RFA II -R

RFA ΙΙ -R™

Rotor Flux Analyzer: a second generation advanced tool to find rotor winding shorted turns in turbine generators and high speed synchronous motors

Capabilities

  • Instant analysis of rotor winding condition at a fixed operating load
  • Where needed, ability to analyze tests at different loads for a more certain prediction of rotor winding condition.
  • Able to perform a spot measurement, or automatically acquire results over days during normal generator load changes, without test operator intervention
  • Works with conventional wedge-mounted flux probe, or the Iris stator tooth-mounted TF Probe™, which can often be retrofitted with the rotor in-place

Synchronous Generator And Motor Rotor Windings

  • The rotor insulation must withstand severe electrical, mechanical, and environmental stresses. Insulation failures can result from many factors including:
  • Mechanical wear, especially that caused by frequent load cycles
  • Distortion, breakage and migration due to centrifugal mechanical loading and thermally induced expansion/ contraction cycles
  • Overheating due to overloading/over excitation or inadequate or diminished cooling
  • Local overheating at high resistance brazed joints and at shorted turns
  • Contamination from ventilation or copper dusting resulting in surface tracking between turns or to ground
  • Over-voltages induced from system events or from firing circuits in static exciters.
  • An insulation failure can translate into electrical connections between turns, and eventually a catastrophic winding to ground fault.

A turn-to-turn short is the most frequent rotor insulation failure mechanism. Turn shorts can result in:

  • Thermal imbalance of the rotor leading to mechanical vibrations
  • Magnetic imbalance in the flux resulting in mechanical vibration of the rotor
  • Increased rotor temperature and subsequent insulation degradation
  • Overheating resulting in insulation failure and a ground fault with the potential for a second catastrophic ground fault
  • Inability to reach the rated MVA rating for the machine
  • Decreased generator power due to higher electrical losses

Flux monitoring via permanently mounted air gap flux probes is a proven technology in synchronous machines to determine if turn-to-turn shorts have occurred in the rotor winding. Flux measurement provides the most direct means of monitoring the condition of rotor windings on-line, yielding the information on the integrity of coils’ inter-turn insulation. This information is critical in planning maintenance, diagnosing abnormal vibrations, and verifying new and rewound rotor integrity.

 

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Product Description

Theory of Flux Monitoring

Flux monitoring relies on measurements of the local magnetic field emanated by each coil in the rotor. The leakage flux is determined by the total ampere-turns from each rotor slot. Any change in the ampere-turns within a coil due to shorted turns produces a change in the leakage flux.

To measure the leakage flux, a flux probe sensor is permanently installed on the stator. During machine operation, the flux from each passing slot will induce a voltage in the flux probe. The difficulty in measuring the leakage flux is that the main radial magnetic flux is orders of magnitude greater than the leakage flux. To maximize the sensitivity to shorted turns in all rotor coils, the signals from the flux probe needs to be measured under different load conditions ranging from no load to full load. At a zero crossing of the total flux (which is the function of the real and reactive load of the machine), the sensitivity to the leakage flux is the highest. Thus the flux readings with the older, first generation equipment must be taken at various load points depending on the number of slots in a pole.

Typical Application

The condition of the rotor winding insulation is difficult to assess during minor or major generator maintenance outages. Access to the winding is severely restricted without removal of the retaining rings and of the winding wedges. The off-line tests for detection of shorted turns and ground fault locations can also be frustratingly ineffective due to frequently intermittent nature of faults at speed and at standstill. Therefore, online measurements are much preferred to off-line tests and inspections.

On-line measurements require the permanent installation of a flux probe on the stator to measure the slot leakage flux. Most machine OEM’s offer a flux probe which is attached to a stator winding wedge protruding into the air-gap. Iris now offersan alternative probe, the TF ProbeTM, which is a small, thin flexible, printed circuit board transducer affixed to a tooth of the stator. The TF Probe measures the total air gap flux, rather than just the leakage flux as do older style probes. The
TF Probe is easy to install, and requires no invasive drilling of stator wedges. Frequently it can be installed with the rotor still in place! In the case of hydrogen-cooled machines, the leads from the flux probe are routed out of the machine through a hermetically sealed feed-thru.

Regardless of the probe technology, data from permanently installed flux probes can be measured via software, the RFA ΙΙ -R TM , or continuous on-line monitors like the FluxTracTM. Once the data is acquired, analysis techniques must be applied to compare the flux measurements across various rotor slots to determine if the turn shorts are present.

Features

RFAII-R™ INSTRUMENT

The RFA ΙΙ-R ™ technology is a second generation rotor flux analyzer that revolutionizes the analysis of the flux data by providing an initial diagnosis of the rotor winding condition even if the generator load is constant! This portable instrument can collect and analyze flux data from any flux probe in real time, providing the user with data on potential shorted turns if any. The RFA ΙΙ -R ™ high-speed acquisition, high resolution capability and totally new analysis algorithms allows it to collect and analyse data over an entire unit shut-down or start-up (with the shaft sync signal connected). Once configured, the RFA ΙΙ -R ™ requires no user intervention and is ideal for profiling the rotor insulation condition before a machine outage or after a refurbishment.

  • Rugged portable instrument with USB and Ethernet computer interfaces
  • Custom built, ultra-high resolution digital data acquisition and on-board switchable attenuation for maximum resolution measurements using any manufacturer’s flux probe
  • High speed acquisition with deep memory for complete and accurate data collection
  • Capable of storing over one hundred and fifty raw flux waveforms
  • Can be synchronized to a power frequency signal, or ideally to an external shaft sync signal (key phasor)
  • Predictions of turn-shorts in any slot, regardless of the generator load point during data acquisitionA High-Speed Acquisition mode creates a table of results covering each coil at each load point, including raw flux waveforms
  • User friendly Windows™ based software for data display, analysis, and trending
  • Analysis software capable of reading and analyzing data files from other manufacturers portable instruments
  • International power supply. Instrument operates from 85 VAC to 260 VAC.

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DISCLAIMER: The technical information contained herein is the result of extensive testing in the manufacturer’s laboratories. In addition any application advice given verbally, in writing or by testing corresponds to the best of our knowledge and belief. Since the use and application of any of our products lies beyond our area of control, it remains the responsibility of the user to verify the product’s suitability for their application.

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