The Challenge
At the Bingen University of Applied Sciences (THB), electric vehicles were charged in parallel at ten charging points. Among other things, it was of interest to measure charging currents and their impact on the THB grid, to determine cycle frequencies and their distribution in the grid and to evaluate grid asymmetries. The evaluation showed that standard violations occurred to a small extent (i.e. the power quality has improved in recent years), that the voltage quality must be scrutinised more closely in future and that the charging electronics should be improved. The study ‘Power Quality in Electromobility’ from 2013 found that around a quarter of all electric vehicles measured in this project generated unacceptable harmonic currents during charging [1].
Introduction
Today, charging technology in e-vehicles has markedly improved. Limits are much more regularly maintained. However, the most frequently infringed condition of connection is asymmetry in the single-phase charging of electric vehicles. In some cases, single-phase charging power up to 7 kW is reached here. In serially produced e-vehicles, almost no infringements arise, except when the charging electrics are defective. The greatest harmonic currents arise from converted e-vehicles. These happen when simple rectified circuits consisting of diodes with capacitive or inductive smoothing are used in a DC circuit. In an assessment to the VDN Technical rules, all relevant harmonic generators in a facility are collated with regard to the emissions to be expected at the shared network connection point. Here, the individual harmonic currents and the combined effect of all harmonics are considered through the distortion factor of the basic oscillation THD. To measure the voltage and current during the charging process, power quality analyzers and fault recorders made by A. Eberle, Nuremberg, are used. The sampling frequency of the PQ-Box 200 is 40.96 kHz, meaning DC frequencies up to 20 kHz can be recorded.
Conclusion
Some e-vehicles are charged on a single phase with a power much greater than 4.6 kVA – during the measurement campaign, up to 7.2 kVA was detected. The vehicles produce very differentiated switching frequencies. However, all the vehicles work as interference sinks, “extracting” this supraharmonic from the network. When switched on, all the vehicles generate a clear voltage transient. This could disturb other consumers. While the current harmonics of various e-mobility manufacturers do add up on the network, but not as strongly as a linear addition. No serial manufacturers infringe on the current harmonic limits up to the 50th harmonic.
