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IEEE Std 1653.1

IEEE Standard Practices andRequirements for Traction Power Rectifier Transformers

VERSION: 1/25/2005

VEHICULAR TECHNOLOGY SOCIETYRAIL TRANSIT VEHICLE INTERFACE STANDARDS COMMITTEE

IEEE Traction Power Substation Standards Subcommittee (TPSSC)Working Group 21

Abstract: This standard is a basis for the establishment of performance, interchangeability, and safety requirements of traction power rectifier transformers and provides assistance in the proper selection of such transformers. Electrical and mechanical design, manufacturing, and testing requirements are set forth for traction power rectifier transformers of all power ratings operating at DC electrification systems at voltages up to 1,500 VDC. The standard covers liquid-immersed and dry-type transformers, including those with encapsulated windings and cast coils.

Keywords: traction power rectifier transformers, electrical requirements, mechanical requirements, service conditions, rating, design, construction, short circuit, protection, indication, design optimization, testing.

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IEEE Std. 1653.1 IEEE Standard Practices and Requirements for

Table of Contents

1 OVERVIEW........................................................................................................................ 11.1 Scope.................................................................................................................... 1

1.2 Purpose................................................................................................................. 1

1.3 Mandatory and Recommended Requirements......................................................1

2 REFERENCES.................................................................................................................. 1

3 DEFINITIONS.................................................................................................................... 2

4 SERVICE CONDITIONS....................................................................................................24.1 General................................................................................................................. 2

4.2 Usual Service Conditions......................................................................................34.2.1 Ambient Temperature...............................................................................34.2.2 Fluctuating Loads......................................................................................34.2.3 Short Circuits............................................................................................34.2.4 Voltage and Current Harmonics................................................................34.2.5 Operation Above Rated Voltage or Below Rated Frequency....................34.2.6 Vibrations..................................................................................................34.2.7 Altitude......................................................................................................3

4.3 Unusual Service Conditions..................................................................................3

5 RATING DATA.................................................................................................................. 45.1 Continuous Rating.................................................................................................4

5.2 Overload Ratings..................................................................................................4

5.3 Load Cycle............................................................................................................45.3.1 General.....................................................................................................45.3.2 Standard Load Cycles...............................................................................55.3.3 Custom Load Cycle...................................................................................5

5.4 Winding Temperature Limits.................................................................................65.4.1 Light Traction Service Load Cycle............................................................65.4.2 Heavy Traction Service Load Cycle..........................................................65.4.3 Extra-Heavy Traction Service Load Cycle................................................75.4.4 Custom or User-Defined Load Cycle........................................................8

5.5 Nameplates...........................................................................................................8

6 DESIGN............................................................................................................................. 86.1 Taps...................................................................................................................... 8

6.2 Impedance............................................................................................................96.2.1 Secondary Winding Coupling....................................................................96.2.2 Standard and Reduced Regulation...........................................................9

6.3 Two-Winding Transformers...................................................................................96.3.1 Winding Connections for Six-Pulse Rectification......................................96.3.2 Winding Connections for Twelve-Pulse Rectification................................9

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Traction Power Rectifier Transformers IEEE Std. 1631.1

6.4 Three-Winding Transformers................................................................................96.4.1 Winding Connections Six-Pulse Rectification...........................................96.4.2 Winding Connections for Twelve-Pulse Rectification..............................106.4.3 Tertiary Winding for Auxiliary Power.......................................................10

6.5 Four-Winding Transformers................................................................................106.5.1 Winding Connections for Twelve-Pulse Rectification..............................106.5.2 Quaternary Winding for Auxiliary Power.................................................10

6.6 Secondary Winding Voltage, Current, and Impedance Differences....................106.6.1 Voltage Difference..................................................................................106.6.2 Current Difference...................................................................................116.6.3 Impedance Difference.............................................................................11

6.7 Design Optimization............................................................................................116.7.1 General...................................................................................................116.7.2 Life Cycle Cost........................................................................................116.7.3 Efficiency................................................................................................116.7.4 Loss Measurement Temperature............................................................12

6.8 Life Expectancy...................................................................................................12

7 CONSTRUCTION............................................................................................................127.1 Core and Winding Construction..........................................................................12

7.2 Connecting Cables..............................................................................................12

7.3 Insulation System................................................................................................12

7.4 Winding Conductors............................................................................................13

7.5 Assembled Coils.................................................................................................13

7.6 Thermocouple Location.......................................................................................13

8 SHORT CIRCUIT CHARACTERISTICS..........................................................................138.1 Liquid-Filled Transformers..................................................................................13

8.2 Dry-Type Transformers.......................................................................................13

9 PROTECTION.................................................................................................................139.1 General...............................................................................................................13

9.2 Overcurrent Protection........................................................................................139.2.1 Overload and Phase-Fault Protection.....................................................139.2.2 Ground-Fault Protection..........................................................................14

9.3 Overtemperature Protection................................................................................14

9.4 Unbalance Protection..........................................................................................14

9.5 Gas Pressure Protection.....................................................................................14

9.6 Gas Pressure Relief Device................................................................................14

9.7 Overvoltage Protection........................................................................................14

9.8 Accessories.........................................................................................................14

10 TESTING.........................................................................................................................14

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IEEE Std. 1653.1 IEEE Standard Practices and Requirements for

10.1 General...............................................................................................................14

10.2 Factory Testing...................................................................................................1510.2.1 General.................................................................................................1510.2.2 Load Cycle Test....................................................................................1510.2.3 Short Circuit Test..................................................................................1510.2.4 No-Load Loss Test................................................................................1510.2.5 Load Losses Test at Various Loadings.................................................1510.2.6 Partial Discharge Test...........................................................................1510.2.7 Commutating Impedance......................................................................15

10.3 Field Testing.......................................................................................................1610.3.1 Pre-Energization Inspection Testing.....................................................1610.3.2 Line-to-Rail Short Circuit.......................................................................1610.3.3 Line-to-Ground Short Circuit.................................................................16

11 TOLERANCES................................................................................................................16

12 WINDING CONNECTION FOR SHIPMENT....................................................................16

13 Bibliography.....................................................................................................................16

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IEEE Standard Practices andRequirements for Semiconductor

Traction Power Rectifier Transformers

1 Overview

1.1 Scope

This standard covers design, manufacturing, and testing unique to application of power rectifier transformers intended to operate in DC rail transportation and trolley bus substation applications.

1.2 Purpose

Provide a supplement to IEEE Std C57.18.10 to cover requirements specific to traction power rectifier transformers supplying power to DC powered rail and trolley bus transportation equipment. The purpose of this standard is to provide minimum uniform transportation industry requirements.

1.3 Mandatory and Recommended Requirements

The words “shall” and “must” indicate mandatory requirements. The words “should” and ”may” indicate requirements that are recommended and permitted, but not mandatory.

2 References

References should be made to the latest versions of the following standards:

IEEE Std C57.12.00, Standard General Requirements for Liquid-Immersed Distribution, Power, and Regulating Transformers (ANSI)

IEEE Std C57.12.01, Standard General Requirements for Dry Type Distribution and Power Transformers Including Those with Solid Cast and/or Resin-Encapsulated Windings

IEEE Std C57.12.80, Standard Terminology for Power and Distribution Transformers

IEEE Std C57.12.90, Standard Test Code for Liquid-Immersed Distribution, Power, and Regulating Transformers and Guide for Short-Circuit Testing of Distribution and Power Transformers (ANSI)

IEEE Std C57.12.91, Test Code for Dry-Type Distribution and Power Transformers

IEEE Std C57.18.10, Standard Practices and Requirements for Semiconductor Power Rectifier Transformers

IEEE Std C57.96, IEEE Guide for Loading Dry-Type Distribution and Power Transformers

IEEE Std C.57.120, IEEE Loss Evaluation Guide for Power Transformers and Reactors

IEEE Std 100, IEEE Standard Dictionary of Electrical and Electronics Terms

3 Definitions

Standard transformer terminology available in IEEE Std C57.12.80 shall apply. Other electrical terms are defined in the IEEE Std 100.

Dry-Type Transformer. Transformer cooled by the natural circulation of air. Dry-type transformers include transformers with encapsulated windings or cast coils.

Life Cycle Cost or Total Ownership Cost: the sum of the procurement cost and the cost of energy losses over the transformer expected life.

Liquid-Immersed Transformer. Transformer having its core and coils immersed in liquid. Cooling is achieved by natural or forced circulation of air over cooling surfaces and by natural or forced circulation of liquid over the core and coils.

Load Factor: Ratio of average power demand to peak power demand in the same interval of time.

Multiple-Feed System: Traction electrification system in which substation feed sections of the distribution system at multiple points. Under normal conditions, the distribution system operates as a continuous bus where all sectioning gaps are bypassed by closed circuit breakers or disconnect switches.

Power Demand Analysis and Load-Flow Study: A computer-aided study using specially written computer program to calculate the combined performance of the traction power supply and traction power distribution systems with operating trains. The study results normally include distribution system voltages, distribution system currents, substation power demand requirements and substation energy consumption.

Rolling Stock: Transit vehicle or train receiving motive power from a traction power distribution system.

Traction Electrification System: Traction power supply, traction power distribution, and traction power return systems.

Traction Power Distribution System: Overhead catenary system, overhead contact wire system, third rail system, four-rail system, or guideway contact busbar system which may be accompanied by parallel along track feeders. The system also includes positive feeders from substation circuit breakers to the trackside.

Traction Power Supply System: Traction power substations located at predetermined spacings along the system route.

Traction Power Return System: Running rails or guideway contact busbar system, impedance bonds, and cross-bonds. The system also includes negative feeders from the trackside to substation negative busbar.

4 Service Conditions

4.1 General

Transformers conforming to this standard shall be suitable for operation at rated kVA under following usual service conditions.

4.2 Usual Service Conditions

4.2.1 Ambient Temperature

Outdoor- and indoor-installed transformers shall be capable of operating in an average daily ambient temperature of 30°C, with a maximum ambient temperature of 40°C and minimum temperature of -20°C.

4.2.2 Fluctuating Loads

Traction power rectifier transformers shall be capable of operating under rapidly changing and fluctuating loads typical of traction power systems. The load fluctuation is caused as rolling stock accelerates, decelerates, encounters alignment gradients, and enters different substation feeding sections.

4.2.3 Short Circuits

Traction power systems are characterized by high incidence of short circuits. On occasions, a system may experience several short circuits in a day.

4.2.4 Voltage and Current Harmonics

Traction power rectifier transformers shall be capable of operating under expected load with voltage and current harmonics caused by substation rectifiers and rolling stock propulsion system electronics. During preparation of transformer specification, the designer should contact the rectifier and rolling stock manufacturers, if known, and obtain the expected harmonic spectra under various loading conditions. As a minimum, the transformer manufacturer must receive the expected harmonic content at 100% loading and 450% loading.

In the event that the rectifier and rolling stock manufacturers are not known, the rectifier harmonic spectrum in accordance with IEEE Std C57.18.10 shall be used with a reasonable adjustment for rolling stock harmonics.

4.2.5 Operation Above Rated Voltage or Below Rated Frequency

Traction power rectifier transformers shall be capable op operation above rated voltage or below rated frequency in accordance with IEEE Std C57.12.00.

4.2.6 Vibrations

Traction power substations are generally located close to the system operating envelope. Therefore, the transformers will be subjected to the vibrations due to passage of trains.

4.2.7 Altitude

The usual service altitude shall not exceed 3,300 ft (1,000 m).

4.3 Unusual Service Conditions

Conditions other than those described in 4.2 are considered unusual service conditions. When prevalent, the conditions should be brought to the attention of those responsible for design and application of the transformer. Unusual service conditions may constitute any one or combination of the following:

Operation at higher than maximum ambient temperature Lower than minimum ambient temperature

Operation above the load cycle Operation in high altitude Abnormal vibration or tilting

Further examples of unusual operating conditions are given in IEEE Std C57.12.00.

5 Rating Data

5.1 Continuous Rating

The traction power rectifier transformer kVA rating shall be on the fundamental basis in accordance with IEEE C57.18.10. All electrical characteristics, such as efficiency, regulation, losses, impedance, and commutating impedance, shall use the fundamental kVA as the base kVA.

The thermal capability of the transformers shall be tested using root-mean-square (rms) kVA in accordance with IEEE C57.18.10. The rms kVA must include the fundamental kVA plus kVA due to the specified harmonics caused by rectifiers and rolling stock.

The transformer continuous rating should take into account planned load growth, compensate for a transformer or substation outage, and accommodate special or unusual transit system operation.

The continuous kVA rating is recommended to be the transformer self-cooled rating designated as follows:

Liquid-Immersed Transformers - Self-Cooled (ONAN) Rating Dry-Type Transformers - Self-Cooled (AA) Rating

5.2 Overload Ratings

In order to provide capability to service planned load growth beyond the current projections, it necessary to specify overload ratings. The overload rating is recommended to be the transformer air-cooled rating designated as follows:

Liquid-Immersed Transformers - Self-Cooled/Forced Air-Cooled (ONAN/ONAF) Rating Dry-Type Transformers - Self-Cooled/Forced Air-Cooled (AA/AF) Rating

It is recommended that the transformers are rated so that they can be initially installed as self-cooled. The fans, if needed, are recommended to be installed in the future when the load increases beyond the projected load growth. This requires that the transformers are specified and manufactured with full provisions for future fan installation.

In unusual situations where space for liquid-immersed transformers is limited, air- and oil-cooled rating may be used, designated as follows:

Self-Cooled/Forced Air-Cooled/Forced Liquid-Cooled (ONAN/ONAF/OFAF) Rating

The designer should be aware that provision of cooling fans and oil pumps introduces additional complexity, maintenance, and possibly remote supervision.

5.3 Load Cycle

5.3.1 General

Because of the traction load fluctuation and high demand peaks, the traction power rectifier transformers must have the capability to supply the rated power continuously with a superimposed overload cycle representing a rush period operation twice a day.

5.3.2 Standard Load Cycles

Normally, the overload cycle is specified in accordance with the IEEE Std C57.18.10. The following standard load cycles are recommended for traction service:

5.3.2.1 Light Traction Service Load Cycle

The Light Traction Service capability is defined as follows:

100% of rated power continuously 150% of rated power for 2 hours 200% of rated power for 1minute

Subject to traction power system load-flow study of the transit system, the Light-Traction Service load cycle can be used for street car, trolley bus, and some people mover transit applications.

5.3.2.2 Heavy Traction Service Load Cycle

The Heavy Traction Service capability is defined as follows:

100% of rated power continuously 150% of rated power for 2 hours 300% of rated power for 1minute

Subject to traction power system load-flow study of the transit system, the Heavy-Traction Service load cycle can be used for street car, trolley bus, people mover, and some light rail transit applications.

5.3.2.3 Extra-Heavy Traction Service Load Cycle

The Extra-Heavy Traction Service capability is defined as follows:

100% of rated power continuously 150% of rated power for 2 hours 300% of rated power for five equally spaced periods of 1 minute 450% of rated power for the final 15 seconds

Subject to traction power system load-flow study of the transit system, the Heavy-Traction Service load cycle can be used for all light rail transit applications and all heavy rail applications.

5.3.3 Custom Load Cycle

In transit applications where the overload cycle is substantially different than the Light-Traction Service, Heavy-Traction Service, or the Extra-Heavy Traction Service as defined by C57.18.10, a custom load cycle may be specified.

The custom load cycle should be developed based on the traction power system load-flow studies, load measurement on existing systems, or equivalent calculation or simulation methods. Unusual overload operation associated with such conditions as special events and adjacent unit failure shall be assessed based on actual operating history of similar systems. This assessment is to be used to define the amount of unusual overload and the portion of total operating time this overload represents.

The results of these analyses shall be applied in accordance with IEEE 57.96 to determine the thermal capacity necessary to meet the required design life. This required design life shall be specified.

Operation, especially during unusual overload conditions, may result in insulation hot spot temperatures exceeding those of the insulation class, which is defined by 20 year life at class temperature. Design tests must be performed to demonstrate that these peak temperatures will not result in mechanical or dielectric discontinuities, which could result in catastrophic failure. Suggested design tests should include thermal shock and dielectric tests at peak temperature on the first unit which would represent stresses in the production units.

5.4 Winding Temperature Limits

5.4.1 Light Traction Service Load Cycle

Winding hottest-spot temperature and average winding temperature shall not exceed the limits given in Table 1 for liquid-immersed transformers and the limits given in Table 2 for dry-type transformers, taking into account the maximum ambient temperature specified. The limits apply at the tap connection resulting in the highest losses.

The hottest-spot temperature shall be determined by calculation or from temperature test data. The average temperature shall be measured by the resistance method in accordance with IEEE Std C57.12.90 and IEEE Std C57.12.91.

Table 1 – Limits of Temperatures for Liquid-Immersed Rectifier Transformer WindingsLight Traction Service

Insulation System

Temperature Class (°C)

Load in % of Rated Power

Winding Hottest-Spot Temperature

(°C)

Average Winding

Temperature (°C)

Operation

120100 110 105 Continuous at rated power until temperature steady-state.

200 140 135 Transformer reached a steady-state temperature at 100% load and operated at 150% overload for two hours, at 200% overload for one minute.

Table 2 – Limits of Temperatures for Dry-Type Rectifier Transformer WindingsLight Traction Service

Insulation System




(°C)

Average Winding

Temperature (°C)

Operation

150100 140 90 Continuous at rated power until temperature reaches steady-state value.






5.4.2 Heavy Traction Service Load Cycle



Table 3 –Limits of Temperatures for Liquid-Immersed Rectifier Transformer WindingsHeavy Traction Service

Insulation System




(°C)

Average Winding

Temperature (°C)

Operation

120100 110 105 Continuous at rated power until temperature steady-state.


Table 4 – Limits of Temperatures for Dry-Type Rectifier Transformer WindingsHeavy Traction Service

Insulation System




(°C)

Average Winding

Temperature (°C)

Operation







5.4.3 Extra-Heavy Traction Service Load Cycle



Table 5 – Limits of Temperatures for Liquid-Immersed Rectifier Transformer WindingsExtra-Heavy Traction Service

Insulation System




(°C)

Average Winding

Temperature (°C)

Operation

120


450 140 135Transformer reached a steady-state temperature at 100% load and operated at150% overload for two hours, at 300% overload for five 5 minute intervals, and at 450% overload for 15 seconds.

Table 6 – Limits of Temperatures for Dry-Type Rectifier Transformer Windings Extra-Heavy Traction Service

Insulation System




(°C)

Average Winding

Temperature (°C)

Operation

150



185



220



5.4.4 Custom or User-Defined Load Cycle

The data in Tables 1 through 6 apply to transformers rated to supply the recommended load cycles defined in IEEE Std C57.18.10. For transformers rated to supply a custom load cycle, similar temperature limits are recommended for specification and design.

5.5 Nameplates

The transformer nameplates shall be in accordance with IEEE Std C57.18.10.

6 Design

6.1 Taps

The traction power rectifier transformers should be equipped with nominal voltage and ±2 x 2.5% full capacity, no-load taps.

In applications where the local utility system voltages between consecutive substations are not equal, transformers with nominal and ±4 x 1.25% full capacity, no-load taps are recommended.

6.2 Impedance

6.2.1 Secondary Winding Coupling

The relationship between transformer impedance, secondary voltage regulation and short circuit current fault level is largely given by the coupling of the transformer wye and delta secondary windings. The more closely coupled the windings, the higher voltage regulation and the lower short circuit current.

It is recommended that the transformer specification clearly states the desired voltage regulation at various loadings and the maximum permissible short circuit current at the substation output. The transformer regulation calculation should take into account the harmonic content of the transformer load current.

The substation equipment includes the traction power rectifier transformer, the rectifier, interphase transformer, and any associated AC and DC busbars. In order to prevent the transformer manufacturer to be put into a position of designing the substation, it is the responsibility of the transformer specification engineer to supply to the transformer manufacturer the voltage drops in the substation equipment or require in the specification coordination between the transformer, rectifier, and interphase transformer manufacturers.

6.2.2 Standard and Reduced Regulation

Normally, transformers are manufactured to produce a standard 6% regulation in the traction power substation equipment.

Lower transformer impedance is beneficial as it decreases substation volt drop, increases voltage available to rolling stock, and decreases the system losses. For new system installations, the feasibility of reducing the impedance to 4% should be investigated taking into account the fault interrupting capability of the DC switchgear and the associated protective relaying.

6.3 Two-Winding Transformers

6.3.1 Winding Connections for Six-Pulse Rectification

For six-pulse rectification, the following transformer primary/secondary winding connections are recommended:

Delta/delta winding connections suitable for rectifier connection as per ANSI circuits 25. Wye/delta winding connections suitable for rectifier connection as per ANSI circuits 26.

6.3.2 Winding Connections for Twelve-Pulse Rectification

For twelve-pulse rectification, two two-winding transformers are required to be connected in parallel with the following transformer primary/secondary winding connections:

Two-transformer arrangement with delta/delta winding connections on one transformer and wye/delta winding connections on the other transformer suitable for rectifier connection as per ANSI circuits 25 & 26.

6.4 Three-Winding Transformers

6.4.1 Winding Connections Six-Pulse Rectification

For six-pulse rectification, using a three-winding transformer, the following transformer primary/secondary winding connections are recommended:

Transformer arrangement with delta/delta winding connections suitable for rectifier connection as per ANSI circuits 25. The third, or tertiary, winding is used for auxiliary power requirements.

Transformer arrangement with wye/delta winding connections suitable for rectifier connection as per ANSI circuits 26. The third, or tertiary, winding is used for auxiliary power requirements.


For twelve-pulse rectification, using a three-winding transformer, the following transformer primary/secondary winding connections are recommended:

Delta primary winding and two secondary windings, one connected in wye and one connected in delta providing a 30° phase shift and suitable for rectifier connection as per ANSI circuit 31.

6.4.3 Tertiary Winding for Auxiliary Power

In general, it is recommended that the auxiliary power requirements be met by a separate auxiliary transformer and not by the use of a tertiary winding part of the traction power transformer. The disadvantages of a separate auxiliary transformer include:

More complex traction power transformer design The substation auxiliary power is deenergized when the AC circuit breaker is opened

6.5 Four-Winding Transformers


For twelve-pulse rectification, using a four-winding transformer, the following transformer primary/secondary winding connections are recommended:

One delta primary winding and two secondary windings, one connected in wye and one connected in delta providing a 30° phase shift and suitable for rectifier connection as per ANSI circuit 31. The fourth, or quaternary, winding is used for auxiliary power requirements.

Two delta primary windings, one wye secondary winding, and one secondary delta winding connected to be suitable for rectifier connection as per ANSI circuits 25 & 26.

6.5.2 Quaternary Winding for Auxiliary Power

In general it is recommended that the auxiliary power requirements be met by a separate auxiliary transformer, and not by use of a quaternary winding part of the rectifier transformers. The disadvantages of using a quaternary winding are the same as the disadvantages of using tertiary winding discussed above.

6.6 Secondary Winding Voltage, Current, and Impedance Differences

6.6.1 Voltage Difference

For a three-winding transformers, the voltage difference between secondary delta and wye windings shall not exceed 0.28% of the transformer secondary winding no-load voltage. This voltage difference shall include the permissible voltage distortion limits in accordance with IEEE Std 519.

When the difference exceeds 0.28% of the transformer secondary winding no-load voltage, use of compensating transformers is recommended to decrease the difference.

6.6.2 Current Difference

For a three-winding transformers, the current difference between secondary delta and wye windings shall not exceed 10% of the transformer secondary winding rated current.

6.6.3 Impedance Difference

For a three-winding transformers, the impedance difference between secondary delta and wye windings shall not exceed 5%.

6.7 Design Optimization

6.7.1 General

Whenever allowable by the Owner’s procurement procedures, the transformer specification should require design and material selection to be optimized to result in units of the lowest possible life cycle cost or total ownership cost. In the event that the procurement procedures do not allow the lowest possible life cycle cost approach, minimum acceptable efficiency can be specified.

6.7.2 Life Cycle Cost

The transformer life cycle cost or total ownership cost should be determined in accordance with IEEE Std C57.120. Alternatively, the following procedure can be used:

1. Since the transformers are required to supply highly fluctuating power demand, the preparer of the transformer specification should provide in the request for bid the duration, in hours/year, of no-load operation and operation at power demand at several transformer loadings, for example at 50%, 100%, 150%, 200%, and 300% of rated power. This data can be developed during the traction power system simulations and load-flow studies.

2. An equation should be provided in the specification that calculates the present worth of the transformer losses over the transformer expected life. This equation should include the number of identical transformers, the total annual energy losses per transformer, energy rate, as well as energy escalation rate and interest rate over the expected life of the transformer. For different transformers, individual calculations are required.

3. The manufacturer should be required to provide in their bids the transformer no-load losses and load losses at the same power demands as indicated above, i.e., 50%, 100%, 150%, 200%, and 300% of rated power.

4. Based on the losses and their duration, the total annual energy loss of the transformer can be determined. Using the equation in the specification, the manufacturer can calculate the present worth of the transformer losses over the transformer expected life. This cost will be required to be presented in the manufacturer’s bid and will become the Guaranteed Cost of Losses.

5. When the first transformer is manufactured, the specification should require tests to determine the no-load and load losses at loadings mentioned in item 1.

6. The manufacturer should recalculate the present worth of the losses based on the measured values.7. Should the present worth of the losses based on the measured values be higher than the

Guaranteed Cost of Losses in the contract, the manufacturer should be required to pay the difference to the customer.

The transformer life expectancy over which the life cycle cost should be calculated is recommended to be 30 years.

6.7.3 Efficiency

The highest efficiency of transformer design occurs when the no-load loss equals load loss, at the load factor demand. Due to highly fluctuating nature of traction loads, the load factor of rectifier transformers is typically low. It is recommended that the specification preparer determines the average load factor for the rectifier transformers and specifies the efficiency at that power demand. The load factor must take into account the night period when the system is not operational, the rush-hour period with peak power demands, the off-peak period, as well as long-term load variations caused by substation outages.

Traction power rectifier transformers rated 500 kVA and below should be designed with minimum efficiency of 98% at load factor power demand.

Traction power rectifier transformers rated 501 kVA and above should be designed with minimum efficiency of 98.5% at load factor power demand.

6.7.4 Loss Measurement Temperature

The loss measurements for the life cycle cost or the efficiency determination should be specified. For transformers with load factor of lower than 0.2, the reporting temperature of ambient plus 10°C is recommended, for transformers with load factor higher than 0.2, the reporting temperature of ambient plus 20°C is recommended

6.8 Life Expectancy

The life expectancy for a new transformer is recommended to be specified as 30 years minimum. The manufacturer shall provide the life expectancy of the transformer on the basis of load cycle provided by the purchaser, the design temperature rise and insulation properties per IEEE Std C57.91 for liquid immersed transformers and IEEE Std C57.96 for the dry type transformers. Life expectancy shall be calculated for the daily load curve provided by the purchaser.

7 Construction

7.1 Core and Winding Construction

The fluctuating load currents and relatively high incidence of heavy fault currents produce pulsating forces and mechanical stresses in the transformer core and windings. These forces and stresses may cause axial and radial movement of the coils and eventual transformer failure. It is recommended that the specifications include the requirement for augmented mechanical strength of the transformer core and include an internal bracing system for windings. Winding and tap connections should be located to minimize their movement and damage.

7.2 Connecting Cables

Cables connected to the transformer primary windings shall be insulated at the same voltage class insulation level at the primary system.

Cables connected to the transformer secondary windings shall be insulated at the same voltage class insulation level at the secondary system.

All primary and secondary cables must be supported and braced to prevent movement during severe overload conditions and short circuits.

7.3 Insulation System

Liquid-immersed transformers are recommended to be constructed with 120°C temperature class insulating materials.

Dry-type transformer insulation temperature class depends on the insulation type used. The following temperature classes are recommended:

Polyester insulation: 220°C Epoxy insulation: 150°C or 185°C Cast coil transformers: 150°C or 185°C

7.4 Winding Conductors

All transformer windings shall be manufactured from high conductivity electrolytic copper. Use of aluminum windings shall not be permitted.

7.5 Assembled Coils

Winding coils for liquid-immersed transformers should be assembled to provide for adequate flow of the cooling liquid.

Winding coils for dry-type transformer should be assembled to provide adequate ventilation by convection or forced air.

7.6 Thermocouple Location

8 Short Circuit Characteristics

8.1 Liquid-Filled Transformers

8.2 Dry-Type Transformers

9 Protection

9.1 GeneralTraction power rectifier transformers rated 500 kVA and above connected to a utility system at 4.16 kV and above shall be protected by a three-phase circuit breaker. A metal-clad, draw-out circuit breaker incorporating vacuum interrupter is recommended.

Traction power rectifier transformers rated below 500 kVA and connected to a power system at 480 V shall be protected by a three-phase circuit breaker. A molded-case circuit breaker is recommended.

9.2 Overcurrent Protection

9.2.1 Overload and Phase-Fault Protection

Transformer overload and phase-fault protection is recommended to incorporate time-overcurrent relay providing adequate protection at currents resulting from overloads above the design load cycle and at currents resulting from phase-to-phase and three-phase short circuits. A relay incorporating an instantaneous element and a custom time-current characteristic capable to be set just below the transformer time-current withstand curve is recommended.

9.2.2 Ground-Fault Protection

Transformer ground-fault protection is recommended to incorporate time-overcurrent relay providing adequate protection at currents resulting from phase-to-ground short circuits. A relay incorporating an instantaneous element and inverse time-current characteristic is recommended.

9.3 Overtemperature Protection

Transformer winding overtemperature protection is recommended to incorporate a two stage device with temperature detection probes embedded in the transformer windings. The setting of each stage shall be adjustable over a range of temperatures. One stage shall be set to provide an alarm and one stage shall be set to trip the transformer circuit breaker

9.4 Unbalance Protection

Unbalance protection shall be provided for transformers being supplied by utility system feeders equipped with fuses. The relay shall be set to protect the transformer when one or two utility system fuses blow.

9.5 Gas Pressure Protection

All liquid-immersed transformers shall be equipped with sudden pressure protection.

9.6 Gas Pressure Relief Device

All liquid-immersed transformers shall be equipped with automatic pressure relief device.

9.7 Overvoltage Protection

Transformers shall be protected against overvoltages resulting from lightning and switching surges. The protection shall incorporate surge arrestors installed as close to the transformer as possible.

9.8 Accessories

Accessories for liquid-immersed transformers shall be selected in accordance with the following standards:

Liquid Immersed Transformers: IEEE Std C57.12.00-1998 and IEEE Std C57.12.90-1993, and IEEE Std C57.18.10-1998

Dry-type Transformers: IEEE Std C57.12.01-1998, IEEE Std C57.12.91-1995, and IEEE Std C57.18.10-1998

10 Testing

10.1 General

All routine, design and other tests are defined in IEEE Std C57.12.80, and shall be performed in accordance with the latest edition of the following standards:

Liquid Immersed Transformerso IEEE Std C57.12.00o IEEE Std C57.12.90o IEEE Std C57.18.10

Dry-type Transformerso IEEE Std C57.12.01o IEEE Std C57.12.91o IEEE Std C57.18.10

In addition to these test, the test described in the following sections are recommended.

10.2 Factory Testing

10.2.1 General

Unless otherwise specified, all test required by IEEE Std C57.12.00, IEEE Std C57.12.01, and IEEE Std C57.18.10 are required.

The tests must be performed in accordance with IEEE Std C57.12.90, IEEE Std C57.12.91, and IEEE Std C57.18.10.

10.2.2 Load Cycle Test

Load cycle test shall be performed as specified. Two options are available:

1. Applying actual load cycle on the transformer by changing the primary voltage with the secondary windings short-circuited

2. Applying a constant equivalent rms load for the duration of the load cycle. For extra-heavy traction service, the equivalent rms is 161% of the continuous rating.

10.2.3 Short Circuit Test

10.2.4 No-Load Loss Test

10.2.5 Load Losses Test at Various Loadings

10.2.6 Partial Discharge Test

Partial discharge test should be performed on the first transformer unit manufactured. At the option of the transformer specifying engineer, the partial discharge test may be performed on all units.

10.2.7 Commutating Impedance

Commutating Impedance shall be measured in accordance with IEEE Std C57.18.10.

10.3 Field Testing

10.3.1 Pre-Energization Inspection Testing

Pre-energization testing should be performed in accordance with the manufacturer’s instructions. As a minimum, following inspections and tests should be performed:

1. Visual inspection of transformer to verify that there is no damage to the transformer or its accessories

2. Verification of the safety aspects such as grounding of the tank, grounding of the neutral (if present), grounding of the secondary terminals of the instrument transformers, and grounding of the surge arrester connections.

3. Verification of the ground grid resistance.4. Removal of CT shorting links, if installed.5. Relay primary injection testing to verify correctness of CT connections.6. Verification of the readiness of the trip circuit from the relays.7. Measurement of winding resistance at all taps and comparison with the factory test values.8. Measurement of insulation resistance of the transformer with a suitable megger.9. Measurement of the winding ratio at all taps.10. Testing of the insulating oil per ASTM-D3487 in case of liquid filled transformers11. Insulation power factor test.

10.3.2 Line-to-Rail Short Circuit

10.3.3 Line-to-Ground Short Circuit

11 Tolerances

12 Winding Connection for Shipment

13 Bibliography

IEEE Std 62-1995, Guide for Diagnostic Field Testing of Electric Power Apparatus (ANSI)1

IEEE Std 259-1994, Standard Test Procedure of Evaluation of Systems of Insulation for Specialty Transformers (ANSI)2

ANSI C57.12.10-1988, Safety Requirements for Transformers, 230 kV and Below, 833/958 Through 8333/10 417kVA, Single-Phase, and 750/862 Through 60 000/80 000/100 000 kVA, Three-Phase Without

1 ANSI standards are available from the Sales Department, American National Standards Institute, 11 West 42nd Street, 13th Floor, New York, NY 10036, USA (http://www.ansi.org/).

2 IEEE publications are available from the Institute of Electrical and Electronics Engineers, 445 Hoes Lane, P.O. Box 1331, Piscataway, NJ 08855-1331, USA (http://www.standards.ieee.org/).

Load Tap Changing; and 3750/4687 Through 60 000/80 000/100 000kVA with Load Tap Changing

ANSI C57.12.20-1997, Standard for Overhead Type Distribution Transformers, 500 kVA and Smaller, High Voltage 34,500 Volts and below, Low Voltage 7970/10 800Y Volts and Below,

ANSI C57.12.22-1989, Requirements for Pad-Mounted, Compartmental-Type, Self-Cooled, Three-Phase Distribution Transformers with High-Voltage Bushings, 2500 kVA and Smaller: High-Voltage, 34 500 GrdY/19 920 Volts and Below; Low Voltage, 480 Volts and Below

ANSI C57.12.24-1992, Standard for Transformers-Underground-Type Three-Phase Distribution Transformers, 2500 kVA and Smaller; High Voltage, 34 500 GrdY/19 920 Volts and Below; Low Voltage, 490 Volts and Below-Requirements

ANSI C57.12.29-1991, Switchgear and Transformers - Pad-Mounted Equipment -Enclosure Integrity for Coastal Environments

ANSI C57.12.50-1981 (Reaff 1989), Requirements for Ventilated Dry-Type Distribution Transformers, 1 to 500 kVA, Single-Phase, and 15 to 500 kVA, Three-Phase, with High-Voltage 601 to 34 500 Volts, Low-Voltage 120 to 600 Volts

ANSI C57.12.51-1981 (Reaff 1989), Requirements for Ventilated Dry-Type Power Transformers, 501 kVA and Larger, Three Phase, with High-voltage 601 to 34 500 Volts, Low-Voltage 208Y/120 to 4160 Volts

ANSI C57.12.52-1981 (Reaff 1989), Requirements for Sealed Dry-Type Power Transformers, 501 kVA and Larger, Three-Phase, with High-Voltage 601 to 34500 Volts, Low-Voltage 208Y/120 to 4160 Volts

ANSI C57.12.55-1987, Conformance Standard for Transformers-Dry-Type Transformers Used in Unit Installations, Including Unit Substations

ANSI C57.12.56-1986 (Reaff 1993), Standard Test Procedure for Thermal Evaluation of Insulation Systems for Ventilated Dry-Type Power and Distribution Transformers

ANSI C57.12.57-1987 (Reaff 1992), Standard for Transformers-Ventilated Dry-Type Network Transformers 2500 kVA and Below, Three-Phase, with High-Voltage 34 500 Volts and Below, Low-Voltage216Y/125 and 480Y/277 Volts – Requirements

ASTM D3487-1988 (Reaff. 1993), Standard Specification for Mineral Insulating Oil Used in Electrical Apparatus3

IEEE Std C57.12.58-1991 (Reaff 1996), Guide for Conducting a Transient Voltage Analysis of a Dry-Type Transformer Coil (ANSI)

IEEE Std C57.12.60-2001, Test Procedure for Thermal Evaluation of Insulation Systems for Solid-Cast and Resin-Encapsulated Power and Distribution Transformers

ANSI Std C57.12.70-1978 (Reaff 1992), Terminal Markings and Connections for Distribution and Power Transformers

IEEE Std C57.93-1995, Guide for Installation of Liquid-Immersed Power Transformers

IEEE Std C57.94-1982 (Reaff 1987), Recommended Practice for Installation, Application, Operation, and Maintenance of Dry-Type General Purpose Distribution and Power Transformers

3 ASTM publications are available from the American Society for Testing and Materials, 100 Barr Harbor Drive, West Conshohocken, PA 19425-2959, USA (http://www.astm/org/).

IEEE Std C57.96-1989, Guide for Loading Dry-Type Distribution and Power Transformers (ANSI)

IEEE Std C57.98-1993, Guide for Transformer Impulse Tests

IEEE Std C57.110-1986 (Reaff 1992), Recommended Practice for Establishing Transformer Capability When Supplying Nonsinusoidal Load Currents (ANSI)

IEEE Std C57.131-1995, Standard Requirement for Load Tap Changers

Draft IEEE Std C57.138-1998, Recommended Practice for Routine Impulse Test for Distribution Transformers

IEEE Std C62.22, Guide for the Application of Metal Oxide Surge Arresters to Alternating Current Systems

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