July 4, 2017

Why is water killing power transformer insulation?

1   Water in power transformers Water is the foundation for life on our planet. It is essential for every living plant or creature. But there...

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1   Water in power transformers

Water is the foundation for life on our planet. It is essential for every living plant or creature. But there is one place where water greatly reduces life: the oil-paper insulation of power transformers. This insulation consists of paper and pressboard elements which provide the mechanical stability of the insulation. The main component of paper and pressboard is cellulose. It consists of glucose molecules which are linked and form a chain (Figure 1). The average number of glucose molecules in a cellulose chain (which is also called “degree of polymerization” or “DP” in short) is about 1,200 for new paper. These large chains give the paper the mechanical strength it requires to fix the windings, even in rough conditions such as when a short circuit has occurred.

Figure 1. Part of a cellulose molecule showing 12 glucose molecules (schematic)

Unfortunately, these chains can be split by water molecules, which reduces the mechanical force they can withstand. A small amount of moisture (typically between 0.3 wt.% to 0.5 wt.%; wt.% being percentage by weight) is always present in the paper of a power transformer, even if it is dried perfectly. At elevated temperatures, the water molecules split the cellulose chains, causing shorter chain lengths and – as a byproduct – even more water. So a self-accelerating process is taking place which increasingly reduces the mechanical stability of the transformer insulation. The speed of this process is highly dependent on the temperature but also on the moisture content (Figure 2). Although the higher temperatures displayed in the chart are unlikely to be reached by the average temperature of the transformer, local hot spots have to be considered.

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