DIAGNOSTIC TESTING AND CONDITION ASSESSMENT OF

LARGE GENERATORS

Introduction

This presentation briefly describes some of the failure mechanisms on large generators, and then describes some of the testing we perform, in our Service Facilities or on-site during an outage, to more accurately assess the condition of a machine.

FAILURE MECHANISMS

40% of failures on generators occur due to problems with the windings. The following is a list of possible causes of winding failure and stator winding failure mechanisms.

Inadequate bonding of the winding insulation. Results in voids in the groundwall insulation, either at the copper surface or within the groundwall insulation itself. Depending on the size of the void, the voltage of the conductor and the pressure of the cooling gas, a spark or partial discharge may occur within the voids, attacking the organic resin holding the insulation together. This results in the voids growing, to the point where the insulation fails and an earth fault occurs.

Introduction

Slot discharges. If the surface corona or semiconductive coating on the coils in the slot section is incorrectly applied or eroded over time due to movement of the coils, discharges will take place between the coil and the core. These too will attack the organic resin, and eventually result in the failure of the winding insulation.

Stress Grading failure. This results in a sharp voltage gradient over the surface of the coil in the endwinding, and consequently corona discharges in air-cooled machines. These are prevalent at the interface between the stress grading and the slot semiconductive coating and will start breaking down the interface area. Eventually they will attack the slot semiconductive coating until it is eroded back into the slot section, resulting in slot discharges and eventual winding insulation failure. Both the above two failure mechanisms are slow, but irreversible once started.

Introduction

Loose windings. If the windings can move in the slots (due to loose wedges, loose packing, ingress of oil, shrinking of resin, etc.), the semiconductive coating can be eroded against the core, resulting in sparking as described above. In addition, the groundwall insulation can then be mechanically eroded, and the combination of the electrical and mechanical erosion mechanisms can result in failure of the winding insulation in a relatively short period (two years or even less).

Inadequate spacing. Inadequate spacing between the coils in the overhang can result in a high voltage stress between bars of different phases, and consequently corona discharges. These will gradually erode the groundwallinsulation until failure occurs, although the failure mechanism is a slow one.

Endwinding failure. Endwindings of high voltage machines are subject to strong twice frequency (i.e. 100Hz in a 50Hz machine) mechanical (from the core/rotor magnetic interaction) and electro-magnetic forces, which can cause vibrations in poorly blocked or inadequately designed machines. These vibrations can abrade the windings against the blocking or support structure, eventually causing the copper to be exposed and a possible phase-to-phase or earth fault. Also, the vibrations can cause the coils to loosen in the slots.

Introduction

Surges. These result in voltage spikes, which stress the interturninsulation in multi-turn coils. Eventually the interturn insulation may fail, resulting in very high level circulating currents flowing in the loop caused by the shorted turn and failure of the groundwall insulation. The main problem here is that it is almost impossible to predict failure in the majority of cases, as the groundwall insulation will appear in good condition until failure occurs – the failure is taking place from within the winding.

Thermal Deterioration. When an insulation system is exposed to overheating, the bonding material loses its mechanical strength, and the insulation layers delaminate. This eventually results in voids and long term failure, depending on the degree of overheating. With asphaltic systems, the black asphalt material sometimes oozes out of the winding.

Introduction

Load Cycling. As copper expands due to increase in temperature under loading of a machine, the insulation bonded to the copper is placed under stress, as it has a different coeffecient of expansion. This can lead to delamination, and can also cause damage to interturn and strand insulation. In older asphaltic systems, a phenomenon called ‘girth cracking’ can occur, where the coils exit the core. This is because when the copper expands, it drags the insulation out of the slot, and when the copper cools and contracts again, the insulation doesn’t contract back into the slot but collects just outside the core, eventually leading to cracking and sometimes splitting of the insulation.

Endwinding Contamination. Any conductive contamination from moisture or oil mixed with dust or dirt can result in electrical tracking and partial discharges on the surface of the insulation. Phase-to-phase and phase-to-ground deterioration may occur, and eventual failure of the groundwall insulation may result.

Introduction

In the core, the following are the main failure mechanisms which occur.

Overheating or hot spots. Failure of the interlaminar insulation can result in circulating currents in the core and localised overheating. This then can degrade more of the insulation, resulting in bigger cicrculatingcurrents and more heat. Eventually, the hot spot will cause the winding insulation near the spot to overheat and fail, and an earth fault will occur.

Loose Laminations. Particularly near the ends of the core, the press plates and press fingers can relax and result in the laminations becoming loose and vibrating in service. These can then abrade the winding insulation, or even fatigue, crack and break off resulting in iron particle contamination of the machine.

Introduction

In the rotor, the main concern is mechanical stresses on the shaft, body and Coil Retaining Rings resulting in possible cracking, problems on the bearings due to contaminated or insufficient oil, out of tolerance clearances or electrical pitting on the surface. The two main electrical failure mechanisms are as follows.

Interturn Faults. Failure of the interturn insulation on one or more of the poles results in a thermal and possibly magnetic unbalance of the rotor and a possible increase in vibrations. In addition, the fault can deteriorate to the point where an earth fault occurs. Interturn faults result in a reduction in the net ampere-turns of the rotor, so more excitation current than prior to the fault may be necessary to reach a certain loading of the machine.

Earth Fault. An earth fault is a failure of the insulation between the rotor winding and the body of the rotor, either in the slot section or the overhang (to the Coil Retaining Ring). A single earth fault may not necessarily negatively impact the operation of the rotor. However, if a double earth fault occurs, there is a path for circular currents to flow, which will result in catastrophic damage to the rotor and potentially cause it to disintegrate at speed. The main reasons for failure of either interturn or groundwall insulation on generator rotors are mechanical abrasion of the insulation, or overheating.

Introduction

• There are a variety of tests,

• Some general to all machines, some specific to certain types (e.g. pneumatic/vacuum, water flow, etc – only to stators with direct water cooled windings)

• We will only be looking at the ‘common’ tests - IR/PI,HV , RSO, volt drop, PD, tan delta, ELCID and wedge tightness

• Mostly harmless, however, HV testing may cause damage

• Condition assessment of generators is difficult, therefore it is best to do as many tests as possible when the opportunity arises

Insulation Assessment Testing

IR / PI

AC / DC / VLF

PD/Tan Delta

Insulation Resistance (IR) Test

• Ratio of DC voltage applied between winding and earth for 1 minute and the resultant current (R=V/I).

• Applicable to stators and to rotors ( or field and armature windings)

Winding Rated Voltage Insulation Resistance Test Voltage

<1000V 500V

1000 – 2500V 500 – 1000V

2501 – 5000V 1000 – 2500V

5001 – 12000V 2500 – 5000V

>12000V 5000 – 10000V

Insulation Resistance (IR) Test - Stators

• From IEEE 43-2013, the applied DC Voltage for the majority of generators is 5000 V

Insulation Resistance (IR) Test -Stators

• The acceptance criteria for stators is usually 100 MΩ

Description Minimum IR Value

Most windings made before 1970, all rotor

windings and others not described below (kV + 1) M

For most DC armature and AC stator

windings after 1970 (form-wound coils) 100 M

Most machines with random-wound coils

and form-wound coils rated below 1kV 5 M

Insulation Resistance (IR) Test

• Results very sensitive to humidity and temperature- they are usually corrected to 40 °C

• Difficult to trend results

• Generally a pass / fail test,

• But very simple and useful

Creepage Paths in the rotor

• Rotors rely on creepage paths for their IR value, so the acceptance limits are much lower

• As can be seen in this photo, the creepage path is the gap between the copper winding and the body

• If it gets contaminated, by moisture or dirt, the IR value drops considerably

Winding Rated Voltage Insulation Resistance Test Voltage

<1000V 500V

1000 – 2500V 500 – 1000V

2501 – 5000V 1000 – 2500V

5001 – 12000V 2500 – 5000V

>12000V 5000 – 10000V

Insulation Resistance (IR) Test - Rotors

• The test voltage for rotors is typically 500 VDC

Description Minimum IR Value

Most windings made before 1970, all rotor

windings and others not described below (kV + 1) M

For most DC armature and AC stator

windings after 1970 (form-wound coils) 100 M

Most machines with random-wound coils

and form-wound coils rated below 1kV 5 M

Insulation Resistance (IR) Test - Rotors

• Acceptance criteria of (kV+1)MΩ – e.g. for a 400V rotor, acceptance is 1.4MΩ

Insulation Resistance (IR) Test - Rotors

• So for most rotors, we’re looking at an IEEE acceptable IR value

of 1.2 to 1.6 MΩ

• However, we use 100 MΩ as an alert value – from experience

most test results are in the GΩ range, so we want to see what’s

happening.

• Some OEMs use 50 MΩ for rotor acceptance

• Some utilities – minimum of 20 MΩ

Polarisation Index (PI) Test

• With a constant DC voltage applied with a Megger, PI is the ratio of the 10 minute reading to the 1 minute reading

• Provides a criteria to ascertain the dryness and cleanliness of the winding

• Ratio, hence relatively insensitive to winding temperature

• With wet/dirty windings, the surface leakage current will dominate, so the total current will not vary much with time

Polarization Index (PI) Test

• IEEE: PI > 2 means the winding is clean and dry

Older machines, 1.5 is acceptable

• General Guideline: <1: dangerous1-2: questionable2-4: good>4: excellent

• BUT if the insulation resistance is very high (5000 M or more), the PI can be considered meaningless:

- some modern insulation systems polarize quickly (within the 1 minute limit for IR);

- also very low leakage current makes the readings unreliable

High Voltage AC Test

• Final proving test - pass / fail

• Often called a “pressure” test - applies pressure to the winding insulation

• Apply 2Un + 1 for new winding, between 1 - 1.5Un (or less – phase voltage in some cases) for old winding

• Requires large power supply, up to 100kVA or greater, to charge the capacitance of the winding

• Ensure that CT’s, VT’s, thermocouples and RTD’s are disconnected or shorted out

• Safety precautions are paramount

High Voltage AC Test Set

High Voltage AC Test

• Apply test voltage for 1 minute - the only aging that will occur is if there is excessive PD, but this will not take much off the life of the machine. DC has no aging effects.

• Leakage current must be monitored and trended.

• The winding must be properly grounded (through a resistor) after the test, and an IR repeated.

• Alternate tests - VLF (0.1Hz) or DC (recommended value 1.7 x AC value). DC places more stress on the overhangs.

High Voltage DC Test

• Major advantage – small test set!

• Plotting leakage current against voltage can give some useful information.

• Stresses slot section almost the same as AC.

• Doesn’t stress the overhang the same –stress grading not designed for DC (higher stress)!

• Can’t do PD or Tan Delta testing.

High Voltage DC Test

High Voltage VLF Test

• Major advantage – small test set, but produces a sinusoidal wave form

• Less stress on the overhang in stator testing than DC

• Stresses slot section the same as AC.

• Can see on the waveform where any breakdown occurs

• Can do PD and Tan Delta testing with specialized equipment

• Often used mainly for rotors

The decisions that need to be made when HV testing:

• Test or not?

• AC / DC / VLF?

• What voltage?

A properly conducted high voltage test:

1) Will not damage a winding that is not already severely deteriorated.

2) Will give good assurance against service failure for a stator winding that

is not experiencing aggressive local or general deterioration.

Partial Discharges

• Partial discharges are tiny sparks (a flow of electrons and ions) occurring

within or around a high voltage insulation system.

• Voltage buildup occurs across gaps or voids in the insulation system as the

voltage waveform builds up, and depending on the gap length or cavity

diameter, gas pressure and nature of the surfaces involved a breakdown

will occur resulting in the spark.

• Partial discharges are also produced in the overhang in air-cooled

machines in the form of corona (discharges to air adjacent to high voltage

conductors).

• PD has a very fast rise time and is generated at high frequencies.

• Bus couplers are 80pF capacitors that detect the high frequency signals and block signals of lower frequencies (specifically power frequency!).

• Lower frequencies (<100 MHz) see ‘deeper’ into the winding, but can be

affected by circuit parameters.

• Higher frequencies (>100 MHz) attenuate more rapidly, but are more

repeatable.

• PD is then grouped and plotted according to number of pulses,

magnitude and phase angle.

• The higher the magnitude the bigger the void that the spark is ‘jumping’

across

• The more pulses, the more voids

Partial Discharges

Linear Plot of PD

-400

-300

-200

-100

0

100

200

300

400

0 45 90 135 180 225 270 315 360

[ Ø° ]

Bipolar Machine PD

Magnitude Plot

Partial Discharge Testing

Interpretation

• No standardization

• Readings > 5000pC indicative of winding deterioration

• IEEE for new stator windings – max PD = 2000 pC

• Only reliable method of interpretation (opinion!) –

TRENDING!

TVA Test (RF probe or Peak Pulse Meter)

Technical background

• PD radiates radio frequency energy

• Uses a modified AM radio loop antenna (tuned coil) at the end of a well insulated rod

Test Setup

• Rotor removed – safety paramount!

Procedure

• Operator stands inside bore of machine, voltage is raised, and each slot is scanned

• Discharges induce a current in the coil proportional to the magnitude of the discharge, which is measured on a peak pulse meter

TVA Test (RF probe or Peak Pulse Meter)

Interpretation

• On a new class F epoxy mica based winding readings should be less than 5 mA

• For older machines, the limits depend on the insulation system

• Included in IEEE standard 1434 (PD testing)

Tan Delta (Power Factor) Testing

• Stator insulation system is a capacitor

• In a perfect capacitor, no losses, I and V exactly 90º out of phase, ie PF = 0

• In the real world, voids and losses in the resin affect the power factor, causing resistive (real) losses in phase with voltage

• Resultant current vector out of phase with ‘perfect’ current vector by angle -losses proportional to tan

• Measured with a Schering / Transformer Ratio bridge

I

V

Perfect Insulation

Ic

V

Ir

I

Real Insulation

Φ

Tan Delta Testing

• Practically the Cos Φ (power factor) = tan at very small angles

• In a very good capacitor, losses would increase linearly with voltage

• In a poor insulation system (poor capacitor) as voids start to discharge with increased voltage the losses increase, so no longer linear relationship

• The more voids, the greater the losses, hence the larger the angle

• Good insulation – power factors (tan deltas) of 0.2 to 1.5 %

• Deteriorating systems – PF/tan delta 5 – 10 %

• Tip up (deviation from linearity) a good indicator as well

Tan Delta Testing

Tan Delta Testing

Rotor Tests

RSO Recurrent Surge Generator/Oscilloscope (RSG/RSO) Testing

• Used to check for inter-turn insulation faults in rotor windings

• Fast rise time, low voltage pulses / surges (5 -10 V) injected into both ends of the winding

• Inter-turn faults, earth faults or breaks in the winding produce discontinuities in the surge impedance.

• These cause part of the pulse to reflect back to the injection point, changing the voltage waveform

RSO

Recurrent Surge Generator/Oscillograph (RSG/RSO) Testing

• The waveforms from both ends of the winding viewed simultaneously on an oscilloscope should theoretically be identical if no problems are present

• A problem shows as a difference in the two traces

RSO 220 Vac

Oscilloscope

Ground wire

Slip-rings

‘Good’ trace

Trace showing fault

Volt Drop Test

• DC or AC Voltage applied (DC more stable, but large current source required. AC more practical)

• Typical voltage 80 – 110 V (to get at least 2 – 5 V drop across each coil)

• Can measure across poles, coils or turns – 2% pole balance, 10% coils

• Looking for shorted turns - less current will flow and voltage will be less (little or no change turn-to-turn)

STATOR

TESTS

Stator Wedge Testing - Two Methods

Simple – done by tapping each wedge with a ball

peen hammer or spanner and listening (and feeling) for

its response.

• No recorded data –manual check sheet filled out.

• High level of experience required.

• Highly Subjective – two different operators can get

completely different results!

Tap Test:

Digital Wedge Tightness Detector

Digital Testing :

• Hand held probe with magnetic adjustable side units.

• Hammer strikes the wedge approximately 10 times

per second.

• Typical scan rate of approximately 3 seconds per wedge

• Accelerometer detects frequency response of wedge

after it is struck

WTD-501 Wedge Tightness Detector

Probe

System Configuration

ProbeSignal Processing Unit

(SPU)

Operation

Operator chooses

first slot/wedge.

Initiates test with

switch on probe.

Results displayed on

PC and stored.

Large PC display

allows remote

reading.

WTD Results

• Results may be displayed in colour coding up to 5 categories.

• Alternate calibrations may be used after test to re-qualify data.

• Multiple tests may be compared to establish trends.

• Results are repeatable and not subjective – two different operators

will get the same results

ELCID Testing Stator Cores

• ELectromagnetic Core Imperfection Detection: a technique used to assess the integrity of inter-laminar insulation for stator cores of large generators and motors

• Only requires a low voltage supply to excite the core to 4% of rated flux

• Use a Chattock Coil to look for axial currents (ie in quadrature to the main currents)

• These represent fault currents. A reference coil tells us the direction and magnitude of the main currents, and any current induced in the Chattock coil 90°out of phase to this is plotted on the computer against the distance from the core end.

• A relatively flat line indicates a healthy core - acceptance usually 100mA or less.

Fault Current

Positioning the Chattock

Adjustments for width (4) Adjustments for bore radius (2)

Screws for cable clamp plate (2)

Clamp screws for Chattock (2) Clamp nuts for side guidance rails (4)

Chattock

Test Trolley

Typical Turbogenerator EL CID Excitation System

Typical Hydrogenerator EL CID Excitation System

Electromagnetic Core Imperfection Detection System Configuration

Digital ELCID - Model 601

Digital ELCID – Evolution (Latest Model)

Scanning

End of slot N Start of Slot N+1

Sample Fault Indication (Turbo)

ELCID Signal and Thermal Response to Faults

Thermal

Imaging

Camera

EL CID

Trace

Trending

• When things are changing inside a machine, without an obvious reason for the change such as increased load or a very hot day, then there is cause for concern.

• And the only way to know if something is changing is to trend machine operating or test data.

Conclusions

• Generators – big, but relatively sensitive and fragile

• They contain and direct huge amounts of power, often sufficient to light up a town. When that power gets out of control within the generator, it can do severe damage in a very short space of time.

• Hence is it vital to know in advance when something is starting to go wrong -regular inspection and testing of the machine, routine maintenance, and good trending of all available information is vital.

Test and inspection on a generator is relatively inexpensive and short duration However, a full rewind or core restack, is extremely expensive and can take from a few months to over a year to complete, depending on the size of the machine and the availability of spares.

• A plant owner can save the cost of testing over the years by not doing the necessary maintenance and information gathering on his machine. But when the machine eventually fails, the cost of a major repair and outage will far outweigh the saved cost of testing.

CUSTOMER: REMACO

LOCATION : JOHOR, MALAYSIA

JOB NO. : P.1621 Aug 2014

Work Scope: Rewind coil No.1 pole A

and Pole B, replace broken and damaged leads.

PASIR GUDANG GT 6A - ROTOR

RATING: 143400KVA

GE FRAME 9 GAS TURBINE GENERATOR

Field Case: Repair of a GT Generator Rotor

Field Case: Repair of a GT Generator Rotor

The slot liners have some fretting damage caused by coil

movement. It was also noticed that there is considerable radial

clearance in the slot. This is apparent by looking at the

clearance between the coils and the bottom of the slot. The

calculated clearance is 4.5 mm.

Field Case: Repair of a GT Generator Rotor

The radial connection pins show evidence of movement and

wear between the pin insulation and the retaining rings in the

shaft.

The leads also show wear marks caused by looseness in the

bores and in the sealing plugs..

Field Case: Repair of a GT Generator Rotor

There was red iron oxide dust on the bottom of the radial

connection studs, in the stud holes and on the connection

leads. The up-shaft leads were removed and found to have

considerable clearance in the shaft.

Suspect water leakage from the stator.

HV Testing would have detected the problem.

The leads were re-installed with additional insulation to achieve

a tight fit in the shaft bores. The new radial connections studs

were shaped to add flexibility to the field leads.

Field Case: Repair of a GT Generator Rotor

A new flexible jumper connection was shaped and brazed to

coils 1 and 2 of pole A.

HV test was done to confirm the validity of the new insulation.

The hydrogen seal plugs were pressure tested to 4 bar for 4

hours.

Field Case: Repair of a GT Generator Rotor

Installing Collector End (Excitation End) Retaining Ring and

Heaters.

Set up rotor for installation of turbine end retaining ring.

Installing the turbine end retaining ring.

Final electrical tests include rotor winding tests, HV tests IR, PI,

RSO.

END