A virtual resistance-based complex dc network pv droop control method

Abstract

The application belongs to the technical field of power equipment, and particularly relates to a complex direct current network PV droop control method based on a virtual resistance. The method is as follows: a complex direct current network system is equivalent to a simple radial system; an equivalent model of a current state and a target state of the direct current network system is given, and a PV droop coefficient is calculated; after the PV droop coefficient is calculated, the PV droop coefficient is added to an outer loop control circuit, the control of a PV droop curve is combined with the outer loop control, a d-axis component of a reference current is obtained, and the control of the entire system is completed through subsequent inner loop control. At present, the optimization methods of complex power grids are basically based on special scenarios, but the complex network simplification method of the application can be applied to general networks, that is, any complex network can be equivalent to and optimized by the method. In terms of droop coefficient calculation, the current measurement link is not required, the number of most current sensors is reduced, and the application has certain advantages in economy.

Description

Technical Field

[0001] This invention belongs to the field of power equipment technology, specifically relating to a PV droop control method for complex DC networks based on virtual resistance. Background Technology

[0002] Currently, the most widely used control loops are vector current control (VCC) and virtual synchronous generator control (VSG). The common implementation of droop control in DC microgrids is as follows: droop curve control is added outside the voltage and current dual closed-loop control of the converter as the outer control loop to obtain the reference value of the converter's output DC voltage. After calculating the droop coefficient according to the PV droop coefficient formula, it is added to the outer control loop. Combining the PV droop curve control and the outer loop control, the d-axis component of the reference current is obtained, and the entire system is controlled through subsequent inner loop control.

[0003] In practical DC microgrids, cable impedance is significant. Therefore, when cable impedance differences are large, the traditional droop control method for current sharing still has limitations. The droop factor can be considered as a virtual resistance, which is the ratio of the voltage difference between the common bus and the converter to the current. The method of obtaining the droop factor by calculating the virtual resistance is widely used in simple radial microgrid systems. However, in complex networks, the virtual resistance cannot be directly calculated in this way.

[0004] Current technologies typically employ two methods to determine the droop coefficient when dealing with complex networks. The first method involves directly installing numerous current and voltage sensors on the complex network to measure the specific current and voltage values ​​at each node, processing the data to obtain the droop coefficient for that node. The second method involves optimizing the complex network into a radial network structure and then calculating the droop coefficient using traditional methods. The first method requires a large number of current and voltage sensors, resulting in excessive computation and significant financial costs. The second method has limited applicability, currently only suitable for networks of extremely low complexity and lacking general applicability. Summary of the Invention

[0005] To address the difficulty in determining the droop coefficient of complex networks, this invention first expands the applicability of equivalent optimization for complex networks from special scenarios to general networks. Secondly, based on this, a formula for calculating the PV droop coefficient is derived, significantly reducing the computational load while eliminating the need for current measurement, thus reducing the number of current sensors and saving substantial funds.

[0006] To achieve the above objectives, the present invention employs the following technical solutions:

[0007] A method for controlling PV droop in complex DC networks based on virtual resistance, the method comprising:

[0008] Step 1: Equivalent the complex DC network system to a simple radial system;

[0009] Step 2: Provide the equivalent models of the current state and the target state of the DC network system, and calculate the PV droop coefficient.

[0010] Step 3: After obtaining the PV droop coefficient, add it to the outer loop control loop. Combine the control of the PV droop curve with the outer loop control to obtain the d-axis component of the reference current, and then complete the control of the entire system through the subsequent inner loop control.

[0011] Furthermore, step 1 effectively includes three steps:

[0012] First, determine the virtual bus voltage value, then classify and process the nodes, and finally obtain the equivalent topology diagram.

[0013] Furthermore, the process of first determining the virtual bus voltage value, then classifying and processing the nodes, and finally deriving the equivalent topology diagram specifically includes:

[0014] The definition of virtual bus, the classification of three types of nodes, the processing of three types of nodes, and the equivalent formulas for the three types of nodes.

[0015] Furthermore, the virtual bus is defined as the virtual bus voltage value V. * The definition and formula are as follows:

[0016]

[0017] In the formula V * V represents the virtual bus voltage value. ave V is the average value of the voltage at each node. mid This represents the median of the voltage values ​​at each node.

[0018] Furthermore, the classification of the three types of nodes specifically includes:

[0019] Nodes that contain virtual nodes in all branches radiating from themselves are classified as first-class nodes;

[0020] The branches radiating out from this node, some of which have virtual nodes and others of which do not, are the second type of nodes.

[0021] All branches radiating from this node, and nodes without any virtual nodes, are classified as third-type nodes.

[0022] Furthermore, the processing of the three types of nodes and their equivalent formulas are as follows:

[0023] The first type of node is processed by directly calculating the equivalent resistance value, and the equivalent formula is:

[0024]

[0025] In the formula r i,z Let V be the resistance from node i to the virtual node. dci and V dcj The node voltages at nodes i and j are respectively, r i Let be the equivalent resistance from node i to the virtual bus;

[0026] The processing of the second type of node involves summarizing the current-voltage relationship of the current circuit node based on Kirchhoff's laws, and then deriving and calculating the target value according to the principle of external equivalence. The equivalent formula is as follows:

[0027]

[0028] In the formula V i V j V * These represent the voltages of node i, node j, and the virtual node, respectively; r vi r is the virtual resistance value between node i and the virtual node; ij r is the resistance value between node i and node j; vj r′ is the virtual resistance value between node j and the virtual node. vi r′ represents the equivalent virtual resistance of node i and the virtual node after the equivalent transformation. vj The equivalent virtual resistance of node j and the virtual node after the equivalent transformation;

[0029] The processing of the third type of node involves first equating it to a second type of node, and then calculating the equivalent resistance value. The equivalent formula is as follows:

[0030]

[0031] In the formula V i V j V * These represent the voltages of node i, node j, and the virtual node, respectively; r vi r′ is the virtual resistance value between node i and the virtual node. vi r′ represents the equivalent virtual resistance of node i and the virtual node after the equivalent transformation. vj r represents the equivalent virtual resistance of node j and the virtual node after the equivalent transformation;vj r′ is the virtual resistance value between node j and the virtual node. ij r is the resistance value between node i and node j; ij The resistance and current values ​​between node i and node j are given.

[0032] During the processing, r′ needs to be set. vj The value of r′ is used to complete the calculation process, but r′ vj It has no impact on the final equivalent result. This variable will be eliminated in the subsequent substitution process; it is merely a process value for convenient calculation.

[0033] Furthermore, the formula for the PV sag coefficient is as follows:

[0034]

[0035] In the formula K v V0 and V are the droop coefficients. * These represent the converter voltage and the virtual bus voltage, respectively; r is the virtual resistance of the converter; r c r0 is the virtual line resistance in the current state; r0 is the virtual line impedance in the rated state.

[0036] Compared with the prior art, the present invention has the following advantages:

[0037] Compared to conventional methods for calculating droop in complex power grids, this invention significantly simplifies the calculation of droop coefficients and reduces the number of computations. Furthermore, while current optimization methods for complex power grids are primarily based on specific scenarios, this invention's simplified method is applicable to general networks; any complex network can be equivalently optimized using this method. Finally, in terms of droop coefficient calculation, this invention eliminates the need for current measurement, reducing the number of current sensors and offering economic advantages. Attached Figure Description

[0038] Figure 1 This is the equivalent graph for the first type of nodes;

[0039] Figure 2 This is the equivalent graph for the second type of nodes;

[0040] Figure 3 Equivalent graph for the third type of node;

[0041] Figure 4 This is an equivalent model of the current state and the target state of the DC system;

[0042] Figure 5 This is a diagram illustrating the application of the PV sag factor. Detailed Implementation

[0043] Example

[0044] A method for controlling PV droop in complex DC networks based on virtual resistance, the method comprising:

[0045] Step 1: Equivalent the complex DC network system to a simple radial system;

[0046] The equivalent method mainly applies to stable DC systems without harmonics, and specifically includes three steps:

[0047] First, the virtual bus voltage value is determined. Then, the nodes are classified and processed. Finally, the equivalent topology is obtained. This includes: the definition of the virtual bus, the classification of the three types of nodes, the processing of the three types of nodes, and the equivalent formulas for the three types of nodes.

[0048] The virtual bus is defined as the virtual bus voltage value V*, and the formula is:

[0049]

[0050] The average value V of the voltage at each node ave and median V mid It is half the sum of the three types of nodes. This definition can effectively reduce the number of third-type nodes and reduce the computational difficulty.

[0051] Points in the system that have the same voltage value as the virtual bus are defined as virtual nodes. At the same time, based on the presence of virtual nodes in branches, system nodes are divided into the following three categories.

[0052] Nodes radiating from themselves that contain virtual nodes are classified as Class I nodes. Class I nodes are processed by directly calculating their equivalent resistance value, using the following formula:

[0053]

[0054] In the formula r i,z Let V be the resistance from node i to the virtual node. dci and V dcj The node voltages at nodes i and j are respectively, r i Let be the equivalent resistance from node i to the virtual bus;

[0055] The equivalent essence of the first type of node is: the parallel connection of the node and the virtual bus, such as... Figure 1 (Equivalent effect diagram of the first type of node) is shown.

[0056] If the branches radiating from this node have some virtual nodes and others without virtual nodes, these nodes are classified as the second type of node; for example... Figure 1 The m and n nodes in the diagram.

[0057] according to Figure 2In the (equivalent graph of the second type of node): I i I j I v V represents the current at node i, node j, and the virtual node, respectively; i V j V * These represent the voltages of node i, node j, and the virtual node, respectively; r vi I vi The virtual resistance and current values ​​between node i and the virtual node; r ij I ij Here are the resistance and current values ​​between node i and node j; r vj I vj Here are the virtual resistance and current values ​​between node j and the virtual node; r′ vi 、I′ vi The equivalent virtual resistance and current of node i and the virtual node after the equivalent transformation; r′ vj 、I′ vj To understand the physical relationship between the equivalent virtual resistance and current of node j and the virtual node after the equivalent transformation, the following conclusions can be drawn:

[0058]

[0059] After equivalent substitution, we get:

[0060]

[0061] This allows us to obtain the equivalent resistance value from the second type of node to the virtual bus.

[0062] If all branches radiating from this node do not contain any virtual nodes, then the node is classified as a third type of node.

[0063] The third type of node cannot be directly calculated using equivalent methods. It is necessary to first use the reduction approach to convert the third type of node into a second type of node, and then process it using the methods for processing second type of nodes.

[0064] according to Figure 3 (Equivalent diagram of the third type of node) reveals the following physical relationships:

[0065]

[0066] After equivalent substitution, we get:

[0067]

[0068] In the formula r′ ij Let r' be the resistance value between node i and node j; during the processing, r' needs to be set. vjThe value of r′ is used to complete the calculation process, but r′ vj It has no impact on the final equivalent result. This variable will be eliminated in the subsequent substitution process; it is merely a process value for convenient calculation.

[0069] Step 2: After completing the above simplification process, the entire complex DC network system is equivalent to a simple radial system. Then, the equivalent models of the current state and the target state of the DC network system are derived (…). Figure 4 Given the given parameters, the PV droop parameters are calculated.

[0070] Figure 4 In this context, V0 and V* represent the converter voltage and virtual bus voltage, respectively; r is the virtual resistance of the converter; r c r0 is the virtual line resistance in the current state; r0 is the virtual line impedance in the rated state; V dc I dc The current state of the converter output voltage and current; V dcref I dcref The converter output voltage and current are under rated conditions. Based on their physical relationships, the following conclusions can be drawn:

[0071]

[0072] Meanwhile, the expressions for traditional IV ptosis and PV ptosis are as follows:

[0073] I dcref -I dc =K c (V dcref -V dc );

[0074] P dcref -P dc =K v (V dcref -V dc );

[0075] In the formula K c These are the traditional droop coefficients. Multiplying each end by V0 and then simplifying, we get:

[0076]

[0077] It is easy to see that the following relationship exists:

[0078]

[0079] In summary, the droop coefficient K v for:

[0080]

[0081] Step 3: After obtaining the droop coefficient according to the PV droop coefficient formula, add it to the outer loop control loop. Combine the control of the PV droop curve with the outer loop control to obtain the d-axis component of the reference current. Then, complete the control of the entire system through subsequent inner loop control, such as... Figure 5 As shown.

[0082] Contents not described in detail in this specification are prior art known to those skilled in the art. Although illustrative specific embodiments of the invention have been described above to facilitate understanding by those skilled in the art, it should be understood that the invention is not limited to the scope of the specific embodiments. Various modifications are readily apparent to those skilled in the art as long as they fall within the spirit and scope of the invention as defined and determined by the appended claims, and all inventions utilizing the concept of this invention are protected.

Claims

1. A method for controlling PV droop in complex DC networks based on virtual resistance, characterized in that, The method includes: Step 1: Equivalent the complex DC network system to a simple radial system; including the classification of three types of nodes, the processing of the three types of nodes, and the equivalent formulas for the three types of nodes, and calculating the virtual equivalent resistance; The classification of the three types of nodes specifically includes: Nodes that contain virtual nodes in all branches radiating from themselves are classified as first-class nodes; The branches radiating out from this node, some of which have virtual nodes and others of which do not, are the second type of nodes. All branches radiating from this node, and nodes without any virtual nodes, are classified as third-type nodes. Step 2: Provide the equivalent models of the current state and target state of the DC network system, and calculate the PV droop coefficient; the formula for the PV droop coefficient is: ； In the formula, K v The droop coefficient is... , These are the converter voltage and the virtual bus voltage, respectively; r is the virtual resistance of the converter. The virtual line resistance in the current state; Virtual line impedance under rated conditions Step 3: After obtaining the PV droop coefficient, add it to the outer loop control loop. Combine the control of the PV droop curve with the outer loop control to obtain the d-axis component of the reference current, and then complete the control of the entire system through the subsequent inner loop control.

2. The PV droop control method for complex DC networks based on virtual resistance according to claim 1, characterized in that, Step 1, in its equivalent form, includes three steps: First, determine the virtual bus voltage value, then classify and process the nodes, and finally obtain the equivalent topology diagram.

3. The PV droop control method for complex DC networks based on virtual resistance according to claim 2, characterized in that, The process first determines the virtual bus voltage value, then classifies and processes the nodes, and finally derives the equivalent topology diagram, specifically including: The definition of virtual bus, the classification of three types of nodes, the processing of three types of nodes, and the equivalent formulas for the three types of nodes.

4. The PV droop control method for complex DC networks based on virtual resistance according to claim 3, characterized in that, The virtual bus is defined as the virtual bus voltage value. The definition and formula are as follows: ； In the formula V represents the virtual bus voltage value. ave V is the average value of the voltage at each node. mid This represents the median of the voltage values ​​at each node.

5. The PV droop control method for complex DC networks based on virtual resistance according to claim 4, characterized in that, The processing of the three types of nodes and their equivalent formulas are as follows: The first type of node is processed by directly calculating the equivalent resistance value, and the equivalent formula is: ， ； In the formula Let V be the resistance from node i to the virtual node. dci and V dcj Let be the node voltages of node i and node j, respectively. Let be the equivalent resistance from node i to the virtual bus; The processing of the second type of node involves summarizing the current-voltage relationship of the current circuit node based on Kirchhoff's laws, and then deriving and calculating the target value according to the principle of external equivalence. The equivalent formula is as follows: ， ； In the formula , , These represent the voltages of node i, node j, and the virtual node, respectively. This represents the virtual resistance value between the i-node and the virtual node; Let be the resistance value between node i and node j; Let be the virtual resistance value between node j and the virtual node; The equivalent virtual resistance of node i and the virtual node after the equivalent transformation; The equivalent virtual resistance of node j and the virtual node after the equivalent transformation; The processing of the third type of node involves first equating it to a second type of node, and then calculating the equivalent resistance value. The equivalent formula is as follows: ， ； In the formula Let be the resistance value between node i and node j.

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