Patent Publication Number: US-9899161-B2

Title: Method and control system for controlling a switching device

Description:
BACKGROUND 
     The present invention relates to a method for controlling a switching device, in particular for synchronizing actuations of the switching device to a reference electrical signal; the present invention also relates to a control system adapted to carry out such method. 
     As known, a switching device is a device conceived for connecting/disconnecting two parts of an electrical circuit into which it is installed. 
     In particular, the switching device comprises one or more electrical phases, each one having at least one couple of contacts which can be switched between a closed condition, where the contacts are coupled to each other, and an open condition, where the contacts are separated from each other. 
     A control system can be provided for controlling the operations of the switching device, in such a way to synchronize the switching of the contacts to a reference waveform of an electrical signal associated to the electrical circuit into which the switching device itself is installed. 
     As known, the control system comprises control means which are adapted to operate by using a sequence of time cycles. The time cycles are set with a predetermined time duration. 
     The control means are adapted to control the actuation of the couple of contacts by using the time cycles with the predetermined time duration. The aim of this control is switching the contacts at a corresponding predetermined electrical angle of the reference waveform. 
     This predetermined electrical angle can be suitably chosen to avoid, or at least reduce, the generation of electrical arcs, inrush currents and transient voltages during the operation of the switching device. 
     However, the control means are adapted to execute the above mentioned control while assuming nominal values of relevant electrical and/or mechanical parameters which are associated to the phase and which could condition the desired synchronization of the contact switchings with the reference waveform. 
     If the real value of such electrical and/or mechanical parameters does not correspond to the presumed nominal value, the control means would fail to keep the desired synchronization as better illustrated with reference to an exemplary known switching device. 
     An exemplary known switching device comprises, for each electrical phase, two couples of contacts which are operatively associated to at least one semiconductor device. 
     The two couples of contacts must be switched in sequence at predetermined electrical angles of the reference waveform, in such a way to correctly use the semiconductor device for the switching tasks. 
     The two couples of contacts are realized by a common movable contact and two corresponding fixed contacts spatially separated from each other. 
     The movable contact can be actuated between a full-open position, where it is separated from both the first and second fixed contacts, and a closed position where it is coupled to the first fixed contact. The second fixed contact is disposed between the first fixed contact and the movable contact in the full-open position, so as to be connected with the movable contact during a portion of its travel path between the first and second fixed contacts. 
     An example of such switching device is disclosed in patent application EP2523203, filed in the name of the same applicant of the subject application. 
     The control means are set to control the actuation of the movable contact using the time cycles with the predetermined time duration, in such a way that:
         the coupling of the movable contact with the second fixed contact starts at a first predetermined point and the coupling of the movable to the first fixed contact occurs at a second, subsequent, predetermined point of the reference waveform;   the separation of the movable contact from the first fixed occurs at a third predetermined point and the separation of the movable contact from the second fixed contact occurs at a fourth, subsequent, predetermined of the reference waveform.       

     However, the control means are set to execute the above control while assuming a frequency value of the reference waveform equal to the frequency nominal value of the electric circuit. 
     In particular, the control means are adapted to apply a delay time between a detection of a predetermined reference point of the waveform and a predetermined starting point of the actuation of the movable contact. 
     This delay time is set according to the nominal frequency value and, hence, if the real frequency value does not correspond to such nominal value, the starting of the actuation of the movable contact will occur too early or too late with respect to the predetermined starting point. 
     More periods of the reference waveform the time delay comprises, and more the starting of the actuation will be far from the predetermined starting point. 
     In addition to such undesired effect, the control means are set to control the actuation of the movable contact while assuming a first preset time interval between the first and second predetermined points, and a second preset time interval between the third and fourth predetermined points of the reference waveform. 
     These first and second preset time intervals are based on the nominal frequency value. 
     Hence, a value difference between the real and nominal frequencies means a stretching or a reduction of the real time interval between the first and second predetermined points with respect to the first preset time interval, and a stretching or a reduction of the real time interval between the third and fourth predetermined points with respect to the second preset time interval. 
     This results in a controlled coupling between the movable contact and first fixed contact occurring too early or too late with respect to the second predetermined point, and in a controlled separation of the movable contact from the second fixed contact occurring too early or too late with respect to the fourth predetermined point of the reference waveform. 
     Furthermore, the control means are set to execute the control of the movable contact while assuming a distance between the first and second fixed contacts having a value corresponding to a nominal value devised in the design of the switching device. 
     However, the real value of such distance can vary in each single realized switching device with respect to the nominal designed value, due for example to mechanical tolerances. Since the control means work presuming the nominal distance value, a value difference between the real and nominal distances results in:
         a coupling of the movable contact with the first fixed contact occurring too early or too late with respect to the second predetermined point of the reference waveform; and   a separation of the movable contact from the second fixed contact occurring too early or too late with respect to the fourth predetermined point of the reference waveform.       

     Hence, all the above exemplary undesired effects combine each other resulting in a missed synchronization between the controlled actuation of the movable contact and the reference waveform. 
     SUMMARY 
     In light of above, at the current state of the art, although known solutions perform in a rather satisfying way, there is still reason and desire for further improvements. 
     Such desire is fulfilled by a method and apparatus as presented in the claims. 
     Another aspect of the present invention is to provide a switching device comprising a control system as defined by the annexed claims and disclosed in the following description. 
     Another aspect of the preset invention is to provide a switchgear comprising a control system and/or a switching device according the annexed claims and disclosed in the following description. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Further characteristics and advantages will become more apparent from the description of some preferred but not exclusive embodiments of the control system, control method and related switching device according to the invention, illustrated only by way of non-limiting examples with the aid of the accompanying drawings, wherein: 
         FIG. 1  is a perspective view of a switching device according to the present description; 
         FIGS. 2, 4 and 6  are section views of one electrical phase of the switching device illustrated in  FIG. 1 , with a movable contact illustrated in different positions; 
         FIGS. 3, 5 and 7  show an electrical scheme of the phase illustrated in  FIGS. 2, 4 and 6 , respectively; 
         FIG. 8  shows a block diagram which schematically illustrates a method according to the present invention; 
         FIG. 9  shows a block diagram which schematically illustrates a control system according to the present invention; 
         FIGS. 10-14  show waveforms and control profiles for illustrating exemplary applications of the control method according to the present invention. 
     
    
    
     DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS 
     It should be noted that in the detailed description that follows, identical or similar components, either from a structural and/or functional point of view, have the same reference numerals, regardless of whether they are shown in different embodiments of the present disclosure; it should also be noted that in order to clearly and concisely describe the present disclosure, the drawings may not necessarily be to scale and certain features of the disclosure may be shown in somewhat schematic form. 
     Further, when the term “adapted” or “arranged” or “configured” is used herein while referring to any component as a whole, or to any part of a component, or to a whole combinations of components, or even to any part of a combination of components, it has to be understood that it means and encompasses correspondingly either the structure, and/or configuration and/or form and/or positioning of the related component or part thereof, or combinations of components or part thereof, such term refers to. 
     With reference to  FIGS. 8 and 9 , the present disclosure is related to a method for controlling a switching device  1  and to a control system for carrying out such method; the control method and system are hereinafter globally indicated with numeral references  100  and  200 , respectively. 
     With reference to  FIGS. 1-7 , the method  100  is adapted to control a switching device  1  for connecting/disconnecting to/from each other two parts  5 ,  6  of an electric circuit into which the switching device  1  itself can be installed. 
     The switching device  1  has at least one phase  2  which comprises at least one couple of contacts  3 ,  4 . This at least one couple of contacts  3 ,  4  can be actuated for switching between a closed condition, where its contacts  10 - 12 ,  10 - 11  are coupled to each other, and an open condition, where its contacts  10 - 12 ,  10 - 11  are separated from each other. 
     For example,  FIGS. 2-4  show one phase  2  of the exemplary switching device  1 . 
     This phase  2  comprises terminals  20 ,  21  for connecting the phase  2  to a power supply  5  and to an associated load  6  of the electrical circuit. 
     Further, the phase  2  comprises:
         at least one semiconductor device  30  adapted to block a current flowing there-through in a first direction, and to allow a current flowing there-through in a second direction opposed to the first direction;   a first couple of contacts  3  which is adapted to cause, through its switching from the open condition to the closed condition, a connection in series of the at least one semiconductor device  30  between the electrical supply and load  5 ,  6 ; and   a second couple of contacts  4  which is adapted to short-circuit, through its switching from the open condition to the closed condition, the least one semiconductor device  30 .       

     In the exemplary embodiment illustrated in  FIGS. 2-4 , the two couples of contacts  3 ,  4  are realized by a common movable contact  10  and two corresponding fixed contacts  11 ,  12  which are spatially separated from each other by a distance X. 
     The movable contact  10  can be actuated, for example through a rotating motor  13 , between a full-open position (illustrated in  FIG. 2 ), where it is separated from both the fixed contacts  11  and  12 , and a closed position where it is coupled to the fixed contact  11  (as illustrated in  FIG. 6 ). 
     The second fixed contact  12  is disposed between the fixed contact  11  and the movable contact  10  in the full-open position, so as to be connected with the movable contact  10  during a travel path thereof between the fixed contacts  11  and  12 . 
     In practice, the actuation of the movable contact  10  between its full-open and closed positions corresponds to an actuation of the couples of contacts  3 ,  4 , resulting in sequential switchings of these couples  3 ,  4 . 
     For sake of simplicity, reference it will be made in the following description only to the controlled actuation of the couples of contacts  3 ,  4  in one phase  2 , since the disclosed control principles can be applied to the couples of contacts  3 ,  4  in the other phases  2 . 
     With reference to  FIG. 8 , the method  100  according to the present invention comprises the step  101  of providing control means  201  for controlling the actuation of the couples of contacts  3 ,  4  in the phases  2 . 
     As illustrated in  FIG. 9 , the control system  200  comprises such control means  201  which are adapted to operate using time cycles  300 ; in practice, the control means  201  are adapted to execute an operation at each time cycle  300 . 
     The time cycles  300  are initially set with a predetermined time duration T P , according to method step  102 . 
     The method  100  further comprises the step  103  of detecting a difference of a value of at least one parameter  150  associated to the phase  2  with respect to a preset value  500 . 
     In order to implement this step  103 , the control system  200  comprises means  202  for detecting the difference between the value of the parameter  150  and the preset value  500 . 
     The method  100  comprises a step  104 , that is:
         if the value of the parameter  150  corresponds to the preset value  500 , controlling the actuation of the at least one couple of contacts  3 ,  4  of the phase  2  through the control means  201  using the time cycles  300  with the predetermined time duration T P .       

     This controlling is such that the switching between the open and closed positions of the at least one couple of contacts  3 ,  4  is controlled to occur at a predetermined electrical angle  351 - 354  of a waveform  350  of an electrical signal associated to the phase  2 . 
     The control means  201  are adapted to execute such method step  104 . 
     If a difference between the value of the parameter  150  and the preset value  500  is detected by means  202 , the control means  201  are advantageously adapted to:
         modify the predetermined time duration T P  of the time cycles  300  according to the detected difference (method step  105 ); and   control the actuation of the at least one couple of contacts  3 ,  4  through the control means  201  using the time cycles  300  with the modified time duration T M  (method step  106 ).       

     The modification of the predetermined time duration T P  is such that the switching of the at least one couple of contacts  3 , 4  is controlled to occur at the same predetermined electrical angle  351 - 354  of the waveform  350  at which such switching is controlled to occur by method step  104 . 
     In practice, the control means  201  are set to control a predetermined synchronization between the switching of the at least one couple of contacts  3 ,  4  and the waveform  350 , by using time cycles  300  with the initially set time duration T P  and under the condition that the value of the parameter  150  corresponds to the preset value  500 . 
     A difference of the parameter  150  with respect to the preset value  500  can influence such predetermined synchronization; for example, the parameter  150  can be an electrical parameter of the waveform  350  or a mechanical parameter associated to the couple of contacts  3 ,  4 . 
     Advantageously, the control means  201  are adapted to modify the initially set time duration T P , of the time cycles  300  so as to keep the desired predetermined synchronization between the switching of the at least one couple of contacts  3 ,  4  and the waveform  350 , even if the actual value of the parameter  150  is not equal to the presumed preset value  500 . 
     Preferably, the method step  103  comprises the following steps  107  and  108 :
         measuring the value of the parameter  150 ; and   comparing the measured valued to the preset value  500 ;
 
and the method step  105  comprises:
   calculating a correcting factor using the preset value  500  and the measured value of the parameter  150  (method step  109 ); and   applying the correcting factor to the predetermined time duration T P  (method step  110 ).       

     According to method steps  107  and  108 , the detecting means  202  are adapted to receive a measure of or measure the value of the parameter  150 , and to compare such measured value to the present value  500 . The control means  201  are adapted to carry out the method steps  109  and  110 . 
     A preferred but not limited way of carrying out the method  100  and a corresponding preferred but not limited embodiment of the control system  200  are hereinafter illustrated by making reference to their application in controlling the exemplary phase  2  illustrated in  FIGS. 2-7 . 
     With reference to  FIGS. 10 and 11 , the control means  201  are adapted to execute the method step  104  or the method steps  105 - 106  for controlling an opening actuation of the movable contact  10  from the closed position to the full-open position, in such a way that:
         the movable contact  10  separates from the fixed contact  11  at a predetermined electrical angle  151  of the waveform  350  (opening switch of the couple of contacts  4 );   the movable contact  10  separates from the fixed contact  12  at a predetermined electrical angle  152  of the waveform  350 , subsequent to the predetermined electrical angel  151  (opening switch of the couple of contacts  3 ).       

     For example, as illustrated in  FIGS. 10 and 11 , the predetermined electrical angle  151  corresponds to a positive going zero-crossing  151  of the waveform  350  of a current flowing through the phase  2 . In this way, the current starts flowing through the at least one semiconductor device  30  at the separation of the movable contact  10  from the fixed contact  11 , without arc generations between the contacts  10  and  11  under separation. 
     The predetermined angle  152  corresponds to the following negative going zero-crossing  152  of the current waveform  350 . In this way, the separation of the movable contact  10  from the fixed contact  12  is advantageously controlled to occur when the at least one semiconductor device  30  starts blocking the current flowing there-through, hence avoiding arc generations between the contacts  10  and  12  under separation. 
     With reference to  FIGS. 12 and 13 , the control means  201  are also adapted to execute the method step  104  or the method steps  105 - 106  for controlling a closure actuation of the movable contact  10  from the full-open position to the closed position, in such a way that:
         the movable contact  10  starts contacting the fixed contact  12  at a predetermined electrical angle  153  of the waveform  350  (closure switch of the couple of contacts  3 );   the movable contact  10  starts contacting the fixed contact  11  at a predetermined electrical angle  154  of the waveform  350 , subsequent to the predetermined electrical angle  153  (closure switch of the couple of contacts  4 ).       

     For example, as illustrated in  FIGS. 12 and 13 , the predetermined electrical angle  153  corresponds to a negative peak instant  153  of the waveform  350  of a voltage signal associated to the phase  2 . In this way, when the voltage amplitude becomes positive the at least one semiconductor device  30  can start conducting the current flowing through the phase  2 , without arcs between the contacts  10  and  12  and without inrush effects. 
     The predetermined electrical angle  154  corresponds to the following positive peak instant  154  of the voltage waveform  350 ; in this way, the current of the phase  2  can start flowing through the coupled contacts  10  and  11  before that the at least one semiconductor device  30  blocks it. 
     According to method step  104 , if the detected value of the parameter  150  corresponds to the preset value  500 , the control means  102  are adapted to execute the above control of the opening or closure actuation of the movable contact  10  while keeping the initially set time duration T P  of the cycles  300 . 
     According to method steps  105  and  106 , if the detected value of the parameter  150  does not correspond to the preset value  500 , the control means  201  are adapted to execute the above control of the opening or closure actuation of the movable contact  10  by using the modified time durations T M  for the time cycles  300 . 
     In this way, the desired synchronization between the switchings of the couple of contacts  3 ,  4  and the corresponding predetermined electrical angles  151 - 154  is kept even if the effective value of the parameter  150  differs from the preset value  500 . 
     Preferably, both method steps  104  and  106  comprise a method step  111  of detecting a reference point  155  of the waveform  350 ; accordingly, the control system  200  comprises detecting means  203  adapted to detect the reference point  155 . 
     According to the examples illustrated in  FIGS. 10-14 , preferably the method steps  104  and  106  further comprise respectively:
         setting for the control means  201  a first predetermined number N 1 , N 3  of time cycles  300  with the predetermined time duration T P , starting from the detection of the reference point  155  (method step  112 );   setting for the control means  201  a second predetermined number N 2 , N 4  of time cycles  300  with the modified time duration T M  starting from the detection of the reference point  155  (method step  113 ).       

     According to method steps  112  and  113 , the control means  201  are adapted to:
         use the first predetermined number N 1 , N 3  of time cycles  300 , when the detected value of the parameter  150  is equal to the preset value  500 ; and   use the second predetermined number N 2 , N 4  of time cycles  300 , when the detected value of the parameter  150  is different with respect to the preset value  500 .       

     The first predetermined number N 1 , N 3  of time cycles  300  having the predetermined time duration T P  is equal to the second predetermined number N 2 , N 4 , of time cycles  300  having the modified time duration T M . 
     Preferably, the first predetermined number N 1 , N 3  of time cycles  300  comprises a predetermined number N 11 , N 31  of first time cycles  300  which are counted to define a delay time T D1 , T D3  between the detection of the reference point  155  and a starting of the actuation of the movable contact  10  between its full-open and closed positions. 
     Also the second predetermined number N 2 , N 4  of time cycles  300  comprises a predetermined number N 21 , N 41  of second time cycles  300  which are counted to define a modified time delay T D2 , T D4 , T D5  between the detection of the reference point  155  and a starting of the actuation of the movable contact  10  between its full-open and closed positions. 
     The first predetermined number N 1 , N 3  of time cycles  300  further comprises a predetermined number N 12 , N 32  of third time cycles  300  which defines a time duration T open1 , T close1  for the actuation of the movable contact  10  between its full-open and closed positions. 
     Also the second predetermined number N 2 , N 4  of time cycles  300  comprises a predetermined number N 22 , N 42  of fourth time cycles  300  which defined a modified time duration T open 2 , T open 3 , T close1  for the actuation of the movable contact  10  between its full-open and closed positions. 
     Preferably, the method steps  112  and  113  executed by the control means  201  comprise respectively:
         controlling, during each third time cycle  300 , the actuation of the movable contact  10  between its closed and full-open positions by using a closed-loop control; and   controlling, during each fourth time cycle  300 , the modified actuation of the movable contact  10  between its closed and full-open positions by using a closed-loop control.       

     For example, with reference to  FIGS. 10-14 , the control means  201  are adapted to cause the actuation of the movable contact  10  by controlling in a closed-loop way the angular position θ of the motor  13 . 
     To this aim, the control system  200  is adapted to use a sequence of set-point values θ′ for the angular positions θ to be assumed by the motor  13  during the actuation of the movable contact  10 . 
     The control algorithm carried out by the control means  201  comprises at least one closed-loop; at each third time cycle  300  and at each fourth time cycle  300 , the closed-loop is set to:
         receive a feed-back measurement related to the actual angular position θ of the motor  13 ;   compare it with a value related to a corresponding set-point angular position θ′, in order to calculate an error; and   generate an output control signal to the motor  13  basing on the calculated error, such as to minimize the error itself.       

     For example, the at least one parameter  150  under consideration at method step  103  can comprise the frequency of the reference waveform  350 . In this case, the corresponding preset frequency value f P  can be the value of the nominal frequency of the electrical circuit into which the switching device  1  is installed, e.g. 50 Hz or 60 Hz. 
       FIG. 10  is related to the controlled opening actuation of the movable contact  10  and it shows a waveform  350  of the current flowing into the phase  2 ; such current waveform  350  has a frequency value corresponding to the preset frequency value f P . 
     It is also assumed that the distance X between the fixed contacts  11 ,  12  of the phase  2  corresponds to a nominal value X N  which is devised in the design of the switching device  1 . 
     As a consequence, the control means  201  are adapted to execute method step  104  by:
         detecting the reference positive peak  155  of the current waveform  150  (method step  111 ); and   using the first predetermined number of time cycles N 1  with the predetermined initially set time duration T P  starting from the detection of the positive peak  155  (method step  112 ).       

     In particular, the control means  201  are adapted to firstly count the predetermined number N 11  of time cycles  300 , so as to define the time delay T D1  between the detection of the positive peak  155  and a starting of the controlled opening actuation of the movable contact  10 . 
     In practice, the duration of the time delay T D1  is initially set in the control means  102  as corresponding to the product T P ×N 11 . 
     Then, the control means  201  are adapted to use the subsequent predetermined number N 12  of time cycles  300  for executing the control of the opening actuation of the movable contact  10 . In practice, the time duration T open1  of the opening actuation of the movable contact  10  is initially set in the control means  102  as corresponding to the product T P ×N 12 . 
     At each time cycle  300  of the predetermined number N 12 , the control means  201  are adapted to use a corresponding set-point value θ′ associated to the opening actuation of the movable contact  10  carried out by the motor  13 . 
     The allocation of a set-point value θ′ to each corresponding time cycle  300  of the predetermined number N 12  results in the control profile  352  of the angular position θ illustrated in  FIG. 10 . For example, in  FIG. 10  there is illustrated how three first set-point values θ′ 1  θ′ 2 , θ′ 3  of the control profile  352  are used for the control tasks executed in corresponding three time cycles  300  of the predetermined number N 12 . The set-point values of the angular position θ at which the motor  13  causes a separation of the movable contact  10  from the fixed contact  11  and from the fixed contact  12  are indicated as θ′ S1  and θ′ S2 , respectively. 
     As illustrated in  FIG. 10 , the predetermined time duration T P , the number of time cycles N 11  and the number of time cycles N 21  are preset in the control means  102  in such a way that, if the actual frequency value of the current waveform  350  corresponds to the preset frequency value f P :
         the set-point value θ′ S1  is controlled to occur at the positive going zero-crossing  151  of the current waveform  350 ; and   the set-point value θ′ S2  is controlled to occur at the following negative going zero-crossing  152 .       

     If the actual frequency value of the current waveform  350  does not correspond to the preset frequency value f P , the control means  102  keeping these initial settings would fail to reach the desired synchronization between the separations of the movable contact  10  from the fixed contacts  11 ,  12  and the current waveform  350 . 
     In particular, under this frequency condition the desired synchronization would fail because:
         the zero crossing  151  occurs earlier or later with respect to the zero-crossing  151  in the current waveform  350  illustrated in  FIG. 10 , while the time delay T D1  remains unchanged; and   the time interval T I2  between the zero-crossings  151  and  152  is stretched or compressed with respect to the same interval T I1  in the current waveform  350  illustrated in  FIG. 10 , while the time duration T open1  of the control profile  352  remains unchanged.       

     For example,  FIG. 11  illustrates a waveform  350  of the current flowing into the phase  2 , where such current waveform  350  has a frequency value lower that the preset frequency value f P . 
     The difference between the actual frequency value and the preset frequency value f P  is detected by the detecting means  202  at method step  103 . 
     As a consequence of this detection, the control means  201  are advantageously adapted to stretch the predetermined time duration T P  of the time cycles  300  as a function of the detected frequency difference (method step  105 ). 
     For example, the control means  201  are adapted to:
         measure or receive a measurement of the actual frequency value of the waveform  350  (method step  107 );   calculate a frequency correcting factor K f  as a ratio between the preset frequency value f P  and the measured frequency value (method step  109 ); and   multiply the frequency correcting factor K f  to the predetermined time duration T P  (method step  110 ).       

     Further, the control means  201  are advantageously adapted to:
         detect the reference positive peak  155  of the current waveform  150  (method step  111 ); and   use the second predetermined number of time cycles N 2  with the stretched time duration T M  starting from the detection of the reference positive peak  155  (method step  113 ).       

     In particular, the control means  201  are adapted to firstly count the predetermined number of time cycles N 21 , so as to define the modified time delay T D2  between the detection of the reference point  155  and a starting of the controlled opening actuation of the movable contact  10 . Preferably, the number N 21  of time cycles  300  for setting the modified time delay T D2  is equal to the number N 11  of time cycles  300  for setting the preset delay time T D1 . 
     Then, the control means  201  are adapted to use the subsequent predetermined number N 22  of time cycles  300  for executing the control of the opening actuation of the movable contact  10 . 
     Preferably, the number N 22  of time cycles  300  is equal to the number N 12  of time cycles  300 . 
     At each time cycle  300  of the predetermined number N 22 , the control means  201  are adapted to use a corresponding set-point value θ′ associated to the opening actuation of the movable contact  10  carried out by the motor  13 . 
     The allocation of a set-point value θ′ to each corresponding time cycle  300  of the predetermined number N 22  results in the stretched control profile  352  of the angular position θ illustrated in  FIG. 11 . 
     In practice, the duration of the modified time delay T D2  is equal to the product T M ×N 21  and the modified control profile  352  has a time duration T open2  equal to the product T M ×N 22 . The stretched time duration T M  is such that:
         the set-point value θ′ S1  is controlled to occur at the positive going zero-crossing  151  of the current waveform  350  illustrated in  FIG. 11 , even if this point  151  occurs later with respect to the positive going zero-crossing  151  of the waveform  350  illustrated in  FIG. 10 ; and   the set-point value θ′ S2  is controlled to occur at the following negative going zero-crossing  152 ,       

     even if the time interval T I2  between the points  151  and  152  in the current waveform  350  illustrated in  FIG. 11  is longer than the time interval T 11  between such points  151 ,  152  in the current waveform  350  illustrated in  FIG. 10 . 
     The above first control condition can occur because the stretching of the time duration T M  results in a stretched delay time T D2  suitable for synchronizing the execution of the time cycle  300  for reaching the set-point value θ′ S1  to the actual positive going zero-crossing  151 . 
     The above second control condition can occur because the stretching of the time duration T M  results in the stretched the time interval T 12  between the control executions for reaching the set-point values θ′ S1  and θ′ S2 . In practice, the control profile  352  is slowed to synchronize the control executions for reaching the set-point values θ′ S1  and θ′ S2  to the corresponding actual positive going and subsequent negative going zero-crossings  151  and  152 . 
       FIG. 12  is related to the controlled closure actuation of the movable contact  10  and it illustrates a waveform  350  of a voltage associated to the phase  2 , e.g. a voltage of the circuit into which the switching device  1  itself is installed. 
     The illustrated voltage waveform  350  has a frequency value corresponding to the preset frequency value f P . 
     It is also assumed that the actual distance X between the fixed contacts  11  and  12  is equal to the nominal distance value X N . 
     As a consequence, the control means  201  are adapted to execute method step  104  by:
         detecting the reference negative going zero-crossing  155  of the voltage waveform  150  (method step  111 ); and   using the first predetermined number N 3  of time cycles  300  with the predetermined initially set time duration T P  starting from the detection of the reference point  155  (method step  112 ), in order to control the closure actuation of the movable contact  10 .       

     In particular, the control means  201  are adapted to firstly count the predetermined number of time cycles N 31 , so as to define the time delay T D3  between the detection of the reference point  155  and a starting of the controlled closure actuation of the movable contact  10 . 
     In practice, the duration of the time delay T D3  is initially set in the control means  102  as corresponding to the product T P ×N 31 . 
     Then, the control means  201  are adapted to use the subsequent predetermined number N 32  of time cycles  300  for executing the control of the closure actuation of the movable contact  10 . 
     In practice, the time duration T close1  of the closure actuation of the movable contact  10  is initially set in the control means  102  as corresponding to the product T P ×N 32 . 
     At each time cycle  300  of the predetermined number N 32 , the control means  201  are adapted to use a corresponding set-point value θ′ associated to the closure actuation of the movable contact  10  carried out by the motor  13 . 
     The allocation of a set-point value θ′ to each corresponding time cycle  300  of the predetermined number N 32  results in the control profile  353  of the angular position θ illustrated in  FIG. 12 . 
     The set-point values of the angular position θ at which the motor  13  causes a contacting between the movable contact  10  and the fixed contact  12  and a contacting between the movable contact  10  and the fixed contact  11  are indicated as θ′ S3  and θ′ S4 , respectively. 
     As illustrated in  FIG. 12 , the predetermined time duration T P , the number of time cycles N 31  and the number of time cycles N 32  are preset in the control means  102  in such a way that, if the actual frequency value of the voltage waveform  350  corresponds to the preset frequency value:
         the set-point value θ′ S3  is controlled to occur at the negative peak instant  153  of the voltage waveform  150 ; and   the set-point value θ′S 4  is controlled to occur at the following positive peak instant  154  of the voltage waveform  350 .       

     When the actual frequency value of the current waveform  350  does not correspond to the preset frequency value f P , the control means  202  keeping these initial settings would fail to reach the desired synchronization between the couplings of the movable contact  10  with the fixed contacts  11 ,  12  and the voltage waveform  350 . 
     In particular, under this frequency condition the desired synchronization would fail because:
         the negative peak instant  153  occurs earlier or later with respect to the negative peak instant  153  in the voltage waveform  350  illustrated in  FIG. 12 , while the time delay T D3  remains unchanged; and   the time interval T I4  between the negative and subsequent positive peak instants  153  and  154  is stretched or compressed with respect to the same interval T I3  of the voltage waveform  350  illustrated in  FIG. 12 , while the time duration T close1  of the control profile  352  remains unchanged.       

     For example,  FIG. 13  illustrates a voltage waveform  350  having a frequency value lower that the preset frequency value f P . 
     This frequency condition is detected by the detecting means  202  at method step  103 . 
     As a consequence of this detection, the control means  201  are advantageously adapted to:
         stretch the predetermined time duration T P  of the time cycles  300  according to the difference between the actual frequency value of the voltage waveform  350  and the preset frequency value f P  (method step  105 );   detect the reference negative going zero-crossing  155  of the voltage waveform  150  (method step  111 ); and   use the second predetermined number of time cycles N 4  with the stretched time duration T M  starting from the detection of the reference point  155  (method step  113 ).       

     In particular, the control means  201  are adapted to firstly count the predetermined number N 41  of time cycles  300 , so as to define the modified time delay T D4  between the detection of the reference point  155  and a starting of the controlled closure actuation of the movable contact  10 . 
     Then, the control means  201  are adapted to use the subsequent predetermined number N 42  of time cycles  300  for executing the control of the closure actuation of the movable contact  10 . 
     At each time cycle  300  of the predetermined number N 42 , the control means  201  are adapted to use a corresponding set-point value θ′ associated to the closure actuation of the movable contact  10  carried out by the motor  13 . 
     The allocation of a set-point value θ′ to each corresponding time cycle  300  of the predetermined number N 42  results in the stretched control profile  353  of the angular position θ illustrated in  FIG. 13 . 
     In practice, the duration of the modified time delay T D4  is equal to the product T M ×N 41  and the modified control profile  353  has a time duration T close2  equal to the product T M ×N 42 . The stretched time duration T M  is such that:
         the set-point value θ′ S3  is controlled to occur at the negative peak instant  153  of the voltage waveform  350  illustrated in  FIG. 13 , even if this instant  153  occurs later with respect to the negative peak instant  153  of the waveform  350  illustrated in  FIG. 12 ; and   the set-point value θ′ S4  is controlled to occur at the following positive peak instant  154  of the voltage waveform  350 , even if the time interval T I4  between the instants  153  and  154  in the voltage waveform  350  illustrated in  FIG. 13  is longer than the time interval T I3  between the instants  153 ,  154  in the voltage waveform  350  illustrated in  FIG. 12 .       

     The above first control condition can occur because the stretching of the time duration T M  results in the stretched delay time T D4  suitable for synchronizing the execution of the time cycle  300  for reaching the set-point value θ′ S3  to the actual negative peak instant  153 . 
     The above second control condition can occur because the stretching of the time duration T M  also results in a stretched time interval T I4  between the control executions for reaching the set-point values θ′ S3  and θ′ S4 . In practice, the control profile  353  is slowed to synchronize the control executions for reaching the set-point values θ′ S3  and θ′ S4  to the corresponding negative peak instant  153  and subsequent positive peak instant  154  of the voltage waveform  350 . 
     An example of how the control system  200  is adapted to execute the method  100  in case of a difference between the value of the actual distance X between the fixed contacts  11  and  12  and the nominal distance value X N  is disclosed below. 
     In particular, reference is made for simplicity only to a controlled opening actuation of the movable contact  10 , where it is assumed that the actual distance X is smaller than its nominal value and that the actual frequency value of the reference waveform  350  is equal to the preset frequency value f P . 
     As disclosed above, the control profile  352  illustrated in  FIG. 10  is executed by the control means  201  by using the predetermined number N 12  of time cycles  300  with the predetermined time duration T P . 
     The control profile  352  is used while presuming a correspondence between the actual distance X and the preset distance value X P . 
     Hence, according to these settings, the control means  201  would control the occurrence of the set-point value θ′ S2  at the corresponding negative going zero-crossing  152 , presuming that such controlled angular position θ′ S2  of the motor  13  is the right angular position θ for causing the separation of the movable contact  10  from the fixed contact  12 . 
     However, the separation of the movable contact  10  from the fixed contact  12  would already be occurred at the negative going zero-crossing  152 , because the actual distance X is smaller than the nominal distance value X N . 
     The detecting means  202  are adapted to detect the difference between the actual distance X and the its nominal X N . 
     For example, the detecting means  202  are adapted to:
         measure or receive a measurement of a time T lapse  lapsed between the separation of the movable contact  10  from the fixed contact  11  and the subsequent separation of the movable contact  10  from the fixed contact  12  (method step  107 ); and   compare the measured elapsed time T lapse  to a preset time interval T IP  (method step  108 ).       

     The lapsed time T lapse  is preferably measured during routing tests of the switching device  1 . 
       FIG. 14  shows the same current waveform  350  as illustrated in  FIG. 10 , i.e. with an actual frequency value corresponding to the preset frequency value f P . 
     When the measured elapsed time T lapse  is not equal to the preset time interval T IP , the control means  201  are advantageously adapted to stretch the predetermined time duration T D  of the time cycles  300  basing on the detected difference between the elapsed time T lapse  and the preset time interval T IP  (method step  105 ). 
     For example, the control means  201  are adapted to:
         calculate a mechanical correcting factor K M  as a ratio between the preset time interval T IP  and the measured elapsed time T lapse  (method step  109 ); and   multiply the mechanical correcting factor K M  to the predetermined time duration T P  (method step  110 ).       

     Whit reference to  FIG. 14 , the control means  201  are further adapted to:
         detect the reference positive peak  155  of the current waveform  150  (method step  111 ); and   use the second predetermined number N 2  of time cycles  300  with the stretched time duration T M  starting from the detection of the reference point  155  (method step  113 ).       

     In particular, the control means  201  are adapted to firstly count the predetermined number N 21  of time cycles  300 , so as to define the modified time delay T D5  between the detection of the reference point  155  and a starting of the controlled opening actuation of the movable contact  10 . Then, the control means  201  are adapted to use the subsequent predetermined number N 22  of time cycles  300  for executing the control of the opening actuation of the movable contact  10 . In particular, the control means  201  are adapted to use a corresponding set-point value θ′ associated to the opening actuation of the movable contact  10  at each time cycle  300  of the predetermined number N 22 . 
     This allocation of a set-point value θ′ to each corresponding time cycle  300  of the predetermined number N 22  results in the stretched control profile  327  illustrated in  FIG. 14 . 
     In practice, the duration of the modified time delay T D5  is equal to the product T M ×N 21  and the stretched control profile  327  has a time duration T open3  equal to the product T M ×N 22 . 
     Without stretching the predetermined time duration T P  of the cycles  300 , the real separation of the movable contact  10  from the fixed contact  12  would be controlled to occur earlier than the zero going reference point  152 , at an angular set-point position θ′ S6 . This is because the actual distance X between the fixed contacts  11  and  12  is smaller than the nominal distance X N . 
     The stretched time duration T M  is such that:
         a set-point value θ′ S5  is controlled to occur at the positive going zero-crossing  151  of the current waveform  350  instead of the set-point value θ′ S1 ; and   the set-point value θ′ S6  is controlled to occur at the following negative going zero-crossing  152  instead of the set-point value θ′ S2 .       

     In practice, the control profile  327  is stretched such that the set-point value θ′ S6  is correctly controlled at the negative going zero-crossing  152  instead of the set-point value θ′ S2 . 
     The above disclosed exemplary applications of the control method  100  and related control system  200  comprise the case of an actual frequency value of the waveform  350  not corresponding to the preset frequency value f P  or the case of an actual distance X between the fixed contacts  11 ,  12  not corresponding to the nominal distance X N . 
     In case that the means  202  detect both the above mentioned difference conditions, the control means  201  are adapted to execute the method steps  105  and  106  by modifying the preset time duration T P  of the time cycles  300  according to both the detected differences. 
     For example, if following routing tests on the switching device  1  it is detected that the value of the actual distance X between the fixed contacts  11 ,  12  does not correspond to the nominal distance value X N , the initially set predetermined time duration T P  of the time cycles  300  is modified by using the mechanical correcting factor K M . 
     When the difference between the value of the actual frequency of the reference waveform  350  and the preset frequency value f P  is further detected, the initially set predetermined time duration T P  is also modified by using the frequency correcting factor K f . 
     In practice, the modified time duration T M  of the time cycles  300  is equal to: T P ×K M  ×K f . 
     It has been seen how the control method  100  and control system  200  allow achieving the intended object offering some improvements over known solutions. 
     In particular, the method  100  and control system  200  allow to keep a desired synchronization between the switchings of the couple of contacts  3 ,  4  and a reference waveform  350 , even if at least one parameter  150  associated to the phase  2  and which can influence the synchronization does not correspond to a preset value  500 . 
     Indeed, the method  100  and control system  200  are adapted to modify the predetermined time duration T P  of the control cycles  300  according to the detected difference between the actual value of the parameter  150  and the preset value  500 . In this way, the control speed is suitably slowed or accelerated for keeping the desired synchronization. 
     For example, it has been seen how the execution of the control method  100  by the control system  200  keeps the desired synchronization even if the actual frequency value of the reference waveform  350  is not equal to the present frequency value f P . 
     In practice, the control speed is dynamically changed according to the variation of the actual frequency value of the reference waveform  350  with respect to the preset frequency value f P , for example by modifying the predetermined time duration T P  of the cycles  300  with the correcting frequency factor K f . 
     For example, it has been seen how the execution of the control method  100  by the control system  200  keeps the desired synchronization even if the actual distance X between the fixed contacts  11  and  12  is not equal to the nominal distance value X N . 
     In practice, following routine tests of the switching device  1 , the control speed is set according to the detected difference between the actual distance X and its nominal value X N , for example by modifying the predetermined time duration T P  of the cycles  300  with the correcting factor K M . 
     The control method  100  and control system  200  thus conceived are also susceptible of modifications and variations, all of which are within the scope of the inventive concept as defined in particular by the appended claims. 
     In particular, the control method  100  can be applied to switching devices of a different type than the switching device  1  illustrated in  FIGS. 1-7 . 
     For example, the method  100  can be applied to a circuit breaker having for each phase one couple of contacts. In this case, the execution of the method  100  would be useful at least for keeping a desired synchronization between an opening switching of this couple of contacts and a predetermined electrical angle of a reference signal waveform associated to the phase, even if the actual frequency value of the reference waveform is not equal to the nominal preset value. 
     The control means  201  may comprise: microcontrollers, microcomputers, minicomputers, digital signal processors (DSPs), optical computers, complex instruction set computers, application specific integrated circuits, a reduced instruction set computers, analog computers, digital computers, solid-state computers, single-board computers, or a combination of any of these. 
     The detecting means  202  can be any electronic device or unit adapted to measure or receive a measurement of the actual value of the parameter  150 , and to compare it with the preset value  500 ; the detecting means  202  can be separated but operatively connected to the control means  201 , or they can be implemented into the control means  201  themselves. 
     The detecting means  203  can be any electronic device or unit adapted to detect the occurrence of the reference pint  155  of the waveform  350 , the detecting means  203  can separated but operatively connected to the control means  201 , or they can be implemented into control means  201 . 
     In practice, all parts/components can be replaced with other technically equivalent elements; in practice, the type of materials, and the dimensions, can be any according to needs and to the state of the art.