1. Vu Hoang Phuong, Le Viet Hung, Nguyen Dinh Ngoc, Tran Anh Dung, Nguyen Van Dua, “A control of stand-alone photovoltaic Water Pumping Systems”, The 9th AUN/SEED-Net Regional Conference on Electrical and Electronics Engineering (RCEEE 2016).

1. A control of stand-alone photovoltaic Water
Pumping Systems
Vu Hoang Phuong1
, Le Viet Hung2
, Nguyen Dinh Ngoc1
, Tran Anh Dung1
, Nguyen Van Dua1
1
Hanoi University of science and technology, Ha Noi
2
Institude for Hydro Power and Renewable Energy, Ha Noi
Abstract—The stand-alone photovoltaic (PV) water pumping
system is one of most important of renewable energy applications
especially in rural areas and islands, where it is difficult or even
impossible to use classical energies to fed water pumping system
[1]. In a PV pumping system, DC voltage of photovoltaic (PV)
array is converted into three-phase AC voltage by a voltage source
inverter without DC/DC converter to drive inductor motor (IM)
for the water pumping. That called a single-stage inverter makes
the PV pumping system increase in efficiency and decrease in cost
[2]. The previous studies have used vector control method for
induction motors and integrated with maximum power point
tracking (MPPT) algorithm to generate speed reference [3][5][5].
This solution has high performance, but it is complex and difficult
to implement in microprocessor. As a result, it is not suitable for
low power stand-alone photovoltaic Water Pumping Systems in
practice.
This paper presents a scalar control strategy to drive inductor
motor (IM) following torque-speed characteristic of the
centrifugal pump in the stand - alone PV water pumping system.
The control strategy, a simple methodology because of not being
depend on motor parameters, is integrated with hybrid MPPT
algorithm to generate frequency references so that the system
tracks maximum power point of the PV array. The correctness of
the proposed control algorithm is proved by Matlab/Simulink
under different operation conditions of PV array.
Keywords— MPPT; solar PV; scalar control; stand-alone
photovoltaic water pumping
I. INTRODUCTION
With the agriculture countries in Asia as Viet Nam, water
is very importance element in producing crop time. Many
applications are created to support this purpose. Recently,
agriculturist apply electrical water pumping system for feeding
water into the agricultural areas. It is very helpful with the areas
where electricity is available, but it is not available for non-
electricity areas such as some rural areas and islands, where
electrical water pumps are unavailable or low-quality. With
renewable energy revolution later, solar energy is apply widely
in more applications, and it is also a good solution for the areas
where water pump system is required. Therefore, this stand-
alone solar pumping systems have received considerable
attention.
In most of these past studies, the DC/DC and DC/AC
power conversion schemes are used, these systems are called
two-stage inverters. MPPT process is carried out by additional
DC/DC converter. With this power conversion stages, it
decreases the total efficiency of the system and increases the
cost. We can optimize it by decreasing the number of power
conversion stages and the number of components involved in
each stage [6]. In a PV pumping system, DC voltage of PV array
is converted into three-phase AC voltage by a voltage source
inverter by a DC-AC inverter without DC/DC converter to drive
the water pump. That called a single-stage inverter makes the
PV pumping system. Although it is complex, it will increase in
efficiency and decrease in cost.
We have more different method which is proposed for
inductor motor, such as: scalar control, vector control, direct
torque control… The scalar controller method called as V/f
control method is very popular with industrial application. The
previous studies have used vector control method for induction
motors and integrated with MPPT algorithm to generate speed
reference. This solution has high performance, but it is complex
and difficult to implement in microprocessor. However, the PV
pumping system don’t require high dynamic performance and
install it easy in the microcontroller. It need to be robust with
perturbation, can be tracking the maximum power point (MPP) of
the PV system to improve efficiency of the system. Therefore,
the control structure is constituted by combining scalar control
with a novel hybrid MPP tracking algorithm to give the best
result. This paper presents a scalar control strategy to drive IM
following torque-speed characteristic of the centrifugal pump in
the stand - alone PV water pumping. The control strategy, a
simple methodology because of not being depended on motor
parameters, is integrated with hybrid MPPT algorithm to
generate frequency references so that the system tracks
maximum power point of the PV array.
II. TOPOLOGY
System configuration is shown in Fig. 1 for the PV water
pumping system. It consists of solar PV array followed by a
Voltage Source Inverter (VSI) and a three phases IM. A novel
hybrid MPP tracking algorithm searches for the MPP which
decides the reference speed for the scalar control algorithm and
remain it in a couple of time.

2. IM
Centrifugal
Pump
PV Array
Isolation
SVM
Calculation
U/f
Ramp
MPPT
dcC
dcI
dcU
6
6
su  su 
U f
*
f
*
f
dcU
dcI
2
Fig. 1. A scalar control and MPPT algorithm with the stand-alone PV water
pumping system
A. Design of Solar PV Generator
Parameters is chosen as follow:
 Induction motor parameters:
1.86 KW (2.5HP), 50 Hz three phase, 1430 rpm, 4 pole,
Line-to-line Voltage: 230V
Motor current: 5A
Resistor of stator and rotor: 0.603 , 0.7s rR R   
Reactance of stator, rotor, mutual induction:
1.007 , 0.9212 , 23.56s r mX X X     
 Parameters of a module in Solar PV Array:
Open circuit voltage 43.5ocV V
Short circuit current 4.9AscI 
Voltage and current at MPP: 35 , 4.58Amp mpV V I 
Number of series cell in each module= 72
As request, we use a three phase induction motor of 1.86kW
power rating. So solar PV is designed with power be 2kW
capacity considering losses of the system. The maximum power
can be obtained from the system:
   * * * 2mp s mp p mpP N V N I kW 
(1)
Here, mppV is voltage of module at MPP, mppI is current of
module at MPP, mppP is maximum power of module, ;s pN N
is the number of serial and parallel PV module. It has been
observed that ocV and scI at peak power are 80% of ocV and scI
values [3]. So mppP is calculated by the equation:
*0,8* * *0,8* 2mp s oc p scP N V N I kW  (2)
Designing the open circuit voltage of the panel as 525(V).
The open circuit voltage of single module is  43.5ocV V , the
short circuit current is  4.9 AscI  . Therefore:
e 525
12
43.5
ocpan l
s
occell
V
N
V
   (3)
The current of module is determined by:
2
4.8( )
0.8* 0.8*525
mpp
mpp
oc
P k
I A
V
   (4)
PV module is connect parallel: *mpp p scI N I ,  4.9scI A
,thus 1pN 
So we will have 12 serial module and 1 parallel module to
obtained the PV array power as 2.4kW
B. Design of DC Link Capacitor
The capacity of Capacitor is calculated by:
2 2
1
1
3
2
dc dc dcC V V VIt    (5)
Where: dcV is the reference DC bus voltage of VSI; 1dcV is
minimum DC link voltage;  is the overloading factor; V and
I are voltage and current per phase of IM; t is time duration in
which voltage decrease to minimum allowable DC link voltage.
   230
5 ; 133
3
I A U V  
 2 21
525 500 3*1.2*133*5*0.005 934
2
dc dcC C F    
We will choose 2 serial capacitor with value of each
capacitor is 450 / 450F V .
C. Design of power switching device
The current which go through the switch has value as the
current per phase IM:  5tbvI A
Choosing the overloading current factor 2iK  , we
calculate the current which go through the switch:
 * 2*5 10v i tbvI K I A  
At one time, 2 serial switches in a branch just have only a
ON-switch, the other switch is OFF. So the maximum inverse
voltage of the switch is capacitor voltage:  max 525ngU V
Choosing the overloading voltage factor 1.5uK 
So:  max. 1,5*525 788v u ngU K U V  
With above work voltage and current condition, we will
choose switch is power module as FNA23512A of
FAIRCHILD.
D. Design of diode
In this circuit, a diode install serially with PV array to
against reverse DC source (PV array). So diode voltage is equal
with capacitor voltage. As the above calculation, diode voltage
is calculated by:

3.  od max* 1,5*520 788di e u ngU K U V  
The diode current is equal with PV current. Choosing the
overloading current factor 2iK  :
 od . 2*4.9 9.8di e i PVI K I A  
So, we will choose diode: (1200V/30A) MUR
III. STRATEGY
A. Torque-Speed Characteristic of the Centrifugal Pump
When the centrifugal pump is started, a breakaway torque
is over about 10%–25% of the nominal torque to overcome the
static friction. A nonlinear relationship of a summation of two
terms is assumed for this breakaway torque similar to that given
in for a dc motor. The first is an exponentially decaying term
representing the transition from static to kinetic friction and the
second is a constant term representing Coulomb friction.
Adding the breakaway torque, the resultant torque-speed
relationship will be as illustrated in (6) showing very good
agreement with the measured torque-speed characteristic [2].
This measured torque-speed relationship is shown in Fig.2
32
1 2 4. . . k w
PT k w sign w k e k (6)
Fig. 2.Torque-Speed Characteristic of the Centrifugal Pump
With this torque-speed relationship, additional, this
problem is balancing energy (keeping energy at tracking point)
without improving control quality, so a Scalar Control has been
proposed for this application.
B. Maximum power point tracking strategy for proposed
system
In a stand-alone PV water pumping system, the pump uses
power directly from the PV array. When the rotor speed of the
pump increases (i.e., the output frequency of the inverter
increases) and the voltage of PV array decreases
. Therefore, the operating point of the PV array can be
controlled by the output frequency of the inverter. The MPP is
tracked by means of increasing frequency in the voltage source
region and decreasing frequency in the current source region.
1) Classical MPPT methods
Classical MPPT method is an algorithm, which uses a
single criterion to track the MPP, based on the output
characteristics of the PV array.
Several MPPT methods have used, such as constant
voltage method (CV) [7], Perturb and Observe method (P&O)
[5] and Incremental Conductance method (INC) [5] [8] [9].
According to CV method, the MPP is tracked approximate
to a constant value, it only changes weakly when weather
varies. With a stand- alone PV water pumping system, the
output frequency increases when the PV terminal voltage is
higher than the reference value. Otherwise, the output
frequency decreases. The output frequency is specified by:
( 1) ( )
( ) ( 1) ( )
( 1) ( )
ref
ref
ref
f n f V n V
f n f n V n V
f n f V n V
(7)
Where ∆f is the step size of the output frequency; Vref is the
MPP voltage.
Advantages of this method are such as: quick response and
robust to disturbance. But it is hard to keep the reference value
Vref exactly equal to the actual MPP voltage, so the tracked point
often deviates from MPP during operation.
Because of the easy operation, P&O has used popularly in
MPPT algorithm. The output frequency is decided following:
( ) ( 1) ( )
( 1) ( ) ( 1)
( )
( 1) ( ) ( 1)
f n f n f n
f n P n P n
f n
f n P n P n
(8)
When the weather condition does not change quickly, P&O
will track MPP point well. However, if the solar radiation
changes fast, this method will regulate fail the speed and even
causes the system be out of the operation.
INC method is used to overcome disadvantages of P&O
method with rapidly changing weather condition. In a stand-
alone PV water pumping system, INC method is applied by:
( 1) / / 0 ( MPP)
( ) ( 1) / / 0
( 1) / / 0
f n I V I V at
f n f n f I V I V
f n f I V I V
(9)
Theoretically, the INC method using system can operate at
MPP with an improved dynamics, but this method must use
divisional calculation to calculate value of I/V+∆I/∆V, it is not
suitable for a fixed-point MCU-based controller [5].
Furthermore, when the solar radiation increases quickly,
causing the increment of current and voltage, namely ∆I >0 and
∆V >0, the frequency is wrongly decreased according to (4).
Therefore, it is necessary to use a novel hybrid MPPT control
strategy.

4. 2) The Novel hybrid MPPT control method
The Novel hybrid MPPT control method is proposed to
improve the stability and dynamics of system. This hybrid
MPPT control strategy include two parts: the judgment of
speed-up or speed-down frequency and the selection of step size
of frequency.
a) The proposed algorithm
The system with the hybrid method is basically controlled
in CV method while its Vref is period updated by Multi-
criterion method (MC).
MC method bases on same principle of INC criterion,
however, it doesn’t use divisional calculation as INC method
and it checks the signs of ∆P, ∆V directly to track the MPP and
a new variable ∆I is used to judge the change of the solar
radiation to improve the system dynamics [10].
Start
Vref, ∆f
Measure V(k);F(k)
V(k)==Vref
V(k)>Vref
F=F(k) F= F(k)+∆f F= F(k)-∆f
N
Y N
Y
Fig. 3. Frequency change according to the CV algorithm
Calculate ∆P, ∆V,∆I
Start
∆P=0
Mesuare Vpv(k), Ipv(k),
Vpv (k-1), Ipv(k-1), F(k)
∆P>0
∆V>0 ∆V>0
∆I>0 ∆I>0 ∆I>0 ∆I>0
N
N Y
N
Y Y
Fref=F(k)+∆f
N
N
Y
Y
Fref=F(k)-∆fFref=F(k)+∆fFref=F(k)+∆fFref=F(k)-∆fFref=F(k)-∆f
N
YY
N
Fref=F(k)
Fig. 4. Frequency change according to the MC algorithm
The overall system efficiency is as good as the MC method
and the stability is as good as CV method.
MC method
Determining MPP and
Vref
CV method
Remaining output voltage as
Vref
MC method
Determining MPP and
Vref
1 period 1 periodn periods
Fig. 5. The novel hybrid MPPT method
b) Selection step size of the output frequency
Step size of output frequency (∆f) is very important during
controlling periods, it affects to dynamics and stability qualities
of system. A large step size leads to quick dynamic response
but a large oscillation around the MPP. A small step size leads
to a good stability, but it takes a long time to approach the MPP.
In order to combine the merits of large and small step size, the
step size is selected, it base on the voltage of the PV array:
1 min 1
min 1 2
2 2
( )
( )
ref
ref
ref ref
k V V f Vref V
f f V V V
k V V V V
(10)
In order to reduce the vibration oscillation around the MPP,
the minimum step size is selected when the voltage is within V1
and V2. Outside (V1; V2), the operating point of the system is
away from the MPP, the larger the step size is selected. So the
convergence time to the MPP is shortened.
∆fmin is determined on the basis of system parameters and
MCU capabilities [10]. The optimization value of ∆fmin is
calculated by (11):
min
2 . . 2 . .
max
. ( )
N N
rate ad rate ad
I mpp V o mpp
f V f V
f
K I K V V
(11)
With system parameters in this paper and the used MCU
capabilities, we selected ∆fmin= 0.01Hz.
IV. SIMULATION RESULTS
The proposed configuration for pump extracting power
from PV arrays is modeled and simulated in
MATLAB/SIMULINK. In this section, performance of the
drive is analyzed in varying solar radiation, temperature based
on the simulated results. Simulated results show that the
systems perform quite satisfactorily.
A. Characteristics of the System with varying temperature
In Fig. 6, the temperature is increased from 298.15 (K) to
313.15 (K) at 6 second, radiation remain G= 1000 (W/m2
) in all
time.

5. Fig. 6. Current, Voltage and Power of PV after tracking
In this simulation, the system implement MC method for
5s at the first time, after that, the system will begin the process:
implement CV tracking method for 4s to keep the PV at
maximum power point, and MC method for 2s to determine
MPP.
We can see that PV voltage is tracked to MPP voltage after
4.5s. This voltage is so accurate with MPP voltage in real. At
t=6s, the temperature increase from 298.15 (K) to 313.15 (K),
so it make the curve change a little of bit. According the theory,
MPP voltage must be decrease. However, at this time, CV
method activate, so it keep MPP voltage be no change. It also
helps the frequency not to change so much when the weather
change fast. After 9s, MC algorithm continuous activate, CV
algorithm is stopped to determine new MPP.
Fig. 7. Frequency, stator current, torque and speed of the pump
At t=6s, the temperature increase from 298.15 (K) to
313.15 (K), the speed, torque, stator current and frequency
pump is reduced soft and not be changed much at that time.
B. Characteristics of the System with varying Radiation
The radiation is increased from 600 W/m2
to 1000 W/m2
and temperature is hold T=298.15 (K)
Fig. 8. Current, Voltage and Power of PV after tracking
The tracking MPP time is about 4s. At t=6s, radiation
increase from 600 to 1000 W/m2
,current, voltage is change soft
and is established quickly, the voltage value is hold at MPP
which MC determine, and current increase follow the theory.
The voltage, current, power simulation value is approximate
value of the real MPP at the different weather conditions.
Fig. 9. Frequency, stator current, torque and speed of pump motor
Fig. 9 show that frequency, stator current, torque and speed
increase while radiation increase from 600 to 1000 W/m2
.
V. CONCLUSION
This paper has presented a control of stand-alone
photovoltaic Water Pumping Systems. This system include: a
single stage scheme for solar PV array fed induction motor
drive utilizing benefits of a scalar control strategy has been
proposed, a novel hybrid MPPT for determine reference speed
corresponding the P-V characteristic of solar PV array and
remain it for a long time. PV array has been operated at
maximum power in the atmospheric conditions. The pump has
been used affinity law and performance has been simulated.

6. Simulated results show that the induction motor drive performs
satisfactorily during starting, dynamic and steady state
conditions.
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