29-05-2012, 05:20 PM
GENERATOR FIELD WINDING SHORTED TURN DETECTION TECHNOLOGY
GENERATOR FIELD WINDING SHORTED TURN DETECTION TECHNOLOGY.pdf (Size: 213.26 KB / Downloads: 73)
ABSTRACT
The use of air-gap magnetic flux probes has proven effective in the detection of generator rotor
winding shorted turns and has helped to improve the quality of predictive maintenance decisions
concerning when or if to perform rotor rework. Analysis of air-gap flux probe data can pinpoint the number
and location (pole and coil) of shorted turns without having to take the generator offline. This paper will
discuss the theory, methodology and benefits of detecting shorted turns in generator rotors. A number of
case studies will be presented.
EFFECTS OF SHORTED TURNS IN ROTOR WINDINGS
The impact of operating a round rotor generator with rotor winding shorted turns depends upon
many factors1. If the percentage of total turns shorted out is small, the generator may be able to run at
rated load for years without further problems. However, larger shorted turn percentages can cause
operating conditions that may limit unit loads. If the problems become severe, forced outages may occur.
Conditions that may result in running a rotor with shorted-turns include:
Rotor / Stator vibration due to unbalanced magnetic force.
Shorted-turns in 4-pole rotors can also cause unbalanced magnetic forces. Shorted turns in 2-pole
rotors do not generally cause an unbalanced magnetic force since the reduction in magnetic flux will
affect both the north and south poles equally. In 4-pole rotors, however, shorted turns in one pole will
reduce the flux generated for the pole and to a lesser extent the adjacent poles, but will have no effect on
the opposite pole. The resulting unbalanced radial magnetic pull between the rotor and stator can cause
vibrations. The vibrations would effect both the rotor and stator at a frequency of once per revolution.
Higher field current is required than previously experienced at a specific load.
When shorted-turns occur, higher field current is required to maintain a specific load. This is
because the same rotor amp-turns must be generated with fewer active turns. Excitation capacity may
limit load if greater than 5-10% of the field winding is shorted out. Decreased efficiency will result in any
case.
The Amp vs. Field-Turn relationship is AS = ANTN/TS, where AS and TS are the field current and
active turns in a rotor with shorted-turns and AN and TN are the same in a rotor with no shorted-turns. The
field winding loss (I2R) at a specific load can be determined by noting that the resistance of the field
winding will decrease by TS/TN, but the I2 component increases by (TN/TS) 2. This results in an increase in
the field winding loss of (TN/TS) for a specific load.
For example, a rotor with 2 turn shorts out of 100 (TN/TS = 100/98 = 1.02) will increase the field
losses by 2%.
Higher field currents result in higher operating temperatures.
The higher field currents required to maintain a given load will result in an increase in I2R loss for the
entire rotor winding. As a result of this higher I2R loss, the total heat generated by the field will be
increased when compared to operating at the same load factors without shorted turns. However, units
which make use of Volts/Amp field temperature instrumentation will falsely indicate lower operating
temperatures after the onset of shorted turns. This is a result of the instrumentation algorithm used to
calculate field temperature assuming a constant field resistance at any given temperature. In fact, shorted
turns will reduce the total field resistance, which will be interpreted by the instrumentation as a drop in
temperature.