Download Fig.1. Schematic of the circuit model designed to simulate PV system

Harmonic Analysis in A Selected Distributed Generator Devices
PRADIPTA KUMAR TRIPATHY
DURGESH MANJURE
ELHAM B. MAKRAM
Clemson University
Clemson University
Clemson University
303 Riggs Hall, Clemson University, Clemson, SC 29634, USA
Abstract: In recent days photovoltaics (PVs) have come
up as viable alternatives to conventional sources of energy
production. Depending upon their size, PVs can operate
as grid-connected or as stand-alone devices. PVs
invariably involve a power-conditioning system, which
consists
of
power
electronic
devices
like
converters/inverters. These devices are highly nonlinear
and distort the current/voltage waveforms to a significant
extent. Power quality issues are of prime concern to the
customer and to the utility. Thus under this scenario, it
has become imperative to study the characteristics of the
PV system in detail. This paper presents results that seek
to quantify the impact of the PV system, as one of the
selected DG devices, on distribution system harmonics.
Many residential loads are inherently nonlinear, and
results of the interaction of the harmonics injected by
these loads and those introduced by the PV system are
also presented. Circuit models for various typical
residential (nonlinear) loads and for the PV system have
been created, and simulations have been carried out in
time domain. Results are presented for different cases,
and are succeeded by a brief analysis.
1 Introduction
Interest in PV energy dates back to 1954, when the
first PV system was designed. PV energy is often referred
to as clean energy, because it is environment-friendly and
doesn’t cause pollution. The recent developments in new
low cost PV material, and the low manufacturing cost of
PV cells have increased the usage of PV source for energy
production throughout the world [1]. Deregulation of
power system has allowed individual power producers to
produce power autonomously for their own usage and for
commercial sale in the energy market. For this purpose
many power producers are installing PV power stations.
These PV systems use a number of PV cells in series and
parallel combinations to produce DC power from the
absorbed solar power. Inverters are used to convert the
DC power into usable AC power. PV systems can
produce power ranging from small generating units of 5
KW to large installations of 0.1 MW. These PV systems
can operate as stand-alone units supplying power to the
residential loads. If the power generated is in excess, then
it can be fed back to the AC grid. Due to the use of an
inverter, the PV system draws nonlinear current thus
resulting in distortion of the current and voltage
waveforms and injection of harmonics into the system.
directly and indirectly in various electrical equipments,
usually used in residences. Household loads such as
refrigerators, television sets, computers and heat pumps
use power supply units with rectifiers as a part of the
circuit. Another common household load, the compact
fluorescent light (CFL) uses electronic ballasts which use
rectifiers to input power to an inverter, which supplies the
necessary AC power to the CFL [4,5]. Thus CFLs also
create harmonic distortion. Recently more and more heat
pumps with adjustable speed drives (ASD) are being
installed as they have the advantage of better efficiency
and better performance over the conventional ones [6].
These ASDs use single-phase bridge rectifiers to convert
the AC power to DC. A three-phase six-pulse inverter
converts this DC to AC and supplies to a three-phase
induction motor that drives the fan load. All these loads
draw nonlinear current and thus are sources of harmonic
distortion.
Harmonics injected into the grid cause severe
problems like damaging the electrical equipments,
interference with power system protection, causing load
unbalance and increasing losses [7]. It has been seen that
harmonic distortion in current doesn’t affect the load as
much as harmonic distortion in voltage. Harmonics in the
voltage impressed at the terminal of the equipment affect
the equipment adversely [2, 3].
The problem of harmonics contributes significantly to
the power quality issues and it needs to be given careful
attention. In order to do so, it has become imperative to
investigate the exact cause and nature of harmonics, their
quantity and their interaction with the power system. This
necessitates accurate modeling [8-10] of the power
system components and study of power system behavior
with respect to harmonics.
This paper presents the study and analysis of
harmonics in the presence of a PV system. At the outset,
modeling of the PV system and a residential system has
been discussed. The residential load also being a source of
harmonics, studying the interaction of harmonics
generated by the residential load and those injected by the
PV system is one of the major aspects. Subsequently, the
study results and the analysis of the harmonic distortion
and the problems caused by the connection of PV system
with the residential system have been presented.
2. PV system description and modeling
The major contribution to the harmonic injection in the
distribution system is due to the increased use of power
electronics devices [2, 3]. These devices are being used
A circuit model of PV system is simulated using
PSCAD software as shown in Fig. 1. The PV system
consists of a set of solar cells in series and parallel
combinations, which produce DC power. While
performing the computer simulation a DC voltage source
(Vpv) was used for simulating the PV cell. The DC power
is fed to a single-phase, full wave, line-commutated
inverter to produce alternating sinusoidal voltage and
current.
The houses are connected through a common bus to the
distribution system. Each house includes six compact
fluorescent lights, each rated at 100 W, a 100 W TV load
and a 150 W incandescent lighting load. The residential
load also comprises of a 2 HP heat pump load.
The main components of the circuit are:
1. Thyristors - T1, T2, T3, T4
2. Feedback diodes - D1, D2, D3, D4
T1
D1
T3
House1
L
C
D4
T2
D2
Isolation Transformer
Filter
Fig.1. Schematic of the circuit model designed to simulate
PV system
A control circuit is designed to periodically trigger the
thyristors into conduction by a pulse train. The control
circuit consists of a Voltage Controlled Oscillator, which
provides the reference ramp for generating the firing pulse
train. Frequency of the output wave is thus decided by the
frequency of the firing pulses. The feedback diodes
conduct when the voltage and current are of opposite
polarities. The output voltage is alternating pulses of
controlled width. The pulse width is modulated here by
adding two square wave voltages, which are shifted in
phase with respect to each other. Thus the inverter output
voltage is smoothly adjusted from zero to a maximum by
either phase advancing or phase retarding the control
signals of one pair of diodes with respect to the other.
This method of pulse control significantly reduces the
harmonic contents in the output voltage and current [11,
12].
A transformer is used at the inverter output which
isolates the potentials of the PV array and the grid and
prevents supplying DC current to the grid. The harmonic
distortion is further reduced by connecting a single-tuned
filter [2, 13] at the output of the inverter. As per IEEE
standard 519, THD, in voltage should not exceed 5 % in
primary distribution lines or 8% on secondary lines [3].
3.
Residential
modeling
system
House4
House3
Fig.2. Schematic of the residential system
D3
Vpv
T4
House2
description
and
A residential system [14] is developed using the
standard library elements available in PSCAD software
[15]. Fig. 2 shows a residential system with four houses.
The CFLs use electronic ballast, which contains a diode
bridge converter circuit, which in turn supplies power to
an inverter. This inverter supplies the usable AC to the
CFL [5]. Converters are also used in power supply units
in electronic equipment like TV. The waveform for CFL
is assumed to be the same as the waveform of TV load
[14]. To model the TV/CFL load a single-phase rectifier
is designed. Resistor R1, inductor L1 and capacitor C are
connected to the rectifier circuit as shown in Fig. 3. The
load is connected to the output of the rectifier circuit. By
adjusting the parameters R1, L1 and C of the circuit to
match the measured harmonic contents of a CFL and TV
[14], the model for CFL and TV is designed. The typical
parameter values for the CFL and TV are:
R1 = 3.5 ohm
L1 = 0.003 H
C = 250 microfarad
R1
L1
C
1-¢ AC
Input
Load
1-¢ Rectifier
Fig.3. Schematic of the circuit used for simulating CFL
load and TV load
The heat pump model is designed as shown in Fig. 4.
The single-phase rectifier is fed from the AC source and
the output DC voltage of the rectifier is converted to AC
voltage by the three-phase inverter. The inverter feeds to
the three-phase induction motor.
1-¢ AC
Input
1-¢
Rectifier
3-¢
Inverter
3-¢
Induction
motor
Fig.4. Schematic of the heat pump circuit
The use of inverter and converter circuits makes CFLs,
TVs and heat pumps sources of harmonics.
4. Simulation Results and Discussion
The system shown in Fig. 5 is a 12.47 kV radial
distribution system. Two transformers step the voltage
down to 4.16 kV and then to 208 V. The residential
system is connected to one of the phases, as shown, at 120
V. The system is considered as balanced. This system is
used for various case studies performed to analyze
harmonic distortion. The analysis is done in time domain
using PSCAD software.
Primary current
Primary voltage
12.47 kV/4.16 kV
4.16 kV/0.208 kV
Single phase loads
12.47 kV
Substation
Fig. 6 shows the waveforms and harmonic spectra for
voltage and current on the load side. It was seen that the
odd harmonic components are dominant in voltage and
current waveforms. Specifically the 3rd and 5th harmonic
components are dominant in the load voltage and amount
to 13.5 % and 8.1 % respectively. THD in voltage was
found to be 27.1 % and that in current was found to be
24.1 %. One reason for the high value of THD in voltage
and current could be the considerable amount of distortion
created due to the nonlinear loads present in each
individual house (voltage distortion was found to be 3.5
% due to a single house). Thus, the harmonics created by
all the houses add up and the total harmonic distortion
from the four houses exceeds the limit. PV system also
supplies nonlinear current due to the internal inverter
circuit and thus causes harmonic distortion.
Load voltage
Three phase loads
Load current
Load voltage
House3
House4
Magnitude(amperes)
House2
Magnitude(volts)
Line reactor
House1
100
50
0
-50
-100
Residential loads
40
20
0
-20
-40
-60
-150
4.82
PV System
Load current
60
150
4.84
4.86
4.88
4.9
4.83 4.84 4.85 4.86 4.87 4.88 4.89 4.9
Time(seconds)
Isolation transformer
Time(seconds)
Harmonic distortion in load voltage
30
25
25
20
Magnitude(%)
Following case studies are done to investigate the
harmonic distortion:
Magnitude(%)
Fig.5. Schematic of test system used for the simulations
Harmonic distortion in load current
20
15
10
5
Cases 2 and 3 are compared to:
(i)
(ii)
Investigate the harmonic injection into the AC
distribution system by the connection of DG and
Investigate the change in harmonic distortion in
the residential system load current and load
voltage.
0
5
10
15
20
25
30
10
5
0
0
5
10
Harmonic Order
15
20
25
30
Harmonic Order
Fig.6. Load current and load voltage distortion for case 1
B) Analysis and comparison of distortion when PV
system is connected to the AC grid in parallel with
the residential system load
Figs. 7 and 8 show the comparison of the load voltage
distortion and load current distortion when the residential
system is fed by the distribution system and PV system
simultaneously.
Load voltage
Load voltage with PV
10
9
8
Magnitude(%)
1) In case 1, the PV system is used as a stand-alone unit
supplying power to the residential system. A singletuned filter is used to reduce the harmonics at the
inverter output, before it is fed to the residential
system.
2) In case 2, the residential load is fed from the
distribution system only.
3) In case 3, the PV system is connected to the AC grid
through a line reactor and residential load is fed
simultaneously by the two sources; the AC grid and
the PV system.
0
15
7
6
5
4
A graphical comparison of the case studies has been
presented here onwards. FFT is applied to a single cycle
of the waveforms and the harmonic spectrum thus
obtained has been plotted.
3
2
1
0
3
A) Analysis of distortion in the load voltage and current
in the residential system due to PV system alone
5
7
9
11
13
15
17
Harmonic order
19
21
23
25
Fig.7. Harmonic spectrum of load voltage for cases 2 and
3
Load current
Load current with PV
45
4
40
3.5
35
30
25
20
Primary Voltage
Primary Voltage with PV
4.5
Magnitude(%)
Magnitude (%)
50
3
2.5
2
1.5
15
1
10
0.5
5
0
0
3
5
7
9
11
13
15
17
Harmonic order
19
21
23
Fig.8. Harmonic spectrum of load current for cases 2 and
3
The plots show a decrease in the magnitude of the
individual harmonic components in the presence of PV.
THD for load current was found to be 49.3 %, when fed
by the distribution system only. However, it decreased to
44.14 % when PV system is also connected to the system.
In this case, the PV inverter acts as a harmonic sink rather
than a source of harmonic generation. So, the current
distortion decreased. However the current distortion
didn’t decrease to a great extent because of the higher
amount of distortion caused due to the nonlinear loads
present in the houses in comparison to the harmonic
distortion caused due to the PV inverter. It was found that
voltage distortion at the load went down from 11.9 % to
7.2 %.
Figs. 9 and 10 show the harmonic spectra for current and
voltage on the high voltage side of the transformer when
PV system is connected with the distribution grid. It was
seen that THD in the primary injected current is 38.4 %
Primary current
Primary current with DG
40
35
Magnitude(%)
30
25
20
15
10
5
0
3
5
7
9
11
13 15 17
Harmonic order
19
21
23
3
5
7
25
25
Fig.9. Harmonic spectrum of primary current for cases 2
and 3
9
11 13 15 17
Harmonic order
19
21
23
25
Fig. 10 Harmonic spectrum of primary voltage for cases 2
and 3
when the residential load is supplied by distribution
system only. However, it goes down to 21.9 %, when the
PV system is also connected to the system. The voltage
distortion on the high voltage side drops from 5.1 % to
3.1 %. When PV system was not connected to the system,
only the distribution system was feeding to the residential
load, so the harmonics injected into the distribution
system were entirely due to the nonlinear residential load.
However, after PV system is connected to the system,
harmonics due to the PV inverter also gets injected to the
distribution system. Though the distortions from both the
sources look quite high in magnitude, resultant distortion
is less. One reason for the decrease in distortion could be
due to phase-angle cancellation.
5. Conclusion
The PV system and the residential system have been
modeled to investigate the harmonic interaction by
performing various case studies. It was found that the PV
system did not add any distortion to the high amount of
distortion on the load side already present due to the
nonlinear load. Thus the harmonics produced on the load
side by the PV system were not significant compared to
the high current distortion caused due to the household
nonlinear loads. The distortion injected to the distribution
system decreased after connecting the PV system due to
harmonic phasor cancellation. However, an accurate
estimate of how the THD is going to change and how the
individual harmonic components will change for
increased size of PV system or expansion of the number
of PV systems needs further investigation and analysis.
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