A weakly connected microgrid and off-grid switching control strategy based on a dynamic calculation method of shelvable loads

Abstract

The present invention discloses a weakly connected microgrid and an off-grid switching control strategy based on a dynamic calculation method for cuttable loads. The strategy can calculate the cuttable load more accurately and quickly, and the implementation method is relatively simple, with strong real-time performance and flexibility. It can effectively reduce the amount of over-cutting and under-cutting of loads, improve economic efficiency, and effectively ensure the power balance, frequency stability and power supply reliability of the microgrid in the isolated grid mode. The strategy is combined with actual engineering applications, and through theoretical research and simple and practical technical means, it realizes the off-grid fast switching control of weakly connected microgrids without increasing a large amount of investment, thereby improving the reliability of power supply. At present, there are a large number of similar areas in China. The promotion and application of this technology can save a lot of investment while improving the reliability of power supply of the power grid, and provide power support for the economic construction and development of remote areas.

Description

Technical Field

[0001] The present invention belongs to the technical field of microgrid control, and in particular relates to a weakly connected microgrid and an off-grid switching control strategy based on a dynamic calculation method of a shelvable load. Background Art

[0002] A microgrid is a small power system typically used to supply power to a specific region or remote area. It is usually composed of distributed power sources, energy storage, loads, and control systems. It generally operates in two modes:

[0003] In the first mode, microgrids connect to the main grid via tie-line switches for power exchange. In the second mode, when a grid fault occurs, the microgrid disconnects from the main grid by disconnecting the tie-line switches, entering isolated grid operation. In some remote mountainous areas, the harsh climate along the power lines and frequent natural disasters pose a serious threat to the safe, reliable, and stable operation of the interconnecting overhead lines. Therefore, microgrids are deployed in areas with concentrated power loads. When a tie-line fault occurs and trips, the microgrid switches from grid-connected to isolated grid operation to ensure reliable power supply for production and daily life in these areas. Due to natural constraints and a lack of wind and solar resources, microgrids in these areas are characterized by relatively simple loads, primarily powered by small hydropower, supplemented by diesel generators, and lacking energy storage systems. Due to the long dynamic response time and slow regulation speed of small hydropower, how to achieve rapid and stable microgrid switching in the absence of energy storage systems in the event of a tie-line fault is a critical issue. To ensure that the switching process does not significantly impact the main grid and that the microgrid operates safely and stably, a sound control strategy must be implemented to achieve smooth transitions between the two operating modes.

[0004] Currently, microgrid on-grid / off-grid switching control strategies primarily include droop control, constant voltage-constant frequency control, constant power control, and virtual synchronous generators (VSGs). Droop control, due to its advantages such as parallel connection without communication lines, voltage and frequency support, and unified control structure across different operating modes, has become a major research focus for microgrid switching control strategies. However, research on droop control has primarily focused on microgrid structures with typical micro-sources such as wind, solar, diesel, and energy storage. Limited research has been conducted on switching control strategies for microgrids that include small hydropower. Furthermore, switching control strategies for microgrids lacking energy storage systems generally include power or load shedding strategies.

[0005] Therefore, the main problems currently existing in weakly connected microgrids are: the interconnected lines are long and the environment is complex, which makes it easy for line failures to cause power outages. Especially during the dry season, if a permanent failure occurs in the interconnecting line, resulting in off-grid switching, due to the poor regulation ability of small hydropower, the microgrid's ability to withstand power in the isolated grid mode is weak, and power supply reliability is difficult to guarantee. Summary of the Invention

[0006] The purpose of the present invention is to overcome the problems of slow response of small hydropower in weakly connected microgrids in the prior art, limited regulation capability of diesel generators, lack of energy storage systems, etc. The present invention proposes a weakly connected microgrid and an off-grid switching control strategy based on a dynamic calculation method of cuttable loads. When a tie line fails, the control method is adopted to coordinate the start and stop of power supply and output as well as load shedding, thereby ensuring that the microgrid system can achieve stable and rapid off-grid switching.

[0007] The technical solution adopted in the present invention is as follows:

[0008] A weakly-connected microgrid, the key of which is: including a main line connected to multiple distributed power sources, the main line having multiple cuttable loads connected in parallel via distribution branch lines, and the main line being weakly connected to the large power grid via tie lines; and also including a control module, current sampling circuits and voltage sampling circuits being installed on the distribution branch lines and tie lines, all of the current sampling circuits and voltage sampling circuits being respectively connected to the control module, the control module being used to collect current and voltage of corresponding lines and to calculate corresponding tie line power and distribution branch line power in real time, and the control module being further used to control off-grid switching of the weakly-connected microgrid.

[0009] As a preferred solution, the distributed power source includes small hydropower and diesel generators, wherein the small hydropower is the main power source and the diesel generator is the cold standby power source.

[0010] As a preferred solution, the distribution branch line is provided with an intelligent circuit breaker corresponding to each of the cuttable loads, and the intelligent circuit breaker is used to control the switching of the corresponding cuttable load.

[0011] An off-grid switching control strategy based on a dynamic calculation method for shelvable loads includes the following steps:

[0012] S1. Use current and voltage sampling circuits to collect and obtain the current and voltage of the tie line and distribution branch line in real time;

[0013] S2. The real-time power of the tie line is calculated based on the current and voltage obtained in step S1, which is P. L , the real-time power corresponding to each distribution branch line is

[0014] S3. Connect each distribution branch line to the corresponding Sort from small to large to get the power sequence

[0015] S4. From Start accumulating, and when the accumulation reaches the mth item (1≤m≤n), you will get a value that is the same as P L The summed power P with the smallest absolute value of the difference T :

[0016]

[0017] S5. According to the real-time power P of the tie line L The positive or negative value of determines the direction of the tie line current and calculates the power value:

[0018] ΔP=P T -P L

[0019] S6. If P L When P < 0, reduce the output of distributed small hydropower. L >0, such as P L >P T Start the diesel generator, such as |P L When |<δ, it enters the isolated grid mode and gradually recovers the cuttable load. L >0, such as P L <P T , small hydropower automatically switches to active power control mode, and the reduction value is ΔP=P T -P L , start the diesel engine, enter the isolated grid mode, and gradually restore the cuttable load, where 0<δ<0.1*Max(P L ) is the preset threshold.

[0020] As a preferred solution, the specific determination steps of step S5 are:

[0021] When P L When >0, the large power grid supplies power to the weakly connected microgrid, indicating that the output of the distributed generation is insufficient. It is necessary to increase the output of the distributed generation or cut off the load. However, since the small hydropower is in the maximum power generation mode when the weakly connected microgrid is connected to the grid and cannot continue to increase its output, the only strategy is to cut off the load.

[0022] According to step S4 Send disconnection command to the corresponding intelligent circuit breaker to complete the power distribution branch removal. and (1≤m+1≤n) and P L When the absolute value of the difference is less than the set threshold γ, considering the economy and power supply reliability, the load is cut as little as possible, so the output of distributed generation is increased;

[0023] When P L When <0, the weakly connected microgrid supplies power to the large power grid, indicating that the output of distributed generation exceeds the power load demand, and it is necessary to reduce the output of distributed generation or increase the power load. However, considering the actual situation of the weakly connected microgrid, it is only necessary to reduce the output of distributed generation.

[0024] As a preferred solution, when PL >0 and P L >P T When the load is under-shedded, it indicates that there is a load under-shedding situation. At this time, the diesel generator is started and works in the active power control mode. The shelved load can be gradually restored. The weakly connected microgrid enters the isolated grid mode and operates stably.

[0025] When P L >0 and P L <P T When the load is over-cut, the small hydropower station automatically switches to the active power control mode, reducing the output. The reduced power value is ΔP, where ΔP=P T -P L Then start the diesel generator and work in the active power control mode, gradually restore the cut-off load, and the weakly connected microgrid enters the isolated grid mode and operates stably.

[0026] As a preferred solution, when P L When <0, the small hydropower automatically switches to the active power control mode and reduces the output. The reduced power value is ΔP, where ΔP=P L , the weakly connected microgrid enters the isolated grid mode and operates stably.

[0027] Beneficial effects: The weakly-connected microgrid and the off-grid switching control strategy based on the dynamic calculation method of the cuttable load of the present invention can calculate the cuttable load more accurately and quickly, and the implementation method is relatively simple, with strong real-time and flexibility; it can effectively reduce the amount of over-cutting and under-cutting of loads, improve economy, and effectively ensure the power balance, frequency stability and power supply reliability of the microgrid in the isolated grid mode; this strategy is combined with actual engineering applications, through theoretical research and simple and practical technical means, to achieve off-grid rapid switching control of weakly-connected microgrids without increasing a large amount of investment, thereby improving the reliability of power supply; there are currently a large number of similar areas in China, and the promotion and application of this technology can save a lot of investment while improving the reliability of power supply of the power grid, and provide power support for the economic construction and development of remote areas. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 Schematic diagram of the weak-link microgrid structure of the present invention;

[0029] Figure 2 Schematic diagram of the flow chart of the weakly connected microgrid on-grid and off-grid switching control method of the present invention;

[0030] Figure 3 1 is a curve showing the change in active power and frequency of small hydropower in scenario 1 of the embodiment;

[0031] Figure 4 The active power change curve and frequency change curve of small hydropower in scenario 2 in the embodiment;

[0032] Figure 5 2. The curves of active power change and frequency change of small hydropower in scenario 3 in the embodiment;

[0033] Figure 6 2. The active power change curve and frequency change curve of small hydropower in scenario 4 in the embodiment;

[0034] Figure 7 The active power and frequency change curves of the small hydropower when the weakly connected microgrid performs off-grid switching in the dry season in the embodiment;

[0035] Figure 8 The active power and frequency change curves of the diesel generator when the weakly connected microgrid performs off-grid switching in a small way during the dry season in the embodiment. DETAILED DESCRIPTION

[0036] The present invention will be further described in detail below with reference to the embodiments and accompanying drawings:

[0037] Example: As shown in the attached Figure 1 As shown, a weakly connected microgrid includes a main line connected to multiple distributed power sources, the main line is connected to multiple cuttable loads in parallel through distribution branch lines, and the main line is weakly connected to the large power grid through a tie line; it also includes a control module, and current sampling circuits and voltage sampling circuits are installed on the distribution branch lines and the tie line. All the current sampling circuits and voltage sampling circuits are respectively connected to the control module. The control module is used to collect the current and voltage of the corresponding line and calculate the corresponding tie line power and distribution branch line power in real time. The control module is also used to control the off-grid switching of the weakly connected microgrid.

[0038] In specific implementation, the distributed power source includes small hydropower and diesel generators, wherein the small hydropower is the main power source and the diesel generator is the cold standby power source; the distribution branch line is respectively provided with an intelligent circuit breaker (Q F1 -Q Fn ), the intelligent circuit breaker is used to control the switching of the corresponding cuttable load.

[0039] As attached Figure 2 As shown, an off-grid switching control strategy based on a dynamic calculation method for shelvable loads includes the following steps:

[0040] S1. Use current and voltage sampling circuits to collect and obtain the current and voltage of the tie line and distribution branch line in real time;

[0041] S2. The real-time power of the tie line is calculated based on the current and voltage obtained in step S1, which is P. L , the real-time power corresponding to each distribution branch line is

[0042] S3. Connect each distribution branch line to the corresponding Sort from small to large to get the power sequence

[0043] S4. From Start accumulating, and when the accumulation reaches the mth item (1≤m≤n), you will get a value that is the same as P L The summed power P with the smallest absolute value of the difference T :

[0044]

[0045] S5. According to the real-time power P of the tie line L The positive or negative value of determines the direction of the tie line current and calculates the power value:

[0046] ΔP=P T -P L

[0047] S6. If P L When P < 0, reduce the output of distributed small hydropower. L >0, such as P L >P T Start the diesel generator, such as |P L When |<δ, it enters the isolated grid mode and gradually recovers the cuttable load. L >0, such as P L <P T , small hydropower automatically switches to active power control mode, and the reduction value is ΔP=P T -P L , start the diesel engine, enter the isolated grid mode, and gradually restore the cuttable load, where 0<δ<0.1*Max(P L ) is a preset threshold value, which can be set according to test or simulation results.

[0048] The specific determination steps of step S5 are as follows:

[0049] When P L When >0, the large power grid supplies power to the weakly connected microgrid, indicating that the output of the distributed generation is insufficient. It is necessary to increase the output of the distributed generation or cut off the load. However, since the small hydropower is in the maximum power generation mode when the weakly connected microgrid is connected to the grid and cannot continue to increase its output, the only strategy is to cut off the load.

[0050] According to step S4 Send disconnection command to the corresponding intelligent circuit breaker to complete the power distribution branch removal. and (1≤m+1≤n) and P L When the absolute value of the difference is less than the set threshold γ, considering the economy and power supply reliability, the load is cut as little as possible, so the output of distributed generation is increased;

[0051] When P L When <0, the weakly connected microgrid supplies power to the large power grid, indicating that the output of distributed generation exceeds the power load demand, and it is necessary to reduce the output of distributed generation or increase the power load. However, considering the actual situation of the weakly connected microgrid, it is only necessary to reduce the output of distributed generation.

[0052] In specific implementation, the specific operation mode of step S5 is as follows:

[0053] When P L >0 and P L >P T When the load is under-shedded, it indicates that there is a load under-shedding situation. At this time, the diesel generator is started and works in the active power control mode. The shelved load can be gradually restored. The weakly connected microgrid enters the isolated grid mode and operates stably.

[0054] When P L >0 and P L <P T When the load is over-cut, the small hydropower station automatically switches to the active power control mode, reducing the output. The reduced power value is ΔP, where ΔP=P T -P L Then, the diesel generator is started and works in the active power control mode. The removed load is gradually restored, and the weakly connected microgrid enters the isolated grid mode and operates stably.

[0055] When P L When <0, the small hydropower automatically switches to the active power control mode and reduces the output. The reduced power value is ΔP, where ΔP=P L , the weakly connected microgrid enters the isolated grid mode and operates stably.

[0056] This embodiment uses a weakly connected microgrid in a mountainous area of Yunnan as an example for off-grid switching. The weakly connected microgrid includes Hydropower Station No. 1 and Hydropower Station No. 2. Hydropower Station No. 1 includes two small hydropower units with a single unit rated power of 320 kW and two diesel generators with a single unit rated power of 1200 kW and a capacity of 1500 kVA. Hydropower Station No. 2 includes three small hydropower units with a single unit rated power of 320 kW. The power factor of all small hydropower units in Hydropower Station No. 1 and Hydropower Station No. 2 is set to 0.9; the load power factor is set to 0.95; the 35 kV interconnected line model is LGJ-150, with a total length of 52 km; the 20 kV overhead line model is LGJ-70, with a total length of 82.2 km; and the 0.4 kV low-voltage distribution line model is LGJ-35, with a total length of 31.75 km.

[0057] When the weakly connected microgrid in the area performs a large-scale off-grid switching during the flood season, the five small hydropower stations are in the maximum power generation mode, and the excess power is transmitted to the large power grid. After the off-grid switching enters the isolated grid mode due to the fault of the interconnection line, the output of the small hydropower is too large, and the output will be automatically reduced through droop control to match the load power consumption. At this time, there is no need to activate the diesel generator, and only the small hydropower can be used to provide power. The study sets the load power in the weakly connected microgrid system in the area to 1013.17kW. For the convenience of analysis, the influence of the power transmission size of the interconnection line on the transient process of the off-grid switching and the operation of the weakly connected microgrid after the off-grid switching are simulated in 4 scenarios. Different small hydropower outputs are set in the 4 scenarios, and different interconnection line transmission powers are obtained:

[0058] Scenario 1: The active power of the small hydropower plant is set to 300kW, and the power transmitted through the tie line is 405.48kW;

[0059] Scenario 2: The active power of the small hydropower plant is set to 280kW, and the power transmitted through the tie line is 303.45kW;

[0060] Scenario 3: The active power of the small hydropower plant is set to 260kW, and the power transmitted through the tie line is 205.17kW;

[0061] Scenario 4: The active power of the small hydropower plant is set to 240 kW, and the power transmitted through the interconnection line is 106.74 kW.

[0062] The active power change curves and frequency change curves of small hydropower in the above four scenarios are shown in the attached figure. Figure 3-6 As shown in the figure, it can be seen that after the tie line switch is disconnected at 20s, the small hydropower can quickly reduce its output to match the load power consumption and achieve power balance in all four scenarios; after the tie line switch is disconnected, the frequency of the small hydropower will suddenly rise, and the frequency peaks and stabilization times in the four scenarios are summarized in Table 1.

[0063] Table 1. Frequency peak and stabilization time under four scenarios

[0064]

[0065] Table 1 shows that the frequency peak decreases as the tie-line power transmission decreases, and the stabilization speed gradually accelerates. After the off-grid handover, the frequency stabilizes to below 50.5 Hz. The weakly connected microgrid in this region eventually enters an isolated grid mode and operates stably, with the small hydropower supply meeting all internal load demands. However, this also indicates that the tie-line power transmission should not be too high, otherwise it will lead to large frequency fluctuations during the handover process.

[0066] When the weakly connected microgrid in the region switched to off-grid operation during the dry season, the five small hydropower units were still operating at maximum power under the grid-connected dry season mode. However, due to insufficient output from the small hydropower units, power was required to be fed from the larger grid to meet the internal loads of the weakly connected microgrid. After the off-grid switchover entered isolated mode due to a tie line failure, the small hydropower units were insufficient, resulting in a significant power shortfall and the need to activate diesel generators, with both the small hydropower units and the diesel generators providing power. At this point, an off-grid switchover control strategy based on a dynamic calculation method for shelvable loads was implemented to ensure the off-grid switchover and stabilize the microgrid's operation after the switchover. In order to facilitate analysis, a simulation study will be conducted on the transient process of off-grid switching and the operation of the microgrid after off-grid switching using an off-grid switching control strategy based on a dynamic calculation method for off-grid loads under the condition of maximum power transmission within the tie line. The study sets the load power within the weakly connected microgrid system in the region to 1777.78kW and the output of small hydropower to 179kW. Through the flow calculation, it can be seen that the power transmission within the tie line is 972.45kW. From 0 to 5s, the small hydropower is used as the main power source, the tie line switch is disconnected at 5s, and the 10 distribution branches obtained by calculation are cut off at 5.2s. The active power and frequency change curves of the small hydropower and the active power and frequency change curves of the diesel generator are obtained (as shown in the attached figure). Figure 7 and attached Figure 8 shown).

[0067] By the attached Figure 7 From the curve of active power change of small and medium hydropower stations, we can see that due to overload, the active power of small hydropower stations automatically drops to 170kW to match the load power consumption; Figure 7 The frequency change curve of small and medium hydropower stations shows that the frequency fluctuation of small hydropower stations is small, fluctuating within the range of 50.4-49.5Hz. After the diesel generator is put into operation at 30s, the disconnected power distribution branches are gradually restored every 15s in the order of power from small to large during the period of 45-180s. Figure 8 From the active power change curve of the diesel generator, it can be seen that the diesel generator can quickly and gradually restore the cut-off distribution branch after it is put into use; Figure 8 The diesel generator frequency curve shows minimal fluctuations, ranging from 50.05 to 49.7 Hz. The weakly connected microgrid in the region eventually entered stable isolated grid mode, with the entire microgrid's load being met by a combination of small hydropower and diesel generators.

[0068] Finally, it should be noted that the above description is only a preferred embodiment of the present invention. Under the guidance of the present invention, ordinary technicians in this field can make various similar expressions without violating the purpose and claims of the present invention. Such changes fall within the scope of protection of the present invention.

Claims

1. An off-grid switching control strategy based on a dynamic calculation method for shelvable loads, characterized in that: The following steps are involved: S1. Use current and voltage sampling circuits to collect and obtain the current and voltage of the tie line and distribution branch line in real time; S2. The real-time power of the tie line is calculated based on the current and voltage obtained in step S1, which is P. L , the real-time power corresponding to each distribution branch line is S3. Connect each distribution branch line to the corresponding Sort from small to large to get the power sequence S4. From P1 * Start accumulating, and when the accumulation reaches the mth item, and 1≤m≤n, you will get a value that is the same as P L The summed power P with the smallest absolute value of the difference T : S5. According to the real-time power P of the tie line L The positive or negative value of determines the direction of the tie line current and calculates the power value: ΔP=P T -P L S6. If P L <0, reduce the output of distributed small hydropower, P L >0, such as P L >P T Start the diesel generator, such as |P L When |<δ, it enters the isolated grid mode and gradually recovers the cuttable load. L >0, such as P L <P T , small hydropower automatically switches to active power control mode, and the reduction value is ΔP=P T -P L , start the diesel engine, enter the isolated grid mode, and gradually restore the cuttable load, where 0<δ<0.1*Max(P L ) is the preset threshold.

2. The off-grid switching control strategy based on the dynamic calculation method of the shelvable load according to claim 1 is characterized in that: The specific determination steps of step S5 are: When P L When >0, the large power grid supplies power to the weakly connected microgrid, indicating that the output of the distributed generation is insufficient. It is necessary to increase the output of the distributed generation or cut off the load. However, since the small hydropower is in the maximum power generation mode when the weakly connected microgrid is connected to the grid and cannot continue to increase its output, the only strategy is to cut off the load. According to step S4 Send disconnection command to the corresponding intelligent circuit breaker to complete the power distribution branch removal. and With P L When the absolute value of the difference is less than the set threshold γ and 1≤m+1≤n, considering the economy and power supply reliability, the load is cut as little as possible, so the output of distributed generation is increased; When P L When <0, the weakly connected microgrid supplies power to the large power grid, indicating that the output of distributed generation exceeds the power load demand, and it is necessary to reduce the output of distributed generation or increase the power load. However, considering the actual situation of the weakly connected microgrid, it is only necessary to reduce the output of distributed generation.

3. The off-grid switching control strategy based on the dynamic calculation method of shelvable loads according to claim 2 is characterized by: When P L >0 and P L >P T When the load is under-shedded, it indicates that there is a load under-shedding situation. At this time, the diesel generator is started and works in the active power control mode. The shelved load can be gradually restored. The weakly connected microgrid enters the isolated grid mode and operates stably. When P L >0 and P L <P T When the load is over-cut, the small hydropower station automatically switches to the active power control mode, reducing the output. The reduced power value is ΔP, where ΔP=P T -P L Then start the diesel generator and work in the active power control mode, gradually restore the cut-off load, and the weakly connected microgrid enters the isolated grid mode and operates stably.

4. An off-grid switching control strategy based on a dynamic calculation method for shelvable loads according to any one of claims 2 or 3, characterized in that: When P L When <0, the small hydropower automatically switches to the active power control mode and reduces the output. The reduced power value is ΔP, where ΔP=P L , the weakly connected microgrid enters the isolated grid mode and operates stably.

5. A weakly connected microgrid, characterized by: It includes a trunk line connected to multiple distributed power sources, the trunk line is connected to multiple cuttable loads in parallel through distribution branch lines, and the trunk line is weakly connected to the large power grid through a tie line; It also includes a control module, and current sampling circuits and voltage sampling circuits are installed on the distribution branch lines and the tie lines. All the current sampling circuits and voltage sampling circuits are respectively connected to the control module. The control module is used to collect the current and voltage of the corresponding lines, and calculate the corresponding tie line power and distribution branch line power in real time. The control module is also used to control the off-grid switching of the weakly connected microgrid and execute the off-grid switching control strategy based on the dynamic calculation method of the cuttable load as described in any one of claims 1 to 4.

6. A weakly-connected microgrid according to claim 5, characterized in that: The distributed power source includes small hydropower and diesel generators, wherein the small hydropower is the main power source and the diesel generator is the cold standby power source.

7. The weakly-connected microgrid according to claim 5, characterized in that: The distribution branch line is provided with an intelligent circuit breaker corresponding to each of the cuttable loads, and the intelligent circuit breaker is used to control the switching of the corresponding cuttable load.

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