- Subject Definition and Objectives
When an electrical signal is sent to an oscilloscope its waveform is observed in the time domain; that is, the screen shows the signal amplitude at each instant in time. If the same signal is applied to a hi-fi amplifier, the resulting sound is a mix of harmonic frequencies that constitute a complete musical chord. The electrical signal, therefore, can be described either by time-domain or frequency-domain information. This book describes the relationships between these two domains in the power system environment, the causes and effects of waveform distortion and the techniques currently available for their measurement, modelling and control. Reducing voltage and current waveform distortion to acceptable levels has been a problem in power system design from the early days of alternating current. The recent growing concern results from the increasing use of power electronic devices and of waveform-sensitive load equipment. The utilisation of electrical energy is relying more on the supply of power with controllable frequencies and voltages, while its generation and transmission take place at nominally constant levels. The discrepancy, therefore, requires some form of power conditioning or conversion, normally implemented by power electronic circuitry that distorts the voltage and current waveforms. The behaviour of circuits undergoing frequent topological changes that distort the waveforms can not be described by the traditional single-frequency phasor theory. In these cases the steady state results from a periodic succession of transient states that require dynamic simulation. However, on the assumption of reasonable periods of steady-state behaviour, the voltage and current waveforms comply with the requirements permitting Fourier analysis , and can, therefore, be expressed in terms of harmonic components. A harmonic is defined as the content of the function whose frequency is an integer multiple of the system fundamental frequency.
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