What Are The Means To protect The
Substation From Surges?
A substation should be designed to
include safeguards against the hazards of abnormally high voltage
surges that can appear across the insulation of electrical equipment
in the station. The most severe overvoltages are caused by lightning
strokes and by switching surges.
The main methods to prevent these
overvoltages from causing insulation failures include:
1. Use of surge arresters
2. Equipment neutral grounding
3. Proper selection of equipment
impulse insulation level
4. Proper selection and coordination of
equipment basic insulation levels
5. Careful study of switching-surge
levels that can appear in the substation
The main device used to prevent
dangerous overvoltages, flashovers, and serious damage to equipment
is the surge arrester. The surge arrester conducts high surge
currents, such as can be caused by a lightning stroke, harmlessly to
ground and thus prevents excessive overvoltages from appearing across
equipment insulation.
The important consideration in applying
surge arresters and in selecting equipment insulation levels depends
greatly on the method of grounding used. Systems are considered to be
effectively grounded when the coefficient of grounding does not
exceed 80%. Similarly, systems are noneffectively grounded or
ungrounded when the coefficient of grounding exceeds 80%.
A value not exceeding 80% is obtained
approximately when, for all system conditions, the ratio of zero
sequence reactance to positive sequence reactance (X0/X1) is positive
and less than 3 and the ratio of zero sequence resistance to positive
sequence reactance (R0/X1) is positive and less than 1.
What this says in effect is that if
neutrals are grounded solidly everywhere and if a ground occurs on
one of the conductors, then the voltage that can appear on the
healthy phases cannot exceed 80% of normal phase-to-phase voltage.
Thus, the coefficient of grounding is
defined as the ratio of maximum sustained line-to-ground voltage
during faults to the maximum operating line-to-line voltage. On many
HV and EHV systems, the coefficient of grounding may be as low as
70%.
Surge-arrester ratings are normally
selected on the basis of the coefficient of grounding; thus, for
effectively grounded systems, the 80% arrester is selected when using
the conventional gap-type arrester. When using the gapless metal
oxide arrester, a lower-value arrester may be selected based on the
maximum continuous operating voltage (MCOV) equal to the maximum
normal line-to-neutral voltage.
For example, a 115-kV system (maximum
operating voltage equals 121 kV) can use a 97-kV conventional
arrester, that is, 80% of 121 kV, when operating on a solidly
grounded system, and can use a gapless-type metal oxide arrester
rated 70 kV. It should be noted that other factors, such as resonant
conditions and system switching, could increase the value of the
coefficient of grounding and thus should be studied in each
individual system.
The impulse insulation level of a piece
of equipment is a measure of its ability to withstand impulse
voltage. It is the crest value, in kilovolts, of the wave of impulse
voltage that the equipment must withstand. However, at EHV, the
switching-surge insulation level may be lower than the corresponding
impulse level, and thus the switching-surge level becomes the
dominant factor in establishing insulation levels.
Basically, the coordination of
insulation in a substation means the use of no higher-rated arrester
than required to withstand the 60-Hz voltage and the choice of
equipment insulation levels that can be protected by the arrester.
Careful study of switching-surge levels
that can occur at the substation as determined, for example, by
transient network analyzer studies also can be used to determine and
coordinate proper impulse insulation and switching-surge strength
required in a substation electrical equipment.
No comments:
Post a Comment