A power balancing method for unit failure of cascaded bidirectional converter

Abstract

The present invention provides a power balancing method for unit failures in a cascaded bidirectional converter device, belonging to the field of bidirectional converter technology. The method comprises: determining the number of faulty power units in the cascaded bidirectional converter device, wherein the cascaded bidirectional converter device has a multi-stage cascade and multi-group parallel topology; when there is at most one faulty power unit in any one or more stages of the cascaded bidirectional converter device, determining the stage containing the faulty power unit as a first target stage and removing the faulty power unit; and determining the allocated current for the normal power units in the first target stage based on the number of normal power units retained in the first target stage and the total current of each stage. After a power unit failure occurs, the present invention achieves power balance between each stage by redistributing current, effectively reducing the impact of the overall operating power derating of the device caused by the failure.

Description

Technical Field

[0001] The present invention belongs to the technical field of bidirectional current conversion, and in particular relates to a power balancing method for unit failure of a cascaded bidirectional current conversion device. Background Art

[0002] Due to the characteristics of cascaded bidirectional converters, the current in each power unit is the same, that is, the power of each power unit in the same level is the same. When a power unit in a level of the cascaded bidirectional converter fails and stops operating, the total power of that level will decrease, causing power imbalance between different levels. The power imbalance between each level will lead to uneven voltage between different levels of the equipment, at which point the equipment will trigger self-protection and stop operation.

[0003] Therefore, in order to ensure that the equipment continues to be put into operation, the faulty power unit will be manually repaired or replaced with a spare power unit, but this method has a low degree of automation and high maintenance costs; or other normal power units in the same group can be directly withdrawn to ensure that the number of power units at each level is consistent and the power balance at each level is guaranteed. However, this method will not only greatly reduce the operating power of the entire machine and cannot meet the operation of subsequent equipment, but also fail to fully utilize the power units that can operate normally, resulting in idle equipment. Summary of the Invention

[0004] In view of the above-mentioned deficiencies in the prior art, the present invention provides a power balancing method for a cascaded bidirectional converter unit failure to solve the above-mentioned technical problems.

[0005] The present invention provides a power balancing method for a cascaded bidirectional converter unit failure, comprising:

[0006] Determining the number of faulty power units in a cascaded bidirectional converter device, wherein the cascaded bidirectional converter device has a topology structure with multiple cascaded stages and multiple parallel groups;

[0007] In a case where there is at most one faulty power unit in any one or more stages of the cascaded bidirectional converter device, determining the stage where the faulty power unit is located as the first target stage, and exiting the faulty power unit;

[0008] determining the allocated current of the normal power units in the first target level according to the remaining number of normal power units in the first target level and the total current of each level;

[0009] In the case that only one stage of the cascaded bidirectional converter device has two or more faulty power units, determining the stage as the second target stage and exiting the faulty power units;

[0010] According to the maximum carrying current of the power unit, the exit number of normal power units in each level except the second target level is determined, and the current allocated to the normal power units in the second target level is determined according to the exit number.

[0011] Furthermore, the cascaded bidirectional converter device is a topology structure of n-stage cascade and m-group parallel connections, and the method further includes:

[0012] The distributed current of the normal power units in the first target level is determined according to a first formula:

[0013] i1′=mi / (m-1), where i is the current of each power unit when the power unit is normal, i1′ is the current allocated to the first target level, and m is the number of power units in each level;

[0014] The current distributed according to the normal power unit in the second target level is determined according to the second formula:

[0015] i2′=(mk)i / (mt), where i is the current of each power unit when the power unit is normal; i2′ is the current allocated to the second target level; m is the number of power units in each level, t is the number of faulty power units in the second target level, and k is the exit number.

[0016] Furthermore, the step of determining the exit number of normal power units in each level except the second target level according to the maximum carrying current of the power unit includes:

[0017] Determine mi / (mt) and the i max The size of i max ≥mi / (mt), then k=0;

[0018] If i max <mi / (mt), then according to i max ≥(mk)i / (mt), determine the minimum value of k;

[0019] Where m is the total number of power units in any one level of the device, t is the number of faulty power units in the second target level, i max is the maximum carrying current, k is a natural number, and k is less than m.

[0020] Furthermore, after determining the exit number of normal power units in each level except the second target level, the method further includes:

[0021] Determine the target group of the exiting normal power units according to the group where the second target level faulty power unit is located, so that the target group and the group where the faulty power unit is located are consistent;

[0022] When there are multiple target groups, the group where the first faulty power unit is located is preferentially selected.

[0023] Furthermore, the situation where there is at most one faulty power unit in any one or more stages includes the following situations:

[0024] The entire device has only one faulty power unit;

[0025] There is a faulty power unit failure in any two stages;

[0026] There is a faulty power unit in any p levels, n is the number of levels of the cascaded bidirectional converter, p is a natural number, and 2<p≤n, wherein n>2.

[0027] Furthermore, the situation where there are two or more faulty power units in only one level includes the following situations:

[0028] There are two or more faulty power units in any one stage, and only the faulty power units are in that stage;

[0029] There are two or more faulty power units in only one stage, and there is also a faulty power unit in a different stage in the same group.

[0030] There are two or more faulty power units in only one level, and there is also a faulty power unit in a different level and in a different group.

[0031] There are two or more faulty power units in only one stage, and all other stages except this stage also have one faulty power unit in the same group;

[0032] There are two or more faulty power units in only one level, and all levels except this level also have a faulty power unit in the group where the faulty power unit is located.

[0033] There is only one stage with two or more faulty power units, and all other stages except this stage have a faulty power unit in a different group from the faulty power unit;

[0034] There is only one stage with two or more faulty power units, and all other stages except this stage have one faulty power unit in the same group and one faulty power unit in a different group.

[0035] The beneficial effects of the present invention lie in that the method for balancing power in the event of a unit failure in a cascaded bidirectional converter device provided by the present invention achieves power balance between each stage by redistributing current after a power unit failure, without increasing hardware costs. This method ensures that the overall power of the device remains unchanged when at most a single power unit fails at the same stage, and that the power derating is lower when two power units fail at the same stage, effectively reducing the impact of the overall operating power derating caused by the failure. Furthermore, the present invention has a reliable design principle, a simple structure, and a very broad application prospect. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art. Obviously, for ordinary technicians in this field, other drawings can be obtained based on these drawings without any creative work.

[0037] Figure 1 This is a topological diagram of a three-stage cascaded five-group parallel bidirectional converter device according to an embodiment of the present invention;

[0038] Figure 2 A schematic flow chart of a power balancing method provided by one embodiment of the present invention. DETAILED DESCRIPTION

[0039] In order to enable those skilled in the art to better understand the technical solutions of the present invention, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the accompanying drawings of the embodiments of the present invention. Obviously, the described embodiments are only 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 making creative efforts should fall within the scope of protection of the present invention.

[0040] One embodiment of the present invention provides a method for balancing power when a cascaded bidirectional converter fails. The method includes:

[0041] Determining the number of faulty power units in a cascaded bidirectional converter device, wherein the cascaded bidirectional converter device has a topology structure with multiple cascaded stages and multiple parallel groups;

[0042] In a case where there is at most one faulty power unit in any one or more stages of the cascaded bidirectional converter device, determining the stage where the faulty power unit is located as the first target stage, and exiting the faulty power unit;

[0043] determining the allocated current of the normal power units in the first target level according to the remaining number of normal power units in the first target level and the total current of each level;

[0044] In the case that only one stage of the cascaded bidirectional converter device has two or more faulty power units, determining the stage as the second target stage and exiting the faulty power units;

[0045] According to the maximum carrying current of the power unit, the exit number of normal power units in each level except the second target level is determined, and the current allocated to the normal power units in the second target level is determined according to the exit number.

[0046] In an embodiment of the present invention, in the event of a power unit failure in a cascaded bidirectional converter, the power of the failed power unit is allocated to other normal power units, or the normal power units of the unfaulted stage are withdrawn to achieve power balance at each stage. When allocating power, the current of the power unit is reset according to the characteristics of the cascaded bidirectional converter. In addition, two methods are provided for the number of failed power units, so that in most cases of a single power unit failure, power balance can be achieved by simply redistributing power without derating. In the event of two or more power units failing, even if the normal power units need to be withdrawn, it is not a simple matter of ensuring that the number of operating power units at each stage is consistent. Instead, after withdrawing the minimum number of power units, power balance is achieved by redistributing current, which can minimize the total power reduction at each stage.

[0047] Optionally, as an embodiment of the present invention, the cascaded bidirectional converter device has a topology structure of n-stage cascade and m-group parallel connections, and the method further includes:

[0048] The distributed current of the normal power units in the first target level is determined according to a first formula:

[0049] i1′=mi / (m-1), where i is the current of each power unit when the power unit is normal; i1′ is the current allocated to the first target level; m is the number of power units in each level;

[0050] The current distributed according to the normal power unit in the second target level is determined according to the second formula:

[0051] i2′=(mk)i / (mt), where i is the current of each power unit when the power unit is normal; i2′ is the current allocated to the second target level; m is the number of power units in each level, t is the number of faulty power units in the second target level, and k is the exit number.

[0052] Optionally, as an embodiment of the present invention, determining the number of normal power units to be withdrawn in each level except the second target level according to the maximum carrying current of the power unit includes:

[0053] Determine mi / (mt) and the i max The size of i max ≥mi / (mt), then k=0; if i max <mi / (mt), then according to i max ≥(mk)i / (mt), determine the minimum value of k; where m is the total number of power units in any level of the device, t is the number of faulty power units in the second target level, i max is the maximum carrying current, k is a natural number, and k is less than m.

[0054] Optionally, as an embodiment of the present invention, after determining the exit number of normal power units in each level except the second target level, the method further includes:

[0055] Determine the target group of the exiting normal power units according to the group where the second target level faulty power unit is located, so that the target group and the group where the faulty power unit is located are consistent;

[0056] When there are multiple target groups, the group where the first faulty power unit is located is preferentially selected.

[0057] Optionally, as an embodiment of the present invention, the situation in which there is at most one faulty power unit in any one or more stages includes the following situations:

[0058] The entire device has only one faulty power unit;

[0059] There is a faulty power unit failure in any two stages;

[0060] There is a faulty power unit in any p levels, n is the number of levels of the cascaded bidirectional converter, p is a natural number, and 2<p≤n, wherein n>2.

[0061] Optionally, as an embodiment of the present invention, the situation where there are two or more faulty power units in only one level includes the following situations:

[0062] There are two or more faulty power units in any one stage, and only the faulty power units are in that stage;

[0063] There are two or more faulty power units in only one stage, and there is also a faulty power unit in a different stage in the same group.

[0064] There are two or more faulty power units in only one level, and there is also a faulty power unit in a different level and in a different group.

[0065] There are two or more faulty power units in only one stage, and all other stages except this stage also have one faulty power unit in the same group;

[0066] There are two or more faulty power units in only one level, and all levels except this level also have a faulty power unit in the group where the faulty power unit is located.

[0067] There is only one stage with two or more faulty power units, and all other stages except this stage have a faulty power unit in a different group from the faulty power unit;

[0068] There is only one stage with two or more faulty power units, and all other stages except this stage have one faulty power unit in the same group and one faulty power unit in a different group.

[0069] To facilitate understanding of the present invention, the power balancing method provided by the present invention is further described below based on the operating principle of a cascaded bidirectional converter device and the process of managing various power unit failure situations in the embodiment.

[0070] The cascaded bidirectional converter device has an n-stage cascaded topology with m parallel groups. The cascaded bidirectional converter device can be a two-stage cascaded, three-stage cascaded, or n-stage cascaded device. Each stage can include one power unit, two power units, or m power units in parallel, meaning the entire device can have an n-stage cascaded, m-group parallel topology. The cascaded bidirectional converter device shown is connected to the DC traction grid at one end and to the AC grid via a transformer at the other end. It consists of three stages of power units connected in series, with each stage consisting of five power units connected in parallel, creating a three-stage cascaded, five-group parallel topology. The power units can operate in either rectification or inverter mode under control of control signals. When the power module operates in rectification mode, electrical energy from the AC grid is input into the DC traction grid. When the power module operates in inverter mode, electrical energy from the DC traction grid is input into the AC grid.

[0071] Continuously monitor whether the power units in the cascaded bidirectional converter device have faults. When receiving a power unit fault, traverse the number of faulty power units in all levels in turn, determine the number of power units with the most faults in each level, and divide the judgment results into two categories: one is that the level with the largest number of power unit faults has only one faulty unit fault, and the other is that the level with the largest number of power unit faults has two or more faulty units fault. On the premise of determining that there is a faulty power unit, the judgment result must fall into one of the two categories, that is, the two categories of results are opposite events.

[0072] For the first type of results, the situation where there is at most one faulty power unit in any one or more stages includes the following situations:

[0073] Case 1.1: There is only one faulty power unit in the entire installation;

[0074] Case 1.2: There is a faulty power unit in each of the two stages;

[0075] Case 1.3: There is a faulty power unit in each of any p stages, n is the number of stages of the cascaded bidirectional converter device, p is a natural number, and 2<p≤n, wherein n>2.

[0076] No matter which level or levels a single faulty power unit exists in, the level where the faulty power unit is located will be determined as the first target level. If a level has a fault, this level will be the first target level. If two levels have faults, both levels will be the first target level. The faulty power unit will be forced to exit so that it does not participate in the work. To ensure the normal operation of the device, the current allocated to the faulty power unit will be evenly redistributed to other normal power units at the same level to ensure that the total current of the level is not affected by the faulty power unit. The allocated current of the normal power units in the first target level is determined based on the retained number of normal power units in the first target level and the total current of each level. The allocated current of the normal power units in the first target level is determined according to the first formula: i1′=5i / (5-1)=5i / 4.

[0077] There is at most a single power unit failure at the same level. The power redistribution control strategy for normal power units includes: when a power unit failure occurs in a certain level of the cascaded bidirectional converter device or a power unit failure occurs between different levels, since the power units all have a certain power margin, in order to ensure the power balance of each level and the power of the whole machine is not reduced, the power borne by this faulty power unit can be evenly distributed to other normal power units at this level, and will not affect the operation of the whole machine; this embodiment realizes power redistribution after the failure by adjusting the current instruction, thereby ensuring the power balance of the cascaded bidirectional converter device.

[0078] This embodiment takes a three-stage cascade of five parallel bidirectional converters as an example to describe the balancing methods for various situations of the first type of results, as shown in Table 1.

[0079] Table 1 Statistics of power redistribution of normal power units when there is at most one power unit failure at the same level

[0080]

[0081] For the second type of result, the situation where there are two or more faulty power units at only one level includes the following situations:

[0082] Case 2.1: If there are two or more faulty power units in any stage, and only this stage has faulty power units, then this stage is determined as the second target stage.

[0083] Case 2.2: There is only one stage with two or more faulty power units. This stage is determined as the second target stage. In addition to this stage, there is another stage with a faulty power unit in the same group.

[0084] Case 2.3: There is only one level with two or more faulty power units. This level is determined as the second target level. In addition to this level, there is another level in a different group that also has a faulty power unit.

[0085] Case 2.4: Only one stage has two or more faulty power units. This stage is determined as the second target stage. All stages except this stage also have a faulty power unit in the same group as the faulty power unit.

[0086] Case 2.5: Only one stage has two or more faulty power units. This stage is determined as the second target stage. All other stages except this stage have a faulty power unit in a different group from the faulty power unit.

[0087] Case 2.6: There is only one stage with two or more faulty power units. This stage is determined as the second target stage. All other stages except this stage have one faulty power unit in the same group and one faulty power unit in a different group.

[0088] This embodiment takes a three-stage cascaded system with five parallel bidirectional converters as an example to describe the balancing methods for various situations of the second type of results. See Table 2 for details:

[0089] Table 2 Statistics of power redistribution of normal power units when two power units at the same level fail

[0090]

[0091]

[0092] The failed power unit is exited, and the exit number of normal power units in each level except the second target level is determined according to the maximum carrying current of the power unit. The specific process is as follows:

[0093] Determine mi / (mt) and the i max In this embodiment, i max =1.5i, m=5, t=2;

[0094] Then i max<mi / (mt), then according to i max ≥(mk)i / (mt), determine the minimum value of k;

[0095] That is, 1.5i≥(5-k)i / (5-2), and the solution is k x ≥0.5, the minimum value of k is 1, and one normal power unit is withdrawn from each of the levels except the second target level.

[0096] The current allocated to the normal power units in the second target level is determined according to the number of exits; wherein the current allocated to the normal power units in the second target level is determined according to the second formula, and i2′=(5-1)i / (5-2)=4i / 3 is obtained. After the faulty power unit exits, the current of all power units in the second target level is 4i / 3.

[0097] The power redistribution control strategy for normal power units when two power units fail at the same level includes: when two power units fail at a certain level of the cascaded bidirectional converter device, and no power units fail at other levels or a single power unit fails, the power units at other levels should exit to ensure the normal operation of four power units at each level. When two power units fail at the same level, the remaining normal units in the level will evenly distribute the operating power of one failed unit, thereby ensuring power balance at each level.

[0098] The method of the present invention has high flexibility. When there is at most a single power unit failure at the same level, it can ensure that the overall power of the equipment is not reduced. When there are two power units failures at the same level, the power reduction is the least. The present invention can minimize the problem of power reduction operation of the entire machine when a power unit failure occurs in the equipment, and the control strategy is simple and the feasibility is high.

[0099] It is worth noting that in a bidirectional converter, when current distribution is implemented, the current in the power unit will increase, but due to the limitation of the maximum carrying current, it will not increase indefinitely, and the probability of two or more power units failing is relatively low. Therefore, when designing the maximum carrying current of the bidirectional converter, technicians try to ensure that when one of the power units fails, the maximum carrying current of the power unit can withstand the redistributed power. When two power units fail, if the currents of the two failed power units are distributed to the other groups of normal power units, the calculated new current i2' will exceed the maximum carrying current. At this time, exiting one normal power unit will inevitably reduce the total current of each level. Compared with exiting two normal power units in the prior art, the total power reduction due to the failure of the present invention is less, and the overall power of the entire bidirectional converter can be guaranteed.

[0100] In one implementation, there is a certain relationship between the maximum carrying current and the normal operating current of the power unit, such that: [normal operating current*m+ / (m-1)]≤maximum carrying current<[normal operating current(m-1) / (m-2)].

[0101] Optionally, as an embodiment of the present invention, after determining the exit number of normal power units in each level except the second target level, it also includes: determining the target group of exiting normal power units based on the group where the faulty power unit of the second target level is located, so that the target group and the group where the faulty power unit is located remain consistent; when there are multiple target groups, the group where the first faulty power unit is located is preferentially selected.

[0102] In this embodiment, for the case where two power units fail at the same level, when only two power units fail at the same level, such as in case 2.1, the other normal power units "n2m1" and "n3m1" of the same group m1 of the first failed unit "n1m1" are withdrawn; when power units fail at different levels in the same group, such as in case 2.2, the groups where the failed unit is located are m1 and m2, and the power unit "n2m2" of the same group at a different level fails, in order to make the target group and the group where the failed power unit is located consistent, m2 is determined as the target group, and the other normal power units n3m2 of the m2 group are withdrawn at the same time; when power units fail at different levels in different groups, such as in case 2.3, the groups where the failed unit is located are The groups are m1 and m3, and the power units "n2m2" of different levels and groups fail. At this time, the target group can select m1, m2, and m2. At this time, the group m1 where the first faulty power unit is located is preferentially selected as the target group, and the other normal power units n3m2 on the m1 group are exited, and the first power unit of the same group at the level where no faulty power unit exists is exited, and then the other normal power units at the same level adjust the current according to the second formula; when there is a single power unit failure in the remaining levels, such as situation 2.4, situation 2.5, and situation 2.6, first exit the faulty power unit and locate the level where two power units at the same level fail, and then the other normal power units at the same level adjust the current according to the second formula.

[0103] This embodiment provides a method for automatically selecting normal power units to exit, so that the power units that are forced to exit and the faulty power units are in the same group as much as possible, thereby ensuring the overall balance of the bidirectional converter device.

[0104] Although the present invention has been described in detail with reference to the accompanying drawings and in combination with preferred embodiments, the present invention is not limited thereto. Without departing from the spirit and essence of the present invention, a person of ordinary skill in the art may make various equivalent modifications or substitutions to the embodiments of the present invention, and such modifications or substitutions shall be within the scope of the present invention. Any person of ordinary skill in the art who can easily conceive of changes or substitutions within the technical scope disclosed in the present invention shall be included within the scope of protection of the present invention. Therefore, the scope of protection of the present invention shall be subject to the scope of protection of the claims.

Claims

1. A method for balancing power when a cascaded bidirectional converter unit fails, characterized in that: include: Determining the number of faulty power units in a cascaded bidirectional converter device, wherein the cascaded bidirectional converter device has a topology structure with multiple cascaded stages and multiple parallel groups; In a case where there is at most one faulty power unit in any one or more stages of the cascaded bidirectional converter device, determining the stage where the faulty power unit is located as the first target stage, and exiting the faulty power unit; determining the allocated current of the normal power units in the first target level according to the remaining number of normal power units in the first target level and the total current of each level; In the case that only one stage of the cascaded bidirectional converter device has two or more faulty power units, determining the stage as the second target stage and exiting the faulty power units; Determining the number of normal power units to be withdrawn in each level except the second target level according to the maximum carrying current of the power unit, and determining the current allocated to the normal power units in the second target level according to the number of withdrawn; The cascaded bidirectional converter device has a topology structure of n cascaded stages and m parallel groups. The current allocated to the normal power units in the first target stage is determined according to a first formula: i1´=mi / (m-1), where i is the current of each power unit when the power unit is normal, i1´ is the current allocated to the first target level, and m is the number of power units in each level; The current distributed according to the normal power unit in the second target level is determined according to the second formula: i2´=(mk)i / (mt), where i is the current of each power unit when the power unit is normal; i 2 ´ Current allocated to the second target level; m is the number of power units in each level, t is the number of failed power units in the second target level, and k is the exit number; The step of determining the exit number of normal power units in each level except the second target level according to the maximum carrying current of the power unit includes: Determine mi / (mt) and i max The size of i max ≥mi / (mt), then k=0; If i max <mi / (mt), then according to i max ≥(mk)i / (mt), determine the minimum value of k; Where m is the total number of power units in any one level of the device, t is the number of faulty power units in the second target level, i max is the maximum carrying current, k is a natural number, and k is less than m.

2. The method according to claim 1, characterized in that After determining the exit number of normal power units in each level except the second target level, the method further includes: Determine the target group of the exiting normal power units according to the group where the second target level faulty power unit is located, so that the target group and the group where the faulty power unit is located are consistent; When there are multiple target groups, the group where the first faulty power unit is located is preferentially selected.

3. The method according to claim 1, characterized in that The situation where there is at most one faulty power unit in any one or more stages includes the following situations: The entire device has only one faulty power unit; There is a faulty power unit failure in any two stages; There is a faulty power unit in any p levels, n is the number of levels of the cascaded bidirectional converter, p is a natural number, and 2<p≤n, wherein n>2.

4. The method according to claim 1, wherein The situation where there are two or more faulty power units at only one level includes the following situations: There are two or more faulty power units in any one stage, and only the faulty power units are in that stage; There are two or more faulty power units in only one stage, and there is also a faulty power unit in a different stage in the same group. There are two or more faulty power units in only one level, and there is also a faulty power unit in a different level and in a different group. There are two or more faulty power units in only one stage, and all other stages except this stage also have one faulty power unit in the same group; There are two or more faulty power units in only one level, and all levels except this level also have a faulty power unit in the group where the faulty power unit is located. There is only one stage with two or more faulty power units, and all other stages except this stage have a faulty power unit in a different group from the faulty power unit; There is only one stage with two or more faulty power units, and all other stages except this stage have one faulty power unit in the same group and one faulty power unit in a different group.

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