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

Abstract

The application discloses a network-constructing type cascade H-bridge converter transient current control strategy based on cut-off negative feedback, and belongs to the field of grid-connected conversion technology of electrical engineering. The control strategy steps are as follows: (1) power loop control; (2) current cut-off negative feedback control; and (3) modulation wave distribution. Compared with the prior art, the application is only enabled during fault overcurrent, does not affect normal network-constructing control performance, and can realize the maximum network-constructing support performance during the fault through reasonable parameter configuration.

Description

Technical Field

[0001] This invention belongs to the field of grid-connected converter technology in electrical engineering, specifically relating to a transient current control strategy for a grid-connected cascaded H-bridge converter based on cutoff negative feedback. Background Technology

[0002] With the high proportion of new energy sources being connected to the grid, the grid is gradually becoming more characterized by weak inertia and low strength. Cascaded H-bridge converters, using grid-based control, provide certain support to the grid. Their advantages include modular structure that is easy to expand, good redundancy, the ability to output multi-level stepped waves, low harmonic content, saving the cost and volume of filters, and better operational stability under weak grid conditions. In addition, the cascaded H-bridge converters, using grid-based control without inner loops, are closer to the characteristics of synchronous generators. Therefore, they have significant research value in the field of grid-connected converters.

[0003] However, the load limitations of grid-type converters restrict their grid support capabilities. The transient current limiting control of the converter is crucial, especially the low switching frequency and distributed controller characteristics of cascaded H-bridge converters, which make their transient overcurrent problems even more severe. There is currently some research on transient current limiting control for grid-connected inverters. The relevant 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." (L. Huang, C. Wu, D. Zhou and F. Blaabjerg, "Adaptive Overcurrent Protection Scheme for Grid-Forming Inverter Based on Power Angle 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 limiting to indirectly limit current, but this strategy relies on phase-locked loops and essentially adopts a grid-based mode during faults.

[0004] The document "T. Qoria, F. Gruson, F. Colas, X. Kestelyn, and X. Guillaud, "Current limiting algorithms and transient stability analysis of grid-forming VSCs," Electric Power Syst. Res., vol. 189, 2020, Art. no. 106726, ISSN 0378-7796." proposes a control strategy of inner loop current command limiting, which effectively limits the fault current. However, this method is for grid-forming control with inner loop, and is difficult to apply in grid-forming control without 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 value to realize current limiting and grid-forming support, but the response speed is limited by inertia.

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

[0007] 1) mostly degenerate into grid-following control mode during fault, difficult to effectively support the power grid;

[0008] 2) mostly based on grid-forming control framework with inner loop, difficult to be directly applied in grid-forming control without inner loop;

[0009] 3) the response speed is limited by the outer loop, and the transient current limiting effect is not good. SUMMARY

[0010] The problem to be solved by this invention is to overcome the limitations of the above-mentioned solutions and propose a transient current control strategy for grid-type cascaded H-bridge converters based on cutoff negative feedback. This control strategy only works when the current exceeds the limit, does not affect the normal grid performance, and can achieve the maximum support performance during faults through parameter configuration.

[0011] To address the technical problem of this invention, this invention provides a transient current control strategy for a grid-connected cascaded H-bridge converter based on cutoff negative feedback. The cascaded H-bridge converter is a three-phase grid-connected converter, and each phase contains N identical H-bridge units. Any one of these H-bridge units is denoted as H-bridge unit T. Xi X represents the phase sequence, X = A, B, C; i represents the sequence number of the H-bridge unit, i = 1, 2, ..., N; and T represents all H-bridge units T in phases A, B, and C. Xi The AC output terminals are connected in series to form three unit strings. One end of each of the three unit strings is connected together to form a common point, and the other end is connected to 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 allocation, and includes the following steps:

[0013] Step 1, Power Loop Control

[0014] Step 1.1: Sample the actual value of the three-phase grid connection point voltage and record it as the three-phase grid connection voltage v. pcc_A v pcc_B v pcc_C Phase-locked loop is performed to obtain the grid connection point voltage amplitude V. pccM and grid frequency ω g For the power grid frequency ω g Integrating, we obtain the grid voltage phase angle θ. PLL For three-phase grid-connected voltage v pcc_A v pcc_B v pcc_C A coordinate transformation from a three-phase stationary coordinate system to a two-phase stationary coordinate system is performed to obtain the grid-connected voltage in the two-phase stationary coordinate system, which is denoted as the two-phase grid-connected 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 The output current of the cascaded H-bridge converter in the two-phase stationary coordinate system is obtained by coordinate transformation from the three-phase stationary coordinate system to the two-phase stationary coordinate system and denoted as the two-phase output current i. g_α i g_β ;

[0015] The instantaneous active power P and the instantaneous reactive power Q of the cascade H-bridge converter are calculated according to the instantaneous power calculation formula, and the filtered active power P and the filtered reactive power Q are obtained by low-pass filtering the instantaneous active power P and the instantaneous reactive power Q of the cascade H-bridge converter fil fil ;

[0016] Step 1.2, the filtered active power P fil and the filtered reactive power Q fil , the grid-forming control output frequency ω GFM is obtained by an active power loop calculation formula GFM The grid-forming control output phase angle θ GFM is obtained by integrating the grid-forming control output frequency ω GFM ;

[0017] Step 1.3, the initial modulation wave voltages e A , e B , e C of the A phase, the B phase and the C phase of the cascade H-bridge converter are calculated according to the grid-forming control output amplitude V GFM and the grid-forming control output phase angle θ GFM ;

[0018] Step 2, current cut-off negative feedback control

[0019] The amplitudes of the three-phase output currents i g_A , i g_B , i g_C are detected, and the average amplitude is recorded as the output current amplitude I amp The current amplitude limit value I lim is calculated, I lim = min(I amp , I dcr ), wherein I dcr is the current cut-off amplitude of the cut-off negative feedback control;

[0020] The current cut-off negative feedback control amount Δe B , Δe B , Δe C is calculated according to the three-phase output current amplitude I amp and the current amplitude limit value I lim , and the calculation formula is:

[0021]

[0022] Wherein, k cutoff ​is a cut-off negative feedback proportional coefficient, θ lead is a cut-off negative feedback control rotation angle, ω ic is a low-pass filter cut-off frequency, s is a Laplace operator;

[0023] Step 3, modulated wave distribution

[0024] According to the initial modulated wave voltage e A , e B , e C and the current cut-off negative feedback control amount Δe A , Δe B , Δe C , the modulated wave voltage e′ A , e′ B , e′ C of the A phase, the B phase and the C phase are calculated, and the 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 the modulated wave distribution of the power output capability of each H-bridge unit T Xi , the modulated voltage e Xi of the H-bridge unit T Xi is: e Xi = k Xi e′ X , wherein k Xi is the power output capability coefficient of the H-bridge unit T Xi .

[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 filtered reactive power Q fil The calculation formula of the filtered reactive power Q is:

[0033]

[0034] ω c is the cut-off 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 a remote grid amplitude drop fault detection signal, Sag = 1 in normal operation, and Sag = 0 when a remote grid voltage amplitude drop occurs; P 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 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 grid, D q is the reactive power damping coefficient, K q is the reactive power inertia coefficient, E lim is the output upper limit value of the reactive voltage loop.

[0040] Preferably, the calculation formula of the initial modulation wave voltages e A , e B , and e C of the A phase, the B phase, and the C phase of the cascaded H-bridge converter in step 1.3 is respectively:

[0041]

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

[0043] 1. Only enabled during fault overcurrent, without affecting the normal network control performance;

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

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

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

[0047] Figure 3 This is a simulated waveform of the current in a cascaded H-bridge converter when a voltage drop fault occurs in the remote power grid. Detailed Implementation

[0048] The present invention will now be described in detail with reference to the accompanying drawings and embodiments.

[0049] Figure 1 This is the grid-connected structure of the cascaded H-bridge converter in this embodiment of the invention. As shown in the figure, the present invention provides a transient current control strategy for a grid-connected cascaded H-bridge converter based on cutoff negative feedback. The cascaded H-bridge converter is a three-phase grid-connected converter, and each phase contains N identical H-bridge units. Any one of these H-bridge units is denoted as H-bridge unit T. Xi X represents the phase sequence, X = A, B, C; i represents the sequence number of the H-bridge unit, i = 1, 2, ..., N; and T represents all H-bridge units T in phases A, B, and C. Xi The AC output terminals are connected in series to form three unit strings. One end of each of the three unit strings is connected together to form a common point, and the other end is connected to the grid-side filter inductor L. f Connected to a three-phase star-connected power grid.

[0050] In addition Figure 1 It can be seen that in each H-bridge unit T Xi Each of the front ends is connected in parallel with a DC bus capacitor C Xi And each DC bus capacitor C Xi Each of them is connected in parallel with a storage battery.

[0051] Figure 2 This is a simplified diagram of the transient current control strategy of this invention. Figure 2 As can be seen, the transient current control strategy of this invention includes power loop control, current cutoff negative feedback control, and modulation wave allocation, and includes the following steps:

[0052] Step 1, Power Loop Control

[0053] Step 1.1: Sample the actual value of the three-phase grid connection point voltage and record it as the three-phase grid connection voltage v. pcc_A v pcc_B v pcc_C Phase-locked loop is performed to obtain the grid connection point voltage amplitude V. pccM and grid frequency ω g For the power grid frequency ω g Integrating, we obtain the grid voltage phase angle θ.PLL ; three-phase grid-connected voltage v pcc_A , v pcc_B , v pcc_C A coordinate transformation from the three-phase stationary coordinate system to the two-phase stationary coordinate system is performed on the three-phase grid-connected voltage v pcc_α , v pcc_β ; the actual value of the three-phase output current of the cascaded H-bridge converter is sampled and recorded as the three-phase output current i g_A , i g_B , i g_C , and a coordinate transformation from the three-phase stationary coordinate system to the two-phase stationary coordinate system is performed to obtain the two-phase stationary coordinate system output current of the cascaded H-bridge converter and record it as the two-phase output current i g_α , i g_β .

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

[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 calculation formula of the filtered active power P fil and the filtered reactive power Q fil is:

[0059]

[0060] Where 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-forming control output frequency ω GFM is obtained by the active power loop calculation formula, the grid-forming control output frequency ω GFM is integrated to obtain the grid-forming control output phase angle θ GFM; the grid-forming control output amplitude V 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 grid amplitude drop fault detection signal, when normal operation Sag = 1, when the remote grid voltage amplitude drops, Sag = 0; P 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 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 grid, D q is the reactive damping coefficient, K q is the reactive inertia coefficient, E lim is the upper limit value of the output of the reactive voltage loop.

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

[0068] In this embodiment, the calculation formula of the initial modulation wave voltage e A , e B , e C of the A phase, B phase and C phase of the cascaded H-bridge converter is respectively:

[0069] e A = V GFM cos(θ GFM )

[0070]

[0071] Step 2, current cut-off negative feedback control

[0072] detecting three-phase output current i g_A , i g_B , i g_C , and taking the average amplitude thereof as output current amplitude I amp , and calculating current amplitude limit value I lim , I lim = min(I amp , I dcr ), wherein I dcr is a current cut-off amplitude for cutting off negative feedback control;

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

[0074]

[0075] wherein k cutoff is a proportional coefficient for cutting off negative feedback, θlead is a cutting-off negative feedback control rotation angle, ω ic is a low-pass filter cut-off frequency, and s is a Laplace operator.

[0076] Step 3, modulated wave distribution

[0077] According to initial modulated wave voltages e A , e B , e C of A phase, B phase and C phase of the cascaded H-bridge converter and current cut-off negative feedback control amount Δe A , Δe B , Δe C , calculate modulated wave voltages e′ A , e′ B , e′ C of A phase, B phase and C phase, and the 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 The output power capability is used for modulated wave distribution, H-bridge unit T Xi modulation voltage e Xi e Xi =k Xi e′ X , where k Xi H-bridge unit T Xi The power output capability 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 =5,000,000, 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 This is a simulated waveform of the cascaded H-bridge converter current when a drop-out fault occurs in the remote power grid, as described in this embodiment of the invention. Figure 3 As can be seen, the voltage of the remote grid dropped to 0.2 pu after 0.1 s of simulation, the fault transient current was effectively limited, and the voltage support effect during the fault was good.

Claims

1. A network configuration type cascade H-bridge converter transient current control strategy based on cut-off negative feedback, the cascade H-bridge converter being a three-phase grid-connected converter, and each phase comprising N identical H-bridge units, any one of which is denoted as H-bridge unit T Xi , X is a phase sequence, X = A, B, C; i is the serial number of the H-bridge unit, i = 1, 2,..., N; the alternating current output ends of all H-bridge units T Xi in the A-phase, B-phase and C-phase are connected in series with each other, forming three unit strings, one end of the three unit strings being connected together to form a common point, and the other end of each of the three unit strings being connected to a three-phase star-connected power grid through a grid-side filter inductance L f , respectively. Its features are, The control strategy includes power loop control, current cutoff negative feedback control, and modulation wave allocation, and includes the following steps: Step 1, Power Loop Control Step 1.1: Sample the actual value of the three-phase grid connection point voltage and record it as the three-phase grid connection voltage v. pcc_A v pcc_B v pcc_C Phase-locked loop is performed to obtain the grid connection point voltage amplitude V. pccM and grid frequency ω g For the power grid frequency ω g Integrating, we obtain the grid voltage phase angle θ. PLL For three-phase grid-connected voltage v pcc_A v pcc_B v pcc_C A coordinate transformation from a three-phase stationary coordinate system to a two-phase stationary coordinate system is performed to obtain the grid-connected voltage in the two-phase stationary coordinate system, which is denoted as the two-phase grid-connected 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 The output current of the cascaded H-bridge converter in the two-phase stationary coordinate system is obtained by coordinate transformation from the three-phase stationary coordinate system to the two-phase stationary coordinate system and denoted as the two-phase output current i. g_α i g_β ; The instantaneous active power P and the instantaneous reactive power Q of the cascade H-bridge converter are calculated according to the instantaneous power calculation formula, and the filtered active power P and the filtered reactive power Q are obtained by low-pass filtering the instantaneous active power P and the instantaneous reactive power Q of the cascade H-bridge converter fil . fil ​ Step 1.2, based on the filtered active power P fi1 and the filtered reactive power Q fi1 The output frequency ω of the network-type control is obtained through the active power loop calculation formula. GFM The output frequency ω of the network control GFM Integrating, we obtain the phase angle θ of the network control output. GFM The output amplitude V of the grid-type control is obtained through the reactive power loop calculation formula. GFM ; Step 1.3, control the output amplitude V according to the network structure. GFM And the phase angle θ of the output of the network control GFM The initial modulation wave voltage e of phases A, B, and C of the cascaded H-bridge converter was calculated. A e B e C ; Step 2, Current cutoff negative feedback control Detecting three-phase output current i g_A i g_B i g_C The amplitude of the output current is calculated, and its average amplitude is denoted as the output current amplitude I. amp The current amplitude limit value I is calculated. lim I lim =min(I amp I dcr ), where I dcr This is the current cutoff amplitude for the negative feedback control. Based on the three-phase output current amplitude I amp and current amplitude limit value I lim The current cutoff negative feedback control quantity Δe is calculated. A Δe B Δe C The formula for its calculation is: Where, k cutoff θlead is the proportional coefficient for cutting off negative feedback, θlead is the rotation angle for cutting off negative feedback control, and ω is the proportional coefficient. ic is the cutoff frequency of the low-pass filter, and s is the Laplace operator; Step 3, Modulation Wave Assignment Based on the initial modulation wave voltage e of phases A, B, and C of the cascaded H-bridge converter A e B e C and current cutoff negative feedback control quantity Δe A Δe B Δe C The modulation wave voltage e′ of phases A, B, and C is calculated. A ,e′ B ,e′ C The formula for its calculation 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 The output power capability is used for modulated wave distribution, H-bridge unit T Xi modulation voltage e Xi e Xi =k Xi e′ X , where k Xi H-bridge unit T Xi The power output capability coefficient.

2. The transient current control strategy for a grid-type cascaded H-bridge converter based on cutoff negative feedback as described in claim 1, characterized in that, The instantaneous power calculation formula mentioned in step 1.1 is as follows: 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 formula for calculation is: Where, ω c This is the cutoff frequency of the low-pass filter.

3. The transient current control strategy for a grid-type cascaded H-bridge converter based on cutoff negative feedback as described in claim 1, characterized in that, The active power loop calculation formula mentioned in step 1.2 is as follows: The formula for calculating the reactive power loop is: Wherein, Sag is the remote power grid amplitude drop fault detection signal. Under normal operation, Sag = 1; when a sudden drop in voltage amplitude occurs in the remote power grid, Sag = 0; P set ω is the reference value for the three-phase output active power of the cascaded H-bridge converter. n D is the rated angular frequency of the three-phase power grid. p Here, J is the frequency droop coefficient, J is the virtual moment of inertia, and k is the frequency droop coefficient. ap k is the proportional coefficient of the frequency deviation PI controller. ai Q is the integral coefficient of the frequency deviation PI controller. set V is the reference value for the three-phase output reactive power of the cascaded H-bridge converter. nAmp D represents the rated phase voltage amplitude of a three-phase power grid. q K is the reactive power damping coefficient. q E is the reactive inertia coefficient. lim This is the upper limit of the reactive voltage loop output.

4. The transient current control strategy for a grid-type cascaded H-bridge converter based on cutoff negative feedback as described in claim 1, characterized in that, Step 1.3 Initial modulation wave voltage e of phases A, B, and C of the cascaded H-bridge converter A e B e C The calculation formulas are as follows: e A =V GFM cos(θ GFM )

Images