DESIGN AND CONSTRUCTION OF A PORTABLE INSTRUMENT FOR THD MEASUREMENT AND REGISTRATION

In this paper harmful effects of existence of harmonics in power networks are discussed briefly. The THD (Totat Harmonic Distortion) parameter which is a mean for indicating the purity of a sine wave is introduced and some international standards about the maximum permissible harmonic levels in various networks are presented. Analog, digital, and combined analog and digital methods for THD measurements are introduced and reasons for selecting digital method in this project are mentioned. In the method used for THD measurement in this paper, 50 Hz input signal is sampled with 8.53 KHz sampling frequency and 12 bit resolution and measurement is advanced by numerical processing on gathered data. The input voltage can be any value between 200 and 400 V rms. The maximum input current can be 5 A rms. The instrument which has been built is capable of showing THD at least every 1.5 seconds which is adjustable by the user. The voltage or current waveforms data, when the device is used on line, can be stored regulary. These waveforms can be recovered and processed later. When the instrument is connected to a computer, the on line additional processing of data is also possible. This way the size of different harmonics as percents of the fundamental frequency component of the waveform can be viewed as a bar chart. At the end of the paper, some of the measurements made by the constructed device are included.