A method and system for riding through a single-pole ground fault of a modular multilevel converter

Abstract

The present invention relates to a method for riding through a single-pole ground fault of a modular multi-level converter. In response to a detection signal of a single-pole ground fault, the fault circuit segment and the fault pole on the DC side are determined, and the output voltage of each bridge arm is changed according to the polarity of the fault pole to ride through the fault circuit segment; then the voltage and current of each phase of the AC output at both ends of the fault circuit segment are obtained, and a circulating current control signal for maintaining energy balance of each bridge arm is calculated; and then the output level of each bridge arm is modulated according to the circulating current control signal to balance the energy between the bridge arm and the phase. The method of the present invention realizes the smooth riding through of a single-pole ground fault on the basis of making full use of the existing three-phase power supply of the AC port and strategically controlling the AC current, and balances the system energy in the single-pole ground fault, and does not require any auxiliary power supply to compensate for the energy, nor does it require additional equipment investment costs.

Description

Technical Field

[0001] The present invention belongs to the technical field of flexible direct current power transmission, and in particular relates to a method and system for riding through a single-pole grounding fault of a modular multi-level converter. Background Art

[0002] The flexible direct current transmission technology based on modular multilevel converter (MMC) (Voltage Source Converter based High Voltage Direct Current, VSC-HVDC) has become a hot topic of research at home and abroad in recent years due to its advantages of not relying on grid commutation, supporting island operation, low switching loss, low output voltage harmonic content and strong scalability, and has been widely used in practical projects such as offshore wind power collection stations. The newly proposed half-wave alternating modular multilevel converter (Half-Wave Alternating MMC, HA-MMC) inherits the advantages of conventional modular multilevel converters while also having fewer sub-modules, reducing the structural scale and improving the system economy.

[0003] However, during the line connection process of the Haifeng Collection Station, DC short-circuit faults may occur, among which single-pole grounding faults are the most likely type of faults. For DC transmission systems using modular multilevel converters, the ability to quickly ride through faults is particularly important. Since the grounding impedance is usually large, the equivalent circuit of a single-pole grounding fault can be approximately regarded as an open circuit, so the impact on the operation of the system is small, and usually no protection action is required. However, the fault will cause a bias of half the rated DC voltage on the AC side and a 2-fold overvoltage on the DC bus of the healthy pole. For the above faults, the negative level output capability of the submodule is generally used to achieve non-locking fault ride-through. In this process, the converter remains in a controllable state, the submodule capacitor voltage can be well balanced during the fault ride-through, and can be quickly restored after the fault is removed. During the non-locking fault ride-through process, the bridge arm of the fault pole usually bypasses the half-bridge submodule, and only the full-bridge submodule is put into use to eliminate the DC bias of the healthy pole bus and the AC side port voltage. However, due to the asymmetric operation of the upper and lower bridge arms during the fault crossing process, the energy between the bridge arms and between the phases is unbalanced, which may cause low-frequency harmonic pollution problems on the AC and DC side currents.

[0004] Like conventional modular multilevel converters, half-wave alternating modular multilevel converters can also experience single-pole grounding faults. When a fault occurs, the half-wave alternating modular multilevel converter can also use the negative level output capability of the full-bridge submodule to achieve non-lockout fault crossing. At the same time, due to the different alternating multiplexing structure and operation mode between the half-wave alternating modular multilevel converter and the conventional modular multilevel converter, its phase-to-phase energy balancing mechanism is more complicated. Therefore, compared with the conventional modular multilevel converter, it may even face more serious problems of energy imbalance between bridge arms and between phases. This imbalance causes low-frequency harmonic pollution in the current on the AC and DC sides, further causing fluctuations in the submodule capacitor voltage. The increase in capacitor voltage will increase the voltage stress of the switching device and shorten its service life; while the decrease in voltage will affect the output performance of the converter.

[0005] Therefore, a fault ride-through method suitable for a half-wave alternating modular multilevel converter is needed to achieve energy balance of each phase during the fault ride-through process and avoid aggravation of system instability. Summary of the invention

[0006] One of the objects of the present invention is to solve at least one or more of the above-mentioned problems existing in the prior art. In other words, one of the objects of the present invention is to provide a method and system for riding through a single-pole ground fault of a modular multilevel converter that meets one or more of the above-mentioned requirements.

[0007] In order to achieve the above-mentioned object of the invention, the present invention adopts the following technical solutions:

[0008] In a first aspect, the present invention provides a single-pole ground fault ride-through method for a modular multilevel converter, which is applied to a half-wave alternating modular multilevel converter, comprising:

[0009] In response to a detection signal of a single-pole ground fault, a faulty circuit section and a faulty pole on the DC side are determined, and converters located at both ends of the faulty circuit section are determined;

[0010] The output voltage of each bridge arm is changed according to the polarity of the fault pole, so that the AC output voltage at both ends of the fault circuit section matches the DC voltage of the fault circuit section, and the fault circuit section is crossed;

[0011] Obtaining the voltage and current of each phase of the AC output at both ends of the fault circuit segment, and calculating the circulating current control signal for maintaining energy balance of each bridge arm;

[0012] The output level of each bridge arm is modulated according to the circulating current control signal to balance the energy between the bridge arms and the phases.

[0013] As a preferred implementation, the step of changing the output voltage of each bridge arm according to the fault pole includes:

[0014] If the positive pole is the fault pole, when the bridge arm is forward-conducted, the total positive level output of all bridge submodules in the bridge arm is controlled to decrease;

[0015] If the negative pole is the fault pole, when the bridge arm is reversely conducted, the total positive level output of all the bridge sub-modules in the bridge arm is controlled to decrease.

[0016] As a preferred implementation, the step of calculating the circulating current control signal for maintaining energy balance of each bridge arm includes:

[0017] The steps of calculating the circulating current control signal for maintaining energy balance of each bridge arm include:

[0018] Based on the magnitude and angular frequency of the voltage and current flowing through each phase, the amplitude and phase angle of the fundamental frequency circulating current and the odd-order frequency multiplication circulating current required for the energy accumulated in each bridge arm to be zero within one cycle are calculated.

[0019] As a further preferred implementation, when calculating the amplitudes and phase angles of the fundamental frequency circulating current and the odd-order frequency multiplication circulating current, the components of the odd-order frequency multiplication circulating current that are higher than three times the frequency are ignored.

[0020] As a preferred implementation, the step of modulating the output level of each bridge arm according to the circulating current control signal to balance the energy between the bridge arm and the phase includes:

[0021] The output level of each bridge arm is modulated according to the circulating current control signal, so that the output level of each bridge arm fits the sum of the original level and the circulating current control signal.

[0022] As a preferred embodiment, the modulation uses the nearest level approximation or pulse width modulation method.

[0023] As a preferred implementation, modulation applies control signals to the full-bridge sub-modules in each bridge arm respectively.

[0024] As a preferred embodiment, the method further comprises:

[0025] In response to the restoration signal of the single-pole ground fault, the full-bridge submodule is controlled to restore to a normal working state.

[0026] On the other hand, the present invention also provides a single-pole ground fault ride-through system for a modular multi-level converter, comprising a main control unit: in response to a detection signal of a single-pole ground fault, determining a fault circuit segment and a fault pole on a DC side, and determining converters located at both ends of the fault circuit segment;

[0027] Level control unit: changes the output voltage of each bridge arm according to the polarity of the fault pole, so that the AC output voltage at both ends of the fault circuit section is the same, and passes through the fault circuit section;

[0028] A circulating current calculation unit: obtains the voltage and current of each phase of the AC output at both ends of the fault circuit segment, and is configured to calculate a circulating current control signal for maintaining energy balance of each bridge arm according to the voltage and current of each phase of the AC output at both ends of the fault circuit segment;

[0029] Valve control unit: modulates the output level of the full-bridge submodule according to the circulating current control signal, and injects the circulating current control signal into the AC side to balance the energy on the AC side.

[0030] As a preferred implementation, the main control unit also controls the full-bridge submodule to resume normal working state in response to a recovery signal of a single-pole grounding fault.

[0031] Compared with the prior art, the present invention has the following beneficial effects:

[0032] The single-pole ground fault ride-through method and system of the modular multi-level converter of the present invention can achieve smooth ride-through of the single-pole ground fault on the basis of fully utilizing the existing three-phase power supply of the AC port and strategically controlling the AC current, and balance the system energy in the single-pole ground fault, and does not require any auxiliary power supply to compensate for the energy, nor does it require additional equipment investment costs. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] Figure 1 1 is a schematic diagram of the topological structure of a half-wave alternating modular multi-level converter according to an embodiment of the present invention;

[0034] Figure 2 Schematic diagram of two operation modes of the A-phase unit during the operation of the half-wave alternating modular multi-level converter according to an embodiment of the present invention;

[0035] Figure 3 is a schematic diagram of an equivalent circuit of an A-phase unit according to an embodiment of the present invention;

[0036] Figure 4 It is a waveform diagram of the fault ride-through process of the A-phase unit according to an embodiment of the present invention. DETAILED DESCRIPTION

[0037] The technical solutions in the embodiments of the present invention will be described clearly and completely below in conjunction with the accompanying drawings in the embodiments of the present invention.

[0038] The following description provides examples and does not limit the scope, applicability or examples set forth in the claims. Changes may be made to the functions and arrangements of the elements described without departing from the scope of the present invention. Various processes or components may be appropriately omitted, substituted or added to each example. For example, the described method may be performed in an order different from the described order, and various steps may be added, omitted or combined. In addition, the features described in some examples may be combined in other examples.

[0039] In order to facilitate the understanding of the present disclosure, the topological structure and basic operating principle of the half-wave alternating modular multi-level converter are first introduced.

[0040] As attached Figure 1 The topology of a half-wave alternating modular multilevel converter is shown, which is composed of three identical phase units, wherein each phase unit is composed of a half-wave generating circuit and a guide switch group.

[0041] Each half-wave generating circuit is composed of a plurality of full-bridge sub-modules cascaded together, and each full-bridge sub-module can be individually switched on and off under valve control, so that the half-wave generating circuit outputs an equivalent step waveform based on a theoretical reference wave.

[0042] Each steering switch group is composed of a plurality of insulated gate bipolar transistors (IGBTs) with anti-parallel diodes connected in series. The steering switch group is used to adjust the state of the switch according to the operating mode, change the direction of current flow, and thus realize the switching of the conduction circuit.

[0043] Taking the A phase unit as an example, the pilot switch group can be divided into the forward switch group (PD a1 , PD a2 ) and the reverse switch group (RD a1 , RD a2 ). Each set of switches is triggered by the same pulse signal, and the two sets of switches are switched on and off in a complementary manner, and the switching frequency is only the AC power grid frequency.

[0044] When N= When the half-wave alternating modular multilevel converter is used, the number of sub-modules in each phase unit is N / 2. Compared with the conventional modular multilevel converter, which contains 2N sub-modules per phase unit, the number of sub-modules is reduced by 75%, thereby effectively achieving the goal of lightweighting.

[0045] According to the above topological structure, the half-wave alternating modular multilevel converter reduces the amplitude of the output voltage required by the half-wave generating circuit by introducing a guide switch, significantly reduces the number of sub-modules under the same output voltage, and realizes the compact and lightweight design of the modular multilevel converter.

[0046] As for its working principle, take the A phase unit as an example, Figure 2Two operation modes of the A phase unit in the operation process of the half-wave alternating modular multilevel converter are provided. In the operation process, a half-wave multilevel waveform with a DC bias is generated by a half-wave generating circuit, and the half-wave is shaped into a sinusoidal full-wave multilevel waveform by a pilot switch control.

[0047] For details, see Figure 2 In part a, when the A phase voltage reference wave is greater than zero, the submodules (SM1…SM N / 2 ) output level, positive switch group PD a1 , PD a2 Conducting, reverse switch group RD a1 , RD a2 Shutdown, the A phase unit operates in operation mode I.

[0048] For another case, see Figure 2 In part b, when the A phase voltage reference wave is less than zero, the submodule (SM1...SM N / 2 ) output level, positive switch group PD a1 , PD a2 Turn off, reverse switch group RD a1 , RD a2 is turned on, and the A phase unit operates in operation mode II.

[0049] The two switch groups in the forward direction and the reverse direction are switched on and off alternately and complementary, and the switching frequencies are both the industrial frequency.

[0050] definition is the operation mode switching function of the A-phase unit, and its expression is:

[0051] ;

[0052] After the half-wave generating circuit is equivalent to a voltage source, Figure 3 The circuit structure shown is an equivalent circuit of the A phase unit, in which the half-wave generating circuit voltage and current Can be expressed as:

[0053] ;

[0054] Among them is DC bus voltage, is the A phase output voltage, is the A phase output current.

[0055] Voltage across the pilot switch group , Can be expressed as:

[0056] ;

[0057] , Same reason.

[0058] Assuming that the AC side output voltage and current do not contain any harmonics, the expressions for defining the A phase voltage and current are:

[0059] ;

[0060] in Indicates the phase voltage of phase A, Indicates the current amplitude of phase A, ω is the angular frequency, is the power factor angle.

[0061] In order to achieve capacitor voltage balance of the submodules, the calculation goal is that the bridge arm should have no energy accumulation in one cycle, that is:

[0062] ;

[0063] Therefore, the power factor is not considered When the current is always 0, the half-wave alternating modular multilevel converter can work stably at The operating point is always 2 / π.

[0064] Based on the half-wave alternating modular multilevel converter operating in the above topology structure, an embodiment of the present invention provides a single-pole ground fault ride-through method of the modular multilevel converter, comprising the following steps:

[0065] Step S101: In response to a detection signal of a single-pole grounding fault, a faulty circuit section and a faulty pole on a DC side are determined.

[0066] It is understandable that this detection can be achieved through a variety of methods, such as using the zero-sequence current method to monitor the zero-sequence current in the DC line at the converter end to determine the pole where the fault occurs. When the zero-sequence current of a pole exceeds the set threshold, it can be determined that the pole is the faulty pole. Alternatively, the bus voltage bias method can be used to detect the DC bus voltage deviation. When a single-pole grounding fault occurs, the DC voltage of the faulty pole will drop significantly, thereby providing a basis for determining the faulty pole. The detection method is not limited to the method provided above, as long as the faulty circuit segment and the faulty pole on the DC side can be determined and the corresponding detection signal can be sent.

[0067] Step S102: According to the determined faulty circuit section, determine the converters located at both ends of the faulty circuit section.

[0068] After determining the faulty circuit segment and the faulty pole, the method of this embodiment executes step S2, sending a control signal to the converter determined in the above step, so that the full-bridge sub-module in each bridge arm changes the output voltage under a specified control mode.

[0069] If the positive pole of the faulty circuit segment is grounded, that is, the positive pole is the fault pole, when each phase unit is in operation mode I, some full-bridge sub-modules in the bridge arm are controlled to output negative level, so that the total positive level output of all full-bridge sub-modules in the bridge arm is reduced by half; when each phase unit is in operation mode II, the total positive level output of all full-bridge sub-modules in the bridge arm remains the same.

[0070] If the negative pole of the faulty circuit segment is grounded, that is, the negative pole is the fault pole, when each phase unit is in operation mode I, the total positive level output of all full-bridge sub-modules in the bridge arm remains the same; when each phase unit is in operation mode II, some full-bridge sub-modules in the bridge arm are controlled to output negative level, so that the total positive level output of all full-bridge sub-modules in the bridge arm is reduced by half.

[0071] Take the positive pole as an example to explain: If a single-pole grounding fault occurs at the positive pole, the DC voltage will drop. According to the above method, the bridge arm voltage in operation mode I will be reduced , at this time, the AC side output voltage and DC output voltage for:

[0072] ;

[0073] The corresponding waveform of the A phase unit during the above fault ride-through process is as follows Figure 4 As shown, after this adjustment, the DC output voltage is reduced , while the AC output voltage will remain unchanged, achieving the crossing of the fault circuit.

[0074] Although the above adjustment process realizes the crossing of the fault circuit, the asymmetric operation of the fault pole and the sound pole will cause the phase energy imbalance. Therefore, the method of this embodiment also balances the bridge arm energy and the phase energy after the fault circuit is crossed.

[0075] This embodiment achieves balancing through the following steps:

[0076] Step S3: obtaining the voltage and current of each phase of the AC output at both ends of the faulty circuit segment, and calculating a circulating current control signal for maintaining energy balance in each bridge arm.

[0077] This embodiment takes the positive pole as the fault pole and phase A as an example to provide a method for calculating the circulating current control signal, and the steps are as follows:

[0078] List the expressions for the fundamental frequency and odd-order harmonics of phase A current:

[0079] ;

[0080] in, is the amplitude of the A-phase AC current, is the power factor angle of phase A AC current, is the amplitude of the fundamental frequency circulating current, is the phase angle of the fundamental frequency circulating current, is the amplitude of the triple frequency circulating current, is the phase angle of the triple frequency circulating current.

[0081] The 3-fold harmonic is sufficient to meet the accuracy requirement of energy balancing. In order to reduce the complexity of calculation and subsequent modulation, the method of this embodiment ignores the components higher than 3-fold.

[0082] List the voltages of the half-wave generating circuit and current The expression is:

[0083] ;

[0084] Among them, S a The expression in one cycle is:

[0085] ;

[0086] Substituting the above formulas into the half-wave generating circuit voltage and current The expression of half-wave generating circuit voltage is obtained and current The intermediate form:

[0087] ;

[0088] By the above u am and i am The energy accumulated by the half-wave circuit in one cycle is calculated as:

[0089] ;

[0090] The DC side voltage is:

[0091] ;

[0092] Let the above , , the fundamental frequency and third harmonic reference values ​​that can maintain the energy balance of the bridge arm can be calculated , , and , the fundamental frequency and the third harmonic reference value are used as circulating current control signals for subsequent modulation.

[0093] Step S401: After obtaining the fundamental frequency and third harmonic reference values ​​that can maintain the energy balance of the bridge arm, injecting the circulating current control signal into the original reference voltage to generate a new reference voltage.

[0094] Step S402, calculating the on / off state of each full-bridge submodule according to the new reference voltage, sending an on / off control signal to the full-bridge submodule, modulating the output level of the full-bridge submodule, thereby injecting the circulating current control signal into the AC output to balance the energy of the AC output.

[0095] In one embodiment, the above modulation uses minimum level modulation or pulse width modulation, and the valve controller connected to the bridge arm receives the bridge arm modulation wave sent by the converter level controller, and the number of sub-modules put into each bridge arm is obtained through the modulation link, and then the specific sub-modules put into use are obtained through the capacitor voltage balancing control link, and finally the pulses of the semiconductor switching device are obtained through the semiconductor switch signal generation link, thereby realizing direct control of the AC output voltage.

[0096] In another embodiment, the method of the present invention also includes step S5, after the single-pole grounding fault is cleared, sending a recovery signal to the converter that has performed the fault ride-through operation, and after the converter receives the recovery signal, the full-bridge sub-module in each bridge arm outputs voltage normally under the specified control mode, thereby restoring the normal working state.

[0097] An embodiment of the present invention further provides a single-pole ground fault ride-through system for a modular multilevel converter, comprising a main control unit, a level control unit, a circulating current calculation unit, and a valve control unit, wherein the main control unit is used to determine a faulty circuit segment and a faulty pole on a DC side in response to a detection signal of a single-pole ground fault, and determine converters located at both ends of the faulty circuit segment;

[0098] The stage control unit is used to change the output voltage of each bridge arm according to the polarity of the fault pole, so that the AC output voltage at both ends of the fault circuit section is the same and passes through the fault circuit section;

[0099] The circulating current calculation unit is used to obtain the voltage and current of each phase of the AC output at both ends of the fault circuit section, and calculate the circulating current control signal for maintaining the energy balance of each bridge arm;

[0100] The valve control unit is used to modulate the output level of the full-bridge submodule according to the circulating current control signal, and inject the circulating current control signal into the AC side to balance the energy on the AC side.

[0101] In another embodiment, the main control unit is further configured to control the full-bridge submodule to resume normal working state in response to a restoration signal of a single-pole ground fault.

[0102] In the above embodiments, the description of each embodiment has its own emphasis. For parts that are not described in detail in a certain embodiment, reference can be made to the relevant descriptions of other embodiments.

[0103] Those skilled in the art will appreciate that the units and algorithm steps of each example described in conjunction with the embodiments disclosed herein can be implemented in electronic hardware, or a combination of computer software and electronic hardware. Whether these functions are performed in hardware or software depends on the specific application and design constraints of the technical solution. Professional and technical personnel can use different methods to implement the described functions for each specific application, but such implementation should not be considered to be beyond the scope of the present invention.

[0104] The above is only an exemplary embodiment of the present disclosure, and the scope of the present disclosure cannot be limited thereto. That is, any equivalent changes and modifications made according to the teachings of the present disclosure are still within the scope of the present disclosure. After considering the specification and practicing the disclosure here, those skilled in the art will easily think of the implementation scheme of the present disclosure. The present invention is intended to cover any variation, use or adaptive change of the present disclosure, which follows the general principles of the present disclosure and includes common knowledge or customary technical means in the technical field not recorded in the present disclosure. The description and examples are regarded as exemplary only, and the scope and spirit of the present disclosure are defined by the claims.

Claims

1. A method for riding through a single-pole ground fault of a modular multilevel converter, applied to a half-wave alternating modular multilevel converter, characterized in that: include: In response to a detection signal of a single-pole ground fault, determining a faulty circuit section and a faulty pole on the DC side, and determining converters located at both ends of the faulty circuit section; Changing the output voltage of each bridge arm according to the polarity of the fault pole, so that the AC output voltage at both ends of the fault circuit segment matches the DC voltage of the fault circuit segment, and passes through the fault circuit segment; Obtaining the voltage and current of each phase of the AC output at both ends of the fault circuit segment, and calculating the circulating current control signal for maintaining energy balance of each bridge arm; Modulate the output level of each bridge arm according to the circulating current control signal to balance the energy between the bridge arm and the phase; The step of calculating the circulating current control signal for maintaining energy balance of each bridge arm comprises: Based on the magnitude and angular frequency of the voltage and current flowing through each phase, the amplitude and phase angle of the fundamental frequency circulating current and the odd-order frequency multiplication circulating current required for each bridge arm to accumulate zero energy in one cycle are calculated; The step of modulating the output level of each bridge arm according to the circulating current control signal to balance the energy between the bridge arm and the phase comprises: The output level of each bridge arm is modulated according to the circulating current control signal, so that the output level of each bridge arm fits the sum of the original level and the circulating current control signal.

2. A method for riding through a single-pole ground fault of a modular multilevel converter according to claim 1, characterized in that: The step of changing the output voltage of each bridge arm according to the fault pole comprises: If the positive pole is the fault pole, when the bridge arm is forward-conducted, the total positive level output of all bridge submodules in the bridge arm is controlled to decrease; If the negative pole is the fault pole, when the bridge arm is reversely conducted, the total positive level output of all the bridge sub-modules in the bridge arm is controlled to decrease.

3. A method for riding through a single-pole ground fault of a modular multilevel converter according to claim 1, characterized in that: When calculating the amplitude and phase angle of the fundamental frequency circulating current and the odd-order multiplied frequency circulating current, the odd-order multiplied frequency circulating current ignores the components higher than the third multiplied frequency.

4. A method for riding through a single-pole ground fault of a modular multilevel converter according to claim 1, characterized in that: The modulation uses either the nearest level approximation or pulse width modulation method.

5. The method for riding through a single-pole ground fault of a modular multilevel converter according to claim 1, characterized in that: The modulation applies control signals to the full-bridge submodules in each bridge arm respectively.

6. A method for riding through a single-pole ground fault of a modular multilevel converter as claimed in claim 5, characterized in that: The method also includes: In response to a restoration signal of a single-pole ground fault, the full-bridge submodule is controlled to restore to a normal working state.

7. A single-pole ground fault ride-through system for a modular multilevel converter, characterized in that: The main control unit comprises: in response to a detection signal of a single-pole ground fault, determining a faulty circuit section and a faulty pole on the DC side, and determining converters located at both ends of the faulty circuit section; Stage control unit: changes the output voltage of each bridge arm according to the polarity of the fault pole, so that the AC output voltage at both ends of the fault circuit segment matches the DC voltage of the fault circuit segment, and passes through the fault circuit segment; A circulating current calculation unit: obtaining the voltage and current of each phase of the AC output at both ends of the fault circuit section, and calculating a circulating current control signal for maintaining energy balance of each bridge arm; Valve control unit: modulate the output level of each bridge arm according to the circulation control signal to balance the energy between the bridge arm and the phase; Obtaining the voltage and current of each phase of the AC output at both ends of the fault circuit segment, and calculating the circulating current control signal for maintaining energy balance of each bridge arm; Modulate the output level of each bridge arm according to the circulating current control signal to balance the energy between the bridge arm and the phase; The circulating current control signal calculated to maintain energy balance of each bridge arm includes: Based on the magnitude and angular frequency of the voltage and current flowing through each phase, the amplitude and phase angle of the fundamental frequency circulating current and the odd-order frequency multiplication circulating current required for each bridge arm to accumulate zero energy in one cycle are calculated; Modulating the output level of each bridge arm according to the circulating current control signal to balance the energy between the bridge arm and the phase includes: The output level of each bridge arm is modulated according to the circulating current control signal, so that the output level of each bridge arm fits the sum of the original level and the circulating current control signal.

8. A single-pole ground fault ride-through system for a modular multilevel converter as claimed in claim 7, characterized in that: The main control unit also controls the full-bridge submodules in each bridge arm to resume normal working state in response to the restoration signal of the single-pole grounding fault.

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