Self-Measurement of the Admittance Matrix of AC-DC Power Converter by Internal Harmonic Injection

Frequency-domain impedance or admittance model is widely applied to analyze the harmonic stability of power electronic converters. Conventionally, the converter impedance is modeled holistically, without exhibiting the sole effect of different control and phase-locked loop (PLL). Besides, harmonic voltage or current injection external to the converter is usually adopted, which involves dedicated equipment and introduces unexpected inner impedance. In this article, the response of voltage perturbation upon ac-dc power converters along various voltage signal flow paths is analyzed, based on the superposition principle. Each part of admittance and its effect on stability can therefore be quantified explicitly. By injecting harmonic perturbation through the identified paths within the converter controller, each part of the admittances, with and without the concerned control or PLL blocks, is measured and summed to be the total admittance. Harmonic voltage mitigation control implemented on a grid-forming converter creates an ideal grid with constant voltage and nearly zero inner impedance at the injected harmonic frequency. The proposed admittance analysis and measurement are successfully applied to the power converter with different PLL, voltage feedforward, output power control, dc voltage control, and grid-forming cascaded control. The effectiveness of the analysis and the proposed admittance measurement approach is validated by comprehensive simulation and hardware tests.