Download International Electrical Engineering Journal (IEEJ) Vol. 6 (2015) No.6, pp. 1917-1924

International Electrical Engineering Journal (IEEJ)
Vol. 6 (2015) No.6, pp. 1917-1924
ISSN 2078-2365
http://www.ieejournal.com/
A Novel DC-Side Current Injection
Technique for 18-Pulse Converter
System to Improve AC Line Condition
N. Venkata Ramana and V. L. N. Sastry
[email protected], [email protected]

Abstract— In this paper, a new compensation strategy for
receiving clean power of a conventional 18-pulse ac/dc converter
formed by three 6-diode bridges is proposed. According to the
proposed strategy, a three-phase current-controlled inverter
injects the compensation currents into the three positive
terminals of the three six-diode bridges. The goal of injecting
currents at dc side is to improve the quality of the ac line
currents. Compared to the conventional active filter deployed at
the ac side, the three-phase inverter used in this paper is with
lower KVA rating, and the 18-pulse converter draws nearly
sinusoidal currents from the ac main by the proposed
compensation strategy. The simulation results demonstrate that
the proposed method improves the line current quality.
Index Terms—Current-controlled inverter, inter phase transformer (IPT), multi pulse transformer, total harmonic distortion,
18-pulse converter.
I. INTRODUCTION
In high electric power conversion applications, single-phase
or three-phase full-bridge rectifiers have been the most popular
converter as the first stage connected to the utility.
Un-fortunately, these rectifiers drew non sinusoidal currents
from the utility and led to harmonic pollution on the grid. To
regulate these problems and maintain power quality, some
recommended standards [2], [3], which listed the acceptable
distortion levels for different utilities, were published for
industry to follow. Many strategies and topologies have been
proposed to deal with these problems so as to meet the
requirement of the standards [4]–[7]. Among them, multi pulse
schemes played an important role due to their reliability,
compactness, and effectiveness. Another advantage of multi
pulse methods was that they could only be implemented by
uncontrollable semiconductor devices, i.e., diodes.
The basic configuration of multi pulse ac/dc converters is to
connect multiple rectifier bridges together, either in parallel or
in series at the dc side, so that some lower order characteristic
harmonics produced by one rectifier can be cancelled by other
rectifiers [8]. Thus, in practical situation, multi pulse
transformers al-ways accompanied to provide the phase-shifted
power supplies for the multiple rectifiers. Also in [8], multi
pulse transformers with different configurations were
introduced and investigated. Moreover, in parallel multi pulse
methods, an inter phase trans-former (IPT) was needed at the dc
side to average the output voltage of each rectifier. To improve
line current quality further, some modified IPTs cooperated
with semiconductor switches were proposed. In [9], an IPT with
taps connected to thyristors was proposed to improve line
condition of an 18-pulse converter system by trigging the
thyristors at predetermined angles. Then, similar strategy was
proposed with replacing thyristors by diodes in [10] and [11].
Although the performance of the later was not better than the
former, it promised the advantages of reliability, simplicity, and
cost effectiveness. However, the improvement in line current
conditions by these multi taped IPT methods was limited. In
[12] and [13], an ac-side current injection mechanism was
proposed to improve the line conditions of a three-phase diode
bridge. The main advantage was only passive elements were
used to achieve the control goal. Although the performance
could compete with 12-pulse converter, the total harmonic
distortion of line currents was highly dependent on the output
load with high THD at low load. In [14], the input-current
shaping method was proposed for a 12-pulse rectifier, in which
a line IPT was deployed at the ac side and two single-switch
boost-type switching-mode rectifiers were parallel at the dc
side. Both sinusoidal input currents and output-voltage
regulation were achieved through complex control scheme.
1917
Ramana and Sastry
A Novel DC-Side Current Injection Technique for 18-Pulse Converter System to Improve AC Line Condition
International Electrical Engineering Journal (IEEJ)
Vol. 6 (2015) No.6, pp. 1917-1924
ISSN 2078-2365
http://www.ieejournal.com/
Fig. 1. Scheme of the proposed injected method applied to an 18-pulse converter
.
In [15] and [16], the concept of an active IPT was first
pro-posed in 12-pulse and 24-pulse diode converters, which
im-proved the line quality by injecting compensation current
into the extra winding of the IPT with a low kilovolt-ampere
rating inverter. However, the control strategy used in [15] only
im-proved one phase line current of the three-phase source
while the other two phase line currents were still highly
distorted. In [17], another active IPT, which resulted in
balanced line currents with improved quality, was proposed in
a 24-pulse converter system. Moreover, the performance of
different load arrangement was investigated and discussed as
well. It has to be noted that, in [15] and [17], the active IPTs
configured extra windings for injecting the compensation
currents. Thus, the IPTs in the original multi pulse converters
had to be replaced by IPTs with extra windings.
A dc-side injection method applied to a conventional 18-pulse
converter was proposed in an earlier version of this paper [18],
in which the ac main currents were improved by directly
injecting the compensation currents into three positive
Ramana and Sastry
terminals of the three six-diode modules. The term “directly
injecting” means that the proposed method provided the
compensation currents without modifying any parts of the
18-pulse converter, even the IPT. However, in [18], the paper
did not investigate the performance of the compensation
strategy under unbalanced power supply. Following the concept
in [18], this paper proposes a strategy to improve the line
condition of a conventional 18-pulse diode converter by
injecting compensation currents at the dc side of the converter.
II. SYSTEM DESCRIPTION
Fig. 1 shows the proposed scheme, which includes a set of
multi pulse transformers, three six-diode bridges, a
three-winding IPT, and a three-phase current-controlled
inverter. The set of multi pulse transformers, including one
delta–delta connection and two delta-polygon connections,
provide three three-phase sources for the three diode bridges
with +20◦, 0◦, and –20◦ phase shift, respectively. It has to
mention that even the delta-polygon connection are used in this
paper, other isolated connections with identical phase shift also
can be used in the proposed scheme, such as delta-fork
1918
A Novel DC-Side Current Injection Technique for 18-Pulse Converter System to Improve AC Line Condition
International Electrical Engineering Journal (IEEJ)
Vol. 6 (2015) No.6, pp. 1917-1924
ISSN 2078-2365
http://www.ieejournal.com/
connection [8].
The secondary sides of the three phase-shift transformers,
denoted as Tr1, Tr2, and Tr3, connect to the three diode
bridges, denoted as Rec1, Rec2, and Rec3, respectively. Then,
all the negative terminals of the diode bridges connect together
while the three positive terminals connect to the three
non-common terminals of the IPT, respectively. The three
output terminals of the three-phase current-controlled inverter
directly connect to the three non-common terminals of the IPT.
The inverter injects the compensation currents into these
terminals according to the proposed strategy that leads to nearly
sinusoidal ac line currents and will be detailed in next section.
Through the direct connection, the proposed scheme can adapt
easily to an existing 18-pulse converter system without any
modification. Moreover, when the inverter is disconnected from
the proposed system, the 18-pulse converter still can resume it
original performance.
The goal of the proposed method is to improve the overall ac
line currents by injecting the compensation currents into the dc
side of the 18-pulse converter. These compensation currents
modify the dc output current waveforms of individual diode
bridge while the load current remains unchanged, and the THDs
of ac line currents can be improved from 6.6% to 1.2%
experimentally. There are two advantages of the proposed
scheme.
1) Through the direct injection mechanism, the proposed
method can apply easily to an existing 18-pulse converter
system without any modification. Moreover, in case of
dis-connecting the inverter from the system, it still can
work as a conventional 18-pulse converter properly.
2) The proposed method employs a traditional three-phase
inverter, which is a mature technology and can be adapted
from the inverter sector conveniently.
.
Fig.2. Three-phase inverter and the control scheme
1919
Ramana and Sastry
A Novel DC-Side Current Injection Technique for 18-Pulse Converter System to Improve AC Line Condition
International Electrical Engineering Journal (IEEJ)
Vol. 6 (2015) No.6, pp. 1917-1924
ISSN 2078-2365
http://www.ieejournal.com/
III. COMPENSATION PRINCIPLE
The
Compensation Principle:
ia 3 of the three transformers are given by [17]
ND
i4
NP
N
N
 L i7  S i8
NP
NP
where
S1 with  1200 ,  1200 ,  200 ,  1000 ,  1400 ,
I  i1 i2 i3 i4 i5 i6 i7 i8 i9 
T
(1)
N P , N L , N S , and N D are the number of
turns of each winding section of the multi pulse transformers.
Summing these three primary line currents, we can obtain the
overall ac source current
 S1
S  0

 0
ia as
0
S6
0
0
0
S7
0
0

S9 
0
0
S8
T
(4.2)
ib and ic , can be obtained by
quasi-square waveform corresponding to
id 1 , id 2 , and id 3 as
i1 of rectifier1
Vab  2 *VLL * sinwt  is selected as the phase
(5)
Let the dc load be well filtered, i.e., the load current
I d is ripple free, and the three windings of the IPT equally
share the load current. Then, after injecting i x1 , ix 2 , and
ix 3
into the positive terminals of the three rectifiers, the dc
currents of the rectifiers can be expressed as
by [15]. Fig. 3 shows the switching function S1 , which is a
reference, where
0
S5
0
T
similar derivation. For representing the ac source currents
with dc-side currents, the switching function S was introduced
when
0
S4
0
I  S.id 1 id 2 id 3 
NS
ND
NL
, k2 =
, and k 3 =
.
NP
NP
NP
The other two source currents,
S3
0
0
currents of the rectifiers,
(2)
k1 =
S2
0
0
(4.1)
Then, these ac currents can be represented by the dc
i a  i a1  i a 2  i a 3  k1 (i1  i 7 )  k
where
i2 - i9 can be found by shifting
rectifiers and their corresponding switching functions in
matrix forms as
ia 2 
ia 3
S9
S2 -
functions
 400 ,  800 , and  1600 , respectively. For convenience,
I and S are used to represent the ac currents of the three
N
NL
i1  S i 3
NP
NP
i a1 
switching
corresponding to currents
Referring to Fig. 2, the primary line currents, ia1 , ia 2 ,
and
other
id 1
id 2
id 3 
T
I
  d  i x1
3
Id
 ix 2
3
Id

 ix 3 
3

T
(6)
VLL is the rms value of the line-to-line
source voltage and ω is the line angular frequency.
Substituting (6) into (5), we have
I
I  S  d  i x1
3
Fig. 3. Wave form of the diode switching function S1
The switching function S1 is given as
1

S1   0
 1

for / 9  t  4 / 9
for 4 / 9  t  11 / 18
for11 / 18  t  19 / 9
(3)
Id
 ix 2
3
Id

 ix 3 
3

T
(7)
Substituting (7) into (2), then the three source
currents are represented by load current and the injection
currents as


Id
S4  k1 S1  S7   k2 S 3  S8  
3
k1S1  k2 S3 ix1  S4ix 2  k1S7  k2 S8 ix 3
ia 
1920
Ramana and Sastry
A Novel DC-Side Current Injection Technique for 18-Pulse Converter System to Improve AC Line Condition
International Electrical Engineering Journal (IEEJ)
Vol. 6 (2015) No.6, pp. 1917-1924
ISSN 2078-2365
http://www.ieejournal.com/




Id
S5  k1 S2  S8   k2 S 1  S9  
3
k1S2  k2 S1 ix1  S5ix 2  k1S8  k2 S9 ix 3
ib 
Id
S6  k1 S3  S9   k2 S 2  S7  
3
k1S3  k2 S2 ix1  S6ix 2  k1S9  k2 S7 ix 3
ic 
(8)
The goal of the proposed method is to draw purely
sinusoidal currents from the ac source. Therefore, the
problem now is how to solve (8) and find the three injection
currents that can meet the goal. Assuming that the proposed
system is lossless and the source currents are fully
compensated, i.e., without distortion, the three source
currents can be given as
3
I d sint   / 6
2
3
ib 
I d sint   / 6  2 / 3
2
3
ic 
I d sint   / 6  2 / 3
2



 

I
sin t  k 3  n  2  9 

2
I

 d  d 

3 
 3
sin 

9 


 t  k   n  2   , k   n  1  
 3

9 3
9 

ixn  


 

sin t  k  8  n  
I


2I
3
9
 d  d 
7 
3 
3
sin 

9 


 t  k   n  1  , k   n  1  
 3
9 3
9 


(10)
ia 
where n=1, 2, 3 and k =0, 1, 2, . . . According to (10), Fig. 4(a)
shows the waveform of ix2 , and the other two injection
currents can obtain by shifting ix2 with +20◦ and –20◦ ,
respectively.
(9)
Solving (8) by substituting (9) into it, we can obtain the three
injection currents that can meet the control goal. However,
because the switching functions are discontinuous, it is
difficult to obtain the closed form of the solutions. In one line
cycle, each solution of the injection currents has 12 pieces
separated by the switching functions. Thus, the three injection
currents were solved piece by piece over one line period and
given as
Fig. 4. Waveforms of the compensation current command ix2.
(a) Precise command. (b) Approximate command
Equation (10) provides the digital controller to calculate the
commands of the injection currents for the inverter. However,
the waveform of ix2 shown in Fig. 4(a) looks similar to a saw
tooth waveform. Therefore, to alleviate the calculation burden
of the controller, the saw-tooth functions are recommended as
an approximate method and given as
1921
Ramana and Sastry
A Novel DC-Side Current Injection Technique for 18-Pulse Converter System to Improve AC Line Condition
International Electrical Engineering Journal (IEEJ)
Vol. 6 (2015) No.6, pp. 1917-1924
ISSN 2078-2365
http://www.ieejournal.com/
ixns
 I d  18 

 
 1   t  k  n  2  

 
3
9 
 3 






t  k  n  2  , k  n  1 


9 3
9
 3

 I d    1  9  t  k   n  2   
 3 

3
9 

 

 
 t  k  n  1 , k  n  1 

9 3
9
 3

(11)
Where n = 1, 2, 3 and k = 0, 1, 2, . . . Fig. 4(b) shows the
approximate waveform of ix2s , and it is obviously a
saw-tooth wave. It has to be noted that after injecting the
compensation currents, the ac-side currents of the three
rectifiers will change from square-shape waveforms to
saw-tooth-shape waveforms, as the solid lines shown in Fig.
5(a) and (b), respectively.
IV MATLAB/SIMULINK MODEL
Fig.6 MATLAB/Simulink Model for Proposed system
Fig.7 Controller for Proposed system
Fig.8 Uncompensated line
Fig. 5. AC current waveforms of the rectifiers affected by
unbalanced supply voltage. (a) Before compensation.
(b) After compensation
Fig.9 Injecting current
1922
Ramana and Sastry
A Novel DC-Side Current Injection Technique for 18-Pulse Converter System to Improve AC Line Condition
International Electrical Engineering Journal (IEEJ)
Vol. 6 (2015) No.6, pp. 1917-1924
ISSN 2078-2365
http://www.ieejournal.com/
[5] I.A.Vargas,
A.J.Forsyth,and
F.J.C.Zabalza,“Capacitor voltage balancing techniques for a
multi pulse rectifier with active injection” IEEE Trans. Ind.
Appl., vol. 47, no. 1, pp. 185–198, Jan./Feb. 2011.
[6] T. Yi, L. P. Chiang, W. Peng, C. F. Hoong, G. Feng,
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shunt active power filter with output LCL filter,” IEEE Trans.
Ind. Electron., vol. 59, no. 3, pp. 1443– 1452, Mar. 2012.
Fig.10 Improved ac line
V.CONCLUSION
This paper proposed a dc-side compensation strategy to
improve the line current condition of an 18-pulse converter
system formed by three six-pulse rectifiers. A three-phase
current controlled inverter injects the compensation currents
quality of the ac source currents. The compensation strategy
was analyzed and the injection commends were derived in this
paper. An approximate method was recommended to simplify
the calculation. The proposed method only needs a rather low
rating (2.4%) three-phase inverter and when the inverter is
disconnected, the proposed system still can work as a
Conventional 18-pulse converter properly.
[7] D. M. Mohan, B. Singh, and B. K. Panigrahi,
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source converter for power quality improvement in HVDC
system,” in Proc. IEEE Int. Conf. Power Syst., 2009, pp.1–6.
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1923
Ramana and Sastry
A Novel DC-Side Current Injection Technique for 18-Pulse Converter System to Improve AC Line Condition
International Electrical Engineering Journal (IEEJ)
Vol. 6 (2015) No.6, pp. 1917-1924
ISSN 2078-2365
http://www.ieejournal.com/
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1924
Ramana and Sastry
A Novel DC-Side Current Injection Technique for 18-Pulse Converter System to Improve AC Line Condition