発電機の位相アンバランスを診断し、100万ドルを節約

explanation

Location: Vermont Yankee Nuclear Power Plant

Plant Equipment 50 HP, 3600 RPM, 480 Volt, Open Dripproof, Cooling Pump Motor

Affected system: 500 MW generator bearing cooling

Cost of failure: $1 million

Savings: > $1 million

 

A 50 HP, 3600 RPM, delta-connected electric motor was installed to check the rotation of the bearing cooling pump on a Vermont Yankee generator. This motor is one of two and will only operate if the primary motor fails. When the primary failed, the motor came online. There was a current unbalance of 11% (pp), and the voltage unbalance was less than 0.5%. The motor exhibited 120 Hz vibration (electrical) and was found to be operating at 90% load from peak current, but the operating temperature was excessive.

 

First Reading

The phase unbalance was measured using ALL-TEST III™, and the phase-to-phase results when the rotor was shifted to the peak unbalance of each phase were 000, -016, and -016 (unbalance %). Furthermore, two motors of the same model and with similar serial numbers were selected and tested using both the ALL-TEST IV PRO™ 2000 and the ALL-TEST III™. The phase unbalance and rotor tests were evaluated (Figure 1 and Table 1 are examples of common results):

The imbalance was found to be significant and related to unbalanced currents, vibrations and motor heating. Various possibilities were explored, ranging from power quality to calibration of the test equipment. All were satisfactory.

 

Next steps

When we contacted the motor manufacturer, we were informed that a process change had been made in a specific location due to the large concentric winding machine. In a motor of this size and speed, the first set of concentric coils (phase 1) would curl under the next phase, reducing the appearance and mechanical strength of the windings on the machine. To address this issue, the manufacturer decided to significantly increase the size of the first coil in the automated process (phase 1). This allowed the coil ends to emerge without any post-winding modifications. Other than the “applied voltage impedance test” to “meet design requirements,” no dynamometer testing, full-load testing, or other tests were performed on the motor design. Electrically, inductance is directly affected by the distance from the rotor, the number of conductors, and the coil dimensions. Improvements in the motor manufacturing process caused the imbalance.

Evaluation of motors from other manufacturers revealed that they were balanced wound, however, some newer motors were found to have rotor casting voids that affected the motor’s ability to produce torque.

Vermont Yankee Nuclear Power Plant is currently implementing a program to test all critical electric motors prior to acceptance using a combination of ALL-TEST III™ and ALL-TEST IV PRO™ 2000.

 

Cost Avoidance

The generators should have been shut down within two minutes of the second motor failure. The shutdown of the emergency generators could have damaged the generator bearings, causing an unplanned power outage. The cost savings from discovering the failure were estimated to be well over $1 million. Subsequent detection of similar motor conditions across new and repaired motors continues to vindicate the incoming testing and inspection program.

 

conclusion

Neither new nor refurbished electric motors are free from defects. These defects can be the result of production/repair errors or design errors. An incoming inspection program using both ALL-TEST III™ and ALL-TEST IV PRO™ 2000 can identify these potentially costly defects before the equipment is installed.