Design, Development and Control of >13 kV Silicon-Carbide MOSFET based Solid
State Transformer (SST)
by
Gangyao Wang
A dissertation submitted to the Graduate Faculty of
North Carolina State University
in partial fulfillment of the
requirements for the Degree of
Doctor of Philosophy
Electrical Engineering
Raleigh, North Carolina
WANG, GANGYAO. Design, Development and Control of >13 kV Silicon-Carbide
MOSFET based Solid State Transformer (SST).
ABSTRACT
Within the advent of the smart grid system, the solid state transformer (SST) will
replace the traditional 60 Hz transformer formed by silicon steel core and copper
windings and provides the interface between the high distribution voltage and low
utility voltage. Other than the smaller size and less weight, SST also brings many
more functionalities including voltage regulation, reactive power compensation, power
management and renewable energy integration. The motivation of this research is to
design a solid state transformer based on the wide band-gap Silicon Carbide (SiC)
power MOSFETs and compare it with the silicon IGBT based SST.
With wider band-gap and higher critical electrical field, the high voltage SiC power
device has advantages over silicon power device for both conduction and switching.
An extensive study and characterization of the SiC MOSFET was first carried out.
It has been found that the MOSFET parasitic capacitors store significant amount of
energy and the MOSFET turn on loss is high but turn off loss is virtually zero with
small enough turn on gate resistor. A method for estimating the MOSFET parasitic
capacitances has been proposed and explained in detail. A PLECS loss simulation
model has been developed for the >13 kV SiC MOSFET which has been verified
through a boost converter with the SiC MOSFET switches under 40 kHz for both soft
switching and hard switching conditions separately.
Widely used full bridge circuit has been chosen as the topology for the SST rectifier
for its simple structure and bidirectional power transfer capability. Form three different
SPWM modulation methods, the bipolar single frequency SPWM method has been
identified as the most suitable control algorithm for the >13 kV SiC MOSFET base
rectifier. With such modulation method, the generated PWM voltage frequency equals
to the switching frequency, the each MOSFET equivalent switching frequency under
hard switching conditions is only 1/4 of the PWM voltage frequency. The SST rectifier
efficiency has been simulated and measured for 6 kHz and 12 kHz switching frequency
with 6 kV dc bus voltage and 3.6 kV ac voltage, which is 99.2% for 6 kHz with 8.8 kW
load and 98.5% for 12 kHz with 8.3 kW load.
The SST DC-DC stage utilize the dual active half bridge (DHB) as the topology, its
zero voltage switching (ZVS) turn on range has been analyzed and it is concluded
that the dead-time and device parasitic capacitances will reduce the ZVS range while
the magnetizing current will increase the ZVS range. Since the SiC MOSFET has
very high turn on loss, it is desired to have ZVS for the full load range. The high
frequency transformer with integrated leakage inductance for the DHB operation has
been designed, the magnetizing inductance has been decreased for increasing the ZVS
range. The DC-DC stage efficiency has been measured as 96.9% for 10 kHz switching
frequency and 10 kW load, and the peak efficiency is 97.5% for 10 kHz switching
frequency and 5 kW load.
LINK VIEW FULL TEXT
https://repository.lib.ncsu.edu/handle/1840.16/9163
URL DIRECT :
https://repository.lib.ncsu.edu/bitstream/handle/1840.16/9163/etd.pdf?sequence=2&isAllowed=y
ALTERNATIVE LINK
http://www.mediafire.com/file/1eno463fcg4kj26/Design%2C_Development_and_Control_of_13_kV_SiliconC.pdf
