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Chapter 1 of 12 тАв Page 1 of 248ЁЯФТ Protected PDF тАв Watermarked
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Protection of rotating machines against lightning surges is carried out by
Lightning arresters
Capacitor
Lightning arresters and capacitor both
Lightning conductor in arresters
Lightning arresters and capacitor both
Rotating machines (motors and generators) have lower impulse strength than static equipment like transformers. Protection is achieved by using a lightning arrester to limit the crest magnitude of the incoming surge and a capacitor to reduce the steepness of the wave front (dv/dt).
Rotating machines (motors and generators) have lower impulse strength than static equipment like transformers. Protection is achieved by using a lightning arrester to limit the crest magnitude of the incoming surge and a capacitor to reduce the steepness of the wave front (dv/dt).
v=LdtdiтАЛ тАФ relationship between voltage surge and rate of change of current
dtdvтАЛ=CIтАЛ тАФ defining how the capacitor limits the wave front steepness
The lightning arrester clips the peak voltage of the incoming surge to a safe level, while the parallel-connected capacitor slopes the front of the wave. By limiting the steepness of the wave (dv/dt), the capacitor ensures that the voltage distribution across the machine's stator coils remains uniform, preventing inter-turn insulation failure.
Rotating machine insulation is significantly more fragile than transformer insulation due to frequent thermal expansion and mechanical stress.
The surge capacitor must be placed as close as possible to the machine terminals.
A lightning arrester alone is insufficient because it does not control the steepness of the wavefront.
Prevents inter-turn insulation failure due to high dv/dt.
Limits peak overvoltage to levels within the Basic Impulse Insulation Level (BIL) of the machine.
Capacitors increase the cost of installation.
Requires precise maintenance of the surge arrester to ensure reliability.
Large industrial motors
Synchronous generators in power plants
The capacitor is specifically intended to limit the 'steepness' (dv/dt), whereas the arrester is intended to limit the 'peak magnitude' (VmaxтАЛ).
Option B is insufficient because it cannot limit the peak voltage of the surge, which could still exceed the dielectric strength of the insulation.
C is correct тАФ Protection of rotating machines requires both lightning arresters to limit the peak voltage and capacitors to reduce the steepness of the wavefront (dv/dt).
Always remember that transformer protection relies primarily on arresters, but rotating machines need the additional 'capacitance' buffer because their coils are more vulnerable to steep wavefronts.