OFF-LOAD MAINTENANCE
The following paragraphs describe the maintenance activities that will be done on every planned generator overhaul and as a result of a problem identified on the machine while operating.
Stator Internal Work.
The removal of hydrogen coolers or access doors on most machines will allow the inspection of the core back, end windings, and cooling connections. An effective clean-conditions system should be set up before entry into the stator to prevent debris from entering the stator and to ensure that no equipment is left in the machine when the work is completed.
The following maintenance activities should be performed when access to the generator internal parts is available:
● Inspect the end winding and check for tightness of the winding and its support structure.
● Inspect the stator bars and wedges.
● Inspect the stator core for evidence of hot spots and damage.
● Clean any contaminated surfaces with an approved solvent. The dirt should not be driven into the winding.
● Identify and remove any debris on the stator.
● Check the phase and neutral connections to bushings.
● Check the stator coolant pipework and hoses. If a leak is discovered, identify its location by one of the following two methods:
Fill and pressurize the winding with water, and inspect for leaks. There is a possibility of contaminating the insulation with water when using this method. Thus, it is not recom- mended by some manufacturers.
Inject a tracer gas into the dried winding, and locate the leak by using a gas detector.
Stator External Work
Stator Water System. A careful inspection of stator water system pipework, pumps, coolers, valves, and filters is needed before shutting down the generator to identify the areas requiring work during the outage. Following isolation of the system, the filter should be checked and the debris identified. This check will help in identifying the areas that need further investigation. For example, if gasket materials are found, the gaskets of the pipework could have deteriorated. Great care should be taken while working on the stator water system to prevent any contamination from entering it.
Following any work on the stator water system, the pipework should be flushed with the winding bypassed. This is done to prevent any debris from entering the winding.
Hydrogen Coolers. Any small water leakage inside the hydrogen coolers can prevent the system from reaching the required hydrogen dew point. If a cooler is suspected of leaking, it should be removed and pressure-tested. If a leak is detected, the unit should be repaired. A pressure-drop test should be performed following the repair.
Hydrogen Driers. The following checks should be done when there is evidence of deterioration in the performance of the hydrogen driers:
● Performance of the heater
● Operation of the valve
● Obstructions in the pipes
● Desiccant contamination
Main Connections. The condition of the main connection enclosures should be monitored to prevent the ingress of moisture, oil, or dirt and subsequent insulation damage. The insulators should be cleaned and inspected regularly for any evidence of damage. All the joints of the conductors should be inspected for any evidence of overheating. The tightness of the joint bolts should be maintained at the specified torque to ensure that the value of the joint resistance is within acceptable limits.
Rotor
The rotor of all generators should be withdrawn every 8 years for inspection and testing. The condition of end rings and winding should be carefully examined during this work. The rotor should also be withdrawn if the on- or off-load tests indicate a rotor fault.
Slip Rings and Brush Gear
The maintenance of the slip rings and brush gear includes the following activities:
● Remove the brushes from the boxes, mark their position on them, and store them. Great care should be given to prevent chipping the brushes during this work. If one or more brushes are near the minimum length, all the brushes in the brush box should be discarded.
● Disconnect and remove the brush gear enclosure.
● If the condition of the slip ring surfaces is satisfactory, wrap the slip rings with mois- ture-absorbent paper and polythene for protection.
● Remove carbon dust, oil, and other contamination from the enclosure, ventilation cir- cuit and connections, slip ring sides, and shaft. However, do not clean the slip ring surface.
● Overhaul the ventilation system.
● Reassemble the brush gear enclosure and bed in any new brushes in situ by using a strip of fine abrasive cloth. Ensure that there are no abrasive particles in the brushes or between the brush and ring. All carbon dust should have been removed.
● Check the spring tension and freedom of the brushes in the boxes.
It is recommended that the brushes remain lifted off the ring until the end of the out- age. Corrosion of the rings should also be prevented by keeping the enclosure dry and warm. The measures listed above are recommended for any prolonged outage.
Refurbishment of the slip ring surfaces is required occasionally to remove marks or uneven wear, or to recut the grooves. Grinding of the slip rings is normally done in situ by using one of the following methods:
● Machine while the shaft is on turning gear (barring). Grind the rings, using a motor- driven grindstone mounted on a sliding baseplate attached to the generator bedplate.
● Machine while the shaft is stationary. Grind the rings, using a specially designed col- lar with ring of grindstones assembled around the slip ring. A pony motor drives the collar, using a belt.
A lathe is used instead of the motor-driven grindstone for extensive in situ machining of slip rings or recutting of grooves.
Exciter and Pilot Exciter
Maintenance of this equipment includes inspection, cleaning, testing the windings, check- ing connections, and performing the maintenance described in the previous section on brush gear and slip rings.
Rectifier
The maintenance activities of rectifiers include testing of the insulation. The test voltage applied should be kept low to prevent damaging of the diodes. If it is necessary to apply a higher voltage, the diodes must be disconnected before performing the test.
Static Rectifiers. The maintenance activities of static rectifiers include cleaning and inspection of all components, diode testing, checking of fuses, insulation testing, and of checking of all connections.
Rotating Rectifiers. The maintenance activities of rotating rectifiers are similar to the ones for static rectifiers. However, particular care should be taken to the following points due to the high centrifugal forces experienced by the components of the rectifier during operation.
● A thorough inspection of the rectifier components is required to confirm their mechanical integrity. The failure of a component during operation could cause extensive damage to the machine.
● Each module should be accurately weighed following maintenance to ensure the assembly is balanced properly. The balance weights of a module should be adjusted, if required, to ensure it is within the weight tolerance (typically ±0.5 g).
Due to the complexity of some designs of rotating rectifiers, it is recommended to follow an inspection system during maintenance. It consists of an independent check follow- ing each stage of maintenance and subsequent rebuild. It is also recommended to test the performance of the measuring and alarm equipment associated with the rectifier following maintenance.
Field Switch
The switchgear maintenance should include the inspection and testing of the field switch and its operating mechanism. The field switch overcurrent trip unit contacts should be inspected for freedom of movement and operation.
Automatic Voltage Regulator
The off-load maintenance of the AVR includes the following activities:
● Cleaning of the equipment with a soft brush
● Vacuum cleaning of the equipment to remove dust
● Examining the components and conductors thoroughly for any sign of damage and overheating
● Checking and cleaning of the relays
● Checking and lubricating of the motorized potentiometers and their cam-operated switches; also checking of their traverse times
● Checking for correct operation of all tripping devices
GENERATOR INSPECTION AND MAINTENANCE 17.7
Supervisory and Protection Equipment
A schedule should be prepared to identify each supervisory and protection device. The schedule should provide a reference to the test, calibration procedures, and calibration date for each device. A check should be done to confirm that the tests and calibration of the devices were completed as required. A plan should also be made to schedule the tests and calibration of the devices in the future. This work is required to maintain the accuracy and reliability of the equipment. A check of the overall protection system should also be carried out annually.