Generator 

Condition Assessment Study

In this condition assessment package, we will assess the condition of total insulation system of electrical rotating machine consisting of high voltage generators.

The main objective of this condition assessment is to generate a report which gives the information to achieve following tasks

 

  • To arrest the deterioration in performance
  • To improve the availability, reliability, efficiency and safety of equipment.
  • To regain lost capacity
  • To extend useful life beyond design life and save investment on new equipment.

Condition Assessment Technique will be implemented as per following techniques listed below, which will assess the condition of Machine Insulation System in total. The various techniques are detailed as below.

1. Visual & Endoscopic Inspection
2. Insulation Resistance &  Polarisation Index
3. Tan Delta & Capacitance Test
4. Partial Discharge Analysis
5. Digital ELCID Test
6. DC/AC High Voltage Tests
7. Wedge Deflection/Mapping Test
8. DC Winding Resistance
9. Natural Frequency Test
10. Coupling Resistance Test
11. Rotor RSO Test
12. Rotor AC Impedence Tests
13. Exciter Tests
14. Diodes & Fuses Tests

Diagnostic Tests

Thermal, Electrical, Ambient (or environmental) and Mechanical (TEAM) stresses act on various components of a Rotating Machine, either singly or in combinations (thermo-mechanical, electro-thermal, electro-mechanical, etc), with varying intensities, depending on design, manufacturing, operating, maintenance and storage conditions, ultimately weakening (ageing, defect formation) the structure of the Rotating Machine component (winding, insulation, bearing, shaft, etc) and ultimately resulting in failure. Based on various international surveys and studies it has been concluded that winding insulation is found to be the one of the maximum potential areas of failures in Rotating Machines.

PROGNOSYS provides Diagnostic Testing services for Generators,Transformers,Motors,Switch gears,etc., includes following techniques listed below, which will assess the condition of Machine Insulation System in total.

1. Visual & Endoscopic inspection
2. Insulation Resistance & Polarisation Index
3. Tan Delta & Capacitance Test
4. Partial Discharge Analysis
5. Digital Elcid Test
6. DC/AC High Voltage Tests
7. Wedge Deflection/Mapping Test

8. DC Winding Resistance
9. Natural Frequency Test
10. Coupling Resistance Test
11. Rotor RSO Test
12. Rotor AC Impedence Tests
13. Exciter Tests
14. Diodes & Fuses Tests

Visual and Endoscopic Inspection

A detailed visual inspection is performed on Genrator components like Stator winding, stator core, rotor etc.

Visual Inspection with an expert eye is usually the powerful tool for assessing the condition of the machine and determining the presence of corona, contamination,any damages developing in insulation and also its root cause.

Endoscopic kit will be used to access all areas which are not visible with normal eye.

Capacitance & Tan Delta Analysis

Measurement of winding capacitance can indicate problems such as thermal deterioration or saturation of the insulation by moisture within the bulk of insulation. Capacitance tip-up test is the indirect partial discharge test and is closely related to tan delta tip-up.

Dissipation factor or Tan delta provides an indication of the dielectric losses within the insulation. Certain deterioration such as thermal deterioration and moisture absorption will increase these losses. Power factor tip-up is also an indirect way of determining partial discharge occurring.

Tan d and Capacitance will be measured both below and above discharge inception voltage. Capacitance and tan delta measurements will be performed using a Tan delta measurement instrument. Maximum test voltage will be VL /Ö 3 r.m.s (VL – Machine Line Voltage ).

Test will be performed on each phase individually by shorting other phases to ground. Test will be performed on slot region and end winding region separately.

The results will be analyzed in order to assess the winding insulation with regard to:

  • Extent of De-lamination (if any)
  • Condition of the corona protection shield
  • Contamination
  • Aging of Insulation

Partial Discharge Analysis

Partial discharges have been known to accelerate the aging process. They cause erosion of insulating material and propagate through treeing mechanism eventually bridging the electrodes and causing insulation breakdowns. While the Capacitance & Tan Delta tests gives us the indication of presence of these discharges, this test records such pd signals.

Partial Discharge test will be done using Partial Discharge Analyser by applying voltage as applied for tan delta test. PD Analyser system is used for partial discharge test that includes analysis software that gives discharge patterns for defect identification and also quantifies the partial discharge in the winding for trend analysis.

We provide Phase Resolved Partial Discharge (PRPD) analysis which will enable us to identify different types of PD.

  • Internal Discharges
  • Surface Discharges
  • Slot Discharges

Wedge Deflection / Mapping

Wedge looseness is a dangerous condition for two reasons, firstly it may foul with rotor causing mechanical damage and secondly the coils are not held tightly in the slots. This may lead to coil surface erosion due to its rubbing with core and eventually partial discharges in slots. While these effects can be detected by the diagnostic test. In the presence of ripple springs the extent of wedge looseness is measured with dial guage deflection by applying calculated pressure.

Alternatively Generator with no ripple springs are tested by performing wedge mapping test to identify which wedges are partially or completely loose/damaged.The tightness of wedges is checked by tapping each wedge in all the slots, with a hammer and listening to the emanating sound. A map is prepared to represent an overall picture of wedge tightness. Prognosys also provides analysis regarding the criticality and looseness percentage. Prognosys gives colorful map display identifying loose wedges and also analysis of the severity of wedge looseness and effect on the machine.

Recurrent Surge Oscillograph

A high frequency DC step voltage impulse (12 V) is repeatedly applied using Progno RSO Pulse generator to the rotor winding at positive slipring and the reflected weaveform is recorded using Digital Oscilloscope. The RSO technique uses time domain reflectometry theory to detect faults in Rotor Winding. The process is repeated for negative slip ring and the two waveforms are superimposed to check for rotor winding abnormalities like shorted turns, earth faults, interturn faults or rotor high resistance areas. The method is largely suited for 2 pole machines for effective detection of rotor winding interturn shorts or high resistance joints.
The results will be analyzed in order to assess the Rotor winding insulation with regard to:
 

  • INTERTURN SHORTS
  • HIGH RESISTANCE JOINTS
  • EARTH FAULTS
  • Contamination

ELCID test & Core Flux

The Electromagnetic Core Imperfection detector is a test for detection of core faults such as inter-laminar short circuits particularly in large generators, where it can be rather cumbersome to perform a standard loop test. Defects in the inter-laminar insulation cause fault currents to flow locally in the core. These currents can produce dangerous local over heating or hot spots in the damaged areas and the damage to the core may become progressively worse. In extreme cases sufficient heat is generated to melt small parts of the core and even modest rises in core temperature adjacent to the winding can result in the premature failure of the winding insulation.

Around 4% flux will be created in the stator core, with the help of a “loop” wound toroidally around the core. A sensing head is passed over the surface of the core to detect magnetically the presence of fault currents themselves rather than the heating effect they produce. The chattock coils output is fed to the SPU and further to the lap top computer, which gives the graph of fault current v/s slot length. ELCID is considered best technique in assessing the core condition.

Core Flux Test

The EL-CID (electromagnetic core imperfection detection) is the preferred test, but in some cases there is a need for a power flux test. The resistance between laminations is not always linear under different voltage levels and a power flux test with a higher axial potential difference between laminations may therefore reveal core faults that are not detectable by an EL-CID test.

Flux of between 80% and 105% of rated flux is normally used to perform a power flux test or to view hot spots due to shorted lamination if any in the core.The temperatures in the core depend on the flux density. The temperature in the teeth will therefore be lower than the temperature in the area at the back of the core. It is important to compare areas with similar flux density levels when evaluating the core. A difference in “hot spot” versus average core temperature of less than 10ºC is normally acceptable when the test is performed at flux levels between 85% and 100% of the rated level.

Natural Frequency Test (NFT)

The Electromagnetic Core Imperfection detector is a test for detection of core faults such as inter-laminar short circuits particularly in large generators, where it can be rather cumbersome to perform a standard loop test. Defects in the inter-laminar insulation cause fault currents to flow locally in the core. These currents can produce dangerous local over heating or hot spots in the damaged areas and the damage to the core may become progressively worse. In extreme cases sufficient heat is generated to melt small parts of the core and even modest rises in core temperature adjacent to the winding can result in the premature failure of the winding insulation.

Around 4% flux will be created in the stator core, with the help of a “loop” wound toroidally around the core. A sensing head is passed over the surface of the core to detect magnetically the presence of fault currents themselves rather than the heating effect they produce. The chattock coils output is fed to the SPU and further to the lap top computer, which gives the graph of fault current v/s slot length. ELCID is considered best technique in assessing the core condition.

We will able to distinguish the different types of NFT problems

  • End winding looseness
  • Wedge Looseness
  • Slot Corona due to Vibrations

Rotor & Exciter Tests

DC Winding resistance and AC Impedance Measurement

The rotor DC winding resistance and AC impedance is measured to check for electrical unbalance due to shorted turns or open circuit faults in the winding. Low resistance Micro-ohmmeter is used for winding resistance measurement where 10 amps DC current is applied and resistance is measured. To measure Impedence AC voltage is applied and current is measured.

Test on Exciter and PMG Machine

The following tests are conducted on exciter armature/field windings PMG stator/rotor windings.
 

  1. Winding resistance and impedance
  2. IR and PI at low voltages

Miscellaneous Tests

Diode and Fuse checks

A diode is reverse-biased when the positive (red) test lead is on the cathode and the negative (black) test lead is on the anode. The reverse-biased voltage of a good diode displays ‘OL’ on a multimeter. The diode is bad if readings are the same in both directions.

DC Tests

IR, PI and Absorption test

The IR / PI test is an excellent means of finding windings for contamination and moisture presence in stator and rotor windings. The test is also good for detecting major flaws where the insulation is cracked. In form wound stators with thermoplastic insulation systems, the test can also detect thermal deterioration. The test is performed by applying megger voltage and finding Insulation Resistance, Polarisation Index and Absorption ratios & graphs.

Winding Resistance Measurement

The stator DC winding resistance is measured to check for electrical unbalance due to shorted turns or open circuit faults in the winding. Low resistance Micro-ohmmeter is used for winding resistance measurement.

Online Partial Discharge Analysis

Partial discharges have been known to accelerate the aging process. They cause erosion of insulating material and propagate through treeing mechanism eventually bridging the electrodes and causing insulation breakdowns. While the Capacitance & Tan Delta tests gives us the indication of presence of these discharges, this test records such pd signals.

Online Partial Discharge test will be done using Partial Discharge Analyser by using pre installed couplers at rated voltage without disturbing the normal running condition of generator. PD Analyser system is used for partial discharge test that includes analysis software that gives discharge patterns for defect identification and also quantifies the partial discharge in the winding for trend analysis.

Prognosys provides services like installation of pd couplers, commissioning, calibration, data acquisition, and analysis for online partial discharge tests.

We provide Phase Resolved Partial Discharge (PRPD) analysis which will enable us to identify different types of PD.
 

  • Internal Discharges
  • Surface Discharges
  • Slot Discharges

Online Rotor Flux Monitoring

The Rotor Flux Monitoring Probes has Proven Effective in the Detection of Generator rotor winding shorted turns and has helped to improve the quality of predictive maintenance decisions concerning when or if rotor rework should be performed. Analysis of Rotor flux probe data can pinpoint the number and location (pole and coil) of shorted turns without having to take the generator off-line.

Shorted turns can have significant effects on a generator and its performance. If the 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, however, can cause operating conditions that may limit unit loads. If the problems become severe, forced outages may occur.

Prognosys provides services like Online Rotor Flux probe Installation, Calibration,Periodic Data Acquisition & Assessment services
We provide Rotor Flux Monitoring analysis which will enable us to identify different Rotor Faults:

  • Rotor unbalance that varies with field current changes (thermal sensitivity),
  • Rotor/stator vibration due to unbalanced magnetic force
  • Higher field current than previously experienced at a specific load
  • Higher operating temperatures due to higher field currents

Thermographic Inspection

The principle of Infrared Thermography is based on the physical phenomenon that any body of a temperature above absolute zero (-273.15 °C) emits electromagnetic radiation. There is clear correlation between the surface of a body and the intensity and spectral composition of its emitted radiation. By determining its radiation intensity the temperature of an object can thereby be determined in a non-contact way.Thermography has proven to be the best tool to find out the hotspots in machine.This technique uses Thermographic cameras to detect hot spots on stator core, temperatures of blocked teflon tubes in rotating machines.