FIELD OF THE INVENTION
The present invention concerns a method and a system for controlling the switching
time of a device including a magnetic circuit and 5 at least one conductive winding.
BACKGROUND OF THE INVENTION
When a power transformer on an electrical transmission or distribution network is re–
energized, it is known that transient over-currents can occur owing to a difference between
10 the residual flux values in each part of the magnetic circuit and the fluxes generated by the
voltages imposed at the terminals of each winding.
These over-currents rich in harmonics may, in some configurations of the network,
exhibit values much higher than the transformer’s permissible levels.
In addition, these over-currents may create major electro-dynamic forces at the
15 windings, leading to accelerate the degradation of the transformer (deformation, winding
displacement).
These problems of over-currents and over-voltages can also be encountered in other
electrical devices including a magnetic circuit and electric windings (start–up of electrical
machines).
20 As an illustration, attention is given below to the case of a single–phase transformer.
Before energizing, the flux Φ in the ferromagnetic material forming the magnetic circuit
has a value Φr called residual flux.
This residual flux is dependent on the de–energizing conditions of the transformer,
which are generally not controlled, on the type of magnetic circuit (e.g. its geometry) and on
25 the intrinsic parameters of its constituent material.
This residual flux is likely to develop over time, in particular on account of outside
stresses which may be exerted upon the de–energized transformer ( e.g. under the influence
of electrical devices in the vicinity of the transformer).
At the time of energizing at t=0, since the applied voltage is an alternating voltage, the
voltage at the terminals of the inductor winding can be written as: 2 cos  0 30 V  V
where:
V is the root mean square of the imposed voltage;
 is the angle representing the phase at the time of energizing.
V0 therefore has a value that is solely dependent on α.
35 To this value there is a corresponding flux Φ0 imposed within the magnetic circuit.
3
The operating equation is therefore the following:
where:
ω is the voltage pulse
R is the total resistance of the electric circuit 5 t including that of the inductor winding
n is the number of turns of the inductor winding
Φ is the mean flux within the magnetic circuit.
It is known that the expression of flux, with some approximations, is the following:
     

  

 t
r e
n
t V
n
t V 
 


 


 2 sin    2 sin
10 with:
 = L / R and L is the inductance of the inductive winding.
It is then possible to determine the current i(t) as a function of the curve B(H) of the
magnetic material of the circuit.
Optimal energizing of the transformer takes place at a given angle  such that the
15 transient flux (and hence transient current) i.e. the maximum current reached after energizing,
is as low as possible in order to protect the transformer.
For example, if Φr = 0 and =0 (i.e. energizing at maximum voltage and no residual
flux), then:
20 which means that there is no inrush current. Energizing is therefore optimal.
On the other hand, if Φr = Φr max and =3/2 (i.e. energizing at 0 voltage and maximum
residual flux), then:
    

 

 t
r e
n
t V
n
t V 
 


 


  2 cos   2
max
In this case the flux reaches very high values leading to high inrush current or causes
25 major temporary harmonic over-voltages on the network.
These two examples show the advantage of having knowledge of the value of residual
flux.
One known solution for evaluating residual flux is based on the fact that voltage is
homogeneous to flux derivation and therefore consists of evaluating residual flux by
30 integrating the voltage at the terminals of the transformer before it is de–energized.
Said method is described for example in document US 2010/0013470.
dt
V 2 cos( t ) R i(t) n d (t)

    
sin( t)
n
(t) V 2 

 
4
Documents DE 196 41 116 and DE 36 14 057 also disclose methods using data on the
state of the device before it is de–energized to estimate an optimal energizing time.
However, said indirect method for determining residual flux may, in some configurations
of the electric network supplying the transformer, prove to be scarcely precise and scarcely
robust since phenomena may have occurred which change the magnetic state 5 e of the
magnetic circuit, and imprecision in measurement of voltage - which is the input data for
calculating flux - makes this calculation little accurate (offset, drift, low voltage level, noisy
signal).
Additionally, a long time may elapse between the de–energizing and energizing of a
10 transformer, which requires the saving of data over a long period and regular measurement of
flux to verify changes thereof.
It is one objective of the invention therefore to allow more precise, simple and reliable
controlling of the switching time of a transformer or of any device comprising a magnetic
circuit and one or more conductive windings through which a current passes when in
15 operation, such as a rotating machine for example.
A further objective of the invention is to provide a simple and reliable method for
energizing a transformer under optimal conditions.
A further objective of the invention is to design a system for determining residual flux in
a magnetic circuit which provides better performance and is more precise than current
20 systems and is easy to implement.
BRIEF DESCRIPTION OF THE INVENTION
According to the invention there is proposed a method for controlling the switching time
of a device comprising a magnetic circuit and at least one conductive winding, characterized
25 in that it comprises the steps of:
- acquiring at least one measurement of the magnetic field generated by the residual
flux in the said magnetic circuit, by means of at least one magnetic field sensor
positioned in the vicinity of the magnetic circuit;
- processing the acquired magnetic field measurements to infer therefrom the
30 residual flux in the magnetic circuit;
- from the residual flux, determining the optimal switching time.
All these steps are performed after de–energizing the device and do not require any
knowledge or memorizing of the state of the device at the time of its de–energizing.
5
Said method advantageously comprises a prior step to calibrate the sensor whereby the
transfer function is determined between the value of the magnetic field measured by the
sensor and the value of residual flux in the magnetic circuit.
For this purpose, according to a first embodiment, at least one pair of sensors is placed
on the magnetic circuit 5 t symmetrically relative to the said magnetic circuit, which allows the
elimination by subtraction of the disturbing field component from the measurements acquired
by the sensors, and the said transfer function is determined in relation to the values of the
magnetic field measured by said pair of sensors and in relation to the relative permeability of
the constituent material of the magnetic circuit.
10 According to one variant of embodiment of the calibration, at least one pair of sensors is
placed in the vicinity of the magnetic circuit symmetrically relative to the said magnetic circuit,
which allows the elimination by subtraction of the disturbing field component from the
measurements acquired by the sensors; the calibration of the sensor then comprises a step
to determine the integral, over one current period, of the voltage at the terminals of the
15 winding when the current crosses zero, a step to determine induction when the current
crosses zero using the hysteresis curve of induction in the magnetic circuit as a function of
the intensity of the current circulating in the winding before de–energizing, and the
determining of the transfer function from the said steps.
According to one particular embodiment of the invention, the said device comprises an
20 enclosure surrounding the magnetic circuit and the winding, and at least one magnetic field
sensor is then placed on an outer surface of said enclosure.
The invention also concerns the application of the preceding method to the energizing
of a transformer whereby the transformer is energized at the optimal time determined by said
method for each of the power input phases.
25 According to one particular embodiment of the invention in which the said device
comprises several input phases, the above method is implemented to determine the value of
the residual flux in the magnetic circuit for each of the phases of said de–energized device,
and the optimal energizing time is calculated for the phase having the highest residual flux.
The invention also concerns the application of said method to the energizing of a three–
30 phase transformer, whereby the input phase is energized which has the highest residual flux
at the optimal time determined by the said method for the said input phase, after which the
other input phases are simultaneously energized at a time when the voltage induced by the
energizing of the first phase crosses a zero value.
A further objective of the invention concerns a system for controlling the switching time
35 of a device including a magnetic circuit and at least one conductive winding.
6
This system is noteworthy in that it comprises:
- at least one magnetic field sensor;
- a system for acquiring the magnetic field measurements made by said sensor;
- a system for processing the data acquired by the acquisition system, to calculate
the residual flux in the magnetic circuit and, from the residual flux, to 5 determine the
optimal switching time of the device.
Finally, the invention also concerns a transformer comprising a magnetic circuit, at least
one primary conductive winding and one secondary conductive winding, the said magnetic
circuit and the said conductive windings being surrounded by an enclosure, the said
10 transformer being provided, on the magnetic circuit and/or on or in the vicinity of an outer
surface of the enclosure, with at least magnetic field sensor belonging to a system such as
described above.
BRIEF DESCRIPTION OF THE DRAWINGS
15 Other characteristics and advantages of the invention will become apparent from the
following detailed description with reference to the appended drawings in which:
- Figure 1 is a schematic view of a single–phase transformer and of the magnetic
field sensors;
- Figure 2A illustrates a digital model of a portion of magnetic circuit and of an
20 enclosure surrounding said circuit;
- Figure 2B illustrates the different values of the magnetic induction generated by the
residual flux in the magnetic circuit shown Figure 2A, outside the enclosure;
- Figure 3 illustrates the induction measured on a path perpendicular to one surface
of the magnetic circuit in Figure 2A, with and without the enclosure;
25 - Figure 4 gives a curve of magnetic flux in the magnetic circuit as a function of the
intensity of the current circulating in the winding before de–energizing;
- Figure 5 gives a curve of induction in the magnetic circuit as a function of the
intensity of the current circulating in the winding before de–energizing.
30 DETAILED DESCRIPTION OF THE INVENTION
The device to which the method applies generally concerns a magnetic circuit, formed
of a ferromagnetic material and one or more conductive windings which may or may not
surround part of the magnetic circuit and through which a current is able to pass.
In addition, the magnetic circuit and the conductive winding(s) may be enclosed in an
35 enclosure which is typically a steel plate enclosure.
7
This enclosure is particularly intended to contain the oil required for cooling the
transformer.
The material of the enclosure does not form a barrier to the leakage field from the
magnetic circuit so that it is possible to measure magnetic flux on the outer surface of the
5 enclosure.
When applied to a transformer, one of the windings is connected to an alternating
electricity supply source, and one or more additional windings are connected to an electric
circuit to be supplied.
In this non–limiting example a description is given of a single–phase transformer,
10 however the method applies in its principle in similar manner to any other device such as a
three–phase transformer, rotating machine, etc.
Method for determining residual flux
In general the method is based on the determination of residual flux in the magnetic
circuit after de–energizing the device, by taking one or more measurements of the magnetic
15 field generated by this residual flux in the vicinity of the magnetic circuit.
Knowledge of residual flux then allows determination of the optimal switching time.
Figure 1 schematically illustrates a single–phase transformer.
This transformer comprises a magnetic circuit 1, a primary conductive winding 2
connected to an alternating electricity supply source and a secondary conductive winding 3
20 connected to an electric circuit to be supplied.
The magnetic circuit comprises a core in ferromagnetic material which in this Figure 1 is
shown to be homogeneous.
However, as is conventional, the magnetic circuit may have a laminated structure
formed of a plurality of parallel sheets isolated from each other.
25 According to one particular embodiment, the magnetic circuit 1 and the primary and
secondary windings 2 and 3 are surrounded by an enclosure 4 in the form of a steel plate
jacket illustrated here in part.
When the transformer is de–energized, the flux circulating in the magnetic circuit is
immobilized after a transient state and the result is a residual flux Φr in the magnetic circuit 1.
30 To determine the value of this residual flux, one or more magnetic field sensors are
placed in the vicinity of the magnetic circuit 1.
By ″ vicinity ″ in the present text is meant that the sensor is placed either on the
magnetic circuit itself or at a sufficiently short distance so that the measurement of the
magnetic field allows precise determination of the magnetic flux in the magnetic circuit.
8
As an example, it is considered here that the sensor is in the vicinity of the magnetic
circuit if it is positioned at a distance therefrom that is shorter than the longest side of the said
magnetic circuit.
There are two main magnetic field sources in a de–energized transformer: first the
residual flux and secondly a disturbing field (mainly due to the earth’s magnetic fiel5 d).
If fields other than the earth’s magnetic field form the disturbing field, then a system can
be used to reject these perturbations. Signal processing methods are known for separating
different magnetic field sources.
When a magnetic field is measured in the vicinity of the magnetic circuit, the measured
10 value is a result of these two sources.
If an enclosure surrounds the magnetic circuit, it is possible to place the sensor(s) on
the outer or inner surface of the enclosure or in the vicinity thereof, this enclosure not
cancelling the leakage field.
It has been shown, in unexpected manner, that the induction measured outside the
15 enclosure is sufficient to allow reliable measurement of the magnetic field generated by
residual flux.
Figures 2A to 3 show simulations performed on a model of a portion of a circuit and of
an enclosure surrounding said portion.
Figure 2A illustrates a digital model of a portion of a magnetic circuit 1 and of an
20 enclosure 4 surrounding said circuit.
Although only one quarter of the circuit is modelled, using Flux 2D software here, the
digital simulation concerns the whole device.
The magnetic circuit is modelled in the form of a set of magnets each thereof having a
specific direction of magnetization.
25 The magnets have a relative permeability of μr1 = 5000 and residual induction Br = 1 T.
The imperfections of the magnetic circuit are modelled by air gaps 1e of 0.5 mm located
in the corners of the magnetic circuit.
The arrows represent the orientation of the magnetic field lines in the magnetic circuit.
The metallic enclosure 4 has a relative permeability μr4 = 100 and thickness of 1 cm.
30 Figure 2B illustrates the different values of the magnetic induction generated by residual
flux in the magnetic circuit 1 thus modelled, outside the enclosure 4.
The scale from a to p corresponds to iso–induction zones having a value of between 0.5
μT and 50 μT.
As can be seen, the magnetic induction in the vicinity of the enclosure has sufficiently
35 high values to allow useful measurement by a magnetic field sensor placed in this zone.
9
This can be seen in Figure 3 which illustrates the magnetic induction B measured on a
path C1 (schematized in Figure 2B) perpendicular to one side of the magnetic circuit 1,
without the enclosure (dotted curve) and with the enclosure (solid line curve) as a function of
the distance d from the magnetic circuit.
Therefore, in the presence of the enclosure, 5 , the induction measured immediately
outside the enclosure is 20 μT, whilst the induction measured at the same point without an
enclosure is 34 μT.
As a result, even if the enclosure alters the value of the magnetic field generated by
residual flux, there is a trace outside the enclosure representing this magnetic field that can
10 be measured and correlated with the real value by means of adequate calibration.
This can be accounted for by the fact that the magnetic fields generated by the device
concerned by the invention are sufficiently high to generate a measurable trace outside the
enclosure.
In the case of the simulation described herein a residual induction of 1 T of the magnetic
15 circuit is represented by induction measured outside the enclosure of 20 μT.
Residual induction in the order of 1 mT would therefore be represented by induction
outside the enclosure in the order of a few nT, which is higher than the precision of the
sensors currently on the market and which is therefore measurable. Measurement using a
sensor placed outside the enclosure can therefore be representative of residual induction
20 provided that outside interferences can be ignored.
With reference to Figure 1, three magnetic field sensors 10a, 10b, 10c were placed on
three outer sides of the enclosure 4.
The preferred positioning and orientation conditions for the sensors are given further on.
Preferably the sensor(s) are positioned in the median plane of the magnetic circuit so
25 that they have the best possible sensitivity to the magnetic field lines around the circuit.
For the positioning thereof any suitable securing mode can be used (adhesive, etc.) for
securing onto the magnetic circuit and/or enclosure, even onto a support separate from the
magnetic circuit or enclosure.
By means of the sensor(s) one or more magnetic field values are obtained in the vicinity
30 of the magnetic circuit.
The said value or values are then processed to infer the value of the residual flux in the
magnetic circuit.
As will be seen below, prior calibration of each sensor allows the determining of the
transfer function between the value of the magnetic field measured by this sensor and the
35 value of the residual flux.
10
Controlling the switching time
Knowledge of residual flux then allows determination of the optimal switching time.
Returning to the example of a single–phase transformer, if the value of Φr is known, the
optimal energizing angle  to minimize current inrush is inferred from the preceding equations
using 5 the formula:
i.e.:
The advantage of this method is that it does not require knowledge of the conditions
10 under which the device was de–energized, which avoids having to archive the operating
conditions of the device.
In addition, this method is more direct than the prior art integration methods, and is
therefore more precise since it allows the overcoming of phenomena which may occur
leading to modification of the magnetic state of the magnetic circuit, and also avoids
15 inaccuracies of voltage measurement used as input data to calculate flux in prior art methods.
If the device is a three–phase transformer with three columns the residual flux is
measured in each column by placing at least one magnetic field sensor on the magnetic
circuit of each of the columns.
To energize the transformer, first the phase is energized for which the residual flux is
20 the highest, the switching time being determined so as to minimize inrush current.
The energizing of the first phase generates a voltage induced in the two other phases,
phase–shifted by 180° relative to the first phase.
After a time corresponding to a few half–periods of the said induced voltage (intended to
reduce the asymmetry of the magnetic flux in the two phases to be energized), these two
25 phases are simultaneously energized at a time corresponding to the zero–crossing time of
the induced voltage.
This method of energizing is described in the article by A. Mercier et al., ″ Transformer
Controlled Switching taking into account the Core Residual Flux - A real case study ″, CIGRE
13–201, 2002, to which reference can be made.
30 Evidently, any other triggering strategy can be chosen without departing from the scope
of the present invention.
r sin
n
V 2   

)
V 2
n
arcsin( r  
 
11
Calibration of the sensor
The measurements made by means of the sensors allow a qualitative evaluation of
residual flux, namely it presence and its direction.
For quantitative evaluation, i.e. to infer the value of the residual flux in the magnetic
circuit from the measured magnetic field values, it is necessary 5 sary beforehand to calibrate the
sensor to determine the transfer function between the measured magnetic field and the
corresponding residual flux.
To illustrate an example, the use is described below of two sensors placed close to a
single–phase transformer not provided with an enclosure.
10 As will be seen below, the advantage of using two sensors arranged symmetrically is
that it is possible to eliminate by subtraction the disturbing field component from the
measurements acquired by the said sensors.
A distinction is made between the case in which the sensors are positioned directly on
the magnetic circuit and the case in which the sensors are placed at a distance from the
15 magnetic circuit.
Case 1: sensors on the magnetic circuit
In this case, the tangential component of the magnetic field is measured, in the main
direction of induction in the magnetic circuit.
The sensor must therefore be oriented in relation to the magnetic circuit so that it can
20 measure this component.
This case is also valid when the sensor is placed sufficiently close to the magnetic
circuit so that it is possible to measure the tangential component of the magnetic field at the
circuit/air interface.
The induction measured by a sensor is defined by the formula:
sensor a B  A  B  A  B 25 1 0 2
where:
A1 is a magnitude depending both on the position of the sensor and on the induction in
the magnetic circuit;
B0 is the component of the disturbing field (chiefly the earth’s magnetic field) in the
30 direction of tangential induction;
Ba is the main component of induction in the magnetic circuit, tangential to the interface
between air and the magnetic circuit;
A2 = 1/μa by maintaining the tangential magnetic field at the circuit/air boundary
(Ampere’s theorem);
35 μa is the relative permeability of the constituent material of the magnetic circuit.
12
If two sensors are positioned symmetrically on the magnetic circuit, the component of
the disturbing field is modified in the same manner by the presence of the magnetic circuit.
The induction measurements taken by the two sensors are therefore written:
sensor a B  A  B  A  B 1 1 0 2
sensor a B  A  B  A  B 5 2 1 0 2
The subtraction of the measurements obtained by the two sensors allows the
elimination of the disturbing field component and this gives:
sensor sensor a B  B  A  B 1 2 2 2
The value of Ba can therefore be inferred from the measurements obtained with the two
10 sensors and from the relative permeability of the material of the magnetic circuit which is
known.
It is then possible to determine the value of the flux circulating in the magnetic circuit
which is given by the equation:
a a a   n  B  S
15 where n is the number of turns of the inductive winding and Sa is the cross–section of
the magnetic circuit.
Case 2: sensors at a distance from the magnetic circuit
When the sensor is not on the magnetic circuit, the induction measured by this sensor is
expressed by:
sensor a B  A  B  A  B 1 0 2 20
where the magnitudes A1, B0 and Ba have the same definitions as in the preceding case.
On the other hand, the coefficient A2 must be determined by the following steps:
1) Measuring the primary or secondary voltage (denoted V), magnetic induction
(denoted B) measured by a sensor and the primary current (denoted I) when the transformer
25 (no–load) is supplied with alternating voltage.
2) Plotting the curve φ(I) where φ is defined as the integral of voltage V as a function of
time over a current period (i.e. 20 ms at 50 Hz). This curve is illustrated in Figure 4. It is
obtained firstly by calculating, over a current period, the integral φ of voltage as a function of
time (which is of sinusoidal appearance phase–shifted by 90° relative to the voltage V) and
30 secondly by measuring the variation in current I over the same period, and from these two
series of data by plotting the hysteresis φ(I) curve. Said curve and the waveforms of voltage,
flux and current are illustrated on page 455 for example of the work: Electrotechnique - 3ème
édition, by Théodore Wildi, De Boeck Supérieur, 2003.
It is to be noted that here integration is performed over a current period with a view to
35 determining the transfer function of the sensor. This integration is therefore not affected by
13
drifts contrary to the prior art methods mentioned above which use integration of voltage over
a long period to infer residual flux therefrom.
Alternatively, the hysteresis curve φ(I) can be constructed from the saturation curve of
the transformer which is supplied by the manufacturer of the transformer. The said saturation
curve, plotted using a series of tests under no–load conditions, is a non–linear 5 curve of
voltage at the terminals of the primary winding as a function of the current. The corresponding
hysteresis curve φ(I) can be plotted using adapted simulation software such as the EMTP
software.
3) On the curve φ(I), measuring the magnitude Δφ which corresponds to the difference
10 between the minimum and maximum values of φ when the current crosses zero (i.e. I = 0).
4) Plotting the curve B(I) illustrated in Figure 5 from the measurements of magnetic
induction obtained by the sensor at different times, and from the measurements of the
primary current at the same times when the transformer (no–load) is fed with an alternating
supply.
15 5) On curve B(I), measuring the magnitude ΔB which corresponds to the difference
between the minimum and maximum values of B when the current crosses zero.
6) Calculating A2 using the following formula:
n Sa
A B
 


2 
where n is the number of turns of the inductive winding and Sa is the cross–section of
20 the magnetic circuit.
When two sensors are positioned symmetrically relative to the magnetic circuit and to
the disturbing field, the component of the disturbing field is modified in the same manner by
the presence of the magnetic circuit.
The induction measurements performed by the two sensors are therefore written:
sensor a B  A  B  A  B 25 1 1 0 2
sensor a B  A  B  A  B 2 1 0 2
Knowing A2, it is possible to infer Ba therefrom using the measurements taken by the
two sensors and by means of a subtraction which allows elimination of the disturbing field
component:
sensor sensor a B  B  A  B 1 2 2 30 2
It is then possible to determine the value of the flux circulating in the magnetic circuit
which is given by the equation:
a a a   n  B  S
14
Magnetic field sensors
The measurement system comprises at least one magnetic field sensor.
Sensors of this type exist on the market and persons skilled in the art are able to
choose a suitable sensor model.
Advantageously the sensors are vector magnetometers of ″ fluxgate ″ 5 type with one or
three axes, adapted for measuring the magnetic field component along the axe or axes under
consideration.
Magnetometers of this type are available from Bartington Instruments for example,
under the reference Mag–03.
10 If the sensor is a single–axis magnetometer, and it is desired to position this sensor
directly on the magnetic circuit, its axis is caused to lie parallel to the main direction of
induction in the magnetic circuit in order to measure the tangential component of the flux.
The sensors may also be scalar magnetometers, measuring the magnetic field module.
It is then necessary to orient these sensors in a direction parallel to the direction of the flux in
15 the magnetic circuit.
Depending on the geometry of the magnetic circuit, it may be expedient to use at least
two sensors arranged so as to facilitate or obtain better precision of residual flux
determination.
For example, it is possible to arrange two sensors symmetrically on each portion of the
20 magnetic circuit (in this example on each leg) of a single–phase or three–phase transformer
having three columns.
As mentioned above, symmetrical positioning of the sensors makes it possible to
overcome the influence of the earth’s magnetic field.
Since the latter induces magnetic flux oriented from top downwards in the magnetic
25 circuit, the positioning of the sensors so as to obtain components of opposite signs for
residual flux makes it possible to eliminate the effect of the disturbing field.
In addition, in particular in complex devices, the fact that the sensor is positioned
directly on the magnetic circuit makes it possible to ignore parasitic phenomena.
The number of sensors can also vary depending on the method used for calculating
30 residual flux.
In principle, a small number of sensors is sufficient, for example one per portion of the
magnetic circuit for a transformer.
However, it is also possible to determine residual flux from a plurality of point
measurements of the magnetic field according to the method described in document
35 WO 02/101405.
15
In this case, a plurality of magnetic field sensors must be placed around the magnetic
circuit.
Also, the magnetic field sensor or sensors can be placed directly on the magnetic
circuit.
However, said configuration 5 ation in some cases may be difficult to implement since the
immediate environment of the magnetic circuit may be unfavourable for the installing and
functioning of the sensors (e.g. presence of fluids, high temperature, etc.).
In this case, it is also possible to place the sensor(s) on or in the vicinity of the
enclosure surrounding the magnetic circuit.
10 When the distribution of the magnetic field around the circuit is known (for example by
means of theoretical or experimental mapping) the sensor(s) are placed at the points where
the magnetic field is the most intense.
In situations in which the distribution of the magnetic field is not known and/or the points
for positioning the sensors are limited, it must be endeavoured to place pairs of sensors
15 symmetrically relative to the disturbing field.
This solution away from the magnetic circuit has the advantage of not being intrusive
and able to be implemented on existing devices without requiring modification thereof.
The sensor or sensors then lie at a distance from the magnetic circuit that is shorter
than the dimension of the circuit, which allows sufficiently precise measurement of the
20 magnetic field.
The sensors can be installed permanently on the enclosure.
When applicable this makes it possible to take real–time measurements of the magnetic
state of the magnetic circuit when the device is energized, for diagnostic purposes.
A further possible application of the invention is the controlling of treatment of a
25 transformer to cancel or modify residual flux in the magnetic circuit by means of an external
source. The measurements performed by the sensors can then be used to verify whether or
not the treatment has effectively allowed the desired flux to be obtained in the magnetic
circuit.
The invention can also be implemented to control the start–up of a rotating machine,
30 allowing measurement of residual flux in a non–powered machine.
Acquisition system
The acquisition system is adapted to collect the data from the different sensors, and to
record and transmit this data to the processing system.
Processing system
16
The processing system is typically a processor provided with means which use the
signals acquired by the acquisition system to compute the value of residual flux in the
magnetic circuit.
In particular the processor can be adapted so that, from the signal associated with each
sensor, it can compute residual flux from the transfer function 5 of said sensor.
The acquisition and processing systems can be integrated in the device to be
controlled, e.g. for a transformer described above in a box secured to the outside of the
enclosure, or installed at a remote site.
The connection between the sensors and the acquisition and processing systems is
10 obtained by means of any suitable electrical connection.
Finally, the examples that have just been given are evidently only particular illustrations
and in no way limit the fields of application of the invention.

I/We Claim:
1. A method for controlling the switching time of a device including a magnetic circuit
(1) and at least one conductive winding (2), characterized in that it comprises the steps of:
- acquiring at least one measurement of the magnetic field generated by the residual
flux in the said magnetic circuit (1) using at least one magnetic field sensor 5 (10a,
10b, 10c) placed in the vicinity of the magnetic circuit (1);
- processing the acquired magnetic field measurements to infer therefrom the
residual flux in the magnetic circuit (1),
- from the residual flux, determining the optimal switching time for energizing the
10 device;
all said steps being performed after de–energizing the device.
2. The method according to claim 1, characterized in that it comprises a prior
calibration step of the sensor (10a, 10b, 10c) whereby the transfer function between the value
15 of the magnetic field measured by the sensor and the value of the residual flux in the
magnetic circuit (1) is determined.
3. The method according to claim 2, characterized in that at least one pair of sensors
is placed on the magnetic circuit symmetrically relative to said magnetic circuit, and in that the
20 transfer function is determined in relation to the values of the magnetic field measured by said
pair of sensors and in relation to the relative permeability of the constituent material of the
magnetic circuit.
4. The method according to claim 2, characterized in that at least one pair of sensors
25 is placed in the vicinity of the magnetic circuit symmetrically relative to said magnetic circuit,
and in that the calibration of the sensor comprises a step to determine the integral, over a
current period, of the voltage at the terminals of the winding when the current crosses zero,
and a determination step, on the hysteresis curve of induction in the magnetic circuit as a
function of intensity of the current circulating in the winding before de–energizing, to
30 determine induction when the current crosses zero, and determining of the transfer function
from said steps.
5. The method according to one of claims 1 to 4, characterized in that said device
comprises an enclosure (4) surrounding the magnetic circuit (1) and the winding (2), and in
18
that at least one magnetic field sensor (10a, 10b, 10c) is placed on an outer surface of the
said enclosure (4).
6. The method according to one of claims 1 to 5, wherein said device comprises
several power input phases, characterized in 5 that it comprises the implementing of the
method according to one of claims 1 to 5 to determine the value of the residual flux in the
magnetic circuit for each of the phases of said de–energized device, and to calculate the
optimal switching time for the phase having the highest residual flux.
10 7. Application of the method according to claim 6 to the energizing of a three–phase
transformer whereby the power input phase having the highest residual flux is energized at
the optimal time determined by the said method for the said input phase, then the other input
phases are simultaneously energized at a time when the voltage induced by energizing the
first phase crosses a zero value.
15
8. A system for controlling the switching time of a device including a magnetic circuit
(1) and at least one conductive winding (2), characterized in that it comprises:
- at least one magnetic field sensor (10a, 10b, 10c);
- a system for acquiring the magnetic field measurements performed by said sensor;
20 - a system for processing the data acquired by the acquisition system, to calculate
the residual flux(es) in the magnetic circuit and, from the residual flux, to determine
the optimal switching time.
9. A transformer comprising a magnetic circuit (1), at least one primary conductive
25 winding (2) and a secondary conductive winding (3), said magnetic circuit and said conductive
windings being surrounded by an enclosure (4) characterized in that, on the magnetic circuit
and/or on or in the vicinity of an outer surface of the enclosure (4), it comprises at least one
magnetic field sensor (10a, 10b, 10c) belonging to a system according to claim 8.