Abstract:
The invention provides a novel 3-phase electronic tap changer commutation and related device. In one embodiment, the invention includes firing a commutation silicon controlled rectifier (SCR), removing a gating signal from a first SCR connected to a first of the plurality of taps, firing a second SCR connected to a second of the plurality of taps, and removing a gating signal from the commutation SCR.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
   This application claims the benefit of U.S. Provisional Patent Application Ser. No. 60/618,829, filed 14 Oct. 2004, which is hereby incorporated herein by reference. 

   BACKGROUND OF THE INVENTION 
   (1) Field of the Invention 
   This invention applies to voltage regulators, and more particularly to a 3-phase alternating current (AC) electronic tap-changing voltage regulator. The present invention provides a specific transformer winding topology and commutation technique that improves performance and reduces cost compared to conventional methods. 
   (2) Background Art 
   Tap changing transformers are commonly used to regulate AC voltage in both low power, low voltage applications, and high power applications at distribution level voltages. Distribution level regulators typically consist of a multi-tapped transformer winding coupled to a mechanical tap changer so that regulation within +/−10% of nominal voltage is possible. These tap changer designs incorporate various mechanisms to ensure that, when transitioning from one tap to the next under load conditions, load current is not interrupted and arcing and inter-tap short circuit current are minimized. 
   In low voltage (e.g., less than about 1000V) and lower power applications (e.g., less than about 1 MVA) mechanical tap changers are often implemented using a simpler design incorporating a sliding commutation brush which can be positioned at arbitrary points along an exposed transformer winding in order to achieve the change in effective turns ratio. This technique has much lower cost than a discrete tap changer of the type used at higher power levels, but does not provide the same performance and also requires more maintenance. 
   Electronic tap changers are also commonly used in low voltage and low (e.g., less than about 1 kVA) to moderate (e.g., about 500 kVA) power levels. Referring now to  FIGS. 1-3 , three known devices are shown. In  FIG. 1 , an electronic tap changer  10  comprises an electronic switch  20 ,  22 ,  24  connected to each tap  12 ,  14 ,  16  of a multi-tapped transformer  40  or auto transformer. Typically, each switch  20 ,  22 ,  24  includes back-to-back connected silicon controlled rectifiers (SCRs)  30 , due to their low cost, simplicity, and ruggedness. By actively selecting which SCRs  30  are firing (e.g., by using appropriate sensing and gating controls, for example), the effective turns ratio of the transformer  40  can be controlled, so that the output voltage may be varied for a constant input voltage (as supplied by an AC voltage source  50 ), or, in the case of a regulator, the output voltage may be made constant within a certain tolerance under conditions of varying input voltage. Tap changer  10  may include other components, as would be recognized by one of ordinary skill in the art, including, for example, ground connections  32 , loads  34 , etc. 
   An alternative implementation to the basic electronic tap changer  10  ( FIG. 1 ) is shown in  FIG. 2 . Here, a series transformer secondary winding  60  reduces the current through the electronic switches  20 ,  22 ,  24 , while increasing the voltage withstand capability of each switch. 
   In any SCR-based on load tap changer, provisions must be made to avoid both load current discontinuity and high inter-tap circulating current when commutating the load current from one set of active SCRs to another (i.e., making a tap change). This is the same fundamental problem which must be addressed in the design of high power, discrete mechanical-on-load tap changers. The unique problem in the case of SCR based tap changers is a result of the gating characteristics of SCRs. That is, SCRs may be turned on at any arbitrary time, but will only cease to conduct when the load current naturally falls to zero (normally once each electrical half cycle). 
   When commutating from an ‘old’ tap to a ‘new’ tap, if the new tap SCR is fired before the old tap SCR has ceased conducting, short circuit current will potentially flow between the two taps until the old tap SCR current flows through current zero. This current overload is potentially damaging to the SCRs and transformer windings, and may result in a voltage drop as the short circuit current flows through the source impedance. Conversely, if a delay is used such that the old tap SCR is allowed sufficient time to turn off and regain its voltage blocking ability before the new tap SCR is activated, the current discontinuity which exists during the delay period may result in damaging or unacceptable voltage transients for inductive loads. 
   Referring now to  FIG. 3 , this problem can be solved by adding a commutating current path  70  through an impedance element (e.g., commutation resistor  80 ). This is a basic representation of one of many methods commonly utilized in high power, mechanical tap changers. In a device according to  FIG. 3 , when commutating from tap  12  to tap  14 , for example, the SCR pair  26  connected to the commutation resistor  80  is first gated, resulting in short circuit current between the two taps  12 ,  14 , which is limited by resistor  80  to an acceptable level. After the tap  12  conducting SCR  20  has naturally ceased to conduct, the tap  14  SCRs  22 ,  26  may be fired after some delay but with no concern for a current discontinuity as the load current may continue to flow through the resistor  80  until the tap  14  SCRs  22 ,  26  are conducting, at which time the gate signals of SCR pair  26  are removed. 
   The wiring scheme of  FIG. 3 , or one of its known derivatives, could be implemented on each tap in a 3-phase regulator in order to implement an acceptable commutation scheme for all possible tap changes. The additional complexity of this scheme, however, results in a substantial additional cost which may render the entire device impractical, and the additional control complexity and parts count reduces the reliability of the device. 
   SUMMARY OF THE INVENTION 
   The invention provides a novel 3-phase electronic tap changer commutation and related device. In one embodiment, the invention includes firing a commutation silicon controlled rectifier (SCR), removing a gating signal from a first SCR connected to a first of the plurality of taps, firing a second SCR connected to a second of the plurality of taps, and removing a gating signal from the commutation SCR. 
   A first aspect of the invention provides a method of commutating between a plurality of taps in a voltage regulating device, the method comprising: firing a commutation silicon controlled rectifier (SCR); removing a gating signal from a first SCR connected to a first of the plurality of taps; firing a second SCR connected to a second of the plurality of taps; and removing a gating signal from the commutation SCR. 
   A second aspect of the invention provides a method for substantially maintaining a voltage in a voltage regulating device, the method comprising: firing a first back-to-back connected pair of silicon controlled rectifiers (SCRs) connected in series to a commutation resistor; removing a gating signal from a second back-to-back connected pair of SCRs, whereby a load current of the second back-to-back connected pair of SCRs is allowed to fall to zero; firing a third back-to-back connected pair of SCRs; and removing a gating signal from the first back-to-back connected pair of SCRs, whereby the commutation resistor and first back-to-back connected pair of SCRs cease to conduct current. 
   A third aspect of the invention provides an alternating current voltage regulating device comprising: a commutation resistor; a back-to-back connected pair of silicon controlled rectifiers (SCRs); and at least one phase transformer including a plurality of taps, wherein the commutation resistor and back-to-back connected pair of SCRs substantially maintain a voltage for a period when none of the plurality of taps is firing. 
   The illustrative aspects of the present invention are designed to solve the problems herein described and other problems not discussed, which are discoverable by a skilled artisan. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other features of this invention will be more readily understood from the following detailed description of the various aspects of the invention taken in conjunction with the accompanying drawings that depict various embodiments of the invention, in which: 
       FIGS. 1-3  show schematic diagrams of illustrative known devices. 
       FIG. 4  shows a schematic diagram of an illustrative embodiment of the invention. 
       FIG. 5  shows a block diagram of an illustrative method according to the invention. 
   

   It is noted that the drawings of the invention are not to scale. The drawings are intended to depict only typical aspects of the invention, and therefore should not be considered as limiting the scope of the invention. In the drawings, like numbering represents like elements between the drawings. 
   DETAILED DESCRIPTION 
   As noted above, the invention provides a novel 3-phase electronic tap changer commutation method and related device. 
   The present invention provides, inter alia, a topology and control method for implementing an acceptable commutation method on a 3-phase AC electronic voltage regulator using only a single commutation resistor and its associated SCR. The topology of the invention is shown in  FIG. 4 . For the sake of brevity,  FIG. 4  shows only three tap selections  120 A,  122 A,  124 A for one (i.e.,  140 A) of the three phases  140 A-C. However, an actual implementation of the invention would typically contain additional taps. This basic topology utilizes series connected transformers  160 A-C and also makes an additional modification to the basic topology by utilizing a tapped winding  142 A that is separate from the main secondary winding  144 A. 
   An analysis of this topology  110  reveals that the SCRs associated with any of the three phases  140 A-C may be allowed to cease conducting as long as the commutation SCR  126  is fired. As such, a boost or buck voltage applied to the phase undergoing the commutation will equal the vectorial sum of the voltage being added to the other two phases, i.e., the sum of the voltage vectors across the other two buck/boost transformers. In a three-phase system, the boost or buck voltage required by all three phases is generally equal. Accordingly, the voltage buck or boost under this condition will generally be similar to the desired buck or boost under the normal condition in which the tap winding SCRs are conducting. 
   A control scheme can be implemented using the topology  110  of  FIG. 4 . Under normal conditions, the commutation SCR  126  is not being fired, so that each tap winding (e.g.,  112 A,  114 A,  116 A) is connected to its corresponding series transformer (e.g.,  160 A), and all of the current flowing through the primary windings of the series transformer (e.g.,  160 A) is carried by the tap windings of the corresponding transformer phase (e.g.,  140 A). 
   Referring now to  FIG. 5 , a block diagram of an illustrative method of commutating from an ‘old’ SCR pair (e.g.,  120 A in  FIG. 4 ) to a ‘new’ SCR pair (e.g.,  122 A in  FIG. 4 ) is shown. First, at step S 1 , the commutation SCR pair  126  ( FIG. 4 ) is fired such that it remains in an AC conductive state. At this point, if the vectorial sum of the three individual phase voltages being applied to the three buck/boost transformers is non zero, a current will flow through the commutating resistor  180  ( FIG. 4 ) equal to the vectorial sum of the three buck/boost voltages divided by the commutating resistance value in Ohms. 
   Next, at step S 2 , the gating signals to the ‘old’ SCR  120 A are removed, so that its load current may be allowed to naturally fall to zero and the old SCR  120 A ceases conducting current. At this point, the primary current of the series transformer  160 A ( FIG. 4 ) is supplied via the path which includes the commutating resistor  180  ( FIG. 4 ) and SCR pair  126  ( FIG. 4 ) and the tap windings of the other two phases  140 B,  140 C ( FIG. 4 ). 
   At step optional step S 3 , a current through the old SCR  120 A is determined, e.g., through any known or later-developed measurement method, to ensure that the current has reached zero. Alternatively, it may be assumed that the current has reached zero after a fixed delay time (typically ½ or more electrical cycle). 
   Next, at step S 4 , the ‘new’ SCR  122 A is fired. Finally, at step S 5 , the gating signal to the commutation SCR  126  is removed, so that after a maximum of approximately ½ electrical cycle, the commutation SCR  126  and resistor  180  cease to conduct current. 
   The purpose of this scheme, as outlined with the single phase example above, is to provide a method for maintaining a continuous current through a series transformer associated with the phase undergoing a tap change and substantially maintaining the voltage across the primary winding during the commutation period, such that the voltage does not differ appreciably from the desired voltage. 
   The topology and method described herein require far fewer components and control complexity than would otherwise be required. That is, the present invention provides equal or similar performance to a scheme that utilizes a commutation resistor and SCR pair in conjunction with each tap winding SCR, but at greatly reduced cost and complexity. 
   It should be understood that the present invention works with switching solid-state semiconductor devices. Theses devices are synonymously know as Silicon Controlled Rectifiers (SCRs), anti-parallel SCRs, back-to-back SCRs, triode AC switches (triacs), gate turn-off thyristors (GTOs), static induction transistor (SITs), static induction thyristor (SITHs) or MOS-controlled thyristors (MCTs) and the present invention should not limited to the above named electronic switching devices.