Shorted Rotor Winding Turns Detection in Salient Pole

Rotor Machines

Outline

• Review salient pole rotor winding design• Insulation deterioration processes • Off-line and On-line testing and monitoring• Case studies

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Off-line vs On-LineOff-line• Machine out of

service• Expensive• No load• Cold winding• No vibration• Testing voltage

source required

On-line• Machine in operation• Inexpensive, saves

time• Normal operation

voltage, temperature and vibrations

• Does not require test voltage source

• Centrifugal force

Synchronous Machines

• Rotor windings provide magnetic field• Windings provide ampere turns to develop

rated generator output within limits of voltage and temperature

• Electrical requirements of insulation minimal• Severe mechanical and thermal requirements• Rotor diameter up to 20 m, weight in hundreds

of tons

Cross section of 2 pole generator

S

N

Cross section of 4 pole generator

SSN

N

Cross section of 12 pole rotor

N

N

N

N

N

N

S

S

S

S

S S

Salient Pole Synchronous Motor Rotor

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Hydro-generator Rotor

Turbo generator salient pole

Typical Laminated Rotor Pole Assembly

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Rotor Pole

Turn voltages

• Large turbo: 5 to 15 volts per turn• 21,200 kVA 3600 rpm generator: 0.52 volts

per turn• 63000kVA 100 rpm generator 0.16 volts per

turn• 800 HP 1200 rpm motor: 0.098 volts per turn• Turn insulation can be subjected to high

transient voltages

Salient Pole Rotor Windings

• High speed machines are 4-8 poles• Up to 100 poles on large slow hydros• Synchronous motors (salient pole rotors)

have lower starting currents than squirrel cage induction motors

• Three types of winding: wire wound, edge wound and picture frame construction

• Voltages: 125-400 V DC

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Winding Insulation

• Typical materials:- pole piece insulation, wrapped or molded:

nomex, fiberglass/resin laminate- Pole tips and rotor body: collars from high

strength glass/resin, slip planes on long poles

- Pre-wound coils potted after fitting on pole pieces

Multi-Layer Wire Wound Pole

• Mainly for smaller high speed and larger slow speed motors

• Use rectangular magnet wire to create the turns which are wound on to pole

• Turn insulation is usually a polyamide-imide film

• Ground insulation used to separate the turns from the rotor body

• Entire pole dipped in or VPI’d with resin

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Multi-Layer Wire Wound Pole

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“Strip-on-Edge” Pole

• For hydro generators and larger motors• Pole winding made from a continuous strip of

copper formed into a “slinky” or use rectangular “picture frames” with brazed connections

• Use strips of NomexTM or epoxy-glass laminates to separate the turns – resin bonded to conductors

• Tape used at the top and bottom few turns to increase creepage distances to pole body

• Sometimes impregnate entire pole with resin, or each pole winding is hot pressed to consolidate it.

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“Strip-on-Edge” Pole

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Rotor Winding – edge wound

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Rotor Winding – picture frame

Rotor Winding Insulation Aging Processes

• Thermal• Electrical• Ambient• MechanicalDeterioration processes normally take many years (or even decades)

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Thermal Aging

• Overloading or high cooling air temperatures• Inadequate cooling from poor design,

manufacture or poor maintenance• Use of materials having inadequate thermal

rating• Over-excitation of rotor winding• Shrinkage of bracing materials, looseness• Can result in both turn and ground insulation

failures

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Thermal Cycling

• Frequent starts and stops or wide variations in load causing:

Relative movement between winding components

Cracking at interfaces due to differential expansion

Distortion of the winding conductors

• Can cause both turn and ground insulation failures

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Repetitive Voltage Surges

• High transient voltages can be induced on the rotor winding from a static exciter or system surges

• Can cause turn to turn insulation faults

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Contamination

• Can be in the form of:Moisture in cooling airOil from bearingsConducting dust in atmosphereChemicals in atmosphereA combination of these

• Both turn and ground insulation failures from tracking as well as winding overheating

• Chemicals can degrade winding insulation to cause failure

Strip on Edge Shorted Turns Due to Contamination

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Abrasive Particles

• Abrasive particles such as coal dust, sand, iron ore (grinding) can enter the cooling air flow

• Particles impinge on the rotor winding they wear away the insulation

• Can cause both turn and ground faults

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Centrifugal Forces• Mechanical failure of the insulation can result

from:

High continuous or cycling centrifugal forces during operation and starts/stops

• These forces can cause rotor winding failures from;

Inadequate intercoil bracing or shrinkage of bracing materials in salient pole and round rotor windings

Shorted Turn from MultilayerWire Wound Pole

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Detection of Shorted Turns

Off-line• AC pole drop test, IR• RSO testOn-line• Increased bearing vibration• Increased excitation to achieve same MVA• Magnetic airgap flux monitoring

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Relay Protection

• If fitted, failure of the ground insulation is normally detected by protective relaying that gives an alarm or trips the machine

• Failure of the turn insulation in one or more spots does not cause a trip – but may lead to increased bearing vibration (at once per revolution frequency) and limit the output

• An increasing number of turn shorts over time may indicate a higher risk of ground failure

• Insulation may fail suddenly or gradually due to aging

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Pole Drop Test

• Apply 120 Vac across field winding and measure voltage drop across each pole

• Measure the 60 Hz inductive impedance of each pole to detect poles with fewer active turns, and thus lower inductance

• Poles with lower than average voltage drop may have shorted turns

• Shorts may disappear when rotor not spinning, and vice versa

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RSO Test

• Recurrent Surge Oscillation• Low voltage (few volts), high frequency

(kHz range) surge injected into both ends of the rotor winding

• Time domain reflectometry based principle• Identical response indicates no shorted

turns

Magnetic Flux Monitoring to Detect Shorted Turns

• On-line method• Well established to detect shorted turns in

round rotor field windings• TF probe installed on a stator core tooth to

measure the induced voltage from the magnetic flux as each pole passes the probe

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Total Flux ™ Probe Installation

Kit connection

Personal Computer

RFA II/Flux Trac II/Guard

Power supply 100-240 VAC

TF probe installed on a stator tooth

Flux Probe

Termination Box

Key Phasor

Monitoring shaft rotation

Iris RFA II

InstallationFlux MonitorTF Probe Termination

Box

Sync Sensor

TF Probe

TF Probe DetailMachine

Local USB port for configuration

Installation

Connection One Machine

Up to 4 Machines (Option)

150 m max.

Remote Communication• RJ45 port• Ethernet (LAN)• Off-the-shelf

converters allowed

• RotorFluxPro SW included– Memory data

downloading– Data displaying

LAN

RFA Pro Software

Signal Analysis

• High resolution magnitude and time A/D conversion

• Measure flux from each pole• Algorithms developed to reduce the

influence of rotor or stator asymmetry, air gap variations, etc

• If have a sensor to detect a specific location on the shaft (“key phasor”), then can identify the pole number

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-Compare pole to average of all poles-Compare pole to its left and right neighbor-Compare pole to poles of same polarity

Three algorithms

Salient pole flux signal

Change of Flux Pattern with Load change

Compare to average

Compare to adjacent

Use of two algorithms 1Compare to average Compare to adjacent

Air gap comparison

Use of two algorithms 2Compare to average Compare to adjacent

Shorted pole graph

Shorted pole identification

Non-circular rotor shape

Square rotor!

Four Pole Motor Result

Conclusions

-In general, disagreements between pole drop test and on-line flux tests are possible due to lack of centrifugal forces in pole drop test

- Rotor winding insulation is very reliableHowever it does age due to thermal, mechanical and contamination stresses

-Shorted turns can cause high bearing vibration and may limit reactive power output