Journal paper: Dynamic Phasor-Based Modeling of Unbalanced Radial Distribution Systems

Authors: Z. Miao, L. Piyasinghe, J. Khazaei, and L. Fan
Accepted by IEEE trans. Power Systems.

The full paper can be accessed at http://power.eng.usf.edu/docs/papers/DP-Paper_final.pdf

Abstract: This paper develops an analytical model of an unbalanced radial distribution system consisting of a single-phase PV, a three-phase induction machine load, a three-phase power factor correction capacitor (PFC) and a load. The analytical model is based on dynamic phasors (DP) for abc phases. The single-phase PV model includes inverter current control (Proportional Resonance (PR) controller), an L or an LCL filter. The induction machine model is based on positive-, negative-,
and zero-sequence components’ dynamic phasors. The sequencebased induction machine model was converted to the DP-abc reference frame and interconnected with other grid components.
The developed analytical model is capable of small-signal analysis and can be used to identify variety of stability and/or harmonic issues in distribution networks, e.g., instability due to weak grid. Impact of unbalance on system dynamic performance can also be investigated using this model. The analytical model is benchmarked with a high-fidelity model built in Matlab/SimPowerSystems
where power electronic switching details are included. The small-signal analysis results are validated via Matlab/SimPowerSystems time-domain simulations.

Journal Paper: DC Impedance-Model-Based Resonance Analysis of a VSC-HVDC System

Authors: L. Xu, L. Fan and Z. Miao
Accepted by IEEE trans. Power Delivery

Abstract:  Resonances can have negative impact on a Voltage Source Converter-High Voltage DC (VSC-HVDC) system. This paper develops dc impedance models for the rectifier and inverter
stations for a VSC-HVDC system when they are viewed from a dc terminal. The impedance models take converter controllers into account. The derived impedance models are validated by comparing frequency responses of the analytical model and the impedance measured at the dc terminal from a detailed VSC-HVDC model simulated in a real-time digital simulator. Resonances are examined in frequency domain (e.g., Bode plots and Nyquist plots) using the derived analytical impedance models. The analysis is verified by time-domain simulations. Real-time digital simulation in RT-Lab demonstrates that the dc capacitor has a significant impact on resonances while power transfer level
has an insignificant impact on resonances.

Index Terms—Admittance, impedance, resonance, VSCHVDC.

Journal paper: Impedance-Model-Based SSR Analysis for TCSC Compensated Type-3 Wind Energy Delivery Systems

This paper was accepted by IEEE Transactions on Sustainable Energy

Authors: L. Piyasinghe, Z. Miao, J. Khazaei, and L. Fan
Abstract:

This paper employs impedance-model-based frequency domain analysis to detect subsynchronous resonances (SSR) in Type 3 wind farms with Thyristor Controlled Series Capacitor (TCSC). The contributions of this paper are (i) the derivation of dynamic phasor based TCSC impedance model and (ii) the application of such impedance model in Type 3 wind energy systems for SSR analysis. Impedance models for TCSC with constant firing angle control and impedance control are derived in
this paper. With the derived impedance models, Nyquist-stabilitycriterion is applied to compare SSR stability in Type 3 wind farm with TCSC or with fixed capacitor compensation. This paper employs analytical models to demonstrate TCSC’s capability in avoiding SSR in Type 3 wind generator interconnection systems. The analytical results obtained through impedance models are validated by detail model-based (with thyristor switch modeled) time-domain simulation in Matlab/SimPowerSystems.

Mixed Integer Programming Based Battery Sizing

Authors: M. Alhaider and L. Fan
Accepted by Energy Systems (Springer)

Abstract
In this paper, mixed integer programming (MIP) formulations are proposed to obtain the optimal capacity of the battery energy storage system (BESS) in a power system. Two optimization problems will be investigated: (i) When the BESS is owned by a utility, the operation cost of generators and cost of battery will be minimized. Generator on/o states, dispatch level and battery power dispatch level will be determined for a 24-hour period. (ii) When the BESS is owned by a community for peak shaving, the objective function will have a penalty component for the deviation of the imported power from the scheduled imported power. The battery sizing parameters, power limit and energy limit, are treated as decision variables in the optimization problems. In both cases, switchable loads are considered. Further, constrains of switchable loads are included in the optimization problem to show their impact on battery sizing. MIP problems are solved by CPLEX. The simulation results present the e ect of switchable load penetration level on battery sizing parameters.

Keywords
Battery Energy Storage System Switchable Loads Mixed Integer Programming

Journal Paper: Multi-Agent Control of Community and Utility Using Lagrangian Relaxation Based Dual Decomposition

This paper was accepted by Electric Power Systems Research
Authors: Vahid R. Disfani, Lingling Fan , Lakshan Piyasinghe, and Zhixin Miao

Abstract:
In multi-agent based demand response program, communities and a utility make decisions independently and they interact with each other with limited information sharing. This paper presents the design of multi-agent based demand response program while considering ac network constraints. This project develops two types of information sharing and iterative decision making procedures for the utility and communities to reach Nash Equilibrium. The distributed algorithms of decision making are based on Lagrangian relaxation, duality, and the concept of upper and lower bounds. The first algorithm is subgradient iteration based distributed decision making algorithm and the second algorithm is based on lower bound and upper bound switching. The two algorithms require different information flow between the utility and communities. With the adoption of distributed algorithms, the utility solves optimal power flow at each iteration while considering ac network constraints, and the communities also conduct optimization. Through information sharing, the utility and the communities update their decisions until convergence is reached. The decision making algorithms are tested against three test cases: a distribution network IEEE 399 system, two meshed networks (IEEE 30-bus system and IEEE 300-bus system). Fast convergence is observed in all three cases, which indicates the feasibility of the demand response design.

Paper TPWRD-00888-2012.R2 : Impedance Model Based Resonance Analysis in a VSC-HVDC System

L. Xu and L. Fan, "Impedance Model Based Resonance Analysis in a VSC-HVDC System", accepted, IEEE transactions on Power Delivery

Abstract:

Resonances can limit power transfer in a Voltage Source Converter-High Voltage DC (VSC-HVDC) system. The objective of this paper is to develop impedance models for the rectifier AC system and the inverter AC system for a VSC-HVDC system. Resonance stability will be examined using Nyquist stability criterion and impedance frequency responses. The impedance models considers the outer control loop and the inner current control loops of VSCs. Impacting factors are then examined. Stability analysis demonstrates that the feed-forward filter, line length and power transfer level have significant impact on resonances. Time-domain simulation results obtained Matlab SimPowerSystems are used to validate the analysis.

Extended Kalman Filtering Based Real-time Dynamic State and Parameter Estimation Using PMU Data

This paper was accepted by Electric Power Systems Research on May 27, 2013
Authors: Lingling Fan and Yasser Wehbe
This paper proposes extended Kalman filtering (EKF) based real-time dynamic state and parameter estimation using Phasor Measurement Unit (PMU) data. In order to reduce computing load, model decoupling technique is used where measurements (real power, reactive power, voltage magnitude and phase angle) from a PMU are treated as inputs and outputs from the system. Inputs are real and reactive powers while outputs are voltage magnitude phase angle. EKF is implemented using a second-order swing equation and a classical generator model to estimate the two dynamic states (rotor angle and rotor speed) and unknown parameters, e.g., mechanical power, inertia constant, damping factor and transient reactance. An EKF algorithm is developed using model decoupling technique for real-time estimation of states and parameters related to electromechanical dynamics. The EKF based estimation can estimate two dynamic states along with four unknown parameters.

Impedance models

In Dr. Jian Sun's 2009 IEEE trans. Power Electronics letter, impedance model (Laplace) application in detecting resonances in circuits is demonstrated. The beauty of this approach is the conversion of a circuit analysis problem into a feedback control stability analysis problem. From that point, Nyquist stability criterion can be applied.

Another advantage of this approach is component level analysis. Unlike eigenvalue based analysis that a whole dynamic system state matrix has to be formulated, impedance models are built component by component. Therefore, sensitivity analysis can be carried at component level which leads to more insights.

Our letter Nyquist-Stability-Criterion-Based SSR Explanation for Type-3 Wind Generators
http://ieeexplore.ieee.org/xpls/abs_all.jsp?arnumber=6197223&tag=1  (IEEE trans. Energy Conversion, Sep. 2012) adopted impedance modeling approach for Type 3 wind with series compensated network resonance analysis. Two impedance models are formulated: one for the RLC circuit, the other for the DFIG circuit.

For the machine circuit, standard textbooks have well-developed circuit model for induction machine The only difference for DFIG circuit from induction machines is the control circuits of the two converters. With assumptions, the control circuits in vector control can be integrated into scalar impedance model. Another trick of modeling is to integrate mechanical system movement (the rotating speed) into the impedance model.

Intuition wise, we can just write slip(s) = 1-j(wm)/s. Theoretically, this can be derived based on dq-based machine modeling and its conversion to phase domain.

With the developed impedance model, analysis results are super clear which shed insights into key impacting factors of resonance.

Paper accepted by IEEE trans. Energy Conversion

Paper Title: Modal Analysis of a DFIG-Based Wind Farm Interfaced with a Series Compensated Network

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Paper accepted by IEEE trans. Power Delivery

Investigation of Microgrids With Both Inverter Interfaced and Direct AC-Connected Distributed Energy Resources
Zhixin Miao, Senior Member, IEEE, Alexander Domijan, Jr., and Lingling Fan, Senior Member, IEEE

Abstract—With the increasing use of renewable energy resources and energy storage devices, inverter-based distributed energy resources (DERs) become the important components in microgrids. As diesel generators with direct ac connections are the current most cost effective and reliable power sources, the stability investigation of microgrids should include both types of DERs. In this paper, dynamics of diesel generation will be included and the interaction of the diesel generators and the inverter-based DERs will be investigated using eigenvalue analysis and time-domain simulations. The significant contributions of this paper include: 1) identification of the stability problem in microgrids with inverter-based DERs and conventional generators and 2) investigation of the interaction problem of inverter-based DERs and conventional generators in islanded microgrids.

On Active/Reactive Power Modulation of DFIG-based Wind Generation for Inter-area Oscillation Damping

Accepted on 10/11/2010 by IEEE trans. Energy Conversion

Abstract—In this paper, the interactions of the torsional dynamics of the wind turbines and power modulation for oscillations in DFIG-based wind generators are investigated. One major concern of active power modulation is its interaction with wind turbine’s torsional dynamics. A study case with wind turbine torsional dynamics modeled is derived from [1]. Both active
power modulation and reactive power modulation controls are developed in this paper. The paper successfully demonstrates the presence of interaction between active power modulation and torsional dynamics and the absence of such interaction in the case of reactive power modulation. Linear system analysis and root loci approach are employed to demonstrate such interaction. Simulation results in MATLAB/Simulink further verifies the analysis. The major contribution of this paper is: i) the demonstration of the potential disadvantage of wind generation’s active power modulation, and ii) the identification of reactive power modulation in wind generation as an alternative for interarea oscillation damping.

Index Terms—Doubly fed induction generator (DFIG), Interarea
Oscillation Damping, Active Power Modulation, Reactive
Power Modulation

Positive-feedback-based Active Anti-islanding Schemes for Inverter-Based Distributed Generators: Basic Principle, Design Guideline and Performance Ana

Positive-feedback-based Active Anti-islanding Schemes for Inverter-Based Distributed Generators: Basic Principle, Design Guideline and Performance Analysis

Du, P.; Ye, Z.; Aponte, E.; Nelson, J.; Fan, L.;
Power Electronics, IEEE Transactions on

Abstract
The positive-feedback-based anti-islanding schemes (AI) invented by authors are highly effective in preventing islanding without causing any degradation in power quality. This paper presents the basic principles of these schemes and their design guidelines. Moreover, their performance is investigated for inverter-based distributed generators (DG). The parametric study reveals the factors significantly influencing the performance of these schemes. The simulation results demonstrate the effectiveness of these schemes.

Hybrid modeling of DFIGs for wind energy conversion systems

L. Fan, Z. Miao, S. Yuvarajan, and R. Kavasseri, "Hybrid modeling of DFIGs for wind energy conversion systems," Simulation Modelling Practice and Theory
Volume 18, Issue 7, August 2010, Pages 1032-1045

Abstract

The synchronous reference frame is widely used in Doubly Fed Induction Generation (DFIG) modeling. In a DFIG, it is usually assumed that slip control is in place at the rotor-side converters to make sure that the injected rotor voltage at slip frequency gives a constant stator frequency (60 Hz or 50 Hz). In the analysis, the injected rotor voltages are directly expressed in the synchronous reference frame. In the cases where slip control is not available, the above modeling will not work. This paper implements a hybrid model (dqabc) of a DFIG. The key modeling idea is to use the frequencies of the injected rotor voltages as explicit inputs and further transform the injected rotor voltage into the synchronous reference frame. This allows one to model and simulate scenarios including: (i) acceleration via rotor injection, (ii) harmonic simulation for six-pulse rotor injection, and (iii) negative sequence compensation using rotor injection. Matlab/Simulink is used to perform the simulations of three cases listed above. Laboratory experiments on two of the cases confirm the validity of the model and simulation results.

Keywords: Wind Generation; Doubly Fed Induction Generator; Harmonics; Hybrid modeling

Wind Farms with HVDC Delivery in Inertial Response and Load Frequency Control

Z. Miao, L. Fan, D. Osborn, and S. Yuvarajan, "Wind Farms with HVDC Delivery in Inertial Response and Load Frequency Control," to appear in IEEE trans. Energy Conversion.

Abstract
This paper develops a coordination control strategy for wind farms with line commutated converter (LCC) based HVDC delivery for participating in inertial response and load frequency control. The coordination philosophy is to feed back the grid frequency and its derivative and adjust the delivery power of the HVDC link according to the feedback signals. The feedback loop employing the derivative of the grid frequency aims to improve the inertial response while the feedback loop employing the grid frequency deviation introduces a droop at the rectifier control loop. When the grid frequency is too high or too low, active power flow through the HVDC link will be ramped down or up. In turn, the wind generation will increase or decrease the blade angles to reduce or increase the captured wind power through pitch control. A case study demonstrates the effectiveness of the inertial enhancement and frequency droop in HVDC control. Simulation results in TSAT are given.

Modeling of DFIG-based Wind Farms for SSR Analysis

L. Fan, R. Kavasseri, Z. Miao, and C. Zhu, Modeling of DFIG-based Wind Farms for SSR Analysis, to appear in IEEE trans. Power Delivery, Oct. 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: i) series compensation level and ii) 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 Novel Control Scheme for DFIG-based Wind Energy Systems under Unbalanced Grid Conditions

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L. Fan, H. Yin, and Z. Miao, "A Novel Control Scheme for DFIG-based Wind Energy Systems under Unbalanced Grid Conditions," Electric Power Systems Research

Abstract:
This paper presents an analysis and a novel control scheme for a doubly-fed induction generator (DFIG) based wind energy generation under unbalanced grid voltage conditions. The control objectives are: (i) to limit the rotor currents, (ii) to suppress ripples in the torque and (iii) to suppress the dc-link voltage fluctuation through converter controls. Negative sequence compensation techniques by one of the converters, namely, the rotor side converter (RSC) or the grid side converter (GSC) in a DFIG are discussed and their limitations are presented. A coordinated control scheme with a concise structure compared to the conventional dual sequence control structure is proposed in this paper. The RSC is controlled to suppress ripples in the torque and the rotor currents while the GSC is controlled to suppress ripples in the dc-link voltage by considering the rotor power effect. The major contributions of the paper include: (i) the presentation of the limitation of negative sequence compensation using one converter; (ii) development of a concise coordination control scheme which is free of low pass filters and uses a reduced number of reference frame transformation. Matlab/Simulink tests for a 2MW DFIG demonstrate the effectiveness of the control scheme.