ACCA-4 ABB Arresters Gamblin

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. © 2010 Doble Engineering Company -77th Annual International Doble Client Conference All Rights Reserved

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. © 2010 Doble Engineering Company -77th Annual International Doble Client Conference All Rights Reserved

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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