Low current stress non-isolated active back-end DC / DC converter device and control method

Abstract

The invention provides low-current stress non-isolated active back-end DC / DC converter equipment and a control method, and the equipment comprises a plurality of converter power modules which are connected in a manner that the front stages are connected in parallel and the rear stages are connected in series; each converter power module comprises a low-current stress non-isolation circuit and an active rear-end circuit; switching tubes of the low-current stress non-isolated circuit are jointly controlled by a controller, switching tubes of the active rear-end circuit are independently controlled, and a plurality of working modes exist in one period.

Description

Technical Field

[0001] The present invention belongs to the technical field of DC / DC conversion, and in particular relates to a low current stress non-isolated active back-end DC / DC converter and a control method. Background Art

[0002] The statements in this section merely provide background information related to the present invention and do not necessarily constitute prior art.

[0003] Offshore wind power has the lowest greenhouse gas emissions (GHG) among all energy technologies, and large-scale development and consumption of offshore wind power will play a key role in the process of zero carbon emissions. The demand for clean and low-carbon energy transformation, the increasing saturation of offshore wind power development, and the installation scenarios of ultra-large capacity wind turbines all require offshore wind power to move from nearshore to offshore.

[0004] At present, there are two schemes for offshore wind farms: AC collection-AC transmission and AC collection-DC transmission. The applicable conditions of the traditional AC collection-AC transmission scheme are usually less than 400MW and within 70km from the shore. Under the conditions of long-distance and large-capacity offshore wind power, the capacity increase effect of the submarine cable is serious, the reactive power is large, and it is necessary to install bulky and expensive reactors, so the AC transmission method is no longer applicable. Although the AC collection-DC transmission scheme meets the conditions for long-distance power transmission, the size and weight of the offshore platform are large, the cost is high, and it puts extremely high requirements on the construction of the offshore converter station platform; moreover, it is necessary to go through multiple levels of voltage conversion from the wind turbine side to the transmission side, resulting in a large power loss; at the same time, with the increase of wind turbine capacity, the distance between wind turbines increases, and the length of the AC collection network submarine cable increases significantly, which will inevitably bring about the problem of capacity increase and reactive power, and it is inevitable to install bulky and expensive reactors. Therefore, the existing solutions for the collection and transmission of large-scale offshore wind power are not ideal. The DC collection and DC transmission solution can solve the shortcomings of current technology and is the development trend for the collection of large offshore wind farms and long-distance transmission and grid connection in the future.

[0005] In the all-DC collection solution, medium-voltage / large-capacity DC / DC technology supports the development of high-power all-DC wind turbines. The DC / DC voltage boost control in the wind turbine cabin replaces the DC / AC inverter control of conventional wind turbines. It does not require grid-side voltage / frequency, and can solve the grid-side support problem faced by the inverter and grid control of the wind turbine side in the AC collection network. The all-DC wind turbine eliminates the need for the traditional direct-drive wind turbine to first invert the voltage and then boost it through the box transformer, which reduces the volume and weight of the wind turbine and tower, avoids multiple rectification, inversion and boost processes of electric energy, thereby reducing power loss and improving transmission efficiency. In addition, the medium-voltage DC collection cable has low cost, low loss, and no capacity increase and reactive power problem. Compared with the AC collection-DC transmission solution, only high-voltage DC / DC is required to raise the voltage on the transmission side, and no converter transformer is required, which greatly reduces the volume and weight of the offshore platform.

[0006] Therefore, the use of a full DC collection solution for offshore wind power can not only avoid the capacity increase and reactive power problems of long-distance AC submarine cables, but also eliminate the problems of grid control and multi-stage voltage conversion required for AC collection. It is an ideal solution for offshore wind power collection and transmission.

[0007] By summarizing and analyzing existing domestic and foreign literature, medium-voltage large-capacity DC / DC topologies are mainly divided into four categories: resonant boost, cascade boost, input-parallel-output-series (IPOS) boost, and MMC boost.

[0008] Among them, the cascade boost type is divided into two categories: switched capacitor boost type and triangle module boost type; the IPOS boost type can be roughly divided into single active bridge / dual active bridge (SAB / DAB) boost type, resonant soft switch boost type, auxiliary circuit boost type, active front end (AFE) boost type and a combination of boost solutions.

[0009] The resonant boost type uses the dead time of the switch tube drive to design the LC parallel resonant circuit, thereby achieving resonant high voltage gain, and at the same time achieving zero voltage conduction of the switch tube and zero current shutdown of the diode, but a single topology cannot meet the demand for medium-voltage DC / DC of offshore wind turbines. Some scholars have proposed to achieve high voltage gain through switch capacitor cascade boost. The absence of magnetic components reduces the weight of the converter, but the cascade of multiple modules in a single topology leads to excessive current stress in the front stage; some scholars have also proposed to improve the converter's boost capability and achieve equal power sharing between modules through triangular module cascade, and reduce voltage and current ripples by using staggered conduction schemes, but this type of topology does not have fault blocking capabilities.

[0010] The full-bridge MMC boost topology solves the problem of high voltage stress in the subsequent stage and has fault limiting capability, but requires a large number of sub-modules and their components.

[0011] The input parallel output series structure IPOS solves the problem of large input current and high output voltage, and has the characteristics of high modularity and high equipment utilization. The typical solution is the dual active bridge DAB, but its disadvantage is that it may cause DC voltage bias.

[0012] Resonant soft switching technology can realize the soft switching function of the switch tube, reduce switching losses and thus improve efficiency, but short-circuit faults and voltage drops will cause the DC capacitor to generate very large overcurrents, and it does not have fault blocking capabilities. The series resonant solid-state transformer realizes the soft switching transition capability of all semiconductor devices, reduces switching losses and improves efficiency. The auxiliary circuit series resonant topology can adjust the time when the auxiliary transformer participates in the boost, thereby adjusting the voltage gain. The semiconductor device works in a soft switching state, but does not have fault current limiting and blocking capabilities.

[0013] Some scholars have proposed the AFE active front end, which acts as a DC circuit breaker to effectively disconnect the medium-voltage DC side when a short-circuit fault occurs on the distribution side, thereby enhancing the power transmission capacity. However, the voltage boosting capacity of a single module is limited, and the large number of modules required leads to problems such as large size and weight, and high cost.

[0014] The full DC collection solution for offshore wind power can avoid the capacity increase and reactive power problem of long-distance AC submarine cables, and does not require the grid control and multi-level voltage conversion required for AC collection. It is an ideal solution for offshore wind power collection and transmission. Medium-voltage large-capacity DC / DC is the core equipment of the full DC collection solution.

[0015] In summary, the inventors have found that existing medium-voltage large-capacity DC / DCs have the following problems: (1) limited boost capability and room for efficiency improvement; (2) lack of fault blocking or fault current limiting capabilities; and (3) large size and weight. Summary of the invention

[0016] In order to overcome the deficiencies of the above-mentioned prior art, the present invention provides a low current stress non-isolated active back-end DC / DC converter. The designed topology requires a small number of series-parallel modules, uses few components, has fault blocking capability, has low device current stress, low cost, small size, and high soft switching efficiency.

[0017] To achieve the above objectives, one or more embodiments of the present invention provide the following technical solutions:

[0018] In a first aspect, a low current stress non-isolated active back-end DC / DC converter device is disclosed, comprising: a plurality of converter power modules, wherein the plurality of converter power modules are connected in a manner of front-stage parallel connection and rear-stage series connection;

[0019] Each of the converter power modules includes a low current stress non-isolated circuit and an active back-end circuit;

[0020] The switch tubes of the low current stress non-isolated circuit are jointly controlled by a controller, and the switch tubes of the active back-end circuit are individually controlled, and there are multiple working modes in one cycle.

[0021] The above adopts one controller for joint control: the control method of each low current stress circuit is consistent, and multiple low current stress circuits are controlled by one controller, and M low current stress circuits only need to be controlled by one controller.

[0022] Individual control: One controller controls one active back-end circuit. M active back-end circuits require M controllers to control.

[0023] As a further technical solution, the low current stress non-isolated circuit includes:

[0024] Low voltage side power supply;

[0025] A first inductor, a second inductor, and a third inductor;

[0026] a first switch tube, a second switch tube, a third switch tube, a fourth switch tube, a fifth switch tube, and a sixth switch tube;

[0027] A first capacitor, a second capacitor, and a third capacitor;

[0028] Wherein, the positive electrode of the first inductor and the positive electrode of the second inductor are connected to form a common end which is connected to the positive electrode of the low-voltage side power supply;

[0029] The negative electrode of the first inductor is connected to the common end of the positive electrode of the first switch tube and the negative electrode of the second switch tube;

[0030] The first switch tube cathode, the third switch tube cathode, the first capacitor cathode, the second capacitor cathode and the fifth switch tube cathode are connected to form a common end and connected to the low-voltage side power supply cathode;

[0031] The positive electrode of the second switch tube is connected to the common end of the positive electrode of the first capacitor, the positive electrode of the second capacitor, the positive electrode of the fourth switch tube, the negative electrode of the third capacitor and the positive electrode of the third inductor;

[0032] The positive electrode of the second inductor is connected to the common end of the positive electrode of the first inductor and the positive electrode of the low-voltage side power supply;

[0033] The negative electrode of the second inductor is connected to the common end of the positive electrode of the third switch tube and the negative electrode of the fourth switch tube;

[0034] The cathode of the third switch tube is connected to the common end of the cathode of the first switch tube, the cathode of the first capacitor, the cathode of the second capacitor, the cathode of the fifth switch tube and the cathode of the low-voltage side power supply;

[0035] The fourth switch tube positive electrode, the second capacitor positive electrode, the second switch tube positive electrode, the third capacitor negative electrode and the third inductor positive electrode are connected to form a common end connected to the first capacitor positive electrode.

[0036] The positive electrode of the third inductor is connected to the common end of the negative electrode of the third capacitor, the positive electrode of the first capacitor, the positive electrode of the second capacitor, the positive electrode of the second switch tube, and the positive electrode of the fourth switch tube;

[0037] The negative electrode of the third inductor is connected to the common end of the positive electrode of the fifth switch tube and the negative electrode of the sixth switch tube;

[0038] The negative electrode of the fifth switch tube, the negative electrode of the first switch tube, the negative electrode of the third switch tube, the negative electrode of the first capacitor, and the negative electrode of the second capacitor are connected to form a common end and connected to the negative electrode of the low-voltage side power supply;

[0039] The anode of the sixth switch tube is connected to the anode of the third capacitor.

[0040] As a further technical solution, the active back-end circuit includes: a seventh switch tube, an eighth switch tube, a ninth switch tube, and a tenth switch tube;

[0041] The anode of the seventh switch tube is connected to the anode of the third capacitor of the low current stress non-isolated circuit;

[0042] The cathode of the seventh switch tube is connected to the anode of the eighth switch tube;

[0043] The cathode of the eighth switch tube is connected to the anode of the ninth switch tube, the cathode of the third capacitor of the low current stress non-isolated circuit and the common end of the anode of the first capacitor;

[0044] The negative electrode of the ninth switch tube is connected to the positive electrode of the tenth switch tube, and the negative electrode of the tenth switch tube is connected to the negative electrode of the first capacitor of the low current stress non-isolated circuit.

[0045] As a further technical solution, the driving signals of the first switch tube, the third switch tube and the fifth switch tube of the low current stress non-isolated circuit are the same; the driving signals of the second switch tube, the fourth switch tube and the sixth switch tube are the same and are complementary to the former.

[0046] As a further technical solution, the working modes of the low current stress non-isolated active back-end DC / DC converter device in one cycle include:

[0047] At time t0, the second switch tube, the third switch tube and the sixth switch tube are disconnected, and the buffer capacitors connected in parallel at both ends of the switch tubes are charged and discharged respectively, the voltages at both ends of the second switch tube, the third switch tube and the sixth switch tube rise, and the voltages at both ends of the first switch tube, the fourth switch tube and the fifth switch tube fall;

[0048] When the voltages across the second switch tube, the third switch tube and the sixth switch tube rise to the switch tube voltage stress and the voltages across the first switch tube, the fourth switch tube and the fifth switch tube drop to zero, this mode ends.

[0049] As a further technical solution, the working mode also includes: at time t1, when the voltage across the first switch tube, the fourth switch tube and the fifth switch tube drops to zero, the body diodes of the switch tubes are turned on.

[0050] As a further technical solution, the working mode also includes: at time t2, the first switch tube, the fourth switch tube and the fifth switch tube are turned on at zero voltage and zero current; the low-voltage side power supply charges the first inductor / the second inductor, the second inductor transfers energy to the second capacitor, the first capacitor charges the third inductor, and the first capacitor and the third capacitor transfer energy to the subsequent active back-end circuit.

[0051] As a further technical solution, the working mode also includes: at time t3, the first switch tube, the fourth switch tube and the fifth switch tube are disconnected, the buffer capacitors connected in parallel at both ends of the switch tubes are charged and discharged respectively, the voltages at both ends of the first switch tube, the fourth switch tube and the fifth switch tube rise, and the voltages at both ends of the second switch tube, the third switch tube and the sixth switch tube drop; when the voltages at both ends of the first switch tube, the fourth switch tube and the fifth switch tube rise to the switch tube voltage stress, and at the same time the voltages at both ends of the second switch tube, the third switch tube and the sixth switch tube drop to zero, this mode ends.

[0052] As a further technical solution, the working mode also includes: at time t4, when the voltage across the second switch tube, the third switch tube and the sixth switch tube drops to zero, the body diode of the switch tube is turned on;

[0053] At time t5, the second switch tube, the third switch tube and the sixth switch tube are turned on at zero voltage and zero current. The low-voltage side power supply charges the first inductor and the second inductor, the first inductor transfers energy to the first capacitor, the third inductor transfers energy to the third capacitor, and the second capacitor discharges in this mode.

[0054] As a further technical solution, the working mode also includes: the seventh switch tube and the eighth switch tube of the active back-end circuit are complementary, the ninth switch tube and the tenth switch tube are complementary, and the circuit can adjust the voltage level to ensure that the output voltage matches the required voltage.

[0055] At time t0, the seventh switch tube and the tenth switch tube are turned on, the eighth switch tube and the ninth switch tube are turned off, and the first capacitor and the third capacitor are discharged;

[0056] At time t1, the tenth switch tube is turned off, the ninth switch tube is turned on, and the third capacitor is charged;

[0057] At time t2, the seventh switch tube and the tenth switch tube are turned on, the eighth switch tube and the ninth switch tube are turned off, and the first capacitor and the third capacitor are discharged;

[0058] At time t3, the eighth switch tube and the ninth switch tube are turned on, the seventh switch tube and the tenth switch tube are turned off, and the first capacitor is charged.

[0059] In a second aspect, a control method for a low current stress non-isolated active back-end DC / DC converter device is disclosed, comprising:

[0060] When a power module fails, after the failure is detected, the seventh and tenth switches of the faulty power module are locked; the eighth and ninth switches of the faulty power module remain continuously turned on, at which point the faulty power module is removed, and the switches of other power modules are still turned on alternately, and the number of power modules is reduced by one;

[0061] When the input voltage on the low-voltage side fluctuates and the voltage change is detected, the duty cycle of the switch tube of the subsequent active back-end circuit is controlled by feedback to adjust the output voltage on the medium-voltage side and maintain the stability of the output voltage;

[0062] When a fault occurs on the medium voltage side, the switch tubes of all subsequent active back-end circuits are locked, there is no DC path discharge current, no overcurrent, and it has the ability to block DC short-circuit faults.

[0063] One or more of the above technical solutions have the following beneficial effects:

[0064] In the DC / DC converter of the technical solution of the present invention, the power modules are connected in parallel at the front stage and in series at the rear stage. The front stage parallel input solves the problem of large input current, and the power module rear stage series connection solves the problem of large output voltage. In the low current stress non-isolated circuit, the capacitors in the traditional boost circuit are reconnected to form a new structure, and the two structures are cascaded together to increase the voltage level, which can reduce the number of input parallel and output series power modules, and reduce the volume, weight and configuration cost.

[0065] The DC / DC converter described in the technical solution of the present invention stacks a module at the front end, that is, a "capacitor-capacitor reconnection to form a new structure" module is cascaded to share the input current, so the current stress of the front-end components is reduced, the problem of large front-end current stress is solved, and the device has low current stress; soft switching of the switch tube is realized, and efficiency is improved.

[0066] The DC / DC converter of the technical solution of the present invention can still work normally through the voltage regulation capability of the active back-end circuit when some power modules fail or the voltage on the low-voltage side fluctuates, thereby improving the reliability of the system; when a medium-voltage side fails, all active back-end circuit switches are turned off, there is no DC path discharge current, no overcurrent, and the converter has the ability to block DC short-circuit faults.

[0067] Advantages of additional aspects of the present invention will be given in part in the following description, and in part will become obvious from the following description, or will be learned through practice of the present invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0068] The accompanying drawings in the specification, which constitute a part of the present invention, are used to provide a further understanding of the present invention. The exemplary embodiments of the present invention and their descriptions are used to explain the present invention and do not constitute improper limitations on the present invention.

[0069] Figure 1 A schematic diagram of the topology of a low current stress non-isolated active back-end DC / DC converter provided in Example 1 of the present invention, wherein (a) is an overall schematic diagram of the topology; (b) is a specific circuit diagram of the module;

[0070] Figure 2 A waveform diagram of key components of a low current stress non-isolated circuit of a low current stress non-isolated active back-end DC / DC converter provided in Example 1 of the present invention within one switching cycle;

[0071] Figure 3 A schematic diagram of the current path of the low current stress non-isolated circuit in the time period t0-t1 provided in Embodiment 1 of the present invention;

[0072] Figure 4 A schematic diagram of the current path of the low current stress non-isolated circuit in the time period t1-t2 provided in Example 1 of the present invention;

[0073] Figure 5 A schematic diagram of the current path in the time period t2-t3 of the low current stress non-isolated circuit provided in Embodiment 1 of the present invention;

[0074] Figure 6 A schematic diagram of the current path in the time period t3-t4 of the low current stress non-isolated circuit provided in Embodiment 1 of the present invention;

[0075] Figure 7 A schematic diagram of the current path in the time period t4-t5 of the low current stress non-isolated circuit provided in Embodiment 1 of the present invention;

[0076] Figure 8 A schematic diagram of the current path in the time period t5-t6 of the low current stress non-isolated circuit provided in Embodiment 1 of the present invention;

[0077] Fig. 9 Waveform diagram of key components of the active back-end circuit provided in Example 1 of the present invention in one switching cycle;

[0078] Fig.10 A schematic diagram of the current path of the active back-end circuit provided in Embodiment 1 of the present invention within a switching cycle;

[0079] Fig.11 The simulation waveform provided in Example 1 of the present invention is shown as follows: Figure 1 ;

[0080] Fig.12 The simulation waveform provided in Example 1 of the present invention is shown as follows: Figure 2 ;

[0081] Fig.13 A 2kW low-voltage experimental platform for a low-current stress non-isolated active back-end DC / DC converter provided in Example 2 of the present invention;

[0082] Fig.14 This is an experimental waveform diagram of a low current stress non-isolated active back-end DC / DC converter provided in Example 2 of the present invention. DETAILED DESCRIPTION

[0083] It should be noted that the following detailed descriptions are exemplary and are intended to provide further explanation of the present invention. Unless otherwise specified, all technical and scientific terms used herein have the same meanings as those commonly understood by those skilled in the art to which the present invention belongs.

[0084] It should be noted that the terms used herein are for describing specific embodiments only and are not intended to be limiting of exemplary embodiments according to the present invention.

[0085] In the absence of conflict, the embodiments of the present invention and the features of the embodiments may be combined with each other.

[0086] Embodiment 1

[0087] This embodiment discloses a low current stress non-isolated active back-end DC / DC converter device, the proposed topology structure is as follows Figure 1 As shown in the figure, the front parallel and rear series structure is composed of M low current stress non-isolated active back-end DC / DC converter (low current stress non-isolated active back-end converter, NIABE) power modules. Each NIABE power module is mainly composed of two parts: low current stress non-isolated converter (NI) and active back-end converter (ABE).

[0088] The sub-technical solution of this embodiment reconnects the capacitor of the traditional boost circuit to form a new structure, and cascades the two structures together to increase the voltage level. In order to solve the problem of large current stress in the front stage, a module is stacked at the front stage. The modular design reduces the design difficulty. The back stage adopts an active back end, and the output voltage can be adjusted by adjusting the duty cycle of the switch tube in the active back end circuit, and it has the ability to block DC short circuit faults. The designed converter uses an input parallel output series structure. The designed topology requires a small number of series-parallel modules, a small number of components used, has fault blocking capability, small device current stress, low cost, small size, and high soft switching efficiency.

[0089] The driving signals of the first switch tube, the third switch tube and the fifth switch tube of the low current stress non-isolated circuit are the same; the driving signals of the second switch tube, the fourth switch tube and the sixth switch tube are the same, and are complementary to the former. By swapping the driving signals of the switch tubes in module [1, 2], the staggered parallel connection of module [1, 1] and module [1, 2] can be realized, thereby achieving low ripple while reducing the input current. Low ripple can improve the stability of the equipment and improve the quality of electric energy. In order to improve the reliability of the circuit and avoid the upper and lower switch tubes from directly damaging the device, the dead time is set.

[0090] The driving signals of the switch tubes in module [1, 2] are swapped, specifically: the driving signals of the switch tubes S[1, 2] and Sa[1, 2] in module [1, 2] are swapped and connected in parallel: in one switching cycle, when the switch tube S[1, 1] is turned on, the inductor L[1, 1] is charged, the current of the inductor L[1, 1] increases, the inductor L[1, 2] is discharged, the current of the inductor L[1, 2] decreases, and the input current is equal to the sum of the inductor L[1, 1] and the inductor L[1, 2]; when S[1, 1] is turned off, the opposite is true. Therefore, the ripple of the input current is offset in one cycle, thereby achieving low ripple.

[0091] Low current stress in non-isolated circuits [1,1] Positive electrode and L [1,2] The positive electrode is connected to the common terminal and connected to the positive electrode of V1; L [1,1] Negative connection S [1,1] Positive electrode and S a[1,1] Negative common terminal; S [1,1] Negative electrode, S [1,2] Negative electrode, C [1,1] Negative electrode, C [1,2] Negative electrode and S [2,1] The negative pole is connected to the common terminal and connected to the negative pole of V1; S a[1,1] Positive connection C [1,1] Positive electrode, C [1,2] Positive electrode, S a[1,2] Positive electrode, C [2,1] Negative electrode and L [2,1] Positive common terminal.

[0092] L [1,2] Positive connection L [1,1] The common terminal of the positive electrode and the positive electrode of V1; L [1,2] Negative connection S [1,2] Positive electrode and S a[1,2] Negative common terminal; S [1,2] Negative connection S [1,1] Negative electrode, C [1,1] Negative electrode, C [1,2] Negative electrode, S [2,1] The common terminal of the negative electrode and the negative electrode of V1; S a[1,2] , C [1,2] Positive electrode, S a[1,1] Positive electrode, C [2,1] Negative electrode and L [2,1] The positive terminal is connected to the common terminal and C [1,1] Positive pole connected.

[0093] L [2,1] Positive connection C [2,1] Negative electrode, C [1,1] Positive electrode, C [1,2] Positive electrode, S a[1,1] Positive electrode, S a[1,2] Positive common terminal; L [2,1] Negative connection S [2,1] Positive electrode and S a[2,1] Negative common terminal; S [2,1] Negative electrode, S [1,1] Negative electrode, S [1,2] Negative electrode, C [1,1] Negative electrode, C [1,2] The negative pole is connected to the common terminal and connected to the negative pole of V1; S a[2,1] Positive connection C [2,1] positive electrode.

[0094] Active back-end circuit X1 positive connection low current stress non-isolated circuit C [2,1] Positive pole; negative pole of X1 is connected to positive pole of X2; negative pole of X2 is connected to positive pole of X3, C [2,1] Negative electrode and C [1,1] The common end of the positive pole; the negative pole of X3 is connected to the positive pole of X4, and the negative pole of X4 is connected to the low current stress non-isolated circuit C [1,1] negative electrode.

[0095] &1: Working principle of NI circuit

[0096] To simplify the theoretical analysis, the analysis of the embodiment is based on the following assumptions: (1) the switch tube is an ideal switch; (2) the passive component is linear, time-invariant, and frequency-independent; (3) the capacitor voltage remains constant during the switching cycle. The waveforms of the key components of the NI circuit in one cycle are as follows: Figure 2As shown, in order to improve circuit reliability and avoid direct conduction of upper and lower switches to damage the device, the dead time is set. There are 6 working modes in one cycle.

[0097] Mode 1 (t0-t1): At time t0, the switch S a[1,1] , S a[2,1] , S [1,2] The current loop is shown in the figure. The buffer capacitors connected in parallel at both ends of the switch tube are charged and discharged respectively. a[1,1] , S a[2,1] , S [1,2] The voltage across the two ends rises, S [1,1] , S [2,1] , S a[1,2] The voltage across the two ends drops, and when the switch tube S a[1,1] , S a[2,1] , S [1,2] The voltage across the terminals rises to V sa / V s , while S [1,1] , S [2,1] , S a[1,2] When the voltage across the terminals drops to 0, this mode ends.

[0098] Mode 2 (t1-t2): At t1, when the switch S [1,1] , S [2,1] , S a[1,2] When the voltage at both ends drops to 0, the body diode of the switch tube is turned on, and the current loop is as shown in the figure. The current flow path of this mode is as follows Figure 4 shown.

[0099] Mode 3 (t2-t3): At t2, the switch tube S [1,1] , S [2,1] , S a[1,2] It is turned on under ZVZCS, and the current loop is as follows Figure 4 As shown. Input power to the inductor L [1,1] , L [1,2] Charging, L [1,2] Transferring energy to C [1,2] , capacitor C [1,1] Give inductor L [2,1] Charging, capacitor C [1,1] and C [2,1] Transfer energy to the next stage.

[0100] Mode 4 (t3-t4): At t3, the switch tube S [1,1] , S [2,1] , S a[1,2] The current loop is shown in the figure. The buffer capacitors connected in parallel at both ends of the switch tube are charged and discharged respectively. [1,1] , S [2,1] , Sa[1,2] The voltage across the two ends rises, S a[1,1] , S a[2,1] , S [1,2] The voltage across the terminals drops. [1,1] , S [2,1] , S a[1,2] The voltage rises to V s / V sa , while S a[1,1] , S a[2,1] , S [1,2] When the voltage drops to 0, this mode ends. The current flow path in this mode is as follows Figure 6 shown.

[0101] Mode 5 (t4-t5): At time t4, when the switch S a[1,1] , S a[2,1] , S [1,2] When the voltage across the two ends drops to 0, the body diode of the switch tube is turned on, and the current loop is as follows: Figure 7 shown.

[0102] Mode 6 (t5-t6): At t5, the switch S a[1,1] , S a[2,1] , S [1,2] It is turned on under ZVZCS, and the current loop is as follows Figure 8 As shown. Input power to the inductor L [1,1] , L [1,2] Charging, inductance L [1,1] Transferring energy to C [1,1] , inductance L [2,1] Transferring energy to C [2,1] , capacitor C [1,2] Discharge.

[0103] How ABE works

[0104] The active back-end circuits X1 and X2 are complementary, and X3 and X4 are complementary. The circuit can adjust the voltage level to ensure that the output voltage matches the required voltage. When one of the NIABE module circuits fails, its ABE circuits X1 and X4 are turned off, and X2 and X3 remain continuously turned on. Then this NIABE circuit is disconnected, and capacitors C3 and C4 are not discharged. After that, the number of normal sub-modules changes from M to M-1. Due to the voltage regulation capability of the active back-end circuit, the system can still maintain normal operation. When the input voltage on the low-voltage side fluctuates, due to the voltage regulation capability of the active back-end circuit, the system can still operate normally. When a fault occurs on the MVDC side, turn off the switch tubes of all active back-end circuits, V C The voltage can remain unchanged, and after the fault is eliminated, it can be put into operation quickly. The waveform of the key components of ABE in one cycle of this embodiment is as follows Fig. 9 As shown, the current flow path of each mode is as follows Fig.10shown.

[0105] Mode I: At t0, X1 and X4 are turned on, X2 and X3 are turned off, and C [1,1] and C [2,1] Discharge.

[0106] Mode Ⅱ: At t1, X4 is disconnected, X3 is turned on, and C [2,1] .

[0107] Mode III: At time t2, X1 and X4 are turned on, X2 and X3 are turned off, and C [1,1] and C [2,1] Discharge.

[0108] Mode IV: At t3, X2 and X3 are turned on, X1 and X4 are turned off, and C [1,1] Charge.

[0109] Gain analysis

[0110] Define the switching period of the NI circuit as T S1 , the switching period of the ABE circuit is T S2 . So D is switch S [1,1] The duty cycle, D B is the duty cycle of switch X1.

[0111] To simplify the analysis process and considering the duration of each operating mode in one switching cycle, modes 1, 2, 4, and 5 can be ignored in the NI circuit.

[0112] In mode 3, we can get

[0113]

[0114] Among them, V L[1,1]-ON , V L[1,2]-ON and V L[2,1]-ON Switch S [1,1] Inductance L when conducting [1,1] , L [1,2] and L [2,1] The voltage across the two ends; V C[1,1] , V C[1,2] and V C[2,1] They are respectively the capacitor C [1,1] , C [1,2] and C [2,1] The voltage of C[1,1]-ON ,i C[1,2]-ON and i C[2,1]-ON Switch S [1,1] When conducting, the capacitance C [1,1] , C [1,2] and C [2,1] current; i1 is the input current.

[0115] In mode 6, we can get

[0116]

[0117] Among them, V L[1,1]-OFF , V L[1,2]-OFF and V L[2,1]-OFF Switch S [1,1] Inductance L when off [1,1] , L [1,2] and L [2,1] The voltage across the two ends; i C[1,1]-OFF ,i C[1,2]-OFF and i C[2,1]-OFF Switch S [1,1] Capacitor C when off [1,1] , C [1,2] and C [2,1] of current.

[0118] According to the inductance L [1,1] , L [1,2] and L [2,1] According to the volt-second balance principle, we can get

[0119]

[0120] According to the capacitance C [1,1] , C [1,2] and C [2,1] According to the ampere-second balance principle, we can get

[0121] In ABE circuit mode I or mode III, we can get

[0122] Mode II

[0123]

[0124] Mode IV

[0125]

[0126] according to Figure 1 The circuit diagram of the front stage in series and the back stage in parallel can be obtained

[0127] According to the volt-second balance principle of inductance L, we can get

[0128]

[0129] According to formulas (1)-(11), we can get

[0130]

[0131] The voltage across the switch can be derived as

[0132]

[0133] The current across the switch can be derived as

[0134]

[0135] Therefore, the voltage gain is

[0136]

[0137] From formula (17), we can see that the voltage gain is determined by the low current stress circuit switch duty cycle D and the active back-end circuit switch duty cycle D B It is determined by the combination of three degrees of freedom: the number of series and parallel power modules M.

[0138] When M = 1, the boost factor of a single series-parallel module is B = V M / V L for

[0139]

[0140] &3: Parameter design

[0141] Number of power modules

[0142] Since the active back-end circuit is a step-down type, the number of NIABE modules required is

[0143]

[0144] In order to ensure the reliability of the system, a redundant module is set up and M+2 redundancy is adopted.

[0145] Inductors and capacitors

[0146] Capacitor C [1,1] , C [1,2] and C [2,1] It can be calculated according to the capacitance calculation formula

[0147]

[0148] Among them, I C is the current on the capacitor, D is the duty cycle of the NI circuit, and f s is the switching frequency of the NI circuit, r C is the ripple factor of the inductor

[0149] The inductor current ripple can be expressed as

[0150]

[0151] Among them, V L-ON For the switch tube S [1,1] The voltage across an inductor in the on state.

[0152] To make the inductor current continuous, the inductance L must satisfy

[0153]

[0154] Among them, ΔI L =r L i L , r L is the inductor ripple factor.

[0155] Active Devices

[0156] The diode and capacitor can be selected based on the voltage and current stress.

[0157] The sub-technical scheme of this embodiment realizes high boost capability and high soft switching efficiency through the above scheme, reduces volume and weight, has fault blocking capability, and solves the problems existing in the existing medium voltage large-capacity DC / DC. Specifically, the capacitors of the traditional boost circuit are reconnected to form a new structure, and the two structures are cascaded together to increase the voltage level. In order to solve the problem of large current stress in the front stage, a module is stacked at the front stage. The modular design reduces the design difficulty. The back stage adopts an active back end, which can adjust the output voltage and has a DC short-circuit fault blocking capability. The designed converter uses an input parallel output series structure. The designed topology requires a small number of series-parallel modules, a small number of components used, has fault blocking capability, small device current stress, low cost, small size, and high soft switching efficiency.

[0158] The requirements for offshore wind power use scenarios are DC / DC converters with high boosting capability, high efficiency, small size and light weight, and fault blocking capability. The DC / DC designed in the present invention meets the requirements for DC / DC in use scenarios.

[0159] Embodiment 2

[0160] The purpose of this embodiment is to provide a control method for a low current stress non-isolated active back-end DC / DC converter device, including:

[0161] Case 1: At t = 0.4s, a power module fails. After the fault is detected, the seventh and tenth switches of the faulty power module are locked; the eighth and ninth switches of the faulty power module remain continuously turned on. At this time, the faulty power module is removed, and the switches of other power modules are still turned on alternately, and the number of power modules is reduced by one;

[0162] Case 2: At t = 0.4s, the input voltage on the low-voltage side fluctuates. After detecting the voltage change, feedback is used to control the duty cycle of the switch tube of the subsequent active back-end circuit to adjust the output voltage on the medium-voltage side and maintain the stability of the output voltage.

[0163] Case 3: When a fault occurs on the medium voltage side, the switch tubes of all subsequent active back-end circuits are locked, there is no DC path discharge current, no overcurrent, and it has the ability to block DC short-circuit faults.

[0164] Simulation Analysis

[0165] The medium voltage DC / DC requirement for wind turbines is 3kV-30~60kV, and the power is greater than 10MW. A 10MW model of 3kV to 60kV is built in MATLAB. The number of modules is designed, and the total gain is 20 times from 3kV to 60kV. For the convenience of design, the duty cycle D is set to 0.5, and the circuit NI is boosted to 4 times. The voltage is increased by the duty cycle D. B Since the active back-end circuit is a step-down type, M≥20 / 4=5, leaving a margin, take M=7, and establish a simulation model.

[0166] At 0.4s, a module fails. After the system detects the failure of this module, it will be removed. Due to the voltage regulation capability of the active back-end circuit, the system can still operate normally. The simulation results are as follows Fig.11 shown.

[0167] When the input voltage on the low voltage side fluctuates at 0.4s, the system detects the voltage change and controls the duty cycle of the active back-end circuit switch tube through feedback to adjust the output voltage on the medium voltage side and maintain the stability of the output voltage. The system can still maintain normal operation. Fig.12 shown.

[0168] A 2kW low-voltage experimental platform was built to verify the theoretical analysis of the present invention, including a NIABE circuit board, a drive circuit board, a control circuit board, and a program running on a controller, and the experimental platform achieved the steady-state analysis voltage gain and soft switching waveform effect as described in Example 1 of the present invention.

[0169] An experimental prototype with a ratio of 100V / 1060V and a power of 2kW was built in the laboratory. Fig.13 As shown in Figure 1, three NIABE modules are connected in series with parallel input and output. The input voltage is 100V, the load is 550Ω, and f S =10 kHz, f B =5k Hz, D = 0.5, D B =0.9. Voltage V C , output voltage V M The experimental waveform of voltage V2 is as follows: Fig.14As shown. It can be seen that the theoretical output voltage V M =

[0170] D B *M*V L / (1-D)2=0.9*3*100 / (1-0.5)2=1080V, experimental voltage gain V M =1060V is slightly less than the calculated theoretical value of 1080V due to the power loss in the parasitic components. Fig.14 Given a voltage V S[1,1] 、V Sa[1,1] and current i S[1,1] 、i Sa[1,1] The experimental waveform of the switch S [1,1] and S a[1,1] The switching loss is reduced when the device is turned on under ZVS condition. The experimental results verify the correctness of the theoretical analysis.

[0171] Those skilled in the art will appreciate that the embodiments of the present invention may be provided as methods or systems. Therefore, the present invention may take the form of hardware embodiments, software embodiments, or embodiments combining software and hardware.

[0172] Although the above describes the specific implementation mode of the present invention in conjunction with the accompanying drawings, it is not intended to limit the scope of protection of the present invention. Those skilled in the art should understand that various modifications or variations that can be made by those skilled in the art on the basis of the technical solution of the present invention without creative work are still within the scope of protection of the present invention.

Claims

1. Low current stress non-isolated active back-end DC / DC converter device, characterized by, include: A plurality of converter power modules, wherein the plurality of converter power modules are connected in parallel at the front stage and in series at the rear stage; Each of the converter power modules includes a low current stress non-isolated circuit and an active back-end circuit; The switch tubes of the low current stress non-isolated circuit are jointly controlled by a controller, and the switch tubes of the active back-end circuit are individually controlled, and there are multiple working modes in one cycle.

2. The low current stress non-isolated active back-end DC / DC converter device according to claim 1, characterized in that: The low current stress non-isolated circuit comprises: Low voltage side power supply; A first inductor, a second inductor, and a third inductor; A first switch tube, a second switch tube, a third switch tube, a fourth switch tube, a fifth switch tube, and a sixth switch tube; A first capacitor, a second capacitor, and a third capacitor; Wherein, the positive electrode of the first inductor and the positive electrode of the second inductor are connected to form a common end which is connected to the positive electrode of the low-voltage side power supply; The negative electrode of the first inductor is connected to the common end of the positive electrode of the first switch tube and the negative electrode of the second switch tube; The first switch tube cathode, the third switch tube cathode, the first capacitor cathode, the second capacitor cathode and the fifth switch tube cathode are connected to form a common end and connected to the low-voltage side power supply cathode; The positive electrode of the second switch tube is connected to the common end of the positive electrode of the first capacitor, the positive electrode of the second capacitor, the positive electrode of the fourth switch tube, the negative electrode of the third capacitor and the positive electrode of the third inductor; The positive electrode of the second inductor is connected to the common end of the positive electrode of the first inductor and the positive electrode of the low-voltage side power supply; The negative electrode of the second inductor is connected to the common end of the positive electrode of the third switch tube and the negative electrode of the fourth switch tube; The cathode of the third switch tube is connected to the common end of the cathode of the first switch tube, the cathode of the first capacitor, the cathode of the second capacitor, the cathode of the fifth switch tube and the cathode of the low-voltage side power supply; The fourth switch tube positive electrode, the second capacitor positive electrode, the second switch tube positive electrode, the third capacitor negative electrode and the third inductor positive electrode are connected to form a common end connected to the first capacitor positive electrode. The positive electrode of the third inductor is connected to the common end of the negative electrode of the third capacitor, the positive electrode of the first capacitor, the positive electrode of the second capacitor, the positive electrode of the second switch tube, and the positive electrode of the fourth switch tube; The negative electrode of the third inductor is connected to the common end of the positive electrode of the fifth switch tube and the negative electrode of the sixth switch tube; The negative electrode of the fifth switch tube, the negative electrode of the first switch tube, the negative electrode of the third switch tube, the negative electrode of the first capacitor, and the negative electrode of the second capacitor are connected to form a common end and connected to the negative electrode of the low-voltage side power supply; The anode of the sixth switch tube is connected to the anode of the third capacitor.

3. The low current stress non-isolated active back-end DC / DC converter device according to claim 1, characterized in that: The active back-end circuit includes: a seventh switch tube, an eighth switch tube, a ninth switch tube, and a tenth switch tube; The anode of the seventh switch tube is connected to the anode of the third capacitor of the low current stress non-isolated circuit; The cathode of the seventh switch tube is connected to the anode of the eighth switch tube; The cathode of the eighth switch tube is connected to the anode of the ninth switch tube, the cathode of the third capacitor of the low current stress non-isolated circuit and the common end of the anode of the first capacitor; The negative electrode of the ninth switch tube is connected to the positive electrode of the tenth switch tube, and the negative electrode of the tenth switch tube is connected to the negative electrode of the first capacitor of the low current stress non-isolated circuit.

4. The low current stress non-isolated active back-end DC / DC converter device according to claim 2, characterized in that: The driving signals of the first switch tube, the third switch tube and the fifth switch tube of the low current stress non-isolated circuit are the same; the driving signals of the second switch tube, the fourth switch tube and the sixth switch tube are the same and are complementary to the former.

5. The low current stress non-isolated active back-end DC / DC converter device according to claim 2, characterized in that: The working modes of the low current stress non-isolated active back-end DC / DC converter device in one cycle include: At time t0, the second switch tube, the third switch tube and the sixth switch tube are disconnected, and the buffer capacitors connected in parallel at both ends of the switch tubes are charged and discharged respectively, the voltages at both ends of the second switch tube, the third switch tube and the sixth switch tube rise, and the voltages at both ends of the first switch tube, the fourth switch tube and the fifth switch tube fall; When the voltages across the second switch tube, the third switch tube and the sixth switch tube rise to the switch tube voltage stress and the voltages across the first switch tube, the fourth switch tube and the fifth switch tube drop to zero, this mode ends.

6. The low current stress non-isolated active back-end DC / DC converter device according to claim 5, characterized in that: The working mode also includes: at time t1, when the voltages across the first switch tube, the fourth switch tube and the fifth switch tube drop to zero, the body diodes of the switch tubes are turned on.

7. The low current stress non-isolated active back-end DC / DC converter device according to claim 6, characterized in that: The working mode also includes: at time t2, the first switch tube, the fourth switch tube and the fifth switch tube are turned on at zero voltage and zero current; the low-voltage side power supply charges the first inductor / the second inductor, the second inductor transfers energy to the second capacitor, the first capacitor charges the third inductor, and the first capacitor and the third capacitor transfer energy to the subsequent active back-end circuit.

8. The low current stress non-isolated active back-end DC / DC converter device according to claim 7, characterized in that: The working mode also includes: at time t3, the first switch tube, the fourth switch tube and the fifth switch tube are disconnected, the buffer capacitors connected in parallel at both ends of the switch tubes are charged and discharged respectively, the voltages at both ends of the first switch tube, the fourth switch tube and the fifth switch tube rise, and the voltages at both ends of the second switch tube, the third switch tube and the sixth switch tube drop; when the voltages at both ends of the first switch tube, the fourth switch tube and the fifth switch tube rise to the switch tube voltage stress, and at the same time the voltages at both ends of the second switch tube, the third switch tube and the sixth switch tube drop to zero, this mode ends.

9. The low current stress non-isolated active back-end DC / DC converter device according to claim 7, characterized in that: The working mode also includes: at time t4, when the voltage across the second switch tube, the third switch tube and the sixth switch tube drops to zero, the body diode of the switch tube is turned on; At time t5, the second switch tube, the third switch tube and the sixth switch tube are turned on at zero voltage and zero current. The low-voltage side power supply charges the first inductor and the second inductor, the first inductor transfers energy to the first capacitor, the third inductor transfers energy to the third capacitor, and the second capacitor discharges in this mode; Preferably, the seventh switch tube and the eighth switch tube of the active back-end circuit are complementary, and the ninth switch tube and the tenth switch tube are complementary, and the circuit can adjust the voltage level to ensure that the output voltage matches the required voltage; Preferably, at time t0, the seventh switch tube and the tenth switch tube are turned on, the eighth switch tube and the ninth switch tube are turned off, and the first capacitor and the third capacitor are discharged; Preferably, at time t1, the tenth switch tube is turned off, the ninth switch tube is turned on, and the third capacitor is charged; Preferably, at time t2, the seventh switch tube and the tenth switch tube are turned on, the eighth switch tube and the ninth switch tube are turned off, and the first capacitor and the third capacitor are discharged; At time t3, the eighth switch tube and the ninth switch tube are turned on, the seventh switch tube and the tenth switch tube are turned off, and the first capacitor is charged.

10. The control method of the low current stress non-isolated active back-end DC / DC converter device according to any one of claims 1 to 9, characterized in that: include: A power module fails, and after the failure is detected, the seventh switch tube and the tenth switch tube of the failed power module are locked; The eighth and ninth switches of the faulty power module remain continuously turned on. At this time, the faulty power module is removed, and the switches of other power modules are still turned on alternately, and the number of power modules is reduced by one. When the input voltage on the low-voltage side fluctuates and the voltage change is detected, the duty cycle of the switch tube of the subsequent active back-end circuit is controlled by feedback to adjust the output voltage on the medium-voltage side and maintain the stability of the output voltage; When a fault occurs on the medium voltage side, the switch tubes of all subsequent active back-end circuits are locked, there is no DC path discharge current, no overcurrent, and it has the ability to block DC short-circuit faults.

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