Grid-off and grid-connected pre-synchronization method and system for constructed-network converter and computer readable storage medium thereof

Abstract

The invention discloses an off-grid and grid-connected pre-synchronization method for a network-forming converter. The method comprises the following steps: establishing control models of an active loop and a reactive loop; and carrying out pre-synchronization control on the grid-side voltage projection-based grid-constructing converter. The method has the technical effects and advantages that the projection of the power grid voltage in the coordinate system of the inverter can be obtained by sampling the voltage of the grid-connected isolation switch on the grid side and utilizing the phase conversion of the grid-forming inverter. The projection reflects phase difference information, a phase jump process does not exist, and successful pre-synchronization can be effectively achieved by controlling the projection amount to be 0 through the PI controller.

Description

Technical Field

[0001] The present invention relates to the field of power electronics technology, and more specifically to a method and system for pre-synchronizing a grid-connected converter and a computer-readable storage medium thereof. Background Art

[0002] With the widespread access of wind power, photovoltaics, and energy storage equipment in the power system, the inverter, as a key device for AC / DC conversion, can realize the conversion from DC to AC and inject the energy of new energy into the power grid. The regional distribution of wind power, photovoltaics and other resources is uneven. In order to improve the local consumption capacity of new energy in my country, it is generally required that the electricity generated by photovoltaics, wind power, etc. be consumed locally. Compared with the grid-type energy storage inverter, the AC side of the grid-type energy storage inverter shows a voltage source characteristic to the outside, which can independently supply power to the local load and has the ability to supply power to an isolated island, which can maximize the local consumption capacity of wind and solar new energy in the central and western regions. Due to the uncertainty of the output of new energy or the power consumption of local loads, the grid-type energy storage inverter often needs to switch off the grid to make up for the power balance demand. From the off-grid to the grid-connected stage, according to relevant standards in my country, the voltage frequency, phase and amplitude on both sides of the grid-connected disconnector must be less than a certain value before the disconnector can be closed to avoid problems such as overcurrent caused by inconsistent voltages at both ends. How to achieve phase consistency control is called pre-synchronization, which is a problem to be solved.

[0003] The traditional pre-synchronization method relies on two phase-locked loops to sample the voltage of the grid-connected disconnector, perform phase-locking and then make a difference, and input the phase error signal into the PI (proportional-integral) controller to achieve phase consistency control. However, due to the limitation of the existing digital control and its definition of variable types, the phase signal must generally be modulo-modified to avoid the micro-signal reaching the upper limit of the value specified by the data type. After the phase signal is modulo-modified, it is actually a sawtooth wave with a period of 20ms and an amplitude of 2πrad. When the two sawtooth waves are subtracted, a phase jump problem may occur. The phase jump will cause the change law of the input signal and the output signal of the PI controller to become a non-monotonic influence law, thus causing the pre-synchronization failure. Summary of the invention

[0004] In response to the above-mentioned phase jump problem, this patent proposes a pre-synchronization method based on the grid-side voltage projection principle.

[0005] The technical solution of the present invention is as follows:

[0006] A grid-connected pre-synchronization method for a grid-connected converter, comprising:

[0007] Establish control models for active loop and reactive loop; provide phase and voltage references for inverter dual-loop control.

[0008] Pre-synchronization control of grid-connected converter based on grid-side voltage projection is performed.

[0009] Furthermore, the hardware topology includes line impedance, isolating switch, power grid, inverter and local load; the control model includes a three-loop cascade control structure of power loop, voltage loop and current loop.

[0010] Furthermore, a virtual synchronization mechanism grid inverter off-grid hardware topology is established, including: a grid-connected isolation switch STS, the initial state of which is the switch off. When the inverter sends a pre-synchronization success signal, the isolation switch is closed; an equivalent line impedance L g , Power grid U g , grid-connected inverter VSC; the grid-side voltage signal of the disconnector is defined as U S ; The filter inductance and filter capacitance of the inverter output LC filter are L f , C f ; Inverter output voltage U GFM , the inverter output current, load current and current flowing to the grid are I GFM ,I L with I g ; DC side voltage U dc It is assumed that the energy storage system controls the DC bus capacitor C to be constant. dc .

[0011] Furthermore, in the control model of the active loop and the reactive loop, the control equations of the active loop and the reactive loop are respectively (1)-(2):

[0012]

[0013] Where δ is the relative power angle between the inverter and the grid, δ = θ GFM -θ g , J is the virtual inertia coefficient, D is the damping coefficient, ω N is the rated frequency, X g is the line reactance; k q is the reactive loop droop coefficient, U N is the inverter rated phase voltage amplitude, U d * is the AC voltage loop d-axis reference voltage, Q ref is the reactive power reference, Q e is the actual inverter output reactive power, P e is the actual inverter output active power, U d is the actual output phase voltage amplitude of the inverter. It is assumed that the AC voltage loop control speed is fast enough. d ≈U d * ; Ignore line dynamics, with Xg =ω N L g , L g is the equivalent circuit inductance; U g is the grid phase voltage amplitude; L f is the inverter filter inductor, C f is the inverter filter capacitor; 1 / s represents the integral link.

[0014] Furthermore, the inner loop of the control model adopts a typical voltage-current dual closed-loop control strategy, where the voltage reference is given by the reactive loop and the dq conversion angle is given by the active loop; the proportional parameter and integral parameter of the current loop PI controller are k pi , k ii , the proportional parameter and integral parameter of the voltage loop PI controller are k pv , k iv ; The modulation wave m in the dq coordinate system is obtained through the current loop d 、m q After the inverse Park transform, the three modulation waves m are obtained. abc Then PWM modulation is performed to obtain the gate drive signal of the inverter switch tube.

[0015] Furthermore, a pre-synchronization control is performed on the grid-type converter based on the grid-side voltage projection, including:

[0016] Introduce grid voltage projection: Introduce park coordinate transformation, and transform the grid-side voltage of the disconnector into the phase θ of the inverter itself GFM Calculate and get the q-axis voltage, which is the projection of the grid-side voltage in its own voltage coordinate system; sd , U sq and Us0 are the dq axis projections of the grid-side voltage of the disconnector in the inverter coordinate system,

[0017]

[0018] The grid voltage projection is controlled to 0 by proportional integral control to achieve phase consistency control; where S pre It is the flag bit for enabling the pre-synchronization controller. When the pre-synchronization control is started, S pre Set to 1, when the pre-synchronization control is not started or the grid voltage projection is determined to be less than a certain value, S pre Set to 0; this paper sets the threshold to 1%U N ,

[0019]

[0020] The present invention also provides a grid-connected converter off-grid pre-synchronization system, comprising:

[0021] A control model building module is used to build control models of active loops and reactive loops;

[0022] The pre-synchronization control module is used to perform pre-synchronization control on a grid-type converter based on grid-side voltage projection.

[0023] Furthermore, the control model building module is used to build the control model of the active loop and the reactive loop; the specific method is: the control equations of the active loop and the reactive loop are respectively (1)-(2):

[0024]

[0025] Where δ is the relative power angle between the inverter and the grid, δ = θ GFM -θ g , J is the virtual inertia coefficient, D is the damping coefficient, ω N is the rated frequency, X g is the line reactance; k q is the reactive loop droop coefficient, U N is the inverter rated phase voltage amplitude, U d * is the AC voltage loop d-axis reference voltage, Q ref is the reactive power reference, Q e is the actual inverter output reactive power, P e is the actual inverter output active power, U d is the actual output phase voltage amplitude of the inverter. It is assumed that the AC voltage loop control speed is fast enough. d ≈U d * ; Ignore line dynamics, with X g =ω N L g , L g is the equivalent circuit inductance; U g is the grid phase voltage amplitude; L f is the inverter filter inductor, C f is the inverter filter capacitor; 1 / s represents the integral link.

[0026] Furthermore, the pre-synchronization control module is used to perform pre-synchronization control on the grid-type converter based on the grid-side voltage projection, and the specific method is as follows:

[0027] Pre-synchronization control of grid-type converter based on grid-side voltage projection includes:

[0028] Park coordinate transformation is introduced to convert the grid-side voltage of the isolating switch into the inverter phase θ GFM Calculate and get the q-axis voltage; where U sd , U sqand Us0 are the dq axis projections of the grid-side voltage of the disconnector in the inverter coordinate system,

[0029]

[0030] The grid voltage projection is controlled to 0 by proportional integral control to achieve phase consistency control; where S pre It is the flag bit for enabling the pre-synchronization controller. When the pre-synchronization control is started, S pre Set to 1, when the pre-synchronization control is not started or the grid voltage projection is determined to be less than a certain value, S pre Set to 0; Set the threshold to 1% U N ,

[0031]

[0032] The technical effects and advantages of the present invention are as follows: by sampling the voltage of the grid-connected disconnector on the grid side and using the phase transformation of the grid-connected inverter itself, the projection of the grid voltage in the inverter's own coordinate system can be obtained. The projection reflects the phase difference information, and there is no phase jump process. By controlling the projection amount to 0 using a PI controller, successful pre-synchronization can be effectively achieved. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] Figure 1 The grid-connected topology of the virtual synchronous machine control type grid-connected converter of the present invention;

[0034] Figure 2 A cascade control loop of a virtual synchronous machine control type grid-connected converter in the system of the present invention;

[0035] Figure 3 The experimental results of off-grid connection of grid-connected inverter using conventional pre-synchronization control are shown;

[0036] Figure 4 The experimental results of grid-connected grid-connected inverter using the proposed pre-synchronization control are shown in Figure 2. DETAILED DESCRIPTION

[0037] The technical solutions in the embodiments of the present invention are described clearly and completely below. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of them. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0038] Embodiment 1

[0039] Step 1: First, establish the control model of the active loop and the reactive loop. The hardware topology includes line impedance, disconnector, power grid, inverter and local load; the control model includes a three-loop cascade control structure of power loop, voltage loop and current loop.

[0040] Step 1.1: Establish the hardware topology of the grid-connected inverter. Figure 1 As shown. STS is the grid-connected disconnecting switch, and the initial state is that the switch is disconnected. When the inverter sends a pre-synchronization success signal, the disconnecting switch is closed. g is the equivalent line impedance, U g is the grid, VSC is the grid-connected inverter: the grid-side voltage signal of the disconnector is defined as U S ; L f , C f are the filter inductance and filter capacitance of the inverter output LC filter respectively. GFM is the inverter output voltage, I GFM ,I L with I g They are the inverter output current, load current and current flowing to the grid. DC side voltage U dc It is considered that it is controlled by the energy storage system to be constant, C dc is the DC bus capacitance.

[0041] Step 1.2: Establish the control model of the active loop and the reactive loop. The control equations of the active loop and the reactive loop are (1)-(2) respectively:

[0042]

[0043] Where δ is the relative power angle between the inverter and the grid, δ = θ GFM -θ g , J is the virtual inertia coefficient, D is the damping coefficient, ω N is the rated frequency, X g is the line reactance. k q is the reactive loop droop coefficient, U N is the inverter rated phase voltage amplitude, U d * is the AC voltage loop d-axis reference voltage, Q ref is the reactive power reference, Q e is the actual inverter output reactive power, P e is the actual inverter output active power, U d is the actual output phase voltage amplitude of the inverter. It is assumed that the AC voltage loop control speed is fast enough. d ≈U d * . Ignore line dynamics, there is X g =ω N Lg , L g is the equivalent circuit inductance. g is the grid phase voltage amplitude. f is the inverter filter inductor, C f is the inverter filter capacitor. 1 / s represents the integral link.

[0044] like Figure 2 As shown, the inner loop adopts a typical voltage and current double closed-loop control strategy, the voltage reference is given by the reactive loop, and the dq conversion angle is given by the active loop. pi , k ii are the proportional and integral parameters of the current loop PI controller, k pv , k iv is the proportional parameter and integral parameter of the voltage loop PI controller. The modulation wave m in the dq coordinate system is obtained through the current loop. d 、m q After the inverse Park transform, the three modulation waves m are obtained. abc Then PWM modulation is performed to obtain the gate drive signal of the inverter switch tube.

[0045] Step 2: Pre-synchronization control of grid-type converter based on grid-side voltage projection.

[0046] Step 2.1: Introducing grid voltage projection

[0047] Park coordinate transformation is introduced to convert the grid-side voltage of the isolating switch into the inverter phase θ GFM Calculate and get the q-axis voltage, which is the projection of the grid-side voltage in its own voltage coordinate system. sd , U sq and Us0 are the dq axis projections of the grid-side voltage of the disconnector in the inverter coordinate system.

[0048]

[0049] Step 2.2: Use proportional integral control to control the grid voltage projection to 0 to achieve phase consistency control. pre It is the flag bit for enabling the pre-synchronization controller. When the pre-synchronization control is started, S pre Set to 1, when the pre-synchronization control is not started or the grid voltage projection is determined to be less than a certain value, S pre Set to 0. This paper sets the threshold to 1% U N .

[0050]

[0051] Embodiment 2

[0052] like Figure 3As shown in the figure, the control effect of the grid-type converter using the traditional phase-locked loop phase difference as the control variable to achieve pre-synchronization is given. The phase synchronization controller is started at 0.5s, that is, S pre Set to 1, between 0s-0.5sS pre Set to 0, bypass the pre-synchronization control unit after the 2nd second, close the grid-connected disconnector, and switch the inverter from off-grid to grid-connected state. Obviously, the phase difference signal obtained after the difference between the two input signals is a sawtooth wave will produce frequent jumps, causing the phase adjustment output signal of the PI control to frequently jump, making its adjustment trend non-monotonic, and thus causing the failure of phase consistency control. The failure of phase consistency control further causes the grid-connected current to generate a huge impact current after the grid-connected disconnector is closed, with a peak value close to 5 times the rated current, which seriously endangers the safe and stable operation of the inverter.

[0053] Embodiment 3

[0054] like Figure 4 As shown in the figure, the proposed method for preventing pre-synchronization based on grid voltage projection is given. The phase synchronization controller is started at 0.5s, that is, S pre Set to 1, between 0s-0.5sS pre Set to 0, bypass the pre-synchronization control unit after the 2nd second, close the grid-connected disconnector, and switch the inverter from off-grid to grid-connected state. By using coordinate transformation to obtain the q-axis voltage projection of the grid-side voltage in the inverter's own operating coordinate system, its phase difference information can be effectively reflected to avoid the phase jump phenomenon caused by the difference between the two sawtooth waves. There is no jump problem in the input signal and output phase adjustment signal of the PI controller. When the disconnector is closed, there is almost no impact current, indicating that the proposed control strategy can achieve successful phase suppression control during off-grid switching, and the impact current caused by phase inconsistency is well suppressed.

[0055] Embodiment 4

[0056] A grid-connected pre-synchronization system for a grid-connected converter, comprising:

[0057] A control model building module is used to build control models of active loops and reactive loops;

[0058] The pre-synchronization control module is used to perform pre-synchronization control on a grid-type converter based on grid-side voltage projection.

[0059] Furthermore, the control model building module is used to build the control model of the active loop and the reactive loop. The specific method is as follows: the control equations of the active loop and the reactive loop are (1)-(2) respectively:

[0060]

[0061] Where δ is the relative power angle between the inverter and the grid, δ = θ GFM -θ g , J is the virtual inertia coefficient, D is the damping coefficient, ω N is the rated frequency, X g is the line reactance; k q is the reactive loop droop coefficient, U N is the inverter rated phase voltage amplitude, U d * is the AC voltage loop d-axis reference voltage, Q ref is the reactive power reference, Q e is the actual inverter output reactive power, P e is the actual inverter output active power, U d is the actual output phase voltage amplitude of the inverter. It is assumed that the AC voltage loop control speed is fast enough. d ≈U d * ; Ignore line dynamics, with X g =ω N L g , L g is the equivalent circuit inductance; U g is the grid phase voltage amplitude; L f is the inverter filter inductor, C f is the inverter filter capacitor; 1 / s represents the integral link.

[0062] Furthermore, the pre-synchronization control module is used to perform pre-synchronization control on the grid-type converter based on the grid-side voltage projection, and the specific method is as follows:

[0063] Pre-synchronization control of grid-type converter based on grid-side voltage projection includes:

[0064] Park coordinate transformation is introduced to convert the grid-side voltage of the isolating switch into the inverter phase θ GFM Calculate and get the q-axis voltage; where U sd , U sq and Us0 are the dq axis projections of the grid-side voltage of the disconnector in the inverter coordinate system,

[0065]

[0066] The grid voltage projection is controlled to 0 by proportional integral control to achieve phase consistency control; where S pre It is the flag bit for enabling the pre-synchronization controller. When the pre-synchronization control is started, S pre Set to 1, when the pre-synchronization control is not started or the grid voltage projection is determined to be less than a certain value, S pre Set to 0; Set the threshold to 1% UN ,

[0067]

[0068] Embodiment 5

[0069] A computer-readable storage medium is also provided, wherein a computer program is stored in the computer-readable storage medium, and when the computer program is executed by a processor, the steps of any one of the methods of claims 1 to 4 are implemented.

[0070] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principle of the present invention should be included in the protection scope of the present invention.

Claims

1. A method for pre-synchronizing a grid-connected converter, characterized in that: include: Establish control models for active loop and reactive loop to provide phase and voltage reference for inverter dual-loop control; Pre-synchronization control of grid-connected converter based on grid-side voltage projection is performed.

2. A method for pre-synchronizing a grid-connected converter according to claim 1, characterized in that: In establishing the control model of the active loop and the reactive loop, the control equations of the active loop and the reactive loop are respectively (1)-(2): Where δ is the relative power angle between the inverter and the grid, δ = θ GFM -θ g , J is the virtual inertia coefficient, D is the damping coefficient, ω N is the rated frequency, X g is the line reactance; k q is the reactive loop droop coefficient, U N is the inverter rated phase voltage amplitude, U d * is the AC voltage loop d-axis reference voltage, Q ref is the reactive power reference, Q e is the actual inverter output reactive power, P e is the actual inverter output active power, U d is the actual output phase voltage amplitude of the inverter. It is assumed that the AC voltage loop control speed is fast enough. d ≈U d * ; Ignore line dynamics, with X g =ω N L g , L g is the equivalent circuit inductance; U g is the grid phase voltage amplitude; L f is the inverter filter inductor, C f is the inverter filter capacitor; 1 / s represents the integral link.

3. A method for pre-synchronizing a grid-connected converter according to claim 2, characterized in that: The inner loop of the control model adopts a typical voltage and current double closed-loop control strategy. The voltage reference is given by the reactive loop, and the dq conversion angle is given by the active loop. The proportional and integral parameters of the current loop PI controller are k pi , k ii , the proportional parameter and integral parameter of the voltage loop PI controller are k pv , k iv ; The modulation wave m in the dq coordinate system is obtained through the current loop d 、m q After the inverse Park transform, the three modulation waves m are obtained. abc Then PWM modulation is performed to obtain the gate drive signal of the inverter switch tube.

4. A method for pre-synchronizing a grid-connected converter according to claim 1, characterized in that: Pre-synchronization control of grid-type converter based on grid-side voltage projection includes: Park coordinate transformation is introduced to convert the grid-side voltage of the isolating switch into the inverter phase θ GFM Calculate and get the q-axis voltage; where U sd , U sq and Us0 are the dq axis projections of the grid-side voltage of the disconnector in the inverter coordinate system, The grid voltage projection is controlled to 0 by proportional integral control to achieve phase consistency control; where S pre It is the flag bit for enabling the pre-synchronization controller. When the pre-synchronization control is started, S pre Set to 1, when the pre-synchronization control is not started or the grid voltage projection is determined to be less than a certain value, S pre Set to 0; Set the threshold to 1% U N , 5. A grid-connected pre-synchronization system for a grid-connected converter, characterized in that: include: A control model building module is used to build control models of active loops and reactive loops; The pre-synchronization control module is used to perform pre-synchronization control on a grid-type converter based on grid-side voltage projection.

6. A grid-connected pre-synchronization system for grid-connected converters according to claim 5, characterized in that: The control model building module is used to build the control model of the active loop and the reactive loop. The specific method is as follows: the control equations of the active loop and the reactive loop are (1)-(2) respectively: Where δ is the relative power angle between the inverter and the grid, δ = θ GFM -θ g , J is the virtual inertia coefficient, D is the damping coefficient, ω N is the rated frequency, X g is the line reactance; k q is the reactive loop droop coefficient, U N is the inverter rated phase voltage amplitude, U d * is the AC voltage loop d-axis reference voltage, Q ref is the reactive power reference, Q e is the actual inverter output reactive power, P e is the actual inverter output active power, U d is the actual output phase voltage amplitude of the inverter. It is assumed that the AC voltage loop control speed is fast enough. d ≈U d * ; Ignore line dynamics, with X g =ω N L g , L g is the equivalent circuit inductance; U g is the grid phase voltage amplitude; L f is the inverter filter inductor, C f is the inverter filter capacitor; 1 / s represents the integral link.

7. The grid-connected pre-synchronization system of a grid-connected converter according to claim 5, characterized in that: The pre-synchronization control module is used to perform pre-synchronization control on the grid-type converter based on the grid-side voltage projection. The specific method is as follows: Pre-synchronization control of grid-type converter based on grid-side voltage projection includes: Park coordinate transformation is introduced to convert the grid-side voltage of the isolating switch into the inverter phase θ GFM Calculate and get the q-axis voltage; where U sd , U sq and Us0 are the dq axis projections of the grid-side voltage of the disconnector in the inverter coordinate system, The grid voltage projection is controlled to 0 by proportional integral control to achieve phase consistency control; where S pre It is the flag bit for enabling the pre-synchronization controller. When the pre-synchronization control is started, S pre Set to 1, when the pre-synchronization control is not started or the grid voltage projection is determined to be less than a certain value, S pre Set to 0; Set the threshold to 1% U N , 8. A computer-readable storage medium, characterized in that: The computer-readable storage medium stores a computer program, and when the computer program is executed by a processor, the steps of any one of the methods of claims 1 to 4 are implemented.

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