Sliding Mode Control Approach for Voltage Regulation in Microgrids with DFIG Based Wind Generation by Rasool Aghatehrani and Rajesh Kavasseri

(to be presented )in Proc. IEEE PES GM-2011, Detroit, July 2011.

This paper presents a direct torque and reactive power control method which addresses the problem of voltage regulation in microgrids including doubly fed induction generator (DFIG) based wind generation. Due to significant line resistances in a microgrid, active power variations produced by wind turbines can lead to significant fluctuations in voltage magnitudes and results in power quality problems. This paper uses a nonlinear sliding mode control scheme to directly control torque and reactive power of a DFIG system. The control system adjusts the reactive power of DFIG to achieve voltage quality improvement in the important central bus of a microgrid. There is no decoupled proportional-integral (PI) control based method, therefore the control system is not highly dependent to the accuracy of system parameters. Also, the method is local and can be implemented in the absence of a wide communication system or remote measurement. The performance of the method is illustrated on the IEEE 13 bus distribution network. Dynamic models are considered for the DFIG, converters and internal controllers along with their operational limits. Stochastic fluctuations in wind speed are modeled with NREL TurbSim while accounting for the tower shadow and wind shear. Dynamic simulations (in Simulink/Matlab) are presented to assess the voltage fluctuation compensation and control system robustness.

Conferences/Workshops

IEEE Power Engineering Society General Meeting, Minneapolis, July 25-29 2010.
Presented two papers on July 29, 2010.


ONR-NSF sponsored Faculty Development Workshop in Electric Energy Systems, June 7-12 2010 - Minneapolis, MN.
Presented educational module on transformers.

Conference Papers

Efficient Extreme Event Screening for Power Systems Using Constrained and Unbalanced Partitioning by Cristinel Ababei and Rajesh Kavasseri (presented), Proc. IEEE PES General Meeting, Minneapolis, MN, July 2010.

Abstract— We design and implement an efficient algorithm for extreme event screening for power systems based on constrained and unbalanced partitioning. The proposed methodology directly addresses the cutsize and power-imbalance oriented partitioning problem. This problem has two objectives: minimization of the cutsize and maximization of the powerimbalance between partitions. The proposed algorithm uses the well known hMetis partitioner as a core partitioning engine. The main advantages of the proposed screening methodology are scalability, efficiency, and ease of implementation. The average CPU runtime is less than 0.1 s and 1.5 s for systems with 2,383 buses and 43,501 buses, respectively. The quality of the partitioning solution is similar to that achieved by the algorithms reported earlier.

Joint optimal placement of PMU and conventional measurements in power systems by Rajesh Kavasseri and Sudarshan K. Srinivasan, http://ieeexplore.ieee.org/xpl/freeabs_all.jsp?arnumber=5537855

Proc. IEEE ISCAS 2010 (May 30 - June 2) Paris, France.

We consider the problem of joint optimal placement of Phasor Measurement Units (PMU) and conventional measurements to ensure full observability in power systems. The problem is first formulated as a nonlinear integer programming problem and then recast in to an equivalent integer linear programming (ILP) problem by introducing auxiliary variables and constraints. The ensuing ILP problem is solved for the IEEE 14, 57 and 118 bus systems considering zero-injection buses. The results provide a far more economical solution to system observability compared to those obtained solely with PMU placement.




Power Smoothing of the DFIG Wind Turbine Using a Small Energy Storage Device, by Rasool Aghatehrani and Rajesh Kavasseri, (presented) Proc. IEEE PES General Meeting, Minneapolis, MN, July 2010


The active power produced by wind generators is subject to fluctuations due to the stochastic nature of wind. Depending upon the size of the wind farm and the network to which it is connected, active power variations may result in frequency deviations. This can trigger frequency protection devices and lead to undesirable unit trippings. This paper proposes the addition of a small supercapacitor at the DC link in the power converted of a doubly fed induction generator (DFIG). Based on this, a new method is proposed which simultaneously balances the MPPT as well as output power smoothing objectives. Frequency variations in the range of 0.1-1 Hz are investigated considering variations in wind speed, tower shadow and wind shear effects. Numerical simulations are carried out in PSCAD/EMTDC to illustrate the effectiveness of the proposed method in limiting frequency deviations.

Speeding-up Network Reconfiguration by Minimum Cost Maximum Flow Based Branch Exchanges
by Cristinel Ababei (presented) and Rajesh Kavasseri, Proc. IEEE PES Transmission and Distribution Conference and Exposition, New Orleans, LA, Apr. 2010.

We propose a novel and efficient heuristic algorithm for solving the distribution network reconfiguration problem for loss reduction. We formulate the problem of finding incremental branch exchanges as a minimum cost maximum flow (MCMF) problem. This novel approach finds the best set of concurrent branch exchanges during each iteration of the algorithm and leads to larger loss reductions and a reduced number of iterations, hence significantly reducing the computational runtime. Experiments using distribution systems with sizes of up to 10476 buses demonstrate that the proposed technique leads to an average speed-up of 2.3× with similar or better solution quality compared to the Baran’s reconfiguration technique.

Journal Papers

Efficient Network Reconfiguration using minimum cost maximum flow based branch exchanges and random walks based loss estimations.

Cristinel Ababei and Rajesh Kavasseri, to appear in IEEE Transactions on Power Systems, 2010


Abstract: The efficiency of network reconfiguration depends on both the efficiency of the loss estimation technique and the efficiency of the reconfiguration approach itself. We propose two novel algorithmic techniques for speeding-up the computational runtime of both problems. First, we propose an efficient heuristic algorithm to solve the distribution network reconfiguration problem for loss reduction. We formulate the problem of finding incremental branch exchanges as a minimum cost maximum flow problem. This approach finds the best set of concurrent branch exchanges yielding larger loss reduction with fewer iterations, hence significantly reducing the computational runtime. Second, we propose an efficient random walks based technique for the loss estimation in radial distribution systems. The novelty of this approach lies in its property of localizing the computation. Therefore, bus voltage magnitude updates can be calculated in much shorter computational runtimes in scenarios where the distribution system undergoes isolated topological changes, such as in the case of network reconfiguration. Experiments on distribution systems with sizes of up to 10476 buses demonstrate that the proposed techniques can achieve computational runtimes shorter with up to 7.78× and with similar or better loss reduction compared to the Baran’s reconfiguration technique.

Modeling of DFIG-Based Wind Farms for SSR Analysis

by Lingling Fan, Rajesh Kavasseri, Z. Miao and C. Zhu (to appear in) IEEE Transactions on Power Delivery, 2010

Abstract: This paper conducts an analysis of subsynchronous resonance (SSR) phenomena in doubly-fed induction generator (DFIG)-based wind farms interconnected with series compensated networks. A dynamic model is developed to analyze the induction generator effect (IGE) and torsional interaction (TI) in such systems. A test system derived from the IEEE first benchmark model is considered for the analysis. The effect of two factors namely: 1) series compensation level and 2) wind speed on the IGE and TI are studied. In addition, impact of the inner current converter controller parameters and turbine parameters on SSR is also addressed. Small signal (eigenvalue) analysis is conducted to assess the damping of network and torsional modes followed by dynamic (time domain) simulations. The major contribution of this paper is the analytical investigation on SSR phenomena presented in DFIG-based wind farms interconnected with series compensated networks. The paper clearly demonstrates that IGE instead of TI is the major reason for SSR in such systems.

Harmonic Analysis of a DFIG for a Wind Energy Conversion System
by Lingling Fan, Subbaraya Yuvarajan and Rajesh Kavasseri, IEEE Transactions on Energy Conversion, 25(1), pp: 181-190, March 2010.


Abstract: This paper develops a framework for analysis of harmonics in a doubly fed induction generator (DFIG) caused by nonsinusoidal conditions in rotor and unbalance in stator. Nonsinusoidal rotor voltages are decomposed into harmonic components and their corresponding sequences are identified. Induced harmonics in stator are analyzed and computed, from which the torques produced by these interactions between stator and rotor harmonic components can be found. During unbalanced stator conditions, symmetric component theory is applied to the stator voltage to get positive-, negative-, and zero-sequence components of stator and rotor currents. The steady-state negative-sequence equivalent circuit for a DFIG is derived based on the reference frame theory. Harmonic currents in the rotor are computed based on the sequence circuits. In both scenarios, the harmonic components of the electromagnetic torque are calculated from the interactions of the harmonic components of the stator and rotor currents. Three case studies are considered, namely: 1) nonsinusoidal rotor injection; 2) an isolated unbalanced stator load scenario; and 3) unbalanced grid-connected operation. The analysis is verified with results from numerical simulations in Matlab/Simulink. For illustration, the second case is verified using experiments. The simulation results and experimental results agree well with the results from analysis.

Pseudorandom bit generation using coupled congruential generators

by R. S. Katti, Rajesh Kavasseri and V. Sai, IEEE Transactions on Circuits and Systems II Express Briefs, 57(3), pp: 203-207, March 2010

In this brief, we propose the generation of a pseudo-random bit sequence (PRBS) using a comparative linear congruential generator (CLCG) as follows. A bit "1" is output if the first linear congruential generator (LCG) produces an output that is greater than the output of the second LCG, and a bit "0" is output otherwise. Breaking this scheme would require one to obtain the seeds of the two independent generators given the bits of the output bit sequence. We prove that the problem of uniquely determining the seeds for the CLCG requires the following: 1) knowledge of at least log2m2 (m being the LCG modulus) bits of the output sequence and 2) the solution of at least log2m2 inequalities, where each inequality (dictated by the output bit observed) is applied over positive integers. Computationally, we show that this task is exponential in n (where n = log2m is the number of bits in m) with complexity O(22n). The quality of the PRBS so obtained is assessed by performing a suite of statistical tests (National Institute of Standards and Technology (NIST) 800-22) recommended by NIST. We observe that a variant of our generator that uses two CLCGs (called dual CLCG) pass all the NIST pseudorandomness tests with a high degree of consistency.

Nonce Generation for the Digital Signature Standard
Rajendra Katti and Rajesh G. Kavasseri, International Journal of Network Security, 11(1), , pp: 23-32, July 2010.

Abstract: Digital Signature Algorithm (DSA) is an underlying algorithm to form a signature in the Digital Signature Standard (DSS). DSA uses a new random number (or nonce) each time a signature is generated for a message. In this paper, we present a Linear Congruential Generator ( LCG) based approach to generate nonce for DSS. LCG has been shown to be insecure for nonce generation. If two message-signature pairs are known along with the parameters of the LCG used to generate the nonce then the private key in the signature scheme can be found, with high probability, by solving three congruences over different moduli. We use a comparison of the output of two LCGs to generate the nonces and show that our approach is secure. We also show that coupled multiple recursive generators which are similar to LCGs are also safe for nonce generation. Congruences can no longer be set up to solve for the private key. The advantage of LCG based schemes for pseudo-random number generation is their efficiency.