RARIC
Shaft current protection
1MRK 502 001-UEN
Page 3
An emf is induced in the shaft of the generators due to the magnetic dis-
similarities in the armature field. The emf normally contains a large
amount of harmonics. Both the wave-shape and the magnitude of the
induced voltage depend on the type and size of the machine and they also
vary with the loading.
Normally, the induced voltage will be in the range 0.5 to 2 volts for turbo-
generators and 10 to 30 volts for hydro-generators.
To prevent the rotor from being electrically charged, the shaft of the turbo-
generators is usually grounded via a slip-ring on the prime mover side. For
hydro-generators, the water in the turbine provides a connection to
ground. If the bearing pedestal at the other side of the rotor is grounded,
the induced voltage will be impressed across the thin oil-film of the bear-
ing. Due to the low impedance of the loop formed by the shaft, the bearing
and the ground structure, a breakdown of the oil-film insulation may give
rise to a heavy current which leads to destruction of the bearing.
Consequently, the bearing pedestal furthest from the prime mover is usu-
ally insulated from ground and the insulation should be supervised by a
suitable protection. Severe damage on the bearings is not expected to
occur if the shaft current is less than one ampere.
Shaft current protection RARIC with a special current transformer ILDD
is used to detect currents that can damage the bearings of a generator. The
relay can be used for generators with shaft diameter up to 2960 mm.
Min operate current is 0.3 to 0.8 ampere, depending on the diameter of the
shaft.
The RARIC protection is available in two versions:
Version 1 has a linear frequency characteristic and operates on the funda-
mental and harmonic components in the shaft current.
Version 2 has a damping filter for the fundamental frequency and operates
on the harmonics in the shaft current. This version is used when the shaft
current transformer is exposed to a large fundamental frequency leakage
flux.
1 APPLICATION
