Efficiency and Power Factor Investigation of Characteristic Converter To- pologies via Simulation. K. Georgakas, A. Safacas. University of Patras, Department of ...
Efficiency and Power Factor Investigation of Characteristic Converter Topologies via Simulation K. Georgakas, A. Safacas
University of Patras, Department of Electrical and Computer Engineering Laboratory of Electromechanical Energy Conversion 26500 Rion-Patras, GREECE Tel. +302 610 996414, Fax +302 610 997362 Abstract The use of power electronic converters to control the power flow from the AC grid to a DC load leads to reactive power increase and affects the efficiency. These two important energy quantities depend on the structure of the converter topology, the operating switching frequency and the power percentage related to the nominal power. In this paper, the quantitative changes of the power factor and the efficiency each as a function of the output active power and for different switching frequencies are investigated, taking into consideration four basic converter topologies. I. INTRODUCTION
It is well known that the power electronic converters inject unavoidably reactive power to the AC power grid, because of the current basic harmonic displacement and of the current higher harmonic content. The higher switching frequencies of these currents lead to the increase of switching losses, reducing so the converter efficiency. On the other hand the decrease of the effective value of the higher order harmonics implies decrease of the current value in the source, in the transformers and in the power transfer components. Consequently the question that arise from the aforementioned facts, is about the optimum operating condition, when both the efficiency and the power factor get the best possible values. This specific condition depends on the converter topology and the switching frequency of the semiconductive elements. An investigation is held considering the efficiency and power factor by using simulation models so as a quantitive perception of the above mentioned facts to be obtained. This simulation is implemented by the program Matlab/Simulink. Specifically in this paper the above mentioned investigation take place by changing the output power of the converter and consequently by changing the speed of a DC motor considering constant torque. The rectification and the DC motor control are achieved through three different single-phase converter topologies. The comparison of them includes the question about an optimum frequency that efficiency and power factor get the best possible values. Also there is a question about the converter topology with optimal efficiency and power factor. An other question that arises is related with the converter Efficiency and Power Factor when the output power increases. Finally, a bidirectional power converter topology
for both rectification and inversion is investigated compared with the conventional structures. II. CONVERTER TOPOLOGIES
The investigated converter topologies are shown in figures 1, 2, 3 and 4. The converter showing in figure 1 consists of an L-C filter placed on the side of the AC grid, a rectifying half-bridge of IGBTs, a smoothing inductance Lo and a capacitor CO at the DC side and a DC machine. In the half-bridge rectifier, by controlling the duration of the IGBT's conduction, the voltage which is applied at the DC machine changes, leading to the realization of its speed control. The control of the conduction time interval of the IGBT elements is accomplished through sinusoidal Pulse Width Modulation (sPWM) technique. This particular control method was chosen, because it is suitable so that high power factor and high efficiency could be achieved. According to the system of figure 2, during the rectification, the diodes of the bridge as well as the IGBT conduct. When the IGBT is off the current flows through the diode Do. The IGBT switching operation is achieved through sPWM control technique, as in the case of the converter in figure 1. The only difference is that in this case only one IGBT is conducted. In the system of figure 3, two antiparallel IGBT elements are placed at the input of the rectifying diode bridge to control the output voltage. During the rectification, the diodes of the bridge as well as the IGBTs conduct. When the IGBTs are off, the current flows through the bridge's diodes. The IGBT switching operation is achieved through sPWM control technique, as in the case of the converter in figure 1 and 2. The converter topology in fig. 4 enables the rectification as well as the power inversion (generator operation) with high power factor. In the case of rectification this topology operates exactly as the topology in figure 2. The only deference is that the IGBT element (IGBT) is replaced by the MOSFET5. That is because a reduced number of elements and high switching frequency are required. The rectification take place through the parasitic diodes of the bridge's MOSFETs and the MOSFET5. During the inversion the switching components of the MOSFETs 1-4 and the parasitic diode of the MOSFET5 are in operation. The switching technique of this converter is based on the hysteresis current control method [1]. The proposed converter has a reduced number of
1422
L
conductive elements compare to the conventional converter shown in figure 5. The efficiency of this converter is high.
1L
R. 3
ure
e
MOSFET 5 MOSFET
I1(~
R
MOSFET 2
L
I
L
I~~~~
Mechanical
RL
ancl
AC
Mtor
MOSFET
MOSFET (
g3
Figure 4. Proposed bidirectional converter structure (CA) L
Figure 1. Half-Bridge rectifier (RI).
AC
Source
Mechanical Load
Motor
Figure 5. Conventional bidirectional converter structure
DC
III.
achine
LATION
Figure 2. Rectifier consisting of 1 IGBT at the DC side (R2
edFani.al
-f
*Di j 'D3 AC
3Lo0ad
LO -
,,4V
DC
facbhlt
3 D2
EFFICIENCY AND POWER FACTOR STUDY VIA SIMU-
It is well known that by increasing the frequency, when the sPWM technique is used, the power factor get higher. Consequently, the current in the AC grid (transformers, line e.t.c.) and the power losses decrease. But, when the frequency get higher the converter power losses increase. Furthermore the converter efficiency and power factor depends on the output power, owing to the fact that conduction time intervals of the switching elements increase as the output power increases. So it is not easy to answer what is the relation between power factor and efficiency. It is clear that a parallel study of the efficiency and the power factor is purposeful. In this work the study is carried out though simulation using the software package Mathlab/simulation. The values of the system parameters of the DC machine and the other components used for the simulation are highlighted in the following table: TABLE 1. VALUES OF THE SYSTEM PARAMETERS.
Figure 3. Rectifier consisting of two IGiBT at the AC side (R3).
DC Motor
UN=220V
R=1,05Q
Filter
IN= 12,5 A La =0mH
UfN=220 V IfN=0,579 A
PN=2700 W
n=2416/min
Lf 5mH Cf 0,41tF
L,=300mH C.=2mF
1423
Source
R,=lOmQ
L,=200 iH
100 95
The simulation is accomplished for the converters on figure 1, 2 and 3 for operating frequencies 5, 10, 15 and 20 kHz and for different values of the electric power that the DC machine demands. The torque of the mechanical load is considered to be constant. The simulation results leads to the curves that are presented in figures 5-16. Also the simulation is accomplished for the converter on figure 4 (operating as inverter) for five values of the hysteresis band (di1-di5) [1] limits concerning the input current, and for different values of the electric power that the DC machine demands. The torque of the mechanical load is considered to be constant too. Some characteristic simulation results are presented in figures 17-20, where ri=efficiency, PF=power factor, PO=converter output power.
380
980 1180 1380 1580
98 l
97,5 t
97 5 kHz 1 0 kHz 15 kHz 20 kHz
96,5 96 __
95,5
97,5
.
780
Figure 9. Power factor (PF) of the converter (R2)
98!.
580
Po [W]
98,5
t
5 kHz 1 0 kHz 15 kHz _20 kHz
(L
97+
96 5 965
/
L _
95
5 kHz 10 kHz 15 kHz 20 kHz
94,5 380
580
780
980 1180 1380 1580 Po [W]
95,55
Figure 10. Efficiency (q) of the converter (R3)
95 380 580 780 980 1180 1380 1580
100
Figure 6. Efficiency (q) of the converter (RI) 100 95 90 85 80
.0. IL
95
IL80' 90
_
S kHz
70 65
10 kHz 15 kHz 20 kHz
55 50
5 kHz 10 kHz 15 kHz 20 kHz
/
75
1
-
60 / 380
580
780
980 1180 1380 1580 Po [W]
380
580
780
Figure 11. Power factor (PF) of the converter (R3)
980 1180 1380 1580
Po [W]
98,4
Figure 7. Power factor (PF) of the converter (RI)
t
98 97,5
97
96
10 kHz
1_
t
t
II
97,86
t
t
t
X
RI 1_ R3
97, 4
R2
96,8
20 kHz
9b,b
380
95 380
t
t 1-
97
15 kHz
-
98
97,2
5 kHz
96,5
98,2
580
780
980
1180 1380 1580 Po [W]
580 780 980 1180 1380 1580
Figure 12. Efficiency (q) for frequency lOkHz
Po [W]
Figure 8. Efficiency (q) of the converter (R2)
1424
t
3Q0(
- RI R3 R2
.0-
u(t)
-
200Io()0-
85 = 83 380
a0V4
.,
-
580
780
0
i
A
.V 9. 9
9
-ioo
980 1180 1380 1580
-200
-
-300
Po [W]J
400-
Figure 13. Power factor (PF) for frequency 1OkHz
a)
Is U:
u
t)
12
a(t)\
a
1
)I
a'1
E
_sIN_
10 _
4
-
2 ....
a)
0 0
1000
1 0000
2000
Frequency (Hz) 20-.
b)
CE
Figure 16. a)Waveforms of input current and voltage by output power P0=1673W for the converter (R3) b)FFT analysis of the input current.
D
o es
-
so' - _+ o
oo 2..
1..
.
-o soo 60
.. oo
. 'I.. soo
e' .
F re q,,enc y (H z)
uO
-
200
-
(t)
b) Figure 14. a)Waveforms of input current and voltage by output power P.=363 W for the converter (RI) b)FFT analysis of the input current.
i(t)
di I_ ;
1-
-
\N,
t
IS]
U
50
-1
00-
In
K
00 ,
Figure 17. Waveforms of input current and voltage for the inverter (CA) by current variation di1=0,32 A
-1 5 0
i(t)
-2 Q 0-
-2S50DQ0
uCt)
.co]
a)
3CO
-
'00 100
i(t) -
A
t~~~~~~~~~~~~i ES]
/
| sF ' 1 | lft e~~~~~ ~ L! ~ ~ ~~~~~.
03 -
-
2
'
0.40o
;Jo.
Q
0'4o+
lao .
20SiI
-
-300
0
0i0
20fj0
3 00
9 60 0
400
9S00
02l00
0000
-
10400
F requ ency (H z)
Figure 18. Waveforms of input current and voltage for the inverter (CA) by current variation di3=4,8 A
b)
Figure 15. a)Waveforms of input current and voltage by output power P0=1 132W for the converter (R2) b)FFT analysis of the input current.
1425
t
di2 di5
dii di4
di3
98,5 98 =
97,5 97
96,5 96
95,5 95
94,5 0
1000
2000
3000
4000
5000
6000 Po [W]
Figure 19. Efficiency (ri) for the inverter (CA) by current variation di1=0.02*Imax, di2=0.1*Imax, di3=0.16*Imax, di4=0.2*Imax, di5=0.4*Imax
100
+
_
99,8 99,6 -
99,4
_
9I2
m- 99,23
PF1 PF2 PF3 PF4 PF5
99
98,8 98,6 2000
4000
6000
Po [WV
Figure 20. Power Factor (PF) for the inverter (CA) by current variation dii=0.02*1ma,, di2=0. 1*1Im,, di3=0. 16*1.a,,
di4=0.2*Imax, di5=0.4*Imax
The simulation results show that when the output power increase the efficiency increase too because the switching losses are independent of the output power. From the figures 6,8,10 we note that when the converter operates with frequency 10 kHz the efficiency get the higher values in comparison to the other investigated frequencies (5, 15, 20kHz). This happens because the power factor get high values and simultaneously the switching frequency is not so high that the switching losses to be significant. We can point out that the value of the efficiency of all the investigated topologies is indeed very high for the switching frequency 10 kHz. Figure 12 shows the efficiency of the three investigated rectifiers by operating frequency 10kHz. The topology with the highest efficiency is that shown in figure 1. As the reactive power increase by decreasing the switching frequency, the efficiency get a minimal value (fig. 6,8,10) by f=5kHz in comparison with the frequencies 1 OkHz, 15kHz and 20kHz.
It is to remark that the power factor of all the investigated converter topologies increase by increasing the output power until the value of about 1200W, while after this value the power factor decrease (fig. 7,9,11). Figure 14 shows the waveforms of the current and voltage in the AC power grid (before the filter Lf, Cf) and the current harmonic spectrum by output power PO=363 W. According to the sPWM switching method the first order current harmonic is in phase with the AC voltage the power factor becomes lower because of the high harmonics. Figure 15 shows that by Po= 1132 W the input current has a closesinusoidal waveform. Taking in account the above remarks one can see why the power factor get the higher value for the output power of 1200W. Figure 16 shows the current waveform and the corresponding harmonic spectrum in case of PO=1673W, where the current harmonic content becomes higher in comparison with the special output power value PO= 1200W. From the above mentioned facts we conclude that by frequency increase the power factor increase too. Also by frequency increase the efficiency increase until a particular frequency value. The aforementioned are valid for the three investigated topologies. The question that arise is about a topology with higher efficiency and power factor in comparison with the other topologies. Power factor is almost the same for all the topologies. Consequently, the topology with the higher efficiency can considered as the best. For our case that is the topology in figure 1. Finally, simulation results for the topology of bidirectional power flow (fig. 4) are drown in figures 17-20 for the case that this converter operates as an inverter. We notice that high power factor and efficiency for wide power band can be achieved. IV. CONCLUSIONS For the three characteristic investigated converter topologies there is an optimal frequency f=lOkHz that the power factor and efficiency get the best possible value. The efficiency increase when the power increases. That is because the switching losses are independent of the output power. Because sPWM switching operation technique is used the power factor increase until a particular output power value. After this value the power factor decrease. For the rectifier (RI) by switching frequency 10kHz the efficiency and the power factor achieve the optimum value. The investigated bidirectional converter topology by inverter operation gets very high power factor and high efficiency. This topology has the advantage of the reduced number semiconductive elements. REFERENCES [1]. K.Georgakas, A. Safacas "Power Factor Correction and Efficiency Investigation of AC-DC Converters Using Forced Commutation Techniques", ISIE, Congress Dubrovnik, June 20-23, 2005.
[2]. Nabil A. Ahmed, Kenji Amei, Masaaki Sakui, "AC chopper voltage controller-fed single-phase induction motor employing sym-
1426
metrical PWM control technique", Elsevier Sciences, Electric Power System Research, 55(2000), pp. 15-20.
[3]. Bhim Singh, Brij N. Singh, Ambrish Chandra, Kamal Al-Haddad, Ashish Pandey, Dwarka P. Kothari, "A Review of Single-Phase Improved Power Quality AC-DC Converters" IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL.50 NO.5, OCTOBER 2003. [4]. Ramesh Srinivasan, Ramesh Oruganti, "A Unity Power Factor Converter Using Half-Bridge Boost Topology" IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 13, NO.3, MAY 1998.
[5]. Nabil A. Ahmed, Kenji Amei, and Masaaki Sakui, "A New Configuration of Single-Phase Symmetrical PWM AC Choper Voltage Controller", IEEE TRANSACTION ON INDUSTRIAL ELECTRONICS, VOL.46, NO.5, October, 1999, pp.942-952.
ules For Low-Temperature Plasma Generators" IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL.14, NO.6, NOVEMBER 1999. [7]. BOR-REN, "High Power Factor AC/DC/AC Converter With Random PWM", IEEE TRANSACTIONS ON AEROSPACE AND ELECTRONIC SYSTEMS, VOL.35, NO.3 July 1999, pp. 935943.
[8]. Jose R. Rodriguez, Juan W. Dixon, Jose R. Espinoza, Jorge Pontt, Pablo Lezana, "PWM Regenerative Rectifiers: State of the Art", IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS, VOL. 52, NO. 1 FEBRUARY 2005.
[6]. Hideaki Fujita, Hirofumi Akagi, Shin-ichi Shinohara, "A 2-MHz 6kVA Voltage-Sourse Inverter Using Low-Profile MOSFET Mod-
1427
