Droop control inverter current limiting control method and system

Abstract

The present application belongs to the grid-connected converter technical field, disclose a kind of droop control inverter current-limiting control method and system, the virtual impedance size and active reference value restriction condition when the fault current is equal to current-limiting value are accurately obtained by phasor analysis, virtual impedance-current difference proportional coefficient and active reference value-current difference proportional coefficient are designed considering the most serious situation of current-limiting, proportional increase virtual impedance and reduce active reference value with the change of adaptive current at fault, can meet the current-limiting requirement under different depth grid voltage drop.The current-limiting control method proposed in the present application is adaptive to fault current, without fault detection device, the transient stability of inverter is enhanced due to the reduction of active reference value at fault, and the fault current is indirectly limited by adjusting virtual impedance and active reference value, the voltage source characteristics of droop control inverter are retained, and the system fault ride-through capability is significantly enhanced.

Description

Technical Field

[0001] The present invention belongs to the technical field of grid-connected converters, and in particular relates to a current limiting control method and system for a droop control inverter. Background Art

[0002] As a major grid-connected inverter, droop-controlled inverters offer rapid voltage and frequency support, island operation, and enhanced fault ride-through capabilities, making them widely used in renewable energy grid-connected scenarios. However, unlike synchronous generators, which boast an overcurrent capability of 6-7 times, power electronic inverters have very limited overcurrent capability, typically less than 2 times the rated current. During a grid fault, the voltage source characteristics of droop-controlled inverters can cause severe overcurrent damage, resulting in damage to the inverter. Therefore, fault current limiting in droop-controlled inverters is crucial for safe and stable system operation. Current current limiting strategies primarily include control switching, current limiters, and virtual impedance.

[0003] The principle of controlled switching current limiting is to switch droop control to grid-following control during a fault. Current limiting is achieved by applying an appropriate reference value to the current loop. While effective, this method requires fast and accurate fault detection to ensure effective control switching, limiting its practicality. Limiter current limiting involves adding a limiter to the current reference value to limit the fault current to a threshold, thereby ensuring inverter overcurrent protection. This method requires no complex parameter design and directly limits current, offering a simple and effective approach. However, a drawback is that when the current limiter is activated, the droop-controlled inverter will maintain current source operation, reducing its voltage support capability. Furthermore, the power angle characteristics during the fault will also shift to current source characteristics, reducing power angle stability. Virtual impedance current limiting, on the other hand, reduces fault current by introducing a voltage drop across a virtual impedance into the voltage loop. During the fault current limiting period, the droop-controlled inverter maintains voltage source characteristics, leading to its widespread application.

[0004] Research on virtual impedance current limiting control can be primarily categorized into fixed virtual impedance, voltage sag-based virtual impedance, and current-variable adaptive virtual impedance. Fixed virtual impedance requires grid-side fault information. The virtual impedance is determined based on the phasor difference between the inverter's internal potential and the grid voltage at the time of the fault, often taking into account power decoupling requirements to design the virtual impedance ratio. Voltage sag-based adaptive virtual impedance designs the virtual impedance based on voltage drop and current limiting requirements, including the maximum voltage drop between the grid voltage and the internal potential, the voltage drop between the internal potential and the terminal voltage when the internal potential amplitude is a given value, and the voltage drop between the internal potential and the terminal voltage when the internal potential phasor is a given value. Current-variable adaptive virtual impedance can reduce the fault current by introducing the difference between the fault current and the threshold current into the voltage loop multiplied by a proportional coefficient, where the proportional coefficient is determined by the maximum phasor difference between the internal potential and the grid voltage. Alternatively, the fault current is limited by multiplying the difference between the instantaneous value of the fault circuit and the required current limiting value by the proportional coefficient.

[0005] The problems and defects of existing virtual impedance current limiting control are as follows: First, although fixed virtual impedance can meet current limiting requirements, it requires fast and accurate fault detection, which limits its application in practical engineering. Second, in terms of adaptive virtual impedance based on voltage sag and current variation, some studies have closed the outer loop of droop control during a fault and designed the virtual impedance based on the maximum voltage difference between the internal potential and the grid voltage. Although this can successfully limit the fault current, it loses the voltage and frequency support capabilities of the grid-type control. At the same time, closing the outer loop of droop control still requires fault information detection. More commonly, the virtual impedance is designed based on the maximum voltage difference between the internal potential and the grid voltage, which does not require fault information detection. However, the problem is that existing studies design larger virtual impedances to reduce the terminal voltage amplitude, ignoring the fact that due to the constraints of system active power balance, the power angle increases as the terminal voltage amplitude decreases. As a result, the fault current varies non-monotonically with the change of virtual impedance and there is a minimum fault current. Moreover, under deep grid sag, the minimum current will be greater than the current limit value, and traditional adaptive virtual impedance control will not be able to meet the current limiting requirements.

[0006] Through the above analysis, the problems and defects of the existing technology are as follows:

[0007] (1) Control switching and current limiter When the fault current reaches the limit, the grid-type inverter changes from a voltage source characteristic to a current source characteristic, and the voltage support capability and fault ride-through capability are weakened.

[0008] (2) Although fixed virtual impedance can meet the current limiting requirements, it requires fast and accurate fault detection, which limits its application in practical engineering.

[0009] (3) Traditional adaptive virtual impedance control will not be able to meet the current limiting requirements under deep fault conditions. Summary of the Invention

[0010] In view of the problems existing in the prior art, the present invention provides a current limiting control method and system for a droop control inverter.

[0011] The present invention is implemented as follows: a droop control inverter current limiting control method includes:

[0012] The fault current is limited by adaptive virtual impedance and adaptive active reference value reduction, both of which are proportional to the difference between the fault current and the rated current.

[0013] Furthermore, the virtual resistance-current difference proportional coefficient is obtained by dividing the virtual resistance that meets the current limiting requirement by the current difference, and its expression is:

[0014]

[0015] Where K r is the virtual resistance-current difference proportional coefficient, is the virtual resistance corresponding to the fault current equal to the current limit value, I lim is the inverter current limiting requirement, I th is the threshold current.

[0016] Furthermore, the internal potential active constraint can be used to obtain the internal potential phasor that makes the fault current equal to the current limiting value, and the internal potential reactive constraint can be combined to obtain the virtual impedance size that makes the fault current equal to the current limiting requirement value.

[0017] Furthermore, the expressions of the internal potential active and reactive constraints are:

[0018]

[0019]

[0020]

[0021] Where E and δ are the internal potential amplitude and power angle, respectively, V g is the grid voltage, R v 、X v are virtual resistance and virtual inductive reactance respectively, R g 、X g are the grid side resistance and grid side inductive reactance, R Σ is the total resistance, X Σ is the total inductive reactance, Z Σ is the total impedance, V0 is the rated voltage, n is the reactive-voltage droop coefficient, P ref , Q ref are the active and reactive reference values ​​respectively.

[0022] Further, the expression of the internal voltage phasor when the fault current is equal to the current limiting requirement value is:

[0023]

[0024]

[0025] wherein, and is the internal voltage phasor when the fault current is equal to the current limiting requirement value.

[0026] Further, by simultaneously making the internal voltage phasor when the fault current is equal to the current limiting requirement value and the internal voltage reactive power constraint, the expression of the virtual impedance size when the fault current is equal to the current limiting requirement value is:

[0027]

[0028]

[0029] wherein, respectively represent the virtual impedance, the virtual resistance and the virtual inductance when the fault current is equal to the current limiting requirement value.

[0030] Further, considering the voltage support effect, according to the phasor analysis, since is selected as the current limiting virtual impedance.

[0031] Further, when the grid voltage is lower than the boundary value, the minimum fault current is greater than the current limiting requirement value, and only using the virtual impedance cannot meet the current limiting requirement, the active reference value needs to be reduced to ensure effective limitation of the fault current, and the range of the active reference value is:

[0032] P ref ≤V g I lim

[0033] Considering that the typical grid voltage drop range specified in IEEE 1159-2019 is 0.1-0.9 p.u., the minimum grid voltage is selected as 0.1 p.u., and considering a certain margin, the minimum value of the active reference value is considered to be 0. Thus, the expression of the active reference value-current difference proportionality coefficient is:

[0034]

[0035] Further, when the fault current is equal to the current limiting requirement value, the virtual impedance needs to be equal to At the same time, the active reference value needs to be less than V g I lim ​To successfully limit the fault current, the worst case for current limiting is considered: the grid voltage drops to the minimum value of 0.1 p.u., and the grid-side impedance is zero, so the expression for this time is

[0036]

[0037] where V gmin is the minimum value of the grid voltage.

[0038] Thus, the expression for the virtual resistance that makes the fault current equal to the current limiting requirement is

[0039]

[0040] where σ v is the virtual impedance ratio.

[0041] The expression for the virtual resistance-current difference proportionality coefficient is

[0042]

[0043] Another object of the present application is to provide a droop control inverter current limiting control system comprising:

[0044] a droop control type inverter for being a control object;

[0045] a total control module for integrating control effects;

[0046] a proportionality parameter calculation module for giving an inverter current limiting value, an active and reactive reference value, a voltage instruction value, a reactive-voltage droop coefficient, and a virtual impedance ratio to output a proportionality coefficient size;

[0047] a current detection module for monitoring an inverter output current to output a fault current and a threshold current difference;

[0048] a virtual impedance control module for outputting a virtual impedance proportional to the current difference;

[0049] an active reference value control module for outputting an active reference value proportional to the current difference.

[0050] In combination with the above technical solutions and the technical problems solved, the technical solutions to be protected by the present application have the following advantages and positive effects:

[0051] First, in view of the technical problems existing in the above-mentioned prior art, some creative technical effects are brought after the problems are solved. The specific description is as follows:

[0052] ​(1) When the existing technology switches to current source control and current limiter, the voltage loop of the droop control inverter loses its regulating function when the current reaches the limit value, and the inverter becomes a current source, losing the voltage and frequency support function of the droop inverter itself as a grid-type voltage source. The current limiting control method proposed in this invention reduces the terminal voltage to limit the current by adjusting the active reference value of the droop control and the virtual impedance equivalent. Its voltage source characteristics are retained in the event of a fault, and the voltage and frequency support capability is significantly improved.

[0053] (2) Existing fixed virtual impedance current limiting technology requires accurate and rapid fault information detection to perform fault switching and effectively limit fault current. However, in actual engineering, the complex and changeable fault conditions limit its application. The current limiting control method proposed in the present invention increases the virtual impedance and reduces the active reference value by adaptively adapting to the change of the fault current. It does not require fault detection. In normal operation, since the current is the rated value, that is, the current difference is 0, the adaptive virtual impedance is 0 and the active reference value is the rated value. In the event of a fault, the virtual impedance is increased and the active reference value is reduced proportionally with the change of the current, thus avoiding the impact of control switching on the inverter during a fault.

[0054] (3) Existing adaptive virtual impedance technology only uses adaptive virtual impedance to reduce the terminal voltage amplitude to limit the fault current. However, due to the constraint of active power balance, the terminal voltage decreases while its phase angle also increases, causing the fault current to present a minimum value as the virtual impedance changes. Under deep grid voltage drops, the minimum fault current is greater than the current limiting requirement, and the existing adaptive virtual impedance technology will not be able to meet the current limiting requirement. The present invention proposes for the first time to simultaneously use adaptive virtual impedance and adaptive active power reference value reduction to limit the fault current. The current limiting requirement can be met under different grid voltage drops, ensuring the safe and stable operation of the inverter.

[0055] (4) Existing current source control, current limiter, fixed virtual impedance, and adaptive virtual impedance current limiting technologies all reduce the transient stability of the system. The former two reduce the output power limit due to the power angle characteristics of the current source during a fault, which reduces transient stability. The latter two increase the power angle change due to the equivalent reduction of the terminal voltage, resulting in increased active power imbalance and thus reduced transient stability. The present invention, however, simultaneously uses virtual impedance and active reference value reduction to limit current. Since the reduction of the active reference value reduces active power imbalance, transient stability is improved.

[0056] Secondly, the application provides a droop control inverter current limiting control method and system based on virtual impedance current limiting, when the power grid fails, the difference between the fault current and the threshold current is detected, the virtual impedance is increased and the active reference value is reduced in proportion, so as to equivalently reduce the phase difference between the terminal voltage and the grid voltage, and meet the current limiting requirements under different depth grid voltage drop, wherein the virtual impedance-current difference proportional coefficient is obtained according to the phase analysis to meet the current limiting requirements, and the active reference value-current difference proportional coefficient is obtained according to the condition that the minimum value of the fault current is equal to the current limiting value. The control method does not need to detect the grid side fault information detection, and is adaptive to the occurrence of faults, greatly reducing the demand for communication; in addition, the virtual impedance size and the constraint on the active reference value when the fault current is equal to the current limiting value are accurately obtained according to the phase analysis, and the virtual impedance-current difference proportional coefficient and the active reference value-current difference proportional coefficient are designed considering the most serious situation of current limiting, so that the method can accurately meet the current limiting requirements under various fault conditions; since the current limiting control method disclosed by the application does not change the structure of the droop control, it only needs to be introduced into the active ring and the voltage ring according to the current difference value multiplied by the corresponding proportional coefficient, so as to realize it, greatly reducing the difficulty of implementation in actual engineering, and the method does not change the voltage source characteristics of the droop control during the fault, and the voltage frequency support capability of the grid-connected inverter is retained, and the fault ride-through capability is significantly improved.

[0057] Thirdly, as the creative auxiliary evidence of the claims of the application, it is also embodied in the following several important aspects:

[0058] (1) The expected income and commercial value of the technical scheme of the application after transformation are:

[0059] The control method disclosed in the present invention can meet the current limiting requirements of the droop control inverter under different depths of grid sag: the difference between the fault current and the threshold current is detected in the controller, and the virtual impedance is proportionally increased and the active reference value is proportionally reduced, wherein the virtual impedance-current difference proportional coefficient is obtained based on the virtual impedance that meets the current limiting requirements, and the active reference value-current difference proportional coefficient is obtained based on the condition that the minimum fault current is equal to the current limiting value. This control method and controller system can be widely used in droop control inverter fault current limiting in renewable energy grid-connected scenarios. Since the voltage source characteristics of the droop control inverter are retained, the voltage-frequency support capability during fault ride-through is enhanced. In addition, the virtual impedance size and the constraint on the active reference value when the fault current is equal to the current limiting value are accurately obtained based on phasor analysis, and the virtual impedance-current difference proportional coefficient and the active reference value-current difference proportional coefficient are designed considering the most serious current limiting situation. Therefore, the proposed method can accurately meet the current limiting requirements under various fault conditions and avoid damage to the inverter due to overcurrent. The proposed current limiting control adapts to the change of current, does not require a fault detection system and communication, greatly reduces costs, and does not change the control structure of the droop control inverter, which greatly simplifies the difficulty of implementation and modification in engineering practice.

[0060] (2) The technical solution of the present invention fills the technical gap in the industry at home and abroad:

[0061] When a fault occurs in the power grid, the droop control inverter is threatened by the fault overcurrent. Existing current limiting control methods include switching to grid-following control, current limiter, fixed virtual impedance and adaptive virtual impedance technology. When the fault current reaches the limit value when switching to grid-following control and current limiter, the inverter will maintain current source operation, losing the active support capability of droop control as a voltage source operation; while the fixed virtual impedance requires accurate and fast fault detection and fault identification to limit the fault current, and its practicality in actual engineering is limited; and although the adaptive virtual impedance technology does not require fault detection, when the deep grid voltage drops, the minimum fault current is greater than the current limiting requirement value, and the current limiting requirement cannot be met. The current limiting control method provided by the present invention is voltage source operation in the event of a fault, does not require fault detection, and can meet the current limiting requirements under different depths of grid voltage drops, filling the gaps in the above existing current limiting technologies and providing a control basis for fault current limiting in scenarios with a high proportion of new energy grid connection.

[0062] (3) The technical solution of the present invention solves the technical problems that people have been eager to solve but have never been able to solve successfully:

[0063] When it comes to fault current limiting in droop-controlled inverters, the ideal goal is to maintain voltage source operation while meeting current limiting requirements under various grid fault conditions without fault information detection. While existing grid-following control / current limiter technologies can effectively limit fault current, when the fault current reaches the limit, the inverter switches to current source operation, losing its voltage-frequency support function as a voltage source. Fixed virtual impedance technologies, while retaining voltage source characteristics, require additional fault information detection devices to limit fault current, making practical engineering applications more difficult. While retaining voltage source operating characteristics and lacking fault information detection devices, researchers have proposed adaptive virtual impedance technology. This technology calculates the virtual impedance parameters by calculating the maximum voltage amplitude difference when the fault current meets the current limiting requirement. However, this approach ignores the fact that virtual impedance reduces the terminal voltage amplitude while increasing the terminal voltage phase angle. This results in a minimum fault current as the virtual impedance changes. Under deep grid voltage sags, the minimum fault current will exceed the current limiting requirement, making adaptive virtual impedance technology incapable of meeting current limiting requirements. The current limiting control method disclosed in the present invention accurately obtains the virtual impedance size when the fault current is equal to the current limiting value through phasor analysis, and obtains the constraint condition of the active reference value that the minimum fault current does not exceed the current limiting requirement value, and designs the virtual impedance-current difference proportional coefficient and the active reference value-current difference proportional coefficient considering the most serious current limiting situation. The change of adaptive current during fault increases the virtual impedance and reduces the active reference value, which can meet the current limiting requirements under different depths of grid voltage drops. In addition, the current limiting control method proposed in the present invention adapts to the change of adaptive current and does not require a fault detection device. Since the fault current is indirectly limited by the change of virtual impedance and active reference value, the voltage source characteristics of the droop control inverter are retained. Therefore, the present invention effectively solves the above-mentioned problems.

[0064] (4) The technical solution of the present invention overcomes technical prejudice:

[0065] For a long time, when scholars / engineers use adaptive virtual impedance technology to limit fault current, the virtual impedance-current difference proportional coefficient is determined by the maximum voltage amplitude difference between the inverter and the grid. This is the inherent understanding of most researchers. However, the essence of virtual impedance technology is to limit current by reducing the terminal voltage. Due to the constraint of active power balance, the virtual impedance will increase its phase angle while reducing the terminal voltage amplitude. According to phasor analysis, the fault current has a minimum value as the virtual impedance changes, and this minimum value is only related to the grid voltage, grid-side resistance, and active power reference value. Therefore, under deep grid voltage sags, the minimum fault current will exceed the current limiting requirement, and the existing adaptive virtual impedance technology will not be able to meet the current limiting requirements. To solve this technical problem, the present invention publicly proposes for the first time an adaptive fault current change, proportionally increasing the virtual impedance while reducing the active power reference value. The virtual impedance-current difference proportional coefficient is obtained based on the virtual impedance that meets the current limiting requirements, and the active power reference value-current difference proportional coefficient is obtained based on the condition that the minimum fault current is equal to the current limiting value. The proposed current limiting method successfully solves the current limiting problem under different depths of grid voltage sags.

[0066] Fourthly, the significant technical advancements brought about by the droop control inverter fault ride-through control method mentioned in the present invention are mainly reflected in the following aspects:

[0067] 1) Effectively limit fault current: When the fault current increases, the virtual impedance increases and the active reference value decreases. This method can effectively limit the fault current under different grid voltage drops, protecting the inverter and grid equipment from damage and improving the safety of the system.

[0068] 2) Enhanced fault ride-through capability: When a grid fault occurs, the terminal voltage phasor is indirectly affected by changes in virtual impedance and active power reference value, retaining the voltage-frequency support capability of the droop control inverter as a voltage source, thereby enhancing the fault ride-through capability.

[0069] 3) Improved transient stability of the inverter: During a fault, the active power reference value decreases proportionally with the change in current, effectively reducing active power imbalance and enhancing the transient stability of the inverter.

[0070] 4) Improve energy efficiency: The inverter can use energy more efficiently during fault ride-through, reduce energy waste, and improve overall energy efficiency.

[0071] 5) Optimize equipment protection mechanism: By monitoring fault current and adaptively responding quickly, without the need for fault information detection and fault switching, the inverter can quickly switch to protection mode when the grid is abnormal, reducing damage to the inverter itself and extending the life of the equipment.

[0072] The droop control inverter current limiting control method provided by the present invention provides a smarter, more reliable and more efficient way to manage grid-connected inverters, especially in the event of a grid fault, which can effectively limit the fault current and significantly improve the voltage and frequency support performance of the inverter and the stability of the grid.

[0073] Fifth, this invention aims to address the problem of poor current limiting effectiveness in existing droop-controlled inverters during fault conditions. Traditional current limiting methods typically rely on fixed virtual impedances and fault information acquisition, making them difficult to dynamically adapt to changes in fault current in actual applications. This approach can cause fault currents to exceed safe limits when faced with grid voltage fluctuations and load variations, increasing the risk of equipment damage and system instability.

[0074] This invention achieves precise fault current limitation by introducing adaptive virtual impedance and active power reference value adjustment methods. Both the virtual impedance and active power reference value are adjusted proportionally to the difference between the fault current and the rated current, dynamically adapting to current fluctuations under different fault conditions. This method not only improves current limiting accuracy but also effectively reduces the impact on other parts of the system, enhancing the safety and stability of the inverter.

[0075] The current-limiting method proposed in this paper, through adaptive adjustment of the virtual resistance-current difference proportional coefficient, can maintain effective current limiting in various complex power grid environments. Combined with the application of internal potential active and reactive power constraints, the virtual impedance setting is further optimized, ensuring effective control of fault current. This method can maintain stable system operation even under grid voltage drops and sudden load changes, avoiding the excessive current problems that can occur with traditional methods.

[0076] The implementation of this invention not only achieves significant technological advancements but also brings significant social and economic benefits. By improving the inverter's current-limiting capability, the risk of equipment damage and system downtime is reduced, extending equipment life and reducing maintenance costs. Furthermore, the improved system stability and safety contribute to improving the overall reliability of the power system, ensuring power supply to users and significantly promoting the development of the power industry. BRIEF DESCRIPTION OF THE DRAWINGS

[0077] Figure 1 1 is a diagram of a current limiting control method for a droop control inverter provided by an embodiment of the present invention;

[0078] Figure 2 is an equivalent circuit diagram considering virtual impedance provided by an embodiment of the present invention;

[0079] Figure 3 is a phasor diagram of internal potential and terminal voltage provided by an embodiment of the present invention;

[0080] Figure 4 is a critical grid voltage diagram provided by an embodiment of the present invention;

[0081] Figure 5 is a terminal voltage phasor diagram provided by an embodiment of the present invention;

[0082] Figure 6 is a diagram of a droop control inverter current limiting control system provided by an embodiment of the present invention;

[0083] Figure 7 1 is a diagram showing the results of a hardware-in-the-loop experiment using the current limiting control method disclosed in the present invention, provided in an embodiment of the present invention;

[0084] Figure 8 This is a diagram showing theoretical calculation results of a fault current using only the adaptive virtual impedance control method provided by an embodiment of the present invention;

[0085] Figure 9 This is a diagram showing the hardware-in-the-loop experiment results using only the adaptive virtual impedance control method provided by an embodiment of the present invention. DETAILED DESCRIPTION

[0086] In order to make the purpose, technical solutions and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with the embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not intended to limit the present invention.

[0087] In view of the problems existing in the prior art, the present invention provides a droop control inverter current limiting control method based on virtual impedance. The present invention is described in detail below with reference to the accompanying drawings.

[0088] like Figure 1 As shown in FIG, the embodiment of the present invention provides a droop control inverter current limiting control method based on virtual impedance, including an adaptive virtual impedance and an active reference value reduction module. In the figure, the droop control inverter is connected to the common coupling point (PCC) through an LC filter, and the line impedance between the PCC and the power grid is represented by Z g Equivalent. The equivalent circuit diagram considering virtual impedance is as follows Figure 2 As shown, it can be seen that the addition of virtual impedance affects the equivalent impedance and also causes the change of the internal potential phasor.

[0089] according to Figure 2 The inverter output power expression is:

[0090]

[0091]

[0092] According to the droop control equation, its expression is:

[0093]

[0094] By combining the output power equation and the droop control equation, the expression of the active power constraint of the internal potential phasor can be obtained as follows:

[0095]

[0096] Similarly, the expression of the internal potential phasor reactive power constraint is:

[0097]

[0098] For the convenience of expression, the expressions of the active constraint and reactive constraint of the internal potential phasor are recorded as E=f p (δ) and E = f q (δ). According to E=f p (δ) It can be seen that the trajectory of the internal potential vector is a part of a circle when considering the virtual impedance, and its parametric equation is expressed as:

[0099]

[0100] The relationship between the internal potential phasor and the terminal voltage phasor during a fault is:

[0101]

[0102]

[0103] According to E=f p The active constraint expression of the terminal voltage can be obtained by the phasor relationship between (δ) and the internal potential and terminal voltage:

[0104]

[0105]

[0106] Similarly, it is recorded as U = f p (θ), such as Figure 3 As shown, it can be seen that the trajectory of the terminal voltage phasor is a circle with the center being The radius is And the grid voltage phasor endpoint coordinates are (V g ,0). Therefore, according to the mathematical knowledge of the circle, when the terminal voltage endpoint is located on the straight line between the center of the circle and the grid voltage endpoint, the distance is the shortest, that is, ΔU=|UV g | minimum, indicating that the fault current is the minimum at this time. The corresponding terminal voltage phasor expression when the fault current is the minimum is:

[0107]

[0108] According to the phasor relationship, the internal potential phasor expression that minimizes the fault current can be obtained as follows:

[0109]

[0110] Therefore, the expression for the minimum fault current is:

[0111]

[0112] The grid voltage when the minimum fault current is equal to the current limiting requirement can be obtained, which is called the grid voltage boundary value, and its expression is:

[0113]

[0114] like Figure 4 As shown in the figure, when the grid voltage is higher than the boundary value, the minimum fault current is less than the current limiting requirement value. Figure 5 From the phasor relationship in the figure, it can be seen that when the fault current is equal to the current limiting requirement value, or Its expression is:

[0115]

[0116]

[0117] According to the phasor relationship, the expression of the internal potential phasor when the fault current is equal to the current limiting requirement value is:

[0118]

[0119]

[0120] Therefore, the virtual impedance expression when the fault current is equal to the current limiting requirement value can be obtained as follows:

[0121]

[0122]

[0123] can meet the current limiting requirements, but according to Figure 5 The voltage phasor in the equation is known, because Considering the voltage support effect, select As a current limiting virtual impedance.

[0124] When the grid voltage is lower than the boundary value, the minimum fault current is greater than the current limiting requirement. Using only virtual impedance will not meet the current limiting requirement. It is necessary to reduce the active reference value to ensure that the fault current is limited. The range of the active reference value is:

[0125] P ref ≤V gI lim

[0126] Considering the typical grid voltage drop range specified in IEEE 1159-2019 is 0.1-0.9pu, the minimum grid voltage is selected as 0.1pu. Considering a certain margin, the minimum active reference value is considered to be 0. When the fault current is equal to the current limiting requirement value, the virtual impedance needs to be equal to At the same time, the active reference value needs to be less than V g I lim To successfully limit the fault current. Consider the most unfavorable working condition for current limiting: the grid voltage drops to the minimum value of 0.1pu and the grid side impedance is 0. At this time, we can get The expression is:

[0127]

[0128] Therefore, the expression of the corresponding virtual resistance is:

[0129]

[0130] The expression of the virtual resistance-current difference proportional coefficient can be further obtained as:

[0131]

[0132] At the same time, the expression of active reference value-current difference proportional coefficient is:

[0133]

[0134] like Figure 6 As shown, the droop control inverter fault ride-through control system provided by the embodiment of the present invention includes:

[0135] Droop control inverter, used as a control object;

[0136] Total control module, used to integrate control effects;

[0137] The proportional parameter calculation module is used to set the inverter current limit value, active and reactive power reference value, voltage command value, reactive power-voltage droop coefficient, and virtual impedance ratio to output the proportional coefficient;

[0138] Current detection module, which monitors the inverter output current and is used to output the fault current and the threshold current difference;

[0139] A virtual impedance control module for outputting a virtual impedance proportional to the current difference;

[0140] The active reference value control module is used to output an active reference value proportional to the current difference.

[0141] Example 1. Reactive power control of wind farm inverter:

[0142] In a wind farm, multiple inverters convert the output power of wind turbines into AC power and inject it into the grid. These inverters require reactive power control to maintain grid stability. Using a droop control inverter current limiting method, the inverter monitors changes in output current and dynamically increases virtual impedance and decreases active power reference value, ensuring that the inverter provides the required reactive power to support the grid within the current limiting requirements. When the grid voltage amplitude decreases or the phase angle changes, the current limiting control system can adjust the inverter's reactive power output accordingly to meet the grid's requirements and maintain grid voltage stability.

[0143] Example 2. Voltage control of photovoltaic inverter:

[0144] In photovoltaic power generation systems, inverters convert the direct current (DC) generated by photovoltaic panels into alternating current (AC) before connecting it to the grid. Using a droop control inverter current limiting method, the inverter monitors changes in local current and dynamically adjusts the virtual impedance and active power reference to effectively influence the voltage amplitude and phase angle at the inverter terminals, ensuring that the inverter's output voltage is synchronized with the grid voltage. If the grid voltage amplitude or phase angle changes, the internal potential control system automatically adjusts the inverter's output voltage to synchronize it with the grid, ensuring stable operation of the photovoltaic power generation system.

[0145] Two embodiments of this invention demonstrate the application of a droop-controlled inverter current-limiting control method in various types of renewable energy power generation systems. This method can help the inverter provide stable power output under varying grid fault depths and effectively control voltage and power to meet grid requirements.

[0146] The present invention provides two embodiments of the invention: first, the hardware-in-the-loop experimental results of the current limiting control method disclosed in the present invention under different depths of grid voltage drops; second, the hardware-in-the-loop experimental results of the existing method that only uses adaptive virtual impedance control under different depths of grid voltage drops.

[0147] The following simulation platform was built on Typhoon Figure 1 The system shown in Table 1 has parameters as shown in Table 1. In the first embodiment, K can be obtained by substituting the parameters in the table into the formula r =0.233pu,K p =2p.u.. In order to fully verify the effectiveness of the proposed current limiting control method, as shown in Figure 4 As shown, the grid voltage drops of 0.6pu and 0.5pu are selected as two fault conditions for verification. The hardware-in-the-loop experimental results of the current limiting control method disclosed in the present invention are as follows: Figure 7As shown. When the grid voltage drops to 0.6 p.u., the fault current is only 1.11 p.u. with the increase of virtual impedance and the decrease of active reference value, and when the grid voltage drops to 0.5 p.u., the fault current is only 1.15 p.u. It can be seen that the fault current is far less than the current limiting requirement value of 1.5 p.u., and the current limiting control method is effective to limit the fault current.

[0148] In the second embodiment, the relationship between the fault current and the virtual impedance under the existing adaptive virtual impedance technology is first obtained by numerical calculation, as shown in the following table. Figure 8 As shown, as consistent with the results of the phasor analysis, the fault current has a minimum value with the change of the virtual impedance, and when the deep grid voltage drop occurs, such as the grid voltage drops to 0.5 p.u., the minimum value of the fault current will be greater than the current limiting requirement value, and the existing adaptive virtual impedance technology cannot meet the current limiting requirement. Further, Figure 9 The hardware-in-the-loop experimental results under different deep grid voltage drops by using only the adaptive virtual impedance control are given, and it can be seen that the fault current has a minimum value with the increase of the virtual impedance parameter, and when the grid voltage drops to 0.5 p.u., the minimum value of the current is 1.563 p.u.>1.5 p.u., which cannot meet the current limiting requirement.

[0149] III. Parameters of the droop control inverter grid-connected system in Table 1

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[0151]

[0152] It should be noted that the embodiments of the present application can be realized by hardware, software or a combination of software and hardware. The hardware part can be realized by using special logic; the software part can be stored in a memory and executed by a suitable instruction execution system, such as a microprocessor or a specially designed hardware. Those skilled in the art can understand that the above-mentioned devices and methods can be realized by using computer executable instructions and / or included in processor control code, such as provided on a carrier medium, such as a magnetic disk, CD or DVD-ROM, a programmable memory, such as a read-only memory (firmware), or a data carrier, such as an optical or electronic signal carrier. The devices of the present application and their modules can be realized by hardware circuit, such as ultra-large-scale integrated circuit or gate array, semiconductor, such as logic chip, transistor, etc., or programmable hardware device, such as field programmable gate array, programmable logic device, etc., or by software executed by various types of processors, or by a combination of the above-mentioned hardware circuit and software, such as firmware.

[0153] The above description is only a specific embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any modifications, equivalent substitutions and improvements made by any technician familiar with this technical field within the technical scope disclosed by the present invention and within the spirit and principles of the present invention should be covered by the scope of protection of the present invention.

Claims

1. A droop control inverter current limiting control method, characterized in that: The method includes: Limiting the fault current by adaptively adjusting the virtual impedance, wherein the adjustment amount of the virtual impedance is proportional to the difference between the fault current and the rated current; At the same time, the fault current is further limited by adaptively reducing the active reference value, wherein the change in the active reference value is proportional to the difference between the fault current and the rated current; The adjustment of the virtual impedance is achieved by the following steps: Determine the virtual impedance value corresponding to when the fault current is equal to the current limiting requirement value; Calculating a virtual impedance-current difference proportional coefficient based on the virtual impedance value and the difference between the fault current and the rated current; The size of the virtual impedance is adjusted in real time using the proportional coefficient; The method further includes: Under the extreme working condition where the grid voltage drops to the minimum value and the grid side impedance is zero, calculate the virtual resistance value that makes the fault current equal to the current limiting requirement value; Based on the virtual resistance value and the difference between the fault current and the rated current, a limit value of the virtual resistance-current difference proportional coefficient is calculated; By combining the internal potential phasor and internal potential reactive power constraint to make the fault current equal to the current limiting requirement, the expression for the virtual impedance size that makes the fault current equal to the current limiting requirement can be obtained as follows: Where, is the virtual impedance when the fault current is equal to the current limiting requirement value, is the virtual resistance when the fault current is equal to the current limiting requirement value, is the virtual inductive reactance when the fault current is equal to the current limiting requirement value; considering the voltage support effect, according to the phasor analysis, it can be seen that due to choose As a current limiting virtual impedance; when the fault current is equal to the current limiting requirement value, the virtual impedance needs to be equal to At the same time, the active reference value needs to be less than V g I lim In order to successfully limit the fault current, consider the most unfavorable working condition for current limiting: the grid voltage drops to the minimum value of 0.1pu, and the grid side impedance is 0. At this time, it can be obtained The expression is: Where V gmin is the minimum value of the grid voltage; Therefore, the virtual resistance expression that makes the fault current equal to the current limiting requirement value can be obtained as follows: Where σ v is the virtual impedance ratio; The expression of the virtual resistance-current difference proportional coefficient is:

2. The droop control inverter current limiting control method according to claim 1, characterized in that: The reduction of the active reference value follows the following principles: Determine the minimum grid voltage and current limiting requirements; Determine the minimum value of the active power reference value based on the minimum value of the grid voltage and the current limit requirement value; The active reference value decreases gradually to the minimum value according to the preset decrease rate as the fault current increases.

3. The droop control inverter current limiting control method according to claim 1, wherein: The expressions of internal potential active and reactive constraints are: Where E and δ are the internal potential amplitude and power angle, respectively, V g is the grid voltage, R v 、X v are virtual resistance and virtual inductive reactance, R g 、X g are the grid side resistance and grid side inductive reactance, R Σ is the total resistance, X Σ is the total inductive reactance, Z Σ is the total impedance, V0 is the rated voltage, n is the reactive-voltage droop coefficient, P ref , Q ref are the active and reactive reference values ​​respectively.

4. The droop control inverter current limiting control method according to claim 1, wherein: The expression of the internal potential phasor that makes the fault current equal to the current limiting requirement value is: Where, and is the internal potential phasor when the fault current is equal to the current limiting requirement value, I lim is the inverter current limit requirement value.

5. The droop control inverter current limiting control method according to claim 1, wherein: When the grid voltage is lower than the boundary value, the minimum fault current is greater than the current limiting requirement. Using only virtual impedance will not meet the current limiting requirement. It is necessary to reduce the active reference value to ensure effective fault current limiting. The range of the active reference value is: P ref ≤V g I lim Considering the typical grid voltage drop range of 0.1-0.9 pu specified in IEEE 1159-2019, the minimum grid voltage is selected as 0.1 pu. Considering a certain margin, the minimum value of the active reference value is assumed to be 0. Therefore, the expression of the active reference value-current difference proportional coefficient is obtained as follows: Among them, I th is the threshold current.

6. A droop control inverter current limiting control system using the droop control inverter current limiting control method according to any one of claims 1 to 5, characterized in that: The droop control inverter current limiting control system includes: Droop control inverter, used as a control object; Total control module, used to integrate control effects; The proportional parameter calculation module is used to set the inverter current limit value, active and reactive power reference value, voltage command value, reactive power-voltage droop coefficient, and virtual impedance ratio to output the proportional coefficient; Current detection module, which monitors the inverter output current and is used to output the fault current and the threshold current difference; A virtual impedance control module for outputting a virtual impedance proportional to the current difference; The active reference value control module is used to output an active reference value proportional to the current difference.

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