Control of Nonlinear Output Inductor Time Constant and Its Applications in DC Distribution Networks 


Student thesis: Doctoral Thesis

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Award date31 Aug 2022


DC distribution system includes groups of cascaded-converter subsystem having a source interface converter (SIC) and load converters. The load converters have constant power load (CPL) characteristic with negative incremental input impedance, destabilizing local or/and global systems. The author’s objective is maximizing capability of SIC in handling CPLs.

At the beginning, based on canonical circuit of DC/DC SIC, a time-constant-ratio (TCR) chart is proposed to study the stability under the variation of CPL power. The stability boundaries that depend on the output inductance are defined. It is observed that the stability region is widened by reducing the output inductance. Thus, a powder-core nonlinear inductor is proposed to be used as output inductor, providing decreasing inductance with the CPL power increasing. A 80W buck-type SIC is built to prove the stability region widening by nonlinear output inductor (NOI).

Then, NOI is further studied, including its materials and its loss mechanism. A grid-tie inverter is built to validate characteristics of powder-core NOI, such as soft saturation and harmonic generation.

Finally, an optimal control of NOI virtual time constant (VTC) is proposed. The critical TCR for maximizing SIC’s capability in handling CPL is tracked by adjusting the equavalent inductor resistance equal to the characteristic impedance of the output LC filter. An inner current loop is added, generating a compensation duty cycle and functioning as an adaptive inductor resistance. A 25W boost-type SIC is built to verify reliable operation of the proposed control strategy.

The proposed NOI VTC control has the merits of linear methods, such as simple applications and low sensitivity to circuit parameter variations. Meanwhile, it has the merits of nonlinear methods, such as high reliability for large-signal stability. The author’s research lays a solid foundation for application in different types of DC/DC SICs. It is expected that this research will open a new perspective for the power electronics community, that is, the application of NOI VTC to enhance the stability of DC distribution networks.