Grounding protection device of transformer neutral point

Abstract

The invention relates to the technical field of power systems, discloses a grounding protection device for a neutral point of a transformer, and aims to solve the problems that an existing protection device is low in sensitivity, high in contact loss of a discharge gap, poor in matching accuracy of the discharge gap and a lightning arrester and large in overvoltage protection dead zone range. The scheme mainly comprises a control module, a voltage transformer, a fast load switch and a lightning arrester, a neutral point of the transformer is connected with a signal input end of the voltage transformer, a signal output end of the voltage transformer is connected with a first signal input end of the control module, and a signal output end of the control module is connected with a control end of the fast load switch. The transformer neutral point is also grounded through the on-off end of the rapid load switch and the lightning arrester. The control module is configured to control the rapid load switch to be switched on when the voltage acquired by the voltage transformer is greater than the minimum switching-on voltage. According to the invention, the sensitivity and accuracy of transformer neutral point grounding protection are improved, and voltage protection dead zones are reduced.

Description

Technical Field

[0001] The present invention relates to the technical field of power systems, and particularly to a grounding protection device for a transformer neutral point. Background Art

[0002] According to the requirements of power grid operation, the neutral points of transformers in a power system are divided into several grounding methods, such as non-grounded neutral point, neutral point grounded through a high resistance, neutral point grounded through an arc suppression coil, and directly grounded neutral point. Among them, the non-grounded neutral point, neutral point grounded through a high resistance, and neutral point grounded through an arc suppression coil are applicable to 6 - 66 kV power systems. For power systems with a voltage of 110 kV and above, the neutral points of transformers mostly adopt two methods: fixed grounding of the neutral point and non-fixed grounding of the neutral point. The non-fixed grounding method of the neutral point is mainly to improve the selectivity of the transformer neutral point grounding, limit the grounding fault current, suppress overvoltage, and thus improve the power supply continuity.

[0003] Traditional grounding protection devices for non-fixed grounding of transformer neutral points usually adopt a combination of a discharge gap and a lightning arrester. The discharge gap consists of two metal rods or electrodes exposed to the air. One is connected to the transformer neutral point (L1 / N), and the other is grounded (PE). The distance between the two electrodes is adjustable to adapt to the protection requirements of different voltage levels. When power frequency overvoltage occurs in the transformer, the electric field strength between the electrodes exceeds the air breakdown threshold, the gap is ionized and broken down, forming a conductive channel to discharge charges to the ground, protecting the insulation of the transformer neutral point; the lightning arrester conducts to the ground when lightning overvoltage and switching impulse overvoltage occur in the transformer, protecting the insulation of the transformer neutral point.

[0004] The traditional non-fixed grounding method of the transformer neutral point using a discharge gap for power frequency overvoltage protection has at least the following defects: 1) The discharge gap has poor arc extinguishing performance, uneven surface resistance, is prone to surface flashover, and the partial discharge inception voltage and corona inception voltage decrease in high altitude areas, causing corona corrosion on the surface of the discharge gap rod electrode. 2) The discharge gap is affected by the natural environment (such as factors like temperature and humidity changes), with low sensitivity and inaccurate coordination of protection parameters with the lightning arrester. For example, when power frequency overvoltage occurs, the discharge gap cannot be effectively broken down, and there is a risk of explosion of the lightning arrester. 3) The discharge has a large dispersion, general protection characteristics, is prone to misoperation, has a relatively large power frequency follow current, and poor arc extinguishing ability; moreover, the gap action will generate carrier waves, which is not beneficial to the insulation of the transformer itself. 4) To avoid power frequency overvoltage exceeding the rated voltage of the lightning arrester at the transformer neutral point and causing the lightning arrester to burn out, the maximum power frequency breakdown voltage of the discharge gap action should be less than the rated voltage of the lightning arrester, which results in a large range of overvoltage protection dead zones between the rated voltage of the lightning arrester and the action voltage of the lightning arrester, causing the transformer neutral point to lose protection within the overvoltage dead zone. Summary of the Invention

[0005] The present invention aims to solve the problems existing in the existing grounding protection device for the neutral point of a transformer, such as low sensitivity, relatively fast contact loss of the discharge gap, poor accuracy in the cooperation between the discharge gap and the lightning arrester, and a large overvoltage protection dead zone, and proposes a grounding protection device for the neutral point of a transformer.

[0006] The technical solution adopted by the present invention to solve the above technical problems is as follows:

[0007] A grounding protection device for the neutral point of a transformer, the device includes: a control module, a voltage transformer, a fast load switch, and a lightning arrester. The neutral point of the transformer is connected to the signal input end of the voltage transformer. The signal output end of the voltage transformer is connected to the first signal input end of the control module. The signal output end of the control module is connected to the control end of the fast load switch. The neutral point of the transformer is also grounded through the on-off end of the fast load switch and the lightning arrester respectively. The control module is configured to control the fast load switch to close when the voltage collected by the voltage transformer is greater than the minimum closing voltage.

[0008] Further, the device also includes a current transformer. The current transformer is connected in series on the side of the fast load switch close to the ground. The signal output end of the current transformer is connected to the second signal input end of the control module. The control module is configured to control the fast load switch to trip when the current collected by the current transformer is less than the current threshold, and to cut off the circuit breakers on each side of the transformer according to the magnitude and duration of the current collected by the current transformer.

[0009] Further, the control module includes a first comparison circuit and a second comparison circuit. The first comparison circuit is used to send a closing control signal to the fast load switch when the voltage collected by the voltage transformer is greater than the minimum closing voltage. The second comparison circuit is used to send a tripping control signal to the fast load switch when the current collected by the current transformer is less than the current threshold.

[0010] Further, the closing operation time of the fast load switch is less than 40 milliseconds, and the tripping operation time is less than 6 milliseconds.

[0011] Further, the device also includes a manual disconnecting switch. The neutral point of the transformer is grounded through the manual disconnecting switch.

[0012] Further, the method for determining the minimum closing voltage includes:

[0013] Determine the highest line voltage at which the system can operate stably for a long time, and determine the corresponding maximum zero-sequence voltage of the neutral point of the transformer according to the highest line voltage;

[0014] Calculate the minimum closing voltage according to the maximum zero-sequence voltage and based on the effective value formula of the power frequency breakdown minimum voltage of the neutral point discharge gap.

[0015] Furthermore, the calculation formula for the maximum zero-sequence voltage is as follows:

[0016]

[0017] where U 0 represents the maximum zero-sequence voltage, U 1 represents the highest line voltage, X 0 represents the zero-sequence reactance, X 1 represents the positive-sequence reactance;

[0018] The calculation formula for the minimum closing voltage is as follows:

[0019]

[0020] where U L represents the minimum closing voltage, K 1 represents the safety factor, K 2 represents the meteorological correction factor, and σ represents the dispersion coefficient of the power-frequency breakdown voltage of the air gap.

[0021] Furthermore, the method for selecting the type of lightning arrester includes:

[0022] Determine the lowest line voltage at which the system can operate stably for a long time, and calculate the maximum closing voltage based on the lowest line voltage;

[0023] Determine the operating voltage of the lightning arrester based on the maximum closing voltage, determine the rated voltage of the lightning arrester based on the operating voltage, and select the corresponding type of lightning arrester based on the rated voltage.

[0024] Furthermore, the calculation formula for the maximum closing voltage is as follows:

[0025]

[0026] where U H represents the maximum closing voltage, U 2 represents the lowest line voltage.

[0027] Furthermore, the lightning impulse residual voltage of the lightning arrester is less than the lightning impulse withstand voltage of the transformer neutral point, the operating voltage of the lightning arrester is greater than the maximum closing voltage, and the relationship between the operating voltage and the rated voltage of the lightning arrester is as follows:

[0028] U ref = U n × k;

[0029] where U ref represents the operating voltage of the lightning arrester, U n represents the rated voltage of the lightning arrester, and k represents the coefficient.

[0030] The beneficial effects of the present invention are as follows: The grounding protection device for the neutral point of the transformer provided by the present invention replaces the discharge gap in the prior art with a control module, a voltage transformer, and a fast load switch. The voltage is accurately measured by the voltage transformer, and when the measured voltage is greater than the minimum value of the set closing voltage, the control module controls the fast load switch to operate, so as to protect the insulation of the neutral point of the transformer. Since the measurement is accurate and the minimum value of the closing voltage can be set according to the characteristics of the power frequency overvoltage, the action sensitivity of the device is very high. When the voltage of the neutral point of the power transformer is within the power frequency overvoltage range, the fast load switch can close quickly. When there is a lightning overvoltage, the lightning arrester is automatically broken down to protect the insulation of the neutral point of the transformer. The fast load switch and the lightning arrester work within their respective protection ranges, can cooperate effectively, ensure the selectivity and accuracy of the component actions, and at the same time, the wiring is simple, greatly reducing the voltage protection dead zone and avoiding the defect of too large a protection dead zone in the combination of the traditional discharge gap and the lightning arrester, improving the safety and reliability of the equipment operation. Description of the Drawings

[0031] Figure 1 FIG. is a schematic structural diagram of a grounding protection device for the neutral point of a transformer provided for an embodiment;

[0032] Figure 2 FIG. is a schematic structural diagram of another grounding protection device for the neutral point of a transformer provided for an embodiment;

[0033] Figure 3 FIG. is a schematic diagram of the working principle of a grounding protection device for the neutral point of a transformer provided for an embodiment;

[0034] Description of the Reference Numerals:

[0035] 1 - control module; 2 - voltage transformer; 3 - fast load switch; 4 - lightning arrester; 5 - current transformer; 6 - manual disconnecting switch. Detailed Embodiments

[0036] In order to enable those skilled in the art to better understand the solution of the present invention, the technical solutions in this embodiment will be clearly and completely described below in conjunction with the drawings in this embodiment.

[0037] Since in the current transformer neutral point ungrounded protection device, the commonly adopted technical solution is to use a discharge gap to protect against power frequency overvoltage in the transformer. The discharge gap has the advantages of simple structure, low cost, and convenient maintenance. However, through research, the inventor found that the above solution has at least the following defects: First, the arc extinguishing ability of the discharge gap is weak, which may cause power frequency follow current; second, the sensitivity of the discharge gap is low, and the accuracy of cooperation with the lightning arrester is poor; third, the gap action of the discharge gap will generate carrier waves, which is unfavorable to the insulation of the transformer itself; fourth, there is an overvoltage protection dead zone in a large range when the discharge gap cooperates with the lightning arrester.

[0038] Based on this, the technical solution of the present invention is proposed. In the present invention, the grounding protection device for the transformer neutral point includes: a control module, a voltage transformer, a fast load switch, and a lightning arrester. The transformer neutral point is connected to the signal input end of the voltage transformer. The signal output end of the voltage transformer is connected to the first signal input end of the control module. The signal output end of the control module is connected to the control end of the fast load switch. The transformer neutral point is also grounded through the on-off end of the fast load switch and the lightning arrester respectively. The control module is configured to control the fast load switch to close when the voltage collected by the voltage transformer is greater than the minimum closing voltage.

[0039] Specifically, the present invention uses a control module, a voltage transformer, and a fast load switch to replace the discharge gap in the prior art. The voltage transformer accurately measures the voltage of the transformer neutral point, and sets the corresponding minimum closing voltage according to the characteristics of power frequency overvoltage. When the voltage of the transformer neutral point is greater than the set minimum closing voltage, the control module automatically controls the fast load switch to perform the closing action, so that when the voltage of the transformer neutral point is within the range of power frequency overvoltage, it is quickly grounded through the fast load switch to protect the insulation of the transformer neutral point. When the voltage of the transformer neutral point is within the range of lightning overvoltage, the lightning arrester will be automatically broken down. Because the measurement is accurate and the minimum closing voltage can be set according to the characteristics of power frequency overvoltage, the action sensitivity of the device is very high. At the same time, the fast load switch and the lightning arrester work within their respective protection ranges, can effectively cooperate, ensure the selectivity and accuracy of the component actions, greatly reduce the voltage protection dead zone, and improve the reliability and safety of the equipment operation.

[0040] Next, the technical solutions in this embodiment will be clearly and completely described in conjunction with the accompanying drawings in this embodiment. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all of the embodiments.

[0041] Figure 1 Shows a grounding protection device for a transformer neutral point. Please refer to Figure 1, the device includes: a control module 1, a voltage transformer 2, a fast load switch 3, and a lightning arrester 4. The neutral point of the transformer is connected to the signal input terminal of the voltage transformer 2. The signal output terminal of the voltage transformer 2 is connected to the first signal input terminal of the control module 1. The signal output terminal of the control module 1 is connected to the control terminal of the fast load switch 3. The neutral point of the transformer is also grounded through the on-off terminal of the fast load switch 3 and the lightning arrester 4 respectively. The control module 1 is configured to control the fast load switch 3 to close when the voltage collected by the voltage transformer 2 is greater than the minimum closing voltage.

[0042] In this embodiment, the voltage transformer 2 is used to collect the voltage of the neutral point of the transformer and send the collected voltage to the control module 1. The voltage transformer 2 can be a capacitive voltage transformer or an electromagnetic voltage transformer.

[0043] The control module 1 is used to obtain the voltage signal collected by the voltage transformer 2 and control the operation of the fast load switch 3 according to the magnitude relationship between the voltage value corresponding to the voltage signal and the minimum closing voltage. Specifically, when the voltage collected by the voltage transformer 2 is greater than the minimum closing voltage, the control module 1 sends a closing control signal to the fast load switch 3 to make the fast load switch 3 close.

[0044] In practical applications, the above functions of the control module 1 can be realized by a first comparison circuit, so there is no need to use a computer program. That is, the minimum closing voltage is used as the reference voltage of the first comparison circuit. When the voltage collected by the voltage transformer 2 is greater than the minimum closing voltage, the first comparison circuit sends a closing control signal (such as a high-level signal) to the fast load switch 3 to make the fast load switch 3 close. Of course, the control module 1 can also be a controller loaded with a computer program. This computer program is used to implement the comparison of voltages, which is a conventional computer program in the art and is not an improvement of this embodiment, so it will not be elaborated here.

[0045] In this embodiment, the fast load switch 3 is used to receive the control signal of the control module 1 to realize the grounding control of the neutral point of the transformer. When the fast load switch 3 closes, the neutral point of the transformer is grounded through the fast load switch 3 to realize the discharge of fault current.

[0046] Please refer to Figure 2 , in this embodiment, the device further includes a current transformer 5. The current transformer 5 is connected in series on the side of the fast load switch 3 close to the ground. The signal output terminal of the current transformer 5 is connected to the second signal input terminal of the control module 1. The control module 1 is configured to control the fast load switch 3 to open when the current collected by the current transformer 5 is less than the current threshold, and cut off the circuit breakers on each side of the transformer according to the magnitude and duration of the current collected by the current transformer 5.

[0047] Specifically, the current transformer 5 is used to collect the zero-sequence current at the neutral point. When the fast load switch 3 is closed, during the discharge process of the fault current at the neutral point, the current collected by the current transformer 5 is relatively large. If the discharge of the fault current is completed, the current collected by the current transformer 5 is relatively small (about 0). Based on this, during the closing process of the fast load switch 3, the current transformer 5 continuously collects the zero-sequence current at the neutral point and sends it to the control module 1. When the current collected by the current transformer 5 is less than the current threshold, it indicates that the discharge of the fault current is completed. At this time, the control module 1 can control the fast load switch 3 to open. After the fast load switch 3 opens, the neutral point of the transformer is no longer grounded through the fast load switch 3.

[0048] In practical applications, the above function of the control module 1 can be realized by a second comparison circuit, so there is no need to use a computer program. That is, the current threshold is used as the reference current of the second comparison circuit. When the current collected by the current transformer 5 is less than the current threshold, the second comparison circuit sends a tripping control signal to the fast load switch 3 (for example, pulling down the original high-level signal to a low-level signal) to make the fast load switch 3 open. Of course, the control module 1 can also be a controller loaded with a computer program. This computer program is used to realize the comparison of currents. It is a conventional computer program in this field and is not an improvement in this embodiment, so it will not be elaborated here.

[0049] In addition, the control module 1 in this embodiment is also respectively connected to the control terminals of the circuit breakers on both sides of the transformer. After applying the corresponding program, the control module 1 can also cut off the circuit breakers on both sides of the transformer according to the magnitude and duration of the current collected by the current transformer 5. Specifically, the duration of the current can be used to represent the power frequency overvoltage fault time. If the power frequency overvoltage fault time exceeds the setting time of the single-phase grounding fault protection delay of the transformer, the zero-sequence overvoltage / overcurrent protection of the transformer acts to trip the circuit breakers on each side of the transformer to further protect the transformer.

[0050] In this embodiment, the closing operation time of the fast load switch 3 is less than 40 milliseconds, and the opening operation time is less than 6 milliseconds. The fast operation of the fast load switch 3 can realize the fast grounding of the neutral point of the transformer. The millisecond-level response speed can isolate the fault point in time, prevent local faults from spreading to the entire power grid system, and effectively avoid cascade tripping or large-area power outages; and it can quickly disconnect the grounding of the neutral point of the transformer after the discharge of the fault current is completed; at the same time, it can also reduce the arcing time between the switch contacts, thereby reducing the risk of carbonization of the insulating material caused by the high temperature of the arc.

[0051] Please refer to Figure 2, in this embodiment, the device further includes a manual disconnecting switch 6, and the neutral point of the transformer is grounded through the manual disconnecting switch 6. In practical applications, the staff manually operates the manual disconnecting switch 6 according to the dispatching order of the power system to achieve fixed or non-fixed grounding of the neutral point of the transformer.

[0052] Based on the above-mentioned grounding protection device for the neutral point of the transformer, in practical applications, first, according to the characteristics of the power frequency overvoltage of the transformer, the minimum closing voltage of the fast load switch 3 is set in the control module 1, so that the control module 1 controls the closing of the fast load switch 3 according to the magnitude relationship between the voltage collected by the voltage transformer 2 and the set minimum closing voltage. Then, the maximum closing voltage of the fast load switch 3 is determined, and accordingly, a lightning arrester 4 of the corresponding model is selected to avoid the overlap of the operating voltage ranges of the fast load switch 3 and the lightning arrester 4, which may cause misoperation of the lightning arrester 4.

[0053] Please refer to Figure 3 , during the actual operation of the protection device for the neutral point of the transformer, the voltage transformer 2 continuously collects the voltage of the neutral point of the transformer and sends it to the control module 1. When the voltage of the neutral point of the transformer is less than the minimum closing voltage, it indicates that the overvoltage is small at this time and the insulation ability of the transformer itself can withstand it. At this time, the fast load switch 3 does not operate and remains in the open state. When the voltage of the neutral point of the transformer is greater than the minimum closing voltage, it indicates that the transformer has a power frequency overvoltage. At this time, the control module 1 controls the fast load switch 3 to close to discharge the fault current to protect the insulation of the neutral point of the transformer. The current transformer 5 continuously collects the zero-sequence current of the neutral point of the transformer and sends it to the control module 1. When the zero-sequence current of the neutral point of the transformer is greater than the current threshold, it indicates that the fault current is being discharged at this time and no action is taken. When the zero-sequence current of the neutral point of the transformer is less than the current threshold, it indicates that the discharge of the fault current is completed. At this time, the control module 1 controls the fast load switch 3 to open and restores the ungrounded state of the neutral point of the transformer. When the voltage of the neutral point of the transformer is in the range of lightning overvoltage, the lightning arrester 4 is automatically broken down to discharge the fault current to protect the insulation of the neutral point of the transformer.

[0054] In this embodiment, the method for determining the minimum closing voltage includes:

[0055] Determine the highest line voltage at which the system can operate stably for a long time, and determine the corresponding maximum zero-sequence voltage of the neutral point of the transformer according to the highest line voltage; calculate the minimum closing voltage based on the maximum zero-sequence voltage and the formula for the effective value of the minimum power frequency breakdown voltage of the neutral point discharge gap.

[0056] It can be understood that the minimum closing voltage of the fast load switch 3 in this embodiment is calculated under the condition that the system operates in an effectively grounded mode and a single-phase ground fault occurs, and the system takes the highest long-term operating voltage. Since the fast load switch 3 is used to replace the discharge gap, the minimum closing voltage of the fast load switch 3 should be calculated according to the effective value of the minimum power frequency breakdown voltage of the neutral point discharge gap.

[0057] In this embodiment, the calculation formula for the maximum zero-sequence voltage is as follows:

[0058]

[0059] Where, U 0 represents the maximum zero-sequence voltage, unit: kilovolt (kV), U 1 represents the highest line voltage, unit: kilovolt (kV), X 0 represents the zero-sequence reactance, unit: ohm (Ω), X 1 represents the positive-sequence reactance (Ω), unit: ohm (Ω), represents the conversion coefficient between the line voltage and the phase voltage.

[0060] Taking the neutral point of the power transformer in the 110 kV system as an example, the highest line voltage at which the system can operate stably for a long time is 126 kV, and the ratio of the zero-sequence reactance to the positive-sequence impedance of the system: Then:

[0061] The calculation formula for the minimum closing voltage is as follows:

[0062]

[0063] Where, U L represents the minimum closing voltage, unit: kilovolt (kV), K 1 represents the safety factor, dimensionless, K 2 represents the meteorological correction factor, dimensionless, σ represents the dispersion coefficient of the power frequency breakdown voltage of the air gap, dimensionless.

[0064] It can be understood that the safety factor K 1 is used to consider uncertain factors such as equipment aging and errors to provide an additional safety margin, and can be selected according to engineering standards (such as IEC or GB). In this embodiment, it is taken as 1.05; the meteorological correction factor K 2 is used to adjust the influence of humidity and temperature on the air breakdown voltage to ensure that the air gap can still be broken down under severe meteorological conditions, and can be corrected by looking up tables or according to empirical formulas based on local meteorological data (temperature, humidity, air pressure). In this embodiment, it is taken as 1.05; the dispersion coefficient σ is used to consider the statistical fluctuation of the breakdown voltage and can be obtained by fitting the breakdown test data or referring to the corresponding standards. In this embodiment, it is taken as 0.025.

[0065] For example, assume that the maximum zero-sequence voltage U 0 = 43.65 kV, then the minimum closing voltage

[0066] Through the above formula, the minimum power frequency voltage at which the air gap can be reliably broken down under specific meteorological conditions and safety margin can be accurately calculated, so that the set minimum closing voltage corresponds to the power frequency overvoltage. Furthermore, when the voltage of the neutral point of the power transformer is within the power frequency overvoltage range, the fast load switch 3 closes quickly to protect the insulation of the neutral point of the transformer.

[0067] To avoid misoperation of the lightning arrester 4 caused by the overlap of the action voltage ranges of the fast load switch 3 and the lightning arrester 4, in this embodiment, the type of the lightning arrester 4 is selected according to the maximum closing voltage, specifically including:

[0068] Determine the lowest line voltage at which the system can operate stably for a long time, calculate the maximum closing voltage according to the lowest line voltage; determine the action voltage of the lightning arrester 4 according to the maximum closing voltage, determine the rated voltage of the lightning arrester 4 according to the action voltage, and select the corresponding type of lightning arrester 4 according to the rated voltage.

[0069] It can be understood that the maximum closing voltage of the fast load switch 3 is calculated under the condition that the system operates in an effectively grounded mode and a single-phase ground fault occurs, and the system takes the lowest long-term operating voltage.

[0070] In this embodiment, the calculation formula of the maximum closing voltage is as follows:

[0071]

[0072] Among them, U H represents the maximum closing voltage, unit: kilovolt (kV), U 2 represents the lowest line voltage, unit: kilovolt (kV), represents the coefficient for converting the effective value to the peak value, represents the conversion coefficient between the line voltage and the phase voltage.

[0073] Taking the neutral point of the power transformer in the 110 kV system as an example, the lowest line voltage U 2 = 110 kV × 95% = 104.5 kV, then the maximum closing voltage

[0074] The residual voltage of the lightning arrester 4 should be lower than the impulse withstand voltage of the transformer neutral point to avoid damage to the transformer. The operating voltage of the lightning arrester 4 should be greater than the maximum closing voltage to avoid the overlap of the operating voltage ranges of the fast load switch 3 and the lightning arrester 4. Among them, the operating voltage of the lightning arrester 4 can be verified through the rated voltage, that is:

[0075] U ref =U n ×k;

[0076] Among them, U ref represents the operating voltage unit: kilovolt (kV), U n represents the rated voltage, unit: kilovolt (kV), k represents the coefficient, dimensionless, and k takes 1.3 in this embodiment.

[0077] For example, the lightning impulse withstand voltage insulation level of the neutral point of a 110 kV transformer is 250 kV, and the maximum closing voltage is 85.31 kV. Then, a lightning arrester of model Y1.5W-84 / 200 can be selected. The residual voltage of the lightning impulse current of this model lightning arrester is 200 kV, which is less than the lightning impulse withstand voltage of 250 kV of the transformer neutral point; and the rated voltage of this model lightning arrester is 84 kV, and its corresponding operating voltage is 84 kV × 1.3 = 109.2 kV, which is greater than the maximum closing voltage of 85.31 kV.

[0078] To sum up, the grounding protection device for the transformer neutral point provided in this embodiment uses the control module 1, the voltage transformer 2 and the fast load switch 3 to replace the discharge gap in the prior art. The voltage transformer 2 accurately measures the voltage, and the control module 1 controls the fast load switch 3 to act when the measured voltage is greater than the set minimum closing voltage, so as to protect the insulation of the transformer neutral point. Due to accurate measurement and the ability to set the minimum closing voltage according to the characteristics of power frequency overvoltage, the operating sensitivity of the device is very high. When the voltage of the power transformer neutral point is within the power frequency overvoltage range, the fast load switch 3 can be quickly closed. When there is a lightning overvoltage, the lightning arrester 4 is automatically broken down to protect the insulation of the transformer neutral point. The fast load switch 3 and the lightning arrester 4 work within their respective protection ranges, can cooperate effectively, ensure the selectivity and accuracy of the component actions, and at the same time have a simple wiring, greatly reducing the voltage protection dead zone, avoiding the defect of too large a protection dead zone in the combination of the traditional discharge gap and the lightning arrester, and improving the safety and reliability of the equipment operation.

Claims

1. A transformer neutral point grounding protection device, characterized in that: The device includes: a control module, a voltage transformer, a fast load switch and a lightning arrester. The neutral point of the transformer is connected to the signal input end of the voltage transformer, the signal output end of the voltage transformer is connected to the first signal input end of the control module, the signal output end of the control module is connected to the control end of the fast load switch, the neutral point of the transformer is also grounded through the on-off end of the fast load switch and the lightning arrester respectively, and the control module is configured to control the fast load switch to close when the voltage collected by the voltage transformer is greater than the minimum closing voltage.

2. The transformer neutral point grounding protection device according to claim 1, characterized in that: The device also includes a current transformer, which is connected in series on a side of the fast load switch close to the ground. The signal output end of the current transformer is connected to the second signal input end of the control module. The control module is configured to control the fast load switch to open when the current collected by the current transformer is less than the current threshold, and to cut off the circuit breakers on each side of the transformer according to the current size and duration collected by the current transformer.

3. The transformer neutral point grounding protection device according to claim 1, characterized in that: The control module includes a first comparison circuit and a second comparison circuit. The first comparison circuit is used to send a closing control signal to the fast load switch when the voltage collected by the voltage transformer is greater than the minimum closing voltage. The second comparison circuit is used to send an opening control signal to the fast load switch when the current collected by the current transformer is less than the current threshold.

4. The transformer neutral point grounding protection device according to claim 1, characterized in that: The closing action time of the fast load switch is less than 40 milliseconds, and the opening action time is less than 6 milliseconds.

5. The transformer neutral point grounding protection device according to claim 1, characterized in that: The device also includes a manual isolating switch, and the neutral point of the transformer is grounded via the manual isolating switch.

6. The transformer neutral point grounding protection device according to claim 1, characterized in that: The method for determining the minimum closing voltage includes: Determine the highest line voltage at which the system can operate stably for a long time, and determine the corresponding maximum zero-sequence voltage of the neutral point of the transformer according to the highest line voltage; The minimum closing voltage is calculated according to the maximum zero-sequence voltage and based on a formula for the minimum effective value of the power frequency breakdown voltage of the neutral point discharge gap.

7. The transformer neutral point grounding protection device according to claim 6, characterized in that: The calculation formula of the maximum zero-sequence voltage is as follows: Among them, U0 represents the maximum zero-sequence voltage, U1 represents the highest line voltage, X0 represents the zero-sequence reactance, and X1 represents the positive-sequence reactance; The calculation formula for the minimum closing voltage is as follows: Among them, U L It represents the minimum closing voltage, K1 represents the safety factor, K2 represents the meteorological correction factor, and σ represents the dispersion coefficient of the power frequency breakdown voltage of the air gap.

8. The transformer neutral point grounding protection device according to claim 1, characterized in that: The method for selecting the model of the arrester comprises: Determine the lowest line voltage at which the system can operate stably for a long time, and calculate the maximum value of the closing voltage according to the lowest line voltage; The action voltage of the arrester is determined according to the maximum closing voltage, the rated voltage of the arrester is determined according to the action voltage, and the arrester of corresponding model is selected according to the rated voltage.

9. The transformer neutral point grounding protection device according to claim 8, characterized in that: The calculation formula of the maximum closing voltage is as follows: Among them, U H It indicates the maximum closing voltage, and U2 indicates the minimum line voltage.

10. The transformer neutral point grounding protection device according to claim 8, characterized in that: The lightning impulse residual voltage of the arrester is less than the lightning impulse withstand voltage of the neutral point of the transformer, the action voltage of the arrester is greater than the maximum closing voltage, and the relationship between the action voltage and the rated voltage of the arrester is as follows: IN ref =U n ×k; Among them, U ref Indicates the operating voltage of the arrester, U n It represents the rated voltage of the arrester and k represents the coefficient.

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