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

Abstract

The present application belongs to the field of flexible direct current transmission, and discloses a transformer magnetic bias current suppression method under MMC parameter imbalance and related equipment. The 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 by four targets of consistent upper and lower bridge arm energy on the same phase unit and suppression of power frequency zero sequence current of direct current, and then calculates the final modulation wave of the upper and lower bridge arms to control the MMC. The method sets control targets from aspects of bridge arm energy, current characteristics and the like aiming at various factors of transformer magnetic bias current caused by MMC parameter imbalance, balances parameters through precise control of the modulation wave. Compared with the existing method, the method avoids the problems of complex control strategy and poor suppression effect, can effectively suppress the transformer magnetic bias current, reduce the harmonic current, reduce the loss, improve the power quality and transmission efficiency, and guarantee the stable operation of the power system.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of flexible direct current transmission, and particularly relates to a transformer magnetic bias current suppression method under MMC parameter imbalance and related equipment. BACKGROUND

[0002] In modern power systems, high-voltage direct current transmission technology has become a key means for realizing renewable energy grid connection and power cross-regional allocation due to its advantages of large capacity, long-distance transmission, and flexible adjustment. Modular multilevel converter (MMC) as the core component of high-voltage flexible direct current transmission system, with its modular structure design, exhibits high scalability and flexibility, which can conveniently adjust the number of sub-modules according to actual demand, adapt to different voltage levels and power capacity requirements. At the same time, the voltage waveform output by MMC is almost sinusoidal, with low harmonic content, which can meet the power quality standard without complex filtering devices, effectively reducing system cost and land area. In addition, its good dynamic response performance and fault ride-through capability can quickly respond when the system fails, ensuring the safe and stable operation of the power system, so it has been widely applied and deeply researched in the field of electric power.

[0003] However, in the actual operation of MMC, due to the complexity of system parameters and the variability of operating environment, various parameter imbalances often occur. For example, the number of sub-modules in the upper and lower bridge arms of the phase unit may be inconsistent due to faults and other reasons; the capacitance value of the sub-module capacitor may deviate due to production process, service time, and environmental factors; the bridge arm reactor may also be imbalanced due to parameter design differences, different installation positions, etc. These parameter imbalances will further cause the converter to output a certain amount of direct current at the AC side, and the direct current output at the AC side will flow through the valve side winding of the transformer, which is easy to cause the magnetic bias and abnormal sound of the transformer, and will also generate a power frequency zero sequence current at the DC side, affecting the output characteristics of the converter AC / DC port. However, the existing control methods have defects such as complex control strategy and poor suppression effect when dealing with the problems caused by parameter imbalance, which is difficult to meet the strict requirements of power systems for efficient and stable operation. SUMMARY

[0004] The application provides a transformer magnetic current suppression method under MMC parameter imbalance and related equipment. The application takes the consistency of the energy stored in all sub-modules of the upper and lower bridge arms of the same-phase unit as a control target, calculates the common-mode power frequency voltage required for each-phase energy balance; takes the content of the power frequency current in the MMC direct current as 0 as a control target, and further controls the zero sequence power frequency output current of the MMC; takes the equality of the direct current components in the same-phase upper and lower bridge arm currents as a control target; calculates the difference of the differential-mode direct current voltage of the upper and lower bridge arms; in order to reduce the operation loss of the converter valve and suppress the double-frequency circulating current between the converter arms; through the energy balance control of the same-phase upper and lower bridge arms of the MMC converter, the zero sequence power frequency current suppression of the direct current and the direct current component deviation control between the same-phase upper and lower bridge arms, the transformer magnetic current suppression under various converter parameter imbalances such as the number deviation of the bridge arm sub-modules, the capacitance value deviation of the sub-module capacitors, the bridge arm reactance value deviation and the bridge arm series resistance value deviation can be realized under the condition that the capacitor energy of each bridge arm of the converter valve is basically consistent.

[0005] In order to achieve the above-mentioned purpose, the application adopts the following technical scheme:

[0006] A transformer magnetic current suppression method under MMC parameter imbalance comprises the following steps.

[0007] Obtain the total energy stored in each bridge arm of the MMC and the circulating current of each phase unit.

[0008] Based on the total energy stored in each bridge arm, take the consistency of the total energy stored in the upper and lower bridge arms of the same-phase unit as a first control target, obtain the common-mode power frequency voltage corresponding to each phase unit; take the suppression of the power frequency zero sequence current in the direct current of the MMC as a second control target, obtain a first modulation voltage; based on the circulating current of each phase unit, take the suppression of the double-frequency circulating current between the MMC arms as a third control target, obtain a second modulation voltage; take the equality of the direct current components between the upper and lower bridge arm currents of the same-phase unit as a fourth control target, obtain a differential-mode modulation wave.

[0009] 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 upper bridge arm final modulation wave and the lower bridge arm final modulation wave are used for controlling the MMC, so as to realize the suppression of the transformer magnetic current.

[0010] Further,

[0011] The total energy stored in each bridge arm of the MMC and the circulating current of each phase unit are obtained, comprising the following steps.

[0012] Obtain the rated capacitance value of each sub-module in each bridge arm of the MMC, the actual working voltage of each sub-module capacitor and the number of sub-modules in each bridge arm.

[0013] Based on the rated capacitance value of each sub-module in the bridge arm, the actual working voltage of each sub-module capacitor, and the number of sub-modules in each bridge arm, the total energy stored in each bridge arm is calculated, and the specific formula is as follows:

[0014]

[0015] In the formula, Earm is the total energy stored in the bridge arm; C is the rated capacitance value of the sub-module; k is the number of sub-modules in each bridge arm, taking a value of 1~N, N is a positive integer; Uc is the actual working voltage of each sub-module capacitor; Uc （k） is the actual working voltage of the kth sub-module capacitor;

[0016] The upper arm current and the lower arm current of each phase unit are obtained;

[0017] Based on the upper arm current and the lower arm current of each phase unit, the circulating current of each phase unit is calculated, and the specific formula is as follows:

[0018]

[0019] In the formula, 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, respectively representing three phase units.

[0020] Further, the total energy stored in the upper arm and the lower arm of the same phase unit is taken as the first control target, and the common-mode power frequency voltage corresponding to each phase unit is obtained, including:

[0021] Taking the total energy stored in the upper arm and the lower arm of the same phase unit as the first control target, a proportional-integral controller is adopted to control the deviation between the total energy stored in the upper arm and the lower arm of the same phase unit, so that the total energy stored in the upper arm and the lower arm of the same phase unit is kept consistent, so as to obtain the output result of the proportional-integral controller.

[0022] The output result of the proportional-integral controller is multiplied by the three-phase power frequency common-mode reference voltage respectively 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 1 / 4 power frequency period of the MMC transformer valve side three-phase voltage.

[0023] Further, the power frequency zero sequence current in the DC current of the MMC is taken as the second control target to obtain the first modulation voltage, including:

[0024] Taking the power frequency zero sequence current in the DC current of the MMC as the second control target, the DC current of the MMC is band-pass filtered in the power frequency band; wherein the resonance frequency of the quasi-PR controller is set to the power frequency.

[0025] The DC current of the band-pass filtered MMC is controlled by a quasi-PR controller, and the first output result of the quasi-PR controller is used as the first modulation voltage.

[0026] Further, the circulating current of each phase unit is taken as a third control target for suppressing the double-frequency circulating current between MMC phases, to obtain a second modulation voltage, comprising:

[0027] The double-frequency circulating current between MMC phases is taken as the third control target, and 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.

[0028] The quasi-PR controller is used to control the band-stop filtered and high-pass filtered circulating current, and the second output result of the quasi-PR controller is used as the second modulation voltage; wherein the resonance frequency of the quasi-PR controller is set to twice the power frequency.

[0029] Further, the DC current component between the upper bridge arm current and the lower bridge arm current of the same phase unit is taken as a fourth control target, to obtain a differential mode modulation wave, comprising:

[0030] The DC current component between the upper bridge arm current and the lower bridge arm current of each phase unit is taken as the fourth control target.

[0031] The DC component in the bridge arm current of each bridge arm is obtained.

[0032] The DC current component of the upper bridge arm current is subtracted from the DC current component of the lower bridge arm current in the same phase unit, to obtain the DC current component deviation between the upper bridge arm current and the lower bridge arm current.

[0033] The 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 the differential mode modulation wave.

[0034] Further, the common-mode power frequency voltage, the first modulation voltage, the second modulation voltage and the differential mode modulation wave of each phase unit are used to calculate the upper bridge arm final modulation wave and the lower bridge arm final modulation wave corresponding to each phase unit, and the specific formula is as follows:

[0035]

[0036]

[0037] In the formula, 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, respectively representing three phase units.

[0038] A transformer magnetizing current suppression system under MMC parameter imbalance, comprising:

[0039] A data acquisition module is configured to acquire total energy stored in each bridge arm of the MMC and circulating current of each phase unit.

[0040] 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 the upper bridge arm and the lower bridge arm of the same phase unit as a first control target; obtain a first modulation voltage based on 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 suppression of the two-frequency circulating current between the MMC phases as a third control target; and obtain a differential-mode modulation wave based on 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.

[0041] A control output module is configured 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, and 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 achieve suppression of the transformer magnetizing current.

[0042] An electronic device, comprising:

[0043] A memory is configured to store a computer program.

[0044] A processor is configured to implement the steps of the above-mentioned transformer magnetizing current suppression method under MMC parameter imbalance when executing the computer program.

[0045] A computer readable storage medium stores a computer program, and the computer program is executed by a processor to implement the steps of the above-mentioned transformer magnetizing current suppression method under MMC parameter imbalance.

[0046] Compared with the prior art, the present application has the following beneficial effects:

[0047] The application provides a transformer magnetic bias current suppression method under MMC parameter imbalance. BRIEF DESCRIPTION OF DRAWINGS

[0048] Figure 1 A flowchart of the transformer magnetic bias current suppression method under MMC parameter imbalance provided by the application is shown in the figure.

[0049] Figure 2 A structural schematic diagram of the transformer magnetic bias current suppression system under MMC parameter imbalance provided by the application is shown in the figure. DETAILED DESCRIPTION

[0050] For a further understanding of the application, the application will be described in detail with reference to the accompanying drawings and specific embodiments. It should be understood that the embodiments are only used to explain the application and are not limiting.

[0051] The technical terms related to the application are explained as follows:

[0052] MMC: Modular Multilevel Converter, i.e. modular multilevel converter, is a new type of voltage source converter topology structure.

[0053] PR controller: Proportional-Resonant Controller, i.e. proportional-resonant controller, is a controller widely used in AC control systems, which combines the characteristics of proportional control (P) and resonant control (R) and can realize no-static tracking control of AC signals.

[0054] Quasi-PR controller: As an improved PR controller, the frequency adaptability and stability of the traditional PR controller are improved by introducing a damping term.

[0055] The embodiment provides a transformer magnetic bias current suppression method under MMC parameter imbalance, which is specifically as follows:

[0056] In this embodiment, the modular multilevel converter (MMC) includes three phase units, A, B and C, each of which is composed of an upper bridge arm and a lower bridge arm, and each bridge arm is composed of a bridge reactor and a plurality of sub-modules in series. The method comprises:

[0057] S1: Obtain the capacitor voltage of the sub-modules in each bridge arm of the MMC (i.e. the actual working voltage of each sub-module capacitor), and calculate the energy stored in each sub-module in each bridge arm, i.e. the total energy stored in each bridge arm;

[0058] Obtain the bridge current of each bridge arm of the MMC, and calculate the circulating current in each phase unit;

[0059] Obtain the direct current output by the MMC;

[0060] Obtain the three-phase voltage on the valve side of the MMC transformer.

[0061] S2: Based on the energy storage deviation of the sub-modules in the upper bridge arm and the lower bridge arm 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 voltage Uac1_A, Uac1_B and Uac1_C required to eliminate the total energy storage deviation of the upper bridge arm and the lower bridge arm in the three-phase unit, respectively.

[0062] In this embodiment, the total energy stored in the bridge arm can be calculated by the following formula:

[0063]

[0064] In the formula, Earm is the total energy stored in the bridge arm; C is the rated capacitance value of the sub-module; k is the number of sub-modules in each bridge arm, taking a value of 1 to N, N being a positive integer; Uc is the actual working voltage of each sub-module capacitor; Uc （k） is the actual working voltage of the kth sub-module capacitor.

[0065] The specific execution steps are as follows:

[0066] S21: The total energy storage deviation of the upper and lower bridge arms of each phase unit is equal to the storage energy of the upper bridge arm minus the storage energy of the lower bridge arm, and the result of the subtraction is filtered by a band-stop filter and a low-pass filter to obtain the total energy storage deviation of 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;

[0067] S22: The total energy storage deviation of the upper and lower bridge arms of each phase is controlled by a proportional-integral controller, respectively;

[0068] S23: Obtain the three-phase power frequency common-mode reference voltage by lagging the three-phase voltage on the valve side of the converter transformer by 1 / 4 power frequency period;

[0069] S24: the output results of the proportional-integral controller of the stored total energy deviation of the upper and lower bridge arms of each phase are respectively multiplied by the three-phase power frequency common mode reference voltage, and the corresponding common mode power frequency voltages Uac1_A, Uac1_B and Uac1_C of the three-phase units are calculated.

[0070] S3: based on the obtained converter output direct current, the power frequency zero sequence current in the direct current of the converter is suppressed, and a modulation voltage Ud1 for suppressing the power frequency component in the direct current is calculated, that is, the first modulation voltage is obtained.

[0071] The specific execution steps are as follows:

[0072] S31: the output direct current of the MMC is band-pass filtered in the power frequency band;

[0073] S32: the band-pass filtered direct current is controlled by a quasi-PR controller, the resonance frequency of the quasi-PR controller is set to the power frequency, and the output result of the quasi-PR controller is used as the modulation voltage Ud1 for suppressing the power frequency component in the direct current, that is, the first modulation voltage Ud1 is obtained.

[0074] S4: based on the obtained bridge arm current, the two-frequency inter-phase circulating current of the converter is suppressed, and the modulation voltages Uac2_A, Uac2_B and Uac2_C for suppressing the two-frequency inter-phase circulating current of the converter are calculated; that is, the second modulation voltage is obtained.

[0075] The calculation formula of the circulating current of each phase unit is as follows:

[0076]

[0077] In the formula, Icir_j is the circulating current of the phase unit; Iarm_top_j is the upper bridge arm current; Iarm_btm_j is the lower bridge arm current; j=A, B, C, respectively representing three phase units.

[0078] The specific execution steps are as follows:

[0079] S41: the circulating current Icir of each phase unit can be removed by band-stop filtering and high-pass filtering, and the power frequency and direct current components in the circulating current are removed;

[0080] S42: the filtered circulating current is controlled by a quasi-PR controller, the resonance frequency of the quasi-PR controller is set to twice the power frequency, and the output results of each quasi-PR controller are used as the modulation voltages Uac2_A, Uac2_B and Uac2_C of the two-frequency inter-phase circulating current, that is, the second modulation voltage corresponding to each phase unit is obtained.

[0081] S5: Based on the obtained bridge arm current, the deviation of the DC current component of the upper and lower bridge arm current on each phase unit of the converter is suppressed, and the differential mode modulation wave Ud2_A, Ud2_B, Ud2_C in which the DC current components of the upper and lower bridge arm current of each phase unit are equal is calculated; that is, the differential mode modulation wave corresponding to each phase unit is obtained.

[0082] The specific implementation steps are as follows:

[0083] S51: The DC component in the bridge arm current of each bridge arm is calculated.

[0084] S52: The DC component in the upper bridge arm current in each phase unit is subtracted from the DC component in the lower bridge arm current to obtain the DC current component deviation of the upper and lower bridge arm current of each phase unit.

[0085] S53: The DC current component deviation of the upper and lower bridge arm current of each phase unit is controlled by a proportional-integral controller, and the output result of each proportional-integral controller is taken as the differential mode modulation wave Ud2_A, Ud2_B, Ud2_C, that is, the differential mode modulation wave Ud2_A, Ud2_B, Ud2_C corresponding to each phase unit is obtained.

[0086] S6: Based on the obtained common mode power frequency voltage, first modulation voltage, second modulation voltage and differential mode modulation wave of each phase unit, that is, Uac1_A, Uac1_B, Uac1_C, Ud1, Uac2_A, Uac2_B, Uac2_B, Ud2_A, Ud2_B, Ud2_C, the final modulation wave of each bridge arm is calculated, and the control of the converter is performed according to the final modulation wave of each bridge arm.

[0087] The specific calculation formula is as follows:

[0088]

[0089]

[0090] In the formula, 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; and j=A, B, C, respectively representing three phase units.

[0091] For example, the suppression method provided in the embodiment is applied to actual engineering, and the specific implementation is as follows:

[0092] Application scenario and parameter setting: In a certain ± 500 kV modular multilevel converter (MMC) high voltage direct current transmission system, the magnetic bias current problem of the connected transformer caused by the unbalanced MMC parameters affects the stable operation of the system.

[0093] Each phase of the MMC system is composed of 100 sub-modules, and the rated capacitance of the sub-modules is 3000 μF.

[0094] The execution steps are as follows:

[0095] Step 1, real-time acquisition of the rated capacitance of each bridge arm sub-module, the actual working voltage of each sub-module capacitor, and the number of sub-modules in each bridge arm, calculation of the total energy stored in each bridge arm.

[0096] At the same time, the upper and lower arm currents of each phase unit are obtained, and the circulating current of each phase unit is calculated.

[0097] Step 2, taking the total energy stored in the upper and lower arms of the same phase unit as the first control target, using a proportional-integral controller to control the total energy stored in the upper and lower arms of the same phase unit, so that they remain consistent, and obtaining the output result of the proportional-integral controller; multiplying the output result by the three-phase common-mode reference voltage obtained by lagging the MMC transformer valve side three-phase voltage by 1 / 4 power frequency, to obtain the corresponding common-mode power frequency voltage of each phase unit.

[0098] Taking the suppression of the power frequency zero sequence current in the DC current of the MMC as the second control target, the DC current of the MMC is band-pass filtered in the power frequency band (the resonance frequency of the quasi-PR controller is set to the power frequency); the band-pass filtered MMC DC current is controlled by a quasi-PR controller, and its first output result is used as the first modulation voltage. Taking the suppression of the two-frequency circulating current between the MMC phases as the third control target, the circulating current of each phase unit is respectively band-stop filtered and high-pass filtered to remove the power frequency and DC components in the circulating current; the processed circulating current is controlled by a quasi-PR controller (the resonance frequency is set to twice the power frequency), and its second output result is used as the second modulation voltage.

[0099] Differential mode modulation wave acquisition: taking the DC current component between the upper and lower arm currents of each phase unit as the fourth control target; obtaining the DC component in the bridge arm current of each bridge arm, calculating 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; using a proportional-integral controller to control the deviation, and the output result is used as the differential mode modulation wave.

[0100] Based on the common-mode power frequency voltage of each phase unit, the first modulation voltage, the second modulation voltage and the differential-mode modulation wave obtained by the above calculation, and the modulation wave and the DC voltage output by the bridge arm obtained by the MMC control, 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.

[0101] Finally, the generated final modulation wave of the upper bridge arm and the final modulation wave of the lower bridge arm are used to control the MMC, and after a period of operation monitoring, the transformer magnetic bias current is significantly reduced, and the system operation stability is significantly improved.

[0102] For example, as shown in the figure, the embodiment also provides a transformer magnetic bias current suppression method under MMC parameter imbalance, including the following steps: Figure 1

[0103] Obtaining the total energy stored in each bridge arm of the MMC and the circulating current of each phase unit;

[0104] Based on the total energy stored in each bridge arm, 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 common-mode power frequency voltage corresponding to each phase unit is obtained; taking the power frequency zero sequence current in the DC current of the MMC as the second control target, the first modulation voltage is obtained; based on the circulating current of each phase unit, taking the suppression of the double-frequency circulating current between the MMC phases as the third control target, the second modulation voltage is obtained; taking the DC current component between the upper bridge arm current and the lower bridge arm current of the same phase unit as the fourth control target, the differential-mode modulation wave is obtained;

[0105] Based on the common-mode power frequency voltage of each phase unit, the first modulation voltage, the second modulation voltage and the differential-mode modulation wave, 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, so as to realize the suppression of the transformer magnetic bias current.

[0106] In the embodiment,

[0107] The obtaining of the total energy stored in each bridge arm of the MMC and the circulating current of each phase unit comprises:

[0108] Obtaining the rated capacitance value of each submodule in each bridge arm of the MMC, the actual working voltage of each submodule capacitor and the number of submodules in each bridge arm;

[0109] Based on the rated capacitance value of each submodule in each bridge arm, the actual working voltage of each submodule capacitor and the number of submodules in each bridge arm, the total energy stored in each bridge arm is calculated, and the specific formula is as follows:

[0110]

[0111] ​In the formula, Earm is the total energy stored in the bridge arm; C is the rated capacitance value of the sub-module; k is the number of sub-modules in each bridge arm, taking a value of 1 to N, N is a positive integer; Uc is the actual working voltage of each sub-module capacitor;

[0112] Obtain the upper bridge arm current and the lower bridge arm current of each phase unit;

[0113] Based on the upper bridge arm current and the lower bridge arm current of each phase unit, the circulating current of each phase unit is calculated, and the specific formula is as follows:

[0114]

[0115] In the formula, Icir_j is the circulating current of the phase unit; Iarm_top_j is the upper bridge arm current; Iarm_btm_j is the lower bridge arm current; j=A, B, C, respectively representing three phase units.

[0116] The total energy of the bridge arm is calculated by the rated capacitance value, the actual working voltage and the number of sub-modules, and the circulating current of the phase unit is calculated by the upper and lower bridge arm currents. This accurate acquisition method provides an accurate data basis for subsequent control, which helps to more accurately achieve each control target and improve the suppression effect of the transformer magnetic bias current.

[0117] In the embodiment, the total energy stored in each bridge arm is used as the first control target to make the total energy stored in the upper and lower bridge arms of the same phase unit consistent, and the common-mode power frequency voltage corresponding to each phase unit is obtained, including:

[0118] The total energy stored in the upper and lower bridge arms of the same phase unit is used as the first control target, a proportional-integral controller is used to control the deviation between the total energy stored in the upper and lower bridge arms of the same phase unit, so that the total energy stored in the upper and lower bridge arms of the same phase unit is consistent, and the output result of the proportional-integral controller is obtained.

[0119] The output result of the proportional-integral controller is multiplied by the three-phase power frequency common-mode reference voltage respectively 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 1 / 4 power frequency period of the MMC transformer valve side three-phase voltage.

[0120] Based on the total energy of each bridge arm, the common-mode power frequency voltage is obtained by taking the total energy of the upper and lower bridge arms of the same phase unit as the control target. The proportional-integral controller is used to control the energy deviation, and then multiplied by the three-phase power frequency common-mode reference voltage to obtain the common-mode power frequency voltage. This method can effectively balance the energy of the upper and lower bridge arms of the same phase unit, reduce the influence of parameter imbalance from the energy angle, and further suppress the transformer magnetic bias current.

[0121] In the embodiment, the second control target is to suppress the power frequency zero sequence current in the direct current of the MMC, and a first modulation voltage is obtained, including:

[0122] The power frequency band pass filtering is performed on the direct current of the MMC with the suppression of the power frequency zero sequence current in the direct current of the MMC as the second control target, and the resonance frequency of the quasi-PR controller is set as the power frequency.

[0123] The direct current of the MMC after the band pass filtering is controlled by using the quasi-PR controller, and the first output result of the quasi-PR controller is taken as the first modulation voltage.

[0124] The first modulation voltage is obtained by taking the suppression of the power frequency zero sequence current in the direct current of the MMC as the second control target. The power frequency zero sequence current can be accurately suppressed by performing the power frequency band pass filtering on the direct current and then controlling by using the quasi-PR controller, the transformer magnetizing current problem caused by the zero sequence current is reduced, and the stability and power quality of the system are improved.

[0125] In the embodiment, the third control target is to suppress the double-frequency circulating current between the MMC phases based on the circulating current of each phase unit, and a second modulation voltage is obtained, including:

[0126] The double-frequency circulating current between the MMC phases is taken as the third control target, and the band stop filtering and high pass filtering are performed on the circulating current of each phase unit respectively to remove the power frequency and direct current components in the circulating current.

[0127] The quasi-PR controller is used to control the circulating current after the band stop filtering and high pass filtering, and the second output result of the quasi-PR controller is taken as the second modulation voltage, wherein the resonance frequency of the quasi-PR controller is set as twice the power frequency.

[0128] The second modulation voltage is obtained by taking the suppression of the double-frequency circulating current between the MMC phases as the third control target. The band stop filtering and high pass filtering are performed on the circulating current to remove the power frequency and direct current components, and then the quasi-PR controller is used for control, which can effectively suppress the double-frequency circulating current, reduce the adverse effects of the circulating current on the system, and reduce the generation of the transformer magnetizing current.

[0129] In the embodiment, the fourth control target is that the direct current components between the upper bridge arm current and the lower bridge arm current of the in-phase unit are equal, and a differential mode modulation wave is obtained, including:

[0130] The direct current components between the upper bridge arm current and the lower bridge arm current of each phase unit are equal as the fourth control target.

[0131] The direct current components in the bridge arm current of each bridge arm are obtained.

[0132] The DC current component of the upper bridge arm current in the in-phase unit is subtracted from the DC current component of the lower bridge arm current to obtain a DC current component deviation between the upper bridge arm current and the lower bridge arm current.

[0133] The proportional-integral controller is adopted 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 taken as the differential mode modulation wave.

[0134] The DC current component of the upper bridge arm current and the lower bridge arm current in the in-phase unit is equalized as the fourth control target to obtain the differential mode modulation wave. By obtaining the DC component of the bridge arm current, calculating the deviation and adopting the proportional-integral controller to control, the DC component of the upper bridge arm current and the lower bridge arm current can be balanced, the operation state of the MMC is further improved, and the transformer magnetic bias current is inhibited.

[0135] In the embodiment, the upper bridge arm final modulation wave and the lower bridge arm final modulation wave 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, and the specific formula is as follows:

[0136]

[0137]

[0138] In the formula, Vref_top_j is the upper bridge arm final modulation wave; Vref_btm_j is the lower bridge arm final modulation wave; Vref_j is the modulation wave obtained by the 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; and j=A, B, C, respectively representing three phase units.

[0139] It can be seen that the modulation wave obtained by comprehensively considering multiple control targets can accurately synthesize the final modulation wave, realize accurate control of the MMC, effectively inhibit the transformer magnetic bias current, and ensure stable operation of the system.

[0140] As Figure 2As shown, the embodiment also provides a transformer magnetizing current suppression system under MMC parameter imbalance, comprising: a data acquisition module, configured to acquire total energy stored by each bridge arm of the MMC and circulating current of each phase unit; a calculation module, configured to obtain common-mode power frequency voltage corresponding to each phase unit based on total energy stored by each bridge arm, taking consistent total energy stored by upper and lower bridge arms of the same phase unit as a first control target, obtaining first modulation voltage based on suppressing power frequency zero sequence current in direct current of the MMC, obtaining second modulation voltage based on suppressing two-frequency circulating current between MMC phases based on circulating current of each phase unit, and obtaining differential-mode modulation wave based on equal direct current component between upper and lower bridge arm currents of the same phase unit; and a control output module, configured to calculate upper bridge arm final modulation wave and lower bridge arm final modulation wave corresponding to each phase unit based on common-mode power frequency voltage, first modulation voltage, second modulation voltage and 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 realize suppression of the transformer magnetizing current.

[0141] The application also provides an electronic device, comprising: a memory configured to store a computer program; and a processor configured to implement the steps of the transformer magnetizing current suppression method under MMC parameter imbalance when executing the computer program.

[0142] The processor implements the steps of the transformer magnetizing current suppression method under MMC parameter imbalance when executing the computer program, for example: obtaining common-mode power frequency voltage corresponding to each phase unit based on total energy stored by each bridge arm, taking consistent total energy stored by upper and lower bridge arms of the same phase unit as a first control target; obtaining first modulation voltage based on suppressing power frequency zero sequence current in direct current of the MMC; obtaining second modulation voltage based on suppressing two-frequency circulating current between MMC phases based on circulating current of each phase unit; obtaining differential-mode modulation wave based on equal direct current component between upper and lower bridge arm currents of the same phase unit; and calculating upper bridge arm final modulation wave and lower bridge arm final modulation wave corresponding to each phase unit based on common-mode power frequency voltage, first modulation voltage, second modulation voltage and 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 realize suppression of the transformer magnetizing current.

[0143] Exemplarily, the computer program can be divided into one or more modules / units, which are stored in the memory and executed by the processor to complete the present application. The one or more modules / units can be a series of computer program instruction segments capable of completing preset functions, which are used to describe the execution process of the computer program in the transformer magnetizing current suppression device under the MMC parameter imbalance. 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: the data acquisition module is used to acquire the total energy stored in each bridge arm of the MMC and the circulating current of each phase unit; the calculation module is used to obtain the common-mode power frequency voltage corresponding to each phase unit based on the total energy stored in each bridge arm, 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; obtaining the first modulation voltage based on the second control target of suppressing the power frequency zero sequence current in the DC current of the MMC; obtaining the second modulation voltage based on the third control target of suppressing the two-frequency circulating current between the MMC phases based on the circulating current of each phase unit; obtaining the differential-mode modulation wave based on the fourth control target of equalizing the DC current component between the upper bridge arm current and the lower bridge arm current of the same phase unit; 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; and the upper bridge arm final modulation wave and the lower bridge arm final modulation wave are used to control the MMC to achieve suppression of the transformer magnetizing current.

[0144] The transformer magnetizing current suppression device under the MMC parameter imbalance can be a desktop computer, a notebook computer, a palm computer and a cloud server, etc. The transformer magnetizing current suppression device under the MMC parameter imbalance can include, but is not limited to, a processor, a memory. Those skilled in the art can understand that the above is an example of the transformer magnetizing current suppression device under the MMC parameter imbalance, and does not constitute a limitation on the transformer magnetizing current suppression device under the MMC parameter imbalance, and can include more components than the above, or combine certain components, or different components, for example, the transformer magnetizing current suppression device under the MMC parameter imbalance can also include an input / output device, a network access device, a bus, etc.

[0145] The processor can be a central processing unit (CPU), and can also be 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 can be a microprocessor or the processor can also be any conventional processor or the like, which is the control center of the transformer magnetizing current suppression under MMC parameter imbalance, and is connected with various parts of the transformer magnetizing current suppression device under MMC parameter imbalance through various interfaces and lines.

[0146] The memory can be used to store the computer program and / or modules, and the processor realizes various functions of the transformer magnetizing current suppression device under MMC parameter imbalance by running or executing the computer program and / or modules stored in the memory, and calling the data stored in the memory.

[0147] The memory can mainly include a program storage area and a data storage area, wherein the program storage area can store an operating system, at least one application required by a function (such as a sound playing function, an image playing function, etc.), etc.; and the data storage area can store data created according to the use of the mobile phone (such as audio data, a phone book, etc.), etc. In addition, the memory can include a high-speed random access memory, and can also include a non-volatile memory, for example, a hard disk, a 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 memory devices.

[0148] The application further provides a computer readable storage medium, which stores a computer program, and the computer program is executed by a processor to realize the steps of the MMC parameter imbalance based transformer magnetizing current suppression method.

[0149] If the modules / units of the MMC parameter imbalance based transformer magnetizing current suppression system are realized in the form of software function units and sold or used as independent products, they can be stored in a computer readable storage medium.

[0150] Based on this understanding, the present invention can implement all or part of the processes in the transformer bias current suppression method under MMC parameter imbalance, or it can be accomplished by a computer program instructing related hardware. The computer program can be stored in a computer-readable storage medium. When executed by a processor, the computer program can implement the steps of the transformer bias current suppression method under MMC parameter imbalance. The computer program includes computer program code, which can be in the form of source code, object code, executable file, or a preset intermediate form, etc.

[0151] The computer-readable storage medium may include: any entity or device capable of carrying the computer program code, recording media, USB flash drive, portable hard drive, magnetic disk, optical disk, computer memory, read-only memory (ROM), random access memory (RAM), electrical carrier signal, telecommunication signal, and software distribution medium, etc.

[0152] It should be noted that the content contained in the computer-readable storage medium may be appropriately increased or decreased according to the requirements of legislation and patent practice in the jurisdiction. For example, in some jurisdictions, according to legislation and patent practice, the computer-readable storage medium does not include electrical carrier signals and telecommunication signals.

[0153] This invention provides a method for suppressing transformer bias current under MMC parameter imbalance, which has the following advantages:

[0154] This method controls the stored energy of the upper and lower arms of the converter phase unit to ensure that the submodule capacitor voltage and energy are controlled within set values ​​under various system parameter asymmetries. These asymmetries include inconsistent submodule numbers, inconsistent submodule capacitors, and imbalances between arm reactors. The method simplifies the calculation of the inter-phase common-mode voltage and reduces the magnitude of the common-mode circulating current by multiplying the output of the proportional-integral controller of the stored energy deviation of the upper and lower arms of each phase with the voltage of the three-phase voltage on the transformer valve side after lagging by 1 / 4 power frequency cycle. Furthermore, it controls the DC output voltage of the converter. Unified control of the power frequency component in the current flow can simplify the suppression of the zero-sequence power frequency current on the DC side caused by the introduction of interphase circulating current; controlling the interphase circulating current of the converter through a quasi-PR controller can avoid the mutual interference of the power frequency circulating current when using proportional-integral control of the circulating current, so that the circulating current of each frequency band of the converter can be controlled independently, and the changes in grid frequency can be taken into account; by using a proportional-integral controller to control the DC current component deviation of the upper and lower bridge arm current of the phase unit, complete control of the DC current component deviation of the upper and lower bridge arm current of the phase unit can be achieved, thereby suppressing the DC component in the transformer valve side current and the transformer valve side winding current.

[0155] The above embodiment is only one of the implementation manners of the technical scheme of the present application, and the scope of the present application is not limited to the above embodiment, but also includes any changes, substitutions and other implementation manners that are easily thought of by those skilled in the art within the technical scope disclosed by the present application.

[0156] Finally, it should be noted that: the above embodiments are only used to illustrate the technical scheme of the present application but not to limit it, although the present application has been described in detail with reference to the above embodiments, those skilled in the art should understand that: the specific implementation manner of the present application can be modified or replaced equivalently without departing from the spirit and scope of the present application, and any modification or equivalent replacement should be covered within the protection scope of the present application.

Claims

1. A method for suppressing transformer magnetizing current under MMC parameter imbalance, characterized in that, The method comprises the following steps: acquiring total energy stored in each bridge arm of the MMC and circulating current of each phase unit; based on the total energy stored in each bridge arm, 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, obtaining common-mode power frequency voltage corresponding to each phase unit; taking suppression of the power frequency zero sequence current in the DC current of the MMC as the second control target, obtaining a first modulation voltage; based on the circulating current of each phase unit, taking suppression of the two-frequency circulating current between the MMC phases as the third control target, obtaining a second modulation voltage; taking the DC current component between the upper bridge arm current and the lower bridge arm current of the same phase unit as the fourth control target, obtaining a differential-mode modulation wave; 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, calculating the upper bridge arm final modulation wave and the lower bridge arm final modulation wave corresponding to each phase unit; the upper bridge arm final modulation wave and the lower bridge arm final modulation wave are used for controlling the MMC, so as to realize suppression of the transformer magnetic bias current; 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, calculating the upper bridge arm final modulation wave and the lower bridge arm final modulation wave corresponding to each phase unit, and the specific formula is as follows: In the formula, Vref_top_j is the upper bridge arm final modulation wave; Vref_btm_j is the lower bridge arm final modulation wave; 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, which respectively represent three phase units.

2. The transformer magnetic bias current suppression method under MMC parameter imbalance according to claim 1, wherein the step of acquiring the total energy stored in each bridge arm of the MMC and the circulating current of each phase unit comprises the following steps: acquiring the rated capacitance value of each sub-module in the bridge arm of the MMC, the actual working voltage of each sub-module capacitor and the number of sub-modules in each bridge arm; based on the rated capacitance value of each sub-module in the bridge arm, the actual working voltage of each sub-module capacitor and the number of sub-modules in each bridge arm, calculating the total energy stored in each bridge arm, and the specific formula is as follows: In the formula, Earm is the total energy stored in the bridge arm; C is the rated capacitance value of the sub-module; k is the number of sub-modules in each bridge arm, taking a value of 1 to N, N is a positive integer; Uc is the actual working voltage of each sub-module capacitor; Uc （k） is the actual working voltage of the kth sub-module capacitor. acquiring the upper bridge arm current and the lower bridge arm current of each phase unit; based on the upper bridge arm current and the lower bridge arm current of each phase unit, calculating the circulating current of each phase unit, and the specific formula is as follows: In the formula, Icir_j is the circulating current of the phase unit; Iarm_top_j is the upper bridge arm current; Iarm_btm_j is the lower bridge arm current; j=A, B, C, which respectively represent three phase units.

3. The method for suppressing transformer magnetizing current under MMC parameter imbalance according to claim 1, characterized in that, the step of 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 based on the total energy stored in each bridge arm, obtaining the common-mode power frequency voltage corresponding to each phase unit comprises the following steps: The total energy stored in the upper bridge arm and the lower bridge arm of the same-phase unit is taken as a first control target, a proportional-integral controller is adopted 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 is kept consistent, and an output result of the proportional-integral controller is obtained; The output result of the proportional-integral controller is multiplied by a three-phase power frequency common-mode reference voltage respectively to obtain a common-mode power frequency voltage corresponding to each phase unit; wherein the three-phase power frequency common-mode reference voltage is obtained by lagging 1 / 4 power frequency period of the MMC transformer valve side three-phase voltage.

4. The method of claim 1, wherein the MMC parameter imbalance is a DC offset imbalance. The first modulation voltage is obtained by taking the power frequency zero sequence current in the DC current of the MMC as a second control target, including: The power frequency zero sequence current in the DC current of the MMC is taken as the second control target, and the DC current of the MMC is band-pass filtered in the power frequency band; wherein the resonance frequency of the quasi-PR controller is set to the power frequency; The DC current of the MMC after the band-pass filtering is controlled by using the quasi-PR controller, and a first output result of the quasi-PR controller is taken as the first modulation voltage.

5. The method of claim 1, wherein the MMC parameter imbalance is a DC offset imbalance. The second modulation voltage is obtained by taking the two times frequency circulating current between the MMC phases as a third control target based on the circulating current of each phase unit, including: The two times frequency circulating current between the MMC phases is taken as the third control target, and the circulating current of each phase unit is respectively band-stop filtered and high-pass filtered to remove the power frequency and DC components in the circulating current; The circulating current after the band-stop filtering and the high-pass filtering is controlled by using the quasi-PR controller, and a second output result of the quasi-PR controller is taken as the second modulation voltage; wherein the resonance frequency of the quasi-PR controller is set to twice the power frequency.

6. The method of claim 1, wherein the MMC parameter imbalance is a DC offset imbalance. The fourth control target is that the DC current component between the upper bridge arm current and the lower bridge arm current of the same-phase unit is equal, and the differential mode modulation wave is obtained, including: The DC current component between the upper bridge arm current and the lower bridge arm current of each phase unit is equal as the fourth control target; The DC component in the bridge arm current of each bridge arm is obtained; The DC current component of the upper bridge arm current in the same-phase unit is subtracted from the DC current component of the lower bridge arm current to obtain the DC current component deviation between the upper bridge arm current and the lower bridge arm current; The proportional-integral controller is adopted 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 taken as the differential mode modulation wave.

7. A transformer magnetizing current suppression system under MMC parameter imbalance, characterized in that, including: The data acquisition module is used to obtain the total energy stored in each bridge arm of the MMC and the circulating current of each phase unit; The calculation module is used to take the consistency of the total energy stored in the upper bridge arm and the lower bridge arm of the same-phase unit as a first control target based on the total energy stored in each bridge arm, to obtain the common-mode power frequency voltage corresponding to each phase unit; The first modulation voltage is obtained by taking the power frequency zero sequence current in the DC current of the MMC as a second control target; the second modulation voltage is obtained by taking the two times frequency circulating current between the MMC phases as a third control target based on the circulating current of each phase unit; and the fourth control target is that the DC current component between the upper bridge arm current and the lower bridge arm current of the same-phase unit is equal, and the differential mode modulation wave is obtained. The control output module is configured 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; and 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 realize the suppression of the transformer magnetic bias current. 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, and the specific formula is as follows: In the formula, 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; and j=A, B, C, respectively representing three phase units.

8. An electronic device, comprising: The computer program is executed by the processor to realize the steps of the transformer magnetic bias current suppression method under the MMC parameter imbalance according to any one of claims 1-6. The computer program is executed by the processor to realize the steps of the transformer magnetic bias current suppression method under the MMC parameter imbalance according to any one of claims 1-6. ​ 9. A computer-readable storage medium storing a computer program, the computer program comprising instructions that, when executed by a computer, cause the computer to perform the method of any one of claims 1 to 8. ​

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