Rotor Common Issues – Large Electrical Generators Troubleshooting
This section includes the correction actions that could be applied to the main components of the generator rotor. Other than the regular checking and cleaning of the field winding, retaining rings, and wedging system, the ageing generators may need to go through a field winding reinsulation. In the next sections, we will only cover the most frequent rotor problems and the associated interventions.
Short Circuit between Turns
One of the most common problems on the rotor is the shorted turns of the field winding. It is due to an electrical breakdown of the inter-turn insulation, or simply caused by a mechanical damage to the insulation between turns.
A sudden increased in rotor vibration may be a sign of such a shorted turns because of the unequal heating distribution in the rotor winding. The traditional way to detect the presence of a short circuit between turns is to measure the winding impedance while the rotor is speeding from standstill to the rated speed. Another method consists in producing an open circuit saturation curve and compares it to the original curve. If the field current needed to produce the rated voltage is higher than that on the original curve, this means that the number of turns has decreased. W e can even estimate the number of the shorted turns by calculating the ratio between the new field current over the design value.
Most of the new generators are equipped with a shorted turn’s detector (STD) to detect any shorts between turns. It consists in a search coil installed on the air gap side of the core. The short turns will show as a decrease in the magnetic field captured signal, due to the decrease of the Amp-turns value.
Bearing Failure Due to Current Pitting: Detection and Correction
Large shaft voltages up to 100 Vpp are measured on large generators shafts. In many cases, they are induced by harmonics in the static excitation system output to the main generator field. The oil film separating the bearing and the bearing babbitt segments cannot withstand voltages higher than approximately 10 Vpp without damaging the babbitt. The high voltages create arcing from the current discharging across the oil film. This arcing melts the babbitt, roughens their surface, bridges the oil film with metal particles which create heat (mechanical pitting erosion), and slowly degrades the babbitt material strength.
Ultimately, the pitted region fails when the surface roughness increases to the point where the runner touches the babbitt surface.
The best way to correct this problem is by improving the direct current quality from the exciter output. This could be implemented either by filtering the rectifier output or by using a higher performance rectifier with low ripple level in the current waveform. Another common practice is to add suitably designed shaft grounding brushes to shunt the induced voltage to earth before it crosses the bearing oil film. Also, the oil quality has to be regularly checked.
Residual magnetism in some parts of the bearing components is also identified in some cases, as a potential cause of the shaft currents. The level of magnetism is sufficient to generate shaft voltages and current pitting.
Reconditionning of Slip Rings, Brushes, and Commutator
Occasionally, the slip rings need rerounding and resurfacing, particularly on high speed machines. On DC machine exciters, it is very common to have burning and grooving of the commutator bars. These need to be fixed to avoid costly failures.
Other common problems may include the following:
• Unevenness of collector surface, and collector insulation degradation
• Brush axial misalignment against the collector
• Brush pressure and fit inside the brush holder
• Current density, and contamination by liquids and/or carbon/metal dust
The overall performance is affected by the brush pressure, which should be set per the manufacturer recommendations. Discoloration of the brush pressure springs can be a sign of overheating. The normal brush pressure is typically between 1.75 and 2.25 psi. In addition, the experience shows that an axial misalignment or an excessive vibration levels will negatively affect the brush-collector performance. For 3600 rpm machines, the vibration should not exceed 1.5 mils peak-to-peak displacement, and 2.5 mils for machines rotating at 1800 rpm.
The collector is mounted on a layer of insulation material. The accumulation of contaminants such as oil, water, or carbon and copper dust, around and on the insulation, can show up as a grounded field.
Re-insulation: General Requirements
A repetitive insulation failures may require a complete reinsulation of the field winding. This can be due to normal ageing and/or exposure to high operating temperatures. The following precautions are to be considered during the field winding reinsulation:
• While removing the rotor, extreme care should be exercised to avoid damage to the armature laminations or windings, the machined surface of the stator and rotor, the exciter and the retaining rings on the rotor, the fans and the journals.
• After its removal, the rotor should be supported on wooden blocks placed under the pole area of the rotor body or under the journals after they are fully protected. The rotor should not be supported in the region of the coil slots or on the retaining rings.
• After removing the coil and insulation, the pole slots shall be cleaned up to metal.
• After the coil insulation is removed, the copper will be cleaned. The cleaning shall not modify the copper mechanical and electrical properties.
• All insulation material shall be Class F or higher.
• The minimum testing should include the following:
Turn and ground wall insulation.
Coil resistance is to be measured.
Surge test between turns.