Nafia Al-Mutawaly

Impacts of modern residential loads on power grids

Modern residential loads use variable frequency drive motors and solid state electronics to improve operational efficiency and reduce energy consumption. The drawback of such energy efficient loads is the introduction of harmonics onto the local distribution grid. Elevated harmonic content due to the increased use of such devices can potentially impact residential distribution networks and adjacent customers. This paper quantifies the current harmonics generated by modern appliances, subsequently fed back to distribution grids. Multiple tests were performed using typical, commercially available appliances and results were analyzed against the IEEE-519-1992 power quality Standard. Research findings will be used to estimate harmonic content on the power grid, model future residential distribution networks and clarify the need to update the IEEE-519-1992 Standard.

The effects of pulse configuration on magnetic stimulation.

Summary A study is presented in which the authors have examined the effects of pulse configuration, stimulation intensity, and coil current direction during magnetic stimulation. Using figure-8 and circular coils, the median nerve was stimulated at the cubital fossa and at the wrist of 10 healthy volunteers, and the response amplitude and site of stimulation were determined. The key findings of this study are in agreement with other researchers’ findings and confirm that biphasic stimulating pulses produce significantly higher M-wave amplitudes than monophasic stimulating pulses for the same stimulus intensity. Mean response amplitudes for biphasic stimuli applied by both coils at the elbow and wrist are consistently higher for the normal current direction. Mean response amplitudes for monophasic pulses are almost always higher for reversed currents. The site for effective stimulation (the position of the virtual cathode) cannot be defined within a fixed distance from the center of the coil (3 to 4 cm), as has been suggested by other researchers, but was found to vary depending on the coil current amplitude and direction as well as the degree of inhomogeneity of the tissues surrounding the nerve. There is a statistically significant relationship between virtual cathode shift and stimulus intensity for biphasic and monophasic pulses. Reversing the coil current direction has no statistically significant effect on the virtual cathode position. Virtual cathode shifts can be measured for biphasic and monophasic stimulations using a figure-8 coil at the wrist and the elbow. However, such a shift is difficult to determine with a circular coil.

Designing and constructing a magnetic stimulator: theoretical and practical considerations

Magnetic nerve stimulation has proven to be an effective non-invasive technique that can be used to excite peripheral and central nervous systems. In this technique, the excitement of the neural tissue depends on exposing the body to a transient magnetic field. This field can be generated by passing a high pulse of current through a coil over a short period of time. This paper presents general guidelines for designing and constructing a magnetic stimulator. These guidelines cover theoretical concepts, hardware aspects and components required to build these systems. The critical points discussed in this paper are based on key findings and difficulties encountered during the process of building the system used for this study. Furthermore, some suggestions were addressed to improve future designs.

Magnetic nerve stimulation: field focality and depth of penetration

Magnetic nerve stimulation is a non-invasive method of exciting neural tissue. The major limitation of using magnetic stimulation is the lack of a focused field. At sufficiently high magnetic pulses the diffused field not only stimulates the target population of neurons, but also stimulates adjacent structures as well. Further, for deeply penetrating fields, as is the case in transcranial stimulation, excessively high amplitude current pulses are required in the coils because a significant fraction of the field energy is spread throughout the tissue under the coil. In this paper we propose two new coil designs that can be used for magnetic stimulation of the peripheral or central nervous system. The purpose of the design was to increase field focality and depth of penetration. The magnetic fields produced by these coils, when driven by biphasic pulses, were simulated using a finite element technique coupled with a transient solver. The resultant field densities and gradients were compared with those obtained from the commonly used Figure-8 coil. Both the air core and the ferromagnetic core designs have superior results when compared to the Figure-8 coil.

A novel coil design for magnetic nerve stimulation

Magnetic nerve stimulation is a non-invasive method of exciting neural tissue. Stimulation can be achieved by exposing the body to a transient magnetic field which is generated by passing a high current through a coil over short period of time. By positioning the coil in a specific orientation over the targeted nerve, the magnetic field will create an electric field in the conductive milieu of the body. Induced currents will result from that electric field. If those currents reach a certain amplitude within a specific time period this will cause a neural depolarization. This depolarization will enable us to test and examine the excited nerve providing the necessary data for an effective treatment. In this paper we describe a new design for the stimulating coil with the aim of achieving a more focused magnetic field. This resultant field will excite deep targeted nerves with minimum excitation to the surrounding nerves.

From transmission to distribution networks-harmonic impacts on modern grid

This paper investigates the effect of harmonics due to renewable generation, energy-saving home appliances and electric vehicle chargers. Harmonics were found to be contained at the transmission level due to the presence of shunt capacitors. These shunt capacitors behave like single-tuned or high-pass harmonic filters, however there will be an increased risk of harmonic resonance making the transmission line susceptible to tripping wind turbines on transient overvoltage. Harmonics generated by solar, energy-saving home appliances and electric vehicle chargers pose a potential threat to the distribution network systems and if allowed to continue unabated could cause fuse failures and premature failure of pole-mount transformers. In addition, neutral current can be excessive due to current harmonic distortion even with loads balanced on three phases which will cause overheating if the neutral is undersized. The paper also identifies some areas where further work is required to understand harmonics and their impacts on power system performances.

Shunt active power filtering for smart appliances

Due to the increasing trend towards energy saving white goods appliances and the commercial viability of power electronic components, there has been an expansion in the use of solid state electronics and variable frequency drive motors in these applications. However, a major drawback of using such energy efficient loads is the introduction of current harmonics onto the local distribution grid. Furthermore, the proliferation of such devices elevates the supply harmonic content and can potentially impact residential distribution networks. This paper investigates the harmonic content generated by some household appliances and suggests a solution to minimize harmonics by means of active filtering. An active filter circuit was simulated to compare filter performance when used as an active front versus a feeder input compensator.

Advanced power quality laboratory

The mass introduction of solid-state converters used in green energy sources (solar/wind) and modern loads (electric vehicles, smart appliances) results in many power quality challenges. Utility assets ranging from distribution apparatus to power protection and control devices can potentially be affected by poor power quality. This includes power transformers and monitoring instruments (current and potential transformers). Additionally, harmonics may contaminate meter measurements and protection/control devices resulting in false or failed relay trips. Considerable progress has been made in understanding power quality impacts on distribution grids, however, much work remains to be done in this area. Mohawk College and McMaster University (B.Tech. Energy) have jointly developed an Advanced power quality laboratory (APQL), a teaching, training and research facility to meet the needs of local utilities.

A test bed to monitor smart grid power quality

Electric vehicles and renewable energy sources typically generate distorted current waveforms (harmonics). As consumer adoption of these technologies increases, it is expected that harmonics will accumulate resulting in poor power quality and degradation of smart grid performance. Such impacts would affect many sectors including utilities, auto manufacturers, and renewable energy producers. For research, training and teaching purposes, Mohawk College and McMaster University have collaborated to produce a comprehensive mobile test bed to study harmonics generated within a smart grid. The fully automated system includes multiple transformers, electric vehicle chargers, a grid tied inverter and data acquisition systems, which allow the user to monitor harmonic content and control power flow within the test bed.

Correlation of weather conditions and power quality for distributed generation and modern loads

With increasing number of renewable energy installations being added to the Ontario electrical grid, many local distribution companies have raised concerns regarding harmonic impact(s) on the distribution system. The majority of such renewable energy sources consist of solid-state inverters, which can potentially cause damage to utility equipment and appliances due to their high harmonic profiles. Also, modern home loads contribute to power quality degradation, due to nonlinear loads and variable frequency drive appliances, which contribute significantly to harmonic content. This paper provides analysis of harmonic profiles for energy efficient homes combined with renewable energy sources, relative to meteorological conditions.