Method for selecting and balancing zinc oxide resistor discs of EM lightning arrester of direct-current power transmission system

Abstract

The invention discloses a direct-current power transmission system EM lightning arrester zinc oxide resistor disc selecting and balancing method, and belongs to the technical field of lightning arrester disc selecting and matching. The disc selecting method comprises the steps that S1, direct-current parameter testing is carried out, and damped and aged resistor discs are removed based on the direct-current parameter testing result; s2, applying lightning and operation impact to the resistor discs, measuring residual voltage, calculating a voltage ratio based on the residual voltage, and rejecting the resistor discs of which the voltage ratio deviation is not within a set voltage ratio threshold range; s3, carrying out square wave energy testing on the resistor discs, and removing the damaged resistor discs in the testing process; s4, performing long-wave residual voltage test on the resistor discs one by one by using the test square wave, recording a long-wave residual voltage value obtained by the test, and removing the resistor discs which are subjected to breakdown, flashover and aluminum surface burning in the test; calculating long-wave residual voltage deviation of the resistor discs under square wave excitation, and removing the resistor discs with the long-wave residual voltage deviation exceeding a set long-wave residual voltage deviation threshold value; and the sheet selection scheme of the EM lightning arrester is perfected, and the accident rate is reduced.

Description

Technical Field

[0001] The invention belongs to the technical field of arrester chip selection and matching, and in particular relates to a method for selecting and balancing zinc oxide resistor chips of an EM arrester of a direct current transmission system. Background Art

[0002] The new power system characterized by the interaction of sources, grids, loads and storage and the complementarity of multiple energies is the key to ensuring energy and electricity and supporting the electricity demand of social development, and is of decisive significance for my country to achieve the "dual carbon goals". DC transmission has unique advantages in supporting the integration and grid connection of distributed new energy, cross-regional grid coordination and interconnection, and improving the efficiency and stability of long-distance transmission. [1] , is the key to determine the success or failure of the construction of a new power system. In order to meet the needs of operation and maintenance, DC transmission converter stations need to regularly perform conversion operations between bipolar operation, single-stage earth return line, single-stage metal return line and other operating modes. The overvoltage generated during the conversion process can cause faults such as converter valve locking and external insulation breakdown, and a metallic return arrester (EM arrester) must be configured to suppress it.

[0003] GB / T 25083-2010: "Metal Oxide Arrester for ±800kV DC System" and other standards have detailed provisions for the selection criteria of EM arresters used in DC systems. As a multi-column parallel arrester, the selection principles mainly include DC reference voltage test, residual voltage test, 2ms square wave test screening and current distribution screening. In fact, the selection criteria are mostly based on the parameters of AC power station arresters in GB / T 11032-2020: "AC Gapless Metal Oxide Arrester". However, there is a big difference between the overvoltage borne by EM arresters and the overvoltage in AC systems. The use of traditional selection and configuration principles to configure DC metal return line arresters has caused multiple arrester malignant damage accidents and caused the DC transmission system to be locked and shut down. Therefore, it is urgent to carry out research on the selection principles for metal return line arresters in high-voltage DC systems to prevent such accidents from recurring.

[0004] At present, scholars have proposed a variety of selection schemes for improving EM arresters. Hu Shuhui et al. suggested increasing the energy design value of the EM arrester on the rectifier side of the DC project to 50 MJ, and proposed connecting a linear resistor in series in the resistor column to form a negative current feedback to achieve a self-balancing effect, which can significantly reduce the uneven distribution coefficient of the impact current. However, the operation process of this method is relatively complicated, and the cost of adding resistors is high. Taking the ±660 kV Ningdong Converter Station as an example, Liu Zhiyuan et al. proposed the parameter requirements of the metal transfer switch arrester through theoretical calculation and simulation analysis, and redesigned the overall structure of the arrester group. This design scheme is complex and has a small scope of application. Summary of the invention

[0005] The present invention provides a method for selecting and balancing zinc oxide resistors of EM arresters in a direct current transmission system, which aims at improving the long-wave tolerance and current sharing characteristics of metal return line arresters in the direct current transmission system and reducing the failure of the EM arrester caused by uneven current distribution.

[0006] In order to achieve the above object, the present invention adopts the following technical solution: A method for selecting zinc oxide resistors for EM lightning arresters in a direct current transmission system comprises the following steps: S1. Perform DC parameter tests on resistors piece by piece, and remove damp or aged resistors based on the DC parameter test results; S2, applying lightning and operating shock to each resistor piece by piece, measuring the residual voltage, calculating the pressure ratio based on the residual voltage, and eliminating the resistor pieces whose pressure ratio deviation is not within the set pressure ratio threshold range; S3, perform square wave energy test on each resistor piece by piece, and remove the resistor pieces damaged during the test; S4, using a test square wave to perform a long-wave residual voltage test on each resistor piece by piece, recording the long-wave residual voltage value obtained in the test, and eliminating the resistor pieces that have breakdown, flashover, or aluminum surface burns during the test; calculating the long-wave residual voltage deviation of the resistor piece under square wave excitation, and eliminating the resistor piece whose long-wave residual voltage deviation exceeds the set long-wave residual voltage deviation threshold; The pulse width and amplitude of the test square wave are determined according to the parameters of the overvoltage that the arrester withstands when it is applied in a direct current power transmission system.

[0007] Further, step S1 includes: Measure the DC reference voltage of each resistor at a current of 1mA and the leakage current at 0.75 times the DC reference voltage for each resistor, and eliminate the resistors whose DC reference voltage deviates from the average value by more than 4%, the resistors whose leakage current deviates from the average value by more than ±10μA at 0.75 times the DC reference voltage, and the resistors whose leakage current is greater than 30μA at 0.75 times the DC reference voltage.

[0008] Further, step S2 includes: Apply lightning impulse and switching impulse to the resistor piece by piece, measure the lightning residual voltage and switching residual voltage of the resistor piece, calculate the ratio of the lightning residual voltage to the average value of the DC reference voltage to obtain the first voltage ratio, calculate the ratio of the switching residual voltage to the average value of the DC reference voltage to obtain the second voltage ratio, and eliminate the resistor pieces whose deviation of the first voltage ratio or the second voltage ratio is not within the set voltage ratio threshold range.

[0009] Furthermore, the pressure ratio threshold range is -1.5% ~ 1.5%.

[0010] Further, step S3 includes: Apply two square wave impulses to each resistor piece one by one. The square wave impulse current amplitude is 2000A. The time interval between the two square wave impulses is no more than 60s. Place the resistors at ambient temperature, apply four square wave shocks to each resistor, the square wave shock current amplitude is 1000A, and the time interval between each two square wave shocks is no more than 60s; Remove the damaged resistor.

[0011] Furthermore, a 2ms square wave is used for square wave impulse, the apparent duration of the impulse current peak is between 2ms and 2.4ms, the oscillation or initial overshoot does not exceed 10% of the current peak, and each current peak is 100% to 110% of the specified value.

[0012] Further, in step S4, the square wave oscillation or initial overshoot is tested to be no more than 10% of the current peak value, and the current peak value is 100% to 110% of the specified value; Calculate the residual voltage deviation of the resistor under square wave excitation and remove the resistors with residual voltage deviation exceeding the limit.

[0013] Further, in step S4, the long-wave residual pressure deviation threshold is 1.0038.

[0014] A method for balancing zinc oxide resistors of EM lightning arresters in a direct current transmission system comprises the following steps: Step 1, selecting films using the method described in claim 1; Step 2: According to the long-wave residual voltage value, the number of designed EM arrester columns and the number of groups, the groups are matched so that the long-wave residual voltage value of each column is close; Step 3, carrying out a long wave impact test on the multi-column resistor, measuring the current of each column of multiple groups of parallel resistors under the long wave impact, and calculating the current non-uniformity coefficient of the multi-column parallel case based on the current of each column; Step 4: Verify whether the current non-uniformity coefficient of each group meets the constraint conditions when multiple columns are connected in parallel: if so, the process ends; if not, replace the resistors with large current differences and re-arrange the groups.

[0015] Furthermore, in step 4, the constraint condition is that the current non-uniformity coefficient of each group is not greater than 1.1.

[0016] Compared with the prior art, the present invention has at least the following beneficial technical effects: The present invention is aimed at EM lightning arresters. On the basis of traditional matching principles, the present invention further performs overvoltage tolerance screening on EM lightning arresters in actual DC projects and further constrains the uneven characteristics of EM lightning arresters in a case of multi-column parallel connection. The present invention fully considers the overvoltage tolerance on EM lightning arresters in actual DC projects, further constrains the uneven characteristics of multi-column parallel connection, screens unqualified resistors under long-wave excitation before assembly, improves the resistor selection scheme for EM lightning arresters, reduces the accident rate, and improves the stability of system operation. Furthermore, since it is difficult to directly screen the uneven discharge flow under multiple columns in parallel during the selection process, the present invention converts the uneven characteristics between the arrester columns into controlling the long-wave residual voltage deviation of the arrester, so that the resistor sheet meets the constraint condition that the uneven coefficient is less than 1.1.

[0017] Furthermore, by verifying the non-uniformity coefficient of the resistors in the same group when multiple columns are connected in parallel, the resistors are further constrained to avoid the problem of damage to the arrester caused by uneven energy distribution between each column. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Figure 1 It is the film selection flow chart of the present invention; Figure 2 It is the impulse current generator circuit; In the picture C is the total capacitance of many capacitors connected in parallel, L and R is the inductance and resistance value including capacitor, loop connection, shunt and ball gap, G is the ball gap, D is the silicon stack, r is the protective resistor, T is the charging transformer, O is the resistor to be tested, C1 and C2 are voltage dividers, and CRO is the oscilloscope; Figure 3 It is the overvoltage waveform that the EM lightning arrester is subjected to in a certain accident, wherein the first overvoltage is caused by a fault and lasts for about 20ms, which supports the determination of the long-wave screening test parameters in the present invention. DETAILED DESCRIPTION

[0019] The present invention is described in detail below with reference to the accompanying drawings and specific embodiments.

[0020] In order to enable those skilled in the art to better understand the technical solutions in the present invention, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work should fall within the scope of protection of the present invention.

[0021] It should be noted that when an element is referred to as being "disposed on" another element, it may be directly on the other element or there may be another element centered thereon. When an element is considered to be "connected to" another element, it may be directly connected to the other element or there may be another element centered thereon at the same time. The terms "upper", "lower", "front", "back", "left", "right", "top", "bottom", "inside", "outside", etc. used herein indicate orientations or positional relationships based on the orientations or positional relationships shown in the accompanying drawings, and are only for the convenience of describing the present invention and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore cannot be understood as a limitation on the present invention.

[0022] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as those generally understood by those skilled in the art of the present invention. The terms used herein in the specification of the present invention are only for the purpose of describing specific embodiments and are not intended to limit the present invention. The term "and / or" used herein includes any and all combinations of one or more related listed items.

[0023] Traditional selection methods for EM arresters mostly refer to AC resistors, but the actual overvoltages vary greatly, resulting in problems with conventional selection and balancing solutions, which have caused accidents many times. To this end, the present invention proposes a new selection and balancing method for DC system metal return line arresters, which fully considers the overvoltage tolerance of EM arresters in actual DC projects and further constrains the uneven characteristics of multi-column parallel connection, improves the selection solution of EM arresters, and improves the stability of system operation.

[0024] Reference Figure 1 , a method for selecting and balancing zinc oxide resistors for metal return line arresters in a DC transmission system, comprising the following steps: S1. DC parameter test: Check whether the resistor is damp or aged by measuring the DC reference voltage and leakage current parameters of the resistor; measure the DC reference voltage of the resistor at a current of 1mA and the leakage current at 0.75 times the DC reference voltage for each resistor, eliminate the resistors whose DC reference voltage deviates from the average value by more than 4%, eliminate the resistors whose leakage current deviates from the average value by more than ±10μA at 0.75 times the DC reference voltage, and eliminate the resistors whose leakage current is greater than 30μA at 0.75 times the DC reference voltage.

[0025] S2. Lightning / operation impulse test: Use a multi-waveform impulse residual voltage test system to apply lightning and operation impulse to each resistor piece, measure the lightning residual voltage and operation residual voltage of the resistor piece, calculate the ratio of the lightning residual voltage to the average value of the DC reference voltage to obtain the first voltage ratio, and calculate the ratio of the operation residual voltage to the average value of the DC reference voltage to obtain the second voltage ratio. In order to make the first voltage ratio and the second voltage ratio of the resistor piece meet the requirements of the technical specifications, eliminate the resistor pieces with the deviation of the first voltage ratio or the second voltage ratio exceeding -1.5% ~ 1.5%.

[0026] S3. Square wave energy test: Use a square wave impact energy screening system to perform square wave energy test, using a 2ms square wave, the apparent duration of the impact current peak is between 2ms and 2.4ms, the oscillation or initial overshoot should not exceed 10% of the current peak, and each current peak should be 100% to 110% of the specified value. Apply two square wave impacts to each resistor, the square wave impact current amplitude is 2000A, and the time interval between the two square wave impacts is no more than 60s. Cool the resistors naturally to ambient temperature, apply four square wave impacts to each resistor, the square wave impact current amplitude is 1000A, and the time interval between each two square wave impacts is no more than 60s. Eliminate resistors that have been damaged by breakdown, flashover, aluminum surface burns, etc. during the test.

[0027] S4. Long-wave residual voltage test: Use a square wave residual voltage test system to carry out a long-wave residual voltage test. Select a test square wave with a pulse width of 20ms and an amplitude of 400A. The oscillation or initial overshoot should not exceed 10% of the current peak value. The current peak value should be 100% to 110% of the specified value. Measure and record the residual voltage piece by piece. This residual voltage is the long-wave residual voltage value. Eliminate resistors that have breakdown, flashover, or aluminum surface burns during the test; The pulse width and amplitude of the test square wave are obtained according to the simulation and actual recording results, which characterize the impact that the EM lightning arrester withstands under real working conditions.

[0028] Secondly, since the EM arrester is a multi-column parallel structure in actual operation, in order to control the current-sharing characteristics of each column, the parameter non-uniformity coefficient is used to characterize its characteristics. The expression is as follows: (1) Where: is the non-uniformity coefficient; n is the number of parallel columns; The maximum current peak value passing through the resistor column or lightning arrester element, in ampere (A); The total current passing through the arrester, in amperes (A).

[0029] The national standard stipulates that the non-uniformity coefficient should be less than 1.1 when multiple columns are connected in parallel, but the current coefficient is difficult to measure in actual measurement. The present invention converts the non-linear relationship between the current and voltage of the resistor into a voltage relationship for measurement. Taking the case where the non-uniformity coefficient is the largest, equation (1) is converted to: (2) Where: ,in .

[0030] along with n Considering the most serious situation, let ， but (3) According to the nonlinear volt-ampere characteristic formula of the resistor, it is as follows: (4) Where: C is a constant related to the geometric dimensions of the arrester resistor; It is a nonlinear coefficient related to the material characteristics of the resistor. When the impulse current is 100~1000 A, it is about 20~30.

[0031] Substituting (4) into (3), we get is 25, so (5) Calculate the residual voltage deviation of the resistor under square wave excitation and eliminate the resistors with residual voltage deviation exceeding 1.0038.

[0032] The present invention adds long-wave residual voltage tolerance and consistency tests on the basis of traditional resistor selection to select resistors that can tolerate long waves and reduce the uneven current of the arrester. S5. Grouping: Sort the resistors by long-wave residual voltage value from small to large and then group them. Group them according to the number of designed columns and groups of EM arrester so that the calculated residual voltage value of each column is close. Carry out long-wave impulse test on multi-column resistors, measure the current of each column of multiple groups of parallel resistors under long-wave impulse, and calculate the current non-uniformity coefficient of multiple columns in parallel based on the long-wave current of each column of each group of parallel resistors.

[0033] S6. Verify whether the grouping satisfies the constraint condition that the current unevenness coefficient is not greater than 1.1 in the case of multi-column parallel connection. If so, the process ends; if not, replace the resistors with large current differences and re-arrange the grouping.

[0034] The main test equipment in the above method includes: 1) DC parameter test system, used to explore whether the resistor in the low current area meets the requirements; 2) The multi-waveform impact residual voltage test system can not only be used to screen resistors, but also meet the overvoltage square wave waveform requirements required for subsequent testing; 3) Square wave impact energy screening system, used for square wave energy testing; 4) The square wave residual voltage test system can be used not only for square wave energy testing, but also for long wave residual voltage testing.

[0035] The following are specific embodiments. It should be noted that these embodiments are preferred examples of the present invention and are used for those skilled in the art to understand the present invention, but the present invention is not limited to these embodiments.

[0036] Example 1 Take the resistor of EM arrester of factory A as an example. The resistor has a diameter of 105mm, a thickness of 22mm and a volume of 190cm 3 .

[0037] S1. First, use the arrester resistor DC parameter tester to check whether the resistor is damp or aged. At the same time, check V 1mA.dc and I 0.75 , ensure that both are within the specified range, and then conduct a DC reference voltage test on each piece, and control the DC reference voltage of the resistor to deviate from the average value by no more than ±4%. Secondly, conduct a leakage current test on the resistor at 0.75 times the DC reference voltage, and the leakage current deviates from the average value by no more than ±10μA, and the leakage current does not exceed 30μA, and remove the resistors that do not meet the requirements.

[0038] S2. Next, operate the residual voltage test system. The impulse current generator circuit is as follows: Figure 2 As shown. Perform 8 / 20μs lightning residual voltage and 30 / 60μs operating residual voltage tests on each resistor piece by piece, and calculate the ratio of lightning residual voltage, operating residual voltage and DC reference voltage value respectively. Control the resistor piece voltage ratio to meet the technical specification requirements and control the error within -1.5% ~ 1.5%, and eliminate the resistor pieces that do not meet the requirements.

[0039] S3. For energy screening, a 2ms square wave is used. The apparent duration of the impulse current peak is between 2ms and 2.4ms. The oscillation or initial overshoot should not exceed 10% of the current peak. Each current peak should be 100% to 110% of the specified value. The test is divided into two parts: (1) Apply two impulses to each resistor piece one by one. The impulse current amplitude is 2000A. The time interval between the two square wave impulses is no more than 60s.

[0040] (2) The resistors are placed at ambient temperature, and then four impulses are applied to each resistor one by one, with the impulse current amplitude changed to 1000A, and the time interval between each two square wave impulses is no more than 60s. After two tests, the resistors damaged by breakdown, flashover, aluminum surface burns, etc. are eliminated.

[0041] S4. According to the recording results of a ±800kV converter station system EM arrester failure, Figure 3 As shown, the overvoltage that the arrester withstands before the fault is a long wave of about 20ms. Use a square wave residual voltage test system to carry out a long wave residual voltage test on the resistor. Place the above resistor at ambient temperature, use a square wave with a pulse width of 20ms and an amplitude of 400A, and control the oscillation or initial overshoot to not exceed 10% of the current peak value. The current peak value should be 100% to 110% of the required value. Measure and record the residual voltage piece by piece. Eliminate resistors that are damaged by breakdown, flashover, aluminum surface burns, etc. due to square wave excitation. Secondly, based on the nonlinear relationship between the current and voltage of the resistor, the constraint condition that the current distribution unevenness coefficient does not exceed 1.1 is converted into a control arrester residual voltage value of no more than 1.0038, which is approximately equal to 1.004. Eliminate resistors with poor long-wave residual voltage consistency.

[0042] Example 2 This embodiment performs balancing based on the film selection result of Embodiment 1, and includes the following steps: S5. Sort the resistors in ascending order according to the residual voltage values ​​of the 20ms square wave, and group them into groups, with 4 resistors in each group. S6. Connect the four resistors in the same group in parallel to the square wave impulse test system, measure the current of each column of each group of resistors under long wave impulse, and calculate the current unevenness coefficient when multiple columns are connected in parallel.

[0043] S7. Verify whether the constraint condition that the current unevenness coefficient should not be greater than 1.1 in the case of multiple columns in parallel holds: if so, the process ends; if not, replace the resistors with large differences and re-arrange the groups.

[0044] The term "consisting of" describing a combination shall include the identified elements, ingredients, parts or steps and other elements, ingredients, parts or steps that do not substantially affect the basic novel characteristics of the combination. The use of the terms "comprising" or "including" to describe a combination of elements, ingredients, parts or steps herein also contemplates embodiments that consist essentially of these elements, ingredients, parts or steps. By using the term "may", it is intended to illustrate that any of the attributes described that "may" be included are optional.

[0045] Multiple elements, ingredients, parts or steps can be provided by a single integrated element, ingredient, part or step. Alternatively, a single integrated element, ingredient, part or step can be divided into separate multiple elements, ingredients, parts or steps. The disclosure "one" or "an" used to describe an element, ingredient, part or step is not intended to exclude other elements, ingredients, parts or steps.

[0046] It should be understood that the above description is for illustration and not for limitation. Many embodiments and many applications beyond the examples provided will be apparent to those skilled in the art upon reading the above description. Therefore, the scope of the present teachings should not be determined with reference to the above description, but rather with reference to the foregoing claims and the full scope of equivalents to which such claims are entitled. For the purpose of comprehensiveness, all articles and references, including disclosures of patent applications and publications, are incorporated herein by reference. The omission of any aspect of the subject matter disclosed herein in the foregoing claims is not intended to be a waiver of such subject matter, nor should it be considered that the applicant has not considered such subject matter to be part of the disclosed inventive subject matter.

Claims

1. A method for selecting zinc oxide resistors for EM arresters in a DC power transmission system, characterized in that: The following steps are involved: S1. Perform DC parameter tests on resistors piece by piece, and remove damp or aged resistors based on the DC parameter test results; S2, applying lightning and operating shock to each resistor piece by piece, measuring the residual voltage, calculating the pressure ratio based on the residual voltage, and eliminating the resistor pieces whose pressure ratio deviation is not within the set pressure ratio threshold range; S3, perform square wave energy test on each resistor piece by piece, and remove the resistor pieces damaged during the test; S4, using a test square wave to perform a long-wave residual voltage test on each resistor piece by piece, recording the long-wave residual voltage value obtained in the test, and eliminating the resistor pieces that have breakdown, flashover, or aluminum surface burns during the test; calculating the long-wave residual voltage deviation of the resistor piece under square wave excitation, and eliminating the resistor piece whose long-wave residual voltage deviation exceeds the set long-wave residual voltage deviation threshold; The pulse width and amplitude of the test square wave are determined according to the parameters of the overvoltage that the arrester withstands when it is applied in a direct current power transmission system.

2. The method for selecting zinc oxide resistors for EM arresters in a DC power transmission system according to claim 1, characterized in that: The step S1 comprises: Measure the DC reference voltage of each resistor at a current of 1mA and the leakage current at 0.75 times the DC reference voltage for each resistor, and eliminate the resistors whose DC reference voltage deviates from the average value by more than 4%, the resistors whose leakage current deviates from the average value by more than ±10μA at 0.75 times the DC reference voltage, and the resistors whose leakage current is greater than 30μA at 0.75 times the DC reference voltage.

3. The method for selecting zinc oxide resistors for EM arresters in a DC power transmission system according to claim 1, characterized in that: The step S2 comprises: Apply lightning impulse and switching impulse to the resistor piece by piece, measure the lightning residual voltage and switching residual voltage of the resistor piece, calculate the ratio of the lightning residual voltage to the average value of the DC reference voltage to obtain the first voltage ratio, calculate the ratio of the switching residual voltage to the average value of the DC reference voltage to obtain the second voltage ratio, and eliminate the resistor pieces whose deviation of the first voltage ratio or the second voltage ratio is not within the set voltage ratio threshold range.

4. The method for selecting zinc oxide resistors for EM arresters in a DC power transmission system according to claim 1 or 3, characterized in that: The pressure ratio threshold range is -1.5% to 1.5%.

5. The method for selecting zinc oxide resistors for EM arresters in a DC power transmission system according to claim 1, characterized in that: The step S3 comprises: Apply two square wave impulses to each resistor piece one by one. The square wave impulse current amplitude is 2000A. The time interval between the two square wave impulses is no more than 60s. Place the resistors at ambient temperature, apply four square wave shocks to each resistor, the square wave shock current amplitude is 1000A, and the time interval between each two square wave shocks is no more than 60s; Remove the damaged resistor.

6. The method for selecting zinc oxide resistors for EM arresters in a DC power transmission system according to claim 5, characterized in that: A 2ms square wave is used for square wave impact, and the apparent duration of the impact current peak is between 2ms and 2.4ms. The oscillation or initial overshoot does not exceed 10% of the current peak, and each current peak is 100% to 110% of the specified value.

7. The method for selecting zinc oxide resistors for EM arresters in a DC power transmission system according to claim 1, characterized in that: In step S4, the square wave oscillation or initial overshoot is tested to be no more than 10% of the current peak value, and the current peak value is 100% to 110% of the specified value; Calculate the residual voltage deviation of the resistor under square wave excitation and remove the resistors with residual voltage deviation exceeding the limit.

8. The method for selecting zinc oxide resistors for EM arresters in a DC power transmission system according to claim 1, characterized in that: In step S4, the long-wave residual pressure deviation threshold is 1.0038.

9. A method for balancing zinc oxide resistors of EM arresters in a DC power transmission system, characterized in that: The following steps are involved: Step 1, selecting films using the method described in claim 1; Step 2: According to the long-wave residual voltage value, the number of designed EM arrester columns and the number of groups, the groups are matched so that the long-wave residual voltage value of each column is close; Step 3, carrying out a long wave impact test on the multi-column resistor, measuring the current of each column of multiple groups of parallel resistors under the long wave impact, and calculating the current non-uniformity coefficient of the multi-column parallel case based on the current of each column; Step 4: Verify whether the current non-uniformity coefficient of each group meets the constraint conditions when multiple columns are connected in parallel: if so, the process ends; if not, replace the resistors with large current differences and re-arrange the groups.

10. The method for balancing zinc oxide resistors of EM arresters in a DC power transmission system according to claim 9, characterized in that: In step 4, the constraint condition is that the current non-uniformity coefficient of each group is not greater than 1.1.

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