The increased number of distributed power generation resources and high capacity loads connected to electricity transmission grids cause power quality degradation. Accurate measurements are necessary to measure and consequently to control transmission grids. Digitized measurements are of great help in carrying out these measurements as well as other low frequency AC metrology. Digitized waveforms can be used for extensive numerical analysis to obtain values for many properties of the measured power, which is not possible for data measured using thermal converters.
VTT MIKES metrology has designed and built a sampling standard of modular structure for three-phase power and power quality measurements in the electrical grid. The digitizer has seven simultaneously sampling floating analog-to-digital converter channels: three for current and three for voltage conductors and one for the neutral conductor. Each channel has a temperature stabilized precision voltage reference. A flexible sample clock is derived from an on-board or external 10-MHz reference by using direct digital synthesis. The sample rate can be adjusted in 4 MHz increments, which facilitates phase locking to measured input signals. A GPS module provides synchronicity to UTC (Coordinated Universal Time) and provides synchronicity to field measurements. All signal processing is running on a PC, which connects to the digitizer via USB 2.0. The software enables signal visualization and real-time calculation of three-phase power in accordance with the latest IEEE and DIN definitions.
Figure 1 shows results from a measurement in which the digitizer was used to measure current and voltage from an advanced electric vehicle charger during charging.
Tests indicate that a relative measurement uncertainty at or below 10-6 can be achieved with careful design. For instance, the digitizer gives a multifold increase in performance in bandwidth and stability. Versatile synchronization and timing of the simultaneous sampling make it possible to apply the system to a variety of demanding AC metrology tasks. Traceable waveform capture enables accurate three-phase power analysis according to international standards, such as IEEE 1459 and DIN 40110. Modern real-time substation automation is supported as well; the device provides a stream of sampled values over Ethernet as per IEC 61850-9-2 LE. The digitizer is intended for everyone in need of superior performance in laboratory or field measurements.