Flexible interconnection equipment with tracking-networking dual modes and control method thereof

Abstract

The invention discloses flexible interconnection equipment with tracking-networking dual modes and a control method thereof, and belongs to the technical field of high-quality power supply. The equipment comprises an AC / DC bidirectional converter, a DC / DC bidirectional converter, a first transformer area, a second transformer area, a first power grid, a second power grid and an energy storage device. The direct current side of the first AC / DC bidirectional converter is connected with the direct current side of the second AC / DC bidirectional converter to form a back-to-back structure, and the output end of the first transformer area and the output end of the second transformer area are connected with the alternating current end of the first AC / DC bidirectional converter and the alternating current end of the second AC / DC bidirectional converter respectively. One side of the DC / DC bidirectional converter is connected with the DC side of the first AC / DC bidirectional converter and the DC side of the second AC / DC bidirectional converter, and the other side of the DC / DC bidirectional converter is connected with the energy storage device; according to the method, flexible self-adaptive switching between the network following control mode and the network construction control mode can be realized according to the strong and weak conditions of the transformer area power grid, the adaptability under the weak power grid condition is enhanced, and high-quality power supply of the load and efficient and stable operation of the transformer area are ensured.

Description

Technical Field

[0001] The present invention relates to the field of high-quality power supply technology, and more specifically, to a flexible interconnection device with a dual mode of following and building a network and a control method thereof. Background Art

[0002] With the large-scale access of distributed power sources, the diversified growth of power loads, and the increasing proportion of DC loads, the structure and operation of traditional distribution networks face huge challenges. These changes have put forward higher requirements on the stability, reliability and power quality of the power grid. Specifically, the access of distributed power sources has changed the path of power flow and increased the complexity of the distribution system; the diversified power load has led to more frequent and unpredictable fluctuations in power demand; and the increase in the proportion of DC loads has introduced new technical requirements and control difficulties. These factors have put a severe test on the traditional distribution network system, prompting the power grid to urgently respond to new challenges through technological innovation.

[0003] In this context, the use of fully controlled power electronic devices to flexibly interconnect the distribution network has become a key means to improve the controllability and adaptability of the power grid. The introduction of fully controlled power electronic devices can provide higher flexibility for the distribution network and improve the uncertainty in the operation of the power grid through precise control of electric energy. Flexible interconnection technology can not only improve the reliability and safety of the system, but also promote the consumption of distributed power sources while meeting the demand for high-quality power supply. This lays the foundation for the future evolution to smart distribution networks, allowing the power grid to respond to the diverse changes in power demand and supply more efficiently and flexibly.

[0004] Among them, grid-connected converters, as important equipment for flexible interconnection of distribution networks, play a core role in distribution networks. Through grid-following control technology, grid-connected converters can comprehensively manage power quality issues such as reactive power, negative sequence and harmonics between substations, and effectively improve the power quality of the substation area. At the same time, the converter can also realize the decoupling control of active power and reactive power between substations, and realize continuous and precise regulation of power transmission between substations, thereby optimizing the power flow distribution in the power grid and improving the overall efficiency of the system. However, with the continuous increase in the penetration rate of new energy and the proportion of power electronic equipment in the power grid, the dynamic characteristics of the power grid have changed significantly.

[0005] As the proportion of synchronous generators in the system decreases, the power grid gradually transforms into a converter-dominated system. This change has brought about a profound change in the system operation mode, especially in the significant reduction of system inertia and a sharp drop in the short-circuit ratio of the point of common coupling (PCC). These factors make the power grid show obvious weak grid characteristics, and the stability and inertia support mechanisms of the traditional synchronous machine-dominated mode gradually fail in this new environment. The emergence of weak grids has increased the resonance and instability risks of grid-connected converters when interacting with the grid, especially under the condition of high penetration of renewable energy generation, this instability is further aggravated, resulting in frequent accidents in renewable energy power generation systems. Summary of the invention

[0006] In order to overcome the shortcomings of the existing technology, a flexible interconnection equipment with dual modes of following and building a network and a control method thereof are proposed, which can flexibly and adaptively switch between the following control mode and the building a network control mode according to the strength of the power grid in the substation area, and enhance the adaptability under weak power grid conditions, thereby ensuring high-quality power supply to the load and efficient and stable operation of the substation.

[0007] The technical solution adopted by the present invention to solve the technical problem is: a flexible interconnection equipment with dual modes of following and building a network, the improvement of which is that it includes an AC / DC bidirectional converter, a DC / DC bidirectional converter, a first substation, a second substation, a first power grid, a second power grid and an energy storage device;

[0008] The AC / DC bidirectional converter includes a first AC / DC bidirectional converter and a second AC / DC bidirectional converter; the first power grid is connected to the input end of the first substation, the second power grid is connected to the input end of the second substation, the DC side of the first AC / DC bidirectional converter is connected to the DC side of the second AC / DC bidirectional converter to form a back-to-back structure, the output end of the first substation is connected to the AC end of the first AC / DC bidirectional converter, the output end of the second substation is connected to the AC end of the second AC / DC bidirectional converter, one side of the DC / DC bidirectional converter is connected to the DC side of the first AC / DC bidirectional converter and the second AC / DC bidirectional converter, and the other side of the DC / DC bidirectional converter is connected to the energy storage device.

[0009] In the above structure, the first substation includes a first distribution transformer and a first load, the first distribution transformer and the first load, the second substation includes a second distribution transformer and a second load, the second distribution transformer and the second load; the AC side of the first AC / DC bidirectional converter and the first load are connected in parallel to the secondary side of the first distribution transformer, and the AC side of the second AC / DC bidirectional converter and the second load are connected in parallel to the second distribution transformer.

[0010] In the above structure, the first AC / DC bidirectional converter or the second AC / DC bidirectional converter can be in either a grid-following control mode or a grid-building control mode.

[0011] In the above structure, when the AC / DC bidirectional converter is in the grid-following control mode, the internal control structure of the AC / DC bidirectional converter includes: a sampling module, a phase-locked loop module, an outer loop power control module, an inner loop current control module and a pulse width modulation signal generator;

[0012] The sampling module is used to update once in each switching cycle to collect power voltage, current, AC voltage amplitude and phase angle data of the AC / DC bidirectional converter;

[0013] The phase-locked loop module is used to obtain the AC voltage amplitude and phase angle by tracking the AC bus voltage of the AC / DC bidirectional converter;

[0014] The power control module of the outer loop is used to control the given signal according to the active power and reactive power output current;

[0015] The current control module of the inner loop is used to make a difference between the current control given signal and the actual current value, the difference is adjusted by a proportional-integral controller to generate a voltage given signal, and the voltage given signal is transformed into a modulation signal of the converter in a three-phase stationary coordinate system;

[0016] The pulse width modulation signal generator is used to generate a trigger signal from the modulation signal of the converter in the three-phase stationary coordinate system, and the trigger signal controls the on-off time of the insulated gate bipolar transistor, thereby controlling the output voltage of the AC / DC bidirectional converter within a preset threshold range.

[0017] In the above structure, when the AC / DC bidirectional converter is in the grid control mode, the internal control structure of the AC / DC bidirectional converter includes: a second sampling module, an outer loop virtual synchronous machine control module, an inner loop voltage control module, an inner loop current control module, and a pulse width modulation signal generator;

[0018] The second sampling module is used to update once in each switching cycle to collect power voltage, current, AC voltage amplitude and phase angle data of the AC / DC bidirectional converter;

[0019] The virtual synchronous machine control module of the outer ring is used to adjust the power of the AC / DC bidirectional converter and output a voltage amplitude given signal and a voltage phase given signal;

[0020] The voltage control module of the inner loop is used to adjust the voltage amplitude given signal and the voltage phase given signal through a proportional-integral controller to obtain a current control given signal of the inner loop;

[0021] The current control module of the second inner loop is used to convert the current control given signal of the inner loop into a modulation signal of the AC / DC bidirectional converter;

[0022] The second pulse width modulation signal generator is used to generate a trigger signal from the modulation signal of the AC / DC bidirectional converter, and the trigger signal controls the on-off time of the insulated gate bipolar transistor, thereby controlling the output voltage of the AC / DC bidirectional converter within a preset threshold range.

[0023] The present invention also discloses a control method for flexible interconnection equipment with dual modes of following and building a network, wherein the method comprises the following steps:

[0024] The first AC / DC bidirectional converter and the second AC / DC bidirectional converter identify the short-circuit ratio of the grid connection point through fundamental impedance, judge the strength of the grid, and adaptively select the operation mode;

[0025] When it is detected that the short-circuit ratio of the secondary-side grid-connected point of the first substation and the second distribution transformer is within the normal range, that is, the first substation and the second substation are both under strong grid conditions, the first AC / DC bidirectional converter and the second AC / DC bidirectional converter both operate in the grid-following control mode;

[0026] In the grid-following control mode, the converter comprehensively controls the negative sequence current, reactive power, harmonics, etc. in the first substation or the second substation by adjusting the power quality, and the grid supplies power to the load;

[0027] When it is detected that the short-circuit ratio of the grid connection point on the secondary side of the first distribution transformer drops sharply, that is, the first substation grid presents weak grid or extremely weak grid characteristics, the first AC / DC bidirectional converter automatically switches to the grid construction control mode, and the first load is jointly powered by the first substation grid, the energy storage device and the second AC / DC bidirectional converter;

[0028] The energy storage device cooperates with the second AC / DC bidirectional converter to provide stable DC voltage support for the first AC / DC bidirectional converter, and the first AC / DC bidirectional converter provides stable voltage and frequency support for the first substation, and also provides virtual inertia and damping to enhance the adaptability of the first substation under weak grid conditions;

[0029] When it is detected that the short-circuit ratio of the grid connection point on the secondary side of the second distribution transformer drops sharply, that is, the second substation grid presents weak or extremely weak grid characteristics, the second AC / DC bidirectional converter automatically switches to the grid construction control mode, and the second load is jointly powered by the second substation grid, the energy storage device and the first AC / DC bidirectional converter;

[0030] The energy storage device cooperates with the first AC / DC bidirectional converter to provide stable DC voltage support for the second AC / DC bidirectional converter, and the second AC / DC bidirectional converter provides stable voltage and frequency support for the second substation, and also provides virtual inertia and damping to enhance the adaptability of the second substation under weak grid conditions;

[0031] When it is detected that the short-circuit ratio of the grid-connected point on the secondary side of the first substation or the second distribution transformer has recovered from a sharply declining state to a normal range, the first AC / DC bidirectional converter or the second AC / DC bidirectional converter is restored to the grid-following control mode, and continues to comprehensively manage the power quality problems such as negative sequence current, reactive power, harmonics, etc. in the first substation or the second substation;

[0032] When it is detected that the short-circuit ratios of the secondary-side grid-connected points of the first substation and the second distribution transformer are both reduced, that is, the power grids of the first substation and the second substation both show weak or extremely weak power grid characteristics, the energy can be flexibly transferred between the first substation, the second substation and the energy storage device through the regulation of the DC / DC bidirectional converter, the first AC / DC bidirectional converter and the second AC / DC bidirectional converter, so as to achieve flexible adjustment of the power demand of the two substations.

[0033] Furthermore, the short-circuit ratio of the grid-connected point is the short-circuit ratio of the first substation or the second substation, which is the ratio of the real-time short-circuit capacity of the AC system to the capacity of the connected equipment. When the short-circuit ratio of the first substation or the second substation is greater than or equal to a preset threshold, the substation is in a strong grid condition; when the short-circuit ratio of the first substation or the second substation is less than the preset threshold, the substation is in a weak grid condition.

[0034] Furthermore, when the power grids of the first substation and the second substation both present weak or extremely weak power grid characteristics, if the short circuit of the grid-connected point on the secondary side of the first distribution transformer is relatively small, the first AC / DC bidirectional converter automatically switches to the grid-building control mode, and the second AC / DC bidirectional converter maintains the grid-following control mode; if the short circuit of the grid-connected point on the secondary side of the second distribution transformer is relatively small, the second AC / DC bidirectional converter automatically switches to the grid-building control mode, and the first AC / DC bidirectional converter maintains the grid-following control mode.

[0035] Furthermore, the energy storage device serves as a backup power source, draws electric energy from the substation when the load is low, and releases electric energy when the load is peak, so as to reduce the load peak-to-valley difference between the substations and optimize the load curve.

[0036] The beneficial effects of the present invention are: it can flexibly and adaptively switch between the grid-following control mode and the grid-building control mode according to the strength of the grid in the substation area, and enhance the adaptability under weak grid conditions, thereby ensuring high-quality power supply to the load and efficient and stable operation of the substation area. BRIEF DESCRIPTION OF THE DRAWINGS

[0037] Figure 1 It is a schematic diagram of the overall structure of a flexible interconnection device with dual modes of following and networking according to the present invention;

[0038] Figure 2 A schematic diagram of a control method for adaptively switching between the following and networking dual modes of a flexible interconnection device having the following and networking dual modes according to the present invention;

[0039] Figure 3 It is a schematic diagram of an overall control method of a virtual synchronous generator control (VSG) in a networking mode of a flexible interconnection equipment with a dual-mode of following and networking according to the present invention;

[0040] Figure 4 It is a schematic diagram of an active power-frequency regulation control block of a virtual synchronous generator control (VSG) of a flexible interconnection equipment with dual modes of following and building a network according to the present invention;

[0041] Figure 5 It is a schematic diagram of reactive power-voltage regulation control block of virtual synchronous generator control (VSG) of flexible interconnection equipment with dual modes of following and building a network according to the present invention;

[0042] Figure 6 It is a schematic diagram of an overall control method of PQ control of a following-networking mode of a flexible interconnection device with a following-networking dual mode according to the present invention;

[0043] Figure 7 This is a schematic diagram of the internal circuit structure of a DC port DC / DC bidirectional converter of a flexible interconnection device with dual modes of following and building a network according to the present invention;

[0044] Figure 8 The present invention is a flow chart of a control method for flexible interconnection equipment with dual modes of following and networking. DETAILED DESCRIPTION

[0045] The present invention is further described below in conjunction with the accompanying drawings and embodiments.

[0046] The following will clearly and completely describe the concept, specific structure and technical effects of the present invention in combination with the embodiments and drawings, so as to fully understand the purpose, characteristics and effects of the present invention. 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, other embodiments obtained by technicians in this field without creative work are all within the scope of protection of the present invention. In addition, all the connection / connection relationships involved in the patent do not refer to the direct connection of components, but refer to the formation of a better connection structure by adding or reducing connection accessories according to the specific implementation situation. The various technical features in the invention can be combined interchangeably without conflicting with each other.

[0047] It should be noted that if the embodiments of the present invention involve directional indications (such as up, down, left, right, front, back, etc.), the directional indications are only used to explain the relative position relationship, movement status, etc. between the components under a certain specific posture (as shown in the accompanying drawings). If the specific posture changes, the directional indication will also change accordingly.

[0048] In addition, if there are descriptions involving "first", "second", etc. in the embodiments of the present invention, the descriptions of "first", "second", etc. are only used for descriptive purposes and cannot be understood as indicating or suggesting their relative importance or implicitly indicating the number of the indicated technical features. Therefore, the features defined as "first" and "second" may explicitly or implicitly include at least one of the features. Secondly, the technical solutions between the various embodiments can be combined with each other, but they must be based on the ability of ordinary technicians in the field to implement them. When the combination of technical solutions is contradictory or cannot be implemented, it should be deemed that such a combination of technical solutions does not exist and is not within the scope of protection required by the present invention.

[0049] The present invention will be further described below in conjunction with the accompanying drawings.

[0050] See also Figure 1 As shown, the present invention provides a flexible interconnection equipment with a dual mode of following and building a network, including a first AC / DC bidirectional converter, a second AC / DC bidirectional converter, a DC / DC bidirectional converter, a first substation, a second substation, a first power grid, a second power grid and an energy storage device;

[0051] The AC / DC bidirectional converter includes a first AC / DC bidirectional converter and a second AC / DC bidirectional converter; the first power grid is connected to the input end of the first substation, the second power grid is connected to the input end of the second substation, the DC side of the first AC / DC bidirectional converter is connected to the DC side of the second AC / DC bidirectional converter, forming a back-to-back structure, and the back-to-back structural design provides higher system redundancy and reliability. When one of the substations fails or the power grid is abnormal, the other substation and the energy storage device can jointly maintain the stable operation of the system. The output end of the first substation is connected to the AC end of the first AC / DC bidirectional converter, the output end of the second substation is connected to the AC end of the second AC / DC bidirectional converter, one side of the DC / DC bidirectional converter is connected to the DC side of the first AC / DC bidirectional converter and the second AC / DC bidirectional converter, and the other side of the DC / DC bidirectional converter is connected to the energy storage device. The loads of the two substations are adjusted by the bidirectional converter, and combined with the energy support of the energy storage device, the load is balanced and shared, avoiding overload of the substation or imbalance of power supply and demand. This is very important for maintaining the stability of the power grid and reducing power fluctuations.

[0052] The first substation includes a first distribution transformer and a first load, the first distribution transformer and the first load, the second substation includes a second distribution transformer and a second load, the second distribution transformer and the second load; the AC side of the first AC / DC bidirectional converter and the first load are connected in parallel to the secondary side of the first distribution transformer, and the AC side of the second AC / DC bidirectional converter and the second load are connected in parallel to the second distribution transformer. Flexible energy management can be achieved by connecting the AC / DC bidirectional converter in parallel with the substation load and connecting the secondary side of the distribution transformer. This equipment can not only obtain electrical energy from the power grid, but also supply power from the energy storage device when needed, to ensure a smooth supply of electrical energy under different working conditions.

[0053] Furthermore, the first AC / DC bidirectional converter or the second AC / DC bidirectional converter can be in either a grid-following control mode or a grid-building control mode.

[0054] When the AC / DC bidirectional converter is in the grid-following control mode, the internal control structure of the AC / DC bidirectional converter includes: a sampling module, a phase-locked loop module, an outer loop power control module, an inner loop current control module and a pulse width modulation signal generator;

[0055] The sampling module is used to update once in each switching cycle to collect power voltage, current, AC voltage amplitude and phase angle data of the AC / DC bidirectional converter;

[0056] The phase-locked loop module is used to obtain the AC voltage amplitude and phase angle by tracking the AC bus voltage of the AC / DC bidirectional converter. This setting can ensure that the output is synchronized with the power grid and prevent power fluctuations caused by frequency deviation.

[0057] The power control module of the outer loop is used to control the given signal according to the active power and reactive power output current; this control method enables the converter to adapt to different load requirements and improve the energy management efficiency of the system.

[0058] The current control module of the inner loop is used to make a difference between the current control given signal and the actual current value. The difference is adjusted by a proportional-integral controller to generate a voltage given signal. The voltage given signal is transformed into a modulation signal of the converter in a three-phase stationary coordinate system. This refined control can ensure the stability of the output current and improve the dynamic response capability and anti-interference ability of the system.

[0059] The pulse width modulation signal generator is used to generate a trigger signal from the modulation signal of the converter in the three-phase stationary coordinate system, and the trigger signal controls the on-off time of the insulated gate bipolar transistor, thereby controlling the output voltage of the AC / DC bidirectional converter within a preset threshold range; this process can achieve precise control of the output voltage, ensure that it is within the preset threshold range, and reduce the potential risk of overvoltage or overcurrent to the equipment.

[0060] Through the collaborative work of the above modules, the entire control structure can maintain the stability and reliability of the system under various working conditions. When the power grid fluctuates or fails, the system can quickly switch modes to ensure that the load power supply is not affected. It not only improves the energy conversion efficiency, but also reduces the operating cost of the system. By dynamically adjusting the output parameters, it can better adapt to different load requirements and power grid conditions, thereby optimizing the overall energy management.

[0061] Furthermore, when the AC / DC bidirectional converter is in the grid control mode, the internal control structure of the AC / DC bidirectional converter includes: a second sampling module, an outer loop virtual synchronous machine control module, an inner loop voltage control module, an inner loop current control module, and a pulse width modulation signal generator;

[0062] The second sampling module is used to update once in each switching cycle to collect power voltage, current, AC voltage amplitude and phase angle data of the AC / DC bidirectional converter;

[0063] The outer loop virtual synchronous machine control module is used to adjust the power of the AC / DC bidirectional converter and output a voltage amplitude given signal and a voltage phase given signal; the outer loop virtual synchronous machine control module enables the converter to imitate the characteristics of a synchronous generator, and by adjusting the output voltage amplitude and phase signal, the converter can better participate in the scheduling of the power system and improve the stability of the power system.

[0064] The voltage control module of the inner loop is used to adjust the voltage amplitude given signal and the voltage phase given signal through a proportional-integral controller to obtain the current control given signal of the inner loop; the inner loop voltage control module combined with the proportional-integral controller can accurately adjust the output voltage, and generate a current control given signal based on the voltage amplitude and phase given signals. This control helps to improve the dynamic performance of the system and maintain the stability of the power grid.

[0065] The current control module of the second inner loop is used to convert the current control given signal of the inner loop into a modulation signal of the AC / DC bidirectional converter; the current control module of the second inner loop can quickly adapt to changes in the load and ensure that the output current operates within a safe range by converting the current control given signal into a modulation signal.

[0066] The second pulse width modulation signal generator is used to generate a trigger signal from the modulation signal of the AC / DC bidirectional converter, and the trigger signal controls the on-off time of the insulated gate bipolar transistor, thereby controlling the output voltage of the AC / DC bidirectional converter within a preset threshold range. The second pulse width modulation signal generator converts the modulation signal into a trigger signal, accurately controls the switching time of the insulated gate bipolar transistor, thereby ensuring that the output voltage is controlled within a preset threshold range. This process reduces the potential damage to the equipment due to overvoltage or overcurrent, and improves the reliability of the converter.

[0067] Through the design under the grid control mode, the AC / DC bidirectional converter can effectively realize power conversion and management, adapt to different power environments, and ensure stability and flexibility under various conditions. The rationality of the overall structure and the efficient coordination of modules provide a guarantee for the reliable operation of the power system.

[0068] Reference Figure 1-8 As shown, the present invention also discloses a control method for flexible interconnection equipment with dual modes of following and building a network, and the improvement thereof is that the method comprises the following steps:

[0069] S11, when the first substation and the second substation are both in a strong grid condition, the first AC / DC bidirectional converter controller and the second AC / DC bidirectional converter are both in a grid-following control mode, and the first load and the second load are powered by the first substation and the second substation respectively;

[0070] S12, the first AC / DC bidirectional converter controller and the second AC / DC bidirectional converter can also perform decoupling control on the active power and reactive power output between the substations, so as to realize continuous and precise regulation of the transmission power between the substations, smooth out fluctuations, and optimize the power flow distribution between the substations;

[0071] S13, the energy storage device acts as a backup power source, and the energy storage device cooperates with the first AC / DC bidirectional converter controller and the second AC / DC bidirectional converter to draw electric energy from the substation during the low load period and release electric energy during the peak load period to reduce the load peak-to-valley difference between the substations and optimize the load curve;

[0072] S14. Increase the capacity or quantity of energy storage devices according to demand to handle power quality problems such as negative sequence, reactive power, harmonics, etc. in the substation in real time.

[0073] The control method of the flexible interconnection equipment with dual modes of following and building a network, the control method also includes:

[0074] S21. When the first substation presents weak or extremely weak grid characteristics, the first AC / DC bidirectional converter automatically switches to the grid control mode to provide voltage and frequency support for the first substation, thereby enhancing the adaptability of the first substation under weak grid conditions; the first load is jointly powered by the first substation, the energy storage device, and the second AC / DC bidirectional converter;

[0075] S22, the first load is powered by the first station, the energy storage device and the second AC / DC bidirectional converter, and the energy storage device and the second AC / DC bidirectional converter cooperate with each other to provide stable DC voltage support for the first AC / DC bidirectional converter.

[0076] The control method of the flexible interconnection equipment with dual modes of following and building a network, the control method also includes:

[0077] S31. When the second substation presents weak or extremely weak grid characteristics, the second AC / DC bidirectional converter automatically switches to the grid control mode to provide voltage and frequency support for the second substation, thereby enhancing the adaptability of the second substation under weak grid conditions. The second load is jointly powered by the second substation, the energy storage device and the first AC / DC bidirectional converter.

[0078] S32, the second load is powered by the second station, the energy storage device and the first AC / DC bidirectional converter. The energy storage device and the first AC / DC bidirectional converter cooperate with each other to provide stable DC voltage support for the second AC / DC bidirectional converter.

[0079] The control method of the flexible interconnection equipment with dual modes of following and building a network, the control method also includes:

[0080] S41. When the grid strength of the first substation or the second substation returns to the normal range, the first AC / DC bidirectional converter or the second AC / DC bidirectional converter automatically switches back to the grid-following control mode, and re-assumes the task of comprehensive management of power quality problems and rapid power regulation in the first substation or the second substation.

[0081] Furthermore, the first AC / DC bidirectional converter and the second AC / DC bidirectional converter identify the short-circuit ratio of the grid connection point through fundamental impedance to determine the strength of the grid. The short-circuit ratio of the grid connection point is the ratio of the real-time short-circuit capacity of the AC system to the capacity of the connected equipment. When the short-circuit ratio of the first or second substation is greater than or equal to a preset threshold, the substation is in a strong grid condition, and when the short-circuit ratio of the first or second substation is less than the preset threshold, the substation is in a weak grid condition. When the grids of the first and second substations both present weak or extremely weak grid characteristics, if the short-circuit of the grid connection point on the secondary side of the first distribution transformer is relatively small, the first AC / DC bidirectional converter automatically switches to the grid control mode, and the second AC / DC bidirectional converter maintains the grid control mode; if the short-circuit of the grid connection point on the secondary side of the second distribution transformer is relatively small, the second AC / DC bidirectional converter automatically switches to the grid control mode, and the first AC / DC bidirectional converter maintains the grid control mode.

[0082] When both the first substation and the second substation are under strong grid conditions, the first load and the second load are powered by the first substation and the second substation respectively; the energy storage branch draws energy from the load side through the first or second AC / DC bidirectional converter using PQ control to ensure that it can release the stored electric energy when needed; at the same time, comprehensive management of power quality issues in the power grid is achieved.

[0083] When the first substation presents the characteristics of a weak or extremely weak grid, the first AC / DC bidirectional converter automatically switches to the grid-building control mode, and the first AC / DC bidirectional converter provides voltage and frequency support for the grid of the first substation through VSG control, and provides rotational inertia and damping for the first substation, thereby enhancing the adaptability of the first substation under weak grid conditions; when the second substation presents the characteristics of a weak or extremely weak grid, the second AC / DC bidirectional converter automatically switches to the grid-building control mode, and the second AC / DC bidirectional converter provides voltage and frequency support for the second substation through VSG control, and provides rotational inertia and damping for the second substation, thereby enhancing the adaptability of the second substation under weak grid conditions. When the grid strength of the first or second substation returns to the normal range, the first or second AC / DC bidirectional converter automatically switches back to the grid-following control mode, and continues to comprehensively manage the power quality problems in the first or second substation and quickly adjust the power.

[0084] The equipment of the present invention is under the condition that both the first substation and the second substation are in a strong power grid. The energy storage device cooperates with the first AC / DC bidirectional converter and the second AC / DC bidirectional converter to respectively undertake the tasks of comprehensive power quality management and rapid power regulation in the first substation and the second substation, thereby achieving comprehensive management of power quality issues such as reactive power, negative sequence, and harmonics, reducing the peak-to-valley difference of the power grid load, optimizing the power grid flow distribution and load curve, and charging the energy storage device at the same time to keep the energy of the energy storage device sufficient and quickly release power when needed.

[0085] When the first substation presents the characteristics of a weak grid or an extremely weak grid, the first AC / DC bidirectional converter provides voltage and frequency support for the first substation, as well as rotational inertia and damping effects, thereby enhancing the adaptability of the first substation under weak grid conditions; when the second substation presents the characteristics of a weak grid or an extremely weak grid, the second AC / DC bidirectional converter provides voltage and frequency support for the second substation, as well as rotational inertia and damping effects, thereby enhancing the adaptability of the second substation under weak grid conditions.

[0086] When the grid strength of the first substation or the second substation returns to the normal range, the first AC / DC bidirectional converter or the second AC / DC bidirectional converter assumes the task of comprehensive power quality management and rapid power regulation of the first substation or the second substation again. The present invention is further described in detail below with reference to the accompanying drawings.

[0087] The primary frequency regulation process of the power grid refers to the process in which the synchronous generator automatically increases or decreases its output power to limit the change in the power grid frequency when the power grid frequency changes. The voltage regulation characteristic of the power system is that when the power grid has insufficient or excessive reactive power, the excitation system of the synchronous generator will adjust the voltage value according to the magnitude of the reactive power change.

[0088] In the scheme of the present invention, the first AC / DC bidirectional converter and the second AC / DC bidirectional converter in the grid control mode use the active-frequency speed regulator and reactive-voltage exciter in the virtual synchronous generator (VSG) technology to simulate the above two characteristics of the synchronous generator. VSG adopts a three-phase voltage source inverter structure, and its overall control strategy diagram is shown in Figure 3 shown.

[0089] The main control process of VSG can be summarized as follows: the three-phase voltage and three-phase current on the grid side / load side measured by the voltage and current sensors are sent to the instantaneous power calculation module to obtain the active power Pe and reactive power Q output by the system, and then the power is sent to the active-frequency speed regulator and reactive-voltage exciter respectively to obtain the required reference phase angle θ and reference voltage amplitude Us respectively, and both are sent to the voltage and current dual closed-loop control module to obtain the pulse width modulation wave, thereby controlling the on and off of the insulated gate bipolar transistor devices in the AC / DC bidirectional converter, thereby completing the entire control process of VSG.

[0090] Figure 3 In, L f is the inverter side filter inductor; C f is the inverter side filter capacitor; L c is the grid side / load side inductance; i af 、i bf 、i cf is the three-phase inductor current on the inverter side; v ao 、v bo 、v co is the three-phase filter capacitor voltage; i ao 、i bo 、i co is the three-phase filter capacitor current;

[0091] The active power-frequency speed regulator of VSG mainly simulates the rotational inertia and frequency regulation of the synchronous generator. Therefore, we first consider the rotor equation of the synchronous generator, namely:

[0092]

[0093] Δω=ω-ω0; (2)

[0094] Among them, J, T m , T e , D, ω are the inertia, mechanical torque, electromagnetic torque, damping coefficient and mechanical angular velocity of the synchronous generator.

[0095] The P-ω droop equation is usually used to simulate the active power-frequency regulation of synchronous generators:

[0096]

[0097] ΔP=P m -P e ; (4)

[0098] Among them, K p is the P-ω droop coefficient, ω is the angular velocity of VSG, P mis the VSG input active power, and Pe is the VSG output active power. Considering equations (1), (2), (3), and (4), we can get:

[0099]

[0100] The design of VSG reactive-voltage exciter is similar to that of active-frequency speed regulator. The relationship between reactive power and voltage also satisfies the droop characteristic, and the EQ droop equation is usually used:

[0101] e=K u (U ref -U0)+K q (Q ref -Q e ) ; (6)

[0102] U ref is the amplitude of the calculated given voltage, U0 is the rated voltage amplitude, Q0 is the rated reactive power, K u is the voltage regulation coefficient, K q is the reactive power regulation coefficient.

[0103] Short circuit ratio K SCR Defined as the ratio of the real-time short-circuit capacity of the AC system to the capacity of the connected equipment:

[0104]

[0105] Sac is the three-phase short-circuit capacity of the AC system, S d The capacity of the connected equipment.

[0106] Figure 4 , Figure 5 They are respectively the VSG active power-frequency regulation control block diagram and reactive power-voltage regulation control block diagram. Figure 3 As shown in the VSG control block diagram, the reference phase angle θ obtained by the active-frequency speed regulator and the reactive-voltage exciter and the reference voltage amplitude e are combined to form v ref Send it to the outer loop voltage control module to get i ref The signal is sent to the inner loop current control module, and then the on and off of the insulated gate bipolar transistor of the AC / DC bidirectional converter is controlled by the pulse width modulation signal generator.

[0107] Compared with the limitations of traditional single-mode converters, especially the advantages and disadvantages of grid-following control and grid-forming control, this method currently needs to design and develop a dual-mode flexible interconnection equipment. In a weak grid environment, traditional grid-following control cannot cope with these problems well. In order to solve the stability challenges in weak grids, researchers have proposed a grid-forming control technology. The grid-forming converter adopts a power synchronization strategy similar to that of a synchronous generator, which can achieve synchronization with the grid without a phase-locked loop. Compared with grid-following control, grid-forming control can better support the grid voltage and frequency and enhance the stability of the system. In addition, in conjunction with energy storage devices, the grid-forming converter can also provide virtual inertia and damping for the system, further enhancing the dynamic support capability of the grid. However, although grid-forming control has obvious advantages in a weak grid environment, it also has some shortcomings, such as slow power regulation, strong coupling of multi-machine systems, and poor stability in a strong grid environment.

[0108] The present invention combines the advantages of grid-following and grid-building control, and flexibly switches the working mode under different grid conditions. When the grid strength is high and the stability is good, the equipment can adopt grid-following control to achieve comprehensive management of the power quality in the substation area and precise regulation of power transmission. In a weak grid with low grid strength and small system inertia, it can switch to the grid-building control mode to enhance the voltage and frequency support capabilities of the grid and improve the stability of the system. The present invention can flexibly and adaptively switch between the grid-following control mode and the grid-building control mode according to the strength of the substation grid, and enhance the adaptability under weak grid conditions, ensuring high-quality power supply to the load and efficient and stable operation of the substation.

[0109] The above is a specific description of the preferred implementation of the present invention, but the invention is not limited to the embodiments. Those skilled in the art can make various equivalent modifications or substitutions without violating the spirit of the present invention. These equivalent modifications or substitutions are all included in the scope defined by the claims of this application.

Claims

1. A flexible interconnection device with dual modes of connection and networking, characterized in that: It includes an AC / DC bidirectional converter, a DC / DC bidirectional converter, a first substation, a second substation, a first power grid, a second power grid and an energy storage device; The AC / DC bidirectional converter includes a first AC / DC bidirectional converter and a second AC / DC bidirectional converter; the first power grid is connected to the input end of the first substation, the second power grid is connected to the input end of the second substation, the DC side of the first AC / DC bidirectional converter is connected to the DC side of the second AC / DC bidirectional converter to form a back-to-back structure, the output end of the first substation is connected to the AC end of the first AC / DC bidirectional converter, the output end of the second substation is connected to the AC end of the second AC / DC bidirectional converter, one side of the DC / DC bidirectional converter is connected to the DC side of the first AC / DC bidirectional converter and the second AC / DC bidirectional converter, and the other side of the DC / DC bidirectional converter is connected to the energy storage device.

2. According to claim 1, a flexible interconnection device with dual modes of tracking and networking, characterized in that: The first substation includes a first distribution transformer and a first load, the first distribution transformer and the first load, the second substation includes a second distribution transformer and a second load, the second distribution transformer and the second load; the AC side of the first AC / DC bidirectional converter and the first load are connected in parallel to the secondary side of the first distribution transformer, and the AC side of the second AC / DC bidirectional converter and the second load are connected in parallel to the second distribution transformer.

3. According to claim 1, a flexible interconnection device with dual modes of tracking and networking, characterized in that: The first AC / DC bidirectional converter or the second AC / DC bidirectional converter can be in either a grid-following control mode or a grid-building control mode.

4. The flexible interconnection equipment with dual modes of tracking and networking according to claim 3 is characterized in that: When the AC / DC bidirectional converter is in the grid-following control mode, the internal control structure of the AC / DC bidirectional converter includes: a sampling module, a phase-locked loop module, an outer loop power control module, an inner loop current control module and a pulse width modulation signal generator; The sampling module is used to update once in each switching cycle to collect power voltage, current, AC voltage amplitude and phase angle data of the AC / DC bidirectional converter; The phase-locked loop module is used to obtain the AC voltage amplitude and phase angle by tracking the AC bus voltage of the AC / DC bidirectional converter; The power control module of the outer loop is used to control the given signal according to the active power and reactive power output current; The current control module of the inner loop is used to make a difference between the current control given signal and the actual current value, the difference is adjusted by a proportional-integral controller to generate a voltage given signal, and the voltage given signal is transformed into a modulation signal of the converter in a three-phase stationary coordinate system; The pulse width modulation signal generator is used to generate a trigger signal from the modulation signal of the converter in the three-phase stationary coordinate system, and the trigger signal controls the on-off time of the insulated gate bipolar transistor, thereby controlling the output voltage of the AC / DC bidirectional converter within a preset threshold range.

5. The flexible interconnection equipment with dual modes of tracking and networking according to claim 3 is characterized in that: When the AC / DC bidirectional converter is in the grid control mode, the internal control structure of the AC / DC bidirectional converter includes: a second sampling module, an outer loop virtual synchronous machine control module, an inner loop voltage control module, an inner loop current control module, and a pulse width modulation signal generator; The second sampling module is used to update once in each switching cycle to collect power voltage, current, AC voltage amplitude and phase angle data of the AC / DC bidirectional converter; The virtual synchronous machine control module of the outer ring is used to adjust the power of the AC / DC bidirectional converter and output a voltage amplitude given signal and a voltage phase given signal; The voltage control module of the inner loop is used to adjust the voltage amplitude given signal and the voltage phase given signal through a proportional-integral controller to obtain a current control given signal of the inner loop; The current control module of the second inner loop is used to convert the current control given signal of the inner loop into a modulation signal of the AC / DC bidirectional converter; The second pulse width modulation signal generator is used to generate a trigger signal from the modulation signal of the AC / DC bidirectional converter, and the trigger signal controls the on-off time of the insulated gate bipolar transistor, thereby controlling the output voltage of the AC / DC bidirectional converter within a preset threshold range.

6. A control method for flexible interconnection equipment with dual modes of following and building a network according to any one of claims 1 to 5, characterized in that: The method comprises the following steps: The first AC / DC bidirectional converter and the second AC / DC bidirectional converter identify the short-circuit ratio of the grid connection point through fundamental impedance, judge the strength of the grid, and adaptively select the operation mode; When it is detected that the short-circuit ratio of the secondary-side grid-connected point of the first substation and the second distribution transformer is within the normal range, that is, the first substation and the second substation are both under strong grid conditions, the first AC / DC bidirectional converter and the second AC / DC bidirectional converter both operate in the grid-following control mode; In the grid-following control mode, the converter comprehensively controls the negative sequence current, reactive power, harmonics, etc. in the first substation or the second substation by adjusting the power quality, and the grid supplies power to the load; When it is detected that the short-circuit ratio of the grid connection point on the secondary side of the first distribution transformer drops sharply, that is, the first substation grid presents weak grid or extremely weak grid characteristics, the first AC / DC bidirectional converter automatically switches to the grid construction control mode, and the first load is jointly powered by the first substation grid, the energy storage device and the second AC / DC bidirectional converter; The energy storage device cooperates with the second AC / DC bidirectional converter to provide stable DC voltage support for the first AC / DC bidirectional converter, and the first AC / DC bidirectional converter provides stable voltage and frequency support for the first substation, and also provides virtual inertia and damping to enhance the adaptability of the first substation under weak grid conditions; When it is detected that the short-circuit ratio of the grid connection point on the secondary side of the second distribution transformer drops sharply, that is, the second substation grid presents weak or extremely weak grid characteristics, the second AC / DC bidirectional converter automatically switches to the grid construction control mode, and the second load is jointly powered by the second substation grid, the energy storage device and the first AC / DC bidirectional converter; The energy storage device cooperates with the first AC / DC bidirectional converter to provide stable DC voltage support for the second AC / DC bidirectional converter, and the second AC / DC bidirectional converter provides stable voltage and frequency support for the second substation, and also provides virtual inertia and damping to enhance the adaptability of the second substation under weak grid conditions; When it is detected that the short-circuit ratio of the grid-connected point on the secondary side of the first substation or the second distribution transformer has recovered from a sharply declining state to a normal range, the first AC / DC bidirectional converter or the second AC / DC bidirectional converter is restored to the grid-following control mode, and continues to comprehensively manage the power quality problems such as negative sequence current, reactive power, harmonics, etc. in the first substation or the second substation; When it is detected that the short-circuit ratios of the secondary-side grid-connected points of the first substation and the second distribution transformer are both reduced, that is, the power grids of the first substation and the second substation both show weak or extremely weak power grid characteristics, the energy can be flexibly transferred between the first substation, the second substation and the energy storage device through the regulation of the DC / DC bidirectional converter, the first AC / DC bidirectional converter and the second AC / DC bidirectional converter, so as to achieve flexible adjustment of the power demand of the two substations.

7. A control method for flexible interconnection equipment with dual modes of following and building a network according to claim 6, characterized in that: The short-circuit ratio of the grid connection point is the short-circuit ratio of the first substation or the second substation, which is the ratio of the real-time short-circuit capacity of the AC system to the capacity of the connected equipment. When the short-circuit ratio of the first substation or the second substation is greater than or equal to a preset threshold, the substation is in a strong grid condition; when the short-circuit ratio of the first substation or the second substation is less than the preset threshold, the substation is in a weak grid condition.

8. The control method of the flexible interconnection equipment with dual modes of following and building a network according to claim 6 is characterized in that: When the power grids of the first substation and the second substation both present weak or extremely weak power grid characteristics, if the short circuit at the grid-connected point on the secondary side of the first distribution transformer is relatively small, the first AC / DC bidirectional converter automatically switches to the grid-building control mode, and the second AC / DC bidirectional converter maintains the grid-following control mode; if the short circuit at the grid-connected point on the secondary side of the second distribution transformer is relatively small, the second AC / DC bidirectional converter automatically switches to the grid-building control mode, and the first AC / DC bidirectional converter maintains the grid-following control mode.

9. The control method of the flexible interconnection equipment with dual modes of following and building a network according to claim 6 is characterized in that: The energy storage device serves as a backup power source, draws electric energy from the substation when the load is low, and releases electric energy when the load is high, so as to reduce the load peak-to-valley difference between the substations and optimize the load curve.

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