Grounding device capable of flexibly switching neutral points of ultrahigh-voltage transformer and grounding method

Abstract

The invention belongs to the technical field of ultrahigh-voltage power systems, and discloses a grounding device capable of flexibly switching neutral points of an ultrahigh-voltage transformer and a grounding method. The device is connected in series between a neutral point of a transformer and the ground, a main loop of the device integrates three parallel branches, namely a direct grounding branch, a small reactor grounding branch and a capacitor blocking branch, and each branch is controlled by an independent high-speed isolation disconnecting link. The electrical quantity of the neutral point is monitored in real time through the high-precision sensor, and the intelligent controller automatically controls the switching of each branch according to the criteria such as short-circuit current level, direct-current magnetic bias, system fault and the like or remote instructions. Therefore, safe and fast switching among various modes such as direct grounding, grounding through small reactance, composite grounding with a direct current magnetic bias suppression function and the like is realized. The problem that a traditional fixed grounding mode cannot dynamically adapt to variable requirements of a power grid is solved, intelligence and flexibility of the grounding mode are achieved, and the operation safety, flexibility and economical efficiency of the ultra-high voltage power grid are improved.

Description

Technical Field

[0001] This invention belongs to the field of high-voltage power system technology, specifically relating to a flexible switchable neutral point grounding device and grounding method for ultra-high voltage transformers. Background Technology

[0002] In power systems with voltage levels of 500kV and above, the grounding method of the transformer neutral point is crucial to the safe, stable, and economical operation of the system. Currently, the mainstream grounding methods and their limitations are as follows: Direct grounding: Advantages include effectively limiting power frequency overvoltage, protecting equipment insulation, and simple relay protection. Disadvantages include extremely high single-phase grounding short-circuit current, stringent requirements on the breaking capacity of equipment such as isolating switches, and susceptibility to DC bias magnetism, leading to transformer vibration and overheating.

[0003] Grounding via a small reactor: Inserting a small impedance reactor in series into the direct grounding circuit can effectively reduce single-phase short-circuit current by 20%-50%, relieving equipment stress. It is currently the mainstream measure to suppress excessive short-circuit current. However, it cannot suppress DC bias magnetization, and its ability to limit overvoltage is slightly weaker than that of direct grounding.

[0004] Grounding via a capacitive DC bias suppression device: The core of this device is a large-capacity DC blocking capacitor, which exhibits high resistance to DC, effectively blocking the DC component that causes transformer bias; it exhibits low resistance to power frequency, not affecting power frequency grounding performance. However, in the event of a single-phase ground fault in the system, an extremely high power frequency voltage will be generated across the capacitor, requiring a fast bypass protection system.

[0005] The shortcomings of existing technologies: Current substations typically select a fixed grounding method and install corresponding equipment based on anticipated conditions during the power grid planning phase. However, the operating conditions of the power grid are dynamic: for example, changes in the grid structure lead to variations in short-circuit current levels; the occurrence of high-voltage direct current (HVDC) unipolar operation or geomagnetic storms is intermittent. Fixed grounding methods cannot adapt to these variable operational demands, either enduring long-term short-circuit current pressure or being unable to suppress bias magnetization when necessary, lacking flexibility and adaptability.

[0006] Therefore, there is an urgent need for an intelligent device and method that can safely, reliably, and quickly switch between multiple grounding modes based on real-time system status to solve the problems existing in the prior art. Summary of the Invention

[0007] The technical problem this invention aims to solve is to overcome the shortcomings of existing fixed neutral point grounding methods for transformers, which cannot dynamically adapt to the changing needs of the system. This invention provides a flexible, switchable neutral point grounding device for ultra-high voltage transformers. This grounding device integrates direct grounding, low-reactor grounding, and DC bias suppression functions, and can intelligently switch between these functions based on system commands or automatic criteria. The specific technical solution of this invention is as follows: A flexible switchable neutral point grounding device for ultra-high voltage transformers is connected in series between the transformer neutral point and the grounding grid. The flexible switching ultra-high voltage transformer neutral point grounding device includes: a main circuit unit and a measurement and control unit; The main circuit unit includes at least three parallel electrical branches, which include a directly grounded branch, a small reactance branch, and a capacitor-blocking DC branch; each branch is equipped with an independent controllable switching device; The measurement and control unit is used to monitor the electrical quantity of the neutral point in real time, and control the opening and closing of the controllable switching device according to preset criteria or remote commands, so as to realize the switching of the flexible switchable ultra-high voltage transformer neutral point grounding device between different grounding modes.

[0008] Preferably, the direct grounding branch includes a first high-speed isolating switch; The small reactor branch includes a second high-speed isolating switch and a small reactor connected in series; The capacitor DC blocking branch includes a third high-speed isolation switch and a DC blocking capacitor connected in series. After the three electrical branches are connected in parallel, one end is connected to the neutral point lead of the transformer, and the other end is connected to the grounding grid.

[0009] Preferably, the small reactor is an adjustable reactor, or multi-level adjustment can be achieved by switching parallel sub-reactors.

[0010] Preferably, the measurement and control unit includes a current sensor, a voltage sensor, and an intelligent controller; Current sensor, used to measure the power frequency RMS value and DC component of neutral point current; Voltage sensor used to measure the voltage of the neutral point to ground; The intelligent controller receives sensor signals and has built-in multiple sets of automatic switching logic, including short-circuit current over-limit judgment, DC bias over-limit judgment, and system fault judgment.

[0011] Preferably, the DC bias limit criterion is: when the DC current component at the neutral point continuously exceeds the set threshold T1, the capacitor DC blocking branch is automatically activated; the threshold T1 is 5-15A.

[0012] Preferably, the system fault criterion is as follows: when the neutral point power frequency current or voltage is detected to rise sharply and exceed the fault threshold in a very short time, the direct grounding branch is closed first to provide a low impedance path for the fault current; the very short time is less than 5ms.

[0013] Preferably, the grounding mode includes at least: Mode 1: Close only the direct grounding branch to achieve direct grounding; Mode 2: Only close the small reactor branch to achieve grounding through the small reactor; Mode 3: Simultaneously close the small reactor branch and the capacitor DC blocking branch to achieve grounding through the small reactor and suppress DC bias.

[0014] A grounding method, employing the aforementioned flexibly switchable ultra-high voltage transformer neutral point grounding device, includes the following steps: Real-time monitoring of transformer neutral point current and voltage; Analyze monitoring data to determine whether the current system status meets the preset switching conditions; If the conditions are met, a switching command is generated, and the corresponding controllable switching devices are operated according to the preset sequence of control logic to switch the device from the current grounding mode to the target grounding mode.

[0015] Preferably, when switching from high impedance mode to low impedance mode, the logic of switching on first and then switching off is adopted to ensure grounding continuity; when a system fault occurs, the direct grounding branch with the lowest impedance is switched on first and quickly.

[0016] The working process and beneficial effects of applying the technical solution of this invention are as follows: The flexible switching neutral grounding device for ultra-high voltage transformers of the present invention has the characteristics of dynamic adaptation and multi-functionality. Specifically, one device replaces the original single-function equipment and can switch between multiple grounding modes according to the real-time needs of the power grid, realizing multiple operating states such as "direct grounding", "small reactance grounding", and "small reactance grounding + DC isolation", which greatly improves the flexibility and adaptability of substations.

[0017] The flexible switching neutral grounding device for ultra-high voltage transformers of this invention features intelligent decision-making and high reliability. Specifically, through real-time monitoring and intelligent criteria, the device can automatically identify system risks (such as large short-circuit current or DC bias) and switch to the optimal grounding mode, transforming passive defense into active adaptation. The rapid bypass logic in fault conditions ensures the safety of the capacitors and the system.

[0018] The flexible switching neutral grounding device for ultra-high voltage transformers of the present invention has the characteristics of saving investment and space. Specifically, it avoids the trouble of installing multiple fixed devices or subsequent modifications to cope with different working conditions, thus saving equipment costs, land resources and installation engineering.

[0019] The flexible switching ultra-high voltage transformer neutral point grounding device of the present invention has the characteristics of being easy to dispatch and manage. Specifically, it provides power grid dispatchers with a flexible means to adjust the grounding operation mode, becoming an important active control node in the smart grid. Attached Figure Description

[0020] Figure 1 This is a schematic diagram of the transformer neutral point grounding principle in an embodiment of the present invention; Figure 2 This is a plan view of the transformer neutral point grounding arrangement in an embodiment of the present invention; Figure 3 This is a diagram of the transformer neutral point grounding connection in an embodiment of the present invention; Wherein: K1-first high-speed isolating switch, K2-second high-speed isolating switch, K3-third high-speed isolating switch, L-small reactor, C-DC blocking capacitor, G-main current branch isolating switch, CT / PT-current / voltage sensor, Controller-intelligent controller. Detailed Implementation

[0021] Example: A flexible switching neutral point grounding device for ultra-high voltage transformers (hereinafter referred to as the "device") is specifically a composite grounding device used at the neutral point of ultra-high voltage, extra-high voltage transformers, or shunt reactors, capable of intelligently switching between multiple grounding modes according to system conditions. See details. Figures 1-3 The details are as follows: This flexible switchable ultra-high voltage transformer neutral point grounding device is connected in series between the transformer neutral point and the grounding grid, and it includes a main circuit unit and a measurement and control unit.

[0022] The main circuit unit adopts a modular, bypass design, including: Main current-carrying branch: Composed of mechanical disconnect switches or grounding switches, serving as mechanical protection nodes and maintenance isolation points for the device.

[0023] The direct grounding branch includes a first high-speed isolating switch K1. Preferably, the direct grounding branch is connected in parallel with the main current-carrying branch and consists of a high-speed vacuum isolating switch or an SF6 circuit breaker. When this branch is closed, the device is in direct grounding mode.

[0024] The small reactor branch includes a second high-speed isolating switch K2 connected in series and a small reactor L. Preferably, the small reactor branch is connected in parallel with the direct grounding branch, and in series with a small reactor with adjustable rated value and a high-speed vacuum isolating switch. When the direct grounding branch is open and this branch is closed, the device is in a grounding mode via the small reactor. More preferably, the small reactor L is a reactor with adjustable reactance, or multi-level adjustment is achieved by switching parallel sub-reactors.

[0025] The capacitor-based DC blocking branch includes a third high-speed isolating switch K3 connected in series and a DC blocking capacitor C. Preferably, the capacitor-based DC blocking branch is connected in parallel with the small reactor branch, and in series with a large-capacity dry-type or oil-immersed DC blocking capacitor and a high-speed vacuum circuit breaker.

[0026] After the three electrical branches are connected in parallel, one end is connected to the neutral point lead of the transformer, and the other end is connected to the grounding grid. Further explanation: The electrical branches can be switched independently or connected in series with a small reactor branch. When only this branch is connected, it exhibits a capacitor-grounded mode (mainly used for testing and specific protection logic); when connected in series with a disconnected small reactor branch, the capacitor provides high impedance for DC, and the small reactor provides a defined impedance for power frequency, together achieving a composite mode of "grounded through a small reactor and suppressing DC bias".

[0027] The measurement and control unit is the brain of the device, and it includes a current sensor, a voltage sensor, and an intelligent controller, as detailed below: The current sensor is used to measure the power frequency RMS value and DC component of the neutral point current; the voltage sensor is used to measure the neutral point voltage to ground; the intelligent controller receives sensor signals and has built-in multiple sets of automatic switching logic, including short-circuit current over-limit criteria, DC bias over-limit criteria, and system fault criteria.

[0028] In this embodiment, both the current sensor and the voltage sensor are high-precision sensors used to monitor the neutral point current (including power frequency components and DC and harmonic components from 0.5Hz to 100Hz), the neutral point-to-ground voltage, and the temperature of the neutral point grounding device of the ultra-high voltage transformer in real time.

[0029] Intelligent Controller: Its core is a high-performance processor with multiple built-in switching criterion logics. The intelligent controller receives sensor signals and incorporates multiple sets of automatic switching logics, including short-circuit current over-limit criteria, DC bias over-limit criteria, and system fault criteria. Short-circuit current exceeding the limit criterion: If the system short-circuit current level calculated by the receiving dispatch master station or locally exceeds the set threshold T1, the system will automatically switch from "direct grounding" to "small reactance grounding" mode. Preferably, the threshold T1 is 5-15A.

[0030] DC bias limit criterion: When the DC current at the neutral point continuously exceeds the transformer's withstand threshold (e.g., 10A), the system automatically switches from the current mode to the "grounded via small reactor and suppressing DC bias" mode.

[0031] Overvoltage protection criteria: When a power frequency overvoltage that may endanger the capacitor is detected, or when a single-phase ground fault occurs in the system, the controller can issue a command within milliseconds (<5ms) to forcibly connect the "direct grounding branch" or "small reactor branch" to provide a reliable path for the fault current and bypass the capacitor for protection.

[0032] Remote / Local Command Reception: Can receive mode switching commands issued remotely or manually on-site.

[0033] In this preferred embodiment, the grounding mode includes at least: Mode 1: Close only the direct grounding branch to achieve direct grounding; Mode 2: Only close the small reactor branch to achieve grounding through the small reactor; Mode 3: Simultaneously close the small reactor branch and the capacitor DC blocking branch to achieve grounding through the small reactor and suppress DC bias.

[0034] A grounding method using the above-mentioned device specifically includes the following steps: Real-time monitoring of transformer neutral point current and voltage; Analyze monitoring data to determine whether the current system status meets the preset switching conditions; If the conditions are met, a switching command is generated, and the corresponding controllable switching devices are operated according to the preset sequence of control logic to switch the device from the current grounding mode to the target grounding mode.

[0035] Preferably, when switching from high impedance mode to low impedance mode, the logic of switching on first and then switching off is adopted to ensure grounding continuity; when a system fault occurs, the direct grounding branch with the lowest impedance is switched on first and quickly.

[0036] Application Case 1 (Normal State Transition): A 500kV substation initially used direct grounding. With the development of the surrounding power grid, the calculated short-circuit current approached the circuit breaker's breaking limit. The dispatch master station sent a command to this device. Upon receiving the "switch to small reactor grounding mode" command, the intelligent controller executed sequential control: first, it closed the isolating switch (K2) of the small reactor branch, and then it opened the isolating switch (K1) of the directly grounded branch. The entire process was completed within 100ms, and the system's zero-sequence impedance changed smoothly without causing any protection maloperation. At this time, the device operated in mode two, effectively reducing the short-circuit current level.

[0037] Application Case 2 (Automatic Suppression of DC Bias): The device operates in "small reactance grounding" mode (K2 closed, K1 and K3 open). When the high-voltage direct current transmission system starts operating on the monopolar return line, the neutral point sensor detects a continuous 8A DC component. Based on the "DC bias limit criterion" (threshold 5A), the intelligent controller automatically initiates the switching process: first, it closes the capacitor-blocking DC branch isolator (K3), at which point the capacitor C is connected in series with the small reactance L and grounded. The DC current is blocked by the high-impedance capacitor, while the power frequency impedance is still determined by the small reactance L, achieving the dual objectives of DC suppression and short-circuit current limitation.

[0038] Application Case 3 (Fault Protection Action): The device operates in "small reactance grounding + DC isolation" mode (K2 and K3 closed). A single-phase ground fault occurs in the system, causing a surge in neutral point current and voltage. The controller detects the fault characteristics within 2ms using a high-speed PT / CT and immediately initiates the protection logic: synchronously issuing a command to close the isolating switch (K1) of the directly grounded branch. After K1 closes, the fault current mainly flows through the extremely low impedance K1 branch, effectively bypassing the series-connected L and C branches, preventing the capacitors from being damaged by overvoltage, and ensuring reliable conduction of the fault current, thus enabling the line protection to operate correctly.

Claims

1. A flexible switching neutral point grounding device for ultra-high voltage transformers, characterized in that, This flexible switchable ultra-high voltage transformer neutral point grounding device is connected in series between the transformer neutral point and the grounding grid; The flexible switching ultra-high voltage transformer neutral point grounding device includes: a main circuit unit and a measurement and control unit; The main circuit unit includes at least three parallel electrical branches, which include a directly grounded branch, a small reactance branch, and a capacitor-blocking DC branch; each branch is equipped with an independent controllable switching device; The measurement and control unit is used to monitor the electrical quantity of the neutral point in real time, and control the opening and closing of the controllable switching device according to preset criteria or remote commands, so as to realize the switching of the flexible switchable ultra-high voltage transformer neutral point grounding device between different grounding modes.

2. The flexible switching ultra-high voltage transformer neutral point grounding device according to claim 1, characterized in that, The direct grounding branch includes a first high-speed isolating switch (K1). The small reactor branch includes a second high-speed isolating switch (K2) and a small reactor (L) connected in series. The capacitor DC blocking branch includes a third high-speed isolating switch (K3) and a DC blocking capacitor (C) connected in series. After the three electrical branches are connected in parallel, one end is connected to the neutral point lead of the transformer, and the other end is connected to the grounding grid.

3. The flexible switching ultra-high voltage transformer neutral point grounding device according to claim 2, characterized in that, The small reactor (L) is a reactor with adjustable reactance, or it can be adjusted in multiple levels by switching parallel sub-reactors.

4. The flexible switching ultra-high voltage transformer neutral point grounding device according to claim 1, characterized in that, The measurement and control unit includes a current sensor, a voltage sensor, and an intelligent controller; A current sensor is used to measure the power frequency RMS value and DC component of the neutral point current. Voltage sensor used to measure the voltage of the neutral point to ground; The intelligent controller receives sensor signals and has built-in multiple sets of automatic switching logic, including short-circuit current over-limit judgment, DC bias over-limit judgment, and system fault judgment.

5. The flexible switching ultra-high voltage transformer neutral point grounding device according to claim 4, characterized in that, The DC bias limit criterion is as follows: when the DC current component at the neutral point continuously exceeds the set threshold T1, the capacitor DC blocking branch is automatically activated; the threshold T1 is 5-15A.

6. The flexible switching ultra-high voltage transformer neutral point grounding device according to claim 4, characterized in that, The system fault criterion is as follows: when the neutral point power frequency current or voltage is detected to rise sharply and exceed the fault threshold in a very short time, the direct grounding branch is closed first to provide a low impedance path for the fault current; the very short time is less than 5ms.

7. The flexible switching ultra-high voltage transformer neutral point grounding device according to any one of claims 1-6, characterized in that, The grounding mode includes at least: Mode 1: Close only the direct grounding branch to achieve direct grounding; Mode 2: Only close the small reactor branch to achieve grounding through the small reactor; Mode 3: Simultaneously close the small reactor branch and the capacitor DC blocking branch to achieve grounding through the small reactor and suppress DC bias.

8. A grounding method, characterized in that, Grounding using the flexible switchable ultra-high voltage transformer neutral point grounding device as described in any one of claims 1-7 includes the following steps: Real-time monitoring of transformer neutral point current and voltage; Analyze monitoring data to determine whether the current system status meets the preset switching conditions; If the conditions are met, a switching command is generated, and the corresponding controllable switching devices are operated according to the preset sequence of control logic to switch the device from the current grounding mode to the target grounding mode.

9. The grounding method according to claim 8, characterized in that, When switching from high impedance mode to low impedance mode, the logic of switching on first and then switching off is adopted to ensure grounding continuity; when a system fault occurs, the direct grounding branch with the lowest impedance is switched on first and quickly.

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