DC magnetization suppression in power converter transformers

A power converting apparatus includes a power converter connected to a power line via a transformer 3, and current detectors 5A and 5B for detecting the currents of the windings of the transformer. The output signals of the current detectors 5A and 5B are mathematically processed to produce an exciting current component of a transformer 3. The exciting current component is mathematically processed to produce a flux density contained DC component. Further, the flux density contained DC component and a flux density contained DC component command value are mathematically processed to produce a voltage command correction value.

BACKGROUND OF THE INVENTION 
The present invention relates to an electric power converting apparatus 
having a power converter connected to an AC power system and loads by way 
of a transformer, and more particularly to a technique for preventing a DC 
magnetization in the transformer. 
FIG. 9 schematically shows an arrangement of a conventional power 
converting apparatus having a power converter connected to an AC power 
system via a transformer. A circuit for preventing a DC magnetization in 
the transformer is incorporated into the power converting apparatus. The 
power converting apparatus is disclosed in Japanese Patent Laid-Open 
Publication No. Hei. 7-28534. 
In FIG. 9: reference numeral 1 is an AC power system as an AC power line; 2 
is a self-excited converter for generating an AC voltage in response to a 
gate drive signal; 3 is a transformer inserted between the AC power system 
1 and the self-excited converter 2; 4 is a DC voltage source for supplying 
a DC voltage to the self-excited converter 2; 5A and 5B are current 
detectors for detecting currents flowing through the windings of the 
transformer 3; 6 is a subtractor for computing a difference between the 
currents output from the current detectors 5A and 5B; 7 is a DC component 
detector for detecting a DC component of an output signal of the 
subtractor 6; 8 is a potential transformer which measures the voltage of 
the AC power system 1; 9 is a voltage reference circuit for producing a 
set voltage of the AC power system 1; 10 is a voltage command value 
generating circuit for generating a voltage command value to the 
self-excited converter 2 in accordance with the output signals of the 
voltage reference circuit 9 and the potential transformer 8; 11 is an 
adder for adding together an output signal of the DC component detector 7 
and an output signal of the voltage command value generating circuit 10; 
12 is a PWM (pulse width modulation) control circuit which determines an 
ignition timing of a self-extinction element in the self-excited converter 
2 in accordance with the output signal of the adder 11, and generates a 
gate pulse on the basis of the determined timing; and 13 is a gate pulse 
amplifying circuit which amplifies an output signal of the PWM control 
circuit 12 and applies a gate drive signal to the self-excited converter 
2. 
An operation of the conventional power converting apparatus shown in FIG. 9 
will be described. 
In the power converting apparatus of FIG. 9, when a DC component is 
contained in the voltage of the AC power system 1 or the-output voltage of 
the self-excited converter 2, an exciting current containing the DC 
component flows into the transformer 3. The DC component contained in the 
exciting current magnetizes the transformer 3 to saturate the iron core of 
the transformer 3. 
Of the winding currents of the transformer 3, the current flowing through 
the winding connected to the AC power system 1 is called a primary winding 
current, and the current flowing through the winding connected to the 
self-excited converter 2 is called a secondary winding current. An 
exciting current of the transformer 3 can be obtained by computing a 
difference between the primary winding current of the transformer 3 
detected by the current detector 5A and the secondary winding current 
detected by the current detector 5B by the subtractor 6. The DC component 
contained in the exciting current, which will magnetize the iron core of 
the transformer 3, is obtained from the DC component detector 7 which is 
for detecting a DC component of the output signal of the subtractor 6. 
The DC component of the exciting current, thus detected, is applied to the 
adder 11. The adder adds together the DC component and a voltage command 
value that is generated by the voltage command value generating circuit 10 
in accordance with the output signals of the potential transformer 8 and 
the voltage reference circuit 9, and applied to the self-excited converter 
2. The resultant signal output from the adder is used as a signal 
representative of a voltage-command-value correction value. 
The PWM control circuit 12 forms a gate pulse signal in accordance with the 
output signal of the adder 11, and the gate pulse amplifying circuit 13 
processes the gate pulse signal from the adder to form a gate drive 
signal. The gate drive signal is applied to the self-excited converter 2. 
In response to the gate drive signal, the self-excited converter 2 
switches self-extinction elements contained therein in accordance with the 
output voltage of the DC voltage source 4, and produces a voltage 
corresponding to the output signal of the adder 11. 
As described above, in the prior art power converting apparatus shown in 
and described referring to FIG. 9, the self-excited converter 2 produces a 
voltage corresponding to the output signal of the adder 11. Therefore, 
when a DC component is contained in the voltage of the AC power system 1 
or the output voltage of the self-excited converter 2, the power 
converting apparatus operates in the following manner. That is, a DC 
component contained in the exciting current of the transformer 3 is 
detected, and applied to the adder 11. The self-excited converter 2 
produces a voltage, which cancels the DC component contained in the 
voltage of the AC power system 1 or the output voltage of the self-excited 
converter 2, whereby to eliminate the DC magnetization of the transformer 
3. 
The prior art power converting apparatus constructed as mentioned above 
detects an exciting current of the transformer 3, and causes the 
self-excited converter 2 to produce a voltage corresponding to the 
detected exciting current. Therefore, a DC component contained in the 
detected exciting current of the transformer 3 is proportional to an 
output signal of the self-excited converter 2 which is representative of a 
voltage-command-value correction value for suppressing the DC 
magnetization. 
A nonlinear correlation is generally present between an exciting current of 
the transformer 3 and a flux density of the iron core of the transformer 
3, as shown in FIG. 10. A linear relation, expressed by a first order 
integration as given by an equation (1), is present between a voltage 
applied to the transformer 3 and a flux density of the iron core of the 
transformer 3. Therefore, a voltage to be output by the self-excited 
converter 2 when it receives the DC component of the detected exciting 
current must correspond to the exciting current/flux density relationship 
shown in FIG. 10. 
##EQU1## 
The conventional power converting apparatus does not include means for 
compensating for the nonlinear relationship between the exciting current 
and the magnetic flux of the transformer 3. Therefore, it is impossible to 
coincide the voltage of the self-excited converter 2 with the voltage 
necessary for suppressing the DC magnetization over the entire range of 
the flux density. Particularly in a flux density region where the DC 
magnetization in the transformer is large and at a point near to its 
saturation point, the difference between those voltages is great. Under 
this condition, it is difficult to sufficiently suppress the DC 
magnetization in the transformer. 
The publication referred to above describes one of the solutions to the 
above problem. In the solution, a magnetic flux of the iron core of the 
transformer is directly detected by use of a Hall element. To this end, it 
is necessary to specially design and manufacture a transformer with the 
Hall element incorporated thereinto. To incorporate the Hall element into 
the transformer already assembled into the apparatus, it is necessary to 
alter the transformer. Sometimes, it is impossible to practically 
incorporate the Hall element into the transformer. If possible, its 
incorporation needs high cost and much time. As the size of the 
transformer becomes large, it is more difficult to incorporate the Hall 
element into the apparatus. 
SUMMARY OF THE INVENTION 
The present invention is made to solve the above problem, and has an object 
to provide a power converting apparatus which is easily realized and can 
suppress the DC magnetization in the transformer irrespective of a 
quantity of the DC magnetization. 
According to the present invention, a power converting apparatus for 
outputting a voltage in accordance with an output voltage command value, 
comprises: a power converter connected to a power line via a transformer; 
current detectors for detecting the currents of the windings of the 
transformer; an exciting current computing circuit for mathematically 
processing the output signals of the current detectors to produce an 
exciting current component of the transformer; a flux density computing 
circuit for mathematically processing an output signal of the exciting 
current computing circuit to produce a flux density of the transformer; a 
flux density contained DC component computing circuit for mathematically 
processing an output signal of the flux density computing circuit to 
produce a DC component contained in the output signal; and a voltage 
command correction value computing circuit for mathematically processing 
an output signal of the flux density contained DC component computing 
circuit and a flux density contained DC component command value to produce 
a voltage command value correction value; whereby the power converting 
apparatus produces a voltage dependent on the output voltage command value 
and the voltage command correction value. With such a construction, the 
nonlinear relationship between the exciting current and the flux density 
of the transformer is compensated for without directly detecting the 
magnetic flux. Therefore, a reliable suppressing of the DC magnetization 
in the transformer is secured. 
Furthermore, in the power converting apparatus, the flux density computing 
circuit includes a magnetic field computing circuit for mathematically 
processing an output signal of the exciting current computing circuit, to 
thereby produce a magnetic field developed from the transformer, and a 
magnetic field--flux density computing circuit for mathematically 
processing an output signal of the magnetic field computing circuit to 
produce a flux density of the transformer. Therefore, the mathematical 
processing of the exciting current output signal to produce a flux density 
is easy and reliable. 
Furthermore, in the power converting apparatus, the flux density computing 
circuit includes a memory table for storing in advance the correspondence 
between the exciting current and the flux density, and a table referring 
circuit for reading out a flux density specified by an output signal of 
the exciting current computing circuit from the memory table. Therefore, 
the mathematical processing of the exciting current output signal to 
produce a flux density is easy and reliable. 
Furthermore, according to the present invention, a power converting 
apparatus for outputting a voltage in accordance with an output voltage 
command value, comprises: a power converter connected to a power line via 
a transformer; voltage detectors for detecting the voltages of the 
windings of the transformer; a flux density contained DC component 
computing circuit for mathematically processing a difference between the 
output signals of the voltage detectors to produce a flux density 
contained DC component of the transformer; and a voltage command 
correction value computing circuit for mathematically processing an output 
signal of the flux density contained DC component computing circuit and a 
flux density contained DC component command value to produce a voltage 
command value correction value; whereby the power converting apparatus 
produces a voltage dependent on the output voltage command value and the 
voltage command correction value. With such a construction, the nonlinear 
relationship between the exciting current and the flux density of the 
transformer is compensated for without directly detecting the magnetic 
flux. Therefore, a reliable suppressing of the DC magnetization in the 
transformer is secured.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
First Embodiment 
A power converting apparatus according to an embodiment 1 of the present 
invention will be described with reference to FIG. 1 schematically showing 
the converting apparatus. 
Like or equivalent portions are designated by like reference numerals in 
FIG. 9 showing the conventional power converting apparatus, for 
simplicity. 
In FIG. 1: reference numeral 14 is a multiplier for multiplying the output 
signal of the current detector 5A by a coefficient that depends on a ratio 
of the number of turns of the primary winding of the transformer 3 and the 
number of turns of the secondary winding; 15 is an exciting current 
computing circuit including a subtractor 6A and the multiplier 14; 16 is a 
flux density computing circuit for mathematically processing an output 
signal of the subtractor 6A to produce a flux density of the transformer 
3; 17 is a flux density contained DC component command value setting 
circuit for producing a flux density contained DC component command value; 
6B is a subtractor 6B; and 18 is a voltage command value correction value 
computing circuit for computing a voltage command correction value. 
FIGS. 2 and 3 show other constructions of the exciting current computing 
circuit 15, which will subsequently be described in detail. FIGS. 4 nd 5 
show constructions of the flux density computing circuit 16. In FIG. 4, 
reference numeral 19 designates a magnetic field computing circuit for 
mathematically processing an exciting current to produce a magnetic field, 
and numeral 20 indicates a magnetic field.fwdarw.flux density computing 
circuit for computing a magnetic flux density using a magnetic field. In 
FIG. 5, numeral 21 represents a memory table for storing the 
correspondence between the exciting current and the flux density, and 
numeral 22 stands for a table referring circuit which refers to the memory 
table 21 and reads out a flux density specified by an input signal 
received thereby, from the memory table 21. 
The operation of the thus constructed power converting apparatus will be 
described. 
A primary winding current of the transformer 3 is detected by the current 
detector 5A, and a secondary winding current of the same is detected by 
the current detector 5B. The multiplier 14 multiplies an output signal of 
the current detector 5A by a coefficient 1, given by an equation (2), 
which is dependent on a ratio of the number of turns of the primary 
winding of the transformer 3 and the number of turns of the secondary 
winding. 
##EQU2## 
A difference between the output signal of the multiplier 14 and the output 
signal of the current detector 5B, produced by the subtractor 6A is an 
exciting current of the transformer 3. Exciting current detecting means 
including the multiplier 14 and the subtractor 6A is used as the exciting 
current computing circuit 15 of the present invention. 
When the ratio of the numbers of turns of the primary and secondary 
windings is 1, the coefficient 1 in the equation (2) is 1. Therefore, in 
this case, the multiplier 14 is omissible. The exciting current computing 
circuit 15 where the multiplier 14 is omitted may be depicted as shown in 
FIG. 2. 
In the exciting current computing circuit shown in FIG. 1, the multiplier 
14 is located between the current detector 5A and the subtractor 6A. As 
shown, the output signal of the current detector 5B may be multiplied by a 
coefficient 2 given by an equation (3), in the multiplier 14. 
##EQU3## 
The flux density computing circuit 16 mathematically processes an exciting 
current of the transformer 3 as an output signal of the subtractor 6A, to 
thereby produce a magnetic flux density of the iron core of the 
transformer 3. Examples of the constructions of the flux density computing 
circuit 16 are shown in FIGS. 4 and 5. 
In FIG. 4, a magnetic field computing circuit 19 mathematically processes 
the output signal of the subtractor 6A as the exciting current of the 
transformer 3 according to an equation (4), to thereby produce a magnetic 
field developed from the winding of the transformer 3. 
##EQU4## 
The magnetic field.fwdarw.flux density computing circuit 20 produces a flux 
density by mathematically processing a magnetic field developed from the 
windings of the transformer 3 that is computed by the magnetic field 
computing circuit 19. An example of the method of computing the flux 
density is known as described in "Theory of Ferromagnetic Hysterisis" in 
"Journal of Magnetism and Magnetic Materials" 61'86, pp 48 to 60. Hence, 
no description on it will be given here. 
In the flux density computing circuit 16 shown in FIG. 5, the 
correspondence between the exciting current and the flux density of the 
transformer 3 is obtained by an experiment or a theoretical computation, 
and is stored in a memory of the computer, and read out when necessary. 
In FIG. 5, the correspondence between the exciting current and the flux 
density is stored in advance in the memory table 21. The table referring 
circuit 22 reads a flux density specified by the exciting current of the 
transformer 3 as the output signal of the subtractor 6A from the memory 
table 21, and outputs it to the DC component detector 7. 
A DC component contained in the thus detected flux density of the iron core 
of the transformer 3 is detected by the DC component detector 7. The DC 
component detector 7 is a detector for extracting only a DC component from 
an AC signal containing the DC component. The detector may be constructed 
with a low-pass filter, an integrator, a moving average filter or the 
like. 
The output signal of the DC component detector 7 is a feedback value of the 
flux density contained DC component. A difference between it and the 
output signal of the flux density contained DC component command value 
setting circuit 17 is calculated by the subtractor 6B, and is applied to 
the adder by way of the voltage command correction value computing circuit 
18, whereby to form a feedback control system. Through the feedback 
control system, the flux density contained DC component of the transformer 
3 may be made to coincide with the flux density contained DC component 
command value. If the flux density contained DC component command value is 
set at 0, the voltage command correction value computing circuit 18 
outputs a voltage command correction value necessary for reducing the DC 
magnetization in the transformer 3 to zero. 
The nonlinear characteristic between the exciting current and the magnetic 
flux of the transformer 3 is compensated for by the flux density computing 
circuit 16. Accordingly, a relation between a flux density of the 
transformer 3 and a voltage for exciting the transformer 3 is expressed by 
a linear relation given by an equation (5). By using a linear feedback 
control system including the voltage command correction value computing 
circuit 18 constructed with a linear controller, such as a PID controller 
based on proportion, integration and differentiation, the DC magnetization 
in the transformer 3 can be sufficiently suppressed without any 
degradation of the control characteristic caused by the nonlinear exciting 
characteristic of the transformer 3. 
##EQU5## 
A voltage command correction value thus formed by the voltage command 
correction value computing circuit 18 is added, by the adder 11, to an 
output voltage command value to the self-excited converter 2, which is 
formed on the basis of the output signal of the potential transformer 8 
and the output signal of the voltage reference circuit 9 in the voltage 
command value generating circuit 10. The resultant signal is used as a 
voltage command correction value to the self-excited converter 2. 
In the embodiment 1, the combination are used for forming the voltage 
command value in the voltage command value generating circuit 10. A 
combination of a power reference and a power feedback value or a 
combination of a current reference and a current feedback value may be 
substituted for the above output voltage combination. 
In the power converting apparatus, the PWM control circuit 12 generates a 
gate pulse signal in accordance with an output signal of the adder 11, and 
the gate pulse amplifying circuit 13 generates a gate drive signal in 
response to the gate pulse signal. The gate drive signal is applied to the 
self-excited converter 2. The self-excited converter 2 switches the 
self-extinction element, such as gate-turn-off thyristors and transistors, 
to thereby produce a voltage corresponding to the output signal of the 
adder 11. 
The embodiment 1 uses the self-excited converter 2 for a power converter to 
produce a voltage in accordance with the output signal of the adder 11. 
The power converter may be any type of converter if it is able to produce 
a voltage in accordance with a voltage command. An example of such a power 
converter is a thyristor power converter. 
As described above, the conventional power converting apparatus shown in 
FIG. 9 does not include the flux density computing circuit 16 as an 
element for compensating for the nonlinear relation between the exciting 
current and the flux density of the transformer 3. Therefore, the 
conventional power converting apparatus cannot make the output voltage of 
the self-excited converter 2 coincident with the voltage necessary for 
suppressing the DC component over the entire range of flux density. 
Particularly in a flux density region where the DC magnetization in the 
transformer is large and at a point near to its saturation point, the 
difference between the voltages is great. Under this condition, it is 
difficult to sufficiently suppress the DC magnetization in the 
transformer. The power converting apparatus of the embodiment 1 shown in 
FIG. 1 mathematically processes an exciting current of the transformer 3 
in the flux density computing circuit 16 to produce a flux density of the 
iron core of the transformer, whereby the nonlinear relation between the 
exciting current and the flux density of the transformer 3 is compensated 
for. When a DC component is contained in the voltage of the AC power 
system 1 or the output voltage of the self-excited converter 2, the 
self-excited converter 2 appropriately produces a voltage necessary for 
canceling the DC component contained in that voltage, to thereby suppress 
the DC magnetization in the transformer 3. 
In the embodiment 1, since there is no need of directly detecting a 
magnetic flux of the iron core of the transformer 3, any alteration is not 
required for the transformer 3 of the hardware, and the construction of 
the DC magnetization suppressing circuit is simple. 
Second Embodiment 
FIG. 6 is a block diagram schematically showing an embodiment 2 of the 
present invention. In FIG. 6, reference numeral 23 designates a flux 
density contained DC component computing circuit for computing a flux 
density contained DC component of the transformer 3. 
The operation of the present embodiment will be described. The embodiment 2 
is different from the embodiment 1 in that the current detectors 5A and 5B 
are replaced with potential transformers 8A and 8B for computing a flux 
density of the iron core of the transformer 3. 
The potential transformer 8A detects a voltage of the AC power system 1, 
and another potential transformer 8B detects an output voltage of the 
self-excited converter 2. The multiplier 14 multiplies the output signal 
of the potential transformer 8A is multiplied by a coefficient 3, given by 
an equation (6), which dependent on a ratio of the number of turns of the 
primary winding of the transformer 3 and the number of turns of the 
secondary winding. 
##EQU6## 
The subtractor 6A arithmetically processes the output signal of the 
multiplier 14 and the output signal of the potential transformer 8B to 
produce a difference therebetween. The difference is a called impedance 
voltage of the transformer 3. The impedance voltage is a perfect AC 
component in a normal state of the transformer 3. When it is averaged, the 
result is 0. When the iron core of the transformer 3 is DC magnetized, the 
averaging of the impedance voltage produces a voltage, not zero, as an 
average value. The DC component detector 7 detects the average value, or a 
DC component. 
The flux density computing circuit 16 stores know various parameters, such 
as the number of turns of each winding of the transformer 3, % impedance, 
and the iron core cross sectional area. The circuit 16 integrates an 
output signal of the DC component detector 7 by using those parameters, to 
produce a flux density contained DC component of the transformer 3. 
The result of the computation by the flux density computing circuit 16 is 
input to the subtractor 6B. The subsequent operations of the embodiment 2 
is the same as of the embodiment 1, and hence description of it will be 
omitted. 
When the ratio of the numbers of turns of the primary and secondary 
windings is 1, the coefficient 3 is 1. Therefore, in this case, the 
multiplier 14 is omissible. The flux density contained DC component 
computing circuit 23 where the multiplier 14 is omitted may be depicted as 
shown in FIG. 7. 
In the circuit of FIG. 6, the multiplier 14 is inserted between the 
potential transformer 8A and the subtractor 6A. As shown in FIG. 8, the 
output signal of the potential transformer BE may be multiplied by a 
coefficient 4 given by an equation (7). 
##EQU7## 
In the FIG. 6 circuit, the DC component detector 7 arithmetically processes 
the output signal of the subtractor 6A to produce a DC component contained 
therein, and then the flux density computing circuit 16 integrates the 
resultant to produce a flux density contained DC component. The above 
computing procedural order may be reversed. That is, the output signal of 
the subtractor 6A is first integrated and then the DC component is 
computed to obtain the flux density contained DC component. 
In the second embodiment, the output signal of the potential transformer BA 
is arithmetically processed to produce a flux density contained DC 
component of the transformer 3, without not detecting the exciting current 
of the transformer, whereby a voltage command correction value can be 
obtained free from the adverse effect by the nonlinear relationship 
between the exciting current and the flux density of the transformer 3. 
Therefore, the DC magnetization in the transformer 3 can be suppressed 
properly. 
Any alteration is not required for the transformer 3 of the hardware since 
there is no need of directly detecting a magnetic flux of the iron of the 
transformer 3. Accordingly, the construction of the DC magnetization 
suppressing circuit is simple.