Thyristor assisted on-load tap changer and method thereof

A terminal I thereof is respectively connected with transition switches, and a terminal II is respectively connected with transition switches; the other terminals of the transition switches are jointly connected and are connected with the terminal through a saturable reactor and a thyristor switch; the other terminals of the transition switches are jointly connected and are connected with the terminal through a linear reactor, a saturable reactor and an overvoltage triggering thyristor circuit; an odd-numbered side main contact is respectively connected with the terminal I and a common terminal, and an even-numbered side main contact is respectively connected with the terminal II and the common terminal; the terminals of non-common terminals of a main path and a transition path are further connected with a bidirectional voltage stabilizing circuit.

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 includes a main path and a transition path; the main path is composed of a switch K1, and the transition path is composed of a linear reactor L1, a saturable reactor L2and an overvoltage triggering thyristor circuit, which are connected in series; one end of the switch K1is switched between a tap selector terminal I and a tap selector terminal II through a change-over switch K5, and one end of the linear reactor L1is switched between the tap selector terminal I and the tap selector terminal II through a change-over switch K6; the switch K1and 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 L4and a thyristor switch controlled by a control switch K10, which are connected in series; the transition path is composed of a linear reactor L1, a saturable reactor L2and an overvoltage triggering thyristor circuit, which are connected in series; one end of the saturable reactor L4is switched between a tap selector terminal I and a tap selector terminal II through a change-over switch K5, and one end of the linear reactor L1is switched between the tap selector terminal I and the tap selector terminal II through a change-over switch K6; 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 L4and a thyristor switch controlled by a control switch K10, which are connected in series; the transition path is composed of a linear reactor L1, a saturable reactor L2and an overvoltage triggering thyristor circuit, which are connected in series; one end of the saturable reactor L4is jointly connected with one ends of transition switches K15, K14, and the other ends of the transition switches K15, K14are respectively connected with tap selector terminals I, II; one end of the linear reactor L1is jointly connected with one ends of transition switches K17, K16, and the other ends of the transition switches K17, K16are 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 K11is further connected between the tap selector terminal I and the common terminal, and an even-numbered side main contact K12is further connected between the tap selector terminal II and the common terminal.

The reactance of the linear reactor L1is larger than zero and is smaller than Z1; Z1is equal to a quotient obtained by dividing a rated voltage between the tap selector terminals I, II by rated load current.

The linear reactor L1and the saturable reactor L2are merged into a reactor L3; the reactor L3is provided with a magnetic flux closed-loop iron core and a coil L3, 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 L3is 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 L3is equivalent to the saturable reactor L2; 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 L3is decreased to a smaller value quickly, and at this time, the coil L3is equivalent to the linear reactor L1.

The overvoltage triggering thyristor circuit includes a fuse FU1, and the fuse FU1is serially connected with a pair of thyristors D1, D2reversely connected in parallel to form a main path of the overvoltage triggering thyristor circuit; a resistor R1and a capacitor C1are connected to the two ends of the thyristors D1, D2reversely connected in parallel after being connected in series; the gate electrodes and the cathodes of the two thyristors D1, D2are respectively connected with capacitors C2, C3, resistors R2, R3and diodes D3, D4; the gate electrodes of the two thyristors D1, D2are further respectively connected with the input terminal of a full-bridge rectifier composed of diodes D5, D6, D7, D8, the output end of the full-bridge rectifier is connected with a constant voltage diode D9, the cathode of the constant voltage diode D9is connected with the output end anode of the full-bridge rectifier, and the anode of the constant voltage diode D9is connected with the output end cathode of the full-bridge rectifier; the stabilized voltage U1of the constant voltage diode D9is equal to k1U2; k1refers to a confidence coefficient and is a value of 1.2-2; U2refers 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 FU1is serially connected with a pair of thyristors D1, D2reversely connected in parallel to form a main path of the thyristor switch; a resistor R1and a capacitor C1are connected to the two ends of the thyristors D1, D2reversely connected in parallel after being connected in series; the gate electrodes and the cathodes of the two thyristors D1, D2are respectively connected with capacitors C2, C3, resistors R2, R3and diodes D3, D4; the gate electrodes of the two thyristors D1, D2are further respectively connected with the input terminal of a full-bridge rectifier composed of diodes D5, D6, D7, D8; a constant voltage diode D11and a constant voltage diode D9are serially connected in the same direction, the serial anodes of the constant voltage diodes D11, D9are connected with the cathode of the full-bridge rectifier, and the serial cathodes of the constant voltage diodes D11, D9are connected with the anode of the full-bridge rectifier; the anode of a diode D10is connected with the anode of the full-bridge rectifier, the cathode of the diode D10is connected with one end of a switch K10, and the other end of the switch K10is connected with the cathode of the full-bridge rectifier; the stabilized voltage value U3of the serially connected constant voltage diode D11and constant voltage diode D9is equal to k2(U1+U2); k2refers to a confidence coefficient and is a value of 1.1-1.5; U1=k1U2, k1refers to a confidence coefficient and is a value of 1.2-2; U2refers to the peak value of a rated working frequency operating voltage connected between the tap selector terminals1,2of the on-load tap changer; the sum of positive tube voltage drops of all semiconductors of a gate electrode trigger loop of the thyristor D1or D2is about 1.5U4, U4refers 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 D1or D2.

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 U2of 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 U1of the constant voltage diode D9.

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 K21-1, a main switch K22-1, an economical thyristor assisted circuit I, an economical thyristor assisted circuit II, a piezoresistor R and three terminals J1, J2, J3;

one end of the main switch K21-1is connected with the terminal J1, and the other end of the main switch K21-1is connected with the terminal J3; the economical thyristor assisted circuit I is connected with the main switch K21-1in parallel;

one end of the main switch K22-1is connected with the terminal J2, and the other end of the main switch K22-1is connected with the terminal J3; the economical thyristor assisted circuit II is connected with the main switch K22-1in parallel;

the end of the economical thyristor assisted circuit I close to the J1and the end of the thyristor assisted circuit II close to the J2are 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 K23-1, and the serial number of KB is K25-1;

the serial number of a normally open switch KA in the economical thyristor assisted circuit II is K24-1, and the serial number of KB is K26-1.

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 K21-1, a main switch K22-1, a switch K27-1, a switch K28-1, an economical thyristor assisted circuit I, an economical thyristor assisted circuit II, a piezoresistor R and three terminals J1, J2, J3;

one end of the main switch K21-1is connected with the terminal J1, and the other end of the main switch K21-1is connected with the terminal J3; one end of the economical thyristor assisted circuit I is connected with the terminal J3, and the other end of the economical thyristor assisted circuit I is connected with the terminal J1through the switch K27-1;

one end of the main switch K22-1is connected with the terminal J2, and the other end of the main switch K22-1is connected with the terminal J3; one end of the economical thyristor assisted circuit II is connected with the terminal J3, and the other end of the economical thyristor assisted circuit II is connected with the terminal J2through the switch K28-1;

the end of the economical thyristor assisted circuit I connected with the switch K27-1and the end of the thyristor assisted circuit II connected with the switch K28-1are 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 K23-1, and the serial number of KB is K25-1;

the serial number of a normally open switch KA in the economical thyristor assisted circuit II is K24-1, and the serial number of KB is K26-1.

The economical thyristor assisted circuit I and the economical thyristor assisted circuit II have the same structure and respectively include:

a pair of thyristors D1, D2are reversely connected in parallel to form a main path of the thyristor assisted circuit;

a resistor R1and a capacitor C1are connected to the two ends of the thyristors D1, D2reversely connected in parallel after being connected in series;

the gate electrodes and the cathodes of the two thyristors D1, D2are respectively connected with capacitors C2, C3, resistors R2, R3and diodes D3, D4; the anodes of the diodes D3, D4are respectively connected with the gate electrodes of the thyristors D1, D2, and the cathodes of the diodes D3, D4are respectively connected with the cathodes of the thyristors D1, D2;

the input terminal of a full-bridge rectifier composed of diodes D5, D6, D7, D8is connected between the gate electrodes of the two thyristors D1, D2after being serially connected with a normally open switch KB, the output end of the full-bridge rectifier is connected with a constant voltage diode D9, the cathode of the constant voltage diode D9is connected with the anode output end of the full-bridge rectifier, and the anode of the constant voltage diode D9is connected with the cathode output end of the full-bridge rectifier;

diodes D13, D14, D15are serially connected in the same direction, diodes D16, D17, D18are 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 D1, D2.

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 T2, and the terminal of a secondary coil of the transformer T2provides 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 T2is connected with the null line of the secondary coil of the transformer T2;

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 J1of the switcher with the common terminal J3to the conduction of the terminal J2with the common terminal J3is as follows:

(1) switching on the switch K23-1and switching on the switch K26-1; (2) switching off the main switch K21-1; (3) switching off the switch K23-1; (4) switching on the switch K24-1; (5) switching on the main switch K22-1; (6) resetting the entire group;

b. the working method of switching the conduction of the terminal J2of the switcher of the on-load tap changer with the common terminal J3to the conduction of the terminal J1with the common terminal J3is as follows:

(1) switching on the switch K24-1and switching on the switch K25-1; (2) switching off the main switch K22-1; (3) switching off the switch K24-1; (4) switching on the switch K23-1; (5) switching on the main switch K21-1; (6) resetting the entire group. When switching the conduction of the terminal J1of the switcher of the on-load tap changer with the common terminal J3to the conduction of the terminal J2with the common terminal J3, the time interval between switching off the switch K23-1and switching on the switch K24-1is larger than 20 milliseconds;

when switching the conduction of the terminal J2of the switcher of the on-load tap changer with the common terminal J3to the conduction of the terminal J1with the common terminal J3, the time interval between switching off the switch K24-1and switching on the switch K23-1is 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.

A further illustration of the present invention will be given below in combination with accompanying drawings and embodiments.

FIG. 1shows 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 inFIG. 2. It includes two tap selector terminals I1,II2, a common terminal3, two change-over switches K5, K6, a main vacuum switch K1, an overvoltage triggering thyristor circuit4, a linear reactor L1, a saturable reactor L2and a bidirectional voltage stabilizing circuit7; one tap terminal of the change-over switch K5and one tap terminal of the change-over switch K6are jointly connected with the tap selector terminal I1, the other tap terminal of the change-over switch K5and the other tap terminal of the change-over switch K6are jointly connected with the tap selector terminal II2; the common terminal of the change-over switch K5is connected with the common terminal3of the on-load tap changer through the main vacuum switch K1to form a main path; the common terminal of the change-over switch K6is serially connected with the common terminal3of the on-load tap changer through the linear reactor L1, the saturable reactor L2and the overvoltage triggering thyristor circuit4to form a transition path; the bidirectional voltage stabilizing circuit7is connected between the common terminal of the change-over switch K5and the common terminal of the change-over switch K6.

The overvoltage triggering thyristor circuit4is as shown inFIG. 3. A fuse FU1is serially connected with a pair of thyristors D1, D2reversely connected in parallel to form a main path. A resistor R1and a capacitor C1are connected to the two ends of the thyristors D1, D2reversely connected in parallel after being connected in series, in order to achieve the oscillation damping in the processes which the thyristors D1,D2are triggered on/off and prevent the wrong trigger caused by the over fast voltage rise at the two ends of the thyristors D1, D2. The gate electrodes and the cathodes of the two thyristors D1, D2are respectively connected with capacitors C2, C3and resistors R2, R3for resisting interference; the anodes of diodes D3, D4are 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 D1, D2are further respectively connected with the input terminal of a full-bride rectifier composed of diodes D5, D6, D7, D8, the output end of the full-bride rectifier is connected with a constant voltage diode D9, the cathode of the constant voltage diode D9is connected with the output end anode of the full-bride rectifier, the anode of the constant voltage diode D9is connected with the output end cathode of the full bridge rectifier circuit, and multiple low voltage level constant voltage diode D9can be serially connected to obtain a high voltage level constant voltage diode.

The stabilized voltage of the constant voltage diode D9should be larger than the peak value of the maximal normal voltage between the tap selector terminals I1, II2, in order to ensure that the constant voltage diode D9is 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 D9is too large, the withstand voltage of the main vacuum switch K1is required to be increased and the withstand voltages of the thyristors D1, D2are 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 D9is too large, the interference of the overvoltage triggering thyristor circuit4with 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 D9is too large, the interference of the overvoltage triggering thyristor circuit4will 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 D9cannot be too large to ensure the reliable operation of the thyristor assisted on-load tap changer. Therefore, the stabilized voltage U1of the constant voltage diode D9is equal to k1U2; k1refers to a confidence coefficient and is a value of 1.2-2; U2refers to the peak value of a rated working frequency operating voltage connected between the tap selector terminals1,2of the on-load tap changer. It is recommended that k1is 1.5.

The conduction of the tap selector terminal I1of the thyristor assisted on-load tap changer with the common terminal3can be switched to the conduction of the tap selector terminal II2with the common terminal3; the conduction of the tap selector terminal II2with the common terminal3can be switched to the conduction of the tap selector terminal I1with the common terminal3.

The working principle of switching the conduction of the tap selector terminal I1of the on-load tap changer with the common terminal3to the conduction of the tap selector terminal II2with the common terminal3is as follows:

(1) the change-over switch K6is switched; the overvoltage triggering thyristor circuit4is switched on, since the stabilized voltage of the constant voltage diode D9is larger than the peak value of the maximal normal AC voltage between the tap selector terminals I1, II2, the constant voltage diode D9is not conducted, and the thyristors D1, D2reversely connected in parallel are not triggered; the overvoltage triggering thyristor circuit4is not conducted;

(2) the main vacuum switch K1is switched off; the main path is switched off, and the potential of the terminal3connected with a load quickly decreases; the voltages at the two ends of the overvoltage triggering thyristor circuit4quickly rise, when the instantaneous value of the voltage is larger than the stabilized voltage of the constant voltage diode D9, the constant voltage diode D9is conducted to trigger the thyristor D1or D2to be conducted, and the transition path is automatically switched on; load current flows in from the tap selector terminal II2and flows out from the common terminal3through the transition path; since the current is alternating current, the overvoltage triggering thyristor circuit4automatically 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 circuit4rise again, and the overvoltage triggering thyristor circuit4is conducted again; the voltages at two ends of the overvoltage triggering thyristor circuit4are 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 D9. 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;

(4) the main vacuum switch K1is switched on; the load current flows through the main vacuum switch K1, and the current of the overvoltage triggering thyristor circuit4is reduced to zero.

When the main vacuum switch K1is not switched off, the overvoltage triggering thyristor circuit4will form short circuit circulation due to interference and wrong conduction. For a large capacity power system, if the reactance of the linear reactor L1is equal to zero, very large short circuit circulation will be formed. At this time, as long as the reactance of the linear reactor L1is 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 L1must be larger than zero. If the reactance of the linear reactor L1is 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 L1may 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 L1is 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 L1should be larger than zero and smaller than Z1; Z1is equal to a quotient obtained by dividing a rated voltage between the tap selector terminals1,2by rated load current. It is recommended that the reactance of the linear reactor L1is about 0.1 Z1. In order to reduce the volume, it is recommended that the reactor L1is a reactor provided with an air gap iron core.

The fuse FU1can cut off the short circuit circulation to serve as the backup protection of the thyristor D1(D2).

One function of the saturable reactor L2is to reduce the rate of rise of the current at the conducting moment of the thyristor D1(D2). The other function of the saturable reactor L2is as follows: the saturable reactor L2cooperates with the resistor R1and the capacitor C1in the overvoltage triggering thyristor circuit4to improve the anti-interference capability of the overvoltage triggering thyristor circuit4. 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 circuit7is to ensure that the voltages at the two ends of the bidirectional voltage stabilizing circuit7do not exceed a voltage stabilizing value, the voltage stabilizing value of the bidirectional voltage stabilizing circuit7is smaller than U2and is smaller than the stabilized voltage U1of the constant voltage diode D9. When the voltage between the tap selector terminals I1, II2is a normal rated voltage, the bidirectional voltage stabilizing circuit7is not conducted; in the case of a higher interference pulse voltage between the tap selector terminals I1, II2, the interference pulse is clipped to ensure that the interference pulse voltage is not larger than the stabilized voltage U1of the constant voltage diode D9in the overvoltage triggering thyristor circuit4, in order to prevent the interference pulse between the tap selector terminals I1, II2form triggering the conduction of the overvoltage triggering thyristor circuit4to generate the short circuit circulation. If other circuits can eliminate the interference pulse between the tap selector terminals I1, II2, the bidirectional voltage stabilizing circuit7can be removed. The bidirectional voltage stabilizing circuit7can 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 D1, D2are inadequate. Multiple overvoltage triggering thyristor circuits4can be serially connected to improve the working voltage.FIG. 4shows serial connection of three stages of overvoltage triggering thyristor circuits. R4refers to a divider resistor; when multiple overvoltage triggering thyristor circuits are serially connected, the R4balances the voltages of the overvoltage triggering thyristor circuits.

The thyristor assisted on-load tap changer ofFIG. 2is provided with a linear reactor L1and a saturable reactor L2. To further simplify the structure, the linear reactor L1and the saturable reactor L2can be merged into a single reactor L3, as shown inFIG. 5. The reactor L3is provided with a magnetic flux closed-loop iron core5and a coil L3, a part of section of the magnetic flux closed-loop iron core5has a larger sectional area, and the sectional area of the rest section of the iron core is smaller; the coil L3is 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 L3is equivalent to the saturable reactor L2. 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 L3is decreased to a smaller value quickly, and at this time, the coil L3is equivalent to the linear reactor L1.

As shown inFIG. 5, one reactor L3can serve instead of the linear reactor L1and the saturable reactor L2, thus the volume of the reactor is reduced.

The second thyristor assisted on-load tap changer in the present invention is as shown inFIG. 6. It includes two tap selector terminals I1,II2, a common terminal3, two change-over switches K5, K6, a thyristor switch6controlled by a control switch K10, an overvoltage triggering thyristor circuit4, a linear reactor L1, two saturable reactors L2, L4and a bidirectional voltage stabilizing circuit7; one tap terminal of the change-over switch K5and one tap terminal of the change-over switch K6are jointly connected with the tap selector terminal I1, the other tap terminal of the change-over switch K5and the other tap terminal of the change-over switch K6are jointly connected with the tap selector terminal II2; the common terminal of the change-over switch K5is serially connected with the common terminal3of the on-load tap changer through the saturable reactor L4and the thyristor switch6controlled by the control switch K10to form a main path; the common terminal of the change-over switch K6is serially connected with the common terminal3of the on-load tap changer through the linear reactor L1, the saturable reactor L2and the overvoltage triggering thyristor circuit4to form a transition path; the bidirectional voltage stabilizing circuit7is connected between the common terminal of the change-over switch K5and the common terminal of the change-over switch K6.

The circuit of the thyristor switch6controlled by the control switch K10is as shown inFIG. 7, andFIG. 7is obtained by changing on the basis ofFIG. 3. As for the part ofFIG. 7the same asFIG. 3, characteristics and parameter requirements are also the same and will not be repeated redundantly herein.

The difference betweenFIG. 7andFIG. 3lies in that: a diode D10and the control switch K10are added. The anode of the diode D10is connected with the anode of a full-bridge rectifier composed of diodes D5, D6, D7, D8, the cathode of the diode D10is connected with one end of the switch K10, and the other end of the switch K10is connected with the cathode of the full-bridge rectifier composed of the diodes D5, D6, D7, D8. It can be seen fromFIG. 7that, if the switch K10is conducted, the thyristor switch6is conducted, and if the switch K10is disconducted, the thyristor switch5is disconducted. After the control switch K10is switched on, the current passing by the control switch K10is thyristor triggering current, and the current is very small. The conduction and disconduction of the high current path of the thyristor switch6can be controlled by the small capacity control switch K10, 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 betweenFIG. 7andFIG. 3also lies in that: a constant voltage diode D11is added. The constant voltage diode D11and the constant voltage diode D9are serially connected in the same direction to replace the original constant voltage diode D9. The constant voltage diode D11is serially connected with the constant voltage diode D9to achieve the following two functions:

(1) overvoltage protection of the thyristor switch6;

(2) when the thyristor switch6is applied to a high voltage level on-load tap changer, the withstand voltages of the existing thyristors D1, D2may be inadequate, and multiple thyristor switches6must be serially connected to improve the working voltage, just as shown inFIG. 4. Since each stage of thyristor switch6is provided with a control switch K10, the actions of the control switches K10may be asynchronous, and under the condition that the actions of the control switches K10are asynchronous, the constant voltage diode D11and the constant voltage diode D9can ensure that the thyristor switches6act correctly.

If the stabilized voltage of the serially connected constant voltage diode D11and the constant voltage diode D9is too small, the pulse generated by the overvoltage triggering thyristor circuit4causes wrong conduction of the thyristor switch6. If the stabilized voltage of the serially connected constant voltage diode D11and the constant voltage diode D9is too large, the withstand voltages of the thyristors D1, D2are 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 D9is too large, the multiple serially connected thyristor switches6cannot achieve the function of the above-mentioned second item. The stabilized voltage U3of the serially connected constant voltage diode D11and the constant voltage diode D9is equal to k2(U1+U2); k2refers to a confidence coefficient and is a value of 1.1-1.5. After the thyristor D1(D2) is conducted, the thyristor D1(D2) has a diode forward voltage drop, the diode forward voltage drop of the thyristor D1(D2) 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 D1(D2) is U4. After the switch K10is switched on, the current firstly passes by the diode D10and the switch K10to trigger the gate electrode of the thyristor D1(D2) to conduct the thyristor D1(D2). The voltages at the two ends of the thyristor D1(D2) 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 D1(D2) is larger than U4, the current of the gate electrode loop of the thyristor D1(D2) 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 D1(D2) is smaller than U4, high current flows by the gate electrode loop of the thyristor D1(D2) to damage the thyristor D1(D2). When the sum of the diode forward voltage drops of all the semiconductors of the gate electrode trigger path of the thyristor D1(D2) is smaller than 1.2U4, multiple diodes can be serially connected to form D10to improve the diode forward voltage drop of the diode D10. If too many diodes D10are 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 D1(D2) is properly about 1.5th Namely, the sum of the diode forward voltage drops of the diodes D4, D7, D10, D6and the gate electrode of the thyristor D1is about 1.5U4, and the sum of the diode forward voltage drops of the diodes D3, D8, D10, D5and the gate electrode of the thyristor D2is about 1.5U4.

In this embodiment, a combination of the saturable reactor L4and the thyristor switch6is used instead of the main vacuum switch K1of the main path in embodiment 1. The anti-interference capability of the main vacuum switch K1is very strong, but the operation of the main vacuum switch K1needs 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 K10of the thyristor switch6can 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 switch6lies in that wrong action may be caused by the pulse interference. To improve the anti-interference capability of the thyristor switch6, the saturable reactor L4is serially connected. One function of the saturable reactor L4is to reduce the rate of rise of the current at the connection moment of the thyristor D1(D2) in the thyristor switch6. The other function of the saturable reactor L4is as follows: the saturable reactor L4cooperates with the resistor R1and the capacitor C1in the thyristor switch6to improve the anti-interference capability of the thyristor switch6.

The working process of switching the conduction of the tap selector terminal I1of the on-load tap changer with the common terminal3to the conduction of the tap selector terminal II2with the common terminal3is as follows: (1) switching the change-over switch K6; (2) switching off the control switch K10; switching off the main path, and automatically switching on the transition path; (3) switching the change-over switch K5; (4) switching on the control switch K10.

The third thyristor assisted on-load tap changer in the present invention is as shown inFIG. 8. It includes two tap selector terminals I1,II2, a common terminal3, an odd-numbered side main contact K11, an even-numbered side main contact K12, four transition switches K14, K15, K16, K17, an overvoltage triggering thyristor circuit4, a thyristor switch6controlled by a control switch K10, a linear reactor L1, two saturable reactors L2, L4and a bidirectional voltage stabilizing circuit7; the tap selector terminal I1is respectively connected with the transition switches K15, K17, and the tap selector terminal II2is respectively connected with the transition switches K14, K16; the other terminals of the transition switches K14, K15are jointly connected and are serially connected with the common terminal3of the on-load tap changer through the saturable reactor L4and the thyristor switch6controlled by the control switch K10to form a main path; the other terminals of the transition switches K16, K17are jointly connected and are serially connected with the common terminal3of the on-load tap changer through the linear reactor L1, the saturable reactor L2and the overvoltage triggering thyristor circuit4to form a transition path; the two ends of the odd-numbered side main contact K11are respectively connected with the tap selector terminal I1and the common terminal3of the on-load tap changer, and the two ends of the even-numbered side main contact K12are respectively connected with the tap selector terminal II2and the common terminal3of the on-load tap changer; the bidirectional voltage stabilizing circuit7is connected between the connecting terminal of the transition switches K14, K15and the connecting terminal of the transition switches K16, K17.

The thyristor switch6is controlled by the switch K10, if K10is conducted, the thyristor switch6is conducted, and if K10is disconducted, the thyristor switch6is disconducted.

The odd-numbered side main contact K11and the even-numbered side main contact K12ofFIG. 8are contactors with locks and are composed of closing coils, breaking (unlocking) coils, main contacts and auxiliary contacts. The four transition switches K14, K15, K16, K17are contactors without locks and are composed of closing coils, main contacts and auxiliary contacts.

The odd-numbered side main contact K11and the even-numbered side main contact K12are responsible for a long-term energization task. The thyristor switch6and the overvoltage triggering thyristor circuit4can work for a short period of time, and the thyristor D1(D2) needs no complicated cooling plate.

The switcher control circuit for switching the conduction of the tap selector terminal I1of the on-load tap changer with the common terminal3to the conduction of the tap selector terminal II2with the common terminal3is as shown inFIG. 9.

M+ refers to a positive bus of a control power supply, and M− refers to a negative bus of the control power supply; K11-T refers to the breaking (unlocking) coil of the K11contactor, and K11-1and K11-2refer to the auxiliary contacts of the K11contactor; K12-H refers to the closing coil of the K12contactor, and K12-1refers to the auxiliary contact of the K12contactor. K14-1, K14-2, K15-1, K15-2, K16-1and K16-2respectively refer to the auxiliary contacts of the transition switches K14, K15, K16, K10-1, K10-2and K10-3refer to the auxiliary contacts of the control switch K10, and KC1, KC2, KC3and KC4refer 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 M1; X1-2refers 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 X1-2contact 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 KC1-2contact, and the other end of the KC1-2contact is connected with an M1bus; a node between the BH contact and the KC1-2contact serially connects the K12-1auxiliary contact, the X1-2contact and the KC1coil to the M− bus; the KC1-1contact is connected with the two ends of the X1-2contact in parallel; M1serially connects the KC3-1contact and the K15coil to the M− bus; M1serially connects the K15-1contact, the KC2-2contact and the K10coil to the M− bus; M1serially connects the KC2-3contact and the K14-1contact to a node between the KC2-2contact and the K10coil; a node between the K15-1contact and the KC2-2coil serially connects the K10-1contact and the K11-T coil to the M− bus; M1serially connects the K16-1contact, the K11-1contact and the K16coil to the M− bus; a node between the K16-1contact and the K11-1contact is connected with the cathode of a diode D12, and a node between the K10-1contact and the K11-T coil is connected with the anode of the diode D12; M1serially connects the K16-2contact and the KC2coil to the M− bus; the KC2-1contact is connected with the K16-2contact in parallel; M1serially connects the K11-2contact, the KC2-4contact, the K10-2contact and the KC3coil to the M− bus; M1serially connects the KC3-2contact to a node between the K10-2contact and the KC3coil; a node between the KC2-4contact and the K10-2contact serially connects the K15-2contact and the K14coil to the M− bus; the node between the KC2-4contact and the K10-2contact serially connects the K14-2contact, the K10-3contact and the KC4coil to the M− bus; a node between the K14-2contact and the K10-3contact serially connects the KC4contact and the K12-H coil to the M− bus.

The working process of switching the conduction of the tap selector terminal I1with the common terminal3to the conduction of the tap selector terminal II2with the common terminal3is illustrated as follows:

when the X1-2contact is switched on, the K12contact and the X1-2contact are switched on, the KC1coil is energized, the KC1-1and KC1-2contacts are switched on, and the control circuit M1transmits power and is self-holding.

The KC3-1normally open contact is switched on, the K15coil is energized, the transition switch K15ofFIG. 8is switched on, and the thyristor switch6controlled by the control switch K10is conducted with the odd-numbered side main contact K12in parallel; the K15-1contact is switched on and the KC2-2normally closed contact is switched on to connect the K10coil, and the thyristor switch6controlled by the control switch K10ofFIG. 8is switched on; the K15-1contact is switched on and the K10-1contact is switched on to conduct the coil K11-T, the odd-numbered side main contact K11ofFIG. 8is switched off, and the load current is transferred to the path of the thyristor switch6; the K15-1contact is switched on, the K10-1contact is switched on and the K11-1contact is switched on to conduct the K16coil; the K16-1contact is switched on to self hold to conduct the K16coil, the transition switch K16ofFIG. 8is switched on, and the overvoltage triggering thyristor circuit4is switched on; the K16-2contact is switched on to conduct the KC2coil, and KC2-1is switched on to self hold to conduct the KC2coil; the KC2-2contact is switched off, the K10coil is de-energized, the thyristor switch6controlled by the control switch K10ofFIG. 8is disconducted, and the load current is transferred to the path of the overvoltage triggering thyristor circuit4; the K10-1contact is switched off, and the diode D12prevents K16-1from transmitting power to the coil K11-T; the K11-2contact is switched on, the KC2-4contact is switched on and the K10-2contact is switched on to conduct the KC3coil; the KC3-2contact is switched on to self hold to conduct the KC3coil; the KC3-1contact is switched off, and the K15coil is de-energized; the transition switch K15ofFIG. 8is switched off, the K15-2contact is switched on to conduct the K14coil, and the transition switch K14ofFIG. 8is switched on to conduct the thyristor switch6controlled by the control switch K10with the overvoltage triggering thyristor circuit4in parallel; the K14-1contact is switched on to conduct the K10coil again, the thyristor switch6controlled by the control switch K10ofFIG. 8is switched on again, and the load current is transferred to the path of the thyristor switch6controlled by the control switch K10again; the K10-3contact is switched on to conduct the KC4coil; the KC4contact is switched on to conduct the K12-H coil, the K12main contact ofFIG. 8is switched on, and the load current is transferred to the path of the K12main contact to connect the tap selector terminal II2with the common terminal3; meanwhile, the K12normally closed contact is switched off, the KC1coil is de-energized, the KC1-1contact and the KC1-2contact 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 K15is switched on firstly, and the control switch K10is switched on; the program is clear. Or, the transition switch K15and the control switch K10can 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 K11is firstly switched off, and after the load current is transferred to the path of the thyristor switch6, the transition switch K16is switched on to access the overvoltage triggering thyristor circuit4; the program is clear. Or, the odd-numbered side main contact K12is switched off and the transition switch K16is 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 II2of the on-load tap changer with the common terminal3to the conduction of the tap selector terminal I1with the common terminal3can 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 circuit4is 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 inFIG. 8, no current flow is generated in the disconduction and conduction processes of the odd-numbered side main contact K11, the even-numbered side main contact K12and the four transition switches K14, K15, K16, K17; 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 circuit4, the transistor switch6, the linear reactor L1and the saturable reactor L2can be added, in this way, the four transition switches K14, K15, K16, K17can be reduced to two, to achieve the purpose of reducing the number of the mechanical switches. (2) The overvoltage triggering thyristor circuit4and the transistor switch6have a large amount of identical elements and circuits;FIG. 3andFIG. 7can 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 I1and the common terminal3in parallel; after being serially connected with another such set of combined circuit, another transition switch is connected with the tap selector terminal II2and the common terminal3in 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 inFIG. 9can 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.

The principle structure and the connecting manner of a thyristor assisted on-load tap changer in the present invention are as shown inFIG. 2. It includes two tap selector terminals I1,II2, a common terminal3, two change-over switches K5, K6, a main vacuum switch K1, an overvoltage triggering thyristor circuit4, a linear reactor L1, a saturable reactor L2and a bidirectional voltage stabilizing circuit7; one tap terminal of the change-over switch K5and one tap terminal of the change-over switch K6are jointly connected with the tap selector terminal I1, the other tap terminal of the change-over switch K5and the other tap terminal of the change-over switch K6are jointly connected with the tap selector terminal II2; the common terminal of the change-over switch K5is connected with the common terminal3of the on-load tap changer through the main vacuum switch K1to form a main path; the common terminal of the change-over switch K6is serially connected with the common terminal3of the on-load tap changer through the linear reactor L1, the saturable reactor L2and the overvoltage triggering thyristor circuit4to form a transition path; the bidirectional voltage stabilizing circuit7is connected between the common terminal of the change-over switch K5and the common terminal of the change-over switch K6.

The main vacuum switch K1and the change-over switches K5, K6are 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 inFIG. 10. M+ refers to a positive bus of a control power supply, and M− refers to a negative bus of the control power supply; K1-T refers to the breaking coil of the K1switch, and K1-H, K5-H and K6-H respectively refer to the closing coils of the K1, K5and K6switches. K1-1, K1-2, K5-1, K5-2, K6-1, K6-2and K6-3respectively refer to the auxiliary contacts of the K1, K5and K6switches, and KC1and KC2refer 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 M1; X1-2refers 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 X1-2contact 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.

The principle structure and the connecting manner of a thyristor assisted on-load tap changer in the present invention are as shown inFIG. 6. It includes two tap selector terminals I1,II2, a common terminal3, two change-over switches K5, K6, a thyristor switch6controlled by a control switch K10, an overvoltage triggering thyristor circuit4, a linear reactor L1, two saturable reactors L2, L4and a bidirectional voltage stabilizing circuit7; one tap terminal of the change-over switch K5and one tap terminal of the change-over switch K6are jointly connected with the tap selector terminal I1, the other tap terminal of the change-over switch K5and the other tap terminal of the change-over switch K6are jointly connected with the tap selector terminal II2; the common terminal of the change-over switch K5is serially connected with the common terminal3of the on-load tap changer through the saturable reactor L4and the thyristor switch6controlled by the control switch K10to form a main path; the common terminal of the change-over switch K6is serially connected with the common terminal3of the on-load tap changer through the linear reactor L1, the saturable reactor L2and the overvoltage triggering thyristor circuit4to form a transition path; the bidirectional voltage stabilizing circuit7is connected between the common terminal of the change-over switch K5and the common terminal of the change-over switch K6.

The control switch K10and the change-over switches K5, K6are 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 inFIG. 11. M+ refers to a positive bus of a control power supply, and M− refers to a negative bus of the control power supply; K10-T refers to the breaking coil of the K10switch, and K10-H, K5-H and K6-H respectively refer to the closing coils of the K10, K5and K6switches. K10-1, K10-2, K5-1, K5-2, K6-1, K6-2and K6-3respectively refer to the auxiliary contacts of the K10, K5and K6switches, and KC1and KC2refer 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 M1; X1-2refers 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 X1-2contact 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.

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 selector10and a switcher11. The tap selector10is connected with the taps of a regulating transformer, the switcher11is connected with the tap selector10, and after the tap selector10selects the tap of the regulating transformer, the switcher11achieves the on-load switch of two taps of the regulating transformer. A tap selector terminal I1and the terminal J1of the switcher11are connected to a point, thus the tap selector terminal I1and the terminal J1of the switcher11can be considered as the same terminal; a tap selector terminal II2and the terminal J2of the switcher11are connected to a point, thus the tap selector terminal II2and the terminal J2of the switcher11can be considered as the same terminal; a common terminal3of the on-load tap changer is actually a switcher terminal J3.

In some application occasions, L1in the switcher (as shown inFIG. 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, L1, L2and L4in the switcher (as shown inFIG. 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 L1, L2and L4in the switcher (as shown inFIG. 8) of the third thyristor assisted on-load tap changer are removed, the thyristor switch6in the main path and the overvoltage triggering thyristor circuit4in the transition path can be replaced by an economical thyristor assisted circuit as shown inFIG. 13; KA in the economical thyristor assisted circuit as shown inFIG. 13represents K10, KB is switched off, and the economical thyristor assisted circuit is equivalent to the thyristor switch6; KA in the economical thyristor assisted circuit as shown inFIG. 13is switched off, KB is switched on, and the economical thyristor assisted circuit is equivalent to the overvoltage triggering thyristor circuit4. In this way, two economical thyristor assisted circuits (an economical thyristor assisted circuit I8and an economical thyristor assisted circuit II9) 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 (K23-1) and KB (K25-1) in the economical thyristor assisted circuit I8and KA (K24-1) and KB (K26-1) in the economical thyristor assisted circuit II9, the same functions of K14, K15, K16,17inFIG. 8are achieved. When the economical thyristor assisted circuit I8is used as the main path, the economical thyristor assisted circuit II9is used as the transition path; when the economical thyristor assisted circuit II9is used as the main path, the economical thyristor assisted circuit I8is used as the transition path.

The structure and the connecting manner of the switcher11of the fourth thyristor assisted on-load tap changer are as shown inFIG. 12. It includes a main switch K21-1, a main switch K22-1, an economical thyristor assisted circuit I8, an economical thyristor assisted circuit II9, a piezoresistor R and three terminals J1, J2, J3; the terminal J1is connected with the odd-numbered terminal of the tap selector, the terminal J2is connected with the even-numbered terminal of the tap selector, and the terminal J3is a common terminal. One end of the main switch K21-1is connected with the terminal J1, and the other end of the main switch K21-1is connected with the terminal J3; the economical thyristor assisted circuit I is connected with the main switch K21-1in parallel; one end of the main switch K22-1is connected with the terminal J2, and the other end of the main switch K22-1is connected with the terminal J3; the economical thyristor assisted circuit II is connected with the main switch K22-1in parallel; the end of the economical thyristor assisted circuit I close to the J1and the end of the economical thyristor assisted circuit II close to the J2are further connected with the piezoresistor R. The functions and requirements of the piezoresistor R are the same as those of7inFIG. 8, and will not be repeated redundantly herein.

The economical thyristor assisted circuit I8and the economical thyristor assisted circuit II9have the same structure and parameters, thus only one schematic diagram is given, as shown inFIG. 13. It includes: a pair of thyristors D1, D2are reversely connected in parallel to form a main path of the economical thyristor assisted circuit; a resistor R1and a capacitor C1are connected to the two ends of the thyristors D1, D2reversely connected in parallel after being connected in series; the gate electrodes and the cathodes of the two thyristors D1, D2are respectively connected with capacitors C2, C3, resistors R2, R3and diodes D3, D4; the anodes of the diodes D3, D4are respectively connected with the gate electrodes of the thyristors D1, D2, and the cathodes of the diodes D3, D4are respectively connected with the cathodes of the thyristors D1, D2; the input terminal of a full-bridge rectifier composed of diodes D5, D6, D7, D8is connected between the gate electrodes of the two thyristors D1, D2after being serially connected with a switch KB, the output end of the full-bridge rectifier is connected with a constant voltage diode D9, the cathode of the constant voltage diode D9is connected with the anode output end of the full-bridge rectifier, and the anode of the constant voltage diode D9is connected with the cathode output end of the full-bridge rectifier; diodes D13, D14, D15are serially connected in the same direction, diodes D16, D17, D18are 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 D1, D2.

KA of the economical thyristor assisted circuit I8is expressed by K23-1inFIG. 12, and KB is expressed by K25-1inFIG. 12; KA of the economical thyristor assisted circuit II9is expressed by K24-1inFIG. 12, and KB is expressed by K26-1inFIG. 12. Under the condition that KB is switched off, the economical thyristor assisted circuits I8and II2are equivalent to a switching circuit controlled by KA. It can be seen fromFIG. 13that, 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 D1, D2can 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 D1(D2) to conduct the thyristor D1(D2). The voltages at the two ends of the thyristor D1(D2) quickly reduce to the diode forward voltage drop of the thyristor D1(D2), if the sum of the diode forward drops of all semiconductors serially connected in the gate electrode trigger path of the thyristor D1(D2) is larger than the diode forward voltage drop of the thyristor D1(D2), the current of the gate electrode loop of the thyristor D1(D2) 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 D1(D2) is smaller than the diode forward voltage drop of the thyristor D1(D2), high current flows by the gate electrode loop of the thyristor D1(D2) to damage the thyristor D1(D2). InFIG. 13, diodes D13, D14, D15are connected in the same direction to form a diode string, diodes D16, D17, D18are 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 D1, D2after 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 D1(D2). The more the serially connected diodes are, the better the effect of ensuring zero current flowing by the switch KA after the thyristors D1, D2are 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 I8and the economical thyristor assisted circuit II9are equivalent to overvoltage triggering thyristor circuits. The stabilized voltage U1of a constant voltage diode D9is equal to k1U2; k1refers to a confidence coefficient and is a value of 1.2-2; U2refers to the peak value of a rated working frequency operating voltage between the connecting terminals J1, J2of the switcher and the tap selector of the thyristor assisted on-load tap changer. It is recommended that k1is 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 I8and II9are simple in structure and are high in reliability.

The conduction of the switcher terminal J1of the on-load tap changer with the common terminal J3can be switched to the conduction of the terminal J2with the common terminal J3; the conduction of the switcher terminal J2of the on-load tap changer with the common terminal J3can be switched to the conduction of the terminal J1with the common terminal J3. The working method of switching the conduction of the terminal J1of the switcher of the on-load tap changer with the common terminal J3to the conduction of the terminal J2with the common terminal J3is described as follows:

before switching, the main switch K21-1is switched on, the main switch K22-1is switched off, and the switches K23-1, K24-1, K25-1, K26-1are switched off. A power system is connected with one odd-numbered tap of a regulating transformer through the common terminal J3, the main switch K21-1, the terminal J1of the switcher11and the tap selector10. The on-load tap changer receives a regulation instruction and firstly commands the tap selector10to select to switch on a corresponding even-numbered tap changer, and the selection of the tap selector10is finished. The working sequence of the switcher11is as follows:

(1) the switch K23-1is switched on; the switch K26-1is switched on. When the switch K23-1is switched on, the economical thyristor assisted circuit I8is used as a conducted switch access circuit. When the switch K26-1is switched on, the economical thyristor assisted circuit II9is 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 D9, the constant voltage diode D9is not conducted, and the overvoltage triggering thyristor circuit is not conducted.

(2) The main switch K21-1is switched off. The load current is transferred to the economical thyristor assisted circuit I8.

(3) The switch K23-1is switched off. The current of the economical thyristor assisted circuit I8is cut off at a zero crossing point of the current, at the current cutoff moment of the economical thyristor assisted circuit I8, the potential of the terminal J3quickly descends (or ascends); the voltages at the two ends of the economical thyristor assisted circuit II9(the overvoltage triggering thyristor circuit) instantaneously generate overvoltages, when the instantaneous value of the overvoltage reaches the stabilized voltage of the constant voltage diode D9, the conduction of the thyristor D1or thyristor D2is triggered, the load current flows in from the terminal J2and flows out from the common terminal J3through the economical thyristor assisted circuit II9; the load current is transferred from the economical thyristor assisted circuit I8to the economical thyristor assisted circuit II9.

(4) The switch K24-1is switched on. The economical thyristor assisted circuit II9is used as a conducted switch access circuit.

(5) The main switch K22-1is switched on. The load current is transferred from the economical thyristor assisted circuit II9to the main switch K22-1.

(6) The entire group is reset.

It can be seen that, the switch K24-1must be only switched on after the switch K23-1is switched off and the current of the economical thyristor assisted circuit I8is cut off at the zero crossing point of the current. Otherwise, the current of the economical thyristor assisted circuit I8is cut off in front of the zero crossing point of the current, the switch K24-1is switched on too early, and the economical thyristor assisted circuit I8and the economical thyristor assisted circuit II9will cause short circuit circulation. The time between switching off the switch K23-1and cutting off the current of the economical thyristor assisted circuit I8at the zero crossing point of the current is uncertain. To ensure that the switch K24-1is switched on after the current of the economical thyristor assisted circuit I8is cut off at the zero crossing point of the current, the time interval between switching off the switch K23-1and switching on the switch K24-1should be larger than 20 milliseconds.

Similarly, the working method of switching the conduction of the terminal J2of the switcher of the on-load tap changer with the common terminal3to the conduction of the terminal J1and the common terminal3is as follows:

before switching, the main switch K22-1is switched on, the main switch K21-1is switched off, and the switches K23-1, K24-1, K25-1, K26-1are switched off; after the tap selector10selects the transformer tap; (1) the switch K24-1is switched on; the switch K25-1is switched on; (2) the main switch K22-1is switched off; (3) the switch K24-1is switched off; (4) the switch K23-1is switched on; (5) the main switch K21-1is switched on; (6) the entire group is reset.

The time interval between switching off the switch K24-1and switching on the switch K23-1should be larger than 20 milliseconds.

The switches K21-1, K22-1, K23-1, K24-1, K25-1and, K26-1can 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 I8and the economical thyristor assisted circuit II9are serially connected with a saturable reactor L2respectively to increase the safety of the switcher I1of 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.

In embodiment 6, the switches K21-1, K22-1, K23-1, K24-1, K25-1and K26-1can be manually operated, and the electric switches are manually operated to sequentially act in order to achieve the on-load switch of the switcher11. Actually, in view of the switches K21-1, K22-1, K23-1, K24-1, K25-1and K26-1, 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 switcher11; a variety of methods can be adopted, thus the application is flexible.

In a variety of applications, the switches K21-1, K22-1, K23-1, K24-1, K25-1and K26-1are controlled by the contactor (relay) contact to sequentially act to achieve the on-load switch of the switcher11more simply and more economically. The main switch K21-1and the main switch K22-1are contactors with locks and are composed of closing coils, breaking (unlocking) coils, main contacts (main switches) and auxiliary contacts, the switches K23-1, K24-1, K25-1, K26-1are 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 switcher11of the fourth thyristor assisted on-load tap changer as shown inFIG. 12, except the main switch, no other large capacity contactor (relay) is needed; the switches K23-1, K24-1, K25-1, K26-1are 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 switcher11of 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) K23-1, K24-1, K25-1, K26-1can also be operated in the arc-free manner.

The control circuit of the switcher11for switching the conduction of the terminal J1of the fourth thyristor assisted on-load tap changer implemented by a contactor (relay) with the common terminal J3to the conduction of the terminal J2with the common terminal J3is as shown inFIG. 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; K21T refers to the breaking (unlocking) coil of a K21contactor, K21-1refers to the main contact of the K21contactor, and K21-2refers to the auxiliary contact of the K21contactor; K22H refers to the closing coil of a K22contactor, K22-1refers to the main contact of the K22contactor, and K22-2refers to the auxiliary contact of the K22contactor. K23-1, K23-2, K23-3refer to the contacts of a relay K23, K24-1, K24-2refer to the contacts of a relay K24, K26-1, K26-2refer to the contacts of a relay K26, K1C-1, K1C-2refer to the contacts of a relay K1C, KC2-1, KC2-2refer to the contacts of a relay KC2, KC3-1refers to the contact of a relay KC3, and KC4-1, KC4-2, KC4-3refer to the contacts of a relay KC4.

A normally open contact K21-2and a relay coil K1C are serially connected between the buses M+ and M−; the two ends of the normally open contact K21-2are also connected with the normally open contact K1C-1in parallel. A normally open contact K1C-2is connected between a collection line A and the bus M+. A normally closed contact KC2-1and a relay coil K23are serially connected between the collection line A and the bus M−. A relay coil K26is serially connected between the collection line A and the bus M−. A normally open contact K26-2, a normally open contact K23-2and the contactor breaking coil K21T are serially connected between the collection line A and the bus M−. A normally closed contact K21-4and a relay coil KC2are serially connected between the collection line A and the bus M−. A normally open contact KC2-2, a normally closed contact K23-3and a relay coil KC3are serially connected between the collection line A and the bus M−. A normally open contact KC3-1and a relay coil KC4are serially connected between the collection line A and the bus M−. A normally open contact KC4-1and a relay coil K24are serially connected between the collection line A and the bus M−. A normally open contact KC4-2, a normally open contact K24-2and the contactor closing coil K22H 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 K21-2is switched on, the relay K1C acts, the contact K1C-1is switched on, and the relay K1C self holds. The contact K1C-2is switched on. The contact KC2-1is switched on, the relay K23acts, the contact K23-1inFIG. 12is switched on, and the thyristor assisted circuit I8is connected to serve as a switch. The relay K26acts, the contact K26-1inFIG. 12is switched on, the thyristor assisted circuit II9is switched on to serve as the overvoltage triggering thyristor circuit, and the overvoltage triggering thyristor circuit is not conducted. The contact K26-2is switched on, the contact K23-2is switched on, the contactor breaking coil K21T is energized, and the main contact K21-1of the contactor inFIG. 12is switched off. The contact K21-4is switched on, and the relay KC2acts. The contact KC2-1is switched off, the relay K23returns, the contact K23-1inFIG. 12is switched off, and the thyristor assisted circuit I8cuts off the current path at the zero crossing point of the current. At the moment when the thyristor assisted circuit I8cuts off the current path at the zero crossing point of the current, the thyristor assisted circuit II9is connected to serve as the overvoltage triggering thyristor circuit. The contact KC2-2is switched on, the contact K23-3is switched on, and the relay KC3acts. The contact KC3-1is switched on, and the relay KC4acts. The contact KC4-1is switched on, the relay K24acts, the contact K24-1of the thyristor assisted circuit II9inFIG. 12is switched on, and the thyristor assisted circuit II9is used as a switch to conduct the current path. Since the action times of the relays KC3, KC4, K24are about 15 milliseconds, it can be ensured that the contact KC4-1is switched on more than 20 milliseconds after the contact K23-1is switched off, thus generating no short circuit circulation. The contact KC4-2is switched on, the contact K24-2is switched on, and the contactor closing coil K22H is energized; the main contact K22-1inFIG. 12is switched on, and the load current is transferred to the path of J3and J2. Similarly, it can be designed that: the switcher control circuit for switching the conduction of the terminal J2of the fourth thyristor assisted on-load tap changer with the common terminal J3to the conduction of the terminal J1with the common terminal J3is as shown inFIG. 15 (b). The working principle ofFIG. 15 (b)is similar to that ofFIG. 15 (a), and will not be repeated redundantly.

The operating power supply of a switcher11of 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 K21-1, K22-1, K23-1, K24-1, K25-1, K26-1and 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 K21-1, K22-1, K23-1, K24-1, K25-1, K26-1are high, the voltage between the contacts of the switches K21-1, K22-1, K23-1, K24-1, K25-1, K26-1and the operating power supply is higher, the contacts of the switches K21-1, K22-1, K23-1, K24-1, K25-1, K26-1and 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 K21-1, K22-1, K23-1, K24-1, K25-1, K26-1and 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 inFIG. 16. It is assumed that, the regulating transformer T1has five tap terminals, which are respectively connected to the input terminals B1, B2, B3, B4, B5of a tap selector10of the thyristor assisted on-load tap changer; the output terminal of the tap selector10is connected with the input terminals J1, J2of the switcher; a common terminal J3of the switcher11is connected with a power system.

In the tap terminals B1, B2, B3, B4, B5of the regulating transformer, the centremost terminal (B3) is defined as a null line and is connected with one terminal of a primary coil of a transformer T2; the tap terminal B2(or B4) of the regulating transformer adjacent to the null line is connected with another terminal of the primary coil of the transformer T2. Terminals B6, B7of a secondary coil of the transformer T2provide an AC control voltage (for example, AC 220V) to the switcher11of 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 T2is connected with the null line of the secondary coil of the transformer T2.

An AC control voltage terminal is used as the input to a DC voltage stabilization power supply module12, the DC voltage stabilization power supply module12outputs a DC voltage (for example, B8, B9DC 24V) or multiple DC voltages. The output of the DC voltage stabilization power supply module12provides a DC control voltage to the switcher11of 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 switcher11of 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 switcher11of 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 switcher11is equal to the ground voltage of a certain terminal in the terminals B1, B2, B3, B4, B5, and the terminals B1, B2, B3, B4, B5have 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 switcher11of the thyristor assisted on-load tap changer is from the transformer T2, the transformer T2only 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 B3, the maximum potential difference between the contact and the coil is equal to the potential difference between B1and B3. 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 B1and B3is 5% of 10 kV, namely AC 500 V. The switcher11of 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 B1, B2, B3, B4, B5, 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 B1and B0of the regulating transformer, in order to avoid insulation breakdown between the null line and the ground.

If the tap selector10of 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 selector10of the on-load tap changer is the structure as shown inFIG. 16. The analysis method is the same as the above and will not be repeated redundantly.

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 inFIG. 12is suitable for the use that only one tap of the terminal J1and the terminal J2of the switcher is connected with the transformer during normal operation. For example, the conduction of the terminal J1of the switcher of the on-load tap changer with a common terminal J3is switched to the conduction of the terminal J2with the common terminal J3. After the switching of the switcher is finished, the tap selector disconnects the connection of J1with the transformer. At this time, the voltages at the two ends of the economical thyristor assisted circuit I8and the economical thyristor assisted circuit II9are zero, thus the safety is good.

During normal operation, if the terminal J1and the terminal J2of 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 K21-1, a main switch K22-1, a switch K27-1, a switch K28-1, a thyristor assisted circuit I, a thyristor assisted circuit II, a piezoresistor R and three terminals J1, J2, J3; one end of the main switch K21-1is connected with the terminal J1, and the other end of the main switch K21-1is connected with the terminal J3; one end of the thyristor assisted circuit I is connected with the terminal J3, and the other end of the thyristor assisted circuit I is connected with the terminal J1through the switch K27-1; one end of the main switch K22-1is connected with the terminal J2, and the other end of the main switch K22-1is connected with the terminal J3; one end of the thyristor assisted circuit II is connected with the terminal J3, and the other end of the thyristor assisted circuit II is connected with the terminal J2through the switch K28-1; the end of the thyristor assisted circuit I connected with the switch K27-1and the end of the thyristor assisted circuit II connected with the switch K28-1are further connected with the piezoresistor R, as shown inFIG. 14.

Within the non-action time period of the on-load tap changer, K27-1and K28-1are switched off, the voltages at the two ends of the economical thyristor assisted circuit I8and the economical thyristor assisted circuit II9are zero. Before the switcher of the on-load tap changer works, K27-1and K28-1are switched on. After the switcher of the on-load tap changer works, K27-1and K28-1are switched off immediately. The actions of the switch contacts K27-1and K28-1can be achieved by an AC contactor. When the coil of an AC contactor K27is energized, the contact K27-1of the AC contactor K27acts, and when the coil of an AC contactor K28is energized, the contact K28-1of the AC contactor K28acts. Before the switcher of the thyristor assisted on-load tap changer works, the coils of the AC contactors K27and K28are 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 K27and K28are 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.