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TheStory f Electrical Energy, Pt.6 As time passes, all alternators either wear out or fail completely. Now though, instead of scrapping worn out machines, it is often worthwhile rebuilding them to better than new. By BRYAN MAHER All over the world, the picture of electricity generation is undergoing a change in emphasis. Projecting future demands is becoming more difficult and in many countries the number of possible new power station sites is severely restricted. So existing stations are being upgraded and improved. In the USA, Canada and Europe, atomic plants tend to provide the base load. Thus, many fossil fuelled power stations, designed for base load 20 or 30 years ago, are today required to cope with cyclic load duty. The changing economic climate forces generating companies to extend the working life of existing machinery rather than purchase new equipment. All the above factors make the decision to recondition and upgrade old alternators, to extend their life by 20 or more years, a sound financial proposition. No longer does it make sense to scrap a power station after 30 years of service, as was done in Australia and overseas until recently. Very large alt~rnators can be retrofitted to obtain higher output, better efficiency and greater reliability, especially under peak load duty. This giant 970MVA alternator, being lifted by the crane, failed and was rebuilt to a much higher standard. 98 SILICON CHIP The electro/mechanical justification for a 30-year-old alternator retrofit may be summarised as follows: (1). The stator insulation may no longer be reliable; (2). Improved design can today raise the efficiency; (3). Alternators designed for steady base load years ago may well fail if used on cyclic peak duty. Two types of peak load Two forms of peak load duty are used, depending on the requirements: (1). Load cycling means that the turboalternators are run at full speed all the time, the load changing from full power during peak hours to some minimum load at other times. (2). In two-shift operation, the machine may run at full speed between 7am and 7pm daily, carrying the full load during peak times and perhaps 3/ 4 load throughout the day. Between 7pm and 7am, all electrical load may be removed, the steam supply shut down and the turbo-alternators kept revolving slowly. This way, thermal equilibrium is more easily restored before the next shift. Sometimes, electrical shaft turning gear is employed for this purpose. Either of the above two forms of cyclic loading has a bad affect on the alternator. The changing power load causes thermal cycling in the stator with consequent expansion and contraction problems in the insulation of the copper coils. Either the copper coils will expand by sliding with respect to the core slots, or the copper will remain still in the slot and suffer elastic compression. The amount of movement or stress depends on the difference between the no-load and full-load temperatures, and the relative expansion coefficients of core and windings. Minimising these stator stresses requires careful choice of copper and Modern alternators are big brutish machines running with very fine tolerances. The rotor of a big machine (say 1 gigawatt) may weigh as much as 70 tonnes and rotates at 3600rpm (60Hz) or 3000rpm (50Hz). Vibration is a very big problem, particularly at the ends of the stator windings. silicon steel alloys. Also (and most importantly), the full load temperature must be kept as low as possible. The best way to do this is to use water cooling. Rotor stresses In two-shift operation, the rotor winding and core teeth suffer cyclic forces which accelerate metal fatigue. At full speed, centrifugal forces on the rotor winding are so strong that the conductors cannot possibly slide axially under thermal expansion. Therefore, the copper remains under axial compressive stress at full load. In load cycling operation, this thermal compression cycles on and off, fatiguing the copper. In two-shift mode, the centrifugal forces alternately range from maximum to zero, accelerating wear on the winding insulation. Rejuvenation Some alternators are rejuvenated when their reliable lifetime is judged to have ended. In other cases, actual failure precipitates a complete retrofit. In this chapter, we trace the story of a very large alternator from burnout to its complete restoration. In June 1986, a 970MVA 60Hz steam turbo-alternator in Florida, USA suffered a stator winding to earth fault. The machine protection automatically opened the alternator circuit breaker and switched off the excitation. This prevented further power from being generated, preventing the escalation of the short circuit into a possible full scale fire. The steam valve was closed and DECEMBER 1990 99 weeks. While on loan, this spare ·alternator produced 2500GWh of electrical energy. The contract involved complete redesign of the alternator using the original stator casing. This required a new stator core with improved slot geometry and provision of laminated end pressure plates. The original hydrogen cooled stator winding was replaced by a new water cooled coil set and end-winding supports were made flexible to accommodate startup expansion stresses. Improved stator insulation was also incorporated and the rotor winding replaced using coil retaining rings of advanced design. Dismantling During assembly, the laminated stator core of an alternator must be thoroughly compacted to minimise vibration. This is achieved by inductive vibration and massive clamps. half an hour later, the machine was at a standstill. When the 69-tonne rotor was removed, inspection 'showed severe burning of the stator coils deep within the core slots. And removal of the ZZkV coils revealed the iron laminations to be severely burnt. It was a mess. Records showed that this machine .had run for almost 20,000 hours, in- eluding nearly 800 stop/start cycles. In fact, even though it was a very large machine, it had been used mostly for peak load duty. The ABB company was awarded the contract to remove, retrofit and replace the 460-tonne alternator. During the repair, a 730MVA alternator was loaned to the power company, being mounted on the original foundations and brought on line in just 11 With the alternator lifted from its foundations, the old stator windings and laminated core were removed. The remaining 113-tonne stator housing was then shipped by rail to ABB's workshops in Richmond, Virginia for cleaning and modification. Simultaneously, at the company's Birr workshops in Switzerland, new stator coils and iron core segments were designed and manufactured. These were delivered in sections tr. the Richmond shops for assembly. With the stator casing mounted vertically over one end press-plate, the first task was stacking the core into the stator casing. The core is composed of low-loss silicon alloy steel segments, each 119th of a circle. Each lamination was punched, deburred and coated on both sides with heat resistant varnish, then placed on the core stack. For long life, it is important that the core laminations be compacted tightly, otherwise vibrating segments would damage the coil insulation. Therefore the core was periodically compressed using hydraulic rams during the stacking process. The completed core was vibrated by inductive currents, while being squeezed hydraulically to settle and compact all the steel segments. Stator winding This photo shows the stator of a 300MVA 2-pole turbogenerator undergoing testing at ABB's Birr works in Switzerland. The stator is cooled by pumping de-ionised water through the windings and end connections. 100 SILICON CHIP With the casing and its core returned to horizontal attitude and the coil support ring fitted, the new 31,800 amp stator windings were installed. The new stator coils were wound using multiple parallel flat copper wer/ng machine• lesaphone• /ml/ea The mOMI CPEP-1 MWUIN 1/ghtn/n(I, po- .UtflN #Id ap/ltN coally aqulpment by do not reach yi,ur c-lantly monitoring lhe PHONE /Ina and lhe IIAINS po- IIM, Simply conMCt to an exlatln(I po-, point, plllfl In your fllx •tc and• MrlH of lndk:ators a/Iowa lhe atatia of Iha ma/na This close-up photo of a rotor shows some of the detail of the ends of the excitation windings. The windings are subjected to considerable stresses, both from the retaining rings which stop them flying out and the centrifugal forces. bars interleaved with stainless steel tubes for the water cooling. The original coils were hydrogen cooled. Because water has much higher specific heat than any gas, less volume is needed when water does the cooling. Hence, less space in each coil is required for the water tubes (compared with the original larger hydrogen tubes). Thus, more of the coil cross section can be copper, even when slightly smaller coils are used. So compared to the old coils, the new coils are smaller in total cross section yet contain more copper. The new smaller coils naturally require smaller slots to accommodate them. Therefore, more iron volume remains. The result is that for the same total magnetic field , the greater core section means lower flux density. Thus, in the refurbished machine, the excitation losses are lower. As core vibration amplitude is proportional to the square of magnetic flux density, the reworked machine (using lower flux density) also has much less vibration. The coil insulation can therefore be expected to last a lot longer. High voltage insulation Micadur, which exhibits high di- electric and mechanical strength, was used for the stator coil insulation. Developed by ABB specifically for turbo-alternators, Micadur is superior to the insulation used in the original windings. The newly renovated stator, winding end connections and cooling water tubes are shown in the photos. The laminated construction of the end-press plates results in reduced iron losses compared to the original design. The method of supporting the stator end connections is just as important as the slot insulation. This is because coil overhangs can be subject to vibration if not suitably clamped. The clamping support used in the refit is rigidly fixed to the stator core. Therefore, no 120Hz vibration can be transferred from the core to the coil end overhangs. This is a considerable improvement over the original design. A further improvement introduced in the retrofit is that the coil ends are allowed to move in the axial direction, yet are restrained radially and tangentially. Axial flexibility accommodates differential thermal expansions which must occur during changing loads. Therefore, the reworked machine is -~JS11•a po-point. Fully Telecom and O.pt of Mlnarala and .n,a __ ,... Enargyapprowd. ELECTRONICS suitable for cyclic peak load duty as well as steady base load operation, an important consideration for its owners. Provision was also made for retightening the end connections as the winding insulation settles and ages. De-ionised cooling water is pumped through the stator windings and end connections, passed through a cooling heat exchanger and filter, then recirculated. On return from the stator, the circulating water is monitored for temperature rise, flow rate, pressure differential and electrical conductivity. Rotor improvements Alternator rotors are made in one piece from high grade steel, with slots milled to receive the insulated copper excitation winding. The steel teeth between these slots are subject many stresses. At the normal speed of 3600rpm, the copper winding tends to fly out, so must be tightly restrained against the enormous centrifugal forces. Wedges driven into keyways in the slots, along with circumferential steel to anlen exlallng Unique powder -a ret»fvin ehlmen excalhln UHF recapllon compared to ollHlr UHF anlennM of almllar aize and price. Two m""'11a.,. a11allable: TVA14-S.nd Four and TVA15-Band R.,,., Both.,. aupplied with back reflectors to pre'llent ghoalln(I aa _,, .. • Mlerproof entry ,& f t .bolt and a lilt adjua1able ELECTRONICS metal mounting bracket. An Tl.'llllft. .n.&\.li'J DECEMBER 1990 101 ·:r ·- This photo shows the rotor of a reconditioned alternator in the process of being refitted. This is a tricky job since the clearances are tight and because of the tendency for rotors to sag under their own weight. Compared to the original design, the reconditioned machine generates 14% more power. bands, are used to hold the winding in place along the length of the rotor body. Restraining the ends of the rotor windings is a big mechanical problem. The usual practice is to fit a steel retaining ring over the insulated winding ends. On this machine, this had ultimately resulted in cracking of the rotor teeth. This happened because when the machine was stationary, .the retaining rings applied considerable force to the steel teeth. Then, at full speed, centrifugal forces exerted by the winding pushed radially outwards, taking tension off the teeth. This reversal of forces in the rotor teeth each time the machine was run up to speed caused metal fatigue, and hence the cracking. Extensive tests with this method have shown that cracks appear after about 200 starts, while 1000 starts can cause dangerous cracking right through a tooth. As this particular machine had already had about 800 starts in its lifetime, a new approach was needed. This involved using a ring set fixed in place on the rotor using a bayonet locking system. This greatly reduces the stress reversal cycle at start-up. In addition, the new rings are made from a manganese chromium steel alloy that's not sensitive to stress-corrosion cracking. These rings and the rotor modifications to accommodate them are designed for 10,000 starts, ensuring a long life for the rebuilt machine. Uprated alternator After the completed rotor had been overspeed tested, the finished alter- TABLE 1 Output voltage Apparent power Stator volts Power factor Increase in power 102 SILICON CHIP Original 22kV 970MVA 22kV 0.89 Retrofitted 22kV 20kV 1050MVA 1100MVA 20kV 22kV 0.89 0.89 71 .2MW 115.?MW Ii' ·' . -- ·' nator was returned to its original site and coupled to the steam turbine. Tests validated the calculated details used during the retrofit. Because of the changes made in the design of the stator coils, 14% more power (116 megawatts) could be taken from the alternator for the same temperature rise. Even then, this increase in available power is limited not by the alternator but by the ratings of the original exciter. The results are shown in Table 1 for two values of alternator voltage: Z0kV and 22kV. Increased efficiency The modifications made during the retrofit resulted in lower alternator losses and hence increased efficiency. The new stator slot and coil design give lower flux density, thus reducing excitation power loss and stator heating. Compared at the original power output, the rebuilt machine has 600kW less losses which is a fantastic saving in energy. Completed in December 1987, the retrofitted alternator has been operating on load ever since - a much cheaper solution than a completely new machine. Acknowledgement Grateful thanks to ABB staff and to ABB Review for data, photos and permission to publish.