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Generator Failure Investigations

Dr Antony Anderson CEng FIEE/FIETOctober 6th 2011

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Turbine Generator System

A Main Turbine System

B Boiler Feedwater Pump System

C Condensing System

D Deaerating & Feedheating System

E Electrical Generation System

138 MW per

metre of active

length

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E Electrical Generation System

EA Generator EB Phase Isolated Busbars

EC Generator Neutral Earthing

ED Generator Output Measurement System

EE Generator Transformer

EG Generator Switchgear

EJ Seal Oil System

EK Hydrogen Supply & Purging System

EM Stator Winding and Cooling Water System

ES Generator Stator & Exciter Drains

EW Static Excitation System

EX Brushless Excitation System

EY AC Excitation System

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EA Generator

EAEA 10 Supports

EA 11 Bearings

EA12 Gas Enclosure System

EA 14 Wound Stator

EA 18 Wound Rotor

EA 20 Brushgear

EA 12 Gas Enclosure SystemEA 11

EA 20

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EA 14 Wound Stator

EA 14 10 10 Stator Frame

EA Generator

EA 14 12 Stator Winding & Supports

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Generator Output Power5 kW to 1200 MW + in 120 years (240,000 x increase)

Output = kBAN D2 L, where:

k constant

B Flux Density at Stator Winding

A Ampere Conductor Loading 300kA/metre

N Rotational Speed (3000-3600 RPM)

D2 Square of Stator Winding Diameter

L Active Length of Stator Iron

MW/metre of active

length138 kW/mm of active length

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Generator RotorLong thin cylinder

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When things go wrong

Rotor End Ring Failure &

resultant stator damage

Stator core fault in 660 MW Nuclear

Power Station600 kg molten metal

Centre of fault? Root causes? One off or type fault? Implications and for whom?

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Generator - Incipient fault damage

Incipient fault damage is easily missed during failure investigation

Interlaminar spot weld

Intermittent micro arcing

rotor winding

Meandering breakdown

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Typology of Generator faults

Rotor faults Multiple field winding short circuits

Stator faults End windings

Loose end winding cording

Coil-to-coil short circuits

Coil-to-earth faults

Active part of winding

Loose slot wedgesbouncing bars

Conductor-to-earth faults Presumed turn-to-turn short circuits

Core damage Location

Bearing Faults Shaft voltages cause bearing currents and damage

Magnetic unbalance, and many other possible factors

Excitation System: AVR/Control related Faults Exciter field freewheel diode failures

Internal AVR faults

Intermittent connection faults (Loss of field control) Protection Faults

Failure of protection results in excessive damage before trip

External System Faults Line clashing

Lightning

Negative sequence operation due to phase unbalance

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Generator is a dynamic system

Failure modes and effects must include

interactions between sub-systems

Rotor Faults

StatorFaults

BearingsFaults

ExcitationFaults

AVR/ContrFaults

ProtectionFaults

Bearings toexcitation

Stator tobearings

Rotor tostator

Excitation toAVR

Bearings toAVR

Bearings toProtection

Excitation toProtection

AVR toProtection

Stator toProtection

Stator toAVR

Stator toexcitation

Rotor toAVR

Rotor toExcitation

Rotor toBearings

AVR toExcitation

Protectionto AVR

Rotor toProtection

AVR toBearings

Excitationbearings

Protectionto Excitation

Protectionto Bearings

Bearings toStator

Excitation toStator

AVR toStator

Protectionto Stator

Bearings toRotor

Stator toRotor

Excitation toRotor

AVR toRotor

Protectionto Rotor

Main Fault Categories showing hypothetical sub-system causal interactions (15+15)

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How not to investigate!

Some act first and think afterwards

Stator winding of small machine cut with

wire-cutters and removed from core before

the failure investigators had arrived on the

scene. Information recovery minimal.

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Managing the Unknown

Requirements:

A disciplined, systematic approach

Open-mindedness on possible causes Imagination looking for the unexpected

Team build up/ training -

Limitation of Scope:

Cannot necessarily investigate every aspect of failure because of time and cost

restraints Adaptability of investigators

Investigation plans may have to change to accommodate changingcircumstances

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Project manage failure investigations

Project Objectives (main, subsidiary etc.)

Clear definition of objectives essential right at the beginning

Investigative Team and Responsibilities Who owns the project? Who will carry it out? What expertise needs to be brought in?

Work breakdown structure

Phases, Timescales, Resources, Deliverables

Data gathering and analysis Photographs, samples, statistical analysis etc.

Experimental work

Archiving

Investigative Results

Reports & Presentations

Clear presentation of the evidence, hypotheses etc.

Determination of causes of failure (if possible)

Remaining unknowns

Preventive measures/recommendations (short, long term)

Lessons learnt

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Carrying out the investigation

Preliminary investigation Establish circumstances of incident and prior history

Have any similar incidents occurred?

Visual, non-intrusive inspection Allows extent of damage to be assessed Enables likely scope of full investigation to be established

Preliminary Report outlines any future investigation necessary Full investigation

Intrusive evidence will be destroyed in examination process

May involve additional work by outside specialists to demonstratepotential failure mechanisms

May involve experimental rig work to test a hypothesis

Main Report Presentations

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Team must agree upon: How to describe location of damage within machine

The kinds of charts and diagrams to be prepared

Prepare masters

The approach to taking photographs of particular damage (general,

local area, close up)

Ensure that close up photographs are taken with sufficient detail usingtripod, small apertures, long exposures

The photographic sequences to be taken for records

Storyboard

Samples methods of preservation, labelling etc.

Sample bags, gloves

Video sequences if any

Extra equipment hire

Boroscope? Q meter? Other test equipment?

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Laying out damaged material

Use of a sand bed to display core fault damage in a

500 MW machine

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Organising damaged material

Patterns of stator slot damage become

immediately obvious

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Display results of statistical

analysis

Heights of red rods indicate cumulative incidence

of breakdowns in stator vent ducts

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Interlaminar damage SEM investigation

It maybe necessary to engage a

specialist laboratory to examine

samples of damage

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Adopt a Physics of failure approachTry to understand failure mechanisms

Demonstration of physical principle Mechanically-induced EMI can cause

interlaminar breakdowns in stator cores and insulation breakdowns in rotor windings

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Physics of failure approachTransient leakage flux from an electric drill

Transient field on start up attractshanging piece of steel coreplate

Transient field moves electrically

driven watch on by 10 seconds

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Back of Core Leakage Flux

Under sudden short circuit leakage flux collapses at centre of

machine and rises at ends of machine

Rotating fluxpattern

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Concluding remarks

Think about failure investigation methodology before

failures occur rather than on the hoof

Build a generalised framework for knowledge

gathering that can be used for all investigations Train investigation teams Build up expertise rather

than leaving matters to chance

Capture the knowledge of staff before they retire sothat it is available in the future

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Project Completion: Colombian style!