A circulating current suppression method under M3C bridge arm inductance asymmetry

Abstract

The application provides a circulating current suppression method under M3C bridge arm inductance asymmetry, and relates to the technical field of power electronics. The method comprises the following steps: combining the circulating current, the grid side and the machine side control outputs of the M3C and modulation to obtain the switching signal of each sub-module, and controlling the switching of the sub-module. The circulating current control comprises the following steps: calculating the deviation value of each circulating current component obtained by double alpha beta 0 coordinate transformation of the M3C bridge arm current from the corresponding reference value; taking the grid side frequency and the machine side frequency as the resonance frequencies of the first and second PR controllers; superimposing the output of the first and second PR controllers and the P controller to which each deviation value is respectively input to obtain the circulating current control output. The method can selectively suppress the circulating current component caused by the bridge arm inductance asymmetry, improve the time domain waveform, reduce the peak value and effective value, reduce the bridge arm current stress, power device on-state loss and thermal load, and improve the operation reliability of the M3C and the service life of the device.

Description

Technical Field

[0001] This invention relates to the field of power electronics technology, and in particular to a method for suppressing circulating current under M3C bridge arm inductance asymmetry. Background Technology

[0002] The Modular Multilevel Matrix Converter (M3C) is considered a promising power electronic converter topology for high-voltage, high-capacity, single-stage energy conversion, and wide-band operation due to its advantages such as high voltage, large capacity, single-stage energy conversion, and wide-band operation.

[0003] Due to the unique topology of the M3C converter, the arm current consists of grid-side current, machine-side current, and circulating current. The circulating current flows only within the converter and does not participate in power transfer between the grid and machine sides, but it increases the RMS value of the arm current and the converter's power loss. Furthermore, excessive circulating current can degrade capacitor voltage equalization performance. Therefore, analyzing and suppressing circulating current characteristics is a crucial aspect to understand when applying the M3C.

[0004] In actual engineering, due to factors such as manufacturing tolerances, installation errors, aging, and differences in heat dissipation conditions, the inductance parameters of each bridge arm of the M3C inevitably have certain deviations. Furthermore, the asymmetry of the bridge arm inductance will disrupt the original voltage balance between the bridge arms, leading to uneven distribution of circulating current among different bridge arms, causing local overheating, changing the circulating current frequency characteristics, and in severe cases, affecting the operating efficiency of the M3C.

[0005] Most existing M3C circulating current analysis and control methods are based on establishing decoupled models using dual αβ0 coordinate transformations to decouple grid-side current, machine-side current, and circulating current. For the four characteristic frequency components contained in the circulating current, strategies such as proportional (P) controllers, proportional-resonant (PR) controllers, and model predictive control are employed to suppress them. For the circulating current characteristics analysis under the condition of bridge arm inductance asymmetry, Modular Multilevel Converters (MMCs) have been extensively studied. Existing technologies mostly use equivalent circuits, average switching function theory, and harmonic state space methods to analyze the circulating current frequency characteristics. Regarding MMC circulating current suppression strategies, in addition to the control strategies similar to those of M3C, some improved control strategies are also included, such as vector resonant control and virtual impedance resonant control.

[0006] It is evident that existing technologies lack sufficient discussion on the circulating current characteristics and suppression issues of M3C under asymmetrical bridge arm inductance conditions. Existing circulating current suppression strategies are all designed for symmetrical bridge arm inductance conditions and cannot effectively suppress the circulating current component introduced by asymmetrical bridge arm inductance. Summary of the Invention

[0007] To address the aforementioned technical problems in the prior art, this invention aims to provide a method for suppressing circulating current under M3C bridge arm inductance asymmetry, thereby suppressing the circulating current component introduced by M3C when the bridge arm inductance is asymmetrical.

[0008] Specifically, the technical solution is as follows:

[0009] Perform circulating current control, grid-side control, and machine-side control on the M3C;

[0010] The circulating current control output, grid-side control output, and machine-side control output are combined and modulated to obtain the switching signals of each submodule, thereby controlling the switching of the submodule.

[0011] The circulating flow control includes:

[0012] Collect the current of each arm of the M3C bridge; perform a double αβ0 coordinate transformation on each arm current to obtain the decoupled circulating current components;

[0013] Calculate the deviation between each circulation component and the corresponding circulation reference value;

[0014] The resonant frequency of the first PR controller is taken as the grid-side frequency, and the resonant frequency of the second PR controller is taken as the machine-side frequency.

[0015] Each deviation value is input into the P controller, the first PR controller, and the second PR controller, and the corresponding outputs are superimposed to obtain the circulating current control output.

[0016] Preferably, the grid-side control includes obtaining the grid-side control output through overall voltage equalization control and current inner loop control in sequence.

[0017] Preferably, the machine-side control includes obtaining the machine-side control output through current inner loop control.

[0018] Preferably, each deviation value is input to the P controller, the first PR controller, and the second PR controller respectively, and the corresponding outputs are superimposed. The corresponding transfer function G(s) is as follows:

[0019] ;

[0020] In the formula, The proportional gain of the P controller, The resonant gain of the first PR controller, The resonant gain of the second PR controller. For the resonant bandwidth, For network-side frequency, For the machine-side frequency, This is the Lagrange operator.

[0021] Preferably, the circulating reference value is obtained through interphase pressure equalization control.

[0022] Furthermore, the inductance asymmetry of the M3C bridge arm is determined by the following method:

[0023] Spectral detection of the circulating components;

[0024] If the amplitudes of the grid-side frequency component and the machine-side frequency component in the circulating current component exceed the corresponding threshold, then the M3C bridge arm inductance is determined to be asymmetrical.

[0025] Compared to existing technologies, the technical solution provided by this invention introduces grid-side and machine-side frequency components on the basis of the original circulating current frequency components when the arm inductance is asymmetrical. It also uses a P+PR composite control method to selectively suppress the circulating current components, which can effectively reduce the amplitude of the new circulating current components caused by the asymmetry of the arm inductance, improve the time-domain waveform of the circulating current components, and reduce the peak and effective values ​​of the circulating current components. This reduces the arm current stress, power device conduction losses and thermal load, and improves the operational reliability and device lifespan of the M3C. Attached Figure Description

[0026] Figure 1 This is a circuit topology diagram of a modular multilevel matrix converter in one embodiment of the present invention.

[0027] Figure 2 This is the network-side loop of the decoupling equivalent circuit diagram of a modular multilevel matrix converter in one embodiment of the present invention.

[0028] Figure 3 This is the machine-side circuit of the decoupling equivalent circuit diagram of a modular multilevel matrix converter in one embodiment of the present invention.

[0029] Figure 4 This is the circulating loop of the decoupling equivalent circuit diagram of a modular multilevel matrix converter in one embodiment of the present invention.

[0030] Figure 5 This is a diagram illustrating the formation mechanism of the additional frequency component in the circulating circuit under the condition of asymmetrical bridge arm inductance in one embodiment of the present invention.

[0031] Figure 6 This is a frequency component composition diagram of the circulating current component under the condition of bridge arm inductance asymmetry in one embodiment of the present invention.

[0032] Figure 7 This is a schematic diagram of a circulating current suppression strategy based on P control under symmetrical conditions of bridge arm inductance in one embodiment of the present invention.

[0033] Figure 8 This is a block diagram of circulating current suppression based on P+PR composite control under the condition of bridge arm inductance asymmetry in one embodiment of the present invention.

[0034] Figure 9 This is a structural block diagram of a circulating flow suppression control system in one embodiment of the present invention.

[0035] Figure 10 This is a structural block diagram of the network-side control in one embodiment of the present invention.

[0036] Figure 11 This is a structural block diagram of phase-to-phase voltage equalization control in one embodiment of the present invention.

[0037] Figure 12 This is a structural block diagram of the machine-side control in one embodiment of the present invention.

[0038] Figure 13 This is a structural block diagram of the intraphase pressure equalization control in an embodiment of the present invention.

[0039] Figure 14 This is a schematic diagram comparing the frequency characteristics of the circulating current component under symmetrical and asymmetrical conditions of the bridge arm inductance in one embodiment of the present invention.

[0040] Figure 15 This is a waveform comparison diagram of the circulating current component in the time domain under symmetrical and asymmetrical conditions of the bridge arm inductance in one embodiment of the present invention.

[0041] Figure 16 This is a schematic diagram comparing the suppression effect of circulating current spectrum before and after P+PR control in one embodiment of the present invention.

[0042] Figure 17 This is a schematic diagram comparing the time-domain waveforms of the circulating current before and after P+PR control in one embodiment of the present invention.

[0043] Figure 18 This is a schematic diagram comparing the circulating current frequency characteristics under symmetrical and asymmetrical conditions of the bridge arm inductance in one embodiment of the present invention.

[0044] Figure 19 This is a schematic diagram comparing the circulating current time-domain waveforms under symmetrical and asymmetrical conditions of the bridge arm inductance in one embodiment of the present invention.

[0045] Figure 20 This is a schematic diagram comparing the suppression effect of circulating current spectrum before and after P+PR control in one embodiment of the present invention.

[0046] Figure 21 This is a schematic diagram comparing the time-domain waveforms of the circulating current before and after P+PR control in one embodiment of the present invention. Detailed Implementation

[0047] The technical solutions provided by the present invention will be further described in detail below with reference to the embodiments and accompanying drawings.

[0048] Example 1:

[0049] This embodiment proposes a circulating current suppression method under M3C bridge arm inductance asymmetry, mainly applied to suppress newly generated circulating current components under M3C bridge arm inductance asymmetry conditions. This circulating current suppression method can reduce the peak and effective values ​​of circulating current, reduce bridge arm current stress, power device conduction losses and thermal load, and improve the operational reliability and lifespan of M3C devices.

[0050] I. Analysis of circulating current characteristics under asymmetrical inductance conditions of M3C bridge arm.

[0051] like Figure 1 As shown, the M3C consists of nine bridge arms, each of which comprises N cascaded full-bridge submodules (SM1 to SM2). N Each one consists of four transistors S a1 S a2 S b1 and S b2 It consists of a bridge arm inductor connected in series, used to connect the grid-side AC system and the machine-side AC system. (See diagram.) , and These are the three-phase voltages on the grid side, corresponding to phase a, phase b, and phase c voltages; , and These are the three-phase voltages on the machine side, corresponding to phase u, phase v, and phase w. , and This represents the three-phase current on the grid side, corresponding to phase a current, phase b current, and phase c current; , and This represents the three-phase current on the machine side, corresponding to the u-phase current, v-phase current, and w-phase current; ( ; () represents the output voltage of the bridge arm cascaded submodule. Let x be the bridge arm current. For the bridge arm inductance of the xy-axis, This represents the voltage difference between the neutral point N of the grid-side AC system and the neutral point W of the machine-side AC system, i.e., the common-mode voltage of the two AC systems.

[0052] according to Figure 1 The M3C topology shown is used to establish the system state equations of M3C based on Kirchhoff's voltage law, as follows:

[0053] ;

[0054] In the formula, For time, This is the differential symbol.

[0055] At this point, the bridge arm current can be... Indicates the current in the foundation arm. Additional circulating current generated by bridge arm inductance asymmetry The sum, the formula is as follows:

[0056] ;

[0057] ;

[0058] At the same time, when the bridge arm inductance is asymmetrical, the bridge arm inductance This can be expressed as the rated bridge arm inductance. With inductance disturbance The sum, the formula is as follows:

[0059] .

[0060] bridge arm current and bridge arm inductor Substituting the expression into the system state equation and rearranging, we obtain the system state equation under the condition of unbalanced bridge arm inductance, as follows:

[0061] ;

[0062] Compared to the case where the bridge arm inductance is symmetrical, the system state equation adds three additional voltage terms, corresponding to the basic bridge arm inductance. Acting on the rate of change of additional circulation The resulting induced voltage and bridge arm inductance disturbance Rate of change of current in the foundation arm The induced voltage and the inductance disturbance of the bridge arm. Acting on the rate of change of additional circulation The induced voltage is due to the inductance disturbance of the bridge arm. Typically much smaller than the inductance of the foundation bridge arm. And additional circulation Relative to the base bridge arm current The disturbance caused by the bridge arm inductance is relatively small. With additional circulation The second-order small quantities generated by the combined effect are negligible. Therefore, the simplified state equation of the system under the condition of bridge arm inductance asymmetry is as follows:

[0063] ;

[0064] Perform a double αβ0 coordinate transformation on the simplified (ignoring second-order minor terms) system state equations, i.e., left multiply. right multiplication The transformed formula is as follows:

[0065] ;

[0066] In the formula, the first term on the left side of the equal sign, This represents the α-axis component of the grid-side voltage. This represents the β-axis component of the grid-side voltage. This represents the α-axis component of the machine-side voltage. The first term represents the β-axis component of the machine-side voltage; the second term on the left side of the equation is the result of the common-mode voltage of the two AC systems after double αβ0 transformation; the first term on the right side of the equation is the cascaded H-bridge of 9 bridge arms (excluding bridge arm inductances). The output voltage is the result after double αβ0 transformation; the second term on the right side of the equation is the basic bridge arm current. In the basic bridge arm inductance The induced voltage on the bridge is the result after double αβ0 transformation; the third term on the right side of the equation is the circulating current when the bridge arm inductance is unbalanced. The induced voltage on the basic bridge arm inductor L is the result after double αβ0 transformation; the fourth term on the right side of the equation is the circulating current when the bridge arm inductance is asymmetrical. In bridge arm inductance disturbance The result of the induced voltage on the surface after double αβ0 transformation.

[0067] This is decomposed into three parts: grid-side component, machine-side component, and circulating current component, thereby establishing the M3C decoupling control equivalent circuit, such as... Figures 2 to 4 As shown, the formula is as follows:

[0068] ;

[0069] ;

[0070] ;

[0071] In the formula, This represents the α-axis component of the grid-side voltage. This represents the β-axis component of the grid-side voltage. This represents the α-axis component of the grid-side current. This represents the β-axis component of the grid-side current. This represents the α-axis component of the machine-side voltage. This represents the β-axis component of the machine-side voltage. This represents the α-axis component of the machine-side current. This represents the β-axis component of the machine-side current.

[0072] Depend on Figure 4 It can be seen that when the bridge arm inductance is asymmetrical, the additional voltage term in the circulating current model... The disturbance caused by the inductance of the bridge arm With the rate of change of the base bridge arm current Jointly determined. Bridge arm inductance disturbance. As time-invariant parameters, they do not introduce new frequency components, only changing the amplitude, phase, and distribution of relevant frequency components in different decoupling channels. Assuming both the grid-side and machine-side AC systems of the M3C operate in an ideal sinusoidal state, with the grid-side current containing only grid-side frequency components and the machine-side current containing only machine-side frequency components, then the foundation arm current... It only includes two current frequencies: grid-side frequency component and machine-side frequency component; for the foundation arm current... Differentiation does not change its frequency composition. Therefore, the additional voltage term... It includes network-side frequency components and machine-side frequency components. For example... Figure 5 As shown, taking the αα circulating circuit as an example, when the bridge arm inductance is asymmetrical, the rate of change of the same base bridge arm current is... Different induced voltages are generated across the inductors of different bridge arms. This voltage difference is represented as an additional voltage term in the dual αβ0 coordinate system. This alters the original voltage balance between bridge arms, causing a redistribution of bridge arm currents and introducing grid-side and machine-side frequency components that were not present under the original symmetrical operating conditions into the circulating current.

[0073] like Figure 6 As shown, when the bridge arm inductances are symmetrical, the circulating current component... Circulation component Circulation component and circulation components Mainly includes , , and Four characteristic frequency components; when the bridge arm inductance is asymmetrical, the circulating current component retains the aforementioned inherent frequency components, and adds grid-side frequency components and machine-side frequency components. Among them, For network-side frequency, This refers to the machine-side frequency.

[0074] II. M3C Circulation Suppression Method.

[0075] like Figure 7 As shown, when the bridge arm inductances are symmetrical, the circulating current of M3C mainly includes , , and The four frequency components can be effectively suppressed using a conventional proportional P controller (P controller).

[0076] However, when the bridge arm inductance is asymmetrical, new grid-side frequency components and machine-side frequency components are added to the circulating current component. Conventional P controllers cannot suppress these two types of newly added components without steady-state error. Figure 8As shown, this embodiment introduces two resonant terms based on a conventional P controller to form a composite circulating current suppression strategy based on a P+PR composite controller: a proportional controller is connected in parallel in the circulating current control loop, and the resonant frequency is the grid-side frequency. The first proportional resonant controller (first PR controller) and the resonant frequency are the machine-side frequencies. The second proportional resonant controller (second PR controller) superimposes the deviation signals between the circulating current reference value and the measured value after they have been adjusted by the three controllers respectively, to obtain four outputs for circulating current control. , , and This allows for the selective suppression of the grid-side and machine-side frequency components caused by bridge arm inductance asymmetry.

[0077] Specifically, the transfer function of the P+PR composite controller is:

[0078] ;

[0079] in, The proportional gain of the P controller, The resonant gain of the first PR controller, The resonant gain of the second PR controller. For the resonant bandwidth, This refers to the Laplace operator. The transfer function. The process involves inputting each deviation value into the P controller, the first PR controller, and the second PR controller, and then superimposing the corresponding outputs.

[0080] The overall structural block diagram of the circulation suppression control system is as follows: Figure 9 As shown in the figure, the outputs of the grid-side control, circulating current suppression, and machine-side control can be superimposed into a third-order matrix, as shown in the following formula:

[0081] ;

[0082] In the formula, The value is generally 0, so we can use 0 to replace it. Multiply the above expression by 0 on the left. right multiplication This allows us to obtain the modulated wave of the nine bridge arms. Modulated wave The value plus the output of phase equalization , For the bridge arm xy The correction amount of the modulated wave for each submodule. For the same bridge arm xy, the modulated wave Add the Modulation correction amount corresponding to each submodule This will give the first xyth segment of the bridge arm. Modulation wave of each submodule Finally, the modulated waves of all submodules are... The switching signal of each submodule is obtained by performing unipolar frequency doubling modulation. The output voltage of the bridge arm is controlled by controlling the switching transistor, thereby suppressing the new circulating current component under the condition of bridge arm inductance asymmetry.

[0083] In the circulating current suppression control system, the grid-side control block diagram is as follows: Figure 10 As shown, this includes overall voltage equalization control and inner-loop current control. Grid-side control enables effective tracking of the grid-side current, ensuring it meets operational requirements; simultaneously, the outer-loop overall voltage equalization control determines the system's energy transfer level—100 MW in this example. (See figure.) This refers to the common-mode components obtained after the voltages of the nine bridge arm capacitors of the M3C are transformed by double αβ0. Common-mode components Reference value, The d-axis component of the grid-side current. d-axis component Reference value, The d-axis component of the grid-side current. q-axis component Reference value, The d-axis component of the grid-side voltage. This represents the q-axis component of the grid-side voltage. and This is the network-side control output in the dq0 coordinate system. and The output of the network-side control in the αβ0 coordinate system is the modulated wave used to synthesize and modulate the switching signal. i.e., network-side frequency .

[0084] The overall grid-side control process is as follows: In the outer loop overall voltage control, the reference value of the common-mode component of the bridge arm capacitor voltage is... Subtract the actual value of the common-mode component The error is then processed by PI control to obtain the reference value of the d-axis component of the grid-side current. In inner-loop current control, the reference value of the d-axis component of the grid-side current. Subtract the actual value of the d-axis component of the grid-side current A voltage output is then obtained through a PI controller. Subtract the voltage value and add the decoupling feedforward term. The d-axis output of the network side control is then obtained. Reference value of q-axis component of grid-side current Subtract the actual value of the q-axis component of the grid-side current A voltage output is then obtained through a PI controller. Subtract the voltage value and subtract the decoupling feedforward term. Then the q-axis output of the network side control is obtained, and the output is... and The network-side control output in the αβ0 coordinate system is obtained by performing a transformation of dq0 / αβ0. and .

[0085] The above-described double αβ0 transformation of the bridge arm capacitor voltage is given by the following formula:

[0086] ;

[0087] In the formula, For bridge arm The capacitor voltage, , , and To obtain the contents of the double αβ0 transformation and Components of frequency fluctuation, and To obtain the contents of the double αβ0 transformation Components of frequency fluctuation, and To obtain the contents of the double αβ0 transformation The components of frequency fluctuation.

[0088] In the circulating current suppression control system, the control block diagram for phase-to-phase voltage equalization control that generates each circulating current reference value is as follows: Figure 11 As shown. Phase-to-phase voltage equalization control can achieve energy balance among the nine arms of the M3C, that is, the overall capacitor voltage of the nine arms. It is balanced. In the diagram, the voltage component of the capacitor voltage is taken. Voltage components Voltage components and voltage components Construct a column matrix and The separation of the two frequencies yields the result of the separated voltage components. Separate voltage components Separate voltage components and separate voltage components The resulting column matrix is ​​formulated as follows:

[0089] ;

[0090] Among them, the voltage components are separated. and separate voltage components Contains Frequency components, separate voltage components and separate voltage components Contains Frequency components.

[0091] The interphase pressure equalization control process is as follows:

[0092] First, it contains 2ω m Frequency component capacitor voltage balance control, also known as horizontal capacitor voltage balance.

[0093] Subtract the voltage component from the zero matrix of the same order. and voltage components The constructed column matrix is ​​then subjected to PI control to obtain the power deviation. This deviation is then multiplied by the expression for the grid-side voltage in the αβ coordinate system to obtain the reference value for this circulating current component. The expression for the grid-side voltage in the αβ coordinate system is as follows:

[0094] .

[0095] Secondly, it contains 2ω s Capacitor voltage balance control based on frequency components, i.e., capacitor voltage balance in the vertical direction. This involves subtracting the voltage component from the zero matrix of the same order. and voltage components The resulting column matrix is ​​then subjected to PI control to obtain the power deviation. This deviation is then multiplied by the expression for the machine-side voltage in the αβ coordinate system to obtain the reference value for this circulating current component. The expression for the machine-side voltage in the αβ coordinate system is as follows:

[0096] .

[0097] Finally, it contains Frequency components and Capacitor voltage balance control for frequency components, i.e., capacitor voltage balance in the diagonal and anti-diagonal directions. Subtracting the zero matrix of the same order from the voltage components... Separate voltage components Separate voltage components and separate voltage components The power deviation is obtained by performing PI control on the constructed second-order matrix, and then multiplied by the voltage matrix. The voltage matrix formula is as follows:

[0098] .

[0099] Perform the multiplication result The corresponding transformation yields the reference value for this circulating current component. This part of the control can achieve capacitor voltage balance in the diagonal and anti-diagonal directions, i.e., it includes... and Balance the capacitor voltage due to frequency fluctuations.

[0100] By superimposing the reference values ​​of the three circulation components mentioned above, a second-order matrix composed of the reference values ​​of each circulation component is obtained, as shown in the following formula:

[0101] .

[0102] like Figure 8 As shown, the circulating current component deviation value is obtained by subtracting the reference value of each circulating current component from the phase-to-phase voltage equalization control output and the corresponding actual circulating current value. The grid-side frequency component is used as the resonant frequency of the first PR controller, and the machine-side frequency component is used as the resonant frequency of the second PR controller. Each deviation value is input to the P controller, the first PR controller, and the second PR controller respectively, and the corresponding outputs are superimposed to obtain the circulating current control output. The formula is as follows:

[0103] .

[0104] In the circulating current suppression control system, the machine-side control block diagram is as follows: Figure 12 As shown, the machine-side control can achieve zero steady-state error tracking of the machine-side current. (See figure.) This represents the d-axis component of the machine-side current. d-axis component Reference value, Let be the q-axis component of the machine-side current. q-axis component Reference value, This represents the d-axis component of the machine-side voltage. This represents the q-axis component of the machine-side voltage. and This refers to the machine-side control output in the dq0 coordinate system. and The machine-side control output in the αβ0 coordinate system is a modulated wave used to synthesize and modulate the switching signal.

[0105] The overall control process on the machine side is as follows: In current control, the reference value of the d-axis component of the machine side current. Subtract the actual value of the d-axis component of the machine-side current A voltage output is then obtained through a PI controller. Subtract the voltage value and add the decoupling feedforward term. The machine-side control d-axis output is then obtained. Reference value of q-axis component of machine-side current Subtract the actual value of the q-axis component of the machine-side current A voltage output is then obtained through a PI controller. Subtract the voltage value and subtract the decoupling feedforward term. The machine-side control q-axis output is then obtained. ,Will and The machine-side control output in the αβ0 coordinate system is obtained by performing a transformation of dq0 / αβ0. and .

[0106] In the circulating current suppression control system, the block diagram of the intra-phase voltage equalization control is as follows: Figure 13 As shown, intra-phase voltage equalization control can achieve energy balance among sub-modules within the bridge arm. (See figure.) The first of the bridge arm xy The actual capacitor voltage values ​​of each submodule, sgn(i xy This refers to an AC current signal after the peak-to-peak value of the bridge arm current has been reduced to within ±1. The first of the bridge arm xy The correction value of the modulation wave of each submodule. The intra-phase voltage equalization control process is as follows: the capacitor voltages of N submodules within the bridge arm xy are added together and the average value is obtained. Then, this average value is compared with the capacitor voltage of each submodule within the bridge arm. The difference is subtracted, and the resulting error is controlled by P before being compared with the sgn(i) of the corresponding bridge arm xy. xy By multiplying the signals, the correction value of the modulated wave of each submodule in the bridge arm xy can be obtained. .

[0107] like Figure 9 As shown, the modulated wave is composed of the reference values ​​of the bridge arm output voltages of the combined circulating current control output, grid-side control output, and machine-side control output. In addition, the modulation wave correction values ​​of each submodule of the phase equalization control output are also included. The modulated wave of each submodule is obtained. And perform single-pole frequency doubling modulation to obtain the switching signals of each submodule.

[0108] The circulation suppression method in this embodiment specifically includes the following steps:

[0109] Step S1: Collect the bridge arm current of each bridge arm of the M3C Then, a double αβ0 coordinate transformation is performed to obtain the decoupled circulation components. Circulation component Circulation component and circulation components .

[0110] Step S2: Determine the additional frequency component introduced into the circulating current by the bridge arm inductance asymmetry, i.e., the grid-side frequency component. and machine-side frequency components In this embodiment, the network-side frequency component... Machine-side frequency components Preferably, the circulating current component can also be spectrum detected; when the grid-side frequency component in the circulating current is detected... Components and machine-side frequency components When the amplitude exceeds the set threshold, the M3C is determined to be in a bridge arm inductance asymmetry condition, and the first PR controller and the second PR controller are activated.

[0111] Step S3: Calculate the deviation between the reference value and the corresponding measured value of each circulating component; send the deviation values ​​to the proportional controller, the first PR controller, and the second PR controller respectively, and superimpose the three outputs to obtain the circulating control output. .

[0112] Step S4: Output the circulating current control. , Network-side control output and machine-side current output The bridge arm voltage reference signal is synthesized and modulated to generate switching signals for each submodule, controlling the switching of the submodules and thus suppressing the grid-side frequency component introduced by the bridge arm inductance asymmetry. and machine-side frequency components .

[0113] It should be noted that the resonant frequencies of the first PR controller and the second PR controller correspond to the system operating conditions and are respectively the grid-side frequency components. and machine-side frequency components ; proportional gain Resonant gain and resonant gain The proportional gain can be tuned according to the system damping requirements and stability constraints. In this embodiment, the proportional gain is... Resonant gain resonant bandwidth .

[0114] Example 2:

[0115] To verify the theoretical analysis results of Example 1 and the effectiveness of the proposed circulating current suppression method, an M3C simulation model was built on the experimental platform in this example. System network-side frequency. Set to 50Hz, machine-side frequency The frequency is set to 37Hz and the rated transmission power is 100MW. The detailed parameters of the model are shown in Table 1.

[0116] Table 1 M3C Simulation Parameters

[0117] .

[0118] To analyze the impact of bridge arm inductance asymmetry on the circulating current characteristics of the M3C, while keeping other bridge arm inductance parameters constant, the bridge arm au inductance was increased by 20% and the bridge arm bv inductance was decreased by 20%, establishing an operating condition with asymmetrical bridge arm inductance. The circulating current component was then analyzed. Circulation component Circulation component and circulation components The grid-side and machine-side currents were analyzed using FFT (Fast Fourier Transform) and compared with the results under the condition of symmetrical bridge arm inductance. Based on this, following the method described in Example 2, a P+PR composite controller (including a first PR controller and a second PR controller for the newly added frequency components 50Hz and 37Hz) was added to the circulating current control loop, and its circulating current suppression performance was verified by simulation using Platform 1.

[0119] The FFT analysis results of the four circulating current components under symmetrical and asymmetrical bridge arm inductance conditions are shown in the figure below, which compares the frequency characteristics of the circulating current components under symmetrical (Sym) and asymmetrical (Asym) conditions. Figure 14 As shown in the figure, when the bridge arm inductances are symmetrical, the amplitudes of the 37Hz and 50Hz components in the circulating current are close to 0, and the main frequency components are 24, 63, 124 and 137Hz; when the bridge arm inductances are asymmetrical, obvious 37Hz and 50Hz components appear in the circulating current, and the amplitudes are relatively large. The simulation results are consistent with the theoretical analysis above.

[0120] The waveforms of the circulating current component in the time domain under symmetrical and asymmetrical conditions of the bridge arm inductance are shown in the figure below. Figure 15 As shown in the figure, the amplitude of the circulating current component increases significantly when the bridge arm inductors are asymmetrical compared to when they are symmetrical. This new circulating current component affects the on-state stress, switching stress, and thermal stress of the power devices; simultaneously, the asymmetrical bridge arms lead to uneven distribution of the circulating current among the different arms, causing localized overheating.

[0121] The effect of adding P+PR composite control on the suppression of circulating current spectrum is as follows: Figure 16 As shown in the figure, after adding the PR controller, the 50Hz and 37Hz frequency components in most circulating current components are effectively suppressed, with the frequency components with larger original amplitudes showing a more significant suppression effect. Due to the coupling between the circulating current components in the dual αβ0 coordinate system, the suppression effect of the same frequency component varies in different circulating current channels. Overall, the P+PR composite controller can effectively reduce the new circulating current components caused by bridge arm inductance asymmetry.

[0122] Comparison of the time-domain waveforms of the four circulating current components before and after adding P+PR control, for example Figure 17 As shown in the figure, after adding P+PR control, the peak values ​​of all four circulating current waveforms are reduced, with the circulating current component... The reduction effect was most significant, with the amplitude decreasing by approximately 20A. Reducing the amplitude of the newly added circulating current component and improving the circulating current waveform through P+PR composite control helps reduce stress on power devices and improve system reliability.

[0123] To further verify the correctness of the theoretical analysis and the effectiveness of the proposed circulating current suppression method, an M3C model was built on the simulation platform (Platform 2). The model parameters are shown in Table 1. The FFT analysis results of the circulating current components under symmetrical and asymmetrical conditions of the bridge arm inductance in the simulation platform, i.e., the comparison diagram of the circulating current frequency characteristics, are shown below. Figure 18 As shown in the diagram, the time-domain waveform comparison of the circulating current is as follows: Figure 19 As shown. By Figure 18 , Figure 19 It can be seen that when the bridge arm inductance is asymmetrical compared to when the bridge arm inductance is symmetrical, the amplitudes of the 37Hz and 50Hz frequency components in the circulating current increase significantly, and the amplitude of the circulating current time-domain waveform increases, which is consistent with the simulation results of Platform 1.

[0124] The FFT analysis results of the circulation components before and after adding P+PR control on Platform 2, i.e., the comparison diagram of the circulation spectrum suppression effect, are shown below. Figure 20 As shown in the diagram, the time-domain waveform comparison of the circulating current is as follows: Figure 21 As shown. By Figure 20 , Figure 21 It can be seen that under bridge arm inductance asymmetry, the introduction of the PR controller effectively suppresses most of the 37Hz and 50Hz frequency components in the circulating current. A slight increase in the amplitude of the 37Hz component in some circulating current components is observed, mainly related to the frequency coupling characteristics between different circulating current channels, discretization errors in real-time simulation, controller digital implementation errors, and simulation step size limitations. Overall, the simulation results on Platform 2 verify that the P+PR control method proposed in this invention can effectively suppress the new circulating current frequency components caused by bridge arm inductance asymmetry.

[0125] It is evident that the real-time simulation results of Platform 2 are in good agreement with those of Platform 1, jointly verifying the correctness of the M3C circulating current frequency change and the coupling mechanism of current frequencies on both sides under the condition of bridge arm inductance asymmetry. At the same time, it verifies the effectiveness of the P+PR composite control method provided by this invention in suppressing the newly added circulating current component.

[0126] As can be seen from the above embodiments and accompanying drawings, compared with the prior art, the technical solution provided by the present invention introduces grid-side frequency components and machine-side frequency components on the basis of the original circulating current frequency components when the bridge arm inductance is asymmetrical. It also uses the P+PR composite control method to achieve selective suppression of the circulating current components, which can effectively reduce the amplitude of the new circulating current components caused by the asymmetry of the bridge arm inductance, improve the time-domain waveform of the circulating current components, and reduce the peak value and effective value of the circulating current components. This reduces the bridge arm current stress, power device conduction losses and thermal load, and improves the operational reliability of the M3C and the service life of the devices.

Claims

1. A method for suppressing circulating current under M3C bridge arm inductance asymmetry, characterized in that, include: Perform circulating current control, grid-side control, and machine-side control on the M3C; The circulating current control output, grid-side control output, and machine-side control output are combined and modulated to obtain the switching signals of each submodule, thereby controlling the switching of the submodule. The circulating flow control includes: Collect the current of each arm of the M3C bridge; perform a double αβ0 coordinate transformation on each arm current to obtain the decoupled circulating current components; Calculate the deviation between each circulation component and the corresponding circulation reference value; The resonant frequency of the first PR controller is taken as the grid-side frequency, and the resonant frequency of the second PR controller is taken as the machine-side frequency. Each deviation value is input into the P controller, the first PR controller, and the second PR controller, and the corresponding outputs are superimposed to obtain the circulating current control output.

2. The circulating current suppression method under M3C bridge arm inductance asymmetry according to claim 1, characterized in that, The grid-side control includes obtaining the grid-side control output through overall voltage equalization control and current inner loop control in sequence.

3. The circulating current suppression method under M3C bridge arm inductance asymmetry according to claim 1, characterized in that, The machine-side control includes obtaining machine-side control output through current inner loop control.

4. The circulating current suppression method under M3C bridge arm inductance asymmetry according to claim 1, characterized in that, Each deviation value is input into the P controller, the first PR controller, and the second PR controller, and the corresponding outputs are superimposed. The corresponding transfer function G(s) is as follows: ； In the formula, The proportional gain of the P controller, The resonant gain of the first PR controller, The resonant gain of the second PR controller. For the resonant bandwidth, For network-side frequency, For the machine-side frequency, This is the Lagrange operator.

5. The circulating current suppression method under M3C bridge arm inductance asymmetry according to claim 1, characterized in that, The circulating reference value is obtained through interphase pressure equalization control.

6. The circulating current suppression method under M3C bridge arm inductance asymmetry according to claim 1, characterized in that, The M3C bridge arm inductance asymmetry is determined by the following method: Spectral detection of the circulating components; If the amplitudes of the grid-side frequency component and the machine-side frequency component in the circulating current component exceed the corresponding threshold, then the M3C bridge arm inductance is determined to be asymmetrical.

Images