Joseph D. Law

Modeling and analysis of a flywheel energy storage system for Voltage sag correction

The U.S. Navy is looking for methods to maximize the survivability of combat ships during battle conditions. A shipboard power distribution system is a stiff isolated power system that is vulnerable to voltage sags, which arise due to faults or pulsed loads, which can cause interruptions of critical loads. A series voltage injection type flywheel energy storage system (FESS) is used to mitigate voltage sags and maximize the survivability of the ship. The basic circuit consists of an energy storage system, power electronic interface, and a series injection transformer. In this case, the energy storage system consists of a flywheel coupled to an induction machine. The stored energy is used for sag correction for the critical load. Indirect field-oriented control (IDFOC) with space-vector pulsewidth modulation (SVPWM) is used to control the induction machine. Sinusoidal PWM is used for controlling the power system side converter. This paper presents the modeling, simulation, and analysis of a FESS with a power converter interface using PSCAD/EMTDC.

Modeling and analysis of a flywheel energy storage system for voltage sag correction

The US Navy is looking for methods to maximize the survivability of combat ships during battle conditions. A shipboard power distribution system is a stiff, isolated power system that is vulnerable to system transients. Power quality problems, such as voltage sags, which arise due to a fault or a pulsed load, can cause interruptions of critical loads. Critical loads include radar systems, pumps and weapon systems. A series voltage injection type flywheel energy storage system is proposed to mitigate voltage sap and maximize the survivability of the ship. The bask circuit consists of an energy storage system, power electronic interface and a series injection transformer. In this case the energy storage system consists of a flywheel coupled to an induction machine. The flywheel stores energy In the form of kinetic energy and the induction machine is used for energy conversion. The power electronic interface facilitates the bi-directional flow of power for charging and discharging the flywheel through the induction machine. The stored energy is used for sag correction when the critical load sees a voltage sag. Indirect field oriented control with space vector PWM is used to control the induction machine. Sinusoidal PWM is used for controlling the power system side converter. This paper presents the modeling, simulation and analysis of a flywheel energy storage system with a power converter interface using PSCAD/EMTDC.

A Single Phase Induction Motor Voltage Controller with Improved Performance

Induction motors inherently operate with nearly constant airgap flux and therefore almost constant iron losses. When the load does not require full flux, conventional voltage controllers utilize thyristors in series with the motor to reduce airgap flux by decreasing the applied voltage. Thereby, iron losses decrease and the overall efficiency increases. However, thyristor voltage controllers tend to introduce harmonics into the current waveform which not only reduces motor efficiency but also causes harmonic pollution of the power lines. An improved voltage controller and control strategy for efficiency improvement of single phase induction motors is presented. In particular, thyristor voltage control by dynamic switching of the winding configuration is presented. Laboratory data for a voltage controller, thus enhanced, demonstrates a significant decrease in input motor current distortion and increase in efficiency below one-quarter load.

Magnetic circuit modelling of the field regulated reluctance machine. Part I: model development

A transient magnetic circuit model of a field regulated reluctance machine is presented. The model is based on flux loop equations rather than node equations as have previously been employed. A key feature of this approach is that magnetic circuit models based on loop equations permit an entire machine to be represented by only one equivalent pole.

Design and performance of the field regulated reluctance machine

A topological configuration for rotating electromagnetic machines that can produce significantly higher force density than an induction machine is investigated. The stator is constructed using full pitch concentrated windings embedded in conventional slots. Rotor saliency is produced using poles constructed of axially oriented laminations. Operation is such that all of the conductors are actively taking part in torque production all of the time. The means for exciting the stator windings allow for independent control of torque and regulation of the rotor flux. A force density comparison is made based on operation with equal surface current density, construction losses, and peak-air-gap flux density. Construction and testing of a prototype 500-r/min, 28-kW laboratory machine, converter, and controller are described.<<ETX>>

Single-Phase Transformer Inrush Current Reduction Using Prefluxing

Power transformers can experience large inrush currents upon energization, the severity of which depends on the source strength, the leakage impedance and residual flux of the transformer, and the angle of the applied voltage at energization. A novel inrush current reduction strategy has been implemented which involves setting a single-phase transformers residual flux to a known polarity after the transformer has been de-energized, a process called “prefluxing,” and controlling the instant of transformer energization based on the flux polarity, seeking not to eliminate inrush current but to substantially reduce it. Unlike a popular suggested solution, this strategy does not require prior knowledge of the transformers flux. The device used for prefluxing is simple in construction and operates at substantially lower voltage levels when compared to the transformers rated voltage. The presented strategy has been successfully implemented on an 18-kVA laboratory transformer with inrush current levels reduced below the rated current of the transformer even when accounting for typical breaker deviations. This paper describes the operation of the reduction strategy, including theory, device sizing, and implementation, and presents the successful laboratory results, all of which provide the basis for implementing inrush current reduction in three-phase transformers using a three-pole circuit breaker.

Using a superconducting magnetic energy storage coil to improve efficiency of a gas turbine powered high speed rail locomotive

The US Federal Railroad Administration has been pursuing the use of locomotives with an on-board prime mover for high speed rail. Such systems would not require the added cost of rail electrification on top of the rail bed modifications. The prime mover runs a synchronous generator, with the output rectified to feed a DC bus. Adjustable speed drives control the traction motors. However, gas turbines run efficiently over a narrow speed range and a relatively narrow power range. The addition of a superconducting magnetic energy storage coil can improve overall system performance. The SMES coil is charged whenever the locomotive is in regenerative braking mode and whenever the prime mover is producing more power than is needed to maintain the desired speed down the track. The chief benefits to such a scheme are: (1) better acceleration at high speeds, (2) reduced prime mover power rating and weight, (3) reduced railbed cost due to reduced weight (4) reduced trip time and (5) improved fuel efficiency.

Magnetic circuit modeling of the field regulated reluctance machine. Part II: saturation modeling and results

This paper explores and offers solutions for two causes of simulation instabilities which occur when modeling machines using magnetic circuits. An algorithm for dynamic simulation of electric machines based on the magnetic circuit model (MCM) developed in part I of this paper is augmented to include the ability to model magnetic saturation without numerical oscillations. The resulting MCM algorithm is implemented in a digital computer program which is well suited for both simulation of transients in nonsinusoidally wound machines such as a field regulated reluctance machine (FRRM), and for design. The model developed is verified using a laboratory prototype FRRM, power converter, and controller. Results of computer simulations using the magnetic circuit model agree well with experimental machine data.

Analysis of synchronous generator internal insulation failures

Winding insulation failures within a synchronous generator cause extensive damage and are expensive to repair. The understanding of the machine behavior during internal failures is very important for the full protection of the machine. Within the literature, little is written relating to modeling, analysis, and experimental testing of synchronous generators during internal fault conditions. This paper provides a summary of an experimental approach taken to capture this phenomenon. These results discuss real-time measurements taken during controlled internal faults on a laboratory scale generator. This data were also compared to data recorded by protective relays during an evolving internal insulation failure of a 145 MVA utility generator

AC/DC/AC Converter Modulation Strategy With Natural Zero sequence Rejection Using Only One Six-Switch Inverter Module

This paper reports an innovative power converter modulation strategy that converts energy from a poly-phase variable frequency source to a single-phase fixed frequency load using only one of the customary two six-switch bridge circuits characteristic of a dc link based power converter. To achieve this, a six-switch dc link CPRWM inverter is creatively modulated with two voltage components. The application is single- phase, fixed frequency power generation from a variable frequency wind energy conversion system, which uses an induction generator, a converter, and a zero sequence filter. Modulating the converter for a three-phase balanced positive sequence voltage component at the induction generators optimum operating frequency transfers energy from the generator to the converter. At the same time, modulating the converter for a three-phase fixed frequency zero sequence voltage component transfers the energy from the converter to the load through a zero sequence filter. The two voltage components are separated by the ungrounded induction motors natural rejection of zero sequence components and the zero sequence filters natural rejection of positive sequence components. Unity power factor output is readily achieved. This paper presents operation, control, theoretical calculations, simulations, and experimental results that reveal important limitations of this energy conversion scheme with a single converter.