A thyristor-based arcless on-load tap changer for distribution transformers

Abstract

The application discloses a thyristor-based arcless on-load voltage regulating switch device for distribution transformers, which comprises three-phase voltage regulating switch circuits connected with three-phase windings of a primary side of the distribution transformer respectively; the three-phase voltage regulating switch circuits have the same topological structure and each comprises five groups of anti-parallel thyristor groups, a first anti-circulating current circuit, a second anti-circulating current circuit, a starting switch and a lightning arrester; the five groups of anti-parallel thyristor groups are connected in series in sequence to form an anti-parallel thyristor string with six connection points, and the six connection points are used for connecting with A / B / C phase winding taps of the corresponding distribution transformer; the first anti-circulating current circuit is connected in parallel with the second anti-parallel thyristor group, and the second anti-circulating current circuit is connected in parallel with the fourth anti-parallel thyristor group; the first anti-circulating current circuit and the second anti-circulating current circuit are controlled by using a half-wave switching control method, and low-circulating current switching between multiple variable ratios of the distribution transformer is realized.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of medium-voltage alternating current switch, and particularly relates to a thyristor-based arcless on-load voltage regulating switch device for distribution transformer. BACKGROUND

[0002] At present, the on-load voltage regulating switch of high-voltage distribution transformer usually adopts mechanical design, but the mechanical design has the defects of arcing in voltage regulation and short service life. With the deepening of the whole county photovoltaic project, a large number of distributed photovoltaic are connected to the low-voltage distribution network, causing wide voltage fluctuation and even overrunning in the transformer area, resulting in the decline of power quality and affecting the normal work of user equipment.

[0003] Therefore, there is an urgent need for an arcless on-load voltage regulating device suitable for 10kV distribution transformer. SUMMARY

[0004] The application aims to solve the problem of transformer area voltage overrun caused by photovoltaic power fluctuation, and provides an arcless on-load voltage regulating switch device suitable for 10kV distribution transformer in a high-proportion photovoltaic grid-connected scene.

[0005] The technical scheme is as follows: a thyristor-based arcless on-load voltage regulating switch device for distribution transformer, comprising A-phase voltage regulating switch circuit, B-phase voltage regulating switch circuit and C-phase voltage regulating switch circuit connected with the three-phase winding of the primary side of the distribution transformer respectively;

[0006] The A-phase voltage regulating switch circuit, the B-phase voltage regulating switch circuit and the C-phase voltage regulating switch circuit have the same topology, and each comprises: a first anti-parallel thyristor group, a second anti-parallel thyristor group, a third anti-parallel thyristor group, a fourth anti-parallel thyristor group, a fifth anti-parallel thyristor group, a first anti-circulation circuit, a second anti-circulation circuit, a starting switch and a lightning arrester;

[0007] The first anti-parallel thyristor group, the second anti-parallel thyristor group, the third anti-parallel thyristor group, the fourth anti-parallel thyristor group and the fifth anti-parallel thyristor group are connected in series in turn to form an anti-parallel thyristor string with six connection points, and the six connection points are connected with the No.2 tap, the No.3 tap, the No.4 tap, the No.5 tap, the No.6 tap and the No.7 tap of the A / B / C-phase winding of the distribution transformer in turn; each anti-parallel thyristor group is composed of a first thyristor and a second thyristor connected in anti-parallel;

[0008] The first and second anti-circulation circuits each include a buffer resistor and a switching thyristor; one end of the buffer resistor is connected with the positive pole of the switching thyristor; the first anti-circulation circuit is connected in parallel with the second anti-parallel thyristor group, and the conduction direction of the switching thyristor in the first anti-circulation circuit is consistent with the conduction direction of the first thyristor in the second anti-parallel thyristor group; the second anti-circulation circuit is connected in parallel with the fourth anti-parallel thyristor group, and the conduction direction of the switching thyristor in the second anti-circulation circuit is consistent with the conduction direction of the first thyristor in the fourth anti-parallel thyristor group;

[0009] The starting switch is connected in parallel with the switching thyristor in the second anti-circulation circuit;

[0010] The lightning arrester is connected in parallel with the anti-parallel thyristor string;

[0011] The first and second anti-circulation circuits are controlled by using a half-wave switching control method, so as to realize low-circulation switching among multiple variable ratios of the distribution transformer;

[0012] The half-wave switching control method comprises the following steps: when the secondary side voltage is 0.95 p.u., the first and second anti-parallel thyristor groups are controlled to be in the off state, and the switching thyristors in the first and second anti-circulation circuits are controlled to be in the on state; when the secondary side voltage is 0.96 p.u., the first anti-parallel thyristor group is controlled to be in the on state, and the switching thyristor in the first anti-circulation circuit is controlled to be in the off state; when the secondary side voltage is 0.97 p.u., the second anti-parallel thyristor group is controlled to be in the on state, and the switching thyristor in the second anti-circulation circuit is controlled to be in the off state; when the secondary side voltage is 0.98 p.u., the first anti-parallel thyristor group is controlled to be in the off state, and the switching thyristor in the first anti-circulation circuit is controlled to be in the on state; when the secondary side voltage is 0.99 p.u., the second anti-parallel thyristor group is controlled to be in the off state, and the switching thyristor in the second anti-circulation circuit is controlled to be in the on state; and when the secondary side voltage is 1.0 p.u., the first and second anti-parallel thyristor groups are controlled to be in the on state, and the switching thyristors in the first and second anti-circulation circuits are controlled to be in the off state.

[0013] Further, when the secondary side voltage needs to be raised from 0.95 p.u. to 0.975 p.u., the half-wave switching control method comprises the following steps:

[0014] The second connection point is kept connected with the No. 3 tap of the A / B / C phase windings of the distribution transformer, the first connection point is disconnected from the No. 2 tap of the A / B / C phase windings of the distribution transformer, and the third connection point is connected to the No. 4 tap of the A / B / C phase windings of the distribution transformer;

[0015] When the current of the second thyristor of the first anti-parallel thyristor group is 0, the second thyristor is turned off;

[0016] When the switching thyristor in the first anti-circulation circuit bears voltage in the forward direction, the switching thyristor is turned on, and the current is transferred to the first anti-circulation circuit;

[0017] When the current of the first thyristor of the first anti-parallel thyristor group drops to 0, the first thyristor of the first anti-parallel thyristor group is turned off, and the second thyristor of the second anti-parallel thyristor group is turned on;

[0018] When the current of the switching thyristor in the first anti-circulation circuit drops to 0, the switching thyristor in the first anti-circulation circuit is turned off, and the first thyristor of the second anti-parallel thyristor group is turned on; the No. 3 tap of the A / B / C phase windings of the distribution transformer is short-circuited with the No. 4 tap of the A / B / C phase windings of the distribution transformer, and the voltage raising process is completed.

[0019] Further, when the secondary voltage needs to be raised from 0.975 p.u. to 1 p.u., the half-wave switching control method comprises the following steps:

[0020] The third connection point is connected to the No. 4 tap of the A / B / C phase winding of the distribution transformer, the second connection point is disconnected from the No. 3 tap of the A / B / C phase winding of the distribution transformer, and the fourth connection point is connected to the No. 5 tap of the A / B / C phase winding of the distribution transformer;

[0021] When the current of the first thyristor of the second anti-parallel thyristor group is 0, the first thyristor is turned off;

[0022] When the switching thyristor in the first anti-circulation circuit is positively subjected to voltage, the switching thyristor is turned on;

[0023] When the current of the second thyristor of the second anti-parallel thyristor group decreases to 0, the second thyristor of the second anti-parallel thyristor group is turned off, and the first thyristor of the third anti-parallel thyristor group is turned on;

[0024] When the current of the switching thyristor in the first anti-circulation circuit is 0, the switching thyristor in the first anti-circulation circuit is turned off, and the second thyristor of the third anti-parallel thyristor group is turned on; the No. 4 tap of the A / B / C phase winding of the distribution transformer is short-circuited with the No. 5 tap of the A / B / C phase winding of the distribution transformer, and the voltage raising process is completed.

[0025] Further, when the secondary voltage needs to be raised from 1 p.u. to 1.025 p.u., the half-wave switching control method comprises the following steps:

[0026] The fourth connection point is connected to the No. 5 tap of the A / B / C phase winding of the distribution transformer, the third connection point is disconnected from the No. 4 tap of the A / B / C phase winding of the distribution transformer, and the fifth connection point is connected to the No. 6 tap of the A / B / C phase winding of the distribution transformer;

[0027] When the current of the second thyristor of the third anti-parallel thyristor group is 0, the second thyristor is turned off;

[0028] When the switching thyristor in the second anti-circulation circuit is positively subjected to voltage, the switching thyristor in the second anti-circulation circuit is turned on, and the current is transferred to the second anti-circulation circuit;

[0029] When the current of the first thyristor of the third anti-parallel thyristor group decreases to 0, the first thyristor of the third anti-parallel thyristor group is turned off, and the second thyristor of the fourth anti-parallel thyristor group is turned on;

[0030] When the current of the switching thyristor in the second anti-circulation circuit is 0, the switching thyristor in the second anti-circulation circuit is closed, and the first thyristor of the fourth anti-parallel thyristor group is opened at the same time; the No. 5 tap of the A / B / C phase winding of the distribution transformer and the No. 6 tap of the A / B / C phase winding of the distribution transformer are short-circuited, and the boosting process is completed.

[0031] Further, when the secondary side voltage per unit needs to be increased from 1.025 p.u. to 1.05 p.u., the half-wave switching control method comprises the following steps:

[0032] The fifth connection point is connected with the No. 6 tap of the A / B / C phase winding of the distribution transformer, the fourth connection point is disconnected with the No. 5 tap of the A / B / C phase winding of the distribution transformer, and the sixth connection point is connected with the No. 7 tap of the A / B / C phase winding of the distribution transformer;

[0033] When the current of the first thyristor of the fourth anti-parallel thyristor group is 0, the first thyristor of the fourth anti-parallel thyristor group is closed;

[0034] When the switching thyristor in the second anti-circulation circuit is subjected to forward voltage, the switching thyristor in the second anti-circulation circuit is turned on, and the current is transferred to the second anti-circulation circuit;

[0035] When the current of the second thyristor of the fourth anti-parallel thyristor group is 0, the second thyristor of the fourth anti-parallel thyristor group is closed; and the first thyristor of the fifth anti-parallel thyristor group is opened at the same time;

[0036] When the current of the switching thyristor in the second anti-circulation circuit is 0, the switching thyristor in the second anti-circulation circuit is closed, and the second thyristor of the fifth anti-parallel thyristor group is opened at the same time; the No. 6 tap of the A / B / C phase winding of the distribution transformer and the No. 7 tap of the A / B / C phase winding of the distribution transformer are short-circuited, and the boosting process is completed.

[0037] Further, when the secondary side voltage per unit needs to be decreased from 1.05 p.u. to 1.025 p.u., the half-wave switching control method comprises the following steps:

[0038] The fifth connection point is connected with the No. 6 tap of the A / B / C phase winding of the distribution transformer; the sixth connection point is disconnected with the No. 7 tap of the A / B / C phase winding of the distribution transformer, and the fourth connection point is connected with the No. 5 tap of the A / B / C phase winding of the distribution transformer;

[0039] When the current of the second thyristor of the fifth anti-parallel thyristor group is 0, the second thyristor of the fifth anti-parallel thyristor group is closed;

[0040] When the switching thyristor in the second anti-circulation circuit is subjected to forward voltage, the switching thyristor in the second anti-circulation circuit is turned on, and the current is transferred to the second anti-circulation circuit;

[0041] When the current of the first thyristor of the fifth anti-parallel thyristor group is 0, the first thyristor of the fifth anti-parallel thyristor group is turned off, and the second thyristor of the fourth anti-parallel thyristor group is turned on;

[0042] When the current of the switching thyristor in the second anti-circulation circuit is 0, the switching thyristor in the second anti-circulation circuit is turned off, and the first thyristor of the fourth anti-parallel thyristor group is turned on; the short circuit of the No. 5 tap of the A / B / C phase winding of the distribution transformer and the No. 6 tap of the A / B / C phase winding of the distribution transformer is changed to the short circuit of the No. 5 tap of the A / B / C phase winding of the distribution transformer and the No. 6 tap of the A / B / C phase winding of the distribution transformer, and the voltage reduction is completed.

[0043] Further, when the secondary side voltage per unit needs to be reduced from 1.025 p.u. to 1 p.u., the half-wave switching control method comprises the following steps:

[0044] The fourth connection point remains connected to the No. 5 tap of the A / B / C phase winding of the distribution transformer, the fifth connection point is disconnected from the No. 6 tap of the A / B / C phase winding of the distribution transformer, and the third connection point is connected to the No. 4 tap of the A / B / C phase winding of the distribution transformer;

[0045] When the current of the first thyristor of the fourth anti-parallel thyristor group is 0, the first thyristor of the fourth anti-parallel thyristor group is turned off;

[0046] When the switching thyristor in the second anti-circulation circuit is positively subjected to voltage, the switching thyristor in the second anti-circulation circuit is turned on;

[0047] When the current of the second thyristor of the fourth anti-parallel thyristor group is 0, the second thyristor of the fourth anti-parallel thyristor group is turned off, and the first thyristor of the third anti-parallel thyristor group is turned on;

[0048] When the current of the switching thyristor in the second anti-circulation circuit is 0, the switching thyristor in the second anti-circulation circuit is turned off, and the second thyristor of the third anti-parallel thyristor group is turned on.

[0049] Further, when the secondary side voltage per unit needs to be reduced from 1 p.u. to 0.975 p.u., the half-wave switching control method comprises the following steps:

[0050] The third connection point remains connected to the No. 4 tap of the A / B / C phase winding of the distribution transformer, the fourth connection point is disconnected from the No. 5 tap of the A / B / C phase winding of the distribution transformer, and the second connection point is connected to the No. 3 tap of the A / B / C phase winding of the distribution transformer;

[0051] When the current of the second thyristor of the third anti-parallel thyristor group is 0, the second thyristor of the third anti-parallel thyristor group is turned off;

[0052] When the switching thyristor in the first anti-circulation circuit is positively subjected to voltage, the switching thyristor in the first anti-circulation circuit is turned on;

[0053] When the current of the first thyristor of the third anti-parallel thyristor group is 0, the first thyristor of the third anti-parallel thyristor group is turned off, and the second thyristor of the second anti-parallel thyristor group is turned on;

[0054] When the current of the switching thyristor in the first anti-circulation circuit is 0, the switching thyristor in the first anti-circulation circuit is turned off, and the first thyristor of the second anti-parallel thyristor group is turned on, and the voltage reduction process is completed.

[0055] Further, when the secondary side voltage standard needs to be reduced from 0.975 p.u. to 0.95 p.u., the half-wave switching control method comprises the following steps:

[0056] The second connection point is connected with the No. 3 tap of the A / B / C phase winding of the distribution transformer, the third connection point is disconnected with the No. 4 tap of the A / B / C phase winding of the distribution transformer, and the first connection point is connected with the No. 2 tap of the A / B / C phase winding of the distribution transformer;

[0057] When the current of the first thyristor of the second anti-parallel thyristor group is 0, the first thyristor of the second anti-parallel thyristor group is turned off;

[0058] When the switching thyristor in the first anti-circulation circuit is positively subjected to voltage, the switching thyristor in the first anti-circulation circuit is turned on;

[0059] When the current of the second thyristor of the second anti-parallel thyristor group is 0, the second thyristor of the second anti-parallel thyristor group is turned off, and the first thyristor of the first anti-parallel thyristor group is turned on;

[0060] When the current of the switching thyristor in the first anti-circulation circuit is 0, the switching thyristor in the first anti-circulation circuit is turned off, and the second thyristor of the first anti-parallel thyristor group is turned on.

[0061] Beneficial effects: compared with the prior art, the present application has the following advantages:

[0062] (1) The 5 groups of anti-parallel thyristors of each phase winding of the present application are connected in series in turn and connected with the step voltage tap, forming a voltage regulating circuit, which is suitable for the winding structure of the standard distribution transformer with step voltage taps;

[0063] (2) The lightning arrester is connected in parallel at both ends of the 5 groups of series-parallel thyristor strings, the voltage of the series-parallel thyristor string is clamped sensitively, while protecting all the thyristors, the voltage difference between the normal working condition and the abnormal working condition of the thyristor is increased, in particular, the maximum voltage stress of the thyristor under the normal working condition is greater than the voltage stress of the lightning arrester, and the sensitivity is greatly improved;

[0064] (3) The anti-circulating current circuit is composed of only one thyristor and a resistor in series, and a half-wave switching control method suitable for the anti-circulating current circuit is provided, so that the low circulating current switching between multiple variable ratios of the distribution transformer is realized, and one thyristor is saved for each phase compared with the traditional anti-parallel thyristor combination mode;

[0065] (4) For each phase winding, the first anti-circulating current circuit is connected between the No. 3 tap and the No. 4 tap of the winding, and the second anti-circulating current circuit is connected between the No. 5 tap and the No. 6 tap of the winding, and the low circulating current switching of multiple variable ratios of the distribution transformer is realized through control;

[0066] (5) For each phase winding, the starting switch is connected in parallel with the switching thyristor in the second anti-circulating current circuit, and the resistance of the anti-circulating current circuit is reused, and one starting resistance is saved for each phase compared with the traditional independent starting switch configuration mode. BRIEF DESCRIPTION OF DRAWINGS

[0067] Figure 1 It is a wiring schematic diagram of the primary winding of phase A;

[0068] Figure 2 It is a wiring schematic diagram of the primary winding of phase B;

[0069] Figure 3 It is a wiring schematic diagram of the primary winding of phase C;

[0070] Figure 4 It is a voltage-time curve of the total thyristors of phase A;

[0071] Figure 5 It is a voltage-time curve of the thyristor 1213, the thyristor 1223, the thyristor 1214, the thyristor 1224 and the switching thyristor 1234 when the output voltage of the transformer is increased from 1p.u. to 1.025p.u.;

[0072] Figure 6 It is a current-time curve of the thyristor 1213, the thyristor 1223, the thyristor 1214, the thyristor 1224 and the switching thyristor 1234 when the output voltage of the transformer is increased from 1p.u. to 1.025p.u.;

[0073] Figure 7 It is an output voltage;

[0074] Figure 8The graph shows the voltage changes of thyristors 1213, 1223, 1212, 1222, and 1232 over time when the transformer output voltage drops from 1 p.u. to 0.975 p.u.

[0075] Figure 9 The curves show the changes in current of thyristors 1213, 1223, 1212, 1222, and 1232 over time when the transformer output voltage drops from 1 p.u. to 0.975 p.u.

[0076] Figure 10 The output voltage is the voltage when the transformer output voltage drops from 1 p.u. to 0.975 p.u. Detailed Implementation

[0077] The technical solution of the present invention will now be further described in conjunction with the accompanying drawings and embodiments.

[0078] Now combined Figures 1 to 3 The illustrated topology further illustrates the circuit structure of the arc-free on-load tap changer device for 10kV distribution transformers proposed in this invention. It mainly includes an A-phase tap changer circuit 1000, a B-phase tap changer circuit 2000, and a C-phase tap changer circuit 3000, each connected to one of the three-phase windings on the primary side of the distribution transformer.

[0079] like Figure 1 As shown, the A-phase voltage regulating switch circuit 1000 includes: A-phase first device winding connector 1101, A-phase second device winding connector 1102, A-phase third device winding connector 1103, A-phase fourth device winding connector 1104, A-phase fifth device winding connector 1105, A-phase sixth device winding connector 1106, A-phase first anti-parallel thyristor group, A-phase second anti-parallel thyristor group, A-phase third anti-parallel thyristor group, A-phase fourth anti-parallel thyristor group, A-phase fifth anti-parallel thyristor group, A-phase first anti-circulating current circuit, A-phase second anti-circulating current circuit, A-phase start switch 1235, and A-phase surge arrester 1236.

[0080] The anti-parallel thyristor groups involved in the A-phase voltage regulating switch circuit 1000 are each composed of two anti-parallel thyristors, i.e., the A-phase first anti-parallel thyristor group is composed of the anti-parallel thyristors 1211 and 1221; the A-phase second anti-parallel thyristor group is composed of the anti-parallel thyristors 1212 and 1222; the A-phase third anti-parallel thyristor group is composed of the anti-parallel thyristors 1213 and 1223; the A-phase fourth anti-parallel thyristor group is composed of the anti-parallel thyristors 1214 and 1224; and the A-phase fifth anti-parallel thyristor group is composed of the anti-parallel thyristors 1215 and 1225. The five groups of anti-parallel thyristors are connected in series and connected with the step voltage tap, thereby forming a voltage regulating circuit, which can be suitable for the winding structure of a standard distribution transformer with a step voltage tap. The lightning arrester is connected in parallel at both ends of the anti-parallel thyristor string formed by the five groups of anti-parallel thyristors connected in series, which can clamp the voltage of the anti-parallel thyristor string and increase the voltage difference between the normal working condition and the abnormal working condition of the thyristor, especially the maximum voltage stress of the thyristor in the normal working condition is greater than the voltage stress of the lightning arrester, thereby greatly improving the sensitivity.

[0081] The anti-circulation circuits involved in the A-phase voltage regulating switch circuit 1000 are each composed of a buffer resistor and a switching thyristor connected in series, i.e., the A-phase first anti-circulation circuit is composed of the buffer resistor 1231 and the switching thyristor 1232 connected in series; and the A-phase second anti-circulation circuit is composed of the buffer resistor 1233 and the switching thyristor 1234 connected in series. The anti-circulation circuit of the present application is composed of only one thyristor and a resistor connected in series, and a half-wave switching control method suitable for the anti-circulation circuit is provided, thereby realizing low-circulation switching between multiple variable ratios of a distribution transformer, and one thyristor is saved for each phase compared with the traditional anti-parallel thyristor combination mode.

[0082] For the convenience of describing the topology structure, the two terminals of the buffer resistor 1231 are denoted as 12311 and 12312, the two terminals of the buffer resistor 1233 are denoted as 12331 and 12332, the two terminals of the A-phase starting switch 1235 are denoted as 12351 and 12352, and the two terminals of the A-phase lightning arrester 1236 are denoted as 12361 and 12362.

[0083] The A-phase first device winding connector 1101 is connected with the A-phase winding "No. 6" tap of the distribution transformer (a typical design of State Grid); the A-phase first device winding connector 1101 is connected with the negative electrode of the switching thyristor 1234, the other terminal 12352 of the A-phase starting switch 1235, the negative electrode of the thyristor 1214, the positive electrode of the thyristor 1224, the positive electrode of the thyristor 1215, and the negative electrode of the thyristor 1225.

[0084] The A-phase second device winding connector 1102 is connected with the No. 4 tap of the A-phase winding of the distribution transformer (national grid typical design). The A-phase second device winding connector 1102 is connected with the negative pole of the switching thyristor 1232, the negative pole of the thyristor 1212, the positive pole of the thyristor 1222, the positive pole of the thyristor 1213, and the negative pole of the thyristor 1223.

[0085] The A-phase third device winding connector 1103 is connected with the No. 2 tap of the A-phase winding of the distribution transformer (national grid typical design). The A-phase third device winding connector 1103 is connected with the positive pole of the thyristor 1211, the negative pole of the thyristor 1221, and one end 12361 of the A-phase arrester 1236.

[0086] The A-phase fourth device winding connector 1104 is connected with the No. 3 tap of the A-phase winding of the distribution transformer (national grid typical design). The A-phase fourth device winding connector 1104 is connected with one end 12311 of the buffer resistor 1231, the negative pole of the thyristor 1211, the positive pole of the thyristor 1221, the positive pole of the thyristor 1212, and the negative pole of the thyristor 1222.

[0087] The A-phase fifth device winding connector 1105 is connected with the No. 5 tap of the A-phase winding of the distribution transformer (national grid typical design). The A-phase fifth device winding connector 1105 is connected with one end 12331 of the buffer resistor 1233, the negative pole of the thyristor 1213, the positive pole of the thyristor 1223, the positive pole of the thyristor 1214, and the negative pole of the thyristor 1224.

[0088] The A-phase sixth device winding connector 1106 is connected with the No. 7 tap of the A-phase winding of the distribution transformer (national grid typical design). The A-phase sixth device winding connector 1106 is connected with the negative pole of the thyristor 1215, the positive pole of the thyristor 1225, and the other end 12362 of the A-phase arrester 1236.

[0089] The other end 12312 of the buffer resistor 1231 is connected with the positive pole of the switching thyristor 1232. The other end 12332 of the buffer resistor 1233 is connected with the positive pole of the switching thyristor 1234 and one end 12351 of the A-phase starting switch 1235.

[0090] The A-phase starting switch 1235 is connected in parallel with the switching thyristor 1234, and is used as the resistor of the A-phase second anti-circulation circuit. Compared with the traditional independent starting switch configuration mode, one starting resistor is reduced for each phase.

[0091] As Figure 2As shown, the topology of the B-phase voltage-regulating switch circuit 2000 is the same as that of the A-phase voltage-regulating switch circuit 1000, i.e., it mainly comprises: a B-phase first device winding connector 2101, a B-phase second device winding connector 2102, a B-phase third device winding connector 2103, a B-phase fourth device winding connector 2104, a B-phase fifth device winding connector 2105, a B-phase sixth device winding connector 2106, a B-phase first anti-parallel thyristor group, a B-phase second anti-parallel thyristor group, a B-phase third anti-parallel thyristor group, a B-phase fourth anti-parallel thyristor group, a B-phase fifth anti-parallel thyristor group, a B-phase first anti-circulation circuit, a B-phase second anti-circulation circuit, a B-phase starting switch 2235, and a B-phase lightning arrester 2236.

[0092] In the B-phase voltage-regulating switch circuit 2000, each anti-parallel thyristor group is composed of two anti-parallel thyristors. Specifically, the B-phase first anti-parallel thyristor group is composed of thyristor 2211 and thyristor 2221 in anti-parallel connection; the B-phase second anti-parallel thyristor group is composed of thyristor 2212 and thyristor 2222 in anti-parallel connection; the B-phase third anti-parallel thyristor group is composed of thyristor 2213 and thyristor 2223 in anti-parallel connection; the B-phase fourth anti-parallel thyristor group is composed of thyristor 2214 and thyristor 2224 in anti-parallel connection; and the B-phase fifth anti-parallel thyristor group is composed of thyristor 2215 and thyristor 2225 in anti-parallel connection. The five anti-parallel thyristor groups are connected in series and connected to the step voltage tap, thereby forming a voltage-regulating circuit, which is suitable for the winding structure of a standard distribution transformer with a step voltage tap. The lightning arrester connected in parallel to the two ends of the anti-parallel thyristor string formed by the five anti-parallel thyristor groups in series connection can sensitively clamp the voltage of the anti-parallel thyristor string, thereby increasing the voltage difference between the normal working condition and the abnormal working condition of the thyristors, and especially, the maximum voltage stress of the thyristors in the normal working condition is greater than the voltage stress of the lightning arrester, thereby greatly improving the sensitivity.

[0093] In the B-phase voltage-regulating switch circuit 2000, each anti-circulation circuit is composed of a buffer resistor and a switching thyristor in series connection. Specifically, the B-phase first anti-circulation circuit is composed of buffer resistor 2231 and switching thyristor 2232 in series connection; and the B-phase second anti-circulation circuit is composed of buffer resistor 2233 and switching thyristor 2234 in series connection. The anti-circulation circuit of the present application is composed of only one thyristor and a resistor in series connection, and a half-wave switching control method suitable for the anti-circulation circuit is provided, thereby realizing low-circulation switching between multiple variable ratios of a distribution transformer, and saving one thyristor per phase compared with the traditional anti-parallel thyristor combination mode.

[0094] For the convenience of explaining the topology, the two terminals of the buffer resistor 2231 are denoted as 22311 and 22312, the two terminals of the buffer resistor 2233 are denoted as 22331 and 22332, the two terminals of the B-phase starting switch 2235 are denoted as 22351 and 22352, and the two terminals of the B-phase arrester 2236 are denoted as 22361 and 22362.

[0095] The B-phase first device winding connector 2101 is connected with the B-phase winding "No. 6" tap of the distribution transformer (national grid typical design); the B-phase first device winding connector 2101 is connected with the negative electrode of the switching thyristor 2234, the other end 22352 of the B-phase starting switch 2235, the negative electrode of the thyristor 2214, the positive electrode of the thyristor 2224, the positive electrode of the thyristor 2215, the negative electrode of the thyristor 2225; the other end 22312 of the buffer resistor 2231 is connected with the positive electrode of the thyristor 2232; the other end 22332 of the buffer resistor 2233 is connected with the positive electrode of the thyristor 2234 and the one end 22351 of the B-phase starting switch 2235.

[0096] The B-phase second device winding connector 2102 is connected with the B-phase winding "No. 4" tap of the distribution transformer (national grid typical design); the B-phase second device winding connector 2102 is connected with the negative electrode of the switching thyristor 2232, the negative electrode of the thyristor 2212, the positive electrode of the thyristor 2222, the positive electrode of the thyristor 2213, and the negative electrode of the thyristor 2223.

[0097] The B-phase third device winding connector 2103 is connected with the B-phase winding "No. 2" tap of the distribution transformer (national grid typical design); the B-phase third device winding connector 2103 is connected with the positive electrode of the thyristor 2211, the negative electrode of the thyristor 2221, and the one end 22361 of the B-phase arrester.

[0098] The B-phase fourth device winding connector 2104 is connected with the B-phase winding "No. 3" tap of the distribution transformer (national grid typical design); the B-phase fourth device winding connector 2104 is connected with the one end 22311 of the buffer resistor 2231, the negative electrode of the thyristor 2211, the positive electrode of the thyristor 2221, the positive electrode of the thyristor 2212, and the negative electrode of the thyristor 2222.

[0099] The B-phase fifth device winding connector 2105 is connected with the B-phase winding "No. 5" tap of the distribution transformer (national grid typical design); the B-phase fifth device winding connector 2105 is connected with the one end 22331 of the buffer resistor 2233, the negative electrode of the thyristor 2213, the positive electrode of the thyristor 2223, the positive electrode of the thyristor 2214, and the negative electrode of the thyristor 2224.

[0100] The B-phase sixth device winding connector 2106 is connected to the "7" tap of the B-phase winding of the distribution transformer (typical design of State Grid). The B-phase sixth device winding connector 2106 is connected to the negative terminal of thyristor 2215, the positive terminal of thyristor 2225, and the other end 22362 of B-phase surge arrester 2236.

[0101] The B-phase start switch 2235 is connected in parallel with the switching thyristor 2234, and the resistor of the B-phase second anti-circulating current circuit is reused. Compared with the traditional independent start switch configuration, one start resistor is reduced per phase.

[0102] like Figure 3 As shown, the topology of the C-phase voltage regulating switch circuit 3000 is the same as that of the A-phase voltage regulating switch circuit 1000, which mainly includes: C-phase first device winding connector 3101, C-phase second device winding connector 3102, C-phase third device winding connector 3103, C-phase fourth device winding connector 3104, C-phase fifth device winding connector 3105, C-phase sixth device winding connector 3106, C-phase first anti-parallel thyristor group, C-phase second anti-parallel thyristor group, C-phase third anti-parallel thyristor group, C-phase fourth anti-parallel thyristor group, C-phase fifth anti-parallel thyristor group, C-phase first anti-circulating current circuit, C-phase second anti-circulating current circuit, C-phase start switch 3235 and C-phase surge arrester 3236.

[0103] In the C-phase voltage regulating switch circuit 3000, each anti-parallel thyristor group consists of two thyristors connected in anti-parallel. Specifically: the first anti-parallel thyristor group in C-phase consists of thyristors 3211 and 3221 connected in anti-parallel; the second anti-parallel thyristor group in C-phase consists of thyristors 3212 and 3222 connected in anti-parallel; the third anti-parallel thyristor group in C-phase consists of thyristors 3213 and 3223 connected in anti-parallel; the fourth anti-parallel thyristor group in C-phase consists of thyristors 3214 and 3224 connected in anti-parallel; and the fifth anti-parallel thyristor group in C-phase consists of thyristors 3215 and 3225 connected in anti-parallel. These five anti-parallel thyristors are connected in series and then connected to the step voltage tap to form a voltage regulating circuit, which is suitable for the winding structure of standard distribution transformers with step voltage taps. A surge arrester is connected in parallel at both ends of an anti-parallel thyristor string formed by five anti-parallel thyristors connected in series. This sensitively clamps the voltage of the anti-parallel thyristor string, protecting all thyristors while increasing the voltage difference between normal and abnormal operating conditions. In particular, the maximum voltage stress of the thyristor under normal operating conditions is greater than the voltage stress of the surge arrester, thus significantly improving sensitivity.

[0104] The anti-circulation circuit involved in the C-phase voltage regulating switch circuit 3000 is composed of a buffer resistor and a switching thyristor in series, that is, the C-phase first anti-circulation circuit is composed of the buffer resistor 3231 and the switching thyristor 3232 in series, and the C-phase second anti-circulation circuit is composed of the buffer resistor 3233 and the switching thyristor 3234 in series. The anti-circulation circuit is composed of only one thyristor and a resistor in series, and a half-wave switching control method suitable for the anti-circulation circuit is provided, so that low-circulation switching between multiple variable ratios of the distribution transformer is realized, and one thyristor is saved for each phase compared with the traditional anti-parallel thyristor combination mode.

[0105] For the convenience of explaining the topology structure, the two terminals of the buffer resistor 3231 are denoted as 32311 and 32312 respectively, the two terminals of the buffer resistor 3233 are denoted as 32331 and 32332 respectively, the two terminals of the C-phase starting switch 3235 are denoted as 32351 and 32352 respectively, and the two terminals of the C-phase lightning arrester 3236 are denoted as 32361 and 32362 respectively.

[0106] The C-phase first device winding connector 3101 is connected with the C-phase winding "No. 6" tap (national grid typical design) of the distribution transformer; the C-phase first device winding connector 3101 is connected with the negative electrode of the switching thyristor 3234, the other end 32352 of the C-phase starting switch 3235, the negative electrode of the thyristor 3214, the positive electrode of the thyristor 3224, the positive electrode of the thyristor 3215, the negative electrode of the thyristor 3225; the other end 32312 of the buffer resistor 3231 is connected with the positive electrode of the thyristor 3232; the other end 32332 of the buffer resistor 3233 is connected with the positive electrode of the thyristor 3234 and one end 32351 of the C-phase starting switch 3235.

[0107] The C-phase second device winding connector 3102 is connected with the C-phase winding "No. 4" tap (national grid typical design) of the distribution transformer; the C-phase second device winding connector 3102 is connected with the negative electrode of the switching thyristor 3232, the negative electrode of the thyristor 3212, the positive electrode of the thyristor 3222, the positive electrode of the thyristor 3213, and the negative electrode of the thyristor 3223.

[0108] The C-phase third device winding connector 3103 is connected with the C-phase winding "No. 2" tap (national grid typical design) of the distribution transformer; the C-phase third device winding connector 3103 is connected with the positive electrode of the thyristor 3211, the negative electrode of the thyristor 3221, and one end 32361 of the C-phase lightning arrester.

[0109] C phase fourth device winding connector 3104 is connected with distribution transformer C phase winding "3" tap (state grid typical design); C phase fourth device winding connector 3104 is connected with one end 32311 of buffer resistance 3231, negative pole of thyristor 3211, positive pole of thyristor 3221, positive pole of thyristor 3212, negative pole of thyristor 3222.

[0110] C phase fifth device winding connector 3105 is connected with distribution transformer C phase winding "5" tap (state grid typical design); C phase fifth device winding connector 3105 is connected with one end 32331 of buffer resistance 3233, negative pole of thyristor 3213, positive pole of thyristor 3223, positive pole of thyristor 3214, negative pole of thyristor 3224.

[0111] C phase sixth device winding connector 3106 is connected with distribution transformer C phase winding "7" tap (state grid typical design). C phase sixth device winding connector 3106 is connected with negative pole of thyristor 3215, positive pole of thyristor 3225 and the other end 32362 of C phase arrester 3236.

[0112] C phase starting switch 3235 is connected in parallel with switching thyristor 3234, and the resistance of C phase second anti-circulation circuit is reused. Compared with the traditional independent starting switch configuration mode, one starting resistance is reduced for each phase.

[0113] The withstand voltage limit of the thyristor between the winding taps is much lower than 10kV. If there is no reliable connection between the two sections of the winding in the starting moment, the voltage loaded on the two ends of the thyristor is 10kV, which exceeds the withstand voltage limit, causing the thyristor to be sequentially broken down. Therefore, a starting switch needs to be added to ensure reliable connection of the two sections of the winding before the transformer is connected to the grid.

[0114] The control method of each phase is consistent, and the A phase voltage regulating switch circuit is taken as an example for detailed description.

[0115] In the step-up voltage regulation control of the distribution transformer, the primary side winding needs to be short-circuited with more turns to reduce the turns ratio between the primary and secondary windings, thereby increasing the secondary side voltage under the condition that the primary side voltage remains unchanged.

[0116] I. When the secondary side voltage is increased from 0.95p.u. to 0.975p.u., the primary side winding is short-circuited between "3" and "4" taps instead of "2" and "3" taps.

[0117] a) When the current of thyristor 1221 is 0, turn off thyristor 1221;

[0118] b) When the switching thyristor 1232 is forward biased, turn on the switching thyristor 1232, divert the current to the anti-circulating circuit, and reduce the circulating current by using the buffer resistor 1231;

[0119] c) When the current of the thyristor 1211 is reduced to 0, turn off the thyristor 1211, and turn on the thyristor 1222;

[0120] d) When the current of the thyristor 1232 is reduced to 0, turn off 1232 and turn on the thyristor 1212, complete the voltage boosting process.

[0121] II. When the secondary side voltage per unit value is raised from 0.975 p.u. to 1 p.u., the primary side winding is changed from short-circuiting between "No. 3" and "No. 4" taps to short-circuiting between "No. 4" and "No. 5" taps.

[0122] a) When the current of the thyristor 1212 is 0, turn off 1212;

[0123] b) When the switching thyristor 1232 is forward biased, turn on the switching thyristor 1232, divert the current to the anti-circulating circuit, and reduce the circulating current by using the buffer resistor 1231;

[0124] c) When the current of the thyristor 1222 is reduced to 0, turn off 1222, and turn on 1213;

[0125] d) When 1232 is reduced to 0, turn off 1232 and turn on 1223, complete the voltage boosting process.

[0126] III. When the secondary side voltage per unit value is raised from 1 p.u. to 1.025 p.u., the primary side winding is changed from short-circuiting between "No. 4" and "No. 5" taps to short-circuiting between "No. 5" and "No. 6" taps.

[0127] a) When the current of 1223 is 0, turn off 1223;

[0128] b) When 1234 is forward biased, turn on 1234, divert the current to the anti-circulating circuit, and reduce the circulating current by using the buffer resistor 1233;

[0129] c) When the current of 1213 is reduced to 0, turn off 1213, and turn on 1224;

[0130] d) When the current of 1234 is reduced to 0, turn off 1234, and turn on 1214, complete the voltage boosting process.

[0131] IV. When the secondary side voltage per unit value is raised from 1.025 p.u. to 1.05 p.u., the primary side winding is changed from short-circuiting between "No. 5" and "No. 6" taps to short-circuiting between "No. 6" and "No. 7" taps.

[0132] a) When 1214 current is 0, turn off 1214;

[0133] b) When 1234 is forward voltage, turn on 1234, divert current to anti-circulation circuit, reduce circulation current by using buffer resistance 1233;

[0134] c) When 1224 current is reduced to 0, turn off 1224, and turn on 1215;

[0135] d) When 1234 is reduced to 0, turn off 1234, and turn on 1225, complete the boost process.

[0136] In the step-down voltage control of distribution transformer, the primary winding needs to be short-circuited with fewer turns to increase the turns ratio between the primary and secondary windings, thereby reducing the secondary voltage under the condition that the primary voltage remains unchanged.

[0137] I. When the secondary voltage per unit value is reduced from 1.05 p.u. to 1.025 p.u., the primary winding is short-circuited between the "5th" and "6th" taps instead of the "6th" and "7th" taps.

[0138] a) When 1225 current is 0, turn off 1225;

[0139] b) When 1234 is forward voltage, turn on 1234, divert current to anti-circulation circuit, reduce circulation current by using buffer resistance 1233;

[0140] c) When 1215 current is reduced to 0, turn off 1215, and turn on 1224;

[0141] d) When 1234 current is reduced to 0, turn off 1234, and turn on 1214, complete the step-down process.

[0142] II. When the secondary voltage per unit value is reduced from 1.025 p.u. to 1 p.u., the primary winding is short-circuited between the "4th" and "5th" taps instead of the "5th" and "6th" taps.

[0143] a) When 1214 current is 0, turn off 1214;

[0144] b) When 1234 is forward voltage, turn on 1234, divert current to anti-circulation circuit, reduce circulation current by using buffer resistance 1233;

[0145] c) When 1224 current is reduced to 0, turn off 1224, and turn on 1213;

[0146] d) When 1234 is reduced to 0, turn off 1234 and turn on 1223, complete the step-down process.

[0147] III. When the secondary voltage per unit is reduced from 1 p.u. to 0.975 p.u., the primary winding is short-circuited between the "3rd" and "4th" taps instead of the "4th" and "5th" taps.

[0148] a) When the current of 1223 is 0, 1223 is turned off;

[0149] b) When 1232 is positively subjected to voltage, 1232 is turned on, the current is transferred to the anti-circulating circuit, and the circulating current is reduced by using the buffer resistance 1231;

[0150] c) When the current of 1213 is reduced to 0, 1213 is turned off, and 1222 is turned on;

[0151] d) When the current of 1232 is reduced to 0, 1232 is turned off and 1212 is turned on, and the voltage reduction process is completed.

[0152] IV. When the secondary voltage per unit is reduced from 0.975 p.u. to 0.95 p.u., the primary winding is short-circuited between the "2nd" and "3rd" taps instead of the "3rd" and "4th" taps.

[0153] a) When the current of 1212 is 0, 1212 is turned off;

[0154] b) When 1232 is positively subjected to voltage, 1232 is turned on, the current is transferred to the anti-circulating circuit, and the circulating current is reduced by using the buffer resistance 1231;

[0155] c) When the current of 1222 is reduced to 0, 1222 is turned off, and 1211 is turned on;

[0156] d) When 1232 is reduced to 0, 1232 is turned off and 1221 is turned on, and the voltage reduction process is completed.

[0157] Embodiment:

[0158] Take a 400 kVA distribution transformer as an example, thyristor 1211, thyristor 1221, thyristor 1212, thyristor 1222, thyristor 1213, thyristor 1223, thyristor 1214, thyristor 1224, thyristor 1215, thyristor 1225, thyristor 1232, switching thyristor 1234, thyristor 2211, thyristor 2221, thyristor 2212, thyristor 2222, thyristor 2213, thyristor 2223, thyristor 2214, thyristor 2224, thyristor 2215, thyristor 2225, thyristor 2232, thyristor 2234, thyristor 3211, thyristor 3221, thyristor 3212, thyristor 3222, thyristor 3213, thyristor 3223, thyristor 3214, thyristor 3224, thyristor 3215, thyristor 3225, thyristor 3232 and thyristor 3234 are all selected as TOP packaged thyristors with a withstand voltage parameter of 1600V and a rated current parameter of 40A. The buffer resistors 1231, 1233, 2231, 2233, 3231 and 3233 are selected as power resistors with a resistance of 15 ohms and a power of 300 watts. The A-phase starting switch 1235, the B-phase starting switch 2235 and the C-phase starting switch 3235 are selected as three-phase AC air switches with a rated voltage of 380V and a rated current of 40A.

[0159] The grid side is a stable 10kV stable voltage source, the primary side is connected to the grid at 0.005 seconds, and the starting switch is turned off at 0.02 seconds. The voltage of all A-phase thyristors during the starting process is as shown in Figure 4 From Figure 4 it can be seen that the voltage stress of the thyristors does not exceed the withstand voltage parameter.

[0160] When the transformer output voltage is raised from 1p.u. to 1.025p.u., the voltage and current of the thyristors 1213, 1223, 1214, 1224 and 1234 are as shown in Figure 5 and Figure 6 , and the output voltage is as shown in Figure 7 .

[0161] When the transformer output voltage is reduced from 1p.u. to 0.975p.u., the voltage and current of the thyristors 1213, 1223, 1212, 1222 and 1232 are as shown in Figure 8 and Figure 9 , and the output voltage is as shown in Figure 10 .

Claims

1. A thyristor-based arc-free on-load tap changer for distribution transformers, characterized in that: This includes the A-phase voltage regulating switch circuit, the B-phase voltage regulating switch circuit, and the C-phase voltage regulating switch circuit, which are respectively connected to the three-phase windings on the primary side of the distribution transformer. The A-phase voltage regulating switch circuit, B-phase voltage regulating switch circuit, and C-phase voltage regulating switch circuit have the same topology, each including: a first anti-parallel thyristor group, a second anti-parallel thyristor group, a third anti-parallel thyristor group, a fourth anti-parallel thyristor group, a fifth anti-parallel thyristor group, a first anti-circulating current circuit, a second anti-circulating current circuit, a start switch, and a surge arrester. The first, second, third, fourth, and fifth anti-parallel thyristor groups are connected in series to form an anti-parallel thyristor string with six connection points. Taps 7, 5, 3, 2, 4, and 6 are sequentially arranged from top to bottom on the A / B / C phase windings of the distribution transformer. These six connection points are connected sequentially to the A / B / C phase windings of the distribution transformer. The phase winding is connected via taps 2, 3, 4, 5, 6, and 7. Each anti-parallel thyristor group consists of a first thyristor and a second thyristor connected in reverse parallel. The conduction direction of the first thyristor is defined as positive, and the direction of current flow from the smaller tap to the larger tap is defined as positive. The conduction direction of the second thyristor is negative. For the first anti-parallel thyristor group, the direction of current flow from tap 2 to tap 3 is positive. The first and second anti-circuit current circuits each include a buffer resistor and a switching thyristor; one end of the buffer resistor is connected to the positive terminal of the switching thyristor; the first anti-circuit current circuit is connected in parallel with the second anti-parallel thyristor group, and the conduction direction of the switching thyristor in the first anti-circuit current circuit is the same as the conduction direction of the first thyristor in the second anti-parallel thyristor group; the second anti-circuit current circuit is connected in parallel with the fourth anti-parallel thyristor group, and the conduction direction of the switching thyristor in the second anti-circuit current circuit is the same as the conduction direction of the first thyristor in the fourth anti-parallel thyristor group. The start switch is connected in parallel with the switching thyristor in the second anti-circulation circuit; The surge arrester is connected in series and parallel with the anti-parallel thyristor; A half-wave switching control method is used to control the first and second anti-circulating current circuits to achieve low circulating current switching between multiple turns ratios of the distribution transformer. The half-wave switching control method involves changing the short-circuit of the taps of the corresponding A / B / C phase windings of the distribution transformer by opening / closing the first and second thyristors in the corresponding anti-parallel thyristor group and opening / closing the switching thyristor in the corresponding anti-circuit current circuit.

2. The thyristor-based arc-free on-load tap changer for distribution transformers according to claim 1, characterized in that: When the per-unit value of the secondary voltage needs to be increased from 0.95 pu to 0.975 pu, the half-wave switching control method includes the following steps: The second connection point is kept connected to tap 3 of the A / B / C phase winding of the distribution transformer, the first connection point is disconnected from tap 2 of the A / B / C phase winding of the distribution transformer, and the third connection point is connected to tap 4 of the A / B / C phase winding of the distribution transformer. When the current of the second thyristor in the first anti-parallel thyristor group is 0, the second thyristor is turned off. When the switching thyristor in the first anti-circulating current circuit is subjected to a positive voltage, the switching thyristor is turned on, and the current is transferred to the first anti-circulating current circuit. When the current of the first thyristor in the first anti-parallel thyristor group drops to 0, the first thyristor in the first anti-parallel thyristor group is turned off, and the second thyristor in the second anti-parallel thyristor group is turned on at the same time. When the current of the switching thyristor in the first anti-circulating current circuit drops to 0, the switching thyristor in the first anti-circulating current circuit is turned off, and the first thyristor of the second anti-parallel thyristor group is turned on; the No. 3 tap of the A / B / C phase winding of the distribution transformer is short-circuited with the No. 4 tap of the A / B / C phase winding of the distribution transformer, thus completing the voltage boosting process.

3. A thyristor-based arc-free on-load tap changer for distribution transformers according to claim 1, characterized in that: When the per-unit value of the secondary voltage needs to be increased from 0.975 pu to 1 p.u., the half-wave switching control method includes the following steps: The third connection point is kept connected to tap 4 of the A / B / C phase winding of the distribution transformer, the second connection point is disconnected from tap 3 of the A / B / C phase winding of the distribution transformer, and the fourth connection point is connected to tap 5 of the A / B / C phase winding of the distribution transformer. When the current of the first thyristor in the second anti-parallel thyristor group is 0, the first thyristor is turned off. When the switching thyristor in the first anti-circulating current circuit is subjected to a positive voltage, the switching thyristor is turned on. When the current of the second thyristor in the second anti-parallel thyristor group drops to 0, the second thyristor in the second anti-parallel thyristor group is turned off, and at the same time the first thyristor in the third anti-parallel thyristor group is turned on. When the current of the switching thyristor in the first anti-circulating current circuit is 0, the switching thyristor in the first anti-circulating current circuit is turned off, and the second thyristor of the third anti-parallel thyristor group is turned on; the No. 4 tap of the A / B / C phase winding of the distribution transformer is short-circuited with the No. 5 tap of the A / B / C phase winding of the distribution transformer, thus completing the voltage boosting process.

4. A thyristor-based arc-free on-load tap changer for distribution transformers according to claim 1, characterized in that: When the per-unit value of the secondary voltage needs to be increased from 1 p.u. to 1.025 p.u., the half-wave switching control method includes the following steps: The fourth connection point remains connected to tap 5 of the A / B / C phase winding of the distribution transformer; the third connection point is disconnected from tap 4 of the A / B / C phase winding of the distribution transformer; and the fifth connection point is connected to tap 6 of the A / B / C phase winding of the distribution transformer. When the current of the second thyristor in the third anti-parallel thyristor group is 0, the second thyristor is turned off. When the switching thyristor in the second anti-circulation circuit is subjected to a positive voltage, the switching thyristor in the second anti-circulation circuit is turned on, and the current is transferred to the second anti-circulation circuit. When the current of the first thyristor in the third anti-parallel thyristor group drops to 0, the first thyristor in the third anti-parallel thyristor group is turned off, and the second thyristor in the fourth anti-parallel thyristor group is turned on at the same time. When the current of the switching thyristor in the second anti-circulating current circuit drops to 0, the switching thyristor in the second anti-circulating current circuit is turned off, and at the same time the first thyristor of the fourth anti-parallel thyristor group is turned on; the No. 5 tap of the A / B / C phase winding of the distribution transformer and the No. 6 tap of the A / B / C phase winding of the distribution transformer are short-circuited to complete the voltage boosting process.

5. A thyristor-based arc-free on-load tap changer for distribution transformers according to claim 1, characterized in that: When the per-unit value of the secondary voltage needs to be increased from 1.025 pu to 1.05 pu, the half-wave switching control method includes the following steps: The fifth connection point remains connected to tap 6 of the A / B / C phase winding of the distribution transformer; the fourth connection point is disconnected from tap 5 of the A / B / C phase winding of the distribution transformer; and the sixth connection point is connected to tap 7 of the A / B / C phase winding of the distribution transformer. When the current of the first thyristor in the fourth anti-parallel thyristor group is 0, the first thyristor in the fourth anti-parallel thyristor group is turned off. When the switching thyristor in the second anti-circulation circuit is subjected to a positive voltage, the switching thyristor in the second anti-circulation circuit is turned on, and the current is transferred to the second anti-circulation circuit. When the current of the second thyristor in the fourth anti-parallel thyristor group is 0, the second thyristor in the fourth anti-parallel thyristor group is turned off; at the same time, the first thyristor in the fifth anti-parallel thyristor group is turned on. When the current of the switching thyristor in the second anti-circulating current circuit is 0, the switching thyristor in the second anti-circulating current circuit is turned off, and at the same time the second thyristor of the fifth anti-parallel thyristor group is turned on; so that the No. 6 tap of the A / B / C phase winding of the distribution transformer is short-circuited with the No. 7 tap of the A / B / C phase winding of the distribution transformer, and the voltage boosting process is completed.

6. A thyristor-based arc-free on-load tap changer for distribution transformers according to claim 1, characterized in that: When the per-unit value of the secondary voltage needs to be reduced from 1.05 pu to 1.025 pu, the half-wave switching control method includes the following steps: The fifth connection point remains connected to tap 6 of the A / B / C phase winding of the distribution transformer; the sixth connection point is disconnected from tap 7 of the A / B / C phase winding of the distribution transformer, and the fourth connection point is connected to tap 5 of the A / B / C phase winding of the distribution transformer. When the current of the second thyristor in the fifth anti-parallel thyristor group is 0, the second thyristor in the fifth anti-parallel thyristor group is turned off. When the switching thyristor in the second anti-circulation circuit is subjected to a positive voltage, the switching thyristor in the second anti-circulation circuit is turned on, and the current is transferred to the second anti-circulation circuit. When the current of the first thyristor in the fifth anti-parallel thyristor group is 0, the first thyristor in the fifth anti-parallel thyristor group is turned off, and the second thyristor in the fourth anti-parallel thyristor group is turned on at the same time. When the current of the switching thyristor in the second anti-circuit current circuit is 0, the switching thyristor in the second anti-circuit current circuit is turned off, and at the same time the first thyristor of the fourth anti-parallel thyristor group is turned on; the short circuit between tap 6 of the A / B / C phase winding and tap 7 of the A / B / C phase winding of the distribution transformer is changed to short circuit between tap 5 of the A / B / C phase winding and tap 6 of the A / B / C phase winding of the distribution transformer, thus completing the voltage reduction.

7. A thyristor-based arc-free on-load tap changer for distribution transformers according to claim 1, characterized in that: When the per-unit value of the secondary voltage needs to be reduced from 1.025 pu to 1 p.u., the half-wave switching control method includes the following steps: The fourth connection point remains connected to tap 5 of the A / B / C phase winding of the distribution transformer; the fifth connection point is disconnected from tap 6 of the A / B / C phase winding of the distribution transformer; and the third connection point is connected to tap 4 of the A / B / C phase winding of the distribution transformer. When the current of the first thyristor in the fourth anti-parallel thyristor group is 0, the first thyristor in the fourth anti-parallel thyristor group is turned off. When the switching thyristor in the second anti-circulating current circuit is subjected to a positive voltage, the switching thyristor in the second anti-circulating current circuit is turned on. When the current of the second thyristor of the fourth anti-parallel thyristor group is 0, the second thyristor of the fourth anti-parallel thyristor group is turned off, and the first thyristor of the third anti-parallel thyristor group is turned on at the same time. When the current of the switching thyristor in the second anti-circulating current circuit is 0, the switching thyristor in the second anti-circulating current circuit is turned off, and the second thyristor of the third anti-parallel thyristor group is turned on.

8. A thyristor-based arc-free on-load tap changer for distribution transformers according to claim 1, characterized in that: When the per-unit value of the secondary voltage needs to be reduced from 1 p.u. to 0.975 p.u., the half-wave switching control method includes the following steps: The third connection point remains connected to tap "4" of the A / B / C phase winding of the distribution transformer; the fourth connection point is disconnected from tap "5" of the A / B / C phase winding of the distribution transformer; and the second connection point is connected to tap "3" of the A / B / C phase winding of the distribution transformer. When the current of the second thyristor in the third anti-parallel thyristor group is 0, the second thyristor in the third anti-parallel thyristor group is turned off. When the switching thyristor in the first anti-circulating current circuit is subjected to a positive voltage, the switching thyristor in the first anti-circulating current circuit is turned on. When the current of the first thyristor in the third anti-parallel thyristor group is 0, the first thyristor in the third anti-parallel thyristor group is turned off, and the second thyristor in the second anti-parallel thyristor group is turned on at the same time. When the current of the switching thyristor in the first anti-circulating current circuit is 0, the switching thyristor in the first anti-circulating current circuit is turned off, and the first thyristor of the second anti-parallel thyristor group is turned on, thus completing the voltage reduction process.

9. A thyristor-based arc-free on-load tap changer for distribution transformers according to claim 1, characterized in that: When the per-unit value of the secondary voltage needs to be reduced from 0.975 pu to 0.95 pu, the half-wave switching control method includes the following steps: The second connection point remains connected to tap 3 of the A / B / C phase winding of the distribution transformer, the third connection point is disconnected from tap 4 of the A / B / C phase winding of the distribution transformer, and the first connection point is connected to tap 2 of the A / B / C phase winding of the distribution transformer. When the current of the first thyristor in the second anti-parallel thyristor group is 0, the first thyristor in the second anti-parallel thyristor group is turned off. When the switching thyristor in the first anti-circulating current circuit is subjected to a positive voltage, the switching thyristor in the first anti-circulating current circuit is turned on. When the current of the second thyristor in the second anti-parallel thyristor group is 0, the second thyristor in the second anti-parallel thyristor group is turned off, and the first thyristor in the first anti-parallel thyristor group is turned on. When the current of the switching thyristor in the first anti-circulating current circuit is 0, the switching thyristor in the first anti-circulating current circuit is turned off, and the second thyristor of the first anti-parallel thyristor group is turned on.

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