Manual closing or autoreclosing without
synchronization supervision at line terminals that are in close electrical
proximity to turbine-generators can subject them to excessive shaft torques and
winding stresses with resultant loss of life of the turbine-generator system.
These effects should be studied and
evaluated before autoreclosing is initiated by tripping. It is preferable to
re-energize a line at a terminal remote from the generator bus, check synchronism
between the generator bus and line, and then close the breaker at the generator
end.
In past years, considerable research and
analysis focused on the stresses in the shafts and components of
turbine-generators due to switching operations. There is little documentation
of actual damage to, or failure of, turbine-generators resulting from
autoreclosing or switching.
The effects of these stresses induced are
cumulative and can be caused by normal switching operations or system faults.
Therefore, autoreclosing can be a contributing factor to machine failure, but
not necessarily the sole contributor.
An unsuccessful autoreclose attempt
(particularly three-phase faults) close in to a generating plant can contribute
to accelerated torsional fatigue on the turbine-generator shafts (ANSI
C30.13-1977 [B1], IEEE Committee Report [B8], and Jackson et al. [B12]). This
can be dealt with by not autoreclosing near generating plants or by blocking
the autoreclose for close-in faults or three-phase faults.
Consideration should be given as well to
the natural oscillatory frequency of the transmission line as autoreclosing can
result in a resonance condition, which could contribute to other system
problems. The operation of closing a breaker in the power system can result in
the creation of power transients and current oscillations, which can stress or
damage generating units located electrically close to that breaker.
These transients effect various components
of the turbine-generator. The concern is the average initial power, dP, which
occurs when the breaker is closed, and its effect in producing torsional
stresses, primarily in the rotational members of the turbine-generator.
For this condition, the permissible limit
for dP or dI at the generator terminals are 0.5 per unit based on the rated
load and power factor. Regardless of the cause of initial disturbance,
autoreclosing times in excess of 10 s appear long enough to allow the
oscillations from the initial disturbance to die out.
Turbine-generators when subjected to
high-speed autoreclosing can resonate at the natural frequencies of the turbine
and shaft. These transient torques will cause cyclic stress variations in the
generator shaft resulting in cumulative fatigue damage when they exceed
material fatigue limits.
This results in reduced component life of
shafts, retaining rings, and rotors. In extreme cases, these torsional
vibrations have led to growing oscillations resulting in shaft damage. Some of
the more recent papers on the subject of shaft fatigue as a result of
high-speed autoreclosing (ANSI C30.13-1977 [B1]) suggest that simple measures
such as dP or dI cannot be correlated directly.
Transient torque studies that quantify the
impact of high-speed autoreclosing can be performed to calculate the impact on
the turbine-generators. This study would require a detailed turbine-mass
representation and is generally performed using the Electro Magnetic Transient
Program (EMTP). This study can then provide a basis for evaluating the need for
torsional monitors/relays on the turbine-generators.
The torsional monitoring devices monitor
the turbine-generator shaft for torsional oscillations by providing torsional
mechanical response evaluation, shaft torsional stress, and fatigue evaluation,
and can be used by the operator to assess torsional impact of an event on a
unit. The torsional protective devices continuously monitor the
turbine-generator shaft and provide trip output contacts when shaft fatigue
reaches predetermined levels.
As a result of the apparent risk to
turbine-generator life, most utilities have modified their autoreclosing
practices to some form of the following:
a) Autoreclose by synchronism check only
b) Allow a minimum of a 10-s delay prior to
any autoreclose attempt
c) Use single pole tripping and allow
autoreclose on single phase faults only
d) Autoreclose lines with tapped generation
only under dead-line conditions
e) Use no autoreclosing near generation
