SCYR 2010 - 10th Scientific Conference of Young Researchers – FEI TU of Košice
Cascade H-bridge Inverter for Photovoltaic System
1
1, 2
Marek PÁSTOR, 2Marcel BODOR
Dept. of Electrical Engineering, Mechatronics and Industrial Engineering, FEI TU of Košice, Slovak Republic
1
2
marek.pastor@student.tuke.sk, marcel.bodor@tuke.sk
Abstract—The main objective of this paper is to describe a
cascade H-bridge inverter with focus on photovoltaic
applications. The cascade H-bridge inverter is compared to a
single H-bridge inverter mainly from THDi and efficiency point
of view.
distortion input current, and lower switching frequency. The
disadvantages are higher number of power semiconductor
switches, more complex control technique and higher
conduction losses.
Keywords—cascade H-bridge inverter, current control voltage
source inverter, multilevel converters, photovoltaics.
B. Cascade H-bridge inverter
A single-phase structure of a 7-level cascade H-bridge
inverter is shown in Fig.1.
I. INTRODUCTION
The solar energy and especially photovoltaics is one of the
fastest growing industries in the world. There is a demand for
high quality electrical energy and thus the use of photovoltaics
is almost impossible without modern power electronics. If we
omit the simplest PV battery charger there always have to be
certain power conditioning unit (PCU) between the PV
generator and the load whether to maximize the energy yield
or to change certain qualities of the electrical energy. Whether
it is a stand alone PV electrical generator or a grid connected
system there is a demand to change the DC voltage to the AC
voltage, to maximize the energy yield and to monitor the
whole system. This is done by the mean of a PV inverter. The
use of the PV inverter is to change the DC voltage to the AC
voltage and to adapt the PV generator to the electrical load as
well as to monitor the whole system. There are several types
of PV inverters according to the topology. If there is a need
for a galvanic isolation between the PV generator and the grid,
the PV inverter with a transformer has to be use. The PV
inverter can utilize a low frequency transformer with sufficient
filter at the inverter’s output or a high frequency transformer.
The PV generator voltage does not always meet the required
value and thus this voltage needs to be changed. This can be
done by a DC/DC converter at the PV inverter’s input. The
DC/DC converter can utilize the high frequency transformer.
This paper describes the cascade H-bridge inverter which can
be used for photovoltaic applications.
Fig. 1. Single-phase cascade H-bridge inverter with three separated DC
sources (UA = 240V, UB = 120V, and UC = 60V), capable to create 15
voltage levels at its output.
The number of output phase voltage levels n is defined by:
II. CASCADE H-BRIDGE INVERTER
A. Basics
Cascade inverters belong to the multilevel power
converters. Multilevel power converters are mainly used for
medium and high power application due to utilization of
several power semiconductor switches with separated DC
sources connected in series. Multilevel power converters have
several advantages over single level power converters [1]:
staircase output voltage, low common mode voltage, low
n = 2d + 1
(1)
where:
d – is the number of separated DC sources.
However it is possible to create more voltage levels at the
output of the cascade inverter. Each H-bridge converter can
create positive, negative or zero voltage on its output with
magnitude equal to the DC source. Thus there are 15 possible
combinations for the cascade H-bridge inverter with 3
separated DC sources.
SCYR 2010 - 10th Scientific Conference of Young Researchers – FEI TU of Košice
C. Output voltage control technique
There are several methods for a voltage control of the
cascade inverter. One of them is the sinusoidal PWM from
high switching frequency PWM modulation strategies [1].
The amplitude modulation index for the multilevel inverter
is defined by:
ma =
Am
(n − 1)AC
(2)
where:
Am – is the modulation signal amplitude,
AC – is the carrier signal amplitude,
n – is the output voltage level number.
The frequency modulation index is defined by:
fC
fm
mf =
(3)
where:
fC – is the frequency of the carrier signal,
fm – is the frequency of the modulation signal.
D. Current control technique
If we consider the DC/DC converter at the inverter’s input
this DC/DC converter acts as a voltage source. Thus the
inverter must be a voltage source inverter (VSI). There are
two main control strategies for VSI: thevoltage control
(VCVSI) and the current control (CCVSI). They vary in the
way they control the power flow. The VCVSI uses the control
of the decoupling inductor voltage to control the power flow
and the CCVSI uses the decoupling inductor current to control
the power flow. The CCVSI is faster, can control active and
reactive power flow independently but can not provide the
voltage support to the load, can not operate without the grid.
The CCVSI can be used for power factor correction due to the
fact, that it can control the reactive power independently [2]. It
also has a limited short circuit current compared to the
VCVSI.
There are various techniques how to archive the current
control in CCVSI. One of them is a predictive current control
for voltage source inverters [3].
The easiest case is to use a simple RL filter to decouple the
grid voltage E and the inverter’s output voltage V. For circuit
in Fig.4 it can be written:
8
V = RI + L
6
dI
+E
dt
(4)
Modulation
4
2
0
-2
-4
-6
-8
0.02
0.022 0.024 0.026 0.028
0.03
0.032 0.034 0.036 0.038
0.04
Fig. 4. The RL filter between the inverter’s output and the grid used to
decouple the output voltage and the grid and to filter higher harmonics.
->time(s)
Fig. 2. Modulation and carrier signals for 15-level cascade H-bridge
inverter (ma = 0.78, mf = 2).
If the consider the sampling period T to be sufficient small
and the vectors V and E are constant between two sampling
periods, current from (4) can be discretized as follows [3]:
400
300
I (k +1)T = e
voltage(V), current(A)
200
R
− T
L
(
− T 
1
 1 − e L  V kT − E kT

R 

R
I kT +
)
(5)
100
The current value I(k+1)T is predicted by the Lagrange
quadratic extrapolation.
0
-100
-200
-300
-400
0
0.005
0.01
0.015
0.02
0.025
0.03
0.035
0.04
->time(s)
Fig. 3. Output voltage and current of 15-level (3 DC sources) cascade Hbridge inverter with voltage control (ma=0.78, mf=30, L=1mH, R=100Ω,
U=232V, THDu=9,4%).
E. Simulation results of the CCVSI
The same current control technique based on (5) was used
for 3-level (one DC source: 400V) and 15-level (three DC
sources: 240V, 120V and 60V) H-bridge inverter. The THDi
of the injected grid current and estimated efficiency of the
inverter disregarding the output filter were investigated.
It is more accurate to consider sampling frequency of the
current controller rather than the switching frequency of the
inverter. The controller chooses the best voltage vector at the
inverter’s output and thus each H-bridge switches with
different frequency, as can be seen in Fig.5.
SCYR 2010 - 10th Scientific Conference of Young Researchers – FEI TU of Košice
sensitive to the grid distortion. On overall, the cascade Hbridge inverter can produce higher quality electrical energy.
u1(V)
500
0
30
-500
0
0.005
0.01
0.015
0.02
0.025
0.03
0.035
actual
desired
0.04
time(s)
20
u2(V)
200
0
0
0.005
0.01
0.015
0.02
0.025
0.03
0.035
0.04
time(s)
I(A)
-200
10
0
u3(V)
100
-10
0
-100
0
0.005
0.01
0.015
0.02
0.025
0.03
0.035
0.04
-20
time(s)
Fig. 5. Partial output voltages of the cascaded H-bridge inverter
(amplitudes: 240, 120 and 60V), sampling frequency of the current
controller f=10kHz.
The actual value of the output current of the inverter with
respect to the desired value and its change is in Fig. 6. From
Fig. 6 it can be seen that actual current tracks the desired
current value and its changes with low THDi. There are no
zero crossing spikes in the output current.
25
actual
desired
20
15
10
I(A)
5
-30
0
-20
0.015
0.02
0.025
0.03
0.035
0.025
0.03
0.035
0.04
TABLE I
CHARACTERISTICS OF COMPARED INVERTERS
single
cascade
In
16
16
A
Sn
3680
3680
VA
UDC
400
240, 120,60
V
IGBT
IRGB4056
IRGB4056
Rfilter
1
1
Ω
Lfilter
5
5
mH
-15
0.01
0.02
On the other hand there is a presumption for a lower
efficiency compare to the single H-bridge inverter due to the
increased number of power semiconductor switches.
The THDi and the efficiency of the single and the cascade
H-bridge inverter were examined. The basic characteristics of
both inverters are in Table I. Both inverters incorporate the
same current control technique (5) and have the same output
filter.
-5
0.005
0.015
Fig. 7. Output current of the single H bridge inverter (desired and
actual) supplied with DC voltage source 400V. The RL filter values:
L=5mH, R=1Ω.
-10
0
0.01
time(s)
0
-25
0.005
0.04
time(s)
The same simulation of the output current that was done for
the single H-bridge inverter is shown in Fig. 7. The output
current THDi is significantly higher (THDi = 14%). Higher
THDi also means higher uncontrolled reactive power. The
response to the change in the desired value is similar as for the
cascade H-bridge converter. This is caused by the same
maximal voltage swing of the output voltage (±420V for
cascade H-bridge, ±400V for single H-bridge).
The THDi of the grid current was simulated for various
sampling frequencies of the current controller. The simplest
RL filter was used at the inverters output. The results are
shown in Fig. 8.
35
30
25
THDi/%
Fig. 6. Output current of the cascade H bridge inverter (desired and
actual) created by combining partial output voltages shown in Fig. 5. The
RL filter values: L=5mH, R=1Ω.
20
15
10
THDi and efficiency of the inverters
The grid connected PV system is an electrical energy
generator. There are two main points of view when
considering such a system. It is important to ensure high
energy yield and to meet standards for generator system
(frequency, THD, EMI).The lower THD is achieved mainly
by improved output filter. However lower THD requires more
bulky and costly filter which has higher power losses. The
cascade H-bridge inverter can achieve much lower THDi
compared to the single H-bridge inverter and has lower EMI
radiation due to the lower du/dt stresses. It can utilize lower
switching frequency and decrease switching losses. It is less
5
0
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
sampling frequency/kHz
H-bridge
cascade
Fig. 8. The THDi of compared inverters versus current controller
sampling frequency. The RL filter values: L=5mH, R=1Ω.
The desired value of THDi = 3% was never met with the
single H-bridge inverter but THDi was lower than 3% for the
cascade H-bridge inverter and frequencies above 10kHz.
SCYR 2010 - 10th Scientific Conference of Young Researchers – FEI TU of Košice
The efficiency was examined for changing output power of
the inverter. The sampling frequency of the current controller
was set to 10kHz.
technique (240V DC source in this case).
400
E
I
V
300
200
98
97
96
E(V),I(A), V(V)
efficiency/%
100
99
95
94
93
92
91
90
100
0
-100
-200
0
20
40
60
80
100
120
-300
nominal power/%
-400
H-bridge
cascade
0
0.005
0.01
0.015
0.02
0.025
0.03
0.035
0.04
time(s)
Fig. 9. The efficiency of compared inverters versus output power. The
RL filter values: L=5mH, R=1Ω.
The efficiency of the inverter is not easy to estimate due to
the changing current of the semiconductor switches. The
calculation is based on the average value of the current.
TABLE II
COMPARISON OF SIMULATED LOSSES IN INVERTERS
Losses
single H-bridge
cascade H-bridge
switching
5
5
W
conduction
36
103
W
Only the losses in the IGBTs are considerd disregarding diode
losses (they are considered in total inverter’s efficiency).
The switching losses are the same for the single H-bridge
and the cascade H-bridge converter. Because there are three
H-bridges in the cascade H-bridge inverter and only one Hbridge in the single H-bridge inverter, the switching losses per
H-bridge are reduced in the cascade H-bridge inverter. On the
other hand, the conduction losses are approximately three
times higher. The VCE(ON) voltage of the IBGT is very
important parameter when considering the efficiency of the
cascade H-bridge inverter. The cascade H-bridge inverter
gives the opportunity to choose the most suitable
semiconductor switches for the each H-bridge an it gives a
further possibility to increase the cascade H-bridge inverter’s
efficiency.
III. CONCLUSION
The cascade H-bridge inverter is an alternative to the single
H-bridge inverter in photovoltaic systems. However cascade
inverters are not popular in photovoltaic systems in nowadays.
They are more expensive, have lower efficiency, require more
complex control techniques. On the other hand they produce
lower THD of the grid current and THD of the output voltage
(Fig. 10), require smaller filters, can transfer more power and
have smaller du/dt stresses.
There is a tradeoff between the number of output voltage
levels and the switching frequency for the same number of DC
sources at the input of the cascade H-bridge inverter. Less
voltage levels mean lower switching frequency but it was
shown that the high switching frequency can be transferred to
the H-bridge with the lowest DC voltage (60V DC source in
this case). Also the bridge with the highest DC voltage can
operate at the fundamental frequency with a proper control
Fig. 10. The operation of the cascade H-bridge inverter with current
control. The RL filter values: L=5mH, R=1Ω. The THDu of the output
voltage V is 10%, THDi is 2%.
There is a need to increase the lifetime of photovoltaic
inverters as well as their reliability. High voltage stresses
decrease the lifetime of many electrical components [4].
Lower du/dt stresses of components in multilevel H-bridge
inverter can help to meet these needs.
ACKNOWLEDGMENT
This work was supported by Slovak Research and
Development Agency under project APVV-0095-07 and by
Scientific Grant Agency of the Ministry of Education of
Slovak Republic under the contract VEGA No. 1/0099/09.
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