Title page for ETD etd-071399-122308

Type of Document Dissertation
Author Lee, Dong-Ho
Author's Email Address dolee2@vt.edu
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
  • VSC
  • SMES
  • Multi-Level
  • PCS
  • Soft-Switcing
Date of Defense 1999-07-06
Availability unrestricted




Dong-Ho Lee

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

control bandwidth.

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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