Interval type-II fuzzy logic control of neutral DC compensation method to moderate DC bias in power transformer

Direct current flow through power transformers in HVDC systems can lead to significant half-cycle saturation issues, putting the power system at risk. The HVDC system can function in monopolar ground return and unbalanced bipolar without earth return conductors. During these two HVDC modes of operation, a substantial direct current flows through the HVDC ground terminals, creating a ground DC potential difference between the neutrally grounded transformers. As a result, DC flows through the neutrals into the transformer windings. The study presents a transformer-neutral DC compensating device incorporating a novel control to solve the issue. Using a proper control strategy, injecting reverse DC into the grounding grid can compensate for direct current flow in transformer windings to mitigate the biased operating flux of power transformers. In this article, an in-depth analysis of transformer response to DC bias was investigated. Then, an Interval type-II fuzzy logic control (IT2FLC) was proposed as an effective control strategy for managing the neutral DC compensating system. Its robustness was assessed and analysed by comparing it with type-I fuzzy logic-based (T1FLC) and a PI-based compensation system. The control performance is examined using MATLAB/Simulink models and validated with rapid control prototype tests conducted with a Speedgoat™ real-time target machine, assessing the transient response, oscillations, and settling time of the compensation device under DC bias voltage variations. The outcomes indicate that the IT2FLC controls the compensation device more effectively than other controllers to mitigate half-cycle saturation. This approach introduces a novel strategy to prevent transformer half-cycle saturation.