Sriram Vaisambhayana

Solid-state transformers (SSTs) operate within the overvoltage and undervoltage limits to ensure safety and power quality (prevent overmodulation). This constrained region of operation creates a reserve of available energy in the system, which is primarily stored in the multiple dc-link capacitors. The energy reserves in the system are essential during load transients as the initial energy demand is compensated by the dc-link capacitors. This article presents a detailed analysis of the energy distribution during load transients for different SST control strategies and the impact of control gains. Subsequently, a reserved-energy-aided control parameter design approach is proposed that effectively distributes the stored energy of the capacitors within prescribed operating limits for a safe and controllable operation. The proposed design approach expands the operational range of SSTs, allowing them to handle larger variations in load power compared to conventional control design strategies. The results are validated on a single-phase, two-stage, two-string 230-V/250-V 1-kVA 50-kHz SST system.

The architecture of solid-state transformers (SSTs) is usually comprised with three-stage cascaded converters. The control of cascaded multistage SSTs is conventionally designed considering decoupled stages. Furthermore, the isolating stage, which is typically implemented with dual active bridge converters, employs a closed-loop control with a much larger bandwidth (BW) than the ac–dc stage. This disparity in the BWs can put a strain on the intermediate dc-link capacitors that connect the ac–dc and isolating stages, resulting in large capacitor voltage excursions when the load changes. In fact, this can result in ac–dc stage overmodulation and even large current overshoots through the isolating stage's high-frequency transformers. This necessitates advanced SST control strategies that can mitigate the aforementioned challenges. This article proposes a control design in which the state variables are selected as the sum of capacitor energies and differences in capacitor energies. Unlike the state-of-the-art SST controllers, the proposed integrated control approach creates a more balanced and safe distribution of energies among the dc-link capacitors during load transients. The control design is simplified as the difference in the control-loop BWs between the stages does not affect this disturbance attenuation ability. The effectiveness of the proposed control is demonstrated by experiments using a 1-kVA SST with two strings.

Medium-frequency (MF) transformer has gained much popularity in power conversion systems. Temperature control is a paramount concern, as the unexpected high temperature declines the safety and life expectancy of transformer. The scrutiny of losses and thermal-fluid behavior are thereby critical for the design of MF transformers. This paper proposes a coupled, semi-numerical model for electromagnetic and thermal-fluid analysis of MF oil natural air natural (ONAN) transformer. An analytical model that is based on spatial distribution of flux density and AC factor is exploited to calculate the system losses, while the thermal-hydraulic behavior is modelled numerically leveraging the computational fluid dynamics (CFD) method. A close-loop iterative framework is formulated by coupling the analytical model-based electromagnetic analysis and CFD-based thermal-fluid analysis to address the temperature dependence. Experiments are performed on two transformer prototypes with different conductor types and physical geometries for validation purpose. Results suggest that the proposed model can accurately model the AC effects, losses, and the temperature rises at different system components. The proposed model is computationally more efficient than the full numerical method but it reserves accurate thermal-hydraulic characterization, thus it is promising for engineering utilization.

A medium/high-power conversion system using power electronic (PE) converter along with a medium/high-frequency transformer, offers many desirable features that are beneficial for present-day power system topologies. Leakage inductance is identified to be one of the key parameters to characterize performances of such a medium-frequency (MF) high-power (HP) transformer. In this paper, existing analytical method to calculate leakage inductance of a concentric winding is further refined employing mean turn length of individual layer and winding porosity factor. Impact of interleaving to reduce leakage inductance is studied. Consequently different winding dispositions (normal and interleaved) and conductor geometries (foil, round) are investigated for a 10 kW, 0.5/2.5 kV, 1 kHz transformer design. Finally generalized method to deduce leakage inductance of fractional-interleaved winding is introduced in scope of this paper. Analytical calculations are corroborated with PEMag simulation results. With proper selection of wire and core geometry, good agreement between analytical and PEMag results is achieved at medium switching frequency, which substantiates aptness of presented methodology.