Energy and Power Engineering, 2013, 5, 1410-1414
doi:10.4236/epe.2013.54B267 Published Online July 2013 (http://www.scirp.org/journal/epe)
Coordinate Control Strategy for Current Stabilization in
an Aluminum Smelter Including on Load Tap Changer
Heping Xu1, Qipin Xu1, Yuan Gao2, Haoming Liu2
1NARI Technology Development Co. Ltd., Nanjing, China
2College of Energy and Electrical Engineering, Hohai University, Nanjing, China
Received March, 2013
ABSTRACT
The performance of the current stabilization control system in an aluminum smelter affects the quality and the quantity
of the electrolytic products. This paper elaborates the power supply, in which the diode rectifiers and the self-saturable
reactors could keep the series current stable, then describes the basic principle of the rectifier unit control and the series
current control. A coordinate strategy is proposed to keep the series current stable, the self-saturable reactors are
controlled by a proportional-integral control scheme and the on load tap changers of transformer are triggered by the
errors between the setting value of the series voltage and the measured values of the series voltage. Simulation results
on PSCAD/EMTDC show that the effectiveness of the proposed strategy to keep the series current stable.
Keywords: Current Stabilization; Diode Rectifier; Self-saturable Reactor; OLTC; Coordinate Control
1. Introduction
The aluminum smelter is one of the important industrial
loads. The dc current should be kept stable to ensure the
efficient production. Not only the voltage’s fluctuation
on the grid side, but the anode effect in the aluminum
smelter can cause the series current unstable. If the
current control doesn’t work well, the heat of the smelter
might be unbalance, and the power consumption to
product the aluminum might increase, both they would
affect the quantity and the quality of the production to a
great extent [1].
Most of the related papers focus on the concept or the
practical application of the current stabilization control
system of the aluminum electrolysis. While few of them
touches the current stabilization control strategy based on
the self-saturable reactor (SR) and the on load tap changer
(OLTC) together. The basic structures of the current
stabilization control system are discussed in [2-4]. The
reasons of the changes of the series current caused by the
smelters load changing, the voltage fluctuation on the
grid side and the anode effect happen are investigated
respectively in [5]. A proportional- integral (PI) control
method is introduced to adjust the inductance of the
self-saturable reactor in [6].
This paper describes the basic structure of the power
supply in the aluminum smelter, and the basic principle
of the rectifier unit control and the series current control
at first. Then a coordinate control strategy is proposed in
order to keep the series current stable, the PI control
method is introduced into the self-saturable reactor, and
the OLTC will be triggered by the big errors between the
setting value of the series current and the measured series
current. The proposed coordinate control strategy is
carried out in PSCAD/EMTDC to validate the
effectiveness.
2. Current Stabilization Control Scheme
In the aluminum smelter, the whole rectification system
is comprised of several separated rectifier units in
parallel. Each unit consists of the OLTC transformer,
phase- shifting rectifier transformer, cophase counter
parallel connection rectifiers, and some related auxiliary
facilities. In general, the cophase counter parallel
connection rectifier is of a 12 pulse circuit, including two
6-pluse’s rectifier cubicles. The power supply schematic
diagram of an aluminum smelter is shown in Figure 1, it
is comprised of 4 dc rectifier units. Each unit consists of
the diode rectifiers and the self-saturable in series.
The “N+1” principle is adopted in the current
stabilization control system. Here, 1 represents the
master control, N represents the number of the rectifier
units, e.g., N equals to 4 in Figure 1. The power of the
electrolytic series is supplied from the N rectifier units,
and the current of the electrolytic series is equal to the
sum of all the rectifier units current. The master control

H. P. XU ET AL. 1411
ensures the series current stable.
A - Filter Compensation Device; B - Grounding Equipments at the Neutral; 1 - Isolating Switch; 2 - Contactor Switch; 3 - OLTC Transformer; 4 -
Phase-shifting Rectifier Transformer; 5 - SR; 6 - cophase counter parallel connection rectifiers; 7 - Choke Inductor; 8 - The Equivalent Model of the Electrolytic
Series
Figure 1. The power supply sche matic diagram of an aluminum smelter.
2.1. Current Stabilization Control of the
Rectifier Unit
The current stabilization control of the rectifier unit is
realized through means of regulation on the self-saturable
reactor, so as to keep the total load current stable, and
ensure the balance of the currents between the two
6-pluse’s rectifier cubicles.
The self-saturable reactor is made with a
ferromagnetic material with nonlinear magnetization
curve and with nonlinear saturation characteristics. It is
composed of offset winding, control winding, and work
winding. SR is connected in series into the rectifier
bridge arm of the unit.
When an anode effect occurs, the series current I
decreases, and the dc currents Idc of every the rectifier
units decrease. The error ΔI between the dc outputting
current and the reference value of the rectifier unit
increases, the duty cycle of the diode is adjusted after the
PI control. In which, the conduction lag time of the diode
is regulated via the work winding of the self-saturable
reactor [7,8]. Finally, the dc outputting current can be
adjusted close to the reference value. The whole control
process of the current stabilization of the rectifier unit is
shown in Figure 2.
2.2. Series Current Stabilization Control
The series current stabilization controller adjusts the self-
saturable reactors and the OLTC to ensure the total
current stable. It depends on the amount of the error
between the measured value of the outputting series
current and the pre-setting value, and determines whether
or not the OLTC need act. If needed, the taps of OLTC is
regulated to a new appropriate position, and then the
current stabilization control of the rectifier unit is
activating to ensure the current stable [9, 10].
3. Coordinate Control Strategy
3.1. Control Strategy of the SR
The flux density curve of the SR is
b
ac
H
dB eHe
dH  (1)
dc
I
ref
dc
I
k
I
Figure 2. Control process of a rectifier unit.
where, a, b and c are the magnetization curve constants
of SR, B is magnetic flux density, H is magnetic field

H. P. XU ET AL.
1412
density.
Taking no account of the magnetic leakage and iron
loss, the reactance of SR is
22
b
ac ac ac
H
An An dB
L
ldH l
eHe (2)
where, A is the area of the section of the core (cm2), ac
is the number of the SR’s work winding, is the
average length of the magnetic circuit (cm) [11].
n
l
For simplify, the SR is considered as a controllable
nonlinear reactor, and it keeps constant if the system is
steady. All setting values of the current of rectifier units
are supposed as equal, the sum of them is the setting
value of the electrolytic series current, that is, ref
I

. The error between
ref
dc
NIref
dc
I
and dc
I
, the measured
value of the series current, is as the input of the PI
controller, and the output from PI controller is added to
the setting value of SR’s inductance , the sum is the
actual inductance after the regulation of SR.
ref
L
The control diagram of the SR is shown in Figure 3.
p
K
and T are the proportional gain and integral time
constant of PI controller.
Due to the magnetic characteristics of the SR, the
regulating range is limited. It makes the regulating range
of the outputting dc voltage on the load bus limited. In
general, the regulator depth is about 70 V, and the
regulation could be carried out continuously.
3.2. Control Strategy of OLTC
In this paper, the voltage range of fineness regulation is
[0, ΔUmax], the range is divided into three sections. The
first section is [0, U1], called up shift section; the second
section is [U1, U2], called unrestrained section; the third
is [U2, ΔUmax], called downshift section. The voltage
range of unrestrained section is almost 1.5 times as the
voltage change one step of OLTC caused, and the
voltage range of up shift or downshift section can meet
the requirements of the common fluctuation. For
example, the voltage range of downshift section should
cover the voltage change an anode effect caused.
The flow chart of the coordinate current stabilization
control in an aluminum smelter is shown in Figure 4. If
there exists a unit failure, unit maintenance or an anode
effect, the series current varies a few from the reference
value, and it exceeds the capability of the fineness
regulation of SR, the upshift or downshift of OLTC
should be
N
ref
I
ref
dc
I
dc
I
ref
L
L
1
p
KTs 
Figure 3. Control diagram of the self-saturable reactor.
Figure 2. Flowchart of current stabilization control in an
aluminum smelter.
taken place to improve the error of the series current.
4. Simulation Example
The simulation case consists of an infinite power supply,
transmission line, OLTC transformer, and the diode
rectifier group with 12 pulses. The system frequency is
50 Hz, the voltage of power supply is 220 kV, the length
of transmission line is 90 m, and the rated capacity of the
OLTC transformers is 250 MVA.
The back electromotive-force of each electrolytic cell
is supposed as 1.7 V [12], then the load consists of 256
electrolytic bathes in series is equivalent to a 0.4352 kV
dc voltage source in series with a variable resistance, that
is in the range of [0.0021083, 0.00238099] Ω. A choke
inductor, about 0.001 H, joins up in series to the dc side
to reduce the fluctuation. The total case is implemented
in the PSCAD/EMTDC, and the model is shown in
Figure 5.
Suppose there are 3 anode effects occur
simultaneously, the electrolytic series current fluctuation
curves with or without the coordinate current
stabilization control are shown in Figure 6.
It is easy to recognize that the series current is terrible
without any control or only with the control on SR. The
reason is that the series current fluctuates seriously when
there are 3 anode effects occur at the same time, while
the regulation capacity of SR is limit. The OLTC can
change discretely the series current significantly. The
proposed coordinate control strategy could ensure the
adjustment range by means of OLTC, and the precision
by means of SR.

H. P. XU ET AL.
1413
series
R=0
rec tifier transf or mer+ S R+ diode
#1 #2
1[Mohm]
Var Compensator
0. 001 [H ]
Iload
PI
COUPLED
SECTION
Tap2#1 #2
1[Mohm]
Var Compensator
rec tifier transf or mer+ S R+ diode
Tap1
Tap3#1 #2
1[Mohm]
Var Compensator
rec tifier transf or mer+ S R+ diode
Tap4#1 #2
1[Mohm]
Var Compensator
rec tifier transf or mer+ S R+ diode
Figure 3. The model of simulation case in PSCAD/EMTDC.
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5. Conclusions
A coordinate current stabilization control in an aluminum
smelt is proposed in this paper. The control precision is
guaranteed via the close loop control of SR, and the
regulation range is large enough with OLTC transformer
included. The proposed control strategy is applied into
the diode rectifier system, and the effectiveness is
verified by a simulation case.
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