Polarity selective switch suspension overvoltage limiting method

Abstract

The application discloses a polar selection switch suspension overvoltage limiting method, comprising the following steps: 1) according to the working principle of the polar selection switch, a transformer winding load voltage expression containing the polar selection switch is established; 2) according to the transformer winding load voltage, a recovery voltage and breaking current expression of each contact of the polar switch is established; 3) according to the recovery voltage and breaking current of each contact of the polar switch, whether the tap winding needs to be connected with a potential resistance for limiting the suspension overvoltage is judged, and the number and connection mode of the connected potential resistance are determined. The application provides a technical scheme for effectively inhibiting the suspension overvoltage of the polar switch, and can effectively avoid the situation that the short circuit or the generation of a large amount of gas due to unextinguished spark discharge of the voltage regulating winding endangers the insulation of the transformer.

Description

Technical Field

[0001] This invention relates to the field of polarity selection switches, specifically a method for limiting floating overvoltage in polarity selection switches. Background Technology

[0002] Many scholars abroad have discussed the potential problems associated with on-load tap changer operation and believe that the polarity selection switch completes the switching at the zero-voltage position. They have not considered the transient effects caused by polarity switching operation. Polarity switching operation affects the parasitic capacitance on the regulating winding, leading to capacitor discharge.

[0003] Since the widespread application of on-load tap changers with reverse adjustment, polarity switch faults have received increasing attention. Scholars have proposed a method for calculating coupling capacitance based on different connection methods of three-phase transformers, suggesting that the contact recovery voltage of a polarity switch typically depends on the voltage of the winding adjacent to the regulating winding, the length and position of the regulating winding, and the geometric arrangement of the windings. Researchers have calculated and analyzed the floating potential during polarity reversal based on the coupling capacitance C1 between the main winding and the regulating winding, and the coupling capacitance C2 between the regulating winding and ground, and have studied methods to reduce the energy of breaking sparks and arcs.

[0004] Domestic researchers focus more on the insulation analysis of polarity selector switches. Frequent switching of the polarity selector can lead to the detection of a certain amount of characteristic gas in the transformer's insulating oil. During operation, the tap winding is temporarily separated from the main winding, and the regulating winding is momentarily suspended. Therefore, based on the transformer's design parameters and insulation dielectric constant, the coupling capacitances C1 and C2 between the windings can be calculated.

[0005] However, at the moment the polarity-changing contact separates, a very small current is interrupted between the moving and stationary contacts. This current causes a spark between the separated contacts, leading to the decomposition of the insulating oil and the generation of bubbles. Simultaneously, a very high recovery voltage is generated at the K+ or K- junction between the contacts, causing a breakdown discharge between the contact joints. In mild cases, this generates gas in the transformer tank; in severe cases, the arc does not extinguish, causing a direct short circuit in the voltage regulating winding. During the closing process of the polarity switch contact, the coupling capacitance causes the moving contact to carry a capacitive current, resulting in the burning of the polarity-changing contact and the generation of gas through decomposition in the transformer tank.

[0006] On-load tap changers equipped with polarity selection switches are widely used in power systems. The polarity switch not only significantly increases the voltage regulation range of the on-load tap changer but also allows for precise matching with phase-shifting transformers to eliminate line overload and circulating current problems caused by fluctuations in renewable energy sources. However, floating overvoltages during polarity selection switching can easily affect the reliability of the tap changer, threatening the safe and stable operation of the system.

[0007] Therefore, it is necessary to limit the floating overvoltage of the polarity selection switch. Summary of the Invention

[0008] The purpose of this invention is to provide a method for limiting overvoltage of a polarity-selective switch floating circuit, comprising the following steps:

[0009] 1) Based on the working principle of the polarity selection switch, establish the expression for the load voltage of the transformer winding containing the polarity selection switch;

[0010] 2) Based on the expression for the transformer winding load voltage, establish the expressions for the recovery voltage and breaking current of each contact of the polarity switch;

[0011] The breaking current expression for the two contacts of the polarity switch is as follows:

[0012]

[0013]

[0014] In the formula, |I Switch1 |、|I Switch2 | represents the breaking current of the first and second contacts of the polarity selection switch, respectively; ω is the power supply frequency;

[0015] The recovery voltages of each contact of the polarity switch are shown below:

[0016]

[0017]

[0018] In the formula, C1 and C2 are the coupling capacitors between the main winding and the voltage regulating winding, and the coupling capacitor between the voltage regulating winding and ground; U R+ U R- These are the first and second contacts of the polarity selection switch; U HV U is the voltage on the high-voltage winding; tap This refers to the voltage of the regulating winding;

[0019] 3) Based on the recovery voltage and breaking current of each contact of the polarity switch, determine whether the tap winding needs to be connected with a potential resistor to limit the floating overvoltage, and determine the number and connection method of the potential resistors to be connected.

[0020] The steps for determining the number and connection method of the potential resistors to be connected include:

[0021] 3.1) Obtain basic transformer data;

[0022] 3.2) Based on the basic data of the transformer, determine the number n of potential resistance elements, i.e.:

[0023]

[0024] In the formula, n is the number of potential-resistive elements; [U R [This refers to the maximum allowable operating voltage for each potentiometer element;]

[0025] 3.3) Determine the resistance value R of the potential resistor. p The number of matched elements [n], i.e.:

[0026]

[0027]

[0028] In the formula, [P d [I] Allows for continuous heat load power dissipation for each resistive element; switch The capacitance current flows through the resistor;

[0029] 3.4) Determine whether the number of matching elements [n] ≤ 5 is true. If yes, the connection method of the potential resistor is constant connection method. If not, proceed to step 3.5).

[0030] 3.5) A potential switch is used to briefly connect a potential resistor. During this time, the potential resistor continuously experiences heat dissipation Pd and resistance R. p The number n is updated as follows:

[0031] P d =P s (8)

[0032]

[0033]

[0034] In the formula, P s The power dissipation is the current flowing through the capacitor across the potential resistor; [P] s [This refers to the allowable short-term heat load power consumption for each resistive element;]

[0035] 3.6) Determine whether the current recovery voltage is less than or equal to the limit of the polarity selector contact. If yes, end the process; otherwise, re-determine the number and connection method of the potential resistors.

[0036] Furthermore, based on the working principle of the polarity selection switch, the steps to establish the expression for the load voltage of a transformer winding containing a polarity selection switch include:

[0037] 1.1) Based on the mechanical structure and working principle of the polarity selection switch, the polarity switch is equivalent to a single-pole double-throw switch;

[0038] 1.2) Establish the expression for the transformer winding load voltage, i.e.:

[0039]

[0040] In the formula, U K U is the load voltage when the regulating winding is disconnected from the main winding. ZG The voltage regulating winding is connected to the circuit with positive polarity, and the highest tap position is connected to the load voltage when the selector tap is carrying current; U FL The load voltage when the voltage regulating winding is connected to the circuit with reverse polarity, and the selector tap is connected at the lowest tap position, is a voltage vector in the expression.

[0041] Furthermore, the moving contact of the single-pole double-throw switch is connected to the main winding of the transformer, and the stationary contact is connected to the tap of the highest position tap selector and the tap of the lowest position tap selector of the voltage regulating winding, respectively.

[0042] Furthermore, in a transformer circuit containing a polarity selection switch, a capacitor is connected between the middle of the main winding, the middle of the voltage regulating winding, and the grounding point.

[0043] Furthermore, the coupling capacitor C1 between the main winding and the voltage regulating winding, and the coupling capacitor C2 between the voltage regulating winding and ground are shown below:

[0044]

[0045]

[0046] In the formula, ε we1 The dielectric constant of the insulation between the main winding and the voltage regulating winding; h1 is the average height of the main winding and the voltage regulating winding; R TN R is the inner radius of the voltage regulating winding; HW Main winding outer radius; ε we2 h1 is the dielectric constant of the insulation between the voltage regulating winding and the oil tank; h2 is the height of the voltage regulating winding; R YN R is the radius of the inner wall of the fuel tank; TW This is the outer radius of the voltage regulating winding.

[0047] Furthermore, the criteria for determining whether the tap winding is connected to a potential resistor used to limit floating overvoltage include: if the recovery voltage exceeds the limit of the polarity selector contact, then the tap winding needs to be connected to a potential resistor.

[0048] Furthermore, the steps for obtaining basic transformer data include:

[0049] a) Obtain transformer performance parameters, including rated capacity, rated voltage, voltage regulation range, winding connection method, and insulation level;

[0050] b) Obtain the winding arrangement, that is, obtain the relative position of the voltage regulating winding and adjacent windings;

[0051] c) Obtain the capacitance of the voltage regulating winding to the adjacent winding or the capacitance of the tap winding to ground or to the adjacent winding grounded:

[0052] d) Obtain the AC operating voltage at both ends of the winding adjacent to the voltage regulating winding or the winding position;

[0053] e) Calculate the expected gradient of the lightning impulse voltage across half of the voltage regulating winding;

[0054] f) Calculate the operating voltage of half of the voltage regulating winding during operation and the power frequency AC voltage during testing.

[0055] Furthermore, after determining the number and connection method of the connected potential resistors, it is determined whether the breaking strength of the polarity selection switch contact and the thermal load strength of the potential resistor are within the allowable limits. If not, the number and connection method of the connected potential resistors are re-determined.

[0056] The technical effects of this invention are undeniable. Based on the mechanical structure and working principle of the polarity selection switch, this invention obtains the equivalent circuit of the polarity switch under various operating states. Using the vector voltages of the regulating winding and the main winding as a basis, a voltage vector diagram of the polarity selection switch is drawn. Combined with geometric research techniques, the expressions for the recovery voltage and breaking current of each contact of the polarity switch are derived. Furthermore, the influencing factors of floating overvoltage of the polarity switch are studied, and the calculation and characterization methods of each electrical parameter are investigated. This provides a theoretical basis for analyzing the reliability and effectiveness of the overvoltage suppression scheme for the polarity switch. Combined with simulation studies of overvoltage suppression measures, this invention provides a technical solution for effectively suppressing floating overvoltage of the polarity switch. This can effectively avoid the short circuit caused by unextinguished spark discharge in the regulating winding or the generation of a large amount of gas that could endanger the transformer insulation. Attached Figure Description

[0057] Figure 1 (a)-(f) illustrate the working principle of the polarity selection switch;

[0058] Figure 2 Voltage of each contact of the polarity selection switch;

[0059] Figure 3 For marking transformer winding voltage;

[0060] Figure 4 This represents the voltage vector of the transformer windings.

[0061] Figure 5 A model for assembling potentiometers. Detailed Implementation

[0062] The present invention will be further described below with reference to embodiments, but it should not be construed that the scope of the present invention is limited to the following embodiments. Various substitutions and modifications made based on ordinary technical knowledge and common practices in the art without departing from the above-described technical concept of the present invention should be included within the scope of protection of the present invention.

[0063] Example 1:

[0064] See Figures 1 to 5 A method for limiting overvoltage of a polarity-selective switch floating circuit includes the following steps:

[0065] 1) Based on the working principle of the polarity selection switch, establish the expression for the load voltage of the transformer winding containing the polarity selection switch;

[0066] 2) Based on the expression for the transformer winding load voltage, establish the expressions for the recovery voltage and breaking current of each contact of the polarity switch;

[0067] The breaking current expression for the two contacts of the polarity switch is as follows:

[0068]

[0069]

[0070] In the formula, |I Switch1 |、|I Switch2 | represents the breaking current of the first and second contacts of the polarity selection switch, respectively; ω is the power supply frequency;

[0071] The recovery voltages of each contact of the polarity switch are shown below:

[0072]

[0073]

[0074] In the formula, C1 and C2 are the coupling capacitors between the main winding and the voltage regulating winding, and the coupling capacitor between the voltage regulating winding and ground; U R+ U R- These are the first and second contacts of the polarity selection switch; U HV U is the voltage on the high-voltage winding; tap This refers to the voltage of the regulating winding;

[0075] 3) Based on the recovery voltage and breaking current of each contact of the polarity switch, determine whether the tap winding needs to be connected with a potential resistor to limit the floating overvoltage, and determine the number and connection method of the potential resistors.

[0076] The steps for determining the number and connection method of the potential resistors to be connected include:

[0077] 3.1) Obtain basic transformer data;

[0078] 3.2) Based on the basic data of the transformer, determine the number n of potential resistance elements, i.e.:

[0079]

[0080] In the formula, n is the number of potential-resistive elements; [U R [This refers to the maximum allowable operating voltage for each potentiometer element;]

[0081] 3.3) Determine the resistance value R of the potential resistor. p The number of matched elements [n], i.e.:

[0082]

[0083]

[0084] In the formula, [P d [I] Allows for continuous heat load power dissipation for each resistive element; switch The capacitance current flows through the resistor;

[0085] 3.4) Determine whether the number of matching elements [n] ≤ 5 is true. If yes, the connection method of the potential resistor is constant connection method. If not, proceed to step 3.5).

[0086] 3.5) Use a potential switch to connect the potential resistor for a short time. Here, "short time" is the opposite of "constant" and means that the connection time of the potential resistor is less than the preset time t.

[0087] At this time, the continuous heat load power consumption Pd and resistance value R of the potential resistor are... p The number n is updated as follows:

[0088] P d =P s (8)

[0089]

[0090]

[0091] In the formula, P s The power dissipation is the current flowing through the capacitor across the potential resistor; [P] s [This refers to the allowable short-term heat load power consumption for each resistive element;]

[0092] 3.6) Determine whether the current recovery voltage is less than or equal to the limit of the polarity selector contact. If yes, end the process; otherwise, re-determine the number and connection method of the potential resistors.

[0093] Based on the working principle of polarity selection switches, the steps to establish the expression for the load voltage of a transformer winding containing a polarity selection switch include:

[0094] 1.1) Based on the mechanical structure and working principle of the polarity selection switch, the polarity switch is equivalent to a single-pole double-throw switch;

[0095] 1.2) Establish the expression for the transformer winding load voltage, i.e.:

[0096]

[0097] In the formula, U K U is the load voltage when the regulating winding is disconnected from the main winding. ZG The voltage regulating winding is connected to the circuit with positive polarity, and the highest tap position is connected to the load voltage when the selector tap is carrying current; U FL The load voltage when the voltage regulating winding is connected to the circuit with reverse polarity, and the selector tap is connected at the lowest tap position, is a voltage vector in the expression. This is the load voltage vector when the voltage regulating winding is disconnected from the main winding; To ensure the voltage regulating winding is connected to the circuit with positive polarity, the load voltage vector is connected at the highest tap position when the selector tap is carrying current. To enable the voltage regulating winding to be connected to the circuit with reverse polarity, the load voltage vector is connected when the selector tap is current-carrying at the lowest tap position.

[0098] The moving contact of the single-pole double-throw switch is connected to the main winding of the transformer, and the stationary contact is connected to the tap of the highest position tap selector and the tap of the lowest position tap selector of the voltage regulating winding, respectively.

[0099] In a transformer circuit containing a polarity selection switch, a capacitor is connected between the middle of the main winding, the middle of the voltage regulating winding, and the grounding point.

[0100] The coupling capacitor C1 between the main winding and the voltage regulating winding, and the coupling capacitor C2 between the voltage regulating winding and ground are shown below:

[0101]

[0102]

[0103] In the formula, ε we1 The dielectric constant of the insulation between the main winding and the voltage regulating winding; h1 is the average height of the main winding and the voltage regulating winding; R TN R is the inner radius of the voltage regulating winding; HW Main winding outer radius; ε we2 h1 is the dielectric constant of the insulation between the voltage regulating winding and the oil tank; h2 is the height of the voltage regulating winding; R YN R is the radius of the inner wall of the fuel tank; TW This is the outer radius of the voltage regulating winding.

[0104] The criteria for determining whether a potential resistor is connected to the tap winding to limit floating overvoltage include: if the recovery voltage exceeds the limit of the polarity selector contact, then the tap winding needs to be connected to a potential resistor.

[0105] The steps to obtain basic transformer data include:

[0106] a) Obtain transformer performance parameters, including rated capacity, rated voltage, voltage regulation range, winding connection method, and insulation level;

[0107] b) Obtain the winding arrangement, that is, obtain the relative position of the voltage regulating winding and adjacent windings;

[0108] c) Obtain the capacitance of the voltage regulating winding to the adjacent winding or the capacitance of the tap winding to ground or to the adjacent winding grounded:

[0109] d) Obtain the AC operating voltage at both ends of the winding adjacent to the voltage regulating winding or the winding position;

[0110] e) Calculate the expected gradient of the lightning impulse voltage across half of the voltage regulating winding;

[0111] f) Calculate the operating voltage of half of the voltage regulating winding during operation and the power frequency AC voltage during testing.

[0112] After determining the number and connection method of the potential resistors to be connected, check whether the breaking strength of the polarity selection switch contact and the thermal load strength of the potential resistor are within the allowable limits. If not, then redetermine the number and connection method of the potential resistors to be connected.

[0113] Example 2:

[0114] See Figures 1 to 5 A method for limiting overvoltage of a polarity-selective switch floating circuit, comprising the following:

[0115] 1. Analysis of Factors Affecting Overvoltage of Polarity Selection Switch Floating

[0116] Based on the mechanical structure and working principle of the tap selector, the equivalent circuit of the polarity selector switch under different switching states is obtained. Using the transformer winding voltage as a basis, voltage vector diagrams of each component are constructed, and the expressions for the recovery voltage and breaking current of each contact of the polarity selector switch are derived. Based on the floating overvoltage expression, parameters affecting the overvoltage characteristics are obtained, and the influencing factors of the floating overvoltage of the polarity switch are identified.

[0117] 1.1 Mechanism and Influencing Factors of Floating Overvoltage in Polarity Selection Switches

[0118] Based on the mechanical structure and working principle of the polarity selection switch, the equivalent circuit of the polarity switch under various operating states is obtained. Using the vector voltages of the regulating winding and the main winding as a basis, a voltage vector diagram of the polarity selection switch is drawn. Combined with geometric research techniques, the expressions for the recovery voltage and breaking current of each contact of the polarity switch are derived. Furthermore, the influencing factors of floating overvoltage of the polarity switch are studied, and the calculation and characterization methods of each electrical parameter are investigated. This provides a theoretical basis for analyzing the reliability and effectiveness of the overvoltage suppression scheme for the polarity switch, and, in conjunction with simulation studies of overvoltage suppression measures, an effective technical solution for suppressing floating overvoltage of the polarity switch is obtained.

[0119] 1.2 Working principle of polarity selection switch

[0120] like Figure 1 (a) When the polarity switch “+” contact is closed, the voltage regulating winding is connected to the circuit with positive polarity, and the contact of the tap selector labeled 16 bears the load current. Figure 1 (b) During this phase, the tap selector disconnects contact number 15 and closes contact K. Figure 1 (c) In stage (c), the changeover switch completes the switching action, and the load current is transferred from contact 16 to contact K, ensuring that the polarity switch performs no-load switching. Figure 1 (d) In stage (a), the "+" contact of the polarity switch is open and the "-" contact is closed. During this stage, the voltage regulating winding and the main winding will be disconnected, resulting in a potential generated by the voltages of the tap winding and the main winding, the coupling capacitance between these windings, and the capacitance to ground. This potential will generate a voltage, i.e., the recovery voltage U, between the opening contacts of the tap selector. R When the tap selector is disconnected, the capacitive current on the tap winding will be cut off; this current is called the breaking current I. S If the recovery voltage is too high, the interrupting current cannot be extinguished, and the tap winding will be short-circuited. Overvoltage during polarity switching can easily cause arc discharge and insulation damage. Figure 1 During stage (e), the tap selector disconnects contact 16 and closes contact 1 to ensure that the voltage regulating winding is correctly connected to the circuit. Figure 1 During stage (f), the changeover switch completes its switching action, and the load current transfers from contact K to contact 1 of the tap selector. The polarity selection switch switching process ends.

[0121] 1.3 Equivalent Circuit Analysis of Polarity Selection Switch

[0122] The polarity selection switch can change the current direction of the voltage regulating winding, thereby expanding the tap range without increasing the number of transformer winding taps. [9] The on-load tap changer is a common structure in power systems. Therefore, in order to establish a simulation model of a polarity-selective on-load tap changer adapted to high-voltage transformers, research and modeling work were carried out on polarity-selective switches.

[0123] Based on the mechanical structure and working principle of the polarity selection switch, the polarity switch is equivalent to a single-pole double-throw switch. The moving contact of this switch is connected to the main winding of the transformer, and the stationary contact is connected to the taps of the highest and lowest positions of the voltage regulating winding, respectively. Equation (16) can be derived from the working principle of the polarity selection switch, where UK is the load voltage when the voltage regulating winding is disconnected from the main winding, U ZG The voltage regulating winding is connected to the circuit with positive polarity, and the highest tap position is connected to the load voltage when the selector tap is carrying current; U FL To enable the voltage regulating winding to be connected to the circuit with reverse polarity, the lowest tap position is connected to the load voltage when the selector tap is carrying current.

[0124]

[0125] When the tap at the highest positive tap position and the tap at the lowest reverse tap position of the voltage regulating winding are both closed, the load voltage is the same. Therefore, the change in the polarity selection switch does not cause a change in the steady-state voltage of the system. However, when the polarity switch changes the connection between the voltage regulating winding and the high-voltage winding, a switching switch is required to transfer the load current. Therefore, during the polarity reversal of the voltage regulating winding, the system voltage still experiences transient voltage fluctuations similar to tap adjustment. A simulation model of the switching switch is needed to support the simulation study of the electrical stress during the polarity reversal of the voltage regulating winding.

[0126] 1.4 Derivation of the expression for the floating overvoltage of the polarity selection switch

[0127] A mathematical model of the polarity switch based on circuit principles is constructed to achieve a direct mapping between the physical entity and the model. The calculation formulas for each parameter are theoretically derived, and the extraction methods for each parameter are obtained to accurately characterize the electromagnetic transient characteristics of floating overvoltage during polarity switch switching. Based on the analysis of the influencing factors of each parameter in the model, the influencing factors of floating overvoltage in the polarity switch are identified.

[0128] When the voltage regulating winding is disconnected from the main winding, a recovery voltage U exists between the polarity selection switch contacts (K+) and K during the transition from the "K+" contact to the "K-" contact. R+ This is caused by the potential of adjacent windings and the winding coupling capacitance. Similarly, when the polarity switch switches from the "K-" contact to the "K+" contact, the recovery voltage U... R- This will also occur. During the polarity switch switching process, a capacitive current exists between the contacts. The floating potential of the voltage regulating winding and the recovery voltage of the contacts will prevent this current from being effectively cut off, causing sparking and low-energy arc discharge, which will erode the contact material and damage its insulation performance.

[0129] Taking a typical delta-connected transformer system with a tap changer as an example, although the actual coupling capacitance is a distributed parameter, lumped electrical parameters are used for the study to simplify the mathematical model. The coupling capacitance C1 between the main winding and the regulating winding, and the coupling capacitance C2 between the regulating winding and ground are set, U... R+ It is the voltage U between contact K+ and K. R- It is the voltage at contact K relative to K. Simultaneously, as... Figure 2 Three special potential points were set for auxiliary calculations. Therefore, the voltage vector diagram was obtained as follows: Figure 3 As shown, when the polarity switch moves from position "K+" to "K-", the recovery voltage and switching current are different compared to the result when it moves from "K-" to "K+", because the voltage values ​​across capacitor C2 are different.

[0130] Based on the analysis and calculation of the vector diagram, we can obtain:

[0131]

[0132] As shown in Figure U HV U is the voltage on the high-voltage winding. tap For the voltage of the regulating winding, U C1 The voltage U that the coupling capacitor between the main winding and the voltage regulating winding withstands. C2 This is the voltage across the capacitor to ground of the voltage regulating winding. U Y Let K be the voltage at contact K relative to ground. For ease of calculation, distributed parameters C1 and C2 are represented by lumped parameters without affecting calculation accuracy. During modeling, the capacitor is connected between the middle of the main winding, the middle of the regulating winding, and the grounding point. The potential at contact K, the potential at the middle position of the regulating winding, and the ground potential are selected as three special auxiliary calculation potential points. Similarly, based on the vector diagram, we can obtain:

[0133]

[0134]

[0135] Based on the circuit principle, the breaking current of each contact is calculated, and the analytical expression for the breaking current is obtained as follows:

[0136]

[0137]

[0138] According to the formula for calculating the contact recovery voltage, the contact recovery voltage when the polarity switch changes from K+ to K- is different from that when it changes from K- to K+. This is because the sign of the voltage value of the regulating winding changes according to the calculated voltage. From the above formula, it can be seen that the calculation of the recovery voltage boils down to the calculation of the inter-winding capacitance and the winding-to-ground capacitance. Consulting relevant materials, the calculation formulas for each capacitance are as follows:

[0139]

[0140]

[0141] Where: ε we1 The dielectric constant of the insulation between the main winding and the voltage regulating winding; h1 is the average height of the main winding and the voltage regulating winding, in cm; R TN R is the inner radius of the voltage regulating winding, in cm; HW Main winding outer radius, cm; ε we2 h1 is the dielectric constant of the insulation between the voltage regulating winding and the oil tank; h2 is the height of the voltage regulating winding, in cm; R YN R is the folded radius of the inner wall of the fuel tank, in cm. TW Let C2 be the outer radius of the voltage regulating winding, in cm. Considering that approximately 25% of the voltage regulating winding lacks a corresponding oil tank, some scholars advocate multiplying C2 by 75%. However, actual measurements show that the value of C2 is too large; without multiplying by the correction factor, the measured value is close. Except for the insulation dielectric constant, all other parameters are determined during the transformer design calculations, and the insulation dielectric constant depends on the insulation structure. Once the transformer design is complete, these parameters cannot be modified. Therefore, additional measures to suppress the floating potential of the polarity switch are required.

[0142] 2. Overvoltage suppression measures for polarity selection switches floating switches

[0143] 2.1 Add a potential resistor

[0144] According to the on-load tap changer principle, when the polarity selector operates, the regulating winding disconnects its electrical connection from the main winding and acquires a potential. The magnitude of this potential depends on the voltage of the main winding and the coupling capacitance between the regulating winding and the main winding, as well as the coupling capacitance between the regulating winding and ground. The difference between the potential across the regulating winding and the potential before the polarity selector switches open is called the offset voltage, which is the maximum voltage of the regulating winding to ground. This voltage forms a recovery voltage between the disconnected contacts of the polarity selector. When the recovery voltage reaches a certain value, it can cause a discharge between the moving and stationary contacts. Therefore, domestic and international on-load tap changer manufacturers impose certain limits on the offset voltage. The commonly used M-type switch in China allows an offset voltage of no more than 35kV. The V-type switch allows an offset voltage of no more than 15kV. When the capacitive current interrupted by the polarity selector is small, the limit for the M-type switch can be relaxed to 45kV, and for the V-type switch to 20kV.

[0145] 2.2 Effect of Potential Resistance on Floating Overvoltage

[0146] A potentiometer is typically installed between the regulating winding and the tap selector to suppress floating overvoltage in the polarity selector switch. If the recovery voltage and switching current exceed the equipment's permissible insulation limits, a potentiometer is often used to reduce the recovery voltage and breaking current. Connected between the tap winding and the tap selector, it improves the transient response during polarity selector switching. Using a potentiometer to suppress floating overvoltage in the polarity selector switch requires determining the potentiometer's parameters. Data calculations often require obtaining all transformer characteristic data, such as: power, transformer regulation range, winding connection method, insulation level, winding design parameters (i.e., tap positions), winding voltage, and the number of windings arranged in the transformer winding adjacent to the regulating winding, etc.

[0147] The technical solution of adding a potential resistor has been validated in numerous engineering practices. However, adding a potential resistor still complicates the structure of the on-load tap changer. As the only movable structure in a transformer, the on-load tap changer has consistently experienced a high rate of failures. With the addition of a potential resistor, it becomes a branch circuit connected in parallel within the on-load tap changer of the on-load tap-changing transformer. Its function is to create a current-dissipating path during the polarity switching of the tap winding, preventing overvoltage from burning out the polarity selector contacts. Due to manufacturing, installation, and other accidental factors during operation, the potential resistor may fail, leading to unsafe events in the transformer. Therefore, the potential resistor is a weak link in the current operation and maintenance of on-load tap-changing transformers. Aside from assembly checks during production, there is a lack of targeted preventative testing and inspection measures during routine maintenance and testing.

[0148]

[0149]

[0150]

[0151] 2.3 Potential-resistance matching calculation

[0152] In both forward and reverse voltage regulation and coarse and fine voltage regulation, the tap changer polarity selector operates with the tap winding temporarily separated from the main winding, resulting in a floating potential in the tap winding. Consequently, spark discharges will occur between the open and closed contacts of the polarity selector, forming gas. To prevent short circuits caused by unextinguished spark discharges or the generation of large amounts of gas in the regulating winding from endangering the transformer insulation, an effective method is to use a potential resistor connection for the tap winding.

[0153] When determining whether a potentiometer needs to be connected, the breaking strength between the polarity selector contacts must first be calculated: the recovery voltage and the breaking current. If the calculated recovery voltage exceeds the limit of the polarity selector contacts, a potentiometer connection is required. Secondly, the correct connection method for the potentiometer must be selected.

[0154] The calculation and design of the recovery voltage and breaking current, as well as the potential resistance, require the following parameters:

[0155] 1) Transformer performance parameters: rated capacity, rated voltage, voltage regulation range, winding connection method, insulation level, etc.;

[0156] 2) Winding arrangement (i.e., the relative position of the voltage regulating winding and adjacent windings);

[0157] 3) The capacitance of the voltage regulating winding to the adjacent winding, or the capacitance of the tap winding to ground or to the adjacent winding grounded:

[0158] 4) The AC operating voltage at both ends of the winding adjacent to the voltage regulating winding or the winding position;

[0159] 5) The expected gradient of the lightning impulse voltage across half of the voltage regulating winding;

[0160] 6) The operating voltage of half of the voltage regulating winding during operation and the power frequency AC voltage during testing.

[0161] Operating voltage of potentiometer element

[0162] The operating voltage allowed by a single potentiometric element is:

[0163] [U R ]=4kV (12)

[0164] Consulting relevant materials reveals that:

[0165] In a constant connection mode, the continuous heat load power allowed for each potential resistive element is:

[0166] [P d =46.2W (13)

[0167] When using a potential switch connection, the continuous heat load power allowed for each potential resistor element is:

[0168] [P s ]=231W (14)

[0169] 2.4 Determine whether a potentiometer needs to be connected.

[0170] For tap changers with different voltage regulation methods, calculate the breaking strength of the polarity selector contacts without a potential resistor to determine whether a potential resistor needs to be connected. If the calculated recovery voltage exceeds the limit of the polarity selector contacts, a potential resistor needs to be connected.

[0171] 1) Initially determine the number of potentiometer elements:

[0172]

[0173] In the formula:

[0174] n represents the number of potentiometric elements, which is limited by the installation location. Low-voltage power transformers often require n ≤ 5.

[0175] [U R [This refers to the maximum allowable operating voltage for each potentiometer element, which is typically 4kV in engineering applications.]

[0176] 2) Determine the resistance value of the potential resistor and the number of matching components.

[0177] R is determined based on the allowable heat load intensity of the resistive element under continuous or short-term heat load. p The resistance value and the number of matching components.

[0178] (1) Constant potential resistor connection method: The resistance value R of the potential resistor is constant under continuous load. p for:

[0179]

[0180] In the formula: [P d [P] represents the allowable continuous heat load power dissipation of each resistive element. d =46.2W

[0181] In the potential resistance R p After the calculation, the number of matching resistors must be verified by the voltage drop across the potential resistor.

[0182] The capacitor current (I) flows through the potential resistor. switch Pressure drop:

[0183] U p =I switch R p (17)

[0184] The required number of matching resistors [n] is then:

[0185]

[0186] If [n] ≤ 5, the constant potential-resistance connection method is valid. In this case, the asymptotic method should be used for calculation until n = [n]. If [n] > 5, the constant potential-resistance connection method cannot be used.

[0187] (2) Connection method using the potential resistor of the potential switch

[0188] In the calculation, when [n] > 5, a potential switch is needed to connect the potential resistor for a short time. The optimal match in this case is:

[0189] P d =P s (19)

[0190]

[0191] The optimal number of resistors is:

[0192]

[0193] In the formula: P s The power dissipation of the capacitor current flowing through the potential resistor.

[0194] [P s Allow for short-term heat load power consumption for each resistive element.

[0195] (3) Based on the calculation results of the potential resistance, select the resistance value of the resistance element according to the standard specifications of the potential resistance element.

[0196] 3) Verify the feasibility of the potential-resistance matching scheme.

[0197] Based on the matching scheme calculated above, it is necessary to verify whether the contact breaking strength of the polarity selector and the potentiometer (if used) and the thermal load strength of the potentiometer are within the allowable limits when a potentiometer (standard specification) is used. If the calculated values ​​meet the limit requirements, the potentiometer matching scheme is confirmed to be feasible.

[0198] 3. Potential resistor fault analysis

[0199] In January 2019, a power supply company, during a special live-line inspection of heavy-load transformers in its jurisdiction, discovered that the C2H2 and H2 levels in the oil sample of a 220kV transformer exceeded the warning values ​​stipulated by the power industry standards. The C2H2 level reached 10.2 μL / L. A follow-up sample taken the next day showed that the CaH content had increased to 15.39 μL / L, an increase of over 50%. Oil chromatography data are shown in Table 1. It can be seen that, compared with normal data from the past two years, the abnormal data also showed a significant increase in CH4, C2H4, and C2H6 gases. Gas analysis revealed that the absolute gas production rates of the main characteristic gases far exceeded the warning values ​​stipulated by the industry standards. Since the gas growth rate is directly related to the fault energy, fault point temperature, and fault range, it was suspected that a high-energy discharge fault existed inside the transformer, necessitating an immediate power outage for inspection. The company subsequently shut down the transformer.

[0200] The faulty transformer was a three-phase oil-immersed, forced-oil, air-cooled, on-load tap changer transformer manufactured in April 2007, model SFP-SZ-150000 / 220. The on-load tap changer used with this transformer adopted a compact layout structure, minimizing space occupation and featuring precise and lightweight components. The transformer was put into operation in April 2008. Since then, it underwent routine maintenance with power outages in June 2009, July 2012, and June 2015, following normal maintenance cycles. All test data from each maintenance were correct and qualified. A review of the load conditions prior to the transformer fault revealed that from November 2018, excluding the 10kV compensation system, the load on the 220kV and 110kV sides of the transformer began to show an upward trend. By December, the high-voltage side load current had stabilized at around 250A, and the load rate remained above 60% of the rated capacity. Before the fault, the load current on the high-voltage side of the transformer was stable at around 220A. Checking the system's voltage regulation records, the on-load tap changer on the high-voltage side of the transformer had been operating at level 1 to 3 for a long time, and had been operating at level 2 for the past 3 months without any voltage regulation operation.

[0201] After relevant tests confirmed the possible cause and location of the internal transformer fault, and to further locate the fault and guide maintenance, the transformer's internal structure was inspected after the insulating oil was drained. Obvious traces of material cracking and debris were found in the passageway below the on-load tap changer, and powdery residue, possibly from carbonized materials, was present in the residual oil. The fault was located in the area surrounding the cracked area, ultimately revealing stress damage to the last resistor tube of one phase in the binding resistor connecting the bottom of the on-load tap changer to the neutral point. Circuit inspection confirmed the faulty phase was C. The porcelain bushing of the damaged resistor tube had detached in the middle, and the two adjacent binding resistor tubes also showed signs of cracking, but the damage was less severe compared to the last one, with cracks only in the middle. In contrast, the binding resistors of phases A and B appeared intact. After discovering the defect during internal inspection, and to confirm there were no other fault points, the transformer was returned to the factory for repair. Upon return, the transformer underwent a lifting inspection, and the results were similar to what the maintenance personnel observed inside the transformer: the binding resistor of phase C of the on-load tap changer was indeed cracked. Meanwhile, visual inspection and testing revealed no other fault points besides the binding resistor.

[0202] For the potentiometer circuit, each phase potentiometer consists of five resistor tubes connected in series. These series resistors are connected in three phases to the neutral point via the on-load tap changer. Based on the potentiometer wiring diagram, observation of the C-phase potentiometer revealed that the resistor tube closest to the neutral point suffered stress fracture, with most of its porcelain bushing detached. Adjacent resistor tubes also showed slight cracks. Surface traces showed a burn mark along the surface of the broken resistor tube, forming a melting channel. Neighboring resistor tubes showed no obvious discharge burn marks. Resistors closer to the broken resistor tube exhibited more severe cracks, with some ceramic tubing detached. Some resistor tubes only had a thin crack, clearly formed by stress impact and compression. It is speculated that an arc discharge occurred in one resistor tube, melting the wooden porcelain bushing of a nearby broken resistor, ultimately causing the broken resistor tube to explode. The fragments from the explosion struck adjacent resistor tubes, causing them to break as well. The potentiometer consists of five ceramic resistance tubes, each with a metal-coated surface and painted, connected in series. The tubes are hollow and mounted on epoxy resin rods, fixed to epoxy resin plates on both sides. Terminals extend from both sides of each tube, connecting to other resistance tubes via wires. The five tubes are connected in series to form a binding resistor. Careful inspection of the terminals revealed that two sets of terminals on the C-phase resistance tube, near the transformer winding, were physically very close, estimated to be less than 2mm apart. The screws on these terminals showed signs of burning and melting, suggesting that the C-phase resistance tubes experienced discharge due to the excessively close terminal spacing. In normal A and B phases, the resistance tube terminals face different orientations and cannot directly contact each other, therefore, the issue of insufficient physical distance does not exist.

[0203] A transformer failure was caused by the failure of the potentiometer in the on-load tap changer. In the tap changer circuit, the tap changer works in conjunction with the tap winding of the transformer, connected in series between the main winding and the neutral point on the high-voltage side of the transformer. When the transformer changes taps, the transition circuit is activated before the moving contact has completely disengaged from a certain tap, ensuring voltage regulation under load. In addition, a potentiometer circuit is connected in parallel in the tap changer circuit, connected in series between the main winding and the neutral point. When the transformer is running, the winding is in an energized state, and there is capacitance between the main winding and the tap winding, and capacitance between the tap winding and ground potential. A capacitive current will flow through the circuit. When the on-load tap changer adjusts the voltage and switches polarity, the main winding and the tap winding are not directly connected to form a circuit for a short time. At this time, a very high voltage will be generated on the polarity selector. To prevent this voltage from burning out the polarity selector contacts, a permanent resistor circuit is connected in series between the main winding and the neutral point to form a pressure relief path. This resistor is the potentiometer.

[0204] Example 3:

[0205] A method for limiting overvoltage of a polarity-selective switch floating circuit includes the following steps:

[0206] 1) Based on the working principle of the polarity selection switch, establish the expression for the load voltage of the transformer winding containing the polarity selection switch;

[0207] 2) Based on the expression for the transformer winding load voltage, establish the expressions for the recovery voltage and breaking current of each contact of the polarity switch;

[0208] The breaking current expression for the two contacts of the polarity switch is as follows:

[0209]

[0210]

[0211] In the formula, |I Switch1 |、|I Switch2 | represents the breaking current of the first and second contacts of the polarity selection switch, respectively; ω is the power supply frequency; C1 and C2 are the coupling capacitors of the main winding to the voltage regulating winding and the voltage regulating winding to ground; U HV U is the voltage on the high-voltage winding; tap This refers to the voltage of the regulating winding;

[0212] The recovery voltages of each contact of the polarity switch are shown below:

[0213]

[0214]

[0215] In the formula, U R+U R- It is the recovery voltage of the first and second contacts of the polarity selection switch;

[0216] 3) Based on the recovery voltage and breaking current of each contact of the polarity switch, determine whether the tap winding needs to be connected with a potential resistor to limit the floating overvoltage, and determine the number and connection method of the potential resistors to be connected.

[0217] The steps for determining the number and connection method of the potential resistors to be connected include:

[0218] 3.1) Obtain basic transformer data;

[0219] 3.2) Based on the basic data of the transformer, determine the number n of potential resistance elements, i.e.:

[0220]

[0221] In the formula, n is the number of potential-resistive elements; [U R [This refers to the maximum allowable operating voltage for each potentiometer element;]

[0222] 3.3) Determine the resistance value R of the potential resistor. p The number of matched elements [n], i.e.:

[0223]

[0224]

[0225] In the formula, [P d [I] Allows for continuous heat load power dissipation for each resistive element; switch The capacitance current flows through the resistor;

[0226] 3.4) Determine whether the number of matching elements [n] ≤ 5 is true. If yes, the connection method of the potential resistor is constant connection method. If not, proceed to step 3.5).

[0227] 3.5) A potential switch is used to briefly connect the potential resistor. During this time, the potential resistor continuously dissipates heat P. d Resistance value R p The number n is updated as follows:

[0228] P d =P s (8)

[0229]

[0230]

[0231] In the formula, P s The power dissipation is the current flowing through the capacitor across the potential resistor; [P]s [This refers to the allowable short-term heat load power consumption for each resistive element;]

[0232] 3.6) Determine whether the current recovery voltage is less than or equal to the limit of the polarity selector contact. If yes, end the process; otherwise, re-determine the number and connection method of the potential resistors.

[0233] Example 4:

[0234] A method for limiting overvoltage of a polarity-selective switch floating circuit is described in Example 3. The steps for establishing the expression for the load voltage of a transformer winding containing the polarity-selective switch, based on its working principle, include:

[0235] 1) Based on the mechanical structure and working principle of the polarity selection switch, the polarity switch is equivalent to a single-pole double-throw switch;

[0236] 2) Establish the expression for the transformer winding load voltage, i.e.:

[0237]

[0238] In the formula, U K U is the load voltage when the regulating winding is disconnected from the main winding. ZG The voltage regulating winding is connected to the circuit with positive polarity, and the highest tap position is connected to the load voltage when the selector tap is carrying current; U FL The load voltage when the voltage regulating winding is connected to the circuit with reverse polarity, and the selector tap is connected at the lowest tap position, is a voltage vector in the expression.

[0239] Example 5:

[0240] A method for limiting overvoltage of a polarity selection switch is described in Embodiment 4. In this method, the moving contact of the single-pole double-throw switch is connected to the main winding of the transformer, and the stationary contact is connected to the tap of the highest position tap selector and the tap of the lowest position tap selector of the voltage regulating winding, respectively.

[0241] Example 6:

[0242] A method for limiting overvoltage of a polarity selection switch floating circuit is described in Example 4. In the transformer circuit containing the polarity selection switch, a capacitor is connected between the middle of the main winding, the middle of the voltage regulating winding, and the grounding point.

[0243] Example 7:

[0244] A method for limiting overvoltage of a polarity-selective switch floating circuit is described in Example 3. The coupling capacitor C1 between the main winding and the regulating winding, and the coupling capacitor C2 between the regulating winding and ground are shown below:

[0245]

[0246]

[0247] In the formula, ε we1 The dielectric constant of the insulation between the main winding and the voltage regulating winding; h1 is the average height of the main winding and the voltage regulating winding; R TN R is the inner radius of the voltage regulating winding; HW Main winding outer radius; ε we2 h1 is the dielectric constant of the insulation between the voltage regulating winding and the oil tank; h2 is the height of the voltage regulating winding; R YN R is the radius of the inner wall of the fuel tank; TW This is the outer radius of the voltage regulating winding.

[0248] Example 8:

[0249] A method for limiting floating overvoltage of a polarity selector switch is described in Embodiment 3. The criteria for determining whether the tap winding is connected to a potential resistor for limiting floating overvoltage include: if the recovery voltage exceeds the limit of the polarity selector contact, then the tap winding needs to be connected to a potential resistor.

[0250] Example 9:

[0251] A method for limiting overvoltage of a polarity-selective switch floating circuit is described in Example 3. The steps for obtaining basic transformer data include:

[0252] 1) Obtain transformer performance parameters, including rated capacity, rated voltage, voltage regulation range, winding connection method, and insulation level;

[0253] 2) Obtain the winding arrangement, that is, obtain the relative position of the voltage regulating winding and adjacent windings;

[0254] 3) Obtain the capacitance of the voltage regulating winding to the adjacent winding or the capacitance of the tap winding to ground or to the adjacent winding grounded:

[0255] 4) Obtain the AC operating voltage at both ends of the winding adjacent to the voltage regulating winding or the winding position;

[0256] 5) Calculate the expected gradient of the lightning impulse voltage across half of the voltage regulating winding;

[0257] 6) Calculate the operating voltage of half of the voltage regulating winding during operation and the power frequency AC voltage during testing.

[0258] Example 10:

[0259] A method for limiting the floating overvoltage of a polarity selection switch is described in Example 3. After determining the number and connection method of the connected potential resistors, it is determined whether the breaking strength of the polarity selection switch contacts and the thermal load strength of the potential resistors are within the allowable limits. If not, the number and connection method of the connected potential resistors are re-determined.

Claims

1. A method for limiting overvoltage of a polarity-selective switch floating circuit, characterized in that, Includes the following steps: Step 1) Based on the working principle of the polarity selection switch, establish the expression for the transformer winding load voltage containing the polarity selection switch; Step 2) Based on the expression for the transformer winding load voltage, establish the expressions for the recovery voltage and breaking current of each contact of the polarity switch; The breaking current expression for the two contacts of the polarity switch is as follows: （1） （2） In the formula, |I Switch1 |、|I Switch2 | These represent the breaking currents of the first and second contacts of the polarity selection switch, respectively; The power supply frequency; C1 and C2 are the coupling capacitors between the main winding and the voltage regulating winding, and the coupling capacitor between the voltage regulating winding and ground; U HV U is the voltage on the high-voltage winding; tap This refers to the voltage of the regulating winding; The recovery voltages of each contact of the polarity switch are shown below: （3） （4） In the formula, U R+ U R- It is the recovery voltage of the first and second contacts of the polarity selection switch; Step 3) Based on the recovery voltage and breaking current of each contact of the polarity switch, determine whether the tap winding needs to be connected with a potential resistor to limit the floating overvoltage, and determine the number and connection method of the potential resistors to be connected. The steps for determining the number and connection method of the potential resistors to be connected include: Step 3.1) Obtain basic transformer data; Step 3.2) Based on the basic data of the transformer, determine the number n of potential resistance elements, that is: （5） In the formula, n is the number of potential-resistive elements; [U R [This refers to the maximum allowable operating voltage for each potentiometer element;] Step 3.3) Determine the resistance value R of the potential resistor. p The number of matched elements [n], i.e.: （6） （7） In the formula, [P d [I] Allows for continuous heat load power dissipation for each resistive element; switch The capacitance current flows through the resistor; Step 3.4) Determine if the number of matching elements [n] ≤ 5 is true. If yes, the connection method of the potential resistor is constant. If not, proceed to step 3.5). Step 3.5) Connect the potential resistor briefly using a potential switch. During this time, the potential resistor continuously dissipates heat P. d Resistance value R p The number n is updated as follows: （8） （9） （10） In the formula, P s The power dissipation is the current flowing through the capacitor across the potential resistor; [P] s [This refers to the allowable short-term heat load power consumption for each resistive element;] Step 3.6) Determine whether the current recovery voltage is less than or equal to the limit of the polarity selector contact. If yes, end the process; otherwise, re-determine the number and connection method of the potential resistors.

2. The method for limiting overvoltage of a polarity-selective switch floating circuit according to claim 1, characterized in that, Based on the working principle of polarity selection switches, the steps to establish the expression for the load voltage of a transformer winding containing a polarity selection switch include: Step 1) Based on the mechanical structure and working principle of the polarity selection switch, the polarity switch is equivalent to a single-pole double-throw switch; Step 2) Establish the expression for the transformer winding load voltage, that is: （11） In the formula, U K U is the load voltage when the regulating winding is disconnected from the main winding. ZG The voltage regulating winding is connected to the circuit with positive polarity, and the highest tap position is connected to the load voltage when the selector tap is carrying current; U FL The load voltage when the voltage regulating winding is connected to the circuit with reverse polarity, and the selector tap is connected at the lowest tap position, is a voltage vector in the expression.

3. The method for limiting overvoltage of a polarity-selective switch floating circuit according to claim 2, characterized in that, The moving contact of the single-pole double-throw switch is connected to the main winding of the transformer, and the stationary contact is connected to the tap of the highest position tap selector and the tap of the lowest position tap selector of the voltage regulating winding, respectively.

4. The method for limiting overvoltage of a polarity-selective switch floating circuit according to claim 2, characterized in that, In a transformer circuit containing a polarity selection switch, a capacitor is connected between the middle of the main winding, the middle of the voltage regulating winding, and the grounding point.

5. The method for limiting overvoltage of a polarity-selective switch floating circuit according to claim 1, characterized in that, The coupling capacitor C1 between the main winding and the voltage regulating winding, and the coupling capacitor C2 between the voltage regulating winding and ground are shown below: （12） （13） In the formula, ε we1 The dielectric constant of the insulation between the main winding and the voltage regulating winding; h1 is the average height of the main winding and the voltage regulating winding; R TN R is the inner radius of the voltage regulating winding; HW Main winding outer radius; ε we2 h1 is the dielectric constant of the insulation between the voltage regulating winding and the oil tank; h2 is the height of the voltage regulating winding; R YN R is the radius of the inner wall of the fuel tank; TW This is the outer radius of the voltage regulating winding.

6. The method for limiting overvoltage of a polarity-selective switch floating according to claim 1, characterized in that, The criteria for determining whether a potential resistor is connected to the tap winding to limit floating overvoltage include: if the recovery voltage exceeds the limit of the polarity selector contact, then the tap winding needs to be connected to a potential resistor.

7. The method for limiting overvoltage of a polarity-selective switch floating circuit according to claim 1, characterized in that, The steps to obtain basic transformer data include: Step 1) Obtain the transformer performance parameters, including rated capacity, rated voltage, voltage regulation range, winding connection method, and insulation level; Step 2) Obtain the winding arrangement, that is, obtain the relative positions of the voltage regulating winding and adjacent windings; Step 3) Obtain the capacitance of the voltage regulating winding to the adjacent winding or the capacitance of the tap winding to ground or to the adjacent winding grounded: Step 4) Obtain the AC operating voltage at both ends of the winding adjacent to the voltage regulating winding or the winding position; Step 5) Calculate the predicted gradient of the lightning impulse voltage across half of the voltage regulating winding; Step 6) Calculate the operating voltage of half of the voltage regulating winding during operation and the power frequency AC voltage during testing.

8. The method for limiting overvoltage of a polarity-selective switch floating circuit according to claim 1, characterized in that, After determining the number and connection method of the potential resistors to be connected, check whether the breaking strength of the polarity selection switch contact and the thermal load strength of the potential resistor are within the allowable limits. If not, then redetermine the number and connection method of the potential resistors to be connected.

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