Calculation and implementation method of the optimal resonance suppression current for suppressing ferroresonance

Abstract

The present invention provides an optimal detuning current calculation and implementation method for suppressing ferromagnetic resonance, including: Step 1, when a single-phase grounding fault occurs in a neutral non-effectively grounded distribution network, a flexible grounding device is connected to obtain a simplified circuit diagram for suppressing ferromagnetic resonance in the flexible grounding distribution network; Step 2, taking the flexible grounding device as the primary side detuning resistor of the neutral non-effectively grounded distribution network, the zero-state response equivalent circuit and the zero-input response equivalent circuit with the detuning resistor added when the fault of the neutral non-effectively grounded distribution network is removed are analyzed respectively, and three damped vibration states are obtained. The present invention enables the neutral non-effectively grounded distribution network to get out of the resonance state in a short time through the flexible grounding device, with a fast detuning speed, effectively avoiding the secondary resonance overvoltage occurring after the detuning measure is removed. The optimal detuning current is obtained based on the critical resistance value, avoiding problems such as serious heating of the detuning resistor and abnormal increase in the voltage at both ends of the open delta caused by improper parameter selection.

Description

Technical Field

[0001] The present invention relates to the technical field of ferromagnetic resonance suppression in a distribution network, and particularly to a method for calculating and implementing an optimal resonance elimination current for suppressing ferromagnetic resonance. Background Art

[0002] [1] Mei Chenglin, Zhang Chaoshu. Analysis of voltage transformer ferroresonance [J]. Power System Technology, 2008, 32(S2): 311-313. Disadvantages: In the method of suppressing ferroresonance by grounding the neutral point of the primary side of the PT in the article, a large resistor is connected. The resonance elimination effect is good, but if the resistor is too large, it will affect the sensitivity of the protection, and high heat will be generated. The heat capacity of the resistor is limited, and it is possible to burn out the resistor; In the method of suppressing ferroresonance by adding a damping resistor to the open delta winding, connecting a resistor at the open delta of the secondary side of the voltage transformer can avoid the saturation of the transformer, but it is difficult to distinguish between fundamental frequency resonance and single-phase grounding, and it cannot suppress the low-frequency saturation current.

[0003] [2] Liang Zhirui, Dong Wei, Liu Wenxuan, et al. Analysis on ferroresonance of potential transformer [J]. High Voltage Apparatus, 2012, 48(11): 18-23. Disadvantages: In the method of suppressing ferroresonance by connecting a resonance eliminator to the neutral point of the primary side of the PT in the article, the resonance elimination effect is good, but there will be a problem that the output voltage of the open delta decreases, affecting the sensitivity of the grounding device; The method of suppressing ferroresonance by the 4PT wiring method has certain operation experience in practice, but it cannot fundamentally eliminate the ferroresonance frequency. The overcurrent caused by low-frequency resonance may last for a long time, which may cause thermal breakdown and damage the equipment.

[0004] [3] Zeng Xiangjun, Yang Xiangui, Wang Wen, et al. Zero-sequence voltage flexible control based ferroresonance suppressing method for distribution networks[J]. Proceedings of the CSEE, 2015, 35(07): 1666-1673. Disadvantage: The ferroresonance suppression method proposed in the paper is to inject zero-sequence current to force the neutral point voltage to zero. It can quickly suppress ferroresonance during the addition of the resonance elimination measure. However, since it cannot consume the resonance energy in a short time, ferroresonance will still occur when the resonance elimination measure is removed. Summary of the Invention

[0005] The present invention provides a method for calculating and implementing the optimal resonance elimination current for suppressing ferroresonance, aiming to solve the problems that the selection criteria for the resonance elimination current of traditional flexible grounding devices are inconsistent and the traditional methods for suppressing ferroresonance cannot consume the resonance energy in a short time, and ferroresonance will still occur when the resonance elimination measure is removed.

[0006] To achieve the above object, an embodiment of the present invention provides a method for calculating the optimal resonance elimination current for suppressing ferroresonance, including:

[0007] Step 1: When a single-phase grounding fault occurs in a non-effectively grounded neutral distribution network, connect a flexible grounding device to obtain a simplified circuit diagram for suppressing ferroresonance in the flexible grounded distribution network.

[0008] Step 2: Take the flexible grounding device as the primary side resonance elimination resistor of the non-effectively grounded neutral distribution network, and analyze the zero-state response operation circuit and the zero-input response operation circuit with the resonance elimination resistor added when the fault of the non-effectively grounded neutral distribution network is removed, respectively, to obtain three damped vibration states.

[0009] Step 3: Based on the three damped vibration states, according to the conclusion that the damped vibration system consumes energy fastest in the critical resistance state, obtain the optimal resonance elimination resistor.

[0010] Step 4: Calculate the optimal resonance elimination current according to the optimal resonance elimination resistor.

[0011] Among them, the specific content of the step 2 includes:

[0012] Analysis is carried out based on the zero-state response operation circuit: Only considering the influence of the three-phase power supply on the neutral-point non-effectively grounded distribution network, the initial voltage values of the three-phase capacitances to the ground and the initial current values of the three-phase PT exciting inductances in the zero-state response operation circuit are both zero. The first zero-sequence voltage in phasor form is calculated according to the nodal voltage method:

[0013]

[0014] Among them, represents the first zero-sequence voltage in phasor form, represents the voltage of phase A, represents the voltage of phase B, represents the voltage of phase C, Y A represents the admittance of phase A, Y B represents the admittance of phase B, Y C represents the admittance of phase C, is the admittance of each phase, Y x represents the admittance of each phase, x = A, B, C; C represents capacitance, L x represents the exciting inductance of each phase PT, x = A, B, C; In a very short time after the fault is removed, the voltage of the PT exciting inductance gradually approaches the inflection point voltage, and the PT exciting inductance transitions from the linear region to the saturation region. During the transition process of the PT exciting inductance, the three-phase voltages still remain symmetrical, and u n1 = 0, where, u n1 represents the first zero-sequence voltage in the time domain; The three-phase power supply has no effect on the neutral-point voltage.

[0015] Among them, the step 2 further includes:

[0016] Analysis is carried out based on the zero-input response operation circuit: Without considering the three-phase power supply, only considering the initial state of the state variables of the neutral-point non-effectively grounded distribution network, as follows:

[0017]

[0018] Among them, R represents the added harmonic elimination resistance, s represents the complex frequency, U n2 (s) represents the second zero-sequence voltage in the complex frequency domain, u A(0_) represents the voltage value of the capacitance of phase A to the ground, u B(0_) represents the voltage value of the capacitance of phase B to the ground, i A(0_) represents the current value of the exciting inductance of phase A PT, i B(0_) represents the current value of the exciting inductance of phase B PT, C Σ represents the total capacitance value to the ground, C Σ = C A + C B + C C = 3C, C A represents the capacitance of phase A to the ground, CB Represents the relative capacitance of phase B, C C Represents the relative capacitance of phase C, L Σ Represents the total PT excitation inductance value, L A Represents the excitation inductance of the PT in phase A, L B Represents the excitation inductance of the PT in phase B, L C Represents the excitation inductance of the PT in phase C.

[0019] Among them, step 2 further includes:

[0020] The zero-input response operation circuit belongs to a damped vibration system. After being disturbed, the zero-input response operation circuit is no longer affected by external excitation. Due to the damping causing the resonant energy consumption, the peak value of the second zero-sequence voltage u n2 decays until it stabilizes at zero;

[0021] Obtain the second zero-sequence voltage U n2 (s) in the complex frequency domain according to formula (3), as follows:

[0022]

[0023] Let s1 and s2 be the two solutions of the equation The discriminant of the equation is as follows:

[0024]

[0025] Among them, Δ represents the discriminant of the equation.

[0026] Among them, step 2 further includes:

[0027] According to formula (4), three damped vibration states are obtained, including:

[0028] The first damped vibration state is when Δ = 0, where R r represents the optimal harmonic elimination resistance. The non-effectively grounded neutral distribution network has a pair of repeated real roots. The zero-input response operation circuit is in the critical state of non-oscillatory discharge. The damping form of the non-effectively grounded neutral distribution network is called critical damping;

[0029] The second damped vibration state is when Δ > 0, R < R r The non-effectively grounded neutral distribution network has two unequal real roots. The zero-input response operation circuit is in the non-oscillatory discharge state. The damping form of the non-effectively grounded neutral distribution network is called over-damping;

[0030] The third damped vibration state is when Δ < 0, R > R rWhen the neutral point of the non-effectively grounded distribution network has a pair of conjugate complex roots, the zero-input response operation circuit is in an oscillatory discharge state, and the damping form of the non-effectively grounded distribution network is called underdamping.

[0031] Among them, step 3 specifically includes:

[0032] In the non-effectively grounded distribution network, from the perspective of energy loss, when the non-effectively grounded distribution network is in a critically damped state, the energy loss is the fastest, and the non-effectively grounded distribution network will return to the balanced and stable state fastest. At this time, the added harmonic elimination resistor R is the required optimal harmonic elimination resistor value. Substitute R = R r into Equation (3), as shown below:

[0033]

[0034] Among them, step 3 also includes:

[0035] Taking the inverse Laplace transform of formula (5), the harmonic elimination resistor added to the neutral point of the non-effectively grounded distribution network is the optimal harmonic elimination resistor R r When, the change of the second zero-sequence voltage u n2 in the time domain is as follows:

[0036]

[0037] Among them, u n represents the neutral point voltage of the non-effectively grounded distribution network, u n = u n1 + u n2 , u n1 = 0, u n = u n2 , t represents time.

[0038] Among them, step 4 specifically includes:

[0039] According to formula (6) and the added harmonic elimination resistor R is the required optimal harmonic elimination resistor value R r = R, calculate the optimal harmonic elimination current injected into the neutral point of the non-effectively grounded distribution network, as follows:

[0040]

[0041] Among them, i n represents the optimal harmonic elimination current.

[0042] The embodiment of the present invention also provides a method for realizing the optimal harmonic elimination current for suppressing ferromagnetic resonance, including:

[0043] Step 1, measure the amplitude of the zero-sequence voltage of the non-effectively grounded distribution network in real time;

[0044] Step 2: Determine whether the measured zero-sequence voltage amplitude is greater than 15% of the rated phase voltage amplitude;

[0045] Step 3: When the measured zero-sequence voltage amplitude is greater than 15% of the rated phase voltage amplitude, a single-phase grounding fault occurs in the non-effectively grounded neutral distribution network. Delay for 10 s and execute Step 4. When the measured zero-sequence voltage amplitude is less than 15% of the rated phase voltage amplitude, the non-effectively grounded neutral distribution network operates normally and jumps to Step 1;

[0046] Step 4: Measure the zero-sequence voltage amplitude of the non-effectively grounded neutral distribution network and determine whether the measured zero-sequence voltage amplitude is greater than 15% of the rated phase voltage amplitude;

[0047] Step 5: When the measured zero-sequence voltage amplitude is less than 15% of the rated phase voltage amplitude, the non-effectively grounded neutral distribution network operates normally and jumps to Step 1; when the measured zero-sequence voltage amplitude is greater than 15% of the rated phase voltage amplitude, a permanent single-phase grounding fault occurs in the non-effectively grounded neutral distribution network. Conduct grounding fault line selection to determine the grounding fault line and issue a tripping command for the circuit breaker of the fault line;

[0048] Step 6: Cut off the determined grounding fault line, inject the optimal harmonic elimination current into the neutral point of the non-effectively grounded neutral distribution network through the flexible grounding device, control the zero-sequence current to be the optimal harmonic elimination current, so that the non-effectively grounded neutral distribution network gets out of the resonance state. After a delay of 10 s, the non-effectively grounded neutral distribution network resumes normal operation and jumps to Step 1.

[0049] Among them, the specific content of Step 6 includes:

[0050] Controlling the zero-sequence current to be the optimal harmonic elimination current includes dividing the measured zero-sequence voltage amplitude by the optimal harmonic elimination resistance to obtain the real-time optimal harmonic elimination current, taking the real-time optimal harmonic elimination current as the given value of the constant current control, taking the collected real-time zero-sequence current as the input signal of the constant current control, inputting the difference between the real-time optimal harmonic elimination current and the real-time zero-sequence current into the hysteresis controller to generate the drive signal of the flexible grounding device. The flexible grounding device injects the optimal harmonic elimination current into the neutral point of the non-effectively grounded neutral distribution network. After a delay of 10 s, the resonant energy of the non-effectively grounded neutral distribution network is consumed, the distribution network voltage returns to zero, the non-effectively grounded neutral distribution network gets out of the resonance state, and after the non-effectively grounded neutral distribution network resumes normal operation, the flexible grounding device stops injecting the optimal harmonic elimination current into the neutral point of the non-effectively grounded neutral distribution network.

[0051] The above solution of the present invention has the following beneficial effects:

[0052] The method for calculating and implementing the optimal detuning current for suppressing ferromagnetic resonance according to the above embodiments of the present invention enables the flexible grounding device to deplete the resonance energy in a short time, thereby enabling the neutral point non-effectively grounded distribution network to get out of the resonance state. The detuning speed is fast, and the secondary resonance overvoltage occurring after the detuning measure is removed can be effectively avoided. The optimal detuning current is obtained based on the critical resistance value, which helps to select the detuning resistor, thereby avoiding problems such as serious heating of the detuning resistor and abnormal increase in the voltage at both ends of the open delta caused by improper parameter selection. Brief Description of the Drawings

[0053] Figure 1 is the flowchart for calculating the optimal detuning current of the present invention;

[0054] Figure 2 is the flowchart for implementing the optimal detuning current of the present invention;

[0055] Figure 3 is the simplified circuit diagram for suppressing ferromagnetic resonance in the flexible grounding distribution network of the present invention;

[0056] Figure 4 is the operational circuit diagram for the zero-state response of the present invention;

[0057] Figure 5 is the simplified operational circuit diagram for the zero-input response of the present invention;

[0058] Figure 6 is the control schematic diagram for injecting the optimal detuning current of the present invention;

[0059] Figure 7 is the ATP-EMTP simulation model of the present invention;

[0060] Figure 8 is the waveform diagram of the over-damped three-phase voltage and zero-sequence voltage of the present invention;

[0061] Figure 9 is the waveform diagram of the under-damped three-phase voltage and zero-sequence voltage of the present invention;

[0062] Figure 10 is the waveform diagram of the critically damped three-phase voltage, zero-sequence voltage and zero-sequence current of the present invention. Detailed Embodiments

[0063] To make the technical problems, technical solutions and advantages to be solved by the present invention clearer, the following will be described in detail with reference to the drawings and specific embodiments.

[0064] Aiming at the problems that the existing flexible grounding device has inconsistent detuning current selection criteria and the existing methods for suppressing ferromagnetic resonance cannot consume the resonance energy in a short time, and ferromagnetic resonance still occurs when the detuning measure is removed, the present invention provides a method for calculating and implementing the optimal detuning current for suppressing ferromagnetic resonance.

[0065] As shown Figures 1 to 10 in the figure, an embodiment of the present invention provides a calculation method for the optimal detuning current to suppress ferroresonance, including: Step 1, when a single-phase grounding fault occurs in a non-effectively grounded neutral distribution network, a flexible grounding device is connected to obtain a simplified circuit diagram for suppressing ferroresonance in the flexible grounded distribution network; Step 2, taking the flexible grounding device as the primary side detuning resistor of the non-effectively grounded neutral distribution network, analyze the zero-state response operation circuit and the zero-input response operation circuit with the detuning resistor added when the fault of the non-effectively grounded neutral distribution network is removed, and obtain three damped vibration states; Step 3, based on the three damped vibration states, according to the conclusion that the damped vibration system consumes energy fastest in the critical resistance state, obtain the optimal detuning resistor; Step 4, calculate the optimal detuning current according to the optimal detuning resistor.

[0066] The calculation method for the optimal detuning current to suppress ferroresonance and the implementation method described in the above embodiment of the present invention Figure 3 is a simplified schematic diagram for suppressing ferroresonance in a non-effectively grounded neutral distribution network, where u A , u B , u C are three-phase power supplies; L A , L B , L C are the excitation inductances of the three-phase PTs. In the unsaturated state, L A = L B = L C = L; C A , C B , C C are the three-phase capacitances to the ground. Ignoring the asymmetry of the distributed parameters, C A = C B = C C = C. The flexible grounding device is connected between the neutral point of the grounding transformer and the ground. The core structure is a single-phase bridge inverter circuit, which adopts an LC-type output filter and injects current i o into the neutral point through a single-phase injection transformer, so as to achieve the purpose of fault arc extinguishing and ferroresonance suppression.

[0067] Among them, the specific content of Step 2 includes: analyzing based on the zero-state response operation circuit: only considering the influence of the three-phase power supply on the non-effectively grounded neutral distribution network, the initial voltage values of the three-phase capacitances to the ground and the initial current values of the three-phase PT excitation inductances in the zero-state response operation circuit are both zero, and calculating the first zero-sequence voltage in phasor form according to the node voltage method:

[0068]

[0069] Among them, represents the first zero-sequence voltage in phasor form, represents the voltage of phase A, represents the B-phase voltage represents the C-phase voltage, Y A represents the A-phase admittance, Y B represents the B-phase admittance, Y C represents the C-phase admittance is the admittance of each phase, Y x represents the admittance of each phase, x = A, B, C; C represents capacitance, L x represents the exciting inductance of the PT for each phase, x = A, B, C; within a very short time after the fault is removed, the voltage of the PT exciting inductance gradually approaches the knee-point voltage, and the PT exciting inductance transitions from the linear region to the saturation region. During the transition process of the PT exciting inductance, the three-phase voltages still remain symmetric, and u n1 = 0, where u n1 represents the first zero-sequence voltage in the time domain; the three-phase power supply has no effect on the neutral point voltage

[0070] For the optimal detuning current calculation and implementation method for suppressing ferromagnetic resonance described in the above embodiments of the present invention, it is known that in a non-effectively grounded neutral distribution network, the knee-point voltage of the exciting characteristic of the PT exciting inductance is greater than 1.9U / √3, where U is the amplitude of the line voltage. Before the knee-point voltage, the change of the PT exciting inductance at different voltages is small and can be approximately considered linear. During a single-phase ground fault, it can be considered that the three-phase PT exciting inductances are in the linear region. Within a very short time after the fault is removed, the voltage of the PT exciting inductance gradually approaches the knee-point voltage, and the PT exciting inductance transitions from the linear region to the saturation region, but still remains in the linear region. After the transition ends, ferromagnetic resonance may occur in the system

[0071] Among them, step 2 further includes: analyzing based on the zero-input response operation circuit: without considering the three-phase power supply, only considering the initial state of the state variables of the non-effectively grounded neutral distribution network, as follows

[0072]

[0073] Among them, R represents the added detuning resistor, s represents the complex frequency, U n2 (s) represents the second zero-sequence voltage in the complex frequency domain, u A(0_) represents the voltage value of the A-phase capacitance to the ground, u B(0_) represents the voltage value of the B-phase capacitance to the ground, i A(0_) represents the current value of the A-phase PT exciting inductance, i B(0_) represents the current value of the B-phase PT exciting inductance, C Σ represents the total capacitance to the ground value, C Σ = C A + C B + C C = 3C, C A represents the A-phase capacitance to the ground, CB Represents the capacitance of phase B to ground, C C Represents the capacitance of phase C to ground, L Σ Represents the total PT exciting inductance value L A Represents the exciting inductance of PT for phase A, L B Represents the exciting inductance of PT for phase B, L C Represents the exciting inductance of PT for phase C.

[0074] Among them, step 2 further includes: The zero-input response operation circuit belongs to a damped vibration system. After being disturbed, the zero-input response operation circuit is no longer externally excited, and due to the damping causing the resonance energy consumption, the peak value of the second zero-sequence voltage u n2 decays until it stabilizes to zero;

[0075] The second zero-sequence voltage U n2 (s) in the complex frequency domain is obtained according to formula (3), as follows:

[0076]

[0077] Let s1 and s2 be the two solutions of the equation , and the discriminant of the equation is as follows:

[0078]

[0079] Among them, Δ represents the discriminant of the equation.

[0080] Among them, step 2 further includes: According to formula (4), three damped vibration states are obtained, including:

[0081] The first damped vibration state is when Δ = 0, where R r represents the optimal harmonic elimination resistance. The non-effectively grounded neutral distribution network has a pair of repeated real roots, and the zero-input response operation circuit is in the critical state of non-oscillatory discharge. The damping form of the non-effectively grounded neutral distribution network is called critical damping;

[0082] The second damped vibration state is when Δ > 0 and R < R r , the non-effectively grounded neutral distribution network has two unequal real roots, the zero-input response operation circuit is in the non-oscillatory discharge state, and the damping form of the non-effectively grounded neutral distribution network is called over-damping;

[0083] The third damped vibration state is when Δ < 0 and R > R r , the non-effectively grounded neutral distribution network has a pair of conjugate complex roots, the zero-input response operation circuit is in the oscillatory discharge state, and the damping form of the non-effectively grounded neutral distribution network is called under-damping.

[0084] Among them, step 3 specifically includes: In an un-effectively grounded neutral distribution network, from the perspective of energy loss, when the un-effectively grounded neutral distribution network is in a critically damped state, the energy loss is the fastest, and the un-effectively grounded neutral distribution network will return to the balanced and stable state the fastest. At this time, the added harmonic elimination resistor R is the required optimal harmonic elimination resistor value. Let R = R r Substitute it into Equation (3), as shown below:

[0085]

[0086] Among them, step 3 also includes:

[0087] Perform the Laplace inverse transform on Equation (5) to obtain that the harmonic elimination resistor added to the neutral point of the un-effectively grounded neutral distribution network is the optimal harmonic elimination resistor R r When, the variation of the second zero-sequence voltage u n2 in the time domain is as follows:

[0088]

[0089] Among them, u n represents the neutral point voltage of the un-effectively grounded neutral distribution network, u n = u n1 + u n2 ,u n1 = 0,u n = u n2 , t represents time.

[0090] Among them, step 4 specifically includes:

[0091] According to Equation (6) and the added harmonic elimination resistor R which is the required optimal harmonic elimination resistor value R r = R, calculate the optimal harmonic elimination current injected into the neutral point of the un-effectively grounded neutral distribution network, as shown below:

[0092]

[0093] Among them, i n represents the optimal harmonic elimination current.

[0094] An embodiment of the present invention also provides a method for realizing the optimal detuning current for suppressing ferroresonance, including: Step 1, measuring the amplitude of the zero-sequence voltage of an uneffectively grounded neutral distribution network in real time; Step 2, determining whether the measured amplitude of the zero-sequence voltage is greater than 15% of the rated phase voltage amplitude; Step 3, when the measured amplitude of the zero-sequence voltage is greater than 15% of the rated phase voltage amplitude, a single-phase grounding fault occurs in the uneffectively grounded neutral distribution network, with a 10-second delay, and Step 4 is executed. When the measured amplitude of the zero-sequence voltage is less than 15% of the rated phase voltage amplitude, the uneffectively grounded neutral distribution network operates normally, and jumps to Step 1; Step 4, measuring the amplitude of the zero-sequence voltage of the uneffectively grounded neutral distribution network, and determining whether the measured amplitude of the zero-sequence voltage is greater than 15% of the rated phase voltage amplitude; Step 5, when the measured amplitude of the zero-sequence voltage is less than 15% of the rated phase voltage amplitude, the uneffectively grounded neutral distribution network operates normally, and jumps to Step 1; when the measured amplitude of the zero-sequence voltage is greater than 15% of the rated phase voltage amplitude, a permanent single-phase grounding fault occurs in the uneffectively grounded neutral distribution network, perform grounding fault line selection, determine the grounding fault line, and issue a tripping command for the circuit breaker of the fault line; Step 6, disconnect the determined grounding fault line, inject the optimal detuning current into the neutral point of the uneffectively grounded neutral distribution network through a flexible grounding device, control the zero-sequence current to be the optimal detuning current, so that the uneffectively grounded neutral distribution network gets out of the resonance state. After a 10-second delay, the uneffectively grounded neutral distribution network resumes normal operation and jumps to Step 1.

[0095] For the method for calculating and realizing the optimal detuning current for suppressing ferroresonance described in the above embodiment of the present invention, the implementation process of the ferroresonance suppression method for a flexible grounding distribution network is as Figure 2 shown. First, online monitor the zero-sequence voltage and each phase voltage of the uneffectively grounded neutral distribution network. When the amplitude of the zero-sequence voltage is higher than 15% of the rated phase voltage amplitude, it is determined that a single-phase grounding fault has occurred; according to the requirements of the distribution network technical guide, to avoid transient grounding faults, a 10-second delay is set. If the amplitude of the zero-sequence voltage is still higher than 15% of the rated phase voltage amplitude after that, it is determined that a permanent single-phase grounding fault has occurred; then perform single-phase grounding fault line selection, determine the grounding fault line, issue a tripping command for the circuit breaker of the fault line, and at the same time inject the optimal detuning current into the neutral point of the uneffectively grounded neutral distribution network through a flexible grounding device to consume the resonance energy of the uneffectively grounded neutral distribution network, so that the distribution network voltage returns to zero, and further the uneffectively grounded neutral distribution network gets out of the resonance state; finally, the distribution network resumes normal operation and stops injecting the detuning current.

[0096] Among them, step 6 specifically includes: Controlling the zero-sequence current to be the optimal resonance damping current includes dividing the measured zero-sequence voltage amplitude by the optimal resonance damping resistance to obtain the real-time optimal resonance damping current, taking the real-time optimal resonance damping current as the given value of the constant current control, taking the collected real-time zero-sequence current as the input signal of the constant current control, inputting the difference between the real-time optimal resonance damping current and the real-time zero-sequence current into the hysteresis controller to generate the driving signal of the flexible grounding device, injecting the optimal resonance damping current into the neutral point of the neutral-point non-effectively grounded distribution network by the flexible grounding device, delaying for 10 s to consume the resonance energy of the neutral-point non-effectively grounded distribution network, restoring the distribution network voltage to zero, the neutral-point non-effectively grounded distribution network getting out of the resonance state, and after the neutral-point non-effectively grounded distribution network resumes normal operation, the flexible grounding device stops injecting the optimal resonance damping current into the neutral point of the neutral-point non-effectively grounded distribution network.

[0097] The method for calculating and implementing the optimal resonance damping current for suppressing ferromagnetic resonance according to the above embodiment of the present invention Figure 6 is a schematic diagram of the constant current control method for the flexible grounding device. The control objective of this control system is to control the zero-sequence current to be the optimal resonance damping current, so as to consume the resonance energy as quickly as possible and eliminate the ferromagnetic resonance. By collecting the zero-sequence voltage in real time, dividing the zero-sequence voltage by the optimal resonance damping resistance to obtain the optimal resonance damping current as the given value of the constant current control, taking the real-time zero-sequence current collected as the input signal of the constant current control, and generating the driving signal of the flexible grounding device after passing the difference between the optimal resonance damping current and the real-time zero-sequence current through the hysteresis controller.

[0098] The method for calculating and implementing the optimal resonance damping current for suppressing ferromagnetic resonance according to the above embodiment of the present invention uses the ATP-EMTP simulation software to establish a ferromagnetic resonance suppression simulation model for a 10 kV flexible grounding distribution network. Taking the permanent grounding fault of phase C as an example, as Figure 7 shown. The transformation ratio of the main transformer is 110 kV / 10.5 kV, the secondary side is connected to the 10 kV bus. The 10 kV bus contains three outgoing lines. The line model is LJ-95, the line length is 20 km, the capacitance to ground of each phase is 0.0074 uF / km, and the JDZX9-10 type 3039042# PT is adopted. In the simulation model, the single-phase capacitance to ground of the distribution network is taken as 0.1 μF to simulate the frequency division resonance. It is set that a single-phase grounding fault occurs at 0.03 s, and at 0.08 s, while removing the single-phase grounding fault, the neutral point of the neutral-point non-effectively grounded distribution network is grounded through the resonance damping resistance R. The resonance damping measures should not be put into operation for a long time, otherwise it will affect the operation mode of the neutral-point non-effectively grounded distribution network, etc. The resonance damping resistance is removed at 0.24 s. From the parameters of the JDZX9-10 type PT, it can be known that when this neutral-point non-effectively grounded distribution network is in critical damping, R≈13.5 kΩ, for over-damping, R = 10 Ω, and for under-damping, R = 0.2 MΩ.

[0099] The optimal detuning current calculation and implementation method for suppressing ferromagnetic resonance described in the above embodiment of the present invention, Figure 8 The three-phase voltage and zero-sequence voltage waveforms of the neutral point non-effectively grounded distribution network when over-damped. It can be seen that when the 10Ω resistor is added, the zero-sequence voltage is quickly clamped to near zero, but when the resistor is removed, the zero-sequence voltage amplitude changes from zero to about 3kV, which means that the short-term limitation of the zero-sequence voltage to zero cannot really make the neutral point non-effectively grounded distribution network out of the resonant state. When over-damped, the damping resistance is too small, resulting in less energy consumed on the resistor, and the ferromagnetic resonance energy cannot be effectively consumed. Therefore, after removing the damping resistor, a resonant overvoltage will still appear. Figure 9 The three-phase voltage and zero-sequence voltage waveforms of the neutral point non-effectively grounded distribution network under underdamping. When the neutral point non-effectively grounded distribution network is in an underdamped state, the zero-sequence voltage oscillates and decays slowly. At 0.24s, the zero-sequence voltage has not decayed to near zero. When the resistance is removed, the zero-sequence voltage amplitude is about 2kV, indicating that the speed at which underdamping consumes the resonant energy is not fast enough. Figure 10 The three-phase voltage, zero-sequence voltage and zero-sequence current waveforms of the neutral point non-effectively grounded distribution network under critical damping. Figure 10 It can be seen that the zero-sequence voltage stabilizes near zero in a short time when critical damping occurs, and after removing the detuning resistor, the zero-sequence voltage is still stable near zero, and the amplitude of the zero-sequence voltage is only about 0.5V in steady state. It can be seen that when the neutral point non-effectively grounded distribution network is in a critical damping state, the resonance energy can be consumed the fastest, so that the neutral point non-effectively grounded distribution network can be out of the resonance state and resume normal operation. Figure 10 The zero-sequence current shown is the corresponding optimal detuning current.

[0100] The method for calculating and implementing the optimal detuning current for suppressing ferromagnetic resonance described in the above-mentioned embodiment of the present invention enables the flexible grounding device to exhaust the resonant energy in a relatively short time, thereby allowing the neutral point non-effectively grounded distribution network to leave the resonant state, with a fast detuning speed, and can effectively avoid the secondary resonant overvoltage that occurs after the detuning measures are removed. The optimal detuning current is obtained based on the critical resistance value, which is helpful for selecting the detuning resistor, thereby avoiding problems such as severe heating of the detuning resistor and abnormal increase in voltage at both ends of the open triangle caused by improper parameter selection.

[0101] The above is a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the principles of the present invention. These improvements and modifications should also be regarded as the scope of protection of the present invention.

Claims

1. An optimal detuning current calculation method for suppressing ferroresonance, characterized in that, Including: Step 1: When a single-phase grounding fault occurs in a neutral non-effectively grounded distribution network, connect a flexible grounding device to obtain a simplified circuit diagram for suppressing ferroresonance in the flexible grounding distribution network; Step 2: Take the flexible grounding device as the primary side harmonic elimination resistor of the neutral non-effectively grounded distribution network, and analyze the zero-state response operation circuit and zero-input response operation circuit with the harmonic elimination resistor added when the neutral non-effectively grounded distribution network fault is removed respectively, to obtain three damped vibration states; Step 3: Based on the three damped vibration states, according to the conclusion that the damped vibration system consumes energy fastest in the critical resistance state, obtain the optimal harmonic elimination resistor; Step 4: Calculate the optimal harmonic elimination current according to the optimal harmonic elimination resistor.

2. The method for calculating the optimal detuning current for suppressing ferromagnetic resonance according to claim 1, wherein The specific content of Step 2 includes: Analysis based on the zero-state response operation circuit: Only consider the influence of the three-phase power supply on the neutral non-effectively grounded distribution network. The initial voltage value of the three-phase capacitance to ground and the initial current value of the three-phase PT excitation inductance in the zero-state response operation circuit are both zero. Calculate the first zero-sequence voltage in phasor form according to the node voltage method: Among them, represents the first zero-sequence voltage in phasor form, represents the phase-A voltage, represents the phase-B voltage, represents the phase-C voltage, Y A represents the phase-A admittance, Y B represents the phase-B admittance, Y C represents the phase-C admittance, is the admittance of each phase, Y x represents the admittance of each phase, x = A, B, C; C represents capacitance, L x represents the excitation inductance of each phase PT, x = A, B, C; in a very short time after the fault is removed, the voltage of the PT excitation inductance gradually approaches the inflection point voltage, and the PT excitation inductance transitions from the linear region to the saturation region. During the transition process of the PT excitation inductance, the three-phase voltages still remain symmetrical, and u n1 = 0, where u n1 represents the first zero-sequence voltage in the time domain; the three-phase power supply has no effect on the neutral point voltage.

3. The optimal detuning current calculation method for suppressing ferromagnetic resonance according to claim 2, characterized in that, The Step 2 also includes: Analysis based on the zero-input response operation circuit: Do not consider the three-phase power supply, only consider the initial state of the state variables of the neutral non-effectively grounded distribution network, as follows: Among them, R represents the added harmonic elimination resistor, s represents the complex frequency, and U n2 (s) represents the second zero-sequence voltage in the complex frequency domain, u A(0_) represents the capacitance voltage value of phase A to ground, u B(0_) represents the capacitance voltage value of phase B to ground, i A(0_) represents the current value of the exciting inductor of phase A PT, i B(0_) represents the current value of the exciting inductor of phase B PT, C Σ represents the total capacitance value to ground, C Σ = C A + C B + C C = 3C, C A represents the capacitance of phase A to ground, C B represents the capacitance of phase B to ground, C C represents the capacitance of phase C to ground, L Σ represents the total PT exciting inductor value, L A represents the exciting inductor of phase A PT, L B represents the exciting inductor of phase B PT, L C represents the exciting inductor of phase C PT.

4. The method for calculating the optimal detuning current for suppressing ferromagnetic resonance according to claim 3, characterized in that, The Step 2 also includes: The zero-input response operation circuit belongs to a damped vibration system. After being disturbed, the zero-input response operation circuit is no longer subject to external excitation. Due to the damping causing the consumption of resonant energy, the peak value of the second zero-sequence voltage u n2 decays until it stabilizes at zero; The second zero-sequence voltage U(s) in the complex frequency domain is obtained according to Equation (3) as follows: n2 (s) is as follows: Let s1 and s2 be the two solutions of the equation The discriminant of the equation is as follows: Where, Δ represents the discriminant of the equation.

5. The method for calculating the optimal detuning current for suppressing ferromagnetic resonance according to claim 4, characterized in that, The Step 2 also includes: According to formula (4), obtain three damped vibration states, including: The first damped vibration state is when Δ = 0, where R r represents the optimal detuning resistance, the neutral non-effectively grounded distribution network has a pair of repeated real roots, the zero-input response operation circuit is in the critical state of non-oscillatory discharge, and the damping form of the neutral non-effectively grounded distribution network is called critical damping; The second damped vibration state is that when Δ>0 and R<R r r , the non-effectively grounded neutral distribution network has two unequal real roots, the zero-input response operation circuit is in a non-oscillatory discharge state, and the damping form of the non-effectively grounded neutral distribution network is called over-damping; The third damped vibration state is that when Δ < 0 and R > R r , the neutral non-effectively grounded distribution network has a pair of conjugate complex roots, the zero-input response operation circuit is in an oscillatory discharge state, and the damping form of the neutral non-effectively grounded distribution network is called underdamping.

6. The method for calculating the optimal detuning current for suppressing ferromagnetic resonance according to claim 5, wherein The specific content of Step 3 includes: In an uneffectively grounded neutral distribution network, from the perspective of energy loss analysis, when the uneffectively grounded neutral distribution network is in a critically damped state, the energy loss is the fastest, and the uneffectively grounded neutral distribution network will return to the balanced and stable state the fastest. At this time, the added harmonic elimination resistor R is the required optimal harmonic elimination resistance value. Substitute R = R r into Equation (3), as shown below:

7. The method for calculating the optimal detuning current for suppressing ferromagnetic resonance according to claim 6, wherein The Step 3 also includes: Taking the inverse Laplace transform of formula (5), the resonance elimination resistor added to the neutral point of the non-effectively grounded neutral distribution network is the optimal resonance elimination resistor R r When n2 the variation of the second zero-sequence voltage u in the time domain is as follows: Among them, u n represents the neutral point voltage of a non-effectively grounded neutral distribution network, u n = u n1 + u n2 , u n1 = 0, u n = u n2 , and t represents time.

8. The method for calculating the optimal detuning current for suppressing ferromagnetic resonance according to claim 7, characterized in that The specific content of Step 4 includes: According to formula (6) and the added harmonic elimination resistor R, the required optimal harmonic elimination resistor value R r = R, calculate the optimal harmonic elimination current injected into the neutral point of a non-effectively grounded distribution network with a neutral point, as follows: Among them, i n represents the optimal detuning current.

9. A method for realizing the optimal resonance suppression current, which is applied to the method for calculating the optimal resonance suppression current for suppressing ferromagnetic resonance as described in claims 1-8, characterized in that Including: Step 1: Measure the amplitude of the zero-sequence voltage of the neutral non-effectively grounded distribution network in real time; Step 2: Judge whether the measured amplitude of the zero-sequence voltage is greater than 15% of the rated amplitude of the phase voltage; Step 3: When the measured amplitude of the zero-sequence voltage is greater than 15% of the rated amplitude of the phase voltage, a single-phase grounding fault occurs in the neutral non-effectively grounded distribution network. Delay for 10 s and execute Step 4. When the measured amplitude of the zero-sequence voltage is less than 15% of the rated amplitude of the phase voltage, the neutral non-effectively grounded distribution network operates normally and jumps to Step 1; Step 4: Measure the amplitude of the zero-sequence voltage of the neutral non-effectively grounded distribution network, and judge whether the measured amplitude of the zero-sequence voltage is greater than 15% of the rated amplitude of the phase voltage; Step 5: When the measured amplitude of the zero-sequence voltage is less than 15% of the rated amplitude of the phase voltage, the neutral non-effectively grounded distribution network operates normally and jumps to Step 1; when the measured amplitude of the zero-sequence voltage is greater than 15% of the rated amplitude of the phase voltage, a permanent single-phase grounding fault occurs in the neutral non-effectively grounded distribution network. Conduct grounding fault line selection to determine the grounding fault line and issue a tripping command for the circuit breaker of the fault line; Step 6: Cut off the determined grounding fault line, inject the optimal harmonic elimination current into the neutral point of the neutral non-effectively grounded distribution network through the flexible grounding device, control the zero-sequence current to be the optimal harmonic elimination current, so that the neutral non-effectively grounded distribution network gets out of the resonance state. After a delay of 10 s, the neutral non-effectively grounded distribution network resumes normal operation and jumps to Step 1.

10. The method for realizing the optimal detuning current for suppressing ferromagnetic resonance according to claim 9, characterized in that, The specific content of Step 6 includes: Controlling the zero-sequence current to be the optimal resonance damping current includes dividing the measured zero-sequence voltage amplitude by the optimal resonance damping resistance to obtain the real-time optimal resonance damping current, taking the real-time optimal resonance damping current as the given value for constant current control, taking the collected real-time zero-sequence current as the input signal for constant current control, inputting the difference between the real-time optimal resonance damping current and the real-time zero-sequence current into a hysteresis controller to generate the driving signal of the flexible grounding device, injecting the optimal resonance damping current into the neutral point of the non-effectively grounded neutral distribution network by the flexible grounding device, delaying for 10 s to consume the resonance energy of the non-effectively grounded neutral distribution network, restoring the distribution network voltage to zero, enabling the non-effectively grounded neutral distribution network to get out of the resonance state, and after the non-effectively grounded neutral distribution network resumes normal operation, the flexible grounding device stops injecting the optimal resonance damping current into the neutral point of the non-effectively grounded neutral distribution network.

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