Inverse time current limiting method, device and medium based on grid-type converter

Abstract

The present invention discloses an inverse time current limiting method, device and medium based on a grid-type converter. The method comprises collecting the three-phase voltage outputted from the grid-type converter at the grid-connected point, obtaining the initial reference values ​​of the d-axis and q-axis components of the output current of the grid-type converter and further obtaining the reference value of the current vector amplitude; calculating the current at the current moment t The difference Δ from the timing time t , update the maximum overcurrent value, compare the reference value of the current vector amplitude with the maximum overcurrent value to determine whether to correct the reference value of the current vector amplitude. Cyclic monitoring of the reference value of the current vector amplitude of the grid-type converter, until the reference value of the current vector amplitude is less than or equal to the per-unit value of the rated value of the grid-type converter, stop limiting. The present invention increases the overcurrent limit value of the converter in the initial stage when the GFM touches the limit, and then reduces it to the steady-state limit value, thereby improving the transient stability margin of the grid-type converter.

Description

Technical Field

[0001] The present invention relates to the field of current control of a grid-type converter, and in particular to an inverse time current limiting method, device and medium based on a grid-type converter. Background Art

[0002] Traditional renewable energy generation (RES) is integrated into the system using grid-following control to maximize the conversion of primary energy into electrical energy for transmission to the grid. With the increase in RES penetration, the inherent inertia of the power system decreases, which deteriorates the operational stability of the power system. To solve this problem, the grid forming converter (GFM) control technology with inertia simulation, frequency construction and voltage support capabilities has attracted widespread attention. After the grid is disturbed, the converter using GFM needs to output a large current to maintain a constant voltage amplitude and phase. However, since the overcurrent capacity of the power electronic converter is usually only 1.1~1.5 times its rated capacity, the GFM is very likely to enter the current limiting mode after the disturbance.

[0003] In the prior art, such as the current limiting method and device (CN117937916A), different control strategies are adopted for the positive-sequence component, negative-sequence component and zero-sequence component in the output current and voltage of the converter, and multiple limiting controls are performed on the dq-axis positive-sequence current component and the dq-axis negative-sequence current component, so as to achieve the control target that the three-phase current reference value output by the converter is less than the overcurrent point, which is beneficial to avoid converter failure caused by overcurrent and maintain the safe and stable operation of the power grid.

[0004] However, in the related art, a preset current threshold is used to limit the output current of the grid-type converter. The preset current threshold is set according to the steady-state overcurrent capacity of the grid-type converter, and the overcurrent capacity of the grid-type converter is not fully utilized.

[0005] In summary, there are technical problems in related technologies that need to be solved urgently. Summary of the invention

[0006] In order to solve at least one of the problems existing in the prior art and improve the transient stability of the system, the present invention provides an inverse time current limiting method, device and medium based on a grid-type converter. According to the duration after the fault occurs, the inverse time characteristics of the converter are fully utilized to limit the output current of the converter to different degrees, which is beneficial to avoid system stability problems caused by the use of steady-state limiting values ​​and improves the transient stability margin of the system.

[0007] In order to achieve the purpose of the present invention, the present invention provides an inverse time current limiting method based on a grid-type converter, the method comprising the following steps:

[0008] Step 1, collecting the three-phase voltage of the grid-connected point of the grid-connected converter, and performing coordinate transformation on the voltage to obtain the d-axis and q-axis components of the voltage;

[0009] Step 2: Obtain the initial reference values ​​of the d-axis and q-axis components of the grid-type converter output current through the voltage controller, and then obtain the reference value of the current vector amplitude. ;

[0010] Step 3: Set the current reference value Per unit value with grid-connected converter rating For comparison, if > , then execute step 4; otherwise, execute step 10;

[0011] Step 4: Determine the timing time Is the value of zero? If it is zero, go to step 5, otherwise go to step 6;

[0012] Step 5: Update the timing time The value of Equal to the current time t The value of

[0013] Step 6: Calculate the current time t With the timing The difference Δ t , according to the difference Δ t Update the maximum overcurrent value to ,in, is the update function, including piecewise functions at different stages , n is a non-zero natural number, The maximum value is greater than the overcurrent value set according to the steady-state overcurrent capacity and The minimum value is the overcurrent value set according to the steady-state overcurrent capability ;

[0014] Step 7: Set the current reference value and the maximum overcurrent value For comparison, if > , then go to step 8, otherwise go to step 9;

[0015] Step 8: Correct the current reference value ,make = , and according to the current limiting strategy, the updated D-axis and Q-axis reference values ​​assigned to the current controller;

[0016] Step 9: Maintain current reference value No change, return to step 1, and repeat the above steps;

[0017] Step 10: Set the timing time = 0, exit clipping.

[0018] Furthermore, in order to convert the three-phase value of the voltage into a DC value in the dq rotating coordinate system, the coordinate transformation matrix in step 1 is constructed using equation (1):

[0019] (1)

[0020] In the formula, represents the coordinate transformation matrix; represents the reference angular frequency; t Indicates the current moment; m Represents an arbitrary constant.

[0021] Furthermore, the mathematical model of the d-axis and q-axis components of the three-phase voltage in step 1 in dq coordinates is:

[0022] (2)

[0023] in, , They are the d-axis and q-axis components of the three-phase voltage at the grid-connected point of the grid-forming converter respectively; is the value of the filter capacitor; , They are the d-axis and q-axis components of the grid-connected point output current, , are the d-axis and q-axis currents output from the grid-type converter port, Indicates the reference angular frequency.

[0024] Use equations (3) and (4) to construct a voltage controller:

[0025] (3)

[0026] (4)

[0027] In the formula, , They represent the initial reference values ​​of the d-axis and q-axis components of the voltage controller integrator output respectively; , They represent the d-axis and q-axis components of the grid-connected point output current respectively; Represents filter capacitor; , They represent the d-axis and q-axis components of the grid connection point voltage respectively; represents the proportional gain of the voltage controller; represents the integral gain of the voltage controller; , They are the d-axis and q-axis components of the voltage controller respectively.

[0028] In step 2, the d and q current components of the output current are used to obtain the reference value of the synthetic output current amplitude: .

[0029] (5)

[0030] Step 4 is to update the maximum overcurrent value using formula (4) :

[0031] (6)

[0032] In the formula, is the maximum overcurrent value; Respectively represent the piecewise functions at different stages of the update function; They represent different stages of the update function respectively; n is a non-zero natural number.

[0033] The update function in step 4 at different stages is expressed by the function of formula (5):

[0034] (7)

[0035] In the formula, Indicates different stages, n Take a non-zero natural number; For Follow t A monotonically decreasing function.

[0036] Furthermore, the update function is a monotonically decreasing function.

[0037] Furthermore, in step 8, the inverse time-limited clipping strategy shown in equation (6) is used to Reference values ​​assigned to the d and q axis current controllers:

[0038] (8)

[0039] In the formula, , They respectively represent the d-axis and q-axis current reference values ​​before the inverse time limiting method takes effect; , They respectively represent the d-axis and q-axis current loop reference values ​​after the inverse time limiting method is applied.

[0040] The present invention also provides an electronic device, comprising a memory, a processor, and a computer program stored in the memory and executable on the processor, wherein the processor executes the program to implement the steps of any one of the methods described in the present invention.

[0041] The present invention also provides a computer-readable storage medium having a computer program / instruction stored thereon, wherein the computer program / instruction is executed by a processor to implement the steps of any method described in the present invention.

[0042] Compared with the prior art, the present invention has at least the following beneficial effects:

[0043] The present invention is based on the fact that the overcurrent capacity of the converter will decrease in proportion to the increase of the junction temperature, thereby showing an inverse time characteristic, and proposes an inverse time current limiting method, device and medium based on the grid-type converter. The present invention makes full use of this inverse time characteristic to increase the overcurrent limit value of the converter in the initial stage when the GFM touches the limit, that is, to meet the requirements in the initial stage. Greater than the limit value set according to the steady-state overcurrent capacity ( ), instead of directly using the limit value set according to the steady-state overcurrent capacity as the preset threshold, thereby improving the transient stability of the grid-type converter under the following conditions. BRIEF DESCRIPTION OF THE DRAWINGS

[0044] Figure 1 An overall flow chart of an inverse time current limiting method based on a grid-type converter provided by an embodiment of the present invention;

[0045] Figure 2 An overall control structure diagram of an inverse time current limiting method based on a grid-type converter provided in an embodiment of the present invention;

[0046] Figure 3 A schematic diagram of an inverse time function curve of an inverse time current limiting method based on a grid-type converter provided in an embodiment of the present invention;

[0047] Figure 4 It is a schematic diagram of simulation results comparing the inverse time limiting method and the conventional limiting method when a three-phase metallic short circuit fault occurs at a grid connection point according to an embodiment of the present invention. DETAILED DESCRIPTION

[0048] In order to make the purpose and technical solution of the present invention clearer and easier to understand, the technical solution of the present invention will be described in detail below in conjunction with the drawings in the embodiments. It should be pointed out that the described embodiments are only some examples of the present invention and do not cover all possible embodiments. Based on these embodiments, other embodiments derived by ordinary technicians in this field without creative work should also be within the scope of protection of the present invention.

[0049] For ease of understanding, please refer to Figure 2 The energy storage power supply single load system using grid-type control is introduced in detail. The primary circuit is an electrochemical battery that maintains a DC voltage through a DC / DC bidirectional converter. The DC side contains a chopper protection circuit to prevent overvoltage on the DC side. The load is then powered by a DC / AC converter through a filter circuit, a step-up transformer, and a transmission line. The secondary control circuit includes an active control loop, a reactive control loop, a d / q axis voltage controller, a d / q axis current controller, an inverse time limiter, a fault monitoring module, and an update function module. The fault monitoring module determines the effective value of the three-phase voltage. Is it less than the low voltage threshold? To decide whether to start the update function and freeze the integrator of the voltage controller, when < Then the update function is started and the integrator of the voltage controller is frozen. freeze_ integrator = 1, otherwise the update function is not started and the integrator of the voltage controller is not frozen. freeze_ integrator = 0. The active power control loop is based on the active power setpoint Output active power of grid-connected converter P The deviation generates a phase angle θ , with phase angle θ As the transformation angle of Park transformation, the three-phase voltage at the grid connection point and current Transform to obtain the d-axis and q-axis components of the grid-connected point voltage , , d-axis and q-axis components of the grid-connected point output current , The reactive power control loop is based on the reactive power setting value. The reactive power output of the grid-connected converter Q The deviation generates the voltage correction amount Δ V , thus setting the voltage Superposition Δ V As a reference value for the d-axis voltage controller The reference value of the q-axis voltage loop is , in some embodiments of the present invention, The value is 0. The initial reference values ​​of the d-axis and q-axis components of the output current of the grid-type converter are obtained through the d-axis and q-axis voltage controllers. , Then, the update function gets and the initial reference values ​​of the d-axis and q-axis components of the output current , , the d-axis and q-axis current reference values ​​are obtained through the inverse time limiter , . d-axis and q-axis current reference values , The three-phase voltage reference value of the grid-connected converter is obtained through the d-axis and q-axis current control loops and Park inverse transformation. .

[0050] Reference Figure 1 The present invention provides an inverse time current limiting method based on a grid-connected converter, the method comprising the following steps:

[0051] Step 1: Collect the three-phase voltage of the grid-connected point of the grid-forming converter, and transform the three-phase voltage by a left-hand coordinate transformation matrix to obtain the d-axis and q-axis components of the three-phase voltage;

[0052] Step 2: Obtain the initial reference values ​​of the d-axis and q-axis components of the output current of the grid-connected converter through the voltage controller. , , and obtain a reference value of the current vector amplitude based on the initial reference value ;

[0053] Step 3: Set the reference value of the current vector amplitude Per unit value with grid-connected converter rating For comparison, if > , then execute step 4; otherwise, execute step 10, wherein, in some embodiments of the present invention, The value is 1.0 pu;

[0054] Step 4: Get the timing time from storage The value of overcurrent is used to determine the overcurrent moment. Is the value of zero? If it is zero, go to step 5, otherwise go to step 6;

[0055] Step 5: Update the timing time The value of Equal to the current time t ;

[0056] Step 6: Calculate the current time t With the timing The difference Δ t , according to the difference Δ t Update the maximum overcurrent value to ,in, is the update function, including piecewise functions at different stages , n is a non-zero natural number, The maximum value is greater than the overcurrent value set according to the steady-state overcurrent capacity and The minimum value is the overcurrent value set according to the steady-state overcurrent capability ;

[0057] Step 7: Set the reference value of the current vector amplitude and the maximum overcurrent value For comparison, if > , then go to step 8, otherwise go to step 9;

[0058] Step 8: Correct the reference value of the current vector amplitude ,make = , and according to the inverse time limiting strategy, the updated reference value of the current vector amplitude D-axis and Q-axis reference values ​​assigned to the current controller;

[0059] Step 9: Maintain the reference value of the current vector amplitude No change, return to step 1, and repeat the above steps;

[0060] Step 10: Set the timing time = 0, exit clipping.

[0061] Among them, in step 1, the coordinate transformation matrix is

[0062] (1)

[0063] In formula (1), represents the coordinate transformation matrix; represents the reference angular frequency. In some embodiments of the present invention, the reference angular frequency The value is 100π rad / s; t Indicates the current moment, taking the simulation time during the simulation process; m represents an arbitrary constant. In some embodiments of the present invention, m The value is 0.5.

[0064] In step 1, the mathematical model of the d-axis and q-axis components of the three-phase voltage at the grid connection point is:

[0065] (2)

[0066] in, , They are the d-axis and q-axis components of the three-phase voltage at the grid-connected point of the grid-forming converter respectively; is the value of the filter capacitor; , are the d-axis and q-axis components of the grid-connected point output current, , are the d-axis and q-axis currents output from the grid-type converter port, Indicates the reference angular frequency.

[0067] Initial reference values ​​of the d-axis and q-axis components of the output current of the grid-type converter , for

[0068] (3)

[0069] (4)

[0070] In the formula, , They represent the initial reference values ​​of the d-axis and q-axis components of the voltage controller integrator output respectively; , They represent the d-axis and q-axis components of the grid-connected point output current respectively; Represents filter capacitor; , They represent the d-axis and q-axis components of the grid connection point voltage respectively; represents the proportional gain of the voltage controller; represents the integral gain of the voltage controller; , They are the d-axis and q-axis components of the voltage controller respectively.

[0071] In step 2, the initial reference values ​​of the d-axis and q-axis components of the output current in equation (3) are converted using equation (3) to obtain the reference value of the current vector amplitude: :

[0072] (5)

[0073] In the formula, , It represents the initial reference value of the d-axis and q-axis components of the output current, which is the reference value of the d-axis and q-axis current before the inverse time limiting method takes effect.

[0074] In step 6, the maximum overcurrent value is updated by the update function of formula (6): :

[0075] (6)

[0076] In the formula, is the maximum overcurrent value; Respectively represent the piecewise functions at different stages of the update function; They represent different stages of the update function respectively;n is a non-zero natural number.

[0077] Among them, the update function is a monotonically decreasing function.

[0078] The different stages of the update function are all represented by the function shown in formula (5):

[0079] (7)

[0080] In the formula, Indicates n The piecewise function representation of the stage is: n Take a non-zero natural number; For the current moment t A monotonically decreasing function.

[0081] In some embodiments of the present invention, The value is 1.1 pu.

[0082] According to the update function described above, in some embodiments of the present invention, the update function shown in formula (8) is taken as an example for specific illustration.

[0083] (8)

[0084] In the formula, is the maximum overcurrent value, in per-unit value; Δ t For the current moment t With the timing The difference.

[0085] In step 8, the current limiting strategy is to use formula (9) to convert the reference value of the current vector amplitude The d and q axis reference values ​​are assigned to the current controller:

[0086] (9)

[0087] In the formula, , Represents the initial reference values ​​of the d-axis and q-axis components of the output current; , They respectively represent the d-axis and q-axis current loop reference values ​​after the inverse time limiting method is applied; is the maximum overcurrent value.

[0088] In some embodiments of the present invention, preferred embodiments of the present invention are described in detail in conjunction with the accompanying drawings. It should be understood that the preferred embodiments are only for illustrating the present invention, rather than for limiting the protection scope of the present invention.

[0089] Build in PSCAD / EMTDC Figure 2 The simulation model of the grid-type centralized energy storage power supply single load shown in the figure sets the simulation time to 5.0 s, and a three-phase metallic ground short circuit occurs at the grid connection point, and the fault lasts for 0.1 s. Figure 3 shown.

[0090] The simulation waveform is as follows Figure 4 shown.

[0091] Depend on Figure 4 Analysis shows that compared with the traditional current limiter, the voltage drop at the grid connection point (PCC) is alleviated after the inverse time limiting method of the present invention is adopted. This improvement is due to the fact that the grid-connected converter injects more reactive power into the system during the transient period to support the grid connection point voltage. After adopting the inverse time limiting method, the minimum PCC voltage is 0.216 pu, while it is 0.122 pu when the traditional current limiter is used. After adopting the inverse time limiting method, the PCC voltage is greatly increased by 92.3%.

[0092] The current amplitude limit of the existing grid-type converter usually adopts the steady-state value amplitude limit, which does not take into account that when a short-circuit fault occurs, the current flowing through the converter will increase, resulting in increased converter losses and increased heat generation. Therefore, the overcurrent capacity of the converter will decrease in proportion to the increase in junction temperature, thereby showing an inverse time characteristic. The present invention utilizes this characteristic to increase the overcurrent limit value of the converter in the initial stage when the GFM touches the amplitude limit, thereby improving the transient stability margin of the grid-type converter.

[0093] In some embodiments of the present invention, an electronic device is also provided, including a memory, a processor, and a computer program stored in the memory and executable on the processor, wherein the processor executes the program to implement the steps of the method of the above embodiment.

[0094] In some embodiments of the present invention, a computer-readable storage medium is provided on which a computer program / instruction is stored, wherein the computer program / instruction is executed by a processor to implement the steps of the method described in the aforementioned embodiment.

[0095] The specific values ​​of the parameters in the above embodiment are only specific examples and do not constitute a limitation on the scope of protection.

[0096] The above description of the disclosed embodiments enables those skilled in the art to implement or use the present invention. Various modifications to these embodiments will be apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the present invention. Therefore, the present invention will not be limited to the embodiments shown herein, but rather to the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. An inverse time current limiting method based on a grid-connected converter, characterized in that: The following steps are involved: (1) collecting the three-phase voltage of the grid-connected point of the grid-connected converter, and performing coordinate transformation on the three-phase voltage to obtain the d-axis and q-axis components of the three-phase voltage; (2) Obtaining initial reference values ​​of the d-axis and q-axis components of the output current of the grid-connected converter through a voltage controller, and obtaining a reference value of the current vector amplitude based on the initial reference value ; (3) Set the reference value of the current amplitude Per unit value with grid-connected converter rating For comparison, if > , then execute step (4); otherwise, execute step (10); (4) Determine the timing time Is the value of zero? If it is zero, execute step (5); otherwise, execute step (6); (5) Update the timing time The value of Equal to the current time t The value of (6) Calculate the current time t With the timing The difference Δ t , according to the difference Δ t Update the maximum overcurrent value to ,in, is the update function, including piecewise functions at different stages , n is a non-zero natural number, The maximum value is greater than the overcurrent value set according to the steady-state overcurrent capacity and The minimum value is the overcurrent value set according to the steady-state overcurrent capability ; (7) Set the current reference value and the maximum overcurrent value For comparison, if > , then execute step (8), otherwise execute step (9); (8) Correction of current reference value ,make = , and the updated current reference value D-axis and Q-axis reference values ​​assigned to the current controller; (9) Maintain current reference value No change, return to step (1), and repeat the above steps; (10) Set the timing time = 0, exit clipping.

2. The inverse time current limiting method based on the grid-connected converter according to claim 1 is characterized in that: The coordinate transformation matrix used in step (1) is: （1） In formula (1), represents the coordinate transformation matrix; represents the reference angular frequency; t Indicates the current moment; m Represents an arbitrary constant.

3. The inverse time current limiting method based on a grid-connected converter according to claim 1 is characterized in that: The mathematical model of the d-axis and q-axis components of the three-phase voltage at the grid connection point in step (1) is: （2） in, , They are the d-axis and q-axis components of the three-phase voltage at the grid-connected point of the grid-forming converter respectively; is the value of the filter capacitor; , They are the d-axis and q-axis components of the grid-connected point output current, , are the d-axis and q-axis currents output from the grid-type converter port, Indicates the reference angular frequency.

4. The inverse time current limiting method based on a grid-connected converter according to claim 1 is characterized in that: The initial reference values ​​of the d-axis and q-axis components of the output current in step (2) are converted using equation (3) to obtain the reference value of the current vector amplitude: : （3） In the formula, , Represents the initial reference value of the d and q axis components of the output current.

5. The inverse time current limiting method based on a grid-connected converter according to claim 1, characterized in that: In step (6), the maximum overcurrent value is updated using equation (4) : （4） In the formula, is the maximum overcurrent value; Respectively represent the piecewise functions at different stages of the update function; They represent different stages of the update function respectively; n is a non-zero natural number.

6. The inverse time current limiting method based on a grid-connected converter according to claim 1, characterized in that: The different stages of the update function are represented by the function shown in formula (5): （5） In the formula, Indicates n Piecewise function of the phase; For the current moment t A monotonically decreasing function.

7. The inverse time current limiting method based on a grid-connected converter according to claim 1, characterized in that: Update Function is a monotonically decreasing function.

8. The inverse time current limiting method based on a grid-connected converter according to claim 1, characterized in that: Step (8) uses the inverse time limiting strategy shown in equation (6) to set the reference value of the current vector amplitude Reference values ​​assigned to d and q axis currents: （6） In the formula, , Represents the initial reference values ​​of the d-axis and q-axis components of the output current; , They respectively represent the d-axis and q-axis current loop reference values ​​after the inverse time limiting method is applied.

9. An electronic device, characterized in that: The invention comprises a memory, a processor and a computer program stored in the memory and executable on the processor, wherein the processor executes the program to implement the steps of the method according to any one of claims 1 to 8.

10. A computer-readable storage medium having a computer program / instruction stored thereon, characterized in that: The computer program / instructions are executed by a processor to implement the steps of the method according to any one of claims 1 to 8.

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