Calibrating digital electrical energy meters

In Germany, PTB is responsible for the metrological traceability of measuring instruments in high-voltage power grids. By tracing measuring instruments to national standards, PTB ensures that the energy transmitted is correctly recorded by means of adequate measuring instruments.

The new calibration setup developed at PTB is based on an SV generator that allows emulated high AC voltages (or currents) to be temporally synchronized and transmitted in accordance with the IEC 61850-2 LE network protocol to an electrical energy meter undergoing verification. The sampling rates between 4000 and 14400 samples per second are defined by the network protocol. In the calibration setup for SV-based measuring instruments, the generator therefore emulates the digital instrument transformers.

The software controlling the SV generator essentially acts as an SV wave form generator, controls the sequential measurement protocol, and calculates electrical power and energy. In doing so, the voltage and current signals generated emulate the power and energy flow of a three-phase grid. Based on this, reference power and reference energy values are calculated for calibration.

The International Electrotechnical Commission (IEC) is currently preparing a special standard for digital electrical energy meters (IEC TS 62053-25 ED1). PTB therefore first used the existing IEC 62053-22 standard for Static meters for AC active energy (classes 0,1S, 0,2S and 0,5S) in order to develop a test schedule for electrical energy meters to be calibrated. It was then possible to use the recently developed setup to perform the first test on a commercially available digital energy meter of accuracy class 0,2S produced by a European manufacturer at a grid frequency of 50 Hz, a nominal voltage of 100 kV and a nominal current of 1000 A. The measurements performed according to the developed test schedule have shown that the measurement errors of the energy meter for active energy were lower by one order of magnitude than the tolerance limits defined by the meter’s accuracy class.

With a view to the expected change in the standardization situation in the next few years, the measurement setup will be constantly adapted to future requirements. Moreover, the greater meter reading efficiency provided by an optical interface will be included in the evaluation in the short to medium term.