Nilanjan Senroy

Primary frequency regulation by deloaded wind turbines using variable droop

This paper introduces a method to improve the primary frequency contribution of grid connected variable speed wind turbine generators (WTGs). Using their energy reserve margins, deloaded WTGs are controlled to provide relief to the grid during depressed frequency conditions. The frequency support from individual WTGs is regulated based on the available reserve, which depends on the prevailing wind velocities. By continuously adjusting the droop of the WTG in response to wind velocities, its primary frequency response is significantly improved in terms of reduced stresses on WTGs during low wind speeds. The impact of variable droop operation on two aspects of WTG operation is investigated-primary frequency contribution and smoothening power fluctuations caused due to changes in wind speed. Also highlighted is the usefulness of this control when adopted by wind farms.

Improving Stability of a DFIG-Based Wind Power System With Tuned Damping Controller

This paper focuses on the implementation of a damping controller for the doubly fed induction generator (DFIG) system. Coordinated tuning of the damping controller to enhance the damping of the oscillatory modes is presented using bacterial foraging technique. The effect of the tuned damping controller on converter ratings of the DFIG system is also investigated. The results of both eigenvalue analysis and the time-domain simulation studies are presented to elucidate the effectiveness of the tuned damping controller in the DFIG system. The improvement of the fault ride-through capability of the system is also demonstrated.

Generator Coherency Using the Hilbert–Huang Transform

Coherency between generators is tracked, by examining the instantaneous phase differences among interarea oscillations and swing curves in disturbed multiarea power systems. Huangs empirical mode decomposition is applied to extract dominant oscillatory modes from interarea oscillations/swing curves. Hilbert transform on these modes yields their instantaneous phase. The coherency is thus revealed by the temporal evolution of the phase difference among interarea oscillations/generator swing curves. The technique is illustrated using simulations on a test system, as well as actual wide-area measurements.

A conceptual framework for the controlled islanding of interconnected power systems

Controlled islanding is a special protection strategy aimed at preventing a system-wide blackout as a result of the cascading of low probability events. This letter addresses the main issues and proposes a conceptual framework for a centralized and proactive controlled islanding strategy.

Two Techniques to Enhance Empirical Mode Decomposition for Power Quality Applications

Joint time-frequency characterization techniques are gaining importance in power quality studies of time-varying waveform distortions. In this context, the performance of the recently introduced empirical mode decomposition (EMD) as a second order filter is examined in this paper. The original EMD technique is incapable of separating components whose frequencies lie within the same octave. Using appropriate masking signals, the EMD can be made more discriminating for closely spaced frequency components. However, limitations still remain, and this paper discusses some aspects thereof. A frequency shifting technique is suggested to further enhance the discriminating property of EMD. Synthetic data similar to signals found in power quality studies are employed for demonstration purposes.

Doubly Fed Induction Generator (DFIG)-Based Wind Farm Control Framework for Primary Frequency and Inertial Response Application

This paper presents a new wind farm control framework for inertial and primary frequency response for a high wind integrated power system. The proposed architecture is unique in the sense that the methodology can be used for frequency regulation support during subsynchronous and super-synchronous operation of the wind turbines (farm). The architecture work with existing wind farm controllers thus avoiding any additional replacement or tuning. The methodology depends on reduced order modeling based on model order reduction (MOR) and subsequent online controller design. The approach is tested on a smaller wind farm and further evaluated on a larger reduced power grid with 39 buses and ten generators. The results show that the proposed architecture provides greater flexibility in wind farm control towards frequency oscillations.

Electromechanical Dynamics of Controlled Variable-Speed Wind Turbines

Variable-speed wind turbines are increasingly penetrating into the electrical grid, replacing the conventional synchronous-generator-based power plants and thus decreasing the available inertial response for primary frequency stability. This paper offers a deeper understanding of variable-speed wind turbine generators (WTGs) in the context of maximum power point tracking and obtaining primary frequency response. Linearized models have been obtained between the wind velocity and the system frequency versus the power output. System complexity has been studied from the point of view of modal analysis of a two-mass drive train model of a WTG, as well as Hankel singular values. Finally, individual WTG models have been combined to form wind farms, whose complexity has again been found to depend on the nature of modeling of the WTG drive trains.

Balanced Truncation Approach to Power System Model Order Reduction

Abstract This article demonstrates the application of balanced truncation based model order reduction to the task of dynamic reduction of power systems. The entire power system is separated into an external area and a study area; dynamic reduction of the external area is conducted. The benefit of applying the balanced truncation technique is that key input–output relationships between these areas are retained during the reduction process. For perturbations originating in the study area, patterns in the dynamic behavior of the external area, such as generator coherency, are well captured by the reduced equivalent. The efficiency of the balanced truncation algorithm is also explored in the context of changing system conditions. Illustrative applications using a test system representative of the northern grid of India have led to some key findings on the coherency of the generators in this grid.

A comparative study of two model order reduction approaches for application in power systems

The objective of this paper, is to compare two mathematical model order reduction (MOR) approaches - Balanced Truncation and Krylov subspace. The application of these approaches to large power systems is studied. The main focus has been to show how the two approaches are effective in finding dynamic patterns such as coherency between generators. The performance has been studied under changing system damping conditions.

Some Techniques for the Analysis and Visualization of Time-varying Waveform Distortions

The time-varying nature of waveform distortions requires a precise analytical tool that is essential in power quality analysis. The time-frequency representation (TFR) provides a powerful method for identification of the time-varying nature of the waveform distortions. The paper presents a comparative study on four techniques for analysis and visualization of waveform distortions with time-varying amplitudes. The techniques include the traditional fast Fourier transform (FFT) and three TFR techniques, which are the short time fourier transform (STFT), the discrete wavelet transform (DWT), and the S-transform.