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Forensic Analysis of Surge Arresters


ARRESTERS

Forensic Analysis of Surge Arresters Arresters are essentially devices to divert lightning and switching surges and can sometimes become overloaded to the point of failure. Whenever this occurs, it’s extremely useful to properly determine the reason why the arrester (or companion arresters) became inoperative. This will allow utility maintenance engineers to determine whether the end-of-life event was simply the arrester performing its expected protective function or if there are other issues that need to be resolved and that might be system wide. Here is where forensics can play a valuable role, just as in television dramas where specialist police investigators try to identify the real culprit behind a crime. This article, prepared by INMR Columnist Jon Woodworth, offers guidance on how best to search for and establish the root cause of an arrester’s overload.

Introduction

Terminal overload is basically any event whereby an arrester is stressed beyond its design capability. The arrester may or may not even be in one piece at that point and electrically represents an open or short circuit.

some form of failure. Forensic analysis is then the only real means to properly determine which of these scenarios really applies.

In most cases, the underlying reason for conducting this type of analysis is that several arresters of similar design While such a situation could be regarded experience terminal overloads and the affected power utility needs to find out as failure to perform its function, this is if the problem is system wide. If system not necessarily the case. For example, wide and therefore a failure scenario, a should a long-serving arrester that protects a high value asset be terminally different mitigation strategy would likely overloaded during an overvoltage event, be needed compared to a situation where it was only an isolated event. this can hardly be termed failure. In Indeed, one of the main purposes of fact, this scenario should really be the analysis is to determine if there are called a success. separate power system issues to resolve that are not directly related to arresters. However, if an arrester experiences a terminal overload within days of energization and with no related surges, Companion Arresters A companion arrester is one of then it’s more likely an indication of

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similar vintage and style that could be located on the same phase close to the overloaded arrester or nearby on a separate phase. The importance of studying these during a forensic analysis is high and, in this regard, it is preferable that the companion arrester not have been similarly overloaded or blown. This is because any arrester that has experienced terminal overload has much of the forensic evidence obscured by the damage from the power frequency fault current. This fault current typically flows off the system to earth along the arrester and raises the temperature of the various components past their melting points. Obtaining a suitable companion arrester therefore becomes a key ingredient in identifying the root cause of the problem.

Relevant System Data

Collecting system data relating to the

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overload event is beneficial to any analysis but is often the most difficult part of the forensic examination. Such data should include: 1. System voltage; 2. Neutral configuration of the source transformer (i.e. grounded, floating, impedance grounded); 3. Magnitude of available fault current; 4. Location of the arrester; 5. Other equipment on the same phase, at the same substation or on the same line (i.e. capacitors, switches, breakers, transformers, inductors) and its status during and after the overload; 6. History at that location (e.g. other overloads during past years); 7. Switching or lightning activity at that time or during prior weeks; 8. Performance history of that arrester vintage and design applied on the system; 9. Existence of any other forensic analysis data that might offer clues to the root cause. In assembling the above, it’s useful to avoid irrelevant or inaccurate information and rely as much as possible on those directly involved in the response to the overload. The persons picking up the pieces usually have the most information such as the arrester’s exact condition following the event. It is also valuable to know if the overloaded arrester was removed only after a severe rainstorm.

is available, perform full-scale electrical tests such as Vref, watts loss, PD and leakage. • If sealed with an internal air volume, take a sample and conduct a gas analysis before the arrester is disassembled. • Disassemble the arrester carefully, labeling all parts and with a camera constantly in play that’s capable of close ups with highresolution images. Involve others in this process to ensure nothing is overlooked. • If the parts are not too damaged, run more electrical tests and go through a checklist of clues • Once testing and physical examination is complete, let the parts sit in place at least until all the photos have been reviewed – ideally on a big screen and with as high a magnification as possible. • Write the forensic report, listing potential root causes. Eliminate any causes deemed unlikely based on the photos and tests.

Leading Causes & Indicators of Failure 1. Moisture Ingress

Routines to observe so as to make each analysis as effective as possible include the following:

Moisture ingress is probably the leading single cause of arrester failures worldwide – both for porcelain-housed arresters and most likely for hollow core polymeric-housed arresters as well. Such ingress takes place either through the metal diaphragms that are part of the venting system or around rubber seals. In the case of polymeric arresters, moisture can also migrate directly through the rubber over time or around end seals.

• Gather as much system data and from as many sources as possible. Use groups of people who can offer different input on the situation.

1a. Typical Causes: Manufacturing defects, mishandling during transport or external flashover that damages the seal.

Analysis

• Inspect the arresters in question and record as much data as possible from tags, shipping documents and nameplates. • Collect original catalogues and literature on the arrester. • Assemble a comprehensive set of photographs of the received parts. • Assuming a complete arrester unit (overloaded or companion)

In most cases, the underlying reason for conducting a forensic analysis is that several arresters of similar design experience terminal overloads and the affected power utility needs to find out if the problem is system wide.

1b. Failure Mechanism: This tends to be a long-term failure mechanism, whereby moisture is drawn past an aged, cracked or otherwise defective seal due to the difference in pressure between the internal and external volumes of the arrester. Once the seal becomes ineffective, this pumping mechanism tends to draw in moist air during the

Gas sampling of arrester.

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1e. Moisture Ingress into Polymeric Designs with No Air Volume Although the polymeric arrester has As moist air is exchanged between the a reputation for offering the ultimate moisture seal, this has not proven true. internal volume and the atmosphere, Polymer-housed arresters with very low the relative humidity on the inside internal air volume are still susceptible reaches the same level as outside. At to moisture ingress at points where metal some point the temperature of this moist internal volume drops below the terminals exit the unit. It’s also a fact that silicone, EPD and other rubbers dew point and moisture condenses along the arrester’s electrically stressed transmit water vapor at various rates. internal components. This leads to dry Over time, this can make the relative humidity of the entire arrester rise to the band arcing and dielectric tracking same level as the ambient atmosphere. along the wetted surfaces and will eventually result in a short circuit of This moisture transmission property the unit. has resulted in development of some arrester designs that are unaffected 1c. Indicators by the phenomenon. This means that When moisture ingress is part of any transmitted water vapor will not the failure mechanism, this is often be allowed to condense on internal indicated by brown rust on metal parts parts, gather in capillaries or condense (or white rust on aluminum parts), increased watts loss at operating voltage, in any form that is detrimental to the arrester’s performance. tracking along the electrically stressed components, elevated temperature in 1f. Failure Mechanism infrared image, green copper oxidation, As with porcelain-housed arresters, the presence of H2O in a gas analysis this tends to be a long-term failure or hardened rubber seals with little mechanism. If water vapor transmission remaining compressive force. is indeed the cause, this vapor collects Old rust covered by carbon by-products in any small voids at the rubber/MOV disk interfaces and condenses during of arcing indicates that the rust was cool temperatures. This results in there prior to failure. dielectric failure along this interface. Another potential failure mechanism 1d. Cautions is for water to penetrate a pressureThe fault current resulting from a assisted seal and be transmitted along moisture-initiated failure can look similar to a temporary voltage overload. the fiberglass filled components in capillaries. If these capillaries are Often, partial discharge activity within an arrester over long periods of time will electrically stressed, it can lead to dielectric and complete failure. result in enough ozone being produced to oxidize parts, similar to what occurs All the same indicators of this problem during moisture ingress.

exist as described above for porcelainhoused arresters. Red die penetrating fluid is effective in locating ingress leaks and capillaries of this type.

Steel component showing pre-failure (dark) and post failure (light) rust with molten aluminum on top of pre-failure rust.

Using red dye penetrating fluid to locate a crack or leaky diaphragm.

evening, when the internal pressure is lower than the atmosphere.

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White oxidation between disks in a polymer housed arrester indicates moisture ingress.

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2. External Flashover

Failure by external flashover is most common among short arresters between 2.5 kV Uc and 25 kV Uc. However, since the arrester housing is essentially a self-restoring insulator, this may or may not ever be detected. This type of situation is generally a 50 or 60 Hz event and not one associated with lightning. 2a. Causes Leading causes of this type of failure are either animals or severe contamination in conjunction with the presence of fog. 2b. Failure Mechanism If the flashover is animal-assisted, the animal is electrocuted and an external arc created over the external housing of the arrester. The fault created from the flashover will then be interrupted by some over-current device on the system. It’s even possible that the arrester is totally unaffected if the ground fault current is limited and the system can generally be re-energized right after the event. 2c. Indicators Arc marks on the high voltage side of the arrester and sometimes also on the ground end (mild to severe arc marks on the housing).

3. Excessive Internal Partial Discharge

It’s generally accepted that some partial

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discharge activity will occur inside an arrester over short periods of its life, e.g. during rain or precipitation, when there is significant radial stress due to different voltage levels on the outside surfaces compared to the dry internal surfaces. If excessive PD is present, this can lead to degradation of the disk as well as arrester dielectric leading to complete failure.

components near edges, corners and minor damage to the dielectrics of the contact points. There may also be multi- arrester, it can fail at a later time due to colored growths on rubber components. other causes described above.

This type of failure mode occurs mainly on units with significant internal air space such as porcelain and hollow core polymeric-housed (tube type) arresters. Since this is a long-term process, it may never cause the arrester to fail if the PD level remains low.

Lightning strokes above the design limits of an arrester can cause terminal overload of its internal dielectrics. Should an arrester experience a direct strike, the resulting failure can be immediate. However, failure can also occur later if the damage during the surge was minimal. For example, minor damage during a lightning surge can eventually trigger partial discharge that in turn lead to tracking and complete electrical failure.

3a. Causes Manufacturing defects, moisture ingress, mishandling, excessive contamination and moisture on the external surfaces. 3b. Failure Mechanism The partial discharge activity starts small and grows away from the initial point. After long exposure, the dielectrics in this area of the arrester degrade and this can lead to flashover of the electrically-stressed parts. Partial discharge can also reduce the oxygen content of the air around the disks and in some cases even change disk characteristics. 3c. Indicators During electrical tests, PD activity is indicated by high frequency spikes on the power frequency trace, high partial discharge readings using RIV or PD equipment and discoloration of

Polymeric housed arrester that experienced an external flashover and seal compromise.

3d. Cautions Low levels of partial discharge over extended periods of time can corrode and oxidize metal parts of the arrester in a manner similar to moisture ingress.

4. Lightning

4a. Causes Causes of such failures can include an excessive single current surge or a significant multi-stroke surge, an improper lightning duty having been specified, successive strokes to the same arrester or a TOV following a surge caused by a fault on the circuit that exceeds the arrester’s capability.

4c. Indicators If an arrester is subject to a significant surge, it is likely that its MOV material will be polarized and this is seen as conductivity in one polarity anywhere from 5 to 20% different from conductivity in the opposite polarity. If the surge has only damaged the arrester’s dielectric and ultimately leads to a long-term dielectric failure, there is no effective method to confirm this scenario. If the fault current available to a polymeric-housed arrester is low, the rubber may not show any signs of failure. Electrical tests will likely show it as shorted or nearly shorted. AC testing of MOV disks can show polarization. If the disks were subjected to a significant surge, this will be shown quite dramatically. 4d. Cautions If the fault current is of sufficient magnitude, it could cover up all evidence of lightning-induced overload.

5. Temporary Overvoltage

A terminal overload caused by a fundamental power frequency voltage 4b. Failure Mechanism beyond the design capability of the If an arrester’s capability is exceeded arrester is not uncommon. Arresters are by a lightning stroke, the MOV disks designed to survive such overvoltage will initially take the surge, heat up scenarios and, if properly dimensioned dramatically, perhaps crack and then when installed, should not overload as flashover. This will lead to a full power a result. However, circuit configurations frequency flashover and fault current sometimes change or breakers not that can cover up all the evidence of the operate or other circuit issues produce event. If the surge only causes relatively a voltage beyond the design limit of

Effect of partial discharge on organic components.

Disk showing polarization likely caused by excessive surge.

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the arrester. A TOV overload can be immediate but, unfortunately from a forensics point of view, it can also occur as a long-term failure triggered by unrelated other causes (as above). 5a. Causes Typical causes of TOV type failure include excessive voltage rise in the unfaulted phase of a three-phase system, misapplication of the arrester, change in the system’s neutral configuration, ferroresonance, loss of neutral on a system, ageing of the disks or contact of lines with those of a higher system voltage. 5b. Failure Mechanism During a TOV overload, the voltage across the arrester rises to a level where the disks conduct much more than during the steady state. The conduction causes the disk to heat significantly which in turn lowers its resistance and leads to more conduction and ultimate failure. If the TOV overload is only marginal, such heating could take place over a long period. 5c. Indicators These include changes in disk characteristics in both polarities, cracking of a few disks in the stack, flashover of the disks, holes blown inside the polymeric housing or open vents in a porcelain-housed arrester equipped with them.

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section of the disk and conduct AC testing on chipped sections to confirm TOV overload or not.

6. Switching Surge Overload

This overload mode can occur if arresters are subjected to surges generated from switching capacitor banks, switching or energizing long lines, switching high voltage lines or other such situations that cause the design limits of the arrester to be exceeded. Typically, this occurs only on systems above 220 kV or on systems having extremely high capacitor banks installed. 6a. Causes These can include re-strike or prestrike of breakers and re or pre strike of capacitor switches. 6b. Failure Mechanism Essentially the same as for lightning mechanisms. 6c. Indicators Numerous small holes in the aluminum electrode of the disks, generally around the circumference of the disk electrode. Another indicator is polarization of the disk at low levels.

this can lead to arrester failure. 7a. Cause Mainly improper manufacturing of the disk. 7b. Failure Mechanism At normal operating voltages, losses gradually increase leading to internal heating. When the heat generated exceeds the arrester’s ability to dissipate it into the environment this will lead to dielectric failure and a fault on the system. 7c. Indicators Main indicators of this type of situation are a hot arrester at normal operating temperature, excessive electrical losses at operating voltage, disks having different losses than others in the same column, non-linearity of the disk reduced (as shown on VI trace).

8. Other Causes of Arrester Failure & Overload External Contamination This type of overload can lead to external flashover of porcelain housings or excessive internal partial discharge.

5d. Cautions Minimal damage to an arrester during a lightning stroke, disk aging or switching surges can all lead to a failure mode that resembles TOV overload. Here is where it is helpful to remove the faulted

Improper Uc or TOV Rating The installation of arresters with a 6d. Cautions Uc value lower than the steady state Fault current damage after the initial system voltage can result in arrester overload can disguise the real cause. failure for what would appear to be only a minor TOV. For example, if the system 7. Ageing of Disks Since introduction of the MOV arrester, has a neutral impedance installed, it requires a higher MCOV arrester than for it has been widely accepted that metal a grounded neutral, and a fault on the oxide disks can experience long-term changes in their characteristics, referred system can lead to arrester failure on the un-faulted phase. to as ageing. If such changes result in more losses at normal operating voltage,

Disk cracked in two pieces from a TOV overload.

A chipped out section of a disk can reveal the surge history of a disk.

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A porcelain arrester broken open for analysis. Note the disks show flashover.

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Unbalanced Electric Field This is a failure mode that can occur on an arrester if it is mounted too close to a ground plane of another phase. Such an improper installation situation can lead to partial discharge in the internal volume of the arrester and ultimately to failure. It can also lead to overheating of the disks due to voltage imbalance. This type of failure can also be caused by the use of inappropriate grading rings on the arrester. Misalignment of the Disk Column In an optimal design, the single column of disks within a porcelainhoused arrester should be centered along the length of the housing. If the disks become misaligned during transport or installation, steady state partial discharge above the acceptable level could result. Misalignment of the column during transport could also result in physical damage to the edges of the disks such that partial discharge or even a flashover occurs readily during a lightning event. Improper Spring Pressure If the spring within an arrester is not

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of sufficient pressure, the disk column can more easily become misaligned. This type of defect can lead to the same misalignment issue discussed above. Similarly, if the pressure of the spring is too low, partial discharges or damage to disks can occur more easily during a surge event. Mechanical Stress Should an arrester be mounted in such a way that it is subject to excessive mechanical stress, it can fail over a long period of time. This misapplication generally leads to failure of the seals that in turn lead to dielectric failure of the internal components. Burrs In the event that a high voltage arrester is assembled with conductive parts that have sharp points or edges, internal partial discharge can result. This design or manufacturing defect can lead to dielectric failure of the arrester. Insufficient Dielectric Strength If any of the materials inside the arrester have internal or surface dielectric strength that is inadequate

for the steady or impulse states, they can track or flashover. This type of manufacturing defect will be worsened by surge events.

Disassembly Considerations

The dissection of an arrester for the purposes of forensic analysis requires some familiarity with its internal components. It is therefore recommended that a drawing or sketch of all the internal components be retained before disassembly. If the arrester is porcelain-housed and cannot easily be disassembled with bolts, the best solution is to simply crack the assembly with a hammer. While the internal spring pressure is usually not high enough to discharge the porcelain more than a few centimeters, it’s best to cover the arrester with a blanket or carton to contain all the parts. Thick gloves should be used at all times to avoid wounds caused by porcelain shards. When dissecting a polymer-housed arrester for forensic analysis, a razor knife is usually sufficient. ď ¸

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Forensic Analysis of Surge Arresters