Power Quality Improvement of a Distributed Generation Power System

Panga Harish1. Int. Journal of Engineering Research and Application www.ijera.com
ISSN : 2248-9622, Vol. 6, Issue 6, ( Part -3) June 2016, pp. 86-91
www.ijera.com 86|P a g e
Power Quality Improvement of a Distributed Generation Power
System
Panga Harish1
, K. Madhavarao2
1
M.tech Scholar, Dept. Of EEE Centurion University
2
Asst. Professor, Dept. Of EEE Centurion University
ABSTRACT
The aim of this work is to improve the power quality for Distributed Generation (DG) with power storage system. Power
quality is the combination of voltage quality and current quality. Power quality is the set of limits of electrical properties that
allows electrical systems to function in their intended manner without significant loss of performance or life. The electrical
power quality is more concerned issue. The main problems are stationery and transient distortions in the line voltage such as
harmonics, flicker, swells, sags and voltage asymmetries. Distributed Generation (DG) also called as site generation, dispersed
generation, embedded generation, decentralized generation, decentralized energy or distributed energy, generates electricity
from the many small energy sources. In recent years, micro electric power systems such as photovoltaic generation systems,
wind generators and micro gas turbines, etc., have increased with the deregulation and liberalization of the power market.
Under such circumstances the environment surrounding the electric power industry has become ever more complicated and
provides high-quality power in a stable manner which becomes an important topic. Here DG is assumed to include Wind
power Generation (WG) and Fuel Cells (FC), etc. Advantages of this system are constant power supply, constant voltage
magnitude, absence of harmonics insupply voltage, un-interrupted power supply. In this project the electric power qualities in
two cases will be compared.
Case I: With the storage battery when it is introduced.
Case II: Without the storage battery.
The storage battery executes the control that maintains the voltage in the power system. It will be found that the Electric power
quality will be improved, when storage battery is introduced. The model system used in this Project work is composed of a
Wind Turbine, an Induction Generator, Fuel Cells, An Inverter and a Storage Battery. A miniature Wind Power Generator is
represented by WG. A fuel cell module is represented by FC. Transmission lines will be simulated by resistors and coils. The
combined length of the lines from synchronous generator to the load terminal is 1.5 km. This model will be simulated using
Matlab/Simulink.
IndexTerms:Distributed generation , fuel cell , battery,squirrel-cage induction generators (SCIGs)
I. INTRODUCTION
The Renewable energy sources have
attracted attention worldwide due to soaring prices of
fossil fuels. Renewable energy sources are considered
to be important in improving the security of energy
supplies by decreasing the dependence on fossil fuels
and in reducing the emissions of greenhouse gases.
The viability of isolated systems using renewable
energy sources depends largely on regulations and
stimulation measures. Renewable energy sources are
the natural energy resources that are inexhaustible,
for example, wind, solar, geothermal, biomass, and
small hydro generation [1]. Among the renewable
energy sources, small hydro and wind energy have
the ability to complement each other. For power
generation by small or micro hydro as well as wind
systems, the use of squirrel-cage induction generators
(SCIGs) has been reported in literature. Although the
potential for small hydroelectric systems depends on
the availability of suitable water flow, where the
resource exists, it can provide cheap clean reliable
electricity. Hydroelectric plants convert the kinetic
energy of a waterfall into electric energy. The power
available in a flow of water depends on the vertical
distance the water falls (i.e., head) and the volume of
flow of water in unit time (i.e., discharge) [2]. The
water powers a turbine, and its rotational movement
is transferred through a shaft to an electric generator.
When SCIG is used for small or micro hydro
applications, its reactive power requirement is met by
a capacitor bank at its stator terminals. The SCIG has
advantages like being simple, low cost, rugged,
maintenance free, absence of dc, brushless, etc., as
compared with the conventional synchronous
generator for hydro applications [3]. As regards wind-
turbine generators, these can be built either as
constant-speed machines, which rotate at a fixed
speed regardless of wind speed, or as variable-speed
machines in which rotational speed varies in
accordance with wind speed.
For fixed-speed wind turbines, energy-
conversion efficiency is very low for widely varying
wind speeds. In recent years, wind turbine technology
has switched from fixed speed to variable speed. The
variable-speed machines have several advantages [4].
They reduce mechanical stresses, dynamically
compensate for torque and power pulsations, and
improve power quality and system efficiency. The
grid-connected variable-speed wind-energy-
RESEARCH ARTICLE OPEN ACCESS

Conversion system (WECS) based on SCIG
use back-to-back connected power converters. In
such systems, the power converter decouples the
SCIG from the grid, resulting in an improved
reliability. In the case of grid-connected systems
using renewable energy sources, the total active
power can be fed to the grid. For standalone systems
supplying local loads, if the extracted power is more
than the local loads (and losses), the excess power
from the wind turbine is required to be diverted to a
dump load or stored in the battery bank[5].
Moreover, when the extracted power is less
than the consumer load, the deficit power needs to be
supplied from a storage element, e.g., a battery bank.
In the case of stand-alone or autonomous systems, the
issues of voltage and frequency control (VFC) are
very important. In, the authors have addressed the
issues of VFC for stand-alone systems using SCIGs.
Some work has also been reported for stand-alone
WECSs using doubly fed induction generator [8]. In a
battery-based controller is proposed for control of
voltage and frequency in the isolated WECS.
However, maximum power tracking (MPT) could not
be realized in this battery-based isolated system
employing SCIG operated at fixed speed. In Singh et
al. have proposed an electronic load controller for
VFC at the stator terminals, and the controller
transfers excess power from the hydropower
generator to a dump load, whenever the load is less
than the generated power. In this paper, a new three-
phase four-wire autonomous (or isolated) wind–small
hydro hybrid system is proposed for isolated
locations, which cannot be connected to the grid and
where the wind potential and hydro potential exist
simultaneously [9].
One such location in India is the Andaman
and Nicobar group of islands. The proposed system
utilizes variable speed wind-turbine-driven SCIG w
(subscript w for wind), and a constant-speed/constant-
power small hydro-turbine-driven SCIG h (subscript
h for hydro). For the rest of this paper, the subscript w
is used to denote the parameters and variables of the
wind-turbine generator, and the subscript h is used to
denote the parameters and variables of the hydro-
turbine generator. A schematic diagram of a three-
phase four-wire autonomous system. Two back-to-
back-connected pulse width modulation (PWM)-
controlled insulated-gate-bipolar transistor (IGBTs)-
based voltage-source converters (VSCs) are
connected between the stator windings of SCIG w
and the stator windings of the SCIG h to facilitate
bidirectional power flow. The stator windings of the
SCIG h are connected to the load terminals. The two
VSCs can be called as the machine (SCIG w) side
converter and the load-side converter [10].
The system employs a battery energy storage
system (BESS), which performs the function of load
leveling in the wake of uncertainty in the wind speed
and variable loads. The BESS is connected at the dc
bus of the PWM converters. The advantage of using
BESS on the dc bus of the PWM converters is that no
additional converter is required for transfer of power
to or from the battery. Further, the battery keeps the
dc-bus voltage constant during load disturbances or
load fluctuations.
An inductor is connected in series with the
BESS to remove ripples from the battery current. A
zigzag transformer is connected in parallel to the load
for filtering zero-sequence components of the load
currents. Further, the zigzag windings trap triple n
harmonic (third, ninth, fifteenth, etc.) currents. As,
the zigzag transformer consists of three single-phase
transformers with a turn ratio of 1: 1. The zigzag
transformer is to be located as near to the load as
possible. The neutral terminal of the consumer loads
is connected to the neutral terminal of the zigzag
transformer. For the hybrid system, a new control
algorithm is proposed that has the capability of MPT,
harmonic elimination, load leveling, load balancing,
and neutral current compensation along with VFC
II. DESIGN OF SCIG-BASED WIND-
HYDRO HYBRID SYSTEM
The system is designed for an isolated location with
the load varying from 30 to 90 kW at a lagging
power factor (PF) of 0.8. The average load of the
system is considered to be 60 kW. The following
subsections describe the procedure for selection of
ratings for SCIGs, battery voltage, battery capacity,
machine-side converter, load-side converter,
specifications of wind turbine, and gear ratio.
A. Selection of Voltage of DC Link and Battery
Design:
The dc-bus voltage (Vdc) must be more
than the peak of the line voltage for satisfactory
PWM control as
𝑉 𝑑𝑐 > 2
2
3
𝑉 𝑎𝑐 𝑀 𝑎 1
Also, where ma is the modulation index normally
with a maximum value of one and Vacis the rms
value of the line voltage on the ac side of the PWM
converter. In this case, there are two PWM
converters connected to the dc bus; therefore, the
constraint on the dc-bus voltage is from the ac
voltages of both the converters. The maximum rms
value of the line voltage at SCIGwterminals as well
as the rms value of the line voltage at the load [6, 7].
The terminals is 415 V. substituting this value in
(1), Vdc should be more than 677.7 V. The voltage

of the dc link and the battery bank is selected as
700 V.
Considering the ability of the proposed
system to supply electricity to a load of 60 kW for
10 h, the design storage capacity of the battery bank
is taken as 600 kW · h. The commercially available
battery bank consists of cells of 12 V. The nominal
capacity of each cell is taken as 150 A · h. To
achieve a dc-bus voltage of 700 V through series
connected cells of 12 V, the battery bank should
have (700/12) = 59 number of cells in series. Since
the storage capacity of this combination is 150 A ·
h, and the total ampere hour required is (600 kW ·
h/700 V) = 857 A · h, the number of such sets
required to be connected in parallel would be (857
A · h/150 A · h) = 5.71 or 6 (selected). Thus, the
battery bank consists of six parallel-connected sets
of 59 series connected battery cells.
Thevenin’s model is used to describe the
energy storage of the battery in which the parallel
combination of capacitance (Cb) and resistance
(Rb) in series with internal resistance (Rin) and an
ideal voltage source of voltage 700 V are used for
modeling the battery in which the equivalent
capacitance Cbis given as equation(2).
𝐶 𝑏 =
𝐾𝑊∗ℎ∗3600∗1000
0.5 𝑉 𝑜𝑐𝑚𝑎𝑥
2−𝑉 𝑜𝑐𝑚𝑖𝑛
2 (2)
Taking the values of Vocmax= 750 V,
Vocmin= 680 V, and kW · h = 600, the value of
Cbobtained is 43 156 F.
B. Selection of Rating of Machine
(SCIGw) Side Converter:
The maximum active-power flow through
the machine side converter Psw= 55 kW, and the
maximum reactive power flow provided from the
machine-side converter (Qsw) is calculated as
equation (3)
𝑄𝑠𝑤 =
𝑉 𝑚𝑠𝑐
2
2𝜋𝐹𝐿𝑚
=18.4Kvar (3)
Where Vmscis the maximum line voltage
generated at the SCIGwterminals, which is 415 V,
at a frequency (f) of 50 Hz generated at a wind
speed of 11.2 m/s. The V A rating (V Amsc) of the
machine-side converter is calculated as V
Amsc=P2sw + Q2sw =√552 + 18.42 =58kVA, and
theMaximum rms machine-side converter current
as equation (4).
Isw= V Amsc/(√3Vmsc) = 80.7 A. (4)
The voltage and current ratings of the
switching devices (IGBTs) are decided by the
maximum voltage across the device and the
maximum current through it. In view of (1), the
voltage rating of the switching devices is decided
by the dc-link voltage, whose maximum value is
750 V. Taking a 25% margin, the voltage rating of
the switching devices of the machine-side converter
should be more than 1.25 *750 V, i.e., 937.5 V.
The maximum current through the
switching device is 1.25{Ir(p−p)msc+
I(peak)msc}where I(peak)mscis the peak line
current through the machine-side converter, and
Ir(p−p)mscis the peak-to-peak ripple current in the
machineside converter, and 1.25 is the safety
margin taken for design. For design purpose, the
ripple in the machine-side converter current is
assumed to be 5% of I(peak)msc.
III. SIMULINK
Simulink (Simulation and Link) is an extension of
MATLAB by Math works Inc. It works with
MATLAB to offer modeling, simulating, and
analyzing of dynamical systems under a graphical
user interface (GUI) environment. The construction
of a model is simplified with click-and-drag mouse
operations. Simulink includes a comprehensive
block library of toolboxes for both linear and
nonlinear analyses. Models are hierarchical, which
allow using both top-down and bottom-up
approaches. As Simulink is an integral part of
MATLAB, it is easy to switch back and forth
during the analysis process and thus, the user may
take full advantage of features offered in both
environments. This tutorial presents the basic
features of Simulink and is focused on control
systems as it has been written for students in my
control system
A. Sim Power Systems
Sim Power Systems is a modern design
tool that allows scientists and engineers to rapidly
and easily build models that simulate power
systems.
Sim Power Systems uses the Simulink
environment, allowing you to build a model using
simple click and drag procedures. Not only can you
draw the circuit topology rapidly, but your analysis
of the circuit can include its interactions with
mechanical, thermal, control, and other disciplines.
This is possible because all the electrical parts of
the simulation interact with the extensive Simulink
modeling library. Since Simulink uses MATLAB®
as its computational engine, designers can also use
MATLAB toolboxes and Simulink block sets. Sim
Power Systems and Sim Mechanics share a
specialPhysical Modeling block and connection
line interface [11].
B. Sim Power Systems Libraries
You can rapidly put Sim Power Systems
to work. The libraries contain models of typical
power equipment such as transformers, lines,
machines, and power electronics. These models are

proven ones coming from textbooks, and their
validity is based on the experience of the Power
Systems Testing and Simulation Laboratory of
Hydro-Québec, a large North American utility
located in Canada, and also on the experience of
Ecolab de Technologies Superiors and University
Laval.
The capabilities of Sim Power Systems for
modeling a typical electrical system are illustrated
in demonstration files. And for users who want to
refresh their knowledge of power system theory,
there are also self-learning case studies.
The Sim Power Systems main library,
power lib, organizes its blocks into libraries
according to their behavior. The power lib library
window displays the block library icons and names.
Double-click a library icon to open the library and
access the blocks. The main Sim Power Systems
power lib library window also contains the
Powerful block that opens a graphical user interface
for the steady-state analysis of electrical circuits.
C. Nonlinear Simulink Blocks for Sim Power
Systems Models
The nonlinear Simulink blocks of the
power lib library are stored in a specialblock
library named powerlibmodels. These masked
Simulink models are used by Sim Power Systems
to build the equivalent Simulink model of your
circuit [12].
IV. MATLAB SIMULINK MODELLING AND
SIMULATION RESULT
Fig.1.Proposed Simulation model without storage
battery
Fig.2.Inverter subsystem with pulse generation
technique
Fig.3.Proposed Simulation model with storage
battery

Fig.4.Fuel cell subsystemwith storage battery
Fig.5.Wind subsystemwith storage battery
Fig.6.Load Voltage, LoadCurrent
&Output Wave Form of wind system voltage
WITHOUT battery
0 0.05 0.1 0.15 0.2 0.25 0.3
-200
-100
0
100
200
TIME
LOADVOLTAGE
0 0.05 0.1 0.15 0.2 0.25 0.3
-1
-0.5
0
0.5
1
Time
LOADCURRENT
0 0.05 0.1 0.15 0.2 0.25 0.3
-100
-50
0
50
100
TIME
OUTPUTVOLTAGE
0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.18 0.2
-1.5
-1
-0.5
0
0.5
1
1.5
x 10
4
TIME
LOADVOLTAGE
0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.18 0.2
-1
-0.5
0
0.5
1
Time
LOADCURRENT

Fig.7.Load Voltage, Load Current & Output
Wave Form of wind system voltage WITHbattery
V. CONCLUSION
In this paper a new multi-input Cuk-
SEPIC rectifier stage for hybrid wind/solar energy
systems has been presented. The features of this
circuit are: 1) additional input filters are not
necessary to filter out high frequency harmonics; 2)
both renewable sources can be stepped up/down
(supports wide ranges of PV and wind input); 3)
MPPT can be realized for each source; 4)
individual and simultaneous operation is supported.
Simulation results have been presented to verify the
features of the proposed topology.
VI. REFERENCES
[1]. L. L. Lai and T. F. Chan, Distributed Generation:
Induction and Permanent Magnet Generators. West
Sussex, U.K.: Wiley, 2007, ch. 1
[2]. [2] E. D. Castronuovo and J. A. Pecas, “Bounding
active power generation.of a wind-hydro power plant,”
in Proc. 8th Conf. Probabilistic Methods Appl. Power
Syst., Ames, IA, 2004, pp. 705–710.
[3]. B. Singh, S. S. Murthy, and S. Gupta, “An improved
electronic load controller for self-excited induction
generator in micro-Hydel applications,”in Proc. IEEE
Annu. Conf. Ind. Electron. Soc., Nov. 2003, vol. 3, pp.
2741–2746.
[4]. J. B. Ekanayake, “Induction generators for small hydro
schemes,” IEEEPower Eng. J., vol. 16, no. 2, pp. 61–67,
2002.
[5]. M. Molinas, J. A. Suul, and T. Undeland, “Low voltage
ride through of wind farms with cage generators:
STATCOM versus SVC,” IEEE Trans. Power Electron.,
vol. 23, no. 3, pp. 1104–1117, May 2008.
[6]. S. Ganesh Kumar, S. Abdul Rahman, and G. Uma,
“Operation of self excited induction generator through
matrix converter,” in Proc. 23rd
Annu. IEEE APEC, Feb.
24–28, 2008, pp. 999–1002.
[7]. G. Quinonez-Varela and A. Cruden, “Modelling and
validation of a squirrel cage induction generator wind
turbine during connection to the local grid,” IET Gener.,
Transmiss. Distrib, vol. 2, no. 2, pp. 301–309, Mar.
2008.
[8]. E. Diaz-Dorado, C. Carrillo, and J. Cidras, “Control
algorithm for coordinated reactive power compensation
in a wind park,” IEEE Trans. Energy Convers., vol 23,
no. 4, pp. 1064–1072, Dec. 2008.
[9]. L. Tamas and Z. Szekely, “Modeling and simulation of
an induction drive with application to a small wind
turbine generator,” in Proc. IEEE Int. Conf. Autom.,
Quality Test., Robot., May 22–25, 2008, pp. 429–433.
[10]. A. Luna, P. Rodriguez, R. Teodorescu, and F.
Blaabjerg, “Low voltage ride through strategies for
SCIG wind turbines in distributed power generation
systems,” in Proc. IEEE Power Electron. Spec. Conf.,
Jun. 15–19, 2008, pp. 2333–2339.
[11]. Introduction To Matlab For Engineering Students,
David Houcque Northwestern University(version 1.2,
August 2005)
[12]. A brief introduction toMATLAB Linear Algebra with
Application to CME200Engineering Computations M.
Gerritsen,Autumn 2006,Handout 3
0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.18 0.2
-1.5
-1
-0.5
0
0.5
1
1.5
x10
4
TIME
OUTPUTVOLTAGE

Power Quality Improvement of a Distributed Generation Power System

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