Patent Publication Number: US-2023141822-A1

Title: On-load tap changer and method for actuating an on-load tap changer

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a U.S. National Phase application under 35 U.S.C. § 371 of International Application No. PCT/EP2021/067691, filed on Jun. 28, 2021, and claims benefit to German Patent Application No. DE 10 2020 119 344.0, filed on Jul. 22, 2020. The International Application was published in German on Jan. 27, 2022 as WO 2022/017732 A1 under PCT Article 21(2). 
    
    
     FIELD 
     The invention relates to an on-load tap-changer for switching, without interruption, between winding taps of a tap-changing transformer under load. 
     BACKGROUND 
     The on-load tap-changer consists of a mechanical step selector for powerlessly pre-selecting the particular winding tap that is to be switched over to, and a diverter switch with semiconductor switching elements as switching means for actually switching, without interruption, from the previous to the pre-selected new winding tap under load. 
     On-load tap-changers of this kind are usually also referred to as hybrid tap changers because they also have mechanical contacts in addition to the power electronic switching means. 
     A hybrid tap changer of this kind is known from EP 2319058 B1. This has two load paths, which each connect a winding tap via a mechanical switch and a series circuit arranged in series thereto, formed of two oppositely switched IGBTs, to a common load take-off lead. A diode is provided parallel to each IGBT. A varistor is, in turn, provided parallel to each individual IGBT. In stationary operation, each of the load paths is bridged with a mechanical main contact. The IGBTs of both sides are controlled by a common IGBT driver. A disadvantage of this solution is that the tap changer does not have a monitoring function, such that the mechanical switching contacts are also then actuated only if the functionality of the semiconductor switching elements has been assured. If the IGBT on one side fails unnoticeably and the switch-over process is continued, this will result in a tap short circuit, which has serious, destructive consequences for the tap changer and the tap-changing transformer. 
     SUMMARY 
     In an embodiment, the present disclosure provides an on-load tap-changer for switching, without interruption, between winding taps of a tap-changing transformer, comprising a diverter switch for performing a switch-over from a first fixed contact to a second fixed contact of the on-load tap changer, a selector for preselecting, without power, the first fixed contact and the second fixed contact, and a first controller, wherein the diverter switch, for the switch-over, has a plurality of semiconductor switching elements and a plurality of mechanical switching elements, the selector has a first selector arm and a second selector arm, which are actuatable independently of one another and can contact each of the fixed contacts, and the first controller is configured to trigger a switch command and to actuate the first selector arm and the second selector arm and the plurality of mechanical switching elements by a motor drive, wherein the on-load tap changer comprises a second controller which is configured to actuate the plurality of semiconductor switching elements, and wherein during the switch-over the first controller actuates the motor drive depending on the second controller. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Subject matter of the present disclosure will be described in even greater detail below based on the exemplary figures. All features described and/or illustrated herein can be used alone or combined in different combinations. The features and advantages of various embodiments will become apparent by reading the following detailed description with reference to the attached drawings, which illustrate the following: 
         FIG.  1    shows a schematic representation of an exemplary embodiment of an on-load tap-changer; 
         FIG.  2    shows an exemplary schematic arrangement of an exemplary embodiment of an on-load tap-changer according to the improved concept in a tap-changing transformer; 
         FIG.  3    shows a schematic representation of an exemplary embodiment of an on-load tap-changer according to the improved concept; 
         FIGS.  4   a - 4   m    show an exemplary switching sequence of the on-load tap-changer from  FIG.  3   ; and 
         FIG.  5    shows an exemplary schematic arrangement of a further exemplary embodiment of an on-load tap-changer according to the improved concept in a tap-changing transformer. 
     
    
    
     DETAILED DESCRIPTION 
     In an embodiment, the present invention provides an improved concept for a hybrid tap changer, by means of which trouble-free and reliable operation of the hybrid on-load tap-changer is made possible. 
     The improved concept is based on the providing the semiconductor switching elements with their own control unit which cooperates with a further control unit which actuates the mechanical switching contacts by means of a motor drive in such a way that the mechanical switching contacts are actuated depending on the functionality of the semiconductor switching elements. 
     In accordance with a first aspect, an on-load tap-changer for switching, without interruption, between winding taps of a tap-changing transformer is described. The on-load tap-changer comprises a diverter switch for performing a switch-over from a first fixed contact to a second fixed contact of the on-load tap changer, a selector for powerlessly pre-selecting the fixed contacts before the actual switch-over under load, a first control unit and a second control unit. The diverter switch, for the switch-over, has a plurality of semiconductor switching elements and a plurality of mechanical switching elements. The selector has a first selector arm and a second selector arm, which are actuatable independently of one another and can contact each of the fixed contacts. Each fixed contact is electrically connected to a winding tap of the tap-changing transformer. The total number of fixed contacts is dependent on the number of winding taps. 
     The first control unit is configured to trigger a switch command and, depending thereon, to actuate the first selector arm, the second selector arm and the plurality of mechanical switching elements by means of a motor drive. The second control unit is configured to actuate the plurality of semiconductor switching elements. During a switch-over process of the on-load tap-changer, the first control unit actuates the motor drive depending on the second control unit. 
     It is thus ensured that the switch-over process in the on-load tap-changer and the actuation of the mechanical switching elements is then continued or performed only if the semiconductor switches have been correctly actuated and thus there is no risk of a tap short circuit. 
     The motor drive can be formed as a DC motor, as a brushless DC motor, or as a servomotor, in particular a torque motor. The motor drive is preferably formed as a stepper motor. 
     In accordance with at least one embodiment, the on-load tap-changer comprises a first sensor for measuring a first measurement value, which represents the voltage drop at a first semiconductor switching element and a second sensor for measuring a second measurement value which represents the voltage drop at a second semiconductor switching element. 
     The first sensor is configured to transmit the first measurement value to the second control unit. The second sensor is configured to transmit the second measurement value to the second control unit. The second control unit is, in turn, configured to transmit a status message to the first control unit depending on the first and/or the second measurement value. 
     In accordance with at least one embodiment, the second control unit is configured to transmit the status message “error” or the status message “OK”. The status message “error” represents that the switching on or off process of the semiconductor switching element was not successful, for example because the semiconductor switching element is defective. The status message “OK” represents that the switching on or off process of the semiconductor switching element was performed error-free. 
     The second control unit is configured to transmit the status message “error” to the first control unit when
         the first sensor transmits a measurement value that exceeds a previously determined first limit value within a predefined time,   the first sensor transmits a measurement value that does not exceed a previously determined second limit value within a predefined time,   the second sensor transmits a measurement value that does not exceed a previously determined third limit value within a predefined time,   the first and/or the second sensor transmits no measurement value within a predefined time.       

     Otherwise, the second control unit transmits the status message “OK”. 
     The first limit value lies preferably between 2 and 10 volts; the first limit value is particularly preferably 5 volts. 
     The second limit value lies preferably between 40 and 80 volts; the second limit value is particularly preferably 50 volts. 
     The third limit value lies preferably between 40 and 80 volts; the third limit value is particularly preferably 50 volts. 
     The second control unit is preferably formed as a microcontroller and is configured to detect and to assess the measurement values via analogue inputs and/or by means of comparators and to output the status messages depending thereon. 
     The first control unit is preferably likewise formed as a microcontroller. 
     The first and the second sensor are formed as voltage splitters with two ohmic resistors. 
     In accordance with at least one embodiment, the first control unit is configured to receive the status message from the second control unit and, depending thereon and depending on the time within the switch-over process at which the status message is received, either to return the motor drive into the starting position or to continue the switch-over. The latter means, specifically, that during the further course of the switch-over the mechanical switching elements of the diverter switch and the first and/or second selector contact are actuated by means of the motor drive for example via a common drive shaft. 
     The first control unit is preferably configured to return the motor drive to the starting position when
         the first sensor transmits to the second control unit a measurement value that exceeds the first limit value within a predefined time,   the first sensor transmits a measurement value to the second control unit that does not exceed the second limit value within a predefined time,   when the first sensor does not transmit a measurement value to the second control unit within a predefined time.       

     The second control unit is preferably also configured to actuate the motor drive and continue the switch-over when
         the second sensor transmits a measurement value to the second control unit that does not exceed the third limit value within a predefined time,   when the second sensor does not transmit a measurement value to the second control unit within a predefined time.       

     In accordance with at least one embodiment, the status message can be transmitted from the second control unit to the first control unit via a fiber-optic cable or wirelessly, for example via Bluetooth or radio. The fiber-optic cable can be molded in plastic, for example into the drive shaft, or can be formed separately, without sheathing. 
     In accordance with at least one further embodiment, the on-load tap-changer comprises a third sensor for measuring at least one third measurement value which represents the temporal course of the current at the semiconductor switching elements. The third sensor is formed as a current sensor, in particular as an alternating current sensor. 
     The third sensor is configured to transmit the third measurement value to the second control unit. The second control unit is in turn configured to switch off the semiconductor switching elements depending on the third measurement value. The expression “depending on the third measurement value” means here, specifically, depending on the temporal course of the current that flows via the semiconductor switching elements. The switch-off is performed preferably at the current zero crossing. 
     In accordance with at least one preferred embodiment, the diverter switch has a first main path, which connects the first selector arm via a first mechanical switching element to a load take-off lead, a second main path, which connects the second selector arm via a second mechanical switching element to a load take-off lead, and also a first auxiliary path with a first semiconductor switching element, which is formed parallel to the first main path, and a second auxiliary path with a second semiconductor switching element, which is formed parallel to the second main path. 
     The mechanical switching elements are preferably formed as main contacts. 
     In accordance with at least one embodiment, a voltage-dependent resistor is arranged parallel to the first and/or the second auxiliary path or parallel to the first and/or second semiconductor switching element. The voltage-dependent resistor is preferably formed as a varistor. 
     In accordance with at least one further embodiment, the on-load tap-changer is formed in such a way that, when performing the switch-over, during the actuation of the first selector arm and/or of the second selector arm, none of the semiconductor switching elements is activated. 
     In accordance with at least one further embodiment, the on-load tap-changer is formed in such a way that, when performing the switch-over, during the actuation of the semiconductor switching elements, the first selector arm and the second selector arm contact different fixed contacts. 
     In accordance with at least one embodiment, the second control unit has an energy accumulator which is charged when the first selector arm and the second selector arm contact different, adjacent fixed contacts. The charging is performed via the step voltage applied between the first selector arm and the second selector arm in the described position. The energy accumulator delivers the necessary energy for the actuation of the semiconductor switching elements and the transmission of the status messages from the second control unit to the first control unit. The second control unit and thus also the semiconductor switching elements are thus operated independently by means of the applied step voltage. An additional energy supply from outside, for example through the first control unit, is therefore unnecessary. 
     The energy store is preferably formed from ceramic capacitors and therefore has a higher temperature resistance. Since it is recharged continuously during the actuation of the second control unit and the semiconductor switching elements, it merely has to absorb any occurring load peaks. To charge the energy accumulator, a switching network part with an extremely wide input voltage range is preferably used, which still functions even at low step voltages. 
     The second control unit is preferably configured to monitor the charging of the energy accumulator via voltage measurement at one of the analogue inputs and to transmit a status message “OK” to the first control unit when the energy accumulator is fully charged. The first control unit is preferably configured to return the motor drive to the starting position when the status message is not received within a predefined time. 
     In accordance with at least one embodiment, the semiconductor switching elements are formed as IGBT switching elements and/or as thyristors and/or as JFET switching elements and/or as MOSFET switching elements and/or as Integrated Gate Commutated Thyristors (IGCT). The semiconductor switching elements are preferably formed in each case as an IGBT with diodes in a bridge circuit, particularly preferably with diodes in a Graetz circuit. 
     In accordance with at least one embodiment, the first control unit can be arranged above the motor drive in relation to a longitudinal axis L of the on-load tap-changer and the second control unit can be arranged below the diverter switch in relation to the longitudinal axis L of the on-load tap-changer. 
     The first control unit is preferably arranged outside a housing of the tap-changing transformer. The motor drive and/or the semiconductor switching elements and/or the second control unit can be arranged outside or inside the transformer housing. 
     In accordance with at least one further embodiment, the on-load tap-changer comprises, for a second and third phase to be controlled of the tap-changing transformer, additionally a second and third diverter switch, a second and third selector, and a second and third second control unit. The plurality of semiconductor switching elements of each diverter switch are each assigned to a second control unit. The first control unit is configured to trigger a switch command and to actuate the first selector arm and the second selector arm of each selector and the plurality of mechanical switching elements of each diverter switch by means of one motor drive. Each second control unit is configured to actuate the plurality of semiconductor switching elements of the diverter switch assigned to it. In this case, during the switch-over the first control unit actuates the motor drive depending on each second control unit. 
     In accordance with at least one further embodiment, the on-load tap-changer additionally comprises, for a second and third phase to be controlled of the tap-changing transformer, a second and third motor drive, a second and third diverter switch, a second and third selector, a second and third selector, and a second control unit. A selector, that is to say a first selector arm and a second selector arm, and a plurality of mechanical switching elements of the diverter switch for actuation is assigned to each motor drive. The assignment is made mechanically, for example via a drive shaft and a transmission. The plurality of semiconductor switching elements of each diverter switch are each assigned to a second control unit. The first control unit is configured to trigger a switch command and to actuate each motor drive, and thus also the assigned first selector arm and second selector arm and the assigned plurality of mechanical switching elements. Each second control unit is configured to actuate the plurality of semiconductor switching elements assigned to it. In this case, the first control unit actuates each motor drive during the switch-over depending on each second control unit. 
     In accordance with a second aspect of the improved concept, a method for actuating an on-load tap-changer which is formed in accordance with the first aspect of the improved concept is described. 
     With regard to the method, reference is made similarly to the above explanations, preferred features and/or advantages, as has already been explained in relation to the first aspect of the improved concept of one of the associated, advantageous embodiments. 
     The method comprises the steps of:
         generating a switch command to switch over from a first fixed contact to a second fixed contact of the on-load tap changer by means of a first control unit,   actuating one or more mechanical switching elements, a first selector arm, and a second selector arm by means of a motor drive and depending on the first control unit,   actuating one or more semiconductor switching elements by means of a second control unit,   wherein the motor drive is actuated by means of the first control unit during the switch-over depending on the second control unit.       

     In accordance with at least one embodiment, none of the semi-conductor switching elements is activated during the actuation of the first selector arm and/or of the second selector arm. 
     In accordance with at least one embodiment, the method comprises the further steps of:
         measuring at least one first measurement value, which represents the voltage drop at the first semiconductor switching element, and transmitting the first measurement value to the second control unit by means of a first sensor,   measuring at least one second measurement value, which represents the voltage drop at the second semiconductor switching element, and transmitting the second measurement value to the second control unit by means of a second sensor,   transmitting a status message to the first control unit depending on the first measurement value and/or the second measurement value by means of the second control unit,   actuating the motor drive by means of the first control unit depending on the status message.       

     In accordance with at least one further embodiment, the actuation of the mechanical switching elements, of the selector arms, and of the semiconductor switching elements after the generation of the switching command comprises the following steps of
         opening the second mechanical switching element and switching over the second selector arm to the second fixed contact by means of the motor drive,   charging the energy accumulator of the second control unit,   switching on the second semiconductor switching element by means of the second control unit,   opening a first mechanical switching element by means of the motor drive,   switching off the first semiconductor switching element by means of the second control unit,   switching on the second semiconductor switching element by means of the second control unit,   closing the second mechanical switching element by means of the motor drive,   switching off the second semiconductor switching element by means of the second control unit,   switching over the first selector arm from the first fixed contact to the second fixed contact,   closing the first mechanical switching element.       

     In accordance with at least one further embodiment, the first semiconductor switching element is disconnected depending on the temporal course of the current. The switch-off is performed preferably at the current zero crossing. 
     In accordance with at least one further embodiment, once the second semiconductor element has been switched on, the switch-over is continued in any case independently of the status message of the second control unit. 
     Further embodiments and implementations of the method are directly evident from the various embodiments of the tap-changer. In particular, individual components or a plurality of the components and/or assemblies described in relation to the tap-changer can be implemented to carry out the method accordingly. 
     In the following, the invention is explained in detail on the basis of exemplary embodiments with reference to the drawings. Components which are identical or functionally identical or which have an identical effect may be provided with identical reference signs. Identical components or components with an identical function are in some cases explained only in relation to the figure in which they first appear. The explanation is not necessarily repeated in the subsequent figures. 
     The figures merely illustrate exemplary embodiments of the invention without, however, limiting the invention to the illustrated exemplary embodiments. 
       FIG.  1    shows an exemplary embodiment of an on-load tap-changer  10  for a tap-changing transformer  20  in schematic representation. The tap-changing transformer  20  has a main winding  21  and a regulation winding  22  with different winding taps N 1 , . . . , N J , . . . , N N , which are connected and disconnected by the on-load tap-changer  10 . For this purpose, the on-load tap-changer  10  comprises a selector  11 , which can contact the winding taps N 1 , . . . , N J , . . . , N N  of the regulation winding  22  by means of two movable selector contacts, and a diverter switch  12 , which performs the actual diverter switch operation from the currently connected regulation winding to the new, pre-selected regulation winding. The load current flows from the currently connected winding tap N J  or N J+1  via the relevant selector contact and the diverter switch  40  to a load take-off lead  17 . 
       FIG.  2    shows an exemplary, schematic arrangement of an exemplary embodiment of an on-load tap-changer according to the improved concept in a tap-changing transformer. 
     The on-load tap-changer  10  has a selector  11  for powerlessly pre-selecting the fixed contacts, a diverter switch  12  for carrying out the actual load switch-over by means of a plurality of mechanical switching elements and semiconductor switching elements, a motor drive  13 , a first control unit  14 , and a second control unit  15 . In addition, the on-load tap-changer  10  has three sensors, which are arranged in the diverter switch  40 . The two sensors  51  and  52  are voltage sensors and are designed to transmit to the second control unit  15  the measurement values M 1  and M 2  that represent the voltage drop at the semiconductor switching elements. The third sensor  53  is a current sensor and is designed to transmit to the second control unit  15  the third measurement value M 3 , which represents the temporal course of the current, to the semiconductor switching elements. Furthermore, the second control unit  15  comprises an energy accumulator  18  which is arranged directly on the second control unit  15 . In this example the first control unit  14  is arranged above the motor drive  13  in relation to a longitudinal axis L of the on-load tap-changer  10  and outside the tap-changing transformer  20 . The remainder of the on-load tap-changer  10  is arranged within the tap-changing transformer  20 , wherein the second control unit  15  and the energy accumulator  18  are arranged below the diverter switch  40  in relation to the longitudinal axis L. 
       FIG.  3    shows a schematic representation of an exemplary embodiment of an on-load tap-changer according to the improved concept. 
     According to the improved concept, the on-load tap-changer  10  comprises at least one first fixed contact  11  and a second fixed contact  12 , which can each be connected to a winding tap of the regulation winding  22  of the tap-changing transformer  20 . The total number of fixed contacts is dependent on the number of winding taps. Each fixed contact  11 ,  12  has a first contact face and a second contact face. Furthermore, the on-load tap-changer  10  comprises a selector having a first selector arm  31  and a second selector arm  32 , which are actuatable independently of one another and can contact each of the fixed contacts. Here, the first movable contact  31  can contact the first contact faces of the fixed contacts  11 ,  12 , but not the second contact faces. Correspondingly, the second movable contact  32  can contact the second contact faces of the fixed contacts  11 ,  12 , but not the first contact faces.  FIG.  3    shows a schematic diagram of an exemplary embodiment of the on-load tap-changer; in particular, the arrangement of the contact faces opposite one another is not absolutely necessary. 
     The on-load tap-changer  10  furthermore comprises a diverter switch  40  for carrying out the actual diverter switch operation between the pre-selected fixed contacts  11 ,  12 . The diverter switch  40  has a total of four current paths. A first main path  41  connects the first selector arm  31  via a first mechanical switching element  43  to the load take-off lead  17 . A second main path  42 , which connects the second selector arm  32  via a second mechanical switching element  44  to the load take-off lead  17 . A first auxiliary path  45  with a first semiconductor switching element  47  is formed parallel to the first main path  41 , and a second auxiliary path  46  with a second semiconductor switching element  48  is arranged parallel to the second main path  42 . Furthermore, a varistor  49  is provided parallel to each of the first and the second auxiliary path  45 ,  46 . 
     The first sensor  51  formed as a voltage sensor is arranged parallel to the first mechanical switching element  43 . Accordingly, the second sensor  52  likewise formed as a voltage sensor is arranged parallel to the second mechanical switching element  44 . The third sensor  53  formed as a current sensor is arranged in the common take-off lead. 
     Two control units are provided for actuation of the on-load tap-changer  10 . A first control unit  14  is configured to trigger a switch command and to actuate the first selector arm  31 , the second selector arm  32  and the first and second mechanical switching element  43 ,  44  by means of the motor drive. A switching command is triggered in order to keep the primary voltage or the secondary voltage of the tap-changing transformer  20  in a predetermined voltage band. For this purpose, for example, a voltage regulator  50  is provided, which monitors whether the primary voltage is being kept within the predetermined voltage band. Furthermore, a second control unit  15  of the on-load tap-changer  10  is configured to actuate the first and the second semiconductor switching element  47 ,  48 . For this purpose, the second control unit  15  comprises an energy accumulator, which is charged via the voltage difference that occurs between the first selector arm  31  and the second selector arm  32  when these contact different, adjacent fixed contacts  11 ,  12 . The first control unit  14  receives from the second control unit  15  status messages S, depending on which it actuates the motor drive. 
     In the illustration in  FIG.  3   , the on-load tap-changer  10  is in a stationary position. The first and the second selector arm  31 ,  32  are both on the fixed contact  11 , such that the second control unit  15  is currentless and thus deactivates the semiconductor switching elements  45  and  46 . The load current IL flows in equal parts from the contacted fixed contact  11  via the two selector arms  31 ,  32 , the first and the second main path  41 ,  42 , and the closed mechanical switching elements  43  and  44  to the load take-off lead  17 . 
       FIGS.  4   a  to  4   m    show an exemplary switching sequence of the on-load tap-changer from  FIG.  3   . 
     Once the first control unit  14  has generated a switching command, the motor drive is actuated and thus firstly opens the second mechanical contact  44  ( FIG.  4   a   ). 
     The second selector arm  32  is then moved from the first fixed contact  11  to the second fixed contact  12  ( FIG.  4   b   ). 
     In  FIG.  4   c    the two selector arms  31 ,  32  are now on different fixed contacts  11 ,  12  and the motor drive  13  stops. The energy accumulator is now charged by the step voltage USP and thus supplies the second control unit  15  with energy for actuation of the semiconductor switching elements  45  and  46 . Following the charging of the energy accumulator, the second control unit  15  sends a status message S “OK” to the first control unit  14 . If this signal is not received within a predefined time, for example 50 ms, the first control unit  14  prompts the motor drive  13  to return to the starting position. 
     If the switch-over process is performed correctly, in the next step, shown in  FIG.  4   d   , the first semiconductor switching element  47  is switched on by the second control unit  15 . At this moment, no significant current flows via this first semiconductor switching element, since the through-resistance of the first semiconductor switching element  47  is much greater than that of the first mechanical switching element  43 . 
     At the same time, the first control unit  14  actuates the motor drive  13  again and the first mechanical switching contact  43  is then opened ( FIGS.  4   e  and  4   f   ). The motor drive  13  is then stopped again. 
     The steps shown in  FIGS.  4   d  to  4   f    are monitored by the second control unit  15  by means of the first voltage sensor  51 . The first voltage sensor  51  measures the voltage dropping at the first semiconductor switching element  47  and transmits this first measurement value M 1  to the second control unit  15 . If the load current flows across the first semiconductor switching element  47 , the voltage is only a few volts, for example at most 5 volts. In this case, the second control unit  16  transmits the status message S “OK” to the first control unit  14  and the switching process is continued correctly. If, however, the semiconductor switching element  47  is defective, an electric arc is created when the first mechanical switching contact  43  is opened. The voltage would then be many times greater, and for example may be 20 volts. In this case the second control unit  16  sends the status message S “error” to the first control unit  14 , whereupon the first control unit  14  prompts the motor drive  13  to return to the starting position. 
     If the switching process is continued correctly, in a next step ( FIG.  4   g   ) the temporal course of the current at the next semiconductor switching element  47  is monitored by the second control unit  15  by means of the current sensor  53 . The first semiconductor switching element  47  is switched off at the current zero crossing ( FIG.  4   g   ). 
     The switch-off process of the first semiconductor switching element  47  is monitored by the second control unit  15  by means of the first voltage sensor  51 . If the first semiconductor switching element  47  has switched off correctly, the load current then flows on via the varistors  49  arranged parallel to the semiconductor switching elements  47  and  48 , as shown in  FIG.  4   h   . The voltage drop at the first semiconductor switching element  47  thus rises sharply, more specifically to the forward voltage of the varistors, which is several hundreds of volts. The second control unit  15  monitors whether the voltage exceeds a defined threshold of, for example, 50 V within a defined time. If this is the case, the second control unit  15  transmits the status message S “OK” to the first control unit  14  and the switching process is continued correctly. Otherwise, if the voltage remains below the defined limit value, this is an indication of the failure of the switching off of first semiconductor switching element  47  and the second control unit  16  sends the status message S “error” to the first control unit  14 , whereupon the first control unit  14  prompts the motor drive  13  to return to the starting position. 
     If the switch-off process of the first semiconductor element  47  was successful, the second semiconductor switching element  48  is switched on immediately by the second control unit  15 . 
     This step is also monitored again by the second control unit  15  in that the voltage drop at the second semiconductor switching element  48  is measured by means of the second voltage sensor  52 . If the voltage drops to the forward voltage of the second semiconductor switching element  48  in the order of a few volts, the switch-on was successful and the load current flows via the second auxiliary path  46 , as shown in  FIG.  4   i   . The second control unit  15  monitors whether the voltage that drops at the second semiconductor switching element  48  drops below a defined threshold of, for example, 50 V within a defined time. If this is the case, the second control unit  15  transmits the status message S “OK” to the first control unit  14  and the switching process is continued correctly. If this is not the case, the second control unit  15  identifies an error and sends the status message S “error” to the first control unit  14 . From this moment in time, however, the switch-over process is no longer aborted, since the process of the diverter switch operation is already half complete and a return to the starting position would require a greater control effort. 
     The first control unit  14  thus prompts the motor drive  13  to continue in order to complete the switch-over. In this case, the second mechanical switching element  44  is firstly closed ( FIG.  4   i   ). 
     The second control unit  15  then switches the second semiconductor switching element  48  off ( FIG.  4   k   ). This can be performed, for example, on the basis of the detection of a reduction of the voltage drop at the second semiconductor switching element  48  as a result of the closure of the second mechanical switching element  44 . The time of switch-off, however, is not critical, since the switch-off occurs at the latest once the second control unit  15  is no longer supplied with voltage and the voltage of the energy accumulator has decreased. 
     In the next step, the first selector arm  31  is moved from the first fixed contact  11  to the second fixed contact  12  as a result of the further actuation of the motor drive  13  ( FIG.  4   l   ). 
     The voltage supply for the second control unit  14  is thus cancelled. Lastly, during the further course of the movement of the motor drive  13 , the first mechanical switching element  43  is also closed again ( FIG.  4   m   ). The switch-over process is thus ended. The on-load tap-changer  10  is then again in a stationary position, in which both selector arms  31 ,  32  are on the fixed contact  12 . 
     The switch-over process in the reverse direction is performed in a similar manner. 
       FIG.  5    shows an exemplary schematic arrangement of a further exemplary embodiment of an on-load tap-changer according to the improved concept in a tap-changing transformer. 
     In this embodiment, the on-load tap-changer  10  additionally comprises, for a second and third phase to be controlled of the tap-changing transformer  20 , a second and third motor drive  13 , a second and third diverter switch  40 , a second and third selector  30 , and a second and third second control unit  15  each with an energy accumulator  18 . A selector  40 , that is to say a first selector arm and a second selector arm, and a plurality of mechanical switching elements of the diverter switch  40  for actuation is assigned to each motor drive  13 . The plurality of semiconductor switching elements of each diverter switch  40  are each assigned to a second control unit  15 . For all three phases a central, first control unit  14  is provided, which is designed to trigger a switching command and to actuate each motor drive  13  depending on the particular second control unit  15  assigned to the corresponding phase. 
     It is assumed that the present disclosure and many of the attendant advantages thereof can be understood from the above description. Furthermore, it is clear that various changes can be made to the shape, construction and arrangement of the components without departing from the disclosed subject matter or without sacrificing all material advantages. The embodiment described is merely explanatory and such changes are intended to be covered by the following claims. Furthermore, it is understood that the invention is defined by the following claims. 
     While subject matter of the present disclosure has been illustrated and described in detail in the drawings and foregoing description, such illustration and description are to be considered illustrative or exemplary and not restrictive. Any statement made herein characterizing the invention is also to be considered illustrative or exemplary and not restrictive as the invention is defined by the claims. It will be understood that changes and modifications may be made, by those of ordinary skill in the art, within the scope of the following claims, which may include any combination of features from different embodiments described above. 
     The terms used in the claims should be construed to have the broadest reasonable interpretation consistent with the foregoing description. For example, the use of the article “a” or “the” in introducing an element should not be interpreted as being exclusive of a plurality of elements. Likewise, the recitation of “or” should be interpreted as being inclusive, such that the recitation of “A or B” is not exclusive of “A and B,” unless it is clear from the context or the foregoing description that only one of A and B is intended. Further, the recitation of “at least one of A, B and C” should be interpreted as one or more of a group of elements consisting of A, B and C, and should not be interpreted as requiring at least one of each of the listed elements A, B and C, regardless of whether A, B and C are related as categories or otherwise. Moreover, the recitation of “A, B and/or C” or “at least one of A, B or C” should be interpreted as including any singular entity from the listed elements, e.g., A, any subset from the listed elements, e.g., A and B, or the entire list of elements A, B and C. 
     REFERENCE SIGNS 
     
         
         
           
               10  on-load tap-changer 
               11  first fixed contact 
               12  second fixed contact 
               13  motor drive 
               14  first control unit 
               15  second control unit 
               16  first fixed contact 
               17  load take-off lead 
               18  energy accumulator 
               20  tap-changing transformer 
               21  main winding 
               22  regulation winding 
               30  selector 
               31  first selector arm 
               32  second selector arm 
               40  diverter switch 
               41  first main path 
               42  second main path 
               43  first mechanical switching element 
               44  second mechanical switching element 
               45  first auxiliary path 
               46  second auxiliary path 
               47  first semiconductor switching element 
               48  second semiconductor switching element 
               49  voltage-dependent resistor 
               50  voltage regulator 
               51  first sensor 
               52  second sensor 
               53  third sensor 
             (N 1 , . . . , N J , . . . , N N ) winding taps 
             S status messages 
             M 1  first measurement value 
             M 2  second measurement value 
             M 3  third measurement value 
             L longitudinal axis