VSG fault ride-through method and system without transient impulse current

Abstract

The invention provides a VSG fault ride-through method and system without transient impact current, and the method comprises the steps: adjusting a transient power factor angle through introducing adaptive virtual impedance, further changing the initial value of a periodic component of VSG current during a fault, controlling the initial value of a non-periodic component at a short circuit moment to be zero, and achieving the smooth transition of VSG output current during the fault period. Under the condition of not additionally arranging or adopting hardware equipment, under different degrees of power grid faults, the VSG output current is in smooth transition, no transient impact current exists, and safe operation of converter equipment is guaranteed.

Description

Technical Field

[0001] The present invention belongs to the technical field of distribution network fault protection, and in particular relates to a VSG fault ride-through method and system without transient impact current. Background Art

[0002] The statements in this section merely provide background information related to the present invention and do not necessarily constitute prior art.

[0003] Virtual Synchronous Generator (VSG) simulates the rotor motion characteristics of synchronous generators, which enables the converter to have strong inertia and frequency support capabilities, improves the goodness of new energy grid connection and the coordinated interaction ability of source and grid, and has been widely studied. However, under large disturbances such as grid disconnection and short-circuit faults, the transient impact current peak of the grid-connected converter based on VSG control strategy is many times larger than the rated current under normal working conditions, which can reach dozens or even hundreds of times. The higher the severity of the grid fault, the greater the transient impact current peak. This high-amplitude current will cause huge current stress on the power electronic devices (such as IGBT, diode, etc.) inside the converter, which may cause damage to the devices if it exceeds the tolerance of the power electronic devices. Therefore, during grid faults of different degrees, ensuring that the VSG transient impact current can be limited to a certain range is of great scientific and engineering significance to ensure the safe, stable and reliable operation of the converter.

[0004] According to the inventors, most of the current methods for suppressing VSG transient impact current are to introduce virtual impedance to reduce the decay time constant of the non-periodic component of the fault current, so that the transient impact current can decay rapidly. However, such methods cannot eliminate the transient impact current, and the grid-connected converter still needs to withstand the transient impact current. In the event of a serious grid fault, the transient impact current of the converter will reach several times its rated value, threatening the safety of power electronic devices. Summary of the invention

[0005] In order to overcome the shortcomings of the above-mentioned prior art, the present invention provides a VSG fault ride-through method and system without transient impact current, by introducing adaptive virtual impedance to adjust the transient power factor angle, thereby changing the initial value of the VSG current periodic component during the fault, and controlling the initial value of the non-periodic component at the moment of short circuit to zero, thereby achieving a smooth transition of the VSG output current during the fault.

[0006] To achieve the above object, a first aspect of the present invention provides a VSG fault ride-through method without transient surge current, comprising:

[0007] Based on the VSG fault full current analysis model, the VSG current periodic component and VSG current non-periodic component related to the transient power factor angle are obtained;

[0008] Adaptive virtual impedance is introduced to adjust the transient power factor angle, change the initial value of the periodic component of the VSG current during the fault, and control the initial value of the non-periodic component of the VSG current at the moment of short circuit to zero, so as to achieve a smooth transition of the VSG output current during the fault period.

[0009] A second aspect of the present invention provides a VSG fault ride-through system without transient surge current, comprising:

[0010] An analysis module, for obtaining a VSG current periodic component and a VSG current non-periodic component related to a transient power factor angle based on a VSG fault full current analysis model;

[0011] The adaptive adjustment module is used to introduce adaptive virtual impedance to adjust the transient power factor angle, change the initial value of the periodic component of the VSG current during a fault, control the initial value of the non-periodic component of the VSG current at the moment of short circuit to zero, and achieve a smooth transition of the VSG output current during the fault period.

[0012] A third aspect of the present invention provides a VSG grid-connected system, which adopts the above-mentioned VSG fault ride-through method without transient impact current.

[0013] The fourth aspect of the present invention provides a computer device, comprising: a processor, a memory and a bus, wherein the memory stores machine-readable instructions executable by the processor, and when the computer device is running, the processor and the memory communicate through the bus, and when the machine-readable instructions are executed by the processor, a VSG fault crossing method without transient surge current is performed.

[0014] A fifth aspect of the present invention provides a computer-readable storage medium having a computer program stored thereon, wherein the computer program, when executed by a processor, executes a VSG fault ride-through method without transient surge current.

[0015] One or more of the above technical solutions have the following beneficial effects:

[0016] In the present invention, an adaptive virtual impedance is introduced to adjust the transient power factor angle, thereby changing the initial value of the VSG current periodic component during the fault, and controlling the initial value of the non-periodic component at the moment of short circuit to zero, so as to achieve a smooth transition of the VSG output current during the fault. The present invention achieves a smooth transition of the VSG output current under different degrees of grid faults without the need for additional hardware equipment, without transient impact current, and ensures the safe operation of the converter equipment.

[0017] The solution of the present invention avoids the problem of the converter exiting operation due to the malfunction of the converter protection mechanism caused by the high-amplitude transient impact current during the power grid fault, thereby improving the robustness and reliability of the grid-connected converter system.

[0018] Advantages of additional aspects of the present invention will be given in part in the following description, and in part will become obvious from the following description, or will be learned through practice of the present invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] The accompanying drawings in the specification, which constitute a part of the present invention, are used to provide a further understanding of the present invention. The exemplary embodiments of the present invention and their descriptions are used to explain the present invention and do not constitute improper limitations on the present invention.

[0020] Figure 1 This is a structural block diagram of a VSG grid-connected system in Embodiment 1 of the present invention;

[0021] Figure 2 The voltage vector diagram of the VSG grid-connected system in the first embodiment of the present invention;

[0022] Figure 3 This is a control block diagram of a VSG fault ride-through method without transient surge current in Embodiment 1 of the present invention. DETAILED DESCRIPTION

[0023] It should be noted that the following detailed descriptions are exemplary and are intended to provide further explanation of the present invention. Unless otherwise specified, all technical and scientific terms used herein have the same meanings as those commonly understood by those skilled in the art to which the present invention belongs.

[0024] It should be noted that the terms used herein are for describing specific embodiments only and are not intended to be limiting of exemplary embodiments according to the present invention.

[0025] In the absence of conflict, the embodiments of the present invention and the features of the embodiments may be combined with each other.

[0026] Embodiment 1

[0027] This embodiment discloses a VSG fault ride-through method without transient surge current, including:

[0028] Based on the VSG fault full current analysis model, the VSG current periodic component and VSG current non-periodic component related to the transient power factor angle are obtained;

[0029] Adaptive virtual impedance is introduced to adjust the transient power factor angle, change the initial value of the periodic component of the VSG current during the fault, and control the initial value of the non-periodic component of the VSG current at the moment of short circuit to zero, so as to achieve a smooth transition of the VSG output current during the fault period.

[0030] The present embodiment proposes a VSG fault ride-through method without transient impact current, which realizes the smooth transition of VSG output current. First, the mathematical expression of the full current of the VSG fault is derived, and then the starting value of the periodic component of the VSG fault current after the short circuit is dynamically adjusted based on the adaptive virtual impedance, and then the initial value of the non-periodic component is controlled to a minimum value near zero, eliminating the transient transition process of the fault current. This method can realize the smooth transition of the VSG output current during the fault period, and prevent the instantaneous transient impact current of the power grid fault from causing damage to the converter.

[0031] This embodiment scheme realizes the smooth transition of VSG output current during grid faults of different degrees without transient transition process. In addition, this embodiment scheme can suppress the transient impact current of VSG during grid fault without hardware equipment (such as fault current limiter, etc.), ensuring the stable and safe operation of the converter during grid fault, improving the robustness and reliability of the grid-connected converter system, and ensuring the stable and non-offline operation of VSG and good output performance.

[0032] The solution of this embodiment achieves a smooth transition of the VSG output current during grid faults of different degrees. The solution of this embodiment will be described in detail below using a two-level converter as an example.

[0033] 1. Structural block diagram of VSG grid-connected system.

[0034] A typical three-phase voltage source converter VSG grid-connected system is as follows: Figure 1 As shown in the figure, the grid-connected system consists of a DC side, a converter, a filter, a grid impedance and a large grid. dc Indicates the voltage at the DC side port of the converter, L f and C f Respectively represent the inductance and capacitance of the LC filter, R g and L g Respectively represent the resistive and inductive components of the equivalent grid impedance of the transmission and distribution lines, i1 and i pcc They represent the converter port output current and the filtered grid-connected current, u pcc and U g Denote the grid connection point voltage and grid voltage, ω0 and U n Respectively represent the rated angular frequency and VSG output rated voltage. e and Q e Denote the active power and reactive power of VSG output, δ, ω and U respectively. ref Respectively represent the VSG output power angle, angular frequency and voltage amplitude reference, P ref and Q ref The active power reference value and reactive power reference value output for VSG are provided by reasonable coordination and flexible adjustment of new energy power generation and energy storage.

[0035] Since VSG uses a low-pass filter when calculating power, the reference voltage generated by the power control loop will not change much in the short period of time when a grid fault occurs. Therefore, the influence of the power control loop is ignored when analyzing transient impact current.

[0036] 2. VSG fault current characteristics.

[0037] Assume that a three-phase symmetrical short circuit fault occurs in the power grid at t = t0, and the effective value of the power grid voltage after the fault is U gF ∠0°, after the fault, the steady-state output voltage of VSG is U pccF ∠δ F , the equivalent grid impedance Z after the fault F =R F +jωL F Taking phase A as an example, the VSG output current is analyzed:

[0038]

[0039] In the formula, R F and L F is the resistance and inductance of the equivalent grid impedance after the fault, U pcc is the effective value of VSG output voltage, δ0 is the VSG power angle before the fault state, i pcc F is the instantaneous value of the current flowing through the VSG during the grid fault, ω represents the VSG angular frequency, ω0 is the rated angular frequency, and ω=ω0 in the steady state.

[0040] Solving the differential equation of formula (1) yields:

[0041]

[0042] like Figure 2 As shown in the voltage vector diagram, when the equivalent resistance component is ignored, the cosine theorem can be used to obtain the expressions of the parameters in equation (2) as follows:

[0043]

[0044] From formula (2), we can see that the current flowing through the VSG after a grid fault contains a periodic component i p and the non-periodic component i np The periodic component ip is the output current of the VSG after the system reaches a new steady state, which does not decay. Its effective value is expressed by I F Represents. Non-periodic component i np Decays exponentially from the initial value, τ = L F / R F is the decay time constant of the non-periodic component, is the transient power factor angle, is the power factor angle before the fault.

[0045] The initial value of the non-periodic component is I np0 ,exist:

[0046]

[0047] From formula (4), we can know that the initial value of the non-periodic component at the moment of short circuit is np0 The short-circuit time t0 and the effective value of the steady-state current after the fault I F , VSG output current effective value I before grid fault N , Power factor angle before fault and transient power factor angle related.

[0048] It can be seen from formula (4) that if the initial value of the non-periodic component at the moment of short circuit can be np0 If the VSG output current drops to zero, there will be no transient transition process after the fault.

[0049] The core point of this embodiment is to adjust the transient power factor angle by introducing adaptive virtual impedance. Then change the initial value of the VSG current period component during the fault Set the initial value of the non-periodic component at the moment of short circuit I np0 Controlled to zero, to achieve a smooth transition of VSG output current during a fault.

[0050] In normal operation, VSG usually operates at unity power factor and outputs reactive power Q e is 0, there exists:

[0051]

[0052] Steady-state effective value of current after fault I F The steady-state current suppression strategy can be used to limit the current to the converter tolerance current, namely:

[0053] I F =n*I N (6)

[0054] In the formula, n is the overcurrent multiple of the converter. Different types of converters have different overcurrent multiples.

[0055] Taking phase A as an example, when a grid fault is detected, the current measurement module records and maintains the instantaneous value of the VSG output current before the fault as i pcca (t 0- ), the expression of short-circuit time t0 can be derived as follows:

[0056]

[0057] Order I np0 =0, we get:

[0058]

[0059] exist:

[0060]

[0061] Where, L v The virtual inductor is derived from the selection criteria of adaptive virtual impedance:

[0062]

[0063] In order to ensure that the grid-connected system damping is always positive and the voltage drop of the VSG on the virtual impedance is within the allowable range, the resistance R of the equivalent grid impedance after the fault is F Generally, the value is not greater than 1. Equivalent grid reactance L g It can be identified by the least square method or the injection method. In summary, the control block diagram of the present invention is as follows Figure 3 shown.

[0064] Different from the traditional VSG transient impact current suppression strategy, the present invention adjusts the starting value of the VSG current period component after the short circuit based on adaptive virtual impedance without the need for additional hardware equipment, fundamentally avoiding the generation of transient impact current, and achieving direct transition of the VSG output current to a new steady state after the fault. The present invention prevents the VSG from being subjected to transient overcurrent shocks during a power grid fault, ensures the safe and stable operation of the VSG during a power grid fault, and improves the robustness and reliability of the grid-connected power conversion system.

[0065] Embodiment 2

[0066] The purpose of this embodiment is to provide a VSG fault ride-through system without transient surge current, including:

[0067] An analysis module, for obtaining a VSG current periodic component and a VSG current non-periodic component related to a transient power factor angle based on a VSG fault full current analysis model;

[0068] The adaptive adjustment module is used to introduce adaptive virtual impedance to adjust the transient power factor angle, change the initial value of the periodic component of the VSG current during a fault, control the initial value of the non-periodic component of the VSG current at the moment of short circuit to zero, and achieve a smooth transition of the VSG output current during the fault period.

[0069] Embodiment 3

[0070] The purpose of this embodiment is to provide a computing device, including a memory, a processor, and a computer program stored in the memory and executable on the processor, wherein the processor implements the steps of the above method when executing the program.

[0071] Embodiment 4

[0072] The purpose of this embodiment is to provide a computer-readable storage medium.

[0073] A computer-readable storage medium stores a computer program, which executes the steps of the above method when executed by a processor.

[0074] The steps involved in the apparatuses of the above embodiments 2, 3 and 4 correspond to the method embodiment 1, and the specific implementation methods can refer to the relevant description part of embodiment 1. The term "computer-readable storage medium" should be understood as a single medium or multiple media including one or more instruction sets; it should also be understood to include any medium that can store, encode or carry an instruction set for execution by a processor and enable the processor to execute any method in the present invention.

[0075] Those skilled in the art should understand that the modules or steps of the present invention described above can be implemented by a general-purpose computer device, or alternatively, they can be implemented by a program code executable by a computing device, so that they can be stored in a storage device and executed by the computing device, or they can be made into individual integrated circuit modules, or multiple modules or steps therein can be made into a single integrated circuit module for implementation. The present invention is not limited to any specific combination of hardware and software.

[0076] Although the above describes the specific implementation mode of the present invention in conjunction with the accompanying drawings, it is not intended to limit the scope of protection of the present invention. Those skilled in the art should understand that various modifications or variations that can be made by those skilled in the art on the basis of the technical solution of the present invention without creative work are still within the scope of protection of the present invention.

Claims

1. A VSG fault ride-through method without transient surge current, It is characterized in that include: Based on the VSG fault full current analysis model, the VSG current periodic component and VSG current non-periodic component related to the transient power factor angle are obtained; Adaptive virtual impedance is introduced to adjust the transient power factor angle, change the initial value of the periodic component of the VSG current during the fault, and control the initial value of the non-periodic component of the VSG current at the moment of short circuit to zero, so as to achieve a smooth transition of the VSG output current during the fault period.

2. A VSG fault ride-through method without transient surge current as claimed in claim 1, It is characterized in that Based on the VSG fault full current analysis model, the instantaneous value of the current flowing through the VSG during a power grid fault includes a periodic component and a non-periodic component; the initial value of the non-periodic component at the moment of short circuit is determined by the short circuit moment, the effective value of the steady-state current after the fault, the effective value of the VSG output current before the power grid fault, the power factor angle before the fault and the transient power factor angle.

3. A VSG fault ride-through method without transient surge current as claimed in claim 1 or 2, It is characterized in that The transient power factor angle is determined by the rated angular frequency, the resistance and inductance of the equivalent grid impedance after the fault.

4. A VSG fault ride-through method without transient surge current as claimed in claim 3, It is characterized in that The inductance of the post-fault equivalent grid impedance is composed of virtual inductance and equivalent grid reactance. An adaptive virtual impedance is introduced to adjust the virtual inductance. The equivalent grid reactance is identified by a least square method or an injection method.

5. A VSG fault ride-through method without transient surge current as claimed in claim 4, It is characterized in that The virtual inductance is: Among them, L v is the virtual inductor, R F is the resistance of the equivalent grid impedance after the fault, L g is the equivalent grid reactance, is the transient power factor angle, ω n is the rated angular frequency.

6. A VSG fault ride-through method without transient surge current as claimed in claim 2, It is characterized in that After a fault, the steady-state effective value of the current is limited to the converter tolerance current using a steady-state current suppression strategy.

7. A VSG fault ride-through system without transient surge current, It is characterized in that include: An analysis module, for obtaining a VSG current periodic component and a VSG current non-periodic component related to a transient power factor angle based on a VSG fault full current analysis model; The adaptive adjustment module is used to introduce adaptive virtual impedance to adjust the transient power factor angle, change the initial value of the periodic component of the VSG current during a fault, control the initial value of the non-periodic component of the VSG current at the moment of short circuit to zero, and achieve a smooth transition of the VSG output current during the fault period.

8. A VSG grid-connected system, adopting a VSG fault ride-through method without transient impact current as claimed in any one of claims 1 to 7.

9. A computer device, It is characterized in that include: A processor, a memory and a bus, wherein the memory stores machine-readable instructions executable by the processor, and when the computer device is running, the processor and the memory communicate via the bus, and when the machine-readable instructions are executed by the processor, a VSG fault ride-through method without transient surge current is performed as described in any one of claims 1 to 6.

10. A computer-readable storage medium, It is characterized in that The computer-readable storage medium stores a computer program, and when the computer program is executed by a processor, the method for VSG fault ride-through without transient surge current as claimed in any one of claims 1 to 7 is executed.

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