Surges associated with switching transmission lines
(overhead, SF6, or cable) include those that are
generated by line energizing, reclosing (three phase and
single phase operations), fault initiation, line dropping (deenergizing), fault
clearing, etc.
During an energizing operation, for example, closing a
circuit breaker at the instant of crest system voltage results in a 1 pu surge
traveling down the transmission line and being reflected at the remote, open
terminal.
The reflection interacts with the incoming wave on the phase
under consideration as well as with the traveling waves on adjacent phases. At
the same time, the waves are being attenuated and modified by losses.
Consequently, it is difficult to accurately predict the
resultant waveshapes without employing sophisticated simulation tools such as a
transient network analyzer (TNA) or digital programs such as the
Electromagnetic Transients Program (EMTP).
Transmission line overvoltages can also be influenced by the
presence of other equipment connected to the transmission line—shunt reactors,
series or shunt capacitors, static var systems, surge arresters, etc. These
devices interact with the traveling waves on the line in ways that can either
reduce or increase the severity of the overvoltages being generated.
When considering transmission line switching operations, it
can be important to distinguish between ‘‘energizing’’ and ‘‘reclosing’’
operations, and the distinction is made on the basis of whether the line’s
inherent capacitance retains a trapped charge at the time of line closing
(reclosing operation) or whether no trapped charge exists (an energizing
operation).
The distinction is important as the magnitude of the
switching surge overvoltage can be considerably higher when a trapped charge is
present; with higher magnitudes, insulation is exposed to increased stress, and
devices such as surge arresters will, by necessity, absorb more energy when
limiting the higher magnitudes.
Two forms of trapped charges can exist—DC and oscillating. A
trapped charge on a line with no other equipment attached to the line exists as
a DC trapped charge, and the charge can persist for some minutes before
dissipating (Beehler, 1964).
However, if a transformer (power or wound potential
transformer) is connected to the line, the charge will decay rapidly (usually
in less than 0.5 sec) by discharging through the saturating branch of the
transformer (Marks, 1969). If a shunt reactor is connected to the line, the
trapped charge takes on an oscillatory waveshape due to the interaction between
the line capacitance and the reactor inductance.
This form of trapped charge decays relatively rapidly
depending on the Q of the reactor, with the charge being reduced by as much as
50% within 0.5 seconds.
The power system configuration behind the switch or circuit
breaker used to energize or reclose the transmission line also affects the
overvoltage characteristics (shape and magnitude) as the traveling wave
interactions occurring at the junction of the transmission line and the system
(i.e., at the circuit breaker) as well as reflections and interactions with
equipment out in the system are important.
In general, a stronger system (higher short circuit level)
results in somewhat lower surge magnitudes than a weaker system, although there
are exceptions. Consequently, when performing simulations to predict
overvoltages, it is usually important to examine a variety of system
configurations (e.g., a line out of service or contingencies) that might be
possible and credible.
Single phase switching as well as three phase switching
operations may also need to be considered. On EHV transmission lines, for
example, most faults (approximately 90%) are single phase in nature, and
opening and reclosing only the faulted phase rather than all three phases,
reduces system stresses.
Typically, the over voltages associated with single phase switching
have a lower magnitude than those
that occur with three phase switching (Koschik et al.,
1978). Switching surge over voltages produced by line switching are statistical
in nature—that is, due to the way that circuit breaker poles randomly close (excluding
specially modified switchgear designed to close on or near voltage zero), the
instant of electrical closing may occur at the crest of the system voltage, at
voltage zero, or somewhere in between.
1 comment:
this article helpful for me,,, so where can I download the IEEE about the substation grounding system?
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