A transformer neutral point DC isolation device and control method

Abstract

The present invention provides a transformer neutral point DC isolation device and a control method, wherein the transformer neutral point DC isolation device comprises an upper capacitor branch, an upper resistor branch, a lower resistor branch, a lower capacitor branch, a first anti-parallel thyristor, a second anti-parallel thyristor, a first DC current transformer, a second DC current transformer, a first voltage transformer, a second voltage transformer, an overvoltage protector, a first isolating switch, a second isolating switch, and a fully controlled power switch. For situations where the DC grounding electrode distribution is complex and the DC current amplitude varies greatly, the transformer neutral point DC isolation device provided by the present invention can be flexibly adjusted to cope with; any fully controlled power switch in the transformer neutral point DC isolation device provided by the present invention is mis-connected, and there is a resistor current limiting, which will not cause impact and vibration to the capacitor, and the reliability is high; the capacitor in the transformer neutral point DC isolation device provided by the present invention does not need to be provided with a separate discharge circuit.

Description

Technical Field

[0001] The invention belongs to the technical field of power transmission and transformation, and in particular relates to a transformer neutral point DC isolation device and a control method. Background Art

[0002] When a DC converter station is in single-pole operation due to a fault or maintenance, its grounding pole will inject DC current into the earth; UHV converter stations, which use DC-driven rail transit, will also inject DC current into the earth if the rail insulation is reduced. These DC currents will flow back to the grid through the neutral point of the transformer within tens of kilometers nearby. The DC current will cause transformer core saturation and excitation current distortion, leading to transformer core vibration, fixture heating, insulation aging and other problems, threatening transformer safety. Usually, the DC current allowed to pass through a power transformer is no more than 4 A.

[0003] At present, the DC bias suppression devices of transformers are mainly divided into capacitor DC isolation type and resistor current limiting type. For example, Chinese invention patent CN104810793A discloses a capacitor DC isolation device, and CN209046250U discloses a resistor current limiting type suppression device. The transformer neutral point voltage offset is low when using a resistor current limiting device, but the DC suppression effect is poor, and it is usually used in situations where the DC bias level is low; the capacitor DC isolation device has a good DC suppression effect, but the accumulation of DC charge will produce a DC voltage offset at both ends of the capacitor. If the voltage generated by the unbalanced current of the transformer is superimposed, the capacitor will withstand a higher voltage. Therefore, it is usually used in situations where the DC bias level is high, and capacitors with large capacitance values ​​are used, but it also brings greater safety hazards.

[0004] With the rapid development of the power grid and rail transit industry, there are more and more occasions where the DC current in the earth fluctuates within a certain range, and the DC current is sometimes large and sometimes small. The use of fixed-parameter capacitive or resistive suppression devices can no longer meet the suppression needs. Summary of the invention

[0005] The first object of the present invention is to provide a transformer neutral point DC isolation device in view of the deficiencies in the prior art.

[0006] To this end, the above-mentioned purpose of the present invention is achieved through the following technical solutions:

[0007] A transformer neutral point DC isolation device, characterized in that: the transformer neutral point DC isolation device comprises an upper capacitor branch, an upper resistor branch, a lower resistor branch, a lower capacitor branch, a first anti-parallel thyristor, a second anti-parallel thyristor, a first DC current transformer, a second DC current transformer, a first voltage transformer, a second voltage transformer, an overvoltage protector, a first isolating switch, a second isolating switch and a fully controlled power switch;

[0008] The upper capacitor branch and the lower capacitor branch are respectively formed by connecting a capacitor and a fully controlled power switch in series;

[0009] The upper resistance branch and the lower resistance branch are respectively formed by connecting a resistor and a full-controlled power switch in series.

[0010] The lower end of the upper capacitor branch is connected to the upper end of the lower resistor branch, and the lower end of the upper resistor branch is connected to the upper end of the lower capacitor branch; the lower end of the lower resistor branch is connected to the lower end of the lower capacitor branch, and the connection point is connected to the ground grid through the first DC current transformer; the upper end of the lower resistor branch is connected to the upper end of the lower capacitor branch through the first anti-parallel thyristor; the upper end of the upper capacitor branch is connected to the upper end of the upper resistor branch, and the connection point is connected to the neutral point of the transformer through the first isolation switch and the second DC current transformer; the upper end of the lower capacitor branch is connected to the upper end of the upper resistor branch through the second anti-parallel thyristor;

[0011] The upper end of the overvoltage protector is connected to the upper end of the upper capacitor branch; the neutral point of the transformer is connected to the grounding grid through the second isolating switch after being led out;

[0012] A voltage transformer is connected in parallel at both ends of the capacitors of the upper capacitor branch and the lower capacitor branch respectively.

[0013] While adopting the above technical solutions, the present invention may also adopt or combine the following technical solutions:

[0014] As the preferred technical solution of the present invention:

[0015] The power frequency impedance X of the capacitor of the lower capacitor branch C2 =0.3~1.0 Ω; resistance of lower resistance branch R2=0.5πX C2 ;

[0016] The power frequency impedance X of the upper capacitor branch capacitor C1 = k X C2 ; Upper resistance branch resistance R1 = 0.5πX C1 ;

[0017] in: k The value range is 3~10.

[0018] As a preferred technical solution of the present invention: the transformer neutral point DC isolation device has two current control limits I F1 and I F2 and 2 voltage control limits V F1 and V F2 ;

[0019] When the transformer is running, the first isolating switch is opened, the second isolating switch is closed, and all anti-parallel thyristors and all fully controlled power devices are in the disconnected state;

[0020] When the second DC current transformer detects that the DC current exceeds I F1 , but less than I F2 When the first isolating switch is closed, the second isolating switch is opened, and the fully controlled power switch, the first anti-parallel thyristor, and the second anti-parallel thyristor of the lower resistance branch are turned on;

[0021] When the second DC current transformer detects that the DC current exceeds I F2 When the first isolating switch is closed, the second isolating switch is opened, and the fully controlled power switch and the second anti-parallel thyristor of the lower capacitor branch are turned on; then a voltage will appear across the capacitor C2 in the lower capacitor branch. When the voltage detected by the first voltage transformer connected in parallel with the capacitor C2 exceeds V F1 When the second anti-parallel thyristor is turned off, the first anti-parallel thyristor and the fully controlled power switch of the upper capacitor branch are turned on; then a voltage will appear across the capacitor C1 in the upper capacitor branch. When the second voltage transformer connected in parallel with the capacitor C1 detects a voltage exceeding V F2 When , the fully controlled power switch of the upper resistance branch is turned on.

[0022] As a preferred technical solution of the present invention: in the operating state where the first isolating switch is turned on and the second isolating switch is turned off, when the DC current detected by the first DC current transformer is greater than I F2 Reduce to I F2 with I F1 When the DC current is further reduced to less than I F1 When the first isolating switch is opened, the second isolating switch is closed.

[0023] As a preferred technical solution of the present invention: in the operating state where the first isolating switch is disconnected and the second isolating switch is turned on, if the voltage across the capacitor C1 in the upper capacitor branch is higher than 3 V, the full-controlled power switch of the upper capacitor branch, the first anti-parallel thyristor, and the full-controlled power switch of the upper resistor branch are turned on; if the voltage across the capacitor C2 in the lower capacitor branch is higher than 3 V, the full-controlled power switch of the lower capacitor branch, the first anti-parallel thyristor, and the full-controlled power switch of the lower resistor branch are turned on; if the voltage across both capacitors C1 and C2 is higher than 3 V, first turn on the full-controlled power switch of the upper capacitor branch, the first anti-parallel thyristor, and the full-controlled power switch of the upper resistor branch until the voltage across the capacitor C1 is lower than 3 V, then turn off the full-controlled power switch of the upper capacitor branch and the full-controlled power switch of the upper resistor branch, turn on the full-controlled power switch of the lower capacitor branch and the full-controlled power switch of the lower resistor branch, until the voltage across the capacitor C2 is lower than 3 V. V, and then disconnect the first anti-parallel thyristor, the fully-controlled power switch of the lower capacitor branch, and the fully-controlled power switch of the lower resistor branch.

[0024] As the preferred technical solution of the present invention:

[0025] I F1 The range is 3 A~5 A; I F2 The value range is 10 A~20 A;

[0026] V F1 The value range is 30 V~100 V; V F2 The value range is 100 V~300 V.

[0027] Another object of the present invention is to provide a control method for the transformer neutral point DC isolation device mentioned above in view of the deficiencies in the prior art.

[0028] To this end, the above-mentioned purpose of the present invention is achieved through the following technical solutions:

[0029] According to the control method of the transformer neutral point DC isolation device described above, it is characterized in that: the control method has two current control limits I F1 and I F2 and 2 voltage control limits V F1 and V F2 ;

[0030] When the transformer is running, the first isolating switch is opened, the second isolating switch is closed, and all anti-parallel thyristors and all fully controlled power devices are in the off state;

[0031] When the second DC current transformer detects that the DC current exceeds I F1 , but less than I F2 When the first isolating switch is closed, the second isolating switch is opened, and the fully controlled power switch, the first anti-parallel thyristor, and the second anti-parallel thyristor of the lower resistance branch are turned on;

[0032] When the second DC current transformer detects that the DC current exceeds I F2 When the first isolating switch is closed, the second isolating switch is opened, and the fully controlled power switch and the second anti-parallel thyristor of the lower capacitor branch are turned on; then a voltage will appear across the capacitor C2 in the lower capacitor branch. When the voltage detected by the first voltage transformer connected in parallel with the capacitor C2 exceeds V F1 When the second anti-parallel thyristor is turned off, the first anti-parallel thyristor and the fully controlled power switch of the upper capacitor branch are turned on; then a voltage will appear across the capacitor C1 in the upper capacitor branch. When the second voltage transformer connected in parallel with the capacitor C1 detects a voltage exceeding V F2 When , the fully controlled power switch of the upper resistance branch is turned on.

[0033] While adopting the above technical solutions, the present invention may also adopt or combine the following technical solutions:

[0034] As a preferred technical solution of the present invention: in the operating state where the first isolating switch is turned on and the second isolating switch is turned off, when the DC current detected by the first DC current transformer is greater than I F2 Reduce to I F2 with I F1 When the DC current is further reduced to less than I F1 When the first isolating switch is opened, the second isolating switch is closed.

[0035] As a preferred technical solution of the present invention: in the operating state where the first isolating switch is disconnected and the second isolating switch is turned on, if the voltage across the capacitor C1 in the upper capacitor branch is higher than 3 V, the full-controlled power switch of the upper capacitor branch, the first anti-parallel thyristor, and the full-controlled power switch of the upper resistor branch are turned on; if the voltage across the capacitor C2 in the lower capacitor branch is higher than 3 V, the full-controlled power switch of the lower capacitor branch, the first anti-parallel thyristor, and the full-controlled power switch of the lower resistor branch are turned on; if the voltage across both capacitors C1 and C2 is higher than 3 V, first turn on the full-controlled power switch of the upper capacitor branch, the first anti-parallel thyristor, and the full-controlled power switch of the upper resistor branch until the voltage across the capacitor C1 is lower than 3 V, then turn off the full-controlled power switch of the upper capacitor branch and the full-controlled power switch of the upper resistor branch, turn on the full-controlled power switch of the lower capacitor branch and the full-controlled power switch of the lower resistor branch, until the voltage across the capacitor C2 is lower than 3 V. V, and then disconnect the first anti-parallel thyristor, the fully-controlled power switch of the lower capacitor branch, and the fully-controlled power switch of the lower resistor branch.

[0036] As the preferred technical solution of the present invention:

[0037] I F1 The range is 3 A~5 A; I F2 The value range is 10 A~20 A;

[0038] V F1 The value range is 30 V~100 V; V F2 The value range is 100 V~300 V.

[0039] The present invention provides a transformer neutral point DC isolation device and control method, which have the following advantages compared with the prior art:

[0040] (1) For situations where the DC grounding electrode distribution is complex and the DC current amplitude varies greatly, the transformer neutral point DC isolation device provided by the present invention can be flexibly adjusted to cope with the situation;

[0041] (2) If any fully controlled power switch in the transformer neutral point DC isolation device provided by the present invention is mis-connected, there is a resistor to limit the current, which will not cause shock to the capacitor and has high reliability;

[0042] (3) The capacitor in the transformer neutral point DC isolation device provided by the present invention does not need to be provided with a separate discharge circuit. BRIEF DESCRIPTION OF THE DRAWINGS

[0043] Figure 1 This is a topological diagram of the transformer neutral point DC isolation device provided by the present invention. DETAILED DESCRIPTION

[0044] The present invention will be further described in detail below in conjunction with the accompanying drawings and specific embodiments.

[0045] A transformer neutral point DC isolation device comprises an upper capacitor branch 1, an upper resistor branch 2, a lower resistor branch 3, a lower capacitor branch 4, a first anti-parallel thyristor 5, a second anti-parallel thyristor 6, a first DC current transformer 7, a first voltage transformer 8, a second voltage transformer 9, an overvoltage protector 10, a first isolating switch 11, a second isolating switch 12, four fully controlled power switches 1.1, and a second DC current transformer 13.

[0046] The capacitor branch is formed by connecting a capacitor and a full-controlled power switch in series; the resistor branch is formed by connecting a resistor and a full-controlled power switch in series.

[0047] The upper capacitor branch 1 and the lower capacitor branch 4 are respectively formed by connecting capacitors C1 / C2 in series with a full-controlled power switch;

[0048] The upper resistance branch 2 and the lower resistance branch 3 are respectively formed by connecting the resistor R1 / R2 in series with the full-controlled power switch.

[0049] The lower end of the upper capacitor branch 1 is connected to the upper end of the lower resistor branch 3, and the lower end of the upper resistor branch 2 is connected to the upper end of the lower capacitor branch 4; the lower end of the lower resistor branch 3 is connected to the lower end of the lower capacitor branch 4, and the connection point is connected to the ground grid through the first DC current transformer 7; the upper end of the lower resistor branch 3 is connected to the upper end of the lower capacitor branch 4 through the first anti-parallel thyristor 5; the upper end of the upper capacitor branch 1 is connected to the upper end of the upper resistor branch 2, and the connection point is connected to the neutral point of the transformer through the first isolating switch 11 and the second DC current transformer 13; the upper end of the lower capacitor branch 4 is connected to the upper end of the upper resistor branch 2 through the second anti-parallel thyristor 6;

[0050] The upper end of the overvoltage protector 10 is connected to the upper end of the upper capacitor branch 1; the neutral point of the transformer is connected to the grounding grid through the second isolating switch 12 after being led out;

[0051] Two ends of the capacitors of the upper capacitor branch 1 and the lower capacitor branch 4 are respectively connected in parallel with a voltage transformer, which are the second voltage transformer 9 and the first voltage transformer 8 .

[0052] The power frequency impedance of the lower capacitor branch capacitor is X C2 =0.5 Ω; resistance of the lower resistance branch R2=0.79 Ω; power frequency impedance of the upper capacitance branch capacitance X C1 =2.5 Ω; upper resistance branch resistance R1=3.9 Ω.

[0053] The transformer neutral point DC isolation device has two current control limits I F1 =3 A and I F2 = 10 A and 2 voltage control limits V F1 =50 V and V F2 =200 V.

[0054] When the transformer is running, the first isolating switch 11 is opened, the second isolating switch 12 is closed, and the anti-parallel thyristors and the fully controlled power devices are both in the off state;

[0055] When the second DC current transformer 13 detects that the DC current exceeds 3 A but is less than 10 A, the first isolating switch 11 is closed, the second isolating switch 12 is opened, and the lower resistance branch full-controlled power switch, the first anti-parallel thyristor 5, and the second anti-parallel thyristor 6 are turned on;

[0056] When the second DC current transformer 13 detects that the DC current exceeds 10 A, the first isolating switch 11 is closed, the second isolating switch 12 is opened, and the lower capacitor branch fully-controlled power switch and the second anti-parallel thyristor 6 are turned on; then a voltage will appear across the capacitor C2, and when the first voltage transformer 8 connected in parallel with the capacitor C2 detects a voltage exceeding 50 V, the second anti-parallel thyristor 6 is turned off, and the first anti-parallel thyristor 5 and the upper capacitor branch fully-controlled power switch are turned on; then a voltage will appear across C1, and when the second voltage transformer 9 connected in parallel with the capacitor C1 detects a voltage exceeding 200 V, the upper resistor branch fully-controlled power switch is turned on.

[0057] In the operating state where the first isolating switch 11 is turned on and the second isolating switch 12 is turned off, when the DC current detected by the first DC current transformer 7 decreases from greater than 10 A to between 3 A and 10 A, the lower resistance branch full-controlled power switch, the first anti-parallel thyristor 5, and the second anti-parallel thyristor 6 are turned on, and the remaining full-controlled power switches are turned off; when the DC current is further reduced to less than 3 A, the first isolating switch 11 is turned off and the second isolating switch 12 is closed.

[0058] In the operating state where the first isolating switch 11 is disconnected and the second isolating switch 12 is turned on, if the voltage across the capacitor C1 is higher than 3 V, the upper capacitor branch fully-controlled power switch, the first anti-parallel thyristor 5, and the upper resistor branch fully-controlled power switch are turned on; if the voltage across the capacitor C2 is higher than 3 V, the lower capacitor branch fully-controlled power switch, the first anti-parallel thyristor 5, and the lower resistor branch fully-controlled power switch are turned on; if the voltage across both capacitors C1 and C2 is higher than 3 V, the upper capacitor branch fully-controlled power switch, the first anti-parallel thyristor 5, and the upper resistor branch fully-controlled power switch are first turned on until the voltage across the capacitor C1 is lower than 3 V, then the upper capacitor branch fully-controlled power switch and the upper resistor branch fully-controlled power switch are turned off, and the lower capacitor branch fully-controlled power switch and the lower resistor branch fully-controlled power switch are turned on until the voltage across the capacitor C2 is lower than 3 V, then the first anti-parallel thyristor 5, the lower capacitor branch fully-controlled power switch, and the lower resistor branch fully-controlled power switch are turned on.

[0059] The above-mentioned specific implementation methods are used to explain the present invention and are only preferred embodiments of the present invention, rather than limiting the present invention. Any modifications, equivalent substitutions, improvements, etc. made to the present invention within the spirit of the present invention and the protection scope of the claims shall fall within the protection scope of the present invention.

Claims

1. A transformer neutral point DC isolation device, Features: The transformer neutral point DC isolation device comprises an upper capacitor branch, an upper resistor branch, a lower resistor branch, a lower capacitor branch, a first anti-parallel thyristor, a second anti-parallel thyristor, a first DC current transformer, a second DC current transformer, a first voltage transformer, a second voltage transformer, an overvoltage protector, a first isolating switch, a second isolating switch and a fully controlled power switch; The upper capacitor branch and the lower capacitor branch are respectively formed by connecting a capacitor and a fully controlled power switch in series; The upper resistance branch and the lower resistance branch are respectively formed by connecting a resistor and a full-controlled power switch in series; The lower end of the upper capacitor branch is connected to the upper end of the lower resistor branch, and the lower end of the upper resistor branch is connected to the upper end of the lower capacitor branch; the lower end of the lower resistor branch is connected to the lower end of the lower capacitor branch, and the connection point is connected to the ground grid through the first DC current transformer; the upper end of the lower resistor branch is connected to the upper end of the lower capacitor branch through the first anti-parallel thyristor; the upper end of the upper capacitor branch is connected to the upper end of the upper resistor branch, and the connection point is connected to the neutral point of the transformer through the first isolation switch and the second DC current transformer; the upper end of the lower capacitor branch is connected to the upper end of the upper resistor branch through the second anti-parallel thyristor; The upper end of the overvoltage protector is connected to the upper end of the upper capacitor branch; the neutral point of the transformer is connected to the grounding grid through the second isolating switch after being led out; A voltage transformer is connected in parallel at both ends of the capacitors of the upper capacitor branch and the lower capacitor branch respectively.

2. The transformer neutral point DC isolation device according to claim 1, Features: The power frequency impedance X of the capacitor of the lower capacitor branch C2 =0.3~1.0 Ω; resistance of lower resistance branch R2=0.5πX C2 ; The power frequency impedance X of the upper capacitor branch capacitor C1 = k X C2 ; Upper resistance branch resistance R1 = 0.5πX C1 ; in: k The value range is 3~10.

3. The transformer neutral point DC isolation device according to claim 1, Features: The transformer neutral point DC isolation device has two current control limits I F1 and I F2 and 2 voltage control limits V F1 and V F2 ; When the transformer is running, the first isolating switch is opened, the second isolating switch is closed, and all anti-parallel thyristors and all fully controlled power devices are in the disconnected state; When the second DC current transformer detects that the DC current exceeds I F1 , but less than I F2 When the first isolating switch is closed, the second isolating switch is opened, and the fully controlled power switch, the first anti-parallel thyristor, and the second anti-parallel thyristor of the lower resistance branch are turned on; When the second DC current transformer detects that the DC current exceeds I F2 When the first isolating switch is closed, the second isolating switch is opened, and the fully controlled power switch and the second anti-parallel thyristor of the lower capacitor branch are turned on; then a voltage will appear across the capacitor C2 in the lower capacitor branch. When the voltage detected by the first voltage transformer connected in parallel with the capacitor C2 exceeds V F1 When the second anti-parallel thyristor is turned off, the first anti-parallel thyristor and the fully controlled power switch of the upper capacitor branch are turned on; then a voltage will appear across the capacitor C1 in the upper capacitor branch. When the second voltage transformer connected in parallel with the capacitor C1 detects a voltage exceeding V F2 When , the fully controlled power switch of the upper resistance branch is turned on.

4. The transformer neutral point DC isolation device according to claim 3, Features: In the operating state where the first isolating switch is turned on and the second isolating switch is turned off, when the DC current detected by the first DC current transformer is greater than I F2 Reduce to I F2 with I F1 When the DC current is further reduced to less than I F1 When the first isolating switch is opened, the second isolating switch is closed.

5. The transformer neutral point DC isolation device according to claim 3, Features: In the operating state where the first isolating switch is disconnected and the second isolating switch is turned on, if the voltage across the capacitor C1 in the upper capacitor branch is higher than 3 V, the full-controlled power switch of the upper capacitor branch, the first anti-parallel thyristor, and the full-controlled power switch of the upper resistor branch are turned on; if the voltage across the capacitor C2 in the lower capacitor branch is higher than 3 V, the full-controlled power switch of the lower capacitor branch, the first anti-parallel thyristor, and the full-controlled power switch of the lower resistor branch are turned on; if the voltage across both capacitors C1 and C2 is higher than 3 V, the full-controlled power switch of the upper capacitor branch, the first anti-parallel thyristor, and the full-controlled power switch of the upper resistor branch are turned on first until the voltage across the capacitor C1 is lower than 3 V, and then the full-controlled power switch of the upper capacitor branch and the full-controlled power switch of the upper resistor branch are disconnected, and the full-controlled power switch of the lower capacitor branch and the full-controlled power switch of the lower resistor branch are turned on until the voltage across the capacitor C2 is lower than 3 V. V, and then disconnect the first anti-parallel thyristor, the fully-controlled power switch of the lower capacitor branch, and the fully-controlled power switch of the lower resistor branch.

6. The transformer neutral point DC isolation device according to claim 3, Features: I F1 The range is 3 A~5 A; I F2 The value range is 10 A~20 A; V F1 The value range is 30 V~100 V; V F2 The value range is 100 V~300 V.

7. The control method of the transformer neutral point DC isolation device according to claim 1, Features: The control method has two current control limits I F1 and I F2 and 2 voltage control limits V F1 and V F2 ; When the transformer is running, the first isolating switch is opened, the second isolating switch is closed, and all anti-parallel thyristors and all fully controlled power devices are in the off state; When the second DC current transformer detects that the DC current exceeds I F1 , but less than I F2 When the first isolating switch is closed, the second isolating switch is opened, and the fully controlled power switch, the first anti-parallel thyristor, and the second anti-parallel thyristor of the lower resistance branch are turned on; When the second DC current transformer detects that the DC current exceeds I F2 When the first isolating switch is closed, the second isolating switch is opened, and the fully controlled power switch and the second anti-parallel thyristor of the lower capacitor branch are turned on; then a voltage will appear across the capacitor C2 in the lower capacitor branch. When the voltage detected by the first voltage transformer connected in parallel with the capacitor C2 exceeds V F1 When the second anti-parallel thyristor is turned off, the first anti-parallel thyristor and the fully controlled power switch of the upper capacitor branch are turned on; then a voltage will appear across the capacitor C1 in the upper capacitor branch. When the second voltage transformer connected in parallel with the capacitor C1 detects a voltage exceeding V F2 When , the fully controlled power switch of the upper resistance branch is turned on.

8. The control method according to claim 7, Features: In the operating state where the first isolating switch is turned on and the second isolating switch is turned off, when the DC current detected by the first DC current transformer is greater than I F2 Reduce to I F2 with I F1 When the DC current is further reduced to less than I F1 When the first isolating switch is opened, the second isolating switch is closed.

9. The control method according to claim 7, Features: In the operating state where the first isolating switch is disconnected and the second isolating switch is turned on, if the voltage across the capacitor C1 in the upper capacitor branch is higher than 3 V, the full-controlled power switch of the upper capacitor branch, the first anti-parallel thyristor, and the full-controlled power switch of the upper resistor branch are turned on; if the voltage across the capacitor C2 in the lower capacitor branch is higher than 3 V, the full-controlled power switch of the lower capacitor branch, the first anti-parallel thyristor, and the full-controlled power switch of the lower resistor branch are turned on; if the voltage across both capacitors C1 and C2 is higher than 3 V, the full-controlled power switch of the upper capacitor branch, the first anti-parallel thyristor, and the full-controlled power switch of the upper resistor branch are turned on first until the voltage across the capacitor C1 is lower than 3 V, and then the full-controlled power switch of the upper capacitor branch and the full-controlled power switch of the upper resistor branch are disconnected, and the full-controlled power switch of the lower capacitor branch and the full-controlled power switch of the lower resistor branch are turned on until the voltage across the capacitor C2 is lower than 3 V. V, and then disconnect the first anti-parallel thyristor, the fully-controlled power switch of the lower capacitor branch, and the fully-controlled power switch of the lower resistor branch.

10. The control method according to claim 7, Features: I F1 The range is 3 A~5 A; I F2 The value range is 10 A~20 A; V F1 The value range is 30 V~100 V; V F2 The value range is 100 V~300 V.

Images