A rectifier and a start-up control method thereof

Abstract

The application provides a rectifier and a starting control method thereof. The starting control method controls the operation of each phase bridge arm of the rectifier to adjust the voltage on the AC filter capacitor in the rectifier according to the AC grid voltage, thereby avoiding the impact of the AC grid voltage on the AC filter capacitor when the AC switch of the rectifier is subsequently controlled to be closed, avoiding the voltage shock of the AC filter capacitor and the overvoltage failure of the DC bus. Moreover, before controlling the operation of each phase bridge arm of the rectifier, the starting control method controls the operation of the second switch unit connected between the AC side of the bridge arm and the midpoint of the DC bus, discharges the electric energy on the AC filter capacitor to the DC bus capacitor, and avoids the problem that the electric energy remaining on the AC filter capacitor when the rectifier is lastly shut down causes the excessive AC inductor current in the voltage adjustment process during the restart.

Description

Technical Field

[0001] This application relates to the field of power electronics technology, and in particular to a rectifier and its startup control method. Background Technology

[0002] Currently, voltage-source PWM (Pulse Width Modulation) rectifiers, due to the characteristics of their topology, require a soft-start circuit to charge their DC bus capacitors during startup. However, after the soft start is complete, when the AC switch is closed, the AC grid voltage impacts the AC filter capacitor, which can easily cause voltage oscillations in the AC filter capacitor, leading to overvoltage faults on the rectifier's DC bus. Summary of the Invention

[0003] In view of this, this application provides a rectifier and its startup control method to avoid AC filter capacitor voltage oscillation and DC bus overvoltage faults that occur during startup in the prior art.

[0004] To achieve the above objectives, this application provides the following technical solution:

[0005] The first aspect of this application provides a startup control method for a rectifier, the rectifier including at least two phase arms; each arm includes: a first switching unit connected between the AC side of the arm and the positive and negative terminals of the DC bus of the rectifier, and a second switching unit connected between the AC side of the arm and the midpoint of the DC bus; the startup control method includes:

[0006] Control the operation of each of the second switching units to discharge the electrical energy on the AC filter capacitor in the rectifier to the DC bus capacitor of the rectifier;

[0007] Control the movement of each phase's bridge arm to adjust the voltage on the AC filter capacitor according to the AC grid voltage;

[0008] The AC switch of the rectifier is closed, and after the DC bus voltage meets the working conditions, the bridge arm of each phase is controlled to work normally.

[0009] Optionally, control the operation of each of the second switching units to discharge the electrical energy on the AC filter capacitor in the rectifier to the DC bus capacitor of the rectifier, including performing the following for each phase-to-phase capacitor in the AC filter capacitor:

[0010] Repeatedly control the second switching unit in each of the two phase arms corresponding to the interphase capacitor to simultaneously open and close until the absolute value of the voltage on the interphase capacitor is less than a preset threshold.

[0011] Optionally, the preset threshold is less than or equal to half of the rated DC bus voltage when the rectifier is operating normally.

[0012] Optionally, the bridge arm is a T-type three-level topology, and the process of controlling the second switching unit to disconnect includes: controlling all the switching transistors in the second switching unit to disconnect;

[0013] The process of controlling the closing of the second switching unit includes: controlling all the switching transistors in the second switching unit to close.

[0014] Optionally, the bridge arm is a type I three-level topology, and the process of controlling the second switching unit to disconnect includes: controlling all the switching transistors in the second switching unit that are connected to the AC side of the bridge arm to disconnect, and controlling all the switching transistors in the second switching unit that are connected to the midpoint of the DC bus to disconnect or close.

[0015] The process of controlling the closing of the second switching unit includes: controlling all the switching transistors connected to the AC side of the bridge arm within the second switching unit to close, and controlling all the switching transistors connected to the midpoint of the DC bus within the second switching unit to open or close.

[0016] Optionally, while controlling the operation of each of the second switching units, the method further includes:

[0017] Control each of the first switch units to disconnect.

[0018] Optionally, before controlling the movement of each phase's arm, the method further includes:

[0019] The rectifier's soft-start circuit is controlled to pre-charge the DC bus capacitor until the pre-charging completion condition is met.

[0020] Optionally, the pre-charge completion conditions include: the DC bus voltage is greater than or equal to a first threshold, and / or the pre-charge duration is greater than or equal to a first preset duration.

[0021] Optionally, the rectifier's soft-start circuit is controlled to pre-charge the DC bus capacitor until the pre-charging completion condition is met, including:

[0022] Control the soft-start switch in the soft-start circuit to close;

[0023] Determine whether the pre-charging completion condition is met;

[0024] If so, then the soft start switch is turned off.

[0025] Optionally, controlling the operation of each phase's bridge arm to adjust the voltage on the AC filter capacitor according to the AC mains voltage includes:

[0026] Phase-locked loop is applied to the AC grid voltage to obtain its frequency, amplitude, and phase.

[0027] Control the operation of the switching transistors in each of the bridge arms so that the voltage on the AC filter capacitor is in the same frequency and phase as the AC mains voltage, and the difference in amplitude is within a preset deviation range.

[0028] Block the drive signals for each switching transistor.

[0029] Optionally, the operating conditions include: the DC bus voltage is greater than or equal to a second threshold, and / or the closing duration of the AC switch is greater than or equal to a second preset duration.

[0030] Optionally, controlling the normal operation of each phase's bridge arm includes:

[0031] Control the operation of the switching transistors in each of the bridge arms to make the DC bus voltage the expected value.

[0032] A second aspect of this application provides a rectifier, comprising: a controller, an AC switch, an AC filter capacitor, an AC inductor, a DC bus capacitor, and at least two phase bridge arms; wherein,

[0033] One side of the AC switch is connected to the AC port of the rectifier;

[0034] On the other side of the AC switch, the AC filter capacitor is connected, and the AC side of each phase bridge arm is connected through the AC inductor.

[0035] The DC side of each phase's bridge arm is connected to the DC bus;

[0036] The bridge arm includes: a first switching unit connected between the AC side of the bridge arm and the positive and negative terminals of the DC bus, and a second switching unit connected between the AC side of the bridge arm and the midpoint of the DC bus;

[0037] The DC bus capacitor is connected between the positive and negative terminals of the DC bus, and the midpoint of the DC bus capacitor is used as the midpoint of the DC bus.

[0038] The positive and negative terminals of the DC bus are used to connect to the corresponding terminals of the DC port of the rectifier.

[0039] The AC switch, the soft-start circuit, and each phase arm are controlled by the controller, which is used to execute the rectifier start-up control method as described in any of the first aspects above.

[0040] Optionally, the bridge arm is a T-type three-level topology, and the first switching unit includes: a first switching transistor and a fourth switching transistor; the second switching unit includes: a second switching transistor and a third switching transistor; wherein,

[0041] The positive terminal of the DC bus is connected to the negative terminal of the DC bus in sequence through the first switch and the fourth switch;

[0042] The connection point between the first switch and the fourth switch serves as the AC side of the bridge arm.

[0043] The second and third switching transistors are connected in series in reverse between the AC side of the bridge arm and the midpoint of the DC bus.

[0044] Optionally, the bridge arm is a type I three-level topology, and the first switching unit includes: a first switch, a second switch, a third switch, and a fourth switch; the second switching unit includes: a second switch, the third switch, a fifth switch, and a sixth switch; wherein,

[0045] The positive terminal of the DC bus is connected to the negative terminal of the DC bus in sequence through the first switch, the second switch, the third switch, and the fourth switch.

[0046] The connection point between the second switch and the third switch serves as the AC side of the bridge arm;

[0047] The connection point of the first switch and the second switch is connected to the midpoint of the DC bus through the fifth switch;

[0048] The midpoint of the DC bus is connected to the connection point of the third and fourth switches via the sixth switch.

[0049] Optionally, the number of bridge arms is 2 or 3.

[0050] Optional features also include: a soft-start circuit;

[0051] The soft-start circuit is used to draw power from the AC port of the rectifier according to the control of the controller, and to precharge the DC bus capacitor.

[0052] Optionally, the soft-start circuit includes: a soft-start switch, a soft-start resistor, and an uncontrolled rectifier bridge;

[0053] The AC side of the uncontrolled rectifier bridge is connected to the AC port of the rectifier via the soft-start switch;

[0054] The soft-start switch is controlled by the controller;

[0055] The DC side of the uncontrolled rectifier bridge is used to connect to the DC bus;

[0056] The slow-start resistor is provided in at least one pole of the transmission branch between the AC side of the uncontrolled rectifier bridge and the AC port of the rectifier, or in at least one pole of the transmission branch between the DC side of the uncontrolled rectifier bridge and the DC bus.

[0057] Optionally, it also includes: a DC switch;

[0058] The DC switch is located between the DC bus and the DC port of the rectifier and is controlled by the controller.

[0059] The rectifier startup control method provided in this application controls the operation of each phase bridge arm of the rectifier to adjust the voltage on the AC filter capacitor in the rectifier according to the AC grid voltage. This avoids the impact of the AC grid voltage on the AC filter capacitor when the AC switch of the rectifier is closed, thus preventing AC filter capacitor voltage oscillation and DC bus overvoltage faults. Furthermore, after the AC switch is closed, the AC grid continues to charge the DC bus capacitor through the anti-parallel diodes of the switching transistors in each phase bridge arm. Once the DC bus voltage meets the operating conditions, each phase bridge arm can be controlled to operate normally, thereby completing the startup process of the rectifier. In addition, before controlling the operation of each phase bridge arm, this startup control method first controls the operation of the second switching unit connected between the AC side of the bridge arm and the midpoint of the DC bus, discharging the energy on the AC filter capacitor to the DC bus capacitor. This prevents residual energy on the AC filter capacitor from the previous shutdown from causing excessive AC inductor current during voltage regulation during restart. Attached Figure Description

[0060] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on the provided drawings without creative effort.

[0061] Figure 1 This is a schematic diagram of a single-phase topology of a rectifier provided in an embodiment of this application;

[0062] Figure 2 This is a topology diagram of a single-phase T-type three-level PWM rectifier provided in an embodiment of this application;

[0063] Figure 3 This is a topology diagram of a single-phase type I three-level PWM rectifier provided in an embodiment of this application;

[0064] Figure 4 A simplified single-phase three-level PWM rectifier topology provided for embodiments of this application;

[0065] Figure 5 A flowchart of a rectifier startup control method provided in an embodiment of this application. Detailed Implementation

[0066] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0067] In this application, the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such a process, method, article, or apparatus. Without further limitation, an element defined by the phrase "comprising one..." does not exclude the presence of other identical elements in the process, method, article, or apparatus that includes that element.

[0068] This application provides a rectifier startup control method to avoid AC filter capacitor voltage oscillation and DC bus overvoltage faults that occur during startup in the prior art.

[0069] The rectifier, such as Figure 1 The rectifier includes: a controller (not shown in the figure), a soft-start circuit 101, an AC switch SW1, an AC filter capacitor (including Cf shown in the figure), an AC inductor (including Lf shown in the figure), a DC bus capacitor (including C1 and C2 shown in the figure), and at least two phase arms (two phases are shown as an example in the figure). The AC port of the rectifier is connected to the AC side of each phase arm in sequence through the AC switch SW1, the AC filter capacitor, and the AC inductor. The DC side of each phase arm is connected to the DC bus, and the DC bus capacitors C1 and C2 are connected between the positive and negative terminals of the DC bus. The midpoint of the DC bus capacitors, which is also the midpoint of C1 and C2, is the midpoint of the DC bus.

[0070] In practical applications, the number of bridge arms in this rectifier can be... Figure 1 The two shown could also be three or more, meaning the rectifier can use a single-phase topology, or a three-phase or even more-phase topology. Furthermore, the AC filter capacitor includes capacitors connected between each phase, i.e., at least one interphase capacitor, such as... Figure 1In the single-phase topology shown, Cf represents the AC filter capacitor. In a three-phase topology, this AC filter capacitor can directly include three interphase capacitors, i.e., three capacitors connected between different phases: phase ab, phase bc, and phase ca; or it can include three phase capacitors: phase a, phase b, and phase c. Each phase capacitor is connected between its corresponding phase line and a common point. Then, every two phase capacitors are connected in series to form the corresponding interphase capacitor, i.e., phase a and phase b are connected in series to form phase ab, phase b and phase c are connected in series to form phase bc, and phase c and phase a are connected in series to form phase ca. Additionally, in a single-phase topology, this AC inductor can include only... Figure 1 The Lf shown is an example; in a three-phase topology, the AC inductor can include inductors respectively disposed in each phase branch. Further details on other phase topologies will not be elaborated upon, depending on the specific application environment.

[0071] The soft-start circuit 101 is powered by the AC port of the rectifier and is used to precharge the DC bus capacitors C1 and C2.

[0072] The AC switch SW1, the soft-start circuit 101, and each phase bridge arm are all controlled by the controller, which can execute the start-up control method for the rectifier.

[0073] If the startup control method includes the following steps:

[0074] Step 1: When the rectifier starts, close the soft start switch SW2. The AC grid precharges the DC bus capacitors C1 and C2 through the soft start resistor R and the uncontrolled rectifier bridge.

[0075] Step 2: Once the pre-charging completion conditions are met, disconnect the soft start switch SW2 and close the AC switch SW1;

[0076] Step 3: The AC power grid continues to charge the DC bus capacitor through the anti-parallel diodes of the switching transistors in each phase arm;

[0077] Step 4: Once the voltage across DC bus capacitors C1 and C2 meets the operating conditions, the switching transistors in each phase bridge arm will be controlled to operate normally.

[0078] The following problems will arise: When the soft-start switch SW2 is opened and the AC switch SW1 is closed, the AC mains and the AC filter capacitor Cf are directly connected in parallel. From the perspective of the AC mains, the sudden connection of a parallel capacitor means a sudden change in its load impedance, which will cause oscillations in the voltage of the AC filter capacitor. This oscillation will be transmitted to the DC bus capacitors C1 and C2 through the anti-parallel diodes of the switching transistors in each phase arm of the rectifier, causing the total voltage on the DC bus capacitors C1 and C2, i.e., the DC bus voltage u, to be affected. dc The sudden increase causes the DC bus voltage u of the rectifier to rise. dc An overvoltage fault has occurred.

[0079] To solve this problem, the voltage on the AC filter capacitor Cf can be processed before closing the AC switch SW1, ensuring that the voltage on the AC filter capacitor Cf is in the same frequency, amplitude, and phase as the mains voltage, with a slight deviation in amplitude allowed. That is, after disconnecting the soft-start switch SW2 in step two, the AC switch SW1 is not closed immediately; instead, the above four steps are modified as follows:

[0080] Step 1: When the rectifier starts, close the soft start switch SW2. The AC grid precharges the DC bus capacitors C1 and C2 through the soft start resistor R and the uncontrolled rectifier bridge.

[0081] Step 2: Once the pre-charging completion conditions are met, disconnect the soft start switch SW2;

[0082] Step 3: By controlling the switching transistors in each phase arm, ensure that the voltage of the AC filter capacitor Cf is in the same frequency, amplitude, and phase as the grid voltage. After this condition is met, close the AC switch SW1 and then block the drive signals for each switching transistor.

[0083] Step 4: The AC power grid continues to charge the DC bus capacitor through the anti-parallel diodes of the switching transistors in each phase arm;

[0084] Step 5: Once the voltage across DC bus capacitors C1 and C2 meets the operating conditions, the switching transistors in each phase bridge arm will be controlled to operate normally.

[0085] However, this startup control method still has the following problem: during frequent rectifier startups, there is a possibility that the AC filter capacitor Cf still has electrical energy / voltage from the previous rectifier shutdown. Therefore, when performing the newly added step three, there is a possibility that the AC filter capacitor Cf is directly short-circuited to the DC bus capacitors C1 and C2 through the AC inductor Lf, which will cause a current i on the AC inductor Lf. Lf Excessive current or even oscillation can trigger an inductor overcurrent fault.

[0086] Therefore, this embodiment provides a startup control scheme that includes AC filter capacitor discharge, which can not only avoid AC filter capacitor voltage oscillation and DC bus overvoltage faults that occur during startup in the prior art, but also suppress inductor inrush current during startup.

[0087] To explain this startup control method in detail, we will first introduce the topologies of common three-level PWM rectifiers, specifically two types: T-type three-level topology and I-type three-level topology. Taking a single-phase topology as an example, the topologies of the two types of three-level PWM rectifiers are as follows: Figure 2 and Figure 3As shown. Regardless of the topology used, the bridge arm in this rectifier includes: a first switching unit connected between the AC side of the bridge arm and the positive and negative terminals of the DC bus, and a second switching unit connected between the AC side of the bridge arm and the midpoint of the DC bus.

[0088] See Figure 2 When the bridge arm is a T-type three-level topology, the first switching unit specifically includes: a first switching transistor Sa1 and a fourth switching transistor Sa4; the second switching unit specifically includes: a second switching transistor Sa2 and a third switching transistor Sa3; wherein, the positive terminal of the DC bus is connected to the negative terminal of the DC bus in sequence through the first switching transistor Sa1 and the fourth switching transistor Sa4; the connection point of the first switching transistor Sa1 and the fourth switching transistor Sa4 serves as the AC side of the bridge arm; the second switching transistor Sa2 and the third switching transistor Sa3 are connected in reverse series between the AC side of the bridge arm and the midpoint of the DC bus.

[0089] See Figure 3 When the bridge arm is a type I three-level topology, the first switching unit specifically includes: a first switch Sa1, a second switch Sa2, a third switch Sa3, and a fourth switch Sa4; the second switching unit specifically includes: a second switch Sa2, a third switch Sa3, a fifth switch Sa5, and a sixth switch Sa6; wherein, the positive terminal of the DC bus is connected to the negative terminal of the DC bus in sequence through the first switch Sa1, the second switch Sa2, the third switch Sa3, and the fourth switch Sa4; the connection point of the second switch Sa2 and the third switch Sa3 serves as the AC side of the bridge arm; the connection point of the first switch Sa1 and the second switch Sa2 is connected to the midpoint of the DC bus through the fifth switch Sa5; the midpoint of the DC bus is connected to the connection point of the third switch Sa3 and the fourth switch Sa4 through the sixth switch Sa6.

[0090] Figure 2 and Figure 3 Both single-phase topologies shown can be simplified to Figure 4 The structure shown is illustrated below; the explanation will take phase a as an example. Figure 2 and Figure 4 The correspondence between them is as follows: Figure 4 The switching transistor Sa1 in the middle corresponds to Figure 2 Sa1 shown in the figure Figure 4 The switching transistor Sa4 in the middle corresponds to Figure 2 Sa4 shown Figure 4 The switch SaH in the middle is also the second switch unit mentioned above, corresponding to Figure 2 Sa2 and Sa3 are shown in the figure; Figure 3 and Figure 4 The correspondence between them is as follows: Figure 4 The switching transistor Sa1 in the middle corresponds to Figure 3 Sa1 and Sa2 shown in the figure Figure 4 The switching transistor Sa4 in the middle corresponds to Figure 3 Sa3 and Sa4 shown in the figure Figure 4 The switch SaH in the middle corresponds to Figure 3 The cases shown are Sa2, Sa3, Sa5, and Sa6. The case of phase b is similar and will not be described further.

[0091] This startup control method, such as Figure 5 As shown, it specifically includes:

[0092] S101: Control the operation of each second switch unit to discharge the electrical energy on the AC filter capacitor in the rectifier to the DC bus capacitor of the rectifier.

[0093] Figure 4 In the middle, the voltage u between points A and B on the AC side of the two bridge arms AB With DC bus voltage u dc Upper bus voltage u C1 and lower half bus voltage u C2 The relationships between them are shown in Table 1. In the table, switch state 1 represents on and 0 represents off.

[0094] Table 1. Relationship between switch on / off state and voltage between the AC sides of the two bridge arms.

[0095] 1 0 0 1 0 0 0 1 0 0 0 1 0 <![CDATA[u dc ]]> 1 0 0 0 0 1 <![CDATA[u C1 ]]> 0 1 0 1 0 0 <![CDATA[-u C1 ]]> 0 1 0 0 1 0 0 0 1 0 0 0 1 <![CDATA[u C2 ]]> 0 0 1 1 0 0 <![CDATA[-u dc ]]> 0 0 1 0 1 0 <![CDATA[-u C2 ]]> 0 0 1 0 0 1 0

[0096] Due to the presence of the anti-parallel diodes of switching transistors Sa1, Sa4, Sb1, and Sb4, when AC switch SW1 is open and in steady state, the voltage u across AC filter capacitor Cf is... Cf With DC bus voltage u dc The following relationship exists: |u Cf |≤u dc Therefore, the energy on the AC filter capacitor Cf cannot be directly discharged to the DC bus capacitors C1 and C2 through the corresponding anti-parallel diodes; however, when the rectifier is frequently started up, causing the AC filter capacitor Cf to still have energy when it was last shut down, due to |u Cf Since |≥0, a current path can be constructed for the AC filter capacitor Cf by controlling the operation of the two second switching units SaH and SbH. For example, controlling the two second switching units SaH and SbH to close allows the electrical energy on the AC filter capacitor Cf to flow through the AC inductor Lf; then controlling the two second switching units SaH and SbH to open allows the current i on the AC inductor Lf to flow through the AC inductor Lf. Lf The property that it cannot change abruptly makes the current i LfThe current flows through the anti-parallel diodes Sa1 and Sb4, or Sb1 and Sa4, to the DC bus capacitors C1 and C2, thereby enabling the AC filter capacitor Cf to discharge the DC bus capacitors C1 and C2. Specifically, the voltage u across the AC filter capacitor Cf... Cf With DC bus voltage u dc When the directions are opposite, that is Figure 4 u shown Cf When the value is negative, the discharge process described above can reduce its difference from the DC bus voltage u. dc The difference between them is used to avoid the current i in the AC inductor Lf in subsequent steps. Lf Excessive current or even oscillation can trigger an inductor overcurrent fault.

[0097] S102. Control the rectifier's soft-start circuit to precharge the DC bus capacitor until the precharge completion condition is met.

[0098] The pre-charge completion condition may specifically include: the DC bus voltage is greater than or equal to a first threshold, and / or the pre-charge duration is greater than or equal to a first preset duration; the specific values ​​of the first threshold and the first preset duration, as well as the specific selection of this condition, can be determined according to the specific application environment, and are not limited here.

[0099] In practical applications, S102 may specifically include: controlling the soft-start switch in the soft-start circuit (e.g., Figure 1 The SW2 switch shown in the figure is closed; it is determined whether the pre-charge completion condition is met; if so, the soft start switch is opened.

[0100] It should be noted that when the rectifier is frequently started, if there is still some energy on the DC bus capacitors C1 and C2 when it was last shut down, it will cause the DC bus voltage u to be affected. dc If the value is greater than the first threshold, then S102 can be omitted, that is, S103 can be executed directly after S101; there is no limitation here, it depends on the specific application environment, and all are within the protection scope of this application.

[0101] S103. Control the operation of each phase bridge arm to adjust the voltage on the AC filter capacitor according to the AC grid voltage.

[0102] Specifically, S103 may include:

[0103] (1) Phase-locked loop is applied to the AC grid voltage to obtain its frequency, amplitude and phase.

[0104] (2) Control the operation of the switching transistors in each bridge arm so that the voltage on the AC filter capacitor is in the same frequency and phase as the AC grid voltage, and the difference in amplitude is within the preset deviation range.

[0105] This step utilizes the electrical energy on the DC bus capacitor to charge the AC filter capacitor, making it the same as the AC grid voltage. This prevents the AC grid voltage from impacting the AC filter capacitor when the AC switch of the rectifier is closed, thus avoiding AC filter capacitor voltage oscillation and DC bus overvoltage faults.

[0106] The preset deviation range can be determined according to the actual application environment, as long as the above-mentioned impact can be avoided. There is no limitation here, and all of them are within the protection scope of this application.

[0107] (3) Block the drive signals for each switch transistor.

[0108] This step will put all the switching transistors in the off state.

[0109] S104. Control the AC switch of the rectifier to close, and after the DC bus voltage meets the working conditions, control each phase bridge arm to work normally.

[0110] Figures 1 to 4 After the AC switch SW1 shown is closed, the AC power grid will continue to charge the DC bus capacitors C1 and C2 through the anti-parallel diodes of the switching transistors in each phase arm of the bridge, until the DC bus voltage u dc If the operating conditions are met, the switching transistors in each phase bridge arm can be controlled to work normally, thereby completing the startup process of the rectifier.

[0111] The operating conditions may specifically include: the DC bus voltage being greater than or equal to the second threshold, and / or the closing time of the AC switch being greater than or equal to the second preset time; the specific values ​​of the second threshold and the second preset time, as well as the specific selection of the conditions, can be determined according to the specific application environment, and are not limited here.

[0112] The control of each phase bridge arm in S104 to operate normally can specifically include controlling the operation of the switching transistors in each bridge arm to ensure that the DC bus voltage is at the expected value. The specific value of this expected value can be determined according to the actual application environment and is not limited here.

[0113] The rectifier startup control method provided in this embodiment adjusts the voltage on the AC filter capacitor to be in the same frequency, phase, and amplitude as closely as possible to the AC grid voltage through S103. This avoids the impact of the AC grid voltage on the AC filter capacitor when the AC switch is closed subsequently, thereby preventing AC filter capacitor voltage oscillation and DC bus overvoltage faults. Furthermore, before S103, this startup control method also discharges the energy on the AC filter capacitor to the DC bus capacitor through S101, preventing residual energy on the AC filter capacitor from the previous shutdown from causing excessive AC inductor current during voltage regulation during restart.

[0114] Based on the previous embodiment, the startup control method provided in this embodiment gives some specific examples of the implementation process of its S101. For example, it may include the following for each phase-to-phase capacitor in the AC filter capacitor: repeatedly controlling each second switch unit in the two phase bridge arms corresponding to the phase-to-phase capacitor to open and close simultaneously until the absolute value of the voltage on the phase-to-phase capacitor is less than a preset threshold.

[0115] for Figures 2 to 4 The single-phase topology shown includes S101 specifically comprising: repeatedly controlling the second switching units SaH and SbH in the two-phase bridge arms corresponding to the AC filter capacitor Cf to open and close simultaneously until the absolute value of the voltage on the AC filter capacitor Cf is less than a preset threshold.

[0116] See Figure 4 The process specifically includes: Step 1, controlling the switches of SaH and SbH to be both open; Step 2, controlling the switches of SaH and SbH to be both closed; repeating steps 1 and 2 until |uCf| ≤ a preset threshold. In practical applications, this preset threshold can be set to ≤ 0.5*udcN; where udcN is the rated DC bus voltage of the rectifier when it is working normally, that is, the rated value of the DC bus voltage when the rectifier is working normally.

[0117] When the rectifier adopts a three-phase or even more-phase three-level topology, refer to Figure 2 and Figure 3 The topologies of the two single-phase three-level PWM rectifiers shown can be defined as follows: the bridge arms from the AC inductor to the DC bus capacitor are sequentially designated as phase a, phase b, phase c, ..., phase x, where x = c, d, e, ... (See reference) Figure 2 The topology diagram of the T-type single-phase three-level PWM rectifier is shown below. For the T-type multi-phase three-level topology, the switching transistor numbers in each phase arm are as follows: Figure 2 As shown, the vertical pipes are numbered 1 and 4 from top to bottom, and the horizontal pipes are numbered 2 and 3 from left to right. (Reference) Figure 3 The topology diagram of the Type I single-phase three-level PWM rectifier is shown below. For the Type I multi-phase three-level topology, the switching transistor numbers in each phase arm are as follows: Figure 3 As shown in the diagram, the outer tubes are numbered 1 and 4 from top to bottom, the inner tubes are numbered 2 and 3 from top to bottom, and the clamping tubes are numbered 5 and 6 from top to bottom.

[0118] At this point, for the AC filter capacitors in the multiphase topology rectifier, the ab phase capacitors can be discharged by repeatedly controlling the two second switching units in the a-phase and b-phase bridge arms to simultaneously open and close until the absolute value of the voltage on the ab phase capacitors is less than the preset threshold; the bc phase capacitors can be discharged by repeatedly controlling the two second switching units in the b-phase and c-phase bridge arms to simultaneously open and close until the absolute value of the voltage on the bc phase capacitors is less than the preset threshold; and so on, the mn phase capacitors can be discharged by repeatedly controlling the two second switching units in the m-phase and n-phase bridge arms to simultaneously open and close until the absolute value of the voltage on the mn phase capacitors is less than the preset threshold.

[0119] For single-phase, three-phase, or even multi-phase topologies:

[0120] When its bridge arm is a T-type three-level topology, the process of controlling the second switching unit to open specifically includes: controlling all switches within the second switching unit to open; the process of controlling the second switching unit to close specifically includes: controlling all switches within the second switching unit to close. Figure 2 Taking the single-phase topology shown above as an example, step 1 is specifically: the switching states of control switches Sa2, Sa3, Sb2, and Sb3 are all open; step 2 is specifically: the switching states of control switches Sa2, Sa3, Sb2, and Sb3 are all closed.

[0121] When the bridge arm is a type I three-level topology, the process of controlling the second switching unit to open specifically includes: controlling all switches connected to the AC side of the bridge arm within the second switching unit to open, and controlling all switches connected to the midpoint of the DC bus within the second switching unit to open or close; the process of controlling the second switching unit to close specifically includes: controlling all switches connected to the AC side of the bridge arm within the second switching unit to close, and controlling all switches connected to the midpoint of the DC bus within the second switching unit to open or close. Figure 3 Taking the single-phase topology shown as an example, step 1 above specifically involves controlling the switching states of switches Sa2, Sa3, Sb2, and Sb3 to be open, while the switching states of switches Sa5, Sa6, Sb5, and Sb6 can be either open or closed. Step 2 above specifically involves controlling the switching states of switches Sa2, Sa3, Sb2, and Sb3 to be closed, while the switching states of switches Sa5, Sa6, Sb5, and Sb6 can be either open or closed.

[0122] In practical applications, the steps executed by S101 on the inter-phase capacitors in the AC filter capacitor may further include: simultaneously controlling the first switching unit to open while controlling the operation of each second switching unit. Figures 2 to 4Taking the single-phase topology shown as an example, while performing steps 1 and 2 above on the interphase capacitors in the AC filter capacitor, the switching transistors Sa1, Sa4, Sb1, and Sb4 are all kept off.

[0123] The startup control method provided in this embodiment can achieve the discharge function of the AC filter capacitor without adding additional hardware circuitry, transferring the energy on the AC filter capacitor to the DC bus capacitor. This facilitates the control of the AC filter capacitor voltage to be in sync with the grid voltage in terms of frequency, amplitude, and phase when the AC switch of the PWM rectifier is not closed, resulting in a smaller inductor current surge and preventing inductor current overcurrent.

[0124] Another embodiment of this application also provides a rectifier, such as Figure 1 The diagram shows: a controller (not shown), a soft-start circuit 101, an AC switch SW1, an AC filter capacitor (including Cf shown in the diagram), an AC inductor (including Lf shown in the diagram), a DC bus capacitor (including C1 and C2 shown in the diagram), and at least two phase bridge arms (two phases are shown as an example in the diagram); wherein:

[0125] One side of the AC switch SW1 is connected to the AC port of the rectifier; the AC switch SW1 can be a relay, circuit breaker, contactor, etc., which is not limited here.

[0126] On the other side of the AC switch SW1, an AC filter capacitor is connected, and it is connected to the AC side of each phase bridge arm through an AC inductor. In practical applications, the specific configuration of the AC filter capacitor and the AC inductor can be found in the above embodiment, and will not be repeated here.

[0127] The DC side of each phase bridge arm is connected to the DC bus; the DC bus capacitor is connected between the positive and negative terminals of the DC bus, and the midpoint of the DC bus capacitor, that is, the midpoint between C1 and C2, is used as the midpoint of the DC bus.

[0128] The bridge arm includes: a first switching unit connected between the AC side of the bridge arm and the positive and negative poles of the DC bus, and a second switching unit connected between the AC side of the bridge arm and the midpoint of the DC bus.

[0129] When the bridge arm is a T-type three-level topology, see [link to relevant documentation]. Figure 2 The single-phase topology shown includes a first switching unit comprising a first switching transistor Sa1 and a fourth switching transistor Sa4; and a second switching unit comprising a second switching transistor Sa2 and a third switching transistor Sa3. The positive terminal of the DC bus is connected to the negative terminal of the DC bus via the first switching transistor Sa1 and the fourth switching transistor Sa4 in sequence. The connection point of the first switching transistor Sa1 and the fourth switching transistor Sa4 serves as the AC side of the bridge arm. The second switching transistor Sa2 and the third switching transistor Sa3 are connected in series in reverse between the AC side of the bridge arm and the midpoint of the DC bus.

[0130] When the bridge arm is a type I three-level topology, see Figure 3 The single-phase topology shown includes the following components: the first switching unit comprises a first switch Sa1, a second switch Sa2, a third switch Sa3, and a fourth switch Sa4; the second switching unit comprises a second switch Sa2, a third switch Sa3, a fifth switch Sa5, and a sixth switch Sa6; the positive terminal of the DC bus is connected to the negative terminal of the DC bus via the first switch Sa1, the second switch Sa2, the third switch Sa3, and the fourth switch Sa4 in sequence; the connection point of the second switch Sa2 and the third switch Sa3 serves as the AC side of the bridge arm; the connection point of the first switch Sa1 and the second switch Sa2 is connected to the midpoint of the DC bus via the fifth switch Sa5; and the midpoint of the DC bus is connected to the connection point of the third switch Sa3 and the fourth switch Sa4 via the sixth switch Sa6.

[0131] Each of the aforementioned switching transistors can be any type of fully controllable semiconductor device, and can be equipped with a corresponding anti-parallel diode or can achieve the anti-parallel diode function with a body diode. Under PWM control, the rectifier is a PWM rectifier, and all optional devices are within the protection scope of this application.

[0132] The positive and negative terminals of this DC bus are used to connect to the corresponding terminals of the rectifier's DC port. Specifically, this can be achieved through... Figure 1 The DC switch SW3 shown in the diagram enables this connection.

[0133] The soft-start circuit 101 is used to draw power from the AC port of the rectifier to pre-charge the DC bus capacitor. Specifically, as shown... Figure 1 As shown, the soft-start circuit 101 includes: a soft-start switch SW2, a soft-start resistor R, and an uncontrolled rectifier bridge; wherein, the AC side of the uncontrolled rectifier bridge is connected to the AC port of the rectifier via the soft-start switch SW2; the soft-start switch SW2 is controlled by a controller; the DC side of the uncontrolled rectifier bridge is used to connect to the DC bus; in at least one transmission branch between the AC side of the uncontrolled rectifier bridge and the AC port of the rectifier (e.g., Figure 1 As shown in the diagram, or in at least one transmission branch between the DC side of the uncontrolled rectifier bridge and the DC bus (not shown), a soft-start resistor R is provided. In practical applications, depending on the specific application environment, this soft-start circuit 101 may also be omitted, all of which are within the protection scope of this application.

[0134] The AC switch SW1, the soft-start circuit 101, the DC switch SW3, and each phase bridge arm are all controlled by a controller, which executes the rectifier startup control method as described in any of the above embodiments. The specific process and principle of this startup control method can be found in the above embodiments and will not be repeated here. Before the rectifier starts, the AC switch SW1, the soft-start switch SW2, and the DC switch SW3 are all in the open state.

[0135] It should be noted that, Figures 1 to 4 The examples shown all use a single-phase topology with two bridge arms. In practical applications, the rectifier can have three or more bridge arms. This startup control method can be extended to multi-phase (≥3 phases) three-level topologies, where each phase bridge arm can be a T-type or I-type three-level topology, as illustrated in the examples above. Further details are omitted here.

[0136] Similar or identical parts between the various embodiments in this specification can be referred to mutually. Each embodiment focuses on describing the differences from other embodiments. In particular, for system or system embodiments, since they are basically similar to method embodiments, the description is relatively simple, and relevant parts can be referred to the description of the method embodiments. The systems and system embodiments described above are merely illustrative. The units described as separate components may or may not be physically separate. The components shown as units may or may not be physical units, that is, they may be located in one place or distributed across multiple network units. Some or all of the modules can be selected to achieve the purpose of this embodiment solution according to actual needs. Those skilled in the art can understand and implement this without creative effort.

[0137] Those skilled in the art will further recognize that the units and algorithm steps of the various examples described in conjunction with the embodiments disclosed herein can be implemented in electronic hardware, computer software, or a combination of both. To clearly illustrate the interchangeability of hardware and software, the components and steps of the various examples have been generally described in terms of functionality in the foregoing description. Whether these functions are implemented in hardware or software depends on the specific application and design constraints of the technical solution. Those skilled in the art can use different methods to implement the described functions for each specific application, but such implementations should not be considered beyond the scope of this invention.

[0138] The features described above in the disclosed embodiments can be substituted for or combined with each other to enable those skilled in the art to implement or use the invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein can be implemented in other embodiments without departing from the spirit or scope of the invention. Therefore, the invention is not to be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A rectifier start-up control method, characterized in that, The rectifier includes at least two phase arms; each arm includes: a first switching unit connected between the AC side of the arm and the positive and negative terminals of the DC bus of the rectifier, and a second switching unit connected between the AC side of the arm and the midpoint of the DC bus; the start-up control method includes: Control the operation of each of the second switching units to discharge the electrical energy on the AC filter capacitor in the rectifier to the DC bus capacitor of the rectifier; Phase-locked loop is applied to the AC grid voltage to obtain its frequency, amplitude, and phase. Control the operation of the switching transistors in each of the bridge arms so that the voltage on the AC filter capacitor is in the same frequency and phase as the AC mains voltage, and the difference in amplitude is within a preset deviation range. Block the drive signals for each switching transistor; The AC switch of the rectifier is closed, and after the DC bus voltage meets the working conditions, the bridge arm of each phase is controlled to work normally. Specifically, controlling the operation of each of the second switching units to discharge the electrical energy on the AC filter capacitors in the rectifier to the DC bus capacitors of the rectifier includes the following steps performed on the inter-phase capacitors of the AC filter capacitors: Repeatedly control the second switching unit in each of the two phase arms corresponding to the interphase capacitor to simultaneously open and close until the absolute value of the voltage on the interphase capacitor is less than a preset threshold.

2. The rectifier start-up control method according to claim 1, characterized in that, The preset threshold is less than or equal to half of the rated DC bus voltage when the rectifier is operating normally.

3. The rectifier start-up control method according to claim 1, characterized in that, The bridge arm is a T-type three-level topology. The process of controlling the second switching unit to disconnect includes: controlling all the switching transistors in the second switching unit to disconnect. The process of controlling the closing of the second switching unit includes: controlling all the switching transistors in the second switching unit to close.

4. The rectifier start-up control method according to claim 1, characterized in that, The bridge arm is a type I three-level topology. The process of controlling the second switching unit to disconnect includes: controlling all the switching transistors in the second switching unit that are connected to the AC side of the bridge arm to disconnect, and controlling all the switching transistors in the second switching unit that are connected to the midpoint of the DC bus to disconnect or close. The process of controlling the closing of the second switching unit includes: controlling all the switching transistors connected to the AC side of the bridge arm within the second switching unit to close, and controlling all the switching transistors connected to the midpoint of the DC bus within the second switching unit to open or close.

5. The rectifier start-up control method according to claim 1, characterized in that, In addition to controlling the operation of each of the second switching units, it also includes: Control each of the first switch units to disconnect.

6. The rectifier start-up control method according to any one of claims 1 to 5, characterized in that, Before controlling the movement of each phase's arm, the following is also included: The rectifier's soft-start circuit is controlled to pre-charge the DC bus capacitor until the pre-charging completion condition is met.

7. The rectifier start-up control method according to claim 6, characterized in that, The pre-charging completion conditions include: the DC bus voltage is greater than or equal to a first threshold, and / or the pre-charging duration is greater than or equal to a first preset duration.

8. The rectifier start-up control method according to claim 6, characterized in that, The rectifier's soft-start circuit is controlled to pre-charge the DC bus capacitor until the pre-charging completion condition is met, including: Control the soft-start switch in the soft-start circuit to close; Determine whether the pre-charging completion condition is met; If so, then the soft start switch is turned off.

9. The rectifier start-up control method according to any one of claims 1 to 5, characterized in that, The operating conditions include: the DC bus voltage is greater than or equal to the second threshold, and / or the closing time of the AC switch is greater than or equal to the second preset time.

10. The rectifier start-up control method according to any one of claims 1 to 5, characterized in that, Controlling the normal operation of each phase's bridge arm includes: Control the operation of the switching transistors in each of the bridge arms to make the DC bus voltage reach the expected value.

11. A rectifier, characterized in that, include: The system includes a controller, AC switch, AC filter capacitor, AC inductor, DC bus capacitor, and at least two phase bridge arms; among which, One side of the AC switch is connected to the AC port of the rectifier; On the other side of the AC switch, the AC filter capacitor is connected, and the AC side of each phase bridge arm is connected through the AC inductor. The DC side of each phase's bridge arm is connected to the DC bus; The bridge arm includes: a first switching unit connected between the AC side of the bridge arm and the positive and negative terminals of the DC bus, and a second switching unit connected between the AC side of the bridge arm and the midpoint of the DC bus; The DC bus capacitor is connected between the positive and negative terminals of the DC bus, and the midpoint of the DC bus capacitor is used as the midpoint of the DC bus. The positive and negative terminals of the DC bus are used to connect to the corresponding terminals of the DC port of the rectifier. The AC switch and each phase arm are controlled by the controller, which is used to execute the rectifier start-up control method as described in any one of claims 1 to 10.

12. The rectifier according to claim 11, characterized in that, The bridge arm is a T-type three-level topology. The first switching unit includes a first switching transistor and a fourth switching transistor; the second switching unit includes a second switching transistor and a third switching transistor. The positive terminal of the DC bus is connected to the negative terminal of the DC bus in sequence through the first switch and the fourth switch; The connection point between the first switch and the fourth switch serves as the AC side of the bridge arm. The second and third switching transistors are connected in series in reverse between the AC side of the bridge arm and the midpoint of the DC bus.

13. The rectifier according to claim 11, characterized in that, The bridge arm is a type I three-level topology. The first switching unit includes: a first switch, a second switch, a third switch, and a fourth switch; the second switching unit includes: a second switch, the third switch, a fifth switch, and a sixth switch; wherein... The positive terminal of the DC bus is connected to the negative terminal of the DC bus in sequence through the first switch, the second switch, the third switch, and the fourth switch. The connection point between the second switch and the third switch serves as the AC side of the bridge arm; The connection point of the first switch and the second switch is connected to the midpoint of the DC bus through the fifth switch; The midpoint of the DC bus is connected to the connection point of the third and fourth switches via the sixth switch.

14. The rectifier according to any one of claims 11 to 13, characterized in that, The number of bridge arms is 2 or 3.

15. The rectifier according to any one of claims 11 to 13, characterized in that, Also includes: Soft-start circuit; The soft-start circuit is used to draw power from the AC port of the rectifier according to the control of the controller, and to precharge the DC bus capacitor.

16. The rectifier according to claim 15, characterized in that, The soft-start circuit includes: a soft-start switch, a soft-start resistor, and an uncontrolled rectifier bridge; The AC side of the uncontrolled rectifier bridge is connected to the AC port of the rectifier via the soft-start switch; The soft-start switch is controlled by the controller; The DC side of the uncontrolled rectifier bridge is used to connect to the DC bus; The slow-start resistor is provided in at least one pole of the transmission branch between the AC side of the uncontrolled rectifier bridge and the AC port of the rectifier, or in at least one pole of the transmission branch between the DC side of the uncontrolled rectifier bridge and the DC bus.

17. The rectifier according to any one of claims 11 to 13, characterized in that, Also includes: DC switch; The DC switch is located between the DC bus and the DC port of the rectifier and is controlled by the controller.

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