GENERATOR FIELD WINDING SHORTED TURN DETECTION TECHNOLOGY
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:
1. Rotor unbalance that varies with field current changes (thermal sensitivity).
Coils with shorted-turns operate at lower temperatures than coils without shorted-turns. This is because the heating resulting from I2R losses are lower in the effected coil (the coil current is traversing a shorter copper path, therefore the coil resistance is lower), while the cooling circuits remain the same. A rotor temperature gradient that can give rise to rotor bowing will be a function of the number of turn-shorts and their location (FIGURE 1 and 2), as well as the total field current. At higher field currents, any rotor temperature gradient due to shorted turns will be greater.
Shorted-turns in coils near the quadrature axis in 2-pole rotors will have little effect on thermal sensitive balance because the effected slots are nearly 180° apart (FIGURE 1). Shorted-turns in the small coils (1, 2 or 3) of one pole will have a greater effect in causing rotor unbalance problems since the asymmetrical location will result in a larger rotor temperature gradient.
Shorted turns in 4-pole rotors will also act to produce a thermal gradient (FIGURE 2). However, since 4-pole rotors tend to be stiffer than 2-pole rotors, thermal gradients tend to be less effective in producing rotor bowing.
2. 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.
3. 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%.
4. 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.
Winding temperature rise above cold gas is proportional to I2. As an example, two turns shorted out of a total of 100 turns will result in a 4% higher hot-spot rise over cold gas. TABLE 1 shows an example at full load for a rotor with 2% and 10% shorted-turns.