Paper: Graeme Robertson, Sulzer Downing and Mills: Generator repair and rewind


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Graeme Robertson, Head of Operations – UK, Sulzer Dowding & Mills delivered this presentation at the 2013 Gas Turbines conference. The event is designed as a platform for discussion on the latest technologies & developments in gas power generation. For more information on the annual event, please visit the conference website:

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Paper: Graeme Robertson, Sulzer Downing and Mills: Generator repair and rewind

  1. 1. Large HV Machine Repair Repair Options & Rewind Good Practice With Examples of More Unusual Repairs However when the damage extends into the endwinding of the coil this type of repair is no longer possible. Author: Eur Ing John S Allen CEng, FIEE, FIMMM, MIEEE Technical Director Sulzer Dowding & Mills Camp Hill, Birmingham. B12 0JJ United Kingdom. 1.2 Abstract HV stator rewind options & best practice. However this will have an effect on the performance of the machine since there will be an unbalance in the three phases. The unbalance will be seen in the phase currents of motors and generators and in the open circuit voltage of generators. Index Terms EASA IEC IEEE EA EN Electrical Apparatus Service Association International Electrotechnical Commission Institute of Electrical and Electronic Engineers Electricity Association European Standard (Euro Norm) 1. Repair Options This unbalance may be acceptable dependant upon the number of coils in the circuit, i.e. the higher the number of coils in series the less effect cutting 1 coil out of circuit will have. Some protection relay limits may have to be adjusted to accommodate the unbalanced current in the three phases. Unbalance can be eliminated by cutting out the same coil, which is in good condition, in the other 2 phases When a stator winding fails there are several partial repair options, which can be considered to keep the machine in service until it is convenient to take the machine out of service to complete a stator rewind. Coils which have been cut out of circuit will still generate a voltage in each conductor (turn) when the machine is in operation. Therefore the coil leads, which have been cut to take the coil out of circuit, need to be insulated The size of the machine and how critical its ongoing operation in service is to the organisation, i.e. contract performance requirements & lost production cost, as well as the availability of finance to fund the repair/ rewind, influence this decision. In the case of the failed coil as well as insulating the cut leads it is necessary to have the conductors (turns) in the coil knuckle cut and insulated to prevent circulating currents within shorted turns. The partial repair options available depend totally on the degree of damage which occurred during the initial fault and any subsequent damage following the initial fault together with age and condition of the stator winding insulation system. When there are parallel circuits in the phase winding and only the one failed coil is cut out of circuit, the unbalance in the phases will be greater and there will be additional parasitic circulating currents within the parallel paths of that phase. Partial repair costs will be significantly higher when the stator winding has been impregnated using a global VPI system and lowest when the stator has been wound with stator bars (half coil sides) rather than multi-turn lap or concentric coils. 1.1 Option 2 – Cutting Coils Out of Circuit If one stator coil, or even if several coils, have failed either inter-turn or to ground, it may be possible to cut that coil out of its phase circuit and re-connect the remaining good part of the phase winding together, i.e. bypassing the failed coil. Unbalance can be eliminated by cutting out the same coil in all parallel paths in all three phases. When selecting coils for cutting out of circuit consideration must be given to the harmonics in the air gap and symmetry of coil sequence around the stator must be maintained as well a balance between phases. Option 1 – Coil Lead or Connection Repair If as a result of insulation failure or voltage surge, the leads, either of a phase or of a coil within a phase, have failed it is possible to splice in new conductor, reinsulate the leads and replace support blocks. However, the more coils which are cut out of the stator winding, the greater the effect on the performance of the machine. When coils are cut out of the circuit the volts per coil, or more importantly the volts per turn of the remaining winding is increased, which increases the flux density in the machine. The secondary damage resulting from arc flash damage can be cleaned up, repaired and re-insulated. This type of repair can provide many years of additional service. 1
  2. 2. In most machines a small increase in flux density can be accommodated without significant adverse affects on machine performance; other than unbalanced currents. The top half of the slot must be cleaned removing all the old coil insulation and impregnation resin to ensure that the new coil side is able to make good electrical contact with the side of the slot preventing partial discharge activity within the slot. However when the original machine design had high flux density this additional increase in flux density from cutting coils out of circuit, can result in a significant deterioration in machine performance due to increased excitation current and increased iron losses, requiring significant reduction in output. A spare coil is cut at the knuckle at each end of the coil to provide a replacement top coil side. The coil endwinding insulation is removed to about 25% of the outhang leg length. The coil side is fitted into the slot and side packed as necessary to ensure a resistance between the coil side and the stator core no greater than 4,000preferably less than 2,000. The conductors are bent back towards the stator core ready for connection. The larger the machine is in diameter and the greater the number of coils, e.g. hydro-generators or low speed motors, i.e. mill motors, will have greater tolerance to coils being cut out of circuit than high speed machines (8 Poles and lower). Each conductor layer is laid down in its correct position aligning the conductors from the original coil and the new top coil side. The conductors are brazed and insulated repeating the process for each layer and where necessary providing turn – turn insulation. When the coil failure occurs in the slot, which results in an earth fault, and there has been secondary damage to the stator core lamination, this could cause a hot spot in the core with sufficient temperature to cause the other coil side in the slot to fail or the coil in the adjacent slot to fail. When all the conductors have been connected insulated and all turns insulated, the endwinding insulation is applied together with the replacement and any blocking and bracing of the endwinding outhang leg. When core damage occurs in the slot this type of repair (i.e. cutting out coils) is more difficult and may require the cutting out and removal of the failed coil side together with remedial action to remove the damage to the stator core, i.e. grinding and etching. Obviously the spliced coil endwinding outhang leg will not be identical to all the other endwinding outhang legs and will project proud of the rest of the endwinding and it is probable the outhang leg where the splice is located will be fatter reducing the air gap between adjacent coils. Generally this will have little affect on the performance of the machine, but subsequent inspection should check around blocking and bracing for evidence of partial discharge (Pd) activity and localised heating. Where a severe earth fault has occurred at the end of the stator core and there has been significant erosion of the stator tooth, as well as preventing hot spots some remedial measures including fitting temporary tooth section using epoxy glass can be considered to maintain coil stability at the end of the stator core. If the failed coil had been in a stator winding with stator bars with connections at the coil knuckles this process is much simpler, faster and lower in costs since all is required is to break the connection at the knuckles, remove the damaged bar, replace the new top bar, remake the connections at the knuckle and replace blocking and bracing. Remember in service the coil side in the end winding is trying to move at 2 x line frequency and under step load changes and fault conditions with high current surges the straight section of the coil can experience significant circumferential force against the side of the tooth. The coil is also continuously experiencing bar bounce forces where the current in each coil produces a repulsion and attraction force causing the coil to try to bounce up and down in the slot. If this is not properly restrained will cause insulation erosion leading to earth faults and in extreme cases cause the coil side to exit the stator slot and abrade against the rotor which rapidly leads to a catastrophic failure. 1.3 1.4 Option 4 - Replace Failed Coils To make a partial repair which will not compromise the performance of machine, the complete failed coil or stator bar is disconnected from its circuit and removed from the stator and a spare coil or stator bar fitted, then connected back into circuit. The problem is that to remove the failed coil from the stator all the adjacent coils with top coil sides in slots, from the failed coil top coil slot to the slot over the bottom coil side of the failed coil, have to be removed from their slots and lifted clear of the stator bore. Option 3 - Splice in a coil side If the failed multi-turn coil happened to fail on the top coil side, i.e. the part of the coil immediately under the wedge, and spare coils are available, it may be possible to splice in a new coil side to replace the coil side which has failed. In the case of a 60 slot 4 pole machine the typical stator coil pitch for a 2 layer lap winding would be 1 -13. This means the bottom coil side will be in slot 1 and the top coil side will be in slot 13. Therefore to remove a failed coil with coil pitch 1 – 13 12 additional top coil sides have to be lifted out of their slots and restrained clear of the stator bore to permit the removal of the failed coil and the fitting of the replacement coil. In this type of repair the wedges will be removed from the slot containing the failed top coil side. The failed top coil side will be cut through at both ends of the straight section and the top coil end-winding outhang legs stripped of endwinding insulation and the conductors bent back towards the coil knuckle. 2
  3. 3. This is where the condition of the winding is critical, since the removal of the top coil sides out of their slots requires manipulation of the coil endwinding outhang legs and coil knuckle, which can result in damage to the endwinding and/or turn insulation if the insulation is aged and has become brittle. Depending upon where in the winding this turn – turn fault occurred the service facility may need to discuss with the machine user the need for surge suppression equipment or lightning arrestors. If the winding failed to ground the service facility can discuss with their coil manufacturer the possibility of increasing the ground wall insulation thickness. If the insulation is in reasonable condition it is practical to replace failed coils assuming spare coils, still in good condition, are available. Depending upon where in the winding this earth fault occurred, the service facility may need to discuss with the machine user the effectiveness of their synchronization systems, or controls to prevent restarting machines whilst still rotating, or controls to limit the frequency of starting. Obviously this process is much easier in stator windings where stator bars have been used and very difficult in small diameter stators with physically large multi-turn coils when the coil side depth exceeds 50mm and the coil is not very flexible. The service facility may also need to review the endwinding bracing and blocking to evaluate its suitability for the service the machine is experiencing. If several coils adjacent to each other, particularly if predominantly in the same phase, have failed this repair option is often the only solution. For each adjacent failed coil you only need to lift 1 additional top coil side out of its slot. If there is evidence of damage to the stator core, a stator core flux test should be conducted, as a minimum at a flux density equal to the machine design flux density, but preferably at the a density capable of verifying the condition of the stator lamination steel and stator lamination insulation i.e. 1.5 Tesla or 85,000 lines/in2. When the stator winding, which is a 2 layer lap winding with multi-turn diamond coils, has been impregnated using a global VPI impregnation this repair option is not possible. In this case a complete rewind is the only option. Using the procedure specified by EASA the ring flux test would be conducted with a flux density of 85,000 lines /ins2 (1.32 T) on 60 Hz machines or more commonly with 50 Hz European machines 1.5 Tesla. Where stator bars have been used and impregnated using a global VPI impregnation system and the failure is in a bottom bar, it can sometimes be possible to lift the top coil sides, but all the top coil sides lifted out of their slots will need to be replaced as well as the failed bottom bar. 2. IEEE 62-2 Clause 7.2.3 provides details of one method of ring flux testing up to 1.5 Tesla and defines a “hot spot” as an area where the local temperature is 100C or more above the mean core temperature. No hot spot with temperature 200C above mean core temperature is permitted and must be rectified before winding. Stator Rewind When the service facility starts to consider a rewind, whether in a service facility or on site, the first thing to do is to test the stator winding and record the insulation resistance, polarization index and resistance of each phase. After cleaning and drying the phases should be surge tested to verify the turn – turn insulation condition. During the assessment of the stator core condition the tightness of the laminations in the core should be checked, this is generally done using a knife blade. Having assessed the condition of the stator and its winding and determined the root cause of failure and additional work identified at this stage can be agreed with the customer. Phase resistance of the rewound stator should be no greater than the resistance of the original winding preferably lower, hence the need to measure the resistance of each phase before stripping out the old winding. Please note that additional work may only be identified after the winding has been stripped from the stator and access to other parts of the stator become accessible revealing defects previously not visible. Before starting a rewind, the service facility, (rewind company), needs to conduct a root cause analysis to determine the root cause of the failure. This is a basic requirement before rewinding to ensure the new winding does not fail prematurely as a result of the same fault that caused the original winding to fail. Only by understanding the root cause of failure can preventative measures be taken to limit the likelihood of reoccurrence. 2.1 Coil Manufacture Coils need to be manufactured to meet the requirements of the original winding with regard to the coil pitch, projection, drop and number of conductors (turns) unless there has been a re-design. All winding will fail in time but if, for example, the original winding failed as a result of a turn – turn fault the service facility can discuss with their coil manufacturer the possibility of increasing the turn insulation. The coil design needs to take into account the operating voltage & environment of the machine. If the root cause of failure has been determined and the coil re-design can eliminate that cause of failure e.g. 3
  4. 4. increased turn or ground wall insulation, this needs to be taken into account during the coil design. turbine & hydro generating companies when approving coil manufacturer’s insulation systems. The physical shape of the coil during coil manufacture is very important and needs to be verified before coils leave the coil shop. The most common of these is a voltage endurance test where test coils have to be capable of withstanding an over voltage, at temperature, for 400 hours. An even more demanding test is to have test coils undergo thermal cycling, 500 cycles typically 400C -120/1550C followed by voltage endurance testing. Thermal cycling stresses the ground wall and the ground wall to conductor bond to determine if the coil will delaminate in service. The length of the endwinding projection and the drop to the knuckle set must ensure that when fitting the coil into the stator slots, the bottom coil side contacts the bottom of the slot before the endwinding bottom leg contacts the bracing rings. Ideally there should be about a 3mm clearance, for packing, between the coil and the bracing rings which the winder packs with epoxy impregnated felt or equivalent. These tests verify the suitability of insulation systems to give extended life but in reality is only applicable where voltage stress on the slot wall is lower than in industrial machines and modern low power (<20MVA) air cooled power generating equipment where for competitive advantage reasons voltage stress levels have increased significantly. Under no circumstances should the coil outhang contact the bracing ring, before the bottom coil side contacts the bottom of the slot, if this occurs, the coil will need to be taken out of the slots and have the knuckle position adjusted by the winder before refitting. Failure to do so, for example, forcing the bottom coil side down to the bottom of the slot which will put mechanical stresses on to the slot cell ground wall insulation and can crack the slot cell. The relevant international standards are, IEC 60034-18, IEC 60505, IEEE 1043, IEEE 1310 & IEEE 1553, 2.2 Stripping, Detailing & Cleaning This is a critical stage of the rewind process, the service facility must record in great detail and accuracy the details of the original winding, connections, transpositions, blocking & bracing. The fit of the coil in the slot is critical to the life of the insulation system. If the stator core manufacturer and/or the coil manufacturer are not able to produce consistent slot and coils widths accurately and has a large tolerance (i.e. >0 .13mm), then the service facility needs to ensure the coil is manufactured slightly undersize to enable conformable conducting material, typically side ripple springs, to be fitted to maintain good electrical contact between the coil side and the slot wall. Any errors at this stage can affect the machine performance. All the coil groups, (pole), and their connections in a phase must be checked and recorded, all the coils in each coil group must be checked and recorded as must the turns of each coil in a group, and obviously any transposition . If the coil manufacturer is able to produce coils within tight tolerances (i.e. < +/-0.13mm) , then the Service Facility needs to ensure the slot width is measured accurately and the coils made to fit the slot with a good fit maximising the insulation and copper content. The dimension of the conductor and sub-conductors must be measured and recorded particularly when there are different turns per coil. The core length and pressure finger projection must be measured and recorded; likewise the coil pitch, straight lengths, projections and drops. The coil manufacturer should manufacture a coil where the insulation has sufficient thickness & integrity to meet the insulation life expectations of the end user and satisfy all test requirements during coil manufacture and winding and maximise the copper content without increasing eddy current losses in the conductor. It is critical that the slot size is measured accurately; this will necessitate the measurement of the slot at several points along the core and several points around the circumference of the core to ensure an accurate size. The preferred method of measuring the width of the slot is using precision ground steel packers and feeler gauges. The slot width must be measured accurately e.g. -0.00mm to +0.025mm. All coils should meet the test requirements for ground wall insulation (voltage withstand), turn insulation (surge test) and strand insulation, in addition the quality of the ground wall insulation should be verified by tan delta testing (tip up testing). Controlled pyrolysis burn-off ovens are the preferred method of winding removal where VPI impregnation systems have been used. The temperature of the stator 0 should not exceed 370 C during burn off. There are a variety of national & international standards which cover testing of coils during manufacture which include: IEC 60034-1, EA 44-5, EA 44-7, IEC 60894, IEC 60034-15, EN 50209, IEEE 286, and IEEE 522. Where possible coils should be removed without the use of heat but when stripping a VPI impregnated stator on site, the application of localised heat during stripping is necessary, particularly when cleaning the slot. There are also a series of international standards to verify the suitability of insulation systems for extended life. These are predominantly used by high power 4
  5. 5. exceed 4,000 and good practice requires 80% of the coils having a contact resistance < 2,000. Packing between top and bottom coil sides must be conducting, typically a conducting epoxy glass laminate and be sufficiently thick that the top & bottom coil sides do not contact each other at the end of the slot straight lengths. Having removed the failed winding the slots in the stator core must be cleaned. All old insulation and impregnating resins must be cleaned from the slot to ensure the new coil will fit into the slot and make good electrical contact between the coil side and the side of the slot. (<2,000) Having cleaned the slots the radial and axial ventilation ducts need to be cleaned and all debris removed from the stator. Coil sides must be solidly wedged into the slot to prevent coil insulation erosion resulting from coil sides moving radially in the slot. Bar bounce forces try to move the coils in the slot at 2 x line frequency. During this process close examination of the stator core will verify the condition of the core and the tightness of the stator laminations. This should include inspection of the stator core frame, welds, lamination supports and winding supports. 2.3 Best practice is to wedge the slot using epoxy glass laminate ripple springs under the wedge. The ripple spring accommodates any in service shrinkage of insulation materials. Winding When winders fit coils into the stator, the shape of the coil can be distorted, the winders need to check this and when necessary correct the coil shape to ensure the axial location of the coil in the slot, the correct axial projection of the endwinding and the correct drop of the coil below the stator bore. The coil endwinding bottom sides are close to the bracing rings and most importantly that the spacing between adjacent top and bottom coil sides is sufficient and symmetrical. Before any winding commences the cleaned stator core must have a core flux test to confirm that the stator core, following winding removal & cleaning, has no hot spots and is in good condition fit for rewinding and further service. The procedures detailed in 2 above apply. All winding must be carried out in clean conditions preferably in a controlled environment (200C – 300C <55% RH) to minimise build up of moisture in the insulation during winding. If the coil shape is not adjusted by the winder it is possible to reduce the space between coil sides in the endwinding and this can affect airflow through the endwinding of air cooled machines resulting in a higher operating temperature. Before winding commences all winding bracing/surge rings should be re-insulated to the same insulation specification as the coil end-winding insulation. The blocking system is important to the stiffness of the endwinding and the blocking pattern of the original manufacturer should be re-produced as a minimum. When coils are flexed to wind into the slot and under the coil pitch coils, they should be warmed to 500C – 600C to soften the endwinding insulation resin and make winding easier. Best practice is to use epoxy glass blocks wrapped in epoxy impregnated felt lashed between coil sides and cross banded using polyester covered glass roving. Alternatively epoxy glass putty can be used between the coil sides before lashing & cross banding. If the core build has produced a flat surface at the bottom of slot, fit the bottom coil side directly into the bottom of the slot, only fit a conducting packer in the bottom of the slot when the core build has resulted in a surface which is not flat. Never use an insulating pad at the bottom of the slot. When epoxy impregnated felt blocks are used on their own without lashing, only on smaller machines, the felt wrapped blocks must allow the felt to bulge over the ends of the coil side like a dog bone to ensure the retention of the block. Positioning of the coil in the slot is very important it is good practice to centralise the coil in the slot axially (i.e. +/- 1mm) and it is easy to do this by equalising the distance from the end of the core to the start of the stress grading tape. Where possible butt joints should be avoided and where unavoidable ferrules should be used to provide support. Lapped joints are preferable with lap length 1.5 x subconductor width. When the coils are fitted in the slot the tolerances in the width of the coil and the width of the slot will give a variation in the gap between side of the coil and the side of the slot. When this gap exceeds 0.13mm it must be packed with a conducting material such as conducting epoxy glass laminate. All connections should be brazed with suitable brazing rods, typically Sil Fos with15% silver. After brazing all connections should be inspected and dressed (filed) to remove any sharp edges. When making up connections, all coil to coil and group to group transpositions must be replaced and the integrity of the strand insulation must be maintained throughout the phase. The fit of the coil in the slot is important to good insulation life but more important is minimising partial discharge (Pd) activity in the slot by ensuring the contact resistance between all coils and the stator core does not Where multi-turn coils with inverted turns or Roebelled bars are being connected i.e. where block connections 5
  6. 6. are being used, strand insulation is not maintained through the connection. this is generally only applicable manufactured before 1970’s. Any voids between sub-conductors in connection or transpositions must be filled with epoxy resin putty or equivalent to eliminate voids. The final proof test, the voltage withstand test as specified in IEC 60034-1 is twice line voltage plus 1,000v which has to be withstood for 1 minute. This test to be conducted for each phase to ground and between phases. All insulation joints must be scarfed (tapered) so coil lead insulation will probably need to be cut back to give an appropriate scarf length (creepage length) and the new insulation built up in layers over the scarf joint. It is good practice that any IR test includes measurement of the winding temperature at the time the test was conducted and correction of the IR to 400C in accordance with IEEE 43. Curing winding It is beneficial for the user to carry out a tan delta test on the complete winding after these tests to create a baseline which can be used during the life of the machine for assessing the condition of the insulation system. Before any voltage withstand tests are conducted the winding will need to be IR & PI tested, impregnated and cured. During the winding process, the insulating materials in the coil end winding absorb moisture, if the IR is low and PI less than 2 the winding must be dried before impregnation & curing otherwise moisture is trapped within the insulation system. It is beneficial to measure the impedance of the three phases as a baseline for condition assessment. 3 Reports It is good practice that the service facility prepares and presents the customer/user a report detailing: Stators can be dried on site by construction of an enclosure and application of electrical hot air blowers or direct electrical heat by application of LV DC supplies to the winding. Winding temperature must be monitored and controlled during drying & curing.      When the IR is good and PI greater than 2 the endwinding should be impregnated to seal the outer surface and stator core coated to minimise corrosion. The condition of the machine as received Reception test results Summary of work carried out Copies of 3rd party test reports Results of tests upon completion 4 Examples of more unusual repairs 4.1 The impregnated winding should be cured for sufficient time to meet the curing requirements of the coil manufacturer’s insulation system. 12MW 11kV 4Pole Synchronous Motor The stator winding of this gas compressor motor failed with multiple failed coils and several earth faults. The user, facing severe supply contract penalties, could not afford an outage of sufficient duration to permit a stator rewind so a repair was proposed to get the machine back into service in the shortest possible time with the intention to rewind the machine several months later when the operations could cope with a 4 week outage. It is at this stage the final voltage withstands test in accordance with IEC 60034-1 is conducted. 2.5 machines It is good practice to measure Insulation resistance and polarisation index before and Insulation resistance after the voltage withstand test. All connections to be braced as the original manufacture’s winding as a minimum requirement. Where inadequate bracing was a contributing factor in stator winding failure bracing should be improved. 2.4 in Stator winding tests It is good practice to test coils after they have been fitted into the stator, wedged and lashed to the bracing ring before connecting. These tests should include voltage withstand and turn to turn. When the machine was stripped, inspected and tested a total of 3 coils had failed. Two coils in U phase and 1 in W phase had failed to earth as a result of attempted restarts. One earth fault had damaged top and bottom coil sides in one slot whilst the second earth fault had damaged the bottom coil side in a different slot. The voltage withstand test would normally be higher than the final proof test but for a short duration, typically 20% higher than the final test voltage for only 10sec. The stator winding is normally withstand tested again after connecting, typically 10% higher than the final test voltage for only 10 sec. Repairing by replacing failed coils was not possible because the original stator winding had been impregnated using a global vacuum pressure impregnation system (VPI). However the repair was done by using a spliced coil, cutting 2 damaged coils and 4 un-damaged coils out of circuit. The additional coils had to be cut out of circuit to balance the 3 phases and because the stator winding had been connected in 2 parallel circuits. The resistance of each phase must be measured and the temperature of the winding recorded. The resistance of phases must be balanced to less than +/- 5% of the mean values of the phase resistance. Occasionally this cannot be achieved where 3 tier concentric winding have be used by the manufacturer; 6
  7. 7. When the coils were cut out of circuit, the knuckles of the damaged coils at the non-connection end were cut to prevent problems caused by shorts between turns. To save time, the same process was applied to the undamaged coils to prevent problems caused by shorts between strands. Due to the global VPI impregnation stripping back insulation on coil leads and re-insulating strands which had been cut would have been very time consuming. When the winding had been stripped and the core dismantled it was found that the “heavy” lamination back iron had fractured probably as a result of the reaction to the impact between the rotor and the protruding tooth. The core clamp rings had also been damaged where the pressure fingers had been vibrating resulting in erosion and production of grooves. The root cause of the failure was probably loss of core compression pressure initially at the bottom of the core where the damage was greatest; i.e. at the end of the core where the OEM started to build the core. This loss of pressure may have been as a result of insufficient core consolidations during core build or loss of pressure in sections of the core due to localised thickness of laminations. The repair was completed within the agreed 8 days and the motor returned to service at a reduced rating of 9MW. A new operating curve was produced for the machine with reduced output and reduced stator coils. Although the number of coils in stator had been reduced by 8.4%, the machine rating had to be reduced by 25%. No core lamination material is perfectly flat and if all laminations are cut with the same orientation to the rolled strip and built without reversing discs the variation in thickness across the laminations can result in a tapered core which when consolidated will have one area tight and another area loose. When the machine was stripped the condition of the rotor was found to be very poor with evidence of over temperature in the end-winding resulting in very significant insulation degradation. This machine had a chequered past, having initially been supplied as a 9MW motor, which could only run at 7.5MW. The OEM had re-designed this machine supplying a new rotor and re-rating it to 12MW. The clamp rings were machined flat and additional laminations added to each end to compensate during the core rebuild and the pressure fingers welded to 3 heavy laminations. As a result the original 9/7.5 MW rotor was available as a spare. The spare rotor had been in storage for several years and required an overhaul before it could be reused. This work was completed prior to the rewind outage and when the rewound stator was returned to service it was with the overhauled rotor but with the machine rating restricted to 7.5MW. The fundamental mechanical design of this machine has a potential problem which will keep re-occurring. The radial depth of the back iron is only 64% of the slot depth. Our re-design manual specifies a minimum radial back iron depth of 80% of the slot depth to ensure mechanical stability of the back iron. The rotor was subsequently rewound and swapped with the spare rotor returning the machine to its 12MW rating, albeit with rotor operating over temperature. 4.3 The 2 pole synchronous gas turbine driven generator had experienced an in service failure of phase lead connection to the stator winding. As a result one bottom of slot stator bars required replacement as did the end of the phase lead The operator had ordered a replacement machine, from a different OEM. When this is supplied the repaired machine will be replaced by the new machine and kept as a spare machine. 4.2 120MW 13.8kV 2 Pole Generator The cause of failure was probably fatigue failure of the mechanical components used to connect the phase lead to the first bottom bar of the phase winding. This resulted in significant localised arc flash damage. 2,200KW 11kV 6 Pole Induction Motor This cement kiln motor was withdrawn from service following an impact between rotor and stator. The stator winding had been manufactured using a global VPI impregnation system, which meant the repair was going to be extremely difficult. When the motor was dismantled, inspected and tested significant problems were identified at one end of the core. We had undertaken our first site rewind of a 108MW global VPI stator some 6 years earlier and had extreme difficulty in stripping out the original winding. Following that project we had developed a new stripping procedure which was used on this stator. It was evident that the core pressure fingers had started to vibrate and move in service until a tooth of the “heavy” (1 mm thick end of core lamination) had fractured, entering the air gap and impacting the rotor winding. Because it was a bottom bar that had failed we had to remove 25 top bars to gain access to remove the failed bottom bar. Detailed examination of the end of the core clearly showed that many of the pressure fingers had been vibrating for some time since there had been erosion of the wedges where teeth had been vibrating circumferentially and erosion on the end of core clamp ring where the teeth had been vibrating axially. Having removed the original stator bars from the stator, the slots had to be cleaned to remove all of the original epoxy resin on the sides of the slot to ensure good 7
  8. 8. electrical contact between the new stator bars and the core. Coils manufactured with poor control of coil width can lead to under compressed side ripple spring allowing coil movement a secondary affect is the erosion of the semiconducting paint on the inside of the slot which is circulated around the machine with the potential risk of initiating Pd activity in the end-winding. The connection between the phase lead and first bottom bar was redesigned to eliminate the cause of failure. This required some adjustment of the position of the bottom bar terminal position and its insulation. 4.5 The top bars were fitted, connected and insulated before the complete winding was painted with a 2 part epoxy resin and the winding cured. The machine was re-built and returned to service and has been in continuous operation for the last 7 years 4.4 15MW 6.6kV 4 Pole Generator The first indication we had of any problem with the Dunlin Alpha 6.6kV 15MW 4Pole generator was when I was sat in an airport waiting for a flight home to Birmingham from Switzerland. I received a telephone call about very low insulation resistance readings (0.2MΩ & No PI with a 1 KV Megger) and a possible earth fault. 110MW 13.8kV 2 Pole Generator The rig had a very significant operational problem with the failure of this generator since they were required to power sea water injection pumps for oil well maintenance. If these pumps could not be powered within weeks the viability of the oil well would be questionable and production lost. An outage of 14 -18 weeks as required for a rewind where no spare coils existed was not acceptable. This 2 pole synchronous generator had failed in service with an earth fault. As a result one top bar had failed. As repairs go this was a relatively simple repair since the machine had been manufactured with resin rich coils with hot pressed slot cells. However the cause of failure was more interesting. The fit of the coil into the slot had been very loose and in line with current North American winding practice the space between the side of the coil and the side of the slot had been filled using a conducting side ripple spring. With very limited information we proposed a repair to dry out the winding and cut out the failed coil. At this stage we assumed we may have to cut out 3 coils to maintain a balanced winding. I calculated that we should be able to get the generator back into service with 90% of its rated output. The clearance between the coil and the slot was in the order of 0.100” and as a result the side ripple spring had not been compressed adequately. We discussed this with the rig operator and agreed to mobilise a site repair team to identify the failed coil and modify the winding to get the generator back into service in the shortest possible time. The site maintenance team ran the generator in an attempt to dry out the stator winding, unfortunately with no success. The imprint of the ripple spring on the failed coil side clearly shows that the coil had been moving circumferentially within the slot i.e. vibrating against the conducting ripple spring. Unfortunately when the ripple spring had been fitted they had been installed with a cut edge with a sharp corner against the side of a coil. Over time this sharp corner wore into the coil slot wall insulation until the voltage stress built up sufficient to break through the coil slot ground wall insulation and connect through the conducting ripple spring to ground. Our site team arrived on the rig and dismantled the connection end of the generator to get access to the stator winding connection. An enclosure was erected to protect the stator for the environment and facilitate drying out of the winding. Some days later in the middle of a round of golf in Nashville I received detailed information on the condition of the winding and where the failed coil was located. There was good and bad news. A replacement top bar was manufactured with thicker ground wall insulation and fitted into the stator with a fully compressed conducting side ripple spring. The bad news was that the stator winding had been connected in 2 parallel circuits STAR / WYE and as a result we would have to cut out 2 coils in each phase. This meant we would have to cut 6 coils out of 66 which would make achieving 90% output much more difficult. Best practice in UK requires HV coils to be manufactured with close tolerance on width to a size which will give a good fit to the slot and when fitting coils into the stator slot any voids greater than 0.008” are packed with rigid conducting material such that all coil sides have a resistance to the stator core no greater than 2,000 ohms. The North American practice of painting slot sides with a semi-conducting paint and no-voids between coil and slot side >0.002” forces re-winders into the use of conformable slot fillers but can as we can see from this example if this is not fitted correctly it can result in premature failure. Fig 1: Earth Fault Fig 2: Jumpers fitted The good news was that the failed coil was in a pole group of 6 coils. This winding had pole groups with 5 8
  9. 9. coils and 6 coils, had the failed coil been in a 5 coil group achieving 90% output would have been almost impossible. [6] EA 44-5, Industry Technical Standard: - Testing the Insulation Systems of Stator Coils in Rotating electrical machines The failed coil was the 2nd coil in a 6 coil group, as a result it was a fairly simple exercise to cut the coil lead between the 1st and 2nd coil and between the 2nd & 3rd coils then join the lead from the 1st coil to the 3rd coil. This had to be repeated 6 times around the stator. [6] EA 44-7, Industry Technical Standard: - Testing the Insulation Systems of Bars (Half Coils) in Rotating electrical machines of Turbine Driven and Hydro-Generator Types [7] EN 50209, European Standard: - Test of insulation of bars and coils of high-voltage machines Unfortunately we have no photographs of the modified winding since the site team were working 24hrs a day to get the generator back into service. [8] Site work was completed in 14 days with all winding modifications completed and the winding dried and varnished including rebuilding the generator and preparing it to return to service. IEEE 43, IEEE Recommended Practice for Testing Insulation Resistance of Rotating Machinery [9] IR was improved to 4 GΩ with a PI of >4 using a 5KV Megger and the phase resistances balanced. Some adjustments were made to the protection & AVR settings IEEE 62.2 IEEE Guide for Diagnostic Field Testing of Electric Power Apparatus – Electrical Machinery [9] IEEE 286, IEEE Recommended Practice for Measurement of Power Factor Tip-Up of Electric Machinery Stator Coil Insulation [10] IEEE 522, IEEE Guide for Testing Turn Insulation for Form-Wound Stator Coils for Alternating Current electric Machines [11] IEEE 1043, IEEE Recommended Practice for Voltage-Endurance Testing of Form-Wound Bars and Coils [12[ IEEE 1310, IEEE Recommended Practice for Thermal Cycle Testing of Form-Wound Stator Bars and Coils for Large Generators [13] IEEE 1553, IEEE Trial-Use Standard for VoltageEndurance Testing of Form-Wound Coils and Bars for Hydrogenerators The generator was brought on load slowly allowing the insulating materials to stabilise and over the first shift load was increased from 2MW to 7MW after which the load was increased to 13.8MW, 92% of rated output. The generator has been running at this load ever since. We returned to site July this year to rewind the sister generator and are scheduled to rewind this generator in 2014 thereby maintaining continuity of supply until all generators have been rewound. By repairing this generator the risk to the oil production was eliminated and the operator was able to have coils manufactured in advance and plan the outage when it was operationally convenient. The work was completed to our projected timeline and budget and the future revenue of the oil well secured. 5 6 The author graduated from UMIST in 1970 with a B Sc in Electrical Engineering & Electronics. He worked as a DC & AC rotating machine design engineer for 15 years in manufacturing before working in the service industry and has been Technical Director of Sulzer Dowding & Mills since 1998. He has managed service facilities including a HV formed coil manufacturing facilities from 1988 – 1994 and over the last 12 years he has been active in the re-design of hydro-generators. References [1] IEC 60034-1, International Standard – Rotating Electrical Machines - Part 1: Rating and Performance [2] IEC 60034-15, International Standard: – Rotating Electrical Machines – Part 15: Impulse voltage withstand levels of rotating a.c. machines with form-wound coils [3] IEC 60505, International Standard: - Evaluation and qualification of electrical insulation systems [5] He is a member of BSI GEL 31 and the IEC TC31J maintenance team for IEC 60079-1, IEC60079-19 & WG27. He has represented EASA at IECEx since 2003 and is a member of IECEx ExMC WG10 (Service Facility) sub-committee & Chairman of the IECEx ExPoPC Scheme. He has been Technical Adviser for EASA Region 9 since 1998 and on the EASA Technical Committee in St Louis since 2002. IEC 60034-18, International Standard:– Rotating Electrical Machines – Part 18: Functional evaluation of insulation systems [4] Vita IEC 60894, IEC Technical Report: - Guide for a test procedure for the measurement of the loss tangent of coils and bard for machine windings 9