Prevention and control method for low-voltage transformer area power distribution network with participation of photovoltaic interaction in safety auxiliary service

Abstract

The invention discloses a prevention and control method for a low-voltage transformer area power distribution network with photovoltaic interaction participating in safety auxiliary service. Comprising the following steps: establishing an N-0 low-voltage transformer area security domain model based on total power constraint and split-phase access constraint of low-voltage transformer area distributed power generation and load; solving a security boundary through an analytical method, screening an effective boundary, and calculating a geometric security distance from an operation point to the boundary in real time; when the minimum safety distance reaches a starting threshold value, starting prevention control; the photovoltaic output is reduced by adjusting the output power of the photovoltaic inverter, each photovoltaic reduction amount is determined by taking the minimum total reduction amount as a target, and an operation point is transferred to a safer position; and when the safety distance of the operation point after adjustment reaches an end threshold value, the prevention control is ended. According to the invention, under the conditions of less distribution storage or no distribution storage and no flexible interconnection of the transformer area, the potential is adjusted by mining the transformer area and the distributed new energy, the safe operation under the high-permeability new energy is realized, and the power grid safety and the asset efficiency are improved.

Description

Technical Field

[0001] The present invention belongs to the field of safe operation of distribution networks in distribution transformer areas with photovoltaic access, and in particular relates to a preventive control method based on a security domain and combined with photovoltaic interactive participation in safety auxiliary services. Background Art

[0002] Distributed energy consumption is a key approach to developing new power systems with a high penetration of renewable energy. While distributed photovoltaics have experienced rapid growth in my country in recent years, high penetration rates of photovoltaics can lead to safety violations such as reverse power flow and voltage violations, posing significant challenges to the safe operation of distribution networks.

[0003] New smart distribution network equipment offers certain technical solutions to address these issues. One approach involves power electronics. For example, soft switching (SOP) allows for flexible interconnection of low-voltage substations. However, SOPs also require active power transmission, limiting their reactive power regulation capabilities. Furthermore, the cost of flexible interconnection equipment is high. Another approach involves deploying energy storage. However, this requires deploying energy storage in each substation, resulting in high investment costs, especially for large-scale deployment. Therefore, leveraging existing distribution network equipment to enable PV to actively participate in grid regulation may be a more economical approach.

[0004] Active participation of distributed photovoltaics in distribution network regulation primarily refers to the distribution network operator issuing control commands directly to distributed photovoltaics based on the grid's operating status. As the regulated object, photovoltaics directly respond to the regulation commands. This type of problem is typically modeled as an optimal power flow problem, using the network-wide voltage deviation as the objective function and the active and reactive power regulation of each node as the decision variables to solve the optimal power flow and address the issue of voltage and power flow safety violations caused by photovoltaics. These studies have improved voltage regulation efficiency while reducing investment costs through direct control of photovoltaics. However, these methods typically focus on the current state of the grid, resulting in only local or immediate voltage issues that cannot be addressed. They are unable to identify potential voltage violations in the grid and thus take proactive measures.

[0005] Reference [1] proposed the concept of preventive control of power systems. Preventive control refers to the control that is carried out to improve the safety margin and restore the power system to a safe state when it is in a state of alert due to some reasons (deterioration of the operating mode or disturbance) during normal operation. [1] For the preventive control of distribution network, existing research mainly focuses on building a self-healing control system architecture, but the results obtained lack specific control information. In the distribution network, since photovoltaics are more easily connected to the distribution transformer area with lower voltage levels, the safety crossing problem of high penetration photovoltaics in the back-end area is most prominent. However, there are currently only preventive control methods for medium and high voltage distribution networks. [2] .

[0006] [References]

[0007] [1]Dy-Liacco T E.Real-time computer control of power system[J].Proceedings of the IEEE,1974,62(7):884-891.

[0008] [2] Xiao Jun, He Qibo, Su Buyun. Safe and efficient operation mode of smart distribution network based on security domain[J]. Automation of Electric Power Systems, 2014, 38(19): 52-60.

[0009] [3] Chen Weihou. Two-stage algorithm for simplifying linear inequality systems[J]. Journal of Minzu University of China: Natural Science Edition, 1995(1): 23-28.

[0010] [4] Xiao Jun, Zhen Guodong, Wang Bo, et al. Safety distance of distribution network: definition and method [J]. Proceedings of the CSEE, 2017, 37(10): 2840-2851. Summary of the Invention

[0011] Existing methods generally rely on adding some kind of equipment to solve the problem of safety crossing the boundary, and the cost is relatively high for large-scale photovoltaic access areas. In addition, existing research generally focuses on the current state of the power grid and cannot discover the potential safety crossing risks of the power grid. For photovoltaic access distribution transformer areas, the present invention proposes a preventive control method based on the safety domain and combined with photovoltaic interactive participation in safety auxiliary services to solve the problem of reverse safety crossing the boundary in the distribution transformer area after photovoltaic access. The present invention analyzes the safety distance to provide a control scheme, and uses photovoltaic interactive response to make the operating point after control located within the safety domain, eliminating the hidden danger of reverse crossing the boundary; fully tap the regulation potential of the distribution network and distributed new energy in the area, achieve safe operation under high penetration of new energy, and reduce dependence on energy storage and flexible interconnection. The method of the present invention can discover potential safety risks in the low-voltage distribution network, take preventive control measures in advance, ensure the elimination of reverse crossing risks, and thus significantly improve the safety level and asset efficiency of the power grid.

[0012] The present invention proposes a preventive control method for low-voltage distribution network in which photovoltaic interactive participation in safety auxiliary service is carried out based on the safety domain and in combination with photovoltaic interactive participation in safety auxiliary service to perform preventive control on photovoltaic-connected distribution transformer areas. The method mainly includes: first, establishing an N-0 low-voltage area safety domain model according to the total power constraints of distributed generation and load in the low-voltage area and the power constraints of distributed generation and load in phase-connected area; then, solving the safety boundary of the N-0 low-voltage area safety domain by analytical method, screening out the effective safety boundary, and calculating the geometric safety distance from the operating point to the effective safety boundary in real time; judging whether the operating point is safe according to the minimum safety distance in the geometric safety distance; starting preventive control when the minimum safety distance meets the starting threshold; for the operating point at this moment, reducing the photovoltaic output by adjusting the output power of the photovoltaic inverter, and determining the reduction amount of each photovoltaic output with the minimum reduction of the total photovoltaic amount at this moment as the objective function, thereby transferring the operating point to a safer position; when the safety distance of the adjusted operating point meets the preventive control end threshold, this preventive control ends.

[0013] Furthermore, the preventive control method for the low-voltage distribution network of the present invention comprises the following specific steps:

[0014] Step 1: Establish the N-0 low-voltage distribution network security domain model shown in the following formula:

[0015]

[0016] Among them, Ω DSSRT Represents the N-0 safety zone of the distribution transformer area; W is the operating point of the area, W L is the load node part of the operating point, S Lx is the outflow power of the xth load node, n is the number of load nodes; W DG is the distributed generation node part of the operating point, S DGy is the injected power of the yth distributed generation node, m is the number of distributed generation nodes; the load / distributed generation node power adopts the apparent power amplitude, and the power flowing from the grid to the load is defined as positive, and the negative sign only indicates the direction; Θ represents the state space, which is defined as the set of all possible operating points;

[0017] and They represent the A phase line j in normal operation. A The collection of all downstream load nodes and distributed generation nodes; and They represent the B phase line j in normal operation. B The collection of all downstream load nodes and distributed generation nodes; and They represent the C phase line j in normal operation. CThe collection of all downstream load nodes and distributed generation nodes; and Represents A, B, C three-phase lines j A 、j B 、j C Capacity; B A 、B B and B C They are the three-phase line sets of A, B and C respectively;

[0018] Indicates low voltage line j φ capacity, Main transformer φ Capacity; B φ is the low voltage line collection, T φ is the collection of all main transformers; and Respectively represent the low voltage line j during normal operation φ The collection of all load nodes and distributed generation nodes connected to the distribution box downstream; and They represent the normal operation of the main transformer i φ The collection of all load nodes and distributed generation nodes connected to the distribution box downstream; and Indicates low voltage line j φ The set of all load nodes and distributed generation nodes connected to the ABC three-phase network after the downstream distribution box; and They represent the main transformer i in normal operation φ The set of all load nodes and distributed generation nodes connected to the ABC three-phase network after the downstream distribution box;

[0019] Step 2: Use analytical methods to solve the safety boundary of the N-0 low-voltage distribution network security domain, and screen out the effective safety boundary by eliminating redundant linear inequality constraints;

[0020] Step 3: Calculate the geometric security distance (GSD) from the operating point to the effective safety boundary in real time, as shown in the following formula:

[0021]

[0022] Among them, GSD j represents the safety distance from the operating point to the jth safety boundary; c represents the rated capacity of the main transformer or feeder; n is the number of monomials in the effective safety boundary polynomial; a x Represents the coefficient of each monomial; S x Indicates the load or distributed generation connected to the feeder;

[0023] Step 4. Select GSD j Minimum safety distance GSD min The criteria for judging whether the operating point is safe are as follows:

[0024] GSD min =min(GSD1,GSD2,...,GSD j )

[0025] If GSD min ≥0, indicating that the operating point is safe; if GSD min <0, indicating that the operating point has crossed the safety limit;

[0026] Step 5: Start preventive control at the moment t when the minimum safety distance meets the start threshold M0:

[0027] When the minimum safe distance GSD of the operating point min When it approaches 0, the system is about to cross the safety boundary. This moment needs to be marked and measures need to be taken in advance to prevent the crossing. The safety distance from the operating point to all safety boundaries is calculated. When the safety distance drops to a certain value, the distribution network initiates preventive control measures. This value is called the preventive control measure initiation threshold M0.

[0028] Use c p Represents the line capacity and main transformer capacity in the safety domain expression, which is called the coefficient related to capacity; the preventive control starting threshold M0 and c p There is a linear relationship between them, so the pth preventive control activation threshold is expressed as:

[0029] M0=ɑ startp c p

[0030] Among them, ɑ startp To prevent and control the startup coefficient, ɑ startp <1;

[0031] If the minimum safe distance at time t satisfies both conditions 1 and 2, preventive control measures are initiated immediately, and the time t is recorded:

[0032] Condition 1: GSD min (t)≤M0, indicating that the minimum safety distance at the current time t begins to reach the threshold for initiating preventive control measures;

[0033] Condition 2: GSD min (t)≤GSD min (t-1), indicating that the trend of the minimum safety distance is further reduced with the increase of photovoltaic output;

[0034] Step 6: For the operating point at time t, reduce the photovoltaic output by adjusting the output power of the photovoltaic inverter, thereby shifting the operating point to a safer position. The specific steps are as follows:

[0035] Step 6-1) Determine the boundary corresponding to the safety distance that triggers preventive control at time t; then, identify the photovoltaic objects that need to be adjusted based on the safety boundary;

[0036] Step 6-2) Determine the photovoltaic reduction amount ΔP of the photovoltaic object that needs to be adjusted k(t) :

[0037] P k(t) ′=P k(t) -ΔP k(t)

[0038] Among them, P k(t) The k-th photovoltaic output at time t before adjustment is known; ΔP k(t) represents the kth photovoltaic reduction amount at time t; P k(t) ' represents the kth photovoltaic output after adjustment at time t;

[0039] Step 6-3) Establish an optimization model to determine the photovoltaic reduction amount ΔP k(t) The optimal value of is to minimize the total photovoltaic power generation. The objective function is expressed as:

[0040]

[0041] Where ΔP k represents the kth PV reduction amount, m is the total number of PV nodes;

[0042] The objective function satisfies three constraints:

[0043] Constraint 1: Low-voltage substation security domain constraint, such as the N-0 low-voltage substation distribution network security domain model established in step 1;

[0044] Constraint 2: PV output adjustable range constraint: in, is the maximum curtailable amount of the k-th PV node;

[0045] Constraint three, safety distance constraint:

[0046] The boundary corresponding to the safety distance that triggers preventive control in step 6-1 should reach the preventive control end threshold M1 after the adjustment of the measures of PV interactive participation in safety auxiliary services. The preventive control end threshold M1 is related to the constant c related to the capacity. p There is a linear relationship between them, so the p-th preventive control end threshold M1 is expressed as:

[0047] M1=ɑendp c p

[0048] Among them, ɑ endp To prevent the end of the control coefficient, ɑ endp <ɑ startp <1;

[0049] Therefore, the safety distance constraint is expressed as: GSD(t) p '≥M1, where GSD(t) p ' represents the safety distance corresponding to the boundary of the safety distance that triggers the preventive control in step 6-1) after adjustment by the measures of the photovoltaic interactive participation safety auxiliary service;

[0050] If the adjusted operating point meets the above safety distance constraints, it means that the operating point at time t has been adjusted to a safer position to cope with the safety crossing problem that may be caused by the increase in photovoltaic output at the next moment;

[0051] Step 7: End of preventive control

[0052] When GSD min The minimum safety distance at the moment (t)≤M0 is adjusted to the operating point to meet GSD(t) p '≥M1, this preventive control is ended.

[0053] 3. The low-voltage distribution network preventive control method according to claim 1 is characterized in that after the current preventive control is completed, the process returns to step 3 to prepare for the next preventive control start-up.

[0054] Compared with the prior art, the present invention has the following beneficial effects:

[0055] The method of the present invention is aimed at distribution and transformation substations with photovoltaic access. Under conditions of little or no distribution storage and no flexible interconnection, it can fully tap the regulation potential of the substation distribution network and distributed new energy to achieve safe operation under high penetration of new energy, thereby improving the safety level and asset efficiency of the power grid. BRIEF DESCRIPTION OF THE DRAWINGS

[0056] Figure 1 This is the low-voltage station area topology structure in the embodiment of the present invention;

[0057] Figure 2 is the daily load power change of the substation area of ​​the present invention;

[0058] Figure 3 is the daily DG power variation in the substation area of ​​the present invention;

[0059] Figure 4 This is the minimum safe distance between the front and back areas for 24 hours for prevention and control of the present invention. DETAILED DESCRIPTION

[0060] Based on the safety domain and combined with photovoltaic interactive participation in safety auxiliary services, preventive control is carried out on the distribution transformer area connected to photovoltaics. The present invention proposes a preventive control method for low-voltage area distribution network with photovoltaic interactive participation in safety auxiliary services, which mainly includes: first, establishing an N-0 low-voltage area safety domain model according to the total power constraints of distributed generation and load in the low-voltage area and the power constraints of distributed generation and load in phase-connected areas; then, solving the safety boundary of the N-0 low-voltage area safety domain by analytical method, screening out the effective safety boundary, and calculating the geometric safety distance from the operating point to the effective safety boundary in real time; judging whether the operating point is safe according to the minimum safety distance in the geometric safety distance; starting preventive control when the minimum safety distance meets the starting threshold; for the operating point at this moment, reducing the photovoltaic output by adjusting the output power of the photovoltaic inverter, and determining the reduction amount of each photovoltaic output with the minimum reduction of the total photovoltaic amount at this moment as the objective function, thereby transferring the operating point to a safer position; when the safety distance of the adjusted operating point meets the preventive control end threshold, this preventive control ends.

[0061] The present invention will be further described below with reference to the accompanying drawings and specific embodiments, but the following embodiments are by no means intended to limit the present invention in any way.

[0062] Compared to preventive control in medium-voltage distribution networks, preventive control in low-voltage substations is unique. In medium-voltage distribution networks, preventive control targets operating points that exceed the N-1 safety constraint but remain within the safety boundary of the N-0 safety constraint. Low-voltage substations, on the other hand, typically utilize the N-0 safety constraint due to their dense user distribution and the ability to quickly locate and repair faults. Therefore, preventive control targets operating points that are about to cross the N-0 safety boundary.

[0063] The security domain model of the low-voltage distribution network and the preventive control method for photovoltaic interactive participation in safety auxiliary services include the following steps:

[0064] Step 1: Establish the N-0 low-voltage distribution network security domain model. The specific contents are as follows:

[0065] Unlike medium-voltage distribution networks, in low-voltage distribution networks, some DGs are connected to the low-voltage line in three phases before the distribution box, while other DGs are connected in single phases after the distribution box. Therefore, in addition to the DG and load power connected after the distribution box needing to meet the capacity constraints of each phase of the ABC three-phase line, the total power of the DG and load connected to the low-voltage area must also meet the low-voltage line capacity constraints and transformer capacity constraints. The simple low-voltage area topology with distributed new energy is shown in the attached figure. Figure 1 shown.

[0066] First, the total power constraints of the DG and load connected to the low-voltage substation are given. The total power of the DG and load in the substation must meet the low-voltage line capacity constraints and transformer capacity constraints, as shown in formula (1):

[0067]

[0068] In the formula Indicates low voltage line j φ capacity, Main transformer φ Capacity; B φ is the low voltage line collection, T φ is the set of all main transformers; S Lx is the outflow power of the xth load node, S DGy is the injection power of the yth DG node; and Respectively represent the low voltage line j during normal operation φ The downstream is connected to all load nodes and DG nodes before the distribution box; and They represent the normal operation of the main transformer i φ The downstream is connected to all load nodes and DG nodes before the distribution box; and Indicates low voltage line j φ The set of all load nodes and DG nodes connected to the ABC three-phase network after the downstream distribution box; and They represent the main transformer i in normal operation φ The downstream distribution box is connected to all load nodes and DG nodes with single-phase access to the ABC three-phase.

[0069] Total power of DG and load connected to the low-voltage area and It consists of the following two parts:

[0070] 1) The total power of the DG and load connected to the low-voltage line in the three phases before the distribution box. In the low-voltage area topology, it is represented as DG1 and L1 (see the attached Figure 1 Its total power is:

[0071] and

[0072] 2) The total power of the DG and load connected to the three phases A, B, and C after the distribution box. In the low voltage area topology, it is represented as DG2, DG3, DG4 and L2, L3, L4 (see the attached Figure 1 Since the low-voltage line current is the sum of the three-phase currents, the low-voltage line voltage is the sum of the three-phase voltages. Therefore, from the perspective of low-voltage lines, the total power of the DG and load of the ABC three-phase is the sum of the power of the DG and load of the single-phase access to the ABC three-phase. times, that is:

[0073] and

[0074] Power constraints of DG and load connected in phase. The power of DG and load connected in single phase after the distribution box needs to meet the capacity constraints of each phase of ABC line, as shown in formula (2):

[0075]

[0076] Where, and They represent the A phase line j in normal operation. A The set of all downstream load nodes and DG nodes; and They represent the B phase line j in normal operation. B The set of all downstream load nodes and DG nodes; and They represent the C phase line j in normal operation. C The set of all downstream load nodes and DG nodes; and Represents A, B, C three-phase lines j A 、j B 、j C Capacity; B A 、B B and B C They are respectively the three-phase line sets of A, B and C.

[0077] Low-voltage substation N-0 safety domain. Based on equations (1) and (2), the N-0 low-voltage substation safety domain model is established considering the ABC three-phase, as shown in equation (3):

[0078]

[0079] Among them, Ω DSSRT Represents the N-0 safety zone of the distribution transformer area; W is the operating point of the area, W L is the load node part of the operating point, S Lx is the outflow power of the xth load node, n is the number of load nodes; W DG is the DG node part of the operating point, S DGy is the injected power into the yth DG node, and m is the number of DG nodes. Both load and DG node powers are expressed in apparent power magnitudes. Power flowing from the grid to the load is defined as positive, with the negative sign indicating direction only. Θ represents the state space, defined as the set of all possible operating points.

[0080] Formula (3) establishes the safety domain of the distribution transformer area with the DG and load connected to the three-phase ABC of the low-voltage area as the research object. When using this model to evaluate the safety of the operating point, it can accurately and quickly determine whether the operating point is within the safety domain, and screen out the operating points that do not meet the safety domain constraints for governance.

[0081] Step 2: Use analytical methods to solve the safety boundary of the low-voltage substation safety zone, and screen out the effective safety boundary by eliminating redundant linear inequality constraints. [4] .

[0082] Step 3: Calculate the safety distance from the operating point to the effective safety boundary in real time.

[0083] In the DSSR assessment of medium voltage distribution networks, there are geometric security distance (GSD) and feeder security distance (FSD). [5] GSD takes into account the simultaneous changes of multiple variables, so GSD is selected.

[0084] The formula for calculating GSD is:

[0085]

[0086] Among them, GSD j represents the safety distance from the operating point to the jth safety boundary; c represents the rated capacity of the main transformer or feeder; n is the number of monomials in the effective safety boundary polynomial; a x Represents the coefficient of each monomial; S x Indicates the load or DG connected to the feeder.

[0087] Step 4. Select GSD j Minimum safety distance GSD min As a criterion for judging whether the operating point is safe:

[0088] GSD min =min(GSD1,GSD2,...,GSD j ) (5)

[0089] If GSD min ≥0, indicating that the operating point is safe; if GSD min <0, indicating that the operating point has crossed the safety limit.

[0090] Step 5: Start preventive control at time t when the minimum safety distance meets the start threshold M0.

[0091] When the minimum safety distance of the operating point is close to 0, the system is about to cross the safety boundary, and it is necessary to prevent the occurrence of safety crossing in advance.p When the safety distance drops to a certain value, the distribution network starts preventive control, which is called the preventive control start threshold M0. p Represents the line capacity and main transformer capacity in the safety domain expression, which is called the coefficient related to capacity; it is observed that the preventive control start threshold and the safety margin β p The constant c related to capacity p There is a linear relationship between them, so the pth preventive control activation threshold can be expressed as:

[0092] M0=ɑ startp c p (6)

[0093] Among them, ɑ startp To prevent and control the startup coefficient, ɑ startp <1;

[0094] When the minimum safety distance at time t satisfies both conditions 1 and 2, preventive control is initiated:

[0095] Condition 1: GSD min (t)≤M0(7)

[0096] This formula shows that the minimum safety distance at the current time t begins to reach the preventive control startup threshold.

[0097] Condition 2: GSD min (t)≤GSD min (t-1)(8)

[0098] This formula shows that the trend of the minimum safety distance is to further decrease as the photovoltaic output increases.

[0099] If the minimum safety distance at time t meets both of the above conditions, it means that as the light intensity increases further, there is a potential risk of crossing the boundary at time t+1. In order to avoid this phenomenon, preventive control needs to be initiated immediately.

[0100] Step 6: Take preventive control measures for the operating point at time t.

[0101] At time t, preventive control measures are implemented through PV interactive participation in safety auxiliary services, that is, by adjusting the output power of the PV inverter to reduce the PV output, thereby shifting the operating point to a safer position. The specific steps are as follows:

[0102] First, determine the boundary corresponding to the safe distance that triggers preventive control at time t; then, identify the photovoltaic objects that need to be adjusted based on the boundary.

[0103] Secondly, determine the photovoltaic reduction amount ΔP of the photovoltaic object that needs to be adjusted k(t) :

[0104] P k(t) '=P k(t) -ΔP k(t) (9)

[0105] Among them, P k(t) The k-th photovoltaic output at time t before adjustment is known; ΔP k(t) represents the kth photovoltaic reduction amount at time t; P k(t) ' represents the kth photovoltaic output after adjustment at time t.

[0106] To achieve preventive control, ΔP k(t) There are many values. Since photovoltaics adjust their own power through the inverter, ΔP k(t) The value of varies within a certain range; the coordinated regulation of multiple photovoltaic nodes also makes ΔP k(t) There are many combinations, so an optimization model needs to be established to solve and find the photovoltaic reduction amount ΔP k(t) The optimal value of . With the goal of minimizing the total amount of photovoltaic power generation, the objective function is expressed as:

[0107]

[0108] ΔP k represents the kth PV reduction amount, and m is the total number of PV nodes.

[0109] The objective function should satisfy the following constraints:

[0110] 1) Low-voltage area safety domain constraint: The low-voltage area N-0 safety domain constraint model is shown in formula (3).

[0111] 2) Constraints on the adjustable range of photovoltaic output:

[0112]

[0113] in, is the maximum curtailable amount of the k-th PV node.

[0114] 3) Safety distance constraints:

[0115] The goal of preventive control is to adjust the operating point to a safer position in advance. In order to ensure that the system is far away from the preventive control start threshold after regulation and avoid repeated triggering of control in a short period of time, it is necessary to adjust the operating point to the safety boundary β p The safety distance is adjusted to a safer threshold M1. Since when the operating point safety distance reaches M1, it also marks the end of the preventive control regulation of the system, this value is also called the preventive control end threshold M1.

[0116] Observation found that the prevention control end threshold and safety margin β pThe constant c related to capacity p There is also a linear relationship between them, so the pth preventive control activation threshold can be expressed as:

[0117] M1=ɑ endp c p (12)

[0118] Where ɑ endp To prevent the end of the control coefficient, ɑ endp <ɑ startp <1;

[0119] After implementing preventive control through PV interactive response, the safety distance of the safety boundary involved in the adjustment should reach the safety margin M1. The safety distance constraint can be expressed as:

[0120] GSD(t) p '≥M1 (13)

[0121] Among them, GSD(t) p ' represents the boundary β involved in the adjustment j The safety distance at time t after adjustment.

[0122] If the adjusted operating point satisfies formula (13), it means that the operating point at time t has been successfully adjusted to a safer position through the PV interactive participation in the safety assistance service, thereby effectively coping with the safety crossing problem that may be caused by the increase in PV output at the next moment.

[0123] In the present invention, ΔP k The solution is transformed into a linear programming problem, and the optimal solution for photovoltaic reduction can be obtained by solving the linear programming problem. The method proposed in the present invention is also applicable to the safety crossing caused by excessive load output. By reducing the load output at the operating point at time t, the operating point is transferred to a safer position. Specifically, in this step, the boundary corresponding to the safe distance that triggers preventive control at time t is determined; then, based on the boundary expression, the load object that needs to be adjusted is identified, and the photovoltaic output P is set as k , ΔP k 、P k ' and P k max Change to load output, so as to effectively deal with the safety violation problem that may be caused by the increase of load output at the next moment.

[0124] Step 7: End of preventive control.

[0125] When the grid area participates in safety auxiliary services through photovoltaic interaction, GSD min (t)≤M0 The minimum safety distance is adjusted to GSD min (t) βj'≥M1 indicates that the operating point at time t has been adjusted to a safer position, effectively responding to the potential safety violations caused by the increase in PV output at the next moment. When the operating point satisfies Equation (13), preventive control ends. Return to step 3 to prepare for the next preventive control initiation.

[0126] Research Materials:

[0127] by Figure 1 The preventive control methods are explained using the high penetration rate distributed photovoltaic low-voltage area as an example.

[0128] 1. Basic Situation of the Case

[0129] The transformer capacity in the substation is 50kVA. The low-voltage feeder is a BV-10 with a capacity of 30kVA. The low-voltage line length before the distribution box is 30m. After the distribution box, the three-phase A, B, and C feeders are laid independently, each with a capacity of 10kVA and line lengths of 50m, 30m, and 40m, respectively. Considering that the PV output fluctuation is usually around 5%, the preventive control startup coefficient ɑ is taken from formula (6). startp =0.05; From formula (12), take the prevention control end coefficient ɑ endp =0.10.

[0130] Typical daily data for each DG and load node is shown in Figure 1. The load exhibits a typical bimodal characteristic, reaching a maximum load of 26.67 kVA between 8 PM and 10 PM. The DG output exhibits a unimodal distribution, reaching a maximum power generation of 44.73 kVA between 12 PM and 1 PM. Figure 2 、 Figure 3 The load and DG power variation curves within a day are given.

[0131] Table 1 Substation load and DG data

[0132]

[0133] The DG connection capacity is 40 kVA, and the DG and load power factors are both 0.90. Table 2 shows the total load, DG output, and penetration rate for a typical day. The negative sign indicates the direction of power flow only. The voltage at the first section of the feeder is 1.0 (pu), and the allowable voltage deviation range is [0.93, 1.07] (pu). Table 2 shows the typical daily data for the DG, load power, and penetration rate of the example.

[0134] Table 2 Typical daily data of DG, load power and penetration rate in the example

[0135]

[0136] 2. Specific steps of the preventive control method for low-voltage distribution network.

[0137] Step 1) Construct the low-voltage substation N-0 safety domain model according to the DC power flow model of formula (3):

[0138]

[0139] Step 2) Using equation (14), solve the safety boundary of the low-voltage station area safety domain using analytical method, and select 20 effective safety boundaries, see equation (15), and use β i represents the i-th safety boundary.

[0140]

[0141] Step 3) Calculate the safety distance from the operating point to the effective safety boundary in real time.

[0142] From formula (4), solve the 24h operating point W = [S L1 ,S L2 ,S L3 ,S L4 ,S DG1 ,S DG2 ,S DG3 ,S DG4 ] to the effective safety margin β i The safety distance is shown in Table 3.

[0143] Table 3 24h safety distance data of the substation area

[0144]

[0145]

[0146] Step 4) According to formula (4), select the minimum safety distance GSD min (t) is used as the safety margin of the substation at time t, and the minimum safety distance of the 24-hour operating point is calculated in real time.

[0147] Step 5) Start preventive control at time t when the conditions are met.

[0148] When the minimum safety distance of the operating point is close to 0, the system is about to cross the safety boundary. In order to prevent the occurrence of safety crossing in advance, 5% of the safety boundary limit value is taken as the preventive control starting threshold M0.

[0149] When the minimum safety distance at time t satisfies the conditions of equations (7) and (8), it indicates that there is a potential risk of exceeding the limit at that moment during the safety monitoring process:

[0150] From Table 3, we can see that the minimum safe distance GSD at 12 o'clock min (t 12 )=0.23kVA, the corresponding boundary limit value is β j=10kVA, M0(t 12 )=5%β j =0.5, meeting the condition -GSD min (t 12 )≤M0(t 12 ); From Table 3, we can see that the minimum safe distance GSD is 11 o'clock min (t 11 )=1.37kVA, meeting condition 2 GSD min (t 12 )≤GSD min (t 11 ). If the minimum safety distance at 12 o'clock satisfies both the conditions of equations (7) and (8), it means that as the light intensity increases further, there is a potential risk of crossing the boundary at time t+1. In order to avoid this phenomenon, preventive control measures need to be taken in advance.

[0151] Step 6) Take preventive control measures for the 12:00 operating point.

[0152] First, we find the boundaries corresponding to the safe distances that reach the preventive control activation threshold at 12:00. Based on these boundaries, we determine the PV systems that require adjustment. Table 4 shows the safe distances from the 12:00 operating point to each boundary and the preventive control activation threshold M0 for each boundary.

[0153] Table 4 Safety distance from the 12:00 operating point to each boundary and the preventive control activation threshold M0 of each boundary

[0154]

[0155] From Table 4, we can see that the boundaries β2, β6, β 10 The safety distance is less than the prevention and control start threshold 0.5; the boundary β 14 The safety distance is less than the prevention and control start threshold of 1.5; the boundary β 19 The safety distance is less than the prevention control start threshold of 2.0. 10 , β 14 , β 19 From the expression (see formula (15)), it can be seen that the photovoltaics that need to be adjusted are DG2, DG3 and DG4.

[0156] Secondly, the photovoltaic reduction amount ΔP of DG2, DG3 and DG4 at 12 o'clock is determined according to formula (11): k(t) , to adjust the operating point to a safer position.

[0157] Due to the ΔP of preventive control k(t) There are many possible values, so an optimization model needs to be established to solve and find the photovoltaic reduction amount ΔP k(t)The objective function (see formula (10)) with the goal of minimizing the total amount of photovoltaic power generation should satisfy the following constraints:

[0158] 1) Low-voltage area safety zone constraints:

[0159] See step 1, formula (14).

[0160] 2) Constraints on the adjustable range of photovoltaic output:

[0161] According to formula (11) and the data in Table 1, the adjustable range of photovoltaic output at 12:00 is constrained as follows:

[0162] Table 5 12:00 PV output adjustable range constraints

[0163]

[0164] In the table, ΔP 12A , ΔP 12B , ΔP 12C They represent the photovoltaic reduction of phases A, B, and C in the substation at 12:00 respectively.

[0165] 3) Safety distance constraints:

[0166] In order to adjust the operating point to a safer position through preventive control, a larger safety margin M1 should be left between the safety distance at 12 o'clock and 0, and M1 should be 10% of the safety boundary limit value.

[0167] According to Table 4, the photovoltaic cells that need to be adjusted are DG2, DG3 and DG4, so it is necessary to set safety distance constraints on the boundaries containing DG2, DG3 and DG4, that is, the boundaries β2, β6, β 10 , β 14 , β 19 , so that the safety distance of these boundaries after the photovoltaic interactive response is greater than or equal to M1. Equation (16) gives the boundary β2, β6, β1 at 12 o'clock after taking preventive control. 10 , β 14 , β 19 Safety distance constraint, where S DG2 ', S DG3 ' and S DG4 'Indicates the photovoltaic output at 12:00 after preventive control.

[0168]

[0169] So for ΔP k The solution is transformed into a linear programming problem. Solving this linear programming problem can obtain the optimal solution for preventing and controlling photovoltaic curtailment at 12 o'clock, as shown in Table 6.

[0170] Table 6 Optimal plan for power reduction by preventive control at 12:00

[0171]

[0172] The minimum total photovoltaic reduction under this plan is 2.73kVA.

[0173] The safety distance from the 12:00 operating point to the relevant boundaries after adjustment and the end threshold M1 of the preventive control at each boundary are shown in Table 7.

[0174] Table 7 Distance from the operating point to the relevant boundaries at 12:00 after preventive control and the end threshold M1 of each boundary preventive control

[0175]

[0176] Table 8 shows the low-voltage feeder outlet load rate and distribution transformer load rate at 12:00 before and after preventive control.

[0177] Table 8 Load rates of distribution transformers and low-voltage feeders before and after preventive control

[0178] 12 o'clock Distribution transformer load factor / % Low voltage feeder load rate / % Before prevention and control 81.64 88.33 After prevention and control 58.25 77.59

[0179] As can be seen from Tables 7 and 8, after the PV output adjustment, the safety distance from the 12:00 operating point to the safety boundary is greater than or equal to the preventive control end threshold, and the distribution transformer load factor and the low-voltage substation export load factor have been significantly improved, indicating that after preventive control, the system has sufficient margin to deal with the potential safety crossing risk of the substation.

[0180] Step 7) Preventive control ends.

[0181] As shown in Table 7, after 12 hours of preventive control measures, GSD min (t 12 ) βj '≥M1, indicating that the operating point has been adjusted to a safer position, which can effectively deal with the potential cross-border risk of the substation at the next moment, and this preventive control is ended.

[0182] At this point, return to step 3 to prepare for the next activation of preventive controls.

[0183] See the 24h minimum safety distance for prevention and control of the front and back areas. Figure 4 .from Figure 4 As can be seen, through preventive control measures, during periods of time when there is a risk of overstepping the boundary (such as 12:00 PM) without control measures, the minimum safe distance has been raised from near zero to above the safety threshold of 10% through PV interactive participation in safety assistance services. This preventive control measure leaves ample margin for further PV expansion and effectively resolves the potential overstepping safety issue in the 1:00 PM area.

[0184] Although the present invention has been described above in conjunction with the accompanying drawings, the present invention is not limited to the above-mentioned specific embodiments. The above-mentioned specific embodiments are merely illustrative and not restrictive. Under the guidance of the present invention, ordinary technicians in this field can make many improvements and changes without departing from the purpose of the present invention, which are all protected by the present invention.

Claims

1. A preventive control method for low-voltage distribution network in photovoltaic interactive participation safety auxiliary service, characterized in that: First, the N-0 low-voltage substation safety domain model is established based on the total power constraints of distributed generation and loads in the low-voltage substation and the power constraints of distributed generation and loads connected in phases; then, the safety boundary of the N-0 low-voltage substation safety domain is solved by an analytical method, the effective safety boundary is screened out, and the geometric safety distance from the operating point to the effective safety boundary is calculated in real time; the safety of the operating point is judged based on the minimum safety distance in the geometric safety distance; preventive control is started when the minimum safety distance meets the starting threshold; for the operating point at that moment, the photovoltaic output is reduced by adjusting the output power of the photovoltaic inverter, and the photovoltaic output reduction amount of each photovoltaic output is determined with the minimum photovoltaic total reduction at that moment as the objective function, so as to shift the operating point to a safer position; when the safety distance of the adjusted operating point meets the preventive control end threshold, this preventive control ends.

2. The low-voltage distribution network preventive control method according to claim 1, characterized in that: The specific steps are as follows Step 1: Establish the N-0 low-voltage distribution network security domain model shown in the following formula: Among them, Ω DSSRT Represents the N-0 safety zone of the distribution transformer area; W is the operating point of the area, W L is the load node part of the operating point, S Lx is the outflow power of the xth load node, n is the number of load nodes; W DG is the distributed generation node part of the operating point, S DGy is the injected power of the yth distributed generation node, m is the number of distributed generation nodes; the load / distributed generation node power adopts the apparent power amplitude, and the power flowing from the grid to the load is defined as positive, and the negative sign only indicates the direction; Θ represents the state space, which is defined as the set of all possible operating points; and They represent the A phase line j in normal operation. A The collection of all downstream load nodes and distributed generation nodes; and They represent the B phase line j in normal operation. B The collection of all downstream load nodes and distributed generation nodes; and They represent the C phase line j in normal operation. C The collection of all downstream load nodes and distributed generation nodes; and Represents A, B, C three-phase lines j A 、j B 、j C Capacity; B A 、B B and B C They are the three-phase line sets of A, B and C respectively; Indicates low voltage line j φ capacity, Main transformer φ Capacity; B φ is the low voltage line collection, T φ is the collection of all main transformers; and Respectively represent the low voltage line j during normal operation φ The collection of all load nodes and distributed generation nodes connected to the distribution box downstream; and They represent the normal operation of the main transformer i φ The collection of all load nodes and distributed generation nodes connected to the distribution box downstream; and Indicates low voltage line j φ The set of all load nodes and distributed generation nodes connected to the ABC three-phase network after the downstream distribution box; and They represent the main transformer i in normal operation φ The set of all load nodes and distributed generation nodes connected to the ABC three-phase network after the downstream distribution box; Step 2: Use analytical methods to solve the safety boundary of the N-0 low-voltage distribution network security domain, and screen out the effective safety boundary by eliminating redundant linear inequality constraints; Step 3: Calculate the geometric security distance (GSD) from the operating point to the effective safety boundary in real time, as shown in the following formula: Among them, GSD j represents the safety distance from the operating point to the jth safety boundary; c represents the rated capacity of the main transformer or feeder; n is the number of monomials in the effective safety boundary polynomial; a x Represents the coefficient of each monomial; S x Indicates the load or distributed generation connected to the feeder; Step 4. Select GSD j Minimum safety distance GSD min The criteria for judging whether the operating point is safe are as follows: GSD min =min(GSD1,GSD2,...,GSD j ) If GSD min ≥0, indicating that the operating point is safe; if GSD min <0, indicating that the operating point has crossed the safety limit; Step 5: Start preventive control at the moment t when the minimum safety distance meets the start threshold M0: When the minimum safe distance GSD of the operating point min When it approaches 0, the system is about to cross the safety boundary. This moment needs to be marked and measures need to be taken in advance to prevent the crossing. The safety distance from the operating point to all safety boundaries is calculated. When the safety distance drops to a certain value, the distribution network initiates preventive control measures. This value is called the preventive control measure initiation threshold M0. Use c p Represents the line capacity and main transformer capacity in the safety domain expression, which is called the coefficient related to capacity; the preventive control starting threshold M0 and c p There is a linear relationship between them, so the pth preventive control activation threshold is expressed as: M0=ɑ startp c p Among them, ɑ startp To prevent and control the startup coefficient, ɑ startp <1; If the minimum safe distance at time t satisfies both conditions 1 and 2, preventive control measures are initiated immediately, and the time t is recorded: Condition 1: GSD min (t)≤M0, indicating that the minimum safety distance at the current time t begins to reach the threshold for initiating preventive control measures; Condition 2: GSD min (t)≤GSD min (t-1), indicating that the trend of the minimum safety distance is further reduced with the increase of photovoltaic output; Step 6: For the operating point at time t, reduce the photovoltaic output by adjusting the output power of the photovoltaic inverter, thereby shifting the operating point to a safer position. The specific steps are as follows: Step 6-1) Determine the boundary corresponding to the safety distance that triggers preventive control at time t; then, identify the photovoltaic objects that need to be adjusted based on the safety boundary; Step 6-2) Determine the photovoltaic reduction amount ΔP of the photovoltaic object that needs to be adjusted k(t) : P k(t) '=P k(t) -ΔP k(t) Among them, P k(t) The k-th photovoltaic output at time t before adjustment is known; ΔP k(t) represents the kth photovoltaic reduction amount at time t; P k(t) ' represents the kth photovoltaic output after adjustment at time t; Step 6-3) Establish an optimization model to determine the photovoltaic reduction amount ΔP k(t) The optimal value of is to minimize the total photovoltaic power generation. The objective function is expressed as: Where ΔP k represents the kth PV reduction amount, m is the total number of PV nodes; The objective function satisfies three constraints: Constraint 1: Low-voltage substation security domain constraint, such as the N-0 low-voltage substation distribution network security domain model established in step 1; Constraint 2: PV output adjustable range constraint: in, is the maximum curtailable amount of the k-th PV node; Constraint three, safety distance constraint: The boundary corresponding to the safety distance that triggers preventive control in step 6-1) should reach the preventive control end threshold M1 after the adjustment of the measures of PV interactive participation in safety auxiliary services. The preventive control end threshold M1 is related to the constant c related to the capacity. p There is a linear relationship between them, so the p-th preventive control end threshold M1 is expressed as: M1=ɑ endp c p Among them, ɑ endp To prevent the end of the control coefficient, ɑ endp <ɑ startp <1; Therefore, the safety distance constraint is expressed as: GSD(t) p '≥M1, where GSD(t) p ' represents the safety distance corresponding to the boundary of the safety distance that triggers the preventive control in step 6-1) after adjustment by the measures of the photovoltaic interactive participation safety auxiliary service; If the adjusted operating point meets the above safety distance constraints, it means that the operating point at time t has been adjusted to a safer position to cope with the safety crossing problem that may be caused by the increase in photovoltaic output at the next moment; Step 7: End of preventive control When GSD min The minimum safety distance at the moment (t)≤M0 is adjusted to the operating point to meet GSD(t) p '≥M1, this prevention and control is ended.

3. The low-voltage distribution network preventive control method according to claim 1, characterized in that: After this preventive control is completed, return to step 3 to prepare for the next preventive control.

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