Patent Publication Number: US-9898019-B2

Title: Thyristor assisted on-load tap changer and method thereof

Description:
FIELD OF THE INVENTION 
     The present invention relates to the technical field of power transmission and transformation of power systems, and particularly relates to a thyristor assisted on-load tap changer and a method thereof. 
     BACKGROUND OF THE INVENTION 
     The operation mode of a power system changes at any time, and the change of the operation mode will cause the fluctuation of a bus voltage. The power system has strict requirements on the fluctuation range of the bus voltage, therefore, a technology for regulating the bus voltage is needed. The most direct manner of regulating the voltage is to change a transformer tap. However, in a load transmission process of the power system, an on-load tap changer with a very high technical content is required for changing the transformer tap without power outage. Reactive on-load tap changers and resistive on-load tap changers are mainly adopted in the world at present. U.S. Pat. No. 3,176,089, U.S. Pat. No. 5,128,605 and U.S. Pat. No. 7,880,341 disclose the reactive on-load tap changers, and U.S. Pat. No. 4,081,741 and U.S. Pat. No. 4,520,246 disclose the resistive on-load tap changers. The reactors of the reactive on-load tap changers are energized for a long term, are relatively large in volumes and are only adopted in the America in the world, and the resistive on-load tap changers are generally adopted in other countries. The resistive on-load tap changers suffer a heating problem, and a significant rise in temperature will be generated by switching the taps of the on-load tap changers for multiple times within a short period of time. Therefore, the switching time of the on-load tap changers within a certain time is strictly limited. 
     The performance of the on-load tap changer is improved by a thyristor circuit in the U.S. Pat. No. 4,622,513. One of the invention points is that when the switch of a switched current path is switched off, an overvoltage triggering thyristor circuit of a switching path is automatically switched on to quickly splice and switch load current. The defect of the overvoltage triggering thyristor circuit lies in that very large pulse interference is generated every 10 milliseconds. Therefore, adequate anti-interference measures and safety measures are needed to ensure the reliable work of the on-load tap changer. Another invention point of the invention is that, a bidirectional parallel thyristor is triggered by a current transducer to assist a mechanical switch to disconduct the switched current path; the bidirectional parallel thyristor is connected with the mechanical switch in parallel, and the bidirectional parallel thyristor may be switched on by the pulse interference by mistake to cause short circuit circulation. Therefore, the overvoltage triggering thyristor circuit in the invention is serially connected with a transition resistor, in order to limit the possible short circuit circulation to improve the operation safety of the thyristor; thus in U.S. Pat. No. 4,622,513, the heating of the transition resistor is only reduced, while the heating problem of the transition resistor is not solved completely. The U.S. Pat. No. 7,595,614 is an improvement of U.S. Pat. No. 4,622,513. In the U.S. Pat. No. 7,595,614, the transition resistor serially connected with the overvoltage triggering thyristor circuit is cancelled, the heating problem of the transition resistor is solved; since the transition resistor limiting the short circuit circulation is cancelled, in the case of the short circuit circulation, the short circuit circulation is very large; in U.S. Pat. No. 7,595,614, protection is only achieved by a fuse, and the reaction speed of fuse protection is slow, so the safety is poor. In the U.S. Pat. No. 7,595,614, the bidirectional parallel thyristor is still triggered by the current transducer to disconduct the switched current path, and no new anti-interference measure is added, thus the reliability is poor. 
     In the U.S. Pat. No. 4,622,513 and U.S. Pat. No. 7,595,614, a bidirectional parallel thyristor switching circuit is triggered by the secondary current of a current transducer to switch on and cut off a bidirectional parallel thyristor, and the reliability of a trigger circuit is poor. In the U.S. Pat. No. 4,622,513 and U.S. Pat. No. 7,595,614, a traditional complicated mechanical cam sliding mechanism and an energy accumulating mechanism are stilled adopted, thus the operation vibration and the noise are large; failure is liable to happen, and more frequent operation cannot be implemented. 
     SUMMARY OF THE INVENTION 
     The purpose of the present invention is to solve the above problems, (1) to overcome the shortcomings and to provide an overvoltage triggering thyristor circuit assisted on-load tap changer, which has no transition resistor, has a short circuit circulation limit measure and is high in safety and high in reliability; (2) to give play to the advantages and to provide an on-load tap changer, which needs no constraint of a mechanical linkage between a tap selector and a switcher and is clear in logical relationship, simple in structure and convenient to cooperate; (3) to provide an on-load tap changer, which is simpler in structure and is more economical. 
     To achieve the above purpose, the present invention adopts the following technical solutions: 
     a thyristor assisted on-load tap changer, including: 
     a thyristor assisted on-load tap changer includes a main path and a transition path; the main path is composed of a switch K 1 , and the transition path is composed of a linear reactor L 1 , a saturable reactor L 2  and an overvoltage triggering thyristor circuit, which are connected in series; one end of the switch K 1  is switched between a tap selector terminal I and a tap selector terminal II through a change-over switch K 5 , and one end of the linear reactor L 1  is switched between the tap selector terminal I and the tap selector terminal II through a change-over switch K 6 ; the switch K 1  and the other end of the overvoltage triggering thyristor circuit are connected with a common terminal. 
     A thyristor assisted on-load tap changer includes a main path and a transition path; the main path is composed of a saturable reactor L 4  and a thyristor switch controlled by a control switch K 10 , which are connected in series; the transition path is composed of a linear reactor L 1 , a saturable reactor L 2  and an overvoltage triggering thyristor circuit, which are connected in series; one end of the saturable reactor L 4  is switched between a tap selector terminal I and a tap selector terminal II through a change-over switch K 5 , and one end of the linear reactor L 1  is switched between the tap selector terminal I and the tap selector terminal II through a change-over switch K 6 ; the thyristor switch and the other end of the overvoltage triggering thyristor circuit are connected with a common terminal. 
     A thyristor assisted on-load tap changer includes a main path and a transition path; the main path is composed of a saturable reactor L 4  and a thyristor switch controlled by a control switch K 10 , which are connected in series; the transition path is composed of a linear reactor L 1 , a saturable reactor L 2  and an overvoltage triggering thyristor circuit, which are connected in series; one end of the saturable reactor L 4  is jointly connected with one ends of transition switches K 15 , K 14 , and the other ends of the transition switches K 15 , K 14  are respectively connected with tap selector terminals I, II; one end of the linear reactor L 1  is jointly connected with one ends of transition switches K 17 , K 16 , and the other ends of the transition switches K 17 , K 16  are respectively connected with the tap selector terminals I, II; the thyristor switch and the other end of the overvoltage triggering thyristor circuit are connected with a common terminal; an odd-numbered side main contact K 11  is further connected between the tap selector terminal I and the common terminal, and an even-numbered side main contact K 12  is further connected between the tap selector terminal II and the common terminal. 
     The reactance of the linear reactor L 1  is larger than zero and is smaller than Z 1 ; Z 1  is equal to a quotient obtained by dividing a rated voltage between the tap selector terminals I, II by rated load current. 
     The linear reactor L 1  and the saturable reactor L 2  are merged into a reactor L 3 ; the reactor L 3  is provided with a magnetic flux closed-loop iron core and a coil L 3 , a part of section of the magnetic flux closed-loop iron core has a larger sectional area, and the sectional area of the rest section of the iron core is smaller; the coil L 3  is winded on the iron core at the section with the larger sectional area; when the current is relatively small, the closed-loop iron core is unsaturated; the coil L 3  is equivalent to the saturable reactor L 2 ; when the current is relatively large, the iron core at the section with the smaller sectional area of the closed-loop iron core is saturated, and the iron core at the section with the larger sectional area is unsaturated; the reactance of the coil L 3  is decreased to a smaller value quickly, and at this time, the coil L 3  is equivalent to the linear reactor L 1 . 
     The overvoltage triggering thyristor circuit includes a fuse FU 1 , and the fuse FU 1  is serially connected with a pair of thyristors D 1 , D 2  reversely connected in parallel to form a main path of the overvoltage triggering thyristor circuit; a resistor R 1  and a capacitor C 1  are connected to the two ends of the thyristors D 1 , D 2  reversely connected in parallel after being connected in series; the gate electrodes and the cathodes of the two thyristors D 1 , D 2  are respectively connected with capacitors C 2 , C 3 , resistors R 2 , R 3  and diodes D 3 , D 4 ; the gate electrodes of the two thyristors D 1 , D 2  are further respectively connected with the input terminal of a full-bridge rectifier composed of diodes D 5 , D 6 , D 7 , D 8 , the output end of the full-bridge rectifier is connected with a constant voltage diode D 9 , the cathode of the constant voltage diode D 9  is connected with the output end anode of the full-bridge rectifier, and the anode of the constant voltage diode D 9  is connected with the output end cathode of the full-bridge rectifier; the stabilized voltage U 1  of the constant voltage diode D 9  is equal to k 1 U 2 ; k 1  refers to a confidence coefficient and is a value of 1.2-2; U 2  refers to the peak value of a rated working frequency operating voltage connected between the tap selector terminals I, II of the on-load tap changer. 
     The thyristor switch includes: a fuse FU 1  is serially connected with a pair of thyristors D 1 , D 2  reversely connected in parallel to form a main path of the thyristor switch; a resistor R 1  and a capacitor C 1  are connected to the two ends of the thyristors D 1 , D 2  reversely connected in parallel after being connected in series; the gate electrodes and the cathodes of the two thyristors D 1 , D 2  are respectively connected with capacitors C 2 , C 3 , resistors R 2 , R 3  and diodes D 3 , D 4 ; the gate electrodes of the two thyristors D 1 , D 2  are further respectively connected with the input terminal of a full-bridge rectifier composed of diodes D 5 , D 6 , D 7 , D 8 ; a constant voltage diode D 11  and a constant voltage diode D 9  are serially connected in the same direction, the serial anodes of the constant voltage diodes D 11 , D 9  are connected with the cathode of the full-bridge rectifier, and the serial cathodes of the constant voltage diodes D 11 , D 9  are connected with the anode of the full-bridge rectifier; the anode of a diode D 10  is connected with the anode of the full-bridge rectifier, the cathode of the diode D 10  is connected with one end of a switch K 10 , and the other end of the switch K 10  is connected with the cathode of the full-bridge rectifier; the stabilized voltage value U 3  of the serially connected constant voltage diode D 11  and constant voltage diode D 9  is equal to k 2 (U 1 +U 2 ); k 2  refers to a confidence coefficient and is a value of 1.1-1.5; U 1 =k 1 U 2 , k 1  refers to a confidence coefficient and is a value of 1.2-2; U 2  refers to the peak value of a rated working frequency operating voltage connected between the tap selector terminals  1 ,  2  of the on-load tap changer; the sum of positive tube voltage drops of all semiconductors of a gate electrode trigger loop of the thyristor D 1  or D 2  is about 1.5U 4 , U 4  refers to the maximum current and includes the transient peak value of the short circuit current possibly flowing by and the positive tube voltage drop flowing by the main path of the thyristor D 1  or D 2 . 
     The terminal of a non-common terminal of the main path and the terminal of a non-common terminal of the transition path are further connected with a bidirectional voltage stabilizing circuit; the voltage stabilizing value of the bidirectional voltage stabilizing circuit is larger than the peak value U 2  of the rated working frequency operating voltage connected between the tap selector terminals I, II of the on-load tap changer and is smaller than the stabilized voltage U 1  of the constant voltage diode D 9 . 
     The switches (contacts) are contactors with locks and are composed of closing coils, breaking coils, main contacts and auxiliary contacts; or are contactors without locks and are composed of closing coils, main contacts and auxiliary contacts; the coils are energized or de-energized to switch on and switch off the switches (contacts). 
     A thyristor assisted on-load tap changer is composed of a tap selector and a switcher; the tap selector is connected with the switcher, and after the tap selector selects the tap of a regulating transformer, the switcher achieves the on-load switch of the tap; wherein the switcher includes a main switch K 21 - 1 , a main switch K 22 - 1 , an economical thyristor assisted circuit I, an economical thyristor assisted circuit II, a piezoresistor R and three terminals J 1 , J 2 , J 3 ; 
     one end of the main switch K 21 - 1  is connected with the terminal J 1 , and the other end of the main switch K 21 - 1  is connected with the terminal J 3 ; the economical thyristor assisted circuit I is connected with the main switch K 21 - 1  in parallel; 
     one end of the main switch K 22 - 1  is connected with the terminal J 2 , and the other end of the main switch K 22 - 1  is connected with the terminal J 3 ; the economical thyristor assisted circuit II is connected with the main switch K 22 - 1  in parallel; 
     the end of the economical thyristor assisted circuit I close to the J 1  and the end of the thyristor assisted circuit II close to the J 2  are further connected with the piezoresistor R; 
     a pair of switches are respectively arranged in the economical thyristor assisted circuit I and the economical thyristor assisted circuit II, for controlling the state switch of the corresponding thyristor assisted circuit, wherein the serial number of a normally open switch KA in the economical thyristor assisted circuit I is K 23 - 1 , and the serial number of KB is K 25 - 1 ; 
     the serial number of a normally open switch KA in the economical thyristor assisted circuit II is K 24 - 1 , and the serial number of KB is K 26 - 1 . 
     A thyristor assisted on-load tap changer is composed of a tap selector and a switcher; the tap selector is connected with the switcher, and after the tap selector selects the tap of a regulating transformer, the switcher achieves the on-load switch of the tap; wherein the switcher includes a main switch K 21 - 1 , a main switch K 22 - 1 , a switch K 27 - 1 , a switch K 28 - 1 , an economical thyristor assisted circuit I, an economical thyristor assisted circuit II, a piezoresistor R and three terminals J 1 , J 2 , J 3 ; 
     one end of the main switch K 21 - 1  is connected with the terminal J 1 , and the other end of the main switch K 21 - 1  is connected with the terminal J 3 ; one end of the economical thyristor assisted circuit I is connected with the terminal J 3 , and the other end of the economical thyristor assisted circuit I is connected with the terminal J 1  through the switch K 27 - 1 ; 
     one end of the main switch K 22 - 1  is connected with the terminal J 2 , and the other end of the main switch K 22 - 1  is connected with the terminal J 3 ; one end of the economical thyristor assisted circuit II is connected with the terminal J 3 , and the other end of the economical thyristor assisted circuit II is connected with the terminal J 2  through the switch K 28 - 1 ; 
     the end of the economical thyristor assisted circuit I connected with the switch K 27 - 1  and the end of the thyristor assisted circuit II connected with the switch K 28 - 1  are further connected with the piezoresistor R; 
     a pair of switches are respectively arranged in the economical thyristor assisted circuit I and the economical thyristor assisted circuit II, for controlling the state switch of the corresponding thyristor assisted circuit, wherein the serial number of a normally open switch KA in the economical thyristor assisted circuit I is K 23 - 1 , and the serial number of KB is K 25 - 1 ; 
     the serial number of a normally open switch KA in the economical thyristor assisted circuit II is K 24 - 1 , and the serial number of KB is K 26 - 1 . 
     The economical thyristor assisted circuit I and the economical thyristor assisted circuit II have the same structure and respectively include: 
     a pair of thyristors D 1 , D 2  are reversely connected in parallel to form a main path of the thyristor assisted circuit; 
     a resistor R 1  and a capacitor C 1  are connected to the two ends of the thyristors D 1 , D 2  reversely connected in parallel after being connected in series; 
     the gate electrodes and the cathodes of the two thyristors D 1 , D 2  are respectively connected with capacitors C 2 , C 3 , resistors R 2 , R 3  and diodes D 3 , D 4 ; the anodes of the diodes D 3 , D 4  are respectively connected with the gate electrodes of the thyristors D 1 , D 2 , and the cathodes of the diodes D 3 , D 4  are respectively connected with the cathodes of the thyristors D 1 , D 2 ; 
     the input terminal of a full-bridge rectifier composed of diodes D 5 , D 6 , D 7 , D 8  is connected between the gate electrodes of the two thyristors D 1 , D 2  after being serially connected with a normally open switch KB, the output end of the full-bridge rectifier is connected with a constant voltage diode D 9 , the cathode of the constant voltage diode D 9  is connected with the anode output end of the full-bridge rectifier, and the anode of the constant voltage diode D 9  is connected with the cathode output end of the full-bridge rectifier; 
     diodes D 13 , D 14 , D 15  are serially connected in the same direction, diodes D 16 , D 17 , D 18  are serially connected in the same direction, and the two diode strings are serially connected with a normally open switch KA after being reversely connected in parallel and are connected between the gate electrodes of the two thyristors D 1 , D 2 . 
     In the tap terminals of the regulating transformer, the centremost terminal is defined as a null line, the null line and an adjacent tap terminal of the regulating transformer are respectively connected with two terminals of a primary coil of a transformer T 2 , and the terminal of a secondary coil of the transformer T 2  provides an AC control voltage to the switcher; one terminal of the AC control voltage is defined as a null line, and the null line of the primary coil of the transformer T 2  is connected with the null line of the secondary coil of the transformer T 2 ; 
     the AC control voltage terminal is further used as the input to a DC voltage stabilization power supply module, the DC voltage stabilization power supply module provides a DC control voltage to the switcher, the low-potential terminal of the DC control voltage is defined as a null line, and the null line of the DC control voltage is connected with the null line of the AC control voltage. 
     The working method of the thyristor assisted on-load tap changer is characterized in that, 
     a. the working method of switching the conduction of the terminal J 1  of the switcher with the common terminal J 3  to the conduction of the terminal J 2  with the common terminal J 3  is as follows: 
     (1) switching on the switch K 23 - 1  and switching on the switch K 26 - 1 ; (2) switching off the main switch K 21 - 1 ; (3) switching off the switch K 23 - 1 ; (4) switching on the switch K 24 - 1 ; (5) switching on the main switch K 22 - 1 ; (6) resetting the entire group; 
     b. the working method of switching the conduction of the terminal J 2  of the switcher of the on-load tap changer with the common terminal J 3  to the conduction of the terminal J 1  with the common terminal J 3  is as follows: 
     (1) switching on the switch K 24 - 1  and switching on the switch K 25 - 1 ; (2) switching off the main switch K 22 - 1 ; (3) switching off the switch K 24 - 1 ; (4) switching on the switch K 23 - 1 ; (5) switching on the main switch K 21 - 1 ; (6) resetting the entire group. When switching the conduction of the terminal J 1  of the switcher of the on-load tap changer with the common terminal J 3  to the conduction of the terminal J 2  with the common terminal J 3 , the time interval between switching off the switch K 23 - 1  and switching on the switch K 24 - 1  is larger than 20 milliseconds; 
     when switching the conduction of the terminal J 2  of the switcher of the on-load tap changer with the common terminal J 3  to the conduction of the terminal J 1  with the common terminal J 3 , the time interval between switching off the switch K 24 - 1  and switching on the switch K 23 - 1  is larger than 20 milliseconds. 
     The beneficial effects of the present invention lie in that: the transition resistor is cancelled and the heating problem of the resistor is solved; measures for limiting the short circuit circulation can be adopted on occasions with high safety requirements to better ensure the safety of the overvoltage triggering thyristor circuit and the transistor switch circuit. The overvoltage triggering thyristor circuit and the transistor switch circuit have stronger anti-interference measures to ensure the reliable work of the thyristor assisted on-load tap changer under a strong pulse interference condition. No current is generated in the disconduction and conduction processes of the mechanical switch; arc-free switch is achieved; and the switch contact is not damaged by frequent action. The energy accumulating mechanism of the traditional on-load tap changer can be eliminated, thus the overall action time of the thyristor assisted on-load tap changer can be shortened. The complicated mechanical linkage mechanism, particularly the energy accumulating mechanism is removed to reduce the volume and weight of the on-load tap changer; the failure rate is decreased. The control circuit in the manner of an intermediate relay (contactor) can be adopted to ensure entering the action program of the next switch after the action of a certain switch is finished, in order to improve the reliability. The action of the tap selector needs no intervention of the switcher, the switcher is started to work after the action of the tap selector is finished, and no intervention of the tap selector is needed in the switch process of the switcher; the tap selector and the switcher need no constraint of a mechanical linkage, and are clear in logical relationship, simple in structure and convenient to cooperate. 
     For the thyristor assisted on-load tap changer, the electric switches can be manually operated to sequentially act to achieve the on-load switch of the switcher; the electric switches can be driven by a mechanical linkage mechanism to sequentially act to achieve the on-load switch of the switcher; the electric switches can be controlled by the contacts of contactors (relays) to sequentially act to achieve the on-load switch of the switcher; a variety of methods can be adopted, thus the application is flexible. The action state of the main contact is reflected by the auxiliary contact of the relay (contactor), namely, it is ensured that the action program of the next switch is entered after the action state of a certain switch is determined and that the action program of the next switch is entered immediately after the action state of the certain switch is determined; a perfect combination of speed and reliability is achieved. Except the main switch, a switcher of the thyristor assisted on-load tap changer needs no other large capacity relay (contactor); the thyristor trigger circuit can be controlled by the on/off of the contact of a small capacity relay (contactor) to switch on/off a high current thyristor, in order to switch the on-load tap changer. The on-load tap changer is simple in structure, convenient to control and low in cost. The main switch and the contact of the small capacity relay (contactor) are operated in an arc-free manner. Within the non-action time period of the on-load tap changer, the thyristor assisted circuit has no voltage, thus the safety of the thyristor assisted circuit is high. The voltage difference between the control power supply potential and the switch contact of the switcher of the thyristor assisted on-load tap changer is small, and the requirements on the withstand voltage of the insulating material there between are low; particularly for the on-load tap changer of a 10 kV system, the on-load tap changer in the present invention can be manufactured by a conventional AC contactor to reduce the manufacturing cost. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a structure and a connecting manner of an existing on-load tap changer. 
         FIG. 2  shows a structure and a connecting manner of a thyristor assisted on-load tap changer. 
         FIG. 3  shows an overvoltage triggering thyristor circuit. 
         FIG. 4  shows serial connection of overvoltage triggering thyristor circuits. 
         FIG. 5  shows a reactor structure. 
         FIG. 6  shows a structure and a connecting manner of a second thyristor assisted on-load tap changer. 
         FIG. 7  shows a thyristor switching circuit. 
         FIG. 8  shows a structure and a connecting manner of a third thyristor assisted on-load tap changer. 
         FIG. 9  shows a switcher control circuit of a third thyristor assisted on-load tap changer. 
         FIG. 10  shows a switcher control circuit of a thyristor assisted on-load tap changer. 
         FIG. 11  shows a switcher control circuit of a second thyristor assisted on-load tap changer. 
         FIG. 12  shows a structure of a switcher of a fourth thyristor assisted on-load tap changer. 
         FIG. 13  shows an economical thyristor assisted circuit. 
         FIG. 14  shows a structure of a switcher of a fifth thyristor assisted on-load tap changer. 
         FIG. 15 a    shows a control circuit of a third switching state. 
         FIG. 15 b    shows a control circuit of a fourth switching state. 
         FIG. 16  shows a power supply structure of a thyristor assisted on-load tap changer. 
     
    
    
     Wherein,  1 . tap selector terminal I,  2 . tap selector terminal II,  3 . common terminal,  4 . overvoltage triggering thyristor circuit,  5 . reactor iron core,  6 . thyristor switch,  7 . bidirectional voltage stabilizing circuit,  8 . economical thyristor assisted circuit I,  9 . economical thyristor assisted circuit II,  10 . tap selector,  11 . Switcher (diverter switch),  12 . DC voltage stabilization power supply module. 
     EMBODIMENT 1 
     A further illustration of the present invention will be given below in combination with accompanying drawings and embodiments. 
       FIG. 1  shows a working principle structure and a connecting manner of an existing on-load tap changer. The on-load tap changer is composed of a tap selector and a switcher. The tap selector is connected with the switcher, and after the tap selector selects the tap of a regulating transformer, the switcher achieves the on-load switch of the tap. The working principle of the tap selector of the on-load tap changer is public knowledge; the on-load tap changer is characterized by the switcher, and the so-called on-load tap changer generally refers to the switcher (diverter switch) of the on-load tap changer. 
     The principle structure and the connecting manner of a thyristor assisted on-load tap changer in the present invention are as shown in  FIG. 2 . It includes two tap selector terminals I 1 ,II 2 , a common terminal  3 , two change-over switches K 5 , K 6 , a main vacuum switch K 1 , an overvoltage triggering thyristor circuit  4 , a linear reactor L 1 , a saturable reactor L 2  and a bidirectional voltage stabilizing circuit  7 ; one tap terminal of the change-over switch K 5  and one tap terminal of the change-over switch K 6  are jointly connected with the tap selector terminal I 1 , the other tap terminal of the change-over switch K 5  and the other tap terminal of the change-over switch K 6  are jointly connected with the tap selector terminal II 2 ; the common terminal of the change-over switch K 5  is connected with the common terminal  3  of the on-load tap changer through the main vacuum switch K 1  to form a main path; the common terminal of the change-over switch K 6  is serially connected with the common terminal  3  of the on-load tap changer through the linear reactor L 1 , the saturable reactor L 2  and the overvoltage triggering thyristor circuit  4  to form a transition path; the bidirectional voltage stabilizing circuit  7  is connected between the common terminal of the change-over switch K 5  and the common terminal of the change-over switch K 6 . 
     The overvoltage triggering thyristor circuit  4  is as shown in  FIG. 3 . A fuse FU 1  is serially connected with a pair of thyristors D 1 , D 2  reversely connected in parallel to form a main path. A resistor R 1  and a capacitor C 1  are connected to the two ends of the thyristors D 1 , D 2  reversely connected in parallel after being connected in series, in order to achieve the oscillation damping in the processes which the thyristors D 1 ,D 2  are triggered on/off and prevent the wrong trigger caused by the over fast voltage rise at the two ends of the thyristors D 1 , D 2 . The gate electrodes and the cathodes of the two thyristors D 1 , D 2  are respectively connected with capacitors C 2 , C 3  and resistors R 2 , R 3  for resisting interference; the anodes of diodes D 3 , D 4  are respectively connected with the cathodes of the thyristors, and the cathodes of the diodes are respectively connected with the gate electrodes of the thyristors to protect the gate electrodes and the cathodes from being broken down by a reverse voltage and to provide a reverse current path. The gate electrodes of the two thyristors D 1 , D 2  are further respectively connected with the input terminal of a full-bride rectifier composed of diodes D 5 , D 6 , D 7 , D 8 , the output end of the full-bride rectifier is connected with a constant voltage diode D 9 , the cathode of the constant voltage diode D 9  is connected with the output end anode of the full-bride rectifier, the anode of the constant voltage diode D 9  is connected with the output end cathode of the full bridge rectifier circuit, and multiple low voltage level constant voltage diode D 9  can be serially connected to obtain a high voltage level constant voltage diode. 
     The stabilized voltage of the constant voltage diode D 9  should be larger than the peak value of the maximal normal voltage between the tap selector terminals I 1 , II 2 , in order to ensure that the constant voltage diode D 9  is not conducted when performing on-load voltage regulation on the voltage of the regulating transformer within the maximal normal fluctuation range. If the stabilized voltage of the constant voltage diode D 9  is too large, the withstand voltage of the main vacuum switch K 1  is required to be increased and the withstand voltages of the thyristors D 1 , D 2  are required to be increased, thus increasing the volume and investment of the on-load tap changer. If the stabilized voltage of the constant voltage diode D 9  is too large, the interference of the overvoltage triggering thyristor circuit  4  with other equipment is increased, and the reliability of the thyristor assisted on-load tap changer is poor. In particular, if the stabilized voltage of the constant voltage diode D 9  is too large, the interference of the overvoltage triggering thyristor circuit  4  will produce a transient DC component to excite the regulating transformer to produce magnetizing rush current to cause protection trip. The stabilized voltage of the constant voltage diode D 9  cannot be too large to ensure the reliable operation of the thyristor assisted on-load tap changer. Therefore, the stabilized voltage U 1  of the constant voltage diode D 9  is equal to k 1 U 2 ; k 1  refers to a confidence coefficient and is a value of 1.2-2; U 2  refers to the peak value of a rated working frequency operating voltage connected between the tap selector terminals  1 ,  2  of the on-load tap changer. It is recommended that k 1  is 1.5. 
     The conduction of the tap selector terminal I 1  of the thyristor assisted on-load tap changer with the common terminal  3  can be switched to the conduction of the tap selector terminal II 2  with the common terminal  3 ; the conduction of the tap selector terminal II 2  with the common terminal  3  can be switched to the conduction of the tap selector terminal I 1  with the common terminal  3 . 
     The working principle of switching the conduction of the tap selector terminal I 1  of the on-load tap changer with the common terminal  3  to the conduction of the tap selector terminal II 2  with the common terminal  3  is as follows: 
     (1) the change-over switch K 6  is switched; the overvoltage triggering thyristor circuit  4  is switched on, since the stabilized voltage of the constant voltage diode D 9  is larger than the peak value of the maximal normal AC voltage between the tap selector terminals I 1 , II 2 , the constant voltage diode D 9  is not conducted, and the thyristors D 1 , D 2  reversely connected in parallel are not triggered; the overvoltage triggering thyristor circuit  4  is not conducted; 
     (2) the main vacuum switch K 1  is switched off; the main path is switched off, and the potential of the terminal  3  connected with a load quickly decreases; the voltages at the two ends of the overvoltage triggering thyristor circuit  4  quickly rise, when the instantaneous value of the voltage is larger than the stabilized voltage of the constant voltage diode D 9 , the constant voltage diode D 9  is conducted to trigger the thyristor D 1  or D 2  to be conducted, and the transition path is automatically switched on; load current flows in from the tap selector terminal II 2  and flows out from the common terminal  3  through the transition path; since the current is alternating current, the overvoltage triggering thyristor circuit  4  automatically cuts off the current loop for one time at a zero crossing point of the current; then, the voltages at the two ends of the overvoltage triggering thyristor circuit  4  rise again, and the overvoltage triggering thyristor circuit  4  is conducted again; the voltages at two ends of the overvoltage triggering thyristor circuit  4  are pulse voltages performing positive and negative transformation every 10 milliseconds; the pulse peak value is equal to the stabilized voltage of the constant voltage diode D 9 . The positive and negative alternating pulse voltage has small influence on the waveform of the load current and has small influence on the waveform of the load voltage; the load current is transferred from the main path to the transition path; 
     (3) the change-over switch K 5  is switched; 
     (4) the main vacuum switch K 1  is switched on; the load current flows through the main vacuum switch K 1 , and the current of the overvoltage triggering thyristor circuit  4  is reduced to zero. 
     When the main vacuum switch K 1  is not switched off, the overvoltage triggering thyristor circuit  4  will form short circuit circulation due to interference and wrong conduction. For a large capacity power system, if the reactance of the linear reactor L 1  is equal to zero, very large short circuit circulation will be formed. At this time, as long as the reactance of the linear reactor L 1  is slightly larger than zero, the effect of limiting the short circuit current is very obvious. Therefore, for the safety of the thyristor assisted on-load tap changer, the reactance of the linear reactor L 1  must be larger than zero. If the reactance of the linear reactor L 1  is large, the advantages lie in that the formed short circuit circulation is small and the safety is good. The defects lie in that the linear reactor L 1  may generate larger interference, in particular the DC component excite an iron core transformer to produce magnetizing rush current, thus the harm is quite large. The value of the linear reactor L 1  is designed to balance the conflict of limiting the short circuit circulation and reducing the interference. Considering that the overvoltage triggering thyristor circuit can limit the short circuit circulation time within a half cycle, as long as the short circuit circulation is not larger than 10 times of rated working current of the thyristor, the safety of the thyristor is guaranteed; the reactance of the linear reactor L 1  should be larger than zero and smaller than Z 1 ; Z 1  is equal to a quotient obtained by dividing a rated voltage between the tap selector terminals  1 ,  2  by rated load current. It is recommended that the reactance of the linear reactor L 1  is about 0.1 Z 1 . In order to reduce the volume, it is recommended that the reactor L 1  is a reactor provided with an air gap iron core. 
     The fuse FU 1  can cut off the short circuit circulation to serve as the backup protection of the thyristor D 1  (D 2 ). 
     One function of the saturable reactor L 2  is to reduce the rate of rise of the current at the conducting moment of the thyristor D 1  (D 2 ). The other function of the saturable reactor L 2  is as follows: the saturable reactor L 2  cooperates with the resistor R 1  and the capacitor C 1  in the overvoltage triggering thyristor circuit  4  to improve the anti-interference capability of the overvoltage triggering thyristor circuit  4 . The narrow voltage pulse interference resistance of the saturable reactor is larger than that of the linear reactor. 
     The function of the bidirectional voltage stabilizing circuit  7  is to ensure that the voltages at the two ends of the bidirectional voltage stabilizing circuit  7  do not exceed a voltage stabilizing value, the voltage stabilizing value of the bidirectional voltage stabilizing circuit  7  is smaller than U 2  and is smaller than the stabilized voltage U 1  of the constant voltage diode D 9 . When the voltage between the tap selector terminals I 1 , II 2  is a normal rated voltage, the bidirectional voltage stabilizing circuit  7  is not conducted; in the case of a higher interference pulse voltage between the tap selector terminals I 1 , II 2 , the interference pulse is clipped to ensure that the interference pulse voltage is not larger than the stabilized voltage U 1  of the constant voltage diode D 9  in the overvoltage triggering thyristor circuit  4 , in order to prevent the interference pulse between the tap selector terminals I 1 , II 2  form triggering the conduction of the overvoltage triggering thyristor circuit  4  to generate the short circuit circulation. If other circuits can eliminate the interference pulse between the tap selector terminals I 1 , II 2 , the bidirectional voltage stabilizing circuit  7  can be removed. The bidirectional voltage stabilizing circuit  7  can be achieved by a piezoresistor and can also be achieved by a pair of high power constant voltage diodes which are reversely connected in series. 
     When the thyristor assisted on-load tap changer is applied to an extra-high voltage level, the withstand voltages of the existing thyristors D 1 , D 2  are inadequate. Multiple overvoltage triggering thyristor circuits  4  can be serially connected to improve the working voltage.  FIG. 4  shows serial connection of three stages of overvoltage triggering thyristor circuits. R 4  refers to a divider resistor; when multiple overvoltage triggering thyristor circuits are serially connected, the R 4  balances the voltages of the overvoltage triggering thyristor circuits. 
     The thyristor assisted on-load tap changer of  FIG. 2  is provided with a linear reactor L 1  and a saturable reactor L 2 . To further simplify the structure, the linear reactor L 1  and the saturable reactor L 2  can be merged into a single reactor L 3 , as shown in  FIG. 5 . The reactor L 3  is provided with a magnetic flux closed-loop iron core  5  and a coil L 3 , a part of section of the magnetic flux closed-loop iron core  5  has a larger sectional area, and the sectional area of the rest section of the iron core is smaller; the coil L 3  is winded on the iron core at the section with the larger sectional area. When the current is relatively small, the closed-loop iron core is unsaturated; the coil L 3  is equivalent to the saturable reactor L 2 . When the current is relatively large, the iron core at the section with the smaller sectional area of the closed-loop iron core is saturated, and the iron core at the section with the larger sectional area is unsaturated; the reactance of the coil L 3  is decreased to a smaller value quickly, and at this time, the coil L 3  is equivalent to the linear reactor L 1 . 
     As shown in  FIG. 5 , one reactor L 3  can serve instead of the linear reactor L 1  and the saturable reactor L 2 , thus the volume of the reactor is reduced. 
     EMBODIMENT 2 
     The second thyristor assisted on-load tap changer in the present invention is as shown in  FIG. 6 . It includes two tap selector terminals I 1 ,II 2 , a common terminal  3 , two change-over switches K 5 , K 6 , a thyristor switch  6  controlled by a control switch K 10 , an overvoltage triggering thyristor circuit  4 , a linear reactor L 1 , two saturable reactors L 2 , L 4  and a bidirectional voltage stabilizing circuit  7 ; one tap terminal of the change-over switch K 5  and one tap terminal of the change-over switch K 6  are jointly connected with the tap selector terminal I 1 , the other tap terminal of the change-over switch K 5  and the other tap terminal of the change-over switch K 6  are jointly connected with the tap selector terminal II 2 ; the common terminal of the change-over switch K 5  is serially connected with the common terminal  3  of the on-load tap changer through the saturable reactor L 4  and the thyristor switch  6  controlled by the control switch K 10  to form a main path; the common terminal of the change-over switch K 6  is serially connected with the common terminal  3  of the on-load tap changer through the linear reactor L 1 , the saturable reactor L 2  and the overvoltage triggering thyristor circuit  4  to form a transition path; the bidirectional voltage stabilizing circuit  7  is connected between the common terminal of the change-over switch K 5  and the common terminal of the change-over switch K 6 . 
     The circuit of the thyristor switch  6  controlled by the control switch K 10  is as shown in  FIG. 7 , and  FIG. 7  is obtained by changing on the basis of  FIG. 3 . As for the part of  FIG. 7  the same as  FIG. 3 , characteristics and parameter requirements are also the same and will not be repeated redundantly herein. 
     The difference between  FIG. 7  and  FIG. 3  lies in that: a diode D 10  and the control switch K 10  are added. The anode of the diode D 10  is connected with the anode of a full-bridge rectifier composed of diodes D 5 , D 6 , D 7 , D 8 , the cathode of the diode D 10  is connected with one end of the switch K 10 , and the other end of the switch K 10  is connected with the cathode of the full-bridge rectifier composed of the diodes D 5 , D 6 , D 7 , D 8 . It can be seen from  FIG. 7  that, if the switch K 10  is conducted, the thyristor switch  6  is conducted, and if the switch K 10  is disconducted, the thyristor switch  5  is disconducted. After the control switch K 10  is switched on, the current passing by the control switch K 10  is thyristor triggering current, and the current is very small. The conduction and disconduction of the high current path of the thyristor switch  6  can be controlled by the small capacity control switch K 10 , in order to reduce the electric arc generated by the cutoff of the load current and improve the control speed and sensitivity of the switch. 
     The difference between  FIG. 7  and  FIG. 3  also lies in that: a constant voltage diode D 11  is added. The constant voltage diode D 11  and the constant voltage diode D 9  are serially connected in the same direction to replace the original constant voltage diode D 9 . The constant voltage diode D 11  is serially connected with the constant voltage diode D 9  to achieve the following two functions: 
     (1) overvoltage protection of the thyristor switch  6 ; 
     (2) when the thyristor switch  6  is applied to a high voltage level on-load tap changer, the withstand voltages of the existing thyristors D 1 , D 2  may be inadequate, and multiple thyristor switches  6  must be serially connected to improve the working voltage, just as shown in  FIG. 4 . Since each stage of thyristor switch  6  is provided with a control switch K 10 , the actions of the control switches K 10  may be asynchronous, and under the condition that the actions of the control switches K 10  are asynchronous, the constant voltage diode D 11  and the constant voltage diode D 9  can ensure that the thyristor switches  6  act correctly. 
     If the stabilized voltage of the serially connected constant voltage diode D 11  and the constant voltage diode D 9  is too small, the pulse generated by the overvoltage triggering thyristor circuit  4  causes wrong conduction of the thyristor switch  6 . If the stabilized voltage of the serially connected constant voltage diode D 11  and the constant voltage diode D 9  is too large, the withstand voltages of the thyristors D 1 , D 2  are required to be increased, thus increasing the volume and investment of the on-load tap changer. If the stabilized voltage of the constant voltage diode D 9  is too large, the multiple serially connected thyristor switches  6  cannot achieve the function of the above-mentioned second item. The stabilized voltage U 3  of the serially connected constant voltage diode D 11  and the constant voltage diode D 9  is equal to k 2 (U 1 +U 2 ); k 2  refers to a confidence coefficient and is a value of 1.1-1.5. After the thyristor D 1  (D 2 ) is conducted, the thyristor D 1  (D 2 ) has a diode forward voltage drop, the diode forward voltage drop of the thyristor D 1  (D 2 ) increases with the increase of the current flowing by. It is assumed that the diode forward voltage drop of the maximum current (including the transient peak value of the short circuit current possibly flowing by) flowing by the thyristor D 1  (D 2 ) is U 4 . After the switch K 10  is switched on, the current firstly passes by the diode D 10  and the switch K 10  to trigger the gate electrode of the thyristor D 1  (D 2 ) to conduct the thyristor D 1  (D 2 ). The voltages at the two ends of the thyristor D 1  (D 2 ) quickly reduce to the diode forward voltage drop, if the sum of the diode forward voltage drops of all semiconductors serially connected in the gate electrode trigger path of the thyristor D 1  (D 2 ) is larger than U 4 , the current of the gate electrode loop of the thyristor D 1  (D 2 ) automatically disappears; if the sum of the diode forward voltage drops of all the semiconductors serially connected in the gate electrode trigger path of the thyristor D 1  (D 2 ) is smaller than U 4 , high current flows by the gate electrode loop of the thyristor D 1  (D 2 ) to damage the thyristor D 1  (D 2 ). When the sum of the diode forward voltage drops of all the semiconductors of the gate electrode trigger path of the thyristor D 1  (D 2 ) is smaller than 1.2U 4 , multiple diodes can be serially connected to form D 10  to improve the diode forward voltage drop of the diode D 10 . If too many diodes D 10  are serially connected, heating will be increased, and the waveform of the zero-crossing current goes bad. The sum of the diode forward voltage drops of all the semiconductors serially of the gate electrode trigger path of the thyristor D 1  (D 2 ) is properly about 1.5th Namely, the sum of the diode forward voltage drops of the diodes D 4 , D 7 , D 10 , D 6  and the gate electrode of the thyristor D 1  is about 1.5U 4 , and the sum of the diode forward voltage drops of the diodes D 3 , D 8 , D 10 , D 5  and the gate electrode of the thyristor D 2  is about 1.5U 4 . 
     In this embodiment, a combination of the saturable reactor L 4  and the thyristor switch  6  is used instead of the main vacuum switch K 1  of the main path in embodiment 1. The anti-interference capability of the main vacuum switch K 1  is very strong, but the operation of the main vacuum switch K 1  needs a larger mechanical force, thus the operation is insensitive; an electric arc exists in the disconnection process of the contact to generate interference with other semiconductor devices. The control switch K 10  of the thyristor switch  6  can be a miniature relay with a contact and can also be a solid-state switch without contact, the control voltage of the solid-state switch without contact is small, and the action is faster and more sensitive; the interference with other semiconductor devices is small. The working principle and the control circuit of the solid-state switch without contact are public knowledge, and are not repeated redundantly. The defect of the thyristor switch  6  lies in that wrong action may be caused by the pulse interference. To improve the anti-interference capability of the thyristor switch  6 , the saturable reactor L 4  is serially connected. One function of the saturable reactor L 4  is to reduce the rate of rise of the current at the connection moment of the thyristor D 1  (D 2 ) in the thyristor switch  6 . The other function of the saturable reactor L 4  is as follows: the saturable reactor L 4  cooperates with the resistor R 1  and the capacitor C 1  in the thyristor switch  6  to improve the anti-interference capability of the thyristor switch  6 . 
     The working process of switching the conduction of the tap selector terminal I 1  of the on-load tap changer with the common terminal  3  to the conduction of the tap selector terminal II 2  with the common terminal  3  is as follows: (1) switching the change-over switch K 6 ; (2) switching off the control switch K 10 ; switching off the main path, and automatically switching on the transition path; (3) switching the change-over switch K 5 ; (4) switching on the control switch K 10 . 
     EMBODIMENT 3 
     The third thyristor assisted on-load tap changer in the present invention is as shown in  FIG. 8 . It includes two tap selector terminals I 1 ,II 2 , a common terminal  3 , an odd-numbered side main contact K 11 , an even-numbered side main contact K 12 , four transition switches K 14 , K 15 , K 16 , K 17 , an overvoltage triggering thyristor circuit  4 , a thyristor switch  6  controlled by a control switch K 10 , a linear reactor L 1 , two saturable reactors L 2 , L 4  and a bidirectional voltage stabilizing circuit  7 ; the tap selector terminal I 1  is respectively connected with the transition switches K 15 , K 17 , and the tap selector terminal II 2  is respectively connected with the transition switches K 14 , K 16 ; the other terminals of the transition switches K 14 , K 15  are jointly connected and are serially connected with the common terminal  3  of the on-load tap changer through the saturable reactor L 4  and the thyristor switch  6  controlled by the control switch K 10  to form a main path; the other terminals of the transition switches K 16 , K 17  are jointly connected and are serially connected with the common terminal  3  of the on-load tap changer through the linear reactor L 1 , the saturable reactor L 2  and the overvoltage triggering thyristor circuit  4  to form a transition path; the two ends of the odd-numbered side main contact K 11  are respectively connected with the tap selector terminal I 1  and the common terminal  3  of the on-load tap changer, and the two ends of the even-numbered side main contact K 12  are respectively connected with the tap selector terminal II 2  and the common terminal  3  of the on-load tap changer; the bidirectional voltage stabilizing circuit  7  is connected between the connecting terminal of the transition switches K 14 , K 15  and the connecting terminal of the transition switches K 16 , K 17 . 
     The thyristor switch  6  is controlled by the switch K 10 , if K 10  is conducted, the thyristor switch  6  is conducted, and if K 10  is disconducted, the thyristor switch  6  is disconducted. 
     The odd-numbered side main contact K 11  and the even-numbered side main contact K 12  of  FIG. 8  are contactors with locks and are composed of closing coils, breaking (unlocking) coils, main contacts and auxiliary contacts. The four transition switches K 14 , K 15 , K 16 , K 17  are contactors without locks and are composed of closing coils, main contacts and auxiliary contacts. 
     The odd-numbered side main contact K 11  and the even-numbered side main contact K 12  are responsible for a long-term energization task. The thyristor switch  6  and the overvoltage triggering thyristor circuit  4  can work for a short period of time, and the thyristor D 1  (D 2 ) needs no complicated cooling plate. 
     The switcher control circuit for switching the conduction of the tap selector terminal I 1  of the on-load tap changer with the common terminal  3  to the conduction of the tap selector terminal II 2  with the common terminal  3  is as shown in  FIG. 9 . 
     M+ refers to a positive bus of a control power supply, and M− refers to a negative bus of the control power supply; K 11 -T refers to the breaking (unlocking) coil of the K 11  contactor, and K 11 - 1  and K 11 - 2  refer to the auxiliary contacts of the K 11  contactor; K 12 -H refers to the closing coil of the K 12  contactor, and K 12 - 1  refers to the auxiliary contact of the K 12  contactor. K 14 - 1 , K 14 - 2 , K 15 - 1 , K 15 - 2 , K 16 - 1  and K 16 - 2  respectively refer to the auxiliary contacts of the transition switches K 14 , K 15 , K 16 , K 10 - 1 , K 10 - 2  and K 10 - 3  refer to the auxiliary contacts of the control switch K 10 , and KC 1 , KC 2 , KC 3  and KC 4  refer to intermediate relays; BH refers to a protection outlet contact, when the action of the on-load tap changer is protected and inhibited, the BH contact is open to cut off the power supply of a control circuit M 1 ; X 1 - 2  refers to a tap selector instruction contact of the on-load tap changer, after the tap selector of the on-load tap changer selects the tap, the X 1 - 2  contact is switched on to notify the control circuit of the on-load tap changer to start working. 
     After being serially connected with the BH contact, the positive bus M+ of the control power supply is connected with one end of the KC 1 - 2  contact, and the other end of the KC 1 - 2  contact is connected with an M 1  bus; a node between the BH contact and the KC 1 - 2  contact serially connects the K 12 - 1  auxiliary contact, the X 1 - 2  contact and the KC 1  coil to the M− bus; the KC 1 - 1  contact is connected with the two ends of the X 1 - 2  contact in parallel; M 1  serially connects the KC 3 - 1  contact and the K 15  coil to the M− bus; M 1  serially connects the K 15 - 1  contact, the KC 2 - 2  contact and the K 10  coil to the M− bus; M 1  serially connects the KC 2 - 3  contact and the K 14 - 1  contact to a node between the KC 2 - 2  contact and the K 10  coil; a node between the K 15 - 1  contact and the KC 2 - 2  coil serially connects the K 10 - 1  contact and the K 11 -T coil to the M− bus; M 1  serially connects the K 16 - 1  contact, the K 11 - 1  contact and the K 16  coil to the M− bus; a node between the K 16 - 1  contact and the K 11 - 1  contact is connected with the cathode of a diode D 12 , and a node between the K 10 - 1  contact and the K 11 -T coil is connected with the anode of the diode D 12 ; M 1  serially connects the K 16 - 2  contact and the KC 2  coil to the M− bus; the KC 2 - 1  contact is connected with the K 16 - 2  contact in parallel; M 1  serially connects the K 11 - 2  contact, the KC 2 - 4  contact, the K 10 - 2  contact and the KC 3  coil to the M− bus; M 1  serially connects the KC 3 - 2  contact to a node between the K 10 - 2  contact and the KC 3  coil; a node between the KC 2 - 4  contact and the K 10 - 2  contact serially connects the K 15 - 2  contact and the K 14  coil to the M− bus; the node between the KC 2 - 4  contact and the K 10 - 2  contact serially connects the K 14 - 2  contact, the K 10 - 3  contact and the KC 4  coil to the M− bus; a node between the K 14 - 2  contact and the K 10 - 3  contact serially connects the KC 4  contact and the K 12 -H coil to the M− bus. 
     The working process of switching the conduction of the tap selector terminal I 1  with the common terminal  3  to the conduction of the tap selector terminal II 2  with the common terminal  3  is illustrated as follows: 
     when the X 1 - 2  contact is switched on, the K 12  contact and the X 1 - 2  contact are switched on, the KC 1  coil is energized, the KC 1 - 1  and KC 1 - 2  contacts are switched on, and the control circuit M 1  transmits power and is self-holding. 
     The KC 3 - 1  normally open contact is switched on, the K 15  coil is energized, the transition switch K 15  of  FIG. 8  is switched on, and the thyristor switch  6  controlled by the control switch K 10  is conducted with the odd-numbered side main contact K 12  in parallel; the K 15 - 1  contact is switched on and the KC 2 - 2  normally closed contact is switched on to connect the K 10  coil, and the thyristor switch  6  controlled by the control switch K 10  of  FIG. 8  is switched on; the K 15 - 1  contact is switched on and the K 10 - 1  contact is switched on to conduct the coil K 11 -T, the odd-numbered side main contact K 11  of  FIG. 8  is switched off, and the load current is transferred to the path of the thyristor switch  6 ; the K 15 - 1  contact is switched on, the K 10 - 1  contact is switched on and the K 11 - 1  contact is switched on to conduct the K 16  coil; the K 16 - 1  contact is switched on to self hold to conduct the K 16  coil, the transition switch K 16  of  FIG. 8  is switched on, and the overvoltage triggering thyristor circuit  4  is switched on; the K 16 - 2  contact is switched on to conduct the KC 2  coil, and KC 2 - 1  is switched on to self hold to conduct the KC 2  coil; the KC 2 - 2  contact is switched off, the K 10  coil is de-energized, the thyristor switch  6  controlled by the control switch K 10  of  FIG. 8  is disconducted, and the load current is transferred to the path of the overvoltage triggering thyristor circuit  4 ; the K 10 - 1  contact is switched off, and the diode D 12  prevents K 16 - 1  from transmitting power to the coil K 11 -T; the K 11 - 2  contact is switched on, the KC 2 - 4  contact is switched on and the K 10 - 2  contact is switched on to conduct the KC 3  coil; the KC 3 - 2  contact is switched on to self hold to conduct the KC 3  coil; the KC 3 - 1  contact is switched off, and the K 15  coil is de-energized; the transition switch K 15  of  FIG. 8  is switched off, the K 15 - 2  contact is switched on to conduct the K 14  coil, and the transition switch K 14  of  FIG. 8  is switched on to conduct the thyristor switch  6  controlled by the control switch K 10  with the overvoltage triggering thyristor circuit  4  in parallel; the K 14 - 1  contact is switched on to conduct the K 10  coil again, the thyristor switch  6  controlled by the control switch K 10  of  FIG. 8  is switched on again, and the load current is transferred to the path of the thyristor switch  6  controlled by the control switch K 10  again; the K 10 - 3  contact is switched on to conduct the KC 4  coil; the KC 4  contact is switched on to conduct the K 12 -H coil, the K 12  main contact of  FIG. 8  is switched on, and the load current is transferred to the path of the K 12  main contact to connect the tap selector terminal II 2  with the common terminal  3 ; meanwhile, the K 12  normally closed contact is switched off, the KC 1  coil is de-energized, the KC 1 - 1  contact and the KC 1 - 2  contact are switched off to cut off the power supply of the control circuit, and the entire group of the control circuit is reset. 
     In the above-mentioned switcher control circuit, the transition switch K 15  is switched on firstly, and the control switch K 10  is switched on; the program is clear. Or, the transition switch K 15  and the control switch K 10  can also be simultaneously switched on to shorten the overall time of the program. In the above-mentioned switcher control circuit, the odd-numbered side main contact K 11  is firstly switched off, and after the load current is transferred to the path of the thyristor switch  6 , the transition switch K 16  is switched on to access the overvoltage triggering thyristor circuit  4 ; the program is clear. Or, the odd-numbered side main contact K 12  is switched off and the transition switch K 16  is switched on at the same time to shorten the overall time of the program. 
     The switcher control circuit for switching the conduction of the tap selector terminal II 2  of the on-load tap changer with the common terminal  3  to the conduction of the tap selector terminal I 1  with the common terminal  3  can refer to the above-mentioned method design, and will not be repeated redundantly. 
     The traditional on-load tap changer adopts a motor rotation driving manner, the overall action time is 4.4 seconds, wherein the action time of the diverter switch is only 40 milliseconds, and most of the time is used as energy accumulating and preparation time of the mechanical mechanism. For the thyristor assisted on-load tap changer in which the overvoltage triggering thyristor circuit  4  is used instead of the transition resistor R, the action time of the diverter switch is prolonged without heating to damage the equipment, in this way, the energy accumulating mechanical mechanism can be eliminated, and the overall action time of the thyristor assisted on-load tap changer can be shortened on the contrary. The complicated mechanical linkage mechanism and energy accumulating mechanical mechanism are eliminated to reduce the volume and weight of the on-load tap changer; the failure rate can be reduced. In particular, the control circuit in the manner of the intermediate relay (contactor) can be adopted to achieve the sequential action of the switcher. The control manner of the intermediate relay (contactor) is adopted to ensure entering the action program of the next switch after the action of a certain switch is finished, in order to improve the reliability. The action of the tap selector needs no intervention of the switcher, the switcher is started to work after the action of the tap selector is finished, and no intervention of the tap selector is needed in the switch process of the switcher; the tap selector and the switcher need no constraint of a mechanical linkage, and are clear in logical relationship, simple in structure and convenient to cooperate. In the thyristor assisted on-load tap changer as shown in  FIG. 8 , no current flow is generated in the disconduction and conduction processes of the odd-numbered side main contact K 11 , the even-numbered side main contact K 12  and the four transition switches K 14 , K 15 , K 16 , K 17 ; arc-free switch is achieved; the switch contact is not damaged by frequent action. 
     According to some preference, the thyristor assisted on-load tap changer can be changed based on this embodiment. For example: (1) a set of the overvoltage triggering thyristor circuit  4 , the transistor switch  6 , the linear reactor L 1  and the saturable reactor L 2  can be added, in this way, the four transition switches K 14 , K 15 , K 16 , K 17  can be reduced to two, to achieve the purpose of reducing the number of the mechanical switches. (2) The overvoltage triggering thyristor circuit  4  and the transistor switch  6  have a large amount of identical elements and circuits;  FIG. 3  and  FIG. 7  can be combined to form a set of new combined circuit, the combined circuit can be switched between the two functions of the main path and the transition path by means of the switch on or switch off of a miniature switch, thus one set of circuit has two functions. After being serially connected with such set of combined circuit, one transition switch is connected with the tap selector terminal I 1  and the common terminal  3  in parallel; after being serially connected with another such set of combined circuit, another transition switch is connected with the tap selector terminal II 2  and the common terminal  3  in parallel, in order to reduce the number of the mechanical switches, reduce the semiconductor elements, reduce the operation steps and shorten the switch time. 
     According to some preference, the switcher control circuit as shown in  FIG. 9  can be changed based on this embodiment. The control circuit with equivalent program and time sequence requirements can be implemented in multiple methods. The control circuit can not only be implemented by the logic cooperation of miniature intermediate relays, but also can be implemented by semiconductor devices. These are public knowledge and will not be repeated redundantly. 
     EMBODIMENT 4 
     The principle structure and the connecting manner of a thyristor assisted on-load tap changer in the present invention are as shown in  FIG. 2 . It includes two tap selector terminals I 1 ,II 2 , a common terminal  3 , two change-over switches K 5 , K 6 , a main vacuum switch K 1 , an overvoltage triggering thyristor circuit  4 , a linear reactor L 1 , a saturable reactor L 2  and a bidirectional voltage stabilizing circuit  7 ; one tap terminal of the change-over switch K 5  and one tap terminal of the change-over switch K 6  are jointly connected with the tap selector terminal I 1 , the other tap terminal of the change-over switch K 5  and the other tap terminal of the change-over switch K 6  are jointly connected with the tap selector terminal II 2 ; the common terminal of the change-over switch K 5  is connected with the common terminal  3  of the on-load tap changer through the main vacuum switch K 1  to form a main path; the common terminal of the change-over switch K 6  is serially connected with the common terminal  3  of the on-load tap changer through the linear reactor L 1 , the saturable reactor L 2  and the overvoltage triggering thyristor circuit  4  to form a transition path; the bidirectional voltage stabilizing circuit  7  is connected between the common terminal of the change-over switch K 5  and the common terminal of the change-over switch K 6 . 
     The main vacuum switch K 1  and the change-over switches K 5 , K 6  are contactors with locks and are composed of closing coils, breaking coils, main contacts and auxiliary contacts. 
     The sequential action of the switches is achieved by a switcher control circuit of the on-load tap changer, as shown in  FIG. 10 . M+ refers to a positive bus of a control power supply, and M− refers to a negative bus of the control power supply; K 1 -T refers to the breaking coil of the K 1  switch, and K 1 -H, K 5 -H and K 6 -H respectively refer to the closing coils of the K 1 , K 5  and K 6  switches. K 1 - 1 , K 1 - 2 , K 5 - 1 , K 5 - 2 , K 6 - 1 , K 6 - 2  and K 6 - 3  respectively refer to the auxiliary contacts of the K 1 , K 5  and K 6  switches, and KC 1  and KC 2  refer to intermediate relays; BH refers to a protection outlet contact, when the action of the on-load tap changer is protected and inhibited, the BH contact is disconducted to cut off the power supply of a control circuit M 1 ; X 1 - 2  refers to a tap selector instruction contact of the on-load tap changer, after the tap selector of the on-load tap changer selects the tap, the X 1 - 2  contact is switched on to notify the control circuit of the on-load tap changer to start working. 
     The switcher control circuit controls the power supply connection sequence of the switch coils according to the action sequence of the contacts, in order to achieve the sequential action of a series of electric switches and achieve the on-load switch of the on-load tap changer. The working method of the switcher control circuit refers to embodiment 3, and will not be repeated redundantly. 
     EMBODIMENT 5 
     The principle structure and the connecting manner of a thyristor assisted on-load tap changer in the present invention are as shown in  FIG. 6 . It includes two tap selector terminals I 1 ,II 2 , a common terminal  3 , two change-over switches K 5 , K 6 , a thyristor switch  6  controlled by a control switch K 10 , an overvoltage triggering thyristor circuit  4 , a linear reactor L 1 , two saturable reactors L 2 , L 4  and a bidirectional voltage stabilizing circuit  7 ; one tap terminal of the change-over switch K 5  and one tap terminal of the change-over switch K 6  are jointly connected with the tap selector terminal I 1 , the other tap terminal of the change-over switch K 5  and the other tap terminal of the change-over switch K 6  are jointly connected with the tap selector terminal II 2 ; the common terminal of the change-over switch K 5  is serially connected with the common terminal  3  of the on-load tap changer through the saturable reactor L 4  and the thyristor switch  6  controlled by the control switch K 10  to form a main path; the common terminal of the change-over switch K 6  is serially connected with the common terminal  3  of the on-load tap changer through the linear reactor L 1 , the saturable reactor L 2  and the overvoltage triggering thyristor circuit  4  to form a transition path; the bidirectional voltage stabilizing circuit  7  is connected between the common terminal of the change-over switch K 5  and the common terminal of the change-over switch K 6 . 
     The control switch K 10  and the change-over switches K 5 , K 6  are contactors with locks and are composed of closing coils, breaking coils, main contacts and auxiliary contacts. 
     The sequential action of the switches is achieved by a switcher control circuit of the on-load tap changer, as shown in  FIG. 11 . M+ refers to a positive bus of a control power supply, and M− refers to a negative bus of the control power supply; K 10 -T refers to the breaking coil of the K 10  switch, and K 10 -H, K 5 -H and K 6 -H respectively refer to the closing coils of the K 10 , K 5  and K 6  switches. K 10 - 1 , K 10 - 2 , K 5 - 1 , K 5 - 2 , K 6 - 1 , K 6 - 2  and K 6 - 3  respectively refer to the auxiliary contacts of the K 10 , K 5  and K 6  switches, and KC 1  and KC 2  refer to intermediate relays; BH refers to a protection outlet contact, when the action of the on-load tap changer is protected and inhibited, the BH contact is disconducted to cut off the power supply of a control circuit M 1 ; X 1 - 2  refers to a tap selector instruction contact of the on-load tap changer, after the tap selector of the on-load tap changer selects the tap, the X 1 - 2  contact is switched on to notify the control circuit of the on-load tap changer to start working. 
     The switcher control circuit controls the power supply connection sequence of the switch coils according to the action sequence of the contacts, in order to achieve the sequential action of a series of electric switches and achieve the on-load switch of the on-load tap changer. The working method of the switcher control circuit refers to embodiment 3, and will not be repeated redundantly. 
     EMBODIMENT 6 
     In embodiments 1, 2, 3, 4, 5, an on-load tap changer switcher is habitually referred to as the on-load tap changer. In the following embodiments 6, 7, 8, 9, in order to express details, the tap selector and the switcher of the thyristor assisted on-load tap changer are specially expressed by a tap selector  10  and a switcher  11 . The tap selector  10  is connected with the taps of a regulating transformer, the switcher  11  is connected with the tap selector  10 , and after the tap selector  10  selects the tap of the regulating transformer, the switcher  11  achieves the on-load switch of two taps of the regulating transformer. A tap selector terminal I 1  and the terminal J 1  of the switcher  11  are connected to a point, thus the tap selector terminal I 1  and the terminal J 1  of the switcher  11  can be considered as the same terminal; a tap selector terminal II 2  and the terminal J 2  of the switcher  11  are connected to a point, thus the tap selector terminal II 2  and the terminal J 2  of the switcher  11  can be considered as the same terminal; a common terminal  3  of the on-load tap changer is actually a switcher terminal J 3 . 
     In some application occasions, L 1  in the switcher (as shown in  FIG. 8 ) of the third thyristor assisted on-load tap changer can be removed, and the rest part can still work. On the premise of small safety loss, the economical efficiency is improved. 
     In some application occasions, L 1 , L 2  and L 4  in the switcher (as shown in  FIG. 8 ) of the third thyristor assisted on-load tap changer can be removed, and the rest part can still work. On the premise of small safety loss, the economical efficiency is further improved. 
     After L 1 , L 2  and L 4  in the switcher (as shown in  FIG. 8 ) of the third thyristor assisted on-load tap changer are removed, the thyristor switch  6  in the main path and the overvoltage triggering thyristor circuit  4  in the transition path can be replaced by an economical thyristor assisted circuit as shown in  FIG. 13 ; KA in the economical thyristor assisted circuit as shown in  FIG. 13  represents K 10 , KB is switched off, and the economical thyristor assisted circuit is equivalent to the thyristor switch  6 ; KA in the economical thyristor assisted circuit as shown in  FIG. 13  is switched off, KB is switched on, and the economical thyristor assisted circuit is equivalent to the overvoltage triggering thyristor circuit  4 . In this way, two economical thyristor assisted circuits (an economical thyristor assisted circuit I 8  and an economical thyristor assisted circuit II 9 ) respectively form two paths, and the two paths respectively have the functions of the main path and the transition path. By means of the control of the four small capacity switches KA (K 23 - 1 ) and KB (K 25 - 1 ) in the economical thyristor assisted circuit I 8  and KA (K 24 - 1 ) and KB (K 26 - 1 ) in the economical thyristor assisted circuit II 9 , the same functions of K 14 , K 15 , K 16 ,  17  in  FIG. 8  are achieved. When the economical thyristor assisted circuit I 8  is used as the main path, the economical thyristor assisted circuit II 9  is used as the transition path; when the economical thyristor assisted circuit II 9  is used as the main path, the economical thyristor assisted circuit I 8  is used as the transition path. 
     The structure and the connecting manner of the switcher  11  of the fourth thyristor assisted on-load tap changer are as shown in  FIG. 12 . It includes a main switch K 21 - 1 , a main switch K 22 - 1 , an economical thyristor assisted circuit I 8 , an economical thyristor assisted circuit II  9 , a piezoresistor R and three terminals J 1 , J 2 , J 3 ; the terminal J 1  is connected with the odd-numbered terminal of the tap selector, the terminal J 2  is connected with the even-numbered terminal of the tap selector, and the terminal J 3  is a common terminal. One end of the main switch K 21 - 1  is connected with the terminal J 1 , and the other end of the main switch K 21 - 1  is connected with the terminal J 3 ; the economical thyristor assisted circuit I is connected with the main switch K 21 - 1  in parallel; one end of the main switch K 22 - 1  is connected with the terminal J 2 , and the other end of the main switch K 22 - 1  is connected with the terminal J 3 ; the economical thyristor assisted circuit II is connected with the main switch K 22 - 1  in parallel; the end of the economical thyristor assisted circuit I close to the J 1  and the end of the economical thyristor assisted circuit II close to the J 2  are further connected with the piezoresistor R. The functions and requirements of the piezoresistor R are the same as those of  7  in  FIG. 8 , and will not be repeated redundantly herein. 
     The economical thyristor assisted circuit I 8  and the economical thyristor assisted circuit II  9  have the same structure and parameters, thus only one schematic diagram is given, as shown in  FIG. 13 . It includes: a pair of thyristors D 1 , D 2  are reversely connected in parallel to form a main path of the economical thyristor assisted circuit; a resistor R 1  and a capacitor C 1  are connected to the two ends of the thyristors D 1 , D 2  reversely connected in parallel after being connected in series; the gate electrodes and the cathodes of the two thyristors D 1 , D 2  are respectively connected with capacitors C 2 , C 3 , resistors R 2 , R 3  and diodes D 3 , D 4 ; the anodes of the diodes D 3 , D 4  are respectively connected with the gate electrodes of the thyristors D 1 , D 2 , and the cathodes of the diodes D 3 , D 4  are respectively connected with the cathodes of the thyristors D 1 , D 2 ; the input terminal of a full-bridge rectifier composed of diodes D 5 , D 6 , D 7 , D 8  is connected between the gate electrodes of the two thyristors D 1 , D 2  after being serially connected with a switch KB, the output end of the full-bridge rectifier is connected with a constant voltage diode D 9 , the cathode of the constant voltage diode D 9  is connected with the anode output end of the full-bridge rectifier, and the anode of the constant voltage diode D 9  is connected with the cathode output end of the full-bridge rectifier; diodes D 13 , D 14 , D 15  are serially connected in the same direction, diodes D 16 , D 17 , D 18  are serially connected in the same direction, and the two diode strings are serially connected with the switch KA after being reversely connected in parallel and are connected between the gate electrodes of the two thyristors D 1 , D 2 . 
     KA of the economical thyristor assisted circuit I 8  is expressed by K 23 - 1  in  FIG. 12 , and KB is expressed by K 25 - 1  in  FIG. 12 ; KA of the economical thyristor assisted circuit II  9  is expressed by K 24 - 1  in  FIG. 12 , and KB is expressed by K 26 - 1  in  FIG. 12 . Under the condition that KB is switched off, the economical thyristor assisted circuits I 8  and II 2  are equivalent to a switching circuit controlled by KA. It can be seen from  FIG. 13  that, if the switch KA is conducted, the economical thyristor assisted circuit is conducted, and if the switch KA is disconducted, the economical thyristor assisted circuit is disconducted. After the switch KA is switched on, the current passing by the switch KA is thyristor triggering current, and the current is very small. The conduction and disconduction of the high current path of the thyristors D 1 , D 2  can be controlled by the small capacity switch KA, in order to reduce the electric arc generated by the cutoff of the load current and improve the control speed and sensitivity of the switch. After the switch KA is switched on, the current passes by the switch KA to trigger the gate electrode of the thyristor D 1  (D 2 ) to conduct the thyristor D 1  (D 2 ). The voltages at the two ends of the thyristor D 1  (D 2 ) quickly reduce to the diode forward voltage drop of the thyristor D 1  (D 2 ), if the sum of the diode forward drops of all semiconductors serially connected in the gate electrode trigger path of the thyristor D 1  (D 2 ) is larger than the diode forward voltage drop of the thyristor D 1  (D 2 ), the current of the gate electrode loop of the thyristor D 1  (D 2 ) automatically disappears; if the sum of the diode forward voltage drops of all the semiconductors serially connected in the gate electrode trigger path of the thyristor D 1  (D 2 ) is smaller than the diode forward voltage drop of the thyristor D 1  (D 2 ), high current flows by the gate electrode loop of the thyristor D 1  (D 2 ) to damage the thyristor D 1  (D 2 ). In  FIG. 13 , diodes D 13 , D 14 , D 15  are connected in the same direction to form a diode string, diodes D 16 , D 17 , D 18  are connected in the same direction to form another diode string, and the two diode strings are connected between the gate electrodes of the two thyristors D 1 , D 2  after being reversely connected in parallel and serially connected with the normally open switch KB, in order to improve sum of the diode forward voltage drop of the trigger circuit of the thyristor D 1  (D 2 ). The more the serially connected diodes are, the better the effect of ensuring zero current flowing by the switch KA after the thyristors D 1 , D 2  are connected is; however, if too many diodes are serially connected, heating will be increased, and the waveform of the zero-crossing current goes bad. It is proper to serially connect three diodes positively and negatively, respectively. 
     Under the condition that KA is switched off and KB is switched on, the economical thyristor assisted circuit I 8  and the economical thyristor assisted circuit II 9  are equivalent to overvoltage triggering thyristor circuits. The stabilized voltage U 1  of a constant voltage diode D 9  is equal to k 1 U 2 ; k 1  refers to a confidence coefficient and is a value of 1.2-2; U 2  refers to the peak value of a rated working frequency operating voltage between the connecting terminals J 1 , J 2  of the switcher and the tap selector of the thyristor assisted on-load tap changer. It is recommended that k 1  is preferably 1.5. The working property of the overvoltage triggering thyristor circuit is the same as that in embodiment 1, and will not be repeated redundantly. The economical thyristor assisted circuits I 8  and II 9  are simple in structure and are high in reliability. 
     The conduction of the switcher terminal J 1  of the on-load tap changer with the common terminal J 3  can be switched to the conduction of the terminal J 2  with the common terminal J 3 ; the conduction of the switcher terminal J 2  of the on-load tap changer with the common terminal J 3  can be switched to the conduction of the terminal J 1  with the common terminal J 3 . The working method of switching the conduction of the terminal J 1  of the switcher of the on-load tap changer with the common terminal J 3  to the conduction of the terminal J 2  with the common terminal J 3  is described as follows: 
     before switching, the main switch K 21 - 1  is switched on, the main switch K 22 - 1  is switched off, and the switches K 23 - 1 , K 24 - 1 , K 25 - 1 , K 26 - 1  are switched off. A power system is connected with one odd-numbered tap of a regulating transformer through the common terminal J 3 , the main switch K 21 - 1 , the terminal J 1  of the switcher  11  and the tap selector  10 . The on-load tap changer receives a regulation instruction and firstly commands the tap selector  10  to select to switch on a corresponding even-numbered tap changer, and the selection of the tap selector  10  is finished. The working sequence of the switcher  11  is as follows: 
     (1) the switch K 23 - 1  is switched on; the switch K 26 - 1  is switched on. When the switch K 23 - 1  is switched on, the economical thyristor assisted circuit I 8  is used as a conducted switch access circuit. When the switch K 26 - 1  is switched on, the economical thyristor assisted circuit II 9  is used as the access circuit of the overvoltage triggering thyristor circuit, since the peak value of the maximum normal AC voltage is smaller than the stabilized voltage of the constant voltage diode D 9 , the constant voltage diode D 9  is not conducted, and the overvoltage triggering thyristor circuit is not conducted. 
     (2) The main switch K 21 - 1  is switched off. The load current is transferred to the economical thyristor assisted circuit I 8 . 
     (3) The switch K 23 - 1  is switched off. The current of the economical thyristor assisted circuit I  8  is cut off at a zero crossing point of the current, at the current cutoff moment of the economical thyristor assisted circuit I  8 , the potential of the terminal J 3  quickly descends (or ascends); the voltages at the two ends of the economical thyristor assisted circuit II  9  (the overvoltage triggering thyristor circuit) instantaneously generate overvoltages, when the instantaneous value of the overvoltage reaches the stabilized voltage of the constant voltage diode D 9 , the conduction of the thyristor D 1  or thyristor D 2  is triggered, the load current flows in from the terminal J 2  and flows out from the common terminal J 3  through the economical thyristor assisted circuit II  9 ; the load current is transferred from the economical thyristor assisted circuit I  8  to the economical thyristor assisted circuit II  9 . 
     (4) The switch K 24 - 1  is switched on. The economical thyristor assisted circuit II  9  is used as a conducted switch access circuit. 
     (5) The main switch K 22 - 1  is switched on. The load current is transferred from the economical thyristor assisted circuit II  9  to the main switch K 22 - 1 . 
     (6) The entire group is reset. 
     It can be seen that, the switch K 24 - 1  must be only switched on after the switch K 23 - 1  is switched off and the current of the economical thyristor assisted circuit I  8  is cut off at the zero crossing point of the current. Otherwise, the current of the economical thyristor assisted circuit I  8  is cut off in front of the zero crossing point of the current, the switch K 24 - 1  is switched on too early, and the economical thyristor assisted circuit I  8  and the economical thyristor assisted circuit II  9  will cause short circuit circulation. The time between switching off the switch K 23 - 1  and cutting off the current of the economical thyristor assisted circuit I  8  at the zero crossing point of the current is uncertain. To ensure that the switch K 24 - 1  is switched on after the current of the economical thyristor assisted circuit I  8  is cut off at the zero crossing point of the current, the time interval between switching off the switch K 23 - 1  and switching on the switch K 24 - 1  should be larger than 20 milliseconds. 
     Similarly, the working method of switching the conduction of the terminal J 2  of the switcher of the on-load tap changer with the common terminal  3  to the conduction of the terminal J 1  and the common terminal  3  is as follows: 
     before switching, the main switch K 22 - 1  is switched on, the main switch K 21 - 1  is switched off, and the switches K 23 - 1 , K 24 - 1 , K 25 - 1 , K 26 - 1  are switched off; after the tap selector  10  selects the transformer tap; (1) the switch K 24 - 1  is switched on; the switch K 25 - 1  is switched on; (2) the main switch K 22 - 1  is switched off; (3) the switch K 24 - 1  is switched off; (4) the switch K 23 - 1  is switched on; (5) the main switch K 21 - 1  is switched on; (6) the entire group is reset. 
     The time interval between switching off the switch K 24 - 1  and switching on the switch K 23 - 1  should be larger than 20 milliseconds. 
     The switches K 21 - 1 , K 22 - 1 , K 23 - 1 , K 24 - 1 , K 25 - 1  and, K 26 - 1  can be manually operated, and the electric switches are manually operated to sequentially act in order to achieve the on-load switch of the switcher. 
     The economical thyristor assisted circuit I  8  and the economical thyristor assisted circuit II  9  are serially connected with a saturable reactor L 2  respectively to increase the safety of the switcher I 1  of the fourth thyristor assisted on-load tap changer, and the economical efficiency is slightly reduced. In practical application, the balance of the requirements on the safety and the economical efficiency can be considered. 
       FIG. 12  is compared with  FIG. 8 . L 1 , L 2 , L 4  in  FIG. 8  are removed in  FIG. 12 , and four large capacity switches K 14 , K 15 , K 16 , K 17  are also removed. K 21 - 1  in  FIG. 12  is equivalent to K 11  in  FIG. 8 , K 22 - 1  in  FIG. 12  is equivalent to K 12  in  FIG. 8 , and a nonlinear resistor R in  FIG. 12  is equivalent to the bidirectional voltage stabilizing circuit  7  in  FIG. 8 . Four small capacity switches K 23 - 1 , K 24 - 1 , K 25 - 1 , K 26 - 1  are adopted in  FIG. 12  to achieve the functions of four large capacity switches K 14 , K 15 , K 16 , K 17  and the K 10  switch in  FIG. 8 . The switcher as shown in  FIG. 12  has better economical efficiency than the switcher as shown in  FIG. 8  and is more convenient to control. 
     EMBODIMENT 7 
     In embodiment 6, the switches K 21 - 1 , K 22 - 1 , K 23 - 1 , K 24 - 1 , K 25 - 1  and K 26 - 1  can be manually operated, and the electric switches are manually operated to sequentially act in order to achieve the on-load switch of the switcher  11 . Actually, in view of the switches K 21 - 1 , K 22 - 1 , K 23 - 1 , K 24 - 1 , K 25 - 1  and K 26 - 1 , the electric switches can also be driven by a mechanical linkage mechanism to sequentially act to achieve the on-load switch of the switcher; the electric switches can also be controlled by a contactor (relay) contact to sequentially act to achieve the on-load switch of the switcher  11 ; a variety of methods can be adopted, thus the application is flexible. 
     In a variety of applications, the switches K 21 - 1 , K 22 - 1 , K 23 - 1 , K 24 - 1 , K 25 - 1  and K 26 - 1  are controlled by the contactor (relay) contact to sequentially act to achieve the on-load switch of the switcher  11  more simply and more economically. The main switch K 21 - 1  and the main switch K 22 - 1  are contactors with locks and are composed of closing coils, breaking (unlocking) coils, main contacts (main switches) and auxiliary contacts, the switches K 23 - 1 , K 24 - 1 , K 25 - 1 , K 26 - 1  are contactors (or relays) without locks and are composed of closing coils, main contacts (switches) and auxiliary contacts; the action state of the main contact is reflected by the auxiliary contact of the contactor (relay), namely, it is ensured that the action program of the next switch is entered after the action state of a certain switch is determined and that the action program of the next switch is entered immediately after the action state of the certain switch is determined; a perfect combination of speed and reliability is achieved. 
     In the structure of the switcher  11  of the fourth thyristor assisted on-load tap changer as shown in  FIG. 12 , except the main switch, no other large capacity contactor (relay) is needed; the switches K 23 - 1 , K 24 - 1 , K 25 - 1 , K 26 - 1  are small capacity switches, the thyristor trigger circuit can be controlled by the on/off of the contact of a small capacity contactor (relay) to switch on/off a high current thyristor, in order to switch the on-load tap changer. The switcher  11  of the on-load tap changer implemented by the contactor (relay) is simple in structure, convenient to control and low in cost. 
     The main switch is switched on and switched off under the condition that the voltages at the two ends of the switch are equal to zero, and the main switch is operated in an arc-free manner. The contacts of the small capacity contactors (relays) K 23 - 1 , K 24 - 1 , K 25 - 1 , K 26 - 1  can also be operated in the arc-free manner. 
     The control circuit of the switcher  11  for switching the conduction of the terminal J 1  of the fourth thyristor assisted on-load tap changer implemented by a contactor (relay) with the common terminal J 3  to the conduction of the terminal J 2  with the common terminal J 3  is as shown in  FIG. 15 ( a ) . 
     M+ refers to a positive bus of a control power supply, and M− refers to a negative bus of the control power supply; K 21 T refers to the breaking (unlocking) coil of a K 21  contactor, K 21 - 1  refers to the main contact of the K 21  contactor, and K 21 - 2  refers to the auxiliary contact of the K 21  contactor; K 22 H refers to the closing coil of a K 22  contactor, K 22 - 1  refers to the main contact of the K 22  contactor, and K 22 - 2  refers to the auxiliary contact of the K 22  contactor. K 23 - 1 , K 23 - 2 , K 23 - 3  refer to the contacts of a relay K 23 , K 24 - 1 , K 24 - 2  refer to the contacts of a relay K 24 , K 26 - 1 , K 26 - 2  refer to the contacts of a relay K 26 , K 1 C- 1 , K 1 C- 2  refer to the contacts of a relay K 1 C, KC 2 - 1 , KC 2 - 2  refer to the contacts of a relay KC 2 , KC 3 - 1  refers to the contact of a relay KC 3 , and KC 4 - 1 , KC 4 - 2 , KC 4 - 3  refer to the contacts of a relay KC 4 . 
     A normally open contact K 21 - 2  and a relay coil K 1 C are serially connected between the buses M+ and M−; the two ends of the normally open contact K 21 - 2  are also connected with the normally open contact K 1 C- 1  in parallel. A normally open contact K 1 C- 2  is connected between a collection line A and the bus M+. A normally closed contact KC 2 - 1  and a relay coil K 23  are serially connected between the collection line A and the bus M−. A relay coil K 26  is serially connected between the collection line A and the bus M−. A normally open contact K 26 - 2 , a normally open contact K 23 - 2  and the contactor breaking coil K 21 T are serially connected between the collection line A and the bus M−. A normally closed contact K 21 - 4  and a relay coil KC 2  are serially connected between the collection line A and the bus M−. A normally open contact KC 2 - 2 , a normally closed contact K 23 - 3  and a relay coil KC 3  are serially connected between the collection line A and the bus M−. A normally open contact KC 3 - 1  and a relay coil KC 4  are serially connected between the collection line A and the bus M−. A normally open contact KC 4 - 1  and a relay coil K 24  are serially connected between the collection line A and the bus M−. A normally open contact KC 4 - 2 , a normally open contact K 24 - 2  and the contactor closing coil K 22 H are serially connected between the collection line A and the bus M−. 
     The working process thereof is as follows: the buses M+ and M− are connected with the power supply. The contact K 21 - 2  is switched on, the relay K 1 C acts, the contact K 1 C- 1  is switched on, and the relay K 1 C self holds. The contact K 1 C- 2  is switched on. The contact KC 2 - 1  is switched on, the relay K 23  acts, the contact K 23 - 1  in  FIG. 12  is switched on, and the thyristor assisted circuit I 8  is connected to serve as a switch. The relay K 26  acts, the contact K 26 - 1  in  FIG. 12  is switched on, the thyristor assisted circuit II 9  is switched on to serve as the overvoltage triggering thyristor circuit, and the overvoltage triggering thyristor circuit is not conducted. The contact K 26 - 2  is switched on, the contact K 23 - 2  is switched on, the contactor breaking coil K 21 T is energized, and the main contact K 21 - 1  of the contactor in  FIG. 12  is switched off. The contact K 21 - 4  is switched on, and the relay KC 2  acts. The contact KC 2 - 1  is switched off, the relay K 23  returns, the contact K 23 - 1  in  FIG. 12  is switched off, and the thyristor assisted circuit I 8  cuts off the current path at the zero crossing point of the current. At the moment when the thyristor assisted circuit I 8  cuts off the current path at the zero crossing point of the current, the thyristor assisted circuit II 9  is connected to serve as the overvoltage triggering thyristor circuit. The contact KC 2 - 2  is switched on, the contact K 23 - 3  is switched on, and the relay KC 3  acts. The contact KC 3 - 1  is switched on, and the relay KC 4  acts. The contact KC 4 - 1  is switched on, the relay K 24  acts, the contact K 24 - 1  of the thyristor assisted circuit II 9  in  FIG. 12  is switched on, and the thyristor assisted circuit II 9  is used as a switch to conduct the current path. Since the action times of the relays KC 3 , KC 4 , K 24  are about 15 milliseconds, it can be ensured that the contact KC 4 - 1  is switched on more than 20 milliseconds after the contact K 23 - 1  is switched off, thus generating no short circuit circulation. The contact KC 4 - 2  is switched on, the contact K 24 - 2  is switched on, and the contactor closing coil K 22 H is energized; the main contact K 22 - 1  in  FIG. 12  is switched on, and the load current is transferred to the path of J 3  and J 2 . Similarly, it can be designed that: the switcher control circuit for switching the conduction of the terminal J 2  of the fourth thyristor assisted on-load tap changer with the common terminal J 3  to the conduction of the terminal J 1  with the common terminal J 3  is as shown in  FIG. 15 ( b ) . The working principle of  FIG. 15 ( b )  is similar to that of  FIG. 15 ( a ) , and will not be repeated redundantly. 
     EMBODIMENT 8 
     The operating power supply of a switcher  11  of an on-load tap-changer is generally from a local 220V low-voltage power supply. If a regulating transformer is connected in a Y shape, a transformer tap is close to a ground wire, and the voltage of the transformer tap is lower; the voltage between the contacts of switches K 21 - 1 , K 22 - 1 , K 23 - 1 , K 24 - 1 , K 25 - 1 , K 26 - 1  and the operating power supply is lower. If the coils of the regulating transformer are connected in a triangle, the voltages of the contacts of the switches K 21 - 1 , K 22 - 1 , K 23 - 1 , K 24 - 1 , K 25 - 1 , K 26 - 1  are high, the voltage between the contacts of the switches K 21 - 1 , K 22 - 1 , K 23 - 1 , K 24 - 1 , K 25 - 1 , K 26 - 1  and the operating power supply is higher, the contacts of the switches K 21 - 1 , K 22 - 1 , K 23 - 1 , K 24 - 1 , K 25 - 1 , K 26 - 1  and the operating power supply must be well insulated, and a high-voltage insulating material is expensive. 
     This embodiment provides a power supply structure of a thyristor assisted on-load tap changer with lower insulating requirements between the contacts of the switches K 21 - 1 , K 22 - 1 , K 23 - 1 , K 24 - 1 , K 25 - 1 , K 26 - 1  and the operating power supply. For convenience of expression, the structure and the connecting manner of a thyristor assisted on-load tap changer with five tap terminals are as shown in  FIG. 16 . It is assumed that, the regulating transformer T 1  has five tap terminals, which are respectively connected to the input terminals B 1 , B 2 , B 3 , B 4 , B 5  of a tap selector  10  of the thyristor assisted on-load tap changer; the output terminal of the tap selector  10  is connected with the input terminals J 1 , J 2  of the switcher; a common terminal J 3  of the switcher  11  is connected with a power system. 
     In the tap terminals B 1 , B 2 , B 3 , B 4 , B 5  of the regulating transformer, the centremost terminal (B 3 ) is defined as a null line and is connected with one terminal of a primary coil of a transformer T 2 ; the tap terminal B 2  (or B 4 ) of the regulating transformer adjacent to the null line is connected with another terminal of the primary coil of the transformer T 2 . Terminals B 6 , B 7  of a secondary coil of the transformer T 2  provide an AC control voltage (for example, AC 220V) to the switcher  11  of the thyristor assisted on-load tap changer; one terminal of the AC control voltage is defined as a null line, and the null line of the primary coil of the transformer T 2  is connected with the null line of the secondary coil of the transformer T 2 . 
     An AC control voltage terminal is used as the input to a DC voltage stabilization power supply module  12 , the DC voltage stabilization power supply module  12  outputs a DC voltage (for example, B 8 , B 9  DC 24V) or multiple DC voltages. The output of the DC voltage stabilization power supply module  12  provides a DC control voltage to the switcher  11  of the thyristor assisted on-load tap changer; the low-potential terminal of the DC control voltage is defined as a null line, and the null line of the DC control voltage is connected with the null line of the AC control voltage. 
     The power supply of the switcher  11  of the original on-load tap changer is from the local low-voltage power supply, and the zero potential of the local low-voltage power supply is equal to the ground potential. If the switcher  11  of the on-load tap changer in the present invention is controlled in the manner of a contactor, the ground voltage of the contact of the switcher  11  is equal to the ground voltage of a certain terminal in the terminals B 1 , B 2 , B 3 , B 4 , B 5 , and the terminals B 1 , B 2 , B 3 , B 4 , B 5  have high voltages; the coil of the contactor is connected with a control power supply, the potential difference between the contact and the coil is very high, thus an expensive high-voltage contactor is needed. 
     In this embodiment, the power supply of the switcher  11  of the thyristor assisted on-load tap changer is from the transformer T 2 , the transformer T 2  only provides power supply to the thyristor assisted on-load tap changer, the capacity is small, thus the transformer is a small capacity transformer. The null line of the power supply has the same potential as B 3 , the maximum potential difference between the contact and the coil is equal to the potential difference between B 1  and B 3 . The requirements on the insulating withstand voltage between the contactor coil and the switch contact are reduced, thus the manufacturing cost can be reduced; in particular, for the on-load tap changer of a 10 kV system, the potential difference between B 1  and B 3  is 5% of 10 kV, namely AC 500 V. The switcher  11  of the thyristor assisted on-load tap changer can be manufactured by a conventional AC contactor to reduce the manufacturing cost. 
     The potential of the null line is equal to the potential of the centremost terminal among B 1 , B 2 , B 3 , B 4 , B 5 , and the potential is very high; therefore, the withstand voltage between the null line and the ground is larger than the maximum normal voltage between the terminals B 1  and B 0  of the regulating transformer, in order to avoid insulation breakdown between the null line and the ground. 
     If the tap selector  10  of the thyristor assisted on-load tap changer is also implemented in the manner of a contactor (relay), the structure of the operating power supply of the tap selector  10  of the on-load tap changer is the structure as shown in  FIG. 16 . The analysis method is the same as the above and will not be repeated redundantly. 
     EMBODIMENT 9 
     The action time of an on-load tap changer of a power system is very short, and the on-load tap changer is at a non-action state at most of time. Within the non-action time period of the on-load tap changer, if the two ends of a thyristor assisted circuit have voltages, the safety is poor; if the two ends of the thyristor assisted circuit have no voltage, the safety is high. The structure of the switcher of the fourth thyristor assisted on-load tap changer as shown in  FIG. 12  is suitable for the use that only one tap of the terminal J 1  and the terminal J 2  of the switcher is connected with the transformer during normal operation. For example, the conduction of the terminal J 1  of the switcher of the on-load tap changer with a common terminal J 3  is switched to the conduction of the terminal J 2  with the common terminal J 3 . After the switching of the switcher is finished, the tap selector disconnects the connection of J 1  with the transformer. At this time, the voltages at the two ends of the economical thyristor assisted circuit I 8  and the economical thyristor assisted circuit II 9  are zero, thus the safety is good. 
     During normal operation, if the terminal J 1  and the terminal J 2  of the switcher are still connected with the transformer and are not disconnected, the structure of the switcher of the fifth thyristor assisted on-load tap changer can be selected, the switcher includes a main switch K 21 - 1 , a main switch K 22 - 1 , a switch K 27 - 1 , a switch K 28 - 1 , a thyristor assisted circuit I, a thyristor assisted circuit II, a piezoresistor R and three terminals J 1 , J 2 , J 3 ; one end of the main switch K 21 - 1  is connected with the terminal J 1 , and the other end of the main switch K 21 - 1  is connected with the terminal J 3 ; one end of the thyristor assisted circuit I is connected with the terminal J 3 , and the other end of the thyristor assisted circuit I is connected with the terminal J 1  through the switch K 27 - 1 ; one end of the main switch K 22 - 1  is connected with the terminal J 2 , and the other end of the main switch K 22 - 1  is connected with the terminal J 3 ; one end of the thyristor assisted circuit II is connected with the terminal J 3 , and the other end of the thyristor assisted circuit II is connected with the terminal J 2  through the switch K 28 - 1 ; the end of the thyristor assisted circuit I connected with the switch K 27 - 1  and the end of the thyristor assisted circuit II connected with the switch K 28 - 1  are further connected with the piezoresistor R, as shown in  FIG. 14 . 
     Within the non-action time period of the on-load tap changer, K 27 - 1  and K 28 - 1  are switched off, the voltages at the two ends of the economical thyristor assisted circuit I 8  and the economical thyristor assisted circuit II 9  are zero. Before the switcher of the on-load tap changer works, K 27 - 1  and K 28 - 1  are switched on. After the switcher of the on-load tap changer works, K 27 - 1  and K 28 - 1  are switched off immediately. The actions of the switch contacts K 27 - 1  and K 28 - 1  can be achieved by an AC contactor. When the coil of an AC contactor K 27  is energized, the contact K 27 - 1  of the AC contactor K 27  acts, and when the coil of an AC contactor K 28  is energized, the contact K 28 - 1  of the AC contactor K 28  acts. Before the switcher of the thyristor assisted on-load tap changer works, the coils of the AC contactors K 27  and K 28  are firstly energized, and then the operation program of the switcher is entered. After the thyristor assisted on-load tap changer finishes the work, the coils of the AC contactors K 27  and K 28  are de-energized. 
     The rest structure and program of the fifth thyristor assisted on-load tap changer are the same as those in embodiment 6 and will not be repeated redundantly. 
     The thyristor assisted on-load tap changer and the method thereof in the present invention can be designed and manufactured by the prior art and can be completely achieved, thereby having a broad application prospect.