Power meter underpins system for ensuring accuracy of EV charging points

Now that EVs account for a rapidly increasing proportion of traffic on European roads, public charging points have to earn the same trust as conventional internal combustion engine vehicles. That means creating new calibration systems that ensure a driver can be confident that when they have paid 50kWh of electricity, their battery has been topped up with that amount of energy.

In Germany, it was natural that manufacturers developing metering products for the EV charging market turned to the non-profit VDE Institute to provide a measurement set-up for calibration that complies with the EU Measuring Instruments Directive.

Creating a measurement system that complies with the EU Measuring Instruments Directive and Germany's Measurement and Calibration Act was challenging. The law specifies that meter readings must be accurate to at least 3% and no higher than the actual amount of energy supplied. This means the room for calibration error is tiny: the accuracy of the power meter used in the test system had to be better than the specification by at least a factor of ten, or ±0.3%.

At the heart of VDE's solution is the WT3000E power meter from Yokogawa Test & Measurement, whose accuracy of ±0.04% is almost an order of magnitude better than the minimum specification. Combined with a programmable power supply controlled by the company's FG420 function generator, the system delivers extremely accurate voltage, current and total energy measurements.

Other features of the solution include a short interval between stored samples of just 50ms and easy interfacing to precision current sensors supplied by LEM/SIGNALTEC.

For new designs, Yokogawa offers the WT3000E's successor product, the WT5000 Precision Power Analyzer, a versatile platform that provides high precision and exceptional performance in the most demanding applications. As well as an increased accuracy of ±0.03%, it offers high-speed operation of up to 10MS/s (18-bit) and a power measurement bandwidth of 5MHz.

During this project, the company also supported developing the Raspberry Pi-based control program and measurement data acquisition system, which supplies an interface to the device under test. This was particularly useful when an absence of relevant libraries or drivers faced VDE engineers.

Following accreditation by Germany's national Physikalisch Technische Bundesanstalt (Federal Physical Technical Institute), the test system is now being used by companies manufacturing energy meters for the country's rapidly expanding network of public EV charging stations. With one million charging points set to be installed across the country by 2030, the VDE seal of approval will play an important part in establishing consumer confidence.

VDE engineer Sven Grünberg, who was part of the team that developed the system, has more than 35 years of experience with different types of test equipment. In this case, a need for ultra-high accuracy that made the choice of power meter crucial to the success of the entire project meant that he automatically turned to Yokogawa.

"To be honest, there is hardly anything else worth considering," says Grünberg. "The Yokogawa WT3000E has been used in our calibration departments for years, and measures so precisely that it is also perfectly suited for the charging station test bench."