Current limiting control method and system for droop control inverter

Abstract

The invention belongs to the technical field of grid-connected converters, and discloses a current-limiting control method and system for a droop control inverter, and the method comprises the steps: accurately obtaining a virtual impedance size and an active reference value constraint condition when a fault current is equal to a current-limiting value through phasor analysis; a virtual impedance-current difference proportionality coefficient and an active reference value-current difference proportionality coefficient are designed in consideration of the most serious condition of current limiting, the virtual impedance is increased and the active reference value is reduced proportionally according to the change of the adaptive current during a fault, and the current limiting requirements under different depths of power grid voltage drops can be met. The current-limiting control method provided by the invention is adaptive to the fault current, does not need a fault detection device, enhances the transient stability of the inverter when the fault occurs due to the reduction of the active reference value, indirectly limits the fault current through the adjustment of the virtual impedance and the active reference value, retains the voltage source characteristics of the droop control inverter, and improves the stability of the droop control inverter. And the system fault ride-through capability is obviously 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, the droop control inverter can provide fast voltage and frequency support, can operate in an isolated island, and has stronger fault ride-through capability, so it is widely used in new energy grid-connected scenarios. However, unlike the synchronous generator with 6-7 times overcurrent capability, the power electronic inverter has a very limited overcurrent capability, usually less than 2 times the rated current. When a grid fault occurs, due to the voltage source characteristics of the droop control inverter, it will suffer severe overcurrent and cause damage to the inverter. Therefore, the fault current limiting control of the droop control inverter is crucial to the safe and stable operation of the system. The current current limiting control strategies are mainly divided into control switching, current limiter and virtual impedance.

[0003] The principle of control switching current limiting is to switch the droop control to grid-following control in case of fault, and to achieve current limiting by giving a suitable reference value through the current loop. Although this method is effective, it requires fast and accurate fault detection to ensure the effectiveness of control switching, which limits its practicality. The use of limiter current limiting is to limit the fault current within the threshold by adding a limit link to the current reference value to ensure that the inverter does not over-current. This method does not require complex parameter design, and directly limiting the current is simple and effective. However, the problem is that when the current limiter is activated, the droop control inverter will maintain current source operation, and its voltage support capacity will be weakened. In addition, the power angle characteristics during the fault will also be converted to current source power angle characteristics, and the power angle stability will be weakened. Virtual impedance current limiting reduces the fault current by introducing the voltage drop on the virtual impedance into the voltage loop. During the fault current limiting period, the droop control inverter is still a voltage source characteristic, so virtual impedance current limiting has been widely used.

[0004] In the research of virtual impedance current limiting control, it can be mainly divided into fixed virtual impedance, virtual impedance based on voltage drop and adaptive virtual impedance based on current change. Fixed virtual impedance requires the acquisition of fault information on the grid side. The size of the virtual impedance is given according to the phasor difference between the internal potential of the inverter and the grid voltage at the time of the fault, and the virtual impedance ratio is usually designed considering the power decoupling requirements. Adaptive virtual impedance based on voltage drop designs the size of virtual impedance according to 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. Some adaptive virtual impedance based on current change introduces the difference between the fault current and the threshold current into the voltage loop to reduce the fault current, where the proportional coefficient is determined according to the maximum phasor difference between the internal potential and the grid voltage. Or the fault current is limited by multiplying the difference between the instantaneous value of the fault circuit and the current limiting requirement value by the proportional coefficient.

[0005] The problems and defects of the existing virtual impedance current limiting control are as follows: First, although the fixed virtual impedance can meet the current limiting requirements, it requires fast and accurate fault detection, and its application in actual engineering is limited; second, in terms of adaptive virtual impedance based on voltage drop and current change, some studies close the droop control outer loop when a fault occurs and design the virtual impedance through the maximum voltage difference between the internal potential and the grid voltage. Although the fault current can be successfully limited, this loses the voltage and frequency support capability of the grid-type control, and at the same time, closing the droop control outer loop 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, without the need for fault information detection, but the problem is that the existing research designs a larger virtual impedance to reduce the terminal voltage amplitude, ignoring that due to the constraint of the system active power balance, the power angle increases while the terminal voltage amplitude decreases, resulting in the fault current changing non-monotonicly with the change of the virtual impedance and there is a minimum fault current, and the minimum current will be greater than the current limiting value under deep grid drop, and the 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 prior art are as follows:

[0007] (1) Control switching and current limiter When the fault current reaches the limit, the grid-connected 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 faults. Summary of the invention

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

[0011] The present invention is implemented in this way: a droop control inverter current limiting control method comprises:

[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 proportionality coefficient is obtained by dividing the virtual resistance that meets the current limiting requirement by the current difference, and its expression is:

[0014]

[0015] In the formula, K r is the virtual resistance-current difference proportional coefficient, is the virtual resistance corresponding to the fault current equal to the current limiting 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] In the formula, 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 inductance respectively, R g , X g are the grid side resistance and grid side inductance, 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 power-voltage droop coefficient, P ref , Q ref They are active and reactive reference values ​​respectively.

[0022] Furthermore, the expression of the internal potential phasor that makes the fault current equal to the current limiting requirement value is:

[0023]

[0024]

[0025] In the formula, and It is the internal potential phasor when the fault current is equal to the current limiting requirement value.

[0026] Furthermore, by combining the internal potential phasor and the internal potential reactive power constraint that make the fault current equal to the current limiting requirement, the expression of the virtual impedance size that makes the fault current equal to the current limiting requirement can be obtained as follows:

[0027]

[0028]

[0029] In the formula, They respectively represent the virtual impedance, virtual resistance and virtual inductive reactance when the fault current is equal to the required current limiting value.

[0030] Further, considering the voltage support effect, according to the phasor analysis, it can be seen that due to choose As a current limiting virtual impedance.

[0031] Furthermore, 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 limitation of the fault current. The range of the active reference value is:

[0032] P ref ≤V g I lim

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

[0034]

[0035] Furthermore, when the fault current is equal to the current limiting requirement, the virtual impedance needs to be equal to At the same time, the active reference value needs to be less than V g I limIn 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 that The expression is:

[0036]

[0037] Where V gmin is the minimum grid voltage.

[0038] Therefore, the virtual resistance expression that makes the fault current equal to the current limiting requirement value can be obtained as:

[0039]

[0040] In the formula, σ v is the virtual impedance ratio;

[0041] The expression of the virtual resistance-current difference proportional coefficient is obtained as follows:

[0042]

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

[0044] A droop control inverter is used as a control object;

[0045] The overall control module is used to integrate the control effects;

[0046] The proportional parameter calculation module is used to set the inverter current limit value, active and reactive reference value, voltage command value, reactive-voltage droop coefficient, and virtual impedance ratio to output the proportional coefficient;

[0047] A current detection module monitors the inverter output current and is used to output the fault current and the threshold current difference;

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

[0049] The active reference value control module is used to output an active reference value proportional to the current difference.

[0050] In combination with the above technical solutions and the technical problems solved, the advantages and positive effects of the technical solutions to be protected by the present invention are as follows:

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

[0052] (1) The existing technology switches to current source control and current limiter. When the current reaches the limit value, the voltage loop of the droop control inverter will lose its regulating function, and the inverter will become a current source, losing the voltage and frequency support function of the droop inverter itself as a grid-forming voltage source. The current limiting control method proposed in the present 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) The existing fixed virtual impedance current limiting technology requires accurate and fast fault information detection to perform fault switching and effectively limit fault current, but in actual engineering, the fault conditions are complex and changeable, which limits 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 fault current, without the need for 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 in proportion to the change of the current, thereby avoiding the impact of control switching on the inverter in the event of a fault.

[0054] (3) The 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, resulting in the fault current presenting a minimum value with the change of virtual impedance. Under deep grid voltage drop, the minimum fault current is greater than the current limiting requirement value, 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, which can meet the current limiting requirements under different grid voltage drops, ensuring the safe and stable operation of the inverter.

[0055] (4) The 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 the transient stability performance. The latter two increase the power angle change because of the equivalent reduction of the terminal voltage, resulting in an increase in active power imbalance and thus reducing transient stability. The present invention uses both virtual impedance and active reference value reduction methods to limit current. Since the reduction of the active reference value reduces the active power imbalance, the transient stability is improved.

[0056] Second, the present invention provides a droop control inverter current limiting control method and system based on virtual impedance current limiting. When a fault occurs in the power grid, the virtual impedance is increased and the active reference value is reduced in proportion to the difference between the fault current and the threshold current detected, thereby equivalently reducing the phasor difference between the terminal voltage and the power grid voltage, and meeting the current limiting requirements under different depths of power grid voltage drops. The virtual impedance-current difference proportionality coefficient is obtained by obtaining the virtual impedance that meets the current limiting requirements based on phasor analysis, and the active reference value-current difference proportionality coefficient is obtained based on the condition that the minimum fault current is equal to the current limiting value. The proposed control method does not need to detect fault information on the grid side, and the occurrence of adaptive faults greatly reduces 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 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. Since the current limiting control method disclosed in the present invention does not change the structure of the droop control, it only needs to be introduced into the active loop and the voltage loop according to the current difference multiplied by the corresponding proportional coefficient, which greatly reduces the difficulty of implementation in actual projects. In addition, the proposed method does not change the voltage source characteristics of the droop control during a fault, and the voltage-frequency support capability of the grid-connected inverter is retained, and the fault ride-through capability is significantly improved.

[0057] Third, as auxiliary evidence of the inventiveness of the claims of the present invention, it is also reflected in the following important aspects:

[0058] (1) The expected benefits and commercial value of the technical solution of the present invention 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, 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. The control method and controller system can be widely used in fault current limiting of droop control inverters 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 the cost, and does not change the control structure of the droop control inverter, which greatly simplifies the difficulty of implementation and transformation 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. The 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 the current source operation, and lose the active support ability of droop control as a voltage source operation; and 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 a 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] In terms of fault current limiting of droop control inverters, how to maintain voltage source operation while meeting current limiting requirements under various grid faults without fault information detection is an ideal goal that people desire. When the existing switching to grid-following control / current limiter technology is adopted, although the fault current can be effectively limited, when the fault current reaches the limit, the inverter will switch to current source operation, losing its voltage frequency support function as a voltage source; when the fixed virtual impedance technology is adopted, although the voltage source characteristics are retained, additional fault information detection devices are required to limit the fault current, and the difficulty of application in actual engineering increases; while retaining the voltage source operation characteristics and the absence of fault information detection devices, scholars have proposed adaptive virtual impedance technology, which calculates the virtual impedance parameters by calculating the maximum voltage amplitude difference when the fault current is the current limiting requirement, but the problem is that the virtual impedance is ignored while reducing the terminal voltage amplitude. Increase the terminal voltage phase angle, resulting in a minimum value of the fault current with the change of virtual impedance. The minimum value of the fault current will be greater than the current limiting requirement value under deep grid voltage drop, and the adaptive virtual impedance technology will not be able to meet the 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 minimum fault current not exceeding the current limiting requirement value on the active reference value, and considers the most serious current limiting situation to design the virtual impedance-current difference proportional coefficient and the active reference value-current difference proportional coefficient, and increases the virtual impedance and reduces the active reference value according to the change of adaptive current during fault, which can meet the current limiting requirements under different depths of grid voltage drop. In addition, the current limiting control method proposed in the present invention adapts to the change of adaptive current, does not require a fault detection device, and indirectly limits the fault current through the change of virtual impedance and active reference value, retains the voltage source characteristics of the droop control inverter, so 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 power grid. This is the inherent cognition of most researchers at present. 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 the phasor analysis, the fault current has a minimum value as the virtual impedance changes, and the minimum value is only related to the grid voltage, the grid side resistance and the active power reference value. Therefore, under deep grid voltage drops, the minimum value of the fault current will be greater than the current limiting requirement value, and the existing adaptive virtual impedance technology will not be able to meet the current limiting requirements. In order to solve this technical problem, the present invention publicly proposes for the first time the change of adaptive fault current, proportionally increasing the virtual impedance while reducing the active power reference value, wherein the virtual impedance-current difference proportional coefficient is obtained according to the virtual impedance that meets the current limiting requirements, and the active power 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 proposed current limiting method successfully solves the current limiting problem under different depths of grid voltage drops.

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

[0067] 1) Effectively limit fault current: When the fault current increases, by increasing the virtual impedance and reducing the active reference value, this method can effectively limit the fault current when different grid voltages drop, protect the inverter and grid equipment from damage, and improve 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 the change of virtual impedance and active 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: When a fault occurs, the active reference value decreases proportionally with the change in current, effectively reducing active 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 responding adaptively and quickly, without the need for fault information detection and fault switching, the inverter can quickly switch to protection mode when the power 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, the present invention aims to solve the problem of poor current limiting effect of droop control inverter in the prior art under fault conditions. Traditional current limiting methods usually rely on fixed virtual impedance and fault information acquisition, and it is difficult to dynamically adapt to changes in fault current in actual applications. This method may cause the fault current to exceed the safe range when facing grid voltage fluctuations and load changes, increasing the risk of equipment damage and system instability.

[0074] The present invention achieves accurate limitation of fault current by introducing an adaptive virtual impedance and an adaptive active reference value adjustment method. The adjustment of virtual impedance and active reference value is proportional to the difference between fault current and rated current, so as to dynamically adapt to the current changes under different fault conditions. This method not only improves the accuracy of current limiting, but also effectively reduces the impact on other parts of the system, and improves the safety and stability of the inverter.

[0075] The current limiting method proposed in the present invention can maintain a good current limiting effect in various complex power grid environments through the adaptive adjustment of the virtual resistance-current difference proportional coefficient. Combined with the joint application of internal potential active and reactive constraints, the setting of virtual impedance is further optimized to ensure the effective control of fault current. This method can still maintain the stable operation of the system under the conditions of grid voltage drop and load mutation, avoiding the problem of excessive current that may occur in traditional methods.

[0076] The implementation of the present invention has not only achieved significant technical progress, but also brought significant social and economic benefits. By improving the current limiting capability of the inverter, the risk of equipment damage and system downtime is reduced, the service life of the equipment is extended, and the maintenance cost is reduced. In addition, the stability and safety of the system are improved, which helps to improve the overall reliability of the power system, ensure the power supply of users, and play an important role in promoting the development of the power industry. BRIEF DESCRIPTION OF THE DRAWINGS

[0077] Figure 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 is a diagram of hardware-in-the-loop experiment results using the current limiting control method disclosed in the present invention provided in an embodiment of the present invention;

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

[0085] Fig. 9 This is a diagram of 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 solution 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 used 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, and the present invention is described in detail below in conjunction with the accompanying drawings.

[0088] like Figure 1 As shown in FIG. 1 , the droop control inverter current limiting control method based on virtual impedance provided by the embodiment of the present invention includes 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 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 reactive power constraint of the internal potential phasor is:

[0097]

[0098] For the convenience of expression, the expressions of active constraint and reactive constraint of 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 virtual impedance, and the expression of its parameter equation is:

[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 (δ) and the phasor relationship between the internal potential and the terminal voltage, the active constraint expression of the terminal voltage can be obtained as follows:

[0104]

[0105]

[0106] Similarly, let it be 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 at 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 of the minimum fault current can be obtained as:

[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, we can see 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 the limiting of the fault current. 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 as 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, 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, it can be obtained 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] A droop control inverter is used as a control object;

[0136] The overall control module is used to integrate the control effects;

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

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

[0139] A virtual impedance control module, used 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] Embodiment 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 need to perform reactive power control to maintain grid stability. Using the droop control inverter current limiting control method, the inverter can monitor the change of output current, dynamically increase the virtual impedance and reduce the active reference value, and then ensure that the inverter provides the required reactive power to provide grid support within the current limiting requirements. When the grid voltage amplitude decreases or the phase angle changes, the current limiting control system can adjust the reactive power output of the inverter accordingly to meet the requirements of the grid and maintain the voltage stability of the grid.

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

[0144] In a photovoltaic power generation system, the inverter converts the direct current generated by the photovoltaic panel into alternating current and then connects it to the grid. Using the droop control inverter current limiting control method, the inverter can monitor the change of the local current, and dynamically adjust the virtual impedance and active reference value to equivalently affect the magnitude of the voltage amplitude and phase angle at the inverter end, thereby ensuring that the output voltage of the inverter is synchronized with the grid voltage. If the grid voltage amplitude or phase angle changes, the internal potential control system can automatically adjust the output voltage of the inverter to keep it synchronized with the grid, ensuring the stable operation of the photovoltaic power generation system.

[0145] The two embodiments provided by the present invention demonstrate the application of the droop control inverter current limiting control method in different types of renewable energy power generation systems. The method can help the inverter provide stable power output under different depth grid faults and effectively control the voltage and power to meet the requirements of the grid.

[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 Typhoon simulation platform was built as follows Figure 1 The system shown in the figure 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, 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 7When the grid voltage drops to 0.6pu, with the increase of virtual impedance and the decrease of active reference value, the fault current is only 1.11pu. When the grid voltage drops to 0.5pu, the fault current is only 1.15pu. It can be seen that the fault current is much smaller than the current limiting requirement of 1.5pu. The proposed current limiting control method effectively limits the fault current.

[0148] In the second embodiment, numerical calculation is first used to obtain the relationship between the fault current and the virtual impedance when only the existing adaptive virtual impedance technology is used, such as Figure 8 As shown in the figure, as the phasor analysis results are consistent, the fault current has a minimum value as the virtual impedance changes, and when a deep grid voltage drop occurs, such as the grid voltage drops to 0.5pu, the minimum fault current will be greater than the current limiting requirement, and the existing adaptive virtual impedance technology cannot meet the current limiting requirements. Fig. 9 The hardware-in-the-loop experimental results of using only adaptive virtual impedance control under different depths of grid voltage drop are given. It can be seen that with the increase of virtual impedance parameters, the fault current has a minimum value, and when the grid voltage drops to 0.5pu, the minimum current value is 1.563pu>1.5pu, which cannot meet the current limiting requirements.

[0149] 3. Table 1 Parameters of Grid-connected Inverter System with Droop Control

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[0152] It should be noted that the embodiments of the present invention can be implemented by hardware, software, or a combination of software and hardware. The hardware part can be implemented using dedicated logic; the software part can be stored in a memory and executed by an appropriate instruction execution system, such as a microprocessor or dedicated design hardware. It can be understood by a person of ordinary skill in the art that the above-mentioned devices and methods can be implemented using computer executable instructions and / or contained in a processor control code, such as a carrier medium such as a 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. Such code is provided on the carrier medium. The device and its modules of the present invention can be implemented by hardware circuits such as very large-scale integrated circuits or gate arrays, semiconductors such as logic chips, transistors, etc., or programmable hardware devices such as field programmable gate arrays, programmable logic devices, etc., can also be implemented by software executed by various types of processors, and can also be implemented by a combination of the above-mentioned hardware circuits and software, such as firmware.

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

2. The droop control inverter current limiting control method according to claim 1, characterized in that: 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 proportionality coefficient based on the virtual impedance value and the difference between the fault current and the rated current; The scale factor is used to adjust the size of the virtual impedance in real time.

3. The droop control inverter current limiting control method according to any one of claims 1 or 2, characterized in that: The reduction of the active reference value follows the following principles: Determine the minimum grid voltage and current limiting requirement; Determine the minimum value of active reference value according to the minimum value of grid voltage and current limiting requirement value; The active reference value gradually decreases to the minimum value at a preset decrease rate as the fault current increases.

4. The droop control inverter current limiting control method according to claim 1, characterized in that: The method further includes: Under the extreme condition that 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 proportionality coefficient is calculated.

5. The droop control inverter current limiting control method according to claim 1, characterized in that: The expressions of internal potential active and reactive constraints are: In the formula, 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 inductance respectively, R g , X g are the grid side resistance and grid side inductance, 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 power-voltage droop coefficient, P ref , Q ref They are active and reactive reference values ​​respectively.

6. The droop control inverter current limiting control method according to claim 1, characterized in that: The expression of the internal potential phasor that makes the fault current equal to the current limiting requirement value is: In the formula, and It is the internal potential phasor when the fault current is equal to the current limiting requirement value.

7. The droop control inverter current limiting control method according to claim 1, characterized in that: By combining the internal potential phasor and the internal potential reactive power constraint that make the fault current equal to the current limiting requirement, the expression of the virtual impedance size that makes the fault current equal to the current limiting requirement can be obtained as follows: In the formula, They respectively represent the virtual impedance, virtual resistance and virtual inductive reactance when the fault current is equal to the required current limiting value.

8. The droop control inverter current limiting control method according to claim 1, characterized in that: 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 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 active reference value is: P ref ≤V g I lim Considering the typical grid voltage drop range specified in IEEE 1159-2019 as 0.1-0.9pu, the minimum grid voltage is selected as 0.1pu, and considering a certain margin, the minimum active reference value is considered to be 0. Therefore, the expression of the active reference value-current difference proportional coefficient is obtained as follows:

9. The droop control inverter current limiting control method according to claim 2, characterized in that: When the fault current is equal to the current limiting requirement, 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 that 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: In the formula, σ v is the virtual impedance ratio; The expression of the virtual resistance-current difference proportional coefficient is obtained as follows:

10. 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 9, characterized in that: The droop control inverter current limiting control system comprises: A droop control inverter is used as a control object; The overall control module is used to integrate the control effects; The proportional parameter calculation module is used to set the inverter current limit value, active and reactive reference value, voltage command value, reactive-voltage droop coefficient, and virtual impedance ratio to output the proportional coefficient; A current detection module monitors the inverter output current and is used to output the fault current and the threshold current difference; A virtual impedance control module, used 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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