text: 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 fig4 . for the sake of brevity , fig4 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 fig4 . 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 fig5 , a block diagram of an illustrative method of commutating from an ‘ old ’ scr pair ( e . g ., 120 a in fig4 ) to a ‘ new ’ scr pair ( e . g ., 122 a in fig4 ) is shown . first , at step s 1 , the commutation scr pair 126 ( fig4 ) 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 ( fig4 ) 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 ( fig4 ) is supplied via the path which includes the commutating resistor 180 ( fig4 ) and scr pair 126 ( fig4 ) and the tap windings of the other two phases 140 b , 140 c ( fig4 ). 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 .