© 2010 Doble Engineering Company -77th Annual International Doble Client Conference

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MANITOBA HYDRO’S EXPERIENCE WITH ABB XPS ARRESTERS

Reg Gamblin

Manitoba Hydro

ABSTRACT

Since 1999, when Manitoba Hydro began purchasing the ABB type XPS arrester, over 700 of them have been

received. The voltage classes for this type of arrester range from 4kV up to 230kV with approximately 600

presently in service. In 2001, less than three months after placing a bank of 230kV XPS arresters into service, tests

on one of the units indicated suspect results. Discussions with the manufacturer lead to design modifications of the

XPS arrester. Since this discussion there have been several other incidents which have lead to replacement of entire

orders, additional manufacturing modifications and an in-service failure of an XPS arrester due to moisture ingress.

Manitoba Hydro stopped buying the XPS arrester in 2006. It has been Manitoba Hydro’s experience that all voltage

classes of the XPS arrester manufactured up to January 2007 are vulnerable to moisture ingress.

INTRODUCTION

For station class surge arresters Manitoba Hydro’s acceptance test program specifies that all units be inspected and

acceptance tested upon arrival, prior to payment of the supplier. Intermediate and distribution class arresters are

sample tested. The tests are 10kV watts-loss, voltage-current (V-I) response, and partial discharge as prescribed in

IEEE C62.11 for Metal-Oxide Surge Arresters for AC Power Circuits (>1kV) Additionally, units rated above 66kV

are field tested prior to energization for 10kV watts-losses and V-I response. Manitoba Hydro’s maintenance

program requires that arresters be tested every eight or ten years depending on make/model and previous experience.

These tests are limited to 10kV watts-loss and when practical metal oxide arresters are V-I response tested and

gapped units are sparkover tested.

On a yearly average Manitoba Hydro acceptance tests over 150 arresters while maintenance tests are performed on

over 400 arresters. Failure rates are below 1% but significant success has been achieved in finding problem batches

within an arrester order. This has prevented large numbers of substandard units from being received and more

importantly, from being placed into service. This program has provided a great amount of information and history

related to whole families of arrester. In many situations, this in turn has lead to quick resolution where an arrester is

in question.

Manitoba Hydro’s XPS arresters were manufactured and assembled at the ABB facility in Youngwood

Pennsylvania. All voltage classes of XPS arresters are assembled in the following manner. Metal-oxide varistor

(MOV) discs are series connected with metal spacers inserted to ensure electrical functionality. Aluminum end caps

are at both ends of the MOV structure and are held in place by fibreglass hoops. These hoops provide mechanical

support for the entire structure while preventing large pieces from escaping when an arrester fails. Tension is

maintained on the hoops via a steel compression screw through the end caps which is torqued to a predetermined

value. The assembled unit is encapsulated with grey silicone via injection moulding except for an area at the ends of

the aluminum end caps. Prior to moulding, adhesion promoter is applied to the aluminum end caps to ensure proper

bonding of the silicone to the end caps.

230kV Ashern Station

In January of 2001 three 230kV class ABB XPS arresters were manufactured, delivered to Manitoba Hydro (MH),

and later that month passed MH acceptance tests. In May of 2001 they were installed and energized at Ashern

Station but contrary to MH protocol they were not tested at site prior to energization. Conflicts with resources lead

to scheduling their acceptance tests in July of 2001, three months after being placed into service. At that time it was

found that the 10kV watts-losses had increased and the voltage to conduct 1mA peak resistive current had reduced.

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It is important to note that these arresters were made up of three individual pieces assembled at the factory which in

effect were three separate arresters designed to function in series as one.

The suspect arresters were removed from service and returned to ABB in Youngwood Pennsylvania for dissection at

their manufacturing facility. Dissection showed that the silicone was not thoroughly adhering to the aluminum end

caps on some arresters. This allowed the opportunity for moisture ingress to occur. Carbonized and rusted

components were observed within several units as well as the presence of moisture. To address this, a red dye was

added to the adhesion promoter to allow visual inspection of its application. Application of the adhesion promoter

was changed from brush-on to dipping the end cap into the promoter.

138kV Ilford Station - Part One

In August of 2005 two new banks of 138kV class XPS arresters were installed at Ilford Station. At site, three of six

showed high 10kV watts-losses and several of the replacement units failed as well. These units were manufactured

in December of 2004 and passed MH acceptance tests in February of 2005. After failing site tests, they were

returned to MH’s main distribution centre for re-acceptance testing. Several of these suspect units passed all high

voltage tests. To investigate this, two of these arresters were immersed overnight in a barrel filled with tap water

and retested. They both showed high 10kV watts-losses.

Four units were selected from storage which had never been delivered to any MH site for installation. These units

were manufactured in December of 2004 and passed MH acceptance tests in February of 2005. All four were 10kV

watts-loss tested and immersed into a large plastic-lined tub of tap water. Prior to water immersion, two showed

increased watts-losses and the other two showed acceptable results. After four hours of soaking, three showed

increased watts-losses. These four units were soaked for a total of 20 hours with three units showing increased

watts-losses and one unit showing normal. Additionally, one section of the soaked units showed 10kV watts-losses

above 12 watts. Acceptable 10kV watts-losses for one section of a 138kV XPS arrester are at or below 100mW.

Additionally, tests between soakings showed one of the units changing from good to bad to good to bad. This

demonstrated that moisture was able to enter and exit the arrester. This correlated to test results where some units

tested OK after failing site acceptance tests. These arresters were from an order of about 40 units rated 115kV and

138kV that were manufactured from late 2004 to early 2005. In May 2006 it was decided that this must be a bad

batch of arresters and the entire order was replaced for that of a different type.

138kV Limestone Construction Power Station & Ilford Station - Part Two

In September of 2006, one of three 138kV XPS arresters failed while in service at 138kV Limestone CP Station

however, there was no lightning activity at the time. This unit was manufactured in 2003, had been in service for

about 15 months, and had always shown acceptable test results. It was decided to send all three phases from this

installation to ABB in Youngwood Pennsylvania for dissection. Several additional units that had been salvaged from

Ilford Station were included in the shipment. These samples were manufactured in late 2004 and had been in

service for about 15 months without incident.

The failed Limestone sample was manufactured in 2003, which predated the Ilford arresters. Dissection showed

some rusted internal components and the MOV discs to be largely intact. Had the discs gone into conduction mode

with the system supplying the current, their appearance would have been altered and many would have been

damaged. Much of the heat damage was observed to be where the fibreglass hoops follow the length of the arrester

in parallel with the MOV discs. This demonstrated that the fault current travelled around the discs and not through,

which is contrary to their designed mode of operation.

The Limestone Station sister-arrester had been installed on the same bank at but on a different phase from the unit

which experienced an in-service failure. This arrester appeared normal, had not failed, and had always shown

acceptable test results. Dissection of it revealed rusted parts which correlated to moisture being able to freely move

in and out of the arrester. Voids within the silicone encapsulation were also found which could provide a location

for moisture to collect once inside.

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The Ilford sample was manufactured in 2004, had not failed, and had always shown acceptable test results.

Dissection of this unit revealed carbonized metal components in addition to rusted internal components. This

correlated to moisture being able to freely move in and out of the arrester. Voids were also present within the

silicone encapsulation of this unit.

To address these findings and Manitoba Hydro concerns, several modifications were made to the manufacturing

process. The steel compression screw was removed from the top aluminum end cap which in turn eliminated a

possible path for moisture ingress via the threaded thru-hole. The remaining steel set screw was replaced with one

plated with zinc chromate. This was to maintain its surface free of machining fluids and promote the bonding of a

sealant/thread-lock between the screw and threaded thru-hole in the end cap.

Additionally a program of infra-red scanning was initiated in 2007 to inspect all in service XPS arresters for signs of

overheating. The scanning was to be performed when the ambient temperature was above 0°C with the following

temperature criteria applied. A variation of >0°C to <5°C requires re-scanning after six months, 5°C to <10°C

requires off-line testing within three months, and 10°C or greater requires immediate removal from service. No

suspect units were reported in relation to this program

230kV Ralls Island Station

In June of 2009 routine maintenance testing was performed on three banks of 230kV XPS arresters at Ralls Island

Station. These nine three-piece arresters were manufactured in January of 2001, passed MH acceptance tests later

that month, and site acceptance tests in August of that year. At this time, six of nine units showed increased 10kV

watts-losses and reduced voltage to produce 1mA peak resistive current. The top section of one arrester showed

10kV watts-losses of 554mW with typical being below 100mW. The bottom section of another arrester required

only about 19kV to produce 1mA peak resistive current with typical being about 59kV. These arresters had been in

service for less than eight years.

Investigative Tests

To investigate this further, three 230kV XPS arresters which had been removed from Ralls Island Station were

returned to Manitoba Hydro’s main shop. Waverley Service Centre is the location where new arresters are

acceptance tested. Upon the arrival of these three arresters, they were stored outdoors so as to not allow them to

“dry-out” indoors. Three months later, in August of 2009, two of the arresters were brought indoors for retesting via

10kV watts-losses, V-I response, and partial discharge activity. As found tests confirmed the Ralls Island results

from three months earlier, however they were not identical.

The arrester whose top section previously showed 10kV watts-losses of 554mW was now over 39W. The voltage to

produce 1mA peak resistive current had been 44kV but was now reduced to only 2.8kV. Dissection of this arrester

revealed rusted internal components and signs of overheating along one of the fibreglass hoops. One leg of this hoop

showed evidence of tracking with the MOV discs and silicone being discoloured where the hoop was against them.

This damage covered the height of the arrester.

The second arrester brought indoors for tests had a bottom section that previously showed 10kV watts-losses of

127mW was now found to be 345mW. Also, the voltage to produce 1mA peak resistive current had been 58kV,

which is an acceptable value, but was now at about 49kV. After 14 days of indoor storage, its 10kV watts-losses

were found to be 107mW which showed the ability of the moisture to freely enter and leave the arrester. However,

the voltage required to produce 1mA peak resistive current was now about 20kV. This seemed to indicate that

damage had been done which was voltage dependant. For all tests the top and middle sections showed acceptable

test results.

Because the voltage distribution across an arrester is dependant on its capacitive and resistive currents there is no

practical method of measuring the voltage across one section of a three piece unit. In order to prevent the addition

of a significant capacitive or resistive load it was decided to add a source of corona at the top of the bottom section

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in hopes of approximating the voltage. A pair of locking pliers and a length of wire provided the corona source

while an ultraviolet camera was used to observe corona activity and measure its corona inception voltage. The top

and middle sections were shorted, the voltage was applied, and the corona inception voltage was recorded. The short

was then removed and the test voltage increased until the corona inception was observed.

It was assumed that each section in a healthy arrester would show a voltage drop of about 1/3 of the total voltage.

When in service a 230kV arrester would experience a line to ground voltage of about 133kV which equates to about

44kV across each section. Applying a test voltage of 41kV to the top of the arrester showed about 11kV across the

bottom section while applying 123kV overall showed about 18kV across the bottom. Applying 165kV overall

showed about 24kV across the bottom and applying 175kV overall showed about 33kV across the bottom. This

implied that a deteriorated section will carry a reduced voltage when in service. This in turn would cause the other

sections to carry an increased voltage which could be above their maximum continuous operating voltage.

It was decided to use this unit to investigate the effects a deteriorated arrester would have on its infra-red images.

Rated line to ground voltage was applied to the entire arrester and almost immediately the bottom unit showed

heating in a very small area about 4°C above the rest of the arrester. This was near the top of the bottom section and

correlated to the location of a fibreglass hoop. The rest of this section showed a normal heat signature that was

comparable to the top and middle sections. However, it is important to note that from the opposite side of the heat

source it was not visible to the infra-red camera. This showed that a proper infra-red scan of an XPS arrester

required inspection through its entire circumference.

About 20 minutes after energization the heat became apparent around the circumference leading to a temperature

increase of 1°C on the opposite side. After 30 minutes the area of localized heating was about 7°C above the rest of

the arrester with the opposite side showing an increase of about 1°C. After about 50 minutes the area of localized

heating was about 5°C above the rest of the arrester with the opposite side showing an increase of about 2 to 3°C.

These results imply that a problem unit could be overlooked during infra-red survey of an entire substation.

Eventually it came to a point where it was impossible to apply voltage to this arrester because the damage became

too extensive due to the excessive testing.

CONCLUSIONS

Moisture freely enters and exits these arresters however, once moisture is inside it appears the fibreglass hoops wick

it; but the moisture will exit if it is in a locale with low humidity. This can make it difficult to diagnose those which

suffer from a poor moisture barrier. Weather could have a significant impact on the longevity of a unit with a poor

moisture barrier.

The climate in Manitoba Canada is quite severe compared to most other places. In the winter temperatures can drop

to below -40°C while in the summer it can rise as high as +38°C. During the spring and fall seasons there are many

days and nights where the temperature fluctuates from above to below freezing. This may be a significant

contributor to acceleration of any moisture ingress situation.

During 10kV watts-loss testing of a multi-section arrester, one section with increased watts-losses will result in a

reduction in watts-losses measured over the entire stack. Additionally, multi piece arresters which have one section

suffering from moisture ingress appear to cause an increased voltage across the otherwise healthy section(s). This

could cause accelerated aging of those units and subsequently lead to an in-service failure.

Effective infra-red scanning of an XPS arrester which suffers from moisture ingress requires a skilled thermographer

who is knowledgeable of its internal structure.

Since early 2007, improvements to ABB’s XPS arresters appear to have effectively addressed the locations where

moisture ingress can occur. The end caps have a strong method of ensuring proper application of the adhesion

promoter and the thru-holes have an effective sealing system. This appears to have addressed the problems

encountered during Manitoba Hydro’s experience with this arrester.

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REFERENCES

Instruction Manual, "Insulation Testing Practices - Field Tests on Surge Arresters," 1991, Manitoba Hydro.

McDermid W, “Testing of Surge Arresters by the Utility,” Presented at CEA Seminar Surge Arresters for the

Protection of EHV Systems, Quebec City, September 1973.

McDermid W, “Testing of Zinc Oxide Arresters,” Minutes of the Fifty-Third Annual International Conference of

DOBLE Clients, 1986, Sec. 9-201.

BIOGRAPHY

Reg J Gamblin presently works in the Insulation Engineering & Testing Department of Manitoba Hydro where he

holds the position of Technical Officer. His current responsibilities include the condition assessment of generators,

transformers, insulating oil, bushings, and instrument transformers. He earned a Diploma in Electrical Engineering

Technology in 1998 from Red River College. Reg is a current member of the Certified Technicians and

Technologists Association of Manitoba and has been a CET since 2005. He has authored and co-authored several

papers which have been presented at various industry conferences on the topic of rotating machines and insulation

testing. Additionally, he is currently the Chair of the Doble Arresters, Capacitors, Cables, and Accessories

Committee.