Quick and easy energy measurement with new metering device

Until recently, large-scale industrial energy users in Germany have benefited from reduced tax rates on the electricity and gas they consume. Now the German government has decided that from 2013 onwards, companies will only be eligible for such tax breaks if they are equipped with an energy management system that provides details of their power consumption. This means companies will have to install individual auxiliary meters to monitor the power consumption of individual loads such as presses and welding machines, or bakers’ ovens, or electric motors. And in many cases, the metering instruments currently available on the market are too large to fit into existing power distribution cabinets.

To remedy this situation, researchers at the Fraunhofer Institute for Integrated Circuits IIS in Erlangen have developed a space-saving metering unit that can be simply clipped onto a power cable like a laundry peg, without even having to disconnect the load. The new ‘energy analyser’ was developed in collaboration with Rauschert GmbH - a manufacturer of advanced ceramic products that require energy-intensive production processes. The research project was funded by the Bavarian Ministry of Economic Affairs, Infrastructure, Transport and Technology as part of its microsystems technology program. The device is based on the HallinOne 3-D magnetic field sensor originally developed by IIS for use in Bosch and Siemens branded washing machines, where it monitors the position and orientation of the rotating drum.

“This new device is the first application in which we have used our 3D magnetic-field sensor technology to measure the magnetic field generated by an electric current as a means of determining the energy consumed by the connected load. As such, it is an entirely novel approach,” said IIS research scientist Michael Hackner.

To build the device, Hackner and his team of engineers mounted eight sensors, in the form of application-specific integrated circuits (ASICs), on a flexible, flat circuit board. What sets these sensors apart from more conventional designs is that they measure the magnetic field not only perpendicular to the surface of the chip but also in tangential directions, which improves measurement accuracy. The recorded data are transmitted to a microcontroller, which forwards them to a central processor via a gateway switch.

“Our power sensor is quick to install and can be integrated online,” said Hackner. He went on to note the fact that the sensor functions in the same way as a Rogowski coil, a component incorporated in many standard instruments used to measure electrical currents. “But the Rogowski coil only measures alternating current, whereas the IIS sensor can also measure direct current - an important consideration when measuring the power consumption of photovoltaic systems that include solar inverters for converting DC output into AC power.”

The metering device is cheap to build, and because the IIS researchers have not used any magnetisable materials in its design, there are no accuracy errors from this source - a recurring problem with other clamp-on amp meters that cease to function correctly after a short circuit. Another advantage is that, unlike clamp ammeters, the device can also measure voltages.

Michael Müller, head of energy management systems at Rauschert GmbH, outlined the benefits of the device: “The new device helps us to rapidly identify production problems. We can immediately detect irregularities in the firing process and avoid having to scrap a whole batch of ceramics.” Müller intends to set up an energy-management consulting service for other plants in the group and equip them with the measurement system.

The sensor devices can even be installed in the power utility’s medium-voltage network (20 kV). As a result of the German Renewable Energy Sources Act (EEG), there are now a large number of small and medium-sized electricity producers feeding energy into the grid, without being obliged (or able) to inform the energy provider exactly how much power is flowing on a specific line at any given time, and in what direction.

“The grid capacity could be utilised much more efficiently if detailed measurement data were available,” said Hacker and Müller.

The two experts reckon there is huge market potential for this application: in Bavaria alone, the 20 kV distribution network contains tens of thousands of transformer stations, which could be equipped with the low-cost device without having to temporarily disconnect the stations or the transmission lines from the grid. A prototype measuring system for low-voltage networks has already been developed.