Design of computerised unbalance measuring system

In a complex network, the connected loads may be of single phase or three phase. The increased trend of traction loads, arc furnace loads and thyristor devices necessitates checks for voltage quality. Harmonics, flicker and unbalance are some of the factors influencing the voltage quality. Evaluation of voltage quality and applying correction, if required, are to be made depending upon the situation prevailing in the system. Under certain load conditions like an arc furnace, traction and thyristor devices, the unbalance factor may vary every few cycles and for the unbalance correction, devices like static VAr compensators (SVC) have to be used. These correcting devices have some time lag when effecting corrections, thus measurement based on short time information will not give the true picture. To avoid erroneous conclusions from the short term data, an instrument based on an integrated value over fairly long duration was designed and fabricated by CPRI. This paper discusses the design and calibration of the instrument and presents typical measured data on an electrical system using this instrument.