The two types of generator maintenance are on-load and off-load maintenance. The on-load maintenance consists mainly of monitoring activities. This type of maintenance is very important because it detects the conditions that could lead to a catastrophic failure.
ON-LOAD MAINTENANCE AND MONITORING
This type of maintenance varies with the design of the machine and the instrumentation installed in it. The following paragraphs describe the standard maintenance and monitoring activities as well as the recent developments in monitoring techniques.
Stator
Temperature Measurement. Thermocouples are installed in the stator to monitor the temperature of the stator bar insulation, core teeth, core end plates, stator water connections, and hydrogen gas. Eighty thermocouples are used for a typical 660-MW generator. The readings of these thermocouples are displayed in the control room. Analysis of the information obtained from them will help to identify developing faults and deficiencies in the cooling circuit and the stator.
End Winding Vibration. A vibrations transducer module is installed in the end windings. It measures the vibration in all directions. It identifies deterioration in the structural integrity of the end winding and stresses in the conductor coil that approach the initiation of fatigue cracking.
Stator Water Flow. The flow rate of the stator water is monitored by sight-glass indicators having a flap in the flow. The angle of the flap provides a rough indication of the flow rate. Regular monitoring of the flow rate will give an indication of a change in the flow rate that requires investigation during an outage.
Hydrogen Dew Point. Hydrogen dew point is checked regularly to ensure that moisture condensation will not occur at the cooler parts inside the generator. An increase in the hydrogen dew point could have any of the following consequences:
● Moisture in oil
● Leaks from the hydrogen heat exchanger
● Leaks from the stator coolant system
● Deterioration in the performance of the hydrogen driers
Hydrogen Entrainment into Stator Water. A small amount of hydrogen enters the water circuit normally due to the pressure differential between the hydrogen and the water. Alarms indicate excessive ingress of hydrogen into the water. The volume of hydrogen detrained from the stator water should be monitored for early detection of a developing leak.
Hydrogen Leakage. Hydrogen leakage checks should be initiated over the whole generator when there is evidence of high hydrogen makeup. Nonflammable gas detectors are used to detect the presence of hydrogen. The exact location of the leak is then identified by using an aerosol foam or a soap solution. The search for hydrogen should include the slip ring cooling circuits and the lubricating oil.
Core Monitor. The core monitor is an online gas sampling device used to detect the decomposition products of overheated insulation. The products are normally released due to a developing fault. The monitor alarms at a specified level of decomposition products in the gas. A gas sample also can be collected in the monitor for future analysis of entrained decomposition products.
Radio-Frequency Monitor. Corona is discharged due to deterioration in the condition of the insulation. This discharge can be detected in the radio-frequency currents in the neutral conductors. Analyzing equipment is used to monitor the level of corona discharge.
Rotor
Vibration Monitoring. The vibration of the turbine-generator rotor should be monitored during normal operation and rundown. The vibration levels are monitored in the axial, vertical, and lateral directions at each bearing. The signals are recorded and subsequently analyzed to identify the cause of the vibration. Any change in the phase or amplitude of the vibration should be investigated. The investigation may include obtaining a rundown vibration plot as soon as the unit can be shut down. This plot should be compared with the rundown vibration plot that was obtained initially to fingerprint the machine. Any anomalies between the two plots should be identified to provide early indication of the possible cause of the problem.
Rotor Ground Fault Detection. The ground fault protection system should detect a serious rotor ground fault. The rotor insulation resistance should be checked periodically, using the measuring circuit of the ground fault protection system to confirm the integrity of the insulation.
Shaft Voltage Measurement. The shaft voltage readings should be taken regularly. Any deviation from the normal readings should be investigated. Some units have a shaft voltage alarm system. It is recommended to seek a specialist’s advice on the cause of such abnormal voltage. However, consideration should be given to the following possible contributory factors:
● Changes in the condition of the bearing insulation
● Problems with the shaft ground brush
● Magnetized shaft
Flux Monitor. The search coil is a device used to identify changes in the leakage flux from the rotor due to interturn faults. It is installed on the stator in the air gap between the rotor and the stator. This device allows regular monitoring of the condition of rotor inter- turn insulation.
Shaft Ground Brush. Contamination buildup normally on the shaft ground brush renders it ineffective. It is recommended to inspect the brush frequently to maintain its cleanliness.
Bearing Pedestal Insulation. The insulation resistance of the generator outboard bearing, shaft seal ring, pipework, and exciter baseplate should be inspected regularly. This insulation can fail due to dirt accumulation, moisture ingress, or a short circuit caused by badly placed conduit. The insulation failure will allow significant currents to go through the bearings, causing damage in the bearing and journal surfaces. On-load measurement of the insulation resistance can be taken in some bearings integral with the generator end plate.
The on-load check of the pedestal bearing insulation can be done as follows:
1. Connect the rotor shaft to ground at the turbine end, using a portable copper gauze brush and lead on an insulated handle.
2. Measure the ac voltage differential between the shaft and ground (using a gauze brush) at the exciter end of the shaft.
3. Develop a short circuit between the rotor shaft and pedestal at the exciter end and measure the differential voltage between the pedestal and ground.
The two voltage readings will be similar if the insulation is in a good condition. If the insu- lation is faulty, the voltage measured in step 3 will be lower than that in step 2. Finally, measure the differential voltage between the exciter end of the shaft and the pedestal. This value should normally be less than 25 percent of that measured in step 2, if the insulation is in good condition.
Excitation System
Brushless System. The brushless excitation system requires minimal on-load maintenance. Leaks from the exciter and rectifier heat exchangers are a great hazard to the system. Thus, regular visual checks for moisture around the cooling circuit are recommended.
If the alarm indicating a diode failure in the rotating rectifier is received, the failed diode should be replaced at the earliest opportunity. This is done to prevent overloading of the remaining diodes and causing a bridge-arm failure.
Slip Ring Systems. Regular checks and maintenance are required to have a trouble-free operation of the rotor and exciter slip ring brush gear. This work is normally done on-load. It includes checks to determine
● Freedom of the brushes within the boxes
● Individual brush length
● Individual brush current
● Condition of brushes
● Evidence of sparking
● Brush vibration
● Brush spring tension
● Oil ingress from adjacent bearings
The condition of the slip ring, exciter cooling circuit including the filters should be checked.
The number of brushes that can be changed on-load within a given period should be limited. In general, when one brush reaches the minimum length, all the brushes in the box should be replaced. However, only the brushes in one box should be replaced within a 24-h period. This allows the brushes to settle and share current equally before additional changes are made.
Strict safety procedures must be followed while performing all on-load brush gear work. They include placing the unit on manual excitation, disconnection of the ground fault protection system, and adoption of ground-free work practice by using rubber mats, insulated tools, rubber gloves, etc.