Zinc Oxide Arrester

Zinc Oxide Arrester is a lightning arrester with good protection performance.

The Zinc Oxide arrester is the most advanced over-voltage protector in the world. This is due to the incorporation of azinc oxide arrester as the core resistor discoThis allows an improvement in the volt-ampere characteristics of the resistor disc and an increase in through-current capability in over-voltage sistuations which is a drastic improvement when compared to conventional silicon carbide arresters.Under normal operating voltage, the current through the arrester is only one micro-ampere degree. When the arrester experiences an aver-voltage situation, the excellent non linear characteristics wil increase the current through the.arrester several thousand times. The arrester is in the conducting state and will release the over-voltage energy to earth and thereby protecting power transmision equipment against the effects of over-voltage.

Zinc oxide arrester is a new type of arrester developed in 1970s, which is mainly composed of zinc oxide varistor.Each varistor has its certain switching voltage (called varistor voltage) from the time it is made. Under the normal working voltage (that is, less than the varistor voltage), the varistor value is very large, which is equivalent to the insulation state. But under the impact voltage (greater than the varistor voltage), the varistor is broken down at a low value, which is equivalent to the short-circuit state.However, after the varistor is hit, it can restore the insulation state;When the voltage above the varistor is withdrawn, it returns to the high resistance state.Therefore, if the zinc oxide arrester is installed on the power line, when lightning strikes, the high voltage of the lightning wave makes the varistor breakdown, and the lightning current flows through the varistor into the earth. The voltage on the power line can be controlled within a safe range, so as to protect the safety of electrical equipment.

Seven characteristics of zinc oxide arrester:

Flow capacity

This is mainly reflected in the lightning arrester has the ability to absorb all kinds of lightning overvoltage, power frequency transient overvoltage and operation overvoltage.

Protective properties

Zinc oxide arrester is used to protect all kinds of electrical equipment in power system from overvoltage damage of electrical products, has good protection performance.Because of the excellent nonlinear voltammetry characteristics of zinc oxide valve sheet, only a few hundred microamps of current pass under the normal operating voltage, it is easy to design a no-gap structure, so that it has the characteristics of good protection performance, light weight and small size.When overvoltage invades, the current flowing through the valve plate increases rapidly, and limits the amplitude of overvoltage and releases the energy of overvoltage. After that, the zinc oxide valve plate restores the state of high resistance to make the power system work normally.

The sealing performance

Lightning arrester components with good aging performance, good air tightness of high-quality composite coat, using control of sealing ring compression and coated sealant and other measures, ceramic coat as sealing material, to ensure reliable sealing, so that the performance of the lightning arrester is stable.

Mechanical properties of

The following three factors are mainly considered: the seismic force to bear;

Maximum wind pressure on the arrester;

The top of the arrester bears the maximum allowable tension of the wire.

Decontamination property

The non – gap zinc oxide arrester has high pollution resistance.

The specific creepage distance stipulated by the national standard is: Class II medium polluted area: specific creepage distance 20mm/ kV;

Class III heavy pollution area: specific creepage distance 25mm/ kV;

Class IV extremely heavy polluted area: specific creepage distance 31mm/ kV.

High operational reliability

The reliability of long-term operation depends on the quality of the products and the reasonable selection of the products.Influence its product quality mainly has the following three aspects: the rationality of the overall structure of the lightning arrester;The voltammetric characteristics and aging resistance of zinc oxide valve;Sealing performance of lightning arrester.

Power frequency tolerance

Due to the power system such as single-phase grounding, long line capacitance effect and load rejection and other reasons, will cause the increase of power frequency voltage or produce high amplitude transient overvoltage, lightning arrester has a certain time to withstand a certain power frequency voltage rise.

Function

Each type of arrester has its own advantages and characteristics, and them needs to be used in different environments to achieve a good insulation effect. Under the rated voltage, the arrester is equivalent to an insulator and will not have any action. In the event of a crisis or high voltage, the lightning arrester will work and lead the current to the ground to effectively protect the power equipment.

Classification

According to voltage level

Zinc oxide arresters are classified according to the rated voltage value and can be divided into three categories:

High voltage category: It refers to the series of zinc oxide arresters above 66KV, which can be roughly divided into seven grades: 1000kV, 750kV, 500kV, 330kV, 220kV, 110kV, and 66kV.

Medium voltage category: It refers to the zinc oxide arrester series products in the range of 3kV to 66kV (excluding 66kV series products), which can be roughly divided into four voltage levels: 3kV, 6kV, 10kV, and 35KV.

Low voltage category: It refers to the zinc oxide arrester series products below 3KV (excluding 3kV series products), which can be roughly divided into four voltage levels: 1kV, 0.5kV, 0.38kV, and 0.22kV.

According to the nominal discharge current

According to the nominal discharge current, zinc oxide arresters can be divided into five classes: 20, 10, 5, 2.5, 1.5kA.

According to use

Zinc oxide arresters can be divided into seven types according to their use: system circuit type, system power station type, system distribution type, shunt compensation capacitor bank protection type, electrified railway type, motor and motor neutral point type, transformer neutral point type.

According to the structure

Zinc oxide arresters can be divided into two categories according to their structure.

Zinc oxide arrester can be divided into four grades according to the pollution resistance performance. Class ⅰ is common type, Class ⅱ is used in medium pollution area (specific creepage distance 20mm/KV), Class ⅲ is used in heavy pollution area (specific creepage distance 25mm/ KV), and class ⅳ is used in extremely heavy pollution area (specific creepage distance 31mm/ KV).

Composite coat: composite coat zinc oxide arrester is made of composite silicone rubber material as the coat, and the selection of high-performance zinc oxide resistance sheet, internal use of special structure, assembled by advanced technology, has the double advantages of silicone rubber material and zinc oxide resistance sheet.In addition to all the advantages of the porcelain zinc oxide arrester, the series of products have good insulation performance, high pollution resistance, good explosion-proof performance and small size, light weight, no maintenance, not easy to damage, reliable sealing, excellent aging resistance and other advantages.

According to the structure performance

Zinc oxide arrester can be divided into three types according to structure and performance: no clearance (W), with series clearance (C), with parallel clearance (B).

1. In the past, the selection and disadvantages of lightning arresters that only considered operating overvoltage and lightning overvoltage levels

The national standard stipulates that the voltage of the power supply terminal of the system should be slightly higher than K times the nominal voltage (or rated voltage) Un of the system, that is, K=Um/Un (Um is the highest voltage of the system). The insulation of electrical equipment should be able to operate for a long time under Un. The K for 220kV and below systems is 1.15, and for 330kV and above systems, K=1.1. The above-mentioned principles are also observed in the initial stage of the arrester design.

2. Selection and necessity of lightning arrester to ensure operation under single-phase grounding overvoltage and safety of power system

From the perspective of safe operation, the selection of rated voltage of the arrester should also comply with the following principles:

(1) The rated voltage of the zinc oxide arrester should be higher than the power frequency transient voltage that may appear at its installation.In neutral grounding system of 110kV and above, it can be selected according to the above method.

(2) in 110 kv and below the neutral indirect grounding system, the electric power department rules under the condition of single-phase grounding allowed to run 2 h, and sometimes even in produced in an intermittent arc grounding overvoltage run more than 2 h to find fault, this kind of system operation characteristics of zinc oxide lightning arrester safe operation under rated voltage 10 s pose a serious threat.And zinc oxide arrester and SiC arrester structure, design is different (the latter is a gap arc extinction, the former has no gap or only separation gap), so that in practice zinc oxide arrester thermal collapse and even serious explosion accidents.Faced with this situation, many power supply bureaux and electric power design institutes put forward many types of rated voltage values (such as 14.4kV, 14.7kV, etc.) according to the local power grid conditions.In the action load test of several national standard discussion papers, the rated voltage tolerance of 10s is increased to 1.2 ~ 1.3 times, so that the adaptability of zinc oxide arrester to the neutral point non-direct grounding system is improved.

Because the rated voltage of zinc oxide arrester is too low, the arrester will work under single-phase grounding overvoltage or even many transient overvoltage.The Department of Safety Supervision and Production Coordination of the Ministry of Electricity proposed the revision of the arrester as early as in the 17th safety bulletin dated 30 October 1993.

In the transition period between the announcement and the revision of the new standard, the following design rules are proposed for the selection of the rated voltage and the continuous operation voltage of the zinc oxide arrester in the neutral non-grounded system: the rated voltage is 1.2-1.3 times of the reference SiC arrester arc extinguishing voltage design, and the continuous operation voltage is the highest line voltage of the system operation

Because there is no unified standard for the above basic data, there will be discrepancy between the lightning arrester manufacturer and the user in the design and manufacture.

3, implement the 2000 edition of the new standard, safe and reasonable selection of the lightning arrester reality

In China’s 2000 new standard (GB11032-2000), the rated voltage selection above 1.2-1.3 times principle has been recognized, but the selection of continuous operation voltage has a new provision: from the reflection of the arrester service life of 1.5Un//U1mA as a reference value to select (design) the arrester continuous operation voltage.With the design and manufacturing level of the domestic arrester, the general?Value is 80%, so the continuous operating voltage is selected as 0.8 times of the rated voltage.This is supported by the small current region of the voltammetry curve.

Therefore, in practice, the overvoltage level of 1h single-phase grounding operation should be taken into account when conducting simulation calculation according to specific conditions or selecting the arrester according to experience and practice.However, there are many rated voltage values slightly different from the above values in the technical agreements of users and even in the drawings of power design institutes, which I think are unnecessary (such as 16.5kV and 16.7kV in 10kV, etc.).The reason is that in the actual design process of the lightning arrester, the rated voltage value in the volt-ampere curve is in the small current area, which is less than the U1mAAC value, the pursuit of subtle difference can not be realized in the actual design of the lightning arrester;In addition, from the following discussion, in accordance with the requirements of the new national standard to select safe use under the permitted overvoltage (this refers to the ungrounded system).

4. Scientific selection of ungrounded zinc oxide arrester according to the new standard of 2000 edition

(1) The rated voltage shall be selected and designed according to the maximum effective value of allowable power frequency voltage applied to the terminals of the arrester, so that it can work correctly under the transient overvoltage determined in the specified action load test.The continuous operating voltage must be selected to allow constant RMS between the arrester terminals.At this time, the power frequency discharge voltage should be high enough to avoid the operation of the insulation of the protected equipment under the operation overvoltage without protection and prolong the service life, and the actual level of charge rate and residual voltage of the zinc oxide arrester manufactured in China must be taken into account.

(2) Where the power frequency voltage rises more seriously or the insulation test voltage of the equipment is higher, the higher rated voltage of the zinc oxide arrester should be selected.The selection of power frequency reference voltage shall be equal to or greater than the rated voltage.These two points in the new NATIONAL standard requirements are better satisfied, the following calculation can also be found to meet the requirements of overvoltage.National standard requirements, to ensure that the single-phase grounding operation 2h without action.In the most serious case, when single-phase grounding and load rejection occur at the same time, the maximum possible overvoltage calculated by theory is 1.99 times, and the allowable continuous operating voltage UC(effective value) of zinc oxide arrester is selected as follows:

The rated voltage (Ur≈1.25 UC) was selected according to the national standard charging rate of 0.8, which all met the requirements.If the arc grounding and resonant overvoltage with high escape probability are used, the rated voltage shall meet: press again?=0.8 Select continuous operating voltage, which also meets the requirements.

To sum up, the main difficulty of the selection of arrester is to determine the scope of temporary overvoltage. It is necessary to ensure safe and accurate operation under high operating overvoltage and atmospheric overvoltage, and to ensure that the valve plate does not operate under temporary overvoltage.

At present, the type selection and design of the arrester must ensure that the system’s highest overvoltage zinc oxide arrester does not work when 2h single-phase grounding, otherwise the zinc oxide arrester will appear thermal collapse or even explosion accident.

Therefore, it is appropriate to select the ungrounded system according to the new requirements.

But in the arc suppression coil grounded capacitor device, grounding overvoltage will be much lower, then can choose low according to the practical simulation calculation of rated voltage and continuous operating voltage to make zinc oxide lightning arrester action under the lower operating overvoltage, capacitor protection device, but if you don’t convenient, also can choose according to no grounding system,

Because the capacitor pole to ground insulation has been considered to meet the single-phase grounding 2h requirements.

Operating at less than rated voltage, the arrester does not operate and will not cause overvoltage damage to the capacitor device.

All in all, this is because the zinc oxide series valve plates without clearance directly in series, in zinc oxide lightning arrester resistors can’t bear even more than 1.99 times the overvoltage, caused by SiC arcing voltage reference choice of zinc oxide lightning arrester rated voltage can’t meet the requirements, is bound to rise to ensure the safety of the arrester, such as no actual simulation data,

The recommended value embodied in the spirit of the national standard is more appropriate because it meets the limit requirements.