Electricity + Control October 2018

all options without oil work and no need to open the transformer tank. The system can be tested using testing equipment – voltage-proof enclosed and connected to a cable system using cable con- nectors or connected to an overhead line utilising pluggable bushings. These pluggable bushings are available up to UM 245 kV and solutions for higher voltages are under development. While pluggable bushings are advantageous with regards to trans- formers, other applications are made possible with this approach. One example is a temporary instal- lation for short- or mid-term applications where downtime is crucial and full flexibility is required. All components are pre-tested and shipped to site and final assembly is but a plugging process. Evolution of cable terminations The major proportion of all cable terminations to- day are oil-insulated and consist of a GFR housing and a silicone stress cone for field control. This ar- rangement is established and still widely in use. However, if referring to assembly requirements, long-term stability and mode of failure, this sys- tem does not comply with most customer ex- pectations. Nowadays, cable terminations up to UM 170 kV are dry and have solid insulation. The assembly process is easier, there is no liquid in- sulation and therefore failure mode is optimised. By way of disadvantage, they are not self-support- ing and therefore the next evolutionary step is dry type terminations that are self-supporting, ena- bling integration of additional functions.

The discs are connected to the male part of the plug-in system and insulated by a silicone body, which includes field control elements. The head armature (flange) includes a bursting disk for pressure relief and a ‘turnable’ head for re-di- recting the gas in the event of failure, according to IEC 60099-4. The housing is made of glass-fibre reinforced resin and allows great mechanical strength as well as protection of the silicone body against the environment. The silicone body itself is protected and touch-proof. A special arrangement of the earthing path allows for connecting monitor- ing devices or discharge counters, if desired.

Figure 6: Cutaway view of surge arrester Size 4.

Pluggable bushings Conventional bushing are mainly used on trans- formers and these days RIP (resin impregnated paper) types are being more widely used. During assembly of such a conventional bushing, the transformer tank has to be opened and therefore its main insulation, in most cases mineral oil, is exposed to the environment. This process could cause difficulties in view of pollution, humidity and assembly time. With reference to a pluggable system, the transformer is enclosed by a socket, which then offers an interface.This interface allows

Dr. Michael Zerrer is a Senior Manager Development and Ruben Grund is a Senior Portfolio Manager at Pfisterer Kontaktsysteme GmbH.

Figure 7: Evolution of cable terminations.

Electricity + Control

OCTOBER 2018

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