**9.1 Orthogonality check**

According to the orthogonality of the trigonometric function, power can be produced when, and only when, voltage and current are at the same frequency. Fundamental voltage and harmonic current do not produce power, a constraint that can be applied to validate the effectiveness of the equipment [Arseneau et al., 1995b, 2001].

Four steps are designed in the experiment to verify the performance of the standard equipment of the NIM. A time division-type power converter with an accuracy of 50 ppm is selected as a reference standard.


Two sets of non-sinusoidal signals are selected on the basis of (a) IEC61000-3-2 [IEC, 2001] (fundamental voltage, 100 V; current, 3 A; 5th order voltage, 10%; 5th order current, 40%), and (b) NRC [Arseneau et al., 1995a, 1997] (voltage, 120 V with *THD* = 9.5%; current, 5 A with *THD* = 19.5%).

With the set of IEC signals at PF = 1.0, the errors are –1.4 and –0.7 W/VA with the 5th order current harmonics and 5th order voltage harmonics included, respectively. With the set of NRC signals, the corresponding errors are –0.6 and –0.4 W/VA, which can be disregarded.

Two other commercial power meters, one operating under the principle of a thermoconverter and the other under the principle of sampling, are also subjected to the same test. Similar results were obtained for these power meters.
