A strong network stability type voltage control method applicable to low line impedance

Abstract

The present invention discloses a strong network stable voltage control method applicable to low line impedance, including: the voltage on the filter capacitor is sampled and subjected to a coordinate transformation of 3s / 2r to obtain a d-axis voltage quantity and a q-axis voltage quantity respectively. After comparing and taking the difference with the dq-axis voltage given reference value, a dq-axis voltage deviation signal is obtained, and then this deviation signal is input into the strong network stable voltage control as the input reference value of the current inner loop; the output result quantity of the dq-axis voltage loop is used as the input reference value of the current inner loop. At the same time, the current on the filter inductor close to the inverter side is sampled, and the difference between the two is sent to the PI regulator. The output of the current inner loop PI regulator serves as the modulation wave of the PWM modulation. After comparing with the triangular carrier wave, six-way PWM drive signals are generated to drive the on-off of the bridge arm switching tubes of the three-phase full-bridge inverter. This method has good compatibility and broadens the application range of the network-forming converter.

Description

Technical Field

[0001] The invention belongs to the field of electric power technology, and in particular relates to a strong network stabilization type voltage control method suitable for low line impedance. Background Art

[0002] In the past decade, the large-scale access of new energy sources such as wind and solar power to the power grid through power electronic converters is one of the main technical features of the new generation of power systems. According to the different control methods of power electronic converters and the differences in equivalent models, converter control technology can be roughly divided into two categories: grid-following control and grid-building control. In grid-following control, most converters use a single current loop to control the output current amplitude of the converter, so its equivalent model is a controlled current source, and its synchronization frequency is generally obtained by sampling the grid voltage through a phase-locked loop. Compared with the grid-following converter, the grid-building converter uses a voltage and current dual inner loop to control the amplitude of the output voltage. The synchronization loop often uses virtual synchronous machine control or active power-frequency droop control, and its equivalent model is a controlled voltage source. At present, in actual engineering applications, most new energy converters adopt grid-following control. However, since the grid-following converter is equivalent to a current source that injects active power current into the grid, its voltage support capability and frequency support capability are naturally insufficient. In addition, in long-distance power transmission (the longer the transmission line, the greater the line impedance, and the weaker the grid strength), it is easy to produce wide-band oscillations in the system. Therefore, grid-forming control technology came into being subsequently. Due to its excellent voltage / frequency support capability and the ability to access extremely weak grids or even operate in isolated grids, as well as the essential requirement that new power systems must control some converters as voltage sources, the control technology related to grid-forming converters has become a hot research issue in the current field.

[0003] Although the grid-connected converter can operate more stably in weak grid conditions than the grid-connected converter, some researchers have noticed that when the transmission line between the grid-connected converter and the external grid is close, that is, the line impedance is small and the grid strength is high, the grid-connected converter grid-connected system is prone to low-frequency oscillation. In addition to optimizing and adjusting the control parameters of the system to avoid the strong grid unstable oscillation of the grid-connected converter as much as possible, the most commonly used method is to add a virtual impedance loop outside the voltage and current loop to increase the output impedance of the converter. However, in actual implementation, the three loops of voltage loop, current loop and virtual impedance loop are coupled with each other, resulting in a cumbersome control architecture and complex parameter design. Summary of the invention

[0004] The purpose of the present invention is to provide a strong network stability voltage control method suitable for low line impedance. Based on the traditional dual proportional integral inner loop network control method, this method only needs to adjust the voltage loop control structure under the original proportional integral, without modifying or adding other control loops and additional design control parameters.

[0005] To achieve the above object, the present invention adopts the following technical solutions:

[0006] The present invention provides a strong network stable voltage control method applicable to low line impedance, based on a network-forming converter system under dual voltage-current inner loop - droop synchronous control. The control method includes the following steps:

[0007] The voltage on the filter capacitor is sampled and subjected to a 3s / 2r coordinate transformation to obtain the d-axis voltage quantity and the q-axis voltage quantity respectively. After comparing and taking the difference with the dq-axis voltage given reference value, the dq-axis voltage deviation signal is obtained, and then this deviation signal is input into the strong network stable voltage control as the input reference value of the current inner loop;

[0008] The output result of the dq-axis voltage loop is used as the input reference value of the current inner loop. At the same time, the current on the filter inductor near the inverter side is sampled, and the difference between the two is sent to the PI regulator. The output of the current inner loop PI regulator serves as the modulation wave of the PWM modulation. After comparing with the triangular carrier wave, six-way PWM drive signals are generated to drive the on-off of the bridge arm switching tubes of the three-phase full-bridge inverter.

[0009] As a further improvement of the present invention, the control system is established in the dq coordinate system, and the required 3s / 2r and 2r / 3s coordinate transformation angles are given by the active power - frequency droop synchronous control.

[0010] As a further improvement of the present invention, the final output of the d-axis voltage control will consist of three parts. The first part is the product of the d-axis voltage deviation signal and the proportional coefficient K p in the original PI control. The second part is the output after the d-axis voltage deviation signal passes through the second-order low-pass filter function . The third part is the output after introducing the q-axis voltage deviation quantity through another second-order low-pass filter function . The sum of the three parts constitutes the final output of the d-axis voltage control;

[0011] where ω g is the rated power frequency corresponding to the external power grid during grid connection, and K p and K i parameters are the proportional integral coefficients of the voltage loop.

[0012] As a further improvement of the present invention, the q-axis voltage control structure is in a dual form with the d-axis. The output result of the q-axis voltage loop also consists of three parts. The first part is the product of the q-axis voltage deviation signal and the original proportional coefficient K p . The second part is the output after the q-axis deviation signal passes through the second-order low-pass filter function . The third part is the output after introducing the d-axis voltage deviation quantity through another second-order low-pass filter function The output, the sum of the three parts constitutes the output of the final q-axis voltage loop, where K i is the integral coefficient in the original PI control;

[0013] Among them, ω g is the rated power frequency corresponding to the external power grid during grid connection, and K p and K i The parameters are the proportional-integral coefficients of the voltage loop.

[0014] As a further improvement of the present invention, the grid-forming converter system under the dual voltage-current inner loop - droop synchronous control includes a DC regulated power supply, a three-phase inverter, an LCL filter, and an external power supply connected in sequence.

[0015] As a further improvement of the present invention, the DC side of the three-phase grid-connected inverter is connected to a constant DC regulated power supply. After being driven by a modulation signal, the voltage at the output port of the three-phase grid-connected inverter contains high-order harmonics. After filtering out the switching ripple by the three-phase LCL filter, the finally output three-phase AC voltage is connected to the external power grid through the AC bus.

[0016] As a further improvement of the present invention, the three-phase grid-connected inverter is composed of six switching tubes.

[0017] As a further improvement of the present invention, the three-phase grid-connected inverter is a three-phase grid-connected inverter based on an LCL filter.

[0018] Compared with the prior art, the present invention has the following advantages:

[0019] The strong grid stability control method proposed by the present invention has the same input sampling part as the traditional proportional-integral voltage control method. It only needs to sample the capacitor voltage to control the output voltage amplitude of the grid-forming converter. No modification is required for any link other than the voltage loop control structure compared with the original grid-forming control system. The voltage control method proposed by the present invention is applicable to most grid-forming converter grid-connected systems. Without additional design of control parameters on the basis of the original proportional-integral voltage control, only the voltage loop control structure needs to be changed correspondingly. The specific parameter values are the same as those of the original traditional proportional-integral voltage control. The control structure is simple and the parameter design is easy. This control method has been verified to enable the grid-forming converter to be connected to a stronger external power grid, allowing the connection impedance between the grid-forming converter and the external power grid to be significantly reduced, and at the same time, it can also significantly alleviate the strong grid instability problem of the grid-forming converter. Compared with the control method proposed by the present invention for the original traditional proportional-integral voltage loop control method, without increasing the difficulty of system hardware design and software parameter design, it has good compatibility, broadens the application range of the grid-forming converter, and enables it to have good strong grid working ability. Brief Description of the Drawings

[0020] The accompanying drawings described herein are for illustrative purposes only and are not intended to limit the scope of the disclosure of the present invention in any way. Additionally, the shapes and proportional dimensions of the components in the figures are only schematic and are used to assist in understanding the present invention, rather than specifically defining the shapes and proportional dimensions of the components of the present invention. In the accompanying drawings:

[0021] Figure 1 is the circuit diagram and simplified control diagram of the grid-forming converter grid-connected system of the present invention;

[0022] Figure 2 is the control block diagram of the traditional classic proportional-integral voltage inner loop and current inner loop;

[0023] Figure 3 is the control block diagram of the active-power-frequency droop synchronization control;

[0024] Figure 4 is the strong-grid stability type voltage control block diagram first proposed by the present invention, which can directly replace Figure 2 the old proportional-integral type voltage loop control in;

[0025] Figure 5 is the schematic diagram of the simulation waveforms of the grid-forming converter grid-connected system of the present invention. Among them, (a) is the waveform diagram of strong-grid unstable oscillation under the classic proportional-integral voltage control, and (b) is the waveform diagram of stable grid connection of the strong-grid stability type voltage control when the line impedance is significantly reduced under the same system parameters. Detailed implementation manners

[0026] In order to enable those skilled in the art to better understand the technical solutions in the present invention, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without making creative efforts shall fall within the protection scope of the present invention.

[0027] It should be noted that when an element is referred to as being "disposed on" another element, it can be directly on the other element or there may also be an intermediate element. When an element is considered to be "connected" to another element, it can be directly connected to the other element or there may be an intermediate element at the same time. The terms "vertical", "horizontal", "left", "right" and similar expressions used herein are for illustrative purposes only and do not represent the only embodiments.

[0028] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the technical field to which this invention belongs. The terms used in the description of the present invention herein are for the purpose of describing specific embodiments only and are not intended to limit the present invention. The term "and / or" as used herein includes any and all combinations of one or more of the associated listed items.

[0029] To enable those skilled in the art to better understand the solution of the present invention, the technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, not all of the embodiments, and are not intended to limit the scope of the present invention disclosed. In addition, in the following description, the descriptions of well-known structures and technologies are omitted to avoid unnecessarily confusing the concepts disclosed in the present invention. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts shall fall within the scope of protection of the present invention.

[0030] The purpose of the present invention is to provide a simple and effective grid-forming converter voltage control method that can greatly alleviate the occurrence of strong grid instability. Based on the traditional grid-forming control method with a double proportional-integral inner loop, this method only needs to adjust the voltage loop control structure under the original proportional-integral, without modifying or adding other control loops and designing additional control parameters. The technical solutions of the control method proposed by the present invention are mainly divided into the following parts:

[0031] The grid-forming converter grid-connected system architecture studied in the present invention is as Figure 1 shown. The inverter is a three-phase grid-connected inverter based on an LCL filter. The DC side of the inverter is connected to a constant DC regulated power supply. After being driven by a modulation signal, the voltage at the output port of the three-phase grid-connected inverter contains high-order harmonics. After the switching ripple is filtered by the three-phase LCL filter, the finally output three-phase AC voltage is connected to the external grid through the AC bus. For the sake of convenience, the equivalent line impedance between the grid side outlet of the converter after LCL filtering and the external grid is also incorporated into the grid side filter inductor. As a further improvement of the present invention, the three-phase grid-connected inverter is composed of switching tubes S1, S2, S3, S4, S5, and S6. The control system is established in the dq coordinate system, and the 3s / 2r and 2r / 3s coordinate transformation angles required in the control system are given by the classical active-frequency droop synchronization control.

[0032] The output voltage amplitude of the traditional grid-forming converter is controlled by the voltage inner loop and the current inner loop, and the control methods inside the loops are all classical proportional-integral controls. The traditional proportional-integral voltage loop and current loop control structures are as Figure 2As shown, by sampling the voltage or current value of the corresponding node, the error signal after comparing it with the reference value and taking the difference is connected to a proportional-integral controller, where the output result of the voltage loop is the reference value input of the current loop. The output result E of the current loop dq determines the amplitude of the modulation wave for the PWM modulation of the final converter. The grid-connected frequency of the output voltage of the grid-forming converter is determined by the active-frequency droop control, and its specific structure is as Figure 3 shown, by sampling the capacitor voltage v o and the grid-side inductor current i o to calculate the grid-connected transmitted active power P out . P out passes through a low-pass filter and then is compared with the reference value of the output power and the difference is taken, and then the grid-connected frequency ω of the linear output adjustment system is correspondingly adjusted according to the magnitude of the deviation.

[0033] The voltage control method proposed by the present invention is a voltage control method for a grid-forming converter system under traditional double-voltage-current inner-loop - droop synchronization control. The inverter is a three-phase grid-connected inverter based on an LCL filter. The DC side of the inverter is connected to a constant DC regulated power supply. After being driven by a modulation signal, the voltage at the output port of the three-phase grid-connected inverter contains high-order harmonics. After filtering the switching ripple through a three-phase LCL filter, the finally output three-phase AC voltage is connected to the external grid through an AC bus. For the sake of convenience, the equivalent line impedance between the grid-side outlet of the converter after LCL filtering and the external grid is also incorporated into the grid-side filter inductor.

[0034] As a further improvement of the present invention, the three-phase grid-connected inverter is composed of switching tubes S1, S2, S3, S4, S5, and S6.

[0035] The control system is established in the dq coordinate system. The 3s / 2r and 2r / 3s coordinate transformation angles required in the control system are given by the classical active-frequency droop synchronization control. The voltage on the filter capacitor of the inverter is respectively obtained as the d-axis voltage quantity and the q-axis voltage quantity after sampling and 3s / 2r coordinate transformation. After comparing with the dq-axis voltage given reference value and taking the difference, the dq-axis voltage deviation signal is obtained, and then this deviation signal is input into the strong-grid stability type voltage control method proposed by the present invention.

[0036] Among them, the final output quantity of the d-axis voltage control will consist of three parts. The first part is the product of the d-axis voltage deviation signal and the proportional coefficient K p in the original PI control. The second part is the output after the d-axis voltage deviation signal passes through a second-order low-pass filter function . The third part is the output after introducing the q-axis voltage deviation quantity through another second-order low-pass filter function The output, the sum of the above three parts constitutes the final output of the d-axis voltage control.

[0037] For the q-axis voltage control, its control structure is dual compared with the d-axis. The output result of the q-axis voltage loop also consists of three parts. The first part is the product of the q-axis voltage deviation signal and the original proportional coefficient K p , the second part is the output after the q-axis deviation signal passes through the second-order low-pass filter function , and the third part is the output after introducing the d-axis voltage deviation amount and passing through another second-order low-pass filter function . The sum of the above three parts constitutes the output of the final q-axis voltage loop, where K i is the integral coefficient in the original PI control.

[0038] Take the output results of the dq-axis voltage loops as the input reference values of the current inner loop. At the same time, sample the current on the filter inductor near the inverter side, subtract the two, and send the result to the PI regulator. The output of the PI regulator in the current inner loop serves as the modulation wave for PWM modulation. After comparing with the triangular carrier wave, six-way PWM drive signals are generated to drive the on and off of the bridge arm switching tubes of the three-phase full-bridge inverter.

[0039] As a specific embodiment, the control of the strong grid stability type voltage control method proposed by the present invention is as Figure 4 shown. The voltage on the inverter filter capacitor is sampled and undergoes a 3s / 2r coordinate transformation to obtain the d-axis voltage quantity and the q-axis voltage quantity respectively. After comparing and subtracting with the dq-axis voltage given reference values, the dq-axis voltage deviation signals are obtained, and then these deviation signals are input into the strong grid stability type voltage control method proposed by the present invention. The final output of the d-axis voltage control will consist of three parts. The first part is the product of the d-axis voltage deviation signal and the proportional coefficient K p in the original PI control, the second part is the output after the d-axis voltage deviation signal passes through the second-order low-pass filter function , and the third part is the output after introducing the q-axis voltage deviation amount and passing through another second-order low-pass filter function . The sum of the above three parts constitutes the final output of the d-axis voltage control.

[0040] For the q-axis voltage control, its control structure is dual compared with the d-axis. The output result of the q-axis voltage loop also consists of three parts. The first part is the product of the q-axis voltage deviation signal and the original proportional coefficient K p , the second part is the output after the q-axis deviation signal passes through the second-order low-pass filter function , and the third part is the output after introducing the d-axis voltage deviation amount and passing through another second-order low-pass filter function The output, the sum of the above three parts constitutes the output of the final q-axis voltage loop, where K i is the integral coefficient in the original PI control. The output results of the dq-axis voltage loops are used as the input reference values of the current inner loop. At the same time, the current on the filter inductor near the inverter side is sampled, and the difference between the two is sent to the PI regulator. The output of the current inner loop PI regulator is used as the modulation wave of the PWM modulation. After comparing with the triangular carrier wave, six-way PWM drive signals are generated to drive the on and off of the bridge arm switching tubes of the three-phase full-bridge inverter.

[0041] Compared with Figure 2 the traditional classic proportional-integral voltage control in Figure 4 the control method, the voltage control method proposed by the present invention can directly replace the original proportional-integral voltage control, and no change is required for the rest of the control links. g in the control method is the rated power frequency corresponding to the external power grid during grid connection (usually fixed at 50Hz or 60Hz), K p and K i parameters are the same as the proportional-integral coefficients of the previous voltage loop. The strong grid stability type voltage control method does not require additional parameter design, and only the control architecture needs to be changed correspondingly. The control method proposed by the present invention retains the original proportional control link, transforms the original integral control link into a second-order transfer function link as shown in Figure 4 , and introduces cross-coupling terms between the d-axis and the q-axis respectively, so as to change the equivalent impedance of the converter to the outside by changing the control method, and finally realizes stable voltage control under strong grid conditions.

[0042] The specific control effects are compared as shown in Figure 5 . Under the same system parameters, as shown in Figure 5 (a) is the grid-connected simulation waveform diagram under proportional-integral voltage control. At 1s, the per-unit value of the line impedance is reduced from 0.25 p.u. to 0.2 p.u. (the distance between the converter and the external power grid is closer, and the power grid strength increases), and the system then undergoes strong grid unstable oscillation. At 2s, when the line impedance returns to 0.25 p.u., the system can return to stability again. Figure 5 (b) shows the simulation waveform diagrams of the output frequency and capacitor voltage of the converter after replacing the classic proportional-integral voltage control with the strong grid stability type voltage control. It can be seen from this that when the per-unit value of the line impedance is reduced from 0.25 p.u. to 0.05 p.u. and the power grid strength increases significantly, the system can still work stably. Comparing Figure 5From (a) and (b), it can be seen that the minimum line impedance acceptable to the system is greatly reduced from 0.2 p.u. of the proportional-integral voltage control to 0.05 p.u., which means that the converter can be closer to the external power grid, and the stability of the converter under strong grid conditions is significantly improved, so that the converter can be connected to a stronger power grid.

[0043] Based on the above analysis, it can be seen that the voltage control method proposed by the present invention is applicable to most grid-forming converter grid-connected systems. Without the need to additionally design control parameters on the basis of the original proportional-integral voltage control, the control structure is simple, parameter design is easy, and it can significantly alleviate the strong grid instability problem, allowing the connection impedance between the grid-forming converter and the external power grid to be significantly reduced.

[0044] Upon reading the above description, many embodiments and many applications other than the provided examples will be obvious to those skilled in the art. Therefore, the scope of this teaching should not be determined with reference to the above description, but should be determined with reference to the full scope of the foregoing claims and the equivalents of these claims. For the sake of completeness, all articles and references including patent applications and published announcements are incorporated herein by reference. The omission of any aspect of the subject matter disclosed herein in the foregoing claims is not intended to abandon such subject matter, nor should the applicant be regarded as not considering such subject matter as part of the disclosed inventive subject matter.

[0045] The above content is a further detailed description of the present invention. It cannot be determined that the specific implementation of the present invention is limited to this. For those of ordinary skill in the technical field to which the present invention pertains, without departing from the concept of the present invention, several simple deductions or substitutions can be made, which should all be regarded as belonging to the protection scope determined by the claims submitted for the present invention.

Claims

1. A strong network stability type voltage control method applicable to low line impedance, characterized in that, based on a network-forming converter system under dual voltage-current inner loop - droop synchronous control, the control method comprises the following steps: The voltage on the filter capacitor is sampled and subjected to a 3s / 2r coordinate transformation to obtain the d-axis voltage quantity and the q-axis voltage quantity respectively. After comparing and taking the difference with the dq-axis voltage given reference value, the dq-axis voltage deviation signal is obtained, and then this deviation signal is input into the strong network stability type voltage control as the input reference value of the current inner loop; Taking the output result quantity of the dq-axis voltage loop as the input reference value of the current inner loop, and at the same time sampling the current on the filter inductor near the inverter side. After taking the difference between the two and sending it to the PI regulator, the output of the current inner loop PI regulator is used as the modulation wave of PWM. After comparing with the triangular carrier wave, six-way PWM drive signals are generated to drive the on-off of the bridge arm switching tubes of the three-phase full-bridge inverter; The final output of the d-axis voltage control will consist of three parts. The first part is the product of the d-axis voltage deviation signal and the proportional coefficient K in the original PI control p . The second part is the output after the d-axis voltage deviation signal passes through the second-order low-pass filter function . The third part is the output after introducing the q-axis voltage deviation amount and passing through another second-order low-pass filter function . The sum of the three parts constitutes the final output of the d-axis voltage control; Among them, ω g is the rated power frequency corresponding to the external power grid during grid connection, and K p and K i parameters are the proportional-integral coefficients of the voltage loop; The q-axis voltage control structure is in a dual form with the d-axis. The output of the q-axis voltage loop also consists of three parts. The first part is the product of the q-axis voltage deviation signal and the original proportional coefficient K p . The second part is the output after the q-axis deviation signal passes through the second-order low-pass filter function . The third part is the output after introducing the d-axis voltage deviation amount and passing through another second-order low-pass filter function . The sum of the three parts constitutes the output of the final q-axis voltage loop. Among them, K i is the integral coefficient in the original PI control; Among them, ω g is the rated power frequency corresponding to the external power grid during grid connection, and K p and K i parameters are the proportional-integral coefficients of the voltage loop.

2. A strong network stability type voltage control method applicable to low line impedance according to claim 1, characterized in that, the network-forming converter system is established in the dq coordinate system, and the required 3s / 2r and 2r / 3s coordinate transformation angles are given by active - frequency droop synchronous control.

3. A strong network stability type voltage control method applicable to low line impedance according to claim 1, characterized in that, the network-forming converter system under dual voltage-current inner loop - droop synchronous control comprises a DC regulated power supply, a three-phase inverter, an LCL filter and an external power supply connected in sequence.

4. A strong network stability type voltage control method applicable to low line impedance according to claim 3, characterized in that, the DC side of the three-phase grid-connected inverter is connected to a constant DC regulated power supply. After being driven by the modulation signal, the voltage at the output port of the three-phase grid-connected inverter contains high-order harmonics. After filtering the switching ripple through the three-phase LCL filter, the finally output three-phase AC voltage is connected to the external power grid through the AC bus.

5. A strong network stability type voltage control method applicable to low line impedance according to claim 4, characterized in that, the three-phase grid-connected inverter is composed of six switching tubes.

6. A strong network stability type voltage control method applicable to low line impedance according to claim 4, characterized in that, the three-phase grid-connected inverter is a three-phase grid-connected inverter based on an LCL filter.

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