Networking type energy storage system stability criterion method considering current amplitude limiting link

Abstract

The invention discloses a stability criterion method considering a current amplitude limiting link for a network construction type energy storage system, and belongs to the field of stability control of a power system. According to the invention, the problem of virtual power angle switching after current saturation of the grid-forming converter is solved. The equivalent acceleration area and the equivalent deceleration area serve as the reflection of the energy increment and the energy decrement in the transient process of the converter, and the transient synchronization stability of the grid-connected system of the grid-forming type energy storage converter can be conveniently judged by comparing the acceleration area and the deceleration area or comparing the fault cut-off angle and the limit cut-off angle; the research result has a certain significance in improving the stability of the network construction type energy storage equipment.

Description

Technical Field

[0001] The present invention belongs to the field of power system stability control, and in particular relates to a stability criterion method for a grid-type energy storage system taking into account a current limiting link. Background Art

[0002] As the global energy transition accelerates, the proportion of renewable energy in the power system continues to rise, posing a series of challenges to the safe and stable operation of the power system. Due to their inherent characteristics, the output of new energy sources is uncertain, seriously affecting the stability of grid voltage and frequency. Under these circumstances, vigorously promoting the development of grid-connected energy storage technology has become a key measure to ensure the safe and stable operation of new power systems. Through the application of this technology, new power systems can provide strong support for grid voltage, frequency, and power angle, effectively improving the overall safe and stable operation of the system. In the energy storage technology system, power electronic converters, as the energy transportation hub connecting energy storage and the grid, bear the important responsibility of precisely controlling the charging and discharging of electrical energy. While the integration of grid-connected energy storage into the system provides strong support for the stable operation of the power system, its inherent characteristics also bring new challenges to the safe and stable operation of the grid.

[0003] The core synchronization unit of a grid-type energy storage converter is the active control loop, which primarily achieves synchronization with the grid by simulating physical processes such as the synchronous machine's rotor motion equations and primary frequency modulation characteristics. Classic control strategies include power synchronization control, droop control, droop control with low-pass filtering, and virtual synchronization control. Power synchronization control and droop control are equivalent to each other and are both first-order systems with simple structures and easy design, but they cannot achieve inertia support. Droop control with low-pass filtering is equivalent to virtual synchronization control and can be classified as a second-order system. It can simulate the rotor motion and excitation characteristics of a synchronous generator, giving it equivalent inertia support and damping capabilities, and stronger anti-interference performance. This paper will study the synchronization stability of a grid-type energy storage converter that uses virtual synchronization control as its active power loop.

[0004] In traditional power systems, synchronous stability is generally dominated by the power angle of the synchronous generator. Synchronous stability between the generator and the system is maintained by adjusting the mechanical and electromagnetic power of the generator. Research on this aspect is relatively mature. However, because the synchronization mechanisms of converters and traditional synchronous machines differ, in AC power grids that coexist with converters and synchronous machines, the synchronization stability between the power source and the grid takes on a new form. This is why it is defined as generalized synchronization stability. This refers to the ability of power sources within a new power system to maintain synchronous operation after a disturbance. If a power source cannot maintain synchronization with the others, it is considered synchronization instability or loss of synchronism.

[0005] Because virtual synchronous control converters simulate the dynamic behavior of traditional synchronous machines, they exhibit similar transient stability issues within the system. Specifically, grid-type converters can also experience issues such as loss of equilibrium or critical fault clearing time after experiencing large disturbances. Since grid-type energy storage converters behave as voltage sources, they often require current limiting in practical engineering applications. The transient synchronous stability of a grid-type converter is significantly affected by its current limiting mechanism. When an external fault disturbance causes its output current to reach its limit, its virtual power angle characteristic curve changes. Therefore, the power angle curve of a traditional synchronous generator is no longer applicable to a grid-type energy storage converter equipped with current limiting. It is necessary to study the transient synchronous stability of grid-type energy storage converters with current limiting considerations and to propose a synchronization stability criterion applicable to grid-type energy storage converters with current limiting considerations. Summary of the Invention

[0006] In response to the above technical problems existing in the prior art, the present invention proposes a stability criterion method for a grid-type energy storage system taking into account the current limiting link. The method has a reasonable design, overcomes the shortcomings of the prior art, and has good effects.

[0007] In order to achieve the above object, the present invention adopts the following technical solutions:

[0008] A stability criterion method for a grid-type energy storage system considering a current limiting link includes the following steps:

[0009] Step 1: Construct a synchronous stability mathematical model of the grid-connected energy storage converter system, which specifically includes the following steps:

[0010] Step 1.1: Construct the active power loop control equation of virtual synchronous control, which is expressed as:

[0011]

[0012] Among them, P m and P e Represent the active power reference value and the actual output active power of the converter respectively; J represents the equivalent inertia coefficient; D is the equivalent damping coefficient; δ VSG The difference between the grid-connected point voltage phase angle of the grid-connected converter and the grid voltage phase angle is called the virtual power angle. VSG is the converter angular frequency; ω0 is the angular frequency reference value;

[0013] Step 1.2: Build a current limiting link model. When the following conditions are met, the current limiting link starts:

[0014]

[0015] Among them, iref is the current reference value; and are the dq axis current reference values respectively; I max is the current limit value of the grid-type converter;

[0016] Step 1.3: Determine the output of the dq axis current limiting link and adopt the d axis current priority limiting control, which is expressed as:

[0017]

[0018] in, It is the output of the d-axis current limiting link; It is the output of the q-axis current limiting link;

[0019] Step 1.4: Determine the actual output active power P of the converter based on whether the current reaches saturation. e Expressions of

[0020] When the current is not saturated,

[0021] Where: V VSG is the converter port voltage; V g is the grid voltage; X g is the line reactance; P um is the maximum output power when the converter is not saturated;

[0022] When the current is saturated, P e =V g I max cosδ VSG =P sm cosδ(5);

[0023] Where: P sm is the maximum output power when the converter is saturated;

[0024] Step 2: Calculate the equivalent acceleration area of the grid-type energy storage converter after the disturbance occurs;

[0025] Step 3: Calculate the deceleration area of the grid-type energy storage converter after the fault is cleared;

[0026] Step 4: Calculate the limit resection angle and determine the system stability.

[0027] Preferably, in step 2, the grid-type converter initially operates in a stable operating state. At a certain moment, a fault disturbance causes a step change in the active power reference value of the grid-type converter. At this time, the active power reference value is greater than the actual output active power, and the converter enters an acceleration state, increasing the active power and the output current. The corresponding acceleration area at this time is expressed as:

[0028]

[0029] Where S1 is the acceleration area before saturation; δ a is the power angle corresponding to the initial stable operating point; δ c is the power angle corresponding to the fault clearing; P0′ is the active power reference value after the power step.

[0030] Preferably, step 3 specifically includes the following steps:

[0031] Step 3.1: Calculate the first part of the deceleration area when the current has not reached saturation:

[0032]

[0033] Where S2 is the deceleration area after fault recovery before saturation; δ O is the virtual power angle after the grid-type converter reaches saturation; P0 is the active power reference value after fault recovery;

[0034] Step 3.2: Calculate the second deceleration area when the current reaches saturation:

[0035]

[0036] Among them, δ d To ensure that the output active power is greater than the maximum power angle allowed by the active power reference value;

[0037] Step 3.3: Calculate the equivalent deceleration area S = S2 + S3.

[0038] Preferably, in step 4, the transient stability of the grid-connected system of the grid-type energy storage converter is judged based on the equivalent acceleration area and the deceleration area. If the equivalent acceleration area is greater than the deceleration area, the system is unstable; if the equivalent acceleration area is less than the deceleration area, the system is stable; if the equivalent acceleration area is equal to the deceleration area, the system is in a critical stability state.

[0039] Preferably, by judging the difference between the fault removal angle and the limit removal angle, a quick judgment on the stability of the system is completed, wherein the limit fault removal angle is calculated based on the acceleration area being equal to the deceleration area.

[0040] Preferably, step 4 specifically includes the following steps:

[0041] Step 4.1: Let the acceleration area be equal to the deceleration area and calculate the fault limit removal angle δ cm ;

[0042] Step 4.2: Compare the actual fault removal angle with the limit removal angle δ cmThe size of the fault removal angle is used to determine the system stability. If the actual fault removal angle is greater than the limit removal angle δ cm , the system becomes unstable if the actual fault removal angle is less than the limit removal angle δ cm , the system is stable.

[0043] Preferably, the grid-type energy storage converter adopts an active power loop control method with virtual synchronous control, where the outer loop is a voltage loop and the inner loop is a current loop.

[0044] Preferably, the value of the equivalent damping coefficient D is zero.

[0045] Preferably, the grid-connected system parameters of the grid-type energy storage converter include: reference frequency / Hz, reference capacity / VA, reference voltage / V, line reactance / pu, line resistance / pu, filter source side resistance / pu, filter source side reactance / pu, converter DC side voltage, converter output active power / pu, current loop proportional gain, current loop integral gain, voltage loop proportional gain, voltage loop integral gain, grid voltage, damping coefficient, inertia coefficient, converter grid side voltage and current limit value.

[0046] The beneficial technical effects brought about by the present invention are:

[0047] This paper proposes a criterion suitable for determining the transient synchronization stability of grid-connected energy storage converters, addressing the issue of virtual power angle switching occurring after current saturation. The equivalent acceleration area and equivalent deceleration area, reflecting the energy gain and loss during a converter's transient state, can be compared to determine the transient synchronization stability of grid-connected energy storage converter systems. The results offer valuable insights into improving the stability of grid-connected energy storage devices. BRIEF DESCRIPTION OF THE DRAWINGS

[0048] Figure 1 Schematic diagram of the parameters of the grid-connected system of a single-grid type energy storage converter.

[0049] Figure 2 Schematic diagram of the change in virtual power angle of the grid-type energy storage converter under active power step.

[0050] Figure 3 This is a dynamic diagram of the power angle of the energy storage converter under active power step.

[0051] Figure 4 The phase trajectory diagram corresponding to the fault removal at different times after the active power step. DETAILED DESCRIPTION

[0052] The present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments:

[0053] The technical problem to be solved by the present invention is a method for judging the transient synchronization stability of a grid-connected power system with a grid-connected energy storage converter under the action of the current limiting link, specifically: the transient synchronization instability mechanism after a large disturbance fault occurs in the grid-connected power system with a grid-connected energy storage converter, and a method for calculating and judging the converter synchronization stability based on the equal area rule.

[0054] The present invention provides a transient stability calculation method for a grid-connected system of a grid-type energy storage converter based on the equal-area rule. Taking the grid-connected converter with the current limiting link as the research object, a mathematical model describing its dynamic characteristics is derived. Based on the obtained mathematical model and combined with the analysis method of the equal-area method, the large-interference synchronous instability mechanism of the grid-connected system of the grid-connected energy storage converter under the action of the current limiting link is analyzed, and the critical stability conditions of the grid-connected converter under large disturbances are proposed. According to the critical stability criterion, it is judged whether the system will suffer from transient synchronous instability. The scheme steps are as follows, and the following steps are carried out in sequence:

[0055] Step 1: Construct the synchronous stability mathematical model of the grid-connected energy storage converter system.

[0056] The control structure of a grid-type energy storage converter consists of two main components: an outer power control loop and an inner voltage and current control loop. The outer power control loop primarily includes the active power loop (APC) and the reactive power loop (RPC). The active power loop in this paper uses virtual synchronous control, which generates frequency by simulating the rotor motion equations of a traditional synchronous machine. Therefore, its equivalent inertia and damping can be set. The corresponding rotor motion equation can be described as:

[0057]

[0058] Where: P m and P e Represent the active power reference value and the actual output active power of the converter respectively; J represents the equivalent inertia coefficient; D is the equivalent damping coefficient; δ VSG It is the difference between the voltage phase angle at the grid-connected point of the grid-connected converter and the grid voltage phase angle, which is called the virtual power angle.

[0059] For both the voltage and current loops, the internal voltage loop typically includes two PI control loops: the d-axis voltage control loop and the q-axis voltage control loop. These loops provide references for current loop control based on the d-axis and q-axis voltage reference values, respectively. The current loop then undergoes PI control to generate the dq components of the reference voltage, which are then converted into a three-phase reference voltage using the dq / abc conversion method.

[0060] In actual engineering, due to the weak overcurrent capability of grid-type converters, a current limiting link is often added between the voltage control loop and the current control loop to limit the dq axis current reference value. When the current reference value output by the voltage control loop reaches the limit value, that is:

[0061]

[0062] Current reference value I ref Will be restricted to I max The current limiting control methods commonly used include d-axis current priority limiting control, q-axis current priority limiting control, and angle priority limiting control. The present invention adopts d-axis current priority limiting control (if other current limiting control methods are adopted, the analysis is similar), and its limiting link can generally be described as:

[0063]

[0064] Among them: I max is the current limit value of the grid-type converter; It is the output of the dq axis current limiting link. When the current has not reached saturation, the current limiting link does not work. If the fault causes the current value to reach I max , the current limiting link is activated. When the current limiting link is started, the external characteristics of the grid-type converter change from a voltage source to a current source.

[0065] When the current limiting link is not started, its output active power can be described as:

[0066]

[0067] After the current reaches saturation, its output current can be expressed as:

[0068] P e =V g I max cosδ VSG =P sm cosδ (5);

[0069] Step 2: Calculate the equivalent acceleration area after the disturbance occurs.

[0070] Generally, fault scenarios can be simply divided into two types: 1. Saturation is reached after the fault is cleared; 2. Saturation is reached before the fault is cleared. The calculation and analysis procedures for these two scenarios are similar, and the resulting critical stability condition expressions are identical. Therefore, only Case 1 is discussed here.

[0071] The grid-type converter initially operates in a stable operating state. At a certain moment, a fault disturbance causes the active power reference value of the grid-type converter to jump. At this time, the active power reference value is greater than the actual output active power. The converter enters the acceleration state, increases the active power, and increases the output current. The corresponding acceleration area at this time can be expressed as:

[0072]

[0073] Where: a is the power angle corresponding to the initial stable operating point; δ c It is the power angle corresponding to the fault clearing.

[0074] Step 3: Calculate the deceleration area after the fault is cleared.

[0075] After the fault is cleared, the output current of the grid-type converter has not yet reached saturation, and the output active power is greater than the active power reference value. The converter enters the deceleration state. However, due to the existence of the virtual synchronous control inertia link, the speed of the grid-type converter is still greater than the synchronous speed, so the virtual power angle is still increasing. At this time, the corresponding deceleration area is:

[0076]

[0077] As the virtual power angle increases, its output active power will reach its maximum value:

[0078]

[0079] At this time, the output current of the grid-type converter reaches the limit value and enters the saturation state. The virtual power angle curve switches. The corresponding deceleration area at this time is:

[0080]

[0081] Among them, δ O is the virtual power angle after the grid-type converter reaches saturation; δ d To ensure that the output active power is greater than the maximum power angle allowed by the active power reference value.

[0082] Step 4: Calculate the limit resection angle and determine the system stability.

[0083] Comparing the calculated equivalent acceleration and deceleration areas of the converter after a fault can determine system stability. If the equivalent acceleration area is larger than the deceleration area, the system is unstable; if the acceleration area is smaller than the deceleration area, the system is stable; and if the acceleration area is equal to the deceleration area, the system is in a critically stable operating state. Therefore, the acceleration area can usually be set equal to the deceleration area to calculate the fault cut-off angle:

[0084]

[0085] In actual operation, by judging the relationship between the fault removal angle and the fault limit removal angle, it is possible to quickly determine whether the system will recover stability after the fault is removed.

[0086] Build a single-grid energy storage converter infinite system, its network topology and internal control structure are as follows Figure 1 The active power loop uses virtual synchronous control, the reactive power loop uses droop control, and the internal control loop is a voltage and current loop, with a current limiting link between the voltage and current loops. The internal system parameters are shown in Table 1.

[0087] Table 1 Parameters of grid-type energy storage converter system

[0088]

[0089] The dynamic process of the system after being disturbed is as follows Figure 2 As shown in the figure, when the current is unsaturated, the converter operates on the unsaturated virtual power angle curve; when the current is saturated, the asynchronous generator operates on the saturated virtual power angle curve. The dashed line portion of the figure is unreachable because the converter's output current in this portion exceeds the limit. When the power reference jumps from P0 to P0', the asynchronous generator undergoes acceleration and deceleration due to virtual inertia. Using transient stability analysis methods similar to those used for traditional synchronous generators, it is found that when the asynchronous generator reaches the new stable equilibrium point a' during its first swing, its kinetic energy will be greater than zero, and it will continue to move beyond a' into the deceleration region. If the virtual synchronous generator has not yet decelerated to zero by the time it reaches point d', the asynchronous generator will enter the unstable region and experience transient synchronous instability.

[0090] According to the above analysis, when there is protection action, the fault clearing time can be roughly divided into two cases, namely: Case 1, fault clearing before saturation, corresponding to Figure 3 (a); Case 2, the fault is cleared after saturation, corresponding to Figure 3 (b) Set the active power reference value to 0.6pu, set the active power step to 1.2pu at t = 0.5s, and calculate the limit resection angle δ according to formula (10): cm ≈0.47, changing the fault removal time can obtain the phase trajectory of the fault removal at different times as follows Figure 4 shown.

[0091] The "*" in the figure indicates the initial operating point, and the dashed line indicates the return of the active power reference value to its initial state at that moment. It is important to note that when verifying the equal-area rule, the equivalent damping is set to zero. Therefore, after being disturbed and stabilizing, the asynchronous generator power supply does not return to its original stable operating point or operate at a new stable operating point. Instead, it oscillates within a certain range. After a step in the active power reference value, the converter deviates from its initial stable operating point. The fault is removed at virtual power angles of 0.37 and 0.44. Since the equivalent damping is zero, the converter operating point oscillates within a certain range but does not diverge. At this time, the asynchronous generator power supply remains stable. When the fault is removed at a virtual power angle of 0.49, the resulting phase trajectory curve diverges, the frequency difference between the converter and the grid increases, and the converter becomes unstable.

[0092] The simulation experimental results are similar to the theoretical derivation results, so this method is effective in judging the stability of the grid-connected energy storage converter system.

[0093] This method is suitable for analyzing large disturbances such as voltage drops and active power steps in single-machine systems. By integrating the difference between the virtual power angle curve of the grid-connected energy storage converter and the active power reference value over the virtual power angle, the magnitude of the acceleration and deceleration areas, or the difference between the fault cut-off angle and the limit cut-off angle, can be quickly determined for system stability.

[0094] Of course, the above description is not a limitation of the present invention, and the present invention is not limited to the above examples. Changes, modifications, additions or substitutions made by technicians in this technical field within the essential scope of the present invention should also fall within the scope of protection of the present invention.

Claims

1. A stability criterion method for a grid-type energy storage system considering the current limiting link, characterized in that: The following steps are involved: Step 1: Construct a synchronous stability mathematical model of the grid-connected energy storage converter system, which specifically includes the following steps: Step 1.1: Construct the active power loop control equation of virtual synchronous control, which is expressed as: Among them, P m and P e Represent the active power reference value and the actual output active power of the converter respectively; J represents the equivalent inertia coefficient; D is the equivalent damping coefficient; δ VSG The difference between the grid-connected point voltage phase angle of the grid-connected converter and the grid voltage phase angle is called the virtual power angle. VSG is the converter angular frequency; ω0 is the angular frequency reference value; Step 1.2: Build a current limiting link model. When the following conditions are met, the current limiting link starts: Among them, i ref is the current reference value; and are the dq axis current reference values respectively; I max is the current limit value of the grid-type converter; Step 1.3: Determine the output of the dq axis current limiting link and adopt the d axis current priority limiting control, which is expressed as: in, It is the output of the d-axis current limiting link; It is the output of the q-axis current limiting link; Step 1.4: Determine the actual output active power P of the converter based on whether the current reaches saturation. e Expressions of When the current is not saturated, Where: V VSG is the converter port voltage; V g is the grid voltage; X g is the line reactance; P um is the maximum output power when the converter is not saturated; When the current is saturated, P e =V g I max cosδ VSG =P sm cosδ (5); Where: P sm is the maximum output power when the converter is saturated; Step 2: Calculate the equivalent acceleration area of the grid-type energy storage converter after the disturbance occurs; Step 3: Calculate the deceleration area of the grid-type energy storage converter after the fault is cleared; Step 4: Calculate the limit resection angle and determine the system stability.

2. The stability criterion method of the grid-type energy storage system considering the current limiting link according to claim 1 is characterized in that: In step 2, the grid-type converter initially operates in a stable operating state. At a certain moment, a fault disturbance causes the active power reference value of the grid-type converter to jump. At this time, the active power reference value is greater than the actual output active power. The converter enters the acceleration state, increases the active power, and increases the output current. The corresponding acceleration area at this time is expressed as: Where S1 is the acceleration area before saturation; δ a is the power angle corresponding to the initial stable operating point; δ c is the power angle corresponding to the fault clearing; P0′ is the active power reference value after the power step.

3. The stability criterion method of the grid-type energy storage system considering the current limiting link according to claim 1 is characterized in that: Step 3 specifically includes the following steps: Step 3.1: Calculate the first part of the deceleration area when the current has not reached saturation: Where S2 is the deceleration area after fault recovery before saturation; δ O is the virtual power angle after the grid-type converter reaches saturation; P0 is the active power reference value after fault recovery; Step 3.2: Calculate the second deceleration area when the current reaches saturation: Among them, δ d To ensure that the output active power is greater than the maximum power angle allowed by the active power reference value; Step 3.3: Calculate the equivalent deceleration area S = S2 + S3.

4. The stability criterion method of a grid-type energy storage system considering the current limiting link according to claim 1 is characterized in that: In step 4, the transient stability of the grid-connected system of the grid-connected energy storage converter is judged based on the equivalent acceleration area and the deceleration area. If the equivalent acceleration area is larger than the deceleration area, the system is unstable; if the equivalent acceleration area is smaller than the deceleration area, the system is stable; if the equivalent acceleration area is equal to the deceleration area, the system is in a critical stability state.

5. The stability criterion method of the grid-type energy storage system considering the current limiting link according to claim 4 is characterized in that: By judging the difference between the fault removal angle and the limit removal angle, a quick judgment of the system stability is completed. Among them, the fault limit removal angle is calculated based on the acceleration area being equal to the deceleration area.

6. The stability criterion method of a grid-type energy storage system considering the current limiting link according to claim 5 is characterized in that: Step 4 specifically includes the following steps: Step 4.1: Let the acceleration area be equal to the deceleration area and calculate the fault limit removal angle δ cm ; Step 4.2: Compare the actual fault removal angle with the limit removal angle δ cm The size of the fault removal angle is used to determine the system stability. If the actual fault removal angle is greater than the limit removal angle δ cm , the system becomes unstable if the actual fault removal angle is less than the limit removal angle δ cm , the system is stable.

7. The stability criterion method of a grid-type energy storage system considering the current limiting link according to claim 1 is characterized in that: The grid-type energy storage converter adopts an active power loop control method with virtual synchronous control, where the outer loop is the voltage loop and the inner loop is the current loop.

8. The stability criterion method of a grid-type energy storage system considering the current limiting link according to claim 1 is characterized in that: The equivalent damping coefficient D is zero.

9. The stability criterion method of a grid-type energy storage system considering the current limiting link according to claim 1 is characterized in that: The grid-connected system parameters of the grid-type energy storage converter include: reference frequency / Hz, reference capacity / VA, reference voltage / V, line reactance / pu, line resistance / pu, filter source side resistance / pu, filter source side reactance / pu, converter DC side voltage, converter output active power / pu, current loop proportional gain, current loop integral gain, voltage loop proportional gain, voltage loop integral gain, grid voltage, damping coefficient, inertia coefficient, converter grid side voltage and current limit value.

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