 Open Access
 Authors : Bisrat Gezahegn Lemma , Minfu Laio , Duan Xiongying
 Paper ID : IJERTV9IS010034
 Volume & Issue : Volume 09, Issue 01 (January 2020)
 Published (First Online): 06022020
 ISSN (Online) : 22780181
 Publisher Name : IJERT
 License: This work is licensed under a Creative Commons Attribution 4.0 International License
A ThreePhase ThreeLevel TType Converter for ThreePhase FourWire (3P4W) Active Power Filter
Bisrat Gezahegn Lemma1, Minfu Laio2*, Duan Xiongying 3
1,2,3, Institute of Electrical and electronics: a school of Electrical Engineering,
Dalian University of Technology Dalian, China
AbstractThreephase fourwire system is commonly used in residential, commercial and educational centers. However, the threephase fourwire system has suffered from harmonic, reactive, excessive neutral current and imbalanced loads. In the proposed control, a modified sensitivity function based RC controller was investigated for a threephase threelevel Ttype converter to mitigate harmonics, reactive and neutral current caused by the everincreasing nonlinear loads. The results were analyzed and presented in collaboration with the MATLAB/Simulink environment and the TMS320F28335 board. The proposed control achieved a harmonic compensation, reactive power correction, reduction of neutral current and balancing loads. Therefore, the threephase three level Ttype converter active power filter can be taken as a good candidate for load harmonics, reactive, and neutral current compensation.
Keywords Threelevel; Ttype converter; Active power filters; power quality.

INTRODUCTION
Threephase fourwire (3P4W) distribution feeders have been used for low voltage systems. The power feeder is taken from the secondary side of the Y distribution transformer to deliver power for residential, commercial, manufacturing offices and educational centers of single/ threephase loads. In the present day, singlephase nonlinear loads have been increasing into the low voltage distribution systems. This is due to the attractive features of the appliances from the customers point of view for the everincreasing requirements of information technology [1] and electric traction [2], [3] [4]. The nonlinear and traction loads have known with an imbalance in the grids. In the case of nonsinusoidal source or unbalanced nonlinear load, the neutral current problem will be serious in the threephase fourwire system. In the threephase fourwire distribution cases, the third harmonic current contributed by each singlephase nonlinear load will add up together and results in three times the zerosequence components of phase current. Moreover, the threephase four wire system is exposed to unbalanced loads due to single phasing and abnormal phase change in the industry.
The distribution engineers are tried to distribute loads at the design stage equally among the phases to maintained low neutralline current and better efficiency. However, this is not possible for large loads moreover singlephase loads switched on/off from the system in an unpredicted manner. Moreover, even the balanced singlephase nonlinear loads contribute a significant neutral current. Therefore, in today's threephase
fourwire distribution systems excess neutral current, load unbalances and high total harmonic distortion (THD) is a challenging practice.
The harmonic currents on the threephase fourwire system have negative effects on phase line(s), /Y distribution transformer and neutral conductor. The harmonic effects on the phase line(s) increase harmonic distortion and power losses. Similarly, harmonic increases the circulation of zero sequences harmonic current on the neutral line and delta winding of distribution transformer. The neutral current depends upon the type and condition of the load in the system. In many cases, the neutralline current exceeds the phase line current. An excessive neutralline current causes overloading of distribution transformer and feeder, rise common mode noise, flat topping voltage waveform. The excessive neutral line current may raise the potential of the neutral line to cause an accident. It's flattopping voltage waveform also affects the normal function of precision devices.
Different researchers have investigated different configurations using the passive filter, stardelta transformer, zigzagtransformer, single/threephase active power filter, and a combination of the above [5],[6],[7],[8],[9]. The passive filter has been proposed in [FBD] with resonance LC filter branches for dominant positive/negative sequence and zero sequences. The passive filter has low cost and simple to implement with the system. However, the passive filter might losses its tuning performance to the target frequency due to component aging, and creates a series/parallel resonance with the system inductance [10]. The Zigzag transformer is a low cost, highreliability option to create a lowimpedance path for zerosequence current and open circuit for positivesequence or negativesequence current. However, the zigzag transformer may not be a good option for a system with an unbalanced utility voltage. Because its lowimpedance path for zerosequence source voltage may aggravate the problem of neutral current and even may burndown the ZigZag transformer, the neutral conductor and the distribution power transformer [11]. Moreover, its neutral line current attenuation level determined by the source side to the zigzag transformer impedance ratio [8, 12]. The DY transformer connected in series with a reduced switched threephase active has been proposed in [12] to achieve a reduction of neutral current. However deltastar transformer is expensive and its filtering performance depends upon its installation location.
The multilevel pulsewidthmodulation has advantages on its higher efficiency up to medium switching frequency range,
better harmonic characteristics and smaller output voltage steps as compared to the twolevel VSIs [13, 14]. Among the multilevel converter, the Ttype topology is much preferred for its reduced number of switching semiconductors than neutral point clamped (NPC). Thus the Ttype converter has lower power losses as compared to the NPC [15]. The authors proposed a sensitivity function based repetitive controller (RC) for a threephase threelevel Ttype converter [16] to mitigate dominant load harmonic frequency, reactive power, and reduced neutral current. Therefore, the research has significant importance for harmonic and reactive compensation in low voltage applications to improve power quality. In many works of literature, the repetitive controller (RC) is preferred for the active power filter controller due to its good tracking ability and high gain for the desired frequencies [17]. The instability of the RC control is limited by applying a modified squaring sensitivity function [18], [19]. This provides a wide and deep notch at the selected frequencies. The rest of the paper is organized as follows: Section 2 describes a threephase fourwire threelevel Ttype converter configuration, section 3 explains the repetitive controller for inner controller section 4 presents simulation and experimental results, and finally, the conclusion is drowned in section 5.

MODEL OF THE SYSTEM
Fig.1 presents the threephase fourwire threelevel Ttype converter with the mixed linear and nonlinear load. The
converter. The statespace model of the Ttype converter is defined as [16]:
= +
1 1 + 2
=
1 1
2 1 + 2
=
1 2
(2)
= +
1 1 + 2
=
1 1
2 1 + 2
=
1 2
(2)
Wherethe converter output voltage, vinvxO = Tx1vC1 Tx2vC2 , converter current, ip = Ta1ica + Tb1icb + Tc1icc and in = Ta2ica + Tb2icb + Tc2icc , and the capacitor voltage v1 and v2 respectively. The neutral load current, io = Ta3ica + Tb3icb + Tc3icc.

REPETITIVE CONTROLLER FOR INNER
CURRENT LOOPS.
The dynamic of the inner current controller GI(s) is design using the transfer function as:
() 1/
() = () = / + 1
, = 2 ( + 0)
2
= 2 (
(3)
, = 2 ( + 0)
2
= 2 (
(3)
The inner controller is discretized with the zeroorder hold (ZOH) discretization method. The source voltages are defined as:
)
converter has two monodirectional switches (Tx1, Tx2) and
,
3
4
Tx1 + Tx2 + Tx3 = 1
(1)
Tx1 + Tx2 + Tx3 = 1
(1)
one bidirectional switch (Tx3) per phase. Threephase four wire active filter is applied to guarantee a unity power factor and to set a balanced threephase current in the network. The PI controllers are used to regulate the dcbus voltage variation and the voltage difference between the capacitor. The RC controller is used to compensate for the harmonics and reactive power for the nonlinear loads. The main objective of this controller is to achieve a unity power factor using the proper operation of two power switches and one ac power switch in each leg of the active power filter. The switching states in each leg to generate a unipolar voltage pattern are given as :
Where Txi = 1 if switch Txi is switched on and Txi = 0 if switch Txi is switched off and i = 1, 2, 3 stands for the upper, bottom and inner legs respectively. Similarly, x = a, b, c stands for the threephase. The power switch Tx1is turned on to generate a positive acside voltage, vx0 which is equal to,
Vdc. This effect reduces the compensation phase current, i as
vs,c = Vs2 sin (r 3 )
In the Fourier series, the steadystate load current of the
threephase fourwire system is usually a periodic signal with only odd harmonics. The current in each phase can be written as:
il,x = an.x sin(r(2n + 1)t + x)
n
+ bn.x cos (r(2n + 1)t + x)
(4)
where an.x, an.x R is the real Fourier series coefficients of the xphase current. The load currents of the threephase nonlinear loads are composed of the fundamental active current, fundamental reactive current, and harmonic current.
il,x = il,x,p() + il,x,q() + il,x,h()
(5)
The active nonlinear load current and the active current of the converter are supplied by the ac source:
isx,p = ic,x,p() + il,x,p()
(6)
When the SAPF control meets its goal, the load can be
2
sx
sx
the boost inductor voltage is negative,L dicx = v
dt
Ia = 2 Im cos(t)
2
Ib = 2 Imcos(t 3 )
2
Ic = 2 Im cos(t + 3 )
(7)
Ia = 2 Im cos(t)
2
Ib = 2 Imcos(t 3 )
2
Ic = 2 Im cos(t + 3 )
(7)
vxo
cx
. If the
seen as a resistive for the supply. Taking the Im as the phase RMS current, the corresponding threephase current can be
compensation current icx needs to be increased Tx2 is switched
on. As a result of this, the boost inductor voltage is positive
written as:
sx
sx
L dicx = v
dt

vxo
and the ac side voltage vx0
is changed to

Vdc . On the other hand, zero voltage on the ac side is
2
achieved with switching on Tx3. Following this switching effect, the compensation current increases for positive phase voltage and decrease for negative phase voltage, vsx . As a result of this switching effect on any phase, a unipolar voltage is obtained on the respective phase of the ac side of the Ttype
The asymmetrical linear and/or nonlinear harmonic in symmetrical nonlinear load causes the imbalance phase current. The imbalance phase current in any phase causes the
neutralpoint current and further aggravates the neutralpoint imbalance.
The system reference and disturbance signals are usually an odd harmonic frequency of the power system. Therefore, the repetitive control is the best choice to introduce an infinite gain at a submultiple of these harmonics below the half of the sampling frequency, i.e. /Ts. The basic block is formed from
Root Locus Editor for Open Loop 1(OL1) 1
0.8
0.6
0.4
Imag Axis
Imag Axis
0.2
0
0.2
OpenLoop Bode Editor for Open Loop 1(OL1) 50
40
Magnitude (dB)
Magnitude (dB)
30
20
10
0 G.M.: 14.6 dB
10 Freq: 6.28e+04 rad/s Stable loop
20
45
0
the feedback a delay of N N sampling periods, where N =
T/Ts, T is the period of the signal to be tracked or rejected, and Ts is the sampling period.
L
0.4
0.6
0.8
1
1 0.5 0 0.5 1
Real Axis
45
Phase (deg)
Phase (deg)
90
135
180 P.M.: 74.7 deg
Freq: 7.94e+03 rad/s
225
0 5
10 10
Frequency (rad/s)
Vsa
Vsb Vsc
sa
Lsb Lsc
nonliner Load
Fig. 3. Openloop design for the RC controller.
Lfa
P
1
= O S_ = 1 + () ()
1 ()()
1 ()()(1 ()())
(9)
1
= O S_ = 1 + () ()
1 ()()
1 ()()(1 ()())
(9)
Lfb
Lfc
Assuming that Gx(z)Gx(z) 1, and = 1 for odd RC and = 1 for conventional RC controller, the modified RC
P
1 V
C 2 DC T
U
U
ip Ta1
T
Tb1
Tb1
sensitivity function can be written as (10):

a 2
Z
io Tb 2
a3
Tb3
Lf
a
Vab
b
ica
1 W(z)H(z)
SMod_rc = 1 W(z)H(z)(1 G (z)G (z))
x o
1 W(z)H(z)
1 W(z)H(z)
(10)
1 W(z)H(z)
SMod_rc = 1 W(z)H(z)(1 G (z)G (z))
x o
1 W(z)H(z)
1 W(z)H(z)
(10)
icb
icc
1 V b
Taking the square of the sensitivity function,
2
2
DC
CB
N
Tb 2
T
T
n
n
i b
b 4
Tb 4
Tb1
2 (1 ()())2
_
Bode Diagram
(11)
2 (1 ()())2
_
Bode Diagram
(11)
300
Fig. 1. Threephase fourwire active power filter with a threephase three
level Ttype converter.
zl
200
Magnitude (dB)
Magnitude (dB)
100
0
100
4
z N
Q(z)
4.620080 x 10
Phase (deg)
Phase (deg)
0
Open Loop Conventional function
r e
Q(z)
2
kr z G (z) G (z)
r z G (z) G (z)
z
z

N l m
c I
4.608
9.216
Open Loop of Modified function
Fig. 2. Robust modified RC structure.
()()
O = 1 + () ()
(8)
()()
O = 1 + () ()
(8)
The inner controller is designed for a phase margin of 74.7 degrees and bandwidth of 6280 rad/sec as depicted in Fig. 3. The closedloop system without the repetitive controller Go(z) is stable, where (8):
The sensitivity function of the closedloop system with RC expressed as (9):
101 102 103 104
Frequency (Hz)
Fig. 4. Comparison of openloop bodeplot for conventional and modified
RC controller.
As one observed from Fig. 4. Comparison of openloop bodeplot for conventional and modified RC controller and Fig. 5. Comparison of sensitivity function for convectional and modified function the modified sensitivity function based RC controller has higher openloop gains and deep notches at targeted harmonic frequencies. This provides better performance for the RC controller as compared to the convention one [18].
Bode Diagram
Magnitude (dB)
Magnitude (dB)
0
100
200
Phase (deg)
Phase (deg)
2.304
0
x 104
Sensitivity of Conventional function Sensitivity of Modified function
2.304
101 102 103 104
Frequency (rad/s)
Fig. 5. Comparison of sensitivity function for convectional and modified
function.





SIMULATION RESULTS AND DISCUSSION The various MATLAB/Simulink toolboxes are used to
simulate and design the proposed system of threephase four wire with threelevel Ttype inverter. The six outer IGBTs form the threephase converter and the other six IGBT form the inner circuits. The simulation incorporates various balanced and unbalanced loads such as threephase as well as singlephase nonlinear loads. The investigation also includes unbalancing source voltage and unbalanced threephase four wire nonlinear load.

Balanced threephase nonlinear loads
A balanced threephase nonlinear RL load is applied to the system to evaluate the proposed controller against harmonic compensation. The system harmonic before and after compensation are presented in fig. 1 and 3 respectively.
6
4
2
is(A)
is(A)
0
2
4
6
0.4 0.405 0.41 0.415 0.42 0.425 0.43 0.435 0.44 0.445 0.45
Fig. 6. Uncompensated source current waveform for balanced threephase nonlinear load with rectifier RL loads of R = 100 and L = 10 mH.
5
Fig. 9. The load current THD for uncompensated balanced threephase nonlinear load with rectifier RL loads of R = 100 and L = 10 mH.

Unbalanced nonlinear loads
Three singlephase nonlinear loads with rectifier RC loads are connected to the system to evaluate its compensation for unbalanced nonlinear loads.
isa
isb isc isn
isa
isb isc isn
6
4
is(A)
is(A)
2
0
2
4
6
0.15 0.16 0.17 0.18 0.19 0.2 0.21 0.2
Fig. 10. Current waveform of the unbalanced three singlephase rectifier with RC loads of phase A = 2 kW, phase B = 1.2 kW, phase C = 0.8 kW.
20
0
20
0.05 0.1 0.15 0.2
6
isource (A)
isource (A)
Mag
Mag
4 Fundamental (50Hz) = 17 , THD= 35.52%
0
5
0.78 0.79 0.8 0.81 0.82 0.83 0.84 0.85 0.86 0.87
a)
Fig. 7. Compensated source Current waveform for balanced threephase nonlinear load with rectifier RL loads of R = 100 and L = 10 mH.
2
0 0 5 10 15 2
iaload
ibload
icload
inload
iaload
ibload
icload
inload
Fig. 11. The FFT analysis for phase A.
15
10
i (A)
i (A)
5
Load
Load
0
0
0.8 0.81 0.82 0.83 0.84 0.85
5
10
15
0.33 0.335 0.34
a)
0.345 0.35
Fig. 12. Uncompensated load current.
Mag (% of Fundamental)
Mag (% of Fundamental)
20
15
Fundamental (50Hz) = 5 , THD= 3.83%
10
5
0 0 10 20 30 40 50
Fig. 8 . Phase current FFT after compensation for threephase fourwire system rectifier with RL loads of R = 100 and L = 10 mH.
isa isb isc isn
isa isb isc isn
4
isa
15 isb 3
isc
10
(A)
(A)
5
source
source
0
i
i
5
10
15
isn
0.36 0.365 0.37 0.375 0.38 0.385 0.39 0.395 0.4 0.405 0.41
2
1
is(A)
is(A)
0
1
2
3
4
0.15 0.16 0.17 0.18 0.19 0.2 0.21 0.22 0.23 0.24
a)
a)
Fig. 13. The current waveforms for compensated source current for
Fig. 14. Compensated source current for unbalanced source voltage of
phase A, vsa = 198 2 sin (r 0) phase B, vs,b = 220
threephase fourwire active power filter with repetitive control for threelevel 2 4
ttype converters.
TABLE I
HARMONIC SPECTRUM WITHOUT AND WITH PROPOSED ACTIVE
H.
Order
Without filter
With APF
IEEE Standard
Source current THD
%age
Source current THD %age
% age Indv THD
% age Indv THD
THD
16.66
4.84
8
3
4.04
4.42
7
5
5
14.48
4.83
7
5
7
6.38
1.72
7
5
9
0.924
1.37
7
5
11
1.82
1.98
7
5
13
1.27
0.31
3.5
5
15
0.42
0.89
3.5
5
17
0.79
0.66
3.5
5
H.
Order
Without filter
With APF
IEEE Standard
Source current THD
%age
Source current THD %age
% age Indv THD
% age Indv THD
THD
16.66
4.84
8
3
4.04
4.42
7
5
5
14.48
4.83
7
5
7
6.38
1.72
7
5
9
0.924
1.37
7
5
11
1.82
1.98
7
5
13
1.27
0.31
3.5
5
15
0.42
0.89
3.5
5
17
0.79
0.66
3.5
5
FILTER
15
10
is(A)
is(A)
5
0
5
10
2 sin (r 3 ) and phase C, vs,c = 242 2si n (r 3 ) and balanced nonlinear load.
isa
isb isc isn
0.12 0.14 0.16 0.18 0.2 0.22 0.24
a)
2
2
4
4
Fig. 15. Uncompensated source current for unbalanced source voltage of phase A, vsa = 198 2 sin (r 0) phase B, vs,b = 220
2 sin ( ) and phase C, v = 242 2si n ( ) and
r 3 s,c r 3
balanced nonlinear load.
TABLE II
Parameters
Units
Source voltage,
380 V
Source inductance,
0.1 mH
Filter inductance,
3 mH
DClink capacitance,
2*3940
Dc voltage ,
580V
Load inductance ,
10 mH
Load resistance ,
100, 80,110 ,
Load Capacitance ,
1100
Parameters
Units
Source voltage,
380 V
Source inductance,
0.1 mH
Filter inductance,
3 mH
DClink capacitance,
2*3940
Dc voltage ,
580V
Load inductance ,
10 mH
Load resistance ,
100, 80,110 ,
Load Capacitance ,
1100
SPECIFICATION FOR PROPOSED ACTIVE FILTER
d) Unbalanced source voltage and nonlinear loads
The system with an unbalanced source voltage of phase
A , vsa
= 210 2 sin (r
2

0) phase B, v
s,b
= 220
3
3
2 sin (r ) and phase C, vs,c = 230 2 sin (r
4
) and unbalanced nonlinear loads are used to investigate its
3
performance for unbalanced source voltage and unbalanced nonlinear loads.


Unbalanced source voltage and balanced nonlinear 5
isa
loads
The system with an unbalanced source voltage of phase A, vsa = 210 2 sin (r 0) phase B, vs,b = 220
2
4 isb
isc
3 isn
2
is(A)
is(A)
1
0
2 sin (r
4
) and phase C, vs,c = 230 2 sin (r 1
3
3
2
3
3
) are used to investigate its performance for unbalanced 3
source voltage.
4
5
0.515 0.52 0.525 0.53 0.535 0.54 0.545 0.5
a)
Fig. 16. Uncompensated unbalanced source and unbalanced nonlinear
load source current.
isa
isb
isc isn
isa
isb
isc isn
4
3
2
1
is(A)
is(A)
0
1
2
3
4
0.35 0.36 0.37 0.38 0.39 0.4 0.41
a)
Fig. 17. Compensated unbalanced source and unbalanced nonlinear load
source current


CONCLUSION
A threephase fourwire active power filter based on three level Ttype converter topology was investigated to perform harmonics elimination, reactive power compensation, neutral line current reduction, and split dc capacitor voltages balance. The dclink voltage and capacitor voltage differences are regulated with the PI controller. A repetitive controller is applied for the inner current loop controller. The loss current is applied for unit power factor regulation. The PWM strategy is applied for the converter. The adopted controller achieves for harmonic compensation, reactive power regulation and minimizes neutral current and unbalance in unbalanced nonlinear or linear loads. The threephase threelevel Ttype converter has a potential advantage for low power applications.

ACKNOWLEDGMENTS
The project is supported by the National Natural Science Foundation of China (No.51777025); and the Fundamental Research Funds for the Central Universities (No.DUT19ZD219).
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