A method for load shedding in a flexible interconnected substation

Abstract

The present invention discloses a method for load shedding in a flexible interconnected substation. The method is as follows: Connect the main transformers in the substation through a B2B-MMC and an energy storage system to form a flexible interconnected substation; when one of the main transformers in the substation cannot work normally due to heavy load, maintenance, fault, etc. and load shedding is required, first, immediately cut the load that needs to be immediately cut; secondly, through controlling and assigning values to the B2B-MMC and the energy storage system, realize the transfer power supply for the load that does not need to be immediately cut; thirdly, realize the delayed cut of the load that needs to be cut with a delay; finally, realize the continuous power supply for the load that needs to maintain power supply; this method can not only realize the continuous power supply for the load that needs to maintain power supply, but also realize the delayed cut of the load that needs to be cut with a delay, and greatly reduce the various losses brought to various load users by the load shedding power outage.

Description

Technical Field

[0001] The present invention belongs to the field of power system control, and particularly relates to a load shedding method for a flexible interconnected substation. Background Art

[0002] When a power system fails and affects the safe and stable operation of the power system, the common control means is to cut off some generators at the power supply end and some loads at the load end. The early load shedding method of the stability control device was to divide the loads into several rounds according to their importance. When a power grid failure occurred, the loads were shed in rounds according to the pre-fault power grid operation mode and the severity of the fault disturbance; with the development of computer and communication technologies, in recent years, the stability control device generally adopts the method of real-time monitoring of load power and shedding loads according to the importance of the loads, and this method has a higher load shedding accuracy. Although these methods can achieve accurate and effective load shedding, they cannot solve the problem that the sudden load shedding power outage causes losses to users because the users have no power outage preparation time. Summary of the Invention

[0003] The technical problem to be solved by the present invention is: to provide a load shedding method for a flexible interconnected substation, which is used to solve the problem that the existing load shedding methods cannot give the load-related users power outage preparation time, thus causing losses to users due to load shedding power outages.

[0004] The technical solution of the present invention is:

[0005] A load shedding method for a flexible interconnected substation, in which the method connects each main transformer in the substation through a back-to-back modular multilevel converter B2B-MMC and an energy storage system to form a flexible interconnected substation; when a certain main transformer in the substation needs to shed loads, by controlling and assigning values to the B2B-MMC and the energy storage system, first, the loads that need to be immediately cut off are immediately cut off; secondly, by controlling and assigning values to the B2B-MMC and the energy storage system, the non-immediately cut-off loads are transferred for power supply; then, the loads that need to be cut off with a delay are cut off with a delay; finally, the loads that need to maintain power supply are maintained for power supply.

[0006] The energy storage system is connected to the DC side of the B2B-MMC, and both ends of the B2B-MMC are connected to the low-voltage side of the main transformer.

[0007] The method specifically includes:

[0008] Step 1: When it is determined that a certain main transformer in the substation cannot continue to work properly due to heavy load, maintenance or fault and load shedding is required, obtain the total load P1 that cannot be continuously powered on the inoperable main transformer, the maximum output power P2 and capacity S of the energy storage system, the remaining available load P3 of the remaining main transformers in the substation when reaching full load, and the importance grading and quantity information of the load that can be shed on the inoperable main transformer; the load importance grading is denoted as priority 1, 2, …, n;

[0009] Step 2: When P1 is less than or equal to P3, there is no need to shed load at this time; by controlling and assigning values to the B2B-MMC, realize that all loads corresponding to P1 are transferred and supplied by other main transformers;

[0010] Step 3: When P1 is greater than P3, calculate the load quantity P4 that needs to be shed at this time, and its calculation formula is P4 = P1 - P3;

[0011] Step 4: When P4 is less than or equal to the maximum output power P2 of the energy storage system, by controlling and assigning values to the B2B-MMC and the energy storage system, realize the transfer and supply of the corresponding loads of P1; then delay the shedding of the loads corresponding to the load quantity of P4 in P1, and denote the delayed shedding load quantity as P 延时 , and the maximum delay time is t max = S / P 延时 ; at the same time, keep supplying power to the loads corresponding to the load quantity of P3 in P1;

[0012] Step 5: When P4 is greater than the maximum output power P2 of the energy storage system, calculate the load quantity P5 that needs to be immediately shed, P5 = P1 - P3 - P2, that is, immediately shed the loads corresponding to the load quantity of P5 in P1; then by controlling and assigning values to the B2B-MMC and the energy storage system, realize the transfer and supply of the non-immediately shed loads corresponding to P2 + P3 in P1; then delay the shedding of the loads corresponding to the load quantity of P2 in P1, and denote the delayed shedding load quantity as P 延时 , and the maximum delay time t max = S / P 延时 ; at the same time, keep supplying power to the loads corresponding to the load quantity of P3 in P1.

[0013] The control mode of B2B-MMC is U dc Q-PQ control, and the control mode of the energy storage system is P control.

[0014] The maximum delay time t max = S / P 延时 . When P4 <= P2, P 延时 = P4; when P4 > P2, P 延时 = P2.

[0015] The specific implementation method of step 4 includes:

[0016] Step 4.1: Implement the delayed removal of the load corresponding to the load amount equal to P4 in P1, and the maximum delay time is t max = S / P 延时 , accumulate the load amounts of each load with load priorities from 1 to k to obtain the load amount P(k) to be removed with delay. When P(k) is less than or equal to P4 and P(k + 1) is greater than P4, the loads with load priorities from 1 to k are obtained as the loads to be removed with delay, and arrange the delayed removal for the loads with load priorities from 1 to k; where the maximum delay time t max = S / P4;

[0017] Step 4.2: Implement the continuous power supply for the load corresponding to the load amount equal to P3 in P1; excluding the loads with load priorities from 1 to k that are removed with delay, the loads with load priorities from k + 1 to n are obtained as the loads to be continuously powered, and arrange the continuous power supply for the loads with load priorities from k + 1 to n.

[0018] The specific implementation method of step 5 includes:

[0019] Step 5.1: Implement the immediate removal of the load corresponding to the load amount equal to P5 in P1. Accumulate the load amounts of each load with load priorities from 1 to k to obtain the load amount P(k) to be immediately removed. When P(k) is less than P5 and P(k + 1) is greater than or equal to P5, the loads with load priorities from 1 to k + 1 are obtained as the loads to be immediately removed, and immediately remove the loads with load priorities from 1 to k + 1;

[0020] Step 5.2: Through the control and assignment of the B2B - MMC and the energy storage system, implement the delayed removal of the load corresponding to the load amount equal to P2 in P1. Accumulate the load amounts of each load with load priorities from k + 2 to m to obtain the load amount P m , when P m is less than or equal to P2 and P m+1 is greater than P2, the loads with load priorities from k + 2 to m are obtained as the loads to be removed with delay, and arrange the delayed removal for the loads with load priorities from k + 2 to m; where the maximum delay time t max = S / P2;

[0021] Step 5.3: Implement the continuous power supply for the load corresponding to the load amount equal to P3 in P1; excluding the loads with load priorities from 1 to k + 1 that are immediately removed and the loads with load priorities from k + 2 to m that are removed with delay, the loads with load priorities from m + 1 to n are obtained as the loads to be continuously powered, and arrange the continuous power supply for the loads with load priorities from m + 1 to n.

[0022] The beneficial effects of the present invention are:

[0023] The present invention connects each main transformer in a substation through a B2B-MMC and an energy storage system to form a flexible interconnected substation; when one of the main transformers in the substation cannot work properly due to heavy load, maintenance, fault, etc. and load shedding is required, first, the loads that need to be immediately shed are immediately shed; secondly, by controlling and assigning values to the B2B-MMC and the energy storage system, the power supply of the non-immediately shed loads is transferred; thirdly, the loads that need to be shed with a time delay are shed with a time delay, giving these load users a certain preparation time to greatly reduce various losses brought by load shedding power outages to users; finally, the loads that need to maintain power supply are maintained with power supply to ensure that these loads do not lose power. This method can not only maintain the power supply of the loads that need to maintain power supply, but also delay the shedding of the loads that need to be shed with a time delay, achieving a large reduction in various losses brought by load shedding power outages to various load users. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 is the load shedding flow chart of the present invention;

[0025] Figure 2 is the load shedding system diagram of the present invention. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0026] The present invention will be described in detail below with reference to the accompanying drawings and specific embodiments.

[0027] The present invention provides a load shedding method for a flexible interconnected substation. As Figure 1 shown, when the present invention is implemented, first, it is determined whether a certain main transformer in the substation cannot continue to work properly due to heavy load, maintenance, fault, etc. and load shedding is required. If load shedding is required due to the above reasons, the subsequent implementation steps are as follows:

[0028] Step 1, obtain the total load amount P1 that cannot be continuously powered on the main transformer that cannot continue to work properly, the maximum output power P2 and capacity S of the energy storage system, the remaining load amount P3 that other main transformers reach full load, and the importance grading and corresponding value information of the load that can be shed by the main transformer that cannot continue to work; among them, the load importance grading is recorded as priority levels 1, 2,..., n.

[0029] Step 2, when P1 is less than or equal to P3, load shedding is not required at this time. By controlling and assigning values to the B2B-MMC, it is realized that all the loads corresponding to P1 are transferred and supplied by other main transformers. Among them, the B2B-MMC control mode is set to U dc Q-PQ control.

[0030] Step 3, when P1 is greater than P3, calculate the load amount P4 that needs to be shed at this time, and its calculation formula is P4 = P1 - P3.

[0031] Step 4, when P4 is less than or equal to the maximum output power P2 of the energy storage system, by controlling and assigning values to the B2B-MMC and the energy storage system, power is transferred to the corresponding loads of P1. Then, the loads corresponding to the load amount of P4 in P1 are removed with a time delay, and the load amount removed with a time delay is denoted as P 延时 , at this time P 延时 = P4; the maximum time delay is t max = S / P 延时 ; meanwhile, the loads corresponding to the load amount of P3 in P1 are kept powered. Among them, the control mode of the B2B-MMC is set to U dc Q-PQ control, and the control mode of the energy storage system is set to P control. The specific implementation process is as follows:

[0032] Realize the load corresponding to the load amount of P4 in P1 is removed with a time delay, and the maximum time delay is t max = S / P 延时 . Specifically, the load amounts of each load with load priorities from 1 to k are accumulated to obtain the load amount P(k) to be removed with a time delay. When P(k) is less than or equal to P4 and P(k + 1) is greater than P4, it can be obtained that the loads with load priorities from 1 to k are the loads to be removed with a time delay. Therefore, it is necessary to arrange a time delay for the loads with load priorities from 1 to k; among them, the maximum time delay t max = S / P4.

[0033] Realize the loads corresponding to the load amount of P3 in P1 are kept powered. Specifically, excluding the loads with load priorities from 1 to k that are removed with a time delay, it can be obtained that the loads with load priorities from k + 1 to n are the loads to be kept powered. Therefore, it is necessary to arrange to keep the loads with load priorities from k + 1 to n powered.

[0034] Step 5, when P4 is greater than the maximum output power P2 of the energy storage system, calculate the load amount P5 to be immediately removed as P5 = P1 - P3 - P2, that is, the loads corresponding to the load amount of P5 in P1 need to be immediately removed; then, by controlling and assigning values to the B2B-MMC and the energy storage system, power is transferred to the non-immediately removed loads corresponding to P2 + P3 in P1. After that, the loads corresponding to the load amount of P2 in P1 are removed with a time delay, and the load amount removed with a time delay is denoted as P 延时 , at this time P 延时 = P2; the maximum time delay is t max = S / P 延时 ; meanwhile, the loads corresponding to the load amount of P3 in P1 are kept powered. Among them, the control mode of the B2B-MMC is set to U dc Q-PQ control, and the control mode of the energy storage system is set to P control. The specific implementation process is as follows:

[0035] Implement immediate load shedding for the load corresponding to the load amount equal to P5 in P1. Specifically, accumulate the load amounts of each load with load priorities from 1 to k to obtain the load amount P(k) that needs to be immediately shed. When P(k) is less than P5 and P(k + 1) is greater than or equal to P5, it can be obtained that the loads with load priorities from 1 to k + 1 need to be immediately shed; therefore, immediate load shedding needs to be arranged for the load priorities from 1 to k + 1.

[0036] By controlling and assigning values to the B2B-MMC and energy storage system, implement delayed load shedding for the load corresponding to the load amount equal to P2 in P1. Specifically, accumulate the load amounts of each load with load priorities from k + 2 to m to obtain the load amount P m , when P m is less than or equal to P2 and P m+1 is greater than P2, it can be obtained that the loads with load priorities from k + 2 to m need to be delayed-shed; therefore, delayed load shedding needs to be arranged for the load priorities from k + 2 to m; where the maximum delay time t max = S / P2.

[0037] Implement maintaining power supply for the load corresponding to the load amount equal to P3 in P1. Specifically, excluding the loads with load priorities from 1 to k + 1 for immediate load shedding and the loads with load priorities from k + 2 to m for delayed load shedding, it can be obtained that the loads with load priorities from m + 1 to n need to have power supply maintained; therefore, maintaining power supply needs to be arranged for the load priorities from m + 1 to n.

[0038] The following is an example to illustrate the specific implementation of the present invention.

[0039] For example: For a certain substation, its system diagram is as Figure 2 shown, the tie switches Q2, Q3, Q22, Q33 are closed; the circuit breakers CB2, CB3 are closed; the tie switches Q1, Q11 are open; the circuit breaker CB1 is open. Assume that one of the main transformers T1 cannot continue to operate normally due to heavy load, maintenance, fault, etc. and load shedding is required. The total load amount P1 that cannot be powered on continuously on T1 = 30 MW, the maximum output power P2 of the energy storage system C = 10 MW, its capacity S = 8 MWh, and the remaining load amount P3 that the other main transformer T2 reaches full load = 8 MW; the importance grading and value information of the load that can be shed on the main transformer T1 are as follows:

[0040] Load that can be shed with priority 1: 1.6 MW

[0041] Load that can be shed with priority 2: 1.1 MW

[0042] Load that can be shed with priority 3: 1.4 MW

[0043] Load that can be shed with priority 4: 5.5 MW

[0044] Load that can be shed with priority 5: 0.6 MW

[0045] Load that can be shed with priority 6: 1.6 MW

[0046] Load that can be shed with priority 7: 2.3 MW

[0047] Load that can be shed with priority 8: 3.4 MW

[0048] Load that can be shed with priority 9: 2.0 MW

[0049] Load that can be shed with priority 10: 1.7 MW

[0050] Load that can be shed with priority 11: 0.9 MW

[0051] Load that can be shed with priority 12: 3.3 MW

[0052] Load that can be shed with priority 13: 1.5 MW

[0053] Load that can be shed with priority 14: 0.8 MW

[0054] Load that can be shed with priority 15: 2.3 MW

[0055] According to the method of the present invention, since the total load P1 = 30 MW that cannot be continuously powered on the main transformer T1 is greater than the remaining load P3 = 8 MW when the other main transformer T2 reaches full load, the load quantity P4 to be cut off at this time is calculated as P4 = P1 - P3 = 22 MW. And P4 = 22 MW is greater than the maximum output power P2 = 10 MW of the energy storage system C. The load quantity P5 to be immediately cut off at this time is calculated as P5 = P1 - P3 - P2 = 30 MW - 8 MW - 10 MW = 12 WM, that is, the load corresponding to P5 = 12 MW in P1 needs to be immediately cut off; through calculation, the total load P(6) of loads with priority 1 to 6 is 11.8 MW, which is less than P5 = 12 MW, and P(7) = 14.1 MW. It can be obtained that the loads corresponding to priorities 1 to 7 are the loads to be immediately cut off; therefore, the loads with priorities 1 to 7 are arranged for immediate cut off.

[0056] Next, by controlling and assigning values to the B2B - MMC and the energy storage system, the non - immediately - cut - off loads corresponding to P2 + P3 = 18 MW in P1 are transferred for power supply; among them, the control mode of the B2B - MMC is set to U dc Q - PQ control, and the control mode of the energy storage system is set to P control.

[0057] After that, the loads corresponding to the load quantity of P2 in P1 are cut off with a time delay; through calculation, the total load P 11 = 8 MW is less than or equal to P2 = 10 MW, and P 12= 11.3 MW is greater than P2 = 10 MW. It can be obtained that the load priorities 8 to 11 are the loads that need to be shed with a time delay. Therefore, the loads with load priorities 8 to 11 are arranged to be shed with a time delay; the maximum time delay t max = S / P 延时 = 8 MWh / 10 WM = 0.8 h, that is, t max is 48 minutes.

[0058] At the same time, the power supply to the load corresponding to the load amount of P3 equivalent in P1 is maintained; excluding the loads with load priorities 1 to 7 that are immediately shed and the loads with load priorities 8 to 11 that are shed with a time delay, it can be obtained that the load priorities 12 to 15 are the loads that need to maintain power supply. Therefore, it is necessary to arrange to maintain the power supply for the loads with load priorities 12 to 15.

[0059] The present invention connects the main transformers in the substation through the B2B - MMC and the energy storage system to form a flexible interconnected substation; by controlling and assigning values to the B2B - MMC and the energy storage system; when shedding loads, it realizes that the loads that need to maintain power supply maintain power supply, and the loads that need to be shed with a time delay are shed with a time delay, and realizes a large - scale reduction of various losses brought to various load users by the load - shedding power outage.

[0060] The above embodiments are only used to illustrate the technical idea of the present invention, and the protection scope of the present invention cannot be limited thereby. Any changes made on the basis of the technical solution according to the technical idea proposed by the present invention fall within the protection scope of the present invention.

Claims

1. A load shedding method for a flexible interconnected substation, characterized in that: The method connects the main transformers in the substation through a back-to-back modular multilevel converter B2B-MMC and an energy storage system to form a flexible interconnected substation; when a certain main transformer in the substation needs to shed load, by controlling and assigning values to the B2B-MMC and the energy storage system, first, the load that needs to be immediately shed is immediately shed; secondly, the non-immediate load is transferred for power supply by controlling and assigning values to the B2B-MMC and the energy storage system; then, the load that needs to be shed with a delay is shed with a delay; finally, the load that needs to maintain power supply is maintained for power supply; the method specifically includes: Step 1: When it is judged that a certain main transformer in the substation cannot continue to work normally due to heavy load, maintenance or fault and needs to shed load, obtain the total load amount P1 that cannot be continuously powered on the inoperable main transformer, the maximum output power P2 and capacity S of the energy storage system, the remaining available load amount P3 of the remaining main transformers in the substation when reaching full load, and the importance grading and quantity information of the load that can be shed on the inoperable main transformer; the load importance grading is recorded as priority 1, 2,..., n; Step 2: When P1 is less than or equal to P3, there is no need to shed load at this time; by controlling and assigning values to the B2B-MMC, it is realized that all the loads corresponding to P1 are transferred and supplied by other main transformers; Step 3: When P1 is greater than P3, calculate the load amount P4 that needs to be shed at this time, and its calculation formula is P4 = P1 - P3; Step 4: When P4 is less than or equal to the maximum output power P2 of the energy storage system, control and assign values to the B2B-MMC and the energy storage system to realize power supply transfer to the corresponding loads of P1; then, perform a delayed cut-off on the load corresponding to the load amount of P4 in P1, and record the delayed cut-off load amount as P 延时 , and the maximum delay time is t max = S / P 延时 ; meanwhile, maintain power supply to the load corresponding to the load amount of P3 in P1; Step 5: When P4 is greater than the maximum output power P2 of the energy storage system, calculate the load amount P5 to be immediately cut off as P5 = P1 - P3 - P2, that is, the load corresponding to the load amount of P5 in P1 is immediately cut off; then, by controlling and assigning values to the B2B-MMC and the energy storage system, realize the power supply transfer for the non-immediately cut-off load corresponding to P2 + P3 in P1; afterwards, perform a delayed cut-off on the load corresponding to the load amount of P2 in P1, and record the delayed cut-off load amount as P 延时 , and the maximum delay time t max = S / P 延时 ; meanwhile, keep the power supply to the load corresponding to the load amount of P3 in P1.

2. A load shedding method for a flexible interconnected substation according to claim 1, characterized in that: The energy storage system is connected to the DC side of the B2B-MMC, and both ends of the B2B-MMC are connected to the low-voltage side of the main transformer.

3. A load shedding method for a flexible interconnected substation according to claim 1, characterized in that: The B2B-MMC control mode is U dc The Q-PQ control is adopted, and the energy storage system control mode is P control.

4. A load shedding method for a flexible interconnected substation according to claim 1, characterized in that: Maximum delay time t max = S / P 延时 ; when P4 <= P2, P 延时 = P4; when P4 > P2, P 延时 = P2.

5. A load shedding method for a flexible interconnected substation according to claim 1, characterized in that: The specific implementation method of step 4 includes: Step 4.1: Implement the delayed shedding of the load corresponding to the load amount with the same value as P4 in P1, and the maximum delay time is t max = S / P 延时 , accumulate the load amounts of loads with load priorities from 1 to k to obtain the load amount P(k) to be shed with delay. When P(k) is less than or equal to P4, and When P(k + 1) is greater than P4, it is obtained that the load priorities 1 to k are the loads to be cut off with delay, and the load priorities 1 to k are arranged to be cut off with delay; among them, the maximum delay time t max = S / P4; Step 4.2: Realize maintaining power supply for the load corresponding to the load amount of P3 in P1; excluding the loads with priorities 1 to k that need to be shed with a delay, the loads with priorities k + 1 to n are the loads that need to maintain power supply, and arrange to maintain power supply for the loads with priorities k + 1 to n.

6. A load shedding method for a flexible interconnected substation according to claim 1, characterized in that: The specific implementation method of step 5 includes: Step 5.1: Realize immediately shedding the load corresponding to the load amount of P5 in P1, accumulate the load amounts of each load with priorities 1 to k to obtain the load amount P(k) that needs to be immediately shed. When P(k) is less than P5 and P(k + 1) is greater than or equal to P5, the loads with priorities 1 to k + 1 are the loads that need to be immediately shed, and immediately shed the loads with priorities 1 to k + 1; Step 5.2: By controlling and assigning values to the B2B-MMC and the energy storage system, achieve the delayed shedding of the load corresponding to the load amount with the same value as P2 in P1, and accumulate the load amounts of each load with load priorities from k+2 to m to obtain the load amount P to be delayed and shed m , when P m is less than or equal to P2, and P m+1 is greater than P2, the loads with load priorities from k+2 to m are obtained as the loads to be delayed and shed, and the loads with load priorities from k+2 to m are arranged for delayed shedding; where the maximum delay time t max = S / P2; Step 5.3: Implement power supply maintenance for the load corresponding to the load amount with the same value as P3 in P1; excluding the load priorities 1 to k + 1 for immediate load shedding and the load priorities k + 2 to m for delayed load shedding, the load priorities m + 1 to n are the loads for which power supply needs to be maintained, and arrange for power supply maintenance for the load priorities m + 1 to n.

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