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A robust 18-pulse rectifier for adjustable speed drive systems
Conference Paper · April 2005
DOI: 10.1109/APEC.2005.1453018 · Source: IEEE Xplore
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A Robust 18-Pulse Rectifier for Adjustable Speed Drive
Systems.
*E.Aeloiza
*Y.Durrani ** M. Rayner **R.Gopinath
* Power Electronics and Power Quality Laboratory*
** Toshiba International Corporation
Electrical Engineering Department
Texas A&M University
College Station TX,77843
Emai I: enjeti@ee.tamu.edu
Abstract- Eighteen pulse rectifiers have been lately utilized
to improve the input current distortion in Adjustable
Speed Drive (ASD). However, when a large capacitor is
connected in the dc-side, the performance is significantly
deteriorated. The input current THD decreases when the
relationship between the dc output current and the short
circuit current of the utility line (Id&),
decreases, i.e. for
lighter loads or stronger utility lines the performance
becomes worse. Similarly, the THD is also affected when
the utility line suffers a voltage unbalance condition or
voltage harmonic distortion. It is not uncommon to
measure 1% to 3% voltage unbalance andlor 2.5% to 5%
pre-existing 5'h and 7'b harmonic voltage distortion.
In this paper, a combination of an 18-pulse
rectifier and an Active Power Filter (APF) to improve the
input current distortion in ASD is proposed. The APF
provides the I8-pulse rectifier with the ability to achieve a
high performance over a wide spectrum of operating
conditions. The approach also has the characteristic of
maintaining a balance of performance, cost and reliability.
Results from a 48OV/30kW ASD prototype will
be discussed in the conference presentation.
1.- INTRODUCTION
Numerous research projects have been
conducted in order to minimize the input current
distortion in adjustable speed drives [l,2]. Years of
investigation have resulted in passive and active
solutions. Active approaches use high frequency
controllable PWM rectifiers, which significantly
decrease the input current harmonic distortion (THD, =
4-5%). Nevertheless, the introduction of high frequency
power electronics devices in the AC/DC conversion
process results in poor reliability. The rectification
process relies on a sophisticated PWM technology and
on a suitable control strategy, which makes the rectifier
more vulnerable to fail, due to the large number of
components involved. Moreover, they cannot achieve
efficiencies higher than 97-98% and the cost is
significantly high. Amongst the passive solution, the 18pulse rectifier topotogy seems to be the one that ensures
0-7803-8975-1/05/$20.00
02005 IEEE.
*P. Enjeti
(979) 846-1761
3608 East 29th St
Bryan, TX 77802
Email: mark.rayner@tic.toshiba.com
better balance of performance (THDi = 56%) and cost.
However, when a large capacitor is connected in the dcside the performance is significantly deteriorated. The
input current THDi decreases when the relationship
between the dc output current and the short circuit
current of the utility line (I,,/Isc), decreases, i.e. for
lighter loads or stronger utility lines the performance
becomes worse. Fig. 1 shows the behavior of the THDi
for differents Id& rates. Similarly, the THDi is also
affected when the utility line suffers a voltage unbalance
or voltage harmonic distortion. It is not uncommon to
measure 1% to 3% voltage unbalance andor 2.5% to
5% pre-existing 5th and 7'h harmonic voltage distortion.
Fig. 2 shows how the THDi changes with voltage
unbalance between 0 and 3%, for several Id/Iscrates.
In this paper, a combination of an 18-pulse
front-end rectifier and an active power filter to reduce
the input current distortion in adjustable speed drives is
proposed. The 18-pulse rectifier is used in the acldc
conversion process and reduces the overall input current
THD. Further reduction in the current THD is
accomplished by using the active power filter. The APF
is capable of providing the 18-pulse rectifier with high
performance over a wide spectrum of load and system's
conditions, and also brings with it the expected added
benefit of reactive power compensation. The fact that
the active filter is responsible for just reactive power
compensation and small portion of the current harmonic
reduction allows for a VA rating of onIy 25% of the
total ASD volt amperage. This strategy improves the
reliability of the whole approach, because the acldc
conversion is performed by using a robust uncontrolled
18 pulse rectifier technology. Also, high efficiency can
be achieved due to APF VA rating is no more than a
quarter of the total volt amperage.
Simulation results for a three phase
480Vi30kW unit under several operating conditions are
presented.
665
%MD
4s
0
0,02
0.N
0.06
0.08
0.1
,
0
O,l2
Fig.1. THDi v/s I&.
0.6
1
1.5
2
2.5
3
3.5
%Volla#e Unbalance
Idllac
Fig 2. THD,v/s ?4 voltage unbalance for different I&% rates
Active Power Filter
A small MF is used in order to mitigate the
remaining current harmonic that the 18 pulse rectifier
could not compensate.Fig, 3 shows the basic schematic
of the approach. The topology employs the conventional
6 IGBT inverter connected, through a power reactor, to
the common connection paint of the AC source and the
rectifier. The APF filter is able to inject the proper
reactive power to achieve power factor near to one and
the suitable current harmonic content to reduce the input
cutrent THD even more. The APT; is controlled by mean
o f a fixed switching frequency hysteresis control. The
control strategy allows providing the suitable APF
current IAP while the voltage dc V,, is kept constant.
The control theory for APF has b e n widely explored,
so i t will not be firther covered in this paper [5,6,7,8].
Fig.3. 18 pulse rectifier with Active Power Filter compensation.
666
--111.- DESIGN EXAMPLE
An industrial prototype is being built in the
facilities of Toshiba -1ntemkonal Corporation. The
rectifier load rated power is 30kw and the ac input
voltage i s 480Vnns. Table I shows a summary of the
parameters to be used in the construction and
simulations.
Input voltage
Rectifier Load Rated Power
Active Filter Inductor
Active Filter DC link Voltage
Switching Frequency
: V,
480V~~s
: Pn=30kW
: Lr= 760pH
: E = 800V
: fsw= 15kHz
The VA rating of the APF depends on both the
total reactive power to be compensated and the
distortion component of the input current of the 18 pulse
rectifier. Thereby, the active filter VA rating will be
specified taking into account the following condition.
a) The input current THD of the I 8 pulse rectifier will
no be higher than 15%.
b) The
power factor (DPF) Of the
rectifier is not lower than 0.98
'' pulse
I, =
41- D P F ~
(5)
And the distortion component o f the rectifier's current
Jm
THD;
(4)
IdJs
=
Thus, from equations (l), (5) and (6) the total active
filter current is:
THD;
1+ THD'
Finally, from eeuation (7)
. , we can conclude that
the active fiiter curreit I A ~will no be higher than
o.25pu. Therefore, the VA rating of GPF c~ be
specified as 25% of the total ASD VA. Furthermore,
from (7) we can see that the component that has higher
impact in the VA rating specification is the reactive
power compensation capacity (PF).
Thereby, the total current that the active filter has to
compensate wil1 be;
rv-SIMULATION RESULTS
Fig.4 shows simulation results for 3OkW/48OV system.
The THD of the rectifier's input current (I,) is 7% and
the THD of the total input current is 2.5%.
where
Iq= The current component of the reactive power to
compensate.
Idis = The distortion component of the current that will
be compensated, I,.
Since
Mmw
,om
And,
imm
THD,=L
4,
(3) and
D P F = -P
.............
mm .......................
o m . .
...
Where, Z,, is the fundamental component of I,
(4)
s
i ..,...........
i
warn
i ............
. .
~~~
....t ......... _______.
+ _...
.__.__.....
..
.....
.. ..
~~
;. ........ ..._._._
c~................_.4
. .... ...
imm
r- mi
im m
~
I
1"
Fig.4. Simulation results. a) Total input current b) Rectifier
input current c) APF current,
From basic electric circuit theory the reactive
component current in per unit is:
667
2"
V - Experimental Results
L
m
n m l.......
d)
~
~~~
.......... ~~~~~i
........... .____....._.......I..___..........................
i..............................
I
A 480V,30kVA 18-pulse ASD is currently under
development. Experimental results with the active
filter concept will be available during the
conference presentation.
VI.- CONCLUSION
E
UP(
om
om
um
IB
Tha
.
all
D15
b
Fig. 5 Simulation results for 50% to IOO?? change in the load
operating condition a)Total input current b) Rectifier input current c)
APF current d) APF dc link voltage.
Fig. 5 shows the response of the approach
during a change in the load operating condition from
50% to 100%. For a load of 50% the THD of the total
input current (Is) is 3.2%.
imm
158
I*
IIC
ow
An 18 pulse rectifier with Active Filter
compensation to improve the input current THD in
adjustable speed drive has been proposed. The fact that
the 18 pulse rectifier is responsible far the main current
distortion compensation allows the APF to be specified
by less than 25% of the total ASD VA rating. The main
parameter that affects the active filter VA rating is its
reactive power compensation capacity.
The APF provides the 18-pulse rectifier with
the ability to achieve a high performance over a wide
spectrum of operating conditions. Simulation results
show that the approach can compensate up to 2.5%
current THD. The approach also has the characteristic of
maintaining a balance of performance, cost and
reliability. The reliability of the system is maximized
due to ACDC conversion process which relies on a
largely proven and robust 18 pulse rectifier technology.
~FERENCES
-yIM
-100 00
8oW
SlW
1WW
llDW
12OW
190M
MOW
rims (mrl
Fig. 6 Simulations results for 2% voltage unbalance. 3-phase input
currents.
Fig. 6 shows the response of the approach for
2% voltage unbalanced condition, The input currents
still present some unbalanced (8%) however, the THDi
is less than 2%.
The performance of the combination of 18
pulse rectifier and APF is significantly high for bath
conditions. The results show that it can be superior to an
active front end. Table I shows a comparison of the
performance for different rectifiers.
Table I
Rectifier Type
18 pulse rectifier & APF
Active Front End
18 pulse rectifier
12 pulse rectifier
6 pulse rectifier
Total THD
2-3%
4-5%
54%
8-1 1%
25-27%
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