A low-frequency transformer differential protection method based on excitation inductance parameter identification

Abstract

The present invention discloses a low-frequency transformer differential protection method based on excitation inductance parameter identification, including: according to three-phase collected data, three-phase calculation data is calculated; in response to any phase calculation data in the three-phase calculation data meeting the preset ratio brake differential protection action condition, it is determined that the low-frequency transformer is faulty and the fault type analysis is performed; otherwise, it is determined that the low-frequency transformer is not faulty and the protection is locked; the fault type analysis includes the following steps: according to the three-phase collected data, the parameters are identified by the least square method, and multiple three-phase excitation inductance average values ​​are calculated; in response to any excitation inductance average value of any phase in the three-phase excitation inductance average value meeting the preset excitation surge current identification criterion, it is determined as an in-zone fault and the protection is activated; otherwise, it is determined as an excitation surge current and the protection is locked. The overall method is easy to implement, has high sensitivity, is easier to act quickly and reliably when a fault occurs, and has good speed and reliability.

Description

Technical Field

[0001] The invention relates to a low-frequency transformer differential protection method based on excitation inductance parameter identification, belonging to the technical field of low-frequency transformers. Background Art

[0002] Offshore wind power usually uses a flexible low-frequency transmission system, which is a new type of AC transmission technology with a frequency lower than the standard industrial frequency. The concept of low-frequency transmission (fractional frequency transmission) was first proposed by Academician Wang Xifan of Xi'an Jiaotong University in 1994, along with the advantages and technical feasibility of reducing the transmission frequency.

[0003] The low-frequency transformer is one of the indispensable core equipment in the flexible low-frequency transmission system. Its safe and stable operation is crucial to the networking and stable operation of the entire system. Therefore, the research on the new principle of low-frequency transformer protection is of great significance for practical engineering applications. At present, a transformer that can adapt to the transmission frequency of 16.7Hz has been successfully developed, with a maximum voltage level of 220kV and a maximum capacity of 180MV·A. However, the existing research mainly focuses on the no-load loss and volume change law of low-frequency transformers, and there is no clear conclusion on the impact of low frequency on the performance of ferromagnetic materials. In addition, under the low-frequency transmission system, the relay protection problem of the transformer also needs further in-depth research. Only by fully understanding the impact of the low-frequency environment on the transformer and its protection system can the system be ensured to operate safely and reliably under various working conditions. Therefore, continuous and in-depth research will provide valuable technical support and theoretical guidance for the further development of flexible low-frequency transmission systems, and promote the entire field to develop in a more sustainable, safe and efficient direction.

[0004] In a flexible low-frequency transmission system, once a transformer fails, the electrical quantities of the system show obvious nonlinear characteristics, resulting in complex and changeable current waveforms during the fault. However, the reliability of traditional transformer longitudinal differential protection is insufficient when applied to low-frequency transmission systems, which has become a key challenge that needs to be solved. Therefore, it is necessary to further study new protection methods to reliably protect transformers in flexible low-frequency transmission systems. Summary of the invention

[0005] The purpose of the present invention is to overcome the shortcomings of the prior art and provide a low-frequency transformer differential protection method based on excitation inductance parameter identification. First, based on the preset ratio-braked differential protection action conditions, fault identification can be quickly performed. Secondly, the pre-constructed excitation surge current identification criterion is used to identify the fault type, distinguish between intra-zone faults and excitation surge currents to determine whether to adopt protection action. The overall method is easy to implement and adjust, has good selectivity and high sensitivity, is easier to act quickly and reliably when a fault occurs, has good speed and reliability, and has good application prospects.

[0006] To achieve the above object, the present invention is implemented by adopting the following technical solutions:

[0007] The present invention discloses a low-frequency transformer differential protection method based on excitation inductance parameter identification, comprising the following steps:

[0008] Obtain three-phase data of low-frequency transformer;

[0009] According to the three-phase collected data, three-phase calculated data is obtained; wherein the three-phase calculated data includes three-phase differential current data and three-phase braking current data;

[0010] In response to any phase calculation data of the three-phase calculation data satisfying the preset ratio-restrained differential protection action condition, it is determined that the low-frequency transformer has a fault and a fault type analysis is performed; otherwise, it is determined that the low-frequency transformer has no fault and the protection is locked;

[0011] The fault type analysis includes the following steps:

[0012] According to the three-phase collected data, the parameters are identified by the least square method to obtain the three-phase excitation inductance values ​​at the current moment and multiple moments in the preset time period;

[0013] Calculate and obtain multiple three-phase excitation inductance average values ​​according to the three-phase excitation inductance values ​​at the current moment and multiple moments in the preset time period;

[0014] In response to any one of the three-phase excitation inductance average values ​​satisfying a preset excitation inrush current identification criterion, the fault type is determined to be an intra-zone fault and the protection is activated; otherwise, the fault type is determined to be an excitation inrush current and the protection is locked.

[0015] Furthermore, the three-phase collected data includes primary three-phase voltage, primary three-phase current, secondary three-phase current, primary three-phase leakage inductance and secondary three-phase leakage inductance.

[0016] Furthermore, the expression of the three-phase differential current data is as follows:

[0017]

[0018] In the formula, Indicates the differential current of phase A; Indicates the B phase differential current; Indicates the C phase differential current;

[0019] Indicates the primary A phase current; Indicates the primary B phase current; Represents the primary C phase current;

[0020] Indicates the secondary side A phase current; Indicates the secondary side B phase current; Indicates the secondary side C phase current;

[0021] Indicates the transformer ratio.

[0022] Furthermore, the expression of the three-phase braking current data is as follows:

[0023]

[0024] In the formula, Indicates the braking current of phase A; Indicates the braking current of phase B; Indicates the braking current of phase C;

[0025] Indicates the primary A phase current; Indicates the primary B phase current; Represents the primary C phase current;

[0026] Indicates the secondary side A phase current; Indicates the secondary side B phase current; Indicates the secondary side C phase current;

[0027] Indicates the transformer ratio.

[0028] Further, the preset ratio-braked differential protection action condition includes a first differential protection action criterion and a second differential protection action criterion;

[0029] In response to any phase calculation data of the three-phase calculation data satisfying the first differential protection action criterion or the second differential protection action criterion, it is determined that the low-frequency transformer is faulty;

[0030] The first differential protection action criterion includes: the differential current of any phase in the three-phase calculation data is greater than or equal to the preset minimum action current setting value, and the braking current of any phase is less than or equal to the preset minimum braking current setting value, and the expression is as follows:

[0031]

[0032] In the formula, Represents the differential current of any phase; Indicates the braking current of any phase; Indicates the minimum operating current setting value; Indicates the minimum braking current setting value;

[0033] The second differential protection action criterion includes: the differential current of any phase in the three-phase calculation data is greater than or equal to the sum of the preset minimum action current setting value and the braking current difference based on the ratio braking coefficient, and the braking current of any phase is greater than the preset minimum braking current setting value, and the expression is as follows:

[0034]

[0035] Where k represents the ratio braking coefficient.

[0036] Further, according to the three-phase collected data, the parameters are identified by the least square method to obtain the three-phase excitation inductance values ​​at the current moment and multiple moments in the preset time period, including the following steps:

[0037] According to the three-phase collected data, based on the preset A-phase identification matrix, the parameters are identified by the least square method to obtain a plurality of phase identification parameters; according to the plurality of phase identification parameters, the A-phase excitation inductance value is calculated;

[0038] According to the three-phase collected data, based on the preset B-phase identification matrix, the parameters are identified by the least square method to obtain a plurality of phase identification parameters; according to the plurality of phase identification parameters, the B-phase excitation inductance value is calculated;

[0039] According to the three-phase collected data, based on the preset C-phase identification matrix, the parameters are identified by the least square method to obtain multiple phase identification parameters; according to the multiple phase identification parameters, the C-phase excitation inductance value is calculated.

[0040] Furthermore, the expression of the A phase identification matrix is ​​as follows:

[0041]

[0042] In the formula, Indicates the primary A phase current; Indicates the primary B phase current; represents a set of moments, n≥3; Indicates the primary A phase voltage; Indicates the primary B phase voltage; represents the first phase identification parameter; represents the second phase identification parameter; represents the third phase identification parameter;

[0043] The A phase excitation inductance value The expression is as follows: ;

[0044] The expression of the B phase identification matrix is ​​as follows:

[0045]

[0046] In the formula, Represents the primary C phase current; Indicates the primary C phase voltage; represents the fourth phase identification parameter; represents the fifth phase identification parameter; represents the sixth phase identification parameter;

[0047] The B phase excitation inductance value The expression is as follows:

[0048] The expression of the C phase identification matrix is ​​as follows:

[0049]

[0050] In the formula, represents the seventh phase identification parameter; represents the eighth phase identification parameter; represents the ninth phase identification parameter;

[0051] The C phase excitation inductance value The expression is as follows: .

[0052] Furthermore, the average value of the excitation inductance of any phase among the three-phase excitation inductance average values ​​is The expression is as follows:

[0053] In the formula, Indicates the average value of the excitation inductance of any phase, that is, the average value of the excitation inductance of phase A, or the average value of the excitation inductance of phase B, or the average value of the excitation inductance of phase C; Represents the excitation inductance value at the current time t; Indicated in The excitation inductance value at time, The value is the preset time period Any sampling time within , where , , T represents the period of the system in which the transformer is located.

[0054] Furthermore, the preset excitation inrush current identification criteria include:

[0055] In response to the fact that any one of the three-phase excitation inductance average values ​​is less than a preset excitation inductance setting value, the excitation inductance characteristic is judged to be stable and without fluctuation, the fault type is determined to be an intra-zone fault, and the protection is activated; otherwise, the excitation inductance characteristic is judged to show obvious fluctuation, the fault type is determined to be an excitation inrush current, and the protection is locked.

[0056] Furthermore, the expression of the setting value of the excitation inductance is as follows:

[0057]

[0058] In the formula, Indicates the setting value of the excitation inductance; Indicates the no-load steady-state excitation inductance value; It indicates the ratio of the magnetic permeability in the non-saturated and saturated states in the magnetization characteristic curve of the transformer; Represents the reliability coefficient.

[0059] Compared with the prior art, the present invention has the following beneficial effects:

[0060] The low-frequency transformer differential protection method based on excitation inductance parameter identification of the present invention first extracts the three-phase differential current data and the three-phase braking current data of the low-frequency transformer, and through the analysis of the current fault waveform difference, the fault can be quickly identified based on the preset ratio braking differential protection action conditions; and further based on the difference in excitation inductance characteristics when different faults of the low-frequency transformer occur, the corresponding fault characteristics are analyzed, and the pre-constructed excitation surge current identification criterion is used to identify the fault type, distinguish between faults within the zone and excitation surge current to determine whether to adopt protection action; the differential protection method of the present invention is easy to implement and set, has good selectivity and high sensitivity, is easier to act quickly and reliably when a fault occurs, has good speed and reliability, and has good application prospects. BRIEF DESCRIPTION OF THE DRAWINGS

[0061] Figure 1 It is a flow chart of a low-frequency transformer differential protection method based on excitation inductance parameter identification provided in Example 1;

[0062] Figure 2 is a T-type equivalent circuit diagram of a YnD-connected three-phase dual-winding transformer provided in Example 1;

[0063] Figure 3This is a schematic diagram of the M3C offshore wind power low-frequency transmission system provided in Example 1;

[0064] Figure 4 It is a schematic diagram of the low-frequency simulation system and fault point setting provided in Example 1;

[0065] Figure 5 It is a schematic diagram of the excitation inductance of a single-phase ground fault provided in Example 2;

[0066] Figure 6 This is a schematic diagram of the single-phase ground fault protection action provided by Example 2;

[0067] Figure 7 It is a schematic diagram of the excitation inductance of a two-phase grounding fault provided in Example 3;

[0068] Figure 8 This is a schematic diagram of the two-phase grounding fault protection action provided by Example 3;

[0069] Fig. 9 It is a schematic diagram of the three-phase ground fault excitation inductance provided in Example 4;

[0070] Fig.10 This is a schematic diagram of the three-phase ground fault protection action provided by Example 4;

[0071] Fig.11 It is a schematic diagram of the excitation inductance of a two-phase short-circuit fault provided in Example 5;

[0072] Fig.12 This is a schematic diagram of the two-phase short-circuit fault protection action provided by Example 5;

[0073] Fig.13 It is a schematic diagram of no-load closing excitation inductance provided in Example 6;

[0074] Fig.14 This is a schematic diagram of the no-load closing protection action provided in Example 6. DETAILED DESCRIPTION

[0075] The present invention will be further described below in conjunction with the accompanying drawings. The following embodiments are only used to more clearly illustrate the technical solution of the present invention, and cannot be used to limit the protection scope of the present invention.

[0076] This embodiment provides a low-frequency transformer differential protection method based on excitation inductance parameter identification. Figure 1 As shown, the following steps are included:

[0077] Obtain three-phase data of low-frequency transformer;

[0078] According to the three-phase collected data, three-phase calculated data is obtained; wherein the three-phase calculated data includes three-phase differential current data and three-phase braking current data;

[0079] In response to any phase calculation data in the three-phase calculation data satisfying the preset ratio-restrained differential protection action condition, it is determined that the low-frequency transformer has a fault and a fault type analysis is performed; otherwise, it is determined that the low-frequency transformer has no fault and the protection is locked;

[0080] Among them, the fault type analysis includes the following steps:

[0081] According to the three-phase collected data, the parameters are identified by the least square method to obtain the three-phase excitation inductance values ​​at the current moment and multiple moments in the preset time period;

[0082] Calculate and obtain multiple three-phase excitation inductance average values ​​according to the three-phase excitation inductance values ​​at the current moment and multiple moments in the preset time period;

[0083] In response to any one of the three-phase excitation inductance average values ​​satisfying the preset excitation surge current identification criterion, the fault type is determined to be an intra-zone fault and the protection is activated; otherwise, the fault type is determined to be an excitation surge current and the protection is locked.

[0084] The technical concept of the present invention is: first, the three-phase differential current data and the three-phase braking current data of the low-frequency transformer are extracted, and by analyzing the difference in current fault waveforms, the fault can be quickly identified based on the preset ratio braking differential protection action conditions; and further based on the difference in excitation inductance characteristics when different faults of the low-frequency transformer occur, the corresponding fault characteristics are analyzed, and the pre-constructed excitation surge current identification criteria are used to identify the fault type, distinguish between faults within the zone and excitation surge currents to determine whether to adopt protection actions; the differential protection method of the present invention is easy to implement and set, has good selectivity and high sensitivity, is easier to act quickly and reliably when a fault occurs, has good speed and reliability, and has good application prospects.

[0085] Specifically, the three-phase collected data includes primary three-phase voltage, primary three-phase current, secondary three-phase current, primary three-phase leakage inductance and secondary three-phase leakage inductance.

[0086] Figure 2 This is the equivalent circuit diagram of the YnD connected three-phase double-winding transformer T type. Figure 2 middle is the primary three-phase current; is the secondary three-phase current. In this embodiment, a braking coefficient differential protection action current I op With the braking current I res The current flowing into the transformer is considered as the positive direction, and the expression of the three-phase differential current data is as follows:

[0087]

[0088] In the formula, Indicates the differential current of phase A; Indicates the B phase differential current; Indicates the C phase differential current;

[0089] Indicates the primary A phase current; Indicates the primary B phase current; Represents the primary C phase current;

[0090] Indicates the secondary side A phase current; Indicates the secondary side B phase current; Indicates the secondary side C phase current;

[0091] Indicates the transformer ratio.

[0092] Furthermore, the expression of the three-phase braking current data is as follows:

[0093]

[0094] In the formula, Indicates the braking current of phase A; Indicates the braking current of phase B; Indicates the braking current of phase C;

[0095] Indicates the primary A phase current; Indicates the primary B phase current; Represents the primary C phase current;

[0096] Indicates the secondary side A phase current; Indicates the secondary side B phase current; Indicates the secondary side C phase current;

[0097] Indicates the transformer ratio.

[0098] The preset ratio-brake differential protection action conditions in this embodiment include a first differential protection action criterion and a second differential protection action criterion;

[0099] In response to any phase calculation data of the three-phase calculation data satisfying the first differential protection action criterion or the second differential protection action criterion, it is determined that the low-frequency transformer is faulty;

[0100] Among them, the first differential protection action criterion includes: the differential current of any phase in the three-phase calculation data is greater than or equal to the preset minimum action current setting value, and the braking current of any phase is less than or equal to the preset minimum braking current setting value, and the expression is as follows:

[0101]

[0102] In the formula, Represents the differential current of any phase; Indicates the braking current of any phase; Indicates the minimum operating current setting value, which is 0.4 times the rated current of the transformer in this embodiment; Indicates the minimum braking current setting value, which is 0.8 times the rated current of the transformer in the present invention;

[0103] The second differential protection action criterion includes: the differential current of any phase in the three-phase calculation data is greater than or equal to the sum of the preset minimum action current setting value and the braking current difference based on the ratio braking coefficient, and the braking current of any phase is greater than the preset minimum braking current setting value. The expression is as follows:

[0104]

[0105] In the formula, k represents the ratio braking coefficient, which is usually 0.5 to 0.8. In this embodiment, the ratio braking coefficient k is 0.5.

[0106] In response to any phase calculation data in the three-phase calculation data meeting the preset ratio-restrained differential protection action conditions, it is determined that the low-frequency transformer has an internal fault or generates an excitation inrush current. We use the characteristics of the excitation inductance to distinguish between the low-frequency transformer excitation inrush current and the internal fault. Figure 1 The YnD connection three-phase double winding transformer T-type equivalent circuit diagram is shown in the figure. is the primary phase A voltage, is the primary B phase voltage, is the primary C phase voltage; is the primary phase A current, is the primary B phase current, is the primary C phase current; is the secondary A phase current, is the secondary B phase current, is the secondary C phase current; is the leakage inductance of the primary phase A, is the leakage inductance of the primary phase B, is the leakage inductance of the primary phase C; is the secondary side A phase leakage inductance, is the leakage inductance of the secondary side B phase, is the leakage inductance of phase C on the secondary side; is the A phase excitation inductance, is the B phase excitation inductance, is the excitation inductance of phase C. When the secondary side is unloaded, the three-phase current of the secondary side , , is 0, the primary three currents , , Not 0, the secondary side has only three equal circulating currents pass.

[0107] When the transformer is closed under no-load condition, Figure 2 We can get:

[0108]

[0109] Assume that the primary three-phase leakage inductance value is , , Equal, set the primary three-phase leakage inductance to be equal , , so the above formula can be transformed into:

[0110]

[0111] Taking the A-phase excitation inductance as an example, the primary A-phase voltage after deformation is The least square method is used to identify parameters. Based on the preset A-phase identification matrix, the least square method is used to identify parameters to obtain multiple phase identification parameters. According to the multiple phase identification parameters, the A-phase excitation inductance value is calculated.

[0112] The expression of the A phase identification matrix is ​​as follows:

[0113]

[0114] In the formula, Indicates the primary A phase current; Indicates the primary B phase current; represents a set of moments, n≥3; Indicates the primary A phase voltage; Indicates the primary B phase voltage; represents the first phase identification parameter; represents the second phase identification parameter; represents the third phase identification parameter;

[0115] A phase excitation inductance The expression is as follows: , as follows: , , .

[0116] Similarly, according to the three-phase collected data, based on the preset B-phase identification matrix, the parameters are identified by the least square method to obtain multiple phase identification parameters; according to the multiple phase identification parameters, the B-phase excitation inductance value is calculated.

[0117] The expression of the B phase identification matrix is ​​as follows:

[0118]

[0119] In the formula, Represents the primary C phase current; Indicates the primary C phase voltage; represents the fourth phase identification parameter; represents the fifth phase identification parameter; represents the sixth phase identification parameter;

[0120] B phase excitation inductance The expression is as follows: , as follows: , , .

[0121] Similarly, according to the three-phase collected data, based on the preset C-phase identification matrix, the parameters are identified by the least square method to obtain multiple phase identification parameters; according to the multiple phase identification parameters, the C-phase excitation inductance value is calculated.

[0122] The expression of the C phase identification matrix is ​​as follows:

[0123]

[0124] In the formula, represents the seventh phase identification parameter; represents the eighth phase identification parameter; represents the ninth phase identification parameter;

[0125] C phase excitation inductance The expression is as follows: , as follows: , , .

[0126] When the fault type is an intra-zone fault, the excitation inductance is characterized by a stable excitation inductance without fluctuation; when the fault type is an excitation inrush current, the excitation inductance is characterized by an excitation inductance value showing obvious fluctuations. Based on the above characteristics of the excitation inductance, an excitation inrush current identification criterion is constructed to ensure that the low-frequency transformer protection can accurately identify the excitation inrush current and intra-zone faults.

[0127] The preset excitation inrush current identification criteria include:

[0128] In response to the fact that the average value of the excitation inductance of any phase among the three-phase excitation inductance average values ​​is less than the preset excitation inductance setting value, the excitation inductance characteristic is judged to be stable and without fluctuation, the fault type is determined to be an intra-zone fault, and the protection is activated; otherwise, the excitation inductance characteristic is judged to show obvious fluctuation, the fault type is determined to be excitation inrush current, and the protection is locked.

[0129] The average value of the excitation inductance of any phase among the three-phase excitation inductance average values The expression is as follows:

[0130] In the formula, Indicates the average value of the excitation inductance of any phase, that is, the average value of the excitation inductance of phase A, or the average value of the excitation inductance of phase B, or the average value of the excitation inductance of phase C; Represents the excitation inductance value at the current time t; Indicated in The excitation inductance value at time, The value is the preset time period Any sampling time within , where , , T represents the period of the system in which the transformer is located. The value is All sampling moments.

[0131] The expression of the setting value of the excitation inductance is as follows:

[0132]

[0133] In the formula, Indicates the setting value of the excitation inductance; Indicates the no-load steady-state excitation inductance value; It represents the ratio of the magnetic permeability in the non-saturated state to the magnetic permeability in the saturated state in the magnetization characteristic curve of the transformer, and in this embodiment, the value is 25; It represents the reliability coefficient, which is usually between 0.8 and 1.0. In this embodiment, the value is 0.8.

[0134] Therefore, the preset excitation inrush current identification criterion can be expressed as:

[0135]

[0136] When the transformer is switched on without load, the transformer core will not work in the saturation zone for more than half a cycle. , considering a certain margin, T1 can be ~ At each sampling moment in , The criterion can be described as follows: After the protection is started, the excitation inductance value L of any phase is used M(t) respectively with the time before the current time ~ At each sampling moment, the excitation inductance value L of any phase M (t-T1) Calculate the average value of the excitation inductance L av , if all the average values ​​obtained are less than L Mset , indicating that the low-frequency transformer excitation inductance is stable, otherwise the low-frequency transformer excitation inductance shows obvious fluctuations. If any phase in the three-phase circuit meets the preset excitation inrush current identification criteria, the transformer will have an internal fault, otherwise the transformer will generate an excitation inrush current.

[0137] In order to further verify the method of the present invention, PSCAD / EMTDC was used to build Figure 3-4 Schematic diagram of the low-frequency simulation system and fault point settings shown.

[0138] like Figure 4 As shown in Figure 1, F is the fault setting point in the transformer area, the fault occurs at 1s, and the sampling frequency is 4000Hz. The power supply is a 4.5MW low-frequency wind turbine, and the transformer simulation parameter settings are shown in Table 1.

[0139] Table 1 Transformer simulation parameters

[0140]

[0141] Combined with Table 2, the above-mentioned excitation inrush current identification criteria can be improved as follows:

[0142] Under no-load closing condition, the transformer core will work in the saturation zone for no more than half a cycle, i.e. 30ms, taking into account a certain margin. The criterion can be described as follows: After the protection is started, the current equivalent excitation inductance is used to calculate the value L M (t) is respectively compared with the equivalent excitation inductance calculated value L at each sampling time within 28.5~31.5ms before the current time M (t-T1) Calculate the average value of the excitation inductance L av , if all the average values ​​obtained are less than L Mset , the low-frequency transformer has an internal fault, otherwise the low-frequency transformer generates an excitation surge current.

[0143] Example 2

[0144] Based on the low-frequency simulation system and fault point setting provided in Example 1, this Example 2 adopts the low-frequency transformer differential protection method based on excitation inductance parameter identification in Example 1 to simulate and verify the single-phase grounding fault in the low-frequency transformer area.

[0145] When a single-phase grounding fault occurs in the low-frequency transformer, Figure 5It shows that the excitation inductance of the fault phase becomes smaller and remains stable without obvious fluctuations; Figure 6 In the example, the action signal of 1 indicates that the protection device has triggered the protection action; and the action signal of 0 indicates that the protection action has not been triggered. All action signals in the subsequent embodiments follow this rule, and the protection device acts at 1.0085s. Figure 5 and Figure 6 From the overall characteristics, it can be concluded that when a single-phase grounding fault occurs in the low-frequency transformer, the protection device can accurately and reliably identify the fault and trigger the action within 10ms after the fault.

[0146] Example 3

[0147] Based on the low-frequency simulation system and fault point setting provided in Example 1, this Example 3 adopts the low-frequency transformer differential protection method based on excitation inductance parameter identification in Example 1 to simulate and verify the two-phase grounding fault in the low-frequency transformer area.

[0148] When a two-phase ground fault occurs in a low-frequency transformer, Figure 7 This indicates that the excitation inductance of the fault phase becomes smaller and remains stable without obvious fluctuations. Figure 8 The medium protection device operates at 1.0075s. Figure 7 and Figure 8 It can be concluded from the overall characteristics that when a two-phase grounding fault occurs in a low-frequency transformer, the protection device can accurately and reliably identify the fault and trigger the action within 10ms after the fault.

[0149] Example 4

[0150] Based on the low-frequency simulation system and fault point setting provided in Example 1, this Example 4 adopts the low-frequency transformer differential protection method based on excitation inductance parameter identification in Example 1 to simulate and verify the three-phase grounding fault in the low-frequency transformer area.

[0151] When a three-phase grounding fault occurs in the low-frequency transformer, Fig. 9 It shows that the three-phase excitation inductance becomes smaller and remains stable without obvious fluctuations; Fig.10 The protection device will be activated at 1.0095s. Fig. 9 and Fig.10 It can be concluded from the overall characteristics that when a three-phase grounding fault occurs in a low-frequency transformer, the protection device can accurately and reliably identify the fault and trigger the action within 10ms after the fault.

[0152] Example 5

[0153] Based on the low-frequency simulation system and fault point setting provided in Example 1, this Example 5 adopts the low-frequency transformer differential protection method based on excitation inductance parameter identification in Example 1 to simulate and verify the two-phase interphase short-circuit fault in the low-frequency transformer area.

[0154] When a two-phase short circuit occurs in a low-frequency transformer, Fig.11 This indicates that the excitation inductance of the fault phase becomes smaller and remains stable without obvious fluctuations. Fig.12 The protection device was activated at 1.0097s. Fig.11 and Fig.12 It can be concluded from the overall characteristics that when a two-phase short circuit fault occurs in the low-frequency transformer, the protection device can accurately and reliably identify the fault and trigger the action within 10ms after the fault.

[0155] Example 6

[0156] Based on the low-frequency simulation system and fault point setting provided in Example 1, this Example 6 adopts the low-frequency transformer differential protection method based on excitation inductance parameter identification in Example 1 to simulate and verify the excitation surge current generated by no-load closing of the low-frequency transformer.

[0157] When the low-frequency transformer is closed without load, Fig.13 It shows that the value of the excitation inductance changes periodically between the normal high value and the saturation low value, showing obvious fluctuations; Fig.14 The protection device did not send out an action signal. Fig.13 and Fig.14 It can be concluded from the overall characteristics that when the low-frequency transformer is closed under no-load condition, the protection device can accurately and reliably identify the no-load closing condition and make the protection lockout inoperative.

[0158] According to the verification results of Example 2 to Example 6, it can be significantly shown that the method of the present invention has high sensitivity, good selectivity, fast action speed and high reliability for low-frequency transformer fault judgment, thereby providing reliable relay protection for transformers in offshore wind power low-frequency transmission systems.

[0159] In summary, the method proposed in the present invention utilizes the T-type equivalent circuit diagram of the low-frequency transformer to identify the excitation inductance of the transformer, utilizes the characteristics of the excitation inductance to construct the excitation surge current identification criterion, and uses the excitation surge current identification criterion as an auxiliary criterion for the ratio-braked differential protection of the low-frequency transformer. The simulation results verify the effectiveness of the proposed method, and the method can reliably identify the faults within the low-frequency transformer area. When the low-frequency transformer has an internal fault, the protection does not refuse to operate. When the low-frequency transformer does not have an internal fault, the protection does not operate incorrectly, and the protection operation time is relatively short. The method proposed in the present invention can reliably and quickly identify the faults within the low-frequency transformer area in the offshore wind power low-frequency transmission system and enable the protection to operate quickly, and has good engineering application value.

[0160] Those skilled in the art will appreciate that the embodiments of the present application may be provided as methods, systems, or computer program products. Therefore, the present application may adopt the form of a complete hardware embodiment, a complete software embodiment, or an embodiment combining software and hardware. Moreover, the present application may adopt the form of a computer program product implemented on one or more computer-usable storage media (including but not limited to disk storage, CD-ROM, optical storage, etc.) containing computer-usable program codes.

[0161] The present application is described with reference to the flowcharts and / or block diagrams of the methods, devices (systems), and computer program products according to the embodiments of the present application. It should be understood that each process and / or box in the flowchart and / or block diagram, as well as the combination of the processes and / or boxes in the flowchart and / or block diagram, can be implemented by computer program instructions. These computer program instructions can be provided to a processor of a general-purpose computer, a special-purpose computer, an embedded processor, or other programmable data processing device to generate a machine, so that the instructions executed by the processor of the computer or other programmable data processing device generate instructions for implementing the processes in the flowchart and / or block diagram. Figure 1 A process or multiple processes and / or boxes Figure 1 A device that provides the functions specified in a block or multiple blocks.

[0162] These computer program instructions may also be stored in a computer readable memory capable of directing a computer or other programmable data processing device to operate in a specific manner, so that the instructions stored in the computer readable memory produce an article of manufacture including an instruction device, which implements the process Figure 1 A process or multiple processes and / or boxes Figure 1 A function specified in one or more boxes.

[0163] These computer program instructions can also be loaded onto a computer or other programmable data processing device so that a series of operating steps are executed on the computer or other programmable device to produce a computer-implemented process, thereby providing instructions for implementing the process. Figure 1 A process or multiple processes and / or boxes Figure 1 The steps for the functions specified in one or more boxes.

[0164] The above is only a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the principle of the present invention. These improvements and modifications should also be regarded as the scope of protection of the present invention.

Claims

1. A low-frequency transformer differential protection method based on excitation inductance parameter identification, characterized in that: The steps include: Obtain three-phase data of low-frequency transformer; According to the three-phase collected data, three-phase calculated data is obtained; wherein the three-phase calculated data includes three-phase differential current data and three-phase braking current data; In response to any phase calculation data of the three-phase calculation data satisfying the preset ratio-restrained differential protection action condition, it is determined that the low-frequency transformer has a fault and a fault type analysis is performed; otherwise, it is determined that the low-frequency transformer has no fault and the protection is locked; The fault type analysis includes the following steps: According to the three-phase collected data, the parameters are identified by the least square method to obtain the three-phase excitation inductance values ​​at the current moment and multiple moments in the preset time period; Calculate and obtain multiple three-phase excitation inductance average values ​​according to the three-phase excitation inductance values ​​at the current moment and multiple moments in the preset time period; In response to any one of the three-phase excitation inductance average values ​​satisfying a preset excitation inrush current identification criterion, the fault type is determined to be an internal fault and the protection is actuated; otherwise, the fault type is determined to be an excitation inrush current and the protection is locked; The preset ratio-brake differential protection action condition includes a first differential protection action criterion and a second differential protection action criterion; In response to any phase calculation data of the three-phase calculation data satisfying the first differential protection action criterion or the second differential protection action criterion, it is determined that the low-frequency transformer is faulty; The first differential protection action criterion includes: the differential current of any phase in the three-phase calculation data is greater than or equal to the preset minimum action current setting value, and the braking current of any phase is less than or equal to the preset minimum braking current setting value, and the expression is as follows: In the formula, I d Represents the differential current of any phase; I res Indicates the braking current of any phase; I op.min Indicates the minimum operating current setting value; I res.min Indicates the minimum braking current setting value; The second differential protection action criterion includes: the differential current of any phase in the three-phase calculation data is greater than or equal to the sum of the preset minimum action current setting value and the braking current difference based on the ratio braking coefficient, and the braking current of any phase is greater than the preset minimum braking current setting value, and the expression is as follows: Where k represents the ratio braking coefficient.

2. The low-frequency transformer differential protection method based on excitation inductance parameter identification according to claim 1 is characterized in that: The three-phase collected data includes primary three-phase voltage, primary three-phase current, secondary three-phase current, primary three-phase leakage inductance and secondary three-phase leakage inductance.

3. The low-frequency transformer differential protection method based on excitation inductance parameter identification according to claim 1 is characterized in that: The expression of the three-phase differential current data is as follows: In the formula, I dA Indicates the differential current of phase A; I dB Indicates the B phase differential current; I dC Indicates the C phase differential current; i A Represents the primary A phase current; i B Represents the primary B phase current; i C Represents the primary C phase current; i a Represents the secondary side A phase current; i b Represents the secondary side B phase current; i c Indicates the secondary C phase current; α represents the transformer ratio.

4. The low-frequency transformer differential protection method based on excitation inductance parameter identification according to claim 1 is characterized in that: The expression of the three-phase braking current data is as follows: In the formula, I resA Indicates the braking current of phase A; I resB Indicates the braking current of phase B; I resC Indicates the braking current of phase C; i A Represents the primary phase A current; i B Represents the primary B phase current; i C Represents the primary C phase current; i a Represents the secondary side A phase current; i b Represents the secondary side B phase current; i c Indicates the secondary C phase current; α represents the transformer ratio.

5. The low-frequency transformer differential protection method based on excitation inductance parameter identification according to claim 1 is characterized in that: According to the three-phase collected data, the parameters are identified by the least square method to obtain the three-phase excitation inductance values ​​at the current moment and multiple moments in the preset time period, including the following steps: According to the three-phase collected data, based on a preset A-phase identification matrix, parameters are identified by a least square method to obtain a plurality of phase identification parameters; According to multiple phase identification parameters, the excitation inductance value of phase A is calculated; According to the three-phase collected data, based on a preset B-phase identification matrix, parameters are identified by a least square method to obtain a plurality of phase identification parameters; According to multiple phase identification parameters, the excitation inductance value of phase B is calculated; According to the three-phase collected data, based on a preset C-phase identification matrix, a least square method is used to identify parameters to obtain a plurality of phase identification parameters; The C-phase excitation inductance value is calculated based on a plurality of phase identification parameters.

6. The low-frequency transformer differential protection method based on excitation inductance parameter identification according to claim 5 is characterized in that: The expression of the A phase identification matrix is ​​as follows: In the formula, i A Represents the primary phase A current; i B Represents the primary B phase current; (t1…t n ) represents the set of moments, n≥3; u A Indicates the primary A phase voltage; u B represents the primary B phase voltage; x1 represents the first phase identification parameter; x2 represents the second phase identification parameter; x3 represents the third phase identification parameter; The A phase excitation inductance value L MA The expression is as follows: The expression of the B phase identification matrix is ​​as follows: In the formula, i C Indicates the primary C phase current; u C represents the primary C phase voltage; x4 represents the fourth phase identification parameter; x5 represents the fifth phase identification parameter; x6 represents the sixth phase identification parameter; The B phase excitation inductance value L MB The expression is as follows: The expression of the C phase identification matrix is ​​as follows: Wherein, x7 represents the seventh phase identification parameter; x8 represents the eighth phase identification parameter; x9 represents the ninth phase identification parameter; The C phase excitation inductance value L MC The expression is as follows:

7. The low-frequency transformer differential protection method based on excitation inductance parameter identification according to claim 1 is characterized in that: The average value of the excitation inductance of any phase among the three-phase excitation inductance average values ​​L av The expression is as follows: Where, L av Indicates the average value of the excitation inductance of any phase, that is, the average value of the excitation inductance of phase A, or the average value of the excitation inductance of phase B, or the average value of the excitation inductance of phase C; L M (t) represents the excitation inductance value at the current time t; L M (t-T1) represents the excitation inductance value at time t-T1, and T1 is any sampling time within the preset time period T2-T3, where, T represents the period of the system in which the transformer is located.

8. The low-frequency transformer differential protection method based on excitation inductance parameter identification according to claim 1 is characterized in that: The preset excitation inrush current identification criteria include: In response to the fact that any one of the three-phase excitation inductance average values ​​is less than a preset excitation inductance setting value, the excitation inductance characteristic is judged to be stable and without fluctuation, the fault type is determined to be an intra-zone fault, and the protection is activated; otherwise, the excitation inductance characteristic is judged to show obvious fluctuation, the fault type is determined to be an excitation inrush current, and the protection is locked.

9. The low-frequency transformer differential protection method based on excitation inductance parameter identification according to claim 8 is characterized in that: The expression of the setting value of the excitation inductance is as follows: Where, L Mset Indicates the setting value of excitation inductance; L M0 Indicates the no-load steady-state excitation inductance value; K bmin Indicates the ratio of the magnetic permeability in the non-saturated and saturated states in the transformer magnetization characteristic curve; K k Represents the reliability coefficient.

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