Patent Publication Number: US-2021165433-A1

Title: Current control apparatus

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a continuation of U.S. patent application Ser. No. 16/531,440, filed Aug. 5, 2019 and titled CURRENT CONTROL APPARATUS, which claims the benefit of U.S. Provisional Application No. 62/719,974, filed on Aug. 20, 2018 and titled CURRENT CONTROL APPARATUS, both of which are incorporated herein by reference in their entirety. 
    
    
     TECHNICAL FIELD 
     This disclosure relates to a current control apparatus. The current control apparatus may be used to control current in a winding of a device used for voltage regulation. 
     BACKGROUND 
     Voltage regulators are used to monitor and control a voltage level in an electrical power distribution network. A voltage regulator includes a main winding and an electromagnetic circuit that delivers current from the main winding to an electric load. The electromagnetic circuit includes electrical contacts, and the main winding includes a plurality of taps. The output voltage of the voltage regulator is determined by which of the plurality of taps are in contact with the electrical contacts. 
     SUMMARY 
     In one general aspect, an apparatus for a load tap changer includes a first primary winding electrically connected to a first contact, the first contact configured to connect to one of a plurality of taps in a load tap changer; a second contact, the second contact configured to connect to one of the plurality of taps in the load tap changer; a magnetic core; and a control circuit including: a secondary winding configured to magnetically couple to the first primary winding and the magnetic core; and an electrical network electrically connected to the secondary winding, the electrical network being configured to prevent magnetic saturation of the magnetic core during switching of the first or second contact. 
     Implementations may include one or more of the following features. The electrical network may prevent magnetic saturation of the magnetic core by reducing the absolute value of magnetic flux in the magnetic core. The absolute value of magnetic flux in the magnetic core may be reduced by allowing the flow of electrical current in the secondary winding. The electrical network may be powered by an alternating current (AC) power source. The AC power source may include a third winding that is electrically connected to the first primary winding. The electrical network may prevent magnetic saturation of the magnetic core by increasing or decreasing electrical current in the secondary winding to increase or decrease the magnetic flux in the magnetic core. The electrical network may include a direct current (DC) bus, and electrical power to increase or decrease electrical current in the secondary winding is provided by the direct current (DC) bus. The direct current (DC) bus also may be coupled to an alternating current (AC) power system to compensate reactive power. The alternating current (AC) power system may be a multi-phase system. The electrical network may include a power source, and the power source may be controllable to increase or decrease electrical current in the secondary winding. The power source may be powered from a voltage transformer. The power source may be powered from a current transformer. The increase or decrease of magnetic flux in the magnetic core may cause a circulating current to flow in a short circuit, the short circuit being formed by the first contact, the second contact, and the primary winding. The circulating current may be equal in amplitude and opposite in phase to a load current carried by the first contact or the second contact. The load tap changer may receive power from an alternating current (AC) power distribution network that operates at a system frequency, and causing the circulating current to flow in the short circuit may result in the net current through the first contact or the second contact being equal to zero more frequently than the system frequency. Causing the circulating current to flow in the short circuit may reduce the root-mean-square of the net current through the first contact or the second contact. 
     In some implementations, the apparatus for the load tap changer also includes: a second primary winding connected to the second contact; a second magnetic core; and a second secondary winding magnetically coupled to the second magnetic core and second primary winding. In these implementations, the electrical network is also connected to the second secondary winding and configured to control the current in the first secondary winding and second secondary winding. Further, the electrical network may control the current in the first primary winding and the second primary winding by controlling the current in the first secondary winding and the second secondary winding. The current through the first contact may be zero while switching taps. The output voltage to a connected load may be controlled by an electrical network connected to the first primary winding and the second primary winding. 
     In another general aspect, an apparatus for controlling voltage output of a transformer includes a first current path including a first primary winding electrically connected to a winding tap; a second current path including a second primary winding electrically connected to a winding tap; and an electrical network magnetically coupled to the first primary winding and second primary winding, the electrical network being configured to control current in the first and the second primary windings. 
     Implementations may include one or more of the following features. The transformer may be a multi-phase transformer. The electrical network may include a first switch; 
     a second switch; and a bypass switch connected between the first switch and the second switch. A direct current (DC) bus may be coupled to the transformer to compensate reactive power from the alternating current (AC) power system. 
     Implementations of any of the techniques described herein may include a voltage regulator, a load tap changer, an apparatus, a current control apparatus, a kit for retrofitting an existing voltage regulator with a current control apparatus, a controller for controlling a voltage regulator, or a process. The details of one or more implementations are set forth in the accompanying drawings and the description below. Other features will be apparent from the description and drawings, and from the claims. 
    
    
     
       DRAWING DESCRIPTION 
         FIG. 1A  is a block diagram of an example of an alternating-current (AC) electrical power system. 
         FIGS. 1B and 1C  are block diagrams of an example of a core. 
         FIG. 2  is a block diagram of an example of a voltage regulator. 
         FIGS. 3A-3E  are block diagrams of another example of a voltage regulator. 
         FIG. 4  is a block diagram of another example of a voltage regulator. 
         FIGS. 5 and 6  are examples of simulated data. 
         FIG. 7A  is a block diagram of another example of a voltage regulator. 
         FIG. 7B  is a block diagram of an example of an electrical network. 
         FIG. 7C  is a block diagram of another example of an electrical network. 
         FIG. 7D  is a block diagram of another example of a voltage regulator. 
         FIG. 8A  is a block diagram of another example of a voltage regulator. 
         FIG. 8B  is a block diagram of an another example of an electrical network. 
         FIG. 9A  is a block diagram of another example of a voltage regulator. 
         FIG. 9B  is a block diagram of an example of a current control apparatus. 
         FIGS. 10A, 10B, and 10E  are block diagrams of other examples of a voltage regulator. 
         FIGS. 10C and 10D  are block diagrams of examples of electrical networks. 
         FIGS. 11 and 12  are block diagram of other examples of a current control apparatus. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1A  is a block diagram of an example of an alternating-current (AC) electrical power system  100 . The electrical power system  100  includes an electrical power distribution network  101  that transfers electricity from a power source  102  to electrical loads  103  through a distribution path  104  and an electrical apparatus  110 . The electrical apparatus  110  is any apparatus that is capable of regulating the voltage to the loads  103 . For example, the electrical apparatus  110  may be a voltage regulator that includes a load tap changer. The electrical power distribution network  101  may be, for example, an electrical grid, an electrical system, or a multi-phase electrical network that provides electricity to commercial and/or residential customers. The electrical power distribution network  101  may have an operating voltage of, for example, at least 1 kilovolt (kV), 12 kV, up to 34.5 kV, up to 38 kV, or 69 kV or higher, and may operate at a system frequency of, for example, 50-60 Hertz (Hz). The distribution path  104  may include, for example, one or more transmission lines, electrical cables, and/or any other mechanism for transmitting electricity. 
     The electrical apparatus  110  includes an electromagnetic circuit  120  and a current control circuit  150 , which controls a current in the electromagnetic circuit  120 . The electromagnetic circuit  120  includes a winding  121 . The winding  121  is an electrical conductor. For example, the winding  121  may be a cable or wire made of an electrically conductive material, such as a metal. Referring also to  FIG. 1B , the winding  121  is wrapped in, for example, a coil or helical shape having a central region  122 . 
     In the example shown in  FIG. 1B , the winding  121  is wrapped around a magnetic core  123  that is in the central region  122 . The magnetic core  123  is made of a ferromagnetic material, such as, for example, iron or steel. In the example of  FIG. 1B , the magnetic core is shown as a rod. However, the magnetic core  123  may have any shape. For example, the magnetic core  123  may be a ring, a square or rectangular shaped annulus, or any other structure that has a ferromagnetic region suitable for attaching a winding. The magnetic core  123  may be a gapped core or an un-gapped core. In implementations in which the core  123  is an un-gapped core, the core  123  is a contiguous segment of ferromagnetic material. A gapped core includes a gap that is not ferromagnetic material. The gap may be, for example, air, nylon, or any other material that is not ferromagnetic. Thus, in implementations in which the core  123  is a gapped core, the core includes at least one segment of a ferromagnetic material and at least one segment of a material that is not a ferromagnetic material. In implementations in which the core has more than one segment of ferromagnetic material, the segments are separated from each other with a material that is not a ferromagnetic material. The regions of non-ferromagnetic material between the segments of ferromagnetic material are referred to as gaps. 
     The current control circuit  150  controls the current in the electromagnetic circuit  120  by controlling an amount of magnetic flux in the magnetic core  123 . Magnetic flux is a measure of the total magnetic field that passes through a surface and is defined as is the surface integral of the normal component of a magnetic field that passes through a surface in units of weber (Wb). The relationship between current, voltage, and magnetic flux in an electromagnetic circuit given various open-circuit and closed-circuit conditions is fundamental to the operation of the current control circuit  150 . For example, the magnetic field generated by a current that is carried in a wire is given by: 
     
       
         
           
             
               
                 
                   
                     B 
                     = 
                     
                       
                         
                           μ 
                           0 
                         
                          
                         I 
                       
                       
                         2 
                          
                         
                           π 
                            
                           r 
                         
                       
                     
                   
                   , 
                 
               
               
                 
                   Equation 
                    
                   
                       
                   
                    
                   
                     ( 
                     1 
                     ) 
                   
                 
               
             
           
         
       
     
     where B is the magnitude of the magnetic field in Teslas (T), μ 0  is the permeability of free space, I is the magnitude of the current that is carried in the wire, and r is the distance from the wire in meters (m). The magnetic field in a material that is not free space (such as the core  123 ) is related to B by the permeability of the material. As noted above, the magnetic flux depends on the magnetic field. Thus, the amount of magnetic flux in the core  123  may be controlled by controlling the amount of current in the winding  121  or by controlling the amount of current in a winding  151 , which is also wrapped around the core  123 . 
     The current control circuit  150  includes the winding  151 , which is an electrical conductor (for example, a metal wire). Referring also to  FIG. 1C , the winding  151  (shown as a dashed line to visually distinguish the winding  151  from the winding  121 ) is also wrapped around the magnetic core  123 . The winding  151  and the winding  121  are electrically isolated from each other but are magnetically coupled via the magnetic core  123 . In a system in which a first coil or winding and a second coil or winding share a common magnetic core, a time-varying electrical current in the first winding generates a time-varying magnetic field in the common core, and the generated time-varying magnetic field induces a corresponding time-varying current in the second winding, and vice versa. Thus, a time-varying current in the winding  151  generates a corresponding time-varying current in the winding  121  of the electromagnetic circuit  120 . Using contemporary power electronic principles, the current control circuit  150  can act as a source or an impedance connected to winding  151  to cause or prevent the flow of current in winding  121 . For example, the current control circuit  150  may be used to cause the current in the winding  121  to decrease or drop to zero prior to separating a contact connected to the winding  121  from a tap. Moreover, the control circuit  150  may be used to control an amount of magnetic flux in the magnetic core  123  during a switching operation to thereby prevent or minimize the likelihood of saturation. Various implementations of the current control circuit  150  are discussed below. Prior to discussing the various implementations of the current control circuit  150 , an overview of a voltage regulator that includes a load tap changer is provided. 
     Referring to  FIG. 2 , a block diagram of a voltage regulator  210  is shown. In the example of  FIG. 2 , the dash-dot lines indicate a data link  259  over which data, such as, for example, information, commands, or numerical data, travel. Solid lines between blocks indicate a path through which current flows between the source  102  and the load  103 . The voltage regulator  210  includes a load tap changer and is an example of an implementation of the electrical apparatus  110  ( FIG. 1A ). The load tap changer includes taps  215  and electrical contacts  224 . The voltage regulator  210  monitors and controls the voltage level at the distribution path  104  such that the voltage delivered to the electrical loads  103  ( FIG. 1A ) is maintained within a desired or acceptable voltage range despite changes in the electrical load  103  and/or changes in the voltage supplied by the source  102  ( FIG. 1A ). 
     The voltage regulator  210  includes a monitoring module  212 , a tap selector  213 , a main winding  214 , and at least two taps  215  electrically connected to the main winding  214 . The monitoring module  212  may be any type of device capable of measuring or determining the voltage on the distribution path  104 . For example, the monitoring module  212  may be a voltage sensor. The tap selector  213  may include, for example, motors, mechanical linkages, and/or electronic circuitry that is capable of connecting the load  103  to the source  102  through any of the taps  215 . The voltage regulator  210  also includes an electromagnetic circuit  220 . Together, the taps  215 , the main winding  214 , the tap selector  213 , and the electromagnetic circuit  220  form a voltage regulation operation module  216  for the voltage regulator  210 . 
     The tap selector  213  is configured to move an electrical contact  224  and place the electrical contact  224  on a particular one of the taps  215 . When one or more of the electrical contacts  224  is connected to one or more of the taps  215 , the electromagnetic circuit  220  electrically connects the main winding  214  to the electrical load  103 . The taps  215  are separated from each other on the main winding  214 , and the output voltage of the voltage regulator  210  depends on the location of the selected tap on the main winding  214 . Thus, by controlling which of the taps  215  is connected to the contact or contacts that carry the load current, the output voltage to the load  103  is also controlled. In this way, the voltage delivered to the electrical load  103  may be kept within the acceptable or desired range even if the voltage delivered from the power source  102  changes. 
     The electromagnetic circuit  220  includes current paths  225 . The current paths  225  are any electrically conductive path that is able to conduct current from the contacts  224  to the load  103 . The current paths  225  may be any type of electrical cable, transmission line, or wire. The electromagnetic circuit  220  also includes a winding  221 , which is wrapped around a magnetic core  223  and is also electrically connected to one of the contacts  224 . The magnetic core  223  is similar to the magnetic core  123  ( FIGS. 1A-1C ), and may be an un-gapped or gapped magnetic core. The electromagnetic circuit  220  also includes a secondary winding  251 . The secondary winding  251  is also wrapped around the magnetic core  223 . Thus, the winding  221  and the secondary winding  251  are magnetically coupled, and, a time-varying current that flows in the secondary winding  251  induces a corresponding time-varying current in the winding  221 . 
     The electromagnetic circuit  220  also includes a current control apparatus  250 . The current control apparatus  250  is electrically connected to the secondary winding  251 . The current control apparatus  250  controls the characteristics of a time-varying current that flows in the secondary winding  251 , thereby controlling an induced current in the winding  221  and thus also controlling the current in the contact  224  electrically connected to the winding  221 . Furthermore, by controlling the current in the secondary winding  251  during a switching operation, saturation of the core  223  may be avoided. The voltage regulator  210  includes an on-load tap changer, meaning that the loads  103  remain connected to the source  102  when an electrical contact  224  is removed from one of the taps  215  and when the electrical contact  224  is connected to one of the taps  215 . Because the loads  103  remain connected, removing and/or connecting an electrical contact  224  may generate an arc, which reduces the lifetime of the electrical contact  224 . The current control apparatus  250  controls the current in the electrical contact  224 . By controlling the current in the electrical contact  224 , the current control apparatus  250  results in reduced or eliminated arcing and a longer lifetime for the voltage regulator  210 . Additionally, the current control apparatus  250  is electrically isolated from the main winding  214 . The electrical isolation allows low-voltage devices (for example, transistors) to be used in the current control apparatus  250 , thus reducing costs and complexity. 
     The voltage regulator  210  also includes a sensor  265  that measures voltage and current in various portions of the electromagnetic circuit  220  and/or to the electrical load  103 . The sensor  265  may be located anywhere along the current paths  225 . In some implementations, the electromagnetic circuit  220  includes more than one sensor  265 . The sensor  265  provides data to a controller  260  via a data link  259 . The data link  259  may be any path capable of transmitting data. For example, the data link  259  may be a network cable (such as an Ethernet cable), or the data link  259  may be a wireless connection that is capable of transmitting data. 
     The controller  260  may be implemented as an electronic controller that includes one or more electronic processors and an electronic memory coupled to the electronic processor. The controller  260  also may include manual or electronic user interface devices that allow an operator of the voltage regulator  210  to communicate with the controller  260 . The controller  260  may store instructions, perhaps in the form of a computer program, on the electronic storage. The instructions may relate to manipulation of data received from the sensor  265 . For example, the instructions may include a program or process that analyzes voltage and/or current data over a period of time to determine a time-rate of change of voltage and/or current. The electronic storage may store threshold data and instructions to compare determined rates of change with thresholds. The controller  260  also may interact with the current control apparatus  250 . For example, the controller  260  may produce signals that, when received by the current control apparatus  250 , are sufficient to cause electronic components within the apparatus  250  to perform certain actions. 
       FIGS. 3A-3E  are block diagrams of an example of a load tap changer  301 . Each of the  FIGS. 3A-3E  shows the load tap changer  301  at a different time. The load tap changer  301  is part of a voltage regulator  310 .  FIG. 3A  shows an example of the load tap changer  301  operating in steady-state.  FIG. 3B  shows the load tap changer  301  at a time just prior to breaking a connection to a tap.  FIG. 3C  shows the load tap changer  301  during a load tap change operation (or a switching operation) and while the primary contact is not connected to a tap.  FIG. 3D  shows another example of the load tap changer  301  operating in steady-state.  FIG. 3E  shows another example of the load tap changer  301  during a switching operation. 
     The voltage regulator  310  may be used in the power distribution network  101  ( FIG. 1A ) to deliver power from the source  102  to the loads  103 . The voltage regulator  310  includes an electromagnetic circuit  320  that delivers a load current  381  to the electrical load  103 , and a current control apparatus  350  that controls the current in the electromagnetic circuit  320  and the magnetic flux in the core  323 . Controlling the current in the electromagnetic circuit  320  enables more efficient operation of the voltage regulator  310  and extends the lifetime of the load tap changer  301 . For example, in some implementations (including the implementation discussed in  FIGS. 3A-3E ), the current control apparatus  350  enables a contact to be separated from a tap with no or minimal arcing. Reducing the amount of arcing increases the lifetime of the contacts. Additionally, in some implementations, controlling the magnetic flux results in no or minimal in-rush currents when a contact is connected to a tap. This also increases the lifetime of the contacts. 
     The voltage regulator  310  includes a shunt winding  312  and a main winding  314  (or series winding  314 ). The shunt winding  312  is in parallel with the source  102 , and the main winding  314  is in series with the load  103 . The main winding  314  includes at least two taps (taps  315   a  and  315   b  are shown in the example of  FIGS. 3A-3E ). The main winding  314  also includes a neutral point  317 . Connecting a contact  324   a  or  324   b  to the neutral point  317  causes the load  103  to be energized at the voltage provided by the source  102  without voltage addition or subtraction from the main winding  314 . A first end  318  or second end  319  of the main winding  314  is electrically connected to the source  102 . As in contemporary voltage regulators, the locations of the source  102  and the load  103  may be reversed, but the voltage regulating function is similar. 
     The voltage regulator  310  also includes an electromagnetic circuit  320  that is electrically connected to the electrical load  103  via a node  380 . The electromagnetic circuit  320  includes a first electrical conductor  321   a  that is electrically connected to a first contact  324   a  and to the node  380 . The electromagnetic circuit  320  also includes a second electrical conductor  321   b  that is electrically connected to a second contact  324   b  and to the node  380 . The first electrical conductor  321   a , the second electrical conductor  321   b , the first contact  324   a , and the second contact  324   b  are made of electrically conductive material. For example, the first electrical conductor  321   a  may be a metal wire or cable, and the first contact  324   a  may be formed at an end of the wire or cable. When either of the first contact  324   a  or the second contact  324   b  is connected to one of the taps  315   a ,  315   b , the electromagnetic circuit  320  electrically connects the main winding  314  to the node  380  and delivers a load current  381  to the electrical load  103 . 
     The voltage regulator  301  includes the taps  315   a ,  315   b  and the contacts  324   a ,  324   b . The output voltage of the voltage regulator  301  is the voltage of the source  102  plus the voltage between the selected tap and the neutral point  317 . Thus, the output voltage of the main winding  314  is determined by which tap  315   a ,  315   b  is connected to the contact that carries the load current  381 . Both of the contacts  324   a ,  324   b  may be movable contacts that are capable of contacting either of the taps  315   a ,  315   b . However, in the example discussed below, the contact  324   b  is the primary contact that generally carries the load current  381 , and the contact  324   b  is moved between the taps  315   a ,  315   b.    
     The electromagnetic circuit  320  also includes the current control apparatus  350 . The current control apparatus  350  includes a secondary winding  351  that is wrapped around a magnetic core  323 , and an electrical network  352  that is configured to control the voltage across the secondary winding  351  and the current through the secondary winding  351 . The current through the secondary winding  351  is referred to as the bias current  382 . By controlling the current through the secondary winding  351 , the electrical network  352  allows control of the magnetic flux in the magnetic core  323 . For example, the electrical network  352  is able to substantially prevent saturation of the magnetic core  323  during switching of the first contact  324   a  or the second contact  324   b , as discussed below. The electrical network  352  may include any type of current source that is able to produce a time-varying current having a particular amplitude and phase. The electrical network  352  is controlled by the controller  260 , which receives data that indicates an amplitude and phase of the current that flows in the second electrical conductor  321   b  from the sensor  265  via the data link  259 . 
     The first electrical conductor  321   a  includes a winding  322  that is also wrapped around the magnetic core  323 . Thus, the secondary winding  351  and the first electrical conductor  321   a  are magnetically coupled, and when the bias current  382  flows in the secondary winding  351 , a corresponding AC current is induced in the first electrical conductor  321   a.    
       FIGS. 3A-3E  show an example of a tap change operation performed to change the output voltage of the voltage regulator  310  by selecting different taps on the main winding  314 . In the example discussed below, the contact  324   b  is removed from the tap  315   b , moved toward the tap  315   a , and placed in contact with the tap  315   a.    
       FIG. 3A , shows an example of steady-state operation of the voltage regulator  310 . Steady-state operation is a normal operating condition in which the contacts  324   a ,  324   b  are stationary and neither of the contacts  324   a ,  324   b  is in the process of being moved to another tap. During steady-state operation of the voltage regulator  310 , the electrical network  352  is controlled such that no current flows in the secondary winding  351  and all of the load current  381  flows through the contact  324   b . For example, the controller  260  may open a switch  366  to create an open circuit in the secondary winding  351 . Alternatively, the function of the switch  366  may be realized in the electrical network  352 . In either case, no current flows in the secondary winding  351 , and the winding  322  and the magnetic core  323  can be designed so that the path through the first electrical conductor  321   a  is a high impedance path. In contrast, the impedance of the second electrical conductor  321   b  is essentially zero (0), so the load current  381  flows through the contacts  324   b  and  321   b.    
       FIG. 3B  shows the voltage regulator  310  at a time just prior to separating the contact  324   b  from the tap  315   b . An arc will form if the load current  381  is flowing through the contact  324   b  when the contact  324   b  is separated from the tap  315   b . However, the current control apparatus  350  prevents or mitigates arc formation. The controller  260  receives a signal indicating that the output voltage of the voltage regulator  310  is to be changed. The controller  260  also receives data that includes a measurement of the amplitude and phase of the current that flows in the second electrical conductor  321   b  (the load current  381 ) from the sensor  265 . In preparation for separating the contact  324   b  from the tap  315   b , the controller  260  closes the switch  366  such that there is no longer an open circuit in the secondary winding  351 , and the controller  260  causes the electrical network  352  to generate a bias current  382 . The controller  260  controls the electrical network  352  such that the bias current  382  flows through winding  351  inducing a circulating current  383  through winding  322  having the same amplitude and phase as the load current  381 . The ratio of bias current  382  to the circulating current  383  depends on the number of turns in the winding  322  and the secondary winding  351 . This relationship is well understood by those who practice the art. The circulating current  383  adds to the current  381  flowing in the second electrical conductor  321   b  such that when the amplitude and phase of the circulating current  383  is properly controlled, the sum of current in the contact  324   b  is zero (0). The contact  324   b  is then separated from the tap  315   b . Because no current is flowing in the contact  324   b  at the time of separation, an arc is not formed. In some implementations, the circulating current  383  is not precise enough to cause the current in the contact  324   b  to be precisely zero (0). However, in these implementations, the presence of the circulating current  383  reduces the current in the contact  324   b  such that the root-mean-square (RMS) current in the contact  324   b  is less than the load current  381  and some performance improvement may still be realized. 
       FIG. 3C  shows the voltage regulator  310  after the contact  324   b  has separated from the tap  315   b  but before the contact  324   b  has joined to the tap  315   a . No current flows in the contact  324   b  when the contact  324   b  is not connected to the tap  315   a  or the tap  315   b . The short circuit path for current for the circulating current  383  has been removed. As a result, the contact  324   a  must carry the entire load current  381  during the period in which the contact  324   b  does not touch one of the taps  315   a ,  315   b . To create a low impedance path through the winding  322 , the flux in the core  323  is controlled to zero (0) by the electrical network  352 . The flux in the core  323  is controlled to zero by controlling the amplitude and phase of the bias current  382 . Alternatively, the electrical network  352  is shorted such that it appears as a low impedance and has a negligible effect on the impedance of winding  322 . 
       FIG. 3D  shows the voltage regulator  310  after the contact  324   b  is connected to the tap  315   a , and the load tap changer  301  returns to steady-state operation. In the example shown in  FIG. 3D , after the contact  324   b  is connected to the tap  315   a , the controller  260  opens the switch  366  to create an open circuit in the secondary winding  351 . The open circuit causes the impedance of the secondary winding  351  and the first electrical conductor  321   a  to be higher than the impedance of the second electrical conductor  321   b . Thus, all the load current begins to flow through the contact  324   b  to the node  380  again. 
     A procedure similar to that discussed above is used to separate the contact  324   b  from the tap  315   a . To move the contact  324   b  back to the tap  315   b , the controller  260  closes the switch  366 , and causes the electrical network  352  to generate the bias current  382 , which induces a circulating current  383  that has the same amplitude and phase as the load current  381  that flows in the second electrical conductor  321   b . The bias current  382  induces the circulating current  383  in the first electrical conductor  321   a , and the circulating current  383  cancels the current that flows in the second electrical conductor  321   b . Thus, current stops flowing through the contact  324   b  and the contact  324   b  may be removed from the tap  315   a  without producing an arc. 
     Referring also to  FIG. 3E , while the contact  324   b  is not contacting the tap  315   a  or the tap  315   b , the electrical network  352  is controlled to adjust the flux in the core  323 . In particular, the electrical network  352  adjusts the magnetic flux in the magnetic core  323  to substantially prevent saturation of the core  323 . Saturation occurs when the magnetic core  323  reaches its flux carrying limit. When saturated, the magnetic core  323  is unable to carry more flux, and additional flux must be carried by the medium that surrounds the magnetic core  323  (free space in this example). The medium that surrounds the magnetic core  323  has a much lower magnetic permeability than the magnetic core  323 . Thus, the effective impedance (for example, the inductance) of the winding  322  is dramatically reduced. If the saturation condition is present when the contact  324   b  is connected to the tap  315   b  (or makes with the tap  315   b ), the relatively low effective impedance caused by the saturation may result in generation of a circulating current that has a much greater amplitude than a typical circulating current, and the large circulating current may cause damage to the contacts  324   a ,  324   b  and/or other components of the voltage regulator  310 . As such, it is desirable to control the magnetic flux in the magnetic core  323  during a switching operation to avoid saturation or lessen the likelihood of saturation occurring. 
     The cause of saturation of the magnetic core  323  during a switching operation (in this example, while the contact  324   b  is not on either the taps  315   a ,  315   b ) is the state of the magnetic flux within the magnetic core  323  just prior to and immediately after the contact  324   b  is connected to the tap  315   b . For example, while the contact  324   b  is transitioning to the tap  315   b  (as shown in  FIG. 3E ), the contact  324   a  carries all of the load current  381 . The impedance of the winding  322  is in series with the impedance of the load  103 , and a voltage drop (Vi) forms across the winding  322 . The current flowing in the winding  322  forms a time-varying magnetic field proportional to (Vi) in the magnetic core  323 . When the contact  324   b  makes with the tap  315   b , the voltage drop across the winding  322  immediately becomes equal to a voltage Vt, which is the voltage difference between the tap  315   a  and the tap  315   b . Because the winding  322  has an impedance that is essentially completely inductive, and the load has an impedance that is mostly resistive with some inductive components, the phase of Vi and Vt is generally not the same. Thus, the flux in the magnetic core  323  may be within saturation limits when the voltage Vi is the voltage across the winding  322 , but the additional change in flux imposed by the change in voltage to the voltage Vt may cause the core  323  to saturate. 
     To prevent saturation, the electrical network  352  controls the flux in the magnetic core  323  during the switching operation. For example, the electrical network  352  controls the magnitude and phase of the current through the winding (the bias current  382 ) to ensure that the flux in the core  323  remains within the saturation limit when the contact  324   b  is connected to the tap  315   b . In other words, prior to the contact  324   b  making with the tap  315   b , the electrical network  352  controls the magnitude and phase of the current  382  such that the flux in the core  323  is adjusted to a phase and magnitude that will prevent saturation when the contact  324   b  makes, thus causing the voltage of the winding between the tap  315   a  and the tap  315   b  to appear across winding  322 . The electrical network  352  may control the flux in the core  323 , by, for example, reducing the absolute value of the magnetic flux in the core  323 . The absolute value of the magnetic flux in the core  323  may be reduced by allowing a current to flow in the winding  322 . The magnetic flux in the core  323  may be reduced by increasing or decreasing the bias current  382 . In some implementations, the electrical network  352  controls the flux in the magnetic core  323  to match an ideal flux profile that is to be achieved after the switching operation is completed. 
     Referring to  FIG. 4 , a block diagram of a voltage regulator  410  is shown. The voltage regulator  410  is another example of a voltage regulating electrical apparatus  110  that may be used with the electrical power distribution network  101  ( FIG. 1A ). The voltage regulator  410  includes the shunt winding  312 , the main winding  314 , and the load tap changer  301 , which includes the taps  315   a ,  315   b , a first contact  324   a , and a second contact  324   b . The voltage regulator  410  also includes an electromagnetic circuit  420  that electrically connects one or both of the taps  315   a ,  315   b  to the electrical load  103  to deliver a load current  481  to the electrical load  103 . A current control apparatus  450  that controls the current in the electromagnetic circuit  420  such that the current flowing through a contact connected to one of the taps  315   a ,  315   b  is driven to zero prior to separating that contact from the tap. 
     The electromagnetic circuit  420  includes a first electrical conductor  421   a , which includes a first winding  422   a  that is wound around a magnetic core  423 . The electromagnetic circuit  420  also includes a second electrical conductor  421   b , which includes a winding  422   b  that is also wound around the magnetic core  423 . Thus, the first electrical conductor  421   a  and the second electrical conductor  421   b  are magnetically coupled and a time-varying current in the first electrical conductor  421   a  induces a corresponding time-varying current in the second conductor  421   b , and vice versa. The first contact  424   a  is electrically connected to the first electrical conductor  421   a , and the second contact  424   b  is electrically connected to the second electrical conductor  421   b . The electrical contacts  424   a ,  424   b  share the load current during steady state operation providing benefits over the implementation shown in  FIGS. 3A-3E , which will be apparent to those skilled in the art. 
     Under steady-state conditions, equal load current flows in the first and second electrical conductors  421   a ,  421   b . A current  483   a  flows in the first electrical conductor  421   a , and a current  483   b  flows in the second electrical conductor  421   b . Because the first and second electrical windings  422   a ,  422   b  are magnetically coupled, the load current  481  divides evenly between the conductors  421   a ,  421   b  when the windings  422   a ,  422   b  have the same number of turns. When the contact  424   a  is connected to the tap  315   a  and the contact  424   b  is connected to the tap  315   b , a circulating current (Ix) flows in the electromagnetic circuit  420  in addition to the load current  481  because of the voltage from main winding  314  existing between taps  315   a ,  315   b . The circulating current travels in opposite directions in each of the electrical conductors  421   a ,  421   b . In the example of  FIG. 4 , the current  483   a  has an amplitude of half of the load current  481  plus the circulating current (Ix), and the current  483   b  has an amplitude of half of the load current  481  minus the circulating current (Ix). Thus, the circulating current balances out and is not delivered to the electrical load  103 . 
     Historically, the magnetic core  423  was designed as a gapped core. A gapped core includes gaps of non-magnetic material between segments of magnetic material. The configuration of the gaps controls the impedance of the windings  422   a  and  422   b  and determines saturation characteristics of the core  423 . Generally, the windings  422   a  and  422   b  have a relatively low impedance when a gapped core is used. The configuration of the gaps is typically selected to produce a trade-off between circulating current and saturation of the core during switching. 
     On the other hand, the configuration and presence of the current control circuit  450  makes it possible to use an un-gapped core or a core with a gap that is smaller than a typical gapped core. The core used for a high-current voltage regulator may have a total core gap of about one (1) inch to achieve the desired impedance. Using the current control circuit  450  may allow the reduction of the core gap to perhaps 1/10th inch or 1/100th inch or less. The result would be circulating current reduction of approximately 90% or 99% or more, respectively. The reduction in circulating current results in lower FR losses and the smaller gap size may additionally reduce stray losses. This may lead to a reduction in losses of tens to hundreds of watts depending on the size of the voltage regulator. The current control apparatus  450  allows the control of magnetic flux during switching to prevent saturation, as discussed above. As a result, the magnetic core  423  may be designed without gaps, thus allowing the windings  422   a  and  422   b  to have a high impedance and to thereby effectively minimize the steady-state circulating current substantially close to zero. The reduced circulating current results in less total current flowing in the contacts  424   a  and  424   b , thereby allowing the contacts  424   a,b  to be designed for lower current than previously required. Moreover, the high impedance of the windings  422   a  and  422   b  reduces the electrical losses of the electromagnetic circuit  420  as compared to a design that uses a gapped magnetic core  423 . 
     The electromagnetic circuit  420  also includes the current control apparatus  450 , which controls the current in the first conductor  421   a  and/or the second conductor  421   b . The current control apparatus  450  includes a secondary winding  451 , which is wound around the magnetic core  423 , and an electrical network  452 . The electrical network  452  may include an AC current source. Because the secondary winding  451  is wound around the same magnetic core as the first and second windings  422   a ,  422   b , the secondary winding  451  is also coupled to the first and second windings  422   a ,  422   b . Thus, a current that flows in the secondary winding  451  induces a corresponding circulating current in the first and second electrical conductors  421   a ,  421   b  with characteristic similar to the circulating current (Ix). 
     The electrical network  452  is coupled to the controller  260 , which receives data that indicates the phase and amplitude of the current that flows in the first electrical conductor  421   a  and the second electrical conductor  421   b . During steady-state operation, the current control apparatus  450  is not used to influence the current in the electromagnetic circuit  420 , and the secondary winding  451  may be open circuited (for example, by opening a switch such as the switch  366  of  FIG. 3A ). Just prior to removing a contact from a tap, the controller  260  closes the switch such that current is able to flow in the secondary winding  451 . 
     An example of the operation of the current control apparatus  450  during a tap change operation in which the contact  424   b  is separated from the tap  315   b  is discussed. The controller  260  receives an indication of an upcoming tap change operation and the controller  260  causes the switch to close so that a bias current  482  from the electrical network  452  flows in the secondary winding  451 . The bias current  482  is controlled to produce a circulating current (Ix) with a magnitude that is the same as the magnitude as the current  483   b , and a phase that is opposite to the phase of the current  483   b . The bias current  482  induces a corresponding current in the second electrical conductor  421   b . The corresponding current cancels the current that flows through the contact  424   b  such that no current flows in the contact  424   b , and the contact  424   b  is removed from the tap  315   b  without generating an arc. 
     Referring also to  FIG. 5 , a plot  500  shows a simulated example of the circulating current (Ix) produced by the bias current  482  relative to the load current  481  and to half of the load current (labeled as  484 ). The curves labeled as Ix,  481 ,  484 ,  485  on the plot  500  represent instantaneous current amplitude (y axis) and phase (x axis) as a function time. In the example of  FIG. 5 , prior to separation, the contact  424   b  carries a current that is labeled as  484 . The current  484  has an amplitude that is half of the amplitude of the load current  481  and has a phase that is the same as the phase of the load current  481 . The bias current  482  induces the circulating current Ix in the windings  422   a ,  422   b . The induced circulating current Ix has an equal amplitude as the load current flowing in the second electrical conductor  421   b  and is 180° out of phase. As such, the induced circulating current Ix drives the current in the second electrical conductor  421   b  to zero (0). Thus, the second contact  424   b  can be removed from the tap  315   b  without generating an arc. The net current that flows in the second electrical conductor  421   b  after the bias current  482  flows in the secondary coil  451  is represented by the data labeled  485  in the plot  500 . 
     Referring also to  FIG. 6 , a plot  600  shows a simulated example based on an implementation in which a low-amplitude, high-frequency current is superimposed on the bias current  482  to form a bias current  482 ′. The current (Ix′) induced in the second electrical conductor  421   b  also has the low-amplitude, high-frequency ripple. The load current  481  is not affected by the presence of the low-amplitude, high-frequency ripple. Thus, the net current that flows in the second electrical conductor  421   b  (labeled as switched current  485 ′) also has the low-amplitude, high-frequency ripple. In the situation in which the bias current  482  is not accurate enough to create a circulating current to completely cancel the current  483   b , the ripple produces more frequent opportunities for zero crossings in the net current  485 . A zero crossing in the net current  485  represents an instance in time when there is no current flowing in the second contact  424   b . A zero crossing is required to extinguish the arc of a traditional load tap changer. Thus, the presence of the ripple reduces the duration of the arc and therefor reduces the arc energy as compared to a situation in which there is no ripple and the bias current is not accurate enough to bring the net current in the second electrical contact  421   b  to zero. 
     The low-amplitude, high-frequency current used to form the bias current  482 ′ and the current Ix′ may have a frequency that is, for example, four to twenty times the system frequency. For example, in an implementation in which the system frequency is 50 Hz, the high-frequency current may have a frequency of 200 Hz to 1000 Hz. In an implementation in which the system frequency is 60 Hz, the high-frequency current may have a frequency of 240 Hz to 1200 Hz. The amplitude of the low-amplitude, high-frequency current used to form the bias current  482 ′ may result in a switched current  485 ′, for example, of 5 to 20 Amperes (A). 
     Referring to  FIG. 7A , a block diagram of another example voltage regulator  710  that includes the load tap changer  301  is shown. The voltage regulator  710  is an example of an implementation of the voltage regulator  410 . The voltage regulator  710  includes an electrical network  752 . The electrical network  752  functions as an AC current source that produces the bias current  482 . The voltage regulator  710  also includes a current control apparatus  750 , which controls an amount of magnetic flux in a magnetic core  423 . 
     In the voltage regulator  710 , the first electrical conductor  421   a  and the second electrical conductor  421   b  are electrically connected to an equalizer winding  780  that is magnetically coupled to the main winding  314 . The equalizer winding  780  is also electrically connected to a node  779  and the electrical load  103 . Additionally, in the voltage regulator  710 , the bias current  482  is generated by the electrical network  752 . The electrical network  752  is electrically connected to a winding  753 , which is magnetically coupled to the shunt winding via a core  790  and draws power from the shunt winding  312 . Thus, the time-varying (AC) current in the shunt winding  312  from the source  102  induces a corresponding time-varying (AC) current  788  in the winding  753 . Together, the winding  753 , the winding  312 , and their common core (the core  790 ) form a voltage transformer. 
     The electrical network  752  includes a rectifier  754 , which converts the AC current  788  that flows in the winding  753  to a direct current (DC), a DC link  755  (or DC bus  755 ), and an inverter  756 , which converts DC energy stored in the DC link  755  into AC current to produce the bias current  482 . The DC link  755  stores DC energy and regulates a current ripple between the rectifier  754  and the inverter  756 . The DC link  755  may include one or more capacitors and/or inductors. 
     The rectifier  754  is any type of electrical network that is capable of converting an AC current into a DC current. The rectifier  754  may utilize controlled switches such that it can return power from the DC link  755  to the AC power system  100  through the winding  753 , which is magnetically coupled to the shunt winding  312 . The controlled switches may be, for example, transistors, such as, MOSFETS, BJTs, and/or IGBTs. Thus, in implementations in which controlled switches are used in the rectifier  754 , the rectifier  754  serves two purposes. First, the rectifier converts AC current into DC current that is supplied to the DC link  755 , which stores energy that the inverter  756  uses to produce the bias current  482 . Second, the rectifier  754  is able to compensate reactive power from the power distribution network  101 . In other words, the rectifier  754  is able to accept reactive power, which may be expressed in units of volt-ampere reactive (VAr), and to provide reactive power to the power distribution network  101 . The ability of the rectifier to compensate reactive power improves the power factor in the power distribution network  101 . Thus, the rectifier implemented with controllable switches allows a single apparatus (the rectifier) to serve more than one purpose, thereby reducing the need for additional components and providing a more efficient design. 
     The inverter  756  is any type of electrical network that converts the DC energy in the DC link into an AC current (the bias current  482 ). In some implementations, the rectifier  754  and the inverter  756  are implemented as two H-bridges.  FIG. 7B  is a block diagram of an example current source  752 B. The current source  752 B is an example implementation of a current source that may be used as the electrical network  752 . The current source  752 B includes a rectifier  754 B and an inverter  756 B coupled to each other by a DC link  755 B. The rectifier  754 B and the inverter  756 B are implemented as H-bridges. 
     An H-bridge is a circuit that includes four (4) switches. The switches may be, for example, transistors, diodes, or any other mechanism that may be configured to allow current to flow or to prevent the flow of current. In the example of  FIG. 7B , the rectifier  754 B includes switches SR_ 1 , SR_ 2 , SR_ 3 , and SR_ 4 . The inverter  756 B includes switches SI_ 1 , SI_ 2 , SI_ 3 , and SI_ 4 . The DC link  755 B is a capacitor that is electrically connected between the rectifier  754 B and the inverter  756 B. 
       FIG. 7C  is a block diagram of a current source  752 C. The current source  752 C may be used as the electrical network  752  ( FIG. 7A ). The current source  752 C generates the bias current  482  directly from the AC electrical power that flows in the winding  753 . As shown in  FIG. 7A , the winding  753  is magnetically coupled to the core  790  ( FIG. 7A ), which draws power from the shunt winding  312 . The source  102  provides an AC current that flows in the shunt winding  312 . Thus, the AC current in the winding  753  is current that is induced due to electrical power that is already present in the core  790 . 
     The current source  752 C includes switches S 1 C, S 2 C. The switches S 1 C, S 2 C have at least two stable states, one state in which the switch conducts current and another state in which the switch does not conduct current. The switches S 1 C, S 2 C may be, for example, transistors, such as MOSFETS, BJTs, and/or IGBTs. The switches S 1 C, S 2 C may be controlled, for example, by controlling the voltage at the gate of the transistor. 
     The winding  753  is electrically connected to the secondary winding  451  via an electrical conductor  727   c . The switch S 1 C is electrically connected to a first terminal  753   a  of the winding  753  via an electrical conductor  727   a . The switch S 2 C is electrically connected to a second terminal  753   b  of the winding  753  via an electrical conductor  727   b . Both of the switches S 1 C, S 2 C are electrically connected to the secondary winding  451  via an electrical conductor  727   d . Controlling the state of the switches S 1 C, S 2 C determines the polarity of the voltage across the secondary winding  451  and the direction of the bias current  482 . For example, the bias current  482  flows in a first direction when the switch S 1 C conducts current and the switch S 2 C does not conduct current, and the bias current  482  flows in the opposite direction when the switch S 1 C does not conduct current and the switch S 2 C conducts current. 
     The current source  752 C uses the AC current that flows in the winding  753  to generate the bias current  482  instead of using an inverter, such as the inverter  756  ( FIG. 7A ). In other words, the winding  753  acts as an AC current source. Thus, in implementations of the voltage regulator  710  ( FIG. 7A ) in which the current source  752 C is used as the electrical network  752 , the electrical network  752  is directly powered by an AC current source. 
     Referring to  FIG. 7D , a block diagram of a voltage regulator  710 D is shown. The voltage regulator  710 D is another example of an implementation of the voltage regulator  410  ( FIG. 4 ). The voltage regulator  710 D is similar to the voltage regulator  710  ( FIG. 7A ), except the voltage regulator  710 D does not include the winding  753 , and the voltage regulator  710 D includes a current control apparatus  750 D instead of the current control apparatus  750 . 
     The current control apparatus  750 D includes a current source  752 D. The current source  752 D is electrically connected to the equalizer winding  780 . The AC current source  752 D includes switches S 1 D, S 2 D. The switches S 1 D, S 2 D may be, for example, transistors. The switch S 1 _D is electrically connected to an electrical conductor  727   a _D. The electrical conductor  727   a _D is connected to the conductor  421   b , which is electrically connected to a terminal  780   a  of the equalizer winding  780 . The switch S 2 D is electrically connected to an electrical conductor  727   b _D, which is electrically connected to a terminal  780   b  of the equalizer winding  780 . The switches S 1 D, S 2 D are also electrically connected to the secondary coil  451 . The equalizer winding  780  is electrically connected to the secondary winding  451  via an electrical conductor  727   c _D. Controlling the state of the switches S 1 D, S 2 D determines the polarity of the voltage across the secondary winding  451  and the direction of the bias current  482 . For example, the bias current  482  flows in a first direction when the switch S 1 D conducts current and the switch S 2 D does not conduct current, and the bias current  482  flows in the opposite direction when the switch S 1 D does not conduct current and the switch S 2 D conducts current. 
     The current source  752 D uses the AC current that flows in the equalizer winding  780  to generate the bias current  482  instead of using an inverter, such as the inverter  756  ( FIG. 7A ). In other words, the equalizer winding  780  acts as an AC current source, and the current source  752 D is directly powered by an AC current source. 
     Referring to  FIG. 8A , a block diagram of a voltage regulator  810  that includes the load tap changer  301  is shown. The voltage regulator  810  may be used in the power distribution network  101  ( FIG. 1A ) to deliver power from the source  102  to the electrical load  103 . The load tap changer  810  includes the electromagnetic circuit  420 , which electrically connects the source  102  to the electrical load  103  by connecting the contact  424   a  and/or the contact  424   b  to a tap. The voltage regulator  810  also includes a current control apparatus  850 , which controls an amount of magnetic flux in a magnetic core  423 . 
     The current control apparatus  850  includes an electrical network  857 , a secondary winding  851  that is electrically connected to the electrical network  857 , and a sensor  265  that is configured to measure the voltage across the secondary winding  851  and/or the current in the secondary winding  851  and/or flux in the core  423 . The electrical network  857  includes one or more electronic components configured to short the secondary winding  851 . For example, the electrical network may include a controllable electronic switch, such as a transistor. Like the windings  422   a ,  422   b , the secondary winding  851  is wrapped around the magnetic core  423 . Thus, the secondary winding  851  is magnetically coupled to the first winding  422   a  and the second winding  422   b  and to the electromagnetic circuit  420 . The sensor  265  is coupled to the controller  260  via a data link  259 . The sensor  265  is configured to provide measurements of the current and/or voltage and or flux to the controller  260 . The controller  260  processes the measurements, and provides command signals to the current control apparatus  850 . 
     The current control apparatus  850  eliminates or greatly reduces losses related to a gapped magnetic core. Moreover, the current control apparatus  850  makes it feasible to use an un-gapped magnetic core or a magnetic core that has a smaller than typical gap as the magnetic core  423 . Using a gapped magnetic core or a magnetic core with a smaller than typical gap as the magnetic core  423  results in a higher impedance for the windings  422   a  and  422   b , leading to lower losses and less circulating current in steady-state. Un-gapped magnetic cores and cores that have a smaller than typical gap are generally more prone to saturation during switching. However, by controlling the magnetic flux in the core  423 , the current control apparatus  850  is also able to prevent saturation of the core  423  during a switching operation in implementations in which an un-gapped magnetic core or a magnetic core with a smaller than typical gap is used as the magnetic core  423 . 
     Under steady-state conditions, both of the contacts  424   a ,  424   b  are connected to the same tap or adjacent taps, the electrical network  857  is an open circuit, and current does not flow in the secondary winding  851 . The contact  424   b  is separated from the tap  315   a  and moved to the tap  315   b . In this position, the output voltage at node  480  is the average of the taps  315   a ,  315   b  if windings  422   a ,  422   b  have the same number of turns. Subsequently, the contact  424   a  may be separated from the tap  315   a  and moved to the tap  315   b  so that both of the contacts  424   a ,  424   b  make contact with the tap  315   b . Only the movement of the contact  424   b  is discussed in the example below. 
     When the contact  424   b  is separated from the tap  315   a , an arc is formed because, unlike the current control apparatuses  350 ,  450 , and  750 , the current control apparatus  850  does not reduce the current in the contact  424   a  prior to separation. After the contact  424   b  separates from the tap  315   a  and the arc is interrupted, all load current is transferred to the contact  424   a , and the voltage across the secondary winding  851  changes (for example, increases) rapidly. The sensor  265  measures the voltage across the secondary winding  851  over time, and provides the measurement to the controller  260 . The controller  260  determines the time-rate-of-change of the voltage (dV/dt) based on at least two voltage measurements taken at different times and compares the dV/dt to a threshold. If the dV/dt exceeds the threshold, the controller  260  causes the electrical network  857  to short the secondary winding  851 . 
     For example, the electrical network  857  may include a transistor that shorts the secondary winding  851  when in an ON state and forms an open circuit when in an OFF state. In this example, the controller  260  generates a trigger signal in response to determining that the dV/dt exceeds the threshold and provides the trigger signal to the gate of the transistor. The trigger signal is sufficient to cause the transistor to turn ON, and the secondary winding  851  is shorted. Shorting the secondary winding  851  provides a very low impedance path for electrical current. Because the secondary winding  851  is magnetically coupled to the magnetic core  423 , the secondary winding  851  draws magnetic flux out of the magnetic core  423  and reduces the impedance of the electromagnetic circuit  420  by conducting current in the secondary winding  851  and the electrical network  857 . 
     When the contact  424   b  makes contact with the tap  315   b , the contact  424   a  is still connected to the tap  315   a , and the secondary winding  851  is still shorted. A voltage difference between the tap  315   a  and the tap  315   b  creates a circulating current in the electromagnetic circuit  420 , and the circulating current induces a current in the secondary winding  851 . Thus, the current in the electromagnetic circuit  420  and the secondary winding  851  changes (for example, increases) rapidly. The sensor  265  measures the current in the secondary winding  851  over a period of time, and provides the current data to the controller  260 . The controller  260  determines the time-rate-of-change of the current (di/dt), and compares the di/dt to a threshold. A di/dt that exceeds the threshold is an indication that the contact  424   a  has connected to the tap  315   b  and that the secondary winding  851  should no longer be shorted. If the di/dt exceeds the threshold, the controller  260  provides a trigger signal to the electrical network  857  that is sufficient to form an open circuit in the electrical network such that no current flows in the secondary winding  851 . Continuing with the example of the electrical network  857  including a transistor, the trigger signal is a signal that is provided to the gate of the transistor and is sufficient to cause the transistor to switch from the ON state to the OFF state. After the transistor is turned OFF, the impedance of the windings  422   a ,  422   b  increases, and the electromagnetic circuit  420  returns to steady-state operation. 
     Although the current control apparatus  850  does not reduce the current that flows in the contact  424   b  to zero (0) prior to removing the contact  424   b  from the tap  315   a , the current control apparatus  850  still increases the lifetime of the contact  424   b  as compared to a conventional load tap changer that lacks the current control apparatus  850 . For example, by shorting the secondary winding  851  during a switching operation when only one of the contacts  424   a ,  424   b  is connected to a tap, the current control apparatus  850  reduces the magnetic flux in the magnetic core  423  and reduces the impedance of the electromagnetic circuit  420  during the switching operation. The reduction in magnetic flux reduces the likelihood of the core  423  saturating when the contact  424   b  is connected to the tap  315   b  and thereby reduces or prevents inrush currents (or surge currents) that would otherwise occur when the contact  424   b  is connected to the tap  315   b . By reducing or preventing inrush currents, the current control apparatus  850  prolongs the lifetime of the contact  424   b  and the voltage regulator  810 . Further, the current control apparatus allows for the use of a high impedance electromagnetic circuit to minimize circulating current, which substantially reduces the amount of current the contacts must interrupt, especially at lower load current levels, such that the contact erosion from arcing is reduced. Moreover, the low circulating current contributes to a less inductive power factor to generally improve arc interruption. Less inductive power factor combined with the shorting of winding  851 , which will reduce recovery voltage after arcing, improves the arc interrupting capability of a load tap changer. 
       FIG. 8B  is a block diagram of an electrical network  857 B. The electrical network  857 B is an example implementation of the electrical network  857 . The electrical network  857 B includes a transistor  891  and a rectifier bridge  892  made from diodes D 1 -D 4 . The transistor  891  may be any type of transistor, for example, a metal-oxide semiconductor field-effect transistor (MOSFET) or an insulated-gate bipolar transistor (IGBT). The transistor  891  receives a trigger at a gate  893 , and the trigger is sufficient to cause the transistor  891  to change state. The trigger may be received from the controller  260 . When the transistor  891  is ON, current flows through the transistor  891  and the diodes D 1 -D 4  conduct current so the secondary winding  851  is shorted. The diodes D 1 -D 4  work in pairs, with the first pair being diodes D 1  and D 3 , and the second pair being diodes D 2  and D 4 . Current flow through one of the two pairs during the positive half of the cycle and the other of the two pairs during the negative half of the cycle. When the transistor is OFF, the electrical network  857 B is an open circuit, and no current flows through the winding  851 . 
       FIG. 9A  is a block diagram of another example of a voltage regulator  910  that includes the load tap changer  301 . The voltage regulator  910  may be used in the power distribution network  101  ( FIG. 1A ) to deliver power from the source  102  to the electrical load  103 . The voltage regulator  910  includes the electromagnetic circuit  420 , which electrically connects the source  102  to the node  480  and the electrical load  103  by connecting the contact  424   a  and/or the contact  424   b  to a tap on the main winding  314 . A current transformer  930  is between the node  480  and the electrical load  103 . The current transformer  930  includes a first current winding  932 , which is electrically connected to the node  480  and the electrical load  103 , and a second current winding  933 . The first current winding  932  and the second current winding  933  are wrapped around a magnetic core  931 . Thus, when the load current  481  flows between the node  480  and the electrical load  103  and in the first current winding  932 , a corresponding time-varying current is induced in the second current winding  933 . 
     The voltage regulator  910  also includes a current control apparatus  950 , which controls the current in the electromagnetic circuit  420 . The current control apparatus  950  includes an electrical network  996 , a phase network  997 , and a secondary coil  951 . The secondary winding  951  is wrapped around the magnetic core  423 , and is thus magnetically coupled to the windings  422   a ,  422   b  of the electromagnetic circuit  420 . The electrical network  996  is electrically connected to the second current winding  933  and to the phase network  997 . The phase network  997  is electrically connected to the secondary winding  951 . 
     The electrical network  996  includes a shorting circuit, which may be closed (or gated on) or opened (gated off). When the shorting circuit is closed, the electrical network  996  reduces the magnetic flux in the magnetic core  931  and prevents saturation of the magnetic core  931 . When the shorting circuit is open, the current that is induced in the second current winding  933  may flow through the phase network  997  to form the bias current  982 . The phase network  997  is one or more electronic components arranged to form an electrical network that determines whether the bias current  982  is able to flow to the secondary winding  951  and also controls the direction that the bias current  982  flows in the secondary winding  951 . 
       FIG. 9B  shows a current control apparatus  950 B, which is an example implementation of the current control apparatus  950 . The current control apparatus  950 B includes an electrical network  996 B, which is implemented in the same manner as the electrical network  857 B of  FIG. 8B , and a phase network  997 B, which is implemented as H-bridge formed from switches S 1 -S 4 . The switches S 1 -S 4  are used to control the direction in which current flows through the secondary winding  951 . When the switches S 1  and S 4  are closed and the switches S 2  and S 3  are open, current flows through the secondary winding  951  with a first phase convention, for instance zero (0) degrees. When the switches S 1  and S 4  are open and the switches S 2  and S 3  are closed, current flows through the secondary winding  951  with the opposite phase convention, for instance 180 degrees. 
     In steady-state operation, both of the contacts  424   a ,  424   b  make contact with one of the taps  315   a ,  315   b . The contact  424   a  conducts the current  483   a , and the contact  424   b  conducts the current  483   b . Each of the currents  483   a ,  483   b  are half of the load current  481 . The phase-inverting network  997  is in a configuration that does not conduct current and the bias current  982  does not flow in the secondary winding  951 . For example, in implementations in which the phase-inverting network  997  is implemented as shown in  FIG. 9B , all of the switches S 1 -S 4  are open during steady-state operation. Additionally, the shorting circuit of the electrical network  996  is gated on (for example, the transistor  891  is ON) such that the current transformer  930  does not saturate or create an impedance between the tap changer  910  and the electrical load  103 . The functionality of the electrical network  996 B may also be realized within the phase network  997 B given the proper switch topology. For instance, switching on only S 1 -S 2  or S 3 -S 4  will short winding  933  and leave winding  951  open. 
     The current control apparatus  950  is able to drive the current in either the contact  424   a  or the contact  424   b  to zero prior to a switching operation by producing the bias current  982  and controlling the direction of the bias current  982 . The bias current  982  is current that is induced in the second current winding  933  and flows into the secondary winding  951  via the phase network  997 . The bias current  982  induces a circulating current in windings  422   a ,  422   b  having an amplitude that is half of the amplitude of the load current  481 . The bias current  982  has the same phase as the load current  481  because the bias current  982  is a current that is induced by the load current  481 . Proper coordination of switches in phase network  997  causes the circulating current to cancel current through contacts  424   a  or  424   b . In preparation for performing a tap change operation, the shorting circuit in the electrical network  996  is opened (for example, the transistor  891  is switched to an OFF state), and the phase-inverting network  997  is configured to allow the bias current  982  to flow through the secondary winding  951 . The bias current  982  induces a corresponding current in the electromagnetic circuit  420 . The corresponding current causes the current on the contact  424   b  to drop to zero, and all of the load current flows in the contact  424   a . The contact  424   b  is then removed from the tap  315   a . An arc is not formed because no current flows through the contact  424   b  immediately prior to separation. 
     After the contact  424   b  has separated from the tap  315   a , a rapid change in voltage occurs in the electrical network  996 , and the electrical network  996  is closed (for example, the transistor  891  is switched to an ON state) to prevent saturation of the magnetic cores  423  and  931 . While contact  424   b  is transitioning from tap  315   a  to  315   b , the electrical network  996  and phase network  997  can be coordinated to control the flux of the magnetic cores  423 ,  931  to zero (0) to avoid saturation. Alternatively, the electrical network  996  and phase network  997  can be coordinated to control the flux of the magnetic core  423  with an amplitude and phase to prevent saturation when contact  424   b  makes on tap  315   b . Once the contact  424   b  makes on tap  315   b , the electrical network  996  and phase network  997  are returned to steady state conditions. 
     Thus, the current control apparatus  950  mitigates arc formation when a contact separates from a tap. Additionally, the current control apparatus  950  prevents or reduces the likelihood of core saturation during switching, and thus also mitigates or prevents in-rush currents when a contact makes contact with a tap. Moreover, the current control apparatus  950  generates the bias current  982  at the correct amplitude and phase without using separate current-generation devices and without using a DC link or bus. 
       FIG. 10A  is a block diagram of another example of a voltage regulator  1010  that includes the load tap changer  301 . The voltage regulator  1010  may be used in the power distribution network  101  ( FIG. 1A ) to deliver power from the source  102  to the electrical load  103 . The voltage regulator  1010  includes an electromagnetic circuit  1020 , which electrically connects the source  102  to the node  1079  and the electrical load  103  by connecting the contact  424   a  and/or the contact  424   b  to a tap on the main winding  314 . The electromagnetic circuit  1020  is the same as the electromagnetic circuit  420 , except the electromagnetic circuit  1020  includes two magnetic cores  1023   a  and  1023   b . The winding  422   a  is wrapped around the core  1023   a , and the winding  422   b  is wrapped around the core  1023   b . Additionally, in the voltage regulator  1010 , the first electrical conductor  421   a  and the second electrical conductor  421   b  are electrically connected to an equalizer winding  1080  that is magnetically coupled to the main winding  314  and electrically connected to the electrical load  103 . 
     The voltage regulator  1010  also includes a current control apparatus  1050  that is configured to magnetically couple to the electromagnetic circuit  1020  to control the current flow in the electromagnetic circuit  1020 . The current is controlled prior to removing a contact from a tap to mitigate or prevent arcing. 
     The current control apparatus  1050  includes a first secondary winding  1051   a , which is wrapped around the core  1023   a , and a second secondary winding  1051   b , which is wrapped around the core  1023   b . Thus, the first secondary winding  1051   a  is magnetically coupled to the first winding  422   a  via the core  1023   a , and the second secondary winding  1051   b  is magnetically coupled to the second winding  422   b  via the core  1023   b . The core  1023   a  and the core  1023   b  are un-gapped magnetic cores or cores that include a smaller than usual gap. 
     The current control apparatus  1050  also includes an electrical network  1052  that is electrically connected to the first secondary winding  1051   a  and the second secondary winding  1051   b . The electrical network  1052  is configured to control an amount of current that flows in and/or voltages across the first winding  422   a  and the second winding  422   b.    
       FIG. 10B  includes an electrical network  1052 B, which is an example of an implementation of the electrical network  1052 . The electrical network  1052 B includes a switch  1037 , which is in parallel with the first secondary winding  1051   a , a switch  1038 , which is in parallel with the second secondary winding  1051   b , and a by-pass switch  1036 , which is electrically connected to the first secondary winding  1051   a  and the second secondary winding  1051   b . The by-pass switch  1036  is electrically connected to the switches  1037  and  1038 , and the by-pass switch  1036  is positioned between the switches  1037  and  1038 . The switches  1036 ,  1037 , and  1038  may be any type of electronic component that may be controlled to permit or allow current flow. For example, the switches  1036 ,  1037 , and  1038  may be transistors. The state of the switches  1036 ,  1037 ,  1038  may be controlled by the controller  260 . 
     During steady-state operation, the switches  1037  and  1038  are open (such that no current flows through these switches), and the switch  1036  is closed (such that current flows through  1036 ). Both of the contacts  424   a  and  424   b  are on the same tap (the tap  315   a  in the example of  FIG. 10B ), and the currents  483   a  and  483   b  flow through the contacts  424   a  and  424   b , respectively. Because the cores  1023   a  and  1023   b  are not gapped cores, there is little to no circulating current in the steady-state. Equal current flows through the contacts  424   a  and  424   b . Thus, the currents  483   a  and  483   b  have an amplitude that is half of the amplitude of the load current  1081 . Moreover, because there is no circulating current, the electromagnetic circuit  1020  and the current control apparatus  1050  have lower losses than a conventional voltage regulator with load tap changer, and the magnitude of current flowing in the contacts  424   a ,  424   b  and in the first and second electrical conductors  421   a ,  421   b  is smaller than in a conventional load tap changer. As a result, the contacts and conductors may be smaller and otherwise designed for operation at a lower than typical current. 
     An operation that moves the contact  424   b  from the tap  315   a  to the tap  315   b  is discussed as an example. The operation begins by removing the contact  424   b  from the tap  315   a  while the load current  1081  is delivered to the load. Just prior to removing the contact  424   b  from the tap  315   a , the controller  260  provides a trigger signal to the switch  1036 , a trigger signal to the switch  1037 , and a trigger signal to the switch  1038 . The trigger signal to the switch  1037  causes the switch  1037  to close. The trigger signals to the switches  1036  and  1038  causes the switches  1036  and  1038  to open. For example, the switches  1036 ,  1037 , and  1038  may be transistors, and the trigger signals may be trigger signals provided to the gate of the transistor that are sufficient to cause the transistor to change state. 
     With the switches  1036 ,  1037 , and  1038  configured in this manner, the first secondary winding  1051   a  is shorted and provides a very low impedance path for the load current  1081  through winding  422   a . Because the first secondary winding  1051   a  is magnetically coupled to the magnetic core  1023   a , the first secondary winding  1051   a  draws magnetic flux out of the magnetic core  1023   a  by conducting current in the first secondary winding  1051   a . At the same time, second secondary winding  1051   b  is open circuited such that winding  422   b  becomes a high impedance path. All of the load current  1081  flows through the first contact  424   a , and the contact  424   b  is removed from the tap  315   a  without forming an arc. While contact  424   b  is transitioning from tap  315   a  to  315   b , switches  1036 ,  1037 ,  1038  are coordinated to control the flux in magnetic cores  1023   a ,  1023   b  to prevent saturation. After the contact  424   b  makes with tap  315   b , switches  1037 ,  1038  are opened and switch  1036  is closed to complete the tap change process. 
     Other implementations of the electrical network  1052  are possible. For example, as shown in  FIG. 10C , the electrical network  1052  may be implemented as an electrical network  1052 C that includes a first AC-AC converter  1061 _ 1  that couples the first secondary winding  1051   a  to the shunt winding  312  and a second AC-AC converter  1061 _ 2  that couples the second secondary winding  1051   b  to the shunt winding  312 . Another example implementation of the electrical network  1052  is shown in  FIG. 10D .  FIG. 10D  includes an electrical network  1052 D. The electrical network  1052 D includes an AC-AC converter  1061 _ 3  that couples the first secondary winding  1051   a  to the second secondary winding  1051   b.    
     The implementations discussed, for example, in  FIGS. 3A-3E, 4, 7A, 8A, and 9A  have a steady state condition with both contacts (for example, contacts  324   a ,  324   b ) connected to taps such that a fixed voltage ratio exists at the voltage regulator between the voltage from the source input to the load output (for instance, the voltage across the shunt winding  312  and the voltage at node  480  of  FIG. 4 ) such that the steady state performance is the same as conventional voltage regulator performance but with reduced losses and also with the aforementioned benefit of tap changing. In the implementations of, for example,  FIG. 4  and  FIG. 7A , the respective electromagnetic circuits  420 ,  720  cause a dependent relationship between the load currents flowing through the contacts  424   a  and  424   b  such that each contact  424   a ,  424   b  carries one half of the load current when windings  422   a  and  422   b  have an equal number of turns. Due to magnetic coupling, equal voltage exists across winding  422   a  and winding  422   b . The effect is that the output voltage at node  480  or  779  (respectively) is the average of the voltage at contact  424   a  and the voltage at  424   b.    
     An additional advantage is realized in the implementation of  FIG. 10A . In particular, the implementation of  FIG. 10A  employs the current control apparatus  1050  to couple and decouple the windings  422   a  and  422   b . For example, as discussed with respect to  FIG. 10B , manipulation of switches  1036 ,  1037  and  1038  influences the current and voltage of the two windings  1051   a ,  1051   b . Manipulating switches  1036 ,  1037  and  1038  at high frequencies and with controlled duty cycles influences the root-mean-square voltage at node  1079 . Whereas the implementations of  FIG. 4 ,  FIG. 7A , and  FIG. 9A  have a fixed ratio between the input and output voltage while the contacts  424   a  and  424   b  are fixed, the implementation of  FIG. 10A  allows for variability of the ratio based on the control of the voltage across windings  1051   a  and  1051   b . Whether by manipulation of switches  1036 ,  1037 ,  1038  (or an alternative configuration of the electrical network  1052 ), the input:output voltage ratio of the voltage regulator  1010  can be manipulated without the moving contacts  424   a ,  424   b . Additional filter components may be added to the system to reduce harmonics in the output voltage. 
     The example in  FIG. 10E  shows another implementation of the current control apparatus  1050 . In the implementation of  FIG. 10E , the current control apparatus  1050  includes a winding  1053 , a rectifier  1054 , a DC link  1055 , and an inverter  1056 . The current control apparatus  1050  is electrically connected to the winding  1053 , which is magnetically coupled to the shunt winding  312  via a core  1090  and draws power from the shunt winding  312 . A time-varying (AC) current in the shunt winding  312  from the source  102  induces a corresponding time-varying (AC) current  1088  in the winding  1053 . The implementation of the current control apparatus  1050  shown in  FIG. 10E , like the current control apparatus  750  discussed with respect to  FIG. 7A , is able to compensate reactive power from the power distribution network  101  in addition to having the functionality of the implementations of  FIGS. 10A and 10B . 
     Other implementations are within the scope of the claims. 
     For example,  FIG. 10A  illustrates the electromagnetic circuit  1020  and the current control apparatus  1050  being used in the voltage regulator  1010 . However, other applications and implementations are possible. For example, in the implementation shown in  FIG. 11 , the load tap changer  301  is removed and the electromagnetic circuit  1020  is connected directly to the main winding  314 . The source  102  may be connected, for instance, in the middle of the main winding  314 , either winding end  318 ,  319 , or at any point along the main winding  314  depending on the desired output range of the voltage regulator. In the configuration shown in  FIG. 11 , when the electromagnetic circuit is in a neutral state, the voltage delivered to node  1079  may be the average of the voltage present at the first end  318  and second end  319  of the winding  314  which may, for example, cause the output voltage to the load  103  be equal to the output voltage from the source  102 . Manipulation of the current control apparatus  1050  may, however, cause the output voltage to be increased or decreased as the voltages of windings  422   a,b  are controlled. 
       FIG. 12  is an example of applying the implementation of  FIG. 11  to a distribution transformer with a primary winding  1212  and a secondary winding  1214 . The distribution transformer delivers a load current  1281  to the load  103 . Manipulation of the current control apparatus  1050  changes the voltages of windings  422   a,b  such that the effective turns ratio of the windings  1212 ,  1214  is adjusted and the voltage delivered to the load  103  may be regulated. Windings  1212 ,  1214  in  FIG. 12  share a common neutral node as in a grounded-wye connection, but the principle also applies to delta-connected or otherwise isolated windings. 
     The rectifier, inverter and DC bus components of the implementation in  FIG. 10E  may be similarly implemented with the topologies in  FIG. 11  and  FIG. 12  to provide reactive power compensation for the power system to improve power factor. In addition to the single-phase power system applications, any of these implementations may be applied to a three-phase system. Within a three-phase application, the application may benefit from economies of scale, especially for the switching or power electronic components, where the number of switches for inverters and rectifiers per phase is reduced and the DC bus performance is thereby improved.