Soft start method for high-capacity direct-current bus

Abstract

The invention discloses a high-capacity direct-current bus soft start method. The soft start process comprises the following steps: 1) a converter receives a start command; (2) the converter controls a soft start relay KM2 to be closed; (3) the converter collects starting loop current through a current detection circuit to control a soft start relay KM2, and when the current reaches a set threshold value after time T1, the soft start relay KM2 is closed; (4) after the interval time T2, the soft start relay KM2 is closed again; 5) repeating the steps 3) and 4); (6) the converter judges the difference value between the bus voltage and the direct current side voltage by collecting the bus voltage, and when the difference value between the bus voltage and the direct current side voltage meets a design threshold value, the main loop relay KM1 is closed, the soft start relay KM2 is disconnected, and therefore the direct current soft start process is completed. According to the soft start method, it can be guaranteed that the impact current in the soft start process is within the controllable range, and meanwhile the starting resistor can be prevented from being burnt in the initial charging state.

Description

Technical Field

[0001] This invention relates to the field of power electronics technology, specifically to a soft-start method for a large-capacity DC bus. Background Technology

[0002] To prevent the large inrush current during DC startup of an ACDC bidirectional DC converter, a soft-start circuit is required. However, as the capacity of ACDC modules increases, so does the capacitance of the DC bus, rendering traditional DC soft-start circuits impractical. Common forms of DC soft-start circuits include... Figure 1 As shown. Figure 1 This is an existing DC soft-start topology, with the starting circuit consisting of a soft-start relay KM2, a soft-start resistor R1, and a main circuit relay KM1. During power-up, KM2 closes first, and after the bus voltage reaches the set threshold voltage, KM1 closes and KM2 opens. However, this circuit has a drawback: when the bus capacitor value is too large, the power consumption generated in resistor R1 will be very large, making resistor selection difficult and increasing costs.

[0003] To address the above problems, a solution is proposed. Summary of the Invention

[0004] The purpose of this invention is to overcome the shortcomings of the prior art and provide a soft-start method for large-capacity DC buses.

[0005] To achieve the above objectives, the technical solution adopted by this invention is: a large-capacity DC bus soft-start method, applied to a large-capacity AC-CDC bidirectional converter. The converter includes a starting circuit, a DC bus capacitor, and an inverter circuit. The starting circuit is connected to the output terminal of the DC power supply and includes a main circuit relay KM1, a soft-start relay KM2, a thermistor NTC1, and a current detection circuit. The soft-start relay KM2, the thermistor NTC1, and the current detection circuit are connected in series and then connected in parallel with the main circuit relay KM1. The DC bus capacitor includes capacitors C1 and C2 connected in series. The input terminal of capacitor C1 is connected to the output terminal of the main circuit relay KM1, and the output terminal of capacitor C2 is connected to the negative terminal of the DC power supply. The input terminal of the inverter circuit is connected to the output terminal of the current detection circuit. The soft-start process includes the following steps: 1) The converter received a start-up command; 2) The converter control soft start relay KM2 closes; 3) The converter collects the starting circuit current through the current detection circuit to control the soft start relay KM2. After time T1, when the current reaches the set threshold, the soft start relay KM2 is turned off. 4) After an interval of time T2, the soft start relay KM2 is closed again, where T2 satisfies the following condition: T1 < T2 < 2T1; 5) Repeat steps 3) and 4). 6) The converter collects the bus voltage and determines the difference between the bus voltage and the DC side voltage. When the difference between the bus voltage and the DC side voltage meets the design threshold, the main circuit relay KM1 is closed and the soft start relay KM2 is opened, thereby completing the DC soft start process.

[0006] In one specific implementation, in step 6), the converter uses a current protection circuit to determine the current in the starting circuit to control the opening and closing of the soft-start relay KM2 in the starting circuit. It also controls the opening and closing of the main circuit relay KM1 by judging the difference between the bus voltage and the DC side voltage. The current protection circuit includes an operational amplifier, a comparator, two resistors R2 and R3, and a regulated power supply VCC. The output of the current sampling chip is connected to the non-inverting input of the operational amplifier, and the output of the reference voltage circuit is connected to the inverting input of the operational amplifier. One of the two resistors R2 is positioned between the output of the current sampling chip and the non-inverting input of the operational amplifier, and the other resistor R2 is positioned between the reference voltage circuit and the inverting input of the operational amplifier. One end of one resistor R3 is connected to the non-inverting input of the operational amplifier and the other end is grounded; the other resistor R3 is connected to the inverting input of the operational amplifier and the other end is connected to the output of the operational amplifier. The output of the operational amplifier is connected to the inverting input of the comparator, and the non-inverting input of the comparator is the designed current protection value V. OCP This refers to the current value corresponding to the maximum power of the selected NTC resistor. The output of the comparator is connected to the input of the converter, and the regulated power supply VCC is connected to the output of the operational amplifier. The specific judgment process is as follows: When the converter detects a low level at the comparator output, it controls the soft-start relay KM2 to turn off. When the converter detects that the comparator output is high, after time T2, the converter re-controls the soft-start relay KM2 to close. The formula for calculating current protection is as follows: , Among them, V IN V represents the output voltage of the current sampling chip. REF This indicates the reference voltage of the current sampling chip; V OUT This represents the output voltage of the operational amplifier; When V OUT >V OCP When this occurs, the comparator outputs a low level GND; When V OUT <V OCP When this occurs, the comparator outputs a high level VCC.

[0007] As one specific implementation, the inverter circuit adopts a three-bridge-arm topology.

[0008] Due to the application of the above technical solution, the present invention has the following advantages compared with the prior art: 1. The present invention proposes a method for soft starting a large-capacity DC bus, which sets a negative temperature coefficient thermistor NTC1 in the DC soft start circuit, and uses the relationship between the temperature and resistance of the thermistor NTC1 to control the starting impedance of the circuit, so as to ensure that the inrush current is within a controllable range during the soft start process. 2. This invention controls the relay switch by detecting the current in the DC soft start circuit, thus avoiding burning out the starting resistor in the initial charging state. Attached Figure Description

[0009] Figure 1 Design an electrical schematic diagram for a traditional DC soft starter circuit. Figure 2 This is a block diagram illustrating the electrical principle of the high-capacity DC bus soft-start method described in this invention. Figure 3 This is a timing diagram of the high-capacity DC bus soft-start method described in this invention. Figure 4 This is a circuit diagram of the current protection circuit in the large-capacity DC bus soft-start method described in this invention. Detailed Implementation

[0010] The following is in conjunction with the appendix Figure 2-4 The technical solution of the present invention will be further described in detail with reference to specific embodiments.

[0011] This invention proposes a soft-start method for large-capacity DC bus, applicable to large-capacity AC-CDC bidirectional converters.

[0012] See Figure 2 As shown, the converter includes a starting circuit, a DC bus capacitor, and an inverter circuit. The starting circuit is connected to the output terminal of the DC power supply and includes a main circuit relay KM1, a soft-start relay KM2, a thermistor NTC1, and a current detection circuit. The soft-start relay KM2, the thermistor NTC1, and the current detection circuit are connected in series and then connected in parallel with the main circuit relay KM1. The DC bus capacitor includes capacitors C1 and C2 connected in series. The input terminal of capacitor C1 is connected to the output terminal of the main circuit relay KM1, and the output terminal of capacitor C2 is connected to the negative terminal of the DC power supply. The input terminal of the inverter circuit is connected to the output terminal of the current detection circuit. In this example, the inverter circuit adopts a three-bridge topology.

[0013] The soft-start process of this converter specifically includes the following steps: 1) The converter received a start-up command; 2) The converter control soft start relay KM2 closes; 3) The converter collects the starting circuit current through the current detection circuit to control the soft start relay KM2. After time T1, when the current reaches the set threshold, the soft start relay KM2 is turned off. 4) After an interval of time T2, the soft start relay KM2 is closed again, where T2 satisfies the following condition: T1 < T2 < 2T1; 5) Repeat steps 3) and 4), see details below. Figure 3 As shown, T1 and T3 are the turn-on times of the soft start relay KM2 until the current threshold is reached; T2 is the turn-off time of the soft start relay KM2 after the first round of reaching the threshold current; T4 is the turn-on time of the soft start relay KM2 until the voltage threshold is reached, closing the main circuit relay KM1 and the soft start relay KM2. 6) The converter collects the bus voltage and determines the difference between the bus voltage and the DC side voltage. When the difference between the bus voltage and the DC side voltage meets the design threshold, the main circuit relay KM1 is closed and the soft start relay KM2 is opened, thereby completing the DC soft start process.

[0014] In step 6), the converter uses a current protection circuit to determine the current in the starting circuit to control the opening and closing of the soft-start relay KM2 in the starting circuit. It also controls the opening and closing of the main circuit relay KM1 by judging the difference between the bus voltage and the DC side voltage. The current protection circuit includes an operational amplifier, a comparator, two resistors R2 and R3, and a regulated power supply VCC. The output of the current sampling chip is connected to the non-inverting input of the operational amplifier, and the output of the reference voltage circuit is connected to the inverting input of the operational amplifier. One resistor R2 is placed between the output of the current sampling chip and the non-inverting input of the operational amplifier, and the other resistor R2 is placed between the reference voltage circuit and the inverting input of the operational amplifier. One end of one resistor R3 is connected to the non-inverting input of the operational amplifier and the other end is grounded; the other resistor R3 is connected to the inverting input of the operational amplifier and the other end is connected to the output of the operational amplifier. The output of the operational amplifier is connected to the inverting input of the comparator, and the non-inverting input of the comparator is the designed current protection value V. OCP This refers to the current value corresponding to the maximum power of the selected NTC resistor. The output of the comparator is connected to the input of the converter, and the regulated power supply VCC is connected to the output of the operational amplifier. The specific judgment process is as follows: When the converter detects a low level at the comparator output, it controls the soft-start relay KM2 to turn off. When the converter detects that the comparator output is high, after time T2, the converter re-controls the soft-start relay KM2 to close. The formula for calculating current protection is as follows: , Among them, V IN V represents the output voltage of the current sampling chip. REF This indicates the reference voltage of the current sampling chip; V OUT This represents the output voltage of the operational amplifier; When V OUT >V OCP When this occurs, the comparator outputs a low level GND; When V OUT <V OCP When this occurs, the comparator outputs a high level VCC.

[0015] This scheme utilizes the temperature characteristics of the thermistor NTC for charging in the DC soft start circuit. Specifically, in the DC soft start circuit, the soft start resistor is a negative temperature coefficient thermistor NTC1. By utilizing the relationship between the temperature and resistance of the NTC, the starting impedance of the circuit is controlled to ensure that the inrush current is within a controllable range during the soft start process.

[0016] Furthermore, this scheme controls the relay switching by detecting the current in the DC soft-start circuit. Specifically, since the converter's maximum voltage, bus capacity, and initial starting resistance of the starting circuit are known, the initial maximum current can be calculated using the voltage-current relationship across the resistance. This current is set as the maximum current value. When the current exceeds the set value, the soft-start relay KM2 is disconnected; when the current is less than the set value, the soft-start relay KM2 is closed after a time interval T2. This operation is repeated.

[0017] The above embodiments are only for illustrating the technical concept and features of the present invention, and are intended to enable those skilled in the art to understand the content of the present invention and implement it accordingly. They should not be construed as limiting the scope of protection of the present invention. All equivalent changes or modifications made in accordance with the spirit and essence of the present invention should be covered within the scope of protection of the present invention.

Claims

1. A method for soft-starting a large-capacity DC bus, applied to a large-capacity AC-CDC bidirectional converter, characterized in that, The converter includes a starting circuit, a DC bus capacitor, and an inverter circuit. The starting circuit is connected to the output terminal of the DC power supply and includes a main circuit relay KM1, a soft-start relay KM2, a thermistor NTC1, and a current detection circuit. The soft-start relay KM2, the thermistor NTC1, and the current detection circuit are connected in series and then in parallel with the main circuit relay KM1. The DC bus capacitor includes capacitors C1 and C2 connected in series. The input terminal of capacitor C1 is connected to the output terminal of the main circuit relay KM1, and the output terminal of capacitor C2 is connected to the negative terminal of the DC power supply. The input terminal of the inverter circuit is connected to the output terminal of the current detection circuit. The soft-start process includes the following steps: 1) The converter received a start-up command; 2) The converter control soft start relay KM2 closes; 3) The converter collects the starting circuit current through the current detection circuit to control the soft start relay KM2. After time T1, when the current reaches the set threshold, the soft start relay KM2 is turned off. 4) After an interval of time T2, the soft start relay KM2 is closed again, where T2 satisfies the following condition: T1 < T2 < 2T1; 5) Repeat steps 3) and 4). 6) The converter collects the bus voltage and determines the difference between the bus voltage and the DC side voltage. When the difference between the bus voltage and the DC side voltage meets the design threshold, the main circuit relay KM1 is closed and the soft start relay KM2 is opened, thereby completing the DC soft start process.

2. The method for soft-starting a large-capacity DC bus according to claim 1, characterized in that, In step 6), the converter uses a current protection circuit to determine the current in the starting circuit to control the opening and closing of the soft-start relay KM2 in the starting circuit. It controls the opening and closing of the main circuit relay KM1 by judging the difference between the bus voltage and the DC side voltage. The current protection circuit includes an operational amplifier, a comparator, two resistors R2 and R3, and a regulated power supply VCC. The output of the current sampling chip is connected to the non-inverting input of the operational amplifier, and the output of the reference voltage circuit is connected to the inverting input of the operational amplifier. One resistor R2 is placed between the output of the current sampling chip and the non-inverting input of the operational amplifier, and the other resistor R2 is placed between the reference voltage circuit and the inverting input of the operational amplifier. One end of one resistor R3 is connected to the non-inverting input of the operational amplifier and the other end is grounded; one end of the other resistor R3 is connected to the inverting input of the operational amplifier and the other end is connected to the output of the operational amplifier. The output of the operational amplifier is connected to the inverting input of the comparator, and the non-inverting input of the comparator is the designed current protection value V. OCP This refers to the current value corresponding to the maximum power of the selected NTC resistor. The output of the comparator is connected to the input of the converter, and the regulated power supply VCC is connected to the output of the operational amplifier. The specific judgment process is as follows: When the converter detects a low level output from the comparator, it controls the soft-start relay KM2 to turn off. When the converter detects that the comparator output is high, after time T2, the converter re-controls the soft-start relay KM2 to close. The formula for calculating current protection is as follows: ， Among them, V IN V represents the output voltage of the current sampling chip. REF This indicates the reference voltage of the current sampling chip; V OUT This represents the output voltage of the operational amplifier; When V OUT >V OCP When this occurs, the comparator outputs a low level GND; When V OUT <V OCP When this occurs, the comparator outputs a high level VCC.

3. The method for soft-starting a large-capacity DC bus according to claim 1, characterized in that, The inverter circuit adopts a three-bridge topology.

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