load level under various possible pre-contingency and contingency states and load uncertainty. Based on the proposed risk index functions, a method of tracing and locating long-term voltage instability risk in power system planning is presented. The methodology can trace the overall system-wide voltage instability risk in a planning timeframe and locate weak branches and buses. The effectiveness of the proposed method is demonstrated using the IEEE 14-bus system and an actual utility system with 171 buses.
    IET Generation Transmission & Distribution 05/2013; 7(5):483-490. DOI:10.1049/iet-gtd.2012.0335 · 1.31 Impact Factor
  • Bruno Leonardi, Venkataramana 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 05/2013; 28(2):615-625. DOI:10.1109/TPWRS.2012.2212253 · 3.53 Impact Factor
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    ABSTRACT: Transient stability constrained optimal power flow (TSCOPF) is one the most computational-intensive applications in power systems. Research efforts were made to utilize high performance computing (HPC) technology to parallelize and accelerate TSCOPF solving process, but challenges still exist in performance analysis to identify bottlenecks and improve efficiency on practical HPC platforms such as Beowulf clusters. Based on first two authors' previous work [8, 12] - two-level parallel TSCOPF with reduced-space interior point method (RIPM), a systematic performance analysis approach is demonstrated in this paper. Comprehensive performance analysis procedures - wall time analysis, MPI/OpenMP profiling and tracing - is employed in order to discover performance bottlenecks. State-of-the-art performance analysis software are employed to generate and visualize performance data, providing guidelines for parallel performance enhancement. Numerical results on a 2746-bus system show effectiveness of the proposed approach and relative low overhead in solving large-scale multi-contingency TSCOPF on a Beowulf cluster.
    North American Power Symposium (NAPS), 2013; 01/2013
  • P. Xu, X. Wang, V. Ajjarapu
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    ABSTRACT: A continuation power flow method with adaptive step size is proposed. Two approaches are used to predict the step size. First, convergence monitor is proposed to predict step size. This step size is compared with the step sizes obtained by the first-order sensitivity of Q-generation and voltage with respect to a parameter. Whichever step size is smaller is chosen to reduce the possibility of divergence and to deal with discrete controls. Convergence monitor is capable of detecting possible divergence in corrector within three iterations. The method of detecting limit-induced bifurcation is proposed by means of sensitivity. The proposed approach is tested on a 300 bus system as well as a practical system. The performance shows that it needs much less time and does not need the critical buses preselected.
    IET Generation Transmission & Distribution 07/2012; 6(7):673-679. DOI:10.1049/iet-gtd.2011.0573 · 1.31 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 03/2012; 6(3):199-206. DOI:10.1049/iet-gtd.2011.0555 · 1.31 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
  • 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 10/2011; 2(4):383-391. DOI:10.1109/TSTE.2011.2148182 · 3.84 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 08/2011; 26(3):1484-1492. DOI:10.1109/TPWRS.2010.2098424 · 3.53 Impact Factor
  • Menghua Fan, Venkataramana Ajjarapu, Chengshan Wang, Dan Wang, Cheng Luo
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    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; 26(1-26):261 - 269. DOI:10.1109/TPWRS.2010.2046656 · 3.53 Impact Factor
  • Bruno Leonardi, Venkataramana 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; 26(1-26):374 - 383. DOI:10.1109/TPWRS.2010.2050155 · 3.53 Impact Factor
  • Hua Bai, Venkataramana 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; 26(1-26):294 - 304. DOI:10.1109/TPWRS.2010.2047279 · 3.53 Impact Factor
  • Cheng Luo, Venkataramana 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; 26(1-26):479 - 480. DOI:10.1109/TPWRS.2010.2052963 · 3.53 Impact Factor

Publication Stats

2k Citations
143.97 Total Impact Points

Institutions

  • 1989–2014
    • Iowa State University
      • Department of Electrical and Computer Engineering
      Ames, Iowa, United States
  • 2008
    • Annamalai University
      • Department of Electrical Engineering
      Chidambaram, State of Tamil Nadu, India
  • 2005
    • EMC Corporation
      Hopkinton, Massachusetts, United States
  • 2002
    • Kepco
      Sŏul, Seoul, South Korea
  • 2000
    • Korea University
      • Department of Electrical Engineering
      Seoul, Seoul, South Korea