Novel AC / DC converter and bridge arm overcurrent prediction and shutdown method

Abstract

The invention provides a novel AC / DC converter and a bridge arm overcurrent prediction and shutdown method, specifically, the novel AC / DC converter comprises a switch valve and a first filtering branch, two ends of the switch valve are respectively connected with a direct current positive bus and a direct current negative bus, the direct current positive bus is connected with a direct current inductor L1 and a direct current inductor L2 in series, and the direct current inductor L2 is connected with the direct current negative bus. The direct-current negative bus is connected in series with direct-current inductors L3 and L4; two ends of the filtering branch are respectively connected to a connection point between the L1 and the L2 and a connection point between the L3 and the L4; the switch valve comprises three switch valve units which are connected in series, and each switch valve unit comprises a switch capacitance valve string and two novel switch valve strings which are connected in parallel. According to the invention, the converter can be controlled to be protectively shut down according to a set method under the condition that overcurrent is predicted.

Description

Technical Field

[0001] The invention belongs to the technical field of power systems, and in particular relates to a novel AC / DC converter and a bridge arm overcurrent prediction and shutdown method. Background Art

[0002] As my country's power grid transforms from a thermal power grid to a green and clean power grid dominated by renewable energy, the penetration rate of renewable energy is increasing. Traditional AC power grids are gradually showing problems in stability and loss under the condition of large-scale renewable energy access. Flexible DC transmission is an effective means for large-scale grid connection of renewable energy. There are many existing flexible DC transmission converter topologies. The topology represented by modular multilevel converter (MMC) has been widely used due to its modular structure, which is conducive to the expansion of voltage and capacity. However, MMC is based on a half-bridge double star-connected circuit, with a large number of modules, huge size, large footprint and high cost, which is the main factor hindering the application of flexible DC transmission, especially in offshore wind power applications.

[0003] The existing new AC / DC converter topology can greatly reduce module capacitance compared to MMC, and significantly reduce cost and volume, especially for offshore wind power DC transmission applications. The cost per unit area of ​​offshore platforms is very high. The new AC / DC converter can significantly reduce the floor space, thereby greatly reducing the area of ​​offshore platforms, thereby significantly reducing the construction cost of offshore wind power, and has good application prospects.

[0004] However, the existing new AC / DC converter topology is significantly different from the traditional flexible DC converter. There is a risk of overcurrent in the switching capacitor valve when the new switching valve is switching state or shut down and locked. Once the overcurrent occurs, the capacitor valve will be damaged.

[0005] Therefore, it is necessary to design a new AC / DC converter and bridge arm overcurrent prediction and shutdown method to solve the above technical problems. Summary of the invention

[0006] In view of the above technical problems, the present invention provides a novel AC / DC converter, wherein the converter comprises a switch valve and a first filter branch, wherein:

[0007] The two ends of the switch valve are respectively connected to a DC positive bus and a DC negative bus, wherein the DC positive bus is connected in series with DC inductors L1 and L2, and the DC negative bus is connected in series with DC inductors L3 and L4; the two ends of the filter branch are respectively connected to the connection point between L1 and L2, and the connection point between L3 and L4;

[0008] The switch valve comprises three switch valve units connected in series, the switch valve units comprise a switch capacitor valve string connected in parallel and two novel switch valve strings, the switch capacitor valve string comprises a plurality of switch capacitor modules connected in series; the novel switch valve string comprises an upper bridge arm and a lower bridge arm connected by a connection midpoint, the upper bridge arm and the lower bridge arm both comprise a power module;

[0009] The two connection midpoints of each switch valve unit are connected to a second filter branch.

[0010] Furthermore, one end of L1 is connected to the positive terminal of the DC port of the converter, and one end of L3 is connected to the negative terminal of the DC port of the converter.

[0011] Furthermore, each second filtering branch is also connected to a transformer, and one end of the same side windings of the three transformers are connected to form an AC port of the converter.

[0012] Further, the switch capacitor module is any one or more combinations of a half-bridge module, a full-bridge module and a hybrid module, wherein:

[0013] The hybrid module includes a first number of half-bridge modules connected in series and a second number of full-bridge modules connected in series. The first number of half-bridge modules connected in series and the second number of full-bridge modules connected in series are connected in series.

[0014] Furthermore, the power module includes a first bridge arm, a voltage-sharing capacitor C1 and an energy-consuming element H1 connected in parallel, wherein:

[0015] The first bridge arm includes a power switch device T1 and a diode D1, wherein the anode of the diode D1 is connected to the anode or collector of the power switch device T1;

[0016] One end of the voltage-equalizing capacitor C1 is connected to the cathode of the diode D1 , and the other end of the voltage-equalizing capacitor C1 is connected to the cathode or emitter of the power switch device T1 . A diode D2 is anti-parallel connected to the power switch device T1 .

[0017] Further, the first filtering branch comprises a first filter, wherein:

[0018] The first filter is any one or a combination of a single tuned filter, a single tuned filter with damping, a first order high pass filter, a first order high pass filter with damping, a second order high pass filter, a double tuned filter, a double tuned filter with damping, and a triple tuned filter.

[0019] Further, the second filtering branch comprises a second filter, wherein:

[0020] The second filter is any one or a combination of an L filter, an LC filter, an LC filter with damping, an LCL filter, and an LCL filter with damping.

[0021] On the one hand, the present invention also provides a novel method for predicting overcurrent in a bridge arm of an AC / DC converter, wherein the method comprises:

[0022] Step S1: before the new switch valve string switches to a different state, detecting the current Idc flowing into the DC port and the rated current Ih flowing into the new switch valve string;

[0023] Step S2: predicting the switch capacitor valve string current Isc* after the new switch valve string is switched to another state according to Idc and Ih, and determining the relationship between the absolute value |Isc*| of Isc* and the switch capacitor valve string predictive overcurrent protection constant IscP;

[0024] Step S3: If |Isc*|>IscP, the predictive overcurrent protection is triggered and the converter is locked and shut down; if |Isc*|≤IscP, the new switch valve string switches normally and jumps to step S1.

[0025] Furthermore, Isc* is calculated by the following formula:

[0026] Isc*=Idc-Ih.

[0027] Further, Ih is calculated by the following formula:

[0028] Ih=sign(UAA)*IAA

[0029] Among them, sign(UAA) is a function for obtaining the sign of UAA, when UAA≥0, sign(UAA)=1, otherwise sign(UAA)=-1, UAA is the voltage value between the midpoints of the two new switch valves connected in series, and IAA is the current value between the midpoints of the two new switch valves connected in series.

[0030] Furthermore, the method for selecting the switch capacitor valve string predictive overcurrent protection constant IscP is:

[0031] Calculate the difference △IAA between the most recent IAA sampling value and the IAA sampling value of the previous cycle;

[0032] According to △IAA, the change rate of IAA △IAA / T is calculated, where T is the preset sampling period;

[0033] IscP is selected according to △IAA / T, the preset delay td and the maximum flow value IscN of the switch capacitor valve string.

[0034] Furthermore, IscP is expressed by the following formula:

[0035] IscP≤IscN-td*△IAA / T

[0036] Among them, IscP at least satisfies: IscP≤IscN.

[0037] On the other hand, the present invention also provides a novel shutdown method of an AC / DC converter, wherein the converter adopts the converter as described above, and the method comprises:

[0038] Step S1: receiving a shutdown instruction;

[0039] Step S2: the switch capacitor valve string is locked;

[0040] Step S3: delay td2, wherein td2 is the time that the switch capacitor valve string is locked before the new switch valve string;

[0041] Step S4: The new switch valve string is locked and the converter is shut down.

[0042] Furthermore, the value of td2 is 10 μs to 100 μs.

[0043] The present invention provides a novel AC / DC converter and a bridge arm overcurrent prediction and shutdown method. The topology of the converter cooperates with the overcurrent prediction and shutdown method to predict whether the switching state of the novel switch valve will cause an overcurrent in the bridge arm of the switch capacitor valve, and when an overcurrent is predicted, the converter is controlled to shut down protectively according to a predetermined method.

[0044] Other features and advantages of the present invention will be described in the following description, and partly become obvious from the description, or be understood by implementing the present invention. The purpose and other advantages of the present invention can be realized and obtained by the structures pointed out in the description and the drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0045] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, a brief introduction is given below to the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings described below are some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0046] Figure 1 A topological diagram of a novel AC / DC converter according to an embodiment of the present invention is shown;

[0047] Figure 2 shows a topological diagram of a switch valve unit according to an embodiment of the present invention;

[0048] Figure 3shows a topological diagram of a power module according to an embodiment of the present invention;

[0049] Figure 4 It shows a topological diagram of the energy dissipation element H1 when the energy dissipation element H1 is a switchable energy dissipation resistor according to an embodiment of the present invention;

[0050] Figure 5 shows a topological diagram of a single tuned filter according to an embodiment of the present invention;

[0051] Figure 6 shows a diagram of a single tuned filter with damping according to an embodiment of the present invention;

[0052] Figure 7 shows a topological diagram of a first-order high-pass filter according to an embodiment of the present invention;

[0053] Figure 8 A topological diagram of a damped first-order high-pass filter according to an embodiment of the present invention is shown;

[0054] Fig. 9 shows a topological diagram of a second-order high-pass filter according to an embodiment of the present invention;

[0055] Fig.10 shows a topological diagram of a double tuned filter according to an embodiment of the present invention;

[0056] Fig.11 A topology diagram of a double-tuned filter with damping according to an embodiment of the present invention is shown;

[0057] Fig.12 shows a topological diagram of a triple tuned filter according to an embodiment of the present invention;

[0058] Fig.13 A topological diagram showing two identical first-order high-pass filters connected in series according to an embodiment of the present invention, with the connection point of the series connection directly grounded;

[0059] Fig.14 A topological diagram showing two identical first-order high-pass filters connected in series according to an embodiment of the present invention, with the connection point of the series connected being grounded via a grounding resistor;

[0060] Fig.15 A topological diagram showing two identical first-order high-pass filters connected in series according to an embodiment of the present invention, wherein the connection point of the series connection is grounded via a grounding switch;

[0061] Fig.16 A topological diagram showing two identical first-order high-pass filters connected in series according to an embodiment of the present invention, wherein the connection point of the series connection is grounded via a grounding resistor and a grounding switch;

[0062] Fig.17shows a topological diagram of an L filter according to an embodiment of the present invention;

[0063] Fig.18 shows a topological diagram of an LC filter according to an embodiment of the present invention;

[0064] Fig.19 A topological diagram of a damped LC filter according to an embodiment of the present invention is shown;

[0065] Fig. 20 shows a topological diagram of an LCL filter according to an embodiment of the present invention;

[0066] Fig.21 A topological diagram of a damped LCL filter according to an embodiment of the present invention is shown;

[0067] Fig. 22 A flow chart of an overcurrent prediction method according to an embodiment of the present invention is shown;

[0068] Fig.23 A flowchart of a shutdown method according to an embodiment of the present invention is shown;

[0069] Fig.24 A flow chart of an overcurrent prediction+shutdown method according to an embodiment of the present invention is shown. DETAILED DESCRIPTION

[0070] In order to make the purpose, technical solution and advantages of the embodiments of the present invention clearer, the technical solution in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0071] The present invention provides a novel AC / DC converter, wherein the converter comprises a switch valve and a first filter branch, which are described in detail below.

[0072] In some embodiments of the present invention, Figure 1 As shown, the two ends of the switch valve are respectively connected to a DC positive bus and a DC negative bus, wherein the DC positive bus is connected in series with DC inductors L1 and L2, and the DC negative bus is connected in series with DC inductors L3 and L4; the two ends of the filter branch are respectively connected to the connection point between L1 and L2, and the connection point between L3 and L4; one end of L1 is connected to the positive terminal (+) of the DC port of the converter, and one end of L3 is connected to the negative terminal (-) of the DC port of the converter; specifically:

[0073] exist Figure 1In the definition, the DC positive bus has points P1 and P3, the DC negative bus has points P2 and P4, point P1 is between L1 and L2, and point P2 is between L3 and L4. The DC inductor L1 is connected between the DC port + end and P1; the DC inductor L2 is connected between P1 and P3; the DC inductor L3 is connected between the DC port - end and P2; the DC inductor L4 is connected between P2 and P4, and the two ends of the switch valve are connected between P3 and P4. In addition, any one or more of the inductors L1 to L4 may not be configured (i.e., replaced by a section of wire), and the first filter branch is connected between P1 and P2.

[0074] In some embodiments of the present invention, Figure 1 As shown, the switch valve includes three switch valve units connected in series. Figure 2 As shown, the switch valve unit includes a parallel switch capacitor valve string and two new switch valve strings, the switch capacitor valve string includes a plurality of switch capacitor modules connected in series; the new switch valve string includes an upper bridge arm and a lower bridge arm connected by a connecting midpoint, and the upper bridge arm and the lower bridge arm both include power modules.

[0075] In some embodiments of the present invention, Figure 1 As shown, the two connection midpoints of each switch valve unit are connected to the second filter branch, and each second filter branch is also connected to the transformer. The two connection midpoints of each switch valve unit are defined as the AA end of the switch valve unit, and the valve side of the transformer is the BB end. The second filter branch is connected between the AA end of each phase and the BB end of the valve side of the transformer. One end of the same side windings of the three transformers is connected to form an AC port of the converter, that is, for each transformer, the corresponding second filter branch is connected to the two ends of the winding on one side of the transformer (that is, one end of the transformer), and one end of the windings on the other side of the three transformers is connected to each other, and the other ends of the windings on the other side of the three transformers respectively form an a-phase AC port, a b-phase AC port, and a c-phase AC port.

[0076] In some embodiments of the present invention, Figure 3 As shown, the power module includes a first bridge arm, a voltage-equalizing capacitor C1 and an energy-consuming element H1 connected in parallel, wherein: the first bridge arm includes a power switching device T1 and a diode D1, the anode of the diode D1 is connected to the anode or collector of the power switching device T1; one end of the voltage-equalizing capacitor C1 is connected to the cathode of the diode D1, and the other end of the voltage-equalizing capacitor C1 is connected to the cathode or emitter of the power switching device T1, and a diode D2 is anti-parallel to the power switching device T1.

[0077] In some embodiments of the present invention, the energy-consuming element H1 is any one or a combination of an energy-taking power supply, a first energy-consuming resistor, and a switchable energy-consuming resistor, wherein the energy-taking power supply is used to obtain energy from the corresponding voltage-equalizing capacitor C1 to power the control circuit of the corresponding power module, and the switchable energy-consuming resistor includes a second energy-consuming resistor and a switch connected in series with the second energy-consuming resistor.

[0078] like Figure 4 As shown, when the energy-consuming element H1 is a switchable energy-consuming resistor, the switchable energy-consuming resistor includes a second energy-consuming resistor and a switch (exemplarily, the switch can be a relay, a contactor, a circuit breaker, a semiconductor switch, etc., which has a current that can be disconnected), one end of the switch is connected to one end of the second energy-consuming resistor, the other end of the switch is connected to the positive electrode of the corresponding voltage-equalizing capacitor, the other end of the second energy-consuming resistor is connected to the negative electrode of the corresponding voltage-equalizing capacitor, and the energy source is connected to both ends of the corresponding voltage-equalizing capacitor. The switching control of the second energy-consuming resistor can be realized by setting the switch.

[0079] In one embodiment of the present invention, in some embodiments of the present invention, the switch capacitor module is any one or more combinations of a half-bridge module, a full-bridge module and a hybrid module, wherein:

[0080] The hybrid module includes a first number of half-bridge modules connected in series and a second number of full-bridge modules connected in series. The first number of half-bridge modules connected in series and the second number of full-bridge modules connected in series are connected in series.

[0081] In one embodiment of the present invention, in this embodiment, the power switching devices T1 and T2 can be fully controlled power semiconductor switching devices such as insulated gate bipolar transistors (IGBT), integrated gate-commutated thyristors (IGCT), and field-effect transistors (FET).

[0082] In some embodiments of the present invention, Figure 3 As shown, a bypass switch K may be configured in the power module, that is, one end of the bypass switch S1 in the power module is connected to the anode or collector of the power switch device T1 , and the other end is connected to the cathode or emitter of the power switch device T1 .

[0083] In some embodiments of the present invention, the first filtering branch includes a first filter, wherein: the first filter is any one or a combination of a single tuned filter, a single tuned filter with damping, a first order high pass filter, a first order high pass filter with damping, a second order high pass filter, a double tuned filter, a double tuned filter with damping, and a triple tuned filter, wherein:

[0084] like Figure 5 As shown, the single tuned filter includes a capacitor C3 and an inductor L5 connected in series.

[0085] like Figure 6 As shown, the single tuned filter with damping includes a resistor R1, a capacitor C3 and an inductor L5 which are sequentially connected in series.

[0086] like Figure 7 As shown, the first order high pass filter includes capacitor C3.

[0087] like Figure 8 As shown, the first-order high-pass filter with damping includes a capacitor C3 and a resistor R1, wherein the resistor R1 is connected in parallel or in series with the capacitor C3.

[0088] like Fig. 9 As shown, the second-order high-pass filter includes an inductor L5, a capacitor C3 and a resistor R1, wherein the capacitor C3 is connected in series with the inductor L5, and the resistor R1 is connected in parallel with the inductor L5.

[0089] like Fig.10 As shown, the double tuned filter includes an inductor L5, a capacitor C3, a capacitor C4 and an inductor L6, wherein the capacitor C3, the inductor L5 and the capacitor C4 are sequentially connected in series, and the inductor L6 is connected in parallel with the capacitor C4.

[0090] like Fig.11 As shown, the double-tuned filter with damping includes an inductor L5, a capacitor C3, a capacitor C4, an inductor L6 and a resistor R1, wherein the capacitor C3, the inductor L5 and the capacitor C4 are sequentially connected in series, the inductor L6 is connected in parallel with the capacitor C4, and the resistor R1 is connected in parallel with the inductor L6 and the capacitor C3 or the inductor L5 and the capacitor C4 are sequentially connected in series and connected in parallel as a whole.

[0091] like Fig.12 As shown in the left figure, the three-tuned filter includes an inductor L5, a capacitor C3, a capacitor C4, an inductor L6, a resistor R1, a capacitor C5 and an inductor L7, wherein the capacitor C3, the inductor L5, the capacitor C4 and the capacitor C5 are connected in series in sequence, the inductor L6 is connected in parallel with the capacitor C4, and the inductor L7 is connected in parallel with the capacitor C5. In addition, as Fig.12As shown in the right figure, based on the left figure, the triple-tuned filter has another topology, that is, the triple-tuned filter can also include a resistor R1 and a resistor R2, wherein the resistor R1 is connected in parallel with the inductor L6, and the resistor R2 is connected in parallel with the inductor L5.

[0092] In the present invention, the DC port and the port formed by P3 and P4 are used as the input and output ports of the passive filter network composed of the first filter and L1 to L4. Assuming the AC grid frequency is f g , then the resonance point of the passive filter network should avoid 2f g , and 6f g and its integer multiple frequency points to avoid harmonic amplification caused by resonance.

[0093] Furthermore, the passband cutoff frequency of the passive filter network should be lower than 6f g , thereby weakening the 6f at both ends of P3 and P4 g The harmonic voltage of the frequency and its multiples can improve the power quality of the DC port.

[0094] Furthermore, for a converter with active damping control, the first filter may not be configured with damping (ie, corresponding resistance).

[0095] In addition, in some embodiments of the present invention, any two first filters of the same type or different types connected in series can form a new first filter, and the series connection point can be directly grounded, grounded via a resistor, grounded via a grounding switch, or grounded via a resistor and a grounding switch, thereby achieving the grounding of the converter. The following takes any two first filters of the same type connected in series to form a new first filter as an example for detailed description, specifically:

[0096] like Fig.13 As shown above, Figure 7 Two identical first-order high-pass filters are shown connected in series, with the connection point of the series connected directly to ground.

[0097] like Fig.14 As shown above, Figure 7 Two identical first-order high-pass filters are shown connected in series, and the connection point of the series connection is grounded via a resistor R3.

[0098] like Fig.15 As shown above, Figure 7 The two identical first-order high-pass filters shown are connected in series, and the connection point of the series connection is grounded via the grounding switch K1.

[0099] like Fig.16 The above is Figure 7 The two identical first-order high-pass filters shown are connected in series, and the connection point of the series connection is grounded via a resistor R and a series grounding switch K1.

[0100] The above-mentioned new filter grounding method does not require additional grounding components, or only adds a grounding resistor R3 and a grounding switch K1, and the steady-state grounding resistor and grounding switch do not bear voltage and current, so the cost is low. And because two identical first filters are connected in series, the electrical parameters of the filters are consistent, so the DC voltage divided by the series connection is consistent, so the above-mentioned grounding method is adopted, for example, the voltage to ground of the DC port is the same and the voltage to ground is the minimum, the insulation difficulty is the lowest, and the cost and volume are the smallest.

[0101] In some embodiments of the present invention, the second filtering branch includes a second filter, wherein: the second filter is any one or a combination of an L filter, an LC filter, an LC filter with damping, an LCL filter, and an LCL filter with damping, wherein:

[0102] like Fig.17 As shown, Fig.17 In (a), the L filter includes a wire and an inductor L8 (the wire and the inductor L8 are parallel and do not intersect each other), or Fig.17 In FIG. (b), the L filter includes two inductors L8 (the two inductors L8 are parallel and non-intersecting), wherein when the L filter includes a wire and an inductor L8, the positions of the wire and the inductor L8 are interchangeable. In the L filter, Fig.17 In Figure (c), inductor L8 can be partially or completely replaced by transformer leakage inductance.

[0103] like Fig.18 As shown, Fig.18 In Figure (a), the LC filter includes capacitor C6 and two inductors L8, or Fig.17 In (b), the LC filter includes an inductor L8, a capacitor C6, and a conductor. When the LC filter includes a capacitor C6 and two inductors L8, the two ends of the capacitor C6 are respectively connected between the same ends of the two inductors L8. When the LC filter includes an inductor L8, a capacitor C6, and a conductor, the two ends of the capacitor C6 are respectively connected between the same ends of the inductor L8 and the conductor. In the LC filter, the inductor L8 near point B can be partially or completely replaced by the transformer leakage inductance.

[0104] like Fig.19 As shown in the figure, the damped LC filter is Fig.18 A resistor R4 connected in series below the capacitor C6 is added to the L filter of Figures (a) and (b).

[0105] like Fig. 20 As shown, Fig. 20 As shown in Figure (a), the LCL filter includes capacitor C6, wires and two inductors L8, or Fig. 20 As shown in Figure (b), the LCL filter includes capacitor C6 and two inductors L8, or Fig. 20 In Figure (c), the LCL filter includes a capacitor C6 and four inductors L8. When the LCL filter includes a capacitor C6, a wire and two inductors L8, one end of the capacitor C6 is connected to the connection point of the two inductors L8 in series, and the other end is connected to the wire.

[0106] When the LCL filter includes a capacitor C6 and two inductors L8, one end of one of the inductors L8 is connected to one of the A ends, the other end of the inductor L8 is connected to one of the B ends, one end of the other inductor L8 is connected to the other A end, the other end of the inductor L8 is connected to the other B end, one end of the capacitor C6 is connected to the other end of one of the inductors L8, and the other end of the capacitor C6 is connected to one end of the other inductor L8. When the LCL filter includes a capacitor C6 and four inductors L8, one end of the capacitor C6 is connected to the connection point where two of the inductors L8 are connected in series, and the other end of the capacitor C6 is connected to the connection point where the other two inductors L8 are connected in series.

[0107] like Fig.21 As shown in the figure, the damped LCL filter is Fig. 20 A resistor R4 is added in series below the capacitor C6 to the LCL filter in FIG.

[0108] In the present invention, if the second filter has a resonance point, the resonance point should avoid f g 、3f g , to avoid overcurrent. The converter will output the third harmonic, so the resonance point must not only avoid the power frequency, but also the third harmonic frequency.

[0109] Furthermore, if the second filter has a cutoff frequency, the cutoff frequency should be higher than f g , to avoid affecting the power frequency output.

[0110] Furthermore, for a converter with active damping control, the second filter may not be configured with passive damping (ie resistance).

[0111] Preferably, the second filter may not be configured, that is, A and B are short-circuited. When the converter is not used for transient working condition processing, such as low voltage ride-through, the AC port will not generate a large amount of harmonic voltage, so there is no need to configure the second filter.

[0112] In summary, when the second filter of the present invention is not configured, the port power quality of the converter cannot meet the national standard requirements. After adopting the filter solution of the present invention, the harmonic voltage and current of the DC port and the AC port can be significantly suppressed, so that the port power quality of the converter meets the national standard requirements. In addition, the filter is also used to achieve low-cost grounding.

[0113] In the present invention, a novel bridge arm overcurrent prediction method of an AC / DC converter is also provided to solve the problems corresponding to the following situations:

[0114] A. During normal operation: For a clearer description, Figure 2 Taking the single-phase topology as an example, when the output voltage UAA and current IAA of the converter AA port are not in phase or in opposite phase (that is, the AA port is not a unity power factor), if the new switch valve string switches state (that is, the 1st and 3rd bridge arms are turned on, the 2nd and 4th bridge arms are turned off (that is, all the fully controlled power semiconductor switching devices in the bridge arms are turned off), the 2nd and 4th bridge arms are turned on, and the 1st and 3rd bridge arms are turned off; or vice versa), the AC current at the AA port is chopped, which is reflected as a sudden change in the DC side current of the new switch valve string - reverse, so the following working conditions exist:

[0115] Before the new switch valve string switches to another state: Taking phase a as an example, assume that the DC port current Idc flows from DC+ into the new AC / DC converter and flows out of the converter from DC- (i.e. in the positive direction), and the DC side current Ih of the new switch valve string flows from the new switch valve string+ into the new switch valve string and flows out of the new switch valve string from the new switch valve string (i.e. in the positive direction). At this time, the switch capacitor valve string current Isc=Idc-Ih.

[0116] After the new switch valve string switches state: the DC side current Ih of the new switch valve string reverses (i.e., it is in the negative direction), and at this time the switch capacitor valve string current Isc=Idc+Ih. If |Isc| (i.e., the absolute value of Isc) is greater than the flow capacity (i.e., the flow value) of the switch capacitor valve string, the switch capacitor valve string is damaged by overcurrent.

[0117] Obviously, in normal operation, the new switch valve string may cause overcurrent in the switch capacitor valve string after switching the state. This situation mainly occurs when the power factor is too low, exceeds the design range, or is overloaded.

[0118] B. When the converter is locked and shut down: the new switch valve string is locked (i.e. bridge arms 1 to 4 are turned off), and the current at the AA port can only flow into the DC side of the new switch valve string after being rectified by the anti-parallel diode of the fully controlled power semiconductor switch device of the new switch valve string. Therefore, the following working conditions exist:

[0119] 1) Before the new switch valve string is locked, the DC port Idc is positive, and the AA port voltage UAA and the AA port current IAA are of the same polarity. IAA flows through the fully controlled power semiconductor switch device of the new switch valve string. At this time, the direction of Ih is positive, and the current Isc of the switch capacitor valve string is Idc-Ih.

[0120] 2) After the new switch valve string is locked, IAA flows through the anti-parallel diode of the new switch valve string fully controlled power semiconductor switch device. At this time, the direction of Ih is negative, and the current of the switch capacitor valve string Isc=Idc+Ih. If |Isc| is greater than the flow capacity of the switch capacitor valve string, the switch capacitor valve string will be damaged by overcurrent.

[0121] Obviously, the locking of the new switch valve series may cause overcurrent in the switch capacitor valve series. This situation mainly occurs in the overcurrent protection shutdown condition.

[0122] Based on the above problems, a novel bridge arm overcurrent prediction and method for an AC / DC converter provided by the present invention can solve the problems, which is described in detail below.

[0123] In some embodiments of the present invention, Fig. 22 As shown, a novel bridge arm overcurrent prediction method for an AC / DC converter is provided, wherein the converter adopts the above-mentioned converter, and the method comprises:

[0124] Step S1: before the new switch valve string switches to a different state, detecting the current Idc flowing into the DC port and the rated current Ih flowing into the new switch valve string.

[0125] Step S2: Based on Idc and Ih, predict the switch capacitor valve string current Isc* after the new switch valve string switches to a different state, and determine the relationship between the absolute value |Isc*| of Isc* and the switch capacitor valve string predictive overcurrent protection constant IscP.

[0126] Step S3: If |Isc*|>IscP, the predictive overcurrent protection is triggered and the converter is locked and shut down; if |Isc*|≤IscP, the new switch valve string switches normally and jumps to step S1.

[0127] Where Isc* is calculated by the following formula:

[0128] Isc*=Idc-Ih.

[0129] Where Ih is calculated by the following formula:

[0130] Ih=sign(UAA)*IAA

[0131] Among them, sign(UAA) is a function for obtaining the sign of UAA, when UAA≥0, sign(UAA)=1, otherwise sign(UAA)=-1, UAA is the voltage value between the midpoints of the two new switch valves connected in series, and IAA is the current value between the midpoints of the two new switch valves connected in series.

[0132] When the upper bridge arm (bridge arm 1) of one novel switch valve string and the lower bridge arm (bridge arm 4) of another novel switch valve string are turned on, and the lower bridge arm (bridge arm 2) of one novel switch valve string and the upper bridge arm (bridge arm 3) of another novel switch valve string are turned off, the voltage UAA between the midpoints of the two novel switch valve strings is +;

[0133] On the contrary, when the lower bridge arm of one new switch valve string and the upper bridge arm of another new switch valve string are turned on, and the upper bridge arm of one new switch valve string and the lower bridge arm of another new switch valve string are turned off, UAA is -.

[0134] Among them, the selection method of the switch capacitor valve series predictive overcurrent protection constant IscP is:

[0135] Calculate the difference △IAA between the most recent IAA sampling value and the IAA sampling value of the previous cycle;

[0136] According to △IAA, the change rate of IAA △IAA / T is calculated, where T is the preset sampling period;

[0137] IscP is selected according to △IAA / T, the preset delay td and the maximum flow value IscN of the switch capacitor valve string.

[0138] Among them, IscP is expressed by the following formula:

[0139] IscP≤IscN-td*△IAA / T

[0140] Among them, IscP at least satisfies: IscP≤IscN.

[0141] In some embodiments of the present invention, Fig.23 As shown, a novel shutdown method of an AC / DC converter is also provided, wherein the converter adopts the above converter, and the method comprises:

[0142] Step S1: receiving a shutdown instruction;

[0143] Step S2: the switch capacitor valve string is locked;

[0144] Step S3: delay (wait) td2, where td2 is the time that the switch capacitor valve string is locked before the new switch valve string;

[0145] Step S4: The new switch valve string is locked and the converter is shut down.

[0146] Among them, the purpose of delay td2 is to allow the switch capacitor valve string to be locked before the new switch valve string. After locking, the current of the switch capacitor valve string can only flow through the anti-parallel diode. The anti-parallel diode has a high surge current bearing capacity. Therefore, the switch capacitor valve string will not be damaged by overcurrent due to the locking of the new switch valve string. Usually, td2 takes a value of 10μs~100μs.

[0147] In summary, the overall process of the overcurrent prediction + shutdown method in the present invention can be summarized as follows: Fig.24 The process shown, that is, after the predictive overcurrent protection is triggered in step S2 of the overcurrent prediction method and the converter is locked and shut down, step S1 in the shutdown method is started. The present invention combines overcurrent prediction + shutdown, on the one hand, avoids the switching state of the new switch valve string causing the switch capacitor valve to overcurrent, and on the other hand, avoids the new switch valve string locking during the shutdown process causing the switch capacitor valve string to overcurrent.

[0148] The above is only a preferred embodiment of the present invention, and does not limit the present invention in any form. Although the present invention has been disclosed as a preferred embodiment as above, it is not used to limit the present invention. Any technician familiar with this profession can make some changes or modifications to equivalent embodiments of equivalent changes using the technical contents disclosed above without departing from the scope of the technical solution of the present invention. However, any simple modification, equivalent change and modification made to the above embodiments based on the technical essence of the present invention without departing from the content of the technical solution of the present invention still fall within the scope of the technical solution of the present invention.

Claims

1. A novel AC / DC converter, wherein: The converter comprises a switch valve and a first filter branch, wherein: The two ends of the switch valve are respectively connected to a DC positive bus and a DC negative bus, wherein the DC positive bus is connected in series with DC inductors L1 and L2, and the DC negative bus is connected in series with DC inductors L3 and L4; the two ends of the filter branch are respectively connected to the connection point between L1 and L2, and the connection point between L3 and L4; The switch valve comprises three switch valve units connected in series, the switch valve units comprise a switch capacitor valve string connected in parallel and two novel switch valve strings, the switch capacitor valve string comprises a plurality of switch capacitor modules connected in series; the novel switch valve string comprises an upper bridge arm and a lower bridge arm connected by a connection midpoint, the upper bridge arm and the lower bridge arm both comprise a power module; The two connection midpoints of each switch valve unit are connected to a second filter branch.

2. A novel AC / DC converter according to claim 1, wherein: One end of L1 is connected to the positive terminal of the DC port of the converter, and one end of L3 is connected to the negative terminal of the DC port of the converter.

3. A novel AC / DC converter according to claim 2, wherein: Each second filtering branch is also connected to a transformer, and one end of the same side windings of the three transformers are connected to form an AC port of the converter.

4. A novel AC / DC converter according to claim 1 or 3, wherein: The switch capacitor module is any one or more combinations of a half-bridge module, a full-bridge module and a hybrid module, wherein: The hybrid module includes a first number of half-bridge modules connected in series and a second number of full-bridge modules connected in series. The first number of half-bridge modules connected in series and the second number of full-bridge modules connected in series are connected in series.

5. A novel AC / DC converter according to claim 1 or 3, wherein: The power module includes a first bridge arm, a voltage-equalizing capacitor C1 and an energy-consuming element H1 connected in parallel, wherein: The first bridge arm includes a power switch device T1 and a diode D1, wherein the anode of the diode D1 is connected to the anode or collector of the power switch device T1; One end of the voltage-equalizing capacitor C1 is connected to the cathode of the diode D1 , and the other end of the voltage-equalizing capacitor C1 is connected to the cathode or emitter of the power switch device T1 . A diode D2 is connected in anti-parallel to the power switch device T1 .

6. A novel AC / DC converter according to claim 1 or 3, wherein: The first filtering branch comprises a first filter, wherein: The first filter is any one or a combination of a single tuned filter, a single tuned filter with damping, a first order high pass filter, a first order high pass filter with damping, a second order high pass filter, a double tuned filter, a double tuned filter with damping, and a triple tuned filter.

7. A novel AC / DC converter according to claim 1 or 3, wherein: The second filtering branch comprises a second filter, wherein: The second filter is any one or a combination of an L filter, an LC filter, an LC filter with damping, an LCL filter, and an LCL filter with damping.

8. A novel method for predicting overcurrent in a bridge arm of an AC / DC converter, wherein the converter is a converter according to any one of claims 1 to 7, and the method comprises: Step S1: before the new switch valve string switches to a different state, detecting the current Idc flowing into the DC port and the rated current Ih flowing into the new switch valve string; Step S2: predicting the switch capacitor valve string current Isc* after the new switch valve string is switched to another state according to Idc and Ih, and determining the relationship between the absolute value |Isc*| of Isc* and the switch capacitor valve string predictive overcurrent protection constant IscP; Step S3: If |Isc*|>IscP, the predictive overcurrent protection is triggered and the converter is locked and shut down; if |Isc*|≤IscP, the new switch valve string switches normally and jumps to step S1.

9. The method for predicting overcurrent in a bridge arm of a novel AC / DC converter according to claim 8, wherein: Isc* is calculated by the following formula: Isc*=Idc-Ih.

10. A novel bridge arm overcurrent prediction method for an AC / DC converter according to claim 8 or 9, wherein: Ih is calculated by the following formula: Ih=sign(UAA)*IAA Among them, sign(UAA) is a function for obtaining the sign of UAA, when UAA≥0, sign(UAA)=1, otherwise sign(UAA)=-1, UAA is the voltage value between the midpoints of the two new switch valves connected in series, and IAA is the current value between the midpoints of the two new switch valves connected in series.

11. A novel bridge arm overcurrent prediction method for an AC / DC converter according to claim 10, wherein: The method for selecting the switch capacitor valve series predictive overcurrent protection setting value IscP is: Calculate the difference △IAA between the most recent IAA sampling value and the IAA sampling value of the previous cycle; According to △IAA, the change rate of IAA △IAA / T is calculated, where T is the preset sampling period; IscP is selected according to △IAA / T, the preset delay td and the maximum flow value IscN of the switch capacitor valve string.

12. A novel bridge arm overcurrent prediction method for an AC / DC converter according to claim 11, wherein: IscP is expressed by the following formula: IscP≤IscN-td*△IAA / T Among them, IscP at least satisfies: IscP≤IscN.

13. A novel shutdown method for an AC / DC converter, wherein the converter is a converter according to any one of claims 1 to 8, and the method comprises: Step S1: receiving a shutdown instruction; Step S2: the switch capacitor valve string is locked; Step S3: delay td2, wherein td2 is the time that the switch capacitor valve string is locked before the new switch valve string; Step S4: The new switch valve string is locked and the converter is shut down.

14. A novel AC / DC converter shutdown method according to claim 13, wherein: The value of td2 is 10μs~100μs.

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