Type of Document Dissertation Author Lee, Dong-Ho Author's Email Address firstname.lastname@example.org URN etd-071399-122308 Title A Power Conditioning System for Superconductive Magnetic Energy Storage based on Multi-Level Voltage Source Converter Degree PhD Department Electrical Engineering Advisory Committee
Advisor Name Title Lee, Fred C. Committee Chair Borojevich, Dushan Committee Member Chen, Dan Y. Committee Member Huang, Alex Q. Committee Member Lai, Jason Committee Member Nelson, Douglas J. Committee Member Keywords
Date of Defense 1999-07-06 Availability unrestricted AbstractA POWER CONDITIONING SYSTEM FOR SUPERCONDUCTIVE
MAGNETIC ENERGY STORAGE BASED ON MULTI-LEVEL
VOLTAGE SOURCE CONVERTER
Fred C. Lee, Chairman
A new power conditioning system (PCS) for
superconductive magnetic energy storage (SMES)
is developed and its prototype test system is
built and tested. The PCS uses IGBTs for
high-speed PWM operation and has a multi-level
chopper-VSC structure. The prototype test system
has three-level that can handle up to 250-kVA
with a 1800-V DC link, a 200-A maximum load
current , and a switching frequency reaching
20-kHz with the help of zero-current-transition
(ZCT) soft-switching. This PCS has a great number
of advantages over conventional ones in terms of
size, speed, and cost.
Conventional PCSs use thyristors, due to the
power capacity of the SMES system. The speed
limit of the thyristor uses a six-pulse operation
that generates a high harmonic. To reduce the
harmonic, multiple PCSs are connected together
with phase-matching transformers that need to be
precise to be effective in reducing the harmonics.
So, the system becomes large and expensive. In
addition, the dynamic range of the PCSs are also
limited by the six-pulse operation, because it
limits the useful area of the PCS applications.
By employing a high-speed PWM, the new PCS can
reduce the harmonics without using the
transformers reducing size and cost, and has wide
dynamic range. However, the speed of a switching
device is generally inversely proportional to its
power handling capacity. Therefore, employing a
multi-level structure is one method of extending
the power-handling capability of the high-speed
device. Switching loss is another factor that
limits the speed of the switch, but it can be
reduced by soft-switching techniques. The 20-kHz
switching frequency can be obtained with the help
of the ZCT soft-switching technique, which can
reduce about 90% of switching losses from the IGBT
during both turn-on and turn-off transients.
There are two different topologies of the PCS; the
current source converter (CSC) type and the
chopper and voltage source converter (VSC) type.
In terms of the SMES system efficiency, the
chopper-VSC type shows a less volt-ampere
requirement of the power device. Therefore, the
new PCS system has a chopper-VSC structure.
Since the chopper-VSC structure consists of
multiple legs that can be modularized, a power
electronics building block (PEBB) leg is a good
choice; all of the system problems caused by the
high frequency can be solved within the PEBB leg.
The VSC is built with three of the PEBB legs.
Three-phase AC is implemented with a three-level
space vector modulation (SVM) that can reduce the
number of switching and harmonic contents from
the output current. A closed-loop control system
is also implemented for the VSC, and shows 600-Hz
The multi-level structure used requires too many
high-speed switches. However, not all of them are
used at the same time during normal multi-level
operation. A new multi-level topology is suggested
that requires only two high-speed switches,
regardless of the number of levels. Other
switches can be replaced with slow-speed switches
that can allow additional cost savings.
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