Abstract
This paper presents the implementation of a single-phase solid-state transformer (SST) for the interface between a 13.2 kV medium voltage alternative current (MVAC) network and a 750 V bipolar DC distribution. The SST has ten cascaded subunits in consideration of the device rating and modulation index (MI). Each subunit consists of an AC/DC stage and a DC/DC stage with a high frequency isolated transformer (HFIT). The AC/DC stage consists of cascaded H-bridges (CHBs) to cope with the MVAC. The DC/DC stage employs a triple active bridge (TAB) converter for bipolar DC distribution. Topology analysis and controller design for this specific structure are discussed. In addition, the insulation of HFIT used in DC/DC converters is also discussed. A simple balancing controller at the AC/DC stage and a current sharing controller at the DC/DC stage are used to prevent DC-link voltage unbalance caused by the cascaded structure. The discussions are validated using a 150 kW single-phase 21-level SST prototype at the laboratory level.
Highlights
In recent decades, DC distribution has received increased attention for many applications because it is considered advantageous and superior to AC distribution in some cases, such as interfacing between renewable energy sources and DC loads, lower line losses, and so on [1,2,3,4,5]
For application to bipolar DC distribution, the high frequency isolated transformer (HFIT) coupled with three windings and the low voltage direct current (LVDC) side is configured as a multi-terminal in which two half-bridge circuits are connected in series
The extended cantilever model (ECM) for the transformer is convenient in terms of the parameter extraction, since each model parameter can be extracted from a single measurement of an open-circuit voltage or a short-circuit current [34]
Summary
DC distribution has received increased attention for many applications because it is considered advantageous and superior to AC distribution in some cases, such as interfacing between renewable energy sources and DC loads, lower line losses, and so on [1,2,3,4,5]. The bipolar DC system can provide different voltage levels to loads, which allows connecting high- and low-power devices at a suited voltage level. This concept reduces the voltage level with respect to ground, which makes the distribution system safer for users [6]. [13], a 15 kV 1.2 MVA single-phase SST prototype was designed for a railway grid. The DC/DC stage employs a specially designed TAB converter for connecting to the bipolar DC distribution system. It is responsible for voltage control, and current sharing in parallel-connected outputs.
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