A method for suppressing open-circuit overvoltage of a current transformer secondary circuit

Abstract

A current transformer secondary circuit open circuit overvoltage suppression method, comprising: parallelly connecting a voltage-dependent resistor, a back-to-back voltage stabilizing diode and a short-circuit switch in the current transformer secondary circuit; setting an action voltage threshold, when the secondary side voltage reaches the action voltage threshold, the voltage-dependent resistor realizes first-stage voltage clamping; measuring the primary side excitation current, calculating the core magnetic energy storage and the voltage-dependent resistor magnetic energy absorption capacity; when the absorption magnetic energy capacity is less than the core magnetic energy storage, setting a clamping voltage threshold, when the secondary side voltage reaches the clamping voltage threshold, the back-to-back voltage stabilizing diode is turned on, and the clamping voltage is started; extracting the combined current peak value, calculating the sliding average current, and setting the current threshold of the short-circuit switch trigger; when the sliding average current is not less than the current threshold and the duration reaches the delay threshold, the short-circuit switch is triggered to close. The present application can realize the effective connection of voltage clamping and energy dissipation, and make the fault state judgment more accurate.

Description

Technical Field

[0001] This invention relates to the field of power system overvoltage control technology, and specifically to a method for suppressing open-circuit overvoltage in the secondary circuit of a current transformer. Background Technology

[0002] In large power systems such as substations, current transformers serve as crucial isolation and transformation devices between the primary side's high current and the secondary side's measurement and protection devices. The stable operation of their secondary circuits directly impacts system safety. When an open circuit occurs in the secondary circuit of a current transformer due to loose wiring, maintenance operations, or faults, the continuous excitation current on the primary side cannot be counteracted by the reverse magnetomotive force generated by the secondary current. This leads to the continuous accumulation of magnetic flux in the core, causing magnetic saturation. This saturation induces high-amplitude, steeply rising overvoltages accompanied by high-frequency oscillations on the secondary side. This can not only damage the insulation of protection equipment and burn out measuring instruments but also pose a serious threat to the safety of maintenance personnel.

[0003] Existing technologies often use simple clamping elements (such as a single varistor) to clip overvoltage peaks, which can easily lead to excessive pressure on a single element and fail to achieve effective connection between voltage clamping and energy dissipation. In addition, existing technologies are not accurate enough in judging fault conditions. Summary of the Invention

[0004] In view of this, the present invention provides a method for suppressing open-circuit overvoltage in the secondary circuit of a current transformer, in order to solve the problems of excessive operating pressure of a single component in the prior art, the inability to effectively connect voltage clamping and energy dissipation, and the insufficient accuracy in judging the fault state.

[0005] A method for suppressing open-circuit overvoltage in the secondary circuit of a current transformer includes:

[0006] Step S1: Connect a varistor, a back-to-back Zener diode, and a short-circuit switch in parallel in the secondary circuit of the current transformer to form a cascaded three-level protection circuit structure.

[0007] Step S2: Based on the magnetic flux accumulation and high-frequency induced voltage generated when the secondary side of the current transformer is open, establish a model of the induced voltage on the secondary side of the current transformer, and set the operating voltage threshold of the varistor. When the secondary side voltage reaches the operating voltage threshold, the varistor achieves primary voltage clamping to limit the secondary side voltage within a preset range.

[0008] Step S3: When the secondary side voltage exceeds the operating voltage threshold of the varistor, measure the primary side excitation current and calculate the magnetic energy storage capacity of the iron core and the magnetic energy absorption capacity of the varistor.

[0009] Step S4: When the magnetic energy absorption capacity of the varistor is less than the magnetic energy storage capacity of the iron core, the clamping voltage threshold of the back-to-back Zener diode is set. When the secondary voltage reaches the clamping voltage threshold, the back-to-back Zener diode is turned on and begins to clamp the voltage to achieve secondary voltage clamping protection.

[0010] Step S5: When the secondary voltage exceeds the clamping voltage threshold, monitor the conduction current of the varistor branch and the conduction current of the back-to-back Zener diode branch, extract the combined current peak of the two branches, calculate the moving average current based on the combined current peak, and set the current threshold for short-circuit switch triggering.

[0011] Step S6: When the moving average current is detected to be not less than the current threshold and the duration reaches the delay threshold, the short-circuit switch is triggered to close, so as to realize the final stage voltage clamping protection.

[0012] The method for suppressing open-circuit overvoltage in the secondary circuit of a current transformer provided by the present invention has the following beneficial effects:

[0013] (1) This invention constructs a three-level protection circuit cascade structure consisting of a varistor, a back-to-back Zener diode, and a short-circuit switch, which serves as the basis for multi-level and phased coordinated suppression of overvoltage at different stages. This structure can operate sequentially at different stages of overvoltage development, which not only avoids excessive pressure on a single component but also achieves effective connection between voltage clamping and energy dissipation, significantly improving the integrity and reliability of the protection.

[0014] (2) This invention sets the operating voltage threshold of the varistor based on magnetic flux accumulation and high-frequency induced voltage, and combines the primary side current to dynamically verify its energy absorption capacity, thereby improving the accuracy and timeliness of overvoltage identification. In addition, this invention extracts the combined current peak by real-time monitoring of the varistor branch conduction current and the back-to-back Zener diode conduction branch current, and sets the current threshold for short-circuit switch triggering, thereby achieving accurate judgment of fault status and enhancing the ability to cope with complex electromagnetic transient processes.

[0015] (3) This invention calculates the moving average current based on the combined peak current and needs to determine whether the duration has reached the delay threshold. Therefore, this invention introduces a moving average filtering and delay triggering mechanism, which effectively suppresses the risk of maloperation caused by instantaneous interference and ensures that the short-circuit switch closes reliably under real overvoltage conditions. Finally, the residual energy is completely discharged through a low-resistance circuit to prevent the voltage from continuing to rise, thus fully ensuring the insulation safety of the current transformer and its secondary equipment and the stable operation of the system. Attached Figure Description

[0016] Figure 1 This is a flowchart illustrating the open-circuit overvoltage suppression method for the secondary circuit of a current transformer provided in an embodiment of the present invention.

[0017] Figure 2 This is a schematic diagram of a cascaded three-level protection circuit. Detailed Implementation

[0018] Embodiments of the present invention are described in detail below, examples of which are illustrated in the accompanying drawings, wherein the same or similar reference numerals denote the same or similar elements or elements having the same or similar functions throughout. The embodiments described below with reference to the accompanying drawings are exemplary and intended to explain embodiments of the present invention, and should not be construed as limiting the present invention.

[0019] Please see Figure 1 The method for suppressing open-circuit overvoltage in the secondary circuit of a current transformer provided by the present invention includes steps S1 to S6:

[0020] Step S1: Connect a varistor, a back-to-back Zener diode, and a short-circuit switch in parallel in the secondary circuit of the current transformer to form a cascaded three-level protection circuit structure.

[0021] For the cascaded structure of the three-level protection circuit, please refer to [link / reference]. Figure 2 This three-stage protection circuit cascade structure enables the parallel clamping and closing response at different stages to be interconnected. In this embodiment, the varistor is a temperature-compensated zinc oxide varistor, the back-to-back Zener diodes are fast-recovery back-to-back combined structures, and the short-circuit switch is a composite switch structure consisting of a magnetic latching relay and a thyristor connected in parallel.

[0022] Step S2: Based on the magnetic flux accumulation and high-frequency induced voltage generated when the secondary side of the current transformer is open, a model of the induced voltage on the secondary side of the current transformer is established, and the operating voltage threshold of the varistor is set. When the secondary side voltage reaches the operating voltage threshold, the varistor achieves primary voltage clamping to limit the secondary side voltage within a preset range.

[0023] The established model of the induced voltage on the secondary side of the current transformer satisfies the following equation:

[0024] ;

[0025] ;

[0026] ;

[0027] in, This refers to the magnetic flux generated in the core of the current transformer. For time, To represent the differential, For the magnetizing inductance of the current transformer, This refers to the number of turns in the primary winding. This refers to the number of turns in the secondary winding. This refers to the primary current of the current transformer. This is the secondary voltage that increases with the rate of change of magnetic flux before the iron core is saturated. This is a secondary side overvoltage. for The maximum value, The damping coefficient is... The resonant frequency is composed of the magnetizing inductance and parasitic capacitance of the transformer.

[0028] To limit the overvoltage amplitude, the varistor's operating voltage should be set to be earlier than... The varistor conducts before the first peak of the oscillation frequency, therefore, the operating voltage threshold of the varistor... for:

[0029]

[0030] in, This indicates taking the maximum value. This refers to the maximum voltage on the secondary side of the current transformer under normal operating conditions. For safety reasons, This is to ensure that it will not malfunction within the normal voltage fluctuation range, but can respond in time when overvoltage occurs.

[0031] When the secondary voltage reaches the operating voltage threshold, the varistor clamps the primary voltage to limit the secondary voltage within a preset range, thus preventing false triggering due to normal voltage fluctuations and enabling rapid response to overvoltage.

[0032] Step S3: When the secondary side voltage exceeds the operating voltage threshold of the varistor, measure the primary side excitation current and calculate the core magnetic energy storage capacity and the varistor magnetic energy absorption capacity.

[0033] When the varistor clamping fails, the clamping voltage of the Zener diode is determined based on the energy absorption capacity of the varistor to form a secondary backup protection.

[0034] In this embodiment, the current transformer core is equivalent to an excitation inductor, and the magnetic energy storage of the core is calculated using the following formula. :

[0035]

[0036] in, To excite the inductor, This is the primary excitation current.

[0037] When the open-circuit voltage surges to the operating voltage threshold When the varistor enters the nonlinear conduction region, it absorbs this portion of magnetic energy and converts it into heat energy for dissipation. Therefore, the varistor's magnetic energy absorption capacity... Satisfy the following formula:

[0038] ;

[0039] in, This is the heat capacity margin coefficient. , For maximum energy storage of the iron core, This is the maximum excitation current on the primary side.

[0040] Step S4: When the magnetic energy absorption capacity of the varistor is less than the magnetic energy storage capacity of the iron core, the clamping voltage threshold of the back-to-back Zener diode is set. When the secondary voltage reaches the clamping voltage threshold, the back-to-back Zener diode is turned on and begins to clamp the voltage to achieve secondary voltage clamping protection.

[0041] Among them, the varistor is at risk of conduction failure and energy absorption weakening under high-frequency overvoltage impacts. Therefore, a redundant protection path is required, namely, setting a back-to-back Zener diode clamping voltage threshold. Satisfy the following formula:

[0042]

[0043] in, For action coefficients, , This is to ensure that the varistor activates preferentially during overvoltage.

[0044] Step S5: When the secondary side voltage exceeds the clamping voltage threshold, monitor the conduction current of the varistor branch and the conduction current of the back-to-back Zener diode branch, extract the combined current peak of the two branches, calculate the moving average current based on the combined current peak, and set the current threshold for triggering the short-circuit switch.

[0045] In cases where both the varistor and the back-to-back Zener diode fail to provide protection and the voltage still cannot be clamped, a short-circuit switch must be triggered as the final protection action. Triggering the short-circuit switch requires monitoring the conduction current in both the varistor branch and the back-to-back Zener diode branch.

[0046] The induced voltage on the secondary side of the current transformer rises rapidly under the effect of rapid magnetic flux accumulation, which in turn triggers the parallel varistor and the back-to-back Zener diode to conduct in sequence. Based on the nonlinear power-law characteristic of the current response of the varistor branch and the conduction current characteristic of the back-to-back Zener diode, the conduction current of the varistor branch... Satisfy the following formula:

[0047]

[0048] in, A constant representing the electrical characteristics of a varistor. For reference voltage, The nonlinear verification exponent determines the clamping steepness. .

[0049] Back-to-back Zener diode conduction branch current Satisfy the following formula:

[0050]

[0051] in, This is the breakdown voltage of the back-to-back Zener diodes. It is the dynamic impedance of the back-to-back Zener diode in the on-state, and has an approximately constant linear characteristic.

[0052] To quantify the maximum current during the overall clamping response, a joint current peak value is introduced. Satisfy the following formula:

[0053] .

[0054] The current threshold triggered by the short-circuit switch is not a fixed value, but should be set according to the dynamic modeling parameters of the current transformer. Specifically, the current threshold triggered by the short-circuit switch... for:

[0055] ;

[0056] in, The magnetoelectric coupling coefficient is... The distributed capacitance of the secondary circuit. As the capacitance effect weighting factor, This is the correction factor for the rated current. This is the rated current on the secondary side.

[0057] when When the current transformer's induced voltage is considered to be in a clamping critical state, it is necessary to switch to the next stage of sliding average and delay logic control to complete the protective closing action of the short-circuit switch.

[0058] Electromagnetic interference, spikes, or initial oscillations commonly found in power systems can cause sudden changes in branch currents within a short period, leading to misjudgments. Therefore, this invention addresses the issue of combined current peak values. Time-domain smoothing filtering is performed to improve the stability of the judgment.

[0059] Specifically, moving average current Satisfy the following formula:

[0060]

[0061] in, This represents the window width, typically ranging from 1ms to 5ms. As the integrating factor, for The corresponding peak value of the combined current.

[0062] Step S6: When the moving average current is detected to be not less than the current threshold and the duration reaches the delay threshold, the short-circuit switch is triggered to close, so as to realize the final stage voltage clamping protection.

[0063] Among them, the delay threshold Satisfy the following formula:

[0064] ;

[0065] in, Maximum window width, The magnetic flux attenuation coefficient of the iron core is... is the secondary time constant of the current transformer.

[0066] Therefore, when And the duration during which the moving average current is not less than the current threshold. When the short-circuit switch is closed, a control signal is triggered, which quickly forms a low-resistance circuit on the secondary side of the current transformer, dissipating the residual energy through the resistor, preventing the voltage from continuing to rise, and avoiding further damage to the relay protection equipment or insulation system.

[0067] In summary, the current transformer secondary circuit open-circuit overvoltage suppression method according to the above embodiments has the following beneficial effects:

[0068] (1) This invention constructs a three-level protection circuit cascade structure consisting of a varistor, a back-to-back Zener diode, and a short-circuit switch, which serves as the basis for multi-level and phased coordinated suppression of overvoltage at different stages. This structure can operate sequentially at different stages of overvoltage development, which not only avoids excessive pressure on a single component but also achieves effective connection between voltage clamping and energy dissipation, significantly improving the integrity and reliability of the protection.

[0069] (2) This invention sets the operating voltage threshold of the varistor based on magnetic flux accumulation and high-frequency induced voltage, and combines the primary side current to dynamically verify its energy absorption capacity, thereby improving the accuracy and timeliness of overvoltage identification. This invention achieves accurate judgment of fault conditions and enhances the ability to cope with complex electromagnetic transient processes by real-time monitoring of the varistor branch conduction current and the back-to-back Zener diode conduction branch current, extracting the combined current peak value, and setting the current threshold for short-circuit switch triggering.

[0070] (3) This invention calculates the moving average current based on the combined peak current and needs to determine whether the duration has reached the delay threshold. Therefore, this invention introduces a moving average filtering and delay triggering mechanism, which effectively suppresses the risk of maloperation caused by instantaneous interference and ensures that the short-circuit switch closes reliably under real overvoltage conditions. Finally, the residual energy is completely discharged through a low-resistance circuit to prevent the voltage from continuing to rise, thus fully ensuring the insulation safety of the current transformer and its secondary equipment and the stable operation of the system.

[0071] The embodiments described above are merely illustrative of several implementations of the present invention, and while the descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the invention patent. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention, and these all fall within the protection scope of the present invention. Therefore, the protection scope of this invention patent should be determined by the appended claims.

Claims

1. A method for suppressing open-circuit overvoltage in the secondary circuit of a current transformer, characterized in that, include: Step S1: Connect a varistor, a back-to-back Zener diode, and a short-circuit switch in parallel in the secondary circuit of the current transformer to form a cascaded three-level protection circuit structure. Step S2: Based on the magnetic flux accumulation and high-frequency induced voltage generated when the secondary side of the current transformer is open, establish a model of the induced voltage on the secondary side of the current transformer, and set the operating voltage threshold of the varistor. When the secondary side voltage reaches the operating voltage threshold, the varistor achieves primary voltage clamping to limit the secondary side voltage within a preset range. Step S3: When the secondary side voltage exceeds the operating voltage threshold of the varistor, measure the primary side excitation current and calculate the magnetic energy storage capacity of the iron core and the magnetic energy absorption capacity of the varistor. Step S4: When the magnetic energy absorption capacity of the varistor is less than the magnetic energy storage capacity of the iron core, the clamping voltage threshold of the back-to-back Zener diode is set. When the secondary voltage reaches the clamping voltage threshold, the back-to-back Zener diode is turned on and begins to clamp the voltage to achieve secondary voltage clamping protection. Step S5: When the secondary voltage exceeds the clamping voltage threshold, monitor the conduction current of the varistor branch and the conduction current of the back-to-back Zener diode branch, extract the combined current peak of the two branches, calculate the moving average current based on the combined current peak, and set the current threshold for short-circuit switch triggering. Step S6: When the moving average current is detected to be not less than the current threshold and the duration reaches the delay threshold, the short-circuit switch is triggered to close, so as to realize the final stage voltage clamping protection. In step S5, the combined current peak value Satisfy the following formula: ； in, For the varistor branch to conduct current, To allow the back-to-back Zener diodes to conduct branch current, This indicates taking the maximum value; Current threshold triggered by short-circuit switch for: ； in, The magnetoelectric coupling coefficient is... This refers to the magnetic flux generated in the core of the current transformer. For time, To represent the differential, The distributed capacitance of the secondary circuit. As the capacitance effect weighting factor, This is a secondary side overvoltage. This is the correction factor for the rated current. This is the rated current on the secondary side.

2. The method for suppressing open-circuit overvoltage in the secondary circuit of a current transformer according to claim 1, characterized in that, In step S2, the established model of the induced voltage on the secondary side of the current transformer satisfies the following equation: ； ； ； in, For the magnetizing inductance of the current transformer, This refers to the number of turns in the primary winding. This refers to the number of turns in the secondary winding. This refers to the primary current of the current transformer. This is the secondary voltage that increases with the rate of change of magnetic flux before the iron core is saturated. for The maximum value, The damping coefficient is... It is the resonant frequency.

3. The method for suppressing open-circuit overvoltage in the secondary circuit of a current transformer according to claim 2, characterized in that, In step S2, the operating voltage threshold of the varistor for: in, This is for the safety factor.

4. The method for suppressing open-circuit overvoltage in the secondary circuit of a current transformer according to claim 3, characterized in that, In step S3, the current transformer core is equivalent to an excitation inductor, and the magnetic energy storage of the core is calculated using the following formula. : in, To excite the inductor.

5. The method for suppressing open-circuit overvoltage in the secondary circuit of a current transformer according to claim 4, characterized in that, In step S3, the varistor absorbs magnetic energy capacity. Satisfy the following formula: ; in, This is the heat capacity margin coefficient. For maximum energy storage of the iron core, This is the maximum excitation current on the primary side.

6. The method for suppressing open-circuit overvoltage in the secondary circuit of a current transformer according to claim 5, characterized in that, In step S4, the back-to-back Zener diode clamps the voltage threshold. Satisfy the following formula: in, This represents the action coefficient.

7. The method for suppressing open-circuit overvoltage in the secondary circuit of a current transformer according to claim 6, characterized in that, In step S5, the varistor branch conducts current. Satisfy the following formula: in, A constant representing the electrical characteristics of a varistor. For reference voltage, It is a non-linear verification exponent; Back-to-back Zener diode conduction branch current Satisfy the following formula: in, This is the breakdown voltage of the back-to-back Zener diodes. This represents the dynamic impedance of the back-to-back Zener diodes in the on-state.

8. The method for suppressing open-circuit overvoltage in the secondary circuit of a current transformer according to claim 7, characterized in that, In step S5, the moving average current Satisfy the following formula: in, For window width, As the integrating factor, for The corresponding peak value of the combined current.

9. The method for suppressing open-circuit overvoltage in the secondary circuit of a current transformer according to claim 8, characterized in that, In step S6, the delay threshold Satisfy the following formula: ； in, Maximum window width, The magnetic flux attenuation coefficient of the iron core is... is the secondary time constant of the current transformer.

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