WO2021134492A1

WO2021134492A1 - Pre-charging circuit, converter, and pre-charging method

Info

Publication number: WO2021134492A1
Application number: PCT/CN2019/130537
Authority: WO
Inventors: 刘泽伟; 巴鲁斯卡·伦纳特; 廖华
Original assignee: 西门子股份公司; 西门子（中国）有限公司
Priority date: 2019-12-31
Filing date: 2019-12-31
Publication date: 2021-07-08

Abstract

The present disclosure relates to a pre-charging circuit, a converter, and a pre-charging method, comprising: a pre-charging unit and a voltage equalising resistance unit, the pre-charging unit being connected in parallel with the input capacitor of a multi-level flying capacitor converter, the voltage equalising resistance unit comprising at least three series connected resistors connected in parallel with the input capacitor, a first resistor amongst the resistors being connected in parallel with a flying capacitor of the multi-level flying capacitor converter, and a second resistor and a third resistor amongst the resistors respectively being connected in parallel with a first power switch and a fourth power switch in the multi-level flying capacitor converter that are not connected in parallel with the flying capacitor.

Description

Pre-charging circuit, converter and pre-charging method

Technical field

The present disclosure generally relates to the field of circuit technology, and more specifically, to a pre-charging circuit, a converter, and a pre-charging method.

Background technique

Three-level converters have attracted great interest for medium/high power applications. Among the various three-level converter topologies, the three-level flying capacitor (TLFC) converter provides some special advantages, such as low voltage stress on semiconductor devices, twice the switching frequency on the output inductance, and input Common ground between output and output. However, in order to avoid damage to the semiconductor due to the initial high voltage, the flying capacitor needs to be precharged to half of the input voltage.

In the prior art on three-level flying capacitor (TLFC) converters, it is usually assumed that the flying capacitor is charged to some extent, and there is no specific discussion on such a pre-charging process and its effectiveness. The pre-charging of the converter capacitors must be done in a controllable manner to avoid stress on power devices and passive components.

At present, in some references, an additional pre-charging unit is used to pre-charge the flying capacitor so that it can reach a high voltage that is half of the input voltage.

Figure 1 is a circuit topology diagram of a three-level flying capacitor converter 1. In FIG. 1, an additional pre-charging unit 102 is included, and the additional pre-charging unit 102 is used to pre-charge the flying capacitor Cf1. Here, other circuit elements of the three-level flying capacitor converter are not described in detail.

However, there are some drawbacks when applying this additional pre-charging unit.

1. The pre-charging unit will take up more space and additional cost. This is especially unacceptable for cost-sensitive and volume-constrained customers.

2. The input of the pre-charging unit usually comes from the power supply of the control system of the converter. When the TFLC converter has a high input voltage, it is necessary to consider the reinforced insulation between the flying capacitor and the input of the control system.

Therefore, it is desired to have a more efficient pre-charging circuit for pre-charging the flying capacitor.

Summary of the invention

A brief overview of the present invention is given below in order to provide a basic understanding of certain aspects of the present invention. It should be understood that this summary is not an exhaustive summary of the present invention. It is not intended to determine the key or important part of the present invention, nor is it intended to limit the scope of the present invention. Its purpose is merely to present some concepts in a simplified form as a prelude to the more detailed description that will be discussed later.

In view of the above, the present disclosure proposes a pre-charging circuit capable of pre-charging the flying capacitor in the multi-level flying capacitor converter.

According to one aspect of the present disclosure, there is provided a pre-charging circuit, including: a pre-charging unit connected in parallel with an input capacitor (Cin) of a converter; and a voltage-sharing resistor unit, including a pre-charging unit connected to the input capacitor At least three resistors connected in parallel and connected in series, wherein the first resistor of the resistors is connected in parallel with the flying capacitor of the multilevel flying capacitor converter, and the second resistor and the third resistor of the resistors are connected in parallel. The resistors are respectively connected in parallel with the first power switch and the fourth power switch in the multi-level flying capacitor converter that are not connected in parallel with the flying capacitor.

Optionally, in an example of the foregoing aspect, the resistance value of the first resistance is equal to twice the resistance value of the second resistance and the third resistance, and the second resistance and the third resistance The resistance values are equal.

Optionally, in an example of the above aspect, the pre-charging circuit further includes: a voltage measuring unit, which is connected in parallel with the flying capacitor, and measures the voltage across the flying capacitor.

Optionally, in an example of the above aspect, the pre-charging unit includes a line DC switch, a pre-charging DC switch, and a pre-charging resistor.

According to one aspect of the present disclosure, there is provided a three-level flying capacitor converter, including: a converter unit; an input capacitor; an output circuit; and a pre-charging circuit according to the above.

Optionally, in an example of the above aspect, the converter unit includes a first power switch, a second power switch, a third power switch, and a fourth power switch, and a flying capacitor, the flying capacitor and the The second power switch and the third power switch are connected in parallel, and the voltage-sharing resistance unit of the pre-charging circuit includes a first resistor, a second resistor, and a third resistor connected in series, wherein the first resistor is connected to the converter The flying capacitors of the units are connected in parallel, the second resistor is connected in parallel with the first power switch of the converter unit, and the third resistor is connected in parallel with the fourth power switch of the converter unit.

According to one aspect of the present disclosure, a method for precharging a three-level flying capacitor converter is provided, including: keeping the line DC switch and the precharging DC switch of the precharging unit off, and keeping the converter The first power switch, the second power switch, the third power switch, and the fourth power switch of the unit are turned off; the first power switch and the fourth power switch are turned on; and the precharge DC switch is closed, the three-level The input voltage of the flying capacitor converter passes through the precharge resistor of the precharge unit, the first power switch, the fourth power switch and the precharge DC switch to charge the input capacitor and the flying capacitor.

Optionally, in an example of the above aspect, the method for precharging a three-level flying capacitor converter further includes: monitoring the voltage of the flying capacitor by a voltage measurement circuit, and when the flying capacitor When the voltage of the capacitor reaches a predetermined ratio of the input voltage, the first power switch and the fourth power switch are turned off.

Optionally, in an example of the foregoing aspect, the method for precharging a three-level flying capacitor converter further includes: monitoring the voltage of the input capacitor, and when the voltage of the input capacitor reaches a predetermined voltage Or when the pre-charging reaches a predetermined time, the line DC switch is closed, and the pre-charging DC switch is opened.

According to the pre-charging circuit of the present disclosure, the existing pre-charging circuit of the input capacitor can be reused, and a voltage equalizing resistor unit can be added. During the pre-charging process, the flying capacitor can be pre-charged by controlling the power switch. In addition, , The increased resistance can also balance the voltage of the flying capacitor.

According to the pre-charging circuit of the present disclosure, a controllable solution can be provided to pre-charge the flying capacitor to half of the input voltage; and the pre-charging circuit according to the present disclosure fails in the external pre-charging circuit or the input capacitor is not required In the case of pre-charging, a backup solution for pre-charging the flying capacitor can be provided.

According to the pre-charging circuit of the present disclosure, using the existing pre-charging circuit for the input capacitor, the added voltage-balancing resistor unit occupies a small space and has a low cost, and there is no need to consider the reinforced insulation between the flying capacitor and the input of the control system .

Description of the drawings

The above and other objects, features and advantages of the present invention will be more easily understood with reference to the following description of the embodiments of the present invention in conjunction with the accompanying drawings. The components in the drawings are only to illustrate the principle of the present invention. In the drawings, the same or similar technical features or components will be represented by the same or similar reference numerals.

Figure 1 Circuit topology diagram of a three-level flying capacitor converter in the prior art;

FIG. 2 is an exemplary circuit topology diagram showing a pre-charging circuit according to an embodiment of the present disclosure;

Fig. 3 shows an exemplary circuit topology diagram of a three-level flying capacitor converter employing a pre-charging circuit according to an embodiment of the present disclosure;

Figure 4 shows the current flow in the process of precharging the three-level flying capacitor converter; and

FIG. 5 is a flowchart illustrating an exemplary process 500 for precharging a three-level flying capacitor converter according to an embodiment of the present disclosure.

Reference number

1, 2: Multilevel flying capacitor converter VDC1+: Positive DC bus

VDC1-: negative DC bus Cin1: input capacitance

Cf1: Flying capacitor Q11, Q21, Q31, Q41: Power on

turn off

Lout1: output inductance Vout1: output voltage

Cout1: output capacitor 102: additional pre-charging unit

VDC+: positive DC bus VDC-: negative DC bus

20: Pre-charge circuit 202: Pre-charge unit

204: Voltage-balancing resistor unit Cin: Input capacitance

Cf: flying capacitor R1: first resistor

R2: Second resistor R3: Third resistor

Q1: The first power switch Q2: The second power switch

Q3: Third power switch Q4: Fourth power switch

206: Voltage measurement circuit 30: Converter unit

40: Output circuit S1: Line DC switch

S2: Pre-charge DC switch Rp: Pre-charge resistance

Lout: output inductance Vout: output voltage

Cout: Output capacitor 500: Enter the three-level flying capacitor converter

Method of pre-charging

S502, S504, S506, S508, S510: 3: Three-level flying capacitor converter

step

Detailed ways

The subject matter described herein will now be discussed with reference to example embodiments. It should be understood that the discussion of these embodiments is only to enable those skilled in the art to better understand and realize the subject described herein, and is not to limit the scope of protection, applicability, or examples set forth in the claims. The function and arrangement of the discussed elements can be changed without departing from the scope of protection of the present disclosure. Various examples can omit, substitute, or add various procedures or components as needed. For example, the described method may be executed in a different order from the described order, and various steps may be added, omitted, or combined. In addition, features described with respect to some examples can also be combined in other examples.

As used herein, the term "including" and its variations mean open terms, meaning "including but not limited to". The term "based on" means "based at least in part on." The terms "one embodiment" and "an embodiment" mean "at least one embodiment." The term "another embodiment" means "at least one other embodiment." The terms "first", "second", etc. may refer to different or the same objects. Other definitions can be included below, whether explicit or implicit. Unless clearly indicated in the context, the definition of a term is consistent throughout the specification.

Taking into account the problems in the prior art, the present disclosure proposes a pre-charging circuit capable of pre-charging the flying capacitor in a multi-level flying capacitor converter. The pre-charging circuit uses the existing pre-charging circuit for the input capacitor of the converter, and adds a voltage-balancing resistor unit, which includes three or more resistors connected in series. During the process, the flying capacitor can be pre-charged by controlling the power switch; in addition, through the increased resistance, even if the external pre-charging circuit fails or the input capacitor does not need to be pre-charged, the flying capacitor can also be provided Carry out a pre-charged backup scheme.

FIG. 2 is an exemplary circuit topology diagram showing a precharge circuit according to an embodiment of the present disclosure.

The pre-charging circuit 20 according to an embodiment of the present disclosure includes a pre-charging unit 202 and a voltage equalizing resistance unit 204.

The pre-charging unit 202 is connected in parallel with the input capacitor Cin of the converter 2 for pre-charging the input capacitor Cin and the flying capacitor Cf. In FIG. 2, the pre-charging unit 202 includes a line DC switch S1, a pre-charging DC switch S2, and a pre-charging resistor Rp. The line DC switch S1 and the pre-charging DC switch S2 may be contactors or other types of switches. Those skilled in the art can understand that the specific circuit topology of the pre-charging unit 202 is not limited to that shown in FIG. 2, and an appropriate circuit topology can be selected according to needs, which will not be described in detail here.

In the pre-charging circuit 20 according to an embodiment of the present disclosure shown in FIG. 2, the voltage-sharing resistance unit 204 includes three resistors connected in series, that is, a first resistor R1, a second resistor R2, and a third resistor R3. Among them, the first resistor R1 is connected in parallel with the flying capacitor Cf of the converter 2, the second resistor R2 is connected in parallel with the first power switch Q1 of the converter 2, and the third resistor R3 is connected with the fourth power switch Q4 of the converter 2. Parallel connection, where the first power switch Q1 and the fourth power switch Q4 are power switches that are not connected in parallel with the flying capacitor Cf.

FIG. 2 shows a specific example of applying the pre-charging circuit according to an embodiment of the present disclosure to a three-level flying capacitor converter, which shows that the pre-charging circuit 20 operates in the three-level flying capacitor converter. For the exemplary connection mode of the inverter unit (in the dashed line box 30 on the right), those skilled in the art can understand that the converter 2 here is not limited to a three-level flying capacitor converter, and it may also be a five-level flying capacitor converter. Other multi-level flying capacitor converters such as flat flying capacitor converter or seven-level flying capacitor converter. In the five-level or seven-level flying capacitor converter, the voltage-balancing resistor unit 204 of the pre-charging circuit 20 includes more resistors, and the number of resistors depends on the power switch in the multi-level flying capacitor converter Number of. The first resistor of the plurality of resistors is connected in parallel with the flying capacitor, and the remaining resistors are respectively connected in parallel with other power switches except the power switch connected in parallel with the flying capacitor.

Those skilled in the art can understand that when the pre-charging circuit 20 according to an embodiment of the present disclosure is applied to other multi-level flying capacitor converters, the connection with the converter unit and the pre-charging process are the same as those described above with reference to FIG. 2 The pre-charging circuit 20 is similar to the case of a three-level flying capacitor converter, which will not be described in detail here.

The input capacitor Cin shown in FIG. 2 and the flying capacitor Cf included in the converter unit 30 of the converter 2, the first power switch Q1, the second power switch Q2, the third power switch Q3, and the fourth power switch Q4 are shown in FIG. It is only used to illustrate the connection relationship between the pre-charging circuit 20 and the converter unit 30, and not as a limitation to the pre-charging circuit 20.

Those skilled in the art can understand that each resistor included in the voltage-sharing resistance unit 204 may also be a resistor row composed of multiple resistors connected in series or in parallel. For the convenience of description, the resistor and the resistor row are collectively referred to herein as resistance.

In FIG. 2, the resistance relationship between the first resistor R1, the second resistor R2, and the third resistor R3 is R2=R3=0.5*R1. Similarly, in a multi-level flying capacitor converter, the resistance of the first resistor connected in parallel with the flying capacitor is equal to twice the resistance of the other resistors, and the resistances of the other resistors are all the same.

In an example, the pre-charging circuit 20 may further include a voltage measuring circuit 206, which is connected in parallel with the flying capacitor Cf, and the voltage measuring circuit 206 may be formed by multiplexing the first resistor R1. The voltage measurement circuit 206 can be used to monitor the magnitude of the pre-charged voltage on the flying capacitor Cf. When the voltage of the flying capacitor Cf reaches about one-half of the input voltage, the pre-charging of the flying capacitor Cf is stopped.

FIG. 3 shows an exemplary topology diagram of the three-level flying capacitor converter 3 using the pre-charging circuit 20 according to an embodiment of the present disclosure.

The three-level flying capacitor converter 3 includes a converter unit 30, an input capacitor Cin, an output circuit 40, and the precharge circuit 20 described above with reference to FIG. 2.

As described above, the pre-charging circuit 20 includes a pre-charging unit 202 and a voltage equalizing resistance unit 204. The pre-charging unit 202 includes a line DC switch S1, a pre-charging DC switch S2, and a pre-charging resistor Rp. The voltage-sharing resistance unit 204 includes a first resistor R1, a second resistor R2, and a third resistor R3 connected in series. The pre-charging circuit 20 is connected in parallel with the input capacitor Cin, and is used to pre-charge the input capacitor Cin and the flying capacitor Cf.

The converter unit 30 includes a first power switch Q1, a second power switch Q2, a third power switch Q3, and a fourth power switch Q4 connected in series, and a flying capacitor Cf, a flying capacitor Cf, and a second power switch Q2 and a second power switch Q2. The three power switches Q3 are connected in parallel.

The first resistor R1 of the precharging circuit 20 is connected in parallel with the flying capacitor Cf of the converter unit 30, and the second resistor R2 of the precharging circuit 20 is connected in parallel with the first power switch Q1 of the converter unit 30. The third resistor R3 is connected in parallel with the fourth power switch Q4 of the converter unit 30.

The output circuit 40 shown in FIG. 3 includes an output inductor Lout and an output capacitor Cout, and can output a voltage Vout. Those skilled in the art can understand that the output circuit 40 may also be an output circuit of other suitable forms, and is not limited to that shown in FIG. 3.

Those skilled in the art can understand that the pre-charging circuit 20 according to an embodiment of the present disclosure can also be used for pre-charging and pre-charging in other multi-level flying capacitor converters such as five-level flying capacitor converters. The resistance of the voltage-balancing resistor unit of the circuit and the flying capacitor and the power switch of the converter unit can have a similar connection mode, which will not be described in detail here.

4 is a circuit topology diagram of a converter similar to the structure of the three-level flying capacitor converter according to an embodiment of the present disclosure shown in FIG. The current flows during the pre-charging of the transcapacitor Cf. FIG. 5 is a flowchart illustrating an exemplary process 500 for precharging a three-level flying capacitor converter according to an embodiment of the present disclosure.

Hereinafter, the process of precharging the three-level flying capacitor converter 3 will be described with reference to FIGS. 4 and 5.

In block S502, the three-level flying capacitor converter 3 is in the initial state, the line DC switch S1 and the precharging DC switch S2 of the precharging unit 202 are kept off, and the first power switch of the converter unit 30 Q1, the second power switch Q2, the third power switch Q3, and the fourth power switch Q4 remain off. At this time, the voltages of the input capacitor Cin and the flying capacitor Cf are both zero.

Next, in block S504, before the pre-charging process starts, the first power switch Q1 and the fourth power switch Q4 are turned on. In this way, the first power switch Q1 and the fourth power switch Q4 will not have high voltage stress.

Then, in block S506, the pre-charging DC switch S2 is closed to start pre-charging. The input voltage of the converter charges the input capacitor Cin and the flying capacitor Cf through the precharge resistor Rp, the first power switch Q1, the fourth power switch Q4, and the precharge DC switch S2. The dashed arrow in FIG. 4 shows the direction of current during the charging process.

Through the above process, the input capacitor Cin and the flying capacitor Cf can be precharged. Preferably, during the pre-charging process, in block S508, the voltage measuring circuit 206 (as shown in FIG. 2) may be used to monitor the voltage of the flying capacitor Cf. The second resistor R2 can be multiplexed, that is, the second resistor R2 is used as a circuit component constituting the voltage measurement circuit 206. The specific circuit topology of the voltage measurement circuit 206 will not be described in detail here.

When the pre-charged voltage of the flying capacitor Cf reaches a predetermined ratio of the input voltage, preferably, when it reaches half of the input voltage, the first power switch Q1 and the fourth power switch Q4 are turned off. In this way, high voltage stress can be avoided.

During the precharging process, in block S510, the voltage of the input capacitor Cin may also be monitored. When the pre-charged voltage of the input capacitor Cin reaches a predetermined voltage or the pre-charge reaches a predetermined time, the line DC switch S1 is closed. The predetermined voltage is a predetermined voltage value that is approximately equal to the input voltage, and the predetermined time is calculated in advance to make the voltage of the input capacitor Cin approximately equal to the input voltage. After the line DC switch S1 is closed, the pre-charging resistor Rp and the pre-charging DC switch S2 are short-circuited. After the pre-charging is completed, the pre-charging DC switch S2 can be kept off until the next pre-charging is to be performed.

When the external pre-charging circuit fails, or when the input capacitor Cin already has an input voltage at both ends and no pre-charging is required, the flying capacitor Cf can still be charged through the first resistor R1 and the third resistor R3. And due to the relationship between the resistance values of the first resistor R1, the second resistor R2, and the third resistor R3, it is ensured that the voltage of the flying capacitor Cf can reach half of the input voltage after a long period of precharging.

In addition, when the converter is turned off, the first resistor R1, the second resistor R2, and the third resistor R3 can also be used as discharge resistors for the input capacitor Cin and the flying capacitor Cf.

Compared with the prior art solutions, the pre-charging circuit according to the present disclosure can provide a controllable solution to pre-charge the flying capacitor to half of the input voltage; and the pre-charging circuit according to the present disclosure is externally pre-charging the circuit In the event of a fault, a backup solution for pre-charging the flying capacitor can be provided.

The pre-charging circuit according to the present disclosure utilizes the existing pre-charging circuit for the input capacitor, and the added voltage-balancing resistor unit occupies a small space, has a low cost, and does not need to consider the reinforced insulation between the flying capacitor and the input of the control system.

The specific implementations set forth above in conjunction with the drawings describe exemplary embodiments, but do not represent all embodiments that can be implemented or fall within the protection scope of the claims. The term "exemplary" used throughout this specification means "serving as an example, instance, or illustration", and does not mean "preferred" or "advantageous" over other embodiments. The detailed description includes specific details for the purpose of providing an understanding of the described technology. However, these techniques can be implemented without these specific details. In some instances, in order to avoid incomprehensibility to the concepts of the described embodiments, well-known structures and devices are shown in the form of block diagrams.

The foregoing description of the present disclosure is provided to enable any person of ordinary skill in the art to implement or use the present disclosure. For those of ordinary skill in the art, various modifications to the present disclosure are obvious, and the general principles defined herein can also be applied to other modifications without departing from the scope of protection of the present disclosure. . Therefore, the present disclosure is not limited to the examples and designs described herein, but is consistent with the widest scope that conforms to the principles and novel features disclosed herein.

Claims

The pre-charging circuit (20) includes:

A pre-charging unit (202), the pre-charging unit (202) is connected in parallel with the input capacitor (Cin) of the multi-level flying capacitor converter (2); and

A voltage equalization resistance unit (204), the voltage equalization resistance unit (204) includes at least three resistors connected in parallel with the input capacitor (Cin) and connected in series, wherein the first resistor (R1 of the resistors) is connected in parallel with the input capacitor (Cin). ) Is connected in parallel with the flying capacitor (Cf) of the multi-level flying capacitor converter (2), and the second resistor (R2) and the third resistor (R3) of the resistors are respectively connected to the multi-level flying capacitor (R3). The first power switch (Q1) and the fourth power switch (Q4) in the transcapacitor converter (2) that are not connected in parallel with the flying capacitor (Cf) are connected in parallel.
The precharge circuit (20) according to claim 1, wherein:

The resistance value of the first resistor (R1) is equal to twice the resistance value of the second resistor (R2) and the third resistor (R3), and the second resistor (R2) and the third resistor The resistance values of (R3) are equal.
The pre-charging circuit (20) according to claim 1 or 2, further comprising: a voltage measuring unit (206) connected in parallel with the flying capacitor (Cf), which measures the The voltage across the flying capacitor (Cf).
The pre-charging circuit (20) according to claim 1 or 2, wherein the pre-charging unit (202) includes a line DC switch (S1), a pre-charging DC switch (S2) and a pre-charging resistor (Rp).
Three-level flying capacitor converter (3), including:

Converter unit (30);

Input capacitance (Cin);

Output circuit (40); and

The precharge circuit (20) according to any one of claims 1-4.
The three-level flying capacitor converter (3) according to claim 5, wherein the converter unit (30) comprises a first power switch (Q1), a second power switch (Q2), The third power switch (Q3) and the fourth power switch (Q4) and the flying capacitor (Cf), the flying capacitor (Cf) and the second power switch (Q2) and the third power switch (Q3) ) Are connected in parallel, and the voltage equalizing resistance unit (204) of the pre-charging circuit (20) includes a first resistor (R1), a second resistor (R2), and a third resistor (R3) connected in series, wherein the first resistor (R1), the second resistor (R2), and the third resistor (R3) are connected in series. A resistor (R1) is connected in parallel with the flying capacitor (Cf) of the converter unit (30), and the second resistor (R2) is connected in parallel with the first power switch (Q1) of the converter unit (30), The third resistor (R3) is connected in parallel with the fourth power switch (Q4) of the converter unit (30).
The method used to precharge (3) the three-level flying capacitor converter includes:

Keep the line DC switch (S1) and the precharge DC switch (S2) of the pre-charging unit (202) disconnected, and keep the first power switch (Q1), the second power switch (Q2), and the second power switch (Q2) of the converter unit (30) The third power switch (Q3) and the fourth power switch (Q4) are turned off;

Turn on the first power switch (Q1) and the fourth power switch (Q4); and

The pre-charging DC switch (S2) is closed, and the input voltage of the three-level flying capacitor converter (3) passes through the pre-charging resistor (Rp) of the pre-charging unit (202) and the first power The switch (Q1), the fourth power switch (Q4) and the pre-charging DC switch (S2) charge the input capacitor (Cin) and the flying capacitor (Cf).
The method according to claim 7, further comprising:

The voltage measurement circuit (206) monitors the voltage of the flying capacitor (Cf), and when the voltage of the flying capacitor (Cf) reaches a predetermined ratio of the input voltage, the first power switch (Q1) is turned off. ) And the fourth power switch (Q4).
The method according to claim 7 or 8, further comprising:

The voltage of the input capacitor (Cin) is monitored, and when the voltage of the input capacitor (Cin) reaches a predetermined voltage or the pre-charge reaches a predetermined time, the line DC switch (S1) is closed, and the pre-charge DC is disconnected Switch (S2).