Distributed generation unit, electrical microgrid system, and method of current-sharing control in electrical microgrid system

Abstract

A distributed generation unit is provided, including an electricity meter and a processor. The electricity meter generates a unit current according to the power rating of the distributed generation unit. The processor is coupled between a control area network (CAN) bus and a circuit block. The processor enables the distributed generation unit to identify a first electrical signal at an initial time and implement a master unit selection procedure. The master unit selection procedure includes: determining a delay time based on the unit current and a maximum unit current; counting from the initial time, identifying the unit current as the maximum unit current after the delay time has elapsed; and broadcasting the maximum unit current and a second electrical signal through the CAN bus. The processor is further configured to: after the second electrical signal is broadcast, implement an attenuation coefficient adjustment procedure to maintain the current state of the distributed generation unit.

Description

Technical Field

[0001] This invention relates to shunt control, and more particularly to shunt control among multiple distributed generation units with less communication load. Prior Technology

[0002] Power management and current / voltage regulation among multiple parallel distributed generation units (DGUs) are crucial for ensuring normal and efficient system operation. In a system containing multiple DGUs, a mismatch may exist between the power rating and output power of each DGU. Communication mechanisms between these DGUs can be used to adjust the output power of each DGU. However, when the system contains many DGUs, the amount of communication data can be enormous, consuming a significant portion of the system's control and communication resources. Summary of the Invention

[0003] One embodiment of the present invention provides a distributed generation unit (DGU), comprising: an electricity meter and a processor. The electricity meter is configured to generate a unit current of a circuit block based on a rated power value of the distributed generation unit; the processor is coupled between a control area network (CAN) bus and the circuit block, wherein the processor is configured to cause the distributed generation unit to: identify a first electrical signal representing a master selection start event at an initial time, and implement a master selection process.

[0004] The master unit selection process includes determining a delay time based on the unit current and a maximum unit current. The master unit selection process further includes, starting from the initial time, determining the unit current as the maximum unit current after the delay time has elapsed. The master unit selection process further includes, after the unit current is determined to be the maximum unit current, broadcasting the maximum unit current and a second electrical signal through the control area network bus. The second electrical signal represents a master unit selection completion event. After the second electrical signal is broadcast, the processor executes a droop coefficient regulation process to maintain the current state of the distributed generation units.

[0005] One embodiment of the present invention provides a distributed power generation unit, including a meter and a processor. The meter is configured to generate a unit current of a circuit block according to a rated power value of the distributed power generation unit. The processor is coupled between a control area network bus and the circuit block, wherein the processor is configured to cause the distributed power generation unit to identify a first electrical signal representing a master unit selection start event at an initial time and to implement a master unit selection process.

[0006] The master unit selection process includes determining a delay time based on the unit current and a maximum unit current. The master unit selection process further includes receiving a second electrical signal representing a master unit selection completion event via the control area network bus, starting from the initial time and before the delay time has elapsed. The master unit selection process further includes pausing the master unit selection process in response to the second electrical signal. The processor, in response to the second electrical signal, executes a decay coefficient adjustment process to adjust the current power value of the distributed generation units.

[0007] One embodiment of the present invention provides a microgrid system including a control area network bus, multiple meters, and multiple processors. The meters are configured to generate a first unit current based on a first rated power value of one of the first distributed generation units, and are configured to generate a second unit current based on a second rated power value of one of the second distributed generation units. The processors are coupled to the control area network bus and are configured to identify a first electrical signal representing a master unit selection start event, determine a first delay time based on the first unit current and a maximum unit current, and determine a second delay time based on the second unit current and the maximum unit current.

[0008] The processor is further configured to recognize the first unit current as the maximum unit current in response to the first delay time being less than the second delay time; broadcast the maximum unit current and a second electrical signal representing a master unit selection termination event from the first distributed generation unit via the control area network bus; receive the second electrical signal and the maximum unit current from the second distributed generation unit via the control area network bus; and after the second electrical signal is broadcast, cause the first distributed generation unit to perform an attenuation coefficient adjustment procedure to maintain a first current power value of the first distributed generation unit, and cause the second distributed generation unit to perform an attenuation coefficient adjustment procedure to adjust a second current power value.

[0009] One embodiment of the present invention provides a current shunt control method in a microgrid system. This method includes determining a first unit current based on a first rated power value of one of a first distributed generation units, and determining a second unit current based on a second rated power value of one of a second distributed generation units. The method further includes identifying a first electrical signal representing a master unit selection start event at an initial time; after identifying the first electrical signal, adding a first delay constant to a multiple of the difference between the first unit current and a maximum unit current to determine a first delay time; and adding a second delay constant to a multiple of the difference between the second unit current and the maximum unit current to determine a second delay time.

[0010] This method further includes identifying the first unit current as the maximum unit current in response to the first delay time passing earlier than the second delay time; broadcasting the maximum unit current and a second electrical signal representing a master unit selection end event when the first delay time passes, starting from the initial time; and generating a third electrical signal (Fstg=2) with a first criterion in response to the second electrical signal to maintain the first current power value of one of the first distributed generation units and adjust the second current power value of one of the second distributed generation units. Simple Explanation of the Diagram

[0011] Figure 1 is a schematic diagram of a microgrid system having two distributed power generation units according to an embodiment of the present disclosure; Figure 2 is an example of a distributed power generation unit according to one embodiment of the present disclosure; Figure 3 is an example unit diagram of a subordinate distributed power generation unit according to one embodiment of the present disclosure; Figures 4A-4D are flowcharts of a shunt control method for a microgrid system implemented with minimal communication, according to an embodiment of this disclosure. Figure 5A is a timing diagram of an electrical signal representing the start event of master unit selection in different distributed power generation units according to an embodiment of this disclosure; Figure 5B is a timing diagram of an electrical signal representing a master unit selection termination event in different distributed power generation units according to an embodiment of this disclosure; Figure 5C is a timing diagram of electrical signals representing the master unit selection start event and the master unit selection end event in the same distributed power generation unit according to an embodiment of the present disclosure. Implementation

[0012] The following description is intended to illustrate the general principles of the invention and should not be construed as limiting. The scope of the invention is best determined by referring to the appended claims.

[0013] In recent decades, power systems have struggled to meet the ever-growing electricity demand. Therefore, microgrids have been proposed to address this challenge. A DC microgrid integrates various distributed generation units (DGUs), such as solar photovoltaic panels and batteries. To achieve the required efficiency (e.g., power shunting), the output power of each DGU should be proportional to its power rating. However, due to the distributed nature of DC microgrids, a mismatch often exists between the output power of the DGUs and their power ratings.

[0014] Figure 1 is a schematic diagram of a microgrid system 100 having two distributed generation units (distributed generation unit 110 and distributed generation unit 120) according to an embodiment of the present disclosure. As shown in Figure 1, distributed generation unit 110 has a line impedance RL1, a circuit block (CB) 112, a digital signal processor (DSP) 114, and a meter 116. Distributed generation unit 120 has a line impedance RL2, a circuit block 122, a DSP 124, and a meter 126. The DSP 114 and DSP 124 can be any processor or processing unit, controlling distributed generation units 110 and 120 respectively. In addition, the microgrid system 100 includes a control area network (CAN) bus 130, a DC bus 140, and multiple loads coupled between the DC bus 140 and ground. The control area network bus 130 is coupled to the distributed generation unit 110 and the distributed generation unit 120, providing a communication gateway for the distributed generation unit 110 and the distributed generation unit 120.

[0015] As stated above, current shunting is an important objective to ensure normal and efficient system operation, and the current shunting required by a distributed generation unit is manifested as output power proportional to its power rating. Referring to Figure 1, distributed generation unit 110 has a line impedance RL1, and distributed generation unit 120 has a line impedance RL2. Therefore, the ratio of the output power (i.e., the current flowing through the load) of distributed generation unit 110 to distributed generation unit 120 should be approximately RL2:RL1, related to the line impedance rather than the power rating. To adjust the current shunting performance, a virtual impedance is introduced.

[0016] Assume that virtual impedances RV1 and RV2 (see Figures 2 and 3) are introduced into distributed generation unit 110 and distributed generation unit 120, respectively, and that virtual impedances RV1 and RV2 are fixed virtual impedances. To achieve ideal shunt performance, the impedance values ​​of the fixed virtual impedances RV1 and RV2 may be proportional to the power rating of distributed generation unit 110 and distributed generation unit 120. However, in order to reduce or eliminate the influence of line impedance on the output power of distributed generation unit 110 and distributed generation unit 120, the fixed virtual impedances RV1 and RV2 must be much larger than the line impedances RL1 and RL2, which will cause an unexpected voltage drop.

[0017] Therefore, this disclosure introduces an adaptive virtual impedance instead of a fixed virtual impedance. Thus, the output power ratio of distributed generation unit 110 to distributed generation unit 120 is (RL2+RV2):(RL1+RV1), where the virtual impedances RV1 and RV2 are variable. By appropriately selecting the values ​​of virtual impedances RV1 and RV2, the output power of distributed generation units 110 and 120 can be proportional to their power ratings, while reducing or eliminating voltage drops caused by high impedance. However, to adaptively adjust virtual impedances RV1 and RV2, a communication mechanism is needed between distributed generation units 110 and 120 (or more distributed generation units connected in parallel), or between the distributed generation units and the central controller (not shown) of the microgrid system 100. Therefore, this disclosure provides a master unit selection (or differential-delay-based) strategy to limit the amount of communication data required.

[0018] Referring to Figure 1, digital signal processor 114 or digital signal processor 124 (or other digital signal processors within a third distributed generation unit in the microgrid system 100) broadcasts or receives a first electrical signal FS to initiate a master unit selection process (i.e., act as a master unit selection start event), selecting a master distributed generation unit based on the delay times of distributed generation units 110 and 120. Specifically, when the master unit selection process is initiated, meters 116 and 126 generate unit currents Ipu1 and Ipu2 respectively, based on the rated power of circuit blocks 112 and 122. Then, digital signal processors 114 and 124 receive unit currents Ipu1 and Ipu2 to calculate the delay time Td1 of distributed generation unit 110 and the delay time Td2 of distributed generation unit 120 (see Figure 2). If the delay time Td1 is less than the delay time Td2, the distributed generation unit 110 is selected as the master distributed generation unit of the microgrid system 100, and the distributed generation unit 120 is identified as a slave distributed generation unit. In another embodiment, the microgrid system 100 may contain more than two distributed generation units, but there will still only be one master distributed generation unit, and the others are slave distributed generation units.

[0019] After selecting distributed generation unit 110 as the primary distributed generation unit, the digital signal processor 114 of distributed generation unit 110 broadcasts a unit current Ipu1 as the maximum unit current Imax of the microgrid system 100, and transmits the maximum unit current Imax to distributed generation unit 120 through the control area network bus 130. Additionally, the digital signal processor 114 broadcasts a second electrical signal FR through the control area network bus 130 to initiate an attenuation coefficient adjustment process, adjusting the output power of distributed generation unit 120. Because the primary distributed generation unit of the microgrid system 100 (i.e., distributed generation unit 110) has been selected, the second electrical signal FR can also stop the primary unit selection process (i.e., act as a primary unit selection stop event).

[0020] Next, the distributed power generation unit 120 implements an attenuation coefficient adjustment process. This is achieved by outputting a control signal S2 from the digital signal processor 124 to the circuit block 122, adjusting the output power of the distributed power generation unit 120. Specifically, the attenuation coefficient adjustment process adjusts the virtual impedance RV2 of the distributed power generation unit 120 to adjust its output power and improve the shunt performance between the distributed power generation units 110 and 120. It should be noted that although the distributed power generation unit 110 can also recognize its own broadcast second electrical signal FR, the control signal S1 output by the digital signal processor 114 does not adjust the virtual impedance RV1 of the distributed power generation unit 110. Instead, the control signal S1 maintains the current state of the distributed power generation unit 110 until the first electrical signal FS is recognized (e.g., received from another distributed power generation unit or broadcast by the distributed power generation unit 110 itself). At this point, a new master unit selection process is executed, and a new master distributed power generation unit is selected. The details of the main unit selection process and the attenuation coefficient adjustment process will be explained in subsequent paragraphs, and please refer to Figures 2, 3 and 4A to 4D.

[0021] When a microgrid system has a large number of distributed generation units, the amount of communication data can be enormous, consuming a significant portion of the system's communication resources. By introducing a master unit selection process, only the master distributed generation unit will transmit data (e.g., maximum unit current Imax) to all slave distributed generation units (e.g., distributed generation unit 120), and none of the slave distributed generation units will transmit any data to other distributed generation units in the microgrid system. Therefore, a significant amount of communication resources are saved.

[0022] To further reduce communication data volume, the first electrical signal FS and the second electrical signal FR serve as part of the event-driven initiation mechanism for the master unit selection process and the attenuation coefficient adjustment process. These processes are only executed when the corresponding signal is broadcast, which reduces the number of times the master unit selection process and the attenuation coefficient adjustment process are executed compared to the conventional periodic initiation mechanism.

[0023] Figure 2 is an example of a main distributed power generation unit 200 according to an embodiment of the present disclosure. Similar to the distributed power generation unit 110 in Figure 1, the main distributed power generation unit 200 is coupled to a control area network (CLAN) bus 210 and includes a circuit block 212, a meter 214, and a digital signal processor 220. The digital signal processor 220 includes a main unit selection module 230 for a main unit selection process, an attenuation coefficient adjustment module 240 for an attenuation coefficient adjustment process, a communication bus 250, and an internal current and voltage adjustment module 260 that generates a control signal S1. The communication bus 250 communicates with other communication buses in other digital signal processors through the CLAN bus 210. The meter 214 generates a unit current Ipu1 according to the rated power of the main distributed power generation unit 200. In addition, the meter 214 further generates an initial unit current in response to a second electrical signal FR. pu1.

[0024] The master unit selection module 230 of the main distributed generation unit 200 identifies the first electrical signal FS at an initial time and begins the master unit selection process. The first electrical signal FS is received from other distributed generation units through the control area network bus 210 and communication bus 250, or broadcast by the attenuation coefficient adjustment module 240 of the main distributed generation unit 200 through the communication bus 250. After identifying the first electrical signal FS, the digital signal processor 220 generates a third electrical signal Fstg with a first criterion (e.g., representing a value of 1) to activate a delay time generator 234, which generates a first delay time Td1 based on a first delay constant Td0_1, a unit current Ipu1, and a maximum unit current Imax. The maximum unit current Imax can be the largest unit current in the past master unit selection process. Specifically, the first delay time Td1 can be generated by the delay time generator 234 according to the following formula: Td1 = kd (Imax – Ipu1) + Td0_1 (1)

[0025] Here, kd is a preset gain factor, and the first delay constant Td0_1 is a preset constant, ensuring an appropriate time interval between each master unit selection process to prevent the next master unit selection process from starting before the previous one is completed. Furthermore, since the first delay constant Td0_1 is preset, it can also represent the order of distributed generation units within the same microgrid. For example, distributed generation units 110 and 120 in Figure 1 may have different delay constants. When the delay constant of distributed generation unit 120 is much larger than that of distributed generation unit 110, distributed generation unit 110 may have a higher probability of being selected as the master distributed generation unit, and therefore has a higher priority than distributed generation unit 120 in the master unit selection process.

[0026] Next, the delay time generator 234 transmits the first delay time Td1 to a timer 232 to determine whether the first delay time Td1 has elapsed since the initial time (i.e., the timestamp when the first electrical signal FS was identified). Assume that the primary distributed generation unit 200 has the largest unit current relative to other distributed generation units in the microgrid. Therefore, according to formula (1), the primary distributed generation unit 200 will have the shortest delay time relative to other distributed generation units, and thus the first delay time Td1 will elapse earlier than the delay times of other distributed generation units in the same microgrid. Since the first delay time Td1 has elapsed since the initial time, the digital signal processor 220 will broadcast the second electrical signal FR to other distributed generation units via the communication bus 250 and the control area network bus 210.

[0027] Because the primary distributed generation unit 200 has the shortest delay time, other distributed generation units are still timing their own delay time when they receive the second electrical signal FR (i.e., the primary unit selection process of other distributed generation units has not yet been completed). Therefore, the second electrical signal FR represents the primary unit selection end event and can pause the primary unit selection process of other distributed generation units. After the second electrical signal FR is broadcast, the digital signal processor 220 generates a third electrical signal Fstg with a second level (e.g., representing the value 2), starts the attenuation coefficient adjustment module 240, and executes the attenuation coefficient adjustment process.

[0028] In response to the second electrical signal FR, meter 214 generates an initial unit current. pu1, a regulator 244 is used to adjust the virtual impedance RV1. The virtual impedance RV1 is adjusted by a rate of change e1(t), which is based on the maximum unit current Imax and the initial unit current. It depends on pu1. Specifically, the relationship between the virtual impedance RV1 and the rate of change e1(t) can be expressed by the following formula: = kc e1(t) (2a) e1(t) = pu1 – Imax (2b) Where kC is a gain coefficient.

[0029] Because the initial unit current pu1 is defined as the unit current Ipu1 when the second electrical signal FR is identified, and the maximum unit current Imax is the unit current Ipu1 when the first electrical signal FS is identified. Therefore, the rate of change e1(t) remains constant until the current attenuation coefficient adjustment process ends. In other words, the virtual impedance RV1 is adjusted with a fixed rate of change. However, in this embodiment, the main distributed generation unit 200 is selected as the main distributed generation unit of a microgrid, meaning that the maximum unit current Imax of the distributed generation unit 200 is its own unit current Ipu1. Therefore, the initial unit current... pu1 equals the maximum unit current Imax, making the rate of change e1(t) equal to 0. In other words, the distributed generation unit selected as the primary distributed generation unit will not adjust its virtual impedance and will maintain its current state (e.g., maintain its virtual impedance).

[0030] Therefore, referring to Figure 2, the control signal S1 generated by the internal current and voltage adjustment module 260 will not adjust the current state of circuit block 212. Instead, the control signal S1 will maintain the current state of circuit block 212 (e.g., maintain output voltage / power). Therefore, it can also be considered that the main distributed power generation unit 200 does not need to implement attenuation coefficient adjustment.

[0031] The adjustment process for each attenuation coefficient may take some time to complete, and the current unit current Ipu1 may differ from the initial unit current. Since pu1 is different, a current increment γ1 is introduced, defined by the following formula: γ1 = pu1 – Ipu1 (3) Next, to determine whether to initiate the next master unit selection process, a startup function ftc is introduced: ftc = (4) in This is a gain coefficient.

[0032] When the relationship between the current increment γ1 and the maximum unit current Imax satisfies a first criterion (ftc greater than or equal to 0), the primary distributed generation unit 200 will no longer operate with the required shunt efficiency. Therefore, the regulator 244 outputs a start signal TE to the start controller 242. The start controller 242 then generates a first electrical signal FS and broadcasts it via the communication bus 250 and the control area network bus 210, causing other distributed generation units to suspend their attenuation coefficient adjustment process and initiate the next primary unit selection process. In another embodiment, the first distributed generation unit in a microgrid to satisfy the first criterion may not be the primary distributed generation unit, but rather one of the subordinate distributed generation units. In other words, the unit broadcasting the first electrical signal FS may not be the primary distributed generation unit 200.

[0033] Figure 3 illustrates an example of a subordinate distributed generation unit 300 according to one embodiment of the present disclosure. Similar to the distributed generation unit 120 in Figure 1, the subordinate distributed generation unit 300 is coupled to a control area network (CLAN) bus 310 and includes a circuit block 312, a meter 314, and a digital signal processor 320. The digital signal processor 320 includes a master unit selection module 330, an attenuation coefficient adjustment module 340, a communication bus 350, and an internal current and voltage adjustment module 360 ​​for generating a control signal S2. The communication bus 350 communicates with other communication buses in other digital signal processors through the CLAN bus 310. The meter 314 generates a unit current Ipu2 based on the rated power of the subordinate distributed generation unit 300. Furthermore, the meter 314 generates an initial unit current in response to a second electrical signal FR. pu2.

[0034] Similar to the distributed generation unit 120, the subordinate distributed generation unit 300, after identifying the first electrical signal FS and the third electrical signal Fstg with the first quasi-value (representing a value of 1), generates a unit current Ipu2 and implements the master unit selection process. Next, a delay time generator 334 generates a second delay time Td2 according to the following relationship: Td2 = kd (Imax – Ipu2) + Td0_2 (5) Where Imax is the maximum unit current in the previous main cell selection process, and Td0_2 is a second delay constant, which may be the same as or different from the first delay constant Td0_1.

[0035] Assuming the master distributed generation unit 200 and the slave distributed generation unit 300 are in the same microgrid, and the second delay time Td2 of the slave distributed generation unit 300 is greater than the first delay time Td1, then while the timer 332 of the slave distributed generation unit 300 is still timing the second delay time Td2, the timer 232 of the master distributed generation unit 200 may have finished timing the first delay time Td1 and broadcast the second electrical signal FR as a master unit selection end event. Therefore, the slave distributed generation unit 300 suspends its master unit selection process (i.e., stops timing the second delay time Td2), and the attenuation coefficient adjustment process is initiated by the third electrical signal Fstg with a second level (representing a value of 2), and the initial unit current is generated at the timestamp broadcast by the second electrical signal FR. pu2.

[0036] Because the initial unit current Since pu2 differs from the maximum unit current Imax (unit current Ipu1 of the main distributed generation unit 200), the virtual impedance RV2 of the subordinate distributed generation unit 300 is adjusted through the regulator 344 using the following formulas (6a) and (6b): = kc e2(t) (6a) e2(t) = pu2 – Imax (6b) Where kc is the gain coefficient. Therefore, the control signal S2 generated by the internal current and voltage adjustment module 360 ​​adjusts the output voltage / power of the circuit block 312 according to the change of the virtual impedance RV2.

[0037] The adjustment process for each attenuation coefficient may take some time to complete, and the current unit current Ipu2 may differ from the initial unit current. Since pu2 is different, a current increment γ2 is introduced, defined by the following formula: γ2 = pu2 – Ipu2 (7) Next, referring to formula (4), the startup function ftc is as follows: ftc = (8) in This is the gain coefficient.

[0038] When the relationship between the current increment γ2 and the maximum unit current Imax reaches a first criterion (ftc greater than or equal to 0), the slave distributed generation unit 300 will no longer operate with the desired shunt performance. Therefore, the regulator 344 outputs a start signal TE to the start controller 342. The start controller 342 then generates a first electrical signal FS and broadcasts the first electrical signal FS through the communication bus 350 and the control area network bus 310, causing other distributed generation units to suspend their attenuation coefficient adjustment process and start the next master unit selection process. In another embodiment, the first distributed generation unit in a microgrid to reach the first criterion may be another DGU besides the slave distributed generation unit 300. Therefore, the slave distributed generation unit 300 suspends its attenuation coefficient adjustment process upon receiving the first electrical signal FS.

[0039] The attenuation coefficient adjustment process is executed to meet the required shunt performance. This means adjusting the virtual impedance of each subordinate distributed generation unit, the unit current Ipu2, and the initial unit current. The difference between pu2 will decrease, resulting in a smaller current increment γ2. Because the maximum unit current Imax will be a constant value during the attenuation coefficient adjustment process, it can be inferred that the value of the startup function ftc will have a high probability of being less than 0 (due to the small current increment γ2). At this point, a current error |δ| is introduced, defined as follows: |δ| = |Ipu2 – Imax| (9)

[0040] The current error |δ| can be used to pause the attenuation coefficient adjustment process in the absence of the first electrical signal FS and maintain the current state of the subordinate distributed generation unit 300. Specifically, according to formula (8), the attenuation coefficient adjustment process continues when the start function ftc is less than 0. However, after adjusting the virtual impedance RV2 multiple times, the start function ftc being less than 0 may indicate that the subordinate distributed generation unit 300 has achieved the desired shunt performance, and the attenuation coefficient adjustment process is no longer able to reduce the current error of the subordinate distributed generation unit 300.

[0041] To further conserve communication resources, the second criterion is: |δ| ξ, where ξ is a preset error range. When the current error |δ| reaches the second criterion (|δ| is within the preset error range), the attenuation coefficient adjustment process is paused, and the digital signal processor 320 determines whether the start function ftc is less than 0. By repeatedly determining whether the current error |δ| is within the preset error range and whether the start function ftc is less than 0, the subordinate distributed power generation unit 300 maintains its current virtual impedance RV2 and output power, without having to continuously execute the attenuation coefficient adjustment process before the next master unit selection process starts.

[0042] Referring to Figures 2 and 3, the internal current and voltage adjustment modules 260 and 360 respectively receive the rated voltage VN1 of the main distributed power generation unit 200 and the rated voltage VN2 of the slave distributed power generation unit 300. After the attenuation coefficient adjustment process, virtual impedances RV1 and RV2 are generated to maintain or adjust the output voltage / power of the main distributed power generation unit 200 and the slave distributed power generation unit 300. In other words, the output voltage Vout1 of the main distributed power generation unit 200 and the output voltage Vout2 of the slave distributed power generation unit 300 can be expressed as follows: Vout1 = VN1 – Ipu1 (RL1 + RV1) (10a) Vout2 = VN2 – Ipu2 (RL2 + RV2) (10b) Therefore, by appropriately adjusting the virtual impedance, the output voltage / power of each distributed power generation unit can be adjusted to meet the required shunt performance.

[0043] Figures 4A-4D are flowcharts of a shunt control method 400 applied to a microgrid system using minimal communication, according to an embodiment of this disclosure. Assume a microgrid contains more than one distributed generation unit. In operation 402, all distributed generation units in the microgrid determine whether a first electrical signal FS has been broadcast. If the first electrical signal FS is not identified, method 400 repeats operation 402 until the first electrical signal FS is identified. If the first electrical signal FS is identified, method 400 proceeds to operation 404, where a third electrical signal Fstg is set to a first criterion (representing a value of 1). At this time (i.e., an initial time), each distributed generation unit begins executing a master unit selection process and generates a delay time based on the unit current and delay parameters of each distributed generation unit, as shown in formula (1) or (5) above.

[0044] Next, in operations 408 and 410, the timer of each distributed generation unit starts timing the delay time and determines whether the delay time has elapsed before the distributed generation unit identifies the second electrical signal FR. If the delay time has elapsed before the second electrical signal FR is identified (i.e., this distributed generation unit has the shortest delay time), method 400 continues to operation 412. In operation 412, this distributed generation unit is identified as the main distributed generation unit, and the unit current of this distributed generation unit is broadcast as the maximum unit current. Then, the main distributed generation unit broadcasts the second electrical signal FR and a third electrical signal Fstg (representing the value 2) with a second level. After broadcasting the second electrical signal FR, method 400 continues to operations 416 and 418, starting the attenuation coefficient adjustment process. In this process, the main distributed generation unit generates an initial unit current (e.g., the unit current when the second electrical signal FR is identified) and calculates the current error between the maximum unit current and the current (or instantaneous) unit current.

[0045] Referring to formulas (2a) and (2b), the virtual impedance of the main distributed generation unit will not be adjusted because the initial unit current of the main distributed generation unit is the same as the maximum unit current. Therefore, method 400 continues to operation 420, maintaining the current state of the main distributed generation unit, and calculating the current increment between the initial unit current and the current unit current using formula (3). Next, in operation 422, the start function ftc is based on the square of the current increment (i.e., and a multiple of one square of the maximum unit current (i.e., The calculation is performed jointly. The main distributed generation unit then determines whether the startup function ftc satisfies a first criterion (i.e., ftc). 0).

[0046] If the startup function is ftc 0. The primary distributed generation unit broadcasts the first electrical signal FS to initiate the next primary unit selection process. If the startup function ftc is less than 0, the primary distributed generation unit determines whether the current error is within an error range (operation 424a). If the current error is within the error range, method 400 returns to operation 424 to determine whether the startup function ftc is less than 0. If the current error exceeds the error range, method 400 returns to operation 420, generating a new current increment (equal to the difference between the initial unit current and the current unit current), while maintaining the virtual impedance of the primary distributed generation unit.

[0047] Returning to operation 410, if the distributed generation unit receives the second electrical signal FR before the delay time expires, method 400 continues to operation 428. In operation 428, the master unit selection process is paused, and the delay time of the distributed generation unit is also paused. In operation 430, this distributed generation unit is identified as a slave distributed generation unit. Next, the slave distributed generation unit receives the maximum unit current from the master distributed generation unit and, in response to the second electrical signal FR and the third electrical signal Fstg with a second level (representing a value of 2), initiates the attenuation coefficient adjustment process. After initiating the attenuation coefficient adjustment process, in operations 432 and 434, the slave distributed generation unit generates an initial unit current upon receiving the second electrical signal FR. Additionally, the slave distributed generation unit calculates the current error between the current unit current and the maximum unit current.

[0048] Because the maximum unit current differs from the initial unit current of the subordinate distributed generation unit, in operation 436, the virtual impedance of the subordinate distributed generation unit is adjusted by a rate of change proportional to the difference between the maximum unit current and the initial unit current of the subordinate distributed generation unit. Next, in operations 438 and 440, the current increment and the starting function ftc are calculated using formulas (7) and (8). In operation 442, the subordinate distributed generation unit determines whether the starting function ftc is less than 0. If the starting function ftc... 0. The subordinate distributed generation unit broadcasts the first electrical signal FS to initiate the next master unit selection process (operation 444). If the start function ftc is less than 0, the subordinate distributed generation unit determines whether the current error is within an error range (operation 446).

[0049] If the current error is within the error range, method 400 returns to operation 442 to determine if the startup function ftc is less than 0. If the current error exceeds the error range, method 400 returns to operation 434. At this point, because the virtual impedance has changed (e.g., by a rate of change proportional to the difference between the maximum unit current and the initial unit current of the subordinate distributed generation unit), the current unit current will also change, causing the current error, current increment, and startup function ftc to all change. Method 400 will continue to adjust the virtual impedance to adjust the current unit current of the subordinate distributed generation unit until the startup function ftc reaches the first criterion (i.e., ftc > 0). 0), and initiate the next master unit selection process through the first electrical signal FS, or until the current error is within the error range.

[0050] Figure 5A is a timing diagram of the first electrical signal FS representing the start event of master unit selection in different distributed generation units according to an embodiment of this disclosure. Electrical signals FS1, FS2, and FS3 are the first electrical signals FS identified by a first distributed generation unit, a second distributed generation unit, and a third distributed generation unit, respectively, all located in the same microgrid. As shown in Figure 5A, electrical signals FS1, FS2, and FS3 do not overlap. Specifically, the first electrical signal FS is selected only when the start function ftc is not less than 0 (ftc... The signal FS will only be broadcast when the signal is 0), and all other distributed generation units will pause their current attenuation factor adjustment process and initiate the next master unit selection upon receiving the first signal FS. In other words, only one distributed generation unit will broadcast the first signal FS.

[0051] Figure 5B is a timing diagram of the second electrical signal FR in different distributed generation units according to an embodiment of the present disclosure. Electrical signals FR1, FR2, and FR3 are the second electrical signals FR identified by the first, second, and third distributed generation units, respectively, which are located in the same microgrid. Unlike electrical signals FS1, FS2, and FS3 in Figure 5A, as shown in Figure 5B, electrical signals FR1, FR2, and FR3 may overlap.

[0052] The master unit selection process requires only one timing delay. However, the virtual impedance may be adjusted multiple times to achieve the desired shunt performance. Therefore, some distributed generation units may require more adjustments, while others may require fewer, resulting in different levels (e.g., logic 0 or 1) for the second electrical signal FR in each distributed generation unit. Furthermore, because the frequency of virtual impedance adjustment is the same throughout the microgrid, all distributed generation units requiring virtual impedance adjustment will adjust simultaneously, causing the second electrical signal FR in different distributed generation units to overlap.

[0053] Figure 5C is a timing diagram of the first electrical signal FS and the second electrical signal FR in the same distributed power generation unit according to an embodiment of this disclosure. Taking the first distributed power generation unit in Figures 5A and 5B as an example, because the attenuation coefficient adjustment process is always executed after the main unit selection process, the electrical signal FR1 also changes to a high level after the electrical signal FS1 becomes high. In other words, the electrical signals FS1 and FR1 will not overlap. In addition, a delay constant (e.g., Td0_1, Td0_2) is added to the delay time of a distributed power generation unit, further ensuring the time interval between the main unit selection process and the attenuation coefficient adjustment process in the same shunt adjustment process. Therefore, a sufficient time period is provided to prevent the second electrical signal FR from immediately becoming high after the first electrical signal FS becomes high, causing identification errors between different electrical signals.

[0054] According to the strategy described above, only one distributed generation unit (DGU) will broadcast the first electrical signal FS to initiate the master unit selection process, and only one DGU (i.e., the master DGU) will broadcast the maximum unit current (or communication data) to other subordinate DGUs. Furthermore, the next master unit selection process will only be initiated when the initiation function ftc meets a first criterion. Compared to the conventional periodic initiation strategy, this further reduces the required communication resources. In other words, in the method disclosed herein, only one DGU transmits communication data to other DGUs within the same shunt control process. Simultaneously, this method further introduces an event-driven initiation mechanism to reduce the number of power adjustments performed before the required shunt performance is achieved.

[0055] Although the invention has been described by way of example and according to preferred embodiments, it should be understood that the invention is not limited to the disclosed embodiments. Rather, it is intended to cover various modifications and similar arrangements (which will be apparent to those skilled in the art). Therefore, the appended claims should be given the broadest interpretation to cover all such modifications and similar arrangements.

[0056] 100: Microgrid System 110, 120, 200, 300: Distributed power generation units 112, 122, 212, 312: Circuit blocks 114, 124, 220, 320: Digital signal processors 116,126,214,314: Electricity meters 130, 210, 310: Control Area Network Buses 140: DC bus RL1, RL2: Line impedance Ipu1, Ipu2, Imax pu1, pu2: Unit current FS,FR,Fstg,FS1,FS2,FS3,FR1,FR2,FR3: Electrical signals S1, S2: Control signals 230, 330: Main Unit Selection Module 232,332: Timers 234,334: Delay time generator 240, 340: Attenuation coefficient adjustment module 242,342: Start the controller 244,344: Regulator 250, 350: Communication Buses 260, 360: Internal current and voltage adjustment module TE: Start Signal RV1, RV2: Virtual impedance Td1, Td2: Delay time Td0_1, Td0_2: Delay constants ER: Error range VN1, VN2: Rated voltage 400: Method 402~446,424a: Operation 500a, 500b, 500c: Timing Diagram

Claims

1. A distributed power generation unit, comprising: An electricity meter is configured to produce a first unit current in a circuit block based on a first rated power value of one of the distributed power generation units; And a processor coupled between a control area network bus and the circuit block, wherein the processor is configured to cause the distributed power generation unit to: identify a first electrical signal at an initial time and execute a master unit selection procedure, wherein the master unit selection procedure includes: determining a first delay time based on the first unit current and a maximum unit current; Starting from the initial time, when the first delay time has elapsed, the first unit current is identified as the maximum unit current; and after the first unit current is identified as the maximum unit current, the maximum unit current and a second electrical signal are broadcast through the control area network bus; and after the second electrical signal is broadcast, an attenuation coefficient adjustment process is executed to maintain the current state of the distributed power generation unit.

2. The distributed power generation unit as described in claim 1, wherein the attenuation coefficient adjustment process includes: Through the meter, in response to the second electrical signal, a first initial unit current is generated in one of the distributed power generation units; Based on the first initial unit current and a first current, a first unit current increment is determined; based on the first unit current increment and the maximum unit current, a first difference is determined. And in response to the first difference reaching a first criterion, maintain the current state of the distributed power generation unit.

3. The distributed power generation unit as described in claim 2, wherein the first criterion is that the first difference is less than 0.

4. The distributed power generation unit as described in claim 2, wherein the first unit current increment is a difference between the first initial unit current and the first current, the first difference being a difference between the square of the first unit current increment and a multiple of the square of the maximum unit current.

5. The distributed power generation unit as described in claim 2, wherein the processor is further configured to cause the distributed power generation unit to: stop the attenuation coefficient adjustment process in response to the first electrical signal, and start the master unit selection process.

6. The distributed power generation unit as described in claim 1, wherein the processor includes: A master unit selection module is configured to generate the maximum unit current and the second electrical signal, and is configured to determine the first delay time in response to the first electrical signal based on the first unit current, the maximum unit current, and a first preset delay constant; and an attenuation coefficient adjustment module is configured to maintain a first virtual impedance of one of the distributed power generation units in response to the second electrical signal based on the maximum unit current, a first initial unit current, and a first current unit current, wherein the first initial unit current is generated in response to the second electrical signal; and wherein the processor receives the first unit current from the meter, transmits the maximum unit current to the control area network bus, and outputs a control signal to the circuit block to maintain a first current power value of one of the distributed power generation units.

7. The distributed power generation unit as claimed in claim 6, wherein before the first unit current is identified as the maximum unit current, the first preset delay constant is added to a multiple of a difference between the first unit current and the maximum unit current to produce the first delay time.

8. The distributed power generation unit as described in claim 6, wherein the processor further comprises: A communication module is configured to broadcast the first electrical signal, the second electrical signal, and the maximum unit current through the control area network bus.

9. The distributed power generation unit as described in claim 6, wherein the processor further comprises: An internal current and voltage regulation module is configured to receive the first unit current, the first virtual impedance, and the first rated power value to generate the control signal to maintain the first current power value of the distributed power generation unit.

10. The distributed generation unit as claimed in claim 1, wherein the processor is further configured to cause the distributed generation unit to: broadcast the maximum unit current and the second electrical signal (FR) to a parallel distributed generation unit via the control area network bus, wherein the parallel distributed generation unit has a second delay time that is longer than the first delay time.

11. The distributed power generation unit as claimed in claim 10, wherein the second delay time is generated based on a second unit current of a second rated power value of one of the parallel distributed power generation units and the maximum unit current before the first unit current is determined to be the maximum unit current.

12. The distributed generation unit as claimed in claim 11, wherein before the first unit current is identified as the maximum unit current, a second delay constant is added to a multiple of a difference between the second unit current and the maximum unit current to produce the second delay time.

13. The distributed generation unit as described in claim 10, wherein after identifying the first electrical signal, the parallel distributed generation unit is configured to: execute the master unit selection procedure, the master unit selection procedure further comprising: The second delay time is determined based on the second unit current of one of the parallel distributed power generation units, wherein the second unit current is based on the second rated power value of one of the parallel distributed power generation units; and before the second delay time has elapsed, starting from the initial time, the second electrical signal representing a master unit selection end event is received, and the master unit selection process is suspended; and the attenuation coefficient adjustment process is implemented, and after the second electrical signal is broadcast, the second current power value of one of the parallel distributed power generation units is adjusted.

14. The distributed generation unit as described in claim 13, wherein the parallel distributed generation unit performs the attenuation coefficient adjustment process in response to the second electrical signal, including: In response to the second electrical signal, a second initial unit current is generated in one of the parallel distributed power generation units; Based on the second initial unit current and a second current unit current, a second unit current increment is determined; based on the second unit current increment and the maximum unit current, a second difference is determined. And in response to the second difference reaching a second criterion, the second current power value of the parallel distributed power generation unit is adjusted.

15. The distributed generation unit as claimed in claim 14, wherein the parallel distributed generation unit is further configured to: adjust a second virtual impedance at a rate of change proportional to a third difference between the second initial unit current and the maximum unit current to adjust the second current power value of the parallel distributed generation unit; determine a current error between the second unit current and the maximum unit current; and, in response to the second difference not meeting the second criterion, or in response to the current error being within a preset error range, suspend the operation of adjusting the second virtual impedance, and broadcast the first electrical signal through the control area network bus.

16. The distributed power generation unit as described in claim 15, wherein the second criterion is that the second difference is less than 0.

17. The distributed generation unit as claimed in claim 14, wherein the second unit current increment is a difference between the second initial unit current and the second current, the second difference being a difference between the square of the second unit current increment and a multiple of the square of the maximum unit current.

18. The distributed generation unit as described in claim 1, wherein the master unit selection process further includes: In response to the first electrical signal, a third electrical signal with a first criterion is generated to initiate the operation of determining the first delay time; And after the second signal is broadcast, a third signal with a second bit is generated to initiate the attenuation coefficient adjustment process.

19. A distributed power generation unit, comprising: An electricity meter is configured to generate a unit current for a circuit block based on the rated power value of one of the distributed generation units; and a processor is coupled between a control area network bus and the circuit block, wherein the processor is configured to cause the distributed generation unit to: identify a first electrical signal at an initial time and implement a master unit selection procedure, wherein the master unit selection procedure includes: determining a delay time based on the unit current and a maximum unit current; receiving a second electrical signal through the control area network bus before the delay time has elapsed, starting from the initial time; suspending the master unit selection procedure in response to the second electrical signal; and implementing an attenuation coefficient adjustment procedure in response to the second electrical signal to adjust the current power value of one of the distributed generation units.

20. The distributed generation unit as described in claim 19, wherein the attenuation coefficient adjustment process includes: Through the meter, in response to the second electrical signal, an initial unit current is generated for one of the distributed power generation units; Based on the initial unit current and a current unit current, determine a unit current increment; based on the unit current increment and the maximum unit current, determine a first difference; And in response to the first difference reaching a first criterion, the current power value of the distributed power generation unit is adjusted.

21. The distributed power generation unit as described in claim 20, wherein the first criterion is that the first difference is less than 0.

22. The distributed generation unit as described in claim 20, wherein the attenuation coefficient adjustment process further includes: A virtual impedance is adjusted by a rate of change proportional to a second difference between the initial unit current and the maximum unit current to adjust the current power value of the distributed generation unit; a current error between the current unit current and the maximum unit current is determined; And in response to the first difference not meeting the first criterion, or in response to the current error being within a preset error range, the operation of adjusting the virtual impedance is suspended, and the first electrical signal is broadcast through the control area network bus.

23. The distributed generation unit as described in claim 20, wherein the unit current increment is a difference between the initial unit current and the current unit current, the first difference being a difference between the square of the unit current increment and a multiple of the square of the maximum unit current.

24. The distributed generation unit as described in claim 19, wherein the master unit selection process further includes: In response to the first electrical signal, a third electrical signal with a first criterion is generated to initiate the operation of determining the delay time; And after receiving the second electrical signal, a third electrical signal with a second bit is generated to initiate the attenuation coefficient adjustment process.

25. A microgrid system, comprising: A control area network bus; a plurality of meters configured to generate a first unit current based on a first rated power value of one of a first distributed generation units, and configured to generate a second unit current based on a second rated power of one of a second distributed generation units; and a plurality of processors coupled to the control area network bus and configured to: identify a first electrical signal representing a master unit selection start event; determine a first delay time based on the first unit current and a maximum unit current, and determine a second delay time based on the second unit current and the maximum unit current; identify the first unit current as the maximum unit current if the first delay time is less than the second delay time; and broadcast the maximum unit current and a second electrical signal representing a master unit selection end event from the first distributed generation unit through the control area network bus. The second distributed power generation unit receives the second electrical signal and the maximum unit current through the control area network bus; and after the second electrical signal is broadcast, the first distributed power generation unit performs an attenuation coefficient adjustment process to maintain a first current power value of the first distributed power generation unit, and the second distributed power generation unit performs the attenuation coefficient adjustment process to adjust a second current power value.

26. The microgrid system as described in claim 25, wherein the operation of causing the first distributed generation unit to perform the attenuation coefficient adjustment process includes: In response to the second electrical signal, a first initial unit current is generated in one of the first distributed power generation units; Based on the first initial unit current and a first current, a first unit current increment is determined; based on the first unit current increment and the maximum unit current, a first difference is determined; in response to the first difference reaching a first criterion, a first virtual impedance of one of the first distributed power generation units is maintained to maintain the first current power value.

27. The microgrid system as described in claim 26, wherein the first criterion is that the first difference is less than 0.

28. The microgrid system as claimed in claim 26, wherein the first unit current increment is a difference between the first initial unit current and the first current, the first difference being a difference between the square of the first unit current increment and a multiple of the square of the maximum unit current.

29. The microgrid system as described in claim 25, wherein the operation of causing the second distributed generation unit to perform the attenuation coefficient adjustment process includes: In response to the second electrical signal, a second initial unit current is generated in one of the second distributed power generation units; Based on the second initial unit current and the second current, a second unit current increment is determined; based on the second unit current increment and the maximum unit current, a second difference is determined; in response to the second difference reaching a second criterion, the second current power value of the second distributed power generation unit is adjusted.

30. The microgrid system as described in claim 29, wherein the second criterion is that the second difference is less than 0.

31. The microgrid system as described in claim 29, wherein the operation of causing the second distributed generation unit to perform the attenuation coefficient adjustment process includes: A second virtual impedance is adjusted by a rate of change proportional to a third difference between the second initial unit current and the maximum unit current to adjust the second current power value; a current error between the second current unit current and the maximum unit current is determined; And in response to the second difference not meeting the second criterion, or in response to the current error being within a preset error range, the operation of adjusting the second virtual impedance is suspended, and the first electrical signal is broadcast from the second distributed power generation unit through the control area network bus.

32. The microgrid system as claimed in claim 29, wherein the second unit current increment is a difference between the second initial unit current and the second current, the second difference being a difference between the square of the second unit current increment and a multiple of the square of the maximum unit current.

33. The microgrid system as described in claim 25, wherein the master unit selection process further includes: Using the first distributed power generation unit or the second distributed power generation unit, a third electrical signal having a first criterion and representing an attenuation coefficient adjustment end event is generated to initiate the operation of determining the first delay time and the second delay time in response to the first electrical signal; and the first distributed power generation unit is used to generate the third electrical signal having a second criterion to initiate the operation of the second distributed power generation unit executing the attenuation coefficient adjustment process after the second electrical signal representing the master unit selection end event is broadcast.

34. A current shunt control method in a microgrid system, comprising: A first unit current is generated based on a first rated power value of one of the first distributed generation units, and a second unit current is generated based on a second rated power of one of the second distributed generation units; at an initial time, a first electrical signal representing a master unit selection start event is identified; after identifying the first electrical signal, a first delay constant is added to a multiple of the difference between the first unit current and a maximum unit current to determine a first delay time, and a second delay constant is added to a multiple of the difference between the second unit current and a maximum unit current to determine a second delay time; since the first delay time has elapsed earlier than the second delay time, the first unit current is identified as the maximum unit current; starting from the initial time, when the first delay time has elapsed, the maximum unit current and a second electrical signal representing a master unit selection end event are broadcast; In response to the second electrical signal, a third electrical signal with a first criterion is generated to maintain the first current power value of one of the first distributed power generation units and adjust the second current power value of the second distributed power generation unit.

35. The shunt control method in a microgrid system as described in claim 34, further comprising: The virtual impedance of one of the second distributed power generation units is changed at a rate proportional to the difference between the maximum unit current and the initial unit current of one of the second distributed power generation units, wherein the initial unit current is generated in response to the broadcast of the second electrical signal; a first difference between the current unit current of one of the second distributed power generation units and the initial unit current is determined; a second difference between the square of the first difference and a multiple of the square of the maximum unit current is determined; a current error between the current unit current of the second distributed power generation unit and the maximum unit current is determined; and the first electrical signal is generated in response to the current error being within a preset error range, or in response to the second difference reaching a criterion, to suspend the operation of maintaining the first current power value and adjusting the second current power value. And in response to the first electrical signal, a third electrical signal with a second bit is generated to initiate the operation of determining the first delay time and the second delay time.

36. The shunt control method in a microgrid system as described in claim 35, wherein the criterion is that the second difference is not less than 0.

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