Motor testing equipment prevents power station downtime

GE Energy provides a wide range of technical services for businesses and authorities that generate, transmit or use electricity; these services include test and inspection of electric motors, transformers and switch gear. The company works in all areas of the energy industry, including coal, oil, natural gas and nuclear energy, as well as with renewable resources such as wind, solar and biogas. Tests are undertaken using a wide range of technologies, from vibration analysis and thermography, to electrical testing.

To support its expanding work programme, in August 2009 GE Energy took delivery of two advanced motor testing units manufactured by the Baker Instrument Company, (an SKF group company), supplied by Baker's UK agent, Whitelegg Machines Limited. The AWA Winding Analyser is an offline test unit that is used to check the integrity of the insulation system within electric motors and transformers. The other instrument in the range is Baker's 'Explorer' Online Monitor, which is used to test motors while they are running to check for a wide range of problems including power quality issues, cracked rotor bars and mechanical problems with the load.

Shortly after completing a training course for the AWA Winding Analyser, GE's Tony Ruane visited a UK power station to perform an annual test and inspection on 45 motors over three days. On testing the fifth motor, a fault was detected.

The AWA unit provides automatic testing capabilities for winding resistance with automatic temperature compensation, meg-ohm, PI, DC step voltage and surge tests. On-site testing should ideally be performed from the motor control cabinet (MCC) through the power cables feeding the motor, as faults can be present in both the cable and the motor. It is typically easier to perform testing from the MCC, as there are usually no major restrictions on access.

When performing the surge test, a fault was detected at 1630V and the test was stopped automatically. The fault was detected using the patented pulse-to-pulse EAR software that is designed to detect turn-to-turn insulation weakness or arcing. However, since the test was performed from the motor control cabinet, Ruane was unsure as to whether the fault lay with the motor itself or the power cables.

The next step was to go to the motor terminal box, which was awkward to reach due to its elevated location. However, it was critical this was done in order to isolate the fault location. The power cables were disconnected from the terminal box and the motor was tested again. This time the pulse-to-pulse software detected no insulation weakness and therefore Ruane's attention turned to the power cable. A third test was performed on the cable itself, with it still disconnected from the motor. This time the pulse-to-pulse EAR fault returned at a similar voltage to the original fault.

Ruane alerted the responsible engineers to this fault but, as there was insufficient time to investigate further, a decision was taken to restart the motor. Since the voltage level at which the fault was detected was relatively high, Ruane was confident that the motor would restart. Motors see the most stress on start-up, hence motors that see a lot of start-ups each day and under heavy load, and which have weak insulation, are more likely to fail in the short term.

The motor in question was a 15kW machine that drives an oil mist eliminator unit. The oil mist eliminator is a critical piece of equipment on the gas turbine unit and works by drawing a partial vacuum on the outer bearing oil seals to extract oil vapour from the bearing cavity into a separator.

If this unit were to fail, the pressure inside the bearing cavity would create an oil leak, forcing oil vapour into the rest of the surrounding environment and creating a fire hazard. The gas turbine unit would then have to be taken offline until a repair had been affected, resulting in reduced power generation capability.

While a 15kW motor will often be deemed as not worthy of regular testing, in this case it is critical to other processes and, if it were to fail, it has been estimated that downtime and lost generating revenue would be in the region of 100,000 to 150,000.

Another myth relating to high-voltage testing is that such testing will damage insulation and cause premature motor failure. However, the voltage at which the fault was detected on numerous occasions was within 100V each time and, when the motor was required to restart, it did so - and remains operational more than two months later. The cable fault will be dealt with at the next planned shutdown.