Solid state transformers could be the next big thing in power electronics
(Energy Efficiency & Technology Via Acquire Media NewsEdge) There's been a lot of talk in the past few years about coming up
with a solid-state version of the distribution transformer that now
sits on utility poles in neighborhoods throughout the land. A
solid-state transformer (SST) would be at least as efficient as a
conventional version but would provide other benefits as well,
particularly as renewable power sources become more widely used.
Among its more notable strong points are on-demand reactive power
support for the grid, better power quality, current limiting,
management of distributed storage devices and a dc bus.
It is starting to look more likely that we'll see a practical
SST design as GaN and SiC power transistors with higher current and
voltage ratings start coming to market and their prices drop. But a
practical SST design could have an impact extending well beyond
transformers for electrical utilities.
One example of where SSTs could also find use is in
variable-frequency drives for big induction motors. In that regard,
Siemens Industry Inc.'s Drive Technologies Div. in New Kensington,
Pa. is keeping an eye on SST work now underway at North Carolina
State University's FREEDM System Center for smart grid research.
Siemens Principal Engineer Mark Harshman says use of SSTs in the
medium-voltage motor drives that Siemens makes could conceivably
reduce the size of the VFDs by 30% and have similar beneficial
effects on their overall efficiency levels.
There have been several topologies suggested for SSTs but most
being evaluated today are based around the idea of a dual active
bridge (DAB) converter. A DAB uses a power bridge to modulate the
incoming ac waveform into a high-frequency square wave. The square
wave gets passed through a small high-frequency transformer to
another power stage. This converter demodulates the square wave and
sends it to another inverter which produces low-voltage ac.
This scheme still uses a conventional transformer, but one
optimized for higher frequencies (typically about 1 kHz). This
makes it much smaller and lighter than transformers optimized for
ac line frequencies.
The high-frequency transformer gives the SST galvanic isolation.
It also has some leakage inductance in its primary and secondary
windings, which also helps synthesize soft switching. During
switching transients, transformer current resonates with the
capacitors in parallel with switching devices, limiting the dv/dt
and di/dt across the switches, thus reducing switching loss and
boosting power efficiency.
The fact that DAB converters have a symmetrical circuit
configuration lets them handle bi-directional power flows,
important when it comes to renewable sources sending power back up
the grid. The power flow of a DAB converter can be controlled by
varying the phase shift between those two bridges which changes the
voltage across the transformer leakage inductance. Power transfers
from the leading bridge to the lagging bridge.
One of the difficulties in fabricating a SST is that the 7.2-kV
line voltages that characterize distribution power lines exceed the
operating voltage of today's IGBTs, 6.5 kV. So multiple devices
must be used in series to keep below the operating maximum. The NC
State prototype, for example, uses a topology that includes a
seven-level cascaded H-bridge for the high voltage rectifier
There are other difficulties as well. One is that the minimum
current rating for the 6.5-kV IGBTs is 200 A. This is too large for
the 20 kVA transformer NC State is building because the input
current is only about 3 Arms. Thermal issues also affect the SST's
operation, which has forced NC State researchers to come up with
special packaging for their 25-A IGBTs. Additionally, the team had
to come up with a way to isolate IGBT drivers for both power supply
and gating signals.
To sense the 7.2 kVac voltage, the researchers devised a sensor
that was compact and which incorporated high-voltage isolation
because existing models were too large and not isolated from the
high input voltage. Finally, they had to get around the fact that
the insulation capability for 6.5-kV IGBT is 10.2 kV, but the
high-voltage-side dc bus voltage is 11.4 kV. They ended up floating
the heatsink for each 6.5-kV IGBT while maintaining ample clearance
and creepage distance between the heatsinks. To keep the voltage
across input inductor down to manageable levels, the team built
eight identical inductors and put them in series so the maximum
voltage stress for each of them is just 0.9 kV.
Researchers have also developed a prototype using 15 kV SiC
MOSFET/JBS diodes. They are not trying to identify other major
issues related to implementing a high-voltage system using SiC
power devices, including the challenges in designing a system to
support high dV/dt and dI/dt, and to design an efficient and
compact high-frequency transformer.
More info from the FREEDM project: http://www.freedm.ncsu.edu/index.php s=3&p=439&i=2
© 2012 Penton Media
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