A method for preventing and responding to the misoperation of a superior line distance protection caused by inrush current

Abstract

This invention relates to a method for preventing maloperation of distance protection on upstream lines caused by inrush current. The method includes: activating the protection device; when the distance protection stage III impedance element operates, extracting the DC, second, and third harmonic components of the phase current; when a sudden change in the DC component of one or more phases is detected and shows a decaying trend, then the following conditions are met in sequence: (1) the protection activation time is 0ms; within 0 to 40ms, one or more phases satisfy a second harmonic content greater than 15% and a third harmonic content less than the second harmonic content; (2) within the 40 to 90ms interval, the second harmonic content decreases and the third harmonic content increases, and after a certain moment, the third harmonic content remains greater than the second harmonic content; (3) after 90ms, the second harmonic further decreases, and the harmonic content is dominated by the third harmonic, accounting for more than 12%; when conditions (1)-(3) are met, the distance protection stage III impedance element is blocked; otherwise, the protection is opened. This invention can effectively prevent maloperation of distance protection in inrush current scenarios.

Description

Technical Field

[0001] This invention relates to the field of power system relay protection technology, specifically to a method for preventing and responding to inrush currents from causing maloperation of the distance protection of upstream lines. Background Technology

[0002] Incidents of transformer and inrush current causing protection maloperation occur frequently. Some effective preventive measures and criteria have been proposed for the main transformer protection maloperation, such as adding current transformer saturation criteria, selecting CT windings with strong anti-saturation capability, and second harmonic restraint. However, the problem of inrush current causing the upper-level line distance section III protection maloperation has not yet received enough attention.

[0003] There are two possible scenarios: First, due to substation maintenance, the power grid operation mode needs to be adjusted to meet the power supply needs of non-maintenance areas, resulting in a single line supplying multiple substations. Second, new energy power plants with two or more parallel-operating transformers are connected to the grid via a single transmission line. When one main transformer in the substation is closed under no-load conditions, generating an inrush current, adjacent parallel-operating transformers will also generate corresponding inrush currents. These two types of inrush currents alternate in time and in opposite directions, superimposing to form a line current, characterized by an increased current amplitude and a slow decay period. Because the two types of inrush currents alternate at different discontinuity angles, the line protection measurement impedance exhibits line impedance characteristics. Due to the distance between sections I and II... The first and second distance protection sections reflect the impedance measurement under low voltage and high current during short-circuit faults. However, in the case of inrush current, the bus voltage does not drop significantly, so the first and second distance protection sections will not malfunction. The third distance protection section, as a backup section, can reflect the measured impedance under higher voltage and lower current on the local side during remote faults. In addition, the line current decays slowly, and the third distance protection section based on the time gradient setting is at risk of malfunction, which could cause substation power loss and disconnection from the grid, seriously endangering the safe and stable operation of the power grid and posing a challenge to the construction of new power systems. Therefore, it is urgent to configure effective methods to prevent the third distance protection section from malfunctioning due to inrush current for the line protection in the above two operating modes. Summary of the Invention

[0004] The purpose of this invention is to solve the above-mentioned problems by extracting harmonic characteristics from the line current formed by the superposition of inrush current and excitation inrush current, and to provide a method to prevent the inrush current from causing maloperation of the distance protection of the upstream line, thereby avoiding maloperation and tripping of the distance III stage impedance element.

[0005] To achieve the above objectives, the technical solution of the present invention is: a method to prevent inrush current from causing maloperation of the distance protection of the upper-level line. After the protection is started, if the distance III stage impedance element operates, the DC component of the three-phase current of the line is detected. If a sudden DC component appears in one or more phases and the DC component shows a decaying trend, the second harmonic and third harmonic content of the phase current is extracted. When the following conditions (1) to (3) are met, the distance III stage impedance element is blocked; otherwise, the protection is opened.

[0006] (1) The protection start time is 0ms. During the time from 0 to T1, one or more phases satisfy the condition that the second harmonic content is greater than K1 and the third harmonic content is less than the second harmonic content.

[0007] (2) During the time interval from T1 to T2, the second harmonic content decreases and the third harmonic content increases, and after a certain moment, the third harmonic content is always greater than the second harmonic content.

[0008] (3) After time T2, the second harmonic further decreases, and the harmonic content is dominated by the third harmonic, which accounts for a larger proportion than K2.

[0009] In one embodiment of the present invention, the method is applied to prevent the distance protection of the upstream line from malfunctioning, wherein the upstream line refers to a single-channel line that supplies power to two or more transformers operating in parallel.

[0010] In one embodiment of the present invention, the second and third harmonic content refers to the ratio of the effective values ​​of the second and third harmonics extracted by the Fourier algorithm to the effective value of the fundamental wave.

[0011] In one embodiment of the present invention, the conditions (1) to (3) are characteristic terms extracted from the line current waveform formed by the superposition of transformer excitation inrush current and the corresponding inrush current.

[0012] In one embodiment of the present invention, the condition (1) is derived based on the characteristic that the inrush current is generated after the excitation inrush current has lasted for a period of time; during the time from 0 to T1, the line current waveform exhibits the characteristics of the excitation inrush current waveform, and the harmonics are mainly second harmonics. According to the field waveform and a large number of simulation tests, the second harmonic content is greater than 15%, so K1 is taken as 15%.

[0013] In one embodiment of the present invention, T1 is 40ms.

[0014] In one embodiment of the present invention, the condition (2) is derived from the characteristics that after the inrush current is generated, the inrush current and the excitation inrush current alternate, are opposite in direction, do not overlap, and the DC components cancel each other out, and the line current gradually enters a symmetrical process; as the inrush current increases rapidly, the second harmonic of the line current decreases rapidly and the third harmonic increases rapidly. Based on the field waveform and a large number of simulation tests, T2 is set to 90ms.

[0015] In one embodiment of the present invention, the condition (3) is derived based on the fact that the surge current rapidly reaches its maximum value after time T2 and then enters a slow decreasing phase, and the line current has the characteristics of high third harmonic content and low second harmonic content; considering that the protection action time of the impedance element of the distance III section is set to no more than 3s, according to the field waveform and a large number of simulation tests, when the surge current decays to 3s, the third harmonic is still greater than 12%, so K2 is set to 12%.

[0016] In one embodiment of the present invention, the conditions (1) to (3) are in a series AND relationship, that is, the impedance element of distance segment III is locked after the above three conditions are met in sequence, otherwise the protection is opened.

[0017] In one embodiment of the present invention, the occurrence of abrupt DC components in one or more phases, and the decaying trend of the DC components, is derived from the characteristics of excitation inrush current, the characteristics of response inrush current, and the result of their interaction.

[0018] Compared to existing technologies, this invention has the following advantages: Upon activation of line protection, the impedance element of distance segment III operates, initiating current harmonic characteristic analysis. If it is confirmed that the impedance element operation is affected by inrush current, then distance segment III protection is blocked. The application of this method can prevent load loss or disconnection from the grid for renewable energy sources. This method is simple, reliable, easy to implement, and the setting value is unaffected. Attached Figure Description

[0019] Figure 1 The flowchart for implementing this method.

[0020] Figure 2 This diagram illustrates an application scenario suitable for this method.

[0021] Figure 3 This is a typical waveform diagram when the line protection malfunctions.

[0022] Figure 4 This refers to the aperiodic component of the line current. Detailed Implementation

[0023] The technical solution of the present invention will now be described in detail with reference to the accompanying drawings.

[0024] like Figure 1 As shown, the present invention provides a method for preventing inrush current from causing maloperation of the distance protection of the upstream line, comprising the following steps:

[0025] After the protection is started, if the distance III impedance element operates, the DC component of the three-phase current of the line is detected. If a sudden DC component appears in one or more phases and the DC component shows a decaying trend, the second and third harmonic content of the phase current is extracted. If the following conditions are met, the distance III impedance element is blocked; otherwise, the protection is opened.

[0026] (1) The protection start time is 0ms. During the time from 0 to T1, one or more phases satisfy the condition that the second harmonic content is greater than K1 and the third harmonic content is less than the second harmonic content.

[0027] (2) During the time interval from T1 to T2, the second harmonic content decreases and the third harmonic content increases, and after a certain moment, the third harmonic content is always greater than the second harmonic content.

[0028] (3) After time T2, the second harmonic further decreases, and the harmonic content is dominated by the third harmonic, which accounts for a larger proportion than K2.

[0029] The above conditions (1) to (3) are characteristic terms extracted from the line current waveform formed by the superposition of transformer excitation inrush current and induced inrush current. T1 is set to 40ms, T2 is set to 90ms, K1 is set to 15%, and K2 is set to 12%.

[0030] The principles of the present invention will be further explained below with reference to specific embodiments:

[0031] like Figure 2 As shown, a 220kV substation contains two main transformers operating in parallel. Due to the need for power outage maintenance of the 220kV bus and the #2 main transformer, the power grid operation mode is adjusted so that the load of the #1 main transformer of the substation is supplied by the upstream 110kV line. The line protection is equipped with distance protection. The following uses the protection on the power supply side of this line as an example to further illustrate the principle of the present invention.

[0032] The 220kV busbar maintenance was not completed, and the #2 main transformer needed to be energized ahead of schedule after maintenance. According to the dispatch order, the 15B switch was closed under no-load conditions, impacting the #2 main transformer, resulting in... Figure 3 As shown in (a), the inrush current generated in the #1 main transformer is as follows: Figure 3 (b) shows the inrush current. Comparing the waveforms, it can be found that the inrush current is generated after the excitation inrush current has been running for a period of time, and it rapidly reaches its maximum value within several cycles, then gradually decreases. Furthermore, the two occur alternately in time, in opposite directions, and superimpose to form the current as shown in Figure (b). Figure 3 The line current waveform is shown in (c). The changes in the two types of inrush current and the DC component of the line current are as follows: Figure 4 As shown, it can be seen that the DC component of the line current undergoes a sudden change and shows a decreasing trend.

[0033] When the #2 main transformer 15B switch is closed, the protection device starts. It calculates the distance impedance value based on the three-phase voltage and current of the line. When the distance III stage impedance element operates, it uses the Fourier algorithm to extract the DC component, second harmonic component, and third harmonic component of the three-phase current. If it is determined that the DC component of one or more phases changes abruptly and shows a decaying trend, it is further determined that within 40ms from the start time, one or more phases meet the condition that the second harmonic content is greater than 15% and the third harmonic content is less than the second harmonic content. If the condition is met, it is further determined that within 40ms to 90ms, the second harmonic content decreases and the third harmonic content increases, and after a certain moment, the third harmonic content is always greater than the second harmonic content. If this condition is also met, it is further determined that after 90ms, the second harmonic further decreases and the third harmonic is dominant, accounting for more than 12%. Then, the distance III stage protection is blocked. If any of the above conditions are not met, the protection is opened.

[0034] This invention has been validated on an RTDS simulation testing system. First, it simulated various scenarios where distance protection stage III malfunctioned. Then, the method proposed in this invention was embedded into the distance protection program. Under the same scenarios, the distance protection stage III malfunctioned no longer occurred, proving the effectiveness of the method. Using the method of this invention, malfunctions in distance protection under inrush current scenarios can be effectively avoided, and the reliability of power supply from single-supply lines and the stability of new energy power generation can be effectively improved.

[0035] While the specific embodiments of the present invention have been described above in conjunction with the accompanying drawings, this is not intended to limit the scope of protection of the present invention. Those skilled in the art should understand that various modifications or variations that can be made by those skilled in the art without creative effort based on the technical solutions of the present invention are still within the scope of protection of the present invention.

Claims

1. A method for preventing and responding to the inrush current causing the misoperation of the line distance protection, characterized in that, After the protection is started, if the distance III section impedance element acts, the direct current component of the three-phase current of the detection circuit is detected, if sudden direct current component appears in certain phase or multiple phases, and the direct current component shows a decay trend, the second harmonic and third harmonic content of the phase current is extracted, when the following conditions (1)-(3) are met, the distance III section impedance element is blocked, otherwise the protection is opened: (1) at the protection starting time of 0 ms, within 0 to T1 time, certain phase or multiple phases meet the second harmonic content greater than K1 and the third harmonic content less than the second harmonic content; (2) within T1 to T2 time interval, the second harmonic content decreases, the third harmonic content increases, and after a certain time, the third harmonic content is always greater than the second harmonic content; (3) after T2 time, the second harmonic further decreases, the harmonic content is mainly the third harmonic, and the proportion is greater than K2; The second harmonic and third harmonic content refers to the ratio of the effective value of the second harmonic and third harmonic to the fundamental value extracted by the Fourier algorithm; The conditions (1)-(3) are characteristic items extracted from the line current waveform formed by the mutual superposition of the transformer excitation inrush current and the sympathetic inrush current; The condition (1) is derived from the characteristic that the sympathetic inrush current is generated after the excitation inrush current lasts for a period of time; within 0 to T1 time, the line current waveform shows the excitation inrush current waveform characteristics, and the harmonic is mainly the second harmonic, according to the field waveform and a large number of simulation tests, the second harmonic content is greater than 15%, therefore K1 is 15%; The condition (2) is derived from the characteristic that after the sympathetic inrush current is generated, the sympathetic inrush current and the excitation inrush current appear alternately, the direction is opposite, they do not overlap, the direct current component is offset, and the line current gradually enters the symmetry process; with the rapid increase of the sympathetic inrush current, the line current second harmonic rapidly decreases, and the third harmonic rapidly increases, according to the field waveform and a large number of simulation tests, T2 is set to 90 ms; The condition (3) is derived from the characteristic that the sympathetic inrush current rapidly reaches the maximum value after T2 time, and then enters the slow decrease stage, and the line current has the characteristics of large third harmonic content and small second harmonic content; considering that the distance III section impedance element protection action time setting is not more than 3 s, according to the field waveform and a large number of simulation tests, when the sympathetic inrush current decays to 3 s, the third harmonic is still greater than 12%, therefore K2 is set to 12%.

2. The method for preventing and responding to the inrush current causing the maloperation of the distance protection of the superior line according to claim 1, characterized in that, The method is applied to prevent the misoperation of the upper line distance protection, and the upper line refers to a single-channel line for supplying power to two or more parallel operation transformers.

3. The method for preventing and responding to the inrush current causing the maloperation of the distance protection of the superior line according to claim 1, characterized in that, T1 is 40 ms.

4. The method for preventing and responding to the inrush current causing the maloperation of the distance protection of the superior line according to claim 1, characterized in that, The conditions (1)-(3) are in series "and" relationship, that is, after the above three conditions are sequentially met, the distance III section impedance element is blocked, otherwise the protection is opened.

5. The method for preventing and responding to the inrush current causing the maloperation of the distance protection of the superior line according to claim 1, characterized in that, The sudden direct current component appears in certain phase or multiple phases, and the direct current component shows a decay trend, which is derived from the characteristics of the excitation inrush current, the characteristics of the sympathetic inrush current and the interaction results of the two.

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