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Review of Common-Mode Voltage in Transformerless Inverter Topologies for PV
Systems
Article · January 2012
DOI: 10.1007/978-3-642-27509-8_49
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Analysis of Harmonics and Common-Mode Voltage in
Transformerless AC Modules Integrated in PV Systems
Tarak Salmi#1, Mounir Bouzguenda*2, Adel Gastli*3 and Ahmed Masmoudi#4
#
Research Unit on Renewable Energies and Electric Vehicles, National Engineering School of
Sfax, P.O.B: W, 3038 Sfax, Tunisia,
1
*
tarak_sel@yahoo.fr
4
a.masmoudi@enis.rnu.tn
Department of Electrical and Computer Engineering, College of Engineering, Sultan Qaboos
University, P.O. Box 33, P.C. 123, Al-Khoudh, Sultanate of Oman.
2
3
buzganda@squ.edu.om
gastli@squ.edu.om
Abstract: When a galvanic connection between the grid and the PV array is made, a commonmode voltage exists which generates common-mode currents. These common-mode currents may
produce electromagnetic interferences, grid current distortion and additional losses in the
system. Therefore, to avoid the leakage currents penalizing transformerless power chains, it is
worth focusing on topologies which do not generate common-mode currents. Some topologies
available in the market touch more or less such a crucial requirement. This is said, a small room
for improvement still exists. The aim of this work is to focus on recently developed topologies
which do not generate common-mode voltage. It was shown by analysis and simulation that the
HERIC topology has a high efficiency and does not generate a common-mode voltage. The focus
is also on the different ways to reduce harmonics in the output inverter waveform and to
maximize the PV cells output power.
Keywords: Photovoltaics, transformerless, AC module integrated, leakage current, commonmode voltage, harmonics.
low. Thanks to the decrease of their cost and the
1. Introduction
Nowadays, the invention and development of
new energy sources are enhanced continuously
because of the gradually critical situation of the
chemical industrial fuels such as oil, gas and
others. In fact, burning oil, coal and natural gas
generates nitrogen oxide, sulfur dioxide and
mercury and other toxic metals in the
atmosphere, polluting air, land and water.
Nuclear fission as an energy source also
produces radioactive waste, material that will
remain deadly for thousands of years. The
poisonous results of the various pollutants
created by the use of these fuels are becoming
increasingly harder to justify. This is why the
renewable energy sources have became a more
important contributor to the total energy
consumed in the world. These sources are
sustainable, and are independent from limited
fossil and nuclear fuels. Therefore, it is expected
that they will become the only satisfying and
reliable energy in the coming years.
It is true that currently, the energy production
contribution of photovoltaic (PV) systems is still
increase of their efficiency and reliability, the
market for PV systems is growing worldwide. To
enhance the application of PV systems, research
activities are carried out in an attempt to gain
further improvement in their cost-effectiveness,
efficiency and reliability. Within this trend,
valuable improvement may be introduced in the
power electronic converters and particularly, the
inverters that are integrated in PV systems.
Section two aims to review the advantages and
disadvantages of the AC module integrated.
Meanwhile a comparison among the most
common topologies available in the market is
included in section three. In the fourth section, a
review of recent ways to reduce harmonics is
presented.
2. Integrated AC Module
The early PV systems used were the string
topologies and the central ones. These two
topologies use long and high voltage DC cables
to feed power from the PV array into the inverter
and then into the utility grid. These long cables
cause power losses. Besides, in these
configurations, only one maximum power point
tracker (MPPT) is used for the whole array. In
this case, mismatch losses will take place and
will reduce the system efficiency [1].
Furthermore, these configurations necessitate
high level power inverters which minimize and
restrict the flexibility of the system to expand.
Recent trends focus on the so called AC module
in which the inverter is fixed on the back of the
panel. Such a configuration totally eliminates
DC cables, reduces maximum power point
(MPP) mismatch losses and therefore increases
the efficiency of the whole system and
significantly reduces the installation cost. In fact,
the PV panel is delivered to the user as a
complete system.
The disadvantage of the AC module integrated
solution is the strict requirement for the design
and components capable of resisting under
serious ambient conditions lasting twenty years
of lifetime similar to the PV panel. It is
reasonable then to choose topologies with
components having long lifetime and good
thermal and electrical stability. It is well known
that among all passive or active components,
electrolytic capacitors have the shortest lifetime.
As a conclusion, the use of topologies with no
high capacitance values merit to be taken into
consideration.
3.
Common-mode voltage
There are two main topology groups used in grid
connected PV systems: with and without
galvanic isolation. This galvanic isolation can be
achieved either in the DC side by the use of a
high frequency DC-DC transformer or in the
grid side by the use of low frequency AC one.
Both cases ensure the safety and galvanic
isolation. However, PV inverters that have an
isolation transformer on the grid side are big in
size making the whole system bulky and hard to
install while topologies that use high frequency
transformer in the DC-DC converter have a
reduction in the overall efficiency due to the
leakage in the transformer [2-8]. In fact, the
elimination of the transformer increases the
efficiency by 1-2% [1]. Fig. 1 gives a
comparison of three different topologies [1]:
with high frequency transformer, with low
frequency transformer, and with no transformer.
The data in Fig. 1 was collected for more than
400 commercially available inverters. It is
clearly shown that transformerless inverters have
smaller weight, size and better efficiency than
their counterpart with galvanic separation. A
considerable reduction in price is also guaranteed
in case the transformer is omitted [1, 9].
Fig.1. Advantages and drawbacks of different
Inverter topologies [9].
As mentioned before, the transformer in a PV
system ensures safety by galvanic isolation
between the PV array and the grid. It also
significantly reduces the leakage current between
the PV system and the ground. Besides, it
guarantees that no DC current is injected into the
grid [6]. As a consequence, transformerless
topologies have to cover all these issues. Safety
can be easily solved by including a ground fault
detector in the inverter. This fault detector which
disconnects the inverter immediately when it
detects any fault in the installation was
previously investigated in [10, 11].
Without transformer, common-mode resonant
circuit (including the DC source ground parasitic
capacitance, the filter, the inverter and the
impedance of the grid) is created when the DC
source is connected to the ground. A leakage
current takes place and will be superimposed to
the grid increasing its harmonics compared to the
inverter with a transformer [5, 12].
The ground parasitic capacitance depends on
many factors such as PV panel and frame
structure, surface of cells, distance between cells,
weather conditions, humidity and dust covering
the panel [13,14].
Still regarding the common-mode resonant
circuit, if the common-mode voltage generated
by the inverter includes frequencies close to
those of the circuit's resonance, large commonmode currents will appear [5], as previously
mentioned. To eliminate these currents,
topologies which do not generate common-mode
voltage are necessary in transformerless PV
inverters.
Many topologies which do not generate
common-mode voltages have been proposed in
the literature. The full-bridge with bipolar PWM
is one of them [4, 7]. However, this topology
causes high switching losses, large current ripple
and does not eliminate DC current injected into
the grid [5]. Some improvements were
introduced on this topology to improve its
efficiency, but they did not solve the problem of
the DC injection [7].
Another topology which avoids the commonmode voltage is the half-bridge inverter.
However, a high input voltage is needed.
Therefore, the use of boost converter on the DC
side is required in this case. This would increase
the cost and decrease the efficiency down to 92%
[12]. That is why the half bridge is not popular.
The common-mode voltage can be avoided using
the full bridge with bipolar PWM [6, 9]. This
topology is being used in some commercial
transformerless inverters but it still presents quite
low efficiency (95.3%) compared with the
expected one due to the losses caused by the
double switching frequency [12].
To improve the inverter’s efficiency, a bypass
branch in the AC side using two IGBTs with
freewheeling diodes shown in Fig. 2 was
proposed [12]. It is used by Sunways AG for its
commercial inverters. In [12], the bypass was
proposed on the DC side. It was found that this
topology reduces both the losses and the DC
currents injected to the grid. In addition, this
topology guarantees an efficiency of up to
96.3%.
Among the many proposed topologies the
HERIC topology combines the advantages of the
unipolar modulation with the reduced varying
common-mode voltage of the bipolar one [9, 15].
Its common-mode current 𝒊𝒄𝒎 generated by the
capacitance between the photovoltaic array and
earth ( 𝑪𝑮𝑷𝑽 in Fig. 2) is:
𝒅𝒗
𝒊𝒄𝒎 = 𝑪𝑮𝑷𝑽 𝒅𝒕𝒄𝒎
(1)
The common-mode voltage 𝒗𝒄𝒎 is:
𝒗𝒄𝒎 =
𝒗𝑨𝑶+𝒗𝑩𝑶
𝟐
(2)
It can be deduced that if 𝒗𝒄𝒎 is kept constant, no
leakage current would appear.
In this topology, voltages 𝒗𝑨𝑶 and 𝒗𝑩𝑶 are
controlled by four switches S1-S4. When the
upper switch is ON, the corresponding voltage is
𝒗𝑰𝑵 . However when the lower switch is ON, the
corresponding voltage is zero. Therefore, during
the positive half-wave, S6 is turned ON and is
used in the freewheeling period of S1 and S4.
During the interval of time when S1 and S4 are
ON (𝑡𝑂𝑁 =dT, where d is the duty cycle, T is the
switching period) the common-mode voltage
applied is:
𝒗
𝒗𝒄𝒎 = 𝑰𝑵
(3)
𝟐
Since 𝑣𝐴𝑂 = 𝑣𝐼𝑁 and 𝑣𝐵𝑂 = 0.
After 𝑡𝑂𝑁 S1 and S4 are turned off. The voltage
𝒗𝑨𝑶 decreases and 𝒗𝑩𝑶 increases until diode of
S5 switches ON. If during the switching process,
the amount of 𝒗𝑩𝑶 increase is exactly the same
amount of decrease in 𝒗𝑨𝑶 then 𝑣𝑐𝑚 is kept
𝑣
equal to 𝐼𝑁 .
2
During 𝑡𝑂𝑓𝑓 ((1-d) T), the inductor current flows
through S6 and the diode of S5. The voltage
applied to the inductor is (- 𝑣𝑔𝑟𝑖𝑑 ) [12]. The
common-mode voltage is still constant:
𝒗𝒄𝒎 =
𝒗𝑰𝑵
𝟐
(4)
Since 𝒗𝑨𝑶 = 𝟎 and 𝒗𝑩𝑶 = 𝒗𝑰𝑵 during 𝑡𝑂𝑓𝑓 .
This analysis means that if the choice of the
switching devices is as serious as the switching
actions would be done simultaneously, the
common-mode voltage would not change.
Thereby the leakage current is kept very small as
shown in Fig.3.
Fig.2. HERIC inverter.
The operating principle during the negative halfcycle is analyzed similarly. The switches of the
𝒗
HERIC commutate with only 𝑰𝑵 which reduces
𝟐
the switching losses and therefore, it increases
the inverter efficiency.
The precision and the exactitude of the switching
process can be achieved by the serious choice of
the semiconductor switching devices as well as
the control strategies.
Compared to the half-bridge with unipolar
modulation (HB-Unip) and other topologies such
as the H5-topology, the HERIC topology stands
among the best ones. To prove this, the H5 has
been simulated and the obtained results (Fig.4)
were compared to those of the HERIC inverter. It
is well clear that the leakage current as well as
the output current ripple is very small in the
HERIC topology. In addition, less harmonics
were found in the inverter output waveform [1,
15, 16]. Therefore, these promising results make
the HERIC topology a suitable solution in case
of transformerless systems.
Fig.3. from top to down: leakage current, load current and
output voltage of the HERIC inverter.
Fig.4. from top to down: leakage current, load current and
output voltage of the H5 inverter.
The major drawback of the HERIC inverter is
that it is only ideal in the case of PV systems that
supply the grid with active power. In fact, in case
of many inverters supplying the grid with active
power at the same time, the voltage at the point
of common coupling can exceed limits and
affects the safety of the inverter.
Also, this process can lead to extra losses
because some produced power would not be fed
to the grid [9].
4. Harmonic distortion
PV generators are connected to the distribution
network through DC-AC inverters and are
therefore able to inject harmonics into the
distribution system, thus, downgrading the
power quality.
The harmonics in the output current of an
inverter can be classified according to their
sources, into two types [17-19]:
 Switching harmonics which are related to
the PWM circuit in each inverter.
 Low frequency harmonics which are due to
the deficiencies in the control of the inverter
output current.
Several techniques aimed to improve the power
quality by reducing or totally eliminating the
harmonics. In [20], it was demonstrated that
certain PWM strategies and techniques could
help eliminate a particular side-band switching
harmonics. Such a work also identified further
opportunities for harmonic elimination in
multilevel cascaded inverter systems. Another
research done by [21] in which a description of
Walsh Functions technique for full-bridge single
phase inverter was presented. This technique
allowed the amplitude of the inverter output
voltage to be expressed as function of the
inverter switching angles. In this case, a series of
algebraic equations could be solved to eliminate
unwanted harmonics.
In order to improve the power quality, it is well
advised to lead a wide survey touching the
fundamental ways to reduce the harmonics by
means of active and passive methods.
Generally, harmonics can be divided into two
types namely voltage harmonics and current
harmonics. Both of them are generated at either
the source or the load side. Harmonics generated
by the load are caused by nonlinear
characteristics of devices. While source
harmonics are always generated by the power
supply with non-sinusoidal waveform. Any
periodic waveform can be shown as the
superposition of the fundamental and some
harmonic components. These components can be
extracted by solving mathematical equations and
leading some Fourier transformations. When the
mathematical description is transferred to the
discrete tense, building algorithms and
implementing them on programmable chips
become easy. The trend of these algorithms is to
reduce as much as possible the different
harmonics content in the inverter output
waveform. Most of them use a Digital Signal
Processor (DSP) to implement these algorithms
and obtain the expected results.
In recent years, it was found that the multilevel
inverter appears as an attractive solution due to
its improved output waveform which approaches
the sinusoidal one, its smaller filter size, its
lower Electromagnetic Interference (EMI) and
reduced total harmonic distortion (THD) [22].
Three common topologies, widely studied in the
literature, are the diode clamped [23], the
capacitor clamped [24] and the cascaded halfbridge inverters [25]. Further, many modulations
and control strategies have been developed for
multilevel inverters. These include sinusoidal
pulse with modulation, selective harmonic
elimination and the space vector modulation
[26].
PV systems also suffer from another important
problem under changing weather conditions. In
fact the quantity of electric power generated by
an inverter is a function of the intensity of the
solar radiation. In this case, an MPPT method or
algorithm, which has a quick response, is needed
to overcome any lack of power [26].
Fig.4 shows an example of such an algorithm,
described in [26], to extract the maximum power
from the PV arrays. The resulting error between
the grid signal and the reference one is compared
with a triangular signal and their intersections
produce the PWM control for the inverter.
Simulation and experimental results indicated
that the quality of the waveforms has improved
from the THD point of view.
efficiency
compared
to
the
common
commercially available topologies. Despite this,
it still has some drawbacks as previously
mentioned.
A matter that still has a room for improvement is
the AC module which was found to be a viable
candidate for improving the performance of PV
systems. Indeed, avoiding long DC cables has an
impact on the cost of the inverter installation and
its efficiency.
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5. Conclusion
This work aimed to give an overview of the best
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