An analysis method for the oil discharge process of oil-immersed transformer accidents

Abstract

The present invention discloses an analysis method for the accident oil drainage process of an oil-immersed transformer, and belongs to the field of volume, flow, mass flow or liquid level measurement. In view of the fact that the current existing specifications, patents, papers and other studies are unable to describe the entire process of transformer oil drainage, the present invention combines microelement analysis to establish control differential equations for the transformer leakage process, the accident oil storage pit oil drainage process, and the oil drainage process when fire water flows in. For differential equations that do not have analytical solutions, the corresponding iterative calculation equations are obtained using the difference method. The analysis method for the accident oil drainage process of an oil-immersed transformer proposed in the present invention can quickly obtain the corresponding oil drainage curve as long as the design parameters of the existing transformer are introduced, which is conducive to the control of the entire oil drainage process and provides a theoretical basis for the design of parameters related to the accident oil drainage of the transformer.

Description

Technical Field

[0001] The invention relates to the field of volume, flow, mass flow or liquid level measurement, and in particular to an analysis method for an accident oil discharge process of an oil-immersed transformer. Background Art

[0002] During the oil-immersed transformer oil drainage process, for outdoor electrical equipment with a single oil volume of more than 1,000 kg, oil storage or oil blocking facilities should be installed. The volume should be designed according to 20% of the equipment oil volume, and the accident oil can be discharged to the total accident oil storage tank. The accident oil storage pit designed by the patent "An oil-immersed transformer accident oil storage tank" can solve the problem that the accident oil of the expanded transformer cannot be discharged, and there is no available space in the station to build a new accident oil tank. The transformer fire accident was analyzed by on-site investigation of the transformer burning situation; the transformer bushing oil leakage incident was investigated, and the main cause of the bushing oil leakage was obtained.

[0003] However, after the transformer explodes, stress concentration locations such as the bushing riser are often damaged and cracks are formed. The transformer oil flows out from the cracks to the accident oil pit at the transformer foundation. The oil in the accident oil pit is discharged to the total accident oil pool through the oil discharge pipe at the oil discharge port. Therefore, the leakage and discharge of transformer oil after the transformer explosion can be divided into two stages: (1) the transformer oil is discharged from the transformer bushing crack into the accident oil pit; (2) the oil in the accident oil pit is discharged to the total accident oil pool through the oil discharge port. In this process, if the leakage flow discharged into the accident oil pit is greater than the discharge volume of the accident oil pit oil, the amount of transformer oil retained in the accident oil pit will increase. When the retained oil volume exceeds the design capacity of the accident oil pit, the transformer oil will overflow to form a flowing fire, and the oil discharge process will fail; on the contrary, the amount of oil retained in the accident oil pit will continue to decrease until it is discharged to the total accident oil pool. The transformer oil will not form a flowing fire, and the oil discharge process will be successful. After a transformer catches fire, fire extinguishing will be carried out. Due to the use of fire-fighting water, the impact of fire-fighting water flowing into the accident oil storage pit needs to be considered during the process. Only relevant on-site investigations on the problem of transformer accident oil drainage have been seen at home and abroad. The relevant specifications have only made relatively principled provisions on the design of the accident oil pool. However, there are relatively few reports on theoretical analysis methods for the transformer accident oil drainage process, and no systematic research has yet been formed. Summary of the invention

[0004] In summary, the present invention provides an analysis method for the oil-immersed transformer accident oil drainage process. As the existing specifications, patents, papers and other studies are unable to describe the entire transformer oil drainage process, the present invention, based on the Bernoulli equation, combines the microelement analysis to establish the corresponding transformer accident oil drainage process control differential equation, forming a set of oil-immersed transformer accident oil drainage process analysis methods, providing a theoretical basis for the design of transformer accident oil drainage related parameters.

[0005] To achieve the above purpose, the specific technical solution is as follows:

[0006] S1. Establish the liquid level change equation, oil leakage volume change equation and transformer oil tank maximum leakage time equation during the transformer oil tank leakage process;

[0007] Specifically, the transformer explosion causes the casing to rupture and the transformer oil level above the opening will change due to transformer oil leakage. According to the Bernoulli equation, t The transformer outflow volume (or the volume change of transformer oil in the tank) during the period is:

[0008]

[0009] Where H1(t) is the height of the transformer oil level from the opening, g is the acceleration due to gravity, and A is K is the opening area at the root of the transformer bushing, Q1(t) is the flow rate at the opening, and C is the flow coefficient;

[0010] After the oil volume of dV1 flows out, the change in transformer oil level dH1 can be expressed as:

[0011]

[0012] Where A0 is the effective cross-sectional area of ​​the transformer box. After sorting, the control differential equation of the transformer oil level in the box is obtained:

[0013]

[0014] After integrating both sides of the differential equation governing the oil level of the transformer in the box, we get the following equation:

[0015]

[0016] In the formula, C0 is the integral constant. When t = 0, H1(t) = H0, where H0 is the initial transformer oil level. Substituting the transformer oil level as the initial condition H0 into formula (4), the integral constant can be obtained: Substituting into formula (4) we get:

[0017]

[0018] After sorting, the liquid level change equation of the transformer oil tank leakage process is obtained. The specific expression is as follows:

[0019]

[0020] The oil leakage volume change equation of the transformer oil tank leakage process is obtained from formula (6), and the specific expression is as follows:

[0021]

[0022] It is worth noting that the condition for formula (7) to be valid is that there is no oil leakage, that is, H1(t) ≥ 0, or according to formula (2) it should satisfy:

[0023]

[0024] After taking the derivative of formula (6), the maximum leakage time equation is obtained according to formula (8):

[0025]

[0026] Where, T 1max is the maximum leakage time of the transformer tank;

[0027] S2. Establish the liquid level change equation of the accident oil storage pit during the oil discharge process in stages, and establish the maximum oil discharge time equation of the accident oil storage pit;

[0028] Before deriving the analytical solution for the oil discharge volume of the accident oil pool, the following three simplifications were made:

[0029] 1. Because the flow time of transformer oil from the accident oil storage pit to the oil outlet of the accident oil storage pit has little effect on the oil discharge problem and can be ignored, the flow time of transformer oil from the accident oil storage pit to the oil outlet of the accident oil storage pit is ignored;

[0030] 2. Since the oil discharge speed of the accident oil storage pit depends on the oil discharge capacity of the oil discharge port, the blocking effect caused by pebbles and grilles is not considered;

[0031] 3. Because the blockage problem is not considered, the impact of fine particle blockage on the oil discharge capacity of the oil discharge port is not considered;

[0032] The staged analysis includes: the first stage, the oil in the transformer oil tank is discharged into the accident oil storage pit, and the accident oil storage pit is also in the oil discharge stage; the second stage, the oil in the transformer oil tank is completely discharged, and only the accident oil storage pit is in the oil discharge stage; the specific analysis process is as follows:

[0033] In the first stage, when the height of the liquid level in the accident oil storage pit from the oil discharge port of the accident oil storage pit (the first stage) is H 2,1 At (t), according to the Bernoulli equation, the volume of oil discharged from the accident oil storage pit during the period dt (or the change in the oil volume in the accident oil storage pit) is:

[0034]

[0035] Where V2 is the volume of oil discharged from the accident oil storage pit, Q2(t) is the flow rate at the oil outlet of the accident oil storage pit, and A P is the effective area of ​​the oil discharge port, g is the acceleration due to gravity;

[0036] The micro-unit volume leaking from the transformer oil tank into the accident oil storage pit is:

[0037] dV1=A0dH1(t), t≤T 1max (11)

[0038] Where, T 1max is the maximum leakage time of the transformer oil tank, which is determined according to formula (9). After flowing into this micro-unit, the change in the transformer oil level in the accident oil storage pit is:

[0039]

[0040] In the formula, A C is the effective cross-sectional area of ​​the accident oil storage pit, A K is the opening area at the root of the transformer bushing. Substituting formula (5) into formula (12), we get:

[0041]

[0042] make Arranging equation (13) yields the equation for the change of liquid level in the accident oil storage pit during this stage:

[0043]

[0044] Equation (14) has no analytical solution.

[0045] In the second stage, it is not difficult to see from formula (12) that formula (13) is valid only when H1(t)>0, or in other words, it is valid only when there is still oil in the transformer tank leaking to the accident oil storage pit. 1max ), H1(t)=0, then according to formula (12):

[0046]

[0047] In the formula, H 2,2 (t) is the second stage height of the liquid level in the accident oil storage pit from the oil discharge port of the accident oil storage pit;

[0048] After sorting, we get:

[0049]

[0050] Solving equation (16) yields:

[0051]

[0052] The starting time of formula (17) is T 1max At this time, the oil level in the accident oil storage pit is H 2max, then we get the integral constant Substituting into equation (17), we get t>T 1max When , the liquid level change equation of the accident oil storage pit in this stage is obtained:

[0053]

[0054] The condition for formula (18) to be valid is that before the accident oil storage pit is emptied, that is,

[0055]

[0056] The maximum oil discharge time equation of the accident oil storage pit is obtained as follows:

[0057]

[0058] S3. Establish the equation for the change of liquid level during the oil discharge process of the accident oil storage pit when considering the inflow of fire water;

[0059] According to formula (9), for a 500kVA transformer, H0 = 5.9m, A0 = 27.1m 2 , A K =0.205m 2 , C = 0.6, then we get T 1max =241s≈4min, at which time most fire-fighting actions have not yet begun;

[0060] Therefore, the fire extinguishing action often occurs when the oil in the transformer tank is drained and only the accident oil pit is drained; assuming that the fire extinguishing action occurs at T m Time (T m >T 1max ), the fire water flow rate into the accident oil storage pit is Q m , then based on formula (15), the fire flow is introduced, and formula (15) is corrected to:

[0061]

[0062] In the formula, V is the distance between the liquid level in the accident oil storage pit and the oil outlet of the accident oil storage pit, and considering the height of the fire water, m The volume of fire water flowing into the accident oil storage pit;

[0063] After sorting, the equation for the change of liquid level in the accident oil storage pit during this stage is obtained:

[0064]

[0065] Equation (22) has no analytical solution;

[0066] S4. Use numerical methods to solve differential equations that have no analytical solutions;

[0067] From the above, it can be seen that equation (14) and equation (22) have no analytical solutions and can only be solved numerically. The numerical method is the difference method, and the specific analysis process is as follows:

[0068] When the oil in the transformer tank is discharged into the accident oil storage pit, and the accident oil storage pit is also in the oil discharge stage, the derivation process is as follows:

[0069] Based on formula (14), the differential equation satisfies the following form:

[0070]

[0071] Considering the difference approximation of the first-order derivative, we get:

[0072]

[0073] Where Δt = t i -t i-1 is the time step, H 2,1i t i The height of the liquid level in the oil storage pit from the oil discharge port at the moment of the accident, H 2,1i-1 t i-1 The height of the liquid level in the accident oil storage pit from the oil discharge port at the moment, i∈(1, N), where N is a non-negative natural number;

[0074] ΔH 2,1i Indicates that at two consecutive moments t i With t i-1 The change in the liquid level in the accident oil storage pit between the two events is shown in the following formula:

[0075] ΔH 2i =H 2i -H 2i-1 (25)

[0076] right In t i-1 Expand it, and we get:

[0077]

[0078] Substituting equations (24) and (26) into equation (23), we obtain:

[0079]

[0080] After sorting, the iterative calculation format is obtained:

[0081]

[0082] because:

[0083]

[0084] Therefore, in H 2,1i When >0, the above iterative process is stable and there is a numerical solution. At the same time, according to formula (14), the condition for formula (28) to be valid is t <T 1max ;

[0085] When the oil in the transformer tank is completely drained and only the accident oil storage pit is drained, and fire water is considered to flow in, the derivation process is as follows:

[0086] Based on formula (22), the differential equation satisfies the following form

[0087]

[0088] Considering the difference approximation of the first-order derivative, we get

[0089]

[0090] In the formula, t i The height of the liquid level in the accident oil storage pit from the oil discharge port at the moment, t i-1 The height of the liquid level in the accident oil storage pit from the oil discharge port at any moment; Indicates that at two consecutive moments t i With t i-1 The change in the liquid level in the accident oil storage pit between the two events is shown in the following formula:

[0091]

[0092] right In t i-1 Expand it, and we get:

[0093]

[0094] Substituting equations (31) and (33) into equation (30), we obtain the following equation:

[0095]

[0096] After sorting, the iterative format is obtained, and the specific expression is as follows:

[0097]

[0098] It is worth noting that the condition for formula (22) to be valid is after the fire extinguishing action occurs. Therefore, the iterative format described in formula (35) is only valid after the water fire extinguishing action occurs, that is:

[0099] t≥T m >T 1max (36)

[0100] Among them, Tm is the start time of fire extinguishing action, which can be determined according to the actual situation. 1max is the maximum leakage time of the transformer, determined according to formula (9);

[0101] S5. Develop calculation parameters for transformer and accident oil storage pit;

[0102] The symbols and descriptions of the transformer and accident oil storage pit design parameters that need to be drawn up are shown in Table 1.

[0103] Table 1 Design parameters and their descriptions

[0104] symbol unit illustrate <![CDATA[H0]]> m Initial transformer oil level <![CDATA[V0]]> <![CDATA[m 3 ]]> Transformer oil volume above the opening <![CDATA[A0]]> <![CDATA[m 2 ]]> <![CDATA[The effective cross-sectional area A0 of the transformer housing = V0 / H0]]> <![CDATA[A K ]]> <![CDATA[m 2 ]]> Transformer bushing root opening area <![CDATA[A C ]]> <![CDATA[m 2 ]]> Effective cross-sectional area of ​​accident oil storage pit <![CDATA[A P ]]> <![CDATA[m 2 ]]> Effective area of ​​oil outlet of accident oil storage pit

[0105] S6. Obtaining by calculation the liquid level change curve and the oil volume change curve during the transformer leakage process and the accident oil storage pit oil discharge process;

[0106] S7. Draw conclusions about the changes in oil and water volumes based on the curve.

[0107] Beneficial effects of the present invention

[0108] 1. Currently, existing specifications, patents, papers and other research cannot describe the entire process of transformer oil drainage. The present invention fills this blank area.

[0109] 2. Compared with the traditional numerical modeling calculation method, the method proposed in the present invention has the advantages of being effective and simple, and can save time and labor costs to a great extent.

[0110] 3. The invention can describe in detail the entire process of transformer oil drainage and provide a theoretical basis for the design of accident oil storage pits. BRIEF DESCRIPTION OF THE DRAWINGS

[0111] Figure 1 It is a flow chart of the present invention;

[0112] Figure 2 This is a simplified schematic diagram of the 220kVA transformer and the accident oil storage pit;

[0113] Figure 3 is the liquid level change curve during transformer leakage process;

[0114] Figure 4 It is the oil volume change curve during transformer leakage process;

[0115] Figure 5 It is the liquid level change curve during the oil discharge process of the accident oil storage pit;

[0116] Figure 6 The oil volume change curve of the accident oil storage pit during the oil discharge process;

[0117] Figure 7Comparison of oil volume change curves of transformer leakage process simulated by the present invention and DualsPhysics software;

[0118] Figure 8 The oil volume change curves of the accident oil storage pit oil discharge process simulated by the present invention and DualsPhysics software are compared. DETAILED DESCRIPTION

[0119] The present invention is further described in detail below in conjunction with specific embodiments.

[0120] Example 1

[0121] like Figure 1 As shown, a method for analyzing the oil discharge process of an oil-immersed transformer accident includes the following steps:

[0122] S1. Establish the liquid level change equation, oil leakage volume change equation and transformer oil tank maximum leakage time equation during the transformer oil tank leakage process;

[0123] Specifically, the transformer explosion causes the casing to rupture and the transformer oil level above the opening will change due to transformer oil leakage. According to the Bernoulli equation, t The transformer outflow volume (or the volume change of transformer oil in the tank) during the period is:

[0124]

[0125] Where H1(t) is the height of the transformer oil level from the opening, g is the acceleration due to gravity, and A is K is the opening area at the root of the transformer bushing, Q1(t) is the flow rate at the opening, and C is the flow coefficient;

[0126] After the oil volume of dV1 flows out, the change in transformer oil level dH1 can be expressed as:

[0127]

[0128] Where A0 is the effective cross-sectional area of ​​the transformer box. After sorting, the control differential equation of the transformer oil level in the box is obtained:

[0129]

[0130] After integrating both sides of the differential equation governing the oil level of the transformer in the box, we get the following equation:

[0131]

[0132] In the formula, C0 is the integral constant. When t = 0, H1(t) = H0, where H0 is the initial transformer oil level. Substituting the transformer oil level as the initial condition H0 into formula (4), the integral constant can be obtained: Substituting into formula (4) we get:

[0133]

[0134] After sorting, the liquid level change equation of the transformer oil tank leakage process is obtained. The specific expression is as follows:

[0135]

[0136] The oil leakage volume change equation of the transformer oil tank leakage process is obtained from formula (6), and the specific expression is as follows:

[0137]

[0138] It is worth noting that the condition for formula (7) to be valid is that there is no oil leakage, that is, H1(t) ≥ 0, or according to formula (2) it should satisfy:

[0139]

[0140] After taking the derivative of formula (6), the maximum leakage time equation is obtained according to formula (8):

[0141]

[0142] Where, T 1max is the maximum leakage time of the transformer tank;

[0143] S2. Establish the liquid level change equation of the accident oil storage pit during the oil discharge process in stages, and establish the maximum oil discharge time equation of the accident oil storage pit;

[0144] Before deriving the analytical solution for the oil discharge volume of the accident oil pool, the following three simplifications were made:

[0145] 1. Because the flow time of transformer oil from the accident oil storage pit to the oil outlet of the accident oil storage pit has little effect on the oil discharge problem and can be ignored, the flow time of transformer oil from the accident oil storage pit to the oil outlet of the accident oil storage pit is ignored;

[0146] 2. Since the oil discharge speed of the accident oil storage pit depends on the oil discharge capacity of the oil discharge port, the blocking effect caused by pebbles and grilles is not considered;

[0147] 3. Because the blockage problem is not considered, the impact of fine particle blockage on the oil discharge capacity of the oil discharge port is not considered;

[0148] The staged analysis includes: the first stage, the oil in the transformer oil tank is discharged into the accident oil storage pit, and the accident oil storage pit is also in the oil discharge stage; the second stage, the oil in the transformer oil tank is completely discharged, and only the accident oil storage pit is in the oil discharge stage; the specific analysis process is as follows:

[0149] In the first stage, when the height of the liquid level in the accident oil storage pit from the oil discharge port of the accident oil storage pit (the first stage) is H 2,1 At (t), according to the Bernoulli equation, the volume of oil discharged from the accident oil storage pit during the period dt (or the change in the oil volume in the accident oil storage pit) is:

[0150]

[0151] Where V2 is the volume of oil discharged from the accident oil storage pit, Q2(t) is the flow rate at the oil outlet of the accident oil storage pit, and A P is the effective area of ​​the oil discharge port, g is the acceleration due to gravity;

[0152] The micro-unit volume leaking from the transformer oil tank into the accident oil storage pit is:

[0153] dV1=A0dH1(t), t≤T 1max (11)

[0154] Where, T 1max is the maximum leakage time of the transformer oil tank, which is determined according to formula (9). After flowing into this micro-unit, the change in the transformer oil level in the accident oil storage pit is:

[0155]

[0156] In the formula, A C is the effective cross-sectional area of ​​the accident oil storage pit, A K is the opening area at the root of the transformer bushing. Substituting formula (5) into formula (12), we get:

[0157]

[0158] make Arranging equation (13) yields the equation for the change of liquid level in the accident oil storage pit during this stage:

[0159]

[0160] Equation (14) has no analytical solution.

[0161] In the second stage, it is not difficult to see from formula (12) that formula (13) is valid only when H1(t)>0, or in other words, it is valid only when there is still oil in the transformer tank leaking to the accident oil storage pit. 1max), H1(t)=0, then according to formula (12):

[0162]

[0163] In the formula, H 2,2 (t) is the second stage height of the liquid level in the accident oil storage pit from the oil discharge port of the accident oil storage pit;

[0164] After sorting, we get:

[0165]

[0166] Solving equation (16) yields:

[0167]

[0168] The starting time of formula (17) is T 1max At this time, the oil level in the accident oil storage pit is H 2max , then we get the integral constant Substituting into equation (17), we get t>T 1max When , the liquid level change equation of the accident oil storage pit in this stage is obtained:

[0169]

[0170] The condition for formula (18) to be valid is that before the accident oil storage pit is emptied, that is,

[0171]

[0172] The maximum oil discharge time equation of the accident oil storage pit is obtained as follows:

[0173]

[0174] S3. Establish the equation for the change of liquid level during the oil discharge process of the accident oil storage pit when considering the inflow of fire water;

[0175] According to formula (9), for a 500kVA transformer, H0 = 5.9m, A0 = 27.1m 2 , A K =0.205m 2 , C = 0.6, then we get T 1max =241s≈4min, at which time most fire-fighting actions have not yet begun;

[0176] Therefore, the fire extinguishing action often occurs when the oil in the transformer tank is drained and only the accident oil pit is drained; assuming that the fire extinguishing action occurs at T m Time (T m >T 1max ), the fire water flow rate into the accident oil storage pit is Q m, then based on formula (15), the fire flow is introduced, and formula (15) is corrected to:

[0177]

[0178] In the formula, V is the distance between the liquid level in the accident oil storage pit and the oil outlet of the accident oil storage pit, and considering the height of the fire water, m The volume of fire water flowing into the accident oil storage pit;

[0179] After sorting, the equation for the change of liquid level in the accident oil storage pit during this stage is obtained:

[0180]

[0181] Equation (22) has no analytical solution;

[0182] S4. Use numerical methods to solve differential equations that have no analytical solutions;

[0183] From the above, it can be seen that equation (14) and equation (22) have no analytical solutions, and can only be solved numerically; the numerical method is the difference method, and the specific analysis process is as follows:

[0184] When the oil in the transformer tank is discharged into the accident oil storage pit, and the accident oil storage pit is also in the oil discharge stage, the derivation process is as follows:

[0185] Based on formula (14), the differential equation satisfies the following form:

[0186]

[0187] Considering the difference approximation of the first-order derivative, we get:

[0188]

[0189] Where Δt = t i -t i-1 is the time step, H 2,1i t i The height of the liquid level in the oil storage pit from the oil discharge port at the moment of the accident, H 2,1i-1 t i-1 The height of the liquid level in the accident oil storage pit from the oil discharge port at the moment, i∈(1, N), where N is a non-negative natural number;

[0190] ΔH 2,1i Indicates that at two consecutive moments t i With t i-1 The change in the liquid level in the accident oil storage pit between the two events is shown in the following formula:

[0191] ΔH 2i =H 2i -H2i-1 (25)

[0192] right In t i-1 Expand it, and we get:

[0193]

[0194] Substituting equations (24) and (26) into equation (23), we obtain:

[0195]

[0196] After sorting, the iterative calculation format is obtained:

[0197]

[0198] because:

[0199]

[0200] Therefore, in H 2,1i When >0, the above iterative process is stable and there is a numerical solution. At the same time, according to formula (14), the condition for formula (28) to be valid is t <T 1max ;

[0201] When the oil in the transformer tank is completely drained and only the accident oil storage pit is drained, and fire water is considered to flow in, the derivation process is as follows:

[0202] Based on formula (22), the differential equation satisfies the following form

[0203]

[0204] Considering the difference approximation of the first-order derivative, we get

[0205]

[0206] In the formula, t i The height of the liquid level in the accident oil storage pit from the oil discharge port at the moment, t i-1 The height of the liquid level in the accident oil storage pit from the oil discharge port at any moment; Indicates that at two consecutive moments t i With t i-1 The change in the liquid level in the accident oil storage pit between the two events is shown in the following formula:

[0207]

[0208] right In t i-1 Expand it, and we get:

[0209]

[0210] Substituting equations (31) and (33) into equation (30), we obtain the following equation:

[0211]

[0212] After sorting, the iterative format is obtained, and the specific expression is as follows:

[0213]

[0214] It is worth noting that the condition for formula (22) to be valid is after the fire extinguishing action occurs. Therefore, the iterative format described in formula (35) is only valid after the water fire extinguishing action occurs, that is:

[0215] t≥T m >T 1max (36)

[0216] Among them, T m is the start time of fire extinguishing action, which can be determined according to the actual situation. 1max is the maximum leakage time of the transformer, determined according to formula (9);

[0217] S5. Develop calculation parameters for transformer and accident oil storage pit;

[0218] The symbols and descriptions of the transformer and accident oil storage pit design parameters that need to be drawn up are shown in Table 1.

[0219] Table 1 Design parameters and their descriptions

[0220] symbol unit illustrate <![CDATA[H0]]> m Initial transformer oil level <![CDATA[V0]]> <![CDATA[m 3 ]]> Transformer oil volume above the opening <![CDATA[A0]]> <![CDATA[m 2 ]]> <![CDATA[The effective cross-sectional area A0 of the transformer housing = V0 / H0]]> <![CDATA[A K ]]> <![CDATA[m 2 ]]> Transformer bushing root opening area <![CDATA[A C ]]> <![CDATA[m 2 ]]> Effective cross-sectional area of ​​accident oil storage pit <![CDATA[A P ]]> <![CDATA[m 2 ]]> Effective area of ​​oil outlet of accident oil storage pit

[0221] Taking a 220kVA transformer as an example, a simplified schematic diagram of the transformer and the accident oil storage pit is shown below: Figure 2 The specific design parameters are shown in Table 2.

[0222] Table 2 Design parameters of 220kVA transformer and its accident oil storage pit

[0223]

[0224]

[0225] S6. The liquid level change curve and oil volume change curve of the transformer leakage process and the accident oil storage pit oil discharge process are obtained by calculation. The specific steps are as follows:

[0226] S6.1. The liquid level change curve and oil volume change curve of the transformer leakage process are obtained by calculation. Specifically, according to the calculation parameters of the 220kVA transformer given in Table 2, the liquid level change curve of the transformer leakage process is obtained by calculation through formula (6). Figure 3 The oil volume change curve of the transformer leakage process is obtained by calculating formula (7). Figure 4 , the maximum leakage time of the transformer oil tank T is calculated by formula (9): 1max =320s;

[0227] S6.2. Calculate the liquid level change curve and oil volume change curve during the oil discharge process of the accident oil storage pit, specifically:

[0228] S6.2.1. According to the calculation parameters given in Table 2, calculate H under this condition. 2max =0.911m, when the oil is discharged from the transformer tank into the accident oil storage pit, the accident oil storage pit is also in the oil discharge stage, that is, t≤T 1max When the oil discharge process is in the stage where the oil in the transformer oil tank is completely discharged and only the accident oil storage pit is discharged, that is, t>T 1max When the liquid level change curve of this stage is calculated by formula (18), the liquid level change curve of the accident oil storage pit during the oil discharge process is obtained by combining the curves of the two stages. Figure 5 .

[0229] S6.2.2. Based on the liquid level change curve of the accident oil storage pit during the oil discharge process, the oil volume change curve of the accident oil storage pit during the oil discharge process can be obtained by the effective cross-sectional area A of the accident oil storage pit. C Get, that is

[0230]

[0231] Where V 2,1 (t) is the volume of oil discharged from the oil storage pit in the first stage of the accident, V 2,2 (t) is the volume of oil discharged from the accident oil storage pit in the second stage; the curves of the two stages are combined to obtain the oil volume change curve of the accident oil storage pit during the oil discharge process. Figure 6 ;

[0232] S6.3. Considering the liquid level change curve and oil volume change curve of the accident oil storage pit during the oil discharge process when using fire water, the specific ones are:

[0233] S6.3.1. According to the calculation parameters given in Table 2, when the oil in the transformer oil tank is discharged into the accident oil storage pit during the oil discharge process, and the accident oil storage pit is also in the oil discharge stage, that is, t≤T 1maxWhen the oil discharge process is in the oil discharge stage of the transformer oil tank, only the accident oil storage pit is discharged and the fire water is considered, that is, t ≥ T m >T 1max When the liquid level change curve of this stage is calculated by formula (35), the liquid level change curve of the accident oil storage pit oil discharge process is obtained by combining the two curves. Figure 5 ;

[0234] S6.3.2. Based on the liquid level change curve of the accident oil storage pit during the oil discharge process, the oil volume change curve of the accident oil storage pit during the oil discharge process when fire water is used can be obtained by calculating the effective cross-sectional area A of the accident oil storage pit. C Obtain, that is:

[0235]

[0236] In the formula, The volume of oil discharged from the accident oil storage pit in the second stage and considering the use of fire water. When analyzing the impact of fire water, according to the "Technical Specifications for Fire Water Supply and Fire Hydrant Systems", the outdoor fire hydrant flow is calculated as 30L / s, and the fire extinguishing action start time is calculated as 350s; after merging the two curves, the oil volume change curve of the accident oil storage pit oil discharge process when considering fire water is obtained. Figure 6 ;

[0237] S7. Draw conclusions on the changes in oil and water volumes based on the curve;

[0238] according to Figure 5 , 6 The analysis results show that:

[0239] 1. The transformer oil volume in the accident oil storage pit will reach its peak at 209s. At this time, the peak liquid level of the transformer oil in the accident oil storage pit is 1.15m and the volume is 48.3m 3 This shows that in the early stage of transformer oil leakage, a large amount of transformer oil quickly flows into the accident oil storage pit, causing the oil volume in the accident oil storage pit to reach a peak in a very short time, and then the oil volume in the accident oil storage pit gradually decreases after being discharged to the total accident oil storage pit through the oil drain port. If the capacity of the accident oil storage pit is 20% of the total oil volume of the transformer according to the current specifications, the current design total oil volume of the 220kVA transformer is 92.7m 3 The capacity of the accident oil storage pit is 18.54m 3 , will overflow 29.76m 3 This result means that under the current design conditions, after the transformer explodes, the accident oil storage pit cannot contain the leaked transformer oil in time, and the transformer oil overflow may cause a large-scale flowing fire.

[0240] 2. Since the transformer oil volume in the accident oil storage pit reached its peak at 209s, earlier than the time when the fire extinguishing action occurred, in other words, the transformer oil had overflowed from the accident oil storage pit when the fire extinguishing action began, and compared with the oil discharge flow rate of the accident oil storage pit, the fire water flow rate is often lower than the oil discharge flow rate. In addition, if the fire water is not considered, the transformer oil level in the accident oil storage pit at 700s is 0.01m and the volume is 0.54m 3 , the transformer oil is basically drained; if the inflow of fire water is considered, the liquid level of the oil-water mixture in the accident oil storage pit is 0.11m at 700s, and the volume is 4.87m 3 Therefore, the amount of water used for fire fighting only has a certain impact on the volume of oil and water in the later stage of oil discharge from the accident oil storage pit, but the impact is relatively small.

[0241] Example 2

[0242] The proposed oil discharge model was compared and verified by the DualsPhysics software simulation method. The calculation parameters are shown in Table 3. Figure 7 , Figure 8 As shown;

[0243] Table 3 Calculation parameters of simulation cases

[0244]

[0245] Figure 6 The time-varying law of the volume of transformer oil flowing out of the oil tank after the transformer explodes is given. Figure 7 The law of the change of transformer oil volume discharged from the accident oil storage pit over time is given, and it can be seen that: (1) the maximum transformer oil volume discharged from the oil tank is 68.85m 3 , DualsPhysics software simulation method is 66.55m 3 , a difference of 2.30m 3 (2) the volume of transformer oil removed from the accident oil storage pit, the present invention is 68.88m 3 , DualsPhysics software simulation method is 65.40m 3 , a difference of 3.48m 3 The calculation results of the two methods are generally consistent, but the DualsPhysics software simulation method shows that a small amount of oil is not discharged. The main reason is that the DualsPhysics software uses oil droplets of limited size (5 cm in diameter) when simulating.

[0246] The present invention can describe the entire process of transformer oil drainage and can accurately judge the fire risk after the transformer explosion. However, the existing specifications and patents are unable to display the entire oil drainage process. The present invention fills this gap, is conducive to full process control, and reduces fire safety risks.

Claims

1. A method for analyzing the oil discharge process of an oil-immersed transformer accident, characterized in that: The following steps are involved: S1. Establishing the liquid level change equation, oil leakage volume change equation and transformer oil tank maximum leakage time equation during the transformer oil tank leakage process; S2. Establish the liquid level change equation of the accident oil storage pit during the oil discharge process in stages, and establish the maximum oil discharge time equation of the accident oil storage pit; The phased method includes: the oil in the transformer oil tank is discharged into the accident oil storage pit, while the accident oil storage pit is also in the oil discharge phase and the oil in the transformer oil tank is completely discharged, and only the accident oil storage pit is in the oil discharge phase; The oil in the transformer tank is discharged into the accident oil storage pit. At the same time, the liquid level change equation of the accident oil storage pit during the oil discharge stage is as follows: In the formula, H 2,1 (t) is the first stage height of the liquid level in the accident oil storage pit from the oil discharge port of the accident oil storage pit, A C is the effective cross-sectional area of ​​the accident oil storage pit, A P is the effective area of ​​the oil discharge port, t is the time, and the equation has no analytical solution; After the oil in the transformer tank is drained, the equation for the change in the liquid level during the oil draining phase of the accident oil storage pit is as follows: In the formula, H 2,2 (t) is the second stage height of the liquid level in the accident oil storage pit from the oil discharge port of the accident oil storage pit; T 1max is the maximum leakage time of the transformer tank, H 2max It is T 1max The oil level in the accident oil storage pit at any moment; The expression of the maximum oil discharge time equation of the accident oil storage pit is as follows: Where, T 2max The maximum oil discharge time of the accident oil storage pit; S3. Establish the equation for the change of liquid level during the oil discharge process of the accident oil storage pit when considering the inflow of fire water; S4. When using numerical methods to solve differential equations that do not have analytical solutions, the numerical method used is the difference method, as follows: When the oil in the transformer tank is discharged into the accident oil storage pit, and the accident oil storage pit is also in the oil discharge stage, the numerical solution of the liquid level change equation is determined according to the difference method, and the iterative calculation expression is as follows: Where Δt = t i -t i-1 is the time step, H 2,1i t i The height of the liquid level in the oil storage pit from the oil discharge port at the moment of the accident, H 2,1i-1 t i-1 The height of the liquid level in the accident oil storage pit from the oil discharge port at the moment, ΔH 2,1i =H 2,1i -H 2,1i-1 Indicates that at two consecutive moments t i With t i-1 The change in the liquid level in the accident oil storage pit between the two events, i∈(1,N), where N is a non-negative natural number; When the oil in the transformer tank is completely drained and only the accident oil storage pit is drained, considering the inflow of fire water, the numerical solution of the differential equation of the liquid level change is determined according to the difference method. The specific iterative calculation expression is as follows: In the formula, t i The height of the liquid level in the accident oil storage pit from the oil discharge port at the moment, t i-1 The height of the liquid level in the accident oil storage pit from the oil discharge port at any moment; S5. Develop calculation parameters for transformer and accident oil storage pit; S6. Obtaining the liquid level change curve and the oil volume change curve during the transformer leakage process and the accident oil storage pit oil discharge process by calculation; S7. Draw conclusions about the changes in oil and water volumes based on the curve.

2. The method for analyzing the oil-draining process of an oil-immersed transformer accident according to claim 1 is characterized in that: The expression of the liquid level change equation during the transformer oil tank leakage process is as follows: Where H1(t) is the height of the transformer oil level from the opening, t is time, A0 is the effective cross-sectional area of ​​the transformer box, C is the flow coefficient, and A K is the opening area of ​​the transformer bushing root, H0 is the initial transformer oil level, and g is the gravitational acceleration; The expression of the oil leakage volume change equation during the transformer oil tank leakage process is as follows: Where V1(t) is the volume of oil leaked from the transformer tank; The expression of the maximum leakage time equation of the transformer tank leakage process is as follows: Where, T 1max is the maximum leakage time of the transformer tank.

3. The method for analyzing the oil-draining process of an oil-immersed transformer accident according to claim 1 is characterized in that: The above-mentioned equation for the change of the liquid level during the oil discharge process of the accident oil storage pit when considering the inflow of fire water is established. The expression is as follows: In the formula, Q is the distance between the liquid level in the accident oil storage pit and the oil outlet of the accident oil storage pit, and considering the height of the fire water, m is the fire-fighting water flow into the accident oil storage pit. The equation has no analytical solution.

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