Causes of Shorted Turns
Some Common Causes of Turn Short Development
Shorted-turns are the result of failed insulation between individual windings in generator rotors. Stop-Start cycles, line disturbances, contamination, moisture, manufacturer error, and damage during retaining ring installation are some of the reasons insulation fails.
The rotor winding carries currents ranging from hundreds to thousands of amperes, causing its turns to heat and expand during startup and then cool and contract during shutdown. Ideally, all turns within a coil stack should move uniformly; however, this does not always occur. When differential turn-to-turn movement takes place, significant shear forces are imposed on the thin insulation separating adjacent turns. This insulation can tear or become displaced, allowing the conductors to contact one another and form a shorted turn. The frequency of machine starts and stops directly increases the likelihood of such turn movement and the resulting insulation failure.
The pictures below insulation damage that allowed turn short contacts to form. Effects of the heat produced at the initial contact point is evident on the insulation layers. Examples of copper melting are also shown. The initial contact point will melt and then expand in size until the current density drops below a critical threshold that allows the copper to solidify while still carrying the entire field current. The result is often a spot weld joining the two turns in a very low resistance short. The resistance of the resulting short will be less than 1% of the resistance of the paralleled turn, so more than 99% of the current will flow through the short, effectively bypassing one turn.
Three separate strips of inter-turn insulation showing holes that resulted in turn shorts.
The high current density from the initial point contact between turns produced a spot weld that provided a low resistance short.
Example showing both the hole in the insulation and the copper damage at a shorted turn contact.
Distortion of the top turns under the retaining ring is fairly common. This pictures shows gross distortion of multiple turns, which resulted in multiple turn shorts. A coil-to-coil short appeared to be imminent.
When turn-to-turn movement occurs, tremendous shear forces are created on the thin insulation layer, This can create tears or displace insulation producing a shorted turn contact.
Insulation migration has been a problem in some rotors where the insulation strips were squeezed out from between the turns, resulting in large numbers of shorts developing over a brief period of time. The example below shows the history of turn shorts in three GE Frame 7A6 generators at a plant. The picture is a view of the side of one coil stack with 17 turns and the migration of the white insulation can be seen between many turns near the middle of the stack. The two gas turbine rotors experienced a rapid increase in turn shorts over 4-5 years before they were replaced. The steam turbine rotor showed a slow increase to 3 shorts before its replacement.
Insulation migration led to a rapid increase in turn shorts in the two gas turbine rotors.
The high centrifugal forces at synchronous speed (3000 or 3600-RPM) strongly compress the turns in the winding, so any manufacturing defects left on the winding (such as bumps from braze joints) can result in turn shorts as the bumps penetrate through the insulation layer. Any bumps will also aggravate insulation damage if turn-to-turn movements occur, making insulation tears more likely.
Any bumps left on the turns (such as braze joint material) can punch through the thin insulation layer when the large centrifugal forces compress the turns in the coil stacks, producing a turn short contact.
Overspeed events have been shown to cause turn shorts. The animation below shows a large 800 MVA rotor than had been tested free of shorts. Six months later, a major grid disturbance led to a 3960-RPM overspeed event (synchronous speed is 3600-RPM). The elevated centrifugal forces on the turns evidently created a new turn short in Coil 6B (only the loads with the best Coils 6 detection sensitivity are shown).
A large 800 MVA generator had tested free of shorts. Six months later, an overspeed event raised rotor speed to 3960 RPM, which results in a new Coil 6B short.
Inadequate or Misplaced Blocking
Problems with the blocking used between coil stacks can allow undesirable turn movement. The example below shows a generator that developed an unexplained jump in both vibration and field excitation levels. The flux probe test very clearly identified a coil-to-coil short between the bottom turns Coils 3A and 4A. That prediction was confirmed when the rotor was rewound. Displaced blocking between Coils 3A and 4A allow the Coils 3A stack to tilt over and the bottom turns of those two coils made contact, bypassing the 20 total turns used in those two coils.
A large jump in vibration was investigated with a flux probe test, which clearly showed a coil-to-coil short between Coils 3A and 4A. The contact was unusual in that it was between the bottom turns of the two coils and it bypassed 20 total turns in the winding.
The misplaced block in this image was originally located at the top of the stacks, but it slid down to the corners of end-turns. This allows the turns in Coil 3A to tilt over, allowing contact between the bottom turns of Coil 3A and 4A, bypassing all turns in those two coils (20 total turns).
Diagram of how current could bypass all turns in Coils 3A and 4A after the coil-to-coil short between their bottom turns.
Disruption to the normal cooling gas flow to the rotor windings can create hot spots that can lead to turn shorts.. The example below shows a 125 MVA generator that was tested free of turn shorts. About 6 months later, vibration increased and the required field current increased about 15 %. A repeat of the flux probe test showed about 30-35 new shorts in seven separate coils. An examination of the rotor during the subsequent outage found a cooling gas baffle had cracked, severely disrupting cooling gas flow to the rotor windings. The turns were overheated and distorted and the insulation was damaged, causing a very large number of turn shorts, with the majority of the shorts affecting the larger rotor coils (Coils 5-7).
Rotor was tested over a wide range of loads and the rotor winding tested free of any shorts.
About six months after the previous test, rotor vibration and field current both increased significantly. The flux probe test showed at least 30-35 new turn shorts. A failed cooling system created hot spots in the winding that resulted in many new shorts.
High moisture content and foreign material ingress can produce issues that promote turn short development.
