PEDS 2007
A Modular Structured Multilevel Inverter Active
Filter with Unified Constant-Frequency
Integration Control for Nonlinear AC Loads
Power
P. Y. Lim
School of Engineering and Information Technology
Universiti Malaysia Sabah
Sabah, Malaysia
[email protected]
N. A. Azli
Department of Energy Conversion
Faculty of Electrical Engineering
Universiti Teknologi Malaysia
Johor, Malaysia
[email protected]
component in the load in order to generate the reference
current for controlling the inverter to produce a current that is
equal to the amplitude and opposite in direction of the reactive
current of nonlinear loads. Those control methods require fast
and real-time calculation which require several high precision
analogue multipliers or high speed DSP chip with fast A/D
converter that yields high cost, complexity and low
stability. [3]
As for the UCI controller, it is based on the theory of one
cycle control [6], which employs an integrator with reset port
as its core component to control the duty ratio of the APF
inverter switches. This method ensures that the current drawn
from the AC main supply will be sinusoidal waveform with
different non-linear AC loads connected to the supply. No
calculation of reference current for the controller is required
which greatly simplifies the control circuit.
This paper is to realize the implementation of UCI
Index Terms--Active power filter, current harmonics
Controller
to Modular Structured Multilevel Inverter (MSMI)
compensation, modular structured multilevel inverter, unified
APF
which
will generate harmonics current that cancels the
constant frequency integration control, nonlinear loads.
harmonics current from a nonlinear load in order to form
better sinusoidal incoming supply. MSMI has an advantage of
I. INTRODUCTION
reducing the voltage stress of switching devices. Furthermore,
Active power filter (APF) has been widely used for the modularized circuit layout of MSMI allows expansion of
harmonics and reactive power compensation and it now the APF structure to be done easily. [7] This will be
becomes a mature technology for improving the power convenient if expansion of APF is required to meet higher
quality. Recently, semiconductor constraints and the voltage power level demand, such as in power transmission line. The
stress on the semiconductor switches have drawn the attraction performance of a single phase simulation of MSMI APF
of the researchers. In order to achieve higher voltage level of which is connected to different AC nonlinear loads will be
the filtering operation, inverter configuration such as Hybrid presented for the purpose of demonstration.
Asymmetric Multilevel Inverter [1] was proposed.
The performance of the APF inverter depends very much
II. OPERATION OF THE MSMI WITH UCI CONTROLLER
on the control scheme. Many technical papers related with
active power filters and their control methods were presented.
MSMI is an inverter structure, which has cascaded
Some researchers spent their effort in developing the APF inverters with Separate DC Sources (SDCs). This topology
system with Unified Constant-Frequency Integration (UCI) allows expansion of the number of levels, which provides
controller [2][3] recently. APF controlled by UCI Controller flexibility in increasing of higher voltage level. This can be
has the attractive advantages comparing to the other control done easily as the inverter has modularized circuit layout.
method such as the PQ Theory [4], Linear Current Control, Furthermore, the cascaded inverter structure help to reduce
Digital Deadbeat Control and Hysterisis Control [5]. Those voltage stress on switches, as lower voltage will be imposed
proposed control approaches need to sense the input voltage, by the DC side capacitor voltage to the switches or in other
load current and then calculate the harmonic reactive words, switches only have to bear on smaller value of voltage.
Abstract- Active power filter (APF) has a vital role for
compensating the reactive power and harmonics from a group of
nonlinear loads connected at the common point of coupling to
ensure that the resulting total current drawn from the main
incoming supply is sinusoidal. This paper is to present the
performance of a Modular Structured Multilevel Inverter
(MSMI) APF with Unified Constant Frequency Integration
(UCI) control technique for compensating the distorted current
caused by various nonlinear ac loads. MSMI is well known for
its capability in reducing the voltage and current stresses on the
semiconductors switches and providing the convenience for
future expansion in order to achieve higher power ratings for the
APF system. Whereas, the UCI control technique is adopted due
to its constant switching frequency operation and simple analog
circuitry. To demonstrate the capability of the single phase
MSMI APF with UCI control technique for compensation of
harmonics caused by various AC nonlinear loads, simulation
results are presented and discussed.
1-4244-0645-5/07/$20.00©2007 IEEE
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Fig.1 shows the structure of a 5-level MSMI APF.[8]
When this multilevel inverter is implemented as an APF, the
phase voltage of this particular APF is listed as in Table I
based on Kirchoff s voltage law, considering the DC capacitor
voltages as Vo1 and Vo2 and they are equal to Vd, respectively.
Incoming
Power
supply
Si S2 S3 84 S5 S6 S7 88
_
S34
S4
lload
i
I iapf
t
Vg
Current
Sig
Q*
Q
S2- Q*
S1i Q
Si
Lapf
Modular Structured Multilevel Inverter
(MSMI)
Vo2
Vmi
Vdc
S74
Q*
S6
Q*
Ss Q
Vphase
Module 1
S5
1
-4
S5s Q
CI K
S84
Vdc
_
III.
SIMULATION RESULTS
Computer simulations using MATLAB / SIMULINK
used to evaluate the performance of the MSMI APF
with UCI controller under different AC load conditions.
The developed system is shown in Fig. 3.
7
-Modul
s _
L2~~~
APF UCI Controller
Fig. 2 Conceptual diagram of the proposed structure of single phase MSMI
APF with UCI Controller.
Vm2
S6
RI
,
were
Module 2
Fig. 1 Modular Structured Multilevel Inverter
Table 1 Switching Function Table
SI
1
S2
0
S3
1
S4
0
0
0
1
O
1
1
1
0
1
1
0
1
0
Vdc
0
1
0
Vdc
S7
1
0
1
1
o
0
0
1
0
0
i
1
0
1
0
1
O
1
1
0
0
1
1
1
0
0
1
0
1
1
0
O
1
0
1
0
1
0
0
-Vdc
0
1
0
0
1
0
1
o
O
O
Vphase
S6
O
1
S8
0
S5
0
11
i
1
0
1
1
1
0
1
1
0
2Vdc
Vdc
0
Fig.3 MSMI APF system blocks
0
0
-Vdc
-2Vdc
In MATLAB / SIMULINK simulations, the nonlinear RC
and RL load will be connected to the AC supply and the
APF performance will be analysed. First, a diode rectifier
with RC load was connected to the AC main inlet as shown
in Fig. 4.
-Vdc
-Vdc
Vdc
The proposed configuration is as shown in Fig. 2. [8]
Through the current transformer, the mains current will be
compared with the integrated voltage signal from each module
of inverter to generate pulses for R-S flip-flops for triggering
the inverter switches.
Fig.4 MATLAB / SIMULINK subsystem blocks of nonlinear diode rectifier
with RC loads.
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Fig.5 shows that without the compensation from the
APF, the AC current waveform was highly distorted by the
nonlinear RC load. The indicated Total Harmonic
Distortion in Fig. 6 is 146.96% based on the first hundred
harmonics components.
Fundamental (55OHz)
100
=
9.894
THD= 9.84%
80
60
40-
0
220
61
40
60
Harmonic order
1 00
Fig.9 Harmonic spectrum of source current after compensation.
Fig.5 Distorted source current caused by diode rectifier with RC load.
Fundamental (0OHz)
106
2
2.381
A load disturbance has been created to test the ability and
flexibility of the proposed APF configuration. At 0.3 second
of the simulation time, the load suddenly changes from 250 Q
to 200 Q. Fig. 10 shows the increment of current peak due to
the reduction of load.
THD= 146.96%
0
-~60
40
11
20
60
401
Harmonic order
6so
100
Fig.6 Harmonic current components of current drawn by RC load without
APF compensation.
Fig. 10 Load changes from 250 Q to 200 Q at 0.3s.
By connecting the shunt MSMI APF in between the
incoming supply and the nonlinear RC load, the harmonic
problem can be mitigated. Compensation current from APF in
Fig.7 will compensate the distorted source current waveform.
The UCI controller able to sense the changes of load at 0.3
second and compensation was done immediately within a
cycle of waveform as in Fig. 11. Fig. 12 shows the
compensated source current from the AC input which remains
sinusoidal under load changing condition.
10
1
a
--------------
----------------------
6
4
2
0
-2
-4
-6
-6
O0
-10
---
Fi20.22 0124
Copn
Fig.7 Compensation current produced by MSMI APF.
0io28
cur
toth ch
---
34
38
load at 0.
4
Fig. I I Compensation current to the changing load at 0.3 s
After compensated by the proposed configuration of MSMI
APF, the current waveform drawn from the AC incoming
supply is now nearly similar to sinusoidal wave shape as
shown in Fig. 8. The THD of AC supply current after
compensation is greatly reduced from 146.96% to 9.84% as in
Fig.9.
Fig 12: Source current drawn from the AC input with change in load at 0.3s
-10
Fig.8 Source current waveforms after compensation.
Fig. 13 is the phase voltage of this particular MSMI APF,
which indicates 5 levels of voltage including the reference
level. The waveform is closer to sinusoidal compared to the
conventional single stage APF that produces square wave
voltages.
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Fig. 17 depicts the compensation current from the APF to
compensate the distorted current caused by nonlinear RL load.
!500
400
T-----
-
300
200
100
11 rn
IU
0
8
-100
r-
.A-- - - -J---------LA~~
-'-----r
6
------
4
-200
------
-300
-------
T:
-400
------
-:
2
-
-- -
- - --
.-
T-
- - - -
-
-_
r
----
----
-2-
1-.Iuu
-
-1
-
--
-
--
-
-
-
--
-
--
-
--
-
--
-
--
-
-4
-
r-
68
.10
0.2
Fig. 13 Phase voltage of APF in 5-level.
The proposed configuration of APF also tested for
compensating the current drawn by diode rectifier with RL
load. Fig.14 is the subsystem block of nonlinear diode
rectifier with RL load connected to the AC main supply in
MATLAB SIMULINK.
Di.d.1
--
dpL
9
6
3
-31
-6
-9
-12
-15
ok
-
--
----
-
-
-I
-------II-
AL
---
-
--
ok
I-----
0.21 0.22 0.23 0.24 0.25 0.26 0.27 0.28 0.29
Time (s)
Fig. 18 Source current waveforms after compensation.
j
22
FL
I
-
0.2
|
DJi.d
r
--
0.22 0.23 0.24 0.25 6.26 0.27 0.26 0.29
Ti m e (s)
Fig. 17 Compensation current produced by MSMI APF.
Fig. 18 and Fig. 19 show the compensated sinusoidal AC
Tttkspl0r
WT
~~~~~~~E
V-
--y-
current where the Total Harmonic Distortion is reduced to
22.42%.
15
12
FqdD
-
- -
Di0d2
Fundamental (0OHz)
I UU
Fig. 14 MATLAB /SIMULINK subsystem block of nonlinear diode rectifier
with RL loads.
=
9.437
THD= 22.42%
***
-~60
0
The cu Drent drawn by the RL load is shown in Fig. 15 and
the Total Harmonic Distortion percentage obtained using FFT
analysis is 42.96%.
40
7;;
-c
20
u
1
0
LI
20
60
40
Harmonic order
80
100
Fig. 19 Harmonic spectrum of source current after compensation.
_40
-
0.21 0.22 0.23 0.24 0.2`5 0.26 0.L27 O.'20 0.29
Time (s)
Fig 15. Distorted source current caused by diode rectifier with RL load.
1.2
1
nn1
80
-m
60
40
~20
n
I
0
Fundamerntal (5Hz)
=
5. 048
TH1= 42. 9%
Referring to Fig.20, it shows that the proposed APF
configuration is connected to compensate the AC incoming
current. Then, it was disconnected from the system at 0.22
seconds and reconnected at 0.32 seconds. There is an increase
in current during the reconnection of APF to the system.
However, it recovered immediately within half cycle. It is
obvious that the proposed configuration of APF with UCI
Controller has fast response in operation.
30o
1
20 F-
U
lIIIIII I 1.
IIII
20
II
40
50
Harrnonic order
80
100
Fig 16. Harmonic current components of current drawn by RL load without
APF compensation.
-vdijT1T .1
-10 ~A
AAA----
--)I
-Z U
0.1
0.1 5
0.2
0.25
0.3
0.35
Time (s)
Fig. 20 Simulated source current waveforms with the APF disconnected at t
0.22ms and reconnected at t = 0.32s.
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=
IV. CONCLUSION
It has been shown in this paper that the MSMI APF with
UCI controller possesses high capability in compensating the
distorted current caused by different load. Fast response of the
controller was observed when the APF was switched on and
off from the system. This is due to the reason that the UCI
controller performs current compensation in the manner of
cycle by cycle. The capability of the MSMI APF with UCI
controller is undoubtedly providing a flexible solution for
power quality control. Due to the simplicity and flexibility of
the circuitry, this configuration of APF is applicable and is
very suitable for industrial power filtering purposes.
V. REFERENCES
[1] Lopez, M. G., Moran, L. T, Espinoza, J. C.; Dixon, J. R. "Performance
Analysis of a Hybrid Asymmetric Multilevel Inverter for High Voltage
Active Power Filter Applications", Industrial Electronics Society, 2003.
IECON '03. The 29th Annual Conference of the IEEE, Vol: 2, 2-6 Nov
2003, Pg 1050-1055
[2] L. Zhou, and K. Smedley. "Unified Constant-frequency Integration
Control of Active Power Filters." APEC2000, p.406-412.
[3] C. M. Qiao, K. M. Smedley, and F. Maddaleno, "A Comprehensive
Analysis and Design of a Single Phase Active Power Filter with Unified
Constant-frequency Integration Control." presented at the 2001 IEEE 32nd
Annual Power Electronics Specialists Conference, PESC, vol. 3, pp.16191625, June 2001.
[4] Haque, M. T. "Single Phase PQ Theory for Active Filters". 2002 IEEE
Region Conference on Computers, Communications, Control and Power
Engineering. Oct 28-31. TENCON '02 Proceedings. 2002. vol. 3: 19411944.
[5] Simone Buso, Luigi Malesani, "Comparison of Current Control
Techniques for Active Filter Applications" IEEE Trans on Industrial
Electronics. Vol. 45. No. 5 October 1998.
[6] Cuk, S. and Smedley,K. "One-cycle Control of Switching Converter",
PESC 1991, p.888-96.
[7] Azli, N.A.; Yatim, A.H.M, " Modular Structured Multilevel Inverter For
High Power AC Power Supply Applications" Industrial Electronics,
2001. Proceedings. ISIE 2001. IEEE International Symposium on
Volume: 2, 12-16 June 2001 Pages:728 - 733 vol.2.
[8] Azli, N.A.: Lim, P.Y., "Modular Structured Multilevel Inverter with
Unified Constant-Frequency Integration Control for Active Power Filters"
Power Electronics and Drives Systems, 2005, PEDS 2005 International
Conference on Volume 2, 28-01 Nov.2005 Pages 1312 -1316.
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