Harmonic stability analysis of multi-paralleled 3-phase PV inverters tied to grid

International Journal of Power Electronics and Drive System (IJPEDS)
Vol. 12, No. 2, Jun 2021, pp. 783~792
ISSN: 2088-8694, DOI: 10.11591/ijpeds.v12.i2.pp783-792  783
Journal homepage: http://ijpeds.iaescore.com
Harmonic stability analysis of multi-paralleled 3-phase PV
inverters tied to grid
R.S. Ravi Sankar, K. K. Deepika, A.V. Satyanarayana
Department of Electrical and Electronics Engineering, Vignan’s Institute of Information Technology, Visakhapatnam,
Andhra Pradesh, India
Article Info ABSTRACT
Article history:
Received Jun 12, 2020
Revised Mar 27, 2021
Accepted Apr 5, 2021
In this paper the harmonic stability is investigated for multi paralleled three-
phase photovoltaic inverters connected to grid. The causes to harmonically
stabilize/destabilize the multi-paralleled PV inverters when tied to the grid is
analysed by the impedance-based stability criterion (IBSC). In this paper
stability of the system is investigated by varying the grid inductance with
constant grid resistance and also by varying load impedance while
maintaining grid inductance constant. Stability of the multiple three phase
inverters tied to the grid with different grid impedance, inductance value in
particular are analyzed. Overall system is stable up to grid inductance of
5mH even though there is change in load admittance. It is concluded that
system stability depends only on grid impedance. It is verified with
MATLAB Simulations.
Keywords:
Distribution generation
Grid impedance
Impedance based stability
criterion
Inverter output impedance
Maximum power point tracking
Minor loop gain
Proportional-resonant current
controller
PV array
This is an open access article under the CC BY-SA license.
Corresponding Author:
R.S. Ravi Sankar
Department of Electrical and Electronics Engineering
Vignan’s Institute of Information Technology
Visakhapatnam, Andhra Pradesh, India
Email: satya_ravi2001@yahoo.com
1. INTRODUCTION
Proliferation of renewable energy sources is the current scenario to the meet the increasing load
demand. When these sources are connected to the grid there is interaction between the LCL filter, grid
impedance, and inner current loop of the inverters. Thereby, it causes the harmonic instability [1], [2]. Which
further leads to amplification of resonance over the range of frequencies. This sometimes leads to unwanted
withdrawal of the PV inverters from the grid [3], [4]. The interaction of the admittances of multiple inverters
causes two issues one is amplification of Resonance and seconed one harmonic instability. These two issues
are analyzed and answered by means of IBSC analysis. IBSC was applied to design the output filter in
chopper circuits [5], [6]. The conversion method for harmonic transfer function-based model for a Voltage
source converter into a simple and efficient Single Input Single Output model [7]. Stability characteristics
were obtained with the frequency coupling effect. In [8], Impedance model in dq-frame was developed for
offshore wind power plant to study the effect of inverters and transmission cables. The analyzed the
harmonic stability in modular multilevel based DC systems. It was verified with hardware experimentation as
well as simulation. DC impedance model of the system was developed by considering the capacitor voltage
fluctuation, harmonic responses [9].

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In this work, IBSC tool is used in the multi-inverters tied to grid to examine the harmonic stability.
The system stability is evaluated by graphical analysis using minor loop gain of the overall system. If the
system obeys the nyquiest stability criteria than the system is stable otherwise it is unstable. The concept of
passivity-based stability analysis has recently gained attention in control system [10] and from this stability
analysis each subsystem has a phase margin lies between -90 to 90. Then the system is stable otherwise
system is unstable.
In this paper, Section 2 deals with the Mathematicall modeling of the PV system, in Section 3, the
calculation of the Output Impedance of inverter and the modeling of the PR current controller. In Section.4.
simulation results of the two case studies are discussed.
2. MATHEMATICALL MODELING OF PV SYSTEM
2.1. Modeling of PV array
In this study PV array provides the DC voltage to boost converter. Single diode model of a PV Cell
is depicated in Figure 1. It represents simple modeling where resistances are neglected [11]-[16]. The
parameters of the PV module are shown in Table.1. These are used to simulate the PV array in matlab
simulation. two modules are connected in series to give the voltage of 700 V and the PV current supplied is
40A. The PV power supplied by the inverter is 2KW even though it is designed for the 14KW. The PV array
voltage, current and power wave forms are depicted in Figure 2 to Figure 4.
2.2. Maximum power point tracking (MPPT)
PV panel output is highly depending on climatic conditions, extracted maximum power from PV
panels. Different MPPT techniques are proposed in literature. For simple implementation, P&O technique is
implemented in this paper [17], [18].
Table 1. Specifications of the PV module
Parameter Variable Value
Current at PMax Im 8.30A
Voltage at PMax Vm 30.2V
O.C Voltage Voc 37.3V
S.C Current Isc 8.71A
Series Resistance Rs 0.217ohm
Ref. Solar Radiation Sref 1000W/m2
Ref. Temperature Tref 300k
PV
I 1
D
I I
V
Figure 1. PV cell equivalent circuit Figure 2. PV voltage (in volts)
Figure 3. PV current (in amps)
Figure 4. PV power (in watts)

Int J Pow Elec & Dri Syst ISSN: 2088-8694 
Harmonic stability analysis of multi-paralleled 3-phase PV inverters tied to grid (R.S. Ravi Sankar)
785
3. MODELING OF INVERTER OUTPUT ADMITTANCE
3.1. Calculation of inverter output admittance
The small signal modeling of grid connected inverter is shown in Figure 5. in that PV inverter is tied
to the grid, for filtering purpose LCL filter is used. The overall system is represented in block diagrame in
Figure 6. In this work PR current controller is taken as current controller [19], [20].
-
+
Figure 5. 1-Ph. depication of system
+
-
+
-
Figure 6. Block diagrame of the system
Where ( c
G ) Current controller, ( d
G ) delay gain and ( PWM
K ) PWM gain are defined as [21], [22]:
1
)
( K
s
GC = ,
s
T
d
S
e
s
G 5
.
1
)
( −
= , 1
=
PWM
K (1)
The converter output admittance is defined as ( )
M
d
C
C
Y
G
G
Y
s
Y
+
=
1
0 (2)
The output admittance of LCL filter is given by
)
(
1
)
( 2
2
0 g
f
g
f
f
f
f
v
PCC
g
o
L
L
L
L
C
s
s
L
C
s
v
i
s
Y
M
+
+
+
=
−
=
=
(3)
The trans admittance of the LCL filter is given by
)
(
1
)
( 2
0 g
f
g
f
f
v
M
g
M
L
L
L
L
C
s
s
v
i
s
Y
PCC
+
+
=
=
=
(4)
Finally, the converter admittance is given by

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s
T
g
f
g
f
f
f
f
M
d
c
O
i
PCC
g
C S
g
Ke
L
L
s
L
L
C
s
L
C
s
Y
G
G
Y
v
i
s
Y 5
.
1
3
2
0 )
(
1
1
)
( −
= +
+
+
+
=
+
=
=

(5)
Here K1 -Gain of controller, s
T - sampling time, g
i -Grid current, VM, VPCC -converter output voltage, PCC
voltages, Lf, Lg - filter inductance value inverter side and grid side.
3.2. Proportional resonant current controller:
A proportional resonant (PR) controller exhibits a better steady state and dynamic performance.
Input- output relation of an ideal PR controller is expressed by (6) [22], [23]. In the same way, for a non-ideal
PR Controller it is given by (7) [24]:
Ideal proportional resonant controller:
2
2
2
)
(

+
+
=
s
s
K
K
s
G r
P
PR
(6)
Non- ideal PR controller: 2
c
2
c
r
P
PR
s
2
s
s
K
2
K
)
s
(
G

+

+

+
= (7)
3.3. Harmonic stability analysis of multilevel inverters tied to the grid
The block diagram of overall system is shown in Figure 8 and its corresponding Matlab-Simulink
diagram is presented in Figure 9. A capacitor bank of 12μF is connected parallel at PCC. Grid specifications
are taken as 400V, 50Hz and impedance 𝑍𝑔 = (0.1 + 𝑗400) 𝛺. A PR controller is used to drive the inverter
and an LCL filter is used to improve the quality of grid current [25]-[28]. PWM technique is implemented to
produce switching pulses to converter. Specifications and parameters of the five inverters are stated in
Table 2.
PV Array
With
MPPT
3-Ø
Inverter-I
Grid
PR Current
Controller
LCL-Filter
PV Array
With
MPPT
3-Ø
Inverter-III
LCL-Filter
PR Current
Controller
PV Array
With
MPPT
3-Ø
Inverter-IV LCL-Filter
PR Current
Controller
PV Array
With
MPPT
3-Ø
Inverter-V LCL-Filter
PR Current
Controller
PV Array
With
MPPT
3-Ø
Inverter-II
LCL-Filter
PR Current
Controller
*
1
R
i
*
2
R
i
*
3
R
i
*
4
R
i
*
5
R
i
400 V,
50HZ
3-Ø AC SUPPLY
Figure 8. Five PV arrays tied to grid

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Figure 9. Single line diagram of a PV inverter tied to grid
Table 2. System specifications
Inverters Parameters INV.1 INV.2 INV.3 INV.4 INV.5
Rating KVA 5.6 3.5 10.5 4.2 7
Switching frequency KHz 10 10 15 10
DC Voltage 600V
Filter Values
Lf [mH] 20 22 24 25 15
Cf[μf]/rd 22/0.2 15/0.4 2/7 3/42 15/0.9
Lg[mH] 0.22 0.3 1.7 1.3 0.2
Parasitic Values
rLf[mH] 11.4 15.7 66.8 49.7 10
rCf[mf] 7.5 11 21.5 14.5 11
rLg[mΩ] 2.9 3.9 22.3 17 2.5
Control Gain
Kp 5.6 8.05 28.8 16.6 5.6
KI 1000 1500 1500 1000 1000
The expression for source admittance is given in the (8).
S
S
SG
sL
R
Y
+
=
1
(8)
where 𝑅𝑆-Grid resistance, 𝐿𝑆-Grid inductance
The expression for source admittance is depicated in (9)
5
4
3
2
1 CL
CL
CL
CL
CL
CPFC
LG Y
Y
Y
Y
Y
SY
Y +
+
+
+
+
= (9)
where 𝑌𝐶𝑃𝐹𝐶 denotes the admittance of capacitor CPFC, minor loop gain 𝑇𝑀𝐺 is obtained as:
𝑌𝐿𝐺 = 𝑌𝐶𝐹𝑃𝐶 + ∑ 𝑌𝐶𝐿𝑋
5
1 , 𝑇𝑀𝐺 =
𝑌𝑀𝐺
𝑌𝐿𝐺
(10)
The minor loop gain (TMG) is highly depending on the load admittance given by (10). In this study,
load impedance is assumed to be constant. It means that all 5 inverters are tied to the grid and grid inductance
is varied from 1mH to 1000mH, with the grid resistance constant. If the MLG of the overall system obeys the
Nyquist stability criteria for different values of grid impedance, then the system is harmonically stable,
otherwise overall system is said to be unstable.
PV
V

a
b
c
PLL
+
-
M
v f
L g
L
Lf
r Lg
r
Cf
r
f
C
d
R
g
i

i
CB PCC
v
0
 

abc
*
g
i
+
-

i
+ 2
0
2

+
s
s
KI
P
K
0


2
1
dc
v
num
den
abc

Limiter
s
TS
e 5
.
1
−
PWM
*

i

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4. SIMULATION RESULTS AND DISCUSSION
Two different case studies are considered to examine the harmonic stability analysis with the above
defined system parameters given in Table 2, where five PV inverters are tied to the grid. In first case study,
grid impedance is variable and load impedance is constant. In the second case study, load impedance are
variable and grid impedance is constant at 𝑍𝑔 = (0.1 + 𝑗400) 𝛺.
4.1. Case study 1: Constant grid resistance and varying grid inductance
4.1.1. Grid inductance up to 5mH
Here grid resistance is kept constant and grid inductance is varied. The system becomes stable upto
LS = 5mH. In this case all the 5 inverters are tied to grid. The grid iducatance 𝐿𝑆 = 5𝑚𝐻, total grid current
is 44A. Nyquist Plot of the minor loop gain and the simulation results of THD of grid current, grid curent
wave form, individual individual inverter currents, %THD of the grid currents for the overal system are
shown in Figure 10 to Figure 13. respectively.
Figure 10. Nyquist plot of minor loop gain Figure 11. THD analysis of the grid current with grid
inducatane upto 5mH
Figure 12. Grid current (in Amp) Figure 13. Individual inverter currents (in Amp)
4.1.2. Grid inductance more than 5mH
This case is used to illustrate when the grid inductance (LS) is more than 5mH. The overall system
becomes unstable. Nyquist Plot of minor loop gain with gird inductance value 𝐿𝑆 = 6𝑚𝐻 is shown in
Figure 14. %THD of the grid current, Grid current and inverter currents are shown in Figure 15, 16, 17
respectively.

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Figure 14. Nyquist plot of minor loop gain with
mH
Ls 6
=
Figure 15. THD response of the grid current
Figure 16. Grid current (Amp) Figure 17. Individual inverter currents (Amp)
4.2. Case study 2: Constant grid impedance with varying load admittance
In in practical suitutions there is is possibility of change in load admittance will changes de to te
disconnection inverter from grid. Thereby affects the overall system stability. To analyze this situation when
INVs are removed from the grid, two cases are considered. In first case, INV 1 is removed and in case B
INVs 1, 5 are removed. The constant grid impedance is 𝑍𝑔 = (0.1 + 𝑗400) and the reference is INV 1 given
in (11).
1
CL
SA Y
Y = (11)
Calculation for MLG in this case is given in (13).
5
CL
4
CL
3
CL
2
CL
CPFC
G
LA Y
Y
Y
Y
Y
Y
Y +
+
+
+
+
= (12)
Minor loop gain of the system
LA
SA
MA
Y
Y
T = (13)
In First case, the INV 2, INV 3, INV4, and INV 5 are injecting the currents of 8A, 5A, 15A, and 6A
respectively into grid. So total grid current 34A. Nyquist plot of MLG of the system is depicated in Figure
18. From IBSC, it is concluded that the system is stable. The quality of the grid current is 2.37% as illustrated
in Figure 19. Total grid current and individual inverter currents are shown in Figure 20 and Figure 21
respectively.

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Figure 18. Nyquist plot of system INV -1 OFF Figure 19. THD response of the grid current
Figure 20. Grid current (Amp) Figure 21. Individual inverter currents (Amp)
In this case INV 1 & INV5 are disconnected from grid. Nyquist plot of MLG of the system is given
in Figure 22. From IBSC, the system is stable. The Quality of the grid current is given in Figure 23. Grid
current and individual inverter currents are shown in Figure 24 and Figure 25 respectively.
.
Figure 22. Nyquist plot of INV 1 & 5 are
disconnected
Figure 23. THD response of the grid current
Figure 24. Grid current (Amp) Figure 25. Individual grid current (Amp)

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5. CONCLUSION
In this work, investigation of harmonic Stability of multiple PV fed 3-Ø INV is tied to grid is
confirmed by new impedance based stability criterion (IBSC). Stability of the system is investigated by
varying the grid inductance with constant grid resistance and also by varying load impedance and
maintaining grid inductance constant. It was observed that overall system is stable up to grid inductance of
5mH and above this value, the system becomes unstable. It is also demonstrated that stability of the system
depends on grid impedance only and does not depend on load impedance. It is substantiated by means of
graphical analysis and Matlab Simulations.
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BIOGRAPHIES OF AUTHORS
Dr. R.S. Ravi Sankar Currently working as Associate Professor in the department of EEE in
Vignan’s Institute of InformationTechnology, Visakhapatnam, Andhra Pradesh, India. Received
his Bachelors, Masters and Doctoral degree from Institution of Engineers, JNTU Hyderabad,
JNTUA Ananthapuram. His research interests include power quality in distribution systems,
Renewable energy Sources, electric drives.
K.K. Deepika is currently pursuing her Ph. D. degree in Electrical Engineering at KLEF,
Vijayawada, Andhra Pradesh, India. She is working as Assistant Professor in the Department of
EEE in Vignan’s Institute of InformationTechnology, Visakhapatnam, Andhra Pradesh, India.
Her research interests include power quality in distribution systems, Renewable energy Sources
and Demand Side Management.
AV Satyanarayana is currently working as Assistant Professor in the Department of EEE in
Vignan’s Institute of InformationTechnology, Visakhapatnam, Andhra Pradesh, India. Received
his Bachelors, Masters and Doctoral degree from Institution JNTU Kainaa. His research interests
include Renewable energy Sources, electric vehicles, optimization techniques.

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