An Advanced Wide-band Model and A Novel Multi-type Insulation Monitoring Strategy for VSC-connected Transformers Based on Common-mode Impedance Response

The insulation monitoring technique for converter transformers has been explored using characteristic harmonics in the voltage source converter (VSC) system by the authors. The previous work enables the monitoring of aged groundwall insulation but cannot cover other insulation components. In order to explore the possibility of multitype insulation aging monitoring, in this article, we first derive an advanced model considering unevenly distributed circuit parameters along transformer windings and their frequency-dependent feature. Model improvements provide an accurate tool to describe multitype insulation in a wide frequency range. Based on the advanced model, a novel wideband common-mode (CM) impedance-response-based monitoring strategy for the VSC-connected transformer is proposed to achieve the evaluation of the aging in different insulation components. An illustrative case is studied on a 15 kVA transformer driven by an industrial inverter. Offline and online experiments are conducted to prove the effectiveness of the advanced model and the proposed strategy. Compared with the existing methods, the CM impedance-response-based strategy enables the online monitoring of multitype insulation components in an effective and noninvasive manner. In addition, the aging severities can be estimated reliably, even at incipient stages.