Resonance damping and current compensation system and method for grid-forming type photovoltaic grid-connected converter

Abstract

The invention provides a resonance damping and current compensation system and method for a grid-forming type photovoltaic grid-connected converter, and relates to the technical field of power systems, photovoltaic power generation and electric energy quality control. The system comprises a network construction type three-port grid-connected converter and a control strategy module, the network construction type three-port grid-connected converter is provided with two direct current ports and an alternating current port, and the two direct current ports and the alternating current port are connected into an energy storage capacitor bank, a photovoltaic power grid and a power grid respectively. In the control strategy module, the parallel side converter control submodule adopts direct voltage synchronous control, reactive droop control and parallel side voltage damping control, and the series side converter control submodule adopts harmonic current control, series side voltage damping control and photovoltaic control. According to the invention, the series-parallel three-port converter structure is combined with a network construction type control strategy, so that the damping of the photovoltaic grid-connected converter to the voltage resonance of the grid-connected point and the harmonic compensation of the power grid current are realized, and the operation stability and the electric energy quality under the weak power grid are improved.

Description

Technical Field

[0001] The present invention relates to the technical fields of power systems, photovoltaic power generation, and power quality, and particularly relates to a resonant damping and current compensation system and method for a grid-forming photovoltaic grid-connected converter. Background Art

[0002] With the large-scale development of photovoltaic power generation stations, there are many problems with traditional photovoltaic grid-connected equipment: on the one hand, traditional photovoltaic grid-connected equipment weakens the grid strength, reduces the stability of photovoltaic grid-connected equipment, has risks of harmonic amplification and broadband oscillation, and causes deterioration of the power quality of the distribution network; on the other hand, the sequential fluctuation of photovoltaic power reduces the capacity utilization rate of the grid-connected converter, but the grid-connected converter has spare capacity to achieve other functions. As a power quality control device, an Active Power Filter (APF) can control more frequencies of harmonics, but the grid-connected converter has limited damping ability for weak grid systems and is prone to form voltage resonance and cause medium- and high-frequency oscillation problems when connected in parallel with other power electronic grid-connected equipment. In addition, the large number of grid-following grid-connected converters used in photovoltaic power plants reduces the grid strength and is prone to problems with the stability of the phase-locked loop under weak grids. Summary of the Invention

[0003] The purpose of the present invention is to provide a resonant damping and current compensation system and method for a grid-forming photovoltaic grid-connected converter, which is used to solve the problems that the poor stability of traditional photovoltaic grid-connected equipment leads to the deterioration of the power quality of the distribution network, the low capacity utilization rate of the grid-connected converter, the occurrence of high-frequency oscillation, and the problem of the stability of the phase-locked loop under weak grids, and has good adaptability under weak grids, can effectively suppress voltage oscillation and current harmonics, and improve the stability of grid-connected equipment and the power quality control ability.

[0004] To achieve the above purpose, in the first aspect, the present invention provides a resonant damping and current compensation system for a grid-forming photovoltaic grid-connected converter, including:

[0005] A grid-forming three-port grid-connected converter, which has two DC ports and one AC port, and is respectively connected to an energy storage capacitor bank, a photovoltaic power source, and a power grid; the grid-forming three-port grid-connected converter includes a shunt-side converter and a series-side converter. The shunt-side converter is connected to the grid through a first inverter; the series-side converter includes a transformer and a second inverter. The primary winding of the transformer is connected in series to the grid-connected line, the secondary winding is connected to the second inverter, and the neutral point is connected to the photovoltaic power source;

[0006] A control strategy module, including a shunt-side converter control sub-module and a series-side converter control sub-module;

[0007] The parallel - side converter control sub - module adopts direct - voltage synchronous control, reactive - power droop control and parallel - side voltage damping control to realize that the parallel - side converter generates an approximately pure - sine voltage waveform and damping support;

[0008] The series - side converter control sub - module adopts harmonic - current control, series - side voltage damping control and photovoltaic control to realize harmonic suppression of the series - side converter and maximum power output of the photovoltaic system.

[0009] According to a grid - forming photovoltaic grid - connected converter resonance damping and current compensation system provided by the present invention, the first inverter and the second inverter are both LC - type three - phase full - bridge inverters, and the neutral point leads out an LC branch to be connected with the photovoltaic system.

[0010] According to a grid - forming photovoltaic grid - connected converter resonance damping and current compensation system provided by the present invention, the direct - voltage synchronous control adopts a direct - voltage droop control structure, and the control formula is:

[0011] θ c =∫[η(U dc -U dcN +dI dc )+ω N dt

[0012] In the formula, θ c is the synchronous phase of the parallel - side converter, η is the voltage - frequency conversion coefficient, d is the synchronous damping coefficient, ω N is the rated angular frequency, U dcN is the rated DC voltage, U dc is the DC voltage, I dc is the DC current, and t is the time.

[0013] According to a grid - forming photovoltaic grid - connected converter resonance damping and current compensation system provided by the present invention, the control formula of the reactive - power droop control is:

[0014] E k =E kn +K q (Q - Q * )

[0015] In the formula, Q * is the specified value of reactive power, Q is the reactive power, K q is the reactive - power droop coefficient, E kn is the rated voltage amplitude, and E k is the voltage amplitude.

[0016] According to a grid - forming photovoltaic grid - connected converter resonance damping and current compensation system provided by the present invention, the calculation formula of reactive power is:

[0017] Q=U kq I Ld -Ukd I Lq

[0018] Wherein, U kd and U kq are the d - axis and q - axis components of the output voltage of the parallel - side converter, and I Ld and I Lq are the d - axis and q - axis components of the inverter current of the parallel - side converter.

[0019] According to a grid - forming photovoltaic grid - connected converter resonance damping and current compensation system provided by the present invention, the control formula of the parallel - side voltage damping control is:

[0020]

[0021]

[0022] Wherein, is the reference value of the parallel - side damping voltage, i ck is the capacitor current of the parallel - side converter, G D1 (s) is the parallel - side damping transfer function, r k is the inductance resistance of the parallel - side converter, L k is the inductor of the parallel - side converter, C k is the capacitor of the parallel - side converter, R v1 is the parallel - side damping coefficient, ω s is the damping frequency, s is the Laplace operator, ω N is the rated angular frequency.

[0023] According to a grid - forming photovoltaic grid - connected converter resonance damping and current compensation system provided by the present invention, the harmonic current control is realized by multiple harmonic current controllers; among them, the specific process of the k - th harmonic current control is:

[0024] The sampled grid current is converted from the primary winding to the secondary winding to obtain the equivalent grid current of the secondary winding;

[0025] After the discrete Fourier transform under the k - th phase, the real and imaginary components of the k - th harmonic current are obtained, and then input into the integral controller to obtain the reference values of the real and imaginary components of the k - th harmonic voltage output by the series - side converter;

[0026] The reference values of the real and imaginary components of the k - th harmonic voltage are subjected to the inverse discrete Fourier transform and added to the reference values output by all harmonic current controllers to obtain the overall harmonic compensation voltage.

[0027] A resonant damping and current compensation system for a grid-forming photovoltaic grid-connected converter provided by the present invention, in which photovoltaic control outputs a photovoltaic reference voltage through a maximum power point tracking module, and the proportional-integral controller of the photovoltaic voltage-current double-loop control obtains the DC component of the modulation voltage. The control formula of the photovoltaic control is as follows:

[0028]

[0029] Wherein, is the DC component of the modulation voltage, U PV is the photovoltaic output voltage, I PV is the photovoltaic output current, U PV * is the photovoltaic voltage reference value, I PV * is the photovoltaic current reference value, k PVup and k PVui are respectively the PI parameters of the photovoltaic voltage control, k PVip and k PVii are respectively the PI parameters of the photovoltaic current control, and t is time.

[0030] For a resonant damping and current compensation system of a grid-forming photovoltaic grid-connected converter provided by the present invention, the control formula of the series-side voltage damping control is:

[0031]

[0032]

[0033] In the formula, is the reference value of the series-side converter damping voltage, G D2 (s) is the series-side damping transfer function, i cr is the series-side converter capacitor current, r r is the series-side converter inductor resistance, L r is the series-side converter inductor, C r is the series-side converter capacitor, R v2 is the series-side damping coefficient, and s is the Laplace operator.

[0034] Second, the present invention provides a method for resonant damping and current compensation of a grid-forming photovoltaic grid-connected converter. Using the system of the first aspect, the method includes:

[0035] Adopting direct voltage synchronous control, reactive power droop control and parallel-side voltage damping control for the parallel-side converter to enable the parallel-side converter to generate an approximately pure sine voltage waveform and damping support;

[0036] Adopting harmonic current control, series-side voltage damping control and photovoltaic control for the series-side converter to achieve harmonic governance of the series-side converter and maximum photovoltaic power output.

[0037] The resonant damping and current compensation system and method for a grid-forming photovoltaic grid-connected converter provided by the present invention at least have the following technical effects:

[0038] 1. Through the series-parallel three-port converter structure and in combination with the grid-forming control strategy, the photovoltaic grid-connected converter realizes the damping of the resonance of the grid connection point voltage and the harmonic compensation of the grid current, improving the operation stability and power quality under weak grids.

[0039] 2. The parallel-side converter adopts direct voltage synchronous control and reactive power droop control, which can quickly respond to the fluctuations of photovoltaic power, maintain the balance of the DC voltage, and adjust the reactive power output according to the voltage amplitude, enhancing the system stability.

[0040] 3. The harmonic current control of the series-side converter can effectively control the harmonics in the grid current, reduce the harmonic content, and improve the power quality; at the same time, the combination of damping control and photovoltaic control not only realizes harmonic suppression but also ensures the maximum power output of the photovoltaic.

[0041] 4. It has good adaptability under weak grids, can effectively suppress voltage oscillations and current harmonics, and improve the stability of grid-connected equipment and the ability to control power quality. Description of the Drawings

[0042] In order to more clearly illustrate the technical solutions in the present invention or the prior art, the following will briefly introduce the drawings required for use in the description of the embodiments or the prior art. Obviously, the drawings in the following description are some embodiments of the present invention. For those of ordinary skill in the art, without creative efforts, other drawings can also be obtained based on these drawings.

[0043] In the drawings:

[0044] Figure 1 is a schematic diagram of the photovoltaic power generation station of the present invention;

[0045] Figure 2 is the control strategy block diagram of the grid-forming three-port grid-connected converter of the present invention;

[0046] Figure 3 is the equivalent circuit diagram introduced by the damping control of the present invention;

[0047] Figure 4 is the waveform diagram of the grid voltage before and after the damping start of the present invention;

[0048] Figure 5 is the waveform diagram of the line current under the harmonic compensation of the present invention;

[0049] Figure 6This is the corresponding line voltage and current waveform diagram before and after the voltage damping start of the converter when the grid connection point voltage oscillates under a weak grid in the present invention;

[0050] Figure 7 This is the voltage and current waveform diagram after phase compensation in the present invention. Detailed implementation manners

[0051] To make the objectives, technical solutions and advantages of the present invention clearer, the technical solutions in the present invention will be clearly and completely described below with reference to the accompanying drawings in the present invention. Obviously, the described embodiments are some but not all of the embodiments of the present invention. All other embodiments obtained by those of ordinary skill in the art based on the embodiments in the present invention without creative efforts shall fall within the protection scope of the present invention.

[0052] The following will describe in detail some implementation manners of the present invention with reference to the accompanying drawings. Without conflict, the following embodiments and the features in the embodiments can be combined with each other.

[0053] Please refer to Figure 1 , the photovoltaic power generation station includes a grid-forming three-port grid-connected converter and a grid-following grid-connected converter. Among them, the grid-forming three-port grid-connected converter has two DC ports and one AC port, which are respectively connected to an energy storage capacitor bank, a photovoltaic array (hereinafter referred to as photovoltaic) and the grid; the grid-forming three-port grid-connected converter includes a shunt-side converter and a series-side converter. The shunt-side converter is grid-connected through a first inverter; the series-side converter includes a transformer and a second inverter. The primary winding of the transformer is connected in series to the grid connection line, the secondary winding is connected to the second inverter, and the neutral point leads out an LC branch to be connected to the photovoltaic. Figure 1 In, in addition to connecting the grid-following grid-connected converter, the grid connection point also connects a non-linear load; both the first inverter and the second inverter are LC-type three-phase full-bridge inverters.

[0054] Based on this, an embodiment of the present invention provides a resonance damping and current compensation system for a grid-forming photovoltaic grid-connected converter, including the aforementioned grid-forming three-port grid-connected converter and a control strategy module. Among them, the control strategy module includes a shunt-side converter control sub-module and a series-side converter control sub-module; the shunt-side converter control sub-module adopts direct voltage synchronization control, reactive power droop control and shunt-side voltage damping control to realize that the shunt-side converter generates an approximate pure sine voltage waveform and damping support; the series-side converter control sub-module adopts harmonic current control, damping control and photovoltaic control to realize harmonic suppression of the series-side converter and maximum power output of the photovoltaic.

[0055] Specifically, in the grid-forming three-port grid-connected converter, let L k and C k be the inductor and capacitor for filtering used by the shunt-side converter respectively; Lr and C r are the inductor and capacitor for filtering used by the series-side converter; C dc is the DC capacitor; I in and I out are the DC input current and output current respectively; L PV and C PV are the filtering inductor and capacitor of the photovoltaic branch respectively; u m is the primary-side voltage (for three phases a, b, and c, denoted as u ma , u mb , u mc respectively, and uniformly denoted as u mabc , and subsequent parameters adopt a similar notation); u k (u kabc ) is the capacitor voltage of the parallel-side converter; u r (u rabc ) is the output voltage of the series-side converter; i s (i sabc ) is the port current; i g (i gabc ) is the grid current; i k (i kabc ) is the inductor output current of the parallel-side converter; i r (i rabc ) is the output current of the series-side converter; i rr (i rrabc ) is the inverter inductor current of the series-side converter; i ck (i ckabc ) is the capacitor current of the parallel-side converter; i cr (i crabc ) is the capacitor current of the series-side converter; U dc is the DC voltage; I dc is the DC current; U PV is the photovoltaic output voltage; I PV is the photovoltaic output current. The transformer capacity of the series-side converter only needs to cover the harmonic voltage component, which is reduced by more than half compared with the traditional isolation transformer; by reusing the control resources and DC bus of the photovoltaic grid-connected converter, the independent APF or UPQC (Unified Power Quality Conditioner) device is omitted, and the hardware cost is reduced.

[0056] Figure 2 is the principle block diagram for the control strategy module to execute the control strategy.

[0057] Among them, the DC voltage synchronization control adopts the DC voltage droop control structure, and the control formula is:

[0058] θ c= ∫[η(U dc -U dcN +dI dc )+ω N dt (1)

[0059] Where, θ c is the synchronous phase of the shunt converter, η is the voltage-frequency conversion coefficient, d is the synchronous damping coefficient, and I dc is the DC current. When the DC voltage U dc corresponds to the frequency, the relationship of Equation (2) should be satisfied. ω N is the rated angular frequency, and U dcN is the rated DC voltage. When the DC voltage U dc corresponds to the frequency, it should satisfy:

[0060]

[0061] The control equation of reactive power droop control is:

[0062] E k = E kn +K q (Q - Q * ) (3)

[0063] Where, Q * is the reactive power setpoint, Q is the reactive power, K q is the reactive power droop coefficient, E kn is the rated voltage amplitude, and E k is the voltage amplitude. The calculation method of reactive power is:

[0064] Q = U kq I Ld - U kd I Lq (4)

[0065] Where, U kd and U kq are the dq-axis components of the output voltage of the shunt converter, and I Ld and I Lq are the dq-axis components of the inverter current of the shunt converter.

[0066] The control equation of shunt side voltage damping control is:

[0067]

[0068] Where, is the shunt side damping voltage reference value, i ck is the capacitor current of the shunt converter, and G D1 (s) is the shunt side damping transfer function.

[0069]

[0070] Among them, r k is the inductance resistance of the parallel-side converter, L k is the inductance of the parallel-side converter, C k is the capacitance of the parallel-side converter, R v1 is the parallel-side damping coefficient, ω s is the damping frequency, and s is the Laplace operator.

[0071] The harmonic current control is implemented by multiple harmonic current controllers. The k-th harmonic current control method is that the sampled grid current i gabc is first converted from the primary winding to the secondary winding to obtain the equivalent grid current i gabcr of the secondary winding, and then the real component ΔI k and the imaginary component ΔI Rabck of the k-th harmonic current obtained by the discrete Fourier transform (DFT) under the k-th phase θ Iabck are input to the integral controller to obtain the reference values U Rabck and U Iabck of the real and imaginary components of the k-th harmonic voltage output by the series-side converter. The reference values are then subjected to the inverse discrete Fourier transform (IDFT) and added to the output reference values of all harmonic current controllers to obtain the overall harmonic compensation voltage Taking phase a as an example, the expression of the overall harmonic compensation voltage of phase a in the frequency domain is:

[0072]

[0073] Among them, k hi is the integral coefficient of the harmonic current control, θ rk is the k-th harmonic phase compensation value, I gar is the grid current I ga of phase a after sampling and converted from the primary winding to the secondary winding to obtain the equivalent grid current of the secondary winding of the series-side converter of phase a. ω is the fundamental frequency.

[0074] The photovoltaic control outputs the photovoltaic reference voltage through the maximum power point tracking module (MPPT), and obtains the DC component of the modulation voltage through the proportional-integral controller of the photovoltaic voltage-current double-loop control The control formula is:

[0075]

[0076] Among them, U PV * is the photovoltaic voltage reference value, I PV * is the photovoltaic current reference value, k PVupWith k PVui are the PI parameters for photovoltaic voltage control, k PVip With k PVii are the PI parameters for photovoltaic current control, and t is time.

[0077] The control equation for the series-side voltage damping control is:

[0078]

[0079]

[0080] Output damping voltage is controlled by feedback of the series-side capacitor voltage. In the formula, is the reference value of the series-side converter damping voltage, G D2 (s) is the series-side damping transfer function, i cr is the series-side converter capacitor current, r r is the series-side converter inductor resistance, L r is the series-side converter inductor, C r is the series-side converter capacitor, R v2 is the series-side damping coefficient, and s is the Laplace operator.

[0081] See Figure 3 , the damping control has a similar structure to the PI control. When the damping control starts, a sudden change in the DC component of the three-phase current will occur, and the DC component will continuously output through the inverter inductor in the equivalent circuit. At the same time, under the influence of the DC component circulating current, a second-harmonic oscillation component will appear in the DC-side voltage. Figure 3 In, Z g is the grid-side impedance, e k is the shunt-side electromotive force, Z total is the equivalent total impedance. L σ1 is the primary winding leakage inductance, L σ2 is the secondary winding leakage inductance. To control the DC component, the shunt-side converter needs to add DC component control. By feedback of the inverter inductor, the low-frequency component is extracted for PI control, and the control equation is shown in Equation (11).

[0082]

[0083] In the formula, is the DC component compensation voltage, T d is the low-pass filter time constant, k p0 With k i0 are the PI parameters for DC component control, i k is the shunt-side converter inductor output current, G k0 is the transfer function.

[0084] Another embodiment of the present invention provides a resonance damping and current compensation method for a network-forming photovoltaic grid-connected converter, which adopts the resonance damping and current compensation system of the foregoing embodiment. The method includes the following steps:

[0085] Step 1: Apply direct voltage synchronization control, reactive power droop control, and parallel-side voltage damping control to the parallel-side converter to enable the parallel-side converter to generate an approximately pure sine voltage waveform and damping support;

[0086] Further, in Step 1, the direct voltage synchronization control realizes the control of the DC voltage by adjusting the synchronization phase. The reactive power droop control adjusts the reactive power output according to the voltage amplitude. The parallel-side voltage damping control generates a parallel-side damping voltage reference value by feeding back the capacitor current of the parallel-side converter to realize the damping support for the grid connection point voltage resonance.

[0087] Step 2: Apply harmonic current control, series-side voltage damping control, and photovoltaic control to the series-side converter to realize harmonic suppression of the series-side converter and maximum power output of the photovoltaic system.

[0088] Further, in Step 2, the harmonic current control samples the grid current, obtains the harmonic current components of each order through conversion and Fourier transform, obtains the harmonic voltage reference value through an integral controller, and then obtains the overall harmonic compensation voltage through inverse transformation. The photovoltaic control realizes the maximum power output of the photovoltaic system through maximum power point tracking and dual-loop voltage and current control. The series-side voltage damping control generates a series-side converter damping voltage reference value by feeding back the capacitor voltage of the series-side converter to realize the damping control of the series-side converter output.

[0089] The following is a specific embodiment of the present invention.

[0090] In this embodiment, the relevant parameters of the system are shown in Table 1.

[0091] Table 1. System parameter table

[0092]

[0093] See Figure 4 For the simulation waveforms of the three-phase grid voltage u v1 when R v2 is 2 and R gabc is 10 before and after the damping start-up, Figure 4Under the undamped control, high-order harmonics such as the 25th and 35th harmonics mainly exist in the voltage, and the voltage THD (Total Harmonic Distortion) is 1.8%. The converter presents high impedance at these frequencies and cannot absorb and suppress them. After starting the damping, the high-order harmonics are quickly suppressed, and the voltage THD drops to 0.3% at this time. It shows that the control strategy of the present invention has the ability to provide harmonic voltage damping, and can improve the impedance characteristics of the grid-forming three-port grid-connected converter (referred to as the proposed converter) provided by the present invention, and improve the damping characteristics of the grid impedance under weak grids.

[0094] See Figure 5 , for the line current simulation waveform under the harmonic compensation operation of the 2nd to 19th harmonics when a nonlinear load is connected to the node. The proposed converter suppresses the harmonics of the line current under the harmonic control operation, and reduces the THD of the line current from 14.8% to 4.92%.

[0095] See Figure 6 , when the traditional grid-connected converter operates and increases the output, the high-frequency oscillation caused by the coupling between it and the grid inductor and the filter capacitor inductor of the proposed converter rapidly increases after several cycles, and the oscillation frequency is about 1 kHz. When the damping control is started, the high-frequency oscillation of the voltage is effectively suppressed. At this time, the THD of the grid voltage u g changes from 5.2% to 2.1%. Therefore, this waveform change can show that the voltage damping control effectively suppresses the voltage oscillation.

[0096] See Figure 7 , after the phase compensation, the harmonic components of the harmonic current are fully suppressed. At this time, the THD of the grid current i g is 4.16%. It can be verified that compared with the traditional APF, the proposed control can not only ensure its own stability under weak grids, but also absorb the oscillation power of other converter devices, improve the stability of grid-connected devices, and at the same time can suppress the system harmonic current with the introduction of phase compensation.

[0097] In summary, compared with the prior art, the present invention has the following advantages:

[0098] The structure of the grid-forming three-port grid-connected converter proposed by the present invention integrates the access of the energy storage capacitor bank, the photovoltaic array and the grid, realizing the integration of multiple functions. This structure can not only perform the grid-connected conversion of photovoltaic power, but also simultaneously perform resonance damping and harmonic current compensation on the grid, improving the comprehensive utilization efficiency of the equipment. It has good adaptability under weak grids.

[0099] The present invention has good economy. By reusing the control resources and DC bus of the photovoltaic grid-connected converter, the independent APF or UPQC device is eliminated, reducing the hardware cost. At the same time, the capacity of the series-side transformer only needs to cover the harmonic voltage component, which is reduced by more than half compared with the capacity of the traditional isolation transformer, further saving the equipment cost.

[0100] The present invention has strong power quality governance ability. The experimental results show that the proposed control strategy well realizes the suppression of voltage oscillation under a weak grid. At the same time, combined with phase compensation, it stably compensates the grid current, reducing the THD of the grid voltage from 1.8% to 0.3% and the THD of the grid current from 14.8% to 4.92%.

[0101] In summary, a resonant damping and current compensation system and method for a grid-forming photovoltaic grid-connected converter proposed by the present invention propose a three-port grid-connected converter structure, integrating the access of the energy storage capacitor bank, photovoltaic array, and grid, and realizing the integration of multiple functions. It can not only perform the grid connection conversion of photovoltaic power, but also simultaneously perform resonant damping and harmonic current compensation on the grid, improving the comprehensive utilization efficiency of the equipment; it has good adaptability under a weak grid. And it has good economy. By reusing the control resources and DC bus of the photovoltaic grid-connected converter, the independent APF or UPQC device is eliminated, reducing the hardware cost. At the same time, the capacity of the series-side transformer only needs to cover the harmonic voltage component, which is reduced by more than half compared with the capacity of the traditional isolation transformer, further saving the equipment cost. And it has strong power quality governance ability. The experimental results show that the present invention well realizes the suppression of voltage oscillation under a weak grid. At the same time, combined with phase compensation, it stably compensates the grid current, reducing the THD of the voltage from 1.8% to 0.3% and the THD of the grid current from 14.8% to 4.92%.

[0102] After considering the specification and practicing the embodiments disclosed herein, those skilled in the art will readily conceive of other embodiments of the present invention. The present invention is intended to cover any variations, uses, or adaptations of the present invention, which follow the general principles of the present invention and include the common general knowledge or conventional technical means in the technical field not disclosed by the present invention. It should be understood that the present invention is not limited to the exact structures described above and shown in the drawings, and various modifications and changes can be made without departing from its scope. The scope of the present invention is only limited by the appended claims.

Claims

1. A resonant damping and current compensation system for a grid-forming photovoltaic grid-connected inverter, characterized in that, Including: A grid-forming three-port grid-connected converter, which has two DC ports and one AC port, and is connected to an energy storage capacitor bank, a photovoltaic power source and the grid respectively; the grid-forming three-port grid-connected converter includes a shunt-side converter and a series-side converter, and the shunt-side converter is grid-connected through a first inverter; the series-side converter includes a transformer and a second inverter, the primary winding of the transformer is connected in series to the grid-connected line, the secondary winding is connected to the second inverter, and the neutral point is connected to the photovoltaic power source; A control strategy module, including a shunt-side converter control sub-module and a series-side converter control sub-module; The shunt-side converter control sub-module adopts direct voltage synchronization control, reactive power droop control and shunt-side voltage damping control to enable the shunt-side converter to generate an approximate pure sine voltage waveform and damping support; The series-side converter control sub-module adopts harmonic current control, series-side voltage damping control and photovoltaic control to achieve harmonic suppression of the series-side converter and maximum power output of the photovoltaic power source.

2. The resonant damping and current compensation system of the grid-forming PV grid-connected converter according to claim 1, wherein Both the first inverter and the second inverter are LC-type three-phase full-bridge inverters, and an LC branch is led out from the neutral point and connected to the photovoltaic power source.

3. The resonant damping and current compensation system of the network-forming photovoltaic grid-connected converter according to claim 1, wherein The direct voltage synchronization control adopts a direct voltage droop control structure, and the control formula is: θ c = ∫[η(U dc - U dcN + dI dc ) + ω N dt Where, θ c is the synchronous phase of the shunt converter, η is the voltage-frequency conversion coefficient, d is the synchronous damping coefficient, ω N is the rated angular frequency, U dcN is the rated DC voltage, U dc is the DC voltage, I dc is the DC current, and t is the time.

4. The resonant damping and current compensation system for a grid-forming PV grid-connected converter according to claim 1, wherein The control formula of the reactive power droop control is: E k = E kn + K q (Q - Q * ) Where Q * is the reactive power setpoint, Q is the reactive power, K q is the reactive droop coefficient, E kn is the rated voltage amplitude, and E k is the voltage amplitude.

5. The resonant damping and current compensation system for a grid-forming PV grid-connected converter according to claim 4, characterized in that, The calculation formula of the reactive power is: Q = U kq I Ld -U kd I Lq Where, U kd and U kq are the d-axis and q-axis components of the output voltage of the shunt converter, and I Ld and I Lq are the d-axis and q-axis components of the inverter current of the shunt converter.

6. The resonant damping and current compensation system for a grid-forming photovoltaic grid-connected converter according to claim 1, wherein The control formula of the shunt-side voltage damping control is: Wherein, is the damping voltage reference value of the shunt side, i ck is the capacitor current of the shunt side converter, G D1 (s) is the damping transfer function of the shunt side, r k is the inductance resistance of the shunt side converter, L k is the inductance of the shunt side converter, C k is the capacitor of the shunt side converter, R v1 is the damping coefficient of the shunt side, ω s is the damping frequency, s is the Laplace operator, ω N is the rated angular frequency.

7. The resonant damping and current compensation system for a grid-forming PV grid-connected converter according to claim 1, wherein The harmonic current control is realized by multiple harmonic current controllers; among them, the specific process of the k-th harmonic current control is: The sampled grid current is converted from the primary winding to the secondary winding to obtain the equivalent grid current of the secondary winding; After discrete Fourier transform under the k-th phase, the real and imaginary components of the k-th harmonic current are obtained, and then input into an integral controller to obtain the reference values of the real and imaginary components of the k-th harmonic voltage output by the series-side converter; The reference values of the real and imaginary components of the k-th harmonic voltage are inversely transformed by discrete Fourier transform and added to the reference values output by all harmonic current controllers to obtain the overall harmonic compensation voltage.

8. The resonant damping and current compensation system for a grid-forming PV grid-connected converter according to claim 1, characterized in that, The photovoltaic control outputs a photovoltaic reference voltage through a maximum power point tracking module, and the proportional-integral controller of the photovoltaic voltage-current double-loop control is used to obtain the DC component of the modulation voltage. The control formula of the photovoltaic control is: Among them, is the DC component of the modulation voltage, U PV is the photovoltaic output voltage, I PV is the photovoltaic output current, U PV * is the photovoltaic voltage reference value, I PV * is the photovoltaic current reference value, k PVup and k PVui are the PI parameters of the photovoltaic voltage control respectively, k PVip and k PVii are the PI parameters of the photovoltaic current control respectively, and t is the time.

9. The resonant damping and current compensation system of the network-forming photovoltaic grid-connected converter according to claim 1, wherein The control formula of the series-side voltage damping control is: Wherein, is the damping voltage reference value of the series-side converter, G D2 (s) is the series-side damping transfer function, i cr is the capacitor current of the series-side converter, r r is the inductance resistance of the series-side converter, L r is the inductor of the series-side converter, C r is the capacitor of the series-side converter, R v2 is the series-side damping coefficient, and s is the Laplace operator.

10. A method for resonant damping and current compensation of a grid-forming photovoltaic grid-connected converter, which adopts the system according to any one of claims 1 to 9. The method includes: Adopting direct voltage synchronization control, reactive power droop control and shunt-side voltage damping control for the shunt-side converter to enable the shunt-side converter to generate an approximate pure sine voltage waveform and damping support; Adopting harmonic current control, series-side voltage damping control and photovoltaic control for the series-side converter to achieve harmonic suppression of the series-side converter and maximum power output of the photovoltaic power source.

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