**2. System**

A block diagram of the NIM harmonic power standard equipment, including some general hardware, is shown in Fig. 2.1. Two commercial high-accuracy digital sampling voltmeters (DVMs) with type of HP3458A are employed as A/D converters (ADCs) to measure the instantaneous values of voltage and current signals. Non-sinusoidal signals are provided by two commercial programmable signal generators, one with the type of Fluke 6100A and another NST3500 from Chinese manufacturer. A set of resistive dividers have been developed to extend voltage ranges to 8, 15, 30, 60, 120, 240, and 500 V using selected resistors with a low time constant and low temperature coefficient. The resistance values are designed in such a way that the operating current does not introduce a significant heating effect. Another set of resistive dividers provide protection to reduce the effect of stray capacitance. A set of shunts (provided by SP of Sweden) are used to extend current ranges to 0.1, 0.2, 0.5, 1, 2, 5, 10, and 20 A, while a current transformer is adopted for 50 A.

Fig. 2.1. Block Diagram of the harmonic power standard equipment; where HPS is harmonic power source, RVD is resistive voltage divider, and DVM is digital voltage meter.

The full-scale output signals of the voltage dividers and shunts are 0.8 V regardless of the voltage/current ranges, which are the input signals of the two corresponding DVMs.

A sampling trigger is developed to enable equal interval sampling between two contiguous sampling data, and synchronization sampling between voltage and current signals.

For the practical sampling strategy, 1680 samples over about four signal periods are computed on a PC, in accordance with the algorithm. The calculated results are voltage, current, and power at the DC, fundamental, and every harmonic order up to the 60th.
