Performance Analysis of Supercapacitor Integrated PV Fed Multistage Converter with SMC Controlled VSI for Different Load Conditions

International Journal of Power Electronics and Drive System (IJPEDS)
Vol. 9, No. 2, June 2018, pp. 757~765
ISSN: 2088-8694, DOI: 10.11591/ijpeds.v9.i2.pp757-765  757
Journal homepage: http://iaescore.com/journals/index.php/IJPEDS
Performance Analysis of Supercapacitor Integrated PV Fed
Multistage Converter with SMC Controlled VSI for Different
Load Conditions
Shruti Pandey1
, Bharti Dwivedi2
, Anurag Tripathi3
Department of Electrical Engineering, Institute of Engineering and Technology, Lucknow, India
Article Info ABSTRACT
Article history:
Received Sep 3, 2017
Revised Feb 25, 2018
Accepted Mar 18, 2018
The proposed work comprises of an MPPT controlled Photovoltaic (PV)
source, in conjunction with a supercapacitor, cascaded with a Sliding Mode
Controlled (SMC) Inverter, supplying variable nonlinear loads. The effects of
varying solar irradiation and its intermittency have been effectively managed
by the MPPT controlled boost converter and charge controlled supercapacitor
respectively. The charge controller bucks and boosts the terminal voltage and
realizes the power flow in a bidirectional manner. Seamless action has been
obtained by the proposed model under varying irradiation and for varying
load conditions. The performance of the SMC controlled Inverter, when
compared with a PI controlled Inverter, has been found to be superior in
terms of power quality and robustness of the supply system
Keyword:
Bidirectional converter
DC-DC boost converter
Mppt
Photo voltaic
Sliding mode control
Supercapacitor
Voltage source inverter Copyright © 2018 Institute of Advanced Engineering and Science.
All rights reserved.
Corresponding Author:
Shruti Pandey,
Departement of Electrical Engineering,
Institute of Engineering and Technology
Lucknow, India.
Email: shrutipandey@ietlucknow.ac.in
1. INTRODUCTION
The photo-voltaic (PV) system is the most admired energy source among all the existing renewable
energy sources. In PV fed microgrid system Inverters are used at the load for various applications. In order to
provide good class stabilized output whether it is in terms of steady state performance such as THD and
voltage regulation or in terms of transient performance such as quick settlement against variations in supply
and load, advanced strategies of control are needed to be deployed with Voltage source Inverters [1].
There are several strategies of voltage source inverter (VSI) control. Conventionally used PI control,
which is incapable of tracking periodic signals .At the same time basic hysteresis current control that
generates a variable switching frequency requiring application of appropriate filter with it. Multiple feedback
loop control scheme with moderate switching frequency makes output voltage waveform purely sinusoidal
[2] however, with non-linear loads it provides distorted output voltage.
All the above reported linear controllers work satisfactorily nearby equilibrium point only, to
guarantee the global stability of the VSI system under large perturbations. By and large the non-linear
controllers play better role. One of the frequently used non-linear controllers is Passivity-Based Control
(PBC) [3,4]. However in PBC method, the global asymptotic stability is guaranteed with the perfect
parameter match assumption which is not valid in practice and leads to steady-state error in the output
voltage. Sliding Mode Control (SMC) is a non-linear control technique which is superior in terms of stability,
simplicity, regulation and robustness under wide range of operating conditions [5]. The major advantage of

 ISSN: 2088-8694
Int J Pow Elec & Dri Syst, Vol. 9, No. 2, June 2018 : 757 – 765
758
SMC’s performance against any disturbance in the system parameters or in the loads [6] is its fast tracking
ability and better stabilization that help in developing high-performance of VSI circuits. [7,8]
The supercapacitor (or ultracapacitor) has acquired a wide attention as complementary source of
power to PV systems due to its higher power density (kW/kg), longer life, larger charge and discharge-
cycles, lower equivalent series resistance, lesser heating losses and wider temperature range as against
batteries [9, 10]. Lower voltage rating and varying terminal voltage issues of supercapacitor are handled
satisfactorily with the use of a Bidirectional dc-dc converter which acts as a controlled charger allowing
charging and discharging currents to flow to and from the supercapacitor as per the need.
2. PROPOSED SYSTEM CONFIGURATION
The schematic representation of proposed system is shown in Figure 1. The block diagram consists
of a PV array with varying solar irradiation whose output is given to DC-DC boost converter, MPPT
algorithm is used to track maximum power. Super capacitor is introduced with a suitable battery charger
(DC-DC buck boost converter) to interface with Dc link of the inverter. Further the PI and SMC VSI
converts the DC link power into a high quality ac power for feeding the loads for which it makes use of a PI
or SMC controller.
Figure 2 shows a single phase Voltage Source Inverter for RL load, along with an RLC filter. The
output of the Inverter is given by V0 and U is the control variable of the single phase inverter.
Q1
Q2
Q3
Q2
Vi = u VDC
C
R C
i
i
c
iz
R
L
VDC
}Z
V0
Figure 1. Block Diagram of Proposed Configuration Figure 2. Inverter
The state space model of the inverter based on the state variables, given below, is used in the sliding
surface in Sliding Mode Controller. Here V0 and i are the state variables.
1 1
1
0
ZC C
F
L
 

 
  

 
 
 
0
DC
H
L
V
 
 

 
 
 
The output equation with V0 as output can be written as C = [1 0]
3. CONTROLLERS USED
3.1. MPPT Control
Fixed type P and O MPPT is used here to make it easily implementable.In this particular MPPT
algorithm ∆D updating is done and in each step ∆D is varied by 20e-6
, Initial value for D=0.6, upper limit for
D (D max=0.9), lower limit of D (D min=0.02).

Int J Pow Elec & Dri Syst ISSN: 2088-8694 
Performance Analysis of Supercapacitor Integrated PV Fed Multistage ... (Shruti Pandey)
759
Figure 3. I-V and P-V Characteristics at 1000 W/m2
and 25 °C
3.2. Supercapacitor Charge Management Control
Charging and discharging process of supercapacitor is performed in two steps. One is through
constant current charging and the other is constant current discharging. [11] Control diagram of the constant
current charging mode and constant current discharging mode.is shown in the Figure 4a and 4b.
+-
+-
Iactual
LP PV
SC
P P
V
 Iref PID
G(s):
Transfer
Function
Buck Mode
+-
+-
Iactual
LP PV
SC
P P
V
 Iref
G(s):
Transfer
Function
Boost Mode
(a) (b)
Figure 4a. Constant Current Charging Method; 4b. Constant Current Discharging Method
3.3. Proposed Sliding Mode Control
Multi loop controllers gives superior results in terms of better system stability and transient
performance, hence in voltage controlled inverters ,it is always good to consider the inner current loop also.
In the proposed SMC the current error derived from the error in voltage is used in the inner loop while the
voltage error control is used in the outer loop. The Sliding Mode Controller operates at very high switching
frequency and in such a way that the capacitor output voltage and the input inductor current track
precisely their respective reference values.

+
+
+-
+ -

+
+
+-
+ -
Comparator
Boolean
Control signal for
inverter
m
Vc_ref
Vc_actual
ZN based PI
Control
ZN based PI
control
IL_actual
IL_ref
Proposed Sliding Mode Control
Voltage
SMC
Figure 5. Proposed Control Scheme
To get global stability, instead of considering direct voltage and current variables, their respective
errors are taken as state variables. The proposed sliding surface is a linear combination of such state variable.

 ISSN: 2088-8694
760
error error
V mI
   (1)
where error Cref C
V
V V
  , 1
Lr
er ef L
ror
I i i
  and m = sliding coefficient
The controller consists of inductor current and output voltage feedback.
= ( i
p
k
k
s
 ) [ ] (2)
where KP is proportional constant and Ki is integral constant of the PI controller.
When SMC controller drives the VSI towards the singular point, it makes
 =0 (3)
Equating  =0 in equation (1), 0 error error
V mI
 
m
1
Desired
Manifold
Verror
Ierror
ϕ
m
ϕ
m
ϕ
m
error error error
error
F x Hu
x
y Cx
 

ϕ
m
error error error
error
F x Hu
x
y Cx
 

Vref
error
V
error
I
Figure 6. Proposed Sliding Plane Figure 7. Closed Loop Control Model of Proposed
System
Therefore,
error error
V mI
  (4)
From closed loop model control effort can be realized as
error error
u V mI
  (5)
 
1 error
error
V
u m
I
 
  
 
(6)
Without loss of generality according to state feedback method
error
u kx
  (7)
Comparing equation (6) and (7)
 
1
k m


761
As a result of new error state variables the state space representation of VSI system can be represented as:
error error error
F H
x x u
 
& (8)
error
y Cx
 (9)
The value of Ferror is obtained as follows [5]
1
( )
error
F I H kH u F

 
 
 
(10)
From equation (10):
1 1
1 1
error
ZC C
F
mZC mC
 

 
  
 

 
 
(11)
Keeping value of (7) in (8)
( )
error error error error
F x H kx
x   
& (12)
 
error error error
F Hk x
x  
& (13)
Let
'
F be denoted by  
error
F Hk

The value of sliding coefficient m can be calculated in form of Eigen values as
'
0
I F

 
 
  (14)
1 1
1 1
DC DC
ZC C
V mV
mZC L mC L
 
  
 
 
 
   
 
 
= 0 (15)
2
0
DC DC DC
mV mV V
mC L ZC LZC C

 
       (16)
The value of m can be obtained by comparing the above equation with the characteristics equation of desired
system where 4
1 10 sec, 0.8
s
T 
   
Then from 3
s
n
T 

, 4
3.75 10
n
  
Then from standard second order system characteristics equation 2 2
2 0
n n
s s
   
Figure 8. Simulation Result of Sliding Plane of Proposed System

 ISSN: 2088-8694
762
Characteristics equation of the desired system becomes
2 4 4 2
(6 10 ) (3.75 10 ) 0
 
     (17)
Substituting values specified below in equation (16) and comparing with equation (17) will yield sliding
mode coefficient m = 14.7.
The parameters of the system used for simulation results are VDC = 500V, Z=528  , L= 1mH. C =100µF
Now the Eigen values obtained by
  0
error
I F
   (18)
1 1
1 1
ZC C
mZC mC
 
  
 
 
 
 
 
 
=0 (19)
2
0
mC ZC
 
    (20)
By taking sliding coefficient as m =14.7 as calculated above Eigen values of comes out to be
 =0,  = -69.9. Therefore it is clearly seen that the Eigen values obtained will always be located on the
left-half of the s-plane denoting the system stability.
4. RESULTS AND DISCUSSION
Figure 9 represents the simulation of the proposed system with SMC and PI fed VSI for non linear
Load. The simulations are performed with PV panel as primary energy source giving DC voltage of 250 Volt.
The PV parameters considered here for Solar Irradiation of 1000W/ are Voc = 64X4=256V, Isc = 6.14A, Impp
=5.76A, Vmpp = 54.7V. This PV voltage is boosted up with the help of MPPT controlled boost converter
which gives the output around 500Volt DC. Here solar irradiation is varied at particular intervals. The PV
panel is initially given irradiation of 1000 W/m2
. It is increased to 1200 W/m2
and reduced to 500 W/m2
at
0.5 sec and 0.85 sec respectively. This results in the variation of PV output voltage. In addition to the
variation of irradiation, the load power is also varied in order to study the stability of the system in the
presence of load disturbances. By connecting a parallel RL load in existing load, the load power is raised
from 2500W to 3000W and brought back to 2500W at 0.24sec and 0.35sec respectively, with this a reference
power is made and actual power is compared in the upcoming waveforms. The output voltage of PV panel as
seen by the DC link changes with change in irradiation, where the supercapacitor, having output of 250V,
managed by its charge controller (bidirectional buck boost converter): acts to maintain a constant output
voltage of 500V at the DC link.
Q1
Q4
Q3
Q2
Lboost
D
Qboost
Lf
Lcc
D1
Q2
D2
Q1
}
MPPT
}
}
Supercapacitor
PV
Source
Ipv
Vpv
Vc
IL
Reference
Voltage
Sliding mode control/
PI control
Cf
DC
Link
(C1)
To Switches Q1, Q2, Q3, Q4
Boost
Converter
VSI Filter
Battery Charger
Ls
D1
D4
D3
D2
Variable Non Linear Load
Sensitive
Load
R1
C1
SW
D1
D2
D3
D4
Ls
R1
C1
Charge Controller
}
Q1
Q2
Figure 9. Multistage Converter with SMC PI Controlled Inverter for Nonlinear Load

763
The reference capacitor voltage is considered as 325sinωt. The load current is multiplied by 2 for the
purpose of clarity. The inductor current is obtained by passing error capacitor voltage through PI Controller,
whose Kp and Ki values are evaluated through Z-N based technique.
4.1 Performance Analysis of PV Fed Multi Stage Converter with PI Controlled VSI for
Non Linear Load
Simulation results shown in Figure 10 reveal that the irradiation based changes in PV output (Figure
10a(c)) are effectively complimented by the supercapacitor Figure 10a (b) to meet the load variation Figure
10a (a). The increased load power between 0.24sec to 0.34sec as shown in Figure 10a (a) is met by
supercapacitor as shown by Figure 10a (b) with the help of its charge controller (buck-boost converter). It is
clear from Figure 10a (a), (b), (c) that an increase in load power in the interval of 0.24sec to 0.34 sec is
exclusively met by the supercapacitor and not by the PV source. Figure 10a (d) corroborates this fact by
showing a decreasing SOC in this duration. Further the irradiation is increased from 1000 W/m2 to 1200
W/m2 at 0.5 sec and decreased from 1200 W/m2 to 500 W/m2 at 0.85sec.
12000
10000
8000
6000
4000
2000
0
Load
Power(W)
10000
6000
0
-4000
2000
SC
Power(W)
4000
3000
2000
1000
0
PV
Power(W)
(c) Time
(b) Time
(a) Time
0.5083 0.5084 0.5085 0.5086
2500
2400
zoomed
Actual Pload
Ref.Pload
37.2
37
36.8
36.6
36.4
SOC(%)
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1
(d) Time
400
300
200
100
0
-100
-200
-300
-400
Load
Voltage
(V)
&
Current
(I)
20
10
0
-10
-20
Error
Voltage(V)
0.3240 0.3250 0.3260
340
320
300
Zoomed
0.22 0.26 0.30 0.34 0.38
0.24 0.28 0.32 0.36 0.40
(b) Time (s)
(a)
0.3270
(a) (b)
Figure 10a. Output Load Power, SC Power and PV Power for PI Controlled VSI.
10b. PI Controlled VSI Output Voltage Waveform Showing Error = 21.54 V
Figure 10a (b) and Figure 10a (c), (d) shows that the supercapacitor gets charged (by buck action of
buck boost converter) in the interval from 0.5sec to 0.85sec from PV panel such that a constant power of
2500 W is maintained across the load similarly from 0.85sec to 1.1sec the PV output is reduced and the
supercapacitor discharges (by boost action of buck boost converter) to maintain a constant power supply to
the load. The initial dip in power output of supercapacitor is attributed to the initial charging of the
supercapacitor. Further the initial delay in the output power from PV array is due to the time taken by PV
array to generate maximum power of 2500W. The zoomed part of load power waveform in Figure 10a (a)
shows the difference between reference load power and the actual load power is 100 Watt. This difference
remains throughout up to 1.1sec.There are two spikes visible at the instant of load variation in the waveform
at 0.24 sec and 0.34 sec. It is due to the presence of instant charging of capacitor in the nonlinear load.
Figure 10b (a) shows the actual output capacitor voltage, reference voltage waveforms, and the
current waveform, Figure 10b (b) shows its error voltage waveform which has error of 21.54Volt. The THD
in the output voltage of PI controlled VSI is 2.17%.
4.2 Performance Analysis of PV Fed Multi Stage Converter with SMC Controlled VSI for
Non Linear Load
Figure 11a reveals that on changing the load the load power settles to reference power with a
difference of only 4Watts in a few milliseconds only. Figure 11b shows the tracking of voltage across
capacitor to its reference voltage along with the error voltage which is equal to 1.2 Volt.

 ISSN: 2088-8694
764
12000
10000
8000
6000
4000
2000
0
Load
Power(W)
10000
5000
0
-4000
SC
Power(W)
4000
3000
2000
1000
0
PV
Power(W)
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1
(c) Time
(a) Time
(b) Time
Ref.Pload
Actual Pload
2500.0
2496.0
2492.0
0.5083 0.5084 0.5085 0.5086
zoomed
400
300
200
100
0
-100
-200
-300
-400
Load
Voltage
(V)
&
Current
(I)
(a)
1.5
1.0
0.5
0
-0.5
-1.0
-1.5
0.22 0.26 0.30 0.34 0.38
0.24 0.28 0.32 0.36 0.40
(b) Time (s)
Vref
0.3240 0.3250 0.3260
316
326
308
Zoomed
Vref Vact
Iact
(a) (b)
Figure 11a. Output Waveforms of Load Power, SC Power and PV Power for SMC Controlled VSI
11b. SMC Controlled VSI Output Voltage Waveform Showing Error = 1.2 Volts
The zoomed portions of Figure 10b and Figure 11b show that tracking of capacitor voltage to its
reference voltage matches perfectly with Sliding Mode Controller as against the PI controller in nonlinear
load.The THD performance of VSI deploying SMC is 0.25%
4.3. Comprehensive Analysis of Findings
The impact of improved (SMC) controller in the performance of multistage converter system is
assist in terms of THD, Verror (max), steady state error in power and peak of fundamental component of
output voltage is shown in Table 1.
Table 1. Comparative Table
Type of Multistage System SMC controlled VSI for Non-linear load PI controlled VSI for Non-linear load
THD 0.25% 2.17%
Verr (max) 1.3 V 21.54V
Steady state error in Power 4W 100W
Fund Component of Voltage 325 320.5
5. CONCLUSION
A novel approach comprising of the integration of multistage converter system deploying MPPT
with boost converter, PI / SMC controllers with VSI for nonlinear loads has been carried out. The analysis of
performance of PV fed multistage converter system deploying SMC controlled VSI shows that both the THD
as well as the output-voltage-error are less in comparison to those obtained with the conventional PI
controlled VSI under identical loading conditions. Also it is found that the steady state error between the
power demand and supply is very high with PI controllers. Thus the SMC Controlled VSI used with MPPT
controlled PV generation can be a superior option for integrating it to the micro grid systems. Further it was
interesting to note that the controllers take due care of satisfactorily integrating the supercapacitor to
compensate the intermittency of PV source.
REFERENCES
[1] Satish Kumar Gudey, Rajesh Gupta, “Second Order Sliding Mode Control for a Single Phase Voltage Source
Inverter, IET Power Electronics 2014
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phase voltage-source UPS inverters’, IEEE Trans. Power Electron., 1996, 11, (4), pp. 532–541
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[5] Gudey, S.K., Gupta, R.: ‘Sliding mode control in voltage source inverter based higher order circuits’, Int. J.
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[9] Suwat Sikkabut,et all Control strategy of solar/wind energy power plant with supercapacitor energy storage for
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