Introduction to the Generator II

1. Generator Construction

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.

2. Cooling and the Seal Oil System

The generator is internally cooled by gaseous hydrogen and demineralised stator water, separate heat exchangers being provided for the hydrogen and stator water systems. In order to maintain the hydrogen pressure within the generator casing shaft seals are made gas-tight by pumping oil through them. This arrangement is known as the seal oil system.

Hydrogen gas is maintained within the generator casing at a pressure of 310 kPa and is circulated within the generator by fans mounted on each end of the generator shaft. The hydrogen circulates primarily through the rotor windings, and also around the rotor, stator, casing and rotor-stator gas gap, and through the four internal gas-water heat exchangers, located at each of the four corners of the generator outer frame.

The temperature of the hydrogen gas is maintained constant within the generator by automatically regulating the flow of cooling water through the heat exchangers. Cooling water is supplied from the Unit Cooling Water System and circulates through the hydrogen coolers via inlet and outlet couplings at the base of the coolers.

The hydrogen coolers comprise a bank of water cooled tubes, vertically installed between two water boxes by means of a tube plate. The complete assembly is housed in a rectangular shell. A liquid leakage drain is fitted at the base of each cooler and connected, by means of suitable pipework, to the generator stator fluid leakage detector so as to provide information regarding leakage of cooling water within the system.

The two basic types of temperature detecting devices used within the generator are resistance temperature detectors and thermocouples. Resistance temperature detectors are installed adjacent to the hydrogen gas coolers for detecting the temperature of the circulating hydrogen gas within the generator. Compensating leads bring the output of the detecting devices through conduits and hydrogen gas tight glands to an external terminal board for connection to recorders located in the UCR.

Twelve resistance temperature detectors (RTD’s) are installed within the stator windings between selected conductor bar assemblies of each phase, for detecting the temperature in areas of maximum temperature rise during normal generator operation.

Ten RTD elements are installed adjacent to the hydrogen gas inlet and outlet passages of the hydrogen gas coolers for monitoring the temperature of the circulating cooling medium within the generator assembly.

A thermocouple is installed in the cooling water outlet of each stator bar for monitoring the temperature of the outlet stator cooling water passing from each manifold.

A thermocouple is installed within each end of the rotor bearing close to the metal lining of the bearings to facilitate accurate monitoring of the temperature at the bearing surfaces. The thermocouple leads are brought out through the generator outer end shields and are connected to terminal boards positioned at the ends of the stator outer frame.

The supply of hydrogen to the generator is maintained by a separate control system, which includes pressure regulators and gas controllers. The system includes a means of supplying carbon dioxide to the generator casing in order to purge it during hydrogen filling and emptying operations.

Due to the combustible nature of a hydrogen-air mixture within the range of 4 to 74% hydrogen, precautions are taken to prevent such a mixture occurring. High purity hydrogen does not support combustion and, as long as the purity is above 95%, there is no danger of explosion.

A stator gas analyser is connected to the generator to measure the purity of the gas, which is normally held between 97 and 99%. A drop in purity below 95% causes an “H₂ PURITY LOW” alarm to be initiated in the UCR and a signal to be sent to the data logger.

The hydrogen gas is dried by a drying unit mounted outside the generator, the drying agent being activated alumina, which must be periodically reactivated. 

Seal Oil System

The seal oil system for maintaining the generator rotor shaft gas-tight is generally located on the ground floor below the generator. The system is normally automatically controlled but can be manually.

Owing to the importance of the seal oil system and the risk of explosion associated with its failure, comprehensive back-up facilities are provided in the even of failure due to a loss of AC power. If a failure occurs, an alarm will be raised in the UCR. The oil used by the seal oil system is taken from the turbine oil system.

To retain the hydrogen gas within the generator, radial type seal assemblies which utilise an oil film sealing technique are mounted on the end shields at each end of the stator. The seal assemblies are installed inboard of the rotor bearings.

The seal assemblies form an annular space around the sealing land faces on the generator rotor, and oil under pressure is admitted to the annular space to prevent the escape of hydrogen or the ingress of air.

The seal assemblies comprise two adjacent seal rings. One seal ring faces inside the stator to the hydrogen gas and the other outwards to the air. The two seal rings are located in a seal casing and are separated axially but held together circumferentially by springs which hold them against the casing sides, assisted by seal oil pressure.

Seal oil pressure is regulated to 50kPa above the hydrogen gas pressure within the generator. In each seal assembly, oil passes through the clearance between the two seal rings, and also between the seal rings and the rotor shaft. The oil discharges partly inside to the hydrogen gas, and partly outwards to the air side, finally returning to the main oil tank via the seal oil system. 

Stator Cooling

As opposed to the rotor conductors, which are cooled by hydrogen gas, the stator conductors are cooled by a separate stator cooling water system. The cooling water is contained in a storage tank and pumped through the hollow conductors of the stator. It enters and leaves the conductors near the turbine end and passes from the upper to the lower part of the stator at the generator collector end.

The temperature of the cooled water entering the stator is maintained at 46°C, cooled by one of two external water-water heat exchangers. Only one heat exchanger is in service at any one time.

There are two stator cooling water pumps. The pumps are normally operated automatically but can be controlled manually from the UCR or the stator water control cubicle, located on the GWOC cubicle. A drop of 69kPa in the stator water pressure causes the stand-by pump to start.

The conductivity of the stator cooling water is maintained by a resin type de-ioniser. A rise in the electrolytic conductivity of the demineralised water used in the stator cooling system will cause an alarm to be raised in the UCR

3. Excitation System

The purpose of the excitation system is to provide direct current to the rotating field winding of the generator in order to produce a magnetic field. The system is designed to allow accurate and sensitive control of the voltage supplied to the field winding, thus controlling the output voltage of the generator as well as the power factor and reactive demand of the system. 

The excitation system is a static excitation system, in which thyristor rectifiers are provided with AC power from the excitation transformers connected to the main phase isolated bus (PIB). The thyristor gate pulse signals are controlled by the pulse generator unit in combination with the automatic voltage regulator (AVR), and are varied in proportion to the load on the generator. The excitation control system consists of two AC-AVR’s for automatic operation and two DC-AVR’s for manual operation. 

Changeover between automatic and manual control can be made by operating the changeover switch on the excitation control cubicle (ECC), located in an air conditioned room on the turbine mezzanine floor. The changeover switch is marked MAN, ADJ and AUTO. The ADJ position of the switch allows the PT and CT circuits for both AC AVR channels to be closed before effecting changeover to automatic control. 

Selection of AVR-1 or AVR-2 can be made by operating the pushbuttons on the Unit Control Console. These pushbuttons are marked AVR-1 and AVR-2, and near them are pushbuttons for AVR-1 LOCK and AVR-2 LOCK, and RAISE/LOWER pushbuttons for varying the generator voltage in either mode of control. 

Excitation Transformers

Three single phase excitation transformers are located on the ground floor. Each single phase transformer is made up of a 1500kVA, 20/3kV:900V excitation transformer and a 20/3kV:100V auxiliary transformer. The neutral bushings on the high voltage side of these transformers are connected together by Busbars in a star configuration. 

Two low voltage bushings are located on each transformer, one for the line side and the other for the neutral side. The Delta connection of the secondary windings for the excitation transformer is made in the transformer itself, while the Delta connection for the auxiliary transformer is made in a junction box mounted on the fireproof wall of the excitation transformer.

Each single phase transformer is a self-cooled type with a radiator, the oil level and oil temperature of which can be monitored. Each transformer is fitted with a pressure relief vent which is designed to operate in the event of a transformer internal fault.

The main rectifiers are thyristor diodes combined with a forced-air cooling system and are housed in the excitation power rectifier cubicle, EPR. The 24 thyristor rectifying elements are connected in three-phase full-wave configuration and are housed in trays in the cubicle. There are sufficient Thyristors to allow the generator to operate continuously at rated output with up to 20% of the diodes out of operation. Failure of any thyristor will initiate an alarm in the UCR.

An initial excitation system is provided in order to supply direct current to the generator field during start-up. It consists of an initial excitation transformer and rectifier stack, housed in the initial excitation transformer rectifier cubicle. The excitation power rectifier cubicle and the initial excitation transformer cubicle are located on the turbine mezzanine floor below the generator.

4. Neutral Earthing of the Generator

The method of earthing the neutral end of each of the generator windings allows earth faults to be detected and the generator tripped. The neutral end of each winding is taken out separately, through a current transformer, to the high-voltage neutral bushings. The three neutral bushings are connected in parallel externally and taken through the primary of the neutral earthing transformer to earth. The secondary of the transformer is connected across the neutral earthing resistor.

Under normal conditions no current flows to ground. However, if a fault such as a ground short, occurs in one of the stator windings of the generator, there will be an imbalance between the three phases causing a large loop current to pass through the primary of the neutral earthing transformer.

This will result in a voltage appearing across the neutral earthing resistor. The stator earth fault relay, wired in parallel with the neutral earthing resistor, will detect this voltage and operate the generator protection Class 2 trip circuits.

5. Generator Capability

The rating of 375MW is applied to the generator at a power factor of 0.85, lagging. At lower power factors the 375MW rating does not apply. The limiting factor is then the maximum current rating of the generator, which is fixed irrespective of the power factor. The current corresponding to a load of 375MW is 12737A. 

Motoring

The generator will act as a synchronous motor if it is supplied with current from the system. Such operation is not harmful to the generator, but it could seriously damage the turbine if the steam flow were cut off.

Instrumentation is provided to monitor the direction of power flow in the generator, so that when taking a turbo-generator off load, the generator 220kV circuit breaker is manually tripped when the low-reading watt meter indicates zero load or a motoring condition.

6. Cubicles

The following cubicles contain equipment used in the operation of the generator:

This cubicle houses the gauges, switches, meters, filters and valving used during generator operation and cooling. 

Generator Potential Transformer Cubicle

The generator potential transformer cubicle houses two sets of 3 single phase transformers which are connected together in star and delta configurations and also incorporate high voltage fuses.

Three of the transformers, PT-1, are used to step down the generator output voltage from 20000V to 110V, as well as providing power for the AVR-1 excitation control system, meters in the UCR, protection relays in the URR and synchronising instruments.

The remaining three transformers, PT-2, step down the generator output voltage from 20000V to 110V, which is then applied to AVR-2 excitation control system and also to the residual voltage protection relay. 

Neutral Earthing Transformer Cubicle

The neutral earthing transformer cubicle houses a single phase earthing transformer and a neutral earthing resistor. The generator windings are connected together to form a star-point. The star-point is connected in series with the primary of the neutral earthing transformer, the secondary of which is loaded by a high capacity resistance bank.

As long as the generator three-phase load is balanced, the neutral current to earth is zero. If the generator three-phase load becomes unbalanced, due to a fault in one phase, a current will flow in the primary of the transformer, inducing a voltage in the secondary winding.

These voltages will cause the turbine master trip solenoids to energise the turbine trips, and hence a “TURBINE TRIP” alarm will be initiated in the UCR. The impedance of the transformer prevents a large neutral current from flowing, thus avoiding damage to the generator windings. 

Excitation Control Cubicle

Contains equipment used to monitor and control the excitation current to the generator rotor field winding. 

Excitation Power Rectifier Cubicle

The Excitation Power rectifier cubicle has three sections, each section containing trays with four Thyristors. The AC output from the excitation transformers is rectified and applied to the generator rotor. 

Field Circuit Breaker Cubicle

The field circuit breaker cubicle contains the field circuit breaker, two shunts, one connected to the rotor temperature transmitter and the other to the millivolt-to-milliampere converter, and a discharge resistor that dissipates the energy in the rotor winding when the field circuit breaker opens. 

Initial Excitation Transformer Rectifier Cubicle

The initial excitation transformer cubicle contains a magnetic contactor, a 90kVA self air cooled transformer, which steps down the AC voltage from 415V to 70V, and rectifiers.

7. Panels

The following panels are used in the operation of the generator:

Seal Oil Panel

The seal oil panel is mounted on the seal oil unit and contains gauges and switches used in the operation of the generator seal oil system. 

Pressure Gauge Panel

The pressure gauge panel is located on the stator cooling water unit and contains a temperature switch, gauges and converters. The temperature switch is used for the turbine trip circuits.

Hydrogen Gas Dryer Cabinet Front Panel

The hydrogen gas dryer control cabinet contains a power on/off switch and lamps to indicate when the dryer heater power is On or OFF and when the thermostatically controlled heater is ON or OFF. The heater is used for reactivating the activated alumina used as the hydrogen gas drying agent.

8. Liquid Leakage Detection

Drains are installed at various points within the generator assembly where cooling or lubricating fluids are likely to accumulate should leakage occur. These are the stator outer frame, the four hydrogen gas coolers, the high voltage bushing terminal box and the three associated terminal plates.

Pipework connects the drains to a liquid leakage detector that facilitates an analysis of any liquid leakage found. In the event of a fluid leak, alarms will be initiated in the UCR.

9. Generator Cooling Systems

A generator operates with a high magnetic field and a high current density producing unwanted heat, due to copper losses in the conductors (both rotor and stator) and due to iron losses in the iron core. This heat must be removed or the insulation will be damaged and failure of the generator will occur. As the size of the machines increases so too does the amount of heat generated and requiring to be dissipated.

Industry standards define the maximum allowable temperatures for stator and rotor windings, and these standards tend to limit the size of the generator, dependent on their means of cooling.

The first generators to go into commercial service were simple air-cooled machines. As the capacity of the machines increased, however, it became necessary to produce generating sets with a high power output and a relatively compact construction, manufacturers have been forced to devise more effective cooling systems.

There are now several methods of generator cooling employed in the industry. These include:

Note:  Between 50 and 160MVA the choice of cooling methods varies between manufacturers above a nominal 160MVA hydrogen cooling is the most prominent method used.

Most modern high-output generators have a combination of cooling systems with the rotor being cooled by hydrogen, which is circulated through the generator frame by fans integral with the generator rotor, and the stator being cooled by a flow of high quality demineralised cooling water, which is circulated by an external pumping system.

Some generators, even as large as 500MW capacity, however, still utilise air cooling.

9.1. Totally Enclosed Air Cooling

A large number of modern, small and medium capacity alternators employ the totally enclosed air cooling method.

Air cooling has the advantage of minimal capital cost due to the absence of a need for special shaft sealing mechanisms and the constant make-up demands of a hydrogen cooling system.

The internal cooling air circuit is sealed to ensure that airborne dust can be excluded from the cooling system and therefore does not deposit on the windings or in the ducts. If dust is allowed to accumulate on the windings the heat transfer process is hindered and the dust layer can absorb moisture and/or oil leading to failure of the insulation.

A totally enclosed system also ensures that moisture contained in normal ambient air is not allowed to enter the alternator provided that the alternator is initially charged with air of a low dewpoint.

The alternator cooling circuit consists of the following components:

The alternator frame not only acts as the support for the stator assembly but also acts to reduce noise transmission and to provide a passage or passages through which the cooling air circulates. Baffles and partition walls direct the cooling air through a path designed to ensure optimum heat transfer is maintained within the alternator. Typical single and double pass cooling circuits are shown in Figure 1. Shaft mounted fans within the alternator frame are used to ensure that a positive cooling air flow is maintained through the cooling circuit while ever the alternator is in service.

Figure 1:  Axial Fan Mounted on a Generator Rotor Shaft 

In some cases counterweighted dampers are used within the cooling circuit to allow the fan to run up to speed under a no load or minimum load condition. As the fan reaches normal operating speed the discharge pressure of the fan forces the damper closed and the entire cooling circuit is then connected to the fan discharge.

Radial ventilation ducts form part of the stator core and allow the cooling medium to pass through the stator from the air gap to the cooling passages within the outer alternator frame.

Axial ventilation ducts are formed at the bottom of the rotor slots below the windings and radial ventilation holes are formed through the winding bundles to allow a passage for the cooling medium throughout the rotor. Figure 2 shows the flow path through the rotor while Figure 6 shows the general pattern of flow throughout the alternator.

Figure 2:  Section through Rotor Showing the Axial and Radial Ventilation Passages.

Heat exchangers are required to remove the heat from the internal cooling circuit and dissipate it to atmosphere through an external cooling medium. This may be through an air to air or an air to water heat exchanger.

Figure 3: End View of Alternator with Air/Air Heat Exchanger.

As the output of the generator increases the choice of water/air heat exchangers becomes more prominent.

The heat exchangers are located within the alternator frame either side or bottom mounted. The air circuit may be directed through a single pass of the heat exchanger or through multiple passes to maximise heat transfer throughout the alternator.

Figure 4:  Showing Finned Tube Water Cooled Heat Exchanger. Note the Drain and Vent Lines Provided on the Inlet and Outlet Manifolds.

The cooling water is normally filtered water supplied from the turbine auxiliary cooling water system, but may be supplied by an independent cooling water system.

The generator air cooler inlet and outlet water-boxes are fitted with vents and drains to allow any entrained air to be vented from the system and to allow the coolers to be drained for maintenance (See flanged connections in Figure 4).

Liquid detectors are provided in the lower points of the frame to monitor any ingress of water into the generator due to tube leakage from the coolers.

The cooling water inlet and outlet temperature and the air temperature within the frame are normally monitored with an alarm being initiated if high temperatures are detected.

Figure 5:  End View of Alternator Enclosure Showing the Bolted Access Doors to the Water-Cooled Heat Exchangers Mounted Within.

Figure 6:  Typical Single and Air Cooling Circuits Showing Passage of Air through the Rotor and Stator Elements.

9.2. Totally Enclosed Hydrogen Cooling

Hydrogen gas has a higher thermal conductivity and heat transfer coefficient than air but a much lower density, (7.00:1 1.35:1 and 0.07:1 against air in each case) therefore it acts as a much better heat transfer medium than air when applied to cooling the internal components of the generator.

In addition, pressurised hydrogen is a better electrical insulator than air, and hydrogen is not an oxidising agent.

Hydrogen is readily obtained in unlimited quantities, is inert, non-explosive and will not support combustion when mixed with air in concentrations less than 4% and greater than 70%.

Increases in generator output of 20% were gained by the replacement of air with hydrogen at a pressure slightly higher than atmospheric pressure 3 kPa (to prevent air ingress into the frame). As the density of the hydrogen was increased through increasing the pressure of the gas, further gains in heat transfer and related generator output were made. Typical hydrogen pressure in modern day generators vary from 200 to 450 kPa. 

Hydrogen Gas Safety

In considering hydrogen as a cooling medium there was a concern for hydrogen fires, a concern, which air and helium did not share.

It was considered, however, that as long as adequate safety precautions were maintained, hydrogen could be used effectively in an enclosed cooling system.

Hydrogen Gas forms an explosive mixture with air in concentrations between 5% and 70% Hydrogen.

The intensity of the explosion caused by ignited hydrogen/air mixture varies according to the percentage of hydrogen in the air/hydrogen mixture having zero values at 0% and 70% Hydrogen.

The actual pressure of the gas mixture also affects the intensity of an explosion should one occur. 

To ensure that a condition does not occur that will create a possible explosion and to reduce the intensity of the explosion, should one occur, the following precautions must be carried out:

Components of the Hydrogen Gas System 

The components of the Hydrogen Gas System include the following:

Hydrogen Gas Supply 

A makeup supply of hydrogen gas must be available to the hydrogen cooling system to allow the pressure and purity of the hydrogen gas to be maintained while ever the generator is in service and/or filled with hydrogen.

Makeup supply is normally taken from storage banks of compressed hydrogen bottles connected to supply manifolds.

The storage bottles themselves may be replenished by an external supplier or refilled from hydrogen generating plant located on site.

The supply pressure from the hydrogen bottles to the alternator frame may be automatically regulated by a manually set pressure regulating valve or, alternatively, by a reducing valve provided in parallel with the pressure regulating valve.

The pressure of the storage bank and the pressure downstream of the pressure reducing station are normally monitored.

Hydrogen Gas Dryer

The dewpoint of hydrogen within a generator is normally maintained in the order of –25° C in order to prevent internal moisture generated corrosion and failure. This would normally necessitate the inclusion of a hydrogen dryer as part of the hydrogen system.

A number of types of dryer are available in the industry, heatless, regenerative dryers, desiccant dryers, and refrigerant dryers. Any one of these could find an application as a hydrogen dryer, however, because water freezes and can no longer be removed from the system at temperatures below 0° C, refrigerant fryers are not normally used.

Within the generator frame, the hydrogen circulating fans create areas of low and high pressure at their suction and discharge respectively.

By locating the hydrogen dryer supply and return pipework between the high and low pressure sections of the generator frame a gas flow is induced through the dryer whenever the alternator shaft is rotating at synchronous speed and the shaft mounted circulating fans are in operation.

Unless a dedicated circulating fan is incorporated into the dryer circuit, drying cannot take place with the generator out of service. 

Hydrogen Gas Pressure and Purity Monitoring

The cooling quality of hydrogen gas is proportional to its density and therefore the pressure it is under within the generator frame.

Reduction of pressure results in a reduction in cooling capacity. The pressure of the gas is therefore constantly monitored to ensure that optimum gas density is maintained within the frame.

Maintaining hydrogen gas purity is of paramount importance due to its loss of performance as a cooling medium and its likelihood of falling into an explosive range if the loss of purity is caused by its mixture with oxygen.

The purity of the hydrogen gas within the generator frame is normally monitored by passing a sample of the gas through an analyser.

As carbon dioxide is used as an interfacing agent when gassing up and degassing the generator, it is most common for the analyser to have several functions and to be able to monitor hydrogen purity and the concentrations of hydrogen and carbon dioxide in air.

The suction side of the analyser can therefore be connected to either of two suction lines, one taken from the upper portion of the generator and used for hydrogen purity analysis during hydrogen filling ,the other, taken from the bottom section of the generator and used for analysis of the gas purity while CO₂ is being used as an interfacing medium between hydrogen and air.

During normal operation the lower detection point should be used as lower purity hydrogen will tend to be more dense than high purity hydrogen and a reduction in purity should be evident first in the lower portion of the frame.

Although different limits may be placed on purity at different power station sites, hydrogen purity should be maintained above 93%.

During normal operation of the generator the small amount of seal oil migrating into the generator frame may carry some entrained air and moisture with it. This air and moisture will be released from the seal oil as it migrates to the defoam tank and will eventually cause a deterioration in hydrogen purity. In order to maintain hydrogen purity it may be necessary to bleed off a portion of the gas while making up the gas volume with pure hydrogen from the cylinder bank.

In some installations a constant, regulated bleed is maintained through bleed lines directed to atmosphere through a flow regulating valve, an oil mist separator and a flowmeter.

Figure 7:  Hydrogen Bleed Station Showing Regulating Valves, Flowmeters and Oil Mist Separators

Carbon Dioxide Supply

Carbon Dioxide is used as an inert interfacing agent when filling the generator frame with hydrogen or air.

The Carbon Dioxide Supply usually consists of banks of storage cylinders attached to a supply manifold. The storage cylinders may be replenished by an external supplier or by a bulk carbon dioxide facility within the station site.

It should be noted that the gas discharge pipework temperature can be below freezing point and contact between skin and the pipework may result in the skin freezing to the pipework causing frostbite injury to the affected area. Appropriate safety precautions should be taken during operation of this equipment.

During purging the introduction of carbon dioxide may be allowed to cease once the carbon dioxide reaches a purity <75%. The detection of carbon dioxide purity must be taken from the upper sampling point in the generator frame. 

Hydrogen Gas Cooling Circuit

The hydrogen cooling circuit includes paths through the stator and rotor windings (see previous diagram of air cooled circulation paths) where heat is gained and a path through the hydrogen coolers where heat is dissipated. To ensure a positive flow through the circuit two fan impellers are mounted on the generator rotor shaft and each draws a portion of the hydrogen from the cooler and passes it through the generator windings and back to the windings. This circuit creates a high and low pressure area within the generator frame while ever the machine is operating at rated speed.

The hydrogen dryer takes advantage of the two different pressure zones to establish a hydrogen flow through the drying chamber.

Banks of hydrogen coolers are normally provided at the side or top of the generator frame. The cooling medium is normally high quality demineralised water from a closed cooling water system.

Contamination of the Generator frame can occur due to a failure of the hydrogen coolers, causing a water leak into the frame, or failure of the hydrogen shaft seal allowing oil ingress into the frame. In either case leakage must be detected. Detection points are normally tapped into the generator frame at each end of the generator adjacent to the seals. These detectors should be regularly checked and the quantity and type of liquid found in them should be logged.

System Monitoring and Alarms

The following parameters associated with the hydrogen gas system would normally be monitored:

9.3. Liquid Cooling

As the output rating of generators increases so the use of water cooling for heat removal in the stator becomes more effective. Stator Water Cooling is additional to the cooling still maintained by hydrogen circulation around the stator and through radial cooling passages within the stator.

In a water-cooled stator, demineralised water is circulated through dedicated pathways built into the stator laminations. The stator cooling circuit passes through the entire length of the stator and is connected to the stator water supply and return manifolds through insulated hoses (PTFE or other suitable plastic).

The stator water system usually consists of the following components:

Stator Water Pumps and Circuit Components

The stator water cooling system is critical for maintaining the generator within the limits of design operating temperatures and would normally demand redundancy in its pumps.

The stator water pumps are usually ac motor driven centrifugal pumps and are provided to circulate the stator water through the closed stator water system at a head pressure slightly lower than that of the hydrogen pressure contained within the generator frame.

This has the effect of allowing gas to leak into the stator water system rather than permitting stator water to migrate into the generator frame.

Small and insidious amounts of hydrogen gas entering the cooling water circuit may be detected by the incorporation of float chambers above the generator inlet and outlet manifolds and at the pump suction. Float operated alarm contacts are made when a set volume of gas displaces the water in the chamber. The frequency of alarms and the amount of gas vented from the system to clear the alarm aids in determining the size of the leak.

Filters are provided to remove any entrained particulate from the system.

Heat from the Stator Water System is dissipated to atmosphere through a heat exchanger. Usually the heat exchangers use lower quality cooling water as the cooling medium. The supply pressure of the auxiliary cooling water is lower than that of the stator water system to reduce the likelihood of stator water contamination if a leak in the heat exchanger occurs. 

Stator Water Flow and Differential Pressure Measurement

In order to monitor the stator water system’s performance it is common to include of flow measuring station in the supply line to the stator. If stator water flow falls momentarily an alarm is initiated and the standby pump is called into service, if the flow fails an alarm and trip is initiated.

A differential pressure monitoring station, located between the generator stator water inlet and outlet manifolds, serves to detect excessive leakage within the generator frame and initiate a trip of the generator. 

Stator Water Conductivity Monitoring

The quality of the demineralised water must be high to prevent current flow through the system. Metal pickup from the stator pipework can result in an increase in conductivity with time. The conductivity of the stator water is normally monitored and the provision of a side-stream, mixed bed, polishing column allows any dissolved solids to be removed and the conductivity to be maintained within design limits. 

Alternatives to Stator Water Cooling

Some modern generators are oil cooled. The stator winding, the core, busbars, terminals, and structural members of the stator are cooled with a fire-resistant dielectric liquid. The cooling medium, special oils, can be mixed and a fire-resistant quality can be applied to the mixture. The rotor winding has direct cooling with distillate.

The fire-resistant dielectric liquid and distillate circulation is provided by pumps, on a closed-loop system, with the cooling liquids being cooled in cooling liquid/water heat exchangers.

Small generators (70MW) have been developed (1970-2000) using high and low temperature superconductors in the stator and rotor windings. These winding have employed liquid and gaseous helium, liquid nitrogen and liquid argon as the cooling medium.

Figure 8:  Stator Water System

10. Seal Oil System

The function of the seal oil system is to provide a regulated supply of oil to the Hydrogen Shaft Seals in order to prevent the hydrogen held within the alternator frame from passing through the same opening in the alternator frame through which the main alternator rotor shaft passes.

In carrying out its task the seal oil provides:

The seal oil system should be placed in service before hydrogen is admitted to the alternator frame and must be retained in service until the frame is degassed to atmospheric conditions. The seal oil system should also be placed in service before the turbine/generator is rotated on barring gear.

10.1. Operation of the System

Filtered and cooled seal oil, regulated to a pressure slightly in excess of the gas pressure within the generator frame, is supplied by any of three supply pumps (2 main and 1 dc emergency) to the hydrogen shaft seal on the exciter and turbine ends of the generator.

The seal oil acts hydraulically to force the sealing surface against the shaft surface while at the same time lubricating the sealing surfaces and dissipating any heat produced between the mating surfaces.

The majority of oil return flow from the seal migrates to the shaft bearing side of the seal and mixes with the lubricating oil in the bearing oil return line.

A small amount of oil will also migrate to the generator side of the seal against the hydrogen pressure. This oil will return to the seal oil system through the Hydrogen Side Drain Regulator. 

The bearing housings associated with each seal are maintained under a negative pressure by a vapour extraction fan and exhaust line, which also incorporates a path for high volume leakage of hydrogen to atmosphere in the event of seal failure.

10.2. Components of the System

The Seal Oil System is typically comprised of the following components:

Shaft Sealing Mechanism

The shaft sealing mechanism is provided for the purpose of preventing hydrogen gas leakage from the generator along the rotor shaft.

Several types of propriety type sealing mechanism are available including:

Oil, supplied at a pressure greater than the pressure of the gas within the generator frame, forces the sealing surfaces together while at the same time providing lubricant to the seal faces.

In the case of the sealing ring a single oil supply provides both the hydraulic pressure to force the seal against the shaft journal and the coolant/lubricant flow to the seal face.

In the case of the collar type seal, two seal oil flows, through separate pressure regulating stations, provide the hydraulic annular pressure to force the ring against the collar (annulus oil) and the coolant/lubricant flow to the seal face (face seal oil).

Figure 9:    Radial Oil Film Type Seal

Radial Type Seal

A radial oil film type seal (see Figure 9) consists of a seal housing containing a pair of segmented bronze alloy or babbitted steel rings. The segments are positioned against the side walls of the seal casing and are held concentric with the shaft by hydraulic pressure from the seal oil.

The seal rings have a bore diameter of only a few thousandths of an inch greater than the shaft journal and are free to float radially but are prevented from rotating with the shaft by stops in the casing.  

Seal Oil, at a pressure of approximately 0.35 bar greater than hydrogen pressure is supplied to the seal casing forcing the seal against the shaft journal face. The oil then passes through a space between the sealing rings to the seal face; from here it flows axially along the shaft in both directions. It is the thin film of oil that actually provides the seal between the hydrogen and atmosphere.

The pressure across the seal face is not uniform, the alternator side of the seal is at frame pressure (200 – 400 kPa), while the bearing side of the seal is under a slightly negative pressure. It therefore follows that the flow path of the majority of the seal oil will be toward the low pressure side, typical flows are in the order of 80 l/min to the bearing side and only 5 l/min to the alternator side.

Due to the fine clearances between the seal and the shaft journal, failure to provide seal oil flow to the seals while the shaft is rotating will result in overheating, binding and seizure of the seals. A flow of seal oil is therefore required to lubricate and cool the seal faces whenever the shaft is rotating, even when the frame is de-pressurised to air at ambient pressure.

To prevent seal oil, which has migrated to the hydrogen side of the seal, from entering the alternator frame, the alternator shaft is also fitted with oil deflectors and labyrinth type oil seals. The hydrogen side seal oil drain is located before the shaft oil seals. 

Axial Collar Type Seal

In seal designs employing a seal ring and shaft collar two oil supplies are provided; annulus oil and face seal oil.

Annulus oil is directed into an annulus in the seal ring and acts as a hydraulic force to drive the seal against the shaft collar. It is important that the annulus oil is supplied first when setting up the seal oil system in order to ensure that the sealing faces mate around the entire seal circumference. If the ring is slightly skewed excessive oil flows will be seen at the seal face. Although the annulus oil is predominantly a hydraulic medium a small flow is allowed to be bled from the annulus to prevent overheating of the oil at the seal.

As the function of the annulus oil is to maintain a hydraulic force on the seal its pressure is regulated to maintain a set positive differential between itself and the hydrogen pressure within the alternator frame (Annulus Oil pressure > Hydrogen pressure). Annulus oil pressure will therefore vary considerably during periods when the gas pressure is allowed to increase or decrease.

The Face Seal Oil is used as a lubricating and heat dissipation medium. The pressure of the face seal oil is constant.  

Seal Oil Pumps

Due to the need to supply a constant seal oil supply while ever the generator frame is filled with hydrogen, redundant seal oil pumps are required. Two 100% duty seal oil pumps are therefore normally provided, (Duty (A) and Standby (B). These pumps may be ac motor driven or a combination of ac motor and turbine shaft driven.

The pumps take their suction from a convenient location in the lubricating oil system.

Each pump is provided with suction and discharge isolating valves and a non-return valve in the pump discharge.

One Emergency dc motor driven Seal Oil Pump is normally provided to ensure seal oil supply is maintained on loss of all ac supply.

The Standby Seal Oil Pump will automatically start on loss of the Main Seal Oil Pump.

The Emergency Seal Oil Pump will start on loss of both the and Standby Seal Oil Pumps and on Seal Oil Differential Pressure falling to a low limit (typically in the order of 30 kPa)

In some cases, should all pumps fail while the lubricating oil system is still in service, low pressure backup oil supply from the Turbine Bearing Oil System may be used to maintain supply to the seals.

In such a situation, however, the hydrogen gas pressure should be reduced to an appropriate value to ensure sufficient differential pressure exists between the seal oil and hydrogen.

 Figure 10:  Main Seal Oil Pump Showing Differential Pressure Regulator, Relief Valve and Associated Pipework

Differential Pressure Regulators

In order to prevent the hydrogen from escaping through the seal the pressure of the seal oil must be higher than that of the hydrogen within the alternator frame.

Seal oil differential pressure regulators are used to control the pressure of the seal oil being supplied to the seals. If the pressure is too high then excessive amounts of oil may pass into the alternator side of the seal; if the pressure is too low then it may not be sufficient to prevent an outflow of hydrogen past the seal.

In the case of collar type seal separate regulators are provided for the anulus and face seal oil supplies.

The Differential Pressure Regulator works on the principle of a diaphragm or metal bellows being subjected to the generator gas pressure on the upper side and the seal oil pressure to the seals on the lower side. The bellows or diaphragm is connected to the regulating valve spindle. An adjustable spring gives the required amount of bias to establish the required differential pressure (in the order of 30 to 50 kPa).

Should the gas pressure in the generator frame increase at any time the pressure on the top side of the bellows will increase, driving the spindle down slightly and reducing the amount of seal oil being returned to the pump suction header. This will cause a corresponding increase in the seal oil supply pressure.

Under normal operating conditions, once the pressure regulator has been adjusted to the required difference between hydrogen and seal oil pressures it will maintain a constant pressure differential through the complete range of hydrogen pressures.

Figure 11:  Side View of Seal Oil Pumps Showing Differential Pressure Regulators, Relief Valves Suction and Discharge Lines

Seal Oil Cooler

Prior to reaching the shaft seal the seal oil passes through a heat exchanger to dissipate heat from the system and to maintain a set oil supply temperature (changes in oil temperature and viscosity will effect the performance of the seal).

Coolers may be shell and tube type with the seal oil flowing through the shell and the auxiliary cooling water flowing through the tubes or they may be plate or air cooled fin type.

Typical outlet temperature from the cooler ranges between 38 and 43º C.  

Seal Oil Filter

Filters are provided to remove any entrained particulate from the system.

Duplex filters that provide for in service change-over from one basket to the other without loss of seal oil flow are the most common type of filter in use. A changeover handle allows in service operation of either strainer basket. Vent and drain valves are provided for venting and draining of the filter prior to removal and cleaning of the strainer basket.

A differential pressure switch is provided to initiate an alarm on a high filter DP (in the order of 50 kPa). Response to high filter DP and filter cleaning should receive a high priority.

Figure 12:Seal Oil Cooler showing Oil Inlet, Outlet and Bypass Valves and Cooling Water Outlet Flow and Temperature Indication.

Seal Oil Return Lines

The oil returning from the seals takes two independent paths, the majority returns through the bearing side drain while a minor amount returns from the hydrogen or generator frame side of the seal.

Oil returning from the bearing housing side of the seal passes to a loop seal oil tank before returning to the seal oil pump suction side (or main oil tank). A vapour extraction fan is connected to the loop seal to maintain the bearing housing under a negative pressure and to remove any oil vapour or hydrogen present within the bearing housing.

 The oil returning from the alternator side of the seal may have hydrogen gas entrained within it, therefore this oil is passed first to a defoam tank or hydrogen side drain regulator before it is allowed to mix with the oil returning from the bearing side of the seal.

The function of the defoam tank is to allow the small quantity of oil returning from the frame to settle for a period of time during which the hydrogen gas is detrained from the oil.

The flow of oil from the defoam tank is regulated to maintain a constant level of oil within the tank. This is done to form a seal between the alternator frame and the seal oil pump suction pipework. Should the seal within this tank be lost the frame can connected to atmosphere through the loop seal oil tank and associated vapour extraction fan resulting in a major loss of hydrogen to atmosphere.

A float valve regulates the oil level to the centreline of the gauge glass fitted to the tank. Excess oil is released to the Seal Oil Pump suction header.

Level switches initiate an alarm if the oil level rises or falls to a point outside the limits set either side of the gauge glass mid point.

Figure 13:  Hydrogen Side Drain Regulator showing Level Indication and Manual Regulating Valve. 

When the gas pressure Hydrogen, CO₂ or Air) in the alternator is reduced the differential pressure across the seal face is reduced and therefore the flow of seal oil to the hydrogen side of the seal may increase. In such cases the level of the hydrogen side drain regulator vessel should be carefully monitored and if necessary, the drain may be manually operated to prevent overflow from the seal drain into the generator frame. 

Seal Oil Drain Level Switches 

The Hydrogen Side Drain Lines from both the exciter and turbine end of the generator are normally fitted with high level switches to provide an alarm in the event of the oil level within the drain lines rising to the alarm setting.

Excessive oil level could result in oil ingress into the generator frame and fouling of the windings. High priority should be given to this alarm especially in conjunction with a high level alarm from the drain regulator.

Figure 14:  Seal Oil Drain Level Switch

Liquid Detectors

Should the shaft seal face be damaged, allowing excessive flow of oil to the hydrogen side of the seal a condition could exist in which oil overflows from the internal drainage system to the alternator frame.

Liquid detectors with alarm contacts are fitted to the lower sections of the alternator frame to initiate an alarm should a quantity of oil or water be detected.  

Seal Oil Vapour Extraction Fan

In order to remove any hydrogen gas released from the seal oil and to ensure oil does not migrate along the shaft out of the bearing housings, the two alternator shaft bearing housings and the bearing side seal oil return line are placed under a negative pressure from a Vapour Extraction Fan.

The fan suction line may be provided with a valve to allow the line vacuum pressure to be regulated to a set pressure.

The discharge of the fan is normally taken to atmosphere at a for safety.

A parallel circuit incorporating a non-return valve is also taken to atmosphere. Normally the check valve is kept closed by the positive discharge pressure of the fan and the associated negative suction pressure on the opposing side. In the event of seal failure or loss of the fan the non-return valve will open to allow hydrogen gas to vent to atmosphere.

Figure 15:  Seal Oil Vapour Extraction Fan and Associated Pressure Gauge and Check Valve

Seal Oil System Monitoring and Control

To assist with local inspection and monitoring of the system, local indication is normally provided at a monitoring station adjacent to the pumps and cooler.

This station typically includes the following:

Figure 16: Typical Seal Oil Local Monitoring Station

 Figure 17:   Seal Oil Local Control Panel

Seal Oil Local Control Panel

Local control panels typically provide control stations for the following:

The Local Seal Oil Control Panel is normally provided with an alarm fascia to enunciate the following alarms:

  Figure 18: Typical Seal Oil System