New era in electrical power measurements from MIKES electronics workshop
The increased number of distributed power generation resources and high capacity loads connected to electricity transmission grids cause power quality degradation. Accurate measurements are a necessity to measure and consequently to control transmission grids. Digitized measurements are of great help in doing these measurements as well as other low frequency AC metrology. For example, the most accurate AC measurements have been based on thermal converters. A thermal converter is a device that converts electrical power into heat. Heat (temperature) is measured by precise thermocouples. The effective value of AC voltage is obtained by comparing the heat to that produced by an accurately measurable DC voltage. Thermal converters are unable to extract phase information from voltage and current signals and thus tell nothing about the spectral content of the measured signal and have no concept of apparent or reactive power. Measurements are also time-consuming. Digitized waveforms, however, can be used for extensive numerical analysis to obtain values for many properties of the measured power
MIKES has designed and built its own sampling standard of modular structure for three-phase power and power quality measurements in the electrical grid. The MIKES 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 (from the available atomic clock in the laboratory) reference by using direct digital synthesis. A provision for GPS module is designed into the system for time stamping samples and to enable precise synchronization on the field. 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 to latest IEEE and DIN definitions.
Tapio Lehtonen examining one of the seven analog-to-digital converter cards of the digitizer.
Tests indicate that a relative measurement uncertainty at or below 10-6 can be achieved with careful design. For instance, the digitizer gives 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. The digitizer is intended for National Metrology Institutes or others in need of superior performance in laboratory or field measurements.
This work was carried out in the Smart Grids and Energy Markets (SGEM) research program coordinated by CLEEN Ltd. with funding from the Finnish Funding Agency for Technology and Innovation, Tekes.
Additional information: research scientist Tapio Lehtonen, firstname.lastname@example.org, tel. + 358 50 511 0037.