INTRODUCTION
Current source inverters have been used in industry and for using them
the effective operational parameters must be investigated carefully. One
of the important application of CSI is harmonic studying in a power system
which for this purpose, the controllable harmonics are needed. The other
application of CSI is speed control of AC machines which in this case
the speed control is done with changing of modulation index and frequency.
Therefore the changing of these parameters must be studied. An induction
motor fed with CSI inverter has investigated by Salo and Tuusa (2005),
Espinoza and Joos (1995) and Wiechmann et al. (2008) the output
load is inductive, so in the output is used a capacitor but haven`t been
discussed the calculation method and effects of this capacitor.
CSI inverters can be used as AC/AC converter (Kazerani, 2003). In this
system, may be the THD of system woken by changing of modulation index
which have not been investigated. The other application of CSI inverter
is reactive power compensation (Dong and Lehn, 2002; Han et al.,
2000, 2001). In this case, also must be considered the load type and modulation
index (THD).
Past research has studied and analyzed control strategies as well as
pulse width modulation (PWM) schemes for voltage and currentsource inverters
and rectifiers (VSIs and CSIs) where significant accomplishments have
been achieved harmonic distortion minimization, highinput power factor
and reduced switching frequencies, among others (Rashid, 2001; Blaabjerg
et al., 1995; Kwak and Toliyat, 2006).
There are different methods for elimination of harmonics. Antunes et
al. (1999), McGrath and Holmes (2008) and Yu et al. (2004)
are used multilevel structure for this purpose. One of the disadvantages
of this method is requiring more devices that are expensive. Selective
harmonic elimination modulation (SHEM) is another method for harmonic
elimination in which, harmonic of output current can be eliminated by
creating appropriate notches.
This study investigated influence of load type, operational mode and
modulation index on the SHEM current source inverter. For this purpose
an inverter designed and constructed which can be operated in single and
three phase mode with any number of harmonics elimination capability.
By using of experimental results, variation of modulation index, load
type and operational mode are investigated on inverter`s operation. Finally,
results have been obtained from PSPICE simulation is verified by experimental
results.
INTRODUCTION OF SHEM SWITCHING METHOD
SHEM is a PWM switching method for harmonic elimination which harmonic
of output current can be eliminated by creating appropriate notches.

Fig. 1: 
The waveform of CSI output current without 5 and 7 harmonics
and capable of controlling amplitude 
In this method, for elimination of N harmonics, N notches must be created
in quarter of the waveform and if control of amplitude is necessary, one
more notch is needed for this purpose. Hence, for having both amplitude
control and harmonic elimination in total N+1 notches are needed.
Using this method, lower order harmonics are cancelled by proper switching
and higher order harmonics are filtered by high pass filter. Moreover,
elimination of harmonics is optional and possible Current waveform of
the output phase is shown in Fig. 1.
According to Fig. 1, the amplitude can be controlled
and the harmonics number 5, 7 can be cancelled using α_{1}α_{3}
angles.
INVERTER OPERATION IN DIFFERENT MODES
The constructed inverter can be operated in single and three phase states
that each of them included many modes. In this study for each of single
or three phase state, the following modes have been investigated:
Mode 1: 
Without amplitude controlling and any harmonic elimination 
Mode 2: 
Amplitude controlling and without any harmonic elimination 
Mode 3: 
Amplitude controlling and elimination of 5 and 7 harmonics 
Mode 4: 
Amplitude controlling and elimination of 5, 7, 11 and 13 harmonics 
Of course, the above modes have been chosen from many different modes
and it is depends to the user and application. Figure 2
shows the output current of three phase CSI inverter in mode 2 with resistive
load, 50 Hz and 100% modulation index.

Fig. 2: 
The output current of CSI in mode 2 of three phase in
resistive load, 50 Hz output frequency and MI = 100% (a) PSPICE simulation
(b) experimental results 
In this waveform by creating a
notch can be controlled the amplitude but all of harmonics (6k ±
1 where k = 1, 2, ..., n) remain in the waveform.
One of the other modes is mode 3 in three phase state. For this mode
the output current and spectral frequency of it are shown in Fig.
3 and 4, respectively. According to Fig.
4 the harmonics 5 and 7 don`t exist in waveform and the frequency
of the first presence harmonic is 550 Hz (11th harmonic). In this mode
the output lineline voltage has three levels and shown in Fig.
5.
EFFECT OF MODULATION INDEX VARIATION
Figure 6, 7 shows the output current and
spectral frequency of it for three phase CSI inverter in mode 3 with resistive
load, 50 Hz and 80% modulation index. By comparing (Fig. 6)
with (Fig. 3), it is clear that remain harmonics` amplitude
has been increased by decreasing of modulation index. For example in 100% modulation
index the amplitude of 13th harmonic is 10A and MI = 80% is 15A. Therefore the
changing of MI must be considered in CSI applications.

Fig. 3: 
The output current of CSI in mode 3 of three phase in
resistive load, 50 Hz output frequency and MI = 100% (a) PSPICE simulation
and (b) experimental results 

Fig. 4: 
Output current spectral frequency of CSI in mode 2 of
three phase in resistive load, 50 Hz output frequency and MI = 100%
(a) PSPICE simulation (b) experimental results 

Fig. 5: 
Linetoline output voltage of CSI in mode 3 of three
phase in resistive load, 50 Hz output frequency and MI = 100% (a)
PSPICE simulation and (b) experimental results 
EFFECT OF LOAD TYPE ON INVERTER OPERATION
In the resistiveinductive loads, over voltages in output voltage waveform
appears which amplitude of this over voltage depends on the amount of
resistance and inductance of load. This over voltage can be appeared on
the switches and will be caused failing of them.
To remove this over voltage, we used capacitor which parallel with load.
The different waveform have been shown for resistiveinductive (R_{L}
= 2.2 Ù, R_{L} = 0.7 mH) by using of capacitor an without
using.
Without using capacitor: Figure 8 shows the
switch voltage. In Fig. 8 can be seen the over voltage
obviously which amplitude of these over voltages increased with increasing
of L/R ratio. This problem leads to failing of switches.

Fig. 6: 
Output current of CSI in mode 3 of three phase in resistive
load, 50 Hz output frequency and MI = 80% (a) PSPICE simulation and
(b) experimental results 

Fig. 7: 
Output current spectral frequency of CSI in mode 2 of
three phase in resistive load, 50 Hz output frequency and MI = 80%
(a) PSPICE simulation and (b) experimental results 

Fig. 8: 
Switch voltage of CSI in mode 3 of three phase in resistiveinductive
load, 50 Hz output frequency and MI = 100% without capacitor (a) PSPICE
simulation and (b) experimental results 
Figure 9 shows the line to line output voltage. The
over voltage is existed. The output current is shown in Fig.
10. According to this, the inductive loads don`t have any effect on
the output current.
By using capacitor: To remove the over voltage we can use a capacitor
in the output. In this study to removing the over voltage has been used
the 100μF capacitors where its advantages are:
• 
Protects the switches by removing of over voltages 
• 
Act such as low pass filter and remover the high order harmonics 
In mode 1, from load view the inverter can be modeled as current source
which is shown in Fig. 11b. If don`t use from capacitor,
the over voltage can be appeared in A point which can be removed these
over voltage by using of capacitor. The output transfer function can be
written as follow by:

Fig. 9: 
Linetoline output voltage of CSI in mode 3 of three
phase in resistiveinductive load, 50 Hz output frequency and MI =
100% without capacitor (a) PSPICE simulation and (b) experimental
results 

Fig. 10: 
Output current of CSI in mode3 of three phase in resistiveinductive
load, 50 Hz output frequency and MI = 100% without capacitor (a) PSPICE
simulation and (b) experimental results 

Fig. 11: 
(a) Equivalent circuit of CSI inverter (b) the output
current 
Where:
According to (2) different parameters have been affected on the over
voltage removing. If damping factor less than one, the over voltage will
be accorded. Also the changing of capacitor doesn`t have any effect on
the settling time of output voltage. Figure 12 shows
this problem.
The frequency response of the output voltage for Fig.
11 is plotted in Fig. 13 which high cutting frequency
is functioned of R, L and C parameters. So, by suitable selecting of capacitor
value furthermore over voltage amplitude controlling the high order frequency
can be cancelled.

Fig. 12: 
Capacitor effect on output voltage settling time R
= 3 Ω and L = 1 mH 

Fig. 13: 
Frequency response of filter by R = 3 Ω, L = 1
mH and C = 100μF 

Fig. 14: 
Linetoline output voltage of CSI in mode 3 of three
phase in resistiveinductive load, 50 Hz output frequency and MI =
100% by using capacitor (a) PSPICE simulation and (b) experimental
results 
Figure 14 shows the effect of capacitor on the coltage. By comparing Fig. 4 with Fig. 9, can be sent that the waveform is approximately near sinusoidal. According tho this fact, by using of capacitor the high order harmonics can be cancelled.
Figure 15 shows the spectral frequency of output linetoline
voltage. The influence of capacitor in removing of over voltages and canceling
of high order harmonics.

Fig. 15: 
Output current spectral frequency of CSI in mode 3 of
three phase in resistiveinductive load, 50 Hz output frequency and
MI = 100% by using (a) PSPICE simulation and (b) experimental results 
CONCLUSION
This study investigated influence parameters such as of load type, operational
mode and modulation index on the SHEM current source inverter. For this
purpose an inverter designed and constructed which can be operated in
single and three phase mode with any number of harmonics elimination capability.
In current source inverters with resistive inductive loads, over voltages
in output voltage waveform appears. By using of capacitor in the output,
these over voltages removed. Variation of modulation index shown that
the amplitude of the harmonics can be varied by changing of MI. also shown
that by optimal choosing of capacitor value can be cancelled high order
harmonics of output voltage. The effect of these parameters on the CSI,
simulated with PSPICE. The comparisons have been done on our laboratorial
system outputs, with simulated results. Then it has been shown that experimental
results verify the simulation results.