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.

  • Visual & Endoscopic Inspection
  • Insulation Resistance & Polarisation Index
  • Tan Delta & Capacitance Test
  • Partial Discharge Analysis
  • Digital ELCID Test
  • Core Flux test
  • DC/AC High Voltage Tests
  • Wedge Deflection/Mapping Test
  • DC Winding Resistance
  • Natural Frequency Test
  • Coupling Resistance Test
  • Rotor RSO Test
  • Rotor AC Impedance Tests
  • Exciter Tests
  • Diodes & Fuses Tests

VISUAL AND ENDOSCOPIC INSPECTION

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

Visual Inspection with an expert eye is usually a 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.

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

        

Capacitance & Tan δ Analysis

The measurement of capacitance and tanδ is one of the most useful, reliable, and effective diagnostic tools for condition monitoring programs. The tanδ, which is a direct indication of power dissipated by insulation is a significant measure of the quality of insulation. The tanδ provides a measure of the overall operating condition of the insulation.

        In the case of the Rotating machines, the C and tanδ Measurement enables the assessment of the total condition of the main slot insulation. The comparisons of measured values of different phases and or of two or more identical machines are done for evaluation. The capacitance and tan delta measurements are performed by applying the test voltages to the high voltage terminals and measuring using AC bridges such as conventional schering bridge.

Depending on the configuration of equipment under test (UST) in which both electrodes of measurements are not connected to the ground and grounded mode (GST) in which one of the electrodes is permanently connected to the ground various connection methods are utilized. 

        

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

The dissipation factor or Tan δ indicates 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 δ and Capacitance will be measured both below and above discharge inception voltage. Capacitance and tan δ measurements will be performed using a Tan delta measurement instrument. The maximum test voltage will be VL /√3 r.m.s (VL – Machine Line Voltage).

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

The results will be analyzed to assess the winding insulation about:

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

Partial Discharge Analysis

Partial discharge testing, also known as PD testing, is performed to assess electrical insulation health. PD is best described as a failure of part of an insulation system to withstand the electrical field applied to it. This can be a result of poor design, poor workmanship, defective materials, contamination, or ageing. 

Partial discharges are electrical sparks which occur in gas voids within the insulation when the voltage is high enough. The spark generates a fast current pulse which travels through the stator winding. The larger the Pd pulse, the higher the current pulse that reaches the terminals of the winding. A high-voltage capacitor can block the power frequency voltage while allowing the high-frequency pulse signals to reach a Pd detector. The pulse signals after further filtering are displayed on a screen.

PD Phenomena in Rotating Machines

  • Loose wedges/bar vibration/slot discharges
  • Cracked and broken conductors
  • End winding surface contamination
  • Damage and corrosion of the corona suppression system
  • Connection ring external discharge due to vibration
  • Interphase discharge
  • Insulation degradation
  • Collector ring, brush sparking
  • Faults in generator high voltage accessories
        

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

A partial Discharge test will be done using the Partial Discharge Analyzer by applying a voltage as applied for the tan delta test. PD Analyzer system is used for partial discharge tests that include 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.
 

  • Surface Discharges
  • Internal Discharges
  • Slot Discharges
        

Wedge Mapping

Wedge looseness is a dangerous condition for two reasons, firstly it may foul with the 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 the core and eventually partial discharges in slots. These effects can be detected by the diagnostic test. In the presence of ripple springs the extent of wedge looseness is measured with dial gauge deflection by applying calculated pressure.

Alternatively, a Generator with no ripple springs is tested by performing a 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 a colourful map display identifying loose wedges and also an analysis of the severity of wedge looseness and its effect on the machine.

 

        

Electro Magnetic Core Imperfection Detection (ELCID) Test

The effect of the magnetic fields is to produce a magnetic potential gradient on the bore’s core surface. The measurement of this magnetic potential difference (mpd) is using a specially wound coil known as a Chattock potentiometer which provides an AC voltage output proportional to the difference in magnetic potential between its two ends.

The Electromagnetic Core Imperfection detector is a test for the 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 overheating 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 coil output is fed to the SPU and further to the laptop computer, which gives the graph of fault current v/s slot length. ELCID is considered the best technique for assessing the core condition.

        

Natural Frequency Test

The main cause for the source of vibrations of end windings is due to electromagnetic forces fluctuating with certain frequencies. Resonance occurs when the frequency of electromagnetic forces is nearer to the line frequency.

It is very important to be away from resonance and safe operation of rotating machines with definite mode structures.  Stator end windings will vibrate 10 times more when compared to the Centre part of the windings. If the vibrations of the machine are larger, then the rubbing between windings occurs which weakens the insulation, and creates cracks on the semiconductor which leads to damage in the mica layer and greater vibration amplitude which causes looseness in the machine. A vibrational alignment problem is associated when there is a misalignment of the contacted parts of the machine.

Abnormal coil movement in the slot can generate cracks and introduce loose parts, Severe vibrations on overhang stator end windings experience difficulties in the long run hence it is necessary to observe the symptoms and signs of vibrations which lead to fractures, cracks and broken ties at the end windings.

Loose windings are to be tightened to reduce vibrations for better performance of the machine. Due to this looseness in the machine, deteriorated windings degraded the performance of the machine.

        

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.

The 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.

        

DC-HIPOT Test

The purpose of this test is to determine if there are any major flaws in the ground wall insulation before a winding enters service (commissioning or acceptance hipot test) or during service (maintenance hipot test). The principle is that if there is a major flaw in the insulation, a high enough voltage applied to the winding will cause insulation breakdown at the flaw.

 

Uniform time voltage step test.

Graded Time voltage step test.

Ramped voltage test

        

Coupling Resistance Measurement

The measurement of the resistance between the semi-conductive coating and the grounded stator core can indicate if the coils are loose in the slot or if the coating has deteriorated. Specifically, the semi-conductive coating prevents partial discharge (also known as slot discharge) between the coil surface and the stator core. As the semi-conductive coating deteriorates, its resistance increases. Also if the coils are loose in the slot, there may be only a few points of contact between the semi-conductive coating and core. In both cases, the contact resistance between the coil surface and core will increase.

This is applicable for large generators and is used to decide the coefficient of friction between coil and core is an input parameter to the stress modelling of coils.

        

DC Tests

I. R, P. I & 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.

        

Miscellaneous Tests

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

Diodes & Fuses Check

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.

Test on Exciter & PMG Machine

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

  • Winding Resistance & Impedance tests
  • I. R. & P. I. Measurement at low voltages
        

Recurrent Surge Oscillograph (RSO)

A high-frequency DC step voltage impulse (12 V) is repeatedly applied using a Progno RSO Pulse generator to the rotor winding at positive slipping and the reflected waveform is recorded using a Digital Oscilloscope. The RSO technique uses time domain reflectometry theory to detect faults in Rotor Winding. The process is repeated for negative slip rings 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 inter-turn shorts or high resistance joints.

The results will be analyzed to assess the Rotor winding insulation about:

  • INTER TURN SHORTS
  • HIGH RESISTANCE JOINTS
  • EARTH FAULTS
  • Contamination
        
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Get In Touch

Contact Info

Phone number

(456) 789 10 12

(456) 789 10 15

Email address

demo@gmail.com

indusinfo@gmail.com

Address info

Prognosys EMS(P) Ltd.,
1st floor, Subishi town center,
Mokila, Shankarpalle Road,
Hyderabad, Telangana – 501203.