Elimination of transformer core saturation in cascaded multilevel converter based T-statcom systems

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Tezin Türü: Doktora

Tezin Yürütüldüğü Kurum: Orta Doğu Teknik Üniversitesi, Fen Bilimleri Enstitüsü, Fen Bilimleri Enstitüsü, Türkiye

Tezin Onay Tarihi: 2014

Öğrenci: CEM ÖZGÜR GERÇEK

Danışman: MUAMMER ERMİŞ

Özet:

This research work is devoted to the description of coupling transformer core saturation phenomenon and its elimination by modulated firing algorithms in Cascaded Multilevel Converter (CMC) based T-STATCOM systems. Transmission- type Static Synchronous Compensator (T-STATCOM) is connected to the transmission voltage level (≥110kV) via a coupling transformer. The coupling transformer may be an existing Medium Voltage (MV) to High Voltage (HV) or Extra High Voltage (EHV), or a specially designed transformer. This thesis work deals with coupling transformer core saturation problem in Cascaded Multilevel Converter (CMC) based T-STATCOM systems. The effects of nonlinear characteristics of the coupling transformer and CMCs on the core saturation phenomenon are investigated, and the associated mechanisms are described. İmbalance in positive and negative cycles of the converter output voltage waveform, 2nd harmonic voltage at the ac primary side, and low operation flux density of the vi transformer may lead to asymmetric saturation of the coupling transformer core. Transformer core saturation results in significant increase in not only in dc current but also in the 2nd harmonic current and hence voltage component at the ac side of the converter. Furthermore, the 2nd harmonic voltage component will be reflected to the dc side of the converter in the form of ac voltage component at supply frequency. Dc current components in the coupling transformer secondary lines are eliminated by modulating one or more than one voltage steps of staircase CMC voltage waveforms via the electronic gating circuitry. The proposed dc current elimination method is implemented on a 154-kV, ±50-MVAr CMC based T-STATCOM system, which is operating either in inter-area oscillation damping or terminal voltage regulation modes at any time in a 400/154-kV transformer substation since March 2010. The effectiveness of recommended dc current elimination method is verified in the field.