PREPARATION OF TRANSFORMER SPECIFICATIONS By VALLAMKONDA SANKAR P.Eng. E-mail:

PREPARATION OF TRANSFORMER SPECIFICATIONS
By
VALLAMKONDA SANKAR P.Eng.
E-mail: powertransformer@hotmail.com
THANKS TO
IEEE NORTHERN CANADA SECTION,
EDMONTON, ALBERTA.
THANKS TO
MR. PETER ROTHWELL
MR. BLAINE LARSON
ACKNOWLEDGEMENTS
Mr. Frank David, FD Consulting Services, Winnipeg.
Mr. Peter Franzen, Manitoba Hydro, Winnipeg.
Mr. Izy Polischuk, Hydro One, Toronto.
Mr. Ronnie (Rashed) Minhaz, McGregor
Construction, Calgary.
THANKS TO
MR. BILL BERGMAN, Consultant, PowerNex
Associates Inc., Calgary, Alberta.
MR. JEFF TENNANT, Senior Engineer, Ontario
Power Generation Inc, Niagara Falls, Ontario.
MR. JOHN van KOOY, van Kooy Transformer
Consulting Services Inc., Hamilton, Ontario.
Presentation Schedule
45 minutes first half of the presentation.
10 minutes interactions (questions/comments).
10 minutes break.
45 minutes second half of the presentation.
10 minutes interactions (questions/comments).
OBJECTIVES
• To procure reliable transformers at economical prices
per schedule.
• To use the standards to achieve above goals.
• To determine transformer parameters that meet system
requirements.
• To prepare the specifications that make the design and
the manufacturing simple and economical.
• To obtain maximum benefits from Globalization.
• To establish effective interactions between users and
manufacturers for mutual benefits.
• To conduct effective and useful Tender Review and
Design Review meetings.
• To initiate innovations.
FUNCTIONS OF TECHNICAL SPECIFICATIONS
• To formally and fairly communicate exactly what the
contractor has to deliver.
• To be able to accurately offer services and products
which provide a satisfactory solution to user.
• To avoid relationship mishaps associated with costly
variation work.
• To give an opportunity for manufacturer to apply
improved design and manufacturing methods and to
use advanced materials and accessories.
• To procure lowest life cycle cost transformers.
• A record of parameters of transformers purchased.
• A chance for users and manufacturers to work
together for advancement of transformer industry.
TOPICS FOR FIRST HALF OF THE PRESENTATION
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Ratings
Standards
Single phase versus three phase
Winding connections
Vector group
Insulation levels
Terminals
Accessories
Types of cooling
Sound levels
Taps (range, location etc.)
Types of taps
Impedance
RATINGS
• Available generation (GSU transformers).
• For security of supply, unit service and system service transformers
are mostly with double LVs and LVs of two transformers are
interconnected.
• Load growth (System transformers).
• Return on investment.
• Difference in definitions in CSA, IEEE and IEC standards.
• LV and HV ratings per CSA and IEEE based on:
Location of the tap-changer (in HV or in LV).
Use of the tap-changer (for input voltage fluctuations, to
compensate for output voltage regulation, step-up or
step-down.
• Specifying the MV.A to be delivered by output windings and the type
of loading (Arithmetic, vectorial or simultaneous).
• Specifying current to which each winding has to be designed for.
• In stations also where security of supply is critical, the transformers
are with double LVs (LVs of two transformers are interconnected).
RATINGS (continued)
When a transformer is specified per CSA C-88 or IEEE
C.57.12.00, calculation of winding ratings.
HV has 1000 turns and current per specified MV.A is 100
Amps.
LV has 100 turns and current per specified MV.A and rated tap
is 1000 Amps.
LTC is in LV for ±10% tap range. LTC turns are 10.
Operation of the transformer is step-down.
Taps are to compensate for regulation.
While delivering the full load (1000Amps), to keep the LV
voltage constant, the tap changer moves to the maximum
turns position.
Ampturns on LV = (100+10)1000 = 110000.
Ampturns on HV must be same as that on LV.
HV winding current rating = 110000/1000 = 110 Amps.
STANDARDS
• Referencing the standards in the specifications.
• Differences in CSA, IEEE and IEC standards:
Normal or usual service conditions
Rated power
Over voltage conditions
Overload capability at low ambient temperature
Suggested impedances
Tests and test levels
Insulation levels
Dimensions of bushings
Air clearances
Types of taps and tap range
Tolerances on impedances, losses and magnetization current
• Use of standards in preparation of specifications.
SINGLE-PHASE VERSUS THREE-PHASE
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Dimensional and weight limitations.
Transport restrictions.
Air clearances.
Reliability (Risk assessment).
Specifications for replacement transformer:
Current ratings of neutral bushing
Current rating of tertiary bushings
Type of core
Design of stabilizing winding
Tertiary delta voltages
Polarity (subtractive or additive)
WINDING CONNECTIONS
• Wye, delta or zig-zag.
• Wye:
Design of windings (graded insulation)
Neutral is available
Many types of LTCs are available
Winding and phase currents same (need for series transformer)
• Delta:
Design of windings
Grounding transformer is required to get a neutral
Limited types of LTCs are available
Winding current is √3 times lower than the phase current
• Zig-zag.
Design of windings
Compared to wye and delta windings difficult and costly to have taps
Zero sequence impedance and need for a grounding reactor
• Limitations and advantages of each connection.
VECTOR GROUP
• Time angle expressed in degrees between line-to-neutral
voltages of two or more systems.
• Per the book ‘Transformer Engineering’ by L.F. Blume
only three phase three legged core form is recommended
for wye/wye vector group. For all the other types of cores,
a delta tertiary is required.
• In delta connected winding, if neutral is required then a
grounding transformer is used.
• Relative costs of Yd, Dy, Yz and Dz vector groups depend
on voltage rating of each winding, current rating of each
winding and in which winding the taps are needed.
• Technical limitations of different vector groups (One
o’clock vectors of Yd1, Dy1 and Yz1, twelve o’clock
vectors of Yy0, Dd0 and Dz0 etc.).
INSULATION LEVELS
• Insulation levels are to be based on insulation coordination
design.
• Higher insulation level for bushings than windings is not
preferred.
• Decide insulation levels prior to inviting the bids, rather
asking alternate bids with different insulation levels.
• Specify chopped wave and front-of-wave tests only when
needed by the system.
• When wave shape of the switching surge in the system is
different to that in the standards then give the wave shape
in the specifications.
• Unless there is a logical reason do not increase the test
levels from those in the standards.
• Users and bidders should discuss and agree on test
connections and test levels in the Design Review meeting.
TERMINALS
• Consider the cost and delivery time before specifying the
bushings to CSA or IEEE or IEC standards.
• Get copies of routine tests from the manufacturer.
• Specify the tests that are required and not covered under
routine tests in the standards.
• Check the suitability of the bushings for Low-ambient
Temperature Load Capability.
• Transformer manufacturer’s approval is recommended to
interchange corona shields of the bushings.
• Specifications should give details of bus-duct and
enclosures.
• Preferable for manufacturer to determine the current
rating.
• Check the withstand value of the peak fault current also.
• Check the design of horizontal mounted terminals (need
for a conservator etc.).
ACCESSORIES
• List all required accessories. If preferred, specify the
manufacturer and type or model.
• Instruction manuals to include pamphlets describing the
function, construction and operation of the accessories.
• Before specifying air bag in the conservator, check the
compatibility with the inhibitor in oil.
• Specify if certification for revenue metering is required.
• For valves specify the size, function and the type.
• In Design Review discuss the details of packing and
shipping.
• Shipping list to include instructions for storage (outdoor,
indoor etc.)
• Tender price to include training of user’s personnel is a
good idea.
TYPES OF COOLING
•Specify 65⁰C rise per CSA and IEEE standards. Avoid
specifying 55⁰ rise or 55/65⁰ rise.
•Do not call OA, FOA etc. designations, many young
engineers may not know them. Specify only the designations
ONAN, ONAF, OFAF, ODAF etc. in the current CSA and IEEE
standards
•Economics of type of cooling based on site location.
•On pumped units, check oil velocities to avoid electrostatic
charge build-up.
•With OFAF cooling, check oil velocities and gradients of the
windings.
•With OD cooling, correlation of time constants of WTI and
windings is to be checked.
•When self-cooled rating is not required, allow the bidders to
quote with industrial type coolers or with radiators.
•Give air flow restriction (fire walls, enclosures etc.) details.
•For water coolers specify leak detector type.
SOUND LEVELS
• Check CSA and NEMA sound levels before specifying as
they are very old.
• Effect of sound level on cost.
• Sound level on bridging position with reactor LTC.
• Series transformer sound level at different tap positions.
• Sound level with constant flux taps and variable flux taps.
• Effects of sound level reduction methods (core binding
etc.) on life of the transformer.
• Sound levels at different stages of cooling.
• Sound level at no-load and at load.
TAPS (range, location etc.)
• Function of the taps.
• IEEE tutorials on www.transformerscommittee.org or on
Google.
“TAPS” – 2004 in Las Vegas by V. Sankar.
“Taps in autotransformers” – 2010 in Toronto by
Dr. T. Kalicki and V. Sankar.
• Tap ranges in C57.12.10 (DETC ±5% and LTC ±10%).
• Avoid DETC taps, specially when there is LTC.
• DETC taps in autotransformers.
• Formation of oxide film on DETC tap changer contacts.
• Adequacy of LTC range to meet regulation.
• Specifying the tap range by the user versus specifying
system requirements for bidders to calculate the tap range.
TAPS (range, location etc. continued)
• Location of the taps:
On input winding.
On output winding.
In wye or delta or zig-zag windings.
Series transformer.
• LTC selection:
Voltage across the tap range.
Number of steps and step voltage.
Step KV.A.
Number of tap turns.
RMS and peak fault currents.
• Autotransformers:
DETC.
DETC and LTC.
Taps in LV line.
Taps in TV.
TAP RANGE
• If system requires +5% to -15% taps, do not specify ±15%
taps.
• Tap range of ±10% in C57.12.10 and system requirement.
• Specify the tap range such that a reversing or linear type
tap changer can be offered.
Instead of 230KV±10% tap range in ±16 steps, specify that
the taps are required for voltage variation from 207KV to
253KV in 32 steps.
• For taps below normal, load loss reduction with linear
taps compared to reversing taps.
• With vacuum LTC no need to meet the minimum
inductance requirement between coarse and fine
windings, where as this requirement must be met with
non vacuum LTC.
OTHER CONSIDERATIONS FOR THE TAPS
• Reliability and cost of non-linear elements (Zno discs)
across the taps verses the designs without the non-linear
elements.
• To specify tap changer make and type only when
specifically needed.
• At tender review to check tie-in elements effect in service.
• Not to state that selector contacts must not be in main oil.
• RCBN and FCBN.
• Type of tap changer influence on buckling forces on tap
winding.
• Current splitting.
• Step-up and step-down operations. Also how the taps are
used.
• Vacuum verses non-vacuum tap changers.
TYPES OF TAPS
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Constant flux taps (CFVV).
Variable flux taps (VFVV).
Mixed regulation taps (Cb.VV).
Change in types of taps based on step-down operation,
step-up operation, used for input voltage fluctuations or
used to compensate for regulation.
• Types of taps effect on the following:
Impedance
Short-circuit current
Sound level
Losses
TV voltage
LTC location and core flux in two winding transformers
Taps Location Operation Constant Voltage Varying Voltage
HV
LV
Core Flux
step-down
HV
LV
LV
HV
variable
constant
step-up
HV
LV
LV
HV
variable
constant
step-down
HV
LV
LV
HV
constant
variable
step-up
HV
LV
LV
HV
constant
variable
LTC location and core flux in autotransformers
LTC
Location
Operation
Series
Step-down
Compensate for
Input Voltage
Compensate for
Load Regulation
Core
Flux
constant
variable
Step-up
variable
almost constant
LV Line
Step-down
variable
almost constant
Step-up
constant
variable
Common
Step-down
variable
variable
Step-up
variable
variable
TYPES OF TAPS (continued)
•
Based on user preference or special requirement the
following should be specified in the specifications.
Resistor type / Reactor type
Tank mounted / In-tank
Vacuum / Non-vacuum with filters
Reversing / Coarse-fine / Linear
Series transformer, acceptable or not
Non-linear devices, acceptable or not
Tie-in elements, acceptable type
Extra continuous current rating, 1.5 p.u. or 2 p.u. etc.
Taps in body of main winding or taps in a separate
winding
TYPES OF TAPS (continued)
• Some other considerations are given below.
Type and design of tap winding.
Leads bring-out.
Eccentric duct winding arrangement.
Series transformer rating based on reversing or
linear taps.
Low ambient temperature capability.
Measurement of tap winding temperature rise.
Lead supports strength.
Clearances between tap leads of different phases
and from tap leads to ground.
Eccentric duct winding arrangement
LV winding
Tap leads
Eccentric
Duct
Tap
winding
Centerline
of
HV & LV
Core
Centerline
of
Tap & Core
IMPEDANCE
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Specifications must give impedance value
Reference MVA
Impedance value on cost
Effects of low impedance and high impedance
Impedance values recommended in standards
Impedance values at extreme taps
Effect of CFVV and VFVV taps on impedance
TV to LV and TV to HV impedances
Impedance of zig-zag transformers
TOPICS FOR SECOND HALF OF THE PRESENTATION
• Capitalization of losses
• Short-circuit withstand
• Special requirements (altitude, over excitation, GIC etc.)
• Overloads
• Parallel operation
• Rating plate
• Alternatives
• Interesting clauses in the specifications
• Globalization
• Data sheets
• Tests
• Tender and Design Review meetings
• Transportation and limiting dimensions/weights
• Conclusions
CAPITALIZATION OF LOSSES
• IEEE C57.12.120 ‘Evaluation Guide for Power
Transformers and Reactors’ is a very good reference.
• Following should be stated in the specifications.
No-load loss $/KW
Load loss $/KW at a reference MV.A.
Cooler losses if different to load loss $/KW
• Do not specify rate per KWH.
• Do not specify no-load and load losses, give only the loss
evaluation values.
• Reference temperatures for losses if different from the
standards.
• Penalty for losses tested above the guaranteed values.
• Incentives for innovations to reduce the losses.
• Users and manufacturers to work together to develop
green-transformers.
SHORT-CIRCUIT WITHSTAND
• Following should be specified in the specifications.
System impedances if different from the standards.
Pre-fault voltage if different from the standards.
System and transformer grounding details.
For transformers directly connected to the generators;
the duration the voltage source is connected.
Details of impedances connected to limit short-circuit
current magnitude.
For three-circuit transformers the in-feed is from which
circuits.
Requirement and details of the devices that relieve the
tank pressure during the short-circuit.
• Short-circuit test is very expensive. Design and
manufacturing practices of the bidders to withstand
short-circuit forces should be thoroughly evaluated in the
Tender Review meeting.
SHORT-CIRCUIT WITHSTAND (continued)
• Manufacturers normally calculate the following stresses
from the short-circuit forces. In the Design Review
meeting user to check the stresses and withstand
strengths.
Mean hoop tensile stress on outer windings.
Mean compressive stress on inner windings.
Radial bending stress on inner windings.
Axial bending stress on all coils.
Compressive stress on radial spacers (duct sticks).
Compressive stress on axial spacers (keyed spacers)
and on the paper insulation of the winding conductors.
Compressive stress on clamping ring (pressure ring).
Tensile stress on clamping structure.
Compressive stresses on inner windings.
Free buckling stresses on inner windings.
Short-circuit stresses on lead structures.
SPECIAL REQUIREMENTS
• Where economical and needs by the system, deviate the
following from the standards.
Ambient temperature.
Low-ambient temperature capability.
System over voltage.
Pre-fault voltage.
LTC current rating.
• Operating temperature, viscosity of the oil in the LTC and
the LTC cut-out temperature setting are to be properly
coordinated.
SPECIAL REQUIREMENTS (continued)
• Requirements from one station to another station
differs. Information to be included in the specifications
of some requirements is given below.
Operating altitude when above 1000 meters.
Polluted environments.
Transport dimensions/weights/profile restrictions.
GIC requirements.
Oil preservation system.
Special oils like FR3.
Fiber optics.
On-line monitoring devices.
Painting.
Wheels and similar requirements.
Positions of control box, conservator etc.
OVERLOADS
• Information below should be included in the
specifications.
Overload profile.
Number of occurrences in a year.
Information per clause 9.7 of C57.91.
Limits of hot-spot temperature and top oil rise.
Loss of insulation life.
Operation (step-down or step-up).
Load on the tertiary (also arithmetic, vector loading etc.).
Ambient temperature at each load.
Requirement of overload test.
Acceptable DGA levels during the overload test.
OVARLOAD (continued)
• Some of the risks during the overload are listed below.
Evaluation of free gas.
Loss of insulation life.
Leaks on gaskets.
Reduced mechanical strength of the insulation.
Permanent deformation of materials.
Tap changer thermal runaway condition.
Operation of relief devices.
Excessive pressure built-up in bushings.
Risk of damage of internal parts (CTs, current limiting
devices, tap changers etc.).
Increase in regulation.
PARALLEL OPERATION
• When parallel operation with the existing transformers is
required following should be included in specifications:
Rated MV.A.
Exact turns ratio of the windings at all the taps.
Impedance on rated and tap extremes.
Diagram of connection and phasor relationship between
windings.
Compatibility of controls to maintain the correct tap
positions on all transformers while minimizing the
circulating current.
Type of paralleling method preferred.
• Impedances should be same at maximum rating (For
transformers of 60/80MV.A and 60/80/100MV.A impedance
of 8% at base rating of 60MV.A is not correct; should be
impedance of 10% at 80MV.A for the first unit and 10% at
100MV.A for the second unit).
PARALLEL OPERATION (continued)
• Some paralleling methods are given below:
Master / Follower
Power factor
Negative reactance
Circulating current
Circulating reactive current
• Paralleling information on the existing units is not
proprietary.
• If the tolerances on impedances lesser than those in the
standards are required, then the required tolerances
should be stated in the specifications.
• Parallel operation is user responsibility. Manufacturer
designs the transformer to the specifications.
• IEEE standard C57.153 ‘Transformer Paralleling Guide’
and IEEE April 2009 tutorial ‘Transformer Paralleling’ are
good references.
RATING PLATE
• Most important and readily accessible record.
• Data inscribed should not fade away after a few years.
• Suggest to include the following in the specification
along with the data stated in CSA C-88:
Tap changer nameplate.
Phasor diagram including hour clock designation (Dyn11,
Dyn1 etc.).
Step-up operation suitability.
Suitability of reversal of power flow.
Current transformers, voltage transformers etc.
No detectable PCB (less than 2ppm).
Standard number including the year.
Impedances on extreme taps.
Nameplates for bushings, tap changers etc.
Vacuum withstand capability of oil circulating parts.
RATING PLATE (continued)
• Users should consider to include the following also:
Diagram of location of emergency man-holes.
Diagram of location of major valves.
Tie-in resistors including their rating.
Make and serial number of LTC reactor.
Make and the impedance of current limiting reactors.
Location diagram and the type of fall arrest system.
Pressure settings on nitrogen pressurized units.
User specification number.
A statement that the voltages and the currents marked
are based on by not considering the regulation.
Zero sequence impedance.
Make and serial numbers of series transformer,
compensating transformer etc., when used.
Common winding maximum current for autotransformers.
Type of oil (Voltesso 35, Luminal etc.)
ALTERNATIVES
• Users should consider the bids with alternatives meeting
the system needs without the main bid per specifications.
• Management of users should nourish the cost saving and
reliability ideas of their engineers.
• Asking the bids for only the required ratings than all the
ratings in the system saves money and time for both
users and manufacturers.
• Consult the manufacturers to determine the parameters
rather asking alternatives with different parameters
(impedance, insulation levels, winding connections etc.).
• Encouraging the alternatives not only saves money to the
user but also helps to the advancement of the industry.
INTERESTING CLAUSES
The clauses that are either ambiguous or will add
considerable cost with almost no benefit should not be
included in the specifications. A few are given below.
• Missing technical information per industry standard.
• Assembled in a manner best suited for the application.
• Best materials should be used.
• Adequate barriers shall be provided.
• LTC selector contacts shall not be in transformer main
oil.
• No exceptions, no deviations and no alternatives will be
accepted.
• Per manufacturer’s standard.
• Life of the transformer must be 40 years.
• Tank should have adequate stiffeners.
• Only bids from the manufacturers with a skilled labour
force will be considered.
• Core should be built with high grade laminations.
INTERESTING CLAUSES (continued)
•Oil should be of good quality.
•Test voltages should be 25% higher than those in the
standards.
•Calculations must be by computer programs only.
•Top oil limit of 105⁰C and hot-spot limit of 120⁰C for over
loads also.
•Design of the taps should be as CFVV and also as VFVV.
•All components shall provide utmost reliability.
•No abnormal deterioration of insulation in service.
•Should conform to high standards of engineering, design
and workmanship.
•Impedance shall be stated in the order.
•Materials used shall be of established quality.
Globalization
• Specifications are the first and the most important tool in
procuring reliable transformers at economical prices
from the global market.
• Many transformer manufacturers are in the countries
whose first language is not English. As such, the
specifications should be in simple English with globally
known terminology.
• For the fear of losing the competitive edge many bidders
do not wish to post the clarifications on the web site.
• Repair cost and time to repair are important on whom to
place the order.
Globalization (continued):
• Due to stiff competition safety margins between stresses
and strengths are reduced by almost all the
manufacturers.
• Provides an opportunity to update the specifications
from the beneficial experiences in other countries (use of
coarse/fine taps, non-linear devices etc.).
• Specifications should include user maintenance
practices, safety requirements and transport limitations.
• Some countries give subsidies for export. This makes
local manufacturers bankrupt and the local knowledge to
evaporate.
• CSA, IEC and IEEE should be combined to develop one
global standard.
DATA SHEETS
•Data sheets should ask the information needed in
evaluating the bids.
•Data sheets should not ask the information that the user
can not explain what to be filled.
•Suggest not to ask the following.
Efficiency and regulation at various loads and power
factors.
Impulse, applied and induced test levels per standards.
Voltages on all DETC tap positions.
No-load and load losses at 20⁰C and at 85⁰C.
Load losses at base MV.A and at maximum MV.A.
•Electronic Data Forms should allow the bidders to fill
different to the drop down menu and enough space to fill.
•Data sheets are not a substitute to the Tender Review
meeting.
TESTS
• Routine and Design tests (Type tests) on fully assembled
and oil filled transformers are listed in CSA, IEEE and IEC
standards.
• Recommend that the specifications specifying the tests
per standards unless the system conditions require to test
differently. (switching surge wave shape, single phase
induced test etc.).
• Obtain test certificates of all accessories and components
(DETC tap changers, bushings, CTs, pressure relief
devices, bolts used for lead clamping structures, glue
used on insulating items etc.).
• Not possible to do sound level test on load in the factory.
• Tests before shipping, after receiving at the station, before
energization should also be specified.
• Specifying the actions to be performed when a
transformer fails in a factory test is a good practice.
TENDER AND DESIGN REVIEWS
• Tender Review meeting is very important and strongly
recommend for all the tenders.
• Tender & Design Reviews cost to the bidders. As such,
the specifications should state these requirements.
• Suggest to discuss the following in Tender Review:
Manufacturing and test capabilities.
Quality and inspections.
Delivery track record (past 3 years).
Shop failures, investigation & corrections (past 5 years).
Field problems and how rectified (past 10 years).
Exceptions/comments in the tender.
Design of core, coils, tank etc.
Taps effects on transformer parameters.
Processing practices of core & coils and oil.
Short-circuit with stand, overload calculations etc.
• User technical consultant should attend Tender review.
TENDER AND DESIGN REVIEWS (continued)
Suggest to discuss the following in Design Review.
Magneic circuit
• Flux density in legs and yokes at maximum system
voltage (specially in 5 legged cores and split cores).
• Type of core (core or shell) and joints (step-lap etc.).
• Laminations surface insulation.
• Clamps design for short-circuit forces and for lifting.
• Cooling ducts and core maximum temperature.
• Core grounding.
• Undesirable hot-spots due to the leakage flux.
Windings
• Design of coils and their construction.
• Types of conductors and their suitability.
• Impact of taps location on short-circuit forces and on
insulation design.
c
b
d
a
e
43%
A
100%
43%
57%
B
100%
C
100%
43%
57%
57%
j
43%
43%
57%
43%
f
i
h
FIGURE 1, TYPE 1
g
50%
50%
FIGURE 5
50%
50%
100 %
100 %
50%
50%
50%
50%
100 %
50%
50%
TENDER AND DESIGN REVIEWS (continued)
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•
•
•
•
•
•
Cooling design to avoid undesirable hot-spots.
Stresses in oil and in solid insulation.
Construction of static shields, stress rings etc.
On pumped units velocities in different parts.
Leads exists.
Off-set, modeling etc. for calculations of forces.
Design and construction of counter shields, interleaving,
transpositions etc..
• Review of purchasing specifications of winding
conductors, all components and materials.
External to the coils
• Design and construction of current limiting reactors, tap
changer reactor, series transformer etc.
• Design of core shunts, clamp shunts, tank shields etc.
• Leads layout, leads structures, clearances etc.
TENDER AND DESIGN REVIEWS (continued)
Accessories
• Principle of operation, construction, mounting etc.
• Safety systems design and location (fall arrest systems,
pressure relief devices, emergency exits etc.).
• Gaskets material and design.
Manufacturing and processing
• Machinery to cut laminations to avoid burrs.
• Air gaps during core assembly and binding of the core.
• Method of core lifting after the assembly.
• Maintaining tightness of the windings.
• Winding methods, removing from the lathe and sizing.
• Workmanship of installation of counter shields, making
interleaved joints, cross-overs, transpositions etc.
• Vapour-phase process, pre-tanking, moisture control etc.
• Painting specifications and painting process.
TENDER AND DESIGN REVIEWS (continued)
Inspection and testing
• Quality standards and procedures.
• Inspection and tests of raw materials and accessories.
• Pre vapour-phase tests (ratio, vector group etc.).
• Oil tests before filling.
• Factory tests on completely assembled unit.
Shipping and installation
Marking, packing and shipment of the parts.
Preparation of the main unit for shipment.
Transportation method and the route of shipment.
Type and location of impact recorders.
Pre-shipment tests (SFRA, core megger, power factor etc.).
Inspection and tests at site before unloading.
Erection steps and processing at site.
Pre-commissioning tests.
TRANSPORTATION AND LIMITING DIMENSIONS/WEIGHTS
• Specifications should include a schedule for civil drawings, control
drawings, outline, rating plate etc.
• Manufacturers’ drawings to have correct weights and dimensions.
• Manufacturers should not use users as checkers for drawings.
• Manufacturers should realize that after receiving the drawings user
has to make ready the foundations, spill containments, control
schemes etc. by the time the transformer arrives in the station.
• Specifications should clearly give the transportation details like
station location, siding details, dimensional and weight restrictions.
• Enquiries for replacement transformer specifications mostly have the
drawings of the old transformer, but often they are not readable or
critical dimensions missing. Users should rectify this.
• Specifications should also give dimensional and weight restrictions
for the parts.
• Manufacturers should ship all the parts in time with proper packing.
• Based on site location, complications of the transformer etc. a pretender site meeting is a good idea.
• Many damages had occurred in the transportation.
• Many users had problems due to the large differences on dimensions
and weights between initial and final drawings of the manufacturers.
CONCLUSIONS
• User’s management should provide adequate
resources and time to their Technical
Departments to prepare the specifications.
• At the time of preparation of specifications users
should interact with manufacturers to finalize on
cost effective parameters meeting system needs.
• Standards are to complement the specifications
and are not to use as specifications.
• CSA-C88 is not updated for a longtime. Canadian
utilities and manufacturers should take initiative
and update CSA-C88 soon.
CONCLUSIONS (continued)
• Specifications should be revised based on the
work done by organizations like IEEE, IEC etc.
and the developments in the transformer world.
• Users should give freedom to manufacturers to
offer alternatives without a main bid exactly to
the specifications.
• Technical Departments of the users and the
manufacturers must have direct, fast and reliable
communication paths.
• Developments are not manufacturers’ arena only.
Users’ management should encourage their
engineers for innovations.
• Users and manufacturers are not just buyers and
suppliers they are a team.
INTERACTIONS
(Comments, questions etc.)