An EVSE external impedance characteristic measuring method for electric vehicles charging resonance risk evaluation

High power electric vehicles supply equipment (EVSE) is the future direction for the advantages of shorter charging time and vehicle user experience friendly. However, high power charging is prone to cause power system instability and resonance fault, which will lead to charging vehicles damage and even large scale power outages. As a result, the resonance risk of charging equipment and charging stations connected to the grid should be analyzed and evaluated. In this paper, a novel EVSE external impedance characteristic measuring method for charging resonance risk evaluation is proposed. Specific frequency harmonic voltage superimposed on power frequency is injected to the tested EVSE. The fundamental and harmonic voltage and frequency shall be under protection limits of EVSE. By sweeping frequency to 5000 Hz and measuring current and voltage at the injecting harmonic frequency, external EVSE impedance characteristic curve can be obtained. Based on measured EVSE impedance, the matching grid impedance at maximum resonance risk and the corresponding resonance frequency range and can be calculated. The experiment result shows that resonance frequency deviation between the calculated result and the real charging test system is smaller than 5%. The proposed method can effectively avoid grid resonance by guiding high power charging station planning.