Power Systems

Wind Energy Research

We are conducting research on modeling and experimental verification of different types of wind turbine generators integrated with a utility grid. This part of the research is mainly focused on two newer types of generators: Doubly Fed Inductor Generator (DFIG) and Permanent Magnet Synchronous Generator (PMSG).

Permanent Magnet Synchronous Generator (PMSG):

  • Modeling of PMSG wind turbines for normal operation, including wind speed change. In this study PMSG is modeled at the machine level and is simulated in a system with grid and load.
  • Modeling and simulation of PMSG generators under short circuit and voltage sags. The generator and grid side current control is modeled under grid short circuit and grid voltage sag.
  • Utilizing energy storage devices to overcome power oscillations due to wind speed variations. An energy storage device, e.g., battery is used to smooth the output power of the wind turbine while the output power of the generator is varying due to wind speed variations.
  • Optimizing output power of the generator by adjusting the rotor speed.

Doubly Fed Inductor Generator (DFIG):

  • Modeling and simulation of machine level and grid level of DFIG. The DFIG is modeled at the machine level and the same as PMSG is simulated in a system connected to a local load and grid.
  • Output power maximization by adjusting the rotor speed. This practice is the same as described for PMSG above.
  • Modeling of wind speed change. In this study, the interactions of DFIG with a grid are studied when the wind speed changes. Active and reactive power are controlled and monitored in this study.

- Steady-state and dynamic analysis of wind turbine integrated with a power grid, to be used in the traditional power system analyses, such as load flow analysis, power factor correction, short circuit study, stability analysis, contingency study.
- Neural Network based Hour Ahead integrated load forecasting method. In this forecasting method, the electricity generated by wind power is considered as a negative load. The proposed artificial neural network based integrated load forecasting combines both conventional load forecasting and wind power forecasting. The integrated forecasting result will be used to schedule the conventional generator units as well as wind power units.

An Economic Dispatch Model Incorporating Wind Power

In solving the electrical power systems economic dispatch (ED) problem, the goal is to find the optimal allocation of output power among the various generators available to serve the system load. With the continuing search for alternatives to conventional energy sources, it is necessary to include wind energy conversion system (WECS) generators in the ED problem. This research develops a model to include WECS in the ED problem, and, in addition to the classic economic dispatch factors, factors to account for both overestimation and underestimation of available wind power are included. With the stochastic wind speed characterization based on the Weibull probability density function, the optimization problem is numerically solved for a scenario involving two conventional and two wind-powered generators. Optimal solutions are presented for various values of the input parameters, and these solutions demonstrate that the allocation of system generation capacity may be influenced by multipliers related to the risk of overestimation and to the cost of underestimation of available wind power.

Fiber Optics Based Fault Detection in Power Systems

A fiber optics based sensing network applicable for fault detection in a power system is researched in this project. The proposed scheme is secure and immune from interferences. At each monitoring location, passive rugged fiber-Bragg-grating-based sensors are deployed. They use fast and compact magnetostrictive transducers instead of current or potential transformers to translate current-induced magnetic field into optical signal. These sensors can be compensated for temperature drift and easily be integrated into an optical sensing network. A broadband light source at a substation scans the change in reflected optical power at a unique frequency band that corresponds to the surge in magnetic field associated with an increased fault current at a certain location. A unique feature of this real-time scheme is that it only requires current information for fault detections in both radial and networked systems with various pole structures and line configurations. It can easily coordinate with other protective devices and is free from any time-current coordination curves. The proposed scheme has been extensively tested by simulations. They confirm that the proposed scheme is able to detect the faults irrespective of the type and location. It also performs well in the presence of harmonics, high impedance, sensor mal-functions as well as sensor noise.

Techniques for Efficient Wireless Transcutaneous Power/Signal Transmission for Implanted Biomedical Devices

Implanted Electronic Devices require an appropriate power source. Cardiac pacemakers, for example, have included primary batteries that provide a rather long operational lifetime of five or more years because of a low duty cycle and an inspired engineering design. Similarly, cardiac defibrillators require a power supply that comes from an implanted primary (non-rechargeable) battery. However, newer technologies and devices such as Implantable Muscle and Nerve Stimulators, Implantable Telemetry Systems, and Left Ventricular Assist Devices, require more power than pacemakers and defibrillators and therefore need a direct connection through the skin or an implanted secondary (rechargeable) battery, to offer a practical operating lifetime. Traditionally, direct connections are made via a “button” connector that passes through the skin to the outside of the body. Experience has shown that this approach is undesirable because of the infection potential, high maintenance cleaning situation, and cosmetic inappropriateness in some applications. Furthermore, a wired battery charging system cannot provide an LVAD patient with freedom of mobility from the power source. A wireless transcutaneous power transfer system is therefore desirable and mandatory.
The aim of this project is to design and implement a transcutaneous energy and information transfer for implanted devices. Many Left Ventricular Assist Devices (LVADs) have been designed and clinically tried in recent years. One of the main challenges in the design of LVADs is transfer of uninterrupted power to run these devices continuously. The major goal of this project is to develop a novel integrated power electronics converter and transcutaneous power transfer topology to minimize the size and loss of the system inside the body. To accomplish this aim, two researchers from the University of Wisconsin-Milwaukee’s (UWM) College of Engineering and Applied Science and one researcher from Marquette University’s Biomedical Engineering Department are collaborating on this project.

The long term goal of the project is to develop a wireless transcutaneous power and motor control system that can lead to innovative commercial products, including future LVAD powering. The short term goal of this project is to realize a prototypical wirelessly powered motor control system that reveals the remaining challenges for subsequent development. Results would lead to insights into methods for adapting the system for additional high power demand implanted devices such as defibrillators, insulin pumps, nerve and muscle stimulators, etc. and the ability to attract future funds, including contributions from industry.

A Fast Switching Method for PM Synchronous Motor Speed Control

We are working on a new control technique for a PM synchronous motor to adjust motor current amplitude and phase to achieve different ranges of speed. The control logic is different for below and above motor nominal speed. In this method, the typical PID controllers, which cause complexity and instability, are removed. This new technique is simple, fast, stable, and reliable to drive PMSM in a wide range of speeds.

We have two control parameters, stator current, , and torque angle, , to adjust output torque. For speeds below nominal speed, to drive the motor at highest efficiency, we keep the torque angle at and adjust the torque by varying the magnitude of the stator current. Since the angle is kept at , the stator current is in the direction of q axis and  is set at zero. Working over the rated speed, the induced electromotive forces (emf) will exceed the maximum input voltage, if the air gap flux is not controlled. To solve this problem, the induced emf is constrained to be less than the applied voltage by weakening the air gap flux linkage. In this condition, we will change our torque by increasing the torque angle to more than 90 degrees. Therefore, for above nominal speed, we use the constant stator current at its rated value and adjust angle, at some point above 90 degrees to reach the desired speed in flux weakening operation.

Electromagnetic Compatibility for Electric and Hybrid Electric Vehicles

In developing a complex power electronic system, proper modeling at the prototype sample level can save the time and cost to develop a final product. Recently, Hybrid Electric Vehicles (HEV) are on their way to a great increase in number. While it reduced fuel consumption, an Electromagnetic Compatibility problem emerged and it should satisfy stringent EMC regulations. HEV uses a high power electric motor as an alternative driving power source and several electric converters as DC/DC converters and DC/AC inverters are used to drive the traction motor and various electric loads. They are the main sources of Electromagnetic Interference (EMI) due to the presence of a high switching frequency.

Considering that there are many restrictions in mounting and wiring the electric power system in an engine room, analysis and modeling of an Electromagnetic Interference (EMI) coupling path is crucial in designing an optimal electric power system satisfying various EMC regulations. In this project, conductive common mode (CM) and differential mode (DM) equivalent models for those electric power converters used in HEV were developed and evaluated through the simulation both in time domain and frequency domain using FFT. Individual devices in converters, such as inductors, capacitors and the switch, are modeled in accordance with the conducted EMI frequency range (150 KHz~30 MHz). From an EMI point of view, modeling of an IGBT switch is one of the most important parts of the simulation. It is well known that high speed IGBT switching produces high dv/dt (di/dt) and it causes the direct source of generation of DM noise in the conduction loop. It also excites the charging and discharging action of the parasitic capacitances in a converter and eventually generates CM conducted EMI noise between the two conduction lines and the ground. Here, a physics-based IGBT model is developed and used in simulation of the actual converter circuit. Also, a high frequency equivalent model (load side) and a low frequency equivalent model (source side) were developed in order to estimate the CM and DM noise generated from an actual converter circuit. In both the high frequency model and the low frequency model, a multiple-slope noise source was applied to obtain the better resolution of the EMI noise. It is confirmed that the suggested model followed the actual circuit from the point of conducted CM and DM EMI view.