Centralized protection method for successive ground faults of small resistance grounded system

Abstract

The application discloses a centralized protection method for phase-to-phase-to-ground fault of a small-resistance grounding system, which utilizes the characteristic of the ratio of the neutral point zero sequence current to the maximum feeder zero sequence current amplitude to judge the fault type; then analyzes the characteristic of the ratio of the feeder zero sequence current of the phase-to-phase-to-ground fault to the minimum feeder zero sequence current and the phase difference between the feeder zero sequence current and the neutral point zero sequence current, sets the criterion by utilizing the characteristic difference between the phase-to-phase-to-ground fault feeder and the healthy feeder, and realizes the feeder selection of the phase-to-phase-to-ground fault. The centralized protection method for phase-to-phase-to-ground fault of the small-resistance grounding system can accurately judge the two-line same-phase phase-to-phase-to-ground fault, has certain reliability and anti-interference ability, and provides a solution and reference for the accurate identification of the phase-to-phase-to-ground fault of the power supply and distribution line.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of fault line selection of power supply and distribution lines, and particularly relates to a centralized protection method suitable for successive grounding faults of a small-resistance grounding system. BACKGROUND

[0002] With the gradual development of urban construction, the cable rate in cities is increasing year by year, and the maximum capacitive current level of the system is rising, which makes it difficult for a small-current grounding system to be applicable. A neutral point grounding system through a small-resistance grounding system accesses a resistance of a certain resistance value, which can suppress system overvoltage faults and has obvious fault characteristics, and has been used in some urban distribution networks. At present, three-section zero-sequence overcurrent protection is used as the main protection of the small-resistance grounding system. However, in extreme situations such as storms and snowstorms, the distribution network is prone to simultaneous or successive single-phase grounding faults of multiple feeders. Since the zero-sequence overcurrent method uses steady-state quantities, the judgment time is more than 20 ms. If simultaneous or nearly simultaneous single-phase grounding faults occur, it is easy to cause misoperation and refusal of the traditional zero-sequence overcurrent protection, and the protection is difficult to trip multiple feeders at the same time, which endangers the safety of the distribution network.

[0003] The fault characteristics and fault zero-sequence current distribution of multiple feeders are different from those of single-line faults, and the two-line same-phase grounding fault is the most difficult to identify among successive fault situations. Therefore, how to improve the sensitivity of the protection and realize the protection of multiple feeder faults is of great significance to ensure the safe and stable operation of the neutral point grounding system through a small-resistance grounding system. SUMMARY

[0004] The purpose of the present application is to provide a centralized protection method suitable for successive grounding faults of a small-resistance grounding system, which solves the problem that if simultaneous or nearly simultaneous single-phase grounding faults occur, it is easy to cause misoperation and refusal of the zero-sequence overcurrent protection, and improves the sensitivity of the protection.

[0005] The technical solution adopted by the present application is as follows: a centralized protection method suitable for successive grounding faults of a small-resistance grounding system, which analyzes the zero-sequence network of the most serious two-line same-phase simultaneous grounding fault of the successive faults, and obtains the difference in zero-sequence characteristics between the successive faults and single feeder grounding faults. The ratio of the neutral point zero-sequence current to the maximum feeder zero-sequence current amplitude is used to judge the fault type. Then the ratio of the feeder zero-sequence current to the minimum feeder zero-sequence current amplitude and the phase difference between the feeder zero-sequence current and the neutral point zero-sequence current of the successive faults are analyzed, and the difference between the characteristics of the successive fault feeder and the healthy feeder is used to set the criterion to realize the line selection of the successive faults; the specific operation steps are as follows:

[0006] Step 1: When the feeder zero-sequence current reaches the starting threshold, proceed to step 2;

[0007] Step 2: Obtain the zero-sequence currents flowing through the neutral point and each feeder from the station measurement device and each feeder zero-sequence current transformer.

[0008] Step 3: calculate the amplitude ratio of the neutral point zero sequence current and the maximum feeder zero sequence current, if the amplitude ratio of the neutral point zero sequence current and the maximum feeder zero sequence current is between the threshold values, it is determined that the two-line same-phase grounding fault occurs, and step 4 is entered; otherwise, the single-line single-phase grounding fault protection process is directly entered;

[0009] Step 4: calculate the amplitude ratio of all feeder zero sequence currents and the minimum feeder zero sequence current and the phase difference between all feeder zero sequence currents and the neutral point zero sequence current;

[0010] Step 5: determine whether the amplitude ratio and the phase difference calculation values of two or more feeders are within the protection threshold value range, if yes, step 6 is entered; otherwise, the single-line single-phase grounding fault protection process is directly entered;

[0011] Step 6: each feeder protection device performs tripping processing according to the collected protection action signals.

[0012] The application also has the characteristics that,

[0013] The specific method for judging the fault type is that: first, when the zero sequence current collected in step 1 meets the starting condition, the zero sequence currents of each feeder and the neutral point are collected. The amplitude ratio of the neutral point zero sequence current and the maximum feeder zero sequence current is calculated

[0014] Among them, is the neutral point zero sequence current, is the maximum zero sequence current of each feeder.

[0015] Then, the specific calculation process of the fault type criterion threshold value determination is as follows:

[0016] The amplitude of the neutral point zero sequence current under the two-line same-phase fault is proportional to the size of the bus zero sequence voltage. Compared with the single feeder grounding fault, under the two-line same-phase fault, the bus zero sequence voltage is as formula (11), and the transition resistance of the two grounding fault points is equivalent to parallel in the fault equivalent network, and its value is 1-1 / 2 times of the same single-phase grounding resistance value of the single feeder. At this time, compared with the single feeder grounding, the amplitude of the bus zero sequence voltage under the two-line same-phase grounding fault is larger, and the change value is related to the transition resistance and the neutral point resistance value, and the range is 1-2, that is, the amplitude of the neutral point zero sequence current is 1-2 times of the single feeder grounding under the same condition.

[0017]

[0018] The amplitude and phase of the fault feeder zero sequence current are not only related to the transition resistance connected to it, but also affected by the self feeder earth capacitance. The expressions of the two fault line zero sequence currents are as shown in formula (14) and formula (15). When the transition resistances of the two grounding fault points are equal, the fault feeder zero sequence current amplitude becomes small, and when the two grounding faults are both metallic grounding faults, the fault feeder zero sequence current amplitude is reduced by 2 times; when the transition resistances of the two grounding fault points are not equal, and the transition resistances of the two grounding fault points are very different, the transition resistance of the fault feeder zero sequence current amplitude and phase will be approximately equal to the healthy feeder zero sequence current.

[0019]

[0020]

[0021] wherein f1 and f2 are respectively the fault point 1 and the fault point 2; and are respectively the zero sequence currents flowing through the two fault points; R f1 and R f2 are respectively the transition resistances corresponding to the two fault points; R N is the neutral point grounding small resistance; C 0∑ is the system total earth capacitance; C ∑ is the sum of all healthy feeder earth capacitances; C 01 ~ C 0m is the sum of the three-phase earth capacitances of the mth feeder; ω is the power frequency angular velocity; is the equivalent virtual voltage source of the fault point.

[0022] The zero sequence current of the fault line is the superposition of the neutral point zero sequence current and the healthy line earth capacitance current. When the transition resistance values of the two fault feeders are the same, the neutral point zero sequence current is approximately equal to 2 times the fault feeder zero sequence current; when the transition resistance values of the two fault feeders are greatly different, the neutral point zero sequence current is approximately equal to the zero sequence current of the fault feeder with smaller transition resistance. It can be obtained that when the two-line same-phase successive faults occur, the amplitude ratio of the neutral point zero sequence current to the zero sequence current of the fault feeder with smaller transition resistance (i.e. the zero sequence current maximum fault feeder, assuming that the transition resistance connected to f1 is the smallest at this time) is:

[0023]

[0024] And for the single grounding fault case, the fault feeder current is the sum of the neutral point zero sequence current and the zero sequence current of each healthy feeder, and the neutral point zero sequence current will be smaller than the fault feeder. The successive fault causes the neutral point zero sequence voltage to rise, and the neutral point zero sequence current is 1-2 times of the multiple fault feeders. Therefore, the multi-feeder fault type discrimination features can be constructed.

[0025] After determining in step 3 that the fault is a successive fault, calculate the zero-sequence current of all feeders according to equation (25). With minimum feeder zero-sequence current amplitude ratio

[0026]

[0027] Then calculate the zero-sequence current θ of all feeders. i With neutral point zero-sequence current θ n phase difference Δθ i :

[0028] Δθ i =|θ n -θ i | (26)

[0029] The specific method for determining the fault feeder judgment threshold is as follows:

[0030] Compare the magnitude ratios of all feeder zero-sequence currents and the minimum feeder zero-sequence current calculated in step 4 with the phase difference between all feeder zero-sequence currents and the neutral point zero-sequence current.

[0031] Because the characteristics of a two-line in-phase ground fault are greatly affected by the ratio of the transition resistances at the two fault points, specific protection methods need to be discussed on a case-by-case basis. When the transition resistances at the two fault points differ significantly, the zero-sequence current characteristics of the fault feeder with the smaller transition resistance satisfy the following: the amplitude ratio of the zero-sequence current of the fault feeder to that of the healthy feeder is greater than 10, and the zero-sequence current of the fault feeder leads the zero-sequence current of the neutral point by 180° to 194.78°. When the transition resistances at the two fault points differ slightly, the zero-sequence current characteristics of the two fault feeders are similar, both satisfying the following: the amplitude ratio of the zero-sequence current of the fault feeder to that of the healthy feeder is greater than 5, and the zero-sequence current of the fault feeder leads the zero-sequence current of the neutral point by 190° to 210°. Simultaneously, considering the phase deviation α′1-α′2=20°~60° of the zero-sequence current of the feeders under a two-line in-phase ground fault, the phase angle difference between the zero-sequence current of the neutral point and the zero-sequence current of the fault feeder is greater than 170°. In summary, under a two-line in-phase ground fault, at least one faulty feeder will have a zero-sequence current whose amplitude and phase satisfy the fault characteristics. Based on this, the protection threshold for the faulty feeder can be constructed as follows:

[0032] k″ i ≥5&Δθ i ≥170° (29)

[0033] It can be seen from the analysis that the amplitude ratio of the fault feeder zero sequence current to the minimum feeder zero sequence current is greater than 5, and the phase difference between the zero sequence current of the fault feeder and the neutral point zero sequence current is less than 170°. Therefore, the calculated amplitude ratio and phase difference can be used for judging the fault feeder. The fault feeder is the one that simultaneously satisfies the conditions of the amplitude ratio being greater than 5 and the phase difference being less than 170°.

[0034] The beneficial effects of the present application are:

[0035] The centralized protection method for successive grounding faults of a small-resistance grounding system provided by the present application analyzes the fault characteristics of the small-resistance grounding system under multi-feeder grounding faults, and respectively derives the steady-state expressions of the bus zero sequence voltage, the neutral point zero sequence current and the feeder zero sequence current by using the superposition method. The amplitude and phase characteristics of the feeder zero sequence current and the neutral point zero sequence current are obtained. A centralized protection method suitable for two-line same-phase grounding faults is proposed, and accurate detection of two-line same-phase grounding faults is realized to avoid overstep tripping of the neutral point transformer protection. BRIEF DESCRIPTION OF DRAWINGS

[0036] Figure 1 Fig. 1 is a network structure diagram of a 10kV small-resistance grounding system under multi-feeder grounding faults, which is drawn by the present application to reflect the fault condition and the flow direction of the zero sequence current;

[0037] Figure 2 Fig. 2 is a zero sequence network distribution diagram of two-line same-phase grounding faults, which is obtained by the present application to analyze the fault type and realize fault feeder selection;

[0038] Figure 3 Fig. 3 is a zero sequence network diagram of two-line same-phase grounding faults when only virtual fault voltage source 1 acts, which is obtained by the present application to analyze the fault type and realize fault feeder selection;

[0039] Figure 4 Fig. 4 is a zero sequence network diagram of two-line same-phase grounding faults when only virtual fault voltage source 2 acts, which is obtained by the present application to analyze the fault type and realize fault feeder selection;

[0040] Figure 5 Fig. 5 is a phasor analysis diagram of the zero sequence current and the bus zero sequence voltage in two-line same-phase grounding faults, which is drawn by the present application to analyze the fault phase angle characteristics;

[0041] Fig. 6(a) is a feeder zero sequence current diagram when R f2 = 0.1Ω and R f3 = 0.1Ω;

[0042] Fig. 6(b) is a feeder zero sequence current diagram when R f2 = 10Ω and R f3 = 1000Ω;

[0043] Fig. 6(c) is a schematic diagram of the feeder zero sequence current when R f2 = 1000 Ω, R f3 = 1000 Ω;

[0044] Fig. 6(d) is a schematic diagram of the feeder zero sequence current when R f2 = 2000 Ω, R f3 = 3000 Ω. DETAILED DESCRIPTION

[0045] The application will be described in detail below in conjunction with the drawings and specific embodiments.

[0046] Embodiment 1

[0047] The specific operation steps of the centralized protection method for adapting to the successive ground faults of the small-resistance grounding system of the application are as follows:

[0048] Step 1: When the feeder zero sequence current reaches the starting threshold value, proceed to Step 2;

[0049] Step 2: Obtain the zero sequence currents flowing through the neutral point and each feeder in the station measurement device and each feeder zero sequence current transformer;

[0050] Step 3: Calculate the amplitude ratio of the neutral point zero sequence current to the maximum feeder zero sequence current. If the amplitude ratio of the neutral point zero sequence current to the maximum feeder zero sequence current is between the discrimination threshold values, determine that a two-line same-phase ground fault occurs, and proceed to Step 4; otherwise, directly proceed to the single-line single-phase ground fault protection process;

[0051] Step 4: Calculate the amplitude ratio of all feeder zero sequence currents to the minimum feeder zero sequence current and the phase difference between all feeder zero sequence currents and the neutral point zero sequence current;

[0052] Step 5: Determine whether the amplitude ratio and the phase difference calculation values of two or more feeders are within the protection threshold value range. If yes, proceed to Step 6; otherwise, directly proceed to the single-line single-phase ground fault protection process;

[0053] Step 6: Each feeder protection device performs tripping processing according to the received protection action signal.

[0054] Embodiment 2

[0055] The difference from Embodiment 1 is that,

[0056] The measurement device positions of the feeder zero sequence currents in Step 1 and Step 2 are at Figure 1 the feeder outlets, Figure 1 is a schematic diagram of a two-line same-phase ground fault network of a 10 kV small-resistance grounding system. Wherein f1 and f2 are fault point 1 and fault point 2 respectively; R f1 and R f2These are the transition resistors corresponding to the two fault points. Figure 1 The dashed line in the figure represents the distribution of zero-sequence current in the system, where and These are the zero-sequence currents flowing through the two fault points, respectively.

[0057] The specific method for sequential fault judgment in step 3 is as follows: First, after the feeder zero-sequence current meets the start-up conditions, the zero-sequence current of each feeder and neutral point is extracted according to step 2. Figure 2 The simplified zero-sequence network of a two-wire in-phase ground fault shows that two virtual fault voltage sources act simultaneously when a two-wire in-phase ground fault occurs. For ease of analysis, based on the superposition theorem of linear circuits, we will... Figure 2 It is split into two networks, each with only one virtual fault voltage source acting as a component. and These are virtual fault voltage source 1 and virtual fault voltage source 2, respectively.

[0058] With only virtual fault voltage sources When in operation, the virtual fault voltage source will be used. Short circuit, only The zero-order network diagram of the function is as follows Figure 3 As shown, the bus zero-sequence voltage under the action of only virtual fault voltage source 1 can be obtained. As shown in equation (1); a sound feeder zero-sequence current As shown in equation (2); neutral point zero-sequence current As shown in equation (3); fault feeder L m zero-sequence current As shown in equation (4); fault feeder L m-1 zero-sequence current As shown in equation (5). Where f1 and f2 are fault point 1 and fault point 2, respectively; R f1 and R f2 R represents the transition resistance corresponding to the two fault points. N For neutral point grounding, small resistance; C 0∑ C is the total capacitance to ground of the system. ∑ The sum of the capacitances of all healthy feeders to ground; C 01 ~C 0m ω represents the capacitance to ground of the m-th feeder; ω represents the power frequency angular velocity.

[0059]

[0060]

[0061]

[0062]

[0063]

[0064] Similarly, the zero sequence network diagram when only the virtual fault voltage source acts can be obtained as shown in Fig. 2. The bus zero sequence voltage under the action of only the virtual fault voltage source 2 can be obtained as formula (6); the healthy feeder zero sequence current Figure 4 can be obtained as formula (7); the neutral point zero sequence current can be obtained as formula (8); and the zero sequence current of the fault feeder L m can be obtained as formula (9); the zero sequence current of the fault feeder L m-1 can be obtained as formula (10).

[0065]

[0066]

[0067]

[0068]

[0069]

[0070] Using the superposition principle, the bus zero sequence voltage under the action of the virtual fault voltage source and under the two-line same-phase fault can be obtained as formula (11); the healthy feeder zero sequence current can be obtained as formula (12); the neutral point zero sequence current can be obtained as formula (13); and the zero sequence current of the fault feeder L m and L m-1 can be obtained as formula (14) and formula (15). Wherein, R′ f is the equivalent transition resistance under the two-line same-phase fault, which is the parallel equivalent resistance of R f1 and R f2 ; k is the ratio of the transition resistance values of the two fault points, k = R f1 / R f2 .

[0071]

[0072]

[0073]

[0074] ​​​​​​

[0075]

[0076] As can be seen from the above formula, the amplitude of the bus zero sequence voltage under the two-line same-phase fault is inversely proportional to the equivalent transition resistance of the fault. The equivalent transition resistance is equivalent to the parallel connection of the two transition resistances, and its value must be smaller than the smaller one of the two transition resistances. Therefore, compared with the single-phase ground fault of a single feeder, the amplitude of the bus zero sequence voltage under the two-line same-phase fault is larger, and when the amplitudes of the two transition resistances are equal, the amplitude of the bus zero sequence voltage under the two-line same-phase fault is doubled.

[0077] The phase β' of the bus zero sequence voltage lagging the virtual fault voltage under the two-line same-phase fault can be calculated by formula (16):

[0078]

[0079] When a single feeder ground fault occurs, the bus zero sequence voltage lags the virtual voltage at the fault point by an angle β.

[0080]

[0081] As can be seen from formula (16) and formula (17), the phase of the bus zero sequence voltage under the single feeder ground fault and the two-line same-phase ground fault is basically unchanged.

[0082] The zero sequence current of the healthy feeder is affected by the bus zero sequence voltage, and its amplitude is relatively larger, but its phase is basically unchanged. The amplitude of the neutral point zero sequence current is proportional to the size of the bus zero sequence voltage, and the amplitude of the neutral point zero sequence current under the two-line same-phase fault is significantly increased, but the phase is basically the same as that under the single feeder ground fault under the same conditions.

[0083] The amplitude and phase of the zero sequence current of the fault feeder under the two-line same-phase fault are not only related to the two transition resistances R f1 and R f2 , but also affected by the sizes of the respective feeder-to-ground capacitances C 0(m-1) and C 0m . Compared with the single feeder ground fault, the amplitude and phase of the zero sequence current of the fault feeder under the two-line same-phase fault change according to the ratio k of the two transition resistances, which can be divided into the following two cases.

[0084] When k or k When the transition resistance is small, the zero-sequence current amplitude of the fault feeder decreases slightly; the zero-sequence current amplitude of the fault feeder with large transition resistance may increase or decrease due to the influence of the feeder's capacitance to ground. From equation (15), it can be seen that when a metallic two-wire in-phase ground fault occurs, the zero-sequence current amplitude decreases by a maximum of about half that under a single-wire ground fault, and when the value of k is extremely small, there exists (C Σ +C 0(m-1) -C 0m When k) < 0, the zero-sequence current of the faulty feeder will flip from the fourth quadrant to the second quadrant. The smaller k is, the closer the zero-sequence current of the faulty feeder is to the zero-sequence current of the healthy feeder. At this time, the phase characteristics of the zero-sequence current of the faulty feeder will be lost. As can be seen from equation (20), when the transition resistance of the two faulty feeders is greatly different, the maximum phase difference between them can reach about 70°, which is enough to make the traditional phase criterion fail.

[0085] When k = 1, the transition resistance values ​​of the two fault feeders are equal, and the zero-sequence current amplitudes of both fault feeders decrease, as shown in equation (18). In particular, when the capacitance values ​​to ground of the two fault feeders are equal, the zero-sequence current amplitudes of the two fault feeders are equal, as shown in equation (19). Substituting k = 1 into equation (14), the phase difference between the zero-sequence current of the fault feeder and the virtual fault voltage is given by equation (20). It can be seen that the phases of the zero-sequence currents of the two fault feeders are approximately equal. Compared with a single-feeder grounding fault, the phase of the zero-sequence current under a two-line in-phase fault lags behind by a certain angle α. i ′(i=1,2), and this angle occurs when a low-resistance ground fault occurs. Take the maximum value (R) N =10Ω). The system capacitance current typically ranges from 50A to 300A, while the single feeder capacitance current does not exceed 50A, i.e., the capacitance to ground of a low-resistance grounding system. Substituting this value into equation (20), it can be seen that the extreme values ​​of α1′ and α2′ decrease rapidly and gradually as the transition resistance changes. Furthermore, when the transition resistance is equal, the phase angle difference between the two faulted feeders, as shown in equation (21), is determined by the system capacitance current and the difference in ground capacitance between the two faulted feeders, with the difference ranging from 20° to 60°. The phase of the zero-sequence current in the faulted feeder differs under two-line in-phase faults and single-feeder grounding faults.

[0086] When a small resistor is connected to the neutral point, the phase of its zero-sequence current is consistent with the phase of the bus zero-sequence voltage, as shown in equation (16). The phase difference between the neutral point zero-sequence current phase and the fault feeder current is then expressed as equation (22), where k = 1, and the phase difference between the two fault transition resistors is... That is, the zero-sequence current of the faulty feeder leads the zero-sequence current of the neutral point by 190° to 210°. Based on this, a phase difference criterion can be set.

[0087]

[0088]

[0089]

[0090] a'1-a'2=2arctan(3ωR N (C Σ +C 0m -C 0(m-1) )) (21)

[0091]

[0092] From the above, it can be seen that under the two-line same-phase grounding fault, the amplitude of the neutral-point zero-sequence current increases and the amplitude of the fault feeder zero-sequence current decreases. When the transition resistances of the two grounding fault points are equal, the amplitude of the neutral-point zero-sequence current increases by two times and the amplitude of the fault feeder zero-sequence current decreases, at this time, the amplitude of the neutral-point zero-sequence current is about 2 times larger than the amplitude of the fault feeder zero-sequence current; when the transition resistances of the two grounding fault points are not equal, the amplitude ratio of the neutral-point zero-sequence current to the smaller transition resistance fault feeder zero-sequence current is greater than 1, as shown in equation (23).

[0093]

[0094] The two-line same-phase grounding fault discrimination threshold k' can be constructed, as shown in equation (24):

[0095]

[0096] The specific method for fault line selection in steps 4 and 5 is as follows:

[0097] If it is judged in step 3 that this time is a successive fault, the amplitude ratio of all feeder zero-sequence currents to the minimum feeder zero-sequence current is calculated according to equation (25):

[0098]

[0099] The phase difference between the zero-sequence currents of all feeders and the neutral-point zero-sequence current is calculated again

[0100] Δθ i =|θ n -θ i | (26)

[0101] The specific determination method of the fault feeder judgment threshold is as follows:

[0102] The amplitude ratio of the zero-sequence current of the fault feeder L m-1 to the zero-sequence current of the healthy feeder is calculated:

[0103]

[0104] Similarly, the fault feeder L m The ratio of the zero-sequence current of the fault feeder to the zero-sequence current of the healthy feeder is:

[0105]

[0106] Affected by the ratio of the two transition resistances, the ratio of the zero-sequence current of the fault feeder to the zero-sequence current of the healthy feeder can be divided into the following two cases according to the value of k:

[0107] When or , the greater the transition resistance, the greater the corresponding fault feeder zero-sequence current amplitude is weakened. When ωC 0∑ ≤ 11.6 mS, ωC 0i ≤ 2.9 mS (common system of small resistance grounding system and single feeder ground capacitance value, which can be modified according to the capacitance current of different systems and the threshold value) is substituted, the fault feeder zero-sequence current amplitude is still greater than 10 times the amplitude of the healthy feeder zero-sequence current, but the fault feeder zero-sequence current amplitude is smaller than the amplitude of the healthy feeder zero-sequence current, and the greater the transition resistance, the smaller the amplitude ratio.

[0108] When k = 1, substituting k = 1 into equation (23) can obtain that the amplitude ratio of the two-line in-phase fault is about half of the single feeder grounding fault amplitude ratio, that is, about 5 times.

[0109] And the phasor diagram of the zero-sequence current of each feeder, the bus zero-sequence voltage and the fault virtual voltage under the two-line in-phase grounding fault of is drawn by combining equation (12), equation (13), equation (16) and equation (19), as shown in Figure 5 . In particular, when k = 1, the zero-sequence current of the feeder and will coincide.

[0110] When the transition resistances of two grounding fault points are quite different, the fault feeder zero sequence current characteristics of the smaller transition resistance satisfy: the ratio of the fault feeder zero sequence current to the healthy feeder zero sequence current is greater than 10, and the fault feeder zero sequence current leads the neutral point zero sequence current by 180°-194.78°; when the transition resistances of two grounding fault points are quite small, the fault feeder zero sequence currents of two fault feeders are similar and satisfy: the ratio of the fault feeder zero sequence current to the healthy feeder zero sequence current is greater than 5, and the fault feeder zero sequence current leads the neutral point zero sequence current by 190°-210°. Meanwhile, considering the phase deviation of the two-line same-phase grounding fault, α'1-α'2=20°-60°, the phase angle difference between the neutral point zero sequence current and the fault feeder zero sequence current is greater than 170°. In summary, under the two-line same-phase grounding fault, at least one fault feeder zero sequence current satisfies the fault characteristics in amplitude and phase. Based on this, the fault feeder criterion can be constructed as:

[0111] k" i ≥5&Δθ i ≥170° (29)

[0112] That is, when the two-line same-phase grounding fault feeder criterion satisfies formula (29), when the feeder zero sequence current amplitude is greater than 5 times the healthy feeder zero sequence current amplitude, and the phase leads the neutral point zero sequence current by 170°, the feeder is determined as a fault feeder.

[0113] Example 3

[0114] A simulation model of a 10 kV small-resistance grounding system distribution network is built by using PSCAD / EMTDC, as shown in Figure 1 . Zero sequence current transformers are set at feeder branches to extract feeder zero sequence currents, and MATLAB is used to process the extracted zero sequence currents to determine the fault type and fault feeder. The capacity of the 110 kV to 10 kV step-down transformer in the system is 100 MVA, the type is YNd11, which is delta-star type, and the secondary side has no neutral point. Therefore, a Z-type transformer needs to be added at the bus to add an artificial neutral point to the secondary side, and a 10 Ω small resistance is connected to the neutral point. Two fault points are set with transition resistances of R f2 =0.1 Ω, R f3 =0.1 Ω, R f2 =10 Ω, R f3 =1000 Ω, R f2 =1000 Ω, R f3 =1000 Ω, and R f2 =2000 Ω, R f3 =3000 Ω to test the performance of the method. The waveform simulation diagram is shown in Figures 6(a)-6(d) . Fig. 6(a) Rf2 = 0.1 Ω, R f3 = 0.1 Ω, R f2 = 10 Ω, R f3 = 1000 Ω, R f2 = 1000 Ω, R f3 = 1000 Ω, R f2 = 2000 Ω, R f3 = 3000 Ω, R It can be seen that when the transition resistance values of the two fault points are equal or approximately equal, the fault feeder zero sequence current and the healthy feeder zero sequence current are quite different, while the fault feeder zero sequence current and the healthy feeder zero sequence current are similar and difficult to distinguish when the transition resistance is large. Meanwhile, through calculation, the k', k i and Δθ i are obtained.

[0115]

[0116] As shown in the following table. From the table, it can be seen that when the transition resistance values of the two fault points are equal and approximately equal, the application can reliably detect the fault type of the two-line same-phase grounding fault of the system, and accurately judge the multiple fault feeders of the successive fault. When the transition resistance values of the two fault points are quite different, the application can accurately find the fault feeder with smaller transition resistance, and after cutting off the fault feeder, the system becomes a single-phase grounding fault protection and cuts off another feeder. The application has good reliability and sensitivity, and can provide a solution and reference for the accurate identification of the successive fault of the power supply and distribution line.

Claims

1. A centralized protection method adapted to successive phase-to-ground faults in a small resistance grounded system, characterized in that, The specific operation steps are as follows: Step 1: when the feeder zero sequence current reaches the starting threshold, step 2 is entered; Step 2: the zero sequence currents flowing through the neutral point and each feeder are obtained in the station measurement device and each feeder zero sequence current transformer; Step 3: the amplitude ratio of the neutral point zero sequence current to the maximum feeder zero sequence current is calculated, if the amplitude ratio of the neutral point zero sequence current to the maximum feeder zero sequence current is between the discrimination threshold, it is determined that a two-line same-phase grounding fault occurs, step 4 is entered, otherwise, a single-line single-phase grounding fault protection process is directly entered; The specific method for determining that a two-line same-phase grounding fault occurs in step 3 is as follows: When the zero sequence voltage in step 1 meets the starting condition, the amplitude ratio of the neutral point zero sequence current to the maximum feeder zero sequence current is calculated according to the extracted feeder and neutral point zero sequence currents in step 2 wherein, is the neutral zero sequence current, is the maximum zero sequence current for each feeder; The specific calculation process of the fault type discrimination threshold is as follows: The amplitude of the neutral point zero sequence current under the two-line same-phase fault is proportional to the size of the bus zero sequence voltage, compared with a single feeder grounding fault, the bus zero sequence voltage under the two-line same-phase fault is as formula (11), the transition resistance of the two grounding fault points is equivalent to parallel in the fault equivalent network, and the value is 1~1 / 2 times of the same single-phase grounding resistance of a single feeder; at this time, compared with single feeder grounding, the amplitude of the bus zero sequence voltage under the two-line same-phase grounding fault is larger, and the change value is related to the transition resistance and the neutral point resistance, and the range is between 1~2, that is, the amplitude of the neutral point zero sequence current is 1~2 times of the single feeder grounding under the same condition; （11） wherein and are the transition resistances corresponding to the two fault points; The expressions of the zero sequence currents of the two fault lines are as formula (14) and formula (15); when the transition resistances of the two grounding fault points are equal, the amplitude of the fault feeder zero sequence current is smaller, and when the two grounding faults are metallic grounding faults, the amplitude of the fault feeder zero sequence current is reduced by 2 times; when the transition resistances of the two grounding fault points are not equal, and the transition resistances of the two grounding fault points are very different, the amplitude and phase of the fault feeder zero sequence current with the healthy feeder zero sequence current are approximately equal; （14） （15） wherein, f 1 and f 2 are fault point 1 and fault point 2, respectively; and are zero sequence currents flowing through the two fault points, respectively; and are transition resistances corresponding to the two fault points, respectively; is a small grounding resistance of the neutral point; is a total grounding capacitance of the system; is a sum of grounding capacitances of all healthy feeders; is a grounding capacitance of the feeder of the m th feeder; is a power frequency angular velocity; is an equivalent virtual voltage at the fault point; The zero sequence current of the fault line is the superposition of the neutral point zero sequence current and the healthy line ground capacitance current; when the transition resistance values of the two fault feeders are the same, the neutral point zero sequence current is approximately equal to 2 times the fault feeder zero sequence current; when the transition resistance values of the two fault feeders are quite different, i.e. more than 10 times, the neutral point zero sequence current is approximately equal to the zero sequence current of the fault feeder with smaller transition resistance; the amplitude ratio of the neutral point zero sequence current to the zero sequence current of the fault feeder with smaller transition resistance can be obtained when two-line same-phase successive faults occur ​ （23） Constructing a two-line phase-to-ground fault identification threshold As shown in (24), （24）； Step 4: the amplitude ratio of all feeder zero sequence currents to the minimum feeder zero sequence current and the phase difference between all feeder zero sequence currents and the neutral point zero sequence current are calculated; The specific calculation process of the amplitude ratio of all feeder zero sequence currents to the minimum feeder zero sequence current and the phase difference between all feeder zero sequence currents and the neutral point zero sequence current in step 4 is as follows: After the fault is judged to be a sequential fault in step 3, all feeder zero sequence currents are calculated according to formula (25) The amplitude ratio of the minimum feeder zero sequence current to the maximum feeder zero sequence current : （25） Recalculate zero sequence current for all feeders Phase difference from neutral zero sequence current :​ （26）； Step 5: it is judged whether there are two or more feeders whose amplitude ratio and phase difference calculation values are within the protection threshold range, if yes, step 6 is entered, otherwise, a single-line single-phase grounding fault protection process is directly entered; The specific method for judging the fault feeder in step 5 is as follows: When the transition resistances of two grounding fault points are quite different, the characteristic of the zero sequence current of the fault feeder with smaller transition resistance is that the ratio of the zero sequence current of the fault feeder to the zero sequence current of the healthy feeder is greater than 10, and the zero sequence current of the fault feeder leads the neutral point zero sequence current by 180°~194.78°. When the transition resistances of two grounding fault points are quite different, the characteristic of the zero sequence current of the fault feeder with smaller transition resistance is that the ratio of the zero sequence current of the fault feeder to the zero sequence current of the healthy feeder is greater than 10, and the zero sequence current of the fault feeder leads the neutral point zero sequence current by 180°~194.78°. When the transition resistances of two grounding fault points are quite different, the characteristics of the zero sequence currents of the two fault feeder lines are similar, that is, the ratio of the zero sequence current of the fault feeder line to the zero sequence current of the healthy feeder line is greater than 5 and less than 10, and the zero sequence current of the fault feeder line leads the neutral point zero sequence current by 190°~210°. When the transition resistances of two grounding fault points are quite different, the characteristics of the zero sequence currents of the two fault feeder lines are similar, that is, the ratio of the zero sequence current of the fault feeder line to the zero sequence current of the healthy feeder line is greater than 5 and less than 10, and the zero sequence current of the fault feeder line leads the neutral point zero sequence current by 190°~210°. The protection threshold of the fault feeder is constructed as: （29） The two-line same-phase grounding fault feeder criterion is to satisfy formula (29), when the amplitude of the feeder zero sequence current is more than 5 times of the amplitude of the healthy feeder zero sequence current, and the phase of the feeder zero sequence current leads the neutral point zero sequence current by 170°, it is determined that the feeder is a fault feeder; Step 6: each feeder protection device performs tripping processing according to the collected protection action signals.

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