MOTOR SURGE PROTECTION STRATEGIES TUTORIALS

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.

Related post



No comments:

PREVIOUS ARTICLES