Protection discrimination method for faults in transformer area of small resistance grounding system

Abstract

The invention discloses a method for protecting and judging faults in a transformer area of a small-resistance grounding system, and relates to the technical field of relay protection of a power system, and the method is executed by a transformer protection device when the electrical quantity of the system meets the starting criterion of a starting element in the transformer protection device. Comprising the following steps: based on a small resistance grounding system single-phase grounding fault model, obtaining high-low voltage side three-phase current of a transformer when a single-phase grounding fault occurs; obtaining a high-low voltage side break variable current component of the transformer according to the high-low voltage side three-phase current of the transformer when the single-phase earth fault occurs, judging whether the high-low voltage side break variable current component of the transformer meets a pre-constructed internal fault break variable current criterion or not, if so, judging that the internal fault occurs, and executing a protection action; otherwise, determining that the fault is an external fault. The method is used under the condition of new energy access, when a transformer has a single-phase earth fault, accurate identification of a fault area is realized, an internal fault and an external fault are distinguished, and a protection action can be quickly performed.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of power system relay protection technology, in particular to a protection discrimination method for transformer internal fault in a small resistance grounding system. BACKGROUND

[0002] With large-scale access of wind power, photovoltaic and other new energy to the power grid, transformer protection is facing new challenges. The short-circuit current characteristics of new energy power sources are different from traditional synchronous generators. The amplitude of the short-circuit current is limited and contains a large amount of harmonics, which reduces the reliability of the protection criterion based on short-circuit current characteristics. The access of distributed new energy changes the structure of the power grid, and the short-circuit current path is complex during a fault, affecting the selectivity and rapidity of the protection.

[0003] In recent years, there have been several events of low-voltage side grounding fault protection not acting in new energy plants, the root cause of which is that the fault current in the small resistance grounding system is small, resulting in a differential current generated by the system that does not meet the protection setting value, thus causing the transformer protection device to refuse to act.

[0004] The intermittency and volatility of new energy generation lead to changes in power flow direction, intensified voltage and frequency fluctuations, and the action threshold and logic of traditional transformer protection are difficult to adapt to. Therefore, it is urgent to research a protection method suitable for transformer internal fault in a small resistance grounding system to solve the problem of traditional refusal to act, in order to ensure the safe and stable operation of the power grid. SUMMARY

[0005] The technical problem to be solved by the present application is to overcome the defects of the prior art and provide a protection discrimination method for transformer internal fault in a small resistance grounding system, aiming to solve the problem of protection device refusal to act when a transformer internal fault occurs in a small resistance grounding system.

[0006] To achieve the above-mentioned purpose, the present application adopts the following technical scheme:

[0007] In a first aspect, the present application provides a protection discrimination method for transformer internal fault in a small resistance grounding system. The method is executed by the transformer protection device when the electrical quantities of the system meet the starting criterion of the starting element in the transformer protection device. The method comprises: obtaining the three-phase current at the high and low voltage sides of the transformer when a single-phase grounding fault occurs; obtaining the transformer high and low voltage side sudden change variable current component according to the three-phase current at the high and low voltage sides of the transformer when a single-phase grounding fault occurs; determining whether the transformer high and low voltage side sudden change variable current component meets the pre-constructed internal fault sudden change variable current criterion; if it meets, it is determined that there is a transformer internal fault, and the transformer protection device acts; otherwise, it is determined that there is a transformer external fault.

[0008] Further, the transformer protection device further comprises: a single-phase grounding fault phase selection discrimination based on the sudden change current components of the high and low voltage sides of the transformer, so as to accurately select the phase of the single-phase grounding fault.

[0009] Further, the single-phase grounding fault phase selection discrimination is based on: if the sudden change current vector of one phase is 0 and the vector sum of the sudden change current of the remaining two phases is 0 within a preset error range, then the phase sequence of the phase that is 120° ahead of the remaining two phases is determined as the fault phase.

[0010] Further, the starting element comprises: a differential current starting element and a phase current sudden change current starting element.

[0011] When the differential current starting element or the phase current sudden change current starting element meets the corresponding starting criterion, the protection discrimination is started.

[0012] Further, the starting criterion of the differential current starting element is that the differential current of any phase is greater than a preset starting threshold.

[0013] Further, the starting criterion of the phase current sudden change current starting element is:

[0014] ;

[0015] wherein, is the sampling point per power frequency cycle, is the current instantaneous value of the current sampling point, one cycle ago, two cycles ago and three cycles ago, is the phase current sudden change starting threshold.

[0016] Further, the sudden change current component is the difference between the current value of the current sampling point and the current effective value of the sampling point two cycles ago, i.e.:

[0017] ;

[0018] wherein, is the sudden change current component of the current sampling point, is the sampling point per power frequency cycle, is the current effective value of the current sampling point, is the current effective value of the sampling point two cycles ago.

[0019] Further, the sudden change current criterion of the zone fault comprises: within a preset error range, the amplitude ratio of the two-phase sudden change current of the high voltage side is 1, the sudden change current of the remaining one phase is 0, and the amplitude ratio of the two-phase sudden change current of the low voltage side is 1.

[0020] Furthermore, the fault current criterion within the zone also includes: within a preset error range, the phase angle difference between any two phase currents on the high-voltage side is 180°, and the phase angles of the three phase currents on the low-voltage side are equal.

[0021] Compared with the prior art, the beneficial effects achieved by the present invention are as follows:

[0022] (1) The method provided by the present invention only reads the three-phase current of the current transformers on the high and low voltage sides of the transformer when a single-phase ground fault occurs, introduces the sudden change current component, and combines it with the fault criterion of the sudden change current component in the zone, so as to effectively distinguish the fault in the zone from the fault outside the zone, accurately identify the fault area, and realize the rapid action of the transformer protection device when the ground fault occurs in the zone.

[0023] (2) The method provided by the present invention also utilizes the abrupt change current component to construct the phase selection criterion for single-phase grounding fault in the zone, thereby achieving accurate phase selection of the fault under the premise of grounding fault in the zone and improving the sensitivity of transformer protection. Attached Figure Description

[0024] Figure 1 This is a diagram of a transformer single-phase grounding fault model provided in Embodiment 3 of the present invention;

[0025] Figure 2 This is a single-phase ground fault sequence diagram provided in Embodiment 3 of the present invention when a ground fault occurs within the fault zone;

[0026] Figure 3 This is the composite sequence network diagram for a phase A ground fault provided in Embodiment 3 of the present invention;

[0027] Figure 4 This is a single-phase ground fault sequence diagram for an external ground fault, as provided in Embodiment 3 of the present invention. Detailed Implementation

[0028] The present invention will be further described below with reference to the accompanying drawings. The following embodiments are only used to more clearly illustrate the technical solution of the present invention, and should not be used to limit the scope of protection of the present invention.

[0029] Example 1

[0030] This embodiment provides a protection discrimination method for transformer zone faults applicable to low-resistance grounding systems. The method is executed by a transformer protection device, which monitors the current value at its installation location in real time. When the electrical quantity of the system meets the starting criteria of the starting element in the transformer protection device, the protection discrimination is initiated.

[0031] It should be noted that the starting elements in the transformer protection device mentioned here include: differential current starting elements and phase current change current starting elements.

[0032] In some embodiments with differential current starting elements, the criterion for the differential current starting element is: if the differential current of any phase is greater than a preset differential current starting threshold, the protection will start. This can be expressed by the formula:

[0033] ;

[0034] in: These are the vector values ​​of the current in each side line after conversion and rotation; This refers to the differential current of phases A, B, and C.

[0035] Optionally, the differential current starting threshold is set to 0.3 times the rated current on the high-voltage side of the transformer.

[0036] In some embodiments that configure phase current surge current initiation elements, the criterion for the phase current surge current initiation element is:

[0037] ;

[0038] in, For each power frequency cycle, sampling points, These represent the instantaneous current values ​​at the current sampling points, one cycle ago, two cycles ago, and three cycles ago, respectively. This is the threshold value for starting the phase current sudden change.

[0039] It should be noted that in embodiments with two types of starting elements, protection discrimination is initiated when either starting element is activated.

[0040] Next, the protection discrimination method provided in this embodiment will be introduced, specifically including:

[0041] S1: Obtain the three-phase current on the high and low voltage sides of the transformer when a single-phase ground fault occurs.

[0042] Generally, transformer protection devices monitor the current value at their installation location in real time. Once the starting element in the transformer protection device is activated, the current three-phase current on the high and low voltage sides of the transformer is obtained by reading the current of the current transformer.

[0043] S2: Obtain the sudden change current components on the high and low voltage sides of the transformer based on the three-phase currents on the high and low voltage sides when a single-phase ground fault occurs.

[0044] When a single-phase ground fault occurs, the three-phase currents on the high and low voltage sides of the transformer obtained by the transformer protection device in S1 are the sum of the load current component and the fault current component. In order to eliminate the influence of the load current component on the protection criterion, the sudden change current component is introduced to characterize the fault current component.

[0045] The abrupt change in current component is the difference between the current sampling point value and the sampling point value two cycles ago, i.e.:

[0046] ;

[0047] in, The abrupt change is the current component. For each power frequency cycle, sampling points, This is the current sampling point.

[0048] S3: Determine whether the sudden change current components on the high and low voltage sides of the transformer meet the pre-built fault sudden change current criteria within the zone. If they do, it is determined to be a fault within the transformer zone, and the transformer protection device will operate; otherwise, it is determined to be a fault outside the transformer zone.

[0049] In some specific embodiments, the fault current criterion within the zone includes: a criterion regarding the current amplitude, specifically: within a preset error range, there are two phases on the high-voltage side with a current amplitude ratio of 1 for the fault current surge, and the remaining phase with a current surge of 0, and the amplitude ratio of any two phases on the low-voltage side with a current surge is 1, expressed by the formula:

[0050] ;

[0051] ;

[0052] in, This refers to the three-phase sudden change current value on the high-voltage side; This represents the abrupt change in current value of the three phases on the low-voltage side. This is the setting value for the sudden change current ratio on the high-voltage side; This is the setting value for the sudden change in current offset on the high-voltage side. This is the low-voltage side sudden change current ratio setting value.

[0053] Optional, and The value ranges from 0.95 to 0.99. The value ranges from 0.01 to 0.05.

[0054] To more accurately identify fault areas, the fault current surge criteria within the area also include: a phase criterion, specifically: within a preset error range, the phase angle difference between any two phase surge currents on the high-voltage side is 180°, and the phase angles of the three phase surge currents on the low-voltage side are equal; the expression is:

[0055] ;

[0056] ;

[0057] in, The phase angle setting value for the sudden change current on the high-voltage side is optional, and its value should be 170~175. The phase angle setting value for the sudden change current on the low-voltage side is optional and should be 0.90~0.95.

[0058] Example 2

[0059] Based on Example 1, this example provides a protection discrimination method for transformer area faults that includes phase selection discrimination for single-phase grounding faults.

[0060] Similarly, single-phase grounding fault phase selection is also achieved by utilizing the abrupt change in current components on the high and low voltage sides of the transformer.

[0061] Specifically, the basis for selecting the phase for a single-phase grounding fault is as follows: if, within the preset error range, the vector of the sudden change current of one phase is 0, and the vector sum of the sudden change currents of the remaining two phases is 0, then the phase with a phase sequence leading by 120° among the sudden change currents of the remaining two phases is determined to be the fault phase.

[0062] Taking a phase A fault as an example, the ideal situation is as follows:

[0063] ;

[0064] Considering potential errors in the sampling process, the actual discrimination condition is:

[0065] when At that time, it was determined to be a phase A ground fault;

[0066] in, These are the abrupt current vectors for phases A, B, and C, respectively. The fault selection setting value is optional, and its value should be 0.01~0.05.

[0067] Similarly:

[0068] when At that time, it was determined to be a phase B ground fault;

[0069] when At that time, it was determined to be a C-phase ground fault.

[0070] The method provided in this embodiment can achieve accurate phase selection for faults, has high practicality, and can be used in the case of new energy access. When a transformer experiences a fault in the area due to grounding through a small resistance, it can act quickly and accurately. At the same time, it can realize the fault phase selection function and improve the sensitivity of transformer protection.

[0071] Example 3

[0072] This embodiment provides the derivation process of the fault mutation current criterion within the region to prove the effectiveness of the criterion.

[0073] It should be noted that the introduction of the sudden change current component in Example 1 is to characterize the fault current component in order to avoid the load current component from affecting the fault criterion and causing insufficient sensitivity of the protection device. In this example, the fault current component is directly calculated by the symmetrical component method, and its effectiveness as a protection criterion is proven.

[0074] In a low-resistance grounding system, the transformer single-phase grounding fault model is as follows: Figure 1 As shown in the figure, when a single-phase ground fault occurs at point K2 in the area, the following results are obtained: Figure 2 The diagrams shown represent the positive-sequence, negative-sequence, and zero-sequence sequence network diagrams for a single-phase ground fault. In this diagram, a represents the positive-sequence network diagram, b represents the negative-sequence network diagram, and c represents the zero-sequence network diagram. These are the positive-sequence, negative-sequence, and zero-sequence fault currents. These are the positive and negative sequence impedance values ​​of the system under fault conditions. These are the positive and negative sequence impedance values ​​of the transformer under fault conditions. This is the zero-sequence impedance value from the low-voltage side of the transformer to the fault point under fault conditions.

[0075] According to the symmetric component method:

[0076] ;

[0077] In the formula, For rotation factor, .

[0078] Taking a phase A ground fault as an example, Figure 3 The diagram shows the composite sequence network diagram for a phase-A ground fault. These are the positive-sequence, negative-sequence, and zero-sequence impedance values ​​under fault conditions. , , .

[0079] When a phase A ground fault occurs, adding boundary conditions results in: ; ,but:

[0080] ;

[0081] We can obtain:

[0082] ;

[0083] ;

[0084] in, This is the equivalent electromotive force. These are the positive-sequence, negative-sequence, and zero-sequence fault current vectors. These are the positive-sequence, negative-sequence, and zero-sequence fault voltage vectors. The fault voltage vectors are phases A, B, and C.

[0085] First, the criteria for current amplitude are demonstrated through theoretical calculations: within the preset error range, the amplitude ratio of the two phase sudden change currents on the high-voltage side is 1, the amplitude ratio of the remaining phase sudden change current is 0, and the amplitude ratio of any two phase sudden change currents on the low-voltage side is 1.

[0086] When a ground fault occurs at point K2, due to the Y / △-11 wiring configuration, only zero-sequence fault current flows on the low-voltage side, while positive and negative sequence fault currents flow on the high-voltage side. Therefore, the three-phase fault current on the low-voltage side of the transformer is:

[0087] ;

[0088] The zero-sequence, positive-sequence, and negative-sequence fault currents on the high-voltage side of the transformer are:

[0089] ;

[0090] The three-phase fault current on the high-voltage side of the transformer is:

[0091] ;

[0092] In the formula, These are the three-phase fault currents on the low-voltage side. These are the three-phase fault currents on the high-voltage side. These are the zero-sequence, positive-sequence, and negative-sequence fault currents on the high-voltage side, respectively. These are the zero-sequence, positive-sequence, and negative-sequence fault currents on the low-voltage side, respectively.

[0093] Therefore, it can be seen that when phase A is grounded, the amplitudes of the three-phase fault currents on the low-voltage side of the transformer are all equal. In other words, within the allowable error range, the amplitude ratio of any two phase fault currents on the low-voltage side is 1, the fault current of one phase on the high-voltage side is 0, and the amplitude ratio of the other two phase fault currents is 1.

[0094] Therefore, when a single-phase ground fault occurs within the affected area, the criterion condition for the magnitude of the sudden change current described in Example 1 is valid.

[0095] Next, this embodiment takes the case of a single-phase ground fault outside the zone as an example to further demonstrate the effectiveness of the criterion for the magnitude of the sudden change current from the opposite perspective.

[0096] when Figure 1 When an external ground fault occurs at point K3, the following information is obtained: Figure 4 The diagrams shown represent the positive sequence, negative sequence, and zero sequence net diagrams for a single-phase ground fault, where a is the positive sequence net diagram, b is the negative sequence net diagram, and c is the zero sequence net diagram.

[0097] When a fault occurs outside the K3 point zone, since the zero-sequence fault current does not flow through the high and low voltage sides, only the positive and negative sequence fault currents flow through the high and low voltage sides.

[0098] The three-phase fault current on the low-voltage side of the transformer is:

[0099] ;

[0100] therefore:

[0101] ;

[0102] The three-phase fault current on the high-voltage side of the transformer is:

[0103] ;

[0104] Therefore, when a single-phase ground fault occurs outside the zone, although the amplitude of the three-phase fault current on the high-voltage side of the transformer meets the criterion condition for the amplitude of the sudden change current described in Example 1, the amplitude ratios of the three-phase fault current on the low-voltage side are 1, -1 / 2 and -2, respectively, which do not meet the criterion condition for the amplitude of the sudden change current.

[0105] As can be seen from the above arguments on both sides, only when a single-phase ground fault occurs within the fault zone, the amplitude of the three-phase fault current on the high and low voltage sides of the transformer satisfies the criterion condition of the sudden change current with respect to amplitude. In other words, if the amplitude of the three-phase fault current on the high and low voltage sides of the transformer satisfies the criterion condition of the sudden change current with respect to amplitude, then it can be determined that a single-phase ground fault occurs within the fault zone.

[0106] The following verifies the criterion regarding phase: within the preset error range, the phase angle difference between any two phase sudden change currents on the high-voltage side is 180°, and the phase angles of the three phase sudden change currents on the low-voltage side are equal.

[0107] Taking a phase-A ground fault at point K2 within the zone as an example, theoretically:

[0108] ;

[0109] ;

[0110] in: The angle of the fault current for phases A, B, and C on the high-voltage side;

[0111] The angle of the three-phase fault currents A, B, and C on the low-voltage side;

[0112] Therefore, considering the possible errors in the sampling process, within the allowable error range, there are two phase fault currents on the high-voltage side with a phase difference of nearly 180°, while the fault currents on the low-voltage side have the same phase.

[0113] When a phase-A ground fault occurs at point K3 outside the zone, the phase ratio of the fault currents on any two low-voltage sides is:

[0114] ;

[0115] Clearly, the criterion condition that the phase angles of the fault currents on the low-voltage side are equal is not met.

[0116] Therefore, it can also be concluded that only when a single-phase ground fault occurs within the fault zone, the phase of the three-phase fault current on the high-voltage and low-voltage sides of the transformer satisfies the criterion condition of the sudden change current with respect to phase. In other words, if the phase of the three-phase fault current on the high-voltage and low-voltage sides of the transformer satisfies the criterion condition of the sudden change current with respect to phase, then it can be determined that a single-phase ground fault occurs within the fault zone.

[0117] In summary, the sudden change current criterion described in Example 1 can accurately identify the fault area and effectively distinguish between faults within and outside the area.

[0118] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the technical principles of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A method for protecting and identifying faults within a transformer zone of a low-resistance grounding system, characterized in that, When the system electrical quantities meet the starting criteria of the starting element within the transformer protection device, the transformer protection device executes the operation, including the following methods: Obtain the three-phase currents on the high and low voltage sides of the transformer when a single-phase ground fault occurs; The sudden change current components on the high and low voltage sides of the transformer are obtained from the three-phase currents on the high and low voltage sides when a single-phase ground fault occurs. Determine whether the sudden change current components on the high and low voltage sides of the transformer meet the pre-constructed fault sudden change current criteria within the transformer zone. If they do, the fault is determined to be within the transformer zone, and the transformer protection device will operate; otherwise, the fault is determined to be outside the transformer zone.

2. The protection and identification method for faults in the transformer area of ​​a low-resistance grounding system according to claim 1, characterized in that, While the transformer protection device operates, it also includes: performing single-phase grounding fault phase selection based on the sudden change current components on the high and low voltage sides of the transformer, so as to achieve accurate phase selection for single-phase grounding faults.

3. The protection and identification method for faults in the transformer area of ​​a low-resistance grounding system according to claim 2, characterized in that, The basis for single-phase grounding fault phase selection is as follows: if, within the preset error range, the vector of the sudden change current of one phase is 0, and the vector sum of the sudden change currents of the remaining two phases is 0, then the phase with a phase sequence leading by 120° among the sudden change currents of the remaining two phases is determined to be the fault phase.

4. The protection and identification method for faults in the transformer area of ​​a low-resistance grounding system according to claim 1, characterized in that, The starting elements include: a differential current starting element and a phase current change current starting element; When the differential current starting element or the phase current change current starting element meets the corresponding starting criterion, the protection is started.

5. The protection and identification method for faults in the transformer area of ​​a low-resistance grounding system according to claim 4, characterized in that, The starting criterion for the differential current starting element is that the differential current of any phase is greater than a preset starting threshold value.

6. The protection and identification method for faults in the transformer area of ​​a low-resistance grounding system according to claim 4, characterized in that, The starting criterion for the phase current mutation current starting element is as follows: ； in, For each power frequency cycle, sampling points, These represent the instantaneous current values ​​at the current sampling points, one cycle ago, two cycles ago, and three cycles ago, respectively. This is the threshold value for starting the phase current sudden change.

7. The protection and identification method for faults in the transformer area of ​​a low-resistance grounding system according to claim 1, characterized in that, The abrupt change in current component is the difference between the current value at the current sampling point and the effective current value at two sampling points two cycles ago, i.e.: ； in, This represents the abrupt change in current component at the current sampling point. For each power frequency cycle, sampling points, This represents the effective value of the current at the current sampling point. The effective value of the current at the sampling points two cycles ago.

8. The protection and identification method for faults in the transformer area of ​​a low-resistance grounding system according to claim 7, characterized in that, The criteria for determining the fault current change rate within the zone include: within a preset error range, the amplitude ratio of the two phase current change rate changes on the high-voltage side is 1, the current change rate change rate of the remaining phase is 0, and the amplitude ratio of any two phase current change rate changes on the low-voltage side is 1.

9. The protection and identification method for faults in the transformer area of ​​a low-resistance grounding system according to claim 8, characterized in that, The criteria for determining the fault transient current within the zone also include: within a preset error range, the phase angle difference between any two phase transient currents on the high-voltage side is 180°, and the phase angles of the three phase transient currents on the low-voltage side are equal.

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