GENERATOR FIELD WINDING SHORTED TURN DETECTION TECHNOLOGY PRES
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02-03-2011, 04:06 PM
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* Use of air gap magnetic flux probes-proven effective in detection of generator winding shorted turns
* Improve quality of predictive maintenance decision when or if to perform rework
* Can pinpoint the no: & location(pole & coil) of shorted turns without having to take generator offline
EFFECTS OF SHORTED TURNS IN ROTOR WINDINGS
* If percentage of total turns shorted out is small, the generator may be able to run at rated load for years without further problems
* Larger shorted turn percentages can cause operating conditions that may limit unit loads
CONDITIONS RESULTING IN RUNNING A ROTOR WITH SHORTED TURNS
1.Rotor unbalance that varies with field current changes (thermal sensitivity)
* Coils with shorted-turns operate at lower temperature than coils without shorted turns
* Rotor temperature gradient that can give rise to rotor bowing will be a function of the number of turn-shorts & their location, as well as total field current.
2.Rotor/stator vibration due to unbalanced magnetic force
* Shorted turns in 4 pole rotors cause unbalanced magnetic forces
* Shorted turns in 2 pole rotors do not cause unbalanced mag force since reduction in mag flux will affect both north & south poles equally
* 4 pole rotors-shorted turns in one pole will reduce flux generated for the pole and to a lesser extent the adjacent poles, but have no effect on opposite pole ,resulting unbalanced magnetic pull between rotor and stator can cause vibrations
3.Higher field current is required than previously experienced at a specific load.
* When shorted turns occur,higher field current is required to maintain a specific load
* Excitation capacity mat limit load if greater than 5-10% of the field winding is shorted out.↓ η will result in any case
* Amp vs field turn relationship is As=AnTn/Ts
* Field wndg loss(I²R) at a specific load can be determined by noting the R of the field wndg will ↓ by Ts/Tn but I² component ↑ by (Tn/Ts)².this results in an increase in field wndg loss of (Tn/Ts) for a specific load.
4.Higher field currents result in higher operating temperatures
* Higher field currents required to maintain a given load will result in an increase in I^2R loss for the entire rotor winding
* As a result of this higher I²R loss, the total heat generated by the field will be increased when compared to operating at the same load factors without shorted turns
* Causes of shorted turns in rotor windings
* Failed insulation between individual windings in a rotor-insulation failure can b a result of turn to turn movement of rotor windings
* Stresses involved in each stop start cycle play an important role in the development of shorted turns
* Metallic contamination can also result in shorted turns by forming conductive bridges b/w turns
* Failure mechanism include coil forshortening, end strap elongation or inadequate end-turn blocking
* Coil Foreshortening,end strap elongation,adequate end-turn blocking
* Coil forshortening refers to a phenomenon where rotor turn copper decreases in length within a rotor slot after a number of start-stop cycles
* End strap elongation is a result of excessive friction between the end-straps and the retaining ring insulation
* Adequate end-turn blocking is required to maintain the positions of the rotor winding end turns.
* shorted turn characteristics
* Detecting rotor winding shorted turns using flux probe measurements
* Main Rotor Flux: flux that crosses the air gap resulting from current through the rotor field windings
* Stator Reaction Flux: the armature reaction flux due to current flowing through the stator armature windings.
* Rotor Slot Leakage Flux: flux that does not cross the air gap to reach the stator windings.
* Air-gap Flux Density: combination of all three of the above fluxes in the generator air-gap.
* Detection of the rotor slot leakage flux
* It is accomplished using a magnetic flux probe positioned in the air-gap of the generator
* A single flux probe is generally mounted on a stator wedge in a position to be over a continuous ring of non-magnetic rotor wedges
* the flux probe cable is routed out of the stator core, through the stator end-windings and out to the generator casing.
* A gland that provides a gas tight penetration for the flux probe cable is welded to the outside of the generator casing.
* An analysis system is used to record flux probe waveforms by connecting to the casing gland BNC connector using a length of coaxial cable.
* The flux probe is sensitive to the time rate-of-change of the radial flux in the air-gap.
* As each rotor slot passes the flux probe, the slot leakage flux from that slot is detected.
* The flux probe waveform displays a peak for each rotor slot(fig) , magnitude of that peak is related to the amp-turns in the slot.
* a coil with shorted turns will display a smaller peak than a coil without shorted turns.
* By comparing the slot peak magnitudes b/w poles,the no: of shorted turns can be calculated for each coil in the rotor
* To determine point of max sensitivity,flux density curve can be obtained through integration of flux probe waveform(fig)
* A series of load points is needed whose flux density curve zero-crossings(FDZC) align with each of the leading coil slot peaks in the flux probe waveform
* The FDZC varies with the load placed on the generator
* To calculate the shorted turns for a particular coil, the load point whose FDZC most closely with the lead slot of the desired coil is selected
* The shorted turn indications for that coil are then calculated
* Optimally,there would be a load point whose FDZC aligned with the lead slot for each coil
* Shorted turn calculations are performed by measuring the magnitude of each lead slot peak and then making a pole to pole comparison for each coil
* Occurence of symmetric shorted turns where the same coil has experienced shorted turns in each pole will mask the detection of shorted turns
* Software and hardware has been developed to automate the analysis of the flux probe waveform
* Hardware consists of a notebook computer,a PC-Card analog-to-digital converter card and a Signal Conditioner
* A Microsoft Windows based software program controls the recording of the flux probe signal and automates the shorted turn analysis of the waveforms
* The flux density curve is calculated by integrating the flux probe signal and the curve’s zero crossing points are noted
* Figure displays two lead slot overlay graphs for a two pole rotor with 1 shorted turn in both Pole A-Coil 4 & Pole B-Coil 6
* Left graph is at 25% of full load and has an FDZC position near the lead slot of coil 6
* Right graph is at 70% of full load and has a FDZC position near the lead slot of coil 4
* This example emphasizes the importance of recording a series of load points from zero to full load
* Second case study presents a two-pole rotor with 7 coils/pole in which 6 and 7 of one pole were shorted out of field circuit
* This result occurred when the top turns of coils 6 and 7 became shorted together
* Figure displays a dramatic absence of signal for the lead slots of coils 6 and 7 of one pole
* The unit was able to run at reduced loads due to excitation limits.
* Shorted turn detection technology provides the ability to define te location and number of shorted turns while the generator remains online
* Can assist plant engineer and operators in making major maintenance decisions about when or whether to perform rotor service
* Has proved itself to be a reliable test for detecting shorted turns
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