This document summarizes the key findings of a study on the impact of integrating AC microgrids with distributed generators (DGs) into the main grid. The study models and simulates radial and meshed microgrid configurations connected to the main grid. It finds that DGs improve voltage profiles by supplying power locally and reduce active and reactive power losses. Fault analysis shows short circuit currents increase with DGs due to reduced system impedance, though voltages are improved. In conclusion, microgrids can meet future electricity needs by generating and consuming power locally while minimizing losses on the main grid.

1. 2016 Bienniallnternational Conference on Power and Energy Systems:Towards Sustainable Energy (PESTSE)
Studying the Impact of AC-Microgrid on the Main
Grid and It's Fault Analysis
Prasanth Korada
Department ofElectrical Engineering
KTH Royal Institute ofTechnology
Stockholm, Sweden
korada@kth.se
Abstract- This paper studies the advantages of using AC-
microgrids and the impact of Distributed Generators (DG's)
on the main grid by studying its voltage, power flow and
calculating the losses in radial as weil as meshed networks.
For this paper, a grid of 4 houses and 3 DG's is modelled and
simulated and the results were presented and analyzed. Fault
analysis is conducted on the meshed system and the inferences
were drawn. It is found out that due to the integration of
Distributed Generators, the power is consumed locally and
hence the active and reactive power losses in the system are
also minimized. Based on the simulation results conclusions
were drawn.
Keywords-Smart grid, microgrid, distributed generators,
fault detection, PSA T.
I. INTRODUCTION
With the increase in global warming and environmental
issues, renewable energy based distributed generators
(DGs) will increasingly playadominant role in the
operation of the grid [1]. Distributed generation is usually
based on solar, wind, biomass and other renewable energy
based generators. A Microgrid [2] is a localized group of
DG's, storage system and load, all operating as a single
collaborative system. The interconnection of the DG to
utility grid raises concerns about fault detection and
protection which is discussed in this paper.
A huge amount of work is presently going on in the
fjeld of controlling microgrids and making it smart with the
integration of internet of things. The focus is also shifting
towards the usage of Distributed generators and it is very
important to understand the impact of DG on the microgrid
and the main grid to be able to carry a safe transition
between being a traditional grid to a smarter micro grid.
The objective of this paper is to understand the effect of
DG on the main grid and in order to analyze this, fault
analysis is conducted on an AC-microgrid which is
integrated with the main AC-grid. The simulations in this
research paper are done using MATLAB PSAT [3] and
NEPLAN [4]. It is assumed that, all quantities (unless
stated) are expressed in p.u.
This paper is organized as folIows. In section 11
theoretical background of PV generators, microgrids and
the different faults are discussed. Section III presents the
978-1-4673-6658-8116/$3l.00 ©2016 IEEE
Aryadevi Remanidevi Devidas
Amrita Center for Wireless Networks & Applications
Amrita Vishwa Vidhyapeetham (AMRITA University)
Kollam, Kerala, India
aryadevird@am.amrita.edu
simulation results and the conclusions are discussed in the
section IV.
11. THEORETICAL ASPECTS
A. Renewable Energy Sources
The energy which comes from the resources from the
nature such as sunlight, tides, waves, geothermal heat, wind
and rain which can be replenished are coined as renewable
resources. Amongst the naturally available energy sources,
solar is the most prominent one. The utilization of solar
photovoltaic (PV) systems has gained a tremendous
momentum due to decreasing costs of PV arrays and
interface systems by as much as 50% during the last five
years. [5]
A simple solar cell consists of asolid state p-n
junction fabricated from a semiconductor material
usually silicon. When the solar energy in the form of
photons hit the solar cell, electrons from the atoms of the
semi conductor material is released hence forming electron-
hole pairs. This is possible when the incident energy is
greater than the band gap energy.
Current proportional to the incident photon radiation is
developed when the charge carriers are moved apart in the
influence of the internal fields of p-n junctions.This
phenomenon is called photovoltaic effect. When the cell
is short circuited, this current flows in the external circuit
but when open circuited, this current is shunted
internally by the intrinsic p-njunction diode.[6]
The output of the set of PV cells is DC and this is
connected to a DC-AC converter to connect to a grid. The
output of the PV source in the grid is considered as
0.5MVA
B. Microgrids
A microgrid is a combination of loads and micro
sources operating as a single controllable system that
provides power to its local area. These grids tend to be
more distributed, intelligent and flexible. They are not only
driven by the growing environmental concern and the
energy security, but also by the liberalization of the
electricity market. Instead of traditional producer-
centralized power systems, the new electrical grid, also

2. called smart grid (SG) tend to be more distributed, and
consequently, energy generation and consumption areas
cannot be conceived separately.
Hence, microgrids are becoming a reality to cope with a
new scenario in which renewable energy, distributed
generation (DG) and distributed energy-storage systems
have to be integrated together [7]. This new concept makes
the final user not to be a passive element in the grid, but an
entity able to generate, storage, control and manage part of
the energy that he/she will consume. Besides, areduction in
cost and an increment in reliability and transparency are
achieved. The observed radical transformation of the
electrical grid entails deep challenges not only on the
architecture of the power system, but also in the control
system. To the utility, the microgrid of a single controllable
load can meet their special needs such as enhancing local
reliability, reducing feeder losses, supporting local
voltages, providing increased efficiency through the use of
waste heat. [8]
Until now, distribution networks are regarded with
unidirectional power flows and as a passive termination of
the transmission network with a radial structure. They often
have had a simple and efficient protection scheme. But in
reality, the presence of a large amount of DG, distribution
networks will gradually change towards a new kind of
active networks. This most change in handling the
increasing distribution of DG over the network, could be
represented by the adoption of a meshed network
architecture. [9]
C. Fault
In an electric power system, [10] a fault is any
abnormal electric current. For example, a short circuit is a
fault in which current bypasses the normal load. An open-
circuit fault occurs if a circuit is interrupted by some
failure. In three-phase systems, a fault may involve one or
more phases and ground, or may occur only between
phases. In a "ground fault" or "earth fault", charge flows
into the earth. There are two types of faults namely,
• Symmetric faults: Line to line to line to ground
(LLLG) fault and Line to line to line (LLL) faults.
• Unsymmetrical faults: Line to ground (LG), Line
to line (LL) and Line to line to ground (LLG)
faults.
During the case of fault in a microgrid, the distributed
generators would contribute to the increase in the fault. So,
it is essential to do fault detection and configure the power
electronic devices to isolate the DG's so that it would
reduce the resulted fault current.
III. SIMULATION - RESULTS
In the simulations conducted, only the fundamental
frequency components of the voltages and currents are
considered (harmonics are neglected). Also, balanced
operation is assumed i.e. only the positive sequence is
considered. For the simulations, the more traditional radial
networks which are a top down representation and a more
practical representation of the interconnected grid networks
is considered and simulated.
A. Radial Configuration
The given system is a radially connected 69KV sub
transmission system with 13.8KV distribution system
which can be observed in the Fig. l. For this simulation,
4.4 MW of load is considered. The specifications of the
system considered are given in TABLE I.
TAßLE I. LOAD SPECIFICATIONS CONSIDERED
Load t Load 2 Load 3 Load 4
O.5MVA IMVA 3MVA 3MVA
13.8KV 13.8KV 2.4 KV 13.8 KV
PV
Fig. I. PSAT setup ofthe microgrid connectied to main grid
Here, two different cases are considered and compared:
1) Base case
In the considered base case, the DGs in Fig.l are not
turned on. This is simulated using PSAT.
2) Base case with DG's
Now the microgrid is simulated in PSAT with DGs
tumed on from the Fig. 1.
By comparing Fig.2 and Fig.5 with Fig.6 and Fig.9 we
can infer that the power consumed from the main grid has
been reduced with the introduction of DG and the real and
reactive power is supplied by the DG's cater to the local
needs. By comparing Fig.3 and Fig.4 with Fig.7 and Fig.8
we can infer that the voltage profiles at the loads have
increased after introducing DG's into the grid. The DG's
supply voltage locally and hence increasing the voltage
profiles at each bus.

3. Real Power Profile
0.08
005
0.04
~
0.02
Eo
CL~
"CL
-0.02
-0.04
-0.06
10 11
Bus #
Fig.2. Real power profile ofbase case w/o DG
Volt age Magnitude Profile
0.9
0.8
0.7
05
~ 0.5
>
0. 4
0.3
02
0.1
10 11
Bus #
Fig.3. Voltage magnitude profile ofbase case w/o DG
Voltage Phase Profile
0.08
0.06
0.04
0.02
'"~
-0.02
-0.04
-0.06
-0.08
10 11
Bus #
Fig_ 4_ Voltage phase profile ofbase case w/o DG
0.06
0 05
0.04
0.03
0.02
~
o~ 0.01
"0
-0.01
-0.02
-0 03
-0.04
Reactive Power Profile
10 11
Bus #
Fig_ 5_ Reactive power profile ofbase case w/o DG
Real Power Profile
0.05 '--~-~~-~~-~~-~~-~~-,
-0.05 L------'-----'-------O-----'---------,O----------;,-----~----;8;--~---;-1O~-1;';1-----'
09
0.8
0.7
0.6
~ 0.5
>
04
0.3
0.2
0.1
Bus #
Fig. 6_ Real power profile ofbase case with DG
Vo ltage Magnitude Profi le
10 11
Bus #
Fig_7. Voltage magnitude profile ofbase case with DG

4. Voltage Phase Profile
0.8
0.6
0.4
~
'" 0.2
-0.4 L----'--------::--------:'--'--------:'--:--------:'--:--------:'------:1-'::-0 -----:1":-1-----'
Bus #
Fig. 8. Vo1tage phase profile ofbase case with DG
Reactive Power Profile
0.05
0.04
0.03
-'0 0.02
o~
0
0.01
0
-0.01
-0.02
-0.03 L----L------'_--'-----
10 11
Bus #
Fig. 9. Reactive power profile ofbase case with DG
B. Meshed Configuration
For this case study 12 bus system with mesh
interconnections is considered which is shown in
Fig.l0.The power tlow analysis of the system can be seen
from the TABLE 11. The blue line in the Fig.ll and Fig.12
indicates 69kV line and the pink line indicated 13.8kV line.
Here, two PV generators and one Wind generator are
considered.
Fig. 10. Line diagram ofthe meshed microrid case
TABLE 11. POWER FLOW ANALYSIS OF THE SYSTEM
BlJS lJ P Load Q Load P Gen QLoad
NO. (kV) (MW) (MVar) (MW) (MVar)
Bus 1 72.45 0 0 49.94 3.21
Bus 2 72.11 21.7 12.7 40 39.58
Bus3 69.88 60 19 45 0
Bus 4 70.12 47.8 4 0 0
BusS 70.41 7.6 1.6 0 0
Bus6 14.49 11.2 7.5 40 0.09
Bus 7 14.22 0 0 0 0
Bus 8 18.9 0 0 40 12.87
Bus 9 14.02 29.5 16.6 0 0
Bus 10 14.26 6.1 1.6 0 0
Bus 11 14.17 13.5 5.8 0 0
Bus 12 14.26 6.1 1.6 0 0
To observe the impact of the distributed generators
(DG) on heavily loaded meshed networks the following
cases are considered and the simulations are conducted.
1) Base Case
[n this case, like the way it was considered in the radial
case, the DG's are switched off and the system is simulated
and the voltage and power profiles at each bus are noted.
As we can see that the red indications in the Fig.ll indicate
that the voltage regulation is lesser than the required
voltage at the corresponding busses.

5. Fig. 11. NEPLAN simulation ofmeshed base case w/o DG
2) Microgrid Case
We can see from Fig.12 that with the implementation of
DG in the system, the voltage regulations are as per the
requirements.
By comparing the voltage profiles at each bus as shown
in the Fig.13 we can infer that by implementing the DG the
voltage profiles at each bus have improved even in the
mesh distributed system.
By calculating the power losses in the system in 69kV
line and 13.8kV line before and after the usage of DG's as
shown in the Fig.14, we can infer that the active power loss
have reduced.
Fig. 12. NEPLAN simulation ofmeshed microgrid case with DGs
80
70
,-..
60>..:.:: 50'-'
(!)
40CIJ
o::l
30..::::
0
20> 10
0 111111 11111111
1 2 3 4 5 6 7 8 9 10 11 12
Bus number
• Base Case • Microgrid Case
Fig. 13. Comparision ofvoltage profiles at each bus
10
,-..
~ 8
::8 6
'-'
Cf] 4Cf]
0 2......l
0 - - -13.8kV 69kV
Line
• Base Case • Microgrid Case
Fig. 14. Active power losses
C. Fault analysis
The most severe of all the existing faults is 3 phases
connected together and grounded, fault currents are highest
of all four and most likely to damage equipment. So, a 3
phase to ground fault case is considered for this paper.
Fault analysis is conducted to the meshed distributed model
since it provides a more practical approach to the real
systems. Fault analysis is done on the system at each bus
and the fault currents are calculated at each bus. The results
are tabulated as shown in TABLE III.
It can be noted from the Table 111, that by including
DG's which are connected in shunt, we reduce the
impedance, which not only improves the voltage levels but
also reduces the effective impedance of the system due to
which the short circuit current increases'.

6. TAßLE 111. FAULT ANALYSIS RESULTS
Bus No. Base case (kA) Microgrid (kA)
1 17.590 17.860
2 204.349 212.033
3 7.658 425.141
4 12.520 15.712
5 12.923 14.821
6 16.658 18.599
7 21.151 23.910
8 8.977 10.279
9 19.006 20.937
10 11.121 11.910
11 14.056 15.298
12 12.635 13.457
IV. CONCLUSION
Microgrid system is an alternative electricity network
that can be used to meet the electricity needs of the future.
Improvised voltage regulation is observed with the
integration of different distributed generations in the main
grid to form microgrid both in radial as weil as meshed
systems. Since the power is generated and consumed
locally the active and reactive power losses in the system
are also minimized. However, it should be noted with the
integration of distributed generation at lower voltage levels
there might be chances of increasing the fault levels of the
system. Thus, by using power electronic devices which can
create electrical isolation, this problem can be evaded. It is
also observed that there will essentiality to change the
protection settings as per the microgrid configuration since
there will be bidirectional power tlow between
transmission and distribution system. Further, detailed
analysis will be required to find the optimal position to set
up new distribution generations in the given AC system to
form microgrids.
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