A method for monitoring inter-turn short circuit of series reactor based on star-shaped neutral point voltage and harmonic changes

Abstract

The present invention provides a method for monitoring series reactor turn-to-turn short circuit based on star-shaped neutral point voltage and harmonic changes, collects the star-shaped connection ungrounded neutral point voltage of a parallel capacitor group, and then performs Fourier transformation to obtain the fundamental wave and harmonic effective value components of the neutral point voltage; respectively calculates the percentage of the effective value components of the 3rd, 5th, and 7th harmonic voltages in the neutral point voltage to the fundamental effective value components, obtains the 3rd, 5th, and 7th harmonic voltage contents, and determines whether the content of each harmonic voltage exceeds the tripping setting of the series reactor turn-to-turn short circuit protection of the parallel capacitor group, and at the same time compares the bus voltage and the neutral point voltage to determine whether there is a significant deviation caused by non-bus grounding, so as to determine the series reactor turn-to-turn short circuit fault. The parallel capacitor group with the series reactor turn-to-turn short circuit fault is tripped and the power supply is disconnected to prevent the series reactor turn-to-turn short circuit fault from continuing to develop, thereby causing a fire and carbon ion smoke that may cause other more serious secondary phase-to-phase accidents.

Description

[Technical field]

[0001] The invention relates to the technical field of online monitoring of power equipment faults, and in particular to a series reactor turn-to-turn short circuit monitoring method based on star-shaped neutral point voltage and harmonic changes. [Background technology]

[0002] Shunt capacitors (abbreviated as "shunt capacitors") are widely used in the field of power transmission and transformation of power systems. Shunt capacitors of appropriate capacity are configured on the low-voltage winding of the transformer to realize on-site compensation of reactive current, improve the power factor of the transformer and the transmission line, and control and stabilize the voltage level. In the shunt capacitor, a series reactor with a certain series resistance rate (abbreviated as "series reactor") is configured to limit the frequent switching shunt inrush current. At present, the commonly used series reactors in the hub substation of the power system are mainly 12% series resistance rate or 5%-6% series resistance rate. In the shunt capacitor under the star ungrounded connection mode, the series reactor usually adopts a dry-type air-core reactor. The main fault statistics of the operation of the dry-type air-core reactor are turn-to-turn short circuits. Although the inter-turn short circuit causes a significant change in the equivalent inductance and resistance, the inter-turn short circuit fault is not a metallic short circuit fault. As the number of short-circuit turns of the inter-turn short circuit fault continues to develop and expand to about dozens of turns, the equivalent inductance of the series reactance will become very small until it disappears completely. As the equivalent inductance and reactance of the series reactance decrease, the impedance of the entire circuit becomes larger, so it cannot be directly monitored using the principle of measuring impedance changes. Therefore, it is necessary to provide a series reactance inter-turn short circuit monitoring method based on the star neutral point voltage and harmonic changes to solve the above problems. [Summary of the invention]

[0003] The present invention discloses a method for monitoring inter-turn short circuit of series reactors based on star-shaped neutral point voltage and harmonic changes. When the harmonic content of the 3rd, 5th and 7th harmonics in the neutral point voltage is monitored to reach a certain value, the relevant capacitor group having the fault is disconnected, so as to prevent the inter-turn short circuit fault of the series reactor from continuously developing into a fire accident of coil spontaneous combustion.

[0004] To achieve the above object, the technical solution of the present invention is:

[0005] A method for monitoring series reactor turn-to-turn short circuit based on star-shaped neutral point voltage and harmonic changes, comprising the following steps:

[0006] S1: Real-time acquisition of the neutral point voltage of the ungrounded star-connected shunt capacitor bank, and the fundamental wave and harmonic effective value components of the neutral point voltage are obtained through Fourier transform;

[0007] S2: Calculate the percentage of the effective value component of the third, fifth, and seventh harmonic voltage in the neutral point voltage to the effective value component of the fundamental wave, and obtain the third, fifth, and seventh harmonic voltage contents HR(3), HR(5), and HR(7);

[0008] S3: The third, fifth and seventh harmonic voltage contents HR(3), HR(5) and HR(7) are respectively compared with the tripping setting HR of the series reactor turn-to-turn short-circuit protection in the parallel capacitor bank. dz Compare, if any one of HR(3), HR(5), HR(7) exceeds HR dz , then execute step S4, otherwise return to step S1;

[0009] S4: Determine whether the duration of the harmonic voltage exceeds the lower time limit threshold Td z1 If yes, then execute step S5; otherwise, return to step S1;

[0010] S5: Determine whether the duration of the harmonic voltage exceeds the high time limit threshold Td z2 If yes, then execute step S6, otherwise, issue an alarm signal to indicate that a series reactor turn-to-turn short circuit has occurred;

[0011] S6: Determine whether the maximum value of the effective value of the neutral point fundamental voltage exceeds one half of the effective value of the normal phase voltage. If so, it is determined that a short circuit has occurred between the turns of the series reactor and a secondary grounding fault has occurred. The protection device issues a non-delayed opening command to directly cut off the entire set of parallel capacitor groups and lock the closing output. Otherwise, it is determined that a short circuit has occurred between the turns of the series reactor and no grounding fault has occurred. The protection device issues a opening command to directly cut off the entire set of parallel capacitor groups and lock the closing output.

[0012] Preferably, before step S1, the method further includes the following steps:

[0013] S0: In the simulation software PSCAD / EMTDC, according to the infinite power generation system and the relevant equipment parameters of the operating status of the real fault case, a star-connected ungrounded model of the parallel capacitor group is constructed; on the fault phase, two anti-parallel thyristors are used to simulate the short-circuit between the turns of the series reactor; the resistor connected in series with the two parallel thyristors is used to simulate the active power loss consumed by the short-circuit circulating current in the series reactor after the short-circuit between the turns of the series reactor occurs; the inductor connected in parallel with the two parallel thyristors is used to simulate the inductance loss, and by changing the trigger conduction angle of the two parallel thyristors, the discharge characteristics of the extinction and reignition of the short-circuit arc of the short-circuit turn of the series reactor are simulated.

[0014] Preferably, the process of Fourier transform in step S1 is as follows:

[0015]

[0016]

[0017]

[0018]

[0019] A0=a0

[0020]

[0021]

[0022] In the formula, f(t) represents the neutral point voltage after Fourier transformation. represents the DC component of f(t), T represents the period of function f(t), ω represents the angular frequency of function f(t), ωT=2π, represents the initial phase, k represents the harmonic order; a k , b k Respectively represent the amplitudes of the kth harmonic cosine component and sine component; represents the kth harmonic component, A k Indicates the amplitude of the kth harmonic, the effective value is When k = 1, Represents the fundamental component, A1 represents the amplitude of the fundamental wave, and the effective value is

[0023] Preferably, the calculation process of the kth harmonic voltage content in step S2 is:

[0024]

[0025] Preferably, the series reactor is a dry-type hollow series reactor with a series reactance ratio of 12% or 5%-6%.

[0026] Preferably, the reactor turn-to-turn short-circuit protection tripping setting HR dz Take 5%-7%, the low time threshold T dz1 =2~10s, high time threshold T dz2 =10~20s.

[0027] Preferably, in step S6, if it is determined that a turn-to-turn short circuit of the series reactor occurs and a ground fault occurs, the following steps are also included:

[0028] An auxiliary judgment method is adopted in which the bus voltage of the fault phase decreases and the bus voltage of the non-fault phase increases to distinguish the fault phase and the non-fault phase.

[0029] Compared with the related art, the present invention collects the neutral point voltage through computer simulation and then performs Fourier transformation to obtain the fundamental and harmonic effective value components of the neutral point voltage; respectively calculates the percentage of the effective value components of the 3rd, 5th and 7th harmonic voltages in the neutral point voltage to the fundamental effective value components, obtains the 3rd, 5th and 7th harmonic voltage contents, and determines whether the 3rd, 5th and 7th harmonic voltage contents exceed the tripping setting of the capacitor group series reactor inter-turn short-circuit protection, so as to determine the series reactor inter-turn short-circuit fault. The parallel capacitor group with the series reactor inter-turn short-circuit fault is disconnected to prevent the series reactor inter-turn short-circuit fault from continuing to develop and causing fires and other more serious secondary phase-to-phase accidents caused by carbon ion smoke.

Brief Description of the Drawings

[0030] In order to more clearly illustrate the technical solutions in the embodiments of the present invention, the following briefly introduces the drawings required for describing the embodiments. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative work, among which:

[0031] Figure 1 This is the system circuit schematic diagram of the inter-turn short-circuit fault of the series reactor of the parallel capacitor bank;

[0032] Figure 2 The simulation result diagram of the neutral point voltage of the series reactor turn-to-turn short-circuit fault;

[0033] Figure 3 The schematic diagram of the simulation model of the inter-turn short-circuit fault of the series reactor with a series resistance ratio of 12% in the 35kV3×20Mvar group;

[0034] Figure 4 for Figure 3 The simulation model shown is the neutral point voltage waveform and its content diagram for turn-to-turn short circuit fault. [Specific implementation method]

[0035] In order to enable those skilled in the art to better understand the technical solutions in the embodiments of the present invention and to make the above-mentioned objects, features and advantages of the present invention more obvious and understandable, the specific implementation modes of the present invention are further described below.

[0036] Please refer to Figure 1-4 The present invention provides a method for monitoring series reactor turn-to-turn short circuit based on star-shaped neutral point voltage and harmonic changes, comprising the following steps:

[0037] S0: In the simulation software PSCAD / EMTDC, according to the infinite power generation system and the relevant equipment parameters of the real fault case operation status, a star-connected ungrounded model of the parallel capacitor group is constructed; on the fault phase, two anti-parallel thyristors are used to simulate the short-circuit between the turns of the series reactor; a resistor connected in series with the two parallel thyristors is used to simulate the active power loss consumed by the short-circuit circulating current generated in the series reactor after the short-circuit between the turns of the series reactor occurs; an inductor connected in parallel with the two parallel thyristors is used to simulate the inductance loss, and by changing the trigger conduction angle of the two parallel thyristors, the discharge characteristics of the extinguishing and reigniting of the short-circuit arc of the short-circuit turn of the series reactor are simulated;

[0038] S1: Real-time acquisition of the neutral point voltage of the ungrounded star-connected shunt capacitor bank, and the fundamental wave and harmonic effective value components of the neutral point voltage are obtained through Fourier transform;

[0039] S2: Calculate the percentage of the effective value component of the third, fifth, and seventh harmonic voltage in the neutral point voltage to the effective value component of the fundamental wave, and obtain the third, fifth, and seventh harmonic voltage contents HR(3), HR(5), and HR(7);

[0040] S3: The third, fifth and seventh harmonic voltage contents HR(3), HR(5) and HR(7) are respectively compared with the tripping setting HR of the series reactor turn-to-turn short-circuit protection in the parallel capacitor bank. dz Compare, if any one of HR(3), HR(5), HR(7) exceeds HR dz , then execute step S4, otherwise return to step S1;

[0041] S4: Determine whether the duration of the harmonic voltage exceeds the lower time limit threshold Td z1 If yes, then execute step S5; otherwise, return to step S1;

[0042] S5: Determine whether the duration of the harmonic voltage exceeds the high time limit threshold Td z2 If yes, then execute step S6, otherwise, issue an alarm signal to indicate that a series reactor turn-to-turn short circuit has occurred;

[0043] S6: Determine whether the maximum value of the effective value of the neutral point fundamental voltage exceeds one half of the effective value of the normal phase voltage. If so, it is determined that a short circuit occurs between the turns of the series reactor and a secondary grounding fault occurs. The protection device issues a non-delayed opening command to directly cut off the entire set of parallel capacitor groups and lock the closing output. Otherwise, it is determined that a short circuit occurs between the turns of the series reactor and no grounding fault occurs. The protection device issues a opening command to directly cut off the entire set of parallel capacitor groups and lock the closing output. If it is determined that a short circuit occurs between the turns of the series reactor and a grounding fault occurs, an auxiliary judgment method is adopted in which the bus voltage of the fault phase is reduced and the bus voltage of the non-fault phase is increased to distinguish the fault phase from the non-fault phase.

[0044] The series reactor is a dry-type hollow series reactor with a series resistance ratio of 12% or 5%-6%. In actual operation scenarios, through analysis of long-term operation data, it is found that the dry-type hollow series reactor with a series resistance ratio of 12% has the highest failure rate, followed by the dry-type hollow series reactor with a series resistance ratio of 5-6% and 1%.

[0045] The fundamental component and harmonic voltage component are obtained by Fourier transforming the neutral point voltage. The Fourier transform process is as follows:

[0046]

[0047]

[0048]

[0049]

[0050] A0=a0

[0051]

[0052]

[0053] In the formula, f(t) represents the neutral point voltage after Fourier transformation. represents the DC component of f(t), T represents the period of function f(t), ω represents the angular frequency of function f(t), ωT=2π, represents the initial phase, k represents the harmonic order; a k , b k Respectively represent the amplitudes of the kth harmonic cosine component and sine component; represents the kth harmonic component, A k Indicates the amplitude of the kth harmonic, the effective value is When k = 1, Represents the fundamental component, A1 represents the amplitude of the fundamental wave, and the effective value is

[0054] The calculation process of the kth harmonic voltage content is:

[0055]

[0056] After long-term operation, the dry-type air-core reactor will have its winding insulation layer aged due to the operating environment, and the encapsulation will be damp due to rainy weather. The reactor will have weak insulation points, which will be broken down at high field strengths. Due to the large current in the short-circuit ring, a local arc discharge short circuit will be formed. The development of the inter-turn short circuit of the series reactor is relatively slow. As the reactance rate decreases, the development speed will become slower, and the fault will last from several minutes to tens of minutes. The development of the inter-turn short circuit of the series reactor mainly has the following stages:

[0057] (1) In the first stage, a local short-circuit discharge arc occurs, with white flashes appearing, followed by light smoke, and then flames appearing accompanied by thick smoke, which gradually develops into a large fire;

[0058] (2) In the second stage, the short circuit continues to develop naturally, causing the reactor to catch fire and the secondary busbar grounding fault;

[0059] (3) In the third stage, the combustion of the reactor produces a large amount of thick smoke. The smoke containing concentrated carbon ions and metal ions after the burning of epoxy resin and aluminum wire will cause external insulation flashover discharge of other surrounding electrical equipment. Simultaneous flashover discharge of multiple phases to ground will cause serious phase-to-phase and three-phase short circuits.

[0060] Under the simultaneous action of AC voltage and large short-circuit current, in the gap (several millimeters to ten millimeters) formed by the expanding short-circuit arc ablation, the arc burns with the intermittent characteristics and phenomenon of periodic extinguishing and reigniting of a non-completely sinusoidal waveform. The unstable burning of the short-circuit circulating current of the inter-turn fault, the periodic intermittent extinguishing and reigniting of the burning arc will stimulate the LC circuit of the parallel capacitor bank to generate the 3rd, 5th, and 7th harmonics, and due to the continuous reduction of the inductance loss of the series reactor, the neutral point voltage offset phenomenon with harmonic voltage as the main component will inevitably occur.

[0061] Construct a system circuit for a short-circuit fault in a series reactor connected to a parallel capacitor bank, such as Figure 1 As shown in the figure, E a 、E b 、E c is a three-phase voltage source, L s Represents the equivalent inductance of the system, L Trepresents the equivalent inductance of the transformer, L is the equivalent inductance of the series reactor of the non-fault phase, L(t) represents the small change in inductance caused by the demagnetization effect of the short-circuit circulating current and the change in the switching effect caused by the intermittent extinguishing and reigniting of the arc, thus forming the equivalent time-varying inductance characteristic of the series reactor of the fault phase, C a , C b , Cc are the three-phase capacitances of the parallel capacitor groups respectively, and U0 is the neutral point of the parallel capacitors.

[0062] Since the neutral point voltage of the parallel capacitor group with star connection and ungrounded neutral point cannot be directly collected and recorded, the voltage waveform data after the fault can only be calculated and displayed by simulation. The simulation of the fault can provide a theoretical basis for the actual fault.

[0063] by Figure 1 Based on the system circuit of the inter-turn short-circuit fault of the series reactor of the parallel capacitor group shown in the figure, in the simulation software PSCAD / EMTDC, according to the infinite power generation system and the relevant equipment parameters of the operating status of the real fault case, a star-connected ungrounded model of the parallel capacitor group is constructed; on the fault phase, two anti-parallel thyristors are used to simulate the inter-turn short-circuit of the series reactor; the resistor connected in series with the two parallel thyristors is used to simulate the active power loss consumed by the short-circuit circulating current generated in the series reactor after the inter-turn short-circuit occurs in the series reactor; the inductor connected in parallel with the two parallel thyristors is used to simulate the inductance loss, and by changing the trigger conduction angle of the two parallel thyristors, the extinguishing and reigniting discharge characteristics of the short-circuit arc of the short-circuit turn of the series reactor are simulated. The fundamental and harmonic effective value components of the neutral point voltage are obtained by Fourier transform, and the kth (k=3, 5, 7) harmonic appears in the neutral point voltage, and HR(k) exceeds HR dz , indicating that the neutral point voltage has shifted. The simulation results show that when a turn-to-turn short-circuit fault occurs in the series reactor with a series reactance ratio of 12% and 5-6%, the 3rd, 5th, and 7th harmonic currents will inevitably appear in the fault recording, which is significant and can be monitored; at the same time, the equivalent inductance and equivalent resistance of the series reactor of the fault phase will change significantly, which also causes a significant voltage shift in the neutral point voltage of the parallel star connection with the neutral point not grounded.

[0064] like Figure 2 As shown, Figure 2 The simulation result diagram of the neutral point voltage of the series reactor turn-to-turn short-circuit fault is shown. Figure 2 The ordinate represents the voltage (unit: kV), and the abscissa represents the simulation time (unit: s). It can be seen from the waveform that the fundamental voltage is the largest, followed by the 5th, 7th and 3rd harmonic voltages, and the harmonic contents are 55.3%, 41.6% and 22.2% respectively.

[0065] Under normal working conditions, when the system harmonic current is large, although the parallel capacitor group will absorb a certain amount of the 3rd, 5th, and 7th harmonics from the system, due to the three-phase symmetry of the system harmonics, it will not cause a significant offset of the neutral point voltage; even under special working conditions, the three-phase asymmetry of the power supply voltage occurs, but the maximum asymmetry level is within ±5%, and the neutral point voltage will only have a slight power frequency voltage offset, and will not have a significant waveform feature dominated by the 3rd, 5th, and 7th harmonics; and when only a ground short circuit occurs in the parallel capacitor branch, the neutral point voltage will only contain the fundamental wave offset, without the 3rd, 5th, and 7th harmonic offset, and it will not affect the monitoring method of the harmonic voltage content rate. Therefore, by using the offset of the neutral point voltage and the characteristics that the offset voltage mainly contains the 3rd, 5th, and 7th harmonics, the series reactor turn-to-turn short circuit fault can be accurately monitored and reliably identified.

[0066] Embodiment 1

[0067] See also Figure 3 , Figure 3 The schematic diagram of the simulation model of the inter-turn short-circuit fault of the 12% series reactor of the 35kV3×20Mvar group. In the figure, Ia, Ib, and Ic are the A, B, and C phase currents of the faulty capacitor group, respectively. U o It represents the measured neutral point voltage of the parallel capacitor. Two anti-parallel thyristors are used to simulate the situation of inter-turn short circuit in the reactor. The resistor in series with the two parallel thyristors represents the active loss consumed by the short-circuit circulating current generated in the reactor after the inter-turn short circuit in the reactor occurs. The loss of the inductance is changed by changing the inductance in parallel with the two parallel thyristors. The trigger conduction angle of the two parallel thyristors is changed to simulate the discharge characteristics of the extinguishing and reigniting of the short-circuit arc of the series reactor short-circuited turn.

[0068] The simulation analysis is carried out under the conditions that the inductance loss generated by the short-circuit circulating current during the inter-turn short-circuit fault of the series reactor with a series reactance ratio of 12% is 20%, 40%, 50%, 80%, and 90%, respectively, and the simulated neutral point voltage waveform and its content of the inter-turn short-circuit fault are obtained, such as Figure 4 As shown, from Figure 4 It can be seen that after the turn-to-turn short-circuit fault occurs, the neutral point voltage of the parallel capacitor group undergoes a significant voltage offset, and the offset voltage contains significant special changes in the 3rd, 5th, and 7th harmonic voltages.

[0069] Tables 1 and 2 below respectively simulate the delayed short-circuit discharge arc reignition time after the series reactor voltage peak in one power frequency cycle according to five conditions of inductance loss of 90%, 80%, 50%, 40% and 20%, depending on whether the series reactor inter-turn short circuit causes a ground fault (during the fault process, the aluminum wire of the short-circuit ring melts and falls due to the short-circuit circulating current, resulting in a flashover discharge of the supporting insulator to the ground, forming a secondary ground fault), and the arc burns with a continuous sine wave at different trigger angles. For phases A, B, and C, the trigger angles of 270 degrees, 30 degrees, and 150 degrees for the thyristor switch to control conduction are respectively the reignition time after the arc burning delay of 0 seconds.

[0070] Table 1112% series reactor turn-to-turn short-circuit simulation results (no ground fault)

[0071]

[0072]

[0073]

[0074]

[0075] As shown in Table 1 above, when the ground fault has not occurred, the 3rd, 5th and 7th harmonic voltage contents of the neutral point are very large in the initial, middle and late stages of the inter-turn short circuit. At each trigger angle in each period of the fault, the harmonic voltage content can almost reach more than 7%. As the trigger angle increases, the harmonic content gradually increases. At a large part of the trigger angles, the harmonic values ​​exceed the fundamental value (that is, the harmonic content exceeds 100%), which is particularly obvious in the middle stage of the fault. With the increase of inductance loss, that is, the continued development of the fault, the content of the 5th harmonic voltage will exceed the content of the 3rd harmonic voltage. In the early stage of the fault, the content of the 3rd harmonic voltage is the largest. In the middle and late stages of the fault, the content of the 5th harmonic voltage is the largest. At most trigger angles in the late stage of the fault, the content of the 3rd harmonic voltage and the 7th harmonic voltage are not much different.

[0076] According to Table 1 above, in general, the harmonic voltage change rate in the neutral point voltage at each fault period is extremely obvious, which is conducive to monitoring the occurrence of faults. The fault can be effectively monitored in the early stage of the fault, and the fault can be cut off in time to avoid the expansion of the fault.

[0077] Table 212% series reactor turn-to-turn short circuit simulation results (ground fault occurred)

[0078]

[0079]

[0080]

[0081] It can be seen from Table 2 above that after the ground fault is caused by the inter-turn short circuit, the fundamental voltage in the neutral point voltage will increase sharply compared with the case when the ground fault is not caused. In the early and middle periods of the inter-turn short circuit fault, the 5th harmonic voltage content does not exceed 4%, and the 7th harmonic voltage content does not exceed 1%. The value changes are small and are not suitable as a basis for judgment. In the early stage of the fault, the 3rd harmonic voltage can reach nearly 50%, and basically can be above 7%. The harmonic content increases with the increase of the trigger angle. After reaching the peak value at a certain trigger angle, it decreases with the increase of the trigger angle. With the increase of inductance loss, the fault continues to develop, and the 3rd harmonic voltage content gradually decreases, but even in the later stage of the fault, it can reach 4%-15% at most trigger angles. According to the harmonic voltage data in Table 2, it can be seen that monitoring the content of the 3rd harmonic voltage can still effectively and accurately identify the occurrence of the inter-turn short circuit fault of the series reactor, especially in the early stage of the fault, it still has good monitoring capabilities.

[0082] It can be seen from Table 1 and Table 2 that when there is a ground fault, the 5th and 7th harmonic voltage content rates in each period of the turn-to-turn short-circuit fault are below 4%, which affects the monitoring accuracy and may cause misjudgment, and is not suitable as a basis for judgment; while the 3rd harmonic content rate can reach nearly 50% at the initial stage of the fault, and is greater than 8% at most trigger angles, and is not less than 4% at most trigger angles in the middle and late stages of the fault, which can be used as a basis for judgment. When there is no ground fault, the 3rd, 5th, and 7th harmonic voltage content rates of the neutral point are very large in the early, middle and late stages of the turn-to-turn short-circuit, and the content rate changes significantly at this time, which can identify the fault very sensitively and accurately. Therefore, regardless of whether a ground fault occurs, a sensitive and accurate judgment can be made at the initial stage of the fault based on the harmonic voltage change rate.

[0083] Comprehensive analysis of Tables 1 and 2 shows that when the series reactor is short-circuited and a ground fault occurs, the harmonic voltage content will decrease, but the absolute effective value will remain unchanged. The 5th and 7th harmonic voltages change very little, but the neutral point fundamental voltage increases sharply, causing HR(5) and HR(7) to drop sharply, with HR(5) between 0% and 4% and HR(7) below 1%. When the series reactor is short-circuited and no ground fault occurs, the monitored turn-to-turn fault trigger angle (the trigger angle is the angle at which the current lags behind the voltage peak) is most likely to be between 40° and 60°, at which time the 3rd harmonic voltage content is between 3% and 240%, the 5th harmonic voltage content is between 50% and 375%, and the 7th harmonic content is between 25% and 210%.

[0084] The simulation of the inter-turn short circuit of the series reactor with a series resistance ratio of 6% can be directly used Figure 3The simulation model is then obtained by reducing the reactance value of the series reactance by half. According to the simulation results, it is found that when there is no grounding fault, the reactance rate is the same as 12%. In the early, middle and late stages of the inter-turn short circuit, the 3rd, 5th and 7th harmonic voltage contents of the neutral point are very large, and the fault can be detected sensitively and accurately; but when a grounding fault occurs, in the early and middle stages of the fault, the 3rd and 5th harmonic voltage contents of the neutral point voltage are both below 3%, and the fault cannot be detected in the early stage. When the fault occurs in the later stage and reaches 80% inductance loss, the 5th harmonic voltage content can reach nearly 40% at most, and the occurrence of the fault can be accurately monitored. At this time, the 3rd, 5th and 7th harmonic currents will resonate and amplify, which is helpful for monitoring the inter-turn short circuit fault.

[0085] The cases of two sets of parallel capacitors in the low-voltage side listed in Tables 1 and 2 are also applicable to the cases of a single set of parallel capacitors in practice. When the series resistance ratio is 6%, the case of a single set versus two sets in parallel can reliably detect the fault at the early stage of the fault based on the harmonic voltage content rate when no ground fault occurs. When a ground fault occurs, the neutral point harmonic voltage content rate and the sudden change of the neutral point fundamental voltage can also quickly remove the fault at the early stage of the fault.

[0086] Compared with the related art, the present invention collects the neutral point voltage through computer simulation and then performs Fourier transformation to obtain the fundamental and harmonic effective value components of the neutral point voltage; respectively calculates the percentage of the effective value components of the 3rd, 5th and 7th harmonic voltages in the neutral point voltage to the fundamental effective value components, obtains the 3rd, 5th and 7th harmonic voltage contents, and determines whether the 3rd, 5th and 7th harmonic voltage contents exceed the tripping setting of the capacitor group series reactor inter-turn short-circuit protection, so as to determine the series reactor inter-turn short-circuit fault. The parallel capacitor group with the series reactor inter-turn short-circuit fault is disconnected to prevent the series reactor inter-turn short-circuit fault from continuing to develop and causing fires and other more serious secondary phase-to-phase accidents caused by carbon ion smoke.

[0087] The above is only a preferred embodiment of the present invention, and the protection scope of the present invention is not limited to the above embodiments. All technical solutions under the concept of the present invention belong to the protection scope of the present invention. It should be pointed out that for ordinary technicians in this technical field, some improvements and modifications without departing from the principle of the present invention should also be regarded as the protection scope of the present invention.

Claims

1. A method for monitoring series reactor turn-to-turn short circuit based on star-shaped neutral point voltage and harmonic changes, characterized in that: The steps include: S0: In the simulation software PSCAD / EMTDC, according to the infinite power generation system and the relevant equipment parameters of the real fault case operation status, a star-connected ungrounded model of the parallel capacitor group is constructed; on the fault phase, two anti-parallel thyristors are used to simulate the short-circuit between the turns of the series reactor; a resistor connected in series with the two parallel thyristors is used to simulate the active power loss consumed by the short-circuit circulating current generated in the series reactor after the short-circuit between the turns of the series reactor occurs; an inductor connected in parallel with the two parallel thyristors is used to simulate the inductance loss, and by changing the trigger conduction angle of the two parallel thyristors, the discharge characteristics of the extinguishing and reigniting of the short-circuit arc of the short-circuit turn of the series reactor are simulated; S1: Real-time acquisition of the neutral point voltage of the ungrounded star-connected shunt capacitor bank, and the fundamental wave and harmonic effective value components of the neutral point voltage are obtained through Fourier transform; S2: Calculate the percentage of the effective value component of the 3rd, 5th and 7th harmonic voltage in the neutral point voltage to the effective value component of the fundamental wave, and obtain the 3rd, 5th and 7th harmonic voltage contents HR3, HR5 and HR7; S3: Respectively compare the 3rd, 5th, and 7th harmonic voltage contents HR3, HR5, and HR7 with the turn-to-turn short-circuit protection tripping setting HR of the series reactor in the parallel capacitor bank. dz For comparison, if any of HR3, HR5, and HR7 exceeds HR dz , then execute step S4, otherwise return to step S1; S4: Determine whether the duration of the harmonic voltage exceeds the lower time limit threshold Td z1 If yes, then execute step S5; otherwise, return to step S1; S5: Determine whether the duration of the harmonic voltage exceeds the high time limit threshold Td z2 If yes, then execute step S6, otherwise, issue an alarm signal to indicate that a series reactor turn-to-turn short circuit has occurred; S6: Determine whether the maximum value of the effective value of the neutral point fundamental voltage exceeds one half of the effective value of the normal phase voltage. If so, it is determined that a short circuit has occurred between the turns of the series reactor and a secondary grounding fault has occurred. The protection device issues a non-delayed opening command to directly cut off the entire set of parallel capacitor groups and lock the closing output. Otherwise, it is determined that a short circuit has occurred between the turns of the series reactor and no grounding fault has occurred. The protection device issues a opening command to directly cut off the entire set of parallel capacitor groups and lock the closing output.

2. The method for monitoring series reactor turn-to-turn short circuit based on star-shaped neutral point voltage and harmonic changes according to claim 1 is characterized in that: The process of Fourier transform in step S1 is as follows: A0=a0 In the formula, f(t) represents the neutral point voltage after Fourier transformation. represents the DC component of f(t), T represents the period of function f(t), ω represents the angular frequency of function f(t), ωT=2π, represents the initial phase, k represents the harmonic order; a k 、b k Respectively represent the amplitudes of the kth harmonic cosine component and sine component; represents the kth harmonic component, A k Indicates the amplitude of the kth harmonic, the effective value is When k = 1, Represents the fundamental component, A1 represents the amplitude of the fundamental wave, and the effective value is 3. The method for monitoring series reactor turn-to-turn short circuit based on star-shaped neutral point voltage and harmonic changes according to claim 2 is characterized in that: The calculation process of the kth harmonic voltage content in step S2 is:

4. The method for monitoring series reactor turn-to-turn short circuit based on star-shaped neutral point voltage and harmonic changes according to claim 1, characterized in that: The series reactor is a dry-type hollow series reactor with a series reactor ratio of 12% or 5%-6%.

5. The method for monitoring series reactor turn-to-turn short circuit based on star-shaped neutral point voltage and harmonic changes according to claim 1, characterized in that: The reactor turn-to-turn short-circuit protection trip setting HR dz Take 5%-7%, the low time threshold T dz1 =2~10s, high time threshold T dz2 =10~20s.

6. The method for monitoring series reactor turn-to-turn short circuit based on star-shaped neutral point voltage and harmonic changes according to claim 1, characterized in that: In step S6, if it is determined that a series reactor turn-to-turn short circuit occurs and a ground fault occurs, the following steps are also included: An auxiliary judgment method is adopted in which the bus voltage of the fault phase decreases and the bus voltage of the non-fault phase increases to distinguish the fault phase and the non-fault phase.

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