An improved secondary frequency control method for island mode cascaded microgrid

Abstract

The present application relates to the technical field of micro-grid control, especially to an improved secondary frequency control method for island mode cascaded micro-grid, and the specific steps are as follows: for the off-grid mode cascaded micro-grid, in the case that the system load changes, the droop control with fixed droop coefficient is used to adjust power distribution, and the system has frequency deviation; in order to avoid frequency deviation when the load is switched, the secondary control needs to be introduced to realize delta omega = 0; according to this target, the output power characteristics of each CVS module in the cascaded system are related to the load characteristics, and are obtained through low bandwidth communication of adjacent cascaded CVS modules. The present application is used in island mode cascaded micro-grid under high voltage and large current occasions, keeps the stability of frequency during the system load switching process, and finally obtains high power quality.

Description

Technical Field

[0001] The present invention relates to the field of microgrid control technology, and in particular to an improved secondary frequency control method for an island mode cascade microgrid. Background Art

[0002] In island mode, using traditional P-ω droop control for a cascaded structure, each cascaded CVS module adjusts its active power output, Pi, by adjusting its own phase, θi, and its reactive power output, Qi, by adjusting its own voltage amplitude, Vi, thereby achieving power balance across all cascaded submodules. However, when the system load fluctuates, using traditional droop control with a fixed droop coefficient for power distribution adjustment can lead to frequency deviation and fluctuation.

[0003] The specific deficiencies of existing technologies can be solved according to Figure 1 Understanding analysis:

[0004] In off-grid mode, there is a frequency deviation when using traditional droop control. The specific analysis is as follows: Figure 1 As shown in Figure 2. Assume that point a is the system's initial steady-state operating point, with coordinates (P*, ω*). When the load increases or decreases, the steady-state operating point moves from point a (P*, ω*) to point b (Pb, ωb) or point c (Pc, ωc), respectively. For resistive-inductive loads, increasing the load increases the system's operating frequency; decreasing the load decreases it. For resistive-capacitive loads, increasing the load decreases the system's operating frequency; decreasing the load increases it. Analysis reveals that under load changes, the system's new operating point always deviates from the system's initial steady-state point a, with the system's operating frequency shifting above and below point ω*. Summary of the Invention

[0005] The purpose of the present invention is to solve the shortcomings of the prior art and to propose an improved secondary frequency control method for an island mode cascaded microgrid.

[0006] In order to achieve the above object, the present invention adopts the following technical solutions:

[0007] An improved secondary frequency control method for cascaded microgrids in island mode is designed. The specific steps are as follows:

[0008] For off-grid cascaded microgrids, when the system load changes, droop control with a fixed droop coefficient is used to adjust power distribution, resulting in system frequency deviation. To avoid frequency offset during load switching, secondary control is introduced to achieve Δω = 0. Based on this goal and considering that the output power characteristics of each CVS module in the cascaded system are related to the load characteristics, the secondary frequency control design is constructed as follows:

[0009] (11)

[0010] Where: Δωi is the correction term of the i-th CVS module controller, which is obtained through low-bandwidth communication between adjacent cascaded CVS modules;

[0011] Preferably, the Δωi correction term is designed as:

[0012] (12)

[0013] Where: d i is the correction coefficient of the i-th correction term; a ki Represents the communication link status between CVS module k and CVS module i, a ki =1 indicates that there is a communication link, a ki =10 means there is no communication link; n represents the number of CVS modules that actually have communication links with CVS module i; P k and Q k are the active power and reactive power output by the kth CVS module respectively.

[0014] Preferably, in formula (12), when Q k = 0, there is a singular point in equation (12), at which point the system will be unstable. To avoid system singularity, the correction coefficient d i Perform the following processing:

[0015] (13)

[0016] Where: c i is a control coefficient, always positive, and equal to the droop coefficient m i ; Q min is a small positive constant.

[0017] Preferably, it is set according to the actual operating requirements of the system to better avoid singularity and ensure good dynamic performance. Q min Cannot be too small, to simplify the analysis, Q min Set to 1% of the rated power of the CVS module.

[0018] The present invention proposes an improved secondary frequency control method for an island mode cascade microgrid, which has the beneficial effect of maintaining frequency stability during system load switching in an island mode cascade microgrid under high voltage and high current conditions, thereby ultimately achieving high power quality. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 This is a schematic diagram of frequency deviation under resistive-inductive and resistive-capacitive load conditions for an improved secondary frequency control method for an island mode cascaded microgrid proposed by the present invention.

[0020] Figure 2 This is a frequency recovery diagram of an improved secondary frequency control method for an island mode cascaded microgrid proposed in the present invention under resistive-inductive and resistive-capacitive load conditions. DETAILED DESCRIPTION

[0021] The technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, rather than all the embodiments.

[0022] Reference Figure 2 , an improved secondary frequency control method for cascaded microgrid in island mode, the specific steps are as follows:

[0023] For off-grid cascaded microgrids, when the system load changes, droop control with a fixed droop coefficient is used to adjust power distribution, resulting in system frequency deviation. To avoid frequency offset during load switching, secondary control is introduced to achieve Δω = 0. Based on this goal and considering that the output power characteristics of each CVS module in the cascaded system are related to the load characteristics, the secondary frequency control design is constructed as follows:

[0024] (11)

[0025] Where: Δωi is the correction term of the i-th CVS module controller, which is obtained through low-bandwidth communication between adjacent cascaded CVS modules;

[0026] Among them, the Δωi correction term is designed as:

[0027] (12)

[0028] Where: d i is the correction coefficient of the i-th correction term; a ki Represents the communication link status between CVS module k and CVS module i, a ki =1 indicates that there is a communication link, a ki =10 means there is no communication link; n represents the number of CVS modules that actually have communication links with CVS module i; P k and Q k are the active power and reactive power output by the kth CVS module respectively.

[0029] In formula (12), when Q k = 0, there is a singular point in equation (12), at which point the system will be unstable. To avoid system singularity, the correction coefficient d i Perform the following processing:

[0030] (13)

[0031] Where: c i is a control coefficient, always positive, and equal to the droop coefficient m i ; Q min is a small positive constant; it is set according to the actual operating requirements of the system to better avoid singularity and ensure good dynamic performance. Q min Cannot be too small, to simplify the analysis, Q min Set to 1% of the rated power of the CVS module.

[0032] For off-grid cascade microgrids, secondary frequency control is used, which essentially moves the droop curve up and down according to the correction term. The frequency recovery characteristics under resistive-inductive and resistive-capacitive loads are as follows: Figure 2 As shown. Also assume that point a is the initial working point of the system, observe Figure 2 (a) It can be seen that for resistive and inductive loads, when the load is increased, the system operating frequency changes, and the steady-state operating point moves from point a to point c1, and under the action of secondary frequency control, the operating point moves to point c2; when the load is reduced, the system operating frequency also changes, and the operating point moves from point a to point b1, and finally under the action of secondary frequency control, the operating point also moves to point b2. When the resistive and capacitive loads change, the frequency recovery process can be based on Figure 2 (b) to analyze.

[0033] For off-grid cascaded microgrids, traditional droop control with a fixed droop coefficient adjusts power distribution when system loads fluctuate, resulting in system frequency deviation. To prevent frequency offset when switching inductive or resistive-capacitive loads, secondary control is introduced to achieve zero frequency deviation when the system is stable. Furthermore, during load fluctuations, the output frequency of each CVS module remains consistent, ensuring high power quality.

[0034] The above description is only a preferred specific embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any technician familiar with the technical field, within the technical scope disclosed by the present invention, who makes equivalent replacements or changes based on the technical solution and inventive concept of the present invention, should be covered by the scope of protection of the present invention.

Claims

1. An improved secondary frequency control method for an island mode cascaded microgrid, characterized in that: The specific steps are as follows: For off-grid cascaded microgrids, when the system load changes, droop control with a fixed droop coefficient is used to adjust power distribution, resulting in system frequency deviation. To avoid frequency offset during load switching, secondary control is introduced to achieve Δω = 0. Based on this goal and considering that the output power characteristics of each CVS module in the cascaded system are related to the load characteristics, the secondary frequency control design is constructed as follows: （ 11 ） Where: Δωi is the correction term of the i-th CVS module controller, which is obtained through low-bandwidth communication between adjacent cascaded CVS modules; Among them, the Δωi correction term is designed as: （ 12 ） Where: d i is the correction coefficient of the i-th correction term; a ki Represents the communication link status between CVS module k and CVS module i, a ki =1 indicates that there is a communication link, a ki =10 means there is no communication link; n represents the number of CVS modules that actually have communication links with CVS module i; P k and Q k are the active power and reactive power output by the kth CVS module respectively.

2. The improved secondary frequency control method for the island mode cascaded microgrid according to claim 1, characterized in that: In formula (12), when Q k = 0, there is a singular point in equation (12), at which point the system will be unstable. To avoid system singularity, the correction coefficient d i Perform the following processing: （ 13 ） Where: c i is a control coefficient, always positive, and equal to the droop coefficient m i ; Q min Set to 1% of the rated power of the CVS module.

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