Condition Monitoring of Rotating MachinesROHIT KAUSHIK

SIBABRATA PRADHAN

SAMRAT ROY

SHYAM KR. SINGH

EE 629 High Voltage and Insulation Engineering

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INTRODUCTION

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Introduction

By condition monitoring we mean continuous evaluation of the health of plant and equipment throughout its serviceable life.

Condition monitoring and protection are closely related functions. The approach to the implementation of each is, however, quite different.

Condition monitoring can, in many cases, be extended to provide primary protection, but its real function must always be to attempt to recognise the development of faults at an early stage.

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WHAT AND WHEN TO MONITOR

Larger electrical drives, which support generating, process or production plant if a high margin of spare capacity exists, will benefit from monitoring, although perhaps not continuous monitoring.

We can include induced and forced-draught boiler fan drives, boiler water feed pump drives, and cooling water pump drives in power stations in this category.

It must be kept in mind, however, that successful monitoring can allow a big reduction in the requirement for on-site spare capacity.

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WHAT AND WHEN TO MONITOR

The parameters to be monitored are essentially those that will provide the operator and maintainer with sufficient details to make informed decisions on operation and maintenance scheduling, but which ensure security of plant operation.

Traditionally quantities, such as line currents and voltages, coolant temperatures, and bearing vibration levels, have been measured and will continue to be used.

Other specialist methods, involving the accurate measurement of rotational speed, or the sensing of leakage fluxes, are being developed in order to monitor a variety of fault conditions.

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WHEN TO MONITOR

One should monitor when it is cost-effective to do so, or when there are over-riding safety considerations to be observed. The assessment of cost-effectiveness can be a relatively complex matter, but in general terms monitoring is worthwhile when the net annual savings are increased by its use.

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FAILURE SEQUENCE AND EFFECT ON MONITORING

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ROOT CAUSES AND FAILURE MODES

Root causes: Defective design or manufacture Defective material or componentDefective installationDefective maintenance or operationAmbient conditionsOverspeedOverloadComponent failureExcessive temperatureWinding over temperatureBearing over temperature.

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FAILURE MODES

Failure modes:

oElectrical:Core insulation failureStator winding insulation failureRotor winding insulation failureBrush gear failureSlip ring failureCommutator failureElectrical trip

◦ Mechanical:Bearing failureRotor mechanical integrity failureStator mechanical integrity failure.

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Stator winding insulation

Stator winding insulation is affected by all of the stresses: thermal, electrical, environmental and mechanical; however, the extent to which these stresses in normal operation will cause problems in the short- or long-term will depend on factors such as the operating mode and type of ambient cooling conditions.

Deterioration may be like delamination and voids, slot discharge etc.

The principal stresses of concern on rotor windings are thermal and mechanical. It may be induction motor rotor faults, turbine generator rotor winding faults, rotor winding faults in dc machine.

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Instrumentation Requirements

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Temperature Measurement

RTD

(Resistance Temperature Detection) Thermocouple

RTD used in Wheatstone bridge Configuration:

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Thermocouple Types

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Vibration Measurement Used for monitoring of components of Gear boxes, Shaft couplings Bearing etc.

Methods: Displacement Transducer (< 100Hz) Velocity Transducer (100-1000Hz) Acceleration Transducer (> 1000 Hz)

Displacement Transducer:

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Velocity Transducer

E = Blv

where,ov is the velocity of magnet in axial

directionoE is induced emf which is the measure

of vibration.

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Acceleration Transducer

When subjected to vibration mass held against piezoelectric material exerts a force upon it which is proportional to the acceleration which produces electric voltage proportionally.

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Electrical & Magnetic measurement

Electrical Quantities like (currents , voltages) are measured by CT s, PT s .

Magnetic Quantities like (flux) are measured by Hall Sensors.

When current flows through the hall sensor in direction perpendicular to applied magnetic field then direction of motion of electron is in direction mutually perpendicular to both axes.

V=KIB/nq.

where, K/nq is hall constant of material.

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Chemical Monitoring The insulating materials used in electrical machines are complex organic materials when degraded by heat or electrical action, produce a very large number of chemical products in the gas, liquid and solid states.

Methods Of Chemical Monitoring:

Particulate Detection (Using Core Monitor)

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Chemical Monitoring Infrared Ananlysis A beam of light is focused through a film of used oil and the wavelengths are then compared to light transmitted through new oil of the same type. The differences in readings provide information with respect to the degradation of the used oil

Image Processing The image processing and computer vision system reveals more information in the form of quantitative data not revealed by the human eye.

This technique is used to collect quantitative information from wear particle images

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Vibration & Electrical monitoring

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Vibration monitoring

The principal sources of vibration in electrical machines are: The response of the stator core to the attractive force developed magnetically between

rotor and stator The dynamic behavior of the rotor in the bearings as the machines rotatesThe response of the shaft bearings, supported by the machine structure and

foundations, to vibration transmitted from the rotorThe response of the stator end windings to the electromagnetic forces on the

conductors.

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Frequency responses of the machine elements

Stator responseThe forces acting on the stator core are the result of the interaction between the air gap flux wave and the currents flowing in the windings embedded in the stator slots.

The forces acting on the end winding are due to the interaction between the end leakage flux and the winding currents. It is apparent, therefore, that the precise nature of the applied force waves will be a function of the form of the current distribution, and the geometry of the air gap and end regions.

The simplest method of calculating the flux wave form is to multiply the magnetomotive force (MMF) distribution because of winding currents, by the permeance of the air gap.

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Frequency responses of the machine elements

Transverse forces are due to asymmetries in the machine, while torsion is primarily due to the driving torque; however, both may be affected by electrical or mechanical faults in the machine itself or electrical or mechanical system disturbances outside the machine.

There will also be a coupling between torsional and transverse effects due to the transfer function or stiffness between these axes of the machine, so torsional effects, like current faults in rotor and stator windings, can cause transverse effects like vibrations, and vice versa.

Rotor responseWe now consider the motion of a rotor in response to:

Transverse force excitation

Torsional torque excitation

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Frequency responses of the machine elements

Bearing response

Rotor vibration force is transmitted to the stator via the air gap magnetic field and the bearings in parallel. It is therefore important to consider the response of the bearings to that vibration force so that its effect is not confused with vibrations generated by faults within the bearings themselves.

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Monitoring techniquesOverall level monitoring

This simple form of monitoring is the most commonly used technique but its efficiency is limited. The measurement taken is simply the rms value of the vibration level on the stator side of the machine over a selected bandwidth. The usual bandwidth is 0.01–1 kHz or 0.01–10 kHz

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Monitoring techniques

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Monitoring techniques Shock pulse monitoring

The shock pulse method is used exclusively for rolling element bearings ,which deteriorate at the moving surfaces, developing small pits or imperfections.

The interaction between such surfaces generates mechanical stress wave or shock pulses, in the bearing material, propagating into the structure of the machine.

These shock pulses are at ultrasonic frequencies and can be detected by piezoelectric transducers with a resonant frequency characteristic tuned to the expected frequency of the pulses, around 32 kHz. The condition of the bearing is assessed by defining a quantity known as the shock pulse value (SPV), defined as

SPV=R/N²*F².

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Monitoring techniquesShock pulse interpretation

Overall vibration level trend Shock pulse value trend Comments

Low and rising Remains low No bearing damage

Low and rising Low but rising at the same rate as the overall vibration level

Bearing damage likely

Low and rising High value but constant Damaged bearing but another

problem is causing the rising vibration

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Current & Flux monitoring

Faults on either rotor or stator disrupt the radial and circumferential patterns of flux in the machine causing changes to the power being fed to the machine, which can be detected via its terminal quantities voltage, current and power measured outside the machine to give an indication of its condition.

A) Generator stator winding fault detection- The most significant technique in this area is on-line discharge detection, which is dealt in further slides.

B) Generator rotor faults detection-Turn-to-turn faults in a generator rotor winding may lead tolocal overheating and eventually to rotor earth faults. Inaddition, the shorting of turns causes unequal heating of therotor leading to bending and an unbalanced pull, whichtogether cause increased vibration

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Current & Flux monitoring A way of detecting them on-line ,is using a stationary search coil fitted in the air gap of the machine. The search coil, of diameter less than the tooth-width of the rotor, is fixed to the stator usually in the air gap, and detects either the radial or circumferential component of magnetic flux.

New techniques have been developed utilising a digital storage oscilloscope connected to the search coil to give an initial indication of the development of an inter-turn fault. The purpose is to identify any asymmetry in the MMF waveform caused by shorted turns.

Photographs of typical search coil installation in large

generators

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Motor rotor faults detection

Stator current monitoring for rotor faults

Any rotor fault in an induction motor will cause a characteristic swing in the supply ammeter reading, Careful measurement of the stator current will therefore enable such a fault to be monitored. Detecting side bands in the supply current of

an induction motor

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Generator and motor comprehensive methods

Shaft flux-

Shaft flux, or axial leakage flux, occurs in all electrical machines. It is produced because no machine can be constructed with perfect symmetry.

Faults, such as winding short circuits, voltage imbalance and broken rotor bars, represent severe disruptions to the internal symmetry of the machine. It is logical to conclude, therefore, that the effect on the production of axial flux will be readily observable.

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Generator and motor comprehensive methods

Shaft voltage or current-

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Current spectrum with typical fault sidebands

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Temperature Monitoring

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Temperature Monitoring The limits to rating of electrical machines are generally set by the maximum permissible temperature that the insulation can withstand.

There are three basic approaches to temperature monitoring.• To measure local temperatures at points in the machine using embedded temperature detectors.• To use a thermal image, fed with suitable variables, to monitor the temperature of what is

perceived to be the hottest spot in the machine.• To measure the bulk temperatures of coolant fluids.

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Local temperature measurement This can be done using thermocouples, resistance temperature detectors or embedded temperature detectors.

To monitor the active part of the machine they are usually embedded in:• The stator winding and in the stator core.

• temperature detectors embedded in the stator winding need to be located close to its hottest part, which may be in the slot portion or end-winding portion.

• The bearings to detect hot running.

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Local temperature measurementLocation of temperature detectors in electrical machines

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Local temperature measurement On a winding the devices have to be embedded in the insulation at some distance from the copper itself.

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Local temperature measurement As a result, the measured temperature will not necessarily be that of the winding itself but an image of it.

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Local temperature measurement The heat flow per unit area, Q, through the insulation system can be described by simple conduction equations as follows

𝑄=h (𝑇 𝑠−𝑇𝑔 )= 𝑘𝑡 2

(𝑇 𝑠−𝑇 𝑡 )=𝑘𝑡1

(𝑇𝑐 −𝑇 𝑡 )

𝐸𝑙𝑖𝑚𝑖𝑛𝑎𝑡𝑖𝑛𝑔𝑇 𝑠

𝑇 𝑡=𝑇 𝑔+𝑄( 𝑡 2

𝑘+ 1

h )

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Local temperature measurement

𝑇 𝑡=𝑇 𝑔+(𝑇 𝑒−𝑇 𝑡 )( 𝑡 2+(𝑘 /h)𝑡 1

)

𝑇 1=

𝑇𝑔+𝑇 𝑐{𝑡 2+(𝑘/h)𝑡 1

}1+{𝑡2+(𝑘/h)

𝑡1}

𝑠𝑜,𝑇 1 𝑇 𝑐 𝑖𝑓 𝑇 𝑔≪𝑇 𝑐∧𝑡 2+(𝑘h )

𝑡 1

≫1

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Local temperature measurement So the measured temperature Tt will approach the temperature of the hottest active component Tc if the thickness of insulation, t2 , applied over the ETD is sufficient compared to the main insulation.

This problem does not occur for devices embedded in the slot portion between two conductors, where there is a low heat flux between the active copper parts.

𝑇 1 𝑇 𝑐 𝑖𝑓 𝑇 𝑔≪𝑇 𝑐∧𝑡 2+(𝑘h )

𝑡 1

≫1

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Hot-spot measurement and thermal imagesThe thermal image technique has not received wide application on rotating elec-trical machines.

The thermal image consists of a dial-type thermometer with its bulb immersed in the region where the transformer oil is hottest.

A small heating coil, connected to the secondary of a current transformer, serves to circulate around the bulb a current proportional to the load current and is such that it increases the bulb temperature by an amount equal to the greatest winding-to-coil temperature gradient.

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Hot-spot measurement and thermal imagesComparison between measurements and the predictions of a thermal image of an electrical machine.

(a) Comparison for 5.5 kw induction motor.

(b) Duty cycle for (a)

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Hot-spot measurement and thermal imagesComparison between measurements and the predictions of a thermal image of an electrical machine.

(a) Comparison for 7.5 kw induction motor.

(b) Duty cycle for (a)

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Bulk measurementThis can be found from the measurement of the internal and external coolant temperature rises, obtained from thermocouples located.

This is done in most large machines

An increase in temperature rise would clearly show: when a machine is being overloaded. the coolant circuits are not performing as they should.

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ConclusionTemperature measurement can yield very valuable bulk indications of the condition of an electrical machine using simple sensors narrow bandwidth (<1 Hz) low-data-rate signals

Temperature rises are important rather than absolute temperature.

There are advances in the application of modern sensors, which will allow temp-erature measurements to be made closer to the active parts of a machine.

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References: [1]. P.TAVNER, L.RAN, J.PENMAN , H.SEDDING, ‘Condition Monitoring of Rotating Electrical Machines’ 2008 – Book

[2]. TAVNER .P.J ‘ Review of condition monitoring of rotating electrical machines’, IET Electric Power Applications, November 2007.

[3]. A. NEGOITA ,Gh. SCUTARU, R.M. IONESCU ‘A brief Review of Monitoring of Rotating Electrical Machines ‘. Bulletin of the Transilvania University of Brasov • Vol. 3 (52) – 2010 Series I: Engineering Sciences.

[4]. NANDI S., TOLIYAT H.A., LI X.: ‘Condition monitoring and fault diagnosis of electrical motors – a review’, IEEE Trans. Energy Convers., 2005, 20, (4), pp. 719–729.

[5]. TAVNER P.J., GAYDON B.G., WARD D.M.: ‘Monitoring generators and large motors’, IEE Proc. B, Electr. Power Appl., 1986, 133, (3), pp. 169–180.