FIELD OF INVENTION:
The invention generally relates to a method of enhancing the range of transductor
control in an existing high current industrial rectiformer system and more particularly
the invention relates to a method for reducing the size and cost of the transductor in
high current industrial rectiformer system being used for process plants.
BACKGROUND OF THE INVENTION:
In a process industry high DC current is required. This is obtained by employing a
rectifier transformer that steps down the incoming 3 phase high voltage AC to a low
voltage high current (several kilo amperes) multiphase system AC (3 or 6 or 12 phase)
with phase shift as per requirement. This output AC is fed to diode rectifier to provide
DC source. The output voltage is governed by changing the taps in the transformer
with the help of OLTC (On Line Tap Changer).
During the operation of the rectiformer there lies a need for voltage control. Since the
tap changer of the transformer can change voltage in discrete steps only, finer
voltage control is not possible. Coarse voltage control is achieved by On-load tap
changer (OLTC) of the transformer whereas the finer voltage control is achieved by
employing the transductor control. To achieve the finer voltage control, a saturable
reactor (known as transductor) is provided in the output (LV terminal) of the
transformer.
The transductor is a saturable reactor made of electro magnetic material (normally
CRGO). It has 2 input coils viz. Bias and Control winding. The transductor is mounted
on the heavy copper bus-bar that inter-connects the transformer and the rectifier and
carries several kilo amperes of current.
The output of the LV coil of the transformer is made to pass through the transductor.
The control and Bias windings are connected to 2 independent DC excitation current
sources. When a current is flown through the Bias (Of Control) winding, it generates
an EMF in the core of the transductor. In a conventional method the control (Bias)
winding is a single turn coil that passes through the transductor core and hence the
ampere turn generated is equal to the amount of excitation current. The change in
the excitation level is decided by the amperes of the current source of the control (or
Bias) winding. By virtue of the ampere turn the control (or Bias) winding, it generates
a flux in the transductor, which cause (saturation to core resulting voltage drop of
output) a voltage. This voltage is super-imposed upon the output LV voltage to
achieve the final desired LV voltage output and regulates the output LV voltage. For a
particular system, the excitation current can control the LV voltage up to a pre-
decided level of load current only (control current). When the load current exceeds
the pre-decided level, the control is not effective owing to the fact that the ampere
turns of the control winding (which is same as the current of the excitation source)
generates a flux in the transductor proportional to the core cross section area, which
is already fixed. For any increase in load current, the arrangement will need a
replacement of the transductor core (or a change in the current capability of the
excitation system), which means additional cost and also additional space and their
mounting, which is a serious limitation in practice.
This puts a limitation on the system for (a) an increased load current - by virtue of
expansion of capacity or (b) more finer control of the system - a system for
refinement of the process (by introducing more OLTC steps, which is impractical).
There is a requirement to overcome this limitation. The present invention addresses
this problem by introducing a system in which the control and bias windings are of
multi turn configuration.
Hence the exciting ampere turn is increased. For single turn configuration, the ampere
turn was say I (equal to the excitation current). In a multi-turn configuration, say N,
for the same excitation current I, the ampere turn becomes NI. The present invention
replaces the method of transductor assembly so that the above bottle neck is
eliminated. Also, by virtue of the increase in range of transductor automatically makes
the OLTC with lesser steps more efficient.
SUMMARY OF THE INVENTION:
The present invention is introducing multi turn coil in control and bias winding of the
transductor core in place of single turn coil of the prior art. Hence the exciting ampere
turn is increased. For single turn configuration if the ampere turn is I which is same as
excitation current, in a multiturn configuration having N turns it becomes NI for the
same excitation current. Since the ampere turn is increased N times, the cross
sectional area is virtually reduced by N. This means the core cross section area has
become 1/N of the original section. This means transductor core area equal to 1/N of
the original core area is required, for the same out put.
Alternatively, if the cross sectional area of core is kept the same then voltage induced
by control and bias winding will be N times the original voltage and hence a larger
range of control can be obtained for same excitation current and same transductor
core cross section area.
OBJECT OF THE INVENTION:
The main object of the invention is to propose a method for enhancing the range of
transductor control for an increased load current.
One purpose of the invention is to propose a method to increase the life of tap
changer by reducing the number of tap changes required owing to higher range of
voltage control by transductor at desired current level.
Another object of the invention is to propose a method for improving the effectiveness
of the system with a smaller source of the excitation current input source and their
connections which implies lesser cost of the system.
A further object of the invention is to propose a method of employing smaller size
(cross section area) of transductor core which means less material cost for achieving
the desired control level of current.
A still further object of the invention is to propose a method of using smaller
transductor core which facilitates ease of mounting. Since the high current LV coil
terminals of the rectiformer pass through the transductor core, the total length of the
bus-bar from the transductor to rectifier reduces, which implies a saving in cost of the
heavy bus-bar that carries several kilo amperes of current.
A still another object of the invention is to propose a method of reducing the length of
bus-bar resulting reduction in load loss in bus-bar and thereby increases in the
efficiency of the total system.
A still another object of the invention is to propose a method reducing the heat
generated in the bus-bar connection from transformer to rectifier and hence reduction
in the cooling arrangements of the bus-bar.
BRIEF DESCRIPTION OF THE ACCOMPAYING DRAWINGS:
The invention can now be described in detail with the help of the figures of the
accompanying drawings in which
Figure 1- shows the arrangement of transductor with rectifier transformer.
Figure 2- shows the internal arrangement of the transductor (existing practice)
Figure 3- shows the proposed (new practice) arrangement of transductor.
DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE
INVENTION:
The transductor (Figure 3) comprises a ring core (1) of electro-magnetic material,
normally CRGO. The inter-connecting bus-bar from transformer LV to Diode rectifier
passes through the I/D of the ring core. There are 2 other coils viz. Control and Bias
winding, which are conventionally single turn coils; which also passes through the I/D
of the ring core. The LV current sets up a voltage in the ring core by virtue of the load
current. The control and the bias winding sets up voltages corresponding to the
excitation current they carry.
The Bias winding sets up a voltage that is in phase with the LV voltage and hence has
a tendency to increase the LV voltage. Where as the control winding sets up a voltage
that is in opposition to the LV voltage and hence has a tendency to decrease the LV
voltage.
In the new method, the Control (3) and Bias winding (2) are of Multi-turn
configuration in-lieu of conventional single turn configuration. Hence the exciting
ampere turn is increased. For single turn configuration, the ampere turn was say I
(equal to the excitation current). In a multi-turn configuration, say N, for the same
excitation current I, the ampere turn becomes NI.
Voltage Induced (Saturation Level) 8 (the ampere turn) X (cross sectional
area of the transductor)
Since the ampere turn is increased by N times, the cross sectional area is reduced by
N. This means a core cross sectional area has become 1/N of the original section. This
means the transductor with smaller core area equal to 1/N of the original core area is
required, for the same output and control.
Alternatively, if the area is kept as the same as the previous one, then the voltage
induced by the Bias/ control winding will be N times the original voltage and hence a
larger range of control can be obtained for same excitation current and same
transductor core cross section area.
WE CLAIM:
1. A method to enhance the range of voltage control of rectiformers for increase
in load current over a predecided level comprising the steps of:
• increasing the number of turns of the Bias (2) and control winding (3) of the
transductor so that corresponding exciting ampere turns is increased,
generating a control voltage of higher range, which on being superimposed
upon the output LV voltage achieves the final desired LV voltage.
2. The method as claimed in claim 1 wherein the bias winding (2) sets up a
voltage that is in phase with the LV voltage and increases the output LV
voltage.
3. The method as claimed in claim 1 wherein the control winding sets up a
voltage that is in opposition to the LV voltage and decreases the output LV
voltage.

A method to enhance the range of voltage control of rectiformers for increase in load
current over a pre decided level by increasing the number of turns of the Bias (2) and
control (3) winding of the transductor. Additional turns generate additional exciting
ampere turns generating a control voltage which on being superimposed upon the
output LV voltage achieves the final desired voltage. Bias winding (2) sets up a
voltage which is in phase with LV voltage, in creases the output LV voltage whereas
control winding (3) sets up a voltage that is in opposition to the LV voltage, decrease
the output LV voltage.