Surge Arrester

Surge Arrester

1. General 1.1 Description of a surge arrester A surge arrester is a protective device for limiting surge voltages on equipment by discharging or bypassing surge current. Surge arresters allow only minimal flow of the 50-hertzpower current to ground. After the high-frequency lightning surge current has been discharged, a surge arrester, correctly applied, will be capable of repeating its protective function until another surge voltage must be discharged. The technology of surge arresters has undergone major changes in the last 100 years. In the early 1900’s, spark gaps were used to suppress over voltages. In the 1930’s, the silicon carbide replaced the spark gaps. In the mid 1970’s, zinc oxide gapless arresters, which possessed superior protection characteristics, replaced the silicon carbide arrester.

1.2 Types of surge arresters Surge arresters used for protection of exterior electrical distribution lines will be either of the metal oxide or gapped silicon carbide type. Expulsion-type units are no longer used. 1.2.1 Metal oxide type A metal oxide surge arrester (MOSA) utilizing zinc oxide blocks provides the best performance, as surge voltage conduction starts and stops promptly at a precise voltage level, thereby improving system protection. Failure is reduced, as there is no air gap contamination possibility; but there is always a small value of leakage current present at power frequencies. Therefore, the arrester’s maximum power-frequency continuous operating voltage (MCOV) can not be exceeded. 1.2.2 Gapped silicon carbide type Silicon carbide has more nonlinearity than zinc oxide. Without a gap the increase in leakage current, because of this nonlinearity, would soon burn out the arrester. A gap prevents burnout, but it does mean that the arrester will not operate until the gap sparks over. Silicon carbide arresters are vulnerable to moisture ingress that leads to failure due to reduction in spark over. Contamination can also upset voltage distribution resulting in spark over reduction. Over a period of time, excessive energy inputs can destroy the ability of the blocks and gaps to interrupt follow current leading to failure of the arrester. The metal oxide arresters are without gaps, unlike the SIC arrester. This “gap-less” design eliminates the high heat associated with the arcing discharges. The MOV arrester has twovoltage rating: duty cycle and maximum continuous operating voltage, unlike the silicone

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carbide that just has the duty cycle rating. A metal oxide surge arrester utilizing zinc oxide blocks provides the best performance, as surge voltage conduction starts and stops promptly at a precise voltage level, thereby improving system protection. Failure is reduced, as there is no air gap contamination possibility; but there is always a small value of leakage current present at operating frequency. Therefore, GECOL uses Metal oxide arrester as surge arrester in the field.

Figure 1 Comparison of silicon Carbide and Metal Oxide arrester 1.2.3 Polymer/Porcelain Arrester Polymer arresters are gaining in popularity over the porcelain arresters. When a reclose operation occurs and the fault has not cleared, the arrester is subjected to a second fault current. This second operation often leads to arrester explosion since the porcelain had already been weakened by the first fault. If the pressure relief rating of the arrester is exceeded, the arrester may fail violently, since it cannot vent the excess gasses. This type of failure can lead to other equipment being damaged or injury to personnel who may be in the vicinity of the failure. Due to the ability of the polymer station arrester to vent out the side, the housing is not weakened when exposed to the fault current. Therefore a polymer arrester can be reclosed on multiple times without the fear of a violent failure. The polymer arresters are less expensive than the porcelain arrester and appear to avoid some of the in service problems, such as moisture ingress, that often occur in porcelain arrester. One manufacturer reports that moisture ingress is the direct cause of failure in 86% of all failures.

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Figure 2 Polymer Arrester

Figure 3 Porcelain Arrester

1.3 Classifications of surge arrester There are three basic arrester classifications recognized by ANSI Standards: distribution, intermediate, and station. The differences in these classifications are in terms of voltage rating, protective characteristics, and the durability in pressure-relief or fault-withstand characteristics. 1.3.1 Distribution class arresters These arresters, the most widely used, are specified by standards as arresters with ratings of 1 through 30kV. In relationship to the other classes of arresters they have the highest discharge voltage (that is, they will allow the highest voltage appear across equipment) for a given incoming surge. There is no requirement for pressure relief. 1.3.2 Intermediate class arresters These arresters are specified as having voltage ratings of 3 through 120kV. They have better protective characteristics than distribution arresters, but generally not as good as station type arresters. Pressure-relief capabilities are required, although some special type of intermediate arresters developed for underground system protection do not have pressure relief. 1.3.3 Station class arresters These arresters offer the lowest discharge voltages (allowing the lowest voltage to appear across equipment) and therefore provide the highest degree of protection. By standards they have ratings between 3 and 684kV and must have pressure-relief capability.

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Characteristics or feature

Arrester Class Distribution

Intermediate

Station

Ratings

1-30 kV

3 – 120kV

3-684kV

Approximate Protective Characteristics (at 10kA)

3.5 p.u

3.0 p.u

2.7 p.u

65kA N.D.

65kA

65kA

5kA

10kA(>550kV)

Current Discharge Requirements High Current, short duration

100kA H.D.. Duty Cycle

5kA N.D. 10kA H.D.

15kA (550kV) 20kA (800kV)

Low Current, Long Duration

75A N.D.

Transmission-Line Discharge

250A H.D.

Test Required

Pressure Relief Hgh Current

Not Required

16.1kA

40-65kA

Low Curent

Not Required

400-600A

400-600A

Table 1. Comparison of Standard Requirements for Surge Arrester Classifications

1.4 Structure

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Figure 4 Gapless metal oxide surge arrester (※This figure will be re-drawn up by Autocad)

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2. Delivery and storage (a) When delivered or moved, surge arresters must be loaded with less than 5 stacks. (b) Falling of surge arresters would have a bad influence on the quality of surge arresters. (c) Surge arresters should be stored in sufficiently dried condition and in a room where dusts don’t occur. Storage in outdoor condition or in corrupted circumstance for long time could have a bad influence on proper performance of them. ※ Handling Suspect Arresters: 1

• A damaged seal-gapped arrester should be handled with care. Due to increased pressure caused by the destruction of internal elements, a defective arrester may become an explosive hazard.

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• If the decision is made to perform an internal inspection of the failed arrester, be assured that the arrester has vented properly

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• Do not “throw away” a defective arrester – the arrester should be properly vented before disposing

3. Inspection 3.1 Relative departments and duties 3.1.1 Inspection for substation class surge arrester (1) Medium voltage maintenance sub-department of regional distribution department (a) Maintenance programming and follow-up division • Scheduling and completion of the corrective repair work • Checking the performing of the inspection on site • Compiling the inspection results (b) Predictive maintenance division • Analysis of the inspection results • Establishing of the countermeasure based on the inspection results (2) Medium voltage regional maintenance centers of regional distribution department (a) Electrical and mechanical maintenance bureau • Conducting of the inspection on site and recording of the results • Repairing or replacing of the inferior materials (equipment) • Reporting of the inspection results and maintaining of the records 3.1.2 Inspection for distribution class surge arrester

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This inspection should be performed by relative departments involved in the department manager assistance for distribution issues in the same way as the substation class surge arrester.

3.2 Inspection methods Modern surge arresters require little operational maintenance and the degree to which such maintenance can be done is normally limited by lack of adequate test equipment. This limits surge arrester maintenance to visual inspection and simple electrical tests. It is recommended that units found to be defective be replaced rather than repaired: Where an arrester is composed of two or more individually complete units, each unit should be tested separately. Thus, a bad unit may readily be replaced and the good units retained. Surge arresters are almost always applied with one terminal connected to an electrically energized source and one terminal to ground. No work should be done, or contact made with surge arresters, when connected to the energized source. Visual inspection will not always detect a damaged arrester. Interior damage may result from a broken element, presence of moisture, a severe direct lightning stroke, or the use of an arrester with an incorrect rating. Sometimes these conditions will cause radio interference. Special inspection, to detect inferior arrester units, may be made either in the field or shop. Tests must be made strictly in accordance with manufacturer’s recommendations, and the results interpreted in line with manufacturer’s criteria. 3.2.1 Substation class arrester 3.2.1.1 Field inspection before operation Before and after installation, the surge arrester should be carefully inspected about following items. (a) Cracks, chips, contamination, damages on the arrester housing (b) Measuring of ground resistance value (c) Insulation condition of the primary and secondary side (insulation resistance measurement) (d) Terminal connections condition on the primary and secondary side (e) Approved design specification 3.2.1.2 Initial inspection (1) Inspection frequency Surge arresters first installed in the field shall be inspected within 1 year since in operation.

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(2) Tools and facility Tools and facility for initial inspection should be applied as the same as those of routine inspection. (3) Inspection methods Inspection items of initial inspection should be applied correspondingly to routine inspection. 3.2.1.3 Routine inspection (1) Inspection frequency All surge arresters shall be inspected on 5-year cycle or whenever necessary. (2) Tools and facility Vernier calipers, Doble tester, Meggar, Leakage current instrument, Infrared thermal vision. (3) Inspection methods Routine inspection should be made periodically using following tests. (a) Design and visual check (b) Doble power factor test (c) Meggar test (d) Leakage current test (e) Infrared analysis 3.2.2 Distribution class arrestor 3.2.2.1 Field inspection before operation Before and after installation, the surge arrester shall be carefully inspected in the same way as the substation class arrester like 3.2.1.1 3.2.2.2 Routine inspection (1) Inspection frequency All surge arresters shall be inspected annually or whenever necessary. (2) Inspection methods 1) Visual inspection should be made to ensure that: (a) The line lead is securely fastened to the line conductor and the arrester. (b) The ground lead is securely fastened to the arrester terminal and ground. (c) The arrester housing is clean and free from cracks, chips, or evidence of external flashover.

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(d) The arrester is not located in such a manner as to be subject to: • Damaging fumes or vapors. • Excessive dirt or other current-conducting deposits. • Excessive humidity, moisture, dripping water, steam, or salt spray. • Abnormal vibrations or shocks. • Ambient temperatures in excess of 40 degrees C. (e) Any external gaps are free from foreign objects and set at proper spacing. 2) Infrared analysis

3.3 Testing methods 3.3.1 Design and visual check (a) Check cracks, chips, defects harmful in use in appearance of surge arrestor. (b) Perform dimensions inspection using vernier calipers. (c) Inspect surge arrestor according to specification and approved design specification.

Figure 5 Design and visual check 3.3.2 Doble power factor test The power-factor test is the most effective known field test procedure for the early detection of arrester’s contamination and deterioration. Each type and class of surge arrester has a specific power factor when new. Periodic testing of a unit will show little deviation from the original (when new) power factor, so long as it remains in good operating condition. A major deviation from the original value indicates that the arrester has been mechanically damaged or contains moisture. Arresters may be tested by one or more of four methods depending upon the type of arrester and the power factor test set available. For more complete detailed instructions on the method of

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test and test procedure, please see the appropriate power-factor test set instruction book. The four test methods are as follows: (1) The GST (grounded specimen test) (2) The hot-guard test (3) The UST (undergrounded-specimen test) (4) The hot-collar test 3.3.2.1 Arrester test results analysis 1

• Refer to published tabulations

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• Compare current and watt-losses obtained for identical units tested under same conditions

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• Any deviation, either higher or lower, should be investigated

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• Compare to previous tests, if available

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• Ratings are based on watts-Loss values and not % power factor calculated

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• No correction factor for arresters

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• If necessary, contact your doble engineer

3.3.2.2 Analysis of abnormal losses (in case of metal oxide arresters) (1) Higher than normal losses 1

• Contaminated by moisture, dirt or dust

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• Corroded gaps in early design (newer designs are gapless) (2) Lower than normal losses

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• Discontinuities in Internal electrical configuration

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Acceptance testing should be performed on all new arresters in order to compare to other like arresters and for future benchmarking. Incorrect assembly at the factory or shipping damage may allow moisture ingress of the “just received” arresters. Higher than normal losses could be moisture, with lower than normal losses may be due to physically damaged internal components caused by incorrect handling during shipment or installation. 3.3.3 Megger test A megger test is the most usually used for testing surge arrestors. It could simply measure the insulation resistance of surge arrestors. Such a test may indicate shorted valve elements in valve-type arresters. This test can be made to provide additional information on the condition of arresters. But the insulation resistance value of deterioration judgment standard depends on manufacturer and type of surge arrestor. Therefore it is necessary to perform the inspection on the basis of the insulation resistance value the manufacturer recommends.

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Figure 6 Meggar Test for Surge Arrester 3.3.4 Leakage current test Any deterioration of the insulating properties of a metal oxide arrester will cause an increase in the resistive leakage current or power loss at given values of voltage and temperature. The majority of diagnostic methods for determining the condition of gapless metal oxide arresters are based on measurements of the leakage current. The measuring procedures can be divided into two groups: on-line measurements, when the arrester is connected to the system and energized with the service voltage during normal operation, and off-line measurements, when the arrester is disconnected from the system and energized with a separate voltage source on site or in a laboratory. Measurements off-line can be made with voltage sources that are specially suited for the purpose, e.g. mobile AC or DC test generators. Good accuracy may be obtained by using the off-line methods, provided that a sufficiently high test voltage is used. The major disadvantages are the cost of the equipment and the need for disconnecting the arrester from the system. Measurements carried out on-line under normal service voltage are the most common methods. For practical and safety reasons, the leakage current is normally accessed only at the earthed end of the arrester. To allow measurements of the leakage current flowing in the earth connection, the arrester must be equipped with an insulated earth terminal. ※ NOTE: The insulation of the earth terminal must, also after long-term degradation, be sufficient to prevent circulating currents caused by electromagnetic induction, since these currents may interfere with the measurement of the leakage current.

On-line leakage current measurements are usually made on a temporary basis using portable or permanently installed instruments. Portable instruments are usually connected to the earth

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terminal of the arrester by means of a clip-on, or permanently installed, current transformer. Long-term measurements of the leakage current may be necessary for closer investigations, especially if significant changes in the condition of an arrester are revealed by temporary measurements. Remote measurements may be implemented in computerized systems for supervision of substation equipment. Measurement of leakage current of metal oxide arresters may be carried out by two methods largely. For more complete detailed instructions on the methods of measurement and procedure, please see the appropriate leakage current measuring instrument manual. The two measurement methods are as follows (1) Measurement method of the total leakage current (2) Measurement method of the resistive leakage current The measured leakage current data may be compared with information supplied by the arrester manufacturer. To utilize this information, it is important that the operating voltage and the ambient temperature are known at the time of measurement. For efficient use of the diagnostic methods described above, the arrester manufacturer may provide information relevant to the various methods. The information may be comprised of the resistive current, third harmonic current and power loss data for each arrester type as functions of voltage and temperature. ※ NOTE: Due to the complexity of the measurement methods, it is recommended that the arrester manufacturer be consulted in order to avoid misinterpretation of the measurement results.

3.2.5 Infrared analysis Infrared analysis of the arrester is gaining in popularity and has been used by several companies with good results in identifying higher than normal current flow through the metal oxide components. Routine testing on a normal basis is recommended in hopes of identifying and replacing defective arresters before equipment damage or personal injury.

Figure 7 Arrester 10° Centigrade above Ambient

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4. Work procedure 4.1 Periodic inspection for substation class surge arrester Inspection management department

Inspection execution department

(Medium voltage maintenance

(Medium voltage regional maintenance centers)

sub-department)

Reporting the status of facilities • Compiling the facilities and reporting Establishing inspection plan

the results

• Establishing the annual inspection plan

Performing inspection

• Notifying the plan to each Inspection execution department

• Electrical and mechanical maintenance bureau • Initial & Routine inspection

Supporting the inspection

• Recording the inspection result

• When requested • Maintenance programming and

Corrective repair work

follow-up division

• Repairing and replacing of the inferior material

Analyzing inspection results & Establishing the countermeasure

Reporting the inspection results

• Predictive maintenance division Compiling inspection results Checking the performance of inspection & compiling the inspection results

• Maintenance programming and follow-up division

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4.2 Periodic inspection for distribution class surge arrester Inspection management department

Inspection execution department

(Distribution networks maintenance

(Maintenance division of regional distribution

sub-department)

sub-department)

Reporting the status of facilities • Compiling the facilities and reporting Establishing inspection plan

the results

• Establishing the annual inspection plan

Performing inspection

• Notifying the plan to each inspection execution department

• Electrical and mechanical maintenance bureau • Routine inspection

Supporting the inspection

• Recording the inspection result

• When requested • Maintenance programming and

Corrective repair work

follow-up division

• Repairing and replacing of the inferior material

Analyzing inspection results & Establishing the countermeasure

Reporting the inspection results

• Predictive maintenance division Compiling inspection results Checking the performance of inspection & compiling the inspection results

• Maintenance programming and follow-up division

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【Appendix 1】

Surge Arrester Inspection Report Inspection Type:

Inspection Date: 2007. . .

Installation Site Type Plant of Manufacture

Whether & Temperature:

Arrester ID Rating Serial Number Item A phase

External Inspection

Testing

,

℃

Year of Manufacture Inspection result B phase C Phase

Condition of arrester housing Connection of line lead Connection of ground lead Connection with equipment Condition of arrester location Condition of Bracket, Basement Ground resistance value Doble test Insulation resistance value (each phase – ground) Leakage current (mA) Infrared Analysis

Remark

Inspector

Chief of inspection maintenance unit

Name :

Name :

Signature :

Signature :

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