The Switched Capacitor Inverter As A MPPT In
A Photovoltaic Application
CC Marouchos
Cyprus University of
Technology/Electrical Engineering,
Limassol, Cyprus
christos.marouchos@cut.ac.cy
M. Darwish
Brunel University/School of Engineering
and Design
Mohamed.Darwish@brunel.ac.uk
Despo Xenofontos
Cyprus University of
Technology/Electrical
Engineering, Limassol, Cyprus
dx.xenofontos@edu.cut.ac.cy
Abstract-The new topology for a dc to ac converter based on
the switched capacitor circuit was introduced recently.
M.S.Moghadam
Brunel University/School of Engineering and
Design
Mansour.salehimoghadam@brunel.ac.uk
Christina Armefti
Cyprus University of Technology
Electrical Engineering, Limassol,
Cyprus
cg.armeftis@edu.cut.ac.c
The promising features of low harmonic content, high
efficiency, MPPT facility and voltage step up characteristics
makes it a good candidate for grid connected-renewable energy
sources. The harmonic content is further minimised in this
paper by introducing multiple current modulators, each
connected to a PV panel. It is also shown that the circuit also
tracks the Maximum Power Point of the output power of the
photovoltaic panel with varying irradiation from the sunand
ambient temperature. The work is carried out by extensive
simulations on PSIM.
Index Terms--Inverters, Multilevel inverters, Grid-connected
Inverters
I.
INTRODUCTION
The topology of the Switched Capacitor Inverter is based
on the Switched Capacitor Circuit. A Current Modulator in
the form of a boost converter is feeding a three level inverter
operating at 50Hz, Fig.1. Two bands of frequencies are
identified in the output voltage when the values of L and C
are chosen in random. A Lower Band, very close to the
fundamental and a Higher Band around the switching
frequency and its multiple. It was shown [1] that with a wise
choice of the values of L and C for a specific load minimises
the harmonics and specifically the Lower band can be
eliminated. The Higher Band of voltage harmonics is almost
completely eliminated now by introducing multiple Current
Modulators. The switching signal of each Current Modulator
is set at the appropriate phase and at least the lower group of
the Higher Band is eliminated. This is achieved either by two
or more Current Modulators. The extra cost of the Current
Modulators is justified in a PV Park application where the PV
modules are usually arranged in panels and each panel is
connected to an inverter. It is suggested to connect each panel
to a Current Modulator.
The new inverter can be used as an element of a multilevel
configuration [2-4]. It is demonstrated that stacking two or
more Switched Capacitor Inverters, greatly reduces the
output voltage harmonics.
The Current Modulator is a boost dc to dc converter and it
can be utilised also for Maximum Power Point Tracking
(MPPT). The ability of the circuit to track the Maximum
Power Point is therefore tested. It is shown that the Maximum
Power Point is tracked for varying values of sun Irradiation
and a range of ambient temperature.
II.
OPERATION OF THE CIRCUIT
The input dc voltage undergoes two conversion processes
in the Switched Capacitor Inverter [1]. First the input dc
voltage is converted to a PWM rectified sinusoidal voltage,
Fig.2a at the required frequency eg. 50Hz and then it is
converted into a proper sinusoidal voltage, Fig.2b. The first
conversion is done by a current modulator and the second is
done by a simple 3-level inverter. The current modulator is a
boost inverter and the switch is controlled by a PWM circuit,
Fig. 3. The 3-level converter simply reverses the even halfcycles of the rectified voltage in order to construct the proper
ac waveform, switching at the very low switching frequency
of 50Hz. The advantages of this topology over other
topologies [2-4] are obvious since only two switches are
employed in the first conversion switching at relatively high
frequency, a transistor and a diode. In the next stage the
losses are even lower because the 3-level inverter is
switching at a low frequency, at 50Hz -for a 50Hz output
voltage- and more importantly switching is taking place at
almost zero voltage and current. In this way the losses of the
new configuration are lower than the 3-level PWM inverter
switching at a relatively high frequency with four transistors
and four diodes.
I(t)
Dc current
Amplitude
Modulator
ID(t)
Cdc
Vodc(t)
Single
pulse 3Level
Inverter
R
Vo(t)
Fig.1 Block Diagram of the Switched Capacitor Inverter
The inductance L and capacitance C, must be chosen for a
range of load values R in order to minimise output voltage
distortion [1]. The switching frequency is also a parameter
and it determines the order and in conjunction with L, C and
load R values, the magnitude of the harmonics.
Fig.3 Generation of the PWM modulation function
The modulation function is a PWM signal and it is
produced by comparing a rectified sinusoidal voltage at 50Hz
to a triangular carrier signal at a frequency which gives the
switching frequency, Fig.3.
III. THE OUTPUT VOLTAGE
Two bands of frequencies are identified in the output
voltage when the values of L and C are chosen in random. A
Lower Band, very close to the fundamental and a Higher
Band around the switching frequency, Fig.4. The presence of
harmonics close to the fundamental -Lower band- is due to
three main reasons. Unsmoothed inductor current, non-zero
values of the voltage across the capacitor and non-unity load
power factor [1]. The relative values of the inductor and the
capacitor, L and C for a specific load R can be chosen in
order to minimise or even eliminate the Lower Band, Fig. 5.
The Higher Band is repeated at decreasing magnitudes for
multiples of the switching frequency.
It is shown in this paper that the Higher Band of
frequencies can be completely eliminated (at least the first set)
by adding more current modulators to the inverter. This is
practically possible and perhaps desirable in PV Parks where
the PV panels are arranged into groups and connected to a
single inverter. In order to eliminate Higher Band frequency
components, the phase of the switching frequency for each
current modulator is arranged accordingly. For two current
modulators the phase difference is 180o and for three it is
120o etc. This is an effective way to cancel harmonics, keep
the switching frequency low and hence reduce the switching
losses.
eliminate them to some degree is to feed the dc capacitor, Cdc,
of Fig.1 with two currents, each with a phase difference of
180o for the higher order harmonics. This is partly achieved
by arranging the phase of their switching signals in each
current Modulator to have a phase difference of 180o.
This can be repeated for three or more Current Modulators,
the phase difference  of the switching signals for each
current modulator is given by
Vodc
Output of
Current
Modolator
400
300
200
100

0
Vinv
400
360o
h
(2)
Output of Inverter 50Hz
200
Where h is the number of current modulators.
0
-200
Two current modulators, Fig.6a, are feeding the dc capacitor. The
modulation functions applied to each modulator are exactly the same
[1] but the carrier signal, Fig. 3, of each modulator is set at 180o phase
difference. The higher order harmonics, the first set m±1, are
cancelled completely, Fig. 6b. Three current modulators are feeding
the dc capacitor in Fig.7a. Again the modulation functions applied to
each modulator are exactly the same but the carrier of each modulator
is set at 120o phase difference. The higher order harmonics, the first
set, are eliminated Fig. 7b. and the second set is reduced.
-400
0.96
0.97
0.98
Time (s)
0.99
1
Fig. 2a. Top trace: Output of current amplitude modulator Vodc(t)
Fig. 2b. Bottom trace: Output of Inverter, Vo(t)
Vinv
L =0.01 C =50uF R =15Ω
400
Switching Frequency 1000Hz
300
200
Lower Band
100
Higher Band
0
Vinv
0
100
200
300
400
500
600
700
800
Frequency (Hz)
900
1000
1100
1200
1300
1400
350
1500
300
250
Fig.4. Frequency Spectrum of the Output voltage, Vo(t)
200
150
100
Fig.6a. Two Modulator Inverter
50
Vinv
0
0
500
1000
800
250
600
200
1500
2000
2500
Frequency (Hz)
300
vo
150
400
100
200
50
0
0
250
500
750
1000
1250
Frequency (Hz)
1500
1750
2000
2250
2500
0
0
100
200
300
400
500
600
700
800
Frequency (Hz)
900
1000
1100
1200
1300
1400
1500
Fig.6b Frequency spectrum of the double current modulator inverter
Fig. 5. Frequency Spectrum of output voltage, Vo(t)of the new Inverter for
proper choice of L and C (L=0.25 C=9uF R =15Ω switching at 1000Hz)
IV. HIGHER VOLTAGE HARMONIC ELIMINATION
The prime source of the Higher Band of voltage harmonics is
the switching signal. These harmonics are present around the
switching frequency and its multiples.
nm±1
(1)
Where n is an integer number and m is the ratio of the
switching frequency to the frequency of the output voltage to
be produced. Obviously the phase of the higher order
harmonics is set by the switching signal. Therefore a way to
Fig.7a. Triple Current Modulator Inverter
Vinv
350
300
250
200
150
100
50
0
0
1000
2000
Frequency (Hz)
3000
4000
The circuit is tested for Maximum Point Power Tracking in two
ways: for different sun irradiations and different ambient
temperatures. It is demonstrated that it tracks the Maximum Point
Power and precisely sets the depth of modulation accordingly.
Three different values of sun irradiations were tested 1000w/m2,
800w/m2 and 600w/m2. For each value the depth of modulation is set
appropriately between 0.81 and 0.82 Fig.9. Four ambient temperatures
were tested 25 oC, 35 oC, 55 oC and 65 oC. For each temperature the
depth of modulation is set appropriately between 0.6 and 0.7 Fig.10.
Fig.7b Frequency spectrum of the triple current modulator inverter
600
Two inverters are stacked together feeding the same load and all
higher order harmonics are eliminated, Fig.8a. This is repeated for
three inverters in series and again all higher order harmonics are
eliminated Fig. 8b.
500
P
o
w
e
r
1000W/m2
400
800W/m2
300
600W/m2
200
vo
(w)
1200
100
1000
800
0
600
400
0.4
200
0.6
0.8
1
Depth Of Modulation
0
0
250
500
750
1000
1250
Frequency (Hz)
1500
1750
2000
2250
2500
Fig. 9 Maximum power point tracking for different levels of irradiation
Fig.8a Frequency spectrum of two inverters stacked together
450
25 oC
400
P
o
w
e
r
35 oC
350
300
50 oC
250
65 oC
200
150
Fig.8 b Frequency spectrum of three inverters stacked together
100
(w)
A range of options are available to the connection of PV panels
to the grid either with more current modulators or more
inverters stacked together.
50
0
0.4
0.5
0.6
0.7
0.8
0.9
1
Depth Of Modulation
Fig. 10 Maximum power point tracking for ambient temperatures
V. MAXIMUM POWER POINT TRACKING
IV. DISCUSSION AND FUTURE WORK
Elimination of the voltage harmonics at the output voltage
of the SC inverter near the fundamental, Lower Band, is
achieved by proper choice of the value of the dc capacitor C.
Unfortunately the magnitudes of the Higher Band are
increased when the dc capacitor value is set for zero
magnitude of the Lower Band of frequency components. By
introducing more Current Modulators, the Higher Band is
greatly reduced. With two Current Modulators, the lower set
of the Higher Band of voltage harmonics at the output of the
inverter is completely eliminated. The phase of the switching
signal of each Current Modulator is arranged accordingly.
With three Current Modulators more components of the
Upper Band are eliminated. The extra cost and complexity of
Current Modulators is justified when this inverter is used in
PV Parks. In PV Parks panels of PV modules are connected
to individual inverters to form the total output power to the
grid.
The ability of the circuit to monitor the output power and
track The Maximum Power Point is demonstrated for a range
of sun irradiation values and temperature. Therefore this new
inverter is a good candidate for PV Parks connected to the
grid. Naturally a more in depth work is necessary to optimise
this inverter and compare it to the other available
configurations.
Future work will also be concentrated in developing a
compact form of a 3-phase circuit.
V. CONCLUSIONS
The ability of the circuit to perform maximum power point
tracking for different values of sun irradiation and ambient
temperatures is demonstrated. Furthermore the output voltage
harmonics are greatly reduced by the introduction of more
current modulators. Both characteristics make this inverter a
good candidate for PV parks connected to the grid.
Future work is proposed for a 3-phase system and
optimisation of the circuit.
REFERENCES
[1] A new Concept for a Multilevel Switched Capacitor sinusoidal Grid
Connected Inverter, CC Marouchos M. Darwish L. Diomidou,
UPEC2013, Dublin Ireland
[2] Cassiano Rech, Humberto, Pinheiro, Hilton and A. Griindling, HClio L,
“Analysis and Comparison of Hybrid Multilevel Voltage Source
Inverters”, Power Electronics Specialists Conference, 2002. pesc 02.
2002 IEEE 33rd Annual
[3] Fang Z. , “Generalized Multilevel Inverter Topology with Self Voltage
Balancing”, Industry Applications Conference, 2000. Conference Record
of the 2000 IEEE
[4] N Benaifa, “Parallel operated Inverers as a Multilevel Case”, Electrical
and Computer Engineering, 2008. CCECE 2008. Canadian Conference o