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A1-205

CIGRE 2006

FAILURE ANALYSIS OF GENERATOR ROTORS – CAUSES AND

PREVENTIONS N. N. MISRA *D. K. Sood

National Thermal Power Corporation Ltd India

SUMMARY Generator rotor failures have contributed considerable forced outages in many utilities. The failures in our case have been analyzed and remedial measures were incorporated for preventing the reoccurrences. In one case 500 MW hydrogen cooled generator went on forced outage due to operation of rotor earth fault protection. Subsequent investigations revealed that fretting powder of the stator core generated had traveled in the ventilation system and had caused fault below the retaining rings. Subsequent dismantling revealed that fault had penetrated and punctured the insulating cover below the retaining rings. The crack was also observed in the bottom surface of the retaining ring at the fault location. Similarly rotor faults were also observed below retaining ring in 200 MW hydrogen cooled units and were analyzed due to shorting of the coils below retaining rings. The leakage of oil from the seals, moisture in the hydrogen along with some dust particles had caused muck formation resulting in creepage fault below the retaining rings. The modifications were undertaken in these rotors and electrical distance between the coils was increased so as to avoid electrical tracking. The necessary precautions were also taken for preventing the dust entry and also proper adjustment of seals during capital overhaul. Dew point measurement for controlling the moisture levels have also been introduced for strict adherence. Unbalance in air cooled rotors has also contributed to the Generator down time. The analysis reveals that the cause of unbalance was shorted turns in the rotor windings. Entry of dust was analyzed to be another cause of multiple shorts especially in air cooled generators. Modifications were undertaken for preventing the dust entry inside the generator and also the schedule for cleaning of make up air filters were revised. After the preventions, mentioned above, no faults have reported in subsequent operations. In few cases inspection had revealed that the rotor ventilating ducts were also partially blocked due to spreading of inter turn insulation in the ventilating canals. The rewinding of such rotors was undertaken at site. During rewinding when the rotor coils were being removed significant bowing of the coils was also observed indicating improper manufacturing tolerances. All the above aspects of rotor failures have been discussed in this paper and will serve as useful feedback for the operating utilities and manufactures for improvement of the designs, manufacturing tolerances and introduction of the additional quality checks. The paper will also be very useful for operating utilities in the world for taking necessary precautions by predicting the rotor faults and also to ensure long term reliability thus minimizing the forced outages. KEYWORDS Generator – Rotor – Inter Turn Short – Bowing of Coils – Insulation – Migration – Unbalance.

*soodntpc@yahoo.com

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FAILURE ANALYSIS 1. One of our 500 MW unit went on forced outage due to operation of rotor earth fault protection. This unit had chronic problem of generating fretting powder due to hammering phenomenon between core and core bars. The fretting powder had traveled and settled below the retaining ring coils resulting in electrical tracking between the coils below the retaining ring. The rotor earth fault protection in this unit was wired for alarm only, which has caused delay in tripping of the unit. The rotor was threaded out and fault was observed to be in coil No 6 & 7 on turbine end after removal of the retaining ring. The top surface of the coil had melted as shown in Fig 1.

Fig 1

The fault had penetrated and punctured the insulating cover below the retaining ring. The crack was also observed in the bottom surface of the retaining ring at the fault location. An attempt was made to gauze the crack location of the retaining ring for clearing the fault, but the deep penetration of the fault up to the depth of approx. 20 mm had finally caused the rejection of the ring. The location of the fault on the retaining ring is as shown in Fig 2

Fig 2.

The damaged portion of copper coils as shown in Fig. 1 was cut and replaced with new copper section. The rotor earth fault protection was also wired for tripping of unit by sensing the rotor fault in two stages. The first stage of the rotor earth fault was hooked up for alarm and second stage of the same fault was hooked up for tripping. In 200 MW hydrogen cooled generator rotor faults were also observed below the retaining rings due to shorting of the coils below the retaining rings. The faults were occurring frequently and it was observed that the coils were also getting involved and damaged in few occasions. Detailed investigations were undertaken and it was revealed that leakage of the oil from the seals, moisture in the hydrogen along with dust particles were the major source for causing such creepage faults

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below the retaining rings. The necessary modifications were undertaken by flattening the top surface of the coil and fixing specially made fibre glass insulating spacers over top surface of the coils below the retaining ring so that electrical distance between the adjacent coils could be increased as shown in Fig3.

Fig 3.

Precautions were also taken by taping of the ventilating holes when the rotor was parked outside and also proper adjustment of the seals during capital overhauls. We had also started dew point measurements of the hydrogen so as to control the moisture content during operation of the units. Dew point is kept 20 Degree below the cooled gas temperature so that condensation of the hydrogen can be avoided during normal operation of the units. All such measures taken resulted in almost zero forced outage on these units on account of rotor earth faults. Unbalance in air cooled Generator rotors had also contributed significant portion of the forced outages and therefore, had been deliberated in details in this paper. The cause of the unbalance was shorted turns in the winding. Entry of dust was analyzed to be another cause of multiple shorts in these rotors. Modifications were undertaken for preventing the dust entry and schedule for cleaning of the make up filters was revised weekly as compared to monthly basis earlier. In few cases inspection had revealed partial blockage of the rotor ventilating canals and spreading of inter turn insulation in the ventilating canals. The spreading of inter turn insulation during normal operation of the machines is a phenomena which requires strict adherence to the manufacturing practices while building the rotors at shop floor. The rewinding of faulted rotors was undertaken at site. During rewinding when the rotor turns were being removed significant bowing of the rotor coils was observed which pointed towards in proper manufacturing tolerances. All such aspects need to be taken care in details during manufacturing stages so that subsequent problems during normal operation of the units can be avoided. The detailed discussions causing unbalance in Generator rotors have been discussed in the following paragraphs: CAUSES OF GENERATOR ROTOR UNBALANCES Generator rotor unbalance can be grouped into three categories a) Rotor unbalance that can be corrected by placing additional balance weights in strategic

areas.

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b) Rotor unbalance requiring correction of problems external to the rotors and are beyond the scope of this paper. c) Rotor unbalance due to partial blockage of the ventilating canals or due to the shorted turns. SHORTED TURNS One of the greatest sources for rotor unbalance is shorted turns in the generator rotor winding. For conventionally cooled coils, shorted turns can occur due to shortening or elongation of individual turns. As the turns move, insulation separating turn to turn may also shift, allowing turns to contact. In this Generator lot of dust ingress was also observed during capital overhaul of this unit. The dust ingress was taking place through the gap between the rotor and the end shields. The modification of pressurization of this gap has been undertaken to prevent the dust entry. Due to dust ingress damage to the retaining ring liner segment had also occurred. Subsequent inspection after removal of retaining rings at works revealed lot of dust ingress which has choked the ventilation passages resulting in the overheating below the retaining ring insulation liners and subsequent damage. The dust ingress and higher temp. below the retaining rings can result in the inter-turn shorts as shown in the following fig.4

Fig 4

Authors experienced problems in Generator rotor unbalances of air cooled Generators which required balancing on six monthly basis. The Generator bearings vibrations used to rise close to alarm levels of 7-11 mm/sec. During capital overhaul of this unit, the peep in through the ventilating holes revealed partial blockage through the ventilating canals. The retainning rings of this Generator were removed and deficiencies like shifting of top turns, inter-turn shorts between copper turns, deposits of soot, symptoms of overheating and blackening of inter-turn insulation etc were observed on both sides of the retainning rings. After noticing of the above symptoms it was decided to rewind the rotor completely by replacing the elongated copper conductors and also the inter-turn insulation. During lifting of the coils it was observed that inter-turn insulation had spread in the ventilating holes and also the coils were getting BOWED while taking out from the slots as indicated in the fig 5.

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Fig 5

The root cause of above appears to be manufacturing deficiencies during built up of this rotor at works. After complete review of the operating logs, OEM could not point out any operational deficiencies. The bowing action of the coils while lifting also points out the deficiencies in the tolerances during slot buildup SLOT TOLERANCE BUILDUP Another source of rotor unbalance occurs during the winding process with the buildup of slot tolerances of the winding insulation components. If this slot buildup is not carefully controlled, copper turns in one slot can extend radially outward beyond those of another slot. At first glance, this may seem to be insignificant, in light of the fact that all of the turns still must fit underneath the retaining ring with constant or tapered inner diameter. In reality, however, a small increment of radial misposition can add up to a large unbalance force. The resulting unbalance force can be calculated with the equation below: F = mxrxw2

Where F = unbalance force m = mass of object w = angular velocity Since the unbalance force is directly proportional to the diameter of the rotor therefore higher rating Generators require more attention during slot build-up process. Authors have also observed BOWING of the copper coils as mentioned earlier while removal during rewinding. The copper turns were getting locked while lifting in the end portion of the rotor slot. COIL FRICTION BINDING Frictional binding of the coils in the slot can be a source of unbalance for generator rotors. If the coils are bound in the slot and cannot freely grow as temperatures increase, they can exert a bending force on the rotor body, because of the differential expansion forces between the copper and steel. For a given temperature increase, copper coils can grow 30 to 40 per cent greater than the steel forging. Coils can then become bound due to excessive radial pressure from the slot wedge, coil deformation, or lack of a sufficient slip plane. Excessive radial pressure is due to incorrect wedge dimension and the above mentioned slots build up tolerances. Top copper turn deformation and slip plane usage are described below. COPPER TURN DEFORMATION Deformation of the top copper turn can lock the copper turns in place, preventing free expansion of the coils. This deformation is most often caused by slot wedges with too large of a radius at

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their ends. Centrifugal forces “squeeze” the filler and top copper turns in between the gap. This deformation then “locks” the coil in place, preventing free expansion and contraction. The lack of free expansion results in the same phenomena as frictional binding, in that thermal bowing forces are set up, which can cause rotor unbalance. This problem may be corrected by machining off the ends of the wedges and reducing their radii. BLOCKED VENTILATION PASSAGES A common problem with direct-cooled generator rotors is blocked ventilation passages. In these machines, copper is in direct contact with air or hydrogen cooling gas. In many cases, slots are machined in the copper turns to allow radial flow of the cooling gas. Turn insulation between copper turns also must have machined or punched openings to allow the cooling gas to flow out. Sometimes this turn insulation can slip or migrate over the vent slits in the copper, preventing normal flow of coolant gas and disrupting the thermal balance of the machine as shown in Fig. 6.

Fig 6 If a vent passage is blocked, inadequate cooling causes an entire area of a coil to run hot. This leads to excessive thermal expansion, even beyond the yield point of the material. This process therefore results in bulges, shorts, and thermal bowing, upsetting the rotor balance. Authors have experienced excessive thermal expansion of the copper coils beyond the yield point below retaining ring area as shown in Fig 7.

Fig 7

In this unit symptoms of partial blockage of the ventilating canals were also observed and also excessive heating of the order of 250 degree centigrade were observed below the retaining rings. The partial blockage was caused due to inter-turn insulation migration over the vent passage.

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Since the thermal circuit of a generator rotor is very important, design changes without proper analysis should be avoided. Small changes in flow, or short circuits in the thermal path, can cause large thermal gradients, inviting rotor balance problems. INADEQUATE EXPANSION SLIP PLANES It is important that designs allow for adequate slip planes, in order that copper turns can freely expand with a minimum of friction restraint. Slip planes, especially for large rotors, should be incorporated into all materials that contact the copper coil, whether it be the slot liner, the top slot fillers, or the end turn liner that fits inside the retaining ring. If the slip planes are not adequate then the free movement of the copper coils can get restricted resulting in the differential expansion of copper turns. In fig 8 the inadequate slip planes resulted in the constrained movement of the copper turns and top turns in this unit were observed to be shifted from rest of the stack as observed during capital overhaul of this unit.

Fig 8

BEARING VIBRATIONS Large generators have flexible rotor shaft system. Residual unbalance in rotors is the primary cause of vibrations. Such unbalances arise due to non-uniform mass of copper and completing items mounted on rotors and machining inaccuracies. Alignment inaccuracies and insufficient pedestal stiffness also give enhanced vibrations. Electromagnetic unbalances in rotor also cause vibrations that vanish, when excitation currents are removed or reduced. Thermal unbalances because of faults or disturbed ventilation also cause load dependant vibrations. Sagging of foundations with time may change rotor system catenary and cause enhanced vibrations. Defects in bearing and its body, sometimes bearing foundations, improper bearing clearances, rubbing of rotor shaft by seal rings also lead to vibrations. Loss of shrink fit in retaining rings can give rise to continuously increasing vibrations. Increased wear of slip rings and brushes also leads to high vibrations due to mechanical unbalances. CAUSES OF BEARING VIBRATIONS Rotor unbalance causes 50 Hz vibrations. For an unbalance of equivalent mass of G kg: centrifugal force: P=G x r (n/300)2 kg, where: r = radius of unbalance in cm & n = speed in rpm. Thus for an unbalance of two kg at r = 40 cm & for a two pole rotor with 1000 mm dia, P= 8000 Kg. Construction and workmanship of rotor winding is important. Any looseness developed during operation can cause some unbalance. When rewinding old rotors, care to be taken to place near equal mass in slots located diagonally opposite. For a normal rotor, component positions to be marked for correct re-assembly.Authors have also experienced increase of bearing vibrations in Gas turbine Generator rotors and subsequent inspection during capital overhaul revealed blockage of the ventilating ducts due to shifting of the liners below slot wedges as shown in Fig 9.

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Fig 9 This rotor was shifted to manufacturing works and twelve liners were found short in length and were replaced with new liners. The magnetic offset was also observed to be zero as compared to the normal setting of five mm. The rectifications of magnetic offset were also undertaken during capital overhaul itself. The average temperature reductions of 5-6 degree centigrade of rotor temp. had also been observed in subsequent operations of this unit. Inter-turn shorts in rotor winding, disturbed magnetic symmetry due to other reasons, non-uniform air gap cause vibrations to appear with field current and cannot be reduced by balancing. Sometimes vibrations increase much at near full load and suddenly. This could be due to thermal unbalance of rotor coils in overhang at full load and inter-turn faults. Non-uniform air gap leads to asymmetrical magnetic fields and magnetic pulls at the two poles. Normally, with + 10% deviation allowed on air gaps, such asymmetry does not cause change in vibrations. During overhaul, air gap values to be always rechecked at both the ends and corrected if required. Wedge shape gauge that is used must rest on tooth of rotor and not on slot wedges while measuring the air gap. Considerable non-uniformity in air gap cause magnetic asymmetry between stator and rotor and results in vibrations. Experience is that with gap of 15 – 50 mm, variation of 0.8 – 1.5 mm is tolerated. Vibration caused due to unequal air gaps cannot be corrected by balancing. Axially, in working condition, magnetic core and rotor center coincide ideally. Rotor critical speeds if close to working speeds give high vibrations. Inadequate static and dynamic balancing or rotors, incorrect centering of rotors, rotor components not tightly fitted like half coupling, slip rings or even retaining ring, defective machining or improper assembly of half coupling cause vibrations. Turbines or exciters may induce vibrations. Careful investigation can reveal if cause is the generator or turbine. In overhaul all these causes to be removed, for which elaborate measurements need to be done before shut down. CONCLUSIONS The case studies presented above have deliberated in details about the problems faced by utilities as well as precautions required during operation of the units. The deficiencies which can be prevented during manufacturing stages have also been deliberated. The case studies will help the operating utilities for revision of the customer hold points during build up of the Generator rotors at shop floor. The important parameters contributing to rotor unbalance have also been discussed with suggested preventive measures. Adequate attention to design materials that are conducive to maximum cooling of copper should be used. Quality workmanship is must in terms of component fit up, ensuring clear ventilation ports. The paper will thus serve very useful guide both for operating utilities during the operation of the units as well as manufacturers for building up adequate margins during manufacturing of the Generator rotors.

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