Rajesh Kavasseri

Modeling of DFIG-Based Wind Farms for SSR Analysis

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.

A multifractal description of wind speed records

In this paper, a systematic analysis of hourly wind speed data obtained from four potential wind generation sites in North Dakota is conducted. The power spectra of the data exhibited a power law decay characteristic of 1/fα processes with possible long range correlations. The temporal scaling properties of the records were studied using the sophisticated multifractal detrended fluctuation analysis MFDFA. It is seen that the records at all four locations exhibit similar scaling behavior which is also reflected in the multifractal spectrum determined under the assumption of a binomial multiplicative cascade model.

Harmonic Analysis of a DFIG for a Wind Energy Conversion System

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.

Joint Placement of Phasor and Power Flow Measurements for Observability of Power Systems

We consider the problem of joint optimal placement of phasor measurement units (PMU) and conventional measurements to ensure full observability in power systems. The formulation is initially posed as a nonlinear integer programming problem and then transformed in to an equivalent integer linear programming (ILP) problem by introducing auxiliary variables and constraints. The resulting ILP problem is solved for the optimal solution on IEEE 14-, 57-, and 118-bus systems considering zero-injection buses. To extend the formulation to large-scale systems, two heuristics are proposed where the nonlinear problem is decomposed into two separate problems of lesser complexity. The heuristics are evaluated on standard IEEE test cases and a large 2383-bus Polish system. The heuristics yield solutions close to optimal (difference of 1 PMU) solutions on standard IEEE systems, while substantially reducing problem complexity and run time. The placement results obtained with the proposed formulation require fewer PMUs for observability compared to systems with fixed locations of conventional measurements. The results thus potentially provide a more economical solution to system observability compared to those obtained solely with PMU placement.

Efficient Network Reconfiguration Using Minimum Cost Maximum Flow-Based Branch Exchanges and Random Walks-Based Loss Estimations

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 times and with similar or better loss reduction compared to the Barans reconfiguration technique .

Evidence of crossover phenomena in wind-speed data

In this paper, a systematic analysis of hourly wind-speed data obtained from three potential wind-generation sites (in North Dakota) is analyzed. The power spectra of the data exhibited a power-law decay characteristic of 1/f/sup /spl alpha// processes with possible long-range correlations. Conventional analysis using Hurst exponent estimators proved to be inconclusive. Subsequent analysis using detrended fluctuation analysis revealed a crossover in the scaling exponent (/spl alpha/). At short time scales, a scaling exponent of /spl alpha//spl sim/1.4 indicated that the data resembled Brownian noise, whereas for larger time scales the data exhibited long-range correlations (/spl alpha//spl sim/0.7). The scaling exponents obtained were similar across the three locations. Our findings suggest the possibility of multiple scaling exponents characteristic of multifractal signals.

Reactive Power Management of a DFIG Wind System in Microgrids Based on Voltage Sensitivity Analysis

This paper addresses the problem of voltage regulation in microgrids that include doubly fed induction generator (DFIG)-based wind generation. Due to significant line resistances in microgrids, active power variations produced by wind turbines can lead to significant fluctuations in voltage magnitudes. This paper proposes a voltage sensitivity analysis-based scheme to achieve voltage regulation at a target bus in such microgrids. The target voltage can be of an important central bus, or a bus with sensitive voltage loads. The method is local and can be implemented in the absence of a widespread communication system or remote measurements. 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 tower shadow and wind shear. Dynamic simulations (in PSCAD/EMTDC) are presented to assess the voltage regulation characteristics under different load conditions and network contingencies.

Sensitivity-Analysis-Based Sliding Mode Control for Voltage Regulation in Microgrids

This paper presents a sliding mode controller to address the problem of voltage regulation in microgrids involving doubly fed induction wind generators (DFIGs). The control objective is to achieve terminal voltage regulation while ensuring maximum power point tracking (MPPT). The control development is based on voltage sensitivity analysis to eliminate the possibility of interference with the other voltage regulation devices in the microgrid. The proposed method: 1) does not require synchronous coordinate transformation, 2) eliminates the need for decoupled proportional-integral (PI) loops, and 3) is local and can be implemented in the absence of widespread communication systems or remote measurements. Additionally, its control performance is not degraded by errors in system parameters. 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 PSCAD/EMTDC) are presented to assess the control performance with voltage fluctuation compensation and control system robustness.

Unified PMU Placement for Observability and Bad Data Detection in State Estimation

This paper presents a unified algorithm for optimal phasor measurement unit (PMU) placement in power system state estimation. Considering the similarity of the cases that the system is unobservable and that the system becomes unobservable if a critical measurement or a critical pair is removed from the measurement set, a unified algorithm of PMU placement for state estimation combining observability analysis and bad data detection into a single iterative procedure is presented. In each iteration of the algorithm, PMU placement is performed to make the system observable, or make critical measurements become non-critical, or make critical pairs become non-critical pairs, respectively. Based on Theorem 3.1 in “A direct numerical method for observability analysis” (Gou and Abur, IEEE Trans. Power Syst., vol. 15, no. 2, pp. 625-630, May 2000), this paper presents an improved optimal PMU placement algorithm wherein each iteration is performed by a significantly smaller integer linear programming (ILP) model whose size is governed by the rank deficiency of the Jacobian matrix resulting from the configuration of conventional measurements. The proposed formulation thus leads to a far more smaller and simpler ILP problem compared to existing methods. Additional results are also presented to identify critical measurement pairs. The proposed approach is illustrated on the IEEE 14- and 118-bus systems.

Minimizing the effect of trends on detrended fluctuation analysis of long-range correlated noise

Detrended fluctuation analysis (DFA) has been proposed as a robust technique to determine possible long-range correlations in power-law processes (Phys. Rev. E 49 (1994) 1685–1989). However, recent studies have reported the susceptibility of DFA to trends [(Phys. Rev. E 64 (2001) 011114–011133)] which give rise to spurious crossovers and prevent reliable estimation of the scaling exponents. Inspired by these reports, we propose a technique based on singular-value decomposition (SVD) of the trajectory matrix to minimize the effect of linear, power-law, periodic and also quasi-periodic trends superimposed on long-range correlated power-law noise. The effectiveness of the technique is demonstrated on publicly available data sets [(Phys. Rev. E 64 (2001) 011114–011133)].