Energy and Power Engineering, 2013, 5, 377-383
http://dx.doi.org/10.4236/epe.2013.55039 Published Online July 2013 (http://www.scirp.org/journal/epe)
Power Quality Consideration for Off-Grid
Renewable Energy Systems
Mojgan Hojabri1, Arash Toudeshki2
1Faculty of Electrical and Electronics Engineering, University Malaysia Pahang (UMP), Pekan, Malaysia
2Department of Electrical and Electronic Engineering, University Putra Malaysia (UPM), Serdang, Malaysia
Received April 11, 2013; revised May 12, 2013; accepted May 20, 2013
Copyright © 2013 Mojgan Hojabri, Arash Toudeshki. This is an open access article distributed under the Creative Commons Attri-
bution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly
cited.
ABSTRACT
Necessity of electricity access in remote area is the main reason for expanding decentralized energy system such as
stand-alone power systems. The best electrical power supply must provide a constant magnitude and frequency voltage.
Therefore, good power quality is an important factor for the reliable operation of electrical loads in a power system.
However, the current drawn by most of electronic devices and non-linear loads are non-sinusoidal, which can result in a
poor power quality, especially in off-grid power systems. Poor power quality is characterized by electrical disturbances
such as transients, sags, swells, harmonics and even interruptions in the power supply. Off-grid power systems world-
wide often struggle with system failures and equipment damage due to poor power quality. In this paper, MAT-
LAB/Simulink is used to model and analyses power quality in an off-grid renewable energy system. The results show
high voltage transient when the inductive loads were switched OFF. The voltage and current harmonics are also deter-
mined and compared for various types of loads.
Keywords: Off-Grid Renewable Energy System; Power Quality; Pulse Width Modulation (PWM) Inverters; Total
Harmonic Distortion; Total Demand Distortion
1. Introduction
Energy demand increasing makes important problems
such as grid instability or even outage. Therefore, more
energy must be generated at the grid. However, energy
generation by big plant is not economically. Moreover,
implementation of distributed generation is rapidly in-
creased. Because of increasing global warming, limita-
tion and high cost of fossil fuel sources, governments
tend to increase use and implementation of renewable
energy sources. The main difference between renewable
energy and fossil fuels systems is up-front cost versus
lifelong energy cost. Currently, in most development
countries, governments as well as utilities provide a vari-
ety of incentives, to help the renewable energy industry
reach to a higher economic scale. Figure 1 shows the
annual growth rates of renewable energies in the world
between 2006 and 2011 [1]. Solar/photovoltaic, wind,
hydro, geothermal, tidal, wave and bio energy are exam-
ples of renewable energy sources which the solar/
photovoltaic and wind are most popular among them.
However, environmental friendly is the principal
advantage of renewable energies, high up-front cost and
uncontrollability are the main disadvantages of it [2].
Renewable energy sources can be used as an off-grid or
on-grid systems. An off-grid renewable energy system is
typically a stand-alone power system located in a remote
area to fulfill residential or commercial power needs.
Because of the high installation price and technical
problems for supplying utility power in remote area, this
type of power system is useful and also isolated from the
power grid system. The main objectives of this paper are
to create a simulation of a typical off-grid system and
study the power quality issues through the simulation.
Moreover, it determines the issues and impacts of poor
power quality issues on various types of linear and
non-linear loads. The organization of this paper follows
the structure of off-grid power system, power quality
considerations, simulation with the concluded remarks.
2. Structure of Off-Grid Power System
A typical off-grid system is shown in Figure 2. As it is
shown in this figure, an off-grid renewable energy
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opyright © 2013 SciRes. EPE

M. HOJABRI, A. TOUDESHKI 378
Figure 1. World average annual growth rates of renewable
energy capacity between 2006-2011 [1].
Figure 2. Block diagram of off-grid renewable energy sys-
tem.
system is included a renewable energy source, charge
controller, battery and inverter. The important of re-
newable energy sources are solar panel, wind turbine,
hydro turbine, diesel or biofuel generator and geothermal
source. Among these renewable energy sources, solar
and wind are more common since their availability.
Photo Voltaic (PV) cells have no moving parts. They
require only sunlight to operate, and do so without
depleting the materials they are made of. PV modules are
encased in strong tempered glass and are tested to
withstand wind, rain, snow, ice and hailstones. After
installation, PV systems generate electricity for decades
at no additional cost while producing no greenhouse
gases. Off-grid systems with higher demand request
might use another energy sources like wind turbine.
Because much more energy is found in faster winds, it is
wise to place wind turbines where the wind is strong,
steady and smooth. Wind turbines placed just above the
ground on a breezy knoll make lovely spinning pieces of
artwork, but they don’t produce much electricity. A good
wind site has consistent, fast wind that pumps genuine
energy into the turbine’s generator for many hours of
every day. Occasional gusty days or steady but light
breezes just don’t cut it.
Charge controller is another component of off-grid
system which is needed to protect the batteries from
over-charging voltage. Off-grid system also needs battery
to provide power when needed. Price and performance,
capacity, cycle life, self-discharge rate, safety, hazards,
requirement maintenance, size and space requirement are
the important features which must be considered to
choose batteries for the system. The heart of the grid-
direct system is a DC to AC inverter which adapts to the
power grid voltage and frequency. Inverter technology
has an important role to have safe and reliable grid in-
terconnection operation of renewable energy systems. It
is also necessary to generate a high quality power to the
grid with reasonable cost [2-5]. Inverters are divided into
three main types: grid-tied inverters, grid-tied inverters
with battery back-up and stand-alone inverters. And also,
inverter control techniques became interesting for power
system researchers [6-8]. The important characteristics of
good off-grid inverters are high efficiency, low standby
losses, low harmonic distortion, easy maintenance and
reliability. Therefore, some standards have been pub-
lished to detect the power quality, grid-connected and
unintentional islanding operation [9-12].
3. Power Quality Considerations
The term “power quality” refers to a wide variety of
electromagnetic phenomena that characterizes voltage
and current waveforms at a given time and at a given
location on the power system [13]. Power quality is used
to describe the electric power that drives an electrical
load and the load’s ability to function properly with that
electric power. In the absence of high quality power,
loads may malfunction, fail prematurely or not operate at
all [14]. The power quality of a system is defined by a
number of parameters, as long-duration, short-duration
voltage variations, harmonic distortion and transients.
Harmonics are current and voltage waveform compo-
nents that represent multiples of the fundamental fre-
quency. Harmonic distortion in an electrical power sys-
tem is the alteration of the original shape or characteris-
tics of the current or voltage waveforms due to the pres-
ence of harmonics. Harmonic distortion is the result of
nonlinear loads and switching power electronic devices
on the system. Some of the problems caused by harmon-
ics include: very high neutral currents, flickering lights,
random tripping of circuit breakers, malfunction of sensi-
tive equipment, fire hazards, reduced power factor,
overheated phase conductors, panels and transformers,
reduced system capacity due to excessive heating, pre-
mature failure of transformers and UPSs. Total Harmonic
Distortion (THD) is used to quantify the presence of
harmonics in a power system. The THD for a voltage
waveform is defined as the ratio of the sum of the voltage
magnitudes of all harmonic components to the voltage
magnitude of the fundamental frequency [15]:
22 2
13
1
THD n
VV V
V



(1)
The harmonic current distortion expressed as a
function of the maximum demand load current using a

M. HOJABRI, A. TOUDESHKI 379
15- or 30-minute demand period is called Total Demand
Distortion (TDD)[16]. The TDD is given by:
TDD
H
L
I
(2)
I

where
L
I
is the maximum demand load current at the
fundamental frequency, measured at the Point of Com-
mon Coupling (PCC) from a 15-minute or 30-minute
billing demand load kW, and
H
I
is the square root of
the sum of the squares of the harmonic currents, h
I
[17].
H
I
is given by:
25 2
2
H
h
h
I
I

 (3)
Maintaining a good power quality is important for the
reliable operation of loads. Different power quality stan-
dards are recommended to ensure every piece of equip-
ment operates well with maximum efficiency, without
causing any deterioration of the equipment itself. Voltage
standards are also set by the National Electrical Code
(NEC). According to NEC standards, a maximum volt-
age drop of 5% from the standard voltage is allowed at
the equipment. This 5% voltage drop includes less than
3% voltage drop in a feeder and an addition of less than
3% voltage drop in individual branch circuits [18].
Table 1 specifies the harmonic current limits based on
the size of the load with respect to the size of the power
system to which the load is connected. The ratio Isc/IL is
the ratio of the short-circuit current (Isc) available at the
PCC to the maximum fundamental load current. It is
recommended that the load current, IL, be calculated as
the average current of the maximum demand for the
preceding 12 months. As seen in Table 1, as the size of
the user load decreases with respect to the size of the
system, the percentage of harmonic current that the user
is allowed to inject into the utility system increases. Most
off-grid systems have a low short circuit current and
would fit into the first, most restrictive category of TDD
less than 5 percent.
Table 1. IEEE standard current distortion limits [6].
Isc/IL <20 20 < 50 50 < 100 100 < 1000>1000
H < 11 4 7 10 12 15
11 ≤ h < 17 2 3.5 4.5 5.5 7
17 ≤ h < 23 1.5 2.5 4 5 6
23 ≤ h < 35 6 1 1.5 2 2.5
35 ≤ h 0.3 0.5 0.7 1 1.4
Total Demand
Distortion
(TDD)
5 8 12 15 20
Isc: Maximum Short- Circuit Current
IL: Maximum Demand Load Current
Table 2 specifies the IEEE voltage distortion limit for
utilities. At bus voltages of 69 kV and below, the THD
should not exceed 5% and the individual harmonic dis-
tortion should not be more than 3% at the PCC. However,
a THD well above the recommended maximum may not
be a problem on a distribution circuit if the load demand
is very low. Thus, the IEEE standard has defined TDD
with its standard limits to take this situation into account.
Different power conditioning devices provide essential
protection against power quality problems. All of these
devices provide isolation from the power quality distur-
bance. Power conditioning equipment can include one or
more of the following: surge suppressor, noise filter
harmonic filter, motor-generator set, and dual feeder with
static transfer, isolation transformer, low-voltage line
reactor, Uninterruptible Power Supply (UPS).
4. Simulation Results
A complete Simulink model for an off-grid power system
with a pure sine wave inverter and a motor load is
presented in Figure 3. The major parameters used in this
Simulink model are shown in Table 3. A variable step
discrete with a sampling time of 1 µs was chosen. A
variable step solver shortens the simulation time of a
Simulink model significantly by reducing the number of
steps as necessary, by adjusting the step size for a given
level of accuracy [19]. A switch was placed before the
motor to analyze the motor performance at the instant of
turning OFF and turning ON. A timer set a fixed timing
for the switch to open and close. Scopes were placed to
view the voltage and current waveforms at different
nodes in the circuit.
Characteristics of a 560 W single-phase induction
motor is shown in Figures 4 and 5. The three subplots
represent the voltage waveform, current waveform and
Table 2. IEEE standard voltage distortion limits [6].
Bus Voltage Individual Voltage
Distortion (%)
Total Voltage
Distortion (THD %)
V < 69 kV 3 5
69 kV< V < 161 kV1.5 2.5
161 kV ≤ V 1 1.5
Table 3. Main parameters of Simulink model.
Device Parameters
Battery 24 V
LC Filter Cut-Off Frequency = 100 Hz,
Switching Frequency = 2 kHz
Transformer 4 kVA, 60 Hz, 20:120
Induction Motor 560 W (0.75 hp), 120 V, 60 Hz

M. HOJABRI, A. TOUDESHKI 380
Figure 3. A Simulink model of an off-grid renewable energy
system.
(a)
(b)
(c)
Figure 4. Output load characteristics of motor: (a) Voltage
waveform; (b) Current waveform; and (c) Active and reac-
tive power.
the power for the motor, respectively. When the switch
was turned ON at T = 0.3 of a second, a current of almost
23.5 A was drawn by the motor while maintaining an
output voltage of 117.2 V. A high amount of power
(active power of 2632 W and reactive power of 801 VAR)
was drawn by the motor at that time, compared to its
(a)
(b)
Figure 5. Motor characteristics: (a) Auxiliary winding cur-
nt waveform; and (b) Capacitor voltage.
power rating of 560 W. The current drawn by the motor
started decreasing after the auxiliary winding of the
motor was disconnected (when the motor speed reached
75% of its synchronous speed of 1800 RPM), and the
motor current settled to normal operating current of 3.25
A (Figure 6).
During the operating condition, more reactive power
(375 VAR) was consumed by the motor than the active
power (65 Watts). When the switch was turned OFF at
(65/60)th of a second, the motor experienced a high
voltage transient, the value even reaching 43 kV, due to
the simulated instantaneous change in current.
The simulation results did not show any voltage sag
caused by the induction motor during normal operation,
but it indicated the high current at start up, almost six
times the normal operating current, which was similar to
the actual water pump drawing a start-up current seven
times higher than its normal operating current. The
simulation also predicted a much larger transient at
turn-off due to the idealized instantaneous change in
current. The gate pulses for each inverter switch were
generated by the PWM generator as shown in Figure 6.
Only a pair of switches (S1 and S4) or (S2 and S3) were
turned ON at any instant of time. For example, at 0.035
sec, switches S1 and S4 were ON while S2 and S3 were
OFF. The widths of the generated pulses were varied to
ensure that their averaged output produced a 60 Hz
inusoidal waveform. s

M. HOJABRI, A. TOUDESHKI 381
Figure 6. Gate pulses for inverter switche s S1, S2, S3 and S4.
The higher frequency components in the PWM output
of the pure sine wave inverter were filtered using a low
pass LC filter with cut-off frequency 100 Hz, and then
the resulting sine wave was passed through a step-up
transformer. The primary and secondary voltage wave-
forms of transformer are shown in Figure 7. The peak
value of the primary voltage was close to the battery
voltage. The voltage transients that occurred at the in-
stant of switching OFF the motor load travelled to the
secondary side of the transformer. The secondary wind-
ing of the transformer experienced a voltage transient of
nearly 1640 V on the positive half cycle and −2080 V on
the negative half cycle at 1 sec. The transient voltage,
however, did not travel to the transformer’s primary side
because of its nearly instantaneous duration, the absence
of parasitic capacitance in the secondary winding in the
model, and also because of the absence of a direct physi-
cal connection between the two windings [20].
Using the FFT analysis tool in Simulink, the THD of
the output voltage and current waveforms were recorded.
Figure 8 shows the voltage harmonics for the 560 W
motor supplied by the pure sine wave inverter, in which
case the THD value was 8.34% for normal operation. As
the low pass filter eliminated the low order harmonics
well, some higher order harmonics were still seen, but
with small magnitudes. The THD for the current wave-
form during the normal operating condition was 8.79%
as shown in Figure 9. Both of these THD values were
well above the IEEE standard of 5%. The generation of
harmonic currents is a cause for equipment failure and is,

M. HOJABRI, A. TOUDESHKI 382
(a)
(b)
Figure 7. Secondary and primary voltage waveforms of a
step-up transformer.
Figure 8. Voltage harmonics of an induction motor during
normal operating conditions.
therefore, a serious issue for off-grid systems.
5. Power Quality Analysis for Resistive and
Inductive Loads
The system power quality effects from other devices
were observed by replacing the motor load with a parallel
RL load of different power ratings. The loads were
turned ON at 0.5 sec to allow time for the inverter tran-
sients to settle down, and the loads were turned OFF at
1.08 sec. When the load switch was turned OFF, large
Figure 9. Current harmonics of an induction motor during
normal operating conditions.
voltage transients were seen with the parallel RL load as
they were with the motor load, due to the unrealistic,
instantaneous change in the modeled current. For exam-
ple, voltage transients of 350 kV occurred when an in-
ductive load (500 W and 750 VAR) was used. This volt-
age transient caused a negative voltage transient of mag-
nitude 1100 V at the secondary side of the transformer.
However, there were no transients seen for a purely re-
sistive load, due to its linear relationship with current.
Inductive loads (which are the dominant reactive load in
a residential setting) can therefore be assumed as the
primary source of voltage transients. The simulated in-
stantaneous change in current through the inductor is
responsible for generating this high voltage transient ac-
cording to the relation [21]:
d
d
i
VL
t
 (4)
where V is voltage induced across the inductor, L, due to
changing current di/dt. High start-up currents are drawn
by the inductive loads to energize their coils. However,
none of the purely resistive loads or loads where active
power dominated the reactive power drew high start-up
currents, as expected. For purely resistive loads, the
simulation results showed that the current and voltage
harmonics decreased slightly with heavy loads. For in-
ductive loads also, voltage harmonics decreased while
the current harmonics increased with heavy loads be-
cause of the high fundamental current draw. Table 4
summarizes the THD of the current and voltage wave-
forms for different simulated loads and the corresponding
voltage transients that appeared in the system. This
simulation shows the off-grid loads had significant cur-
rent and voltage harmonics due to the inverter’s voltage
along with the power electronics used in those loads. So
the load’s voltage and current harmonic distortion in-
crease by increasing the pure resistance load. For induc-

M. HOJABRI, A. TOUDESHKI 383
Table 4. Simulation results for current and voltage har-
monics for different loads.
Load Voltage THD% Current THD %
No Load 7.45 -
150 W 7.68 7.68
500 W 8.17 8.17
700 W 8.56 8.56
100 W, 200 VAR 7.63 3.68
300 W, 500 VAR 7.87 3.91
500 W, 1000 VAR 8.18 3.93
tive loads, the total current harmonic distortion is in
IEEE standard range. But the voltage THD is out of
IEEE standard range (more than 5%).
6. Conclusion
In most cases, off-grid renewable energy systems are
cheaper than extending the power grid to provide elec-
tricity for remote areas. Therefore, most countries are
developing aims for electrification that includes renew-
able off-grid options and/or renewably powered mini-
grids. However, good power quality is an important fac-
tor for the reliable operation of this system. In this paper,
the MATLAB/Simulink software was used to model and
analyze power quality for additional configurations of the
typical off-grid system. The simulation results show the
high voltage transient and high start-up current for induc-
tive loads. Moreover, the results show the total harmonic
distortion of current and voltage for pure resistance loads
is significant. However, for inductive load, total demand
distortion is in the acceptable range (less than 5%), but
the total harmonic distortion of voltage is still more than
IEEE standard limitation.
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