testing of turbogenerator

5/28/2018 Testing of Turbogenerator

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TESTING OF TURBO GENERATOR

Dept. of EEE 1 AITS- HYD

CHAPTER-1

INTRODUCTION

1.1. INTRODUCTION

A Generator is a rotating Electromagnetic device producing electrical power taking

mechanical input from prime mover (Gas Turbine / Steam Turbine) and magnetic energy

from excitation.

Generator Design will be conforming to International Standards like IEC & National

standards like BS, VDE, IS etc.

Generators driven by steam or gas turbines have cylindrical/ round rotors with slots

into which distributed field windings are placed. These round rotor generators are usually

referred to as turbo generators and they usually have 2 or 4 poles. Generators driven by

hydraulic turbines have laminated salient pole rotors with concentrated field winding and a

large number of poles.

Testing is the most important process to be done on a machine after it is designed.

The testing of machine is necessary primarily to establish that the machine performance

complies with customer specifications. Tests ensure that the piece of equipment concerned is

suitable for and capable for performing duty for which it is intended.

Testing has to be done on a machine at every step in its manufacturing process for the

company to certify it to be a deliverable good. Test brings out the impact of process

variations. Testing is done in simulations which tend to closely resemble the practical

scenario under which the machine works.

Testing provides the experimental data like the efficiency, losses, characteristics,

temperature limits etc. for the use of design office, both as confirmation of design forecast

and also as basic information for the production of future designs.

1.2 NECESSITY OF TESTING:

To ensure that all functional requirements are fulfilled, and to estimate the

performance of generator, the turbo generators are required to undergo some tests. For

testing, the turbo generator was mechanically coupled to a drive motor-motor generator set


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Dept. of EEE 2 AITS - HYD

with gearbox. The rotor was excited by thyristor converter system located in an independent

test room and the operation was controlled from the test gallery.

The following first two tests will be conducted on the stator and rotor before assembling

and the third and final routine tests will be conducted after assembling the turbo generator.

a. Tests conducted on Statorb. Tests conducted on Rotor

1.3. OBJECTIVE OF TESTING:

Testing is the most important process to be conducted on a machine after it is

designed. The testing of machine is necessary primarily to establish that the machine

performance complies with the customer specifications. Tests ensure that the piece of

equipment concerned is suitable for and capable for performing duty for which it is intended.

Testing is done under condition simulating closely as possible to those, which will apply

when the set is finally installed with a view to demonstrate to purchasers representative its

satisfactory operation.

Test provides the experimental data like efficiency, losses, characteristics,

temperature limits, etc. for the use of design office, both as confirmation of design forecast


With ever increasing rating of the modern turbo generators and reliability of service

expected, testing at manufacturers works has become of paramount importance. The

machine performance is evaluated from the results of the equivalent tests.

Advantages of testing

1. Provides data for optimization of design2.

Provides quality assurance

3. Meets the requirement of legal and contract requirements.4. Reduction in rework cost.5. Ensures process capability and develops checklist.6. Increases confidence levels in manufacture.7. Establishes control over raw materials.8. Helps in building of safety and general operation and manual.


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1.4. THEME

Testing is an activity, which basically evaluates a component, and or a product (built

up of component assemblies) as to whether it has the technical capability that has been built

into it by way of design, materials, and technological processes employed while

manufacturing and workmanship.

As such, testing activities can broadly be classified in to a number of categories as follows:

a. Type tests.b. Routine tests.c. Process tests

The characteristics of testing:

1) Provides quality assurance.2) Meets the requirements of legal & contract requirements.3) Ensures process capability & develops checklist.4) Have an approved procedure.5) Check the equipment before use.6) Calibrate the test equipment & instruments.7) Ensure interlocks of the equipment

1.5.ORGANISATION

The definition and objective of the project as well as the design of the project which is

followed by the implementation and testing phases is studied in detail. Finally the project has

been concluded successfully and also the future enhancements of the project were shown.The organization of the project is as follows by

1. Introduction2. Literature survey3. System development4. Analysis of a turbo generator is studied and the precise results are shown in order to

ensure that the turbo generator chosen is deliverable good.


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CHAPTER -2

LITERATURE SURVEY

2.1. INTRODUCTION

Testing is the most important process to be conducted on a machine after it is

designed. The testing of machine is necessary primarily to establish that the machine

performance complies with the customer specifications. Tests ensure that the piece of

equipment concerned is suitable for and capable for performing duty for which it is intended.

Testing is done under condition simulating closely as possible to those, which will

apply when the set is finally installed with a view to demonstrate to purchasersrepresentative its satisfactory operation.

Test provides the experimental data like efficiency, losses, characteristics,

temperature limits, etc. for the use of design office, both as confirmation of design forecast


With ever increasing rating of the modern turbo generators and reliability of service

expected, testing at manufacturers works has become of paramount importance. The

machine performance is evaluated from the results of the equivalent tests.

Advantages:a. Provides data for optimization of design

b. Provides quality assurancec. Meets the requirement of legal and contract requirements.d. Reduction in rework cost.e. Ensures process capability and develops checklist.f. Increases confidence levels in manufacture.g. Establishes control over raw materials.h. Helps in building of safety and general operation and manual.

2.2. EXISTING SYSTEM

The existing system of a turbo generator and their inspection is as shown follows


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2.2.1. Generator Testing & Inspection Service

Generally generator testing and inspection services will be done for all types of fossil and

nuclear generators working with nearly all types of equipment and can offer complete and

accurate testing services. They will inspect and test all parts of your equipment during your

generator testing service. This thorough inspection will allow to generate accurate reports and

make complete recommendations in order to keep your equipment working properly and at

maximum efficiency.

Generator testing and inspection services are available for all types of turbine generators

including:

a) Fossil Steam Turbine Generatorsb) Nuclear Steam Turbine Generatorsc) Gas Turbine Generatorsd) Industrial Turbine Generator

2.2.2. About Generator Testing Services:

Generally experts offer generator testing services for all types, sizes, and brands of

equipment and worked with a variety of customers and are familiar with nearly any type of

generator including fossil steam turbine generators and nuclear steam turbine generator and

have an complete selection of test equipment available for generator testing including

equipment for routine low-voltage generator testing. Routine low-voltage testing services

include:

1. RTD resistance testing with temperature conversion and 500 volt megger2. 500 volt megger of the field with Polarization Index (P.I.)3. Impedance testing of the field4. Copper resistance of the field with temperature conversion to factory test temperature5. Copper resistance of stator 3 phases converted to factory test temperature6. Megger of stator 3 phases with P.I. up to 5000 volts7. Visual inspection of all accessible areas8. Comprehensive report including photos, recommendations and data sheets


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We also offer a variety of optional tests as well. These optional generator testing

services can be completed as required to meet your individual needs. Our optional tests

include:

A. Pressure / Vacuum TestingB. El Cid TestingC. DC Leakage and Hi Pot TestingD. Capacitance Mapping

2.2.3. The Importance of Generator Testing: Keeping your Systems Working

Both the electrical tests and the visual inspections, which are included in our generator

testing services, are important for ensuring proper generator performance. These generator

testing and inspection services will allow us to generate accurate recommendations, which

can be used when planning and scheduling for outages and turbine or generator repairs. It will

also help you evaluate the condition of your equipment in order to determine if replacement

or modernization projects are necessary.

The main goal of our generator testing and turbine generator service is to optimize your

equipment so that it will run reliably and efficiently. We are familiar with all types of

equipment and understand the intricate details of the inside of your machine. This allows us

to provide thorough service to help you achieve the best results.

i. Complete Turbine Generator Testing & Inspection ServicesWe can inspect all parts and aspects of your equipment when performing our turbine

generator testing and inspection services. This includes performing testing for generators,

exciters, and other related equipment. These complete turbine and generator testing services

will ensure that your entire system is working properly and efficiently.

2.2.4. Turbo Generator Testing Procedure and Manufacturing Process:

A sequential approach is followed here in implementing turbo generator assembly.

Here the process of manufacturing closed circuit air cooled turbo generator is explained, the

implementation is carried out in, Preparing a design layout manufacturing parts of stator

section like stator frame, stator core, stator windings, and end covers providing insulation


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for stator elements assembly of rotor with rotor windings and rotor retaining rings

positioning the shaft based on equivalent weigh concept desired tests are performed under

manufacturing time and after assembly.

The stator assembly involves preparation of laminations compounding operation

blanking and notching operationsvarnishingdebuggingcore assemblyslot discharges

stator windings assembly tapping stator end covers fixing resistance temperature

sensors phase connections bottom bar laying top bar laying connected rings

insulation.

The insulation for lamination is carried out in Vacuum pressure impregnation

The rotor assembly carried out by placing the rotor shaftrotor windingsrotor slot wedges

end winding bracingrotor retaining ringsrotor fan assemblyfixing bearingsbearing

insulationLubrication system- SkewingScavenging.

Ventilation for the turbo generator is basically three types :

Radial ventilation system, Axial Ventilation system, and multiple inlet Ventilation

system.

2.2.5. Recent technologies implemented at BHEL:

Vacuum press impregnated moralistic high voltage insulation, polyester fleece tape

impregnation for outer corona protection are two latest technologies implementing in

insulation section to provide high quality insulation for turbines with high standards and life

time.

2.3. PROPOSED SYSTEM

2.3.1. TESTING METHODS

1. EL CIDTo detect failures between laminations of stator cores

2. RSO TestingTo test both the turn and ground insulations of generator rotors


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3. IMCA(Induction Motor Current Analyzer)To test for cracked rotor bars while machines are running in service

4. Insulation ResistanceTesting of all electrical equipment before any high voltage testing is commenced

5. Partial Discharge AnalyzerTo test the condition of a machines insulation by measuring the levels of partial

discharges at operating voltages

6. Tan Delta TestingMain cell wall insulation of all coils above 4.0V AC are tested using an inductively

coupled capacitive bridge to measure tan delta

7. TVA Probe TestingTo locate areas of localized discharge within the stator slots of high voltage stators

8. Underwater TestingIt can be used after the VPI (Vacuum Pressure Impregnation) Process before being

returned to site.

2.3.2. SCOPE OF WORK:

The following are the broad scope of work (detailed scope of work enclosed), but not

limited to:-

1. Decoupling, Opening of end covers and pulling out the rotor from the position andplacing, it on the proper stand

2. Cleaning of stator winding portion, slots over hang portion, etc., using the appropriateCleaning, Agent-

3. Replacement of damaged wedges4. Cleaning of rotor portioned.5. Inspection of bearings and measuring bearing clearances.

The above tests have to be conducted before assembling the machine. Necessary epoxy

spray coating is to be applied, wherever required. CPCL scope is limited only to

disconnection of all cables connected to the machine. All consumables, special tools and

tackles required for the above jobs is to be brought by the contractor.


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The following tests have to be conducted:-

Stator:-

1. Stator core - ELCID Test or Flux, Loop Test

2. Stator windings - IR & PI

- DC hipot step voltage

- Partial discharge

- Capacitance

- Winding DC resistance

Rotor:

1. Rotor winding - IR

- Winding DC resistance

- Impedance

- RSO (earth fault /inter- turn short)

Detailed scope of work-.

Apart from the broad scope of work as detailed above, the following needs to be carried out:

I. Generator Rotor Removal

i. De-coupling the Generator / Turbine and Pilot exciter, Removal of Pilot exciter from the

bed. Remove slip-ring brush holder assembly measure and record diameter of both positive

and negative rings, check for any abnormal wear / pitting on the surface.

ii. Replace the shaft seal at outer covers.

iii. Disconnect and tag the slip ring terminals.

iv. Measure air gap between the stator and rotor at 4 points diametrically opposite at right

angle. This should be done for both turbine and exciter end.

v. Open bearing cover check for clearances and abnormality, if any, on the bearing surface.


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vi. Decouple the generator and record alignment readings.

vii. Remove bearings after ensuring that stator is not jammed by threading out of rotor by

inserting packing material (such as leatheroid, etc).

viii. Remove and place the rotor on the stand specialty provided for.

ix. Check the rotor for any sign of overheating, mechanical abrasion, loose wedges, etc., and

clean it with compressed air and cloth.

x. Check the rotor end rings for any damage or check by ultrasonic inspection method.

xi. Check fan blades and hubs for erosion and cracks.

xii. Check that balancing weight are secured firmly.

xiii. Measure field and insulation resistance of the rotor and compare it with design data.

xiv. Clean the rotor and apply finish coat as recommended by the manufacturer. Dry up the

rotor.

II. Generator Stator:

1 .Clean stator windings, ventilating ducts with dry compressed air (compressed air will be

supplied by CPCL).

2. Inspect for defects like

i. Discoloration of winding (for hot spots)ii. Loose missing slot wedges

iii. Inter coil spacers on overhangsiv. Broken overhaul coil bindings for end supportsv. Protective coatings on the core steps at slot ends

3. Replace any broken wedges as required.

III. Generator Assembly:

1. Insert rotor inside the stator carefully. Put packing material (such as leatheroid etc.,)

in the air gap between stator and rotor for protection and assemble all removed parts. Fix the

pilot exciter in bed. Assemble bearing pedestal.


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2. Ensure that the bearing has been cleaned, necessary scrapping has been done to

remove any uneven surface. Bearing insulation should be taken care, wherever provided

assemble the bearings.

3. Alignment and coupling of the generator with Turbine and pilot exciter with

Generator. Check air gap and ensure it matches with original gap.

4. Check the pedestal pipe flange insulation and also the same for pipe connection and

bolt. Replace if necessary. Box up the bearing.

5. Fix inner and outer end covers.

Generator details

Make Ercole marelli,Italy

Year of Commissioning 1969

Apparent power output 14MVA

Voltage 6.6KV

Rated current 1225A

Power factor 0.8 lag

Speed 3000 RPM, Directly coupled

Class of insulation B

Type of cooling Air cooled


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CHAPTER - 3

SYSTEM DEVELOPMENT

3.1. INTRODUCTION

3.1.1. DEVELOPMENT IN TURBO GENERATOR TECHNOLOGIES

Since the 1901 invention of the cylindrical rotor of Charles Brown for a high-speed

generator, the turbo generator has been the unique solution for converting steam turbine

power into electrical power. The continuously transposed stator bar, invented by Ludwig

Roebel in 1912, opened the door for large scale winding application. Up to the 1930ies thegenerators were designed in 2-, 4- and even 6- pole, in accordance with the speed optimums

of the steam turbines in those days.

The 1920 ended with impressive power generation plants, having generator units in

the 100 MVA range. The stator winding insulation consisted in the beginning of plied-on

mica-paper, compounded by Shellac varnish, later substituted by asphalt. Voltages were up to

12 kV.

In the early 1930s two European manufacturers were introducing 36 kV stator

windings, thus eliminating the machine transformer. All such designs were suffering of

continuous heavy electrical discharges, and were soon discontinued. After a 60-year time-out,

a manufacturer surprised the world in 1998 with a cable-based high-voltage generator up to

400 kV.

However again, the cable technology was not ready for turbo generator requirements,

and a breakthrough for commercial application was not achieved. In the 1930 US

manufacturers were introducing hydrogen as coolant. When combined with direct conductor

hydrogen cooling in the rotor, and later in the stator, this allowed a considerable increase in

specific utilization and efficiency.

By early 1960s the unit ratings were achieving 500 MVA. At that time deionized

water cooling in the stator winding was introduced. Around 1960 all major manufacturers

changed their insulation system to mica tape with synthetic resin impregnation, a technology

for thermal qualification at 155C, and which has been lasting into these days. By end of the

1960, with the power semiconductors becoming mature, the dc machine excitation was

superseded by the static excitation, and by an ac exciter machine with rotating diodes.


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The 1970 brought again a tremendous growth in unit ratings, going along with the

introduction of nuclear power. Units of 1200 MVA at 3000 rpm and 1600 MVA at 1500 rpm

at up to 27 kV were designed and put in operation. The rotor diameters were arriving at their-

physical limits.

Water-cooling of the rotor winding was introduced. Along with plans for 2000 MVA

and beyond, superconducting rotor windings and stator air-gap windings were studied.

However, in early 1980 the market focus was shifting to gas turbine technology, with some

100 MW beginning to grow into the area of large power plants, and initiating a new round of

up rating the simple and robust air-cooling technology in the 300 MVA range by 1996.

The generator has for a long time been developed by repeating the cycle: designtest

adjust design tools extrapolate design. A tremendous breakthrough came with the large

computers in the 1960ies, immediately being used for the key competences, such as magnetic

field calculations, nonlinear coolant flow networks and mechanical turbine generator shaft

calculations. Some programs of that area are even in use in the todays PC environment. As

an example, magnetic equivalent circuits were established to determine excitation currents.

Once these programs were calibrated on measured data, they have been proven very accurate

and still today, for most applications make obsolete any FEM method.

3.1.2. TODAYSTURBOGENERATOR TECHNOLOGIES

A.SMALL UNITS UPTO 150MVAThe size of these small air cooled units has evolved quite quickly. These machines are

mainly devoted for gas turbines and steam turbines accepting cycling expansion. The gas

turbines market has led to a very standardized range of machine based on the evolution of the

turbine technologies and on the market requests. The models developed in 1980 for 40 MW

50 Hz/60 Hz; same generator for 50 Hz and 60 Hz with a gear box wheel and pinion

adaptation; are nowadays joined by models in the 130-150 MW range.

These generators are always designed using the simplest solution in order to reach low

costs using modular solutions. For example the stator is cooled using one chamber and the

excitation system does not need a third bearing and no pilot exciter. By this way, the models

used for gas turbines are easily adapted for steam turbine or double drive solutions.

All these machines are easy to transport and to mount on site and are very often mounted and

coupled to the turbine by the turbine manufacturer. They are delivered in a short time and a


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lot of engineering is done to improve the through put time of these models. The maintenance

of these groups is quite simple requiring a small storage of spare parts.

A recent trend is the increase of the power of the electrical drives used in the oil and gas

industry, mainly for liquid natural gas pumps. Such drive motors require options similar to

those developed for the generators, however having a variable speed drives controlled by

static frequency converters. The performance is evolving quite strongly: a world record for

this kind of motor at 21 MW 5900 rpm in 1985, seems modest in view of todays 100 MW.

The speed values are close to generation with values between 3600 and 4200 rpm.

B.MEDIUM RANGE UPTO 500MVASince the introduction of the 300 MVA class ten years ago, subsequent development has

extended the rating up to the 400 MVA range. One of the main technology drivers has been

the improvement of the rotor axial cooling and winding indirect cooling using a modular

stator multi-chamber airflow. These generators are characterized by their simplicity and ease

of operation and maintenance.

They have also proven their maturity in GT24/GT26 gas turbine applications as well as

on numerous steam turbines and turbines of other manufacturers. The new ratings of the air-

cooled generator series allow for the application of air-cooled technology in power ranges

where hydrogen cooled generators were used previously.

As a result of electrical and cooling optimization the present air-cooled turbo generators

achieve efficiency up to 98.8 % and are used with a maximum voltage of 21 kV. Air cooled

turbo generators technology with highest ratings has now accumulated more than 1.8 million

of successful operating hours with more than 100 units in operation.

In two decades the power output of air-cooled generators has been increased from 200

MVA to 400 MVA. Fig.4 shows this exceptional increase in generator power as a function of

the time. It is clear that this strong increase in power that has occurred in the last decade was

a direct response to the market demands.

Recently, the increase of air-pressure inside the generator was realized. This measure

allows a better cooling and consequently enhances the capability of the air-cooled turbo

generators. The hydrogen-cooled types have hydrogen filling up to 5.5 bar. They are

designed for single-shaft and combined-cycle applications and are increasingly used with

steam turbines. The main features of the gas-cooled design are the same as the air-cooled.


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The cooling principle, end winding support system, the retightening system and the

aluminum press plate are excellent examples of the design similarities. The hydrogen-cooled

types are setting the benchmark for efficiency, large units commonly achieving 99.0 %. Since

1996, ALSTOM has supplied more than 50 unitshydrogen-cooled turbo generators of the

500 MVA range. However, the achievable power is much higher and will be soon at 600

MVA

Fig 3.1 Evolution of the air-cooled turbo generators in the last decades.

C. LARGE UNITS UP TO 2000MVA

These generators are driven by steam turbines in large coal fired power plants and

nuclear power plants. They are all equipped with hydrogen-cooling with up to 6 bar

overpressure, and with direct water cooling in the stator winding bars. The two-pole

generator series begins at 500 MVA, and units up to 1300 MVA are in commercial operation.

They are of highest specific utilization and therefore need complete direct cooling.

Depending on the size the rotor, cooling is performed by axial flow of hydrogen through all

conductors of a slot, either in one path over half-length of the rotor, or in two paths,

supported by a sub slot. The stator core is axially flown by hydrogen, symmetrically fed from

both ends driven by a radial fan, arranged on the non-driving end of the rotor shaft. The stator

winding is cooled by water-flown stainless steel tubes embedded in the Roebel bars. Thanks

to the water cooling the stator winding has ever been open factor for up ratings. The rotor


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winding has revealed to be the limiting part for up ratings. At 1.25 m for 50 Hz, the rotor

diameter is at the limits of mechanical stress. Any extension in active length beyond 8m

needs careful consideration of the shaft line dynamics. Potential lies in multi-zone cooling

concepts for the rotor winding, in an increase of hydrogen absolute pressure and fan pressure.

All the described measures will lead to a consolidation at 1400 MVA unit rating.

Any higher unit rating must go along with a break in rotor winding cooling, and the

parasitic effects due to stray flux will remain a challenge as such. The four poles machines

are running at 1500 rpm up to 1700 MVA. This is a key advantage for nuclear units, where

the temperature of the steam is relatively low and its flow in the low pressure parts of the

turbines huge.

This allows the turbine to have very large diameter by using very long blades. The

hydrogen/water-cooled generators coupled to these turbines are the largest electric turbo

machines both in term of size and performance. This type of machine is ensuring 80% of the

electrical production in France, which is a country with a very high electrical nuclear

production. Some 50 machines in operation of this type have shown a very good reliability in

operation and have a potential of improvement in performance.

Based on this situation, the solutions preferred in the nuclear market are not based on

new technologies, but, more safely, they tend to still improve the existing validated well-

running units. The 2000 MVA limit for turbo generators for the 3rdgeneration of reactors is

now close to be reached with improved life time and reliability. In order to reach this level of

power, following choices have been done:

a. Use the basic solutions validated by years of operation on running nuclearunits

b. Analyze those parts which have led to the faults on existing machinesc. Implement improvements validated on full-speed hydrogen and water-cooled

machines in the last decades.

d. Adapt the cantilever type of excitation technology and adapt it to be even lesssensitive to diode aging.

e. Implement an improved type of cooling in the rotor copper ducts.


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The maintenance of such a machine has to be done very carefully in order to reach the

guaranteed lifetime. The periodic stops to refuel the reactor are to be used for optimum

maintenance. The trend on the modern reactors is also to reduce the time between refueling

and the maintenance has to be adapted accordingly. A wide experience has been accumulated

on the existing machines.

III. AN OUTLOOK INTO THE NEXT 10 YEARS

A. MARKETTRENDS

As a part of the energy chain, the turbo generator requires present and future

developments that have to comply with the market requirements as following:

a. Higher efficiencyb. Higher reliabilityc. Low cost energy productiond. Grid stability enhancement

To fulfill continuously these requirements huge developments are in progress as

presented in the following sections.

B. SUBSTITUTION OF HYDROGEN-COOLED UNITS BY AIR COOLED UNITS

BY FOR HIGHER RELIABILITY AND LOW COST ENERGY PRODUCTION .

The substitution of hydrogen-cooled units by air-cooled and of hydrogen/water-

cooled by hydrogen-cooled will be continuing to shift the ratings upwards. The limits are

given by transport dimensions, by the established temperature classes, and by the degree of

complexity of design. The engineering will further exploit these limits involving mainly

cooling and insulation materials developments. Air-cooled turbo generators offer many

benefits to the operator. Some of which are listed below:

a) Excellent reliabilityb) Less civil work, simpler foundationc) No hydrogen treatment systemd) No seal oil system and less sealinge) Less pipingf) Simple engineering work due to its advanced technology


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These advantages are the consequences of not using hydrogen gas as a cooling

medium. This results in much simpler and shorter maintenance periods as well as a shorter

delivery time and an increased reliability. The good experience with large air-cooled turbo

generators demonstrates the high potential of these generators. The largest air-cooled

generator was designed for 500 MVA. This design has been proven by tests and represents

the maximum achievable capability of air-cooled generators.

C. EFFICIENCY ENHANCEMENT

The improvement of the efficiency is of first importance for the turbo generator of all

kind in particular in air-cooled 60 Hz units for closing the gap to the benchmark values of

hydrogen-cooled units. Actually, it is one of the first issues considered in any new turbo

generator development. In this section, some examples of new design solutions and new

technologies implementation to increase the efficiency will be described.

3.2. TESTING METHODS

A 3-phase, 4 pole micro-alternator system was used for practical tests. The micro-

alternator field is driven through a time constant regulator; a setting of 6 seconds was used in

these tests. The DC motor drive to the micro-alternator can also be electronically controlled

to represent the turbine and its governor, if needed. All the major system variables are

accessible for testing. Initial tests probed controller performance during normal operation,

these were later extended to cover behavior with power system faults.

A specially written C code standard two-axis theory flux linkage based state space

simulation allowed tests beyond the capability of the micro-alternator system, including wide

ranging fault simulation studies. A 10th order model with constant reactance values was used

for much of the work, with single damper coils on each d-q axis, and lumped rotor inertia.

Other model complexities are possible.

3.2.1. STEP RESPONSE TESTS

Often the specification on desired behavior includes TG open circuit response, this

was certainly the case here. Frequently an AVR is site tuned on open circuit. Consequently

the first tests used the micro-alternator in this condition, at rated voltage and speed. Each

controller design was evaluated by standard tests, including applying a 3% positive step. A


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small number of specifications were set as controller design goals. The 'fast' design

specification is typical where rapid action is required for dynamic response control. This set

aims of: overshoot 4.3%, rise time 130ms, settling time 230ms, and closed-loop system

bandwidth 4.0Hz.

A conventional digital AVR which attempts the design specification for the 'fast'

excitation control system was produced for comparisons. Such AVRs are the digital

equivalents of the sort of controller in use for many years, and offer a good standard of

performance. An approximate design using simulation studies fine-tuned by trial and error on

the micro-alternator gave the parameters of this digital AVR as: loop gain with generator on

open circuit = 325; lag time constants = 9.0 and 0.025 seconds; lead time constant = 3.0

seconds. For future reference, this design is termed DGAVRF.

The frequency response of the micro-alternator system was obtained by a Dynamic

Signal Analyzer using Fast Fourier Transforms. These tests used a small (3%) set point

change in output; the eventual field demand settles to a new steady state value also close to a

3% change showing operation is close to magnetically linear here.

The terminal voltage response given had overshoot 4%,, rise time 175ms, settling

time 350ms and bandwidth 2.8Hz, considered acceptably close to the design aims. GPC has

various parameters or 'tuning knobs' which can be chosen to vary the behavior. One such is

the control horizon Nu which specifies the number of steps over which the demand

increments are varied. Initial trials used values of 1-3, with large values causing a faster

response. Nu = 2 is a good compromise giving a terminal voltage step response similar but

slightly improved over the previous test under identical conditions.

Values given were: overshoot 1%, rise time 180ms, settling time 300ms and

bandwidth 3.0Hz. This and the previous DGAVR-F result are shown. As Nu approaches the

prediction horizon Ny(the number of steps over which the output directly influences the

controller, typically set to 10), the step response gets closer to the design values. The chosen

value yields a reasonably 'fast' response which is not very different from the design values,

without the possible reduction in the controller robustness and additional computational

burden imposed by higher values. For future reference, this 'fast' design using Nu = 2 is

termed STAVR-F.

3.2.2. STEP RESPONSE: GENERATOR ON LOAD

As mentioned earlier, it is the response of the excitation control system when the

turbine generator is on load that is really important since the system operates in this mode


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most of its life. The responses of the different types of AVR obtained on open circuit in the

previous section cannot normally be achieved when the generator is on load. This is due to

the significant changes that the generator characteristics undergo when the operating mode is

changed from open circuit to the loaded state.

It was observed during an evaluation of the ST parameter estimator that the steady

state gain and dominant time constant with load are considerably lower than their open circuit

values and the system can exhibit some degree of oscillatory behavior at high load

conditions.

The step response obtained when using the different AVRs on the TG simulator,

representing a typical 660MW set, has also been investigated. A positive step of 3% was used

with an operating point of P = 0.8pu; Q = 0pu. These tests confirmed that performance

similar to that obtained with the micro alternator can be repeated with the TG simulator.

The terminal voltage step response with the STAVR-F, values given were: overshoot

3.5%, rise time 500ms, settling time 500ms; also shown is the variation in real power. The

rise time differs markedly from OC conditions, since the alternator systems steady state gain

has changed by about 5.

The bandwidth of 3.6Hz is similar to the OC case. The corresponding step response

with the conventional DGAVR-F gave overshoot 3%, rise time 840ms, settling time 840ms,

bandwidth 1.7Hz, showing considerable changes from the OC values. These results clearly

indicate that the STAVR is able to maintain its response characteristics under changing

system conditions, while a fixed AVR fails to do so. These responses are comparable to those

on the simulator, a useful confirmation.

3.2.3 RESPONSE TO POWER SYSTEM FAULTS

Major disturbances that occur in the power system from time to time can seriously

affect the smooth operation of the excitation control system. These disturbances which are

transient in nature are classed as abnormal operating conditions of the generator. Although

the occurrence of these abnormal operating conditions is very infrequent, the performance of

an AVR during these events should be evaluated to assess whether the controller is able to

cope with such situations satisfactorily. In the case of the STAVR, the GPC cost function

considers only the deviations of the terminal voltage from its set point and the liveliness of

the control signal. However, during major disturbances the rotor angle of the generator with

respect to the infinite bus bar of the power system is disturbed significantly and can take

some time to settle down following the event.


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It is generally well known that a fast acting AVR such as the ST controller can reduce

the damping torque of the generator if it uses only the terminal voltage as its feedback signal.

The consequence of this is the reduction in the damping of rotor oscillations following a

major disturbance. This aspect should therefore be examined in detail to ensure that sufficient

damping of rotor oscillations is provided.

The response of a turbine generator to severe disturbances depends very much on its

severity as well as the conditions of the power system at which the disturbance occurs. A

severe disturbance, regarded as a standard test, is a 3-phase short circuit. The performance of

the new AVR is now examined under these conditions using the simulator.

3.2.4. THREE PHASE SHORT CIRCUIT

During this test, a sudden short circuit is applied to the stator terminals of the

generator and is removed after a period of 100 ms The operating point of the generator has

been chosen as P = 0.8 pu, Q = 0 pu to obtain a large rotor load angle. The greater the rotor

angle the more severe is the test since the stability margin of the rotor is lesser in that case.

For comparisons on the damping available to the rotor during the disturbance, a factor called

the 'Effective Damping Ratio' (EDR) has been used.

This factor is widely used in the industry and is defined as the ratio of the peak-to-

peak amplitude between the first undershoot of a signal following a disturbance and the

second over-shoot to the peak-to-peak amplitude between the first undershoot and the first

overshoot.

A lower value of the EDR indicates higher damping. gives the response with STAVR-F.

The EDR of the rotor angle signal is 0.64 and its settling time to within 2% is found to be

1.25 seconds; the terminal voltage settles down in 0.39 seconds, a satisfactory performance.

The test was repeated with the conventional AVR, DGAVR-F, and a rather similar response

was obtained. The EDR and the settling time of the rotor angle found were 0.73 and 2.14

seconds respectively and the settling time of the terminal voltage is 0.6 seconds. This

performance indicates that the STAVR has improved the rotor damping in this case

3.3. SUCCESSFUL PROTOTYPE TEST RUNS

A first generator from the new series was set up in the test bay in the summer of 1995 and

put through exhaustive development and type tests. These tests concentrated mainly on the

following:


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i. Measurement of the open-circuit and short-circuit characteristicsii. Determination of the temperature rise in the windings and of the losses

iii. Determination of reactances and time constants (including those for the quadratureaxis) and verification of the short-circuit strength, each by means of sudden short

circuits starting from

iv. No-load and preload conditionsv. Determination of negative-sequence reactance and resistance by means of a sustained

two-phase short circuit.

vi. Standstill tests in order to determine the sub transient reactances in the direct andquadrature axis.

vii. Additional temperature measurements on the press plate and press fingers, on clampsand end connections of the stator winding as well as measurements of the cooling-air

temperature at different locations in the generator

viii. Pressure measurements to verify the distribution of the cooling airix. Measurement of the mechanical vibration in the shaft and bearing pedestals, stator

core and housing, and the winding overhangs

x. Noise measurements to determine the sound level.

A total of approximately 70 vibration pick-ups, 80 pressure and 200 temperature

measuring probes were used for the tests. The results of the test runs will be looked at in

detail in a future article.

The prototype fulfilled the requirements covering the running quality and vibration as

well as temperature rises and lossesin every respect, even exceeding the high expectations

in certain areas.

The rated data of the prototype together with some of the more important measured

values. Special mention has to be made of the excellent efficiency, which lies only marginally

below that of the hydrogen-cooled generators.

Based on the partial temperature rises measured under open-circuit and short-circuit

conditions, the temperature rise during full-load operation at 300 MVA will lie below the

limit for temperature class B by a sufficient margin of safety.

Given the information available today and looking to the future and further innovations (eg,

in the stator winding insulation), it is evident that air-cooled generators are potentially

capable of another increase in output.


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Fig 3.2 Layout of Turbo generator

3.4.TEST METHOD

1. INSTRUMENT

IR is measured with a mega-ohmmeter. Sometimes this is called Megger Tester after

the name of the instrument first developed for this purpose (Megger is a trade name of AVO).

Mega-ohmmeter generates and applies a regulated DC supply.

It measures the flow of current and IR is directly read on its dial. Hand driven and

motorized mega-ohmmeters are available. But for constant rotation and steady DC voltage, a

motorized meter is preferred. Modern mega-ohmmeter can apply voltages exceeding 10 KV,

and measure resistance higher than 100G

2. TEST VOLTAGE

Test voltage should be well below the rated peak line-to-ground voltage of the

winding as it is not a high potential test. But the voltage should be high enough to find

defects such as cuts though the insulation in the windings.


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Table 3.1: Guidelines for DC Voltage to be applied for IR test

Guidelines for DC voltages to be applied for the test are given in Table-1

Rated voltage (line-to-line) of the winding in volts Test voltage (DC) in volt

5000

500500-1000

1000-2500

2500-5000

.

3. TEST PROCEDURE

IR & PI tests shall be done simultaneously. If the winding temperature is below thedew point, the winding may be heated to dry off the moisture that has condensed on the

winding. If the temperature is below the dew point, there is no way to correct the IR & PI for

humidity Otherwise, the test is very simple. The procedure is as follows:

Remove all external connections to the machine and completely discharge the windings to the

grounded machine frame

Apply required DC voltage between the winding and ground using a direct indicating, motor

driven mega-ohmmeter.

Continue to apply the voltage for 10 minutes.

Measure the insulation resistance after 1 minute and 10 minutes. Switch off supply to

the meter and completely discharge the windings to the grounded machine frame.

Calculate the polarization index by dividing the 10-minute insulation resistance by the

1-minute insulation resistance. Note the winding temperature.

If test is carried out only on one winding of three phase equipment, then other

windings should be grounded during the test.

If IR is below the above recommended value, the winding should not be subjected to

high potential test or be taken to service, since failure may occur. However, if historical

record indicates that a low IR value is always obtained on a particular winding, then the

machine can probably be returned to service with little risk of failure.

If IR or PI is below the minimum value in a modern stator winding, it is an indication

that the winding is contaminated or soaked with water. Interpretation of PI value.


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For modern form wound stators, if a very high value of IR is measured (say greater

than 5G) then PI is not likely to indicate anything about the winding. Thus the test may be

stopped after one minute to save time.

If a high PI result is obtained on an older stator winding, then there is a possibility that

the insulation has suffered thermal deterioration. This occurs because thermal deterioration

fundamentally changes the nature of insulation and thus the polarization currents that flow.

In general IR & PI test are an excellent means of ascertaining winding conditions that

are contaminated or soaked with moisture. The tests are also good detecting major flaws

where the insulation is cracked or has been cut through. The test can also detect thermal

deterioration for form wound stators using thermoplastic insulation system.

Figure3.3.Layout of turbo generator foundation

NISA/McFdn, customized software from Cranes Software International Limited, offers CAD

based solutions to different power house structures such as Turbo generator foundations with

or without VIS and Block foundations. Backed by powerful NISA II Analysis and DISPLAY

III/IV the graphical Pre and Post processor of NISA suite of programs, NISA/McFdn

provides seamless interface for modeling, Static, Eigen, Shock & Forced Vibration analysis

and design of TG Foundation.


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2.PROPORTIONING CRITERION:

Proportioning of the components of TG foundation is carried out based on the

following criteria:

a. Shape of top deck, number of level of beams & their sizing based on TG configuration and

its auxiliary units

b. To separate the frequency of machine with natural frequency of foundation,

c. To limit maximum amplitude of structure as per codal provisions and functional

specifications of equipments.

d. To carry Dead loads, thermal loads, equipment loads, operating loads, erection loads,

unbalanced loads, loss of blade, short circuit and seismic loads.

3. TYPES OF TG FOUNDATIONS:

Different types of TG foundations are considered based on power generation capacity

& supported with or without vibration isolators.

Types of TG foundations are:

1) Top Deck with Vibration Isolation System with power generation capacity 210MW,

250MW, 500MW and support Frame. In this type top deck and supporting structure are

modeled separately. The VIS is modeled using spring elements.

2) Top Deck without VIS with power generation capacity 210MW, 250MW, 500MW. In this

an integrated model of Top deck, supporting structure and foundation is generated.

4. GUI DESIGN:

NISA/McFdn as a tool gives an end to end solution with a user friendly Interface for

input of Geometric, Loading and other important connection details such as Insert plates and

Embedded Parts as per TG manufactures data without sacrificing the flexibility for possible

variations in data. Friendly features such as import of data in the Excel format are also

provided. Soil parameters as per site conditions are also considered for computation of spring

stiffness. User interface also provides direct specification of input details like geometry and

elevations. Figure 2 & Figure 3 shows a typical UI text input for McFdn.

5. Finite Element Modeling:

Based on the geometry and loadings input, FE models are generated automatically.

Two types of FE models are generated i.e. Beam/shell model and a detailed 3D solid model.


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Beam/Shell integrated model is generated for the design of RC structural components

conforming to Indian standards.

This model is also used for arriving at the required foundation size. Beam elements are

modeled with a two noded NKTP 12, 3D beam element having six degrees of freedom with 3

translations and 3 rotations at each node. Shell elements are modeled with a four noded

NKTP 20, 3D General Shell element having six degrees of freedom with 3 translations and 3

rotations at each node.

All these elements belong to NISA element Library. A detailed 3D solid model is

used to evaluate the dynamic behavior of the structure. Solid model uses an eight node

NKTP4, 3D Solid element having three degrees of freedom with 3 translations at each node.

This model is used to evaluate the natural frequency of the structure and perform the

frequency response analysis due to harmonic loads on the structure.

Vibration isolators (VIS) used to isolate top deck with rest of the supporting structure

which eliminates dependency on approximate soil properties, to avoid disturbance on to the

surrounding structure. In this case of Top deck supported on VIS, the vibration isolators are

modeled using spring elements using a two node NKTP-38, 3-D general spring element with

six independent spring rates and six degrees of freedom (UX, UY, UZ, ROTX, ROTY,

ROTZ) per node.

The spring constants are computed based on Standard specifications. McFdn has an

extensive database of the Isolators from which relevant spring data are extracted and applied

on the FE Model for analysis. A facility of automatic selection of Isolation springs is also

available. The soil base below foundation is also modeled using the spring elements and

corresponding constants are modeled using the soil data.

Loads and boundary conditions are applied on the FE models and a typical FE model

auto generated by McFdn are given in Figure-4 through Figure 9. Equipment loads are

modeled using 3D mass elements @ the c.g locations and connected to the anchoring points

in the FE model by rigid links. Application of the loadings are based on user specified input

data and provisions of IS 2974Parts III are also considered.

3.5. DIAGNOSTIC TEST, GENERATOR, PARTIAL DISCHARGE, SLOT

DISCHARGE, STATOR INSULATION.

3.5.1. INTRODUCTION

The rate of occurrence and the consequences of service failures in high-voltage

generator stator insulation systems can be reduced by the use of sensitive diagnostic tests


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designed to detect the early stages of insulation deterioration. Degradation processes include

insulation delamination, shrinkage of wedges and/or side packing permitting vibration and

abrasion, and loss of function of gradient control coatings.

All these processes are almost invariably accompanied by partial discharges which

increase in severity as the deterioration progresses, usually making an additional contribution

to the insulation damage rate. Especially important with modem insulation systems are

discharges occurring in the slot between the electrical shield of the stator bar and the core,

usually referred to as slot discharge, which can attain levels of energy ( > 5000 pC) sufficient

to cause damage in times as short as several months.

Detection of discharges at the earliest practical stage, and proper interpretation of test

results can permit corrective action to be taken before a winding deteriorates beyond the point

of economic salvage, and particularly before the risk of a failure in service becomes

unacceptably high.

Reliable early warning from a suitable diagnostic test may permit relatively

inexpensive repairs, such as re-establishing ground connections, side-packing, rewedging or

touch-up of stress grading paint, to be accomplished during a scheduled outage. In many

power systems with mixed generation, hydraulic machines are reluctantly removed from

service for discharge tests because of the relatively high cost of replacement fossil fuel

energy.

Thus, a diagnostic test that, at least for screening purposes, can be performed without

service interruption, presents a distinct advantage. For large thermal machines, such a test

also offers advantages in that the long time restraints for shedding and picking up load may

be avoided. The new test methods described later in this Paper have been designed to respond

to partial discharges originate in the stator insulation system, using signal- coupling

techniques inherently insensitive to system noise.

The signal to-noise ratio is further improved by electronic processing of the detected

signals. To put into perspective the diagnostic tests described in this Paper, a review follows

of the most widely employed tests to ascertain stator insulation condition.

3.5.2. REVIEW OF DIAGNOSTIC TEST TECHNIQUES

3.5.2.1. MEASURABLE QUANTITIES WHICH CORRELATE WITH DAMAGE

Mechanical vibration, gaseous products and partial discharges are three quantities

which can be monitored readily with negligible service interruption, while providing

information with respect to the total stator insulation condition. The first two quantities have


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only relatively recently been under study and have yet to demonstrate their sensitivity and

resolution, since data correlating measurements with visual inspections of stator condition are

sparse.

3.5.2.2. VIBRATION

If a bar or coil side loosens within a stator slot, vibration can cause ground wall

erosion and wear, contributing to ultimate insulation failure. Hence, the presence of a

vibrating bar indicates that the winding is loose and may eventually fail. By measuring the

magnitude and rate of increase of these vibrations by means of accelerometers attached to the

stator frame, the expected remaining useful life might be estimated.

3.5.3. DIAGNOSTIC TESTING OF GENERATOR INSULATION WITHOUT

SERVICE INTERRUPTION

3.5.3.1 GASEOUS BY PRODUCTS

Certain dielectrics, exposed to partial discharges or to heat, evolve various gaseous

products. For insulations commonly used in stator windings of hydrogen-cooled units, some

of these products can be readily distinguished against the background by gas chromatography

or spectroscopy. The quantity of evolved gas car indicates the degree of degradation.

3.5.4. PARTIAL DISCHARGES

a. TIP UP TEST

The tip-up testprovides a measure of the void content and partial discharge activity

in a dielectric by measuring the change in dissipation factor between two discrete voltage

stress levels, usually 50 percent and 100 percent of operating voltage. Unfortunately, this test

tends to be insensitive to localized partial discharges because the loss component is averaged

throughout the entire test sample, unless individual coils are isolated for testan expensive

procedure. Also, the end-grading material will distort results for in-situ measurements. The

tip-up test requires an external supply to energize the winding, thus applying maximum

voltage stresses to the entire winding which is not representative of operating conditions.

b. DIELECTRIC LOSS ANALYZER

The dielectric Loss Analyzer reacts to the power loss in an insulation system as a

function of voltage per cycle, thus indirectly measuring the presence and effects of partial


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discharge. This test method, through more sensitive than the tip-up test, can identify a

number of weaknesses, but cannot detect the presence of a small number of intense discharge

events in a background of many more moderate discharges. This test also requires an external

supply, though the duration of the test outage may be comparatively short.

c. INDUCTIVE PROBE

Inductively coupled radio frequency probes have been employed to detect local

discharges. This test requires a lengthy service interruption and an external high-voltage

supply, though it does have the capability of pin-pointing those bars or coil sides suffering the

most intense internal or slot discharges.

d. ULTRASONIC DETECTOR

Signals from an ultrasonic probe have been introduced into a conventional partial

discharge measuring circuit with some success, especially for locating specific discharge

sites. This procedure does not provide any advantage over the Inductive Probe technique and

is probably less quantitative.

e. PULSE DETECTION

Detection of individual partial-discharge pulses by direct capacitive or inductive

coupling to a machine winding, with the generator self-excited and thus supplying its own

high voltage with normal voltage distribution has been implemented in various measurement

systems.

In this class of tests, the pattern of individual pulses can be displayed on an

oscilloscope or quantified by a pulse-height analyzer.

In the early days of this type of measurement, the high partial-discharge repetition rate

from the many sites in a generator could result in the superposition of pulses since tests were

often performed with pulse shaping circuitry to lengthen the duration of the individual

pulses for easier observation. However, with modem wideband storage oscilloscopes and flat-

response filters, it is observed that actual superposition of pulses rarely occurs.

The rise times of partial- discharge pulses measured with such equipment are about 10

ns or less. Ringing frequencies, which depend only on generator winding parameters and the

measuring system, vary from about 1 to over 50 MHz and are second order effects initiated

by the original partial discharge event.


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Pulse durations, including ringing, are typically less than 1 psec., and consecutive

partial discharges are rarely observed at intervals less than 10 psec. An inexpensive version

of this test has been in routine use within Ontario Hydro for more than 20 years, employing

HV capacitors temporarily connected to the generator, a high-pass filter and an oscilloscope

for display.

This test has demonstrated that the condition of the stator ground wall insulation is

correlated with the magnitude of the highest discharge pulse observed on the oscilloscope.

However, distinguishing between generator insulation partial discharges and external noise is

sometimes difficult, requiring an experienced operator.

Additional difficulties arise because of the nature of the partial-discharge pulses.

Since these pulses are extremely rapid, the peak magnitude is difficult to determine at the

slow oscilloscope sweep speeds required to recognize partial discharges by their phase

position in the power frequency cycle, making the test highly subjective.

A further drawback to the test is that, in practice, only the magnitude of the highest

pulses is recorded. Information such as the number of pulses and the distribution of pulse

magnitudes, that is, the relative abundance of large pulses compared to small pulses, can only

be noted qualitatively. Yet, significant information about the nature and extent of insulation

degradation must be present in the total pulse pattern.

3.6. IMPROVED GENERATOR TESTS

Although the partial discharge test is successful in quickly predicting stator insulation

condition, the above limitations have restricted use of the test outside Ontario Hydro. As a

consequence, CEA and one manufacturer began separately the development of more

sophisticated procedures for observing and quantifying partial discharge activity in generator

stator insulation systems.

The test improvements described below comprise better methods of acquiring and

treating partial discharge data with permanently installed coupling devices. The coupler or

antenna is mounted on the rotor in one system, whilecouplers are installed on the stator in

the system developed for CEA. Both coupling techniques respond to the high frequency

energy in an actual discharge.

Means for reducing the influence of electrical noise are incorporated into both

coupling techniques, thus permitting diagnostic testing while the generator is operating

normally. Methods for quantifying the signals from either coupling system differ, although in

principle both are based on pulse magnitude analysis.


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3.6.1. STATOR-MOUNTED COUPLING SYSTEM

The partial discharge signals are acquired using rugged high-voltage capacitors of 50

to 100 pF which are solidly connected to the stator winding. The low-voltage sides of these

couplers are connected to a convenient location external to the generator housing by

terminated 50 ^l coaxial signal cable. The couplers are sensitive only to the high frequency

components of a discharge pulse. The placing and functioning of the couplers depends on

whether the stator winding is in a hydraulic or a turbine generator.

3.7. HYDRAULIC GENERATORS

In hydraulic generators, the couplers are often placed at or close to the connection

point of the circuit ring bus to each split or parallel of each phase in the winding. Since noise

pulses entering the generator from the power system are first attenuated by surge capacitors

and transformers and maybe further reduced by impedance mismatches as the pulse travels

along the circuit ring buses, a measure of external noise immunity is inherently present.

Additional attenuation of noise, including power frequency and solid-state dc exciter

noise, is afforded by connecting pairs of capacitive couplers to a differential amplifier in such

a way as to cancel common-mode signals, taking into account the pulse travel time from the

machine terminal to each coupler. For example, it shows two capacitors installed on a

hydraulic generator with asymmetrical winding.

When a noise pulse enters the winding, voltage pulses travel along the ring bus and

reach both couplers about 25 ns, say, after signal injection. Since the response is the same at

each coupler, if these two signals are combined in a differential amplifier there will be no

output, at least not until pulse reflections within the generator winding start to build up. For

partial discharges, which usually occur near the high-voltage end of each parallel, a net

response is obtained since the signal reaches one coupler almost 50 ns before it reaches the

other coupler in the pair.

This system works because the partial discharge pulse rise times are typically only 10

ns, much less than the pulse travel times along the transmission-line-like path of the circuit

ring bus. Practical hydraulic-machine windings are rarely symmetrical about the terminals.

However, by the careful placement of the couplers and the use of delay lines, such permanent

differential couplers can be installed on themajority of windings currently in use. More

than twenty installations have been made in a number of Canadian utilities. These


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installations are usually on generators which either are difficult to obtain for test purposes

because of outage restraints or are subject to a high degree of external noise interference.

3.8. TURBO-ALTERNATORS

Because of the much smaller rotor radius involved, the type of differential coupling

system described above is not possible for most turbo-alternators since the electrical length of

circuit ring bus is often shorter than the discharge-pulse rise time. Sensitivity to external

noise can be reduced, however, by the use of two permanently installed directional

couplers per phase on the output bus of the generator. External noise can be greatly

attenuated by differential sensitivity to the direction of pulse travel on the bus, that is, either

from the generator (assumed to be partial discharges) or from the power system (assumed to

be noise).Retrofitting of the directional coupler can often be readily implemented on

Isolated Phase Bus, since the inspection covers, which are regularly placed in the bus sheath,

can provide sufficient capacitive coupling when grounded through a suitable impedance.

3.9. ELECTRONIC ANALYSIS

Partial Discharge Analyzers (PDA) have been constructed to process the voltage

pulses from pairs of couplers into information about the repetition rate and magnitude of the

discharges. An analyzer consists of an 80-MHz bandwidth differential amplifier driving a

single channel, dual polarity, pulse-height analyzer fabricated with ECL integrated circuits.

The pulse-height analyzer is designed to handle generator partial discharges.

Specifically, it responds to pulse rise times of less than 10 ns, ignores ringing, inhibits the

counting of pulse overshoots such as the negative overshoot of a positive pulse which can

cause a false indication in the negative channel, ignores reflections in noise signals, and

accepts consecutive discharge pulses more than 3 psec. apart.

This single-channel pulse-height analyzer provides multichannel operation by

sequential variation of the threshold levels. Fifteen 100-mV-wide channels with lower

thresholds ranging from 100 mV to 1500 MV have been found satisfactory for completely

determining pulse magnitude spectra.

The PDA is controlled by a microprocessor which automatically steps the pulse height

analyzer through the 15 voltage channels. The microprocessor also controls the counters

which total the number of positive and negative pulses per second which occur in each

channel and at the same time supervises a digital printer which provides the pulse magnitude

spectra.


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Also produced is information on the generators operating voltage at the time of test,

which is obtained from the 10-mV power frequency signal appearing on the couplersoutput.

The source coupler of the partial discharges is identified automatically by comparing the

polarity of the discharges with the phase of the power frequency voltage. Facilities are also

included in the PDA for analyzing data from temporary couplers.

These include requisite filters and a circuit which removes the very strong

interference caused by thyristor excitation systems. Various versions of the PDA have been in

use for more than 4 years and improvements are constantly being made. Several of the

PDAS described above have now beencommercially manufactured and are in routine use by

a number of Canadian utilities.

3.10. TEST RESULTS

Test data on many operating hydraulic generators have shown that external noise and

interference caused by thyristor excitation systems are reduced by more than 20 dB when the

permanent couplers and the PDA are employed, whereas sensitivity to generator insulation

partial discharges is maintained. Results are consistent with those obtained by skilled

personnel using the conventional test.

Particularly the magnitude of pulses corresponding to a partial discharge repetition

rate of about 10 Hz was found to correlate well with the magnitude of the peak discharge

pulse observed from the oscilloscope trace in the conventional test.

It indicates pulse magnitude spectra observed on two of the parallels of a modern 200-

MVA hydraulic generator. The stator winding in this machine has been visually examined

and the parallel corresponding to the line on the right side was found to be damaged by slot

discharge deterioration.

3.11. MICRO TURBINES LATEST AND PAST TECHNOLOGY

High-speed micro-turbines and mini-turbines play a significant role in the Distributed

Power Systems that provide dependable electric power close to the user. Several high-speed

turbo-generators manufactured by various corporations are now available in the 30 kW to 90

KW range. These systems operate at speeds from 50000 RPM to 120000 RPM. The generator

is directly coupled to the turbine shaft. This obviates the need for a gearbox, helps reduce the

size of the generator, and lowers the cost of the overall system. The output power is


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electronically processed and conditioned to provide constant voltage dc or multi-phase ac

power at constant frequency.

Technology of micro-turbines is moving forward to address ratings above 100 kW

due to the growing demand for larger units. There is a tendency to use multiple units of the

existing 30to 90 kW packages to satisfy this demand for higher power capacity. However, use

of turbo generators of higher ratings is likely to be beneficial to the user for the following

reasons:

a) Lower cost of investment per kW for purchase and installation

b) Lower cost of maintenance because of reduced parts count

c) Higher efficiency

d) Safer operation.

At the present time most generators used with micro-turbines are based on permanent

magnet technology. It is the objective of this paper to compare alternatives to the PM

generator technology, and introduce induction generator technology as a more viable

alternative in the power range exceeding 100 kW.

The approach in this paper is to present the concept in all its dimensions including the

issues of generator and controller design. The authors are currently engaged in the

development of the high-speed induction generator systems. Their experience in the field ofthe technology forms the basis supporting the discussions in this paper.

1) SYSTEM DEFINITION AND CONSTRAINTSIt is realized that one specific technology does not necessarily provide the best answer

under all situations. We must therefore limit our discussions to applications within certain

constraints. At this time the following broad limits are applicable for the technology under

consideration:

i) The micro or mini turbine systems considered here are in the 100 kW to 500 kW power

range. The system comprises mainly of high-speed turbine, generator, controller, protection,

and instrumentation.

ii) The prime mover operates at speeds between 30000 to 80000 RPM depending upon the

rated output. Typically, the operating speed of the prime mover varies inversely with the

rated output.

iii) Constant speed of operation is considered. However, certain narrowly defined operating

speed range may be required in specific applications.


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iv) The generator must be designed for a cooling system that is compatible with the system

requirements. Typically either air, or lubricant oil, or water glycol mixture is used.

v) The integrated power system is located close to the user such as in a factory building,

hospital, department store, and office complex. Alternatively, vehicle mounted applications in

airborne, land based or marine situations are also considered. These mobile applications are

valuable particularly for military requirements.

vi) The electrical power output is typically 3-phase ac with single or multiple voltages.

Alternatively, DC output may be required. In case of AC power systems, 50/60 Hz frequency

is common for commercial applications, and 400 Hz. frequency is used in military /

aerospace applications.

vii) Compatibility with utility power systems may or may not be required. In most situations

stand- alone capability in isolation from a utility system is required. In some other situations,

power transfer from utility to the turbo-generator and vice versa may be necessary.

viii) The generator must also provide electric start capability during the initial start up of the

turbine.

ix) The system must provide protection against hazards. Safety of operation is an important

consideration.

In approaching various issues, we have considered the following issues to define relative

merits:

i) Cost: Investment and Operational

ii) Reliability and safety

iii) Size, Power Density.

The issues listed above are not necessarily listed in the order of their importance.

2) GENERATOR TECHNOLOGIESWe plan to review three different generator technologies for comparison: permanent

magnet (PM), induction, and switched reluctance (SR). All these three are suitable for high

speed operation in the speed range considered here. There are other technologies such as

synchronous reluctance and homo polar that are suitable for high-speed operation but are not

considered in this paper. We also limit our discussion to radial geometric configurations for

the three technologies. Axial gap geometric configurations are not considered.


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A. PERMANENT MAGNET (PM)

Micro power systems currently in the market use the generator designs based on the

PM technology. The generator itself has two electromagnetic components: the rotating

magnetic field constructed using permanent magnets; and the stationary armature constructed

using electrical windings located in a slotted iron core.

The PMs are made using high-energy rare earth materials such as Neodymium Iron

Boron or Samarium Cobalt. Retention of the PMS on the shaft is provided by high strength

metallic or composite containment ring. The stationary iron core is made of laminated

electrical grade steel. Electrical windings are made from high purity copper conductors

insulated from one another and from the iron core. The entire armature assembly is

impregnated using high temperature resin or epoxy.

The voltage output from the generator is unregulated, multiple phase ac. This voltage

varies as a function of the speed and load. This voltage output is connected to a solid state

power conditioning system. Typically, the solid state power conditioning system uses

buck/boost techniques and regulates the entire power output.

B. INDUCTION

The technology of induction generator is based on the relatively mature electric motor

technology. Induction motors are perhaps the most common types of electric motors used

throughout the industry. Early developments in induction generators were made using fixed

capacitors for excitation, since suitable active power devices were not available. This resulted

in unstable power output since the excitation could not be adjusted as the load or speed

deviated from the nominal values. This approach became possible only where a large power

system with infinite bus was available, such as in a utility power system. In this case the

excitation was provided from the infinite bus. With the availability of high power switching

devices, induction generator can be provided with adjustable excitation and operate in

isolation in a stable manner with appropriate controls.

Induction generator also has two electromagnetic components: the rotating magnetic

field constructed using high conductivity, high strength bars located in a slotted iron core to

form a squirrel cage; and the stationary armature similar to the one described in the previous

paragraph for PM technology.

The voltage output from the generator is regulated, multiple phase ac. The control of

the voltage is accomplished in a closed loop operation where the excitation current is adjusted

to generate constant output voltage regardless of the variations of speed and load current.


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The excitation current, its magnitude and frequency is determined by the control

system. The excitation current is supplied to the stationary armature winding from which it is

induced into the short circuited squirrel cage secondary winding in the rotor.

C. SWITCHED RELUCTANCE (SR)

The technology of SR generator is based on the concepts that magnetically charged

opposite poles attracts. Typically, there are unequal number of salient poles on the stator and

rotor. Both are constructed of laminated electrical grade steel. A cross sectional view of the

construction of the SR generator. The number of poles shown on the stator is 6. The number

of poles shown the rotor is 4. Other pole combinations such as 8/6, 10/8 are possible.

There is no winding on the rotor. Armature coils located on stator poles are concentric

and are isolated from one another. When the coils on opposite poles such as 1 and 1 shown in

are excited the corresponding stator poles are magnetized. The rotor poles A-A are closest to

the stator poles 1 and 1. These are magnetized to opposite polarity by induction and are

attracted to the stator poles.

If the prime mover drives the rotor in the opposite direction, voltage is generated in

the stator coil to produce power. The voltage output from the SR generator is DC and has

high ripple content. The voltage output can be filtered, and is regulated by adjusting the

duration of the excitation current. The commutation of the stator coil is accomplished by the

controller.

D. INDUCTION GENERATOR OPERATION

The speed torque characteristics of an induction motor operating from a constant

frequency power source. Most readers are familiar with this characteristic of the induction

motor operation. The operation of the induction motor occurs in a stable manner in the region

of the speed torque curve. The torque output as well as the power delivered by the motor

varies as the motor speed changes. At synchronous speed no power is delivered at all. The

difference between the synchronous speed and the operating speed is called the slip. The

output torque and power vary linearly with the slip.

If the induction motor is driven to a speed higher than the synchronous speed, the

speed torque curve reverses. In the stable region of this curve, electric power is generated

utilizing the mechanical input power from the prime mover. Once again the generated power

is a function of the slip, and varies with the slip itself.


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In the generator mode, if the slip is controlled in accordance with the load

requirements, the induction generator will deliver the necessary power. It must be

remembered that the synchronous speed is a function of the electrical frequency applied to

the generator terminals. On the other hand, the operating shaft speed is determined by the

prime mover. Therefore to generate power, the electrical frequency must be adjusted as the

changes in the load and the prime mover speed occur.

In addition to the requirement stated above, the excitation current must be provided to

the generator stator windings for induction into the rotor. The magnitude of the excitation

current will determine the voltage at the bus. Thus the excitation current must be regulated at

specific levels to obtain a constant bus voltage. The controller for the induction generator has

the dual function as follows:

i) Adjust the electrical frequency to produce the slip corresponding to the load requirement.

ii) Adjust the magnitude of the excitation current to provide the desirable bus voltage.

A number of torque speed characteristic curves in the stable region of operation are

shown to explain the operation. As an example, consider the situation when the prime mover

is at the nominal or 100% speed. The electrical frequency must be adjusted to cater for load

changes from 0 to 100% of the load. If a vertical line is drawn along the speed of 100%, it

can be observed that the electrical frequency must be changed from 100% at no load to about

95% at full load if the prime mover speed is held at 100%.

3) BENEFITS OF INDUCTION GENERATOR TECHNOLOGY

Induction generator has several benefits to offer for the micro, mini power systems under

consideration. These benefits relate to the generator design as follows:

i) Cost of Materials: Use of electromagnets rather than permanent magnets means lower cost

of materials for the induction generator. Rare earth permanent magnets are substantially more

expensive than the electrical steel used in electromagnets. They also must be contained using

additional supporting rings.

ii) Cost of Labor: PMs require special machining operations and must be retained on the

rotor structure by installation of the containment structure. Handling of permanent magnets

that are pre-charged is generally difficult in production shops. These requirements increase

the cost of labor for the PM generator.

iii) Generator Power Quality: The PM generator produces raw ac power with unregulated


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voltage. Depending upon t

testing of turbogenerator

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