closed-loop control of dc–dc dual active-bridge converters driving

8/18/2019 Closed-Loop Control of DC–DC Dual Active-Bridge Converters Driving

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Closed-Loop Control of DC–DC Dual

Active-Bridge Converters DrivingSingle-Phase Inverters


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Abstract

A solid-state transformer (SST) is a high-frequency power electronic converter that is used as a

distribution power transformer. A common three-stage configuration of an SST consists of ac–

dc rectifier isolated dc–dc dual-active-bridge (!A") converter and dc–ac inverter. This study

addresses the controller design issue for a dc–dc !A" converter when driving a regulated

single-phase dc–ac inverter. Since the switching frequency of the inverter stage is much

higher than that of the !A" stage the single phase inverter is modeled as a double-line-

frequency (e.g. #$% &') current sin. The effect of #$%-&' current by the single-phase

inverter is studied. The limitation of a *-controller low gain at #$% &' is investigated. Two

methods are proposed to improve the regulation of the output voltage of !A" converters. The

first one uses a bands top filter and feed forward while the second method uses an additional

proportional-resonant controller in the feedbac loop. Theoretical analysis simulation and

e+periment results are provided.


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Existing syste•

The multistage ac–dc–ac–dc–ac configuration is one of many feasible

single-phase SST topologies ,#–,. An SST is used to interface between

a medium-voltage distribution networ (e.g. /.$ 0) and a low-voltage

distribution networ (e.g. $% 0). The output voltage and the output

current of the inverter stage are grid frequency while the input and

output voltages of the dual-active-bridge (!A") converter stage are dc.

As a result the instantaneous output power fluctuates at twice the line

frequency and there is significant second-order harmonic current at the

input side of the inverter stage.


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Existing syste


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Proposed syste

• 1hen a dc–dc !A" converter is driving such an inverter it is common to

select a sufficiently large output capacitor ban to absorb the double-

frequency harmonic current and to minimi'e the output voltage ripple.

"ecause the second-order harmonic frequency is relatively low this might

result in a large capacitor ban at the output side of the !A" converter

which increases cost weight and volume. 2urthermore large electrolytic

capacitors are one ma3or factor affecting the reliability of power

converters.


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Cont!!


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S"S#E$ C%&'I()*A#I%& A&DAPP*%+I$A#I%&

• The schematic of a single-phase SST is shown in 2ig. #. The first

stage is a single-phase active front-end rectifier which converts the

grid ac voltage to a fi+ed dc voltage. The second stage is an isolated

dc–dc !A" converter which transfers power between two dc buses

with a high-frequency transformer.

• The 4output5 dc bus is defined as the bus whose voltage is regulated

by the !A" converter which is the low-voltage bus on the right in

2ig. #. The last stage is a single-phase dc–ac inverter which generates

an ac sinusoidal voltage as the SST6s output voltage.


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Cont !!

• #he conventional po,er o, diagra of a ultistage SS# is given in 'ig! .! Ideally/ the

voltage and current at 0oth the input of the recti1er and the output of the inverter are line

fre2uency/ e!g!/ 34 56! #herefore/ the input po,er of the recti1er and the output po,er of the

inverter consist ainly of average po,er 7,hich is in dc8 and ripple po,er 7,hich is the

second-order haronic content at 9.4 568!

• ,here V represents input/output RMS voltage, I represents input:output *$S current/ ωs =

2πfs , and fs = 60 Hz is te grid fre2uency! %n the other hand/ the conventional control

ethod for a dc–dc DAB converter can only process dc po,er! #his eans that the t,o dc

0uses ust a0sor0 the 9.4 56 ripple po,er/ as visuali6ed in 'ig! . 7solid lines represent dc

po,er ,hile dash lines represent 9.4 56 ripple po,er8! Because 9.4 56 is a relatively lo,

fre2uency/ large 0us capacitance is needed to reduce the 0us voltage uctuation and achieve

su;ciently high ripple current ratings!


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Assuptions

• The focus of this study is on the !A" converters in SSTs the following

assumptions are made7

#) The rectifier stage has been well designed to provide a stable input dc voltage

for the !A" stage8

$) The inverter stage has been well designed to provide a #$% 0 sinusoidal output

voltage at 9% &'8 and

:) The switching frequency of the inverter is higher than that of the !A" converter

and the switching frequencies of all converter stages are much higher than the

grid frequency


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$%DEL A&AL"SISThe control-to-output transfer function of a

!A" converter is given by

;iven the circuit parameters described in

Tables * ** and ***


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Cont!!

The power transferred by a !A" converter is given by

where vs is the input voltage. Given a specified power rating, a range of preferred

phase-shift ratio d, and a specific range of soft switching the product of transformer

leaage inductance Lt , and switching frequency fs is fixed. Therefore, increase of fs

would result in decrease of Lt , canceling the effect of increasing switching frequency.


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#


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Cont=


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SI$)LA#I%& A&D E+PE*I$EAL *ES)L#S


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C%&CL)SI%&

• The cascaded connection of power converters poses a challenge for the closed

loop controller design. A single-phase inverter has significant second-order (#$%

&') harmonic current in its input side. A conventional * controller has limited

bandwidth at #$% &' because of the relatively low switching frequency of the

!A" converter. 1hat is more simply increasing switching frequency would not

result in higher bandwidth. Two control methods are proposed to solve the

aforementioned challenge without using a large dc bus capacitor between the

!A" converter and the inverter. Simulation and e+perimental results confirm the

effectiveness of the proposed methods. The *-= controller is more robust to

changing load power factor whereas the feed forward method provides better

transient response. "oth achieve ripple reduction similar to increasing bus

capacitance by a factor of four.


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*E'E*E&CES,# A. &uang >. ?row ;. &eydt @. heng and S. !ale 4The future renewable electric energy delivery and management (freedm) system7 The energy

internet5 Proc. IEEE, vol. 99, no. , pp. !!"#$, %an. &'.

,$ B. =. =onan S. !. Sudhoff S. 2. ;lover and !. C. ;alloway 4A power electronic-based distribution transformer5 IEEE Power Eng. (ev., vol. &&, no. :

pp. 9#–9# >ar. $%%$.

,: S. "hattacharya T. hao ;.1ang S. !utta S. "aeD.!u ". arhideh E. hou and A. F. &uang 4!esign and development of generation-* silicon

based solid state transformer5 in Proc. IEEE )ppl. Power Electron. *onf. Expo., +e. &'', pp. ---"-!.

, &. Fin and @. Gimball 4A comparative efficiency study of silicon-based solid state transformers5 in Proc. IEEE Energ. *onvers. *ongr. Expo., Sep. $%#%

pp. #HI–#9:.

,H !. &olmes T. Cipo ". >c;rath and 1. Gong 4


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closed-loop control of dc–dc dual active-bridge converters driving

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