Balarko Chaudhuri

Improvement of Stability and Load Sharing in an Autonomous Microgrid Using Supplementary Droop Control Loop

This paper investigates the problem of appropriate load sharing in an autonomous microgrid. High gain angle droop control ensures proper load sharing, especially under weak system conditions. However, it has a negative impact on overall stability. Frequency-domain modeling, eigenvalue analysis, and time-domain simulations are used to demonstrate this conflict. A supplementary loop is proposed around a conventional droop control of each DG converter to stabilize the system while using high angle droop gains. Control loops are based on local power measurement and modulation of the d-axis voltage reference of each converter. Coordinated design of supplementary control loops for each DG is formulated as a parameter optimization problem and solved using an evolutionary technique. The supplementary droop control loop is shown to stabilize the system for a range of operating conditions while ensuring satisfactory load sharing.

Wide-area measurement-based stabilizing control of power system considering signal transmission delay

Recent technological advances in the area of wide-area measurement systems (WAMS) has enabled the use of a combination of measured signals from remote locations for centralized control purpose. The transmitted signals can be used for multiple swing mode damping using a single controller. However, there is an unavoidable delay involved before these signals are received at the controller site. To ensure satisfactory performance, this delay needs to be taken into account in the control design stage. This paper focuses on damping control design taking into account a delayed arrival of feedback signals. A predictor-based H/sub /spl infin// control design strategy is discussed for such time-delayed systems. The concept is utilized to design a WAMS-based damping controller for a prototype power system using a static var compensator. The controller performance is evaluated for a range of operating conditions.

Mixed-sensitivity approach to H/sub /spl infin// control of power system oscillations employing multiple FACTS devices

This paper demonstrates the enhancement of inter-area mode damping by multiple flexible AC transmission systems (FACTS) devices. Power system damping control design is formulated as an output disturbance rejection problem. A decentralized H/sub /spl infin// damping control design based on the mixed-sensitivity formulation in the linear matrix inequality (LMI) framework is carried out. A systematic procedure for selecting the weights for shaping the open loop plant for control design is suggested. A 16-machine, five-area study system reinforced with a controllable series capacitor (CSC), a static VAr compensator (SVC), and a controllable phase shifter (CPS) at different locations is considered. The controllers designed for these devices are found to effectively damp out inter-area oscillations. The damping performance of the controllers is examined in the frequency and time domains for various operating scenarios. The controllers are found to be robust in the face of varying power-flow patterns, nature of loads, tie-line strengths, and system nonlinearities, including device saturations.

Adaptive Droop Control for Effective Power Sharing in Multi-Terminal DC (MTDC) Grids

Following a converter outage in a Multi-Terminal DC (MTDC) grid, it is critical that the healthy converter stations share the power mismatch/burden in a desirable way. A fixed value of power-voltage droop in the DC link voltage control loops can ensure proper distribution according to the converter ratings. Here a scheme for adapting the droop coefficients to share the burden according to the available headroom of each converter station is proposed. Advantage of this adaptive (variable) droop scheme for autonomous power sharing is established through transient simulations on an MTDC grid with four bipolar converters and DC cable network with metallic return. Results for both rectifier and inverter outages under two different scenarios are presented. Post-contingency steady-state operating points obtained from transient simulation are shown to be consistent with those derived analytically. Impact of varying droop coefficients on the stability of the MTDC grid is established. An averaged model in Matlab/SIMULINK which has been validated against detailed switched model in EMTDC/PSCAD is used for the stability and modal analysis.

Coherency identification in power systems through principal component analysis

In this letter, a new technique to identify coherent generators in large interconnected power system using measurements of generator speed and bus angle data has been presented. This is based on the application of principal component analysis (PCA) to measurements obtained from simulation studies that represent examples of interarea events. The results of application of PCA separately to data sets of generator speeds and bus angles, respectively, are presented. The approach of PCA was able to highlight clusters of generators showing common features when compared with the conventional modal analysis technique.

Stability Analysis of VSC MTDC Grids Connected to Multimachine AC Systems

Interaction between multimachine ac systems and a multiterminal dc (MTDC) grid and the impact on the overall stability of the combined ac-MTDC system is studied in this paper. A generic modeling framework for voltage-source converter (VSC)-based MTDC grids, which is compatible with standard multimachine ac system models, is developed to carry out modal analysis and transient simulation. A general asymmetric bipole converter configuration comprising positive and negative pole converters and dc cable network with a positive, negative, and metallic return circuit is considered to enable different types of faults and dc-side unbalance studies. Detailed dynamic representation of the dc cables with distributed pi-section models is used along with the averaged model and decoupled control for the converter stations. An averaged model in Matlab/SIMULINK is validated against the detailed switched model in EMTDC/PSCAD by comparing the responses following small and large disturbances (e.g., faults on the dc side). Modal analysis is performed to identify the nature and root cause of the dynamic responses. Interaction between a multimachine ac system and an MTDC grid is examined following faults on the ac and dc sides and outage of converters. It is shown that the cause of instability in certain cases could only be attributed to the dc-side state variables. An averaged model of the converter along with the dc cable network is shown to be essential to analyze the stability and dynamics of combined ac-MTDC grids.

System Frequency Support Through Multi-Terminal DC (MTDC) Grids

Control of the converter stations in a multi-terminal DC (MTDC) grid to provide frequency support for the surrounding AC systems is the subject matter of this paper. The standard autonomous power sharing control loop for each converter is modified with a frequency droop control loop. The objective is to minimize the deviation from nominal AC system frequency and share the burden of frequency support among the converter stations of the MTDC grid. The effectiveness of the frequency support is demonstrated through nonlinear simulation of a test system consisting of three isolated AC systems interconnected through an MTDC grid with four converter stations. An averaged model of the MTDC grids is developed to carry out modal analysis of combined multi-machine AC-MTDC systems. Modal analysis is used to characterize and substantiate the time domain behavior in presence of frequency droop control. It is established that appropriate droop control loop for the MTDC grid converters could be effective in reducing the deviation from nominal AC system frequency provided the sensitivity of the system eigen-values to changes in control parameters (e.g., droop coefficients) is accounted for a priori through modal analysis.

A New Approach to Continuous Latency Compensation With Adaptive Phasor Power Oscillation Damping Controller (POD)

Latency or delay in remote feedback signals can adversely affect the closed-loop damping performance. Accurate time-stamp information at both (PMU location and control center) ends offers a possibility to continuously compensate for time-varying latency. In this paper, an adaptive phasor power oscillation damping controller (APPOD) is proposed wherein the rotating coordinates for phasor extraction are adjusted to account for the change in phase caused due to the delay. The oscillatory component of the original signal is thus retrieved out of the delayed signal received at the control center. Unlike conventional phasor POD, which uses a fixed phase shift to generate damping control signal, an adaptive phase shift algorithm is used here to suit varying signal locations and operating conditions. Case studies confirm the effectiveness of the proposed technique, both in terms of robustness and handling continuously varying delays. A comparison with a conventional gain scheduled POD (CGPOD) and a Unified Smith Predictor (USP) approach is also presented.

A unified Smith predictor approach for power system damping control design using remote signals

This brief illustrates a control design procedure for handling time delays encountered in transmitting the remote signals in power systems. A unified Smith predictor (USP) approach is used to formulate the control design problem in the standard mixed sensitivity framework. The solution to the problem is sought numerically using linear matrix inequalities (LMIs) with additional pole-placement constraints. The predictor based method is applied for designing a damping controller for a prototype power system. Simulation results show that the controller performs satisfactorily even though the feedback signals arrive at the control site after a finite time delay.

Hardware and Control Implementation of Electric Springs for Stabilizing Future Smart Grid With Intermittent Renewable Energy Sources

In this paper, the details of practical circuit and control implementation of an electric spring for reactive power compensation and voltage regulation of the ac mains are presented. With Hookes law published three centuries ago, power electronics-based reactive power controllers are turned into electric springs (ESs) for regulating the ac mains of a power grid. The proposed ES has inherent advantages of: 1) ensuring dynamic load demand to follow intermittent power generation; and 2) being able to regulate the voltage in the distribution network of the power grid where numerous small-scale intermittent renewable power sources are connected. Therefore, it offers a solution to solve the voltage fluctuation problems for future power grids with substantial penetration of intermittent renewable energy sources without relying on information and communication technology. The proof-of-concept hardware is successfully built and demonstrated in a 10-kVA power system fed by wind energy for improving power system stability. The ES is found to be effective in supporting the mains voltage, despite the fluctuations caused by the intermittent nature of wind power.