A method for monitoring the capacitance of an MMC sub-module based on pre-charging of a direct current side

Abstract

The application discloses a kind of based on DC side pre-charging's MMC submodule capacitor monitoring method, the capacitor monitoring method includes the following steps;All submodule capacitors in bridge arm are discharged to zero voltage;Charging is carried out to each submodule in turn;The time constant of RC charging loop is calculated to obtain submodule capacitor capacity value;Distinguish whether each submodule capacitor capacity value is less than threshold value, decide whether the submodule capacitor needs to be replaced, until the pre-charging and capacitor monitoring of all submodules are completed.The application can carry out submodule capacitor monitoring synchronously in the pre-charging link before MMC is put into use, so as to effectively monitor the state of each submodule capacitor and replace the fault capacitor in time.Compared with conventional capacitor monitoring method, the application does not involve additional measurement circuit, sensor and complex control algorithm, avoids heavy calculation burden, and can reduce the influence of measurement error, improve capacitor monitoring precision.

Description

Technical Field

[0001] This invention relates to the field of multilevel power electronic converter technology, specifically a method for monitoring the capacitance of an MMC submodule based on DC-side pre-charging. Background Technology

[0002] Modular multilevel converters (MMCs) are composed of multiple cascaded sub-modules with identical structures. They have superior characteristics such as high modularity, the ability to use low-voltage devices, high energy transmission efficiency, low harmonic content of AC-side output voltage, and the ability to achieve redundant control. They have broad prospects in flexible DC transmission, medium-voltage motor drive, and large-scale AC / DC grid interconnection, and are one of the most promising topologies for high-voltage and high-power applications.

[0003] Modular multilevel converters (MMCs) contain a large number of floating submodule capacitors, which play a crucial role in the operation of the MMC. However, with prolonged use and operation, these submodule capacitors often face the risk of failure and capacitance reduction, which can lead to MMC system malfunction and shutdown in severe cases. Therefore, to improve the reliability of the entire MMC system, it is essential to monitor the status of the MMC submodule capacitors.

[0004] Currently, some conventional methods have been proposed for monitoring the capacitance of submodules in modular multilevel converters (MMCs). However, these methods are often too complex, requiring additional measurement hardware and incurring heavy computational loads, which significantly increases the operating cost of the MMC system. Summary of the Invention

[0005] To address the shortcomings mentioned in the background technology, the present invention aims to provide a method for monitoring the capacitance of MMC submodules based on DC-side precharging. This method monitors the capacitance of submodules synchronously during the precharging process before the MMC is put into use. By calculating the time constant of the RC charging circuit formed during precharging, the capacitance value of the submodules can be obtained. This allows for effective monitoring of the capacitance status of each submodule before each use of the MMC, without the need for additional sensors or heavy computational burden.

[0006] The objective of this invention can be achieved through the following technical solutions:

[0007] A method for monitoring the capacitance of an MMC submodule based on DC-side pre-charging is described, wherein in the pre-charging circuit, all bridge arm inductors in the MMC are in a bypass state, and the DC side of the MMC is connected to a DC voltage source, a charging switch, and a charging resistor. The power supply charging voltage is the nominal value U of the submodule capacitor voltage. sm * .

[0008] Furthermore, the method for monitoring the capacitance of the MMC submodule based on DC-side pre-charging includes the following steps:

[0009] S1: Select any bridge arm to be charged and discharge all submodule capacitors on that bridge arm to zero voltage;

[0010] S2: Bypass the remaining sub-modules of this phase, sequentially engage each sub-module on the bridge arm, and calculate the equivalent time constant τ of charging for each sub-module on the bridge arm based on the changes in its capacitor voltage and charging time. i ;

[0011] S3: Using C i =τ i / R s Calculate the capacitance value C of each submodule. i , where R s This is the charging resistor.

[0012] S4: Determine the obtained submodule capacitance monitoring value C i If the measured value is less than the capacitance threshold, the capacitor of the corresponding submodule is replaced, and then the next submodule is pre-charged and its capacitance is monitored. Otherwise, the next submodule is pre-charged and its capacitance is monitored directly until all submodules have completed pre-charging and capacitance monitoring.

[0013] Furthermore, in step S2, the specific steps for calculating the equivalent charging time constant of the submodule are as follows:

[0014] S21: Record the submodule capacitor voltage as it charges from zero to 0.632U. sm * 0.865U sm * 0.950U sm * The charging times t1, t2, and t3;

[0015] S22: According to the RC charging equation U sm =U sm * (1-e -t / τ The average value of the actual equivalent time constant of the submodule is calculated to be τ. i = (t1+t2+t3) / 6.

[0016] Furthermore, the submodule capacitance monitoring process is completed synchronously with the MMC pre-charging process. After completion, the charging power supply and charging resistor should be removed, and the bridge arm inductor should be reconnected.

[0017] The beneficial effects of this invention are:

[0018] 1. By synchronously monitoring the submodule capacitors during the pre-charging stage before the MMC is put into use, the status of each submodule capacitor can be effectively monitored before each use of the MMC and faulty capacitors can be replaced in time. This will not only not affect the normal startup of the MMC, but also effectively avoid startup with faults.

[0019] 2. Compared with conventional capacitance monitoring methods, this invention does not involve complex control and calculation, avoiding a heavy computational burden. Moreover, it does not require additional measurement circuits, sensors, and auxiliary sub-modules, which means that the MMC system will not introduce additional reliability issues.

[0020] 3. In the pre-charging process, the present invention monitors the capacitor of each sub-module sequentially and averages the measurement results of the three charging time constants to reduce the impact of measurement error. Compared with other existing methods, it can improve the accuracy of capacitor monitoring. Attached Figure Description

[0021] The invention will now be further described with reference to the accompanying drawings:

[0022] Figure 1 This is a flowchart illustrating the implementation of the MMC submodule capacitance monitoring method described in this invention.

[0023] Figure 2 This is a circuit diagram of the DC-side pre-charge circuit of the MMC involved in this invention. Detailed Implementation

[0024] 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.

[0025] like Figure 2 As shown, in the pre-charge circuit, all bridge arm inductors in the MMC are in a bypass state, i.e., K a K b K c Close the circuit, connect the DC voltage source and charging switch K to the DC side of the MMC. s and charging resistor R s The power supply charging voltage is the nominal value U of the submodule capacitor voltage. sm * .

[0026] like Figure 1 As shown, the method for monitoring the capacitance of an MMC submodule based on DC-side pre-charging includes the following steps:

[0027] S1: Select any bridge arm to be charged and discharge all submodule capacitors on that bridge arm to zero voltage;

[0028] S2: Bypass the remaining sub-modules of this phase, sequentially engage each sub-module on the bridge arm, and calculate the equivalent time constant τ of charging for each sub-module on the bridge arm based on the changes in its capacitor voltage and charging time. i ;

[0029] S3: Using C i =τ i / R s Calculate the capacitance value C of each submodule. i , where R s This is the charging resistor.

[0030] S4: Determine the obtained submodule capacitance monitoring value C i If the measured value is less than the capacitance threshold, the capacitor of the corresponding submodule is replaced, and then the next submodule is pre-charged and its capacitance is monitored. Otherwise, the next submodule is pre-charged and its capacitance is monitored directly until all submodules have completed pre-charging and capacitance monitoring.

[0031] The specific method for calculating the equivalent charging time constant in step S2 is as follows:

[0032] S21: Record the submodule capacitor voltage as it charges from zero to 0.632U. sm * 0.865U sm * 0.950U sm * The charging times t1, t2, and t3;

[0033] S22: According to the RC charging equation U sm =U sm * (1-e -t / τ The average value of the actual equivalent time constant of the submodule is calculated to be τ. i = (t1+t2+t3) / 6.

[0034] Furthermore, the submodule capacitance monitoring process described in this invention is completed synchronously with the MMC pre-charging process. After completion, the charging power supply and charging resistor should be removed, and the bridge arm inductor connection should be restored, i.e., K should be disconnected. a K b K c .

[0035] The above-described MMC submodule capacitor monitoring method based on DC-side pre-charging can simultaneously monitor the submodule capacitors during the pre-charging stage before the MMC is put into use, thereby effectively monitoring the capacitor status of each submodule and replacing faulty capacitors in a timely manner. Compared with conventional capacitor monitoring methods, this invention does not involve additional measurement circuits, sensors, and complex control algorithms, avoiding heavy computational burdens, and can also reduce the impact of measurement errors and improve capacitor monitoring accuracy.

[0036] In the description of this specification, references to terms such as "an embodiment," "example," "specific example," etc., indicate that a specific feature, structure, material, or characteristic described in connection with that embodiment or example is included in at least one embodiment or example of the invention. In this specification, illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples.

[0037] The foregoing has shown and described the basic principles, main features, and advantages of the present invention. Those skilled in the art should understand that the present invention is not limited to the above embodiments. The embodiments and descriptions in the specification are merely illustrative of the principles of the invention. Various changes and modifications can be made to the invention without departing from its spirit and scope, and all such changes and modifications fall within the scope of the claimed invention.

Claims

1. A method for monitoring the capacitance of a sub-module of a modular multilevel converter (MMC) based on pre-charging of a DC side, characterized in that, In the pre-charging circuit, all bridge arm inductors in the MMC are in bypass state, the DC side of the MMC is connected with a DC voltage source, a charging switch and a charging resistor, and the output voltage of the power supply is the nominal value of the capacitor voltage of the sub-module ; The monitoring method comprises the following steps: S1: selecting an arbitrary bridge arm to be charged, and discharging all sub-module capacitors on the bridge arm to zero voltage; S2: bypass the bridge arm the rest of the sub-module, in turn put on each sub-module of the bridge arm, and according to its capacitor voltage and charging time change to calculate the equivalent time constant of each sub-module charging on the bridge arm ; S3: Utilizing The capacitance of each sub-module capacitor is calculated wherein is the charging resistance; S4: judging the obtained sub-module capacitor monitoring value whether less than a capacitor capacity threshold value, if the obtained monitoring value is less than the capacitor capacity threshold value, the corresponding sub-module capacitor is replaced, and then the next sub-module is pre-charged and the capacitor is monitored, otherwise the next sub-module is directly pre-charged and the capacitor is monitored, until all sub-modules complete pre-charging and capacitor monitoring.

2. The DC-side pre-charging based MMC sub-module capacitor monitoring method of claim 1, wherein, In the step S2, the specific steps of calculating the equivalent time constant of sub-module charging are as follows: S21: record the charging time of the sub-module capacitor voltage from zero to 0.632 , 0.865 , 0.950 , , ;​ S22: According to the RC charging equation The average value of the actual equivalent time constant of the sub-module is calculated as .

3. The DC-side pre-charging based MMC sub-module capacitor monitoring method of claim 1, wherein, The sub-module capacitor monitoring process is completed synchronously with the MMC pre-charging process, and after completion, the charging power supply and the charging resistor should be removed, and the bridge arm inductor is connected again.

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