Technical Considerations
On Operating with Turn Shorts
Although isolated shorted turns are undesirable, their existence in a rotor winding does not necessarily indicate a condition that needs to be repaired. Generatortech estimates 30% of the rotors in operation may have one or more shorted turns. Many rotors with turn shorts have no operational problems associated with those shorts. These “silent shorts” will raise the temperature of the rotor winding, which makes additional turn shorts more likely to develop, but if the condition is stable and the shorts are not affecting operation, then no corrective actions need be taken. Periodically monitoring for shorted turns will help establish whether new “silent” turn shorts are developing and when corrective action should be taken. If turn shorts are increasing, then repairs need to be considered, since this is a trend that gets worse over time, and problems with operation will eventually occur as turn short numbers increase.
When a generator rotor experiences a jump in vibration or an unexplained increase in the required field current, a flux probe test can quickly determine if new turn shorts are responsible for the changes. A result of no new shorts allows the rotor winding to be eliminated and helps focus the investigation on other causes for the vibration increase.
No Such Thing as Partial Shorted Turns
A partial turn short exist when a significant percentage of the field current continues through the partially shorted turn. However, in the vast majority of cases, partial shorts can not exist for long. With the 100’s to 1,000 of amps used in large generator fields, the very high temperatures generated at the initially very small contact spot will cause copper melting that quickly produces a low resistance spot weld. The low resistance short will effectively bypass a complete turn in the coil by carrying 99.9% or more of the field current. Confusion about partial shorts sometimes arises because the calculated turn short indications produced in the Generatortech program are not normally integer values. That calculated turn short indications are effected by multiple factors. In some cases, engineering judgment must be applied to estimate the integer number of turns shorted out of a coil when using the calculated turn short indication.
Some Turn Shorts Will Clear
Turn shorts will occasionally clear. The same turn-to-turn movement that can damage insulation and produce a turn short can sometimes break the contact responsible for the short. However, when turn shorts clear, there is a high probability that the short will re-develop in the future since the insulation was damaged at the initial contact point. At other times, the apparent clearing of a turn short is due to the development of a symmetrical turn short in the same coil on the opposite pole. Comparison with previous test data recorded before the potential symmetrical turn short condition developed is often to distinguish between the clearing of a short and the development of a symmetrical short. Rapidly Increasing Magnetization (RIM) waveforms can also be used to distinguish between those two options.
More on Thermal Sensitivity Rotor Unbalance
Slots with shorted turns operate at lower temperatures than slots without shorted turns. The I2R losses are lower, but the cooling circuits remain the same. The rotor temperature gradient that causes rotor bows is a function of the number of shorts and their location. Shorted turns in coils near the quadrature axis will have little effect on thermal sensitivity balance because the effected slots are 180 degrees apart. Shorted turns in the small coils 1, 2 and 3 on one pole and not the other pole will have a greater effect in causing rotor unbalance problems.
Anything that produces a temperature gradient across a rotor can potentially result in a rotor bow. Asymmetrical heat production caused by turn shorts in the most common cause of temperature gradients, however, asymmetrical cooling caused by blocked cooling passageways has also caused temperature gradient that caused significant vibration issues.
For a given temperature gradient, the amount of rotor bow is determined by the rotor stiffness, which is strongly related to the (rotor length / rotor diameter) ratio. Large two-pole rotors (500 MVA or larger) are especially vulnerable to turn short induced vibration. A single short in one of the smaller coils (Coils 1, 2 or 3) has produce significant vibration problems in these types of rotors.
