Power-Sharing of Parallel Inverters in Micro-Grids via Droop control and Virtual Impedance

International Research Journal of Engineering and Technology (IRJET) e-ISSN: 2395-0056
Volume: 09 Issue: 11 | Nov 2022 www.irjet.net p-ISSN: 2395-0072
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Power-Sharing of Parallel Inverters in Micro-Grids via Droop control
and Virtual Impedance
Samia Abdalfatah1, Mohammed Gamal2, E.E. Elkholy3, Hilmy Awad4
1Faculty of Technology and Education, Helwan University, Egypt,
2 Electricity Teacher, Industrial Secondary School, Menoufia,
3 Engineering Department, Faculty of Engineering, Menoufia University, Egypt,
4Faculty of Technology and Education, Helwan University, Egypt.
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Abstract – The inverter based small networks connected in
parallel, the inverters can operate in connected or separate
network mode, and in the connected mode, the set points for
each inverter are created by processing data on the active
output powers and placing all the inverters in a principal
controller based on the needed output power ratios. Here, two
proposed power-sharing structures are used to provide a fast
and accurate dynamic response to a low circulating current
between parallel inverters and the ability to adapt to the
required changes in the system to make it stable and make
each inverter give its full capacity to the loads. In this article,
two different types of system control are used to share energy
through electrical inverters. The droop control and virtual
impedance are used, and each system improves the
performance of the control in a better way to share the load
energy. The proposed energy-sharing control systems
strategies have been validated using mat-lab/Simulink
simulation results
Keywords: Droop Control; Micro-grid control; Power
Sharing; parallel inverter; line impedance.
1. INTRODUCTION
The integration of several distributed energy resources
(DERs) that are linked in parallel, such as parallel inverters
in micro-grid operation, is necessary to meet the growing
need for large-scale power supply with high dependability
[1]. Advanced control techniques are necessary for parallel
inverters to operate properly. Many of these methods were
first presented decades ago, and they are still developing
today [2]. a frequency-voltage droop approach is a well-
recognized widely used,and successful methodforoperating
parallel inverters [3]. this method simulates how a large-
scale power system works by using a pre-set droop feature
that links improvements in generator speed and output
active power. This technology is known as wireless control
since no communication is necessary between the inverters
making it simple to deploy and dependable [4]. But, it has
various drawbacks that might hinder its effectiveness.Some
of its limitations were just as described in the following: its
frequency and amplitude differences are load dependent,
resulting in poor load voltage regulation performance; an
inherent trade-off between voltage regulation and power
sharing between inverters, and impedancemismatchamong
inverters affects power sharing performance [5].
Many improvements have been proposed in recent years to
increase the effectiveness of the droop control approach in
order to satisfy the rising needs of micro-grids. Modified
droop [6-8], adaptive droop [9-11], mixed droop [12-14],
and interconnected droop working principle [15-18] are
some of the suggested changes. A standard droop system is
given a boost in transient responsiveness in [6] by the
addition of power derivative-integral terms. Selecting the
proper coefficients forthederivativeterm,however,inorder
to guarantee stable system performance, is challenging. The
authors of [11] suggested combining static droop features
with an adaptive transient droop function to enable active
dampening of power oscillations. The authors did not,
however, provide experimental confirmation for this
method. The authors of [16] presented an enhanced droop
method that employs web-based limited bandwidth
connection to enhance load-sharing ability. This
performance realizes efficient electric power-sharing with
the micro grid. A droop control with an optimization system
is presented in [13]. It uses particle cloud optimization to
optimize the (v-f) constant. It shows respectable active and
reactive power sharing in simulation, but there is no
hardware confirmation.
Inverters linked in parallel have lately come to understand
that exchanging certain information among them may help
accomplish great current sharing and voltage management
in a parallel system. Active load-sharing methods are a few
examples of control strategies that make use of
communication between parallel inverters. These include
the average current configuration [21, 22], the master-slave
system [19, 20], and the spherical current technique.
In the circular-chain current approach, succeeding inverter
modules follow the current of the preceding inverter to
achieve equal current circulating.Thefundamental flawwith
this strategy is that it significantlyreliesoncommunications,
which introduces substantial uncertainties into the system.
The master/slave approach employs one inverter to control
the amplitude and frequency, while the remaining inverters
serve as slaves that inject currents. All of the micro grid's
inverters participate in the typical current-sharing
mechanism, which regulates voltage,frequency,andcurrent.

In particular, the inverter current injection averaged over a
common bus is taken into account as a reference for each
module when determining the average current data.
Active load sharing approach has unique benefits and
drawbacks. There are many different ways to solve the
problem, from communication to techniques that need less
communication. To avoid overheating and maintaina longer
lifespan, some applications have strict loadingrequirements
for the inverters, and correct active and reactive power
sharing is essential. These implementations will be made
easier by having the ability to change the power references
while maintaining precise sharing and damped
responsiveness. Therefore, a control scheme that is
adaptable, trustworthy, and has strong voltage stability,
current sharing, and decreased current flowing features is
preferred. It should also have minimal reduced
communications and be resistant to communication
overhead.
In this article, a comparison will be made between two
different power-sharing technologies. In this paper, a
comparison will be made between two other power-sharing
techniques (droop controller and virtual impedance), which
uses fuzzy logic, and necessarily requires a low-bandwidth
connection to a central controller.All oftheparallel inverters
that are connected to the central control unit send
information about their active and reactive power, and it
uses that data to calculate each inverter's activeandreactive
power references. Based on a particular ratio of output
power to each inverter, these references are computed.This
information is used by each inverter to modify the voltage
and amplitude reference phase in relation to the shared AC
vector in order to achieve the required output power. By
adjusting the phase rather than the frequency, as a result, it
is possible to achieve effective frequency regulationwithout
affecting the nature of the various filters LCL and LC used to
share power between inverters connected in parallel with
the grid between power and load sources for system
stability. The overall design of the inverterisshowninfigure
1. The voltage controller transmits modulated sine waves to
its IGBT switching components, which begin receivingthem.
The system contains two solar cell networks, in which the
maximum power is obtained through Perturb and Observe
(o & p) [33]. The two networks are connected from the side
of the inverter output. The inverter is controlledthroughthe
two techniques .Below is a simplified explanation of each
part of the system.
five sections make up the structure of this term paper
Section (2 and 3) discusses the system architecture One of
the fundamental problems of parallel inverters is the robust
design of each inverter operating in parallel with the power
grid and the proposed power -sharing method, as A stability
study of the proposed technologies for virtual impedance
and system control algorithm, which has a maximum power
point tracking MPPT photoelectric controller,ispresentedin
Section (4). Analysed in a mat-lab environment Simulink
simulation results in variable weather conditionsareshown
in Sections (5) to support the feasibility and efficacy of the
proposed method respectively, the conclusions emphasize
the main results and the ability to contribute to this article.
Fig-1: General micro grid Structure
1.1 Photo voltaic (P.V) System
A PV module consists of several solar cells connected in
series and parallel to obtain the desired voltage & current
output levels shown in Fig 2. PV consists of a photocurrent
source 𝐼ph, diode, and internal resistances 𝑅𝑆andRp[35,36].
Iph: Light produced current (A).
ID: Diode saturation current (A).
Io: reverse saturation current of the diode
q :electron charge( )
K: Boltzmann constant in 1.3865×10^-23 (J/K)
µ: conversion efficiency (%)
Id: current through the diode
Tc: operating temperature (ºc)
o
o
Rsh
I
Rsh
R
s IR
s=Io
VL
oad = Voc
I
D
I
pv
Ir
C
omplete p.v cell model
P
o
PMAX
d
Diod
Fig-2: Model for a solar cell
Parasite resistivity is a component of a functional
photovoltaic array.
Due to the continued resistance of the interconnections
metal grid, p and n layer, there is series resistance

Shunt resistivity brought on by p-n connection leakage
current.
A PV system is physically described in the following
equations [35, 36].
The solar module's eﬃciency is determined by dividing the
greatest amounts of electricity it can generate by solar
irradiation [28].
µ = (5)
1.2 Maximum Power Point Tracking (MPPT)
Tracking the PV array's greatest peak power in order to
produce the most electrical power is the crucial step for
photovoltaic arrays. The best procedure should be followed
during the design of the PV system, and various MPPT
methods can be used to ensure this [23]. It depends on
irradiance and temperature. Therearevariouslayersin each
strategy. Application characteristics, including those of the
most iconic one, the hill climbing technique on the resilience
of two points, can have a significant impact on the choice of
MPPT control systems, including complexity sensors,
amount of digital or analogue applications, rapid
convergence traceability, and financial impact. P(k), p (k-1)
Start comparing MPPT and MPP side by side.
The primary benefit of strategic is that it is inexpensive,
simple to implement, does not require a control scheme or
micro - controller, and only requiresonevoltagesensor[17].
As long as the radiation from the sun does not fluctuate too
much throughout the day, this method works well [28].
As shown in Fig.3 the suggested MPPT algorithm is
perturbative and controls the dc - dc converter using
feedback. A boost converter It increases the voltage value at
the expense of the current value and deals with the constant
current only and makes a comparison process between the
voltage of a solar cell Vpv and the voltage inside the inverter
Vref.
Fig-3: Schematic diagram of boost converter control.
In Fig.4 the photo - voltaic output power is plotted against
the plate voltage for specific radiation. Traditional P&O
technology [27,30] uses photovoltaic panels or current
disruptions to continue operating and compares the power
rating of the photovoltaic cell to the priorperturbationcycle.
Features 1- Initial Restorative Dynamics
2- Analog /digital implementation
3- Minimal software/hardwarerequirements.Theoperating
voltage in between the PV array and the converter is
perturbed by the P&o method.
Fig -4: P&O Basic Idea Algorithm
P&O It consists of two main parts, Point A: dP/dV > 0 If the
operating point on the left MPP and Point B: dP/dV< 0 If
the operating point on the right MPP, dP/dv = 0 At the
operating point on the MPPTurbulentisemployedinvoltage
and power measurement methods, and the high value
utilization of the turmoil direction is calculated by
P . If the polarization of a power output is
positive, a next disturbance will travel in same directions as
the prior one. If the depolarization of a power output is
negative, a next disturbance is in the reverse directionasthe
prior one. When MPP is attained, the operation is repeated.
1.3 DC/AC Inverter and LCL Filter
the inverter as shown in the image(5) converts dc to ac
current and must be included in any micro-network that
uses electrical power electronics to serve single or three-
phase loads consisting of high frequency solid-state
electronics and an L - LCL low-pass filter it is in charge of
smoothing the output wave in order to achieve a sinusoidal
signal free of harmonics the inverter has a steady current
MPPT
Controlle
r
PI
Controll
er
PV& Boost
converter
Vref
Vpv
VPV
IPV
D

input and is linked to a capacitor which stabilizes and
regulates the voltage energy from solar cells and raises it to
the inverter the output is the ac current produced at the
inverters output signals are sent by the voltage controller to
the switching devices which are isolated gated bipolar
transistors( IGBT) Vref and P.W.M signals are generated in
addition when the A.C signal of the power electronic
switches changes harmonic signals are created theLCLfilter
is widely used in conjunction with inverse networks to
enhance current excellence and supply optimum sinusoidal
power to a power grid while minimizing harmonics.
Fig -5: inverter and LCL filter
2. Virtual impedance technology
A voltage controller loop is used in basic voltage source
inverters VSI to track the desired input signal and reduce its
error and the measured output voltage. A proportional
controller, Kv, is utilized in this study, backed by a feed-
forward loop. The feed-forward loop reduces steady-state
error while allowing for a broader control band [1,3].
Fig.4 depicts one inverter phase.Theirinnerloopcontrollers
are shown in this block diagram [12,14]. In this paper,
(“Virtual ImpedanceImpactonInverterControl Topologies”)
This impedance mimics the behavior of an inductor or
resistor in the program. Using a programmable impedance
rather than a physical one reduces the losses and costs
[19,22]. In addition, being programmable presents adaptive
operation and increases the inverter’s robustness against
network impedance variations [23,27]. Fig. 6 shows the
block diagram of the voltage controller with the virtual
impedance Zv(s).
The output impedance with virtual impedance can be
derived as, (“Virtual Impedance Impact on Inverter Control
Topologies”)
The nature of Zv could be chosen to be resistive as,
Where is the resistance of the virtual impedance, or it
can be inductive as
Where is the inductance of the virtual impedance and is
the time constant of the high pass filter used to
approximate the derivative in the transfer function of the
ideal virtual inductance [25].
Physical
part
core
control
_
1
L1S
+
+ 1
CS
L2 S
Ke
KV
VO
VC
Feedforward
Virtualimpedance
V* _
+
_
+
+
_
IO
IC
-
- -
Zv(s)
+
-
Fig -6: The model of basic double-loop voltage controller
3. Droop control technology
In this paper, a micro-grid consisting of two inverters as
shown in Fig.1 is considered. The circuit diagram of each
inverter and its LCL filter and controller is illustrated in Fig.
7. This is the basis for the conventional droop characteristic,
as represented in (8) and (9) [6].
Where 𝜔 ∗, 𝑉 ∗ are the nominal frequency and nominal
voltage references, m and n are the frequency- proportional
drooping coefficient and voltage-proportional drooping
coefficient, respectively. The droop slopes are determined
according to the power rating of the inverter and according
to the maximum allowable variations in output frequency
and voltage [11,13]. In grid-connected mode, the active and
reactive power set-points and areadjustedtobe equal
to the reference power values. The conventional droop
characteristic is summarized in Fig.8. In this figure, the
loaded active- and reactive power is denoted along the x-
axes. As shown, an increased active load leads to a reduction
in the frequency, while the output voltage is reduced if a
more reactive load is addedtothesystem.Thischaracteristic
can be utilized in the conventional droop control method,
where the active power fed into the line is controlled based
on frequency deviations, while deviations in voltage control
the reactive power supply. Thedeviationsaredeterminedby

the droop coefficients, which are given based on (10) and
(11) [36,26].
(13)
∆ωmax is the maximum allowed frequencydeviationdue toa
load change in active power, Pchange. The maximum allowed
voltage change, ∆Vmax, is in a similar way related to a load
change in reactive power, Qchange. Often, ∆ωmax and ∆Vmax are
decided based on the maximum deviations allowed in the
grid. The droop coefficients also affect the power sharing
among inverters, where larger droop coefficients lead to
better power sharing. However, the coefficients have upper
limits, where increasing them would lead toinstabilityin the
system. Within the stability limits,
the choice of droop coefficients is a trade-off between the
power sharing performance and the deviations in voltage
and frequency. [15,17].
Fig -7: Implementation of the conventional droop control
Fig -8: The characteristic of the conventional droop
control
4. SIMULATION RESULTS AND DISCUSSION
This section illustrated the performance of PV module and
performance of perturb and observe algorithm to track
maximum power point of PV module under various weather
conditions. The PV module simulation is implemented in
MATLAB Simulink. The parameter of the PV module (330
Sun Power modules). is displayed for one module is:
Number of parallel-connected cells: 39 cell
Number of series-connected cells: 5 cell
shows Fig.9 shows the power-voltage characteristic of PV
module in four cases, and Fig.01 shows the sun'sradiationof
value.
To verify the performance of the simulation that was
conducted on MATLAB / Simulink, to compare two different
technologies, namely, the virtual impedance and the
traditional droop control to control the electric power
output to share the electricloadswiththemain network.The
micro-grid contains two electrically identical stations
conditioned by variable solarradiationrangingbetween250
and 1000 W/ at a constant temperature of 25 °C.
Fig-9: Characterizes of photovoltaic
The efficiency of the P.V system in various weather
conditions and turbulence is also as a monitoring method
used to track the highest energy point. The photovoltaic
module characteristic shows the(60)kWphotovoltaiccell in
the proposed model consisting of (330) solar modules. The
PV module parameters for a single module are shown as
follows:
It is clear from Fig .10 that the number of panels connected
in series (5) successive stages
• T= (0-0.5) sec: with a radiation intensity of (200) W/
at a constant temperature (25 °C), then the intensity of
solar radiation increases to 1000 W/ gradually
• T= (0.5-2) sec: the radiation intensity remains constant,
• T= (2-4) sec: the radiation intensity gradually decreases
until it reaches (300) W/
• T= (4-5) sec: the radiation intensity gradually decreases
from 300 to zero.
• T= (5-6) sec: the radiation intensity becomes (0), meaning
that the two stations exit the system and give electrical
energy = 0, and the load derives itsenergyfromthe electrical
grid.

Fig -01: Irradiance input to the PV array
The active power output curve of the photovoltaic cells over
time with a variable solar irradiance value from 200 to1000
W/ as in Figure11 and at a constant 25°C. The maximum
value of the capacity of the solar cells is 60 kW, and with the
change of solar radiation over time, when the system starts
in the first 0.5 sec, a sudden change in the value of electric
energy occurred when using the virtual impedance
technology to share the electric power of the loads with the
micro grid.
Fig -01: Active power mean of the two PV
Case study (1): Simulation results if no droop control
technique or no virtual impedance is applied to share the
output power of two identical micro-grid inverters.
Figure 01 illustrates the output power of each inverter. It is
assumed that both inverters arealreadyoperatinginparallel
and that the without power sharing. We note that theoutput
of the second reflector (Ppv2) gives its full value, while the
first (Ppv1) reflector gives a much lower value with the
change in solar radiation. the power responses lost the
stability.
Fig-01: Active power output of the two inverters
(without power sharing)
Case study (2): Simulation results when using the virtual
impedance control technique to share the active power of
two Identical inverters connected to the micro-gridhave the
same output power.
Fig.02 shows the output power of each inverter. It is
assumed that both inverters work together in parallel as the
inverter is controlled by controlling the voltagesourceusing
the visual impedance. Where it was found from the
operation that the output capacity of the first inverter
(which output is connected to a filter of type LCL) does not
reach its maximum power, but is lowered from it bya simple
value
Fig-13: Active power output of the two inverters
(with virtual impedance)
Case study (3): Simulation results when using the droop
control technique to share the output poweroftwoIdentical
inverters connected to the micro-grid.
Fig.14 shows the output power of each inverter. It is
assumed that both inverters work together in parallel as the
inverter is controlled by voltage source control using droop
control. The process shows that the output capacitance of
the first inverter (which output is connected to a LCL-type
filter) has a fast response and achieves cooperation with the
change in the value of the output power of the solar cells.

Fig -14: Active power output of the two inverters
(with droop control)
5. Conclusion
In this article, two methods of output power sharing
between inverters connected in parallel withthe micro-grid,
namely droop control and virtual impedance, are proposed
to be applied to AC networks to ensure high accuracy and
flexibility in electric power sharing. And we conclude from
the results obtained from simulations in the Mat-lab /
Simulink environment for two parallel inverters can be
painted:
• When virtual impedance control is applied in a small grid
system as a technique for sharing electrical energy between
inverters with fuzzy used to improve the output shapeofthe
effective power, it does not work well under these changing
conditions, as the effective power sharing is affected by the
virtual impedances in the small grid between the inverters.
Transformers and PCC during the sharing process.
• To overcome this problem, the proposed droop control
technique is applied to adjust the line resistance value to
properly share the effective power between the inverters
and work better under the same conditionsevenwhenthere
are differences in the line resistance in a small network.
• The voltage droop can be reduced in the proposed droop
control technique.
• Swing control The ability to quickly switchaccordingtothe
proportions of the active electrical power accurately.
When implementing a proposed micro-grid with VSI
transformers parallel to the electrical network forsupplying
connected electrical loads, consideration should begivento:
DC sources with galvanic isolation; Determines the total
electrical capacity of the load. In addition, the controller
controls each of the VSI switches connected to the micro-
network so that no interference occurs and the system is
balanced, stable, and works well.
It has been scientifically proven from the simulation results
that the proposed droop control method is superior and
works better and more accurately while making the system
balanced and stable ineffective force sharing when the line
resistance is actually changed from the default impedance
technology, plus it does not need human intervention.
Table -1: Comparison between Droop control and Virtual
impedance
Comparison
scheme
(v/f)Droop
control
technique
Virtual
impedance
scheme
Capability to set
power ratio
Yes, without the
need for fuzzy
Yes, but needs a
fuzzy
Advantage
- Fast dynamic
response at the
start of the
operation
- Very flexible
and has the
ability to expand.
-System stability
and power
sharing accuracy
-It is not affected
by any
fundamental
change in the
system
-Ease of
implementation
in the absence of
contact between
the inverters
-Can be used
with linear and
non-linear loads
when electric
power is shared
- Reduces
harmonic
voltage
-Decoupled
active and
reactive power
controls
Dis advantage
-Inaccurate
voltage and
frequency
regulation
-Low dynamic
response
-It is necessary
to know the
physical
variables first
-Inaccurate
voltage and
frequency
regulation
-Affected by any
physical change
in the system
- The system is
unstable,
especially when
starting, and
less accurate in
sharing electric
power

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