The PTB Primary Standard for Electrical AC Power

The present PTB primary standard allows power measurements with a measurement uncertainty of about 2.5 μW / VA (k = 2). It operates at voltages up to 240 V and currents up to 10 A in the frequency range from 16 Hz up to some kHz. This paper gives a short overview of the working principle of the standard and recent improvements. © Metrology Society of India, All rights reserved. 1. Basic Operating Principle The scheme of the basic PTB ac power sampling standard [1] is shown in Fig. 1. The key to reach low uncertainties is due to the use of a single clock fClock, which is derived from a highly precise digital sampling voltmeter (DVM). It serves as master clock for the sampling process and for the generation of the test signals with a two-channel ac waveform synthesizer. This synthesizer generates two sinusoidal voltages UA and UB with very low distortion (< -100 dBc), high stability (< 10-6 / hour) and any phase angle γ within ± 180°. The very low distortion voltage and transconductance amplifiers generate the test signals U and I . These quantities are fed to the device / meter under test, to a calibrated voltage divider and a two-stage current transformer with its ac shunt, respectively. The output voltages U1 and U2 of the instrument transformers are voltages proportional to the test quantities U and I defining the power to be measured. The DVM samples the voltages U1 and U2 alternately via the signal switch. This fully synchronized operation eliminates sampling errors of the measurement process, as it MAPAN Journal of Metrology Society of India, Vol. 24, No. 1, 2009; pp. 15-19 allows to set the sampling frequency of the DVM equal to an exactly integer multiple of the signal frequency. Under the condition that the Nyquist theorem is fulfilled and an integer number of periods of the signals are measured, the complex voltages U1 and U2 of the fundamental signal frequency can exactly be reconstructed from the sampled dataset using the Discrete Fourier Transform (DFT). With the ratio of the voltage divider Fu = U1/U and the ratio of the current-to-voltage transducer Fi = U2 / I , the power quantities P (active power), Q (reactive power) and S (apparent power) can be calculated according to: