Using filters when charging electromobiles – an interview with Dr. Dennis Kampen

Today, the quality of the charging technology for electric vehicles is much improved. But what is the future for e-mobility?

The study “Power quality in electro mobility – impact of the charging infrastructure for electric vehicles on grid quality” revealed that, in 2013, approximately 25% of all electromobiles measured in this project generated impermissible harmonic currents during the charging process. Today, the quality of the charging technology for electric vehicles is much improved. Violations of limits and deviations from standards have also decreased considerably. The condition for connecting to the public grid that is most frequently not met today is that of voltage unbalance.

Power Quality in electro mobility

The greatest harmonic currents occur with converted electromobiles. They are produced when simple rectifier circuits are used from diodes with capacitor or inductive smoothing in the DC circuit. The capacitor smoothing occurs with a 2 kW single-phase connection, and with 16 A three-phase connections and charging capacities of 7 kW. The inductive smoothing is used with greater charging capacities, such as from 20 kW. With respect to the applicable standards, electromobiles that were originally designed as electric vehicles only generally violate harmonic limits to a minor extent during the charging process.

Reducing the harmonics

During all of these analyses, the question became increasingly louder as to whether the slightly standard-breaching charging operations could be corrected by known means. This primarily involves the reduction of harmonics by means of passive filtering. The use of a so-called 4% uk line reactor is cited as the most simple option. EMC filters and harmonic filters are considered as another approach.

In any case, it is clear that these three pieces of equipment cannot completely prevent asymmetrical charging from the mains. Astonishingly, there are a few electric vehicles that are charged with single phase with a charging current of approx. 30 A, equating to a single-phase charging capacity of approx. 7 kW. The charging current meets the conditions as laid out in VDN technical rules, abbreviated D-A-CH-CZ A filter is therefore not compulsory.

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My summary

Studies have also shown that the filters do not interfere or breach the standards, e.g. with sinusoidal charging currents. Power Quality analyzers and disturbance recorders from A Eberle were used to measure the currents and voltages during the charging processes. PQ Boxes were deployed for mobile use while PQIDAs were used for stationary installations. The highest sampling frequency was 41 kHz with the PQ-BOX-200, meaning that only frequencies up to 20 kHz could be evaluated.

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