MMC ac side voltage boosting method independent of transformer tap changer

Abstract

The application discloses an MMC AC side voltage boosting method independent of transformer tap changers and relates to the technical field of direct current transmission. The method comprises the following steps: when full-bridge sub-modules output negative voltage, additional half-bridge sub-modules are additionally arranged in each bridge arm of a converter to cooperate with the full-bridge sub-modules to work, so that the upper limit of the modulation ratio of the converter is increased without reducing the direct current voltage, and the output margin of the AC voltage of the converter after the tap changer of the converter transformer is cancelled is ensured to be equivalent to that when the tap changer is configured. The application economically and effectively improves the AC voltage output capacity of the converter by adding the additional half-bridge sub-modules, enhances the ability of the converter to cope with different working conditions under the condition that the cost and failure rate of the converter transformer are reduced, and the control principle is simple and easy to realize.

Description

Technical Field

[0001] This invention relates to the field of DC power transmission technology, and in particular to a method for boosting the AC side voltage of an MMC without relying on a transformer tap changer. Background Technology

[0002] In recent years, the development of new energy power generation has been rapid, and it has become an important strategic direction for various countries. However, due to the intermittency and instability of new energy power generation, grid fluctuations are more frequent during the grid connection process. In existing projects, in addition to PI feedback regulation on both sides, another effective method to maintain the AC voltage at the converter grid connection point is the slow regulation of the converter transformer tap changer. Most of the existing flexible DC transmission projects use converter transformers with on-load tap changers.

[0003] However, the converter transformer tap changer, as a mechanical structure, suffers from slow response speed, susceptibility to failure, and high manufacturing costs. Eliminating the tap changer could not only reduce the occurrence of failures but also save on construction costs. However, eliminating the converter tap changer would cause voltage fluctuations on the AC system side of the converter to be directly reflected on the AC side of the converter. Therefore, certain measures are needed to address the impact of eliminating the tap changer.

[0004] To increase the upper limit of the converter modulation ratio, existing technologies mainly involve injecting third harmonics or negative input of full-bridge submodules, but both have their own shortcomings. First, the improvement capability of third harmonic injection has an upper limit, with the modulation ratio increasing to a maximum of 1.15; while simply using the negative input function of full-bridge submodules can increase the AC voltage output, it will cause DC voltage drop, making it unsuitable for steady-state operating conditions.

[0005] To address the aforementioned problems, this invention proposes a method for boosting the AC side voltage of an MMC that does not rely on a transformer tap changer. Summary of the Invention

[0006] The purpose of this invention is to provide a method for boosting the AC side voltage of a converter without relying on a transformer tap changer, thereby solving the problems mentioned in the background art. This invention, by adding an additional half-bridge submodule to the bridge arm of the semi-full hybrid MMC and coordinating with the negative input of the full-bridge submodule of the other bridge arm of the same phase, can improve the AC voltage output capability of the converter without DC voltage reduction.

[0007] To achieve the above objectives, the present invention adopts the following technical solution:

[0008] A method for boosting the AC side voltage of an MMC without relying on a transformer tap changer is proposed. When the full-bridge submodule outputs a negative voltage, an additional half-bridge submodule is added to each arm of the converter (MMC) to cooperate with the full-bridge submodule. This method increases the upper limit of the modulation ratio of the converter (MMC) without reducing the DC voltage, ensuring that the converter (MMC) maintains an AC voltage output margin comparable to that when the tap changer is configured after the converter transformer tap changer is removed.

[0009] Preferably, the method for calculating the number of additional half-bridge submodules is as follows:

[0010] The relationship between the AC output voltage of the converter (MMC) and the number of submodules after adding additional half-bridge submodules is defined as follows:

[0011]

[0012] Among them, U′ diff Indicates the maximum line voltage that the converter (MMC) can output after adding the additional half-bridge submodule; N′ represents the total number of submodules on the bridge arm after adding the additional half-bridge submodule; N represents the original number of submodules on the bridge arm; U c Indicates the rated voltage of the submodule capacitor; U diff This indicates the maximum AC voltage that the converter (MMC) can output without adding any additional half-bridge sub-modules;

[0013] After canceling the tap changer, U pcc Voltage fluctuations occur. To ensure that the AC voltage output capability of the converter after adding an additional half-bridge submodule has the same margin as before removing the tap changer, the voltage relationship must satisfy:

[0014]

[0015] Among them, U pcc This indicates the line voltage at point PCC when the converter transformer tap changer is not disabled; U′ pcc This indicates the fluctuating voltage at point PCC after the converter tap changer is cancelled;

[0016] Including the addition of extra submodules, the relationship between the number of bridge arm submodules and the modulation ratio is as follows:

[0017]

[0018] Substitute the system parameters into equation (3) and round up the number of submodules to obtain the additional number of half-bridge submodules.

[0019] Compared with the prior art, the present invention provides a method for boosting the AC side voltage of MMC without relying on a transformer tap changer, which has the following advantages:

[0020] This invention improves the AC voltage output capability of the converter without DC voltage reduction by adding an additional half-bridge submodule to the bridge arm of the semi-full hybrid MMC, in conjunction with the negative input of the full-bridge submodule of the other bridge arm of the same phase. This invention economically and effectively improves the AC voltage output capability of the converter, enhances the converter's ability to cope with different operating conditions while reducing the cost and failure rate of the converter transformer, and its control principle is simple and easy to implement. Attached Figure Description

[0021] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the accompanying drawings involved in the embodiments are now briefly described. Obviously, the drawings in the following description are merely illustrative of some embodiments of the present invention. For those skilled in the art, other forms of drawings can be constructed based on these drawings without creative effort.

[0022] Figure 1 This is a single-phase equivalent circuit diagram of the single-ended MMC-HVDC used in Embodiment 1 of the present invention;

[0023] Figure 2 This is a block diagram of the voltage equalization modulation strategy of the submodule in Embodiment 1 of the present invention;

[0024] Figure 3 This is a block diagram of the converter station control strategy in Embodiment 1 of the present invention. Detailed Implementation

[0025] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0026] A method for boosting the AC side voltage of an MMC without relying on a transformer tap changer is proposed. When the full-bridge submodule outputs a negative voltage, an additional half-bridge submodule is added to each arm of the converter (MMC) to cooperate with the full-bridge submodule. This method increases the upper limit of the modulation ratio of the converter (MMC) without reducing the DC voltage, ensuring that the converter (MMC) maintains an AC voltage output margin comparable to that when the tap changer is configured after the converter transformer tap changer is removed.

[0027] The method for calculating the number of additional half-bridge submodules is as follows:

[0028] The relationship between the AC output voltage of the converter (MMC) and the number of submodules after adding additional half-bridge submodules is defined as follows:

[0029]

[0030] Among them, U′ diff Indicates the maximum line voltage that the converter (MMC) can output after adding the additional half-bridge submodule; N′ represents the total number of submodules on the bridge arm after adding the additional half-bridge submodule; N represents the original number of submodules on the bridge arm; U c Indicates the rated voltage of the submodule capacitor; U diff This indicates the maximum AC voltage that the converter (MMC) can output without adding any additional half-bridge sub-modules;

[0031] After canceling the tap changer, U pcc Voltage fluctuations occur. To ensure that the AC voltage output capability of the converter after adding an additional half-bridge submodule has the same margin as before removing the tap changer, the voltage relationship must satisfy:

[0032]

[0033] Among them, U pcc This indicates the line voltage at point PCC when the converter transformer tap changer is not disabled; U′ pcc This indicates the fluctuating voltage at point PCC after the converter tap changer is cancelled;

[0034] Including the addition of extra submodules, the relationship between the number of bridge arm submodules and the modulation ratio is as follows:

[0035]

[0036] Substitute the system parameters into equation (3) and round up the number of submodules to obtain the additional number of half-bridge submodules.

[0037] The following description, in conjunction with relevant accompanying drawings and specific examples, illustrates a method for boosting the AC side voltage of an MMC that does not rely on a transformer tap changer, as proposed in this invention.

[0038] Example 1:

[0039] Please see Figure 1 ,like Figure 1 The diagram shown is a single-phase equivalent circuit diagram of the single-ended MMC-HVDC used in this embodiment. The AC system is equivalent to a voltage source U. S Series equivalent internal resistance Z sys The AC system and the converter transformer share a common coupling point (PCC), which is then connected via the equivalent reactance X of the converter transformer. T Connected to the converter valve, the equivalent reactance of the upper and lower bridge arms is X. L / 2 is ultimately connected to the equivalent converter valve.

[0040] The converter valve parameters used in this embodiment are S = 2000 MVA, U dc =400kV, submodule capacitor voltage Uc =2.2kV, AC side voltage U of the converter pcc =230kV, converter transformer tap changer number of taps +7 / -7, tap changer step size of 1.25% per tap.

[0041] The bridge arm submodule uses capacitor voltage equalization control to maintain the capacitor voltage balance of the submodule. Its control strategy is as follows: Figure 2 As shown.

[0042] After the tap changer is removed, the converter needs to directly face the fluctuations of the AC system, losing the buffer of voltage regulation provided by the transformer. To cope with AC voltage fluctuations, the AC voltage output capability of the converter needs to be enhanced to achieve higher output reactive power. In this embodiment, the converter station control strategy is as follows: Figure 3 As shown.

[0043] According to the tap data of the converter transformer in the embodiment, the converter transformer is sufficient to maintain the voltage stability of the point of common coupling within a range of 8.75% positive and negative fluctuations in the AC system. If the converter transformer tap changer is removed, the additional half-bridge sub-module should also be able to cope with the impact of 8.75% positive and negative fluctuations in the AC system.

[0044] When the AC system experiences negative fluctuations, the difference between the AC system voltage and the converter output AC voltage increases, but the impact on converter stability is small. Therefore, the main focus is on positive fluctuations in the AC system. When the AC system experiences a positive fluctuation of 8.75%, the PCC point voltage increases to 1.0875U. pcc Substitute the converter parameters into the following formula:

[0045]

[0046] The value of ΔN / N can be calculated to be 4.375%.

[0047] Further based on the formula:

[0048]

[0049] We can calculate N′ = 190, so we need to add 8 additional half-bridge sub-modules. After adding the additional half-bridge sub-modules, the AC output of the converter increases to 267kV, and still retains a 6.7% margin when the AC system experiences maximum positive fluctuations.

[0050] For ease of description, the various parts of the device described above are divided into modules or units according to their functions. Of course, in implementing this application, the functions of each module or unit can be implemented in one or more software or hardware components.

[0051] Those skilled in the art will understand that embodiments of this application can be provided as methods, systems, or computer program products. Therefore, this application can take the form of a completely hardware embodiment, a completely software embodiment, or an embodiment combining software and hardware aspects. This application can take the form of a computer program product embodied on one or more computer-usable storage media (including, but not limited to, disk storage, CD-ROM, optical storage, etc.) containing computer-usable program code.

[0052] This application is described with reference to flowchart illustrations and / or block diagrams of methods, apparatus (systems), and computer program products according to embodiments of this application. It will be understood that each block of the flowchart illustrations and / or block diagrams, and combinations of blocks in the flowchart illustrations and / or block diagrams, can be implemented by computer program instructions. These computer program instructions can be provided to a processor of a general-purpose computer, special-purpose computer, embedded processor, or other programmable data processing apparatus to produce a machine, such that the instructions, which execute via the processor of the computer or other programmable data processing apparatus, generate instructions for implementing the flowchart... Figure 1 One or more processes and / or boxes Figure 1 A device that provides the functions specified in one or more boxes.

[0053] These computer program instructions may also be stored in a computer-readable storage medium that can direct a computer or other programmable data processing device to function in a particular manner, such that the instructions stored in the computer-readable storage medium produce an article of manufacture including instruction means, which are implemented in a process Figure 1 One or more processes and / or boxes Figure 1 The function specified in one or more boxes.

[0054] These computer program instructions may also be loaded onto a computer or other programmable data processing equipment to cause a series of operational steps to be performed on the computer or other programmable equipment to produce a computer-implemented process, thereby providing instructions that execute on the computer or other programmable equipment for implementing the process. Figure 1 One or more processes and / or boxes Figure 1 The steps of the function specified in one or more boxes.

[0055] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit them. Those skilled in the art can still make modifications or equivalent substitutions to the specific implementation of the present invention by referring to the above embodiments. Any modifications or equivalent substitutions that do not depart from the spirit and scope of the present invention are within the protection scope of the claims of the present invention pending approval.

Claims

1. A transformer tap changer independent MMC AC side voltage boosting method, characterized in that, When the full-bridge submodule outputs a negative voltage, an additional half-bridge submodule is added to each bridge arm of the semi-full hybrid MMC to work in conjunction with the full-bridge module. This increases the modulation ratio limit of the semi-full hybrid MMC without reducing the DC voltage, ensuring that the semi-full hybrid MMC maintains the same AC voltage output margin as when the tap changer is configured after the converter transformer tap changer is removed. The method for calculating the number of additional half-bridge submodules is as follows: With the addition of an extra half-bridge submodule, the relationship between the output AC voltage of the semi-full hybrid MMC and the number of submodules is defined as follows: (1) wherein, represents the maximum line voltage that can be output by the semi-full hybrid MMC with the additional half bridge submodules added; represents the total number of submodules on the bridge arm with the additional half bridge submodules added; N represents the number of submodules on the original bridge arm; U c represents the rated voltage of the submodules; U diff represents the maximum AC voltage that can be output by the semi-full hybrid MMC without the additional half bridge submodules added; After canceling the tap changer U pcc The point voltage fluctuates. To ensure that the half-full hybrid MMC AC voltage output capability after adding the additional half-bridge sub-module has the same margin as before the tap changer is canceled, the voltage relationship satisfies: (2) wherein, U pcc represents the PCC point line voltage when the converter transformer tap changer is not cancelled; represents the PCC point fluctuation voltage after the converter transformer tap changer is cancelled; Including the addition of extra submodules, the relationship between the number of bridge arm submodules and the modulation ratio is as follows: (3) Substitute the system parameters into equation (3) and round up the number of submodules to obtain the additional number of half-bridge submodules.

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