A measurement system for the characterization of wireless charging stations for electric vehicles

This paper describes a new traceable measurement system, designed for the characterization of inductive charging stations for electric vehicles. The measuring system is able to measure on-site the performance and efficiency of the charging station and converters. The basic relative uncertainty is 10−3, but actual measurement conditions could worsen this figure. The measurement system aims at accurately measure the power at the batteries and the power transferred from the ground to the on board resonant circuit and makes possible a benchmarking between station measuring systems. Finally, it allows the characterization of the magnetic emissions by correlating them with the electric current in the coils.


I. INTRODUCTION
Inductive charging or wireless charging of electric vehicles is one of the charging technologies currently being tested for electric vehicles (EVs). This technology, commonly called inductive power transfer (IPT), or wireless power transfer (WPT), has a number of obvious advantages: it is perfect for autonomous vehicles and excellent to implement in large urban centers, i.e. at the traffic lights, at work, in parking facilities and so forth. It also makes recharging easy for people with moving disabilities and does not require the use of cables. A more futuristic version of WPT charging is this dynamic (DWPT) which provides for charging with a moving vehicle, for example in a road lane. This short paper deals only with WPT since DWPT requires a more extended analysis. As part of the charging of electric vehicles with WPT, two coils are used, one on the ground and one on board, which are coupled as resonant circuits having the same frequency. The latter is unified at 85 kHz for light vehicles [1] and decreases for heavy vehicles.
Many experiments, test, and research projects are currently taking place to analyze the various aspects of the WPT charge (see e.g https://www.electreon.com/projects and https://www.nes.uni-due.de/projects/talako) and to lay the foundations for future pricing and compliance with human exposure to magnetic fields. This (short) paper aims to describe a traceable measurement system developed within the Metrology for Inductive Charging of Electric Vehicles (MICEV) project [2], which was designed to perform the following functions: • to measure the power absorbed from the batteries; • to measure the power absorbed by the electric grid • to determine synchronized ratio between the two power quantities mentioned above (efficiency); • to determine the magnetic flux density levels by contemporary measure the electrical currents at the coils; • to record all measured values and waveforms. The measurement system, called Power Measurement Unit, (PwMU) has been designed for the following measurement range: on-board for voltages up to 1000 V and currents up to 200 A in the DC section. For the grid side (three phase) phase voltage up to 400 V and phase currents up to 200 A. In both cases the target power is 200 kW.

II. MEASUREMENT SYSTEM
The PwMU is constituted by three units plus numerous accessories and transducers: • one unit designed to stay on the ground side and perform measurement of the power, voltage and current adsorbed, which we will call the "Grid Unit" (GU) • one unit that can be embarked on board the vehicle that we will call it "Board Unit" (BU) • one unit that an operator can easily move in the charging station, around and inside vehicles, for magnetic measurements, which we will call "Magnetic Unit" (MU).
• In addition to the three (logical) units, the PwMU is accompanied by 2 laptops, sensors and transducers, which include connectors, shunts, cables and three GPS antennas, having a non-negligible importance.
In static measurements, which mainly concern this paper, BU and GU can be stationed in the same place. For their housing two racks have been designed for the purpose, so that the two units can be stacked and moved on wheels. Fig. 1 shows the overall measurement system.
A specific constructive choice was made to improve reliability. In fact, the measurement system is designed for measurements at the charging stations. Field operations and measurement complexity require additional reliability verification. Thus, also to avoid doubts about the measurement, the presence of a second measurement system may be useful. For this reason, GU and BU are equipped with a double measuring system

A. Grid Unit
In the GU, the first measurement system consists of a high accuracy power analyzer, which is used with current transducers and direct voltage input. The second measurement system consists of a National instruments (NI) PXI system, based essentially on a NI PXIe-6366 card that collects the three voltages and the three currents from the grid through the use of transducers. A NI PXI-6683H -synchronization module is used to synchronize the acquisition systems. The whole system is operated by means of a user interface (UI) developed in Labview language. The user interface allows the user to enter the calibration parameters of the transducers, to correct their response [3], and to save all measurement data, including waveforms. Moreover, it allows the user to program and then save a measurement synchronized with GPS.

B. Board Unit
Also for the BU, the first measurement system consists of a high accuracy power analyzer, which is used with coaxial shunt and direct voltage input. This system operates on a circuit at the frequency of 85 kHz and the use of other current transducers is not desirable, as highlighted in [4]. The second measurement system consists of a National instruments (NI) PXI system, based essentially on two synchronized NI PXIe-5922 cards, that measures through suitable transducers, the voltage and current at the batteries or in one of the resonant circuits and, at the same time, the currents at the resonant coils (board and ground). Also here, a NI PXI-6683H synchronization module is used to synchronize the acquisition systems. The whole system is operated by means of a user interface (UI) similar to that of the GU, allowing to set the measurement parameters, to save data and waveforms and to program synchronized measurements.

C. Magnetic Unit
A Narda ELT-400 magnetic field meter with two probes, one of 100 cm 2 and one of 3 cm 2 is implemented in the system. An acquisition system, based on a PXI chassis that houses two synchronized NI PXI 5922 oscilloscope cards that provide three channels for acquiring the magnetic induction components from the ELT 400 meter. Also here, a NI PXI 6683H GPS card with GPS antenna manages the synchronization functions. A laptop and a LabVIEW program developed for the purpose handles the user Interface, the synchronization, the acquisition and data saving.

III. UNCERTAINTY OF THE MEASUREMENT SYSTEM
The measurement uncertainty of the GU is limited by the accuracy of the power analyzer combined with the one of the current transducers (better than 200 ppm at the grid frequency). The grid harmonic distortion, which is likely generated by the charging system interacting with the grid, is limited to few significant harmonic components. For on-board measurements at the batteries the BU must perform dc + ac (85 kHz fundamental) ripple measurements. In the resonant circuit the current is rather sinusoidal, while the voltage is usually a squared waveform [2]. The BU has been calibrated both in ac and ac + dc up to 150 kHz. The efficiency of the charging station can be assessed with a relative uncertainty within 10 -3 in temperature controlled conditions. The GPS synchronizes the clocks of the different PXI. PXI-6683H cards can be programmed to provide a trigger start signal on the PXI backplane at a certain time. All the systems are started together and inherit the GPS uncertainty of +/-50 ns.

VI. CONCLUSION
The structure and measurement capabilities of a traceable measuring system specifically designed for the characterization of WPT systems and charging stations for electric vehicles has been sketchily described. The system here presented shows a particular reliability in electrical measurements obtained by doubling the embedded power analyzers. The PwMU is a measurement system which improves the ease and ability to characterize WPT charging stations both in field and in laboratory environment.