The effects of nonsinusoidal voltages and currents on the performance of static underfrequency and overcurrent relays were experimentally studied. The tests were conducted such t h a t the frequency, amplitude and phase shift of individual harmonics could be adjusted in a controlled manner by employing a waveform generator with a phase-locking circuit. The relation between the harmonic currents and voltages was modelled through the power system impedances within residential distribution systems. I t was found t h a t for harmonic voltage and current amplitudes, as they occur in distribution systems, underfrequency relays and the time delay operation of overcurrent relays show a marked deterioration in their performance. The instantaneous operating characteristics of overcurrent relays, however, are hardly affected by the presence of harmonic currents. The work described has been supported by the U.S. Department of Energy.
The impact of nonlinear loads on anisotropic transformers is investigated. The saturation of the iron-core materials, the anisotropy of the with-and cross-grain laminations, the dependency of the iron-core losses on the amplitudes of the fundamental and harmonics of the flux density and the dependency of the iron-core losses on the waveshape (harmonic phase shifts) of the flux density are included in this analysis. The impact of nonlinear loads on anisotropic transformers exhibits itself as an increase in losses (%) and consequently a decrease in the performance or derating of the transformer.The approach to obtain the increases in losses (%) requires the computation of the appropriate input voltage and the core-loss resistances assuming that the fundamental components of the output voltage and current have rated quantities and that the secondary current contains a certain amount of low-order harmonics, caused by the nonlinear loads. The determination of derating factors of transformers requires a three-step iterative method assuming rated fundamental output voltage and rated secondary apparent power while maintaining rated losses.
INTRODUCTIONTo obtain the additional losses due to multipleharmonic excitation of transformers, it is necessary to first compute the required input voltage, the total losses, the magnetizing and core-loss currents and the core-loss resistances. It is also assumed that the output current contains a certain amount of low-order harmonics caused by nonlinear loads. Therefore, the resulting input voltage is identical to the voltage at the terminals of the transformer (supplying nonlinear loads) if they were connected to an infinite bus having a nonsinusoidal voltage. The model used for the computation has been previously developed[l] and has been used to cornpu:e the transformer steady-state responses at no load [2] and linear rated load conditions [3] for a single-phase anisotropic transformer with the name plate data High Voltage Side: 240/4805'; (Secondary) (Primary) kVA : 1.00 ; This model includes the saturation behavior of the transformer magnetic core, the anisotropy of the with-and cross-grain laminations, the dependency of the iron-core losses on the amplitudes of the fundamental and harmonic components of the flux density and the fact that the iron-core losses are functions of the harmonic phase shifts of the flux density with respect to its fundamental. The above references address single-harmonic excitations only. It is the topic of this paper to use this large-signal nonlinear model to investigate the impact of various nonlinear loads on anisotropic transformers. 1) a resistor with current controlled by a triac through the firing angle Ttriac; 2) a resistor with a trapezoidally controlled current, where the slopes are defined by the rise time angle T~~~~~~; 3) a resistor with a rectangularly controlled current, where the [I]: Low Voltage Side: 120/240V; Frequenq : 50/60Iiz. Five types of nonlinear loads are considered: delay angle is given by -rrect; 4) transformer secondary cu...
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