Future Hybrid MicrogridsYusi Liu, H. Alan Mantooth, Juan Carlos Balda

Department of Electrical Engineering

University of Arkansas

Hybrid Microgrid Concept and Test Bed at NCREPT

GRAPES Confidential

Proposing A New Start-Up Procedure of AC-DC Converter

Microgrids

Integrates distributed generation

and energy storage

Smart controls

Increased reliability

More resilient system

Faster dynamic response

1 MVA Prototype Validation

High Current Variable-Value AC Filter Inductor

Analysis of general reasons behind the inrush

current during ac-dc converter start-up

procedure

Simple open-loop duty cycle control as a dc-dc

boost converter

All IGBTs are enabled after saturation period

Optimal design of ramp time considering the dc

capacitor value

Validation on a 1-MVA ac-dc rectifier prototype

– Inrush current during duty-cycle soft-start is

less than 25 A

Recent publication:

Y. Liu, C. Farnell, H. A. Mantooth, J. C. Balda, "Realization of High-Current Variable AC Filter

Inductor Using 6.5% Silicon Iron Powder Magnetic Core," in Proc. Applied Power Electronics

Conference and Exposition (APEC), Tampa, FL, March 2017.

Design of 6.5% Si-Fe powder core, µe0=60,

N=24, L = 300 µH @ 0A, 100 µH @ 2000A

24 e e

e

A NL

l

AC Distribution Substation

13.8 kV AC

Non-critical load CHPCritical load

=~ =~ =~

Electric carcharger

Fuel cell(DG)

Solar & windPower (DG)

Li-ion batterystation

=~

PCC

Utility Input

12.47kV – 480VUTCB1

480VMSB1

LVB1

LVCB4

480V

MVCB3

4.16/13.8 kVMVB2

VVVF

T6

MVB1

LVCB5 LVCB6

LVCB9

LVCB18

LVCB12 LVCB13 LVCB14

MVCB2 MVCB1

MVCB5MVCB4 MVCB6

MVCB8 MVCB9 MVCB10

LVCB16

T1 T2 T3

T5

MVCB12

LVCB8

LVCB15

T4

MVCB13

MV microgrid

LV microgrid

~=~

MVCB13

LVCB7

~=~

~

=

~

LVCB3

LVCB10

4.16/13.8 kV

Regen 1 Regen 2

UT

Microgrid

LVCB11

LB

NCREPT as a microgrid test bed

for verifying converter design

and control algorithms of

commercial-scale high-power

ac-dc converters in:

Grid-connected mode

Islanded mode

VDC

Lac

VDC

Lac

480-V duty-cycle soft-start experimental results

Compare four feasible magnetic core solutions

Variable inductor reduces the current ripple at low current

(a) Traditional silicon steel three-phase one-core. (b) Powder material three-phase

one-core.

(c) Powder material single-phase core. (d) Hybrid three-phase one-core (silicon

steel + powder material )

Solution Material

volume

Core

loss

Structure

complex

Unbalance

current

Size fit into

cabinet

Fringing

effect

Variable

inductance

(a) + -- ++ - + -- --

(b) ++ ++ --- - -- + +

(c) - + + + ++ ++ +

(d) ++ - -- - + + -

Cdc

Rb

L1

RSS

Lg

FU

Cf

Rcf

EMI filter

MOV

iabc2 iabc1Soft-start relay

Sap

San

Sbp

Sbn

Scp

Scn

DS vabc

CB

To MV

Microgrid

480 V

T

vPWM

vabc1 To DG load/

source, dc-dc

converter, dc

microgrid

Start

Is Cdc voltage satisfy

β ∙ Vdc* < Vdc < γ ∙ Vdc

*

?

Voltage

error

No

Is Vdc > α1∙ Vdc*?

Duty-cycle control

Disable upper switches

Increase duty-

cycle Tss*

No

Yes

Yes

Complimentary operation

enables

Is VSS_dc* = Vdc

*?

Yes

Increase ramp

reference VSS_dc*

No

Steady state480-V resistor-relay-bypass

experimental results

500 A 1000 A 1800 A

MATLAB-PLECSTM simulation allows

to model variable magnetic material

480-V grid connect mode hardware

test @ 200 kVA

Preliminary 300-V islanded mode

hardware test @ 100 kVA (future work

will increase ac input voltage and

improve current THD)

Vdc = 760 V

vac = 480 V

Vdc = 670 V

vac = 480 V

Vdc = 760 V

vac = 483 V

ia1 = 220 A

ib1 = 220 A

Vdc = 760 V

vac = 310 V

ia1 = 100 A

ib1 =100 A

Grid-connected mode

Islanded mode

Converter-side current

Converter-side current

Vdc

Id*

abc

dq

iabc1-

+

-

ωL/Zb

ωL/Zb

ed

abc

dq

vabc

eq

PLLΘ

ePWM

Sd*

Sq*

S1~6

Θ

Iq*

Id

Iq+-

PIiq

-+

+

+- PIid

Hv

Hi

Hv

÷

+- PIv

Vdc*

vd

vq

SV

GEN

DCSS

GEN

Soft Start

Selector

ta*

tb*

tc*

Tss*

tA*

tB*

tC*

SAT

vab

ia1

Vdc

ib1

t0 t1 t2

t0 t1

vab

ia1

Vdc

ib1