Introduction to the Generator II

The main features of the generator are a rotor surrounded by a two part stator, rotor current collecting rings, known as slip rings, and an assembly of bushings forming the output voltage terminals.

Six high voltage bushings are attached, through terminal plates, to a terminal box located on the underside of the generator collector end. These bushings provide the means for the generator to be electrically connected to the external buses. Each high voltage bushing is internally cooled by the circulation of hydrogen gas.

End shields at either end of the generator stator are of gas tight construction and support the generator bearings and shaft seals which prevent hydrogen gas leaking from the generator.

The entire assembly is gas tight to allow hydrogen gas to be forced through the internal parts for cooling. The stator windings are also cooled by demineralised water. 

The generator is of the conventional form for large power stations, with an inner rotating two-pole field and outer fixed conductors. The rotating field is D.C. excited from a static thyristor excitation system.

Excitation current is supplied to the field windings of the rotor from the collector rings. Two semi-circular conductors run through the drilled-out centre of the rotor forging to connect the collector rings with the field windings.

There are four hydrogen coolers, one vertically mounted at each corner of the generator stator.

Resistance Temperature Detectors (RTD’s), thermocouples and stator liquid drains are positioned within the generator assembly for monitoring generator operation.

Current transformers encircle the six individual high voltage bushings to measure the amount of current supplied by the generator and provide protection for the generator. The output of each current transformer is monitored in the Unit Control Room (UCR). 

Rotor

The rotor is machined from a single forging of steel alloy. The shaft includes a coupling for connecting to the turbine shaft and also a turbine end fan ring. Slots for the rotor conductors are machined in the forging surface.

Axial flow fans are bolted to the fan bosses. The fans furnish the force necessary to move the hydrogen gas for internal cooling of the generator. At the generator collector end of the rotor are the brush assemblies and collector rings, which convey the excitation current to the rotor winding via an internal connector installed in the rotor shaft centre.

Copper conductors, carrying the excitation current, run in slots in the rotor body. The conductors are in the form of bars. Each bar has a series of holes in it to permit the circulation of the cooling hydrogen. The hydrogen is force circulated by means of the fans mounted on the rotor shaft. There are several hydrogen gas flow passages throughout the length of the rotor, providing multiple cooling paths, thus maintaining the field conductors at a near-constant temperature.

The rotor winding temperature is indirectly monitored by continual measurement of the winding resistance, which varies in direct proportion to the temperature. The electrical pick-up for the monitoring system is made across the collector rings and the information is transmitted to the UCR.

The rotor, weighing 53.5 tonne, is balanced to ensure uniform rotation. Warning of excess rotor vibration during generator operation is provided by a detector positioned at each end of the rotor. Vibration sensing devices are inserted through the stator outer end shields. An electrical output proportional to the rotor vibration is produced by the detectors and transmitted to the UCR. 

Collector Rings and Brushes

Field excitation current is provided by the output of the thyristor rectifiers, and passed to the generator rotor via brushes, collector rings and internal conductors. The collector rings are mounted on the generator rotor shaft at the opposite end to the turbine coupling, outward of the stator outer end shield. The rotor is supported on the outer side of the collector ring by an auxiliary bearing which is enclosed within a housing having its own air ventilation system and is forced air cooled by a fan mounted on the shaft.

Excitation current is passed from the collector rings to the field winding by insulated semi-circular copper bars, located in the drilled-out centre of the rotor shaft. After passing through the rotor centre, the copper bars are connected to internal winding terminals.

Positive and negative collector rings are shrunk onto the insulated collector drum. Cooling air is passed through holes through the collector rings by means of a fan mounted on the rotor shaft between the collector rings. The collector rings are spirally grooved to promote self-cleaning and long life.

Altogether, fifty brushes are used, housed in ten magazines, radially disposed in a 240° arc about each collector ring. The brushes and spring assemblies maintain a constant pressure on the brushes so that the only adjustment necessary is to compensate for brush wear. To remove a brush and spring assembly from one of the magazines, the insulated handle must be turned and the magazine removed from the housing. This can be done when the generator is on load.

Cooling air is forced into the collector assembly housing from air filters directly below the assembly. The air from the duct cools the main generator collector rings and brushes. It is drawn up through the duct by the low pressure in the collector housing created by the fan.

Resistance Temperature Detectors (RDT’s) are used for temperature detection in the collector assembly. 

Stator

The stator assembly consists of two basic parts, an outer frame and an inner core. The stator is mounted on its foundations by means of leg plates welded to the outer frame. These rest on sole plates grouted to the foundations. Between the legs and sole plates are liners and stator centering shims. The leg plates move along the sole plates as the stator expands and contracts during operation. The generator collector end of the stator is positively located by keys between the leg and sole plates on both sides of the stator, while bolts clamp the leg and sole plates together to form a fixed point, thus ensuring that expansion is always towards the turbine. The remaining leg plates are retained by bolts tightened just sufficiently so that they do not restrain stator movement.

The gas-tight outer frame is constructed of welded steel plate and is made into a hydrogen tight vessel capable of withstanding an explosive (internal) pressure of 690kPa. Partitioning walls and ventilation tubes are welded internally to increase structural rigidity and to form ventilation passages for the hydrogen gas cooling.

Hydrogen gas cooler boxes are welded at each corner, and also a terminal box is welded under the generator at the collector end for the high voltage bushing assemblies. The leg plates to be installed on the foundation are adjacent to the four hydrogen cooler boxes. The outer frame partitioning walls which form the high and low pressure gas passages are sealed by neoprene seals

Liquid leakage drains are fitted along the bottom of the outer frame, and together with associated liquid leakage detectors, provide information regarding cooling or lubrication fluid leakages within the generator casing assembly. 

The stator coil and stator winding assembly are contained in the inner cage to form the core part of the generator. The inner cage is a four part bolted cage structure which is designed to tighten the stator core after lamination. The stator core is constructed from segmented laminations punched from silicon steel. Each lamination is manufactured by punching out coil slots and core assembly slots, and is coated with resin to prevent the circulation of eddy currents between individual laminations.

The prepared laminations are assembled and secured by dovetail shaped wedges on key bars welded to the inner cage assembly. The laminated core is compressed at both ends by bolted stator flanges, to form a strong cylindrical structure.

The stator core is flexibly mounted inside the frame in order to prevent vibrations induced in the core from being transmitted to the frame. These vibrations may be generated in the following manner. The rotor winding creates a magnetic field having two poles and, as the rotor turns, each of the two poles exerts a pull on the stator. Any point on the stator will be pulled towards the rotor twice during each revolution of the rotor. Consequently, a vibration will be set-up at twice the frequency of the current induced in the stator winding.

Flexible mounting supports are used between the stator core and the frame, as dampers, to reduce the transmission of this double frequency vibration to the frame of the stator. This arrangement ensures a very low level of vibration.

The stator core is made of high-quality steel laminations containing slots for the conductor bars. The conductor bars are hollow, to allow them to be cooled internally by the stator cooling water. The stator winding is a double-layer bar assembly, comprising hollow conductors. The assembled bars are fitted into the laminations of slots of the stator core, joined at the core ends to form the winding, and then formed into phase belts by means of connecting rings.

Each phase consists of two groups of coils, and each group comprises a pole. The stator bars are then impregnated with an epoxy resin. After the stator bars are completed, they are secured into the laminated slots of the stator core by means of dovetail shaped wedges.

The individual hollow conductors are internally cooled by the circulation of demineralised water. At each end of the stator bars, the individual conductors are connected together by a manifold. All conductors are brazed into the manifolds, which have one tube connection to carry the combined water flow of all conductors. Separate brazed connections complete the electrical circuit to the associated bars.

The cooling water enters and leaves the conductor manifolds at the generator from the inlet (upper bar) and outlet (lower bar) conductor manifolds, which are attached to inlet and outlet headers. At the generator collector end, the cooling water flows from the upper bar to the lower bar through a U-shaped tube. Flanged joints provide the interface between the generator and the external cooling water system. Resistance temperature detectors, RTD’s, are embedded in the stator windings for measuring temperatures at the hottest points during normal generator operation. Stator water outlet temperature is normally 80°C.

The stator frame is divided into four quadrants by means of longitudinal plates, forming high pressure and low pressure chambers. Two of these chambers are cold gas supply manifolds, running along the top and bottom of the frame. The other two are hot gas or return manifolds, running along opposite sides of the frame. The outer partitioning wall and arched plates which form the high and low pressure gas passages are sealed at the inner cage by means of neoprene seals.

Connected to one of the high pressure chambers are ventilation tubes, through which cooled hydrogen gas, compressed by the axial flow fans installed on both ends of the rotor shaft inside the generator casing, is blown in. The stator ventilation ducts in the core are partitioned in accordance with these divided chambers to ensure effective stator ventilation.

Hydrogen gas which enters the high pressure chambers passes through the radial ducts from the inner side of the core and the surface of the rotor, through the rotor slot from the inlet region to the outlet region of the rotor slot, and then through the radial core ducts in the low pressure chambers.

High temperature hydrogen gas in the low pressure chambers is fed through ventilation holes in the partition walls into the gas coolers at the four corners of the stator frame to be cooled again before it enters the suction side of the fans. The radial passage of hydrogen through the partitioned core ducts assures uniform cooling of the core and the winding, reduces the thermal stress in various parts of the generator and prevents local heating. 

High Voltage Bushings

The stator winding is formed into phase belts by means of connecting rings which are connected to high voltage bushings at three live and three neutral points, giving a total of six bushings. The bushings of each phase are supported by a terminal plate bolted onto the generator terminal box.

A liquid leakage drain is fitted in the upper part of the terminal box and also in each of the three terminal plates. These drains are connected by means of associated pipework to the generator stator fluid leakage detector to provide information regarding leakage of cooling or lubricating fluid in the generator.

Each bushing assembly comprises a one piece porcelain insulator, a built in hollow conductor and terminal clamps at each end of the bushing. Current transformers encircle the body of each bushing to monitor individual phase currents.

The bushings are cooled by the internal circulation of hydrogen gas. Ventilation tubes are inserted into the hollow conductors. Cool gas from the high pressure gas chamber within the terminal box is forced down through the ventilation tubes, up through the space between the tubes and the conductors, and through connecting pipes to a low pressure area within the stator body.

The high-voltage bushings are mounted as a group on the outer frame and are connected to the phase-isolated buses that run from the generator to the transformer yard outside the station.

Current Transformers (CT’s) are mounted on each high-voltage bushing assembly. These monitor the individual phase currents of the generator and form part of the generator protection system. A total of twenty-three CT’s are provided, four for each bushing with the exception of the centre bushing (white phase) at the live side bushing, which has only three CT’s.