Turbo In Brief Document (2)

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Bharat Heavy Electrical Limited (BHEL) is the largest Engineering and manufacturing Enterprise of it kind in the Public Sector in India. It ranks among the top twelve organizations in the world, engaged in the manufacture of power generation equipment. BHEL, Hyderabad manufactures turbo generators of ratings upto 200MW for industrial applications and for power generation. These turbo generators are supplied together with the turbines and matching excitation systems, and are used mostly in paper, sugar, cement, petrochemical, fertilizers, rayon industries etc, and thermal power stations. TURBO GENERATORS The general components of a turbo generator are # Stator - Stator Frame - Stator Core - Stator Windings - End Covers # -

Rotor Rotor Shaft Rotor Windings Rotor Retaining Rings

# Bearings # Cooling System The following auxiliaries are required for operation: # Oil Supply system # Excitation System Quality Assurance checks being followed right from procurement stage to dispatch stage are enumerated in the following flow chart STATOR 1.1 STATOR FRAME

The stator frame is of welded steel single piece construction. It supports the laminated core and winding. It has radial and axial ribs having adequate strength and rigidity to minimize core vibrations and suitably designed to ensure efficient cooling. Guide bars are welded or bolted inside the stator frame over which the core is assembled. Footings are provided to support the stator foundation. 1.2 STATOR CORE The stator core is made of silicon steel with high permeability and low hysteresis and eddy current Losses. The sheets are suspended in the stator frame from insulated guide bars. Stator laminations are coated with synthetic varnish; are stacked and held between sturdy steel clamping plates with non-magnetic pressing fingers which are fastened or welded to the stator frame. In order to minimize eddy current losses of rotating magnetic flux which interact with the core is built of thin laminations. Each lamination layer is made of individual segments. The segments are punched in one operation from electrical sheet steel lamination having a high silicon content and are carefully deburred. The stator laminations are assembled as separate cage core without the stator frame. The segments are staggered from layer to layer so that a core of high mechanical strength and uniform permeability to magnetic flux is obtained. On the outer circumference the segments are stacked on insulated rectangular bars which hold them in position. To obtain optimum compression and eliminate looseness during operation the laminations are hydraulically compressed and heated during the stacking procedure. To remove the heat, spaced segments are placed at intervals along the core length which divide the core into sections to provide wide radial passages for cooling air to flow. 1.3 STATOR WINDING CONSTRUCTION The stator windings consist of two layers made of individual bars. To minimize losses, bars are composed of separately insulated strands which are transposed by 360 degrees. To minimize stray losses in end windings, strands of top and bottom bars are separately brazed and insulated from each other. Each bar consists of a large number of separately insulated strands to reduce the skin effect losses. In straight slot portion, the strands are transposed by 360 degrees. The transposition provides for mutual neutralization of voltages induced in the individual strands due to slot cross field and ensures that no or small circulating currents exist in the bar interior. The current flowing through the bar is thus distributed uniformly over the entire cross section of a bar so that the current dependent losses will be reduced. INSULATION OF BARS High voltage insulation is provided with thermosetting system. A voltage insulation obtained by vaccum press impregnation is particularly void free with excellent electrical, mechanical and thermal properties. To prevent corona discharge between the insulation and slot wall, a final layer of conductive tape is applied to the surface of all bars within the slot range. All bars are

conditionally provided with an end corona protection to control the electric filed at the transition from slot to the end winding portion and to prevent the formation of creepage sparks. (A) VACCUM PRESS IMPREGNATED MICALASTIC HIGH VOLTAGE INSULATION The high voltage insulation is provided according to the proven resin poor mica base of thermosetting epoxy system. Several half overlapped continuous layer of resin poor mica type are applied over the bars. The number of layers or thickness of insulation depends on machine voltage. The bars are inserted into the slots with very small lateral clearance and wedged with packers. To prevent moment of end windings in circumferential direction, spacer blocks are arranged between the bars and firmly with treated glass tapes. To minimize the effect of radial forces, winding holders and insulated rings are used to support the overhang. The stator is impregnated in a tank under vaccum and pressure with low viscosity epoxy resin that penetrates the winding thoroughly. After impregnation, the stator is cured at at appropriate temperature in an oven. The high voltage insulation thus obtained is characterized by its excellent electrical, mechanical and thermal properties. Its moisture absorption is extremely low and it is oil resistant. The behavior of the insulation is far superior to any other conventional mica tape insulation system. (B) CORONA PROTECTION To prevent a potential difference and possible corona discharges between the insulation and slot wall, the slot sections of bars are provided with an outer corona protection. This protection consists of polyester fleece tape impregnated in epoxy resin with carbon and graphite as filters. At the transition from slot to the end winding portion of stator bars a semiconductive tape made of polyester fleece is impregnated with silicon carbide as filler is applied for a specific length. This ensures uniform control of the electric field and prevents the formation of corona discharge during operation and performance of HV tests. 1.4 END COVERS The end covers are made of fabricated steel or aluminum alloy castings. They are employed with guide vanes on inner side for ensuring uniform distribution of cooling air or gas. ROTOR Solid rotors are manufactured from forged alloy steel with suitable alloying elements to achieve very high mechanical and superior magnetic properties. Rectangular or trapezoidal rotors slots are accurately machined to close tolerances on slot milling machine. For indirectly cooled generator rotors, ventilation slots are machined in the teeth.

For directly cooled rotors, Sub slots are provided for cooling Generators rotors of 1500 RPM are of round laminated construction. Punched and varnished laminations of high tensile steel are mounted over machined shaft are firmly clamped by end clamping plates. 2.1 ROTOR SHAFT Rotor shaft is a single piece solid forming manufactured form a vaccum casting. It is forged from a vaccum cast steel ignot. Slots for insertion or the field winding are milled into rotor body. The longitudinal slots are disturbed over the circumference such that two solid poles are obtained. To ensure that only a high quality product is obtained, strength tests, material analysis and ultrasonic tests are performed during the manufacture of rotor. The high mechanical stresses resulting from the centrifugal forces and short circuit torque call for a high specified mechanical and magnetic properties as well as homogeneous forging. After completion, the rotor is balanced in various planes at different speeds and then subjected the rotor is balanced in various planes at different speeds and then subjected to an over speed test at 120% of the rated speed for two minutes. The rotor consists of electrically active portion and two shaft ends. Approximately 60% of rotor body circumference have longitudinal slots which hold the field winding. Slot pitch is selected so that the two solid poles are displaced by 180 degrees. The rotor wedges act as damper winding within the range of winding slots. The rotor teeth at the ends of rotor body are provided with axial and radial holes enabling the cooling air to be discharged into the air gap after intensive cooling of end windings 2.2 ROTOR WINDINGS The windings consist of several coils inserted into the slots and series connected such that two coil groups form one pole. Each coil consists of several series connected turns, each of which consists of two half turns connected by brazing in the end section. The rotor bearing is made of silver bearing copper ensuring an increased thermal stability. The individual turns of coils are insulated against each other by interlayer insulation. L-shaped strips of laminated epoxy glass fiber fabric with nomex filter are used for slot insulation. The slot wedges are made o high electrical conductivity material and thus act as damper windings. At their ends the slot wedges are short circuited through the rotor body. CONSTRUCTION The field winding consists of several series connected coils inserted into the longitudinal slots of rotor body. The coils are wound so that two poles are obtained . The solid conductors have a rectangular cross section and are provided with axial slots for radial discharge or cooling air. All conductors have identical copper and cooling duct cross section. The individual bars are bent to obtain half turns. After insertion into one slot constitute one coil. The individual coils of rotor are connected in a way that one north and one south pole is obtained.

CONDUCTOR MATERIAL The conductors are made of copper with a silver content of approximately 0.1%. As compared to electrolytic copper, silver alloyed copper features high strength properties at high temperatures so that coil deformations due to thermal stresses are eliminated. INSULATION The insulation between the individual turns is made of layer of glass fiber laminate. The coils are insulated from the rotor body with L- shaped strips of glass fiber laminate with nomex interlines. To obtain the required leakage paths between the coil and rotor body thick top strips of glass fiber laminate are inserted below top wedges. The top strips are provided with axial slots of the same cross section and spacing as used on the rotor winding. ROTOR SLOT WEDGES To protect the winding against the effects of centrifugal forces, the winding is secured in the slots with wedges. The slot wedges are made of copper alloy featuring high strength and good electrical conductivity. They are also used as damper winding bars. The slot wedges extend beyond the shrink seats of retaining rings. The wedge and retaining rings act on the damper winding in the event of abnormal operations. The rings act as a short circuit rings in the damper windings. END WINDING BRACING The spaces between the individual coils in the end winding are filled with insulated members that prevent coil movement. Two insulation plates held by HGLhigh glass laminate plates separate the different cooling zones the overhangs on either sides. 2.3 ROTOR RETAINING RINGS The centrifugal forces of the rotor end winding are contained by single piece rotor retaining rings. Retaining rings are made of non-magnetic high strength steel in order to reduce stray losses. Each retaining ring with its shrink fitted. Insert ring is shrunk on the rotor in an overhang position. The retaining ring is secured in the axial position by snap rings. The rotor retaining rings withstand the centrifugal forces due to end windings. One end of each ring is shrunk fitted on the rotor body while the other end overhangs the end windings without contact on the rotor shaft. This ensures an unobstructed shaft deflection at end winding. The shrunk on hub on the end of the retaining ring serves to reinforce the retaining ring and secures the end winding in the axial direction at the same time. A snap ring is provided against axial displacement of retaining ring. The shrunk seat of currents. To reduce the stray losses and have high strength, The rings are made of non magnetic, cold worked materials.

2.4 ROTOR FANS The cooling air in generator is circulated by two axial flow fans located on the rotor shaft one at each end. To augment the cooling of the rotor winding, the pressure established by the fan works in conjunction with the air expelled from the discharge parts along the rotor. The blades of the fan have threaded roots for being screwed into the rotor shaft. The blades are drop forged from aluminium alloy. Threaded root fastenings permit angle to be changed. Each blade is secured at its root with a threaded pin. BEARINGS The turbo generators are provided with pressure lubricated self - aligning elliptical type bearings to ensure higher mechanical stability and reduced vibration in operation. The bearings are provided with suitable temperature element devices to monitor bearing metal temperature in operation. The temperature of each bearing is monitored with two RTDs (Resistance Thermo Detectors) embedded in the lower bearing sleeve such that the measuring point is located directly below the babbit. These RTDs are monitored a temperature scanner in the control panel and annunciated if the temperature exceeds the prescribed limits. All bearings have provisions for fitting vibration pickups to monitor shaft vibrations. To prevent damage to the journals due to shaft currents, bearings and oil piping on either side of the non-drive end bearings are insulated from the foundation frame. For facilitating and monitoring the healthiness of bearing insulation, split insulation is provided. VENTILATION AND COOLING Turbo generators are designed with the following ventilation systems: Closed circuit air cooling with water or air coolers mounted in the pit. Closed circuit hydrogen cooling with water or hydrogen coolers mounted axially on the stator frame. The fan design usually consists of two axial fans on either made of cast aluminum with integral fan blades or forged and machined aluminum with integral fan blades or forged and machined aluminum alloy blades screwed to the rotor. In case of 1500 RPM generators, fabricated radial fans are provided. EXCITER The exciter is brushless mainly consisting of: -

Rectifier wheels Three phase main exciter Three phase pilot exciter Metering and supervisory equipment

The brushless exciter is an AC exciter with rotating armature and stationary field. The armature is connected to rotating rectifier bridges for rectifying AC voltage induced in the armature to DC voltage.

The pilot exciter is a PMG(Permanent Magnet Generator). The PMG is also an AC machine with stationary armature and rotating field ( the permanent magnets).When the generator rotates at the rated speed, the PMG generates 220v at 150 Hz to provides power supply to automatic voltage regulator. A common shaft carries the rectifier wheels the rotor of the main exciter and the permanent magnet rotor of the pilot exciter. The shaft is rigidly coupled to the generator rotor and exciter rotors are then supported on three bearings.