CN-115051352-B - Control method of cascading type photovoltaic inverter under power grid interphase short-circuit fault condition

Abstract

The invention discloses a control method of a cascade photovoltaic inverter under a power grid interphase short-circuit fault condition, and belongs to the photovoltaic power generation technology in the field of electrical engineering. The method comprises the steps of judging the fault type of a three-phase power grid, detecting the ratio of line voltage amplitude values of two fault phases after falling to line voltage amplitude values of two fault phases before falling, controlling grid-connected current by adopting a proportional resonance controller, adaptively compensating zero sequence voltage according to the maximum positive sequence active current actually output by an inverter by adopting an adaptive zero sequence voltage compensation control strategy based on active current injection, and controlling a two-level full-bridge LLC converter input into m paths to control the average value of direct current bus capacitor voltage of the H-bridge converter. When the power grid has interphase short circuit fault, the control method improves the adaptability of the cascade photovoltaic grid-connected inverter to different output powers and different power grid drop depths, and improves the low voltage ride through capability of the system.

Inventors

• LIN SHAN
• XIANG DONG
• ZHAO TAO
• NONG XINGZHONG
• WANG CHUNFANG
• JIN HUI
• LEI ZHENYU
• CHEN JIANDANG
• RAO MEIWAN
• LUO XU

Assignees

• 广州地铁设计研究院股份有限公司
• 青岛大学

Dates

Publication Date

20260505

Application Date

20220506

Claims (1)

1. 1. A control method of a cascading type photovoltaic inverter under a power grid interphase short-circuit fault condition is characterized in that the cascading type photovoltaic inverter is a photovoltaic grid-connected inverter based on a three-phase isolation type common direct current bus cascading H-bridge topology, and consists of an A phase, a B phase and a C phase; the system comprises a phase A, a phase B and a phase C, wherein the phase A, the phase B and the phase C comprise n modules, the structures of all the modules in the phase A, the phase B and the phase C are completely the same, n is a positive integer larger than 1, each module in the phase A, the phase B and the phase C consists of a two-level full-bridge LLC converter and m H-bridge converters which are input into the phase A, the phase B and the phase C respectively, the m output ports of the two-level full-bridge LLC converter which are input into the phase M are respectively connected with one H-bridge converter, m is a positive integer larger than 1, the input port of each H-bridge converter is connected with an H-bridge converter direct current bus capacitor in parallel, and the output ports of all the modules in the phase A, the phase B and the phase C are connected in series to form a unit alternating current output port; The control method is characterized by comprising the steps of fault detection and judgment of three-phase power grid voltage, grid-connected current control, active current injection and self-adaptive zero sequence voltage compensation control, and average value control of direct current bus capacitor voltage of the H-bridge converter, and specifically comprises the following steps: Step 1, fault detection and judgment of three-phase power grid voltage Step 1.1, sampling the three-phase power grid voltage to obtain a sampling value of the three-phase power grid voltage , , ; Step 1.2, sampling values of the three-phase power grid voltage obtained in step 1.1 are obtained by using a frequency locking ring of a double-2-order generalized integrator , , Phase locking is carried out to obtain the phase angle of the power grid voltage Angular frequency Sampling value of three-phase network voltage is converted through synchronous rotation coordinates , , Conversion into positive-sequence active components of grid voltage in synchronous rotation coordinate system Reactive component of positive sequence of grid voltage Negative sequence active component of grid voltage And a grid voltage negative sequence reactive component The calculation formulas are respectively as follows: in addition, the amplitude of the positive sequence phase voltage of the power grid Amplitude of negative sequence phase voltage of power grid The calculation formula of (2) is as follows: step 1.3, calculating the power grid voltage negative sequence active component according to the step 1.2 Negative sequence reactive component of grid voltage Amplitude of positive sequence phase voltage of power grid Amplitude of negative sequence phase voltage of power grid Judging the fault type of the three-phase power grid: When (when) And is also provided with When in use, if If the power grid is in the two-phase interphase short circuit fault, otherwise, other types of faults occur in the power grid; When (when) And is also provided with When in use, if The grid is subjected to two-phase interphase short circuit faults, otherwise the grid is subjected to other types of faults, wherein, A nominal value representing the amplitude of the grid phase voltage; Step 1.4, according to the grid fault type detection method of step 1.3, when a short circuit fault occurs between two phases in the grid, the ratio D of the line voltage amplitudes of the two fault phases after the drop to the line voltage amplitudes of the two fault phases before the drop can be calculated as: Step 2, grid-connected current control Step 2.1, sampling the three-phase power grid current to obtain a sampling value of the three-phase power grid current , , ; Step 2.2, sampling values of the three-phase network current obtained in step 2.1 are converted by synchronous rotation coordinates , , Conversion to grid current alpha-axis component in two-phase stationary vertical coordinate system And grid current beta-axis component The calculation formula is as follows: Step 2.3, calculating the instruction value of the positive sequence reactive current The calculation formula is as follows: In the formula, A nominal value representing the current amplitude of the three-phase network; Step 2.4, sampling the voltage of the common direct current bus and the output current of the photovoltaic array to obtain a sampling value of the voltage of the common direct current bus And the sampled value of the output current of the photovoltaic array And calculate the actual output power of the photovoltaic array The calculation formula is as follows: Step 2.5, calculating the instruction value of the positive sequence active current The calculation formula is as follows: In the formula, Representation of And (3) with Is set to be a minimum value of (c), Representation of Square root of (2); step 2.6, calculating the command value of the alpha-axis component of the power grid current And command value of grid current beta-axis component The calculation formula is as follows: step 2.7, the power grid current alpha-axis component is respectively controlled by the proportional resonance controller alpha and the proportional resonance controller beta Command value controlled as alpha-axis component of grid current The power grid current beta-axis component Command value controlled as beta-axis component of grid current Obtaining the output value of the proportional resonance controller alpha And the output value of the proportional resonant controller beta The calculation formula is as follows: Wherein, the Is a coefficient of proportionality and is used for the control of the power supply, Is the resonance coefficient; step 2.8, sampling value of the three-phase network voltage obtained in the step 1.1 , , Output value of proportional resonance controller alpha calculated in step 2.7 And the output value of the proportional resonant controller beta Three voltages under a three-phase coordinate system are obtained through coordinate transformation , , The calculation formula is: Step 3, active current injection and self-adaptive zero sequence voltage compensation control Step 3.1, grid current alpha-axis component according to step 2.2 And grid current beta-axis component Calculating the positive sequence active component of the power grid current under the rotation coordinate system Grid current positive sequence reactive component The calculation formula is as follows: step 3.2, according to the ratio D of the line voltage amplitude of the two fault phases after the drop and the line voltage amplitude of the two fault phases before the drop calculated in the step 1.4, and the positive sequence active component of the power grid current under the rotating coordinate system calculated in the step 3.1 Grid current positive sequence reactive component Calculating adaptive compensation coefficients The calculation formula is as follows: step 3.3, according to the ratio D of the line voltage amplitude of the two fault phases after the drop and the line voltage amplitude of the two fault phases before the drop calculated in the step 1.4, three voltages under the three-phase coordinate system calculated in the step 2.8 , , Step 3.1 calculating the positive sequence active component of the power grid current under the rotation coordinate system And a grid current positive sequence reactive component And the adaptive compensation coefficient calculated in the step 3.2 Calculating modulation voltage of cascade photovoltaic inverter , , The calculation formula is as follows: In the formula, when And is also provided with In the time-course of which the first and second contact surfaces, When (1) And is also provided with In the time-course of which the first and second contact surfaces, When (1) And is also provided with In the time-course of which the first and second contact surfaces, ; Representation of Is the arctangent value of (2); step 3.4, modulating voltage of the cascade photovoltaic inverter calculated in the step 3.3 , , Dividing the number of H-bridge converters of A phase, B phase and C phase respectively Obtaining the modulation voltage of the A-phase H-bridge converter Modulation voltage of B-phase H-bridge converter Modulation voltage of C-phase H-bridge converter The calculation is as follows: Step 3.5, sampling the DC bus capacitance voltage of all H-bridge converters in the A phase, the B phase and the C phase respectively to obtain the following data, namely the sampled value of the DC bus capacitance voltage of the j-th H-bridge converter of the i-th module of the A phase DC bus capacitor voltage sampling value of the j H-bridge converter of the i-th module of the B phase DC bus capacitor voltage sampling value of jth H-bridge converter of ith module of C phase , , ; Step 3.6, calculating the modulation wave of all H bridge converters in A phase, B phase and C phase, and recording the modulation wave of the j H bridge converter of the i th module of A phase as Modulated wave of the j H bridge converter of the i-th module of the B phase is Modulated wave of the j H bridge converter of the i-th module of the C phase is , , Then , , The formula of (2) is as follows: step 4, average value control of DC bus capacitor voltage of H-bridge converter Step 4.1, sampling value of DC bus capacitor voltage of the jth H bridge converter of the ith module of the A phase obtained in step 3.5 DC bus capacitor voltage sampling value of jth H-bridge converter of ith module of B phase DC bus capacitor voltage sampling value of jth H-bridge converter of ith module of C phase Calculating to obtain the average value of the direct current bus voltages of m H-bridge converters in the ith module of the A phase Average value of m H bridge converter DC bus voltages in ith module of B phase Average value of direct current bus voltages of m H-bridge converters in ith module of C phase The calculation formulas are respectively as follows: Step 4.2, using three identical two-level full-bridge LLC voltage controllers to respectively average the direct current bus voltages of m H-bridge converters in the ith module of the A phase obtained in step 4.1 Average value of m H bridge converter DC bus voltages in ith module of B phase Average value of direct current bus voltages of m H-bridge converters in ith module of C phase Controlled as Obtaining the switching frequency of the two-level full-bridge LLC converter with the ith input and m output of the A phase Switching frequency of two-level full-bridge LLC converter with ith input and m output of B phase Switching frequency of two-level full-bridge LLC converter with ith input/output of C phase The calculation formula is as follows: In the formula, Is the turn ratio of the primary side winding to the secondary side winding of the high-frequency transformer of the two-level full-bridge LLC converter with one input and m outputs, Is the proportionality coefficient of the two-level full-bridge LLC voltage controller, Is an integral coefficient of the two-level full-bridge LLC voltage controller.

Description

Control method of cascading type photovoltaic inverter under power grid interphase short-circuit fault condition Technical Field The invention belongs to the photovoltaic power generation technology in the field of electrical engineering, and particularly relates to a control method of a cascade photovoltaic inverter under a power grid interphase short circuit fault condition. Background The data of the global renewable energy status report of 2020 shows that the installed capacity of the photovoltaic power generation new energy in 2019 is about 115GW, which accounts for 57.79% of the capacity of the renewable energy new installation machine. With a large number of photovoltaic power stations connected into a power system, medium-and large-sized photovoltaic grid-connected inverters are required to have low voltage ride through capability at home and abroad, namely, during the voltage drop period of a power grid, the photovoltaic power stations need to inject reactive current which is in a certain proportion with the voltage drop to support the voltage of the power grid, and the residual capacity is sent out in the form of active power so as to prevent the power grid from large active shortage to influence the frequency stability of the system. Compared with the scheme that the traditional centralized inverter is connected to a medium-voltage power grid through a power frequency transformer, the grid-connected inverter based on three-phase isolated common-direct-current bus cascading H-bridge topology has more prominent advantages in application to a large-scale photovoltaic power station, such as a modularized structure, low switching frequency, small filter size, high conversion efficiency, low output current harmonic content, no heavy power frequency transformer, and the like, and can be directly connected with the medium-voltage power grid. Therefore, the modularized cascading type photovoltaic inverter can realize high-efficiency ultra-high-power medium-voltage direct-hanging grid-connected access, and has wide development prospect and market potential. However, unlike conventional three-phase inverters, the topology of cascaded photovoltaic inverters is relatively special and the converters present active power reflux problems during low voltage ride through. Active reflux is a problem specific to a three-phase modular cascaded grid-connected inverter and is premised on a three-phase grid voltage asymmetric fault. The method is characterized in that a certain phase of the converter absorbs active power from a power grid (active power is emitted to the power grid from the other two phases), so that a system has no balance operation point in a fault period, the voltage of an H-bridge direct-current bus continuously rises, and the inverter stops off-grid due to overvoltage faults. Therefore, it is necessary to suppress the active power backflow of the three-phase cascade photovoltaic inverter during grid sag, which is a necessary condition for achieving low voltage ride through. The literature H.D.Tafti,A.I.Maswood,G.Konstantinou,C.D.Townsend,P.Acuna,and J.Pou,"Flexible control of photovoltaic grid-connected cascaded H-bridge converters during unbalanced voltage sags,"IEEE Trans.Ind.Electron.,vol.65,no.8,pp.6229-6238,Aug.2018.(H.D.Tafti,A.I.Maswood,G.Konstantinou,C.D.Townsend,P.Acuna,and J.Pou, provides a flexible control of unbalanced voltage drop of a photovoltaic grid-connected cascade H-bridge converter, an IEEE industrial journal of electronics, 8 th month of 2018, 65 th volume of 8, 6229 pages to 6238 pages), and proposes an unbalanced current injection control strategy aiming at inhibiting active power fluctuation, and a proper amount of negative sequence current is injected to realize stable output of active power. The literature "R.Sharma,and A.Das,Analysis of solar PV fed cascaded H-bridge converter during low voltage ride operation,2020IEEE International Conference on Power Electronics,Drives and Energy Systems(PEDES),Jaipur,India,Dec.16-19,2020.(R.Sharma,and A.Das, discloses analysis of low-voltage ride through operation of a solar photovoltaic power supply cascade H-bridge converter, international conference of IEEE power electronics, driving and energy systems in 2020, indomeply, 12 months in 2020, 16 days to 19 days) "deduces an expression of proper zero sequence voltage, and ensures that the system can balance direct current bus voltage and inter-phase power of a cascade H-bridge photovoltaic grid-connected inverter connected in a three-phase star-shaped manner under the condition of asymmetric grid dip. Document "H.Li,Z.Gao,S.Ji,Y.Ma,and F.Wang,An inrush current limit method for SiC-based multi-level grid-connected converter during Low voltage ride through,2021IEEE Applied Power Electronics Conference and Exposition(APEC),Phoenix,AZ,USA,Jun.14-17,2021.(H.Li,Z.Gao,S.Ji,Y.Ma,and F.Wang, discloses a surge current limiting method in a low-voltage ride through process of a SiC-based multi-lev