Original Theory and Design of a Wide-Bandgap Semiconductor Based Solid-State Transformer for Nigerian Distribution Network Applications

This research develops original theory and the complete design methodology for a wide-bandgap semiconductor based solid-state transformer specifically engineered for Nigerian medium voltage distribution network applications, making foundational contributions to solid-state power transformer science and application engineering for developing-country grid contexts. The solid-state transformer replaces the conventional low-frequency iron-core distribution transformer with a power electronics based conversion stage operating at high frequency, enabling real-time voltage regulation, reactive power compensation, harmonic filtering, and seamless integration of distributed PV and battery storage within a single compact device. These multifunctional capabilities address multiple critical Nigerian distribution network deficiencies simultaneously and at lower life-cycle cost than deploying separate conventional devices, but the SST's technical development for Nigerian 11kV distribution voltage levels and tropical operating conditions requires original engineering theory specific to this context. Theoretical contributions include a novel modular multi-level converter topology for the 11kV AC side stage that achieves fundamental frequency switching using SiC MOSFET devices while meeting the harsh surge voltage requirements associated with Nigerian lightning-prone network environments; derivation of generalised switching frequency optimisation theory that simultaneously minimises core losses, device switching losses, and conducted EMI under the non-sinusoidal voltage conditions of Nigerian distribution feeders; and a thermal management design theory for the SST's SiC power modules in the Nigerian tropical ambient temperature environment that ensures junction temperatures remain within safe limits under the overload conditions common in Nigerian distribution networks. Keywords: solid-state transformer, wide-bandgap semiconductor, silicon carbide, distribution network, Nigeria.