Method and device for suppressing dc current harmonics in a flexible dc back-to-back system

Abstract

A method and device for suppressing DC current harmonics in a flexible DC back-to-back system, which realizes suppression of DC current harmonics by controlling the sum of the voltages of the phase input sub-modules. The method comprises the following steps: calculating the instantaneous value of the sum of the voltages of the phase input sub-modules; subtracting the instantaneous value of the sum of the voltages of the phase input sub-modules from the rated DC bus voltage, and generating a reference voltage superimposition amount of each bridge arm through a proportional controller; superimposing the generated superimposition amount with the original reference voltage of each bridge arm to generate a final reference voltage, and generating a trigger pulse of a switching device, thereby realizing suppression of DC current harmonics. The method can realize suppression of DC current harmonics without increasing the switching frequency.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of back-to-back flexible DC power transmission, and particularly relates to a DC current harmonic suppression method and device for a back-to-back flexible DC system. BACKGROUND

[0002] The back-to-back flexible DC system has the advantages of fast decoupling and independent control of active power and reactive power, can supply power to a passive system, and does not have the problem of commutation failure of conventional DC power transmission technology, and therefore has been widely concerned. During system operation, due to the input of the switching frequency optimization strategy, the imbalance degree between the capacitor voltages of the sub-modules in each bridge arm is aggravated, which leads to the occurrence of DC current harmonics and affects the system operation stability. Therefore, it is urgent to propose a DC current harmonic suppression method for a back-to-back flexible DC system. SUMMARY

[0003] The present application aims at the deficiencies of the prior art, and provides a DC current harmonic suppression method and device for a back-to-back flexible DC system, which is used to solve the problem of harmonics in the DC current during system operation.

[0004] To achieve the above-mentioned purpose, the present application adopts the following technical solutions:

[0005] The first aspect of the present application provides a DC current harmonic suppression method for a back-to-back flexible DC system, comprising:

[0006] calculating the instantaneous value of the sum of the input sub-module cascade voltages of each phase;

[0007] subtracting the instantaneous value of the sum of the input sub-module cascade voltages of each phase from the rated DC bus voltage, and generating the reference voltage superposition amount of each bridge arm through a proportional controller;

[0008] generating the final reference voltage by superimposing the generated reference voltage superposition amount of each bridge arm and the original reference voltage of each bridge arm, and generating the trigger pulse of the switching device, so as to suppress the DC current harmonics of the system.

[0009] Further, the calculation method of the instantaneous value U sumj of the sum of the input sub-module cascade voltages of each phase is as follows:

[0010]

[0011] In the formula:

[0012] N sum is the number of all sub-modules of each phase;

[0013] U sm_ij is the capacitor voltage of the i-th sub-module of the j-th phase, i=1, 2, 3, …, N sum , j=a, b, c;

[0014] e ij is the state variable of the i-th sub-module of the j-phase, when the sub-module is in the cut-off state, the value is 0, when the sub-module is in the put-in state and the output voltage shows positive voltage, the value is 1, and when the sub-module is in the put-in state and the output voltage shows negative voltage, the value is -1.

[0015] Further, the calculation method of the reference voltage superimposition amount of each bridge arm is:

[0016]

[0017] In the formula: Δu pj is the reference voltage superimposition amount of the j-phase upper bridge arm generated by the harmonic suppressor; Δu nj is the reference voltage superimposition amount of the j-phase lower bridge arm generated by the harmonic suppressor; U dc is the rated DC bus voltage; k p is the parameter of the proportional controller, and the recommended value range is 0.4 p < 0.7; U sumj is the instantaneous value of the voltage sum of the put-in sub-modules of each phase.

[0018] Further, the final reference voltage is calculated by the following formula:

[0019]

[0020] In the formula, u' pj is the final reference voltage of the j-phase upper bridge arm; u pj is the original reference voltage of the j-phase upper bridge arm; u' nj is the final reference voltage of the j-phase lower bridge arm; u nj is the original reference voltage of the j-phase lower bridge arm.

[0021] The second aspect of the application provides a DC current harmonic suppression device of a flexible DC back-to-back system, comprising:

[0022] A voltage sum instantaneous value calculation module is configured to calculate the instantaneous value of the voltage sum of the put-in sub-modules of each phase.

[0023] A reference voltage superimposition amount generation module is configured to subtract the rated DC bus voltage from the instantaneous value of the voltage sum of the put-in sub-modules of each phase, and generate the reference voltage superimposition amount of each bridge arm through a proportional controller.

[0024] A final reference voltage generation module is configured to superimpose the generated reference voltage superimposition amount and the original reference voltage of each bridge arm to generate the final reference voltage.

[0025] A trigger pulse generation module is configured to generate a trigger pulse of a switching device to suppress the DC current harmonic of the system.

[0026] Further, the voltage and instantaneous value calculation module calculates the instantaneous value U of the voltage sum of the input sub-modules of each phase according to the following formula sumj :

[0027]

[0028] In the formula,

[0029] N sum is the number of all sub-modules of each phase;

[0030] U sm_ij is the capacitor voltage of the i-th sub-module of the j-th phase, i = 1, 2, 3, …, N sum , j = a, b, c;

[0031] e ij is the state variable of the i-th sub-module of the j-th phase, the value is 0 when the sub-module is in the cut-off state, the value is 1 when the sub-module is in the input state and the output voltage shows positive voltage, and the value is -1 when the sub-module is in the input state and the output voltage shows negative voltage.

[0032] Further, the reference voltage superposition amount generation module calculates the reference voltage superposition amount of each bridge arm according to the following formula:

[0033]

[0034] In the formula, Δu pj is the reference voltage superposition amount of the upper bridge arm of the j-th phase generated by the harmonic suppressor; Δu nj is the reference voltage superposition amount of the lower bridge arm of the j-th phase generated by the harmonic suppressor; U dc is the rated DC bus voltage; k p is the parameter of the proportional controller, and the recommended value range is 0.4 < k p < 0.7; U sumj is the instantaneous value of the voltage sum of the input sub-modules of each phase.

[0035] Further, the final reference voltage generation module calculates the final reference voltage according to the following formula:

[0036]

[0037] In the formula, u' pj is the final reference voltage of the upper bridge arm of the j-th phase; u pj is the original reference voltage of the upper bridge arm of the j-th phase; u' nj is the final reference voltage of the lower bridge arm of the j-th phase; u nj is the original reference voltage of the lower bridge arm of the j-th phase.

[0038] In summary, the present application relates to a method and device for suppressing DC current harmonics in a flexible DC back-to-back system, which realizes suppression of DC current harmonics by controlling the sum of the voltages of the cascaded sub-modules of each phase. The method comprises the following steps: calculating the instantaneous value of the sum of the voltages of the cascaded sub-modules of each phase; subtracting the instantaneous value of the sum of the voltages of the cascaded sub-modules of each phase from the rated DC bus voltage, and generating the reference voltage superimposition of each bridge arm through a proportional controller; superimposing the generated superimposition and the original reference voltage of each bridge arm to generate the final reference voltage, and generating the trigger pulse of the switching device, thereby realizing suppression of DC current harmonics. The method can realize suppression of DC current harmonics without increasing the switching frequency. BRIEF DESCRIPTION OF DRAWINGS

[0039] Figure 1 Fig. 1 is a flowchart of the method for suppressing DC current harmonics in a flexible back-to-back system according to an embodiment of the present application;

[0040] Figure 2 Fig. 2 is a logic control diagram of the method for suppressing DC current harmonics in a flexible back-to-back system according to an embodiment of the present application. DETAILED DESCRIPTION

[0041] To make the objectives, technical solutions and advantages of the present application clearer, further detailed description will be made to the present application with reference to the specific embodiments and the accompanying drawings. It should be understood that these descriptions are only exemplary and are not intended to limit the scope of the present application. In addition, in the following description, the description of the known structures and technologies is omitted to avoid unnecessary confusion of the concept of the present application.

[0042] The first aspect of the present application provides a method for suppressing DC current harmonics in a flexible DC back-to-back system, as shown in Figure 1 , comprising:

[0043] Step S100: calculating the instantaneous value of the sum of the voltages of the cascaded sub-modules of each phase. The control object of the present application is the sum of the voltages of the cascaded sub-modules of all the phases, and the sum is compared with the rated DC voltage value, so the instantaneous value of the sum of the voltages of the cascaded sub-modules of each phase needs to be calculated first. Specifically, the instantaneous value of the sum of the voltages of the cascaded sub-modules of each phase is defined as the sum of the instantaneous values of the voltages of the sub-modules of each phase in the input state, so the instantaneous value of the sum of the voltages of the cascaded sub-modules of each phase U sumj can be calculated.

[0044]

[0045] In the formula:

[0046] N sum is the number of sub-modules of each phase;

[0047] U sm_ij Vci,j is the capacitor voltage of the i-th submodule in phase j, i = 1, 2, 3, …, N sum , j = a, b, c;

[0048] e ij is the state variable of the i-th submodule in phase j, the value is 0 when the submodule is in the cut-off state, the value is 1 when the submodule is in the put-in state and the output voltage shows positive voltage, and the value is -1 when the submodule is in the put-in state and the output voltage shows negative voltage.

[0049] In step S200, the reference voltage superimposed amount of each bridge arm is generated by subtracting the instantaneous value of the sum of the cascade voltages of the input submodules in each phase from the rated DC bus voltage and passing through a proportional controller. After the instantaneous value of the sum of the cascade voltages of the input submodules in each phase is calculated, the superimposed amount superimposed on the reference voltage of each bridge arm needs to be calculated. According to the harmonic suppression principle, a proportional controller is designed to obtain the superimposed amount of the reference voltage of each bridge arm by multiplying the difference between the rated DC voltage value and the instantaneous value of the sum of the cascade voltages of the input submodules in each phase by a specific coefficient. Specifically, the calculation method of the reference voltage superimposed amount of each bridge arm is as follows:

[0050]

[0051] In the formula, Δu pj is the reference voltage superimposed amount of the upper bridge arm in phase j generated by the harmonic suppressor; Δu nj is the reference voltage superimposed amount of the lower bridge arm in phase j generated by the harmonic suppressor; U dc is the rated DC bus voltage; k p is the parameter of the proportional controller, and the recommended value range is 0.4 < k p < 0.7; U sumj is the instantaneous value of the sum of the cascade voltages of the input submodules in each phase.

[0052] In step S300, the generated reference voltage superimposed amount and the original reference voltage of each bridge arm are superimposed to generate the final reference voltage, and the trigger pulse of the switching device is generated to suppress the harmonic of the system DC current. The suppression of the harmonic of the DC current is realized by superimposing the superimposed amount on the reference voltage and then adjusting the number of input submodules in real time. Therefore, the superimposed amount of the reference voltage of each bridge arm calculated is superimposed on the initial reference voltage of each bridge arm to generate the final reference voltage, and then the suppression of the harmonic of the DC current is realized. Specifically, the final reference voltage is calculated by the following formula:

[0053]

[0054] In the formula, u' pj is the final reference voltage of the upper bridge arm in phase j; u pjis the original reference voltage of the j-phase upper bridge arm; u nj is the final reference voltage of the j-phase lower bridge arm; u nj is the original reference voltage of the j-phase lower bridge arm.

[0055] The suppression of the DC current harmonics is realized by superimposing the superimposed amount on the reference voltage and then adjusting the number of input sub-modules in real time.

[0056] When the instantaneous value of the sum of the cascade voltages of all the input sub-modules of each phase is greater than the rated DC voltage value, the number of input sub-modules needs to be reduced to reduce the instantaneous value of the sum of the cascade voltages of the input sub-modules, so as to reduce the fluctuation rate of the DC bus voltage, and at this time, the difference between the rated DC voltage value and the instantaneous value of the sum of the cascade voltages of the input sub-modules is negative, that is, the calculated superimposed amount is negative, so the superimposed amount is directly superimposed on the reference voltage.

[0057] When the instantaneous value of the sum of the cascade voltages of all the input sub-modules of each phase is less than the rated DC voltage value, the number of input sub-modules needs to be increased to increase the instantaneous value of the sum of the cascade voltages of the input sub-modules, so as to reduce the fluctuation rate of the DC bus voltage, and at this time, the difference between the rated DC voltage value and the instantaneous value of the sum of the cascade voltages of the input sub-modules is positive, that is, the calculated superimposed amount is positive, so the superimposed amount is directly superimposed on the reference voltage.

[0058] The final reference voltage is calculated, and the trigger pulse of each switching device is calculated, so as to realize the control of the switching device and finally realize the suppression of the DC current harmonics.

[0059] Thus, the suppression of the DC current harmonics of the flexible DC back-to-back system is realized. The method can realize the suppression of the DC current harmonics of the flexible DC back-to-back system without increasing the switching frequency of the switching device.

[0060] The second aspect of the present application provides a DC current harmonic suppression device for a flexible DC back-to-back system, comprising:

[0061] A voltage and instantaneous value calculation module is configured to calculate the instantaneous value of the sum of the cascade voltages of the input sub-modules of each phase.

[0062] A reference voltage superimposed amount generation module is configured to subtract the instantaneous value of the sum of the cascade voltages of the input sub-modules of each phase from the rated DC bus voltage, and generate the reference voltage superimposed amount of each bridge arm through a proportional controller.

[0063] A final reference voltage generation module is configured to superimpose the generated reference voltage superimposed amount and the original reference voltage of each bridge arm to generate the final reference voltage.

[0064] A trigger pulse generation module is configured to generate the trigger pulse of the switching device to suppress the DC current harmonics of the system.

[0065] As Figure 2 shown, Figure 2 is the flexible back-to-back system DC current harmonic suppression method block diagram of the application, half of the DC bus voltage and the double closed loop controller output quantity through the addition and subtraction operation to get each bridge arm original reference voltage u pj , u nj , the DC bus voltage U dc as a reference value and each phase input sub-module capacitor voltage U sumj After the difference, then through the proportional controller to get each bridge arm reference voltage superposition amount Δu pj , Δu nj , the superposition of each bridge arm reference voltage superposition amount and the original reference voltage, get the final reference voltage, and through the trigger pulse generation module generates trigger pulse control switch device's opening and closing, realize the suppression of DC current harmonic.

[0066] In summary, the present application relates to a kind of flexible DC back-to-back system DC current harmonic suppression method and device, the method is controlled by each phase input sub-module cascade voltage sum and realizes DC current harmonic suppression.The implementation of the method includes the following steps: calculate the instantaneous value of each phase input sub-module cascade voltage sum;Difference between the instantaneous value of rated DC bus voltage and each phase input sub-module cascade voltage sum, and generate each bridge arm reference voltage superposition amount through proportional controller;The final version of reference voltage is generated by superimposing each superposition amount and the original reference voltage of each bridge arm, and the trigger pulse of switch device is generated, to realize the suppression of system DC current harmonic.The method can realize the suppression of DC current harmonic without increasing switching frequency.

[0067] It should be understood that the above specific embodiments of the present application are only used for illustrative or explanatory purposes of the principles of the present application, and do not constitute a limitation on the present application. Therefore, any modification, equivalent replacement, improvement, etc. made without departing from the spirit and scope of the present application shall be included within the scope of protection of the present application. In addition, the appended claims of the present application are intended to cover all variations and modifications falling within the scope and boundary of the appended claims, or the equivalent forms of such scope and boundary.

Claims

1. A method for DC current harmonic suppression in a flexible HVDC back-to-back system, characterized by, Comprise calculating the instantaneous value of the voltage sum of each phase input submodule; differencing the instantaneous value of the voltage sum of each phase input submodule from the rated DC bus voltage and generating the reference voltage superposition amount of each bridge arm through a proportional controller; adding the generated reference voltage superposition amount to the original reference voltage of each bridge arm to generate the final reference voltage, and generating the trigger pulse of the switching device to suppress the system DC current harmonics, wherein the final reference voltage is calculated by the following formula: In the formula, u' pj This is the final reference voltage for the upper arm of phase j; u pj The original reference voltage for the upper arm of phase j; u' nj is the final reference voltage for the lower bridge arm of phase j; u nj is the original reference voltage for the lower bridge arm of phase j.

2. The method of claim 1, wherein, the instantaneous value of the sum of the voltages of the submodules of the phases sumj The calculation method is: In the formula: N sum Number of submodules for each phase; U sm_ij Cji,j= 1,2,3,...,N sum , j = a, b, c; e ij Xi is the state variable of the jth sub-module of the ith phase, when the sub-module is in the off state, the value is 0, when the sub-module is in the on state and the output voltage shows positive voltage, the value is 1, and when the sub-module is in the on state and the output voltage shows negative voltage, the value is -1.

3. The method of claim 1 or 2, wherein the calculation method of the reference voltage superposition amount of each bridge arm is as follows: where: Δu pj is the j-phase upper bridge arm reference voltage superimposition generated by the harmonic suppressor; Δu nj is the j-phase lower bridge arm reference voltage superimposition generated by the harmonic suppressor; U dc is the rated DC bus voltage; k p is the parameter of the proportional controller; U sumj is the instantaneous value of the sum of the phase input sub-module voltages.

4. A device for harmonic current suppression in a flexible DC back-to-back system, characterized in that Comprise a voltage sum instantaneous value calculation module, configured to calculate the instantaneous value of the voltage sum of each phase input submodule; a reference voltage superposition amount generation module, configured to difference the instantaneous value of the voltage sum of each phase input submodule from the rated DC bus voltage and generate the reference voltage superposition amount of each bridge arm through a proportional controller; a final reference voltage generation module, configured to add the generated reference voltage superposition amount to the original reference voltage of each bridge arm to generate the final reference voltage; the final reference voltage generation module calculates the final reference voltage according to the following formula: In the formula, u' pj This is the final reference voltage for the upper arm of phase j; u pj The original reference voltage for the upper arm of phase j; u' nj is the final reference voltage for the lower bridge arm of phase j; u nj is the original reference voltage for the lower bridge arm of phase j; a trigger pulse generation module, configured to generate the trigger pulse of the switching device to suppress the system DC current harmonics.

5. The flexible DC back-to-back system DC current harmonic suppression device of claim 4, wherein, The voltage and instantaneous value calculation module calculates the instantaneous value U of the cascade voltage sum of each phase input sub-module according to the following formula sumj : In the formula: N sum Number of submodules for each phase; U sm_ij Cj,i is the capacitor voltage of the jth phase ith sub-module, i = 1, 2, 3, …, N sum , j = a, b, c; e ij Xi is the state variable of the jth submodule of the ith phase, the value of which is 0 when the submodule is in the off state, 1 when the submodule is in the on state and the output voltage assumes a positive voltage, and -1 when the submodule is in the on state and the output voltage assumes a negative voltage.

6. The flexible DC back-to-back system DC current harmonic suppression device according to claim 4 or 5, characterized in that, the reference voltage superposition amount generation module calculates the reference voltage superposition amount of each bridge arm according to the following formula: where: Δu pj is the j-phase upper bridge arm reference voltage superimposition generated by the harmonic suppressor; Δu nj is the j-phase lower bridge arm reference voltage superimposition generated by the harmonic suppressor; U dc is the rated DC bus voltage; k p is the parameter of the proportional controller; U sumj is the instantaneous value of the sum of the phase input sub-module voltages.

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