Circulation control method and device of modular multilevel converter

Abstract

The invention discloses a circulating current control method and device for a modular multilevel converter, and the method comprises the steps: detecting a plurality of bridge arm current instantaneous values when a controller detects that the modular multilevel converter is overloaded for a short time; respectively calculating a three-phase bridge arm circulation measurement value and a three-phase output current instantaneous value according to the current instantaneous value of each bridge arm; performing weighted stacking on the absolute values of the three-phase output current instantaneous values to obtain a three-phase bridge arm circulation target value; calculating a three-phase loop current difference value between the three-phase bridge arm loop current target value and the three-phase bridge arm loop current measurement value; and performing proportional control on the modular multilevel converter according to the three-phase circulation difference value so as to reduce the bridge arm current peak value of the modular multilevel converter. Therefore, by generating and controlling the three-phase bridge arm circulating current tracking target value, the positive and negative peak values of each bridge arm current are balanced, the maximum peak value of the bridge arm current is reduced, and the short-time overload capacity of the MMC can be improved under the condition that extra cost is not increased.

Description

Technical Field

[0001] The present invention relates to the technical field of circulating current control, and in particular to a circulating current control method and device for a modular multi-level converter. Background Art

[0002] Modular multilevel converter (MMC) has the advantages of easy realization of a large number of levels, highly modular structure, low harmonics and low losses, and is suitable for applications in high-voltage and large-capacity AC / DC power conversion.

[0003] At present, MMC has become the main topology of grid-connected voltage source converters in transmission and distribution networks, and has been widely used in flexible DC transmission, flexible AC transmission, energy storage systems and other fields. For voltage source converters connected to AC grids, the AC grid has higher and higher requirements on the active support capability of the converters, requiring the converters to have stronger short-term overload capabilities.

[0004] In order to improve the short-term overload capacity of MMC, in the prior art, a higher current level switch device is usually used to form MMC, or more full-bridge sub-modules are cascaded to increase the set value of the MMC AC side voltage and reduce the AC component of the bridge arm current. However, the above solutions all require adjustments to the specific hardware architecture of MMC, resulting in an increase in the cost of improving the short-term overload capacity. Summary of the invention

[0005] The present invention provides a circulating current control method and device for a modular multi-level converter, which solves the technical problem that existing solutions all require adjustment and addition of the specific hardware architecture of the MMC, resulting in increased costs for improving short-term overload capacity.

[0006] The present invention provides a circulating current control method for a modular multilevel converter, which is applied to a controller of the modular multilevel converter. The modular multilevel converter is connected in parallel to an AC power grid. The method includes:

[0007] When the modular multilevel converter is overloaded for a short time, detecting instantaneous values ​​of multiple bridge arm currents of the modular multilevel converter;

[0008] According to the instantaneous values ​​of the bridge arm currents, respectively calculating the three-phase bridge arm circulating current measurement values ​​and the three-phase output current instantaneous values;

[0009] Performing weighted superposition on the absolute values ​​of the instantaneous values ​​of the three-phase output currents to obtain a target value of the three-phase bridge arm circulating current;

[0010] Calculating a three-phase circulating current difference between the three-phase bridge arm circulating current target value and the three-phase bridge arm circulating current measured value;

[0011] The modular multilevel converter is proportionally controlled according to the three-phase circulating current difference to reduce the peak value of the bridge arm current of the modular multilevel converter.

[0012] Optionally, the calculating the three-phase bridge arm circulating current measurement value and the three-phase output current instantaneous value respectively according to the instantaneous value of each bridge arm current includes:

[0013]

[0014] Among them, i ap is the instantaneous value of the upper bridge arm current of phase a, i an is the instantaneous value of the lower bridge arm current of phase a, i bp is the instantaneous value of the upper bridge arm current of phase b, i bn is the instantaneous value of the lower bridge arm current of phase b, i cp is the instantaneous value of the upper bridge arm current of phase c, i cn is the instantaneous value of the lower bridge arm current of phase c, i cira is the measured value of the circulating current of phase a, i cirb is the measured value of the circulating current of phase b, i circ is the measured value of the c-phase circulating current, i a is the instantaneous value of the output current of phase a, i b is the instantaneous value of the output current of phase b, i c is the instantaneous value of the c-phase output current.

[0015] Optionally, performing weighted superposition on the absolute values ​​of the instantaneous values ​​of the three-phase output currents to obtain the target value of the three-phase bridge arm circulating current includes:

[0016]

[0017] Among them, i cira_ref is the target value of the circulation current of phase a, i cirb_ref is the target value of the circulation current of phase b, i circ_ref is the target value of the c-phase circulation current.

[0018] Optionally, the proportionally controlling the modular multilevel converter according to the three-phase circulating current difference to reduce a bridge arm current peak value of the modular multilevel converter includes:

[0019] Calculate the multiplication value between the three-phase circulating current difference and the preset proportional control coefficient to obtain the three-phase circulating current internal potential reference value;

[0020] Generate a pulse signal according to the reference value of the internal potential of the three-phase circulating current;

[0021] The pulse signal is used to adjust the conduction time of the bridge arm power device in the modular multilevel converter to reduce the bridge arm current peak value of the modular multilevel converter.

[0022] Optionally, the calculating the multiplication value between the three-phase circulating current difference and a preset proportional control coefficient to obtain the three-phase circulating current internal potential reference value includes:

[0023]

[0024] Among them, k p is the preset proportional control coefficient, e cira_ref is the reference value of the potential in the a-phase circulating current, e cirb_ref is the reference value of the potential in the b-phase circulating current, e circ_ref is the reference value of the potential in the c-phase circulating current, ∆i cira is the circulating current difference of phase a, ∆i cirb is the circulating current difference of phase b, ∆i circ is the circulation difference of phase c.

[0025] The present invention also provides a circulating current control device for a modular multilevel converter, which is applied to a controller of the modular multilevel converter, wherein the modular multilevel converter is connected in parallel to an AC power grid, and the device comprises:

[0026] A bridge arm current instantaneous value detection module, used for detecting instantaneous values ​​of multiple bridge arm currents of the modular multilevel converter when the modular multilevel converter is short-term overloaded;

[0027] An output current instantaneous value calculation module is used to calculate the three-phase bridge arm circulating current measurement value and the three-phase output current instantaneous value according to the instantaneous value of each bridge arm current;

[0028] A circulating current target value calculation module, used for performing weighted superposition on the absolute values ​​of the instantaneous values ​​of the three-phase output currents to obtain a three-phase bridge arm circulating current target value;

[0029] A circulation difference calculation module, used for calculating the three-phase circulation difference between the three-phase bridge arm circulation target value and the three-phase bridge arm circulation measurement value;

[0030] A proportional control module is used to perform proportional control on the modular multilevel converter according to the three-phase circulating current difference, so as to reduce the peak value of the bridge arm current of the modular multilevel converter.

[0031] Optionally, the output current instantaneous value calculation module is specifically used to perform the following process:

[0032]

[0033] Among them, i ap is the instantaneous value of the upper bridge arm current of phase a, i an is the instantaneous value of the lower bridge arm current of phase a, i bp is the instantaneous value of the upper bridge arm current of phase b, i bnis the instantaneous value of the lower bridge arm current of phase b, i cp is the instantaneous value of the upper bridge arm current of phase c, i cn is the instantaneous value of the lower bridge arm current of phase c, i cira is the measured value of the circulating current of phase a, i cirb is the measured value of the circulating current of phase b, i circ is the measured value of the c-phase circulating current, i a is the instantaneous value of the output current of phase a, i b is the instantaneous value of the output current of phase b, i c is the instantaneous value of the c-phase output current.

[0034] Optionally, the circulation target value calculation module is specifically used to perform the following process:

[0035]

[0036] Among them, i cira_ref is the target value of the circulation current in phase a, i cirb_ref is the target value of the circulation current of phase b, i circ_ref is the target value of the c-phase circulation current.

[0037] Optionally, the proportional control module includes:

[0038] An internal potential reference value calculation submodule, used for calculating the multiplication value between the three-phase circulating current difference and a preset proportional control coefficient to obtain a three-phase circulating current internal potential reference value;

[0039] A pulse signal generating submodule, used for generating a pulse signal according to the reference value of the internal potential of the three-phase circulating current;

[0040] The power device control submodule is used to use the pulse signal to adjust the conduction time of the bridge arm power device in the modular multilevel converter to reduce the bridge arm current peak value of the modular multilevel converter.

[0041] Optionally, the internal potential reference value calculation submodule is specifically used to perform the following process:

[0042]

[0043] Among them, k p is the preset proportional control coefficient, e cira_ref is the reference value of the potential in the a-phase circulating current, e cirb_ref is the reference value of the potential in the b-phase circulating current, e circ_ref is the reference value of the potential in the c-phase circulating current, ∆i cira is the circulating current difference of phase a, ∆i cirb is the circulating current difference of phase b, ∆i circ is the circulation difference of phase c.

[0044] It can be seen from the above technical solutions that the present invention has the following advantages:

[0045] When the controller connected to the modular multilevel converter determines that the modular multilevel converter has a short-term overload, the instantaneous values ​​of multiple bridge arm currents of the modular multilevel converter are detected; according to the instantaneous value of each bridge arm current, the three-phase bridge arm circulating current measurement value and the three-phase output current instantaneous value are calculated respectively; the absolute values ​​of the three-phase output current instantaneous values ​​are weighted superimposed to obtain the three-phase bridge arm circulating current target value; the three-phase circulating current difference between the three-phase bridge arm circulating current target value and the three-phase bridge arm circulating current measurement value is calculated; and the modular multilevel converter is proportionally controlled according to the three-phase circulating current difference to reduce the bridge arm current peak value of the modular multilevel converter. Thus, by generating the three-phase bridge arm circulating current target value and controlling the three-phase bridge arm circulating current to track the target value, the positive and negative peak values ​​of each bridge arm current are balanced, the maximum peak value of the bridge arm current is reduced, and the short-term overload capacity of the MMC can be improved without increasing additional costs. BRIEF DESCRIPTION OF THE DRAWINGS

[0046] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative labor.

[0047] Figure 1 A flowchart of a circulating current control method for a modular multi-level converter provided by an embodiment of the present invention;

[0048] Figure 2 A topological schematic diagram of a modular multilevel converter provided in an embodiment of the present invention;

[0049] Figure 3 A flowchart of a circulating current control method of a modular multi-level converter in the implementation of the present invention;

[0050] Figure 4 is a comparison diagram of bridge arm current waveforms of a modular multilevel converter in an embodiment of the present invention;

[0051] Figure 5 A structural block diagram of a circulating current control device for a modular multi-level converter provided by an embodiment of the present invention. DETAILED DESCRIPTION

[0052] For voltage source converters connected to the AC grid, the AC grid has increasingly higher requirements for the active support capability of the converter, requiring the converter to have a stronger short-term overload capability. For example, for energy storage systems and flexible DC transmission systems, when a large disturbance occurs in the grid, the converter needs to provide the grid with high active power for about 10-20 seconds, provide inertia support for the grid and participate in frequency modulation. However, unlike conventional rotating generators, the overcurrent capability of voltage source converters based on power electronic devices is limited by both the current effective value flow capacity and the instantaneous current shutdown capability. From the perspective of the current effective value, the junction temperature rise of the switching device caused by the short-term overload of the converter should not exceed the limit, which requires a certain period of heat accumulation before the temperature rise exceeds the limit. From the perspective of current instantaneous, the current peak of each switch of the converter cannot exceed the shutdown capability limit of the device. When the voltage source converter is overloaded, the output current of the converter increases, and the problem of the required shutdown current increase will immediately appear, which becomes the main factor limiting the short-term overload capability of the MMC.

[0053] Compared with conventional two-level voltage source converters, MMC faces greater challenges in short-time overload capacity. Under the output active power condition, there is a DC bias in the bridge arm current of the MMC, which causes the positive and negative peaks of the bridge arm current waveform to be asymmetric within one cycle. The shutdown capability of power electronic devices is affected by the peak with a larger absolute value among the positive and negative peaks, and is more susceptible to limitations in short-time overload capacity. In order to improve the short-time overload capacity of MMC, one solution is to use switching devices with higher current levels to form MMC. In normal operation, the current output by the MMC is low relative to the rated current of the switching device. During short-time overload, the MMC output current increases, but is still within the rated current limit of the switching device. Another solution is to reduce the AC component of the arm current by increasing the design value of the AC side voltage of the MMC. In this way, the peak current of the MMC arm during normal operation can be reduced and space can be provided for short-time overload operation. This method requires multiple additional full-bridge sub-modules to be connected in series in each arm to keep the MMC operating in its linear modulation region.

[0054] However, in the above scheme, for the scheme using a higher current level switch device, the high current capacity of the switch device can only be used during a short-term overload period, and such a large current level is not required during normal operation, which greatly increases the cost of the switch device required per unit capacity. For the method of increasing the design value of the MMC AC side voltage, in order to maintain linear modulation when the AC side voltage is increased, more full-bridge sub-modules need to be cascaded, resulting in a significant increase in the cost and volume of the device.

[0055] To this end, the embodiment of the present invention provides a circulating current control method and device for a modular multi-level converter. During a short-term overload period, an active circulating current injection target value is generated in a specific manner, and the circulating current in the three-phase bridge arm current is controlled to track the target value, so that the bias of the MMC bridge arm current can be flexibly controlled, so that the bias of the MMC bridge arm current is offset, and the positive and negative peak currents of the MMC bridge arm are balanced, that is, the peak current with a larger absolute value is reduced, and the peak current with a smaller absolute value is increased, so as to achieve the effect of reducing the maximum absolute value of the peak current. Thereby, the short-term overload capacity of the MMC is improved without increasing the current level of the switching device and without increasing the number of additional sub-modules.

[0056] In order to make the purpose, features and advantages of the present invention more obvious and easy to understand, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the embodiments described below are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0057] See also Figure 1 , Figure 1 A flowchart of a circulating current control method for a modular multilevel converter provided by an embodiment of the present invention.

[0058] The present invention provides a circulating current control method for a modular multilevel converter, which is applied to a controller of the modular multilevel converter. The modular multilevel converter is connected in parallel to an AC power grid, and includes:

[0059] Step 101, when a short-term overload occurs in the modular multilevel converter, detecting instantaneous values ​​of a plurality of bridge arm currents of the modular multilevel converter;

[0060] Modular Multilevel Converter (MMC) is a power electronic converter. MMC consists of six bridge arms, the upper bridge arm and the lower bridge arm of each phase are each an independent bridge arm, and each independent bridge arm is composed of multiple sub-modules (SM 1, SM 2...SM N) connected in series, such as Figure 2 As shown, the AC voltage includes , is the inductor, and n is the AC power grid. Each submodule is usually composed of a half-bridge or full-bridge circuit and an energy storage capacitor. The half-bridge circuit consists of two power switching devices (such as IGBT or IGCT) and two capacitors. The full-bridge circuit consists of four power switching devices (such as IGBT or IGCT). The four switching devices are connected in series to form two bridge arms, and the middle node is the output end. The DC power supply is connected to both ends of the two bridge arms, and the load is connected between the output end and the ground.

[0061] In the embodiment of the present invention, a current sensor can be installed on each bridge arm of the modular multilevel converter, and each current sensor can be connected to a controller to monitor the current of the modular multilevel converter in real time through the controller. When the current exceeds a certain percentage of the normal operating range (such as 120% to 150%), it can be determined that the modular multilevel converter is short-term overloaded. The instantaneous value of the bridge arm current of each bridge arm of the modular multilevel converter at the current moment is detected as the data basis for the subsequent adjustment of the modular multilevel converter.

[0062] Step 102, according to the instantaneous value of each bridge arm current, respectively calculate the three-phase bridge arm circulating current measurement value and the three-phase output current instantaneous value;

[0063] In the embodiment of the present invention, due to the short-term overload of the modular multilevel converter, the circulating current may increase sharply and exceed the normal range. In order to accurately and effectively switch the power modules in the modular multilevel converter, the three-phase bridge arm circulating current measurement value can be calculated according to the instantaneous value of each bridge arm current to determine the current circulating current in the modular multilevel converter. At the same time, the change of the instantaneous value of the three-phase output current can reflect the dynamic characteristics of the modular multilevel converter. In order to improve the subsequent adjustment accuracy, the instantaneous value of the three-phase output current is calculated according to the difference between the instantaneous values ​​of the bridge arm current between the upper and lower bridge arms.

[0064] In one example of the present invention, step 102 may be implemented by the following calculation process:

[0065] After detecting the instantaneous value of the bridge arm current of each bridge arm, weighted superposition is performed according to the current direction of the instantaneous value of the bridge arm current to calculate the measured value of the three-phase bridge arm circulating current:

[0066]

[0067] At the same time, according to the difference between the instantaneous values ​​of the bridge arm currents between the upper bridge arm and the lower bridge arm of each phase, the instantaneous values ​​of the three-phase output currents are calculated:

[0068]

[0069] Among them, i ap is the instantaneous value of the upper bridge arm current of phase a, i anis the instantaneous value of the lower bridge arm current of phase a, i bp is the instantaneous value of the upper bridge arm current of phase b, i bn is the instantaneous value of the lower bridge arm current of phase b, i cp is the instantaneous value of the upper bridge arm current of phase c, i cn is the instantaneous value of the lower bridge arm current of phase c, i cira is the measured value of the circulating current of phase a, i cirb is the measured value of the circulating current of phase b, i circ is the measured value of the c-phase circulating current, i a is the instantaneous value of the output current of phase a, i b is the instantaneous value of the output current of phase b, i c is the instantaneous value of the c-phase output current.

[0070] Step 103, performing weighted superposition on the absolute values ​​of the instantaneous values ​​of the three-phase output currents to obtain a target value of the three-phase bridge arm circulating current;

[0071] In some cases, the current may flow in the opposite direction, but the size and characteristics of the circulating current still need to be accurately calculated and controlled. In order to avoid the influence of the current direction and simplify the calculation difficulty and complexity, the absolute value of the instantaneous value of the three-phase output current can be used for weighted superposition to calculate the target value of the three-phase bridge arm circulating current. The use of absolute value can make the calculation formula unaffected by the change of current direction and is applicable to various working conditions.

[0072] In one example of the present invention, the calculation process of the three-phase bridge arm circulating current target value in step 103 can be implemented by the following process:

[0073]

[0074] Among them, i cira_ref is the target value of the circulation current of phase a, i cirb_ref is the target value of the circulation current of phase b, i circ_ref is the target value of the c-phase circulation current.

[0075] Step 104, calculating the three-phase circulating current difference between the three-phase bridge arm circulating current target value and the three-phase bridge arm circulating current measured value;

[0076] In this embodiment, in order to improve the stability and performance of the modular multilevel converter, it is necessary to determine the amount of circulation that needs to be adjusted at this time. In order to improve the control accuracy, the three-phase circulating current difference between the three-phase bridge arm circulating current target value and the three-phase bridge arm commutation measurement value is calculated as a feedback signal for subsequent control to adjust the control strategy.

[0077] Specifically, the circulating current difference of each phase can be calculated in the following way:

[0078]

[0079] Among them, ∆i cira is the circulating current difference of phase a, ∆i cirb is the difference of circulating current in phase b, ∆i circ is the circulation difference of phase c.

[0080] Step 105: proportionally control the modular multilevel converter according to the three-phase circulating current difference to reduce the peak value of the bridge arm current of the modular multilevel converter.

[0081] In one example of the present invention, step 105 may include the following sub-steps:

[0082] Calculate the multiplication value between the three-phase circulating current difference and the preset proportional control coefficient to obtain the three-phase circulating current internal potential reference value;

[0083] Generate a pulse signal according to the potential reference value of the three-phase circulating current;

[0084] A pulse signal is used to adjust the on-time of the bridge arm power devices in the modular multilevel converter to reduce the bridge arm current peak of the modular multilevel converter.

[0085] In this embodiment, after the controller obtains the three-phase circulating current difference, it calculates the multiplication value between the circulating current difference and the preset proportional control system for each phase to obtain the three-phase circulating current internal potential reference value, and then converts the corresponding circulating current internal potential reference value into a pulse signal of the power device through pulse width modulation (PWM) technology or nearest level modulation (NLM) technology according to the three-phase circulating current internal potential reference value.

[0086] It should be noted that the pulse signal can control the on-time and off-time of the power device by adjusting the pulse width, thereby realizing the current and circulating current control of each phase bridge arm to change the output voltage.

[0087] Furthermore, the reference value of the internal potential of the three-phase circulating current can be realized by the following calculation process:

[0088]

[0089] Among them, k p is the preset proportional control coefficient, e cira_ref is the reference value of the potential in the a-phase circulating current, e cirb_ref is the reference value of the potential in the b-phase circulating current, e circ_ref is the reference value of the potential in the c-phase circulating current, ∆i cira is the circulating current difference of phase a, ∆i cirb is the difference of circulating current in phase b, ∆i circ is the circulation difference of phase c.

[0090] See also Figure 3 , Figure 3The present invention is a flowchart of a circulating current control method of a modular multi-level converter in the implementation of the present invention.

[0091] In the embodiment of the present invention, the controller generates a corresponding pulse signal to control the switching of the power devices of each bridge arm in the modular multilevel converter. Specifically, when the controller detects the instantaneous value of the bridge arm current (i ap ,i an ,i bp ,i bn ,i cp ,i cn ), and then the three-phase bridge arm circulating current measurement value (i cira ,i cirb ,i circ At the same time, the instantaneous value of the bridge arm current (i ap ,i an ,i bp ,i bn ,i cp ,i cn ), calculate the instantaneous value of the three-phase output current (i a ,i b ,i c ); Using the instantaneous value of the three-phase output current (i a ,i b ,i c ) is weighted summed to calculate the target value of the three-phase bridge arm circulating current (i cira_ref ,i cirb_ref ,i circ_ref ); Using the three-phase bridge arm circulating current target value (i cira_ref ,i cirb_ref ,i circ_ref ) and the three-phase bridge arm circulating current measurement value (i cira ,i cirb ,i circ ) is input into the subtractor for each phase, and then input into the proportional unit to calculate the potential reference value (e cira_ref , e cirb_ref , e circ_ref ); finally, a corresponding pulse signal is generated according to the potential reference value of the three-phase circulating current to control and adjust the power devices in the corresponding bridge arm, thereby completing the circulating current control of the modular multi-level converter.

[0092] See also Figure 4 , Figure 4 4 is a comparison diagram of bridge arm current waveforms of a modular multilevel converter in an embodiment of the present invention.

[0093] In this embodiment, a 5000MW modular multilevel converter MMC is taken as an example. Figure 4 The A waveform in the figure reflects the bridge arm current waveform of the 5000MW MMC. It can be seen that when the control method provided by the embodiment of the present invention is not used, due to the bias of the DC component, the negative peak value of the bridge arm current is much larger than the positive peak value, and the maximum absolute peak value of the bridge arm current reaches 3420A. Figure 4 The B waveform in the figure reflects the bridge arm current waveform of the 5000MW MMC regulated by the control method of the present invention. It can be seen that the bias effect of the circulating current actively injected by the MMC offsets the bias effect of part of the DC current, so that the difference in the positive peak value of the bridge arm current tends to be reduced, and the maximum absolute peak value of the bridge arm current is only 2600A. It can be seen that the peak value of the absolute value of the bridge arm current can be reduced from 3420A to 2600 by the circulating current control method provided in the embodiment of the present invention. When the shutdown current limit of the power electronic device is the same, the short-term overload capacity of the MMC can be improved by about 30% by using the method of the present invention.

[0094] In an embodiment of the present invention, when the controller connected to the modular multilevel converter determines that the modular multilevel converter has a short-term overload, the instantaneous values ​​of multiple bridge arm currents of the modular multilevel converter are detected; according to the instantaneous value of each bridge arm current, the three-phase bridge arm circulating current measurement value and the three-phase output current instantaneous value are calculated respectively; the absolute values ​​of the three-phase output current instantaneous values ​​are weighted superimposed to obtain the three-phase bridge arm circulating current target value; the three-phase circulating current difference between the three-phase bridge arm circulating current target value and the three-phase bridge arm circulating current measurement value is calculated; and the modular multilevel converter is proportionally controlled according to the three-phase circulating current difference to reduce the bridge arm current peak value of the modular multilevel converter. Thus, by generating the three-phase bridge arm circulating current target value and controlling the three-phase bridge arm circulating current to track the target value, the positive and negative peak values ​​of each bridge arm current are balanced, the maximum peak value of the bridge arm current is reduced, and the short-term overload capacity of the MMC can be improved without increasing additional costs.

[0095] See also Figure 5 , Figure 5 A structural block diagram of a circulating current control device of a modular multi-level converter in an embodiment of the present invention is shown.

[0096] An embodiment of the present invention provides a circulating current control device for a modular multilevel converter, which is applied to a controller of the modular multilevel converter. The modular multilevel converter is connected in parallel to an AC power grid. The device includes:

[0097] The bridge arm current instantaneous value detection module 501 is used to detect the instantaneous values ​​of multiple bridge arm currents of the modular multilevel converter when the modular multilevel converter is short-term overloaded;

[0098] The output current instantaneous value calculation module 502 is used to calculate the three-phase bridge arm circulating current measurement value and the three-phase output current instantaneous value according to the instantaneous value of each bridge arm current;

[0099] The circulating current target value calculation module 503 is used to perform weighted superposition on the absolute values ​​of the instantaneous values ​​of the three-phase output currents to obtain the circulating current target values ​​of the three-phase bridge arms;

[0100] A circulation difference calculation module 504, used for calculating a three-phase circulation difference between a three-phase bridge arm circulation target value and a three-phase bridge arm circulation measurement value;

[0101] The proportional control module 505 is used to perform proportional control on the modular multilevel converter according to the three-phase circulating current difference, so as to reduce the peak value of the bridge arm current of the modular multilevel converter.

[0102] Optionally, the output current instantaneous value calculation module 502 is specifically configured to perform the following process:

[0103]

[0104] Among them, i ap is the instantaneous value of the upper bridge arm current of phase a, i an is the instantaneous value of the lower bridge arm current of phase a, i bp is the instantaneous value of the upper bridge arm current of phase b, i bn is the instantaneous value of the lower bridge arm current of phase b, i cp is the instantaneous value of the upper bridge arm current of phase c, i cn is the instantaneous value of the lower bridge arm current of phase c, i cira is the measured value of the circulating current of phase a, i cirb is the measured value of the circulating current of phase b, i circ is the measured value of the c-phase circulating current, i a is the instantaneous value of the output current of phase a, i b is the instantaneous value of the output current of phase b, i c is the instantaneous value of the c-phase output current.

[0105] Optionally, the circulation target value calculation module 503 is specifically used to perform the following process:

[0106]

[0107] Among them, i cira_ref is the target value of the circulation current in phase a, i cirb_ref is the target value of the circulation current of phase b, i circ_ref is the target value of the c-phase circulation current.

[0108] Optionally, the ratio control module 505 includes:

[0109] The internal potential reference value calculation submodule is used to calculate the multiplication value between the three-phase circulating current difference and the preset proportional control coefficient to obtain the three-phase circulating current internal potential reference value;

[0110] A pulse signal generation submodule, used to generate a pulse signal according to a reference value of the internal potential of the three-phase circulating current;

[0111] The power device control submodule is used to adjust the conduction time of the bridge arm power device in the modular multilevel converter by using a pulse signal to reduce the bridge arm current peak value of the modular multilevel converter.

[0112] Optionally, the internal potential reference value calculation submodule is specifically used to perform the following process:

[0113]

[0114] Among them, k p is the preset proportional control coefficient, e cira_ref is the reference value of the potential in the a-phase circulating current, e cirb_ref is the reference value of the potential in the b-phase circulating current, e circ_ref is the reference value of the potential in the c-phase circulating current, ∆i cira is the circulating current difference of phase a, ∆i cirb is the difference of circulating current in phase b, ∆i circ is the circulation difference of phase c.

[0115] Those skilled in the art can clearly understand that, for the convenience and brevity of description, the specific working processes of the above-described devices, modules and sub-modules can refer to the corresponding processes in the aforementioned method embodiments and will not be repeated here.

[0116] In the several embodiments provided by the present invention, it should be understood that the disclosed devices and methods can be implemented in other ways. For example, the device embodiments described above are only schematic. For example, the division of the modules is only a logical function division. There may be other division methods in actual implementation, such as multiple modules or components can be combined or integrated into another system, or some features can be ignored or not executed. Another point is that the mutual coupling or direct coupling or communication connection shown or discussed can be through some interfaces, indirect coupling or communication connection of devices or modules, which can be electrical, mechanical or other forms.

[0117] The modules described as separate components may or may not be physically separated, and the components shown as modules may or may not be physical modules, that is, they may be located in one place or distributed on multiple network modules. Some or all of the modules may be selected according to actual needs to achieve the purpose of the solution of this embodiment.

[0118] As described above, the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit the same. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that the technical solutions described in the aforementioned embodiments may still be modified, or some of the technical features thereof may be replaced by equivalents. However, these modifications or replacements do not deviate the essence of the corresponding technical solutions from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A circulating current control method for a modular multilevel converter, characterized in that: A controller applied to a modular multilevel converter, wherein the modular multilevel converter is connected in parallel to an AC power grid, and the method comprises: When the modular multilevel converter is overloaded for a short time, detecting instantaneous values ​​of multiple bridge arm currents of the modular multilevel converter; According to the instantaneous values ​​of the bridge arm currents, respectively calculating the three-phase bridge arm circulating current measurement values ​​and the three-phase output current instantaneous values; Performing weighted superposition on the absolute values ​​of the instantaneous values ​​of the three-phase output currents to obtain a target value of the three-phase bridge arm circulating current; Calculating a three-phase circulating current difference between the three-phase bridge arm circulating current target value and the three-phase bridge arm circulating current measured value; The modular multilevel converter is proportionally controlled according to the three-phase circulating current difference to reduce the peak value of the bridge arm current of the modular multilevel converter.

2. The method according to claim 1, characterized in that The three-phase bridge arm circulating current measurement values ​​and the three-phase output current instantaneous values ​​are calculated respectively according to the instantaneous values ​​of the bridge arm currents, including: Among them, i ap is the instantaneous value of the upper bridge arm current of phase a, i an is the instantaneous value of the lower bridge arm current of phase a, i bp is the instantaneous value of the upper bridge arm current of phase b, i bn is the instantaneous value of the lower bridge arm current of phase b, i cp is the instantaneous value of the upper bridge arm current of phase c, i cn is the instantaneous value of the lower bridge arm current of phase c, i cira is the measured value of the circulating current of phase a, i cirb is the measured value of the circulating current of phase b, i circ is the measured value of the c-phase circulating current, i a is the instantaneous value of the output current of phase a, i b is the instantaneous value of the output current of phase b, i c is the instantaneous value of the c-phase output current.

3. The method according to claim 2, characterized in that The step of weighted superposition of the absolute values ​​of the instantaneous values ​​of the three-phase output currents to obtain the target value of the three-phase bridge arm circulating current includes: Among them, i cira_ref is the target value of the circulation current in phase a, i cirb_ref is the target value of the circulation current of phase b, i circ_ref is the target value of the c-phase circulation current.

4. The method according to claim 1, characterized in that The method of proportionally controlling the modular multilevel converter according to the three-phase circulating current difference to reduce the peak value of the bridge arm current of the modular multilevel converter includes: Calculate the multiplication value between the three-phase circulating current difference and the preset proportional control coefficient to obtain the three-phase circulating current internal potential reference value; Generate a pulse signal according to the reference value of the internal potential of the three-phase circulating current; The pulse signal is used to adjust the conduction time of the bridge arm power device in the modular multilevel converter to reduce the bridge arm current peak value of the modular multilevel converter.

5. The method according to claim 4, characterized in that The calculating the multiplication value between the three-phase circulating current difference and the preset proportional control coefficient to obtain the three-phase circulating current internal potential reference value includes: Among them, k p is the preset proportional control coefficient, e cira_ref is the reference value of the potential in the a-phase circulating current, e cirb_ref is the reference value of the potential in the b-phase circulating current, e circ_ref is the reference value of the potential in the c-phase circulating current, ∆i cira is the circulating current difference of phase a, ∆i cirb is the difference of circulating current in phase b, ∆i circ is the circulation difference of phase c.

6. A circulating current control device for a modular multilevel converter, characterized in that: A controller applied to a modular multilevel converter, wherein the modular multilevel converter is connected in parallel to an AC power grid, the device comprising: A bridge arm current instantaneous value detection module, used for detecting instantaneous values ​​of multiple bridge arm currents of the modular multilevel converter when the modular multilevel converter is short-term overloaded; An output current instantaneous value calculation module is used to calculate the three-phase bridge arm circulating current measurement value and the three-phase output current instantaneous value according to the instantaneous value of each bridge arm current; A circulating current target value calculation module, used for performing weighted superposition on the absolute values ​​of the instantaneous values ​​of the three-phase output currents to obtain a three-phase bridge arm circulating current target value; A circulation difference calculation module, used for calculating the three-phase circulation difference between the three-phase bridge arm circulation target value and the three-phase bridge arm circulation measurement value; A proportional control module is used to perform proportional control on the modular multilevel converter according to the three-phase circulating current difference, so as to reduce the peak value of the bridge arm current of the modular multilevel converter.

7. The device according to claim 6, characterized in that The output current instantaneous value calculation module is specifically used to perform the following process: Among them, i ap is the instantaneous value of the upper bridge arm current of phase a, i an is the instantaneous value of the lower bridge arm current of phase a, i bp is the instantaneous value of the upper bridge arm current of phase b, i bn is the instantaneous value of the lower bridge arm current of phase b, i cp is the instantaneous value of the upper bridge arm current of phase c, i cn is the instantaneous value of the lower bridge arm current of phase c, i cira is the measured value of the circulating current of phase a, i cirb is the measured value of the circulating current of phase b, i circ is the measured value of the c-phase circulating current, i a is the instantaneous value of the output current of phase a, i b is the instantaneous value of the output current of phase b, i c is the instantaneous value of the c-phase output current.

8. The device according to claim 7, characterized in that The circulation target value calculation module is specifically used to perform the following process: Among them, i cira_ref is the target value of the circulation current in phase a, i cirb_ref is the target value of the circulation current of phase b, i circ_ref is the target value of the c-phase circulation current.

9. The device according to claim 6, characterized in that The proportional control module comprises: An internal potential reference value calculation submodule, used for calculating the multiplication value between the three-phase circulating current difference and a preset proportional control coefficient to obtain a three-phase circulating current internal potential reference value; A pulse signal generating submodule, used for generating a pulse signal according to the reference value of the internal potential of the three-phase circulating current; The power device control submodule is used to use the pulse signal to adjust the conduction time of the bridge arm power device in the modular multilevel converter to reduce the bridge arm current peak value of the modular multilevel converter.

10. The device according to claim 9, characterized in that The internal potential reference value calculation submodule is specifically used to perform the following process: Among them, k p is the preset proportional control coefficient, e cira_ref is the reference value of the potential in the a-phase circulating current, e cirb_ref is the reference value of the potential in the b-phase circulating current, e circ_ref is the reference value of the potential in the c-phase circulating current, ∆i cira is the circulating current difference of phase a, ∆i cirb is the difference of circulating current in phase b, ∆i circ is the circulation difference of phase c.

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