Voltage and Frequency Control of Microgrid Systems with Demand Response

Abstract

This paper describes and present the operation of a microgrid comprising photovoltaic, fuel cell and battery bank. Photovoltaic cells represent variable resources and fuel cell represent dispatchable resources. All these sources are coupled to grid through power electronic converter. The inverters are controlled by a decoupled control method. The frequency and voltage inside this Microgrid is controlled by performing fast demand response. The Microgrid and simulation study are performed in MATLAB/SIMPOWERSYSTEM. The simulation results show that fast demand response is capable in controlling the voltage and frequency inside a Microgrid.

Full text

ALIAS KHAMIS et al: VOLTAGE AND FREQUENCY CONTROL OF MICROGRID SYSTEMS WITH DEMAND …
DOI 10.5013/IJSSST.a.17.41.25 25.1 ISSN: 1473-804x online, 1473-8031 print
Voltage and Frequency Control of Microgrid Systems with Demand Response
Alias Khamis
1
, Mohd Ruddin Ab. Ghani
2
, Gan Chin Kim
3
, Muhammad Nizam Kamarudin
4
, Mohd Shahrieel
Mohd Aras
5
Faculty of Electrical Engineering
Universiti Teknikal Malaysia Melaka
Malacca Malaysia
1
alias@utem.edu.my,
2
dpdruddin@utem.edu.my,
3
ckgan@utem.edu.my,
4
nizamkamarudin@utem.edu.my,
5
shahrieel@utem.edu.my
Abstract - This paper describes and present the operation of a microgrid comprising photovoltaic, fuel cell and battery bank.
Photovoltaic cells represent variable resources and fuel cell represent dispatchable resources. All these sources are coupled to grid
through power electronic converter. The inverters are controlled by a decoupled control method. The frequency and voltage inside
this Microgrid is controlled by performing fast demand response. The Microgrid and simulation study are performed in
MATLAB/SIMPOWERSYSTEM. The simulation results show that fast demand response is capable in controlling the voltage and
frequency inside a Microgrid.
Keywords - Battery Storage, Fuel Cell, Microgrid, Photovoltaic, Matlab/Simpowersystem.
I. INTRODUCTION
Nowadays, microgrid technology using renewable
energy based on distributed power generation system
combined with power electronic system will produce the
concept of future network technologies [1]. The integration
of renewable energy sources and energy storage systems
has been one of the new trends in power electronic
technologies [2]. The main advantages of microgrid
development is to provide the best solution to supply power
in case of an emergency and power outages during power
interruption in the main grid. Microgrids comprise of low
voltage distribution system with distributed energy
resources, such as photovoltaic power system and wind
turbines, together with storage devices [3].
Currently, Photovoltaic generators are designed in
order to generate maximum power to the grid. Because of
the stochastic nature of the PV power output, large
developments of grid connected PV systems involve large
fluctuations of the frequency, power and voltage in the grid
[4]. However, the disadvantage is that PV generation is
intermittent, depending upon weather condition. Thus, the
MPPT makes the PV system providing its maximum power
and that energy storage element is necessary to help get
stable and reliable power from PV system for both loads
and utility grid, and thus improve both steady and dynamic
behaviors of the whole generation system [5]. Because of
its low cost and high efficiency, the battery can be
integrated into the PV generation system to make the
system more stable and reliable.
In this paper, a microgrid testbed using renewable
energy based power generation system which is composed
of PV array, fuel cell, battery, power electronic converters,
filter, controllers, local loads and utility grid is constructed
as shown in Figure 1. The detailed modeling of grid
connected PV, fuel cell and battery generation system will
be conducted. PV array is connected to the utility grid by a
boost converter to optimize the PV output and a DC/AC
inverter to convert the DC output voltage of the solar
modules into the AC system. Meanwhile, the battery is
connected to the common DC bus via a charge controller
to provide regulated PV/fuel cell voltage. The proposed
model of the entire components and control system are
simulated under
MATLAB/SIMPOWERSYSTEM
software.
Simulation results have verified the validity of the models
and effectiveness of the control method.
Figure 1. Configuration of the microgrid tesbed using PV, fuel cell and
battery based power generation.

ALIAS KHAMIS et al: VOLTAGE AND FREQUENCY CONTROL OF MICROGRID SYSTEMS WITH DEMAND …
DOI 10.5013/IJSSST.a.17.41.25 25.2 ISSN: 1473-804x online, 1473-8031 print
II. MICROGRID SYSTEM MODELING
A. Photovoltaic (PV) Model
In this project the PV system is modeled based on the
equivalent circuit model which has already been stated
above. The photocurrent generated when the sunlight hits
the solar cell can be represented by a current source and
the P-N transition area of the solar cell can be represented
by a diode. The shunt and series resistances represent the
losses due to the body of the semiconductor.
The electrical model of the PV system was simulated
in
MATLAB/SIMPOWERSYSTEM
based on the equivalent
circuit model in Figure 2 [6]. The circuit model is using
one current source  and two resistors Rs and Rp. The
value of current Im is calculated by the computational
block that has V, I,  and Ipv as inputs. All the input
parameters to the PV model were developed using
mathematical functions.
Figure 2. Equivalent model of PV system in
MATLAB/SIMPOWERSYSTEM with input  and output port that
connect to output of the subsystem.
In order to create the input supply or model current
, to the equivalent circuit of PV, firstly the saturation
current of  was developed. This is done by using
equation of 1 and 2 with the selected parameters.

,

∆

,∆


(1)







(2)
Then the light generated current  was developed
by using equation 3 with the selected parameters.
󰇛,

∆󰇜



(3)
Finally both  and , also with the selected
parameters are substituted inserted in equation 4 in order to
obtain the value of the input supply Im.
󰇩󰇧
󰇡


󰇢

󰇨1󰇪 (4)
B. Fuel Cell (FC) Model
The detailed model represents a particular fuel cell
stack when the parameters such as pressures, temperature,
compositions, and flow rates of fuel and air vary. These
variations affect the open circuit voltage. The open circuit
voltage is modified as follows:
󰇛󰇛󰇜󰇜 (5)
and 


(6)
where
R = 8.3145 J/(mol K)
F = 96485 A s/mol
z = Number of moving electrons
E
n
= Nernst voltage, which is the thermodynamics voltage
of the cells and depends on the temperatures and partial
pressures of reactants and products inside the stack
i
0
= Exchange current, which is the current resulting from
the continual backward and forward flow of electrons from
and to the electrolyte at no load. It depends also on the
temperatures and partial pressures of reactants inside the
stack.
α = Charge transfer coefficient, which depends on the type
of electrodes and catalysts used
T = Temperature of operation
The equivalent circuit is the same as for the
simplified model, except that the parameters E
oc
and Α as
shown in Figure 3:
Figure 3. Fuel Cell Model [7]
C. Battery Model
The battery block implements a generic dynamic
model parameterized to represent most popular types of
rechargeable batteries is shown in Figure 4.

ALIAS KHAMIS et al: VOLTAGE AND FREQUENCY CONTROL OF MICROGRID SYSTEMS WITH DEMAND …
DOI 10.5013/IJSSST.a.17.41.25 25.3 ISSN: 1473-804x online, 1473-8031 print
Figure 4. Battery model lead acid type in
MATLAB/SIMPOWERSYSTEM.
Lead acid model for discharge model with selected
parameters was inserted in equation 7. While for fully
charged model with selected parameters was inserted in
equation 8.
Discharge model (i* > 0)
1󰇛,∗,,󰇜0.


.∗.


.
1󰇝
󰇛󰇜
󰇛󰇜
.0󰇞 (7)
Charge Model (i* < 0)
1󰇛,∗,,󰇜0.

.
.∗.


.
1󰇝
󰇛󰇜
󰇛󰇜
.0󰇞 (8)
D. Inverter Model
Three phase inverters are used at the DC output
voltage of the PV/Battery/Fuel Cell into AC voltage to be
connected to the electric utility grid. The three phase full
bridge voltage source inverter circuit configuration is
shown in Figure 5. It is composed of a DC voltage source
an input decoupling capacitor and six power switching
blocks. A capacitor is used to filter the noise on the DC
bus. After the inverter an LC harmonics filter is used to
eliminate the high frequencies in the output inverter
voltage. Each block of the switching block consists of a
semiconductor switch (IGBT) and anti-parallel diode. To
create proper gating signals for switches, pulse-width-
modulation (PWM) is used. The functions of PWM are the
control output voltage amplitude and fundamental
frequency[8].
Figure 5. Inverter Model
Figure 6 shows
Inverter
control that connects to filter
and grid system from distribution generation. Three phase
PLL measurement are used to sense voltage and current
from power control system. Vdc regulator sense DC
voltage as a reference voltage before to connect to the
filter. Filter is used for pure sine wave voltage output from
inverter circuit.
Figure 6. Inverter control that connects to filter and grid system.
III. VOLTAGE AND FREQUENCY CONTROL OF
MICROGRID BY THE DEMAND RESPONE
Technical issues such as control, power balance
strategies, operations and storage techniques differ from
one microgrid to another. The main reasons are the
integration of high number of distributed power generation
units near to the electrical load, the nature and size of the
micro generation units, and availability of primary energy
sources for renewable power generation units.
The diverse micro generation units in a microgrid
system and the desire to integrate more clean power in the
future network has led to a focus on a microgrid system
based on renewable power generation units in this
research. As a whole, the characteristics of a microgrid
system depend on the size and nature of the micro
generation units in the microgrid, as well as the site, and
the availability of the primary energy resources on the site,
especially for renewable power source. Therefore, taking
an existing real system in the better approach to investigate
the microgrid issues rather than assuming or taking a
hypothetical system. The objective of this research is to
investigate the system behavior and technical issues of a
microgrid system contain renewable power generation
units in TNB Research.
Figure 7 shows load shedding frequency control in
microgrid design [9]. Here, three loads are considered that
are main load 50kW with additional load 25kW and
secondary load 5kW to 25kW. From the changing of load
demand will affect the frequency performance. The
control of frequency in
MATLAB/SIMPOWERSYSTEM is
design to be maintained at the system frequency of 50 Hz.

ALIAS KHAMIS et al: VOLTAGE AND FREQUENCY CONTROL OF MICROGRID SYSTEMS WITH DEMAND …
DOI 10.5013/IJSSST.a.17.41.25 25.4 ISSN: 1473-804x online, 1473-8031 print
Figure 7. Load shedding frequency control.
IV. RESULT AND DISCUSSION
Considering these reason, the technical challenges
and methods for addressing them for the system shown in
Figure 1. This paper investigates the technical challenges
associated with the PV, Fuel Cell and Battery based
microgrid system. In order to classify the technical
challenges for the microgrid system under investigation,
three operational modes are considered:
1. Grid connected system
2. Isolated system with PV generation
3. Isolated system without PV generation
A. Grid Connected Mode
Figure 8 shows output from the PV inverter after the
filter converted to sine wave. The inverter works using a
pulse-width-modulation technique. The output voltage of
the inverter is shown as sine wave.
Figure 8. Outputs from inverter voltage and current

Figure 9 shows the frequency performance when the
fault and load shedding happened in the system. When the
additional load is added the maximum frequency is
50.5Hz, then the secondary load on the frequency change
minimum at 49.8Hz. When the system is run normally the
frequency is going to stable at 50Hz.
Figure 9. Frequency and Load changing
B. Isolated System with PV Generation
Figure 10 shows output from the PV inverter after the
filter converted to sine wave. The inverter works using
pulse-width-modulation technique. The output voltage of
the inverter is shown as low when the fault happen in the
system at 1.2 sec.
Figure 10. Outputs from inverter voltage and current fault

ALIAS KHAMIS et al: VOLTAGE AND FREQUENCY CONTROL OF MICROGRID SYSTEMS WITH DEMAND …
DOI 10.5013/IJSSST.a.17.41.25 25.5 ISSN: 1473-804x online, 1473-8031 print
Figure 11 shows the frequency performance when the
fault and load shedding happened in the system. When the
additional load is added the maximum frequency is
50.5Hz, then the secondary load on the frequency change
minimum at 49.8Hz. When the system is run normally the
frequency is going to stable at 50Hz.
At 1.2s to 1.4s, the system is fault shows the
frequency changing between 50.15Hz to 49.75Hz. After
transaction mode happen the system also going to stable
again.
Figure 11. Frequency and Load changing
C. Isolated System without PV Generation
Figure 12 shows output from the PV inverter after the
filter converted to sine wave. The inverter works using
pulse-width-modulation technique. The output voltage of
the inverter is shown as sine wave with almost have
harmonic content.
Figure 12. Outputs from inverter voltage and current fault
Figure 13 shows the frequency performance when the
fault and load shedding happened in the system. When the
additional load is added the maximum frequency is
50.5Hz, then the secondary load on the frequency change
minimum at 49.8Hz. When the system is run normally the
frequency is going to stable at 50Hz.
At 1.2s to 1.4s, the system is fault shows the frequency
changing between 50.15Hz to 49.75Hz. After transaction
mode happened the system also going to stable again.
Figure 13. Frequency and Load changing

ALIAS KHAMIS et al: VOLTAGE AND FREQUENCY CONTROL OF MICROGRID SYSTEMS WITH DEMAND …
DOI 10.5013/IJSSST.a.17.41.25 25.6 ISSN: 1473-804x online, 1473-8031 print
IV. CONCLUSION
In this paper the mathematical model of microgrid
system components was introduced in order to investigate
the dynamic behavior of each system. Also the proposed
control technique of the system was presented. This
includes On/Off switch control of the system modes of
operation and inverter control system. The proposed
system is implemented in MATLAB/SIMPOWERSYSTEM
environment interfaced with SimPowerSystem toolbox.
The dynamic behavior of each subsystem is investigated
showing the interaction between different components of
grid connected system. Renewable energy based power
generation with a photovoltaic (PV), Fuel Cell with battery
storage for microgrid system are simulated. Simulation is
focused on the parameter of the each component to
consider the outputs and effectiveness of the inverter. Most
of the results can be used for designing a small scale
microgrid system for practical applications.
ACKNOWLEDGEMENT
We wish to express our gratitude honorable
Unversity, Universiti Teknikal Malaysia Melaka (UTeM)
especially for Faculty of Electrical Engineering from
UTeM to give the financial budget from grant
PJP/2015/FKE(3A)/S01401 as well as moral support for
complete this project successfully.
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