The ultimate surge voltage protection
is obtained through arrester voltage ratings as low as system
grounding conditions will permit during normal and abnormal system
conditions. Initially, however, when the surge arrester was adopted
as the basic protection device, the equipment design (coordination of
major insulating structures) assumed that an “ungrounded neutral”
or “100% rated” arrester would be used, unless otherwise
specified.
In time, after successful service
experience with 100% rated arresters (100% of maximum line-line
voltage), it was reasoned that lower rated arresters would be
suitable on grounded neutral systems. On these systems, the TOV on
the unfaulted phases during a line-to-ground fault would bear the
same relationship to arrester rating as “maximum line-line voltage”
in an ungrounded system.
An “effectively grounded” system
was then defined in terms of the symmetrical-component sequence
resistances and reactances, for which the TOV on an unfaulted phase
does not exceed 80% of the maximum line-to-line voltage. Under this
condition, an arrester rated at 80% of maximum line-to-line voltage
was deemed applicable, and it was classified as a “grounded
neutral” arrester.
The use of a “grounded neutral”
arrester with lower protective levels enabled designs in some
electrical equipment, such as transformers, to have reduced
insulation levels with adequate protection. Reduced insulation
allowed reduction in size, weight, and cost. Subsequently, still
lower rated arresters were commonly applied whenever the grounding
was significantly better than “effective,” particularly at system
voltages where these reductions were significant (above 230 kV).
Usually the TOV produced by a system
ground fault is greater than that produced by other causes (generator
overspeed, ferroresonance, harmonics, etc.). An exception to this
might occur on systems where the coefficient of grounding is less
than 80%. The rating of gapped silicon-carbide surge arresters
generally exceeded the TOV due to a phase-to-ground fault on the
system where it was applied.
This criterion was based on the
assumptions that the maximum TOV is produced by a ground fault and
that the arrester might operate due to a surge while there was a
ground fault on another phase. The arrester had then to seal off
against the TOV, which was sustained until the fault was interrupted.
There were some arresters that sealed
off against voltages higher than their rating. Overvoltage
characteristics for these arresters were published in the late 1960s
or early 1970s. This feature has sometimes been utilized to provide
lower protective levels.
An important consideration for
selecting a metal-oxide arrester is the maximum continuous operating
voltage (MCOV); however, the arrester will also be subjected to TOVs.
A conservative criterion is that the TOV should not exceed the duty
cycle voltage rating of the arrester. However, metal-oxide arresters
can have thermal capability for TOVs in excess of the duty-cycle
rating for specified times, and data and curves of TOV versus
allowable time of the overvoltage are available.
