Control and protection cooperative control method for flexible interconnection device based on energy storage Crowbar

Abstract

The invention provides a control and protection cooperative control method for a flexible interconnection device based on an energy storage Crowbar, and aims to improve the low voltage ride through capability of a grid-connected power generation system. The method comprises the following steps: (1) connecting a direct current bus and an energy storage device by adopting a voltage reversible chopper circuit, realizing bidirectional flow of energy through a step-down / step-up chopper circuit, and performing interlocking control on a switch tube trigger signal by utilizing a double-PI controller; (2) establishing a coupling inductance model based on the magnetic coupling coefficient, and describing a current dynamic response; (3) limiting the speed increase of the fault current by using an inductance obstruction characteristic, and analyzing the current change in combination with an RL circuit differential equation; and (4) carrying out constant current control on an inverter side converter, and maintaining the stability of the current by adjusting the output voltage. According to the invention, through stored energy management, inductive coupling modeling and fault current limitation, stable operation of the system can be maintained and reactive power support can be provided when the power grid fails.

Description

Technical Field

[0001] This invention relates to the field of power electronics technology, specifically a control and protection coordination method for flexible interconnected devices based on energy storage Crowbar. Background Technology

[0002] With the increasing global demand for renewable energy, the integration of high-proportion renewable energy sources such as wind and solar power into microgrids has become an inevitable trend, leading to a further increase in power plant installed capacity. However, large-scale power plant grid connection poses a significant challenge to the safe operation of the entire power system. A fault in a distribution line can cause a momentary peak in grid current, severely impacting converters and other components. Passive protection measures may result in a substantial decrease in the active power output of the power plant, and in severe cases, the fault may spread, causing other power plants to disconnect and triggering large-scale blackouts. Therefore, after introducing new power plant grid connection rules, my country requires power plants to possess strong low-voltage ride-through capabilities to ensure the continuity of grid-connected operation during grid faults.

[0003] Low-voltage ride-through (LVR) technology refers to the ability of grid-connected renewable energy generation systems (such as wind and solar power) to maintain their connection to the grid even when the grid voltage drops. In severe cases, these systems can even contribute power back to the grid, helping it return to normal operation. LVR technology is essential for grid-connected inverters to maintain stable operation during grid-side faults. In Europe and the United States, LVR technology is relatively mature, and comprehensive technical specifications have been established. These specifications vary slightly between countries to ensure that grid-connected power generation systems meet standard requirements. In contrast, China's grid-connection technical specifications started later and are relatively more lenient.

[0004] Existing technologies address this issue by proposing low-voltage ride-through (LVRT) through adjustments to inverter control strategies. For example, a dual-loop voltage and current control strategy can be employed, where the outer voltage loop uses PI control and the inner current loop uses a sliding mode variable structure control strategy, combined with a DC unloading circuit, to ensure stable operation of the photovoltaic grid-connected power generation system during grid faults. LVRT also includes reactive power control strategies, such as fixed power factor, constant active power control, and constant reactive power control. These control techniques can inject more reactive power into the grid during voltage dips. However, these strategies only guarantee basic LVRT capability. Therefore, exploring and proposing a control strategy for a control-protection coordinated flexible interconnection device that can improve LVRT capability is of great significance. Summary of the Invention

[0005] To achieve the above objectives, this invention provides a control and protection coordination method for flexible interconnected devices based on energy storage Crowbar, which addresses the current need for grid-connected power generation systems to improve their low-voltage ride-through capability.

[0006] To solve the above-mentioned technical problems, the present invention adopts the following technical method:

[0007] A control and protection coordination method for a flexible interconnect device based on energy storage Crowbar is proposed. The flexible interconnect device based on energy storage Crowbar includes a rectifier-side converter CSC, a DC-side converter CSC, and an inverter-side converter CSC. Each phase of the converter CSC includes an IGBT and a driver. The DC-side CSC is equipped with an energy storage device and two coupled inductors. and The method includes the following steps:

[0008] (1) Energy storage Crowbar low voltage ride-through foundation control: A voltage reversible chopper circuit is used to connect the DC bus and the energy storage device, wherein:

[0009] When the DC side current is too high, the excess energy is stored in the energy storage device through the buck chopper circuit, which includes a first switching transistor V1 and a second rectifier diode VD2.

[0010] When the DC side current is insufficient, the energy of the energy storage device is fed back to the DC bus through the boost chopper circuit. The boost chopper circuit includes a second switching transistor V2 and a first rectifier diode VD1.

[0011] A dual PI controller is used to interlock the trigger signals of V1 and V2 to prevent V1 and V2 from being turned on at the same time;

[0012] (2) Modeling of inductive coupling effect: for two coupled inductors in a microgrid and An inductive coupling model described by matrix equations is established based on the magnetic coupling coefficient k to describe its current dynamic response;

[0013] (3) Current limitation during faults: Based on the inductive coupling model, the current growth rate during faults is limited by the resistance of the inductor to the change of current. The current change characteristics are described by the differential equation of the RL circuit and the exponential growth model.

[0014] (4) Constant current control on the inverter side: Based on the current limiting results, constant current control is adopted for the CSC on the inverter side, and the output current on the inverter side is set. To maintain a constant value, the inverter output voltage V is adjusted. inv (t) Keep Unaffected by voltage fluctuations.

[0015] Furthermore, the energy storage device mentioned in step (1) is a battery or a supercapacitor.

[0016] Furthermore, in step (1), a dual PI controller is used to interlock the trigger signals of V1 and V2 to prevent V1 and V2 from being turned on simultaneously. Specifically, this includes:

[0017] A voltage reversible chopper circuit is used to connect the DC bus and the energy storage device. V1 and VD2 form a step-down chopper circuit, which is used to store excess energy in the energy storage device when the DC side current is too high. V2 and VD1 form a step-up chopper circuit, which is used to release the energy of the energy storage device to charge the bus capacitor when the DC side current is insufficient.

[0018] To prevent a short circuit in the DC bus caused by simultaneous conduction of V1 and V2, two PI controllers are used to control the upper and lower limits of the DC bus current respectively, and the trigger signals of V1 and V2 are interlocked to ensure that they do not conduct at the same time.

[0019] Furthermore, in step (2), the two inductors in the microgrid and This coupling effect is described using current and voltage equations for multiple inductors, which interact through different current paths.

[0020] Assuming inductance and There exists a coupling coefficient k that describes the magnetic coupling between the two, and the change in inductor current is described by the following matrix equation:

[0021] (1)

[0022] in:

[0023] and They are respectively through inductors and The current; and It is the resistance in the circuit; and It is a voltage source caused by a fault.

[0024] Furthermore, by connecting a DC-side inductor and a crowbar, the charging and discharging of the energy storage device can be controlled by adjusting the inductor current. This allows for better prediction of current trends under fault conditions and the design of more efficient protection strategies. For microgrid systems, accurate inductor modeling helps optimize current limiting mechanisms, ensuring the system can respond quickly and resume normal operation in the event of a fault.

[0025] Furthermore, step (4) specifically includes:

[0026] Set the inverter-side output current To maintain a constant value, the system algorithm is used to control the system. Unaffected by voltage fluctuations, the change in current is described by the following equation:

[0027] (5)

[0028] in, It is the output voltage of the inverter. It is the system impedance, under voltage sag conditions. It is controlled to a constant value, while Adjusts to changes in grid voltage.

[0029] The beneficial effects of this invention are as follows:

[0030] 1. Enhance low voltage ride-through capability: Through bidirectional energy conversion (buck storage / boost feedback) of energy storage Crowbar, dynamic adjustment of DC side energy is achieved to ensure continuous grid-connected operation of the system during faults.

[0031] 2. Ensure system safety: The dual PI controller interlock mechanism prevents the switching transistors from conducting simultaneously, thus preventing the risk of DC bus short circuit and improving device reliability.

[0032] 3. Precise dynamic current control: Based on the magnetic coupling coefficient, an inductor model is established, and combined with the characteristics of the RL circuit, the fault current growth rate is accurately described, the current limiting strategy is optimized, and the fault impact is reduced.

[0033] 4. Stable output characteristics: The inverter side constant current control adjusts the output voltage in real time to ensure that the current is not affected by grid voltage fluctuations, maintains system voltage and frequency stability, and enhances microgrid resilience. Attached Figure Description

[0034] Figure 1 This is a design block diagram of the flexible interconnection device, microgrid structure, microgrid protection device and its fault location involved in the present invention.

[0035] Figure 2 This is a flowchart of a control and protection collaborative control method for a flexible interconnected device based on energy storage Crowbar according to an embodiment of the present invention;

[0036] Figure 3 This is a control block diagram of the PQ control method of the rectifier-side CSC of the flexible interconnect device according to an embodiment of the present invention;

[0037] Figure 4 This is a control block diagram of the constant current control method of the CSC on the inverter side of the flexible interconnect device according to an embodiment of the present invention;

[0038] Figure 5 This is a schematic diagram of the energy storage Crowbar low-voltage ride-through scheme according to an embodiment of the present invention;

[0039] Figure 6This is a schematic diagram of the energy storage Crowbar control principle according to an embodiment of the present invention;

[0040] Figure 7 This is a waveform diagram of the grid-side voltage change during a fault process according to an embodiment of the present invention;

[0041] Figure 8 This is a waveform diagram of the grid-side current change during a fault process according to an embodiment of the present invention;

[0042] Figure 9 This is a schematic diagram of DC-side voltage changes according to an embodiment of the present invention. Detailed Implementation

[0043] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0044] like Figure 1 As shown, the flexible interconnect device based on energy storage Crowbar includes a rectifier-side converter CSC, a DC-side converter CSC, and an inverter-side converter CSC. Each phase of the converter CSC includes an IGBT and a driver. The DC-side CSC is equipped with an energy storage device and two coupled inductors. and .

[0045] The microgrid connected to the rectifier side of the flexible interconnect device, such as Figure 1 As shown, PV, WV, DV, and Load represent photovoltaic, wind power, diesel generators, and loads, respectively. Each distributed power source and load is connected to the microgrid through directional current protection.

[0046] like Figure 2 As shown, this embodiment of the invention provides a control and protection coordination method for flexible interconnected devices based on energy storage Crowbar, including the following steps:

[0047] (1) Low-voltage ride-through foundation control based on energy storage crowbar: A voltage reversible chopper circuit is used to connect the DC bus and the energy storage device. The energy storage device can be a battery or a supercapacitor, such as... Figure 5As shown in the diagram, in this circuit, rectifier switch V1 and rectifier diode VD2 form a buck chopper circuit, supplying power to the energy storage device from the DC bus. When the DC current is too high, the excess energy is stored in the energy storage device. Switch V2 and rectifier diode VD1 form a boost chopper circuit, feeding the energy from the energy storage device back to the DC bus. When the DC current is insufficient, the energy stored in the energy storage device can be released to charge the bus capacitor and increase the DC current. However, it should be noted that when V1 and V2 are both on, it will cause a short circuit in the DC bus, which may damage the entire system.

[0048] The trigger signals of V1 and V2 can be triggered using a hysteresis comparison method, and the trigger signals of V1 and V2 are interlocked to prevent them from conducting simultaneously. However, this method cannot effectively control the switching frequency and is prone to damaging the switching elements. Two PI controllers are used to control the upper and lower limits of the bus current respectively, and the trigger signals of the two elements are interlocked, such as... Figure 5 As shown.

[0049] Where Ldci and Idci (i=1,2).

[0050] (2) Modeling of inductive coupling effect: There are two inductors in the microgrid. and They interact through different current paths. To describe this coupling effect, current and voltage equations for multiple inductors are used.

[0051] For simplicity, assume an inductance. and There exists a certain coupling coefficient k between them, which describes the magnetic coupling between them.

[0052] In this case, the change in inductor current can be described by the following matrix equation:

[0053] (1)

[0054] in:

[0055] and They are respectively through inductors and The current; and is the resistance in the circuit; k is the coupling coefficient, which describes the mutual influence between inductors. and It is a voltage source caused by a fault.

[0056] In this model, the coupling between inductors affects the current change in each inductor. For example, the larger the value of k, the stronger the interaction between inductors, thus affecting the dynamic response of the entire system.

[0057] (3) Current limiting during faults: When a fault occurs, the inductor will prevent the current from rising rapidly by limiting the rate of increase of the current. To describe this process in more detail, it can be solved based on the properties of the inductor and the changes in the voltage source.

[0058] Suppose a short-circuit fault occurs in a circuit, causing a sudden and drastic change in current. Due to the inductance's opposition to this change, the rate of current change is limited. This limiting effect of the inductance on the current can be modeled using its time constant and voltage fluctuations.

[0059] For a typical RL circuit, the change in current follows the differential equation:

[0060] (2)

[0061] In fault conditions, voltage The pulse waveform may be caused by a sudden change in power supply or circuit. By integrating the above equation, the change of current i(t) over time can be obtained:

[0062] (3)

[0063] If we assume the voltage after the fault occurs If the current is a pulse or step waveform, then the change in current will have a certain lag and a growth curve. For example, the growth of current can be represented by an exponential function:

[0064] (4)

[0065] in It is the time constant of the inductor circuit, which controls the rate at which the current increases. Due to the presence of inductance, the current does not increase instantaneously, but rises gradually.

[0066] Furthermore, by connecting an inductor and a crowbar in series on the DC side, the charging and discharging of the energy storage device can be achieved by controlling the inductor current. This allows for better prediction of current trends under fault conditions and the design of more efficient protection strategies. For microgrid systems, accurate modeling of the inductor can help optimize current limiting mechanisms, ensuring that the system can respond quickly and resume normal operation in the event of a fault.

[0067] (4) Inverter-side constant current control: Constant current control is used to control the inverter-side CSC of the flexible interconnected microgrid interlocking fault management device, such as... Figure 3 and Figure 4 As shown. Set the inverter-side output current. To maintain a constant value, the system algorithm is used to keep it constant. Unaffected by voltage fluctuations. Changes in current can be described by the following equation:

[0068] (5)

[0069] in, It is the output voltage of the inverter. It is the system impedance. Under voltage sag conditions, It is controlled to a constant value, while Adjusts to changes in grid voltage.

[0070] This design can avoid the impact of sudden current changes and voltage drops on the power grid system, ensuring that the microgrid can continue to operate stably under low voltage conditions and further improving low voltage ride-through capability.

[0071] To verify the proposed flexible interconnection device control and protection coordination method based on energy storage Crowbar proposed in this invention, this embodiment refers to... Figure 1 A simulation model of a high-proportion distributed energy distribution network and microgrid operating in coordination at 0.4kV / 10kV was built in Matlab / simulink, and protection was configured and faults were designed. The system simulation parameters are shown in Table 1 below. The implementation method of this invention is illustrated by taking a single-phase voltage drop at the microgrid as an example.

[0072] Table 1 System Simulation Parameters

[0073]

[0074] To address single-phase voltage dips, the simulation is set to cause phase A voltage to dip to 0.6 pu. The grid is assumed to experience a single-phase voltage dip at 1 second, with the fault duration also set to 1 second. Local magnifications of the grid-side voltage and current waveforms are shown below. Figure 7 , 8 As shown.

[0075] Figure 9 The DC voltage changes before and after the addition of the hybrid energy storage device were compared. The hybrid energy storage system on the DC side can still maintain voltage stability under asymmetric fault operation, which improves the fault ride-through capability of the system and proves the effectiveness of the control and protection coordination method of flexible interconnection device based on energy storage Crowbar.

[0076] The above description is merely a specific embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any variations or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in the present invention should be included within the scope of protection of the present invention. Therefore, the scope of protection of the present invention should be determined by the scope of the claims.

Claims

1. A method for coordinated control of a flexible interconnection device (FID) and a crowbar based energy storage (CBES), characterized in that: The flexible interconnection device based on energy storage Crowbar includes a rectifier side converter CSC, a DC side converter CSC and an inverter side converter CSC, each phase of the converter CSC includes an IGBT and a driver, the DC side CSC is configured with an energy storage device and two coupled inductors and ; the method comprises the following steps: (1) Energy storage Crowbar low voltage ride through basic control: a voltage reversible chopper circuit is used to connect the DC bus and the energy storage device, wherein: When the DC side current is too high, the excess energy is stored in the energy storage device through the voltage reduction chopper circuit, and the voltage reduction chopper circuit includes a first switch tube V1 and a second rectifier diode VD2; When the DC side current is insufficient, the energy storage device energy is fed back to the DC bus through the voltage boosting chopper circuit, and the voltage boosting chopper circuit includes a second switch tube V2 and a first rectifier diode VD1; A double PI controller is used to interlock control the trigger signals of V1 and V2 to avoid V1 and V2 being turned on at the same time. (2) Inductive coupling effect modeling: For two coupled inductors in the microgrid and , an inductive coupling model described by a matrix equation is established based on the magnetic coupling coefficient k to describe its current dynamic response; (3) Current limiting during fault: based on the inductive coupling model, the inductance hinders the change of current, limits the increase of current during fault, and uses the RL circuit differential equation and exponential growth model to describe the current change characteristics. (4) Inverter side constant current control: based on the current limiting result, the inverter side CSC adopts constant current control, and the inverter side output current is set is a constant value, and the inverter output voltage V inv (t) to maintain not affected by voltage fluctuations.

2. The method of claim 1, wherein: The energy storage device in step (1) is a storage battery or a super capacitor.

3. The method of claim 1, wherein: In step (1), a double PI controller is used to interlock control the trigger signals of V1 and V2 to avoid V1 and V2 being turned on at the same time, which specifically includes: A voltage reversible chopper circuit is used to connect the DC bus and the energy storage device, wherein V1 and VD2 constitute a voltage reduction chopper circuit for storing excess energy in the energy storage device when the DC side current is too high; V2 and VD1 constitute a voltage boosting chopper circuit for releasing energy storage device energy to charge the bus capacitor when the DC side current is insufficient; In order to avoid the short circuit of DC bus caused by V1 and V2 being turned on at the same time, two PI controllers are used to control the upper and lower limits of the DC bus current, and the trigger signals of V1 and V2 are interlocked to ensure that they are not turned on at the same time.

4. The energy storage crowbar based flexible interconnection device control reserve collaborative control method of claim 1, wherein: In step (2), two inductances in the microgrid and This coupling effect is described using the current and voltage equations of the multiple inductances interacting through different current paths. Assume inductance And There is a coupling coefficient k between them that describes the magnetic coupling between them, and the change in inductance current is described by the following matrix equation: （1） Wherein: and are the currents through the inductances and respectively; and are the resistances in the circuit; and are the voltage sources caused by the fault.

5. The energy storage Crowbar-based flexible interconnection device control and protection collaborative control method of claim 1, wherein: The DC side inductance and Crowbar are connected in series, and the charging and discharging of the energy storage device is realized by controlling the inductance current, which can better predict the trend of current change in fault conditions and design more efficient protection strategies. For microgrid systems, accurate modeling of inductance can help optimize the current limiting mechanism to ensure that the system can respond quickly and resume normal operation when a fault occurs.

6. The energy storage crowbar based flexible interconnection device control reserve collaborative control method of claim 1, wherein: Step (4) specifically includes: Setting the inverter-side output current is constant, and is maintained by the control system algorithm The change in current, unaffected by voltage fluctuations, is described by the equation: （5） wherein, is the output voltage of the inverter, is the impedance of the system, in case of a voltage dip, is controlled to a constant value, while is adjusted with the variation of the grid voltage.

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