An interesting new test developed at STRI and referred to as the dry salt layer (DSL) test is now being promoted as an adjunct and improvement to the conventional IEC 60507 salt fog test which has for years been the industry standard for coastal pollution testing. According to researchers at STRI, the salt fog test does not always accurately simulate a coastal environment due to the large amounts of water used and is therefore not necessarily representative of the actual service conditions an insulator will experience in a maritime environment.

“Many utilities simply have no idea of what the ESDD levels are in problem service areas.”

Part 1 of this article, which appeared in INMR Mar-Apr 2000 (Vol. 8 No. 2), dealt with the rationale and key elements of the DSL test set-up. Part 2 now focuses on how pollution severity is treated in the test design as well as on greater specifics of a recent such test involving an 800 kV silicone-housed bushing being tested for service at a large Asian utility.

One of the areas which is viewed by many as especially problematic when carrying out laboratory-based pollution tests is how best to accurately define and quantify the specific pollution environment to be simulated. Such information is first of all of critical importance for a power company in order to be able to properly dimension and specify insulators for that environment. However, it is also necessary to ensure that any laboratory pollution tests being conducted on these insulators are indeed representative of the actual service environment.

“ESDD often peaks just as a storm is at its worst while the ensuing rains can wash off up to 90 per cent of accumulated pollution.”

According to HV Research Manager at Sweden’s STRI, Ralf Hartings, many utilities worldwide simply have no idea of what the ESDD levels are in their problem service areas. For this reason, he claims that these users often have great difficulty in clearly defining the severity level of their various installations. Hartings sees in this dilemma an important problem to be solved but also a unique opportunity for test laboratories such as STRI to offer an added value over simply conducting standard pollution tests.

Says Hartings, “because of this situation, we feel the need to offer a much more comprehensive package to the customer and not simply the test itself. Therefore, as part of the new DSL test, we also assist the client to specify the correct ESDD levels in the service area as well as to decide whether or not it will be advisable to rotate the test object as the test progresses.”

“The major problem in quantifying pollution environments,” notes Hartings, “is that in most instances there are either no or only a few measurements taken of ESDD in severe service areas. To make matters worse, sometimes any measurements which are taken are performed at the wrong time, for example, shortly after a rainstorm. Actually, ESDD often peaks just as a storm is at its worst while the ensuing rains can wash off up to 90 per cent of the accumulated pollution. This is especially problematic since to correctly dimension an insulator you need to establish the correct peak ESDD levels.”