Operation and Control of Grid Connected Hybrid AC/DC Microgrid using Various RES

International Journal of Power Electronics and Drive System (IJPEDS)
Vol. 5, No. 2, October 2014, pp. 195~202
ISSN: 2088-8694  195
Journal homepage: http://iaesjournal.com/online/index.php/IJPEDS
Operation and Control of Grid Connected Hybrid AC/DC
Microgrid using Various RES
Kodanda Ram R B P U S B, M. Venu Gopala Rao
Department of Electrical and Electronics Engineering, K L University, Guntur, AP, INDIA
Article Info ABSTRACT
Article history:
Received Apr 30, 2013
Revised Jan 10, 2014
Accepted Feb 24, 2014
This paper proposes a Hybrid AC/DC Microgrid in alliance with Photo
Voltaic (PV) energy, Wind Energy and Proton Exchange Membrane (PEM)
Fuel cells. Microgrids are becoming increasingly attractive to the researchers
because of the less greenhouse gases, low running cost, and flexibility to
operate in connection with utility grid. The Hybrid AC/DC Microgrid
constitutes independent AC and DC subgrids, where all the corresponding
sources and loads are connected to their respective buses and these buses are
interfaced using an interfacing converter. The Hybrid AC/DC Microgrid
increases system efficiency by reducing the multiple reverse conversions
involved in conventional RES integration to grid. A Small Hybrid AC/DC
Microgrid in grid connected mode was modeled and simulated in MATLAB-
SIMULINK environment. The simulation results prove the stable operation
considering the uncertainty of generations and loads.
Keyword:
Fuel Cells
Hybrid Power Systems
Interfacing Converter
Micro grids
Solar Energy
Wind Energy Copyright © 2014 Institute of Advanced Engineering and Science.
All rights reserved.
Corresponding Author:
Kodanda Ram R B P U S B,
Department of Electrical and Electronics Engineering,
K L University, Vaddeswaram
Guntur District, Andhra Pradesh, INDIA – 521456
Email: balajir1986@kluniversity.in
1. INTRODUCTION
A Microgrid is a small grid formed by banking multiple energy resources and loads to enhance
overall reliability and independent advantages. Now-a-days, it is more preferred to integrate renewable
energy resources to Microgrid to lessen the CO2 emission and fossil fuel consumption. The banked Microgrid
can be operated either in connection to main grid or operated like isolated “islanded” [1]. Now-a-days, DC
loads like LED’s, Electric Vehicles and other Electronic Gadgets are being increasingly used due to their
inherent advantages. Three Phase AC Power systems have existed for over 100 years due to their efficient
transformation at different voltage levels and transmission over long distances. The inherent characteristics of
rotating machines make it feasible for larger period.
To connect the conventional AC system to the renewable resources, AC Microgrids have been
proposed and DC power from the various resources like PV panel, Fuel cells etc., are converted into AC in
order to connect to an AC grid, which are implanted by AC/DC Converters and DC/DC Converters [2]. In an
AC Grid, several converters are required for various home and office facilities to provide required DC
voltages. AC/DC/AC converters are commonly used as drives in order to control the speed of AC motors in
industries.
Recently DC grids are resurging due to development and deployment of renewable DC resources
and their inherent advantage for DC loads in residential, commercial and industrial applications. The DC
Microgrid has been proposed [3]. However, for conventional AC loads DC/AC inverters are required and AC
sources are connected using AC/DC Converters.
Multiple reverse conversions required in individual AC or DC grid may add additional loss to the
system operation and will make the current home and office appliances more complicated in design and

 ISSN: 2088-8694
IJPEDS Vol. 5, No. 2, October 2014 : 195 – 202
196
operation [4]. The current research in the electric power industry is smart grid. One of the most important
futures of a smart grid is the advanced structure which can facilitate the connections of different AC and DC
generation systems, energy storage options and various AC and DC loads with the optimal asset utilization
and operational efficiency. The power electronics converter plays a most important role to interfacing AC
and DC grids, which makes future grid much smarter.
A Hybrid AC/DC Microgrid is proposed to reduce processes of multiple reverse conversions in an
individual AC or DC grid and to facilitate the connection of various energy sources, storage devices and
loads [5]. The advanced power electronic devices and control techniques are used to harness maximum
power from renewable power sources, to minimize power transfer between AC and DC networks. PV system,
PEMFC constitutes the DC Energy sources; Wind system constitutes the AC energy source, whereas Battery
and Conventional Grid are used as storage devices whenever required.
2. SYSTEM CONFIGURATION AND MODELING
Figure 1 illustrates the compact representation of proposed Hybrid Microgrid Configuration. The
Hybrid Microgrid was formed by a DC sub grid and an AC sub grid. Each sub grid has its own sources
elements, storage elements and loads of the same category grouped together so as to reduce the amount of
power conversion required. Both sub grids are interfaced using interfacing converters. Interfacing converters
are the bidirectional converters, and their major role is to provide bidirectional energy transfer between the
sub grids, depending on the prevailing internal supply – demand conditions.
The formed Hybrid grid can be tied to Utility grid using an Intelligent Transfer Switch at point of
common coupling as in conventional AC grids. In grid tied mode of operation, surplus energy in the internal
sub grids if any can be injected to the utility grid without violating the local utility rules. Similarly, the
shortfall in both the sub grids if any can be absorbed from the utility grid.
Figure 1. A Compact representation of the proposed Hybrid Microgrid
2.1. Proposed Hybrid Microgrid Configuration
PV Array(40kW) and PEM Fuel Cell (50kW) are connected to DC bus through independent DC/DC
boost converter to simulate DC sources. Capacitors Cpv and Cfc are used to suppress the high frequency
ripples of the PV and FC output voltage.
Also, a wind turbine generator (WTG) with DFIG (50kW) and utility grid are connected to AC bus
to simulate AC Sources. In addition, a battery (65Ah) and super capacitor (0.5F) are individually connected
as energy storages to DC bus through buck-boost (DC/DC) converter. The DC load was considered as pure
resistive load and AC loads are considered with RLC which are dynamic in nature. Both the loads are
variable between 20kW – 40kW. Rated voltages for both buses are considered as 400V. The parameters of
the Hybrid Microgrid are tabulated in Table 3
.
2.2. Modeling of PV Panel
Figure 2 shows the equivalent circuit of a PV Panel modeled by a controlled current source. Ipv and Vpv
are the terminal current and voltage of the PV panel, respectively. The current output of the panel is modeled
using three Equation (1), (2), (3) [6]-[7]. The parameters that were taken into consideration for simulation are
shown in Table 1.

IJPEDS ISSN: 2088-8694 
Operation and Control of Grid Connected Hybrid AC/DC Microgrid using… (Kodanda Ram R B P U S B)
197
1 (1)
. (2)
. (3)
Figure 2. Equivalent circuit of a PV Panel
Table 1. Parameters of Photovoltaic Panel
Symbol Description Value
Voc Rated open circuit voltage 403 V
Iph Photocurrent
Isat Module reverse saturation current
Q Electron charge 1.602 x 10-19
C
A Ideality factor 1.50
K Boltzmann Constant 1.38 x 10-23
J/K
Rs Series resistance of a PV cell
Rp Parallel resistance of a PV cell
Isso Short-circuit current 3.27 A
ki SC Current temperature Coefficient 1.7 e-3
Tr Reference Temperature 301.18 K
Irr Reverse Saturation current at Tr 2.0793e-6
A
Egap Energy of the band gap for silicon 1.1 eV
np Number of cells in parallel 40
ns Number of cells in series 900
S Solar Irradiation Level 0 ~ 1000 W/m
T Surface temperature of the PV
2.3. Modeling of Fuel Cell
Figure 3 shows the equivalent circuit of PEM Fuel cell. The ohmic, activation and concentration
resistances are represented with Rohmic, Ract, Rconc respectively. C is the membrane capacitance. The
Membrane voltage equation is given by Equation (4).
1 (4)
The output voltage of the PEMFC is given by (5):
(5)
-
Vc
+
E
Rconc
Ract
C
Rohmic
Vfc
Ifc
Figure 3. Equivalent circuit of PEM Fuel cell

 ISSN: 2088-8694
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2.4. Modeling of Battery
Battery is not very important in grid-tied mode. But, provides an energy storage in DC subgrid,
which can reducethe multiple reverse conversion, whenever required. In emergency i.e., Grid Failed
Condition, they plays a vital role in power balance and voltage stability. The battery was modeled using a
controlled nonlinear source in series with a constant resistance. The State Of Charge (SOC) of the battery is
given by Equation (6).
% 100 1 (6)
Where is the extracted capacity and Q is the Maximum capacity if battery.
2.5. Modeling of Wind Turbine Generator with DFIG
In this paper, DFIG was considered as a wound rotor induction machine. The power output Pm from
a WT is determined by [3]. A 50kW DFIG parameters, used in this paper are shown in Table 2.
0.5 , (7)
Table 2. Parameters of DFIG
Pnom Nominal power 50 kW
Vnom Nominal Voltage 400 V
Rs Stator resistance 0.00706 pu
Ls Stator inductance 0.171 pu
Rr Rotor resistance 0.005 pu
Lr Rotor inductance 0.156 pu
Lm Mutual inductance 2.9 pu
J Rotor inertia constant 3.1 s
np Number of poles 6
Vdc_nom Nominal DC voltage of AC/DC/AC converter 800 V
Pm Nominal Mechanical power 45 W
3. CONTROLLERS
The Hybrid Microgrid contains six types of converters. All the converters have to be coordinately
controlled with the utility grid to supply reliable, high efficiency, high quality power for variable DC and AC
loads. The controllers are presented in this section are coordinated successfully in both grid-tied. A Direct
Torque Control Strategy(DTC) with feed forward voltage compensation is selected for DFIG control system
[9].
3.1. Boost Converter
In grid tied mode, the control objective of the boost converter is to track the MPPT of the PV panel
and Fuel Cell. The PV Panel and Fuel Cell boost converters are designed to support the DC bus voltage as
shown in Figure 4. To achieve maximum power, P & O Method proposed in [6].
Figure 4. Control Scheme of PV Cell and PEM Fuel cell
3.2. Control of Battery
Battery has high energy density with slow charging and discharging speeds. Control scheme of
Battery is shown in Figure 5.

199
Figure 5. Control Scheme for Battery
3.3. Interfacing Converter
The objective of the interfacing converter is to interface both the sub grids i.e., AC grid and DC
grid. The major role of the interfacing converter is to exchange power between the AC bus and DC bus.
When operating in grid tied mode, the converter supplies given active and reactive power. The interfacing
converter acts DC/AC inverter when supplying power from DC grid to AC grid and acts as AC/DC rectifier
when supplying power from AC grid to DC grid whenever required. The interfacing converter works based
on droop control [11]. The control scheme of interfacing converter is shown in Figure 6.
The advantages of interfacing converter cannot be realized by just relying on the droop controlled
sources. The interlinking control challenges has to be carefully addressed [12].
a) Unlike unidirectional sources, the interlinking converters has to manage bidirectional active and
reactive power flows between sub grids.
b) At any one instant, the interlinking converters have two roles to play. They appear as load to
one sub grid where energy is absorbed and appear as source to other grid where energy is
injected.
Figure 6. Control Scheme for Interfacing Converter
Table 3. Parameters for the Hybrid Grid
Cpv Capacitor across the solar panel 110 µF
L1 Inductor for the boost converter 2.5 mH
Cd Capacitor across the dc-link 4700 µF
L2 Filtering inductor for the inverter 0.43 mH
R2 Equivalent resistance of the inverter 0.3 ohm
C2 Filtering capacitor for the inverter 60 µF
L3 Inductor for the battery converter 3 mH
R3 Resistance of L3 0.1 ohm
F Frequency of AC grid 60 Hz
fs Switching frequency of power converters 10 kHz
Vd Rated DC bus voltage 400 V
Vll_rms Rated AC bus line voltage (rms value) 400 V
n1/n2 Ratio of the transformer 2:1
C Capacity of Super Capacitor 0.5 F
4. SIMULATION RESULTS
The operation of Grid Connected Hybrid AC/DC Microgrid under various source and load
conditions are simulated to verify the reliability.
DC RES power is supplied directly to the DC loads and AC RES power is supplied directly to AC
loads. Power is balanced directly by the utility grid on AC bus and on DC bus through interfacing converter.
The battery is assumed to be fully charged and operated in rest mode. DC bus voltage is controlled and
maintained by utility grid through interfacing converter. AC bus voltage is directly maintained by utility grid.
The terminal voltage for change in solar irradiation is shown in Figure 7. Optimal terminal voltage

 ISSN: 2088-8694
200
of PV panel is obtained by using the standard P&O algorithm. The solar irradiance was set as 400W/m2
from
0.0s to 0.1s, later it was linearly increased to 1000W/m2
until 0.2s, kept constant to 0.3s, decreased to
400W/m2
by 0.4s and keeps that value until final time 0.5s. The slow tracing speed of the standard P&O
algorithm is optimized by using fuel cell in DC subgrid.
Figure 8 and Figure 9 shows the curves of the PV panel power output and solar irradiation
respectively. The power output varies from 4.85kW to 13.5kW, which closely follows the solar irradiation
curve assuming the fixed ambient temperature.
Figure 10 shows the voltage and current responses on AC side of interfacing converter with a fixed
DC load of 20kW. It was observed that the current direction of interfacing converter was reversed before 0.3s
and after 0.4s.
Figure 11 shows the voltage and current responses on AC side of interfacing converter with variable
DC load from 20kW to 40kW at 0.25s with fixed solar irradiation at 750W/m2
. It can be seen that current
direction was reversed at 0.25s.
Figure 12 shows the voltage response at DC bus of interfacing converter with Fuel cell shows an
improved transient response when compared Figure 13 without fuel cell under same conditions
Figure 7. Terminal Voltage of PV Panel
Figure 8. Power output of PV Panel
Figure 9. Solar Irradiation
0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.5
275
280
285
290
295
300
305
Time in Sec (s)
Voltageinvolts(V)
0 0.1 0.2 0.3 0.4 0.5
0
2000
4000
6000
8000
10000
12000
14000
Time in sec (s)
PVPowerOutputinWatts(W)
0 0.1 0.2 0.3 0.4 0.5
0
200
400
600
800
1000
1200
Time in sec (s)
SolarIrradiation(W/m2)

201
Figure 10. AC side Voltage and Current of the Interfacing Converter with Variable Solar Irradiation and
Constant DC Load
Figure 11. AC side Voltage and Current of the Interfacing Converter with Constant Solar Irradiation and
Variable DC Load
Figure 12. DC Bus Transient Response with Fuel Cell
5. CONCLUSION
A Hybrid AC/DC Microgrid is proposed and comprehensively studied in this paper. The control
strategies are concisely stated to maintain stable system operation under various load and resource
conditions. The control strategies are verified by using MATLAB/Simulink. Various control methods are
incorporated to harness the maximum power from RES during grid connected mode and resembles stable
operation.
The Interfacing Converter shows stable operation during load variations. However there will be
some practical limitations, because of fast and continuous load variations. Even-though, the proposed Hybrid
grid reduces the processes of DC/AC and AC/DC conversions in an individual sub grids, the theory is still
challenging in the AC dominated infrastructure. The Hybrid AC/DC Microgrid has to be tested for various
faults on subgrids and their effects on the other grid. The Hybrid AC/DC Microgrid is only feasible for new
construction either in remote location or industries.
0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.5
-100
-50
0
50
100
Time in Sec (s)
Volatge(V)andCurrent(A)
Voltage
Current
0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.5
-100
-50
0
50
100
Time in sec (s)
Voltage(V)andCurret(A)
Voltage
Current
0.15 0.2 0.25 0.3 0.35 0.4 0.45
380
385
390
395
400
405
410
Time in sec (s)
Voltage(V)

 ISSN: 2088-8694
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BIOGRAPHIES OF AUTHORS
Kodanda Ram R. B. P. U. S. B. received B.Tech degree from JNT University Kakinada, India in
2010. He is currently pursuing M.Tech in Power Systems from K L University, Guntur, India.
He is having 2 years of industrial experience in design, manufacturing, erection and commission
of LT Control Panels from Hyderabad. His special fields of interest included Smart grids,
Microgrids, Hybrid Grids.
Dr. M Venu Gopala Rao at present is Professor & Head, Department of Electrical & Electronics
Engineering, KL University, Guntur, Andhra Pradesh, India. He obtained M.E in Electrical Power
Engineering from M S University, Baroda, India in 1999, M.Tech in Computer Science from
JNTU College of Engineering, Kakinada, India in 2004 and Doctoral Degree in Electrical &
Electronics Engineering from JNT University, Hyderabad, India in 2009. He published more than
42 papers in various National, International Conferences and Journals. His research interests
accumulate in the area of Power Quality, Smart Electric Grid, Distribution System and Electrical
Machines.

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