A fault current limiting system suitable for an over-arched grid type energy storage system

Abstract

The application provides a fault current limiting system suitable for an over-configuration network type energy storage system, which comprises: a voltage loop connected with an energy storage unit; a current limiter connected with the voltage loop; the current limiter has an asymmetric limiting boundary, that is, the maximum allowable values of d-axis and q-axis currents in a current d-q coordinate system are different; and a current loop connected with the current limiter and a power grid. The application can realize differentiated constraints of AC side reactive current and DC side active current, and improve the voltage support capability and low voltage ride through performance of the system.

Description

Technical Field

[0001] This invention relates to a fault current limiting system suitable for oversized grid-type energy storage systems. Background Technology

[0002] With the continuous growth of new energy installed capacity, grid-based energy storage systems are widely used in new power systems due to their voltage source characteristics and active support capabilities. During grid-connected operation, grid-based energy storage systems need to provide transient support to the system during grid faults to maintain the stability of voltage and frequency at the point of common coupling.

[0003] To meet the power grid's reactive power support requirements during faults, oversized grid-type energy storage systems are often used in engineering projects. This involves configuring converters with a larger capacity (e.g., three times the rated capacity) on top of the rated energy storage capacity, thereby providing a higher current output capability during short-term faults. However, in this type of system, there are significant differences in the physical constraints between the AC and DC sides: the AC side is expected to allow for a large reactive current output in short periods, while the DC side is limited by the safety requirements of the energy storage units, and its active current must be strictly limited.

[0004] Existing grid-connected energy storage systems mostly employ a symmetrical current limiting strategy, applying uniform limits to the d-axis and q-axis currents in current control. This approach struggles to simultaneously meet the dual requirements of reactive power support and DC-side safety constraints in oversized grid-connected energy storage systems, potentially leading to insufficient reactive power support or DC-side overcurrent risks during grid faults.

[0005] Taking existing grid-connected energy storage systems as an example, when a short-circuit fault or severe voltage drop occurs in the power grid, the voltage control circuit will drive the system to output a large current to maintain the voltage at the grid connection point. If a symmetrical current limiting strategy is adopted, in order to ensure the safety of the DC side, the current limit usually needs to be set low (such as 1.2 times the rated current), which limits the reactive current output capability of the AC side, resulting in insufficient voltage support effect.

[0006] On the other hand, increasing the symmetrical current limit to enhance reactive power support (e.g., three times the rated current) may cause the DC active current to exceed the allowable range of the energy storage unit, affecting the safe operation of the system. This contradiction is particularly prominent in oversized grid-type energy storage systems.

[0007] Traditional symmetrical current limiting methods are ill-suited to the operational requirements of oversized grid-connected energy storage systems under fault conditions, exhibiting significant limitations in engineering applicability. Therefore, an asymmetric fault current limiting method suitable for oversized grid-connected energy storage systems is urgently needed to improve the system's safety and stability under fault conditions.

[0008] This invention achieves differentiated constraints on the reactive current on the AC side and the active current on the DC side by performing asymmetric limiting control on the output current of the grid-type energy storage system, thereby improving the system's voltage support capability and low-voltage ride-through performance. Summary of the Invention

[0009] To address the shortcomings of existing technologies, the purpose of this invention is to provide a fault current limiting system suitable for oversized grid-type energy storage systems. This system can achieve differentiated constraints on AC-side reactive current and DC-side active current, thereby improving the system's voltage support capability and low-voltage ride-through performance.

[0010] To achieve the above objectives, the present invention provides a fault current limiting system suitable for over-grid energy storage systems, the fault current limiting system comprising: Voltage loop, the voltage loop connects to the energy storage unit; A current limiter is connected to a voltage loop; the current limiter has an asymmetric limiting boundary, that is, the maximum allowable values ​​of the current along the d-axis and q-axis are different in the current d-q coordinate system. The current loop connects the current limiter to the power grid.

[0011] To address the shortcomings of existing technologies, this invention proposes an asymmetric fault current limiting system suitable for oversized grid-connected energy storage. During grid faults, by performing asymmetric limiting control on the output current of the grid-connected energy storage system, differentiated constraints are achieved between the reactive current on the AC side and the active current on the DC side, thereby improving the system's voltage support capability and low-voltage ride-through performance while ensuring the safety of the energy storage unit.

[0012] According to another specific embodiment of the present invention, the current limiting boundary is discontinuous, and the mathematical expression for current limiting is:

[0013] in, This represents the maximum allowable value of the d-axis current. This represents the maximum allowable value for the q-axis current. The current command value generated by the voltage loop is then passed through a current limiter to generate a limited current command value. ; and respectively The components are on the d-axis and q-axis. and They are respectively Components on the d-axis and q-axis.

[0014] According to another specific embodiment of the present invention, the limiting boundary is continuous, and the maximum allowable values ​​of the d-axis and q-axis currents are respectively and The mathematical expression for the amplitude limit boundary is:

[0015] in, and These represent the d-axis and q-axis components of the current at a point on the limiting boundary.

[0016] According to another specific embodiment of the present invention, and Determined based on the safety constraints of the energy storage unit and the capacity configuration of the converter, and > .

[0017] According to another specific embodiment of the present invention, the current limiter is an elliptical amplitude limiter, in which the direction of the current vector relative to the d-axis remains unchanged before and after limiting, and only its amplitude is adjusted so that the current vector after limiting is constrained by the elliptical boundary; the mathematical expression for current limiting is:

[0018] in, The current command value generated by the voltage loop is then passed through a current limiter to generate a limited current command value. ; and respectively These are the components along the d-axis and q-axis.

[0019] According to another specific embodiment of the present invention, the current limiter is an elliptic angle-priority limiter, wherein the direction of the current vector can be preferentially adjusted to a preset angle while satisfying the elliptic amplitude constraint. The mathematical expression for current limiting is:

[0020] in, The current command value generated by the voltage loop is then passed through a current limiter to generate a limited current command value. ; and respectively The components are on the d-axis and q-axis. and They are respectively Components on the d-axis and q-axis; I max Preset angle The current value at the corresponding point on the upper limit boundary of the direction.

[0021] According to another specific embodiment of the present invention, a preset angle The parameters are settable and can be adjusted as needed.

[0022] The key challenge for oversized grid-connected energy storage systems during grid faults lies in how to fully utilize the reactive current support capacity of the AC side while meeting DC-side current safety constraints. Since grid-connected energy storage systems exhibit voltage source characteristics, voltage control mechanisms drive the system to output a large current during voltage dips or short-circuit faults, thus placing higher demands on current limiting strategies.

[0023] Figure 1 The oversized grid-type energy storage system topology used in this invention includes energy storage units, a grid-type converter, a grid-connected filter, and a grid connection line. The DC side of the grid-type converter is connected to the energy storage unit, and the AC side is connected to the common coupling point (PCC) through an LC filter. The power grid is connected to the PCC point through a transmission line. and These represent the converter port voltage and the current flowing through the filter inductor, respectively. and These represent the voltage at point PCC and the current flowing through the power grid transmission line, respectively. This indicates the reference voltage value at the PCC point. This system employs a dual-loop voltage and current structure. The current command value generated by the voltage loop is passed through the current limiter to generate a limited current command value. The active power synchronization loop of a grid-type converter can be controlled using a virtual synchronous machine, and its dynamic relationship can be described by the corresponding synchronization equations, which are used to generate the voltage phase angle. It is the virtual power angle output by the synchronization loop, and also the angle between the dq coordinate system and the abc coordinate system; and These are the angular frequency output by the VSG and the rated angular frequency of the system, respectively. and These are the converter output active power and its reference value, respectively.

[0024] Figure 1 The current limiter in the circuit can limit the large short-circuit current generated on the line when the grid voltage drops sharply. Because the current loop tracks very quickly, the current reference value can be considered equal to the actual current value. The current limiter limits the short-circuit current on the line by limiting the current reference value generated by the voltage loop. In this system, the grid-connected converter capacity is over-configured relative to the rated capacity of the energy storage, enabling the system to have a high current output capability during short-term faults (e.g., a 1 p.u. energy storage battery equipped with a 3 p.u. energy storage converter). Figure 1 When an energy storage system is equipped with an overcapacity converter, the current limiter's limiting strategy needs to be optimized.

[0025] Based on the above system topology, the fault current limiting module can adopt different limiting strategies, including circular limiting, circular truncation limiting, and elliptical limiting.

[0026] (1) Circular limiter The circular limiting method uses a fixed radius in the current d-q coordinate system. The circular boundary uniformly limits the current amplitude. Depending on whether the current vector direction is adjusted during the limiting process, circular limiting methods can be further divided into circular amplitude limiters and circular angle priority limiters.

[0027] Under the circular amplitude limiting method, the direction of the current vector relative to the d-axis remains unchanged before and after limiting; only the amplitude of the current vector is scaled to constrain it to a circular boundary. Its mathematical expression is shown in equation (1), and its shape is as follows: Figure 2a As shown.

[0028] Under the circular angle-priority limiting method, while satisfying the circular amplitude constraint, the direction of the current vector can be adjusted so that the direction of the current vector after limiting is equal to the preset angle. Its mathematical expression is shown in equation (2), and its shape is as follows: Figure 2b As shown.

[0029] (1) (2) Circular limiting is simple to implement and has some applicability in scenarios where the current constraints on the AC and DC sides are similar. However, in oversized grid-type energy storage systems, due to the differences in d-axis and q-axis current constraints, circular limiting is difficult to reflect the asymmetric operating characteristics of the system.

[0030] (2) Circular cutoff limiter Based on the circular limiting method, a truncation constraint can be introduced in the d-axis direction to trim the limiting region, thereby forming an asymmetric limiting region, the schematic shape of which is shown below. Figure 2c As shown, the mathematical expression is shown in equation (3).

[0031] (3) Circular truncation limiting can more strictly restrict the d-axis current, which is beneficial for suppressing DC-side active current overload. However, the limiting boundary of this method has discontinuous characteristics in geometry, which may cause uneven changes in current command during the limiting switching process, thus affecting the transient operating performance of the system.

[0032] (3) To adapt to the asymmetrical current constraints between the AC and DC sides in oversized grid-type energy storage systems, the limiting boundary can be designed as an ellipse. In the elliptical limiting method, the maximum allowable values ​​of the current along the d-axis and q-axis are set in the current d-q coordinate system, respectively. and By constructing asymmetric and continuous limiting boundaries, differentiated constraints on the current can be achieved.

[0033] The elliptical limiting boundary can be expressed as equation (4), where, and Determined based on the safety constraints of the energy storage unit and the capacity configuration of the converter, and > In this way, the d-axis current is limited by DC-side safety constraints, while the q-axis current is allowed to vary over a wider range, thereby improving the reactive current output capability during faults.

[0034] (4) Similar to circular limiting, elliptical limiting can also be divided into elliptical amplitude limiters and elliptical angle priority limiters, depending on whether the current vector direction is adjusted during the limiting process.

[0035] Under the elliptical amplitude limiting method, the direction of the current vector relative to the d-axis remains unchanged before and after limiting; only its amplitude is adjusted so that the current vector after limiting is constrained by the elliptical boundary. Its mathematical expression is shown in equation (5), and its shape is as follows: Figure 2d As shown. This method is simple to implement, but its ability to boost reactive current is limited during periods of sudden voltage drops in the power grid.

[0036] (5) Under the elliptic angle-priority limiting method, the direction of the current vector can be preferentially adjusted to a preset angle while satisfying the elliptic amplitude constraint. Its mathematical expression is shown in equation (6), and its shape is as follows: Figure 2e As shown. The preset angle is a settable parameter. By adjusting this parameter, the reactive current output capability during faults can be improved while meeting the safety constraints of the energy storage unit.

[0037] (6) An elliptical angle-priority limiter is a preferred embodiment of the present invention. Its elliptical boundary naturally fits the asymmetric constraint of overfitting, and the smooth boundary ensures the stability of control. The current limiting angle is set... It can enhance reactive current output capability during faults while ensuring the safe operation of energy storage units.

[0038] To verify the feasibility of the asymmetric fault current limiting method described in this invention, a system was built in Matlab / Simulink as follows: Figure 1 The model of the over-configured grid-type energy storage system is shown (simulation parameters are shown in Table 1). In the event of a voltage drop or short-circuit fault in the power grid, the system operates using the asymmetric fault current limiting strategy of this invention.

[0039] Table 1 Simulation Parameters

[0040] Simulation results are as follows Figure 3-5 As shown, the results indicate that: (1) such as Figure 3 As shown, when a voltage drop fault occurs in the power grid, the grid-type energy storage system operates with an elliptical limiter, which ensures that the DC side active power does not become overloaded and the system operates stably. (2) such as Figure 4 As shown, under the same fault conditions, the elliptical angle-priority limiter can adjust the preset angle... Output more reactive power to provide voltage support; (3) such as Figure 5 As shown, after adding transient additional control, the system operates continuously without significant oscillations during the fault occurrence and fault clearance process, and successfully completes fault crossing.

[0041] The above results demonstrate that the present invention can meet the operational requirements of oversized grid-type energy storage systems under fault conditions.

[0042] Compared with the prior art, the present invention has the following beneficial effects: (1) Adapted to oversized grid-type energy storage system: Designed to address the inconsistent constraints between the AC and DC sides of the oversized grid-type energy storage system, making it highly applicable to engineering projects.

[0043] (2) Achieve asymmetric current limiting: By limiting the d-axis and q-axis currents respectively, the coordination between DC side safety protection and AC side reactive power support capability is achieved.

[0044] (3) Enhance voltage support capability during faults: By changing the limiting angle of the limiter, reactive current output can be increased during grid faults, which is beneficial to maintaining the voltage stability at the grid connection point.

[0045] The present invention will now be described in further detail. Attached Figure Description

[0046] Figure 1 This is a schematic diagram of the topology of a grid-connected energy storage system with a current limiter; Figure 2 is an illustration of various types of current limiters, in which: Figure 2aIndicates a circular amplitude limiter. Figure 2b This indicates a circular angle-priority limiter. Figure 2c This indicates a circular current cutoff limiter. Figure 2d This indicates an elliptic amplitude limiter. Figure 2e This indicates an elliptic angle-priority limiter. Figure 3 This indicates the transient performance of a grid-type energy storage system equipped with different current limiters; Figure 4 This indicates the transient performance of a grid-type energy storage system equipped with different elliptic current limiters. Figure 5 Indicates that the grid-type energy storage system is equipped with The elliptic angle priority limiter, and the transient performance of the converter after adding additional transient control. Detailed Implementation

[0047] Example 1

[0048] This embodiment provides a fault current limiting system suitable for oversized grid-type energy storage systems, such as... Figure 1 As shown, it includes: voltage loop 1, current limiter 2 and current loop 3.

[0049] Voltage loop 1 is connected to the energy storage unit.

[0050] Current limiter 2 is connected to voltage loop 1; current limiter 2 has asymmetric limiting boundaries, that is, the maximum allowable values ​​of the current along the d-axis and q-axis are different in the current d-q coordinate system.

[0051] Current loop 3 connects current limiter 2 and the power grid.

[0052] like Figure 2c As shown, in this embodiment, the current limiter is a circular cutoff current limiter with discontinuous limiting boundaries. The mathematical expression for current limiting is:

[0053] in, This represents the maximum allowable value of the d-axis current. This represents the maximum allowable value for the q-axis current. The current command value generated by the voltage loop is then passed through a current limiter to generate a limited current command value. ; and respectively The components are on the d-axis and q-axis. and They are respectively Components on the d-axis and q-axis.

[0054] Example 2 like Figure 2d As shown, the difference between this embodiment and Embodiment 1 is that the limiting boundary is continuous, and the maximum allowable values ​​of the d-axis and q-axis currents are respectively... and The mathematical expression for the amplitude limit boundary is:

[0055] in, and These represent the d-axis and q-axis components of the current at a point on the limiting boundary.

[0056] and Determined based on the safety constraints of the energy storage unit and the capacity configuration of the converter, and > .

[0057] In this embodiment, the current limiter is an elliptical amplitude limiter. The direction of the current vector relative to the d-axis remains unchanged before and after limiting; only its amplitude is adjusted so that the current vector after limiting is constrained by the elliptical boundary. The mathematical expression for current limiting is:

[0058] in, The current command value generated by the voltage loop is then passed through a current limiter to generate a limited current command value. ; and respectively These are the components along the d-axis and q-axis.

[0059] Example 3 like Figure 2e As shown, the difference between this embodiment and Embodiment 2 is that the current limiter is an elliptical angle-priority limiter, and the direction of the current vector can be preferentially adjusted to a preset angle while satisfying the elliptical amplitude constraint. The mathematical expression for current limiting is:

[0060] in, The current command value generated by the voltage loop is then passed through a current limiter to generate a limited current command value. ; and respectively The components are on the d-axis and q-axis. and They are respectively Components on the d-axis and q-axis; I max Preset angle The current value at the corresponding point on the upper limit boundary of the direction.

[0061] Preset angle The parameters are settable and can be adjusted as needed.

[0062] While the present invention has been disclosed above with reference to preferred embodiments, it is not intended to limit the scope of the invention. Any person skilled in the art can make modifications without departing from the scope of the invention; all equivalent modifications made in accordance with the invention should be covered by the scope of the invention.

Claims

1. A fault current limiting system suitable for oversized grid-type energy storage systems, wherein, The fault current limiting system includes: Voltage loop, which is connected to the energy storage unit; A current limiter is connected to the voltage loop; the current limiter has an asymmetric limiting boundary, that is, the maximum allowable values ​​of the current along the d-axis and the q-axis are different in the current d-q coordinate system; A current loop is provided, which connects the current limiter to the power grid.

2. The fault current limiting system as described in claim 1, wherein, The current limiting boundary is discontinuous, and the mathematical expression for the current limiting is: in, This represents the maximum allowable value of the d-axis current. This represents the maximum allowable value for the q-axis current. The current command value generated by the voltage loop is passed through the current limiter to generate a limited current command value. ; and respectively The components are on the d-axis and q-axis. and They are respectively Components on the d-axis and q-axis.

3. The fault current limiting system as described in claim 1, wherein, The limiting boundary is continuous, and the maximum allowable values ​​for the d-axis and q-axis currents are respectively... and The mathematical expression for the limiting boundary is: in, and These are the components of the current at a point on the limiting boundary along the d-axis and q-axis, respectively.

4. The fault current limiting system as described in claim 3, wherein, and Determined based on the safety constraints of the energy storage unit and the capacity configuration of the converter, and > .

5. The fault current limiting system as described in claim 4, wherein, The current limiter is an elliptical amplitude limiter. The direction of the current vector relative to the d-axis remains unchanged before and after limiting; only its amplitude is adjusted so that the current vector after limiting is constrained by the elliptical boundary. The mathematical expression for current limiting is: in, The current command value generated by the voltage loop is passed through the current limiter to generate a limited current command value. ; and respectively These are the components along the d-axis and q-axis.

6. The fault current limiting system as described in claim 4, wherein, The current limiter is an elliptic angle-priority limiter, whereby the current vector, while satisfying the elliptic amplitude constraint, can be preferentially adjusted to a preset angle. The mathematical expression for current limiting is: in, The current command value generated by the voltage loop is passed through the current limiter to generate a limited current command value. ; and respectively The components are on the d-axis and q-axis. and They are respectively Components on the d-axis and q-axis; I max For the preset angle The current value at the corresponding point on the limiting boundary in the specified direction.

7. The fault current limiting system as described in claim 6, wherein, The preset angle These are configurable parameters.

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