Jeffrey W Moscrop

Experimental Analysis of the Magnetic Flux Characteristics of Saturated Core Fault Current Limiters

A fault current limiter (FCL) is a device that is designed to reduce the impact of fault currents on electricity networks and increase the availability of such networks to consumers. One particular FCL technology that is currently attracting worldwide attention, from both researchers and commercial engineering companies, is the saturated core FCL. This device utilizes the change in permeability between the saturated and unsaturated states of a magnetic core to provide both low steady state losses and effective fault current limiting. Typically the core is saturated using an electromagnetic coil, which can be either superconducting or non-superconducting. Although there have been several studies on the electrical characteristics of this device, the transient magnetic behavior has been largely overlooked. In this paper the magnetic flux characteristics of saturated core FCLs are experimentally analyzed. The study includes the magnetic behavior during both the initial biasing of the cores and during transient fault conditions. The influence of FCL topology and alternative low-cost core materials, on the flux characteristics and overall device performance, is also discussed.

Increasing Energy Efficiency of Saturated-Core Fault Current Limiters With Permanent Magnets

Fault current limiters are devices that limit currents in electrical networks during a fault event, such as power lines touching or arcing due to tree branches. In this paper, two designs for small-scale saturated-core fault current limiters are discussed: one that uses DC coils to saturate the cores, and one that uses a hybrid design with DC coils and permanent magnet blocks. The hybrid design is found to meet the same performance criteria with a 50% reduction in energy consumption when compared to the DC coil only design.

Analytical Nonlinear Reluctance Model of a Single-Phase Saturated Core Fault Current Limiter

A saturated core fault current limiter (FCL) is a device that is designed to limit the fault currents in electrical energy networks and consequently, protect existing network equipment from damage. Due to complex nonlinear magnetic properties, the performance of saturated core FCLs has largely been characterized through experimentation and finite-element analysis simulations. Although both of these techniques are quite accurate, they are time consuming and do not describe the behavior of FCLs in actual electrical networks. This has led to an increasing demand for an accurate analytical model that is suitable for transient network analyses. This paper presents the development of an analytical model of a single-phase open-core FCL, which accurately describes the nonlinear magnetic properties of the FCL through a reduced reluctance approach. The extension of this model to other saturated core FCL arrangements (such as closed core) is also discussed.

Control of servo systems in the presence of motor-load inertia mismatch

There are a number of performance limiting factors that concern the designers of machine tool servo systems. One such factor is the mismatch that often exists between motor and load inertias. This paper briefly discusses the results of a previous study on the factors that interact to introduce stability problems in the presence of a high motor-load inertia mismatch. The effects of such an inertia mismatch, on servo system performance, are then discussed and analysed using closed loop frequency responses. Various methods of improving the system response using fast feedback control are presented, including traditional PI control and modern methods incorporating full order state observation. The effectiveness of these control methods are then compared after comprehensive testing on a specially constructed experimental rig.

Design considerations in MgB2-based superconducting coils for use in saturated-core fault current limiters

Saturated-core fault current limiters (FCLs) are devices that have many applications and potential for use within power networks. At a commercial scale, these devices require high H-field magnets to saturate the steel core, which can typically only be achieved through the use of superconducting coils. Here, we present several challenges that arise in the application of superconducting coils in FCLs and discuss how to address these issues through a case study MgB2-based coil. It is found that significant ac magnetic fields, Lorentz forces, and Joule heating in components occur during normal and fault operations; however, these issues can be mitigated when properly addressed.

Three phase saturated core fault current limiter performance with a floating neutral

High power demands in electricity grids are continually increasing. This increasing trend coupled with the introduction of renewable energy sources, which require energy storage devices, poses significant problems to fault current levels. To improve network availability and grid resilience, superconducting saturated core Fault Current Limiters (FCL) are a suitable solution to reduce high fault currents in distribution level electricity grids. These devices have the characteristic of low impedance to the network during normal operation and high impedance during a fault event. However, this change in impedance is nonlinear and can lead to an imbalance in a three phase saturated core FCL. In this paper, the effects on fault current limiting performance due to the unbalanced instantaneous impedance between the 3 phases are investigated. The FCL fault transients of a grounded fault and a floating fault are simulated and compared using Finite Element Analysis (FEA) techniques. The presented results are also validated against a real world distribution level saturated core FCL under high power testing.

Novel tooth design for a tubular linear motor for machine tool axis

Permanent magnet tubular linear motors have shown potential for use as direct drives in high precision machine tools. These devices are capable of replacing the traditional ball screw, given their ability to overcome or reduce many of the fundamental mechanical properties that limit the precision performance of the ball screw. However, given the desire from industry to increase speed and acceleration, the physical size of the motor must be minimised while maintaining high force output to increase bandwidth in high precision applications. This paper presents a new optimal tooth design to reduce the frame size of a slotted tubular linear motor. Several design aspects of the optimal tooth are considered and general solutions are derived. Data from finite element analysis and from an experimental machine is presented to validate the new designs.

Transient Modeling of Saturated Core Fault Current Limiters

A saturated core fault current limiter (FCL) essentially utilizes the dynamic and nonlinear magnetic behavior of steel cores to operate as a variable reactor. However, the nonlinear characteristic of magnetic materials has made modeling this unique device a difficult task. Hence, experimental measurements and finite-element method (FEM) analysis are the most common techniques used to characterize the transient behavior of the device. Both of these techniques, while accurate, cannot be used to analyze the transient electrical behaviour of FCLs in complex power systems, particularly when investigating power system switchgear behavior during fault events. FEM-based FCL modelling, despite its usefulness as a design verification tool, cannot be easily coupled to all electromagnetic transient programs that are in use today. This paper presents two modeling approaches to represent the saturated core FCL in transient network simulators: 1) the nonlinear reluctance model and 2) the nonlinear inductance model. Both models are implemented in PSCAD/EMTDC and are validated by experimental results of a single-phase prototype saturated core FCL, where excellent agreement between the experimental and the modelling approaches is achieved.

An Analysis of Motor/Load Inertia Mismatch in Machine Tool Servo Systems

Abstract One of the factors that concerns designers of machine tool servo systems is the mismatch that often exists between motor and load inertias. This paper provides a thorough mathematical analysis of the factors that interact to introduce stability problems in the presence of a high motor/load inertia mismatch. As a result of this analysis, the relative effects of inertia mismatch are discussed with reference to three commonly used servo system configurations. Methods of improving the system response using fast feedback control are also presented and experimentally verified.

Applicability of nonlinear reluctance model to a closed core Fault Current Limiter

A saturated core Fault Current Limiter (FCL) is a fault current reducing device that has attracted significant attention from both researchers and electricity utilities. The behaviour of saturated core FCLs has so far been characterised largely through experimentation and/or Finite Element Analysis (FEA) simulations, primarily due to the intricate nonlinear magnetic characteristics of the device. Both these techniques, while accurate, fail to demonstrate the behaviour of FCLs in actual electrical networks. Hence there is an increasing need for an accurate model of the FCL that can be easily incorporated into transient network analysers. This paper presents the development and applicability of an analytical model of a single-phase FCL with a closed core topology, which accurately describes the nonlinear magnetic properties of the FCL through a reduced reluctance approach. The model is implemented in an electromagnetic transient network simulation package and the simulation results are validated against experimentally measured data.