Indonesian J
ournal of Ele
c
trical Engin
eering and
Computer Sci
e
nce
Vol. 1, No. 2,
February 20
1
6
, pp. 238 ~
248
DOI: 10.115
9
1
/ijeecs.v1.i2.pp23
8-2
4
8
238
Re
cei
v
ed Se
ptem
ber 22, 2015; Revi
se
d No
vem
ber
29, 2015; Accepted Decem
ber 18, 20
15
High Penetration PV in Distribution Networks, Design
and Control
Ashk
an Moh
a
mmadi, Saman Hoss
eini
Eslama
bad-E-
Gharb Branc
h, Islamic Azad
U
n
iversit
y
, Esla
maba
d-E-Ghar
b, Iran
Corresp
on
din
g
author, e-mai
l
: Ashkan.m@l
i
ve.com
A
b
st
r
a
ct
Globa
l w
a
rmi
n
g is a d
i
rect c
onse
q
u
ence
of cons
u
m
ption
of fossil fuels
w
h
ich e
m
it gre
enh
ous
e
gasses as they
produc
e en
ergy. Solar en
er
gy is the mo
st
avail
a
b
l
e e
ner
gy throug
ho
ut the w
o
rld in w
h
ich
regar
dless
of
capita
l i
n
vest
ment is fr
ee
an
d
most i
m
port
antly cl
ea
n a
n
d
e
m
iss
i
o
n
fre
e
a
nd c
oul
d
b
e
a
soluti
on for
gl
o
bal w
a
r
m
i
ng
al
ong
w
i
th other
renew
a
b
le
so
urces of
en
erg
y
. But as p
hot
ovolta
ic e
nerg
y
i
s
beco
m
ing w
i
d
e
s
prea
d an
d pe
netratio
n
lev
e
l
of photov
oltaic
pow
er pla
n
ts increas
e, sever
a
l issu
es rise i
n
distrib
u
tion n
e
t
w
orks. In this
pap
er, a hig
h
pen
etrati
o
n
ph
otovolta
ic pow
er pla
n
t is des
ign
ed a
nd issu
es
associ
ated w
i
t
h
it ar
e thor
ou
ghly
disc
ussed
.
Voltag
e
ris
e
and
clo
ud
pas
sage
effect are
a
m
on
gst the
mos
t
chall
e
n
g
in
g is
sues i
n
des
ig
n an
d i
m
p
l
e
m
entatio
n of
a
hig
h
pe
netrati
on p
hotov
oltai
c
pow
er pl
ant
i
n
distrib
u
tion
net
w
o
rks. T
r
ansient effects of cloud p
a
ssag
e
co
uld l
e
a
d
to un
a
cceptab
ly low
v
o
ltag
e in P
o
int
of
Co
mmon
Co
u
p
lin
g a
nd
maxi
mu
m
pe
netrati
on l
e
vel
must
be set
accord
i
ng to th
ese
is
sues. An
effici
ent
Maxi
mu
m Pow
e
r Point T
r
ack
i
ng (MPPT
) and a DC l
i
nk
voltag
e contro
l
scheme are
also pr
esent
e
d
.
Simulati
ons h
a
v
e bee
n do
ne i
n
Matlab/Si
mul
i
nk env
iron
men
t
.
Copy
right
©
2016 In
stitu
t
e o
f
Ad
van
ced
En
g
i
n
eerin
g and
Scien
ce. All
rig
h
t
s reser
ve
d
.
1. Introduc
tion
The co
ntinuo
us in
cre
a
se in the level of
gr
ee
nho
use gas emi
s
sion
s and the cli
m
b in fuel
prices are the main
driving forc
es behi
nd effort
s to
utilize vari
ou
s sources of r
enewable energy
[1, 2]. In rece
nt years there
has
bee
n a g
r
owi
ng
attenti
on toward
s u
s
e of
sola
r e
n
e
rgy. The
mai
n
advantag
es
of photovolta
ic (PV)
syste
m
s em
ploye
d
for ha
rne
s
sing
sola
r en
ergy a
r
e la
ck of
gree
nho
use gas e
m
ission
, low mainte
nan
ce cost
s, fewer limita
t
ions with
re
gard to
site of
installatio
n
an
d absen
ce of mech
ani
cal n
o
ise a
r
isi
ng from moving p
a
rts.
The ma
rket of PV energy h
a
s be
en in a
n
increa
sing trend with th
e annu
al gro
w
t
h
rate of
25-3
5
% over
the last ten y
ears an
d 15
6
%
and 85%
a
nnual g
r
o
w
th
rate in
US on
ly in 2012 a
n
d
2013
re
spe
c
t
i
vely. As the use
of sola
r PV conti
nue
s to expa
nd,
con
c
e
r
n a
b
out its pote
n
tial
impact on the
stability and operation of the grid
g
r
o
w
too. Utilities and
power sy
stem operato
r
s
are
prepa
ring
for
ch
ang
es
to integ
r
ate
a
nd m
anag
e
more
of thi
s
rene
wable
el
e
c
tri
c
ity so
urce in
their sy
stems.
The p
enetration level i
s
defined
as the ratio of n
a
meplate
PV power
ratin
g
to th
e
maximum loa
d
se
en o
n
the
distrib
u
tion f
eede
r. The v
o
ltage ri
se i
s
sue
ha
s be
en
repo
rted
as
one
of the co
nce
r
ns
und
er hi
gh pen
etratio
n
of ren
e
wa
ble Di
strib
u
te
d Gen
e
ratio
n
s (DG
)
[3]. The
reverse
po
we
r flow
ca
used
by larg
e am
o
unts of
DG
m
a
y cau
s
e
voltage
rise to
which
dist
ributi
o
n
netwo
rk
ope
rators
co
ntrol
can
not effecti
v
ely resp
ond
sin
c
e the tra
d
i
tional gri
d
ha
s bee
n pla
n
n
e
d
to deliver po
wer to the loa
d
at satisfa
c
tory voltage ra
nge [4].
Reference [5, 6] concentrated on dist
ri
buted
generators interfaced
to utilities through
inverters, a
n
d
larg
er-scal
e
syst
em i
m
pa
cts an
d rotatin
g
di
stribute
d
gene
ration
(DG), but
still
wi
th
several re
sul
t
s on inverte
r-b
ased DG. The first stu
d
y [5] conclu
ded that for
DG pe
netrati
on
levels of 4
0
%
, such that
the sy
stem
is he
avily de
pend
ent on
DG
s to
satisf
y loads, volt
age
regul
ation ca
n
be
come a seri
ou
s
p
r
obl
em.
The sud
den
lo
ss of DGs, pa
rticul
arly as a result of
false tri
pping
durin
g volta
ge o
r
freq
ue
ncy event
s, can l
ead to
una
cceptably
low voltag
e
s
in
portion
s of the syste
m
. The same m
a
y occur
in
high pe
netrat
i
on PV syste
m
s in which
the
microgri
d
is
heavily depe
ndent on ph
otovoltaic en
er
gy to provi
de the local
load with a
c
tive
power. In this situation, tra
n
sie
n
t effect of
cloud p
a
ssage could le
a
d
to low voltage issue
sin
c
e
the re
sp
on
se
time of O
n
-L
o
ad Ta
p
Cha
n
g
ing
(OL
T
C) t
r
an
sform
e
rs
have fe
w to
several
se
co
n
d
s
delay.
Evaluation Warning : The document was created with Spire.PDF for Python.
IJEECS
ISSN:
2502-4
752
High Pen
e
trat
ion PV in Dist
ribution
Networ
ks, Desig
n
and Control
(Ashkan Mo
ha
mm
adi)
239
Referem
c
e [7
] examined cl
oud tran
sie
n
t effects if the PV were de
ployed a
s
a central-
station
plant,
and it
wa
s fo
und th
at the
maximum tol
e
rabl
e
syste
m
level
penet
ration
level of
PV
was approximately 5%, t
he limit bei
ng imposed by the tran
sient following
capabilities (ramp
rates)
of
the conve
n
tional gene
rato
rs. Referen
c
e [8]
focus o
n
the
operating ex
perie
nce of th
e
Southern California Edi
s
o
n
ce
ntral
-
stat
ion PV pl
ant at
He
spe
r
ia, CA,
whi
c
h re
ported no su
ch
probl
em
s, but
sug
g
e
s
ts th
a
t
this plant
ha
d a very
‘‘s
t
iff’’ c
o
nn
ec
tio
n
to
th
e
gr
id
an
d r
e
pr
es
e
n
t
ed
a
very low PV penetratio
n
level at its point of interco
nne
ction.
Referen
c
e [9
] dealt with voltage regula
t
ion issue
s
o
n
the Public
Service
Com
pany of
Okla
homa
system duri
ng the passa
ge o
f
clouds ove
r
an are
a
with high PV pene
tration levels,
if
the PV were
distrib
u
ted ov
er a
wide
are
a
. At
penetra
tion levels
of 15%, clou
d transi
ents
we
re
found to cau
s
e si
gnifica
nt
but solvable
powe
r
swin
g
issu
es at th
e system lev
e
l, and thus
15%
wa
s deem
ed
to be the maximum syste
m
level penetra
tion level.
In this
pap
er
a hig
h
pe
netration PV p
o
wer pl
ant
con
n
e
cted
to the
distrib
u
tion
n
e
twork
feeder
will be
design
ed an
d controlled. The org
ani
zat
i
on of the rest
of this paper is as follo
ws: in
section
2 high penetration leve
ls of P
V
energy an
d its
consequen
ces will be
di
scussed.
In
se
ction 3 eq
uipment
mod
e
ls used
in
t
h
is re
se
arch
are
presente
d
. The
de
sig
n
p
r
o
c
ed
ure
and
control meth
o
d
will b
e
di
scussed i
n
secti
on 4.
In secti
on 5 the
sim
u
lation re
sult
s
and di
scu
ssi
o
n
s
are p
r
e
s
ente
d
and finally concl
u
si
on
s cl
ose the p
ape
r.
2.
Impact of
High Penetr
ati
on PV on Distribu
tion Net
w
o
r
k
As the pen
etration level of distrib
u
ted en
ergy
re
so
ur
c
e
s in
cr
eas
e,
sev
e
r
a
l is
sue
s
ri
se in
distrib
u
tion n
e
tworks with
regard to control,
ope
rat
i
on, prote
c
tio
n
and po
wer quality. Voltage
rise,
clo
ud transi
ent effect
and
high
er
Total Harm
o
n
ic
Disto
r
tion
(T
HD) a
r
e th
e mo
st impo
rtant
issue
s
asso
ci
ated with hig
h
penetration
PV power pl
a
n
ts.
ANSI C84.1
spe
c
ifie
s utilization volta
g
e
, whi
c
h
refe
rs to the volt
age at the p
o
i
nt of use
whe
r
e the
ou
tlet equipme
n
t is plug
ged
in. Furthe
rm
ore, two
ran
ges
are
defin
ed, Ran
ge A
is
recomme
nde
d for norm
a
l operatin
g con
d
ition
s
, while Rang
e
B corre
sp
o
nds to unu
sual
con
d
ition
s
, so the o
c
cu
rre
nce
ha
s to
b
e
limited i
n
ti
me du
ration
and frequ
en
cy. Recomme
nded
servi
c
e and utilization voltage limit
s according to A
N
SI C84.1
are
shown in
Tabl
e
1. Utilities are
gene
rally con
c
erned
with
maintainin
g
t
he servi
c
e
vo
ltage withi
n
a
c
ceptabl
e limi
t
s; the utilization
voltage then follows autom
atically, provi
ded that
the hou
se wi
ring
is don
e acco
rding to buildi
n
g
cod
e
s.
Table 1. ANS
I
C84.1 voltage ran
g
e
Ser
v
ice
Utilizati
o
n
Min Max
Min
Max
Range
A
(nor
mal)
-
5
% +5
%
-
8
.
3
%
+4
.
2
%
Range
B (e
merg
enc
y
)
-8.3%
+5.8%
-1
1.7
%
+5.8%
Figure 1. Voltage limits in d
i
stribut
io
n net
works acco
rdi
ng to [10]
Evaluation Warning : The document was created with Spire.PDF for Python.
ISSN: 25
02-4
752
IJEECS
Vol.
1, No. 2, February 201
6 : 238 – 248
240
Figure 1 sho
w
s an exa
m
p
l
e of
voltage
limits fo
r prim
ary circuit, se
rvice ent
ran
c
e, and
utilization b
a
s
ed o
n
one
utility’s guidel
ines [10, 11]
.
It reflects the adju
s
tment
for assumpti
ons
about a
dditio
nal voltage
d
r
op i
n
the se
con
dary
ci
rcu
i
t and allo
ws
for the n
e
cessary m
a
rgin.
In
this study, the prima
r
y voltage and servi
c
e entran
c
e voltage
limits shown in Figure 1 were u
s
e
d
as target limits.
3. PV and In
v
e
rter Model
3.1. PV Model
Figure 2 sho
w
s the
equiv
a
lent ci
rcuit of a
sola
r pa
nel. A sola
r panel i
s
co
m
posed of
several p
hot
ovoltaic
cell
s that have
serie
s
o
r
para
llel or serie
s
-parall
e
l external
con
n
e
c
tions.
Equation (1)
sho
w
s V-I ch
ara
c
teri
stic of
a solar p
anel
[12].
Figure 2. Equivalent circuit of Solar Pane
l
-[
e
x
p
(
)
-
1
]
-
SS
pv
o
tp
VR
I
V
R
I
II
I
aV
R
(1)
Whe
r
e I
pv
is
the photovoltaic current, I
o
is saturate
d reverse cu
rre
nt, 'a' is the ideal dio
d
e
c
o
ns
tant,
S
t
NK
T
V
q
is t
he the
r
mal
voltage,
N
s
is
th
e
nu
mb
er
o
f
s
e
r
i
es
c
e
lls
,
q
is th
e e
l
ec
tr
o
n
cha
r
ge, K i
s
t
he Boltzm
an
n co
nsta
nt, T
is the
tempe
r
ature of p
-
n j
unctio
n
, R
S
a
nd R
p
are
series
and p
a
rall
el e
quivalent resi
stan
ce of th
e
sola
r p
anel
s.
I
pv
has a linear relation
wit
h
light intensi
t
y
and al
so vari
es with temp
eratu
r
e variat
ions. I
o
is de
pend
ent on tempe
r
ature variation
s
. Val
ues
of I
pv
and I
o
are cal
c
ulate
d
according to the followi
ng e
q
uation
s:
,
()
pv
pv
n
I
n
G
II
K
T
G
(2)
,
,
exp(
)
/
1
sc
n
I
o
oc
n
V
t
IK
T
I
VK
T
a
V
(3)
In whic
h I
pv
,n
, I
sc,n
and V
oc,n
are p
hotovolt
a
ic
curre
n
t, short ci
rcuit cu
rre
nt and
ope
n circuit
voltage in
st
anda
rd
co
ndi
tions
(T
n
= 2
5
C
an
d G
n
= 1000
W /
m ^ 2)
res
p
ec
tively. K
I
is the
coeffici
ent of
sho
r
t-circuit current to tem
peratu
r
e,
n
TT
T
is the tempe
r
atu
r
e deviatio
n
from
stand
ard tem
peratu
r
e, G i
s
the light in
tensity and
K
V
is the rati
o coeffi
cient
of open
circuit
voltage to temperature.
Open
ci
rcuit
voltage, sho
r
t circuit
cu
rre
n
t and
voltag
e –
current
correspon
ding
to the
maximum p
o
w
er a
r
e th
ree
impo
rtant p
o
ints of I-V
cha
r
acte
ri
stic of Solar
Pa
nel. These
p
o
ints are
cha
nge
d by variation
s
of atmosp
he
ric
co
ndition
s. Usi
n
g Equation (4
) and (5) whi
c
h are de
rived
from PV model equatio
ns, short circuit
current and
open ci
rcuit voltage can
be cal
c
ulate
d
in
different atmo
sph
e
ri
c co
ndi
tions.
,
()
sc
sc
n
I
n
G
II
K
T
G
(4)
,
oc
oc
n
V
VV
K
T
(5)
Evaluation Warning : The document was created with Spire.PDF for Python.
IJEECS
ISSN:
2502-4
752
High Pen
e
trat
ion PV in Dist
ribution
Networ
ks, Desig
n
and Control
(Ashkan Mo
ha
mm
adi)
241
3.2. H
y
steresis Curre
nt
Con
t
rol (HCC) o
f
Po
w
e
r
Electronic
Unit
Hysteresi
s
co
ntrol p
r
e
s
ent
s an
alternative method fo
r
prod
uci
ng a
sinusoidal a
c
curre
n
t
waveform from a dc
voltage s
o
urc
e
. Wit
h
this
meth
od
, the controlle
r maintai
n
s
a
n
output
current
that stays within a given tole
ra
nce of the refe
ren
c
e
waveform
. T
he tolera
nce that the output
stays
within
i
s
calle
d the
“hystere
si
s b
a
nd”.
Unli
ke th
e PWM
switching te
ch
niqu
e, the met
h
o
d
of
hystere
s
i
s
co
ntrol
depe
nd
s o
n
fee
dba
ck from the
o
u
t
put cu
rrent t
o
contro
l the
inverter sy
ste
m
.
The clo
s
e
d
-l
o
op co
ntrol me
thod ena
ble
s
the inverter
with hystere
s
is control to ad
apt instantly to
cha
nge
s in th
e output loadi
ng.
The co
ncept
of hysteresis cont
rol can
be appli
ed to a wid
e
rang
e of inverter
config
uratio
n
s
and to
polo
g
i
es. Both si
ng
le-ph
a
se
an
d three
-
ph
ase inverters
can
be co
ntroll
ed
by
the hystere
s
i
s
method a
s
well. A com
m
on topolo
g
y for single
-
ph
ase inve
rters is the H-b
r
id
ge
becau
se it offers
more con
t
rollability tha
n
the hal
f-bri
dge type. It allows the u
s
e
of three o
u
tp
ut
states inste
a
d
of two
and
requi
re
s h
a
lf
of the d
c
bu
s voltage to p
r
odu
ce the
sa
me pe
ak
out
pu
t
voltage [13].
Figu
re
3 ill
ustrate
s
the
fundame
n
ta
l con
c
e
p
t
of o
peratio
n
fo
r the
hyste
r
e
s
i
s
-
controlled inv
e
rter.
The referen
c
e current,
, re
pre
s
ent
s the desi
r
ed wave
form for the output load current.
The top
an
d
bottom hy
stere
s
is limits
form the
hysteresi
s
ban
d, whi
c
h
corre
s
po
nd
s to th
e
toleran
c
e limi
t
of the inverter co
ntroll
er.
Figure 3. Con
c
ept of hyste
r
esi
s
ban
d an
d hystere
s
i
s
controlle
r
The two-level
inverter
cont
roller will appl
y t
he positive or negative dc
bus voltage to the
load in o
r
de
r to keep the
output cu
rre
n
t within
the
hystere
s
i
s
b
and. For
exa
m
ple, wh
en the
output current rises
above the t
op hysteresis limi
t, the inverter
controll
er
w
ill respond by
swit
chin
g the
tran
sisto
r
s to
apply the
ne
gative d
c
bu
s voltage to th
e loa
d
an
d ef
fectively red
u
c
e
the value of t
he outp
u
t cu
rrent to b
r
ing i
t
below
th
e top hyste
r
e
s
is limit. The in
verter
cont
rol
l
er
will kee
p
the negative dc b
u
s voltage a
c
ross the
load
until the output current re
a
c
he
s the bottom
hystere
s
i
s
lim
it. After the output curre
n
t drop
s bel
ow t
he bottom lim
it, the inverter cont
rolle
r wi
ll
sen
d
the app
rop
r
iate gatin
g sign
als to t
he tran
si
st
ors to switch the
m
to apply the positive d
c
bus
voltage a
c
ross the l
oad. T
h
is
will b
r
ing
the output
cu
rre
nt ba
ck up
above th
e b
o
ttom hyste
r
e
s
i
s
limit and withi
n
the allo
wab
l
e tolera
nce b
and a
r
ou
nd t
he refe
re
nce waveform. Th
e co
ntrolle
r will
contin
uou
sly repe
at this cy
cle to maintai
n
the
output load current within the hystere
s
is b
and.
Unli
ke othe
r
high-fid
e
lity inverter
cont
rol st
rate
gies, the
hysteresi
s
co
ntrolle
r will operate
at a variable
switching freque
ncy that
is sp
read
a
c
ro
ss the sp
ectru
m
. The
instantan
eo
us
swit
chin
g freq
uen
cy
at any point on the current wave
fo
rm ca
n be predicte
d
by [14]:
(
6
)
Whe
r
e
is the
dc b
u
s voltage,
is the
inst
antane
ou
s vo
ltage of reference current
sign
al, L
is the
loa
d
in
ducta
nce, an
d h i
s
the
wi
dth of th
e hy
stere
s
i
s
band
. As
refle
c
ted
in Eq
uation
(6),
the hysteresi
s
inverte
r
wil
l
swit
ch fast
er at
poi
nts
in the cycl
e
whe
r
e the
re
feren
c
e
cu
rre
nt
rea
c
he
s its maximum and minimum values and switch much slo
w
er whe
n
is close to zero in
magnitud
e
. A
larger loa
d
i
ndu
ctan
ce
wi
ll allo
w th
e
i
n
verter to
switch at
a l
o
wer frequ
en
cy to
maintain the
curre
n
t within
the same hy
stere
s
i
s
ban
d
.
Since
will diverge to infini
ty if L is equal
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242
to ze
ro, th
ere
mu
st be
som
e
ind
u
cta
n
ce
pre
s
ent i
n
the
load
for th
e
hystere
s
i
s
-co
n
trolled
inve
rter
to work. Th
e swit
chin
g fre
quen
cy is
al
so inversely p
r
opo
r
tion
al to
. The inverter will switch
at
highe
r rate
s o
v
erall to achi
eve a highe
r fidelity out
put curre
n
t within
a smalle
r hystere
s
i
s
ban
d
.
Figure 4 sho
w
s a
HCC for a sin
g
le p
hase
VSI. Assume the V
S
I terminal voltage V
con
n
e
c
ts to a
sinu
soid
al voltage so
urce
e thro
u
gh an
equivalent in
ducta
nce L a
n
d re
si
stance
R.
If we want to
control outp
u
t current i to
track a
ce
rta
i
n referen
c
e
curre
n
t i*, according to Fi
gure
4(a
)
we h
a
ve instanta
neo
u
s
value eq
uat
ion as:
(
7
)
Whe
n
the SO
FC outp
u
t cu
rre
nt is equ
al
to refere
nce curre
n
t i
*
, the corre
s
po
ndin
g
equatio
n wi
ll
be:
∗
∗
∗
(
8
)
Whe
r
e V*
is the referen
c
e
VSI terminal
voltage
co
rre
s
po
ndin
g to i*
. If we d
e
fine
curre
n
t tra
cki
n
g
error
∆
i
=
i
-
i*, it is clea
r that whe
n
R=0, we have:
∆
∗
(
9
)
Whe
r
e VSI terminal voltag
e V is:
1
0
(
1
0
)
Whe
r
e
E i
s
th
e VSI d
c
volt
age
and
s th
e solid
-state
swit
ch
statu
s
. Wh
en
∆
i is greater than
zero
and beyo
nd the tolera
nce, s is co
ntrolle
d to
be at lower level
s=0
and therefo
r
e (V-V
*
)<0 (n
ote
the dc volta
g
e
sh
ould
be b
i
g eno
ugh fo
r effe
ctive cu
rrent tra
cki
ng)
whi
c
h ma
ke
s
∆
i to reduc
e
.
In
the sam
e
wa
y if
∆
i<0 a
n
d
beyond th
e
toleran
c
e,
s i
s
controll
ed t
o
be at hi
gh
er-l
evel s=1
and
therefo
r
e (V
-V
*
)>0 which make
s
∆
i t
o
incr
ea
se.
Th
e co
rre
sp
ond
i
ng hysteresi
s
cu
rr
ent co
n
t
rol
block dia
g
ra
m is sh
own in
Figure 4.
(a)
(b)
Figure 4. Single pha
se VS
I and HCC
4. Contr
o
l and Coordina
ti
on Scheme
4.1. MPPT Algorithm
In [15] a
sim
p
le hyb
r
id
m
e
thod
ha
s b
een
pro
p
o
s
e
d
for MPPT
of sol
a
r arra
ys. Thi
s
algorith
m
con
s
ist
s
of two
st
age
s; the first one i
s
to e
s
timate the volt
age of m
a
ximum po
we
r poi
nt
(V
MP
P
) and th
e second i
s
to track the exact maximu
m
powe
r
point usin
g the cla
ssi
c Pertu
r
bat
ion
and O
b
servat
ion (P&O
) wit
h
a sm
all am
plitude a
nd fr
eque
n
cy of p
e
rturbation
s
. In the first
sta
ge,
V
MPP
is cal
c
ul
ated u
s
ing E
quation
(5
) which i
s
a
lin
e
a
r e
quation
i
n
term
s of te
mperature.
Using
that method, there i
s
no n
eed to disco
n
nect the
sola
r panel in o
r
d
e
r to mea
s
u
r
e the open
ci
rcuit
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243
voltage. Thi
s
method
ha
s lowe
r
power oscillati
on
s
and hi
ghe
r e
fficiency
also
better tracki
ng
perfo
rman
ce i
n
rapid
cha
n
g
e
s of light intensity and te
mperature.
In this sectio
n, an improv
ed hybrid me
thod
for MPPT will be pro
posed. In this method
instea
d of
ca
lculatin
g V
MP
P
, the cu
rrent
of the m
a
ximum p
o
we
r
point (I
MP
P
) is cal
c
ulate
d
.
This
lead
s to an improve
d
efficiency an
d hig
her a
c
curacy
[16]. The overall algo
rithm
of the improv
ed
hybrid MPPT
method ha
s been sh
own
in Figure 5.
The relation
between o
u
tput current a
n
d
voltage of PV array ha
s be
en sho
w
n in
Equation
(1).
In this eq
uati
on, three
pa
rameters of ‘a’
,
R
S
and R
P
a
r
e
n
o
t given by t
he ma
nufa
c
ture
r. In data
s
he
et of a
given PV array
there
is
usu
a
lly
three pi
nts of
V-I cha
r
a
c
teristics given
b
y
the
manufa
c
ture
r whi
c
h are sho
r
t
ci
rcuit
curre
n
t,
open
circuit voltag
e and volta
g
e
and
cu
rre
n
t of maximum po
we
r p
o
int in sta
n
d
a
rd atm
o
sph
e
ric
con
d
ition of 1
000
w/m
2
and
25
°
C. By sub
s
tituting the
s
e three
point
s in Equation
(12) a
nd
solvi
n
g
set of Equatio
n of (1) th
ree
para
m
eters o
f
‘a’, R
S
and R
P
will be det
ermined.
,,
,,
,
,
,,
,,
,
,,
,,
,
,
exp
(
)
1
0e
x
p
(
)
1
,
,
ex
p
(
)
1
SC
n
S
SC
n
S
SC
n
P
V
n
O
n
Tn
P
OC
n
O
C
n
P
Vn
O
n
S
P
Tn
P
MP
P
n
S
M
P
P
n
S
MP
P
n
P
V
n
O
n
Tn
P
IR
IR
II
I
aV
R
VV
II
a
R
R
aV
R
IR
IR
II
I
aV
R
(
1
1
)
To estimate the maximum
powe
r
point curre
n
t in each atmo
sph
e
r
ic conditio
n
we nee
d
the sho
r
t circuit current in
that atmosp
heri
c
co
nditio
n
. In previou
s
method
s, the mea
s
ure
m
en
t
wa
s d
one
by
disco
nne
ctin
g the l
oad
a
c
tually sh
ort
ci
rcuitin
g
the
t
e
rmin
als of t
he p
anel. In
this
proposed M
PPT method, the shor
t circuit current will be cal
c
ulated using mathemati
c
al
equatio
ns
an
d mea
s
u
r
em
ent of outp
u
t
voltage an
d
current of PV.
The in
stanta
n
eou
s value
s
o
f
voltage, current an
d temp
eratu
r
e
of the solar pa
nel
are
mea
s
u
r
e
d
and
I
PV
in
whi
c
h i
s
the
only
variable
de
p
ende
nt on li
ght inten
s
ity in an
d al
so
to tempe
r
at
ure
will
be
cal
c
ulate
d
u
s
ing
Equation
(1
2). In this
equ
a
t
ion, V
T
and
I
O
whi
c
h
are
tempe
r
ature d
epen
dent
are
upd
ated
usi
n
g
tS
VN
K
T
q
and Equatio
n (3).
e
xp(
)
1
SS
PV
O
TP
VI
R
V
I
R
II
I
aV
R
(
1
2
)
Knowin
g I
PV
,
the nonlin
ear Equation (12
)
will be
solv
ed iteratively to calculate I
SC
. This
iterative equa
tion will be re
peated m ti
mes an
d in each iteration, I
SC
of the previous iteration
will
be sub
s
tituted (Equ
ation
(13
)). After
m iteration
I
SC
no long
er varie
s
which is in
dicative of
conve
r
ge
nce
of the short
circuit curr
e
n
t. ‘m' is a small integer
becau
se I
PV
,
whi
c
h is the
first
estimation of I
SC
, is very cl
ose to it. In o
t
her
words i
n
a few ite
r
ati
ons I
SC
will
b
e
found
with
a
n
accepta
b
le a
pproxim
ation.
,1
,,
,1
,1
e
xp(
)
1
1
,
2
,
...
,
SC
P
V
SC
m
S
S
C
m
S
SC
m
P
V
O
TP
SC
S
C
m
II
IR
I
R
II
I
aV
R
mm
I
I
(
1
3
)
In the pro
p
o
s
ed metho
d
, the fine tunin
g
loop is
used
to corre
c
t the
cal
c
ulation
o
f
the I
SC
to compe
n
sate the effect of the measu
r
e
m
ent er
ror a
n
d
possibl
e model mismatch of solar p
a
n
e
l.
In this meth
o
d
, In ca
se
of small va
riatio
ns of tem
perature
and I
PV
, the fine tunin
g
loop
re
gulat
es
output p
o
we
r.
Since
I
PV
varies
with
radia
t
ion inten
s
ity, it can
be
infe
rre
d that the
fine tunin
g
lo
o
p
will be run
when atmo
sp
h
e
ric
co
ndition
s are ap
prox
i
m
ately con
s
t
ant. Con
s
eq
u
ently beca
u
se in
rapid
cha
nge
s of atmosp
h
e
ric
con
d
ition
s
the fi
ne tuning loop i
s
not run, the amplitude of
the
perturbations of the P&O
algorithm
does not need
to be great
whi
c
h will in
turn will resul
t
i
n
small vari
atio
ns of po
wer i
n
steady stat
e con
d
ition
s
arou
nd the o
p
timal value.
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6 : 238 – 248
244
Figure 5. The
flowch
art of the pro
p
o
s
ed
MPPT metho
d
4.2. DC Link
Voltage Con
t
rol
The ove
r
all
al
gorithm
of th
e DC lin
k volt
age
cont
rol
st
rategy h
a
s b
een
sho
w
n
in
Figu
re
6. This al
go
ri
thm con
s
i
s
ts of two main
mode
s. One
mode i
s
wh
en the PV g
enerates
po
wer
(P
PV
>P
min
). This p
o
we
r
wi
ll be delive
r
ed to the n
e
twork th
rou
gh the Hy
st
ere
s
is
Co
ntrolled
Inverter. The
other m
ode i
s
whe
n
the po
wer
gen
erate
d
by the PV is le
ss th
an th
e thre
shol
d P
min
.
In both mod
e
s
, V
DC
must
be greate
r
th
an V
min
in order to h
a
ve
a sati
sfacto
ry
operation of
th
e
inverter.
In the first m
ode, the ge
n
e
rated
po
wer of the PV is delivere
d
to the DC lin
k throug
h a
boo
st co
nvert
e
r. When V
DC
is le
ss th
an t
he thre
sh
old
(V
DC-min
), accordin
g to the
control
strate
gy
the po
wer
of the PV is fed
to the DC lin
k to mainta
i
n
in acce
ptabl
e ran
ge. In th
is situ
ation, the
power
delivered to P
g
is
ze
ro, in oth
e
r
word
s the
po
wer of the
PV is solely d
edicated to
cha
r
g
e
the capa
citor
of the
DC lin
k. In a
situatio
n whe
r
e V
DC
is
gr
ea
te
r
than
V
DC-min
a po
rtion
of the P
PV
is used to
charge the
capacitor
and the rest will
be de
livered to
the grid
through the invert
er.
Whe
n
V
DC
reach
e
s maxim
u
m allowable
voltage V
DC-m
ax
, the power of the PV is a
ll fed to the grid.
Whe
n
V
DC
is
betwe
en V
DC-min
and V
DC-ma
x
there is a li
near
rel
a
tion
ship bet
wee
n
the po
we
r u
s
e
d
to charge the
cap
a
cito
r and
the V
DC-ma
x
-V
DC
.
Figure 6. The
overal algo
rit
h
m
of the DC link voltage control
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4.3. Safe Op
eration Zo
ne
(SOZ)
In this pa
pe
r, we a
s
sume
that the no
minal
voltage
of the di
stri
bution n
e
two
r
k i
s
the
voltage which
is me
asure
d
in ze
ro
pen
e
t
ration level.
Any highe
r voltage i
s
ove
r
voltage an
d a
n
y
lowe
r volta
g
e
is un
dervolt
age.
The
mi
cro
g
ri
d in
thi
s
re
sea
r
ch
compri
se
s
of
a PV p
o
wer
plant
with b
o
o
s
t co
nverter an
d h
y
stere
s
iscu
rrent controll
ed
inverte
r
a
nd
a DC li
nk an
d
also a
con
s
tant
load. This mi
cro
g
ri
d is con
necte
d to the grid
via an O
n
-loa
d Tap
Changi
ng tran
sformer
(OL
T
C).
The traditio
n
a
l distri
bution
system h
a
s
been
d
e
si
gn
ed as
a unid
i
rectio
nal p
o
w
er flo
w
netwo
rk. A
s
more
and
m
o
re
distri
bute
d
re
ne
wable
sou
r
ce
s a
r
e
con
n
e
c
ted t
o
the g
r
id, t
he
origin
al unidi
rection
a
l network
will be
chang
ed towa
rd the bi
dire
ctional network in the future.
This chan
ge
bring
s
utility operation issues
su
ch
as the voltage rise p
r
obl
em
cau
s
ed by the
reverse
po
wer flo
w
f
r
om
the di
strib
u
te
d rene
wa
ble
energy g
ene
ration. Fig
u
re
7 illu
strate
s t
h
e
one li
ne
dia
g
ram
of the
simplifie
d d
i
stributio
n
n
e
t
work. Th
ere
is
a di
strib
u
ted g
ene
rat
o
r
con
n
e
c
ted to the load si
de.
The gene
rat
o
r voltage V
G
can b
e
app
ro
ximately expressed in:
(
1
4
)
Whe
r
e V
2
i
s
the su
bstatio
n
se
co
nda
ry
bus volta
ge,
X is the fe
ed
er lin
e re
act
ance an
d R
is
feeder li
ne
re
sista
n
ce. P
G
and Q
G
a
r
e t
he re
al an
d reactive p
o
we
r provided
by the gen
erato
r
,
r
e
spec
tively. P
L
and Q
L
are
the real and
rea
c
tive power co
nsume
d
by the load.
Figure 7. One
line diagram
of
a typical grid con
n
e
c
ted
DG
Equation (14
)
sho
w
s th
at the gen
erator
voltage
may
be high
er tha
n
the upp
er-li
m
it if th
e
netwo
rk X/R
ratio is
relatively low and t
here i
s
a
si
gn
ificant reve
rse power flo
w
. One solution
is
that the g
e
n
e
rato
rs can
absorb
a
rel
a
tively la
rge
rea
c
tiv
e
po
w
e
r to
co
mpe
n
sate
the
re
v
e
rse
power flo
w
.
The alte
rn
ative solutio
n
is th
at th
e su
bstatio
n
se
con
d
a
r
y voltage can
be
corre
s
p
ondin
g
ly controll
ed
or the real p
o
we
r inje
ction
to the grid ca
n be de
cre
a
sed.
Currently,
standards such as
IEEE 1547 and
UL1741 state that t
he PV invert
er “shall
not actively regulate the voltage
at the PCC.” Th
erefor
e, PV syste
m
s are desi
g
ned to ope
rat
e
at
unity powe
r
factor
(i.e., provide only active pow
er) b
e
ca
use this condition
will produ
ce the m
o
st
real p
o
wer a
n
d
ene
rgy. Thi
s
limitation i
s
a matte
r of a
g
ree
m
ent in
st
ead of a te
ch
nical o
ne; ma
ny
inverters h
a
ve the capa
bil
i
ty of
providing re
active p
o
we
r to the
grid in
additi
on to the a
c
t
i
ve
power ge
nera
t
ed by their PV cells. The a
m
ount of rea
c
tive powe
r
(
Q
) availabl
e from the inverter
depe
nd
s o
n
it
s
rating
s
(
S
)
and th
e a
c
tive po
we
r
(
Ppv
)
sup
p
lied
by
the PV a
rray.
Co
nsequ
ently,
the inverter
can use its ent
ire ratin
g
to supply
Q
if
Ppv
equ
als
ze
ro
(there i
s
no
sun
)
, and at the
other extre
m
e, it has no
Q
capability if
Ppv
eq
ual
s
S
. Some
Q
capability can
al
ways be retai
ned
by over-sizin
g the inverter. In addition to the c
ontinu
ous rea
c
tive power supp
o
r
t, inverters
can
operate very fast (millise
c
on
ds to micro
s
e
c
o
n
d
s
with high switchi
ng freq
uen
cy inverters)
comp
ared to cap
a
cito
rs, which
can
cau
s
e swit
chin
g transi
ents.
In this paper, a safe operation
zone (SOZ
) will be proposed accordi
ng to the load
impeda
nce o
f
grid
and
tra
n
sformers
an
d a
safe
pe
n
e
tration l
e
vel
will
be
deriv
ed. SOZ i
s
t
he
zon
e
in which a
c
cording
to nominal
irradi
an
ce
and temp
era
t
ure conditio
n
s of a cert
ain
geog
rap
h
ic site, load leve
l and
grid
ch
ara
c
teri
stics,
the devised
penetration l
e
vel wo
uld
n
o
t
cau
s
e ove
r
voltage
s abov
e the ANSI standa
rd
s.
5. Simulation and Discu
ssions
5.1. Sy
stem
Des
c
ription
The mo
del of
PV array ha
s be
en p
r
e
s
e
n
ted in
se
ctio
n 3. The
PV array is
con
n
e
cted to
th
e
in
ve
r
t
er
via
a
bo
os
t c
onve
r
te
r
.
A
c
a
pa
c
i
to
r is
co
nn
ected to
the
output
of the
boo
st co
nvert
e
r
to provid
e tra
n
sie
n
t ene
rgy
storage
ca
p
ability. A hy
stere
s
is control
l
ed inve
rter
conne
cts
DC li
nk
to the PCC via a RL filter. Distrib
u
tion netwo
rk
lin
es are simul
a
te
d as RL imp
edan
ce
s whi
c
h
Evaluation Warning : The document was created with Spire.PDF for Python.
ISSN: 25
02-4
752
IJEECS
Vol.
1, No. 2, February 201
6 : 238 – 248
246
con
n
e
c
t grid
voltage sou
r
ce to the OLTC. A schem
at
ic of the overall powe
r
sy
stems is
sho
w
n
in
Figure 8.
Figure 8. The
block diag
ra
m of the grid con
n
e
c
ted ph
otovoltaic po
wer pl
ant with
high
penetration le
vel
5.2. Simulati
on Results
Table
2
and
3 sho
w
the
PV and l
oad
po
we
r, volta
ge of
the
gri
d
at th
e PCC a
nd th
e
corre
s
p
ondin
g
pen
etratio
n
level for th
re
e differe
nt
lo
ading
s. As
ca
n be
se
en, a
s
the
pen
etra
tion
level in
cre
a
ses, the
voltag
e of
comm
on
co
uplin
g al
so raise
s
. T
h
e
maximum
a
c
cepta
b
le volt
age
in this mi
crog
rid i
s
39
5v an
d co
nsequ
ent
ly t
he maximum pen
etrati
on level fo
r 1
00kw lo
adin
g
in
this microg
rid
is 28%. For
70kw loa
d
ing,
maximum pe
netration leve
ls is 38%.
Table 2. Diffe
rent pen
etrati
on levels of PV for 100kw n
o
minal lo
cal l
oad
PV po
w
e
r
Load
po
w
e
r
v
o
lta
g
e
PL
0
100.05
380.1
0
10.5
102.8
385.3
10.21401
22.8
106 391.3
21.50943
35.25
109.2
397.2
32.28022
41.5
1
10.8
400.13
37.45487
47.7
1
12.4
403
42.43772
58.6
1
15.5
407.6
50.73593
Table 3. Diffe
rent pen
etrati
on levels of PV for 100kw n
o
minal lo
cal l
oad
PV po
w
e
r
Load
po
w
e
r
v
o
lta
g
e
PL
0
70.04
380.39
0
10.5
72.15
385.8
14.55301
19.72
73.95
390.5
26.66667
29.05
75.6 395.15
38.42593
38.4
77.4 399.75
49.6124
47.75
79.1 404.19
60.36662
Evaluation Warning : The document was created with Spire.PDF for Python.
IJEECS
ISSN:
2502-4
752
High Pen
e
trat
ion PV in Dist
ribution
Networks
, Desig
n
and Control
(Ashkan Mo
ha
mm
adi)
247
The amo
unt of penetration level coul
d
be more if the OLT
C
is
set acco
rdin
gly. For
example if
de
sire
d m
a
ximu
m pen
etratio
n
level i
s
6
0
%
the outp
u
t vol
t
age of
OLT
C
sh
ould
be
set
so that in n
o
rmal ope
ratio
n
of the microgrid
(no
m
in
al load a
nd
PV penetrati
on) the volta
ge of
PCC remain
s in allowed
range. Since the amou
nt
of powe
r
gen
erated by PV is a functio
n
of
sola
r irradi
an
ce a
nd it is n
o
t con
s
tant th
r
oug
hout the
day, the outp
u
t voltage of
OLT
C
should
be
desi
gne
d for
maximum ex
pecte
d solar i
rra
dian
ce
(m
aximum expe
cted p
enetration level). In t
h
is
situation, the
limiting facto
r
would not be
the overv
o
ltage
p
r
o
b
le
m issue
s
li
ke
clou
d p
a
ssa
ge,
load chan
ge
and THD rate
could limit the penetration
level.
Clou
d pa
ssa
ge whi
c
h im
p
o
se
s a
sha
d
o
w
on PV arra
ys su
ddenly
decrea
s
e
s
th
e amou
nt
of irra
dian
ce
on PV array.
Con
s
e
que
ntly T
he am
oun
t of power
g
enerated
by PV powe
r pl
ant
decrea
s
e
s
. T
he effect
of this d
e
crea
se
in gen
er
ate
d
power i
s
p
o
wer d
e
ficien
cy
whi
c
h n
e
ed
s
to
be
comp
en
sated by a
ddit
i
onal g
r
id
po
wer. In
crea
se of g
r
id p
o
w
er will
also
increa
se
the
grid
curre
n
t an
d
will result in
voltage d
r
o
p
in P
C
C du
e to di
strib
u
tion n
e
two
r
k i
m
peda
nce. T
he
amount of voltage dro
p
d
epen
ds on th
e distrib
u
ti
on
network imp
e
dan
c
e, loadi
n
g, penetration
level and cha
r
acte
ri
stics of the cloud.
Figure 9
and
Figure 1
0
sh
ow th
e si
mul
a
tion of a
cl
o
ud p
a
ssag
e o
v
er the
micro
g
rid
and
its effe
cts
on
voltage
profile an
d p
o
we
r b
a
la
n
c
e
of
microg
rid
a
nd di
stri
butio
n net
wo
rk.
T
h
e
sha
d
o
w
of th
e clo
ud
ha
s b
een
simul
a
te
d as a 2
5
%
d
e
crea
se i
n
so
lar irra
dian
ce
over the
pe
ri
od
of 1s. As
ca
n be
see
n
, the voltage of
PCC
sud
d
e
n
ly decrea
s
e
s
a
s
the p
o
wer inje
cted
b
y
PV
decrea
s
e
s
d
ue to de
cre
a
se in
sola
r irradi
an
ce.
The short
c
o
m
ing of po
wer ne
ed
s to be
comp
en
sated
by grid po
wer and
as th
e cu
rre
nt flow
s from the
grid to PCC, it will cau
s
e
more
voltage d
r
op
on imp
edan
ce of the g
r
id.
In this
simula
tion it is a
s
su
med that the
nominal
voltage
of PCC ha
s b
een set according to 60% of penetrati
o
n
level by means of OLT
C
.
In other words,
in 60% of
pe
netration
leve
l, and
with 10
0kw of
lo
adin
g
, the voltage
of PCC ha
s
been
set to
3
80v
usin
g OLT
C
.
The
power of
grid,
load
an
d PV is sho
w
n in
Figu
re
9. As
ca
n b
e
seen, the
sho
r
tcoming
power i
s
provided
by gri
d
i
mmediat
ely. I
n
ph
otovoltai
c
p
o
wer
plant
s
with hi
ghe
r
power
rate
s, t
h
e
respon
se tim
e
of grid’
s
synch
r
on
ou
s g
enerators
co
uld al
so p
u
t
a co
nstraint
on the p
enetration
level as the
shortcomin
g p
o
we
r could n
o
t immediat
el
y be provided by grid. Thi
s
limit is imposed
by the tra
n
si
ent follo
wing
cap
abilities (ramp
rate
s)
of the
conventi
onal
gene
rat
o
rs [70]. As can
be se
en in fi
g (10
)
, in 25
% decrea
s
e i
n
sola
r irradi
ance, the voltage of PCC drop
s to 3
70.
5v
whi
c
h is 9
7
.5
% of nominal voltage whi
c
h
is in
accepta
b
le ran
ge a
c
cordin
g to utility standards.
Figure 9. Power b
a
lan
c
e
of micro
g
rid a
s
imposed by cl
oud pa
ssag
e with 25% sol
a
r
irra
dian
ce de
cre
a
se
Figure 10. Voltage of PCC
in 25% sola
r
irra
dian
ce de
cre
a
se
Figure 11 sh
o
w
s the po
we
r balance of micro
g
ri
d
in 50% decrease in solar irradiance. As
can
be
seen,
from 0.5
s
to
1
.
5s
sola
r i
rra
d
i
ance
d
e
crea
se
s 5
0
% an
d
photovoltai
c
power
provid
ed
by PV also decrea
s
e
s
. Th
e corre
s
p
ond
ing PCC volt
age is sho
w
n
in Figure 12.
By increase of
grid po
we
r, the voltage drop also incre
a
se
s an
d
will
result in voltage sag in PCC. The volt
age
sag a
c
cou
n
ts for 5.2% of nominal voltag
e.
Evaluation Warning : The document was created with Spire.PDF for Python.