Surge Arrester silicone-housed PEXLIM

In order to protect other power equipment from dangerous overvoltages, the arresters themselves are employed in the most exposed locations. Hence, under extreme adverse conditions they may be overstressed. 

Surge arrester housings have traditionally been made of porcelain. However, today there is a strong trend, and often even a preference, towards the use of silicone insulators for arresters at all system voltages. There are a number of reasons why silicone is seen as an attractive alternative to porcelain, including potentially better short-circuit capability with increased safety for other equipment and personnel if correctly designed. 
It is false, however, to believe that safer short-circuit performance is automatically achieved simply by replacing the porcelain housing with one made of polymer. In the past, there has been the incorrect belief that all polymer-housed arresters, irrespective of design, were capable of carrying enormous short-circuit currents. This is not the case, and the design must be scrutinized carefully for each specific type. Please read the enclosed article (links section, right hand side) where you will see how Hitachi Energy's patented safety belt winding results in an excellent short-circuit performance.

The most important component for the surge arresters is the ZnO blocks, the varistors. Stacked in the center of the surge arrester, the varistor is the heart of the surge arrester. The varistor consist of a mix of zink oxide and other metallic powders that are blended and pressed into cylindrical blocks. 

Hitachi Energy manufactures ZnO varistors with full control of the process from the raw material to the finished electrically characterized varistors. A high degree of automation and a large number of quality tests at all stages, from raw material to ZnO blocks, approved for arrester manufacture, ensure that they fulfil their stated data.

Type tests has been performed for every size of ZnO varistor. Each varistor is verified by rigorous testing – over and above both IEC & ANSI standard requirements:

Energy withstand test on all blocks - The blocks pass three energy test cycles with cooling in-between. In each cycle, the injected energy is far in excess of the single impulse energy capability. Blocks with insufficient energy capability are automatically rejected.

The blocks are classified at 1 mA (d.c.) and 10 kA (8/20 μs) and the residual voltages are printed on each block together with a batch identification. Finally all blocks are visually inspected. 

Power losses after 1 000 hours calculated from a test with shorter duration (approximately 300 hours) at an elevated temperature of 115 °C at 1.05 times Uc shall not exceed the losses at start of the test. Batches in which unapproved blocks appear are rejected.

Blocks are subjected to high current impulses (4/10 μs) and long duration current impulses (2 500 μs) of amplitudes verifying catalogue data.

Read more about the testing of ZnO blocks here (link to pdf-file)