IEEE Power Electronics Magazine Compendium - March 2018 - 80

devices are enabling much simpler converter topologies
and increased efficiency and reliability, with dramatic
reductions of the size and weight of the MV power-conversion systems.
This article presents the first-ever demonstration
results of a three-phase MV grid-connected 100-kVA SST
enabled by 15-kV SiC n-IGBTs, with an emphasis on the
system design and control considerations. The 15-kV
SiC n-IGBTs were developed by Cree and packaged by
Powerex. The low-voltage (LV) side of the SST is built
with 1,200-V, 100-A SiC MOSFET modules. The galvanic
isolation is provided by three single-phase 22-kV/800-V,
10-kHz, 35-kVA-rated high-frequency (HF) transformers.
The three-phase all-SiC SST that interfaces with 13.8-kV
and 480-V distribution grids is referred to as a transformerless intelligent power substation (TIPS). The characterization of the 15-kV SiC n-IGBTs, the development
of the MV isolated gate driver, and the design, control,
and system demonstration of the TIPS were undertaken
by North Carolina State University's (NCSU's) Future
Renewable Electrical Energy Delivery and Management
(FREEDM) Systems Center, sponsored by an Advanced
Research Projects Agency-Energy (ARPA-E) project.

Background
Si-based power semiconductor devices have been widely
used for several decades over a wide range of voltage and
power levels [1]. Si power devices have almost hit the theoretical limits in terms of their voltage rating and switching
capabilities. The HV Si IGBTs are typically limited to 6.5 kV,

with a switching frequency less than 1 kHz [2]. These limitations pose challenges in the MV applications of Si IGBTbased power converters due to either the required series
connection of the IGBTs or the complex multilevel converter topologies resulting in relatively inefficient and
bulky systems.
Due to its much higher critical electric field, better
thermal conductivity, and higher temperature capability
in comparison to Si, 4H-SiC has been found to be a viable
alternative in its two-decade-long development [3]. An
SST application based on the series connection of 1,500-V
SiC JFETs (super cascode), with a dc-link voltage of 5 kV,
has been presented by ETH Zurich [4]. The challenge in
the series connection of LV devices lies in the complex
static and dynamic voltage balancing across each single
device. In recent years, fast-switching (25 kHz) SiC MOSFETs up to 15 kV [5] and SiC IGBTs beyond 20 kV have
been developed [6]. The 10-kV SiC MOSFETs have been
demonstrated on a single-phase 1-MVA soft-switched
solid-state power substation by General Electric in 2011
[7]. The 13-kV SiC MOSFETs have been demonstrated on a
20-kVA single-phase SST by the FREEDM Systems Center
[8]. The objective of this article is to present the first-ever
SST demonstration of SiC 15-kV IGBTs by developing a
three-phase SST capable of interfacing a 13.8-kV distribution grid with an isolated 480 V load or grid connection.
The proposed three-phase SST topology, as shown
in Figure 1, is referred to as a TIPS [9] and interfaces the
three-phase, 13.8-kV and 480 V distribution grids. The TIPS
has three power-conversion stages. The active front-end

Reactive Power Flow

Reactive Power Flow
Bidirectional Active Power Flow

Front-End
Converter
L1

22-kV dc Bus
Zout-rect (S)

Dual Active
Bridge

800-V dc Bus

L2
480-V,
Three-Phase,
60-Hz Grid

C
13.8-kV,
Three-Phase,
60-Hz Grid

Zin-dab (S)

15-kV/20-A SiC
IGBT/JBS Diode
Copack Module

1,200-V/100-A SiC
MOSFET/JBS Diode
Copack Module

fig 1 A schematic of the TIPS-a 13.8-kV to 480-V grid-interfaced three-phase SST [30].

LV-Side
Inverter

IEEE PowEr ElEctronIcs MagazInE

z September 2015

Table of Contents for the Digital Edition of IEEE Power Electronics Magazine Compendium - March 2018