Energy and Power Engineering, 2013, 5, 31-35
doi:10.4236/epe.2013.54B006 Published Online July 2013 (http://www.scirp.org/journal/epe)
Research on the Three-phase Photovoltaic
Grid-connected Control Strategy*
Yukai Liu, Zhijian Hu, Ziyong Zhang, Jianglei Suo, Kaibin Liang
School of Electrical Engineering, Wuhan University, Wuhan, China
Received February, 2013
ABSTRACT
This paper mainly studies scheduling type photovoltaic generation system, and establishes a three-phase photovoltaic
grid-connected model in Matlab/Simulink, which uses the MPPT control that can make full use of the solar energy. At
the same time, energy storage device is added. The inverter of the energy storage device adopts V/f control. In the run-
ning state of the islanding because of a certain power failure, it can maintain a constant voltage and frequency. The si-
mulation shows that as the output of the photovoltaic power increases, harmonic rate decreases under the same condi-
tions, and the en ergy stor age device can increase the stab ility of pho tovoltaic gr id and reduce harmonic co ntents. So it’s
very necessary to add energy st orage device in the phot ov oltaic system.
Keywords: Photovoltaic Grid; Energy Storage Device; PQ Control; V/f Control; Harmonic Rate
1. Introduction
Nowadays the fossil fuel has been consumed increasingly.
Solar energy, as one kind of clean and renewable energy,
because of its rich reserves of raw materials-silicon, a
decline in the cost of production and improving of the
conversion efficiency, has prompted the photovoltaic
power generation to become an important role in the fu-
ture development. Whether from the point of environ-
mental protection or solving the energy problem, the de-
velopment and utilization of solar energy are of great
strategic significance.
As we know, the electric energy can't be stored on a
large scale, and islanding phenomenon may occur
whenever the power system breaks down, which requires
that the photovoltaic power system store electrical energy
in normal circumstances and release it in case of island-
ing status, thus maintaining the voltage and frequency
stable. So it’s necessary to add storage device to the
photovoltaic grid-connected system. Energy storage de-
vice introduced into the system can realize management
of demand side, eliminate valley-to-peak difference be-
tween day and night and smooth load [1]. Compared with
other energy storage device, super capacitor has greater
power density and higher energy storage efficiency, etc
[2]. When there is an emergency of energy insufficiency,
the super capacitor releases the energy stored, playing the
role of instantaneous power compensation. On the other
hand, energy storage device can be used as a backup
power supply, so as to improve the stability and reliabil-
ity of power supply, thus realizing the stable and bal-
anced control of power [3].
2. Model of Photovoltaic Power Generation
System
The photovoltaic cell is a kind of semiconductor compo-
nents, in which electrical energy is generated by the sun
directly. The photo-generated current expression is :
00
0()
{exp[] 1}
ss
ph satsh
qVIRIRV
II IAKT R

 (1)
where 0
I
is the output current,
p
h is the photovoltaic
current, I
s
at
I
is the reverse saturation curren t,
s
R is the
series resistance,
s
h is the shunt resistance, T is the
absolute temperature, is the electron charge,
Rq
A
is
the ideality factor of PN junction, is the Boltz-
mann constant. K
In the photovoltaic power generation, in order to gen-
erate more electricity and improve efficiency, the Maxi-
mum Power Point Tracking control algorithm [4], namely
MPPT is often used, which can adjust the working point
of photovoltaic array in real-time, and work it on the
maximum power point nearby [5]. The photovoltaic out-
put DC current goes through the DC-DC booster trans-
formation, inverter, filter and isolation transformer,
*This work was financially supported by the Ph.D. Programs Founda-
tion of Ministry of Education of China (20110141110032) and the
Fundamental Research Funds for the Central Universities (2012207
020205).

Y. K. LIU ET AL.
32
eventually merges into the power grid. The inverter
adopts the PQ control [6], which uses the maximum
photovoltaic output as the assigned active power value
and specifies the reactive power to zero, with the result
that the power factor is 1, ensuring the full use of the
green energy [7]. The equivalent circuit is shown in Fig-
ure 1.
The photovoltaic grid parameters are set to
s
c
I
=
14.88 A, m
I
= 13.88 A, oc = 708 A, = 576 V,
= 25℃, ref = 1, LC filter L = H, C =
F, PWM carrier frequency
Vm V
*10
ref
T
100 S3
10
6
*10
s
f
=8000 Hz,
current loop PI control
p
K
= 173, i
K
= 10, distribu-
tion network vo ltage leve l 35 kV, the simula tio n step size
, simulation algorithm: ode23tb.
6
*1050For the photovoltaic battery and MPPT maximum
power tracking, the simulation results are shown in Fig-
ure 2. It can be seen that the photovoltaic current main-
tains at 14.8 A. After a transition process of 0.35 s, the
output voltage maintains at 560 V and the output power
ultimately stabilizes at 8 kW. From here we see that the
structure of photovoltaic model can ensure a maximum
power output.
The photovoltaic DC current becomes a PWM wave
after the inverter, which contains the harmonics.
After going through the RLC filter, high order harmonics
are filtered, and only fundamental component is left. The
voltage and current are shown in Figure 3.
6n1
Increase the photovoltaic power output using the FFT
analysis tools of PowerGUI in Matlab/Simulink. When
the photovoltaic power outputs are 8 kW, 16 kW and 24
Figure 1. Main circuit of three-phase photovoltaic system.
Figure 2. Photovoltaic output current, voltage and power.
Figure 3. Voltage and current after filtering.
kW respectively, the harmonic contents of voltage and
current are observed, and made a comparison with later
simulation within which the energy storage device is
added, as listed in Table 1. As seen from the table, the
harmonic contents increase as the photovoltaic power
increases without energy storage. Supposing that the
photovoltaic power output is too high, it will exceed the
access capability of distribution network.
3. Control Strategy of Super Capacitor
Energy Storage
In the engineering practice, single super capacitor is of-
ten used in series or in parallel to form a super capacitor
group (super capacitor array), so that it can overcome
shortcomings such as a low voltage and small energy
storage capacity [8]. The circuit of super capacitor group
can be equivalent to a RC structure, of which the equiva-
lent resistance is:
s
s
sp
NR
RN

 (2)
Super capacitor group of equivalent capacity is:
p
s
NC
CN

 (3)
where
s
N is the series device number,
p
N is the par-
allel device number,
s
R is the equivalent resistance, C
is the equivalent resistance.
The super capacitor group also need the inverter, filter
and isolation transformer when connected to the grid,
However, the inverter adopts a different control strategy
—V/f control (voltage/frequency control). When island-
ing phenomenon occurs because of a system fault, the
energy storage device should maintain the voltage and
frequency stable. Once power vacancy appears, the sec-
ondary load should be removed in time to ensure that the
sensitive load works. So V/f control can response and
track load switching, and have a certain dynamic respon-
sible performance. At the same time, the photovoltaic
power output is greatly influenced by the external envi-
ronmental, such as temperature and illumination. As the
photovoltaic power output falls, the energy storage de-
vice can have an effect of instantaneous power compen-

Y. K. LIU ET AL. 33
sation.
3.1. Control Principle of V/f
n is the three-phase fundamental wave voltage (grid
voltage), m is the amplitude of phase voltage, which
can be expressed as:
uU
cos( )
2
cos( )
3
2
cos( )
3
m
a
bm
c
m
Ut
u
uU t
uUt


















(4)
The formula from static abc coordinates to the rotary
dq coordinates (called the Park transformation, or dq
transformation) is defined as:
22
cos() cos() cos()
33
22
sin() sin() sin()
33
abc dq
tt t
T
tt t


 

 








 
(5)
d
u and after dq transformation are:
q
u
00
a
dm
abc dqb
q
c
u
uU
Tu
uu


 


 

 

(6)
From (6), it is known that the three-phase grid- con-
nected voltage is converted into two parts via Park trans-
formation, component for amplitude of phase volt-
age, while q u component is 0. Grid voltage id
V and
capacitor current
d
u
c
I
are extracted and decomposed into
idd , idq , 0id , cd
VVV
I
, cq , 0c I
I
after abc/dq0 transfor-
mation, where the reference sin_cos is provided by the
virtual PL L.(Figu r e 4)
V/f control strategy is to regulate AC voltage through
the feedback voltage after inverter, so as to ensure the
output voltage stable. The double loop control scheme of
voltage outer ring and current inner ring is taken. The
outer ring uses the PI controller, whose inputs are the
load voltage, d and q axis components of reference volt-
age, with the purpose of making the steady state accuracy
of load voltag e 0 and stabilizing the load voltage; Output
of voltage ring serves as the reference value of inner ring.
The current servo system formed by the inner ring can
speed up dynamic process of disturbance resistance. The
inner ring uses the proportional controller P, which can
improve the dynamic response speed of system.(Figure
5)
3.2. Model of V/f Control
The photovoltaic system with storage device is shown in
Figure 6, the photovoltaic system is packaged as a PV
subsystem in Simulink, as the shadow parts shown in
Figure 7. The super capacitor is used as equivalent mod-
el of storage device, and merges into distribution network
with photovoltaic system where the inverter adopts V/f
control, which can stabilize the output voltage at a speci-
fied value by assigning the voltage d and q axis compo-
nents [9]. When the three-phase circuit breaker is closing,
the microgrid [10] is in parallel operation whose voltage
and frequency are provided by the grid; However, when
the three-phase circuit breaker is opened, the microgrid
runs separately, and the energy storage control units
maintain the voltage an d frequency stable.
Figure 4. Dq transformation model.
Figure 5. Double loop control model.
4%, 1%
kk
Ur
10 /0.4, 50kV Hz
Figure 6. The simulation example this paper uses.
The equation of the converter and the transfer function
of current inner ring and voltage outer ring are:

Y. K. LIU ET AL.
34
0
(,,
()
Ln nn
nLnn Zn
di
Lvu
dt n abc
du
Ciii
dt




 


)
(7)
2
2
0
2
1
()
1
pwm
CnCn ref
pwm
nZn
pwm
kk CS
ii
LCSkkCS
LCS ii
LCSkkCS





(8)
2
32
2
0
32
(1 )
()
(1 )
pwmpwm uppwm ui
nnref
pwmpwm uppwm ui
nZn
pwmpwm uppwm ui
kkCSkkk CSkkk
uu
LCSkkCS kkkSkkk
LS ii
LCSkkCSkkkS kkk


 

 
(9)
where L is the filter inductance, C is the Filter capaci-
tance,
L
n is the current through inductive, n is the
load voltage, 0n is the grid current,
i u
i
Z
n
i is the load
current, Cn ref and nref are respectively reference
current and reference voltage ,
iu 
p
wm
k is the frequency of
PWM carrier wave.
The Simulation example is shown in figure 6. Settings
of simulation parameters: equivalent capacitance of super
capacitor group C=1 F, PWM carr ier frequen cy 6000 Hz,
RLC filter L= H, R = 0.01, C=
3
50*103 150*10
F
,
double loop of voltage and current
p
K
=0.1, i
K
=10,
K=5, reference voltaged=691 Vq U=0, and the
same goes for the above photovoltaic system parameters.
The circuit breaker maintains closing before 1/6 s with
the network in parallel operation. As the circuit breaker
opens at 1/6 s, the microgrid goes into the islanding sta-
tus. The simulation step:, the simulation algo-
rithm: ode23tb.
U
50
,
6
*10
4. Analysis of the Simulation Results
The simulation results of islanding phenomenon are
shown in Figures 8, 9. It can be seen that after a short
transition process, the grid-connected voltage maintains
at 700 V. When the microgrid breaks away from the dis-
tribution network at 1/6 s, the voltage value of load point
drops slightly as a result of the voltage—frequency con-
trol, but remains unchanged basically, and frequency
deviation is maintained within 0.5 Hz.
The photovoltaic system adopts the PQ control strat-
egy, which can keep a constant active power output, the
output power maintains at 6 kW into the islanding, as
shown in Figure 10.
As the grid cannot provide active power to the load
point in island state, the energy storage device will sup-
ply to the load instead, as shown in Figure 11. The out-
put power of storage device has a sudden increase in the
islanding status, and finally keeps at 16kW. The simula-
tion results show that there appears an active power va-
cancy when the microgrid disconnects from the distribu-
tion network, and that energy storage device will export
more active power in order to maintain power balance.
Increase the photovoltaic power output after adding
storage device as well, and carry on a simulation analysis
on the grid voltage and current harmonic contents. The
results are listed in Table 1 using FFT Analysis tools,
where 2abc is the grid voltage after filter, U2abc
I
is grid
current after filter.
Figure 7. Main circuit of photovoltaic energy storage sys-
tem.
00.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.5
-800
-600
-400
-200
0
200
400
600
800 Uabc 2
Figure 8. Instantaneous values of grid voltage and fre-
quency.
Figure 9. Effective values of grid voltage and frequency.

Y. K. LIU ET AL.
35
Figure 10. Active power output of photovoltaic.
Figure 11. Active power output of energy storage device.
Table 1. Harmonic Contents with and without Energy Sto-
rage.
Photovoltaic power
8 kW 16 kW 24 kW
Harmonic
contents without with without with withoutwith
2abc
U 0.02% 0.03% 0.03% 0.04% 0.04%0.04%
2abc
I 3.92% 3.68% 4.41% 4.08% 5.78%5.71%
From the table we can see that whether to join energy
storage device or not, the voltage distortion rate is small,
and remains unchanged basically. Nevertheless, the cur-
rent distortion rate is large. When the energy storage de-
vice is added, the current harmonic contents are de-
creased obviously.
5. Conclusions
This paper builds the three-phase photovoltaic model,
and proves that the Maximum Power Point Tracking al-
gorithm and PQ control method can ensure the maximum
utilization of renewable energy through the simulation
results.
Use super capacitor as the energy storage device and
set up the main circuit of microgrid, through which the
island phenomenon is simulated. When storage device is
added, it can maintain voltage and frequency of load
point at a constant value through the V/f control. Besides,
as photovoltaic power output fluctuations occur because
of changes in the external environment such as tempera-
ture and illumination, it can have function of instantane-
ous power compensation, thus improving the stability of
microgrid.
Increase the photovoltaic power output, and conduct
the FFT analysis using the Power GUI in Simulink.
Compared with independent photovoltaic grid, the cur-
rent harmonic contents are decreased as storage device is
added, which improves the power quality of grid to a
certain extent.
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