What are the origin of transients in
substation?
a) High-voltage switching.
Opening or closing a switching device
to de-energize or energize a section of substation bus is generally
accompanied by arcing and will initiate a high-frequency transient.
The frequency will be determined by the self-inductance and shunt
capacitance of the high-voltage conductors involved. The resulting
overvoltages can exceed two per unit. Both electric and magnetic
coupling between high-voltage and low-voltage conductors can result
in high-level transients in the low-voltage system.
b) Capacitor switching.
Switching a capacitor bank causes a
current transient which is a function of the bank size and the
circuit constants back to the source. If other capacitors are already
connected nearby to the same line or bus, they lower the impedance
seen by the switched capacitor, increasing the magnitude and
frequency of the transient. Energy stored in the nearby bank may
contribute further to the severity.
The circuit between banks is likely to
ring at a high frequency because of the low inductance in the short
line connecting the banks and the reduced effective capacitance
considering the banks in series. This phenomenon further enhances the
tendency of the transient to interfere with nearby circuits.
c) Transmission line switching.
This phenomenon is similar to capacitor
bank switching, with the difference being the distributed nature of
the inductance and capacitance of the line. The magnitude of the line
charging current tends to be substantially less than that for
capacitor bank switching. The frequency of the transient current or
voltage is inversely proportional to the line length.
d) Coupling capacitor voltage
transformers (CCVT).
The capacitors in these devices, in
conjunction with inductances of the power system conductors,
constitute a resonant circuit whose frequency can be in the megahertz
range. Unless the base of the CCVT has a low-surge impedance to the
substation ground grid, a high voltage can appear between the CCVT
secondary terminals and the grid. The high voltage will be generated
primarily during air-break switching operations.
e) Ground voltage rise (GVR).
GVR is the voltage rise proportional to
the magnitude of the ground current and to the ground resistance.
Under normal conditions, the grounded electrical equipment operates
at essentially zero ground voltage within the substation yard. During
a fault, the portion of fault current which is conducted by a ground
electrode in the earth causes a rise of the electrode voltage with
respect to remote earth (see IEEE Std 80-1986 and [B26]).
f) Ground voltage rise differences.
Both electromagnetic coupling and
conduction can contribute to substantial ground voltage rise
differences, particularly at the higher frequencies typical of many
transients occurring on a high-voltage power system. Even well
designed grounding grids that extend over the large areas needed for
high-voltage switchyards have sufficient inductance to cause high
voltage differences.
Electromagnetic coupling to the ground
grid is directly proportional to the rate of change of flux and the
length and orientation of the current-carrying conductor and
inversely proportional to the height of the conductor above the
ground grid.
Conduction of power system transients
to the ground grid is typically provided through metallic grounding
of transformer neutrals and capacitive paths, such as bushings,
coupling capacitors, and CCVTs. These are low-impedance high-energy
paths that can induce common-mode voltages on control circuits (see
IEEE Std 367-1987 ).
g) Other transient sources.
Other phenomena that generate
transients occur in power systems. Some examples are undesirable time
spans between the closing of the poles of a circuit breaker, fault
occurrence, fault clearing, load tap changing, line reactor
de-energizing, series capacitor gap flashing, arcing ground faults,
failing equipment, lightning, GIS surges, and capacitor reinsertion.
Normally, the magnitudes of such transients are less than those of
other phenomena described herein.
