Alumni – Ph.D. Graduates

Seyed Ahmad Hamidi
Ph.D. Graduate, 
May 2017

Dissertation: “DC LINE-INTERACTIVE UNINTERRUPTIVLE POWER SUPPLE (UPS) WITH LEVELING FOR CONSTANT POWER AND PULSE LOADS”

Abstract: Uninterruptible Power Supply (UPS) systems are usually considered as a backup power for electrical systems, providing emergency power when the main power source fails. UPS systems ensure an uninterruptible, reliable and high quality electrical power for systems with critical loads in which a continuous and reliable power supply is a vital requirement.  A novel UPS system topology, DC line-interactive UPS, has been introduced. The new proposed UPS system is based on the DC concept where the power flow in the system has DC characteristic. The new DC UPS system has several advantageous with respect to the on-line 3-phase UPS which is extensively used in industry, such as lower size, cost and weight due to replacing the three-phase dual converter in the on-line UPS system with a single stage single phase DC/DC converter and thus higher efficiency is expected.

The proposed system will also provide load leveling feature for the main AC/DC rectifier which has not been offered by conventional AC UPS systems. It applies load power smoothing to reduce the rating of the incoming AC line and consequently reduce the installation cost and time. Moreover, the new UPS technology improves the medical imaging system up-time, reliability, efficiency, and cost, and is applicable to several imaging modalities such as CT, MR and X-ray as well.

BSEE from Shiraz University, Shiraz, Iran, 2006.
MSEE from K. N. Toosi University of Technology, Tehran, Iran, 2009.

Senior Design Engineer
Milwaukee Tool

hamidis@uwm.edu

 


Luke Weber
Ph.D. Graduate, August 2016

Dissertation: “TRANSIENT CONTROL OF SYNCHRONOUS MACHINE ACTIVE AND REACTIVE POWER IN MICRO-GRID POWER SYSTEMS”

Abstract: There are two main topics associated with this dissertation. The first is to investigate phase–to–neutral fault current magnitude occurring in generators with multiple zero–sequence current sources. The second is to design, model, and tune a linear control system for operating a micro–grid in the event of a separation from the electric power system. In the former case, detailed generator, AC8B excitation system, and four–wire electric power system models are constructed. Where available, manufacturers data is used to validate the generator and exciter models. A gain–delay with frequency droop control is used to model an internal combustion engine and governor. The four wire system is connected through a transformer impedance to an infinite bus. Phase–to–neutral faults are imposed on the system, and fault magnitudes analyzed against three–phase faults to gauge their severity. In the latter case, a balanced three–phase system is assumed. The model structure from the former case – but using data for a different generator – is incorporated with a model for an energy storage device and a net load model to form a micro–grid. The primary control model for the energy storage device has a high level of detail, as does the energy storage device plant model in describing the LC filter and transformer. A gain–delay battery and inverter model is used at the front end. The net load model is intended to be the difference between renewable energy sources and load within a micro–grid system that has separated from the grid. Given the variability of both renewable generation and load, frequency and voltage stability are not guaranteed. This work is an attempt to model components of a proposed micro–grid system at the University of Wisconsin Milwaukee, and design, model, and tune a linear control system for operation in the event of a separation from the electric power system.

MSEE, University of Wisconsin-Milwaukee, Milwaukee, Wisconsin, 2009
BSEE, Marquette University, Milwaukee, Wisconsin, 1985

Assistant Professor
Milwaukee School of Engineering

lgweber@uwm.edu

 


Ashishkumar Solanki
Ph.D. Graduate, January 2015

Dissertation: “VIRTUAL DROOP CONTROL FRAMEWORK AND STABILITY ANALYSES FOR MICROGRIDS WITH HIGH PENETRATION OF RENEWABLES”

Abstract: Microgrids can provide the most promising means of integrating large amounts of distributed sources into the power grid and can supply reliable power to critical loads. However, managing distributed sources and loads within a microgrid during island and grid-tie modes and during transitions is a challenge. Stable operation of a microgrid is a concern specifically during the starting of motor loads, switching of large loads, and in presence of high penetration of renewable resources. Hence, a generalized control framework is required to regulate microgrid voltage and frequency, maintain power quality, manage Distributed Generations (DG) and ensure microgrid stability. Several control methods have been developed for microgrid control. Majority of these techniques are based on natural droop control or modified natural droop control, which rely on voltage and frequency variations as inputs to control algorithms. At present, there are no methods available for sizing the capacities needed to ensure reliable operation and stability. A new microgrid control framework, Virtual Droop Control (VDC), for power management as well as for voltage and frequency regulation is proposed in this thesis. The proposed control method analyzes the effect of intermittent resources and dispatches the power commands to individual generation assets ensuring stable operation of the microgrid.

M.S. degree from Gannon University, Erie, PA,2008.
B.S. degree from S.P. University, Gujarat, India,2007.

Project Engineer
S&C Electric Company

solanki@uwm.edu

 


Emad Manla
Ph.D. Graduate, December 2012

Dissertation: “INTEGRATED LI-ION ULTRACAPACITOR WITH LEAD ACID BATTERY FOR VEHICULAR START-STOP”

Abstract: Advancements in automobile manufacturing aim at improving the driving experience at every level possible. One improvement aspect is increasing gas efficiency via hybridization, which can be achieved by introducing a feature called start-stop. This feature automatically switches the internal combustion engine off when it idles and switches it back on when it is time to resume driving. This application has been proven to reduce the amount of gas consumption and emission of greenhouse effect gases in the atmosphere. However, the repeated cranking of the engine puts a large amount of stress on the lead acid battery required to perform the cranking, which effectively reduces its life span. This dissertation presents a hybrid energy storage system assembled from a lead acid battery and an ultracapacitor module connected in parallel. The Li-ion ultracapacitor was tested and modeled to predict its behavior when connected in a system requiring pulsed power such as the one proposed. Both test and simulation results show that the proposed hybrid design significantly reduces the cranking loading and stress on the battery. The ultracapacitor module can take the majority of the cranking current, effectively reducing the stress on the battery. The amount of cranking current provided by the ultracapacitor can be easily controlled via controlling the resistance of the cable connected directly between the ultracapacitor module and the car circuitry.

MSEE from University of Wisconsin-Milwaukee, WI. 2009
BSEE from the American University of Sharjah, United Arab Emirates. 2006

Visiting Professor
University of New Haven

emanla@uwm.edu

 


Yogesh Patel
Ph.D. Graduate, December 2012

Dissertation: “MULTI-LEVEL MEDIUM VOLTAGE INVERTER FOR DC DISTRIBUTED WIND FARM TO ESTABLISH GRID INTERFACE AND PROVIDE ANCILLARY SUPPORT”

Abstract: Wind energy has gained in popularity in recent years due to cost, security and environmental concerns associated with conventional energy sources like fossil fuels. However, the utilization of wind energy in power systems creates many technical and non-technical challenges that need to be addressed for successful integrations. The main technical issues related to wind energy are its uncertainty and variability and their impacts on stability, reliability and quality of the electric power. In systems with high wind energy penetrations, unlike conventional generations, sudden changes in active and/or reactive power demand cannot be supported by wind energy. This lack of demand support may create unwanted voltage and frequency variations in the grid. On the hand, the existing AC distributed wind farms have several drawbacks including complexity, higher cost, and lower efficiency.

In this dissertation, a medium voltage direct current (MVDC) distribution system for wind farms is investigated. The proposed system offers higher reliability, lower cost, higher efficiency and more importantly grid support. It also allows for easier integration of energy storage systems at DC level. Design, control, implementation, and testing of a three-level medium voltage inverter are presented. The inverter can provide active and reactive power support to the grid in case of frequency and voltage droops. Simulation and experimental results are presented to verify the viability of the proposed system and control techniques.

MSEE from Illinois Institute of Technology, Chicago. 2003
BSEE from Maharaja Sayajirao University of Baroda, India. 2000

Hardware Engineer
Rockwell Automation.

yppatel@ra.rockwell.com

 



 

Goran Mandic
Ph.D. Graduate, July 2012

Dissertation: “LITHIUM-ION ULTRACAPACITOR ENERGY STORAGE INTEGRATED WITH A VARIABLE SPEED WIND TURBINE FOR IMPROVED POWER CONVERSION CONTROL”

Abstract: The energy of wind has been increasingly used for electric power generation worldwide due to its availability and ecologically sustainability. Utilization of wind energy in modern power systems creates many technical and economical challenges that need to be addressed for successful large scale wind energy integration. Variations in wind velocity result in variations of output power produced by wind turbines. Variable power output becomes a challenge as a number of wind turbines integrated into power systems increase. Power variations cause voltage and frequency disturbances that may lead to activation of relay protective equipment, that sense these disturbances, which may result in power outages. While a majority of power produced in modern power systems comes from synchronous generators that have large inertias and whose control systems can compensate for slow power variations in the system, faster power variations at the scale of fraction of a second to the tens of seconds can seriously reduce reliability of power system operation. Energy storage integrated with wind turbines can address this challenge. In this dissertation, lithium-ion ultracapacitors are investigated as a potential solution for filtering power variations at the scale of tens of seconds. Another class of issues related to utilization of wind energy is related to economical operation of wind energy conversion systems. Wind speed variations create large mechanical loads on wind turbine components, which lead to their early failures. One of the most critical components of a wind turbine is a gearbox that mechanically couples turbine rotor and generator. Gearboxes are exposed to large mechanical load variations which lead to their early failures and increased cost of wind turbine operation. This dissertation proposes a new critical load reduction strategy that removes mechanical load components that are the most dangerous in terms of harmful effect they have on a gearbox, resulting in more reliably operation of a wind turbine.

BSEE, University of Belgrade, Belgrade, Serbia, 2004.

Lead Engineer
Eaton Corporation, Raleigh, NC

gmandic@uwm.edu

 


 

Ravi Nanayakkara
Ph.D. Graduate, December 2012

Dissertation: “SCR-BASED WIND ENERGY CONVERSION CIRCUITRY AND CONTROLS FOR DC DISTRIBUTED WIND FARMS”

Abstract: The current state of art for electrical power generated by wind generators are in alternating current (AC). The wind farms distribute this power as 3-phase AC. There are inherent stability issues with AC power distribution. The grid power transfer capacity is limited by the distance and characteristic impedance of the lines. Furthermore, wind generators have to implement complicated, costly, and inefficient back-to-back converters to implement AC generation. AC distribution does not offer an easy integration of energy storage. To mitigate these drawbacks with AC generation and distribution, direct current (DC) generation and high voltage direct current (HVDC) distribution for the wind farms is proposed. DC power distribution is inherently stable. The generators convert AC power to DC without the use of a back-to-back converter. DC grid offers an easy integration of energy storage.

The proposed configuration for the generator is connected to a HVDC bus using a 12 pulse thyristor network, which can apply Maximum Power Point Tracking (MPPT). To properly control the system, several estimators are designed and applied. This includes a firing angle, generator output voltage, and DC current estimators to reduce the noise effects. A DSP-based controller is designed and implemented to control the system and provide gate pulses. Performance of the proposed system under faults and drivetrain torque pulsation have been analyzed as well. Converter paralleling when turbines operate at different electrical power levels are also studied. The proposed new Wind Energy Conversion System (WECS) is described in details and is verified using MATLAB®/ Simulink® simulation and experimental test setup. The proposed solution offers higher reliability, lower conversion power loss, and lower cost. The following is proposed as future work. Study different control methods for controlling the SCR’s. Investigate reducing torque pulsations of the PMSG as well as using the proposed power conversion method for DFIG turbines. Explore options for communication/control between PMSG, circuit protection and grid-tied inverters. Investigate the best possible configuration for DC storage/connection to the HVDC/MVDC bus. Study the filtering needed to improve the DC bus voltage at the generator.

MSEE from Marquette University, Milwaukee, WI, 2001.
BSEE University of Wisconsin-Milwaukee, Milwaukee, WI, 1997.

Electrical Engineer
Rockwell Automation, Milwaukee, WI

nanayak3@uwm.edu

 


 

Ali Esmaili
Ph.D. Graduate, May 2012

Dissertation: “ENERGY STORAGE FOR SHORT-TERM AND LONG-TERM WIND ENERGY SUPPORT”

Abstract: Wind farm output power fluctuations create adverse effects on the voltage, frequency, and transient stability of the utility grid. Short term wind farm power variations and steep ramp rates cause voltage instability, especially if the farm is located in weak grid areas. In this diisertation, integration of wind energy with energy storage devices to support the short-term shortcomings of wind energy is discussed. A turbine level hybrid configuration of an energy storage system is used to limit the power ramp rate and apply power smoothing. The utilized energy storages devices are zinc bromide flow battery and Lithium-Ion Capacitors (LIC). The actual models of the battery and capacitor, which are derived from testing, are used in this study. The wind farm power is also modeled using measured wind speed data. FCP and ACP as two new concepts have been introduced to evaluate the effect of energy storage system for wind energy support. The analyses show that significant improvements can be made to shape the output power of the farm using practical and turbine level energy storage systems.

Moreover, the dissertation studies a wind farm integrated with a storage device in order to shift the wind power generation to the peak demand periods. A model of system including practical utility level battery integrated with wind farm at wind turbine level is developed and presented. In order to measure the effect of this integration, actual profiles of load, wind energy, and other power sources is studied and the Effective Load Carrying Capability (ELCC) for wind energy, after and before adding the battery, is calculated. Results show that the battery can make some improvements in the ELCC of wind energy.

A wind turbine emulator is built in the lab, and was connected to a storage device, which is tied to the grid. Different tests were conducted at various situations. The wind emulator power data indicates the need for a mechanism to reduce the uncertainty of the wind power. The analysis and figures show that wind power variations and fluctuations can be mitigated using a storage system with a proper control algorithm.

MSEE from Sharif University of Technology, Tehran, Iran, 2005.
BSEE from Shiraz (Pahlavi) University , Shiraz, Iran, 2003.

Senior Design Engineer,
Caterpillar Inc, South Milwaukee, WI.

aesmaili@uwm.edu

 


 

Omar Abdel-baqi
Ph.D. Graduate, July 2010

Dissertation: “SERIES VOLTAGE COMPENSATION FOR DOUBLY-FED INDUCTION GENERATOR WIND TURBINE LOW VOLTAGE RIDE-THROUGH SOLUTION”

Abstract: Wind is clean and unlimited free source of energy. But in order for the wind energy to be effectively and efficiently harvest without interruption, the wind generators required to ride-through grid disturbances and support the grid during voltage sag events. The Doubly-Fed Induction Generator (DFIG) is known for its poor response to the voltage sags. This work develops an approach to provide a robust ride through technique for the DFIG during different types of voltage sags.
In this work, a mathematical model for the DFIG is developed and used to analyze the voltage sags effects. The DFIG stator flux is the main state that affects the system during the voltage sags. A component of the stator flux declines with the slow time constant of the generator during sudden voltage sag and forces a large rise of current in the rotor windings. This current rise damages double conversion power electronics converter connected to the rotor winding.

The existing techniques for Low Voltage Ride Through (LVRT) solutions have many drawbacks. In brief, they introduce undesirable spikes in generator torque and currents. In addition, they do not provide support to the grid during low voltage conditions.

In order to mitigate the effect of the slow declining component of the stator flux during voltage sags, the system is augmented with a series converter on stator circuitry. The converter injects a voltage on the stator to correct the air gap flux and to ultimately prevent the rotor current rise. A dead-beat control technique is developed to adjust the converter. In addition to the extensive computer simulation, a laboratory scale prototype is developed to validate the proposed solution. The size of the energy storage system required for the converter is also discussed in this thesis.
Future work directions are proposed including more robust control technique, minimizing number of additional parts and bigger laboratory scale prototype.

MSEE in Signal and System, University of Detroit Mercy, Detroit, USA, 2005.
BS in Industrial Automation, Palestine Polytechnic University, Palestine.

Sr. Engineering Specialist
Caterpillar Global Mining LLC

oja@uwm.edu

 


 

Tin T. Luu
Ph.D. Graduate, December 2010

Dissertation: “TRANSIENT STABILITY IMPROVEMENT FOR DFIG WIND TURBINES USING ULTRACAPACITOR”

Abstract: Wind energy source is characterized as variable and unpredictable. A random wind speed and blade rotational turbulence can produce fluctuations on the voltage and power supplied into the system. This fluctuated power makes the wind power undispatchable, causing frequency deviations and power outage when wind power penetration is significant. The fluctuating power will impact on power balance and voltage at the point of common coupling. Output power of wind turbine is cubic function of the wind speed. Since wind speed is a nearly random parameter, the output power of the wind turbine is also random process. Even a small variation of wind speed could cause a large variation in the output power. As a result, a large voltage fluctuation may cause voltage variations outside the regulation limit at connection point.

Energy storage devices such as batteries, ultra capacitors, super inductors, and flywheels can be utilized in a hybrid system to solve this problem. A selective control method to mitigate the power fluctuations using the rotor inertia is introduced in the literature. This method is also modified to obtain better energy capturing efficiency. The energy extracting capability using this method is comparable with other methods such as Maximum Power Extraction (MPE) algorithm. In this thesis, a new integrated topology of DFIG wind turbine and Ultracapcitor is introduced. The Ultracapcitor is directly placed on the DC bus of the power conversion device on the rotor. A control technique is developed to adjust the active and reactive power of the turbine, apply power smoothing, and keep the DC bus voltage within an acceptable range.

Matlab Simulink simulation results for various cases are performed on a doubly fed induction generator that verifies the theoretical analysis.

MSEE University of Wisconsin-Milwaukee, WI, 2005
BSEE University of Wisconsin-Milwaukee, WI, 2003

trungtin@uwm.edu

 


 

YounHee Lee
Ph.D. Graduate, Aug 2008

Dissertation: Modeling and Simulation of Conducted EMI Noise Generated in the Electric Power Converters of the Electric and Hybrid Electric Vehicles

Abstract: In developing a complex power electronic system, proper modeling at prototype sample level can save the time and cost to develop final product. Hybrid Electric Vehicle (HEV) is equipped with several high-powered electric devices such as DC/DC converters and DC/AC inverters. Considering that there are many restrictions in mounting and wiring the electric power system in engine room, analysis and modeling of Electromagnetic Interference (EMI) coupling path is crucial in designing optimal electric power system satisfying various EMC regulations. In this thesis, conductive common mode (CM) and differential mode (DM) equivalent models for those electric power converters used in Hybrid Electric Vehicles (HEV) were developed and evaluated through the simulation both in time domain and frequency domain. The simulation results were compared to those of real experimental tests performed in EMC testing Laboratory. It is confirmed that the suggested noise equivalent models followed the actual power electric systems in the point of conducted CM and DM EMI view.

BSEE from Kookmin University, Seoul, Korea, 1985
MSEE from Polytechnic University, NY, 1994

Senior Project Engineer
at General Motors, Hybrid Power Electronics

younhee.lee@gm.com

 


 

Asghar Abedini
Ph.D. Graduate, December 2008

Dissertation: Integration of Permanent Magnet Synchronous Generator Wind Turbines into Power Grid.

Abstract: Wind energy is becoming a major source of generating electrical energy. High price of oil and environmental issues are the most incentives to develop wind power plants. Due to high price of foundation of wind farms, employing variable speed wind turbines to maximize the extracted energy from blowing wind is more beneficial.
On the other hand, since wind power is intermittent, integrating energy storage systems with wind farms has attracted a lot of attention.

These two subjects are addressed in this dissertation in detail. Permanent Magnet Synchronous Generators (PMSG) are used in variable speed wind turbines. In this thesis, the dynamic of the PMSG is investigated and a power electronic converter is designed to integrate the wind turbine to the grid. The risks of PMSG wind turbines such as low voltage ride through and short circuits, are assessed and the methods of mitigating the risks are discussed.
In the second section of the thesis, various methods of smoothing wind turbine output power are explained and compared. Two novel methods of output power smoothing are analyzed: Rotor inertia and Super capacitors. The advantages and disadvantages of each method are explained and the dynamic model of each method is developed. The performance of the system is evaluated by simulating the wind turbine system in each method.

The concepts of the methods of smoothing wind power can be implemented in other types of wind turbines such as Doubly Fed Induction Generator (DFIG) wind turbines.

MSEE from Sharif University of Technology, Tehran, Iran, 2003.
BSEE from Isfahan University of Technology, Isfahan, Iran, 2001.

Assistant Professor of Electrical and Computer Engineering Department,
Khaje Nasir Toosi University of Technology, Tehran, Iran.

aabedini@uwm.edu