Frequency-doubled circulating current suppression method of energy storage type MMC (Modular Multilevel Converter) and related device

Abstract

The invention relates to the technical field of energy storage type MMC control, and particularly provides a second harmonic generation circulating current suppression method of an energy storage type MMC and a related device, which can calculate a three-phase fundamental frequency current compensation value according to a three-phase energy storage output value of each bridge arm in the energy storage type MMC, and determine a three-phase second harmonic generation circulating current in the energy storage type MMC according to the three-phase fundamental frequency current compensation value. According to the method, the three-phase double-frequency circulating current is taken as a control rate for realizing double-frequency circulating current suppression, the three-phase double-frequency modulation voltage is determined, and the three-phase double-frequency modulation voltage is superposed to the three-phase modulation voltage of the energy storage type MMC. Therefore, the additional three-phase double-frequency modulation voltage can be utilized to offset a double-frequency circulating current excitation source caused by unbalanced energy storage output in the energy storage type MMC, and double-frequency circulating current suppression control of the energy storage type MMC is realized. Therefore, under the condition that the energy storage output of the upper and lower bridge arms of the energy storage type MMC is unbalanced, the second harmonic content of the bridge arm current in the converter can be quickly and effectively reduced, and the bridge arm current stress is reduced.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of energy storage type MMC control, and particularly relates to a method for suppressing double-frequency circulating current of an energy storage type MMC and a related device. BACKGROUND

[0002] Under the background that new energy power generation is gradually moving towards large-scale base and remoteness, an energy storage type flexible DC system combining a modular multilevel converter (MMC) and an energy storage system emerges as the times require. The energy storage type flexible DC system uses an energy storage type MMC as a converter unit, combines the control flexibility of the MMC and the energy abundance of the energy storage, can be used to build a large-scale power transmission channel and suppress new energy output uncertainty, and can also provide transient support for a power system, and is a new technical path for safe and efficient transmission of large-scale clean energy base power.

[0003] Unlike other types of voltage source converters, the energy storage type MMC often has internal unbalanced currents between three-phase bridge arms during operation. The unbalanced currents include a DC component, a fundamental component, a double-frequency component and other small components, wherein the DC component represents the power exchange between the sub-module capacitor inside the converter and the DC system, and the fundamental component represents the power exchange between the sub-module capacitor and the AC system. Through a large number of sub-module capacitors as carriers, electric energy can be transmitted between the AC side and the DC side, so the DC component and the fundamental component are necessary in the internal unbalanced current. However, the double-frequency component and other small components will bring additional requirements to the current passing capacity of the converter, so this part of the component is unnecessary.

[0004] Existing researches have proposed quite perfect suppression schemes for internal asymmetric circulating current and sub-module voltage ripple in the MMC, including circulating current suppression control and harmonic injection control schemes. However, in the case of output imbalance of the upper and lower bridge arms of the energy storage type MMC, the system will additionally introduce asymmetric fundamental bridge arm currents, and under the influence of this, the typical circulating current suppression strategy will not work. Therefore, it is urgent to provide a double-frequency circulating current suppression scheme for the case of output imbalance of the upper and lower bridge arms of the energy storage type MMC. SUMMARY

[0005] The purpose of the present application is to at least solve one of the above technical defects, in particular, the technical defect that the double-frequency circulating current of the energy storage type MMC is difficult to suppress in the case of energy storage output imbalance in the prior art.

[0006] In a first aspect, an embodiment of the present application provides a method for suppressing double-frequency circulating current of an energy storage type MMC, comprising:

[0007] acquire an ac voltage, a dc current and a bridge arm current of each bridge arm of the energy storage MMC, respectively;

[0008] determine a three-phase energy storage output value of each bridge arm;

[0009] calculate a three-phase fundamental frequency current compensation value according to the ac voltage and the three-phase energy storage output value;

[0010] calculate a three-phase double-frequency circulating current according to the three-phase fundamental frequency current compensation value, the dc current and each bridge arm current, and determine a three-phase double-frequency modulation voltage based on the three-phase double-frequency circulating current;

[0011] superimpose the three-phase double-frequency modulation voltage to a three-phase modulation voltage of the energy storage MMC.

[0012] In some embodiments, the calculating a three-phase double-frequency circulating current according to the three-phase fundamental frequency current compensation value, the dc current and each bridge arm current comprises:

[0013] calculating the three-phase double-frequency circulating current based on the following expression:

[0014]

[0015] wherein, is a j-phase double-frequency circulating current, is a bridge arm current of a j-phase lower bridge arm, is a bridge arm current of a j-phase upper bridge arm, is a j-phase fundamental frequency current compensation value, is the dc current; j=a, b, c.

[0016] In some embodiments, the calculating a three-phase fundamental frequency current compensation value according to the ac voltage and the three-phase energy storage output value comprises:

[0017] calculating the three-phase fundamental frequency current compensation value based on the following expression:

[0018]

[0019] wherein, is a phasor form of a j-phase fundamental frequency current compensation value, is a j-phase upper bridge arm energy storage output value, is a j-phase lower bridge arm energy storage output value, is a phasor form of a j-phase ac voltage.

[0020] In some embodiments, the determining a three-phase double-frequency modulation voltage based on the three-phase double-frequency circulating current comprises:

[0021] determining a phase reference value;

[0022] performing Park transformation on the three-phase double-frequency circulating current according to the phase reference value to obtain a d-axis circulating current component and a q-axis circulating current component;

[0023] determining a d-axis voltage component and a q-axis voltage component based on the d-axis circulating current component and the q-axis circulating current component, respectively;

[0024] performing inverse Park transformation on the d-axis voltage component and the q-axis voltage component according to the phase reference value to obtain the three-phase double-frequency modulation voltage.

[0025] In some embodiments, the determining the d-axis voltage component and the q-axis voltage component based on the d-axis circulating current component and the q-axis circulating current component, respectively, comprises:

[0026] calculating the d-axis voltage component and the q-axis voltage component based on the following expressions, respectively:

[0027]

[0028] wherein, is the d-axis voltage component, is the q-axis voltage component, L is an inductance value of a bridge arm of the energy storage type MMC, is an actual frequency value of the alternating voltage, is the d-axis circulating current component, is the q-axis circulating current component, is a proportional parameter of a current controller, is an integral parameter of the current controller, and s is a Laplace operator.

[0029] In some embodiments, the determining the phase reference value comprises:

[0030] if the energy storage type MMC adopts a grid-following control strategy, obtaining the phase reference value from a phase-locked loop of the energy storage type MMC, otherwise, taking a preset frequency value as the phase reference value.

[0031] In some embodiments, the determining the three-phase energy storage output value of each bridge arm comprises:

[0032] obtaining an energy storage voltage and an energy storage current of each energy storage unit in the energy storage type MMC;

[0033] calculating the three-phase energy storage output value based on the following expression:

[0034]

[0035] wherein, r=p, n, p represents an upper bridge arm, n represents a lower bridge arm, and j=a, b, c. is the energy output value of the j-phase r-bridge arm, N is the total number of energy storage units of the j-phase r-bridge arm, is the energy storage voltage of the kth energy storage unit in the j-phase r-bridge arm, is the energy storage current of the kth energy storage unit in the j-phase r-bridge arm.

[0036] In a second aspect, the embodiments of the present application provide a double-frequency circulating current suppression device of an energy storage type MMC, comprising:

[0037] a voltage and current acquisition module, configured to acquire an alternating voltage, a direct current and bridge arm currents of each bridge arm in the energy storage type MMC respectively;

[0038] an energy output value determination module, configured to determine three-phase energy output values of the bridge arms;

[0039] a fundamental frequency current compensation value calculation module, configured to calculate three-phase fundamental frequency current compensation values according to the alternating voltage and the three-phase energy output values;

[0040] a double-frequency modulation voltage determination module, configured to calculate three-phase double-frequency circulating currents according to the three-phase fundamental frequency current compensation values, the direct current and the bridge arm currents, and determine three-phase double-frequency modulation voltages based on the three-phase double-frequency circulating currents;

[0041] a voltage superposition module, configured to superimpose the three-phase double-frequency modulation voltages into three-phase modulation voltages of the energy storage type MMC.

[0042] In a third aspect, the embodiments of the present application provide a storage medium, which stores computer readable instructions. When the computer readable instructions are executed by one or more processors, the one or more processors perform the steps of the double-frequency circulating current suppression method of the energy storage type MMC according to any of the above embodiments.

[0043] In a fourth aspect, the embodiments of the present application provide a computer device, which comprises one or more processors and a memory.

[0044] The memory stores computer readable instructions. When the computer readable instructions are executed by the one or more processors, the steps of the double-frequency circulating current suppression method of the energy storage type MMC according to any of the above embodiments are performed.

[0045] In the energy storage type MMC double frequency circulating current suppression method and related device provided by some embodiments of the present application, three-phase double frequency circulating current values in the energy storage type MMC can be calculated according to three-phase energy storage output values of each bridge arm in the energy storage type MMC, and three-phase double frequency circulating currents in the energy storage type MMC can be determined accordingly. In the present application, the three-phase double frequency circulating current is taken as the control rate for realizing double frequency circulating current suppression, three-phase double frequency modulation voltages are determined, and the three-phase double frequency modulation voltages are superimposed into three-phase modulation voltages of the energy storage type MMC. In this way, the additional three-phase double frequency modulation voltages can be used to offset the double frequency circulating current excitation source caused by the unbalanced energy storage output in the energy storage type MMC, so as to realize double frequency circulating current suppression control of the energy storage type MMC. As can be seen, the present application can quickly and effectively reduce the second harmonic content of the bridge arm current in the converter and reduce the bridge arm current stress in the case of unbalanced energy storage output of the upper and lower bridge arms of the energy storage type MMC, so as to improve the stability of the energy storage type MMC in the case of unbalanced energy storage output and avoid overcurrent in the converter to damage the equipment. Compared with the existing double frequency circulating current suppression strategy, the present application can be applied to the energy storage type MMC with unbalanced energy storage output, has a wider application scenario, and has a more perfect double frequency circulating current suppression function. BRIEF DESCRIPTION OF DRAWINGS

[0046] In order to more clearly illustrate the technical solutions in the embodiments of the present application or the prior art, the drawings needed to be used in the embodiments or the prior art description will be briefly introduced below. Obviously, the drawings in the following description are only some embodiments of the present application, and other drawings can be obtained by those skilled in the art without creative labor.

[0047] Figure 1 For some embodiments, one of the flowcharts of the double frequency circulating current suppression method of the energy storage type MMC;

[0048] Figure 2 For some embodiments, the second flowchart of the double frequency circulating current suppression method of the energy storage type MMC;

[0049] Figure 3 For some embodiments, one of the response graphs of the converter bridge arm current and the double frequency circulating current when the suppression method provided by the present application is not used;

[0050] Figure 4 For some embodiments, one of the response graphs of the converter bridge arm current and the double frequency circulating current after the suppression method provided by the present application is used;

[0051] Figure 5 For some embodiments, the second response graph of the converter bridge arm current and the double frequency circulating current when the suppression method provided by the present application is not used;

[0052] Figure 6Fig. 2 is a response diagram of the converter bridge arm current and the double-frequency circulating current after the suppression method is adopted in some embodiments of the present application;

[0053] Figure 7 Fig. 4 is a structural schematic diagram of the double-frequency circulating current suppression device of the energy storage MMC in some embodiments of the present application;

[0054] Figure 8 Fig. 5 is an internal structural diagram of the computer device in some embodiments of the present application. DETAILED DESCRIPTION

[0055] The technical solutions in the embodiments of the present application will be clearly and completely described in combination with the drawings in the embodiments of the present application. Obviously, the described embodiments are only some of the embodiments of the present application, but not all the embodiments of the present application. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative work fall within the scope of protection of the present application.

[0056] In some embodiments, as shown in Fig. 1, Figure 1 The present application provides a double-frequency circulating current suppression method of an energy storage MMC, which comprises the following steps:

[0057] S102: Obtain the AC voltage of the energy storage MMC, the DC current of the energy storage MMC, and the bridge arm current of each bridge arm of the energy storage MMC, respectively.

[0058] Specifically, the energy storage MMC can comprise a three-phase circuit, and the three-phase circuit corresponds to phase A, phase B and phase C, respectively. Each phase circuit can comprise an upper bridge arm and a lower bridge arm. Therefore, the energy storage MMC can comprise six bridge arms, which are the upper bridge arm of the phase A circuit, the lower bridge arm of the phase A circuit, the upper bridge arm of the phase B circuit, the lower bridge arm of the phase B circuit, the upper bridge arm of the phase C circuit and the lower bridge arm of the phase C circuit.

[0059] It can be understood that the specific circuit structure of each bridge arm in the energy storage MMC can be determined according to actual conditions, and the present application does not make specific limitations thereon. In some examples, each bridge arm can comprise a plurality of sub-modules comprising a half-bridge converter and an energy storage unit, and the circulating current is realized through the synergistic effect of the half-bridge converter and the energy storage unit.

[0060] In this step, the AC voltage of the energy storage MMC, the DC current of the energy storage MMC and the bridge arm current of each bridge arm can be obtained, respectively. The AC voltage can be the voltage at the connection between each phase circuit and the power grid in the three-phase circuit of the energy storage MMC. The DC current can be the transmission current between the energy storage MMC and the DC system. The bridge arm current can be the current flowing through the bridge arm. Since the structures and working states of the upper and lower bridge arms of the energy storage MMC can be different, the bridge arm currents of different bridge arms can also be different.

[0061] S104: Determine the three-phase energy output value of each bridge arm.

[0062] In this step, the three-phase energy output value of each of the six bridge arms can be determined to reflect the real-time output value of the energy storage system of the bridge arm through the three-phase energy output value. After calculation, six three-phase energy output values corresponding to the six bridge arms can be obtained, so that the energy storage state of each bridge arm can be determined accordingly.

[0063] It can be understood that the three-phase energy output value can be determined in various ways in the present application. In some examples, S104 can include the following steps:

[0064] Step A1: Obtain the energy storage voltage and current of each energy storage unit in the energy storage MMC;

[0065] Step A3: Calculate the three-phase energy output value based on the following expression:

[0066]

[0067] In the formula, r=p, n, p represents the upper bridge arm, and n represents the lower bridge arm; j=a, b, c; is the energy output value of the j-phase r-bridge arm, N is the total number of energy storage units of the j-phase r-bridge arm, is the energy storage voltage of the kth energy storage unit in the j-phase r-bridge arm, is the energy storage current of the kth energy storage unit in the j-phase r-bridge arm.

[0068] For example, represents the energy output value of the A-phase circuit lower bridge arm, represents the energy storage current of the 3rd energy storage unit in the A-phase circuit lower bridge arm.

[0069] In this example, the three-phase energy output value can include the energy output value of the A-phase upper bridge arm, the energy output value of the A-phase lower bridge arm, the energy output value of the B-phase upper bridge arm, the energy output value of the B-phase lower bridge arm, the energy output value of the C-phase upper bridge arm, and the energy output value of the C-phase lower bridge arm. The present example can accumulate the energy storage power of each energy storage unit of the same bridge arm, so that the three-phase energy output value of the bridge arm can be more accurately determined, and the subsequent double-frequency circulating current suppression effect can be improved.

[0070] S106: Calculate the three-phase fundamental frequency current compensation value according to the AC voltage and the three-phase energy output value.

[0071] The fundamental frequency current compensation value refers to the current value used to compensate for the target fundamental frequency component, and the target fundamental frequency component can be the current fundamental frequency component generated in the bridge arm current due to the imbalance of the energy output. By introducing the fundamental frequency current compensation value, the fundamental frequency deviation in the bridge arm current can be effectively corrected, laying a foundation for the accurate calculation of the subsequent double-frequency circulating current.

[0072] In this step, the base frequency current compensation values of the three phases A, B and C can be calculated according to the AC voltage of the energy storage type MMC and the three-phase energy storage output values by using a preset mathematical expression. This process takes into account the circuit characteristics of the energy storage type MMC and the output state of the energy storage unit, and can ensure the accuracy and effectiveness of the base frequency current compensation values.

[0073] It can be understood that the specific calculation formula of the three-phase base frequency circuit compensation value can be set according to the actual situation. In some examples, the three-phase base frequency current compensation value can be calculated based on the following expression:

[0074]

[0075] In the formula, is the phasor form of the j-phase base frequency current compensation value, is the energy storage output value of the upper bridge arm of the j-phase, is the energy storage output value of the lower bridge arm of the j-phase, is the phasor form of the j-phase AC voltage.

[0076] For example, is the phasor form of the A-phase base frequency current compensation value, which can be calculated according to the energy storage output value of the upper bridge arm of the A-phase circuit, the energy storage output value of the lower bridge arm of the A-phase circuit and the A-phase AC voltage of the energy storage type MMC. The calculation process of the B-phase base frequency current compensation value and the C-phase base frequency current compensation value is similar to that of the A-phase base frequency current compensation value.

[0077] By using the above expression for calculation, more accurate three-phase base frequency current compensation values can be obtained, thereby improving the calculation accuracy of the subsequent double-frequency circulating current, and further improving the double-frequency circulating current suppression effect.

[0078] S108: Calculate the three-phase double-frequency circulating current according to the three-phase base frequency current compensation value, the DC current and the bridge arm current, and determine the three-phase double-frequency modulation voltage based on the three-phase double-frequency circulating current.

[0079] The three-phase double-frequency circulating current refers to the current component oscillating at twice the fundamental frequency generated in the bridge arm current due to factors such as unbalanced energy storage output of the upper and lower bridge arms in the energy storage type MMC. The existence of such double-frequency circulating current can cause an increase in bridge arm current stress, affecting the stable operation of the energy storage type MMC, and even damaging the equipment. The three-phase double-frequency modulation voltage can be used to offset the voltage component corresponding to the three-phase double-frequency circulating current.

[0080] In this step, firstly, the three-phase double-frequency circulating current of the energy storage type MMC can be calculated according to the three-phase fundamental frequency current compensation value, the direct current and the bridge arm current by using a specific algorithm or mathematical model. This process takes into account the circuit structure of the energy storage type MMC, the energy storage output state and the dynamic change of the current, ensuring the accuracy of the calculation of the three-phase double-frequency circulating current. After obtaining the three-phase double-frequency circulating current, the three-phase double-frequency modulation voltage for suppressing the double-frequency circulating current can be determined based on the circulating current value through a preset conversion relationship or control strategy.

[0081] It can be understood that the specific calculation formula of the three-phase double-frequency circulating current and the specific calculation formula of the three-phase double-frequency modulation voltage can be determined according to the actual situation. In some examples, in order to obtain more accurate three-phase double-frequency circulating current and further improve the double-frequency circulating current suppression effect, the three-phase double-frequency circulating current can be calculated based on the following expression:

[0082]

[0083] In the formula, is the j-phase double-frequency circulating current, is the bridge arm current of the lower bridge arm of the j-phase, is the bridge arm current of the upper bridge arm of the j-phase, is the j-phase fundamental frequency current compensation value, is the direct current; j = a, b, c.

[0084] In this example, the three-phase double-frequency circulating current can include A-phase double-frequency circulating current, B-phase double-frequency circulating current and C-phase double-frequency circulating current. The A-phase double-frequency circulating current can be calculated according to the bridge arm current of the lower bridge arm of the A-phase circuit, the bridge arm current of the upper bridge arm of the A-phase circuit, the A-phase fundamental frequency current compensation value and the direct current of the energy storage type MMC. Similarly, the B-phase double-frequency circulating current can be calculated according to the bridge arm current of the lower bridge arm of the B-phase circuit, the bridge arm current of the upper bridge arm of the B-phase circuit, the B-phase fundamental frequency current compensation value and the direct current of the energy storage type MMC; the C-phase double-frequency circulating current can be calculated according to the bridge arm current of the lower bridge arm of the C-phase circuit, the bridge arm current of the upper bridge arm of the C-phase circuit, the C-phase fundamental frequency current compensation value and the direct current of the energy storage type MMC.

[0085] In some examples, determining the three-phase double-frequency modulation voltage based on the three-phase double-frequency circulating current can include:

[0086] Step B1: determining a phase reference value;

[0087] Step B3: performing Park transformation on the three-phase double-frequency circulating current according to the phase reference value to obtain a d-axis circulating current component and a q-axis circulating current component;

[0088] Step B5: determining a d-axis voltage component and a q-axis voltage component based on the d-axis circulating current component and the q-axis circulating current component, respectively;

[0089] Step B7: performing a Park inverse transform on the d-axis voltage component and the q-axis voltage component according to the phase reference value to obtain a three-phase double-frequency modulation voltage.

[0090] Specifically, in the process of determining the three-phase double-frequency modulation voltage, the phase reference value of the energy storage type MMC can be determined first. In some examples, when the phase reference value is determined, it can be judged whether the converter adopts a grid-following control strategy, which determines the way of obtaining the phase reference value. If the converter adopts the grid-following control strategy, the phase reference value can be obtained through the phase-locked loop of the energy storage type MMC. If the grid-following control strategy is not adopted, the preset frequency value can be taken as the phase reference value. In this way, a more accurate phase reference value can be obtained to ensure the accuracy of the results obtained by the Park transform and the Park inverse transform, so as to further improve the double-frequency circulating current suppression effect.

[0091] After obtaining the phase reference value, the present application can perform a Park transform on the three-phase double-frequency circulating current on the basis of the phase reference value, convert the double-frequency circulating currents of phases A, B and C to the dq coordinate system, and obtain a d-axis circulating current component and a q-axis circulating current component. In this way, the three-phase alternating current value can be converted into a direct current value that does not change, which facilitates the double-frequency circulating current suppression. In some examples, the present application can perform the Park transform on the basis of the following expression:

[0092]

[0093] In the formula, is the d-axis circulating current component, is the q-axis circulating current component, is the phase reference value, is the double-frequency circulating current of phase A, is the double-frequency circulating current of phase B, and is the double-frequency circulating current of phase C.

[0094] After obtaining the d-axis circulating current component and the q-axis circulating current component, the present application can determine a d-axis voltage component and a q-axis voltage component on the basis of a current loop control strategy. In some examples, in order to quickly adjust and suppress the double-frequency circulating current, eliminate the three-phase double-frequency circulating current of the converter, and avoid overcurrent of the converter to damage the equipment, the present application can calculate the d-axis voltage component and the q-axis voltage component on the basis of the following expressions, respectively:

[0095]

[0096] In the formula, is the d-axis voltage component, is the q-axis voltage component, and L is the bridge arm inductance value of the energy storage type MMC, is the actual frequency value of the alternating voltage, is the d-axis circulating current component, is the q-axis circulating current component, a proportional parameter of the current controller, an integral parameter of the current controller, s is a Laplace operator.

[0097] Since the control signal ultimately required by the energy storage type MMC is the modulated voltage in the abc stationary coordinate system, after obtaining the modulated voltage component in the dq coordinate system, the application can convert the modulated voltage from the dq coordinate system to the abc coordinate system through the Park inverse transformation, and obtain the corresponding three-phase double-frequency modulated voltage in the abc coordinate system. In some examples, the three-phase double-frequency modulated voltage includes an A-phase double-frequency modulated voltage, a B-phase double-frequency modulated voltage and a C-phase double-frequency modulated voltage, and the application can perform the Park inverse transformation based on the following expression:

[0098]

[0099] In the formula, is the A-phase double-frequency modulated voltage, is the B-phase double-frequency modulated voltage, is the C-phase double-frequency modulated voltage.

[0100] S110: superimpose the three-phase double-frequency modulated voltage into the three-phase modulated voltage of the energy storage type MMC.

[0101] In this step, the three-phase double-frequency modulated voltage can be added to the three-phase modulated voltage of the energy storage type MMC. That is, the final modulated voltage of the j phase is:

[0102]

[0103] In the formula, is the final modulated voltage of the j phase, is the j-phase modulated voltage generated by the energy storage type MMC without using the suppression method described in the application.

[0104] In this way, the double-frequency circulating current suppression function can be added to the energy storage type MMC, thereby achieving rapid suppression of the double-frequency circulating current when the energy storage output is unbalanced.

[0105] In the present application, the three-phase fundamental current compensation values can be calculated according to the three-phase energy output values of each bridge arm in the energy storage type MMC, and the three-phase double-frequency circulating currents in the energy storage type MMC are determined accordingly. The present application takes the three-phase double-frequency circulating currents as the control rate for realizing double-frequency circulating current suppression, determines the three-phase double-frequency modulation voltage, and superimposes the three-phase double-frequency modulation voltage into the three-phase modulation voltage of the energy storage type MMC. In this way, the additional three-phase double-frequency modulation voltage can be used to offset the double-frequency circulating current excitation source caused by the energy output imbalance in the energy storage type MMC, so as to realize double-frequency circulating current suppression control of the energy storage type MMC. As can be seen, the present application can quickly and effectively reduce the second harmonic content of the bridge arm current in the converter under the condition of energy output imbalance of the upper and lower bridge arms of the energy storage type MMC, and reduce the bridge arm current stress, so as to improve the stability of the energy storage type MMC under the condition of energy output imbalance, and avoid overcurrent in the converter to damage the equipment. Compared with the existing double-frequency circulating current suppression strategy, the present application can be applied to the energy storage type MMC with energy output imbalance, has a wider application scenario, and has a more perfect double-frequency circulating current suppression function.

[0106] In some embodiments, as shown in Figure 2 The present application provides a double-frequency circulating current suppression method for an energy storage type MMC, comprising:

[0107] S202: acquiring the AC voltage and DC current of the energy storage type MMC; acquiring the energy storage voltage and energy storage current of the energy storage unit in each sub-module of the energy storage type MMC; and acquiring the bridge arm current of the six bridge arms;

[0108] S204: calculating the energy output values of the six bridge arms; for the specific calculation process of this step, please refer to the above embodiments, which will not be repeated here;

[0109] S206: calculating the A-phase fundamental current compensation value, the B-phase fundamental current compensation value and the C-phase fundamental current compensation value; for the specific calculation process of this step, please refer to the above embodiments, which will not be repeated here;

[0110] S208: calculating the A-phase double-frequency circulating current, the B-phase double-frequency circulating current and the C-phase double-frequency circulating current; for the specific calculation process of this step, please refer to the above embodiments, which will not be repeated here;

[0111] S210: determining whether the energy storage type MMC adopts a grid-connected control strategy, if yes, performing S212, otherwise, performing S214;

[0112] S212: acquiring the phase reference value through a phase-locked loop;

[0113] S214: acquiring the phase reference value through a frequency set value;

[0114] S216: the d-axis component and the q-axis component of the three-phase double-frequency circulating current are calculated to obtain the d-axis circulating current component and the q-axis circulating current component; for the specific calculation process of this step, please refer to the above embodiments, which will not be repeated here;

[0115] S218: the d-axis component and the q-axis component of the three-phase double-frequency modulation voltage are calculated to obtain the d-axis voltage component and the q-axis voltage component; for the specific calculation process of this step, please refer to the above embodiments, which will not be repeated here;

[0116] S220: the three-phase double-frequency modulation voltage is calculated.

[0117] S222: the three-phase double-frequency modulation voltage is added to the three-phase modulation voltage.

[0118] For the double-frequency circulating current suppression method of the energy storage type MMC provided in the above embodiments, the energy storage type flexible DC power transmission test system containing 2 energy storage type MMCs is used for simulation verification. All the bridge arms of the energy storage type MMCs use half-bridge two-stage energy storage submodules.

[0119] It is assumed that the energy storage output of the upper and lower bridge arms is 0 in the steady state, and at time t=0.5s, the energy storage output of the upper bridge arm changes to-50MW, and the energy storage output of the lower bridge arm changes to 15MW. Figure 3 The response of the converter bridge arm current and the double-frequency circulating current when the suppression method provided by the present application is not used is shown. Figure 4 The response of the converter bridge arm current and the double-frequency circulating current after the suppression method provided by the present application is used is shown. From Figure 3 and Figure 4 It can be seen from the above figures that, whether the energy storage output of the upper and lower bridge arms is balanced or unbalanced, the secondary circulating current of the converter can be suppressed to zero by the method provided by the present application.

[0120] Figure 5 and Figure 6 The result figures of the simulation verification using the energy storage type flexible DC power transmission test system containing 2 energy storage type MMCs are shown. The rated capacity of the energy storage type MMC is 2500MW, the lower bridge arm uses half-bridge two-stage energy storage submodules, and the upper bridge arm uses half-bridge submodules.

[0121] It is assumed that the energy storage output of the lower bridge arm is 0 in the steady state, and at time t=1.0s, the energy storage output of the lower bridge arm changes to 500MW, and the energy storage output of the upper bridge arm is still 0MW because the upper bridge arm does not have an energy storage system. Figure 5 The response of the converter bridge arm current and the double-frequency circulating current when the suppression method provided by the present application is not used is shown. Figure 6 The response of the converter bridge arm current and the double-frequency circulating current after the suppression method provided by the present application is used is shown. From Figure 5 andFigure 6 It can be seen that, whether in the case of balanced or unbalanced energy output of upper and lower arms, the secondary circulating current of the converter can be inhibited to zero through the method provided in the application.

[0122] The device for suppressing the double-frequency circulating current of the energy storage MMC provided in the embodiments of the application is described below, and the device for suppressing the double-frequency circulating current of the energy storage MMC described below can be correspondingly referred to the method for suppressing the double-frequency circulating current of the energy storage MMC described above.

[0123] In some embodiments, as shown in Figure 7 The application provides a device 500 for suppressing the double-frequency circulating current of an energy storage MMC, comprising:

[0124] A voltage and current acquisition module 502 is configured to acquire the AC voltage of the energy storage MMC, the DC current of the energy storage MMC, and the bridge arm current of each bridge arm of the energy storage MMC, respectively.

[0125] An energy output value determination module 504 is configured to determine the three-phase energy output value of each bridge arm.

[0126] A fundamental frequency current compensation value calculation module 506 is configured to calculate the three-phase fundamental frequency current compensation value according to the AC voltage and the three-phase energy output value.

[0127] A double-frequency modulation voltage determination module 508 is configured to calculate the three-phase double-frequency circulating current according to the three-phase fundamental frequency current compensation value, the DC current, and each bridge arm current, and determine the three-phase double-frequency modulation voltage based on the three-phase double-frequency circulating current.

[0128] A voltage superposition module 510 is configured to superimpose the three-phase double-frequency modulation voltage into the three-phase modulation voltage of the energy storage MMC.

[0129] In some embodiments, the double-frequency modulation voltage determination module 508 of the application comprises:

[0130] A first calculation unit is configured to calculate the three-phase double-frequency circulating current based on the following expression:

[0131]

[0132] In the expression, is the j-phase double-frequency circulating current, is the bridge arm current of the lower bridge arm of the j phase, is the bridge arm current of the upper bridge arm of the j phase, is the j-phase fundamental frequency current compensation value, is the DC current; j = a, b, c.

[0133] In some embodiments, the fundamental frequency current compensation value calculation module 506 of the present application comprises:

[0134] a second calculation unit, configured to calculate the three-phase fundamental frequency current compensation value based on the following expression:

[0135]

[0136] wherein, is a phasor form of a j-phase fundamental frequency current compensation value, is an energy storage output value of an upper bridge arm of the j-phase, is an energy storage output value of a lower bridge arm of the j-phase, is a phasor form of an alternating current voltage of the j-phase.

[0137] In some embodiments, the double-frequency modulation voltage determination module 508 of the present application comprises:

[0138] a reference value determination unit, configured to determine a phase reference value;

[0139] a Park transformation unit, configured to perform Park transformation on the three-phase double-frequency circulating current according to the phase reference value, to obtain a d-axis circulating current component and a q-axis circulating current component;

[0140] a voltage component determination unit, configured to determine a d-axis voltage component and a q-axis voltage component based on the d-axis circulating current component and the q-axis circulating current component, respectively;

[0141] a Park inverse transformation unit, configured to perform Park inverse transformation on the d-axis voltage component and the q-axis voltage component according to the phase reference value, to obtain the three-phase double-frequency modulation voltage.

[0142] In some embodiments, the voltage component determination unit of the present application comprises:

[0143] a third calculation unit, configured to calculate the d-axis voltage component and the q-axis voltage component based on the following expression, respectively:

[0144]

[0145] wherein, is the d-axis voltage component, is the q-axis voltage component, L is an inductance value of a bridge arm of the energy storage type MMC, is an actual frequency value of the alternating current voltage, is the d-axis circulating current component, is the q-axis circulating current component, is a proportional parameter of a current controller, is an integral parameter of the current controller, s is a Laplace operator.

[0146] In some embodiments, the reference value determination unit of the present application comprises:

[0147] The phase acquisition unit is configured to acquire the phase reference value from a phase-locked loop of the energy storage MMC if the energy storage MMC adopts a grid-connected control strategy, or to take a preset frequency value as the phase reference value.

[0148] In some embodiments, the energy storage output value determination module 504 of the present application comprises:

[0149] The energy storage voltage and current acquisition unit is configured to acquire the energy storage voltage and the energy storage current of each energy storage unit in the energy storage MMC.

[0150] The fourth calculation unit is configured to calculate the three-phase energy storage output value based on the following expression:

[0151]

[0152] In the expression, r = p, n, p represents an upper bridge arm, and n represents a lower bridge arm; j = a, b, c; is the energy storage output value of the j-phase r bridge arm, N is the total number of energy storage units of the j-phase r bridge arm, is the energy storage voltage of the kth energy storage unit in the j-phase r bridge arm, is the energy storage current of the kth energy storage unit in the j-phase r bridge arm.

[0153] In one embodiment, the present application further provides a storage medium having computer readable instructions stored therein, and the computer readable instructions, when executed by one or more processors, cause the one or more processors to perform the steps of the method for suppressing double-frequency circulating current of the energy storage MMC according to any one of the embodiments.

[0154] In one embodiment, the present application further provides a computer device having computer readable instructions stored therein, and the computer readable instructions, when executed by one or more processors, cause the one or more processors to perform the steps of the method for suppressing double-frequency circulating current of the energy storage MMC according to any one of the embodiments.

[0155] Schematically, Figure 8 is a schematic diagram of an internal structure of a computer device provided by an embodiment of the present application, which may, in one example, be a server. Referring to Figure 8The computer device 900 includes a processing component 902, which is further composed of one or more processors, and a memory resource represented by the memory 901 for storing instructions, such as an application program, executable by the processing component 902. The application program stored in the memory 901 can include one or more than one module each corresponding to a set of instructions. In addition, the processing component 902 is configured to execute the instructions to perform the steps of the method for suppressing double-frequency circulating current of energy storage MMC as described in any of the above embodiments.

[0156] The computer device 900 can further include a power supply component 903 configured to perform power management of the computer device 900, a wired or wireless network interface 904 configured to connect the computer device 900 to a network, and an input / output (I / O) interface 905. The computer device 900 can operate based on an operating system stored in the memory 901, such as Windows Server TM, Mac OS X TM, Unix TM, Linux TM, Free BSD TM, or the like.

[0157] Those skilled in the art can understand that the internal structure of the computer device shown in the present application is only a block diagram of part of the structure related to the scheme of the present application, and does not constitute a limitation on the computer device to which the scheme of the present application is applied. A specific computer device can include more or fewer components than those shown in the figure, or combine certain components, or have a different arrangement of components.

[0158] Finally, it should be noted that, in this document, the terms such as first and second are only used to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply that there is any such actual relationship or order between the entities or operations. Moreover, the terms "include", "contain" or any other variant thereof are intended to cover non-exclusive inclusion, so that the process, method, article or device including a series of elements not only includes those elements, but also includes other elements not explicitly listed or inherent to such process, method, article or device. Without more limitation, the element defined by the statement "including a" does not exclude the presence of another identical element in the process, method, article or device including the element. In this document, "a", "one", "said", "the" and "it" can also include plural forms, unless the context clearly indicates otherwise. A plurality means at least two, such as 2, 3, 5 or 8, etc. "And / or" includes any and all combinations of the related listed items.

[0159] The various embodiments described in this specification are intended to be combinable unless otherwise indicated herein. The various embodiments described in this specification are described in the progressions noted, with each embodiment emphasizing different aspects over others. The various embodiments can be combined according to need and similar parts from each embodiment refer to each other as appropriate.

[0160] The foregoing description of the disclosed embodiments enables a person skilled in the art to make or use the application. Modifications of these embodiments will occur to those skilled in the art, and all such modifications are believed to be within the scope of the application. As will be apparent, certain embodiments of the application can be practiced without all the features presented. Therefore, the foregoing description should not be taken as limiting the scope of the application which is defined by the appended claims.

Claims

1. A method for suppressing second-harmonic circulating current in an energy storage MMC, characterized in that, include: The AC voltage, DC current, and bridge arm current of each bridge arm in the energy storage type MMC are obtained respectively. Determine the three-phase energy storage output value of each bridge arm; Calculate the three-phase fundamental frequency current compensation value based on the AC voltage and the three-phase energy storage output value; Based on the three-phase fundamental frequency current compensation value, the DC current and each of the bridge arm currents, calculate the three-phase second harmonic circulating current, and determine the three-phase second harmonic modulation voltage based on the three-phase second harmonic circulating current; The three-phase double-frequency modulation voltage is superimposed on the three-phase modulation voltage of the energy storage type MMC.

2. The method according to claim 1, characterized in that, The calculation of the three-phase second-harmonic circulating current based on the three-phase fundamental frequency current compensation value, the DC current, and each of the bridge arm currents includes: The three-phase second-harmonic circulating current is calculated based on the following expression: In the formula, For the j-phase second harmonic circulating current, Let J be the bridge arm current of the lower bridge arm. Let J be the bridge arm current of the upper bridge arm. This is the compensation value for the fundamental frequency current of phase j. Let j be the DC current; j = a, b, c.

3. The method according to claim 1, characterized in that, The step of calculating the three-phase fundamental frequency current compensation value based on the AC voltage and the three-phase energy storage output value includes: The three-phase fundamental frequency current compensation value is calculated based on the following expression: In the formula, This is the phasor form of the j-phase fundamental frequency current compensation value. Let J be the energy storage output value of the upper arm of the bridge in phase j. Let J be the energy storage output value of the lower arm of the bridge. This represents the phasor form of the j-phase AC voltage.

4. The method according to claim 1, characterized in that, The determination of the three-phase second-harmonic modulation voltage based on the three-phase second-harmonic circulating current includes: Determine the phase reference value; Based on the phase reference value, a Parker transformation is performed on the three-phase second harmonic circulating current to obtain the d-axis circulating current component and the q-axis circulating current component. Based on the d-axis circulating current component and the q-axis circulating current component, the d-axis voltage component and the q-axis voltage component are determined respectively; The three-phase second-harmonic modulation voltage is obtained by performing Parker inverse transformation on the d-axis voltage component and the q-axis voltage component based on the phase reference value.

5. The method according to claim 4, characterized in that, The step of determining the d-axis voltage component and the q-axis voltage component based on the d-axis circulating current component and the q-axis circulating current component, respectively, includes: The d-axis voltage component and the q-axis voltage component are calculated based on the following expressions: In the formula, The d-axis voltage component, Let L be the q-axis voltage component, and L be the bridge arm inductance value of the energy storage MMC. The actual frequency value of the AC voltage. The d-axis circulating component, This refers to the q-axis circulating component. For the proportional parameters of the current controller, Let be the integral parameter of the current controller, and s be the Laplace operator.

6. The method according to claim 4, characterized in that, The determination of the phase reference value includes: If the energy storage MMC adopts a grid-following control strategy, the phase reference value is obtained from the phase-locked loop of the energy storage MMC; otherwise, a preset frequency value is used as the phase reference value.

7. The method according to any one of claims 1 to 6, characterized in that, Determining the three-phase energy storage output value of each bridge arm includes: Obtain the energy storage voltage and energy storage current of each energy storage unit in the energy storage type MMC; The output power of the three-phase energy storage is calculated based on the following expression: In the formula, r = p, n, where p represents the upper bridge arm and n represents the lower bridge arm; j = a, b, c; Let N be the energy storage output of phase j-r bridge arm, and N be the total number of energy storage units in phase j-r bridge arm. Let be the energy storage voltage of the k-th energy storage unit in the j-phase r-arm bridge. Let be the energy storage current of the k-th energy storage unit in the j-phase r-arm bridge.

8. A second-harmonic circulating current suppression device for energy storage type MMC, characterized in that, include: The voltage and current acquisition module is used to acquire the AC voltage, DC current and bridge arm current of each bridge arm in the energy storage type MMC respectively. The energy storage output value determination module is used to determine the three-phase energy storage output value of each bridge arm; The base frequency current compensation value calculation module is used to calculate the three-phase base frequency current compensation value based on the AC voltage and the three-phase energy storage output value. The second harmonic modulation voltage determination module is used to calculate the three-phase second harmonic circulating current based on the three-phase base frequency current compensation value, the DC current and each of the bridge arm currents, and determine the three-phase second harmonic modulation voltage based on the three-phase second harmonic circulating current. A voltage superposition module is used to superimpose the three-phase double frequency modulation voltage onto the three-phase modulation voltage of the energy storage type MMC.

9. A storage medium, characterized in that, The storage medium stores computer-readable instructions that, when executed by one or more processors, cause the one or more processors to perform the steps of the second harmonic circulating current suppression method for the energy storage type MMC as described in any one of claims 1 to 7.

10. A computer device, characterized in that, include: One or more processors, and memory; The memory stores computer-readable instructions, which, when executed by the one or more processors, perform the steps of the second harmonic circulating current suppression method for the energy storage type MMC as described in any one of claims 1 to 7.

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