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Sensor less voltage control of CHB Multilevel inverter fed three phase
induction motor
M.Kondalu, T.Rajesh
Dept. of EEE, Malla Reddy Engineering College
Abstract— In this paper a single DC source per phase cascaded h bridge (CHB) three
phase five level inverter fed induction motor with minimum number of switches and a single
capacitor is proposed. Maximum all available switching states are evaluated and a sensor
less voltage balancing technique is suggested which regulates the second bus voltage as half
of the applied single DC voltage source. Voltage levels at the output are zero, the voltage
across the capacitor and voltage of DC voltage source. The switching method is mixed with
a simple voltage balancing technique which can be possible to implement even with simple
cheap microcontrollers. Simulation results show the dynamic performance of this method in
regulating the second bus capacitor voltage. The low harmonic desired value of five level
voltage is regulated by the voltage across the capacitor.
Keywords— Three phase CHB inverter; Single DC voltage source; Sensor less voltage
balancing technique; Regulating capacitor; Induction motor load.
I. INTRODUCTION
The high power demand in the global energy market leads to redesign of power
converters. The complications with the two level inverter topology are low efficiency and
high power losses, which leads to the development of Multi level inverters (MLI). Now a
day’s the use of multilevel inverters is increasing due their advantages and attraction by
industries. MLI produces a number of voltage levels at output with the use of many switches
with different configurations and DC links, so that the output quasi sine wave has low
harmonic distortion [1-3]. The researchers introduce so many types of MLI, among CHB
and Neutral point clamped (NPC) inverters are the best ones [4-7]. NPC inverters are finest
ones which are attracted by many industries, and provides common DC bus for the
application of three phase loads [8]. The CHB inverters have interesting structures and it
provides more levels for high power applications, but suffers with many separated DC
supplies [9]. Many of the MLI are facing the above mentioned problem [10-20]. Freshly, an
attractive structure is designed with the modification of the flying capacitor (FC) inverter,
but, suffering with separate voltage ratings and switching frequency [15-17, 21- 27]. With
the advantages of h bridge inverters a single DC source three phases MLI is designed in
Fig.1 [28], which has two cells, one cell is connected to DC source and other cell is
connected to a charging capacitor. So many studies are implemented for balancing the
capacitor voltage for different loads [29-34]. To track the voltage of the source and capacitor
it needs a voltage sensor
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In this paper a new CHB sensor three less inverter fed by induction motor is
proposed for generating three phase five level output with single DC source per phase and a
single capacitor phase. In each phase when the capacitor is in series with the source and
load, the charged upto half of the source voltage. When it is in series with load the energy is
discharged through the load. It has the drawback of reducing voltage levels from seven
levels to five levels, but has an advantage of removing sensors at DC source and capacitor.
II. FIVE LEVEL THREE PHASE CHB INVERTER
The block diagram of the three phase CHB inverter is presented in Fig.1. It has three
single phase CHB inverters, each one is fed by one DC source and one capacitor. Each
inverter will act as a single phase CHB inverter, but when connected to induction motor they
has a phase difference of 120 degrees. Each one has eight switches and two H- bridge cells
in which one cell is connected to supply DC voltage source and other is connected to storage
capacitor shown in Fig.2. The voltage across capacitor should be managed as half of the
applied voltage source. If supply source voltage is 2E, then the capacitor voltage was E. In
the fast published works, the CHB inverter is employed as a seven level inverter with
distinct modulation technique, but it is facing voltage balancing problem [32]. But in this
paper, we are presenting a sensor less voltage balancing technique which can produce five
level output. The switching states of five level voltage waveforms are indexed in the Table.
I. Due to no effect on charging and discharging of capacitor voltage some switching states
which produce zero voltage output are not considered. These switching states are used for
reducing the frequency of the inverter. From the switching states listed in Table. I, with
paths 2, 3, 5 & 6 we can analyze whether the capacitor is charging or discharging. With
paths 2 & 6, the capacitor is connected in series with the DC voltage source and load, hence
the capacitor would charge up to E and delivers power to a load. In the sequence 3 & 5 the
capacitor is only connected to load so it will discharges power to the load. By introducing
the voltage balancing technique into switching techniques, the controller structure gets
simple which can be easy to implement by using cheap microcontrollers. The charging and
discharging effects of a capacitor after introducing voltage balancing techniques into
switching techniques are listed in Table. I
CHB Inverter R Phase CHB Inverter Y Phase CHB Inverter B Phase Induction Motor Vdc1
C1
Vdc2
C2
Vdc3
C3
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Fig.1. Block diagram of the three phase CHB five level inverter fed Induction motor
Fig.2. Single DC source multilevel inverter R Phase
Table I: Switching states, Capacitor charging/ discharging states and output voltage
of five levels CHB inverter
State S1 S2 S3 S4 S5 S6 S7 S8 V output V out
value
Capacitor
voltage
status
1 1 0 0 1 1 0 1 0
VDC
+2E No Effect
2 1 0 0 1 0 1 1 0
VDC
-VC
+E Charging
3 1 0 1 0 1 0 0 1
VC
+E Discharging
4 1 0 1 0 0 1 0 1 0 0 No Effect
5 1 0 1 0 0 1 1 0 -VC -E Discharging
6 0 1 1 0 1 0 0 1
VC
-VDC -E Charging
7 0 1 1 0 1 0 1 0 -VDC -2E No Effect
III. SENSOR-LESS VOLTAGE BALANCING TECHNIQUE
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From Table. I, it is well known that the capacitor may be charged or discharged in
any one half cycle, But to keep the capacitor voltage fixed, the switching technique of the
capacitor is designed in such a way that it should be charged during the positive half cycle
and discharged in the negative half cycle. Due to switching technique of capacitor and
output waveform frequency the capacitor is charged to half of DC supply. The capacitor is
charged when it is connected in series with the load and supply DC source, the charging
states of capacitor are 2 and 6, and the load voltage is ±E. These charging and discharging
states are mathematically represented in the equation (1)
2
1 2 2
2
2
2
out
E V E
V V V V E
E V E
(1)
If the source voltage is 2E, to produce the desired load voltage the capacitor must be
charged up to E. The charging and discharging time of the capacitor will force the capacitor
voltage to ±E. Hence, to have equivalent charging and discharging times, in the charging
state 2 the capacitor is connected in series with the voltage source in the positive half cycle
and from switching state 5 the capacitor is discharged in the negative half cycle by
connecting in series with the load. It should be known that the capacitor charging and
discharging depends on the type of load only, but not on the output frequency or switching
frequency. The type of load connected will directly affect the size of the capacitor. The self
voltage balancing procedure is mathematically proved with energy storage relations of the
capacitor. The output voltage and current waveforms of a five level single DC source CHB
inverter is shown in Fig.2.
Mathematically the output voltage and current waveforms can be written as an equation (2)
and (3)
( ) ( ) V t V Sin t l m
(2)
( ) ( ) l m i t I Sin t
(3)
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+2E
+E 2
0
-E
-2E t t t
+E
-E
A
V
0
0
Im
1 2 3 4 5 6 7 8
Vout
i
l
V2
Where Vm, Im and
θ
are the maximum value of voltage, current and phase angle between
voltage and current. The load current flowing through capacitor can be written as
dq I
dt
dU Vdq VIdt
U VIdt
(4)
Where I, V, q and U are the current flowing through the capacitor, the voltage across the
capacitor, the charge on capacitor plates and energy stored in the capacitor respectively.
From equations (3) and (4) the charging energy of the capacitor can be written as
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1
2
1
3
2
4
3
4
0
0
0
0
0
0
0
0
( ) ( )
0 ( ) ( )
( ) ( )
0 ( ) ( )
( ) ( )
0 ( ) ( )
C
m
U V Sin t d t
Sin t d t
E Sin t d t
U I Sin t d t
E Sin t d t
Sin t d t
1 0 2 0
3 0 4 0
( ) ( )
( ) ( ) m
Cos Cos
EI
Cos Cos
(5)
In the same way the discharging energy of the capacitor can be written as
6 8
5 7
2
0
2
0
0 0
6 0 5 0
8 0 7 0
( ) ( )
( ) ( )
( ) ( )
( ) ( )
( ) ( )
C m
m C
m m
m
U V I Sin t d t
I V Sin t d t
EI Cos t EI Cos t
Cos Cos
EI
Cos Cos
(6)
From equation (5) and (6), we can observe that the output voltage is symmetric about
positive and negative half. Hence we can assume an equation (7) as
5 1
6 2
7 3
8 4
(7)
The energy stored in positive and negative half cycles is same but has opposite in sign
U U
(8)
From equation (8) the energy stored or discharged by the capacitor is balanced and constant
and also it keeps the capacitor output voltage constant irrespective of all conditions. For
preparing the hardware setup the sensor less voltage balancing technique is integrated with
modulation technique. The Multi carrier switching technique is used as modulation
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technique [17]. For a five level inverter PWM scheme is implemented with four carrier
waveforms (Cr1, Cr2, Cr3, and Cr4) and reference sine wave are shown in Fig. 3.
Fig.3. Five-level PWM scheme using four vertically shifted carrier waves
VVrreeff
CCrr11 AANNDD Pulse generation based on Table I
Pulse generation based on Table I ≥≥ CCrr22 AANNDD ≥≥ CCrr33 AANNDD ≥≥ CCrr44 ≥≥
Yes
No
Yes
No
Yes
No
Yes
No
State 1 State 2
State 4
State 5 State 7
Switching Pulses
Fig.4. Proposed sensor-less voltage balancing approach integrated into switching technique
The four carrier waves are chipped vertically for modulating the reference sine wave. The
firing pulses related to Table.I are produced after comparing the carrier waves with the
reference waveform. The algorithms for producing the firing pulses are presented in Fig.4.
This algorithm produces the five level output after fixing the capacitor voltage at a desired
value without feedback sensor. This technique does not depend on the type of the system
model (e.g. average modelling), feedback sensors, modulation index, switching frequency
and grid frequency. It can operate the system starting with zero voltage to any arbitrary
value and also at varying DC source conditions.
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IV. SIMULATION RESULTS AND DISCUSSION
The CHB inverter depicted in Fig.1 is simulated with Matlab/ Simulink
environment, the results shows its performance in standalone mode with induction motor as
a load. We can use the standalone inverters as power supply units for motor drives. The
simulation parameters of the test system are listed in Table. II. To evaluate the behavior of
the proposed method induction motor load is connected to the inverter. When the capacitor
is connected with the source and induction motor capacitor voltage starts rising and it
reaches the reference value which is half of the source value within 20 cycles. From Fig.5,
when the source voltage is 200 V, the capacitor voltage starts rising and tracks the reference
value which is half of the source voltage is 100 V. To observe the changes in the voltage
and currents, in Fig.5, the corresponding waveforms are captured.
Fig 5. Voltage across the capacitor voltage
Fig.6. Output voltage of the proposed CHB three phase inverter
V. CONCLUSION
In this paper a new sensor less voltage balancing technique is proposed for the
multilevel inverter fed induction motor with single DC source and a capacitor for each
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phase. The capacitor is charged in the second bus up to half of source voltage, when it is
connected to the DC source and an induction motor. Without having any feedback from DC
links and loads it will provide five level output voltage. By integrating it with the switching
technique industrial products are implemented with a very less number of switches and one
DC source and capacitor per phase. The demerits of diode clamped and flying capacitor
inverters like capacitor voltage balancing, isolated DC sources are eliminated by this
converter. This method is simulated in Matlab, the results shows the good dynamic
performance of this method for induction motor load. The power quality is improved.
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