Design Considerations for End-Winding Vibration Monitoring
For online end winding vibration monitoring, it is necessary to permanently install fiber optic accelerometers on the motor or generator stator end windings.
Typical end-winding vibration accelerometer kits include accelerometers with fiber optic cables, generator frame feed-through with hydrogen tight penetration for hydrogen‐cooled generators, and electro‐optical converters. The type of accelerometers used should be rated 100mV/g, 5‐1000Hz (±1dB), 0‐50g. This will ensure sufficient signal output sensitivity, frequency range, and amplitude range to obtain meaningful results. The following design features should be incorporated to ensure signal accuracy:
- The accelerometer design should have extremely low cross axis sensitivity meaning that the influence of the vibration perpendicular to the direction of measurement is limited.
- The accelerometer cables should not be sensitive to vibration. A cable effect test can be performed on a fiber optic cable design to ensure erroneous data is not being introduced into the signal.
- The fiber optic signal needs to be converted to a voltage in order for the data to be processed. Electronics used to perform this conversion should be located outside of the hydrogen seal to ensure longer life and easier maintenance. A test point allows access for calibration verifications to be performed.
Since the accelerometers are permanently connected to the endwindings, assurance is required that they will not in themselves pose a risk of stator endwinding failure. In addition, they must be very reliable. The following tests should be performed to ensure an extremely low risk of failure.
Partial Discharge Test
A Partial Discharge (PD) test should be performed on the accelerometers because they are installed on high voltage windings. The results of the test should show no PD activity up to 34 kV AC which would indicate that the materials of the accelerometer are not sensitive to magnetic and electrical fields.
Accelerated Life Test
Due to the permanent nature of the accelerometer installation in the endwinding area they must have a long life. An accelerated life test comprised of high temperature, thermal cycling, and vibration testing should be performed on a fiber optic accelerometer design to simulate at least 10 years of continuous service operation. A test procedure to achieve this can be:
- High Temperature Test: place accelerometer in an oven at +200 °C and maintain temperature for at least 450 hours.
- Thermal Fatigue Test: place accelerometer in an environmental chamber thermal cycling from +20 °C to +120 °C for at least 50 cycles at 1 cycle per hour.
- Vibration Test: place accelerometer on a shaker table and apply 150 m/s2 at 120 Hz continuously for more than 12 hours.
- Performance checks should be performed before and after the testing and result in minimal signal output change. This would indicate that the accelerometer would still be operational after 10 years of continuous service operation.
In hydrogen cooled generators the accelerometers are installed in a pressurized environment. Accelerometer performance should be evaluated in a pressure chamber with minimal signal variation.
Feed-through Pressure Test
For hydrogen‐cooled generators a penetration and fiber optic feedthrough cable bundle are used to pass the signal through the hydrogen seal. Pressure testing of the feedthrough cable should show no leaks up to 2.75 MPa‐g (400 psi‐g). In addition, accelerated life testing similar to the accelerometers can be performed to simulate at least 10 years of continuous service operation. The following procedure to achieve this is modified from the accelerometer testing due to different environmental conditions:
- High Temperature Test: place feed-through in an oven at +150 °C and maintain temperature for at least 350 hours.
- Thermal Fatigue Test: place feedthrough in an environmental chamber thermal cycling from +20 °C to +150 °C for at least 35 hours at a rate of 1 cycle per hour.
- Vibration Test: place feedthrough on a shaker table and apply 220 m/s2 at 120 Hz continuously for more than 85 hours.
- A leak check under pressure should be performed before and after the test procedure. No leaks would indicate that the feedthrough would still be operational after 10 years of continuous service operation.
Monitoring vibration in motor and generator stator end windings requires the use of specialized sensing equipment. Certain design considerations should be made to ensure signal accuracy and rigorous testing should be performed to ensure that the sensors do not pose a risk of machine failure.