Patent Publication Number: US-7595614-B2

Title: Load tap changer

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   Not applicable. 
   STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH 
   Not applicable. 
   NAMES OF PARTIES TO A JOINT RESEARCH AGREEMENT 
   Not applicable. 
   SEQUENCE LISTING 
   Not applicable. 
   BACKGROUND 
   1. Technical Field 
   The disclosed embodiments generally relate to the field of voltage regulating or control systems. More particularly, the disclosed embodiments relate to an improved tap changing method and system for power delivery. 
   2. Description of the Related Art 
   A tap changer is a device used to change the load voltage or phase angle of a power delivery system. Typically, the selection of a tap adjusts the number of turns used in one or more of a transformer&#39;s windings. Tap changers most commonly are used to permit the regulation of the output voltage of a transformer or step voltage regulator to a desired level. 
   Tap changing may occur either while the transformer is energized (i.e., under load) or while the transformer is not energized (i.e., offline). A mechanical switching assembly is typically used to accomplish tap changing under load (TCUL) in power transformers and step-voltage regulators. To accomplish a tap change, older design load tap changers (LTCs) simply interrupt the load current, which is sometimes more than 1,000 amperes, with the simple parting of contacts under oil. This practice continues today. 
   The interruption of a high load current can lead to an arc between the contacts. To avoid this arc and the consequential deleterious effects of contact burn and oil decomposition, which leads to early failure or the need for maintenance, newer tap changers include contacts that are immersed in oil with the inclusion of a vacuum switch. In these designs, the current is commutated to a path through the vacuum switch for the current interruption. An early description of such a design, which is still commonly used today, is found in H. A. Fohrhaltz,  Load - Tap Changing with Vacuum Interrupters , IEEE Transactions on PAS, vol. PAS-86, No. 4, April 1967, pp. 422-428. Vacuum switch technologies usually require the use of bridging reactors. The bridging reactor is itself a transformer, of perhaps one quarter of the size of the main transformer. It significantly adds to the cost and weight of the total assembly. It typically will also add to total internal losses, the resulting heat having to be dissipated with additional tank cooling provisions. 
   Various attempts have been made to replace the vacuum switch in a LTC with power thyristors. Some of these attempts can be categorized as “solid-state” technology, and the rest can be categorized as “hybrid” technology. Solid state LTCs can be characterized by the elimination of any mechanical switching assembly. The motivation is very high speed operation, i.e., one to three cycles (typically less than 50 milliseconds), and the opportunity to span multiple tap steps in a single operation. This feature provides more speed than is typically required for run-of-the-mill power distribution transformer applications, which typically involve a 30 second intentional time delay. Thus, it often adds an unnecessary expense. In addition, reliability issues can be extensive since the thyristors must be continuously active. An illustrative early embodiment of a solid-state implementation is found in U.S. Pat. No. 3,195,038, issued Jul. 13, 1965 to Fry. 
   In contrast, “hybrid” technology includes both mechanical switches and solid-state components (e.g., thyristors). In these designs, mechanical switches accomplish the tap position selection, while the thyristors only assist during the actual tap change event, which will typically occur less than 40 times per 24 hours. Because the mechanical switch is doing the actual tap position selection, fewer thyristors are required. One implementation of a hybrid LTC is found in U.S. Pat. No. 4,363,060, issued Dec. 7, 1982 to Stich. 
   A problem with hybrid designs is that they involve the use of a means (usually a power resistor) to limit the magnitude of a current that may circulate in the electronic circuit during the tap change. Others have attempted to avoid such a need with the use of more complex (and expensive) circuitry and gate-turn-off thyristors. Prior hybrid systems also require multiple power switches, further adding to the cost of the circuit. 
   The disclosure contained herein describes attempts to address one or more of the problems described above. 
   SUMMARY 
   In an embodiment, a load tap changer includes a first semiconductor device connected to a first gate trigger circuit, a second semiconductor device connected to a second gate trigger circuit, and a mechanical switch. The mechanical switch shunts the first semiconductor device when the mechanical switch is in a first conducting position, and it shunts the second semiconductor device when the mechanical switch is in a second conducting position. The mechanical switch creates a first circuit between a source and a load through a first contact when the mechanical switch is in the first conducting position, and mechanical switch creates a second circuit between the source and the load through a second contact when the mechanical switch is in the second conducting position. Movement of the mechanical switch from the first conducting position to the second conducting position causes one of the gate trigger circuits to trigger its corresponding semiconductor device and momentarily complete a circuit through the corresponding semiconductor device for effecting engagement of a transformer tap. 
   Optionally, each semiconductor device may include one or more semiconductive components electrically connected as an alternating current switch, such as a thyristor pair. The first thyristor pair and the second thyristor pair may be the only thyristor pairs in the load tap changer that are required to change from the first circuit to the second circuit. In some embodiments, the triggering is responsive to the detection of current in one of the mechanical switch conducting positions by a gate control. The gate control may be a single gate control that controls both the first gate trigger circuit and the second gate trigger circuit. In addition, the mechanical switch may be the only mechanical switch in the circuit that is required to change from the first circuit to the second circuit. 
   In an alternate embodiment, a load tap changer includes a single mechanical switch that is movable to create, in a first position, a first conducting path between a first transformer tap and a load. When the switch is in a second position, the switch creates a second conducting path between a second transformer tap and the load. A first thyristor pair creates a first alternate conducting path between the first transformer tap and the load when the switch is disengaged from the first position. A second thyristor pair creates a second alternate conducting path between the second transformer tap and the load when the mechanical switch is disengaged from the second position. A gate trigger circuit may be included for each thyristor pair, and a gate control circuit may control each of the gate trigger circuits. 
   Optionally, in the above-described embodiment, a current limiting device may not be required to be electrically connected between the switch and either of the thyristor pairs. During long-term operation, each thyristor pair may receive substantially the same level of duty. In some embodiments, the mechanical switch may be a two-pole, single-throw switch. In addition to the mechanical switch, the changer also may include a Geneva wheel, a first moving contact, and a second moving contact. In such an embodiment, the Geneva wheel initiates movement of the first moving contact toward and away from the first transformer tap, and the Geneva wheel also initiates movement of the second moving contact toward and away from the second transformer tap. In some embodiments, the changer also may include a first snubber circuit that is electrically connected in parallel with the first thyristor pair, and a second snubber circuit that is electrically connected in parallel with the second thyristor pair. 
   In an alternate embodiment, a method of delivering power to a load includes operating a circuit having a mechanical switch in a first conducting position and a first moving contact in electrical connection with a first tap of a transformer, so that the mechanical switch and first moving contact create a first circuit between a source and a load. A second contact is moved to create an electrical connection between the second contact and a second tap of the transformer. The mechanical switch is moved away from the first conducting position and toward a second conducting position. A gate trigger circuit triggers a first semiconductor device to complete a circuit between the source and the load through the first semiconductor device. A second gate trigger circuit triggers a second semiconductor device to complete a circuit between the source and the load through the second semiconductor device. The first gate trigger circuit then removes the trigger from the first semiconductor device, which subsequently stops conducting. When the mechanical switch reaches the second conducting position, a second circuit is created between the source and the load by shunting the second semiconductor device and passing current through the mechanical switch. The method may maintain a level of current delivered to the load at a substantially constant level as the mechanical switch moves from the first conducting position to the second conducting position. The event of the first semiconductor device turning off may be responsive to a current zero through the mechanical switch. Triggering and conduction of the second semiconductor device may be responsive to detection of voltage across either the first semiconductor device or the second semiconductor device in excess of a peak voltage of a winding of the transformer. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  illustrates an exemplary single phase regulator circuit in a quiescent state. 
       FIG. 2  illustrates an alternate position of the regulator circuit of  FIG. 1 . 
       FIG. 3  illustrates an alternate position of the regulator circuit of  FIG. 1 . 
       FIG. 4  illustrates an alternate position of the regulator circuit of  FIG. 1 . 
       FIG. 5  illustrates an alternate position of the regulator circuit of  FIG. 1 . 
       FIG. 6  illustrates an alternate position of the regulator circuit of  FIG. 1 . 
       FIG. 7  illustrates an alternate position of the regulator circuit of  FIG. 1 . 
       FIG. 8  illustrates an alternate position of the regulator circuit of  FIG. 1 . 
       FIG. 9  illustrates an alternate position of the regulator circuit of  FIG. 1 . 
   

   DETAILED DESCRIPTION 
   Before the present methods, systems and materials are described, it is to be understood that this disclosure is not limited to the particular methodologies, systems and materials described, as these may vary. It is also to be understood that while all circuit descriptions reveal a single-phase implementation, the methods and systems described herein also include multi-phase applications. It is also to be understood that the terminology used in the description is for the purpose of describing the particular versions or embodiments only, and is not intended to limit the scope. For example, as used herein and in the appended claims, the singular forms “a,” “an,” and “the” include plural references unless the context clearly dictates otherwise. In addition, the word “comprising” as used herein is intended to mean “including but not limited to.” Unless defined otherwise, all technical and scientific terms used herein have the same meanings as commonly understood by one of ordinary skill in the art. 
   As used herein, the term “semiconductor device” means an electronic component made of semiconductive materials such as silicon, germanium or gallium arsenide. Examples of semiconductor devices include thyristors, which include at least four layers of alternating N-type and P-type materials and which act as a switch. A semiconductor device may also include a thyristor pair. Other examples include insulated gate bipolar transistors (IGBTs), or particular thyristor types such as gate turnoff (GTO) thyristors and silicon controlled rectifiers (SCRs). 
   As used herein, the term “connected” means electrically connected, either directly or via one or more intervening devices. The term “shunt” as used herein refers to the ability of a device to allow electrical current to pass around the device as a short circuit in parallel to one or more other devices. 
     FIG. 1  illustrates an exemplary single phase regulator circuit  10  in a quiescent state. The circuit  10  regulates a voltage across a load (not illustrated, but connected between terminal  48  and terminal SL  11 ), which is commonly connected to ground. Stationary contacts  12 ,  14  are connected to taps on the series winding of the transformer, of which only one section  18  is shown. In the embodiment, each such tap section represents approximately one and one-quarter percent (1¼%) of line voltage, although other percentages are possible. The shunt winding  44  is excited, generally at about 100% of source or load voltage, although other voltage levels are possible. In a quiescent state, a circuit is made from a power source through terminal S  13 , through neutral stationary contact  12  and a first moving contact  17 , such as a finger or other shaped contact, through current transformer  24 , switch  28  and optional current transformer  42 . The first moving contact  17 , bus bars, copper or aluminum wire and/or other conductive materials may make up the circuit. In the quiescent state, switch  28  may be a two-pole, single-throw switch that is in electrical contact with first pole  32 . 
   Referring to  FIG. 2 , prior to full movement of the mechanical switch  28 , a first semiconductor device  34  is gated on by the control signal derived from the current traveling through current transformer  24 . Gate control  25  senses the current in current transformer  24  to initiate a gate signal at the first semiconductor device  34  after both moving fingers  16  and  17  make contact. Because the semiconductor device  34  is shorted by switch  28 , first semiconductor device  34  is not conducting in the quiescent state. In the quiescent state, only one finger makes contact, and the semiconductor devices are not triggered. The semiconductor device is, in some embodiments, a thyristor pair as shown in  FIG. 2 . However, other semiconductor devices are possible.  FIGS. 1 and 2  show an option in which a common gate control  25  controls both gate trigger circuits  36  and  41 . However, separate gate controls for each gate trigger circuit may be used in alternate embodiments. Although not shown in the figures, gate control  25  may have electrical connections with one or both of pole  30  (and corresponding current transformer  22 ) and pole  32  (and corresponding current transformer  24 ). Gate control  25  also may make an electrical connection with load terminal  48 . 
   The poles  30  and  32  of the mechanical switch  28  may have a relatively low voltage, such as approximately 50 to approximately 150 volts, across them at nominal distribution system voltages. However, the system line-to-ground voltage driving the load current through the switch  28  may be of a much higher voltage such as that used in the electrical power distribution system, including but not limited to common nominal voltages of about 7,200, about 14,400 or about 19,920 volts. A tap change sequence will include movement of the mechanical switch  28  from one pole position to the other along with tap switching to effect at least a substantially non-arcing tap transition while maintaining substantially continuous load current. 
   A Geneva wheel, sometimes known as a Geneva gear, or other mechanical means may be used to initiate movement of moving contacts  16  and  17  and mechanical switch  28 . Second moving contact  16 , such as a moving finger or other shaped contact, moves from a floating position (as shown in  FIG. 1 ) to a bridging position (as shown in  FIG. 2 ) to contact the series winding  18  at transformer tap  14 . Thus, the ability for conduction of current from the source through terminal S  13  through windings  18  and  20  to load at terminal  48  is possible although in this step current does not yet flow through first thyristor pair  34 , as switch  28  remains in contact with first pole  32 . Thus, current passes through first current transformer  24  but not second current transformer  22 , and thus the second semiconductor devices shown as a second thyristor pair  40  is gated off. Accordingly, in this step the load current passes through first pole  32  across switch  28  to the load terminal  48 . 
   In  FIG. 3 , switch  28  moves away from first pole  32  and towards second pole  30 . First thyristor pair  34  may be gated on prior to movement of switch  28 , as it is triggered by a gate trigger circuit  36  in response to the current in the current transformer  24  and the switch  28 . Thyristor pair  34  will remain gated on until gate control  25  detects a current zero as measured at current transformer  24 . 
   Referring to  FIG. 4 , switch  28  then contacts neither of the poles  32  and  30 . First thyristor pair  34  turns off at the next current zero after the gate signal is removed. The removal of the gate signal is controlled by the lack of current in pole  32  and may be instantaneous or after a predetermined delay. For a moment, conduction occurs through the snubbers  39  and  49 . Voltage then builds across first thyristor pair  34  and second thyristor pair  40 . When instantaneous voltage across either thyristor pair exceeds the normal peak operating voltage of winding  20  and winding  18  in series opposition by some predetermined margin, second thyristor pair  40  is gated into conduction, and current travels from source through S  13  to load through terminal  48  through second thyristor pair  40 . The load path continues in a similar manner with a voltage triggering based on an amount which takes into account the maximum voltage that may appear on winding  20 . 
   Referring to  FIG. 5 , switch  28  moves toward second pole  30 , and second thyristor pair  40  continues to carry the load until the switch electrically contacts second pole  30 . When contact occurs, current path is through second pole  30 , through switch  28  to load terminal  48 . When electrical contact at second pole  30  is solid, the current is shunted away from second thyristor pair  40 . 
   At this point, referring to  FIG. 6 , as the Geneva wheel continues to move, the first moving contact  17  moves away from neutral position  12  to a floating position. 
   Referring to  FIG. 7 , to eliminate the voltage buck or boost provided by directing load current to include the path of winding  20 , current may be redirected by movement of contact  17 , optionally initiated by movement of the Geneva wheel, from a floating position to contact winding  14 . First thyristor pair  34  may initially remain off as the voltage (corresponding to the voltage triggering value of winding  20 ) is not enough to gate first thyristor pair  34  on. Switch  28  may then be moved away from second pole  30 , causing a voltage across second gate trigger circuit  41  to build a voltage across the mechanical and semiconductor switches and the control which occurs after semiconductor switch  40  stops conducting. Since the second thyristor pair  40  has already been triggered on by the control responsive to the current through current transformer  22  and switch  28  in contact at pole  30 , thyristor pair  40  conducts to the next current zero or for a predetermined delay, such as a period of time or a number of half cycles, after the current through  30  ends. When current stops passing through mechanical switch  28 , second thyristor pair  40  stops conducting at the next current zero or for an additional predetermined delay, and the current path is momentarily maintained through snubber circuits  39  and  49  across thyristor pair  34  and  40 , respectively. 
   Referring to  FIG. 8 , mechanical switch  28  may then be moved toward first pole  32 , at which point first thyristor pair  34 , triggered by voltage, will carry the load until electrical contact is made at first pole  32 . 
   Referring to  FIG. 9 , when current is shunted by way of first pole  32  through switch  28 , current ceases to pass through first thyristor pair  34 , and the Geneva wheel continues to turn until the second moving contact  16  moves to a floating position. Accordingly, the thyristors and associated control and trigger circuits are only powered during a tap change, which may help to improve reliability. The power for the trigger and control circuits may be derived from voltage across contact poles  30  and  32 . In the quiescent state, substantially no voltage is present across those poles. 
   Current sensing is used to provide the selective triggering of the thyristor pair  34 , which is in parallel with the then current-conducting mechanical switch  28 . The removal of load current flowing through the switch  28  may cause the thyristor pair  34  to turn off based on removal of triggering from the thyristor pair  34 , optionally after a short time delay to ensure that the contact separation is enough to block impressed voltage. 
   Voltage sensing may be provided by control  25  and the gate controls  36  and  41  with the signal that informs that there is a rapid buildup of voltage across the thyristor switches resulting from both of the contacts  30  and  32  and both thyristor pairs  34  and  40  being open. Load current must then be commutated to the second thyristor pair  40 . During this time, load current flows through the thyristor snubber circuits driven by the regulator line voltage. The snubber circuits  39  and  49 , each including one or more series resistors and capacitors, charge rapidly but allow enough time to ensure that the first conducting thyristor has recovered its blocking state. When this voltage reaches a certain voltage, for example 400 volts, the control  25  causes the gate trigger circuit  41  to trigger the second thyristor pair  40  to maintain a load current path. The control performs this function by using the voltage level information plus the switch  28  direction information to trigger the correct thyristor pair, which will then continue to be triggered until the mechanical switch  28  closes. Directional information about switch  28  helps to ensure that the correct semiconductor device or devices turn on. Methods of obtaining the directional information may include use of electrical signals to set an electronic latch, or to use microswitch inputs from the Geneva wheel. 
   Referring to  FIGS. 1-9 , a first fuse  38  is associated with first thyristor pair  34 , and a second fuse  26  is associated with second thyristor pair  40 . Thus, if any of the semiconductor devices fail and short in a current-conducting position, the associated fuse will protect the circuit. 
   Thus, the hybrid design embodiments described herein provide numerous advantages over the prior art. For example, a bridging reactor that would be required if a vacuum interrupter or conventional tap changers were used is not required in non-vacuum embodiments such as those described herein. Contrasted with pure solid state tap changers, fewer thyristors are required. For example, the embodiments shown in the Figures described above only require two electrical switches, each including two semiconductor devices (e.g., thyristor pairs). Unlike previous hybrid designs, in various embodiments described herein only one mechanical auxiliary switch is required, and no power resistor or other current limiting device is required. The use of gate controls which derive signals from the referenced power circuit current and voltage may improve timing accuracy. Further, the configuration described herein may substantially even out the duty requirement for each thyristor pair on alternating tap changes. For transformers with more than two taps, the circuit may simply alternate between the two thyristor pairs as the contacts move from tap to tap. 
   It will be appreciated that some or all of the above-disclosed and other features and functions, or alternatives thereof, may be desirably combined into many other different systems or applications. Also that various presently unforeseen or unanticipated alternatives, modifications) variations or improvements therein may be subsequently made by those skilled in the art which are also intended to be encompassed by the following claims.