Soft-Start Procedure for a Three-Stage Smart Transformer Based on Dual-Active Bridge and Cascaded H-Bridge Converters

Abstract

Power electronics-based three-stage smart transformers (ST) can be seriously damaged by inrush currents and overvoltages during the start-up phase if the control of the stages is not correctly coordinated. Hence, it is crucial to design properly the start-up procedure, especially in case of modular architectures with distributed DC-links. The design of the start-up procedure depends on the ST power stages topologies, their control systems and the operation modes. This work proposes a soft-shift start modulation technique that allows to limit the inrush current in the DC/DC isolation stage during the DC-link capacitors pre-charging. A fast voltage balancing control, performed by the DC/DC isolation stage, is introduced to avoid overvoltages and unbalanced voltage conditions among the different power cells. Under the proposed method, fast control dynamics is guaranteed thanks to the high frequency bandwidth of the DC/DC isolation stage converters. Theoretical analysis, based on a detailed small signal model of the ST, and simulations are used to demonstrate the principle of the operation. Experimental results, carried out in a ST prototype, confirm the performances of proposed solution in realizing a smooth start-up without voltage/current overshoots.

Reduced-Order Aggregate Model for Large-Scale Converters With Inhomogeneous Initial Conditions in DC Microgrids

“…This scenario illustrates that the proposed approach in this paper can be suitable for different cases, which is an important advantage. Due to the softstarting operation for DC microgrids, there is no doubt that the power system initial values are not always zero [5]. If initial values are wrongly ignored, the input-output maps error will be large, which is difficult for real-time simulators of DC microgrids to adopt this reduced order model.…”

“…Although model order reduction approaches have been widely used for the system consisting of large-scale converters, the initial values requirement is always ignored, or it is avoided through making the restrictive assumption that the initial values are zero [3]- [4]. Due to the soft-starting operation, there is no doubt that the system initial values are not always zero [5]. If initial values are wrongly ignored, the input-output maps error will be large, which is difficult for real-time simulators to adopt this reduced order model.…”

The Phase Shifting Soft Startup of L-LLC Resonant Bidirectional DC-DC Converter Based on Current-Limiting Curve

“…Reference [25] uses analog circuit to realize soft start, and although the starting speed of output voltage is fast and the current inrush is small, it is not desirable for resonant converter with high power density. Reference [26] proposes a three-step soft-shift start modulation technique that allows to limit the inrush current in the DC/DC isolation stage during the DC-link capacitors precharging for a three-stage smart transformer, but it is for nonresonant circuits only.…”

Unified Cost Function Model Predictive Control for a three-stage Smart Transformer

“…Depending on the specific topology, additional controls as the cell balancing [17] should be considered. At higher level, a series of ancillary controls [2], [16], [18], which include reverse power flow control, load identification, storage control, harmonics suppression and constant power load instability mitigation, have been proposed.…”

“…where A represents the control gain. In this case, the technique of symmetrical optimum [17] can be used to maximize the phase margin of open loop gain, the frequency at maximum phase margin can be given as…”