Neutral point grounding system based on negative temperature coefficient resistor

Abstract

The invention relates to a neutral point grounding system based on a negative temperature coefficient resistor. The neutral point grounding system comprises a three-phase power supply, the negative temperature coefficient resistor, a circuit breaker and a zero sequence current transformer, when a system has a single-phase grounding short circuit fault, fault current flows through the negative temperature coefficient resistor, the temperature of the negative temperature coefficient resistor is increased, the resistance value of the negative temperature coefficient resistor is reduced, zero-sequence current of the system is increased, the zero-sequence current transformer monitors the zero-sequence current, and when the zero-sequence current reaches a preset protection threshold value of the zero-sequence current transformer, the zero-sequence current transformer stops working. And starting the circuit breaker to cut the fault current.

Description

Technical Field

[0001] The invention relates to a neutral point grounding system based on a negative temperature coefficient resistor, belonging to the technical field of neutral point grounding of power systems. Background Art

[0002] In the power system, the neutral point grounding method has a vital impact on the system's operating stability, equipment safety and power supply reliability.

[0003] Neutral point grounding methods are divided into two categories: effective grounding method and ineffective grounding method; ineffective grounding method includes neutral point ungrounded method, neutral point low resistance grounding method, neutral point high resistance grounding method and neutral point resonant grounding method.

[0004] The grounding resistance used in the traditional neutral point low resistance grounding method and the neutral point high resistance grounding method is a fixed value.

[0005] (1) The neutral point high resistance grounding method has the following disadvantages:

[0006] Fault current detection and positioning accuracy is limited: In a high-resistance grounding system, the ground fault current is usually small, which makes it more difficult for the zero-sequence current transformer to detect weak zero-sequence current signals and is easily affected by other interference signals in the system, which in turn makes it difficult for the fault selection device to accurately determine the fault line. The accuracy of fault positioning is not high, which increases the time and difficulty of operation and maintenance personnel to troubleshoot faults.

[0007] System stability is affected: When a single-phase grounding fault lasts for a long time, the protection device cannot act quickly to clear the fault due to the small fault current. The system will be in a non-full-phase operation state for a long time, which will cause the three-phase voltage imbalance of the system to increase, affecting the stability of the power system and may have adverse effects on some sensitive loads with high voltage quality requirements.

[0008] (2) The neutral point low resistance grounding method has the following disadvantages:

[0009] Large short-circuit current impact: At the moment of a single-phase grounding fault, low-resistance grounding will generate a large grounding fault current, which will form a strong current impact on the electrical equipment on the fault line and adjacent lines, such as transformers, circuit breakers, cables, etc., and easily cause insulation damage to the equipment, shorten the service life of the equipment, and increase the cost of equipment maintenance and replacement.

[0010] High requirements for circuit breakers: In order to reliably cut off larger fault currents, large-capacity, high-breaking-capacity circuit breakers are required. This not only significantly increases the purchase cost of circuit breakers, but also their complex operating mechanisms and maintenance requirements increase the difficulty and cost of system operation and maintenance. Summary of the invention

[0011] In order to solve the above problems existing in the prior art, the present invention proposes a neutral point grounding system based on a negative temperature coefficient resistor.

[0012] The technical solution of the present invention is as follows:

[0013] The present invention provides a neutral point grounding system based on a negative temperature coefficient resistor, comprising a three-phase power supply, a negative temperature coefficient resistor, a circuit breaker and a zero-sequence current transformer;

[0014] One end of the three-phase power supply is connected to one end of the circuit breaker, the other end of the circuit breaker is connected to the power load end, the other end of the three-phase power supply is connected to one end of a negative temperature coefficient resistor, the other end of the negative temperature coefficient resistor is connected to one end of a zero-sequence current transformer, and the other end of the zero-sequence current transformer is grounded;

[0015] When a single-phase ground short circuit fault occurs in the system, the fault current flows through the negative temperature coefficient resistor. The temperature of the negative temperature coefficient resistor increases and the resistance value decreases. The zero-sequence current of the system increases. The zero-sequence current transformer monitors the zero-sequence current. When the zero-sequence current reaches the preset protection threshold of the zero-sequence current transformer, the circuit breaker opens to cut off the fault current.

[0016] As a preferred embodiment of the present invention, the calculation formula of the rated voltage of the negative temperature coefficient resistor is:

[0017]

[0018] Among them: U R Indicates the rated voltage of negative temperature coefficient resistor; U N Indicates the rated line voltage of the system.

[0019] As a preferred embodiment of the present invention, when the system is operating normally, the calculation formula of the resistance value of the negative temperature coefficient resistor is:

[0020]

[0021] Where: R represents the resistance value of the negative temperature coefficient resistor during normal operation; IR represents the resistive current initially flowing through the negative temperature coefficient resistor during a fault;

[0022] I R =KI C

[0023] Where: K represents the ratio of the resistance current to the capacitance current when the system is single-phase to ground disconnected; I C Indicates the capacitive current when a single phase of the system is short-circuited to ground.

[0024] As a preferred embodiment of the present invention, when a single-phase ground short circuit fault occurs in the system, the instantaneous power calculation formula of the negative temperature coefficient resistor is:

[0025]

[0026] Where: P R It represents the instantaneous power of the negative temperature coefficient resistor when a single-phase ground short circuit fault occurs in the system.

[0027] As a preferred embodiment of the present invention, when a single-phase ground short circuit fault occurs in the system, the energy value consumed by the negative temperature coefficient resistor is calculated as follows:

[0028] W R =∫P R dt=c×m×ΔT

[0029] Where: W R It represents the energy consumed by the negative temperature coefficient resistor when a single-phase ground short circuit fault occurs in the system; c represents the specific heat capacity of the negative temperature coefficient resistor material; m represents the mass of the negative temperature coefficient resistor material; ΔT represents the temperature rise of the negative temperature coefficient resistor material.

[0030] As a preferred embodiment of the present invention, when a single-phase ground short circuit fault occurs in the system, the resistance value calculation formula of the negative temperature coefficient resistor is:

[0031] R d =f(T d )

[0032] Where: R d It indicates the resistance value of the negative temperature coefficient resistor when a single-phase ground short circuit occurs in the system; T d It represents the temperature of the negative temperature coefficient resistor when a single-phase ground short circuit fault occurs in the system; f represents the relationship function between temperature and resistance.

[0033] As a preferred embodiment of the present invention, when a single-phase ground short circuit fault occurs in the system, the temperature calculation formula of the negative temperature coefficient resistor is:

[0034] T d =ΔT+T

[0035] Where: T represents the initial temperature of the negative temperature coefficient resistor.

[0036] As a preferred embodiment of the present invention, when a single-phase ground short circuit fault occurs, the calculation formula of the system zero-sequence current is:

[0037]

[0038] Where: I d Indicates the system zero-sequence current when a single-phase ground short circuit fault occurs.

[0039] The present invention has the following beneficial effects:

[0040] 1. The present invention can realize adaptive current limiting protection based on negative temperature coefficient resistor, limit short-circuit current at the initial stage of fault, and reduce damage to equipment. As the fault continues, the resistor temperature increases, the resistance value decreases, and the ground fault current gradually increases. Since the growth of fault current is controllable, line tripping caused by instantaneous faults can be avoided, which improves power supply reliability to a certain extent. It is particularly suitable for occasions with high requirements for power supply continuity.

[0041] 2. In the present invention, as the fault develops, the ground fault current increases steadily, and the zero-sequence current transformer can detect the change of zero-sequence current more accurately, providing a more reliable signal basis for the fault line selection device, which is beneficial to improve the accuracy of fault detection and positioning, shorten the fault processing time, and speed up the system to resume normal operation.

[0042] 3. The present invention can make the ground fault current increase gradually from the initial small value to the protection setting value according to a regular pattern. Compared with the complex and changeable fault current conditions in the traditional grounding method, the controller only needs to set the protection setting value according to the normal operating parameters of the system and a certain safety margin, without considering many uncertain factors, which greatly reduces the difficulty of protection setting, improves the accuracy and reliability of protection action, and reduces the occurrence of false operation and refusal to operate. BRIEF DESCRIPTION OF THE DRAWINGS

[0043] Figure 1 Schematic diagram of the system structure of the present invention. DETAILED DESCRIPTION

[0044] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0045] It should be understood that the step numbers used in this document are only for convenience of description and are not intended to limit the order in which the steps are executed.

[0046] It should be understood that the terms used in the present specification are only for the purpose of describing specific embodiments and are not intended to limit the present invention. As used in the present specification and the appended claims, unless the context clearly indicates otherwise, the singular forms "a", "an" and "the" are intended to include plural forms.

[0047] The terms “include” and “comprising” indicate the presence of described features, integers, steps, operations, elements and / or components, but do not exclude the presence or addition of one or more other features, integers, steps, operations, elements, components and / or combinations thereof.

[0048] The term "and / or" means and includes any and all possible combinations of one or more of the associated listed items.

[0049] Example:

[0050] See also Figure 1 , a neutral point grounding system based on a negative temperature coefficient resistor, comprising a three-phase power supply, a negative temperature coefficient resistor, a circuit breaker and a zero-sequence current transformer;

[0051] The selection of the negative temperature coefficient resistor should be determined comprehensively based on factors such as the voltage level of the system, the expected ground fault current range, and the system operating environment temperature;

[0052] One end of the three-phase power supply is connected to one end of the circuit breaker, the other end of the circuit breaker is connected to the power load end, the other end of the three-phase power supply is connected to one end of a negative temperature coefficient resistor, the other end of the negative temperature coefficient resistor is connected to one end of a zero-sequence current transformer, and the other end of the zero-sequence current transformer is grounded;

[0053] In this embodiment, when installing the zero-sequence current transformer, it should be accurately installed between the negative temperature coefficient resistor and the ground to ensure that the change of the zero-sequence current can be accurately detected; the installation position of the circuit breaker should be selected at a key branch node or a cut-off node of the main line according to the topological structure of the system, so that the fault line can be quickly isolated when a trip command is received;

[0054] When a single-phase ground short circuit fault occurs in the system, the fault current flows through the negative temperature coefficient resistor. The temperature of the negative temperature coefficient resistor increases and the resistance value decreases. The zero-sequence current of the system increases. The zero-sequence current transformer monitors the zero-sequence current. When the zero-sequence current reaches the preset protection threshold of the zero-sequence current transformer, the circuit breaker opens to cut off the fault current.

[0055] As a preferred implementation of this embodiment, the calculation formula for the rated voltage of the negative temperature coefficient resistor is:

[0056]

[0057] Among them: U R Indicates the rated voltage of negative temperature coefficient resistor; U N Indicates the rated line voltage of the system.

[0058] As a preferred implementation of this embodiment, when the system is operating normally, the calculation formula for the resistance value of the negative temperature coefficient resistor (temperature is 25° C.) is:

[0059]

[0060] Where: R represents the resistance value of the negative temperature coefficient resistor during normal operation; IR represents the resistive current initially flowing through the negative temperature coefficient resistor during a fault;

[0061] I R =KI C

[0062] Where: K represents the ratio of the resistance current to the capacitance current when the system is single-phase to ground disconnected (can be 1.1 to 2); I C Indicates the capacitive current when a single phase of the system is short-circuited to ground.

[0063] As a preferred implementation of this embodiment, when a single-phase ground short circuit fault occurs in the system, the instantaneous power calculation formula of the negative temperature coefficient resistor is:

[0064]

[0065] Where: PR represents the instantaneous power of the negative temperature coefficient resistor when a single-phase ground short circuit fault occurs in the system.

[0066] As a preferred implementation of this embodiment, when a single-phase ground short circuit fault occurs in the system, the energy value consumed by the negative temperature coefficient resistor is calculated as follows:

[0067] W R =∫P R dt=c×m×ΔT

[0068] Where: W R It represents the energy consumed by the negative temperature coefficient resistor when a single-phase ground short circuit fault occurs in the system; c represents the specific heat capacity of the negative temperature coefficient resistor material; m represents the mass of the negative temperature coefficient resistor material; ΔT represents the temperature rise of the negative temperature coefficient resistor material.

[0069] As a preferred implementation of this embodiment, when a single-phase ground short circuit fault occurs in the system, the resistance value calculation formula of the negative temperature coefficient resistor is:

[0070] R d =f(T d )

[0071] Where: R d It indicates the resistance value of the negative temperature coefficient resistor when a single-phase ground short circuit occurs in the system; T d It indicates the temperature of the negative temperature coefficient resistor when a single-phase ground short circuit fault occurs in the system; f indicates the relationship function between temperature and resistance;

[0072] In this embodiment, the relationship function between temperature and resistance is shown as follows:

[0073]

[0074] Where: B represents negative temperature coefficient;

[0075] As a preferred implementation of this embodiment, when a single-phase ground short circuit fault occurs in the system, the temperature calculation formula of the negative temperature coefficient resistor is:

[0076] T d =ΔT+T

[0077] Where: T represents the initial temperature of the negative temperature coefficient resistor.

[0078] As a preferred implementation of this embodiment, when a single-phase ground short circuit fault occurs, the calculation formula of the system current is:

[0079]

[0080] Where: I d Indicates the system current when a single-phase ground short circuit fault occurs.

[0081] In the embodiments of the present application, "at least one" refers to one or more, and "more than one" refers to two or more. "And / or" describes the association relationship of associated objects, indicating that three relationships may exist. For example, A and / or B can represent the existence of A alone, the existence of A and B at the same time, and the existence of B alone. Among them, A and B can be singular or plural. The character " / " generally indicates that the previous and next associated objects are in an "or" relationship. "At least one of the following" and similar expressions refer to any combination of these items, including any combination of single or plural items. For example, at least one of a, b and c can be represented by: a, b, c, a and b, a and c, b and c, or a and b and c, where a, b, c can be single or multiple.

[0082] Those of ordinary skill in the art will appreciate that the various units and algorithm steps described in the embodiments disclosed herein can be implemented in a combination of electronic hardware, computer software, and electronic hardware. Whether these functions are performed in hardware or software depends on the specific application and design constraints of the technical solution. Professional and technical personnel can use different methods to implement the described functions for each specific application, but such implementation should not be considered to be beyond the scope of this application.

[0083] Those skilled in the art can clearly understand that, for the convenience and brevity of description, the specific working processes of the systems, devices and units described above can refer to the corresponding processes in the aforementioned method embodiments and will not be repeated here.

[0084] In several embodiments provided in the present application, if any function is implemented in the form of a software functional unit and sold or used as an independent product, it can be stored in a computer-readable storage medium. Based on this understanding, the technical solution of the present application, or the part that contributes to the prior art or the part of the technical solution, can be embodied in the form of a software product, which is stored in a storage medium and includes several instructions for a computer device (which can be a personal computer, a server, or a network device, etc.) to perform all or part of the steps of the method described in each embodiment of the present application. The aforementioned storage medium includes: U disk, mobile hard disk, read-only memory (Read-Only Memory; hereinafter referred to as: ROM), random access memory (Random Access Memory; hereinafter referred to as: RAM), disk or optical disk, and other media that can store program codes.

[0085] The above descriptions are merely embodiments of the present invention and are not intended to limit the patent scope of the present invention. Any equivalent structure or equivalent process transformation made using the contents of the present invention specification and drawings, or directly or indirectly applied in other related technical fields, are also included in the patent protection scope of the present invention.

Claims

1. A neutral point grounding system based on negative temperature coefficient resistor, characterized in that: It includes a three-phase power supply, a negative temperature coefficient resistor, a circuit breaker and a zero-sequence current transformer; One end of the three-phase power supply is connected to one end of the circuit breaker, the other end of the circuit breaker is connected to the power load end, the other end of the three-phase power supply is connected to one end of a negative temperature coefficient resistor, the other end of the negative temperature coefficient resistor is connected to one end of a zero-sequence current transformer, and the other end of the zero-sequence current transformer is grounded; When a single-phase ground short circuit fault occurs in the system, the fault current flows through the negative temperature coefficient resistor. The temperature of the negative temperature coefficient resistor increases and the resistance value decreases. The zero-sequence current of the system increases. The zero-sequence current transformer monitors the zero-sequence current. When the zero-sequence current reaches the preset protection threshold of the zero-sequence current transformer, the circuit breaker opens to cut off the fault current.

2. A neutral point grounding system based on negative temperature coefficient resistor according to claim 1, characterized in that: The formula for calculating the rated voltage of a negative temperature coefficient resistor is: Among them: U R Indicates the rated voltage of negative temperature coefficient resistor; U N Indicates the rated line voltage of the system.

3. A neutral point grounding system based on negative temperature coefficient resistor according to claim 2, characterized in that: When the system is operating normally, the calculation formula for the resistance value of the negative temperature coefficient resistor is: Where: R represents the resistance value of the negative temperature coefficient resistor during normal operation; I R It indicates the resistive current initially flowing through the negative temperature coefficient resistor during a fault; I R =TO C Where: K represents the ratio of the resistance current to the capacitance current when the system is single-phase to ground disconnected; I C Indicates the capacitive current when a single phase of the system is short-circuited to ground.

4. A neutral point grounding system based on negative temperature coefficient resistor according to claim 3, characterized in that: When a single-phase ground short circuit fault occurs in the system, the instantaneous power calculation formula of the negative temperature coefficient resistor is: Where: P R It represents the instantaneous power of the negative temperature coefficient resistor when a single-phase ground short circuit fault occurs in the system.

5. A neutral point grounding system based on negative temperature coefficient resistor according to claim 4, characterized in that: When a single-phase ground short circuit fault occurs in the system, the energy consumed by the negative temperature coefficient resistor is calculated as follows: W R =∫P R dt=c×m×ΔT Where: W R It represents the energy consumed by the negative temperature coefficient resistor when a single-phase ground short circuit fault occurs in the system; c represents the specific heat capacity of the negative temperature coefficient resistor material; m represents the mass of the negative temperature coefficient resistor material; ΔT represents the temperature rise of the negative temperature coefficient resistor material.

6. A neutral point grounding system based on negative temperature coefficient resistor according to claim 5, characterized in that: When a single-phase grounding short circuit fault occurs in the system, the calculation formula for the resistance value of the negative temperature coefficient resistor is: R d =f(T d ) Where: R d It indicates the resistance value of the negative temperature coefficient resistor when a single-phase ground short circuit occurs in the system; T d It represents the temperature of the negative temperature coefficient resistor when a single-phase ground short circuit fault occurs in the system; f represents the relationship function between temperature and resistance.

7. A neutral point grounding system based on negative temperature coefficient resistor according to claim 6, characterized in that: When a single-phase grounding short circuit fault occurs in the system, the temperature calculation formula of the negative temperature coefficient resistor is: T d =ΔT+T Where: T represents the initial temperature of the negative temperature coefficient resistor.

8. A neutral point grounding system based on negative temperature coefficient resistor according to claim 7, characterized in that: When a single-phase ground short circuit fault occurs, the calculation formula of the system zero-sequence current is: Where: I d Indicates the system zero-sequence current when a single-phase ground short circuit fault occurs.

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