An automatic calibrator for wattmeters with harmonic analysis capability

This paper deals with a three-phase phantom-power generator, which is designed for calibrating digital wattmeters with harmonic analysis capability. The calibration of such instruments is a critical task due to the lack of suitable calibrators. Fortunately, the calibration procedure can be limited to the first harmonic components when digital wattmeters are employed to characterise electrical machines, so that also simple calibrator design can be adopted. This paper describes a three-phase calibrator arrangement which is able to generate an arbitrary spectrum up to 5 kHz and a phantom power variable up to 3 kVA per phase. The uncertainty on the power associated to the first harmonic components is of about 0.1 % in nonsinusoidal conditions. The calibrator employs a Direct Digital Synthesis technique, so that a frequency change of the output signal can be obtained without modifying the sampling frequency. Such a technique also permits to generate the frequency modulated signals that are often encountered at the output of static power converters that feed electrical machines.