machenical disign of hydrogen

HYDROGENERATOR ENGINEERING DIVISIONBHEL, BHOPAL

MECHANICAL DESIGN ASPECT OF HYDROGENERATORS


MECHANICAL DESIGN ASPECT OF HYDROGENERATORS

HYDROGENERATOR ENGG.DIVN.BHEL, BHOPAL.

1. INTRODUCTION

The design of hydroelectric generators differs in many respects from that of other electrical machines. The output of these generators may range from a few megawatts to hundreds of megawatts and the speed of rotation may vary from about 60 rpm to 1500 rpm. The output, speed, inertia and other parameters of hydro generators vary widely with site conditions. Even the detail constructions are greatly influenced by such factors as the transport facilities available, machine tool capacity, ease in erection and dismantling etc. The result is that each project has its own unique machines and the exact duplicate for two projects are rare.

Hydro generators are slow speed, large diameter synchronous machines and therefore, the rotor is normally of salient pole construction. In this paper, the selection of bearing arrangement, method of rim construction, construction of bearing and ventilation aspect of hydro generator will be discussed.

2. BEARING ARRANGEMENTS

In vertical shaft hydro generators, 4 well recognized arrangements as discussed below are usually adopted.

2.1 SUSPENDED ROTOR OR CONVENTIONAL ARRANGEMENT (SEE FIG. 1)

In this arrangement, the thrust and one guide bearing are above the rotor and one guide bearing below it. The thrust bearing bracket spans across the stator in this arrangement. The thrust bearing coolers are more readily accessible. This arrangement results in taller machines and where the stator diameter is larger, the thrust bearing bracket becomes massive. Bearings are insulated against circulating shaft currents.This arrangement is used for higher speed units where stator & rotor diameter are small.

2.2 CLOSE COUPLED SUSPENDED ROTOR ARRGT. (SEE FIG. 2)

This is similar to suspended rotor arrgt. except that the bottom guidebearing is omitted. It is sometimes possible to extent turbine shaft right upto the generator rotor.This arrgt. is normally adopted for small high speedmachines

2.3 SEMI UMBRELLA ARRGT. (SEE FIG. 3)

In this arrgt. the thrust bearing is provided below the rotor. There is oneguide bearing above & one below the rotor.



The thrust bearing housing has to span smaller pit, the dimensions ofwhich are decided by the largest turbine component which has to belowered into the pit.

The arrangement is normally adopted for the machines whereumbrella arrangement cannot be adopted and suspended rotorarrangement is uneconomical.

2.4 UMBRELLA ARRGT. (SEE FIG. 4)

In this arrangement, one guide & thrust bearing is below the rotor and there is no guide bearing at the top of the rotor.

This type of construction is usually preferred where ever possible on account of its simplicity and economy. It is practicable to have this arrangement for generators with large diameter and short core length (slow speed machines). Such type rotor is more stable while revolving than one with a small diameter and long core.

3.0 STATOR

3.1 STATOR FRAME

The stator frame is built of welded steel structure and to facilitate transport, it is dispatched from the factory in segments. It has adequate depth to prevent distortion during transport and under any operating conditions.

The joints between the sections of the frame are heavily flanged internally and coupled by a number of short bolts. Each joint is dowelled and welded with number of ‘Ι ‘ shaped anchor plate at site.


3.2 CORE

The stator core consists of segmental, varnish-insulated stampings of cold-rolled silicon alloy steel, is built up as continuous ring at site. The segments are secured to the frame by dovetail notches engaging with corresponding dovetail bars subsequently the bars are fitted in the horse shoe plates which are welded into the frame and are clamped between segmental steel end plates.

The clamping pressure is transmitted to the stator teeth by means of radial stainless steel fingers securely welded to the end plates. The core is securely clamped by a large number of outer core studs located outside the active region of the core and extending over its full length. In addition to these outer studs, insulated studs are also provided throughout length of the core to provide additional clamping pressure. Combination of disc springs have been provided at the top & bottom ends of the inner and outer core studs to avoid loosening of core during service. Ventilation ducts are provided at intervals along the stator core, being formed by means of non-magnetic steel spacing bars securely welded to adjacent steel stampings. Jacking screws are provided at the outer edge of end plates to enable adjustment of pressure on the teeth.


3.3 WINDINGS The stator winding is of two-layer bar type lap winding.

All the bars are formed, insulated and tested before being placed in the slots. Each bar consists of a number of individual copper strands of rectangular section to minimize eddy current losses.

Each strand is insulated with polyesterimide varnished glass braiding. Each bar has “Roebel” type transposition in slot portion to reduce losses due to circulating currents. The bars are insulated along the slot portion by adequate number of half lap layers of epoxy resin mica paper tape. After this, the straight part of bars are pressed and consolidated in a heated press. This ensures complete elimination of voids and a high factor of safety against breakdown. The anti-corona shield consists of a butt layer of conducting polyester fleece tape. The end portions of the bars have flexible insulation consisting of polyester film and glass backed mica flake tape, reinforced at intervals with layers of varnish treated terylene tape and with glass tape for protection and finish. The overhang of the winding is held rigidly by non-metallic support rings carried on brackets attached to the stator core end plates.The bars are held firmly in place in open type slots by wedges. The joints between bars are made by brazing and are insulated.


Packing blocks are taped between adjacent bars to give added rigidity to the end windings as a precaution against the forces of short circuit. All connections between bars and terminals are securely clamped. Both ends of each phase winding are brought out to suit the terminals near the top of stator frame.

3.4 ANTI-CONDENSATION HEATERS

Low temperature heaters to prevent condensation on the windings during periods of shutdown are mounted inside generator barrel. The heaters are of box type consisting of a coiled resistance wire embedded in an electrically insulating and heat-conducting compound and protected by a metal sheath. They are connected together to form a balanced 3-phase load.

3.5 TEMPERATURE DETECTORS

Duplex type Pt-100, Resistance temperature detectors are built into the generator stator. The leads from the detectors are terminated on a metal clad terminal box located in a convenient accessible position and finally terminated on marshalling box located outside on generator barrel.


4. ROTOR 4.1 TOP SHAFT, THRUST BLOCK & BOTTOM SHAFT

The shafts and thrust block are forged individually from high quality carbon manganese steel. The shafts have been accurately machined and have a hole bored right through their centers for inspection purposes. The top shaft and thrust block are assembled on a taper matching surface under hydraulic pressure. A separate highly polished runner disc acting as rotating bearing surface is bolted with thrust block, properly insulated, to prevent flow of shaft current through thrust pads.

A solid coupling flange is provided at the lower end of the top shaft for connecting to the top of the spider. Solid coupling flanges are provided at both ends of the bottom shaft, upper flange being connected to the bottom side of the spider & the lower flange being connected to the turbine shaft.

4.2 SPIDER AND ROTOR RIM

The rotor spider will be of fabricated structure and will be bolted to the top & bottom shaft. The spider will have heavy steel bars welded on to the outer periphery to support the rotor rim. The rotor rim assembled around rotor spider will be built-up from thin sheet laminations each covering two poles pitches and successive layers of laminations overlapped to give adequate strength to the rim.


The laminations will be clamped by means of fine clearance studs. "T" shaped slots in the outer periphery of the rim will receive similar shaped projections on the poles.

4.3 POLESThe pole piece consists of steel stampings clamped between heavy steel end plates by a number of steel bolts. Both stampings and end plates have T-shaped projections which engage with corresponding slots in the rotor disc referred above. Preformed field coils are mounted on the pole piece insulated with epoxy glass fabric sheets conforming to class ‘F’ insulation system. Each pole is firmly secured by means of pairs of taper keys. The pole face profile is circular, the radius of the profile being chosen to ensure a suitable grading of the air gap.

4.4 DAMPER WINDING The rotor is equipped with a damping winding. The damping bars are of circular section copper embedded in semi-closed slots in the pole faces. The ends of the bars are short circuited together by copper stampings. The damper winding is not interconnected between poles.

4.5 FIELD WINDINGThe field coils are square ended being fabricated from straight length of copper strips, dovetailed and brazed at the ends.

At intervals down each coil the copper is increased in width to give fins for cooling purposes. The insulation between turns consists of epoxy treated Nomex paper and the insulation to ground is of epoxy glass laminates conforming to the class ‘F’ insulation system. The completed field coil with insulation is consolidated in a hydraulic press at high temperature to produce a monolithically bonded insulated coil. All connections between adjacent field coils and also between the field coils and slip rings are firmly secured to the rotor.

4.6 COLLECTOR & BRUSHGEAR

The generator collector assembly is of mild steel assembled on the tubular shaft, which in turn is assembled on generator top shaft. The leads from the collector to the generator field run through the special holes in the top shaft and are joined at suitable points to facilitate dismantling of the rotor. The brush gear for the collector is mounted on insulated studs inside the brush gear casing. The brush gear is in an easily accessible position above the generator.



5. BEARINGS

5.1 THRUST BEARING

The thrust bearing is of a spring-supported type in which the stationary part consists of white metalled segmental pads supported on a mattress of helical springs. The rotating bearing surface is machined accurately perpendicular to the axis of the shaft. The bearing surface is polished to fine surface finish. The thrust pads are of stress relieved mild steel and are faced with a high quality white metal. Each pad rests on a number of springs, which are pre-compressed by a permanently locked centre screw and finished to a standard overall length. The springs are assembled on a heavy fabricated spring plate. which is an integral part of the top bearing housing. The thrust pads are prevented from moving circumferentially by pad stops secured to the spring plate. Radial movement is prevented by dog clamps which would also prevent the pads from rising with the thrust block during the rotor jacking. The thrust bearing pads are completely immersed in an oil bath and oil is cooled by Plug-in oil coolers.

5.2 GUIDE BEARING

The top and bottom guide bearings are of the pivoted pad type consisting of a row of white metalled pads arranged in a support ring.


Top guide bearing is located above the thrust bearing, on a journal surface of thrust collar shrink fitted on to the top shaft. Bottom guide bearing is located on a journal surface of the integrally forged collar of the bottom shaft. A pivot bar is bolted to the back of each guide-bearing pad, to enable the pad to rock slightly to take up a suitable position and facilitate formation of the oil film, when running. The clearance between individual pads and the journal is set by adjusting the shims between the back of the pad and the pivot bar. This robust construction would enable the guide bearings to support the lateral forces without damage. The air space above oil surface is vented to an oil tundish and oil vapour seal is fitted to prevent the escape of oil vapour into the generator air circuit.

5.3 ACCESS TO THRUST BEARING

The thrust bearing housing is designed to give the best possible accessibility to the thrust bearing. The thrust pads and thrust face can be inspected through oil cooler openings after the housing has been drained off, oil coolers removed as required and the rotor jacked up. Special tackle is provided to enable the pads to be withdrawn through the openings, if necessary. It is thus possible to inspect the pads and spring units without dismantling the whole of the bearing or removing the generator rotor.


6. BOTTOM BRACKET

The bottom bracket is of a fabricated steel structure and consists of the bearing housing supported integral arms. It supports the stationary parts of bottom guide bearing. An oil vapour seal and the brake/jack units are also mounted on the bottom bracket. The bracket is designed to support the complete rotor weight. The bracket arms rest on sole plates and shims are provided for leveling purpose. Jacking screws are provided for use in leveling and centering the whole bracket. Sheet steel covers bolted to the underside of the bracket seal the machine enclosure from the turbine pit. The bracket is designed such that it can be lifted out through the stator bore.

7. TOP BRACKET

The brackets are of fabricated steel structure. The top bracket supports the stationary parts of thrust bearing, top guide bearing, flooring etc. It consists of the bearing housing, supported on split arms mounted on top of the stator frame. Jacking screws are provided for adjusting the bracket during leveling & centering. The bracket is designed to support the complete thrust-bearing load of the machine, including hydraulic thrust. Thrust bearing housing provides a cover to the thrust bearing & supports the vapour seal, brush gear casing & top dome.


8. OIL VAPOUR SEAL

Oil Vapour Seals are provided for both the bearing housings to prevent escape of oil vapour to the generator air circuit and its deposition on rotor and stator. It consists of air and oil vapour chambers. The inner diameter is serrated to act as labyrinth to minimize vapour leakage. Pressurized air from the generator air circuit is fed to the air chamber via a control valve. The air pressure being higher than oil vapour pressure of the housing, the valve is to be controlled to strike a balance, such that the oil vapour can escape to an oil tundish through the piping provided for the purpose. For satisfactory performance of the vapour seal, it is essential to maintain the diametrical shaft clearance as 0.25 to 0.4 mm. Also, the air control valve shall be just sufficiently set such that air pressure pushes the oil vapour to the tundish provided for it.

9. BRAKES & JACKS

The generator brakes consist of a number of “Asbestos free brake liners, which operate against a polished circular steel brake track to the underside of the rotor spider hub. Each brake shoe is mounted on a vertical piston moving in a small cylinder. To apply/release the brakes, air would be forced into the brake cylinders in appropriate direction from the station compressed air supply. The brake cylinders are mounted on the bottom bracket.


The brakes also provide a convenient means for lifting the rotor for maintenance purposes. For this duty a high-pressure oil pump (power pack) is provided. The brakes and oil pipes are made suitable for the high pressure needed for lifting the rotor. Pipe connections and valves for the attachment of the pump are also provided. In order that the rotor might be held in the raised position for an extended period of time, a loose locking sleeve with a helical surface is provided around each cylinder with a corresponding fixed sleeve attached to the jack piston. The arrangement of piping is such that, after the jacking system has been in use, air under pressure can be applied to the system to clear the pipes of oil.

10. TOOTHED WHEEL FOR SPEED SIGNALGENERATOR The toothed wheel is mounted on tubular shaft for speed

sensing. The magnetic pick-ups are mounted opposite to toothed wheel to detect the speed of the machine and convert into electrical signals. The output of magnetic pick-ups is a square wave and is fed to the “frequency to voltage converter” of governor regulating circuit as well as comparator card to energies various speed relays.

11.VENTILATION The generator has a closed circuit system of ventilation. Air cooler units are assembled to the outer periphery of the stator frame and the cooled air is discharged into the


annular space surrounding the stator, inside the generator barrel. Half of this air then returns to the fan below the rotor and the remainder of the cooled air returns to the fan above the rotor. The air is circulated through the closed system by the combined action of the rotor poles and the fans. The water supply to each of the air coolers is controlled by separate valves so that any one of the coolers can be isolated if necessary. The inlet and outlet connections are made to bus pipes running round the machine.

12.BEARING PROTECTION

Bearing protection brushes are fitted on the generator above and below the rotor. The top bearing system is insulated from the housing. The top guide bearing support ring is shorted to the shaft with a bearing protection brush. This arrangement blocks the low frequency harmonic currents in the D.C. output from the excitation equipment, while high frequency harmonic currents flow through the bearing protection brush thus bypassing the thrust & guide bearing pads and thereby avoiding damages to the bearing pads. the pads is immersed in oil for lubrication. The guide brg. clearance is adjusted by adjusting the shims between guide pads and the pivot bar. The upward & downward movement of pad segment is restricted by keep plates at top and bottom. In circumferential direction, a limited freedom is allowed to the guide pad segments by the guide pad stop, to facilitate its rocking.

machenical disign of hydrogen

Documents

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thrust bearing coolers