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 present invention belongs to the technical field of selecting and matching varistors for lightning arresters, and particularly relates to a method for selecting and balancing zinc oxide varistors for EM lightning arresters in a DC transmission system. Background Art

[0002] A new power system characterized by the interaction of power sources, grids, loads, and energy storage and multi-energy complementarity is the key to ensuring energy and power and supporting the power demand of social development. DC transmission has unique advantages in supporting the integration and grid connection of distributed new energy, the collaborative interconnection of cross-regional power grids, and improving the efficiency and stability of long-distance power transmission [1] , and is the key to determining the success or failure of the construction of the new power system. To meet the needs of operation and maintenance, the DC transmission converter station needs to regularly perform mutual conversion operations of operation modes such as bipolar operation, single-pole ground return, and single-pole metal return. The overvoltage generated during the conversion process can cause faults such as converter valve locking and external insulation breakdown, and it is necessary to configure a metallic return arrester (abbreviated as EM arrester) to suppress it.

[0003] Standards such as GB / T 25083-2010: "Metal Oxide Arresters for ±800 kV DC Systems" have specified in detail the varistor selection criteria for EM arresters used in DC systems. As an arrester with multiple columns in parallel, the selection and matching principles mainly include DC reference voltage tests, residual voltage tests, 2 ms square wave test screening, and current distribution screening tests. In fact, this selection and matching criterion mainly refers to the parameters of arresters in AC power stations in GB / T 11032-2020: "AC Non-Clearance Metal Oxide Arresters". However, the overvoltage borne by EM arresters is quite different from that in AC systems. Using the traditional varistor selection and configuration principles to configure DC metallic return arresters has caused multiple serious arrester damage accidents and led to the shutdown of DC transmission systems. Therefore, it is urgent to conduct research on the selection and matching principles for metallic return arresters in high-voltage DC systems to prevent such accidents from occurring repeatedly.

[0004] Currently, scholars have proposed various varistor 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 to connect a linear resistor in series in the varistor column to form a current negative feedback to achieve a self-balancing effect, which can significantly reduce the uneven coefficient of impulse current distribution. However, this method has a relatively complex operation process and a relatively high cost of adding varistors. Liu Zhiyuan et al. took the ±660 kV Ningdong converter station as an example, and proposed the parameter requirements for the metal transfer switch arrester through theoretical calculations and simulation analyses, 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 varistors of EM arresters in a DC transmission system, aiming at the metal return line arresters in the DC transmission system, improving the long-wave tolerance and current sharing characteristics, and reducing the failures of EM arresters caused by uneven current sharing.

[0006] To achieve the above object, the present invention adopts the following technical solutions: A method for selecting zinc oxide varistors of EM arresters in a DC transmission system, comprising the following steps: S1. Test the DC parameters of the varistors one by one. Based on the test results of the DC parameters, reject the damp and aged varistors; S2. Apply lightning and switching impulses to the varistors one by one, and measure the residual voltage. Calculate the voltage ratio based on the residual voltage, and reject the varistors whose voltage ratio deviation is not within the set voltage ratio threshold range; S3. Conduct square-wave energy tests on the varistors one by one, and reject the varistors damaged during the test; S4. Conduct long-wave residual voltage tests on the varistors one by one with a test square wave, record the long-wave residual voltage values obtained from the tests, and reject the varistors that break down, flash over, or have their aluminum surfaces burned during the test; calculate the long-wave residual voltage deviation of the varistors under square-wave excitation, and reject the varistors 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 applied in the DC transmission system.

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

[0008] Further, step S2 includes: Apply lightning impulses and switching impulses to the varistors one by one, measure the lightning residual voltage and switching residual voltage of the varistors, 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 reject the varistors whose first voltage ratio or second voltage ratio deviation is not within the set voltage ratio threshold range.

[0009] Further, the voltage ratio threshold range is -1.5% to 1.5%.

[0010] Further, step S3 includes: Apply a square-wave impulse to each resistor chip. The amplitude of the square-wave impulse current is 2000 A, and the time interval between two square-wave impulses is no more than 60 s. Place the resistor chip at ambient temperature, and apply a 4th-order square-wave impulse to each resistor chip. The amplitude of the square-wave impulse current is 1000 A, and the time interval between every two square-wave impulses is no more than 60 s. Reject the damaged resistor chips.

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

[0012] Furthermore, in step S4, the test shows that the oscillation or initial overshoot of the square wave does not exceed 10% of the current peak, and the current peak is 100% - 110% of the specified value. Calculate the residual voltage deviation of the resistor chip under square-wave excitation, and reject the resistor chips with excessive residual voltage deviation.

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

[0014] A method for trimming zinc oxide resistor chips of an EM lightning arrester in a DC transmission system includes the following steps: Step 1: Select chips using the method described in claim 1. Step 2: Group the chips according to the long-wave residual voltage value, the number of columns and groups designed for the EM lightning arrester, so that the long-wave residual voltage value of each column is close. Step 3: Conduct a long-wave impulse test on multiple columns of resistor chips, measure the current of each column of multiple groups of parallel-connected resistor chips under long-wave impulse, and calculate the current non-uniformity coefficient in the case of multiple columns in parallel based on the current of each column. Step 4: Verify whether the current non-uniformity coefficients of each group in the case of multiple columns in parallel meet the constraint conditions: if they meet, the process ends; if not, replace the resistor chips with large current differences and reorder and group them.

[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 directed to EM lightning arresters. Based on the traditional selection principles, overvoltage withstand screening of EM lightning arresters in actual DC projects is also carried out, and the uneven characteristics in the case of multi-column parallel connection of EM lightning arresters are further restricted. The overvoltage withstand of EM lightning arresters in actual DC projects is fully considered, and the uneven characteristics in the case of multi-column parallel connection are further restricted. Before assembly, defective resistor chips under long-wave excitation are screened, the chip selection scheme of EM lightning arresters is improved, the accident rate is reduced, and the stability of system operation is improved. Furthermore, since it is difficult to directly screen the uneven discharge of multi-column parallel connection during the chip selection process, the present invention converts the uneven characteristics between the columns of the lightning arrester into controlling the deviation of the long-wave residual voltage of the lightning arrester, so that the resistor chips meet the constraint condition that the uneven coefficient is less than 1.1.

[0017] Furthermore, by verifying the uneven coefficient of the resistor chips in the same group under the condition of multi-column parallel connection, the resistor chips are further restricted to avoid the problem of damage to the lightning arrester caused by uneven energy distribution between columns. Description of the Drawings

[0018] Figure 1 is the chip selection flow chart of the present invention; Figure 2 is the impulse current generator circuit; In the figure C is the total capacitance of many parallel capacitors, L and R are the inductance and resistance values including capacitors, loop connections, shunts and spark gaps. G is the spark gap, and D is the silicon stack. r is the protective resistor, T is the charging transformer, O is the resistor chip to be measured, C1 and C2 are voltage dividers, and CRO is the oscilloscope; Figure 3 is the overvoltage waveform endured by the EM lightning arrester in a certain accident. The first overvoltage is caused by a fault and lasts for about 20 ms, which supports the determination of the long-wave screening test parameters in the present invention. Detailed Embodiments

[0019] The present invention will be described in detail below in conjunction with the 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 a part of the embodiments of the present invention, rather than all the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts shall 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 can be directly on the other element or there can also be another element in the middle. When an element is considered to be "connected to" another element, it can be directly connected to the other element or there may be another element in the middle at the same time. The orientation or positional relationship indicated by the terms "upper", "lower", "front", "rear", "left", "right", "top", "bottom", "inner", "outer", etc. used in this article is based on the orientation or positional relationship shown in the drawings. It is 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 to the present invention.

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

[0023] In the traditional selection and matching method of EM lightning arresters, the AC resistor chips are mostly referred to, but the actual overvoltage withstand is quite different, resulting in problems in the conventional chip selection and trimming schemes, and accidents have been caused many times. For this reason, the present invention proposes a new method for selecting chips and trimming for metal return line lightning arresters in DC systems, fully considering the overvoltage withstand of EM lightning arresters in actual DC projects and further constraining the uneven characteristics in the case of multi-column parallel connection, improving the chip selection scheme of EM lightning arresters, and enhancing the stability of system operation.

[0024] Referring to Figure 1 , a method for selecting and trimming zinc oxide resistor chips for metal return line lightning arresters in a DC transmission system includes the following steps: S1. DC parameter testing: By measuring the DC reference voltage and leakage current parameters of the resistor chips, check whether the resistor chips are damp or aged; measure the DC reference voltage of each resistor chip at a current of 1 mA and the leakage current at 0.75 times the DC reference voltage one by one, and eliminate the resistor chips whose DC reference voltage deviates from the average value by more than 4%, eliminate the resistor chips whose leakage current at 0.75 times the DC reference voltage deviates from the average value by more than ±10 μA, and eliminate the resistor chips whose leakage current at 0.75 times the DC reference voltage is greater than 30 μA.

[0025] S2. Lightning / switching impulse test: Use a multi-waveform impulse residual voltage test system to apply lightning and switching impulses to each resistor chip one by one. Measure the lightning impulse residual voltage and switching impulse residual voltage of the resistor chip, calculate the ratio of the lightning impulse residual voltage to the average value of the DC reference voltage to obtain the first voltage ratio, and calculate the ratio of the switching impulse residual voltage to the average value of the DC reference voltage to obtain the second voltage ratio. To ensure that the first voltage ratio and the second voltage ratio of the resistor chip meet the technical specification requirements, reject the resistor chips with a deviation of the first voltage ratio or the second voltage ratio exceeding -1.5% to 1.5%.

[0026] S3. Square wave energy test: Use a square wave impulse energy screening system to conduct a square wave energy test. Adopt a 2 ms square wave, the apparent duration of the impulse current peak is between 2 ms and 2.4 ms, and the oscillation or initial overshoot should not exceed 10% of the current peak. Each time the current peak should be 100% to 110% of the specified value. Apply 2 square wave impulses to each resistor chip one by one, the amplitude of the square wave impulse current is 2000 A, and the time interval between the 2 square wave impulses is no more than 60 s. Cool the resistor chip naturally to the ambient temperature, and apply 4 square wave impulses to each one by one, the amplitude of the square wave impulse current is 1000 A, and the time interval between every two square wave impulses is no more than 60 s. Reject the resistor chips damaged such as breakdown, flashover, and aluminum surface burn 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 20 ms and an amplitude of 400 A. The oscillation or initial overshoot should not exceed 10% of the current peak, and the current peak should be 100% to 110% of the specified value. Measure and record the residual voltage for each chip one by one, and this residual voltage is the long wave residual voltage value. Reject the resistor chips with breakdown, flashover, and aluminum surface burn during the test; Among them, the pulse width and amplitude of the test square wave are obtained according to the simulation and actual waveform recording results, which characterize the impulse that the EM lightning arrester withstands under real working conditions.

[0028] Secondly, since the EM lightning arrester is a multi-column parallel structure during actual operation, to control the current sharing characteristics of each column, its characteristics are characterized by the parameter unevenness coefficient, and the expression is as follows: (1) In the formula: is the unevenness coefficient; n is the number of parallel columns; is the maximum current peak value passing through the resistor chip column or the lightning arrester element, with the unit of ampere (A); The total current passing through the lightning arrester, with the unit of ampere (A).

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

[0030] As n increases, considering the most severe case, let ， Then (3) According to the non-linear volt-ampere characteristic formula of the resistor chip, as follows: (4) In the formula: C is a constant related to the geometric size of the arrester resistor chip; is the non-linear coefficient related to the material characteristics of the resistor chip. When the impulse current is 100 - 1000 A, it is about 20 - 30.

[0031] Substitute (4) into (3), take as 25, and we can get (5) Calculate the residual voltage deviation of the resistor chip under square-wave excitation, and eliminate the resistor chips with residual voltage deviation exceeding 1.0038.

[0032] Based on the traditional chip selection, the present invention adds the tests of long-wave residual voltage tolerance and consistency, screens the resistor chips that can withstand long waves, and at the same time reduces the non-uniform current distribution of the arrester; S5. Grouping: Sort the resistor chips according to the ascending order of the long-wave residual voltage values, and group them according to the designed number of columns and groups of the EM arrester, so that the calculated residual voltage values of each column are close. Conduct long-wave impulse tests on multiple columns of resistor chips, measure the current of each column of multiple groups of parallel-connected resistor chips under long-wave impulse, and calculate the current non-uniformity coefficient in the case of multiple columns of parallel connection based on the long-wave current of each column of each group of parallel-connected resistor chips.

[0033] S6. Verify whether the grouping meets the constraint condition that the current non-uniformity coefficient in the case of multiple columns of parallel connection is not greater than 1.1. If it holds, the process ends; if not, replace the resistor chips with large current differences and re-sort and group them.

[0034] The main test equipment in the above method includes: 1) DC parameter test system, used to explore whether the resistor chips in the small current region meet the requirements; 2) Multi-waveform impulse residual voltage test system, which can not only be used for the screening of resistor chips, but also meet the requirements of the over-voltage square-wave waveform required for subsequent tests; 3) Square wave impact energy screening system, used for square wave energy testing; 4) Square wave residual voltage testing system, which can not only be used 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 relatively excellent examples of the present invention and are used for those skilled in the art to understand the present invention. However, the present invention is not limited to these embodiments.

[0036] Embodiment 1 Taking the resistor chip of the EM lightning arrester in Factory A as an example, the diameter of the resistor chip is 105 mm, the thickness of the resistor chip is 22 mm, and the volume of the resistor chip is 190 cm 3 .

[0037] S1. First, use the DC parameter tester for lightning arrester resistor chips to check whether the resistor chips are damp or aged, and at the same time detect V 1mA.dc and I 0.75 . Ensure that both are within the specified range, and then conduct the DC reference voltage test for each chip one by one, controlling the deviation of the DC reference voltage of the resistor chip from the average value not exceeding ±4%. Secondly, conduct the leakage current detection of the resistor chip at 0.75 times the DC reference voltage. The leakage current deviation from the average value does not exceed ±10 μA, and the leakage current does not exceed 30 μA. Reject the resistor chips that do not meet the requirements.

[0038] S2. Secondly, operate the residual voltage testing system, and its impulse current generator loop circuit is as Figure 2 shown. Conduct the 8 / 20 μs lightning residual voltage and 30 / 60 μs switching impulse residual voltage tests for each resistor chip one by one, and calculate the ratios of the lightning residual voltage and the switching impulse residual voltage to the DC reference voltage value respectively. Control the voltage ratio of the resistor chip to meet the technical specification requirements and the error is controlled within -1.5% to 1.5%, and reject the resistor chips that do not meet the requirements.

[0039] S3. For energy screening, a 2 ms square wave is used. The apparent duration of the impulse current peak is between 2 ms and 2.4 ms, and 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. It is divided into two tests: (1) Apply 2 impulses to each resistor chip, and the impulse current amplitude is 2000 A. The time interval between the two square wave impulses is no more than 60 s.

[0040] (2) Place the resistor chip at ambient temperature, and then apply 4 impulses to each resistor chip one by one, changing the impulse current amplitude to 1000 A. The time interval between every two square wave impulses is no more than 60 s. After the two tests, reject the resistor chips damaged due to breakdown, flashover, aluminum surface burning, etc.

[0041] S4. According to the oscillogram results when the EM lightning arrester in a certain ±800 kV converter station system fails, as Figure 3 shown, the overvoltage borne by the lightning arrester before the failure is a long wave of about 20 ms. Use a square wave residual voltage test system to conduct a long wave residual voltage test on the varistors. Place the above-mentioned varistors at ambient temperature, use a square wave with a pulse width of 20 ms and an amplitude of 400 A, and control the oscillation or initial overshoot should not exceed 10% of the current peak value, and the current peak value should be 100% - 110% of the required value. Measure and record the residual voltage of each varistor one by one. Eliminate the varistors damaged due to breakdown, flashover, aluminum surface burn, etc. caused by the square wave excitation. Secondly, according to the non-linear relationship between the varistor current and voltage, convert the constraint condition that the current distribution unevenness coefficient does not exceed 1.1 into controlling the residual voltage value of the lightning arrester not to exceed 1.0038, approximately equal to 1.004. Eliminate the varistors with poor long wave residual voltage consistency.

[0042] Example 2 This example is balanced based on the varistor selection results of Example 1, including the following steps: S5. Sort the varistors in ascending order according to the 20 ms square wave residual voltage value, and group the varistors in order, with 4 varistors in each group; S6. Connect the 4 varistors in the same group in parallel to the square wave impulse test system, measure the current of each column of the varistors under the long wave impulse, and calculate the current unevenness coefficient in the case of multi-column parallel connection.

[0043] S7. Verify whether the constraint condition that the current unevenness coefficient in the case of multi-column parallel connection should not be greater than 1.1 holds: If it holds, the process ends; if it does not hold, replace the varistors with large differences and re-sort and group them.

[0044] The term "comprising" used to describe the combination should include the identified elements, components, parts or steps and other elements, components, parts or steps that do not substantially affect the basic novel features of the combination. Using the terms "including" or "comprising" to describe the combination of elements, components, parts or steps here also contemplates embodiments consisting essentially of these elements, components, parts or steps. Here, by using the term "may", it is intended to indicate that any attribute described as "may" include is optional.

[0045] Multiple elements, components, parts or steps can be provided by a single integrated element, component, part or step. Alternatively, a single integrated element, component, part or step can be divided into multiple separate elements, components, parts or steps. The disclosure of "a" or "an" used to describe an element, component, part or step does not mean to exclude other elements, components, parts or steps.

[0046] It should be understood that the above description is for illustrative purposes and not for limitation. Upon reading the above description, many embodiments and many applications other than the provided examples will be apparent to those skilled in the art. Accordingly, the scope of the present teachings should not be determined with reference to the above description, but should be determined with reference to the full scope of the foregoing claims and the equivalents thereof to which those claims are entitled. For the purposes of completeness, all articles and references, including patent applications and publications, are incorporated herein by reference. The omission of any aspect of the subject matter disclosed herein from the foregoing claims is not intended to abandon such subject matter, nor should it be considered that the applicant has not considered such subject matter to be a part of the disclosed inventive subject matter.

Claims

1. Method for selecting zinc oxide varistors of EM lightning arresters for DC transmission systems, characterized in that, It includes the following steps: S1. Conduct DC parameter tests on the resistor chips one by one. Based on the DC parameter test results, reject the moisture-damaged and aged resistor chips; S2. Apply lightning and switching impulses to the resistor chips one by one, and measure the residual voltage. Calculate the voltage ratio based on the residual voltage, and reject the resistor chips whose voltage ratio deviation is not within the set voltage ratio threshold range; S3. Conduct square wave energy tests on the resistor chips one by one, and reject the resistor chips damaged during the tests; S4. Conduct long wave residual voltage tests on the resistor chips one by one with the test square wave, record the obtained long wave residual voltage values, and reject the resistor chips that are punctured, flashovered, or have aluminum surface burns during the tests; Calculate the long wave residual voltage deviation of the resistor chips under square wave excitation, and reject the resistor chips 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 endured by the lightning arrester when applied to the DC transmission system.

2. The method for selecting zinc oxide varistors of the DC transmission system EM lightning arrester according to claim 1, characterized in that The step S1 includes: Measure the DC reference voltage of the resistor chip at a current of 1 mA and the leakage current at 0.75 times the DC reference voltage one by one. Reject the resistor chips whose DC reference voltage deviates from the average value by more than 4%, reject the resistor chips whose leakage current at 0.75 times the DC reference voltage deviates from the average value by more than ±10 μA, and reject the resistor chips whose leakage current at 0.75 times the DC reference voltage is greater than 30 μA.

3. The method for selecting zinc oxide varistors of the EM lightning arrester in the DC transmission system according to claim 1, wherein The step S2 includes: Apply lightning impulse and switching impulse to the resistor chips one by one, measure the lightning residual voltage and switching residual voltage of the resistor chips, 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 reject the resistor chips whose first voltage ratio or second voltage ratio deviation is not within the set voltage ratio threshold range.

4. The method for selecting zinc oxide varistors of the DC transmission system EM lightning arrester according to claim 1 or 3, characterized in that The voltage ratio threshold range is -1.5% to 1.5%.

5. The method for selecting zinc oxide varistors of the EM lightning arrester in the DC transmission system according to claim 1, characterized in that The step S3 includes: Apply 2 square wave impulses to each resistor chip, the amplitude of the square wave impulse current is 2000 A, and the time interval between the two square wave impulses is no more than 60 s; Place the resistor chip at the ambient temperature, apply 4 square wave impulses to each resistor chip, the amplitude of the square wave impulse current is 1000 A, and the time interval between every two square wave impulses is no more than 60 s; Reject the damaged resistor chips.

6. The method for selecting zinc oxide varistors of the EM lightning arrester in the DC transmission system according to claim 5, characterized in that, When applying the square wave impulse, use a 2 ms square wave, the apparent duration of the impulse current peak is between 2 ms and 2.4 ms, 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 varistors of the EM lightning arrester in the DC transmission system according to claim 1, characterized in that, In the step S4, the oscillation or initial overshoot of the test square wave does not exceed 10% of the current peak, and the current peak is 100% to 110% of the specified value; Calculate the residual voltage deviation of the resistor chips under square wave excitation, and reject the resistor chips whose residual voltage deviation exceeds.

8. The method for selecting zinc oxide varistors of the DC transmission system EM lightning arrester according to claim 1, characterized in that, In the step S4, the long wave residual voltage deviation threshold is 1.0038.

9. A method for balancing zinc oxide varistors of an EM lightning arrester in a DC transmission system, characterized in that, It includes the following steps: Step 1. Select chips by using the method described in claim 1; Step 2. Group them according to the long wave residual voltage value, the number of columns and groups designed for the EM lightning arrester, so that the long wave residual voltage values of each column are close; Step 3: Conduct a long-wave impulse test on the multi-column resistor chips, measure the current of each column of the multi-group parallel-connected resistor chips under the long-wave impulse, and calculate the current non-uniformity coefficient in the case of multi-column parallel connection based on the current of each column; Step 4: Verify whether the current non-uniformity coefficients of each group in the case of multi-column parallel connection meet the constraint conditions: if they meet, the process ends; if they do not meet, replace the resistor chips with large current differences and re-order and group them.

10. The ZnO resistor trimming method for the DC transmission system EM lightning arrester according to claim 9, characterized in that, In the said Step 4, the constraint condition is that the current non-uniformity coefficients of each group are not greater than 1.1.

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