If one or more of the following surge protective elements exist for a
particular motor application, the need for additional surge protection may not
be necessary. Some of these listed elements will be effective for reducing
stress on the groundwall insulation and some will be effective for protecting
the turn insulation.
Some of the elements will be effective for motor starting surges while
others will be effective in limiting stress from lightning surges and system
switching surges. For a particular machine installation a quantitative
evaluation such as is presented in this guide is required to determine whether
protective coordination with the insulation withstand is achieved.
a) Effective shielding from lightning strokes to overhead lines
supplying the building or plant can
reduce the probability of a lightning surge overstress.
b) Gapless metal-oxide surge arresters at the motor terminals can limit
the magnitude of voltage stress without creating a steep-front as caused by
sparkover of a gapped arrester.
c) Surge capacitors at the motor terminals. (NOTE—Three-phase
capacitors have failed much more frequently than single-phase capacitors);
capacitor internal inductance plus the inductance of leads as long as one meter
can isolate the capacitor from the motor during steep-front starting surges,
and may not be effective in wavefront sloping. Surge arrester lead length is
not as critical when machine protective arresters are applied together with
short lead length capacitors, because the capacitors will lengthen the rise
time applied to the arrester lead inductance.
d) Low grounding resistance at the motor-starting switchgear, in the
order of one-fifth of the phase
mode surge impedance, Zc+, of the motor supply cable. (Table 6 lists
Zc+ for large and small sizes of 5 kV cable: triplexed shielded in tray,
triplexed unshielded in conduit, belted unshielded in tray, and single phase
unshielded in tray. This list indicates that for usual cable surge impedances,
Zc+ will vary between 7 to 70 Ω. Low grounding resistance, to be effective,
should be in the order of 1.5 Ω for low
Zc+ constructions, and less than 15 Ω for high Zc+ constructions).
e) Interconnected bonds to ground between the motor frame, the surge
arrester, and the surge capacitor.
f) Motor supply cables individually shielded with outer jackets that
effectively isolate the shields from the raceway, and the shields bonded at
only one end (only at the motor end) to the metallic raceway and to the motor
frame and to a low impedance ground or earthing system. (This shield bonding
configuration can reduce the surge at the motor by as much as 60% compared to
bonding the shields at both ends).
The surge arrester should be selected to limit the magnitude of the
surge voltage to a value less than the motor insulation surge withstands, BIL.
The steepness of the surge wavefront at the motor terminals is influenced by
two time constants: at the supply end by the effect of system inductance, grounding
resistance, and motor cable impedance; at the motor end by cable impedance and
motor capacitance.
Surge capacitors at the motor increase the time constant and lengthen
the time to crest, reducing the steepness of the surge voltage wavefront. As
the surge voltage wavefront travels through the winding, the surge voltage
between adjacent turns of the same phase will be less for a wavefront having a
longer rise time.
The surge arrester should be selected to limit the magnitude of the
surge voltage to a value less than the motor insulation surge withstands, BIL.
The steepness of the surge wavefront at the motor terminals is influenced by
two time constants: at the supply end by the effect of system inductance,
grounding resistance, and motor cable impedance; at the motor end by cable
impedance and motor capacitance.
Surge capacitors at the motor increase the time constant and lengthen
the time to crest, reducing the steepness of the surge voltage wavefront. As
the surge voltage wavefront travels through the winding, the surge voltage
between adjacent turns of the same phase will be less for a wavefront having a
longer rise time.
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