8.2.1 Protection with fuses
As mentioned earlier, fuses are not used to protect transformers with ratings above 2.5 MVA. The
basic philosophy used in the selection of fuses for the high-voltage side of a power transformer
is similar to that used in other applications of fuses. Clearly, the fuse interrupting capability must
exceed the maximum short-circuit current that the fuse will be called upon to interrupt. The continuous
rating of the fuse must exceed the maximum transformer load. Typically, the fuse rating
should be greater than 150% of the maximum load. The minimum melt characteristic of a fuse
indicates that the fuse will be damaged if conditions to the right of (greater than) the characteristic
are obtained. The minimum melt characteristic of the fuse must coordinate with (i.e. should be well
separated from) the protective devices on the low side of the power transformer. In considering
the coordination, the ambient temperature, prior loading and reclosing adjustment factors should be
taken into account. All of these factors influence the prior heating of the fuse, and cause it to melt
at different times than the specifications for a ‘cold’ fuse would indicate. Example 8.1 provides
further explanation of this procedure. It is also clear that the transformer magnetizing current should
not cause damage to the fuse. This calls for a longer duration current which must be lower than
the minimum melt characteristic. The ‘speed ratio’ of the fuse is defined as the ratio between the
minimum melt current values at two widely separated times: for example, 0.1 and 100 s. A smaller
speed ratio would mean a more sloping characteristic, and for proper degree of coordination such
a fuse would have to be set with a smaller sensitivity. It is desirable to have a fuse with as high a
speed ratio as possible. Finally, if a current-limiting fuse is used, the lightning arresters on the line
side of the fuse should have a rating equal to, or higher than, the overvoltages that the fuse may
create, in order to avoid the arrester flashing over due to the operation of the fuse.