Page 1/73

Transformer Specifications

Power Transformer, All Ratings

The City of Palo Alto

Utilities Department

Page 2/73

Table of contents

1 SCOPE

1.1 Definitions

2 REFERENCE STANDARDS

2.1 Industry Publications

2.2 Other Relevant Standards

2.3 Company Publications

3 GENERAL

3.1 Application Information

4 STANDARD CONDITIONS, AND OTHER SERVICE FACTORS

4.1 Type

4.2 Seismic Capability

5 RATING DATA

5.1 Phase Designation

5.2 Kilovolt Ampere (kVA) Ratings

5.3 Cooling Class

5.4 Polarity or Angular Displacement

5.5 Impedance

5.6 Ratings of Transformer Taps

5.7 Transformer Bank and Parallel Operation

5.8 Losses

5.8.1 Loss Cost Multipliers

5.8.2 Loss Evaluation Method

5.8.3 Loss Penalty

6 CONSTRUCTION

6.1 General Requirements

6.2 Core Design Requirements

6.2.1 Steel Specification

6.2.2 Flux Density

6.2.3 Audible Sound Level

6.2.4 Core Temperature

6.2.5 Core Construction and Tie Plate Stress

6.2.6 Core, Frame and Tank Wall Shunt Ground

6.2.7 Core Cooling Ducts

6.3 Winding and Insulation Design Requirements

6.3.1 Conductors

6.3.2 Insulation Structures

6.3.3 Oil Gap Design

6.3.4 Winding Design

6.3.5 Winding Leads and Supports

6.3.6 Bolted Connections

Page 3/73

6.3.7 Drying and processing

6.3.8 Preventative Autotransformer

6.4 Tap Changers

6.4.1 De-Energized Tap Changers, Switches and Terminal Boards

6.4.1.1 Tap Changers and Reconnection Switches

6.4.1.2 Reconnection Terminal Boards

6.4.2 Load Tap Changer (LTC)

6.4.2.1 Tap Voltages

6.4.2.2 Change-over Selector Switch

6.4.2.3 Tap Selector, Bypass and Vacuum Interrupter Switches

6.4.2.4 Diverter Switch

6.4.2.5 Vacuum Interrupting Bottles

6.4.2.6 Motor Drive Unit

6.4.2.7 Oil-Filled Tap Changer Compartment

6.4.2.8 Operating mechanism

6.4.2.9 LTC Controls

6.5 Tanks

6.5.1 Shipping Cover

6.5.2 Tank Manufacturing

6.5.3 Welding

6.5.4 Tank Openings

6.5.5 Gaskets

6.5.6 Tank Grounding

6.5.7 Tank Exterior Finish

6.5.8 Tank Interior Finish

6.5.9 External Bolted Connections

6.5.10 Moving Facilities

6.5.11 Fall Protection System

6.6 Auxiliary Components

6.6.1 Bushings

6.6.2 Bushings for Buried Tertiary-Terminals

6.6.3 Surge/Lightning Arresters

6.6.4 Instrumentation and Monitoring Equipment

6.6.4.1 Indicators

6.6.4.2 Monitor

6.6.5 Cooling Equipment

6.6.5.1 Radiators or Coolers

6.6.5.2 Pumps (if specified)

6.6.5.3 Fans

6.6.5.4 Fan and Pump Control

6.6.5.5 Auxiliary Relay

6.6.5.6 Location of Cooling Control Devices

6.6.6 Bushing Current Transformers

6.6.7 Liquid Insulation and Preservation System

6.6.7.1 Insulating Liquids

6.6.7.2 Insulating Liquid Preservation System

6.6.7.3 Gas Collection System

6.6.8 Valves

6.6.9 Pressure-Relief Device (PRD)

6.6.10 Rapid Pressure Rise Relay/Sudden Pressure Device

Page 4/73

6.6.11 Alarm Monitor

6.6.12 Power Supply for Transformer Auxiliary Equipment

6.6.12.1 AC Power Supply

6.6.12.2 DC Power Supply

6.6.13 Control Cabinet

6.6.14 Supervisory Control and Data Acquisition (SCADA)

6.6.15 Neutral Connections

6.6.16 External Clearances

6.7 Nameplate

6.7.1 Main Transformer Nameplate

6.7.2 Valve Identification and Location Nameplate

6.7.3 Bushing Nameplates

6.7.4 Auxiliary Device Nameplates

7 TESTS

7.1 General

7.2 Routine Tests

7.2.1 Performance Tests

7.2.1.1 Winding Resistance Test

7.2.1.2 No-Load Loss and Excitation Current Test

7.2.1.3 Positive-Sequence Impedance Test Clarification

7.2.1.4 On-Load Tap Changer Operation Test

7.2.1.5 Zero Sequence Impedances Test

7.2.1.6 Temperature Rise Test

7.2.1.7 Dissolved Gas Analysis (DGA)

7.2.1.8 Audible sound level test

7.2.2 Dielectric Tests

7.2.2.1 Winding Insulation Resistance Test

7.2.2.2 Core Insulation Resistance Test

7.2.2.3 Insulation power factor and capacitance test

7.2.2.4 Single phase excitation current test

7.2.2.5 Low frequency test on auxiliary wiring

7.2.2.6 Lightning Impulse Tests

7.2.2.7 Switching Impulse Test

7.2.2.8 Partial discharge test

7.2.3 Tank Pressure Test

7.2.4 Gas Collection Test

7.2.5 Sweep Frequency Response (SFRA)

7.3 Test Sequence

7.4 Surge Protection Devices

7.5 Test Bushings

8 SITE COMMISSIONING TESTS

8.1 Oil Tests

8.2 Electrical Tests

9 TECHNICAL DOCUMENTATION

9.1 Drawing Types

9.1.1 Review Drawings

9.1.2 Final-for-Manufacturing Drawings

Page 5/73

9.1.3 As-Built Drawings

9.1.4 AutoCAD Drawings

9.2 Technical Documentation Description

9.2.1 Bid Documentation

9.2.2 Design and Manufacturing Documentation

9.2.2.1 Certification of Insulating Oil

9.2.2.2 Certified Test Report

9.2.2.3 Outline Drawing

9.2.2.4 Nameplate and Instruction Plate Drawings

9.2.2.5 Bushing Outline Drawings

9.2.2.6 Surge Arrester Outline Drawings

9.2.2.7 Schematic and Wiring Diagrams

9.2.2.8 Current Transformer Nameplate Drawings

9.2.2.9 Current Transformer Information

9.2.2.10 Instruction Manuals

9.2.2.11 Spare Parts

9.2.2.12 Site Installation Drawings

9.2.2.13 Final Drawings

9.2.2.14 Geomagnetic Disturbance Withstand Calculations

10 SHIPPING REQUIREMENTS

10.1 Tests prior to Shipping

10.1.1 Sweep Frequency Response (SFRA)

10.1.2 Dew Point Measurement

10.1.3 Core and Components Insulation Resistance Test

10.2 Supplier Representative

10.3 Method of Shipment

10.4 Factory Assembly and Component Location Marking

10.5 Shipping Crates

10.6 Notification of Shipment

10.7 Impact Recorders

10.8 Acceleration Forces

11 OTHER INSPECTION REQUIREMENTS

11.1 Design Review

11.2 Quality Assurance Inspections and Surveillance

11.3 Cooperation with Quality Surveillance Representative

11.4 Inspection Photographs

12 OPERATIONAL WARRANTY

13 ACCEPTANCE

14 REJECTED MATERIAL

15 MATERIAL SAFETY DATA SHEETS

16 ANNEXED FILES

17 APPENDIX B: SELLER QUALITY PROGRAM REQUIREMENTS

17.1 Introduction

17.2 Quality program

Page 6/73

17.3 Quality program elements

17.3.1 Procurement Control

17.3.2 Document Control

17.3.3 Identification and Control of Material, Parts, and Components

17.3.4 Inspection and Test Control

17.3.5 Measurement and Test Equipment Control (M&TE)

17.3.6 Handling, Storage, and Shipping Control

17.4 Non-conformance Control

17.4.1 Records

17.4.2 Special Process Control

17.4.3 Fabrication/Construction Flow

Page 7/73

Page 8/73

1 SCOPE

This specification covers the requirements for substation power transformers of all ratings purchased by the company. It supplements project specific Transformer Requirements Sheet. Provisions in this specification may be overruled by exceptions stated within the project specific Transformer Requirements Sheet.

1.1 Definitions

The term "Bidder" as used in this specification, shall refer to the party or parties proposing to do the work as hereinafter specified.

The term "Seller" as used in this specification, shall refer to the party or parties awarded the purchase order to do the work as hereinafter specified.

The term "Purchaser", “Company” as used in this specification, shall refer to the City of Palo Alto Utilities

Department (CPAU).

2 REFERENCE STANDARDS

All reference standards included in the latest NEMA, IEEE, ANSI/IEEE, ASTM, AWS, NESC, NEC, OSHA and

ANSI standards (including but not limited to C57), form part of this specification. The following standards, publications and references contained therein form part of this specification. The latest revisions of these reference publications shall be used and are listed in the order of precedence. The equipment proposed by the contractor must meet the requirements of the standards specified in the technical specification or equivalent.

In case of conflict between any of the standard mentioned in this specification and the contents of this specification, the specification shall govern.

2.1 Industry Publications

Standards to be followed include the most recent editions of:

[1] IEEE C57.12.00: IEEE Standard for General Requirements for Liquid-Immersed Distribution, Power, and Regulating Transformers

[2] IEEE C57.12.10: IEEE Standard Requirements for Liquid-Immersed Power Transformers

[3] IEEE C57.12.70: IEEE Standard for Standard Terminal Markings and Connections for Distribution and Power Transformers

[4] IEEE C57.12.90: IEEE Standard Test Code for Liquid-Immersed Distribution, Power, and Regulating

Transformers

[5] IEEE C57.13: IEEE Standard Requirements for Instrument Transformers

[6] IEEE C57.19.00: IEEE Standard General Requirements and Test Procedure for Power Apparatus

Bushings

[7] IEEE C57.19.01: IEEE Standard for Performance Characteristics and Dimensions for Power

Transformer and Reactor Bushings

Page 9/73

[8] IEEE C57.19.100: IEEE Guide for Application of Power Apparatus Bushings

[9] IEEE C57.91: IEEE Guide for Loading Mineral-Oil-Immersed Transformers and Step-Voltage

Regulators

[10] IEEE C57.98: IEEE Guide for Transformer Impulse Tests

[11] IEEE C57.100: IEEE Standard Test Procedure for Thermal Evaluation of Insulation Systems for

Liquid-Immersed Distribution and Power Transformers

[12] IEEE C57.106: IEEE Guide for Acceptance and Maintenance of Insulating Mineral Oil in Electrical

Equipment

[13] IEEE C57.109: IEEE Guide for Liquid-Immersed Transformer Through Fault Current Duration

[14] IEEE C57.110: IEEE Recommended Practice for Establishing Liquid Immersed and Dry-Type Power and Distribution Transformer Capability when Supplying Nonsinusoidal Load Currents

[15] IEEE C57.113: IEEE Recommended Practice for Partial Discharge Measurement in Liquid Filled

Power Transformers and Shunt Reactors

[16] IEEE C57.119: IEEE Recommended Practice for Performing Temperature Rise Tests on Liquid-

Immersed Power Transformers at Loads Beyond Nameplate Ratings

[17] IEEE C57.120: IEEE Guide for Loss Evaluation of Distribution and Power Transformers and Reactors

[18] IEEE C57.123: IEEE Guide for Transformer Loss Measurement

[19] IEEE C57.127: IEEE Guide for the Detection, Location and Interpretation of Sources of Acoustic

Emissions from Electrical Discharges in Power Transformers and Power Reactors

[20] IEEE C57.131: IEEE Standard Requirements for Tap Changers

[21] IEEE C57.148: IEEE Standard for Control Cabinets for Power Transformers

[22] IEEE C57.149: IEEE Guide for the Application and Interpretation of Frequency Response Analysis for Oil-Immersed Transformers

[23] IEEE C57.150: IEEE Guide for the Transportation of Transformers and Reactors rated 10,000 kVA or Higher

[24] IEEE 693 Recommended Practice for Seismic Design of Substations

[25] IEEE 1276: IEEE Guide for the Application of High-Temperature Insulation Materials in Liquid-

Immersed Distribution, Power, and Regulating Transformers

[26] IEC 60076-1: Power transformers - Part 1: General

2.2 Other Relevant Standards

Standards to be followed include the most recent editions of:

[1] AWS D1.1 Structural Welding Code—Steel

[2] NEMA TR1 Transformers, Regulators, and Reactors

[3] NFPA 70 National Electrical Code

2.3 Company Publications

Applicable company documents include, but shall not necessarily be limited to, those listed below:

[1] Palo Alto-1 Transformer Oil Specification

[2] Palo Alto-2 Transformer Seismic Withstand Requirements

Page 10/73

3 GENERAL

3.1 Application Information

This specification states both the general requirements for transformers and the transformer-specific requirements that vary depending on the installation and intended use.

4 STANDARD CONDITIONS, AND OTHER SERVICE FACTORS

Transformers operating under standard service conditions shall be in accordance with the usual service conditions specified in Section 4.1 of IEEE Standard C57.12.00.

All transformers specified may be exposed to an emergency loading in excess of nameplate rating, in accordance with IEEE C57.91. The transformer may be loaded above nameplate ratings in emergency situations with the knowledge that there will be some sacrifice of life.

The following site-specific factors may affect the installation, and are further described in Annex 1:

• Ambient temperature (Annex 1, Section 4.1)

• Elevation (Annex 1, Section 4.2)

• Other unusual service conditions (Annex 1, Section 4.3)

• Step-up, step-down, or system tie applications (Annex 1, Section 5.1)

• Geomagnetic disturbance (Section 9.2.2.1 of this document)

4.1 Type

The transformer shall be outdoor, 60 Hz, oil-immersed, with 65°C average winding temperature rise, 80°C hot-spot winding temperature rise, and 65°C top-oil temperature rise, suitable for the class of service specified in Annex 1, Section 5.1.

4.2 Seismic Capability

The transformer shall be designed and manufactured for seismic capability in accordance with “Palo Alto-2

Transformer Seismic Withstand Requirements”.

5 RATING DATA

The transformer shall be designed to meet the rating in Section 5 of IEEE Standard C57.12.00 and Section 4 of IEEE Standard C57.12.10. Selections from applicable tables are given in Annex 1. This section lists additional requirements.

5.1 Phase Designation

The phase of the transformer shall be single-phase or three-phase, as specified in Annex 1, Section 5.2

5.2 Kilovolt Ampere (kVA) Ratings

1. Transformers shall be kVA-rated in accordance with Section 4.2.1 of IEEE Standard C57.12.10.

Page 11/73

2. The rating of the transformer shall be as per Annex 1, Section 6.3.

3. The complete transformer, including all components and accessories, shall be in accordance with

IEEE Standard C57.91.

4. No auxiliary component shall limit the transformer windings and cooling system capacities.

5. If a three-winding transformer or a three-winding autotransformer is specified in Annex 1 (Sections

5.3, 6 and 6.3) to have the TV-terminals (tertiary) brought out, the transformer shall be designed such that the tertiary, can be simultaneously loaded by either vectoral or arithmetic, whichever is the worst-case, with the HV fully loaded for Step-Down operation, the LV fully loaded for Step-Up operation, or either the HV or LV fully loaded for respective Step-Down or Step-Up operation as identified in Annex 1, Section 5.1.

5.3 Cooling Class

The cooling class shall be as specified in Annex 1, Section 5.4.

5.4 Polarity or Angular Displacement

If the transformer is single-phase, the polarity shall be subtractive. If the transformer is three-phase, the angular displacement shall be as specified in Annex 1, Section 6.1.

5.5 Impedance

The transformer design shall demonstrate sufficient short-circuit withstand capability (for calculation purpose, the transformer won’t be short-circuit tested) so as to be able to pass a short-circuit test as described in IEEE Standard C57.12.90. The total impedances used for the short-circuit-withstand design of the transformer shall be the transformer impedances only, without considering any benefit of system impedances.

The impedance of the transformer shall be as per Annex 1, Section 6.4.

5.6 Ratings of Transformer Taps

If specified, the de-energized tap changer shall have the tap ratings/range and capacity listed in Annex 1, Section 7.1.

If specified, the load tap changer shall have the tap range and capacity listed in Annex 1, Section 8.1.

5.7 Transformer Bank and Parallel Operation

The following requirements for transformer bank and parallel operation shall apply to all de- energized and load tap positions, with impedances on all tap positions in compliance with IEEE tolerances.

If the transformer is single-phase, and if specified in Annex 1, Section 6.6.1, the transformer shall be suitable for operation in a three-phase bank with the identified similar transformers.

Page 12/73

If the transformer is three-phase, and if specified in Annex 1, Sections 6.6.2, the transformer shall be suitable for step-down operation in parallel with the identified similar three-phase transformer(s) or three-phase transformer bank(s).

5.8 Losses

The manufacturer shall measure the no-load, load, and auxiliary losses as specified in Section 5.9 in IEEE

Standard C57.12.00.

5.8.1 Loss Cost Multipliers

The loss cost multipliers to be used in the loss evaluation method shall be as specified in Annex 1, Section

12.

5.8.2 Loss Evaluation Method

An Equivalent Total Owning Cost (ETOC) will be calculated from the bid price and the present value of the supplier’s guaranteed maximum losses as shown below. The ETOC will be used in determining bid awards.

where:

ETOC = Equivalent total owning cost in dollars

BID PRICE = supplier-quoted bid price in dollars

A = Loss cost multiplier for no-load losses in dollars per watt

B = Loss cost multiplier for load losses in dollars per watt

C = Cost multiplier for auxiliary power in dollars per watt

NL = Guaranteed maximum no-load losses at 20°C in watts

LL = Guaranteed maximum load losses at 85°C in watts, excluding auxiliary power (The load losses shall be quoted at the maximum forced-cooled rating.)

AP = Guaranteed maximum auxiliary power requirement in watts (with all forced-cooling equipment in service)

5.8.3 Loss Penalty

Values of no-load loss and excitation current measured after the completion of Dielectric tests shall be the values used in determining compliance with the Supplier’s quoted loss and excitation performance. These values shall not exceed the values measured before the Dielectric tests by more than 4%.

IEEE tolerances from the supplier’s performance quotation for no-load loss at the nominal rated voltage shall also apply to the excitation current at the nominal rated voltage.

If load tap changing (LTC) equipment is specified, both no-load (NL) and load losses (LL) quoted in the

Supplier’s proposal shall be the average of respective losses at five LTC positions: (1) neutral (nominal rated voltage), (2) maximum lower, (3) one position above maximum lower, (4) maximum raise, and (5) one position below maximum raise position.

Page 13/73

In the event that the combined evaluated cost of actual tested no-load losses (NL), load losses (LL), and auxiliary power (AP) exceeds the combined evaluated cost of the respective guaranteed maximum losses and auxiliary power, credit shall be given to the company for the dollar difference. Any dollar difference shall be deducted from the transformer invoice by the supplier.

No credit will be given for tested losses that are less than guaranteed. There are no allowed tolerances from the quoted guarantee.

Without prior written approval from an authorized company representative, no Supplier shall ship a transformer to the company that exceeds the quoted loss value by:

- by 10% or more for No-load Losses (NL) or Load Losses (LL) or

- by 6% or more total losses (NL + LL)

All guaranteed load losses and no-load losses shall be corrected to reference temperature in accordance with Section 5.9 of IEEE Standard C57.12.00. No other corrections are acceptable. Bidder shall submit method of temperature correction calculations.

Certified loss test data shall be supplied to the Purchaser prior to shipment of the transformer.

6 CONSTRUCTION

6.1 General Requirements

Non-painted steel constructions of transformer shall be made of stainless steel or hot-dip galvanized.

Bolts, nuts, washers and other metal fixing elements, not specified below, shall be made of stainless steel.

6.2 Core Design Requirements

6.2.1 Steel Specification

All cores including main, series, and preventative auto, shall be constructed using high-quality, non-aging, low-loss, cold-rolled, grain-oriented, silicon steel free from rust. Steel is to be slit to width, annealed and coated with inorganic insulating material. All slits and cuts must be free of burrs.

All core steel intended for an individual core/all cores including main, series, and preventative auto, shall be the same core steel type, from the same manufacturer; steel from multiple manufacturing sources shall not be mixed.

6.2.2 Flux Density

With the transformer energized at no-load on any tap position and at any type of tap function such as constant flux, variable flux, combined voltage variation taps etc., at 100% voltage, the maximum flux density in any part of the core shall not exceed 1.7 Tesla.

Page 14/73

At the maximum forced-cooled rating, the maximum flux density in magnetic shunts shall not exceed 1.2

Tesla.

The no-load current at the rated voltage shall be ≤ 0.2 % of the rated current.

On over-excitation condition, with the magnetic flux increased by 10 % of the nominal flux, the no-load current shall be ≤ 0.5 % of the rated current.

On load tap changing transformers with the tertiary (LV) terminals brought out, the transformer shall be designed such that variation of the voltage output of the tertiary winding is allowed, unless otherwise stated in Annex 1 (Section 8.1) that the voltage output must be constant.

6.2.3 Audible Sound Level

The guaranteed combined (no-load and load) sound level for operation on any tap position and at any type of tap function such as constant flux, variable flux, combined voltage variation taps etc. for 100% voltage shall be as specified in Annex 1, Section 6.7.

Special coatings or the addition of sound-absorbing or sound-insulating materials shall not be used to reduce the audible sound level.

6.2.4 Core Temperature

The core internal hot-spot temperature shall be limited to a maximum of 125° C, and a maximum core surface temperature of 120° C (at maximum ambient temperature) at:

1. 100% secondary voltage at no load

2. 105% secondary voltage with the transformer at maximum rated kVA load condition and at 80% power factor.

The surface temperature of 120° C is in consideration of both the flux density in the core, and the heating effects of magnetic field leakage. The insulation material between tie bars and the core, and the core frames and core, shall be a high-temperature material (tolerant of 150° C minimum) that coordinates with the surface temperatures in the locations where this insulation is to be applied. A minimum material thickness of 2 mm shall be provided.

6.2.5 Core Construction and Tie Plate Stress

The core steel quoted at bid shall be used in design and production, any variance must have written approval from the Company. No other metallic material except the core steel and copper strip for core ground shall be part of core.

Step-lap core construction is required. All cores must use a mitered-core design. Lamination edge on the miter joint shall not have any kind of notch.

The edges of the laminations on the core legs shall be protected against rust with a permanent rust-inhibiting coating such as epoxy or varnish; however, the top yoke shall not have epoxy or other bonding coatings applied to it.

Page 15/73

The top and bottom yokes shall be continuous (one single piece yoke/lamination in yoke without any split).

Every core step shall be supported by inserting a nonconductive material between the core step and the base bar that connects the core clamps. The bottom and top of every core step, in every direction, shall be supported from a base bar that connects to the low-voltage and high-voltage core clamps.

Every core step including the step in the core leg shall also be supported by a nonconductive material, such as a dowel, etc (see figure 6.2.5 for reference).

Figure 6.2.5: Core cross-section highlighting the requirement of dowels and nomex under bands

The core shall be properly braced and supported to prevent displacement and deformation during the whole life of the transformer. Except for the cooling channels in the core stack and for the slotted outer core packets, no other separation in the core stack, including partial separation between core steps or within one core step, is allowed. Wherever insulation is installed between core steps visible bonding shall be supplied.

Densified laminated wood with a minimum density of 1.10 g/ cm3 is an approved material for core step blocks. High temperature material (tolerant of 150 °C minimum, such as fiberglass or other equivalently rated material) is also acceptable for core step blocks. The use of non-laminated wood, such as maple is not allowed. Step blocks shall be cut with sufficient precision that gaps between each step and the core laminations shall be limited to 2 mm and the gaps shall be filled with High temperature material (tolerant of

150 °C minimum) during the core construction.

Any material in contact with the core steel shall be a high temperature rated material (tolerant of 150° C minimum, such as fiberglass or NOMEX). The high temperature rated material shall be one piece with a minimum material thickness of 2 mm (0.079 inch). Multiple thin strips of insulation totaling 2 mm thick insulation are not allowed. No cellulose-based material shall be used between core and clamp as cannel plate.

Page 16/73

A minimum NOMEX thickness of 2 mm shall be installed under each core limb band/bandage (see figure

6.2.5 for reference).

A minimum of 75% of the core shall be supported by step blocks. Minimum of (4) support blocks for the center of the core and (2) for the ends of the core.

All the step blocks shall be held in place by clamping pressure.

The entire core must be rust free to ensure its maximum life-cycle performance.

Bolting through the core steel is not acceptable except for preventative autotransformers.

Use of sound proof layer in core to reduce audible sound level is not acceptable.

The design of the bottom core clamps and the tank shall allow inspection of the underside of the bottom yoke after assembly. A design in which the bottom yoke is in a bottom tank trough is not allowed.

The axial mechanical support structure for the core and coils shall not be stressed more 65% of the elastic limit of the material during the worst-case conditions of lifting or a short-circuit.

The core-clamping structure shall rigidly hold the core in place to form a sturdy unit structure, which shall protect the core and the coils from mechanical damage due to shipping accelerations or repeated short-circuits. The core-clamping structure shall be electrically and magnetically insulated from the core.

All steel components of the core-clamping structure shall be shot-blasted prior to construction to remove all rust and mill scale. All corners of the core-clamping structure which are in close proximity to the coils shall be rounded to minimize Radio Interference Voltage (RIV).

6.2.6 Core, Frame and Tank Wall Shunt Ground

For each core and associated clamping structure, a separate insulated cable shall be furnished for grounding.

If tank wall shunts are used and they are not welded to the tank, a separated insulated cable shall be furnished for grounding of the shunts.

A core separated by sections shall have a separate insulated cable for each core section and be brought up to a location near the top of the tank and be connected together internally. This connection shall be removable and shall be designed with captive hardware. The connection location shall be easily accessible from a manhole or handhole on the transformer cover and the location shall be clearly identified on the transformer nameplate and marked on the appropriate manhole or handhole cover. Use of any type of jumper to connect the core separated by sections for grounding purpose shall be avoided.

Core, clamp and shunt grounds shall be individually grounded outside the tank (installed in separate box) by bringing each insulated cable through the tank wall near the top of the tank using bushings that shall be rated to withstand a minimum of 2.5 kV for one (1) minute.

Each core ground and core clamp shall be separately grounded outside the main tank. They shall be labeled

Page 17/73 to avoid confusion with other bushings, and shall be protected with a removable, weatherproof metal cover.

All core and clamp ground bushings shall be located in the same area and be clearly identified on the transformer nameplate. All ground bushings shall be separately grounded outside the main tank.

An instruction nameplate shall be furnished and mounted near the core ground bushing(s) specifying that the external bushing terminal must be connected to the tank whenever the transformer is energized.

The transformer shall be shipped with the ground bushing(s) installed and connections made.

If tank wall shunts are used and they are secured to the tank wall with bolts or clips, they shall be isolated from the tank wall. Groups of the shunts shall be connected together and one or more leads shall be brought up to a cover mounted bushing for external grounding. Shunts that are welded to the tank wall are acceptable.

6.2.7 Core Cooling Ducts

Material used to form cooling ducts in the core shall be held in place by clamping pressure and shall not be cellulose; shall be a high temperature non-metallic material (tolerant of 150 °C minimum, such as fiberglass or other equivalently rated material). Use of glue to hold the core cooling duct material in place is not acceptable. Core cooling duct material shall not be moveable either after the application of final clamping pressure or over time due to release of clamping pressure.

6.3 Winding and Insulation Design Requirements

6.3.1 Conductors

1. All conductor material shall be copper and shall be either rectangular Magnet Wire (MW) or

Continuously Transposed Cable (CTC).

2. All conductors shall be clean, shall be free from oxide or scale, and shall have all rough or high points smoothed down to remove burrs and slivers.

3. All conductors must have cellulose or aramid paper as defined by IEEE 1276 or Netting-type or

Perforated aramid paper type of insulation on it. Bare or enamel coated conductors without any type of insulation on it is not acceptable.

4. All conductor cellulose paper insulation shall be thermally upgraded kraft paper as defined by

ASNI/IEEE C57.100.

5. The use of Netting-type or Perforated aramid paper type of insulation on conductors shall be limited to only Helical type winding up-to 46kV operating voltage. Disc type winding shall not use Netting-type or Perforated aramid paper type of insulation on conductors.

6. The insulating paper shall be applied in either single or multiple strands in such a manner that there are 30% overlap between paper layers.

7. All brazed connections in CTC (Continuously Transposed Conductor) shall be strand-to-strand, i.e., each strand shall be individually brazed and installed.

8. Conductors insulated with Formvar (Vinylec) enamels (or an equivalent type) are not acceptable, except in CTC.

Page 18/73

9. The winding hot-spot shall be the most limiting factor that determines transformer loading. No other accessory or component such as leads or connections shall limit the loading capability. The hot-spot of the leads for an individual winding shall not exceed the maximum hot-spot in that winding.

10. The epoxy used for CTC shall be B stage epoxy.

11. Brazes in all windings, regardless of conductor type, shall be avoided unless absolutely necessary. In cases where brazes cannot be avoided, the braze shall be located on the outside turn of the winding.

6.3.2 Insulation Structures

1. The density of all pressboard spacers and barriers shall be 1.15 g/cm3. The minimum density of the pressboard used for formed parts shall be 0.95 g/cm3.

2. All windings and leads shall have paper insulation that is thermally upgraded (suitable for a hot-spot temperature up to 120° C under daily cyclic loading).

3. Each pressure ring, top and bottom, shall be one piece, of uniform thickness and provide clamping to 100% of the winding circumference. The bottom clamping ring shall be fully supported from below to handle the weight of the windings and the clamping force. The bottom rings shall have a maximum deflection of 2 mm, and the top ring shall have a maximum deflection of 3 mm, with full clamping pressure applied. Coils shall have their full bottom ring circumference sufficiently supported by the frame. There shall be no reduction in the thickness of the top clamping ring in the core window. There shall be no support from the top clamping ring to the top yoke. All clamping rings shall be laminated high density pressboard.

4. Care shall be taken to prevent chafing of the winding insulation due to contact with the lead support structure, for example, by rounding the lead support structure material.

5. The winding cylinders shall be made from a single piece of high-density material (with one seam).

All insulating materials and structures shall be protected from contamination and the effects of humidity during and after fabrication, and after receipt, by storing them in a separate, climate-controlled area.

6. All blocks shall be installed such that the grain is oriented in the horizontal direction, perpendicular to the winding compressive forces.

7. The maximum axial key spacer height shall not be more than 50 mm for all windings with a radial build of 50 mm or greater. If the winding radial build is less than 50 mm, the axial spacer/block height shall be no more than the radial build dimension. Blocks are preferred over a column of key spacers.

When multiple blocks are used, the blocks shall be separated with glued washers. If the axial key spacer height space is more than 50 mm, it shall be separated by full circumference washers to maintain the mechanical stability of the winding.

8. For a tap winding, the total height of a key spacer stack including any washers shall not exceed the lesser of two times the radial build of the winding or 38 mm, otherwise, collars shall be used. The collars shall be made of high-density material with horizontal grain. Rolled type collars constructed of high-density material with the sheets glued together cannot be used without the approval of the

Company. For a layer type tap winding, only collars are allowed at the ends and at the center.

9. Key spacer stacks of 50 mm or greater are not acceptable. Solid rings or end collars must be used.

10. Weidmann Klackband or equivalent (with paper on both sides of the blocks) shall be used for cooling ducts in a winding, where applicable.

Page 19/73

6.3.3 Oil Gap Design

1. For nitrogen pressure units, the oil gap stress shall be limited to 80% of the Weidmann saturated curves.

2. For conservator units the oil gap stress shall be limited to 80% of the degassed curve, as published by Weidmann, for oil gap partial discharge inception.

3. The insulation system shall be designed with a ratio of 2.5 or less between the impulse voltage and the one-minute AC voltage (this is also referred to as the BIL-to-power frequency ratio).

4. The average dielectric stress at any location in the core-and-coil assembly shall not exceed 2.65 kVRMS/mm with the transformer energized at 100% of the rated voltage on the maximum-stress tap position(s). Applicable stresses include, but are not limited, turn-to-turn, winding-to-winding, winding-to-ground, phase-to-phase, and lead-to-lead. However, if the configuration is similar to a plane-to-plane stress, such as a phase-to-phase stress for a center line entry, then the maximum stress of 3.0 kVRMS/mm may be permissible. The stress shall be calculated accurately using a verifiable computer modeling technique.

5. The Supplier shall design the radial cooling ducts with sufficient radial spacer thickness to ensure that cooling and adequate oil flow requirements are met. The design will be validated during the factory acceptance testing (including heat run testing).

6. When using CTC that is fully-insulated with paper, the transformer Supplier shall design the radial cooling ducts to have sufficient radial spacer thickness to meet the cooling requirements. A thickness of 5 mm is preferred, but where experience and modeling demonstrate a lower value is adequate, a reduced thickness of 4.5 mm will be accepted.

7. When using Netting-type or Perforated NOMEX type CTC, the transformer Supplier shall design the radial cooling ducts to have a minimum radial spacer thickness of 3 mm.

6.3.4 Winding Design

For main, series, and preventative auto, the winding design shall be circular.

1. The winding design shall not utilize internal surge protection devices. It is recognized that in the special case of the regulating winding located on the HV-side with a relatively high lightning impulse rating (850 kV or higher applied to the terminal that is directly connected to the regulating winding) or fault duty limitations on tertiaries, where, these devices may be necessary and may be acceptable.

2. The windings shall be disc or helical construction; layer winding will require prior approval from the company. Barrel winding is not acceptable.

3. The maximum current density at the ONAN rating shall not exceed 2.0 amps per square mm.

4. Turns per disc shall be selected in such a way that packing can be avoided or minimized. In any disc within the winding implementation of packing more than 2% of the radial build of the winding will require prior approval from the company.

5. When a layer winding is used, the radial build shall be a minimum of 10 mm for transformers with a self-cooled rating below 75000 kVA. For transformers with a self-cooled rating of 75000 kVA and above, the radial build shall be a minimum of 12 mm. Only one conductor in the radial direction is allowed unless the cable used is CTC.

6. When layer windings are used, each layer shall be designed for “free” buckling, independent of the other layers.

Page 20/73

7. Transpositions in windings or coils wound with multi-conductors in parallel shall be arranged so they will not deform the shape of a coil, and also be so arranged that the voltage difference between adjacent conductors will be kept at a minimum value at the point of transposition.

8. All winding crossovers shall be made in between the key spacer columns. If a manufacturer decides that manufacturing the winding with all crossovers in between the key spacer column is not possible, this shall be listed as an exception to the specification during the bidding stage. An explanation for why the manufacturer is unable to meet this requirement shall be included with the bid documentation, in addition to an explanation of the manufacturer’s proposed method.

9. The crossover conductors between coil groups of the high-voltage winding shall have additional insulation installed between the crossover conductor and the adjacent windings and leads.

10. Crossovers shall not protrude more than 3mm out from the nominal build of the winding as this may endanger the dielectric spacing. Other equipment such as ramps that fill in space leading up to a crossover may also not protrude more than 3mm as this may impede the flow of oil through the winding.

11. All windings subject to inward radial buckling shall be designed to withstand “free” (unsupported) buckling in addition to “forced” (supported) buckling. The control of inward radial forces shall not depend upon bracing to the core. The calculated free buckling and forced buckling stresses shall not exceed 65% of the 0.2% yield stress of the conductor for resin-bonded CTC, and shall not to exceed

35% of the 0.2% yield strength for non-bonded magnet wire. Short-circuit calculations shall be based on 110% of the nominal voltage. Upon completion of the transformer design, the manufacturer shall furnish to the company the calculated free and forced buckling forces and the withstand values, clearly indicating the factors of safety based on worst-case fault conditions. The short circuit calculations shall take into account the mechanical tolerances (offset) of the windings for worst fault condition. The worst fault condition shall be based on the fault current limited only by the impedance of the transformer itself (infinite bus) and fault level currents shall be indicated. The offset used in the calculations shall be per the manufacturer’s tolerances, but no less than 6mm.

12. The regulating winding for reversing type tap changer application shall be separate winding.

13. The regulating winding shall be fully distributed. Use of a brazed-on-tap design with upper and lower parallel groups will require prior approval from the company.

14. In Delta connection the regulating winding/tapping sections shall be electrically connected to the center of the Delta connected winding. Any tap changer electrical connection to the Line END of

Delta connection is not acceptable.

15. The regulating winding shall have equal number of turn(s) for each tap.

16. Multi-start type tap windings shall not be used unless there are at least two turns per tap. Multi-start type tap windings that are located between the core and the LV winding with a current summation of 3000 amps or more (current summation is defined as the number of tap groups per layer times the maximum current at the top nameplate rating) shall be designed as two separate windings with opposite current flow. The insulation between the two tap windings shall be oil duct, barrier, and oil duct. The oil ducts shall be of sufficient size for proper oil flow.

17. LV or TV windings located next to the core shall be designed as two separate windings with opposite current flow when the winding current at the top rating exceeds 3000 amps.

18. Conductor(s) within winding shall not be spliced.

19. All coil spacers shall be keyed using dovetailed “key” spacers to the winding cylinder and to vertical key strips on the outside of the coil (except the outside winding). The sticks are to be captured into the key spacers.

Page 21/73

20. The final coil clamping pressure that shall be applied after vapor phase and prior to tanking shall be equal to or greater than 4N per millimeter squared.

21. If the design requires a series transformer, the core and coil design shall be constructed the same as the main core and coil assembly.

6.3.5 Winding Leads and Supports

1. If there is separate regulating winding on the Low Voltage side, then all the leads from regulating winding shall be brought out on the Low Voltage side. If there is separate regulating winding on the

High Voltage side, then all the leads from regulating winding shall be brought out on the High Voltage side.

2. All winding leads must be sized such that the temperature rise of the leads shall not exceed that of the winding to which they are associated with.

3. All leads shall be clamped to blocks secured by non-metallic bolts. Pads shall be used to protect the leads clamped by lead supports and blocks. There shall be no gap between clamp and lead. Ties and tie-wraps shall not be used. In areas where it is not possible to use lead supports and blocks, pads shall be used underneath the ties or tie-wraps.

4. Lead supports shall be supported from the active part or from the cover, if the cover is an integral part of the active part. In no case shall lead supports, either permanent or temporary, be supported from the tank bottom.

5. All connections shall be firmly supported with reasonable clearances between connections of unlike voltages. Sharp bends shall not be used in interties or terminal leads. Supports for insulated transformer leads shall have rounded edges, either from machining or cardboard angles, to prevent damage to cable insulation during normal movement in transportation or operation.

6. Openings in insulated tubing shall be flared or rounded smooth when used for entrances or exits of insulated leads run inside the tubing.

7. Internal non-metallic support structures provided for support of leads and other internal devices shall be bolted together using non-metallic threaded rods with nuts at each end. Nuts shall be locked in place, either by disrupting threads, tying off threads or epoxying threads, so that nuts cannot come loose due to any amount of vibration or any amount of thermal expansion and contraction while still allowing for disassembly by the application of deliberate force on the nut.

8. Fiberglass studs and nuts are preferred for assembling pressboard support components. If Nylon studs and nuts are used, they shall be at least 16 mm (5/8") in diameter. Every effort shall be made to align holes in the supports to avoid placing undue pressure on the studs. In either case, if the support carries a heavy load such as tap leads or tap switches, steel bolts shall be used at the base of the support where the support connects to a steel member.

9. A minimum of two (2) bolt threads extend beyond the end of the nut for any cable routing structure.

10. All leads in the structure shall be double cleated and all lead support distances shall be such (see reference drawing below) that no lead shall be moveable by hand.

Page 22/73

Figure 6.3.5: Lead structure side view illustrating “double cleating” as required

11. All leads are supported with a maximum span of 70 cm.

12. Each LTC switch lead shall have a maximum unsupported length of 70 cm from the bolted terminal connection on the LTC switch to the location where the lead is clamped with blocks to a fixed lead support.

13. For radial leads that exit above the bottom clamping ring or below the top clamping ring, clamping pressure shall not be applied to the leads and the radial spacer columns in the lead area shall be supported.

14. All leads running from the winding to the OLTC tap changer, DETC tap changer or bushing connector shall not have any splices or joints and shall not contribute to excessive stray loss (such as due to running the high current carrying lead(s) close to metallic frame).

15. Internal drawings (3D model preferred) showing support structures shall be provided for the

Company’s review and comments.

6.3.6 Bolted Connections

All internal, bolted electrical connections shall use two bolts. The only exception is for bolting leads to tap changers where only one bolt connection is provided by the tap changer manufacturer or for bolting leads to terminal boards. Each bolt shall have a compression type washer such as a Belleville washer in addition to the flat washers and double nuts for locking. Split-lock washers shall not be used.

6.3.7 Drying and processing

The manufacturer shall provide detailed description of all preliminary coil sizing and stabilization procedures, including the preload clamping pressure applied to windings and the clamping system used. This description shall include:

• pressures,

• temperatures and drying processes, Page 23/73

• time,

• dimensional tolerances allowed, and

• purpose for each step in the winding preparation.

The core and coil assembly, along with all other internal components, shall be dried by means of vapor-phase processing to assure that all of the insulation material has attained proper dryness and treated prior to being placed in the transformer tank.

6.3.8 Preventative Autotransformer

Helical windings with radial spacers shall be used. Sheet windings are not acceptable. Copper conductor shall be used for winding.

Independent clamping of the windings and core is required. It is preferred that the core shall be clamped first and then the windings (as required on the main core and coil clamping system). The clamping force on the windings shall be considered when calculating the clamping force on the core legs.

The use of a top slab instead of individual top clamping rings and the bottom clamping ring can be omitted if the winding is fully supported at the bottom.

The top and bottom yokes shall be flat.

All the insulation between the top and bottom press beams including the core gap material shall be non-hydroscopic such as fiberglass. Exception: A maximum of 2 mm of high-density pressboard may be used if a stack of Belleville washers of sufficient size is provided on each axial clamping rod.

If steel yoke bolts are used, the HV and LV core clamps shall be isolated from each other and from the core and the tank and a ground lead from each clamp shall be brought to a separate cover-mounted bushing in an enclosure for external grounding. If steel yoke bolts are not used, then it is not necessary to isolate the

HV and LV core clamps from each other.

The preventative autotransformer (PA) shall be tested in air prior to assembly to the main unit at 100% maximum step voltage. The phase voltages and phase currents shall be recorded. Full clamping pressure shall be applied to the core legs for the test. In addition, the noise shall be measured on both sides at a distance of three (3) feet from the PA. The loss, phase voltages, phase currents and average noise shall be reported to the customer after completion and shall also be included in the certified test report.

During final factory acceptance tests, the no-load losses shall be tested in the tap position corresponding to the maximum step voltage and also in the adjacent tap position. The measured no-load losses and excitation currents shall be included in the certified test report.

Page 24/73

6.4 Tap Changers

6.4.1 De-Energized Tap Changers, Switches and Terminal Boards

If specified in Annex 1, Section 7, the de-energized tap changers, switches and terminal boards shall be designed in accordance with the following requirements.

6.4.1.1 Tap Changers and Reconnection Switches

If de-energized voltage taps are specified for the H-winding, X-winding, or both windings of a single-phase transformer, a de-energized tap changer shall be furnished for each specified winding. Each tap changer shall be operated by one external handle.

If de-energized voltage taps are specified for the H-winding, X-winding, or both windings of a three-phase transformer, each tap changer shall be three-phase, or a three-phase internally-ganged assembly, operated by one external handle.

If de-energized series-parallel or wye-delta reconnection by means of a switch is specified for the H-winding, X-winding, or both windings of a three-phase transformer, a de-energized switch shall be furnished for each specified reconnection. Each switch shall be three-phase or a three-phase, internally-ganged assembly, operated by one external handle.

Each tap changer or reconnection switch shall be located under oil, contacts shall be silver plated to minimize coking, and shall be designed to ensure positive positioning and correct external position indication. Each external operating handle, with its associated position indication plate, shall be mounted at a height between one and five feet above foundation level, and shall be furnished with provisions for padlocking in any position.

An identification nameplate shall be furnished and mounted adjacent to each operating handle.

Two bolt connections are preferred, but at a minimum, the connection shall be a locking type, such as a beveled washer or lock nut.

6.4.1.2 Reconnection Terminal Boards

If de-energized series-parallel or wye-delta reconnection by means of a terminal board is specified for the

H-winding, X-winding, or both windings of a three-phase transformer, a terminal board shall be furnished for each specified winding. Each terminal board shall be located under oil, on top of the core and-coil assembly, and shall be arranged for convenient access through a handhole or manhole.