Power distribution network grounding protection method based on zero sequence current phase difference variable quantity

Abstract

The invention belongs to the field of power distribution network grounding protection, and particularly discloses a power distribution network grounding protection method based on zero-sequence current phase difference variable quantity, which comprises the following steps of: judging a fault and a sound line based on the difference between the zero-sequence current phase difference variable quantity of a line and a neutral point before and after a parallel small resistor is put into use; the method effectively solves the problem that traditional zero-sequence over-current protection of a high-resistance grounding fault refuses to operate, can meet the requirements of relay protection sensitivity and reliability, does not need to use a centralized protection device to compare outgoing lines, remarkably reduces the calculation complexity caused by one-to-one longitudinal comparison when the number of outgoing lines of a power distribution network cable is large, and improves the reliability of the power distribution network cable. And the practicability is higher.

Description

Technical Field

[0001] The present invention belongs to the field of distribution network grounding protection, and particularly relates to a distribution network grounding protection method based on the change amount of zero-sequence current phase difference. Background Technique

[0002] With the rapid development of urban distribution networks, the advantages of cable lines, such as relatively high personal safety, less land occupation, and no impact on environmental beauty, have become increasingly prominent. The proportion of cable lines in urban distribution systems has gradually increased, and the capacitive current has increased sharply. This may lead to high-amplitude arc grounding overvoltage when a single-phase grounding fault occurs in the system, thus posing a threat to personal safety and the safe operation of equipment. As the capacitive current of the distribution network to the ground has increased significantly, it is difficult to compensate the capacitive current during a single-phase grounding fault. The neutral grounding through a small resistor can effectively reduce the arc overvoltage and resonance overvoltage, and is gradually widely adopted in distribution networks.

[0003] The zero-sequence overcurrent protection is set according to avoiding the maximum capacitive current of the line to the ground, and can withstand a transition resistance of about 100 Ω at most. Moreover, the farther the fault point is from the bus, the lower the sensitivity. In the case of high-resistance grounding faults, the value of the transition resistance may be as high as thousands of ohms, resulting in the zero-sequence current of the faulty line may be only a few amperes, and the zero-sequence overcurrent protection is prone to refusal to operate. Although the current of high-resistance grounding faults is weak, if it is not removed for a long time, it may evolve into a more serious short-circuit fault, expanding the fault range and even causing fires and casualties. Therefore, how to effectively improve the ability of zero-sequence protection to withstand the transition resistance is a difficult problem that urgently needs to be solved in the small-resistance grounding system. Summary of the Invention

[0004] In order to solve the deficiencies of the prior art, the present invention aims to provide a distribution network grounding protection method based on the change amount of zero-sequence current phase difference. This method determines the faulty line and the sound line based on the difference in the change amount of the zero-sequence current phase difference between the line and the neutral point before and after the parallel small resistor is put into operation, effectively solving the problem of refusal to operate of the traditional zero-sequence overcurrent protection for high-resistance grounding faults, meeting the requirements of the sensitivity and reliability of relay protection, and not requiring the use of a centralized protection device to compare between each outgoing line, significantly reducing the calculation complexity caused by longitudinal comparison one by one when there are many cable outgoing lines in the distribution network, and having stronger practicability.

[0005] In order to achieve the above purpose, the technical solution adopted by the present invention is as follows:

[0006] A distribution network grounding protection method based on the change amount of zero-sequence current phase difference, which is realized based on the following steps:

[0007] 1) A group of small resistors is connected in parallel at the neutral point of the distribution network; based on the unit zero-sequence capacitance parameter C of the line 0and the total length L of the remaining lines after removing the longest feeder. By analyzing the characteristics of zero-sequence current and neutral-point current, and calculating the phase difference change before and after the small resistance is put into operation;

[0008] 2) When the measuring equipment detects that the magnitude of the zero-sequence current of the feeder is greater than the starting current I S , the protection starts, and records and processes the zero-sequence current signal of the neutral point and the zero-sequence current signal of the feeder;

[0009] 3) For low-resistance grounding faults, a setting value I min is set for the fault identification method based on the phase difference of zero-sequence current. When the detected zero-sequence current is greater than the setting value I min , directly determine that the line where it is located is the fault feeder, and do not put the shunt small resistance at the neutral point to calculate the phase difference of zero-sequence current, and quickly cut off the fault as soon as possible;

[0010] 4) When the zero-sequence current is less than the setting value I min , use the fault identification method based on the phase difference of zero-sequence current to judge whether it is a sound line with a metallic grounding fault or a fault line with a high-resistance grounding fault;

[0011] 5) The high-resistance grounding fault protection criterion based on the change amount of the phase difference of zero-sequence current before and after changing the small resistance at the neutral point is as follows:

[0012]

[0013] In the formula, Δθ is the change amount of the phase difference between the zero-sequence current of the feeder and the zero-sequence current of the neutral point before and after the small resistance is put into operation; if θ set < Δθ < 18°, it is determined that a high-resistance grounding fault has occurred on the line where the protection is located, and the protection acts on tripping; if -1.2° < Δθ < 1.2°, it is determined to be a sound feeder, and the protection does not act.

[0014] Furthermore, in step 1), after combining the original 20Ω small resistance at the neutral point with the newly added shunt 20Ω small resistance, the equivalent resistance value of the neutral point is reduced to 10Ω.

[0015] Furthermore, in step 2), in order to ensure that the protection does not malfunction under normal operating conditions of the distribution system, the setting of the starting current I S should avoid the maximum unbalanced current estimated to appear in the system; the maximum unbalanced currents of the overhead line and the cable line of the 10kV small resistance grounding system during normal operation are 0.37A and 0.26A respectively, and the protection device is often connected to 3 times the zero-sequence current, and the starting current should be greater than 1.11A.

[0016] Furthermore, the starting current I Sis 1.5A.

[0017] Further, in step 3), considering the maximum zero-sequence current amplitude of the sound outgoing line and the protection margin factor, the setting value I min is set to 30A.

[0018] Further, step 4) specifically includes: after the parallel small resistor at the neutral point is put into operation, the equivalent resistance of the neutral point changes from 20Ω to 10Ω, and the zero-sequence current of the feeder and the zero-sequence current of the neutral point at this time are recorded and the phase difference between them is calculated. After calculating the change amount Δθ of the phase difference between before and after the small resistor is put in, the parallel small resistor is disconnected to restore the neutral point resistance to 20Ω. and between them.

[0019] Further, in step 5), under normal circumstances, Δθ will not fall into the area outside the two intervals of the judgment formula. If the criteria for both the faulty and sound feeders are not met, there is a measurement or calculation error, and the protection will recalculate and judge.

[0020] Further, in step 5), considering the measurement equipment error, the error limit of the phase difference of the 5P-class current transformer for steady-state protection under the rated primary current is 60′. In addition, considering the errors caused by uncertain factors such as line parameters and calculation processes, in order to ensure the reliability of the high-resistance grounding fault identification result, a certain margin is reserved. Therefore, the criterion for the sound feeder is set as -1.2° < Δθ < 1.2°;

[0021] For the lower limit of the value taken for the faulty feeder, it is obtained according to the line parameters of the distribution network. The specific expression is as follows:

[0022]

[0023] where θ set is the lower limit of the value of Δθ for the faulty feeder, K rel is the set margin coefficient, taking 0.9; R n represents the resistance value of the parallel small resistor, and R' n represents the equivalent small resistance value of the neutral point after the parallel small resistor is put in; ω is the power frequency angular frequency; when R n and R' n are 20Ω and 10Ω respectively, only by obtaining the line length and ground capacitance parameters of the distribution network, θ set can be set;

[0024] For the upper limit of the value taken for the faulty feeder, a certain margin is also reserved, taking 18°.

[0025] The beneficial effects of the present invention are:

[0026] The present invention proposes a method for grounding protection of a distribution network based on the change in the phase difference of zero-sequence current. By using the difference in the change in the phase difference of zero-sequence current between the line and the neutral point before and after the parallel small resistor is inserted, it can effectively judge the faulty line and the healthy line, effectively solve the problem of refusal to operate of the traditional zero-sequence over-current protection for high-resistance grounding faults, meet the requirements of sensitivity and reliability of relay protection, and does not require the use of a centralized protection device to compare between each outgoing line, significantly reducing the calculation complexity caused by one-by-one longitudinal comparison when there are many cable outgoing lines in the distribution network, and having stronger practicability. Brief Description of the Drawings

[0027] Figure 1 It is the flowchart of the method of the present invention;

[0028] Figure 2 It is the simulation topology diagram of a neutral point small resistor grounded distribution network with IIDG of the present invention;

[0029] Figure 3 In the simulation experiment of the present invention, the zero-sequence current waveform diagram at the fault of the l 2 midpoint;

[0030] Figure 4 In the simulation experiment of the present invention, the l 2 zero-sequence current phase diagram at the fault of the midpoint. Detailed Embodiment

[0031] The principle and features of the present invention will be described below with reference to the accompanying drawings. The examples given are only for explaining the present invention and do not limit the scope of use of the present invention.

[0032] As Figure 1 shown, the present invention proposes a method for grounding protection of a distribution network based on the change in the phase difference of zero-sequence current. Aiming at the problem of refusal to operate of the traditional zero-sequence over-current protection for high-resistance grounding faults, by inserting a parallel small resistor at the neutral point, based on the difference in the change in the phase difference between the zero-sequence current of the feeder and the neutral point current before and after the change of the neutral point small resistor, a protection scheme suitable for high-resistance grounding faults is proposed. The specific steps are as follows:

[0033] 1) In order to solve the problem of high-resistance grounding faults in the distribution network, by connecting a group of small resistors in parallel at the neutral point, after combining the original neutral point small resistor (20 Ω) and the newly added parallel small resistor (20 Ω), the equivalent resistance value of the neutral point is reduced to 10 Ω; based on the unit zero-sequence ground capacitance parameter C of the line 0 and the total length L of the remaining lines after excluding the longest feeder, by analyzing the characteristics of the zero-sequence current and the neutral point current, calculate the change in the phase difference before and after inserting the small resistor, providing a basis for the protection action.

[0034] 2) When the measuring device detects that the magnitude of the zero-sequence current of the feeder is greater than the starting current I SWhen the protection is activated, the zero-sequence current of the neutral point and the zero-sequence current signal of the feeder are recorded and processed.

[0035] Affected by factors such as three-phase asynchronous closing, unbalanced three-phase ground parameters of the line, poor transposition of the overhead line, and single-phase power supply, unbalanced current will be generated in the normally operating system. To ensure that the protection does not malfunction under normal operating conditions of the distribution system, the starting current I S shall be set to avoid the maximum unbalanced current that may occur in the system. The maximum unbalanced currents of the overhead line and the cable line in the 10kV low-resistance grounded system during normal operation are approximately 0.37A and 0.26A respectively. The protection device usually accesses three times the zero-sequence current, and the starting current should be greater than 1.11A.

[0036] It should be noted that the magnitude of the zero-sequence current after a fault decreases with the increase of the transition resistance. Therefore, the magnitude of the starting current setting value will affect the ability of the protection scheme to withstand the transition resistance. The higher the accuracy of the zero-sequence current transformer, the greater the transition resistance value that the protection can withstand, but the higher the economic cost of the corresponding measuring equipment. The setting of the starting current depends on the trade-off between the economic cost of the current transformer and the effect of the protection against high-resistance grounding faults.

[0037] In practical applications, the starting current I S can be set according to the actual situation of the distribution network where the protection is located, considering the importance of the power supply area and economic factors. Based on the above two aspects, considering the influence of uncertain factors such as transformation error and retaining a certain margin, the starting current I S of the present invention is set to 1.5A.

[0038] 3) The fault identification method based on the phase difference of zero-sequence current is to solve the problem of refusal to operate of the traditional zero-sequence overcurrent protection for high-resistance grounding faults. Therefore, for low-resistance grounding faults, a setting value I min can be set with reference to the existing zero-sequence overcurrent protection. When the detected zero-sequence current is greater than this value, it is directly determined that the line where it is located is the faulty feeder, and the parallel small resistor at the neutral point is not put into operation to calculate the phase difference of the zero-sequence current, so that the fault can be quickly cut off as soon as possible.

[0039] The present invention comprehensively considers factors such as the maximum zero-sequence current amplitude of the healthy outgoing line and the protection margin, and sets the setting value I min to 30A.

[0040] 4) When the zero-sequence current is less than I min , it is necessary to use the proposed method to judge whether it is a healthy line with a metallic grounding fault or a faulty line with a high-resistance grounding fault.

[0041] Specifically: After the parallel small resistor at the neutral point is put into operation, the equivalent resistance of the neutral point changes from 20Ω to 10Ω, and the zero-sequence current of the feeder is recorded at this time and the phase difference between the zero-sequence current of the neutral point After calculating the change amount Δθ of the phase difference between before and after the small resistance is put in and Disconnect the shunt small resistance to restore the neutral point resistance to 20 Ω.

[0042] 5) Usually, there are many outgoing lines and branches in the distribution network, the line topology structure is relatively complex, and the total length of the line is relatively long; on the other hand, in the distribution network with small resistance grounding, the proportion of cables is large, and the zero-sequence capacitance to the ground of the cable line is relatively large, so the total capacitance to the ground of the sound line is relatively large. Different from the sound feeder, the value of Δθ of the faulty line will have a large difference from 0°, and the fault and the sound feeder can be distinguished based on this.

[0043] Considering the measurement equipment error, the error limit of the phase difference of the 5P-class current transformer for steady-state protection is 60′ under the rated primary current. In addition, considering the possible error caused by uncertain factors such as line parameters and calculation processes in actual applications, in order to ensure the reliability of the identification result of the high-resistance grounding fault and retain a certain margin, the criterion for the sound feeder is set as -1.2° < Δθ < 1.2°.

[0044] For the lower limit of the value of the faulty feeder, it can be obtained according to the line parameters of the distribution network, and the specific expression is as follows:

[0045]

[0046] where θ set is the lower limit of the value of Δθ of the faulty feeder, K rel is the set margin coefficient, taking 0.9; L is the total length of all feeders after removing the longest feeder in the distribution network, C 0 is the zero-sequence capacitance to the ground per unit length of the line; R n represents the shunt small resistance value, R' n represents the equivalent small resistance value of the neutral point after the shunt small resistance is put in, and ω is the power frequency angular frequency.

[0047] It can be seen that when R n and R' n are 20 Ω and 10 Ω respectively, only by knowing the line length and capacitance to the ground parameters of the distribution network, θ set can be set.

[0048] For the upper limit of the value of the faulty feeder, a certain margin is also retained, taking 18°. It should be noted that the premise of all the above values is that the original small resistance of the neutral point and the later put-in shunt small resistance are both 20 Ω. For the case where the original neutral point small resistance value is other, the upper and lower limits of Δθ of the faulty feeder need to be modified accordingly, such as R n and R'n When they are 10Ω and 5Ω respectively, in the formula for judging whether the feeder is faulty, the upper limit of Δθ of the faulty feeder is modified from 15.42° to 9.25°, and other situations are not listed one by one.

[0049] Based on the above analysis, the high-resistance grounding fault protection criterion based on the change in the phase difference of zero-sequence current before and after changing the small neutral resistance is as follows:

[0050]

[0051] Specifically, if Δθ satisfies θ set <Δθ<18°, it is determined that a high-resistance grounding fault has occurred on the line where the protection is located, and the protection acts on tripping; if -1.2°<Δθ<1.2°, it is determined as a sound feeder and the protection does not act. Among them, theoretically Δθ will not fall into the area outside these two intervals. If the criteria for both faulty and sound feeders are not met, there may be measurement or calculation errors, and the protection will recalculate and judge.

[0052] Simulation experiment:

[0053] As Figure 2 shown, in order to verify the effectiveness of the proposed high-resistance grounding protection scheme, a small-resistance grounded distribution network with IIDG is built in PSCAD / EMTDC. The original small neutral resistance and the shunt small resistance R n of the distribution network are both 20Ω. The lengths of lines l 1 , l 2 are 5km, the length of line l 3 is 15km, and the lengths of lines l 4 ~l 6 are 10km. Protections 1 - 6 are installed at the heads of the lines respectively. The lines all adopt cable lines with the model of YJV22 - 6 / 10kV - 3*70mm2, and the unit zero-sequence capacitance to ground parameter is C 0 =124.28×10-9F / km. The photovoltaic model with PQ control is encapsulated as an IIDG module and is T-connected to the line, and constant impedance load models of 1MW + 0.1MVar per phase are connected to the ends of each outgoing line respectively.

[0054] Figure 2 In the shown topology, after removing the longest l 3 feeder, the total line length L is 40km, and the calculated value of θ set is 2.4°. The fault time is set to 0.1s, and the input time of the shunt small resistance at the neutral point is 0.2s.

[0055] To verify the accuracy of the high-resistance grounding fault identification method based on the change in phase difference, in the case where the distributed power source is not connected, on line l 2A single-phase grounding fault with a transition resistance of 500Ω is set at the midpoint. The waveforms of the zero-sequence currents flowing through each protection and the zero-sequence current at the neutral point are as shown in Figure 3 the figure.

[0056] Among them, 3I 01 ~3I 06 are the three times zero-sequence currents at protection points 1 to 6 respectively, and I 0R is the zero-sequence current at the neutral point. It can be seen that at 0.2s, the amplitudes and phases of each zero-sequence current have changed. The change in the phase relationship between the zero-sequence currents at the neutral point and the line before and after the parallel small resistor is put into operation is as shown in Figure 4 the figure.

[0057] In order to more clearly display the amplitudes and phases of the zero-sequence currents, the simulation data is recorded in Table 1 as follows.

[0058]

[0059] Table 1 l 2 Simulation data of each current for the fault at the midpoint

[0060] Among them, R n =20Ω represents before the parallel small resistor is put into operation, R ' n =10Ω represents after the parallel small resistor is put into operation, that is, the equivalent small resistor at the neutral point is 10Ω. 3I 01 ~3I 06 are the three times zero-sequence currents at protection points 1 to 6 respectively, 3I 01B ~3I 06B represents the corresponding zero-sequence current with the neutral point current I 0R as the reference. Its phase angle is the included angle θ between the zero-sequence current of the line and the neutral point current. Δθ is the change amount of θ before and after the change of the resistance value of the neutral point small resistor. It can be seen that for lines l 3 and l 4 ~l 6 , the Δθ values are -0.31° and -0.32° respectively, which are within the interval (-1.2°, 1.2°), and are determined as healthy feeders; for lines l 1 and l 2 , the Δθ values are 3° and 3.34° respectively, which are within the interval (2.4°, 18°), and are determined as faulty lines. Since the operating time limit of protection 1 is Δt more than that of protection 2, protection 2 will operate to trip and selectively cut off the high-resistance grounding fault, verifying the effectiveness of the proposed protection scheme.

[0061] The zero-sequence current phase difference variation-based high-resistance grounding fault protection method proposed by the present invention solves the problem of refusal to operate of traditional zero-sequence overcurrent protection for high-resistance grounding faults, meets the requirements for the sensitivity and reliability of distribution network relay protection, does not require the use of a centralized protection device to compare between outgoing lines, reduces the computational complexity caused by one-by-one longitudinal comparison when there are many cable outgoing lines in the distribution network, and has better practicability.

[0062] Obviously, the embodiments described above are only a part of the embodiments of the present application, rather than all embodiments. The accompanying drawings show preferred embodiments of the present application, but do not limit the patent scope of the present application. The present application can be implemented in many different forms. On the contrary, the purpose of providing these embodiments is to make the understanding of the disclosed content of the present application more thorough and comprehensive. Although the present application has been described in detail with reference to the foregoing embodiments, those skilled in the art can still modify the technical solutions described in the foregoing specific embodiments, or perform equivalent replacements for some of the technical features. Any equivalent structure directly or indirectly using the content of the specification and drawings of the present application in other related technical fields shall be within the scope of the patent protection of the present application by the same token.

Claims

1. A distribution network grounding protection method based on zero-sequence current phase difference variation, characterized in that: The method is implemented based on the following steps: 1) A group of small resistors are connected in parallel at the neutral point of the distribution network; based on the unit zero-sequence ground capacitance parameter C0 of the line and the total length L of the remaining lines after removing the longest feeder, the characteristics of the zero-sequence current and the neutral point current are analyzed, and the phase difference change before and after the small resistor is put into use is calculated; 2) When the measuring device detects that the zero-sequence current of the feeder is greater than the starting current I S When the protection starts, the neutral point zero-sequence current and feeder zero-sequence current signals are recorded and processed; 3) For low-resistance grounding faults, a setting value I is set based on the fault identification method of zero-sequence current phase difference. min , when zero sequence current is detected Greater than the set value I min When the fault occurs, the line is directly determined to be the fault feeder, and a small resistor is no longer connected in parallel with the neutral point to calculate the zero-sequence current phase difference, so the fault can be quickly removed as soon as possible. 4) When the zero-sequence current is less than the set value I min When the fault is detected, the fault identification method of zero-sequence current phase difference is used to determine whether it is a sound line with metallic grounding fault or a faulty line with high-resistance grounding fault. 5) The high-resistance grounding fault protection criterion based on the change in the zero-sequence current phase difference before and after changing the neutral point small resistance is as follows: Where Δθ is the zero-sequence current of the feeder before and after the small resistor is put into operation. and neutral point zero sequence current The change in phase difference between set <Δθ<18°, it is determined that a high-resistance grounding fault has occurred in the protection line, and the protection is tripped; if -1.2°<Δθ<1.2°, it is determined to be a healthy feeder and the protection is not activated.

2. The method for grounding protection of a distribution network based on a change in zero-sequence current phase difference according to claim 1, characterized in that: In step 1), the original 20Ω small resistor at the neutral point is combined with the newly added 20Ω small resistor in parallel, so that the equivalent resistance of the neutral point is reduced to 10Ω.

3. The method for grounding protection of a distribution network based on a change in zero-sequence current phase difference according to claim 2, characterized in that: In step 2), in order to ensure that the protection does not malfunction when the power distribution system is operating normally, the starting current I S The setting should avoid the maximum unbalanced current estimated in the system; the maximum unbalanced current of the overhead line and cable line of the 10kV low-resistance grounding system during normal operation is 0.37A and 0.26A respectively. The protection device is often connected to 3 times the zero-sequence current, and the starting current should be greater than 1.11A.

4. The method for grounding protection of a distribution network based on a change in zero-sequence current phase difference according to claim 3 is characterized in that: The starting current I set in step 2) S is 1.5A.

5. The method for grounding protection of a distribution network based on a change in zero-sequence current phase difference according to claim 2, characterized in that: Step 3) Taking into account the maximum zero-sequence current amplitude of the healthy outgoing line and the protection margin factors, set the setting value I min Set to 30A.

6. The method for grounding protection of a distribution network based on a change in zero-sequence current phase difference according to claim 2, characterized in that: Step 4) specifically includes: after a small resistor is connected in parallel to the neutral point, the neutral point equivalent resistance changes from 20Ω to 10Ω, and the feeder zero-sequence current at this time is recorded and neutral point zero sequence current The phase difference between the two is calculated before and after the small resistor is put into operation. and After the phase difference changes by Δθ, the small parallel resistor is disconnected to restore the neutral point resistance to 20Ω.

7. The method for grounding protection of a distribution network based on a change in zero-sequence current phase difference according to claim 2, characterized in that: In step 5), under normal circumstances, Δθ will not fall into the area outside the two intervals of the judgment formula. If the judgment criteria of both the faulty and sound feeders are not met, a measurement or calculation error has occurred, and the protection will be recalculated and judged.

8. The method for grounding protection of a distribution network based on a change in zero-sequence current phase difference according to claim 2, characterized in that: In step 5), the measurement equipment error is considered. The error limit of the phase difference of the 5P-level current transformer for steady-state protection is 60' at the rated primary current. In addition, considering the errors caused by line parameters and uncertainties in the calculation process, in order to ensure the reliability of the high-resistance grounding fault identification result, a certain margin is reserved, so the criterion for a healthy feeder is set to -1.2°<Δθ<1.2°; The lower limit of the fault feeder value is obtained according to the distribution network line parameters. The specific expression is as follows: Among them, θ set is the lower limit of Δθ of the faulty feeder, K rel is the set margin coefficient, which is 0.9; R n Indicates the resistance of a small parallel resistor, R' n Represents the value of the equivalent small resistance at the neutral point after the parallel small resistance is put into use; ω is the angular frequency of the power frequency; when R n and R' n When the values ​​of θ are 20Ω and 10Ω respectively, it is only necessary to obtain the line length of the distribution network and the ground capacitance parameters to calculate θ. set Perform adjustment; For the upper limit of the fault feeder, a certain margin is also retained, which is 18°.

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