Kevin D. Bachovchin

Stable Levitation of a Passive Magnetic Bearing

A design for a passive magnetic bearing system that can stably levitate a rotor in all directions is described. The bearing system consists of levitation magnets coupled with a Halbach array stabilizer, which induces currents in stabilization coils, in order to overcome the inherent instability of a system composed only of permanent magnets. The levitation magnet system consists of two pairs of annular ring magnets which provide an upward magnetic levitation force to counteract the downward gravitational force of the rotor. The Halbach array stabilizer consists of two stabilization coils shifted in angular position with respect to one another and centered in the vertical direction between two rotating Halbach arrays. Magnetic fields from permanent magnets are calculated using superposition of fields due to patches of magnetization charge at surfaces where the magnetization is discontinuous. Induced currents in the stabilization coils are calculated by computing the time derivative of the magnetic flux through those coils. Magnetic forces on the rotor are computed using a superposition of forces on each patch of magnetization charge. The entire magnetic bearing system, consisting of both the levitation magnets and the Halbach array stabilizer, is stable to both vertical and lateral displacements. Results are compared with a simpler straightened approximation of the Halbach array stabilizer.

Magnetic Fields and Forces in Permanent Magnet Levitated Bearings

Magnetic fields and magnetic forces from magnetic bearings made of circular Halbach permanent-magnet arrays are computed and analyzed. The magnetic fields are calculated using superposition of fields due to patches of magnetization charge at surfaces where the magnetization is discontinuous. The magnetic force from the magnetic bearing is computed using superposition of forces on each patch of magnetization charge. The magnetic force from a Halbach array magnetic bearing is compared to an annular ring bearing of the same dimensions. A comparison is also made between the results obtained using the magnetic surface charge method and the simpler approximate method using a 2-D analytic representation of the Halbach array fields.

Physics-based foundations for cyber and market design in complex electric energy systems

In this paper we propose a Dynamic Modeling and Decision Systems (DyMonDS) framework for modeling, simulating and designing cyber in the changing electric energy systems. The approach is fundamentally physics-based, which helps identify the relevant multi-layered structure of their complex dynamics. The common thread throughout the paper is the idea that coupling within a complex interconnected system can be represented using energy stored in different components and the rate of energy exchange with the rest of the system. This idea supports modeling in a transformed state space which makes it possible to systematically design interactive cyber for managing dynamics of energy exchange for provable performance in the evolving multi-physics energy systems. We describe a general scalable DyMonDS simulator currently under the development by our group for demonstrating potential of newly proposed cyber.

Transient stabilization of power grids using passivity-based control with flywheel energy storage systems

A novel passivity-based controller using three time-scale separations is introduced for variable speed drives of flywheel energy storage systems. The time-scale separation between the power electronics, the electrical machine variables, and the mechanical machine variables is used in order to design a nested three-layer controller. Since the dynamics of the variable speed drive are non-linear, the controller uses passivity-based control logic to achieve regulation of both the flywheel speed and the currents into the power electronics. Finally, the controller is demonstrated for transient stabilization of an interconnected power system in response to a large sudden wind generator disturbance.

Toward a systems approach to power-electronically switched T&D equipment at value

This paper concerns the difficult problem of dynamic management of T&D controllable equipment. This approach is likely to be necessary in order to ensure reliable operations as the system inputs are becoming very dynamic and hard to predict accurately. Complexity of congestion caused by voltage- and stability-related operating problems will require systematic software tools for managing slow mechanically-switched as well as fast power-electronically-controlled T&D equipment. For the first time sensing and communications technology is available to enable genuine automation of Flexible AC Transmission Systems (FACTS). Systematic design of widely dispersed power-electronically controlled T&D equipment, together with software-enabled on and off line switching and dispatch of set points on mechanically controlled T&D equipment jointly could enable highly efficient and clean utilization in future electric energy systems without endangering system reliability. To the contrary, it is illustrated using small examples in this paper that, unless this approach is implemented, the system may become highly vulnerable during previously unexperienced operating conditions. In addition to the technical challenge underlying systematic dynamic T&D equipment management, major open R&D questions remain concerning the design of protocols for dynamic and interactive risk management by both the T&D owners and operators and the system users themselves.

Costs and benefits of transmission congestion management

Transmission congestion is a major economic and reliability concern in todays electric power systems. However, as the industry undergoes changes, it becomes critically important to better understand the fundamental role of transmission and its technical and financial effects on the electricity provision. Thermal limits, voltage drop regulation, and voltage stability limits are among the most common causes of transmission congestion. In order to relieve congestion, new transmission can be built or reactive power control can be installed. In this paper, simple power systems are analyzed in order to compare costs of different ways of reducing congestion. The length of a transmission line determines the cause and the most cost effective solution. For short lines, the thermal limit is the limiting factor in determining the maximum power flow and building new transmission is the only effective solution. For longer lines, the voltage drop regulation is the limiting factor and adding a shunt capacitor is the most cost effective solution.

Ultimate limits to the fully decentralized power inverter control in distribution grids

As low-voltage distribution grids and microgrids emerge, their decentralized control design is a major issue and needs rethinking, as several recent examples of oscillatory behavior in voltage and reactive power have been reported. This paper analyzes a few possible reasons for these oscillations. Oscillations can occur as a result of connecting distributed energy resources to a single phase only, which creates unbalanced conditions. In order to avoid these oscillations, a novel master controller for assigning the slow set points to the inverters is proposed, so that the interdependence between unbalanced phases is accounted for. Oscillations can also occur due to the lack of a master controller to ensure the existence of an equilibrium or even as a result of inaccurate power set points used by droop-based controllers. Finally, even the very fast power electronic dynamics, which are usually assumed to be instantaneous in literature, may cause high frequency instabilities because the power electronics are not an ideal power source and have stability and switch saturation limits.

Transient Stabilization in Systems with Wind Power

This chapter analyzes transient stability problems in networks with high wind penetration. Disturbances caused by wind power perturbations and major equipment failures are considered, and their impact on system dynamics is analyzed. Next, the potential of power-electronically switched devices for the stabilization of system dynamics during high energy perturbations is considered. In particular, the use of shunt Flexible AC Transmission Systems (FACTS) devices, which are the most common power-electronically switched devices in today’s power grids as well as the use of flywheel energy storage systems is analyzed. The impact of nonlinear FACTS and flywheel control on the system stability is quantified by benchmarking it against today’s FACTS control using the electrical power system on the Flores Island. Major contributions are made in exploring the potential of nonlinear control logic using the dynamical model of the interconnected system.

Modeling, analysis and control design complexities in future electric energy systems

In this paper we suggest that a successful evolution of future electric energy systems will be critically dependent on having a systematic framework for communications and control architectures. These must be capable of ensuring stability and quality of frequency and voltage response in qualitatively different ways than it is currently being done. The main drivers of such change are described first. In particular, the focus of this paper is on the stability problems caused by the wind turbulence in systems with significant wind penetration. Next, solutions to the small signal stability problem, frequency regulation and large wind surge problem are proposed. The solutions are based on the concepts which honor the change in system dynamics caused by the new technologies. The claims are illustrated using realistic data for a small island power system in Flores, Portugal.