Voltage inertia control method for improving transient voltage stability of network-forming converter

Abstract

The invention relates to a voltage inertia control method for improving transient voltage stability of a grid-forming converter, and the method comprises the steps: setting a voltage control architecture which comprises an outer reactive loop, a middle voltage loop and an inner current loop; the outer reactive loop comprises a low-pass filter with a proportionality coefficient, and the proportionality coefficient is set according to the ratio of the voltage disturbance quantity to the maximum adjustable reactive power; inputting reactive power of a power grid side to obtain a reactive loop control feed-forward item, and performing feed-forward adjustment on an outer-layer reactive loop according to the reactive loop control feed-forward item; and calculating a voltage control framework according to the outer reactive loop after feedforward regulation so as to control the converter.

Description

Technical Field

[0001] The present invention belongs to the technical field of converter control, and in particular relates to a voltage inertia control method for improving transient voltage stability of a grid-connected converter. Background Art

[0002] The penetration rate of renewable energy is increasing rapidly around the world. At the same time, the number of energy storage devices and electric vehicles connected to the grid through power converters is also increasing significantly. As the global energy transition continues, the modern power grid is gradually changing from a traditional power system to a power system with electronic equipment as the core, and may eventually evolve into a pure power electronic system. In this process, the control method of the grid-connected power converter plays a vital role in ensuring the safe and stable operation of the electronic power system.

[0003] Among them, the grid-type converter is considered to be an ideal technical solution for the future scenario where a high proportion of power electronic equipment is connected to the grid because of its ability to support the grid voltage and frequency. However, the power electronic devices used in the grid-type converter have weak overcurrent capabilities and cannot withstand large current shocks like traditional rotating motors. This feature leads to its current priority strategy during operation, which makes its voltage control present an indirect control mode. Compared with existing synchronous generators, the voltage support capability of the grid-type converter is relatively weak.

[0004] In order to improve the voltage control capability of grid-type converters in future power electronic systems and ensure the stability of grid voltage, it is urgent to improve the voltage control structure of grid-type converters. At the same time, its control parameters should be optimized to enhance its responsiveness to grid fluctuations. This will help ensure the reliability and stable operation of the grid when a high proportion of power electronic equipment is connected, and provide technical support for the construction of future electronic power systems. Summary of the invention

[0005] One of the objects of the present invention is to solve at least one or more of the above-mentioned problems existing in the prior art. In other words, one of the objects of the present invention is to provide a voltage inertia control method that meets one or more of the above-mentioned requirements and improves the transient voltage stability of a grid-type converter.

[0006] In order to achieve the above-mentioned object of the invention, the present invention adopts the following technical solutions: In a first aspect, the present invention provides a voltage inertia control method for improving transient voltage stability of a grid-connected converter, comprising: Setting a voltage control architecture, the voltage control architecture includes an outer reactive loop, a middle voltage loop, and an inner current loop; Set up a low-pass filter for reactive power; Obtain the voltage disturbance and the maximum adjustable reactive power on the grid side, and set the proportionality coefficient according to the ratio of the voltage disturbance and the maximum adjustable reactive power; The reactive power on the grid side is input into the low-pass filter to calculate the voltage inertia, and the proportional coefficient is multiplied by the voltage inertia to obtain the reactive loop control feedforward term; Perform feedforward adjustment on the outer reactive loop according to the reactive loop control feedforward item; The voltage control architecture is calculated based on the outer reactive loop after feedforward regulation to control the converter.

[0007] As a preferred implementation, the lower limit of the cutoff frequency of the low-pass filter is calculated based on the time domain analytical solution of the step response, and the upper limit of the cutoff frequency of the low-pass filter is calculated based on the gain of the open-loop transfer function at twice the operating frequency.

[0008] As a further preferred implementation, the method for calculating the lower limit of the cutoff frequency of the low-pass filter is specifically as follows: The reactive steady-state value and time-domain response of the outer reactive loop are calculated, and the lower limit of the cut-off frequency is calculated according to the ratio of the reactive steady-state value and the time-domain response.

[0009] As a further preferred embodiment, it is characterized in that: As a preferred implementation, the converter responds to the control of the voltage control architecture and uses a pulse width modulation method to modulate the output voltage of the converter.

[0010] As a preferred implementation, the output current frequency of the converter is calculated by substituting the inertia coefficient and the damping coefficient into the difference between the active power and the active control instruction through a virtual synchronous control link.

[0011] On the other hand, the present invention provides a voltage inertia control system for improving transient voltage stability of a grid-type converter, comprising a voltage inertia calculation unit, a voltage control architecture generation unit, and a control unit; The voltage inertia calculation unit is used to set a low-pass filter with a proportionality coefficient according to the voltage disturbance amount and the maximum adjustable reactive power of the power grid, and calculate the voltage inertia according to the low-pass filter, and the proportionality coefficient is calculated according to the ratio of the voltage disturbance amount to the maximum adjustable reactive power; The voltage control architecture generation unit is used to generate a voltage control architecture, the voltage control architecture includes an outer reactive loop, a middle voltage loop, and an inner current loop, and the outer reactive loop uses voltage inertia for feedforward regulation; The control unit is used to control the converter according to a three-level voltage control architecture.

[0012] As a preferred implementation, the voltage inertia calculation unit calculates the lower limit of the cutoff frequency of the low-pass filter according to the time domain analytical solution of the step response, and calculates the upper limit of the cutoff frequency of the low-pass filter according to the gain of the open-loop transfer function at twice the power frequency.

[0013] As a preferred implementation, the voltage inertia calculation unit calculates the lower limit of the cut-off frequency of the low-pass filter in the following manner: The reactive steady-state value and time-domain response of the outer reactive loop are calculated, and the lower limit of the cut-off frequency is calculated according to the ratio of the reactive steady-state value and the time-domain response.

[0014] As a preferred implementation, the converter responds to the control of the voltage control architecture and uses a pulse width modulation method to modulate the output voltage of the converter.

[0015] Compared with the prior art, the present invention has the following beneficial effects: The method and system of the present invention calculate the proportional coefficient in the voltage inertia control and the cut-off frequency of the low-pass filter, and substitute them into the reactive loop in the three-level voltage control architecture, so that the voltage inertia control has a larger feasible domain of stable equilibrium points, and enhances the anti-interference ability of the reactive control link in the transient process, so that the system can operate safely and stably. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Figure 1 is a schematic diagram of a feasible region of a stable equilibrium point of an embodiment of the present application; Figure 2 is a schematic diagram of time domain simulation of a comparison method of an embodiment of the present application under a fault; Figure 3 It is a schematic diagram of time domain simulation of the method of an embodiment of the present application under fault conditions. DETAILED DESCRIPTION

[0017] The technical solutions in the embodiments of the present application will be clearly and completely described below in conjunction with the drawings in the embodiments of the present application.

[0018] The following description provides examples and does not limit the scope, applicability or examples set forth in the claims. Changes may be made to the functions and arrangements of the elements described without departing from the scope of the present application. Various processes or components may be appropriately omitted, substituted or added to each example. For example, the described method may be performed in an order different from the order described, and various steps may be added, omitted or combined. In addition, features described in some examples may be combined in other examples.

[0019] This embodiment provides a voltage inertia control method for improving transient voltage stability of a grid-connected converter, including: S1. A voltage control architecture for voltage inertia control of a grid-type converter is preset. The voltage control architecture includes three levels, namely an outer reactive loop, a middle voltage loop, and an inner current loop.

[0020] The outer reactive loop adopts a voltage inertia control structure, and a low-pass filter with a proportional coefficient is set to filter the voltage inertia transmitted to the reactive loop.

[0021] Specifically, the above low-pass filter expression is: ; Where k is the proportionality coefficient, is the cutoff frequency of the low-pass filter.

[0022] The above cut-off frequency The lower bound of is determined by the gain of the open-loop transfer function at twice the power frequency, and the upper bound is determined by the time domain analytical solution of the step response.

[0023] Specifically, the lower bound of the cutoff frequency is calculated as:

[0024] Where ln is the logarithmic function, is the d-axis voltage of the controller, E is the equivalent voltage of the external power grid, is the phase angle difference between the voltage at the converter connection point and the equivalent voltage of the external grid, is the equivalent reactance of the external power grid, t represents time in seconds, is the reactive steady-state value, is the time domain response of the outer reactive loop.

[0025] The calculation formula is:

[0026] The calculation formula is: .

[0027] The reactive outer loop constructed with the above low-pass filter can make the grid-type converter show characteristics similar to synchronous machines in reactive power regulation. When the system voltage is disturbed, the outer loop reactive layer can make the converter suppress voltage fluctuations by adjusting reactive output like a synchronous generator. This response can quickly stabilize the voltage and improve the system's anti-disturbance ability.

[0028] S2. Receive grid operation data, obtain the voltage disturbance and maximum adjustable reactive power on the grid side, and set a proportional coefficient according to the ratio of the voltage disturbance and the maximum adjustable reactive power.

[0029] The proportional coefficient is set by the following formula: ; in, is the disturbance of the grid-connected grid voltage of the grid-connected converter, It is the maximum adjustable reactive power allowed by the grid-connected converter.

[0030] S3. After the proportionality coefficient is calculated, the voltage inertia is calculated using the reactive outer loop with the low-pass filter set in step S1.

[0031] During the control process, the reactive power on the grid side at each moment is input into the low-pass filter, the voltage inertia is output from the low-pass filter, and then the proportional coefficient is multiplied by the voltage inertia at this moment to obtain the reactive loop control feedforward term at the next moment.

[0032] S4, feedforward adjustment is performed on the outer reactive loop according to the reactive loop control feedforward item, and then step S5 is performed, a voltage control architecture is calculated according to the outer reactive loop after the feedforward adjustment, so as to control the converter.

[0033] The method of this embodiment calculates the proportional coefficient in the voltage inertia control and the cut-off frequency of the low-pass filter, and substitutes them into the reactive loop in the three-level voltage control architecture, so that the voltage inertia control has a larger feasible domain of stable equilibrium points, and enhances the anti-interference ability of the reactive control link in the transient process, so that the system can operate safely and stably.

[0034] As an example, when feedforward regulation is performed on the outer reactive loop, the transfer function of the reactive outer loop is: ,in, is the control command value, and Q is the reactive power output by the converter.

[0035] The reactive power Q on the grid side is specifically: , is the inductive impedance. Combining the above two formulas, we get:

[0036] Let the above formula be equal to 0, we can find There are two values ​​of, that is, two equilibrium points, one of which is less than 0, so the feasible range of stable equilibrium points is from 0 to the other The value of Figure 1 shown.

[0037] After taking t=1s and setting the proportional coefficient and cut-off frequency according to this embodiment, the time domain simulation comparison of the grid-connected converter under fault conditions using the method of this embodiment and the conventional method is shown in FIG. Figure 2 and Figure 3 . Figure 2Demonstrates transient stability of reactive PI control under fault conditions, Figure 3 The transient stability of the voltage inertia control strategy of this embodiment under fault conditions is demonstrated. As shown in the figure, compared with reactive PI control, the control method based on voltage inertia of this embodiment expands the stability region. Therefore, when a power grid fails, the grid-forming equipment controlled by the method of this embodiment can operate more stably and is less likely to cause transient voltage instability.

[0038] On the other hand, the present invention provides a voltage inertia control system for improving transient voltage stability of a grid-type converter, comprising a voltage inertia calculation unit, a voltage control architecture generation unit and a control unit; The voltage inertia calculation unit is used to execute steps S1-S3 of the method in the above embodiment, set a low-pass filter with a proportional coefficient according to the voltage disturbance of the power grid and the maximum adjustable reactive power, and calculate the voltage inertia according to the low-pass filter.

[0039] The voltage control architecture generation unit is used to generate the voltage control architecture of the method in the above embodiment, and the voltage control architecture includes an outer reactive loop, a middle voltage loop, and an inner current loop.

[0040] The control unit is used to calculate control instructions according to the three-level voltage control architecture generated by the voltage control architecture, so as to control the converter.

[0041] Those of ordinary skill in the art will appreciate that the units and algorithm steps of each example described in conjunction with the embodiments disclosed herein can be implemented in electronic hardware, or a combination of computer software and electronic hardware. Whether these functions are performed in hardware or software depends on the specific application and design constraints of the technical solution. Professional and technical personnel can use different methods to implement the described functions for each specific application, but such implementation should not be considered to be beyond the scope of this application.

[0042] The above is only an exemplary embodiment of the present disclosure, and the scope of the present disclosure cannot be limited thereto. That is, any equivalent changes and modifications made according to the teachings of the present disclosure are still within the scope of the present disclosure. After considering the specification and practicing the disclosure here, those skilled in the art will easily think of the implementation scheme of the present disclosure. This application is intended to cover any modification, use or adaptation of the present disclosure, which follows the general principles of the present disclosure and includes common knowledge or customary technical means in the technical field not recorded in the present disclosure. The description and examples are regarded as exemplary only, and the scope and spirit of the present disclosure are defined by the claims.

Claims

1. A voltage inertia control method for improving transient voltage stability of a grid-connected converter, characterized in that: include: Setting a voltage control architecture, wherein the voltage control architecture includes an outer reactive loop, a middle voltage loop, and an inner current loop; Set up a low-pass filter for reactive power; Acquire the voltage disturbance amount and the maximum adjustable reactive power on the grid side, and set the proportionality coefficient according to the ratio of the voltage disturbance amount and the maximum adjustable reactive power; Inputting the reactive power of the grid side into the low-pass filter to calculate the voltage inertia, and multiplying the proportional coefficient by the voltage inertia to obtain a reactive loop control feedforward term; Performing feedforward adjustment on the outer reactive loop according to the reactive loop control feedforward item; The voltage control architecture is calculated according to the outer reactive loop after feedforward adjustment, so as to control the converter.

2. A voltage inertia control method for improving transient voltage stability of a grid-connected converter as claimed in claim 1, characterized in that: The lower limit of the cutoff frequency of the low-pass filter is calculated according to the time domain analytical solution of the step response, and the upper limit of the cutoff frequency of the low-pass filter is calculated according to the gain of the open-loop transfer function at twice the power frequency.

3. A voltage inertia control method for improving transient voltage stability of a grid-connected converter as claimed in claim 2, characterized in that: The calculation method of the lower limit of the cut-off frequency of the low-pass filter is specifically as follows: The reactive steady-state value and the time-domain response of the outer reactive loop are calculated, and the lower limit of the cut-off frequency is calculated according to the ratio of the reactive steady-state value and the time-domain response.

4. A voltage inertia control method for improving transient voltage stability of a grid-connected converter as claimed in claim 1, characterized in that: The converter modulates the output voltage of the converter using a pulse width modulation method in response to the control of the voltage control architecture.

5. A voltage inertia control method for improving transient voltage stability of a grid-connected converter as claimed in claim 1, characterized in that: The output current frequency of the converter is calculated by substituting the inertia coefficient and the damping coefficient into the difference between the active power and the active control instruction through a virtual synchronous control link.

6. A voltage inertia control system for improving transient voltage stability of a grid-connected converter, characterized in that: It includes a voltage inertia calculation unit, a voltage control architecture generation unit, and a control unit; The voltage inertia calculation unit is used to set a low-pass filter with a proportionality coefficient according to the voltage disturbance amount and the maximum adjustable reactive power of the power grid, and calculate the voltage inertia according to the low-pass filter, wherein the proportionality coefficient is calculated according to the ratio of the voltage disturbance amount to the maximum adjustable reactive power; The voltage control architecture generation unit is used to generate a voltage control architecture, wherein the voltage control architecture includes an outer reactive loop, a middle voltage loop, and an inner current loop, wherein the outer reactive loop uses the voltage inertia for feedforward regulation; The control unit is used to control the converter according to the three-level voltage control architecture.

7. A voltage inertia control system for improving transient voltage stability of a grid-connected converter as claimed in claim 6, characterized in that: The voltage inertia calculation unit calculates the lower limit of the cutoff frequency of the low-pass filter according to the time domain analytical solution of the step response, and obtains the upper limit of the cutoff frequency of the low-pass filter according to the gain of the open-loop transfer function at twice the power frequency.

8. A voltage inertia control system for improving transient voltage stability of a grid-connected converter as claimed in claim 6, characterized in that: The voltage inertia calculation unit calculates the lower limit of the cut-off frequency of the low-pass filter in the following manner: The reactive steady-state value and the time-domain response of the outer reactive loop are calculated, and the lower limit of the cut-off frequency is calculated according to the ratio of the reactive steady-state value and the time-domain response.

9. A voltage inertia control system for improving transient voltage stability of a grid-connected converter as claimed in claim 6, characterized in that: The converter modulates the output voltage of the converter using a pulse width modulation method in response to the control of the voltage control architecture.

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