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.
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.
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.