Capacitor energy equalization system and method for modular multilevel converter

Abstract

The invention provides a capacitor energy equalization system and method for a modular multilevel converter. The system comprises a modular multilevel converter access module, an average capacitor voltage controller and a circulating current controller, the modular multilevel converter access module is used for generating a circulating current and inputting the circulating current to the circulating current controller; the average capacitor voltage controller is used for controlling the average value of the capacitor voltages of the sub-modules in the phase units to track a preset voltage reference value, generating a circulating current control reference current and inputting the circulating current control reference current to the circulating current controller; and the circulating current controller is used for receiving the circulating current and the circulating current control reference current, generating a compensating current which is opposite to the double-frequency circulating current in phase, and adjusting the circulating current through the compensating current so as to equally divide the capacitance energy. According to the invention, the loop current controller is added to suppress the frequency doubling loop current, and the operation reliability of the modular multilevel converter is improved.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of power electronics, and particularly relates to a capacitor energy equalization system and method for a modular multilevel converter. BACKGROUND

[0002] As a core device of a high-voltage direct current transmission system, a modular multilevel converter (MMC) is usually realized by a series combination of sub-module capacitors for high-voltage output. How to balance the sub-module capacitor voltage and reduce the second harmonic component in the bridge arm circulating current during MMC operation is a key problem to be solved at present.

[0003] At present, the existing circulating current control mainly relies on a traditional PI controller. However, the second harmonic circulating current generated during MMC operation will significantly affect the capacitor voltage balancing effect. This circulating current not only causes the distortion of the bridge arm current waveform, but also causes the fluctuation of the sub-module capacitor voltage, thereby affecting the stability and reliability of the entire system. In particular, in a high-voltage and large-capacity application scenario, the problem of uneven energy distribution caused by the circulating current is more prominent, which seriously restricts the performance of the MMC. As can be seen, the PI controller used in the prior art is affected by the second harmonic circulating current, resulting in poor operation reliability of the modular multilevel converter. SUMMARY

[0004] In order to solve the above problems, the present application provides a capacitor energy equalization system and method for a modular multilevel converter, which realizes the suppression of the second harmonic circulating current by adding a circulating current controller, and improves the operation reliability of the modular multilevel converter.

[0005] To achieve the above purpose, the embodiment of the present application provides a capacitor energy equalization system for a modular multilevel converter, comprising: a modular multilevel converter access module, an average capacitor voltage controller and a circulating current controller; the output end of the modular multilevel converter access module is connected with the input end of the circulating current controller; the input end of the modular multilevel converter access module is connected with a DC bus; the output end of the average capacitor voltage controller is connected with the input end of the circulating current controller; the modular multilevel converter access module is used for generating a circulating current and inputting to the circulating current controller; the average capacitor voltage controller is used for controlling the average value of the sub-module capacitor voltage in the phase unit to track a preset voltage reference value, generating a circulating current control reference current and inputting to the circulating current controller; the circulating current controller is used for controlling the circulating current to track the circulating current control reference current, and generating a compensation current opposite to the second harmonic circulating current, and adjusting the circulating current by the compensation current to equalize the capacitor energy.

[0006] The embodiment of the present application provides a capacitor energy sharing system for a modular multilevel converter, obtains a circulating current from a modular multilevel converter access module, obtains a circulating current control reference current from an average capacitor voltage controller, introduces a circulating current controller to generate a compensation current opposite to the double-frequency circulating current, adjusts the circulating current to share capacitor energy, thereby maintaining the PI controller DC regulation capability, adding the circulating current controller to retain the voltage reference tracking characteristic, suppressing the double-frequency circulating current, and improving the operation reliability of the modular multilevel converter.

[0007] Further, the modular multilevel converter access module comprises a plurality of phase units, and the plurality of phase units are commonly connected to a DC bus to form a parallel loop. Further, the phase unit comprises an upper bridge arm and a lower bridge arm, one end of the upper bridge arm is connected to the positive pole of the DC bus, the other end of the upper bridge arm is connected to one end of the lower bridge arm, and the other end of the lower bridge arm is connected to the negative pole of the DC bus. The circulating current is calculated from the upper bridge arm current and the lower bridge arm current.

[0008] In the above scheme, the parallel loop is formed by the plurality of phase units, the circulating current is calculated by the symmetrically designed upper arm bridge and lower arm bridge of each phase unit, the voltage mismatch risk between the phase units is reduced by the voltage sharing characteristic of the parallel loop, and the circulating current is directly calculated from the bridge arm current, the signal processing process is simplified, and the error accumulation can be reduced. Therefore, the efficient and accurate extraction of the circulating current component provides a reliable circulating current for capacitor energy sharing control, and the operation reliability of the modular multilevel converter is improved.

[0009] Further, the average capacitor voltage controller comprises a voltage average calculation unit, an error calculation unit and a first PI controller. The voltage average calculation unit is used to calculate the average value of the capacitor voltage of the sub-module in the phase unit. The error calculation unit is used to calculate the tracking error of the average value of the capacitor voltage of the sub-module in the phase unit and the preset voltage reference value. The first PI controller is used to generate a circulating current control reference current according to the tracking error. The PI control unit is also used to adjust the input or output of energy.

[0010] In the above scheme, the average value of the capacitor voltage of the sub-module in the phase unit is calculated by the voltage average calculation unit, the tracking error of the average value of the capacitor voltage of the sub-module in the phase unit and the preset voltage reference value is calculated by the error calculation unit, and the circulating current control reference current is generated in combination with the dynamic adjustment capability of the first PI controller. Therefore, the capacitor voltage average value is combined with the first PI controller for closed-loop control, the voltage fluctuation caused by load mutation or parameter drift is effectively suppressed, the dynamic balance of energy distribution of each sub-module is ensured, and the voltage reference is generated by hierarchical processing and error adjustment, thereby providing an accurate circulating current control reference current for subsequent circulating current control, and the operation reliability of the modular multilevel converter is improved.

[0011] Further, the circulating current controller comprises a second PI controller and a QR controller; the second PI controller is used for controlling the circulating current to track a circulating current control reference current; and the QR controller is used for generating a compensation current opposite to a double-frequency circulating current, and the circulating current is adjusted by the compensation current to suppress the double-frequency circulating current. Further, the QR controller comprises: obtaining a target frequency according to a tracking error of the circulating current and the circulating current control reference current; constructing a high-gain control channel according to the target frequency; and generating the compensation current opposite in phase and same in amplitude to the double-frequency circulating current based on the high-gain control channel.

[0012] In the above scheme, the circulating current controller is composed of the second PI controller and the QR controller, the circulating current is controlled by the second PI controller to track the circulating current control reference current, the high-gain control channel is constructed at the target frequency by the QR controller, and the compensation current opposite in phase and same in amplitude to the double-frequency circulating current is generated to suppress the double-frequency circulating current. Under the premise that the existing PI controller can only adjust the direct current component of the circulating current and the suppression effect on the alternating current component is limited, the high-gain characteristic formed at the target frequency by the added QR controller is used to suppress the double-frequency circulating current. Thus, the double-frequency circulating current disturbance is directly eliminated in the current compensation mode, the uncontrollable fluctuation of the switching frequency is avoided, and the operation reliability of the modular multilevel converter is improved.

[0013] The embodiment of the present application also provides a capacitor energy equalization method for a modular multilevel converter, comprising: obtaining a circulating current and a circulating current control reference current; generating a compensation current opposite to a double-frequency circulating current according to the circulating current and the circulating current control reference current; and adjusting the circulating current by the compensation current to equalize capacitor energy.

[0014] The embodiment of the present application provides a capacitor energy equalization method for a modular multilevel converter, obtains a circulating current and a circulating current control reference current, and then generates a compensation current opposite to a double-frequency circulating current to adjust the double-frequency circulating current, so as to equalize capacitor energy. Thus, by accurately obtaining the circulating current and the circulating current control reference current, the capacitor voltage fluctuation caused by the double-frequency circulating current is effectively suppressed, and the operation reliability of the modular multilevel converter is improved.

[0015] Further, the obtaining of the circulating current and the circulating current control reference current comprises: obtaining an upper bridge arm current and a lower bridge arm current; and calculating half of the sum of the upper bridge arm current and the lower bridge arm current to obtain the circulating current.

[0016] In the above scheme, the circulating current is directly calculated by the bridge arm current, the signal processing procedure is simplified, and the error accumulation can be reduced. Thus, the circulating current component is efficiently and accurately extracted, a reliable circulating current is provided for capacitor energy equalization control, and the operation reliability of the modular multilevel converter is improved.

[0017] Further, the circulating current and the circulating current control reference current are obtained by: calculating an average value of the capacitor voltage of the sub-module in the phase unit; calculating a tracking error of the average value of the capacitor voltage of the sub-module in the phase unit and a preset voltage reference value; and generating the circulating current control reference current according to the tracking error.

[0018] In the above scheme, the average value of the capacitor voltage of the sub-module in the phase unit and the tracking error of the average value of the capacitor voltage of the sub-module in the phase unit and the preset voltage reference value are calculated to generate the circulating current control reference current, so as to ensure the dynamic balance of the energy distribution of each sub-module, and to generate the voltage reference and the error adjustment through the hierarchical processing, thereby providing an accurate circulating current control reference current for subsequent circulating current control, and improving the operation reliability of the modular multilevel converter.

[0019] Further, the compensation current opposite to the double-frequency circulating current in phase is generated according to the circulating current and the circulating current control reference current, including: calculating a tracking error of the circulating current and the circulating current control reference current, obtaining a target frequency; generating a high-gain control channel according to the target frequency; and generating the compensation current opposite to the double-frequency circulating current in phase and having the same amplitude based on the high-gain control channel.

[0020] In the above scheme, under the premise that the existing PI controller can only adjust the direct current component of the circulating current and the alternating current component suppression effect is limited, the high-gain characteristic formed by the QR controller at the target frequency is added to suppress the double-frequency circulating current. Thus, the double-frequency circulating current disturbance is directly eliminated in the current compensation mode, the uncontrollable fluctuation of the switching frequency is avoided, and the operation reliability of the modular multilevel converter is improved. BRIEF DESCRIPTION OF DRAWINGS

[0021] Figure 1 A module structure diagram of a capacitor energy distribution system for a modular multilevel converter is provided for an embodiment of the present application Figure 1 ;

[0022] Figure 2 A module structure diagram of a capacitor energy distribution system for a modular multilevel converter is provided for an embodiment of the present application Figure 2 ;

[0023] Figure 3 A frequency response curve comparison diagram of a capacitor energy distribution system for a modular multilevel converter is provided for an embodiment of the present application

[0024] Figure 4 A simulation result analysis diagram of a capacitor energy distribution system for a modular multilevel converter is provided for an embodiment of the present application Figure 1 ;

[0025] Figure 5A simulation result analysis schematic of a capacitor energy sharing system for a modular multilevel converter provided by an embodiment of the present application Figure 2 ;

[0026] Figure 6 A step flowchart of a capacitor energy sharing method for a modular multilevel converter provided by an embodiment of the present application DETAILED DESCRIPTION

[0027] The technical solutions in the embodiments of the present application will be described clearly and completely below with reference to the drawings in the embodiments of the present application. Obviously, the described embodiments are only part of the embodiments of the present application, rather than 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 protection scope of the present application.

[0028] Embodiment 1

[0029] Reference is made to Figure 1 , Figure 1 A module structure schematic of a capacitor energy sharing system for a modular multilevel converter provided by an embodiment of the present application Figure 1 . As shown in Figure 1 , the present application provides a capacitor energy sharing system for a modular multilevel converter, which comprises a modular multilevel converter access module, an average capacitor voltage controller and a circulating current controller. The output end of the modular multilevel converter access module is connected with the input end of the circulating current controller. The input end of the modular multilevel converter access module is connected with a DC bus. The output end of the average capacitor voltage controller is connected with the input end of the circulating current controller. The modular multilevel converter access module is used to generate a circulating current and input the circulating current to the circulating current controller. The average capacitor voltage controller is used to control the average value of the capacitor voltage in the phase unit to track a preset voltage reference value, generate a circulating current control reference current and input the circulating current control reference current to the circulating current controller. The circulating current controller is used to control the circulating current to track the circulating current control reference current, generate a compensation current opposite to the double-frequency circulating current, and adjust the circulating current through the compensation current to share the capacitor energy.

[0030] In a specific embodiment, the MMC runtime first calculates the circulating current through the bridge arm current of the MMC access module, and the average capacitor voltage controller calculates the average value of the capacitor voltage of each submodule and compares it with the voltage reference value to generate a tracking error, which is adjusted by a proportional integral regulator to form a circulating current reference value, which is input into the circulating current controller together with the circulating current. The circulating current controller generates a compensating current with opposite phase at the target frequency through a quasi-resonant unit, which forms a reverse offset effect after being injected into the converter, thereby eliminating the second harmonic component in the circulating current. More specifically, the MMC access module is a topology structure for electrically connecting the converter to the DC bus, which can be implemented by a three-phase bridge circuit structure. Its function is to provide an energy transmission channel for the MMC and generate a circulating current signal representing energy distribution. The average capacitor voltage controller is a regulating device for maintaining the average value of the capacitor voltage of the submodule, which can be implemented by combining an error calculation unit with a proportional integral regulator (i.e., a PI controller). Its function is to ensure the stability of the overall energy reference through closed-loop control. The circulating current controller is an execution mechanism for regulating the inter-phase circulating current, which can be implemented by a quasi-resonant controller (i.e., a QR controller) in parallel with a proportional integral controller (i.e., a PI controller). It is used to eliminate specific harmonic components through frequency-selective control, which is represented as suppressing the second harmonic circulating current in the embodiment.

[0031] The embodiment of the present application provides a capacitor energy equalization system for a modular multilevel converter. The circulating current is obtained from the MMC access module, and the circulating current control reference current is obtained from the average capacitor voltage controller. The compensating current with opposite phase to the second harmonic circulating current is generated by the circulating current controller to regulate the circulating current, so as to equalize the capacitor energy. Thus, on the basis of maintaining the DC regulation capability of the PI controller, the voltage reference tracking characteristic is reserved by adding the circulating current controller, and the second harmonic circulating current is suppressed, thereby improving the operation reliability of the modular multilevel converter.

[0032] In a preferred scheme, the MMC access module comprises a plurality of phase units, and the plurality of phase units are connected to the DC bus to form a loop. Further, the phase unit comprises an upper bridge arm and a lower bridge arm. One end of the upper bridge arm is connected to the positive electrode of the DC bus, the other end of the upper bridge arm is connected to one end of the lower bridge arm, and the other end of the lower bridge arm is connected to the negative electrode of the DC bus. The circulating current is calculated from the upper bridge arm current and the lower bridge arm current.

[0033] In an embodiment of the preferred scheme, the circulating current controller comprises a quasi-resonant controller (i.e., a QR controller) and a proportional integral controller (i.e., a PI controller) in parallel. Figure 2 , Figure 2 A module structure diagram of the capacitor energy equalization system for the modular multilevel converter is provided for the embodiment of the present application Figure 2As shown in Figure 2, in the topology of the MMC converter, the three phase units are connected to the DC bus to form a parallel loop. However, in actual operation, it is difficult for each phase unit to achieve a completely consistent DC voltage output, which inevitably leads to the flow of circulating current between phases. Therefore, a topology as shown in Figure 2 is constructed. Figure 2 The equivalent circuit model of the rectifier side is shown for solution. In this embodiment of the invention, a phase unit refers to an electrical circuit composed of upper and lower bridge arms. Specifically, it can be implemented using an insulated gate bipolar transistor and a submodule capacitor in series structure. Each bridge arm contains multiple submodules to achieve voltage superposition function. The upper and lower bridge arms of the phase unit are connected to the positive and negative terminals of the DC bus through bridge arm reactors, respectively. The electrical connection points of each phase unit are at the same potential. When the converter is running, the DC bus provides a unified voltage reference for each phase unit, so that each phase unit has a consistent DC voltage output condition in the parallel structure. Since the DC side voltage of all phase units in the parallel circuit is forced to be balanced, the circulating current path caused by voltage difference between phase units is effectively constrained, thereby reducing the energy distribution deviation caused by voltage imbalance. Taking phase a as an example, according to the symmetry of MMC, the output current i of phase a is... a Provided by the combined currents of the upper and lower bridge arms, assuming the current direction is as follows: Figure 2 As shown, the output current i a It can be represented as:

[0034] i a =i a1 -i a2 ;

[0035] In the formula, i a1 For the upper bridge arm current, i a2 This refers to the current in the lower bridge arm.

[0036] Due to the circulating current i cir1 It is the result of separating the DC and AC components within the phase unit, according to Figure 3 The current path shown has a circulating current i. cir1 The following relationship exists and satisfies in both the upper and lower bridge arms:

[0037]

[0038] Substituting the expression for the output current ia into the above relationship, we obtain the circulating current i. cir1 The expression is as follows:

[0039] i cir1 =(i a1 +i a2 ) / 2;

[0040] The processing methods for other phases are similar and will not be elaborated here. In addition, in the embodiments of the present invention, the number of phase units can be flexibly configured according to the MMC capacity requirements.

[0041] In the above scheme, the parallel circuit is formed by several phase units and the circulating current is calculated by the symmetrical upper arm bridge and lower arm bridge of each phase unit. By the parallel circuit topology, the voltage mismatch risk between phase units is reduced by the voltage equalization characteristic of the parallel circuit, and the circulating current is directly calculated by the bridge arm current, which simplifies the signal processing procedure and can reduce error accumulation. Therefore, the efficient and accurate extraction of the circulating current component provides a reliable circulating current for the capacitor energy sharing control, and improves the operation reliability of the modular multilevel converter.

[0042] In a preferred scheme, the average capacitor voltage controller comprises a voltage average calculation unit, an error calculation unit and a first PI controller; the voltage average calculation unit is configured to calculate the average value of the capacitor voltage of the sub-modules in the phase unit; the error calculation unit is configured to calculate the tracking error of the average value of the capacitor voltage of the sub-modules in the phase unit and a preset voltage reference value; and the first PI controller is configured to generate a circulating current control reference current according to the tracking error, and the PI control unit is further configured to adjust the input or output of energy.

[0043] In an implementation manner of the preferred scheme, the voltage average calculation unit first collects the capacitor voltage signals of all the sub-modules in the phase unit, and calculates the average value v c of the current capacitor voltage according to the following formula:

[0044]

[0045] In the formula, N is the number of bridge arm sub-modules, u Cja is the capacitor voltage of the jth sub-module;

[0046] Then, the error calculation unit compares the average value with a preset voltage reference value v Cref and outputs the deviation signal between the two, i.e. the tracking error. After receiving the tracking error, the first PI controller quickly responds to voltage fluctuations through the proportional element, and gradually eliminates residual errors through the integral element to generate a circulating current control reference current The circulating current control reference current will be input to the circulating current controller to realize dynamic allocation of energy by adjusting the DC component of the bridge arm current. For example, when it is detected that the average capacitor voltage is lower than the reference value, the first PI controller will increase the circulating reference value to prompt the sub-modules to absorb energy from the DC bus to raise the voltage level.

[0047] In the above scheme, the average value of the capacitor voltage of the sub-module in the phase unit is calculated by the voltage average value calculation unit, and the tracking error of the average value of the capacitor voltage of the sub-module in the phase unit and the preset voltage reference value is calculated by the error calculation unit, and the dynamic adjustment capability of the first PI controller is combined to generate the circulating current control reference current. Thus, the capacitor voltage average value is combined with the first PI controller for closed-loop control, effectively suppressing the voltage fluctuation caused by load mutation or parameter drift, ensuring the dynamic balance of energy distribution of each sub-module, and providing accurate circulating current control reference current for subsequent circulating current control through hierarchical processing of voltage reference generation and error adjustment, thereby improving the operation reliability of the modular multilevel converter.

[0048] In a preferred scheme, the circulating current controller comprises a second PI controller and a QR controller; the second PI controller is used to control the circulating current to track the circulating current control reference current; the QR controller is used to generate a compensation current opposite to the double-frequency circulating current, and the circulating current is adjusted by the compensation current to suppress the double-frequency circulating current. Further, the QR controller comprises: obtaining a target frequency according to the tracking error of the circulating current and the circulating current control reference current; constructing a high-gain control channel according to the target frequency; and generating a compensation current opposite in phase and equal in amplitude to the double-frequency circulating current based on the high-gain control channel.

[0049] In an implementation manner of the preferred scheme, the circulating current controller adopts a composite control structure, the second PI controller is used as a basic adjustment unit, the input end of the second PI controller receives a deviation signal of the circulating current i cir1 from the circulating current control reference current I , and the output end generates an adjustment instruction of the fundamental component; the QR controller is connected in parallel with the second PI controller, the resonance frequency of the QR controller is set to be double the power frequency, a compensation current opposite in phase and equal in amplitude is generated by real-time detection of the double-frequency component in the circulating current, and the output signals of the two controllers form a total control quantity v aref after superposition, the control quantity is converted into a bridge arm voltage instruction through a modulation link to drive the sub-module switching device to act, thereby maintaining the stability of the circulating current average value while eliminating the double-frequency harmonic. For example, when a 100Hz component is detected in the circulating current, the QR controller immediately generates a 100Hz opposite-phase current to inject into the system, so that the double-frequency components in the bridge arm current cancel each other out. In this embodiment, the quasi-resonant (QR) control is a control method for accurately adjusting a specific frequency signal, and the core principle is to construct a high-gain control channel at the target frequency to realize tracking or suppression of signals in a specific frequency band, and the transfer function is:

[0050]

[0051] In the formula, ω0 is the target resonance frequency, ω c is the control bandwidth, and ξ is the damping coefficient.

[0052] The circulating current controller adopts a composite control structure of PI controller + QR controller. The PI regulator ensures the circulating current i cir1 Fast follow-circulating current control reference current The change in frequency, while QR control ensures that the second harmonic is suppressed, its transfer function is expressed as:

[0053]

[0054] In the formula, K P K represents the proportional gain in a PI controller. I This refers to the integral gain in the PI controller.

[0055] See Figure 3 , Figure 3 This is a comparative schematic diagram of the frequency response curves of a capacitor energy sharing system for a modular multilevel converter, provided in a certain embodiment of the present invention; as shown below. Parameter name As shown in the analysis results of the frequency response curves of the circulating current controller and the single PI controller, in the low frequency range (<10Hz), the gain characteristics of the circulating current controller and the single PI controller are completely overlapping. For example, at 1Hz, they overlap at 16.17dB, ensuring the accurate and stable DC component. The DC steady-state error-free regulation achieved through the integral term can control the average error of the capacitor voltage within a small range. Secondly, at the critical 100Hz second harmonic, the circulating current controller exhibits a sharp resonant peak of 53dB, while the single PI controller drops from 16.17dB to 3.53dB. At the 100Hz second harmonic, the physical essence of the circulating current controller is to generate a compensation current that is opposite to the circulating current through the QR link, forming an equivalent "100Hz current notch filter", which can effectively suppress capacitor voltage ripple while maintaining a phase margin of >45° in the frequency range above 50Hz.

[0056] In the above scheme, the circulating current controller consists of a second PI controller and a QR controller. The second PI controller controls the circulating current to track the circulating current control reference current, and the QR controller constructs a high-gain control channel at the target frequency and generates a compensation current with the opposite phase and the same amplitude as the second harmonic circulating current to suppress the second harmonic circulating current. Given that the existing PI controller can only adjust the DC component of the circulating current and has limited suppression effect on the AC component, the addition of the high-gain characteristic formed by the QR controller at the target frequency further suppresses the second harmonic circulating current. Thus, the second harmonic circulating current disturbance is directly eliminated through current compensation, avoiding uncontrollable fluctuations in the switching frequency and improving the operational reliability of the modular multilevel converter.

[0057] To further verify the suppression effect of the second harmonic component after adding the QR controller in the embodiments of the present invention, a 3000V output MMC rectifier system was built on the Simulink platform to verify the theoretical analysis. The parameters of the MMC rectifier system are shown in Table 1:

[0058] Table 1 Parameters of MMC Rectifier System

[0059] Parameter value Three-phase input voltage effective value Output voltage V out ]] 6000V Number of sub-modules N per bridge leg 3300V Figure 4 6 Submodule capacitor voltage V C ]]> 1000V

[0060] See Figure 5 and Figure 4 , Figure 1 This invention provides a schematic diagram of simulation results analysis for a capacitor energy sharing system in a modular multilevel converter, according to a certain embodiment of the present invention. Figure 5 ; Figure 2 This invention provides a schematic diagram of simulation results analysis for a capacitor energy sharing system in a modular multilevel converter, according to a certain embodiment of the present invention. Figure 5 ;like Figure 6 and Figure 5 As shown, Fundamental represents the 50Hz fundamental frequency, THD represents the total harmonic distortion, Frequency represents the frequency, and the vertical axis represents the amplitude of each frequency component (harmonic), expressed as a percentage of the fundamental frequency (50Hz) amplitude. Figure 6 To achieve capacitor balancing without QR control, only PI control is used. Figure 6 To incorporate QR control, the circulating current i is obtained from the simulation results. cir1 The FFT analysis revealed that the addition of QR control significantly reduced the second harmonic component, which effectively reduced the ripple of the submodule capacitor and had a very significant effect on extending the submodule's lifespan.

[0061] Example 2

[0062] See Figure 6 , Figure 6 This is a flowchart illustrating the steps of a capacitor energy sharing method for a modular multilevel converter according to a certain embodiment of the present invention. Figure 2 As shown, this embodiment of the invention also provides a capacitor energy sharing method for a modular multilevel converter, including steps 101 to 103, the specific steps of which are as follows: Step 101, obtain the circulating current and the circulating current control reference current;

[0063] Step 102: Generate a compensation current that is in the opposite phase to the second harmonic circulating current based on the circulating current and the circulating current control reference current.

[0064] Step 103: Adjust the circulating current by compensating for the energy of the capacitor to distribute it evenly.

[0065] In a specific embodiment, the MMC runtime first calculates the circulating current through the bridge arm current of the modular multilevel converter access module, and the average capacitor voltage controller calculates the average value of the capacitor voltage of each submodule and compares it with the voltage reference value to generate a tracking error, which is adjusted by a proportional integral regulator to form a circulating reference value, which is input to the circulating controller together with the circulating current. The compensation current with opposite phase is generated at the target frequency by the quasi-resonant unit inside the circulating controller, which forms a reverse offset effect after being injected into the converter, thereby eliminating the second harmonic component in the circulating current. More specifically, the modular multilevel converter access module is a topological structure that realizes the electrical connection of the converter and the DC bus, which can be implemented by a three-phase bridge circuit structure. Its function is to provide an energy transmission channel for the MMC and generate a circulating current signal representing energy distribution. The average capacitor voltage controller is a regulating device that maintains the average value of the capacitor voltage of the submodule. It can be implemented by combining an error calculation unit with a proportional integral regulator (i.e. PI controller). Its function is to ensure the stability of the overall energy reference through closed-loop control. The circulating controller is an execution mechanism that regulates the inter-phase circulating current. It can be implemented by connecting a quasi-resonant controller (i.e. QR controller) and a proportional integral controller (i.e. PI controller) in parallel. It is used to eliminate specific harmonic components through frequency-selective control, which is represented as suppressing the second harmonic circulating current in the embodiment.

[0066] The embodiment of the present application provides a capacitor energy equalization method for a modular multilevel converter. The circulating current and the circulating control reference current are obtained, and a compensation current opposite to the second harmonic circulating current is generated to adjust the second harmonic circulating current, so that the capacitor energy is equalized. Therefore, by accurately obtaining the circulating current and the circulating control reference current, the capacitor voltage fluctuation caused by the second harmonic circulating current is effectively suppressed, and the reliability of the operation of the modular multilevel converter is improved.

[0067] Further, the circulating current and the circulating control reference current are obtained, including: obtaining the upper bridge arm current and the lower bridge arm current; and calculating half of the sum of the upper bridge arm current and the lower bridge arm current to obtain the circulating current.

[0068] In one embodiment, the circulating current and the circulating control reference current are obtained, including: obtaining the upper bridge arm current and the lower bridge arm current; and calculating half of the sum of the upper bridge arm current and the lower bridge arm current to obtain the circulating current. Figure 2 , Figure 2 A module structure diagram of a capacitor energy equalization system for a modular multilevel converter is provided for an embodiment of the present application Figure 2 ; as shown in FIG. 2, in the topological structure of the MMC converter, three phase units are connected to the DC bus to form a loop, but it is difficult to realize the same DC voltage output in actual operation, which inevitably causes circulating current to flow between the phases, so as to construct Figure 2The equivalent circuit model of the rectifier side is shown for solution. In this embodiment of the invention, a phase unit refers to an electrical circuit composed of upper and lower bridge arms. Specifically, it can be implemented using an insulated gate bipolar transistor and a submodule capacitor in series structure. Each bridge arm contains multiple submodules to achieve voltage superposition function. The upper and lower bridge arms of the phase unit are connected to the positive and negative terminals of the DC bus through bridge arm reactors, respectively. The electrical connection points of each phase unit are at the same potential. When the converter is running, the DC bus provides a unified voltage reference for each phase unit, so that each phase unit has a consistent DC voltage output condition in the parallel structure. Since the DC side voltage of all phase units in the parallel circuit is forced to be balanced, the circulating current path caused by voltage difference between phase units is effectively constrained, thereby reducing the energy distribution deviation caused by voltage imbalance. Taking phase a as an example, according to the symmetry of MMC, the output current i of phase a is... a Provided by the combined currents of the upper and lower bridge arms, assuming the current direction is as follows: Figure 2 As shown, the output current i a It can be represented as:

[0069] i a =i a1 -i a2 ;

[0070] In the formula, i a1 For the upper bridge arm current, i a2 This refers to the current in the lower bridge arm.

[0071] Due to the circulating current i cir1 It is the result of separating the DC and AC components within the phase unit, according to Figure 3 The current path shown has a circulating current i. cir1 The following relationship exists and satisfies in both the upper and lower bridge arms:

[0072]

[0073] Output current i a Substituting the expression into the above relationship, we obtain the circulating current i. cir1 The expression is as follows:

[0074] i cir1 =(i a1 +i a2 ) / 2;

[0075] The processing methods for other phases are similar and will not be elaborated here. In addition, in the embodiments of the present invention, the number of phase units can be flexibly configured according to the MMC capacity requirements.

[0076] In the above scheme, the circulating current is directly calculated by the bridge arm current, the signal processing procedure is simplified, and error accumulation can be reduced. Thus, the circulating current component is efficiently and accurately extracted, a reliable circulating current is provided for the capacitor energy sharing control, and the operation reliability of the modular multilevel converter is improved.

[0077] Further, the circulating current and the circulating current control reference current are obtained, including: calculating an average value of the capacitor voltages of the submodules in the phase unit; calculating a tracking error of the average value of the capacitor voltages of the submodules in the phase unit and a preset voltage reference value; and generating the circulating current control reference current according to the tracking error.

[0078] In one implementation, the voltage average value calculation unit first collects the capacitor voltage signals of all the submodules in the phase unit, and calculates the average value v of the current capacitor voltages c , and the calculation formula is as follows:

[0079]

[0080] In the formula, N is the number of the bridge arm submodules, u is the number of the submodules in the phase unit, and v is the capacitor voltage of the jth submodule. Cja

[0081] Then, the error calculation unit compares the average value with the preset voltage reference value v Cref , and outputs a deviation signal between the two, i.e., a tracking error. After receiving the tracking error, the first PI controller quickly responds to voltage fluctuations through a proportional element, and gradually eliminates residual errors through an integral element, to generate the circulating current control reference current The circulating current control reference current will be input to the circulating current controller, to realize dynamic energy distribution by adjusting the direct current component of the bridge arm current. For example, when it is detected that the average value of the capacitor voltages is lower than the reference value, the first PI controller will increase the circulating current reference value, to prompt the submodules to absorb energy from the DC bus to increase the voltage level.

[0082] In the above scheme, the average value of the capacitor voltages of the submodules in the phase unit and the tracking error of the average value of the capacitor voltages of the submodules in the phase unit and the preset voltage reference value are calculated, to generate the circulating current control reference current, ensure the dynamic balance of the energy distribution of the submodules, and generate the voltage reference value and the error adjustment through hierarchical processing, to provide an accurate circulating current control reference current for subsequent circulating current control, and improve the operation reliability of the modular multilevel converter.

[0083] Further, according to the circulating current and the circulating current control reference current, a compensation current opposite to the double-frequency circulating current in phase is generated, including: calculating a tracking error of the circulating current and the circulating current control reference current, obtaining a target frequency; generating a high-gain control channel according to the target frequency; and generating the compensation current opposite to the double-frequency circulating current in phase and having the same amplitude based on the high-gain control channel.​

[0084] In one embodiment, the circulating current controller adopts a compound control structure, a second PI controller as a basic regulating unit, the input end of which receives the circulating current i cir1 and the deviation signal of the circulating current control reference current , and the output end generates the regulating instruction of the fundamental component, and a QR controller is connected in parallel with the second PI controller, the resonant frequency of which is set as twice the power frequency, and through real-time detection of the twice frequency component in the circulating current, a compensation current with opposite phase and equal amplitude is generated, and the output signals of the two controllers form the total control quantity v aref after superposition, which is converted into the bridge arm voltage instruction through a modulation link to drive the switching devices of the sub-modules to eliminate the twice frequency harmonic while maintaining the stability of the average value of the circulating current. For example, when a 100Hz component is detected in the circulating current, the QR controller immediately generates a 100Hz anti-phase current to inject into the system, so that the twice frequency components in the bridge arm current are offset each other. In this embodiment, the quasi-resonant (QR) control is a control method for accurate regulation of specific frequency signals, and the core principle is to realize the tracking or suppression of signals in a specific frequency band by constructing a high-gain control channel at the target frequency, and the transfer function is:

[0085]

[0086] In the formula, ω0 is the target resonant frequency, ω c is the control bandwidth, and ξ is the damping coefficient.

[0087] The circulating current controller adopts a compound control structure of PI controller + QR controller, the PI regulator ensures that the circulating current i cir1 quickly follows the change of the circulating current control reference current , and the QR control ensures that the twice frequency is suppressed, and the transfer function is represented as:

[0088]

[0089] In the formula, K P is the proportional gain in the PI controller; K I is the integral gain in the PI controller.

[0090] Referring to Figure 3 , Figure 3 is a frequency response curve comparison schematic diagram of a capacitor energy sharing system for a modular multilevel converter provided by one embodiment of the application; as ​As shown in the frequency response curve of the loop current controller and the analysis result of the frequency response curve of the PI controller alone, at a low frequency band (<10Hz), the gain characteristics of the loop current controller and the PI controller alone are completely coincident, for example, at 1Hz, both are coincident at 16.17dB, which ensures accurate stabilization of the DC component, and the DC zero static adjustment realized by the integral term can control the average error of the capacitor voltage within a small range; secondly, at the key 100Hz double frequency, the loop current controller presents a sharp resonance peak of 53dB, while the PI controller alone is reduced from 16.17dB to 3.53dB; at the 100Hz double frequency, the physical nature of the loop current controller is to generate a compensation current opposite to the loop current through the QR link, forming an equivalent "100Hz current notch filter", which can effectively suppress the capacitor voltage ripple while maintaining a phase margin >45° above 50Hz.

[0091] In the above scheme, under the premise that the existing PI controller can only adjust the DC component of the loop current and has limited suppression effect on the AC component, the high gain characteristics formed by the QR controller at the target frequency suppress the double frequency loop current. Thus, the double frequency loop current disturbance is directly eliminated in a current compensation manner, avoiding uncontrollable fluctuations of the switching frequency and improving the operation reliability of the modular multilevel converter.

[0092] The above only describes the preferred embodiments of the present application, and it should be noted that those skilled in the art can make several improvements and modifications without departing from the technical principles of the present application, and these improvements and modifications should also be considered as the protection scope of the present application.

[0093] In the description of the present specification, the description of the terms "one embodiment", "some embodiments", "example", "specific example" or "some examples" means that the specific features, structures, materials or characteristics described in connection with the embodiment or example are included in at least one embodiment or example of the present application. Moreover, the specific features, structures, materials or characteristics described can be combined in any suitable manner in any one or more embodiments or examples. In addition, those skilled in the art can combine and combine the different embodiments or examples described in the present specification and the features of the different embodiments or examples without contradiction.

[0094] In addition, the terms "first", "second" are only for descriptive purposes, and cannot be understood as indicating or implying relative importance or implicitly indicating the number of indicated technical features. Therefore, the features limited by "first", "second" can explicitly or implicitly include at least one feature. In the description of the present application, the meaning of "multiple" is two or more, unless otherwise specifically limited.

Claims

1. A capacitor energy sharing system for a modular multilevel converter, characterized by, The application relates to a capacitor energy equalization system for a modular multilevel converter, comprising: a modular multilevel converter access module, an average capacitor voltage controller and a circulating current controller; an output end of the modular multilevel converter access module is connected with an input end of the circulating current controller; an input end of the modular multilevel converter access module is connected with a DC bus, and an output end of the average capacitor voltage controller is connected with an input end of the circulating current controller; the modular multilevel converter access module is used for generating a circulating current and inputting the circulating current into the circulating current controller; the average capacitor voltage controller is used for controlling an average value of capacitor voltages in a phase unit to track a preset voltage reference value, generating a circulating current control reference current and inputting the circulating current control reference current into the circulating current controller; the circulating current controller is used for controlling the circulating current to track the circulating current control reference current, generating a compensation current opposite to a double-frequency circulating current and adjusting the circulating current through the compensation current to equally divide capacitor energy.

2. A system for sharing of capacitor energy for a modular multilevel converter according to claim 1, characterized in that, The modular multilevel converter access module comprises a plurality of phase units, and the plurality of phase units are commonly connected to a DC bus to form a loop.

3. A system for sharing of capacitor energy for a modular multilevel converter according to claim 2, characterized in that, The phase unit comprises an upper bridge arm and a lower bridge arm, one end of the upper bridge arm is connected with a positive electrode of the DC bus, the other end of the upper bridge arm is connected with one end of the lower bridge arm, the other end of the lower bridge arm is connected with a negative electrode of the DC bus, and the circulating current is calculated from upper bridge arm current and lower bridge arm current.

4. A system for sharing of capacitor energy for a modular multilevel converter according to claim 1, characterized in that, The average capacitor voltage controller comprises a voltage average value calculation unit, an error calculation unit and a first PI controller. The voltage average value calculation unit is used for calculating the average value of capacitor voltages in the phase unit. The error calculation unit is used for calculating a tracking error of the average value of capacitor voltages in the phase unit and the preset voltage reference value. The first PI controller is used for generating the circulating current control reference current according to the tracking error, and the PI control unit is also used for adjusting energy input or output.

5. A system for sharing of capacitor energy for a modular multilevel converter as claimed in claim 1, wherein, The circulating current controller comprises a second PI controller and a QR controller. The second PI controller is used for controlling the circulating current to track the circulating current control reference current. The QR controller is used for generating the compensation current opposite to the double-frequency circulating current, adjusting the circulating current through the compensation current and inhibiting the double-frequency circulating current.

6. A system for sharing of capacitor energy for a modular multilevel converter according to claim 5, characterized in that, The QR controller comprises: a target frequency is obtained according to a tracking error of the circulating current and the circulating current control reference current; a high-gain control channel is constructed according to the target frequency; a compensation current opposite in phase and same in amplitude to the double-frequency circulating current is generated based on the high-gain control channel.

7. A method for capacitor energy sharing for a modular multilevel converter, characterized in that, The application is applied to the capacitor energy equalization system for the modular multilevel converter, and comprises: a circulating current and a circulating current control reference current are obtained; a compensation current opposite to a double-frequency circulating current is generated according to the circulating current and the circulating current control reference current; the circulating current is adjusted through the compensation current to equally divide capacitor energy.

8. A method for sharing of capacitor energy for a modular multilevel converter as claimed in claim 7, wherein, The circulating current and the circulating current control reference current are obtained, comprising: an upper bridge arm current and a lower bridge arm current are obtained; half of the sum of the upper bridge arm current and the lower bridge arm current is calculated to obtain the circulating current.

9. A method for sharing of capacitor energy for a modular multilevel converter as claimed in claim 7, wherein, The acquisition of the circulating current and the circulating current control reference current comprises: calculating the average value of the capacitor voltage of the sub-module in the phase unit; calculating the tracking error of the average value of the capacitor voltage of the sub-module in the phase unit and the preset voltage reference value; generating the circulating current control reference current according to the tracking error.

10. A method for sharing of capacitor energy for a modular multilevel converter as claimed in claim 7, wherein, According to the circulating current and the circulating current control reference current, a compensation current opposite to the double-frequency circulating current is generated, comprising: calculating the tracking error of the circulating current and the circulating current control reference current to obtain a target frequency; generating a high-gain control channel according to the target frequency; generating a compensation current opposite to the phase of the double-frequency circulating current and having the same amplitude based on the high-gain control channel.

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