Power optimization method for flexible interconnection of low-voltage side of distribution transformer

Abstract

The present application relates to a kind of power optimization method of distribution transformer low voltage side flexible interconnection, comprising, by SNOP, the low voltage side of adjacent transformer is interconnected;Determine the standard deviation of substation load rate, transformer comprehensive power loss and transformer comprehensive power loss rate as the reference target of power coordination optimization model;With minimum substation load rate standard deviation and transformer comprehensive power loss minimum as target, power coordination optimization model is constructed;Determine whether the optimization objective function and constraint condition of power coordination optimization control model or feeder fault recovery strategy model are judged feeder fault;Analytic hierarchy process is used to process objective function;The result is output and the output power of each port of SNOP is regulated and controlled.The present application is solved by solving power coordination optimization control model, in guaranteeing substation load balancing regulation and control, effectively avoids the light and heavy load operation of substation main transformer, solves the problem that each substation load cannot be balanced in prior art, and the problem of endangering the safe operation of distribution network.

Description

Technical Field

[0001] This invention relates to the field of flexible interconnection system technology, and in particular to a power optimization method for flexible interconnection on the low-voltage side of a distribution transformer. Background Technology

[0002] Against the backdrop of promoting renewable energy substitution, the large-scale, high-penetration integration of distributed energy sources into distribution systems presents new challenges to traditional distribution networks. Traditional distribution networks typically employ a "closed-loop design, open-loop operation" mode. Under normal operation, the power flow in each distribution substation is naturally distributed, making it impossible to actively balance the load. Extreme load imbalance can jeopardize the safe and stable operation of the distribution network. With the large-scale integration of distributed power sources and the widespread adoption of fluctuating loads, transformers in distribution substations often experience light or heavy load operation, increasing overall transformer losses. When a feeder fault occurs, traditional distribution substations are usually equipped with normally open tie switches to transfer power to the faulty substation. However, these normally open tie switches cannot regulate power, further exacerbating the load imbalance problem. Therefore, traditional distribution networks cannot solve problems such as load balancing between substations, light or heavy load operation of main transformers, and fault transfer.

[0003] Soft Normally Open Point (SNOP) switches can effectively replace traditional mechanical switches. They are power electronic devices that flexibly interconnect distribution substations via DC buses. SNOPs integrated into the distribution system can reduce power outage time. In the event of a fault, they can interrupt the fault current through interlocking and, in conjunction with relay protection devices, quickly clear the fault. SNOPs can also transfer power to faulty substations, aiming to minimize the power loss in the faulty substation. However, traditional SNOPs do not consider the load balance of the substations and the overall power loss of the transformer when implementing this transfer objective.

[0004] The flexible interconnection system for distribution substations is formed by interconnecting DC and AC distribution substations using a SNOP (Signal-Only Power Adapter). By controlling the power of voltage source converters, it coordinates and allocates various dispatchable source load resources in the flexible interconnection system to avoid problems such as load imbalance in substations and light or heavy loads on transformers. Currently, it is not possible to simultaneously achieve control objectives such as load balancing in substations, avoiding light or heavy load operation of main transformers, and considering the access of new energy sources and rapid power restoration in substations with feeder faults. Each substation in the system carries different loads, and before the SNOP is connected, it is impossible to balance the loads of each substation. In severe cases, this can endanger the safe operation of the distribution network. Summary of the Invention

[0005] To address this issue, the present invention provides a power optimization method for flexible interconnection on the low-voltage side of distribution transformers, which overcomes the problem in the prior art that the load of each transformer area cannot be balanced, thus endangering the safe operation of the distribution network.

[0006] To achieve the above objectives, the present invention provides a power optimization method for flexible interconnection on the low-voltage side of a distribution transformer, comprising:

[0007] Step S1: Interconnect the low-voltage sides of adjacent transformers via SNOP;

[0008] Step S2: Determine the standard deviation η of the load rate of the transformer area, the comprehensive power loss of the transformer, and the comprehensive power loss rate of the transformer as the reference targets for the power coordination optimization model;

[0009] Step S3: Determine if the feeder is faulty. If the feeder is faulty, set the objective function and constraints of the feeder fault recovery strategy model. If the feeder is not faulty, set the objective function and constraints of the power coordination optimization control model.

[0010] Step S4: Use the analytic hierarchy process (AHP) to process the objective function of the feeder fault recovery strategy model or the objective function of the power coordination optimization control model, in order to achieve the transformation from multiple objectives to a single objective;

[0011] Step S5: Output the optimization scheme and adjust the output power of each port of SNOP.

[0012] Furthermore, the standard deviation η of the load rate of the transformer area can be expressed as:

[0013]

[0014]

[0015]

[0016] In the formula: α i β is the load factor of transformer area i; β is the average load factor of transformer area i; P aLi P represents the load power of transformer area i; VSCi For the converter power connected to transformer area i, the inflow to SNOP is taken as positive and the outflow as negative; P Ni The active power capacity transmitted by the transformers within the i-th distribution area.

[0017] Furthermore, the calculation formulas for the overall power loss of the transformer and the overall power loss rate of the transformer are as follows:

[0018] ΔP Zi =P 0Zi +δ i 2 P KZi

[0019]

[0020] In the formula: P0Zi P represents the no-load loss of the transformer. KZi Indicates the rated load power loss of the transformer; ΔP Zi The total power loss of the transformer; δ i For transformer load factor; S Ni Indicates the rated capacity of the transformer; ΔP Zi % represents the overall power loss rate of the transformer. This is the power factor angle.

[0021] Furthermore, the objective functions of the power coordination optimization control model include minimizing feeder load balancing f1 and minimizing overall transformer power loss f2:

[0022] f1 = min(η)

[0023] f2=min(ΔP Zi )

[0024] Furthermore, the constraints of the power coordination optimization control model include,

[0025] Optimal economic operating constraints for transformers;

[0026] Feeder power balance constraints:

[0027] P aci -P aLi =P VSCi

[0028] SNOP execution constraints:

[0029]

[0030] SNOP capacity constraints:

[0031]

[0032] In the formula: P DC For combined photovoltaic and energy storage output power, P represents the average capacity of the converters at each port of the SNOP. aci The active power of transformer area i.

[0033] Furthermore, the constraints of the feeder fault power supply recovery model include the transformer optimal economic operation constraints, the SNOP operation constraints, and the SNOP capacity constraints, and also include...

[0034] Power balance constraints for faulty feeders:

[0035] P acj -P aLj =P VSCj

[0036] PaLi +P VSCi =0

[0037] Uninterruptible power supply constraints for critical loads:

[0038] P VSCi +P aLi，Im ≤0

[0039] In the formula: P aLk Let P be the load power of the faulty transformer area k. VSCk P represents the power of the converter connected to the faulty transformer area k. aLk，Im For the critical load in faulty transformer area k, P aLj For the normal load power of transformer area j, P VSCj P represents the power of the converter connected to the normal distribution area j. acj The active power of normal transformer substation j.

[0040] Furthermore, the formula for the Analytic Hierarchy Process (AHP) is:

[0041] f=min(λ1f1′+λ2f2′+λ3f3′)

[0042] In the formula: λ1, λ2, and λ3 are the weight coefficients corresponding to the optimization objective function of the power coordination optimization control model or the optimization objective function of the feeder fault recovery strategy model, respectively; f1′, f2′, and f3′ are the normalized values ​​of f1, f2, and f3, respectively, used to eliminate the influence of the different orders of magnitude and dimensions of the optimization objective functions of each power coordination optimization control model or each feeder fault recovery strategy model on the optimization results.

[0043] Compared with the prior art, the beneficial effects of the present invention are that it interconnects the low-voltage sides of adjacent transformers through SNOP, allowing power to flow bidirectionally through each port of the SNOP, thereby achieving the purpose of balancing the load of each distribution area; and it achieves the goal of minimizing the standard deviation of the distribution area load rate η and minimizing the overall transformer power loss ΔP. Zi A power coordination optimization model was constructed to achieve the objective, and the relationship curve between the transformer's comprehensive power loss rate and the load rate was plotted. Considering the differences in the optimal economic operating range of different transformers, the economic operating range of the transformer in the distribution area, the power balance constraint of the feeder, the SNOP operation constraint, and the SNOP capacity constraint were used as constraints. The model was solved with the control objectives of feeder load balancing and minimizing the comprehensive power loss of the transformer. While ensuring the load balancing regulation of the distribution area, the model effectively avoided the light and heavy load operation of the main transformer in the distribution area, and improved the economy of the system. Attached Figure Description

[0044] Figure 1 This is a schematic diagram of the overall process of the power optimization method for flexible interconnection on the low-voltage side of a distribution transformer according to an embodiment of the present invention;

[0045] Figure 2 The waveform of the transformer load rate before power coordination optimization control is shown in this embodiment of the invention.

[0046] Figure 3 The waveform diagram of transformer load rate after power coordination optimization control in an embodiment of the present invention is shown.

[0047] Figure 4 This is a schematic diagram comparing the standard deviation of load rates in the front and back areas using power coordination optimization control in an embodiment of the present invention.

[0048] Figure 5 The above is a waveform diagram of the transformer load rate before adopting the feeder fault recovery strategy model in an embodiment of the present invention.

[0049] Figure 6 The image shows the transformer load rate waveform after adopting the feeder fault recovery strategy model in this embodiment of the invention.

[0050] Figure 7 This is a schematic diagram comparing the standard deviation of load rates in the front and back areas of the feeder fault recovery strategy model according to an embodiment of the present invention. Detailed Implementation

[0051] To make the objectives and advantages of the present invention clearer, the present invention will be further described below with reference to embodiments; it should be understood that the specific embodiments described herein are merely for explaining the present invention and are not intended to limit the present invention.

[0052] Preferred embodiments of the present invention will now be described with reference to the accompanying drawings. Those skilled in the art should understand that these embodiments are merely illustrative of the technical principles of the present invention and are not intended to limit the scope of protection of the present invention.

[0053] It should be noted that in the description of this invention, the terms "upper", "lower", "left", "right", "inner", "outer", etc., which indicate directions or positional relationships, are based on the directions or positional relationships shown in the accompanying drawings. This is only for the convenience of description and is not intended to indicate or imply that the device or element must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, it should not be construed as a limitation of this invention.

[0054] Furthermore, it should be noted that, in the description of this invention, unless otherwise explicitly specified and limited, the terms "installation," "connection," and "linking" should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral connection; they can refer to a mechanical connection or an electrical connection; they can refer to a direct connection or an indirect connection through an intermediate medium; and they can refer to the internal connection of two components. Those skilled in the art can understand the specific meaning of the above terms in this invention according to the specific circumstances.

[0055] Please see Figure 1 The diagram shows the overall flow of the power optimization method for flexible interconnection on the low-voltage side of a distribution transformer. The specific steps are as follows:

[0056] Step S1: Interconnect the low-voltage sides of adjacent transformers through SNOP. Power can flow bidirectionally through each port of SNOP to achieve the purpose of balancing the load of each distribution area.

[0057] Step S2: Determine the standard deviation η of the load rate of the transformer area, the comprehensive power loss of the transformer, and the comprehensive power loss rate of the transformer as the reference targets for the power coordination optimization model;

[0058] Step S3: Determine if the feeder is faulty. If the feeder is faulty, set the objective function and constraints of the feeder fault recovery strategy model. If the feeder is not faulty, set the objective function and constraints of the power coordination optimization control model.

[0059] Step S4: Use the analytic hierarchy process (AHP) to process the objective function of the feeder fault recovery strategy model or the objective function of the power coordination optimization control model, in order to achieve the transformation from multiple objectives to a single objective;

[0060] Step S5: Output the optimization scheme and adjust the output power of each port of SNOP.

[0061] Furthermore, this invention uses the standard deviation of the transformer area load rate to measure the load balance of the transformer area. The smaller the standard deviation, the higher the load balance of the transformer area. The standard deviation η of the main transformer area load rate can be expressed as:

[0062]

[0063]

[0064]

[0065] In the formula: α i β is the load factor of transformer area i; β is the average load factor of transformer area i; P aLi P represents the load power of transformer area i; VSCi For the converter power connected to transformer area i, the inflow to SNOP is taken as positive and the outflow as negative; P Ni The active power capacity transmitted by the transformers within the i-th distribution area.

[0066] Furthermore, SNOP interconnects the low-voltage sides of transformers, making line losses negligible. As key equipment in each distribution area, the main transformer in each area accounts for the majority of the area's losses, approximately 70%. Moreover, the overall transformer loss is closely related to the transformer load rate. Flexible power matching can significantly reduce transformer losses, which is of great importance to the entire system. Therefore, it is necessary to consider the differences in economic operation of transformers of different models and capacities. GB / T13462 "Economic Operation of Power Transformers" introduces the concepts of overall transformer power loss and overall power loss rate, calculated using the following formula:

[0067] ΔP Zi =P 0Zi +δ i 2 P KZi

[0068]

[0069] In the formula: P 0Zi P represents the no-load loss of the transformer. KZi Indicates the rated load power loss of the transformer; ΔP Zi The total power loss of the transformer; δ i For transformer load factor; S Ni Indicates the rated capacity of the transformer (kVA); ΔP Zi % represents the overall power loss rate of the transformer.

[0070] Furthermore, it is necessary to determine whether the feeder is faulty. If the feeder is not faulty, during normal system operation, both the load balancing of the distribution area and the overall power loss of the transformer must be considered. The minimum standard deviation of the distribution area load rate η and the minimum overall power loss of the transformer ΔP should be used as the criteria. Zi To achieve the goal, a power coordination optimization model is constructed. The objective function of the power coordination optimization control model is:

[0071] f1 = min(η)

[0072] f2=min(ΔP Zi )

[0073] The constraints of the power coordination optimization control model include transformer optimal economic operation constraints, feeder power balance constraints, SNOP operation constraints, and SNOP capacity constraints. Among these, the transformer optimal economic operation constraint is given by: (Equation omitted for brevity) A curve showing the relationship between the transformer's overall power loss rate and load rate can be plotted. This curve is a concave function; both excessively low and excessively high load rates in each transformer substation will increase the transformer's loss rate. Furthermore, the optimal economic operating range varies between different transformer series. Taking the S9 series transformer as an example, its optimal economic operating range is [0.25, 0.75]. Therefore, the optimal economic operating constraints for the transformer can be derived:

[0074]

[0075] Feeder power balance constraints:

[0076] P aci -P aLi =P VSCi

[0077] SNOP execution constraints:

[0078]

[0079] SNOP capacity constraints:

[0080]

[0081] In the formula: P DC For combined photovoltaic and energy storage output power, The capacity of each port converter in the SNOP;

[0082] The model is solved with constraints such as the economic operating range of transformers in the distribution area, power balance constraints of feeders, SNOP operating constraints and SNOP capacity constraints. The control objectives are to minimize the standard deviation of the load rate of the distribution area and minimize the comprehensive power loss of the transformer. While ensuring the balanced regulation of the distribution area load, the model effectively avoids the operation of the main transformer in the distribution area under light or heavy load, and improves the economy of the system. The optimal economic operating constraint of the transformer only considers the transformers in the distribution area where no fault has occurred.

[0083] If a feeder fault occurs, the objective function and constraints of the feeder fault recovery strategy model are set. SNOP (Single-Stage Operator) integration into the distribution system can reduce power outage time. In the event of a fault, it can quickly clear the fault by blocking the fault current and cooperating with relay protection devices. SNOP can also transfer power to the faulty transformer area, aiming to minimize the power loss in the faulty transformer area. When implementing the power transfer objective, the load balance of the transformer area and the overall power loss of the transformer should also be considered. Therefore, taking a fault in transformer area i as an example, the objective function of the feeder fault recovery strategy model is: minimum feeder load balance f1, minimum overall transformer power loss f2, and minimum power loss in the faulty transformer area f3, where f3 = min(P... aLi +P VSCi );

[0084] After the fault in transformer area i is cleared, the system topology changes, and the constraints that the feeder fault recovery strategy model must satisfy need to be modified. Therefore, the constraints of the feeder fault power supply recovery model include:

[0085] Power balance constraints for faulty feeders:

[0086] P acj -P aLj =P VSCj

[0087] P aLi +P VSCi =0

[0088] In addition, it should be considered that the power supply to important loads within the fault area cannot be interrupted, and an uninterrupted power supply constraint for important loads should be introduced:

[0089] P VSCi +P aLi，Im ≤0

[0090] In the formula: P aLi，Im For important loads in transformer area i; the SNOP capacity constraint and the optimal economic operation constraint of the transformer are the same as those in the power coordination optimization control model, where the optimal economic operation constraint of the transformer only considers transformers in transformer areas where no faults have occurred.

[0091] Furthermore, both optimization models are multi-objective optimization models. The Analytic Hierarchy Process (AHP) is used to handle the objective function of the feeder fault recovery strategy model or the power coordination optimization control model, thus transforming multiple objectives into a single objective. The AHP formula is as follows:

[0092] f=min(λ1f′1+λ2f′2+λ3f′3)

[0093] In the formula: λ1, λ2, and λ3 are the weight coefficients corresponding to the optimization objective function; f′1, f′2, and f′3 are the normalized values ​​of f1, f2, and f3, that is, the values ​​after transformation to the interval [0, 1], which are used to eliminate the influence of the different orders of magnitude and dimensions of the optimization objective function of each power coordination optimization control model or each feeder fault recovery strategy model on the optimization results;

[0094] The objective functions during normal operation are minimizing the standard deviation of the transformer load rate and minimizing the overall power loss of the transformer. This paper considers objective function 1 and objective function 2 to be of equal importance, and sets the weight vector for each objective as follows:

[0095] [λ1 λ2]=[0.5 0.5]

[0096] When a feeder fault occurs and the system operates in the load transfer phase, the objective function includes minimizing the power loss in the fault area, minimizing the standard deviation of the load factor in the distribution area, and minimizing the overall power loss of the transformer. Among these, minimizing the power loss in the fault area is more important than the other two objectives, which are considered equally important. Based on this, a discrimination matrix is ​​obtained, and after normalization, the weight vectors of each objective can be obtained:

[0097] [λ1 λ2 λ3]=[0.478 0.350 0.172]

[0098] By solving the objective function, the active power reference values ​​of each converter in the multi-state switch are obtained, and then the system is coordinated and controlled.

[0099] The system's multi-mode operation can be divided into normal operation mode and fault operation mode, with corresponding control strategies of power coordination optimization strategy and feeder fault power supply recovery strategy, respectively. The former is used to achieve load balancing and economical operation of the main transformer, while the latter, compared to the former, also needs to consider load transfer to achieve the control objective of minimizing the power loss in the faulty transformer area. The optimization strategy can not only effectively dispatch photovoltaic output to participate in system power mutual assistance, but also eliminates the need to switch control strategies when a feeder fault occurs. Virtual synchronous machine control technology can effectively provide support for the faulty line and ensure power supply to important loads.

[0100] like Figure 2 and Figure 3 The figures show the transformer load rate waveforms before and after power coordination optimization control, respectively. Before power coordination optimization control, the transformer was stuck in a light-heavy load operation. After power coordination optimization control, the load rate of the main transformer in each distribution area can be kept in the optimal economic operating range.

[0101] like Figure 4 As shown, the standard deviation of the load rate of the distribution substations before and after power coordination optimization control is compared. Before optimization, the load distribution of each distribution substation was unbalanced, which would endanger the safe operation of the power grid. After optimization, the standard deviation of the load rate of the distribution substations was significantly reduced, which effectively improved the load balance of the distribution substations and ensured the safe and economical operation of the flexible interconnection system.

[0102] like Figure 5 and Figure 6 The figures show the transformer load rate waveforms before and after adopting the feeder fault recovery strategy model, respectively. After the fault occurs, the main transformer load rate of transformer area 2 drops to 0, all loads in transformer area 2 lose power, transformer areas 1 and 3 are not regulated, and the main transformers in the non-faulty transformer areas have serious problems with light and heavy loads. After adopting the control strategy in this paper, the load of transformer area 2 is reasonably distributed to transformer areas 1 and 3 for load transfer, effectively avoiding the transformer from falling into light and heavy load operation.

[0103] like Figure 7 As shown, the standard deviation of the load rate of the transformer substations is compared before and after the feeder fault recovery strategy model. When the transformer substation 2 fails, the load supplied by transformer substations 1 and 3 to transformer substation 2 can still be kept at a relatively low level. It can be seen that the method in this paper can maintain the load balance of the transformer substations at a relatively high level while ensuring the economic operation of the transformer and the absence of power loss for important loads.

[0104] The technical solution of the present invention has been described above with reference to the preferred embodiments shown in the accompanying drawings. However, it will be readily understood by those skilled in the art that the scope of protection of the present invention is obviously not limited to these specific embodiments. Without departing from the principles of the present invention, those skilled in the art can make equivalent changes or substitutions to the relevant technical features, and the technical solutions after these changes or substitutions will all fall within the scope of protection of the present invention.

[0105] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A power optimization method for flexible interconnection on the low-voltage side of a distribution transformer, characterized in that, include, Step S1: Interconnect the low-voltage sides of adjacent transformers via SNOP; Step S2: Determine the standard deviation of the load rate η of the transformer substation and the comprehensive power loss Δ of the transformer. The overall power loss rate of the transformer is a reference target for the power coordination optimization model; Step S3: Determine if the feeder is faulty. If the feeder is faulty, set the objective function and constraints of the feeder fault recovery strategy model. If the feeder is not faulty, set the objective function and constraints of the power coordination optimization control model. Step S4: The Analytic Hierarchy Process (AHP) is used to process the objective function of the feeder fault recovery strategy model or the objective function of the power coordination optimization control model, in order to achieve the transformation from multiple objectives to a single objective. The AHP formula is as follows: f=min( + + ); Step S5: Output the optimization scheme and adjust the output power of each port of the SNOP; Set the weight vectors for each objective. The objective function of the power coordination optimization control model is to minimize feeder load balancing. Minimizes overall power loss of transformer : =min(n); =min(Δ) ); The objective function of the feeder fault recovery strategy model is to minimize feeder load balancing. Minimize the overall power loss of the transformer Minimum power loss in faulty distribution areas ,in, =min( ); in, , , These are the weighting coefficients corresponding to the objective function of the power coordination optimization control model or the objective function of the feeder fault recovery strategy model, respectively. , , They are respectively , , The normalized value is used to eliminate the influence of differences in the order of magnitude and dimensions of the objective functions of the power coordination optimization control models or the feeder fault recovery strategy models on the optimization results. Let i be the load power of transformer area i; The power of the converter connected to transformer area i.

2. The power optimization method for flexible interconnection on the low-voltage side of a distribution transformer according to claim 1, characterized in that, The standard deviation of the load rate of the transformer area, η, is expressed as: or ; = ； = ； In the formula: Let i be the load rate of the transformer area. The average load factor of the transformer area; inflow to SNOP is taken as positive, and outflow as negative; The active power capacity transmitted by the transformers within the i-th distribution area.

3. The power optimization method for flexible interconnection on the low-voltage side of a distribution transformer according to claim 2, characterized in that, The calculation formulas for the transformer's overall power loss and the transformer's overall power loss rate are as follows: D = + ; D % 100%; In the formula: This indicates the no-load loss of the transformer; Indicates the rated load power loss of the transformer; Δ This refers to the total power loss of the transformer; Transformer load factor; Indicates the rated capacity of the transformer; Δ % represents the overall power loss rate of the transformer. This is the power factor angle.

4. The power optimization method for flexible interconnection on the low-voltage side of a distribution transformer according to claim 3, characterized in that, The constraints of the power coordination optimization control model include: Optimal economic operating constraints for transformers; Feeder power balance constraints: - = ； SNOP execution constraints: + =0； SNOP capacity constraints: ≤ ； In the formula: For combined photovoltaic and energy storage output power, For the capacity of each port converter in SNOP, The active power of transformer area i.

5. The power optimization method for flexible interconnection on the low-voltage side of a distribution transformer according to claim 4, characterized in that, The constraints of the feeder fault recovery strategy model include the transformer optimal economic operation constraints, the SNOP operation constraints, and the SNOP capacity constraints, and also include... Power balance constraints for faulty feeders: - = ； + =0； Uninterruptible power supply constraints for critical loads: + ≤0； In the formula: Let k be the load power of the faulty transformer area. The power of the converter connected to the faulty transformer area k. For the critical load in faulty transformer area k, This represents the normal load power of transformer substation j. The power of the converter connected to the normal distribution area j. The active power of normal transformer substation j.

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