Transformer bias current suppression method under MMC parameter imbalance and related equipment

Abstract

The invention belongs to the field of flexible direct current power transmission, and discloses a transformer bias current suppression method under MMC parameter imbalance and related equipment. Common-mode power frequency voltage, first modulation voltage, second modulation voltage and differential-mode modulation waves are respectively obtained by four targets of consistent energy of upper and lower bridge arms of an in-phase unit and suppression of direct-current power frequency zero-sequence current, and final modulation waves of the upper and lower bridge arms are calculated to control the MMC. According to the method, aiming at various factors of transformer magnetic bias current caused by MMC parameter imbalance, control targets are set from multiple aspects of bridge arm energy, current characteristics and the like, and parameters are balanced through accurate control of modulation waves. Compared with an existing method, the method has the advantages that the problems of complex control strategy and poor suppression effect are avoided, the bias current of the transformer can be effectively suppressed, the harmonic current is reduced, the loss is reduced, the electric energy quality and transmission efficiency are improved, and stable operation of a power system is guaranteed.

Description

Technical Field

[0001] The present invention belongs to the technical field of flexible direct current transmission, and in particular relates to a method for suppressing transformer bias current under MMC parameter imbalance and related equipment. Background Art

[0002] In modern power systems, high-voltage direct current (HVDC) transmission technology, with its advantages of large capacity, long-distance transmission, and flexible regulation, has become a key means of integrating renewable energy into the grid and allocating power across regions. As a core component of HVDC systems, the modular multilevel converter (MMC) boasts a high degree of scalability and flexibility thanks to its modular design. The number of submodules can be easily adjusted based on actual needs, adapting to different voltage levels and power capacity requirements. Furthermore, the MMC output voltage waveform is nearly sinusoidal with low harmonic content, meeting power quality standards without the need for complex filtering devices, effectively reducing system costs and footprint. Furthermore, its excellent dynamic response performance and fault ride-through capability enable rapid response to system faults, ensuring the safe and stable operation of the power system. Consequently, it has been widely used and studied in the power sector.

[0003] However, during the actual operation of MMC, various parameter imbalances often occur due to the complexity of system parameters and the variability of the operating environment. For example, the number of submodules in the upper and lower bridge arms of a phase unit may be inconsistent due to faults or other reasons; the capacitance of the submodules may deviate due to production processes, usage time, and environmental factors; and the bridge arm reactors may also be unbalanced due to differences in parameter design and installation locations. These parameter imbalances will cause the converter to output a certain amount of DC current on the AC side. The DC current output on the AC side will flow through the valve-side winding of the transformer, which can easily cause transformer demagnetization and abnormal noise. At the same time, it will generate power frequency zero-sequence current on the DC side, affecting the output characteristics of the converter's AC and DC ports. Existing control methods have the disadvantages of complex control strategies and poor suppression effects when dealing with problems caused by these parameter imbalances, making it difficult to meet the stringent requirements of power systems for efficient and stable operation. Summary of the Invention

[0004] The present invention provides a transformer bias current suppression method under MMC parameter imbalance and related equipment. The method takes the energy stored in the capacitors of all submodules in the upper and lower bridge arms of the same-phase unit as the consistency as a control target, and obtains the common-mode power frequency voltage between phases required for energy balance of each phase; takes the power frequency current content in the MMC direct current as 0 as a control target, and then controls the zero-sequence power frequency output current of the MMC; takes the DC component in the current of the upper and lower bridge arms of the same phase of the MMC as the control target; obtains the differential-mode DC voltage deviation of the upper and lower bridge arms; suppresses the double frequency circulating current between the converter phases to reduce the operating loss of the converter valve; and through energy balance control of the upper and lower bridge arms of the same phase of the MMC converter, zero-sequence power frequency current suppression of the DC current, and DC component deviation control of the current between the upper and lower bridge arms of the same phase, it is possible to achieve transformer bias current suppression under various converter parameter imbalances such as deviation in the number of bridge arm submodules, deviation in the capacitance value of the submodules, deviation in the reactance value of the bridge arm, and deviation in the series resistance value of the bridge arm, while ensuring that the energy of the capacitors of each bridge arm of the converter valve is basically consistent.

[0005] In order to achieve the above object, the present invention adopts the following technical solutions: A method for suppressing transformer bias current under MMC parameter imbalance, comprising: Obtain the total energy stored in each bridge arm of the MMC and the circulating current of each phase unit; Based on the total energy stored in each bridge arm, the first control target is to make the total energy stored in the upper bridge arm and the lower bridge arm of the same-phase unit consistent, and the common-mode power frequency voltage corresponding to each phase unit is obtained; the second control target is to suppress the power frequency zero-sequence current in the DC current of the MMC, and the first modulation voltage is obtained; based on the circulating current of each phase unit, the third control target is to suppress the doubled frequency circulating current between the MMC phases, and the second modulation voltage is obtained; the fourth control target is to make the DC current component between the upper bridge arm current and the lower bridge arm current of the same-phase unit equal, and the differential-mode modulation wave is obtained; Based on the common-mode power frequency voltage, the first modulation voltage, the second modulation voltage and the differential-mode modulation wave of each phase unit, the final modulation wave of the upper bridge arm and the final modulation wave of the lower bridge arm corresponding to each phase unit are calculated; the final modulation wave of the upper bridge arm and the final modulation wave of the lower bridge arm are used to control the MMC to suppress the bias current of the transformer.

[0006] Further, The obtaining of the total energy stored in each bridge arm of the MMC and the circulating current of each phase unit includes: Obtain the rated capacitance value of the submodule in each bridge arm of the MMC, the actual operating voltage of the capacitor of each submodule, and the number of submodules in each bridge arm; Based on the rated capacitance of the submodules in each bridge arm, the actual operating voltage of the capacitors in each submodule, and the number of submodules in each bridge arm, the total energy stored in each bridge arm is calculated. The specific formula is as follows:

[0007] Where Earm is the total energy stored in the bridge arm; C is the rated capacitance of the submodule; k is the number of submodules in each bridge arm, ranging from 1 to N, where N is a positive integer; Uc is the actual working voltage of the capacitor of each submodule; Uc （k） is the actual working voltage of the kth submodule capacitor; Obtain the upper arm current and lower arm current of each phase unit; Based on the upper arm current and lower arm current of each phase unit, the circulating current of each phase unit is calculated. The specific formula is as follows:

[0008] Where Icir_j is the circulating current of the phase unit; Iarm_top_j is the upper arm current; Iarm_btm_j is the lower arm current; j=A, B, C, representing the three phase units respectively.

[0009] Furthermore, the total energy stored in each bridge arm is based on the total energy stored in the upper bridge arm and the lower bridge arm of the same-phase unit, and the common-mode power frequency voltage corresponding to each phase unit is obtained by taking the consistency of the total energy stored in the upper bridge arm and the lower bridge arm of the same-phase unit as the first control target, including: The first control goal is to make the total energy stored in the upper bridge arm and the lower bridge arm of the same-phase unit consistent. A proportional-integral controller is used to control the deviation between the total energy stored in the upper bridge arm and the lower bridge arm of the same-phase unit so that the total energy stored in the upper bridge arm and the lower bridge arm of the same-phase unit remains consistent, thereby obtaining the output result of the proportional-integral controller. The output result of the proportional-integral controller is multiplied by the three-phase power frequency common-mode reference voltage to obtain the common-mode power frequency voltage corresponding to each phase unit; wherein, the three-phase power frequency common-mode reference voltage is obtained by lagging the three-phase voltage on the valve side of the MMC transformer by 1 / 4 of the power frequency period.

[0010] Furthermore, the method of obtaining the first modulation voltage by taking the suppression of the power frequency zero-sequence current in the direct current of the MMC as the second control target includes: The second control objective is to suppress the power frequency zero-sequence current in the DC current of the MMC, and perform band-pass filtering of the DC current of the MMC in the power frequency band; wherein the resonant frequency of the quasi-PR controller is set to the power frequency; The direct current of the MMC after band-pass filtering is controlled by a quasi-PR controller, and a first output result of the quasi-PR controller is used as a first modulation voltage.

[0011] Furthermore, the second modulation voltage is obtained based on the circulating current of each phase unit with the third control target of suppressing the double frequency circulating current between the MMC phases, including: The third control objective is to suppress the double frequency circulating current between MMC phases. The circulating current of each phase unit is subjected to band-stop filtering and high-pass filtering to remove the power frequency and DC components in the circulating current. A quasi-PR controller is used to control the circulating current after band-stop filtering and high-pass filtering, and a second output result of the quasi-PR controller is used as a second modulation voltage; wherein the resonant frequency of the quasi-PR controller is set to twice the power frequency.

[0012] Furthermore, the fourth control target is to make the DC current component between the upper arm current and the lower arm current of the in-phase unit equal to each other, thereby obtaining a differential mode modulation wave, including: Taking the DC current component between the upper bridge arm current and the lower bridge arm current of each phase unit as the fourth control target; Obtaining a DC component in the bridge arm current of each bridge arm; Subtracting the DC current component of the lower bridge arm current from the DC current component of the upper bridge arm current in the same-phase unit to obtain a DC current component deviation between the upper bridge arm current and the lower bridge arm current; A proportional-integral controller is used to control the DC current component deviation between the upper bridge arm current and the lower bridge arm current of each phase, and the output result of the proportional-integral controller is used as a differential-mode modulation wave.

[0013] Furthermore, based on the common-mode power frequency voltage, the first modulation voltage, the second modulation voltage and the differential-mode modulation wave of each phase unit, the upper bridge arm final modulation wave and the lower bridge arm final modulation wave corresponding to each phase unit are calculated. The specific formula is as follows:

[0014]

[0015] Where Vref_top_j is the final modulation wave of the upper bridge arm; Vref_btm_j is the final modulation wave of the lower bridge arm; Vref_j is the modulation wave obtained by MMC control; Udc_set is the DC voltage output by the bridge arm; Uac1_j is the common-mode power frequency voltage; Ud1 is the first modulation voltage; Uac2_j is the second modulation voltage; Ud2_j is the differential-mode modulation wave; j = A, B, C, representing the three phase units respectively.

[0016] A transformer bias current suppression system under MMC parameter imbalance, comprising: Data acquisition module, used to obtain the total energy stored in each bridge arm of the MMC and the circulating current of each phase unit; A calculation module is configured to obtain a common-mode power frequency voltage corresponding to each phase unit based on the total energy stored in each bridge arm, with the total energy stored in the upper bridge arm and the lower bridge arm of the same-phase unit being consistent as a first control target; obtain a first modulation voltage based on the suppression of the power frequency zero-sequence current in the DC current of the MMC as a second control target; obtain a second modulation voltage based on the circulating current of each phase unit, with the suppression of the doubled frequency circulating current between the MMC phases as a third control target; and obtain a differential-mode modulation wave based on the equality of the DC current component between the upper bridge arm current and the lower bridge arm current of the same-phase unit as a fourth control target; The control output module is used to calculate the final modulation wave of the upper bridge arm and the final modulation wave of the lower bridge arm corresponding to each phase unit based on the common-mode power frequency voltage, the first modulation voltage, the second modulation voltage and the differential-mode modulation wave of each phase unit; the final modulation wave of the upper bridge arm and the final modulation wave of the lower bridge arm are used to control the MMC to suppress the bias current of the transformer.

[0017] An electronic device, comprising: memory for storing computer programs; A processor is configured to implement the steps of the transformer bias current suppression method under MMC parameter imbalance when executing the computer program.

[0018] A computer-readable storage medium stores a computer program, which, when executed by a processor, is used to implement the steps of the transformer bias current suppression method under MMC parameter imbalance.

[0019] Compared with the prior art, the present invention has the following beneficial effects: The present invention provides a method for suppressing transformer bias current under MMC parameter imbalance. This method obtains the total energy of each bridge arm of the MMC and the circulating current of each phase unit, and obtains the common-mode power frequency voltage, the first modulation voltage, the second modulation voltage and the differential-mode modulation wave respectively with the four goals of consistent energy of the upper and lower bridge arms of the same-phase unit and suppression of the DC current power frequency zero-sequence current, and then calculates the final modulation wave of the upper and lower bridge arms to control the MMC. This method targets various factors that lead to transformer bias current due to MMC parameter imbalance, sets control targets from multiple aspects such as bridge arm energy and current characteristics, and balances parameters through precise control of the modulation wave. Compared with existing methods, this method avoids the problems of complex control strategies and poor suppression effects, and can effectively suppress transformer bias current, reduce harmonic currents, reduce losses, improve power quality and transmission efficiency, and ensure stable operation of the power system. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 A flowchart of a method for suppressing transformer bias current under MMC parameter imbalance provided by the present invention; Figure 2This is a structural schematic diagram of a transformer bias current suppression system under MMC parameter imbalance provided by the present invention. DETAILED DESCRIPTION

[0021] In order to further understand the content of the present invention, the present invention is described in detail below in conjunction with the accompanying drawings and specific embodiments. It should be understood that the embodiments are only for explaining the present invention and are not intended to limit it.

[0022] The following is an explanation of the technical terms involved in the present invention: MMC: Modular Multilevel Converter, a modular multilevel converter, is a new type of voltage source converter topology.

[0023] PR controller: Proportional-Resonant Controller, also known as proportional resonant controller, is a controller widely used in AC control systems. It combines the characteristics of proportional control (P) and resonant control (R) and can achieve zero-static error tracking control of AC signals.

[0024] Quasi-PR controller: As an improved PR controller, it improves the frequency adaptability and stability problems of the traditional PR controller by introducing a damping term.

[0025] This embodiment provides a method for suppressing transformer bias current under MMC parameter imbalance, which is specifically as follows: In this embodiment, a modular multilevel converter (MMC) includes three phase units, namely phases A, B, and C. Each phase unit consists of an upper bridge arm and a lower bridge arm, and each bridge arm is composed of a bridge arm reactor and several submodules connected in series. The method includes: S1: Obtain the capacitor voltage of the submodule in each bridge arm of the MMC (i.e., the actual working voltage of the capacitor of each submodule), and calculate the energy stored in the submodule in each bridge arm, i.e., the total energy stored in each bridge arm; Obtain the arm current of each bridge arm of the MMC and calculate the circulating current in each phase unit; Get the DC current output by the MMC; Obtain the three-phase voltage on the valve side of the MMC transformer.

[0026] S2: Based on the submodule stored energy deviation of the upper and lower bridge arms of each phase unit, which refers to the difference between the total energy stored in the two bridge arms of the same phase unit; calculate the common-mode power frequency voltages Uac1_A, Uac1_B, and Uac1_C required to eliminate the total energy deviation stored in the upper and lower bridge arms of the three-phase unit.

[0027] In this embodiment, the stored energy of the bridge arm can be calculated using the following formula:

[0028] Where Earm is the total energy stored in the bridge arm; C is the rated capacitance of the submodule; k is the number of submodules in each bridge arm, ranging from 1 to N, where N is a positive integer; Uc is the actual working voltage of the capacitor of each submodule; Uc （k） is the actual working voltage of the kth submodule capacitor.

[0029] The specific execution steps are as follows: S21: The total energy deviation stored in the upper and lower bridge arms of each phase unit is equal to the stored energy of the upper bridge arm minus the stored energy of the lower bridge arm, and the subtraction result is filtered through a band-stop filter and a low-pass filter to obtain the total energy deviation stored in the upper and lower bridge arms of each phase after filtering, wherein the center frequency of the band-stop filter is set to the power frequency; S22: The total energy stored in the upper and lower bridge arms of each phase is controlled separately by a proportional-integral controller; S23: lag the three-phase voltage on the valve side of the converter transformer by 1 / 4 of the power frequency cycle to obtain the three-phase power frequency common mode reference voltage; S24: Multiply the output results of the proportional-integral controller of the total energy stored in the upper and lower bridge arms of each phase by the three-phase power frequency common-mode reference voltage respectively, and calculate the common-mode power frequency voltages Uac1_A, Uac1_B, and Uac1_C corresponding to the three-phase unit.

[0030] S3: Based on the acquired converter output DC current, suppress the power frequency zero-sequence current in the converter DC current, and calculate a modulation voltage Ud1 for suppressing the power frequency component in the DC current, that is, obtain a first modulation voltage.

[0031] The specific execution steps are as follows: S31: performing band-pass filtering on the output DC current of the MMC in the power frequency band; S32: The bandpass filtered DC current is controlled by a quasi-PR controller, the resonant frequency of the quasi-PR controller is set to the industrial frequency, and the output result of the quasi-PR controller is used as the modulation voltage Ud1 for suppressing the industrial frequency component in the DC current, that is, the first modulation voltage Ud1 is obtained.

[0032] S4: Based on the acquired bridge arm current, suppress the double frequency interphase circulating current of the converter, and calculate the modulation voltages Uac2_A, Uac2_B, and Uac2_C for suppressing the double frequency interphase circulating current of the converter; that is, obtain the second modulation voltage.

[0033] The calculation formula for the circulation of each phase unit is as follows:

[0034] Where Icir_j is the circulating current of the phase unit; Iarm_top_j is the upper arm current; Iarm_btm_j is the lower arm current; j=A, B, C, representing the three phase units respectively.

[0035] The specific steps are as follows: S41: The circulating current Icir of each phase unit can be subjected to band-stop filtering and high-pass filtering respectively to remove the power frequency and DC components in the circulating current; S42: The filtered circulating current is controlled by a quasi-PR controller. The resonant frequency of the quasi-PR controller is set to twice the operating frequency. The output results of each quasi-PR controller are used as the modulation voltages Uac2_A, Uac2_B, and Uac2_C of the doubled-frequency interphase circulating current, thereby obtaining the second modulation voltage corresponding to each phase unit.

[0036] S5: Based on the acquired bridge arm current, the deviation of the DC current component of the upper and lower bridge arm currents of each phase unit of the converter is suppressed, and the differential mode modulation waves Ud2_A, Ud2_B, and Ud2_C with equal DC current components in the upper and lower bridge arm currents of each phase unit are calculated; that is, the differential mode modulation waves corresponding to each phase unit are obtained.

[0037] The specific steps are as follows: S51: Calculate the DC component of the bridge arm current of each bridge arm; S52: Subtract the DC component of the lower bridge arm current from the DC component of the upper bridge arm current in each in-phase unit to obtain a DC current component deviation of the upper and lower bridge arm currents of each phase unit; S53: The DC current component deviation of the upper and lower bridge arm currents of each phase unit is controlled by a proportional-integral controller, and the output results of each proportional-integral controller are used as differential-mode modulation waves Ud2_A, Ud2_B, and Ud2_C, that is, the differential-mode modulation waves Ud2_A, Ud2_B, and Ud2_C corresponding to each phase unit are obtained.

[0038] S6: Based on the acquired common-mode power frequency voltage, first modulation voltage, second modulation voltage and differential-mode modulation wave of each phase unit, i.e., Uac1_A, Uac1_B, Uac1_C, Ud1, Uac2_A, Uac2_B, Uac2_B, Ud2_A, Ud2_B, Ud2_C, calculate the final modulation wave of each bridge arm, and control the converter according to the final modulation wave of each bridge arm.

[0039] The specific calculation formula is as follows:

[0040]

[0041] Where Vref_top_j is the final modulation wave of the upper bridge arm; Vref_btm_j is the final modulation wave of the lower bridge arm; Vref_j is the modulation wave obtained by MMC control; Udc_set is the DC voltage output by the bridge arm; Uac1_j is the common-mode power frequency voltage; Ud1 is the first modulation voltage; Uac2_j is the second modulation voltage; Ud2_j is the differential-mode modulation wave; j = A, B, C, representing the three phase units respectively.

[0042] For example, the suppression method provided in this embodiment is applied to a practical project, and the specific implementation is as follows: Application scenario and parameter setting: In a ±500kV modular multilevel converter (MMC) HVDC transmission system, unbalanced MMC parameters can cause magnetic bias current in the connected transformer, affecting system stability.

[0043] Each phase of the upper and lower bridge arms of this MMC system consists of 100 sub-modules, and the rated capacitance of the sub-module is 3000μF.

[0044] The execution steps are as follows: The first step is to collect the rated capacitance value of the submodule in each bridge arm, the actual operating voltage of the capacitor of each submodule, and the number of submodules in each bridge arm in real time, and calculate the total energy stored in each bridge arm.

[0045] At the same time, the upper arm current and the lower arm current of each phase unit are obtained to calculate the circulating current of each phase unit.

[0046] In the second step, the first control goal is to ensure that the total energy stored in the upper and lower arms of the same-phase unit is consistent. A proportional-integral controller is used to control the total energy deviation stored in the upper and lower arms of the same-phase unit to keep them consistent, and the output result of the proportional-integral controller is obtained. This output result is multiplied by the three-phase power frequency common-mode reference voltage obtained by lagging the three-phase voltage on the valve side of the MMC transformer by 1 / 4 of the power frequency period to obtain the common-mode power frequency voltage corresponding to each phase unit.

[0047] The second control objective is to suppress the power frequency zero-sequence current in the MMC's DC current. The MMC's DC current is bandpass filtered at the power frequency band (the quasi-PR controller's resonant frequency is set to the power frequency). This bandpass-filtered MMC DC current is controlled using a quasi-PR controller, with the first output serving as the first modulation voltage. The third control objective is to suppress the doubled-frequency circulating current between the MMC phases. The circulating current of each phase unit is subjected to band-stop filtering and high-pass filtering to remove the power frequency and DC components. This processed circulating current is then controlled using a quasi-PR controller (with the resonant frequency set to twice the power frequency), with the second output serving as the second modulation voltage.

[0048] Differential-mode modulation wave acquisition: The fourth control target is to make the DC current component between the upper arm current and the lower arm current of each phase unit equal; the DC component in the arm current of each bridge arm is obtained, and the deviation of the DC current component of the upper arm current minus the DC current component of the lower arm current in the same-phase unit is calculated; the deviation is controlled by a proportional-integral controller, and the output result is used as the differential-mode modulation wave.

[0049] Based on the common-mode power frequency voltage, first modulation voltage, second modulation voltage and differential-mode modulation wave of each phase unit obtained by the above calculation, as well as the modulation wave obtained by MMC control and the DC voltage output by the bridge arm, the final modulation wave of the upper bridge arm and the final modulation wave of the lower bridge arm corresponding to each phase unit are calculated according to the above-obtained results.

[0050] Finally, the generated upper bridge arm final modulation wave and lower bridge arm final modulation wave are used to control the MMC. After a period of operation monitoring, the transformer bias current is significantly reduced and the system operation stability is significantly improved.

[0051] For example, Figure 1 As shown, this embodiment also provides a method for suppressing transformer bias current under MMC parameter imbalance, including the following steps: Obtain the total energy stored in each bridge arm of the MMC and the circulating current of each phase unit; Based on the total energy stored in each bridge arm, the first control target is to make the total energy stored in the upper bridge arm and the lower bridge arm of the same-phase unit consistent, and the common-mode power frequency voltage corresponding to each phase unit is obtained; the second control target is to suppress the power frequency zero-sequence current in the DC current of the MMC, and the first modulation voltage is obtained; based on the circulating current of each phase unit, the third control target is to suppress the doubled frequency circulating current between the MMC phases, and the second modulation voltage is obtained; the fourth control target is to make the DC current component between the upper bridge arm current and the lower bridge arm current of the same-phase unit equal, and the differential-mode modulation wave is obtained; Based on the common-mode power frequency voltage, the first modulation voltage, the second modulation voltage and the differential-mode modulation wave of each phase unit, the final modulation wave of the upper bridge arm and the final modulation wave of the lower bridge arm corresponding to each phase unit are calculated; the final modulation wave of the upper bridge arm and the final modulation wave of the lower bridge arm are used to control the MMC to suppress the bias current of the transformer.

[0052] In this embodiment, The obtaining of the total energy stored in each bridge arm of the MMC and the circulating current of each phase unit includes: Obtain the rated capacitance value of the submodule in each bridge arm of the MMC, the actual operating voltage of the capacitor of each submodule, and the number of submodules in each bridge arm; Based on the rated capacitance of the submodules in each bridge arm, the actual operating voltage of the capacitors in each submodule, and the number of submodules in each bridge arm, the total energy stored in each bridge arm is calculated. The specific formula is as follows:

[0053] Where Earm is the total energy stored in the bridge arm; C is the rated capacitance of the submodule; k is the number of submodules in each bridge arm, ranging from 1 to N, where N is a positive integer; Uc is the actual operating voltage of the capacitor of each submodule; Obtain the upper arm current and lower arm current of each phase unit; Based on the upper arm current and lower arm current of each phase unit, the circulating current of each phase unit is calculated. The specific formula is as follows:

[0054] Where Icir_j is the circulating current of the phase unit; Iarm_top_j is the upper arm current; Iarm_btm_j is the lower arm current; j=A, B, C, representing the three phase units respectively.

[0055] The total bridge arm energy is calculated using the rated capacitance, actual operating voltage, and number of submodules, and the phase unit circulating current is calculated using the upper and lower bridge arm currents. This precise acquisition method provides an accurate data foundation for subsequent control, helping to more accurately achieve various control objectives and improve the suppression of transformer bias current.

[0056] In this embodiment, the total energy stored in each bridge arm is based on the total energy stored in the upper bridge arm and the lower bridge arm of the same-phase unit, and the total energy stored in the upper bridge arm and the lower bridge arm of the same-phase unit is consistent as the first control target to obtain the common-mode power frequency voltage corresponding to each phase unit, including: The first control goal is to make the total energy stored in the upper bridge arm and the lower bridge arm of the same-phase unit consistent. A proportional-integral controller is used to control the deviation between the total energy stored in the upper bridge arm and the lower bridge arm of the same-phase unit so that the total energy stored in the upper bridge arm and the lower bridge arm of the same-phase unit remains consistent, thereby obtaining the output result of the proportional-integral controller. The output result of the proportional-integral controller is multiplied by the three-phase power frequency common-mode reference voltage to obtain the common-mode power frequency voltage corresponding to each phase unit; wherein, the three-phase power frequency common-mode reference voltage is obtained by lagging the three-phase voltage on the valve side of the MMC transformer by 1 / 4 of the power frequency period.

[0057] Based on the total energy of each bridge arm, the common-mode power frequency voltage is generated with the goal of achieving uniform total energy between the upper and lower bridge arms of the in-phase unit. A proportional-integral controller is used to control the energy deviation, which is then multiplied by the three-phase power frequency common-mode reference voltage to obtain the common-mode power frequency voltage. This approach effectively balances the energy between the upper and lower bridge arms of the in-phase unit, minimizing the impact of parameter imbalance from an energy perspective and thereby suppressing transformer bias current.

[0058] In this embodiment, the second control objective is to suppress the power frequency zero-sequence current in the DC current of the MMC to obtain the first modulation voltage, including: The second control objective is to suppress the power frequency zero-sequence current in the DC current of the MMC, and perform band-pass filtering of the DC current of the MMC in the power frequency band; wherein the resonant frequency of the quasi-PR controller is set to the power frequency; The direct current of the MMC after band-pass filtering is controlled by a quasi-PR controller, and a first output result of the quasi-PR controller is used as a first modulation voltage.

[0059] The first modulation voltage is derived with the secondary control objective of suppressing the power-frequency zero-sequence current in the MMC DC current. By bandpass filtering the DC current at the power-frequency band and then controlling it with a quasi-PR controller, the power-frequency zero-sequence current can be precisely suppressed, reducing transformer bias current issues caused by zero-sequence current, and improving system stability and power quality.

[0060] In this embodiment, the second modulation voltage is obtained based on the circulating current of each phase unit with the third control objective of suppressing the double frequency circulating current between the MMC phases, including: The third control objective is to suppress the double frequency circulating current between MMC phases. The circulating current of each phase unit is subjected to band-stop filtering and high-pass filtering to remove the power frequency and DC components in the circulating current. A quasi-PR controller is used to control the circulating current after band-stop filtering and high-pass filtering, and a second output result of the quasi-PR controller is used as a second modulation voltage; wherein the resonant frequency of the quasi-PR controller is set to twice the power frequency.

[0061] The second modulation voltage is generated with the third control objective of suppressing the double-frequency circulating current between the MMC phases. Band-reject and high-pass filtering of the circulating current removes the power frequency and DC components, and then controls it using a quasi-PR controller. This effectively suppresses the double-frequency circulating current, reduces its adverse effects on the system, and reduces the generation of transformer bias current.

[0062] In this embodiment, the fourth control target is to obtain a differential-mode modulated wave by taking the DC current component between the upper arm current and the lower arm current of the in-phase unit to be equal, including: Taking the DC current component between the upper bridge arm current and the lower bridge arm current of each phase unit as the fourth control target; Obtaining a DC component in the bridge arm current of each bridge arm; Subtracting the DC current component of the lower bridge arm current from the DC current component of the upper bridge arm current in the same-phase unit to obtain a DC current component deviation between the upper bridge arm current and the lower bridge arm current; A proportional-integral controller is used to control the DC current component deviation between the upper bridge arm current and the lower bridge arm current of each phase, and the output result of the proportional-integral controller is used as a differential-mode modulation wave.

[0063] A differential-mode modulation wave is generated by ensuring the DC current components of the upper and lower bridge arms of the in-phase unit are equal, as the fourth control objective. By obtaining the DC component of the bridge arm current, calculating the deviation, and using a proportional-integral controller, the DC components of the upper and lower bridge arm currents can be balanced, further improving the MMC's operating state and suppressing the transformer's bias current.

[0064] In this embodiment, the final modulation wave of the upper bridge arm and the final modulation wave of the lower bridge arm corresponding to each phase unit are calculated based on the common-mode power frequency voltage, the first modulation voltage, the second modulation voltage and the differential-mode modulation wave of each phase unit. The specific formula is as follows:

[0065]

[0066] Where Vref_top_j is the final modulation wave of the upper bridge arm; Vref_btm_j is the final modulation wave of the lower bridge arm; Vref_j is the modulation wave obtained by MMC control; Udc_set is the DC voltage output by the bridge arm; Uac1_j is the common-mode power frequency voltage; Ud1 is the first modulation voltage; Uac2_j is the second modulation voltage; Ud2_j is the differential-mode modulation wave; j = A, B, C, representing the three phase units respectively.

[0067] It can be seen that this method comprehensively considers the modulation waves obtained by multiple control objectives, can accurately synthesize the final modulation wave, realize precise control of MMC, effectively suppress the transformer bias current, and ensure the stable operation of the system.

[0068] like Figure 2 As shown, this embodiment also provides a transformer bias current suppression system under MMC parameter imbalance, including: a data acquisition module, used to obtain the total energy stored in each bridge arm of the MMC and the circulating current of each phase unit; a calculation module, used to obtain the common-mode power frequency voltage corresponding to each phase unit based on the total energy stored in each bridge arm, with the total energy stored in the upper bridge arm and the lower bridge arm of the same-phase unit being consistent as the first control target; to obtain the first modulation voltage with the power frequency zero-sequence current in the DC current of the MMC as the second control target; based on the circulating current of each phase unit, to suppress the MMC The doubled frequency circulating current between the MC phases is used as the third control target to obtain the second modulation voltage; the DC current component between the upper bridge arm current and the lower bridge arm current of the same-phase unit is equal as the fourth control target to obtain the differential-mode modulation wave; the control output module is used to calculate the upper bridge arm final modulation wave and the lower bridge arm final modulation wave corresponding to each phase unit based on the common-mode power frequency voltage, the first modulation voltage, the second modulation voltage and the differential-mode modulation wave of each phase unit; the upper bridge arm final modulation wave and the lower bridge arm final modulation wave are used to control the MMC to suppress the transformer bias current.

[0069] The present invention also provides an electronic device, comprising: a memory for storing a computer program; and a processor for implementing the steps of the transformer bias current suppression method under MMC parameter imbalance when executing the computer program.

[0070] When the processor executes the computer program, it implements the steps of suppressing the bias current of the transformer under the above-mentioned MMC parameter imbalance, for example: based on the total energy stored in each bridge arm, the total energy stored in the upper bridge arm and the lower bridge arm of the same-phase unit is consistent as the first control target, and the common-mode power frequency voltage corresponding to each phase unit is obtained; the power frequency zero-sequence current in the direct current of the MMC is suppressed as the second control target, and the first modulation voltage is obtained; based on the circulating current of each phase unit, the second modulation voltage is suppressed between the two-fold frequency circulating current of the MMC phases as the third control target, and the second modulation voltage is obtained; the DC current component between the upper bridge arm current and the lower bridge arm current of the same-phase unit is equal as the fourth control target, and the differential-mode modulation wave is obtained; based on the common-mode power frequency voltage, the first modulation voltage, the second modulation voltage and the differential-mode modulation wave of each phase unit, the final modulation wave of the upper bridge arm and the final modulation wave of the lower bridge arm corresponding to each phase unit are calculated; the final modulation wave of the upper bridge arm and the final modulation wave of the lower bridge arm are used to control the MMC to suppress the bias current of the transformer.

[0071] Exemplarily, the computer program can be divided into one or more modules / units, and the one or more modules / units are stored in the memory and executed by the processor to complete the present invention. The one or more modules / units can be a series of computer program instruction segments that can complete preset functions, and the instruction segments are used to describe the execution process of the computer program in the transformer bias current suppression device under the imbalance of MMC parameters. For example, the computer program can be divided into a data acquisition module, a calculation module and a control output module; the specific functions of each module are as follows: a data acquisition module, used to obtain the total energy stored in each bridge arm of the MMC and the circulating current of each phase unit; a calculation module, used to obtain the common-mode power frequency voltage corresponding to each phase unit based on the total energy stored in each bridge arm, with the total energy stored in the upper bridge arm and the lower bridge arm of the same-phase unit being consistent as the first control target; a first modulation voltage is obtained with the suppression of the power frequency zero-sequence current in the DC current of the MMC as the second control target; a first modulation voltage is obtained based on the circulating current of each phase unit. The third control target is to suppress the double frequency circulating current between the MMC phases to obtain the second modulation voltage; the fourth control target is to make the DC current component between the upper bridge arm current and the lower bridge arm current of the same-phase unit equal to each other to obtain the differential-mode modulation wave; the control output module is used to calculate the upper bridge arm final modulation wave and the lower bridge arm final modulation wave corresponding to each phase unit based on the common-mode power frequency voltage, the first modulation voltage, the second modulation voltage and the differential-mode modulation wave of each phase unit; the upper bridge arm final modulation wave and the lower bridge arm final modulation wave are used to control the MMC to suppress the transformer bias current.

[0072] The transformer bias current suppression device under MMC parameter imbalance can be a computing device such as a desktop computer, a notebook computer, a PDA, and a cloud server. The transformer bias current suppression device under MMC parameter imbalance can include, but is not limited to, a processor and a memory. Those skilled in the art will understand that the above is an example of a transformer bias current suppression device under MMC parameter imbalance and does not constitute a limitation on the transformer bias current suppression device under MMC parameter imbalance. It can include more components than the above, or a combination of certain components, or different components. For example, the transformer bias current suppression device under MMC parameter imbalance can also include input and output devices, network access devices, buses, etc.

[0073] The processor may be a central processing unit (CPU), or other general-purpose processors, digital signal processors (DSP), application-specific integrated circuits (ASIC), field-programmable gate arrays (FPGA), or other programmable logic devices, discrete gate or transistor logic devices, discrete hardware components, etc. The general-purpose processor may be a microprocessor, or the processor may be any conventional processor, etc. The processor serves as the control center for the transformer bias current suppression under MMC parameter imbalance, and utilizes various interfaces and lines to connect various parts of the entire MMC parameter imbalance transformer bias current suppression device.

[0074] The memory can be used to store the computer program and / or module, and the processor implements various functions of the transformer bias current suppression device under MMC parameter imbalance by running or executing the computer program and / or module stored in the memory and calling the data stored in the memory.

[0075] The memory may primarily include a program storage area and a data storage area. The program storage area may store an operating system and at least one application required for a function (such as sound playback or image playback); the data storage area may store data generated based on the use of the mobile phone (such as audio data and a phone book). Furthermore, the memory may include high-speed random access memory (RAM) and non-volatile memory, such as a hard disk, internal memory, a plug-in hard disk, a Smart Media Card (SMC), a Secure Digital (SD) card, a flash card, at least one disk storage device, a flash memory device, or other volatile solid-state storage device.

[0076] The present invention also provides a computer-readable storage medium, which stores a computer program. When the computer program is executed by a processor, the steps of the transformer bias current suppression method under MMC parameter imbalance are implemented.

[0077] If the integrated module / unit of the transformer bias current suppression system under MMC parameter imbalance is implemented in the form of a software functional unit and sold or used as an independent product, it can be stored in a computer-readable storage medium.

[0078] Based on this understanding, the present invention implements all or part of the process steps of the above-mentioned method for suppressing the bias current of a transformer under MMC parameter imbalance, and can also be completed by instructing related hardware through a computer program. The computer program can be stored in a computer-readable storage medium. When the computer program is executed by a processor, it can implement the steps of the above-mentioned method for suppressing the bias current of a transformer under MMC parameter imbalance. The computer program includes computer program code, which can be in source code form, object code form, executable file, or preset intermediate form.

[0079] The computer-readable storage medium may include: any entity or device capable of carrying the computer program code, recording medium, USB flash drive, mobile hard disk, magnetic disk, optical disk, computer memory, read-only memory (ROM), random access memory (RAM), electrical carrier signal, telecommunication signal, and software distribution medium, etc.

[0080] It should be noted that the content contained in the computer-readable storage medium can be appropriately increased or decreased according to the requirements of legislation and patent practices in the jurisdiction. For example, in some jurisdictions, according to legislation and patent practices, computer-readable storage media do not include electrical carrier signals and telecommunication signals.

[0081] The present invention provides a method for suppressing transformer bias current under MMC parameter imbalance, which has the following advantages: This method can ensure that the submodule capacitor voltage and energy of the upper and lower bridge arms of the converter phase unit are controlled at set values ​​under various system parameter asymmetries by controlling the energy storage energy of the upper and lower bridge arms in the phase unit; wherein, the system parameter asymmetry includes inconsistent number of submodules, inconsistent capacitance of submodules, imbalance between bridge arm reactors, etc.; this method simplifies the calculation of the inter-phase power frequency common mode voltage and reduces the size of the power frequency common mode circulating current by multiplying the output result of the proportional integral controller of the stored energy deviation of the upper and lower bridge arms of each phase with the voltage of the three-phase voltage on the transformer valve side after 1 / 4 power frequency cycle lag; and also by controlling the DC output voltage of the converter The unified control of the power frequency components in the DC current can simplify the suppression of the zero-sequence power frequency current on the DC side caused by the introduction of the inter-phase circulating current; controlling the inter-phase circulating current of the converter by a quasi-PR controller can avoid the mutual interference of the power frequency circulating current when the proportional-integral control of the circulating current is adopted, so that the circulating current of each frequency band of the converter can be controlled independently and the change of the grid frequency can be taken into account; by adopting the proportional-integral controller to control the DC current component deviation of the upper and lower bridge arm currents of the phase unit, the DC current component deviation of the upper and lower bridge arm currents of the phase unit can be fully controlled, and the DC component in the valve-side current of the transformer and the valve-side winding current of the transformer can be suppressed.

[0082] The above embodiment is only one of the implementation methods that can realize the technical solution of the present invention. The scope of protection claimed by the present invention is not limited only to this embodiment, but also includes changes, replacements and other implementation methods that can be easily thought of by any technician familiar with this technical field within the technical scope disclosed by the present invention.

[0083] 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 it. Although the present invention has been described in detail with reference to the above embodiments, ordinary technicians in the field should understand that the specific implementation methods of the present invention can still be modified or replaced by equivalents. Any modification or equivalent replacement that does not depart from the spirit and scope of the present invention should be included in the scope of protection of the present invention.

Claims

1. A method for suppressing transformer bias current under MMC parameter imbalance, characterized in that: include: Obtain the total energy stored in each bridge arm of the MMC and the circulating current of each phase unit; Based on the total energy stored in each bridge arm, the first control target is to make the total energy stored in the upper bridge arm and the lower bridge arm of the same-phase unit consistent, and obtain the common-mode power frequency voltage corresponding to each phase unit; The first modulation voltage is obtained by suppressing the power frequency zero-sequence current in the DC current of the MMC as the second control target; the second modulation voltage is obtained by suppressing the double frequency circulating current between the MMC phases as the third control target based on the circulating current of each phase unit; and the differential mode modulation wave is obtained by making the DC current component between the upper bridge arm current and the lower bridge arm current of the same phase unit equal as the fourth control target. Based on the common-mode power frequency voltage, the first modulation voltage, the second modulation voltage and the differential-mode modulation wave of each phase unit, the final modulation wave of the upper bridge arm and the final modulation wave of the lower bridge arm corresponding to each phase unit are calculated; the final modulation wave of the upper bridge arm and the final modulation wave of the lower bridge arm are used to control the MMC to suppress the bias current of the transformer.

2. The transformer bias current suppression method under MMC parameter imbalance according to claim 1, characterized in that: The obtaining of the total energy stored in each bridge arm of the MMC and the circulating current of each phase unit includes: Obtain the rated capacitance value of the submodule in each bridge arm of the MMC, the actual operating voltage of the capacitor of each submodule, and the number of submodules in each bridge arm; Based on the rated capacitance of the submodules in each bridge arm, the actual operating voltage of the capacitors in each submodule, and the number of submodules in each bridge arm, the total energy stored in each bridge arm is calculated. The specific formula is as follows: Where Earm is the total energy stored in the bridge arm; C is the rated capacitance of the submodule; k is the number of submodules in each bridge arm, ranging from 1 to N, where N is a positive integer; Uc is the actual working voltage of the capacitor of each submodule; Uc （k） is the actual working voltage of the kth submodule capacitor; Obtain the upper arm current and lower arm current of each phase unit; Based on the upper arm current and lower arm current of each phase unit, the circulating current of each phase unit is calculated. The specific formula is as follows: Where Icir_j is the circulating current of the phase unit; Iarm_top_j is the upper arm current; Iarm_btm_j is the lower arm current; j=A, B, C, representing the three phase units respectively.

3. The transformer bias current suppression method under MMC parameter imbalance according to claim 1, characterized in that: The method of obtaining the common-mode power frequency voltage corresponding to each phase unit based on the total energy stored in each bridge arm and taking the total energy stored in the upper bridge arm and the lower bridge arm of the same-phase unit as the first control target to be consistent includes: The first control goal is to make the total energy stored in the upper bridge arm and the lower bridge arm of the same-phase unit consistent. A proportional-integral controller is used to control the deviation between the total energy stored in the upper bridge arm and the lower bridge arm of the same-phase unit so that the total energy stored in the upper bridge arm and the lower bridge arm of the same-phase unit remains consistent, thereby obtaining the output result of the proportional-integral controller. The output result of the proportional-integral controller is multiplied by the three-phase power frequency common-mode reference voltage to obtain the common-mode power frequency voltage corresponding to each phase unit; wherein, the three-phase power frequency common-mode reference voltage is obtained by lagging the three-phase voltage on the valve side of the MMC transformer by 1 / 4 of the power frequency period.

4. The transformer bias current suppression method under MMC parameter imbalance according to claim 1, characterized in that: The method of obtaining a first modulation voltage by taking the suppression of the power frequency zero-sequence current in the direct current of the MMC as the second control target includes: The second control objective is to suppress the power frequency zero-sequence current in the DC current of the MMC, and perform band-pass filtering of the DC current of the MMC in the power frequency band; wherein the resonant frequency of the quasi-PR controller is set to the power frequency; The direct current of the MMC after band-pass filtering is controlled by a quasi-PR controller, and a first output result of the quasi-PR controller is used as a first modulation voltage.

5. The transformer bias current suppression method under MMC parameter imbalance according to claim 1, characterized in that: The second modulation voltage is obtained based on the circulating current of each phase unit with the third control goal of suppressing the double frequency circulating current between the MMC phases, including: The third control objective is to suppress the double frequency circulating current between MMC phases. The circulating current of each phase unit is subjected to band-stop filtering and high-pass filtering to remove the power frequency and DC components in the circulating current. A quasi-PR controller is used to control the circulating current after band-stop filtering and high-pass filtering, and a second output result of the quasi-PR controller is used as a second modulation voltage; wherein the resonant frequency of the quasi-PR controller is set to twice the power frequency.

6. The transformer bias current suppression method under MMC parameter imbalance according to claim 1, characterized in that: The fourth control target is to obtain a differential-mode modulated wave by taking the DC current component between the upper bridge arm current and the lower bridge arm current of the in-phase unit as equal, including: Taking the DC current component between the upper bridge arm current and the lower bridge arm current of each phase unit as the fourth control target; Obtaining a DC component in the bridge arm current of each bridge arm; Subtracting the DC current component of the lower bridge arm current from the DC current component of the upper bridge arm current in the same-phase unit to obtain a DC current component deviation between the upper bridge arm current and the lower bridge arm current; A proportional-integral controller is used to control the DC current component deviation between the upper bridge arm current and the lower bridge arm current of each phase, and the output result of the proportional-integral controller is used as a differential-mode modulation wave.

7. The transformer bias current suppression method under MMC parameter imbalance according to claim 1, characterized in that: Based on the common-mode power frequency voltage, the first modulation voltage, the second modulation voltage and the differential-mode modulation wave of each phase unit, the upper bridge arm final modulation wave and the lower bridge arm final modulation wave corresponding to each phase unit are calculated. The specific formula is as follows: Where Vref_top_j is the final modulation wave of the upper bridge arm; Vref_btm_j is the final modulation wave of the lower bridge arm; Vref_j is the modulation wave obtained by MMC control; Udc_set is the DC voltage output by the bridge arm; Uac1_j is the common-mode power frequency voltage; Ud1 is the first modulation voltage; Uac2_j is the second modulation voltage; Ud2_j is the differential-mode modulation wave; j = A, B, C, representing the three phase units respectively.

8. A transformer bias current suppression system under MMC parameter imbalance, characterized in that: include: Data acquisition module, used to obtain the total energy stored in each bridge arm of the MMC and the circulating current of each phase unit; a calculation module, configured to obtain a common-mode power frequency voltage corresponding to each phase unit based on the total energy stored in each bridge arm and taking the total energy stored in the upper bridge arm and the lower bridge arm of the same-phase unit as the first control target; The first modulation voltage is obtained by suppressing the power frequency zero-sequence current in the DC current of the MMC as the second control target; the second modulation voltage is obtained by suppressing the double frequency circulating current between the MMC phases as the third control target based on the circulating current of each phase unit; and the differential mode modulation wave is obtained by making the DC current component between the upper bridge arm current and the lower bridge arm current of the same phase unit equal as the fourth control target. The control output module is used to calculate the final modulation wave of the upper bridge arm and the final modulation wave of the lower bridge arm corresponding to each phase unit based on the common-mode power frequency voltage, the first modulation voltage, the second modulation voltage and the differential-mode modulation wave of each phase unit; the final modulation wave of the upper bridge arm and the final modulation wave of the lower bridge arm are used to control the MMC to suppress the bias current of the transformer.

9. An electronic device, characterized in that: include: Memory for storing computer programs; A processor is configured to implement the steps of the transformer bias current suppression method under MMC parameter imbalance as described in any one of claims 1 to 7 when executing the computer program.

10. A computer-readable storage medium storing a computer program, characterized in that: When the computer program is executed by a processor, it is used to implement the steps of the transformer bias current suppression method under MMC parameter imbalance described in any one of claims 1 to 7.

Images