IWJMYAR POWER CONDITIONING UNIT FOR PHOTOVOLTAIC APPLICATIONS.
Parag B h i d e and S.R.Bhat
C e n t r e f o r E l e c t r o n i c s D e s i g n and Technology,
I n d i a n I n s t i t u t e o f Science,
560012.
Bangalore,
INDIA.
2.PHOTOVOI T A I C ARRAY CHflRACTERISTICS
ABSTRACT
T h i s paper d e a l s w i t h
some d e s i g n i s s u e s
involved
i n a p h o t o v o l t a i c power
conditioning
system. The
power
conditioning
an0
u n i t (PCUI i s b u i l t i n a mooular way
microcontroller
for
i s c o n t r o l l e d by a
The
maximum p o w e r p o i n t t r a c k i n g (MPPT).
choice
o f t h e power c o n v e r t e r and
suitable
pv-array
configuration
are
also
discussed.
Power
MOSFETs a r e used as
s w i t c h i n g e l e m e n t s . E x p e r i m e n t a l system
1kW c a p a c i t y i s a l s o e x p l a i n e d ,
with
of
battery
c h a r g i n g as a s p e c i f i c
application.
1.INTRODUCTION
This
paper
discusses the
design
of
a
modular power c o n d i t i o n i n g system used i n
photovoltaic
applications.
This
power
c o n d i t i o n i n g system a l s o i n c o r p o r a t e s t h e
MPPT c o n t r o l l e r
t o obtain
the
maximum
p o s s i b l e energy f r o m t h e pv
array.
The
system i s d e s i g n e d i n s u c h a way t h a t t h e
total
power o u t p u t i s s h a r e d by
several
power
c o n d i t i o n i n g u n i t s (PCUs)
running
i n p a r a l l e l . The power c o n d i t i o n i n g u n i t s
are
used f o r m a t c h i n g t h e l o a d w i t h
the
pv a r r a y . Each o f t h e PCU i s powered f r o m
a s e p a r a t e pv a r r a y . The PCUs d e l i v e r t h e
40
power
i n t o t h e same l o a d , w h i c h i s a
volt
battery
bank.
A l l
the
PCUs a r e
for
controlled
by a pP based c o n t r o l l e r
optimum u t i l i z a t i o n o f
the
available
energy.
The
parallel
configuration
of
PCUs o f f e r s s e v e r a l advantages. The power
required
to
be h a n d l e d by
one PCU i s
reduced,, w h i c h makes t h e
PCU
design
This
s i m p l e r and make i t more e f f i c i e n t .
c o n f i g u r a t i o n o f f e r s t h e system e x p a n s i o n
D p t i o n a t a v e r y low c o s t , f o r i n c r e a s i n g
systhe
power h a n d l i n g c a . p a c i t y o f t h e
tem. The system r e l i a b i l i t y i s
increased
as
total
s h u t down i s n o t
required
in
zase o f a f a i l u r e . A c e n t r a l i z e d c o n t r o l l e r o f f e r s t h e ease o f c o n t r o l and s u p e r 1
vision
of
t h e e n t i r e system.
Figure
shows
an
e x p e r i m e n t a l system o f
1 kW
c a p a c i t y which
i s explained
in
this
paper.
0-7803-0695-3/92
$3.00
The
power o u t p u t o f a p v a r r a y
depends
gharacterissignificantly
on t h e s r r s v
.
.
and t h e L g b d i n a E p n d i t i o ns.
Typical
I-V
and P-V c h a r a c t e r i s t i c s o f
a
solar
cell
a r e shown i n F i g u r e 2. I t i s seen
cell
that
t h e power o u t p u t f r o m a s o l a r
i s maximum o n l y i f t h e l o a d i s o p e r a t e d
a t maximum power p o i n t (MPP). The MPP
i s
not
a f i x e d p o i n t . The 1-V
characterist i c s o f s o l a r c e l l s h i f t w i t h changes
i n
i n s o l a t i o n ( s o l a r r a d i a t i o n i n kw/rn2) and
the
cell
temperature.
Hence MPP a l s o
becomes a f u n c t i o n o f i n s o l a t i o n and c e l l
t e m ' p e r a t u r e Cl]. F i g u r e 3 shows t h e l o c u s
o f MPP as t h e i n s o l a t i o n changes. As
the
MPP changes t h e power d e l i w r e d
to
the
l o a d i e reduced. T h e r e f o r e a power c o n d i point
t i o n i n g u n i t , c a l l e d m a x i m u m power
t r a c k e r (MPPT), i s r e q u i r e d t o match
the
l o a d t o t h e pv a r r a y i r r e s p e c t i v e o f
the
i n the
load
variations
in
t h e MPP o r
i t s e l f . The maximum power i s
transferred
o n l y u n d e r t h e matched c o n d i t i o n s .
a
3.POWER CONDITIONINE UNIT (PCU)
T h i s p a r t i c u l a r system i s used f o r c h a r g for
i n g a 48 v o l t b a t t e r y bank. The need
an
MPPT,
i n t h i s application
i s
quite
c l e a r as b o t h MPP and t h e b a t t e r y v o l t a g e
a r e c h a n g i n g C4J. A PWM c o n t r o l l e d DC-DC
c o n v e r t e r can be used i n t h i s a p p l i c a t i o n
for
power c o n d i t i o n i n g and m a t c h i n g .
In
1 KW i s
this
system a t o t a l power o f
s h a r e d by two PCUs o p e r a t i n g i n p a r a l l e l .
The
power s h a r e d by each PCU i s d e c i d e d
by
t h e MPP o f t h e pv a r r a y c o n n e c t e d
to
i t . PCU i s a PWM c o n t r o l l e d DC-DC
converter.
The o p e r a t i n g p o i n t on pv
array
i s adjusted
t o MPP by
controlling
the
d u t y c y c l e o f t h e PCU. V a r i o u s
converter
t o p o l o g i e s can be employed f o r
PCU.
In
the
p r e s e n t work, a buck
converter
i s
comparaused
f o r power c o n d i t i o n i n g . A
t i v e study of various
converters
shows
t h a t buck c o n v e r t e r r e q u i r e s l o w e r d e v i c e
ratings
than o t h e r
configurations,
for
this
application.
The o u t p u t
current
for
( c h a r g i n g c u r r e n t ) i s r o u g h l y 10 A
each PCU a t Deak i n s o l a t i o n c o n d i t i o n .
'1992 IEEE
I t s h o u l d be n o t e d t h a t
the i n p u t current
( a r r a y c u r r e n t ) f o r t h e buck c o n v e r t e r i s
o f discontinuous (switching) nature. This
* ' e s u l t s i n an u n s t a b l e o p e r a t i n q p o i n t on
pv
a r r a y w h i c h s w i t c h e s between MPP and
t h e open c i r c u i t and r e s u l t s i n t h e
loss
of
energy.
Under
such c o n d i t i o n
true
maximum power p o i n t o p e r a t i o n c o u l d n e v e r
be a c h i e v e d .
To a v o i d
this
problem a
l a r g e c a p a c i t o r i s used a t t h e i n p u t o f
offers
t h e power c o n d i t i o n i n g u n i t . T h i s
a
s t a b l e o p e r a t i n g p o i n t on
the
array.
The c a p a c i t o r s t i l l i n t r o d u c e s a
switchi n g f r e q u e n c y r i p p l e on t h e a r r a y v o l t a g e
in
because o f c h a r g i n g arid
discharging
e v e r y c y c l e . T h i s r i p p l e can be k e p t t o a
by
using
a
minimum a c c e p t a b l e
value
pF
l a r g e r c a p a c i t o r a t t h e i n p u t . A 2000
capacitor gives the satisfactory results.
3.1
3.2
NOSFET HASEU POWER CONVEHl-EH
FI MOSFET i s used a s a s w i t c h i n g d e v i c e a s
i t 1 5 easy t o c o n t r o l and can be o p e r a t e d
a s compared
to
at
higher
frequencies
BJTs. F o r t h e s e l e c t e d l o a d and pv
arr-ay
c o n f i g u r a t i o n , i t can be shown t h a t
the
to
duty
c y c l e m u s t be v a r i e d f r o m 0.33
0.90 t o a c h i e v e MPP o p e r a t i o n . The f l o a t ing
g a t e d r i v e used i s shown i n F i g u r e
5.
The d r i v e r t r a n s i s t o r s
(01.02) a r e
cycle.
d r i v e n o u t o f phase, i n a l t e r n a t e
The
diity c y c l e r a t i o f o r the
individual
The
t r a n s i s t o r v a r i e s between 0 and 0 . 5 .
that
the
two
secondaries
a r e ORed so
r e s u l t a n t d u t y c y c l e can be v a r i e d f r o m 0
to
1 . 0 . The g a t e d r i v e c i r c u i t
greatly
a f f e c t s the e f f i c i e n c y o f the converter.
A f a s t t u r n o f f c i r c u i t i s used t o
minithe
gate
mize
t h e t u r n o f f losses.. A l l
d r i . v e c i r c u i t components a r e o p t i m i z e d
using s p i c e s i m u l a t i o n t o achieve
higher
e f f i c i e n c y . The s w i t c h i n g d e v i c e must
be
carefully
selected
to
obtain
a
high
92% ( a t
efficiency.
The e f f i c i e n c y o f
f u l l l o a d ) and 83% ( a t 30% l o a d ) has been
a c h i e v e d d u r i n g t h e e x p e r i m e n t a l phase o f
the project.
PV ARRAY SIZING
It i s
important t o note t h a t
the
buck
converter
can match t h e l o a d t o
the
pv
a r r a y ift h e l o a d i s l e s s t h a n t h e o p t i mum
load
f o r t h e pv a r r a y
Therefore
pv
a r r a y s must be c o n f i g u r e d s u c h
that
the
w o r s t c a s e ( l o w e s t ) MPP v o l t a g e
i s
more
than
t h e maximum o u t p u t
voltage.
Kyocera p o l y c r y s t a l l i n e
pv
panels are
used
i n t h i s work. Each p a n e l
i s
rated
4 4 peak w a t t w i t h t h e open
circuit
for
simuv o l t a g e o f 20 v o l t s . S p i c e c i r c u i t
l a t i o n was
used
to
simulate
the
i-v
characteristics of
t h e pv
panel
under
different
insolation
and
temperature
c o n d i t i o n s . The S p i c e model used i s shown
i n f i g u r e 4. I n t h i s simulation a c u r r e n t
s o u r c e whose v a l u e i s changed a c c o r d i n g
to
t h e i n s o l a t i o n , i s used
i n
parallel
PV
with
a diode
s t r i n g t o model
the
module. The d i o d e p a r a m e t e r s i n t h e s p i c e
that
the
model
a r e chosen c a r e f u l l y so
s i m u l a t e d c h a r a c t e r i s t i c s match w i t h
the
c h a r a c t e r i s t i c s s u p p l i e d by t h e rnanutacturer.
The S p i c e s i m u l a t i o n a l s o
takes
care o f
t h e temperature v a r i a t i o n e f t h e PV
fects.
The
r a t e d peak power o f
module
i s available only a t the
refer1
ence c o n d i t i o n s i . e . an i n s o l a t i o n o f
kw/mz and c e l l t e m p e r a t u r e o f 27 O C .
But
in
p r a c t i c e , h i g h i n s o l a t i o n i s always
accompanied by h i g h c e l l t e m p e r a t u r e s , i n
w h i c h c a s e t h e MPP v o l t a g e o f a pv
array
i s found t o be t h e l o w e s t . T h i s f a c t must
.
3.3 M
E CONTROLLER
Various
schemes have been
proposed t o
implement t h e maximum power p o i n t
trackbased on
ing
function [ Z ] . A technique
the
dP/dV method i s implemented i n
this
work. The pP based a p p r o a c h used i n
this
project,
o f f e r s a s i m p l e r and
a stable
I t also o f f e r s several
other
solution.
useful
f e a t u r e s l i k e good
steady
state
in
and
t r a n s i e n t response,
flexibility
the c o n t r o l a l g o r i t h m which a r e discussed
i n t h i s paper.
The
system i s b u i l t around
80C31 m i c r o
c o n t r o l l e r w h i c h has a p o w e r f u l
instruction
set, with b u i l t i n multiply
funcbit,
8
tion.
The c o n t r o l l e r u s e s an 0
ADC
interface for
reading the
channel
voltage
and c u r r e n t o f each o f
the
PV
array.
These v a l u e s a r e used f o r
calcul a t i n g t h e i n s t a n t a n e o u s power o u t p u t
of
for
the
pv a r r a y . DOC i n t e r f a c e i s used
g i v i n g t h e c o n t r o l s i g n a l s t o t h e PCUs.
3.4
be c o n s i d e r e d b e t o r e s i z i n g t h e pv
array
for
the
required application.
Spice
s i m u l a t i o n r e s u l t s a r e used t o c o n f i g u r e
500 w a t t p v a r r a y s u c h t h a t
MPP
the
o p e r a t i o n i s p o s s i b l e a t an i n s o l a t i o n o f
1 kw/m2 and t h e c e l l t e m p e r a t u r e o f 60 O C
(Vmp = 12 v o l t ) . T h i s r e s u l t s i n c o n n e c t Two
such
ing six
pv p a n e l s i n s e r i e s .
get
s t r i n g s a r e connected i n p a r a l l e l t o
t h e r e q u i r e d power and v o l t a g e r a t i n g s .
CONTROL CILGORITHM
FI
t i m e r i n t e r r u p t i s generated a t
every
action.
lOOmS f o r i n i t i a t i n g t h e c o n t r o l
I n i t i a l l y on s t a r t u p , t h e d u t y c y c l e
is
set
t o ZERO and i s g r a d u a l l y
increased.
A t
each i n t e r r u p t t h e s i g n o f dP/dV
i s
2
c a l c u l a t e d . I t can be seen f r o m f i g u r e
that
dP/dV i s p o s i t i v e on t h e l e f t
hand
side
(short c i r c u i t side)
of
t h e MPP
i t i s n e g a t i v e on t h e r i g h t
hand
while
side
and
z e r o a t MPP. T h e r e f o r e
for
a
buck
converter,
the
pulse width
(D)
709
should
be i n c r e a s e d f o r
n e g a t i v e dP/dV
D s h o u l d be reduced f o r
positive
and
dP/dV so as t o s h i f t t h e o p e r a t i n g p o i n t
towards t h e MPP C i t is i m p o r t a n t t o n o t e
dP
that
a d e c i s i o n based on t h e s i g n o f
a l o n e does n o t r e s u l t i n a s t a b l e o p e r a t ing
point].
I n the
steady
state,
the
o p e r a t i n g p o i n t o s c i l l a t e s a b o u t t h e MPP.
The
change
i n the duty
cycle d i r e c t l y
a change i n
the
operating
r e f l e c t s as
p o i n t . I n t h e s t e a d y s t a t e t h e change
in
D i . e . s t e p s i z e 1s k e p t v e r y s m a l l t o
keep t h e o p e r a t i n g p o i n t as c l o s e t o MPP
as p o s s i b l e . A f a s t r e s p o n s e i s n e c e s s a r y
when t h e system i s powered up o r i n c a s e
o f p a s s i n g c l o u d s , so t h a t t h e system can
prevent
the
quickly
l o c k on to MPP and
l o s s o f e n e r g y . The c o n t r o l l e r s t o r e s t h e
s i g n o f dP/dV f o r t h e p a s t few c y c l e s
to
d e t e c t whether t h e system i s b e i n g o p e r system
ated
i n the steady s t a t e . I f the
i s n o t i n the steady s t a t e , the step s i z e
i s t h e s u i t a b l y m o d i f i e d t o l o c k back on
t o MPP as f a s t a s p o s s i b l e . T h i s dynamic
s t e p s i z e change i m p r o v e s
the
transient
response o f t h e system. The same
procedure
i s repeated f o r a l l the
PCUs connected
to
the
system.
The c o n t r o l l e r
structure
for
m a i n t a i n s a 16 b y t e d a t a
each o f t h e PCU. ( f i g u r e 6 ) T h i s d a t a
i s
PCU.
used f o r c o n t r o l l i n g t h e i n d i v i d u a l
The
flow
chart
depicting
the control
s u b - r o u t i n e i s g i v e n i n f i g u r e 7.
4.cONCLUSIO
NS
Concept o f
modular
power
conditioning
systems f o r p h o t o v o l t a i c a p p l i c a t i o n s has
w i t h i n t h i s paper,
with a
bren
dealt
as
the
particular
r e f e r e n c e to b a t t e r y
A MOSFET based power C o n d i t i o n i n g
load.
unit o f
LKW c a p a c i t y
i s discussed i n
to
d e t a i l along w i t h a c o n t r o l algorithm
track
t h e maximum power p o i n t .
Maximum
power f r o m each PV a r r a y i s e x t r a c t e d
in
chars p i t e o f any mismatch i n t h e a r r a y
acteristics.
One system c o n t r o l l e r
i s
c a p a b l e o f h a n d l i n g s e v e r a l PCUs, s i m p l i f y i n g t h e d e s i g n and r e s u l t i n g i n a l o w e r
that
c o s t o f t h e system. I t was o b s e r v e d
the
MPPT i n c r e a s e s t h e power
output
by
the
a b o u t 10 % o v e r a day as compared t o
d i r e c t connection.
REFERENCES
C1l.Solar
c e l l from b a s i c s
t o advanced
White.
systems,
by Chenming Hu and R.M.
McGraw-Hill p u b l i c a t i o n .
C2l.Step
up maximum power p o i n t
tracker
for
photovoltaic arrays,
Ziyad
Salameh
and
Daniel Taylor,
Solar
Energy V o l .
44 No.1, pp 57
61, 1990.
-
[3].Desiqn,
c o n s t r u c t i o n and f i e l d
experience o f
an I n d i a n s t a n d a l o n e
rural
p h o t o v o l t a i c s o l a r e n e r g y c e n t r e . A.Saha,
C. B h a t t a c h a r y a . S i x t h EC PVSEC
P.Basu,
537.
1985 pp 533
I t may be n o t e d t h a t t h i s t e c h n i q u e can
be used w i t h o t h e r t y p e s o f
dc
loads
( o t h e r t h a n b a t t e r y ) where t h e
resulting
l o a d v o l t a g e i s l o w e r t h a n t h e MPP v o l t age, as a buck c o n v e r t e r i s used here. I n
the
battery
charging application
the
system power s u p p l y i s d e r i v e d
using a
SMPS powered by
t h e b a t t e r y . I n t h e case
of
battery-less
systems,
separate arpowering
rangements
s h o u l d be made f o r
t h e c o n t r o l system.
-
C43 .Mismatch between b a t t e r i e s and two
module t y p e s pv a r r a y s , I n t e r e s t o f DC-DC
c o n v e r t e r s , by P. Gucher, J. A. Roger, S .
Massad,
J . P o s b i c , J. P i v o t ,
F o u r t h EC
PVSEC 1982 ,pp 330-334.
A11
the
protections
and
the
battery
status
a r e checked a t 1 second
interval
in
the
main program. The
main
program
a l s o d e t e r m i n e s t h e o p e r a t i n g mode o f t h e
system and
takes
care o f
the
status
i n d i c a t i o n s . The c o n t r o l l e r
monitors the
When
the
battery
i s
battery
voltage.
F u l l y charged t h e c o n t r o l l e r e n t e r s
into
3
F l o a t - c h a r g i n g mode i n w h i c h a
small
of
power i s t r a n s f e r r e d
to
the
amount
battery
bank
for
float
charging.
The
controller
also
indicates
the
undercharged s t a t e o f t h e b a t t e r y and p r o v i d e s
3 s i g n a l t o d i s c o n n e c t t h e l o a d from
the
A low power c o n d i t i o n
i s also
Jattery.
indicatt 1
on t h e f r o n t p a n e l
when
the
insolation
i s
poor,
say,less
than
3 0 W / sqm ,
710
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
4
CIRCUITS
1
I
I
t
3r
l
'r
lKW/m2
a
I
I
I-
za 2
a
3
0
$ 1
z
a
n
PANEL VOLTAGE (V 1
-
5
10
15
PANEL VOLTAGE (V 1
M
Fig.3 VARIATION I N M P P WITH INSOLATION.
Fig, 2 I - V CHARACTERISTICS OFA PV PANEL.
$.SUBCKT d e f i n i t i o n o f a
rSUBCKT x p a n e l a b
D1
D2
D3
D4
D5
s o l a r panel
a 1 diode
1 2 diode
2 3 diode
3 4 diode
4 5 diode
DJ4 33 34 d i o d e
D35 34 35 d i o d e
D36 35 b d i o d e
F i g , & SPICE MODEL.
-
.model
ends
711
diode DC(is=1.5e-9)]
+15V 0
--Nq:
TR 1
R32
P
CR 6
1
L
SOUR E
CR2
+
CR 5
+15V
ie
CR3
Fig, 5 GATE DRI VE CIRCUIT.
OC-OD
D A C P O R T ADDR
(CONTROL )
-
(-IQ
-
oa
Priew
~
Pold
OS
04
ARRAY CURRENT
--_____
STEP S I Z E
~~~
(31
GATE
I N C R E A S E / D E C R E A S E I.
-____
ZONTROL S I G N A L
F1g.6 : D 4 T A STRUCTURE
712
(D)
T
ISR-100ms
U
Initialise timers
6
1
1 PCUs orcessed
I
E-3
Increment PCU
Load Database Pointers
Float
*
aA
calculate Array
Power
by
by the step
the step
size
i
I
Operate at
fixed, low
dutv cycle
o/p control
signa 1 toPCU
Calculate new
Step size
-
t
Fiq.7 CONTROL ROUTINE FLOW CHART
713
-.
-