Transient current control strategy based on cut-off negative feedback for grid-forming type cascaded H-bridge converter

Abstract

The invention discloses a grid construction type cascaded H-bridge converter transient current control strategy based on cut-off negative feedback, and belongs to the technical field of grid-connected conversion of electrical engineering. The control strategy comprises the following steps: (1) power loop control; (2) current cut-off negative feedback control; and (3) modulating wave distribution. Compared with the prior art, the method is only started in the fault overcurrent period, the normal networking control performance is not affected, and the maximum networking support performance in the fault period can be achieved through reasonable parameter configuration.

Description

Technical Field

[0001] The invention belongs to the technical field of grid-connected current conversion in electrical engineering, and in particular relates to a transient current control strategy of a grid-connected cascade H-bridge converter based on cut-off negative feedback. Background Art

[0002] With the access of a high proportion of renewable energy to the power grid, the power grid is gradually tending towards weak inertia and low strength characteristics. The cascaded H-bridge converter adopts grid-type control, which has a certain supporting effect on the power grid. Its advantages include modular structure, easy expansion, good redundancy, ability to output multi-level step waves, low harmonic content, saving the cost and volume of the filter, and better operating stability under weak power grids. In addition, the cascaded H-bridge converter adopts non-inner-loop grid-type control, which is closer to the characteristics of synchronous generators. Therefore, it has very important research value in the field of grid-connected conversion.

[0003] However, the load limitation of the grid-forming converter restricts its grid-forming support capability, and the transient current limiting control of the converter is very critical, especially the low switching frequency and decentralized controller characteristics of the cascaded H-bridge converter, which make its transient overcurrent problem more severe. At present, there have been relevant studies on transient current limiting control of grid-forming inverters. The literature is "L. Huang, C. Wu, D. Zhou and F. Blaabjerg, "A Power-Angle-Based Adaptive Overcurrent Protection Scheme for Grid-Forming Inverter Under Large Grid Disturbances," IEEE Transactions on Industrial Electronics, vol. 70, no. 6, pp. 5927-5936, June 2023, DOI 10.1109 / TIE.2022.3199906." 10.1109 / TIE.2022.3199906) uses power angle limitation to indirectly limit current, but this strategy relies on a phase-locked loop and essentially adopts a grid-following mode during a fault.

[0004] The document "T.Qoria, F.Gruson, F.Colas, X.Kestelyn, and X.Guillaud, "Current limiting algorithms and transient stability analysisof grid-forming VSCs," Electric Power Syst.Res., vol.189, 2020, Art.no.106726, ISSN 0378-7796." proposes a control strategy for inner-loop current command limitation, which effectively limits the fault current. However, this method is aimed at grid control with an inner loop and is difficult to apply to grid control without an inner loop.

[0005] The document “J.Wang and X.Zhang,″Active Power and Voltage Cooperative Control for Improving Fault Ride-Through Capability of Grid-Forming Converters,″IEEE Transactions on Industrial Electronics,DOI 10.1109 / TIE.2023.3340202.” proposes a method of superimposing frequency deviation and current deviation on power and voltage set values ​​to achieve current limitation and grid forming support, but there is a response speed limited by inertia.

[0006] In summary, the existing solutions still have the following disadvantages:

[0007] 1) It often degenerates into the grid-following control mode during faults, making it difficult to effectively support the grid;

[0008] 2) Most of them are based on the network control framework with inner loops, which makes it difficult to directly apply them to network control without inner loops;

[0009] 3) The response speed is restricted by the external loop, and the transient current limiting effect is poor. Summary of the invention

[0010] The problem to be solved by the present invention is to overcome the limitations of the above-mentioned scheme and propose a transient current control strategy for a meshed cascade H-bridge converter based on cutoff negative feedback. This control strategy only takes effect when the current exceeds the limit value, does not affect the normal meshing performance, and can achieve maximum support performance during faults through parameter configuration.

[0011] In order to solve the technical problem of the present invention, the present invention provides a transient current control strategy of a grid-connected cascade H-bridge converter based on cutoff negative feedback, wherein the cascade H-bridge converter is a three-phase grid-connected converter, and each phase includes N identical H-bridge units, and any one of the H-bridge units is denoted as H-bridge unit T Xi , X is the phase sequence, X = A, B, C; i is the serial number of the H-bridge unit, i = 1, 2..., N; all H-bridge units T in phase A, phase B and phase C Xi The AC output ends of the three cell strings are connected in series to form three cell strings. One end of the three cell strings is connected together to form a common point, and the other end is connected through the grid-side filter inductor L f Connected to a three-phase star-connected power grid;

[0012] The control strategy includes power loop control, current cutoff negative feedback control and modulation wave distribution, and includes the following steps:

[0013] Step 1: Power loop control

[0014] Step 1.1, sample the actual value of the three-phase grid-connected point voltage and record it as the three-phase grid-connected voltage v pcc_A , v pcc_B , v pcc_C , and phase-lock it to obtain the grid-connected point voltage amplitude V pccM and grid frequency ω g , for the grid frequency ω g Integrate to get the grid voltage phase angle θ PLL ; For three-phase grid-connected voltage v pcc_A , v pcc_B , v pcc_C The coordinate transformation from the three-phase stationary coordinate system to the two-phase stationary coordinate system is performed to obtain the grid connection point voltage of the two-phase stationary coordinate system and record it as the two-phase grid connection voltage v pcc_α , v pcc_β ; Sample the actual value of the three-phase output current of the cascaded H-bridge converter and record it as the three-phase output current i g_A ,i g_B ,i g_C And through the coordinate transformation from the three-phase stationary coordinate system to the two-phase stationary coordinate system, the output current of the cascaded H-bridge converter in the two-phase stationary coordinate system is obtained and recorded as the two-phase output current i g_α ,i g_β ;

[0015] The instantaneous active power P and instantaneous reactive power Q of the cascaded H-bridge converter are calculated according to the instantaneous power calculation formula. The instantaneous active power P and instantaneous reactive power Q of the cascaded H-bridge converter are low-pass filtered to obtain the filtered active power P fil And the filtered reactive power Q fil ;

[0016] Step 1.2, according to the filtered active power P fil And the filtered reactive power Q fil , the grid-type control output frequency ω is obtained through the active power loop calculation formula GFM , for the network type control output frequency ω GFM Integrate to obtain the grid-type control output phase angle θ GFM ; The grid-type control output amplitude V is obtained through the reactive power loop calculation formula GFM ;

[0017] Step 1.3, control the output amplitude V according to the network type GFM And the grid-type control output phase angle θ GFM The initial modulation wave voltage e of phase A, phase B, and phase C of the cascaded H-bridge converter is calculated A , e B , e C ;

[0018] Step 2: Current cutoff negative feedback control

[0019] Detect three-phase output current i g_A ,i g_B ,i g_C The amplitude of the output current is recorded as the average amplitude of the output current I amp , calculate the current amplitude limit value I lim , I lim =min(I amp , I dcr ), where I dcr is the current cutoff amplitude of the cutoff negative feedback control;

[0020] According to the three-phase output current amplitude I amp and current amplitude limit value I lim , calculate the current cutoff negative feedback control amount Δe B , Δe B , Δe C , and its calculation formula is:

[0021]

[0022] Among them, k cutoffis the proportional coefficient of the cut-off negative feedback, θlead is the cut-off negative feedback control rotation angle, ω ic is the cutoff frequency of low-pass filtering, s is the Laplace operator;

[0023] Step 3: Modulation wave allocation

[0024] According to the initial modulation wave voltage e of phase A, phase B, and phase C of the cascaded H-bridge converter A , e B , e C and current cutoff negative feedback control amount Δe A , Δe B , Δe C , calculate the modulation wave voltage e′ of phase A, phase B and phase C A , e′ B , e′ C , and its calculation formula is:

[0025] e′ A =e A -Δe A

[0026] e′ B =e B -Δe B

[0027] e′ C =e C -Δe C

[0028] According to each H-bridge unit T Xi Output power capability for modulated wave distribution, H-bridge unit T Xi The modulation voltage e Xi For: e Xi =k Xi e′ X , where k Xi For H bridge unit T Xi The power output capacity coefficient.

[0029] Preferably, the instantaneous power calculation formula in step 1.1 is:

[0030] P=v pcc_α i g_α +v pcc_β i g_β

[0031] Q=v pcc_β i g_α -v pcc_α i g_β

[0032] The filtered active power P filAnd the filtered reactive power Q fil The calculation formula is:

[0033]

[0034] Among them, ω c is the cutoff frequency of the low-pass filter.

[0035] Preferably, the active power loop calculation formula in step 1.2 is:

[0036]

[0037] The reactive power loop calculation formula is:

[0038]

[0039] Wherein, Sag is the remote power grid amplitude drop fault detection signal. In normal operation, Sag = 1. When the remote power grid voltage amplitude suddenly drops, Sag = 0. set is the three-phase output active power reference value of the cascaded H-bridge converter, ω n is the rated angular frequency of the three-phase power grid, D p is the frequency droop coefficient, J is the virtual moment of inertia, k ap is the proportional coefficient of the frequency deviation PI regulator, k ai is the integral coefficient of the frequency deviation PI regulator, Q set is the three-phase output reactive power reference value of the cascaded H-bridge converter, V nAmp is the rated phase voltage amplitude of the three-phase power grid, D q is the reactive damping coefficient, K q is the reactive inertia coefficient, E lim It is the output upper limit of the reactive voltage loop.

[0040] Preferably, the initial modulation wave voltage e of the A phase, B phase and C phase of the cascaded H-bridge converter in step 1.3 is A , e B , e C The calculation formulas are:

[0041]

[0042] The beneficial effects of the present invention relative to the prior art are:

[0043] 1. It is only enabled during fault overcurrent and does not affect the normal network control performance;

[0044] 2. Through reasonable parameter configuration, the maximum network support performance can be achieved during faults. BRIEF DESCRIPTION OF THE DRAWINGS

[0045] Figure 1 This is the grid-connected structure of the cascaded H-bridge converter in the embodiment of the present invention.

[0046] Figure 2 It is a simplified diagram of the transient current control strategy of the present invention.

[0047] Figure 3 It is the simulated current waveform of the cascaded H-bridge converter when a remote grid voltage drop fault occurs. DETAILED DESCRIPTION

[0048] The present invention is described in detail below with reference to the accompanying drawings and embodiments.

[0049] Figure 1 It is a cascaded H-bridge converter grid-connected structure in an embodiment of the present invention. As can be seen from the figure, the present invention provides a grid-connected cascaded H-bridge converter transient current control strategy based on cutoff negative feedback, wherein the cascaded H-bridge converter is a three-phase grid-connected converter, and each phase includes N identical H-bridge units, and any one of the H-bridge units is denoted as H-bridge unit T Xi , X is the phase sequence, X = A, B, C; i is the serial number of the H-bridge unit, i = 1, 2..., N; all H-bridge units T in phase A, phase B and phase C Xi The AC output ends of the three cell strings are connected in series to form three cell strings. One end of the three cell strings is connected together to form a common point, and the other end is connected through the grid-side filter inductor L f Connect to a three-phase star-connected grid.

[0050] In addition, Figure 1 It can be seen that in each H-bridge unit T Xi A DC bus capacitor C is connected in parallel to the front end of Xi , and each DC bus capacitor C Xi They are each connected in parallel with a storage battery.

[0051] Figure 2 It is a simplified diagram of the transient current control strategy of the present invention. Figure 2 It can be seen that the transient current control strategy of the present invention includes power loop control, current cutoff negative feedback control and modulation wave distribution, including the following steps:

[0052] Step 1: Power loop control

[0053] Step 1.1, sample the actual value of the three-phase grid-connected point voltage and record it as the three-phase grid-connected voltage v pcc_A , v pcc_B , v pcc_C , and phase-lock it to obtain the grid-connected point voltage amplitude V pccM and grid frequency ω g , for the grid frequency ω g Integrate to get the grid voltage phase angle θPLL ; For three-phase grid-connected voltage v pcc_A , v pcc_B , v pcc_C The coordinate transformation from the three-phase stationary coordinate system to the two-phase stationary coordinate system is performed to obtain the grid connection point voltage of the two-phase stationary coordinate system and record it as the two-phase grid connection voltage v pcc_α , v pcc_β ; Sample the actual value of the three-phase output current of the cascaded H-bridge converter and record it as the three-phase output current i g_A ,i g_B ,i g_C And through the coordinate transformation from the three-phase stationary coordinate system to the two-phase stationary coordinate system, the output current of the cascaded H-bridge converter in the two-phase stationary coordinate system is obtained and recorded as the two-phase output current i g_α ,i g_β .

[0054] The instantaneous active power P and instantaneous reactive power Q of the cascaded H-bridge converter are calculated according to the instantaneous power calculation formula. The instantaneous active power P and instantaneous reactive power Q of the cascaded H-bridge converter are low-pass filtered to obtain the filtered active power P fil And the filtered reactive power Q fil .

[0055] In this embodiment, the instantaneous power calculation formula is:

[0056] P=v pcc_α i g_α +v pcc_β i g_β

[0057] Q=v pcc_β i g_α -v pcc_α i g_β

[0058] In this embodiment, the filtered active power P fil And the filtered reactive power Q fil The calculation formula is:

[0059]

[0060] Among them, ω c is the cutoff frequency of the low-pass filter.

[0061] Step 1.2, according to the filtered active power P fil And the filtered reactive power Q fil , the grid-type control output frequency ω is obtained through the active power loop calculation formula GFM , for the network type control output frequency ω GFM Integrate to obtain the grid-type control output phase angle θ GFM; The grid-type control output amplitude V is obtained through the reactive power loop calculation formula GFM .

[0062] In this embodiment, the active power loop calculation formula is:

[0063]

[0064] In this embodiment, the reactive power loop calculation formula is:

[0065]

[0066] Wherein, Sag is the remote power grid amplitude drop fault detection signal. In normal operation, Sag = 1. When the remote power grid voltage amplitude suddenly drops, Sag = 0. set is the three-phase output active power reference value of the cascaded H-bridge converter, ω n is the rated angular frequency of the three-phase power grid, D p is the frequency droop coefficient, J is the virtual moment of inertia, k ap is the proportional coefficient of the frequency deviation PI regulator, k ai is the integral coefficient of the frequency deviation PI regulator, Q set is the three-phase output reactive power reference value of the cascaded H-bridge converter, V nAmp is the rated phase voltage amplitude of the three-phase power grid, D q is the reactive damping coefficient, K q is the reactive inertia coefficient, E lim It is the output upper limit of the reactive voltage loop.

[0067] Step 1.3, control the output amplitude V according to the network type GFM And the grid-type control output phase angle θ GFM The initial modulation wave voltage e of phase A, phase B, and phase C of the cascaded H-bridge converter is calculated A , e B , e C .

[0068] In this embodiment, the initial modulation wave voltages e of the A phase, B phase, and C phase of the cascaded H-bridge converter are A , e B , e C The calculation formulas are:

[0069] e A =V GFM cos(θ GFM )

[0070]

[0071] Step 2: Current cutoff negative feedback control

[0072] Detect three-phase output current i g_A ,i g_B ,i g_C The amplitude of the output current is recorded as the average amplitude of the output current I amp , calculate the current amplitude limit value I lim , I lim =min(I amp , I dcr ), where I dcr is the current cutoff amplitude of the cutoff negative feedback control;

[0073] According to the three-phase output current amplitude I am p and current amplitude limit value I lim , calculate the current cutoff negative feedback control amount Δe A , Δe B , Δe C , and its calculation formula is:

[0074]

[0075] Among them, k cutoff is the proportional coefficient of the cut-off negative feedback, θlead is the cut-off negative feedback control rotation angle, ω ic is the cutoff frequency of the low-pass filter, and s is the Laplace operator.

[0076] Step 3: Modulation wave allocation

[0077] According to the initial modulation wave voltage e of phase A, phase B, and phase C of the cascaded H-bridge converter A , e B , e C and current cutoff negative feedback control amount Δe A , Δe B , Δe C , calculate the modulation wave voltage e′ of phase A, phase B and phase C A , e′ B , e′ C , and its calculation formula is:

[0078] e′ A =e A -Δe A

[0079] e′ B =e B -Δe B

[0080] e′ C =e C -Δe C

[0081] According to each H-bridge unit T Xi Output power capability for modulated wave distribution, H-bridge unit T Xi The modulation voltage e Xi For: e Xi =k Xi e′ X , where k Xi For H bridge unit T Xi The power output capacity coefficient.

[0082] In this embodiment, ω c =100π,ω n =100π, D p =5066.1, J=0.057, V nAmp =8164,D q =25000, K q =3583,P set =5000000,Q set =0, k ap =21, k ai =8674,E lim =8980,I dcr =572, k cutoff =0.2,θlead=1.5,ω ic =300π, k Xi =1 / 30.

[0083] Figure 3 is the current simulation waveform of the cascaded H-bridge converter when a drop fault occurs in the remote power grid in the embodiment of the present invention. Figure 3 It can be seen that when the simulation runs for 0.1s, the voltage of the remote grid drops to 0.2pu, the fault transient current is effectively limited, and the voltage support effect during the fault is good.

Claims

1. A transient current control strategy for a grid-connected cascade H-bridge converter based on cutoff negative feedback, wherein the cascade H-bridge converter is a three-phase grid-connected converter, and each phase includes N identical H-bridge units, and any one of the H-bridge units is denoted as H-bridge unit T Xi , X is the phase sequence, X = A, B, C; i is the serial number of the H-bridge unit, i = 1, 2..., N; all H-bridge units T in phase A, phase B and phase C Xi The AC output ends of the three cell strings are connected in series to form three cell strings. One end of the three cell strings is connected together to form a common point, and the other end is connected through the grid-side filter inductor L f Connected to a three-phase star-connected power grid; It is characterized in that The control strategy includes power loop control, current cutoff negative feedback control and modulation wave distribution, and includes the following steps: Step 1: Power loop control Step 1.1, sample the actual value of the three-phase grid-connected point voltage and record it as the three-phase grid-connected voltage v pcc_A , v pcc_B , v pcc_C , and phase-lock it to obtain the grid-connected point voltage amplitude V pccM and grid frequency ω g , for the grid frequency ω g Integrate to get the grid voltage phase angle θ PLL ; For three-phase grid voltage v pcc_A , v pcc_B , v pcc_C The coordinate transformation from the three-phase stationary coordinate system to the two-phase stationary coordinate system is performed to obtain the grid connection point voltage of the two-phase stationary coordinate system and recorded as the two-phase grid connection voltage v pcc _ α , v pcc_β ; Sample the actual value of the three-phase output current of the cascaded H-bridge converter and record it as the three-phase output current i g_A ,i g_B ,i g_C And through the coordinate transformation from the three-phase stationary coordinate system to the two-phase stationary coordinate system, the output current of the cascaded H-bridge converter in the two-phase stationary coordinate system is obtained and recorded as the two-phase output current i g_α ,i g_β ; The instantaneous active power P and instantaneous reactive power Q of the cascaded H-bridge converter are calculated according to the instantaneous power calculation formula. The instantaneous active power P and instantaneous reactive power Q of the cascaded H-bridge converter are low-pass filtered to obtain the filtered active power P fil And the filtered reactive power Q fil ; Step 1.2, according to the filtered active power P fi1 And the filtered reactive power Q fi1 , the grid-type control output frequency ω is obtained through the active power loop calculation formula GFM , for the network type control output frequency ω GFM Integrate to obtain the grid-type control output phase angle θ GFM ; The grid-type control output amplitude V is obtained through the reactive power loop calculation formula GFM ; Step 1.3, control the output amplitude V according to the network type GFM And the grid-type control output phase angle θ GFM The initial modulation wave voltage e of phase A, phase B, and phase C of the cascaded H-bridge converter is calculated A , e B , e C ; Step 2: Current cutoff negative feedback control Detect three-phase output current i g_A ,i g_B ,i g_C The amplitude of the output current is recorded as the average amplitude of the output current I amp , calculate the current amplitude limit value I lim , I lim =min(I amp , I dcr ), where I dcr is the current cutoff amplitude of the cutoff negative feedback control; According to the three-phase output current amplitude I amp and current amplitude limit value I lim , calculate the current cutoff negative feedback control amount Δe A , Δe B , Δe C , and its calculation formula is: Among them, k cutoff is the proportional coefficient of the cut-off negative feedback, θlead is the cut-off negative feedback control rotation angle, ω ic is the cutoff frequency of low-pass filtering, s is the Laplace operator; Step 3: Modulation wave allocation According to the initial modulation wave voltage e of phase A, phase B, and phase C of the cascaded H-bridge converter A , e B , e C and current cutoff negative feedback control amount Δe A , Δe B , Δe C , calculate the modulation wave voltage e′ of phase A, phase B and phase C A , e′ B , e′ C , and its calculation formula is: e′ A =e A -Δe A And' B =and B -△e B And' C =and C -△e C According to each H-bridge unit T Xi Output power capability for modulated wave distribution, H-bridge unit T Xi The modulation voltage e Xi For: e Xi =k Xi e′ X , where k Xi For H bridge unit T Xi The power output capacity coefficient.

2. The transient current control strategy of the grid-type cascaded H-bridge converter based on cut-off negative feedback according to claim 1 is characterized in that: The instantaneous power calculation formula described in step 1.1 is: P=v pcc_α i g_α +v pcc_β i g_β Q=v pcc_β i g_α -v pcc_α i g_β The filtered active power P fil And the filtered reactive power Q fil The calculation formula is: Among them, ω c is the cutoff frequency of the low-pass filter.

3. The transient current control strategy of the grid-type cascaded H-bridge converter based on cut-off negative feedback according to claim 1 is characterized in that: The active power loop calculation formula in step 1.2 is: The reactive power loop calculation formula is: Wherein, Sag is the remote power grid amplitude drop fault detection signal. In normal operation, Sag = 1. When the remote power grid voltage amplitude suddenly drops, Sag = 0. set is the three-phase output active power reference value of the cascaded H-bridge converter, ω n is the rated angular frequency of the three-phase power grid, D p is the frequency droop coefficient, J is the virtual moment of inertia, k ap is the proportional coefficient of the frequency deviation PI regulator, k ai is the integral coefficient of the frequency deviation PI regulator, Q set is the three-phase output reactive power reference value of the cascaded H-bridge converter, V nAmp is the rated phase voltage amplitude of the three-phase power grid, D q is the reactive damping coefficient, K q is the reactive inertia coefficient, E lim It is the output upper limit of the reactive voltage loop.

4. The transient current control strategy of the grid-type cascaded H-bridge converter based on cut-off negative feedback according to claim 1 is characterized in that: The initial modulation wave voltage e of phase A, phase B, and phase C of the cascaded H-bridge converter in step 1.3 A , e B , e C The calculation formulas are: e A =V GFM cos(θ GFM )

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