Inrush current suppression method based on RC loop and phase control closing

Abstract

The invention relates to the technical field of power system relay protection, in particular to an inrush current suppression method based on an RC loop and phase control closing, which comprises the following steps: connecting a pre-magnetizing branch formed by connecting a pre-magnetizing contactor, a current limiting resistor and an energy storage capacitor in series into two ends of a primary side winding of a main transformer in parallel; a pre-magnetizing contactor is controlled to be closed, and current is injected into a winding of the main transformer through a current limiting resistor and an energy storage capacitor so as to establish pre-magnetizing flux; collecting a current signal of the pre-magnetizing branch and voltage signals at two ends of the energy storage capacitor, determining a current change rate according to the current signal so as to determine an impedance state of a power supply loop system of the main transformer, and determining a closing mode according to the impedance state; according to the impedance state, the voltage signal and the resistance value of the current-limiting resistor, the residual magnetism and the residual magnetic polarity of the iron core in the main transformer are determined, then the closing time is determined, and after the closing action of the main circuit breaker is completed, the pre-magnetizing contactor is disconnected. According to the invention, efficient suppression of the magnetizing inrush current of the main transformer can be realized.

Description

Technical Field

[0001] This invention relates to the field of power system relay protection technology, and in particular to an inrush current suppression method based on RC circuit and phase-controlled closing. Background Technology

[0002] In power systems, transformers are the core equipment for power transmission and distribution. When a transformer is switched on under no-load conditions or when power is restored after an external fault is cleared, the iron core is prone to entering a deep saturation state due to a sudden change in magnetic flux, resulting in an inrush current with an amplitude that can be several to tens of times the rated current. This inrush current can not only cause relay protection devices to malfunction, but may also cause mechanical stress damage to the transformer windings and reduce the stability of the power grid. Therefore, the suppression technology of transformer inrush current is one of the key research focuses in the field of power systems.

[0003] The mainstream inrush current suppression methods in the industry mainly include the following categories: (1) Connecting a small-capacity pre-magnetized transformer in series in the transformer substation line, and putting the pre-magnetized transformer into operation before the main transformer is closed, so that the main transformer winding establishes a steady-state magnetic flux in advance, thereby reducing the peak value of the inrush current after closing. However, this method requires an additional small-capacity transformer in series, which is costly and has limited inrush current suppression effect; (2) By detecting the phase of the grid voltage, controlling the transformer to close at a specific phase of the voltage waveform (usually the voltage peak or the magnetic flux passing through zero point), so as to minimize the amplitude of the core magnetic flux change, thereby reducing the generation of excitation inrush current. This method requires extremely high accuracy in voltage phase detection. Once the grid voltage fluctuates or the closing action is delayed, the suppression effect will drop significantly; (3) Connecting an absorption circuit composed of a resistor (R) and a capacitor (C) in parallel at both ends of the transformer winding, using the impedance characteristics of the RC element to consume the inrush current energy, and suppressing the transient overvoltage of the winding, thereby indirectly reducing the inrush current amplitude. The RC circuit can only passively consume the inrush current energy, and the suppression effect is affected by the matching degree of the RC parameters.

[0004] Therefore, there is an urgent need for an inrush current suppression method that can effectively suppress excitation inrush current while being low in cost and having less stringent requirements for phase detection accuracy. Summary of the Invention

[0005] Therefore, this invention provides an inrush current suppression method based on RC circuit and phase-controlled closing, which overcomes the problems of high cost, unstable suppression effect, stringent requirements for detection accuracy or poor adaptability of existing transformer excitation inrush current suppression methods.

[0006] To achieve the above objectives, the present invention provides an inrush current suppression method based on RC circuit and phase-controlled closing, comprising: The pre-magnetized branch, which is formed by the pre-magnetized contactor, the current-limiting resistor and the energy storage capacitor connected in series, is connected in parallel to both ends of the primary winding of the main transformer. The pre-charge contactor is controlled to close, and current is injected into the windings of the main transformer through the current-limiting resistor and the energy storage capacitor to establish pre-charge flux; The current signal of the pre-magnetized branch and the voltage signal across the energy storage capacitor are collected. The current change rate is determined based on the current signal to determine the impedance state of the main transformer power supply circuit system, and the closing method is determined based on the impedance state. The magnitude and polarity of the residual magnetism of the core inside the main transformer are determined based on the current signal, the voltage signal, and the resistance value of the current limiting resistor, and the timing of closing the circuit is determined based on the magnitude and polarity of the residual magnetism. The circuit breaker is closed according to the closing method and the closing timing. After the main circuit breaker closes, the pre-charged contactor is disconnected.

[0007] As a preferred method for inrush current suppression based on RC circuits and phase-controlled closing, the process of establishing pre-charge flux includes: The peak steady-state magnetic flux of the main transformer when it is closed under no-load conditions is determined based on the rated parameters of the main transformer. The expected closing time of the pre-charged magnetic contactor is determined based on the steady-state magnetic flux peak value, resistance value, and rated voltage of the energy storage capacitor. The rated parameters include rated voltage and rated frequency.

[0008] As a preferred option for inrush current suppression methods based on RC circuits and phase-controlled closing, the impedance state of the main transformer power supply circuit system is determined, including: The rate of change of current is determined based on the current signal and the preset sampling period; The impedance state is determined based on the current change rate and the preset change rate threshold. The impedance states include low impedance states and high impedance states.

[0009] As a preferred embodiment of the inrush current suppression method based on RC circuit and phase-controlled closing, the closing mode is determined according to the impedance state, including: In response to the high impedance state, the main circuit breaker is controlled to complete the closing action in one go. In response to the impedance state being low impedance, the main circuit breaker is controlled to first close to a preset travel position and remain there for a preset duration before fully closing.

[0010] As a preferred method for inrush current suppression based on RC circuits and phase-controlled closing, the magnitude and polarity of the residual magnetism of the main transformer core are determined, including: The current curve and voltage curve are established based on the current signal of the pre-charged magnetic branch and the voltage signal across the energy storage capacitor, respectively. The total change in magnetic flux of the core inside the main transformer during the pre-magnetization process is determined based on the current curve, voltage curve, expected closing time, resistance value of the current limiting resistor, and number of turns of the primary winding of the main transformer. The magnitude and polarity of remanence are determined based on the total change in magnetic flux.

[0011] As a preferred embodiment of the inrush current suppression method based on RC circuit and phase-controlled closing, the grid voltage signal on the incoming side of the main circuit breaker is obtained to determine the voltage zero-crossing point. The closing timing is determined based on the voltage zero-crossing point and a comparison between the residual magnetism polarity and the magnetic flux polarity corresponding to the main grid voltage, including: In response to the fact that the residual magnetism polarity is the same as the flux polarity of the main grid voltage, the closing is delayed after the voltage zero-crossing point. In response to the result that the magnetic flux polarity of the residual magnetism is opposite to that of the main grid voltage, the circuit is closed in advance before the voltage crosses zero. The closing delay time and the closing advance time are both proportional to the magnitude of the residual magnetism.

[0012] As a preferred embodiment of the inrush current suppression method based on RC circuit and phase-controlled closing, after the main circuit breaker closing operation is completed, the pre-magnetizing branch exits control including: After the main circuit breaker completes closing, the time delay relay is activated to keep the pre-magnetized branch in a closed state. While the pre-magnetized branch remains closed, the excitation current of the main transformer is detected; The timing for disconnecting the pre-charged contactor is determined based on the excitation current.

[0013] As a preferred embodiment of the inrush current suppression method based on RC circuit and phase-controlled closing, during the pre-magnetizing process, the current signal of the pre-magnetizing branch and the voltage signal across the energy storage capacitor are monitored in real time. Based on the comparison result of the current signal with a preset overcurrent threshold and / or the comparison result of the voltage signal with a preset undervoltage threshold, the following operations are performed: In response to a current signal greater than the preset overcurrent threshold and / or a voltage signal lower than the preset undervoltage threshold, the pre-magnetizing contactor is immediately disconnected, and pre-magnetizing is stopped.

[0014] As a preferred embodiment of the inrush current suppression method based on RC circuit and phase-controlled closing, the peak inrush current and corresponding pre-magnetization parameters during each closing process are recorded and a sample database is constructed. The offset between the actual magnetization curve and the standard magnetization curve is determined based on the sample database, and the expected closing time is corrected based on the offset. The pre-magnetization parameters include the pre-magnetization duration, the resistance value of the current-limiting resistor, the rated voltage of the energy storage capacitor, the core temperature of the main transformer, and the service life.

[0015] As a preferred embodiment of the inrush current suppression method based on RC circuit and phase-controlled closing, correcting the expected closing time according to the offset includes: The magnetic property aging coefficient is determined based on the offset. The steady-state magnetic flux peak value is corrected in real time according to the magnetic property aging coefficient to obtain the steady-state corrected magnetic flux peak value. The expected closure duration is corrected based on the peak steady-state correction magnetic flux.

[0016] Compared with existing technologies, the advantages of this invention lie in its ability to achieve precise linkage across the entire process of pre-magnetization, impedance identification, residual magnetism quantification, and closing timing by constructing a coordinated control mechanism between the RC pre-magnetization branch and phase-controlled closing. This eliminates the need for modifications to the main transformer, offering strong adaptability and low implementation costs. Furthermore, it uses pre-magnetic flux to offset residual magnetism and dynamically adjusts the closing method based on impedance status, fundamentally suppressing inrush current peaks. This effectively reduces the mechanical impact and electrical damage of inrush current on transformer windings and circuit breakers, reduces the risk of relay protection malfunctions, ensures stable and reliable operation during power system start-up and shutdown, and solves the problems of poor suppression effect and insufficient adaptability of traditional single control methods.

[0017] In particular, this invention determines the expected closing time of the pre-charge contactor based on the rated parameters of the main transformer and the parameters of the RC branch, thereby achieving quantitative control of the pre-charge flux. This avoids the problems of insufficient or excessive pre-charge caused by the traditional method of setting the pre-charge duration based on experience, ensuring that the pre-charge flux accurately matches the peak value of the transformer's steady-state flux.

[0018] In particular, this invention calculates the rate of change of current by using the current signal and a preset sampling period, and combines this with a dual-threshold impedance state to achieve real-time and accurate identification of system impedance. This solves the problem of poor adaptability of closing strategies caused by neglecting dynamic impedance changes in traditional methods. At the same time, this invention dynamically adjusts the closing method according to the impedance state, avoiding both the instantaneous surge caused by a one-time closing in a low impedance state and the response lag caused by step-by-step closing in a high impedance state.

[0019] In particular, this invention determines the total change in magnetic flux of the core inside the main transformer during pre-magnetization based on the current and voltage curves, the expected closing time, the resistance value of the current-limiting resistor, and the number of turns of the primary winding of the main transformer, thus achieving accurate identification of the magnitude and polarity of residual magnetism. This solves the technical problems of large estimation errors and the inability to directly measure residual magnetism in traditional methods, providing data support for the precise selection of closing timing.

[0020] In particular, this invention combines the zero-crossing point of the grid voltage with the polarity and magnitude of the residual magnetism to dynamically determine the closing timing. When the polarities are the same, the closing is delayed; when the polarities are opposite, the closing is advanced. Moreover, the duration is proportional to the magnitude of the residual magnetism. This achieves precise matching between the closing phase and the characteristics of the residual magnetism and the magnetic flux of the grid, effectively avoiding core saturation caused by magnetic flux superposition, and further reducing the peak inrush current from a phase perspective.

[0021] In particular, this invention maintains the pre-magnetized branch closed by using a time-delay relay after the main circuit breaker is closed, and determines the disconnection timing by combining excitation current detection. This solves the problem of secondary inrush current caused by the sudden drop in pre-magnetized flux due to the immediate disconnection of the branch in traditional circuits. Extending the effective operating time of the pre-magnetized branch ensures a stable transition of the core flux after the main circuit is closed.

[0022] In particular, this invention addresses the risk of equipment damage caused by branch faults and capacitor abnormalities during the pre-magnetization process by real-time monitoring of the pre-magnetization branch current and capacitor voltage and setting an overcurrent / undervoltage threshold trigger protection mechanism.

[0023] In particular, by constructing a sample database of inrush peak and pre-charge magnetization parameters, the present invention uses the magnetization curve offset to correct the expected closing time, so that the expected closing time can be adapted in real time to dynamic factors such as core aging and temperature changes, ensuring that the pre-charge magnetic flux always matches the actual magnetic characteristics, and further improving the inrush suppression effect. Attached Figure Description

[0024] Figure 1 This is a flowchart illustrating the inrush current suppression method based on RC circuit and phase-controlled closing according to an embodiment of the present invention. Figure 2 This is a wiring diagram of the 35kV collection system and main transformer according to an embodiment of the present invention; Figure 3 A flowchart illustrating the steps for determining the expected closing time of a pre-charged contactor according to an embodiment of the present invention; Figure 4 A flowchart illustrating the steps for determining the magnitude and polarity of the residual magnetism of the main transformer core in an embodiment of the present invention; Figure 5 A flowchart illustrating the steps for correcting the expected market in an embodiment of the present invention. Detailed Implementation

[0025] To make the objectives and advantages of the present invention clearer, the present invention will be further described below with reference to embodiments; it should be understood that the specific embodiments described herein are merely for explaining the present invention and are not intended to limit the present invention.

[0026] Preferred embodiments of the present invention will now be described with reference to the accompanying drawings. Those skilled in the art should understand that these embodiments are merely illustrative of the technical principles of the present invention and are not intended to limit the scope of protection of the present invention.

[0027] Please see Figure 1 and Figure 2 , Figure 1 This is a flowchart illustrating the inrush current suppression method based on RC circuit and phase-controlled closing according to an embodiment of the present invention. Figure 2 This is a wiring diagram of a 35kV collection system and main transformer according to an embodiment of the present invention. The diagram shows a pre-magnetized branch, formed by a pre-magnetized contactor (not shown), a current-limiting resistor, and an energy storage capacitor connected in series, connected in parallel to both ends of the primary winding of the main transformer. To effectively suppress inrush current, the present invention provides an inrush current suppression method based on an RC circuit and phase-controlled closing, including: Step S1: Connect the pre-magnetized branch, which is formed by the pre-magnetized contactor, the current-limiting resistor and the energy storage capacitor connected in series, in parallel to the two ends of the primary winding of the main transformer. Step S2: Control the pre-charge contactor to close, and inject current into the winding of the main transformer through the current-limiting resistor and the energy storage capacitor to establish pre-charge flux; Step S3: Collect the current signal of the pre-magnetized branch and the voltage signal across the energy storage capacitor; determine the current change rate based on the current signal to determine the impedance state of the main transformer power supply circuit system; and determine the closing mode based on the impedance state. Step S4: Determine the magnitude and polarity of the residual magnetism of the core in the main transformer based on the current signal, the voltage signal, and the resistance value of the current limiting resistor; and determine the closing timing based on the magnitude and polarity of the residual magnetism. Step S5: Perform closing according to the closing method and the closing timing. After the main circuit breaker closes, disconnect the pre-charged contactor.

[0028] In detail, traditional inrush current suppression methods often rely solely on phase-controlled closing or pre-magnetizing, which suffers from inaccurate residual magnetism identification and mismatch between closing timing and system impedance. This leads to unstable inrush current suppression effects, easily damaging equipment or causing protection malfunctions. Since the initial residual magnetism of the iron core cannot be measured, this invention uses a pre-magnetizing contactor, a current-limiting resistor, and an energy storage capacitor connected in series to form a pre-magnetizing branch. The pre-magnetizing contactor enables precise control of the pre-magnetizing branch's on / off state, the current-limiting resistor limits the peak pre-magnetizing current to prevent branch overload, and the energy storage capacitor provides a stable pre-magnetizing current, establishing a controllable pre-magnetizing flux. This allows for adaptation to existing power systems without altering the main circuit structure. By injecting controllable pre-magnetizing flux into the pre-magnetizing branch, the iron core flux transitions from the initial residual magnetism state to the pre-magnetizing flux state. The unknown residual magnetism is deduced from the known changes in pre-magnetizing flux, laying the magnetic circuit foundation for subsequent closing.

[0029] In detail, pre-magnetization is performed when the main circuit breaker is open. After the pre-magnetization contactor closes, a complete closed loop is formed, consisting of the energy storage capacitor, the current-limiting resistor, the transformer winding, and the energy storage capacitor. The energy storage capacitor discharges through this closed loop, converting the pre-stored electric field energy into electrical energy, which in turn generates a discharge current. Simultaneously, the voltage across the energy storage capacitor decreases due to the discharge, allowing the detection of the voltage signal of the energy storage capacitor in the loop. The current signal exhibits a negative correlation with the loop impedance. By collecting the current signal and analyzing the rate of current change, the real-time impedance state of the main transformer power supply circuit system can be deduced, thereby dynamically matching the closing mode. The voltage signal of the energy storage capacitor directly reflects its discharge process, and the discharge current is linearly related to the change in magnetic flux of the main transformer winding. Therefore, the voltage signal can indirectly characterize the dynamic change law of the core magnetic flux. The current-limiting resistor can serve as the core calculation basis. Combined with the impedance state, the magnitude and polarity of the residual magnetism in the main transformer core can be calculated to determine the optimal closing time, ensuring that the grid magnetic flux and the pre-magnetic flux achieve reverse cancellation or a smooth transition at the moment of closing, avoiding magnetic flux saturation caused by impedance mismatch, and accurately suppressing inrush current.

[0030] In this embodiment, the main transformer is a medium-high voltage power transformer connected to the power system to perform power transmission and voltage transformation functions. The energy source of the main circuit (i.e., the transformer) is the 35kV power grid. The energy source of the pre-magnetizing circuit is the pre-stored energy of the energy storage capacitor, which is a one-time discharge and does not depend on the power grid. The resistance value of the current limiting resistor and the capacitance value of the energy storage capacitor need to be determined based on various factors such as the voltage level, capacity, core material, and DC resistance of the winding of the main transformer. They can be determined based on several pre-magnetizing tests, so no specific limitation is made here.

[0031] In this embodiment, the target preset magnetic flux of the pre-charge flux is 60% to 80% of the peak value of the steady-state magnetic flux when the main transformer is closed under no-load conditions. Preferably, the target value of the pre-charge flux is 75% of the peak value of the steady-state magnetic flux when the main transformer is closed under no-load conditions.

[0032] This invention achieves precise linkage across the entire process of pre-magnetization, impedance identification, residual magnetism quantification, and closing timing by constructing a coordinated control mechanism between an RC pre-magnetization branch and phase-controlled closing. It requires no modification to the main transformer, offering strong adaptability and low implementation cost. Furthermore, it can counteract residual magnetism through pre-magnetization flux and dynamically adjust the closing mode based on impedance status, fundamentally suppressing inrush current peaks. This effectively reduces the mechanical impact and electrical damage of inrush current on transformer windings and circuit breakers, reduces the risk of relay protection malfunctions, ensures stable and reliable operation during power system start-up and shutdown, and solves the problems of poor suppression effect and insufficient adaptability of traditional single control methods.

[0033] Specifically, such as Figure 3 As shown, in step S2, the process of establishing the pre-charge flux includes: Step S21: Determine the peak steady-state magnetic flux of the main transformer when it is closed under no-load conditions based on the rated parameters of the main transformer. Step S22: Determine the expected closing time of the pre-charged magnetic contactor based on the steady-state magnetic flux peak value, resistance value, and rated voltage of the energy storage capacitor; The rated parameters include rated voltage and rated frequency.

[0034] In detail, if the expected closing time of the pre-charge contactor is too short, insufficient pre-charge flux will result in an inability to offset residual magnetism; if the expected closing time is too long, it may lead to over-magnetization of the iron core, which will exacerbate inrush current. Therefore, this invention selects the rated voltage of the main transformer and the rated frequency of the associated flux change period, which determine the reference for the rated flux of the iron core, as core parameters. Combined with the current-limiting resistor and energy storage capacitor, the expected closing time is determined. A steady-state flux peak reference is established through the rated voltage and rated frequency parameters to ensure that the pre-charge time matches the transformer's magnetic characteristics, so that the pre-charge flux reaches the optimal value and avoids insufficient or excessive pre-charge.

[0035] In practice, the peak steady-state magnetic flux = rated voltage / (4.44 × rated frequency × number of turns of the primary winding of the main transformer). , where r is the product of the resistance value of the current-limiting resistor and the capacitance value of the energy storage capacitor.

[0036] Based on the aforementioned technical advantages, this invention determines the expected closing time of the pre-charge contactor by combining the rated parameters of the main transformer with the parameters of the RC branch, thereby achieving quantitative control of the pre-charge flux. This avoids the problems of insufficient or excessive pre-charge caused by traditional pre-charge duration settings based on experience, ensuring that the pre-charge flux accurately matches the peak value of the transformer's steady-state flux.

[0037] Specifically, in step S3, determining the impedance state of the main transformer power supply circuit system includes: The rate of change of current is determined based on the current signal and the preset sampling period; The impedance state is determined based on the current change rate and the preset change rate threshold. The impedance states include low impedance states and high impedance states.

[0038] In implementation, the current change rate is determined based on the current signal and the preset sampling period; the time for the rise of the pre-magnetizing current of the main transformer is usually tens of milliseconds, so the value of the preset sampling period is in the range of 0.5 to 1 ms, preferably 0.8 ms; the current change rate is the difference between two adjacent current signals / preset sampling period.

[0039] Specifically, in step S3, determining the closing method based on the impedance state includes: In response to the high impedance state, the main circuit breaker is controlled to complete the closing action in one go. In response to the impedance state being low impedance, the main circuit breaker is controlled to first close to a preset travel position and remain there for a preset duration before fully closing. The preset travel position refers to a specific proportion of the total closing travel position when the contacts are not fully closed during the main circuit breaker closing process.

[0040] In detail, the impedance of the power supply circuit system is dynamically affected by line length, equipment aging, and load conditions. The current surge risk and response requirements during the closing process differ under different impedance conditions. For example, at low impedance, the current rise rate is fast at the moment of closing. If a one-time closing is used, the sudden increase in current can easily cause rapid saturation of the core flux, resulting in a large inrush current. At high impedance, the current rise rate is slow and the current surge risk is low. If a step-by-step closing is used, the closing response time will be prolonged, which may lead to the decay of the pre-charge flux and affect the inrush current suppression effect.

[0041] In implementation, if the current change rate is less than the preset change rate threshold, the impedance state is a low impedance state; if the current change rate is greater than or equal to the preset change rate threshold, the impedance state is a high impedance state. The preset change rate threshold ranges from 5 to 10 A / ms, preferably 7.5 A / ms; the preset travel position is 50% to 70% of the total closing travel, preferably 65% ​​of the total closing travel; the preset duration ranges from 10 to 30 ms, preferably 15 ms.

[0042] Based on the aforementioned technical effects, this invention calculates the current change rate using a current signal and a preset sampling period, and combines this with a dual-threshold impedance state to achieve real-time and accurate identification of system impedance. This solves the problem of poor adaptability of closing strategies caused by traditional methods neglecting dynamic impedance changes. Furthermore, this invention dynamically adjusts the closing method according to the impedance state, avoiding both the instantaneous surge caused by a single closing operation in a low-impedance state and the response lag caused by step-by-step closing in a high-impedance state.

[0043] Specifically, such as Figure 4 As shown, in step S4, determining the magnitude and polarity of the residual magnetism of the main transformer core includes: Step S41: Establish current curves and voltage curves based on the current signal of the pre-magnetized branch and the voltage signal across the energy storage capacitor, respectively. Step S42: Determine the total change in magnetic flux of the core inside the main transformer during the pre-magnetization process based on the current curve, voltage curve, expected closing time, resistance value of the current limiting resistor, and number of turns of the primary winding of the main transformer. Step S43: Determine the magnitude and polarity of the remanence based on the total change in magnetic flux.

[0044] In detail, the magnitude and polarity of the residual magnetism in the main transformer core cannot be directly measured. The pre-magnetizing process involves the energy storage capacitor discharging into the transformer windings through a current-limiting resistor. During this process, the energy storage capacitor, the current-limiting resistor, and the primary winding of the main transformer are connected in series to form an RC discharge circuit, i.e., the pre-magnetizing circuit. The decay of the capacitor voltage, the change in the circuit current, and the change in the core magnetic flux strictly follow electromagnetic laws. The magnetic flux can be indirectly mapped through circuit parameters. Therefore, the magnitude and polarity of the residual magnetism can be deduced inversely from the current and voltage curves.

[0045] In implementation, after the pre-magnetized contactor is closed, current curves with time as the abscissa are established based on the collected current signal of the pre-magnetized branch and the voltage signal across the energy storage capacitor. and voltage curve Total change in magnetic flux The primary winding of the main transformer has N turns, and the current-limiting resistor has a resistance value of [value missing]. , This is the start time of pre-magnetization. The pre-magnetization end time can be calculated based on the expected closing time and the pre-magnetization start time. The absolute value of the difference between the target preset magnetic flux and the total magnetic flux change is the magnitude of the residual magnetism. If the difference between the target preset magnetic flux and the total magnetic flux change is greater than 0, the residual magnetism polarity is positive, and the closing is delayed; otherwise, the residual magnetism polarity is negative, and the closing is advanced.

[0046] Based on the aforementioned technical effects, this invention determines the total magnetic flux change of the core in the main transformer during pre-magnetization based on the current and voltage curves, the expected closing time, the resistance value of the current-limiting resistor, and the number of turns in the primary winding of the main transformer, thus achieving accurate identification of the magnitude and polarity of residual magnetism. This solves the technical problems of large estimation errors and the inability to directly measure residual magnetism in traditional methods, providing data support for the precise selection of closing timing.

[0047] Specifically, in step S5, the grid voltage signal on the incoming side of the main circuit breaker is acquired to determine the voltage zero-crossing point. The closing timing is determined based on the voltage zero-crossing point and the comparison result between the residual magnetism polarity and the magnetic flux polarity corresponding to the main grid voltage, including: In response to the fact that the residual magnetism polarity is the same as the flux polarity of the main grid voltage, the closing is delayed after the voltage zero-crossing point. In response to the result that the magnetic flux polarity of the residual magnetism is opposite to that of the main grid voltage, the circuit is closed in advance before the voltage crosses zero. The closing delay time and the closing advance time are both proportional to the magnitude of the residual magnetism.

[0048] In detail, traditional phase-controlled closing technology ignores the differences in residual magnetism between different transformers, mechanically fixing the closing operation at the voltage zero-crossing point. However, in reality, the magnitude and polarity of residual magnetism vary for each transformer. Fixed-phase closing cannot adapt to all scenarios. For example, when the polarity of the residual magnetism is opposite to that of the grid flux, closing at the zero-crossing point can actually exacerbate flux superposition, causing the core to quickly enter a deep saturation state. The essence of the inrush current in the main transformer is caused by the core flux exceeding the saturation point at the moment of closing. The core cause of flux saturation is the superposition of the residual magnetism in the core and the new flux generated by the grid voltage. The polarity of the grid voltage flux changes periodically with phase. Therefore, the timing of closing needs to be selected through personalized control of early / delayed closing to achieve reverse cancellation or smooth transition between the grid flux and the residual magnetism, thereby dynamically matching the residual magnetism phase of different main transformers. Ultimately, this ensures that the total flux is always below the core saturation value, suppressing inrush current at its source.

[0049] In practice, the grid voltage signal is first filtered and then compared with 0V. When the signal changes from negative to positive, that is, when the phase crosses 0°, it is determined to be a positive zero-crossing point; when it changes from positive to negative, that is, when the phase crosses 180°, it is determined to be a reverse zero-crossing point. This is how the time of the voltage zero-crossing point is determined.

[0050] Both the closing delay time and the closing advance time are considered as compensation time. When the residual magnetism polarity is the same as the grid flux polarity, the closing should be delayed by the compensation time after the voltage crosses zero. When the residual magnetism polarity is opposite to the grid flux polarity, the closing should be advanced by the compensation time before the voltage crosses zero. Compensation time = (residual magnetism magnitude / steady-state flux peak value) × 0.25 × grid cycle. The grid cycle is the time for a sinusoidal alternating current to complete one full positive and negative half-cycle change, which is inversely related to the grid frequency. In this embodiment, the grid cycle is 20ms. Within 1 / 4 of the grid cycle, the grid flux is in the attenuation / cancellation range and will not enter the reverse superposition stage.

[0051] In a specific embodiment, the optimal closing time includes a first closing time and a second closing time. The first closing time is the voltage zero-crossing point plus compensation time, and the second closing time is the voltage zero-crossing point minus compensation time. The main circuit breaker has an inherent mechanical delay when performing the closing action. If the phase-controlled closing command is issued directly at the optimal closing time, the actual contact closing time will deviate from the target phase due to the mechanical lag, weakening the inrush current suppression effect. Therefore, a lead-time compensation mechanism is needed to eliminate the influence of mechanical delay. Thus, the command issuance time of the phase-controlled closing command is determined based on the optimal closing time and the basic lead-time. Specifically: if closing is delayed, the command issuance time = first closing time - basic lead-time; if closing is advanced, the command issuance time = second closing time - basic lead-time. The delay time from the issuance of the main circuit breaker's phase-controlled closing command to contact closing is measured through a finite number of tests, and the average of several delay times is used as the basic lead-time. To ensure that the main circuit breaker contacts close at the optimal closing time, the grid magnetic flux and residual magnetism achieve reverse cancellation or smooth transition, and the total magnetic flux is always lower than the core saturation value, thereby maximizing the suppression of excitation inrush current.

[0052] Based on the above embodiments, the present invention combines the zero-crossing point of the grid voltage with the polarity and magnitude of the residual magnetism to dynamically determine the closing timing. When the polarities are the same, the closing is delayed; when the polarities are opposite, the closing is advanced. The duration is proportional to the magnitude of the residual magnetism. This achieves precise matching between the closing phase and the residual magnetism characteristics and the grid magnetic flux, effectively avoiding core saturation caused by magnetic flux superposition, and further reducing the peak inrush current from a phase perspective.

[0053] Specifically, in step S5, after the main circuit breaker closes, the pre-magnetizing branch exit control includes: After the main circuit breaker completes closing, the time delay relay is activated to keep the pre-magnetized branch in a closed state. While the pre-magnetized branch remains closed, the excitation current of the main transformer is detected; The timing for disconnecting the pre-charged contactor is determined based on the excitation current.

[0054] In detail, if the pre-charged magnetic branch is disconnected immediately after the main circuit breaker is closed, the pre-charged magnetic flux in the main circuit may decay rapidly at the moment of closing, failing to continuously offset the residual magnetism and thus triggering a secondary inrush current. Activating a time-delay relay after the main circuit breaker is closed keeps the pre-charged magnetic branch closed, delaying its exit and ensuring the magnetic flux transitions from the pre-charged state to a steady state, completely avoiding secondary inrush current caused by a sudden drop in magnetic flux. In detail, the excitation current is the no-load current in the circuit after closing. By detecting the excitation current, it is determined whether the core flux is stable and thus the timing of disconnection can be determined. This can extend the effective action time of the pre-magnetized branch while ensuring that the core flux is in a stable state after the main circuit is closed.

[0055] In implementation, the time-delay relay can keep the pre-magnetized branch closed. The time-delay relay can be integrated into the control and protection cabinet of the pre-magnetized branch, without occupying additional space in the main circuit. After the time-delay relay is activated, the controller synchronously begins to collect the excitation current of the main transformer. The excitation current needs to drop below the steady-state threshold and remain there for a certain period of time to meet the disconnection condition, because a single detection of the excitation current below the steady-state threshold is insufficient to determine magnetic flux stability. Therefore, the steady-state threshold = safety factor × rated no-load current of the main transformer; the safety factor ranges from 1.2 to 1.5, preferably 1.3. The stability duration ranges from 20 to 50 ms, preferably 30 ms, meaning that when the excitation current drops below the steady-state threshold and this state is maintained for more than the stability duration, the pre-magnetized contactor can be disconnected.

[0056] Based on the aforementioned technical advantages, this invention maintains the pre-magnetized branch closed via a time-delay relay after the main circuit breaker is closed, and determines the disconnection timing by combining excitation current detection. This solves the problem of secondary inrush current caused by a sudden drop in pre-magnetized flux due to the immediate disconnection of the branch in traditional circuits. Extending the effective operating time of the pre-magnetized branch ensures a stable transition of the core flux after the main circuit is closed.

[0057] Specifically, in step S2, during the pre-magnetization process, the current signal of the pre-magnetization branch and the voltage signal across the energy storage capacitor are monitored in real time. Based on the comparison result of the current signal with a preset overcurrent threshold and / or the comparison result of the voltage signal with a preset undervoltage threshold, the following operations are performed: In response to a current signal greater than the preset overcurrent threshold and / or a voltage signal lower than the preset undervoltage threshold, the pre-magnetizing contactor is immediately disconnected, and pre-magnetizing is stopped.

[0058] In detail, during the pre-magnetization process, excessive branch current may occur due to circuit faults or capacitor abnormalities, posing a risk of equipment burnout. By continuously monitoring the current and voltage signals during the pre-magnetization process, it is possible to determine whether the current is overcurrent or the capacitor is undervoltage. Setting a preset overcurrent threshold can prevent branch overload, and setting a preset undervoltage threshold can prevent ineffective capacitor discharge. The pre-magnetization contactor is immediately disconnected after the threshold is triggered, thus achieving safety protection during the pre-magnetization process.

[0059] In implementation, the maximum allowable current when the pre-magnetized branch is working normally, i.e. the rated pre-charging current, is the rated voltage of the energy storage capacitor / the resistance value of the current limiting resistor. The preset overcurrent threshold value ranges from 3 to 5 times the rated pre-charging current. Preferably, the preset overcurrent value is 3 times the rated pre-charging current. In practice, the preset undervoltage threshold is 30% to 40% of the rated voltage of the energy storage capacitor. Preferably, the preset undervoltage value is 35% of the rated voltage of the energy storage capacitor.

[0060] Based on the above-mentioned technical effects, this invention solves the risk of equipment damage caused by branch faults and capacitor abnormalities during the pre-magnetization process by real-time monitoring of the pre-magnetization branch current and capacitor voltage and setting an overcurrent / undervoltage threshold trigger protection mechanism.

[0061] Specifically, based on steps S1 to S5, the peak inrush current and corresponding pre-magnetization parameters during each closing process are recorded and a sample database is constructed. The offset between the actual magnetization curve and the standard magnetization curve is determined according to the sample database, and the expected closing time is corrected according to the offset. The pre-magnetization parameters include the pre-magnetization duration, the resistance value of the current-limiting resistor, the rated voltage of the energy storage capacitor, the core temperature of the main transformer, and the service life.

[0062] In detail, the standard magnetization curve is the magnetic field strength-magnetic flux density relationship curve measured under standard operating conditions when the main transformer leaves the factory. This curve reflects the magnetic characteristics of the iron core under ideal conditions. The actual magnetization curve is the magnetic field strength-magnetic flux density relationship curve measured during the actual operation of the main transformer. As the operating time increases, the iron core material will experience accumulated hysteresis loss and changes in grain structure. At the same time, temperature fluctuations will directly affect the permeability, causing the actual magnetization curve to deviate from the standard magnetization curve. The initial setting of the expected closing time is based on the standard magnetization curve. Current is injected through the pre-magnetizing branch to ensure that the pre-charge flux established by the iron core accurately cancels the residual magnetism and avoids flux superposition saturation during closing. However, due to the deviation between the actual magnetization curve and the standard curve, if a fixed expected closing time is still used, the pre-charge flux will not reach the design target, and the inrush current suppression effect will gradually decrease.

[0063] In detail, by quantifying the offset between the actual magnetization curve and the standard curve through a sample database, and adjusting the effective action time of the pre-magnetization in reverse, the pre-magnetization flux can always accurately match the actual magnetic characteristics of the iron core.

[0064] In implementation, a standard magnetization curve is obtained from the main transformer's manufacturer's technical manual and discretized into several reference coordinate points. The horizontal axis of each reference coordinate point represents the standard magnetic field strength, and the vertical axis represents the standard magnetic flux density. Based on the pre-magnetization parameters in the sample database, the actual magnetic field strength and actual magnetic flux density corresponding to each closing operation are calculated to determine the actual coordinate points, which are then fitted to form the actual magnetization curve. The actual magnetic field strength is calculated as the number of winding turns × the actual discharge current of the pre-magnetization branch / the magnetic circuit length of the main transformer core. The actual discharge current of the pre-magnetization branch is calculated based on the resistance value of the current-limiting resistor, the rated voltage of the energy storage capacitor, and the pre-magnetization duration. The actual magnetic flux density is calculated as the actual magnetic flux established by pre-magnetization / the effective cross-sectional area of ​​the main transformer core. Under the same magnetic field strength, the difference between the actual magnetic flux density and the standard magnetic flux density is determined to obtain the offset. In implementation, the effectiveness of the actual magnetization curve can be verified using the inrush current peak value.

[0065] Specifically, such as Figure 5 As shown, correcting the expected closure duration based on the offset includes: The magnetic property aging coefficient is determined based on the offset. The steady-state magnetic flux peak value is corrected in real time according to the magnetic property aging coefficient to obtain the steady-state corrected magnetic flux peak value. The expected closure duration is corrected based on the peak steady-state correction magnetic flux.

[0066] In implementation, the offset can quantify the aging degree of the core's magnetic properties. The magnetic property aging coefficient is determined by the offset, which then corrects the steady-state magnetic flux peak value, ultimately updating the expected closing time. This ensures that even when the core's magnetic properties change, the pre-charge flux can accurately offset residual magnetism, maintaining optimal inrush current suppression. The magnetic property aging coefficient = offset / preset offset threshold, where the preset offset threshold is 0.05 Wb. If the magnetic property aging coefficient exceeds 1.3, it is calculated as 1.3. The magnetic property aging coefficient × steady-state magnetic flux peak value = steady-state corrected magnetic flux peak value. Using the relationship described in the above embodiment, the steady-state corrected magnetic flux peak value replaces the steady-state magnetic flux, thus calculating the corrected expected closing time.

[0067] Based on the above embodiments, the present invention constructs a sample database of inrush peak and pre-magnetization parameters, and uses the magnetization curve offset to correct the expected closing time, so that the expected closing time can be adapted in real time to dynamic factors such as core aging and temperature changes, ensuring that the pre-magnetic flux always matches the actual magnetic characteristics, and further improving the inrush suppression effect.

[0068] The technical solution of the present invention has been described above with reference to the preferred embodiments shown in the accompanying drawings. However, it will be readily understood by those skilled in the art that the scope of protection of the present invention is obviously not limited to these specific embodiments. Without departing from the principles of the present invention, those skilled in the art can make equivalent changes or substitutions to the relevant technical features, and the technical solutions after these changes or substitutions will all fall within the scope of protection of the present invention.

[0069] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for suppressing inrush current based on RC circuit and phase-controlled closing, characterized in that, include: The pre-magnetized branch, which is formed by the pre-magnetized contactor, the current-limiting resistor and the energy storage capacitor connected in series, is connected in parallel to both ends of the primary winding of the main transformer. The pre-charge contactor is controlled to close, and current is injected into the windings of the main transformer through the current-limiting resistor and the energy storage capacitor to establish pre-charge flux; The current signal of the pre-magnetized branch and the voltage signal across the energy storage capacitor are collected. The current change rate is determined based on the current signal to determine the impedance state of the main transformer power supply circuit system, and the closing mode is determined based on the impedance state. The magnitude and polarity of the residual magnetism of the core inside the main transformer are determined based on the current signal, the voltage signal, and the resistance value of the current limiting resistor, and the timing of closing the circuit is determined based on the magnitude and polarity of the residual magnetism. The circuit breaker is closed according to the closing method and the closing timing. After the main circuit breaker closes, the pre-charged contactor is disconnected.

2. The inrush current suppression method based on RC circuit and phase-controlled closing according to claim 1, characterized in that, The process of establishing pre-charge flux includes: The peak steady-state magnetic flux of the main transformer when it is closed under no-load conditions is determined based on the rated parameters of the main transformer. The expected closing time of the pre-charged magnetic contactor is determined based on the steady-state magnetic flux peak value, resistance value, and rated voltage of the energy storage capacitor. The rated parameters include rated voltage and rated frequency.

3. The inrush current suppression method based on RC circuit and phase-controlled closing according to claim 1, characterized in that, Determine the impedance state of the main transformer power supply circuit system, including: The rate of change of current is determined based on the current signal and the preset sampling period; The impedance state is determined based on the current change rate and the preset change rate threshold. The impedance states include low impedance states and high impedance states.

4. The inrush current suppression method based on RC circuit and phase-controlled closing according to claim 3, characterized in that, Determining the closing method based on the impedance state includes: In response to the high impedance state, the main circuit breaker is controlled to complete the closing action in one go. In response to the impedance state being low impedance, the main circuit breaker is controlled to first close to a preset travel position and remain there for a preset duration before fully closing.

5. The inrush current suppression method based on RC circuit and phase-controlled closing according to claim 4, characterized in that, Determine the magnitude and polarity of the residual magnetism of the main transformer core, including: The current curve and voltage curve are established based on the current signal of the pre-charged magnetic branch and the voltage signal across the energy storage capacitor, respectively. The total change in magnetic flux of the core inside the main transformer during the pre-magnetization process is determined based on the current curve, voltage curve, expected closing time, resistance value of the current limiting resistor, and number of turns of the primary winding of the main transformer. The magnitude and polarity of remanence are determined based on the total change in magnetic flux.

6. The inrush current suppression method based on RC circuit and phase-controlled closing according to claim 5, characterized in that, Acquire the grid voltage signal on the incoming side of the main circuit breaker to determine the voltage zero-crossing point. Determine the closing timing based on the voltage zero-crossing point and the comparison result between the residual magnetism polarity and the magnetic flux polarity corresponding to the main grid voltage, including: In response to the fact that the residual magnetism polarity is the same as the flux polarity of the main grid voltage, the closing is delayed after the voltage zero-crossing point. In response to the result that the magnetic flux polarity of the residual magnetism is opposite to that of the main grid voltage, the circuit is closed in advance before the voltage crosses zero. The closing delay time and the closing advance time are both proportional to the magnitude of the residual magnetism.

7. The inrush current suppression method based on RC circuit and phase-controlled closing according to claim 1, characterized in that, After the main circuit breaker closes, the pre-magnetized branch exit control includes: After the main circuit breaker completes closing, the time delay relay is activated to keep the pre-magnetized branch in a closed state. While the pre-magnetized branch remains closed, the excitation current of the main transformer is detected; The timing for disconnecting the pre-charged contactor is determined based on the excitation current.

8. The inrush current suppression method based on RC circuit and phase-controlled closing according to claim 1, characterized in that, During the pre-magnetization process, the current signal of the pre-magnetization branch and the voltage signal across the energy storage capacitor are monitored in real time. Based on the comparison result of the current signal with a preset overcurrent threshold and / or the comparison result of the voltage signal with a preset undervoltage threshold, the following operations are performed: In response to a current signal greater than the preset overcurrent threshold and / or a voltage signal lower than the preset undervoltage threshold, the pre-magnetizing contactor is immediately disconnected, and pre-magnetizing is stopped.

9. The inrush current suppression method based on RC circuit and phase-controlled closing according to claim 2, characterized in that, Record the peak inrush current and corresponding pre-magnetization parameters during each closing process and construct a sample database. Determine the offset between the actual magnetization curve and the standard magnetization curve based on the sample database, and correct the expected closing time based on the offset. The pre-magnetization parameters include the pre-magnetization duration, the resistance value of the current-limiting resistor, the rated voltage of the energy storage capacitor, the core temperature of the main transformer, and the service life.

10. The inrush current suppression method based on RC circuit and phase-controlled closing according to claim 9, characterized in that, Correcting the expected closure duration based on the offset includes: The magnetic property aging coefficient is determined based on the offset. The steady-state magnetic flux peak value is corrected in real time according to the magnetic property aging coefficient to obtain the steady-state corrected magnetic flux peak value. The expected closure duration is corrected based on the peak steady-state correction magnetic flux.

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