Faulty substation and network coordination and stability control method and system for inverter cluster

Abstract

The application discloses a kind of fault under substation follow network inverter cluster coordination stable control method and system, after substation decouples into island operation, by network type converter establishes bus voltage and frequency reference, and network type converter is according to substation load demand and participates active support.For the problem that active reference of network type converter is prone to output current increase, phase-locked loop coupling enhancement and power oscillation in rising process, the application takes stability margin perception damping enhancement module and multi-constraint adaptive damping compensation module as core, assesses system stability margin by stability margin perception damping enhancement module, and outputs active reference slope limiting, current limiting and damping compensation input instruction;Through multi-constraint adaptive damping compensation module, damping compensation amount is generated and the voltage reference of network type converter is corrected, so as to improve the equivalent damping of system, inhibit power oscillation, and guarantee the continuous power supply of important load of substation.

Description

Technical Field

[0001] This invention relates to the field of distributed energy and islanded microgrid stability control technology, and in particular to a method and system for coordinated stability control of transformer substations and grid-connected inverter clusters under fault conditions. Background Technology

[0002] As the proportion of distributed photovoltaic, energy storage systems, and flexible loads connected to low-voltage distribution areas continues to increase, these areas are gradually exhibiting characteristics of power electronics and multi-source collaborative operation. During normal grid-connected operation, the voltage and frequency of the distribution area are supported by the upstream power grid. When the upstream power grid fails, the grid-connected switch is disconnected, or the distribution area actively disconnects from the grid, the area needs to switch to islanded operation. The grid-connected converter establishes the bus voltage and frequency, and the grid-connected converter participates in load power support to ensure continuous power supply to important loads.

[0003] In parallel operation scenarios of grid-connected and grid-linked converters, the grid-linked converter relies on a phase-locked loop (PLL) to track the phase of the point of common coupling (PCC) voltage and outputs current according to the power reference. When the load increases after islanding operation or the output pressure of the grid-connected converter increases, the grid-linked converter needs to increase its active power output to share the load. However, an increase in the active power reference leads to an increase in its output current, a stronger tie-line impedance voltage drop, and a greater phase angle shift, which intensifies the coupling between the PLL and the PCC voltage, easily causing power oscillations or even system instability.

[0004] Existing methods primarily focus on droop control of grid-connected inverters, energy storage power distribution, or centralized coordination control. Droop control is prone to voltage and frequency deviations during load surges, while centralized coordination control relies on communication links and faces delays or failure risks in fault-clearing scenarios. Simply limiting current in grid-connected inverters can restrict overcurrent, but it cannot provide additional damping and is insufficient to suppress power oscillations. Therefore, there is an urgent need for a coordinated and stable control method for the grid-connected inverter cluster under extreme fault conditions, which can improve system damping and stability margin while ensuring power support. Summary of the Invention

[0005] The technical problem to be solved by the present invention is to provide a method and system for coordinated and stable control of inverter clusters in the grid under fault conditions, which addresses the shortcomings of the existing technology and improves system damping and stability margin while ensuring power support.

[0006] To solve the above-mentioned technical problems, the technical solution adopted by the present invention is: a method for coordinated and stable control of inverter clusters in a fault zone and grid, comprising the following steps: S1, Three-phase voltage u at the common connection point of the sampling station area PCC,a u PCC,b u PCC,c The three-phase output voltage u of the grid converter GFL,a u GFL,b uGFL,c The output three-phase current of the grid converter is i GFL,a i GFL,b i GFL,c Three-phase load voltage u L,a u L,b u L,c Three-phase load current i L,a i L,b i L,c System frequency f and grid connection switch status S br ; S2, based on the three-phase voltage u at the point of common coupling. PCC,a u PCC,b u PCC,c The effective value of the voltage at the point of common coupling, U, is obtained. PCC The system frequency is obtained based on the phase-locked loop output or frequency detection circuit; when the common coupling point voltage deviation, system frequency deviation, or grid-connected switch status meets the disconnection criterion and continues for more than a set time T. iso When this occurs, the transformer area is determined to have entered islanded operation mode. Specifically, the voltage deviation ΔU = |U PCC -U ref The frequency deviation Δf = |f - f0|. When ΔU > U th Δf>f th or S br If any of the conditions in the condition = 0 are met, and the duration is greater than T, then... iso At that time, it is determined that the station area has entered an islanded operation state; S3. After the transformer area enters islanded operation, the grid-type inverter cluster switches to voltage and frequency support mode to provide synchronous voltage and frequency references for the subsequent grid-connected inverter cluster; the grid-connected inverter cluster switches to power support mode and enters the subsequent active power reference generation, current limiting and damping compensation control process. S4. Based on the output three-phase voltage u of the grid-connected converter GFL,a u GFL,b u GFL,c and the output three-phase current i of the grid-type converter GFL,a i GFL,b i GFL,c Calculate the active power P output of the grid converter. GFL and reactive power Q GFL According to the three-phase voltage u of the load L,a u L,b u L,c and load three-phase current i L,a i L,b i L,c Calculate the active power P of the load in the transformer area. Load ; S5. Based on the active power P of the transformer area load.Load The output active power P of the grid-type converter GFM And the active power output P of the grid-type converter GFL Calculate the active power deficit under islanded operation; S6. Based on the active power support reference of the grid-connected converter. Reactive power reference Common coupling point voltage U PCC and the upper limit of current I max The d-axis and q-axis current references after limiting are obtained. , ; S7, Limit the current reference , Relative to the actual d-axis and q-axis currents i GFL,d i GFL,q By comparison, and after PI regulation and dq-axis feedforward decoupling, the original d- and q-axis voltage references e of the grid converter are obtained. d e q Among them, the active power support reference of the grid-connected converter Determined based on the aforementioned active power deficit; S8. Based on the active power P output of the grid-connected converter GFL Phase-locked loop frequency f PLL Common coupling point voltage U PCC and output current amplitude I GFL The system stability evaluation index S is calculated, and the coordinated adjustment parameters are output based on the stability evaluation index; the coordinated adjustment parameters include the upper limit of the active power reference change rate. Current limit value and compensation investment mark ; S9. Based on the compensation input indicators Combined with the phase θ of the phase-locked loop PLL The output three-phase current of the grid converter is i GFL,abc Output active power P GFL The original d-axis and q-axis voltage references of the grid-type converter are e. d e q and modulation margin M margin This generates damping voltage and corrects the three-phase voltage reference of the grid converter; S10. PWM modulation is applied to the corrected three-phase voltage reference to obtain the grid-connected converter switching drive signal, which controls the on / off state of the grid-connected converter switching devices.

[0007] This invention addresses the islanded operation scenario after a transformer substation is disconnected from the grid. It establishes a voltage and frequency reference through a grid-connected converter and supports load power through a grid-connected converter. When the grid-connected converter increases its active power output, it avoids overcurrent and current loop saturation by limiting current, increases the equivalent damping at the output end through virtual resistance, and adaptively adjusts control parameters in conjunction with stability evaluation indicators. This suppresses or delays power oscillations and improves the continuity of power supply to important loads and the system stability margin during islanded operation after a transformer substation is disconnected from the grid under extreme fault conditions in rural areas.

[0008] The active power P of the transformer area load Load The calculation formula is: ; Among them, u a (n), u b (n), u c (n) represent the three-phase voltage at the nth sampling point, i a (n), i b (n), i c (n) represents the three-phase current at the nth sampling point, and N represents the number of sampling points in one power frequency cycle.

[0009] Active power deficit The calculation formula is: .

[0010] when When >0, generate active power support reference for grid-type converters. : ; Among them, P GFL,ref0 To provide a reference for the active power of the grid-connected inverter before disconnection, k P P is the power sharing factor. GFL,max The maximum active power allowed for grid-connected inverters.

[0011] The expression for the system stability evaluation index S is: Among them, P GFL,avg P represents the average active power of the grid-connected converter after low-pass filtering. N and I N These represent the rated power and rated current of the grid-connected converter, respectively, f0 and U. ref These represent the rated frequency and rated voltage, respectively, with k1~k4 being weighting coefficients. The voltage at the point of common coupling, This refers to the output current amplitude of the grid-connected converter.

[0012] This stability evaluation index considers the output power oscillation of the grid-connected inverter, the frequency deviation of the phase-locked loop, the voltage deviation of the point of common coupling, and the output current load factor. It can characterize the system stability margin from four dimensions: power, voltage, frequency, and current. Compared with a single voltage or frequency threshold criterion, it can identify the oscillation trend that occurs during the active power support process of the grid-connected inverter earlier.

[0013] The compensation investment mark The specific determination process includes: when hour, Subsequent damping compensation is then implemented; when hour, No damping compensation is provided; To stabilize the evaluation threshold.

[0014] Compensation Investment Mark Based on system stability evaluation indicators With stability evaluation threshold The comparison results determine the on-demand input used to achieve damping compensation. When At that time, the system stability margin meets the requirements, making To avoid the long-term impact of damping compensation on the steady-state voltage reference and power point tracking accuracy of grid-connected inverters; when At that time, the system stability margin was insufficient, causing Damping compensation is activated promptly to suppress power oscillations. This compensation activation flag allows for a coordinated switching between steady-state operating accuracy and transient oscillation suppression.

[0015] The corrected three-phase voltage reference is expressed as follows: ; in, , K represents the d-axis and q-axis voltage references after damping compensation, i.e., the corrected three-phase voltage references. m K represents the modulation margin coefficient. m =M margin / (M margin +M0), where M0 represents the margin smoothing constant, u damp,d u damp,q These represent the d-axis and q-axis damping voltages, respectively.

[0016] The corrected three-phase voltage reference is first damped and compensated in the dq synchronous rotating coordinate system, and then obtained through dq / abc coordinate transformation. The technical advantage of this calculation method is that, on the one hand, the damping compensation amount u... damp,d u damp,q Compared with the original voltage reference e d e qBeing in the same coordinate system and phase reference can avoid miscompensation caused by inconsistent compensation voltage phase; on the other hand, by using the compensation activation flag... Whether damping compensation is engaged is controlled by adjusting the margin coefficient K. m Limiting the compensation voltage amplitude can suppress power oscillations while avoiding PWM modulation saturation. Therefore, this correction method can balance transient oscillation suppression, steady-state power tracking, and modulation safety margin.

[0017] The formulas for calculating the d-axis and q-axis damping voltages are as follows: ; where k p k represents the active oscillation damping coefficient. q Represents the active oscillation damping coefficient. Represents adaptive virtual resistance, i GFL,d i GFL,q These are the d-axis and q-axis currents, respectively, after the three-phase output current of the grid converter has been transformed into a synchronous rotating coordinate system. GFL,avg This represents the average active power of the grid-connected converter after low-pass filtering. This represents the average reactive power of the grid-connected converter after low-pass filtering.

[0018] on the one hand, and Virtual resistor voltage drops in phase with the output current are introduced on the d and q axes respectively, thereby increasing the equivalent damping at the output of the grid-connected inverter; on the other hand, k p (P GFL -P GFL,avg The active power oscillation is fed back to the d-axis damping voltage to suppress power oscillations generated during active power support. q (Q GFL -Q GFL,avg The reactive power oscillation is fed back into the q-axis damping voltage to suppress reactive power fluctuations and point of common coupling voltage disturbances. This calculation method enables composite compensation of current damping and power oscillation damping, providing stronger oscillation suppression compared to single virtual resistance compensation.

[0019] Among them, R v0 k represents the initial virtual resistance. R M represents the virtual resistance adjustment coefficient. margin M0 represents the modulation margin, and M0 represents the modulation margin smoothing constant, used to avoid M margin When the value is small, the denominator is too small, causing a sudden increase in virtual resistance; R v,min and R v,max These represent the lower and upper limits of the virtual resistance, respectively. =max(SS th ,0) / (S max -Sth ) represents the degree of inadequacy in stability margin; S max Represents the maximum compensation threshold, when S S max At that time, the damping compensation reaches its maximum level of investment.

[0020] first, Based on the degree of inadequacy of stability margin α S Adaptive adjustment is implemented: when the system has sufficient stability margin, a small compensation is maintained to avoid affecting steady-state power tracking; when system power oscillations increase, the virtual resistance is increased to improve the equivalent damping at the output. Secondly, a modulation margin coefficient M is introduced. margin / (M margin +M0) makes the virtual resistance compensation constrained by the current PWM modulation margin, avoiding modulation saturation due to excessive compensation when the DC bus voltage utilization is high or the modulation margin is insufficient. Finally, through R v,min and R v,max The virtual resistance is limited to ensure that the compensation parameters are always within the achievable range, thereby balancing the damping enhancement effect, steady-state operation accuracy, and modulation security.

[0021] As an inventive concept, the present invention also provides a coordinated and stable control system for a fault-prone area and a grid-connected inverter cluster, including a memory, a processor, and a computer program stored in the memory; the processor executes the computer program to implement the steps of the above method.

[0022] Compared with the prior art, the beneficial effects of this invention are as follows: This invention is aimed at the islanded operation scenario after the distribution area is disconnected from the grid. It establishes a voltage and frequency reference through a grid-type converter and participates in load power support with a grid-connected converter. When the grid-connected converter increases the active power output, it avoids overcurrent and current loop saturation by current limiting, increases the equivalent damping at the output end by virtual resistance, and adaptively adjusts the control parameters in combination with the stability evaluation index, thereby suppressing or delaying power oscillation and improving the power supply continuity of important loads and the system stability margin during the islanded operation of the distribution area under extreme faults in rural areas. Attached Figure Description

[0023] Figure 1 Topology diagram of GFM-GFL parallel system under islanded operation of transformer substations in extreme fault conditions; Figure 2 Block diagram of stability margin sensing and multi-constraint adaptive damping compensation control for grid-connected converters; Figure 3 Comparison of power response of grid-connected converter before and after the damping module is put into operation; (a) power of grid-connected converter without the control method of Embodiment 1 of the present invention, (b) power of grid-connected converter with the control method of Embodiment 1 of the present invention. Detailed Implementation

[0024] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0025] Example 1 like Figure 1 As shown, an embodiment of the present invention provides a coordinated stability control method for distribution transformer areas and grid-connected inverter clusters under extreme fault conditions. This method is applicable to low-voltage islanded microgrids in distribution transformer areas where grid-connected converters (GFM) and grid-connected converters (GFL) are connected in parallel to the point of common coupling. During grid-connected operation, the distribution transformer area receives voltage and frequency support from the upstream grid. When the upstream grid fails or the grid connection switch is disconnected, the distribution transformer area enters islanded operation. The grid-connected converter switches to voltage and frequency support mode, while the grid-connected converter switches to power support mode. During the active power reference rise process, the grid-connected converter is prone to increased output current, enhanced phase-locked loop coupling, and power oscillations. Therefore, this invention introduces slope limiting, current limiting, stability margin sensing, and multi-constraint adaptive damping compensation in the grid-connected converter control to achieve stable support under islanded operation of the distribution transformer area under extreme fault conditions.

[0026] This embodiment provides a coordinated stability control method for distribution transformer areas and grid-connected inverter clusters under extreme fault conditions. It is applicable to isolated microgrids in rural distribution transformer areas that include both grid-connected and grid-connected inverters. Figure 2 As shown, an embodiment of the present invention, a grid-type converter stability margin sensing-multi-constraint damping compensation control block diagram, includes a slope limiting stage, a current limiting stage, a stability margin sensing damping enhancement module, a multi-constraint adaptive damping compensation module, a phase-locked loop, and a coordinate transformation module. The grid-type converter's active power reference P... ref First, the slope limiting process is obtained. This is used to avoid abrupt changes in the active power reference; subsequently, the outer power loop is based on... With P GFL The deviation generates the d-axis current reference, based on Q. ref With Q GFL The deviation generates a q-axis current reference. This current reference, constrained by a current limiting circuit, is input into the inner current loop. The inner current loop, after PI regulation and decoupling from the d-q axis feedforward, yields the original d- and q-axis voltage references e. d e q The control method in this embodiment specifically includes the following steps: 1) Three-phase voltage u at the common connection point of the sampling station area PCC,abc The three-phase output voltage u of the grid converterGFL,abc The output three-phase current of the grid converter is i GFL,abc Three-phase load voltage u L,abc Three-phase load current i L,abc System frequency f and grid connection switch status S br The sampling period is T. s N points are sampled per power frequency cycle. Specifically, the number of sampling points per power frequency cycle is obtained by the following formula: ; Where f0 represents the rated frequency of the substation, T s This represents the sampling period.

[0027] 2) Using a voltage and frequency detection module, based on the three-phase voltage u at the point of common coupling... PCC,abc The effective value of the voltage at the point of common coupling, U, is obtained. PCC When the voltage deviation at the point of common coupling, the system frequency deviation, or the grid-connected switch status meets the disconnection criterion and continues for more than a set time T. iso At that time, it was determined that the station area had entered an islanded operation state.

[0028] 3) After the distribution area enters islanded operation, the grid-type converter is controlled to enter the voltage and frequency support mode to establish the islanded bus voltage and frequency of the distribution area; the grid-type converter is controlled to enter the power support mode to participate in the active power support of the distribution area load.

[0029] 4) Using the instantaneous power calculation module, based on the three-phase output voltage u of the grid-connected converter... GFL,abc and output three-phase current i GFL,abc Calculate the active power P output of the grid converter. GFL and reactive power Q GFL According to the three-phase voltage u of the load L,abc and load three-phase current i L,abc Calculate the active power P of the load in the transformer area. Load Specifically, active power is obtained using the following formula: ; Where P represents the average active power over one power frequency cycle, u a (n), u b (n), u c (n) represent the three-phase voltage at the nth sampling point, i a (n), i b (n), i c (n) represents the three-phase current at the nth sampling point, and N represents the number of sampling points in one power frequency cycle.

[0030] 5) Based on the active power P of the transformer area load Load The output active power P of the grid-type converterGFM And the active power output P of the grid-type converter GFL Calculate the active power deficit under islanded operation. Specifically, the active power deficit is obtained using the following formula: ; in, P represents the active power deficit under extreme fault conditions where the transformer substation is isolated and disconnected from the grid. Load P represents the active power of the load in the transformer area. GFM P represents the output active power of a grid-connected converter. GFL This represents the active power output of the grid-connected converter. When When >0, generate the active power support reference for the grid-connected converter. .

[0031] 6) Utilize the islanded power support constraint generation module, based on the active power support reference of the grid-connected converter. Reactive power reference Common coupling point voltage U PCC and the upper limit of current I max The d-axis and q-axis current references after limiting are obtained. , This ensures that the output current of the grid-connected converter does not exceed the allowable range when participating in power support.

[0032] 7) Limit the current reference , The input current inner loop of the grid-type converter is related to the actual d-axis and q-axis currents i. GFL,d i GFL,q By comparison, and after PI regulation and dq-axis feedforward decoupling, the original d- and q-axis voltage references e of the grid-type converter are obtained. d e q Among them, e d e q It serves as the voltage reference input for subsequent multi-constraint adaptive damping compensation modules.

[0033] 8) Utilizing the stability margin sensing damping enhancement module, based on the output active power P of the grid-connected converter... GFL Phase-locked loop frequency f PLL Common coupling point voltage U PCC and output current amplitude I GFL The system stability evaluation index S is calculated, and the coordinated adjustment parameters are output based on the stability evaluation index. The coordinated adjustment parameters include the upper limit of the active power reference rate of change. Current limiting value and compensation investment mark .

[0034] 9) Utilize the multi-constraint adaptive damping compensation module to sense the compensation input flag output by the damping enhancement module based on the stability margin. And combined with phase-locked loop phase The output three-phase current of the grid converter is i GFL,abc Output active power P GFL Original d-axis and q-axis voltage reference e d e q and modulation margin M margin This generates a damping voltage and corrects the voltage reference of the grid converter. Specifically, the corrected d-axis and q-axis voltage references are obtained using the following formula: ; in, , These represent the d-axis and q-axis voltage references after damping compensation, respectively, e d e q These represent the original d-axis and q-axis voltage references of the grid converter, respectively. K represents the indicator of compensation input. m Represents the modulation margin coefficient, u damp,d u damp,q These represent the d-axis and q-axis damping voltages generated by the multi-constraint adaptive damping compensation module, respectively. Then, , The corrected three-phase voltage reference is obtained after dq / abc coordinate transformation. And input to the PWM modulation module.

[0035] 10) The corrected three-phase voltage reference Input the PWM modulation module to obtain the switching drive signal of the grid-connected converter, drive the output of the grid-connected converter, so that it can achieve power support, current constraint and oscillation suppression during the islanding operation of the transformer area under extreme fault conditions and the rise of the active power reference.

[0036] Accordingly, in this embodiment of the invention, the stability margin sensing damping enhancement module is used to calculate stability evaluation indicators based on the operating status of the grid-connected converter and output coordinated adjustment parameters.

[0037] Specifically, the inputs of the stability margin sensing damping enhancement module include the active power P output from the grid-type converter. GFL Phase-locked loop output frequency f PLL Common coupling point voltage U PCC and the output current amplitude I of the grid-type converter GFL The output of the module includes the stability evaluation index S and the upper limit of the active power reference rate of change. Current limiting value and compensation investment mark .

[0038] First, construct stability evaluation indicators: ; Among them, P GFL,avg P represents the average active power of the grid-connected converter after low-pass filtering. N and I N These represent the rated power and rated current of the grid-connected converter, respectively, f0 and U. ref These represent the rated frequency and rated voltage, respectively, with k1~k4 being weighting coefficients.

[0039] when When the system stability margin is satisfied, the current control parameters are kept unchanged; when If the system is deemed to have insufficient stability margin, the upper limit of the active power reference rate of change is lowered and the current limit is tightened. Specifically: ; Where, α S =max(SS th ,0) / (S max -S th ), r P0 and I max0 These represent the upper limit of the initial active power reference rate of change and the initial current limit, respectively, with sat(.) denoteing the limiting function. r The active power reference rate of change adjustment coefficient is used to adjust the rate of increase of the active power reference according to the degree of insufficient stability margin; k I The current limiting adjustment coefficient represents the current limiting value of the grid-connected inverter, used to adjust the current limiting value according to the degree of insufficient stability margin; r P,min r P,max These represent the lower and upper limits of the allowable rate of change of the active power reference, respectively; I min Represents the lower limit of current limiting; I max0 This represents the initial current limit value and serves as the upper limit value during the current limit adjustment process.

[0040] The compensation investment mark According to S and S th The comparison results are determined; when hour, Subsequent damping compensation is then implemented; when hour, No damping compensation is provided.

[0041] Accordingly, the multi-constraint adaptive damping compensation module of the present invention is used to generate a damping voltage and correct the grid converter voltage reference based on the coordinated adjustment parameters output by the stability margin sensing damping enhancement module.

[0042] Specifically, the inputs of the multi-constraint adaptive damping compensation module include the stability evaluation index S and the compensation input flag. Phase-locked loop phase Phase-locked loop angular frequency The output three-phase current of the grid converter is i GFL,abc The active power output P of the grid-type converter GFL Original d-axis and q-axis voltage reference e d e q DC bus voltage U dc and modulation margin M margin The module's output includes corrected d-axis and q-axis voltage references. , and the corrected three-phase voltage reference .

[0043] The module generates an adaptive virtual resistor based on stability evaluation indicators and modulation margin: ; in, Represents the adaptive virtual resistance, R v0 k represents the initial virtual resistance. R M0 represents the virtual resistance adjustment coefficient, and M0 represents the margin smoothing constant.

[0044] Subsequently, using the phase θ of the phase-locked loop PLL Transform the three-phase output current of the grid converter to a synchronous rotating coordinate system to obtain i GFL,d i GFL,q And generate a damping voltage: ; Among them, u damp,d u damp,q Representing the d-axis and q-axis damping voltages respectively, k p k represents the active oscillation damping coefficient. q This represents the active oscillation damping coefficient.

[0045] To avoid PWM modulation saturation caused by the compensation voltage, the module constrains the compensation amount based on the modulation margin and obtains a corrected voltage reference: ; Among them, K m K represents the modulation margin coefficient. m =M margin / (M margin +M0), M margin M0 represents the PWM modulation margin; M0 represents the modulation margin smoothing constant, used to avoid M margin When the value is small, the compensation coefficient changes abruptly. d eq These represent the original d-axis and q-axis voltage references of the inner current loop output of the grid-connected inverter, respectively. , These represent the corrected d-axis and q-axis voltage references, respectively. Then, , The corrected three-phase voltage reference is obtained after dq / abc coordinate transformation. And send it to the PWM modulation module; Indicates the input of damping compensation; u damp,d u damp,q These represent the d-axis and q-axis damping compensation voltages, respectively.

[0046] like Figure 3 As shown, this is a comparison diagram of the power response of a grid converter before and after the compensation damping module is put into operation, according to an embodiment of the present invention. Figure 3 In the figure, (a) shows the simulation results without the introduction of the compensation damping module. A sudden load increase occurs at 2s. After the active power reference of the grid converter rises at 3s, the output active power P of the grid converter is... GFL and reactive power Q GFL (a) The oscillations gradually amplified and eventually became unstable. (b) The simulation results after introducing the compensation damping module show that, under the same operating conditions, the active power output of the grid-connected converter can stably transition to the new operating point, and the reactive power does not exhibit divergent oscillations. The simulation results show that the control method of this embodiment can improve system damping, suppress power oscillations, and enhance the power supply stability of important loads in the distribution area during islanded operation of the distribution area and active power support of the grid-connected converter under extreme fault conditions.

[0047] Example 2 Embodiment 2 of the present invention provides a system corresponding to Embodiment 1 above, including a memory, a processor, and a computer program stored in the memory; the processor executes the computer program in the memory to implement the steps of the method of Embodiment 1 above.

[0048] In some implementations, the memory may be high-speed random access memory (RAM), and may also include non-volatile memory, such as at least one disk storage device.

[0049] In other implementations, the processor can be any type of general-purpose processor, such as a central processing unit (CPU) or a digital signal processor (DSP), and there is no limitation here.

[0050] Although preferred embodiments of this application have been described, those skilled in the art, upon learning the basic inventive concept, can make other changes and modifications to these embodiments. Therefore, the appended claims are intended to be interpreted as including the preferred embodiments as well as all changes and modifications falling within the scope of this application.

[0051] Obviously, those skilled in the art can make various modifications and variations to this application without departing from the spirit and scope of this application. Therefore, if such modifications and variations fall within the scope of the claims of this application and their equivalents, this application also intends to include such modifications and variations.

Claims

1. A method for coordinated stability control of inverter clusters and grid-connected transformer substations under fault conditions, characterized in that, Includes the following steps: S1, Three-phase voltage u at the common connection point of the sampling station area PCC,a u PCC,b u PCC,c The three-phase output voltage u of the grid converter GFL,a u GFL,b u GFL,c The output three-phase current of the grid converter is i GFL,a i GFL,b i GFL,c Three-phase load voltage u L,a u L,b u L,c Three-phase load current i L,a i L,b i L,c System frequency f and grid connection switch status S br ; S2, based on the three-phase voltage u at the point of common coupling. PCC,a u PCC,b u PCC,c The effective value of the voltage at the point of common coupling, U, is obtained. PCC ; Determine whether the station area has entered islanded operation mode; The specific process for determining whether a transformer area enters islanded operation includes: defining voltage deviation ΔU = |U PCC -U ref |, frequency deviation Δf=|f-f0|, when ΔU>U th Δf>f th or S br If any of the conditions in condition = 0 is met, and the duration is greater than the set time T, then... iso At that time, it was determined that the station area had entered an islanded operation state; among them, U th f is the voltage deviation threshold. th S is the frequency deviation threshold. br S is the status variable of the grid-connected switch. br =0 indicates that the grid connection switch is open, S br =1 indicates that the grid-connected switch is closed, f0 and U ref These represent the rated frequency and rated voltage, respectively. S3. After the transformer area enters islanded operation, the grid-type inverter cluster switches to voltage and frequency support mode; the grid-connected inverter cluster switches to power support mode and proceeds to step S4. S4. Based on the output three-phase voltage u of the grid-connected converter GFL,a u GFL,b u GFL,c and the output three-phase current i of the grid-type converter GFL,a i GFL,b i GFL,c Calculate the active power P output of the grid converter. GFL and reactive power Q GFL According to the three-phase voltage u of the load L,a u L,b u L,c and load three-phase current i L,a i L,b i L,c Calculate the active power P of the load in the transformer area. Load ; S5. Based on the active power P of the transformer area load. Load The output active power P of the grid-type converter GFM And the active power output P of the grid-type converter GFL Calculate the active power deficit under islanded operation; S6. Based on the active power support reference of the grid-connected converter. Reactive power reference Common coupling point voltage U PCC and the upper limit of current I max The d-axis and q-axis current references after limiting are obtained. ; S7, Limit the current reference Relative to the actual d-axis and q-axis currents i GFL,d i GFL,q By comparison, and after PI regulation and dq-axis feedforward decoupling, the original d- and q-axis voltage references e of the grid converter are obtained. d e q Among them, the active power support reference of the grid-connected converter Determined based on the aforementioned active power deficit; S8. Based on the active power P output by the grid-connected converter GFL Phase-locked loop frequency f PLL Common coupling point voltage U PCC and output current amplitude I GFL The system stability evaluation index S is calculated, and the coordinated adjustment parameters are output based on the stability evaluation index; the coordinated adjustment parameters include the upper limit of the active power reference change rate. Current limit value and compensation investment mark ; S9. Based on the compensation input indicator Combined with the phase θ of the phase-locked loop PLL The output three-phase current of the grid converter is i GFL,abc Output active power P GFL The original d-axis and q-axis voltage references of the grid-type converter are e. d e q and modulation margin M margin This generates damping voltage and corrects the three-phase voltage reference of the grid converter; S10. PWM modulation is applied to the corrected three-phase voltage reference to obtain the grid-connected converter switching drive signal, which controls the on / off state of the grid-connected converter switching devices.

2. The method for coordinated stability control of inverter clusters in fault-prone areas according to claim 1, characterized in that, The active power P of the transformer area load Load The calculation formula is: ； Among them, u a (n), u b (n), u c (n) represent the three-phase voltage at the nth sampling point, i a (n), i b (n), i c (n) represents the three-phase current at the nth sampling point, and N represents the number of sampling points in one power frequency cycle.

3. The method for coordinated stability control of inverter clusters in fault-prone areas according to claim 1, characterized in that, Active power deficit The calculation formula is: .

4. The method for coordinated stability control of inverter clusters in fault-prone areas according to claim 3, characterized in that, when When >0, generate active power support reference for grid-type converters. ; ； Among them, P GFL,ref0 Before the transformer area enters islanded operation, the active power reference of the grid converter is k. P P is the power sharing factor. GFL,max The maximum active power allowed for grid-type converters.

5. The method for coordinated stability control of inverter clusters in fault-prone areas according to claim 1, characterized in that, The expression for the system stability evaluation index S is: Among them, P GFL,avg P represents the average active power of the grid-connected converter after low-pass filtering. N and I N These represent the rated power and rated current of the grid-connected converter, respectively, f0 and U. ref These represent the rated frequency and rated voltage, respectively, with k1~k4 being weighting coefficients. The voltage at the point of common coupling, This refers to the output current amplitude of the grid-connected converter.

6. The method for coordinated stability control of inverter clusters in fault-prone areas according to claim 1, characterized in that, The compensation investment mark The specific determination process includes: when hour, Subsequent damping compensation is then implemented; when hour, No damping compensation is provided; To stabilize the evaluation threshold: ； Where, ΔP th The permissible active power oscillation threshold, Δf th The allowable phase-locked loop frequency deviation threshold, ΔU th For the allowable common point of connection voltage deviation threshold, I th λ is the threshold value for the allowable output current amplitude of a grid-connected inverter. s P represents the stability margin coefficient, k1~k4 are the weighting coefficients, and P is the stability margin coefficient. N and I N These represent the rated power and rated current of the grid-connected converter, respectively.

7. The method for coordinated stability control of inverter clusters in fault-prone areas according to claim 1, characterized in that, The corrected three-phase voltage reference is expressed as follows: ； in, K represents the d-axis and q-axis voltage references after damping compensation, i.e., the corrected three-phase voltage references. m K represents the modulation margin coefficient. m =M margin / (M margin +M0), where M0 represents the margin smoothing constant, u damp,d u damp,q These represent the d-axis and q-axis damping voltages, respectively. This is a sign to compensate for the investment.

8. The method for coordinated stability control of fault-prone areas and grid-connected inverter clusters according to claim 7, characterized in that, The formulas for calculating the d-axis and q-axis damping voltages are as follows: ; where k p k represents the active oscillation damping coefficient. q Represents the active oscillation damping coefficient. Represents adaptive virtual resistance, i GFL,d i GFL,q These are the d-axis and q-axis currents after the three-phase output current of the grid converter has been transformed into a synchronous rotating coordinate system, respectively. GFL,avg This represents the average active power of the grid-connected converter after low-pass filtering. This represents the average reactive power of the grid-connected converter after low-pass filtering.

9. The method for coordinated stability control of fault-prone areas and grid-connected inverter clusters according to claim 8, characterized in that, Among them, R v0 k represents the initial virtual resistance. R R represents the virtual resistance adjustment coefficient; v,min and R v,max These represent the lower and upper limits of the virtual resistance, respectively. This represents the degree of insufficient stability margin. =max(SS th ,0) / (S max -S th ) represents the degree of inadequacy in stability margin; S max Represents the maximum compensation threshold, when S S max At that time, the damping compensation reaches its maximum level of investment. To stabilize the evaluation threshold.

10. A coordinated stability control system for a fault-prone area and grid-connected inverter cluster, comprising a memory, a processor, and a computer program stored in the memory; characterized in that, The processor executes the computer program to implement the steps of the method according to any one of claims 1 to 9.

Images