BP Standard GP 12 05 Power Transformer & Reactor

Document No.

GP 12-05

Applicability

Group

Date

10 November, 2003

Guidance on Practice for Power Transformers & Reactors

GP 12-05

BP GROUP ENGINEERING TECHNICAL PRACTICES

10 November, 2003

GP 12-05 Guidance on Practice for Power Transformers & Reactors

Foreword This is the first issue of Engineering Technical Practice (ETP) GP 12-05. This Guidance on Practice (GP) is based on the following heritage documents from merged BP companies:

Amoco (ACES) A EL-TR-00-G A EL-TR-00-E A EL-TR-LI-P A EL-TR-OH-P A EL-TR-PM-P

Electrical—Transformer—Guide Electrical—Transformers—Engineering Specification Electrical—Transformers—Liquid-Immersed—Supply Specification Electrical—Transformers—Overhead Type—Supply Specification Electrical—Transformers—Pad-Mounted—Supply Specification

Arco (APCES) ES 407-93

Outdoor Power Transformer

BP GOMDW 1400-20-EL—SP-4018 Liquid Filled Power Transformers

BP Chemicals US CP 17-4-1

Power Transformers

BP (RPSE) RP 12-9 GS 112-5

Electrical Systems and Installations – Transformers and Reactors Guidance for Specification GS 112-5 Transformers and Reactors

Copyright  2003, BP Group. All rights reserved. The information contained in this document is subject to the terms and conditions of the agreement or contract under which the document was supplied to the recipient’s organization. None of the information contained in this document shall be disclosed outside the recipient’s own organization without the prior written permission of Manager, Standards, BP Group, unless the terms of such agreement or contract expressly allow.

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Table of Contents ............................................................................................................................................. Page Foreword .............................................................................................................................................. 2 1.

Scope.......................................................................................................................................... 5

2.

Normative references ................................................................................................................. 5

3.

General transformer classification.............................................................................................. 6 3.1. Distribution and power transformers............................................................................... 6 3.2. Medium and large power transformers ........................................................................... 6

4.

Transformer ratings .................................................................................................................... 7 4.1. Standard ratings .............................................................................................................. 7 4.2. Liquid immersed transformer cooling.............................................................................. 8

5.

Design considerations ................................................................................................................ 9 5.1. Existing systems.............................................................................................................. 9 5.2. Type of transformer......................................................................................................... 9 5.3. Voltage rating .................................................................................................................. 9 5.4. Voltage taps .................................................................................................................... 9 5.5. Impedance (base rating) ............................................................................................... 10 5.6. Basic impulse insulation level (BIL) .............................................................................. 10 5.7. Ambient temperature..................................................................................................... 11 5.8. Winding hotspot............................................................................................................. 12 5.9. Altitude........................................................................................................................... 12

6.

Construction details .................................................................................................................. 13 6.1. Windings........................................................................................................................ 13 6.2. Insulation medium, dry or liquid type ............................................................................ 13 6.3. Accessories ................................................................................................................... 14 6.4. Type and location of termination facilities..................................................................... 15 6.5. Audible noise................................................................................................................. 15 6.6. Earthing/Grounding requirements................................................................................. 16 6.7. Radiators ....................................................................................................................... 16 6.8. Tank Design .................................................................................................................. 17 6.9. Energy conservation ..................................................................................................... 17

7.

Hazardous locations ................................................................................................................. 18

8.

Testing ...................................................................................................................................... 18 8.1. General.......................................................................................................................... 18 8.2. Type tests ...................................................................................................................... 18 8.3. Special tests ..................................................................................................................19

9.

Special transformer types......................................................................................................... 19 9.1. Distribution (overhead) transformer .............................................................................. 19 9.2. Padmount (distribution) transformers ........................................................................... 21

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1.


Scope This GP document provides guidance for engineering design, installation, operation, and maintenance of power transformers and reactors.

2.

Normative references The following normative documents contain requirements that, through reference in this text, constitute requirements of this technical practice. For dated references, subsequent amendments to, or revisions of, any of these publications do not apply. However, parties to agreements based on this technical practice are encouraged to investigate the possibility of applying the most recent editions of the normative documents indicated below. For undated references, the latest edition of the normative document referred to applies.

American National Standards Institute (ANSI) C57.12.10

C57.12.20

C57.12.21

C57.12.22

American National Standard, For Transformers-230 kV and Below, 833/958 through 8333/10417 kVA, Single-Phase, and 750/862 through 60 000/ 80 000/ 100 000 kVA, Three-Phase Without Load Tap Changing; and 3750/4687 through 60 000/ 80 000/ 100 000 kVA with Load Tap Changing-Safety Requirements American National Standard, For Transformers-Standard for Overhead Type Distribution Transformers, 500 kVA and Smaller High Voltage, 34500 Volts and below Low voltage, 7970/13800Y Volts and Below American National Standard, For Transformers-Pad-Mounted, Compartmental-Type, Self-Cooled, Single-phase Distribution Transformers with High-Voltage Bushings; High Voltage, 34500/19920 Volts and below Low voltage, 240/120 Volts and smaller American National Standard, For Transformers-Pad-Mounted, Compartmental-Type, Self-Cooled Distribution Transformers with HighVoltage Bushings, 2500 kVA and Smaller High Voltage, 34 500 GRD Y/19 920 Volts and below Low Voltage, 480 Volts and below

BP GIS 12-051 GIS 12-052 GP 12-60

Guidance on Industry Standard for Power Transformers (IEC) Guidance on Industry Standard for Power Transformers (ANSI) Guidance on Practice Hazardous Area Electrical Installations

International Electrotechnical Commission (IEC) IEC 60076-1 IEC 60076-2 IEC 60076-3 IEC 60076-5 IEC 60076-10 IEC 60354 IEC 60726

Power transformers – Part 1: General Power transformers – Part 2: Temperature Rise Power transformers – Part 3: Insulation levels, dielectric tests and external clearances in air Power transformers – Part 5: Ability to withstand short circuit Power transformers – Part 10: Determination of sound levels Loading guide for oil-immersed power transformers Dry-type power transformers

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Institute of Electrical and Electronic Engineers (IEEE) C57.12.00

IEEE Standard General Requirements for Liquid-Immersed Distribution, Power, and Regulating Transformers IEEE Standard General Requirements for Dry-Type Distribution and Power Transformers including those with Solid-Cast and/or ResinEncapsulated Windings

C57.12.01

National Fire Protection Association (NFPA) NFPA 70

3. 3.1.

General transformer classification Distribution and power transformers a.

b.

c. 3.2.

National Electrical Code (NEC)

Both IEC and ANSI classify single-phase and three-phase transformers with ratings up to 2 500 kVA and a high voltage rating up to 36 kV as “distribution transformers”. ANSI further classifies distribution transformer into two specific groups: 1.

Overhead transformers - up to 500 KVA

2.

Pad-mounted - up to 2 500 kVA

Transformers above 2 500 kVA are classified as “power transformers” and are also subclassified into two specific groups: 1.

Medium power transformers – above 2 500 kVA up to 100 000 kVA

2.

Large power transformers – above 100 000 kVA

These classifications are not recognised as specific types in IEC but are roughly equivalent to pole-mounted and ground-mounted types, respectively.

Medium and large power transformers a.

IEC classifies power transformers into three categories based on ratings described in Table 1A. Classification (IEC 60354) Distribution Medium Power Large Power

b.

Table 1A – IEC Classifications Category Rating Sr (kVA) (IEC 60076-2) I Sr ≤ 2 500 II 2 500 < Sr ≤ 100 000 III Sr > 100 000

ANSI transformer categories with their rating limits are given in Table 1B. Table 1B – ANSI classifications Transformer Type

Rating (kVA)

Overhead - C57.12.20

500 and Smaller

Pad-mounted – C57.12.22

2 500 and Smaller

Medium Power – C57.12.10

Above 2 500 up to 100 000

Large Power -

100 000 and above

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4.


Transformer ratings

4.1.

Standard ratings a.

IEC and ANSI standard kVA ratings for transformers, up to 10 MVA, are shown in tables 2A and 2B. It will be necessary to select a rating appropriate to the particular standard area.

Table 2A Standard IEC ratings R10

100

Power (kVA)

125

160

200

250

315

400

500

630

5

6.3

800 etc. 8

10

12.5

16

20

25

31.5

40

50

63

80

100

125

160

200

250

300

400

500

630

800

1000

1250

1600

2000

2500

3000

4000

5000

6300

8000

10 000 R10 numbers and rating (IEC 60076 & CENELEC)

Bold Values are preferred for Distribution Transformers in CENELEC countries (to HD 428 Standards). For Industrial purposes there may be merit in adopting common practices. For the larger ratings consideration should also be given to transformer efficiency in order to minimise the cost of losses over the lifetime of the plant.

Table 2B Standard ANSI ratings (3-phase) TYPE Standard Rating (kVA)

Overhead C57.12.20 30, 45, 75, 112.5, 150, 225, 300. 500

Pad-mounted C57.12.22 75, 112.5, 150, 225, 300. 500, 750, 1 000, 1 500, 2 000, 2 500

Oil-immersed C57.12.10 15, 30, 45, 75, 112.5, 150, 225, 300. 500, 750, 1 000, 1 500, 2 000, 2 500, 3 750, 5 000, 7 500, 10 000(1)

Note: C57.12.10 includes transformers rated up to 60 000 kVA (ONAN) and 100 000 kVA (ONAF), where the rating should be selected on economical basis.

b.

Transformers should be rated for continuous operation. When determining transformer kVA rating, and load is firm, a minimum of 125% of connected load should be specified as an ONAN rating. A consideration for load growth, where the more conservative ONAN rating is considered to be unduly costly, is to use transformers equipped for future forced-air cooling. Forced air (ONAF1), 2nd stage forced air (ONAF2) or forced oil and air (OFAF) cooling ratings allow for future load growth while meeting the temperature limits. Winter/summer load variations on transformer should be considered. Summer electrical loading at ambient temperature conditions is most often the primary factor in sizing a transformer. It may also be specified that winding temperature rise should not exceed 55°C when the transformer is loaded to base kVA rating (ONAN). An additional 12% of load can be added resulting in the winding temperature rising to 65°C. This ‘dual rating has in the past been used to allow for additional load growth without replacing transformers or having a negative impact on insulation life.

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Consideration in determining loadings should include: • • • • • 4.2.

Source circuit loading Future load growth Voltage drop on motor start Voltage droop on feeder circuits Economic evaluation of equipment required for each choice

Liquid immersed transformer cooling a.

Forced cooling allows a substantial increase in rating of a transformer compared with natural cooling. Above 10 000 kVA, the transformer cooling class should be ONAN/ONAF1/ONAF2 (i.e. self-cooled/forced-air stage 1/ forced-air stage 2) unless otherwise approved. The two stages of forced air-cooling allow the transformer to handle additional load growth. ANSI standards generally indicate that forced air-cooling provides an additional 15% capacity on ratings of 750 kVA through 2 000 kVA and a minimum of 25% additional kVA on ratings 2 500 kVA through 10 000 kVA. This percentage increase in transformer capacity continues to the 60 000 kVA transformer limit of this specification and results in up to a 67% (1) increase over base rating. Over the installed life of the transformer it is probable that the load will increase significantly. Note-1: A general ‘rule-of-thumb’ within IEC for medium and large transformers for rating of a base transformer is as follows: ONAN

100%

OFAN

130%

ONAF

160%

OFAF

200%

(These rating increases are approximate and require verification with the manufacturer.) b.

Natural cooling (ONAN) methods are the preferred methods of transformer and reactor cooling. Natural cooling methods are preferred since these provide high reliability with minimum maintenance. However, this represents the highest cost cooling arrangement. ANSI specifies that each stage of forced-air cooling will provide an additional 15% capacity on ratings of 750 kVA through 2 000 kVA and 25% on ratings 2 500 kVA through 10 000 kVA. For transformers above 10 000 kVA, each stage of forced-air cooling provides 33.3% additional capacity above the base rating.

c.

Forced cooling or provision for forced cooling should only be considered under the following circumstances: 1.

Where it is anticipated that there could be a future plant load increase.

2.

Where transformers for use on a triple radial system could under certain operating conditions experience higher-than normal rated loads.

3.

Where a plant, which usually runs as a base load has occasional periods of high load demand that would be outside the normal cyclic loading capabilities or the transformer or reactor.

4.

Where a seasonal high ambient temperature would cause transformers or reactors to be de-rated.

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5. 5.1.


Design considerations Existing systems When a transformer will be paralleled with an existing transformer, all rating information should be obtained from the existing transformer nameplate. A photocopy of existing transformer nameplate should be attached to data sheet for new transformer.

5.2.

Type of transformer The basic plant load will be three-phase in nature, however, single-phase transformers may be used for lighting, instrumentation, control and plant auxiliaries. Single-phase transformer installation should be distributed evenly on the three-phase supply circuit(s) to avoid a loading unbalance the three-phase primary.

5.3.

Voltage rating Primary voltage may be determined by connected load, i.e. very large motor, but is most often dictated by existing electrical system voltage. Note that it is common practice to specify the secondary voltage rating of a transformer as 1.05 times nominal when on zero tap. This accounts for transformer regulation on full load.

5.4.

Voltage taps a.

Off circuit tap-changers (i.e. a tap-changer operated with the transformer de-energised) with a tapping range of ± 5 % in 2½ % tap steps should be fitted to all transformers where it is anticipated that after the optimum tapping is selected, there will be no further requirement to alter the tapping except under exceptional circumstances such as after plant modifications which result in a significant load change. Off-circuit tap-changers should be specified where it is acceptable for tap-changing to be carried out when the transformer is not energised.

b.

To increase safety it is recommended that access to an off circuit tap change mechanism be limited by mechanical interlock with the HV circuit breaker. An additional accessory for liquid immersed transformers may be a key interlock on the off circuit tap changer and the transformer primary breaker to assure transformer is deenergized before the taps are changed. This is often not specified because historically padlocks have been used and procedures reflect the need for the circuit to be deenergised. However, there have been cases where mal-practice has occurred and a simple mechanical interlock offers a foolproof way of achieving a safe practice.

c.

On-load tap-changers fitted to site distribution transformers should be provided where if it is anticipated that plant non-transient voltage variations of greater than ± 5% could occur as a result of normal process operations (maximum to minimum load conditions) and where an off circuit tap change operation would cause unacceptable disruption. An on-load tap-changer has a much more difficult duty than an off-circuit type. Experience shows that on-load tap-changers are a significant source of transformer faults and hence careful consideration should be given to the use of such devices. Note that on load tap changing should be provided at HV intake points in order that voltages lower in the system will remain within specified tolerance limits (normally ± 5% but for some applications ±6% is often used) for load swings between 0 and 100%. Should the voltage swing not be a problem nor be expected to be a future problem, a manual off circuit tap changer would be satisfactory.

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d.

On-load tap changers should be automatic voltage control type and manually operable either at the transformer or from a remote servomechanism control located in the appropriate substation or control room.

e.

Where automatic voltage control of transformer on-load tap-changers is provided, the method of control should be compatible with the automatic voltage control of any in plant generation or synchronous motors to prevent 'hunting'. Similarly, where transformers with automatic voltage control of on-load tap-changers are required to work in parallel, the controls should be compatible to prevent 'hunting' and to prevent circulating currents due to inconsistent taps. The automatic voltage control scheme should incorporate a built-in adjustable time delay to prevent tap-changing under transient voltage fluctuations, and means for preventing parallel transformers having a disparity of more than one tap-step during any tapping operation. The term 'hunting' in this context is defined as the action of two or more control systems failing to reach an equilibrium state and as a result acting in opposition. The control system should also prevent paralleled transformers operating with greater than two step positions apart.

f.

5.5.

On-load tap changers should have a tapping range of +10% to -10% with 8 tap-steps of 1.25%, above and below nominal unless voltage variations dictate that a wider range is required.

Impedance (base rating) Transformers generally conform to “standard” impedance ratings. However non-standard impedances can be specified to provide an economical solution to high fault current. Standard impedance for a given kVA rating of an overhead, pad mount or liquid immersed transformer is specified in IEC 60076-5 (ANSI/IEEE C57.12.00). Nonstandard (special) impedance can be specified to provide reduced short-circuit current or less voltage drop on motor starting. A disadvantage of non-standard impedance is the possibility of reduced inter-changeability.

5.6.

Basic impulse insulation level (BIL) a.

Phase bushings shall be rated for line-to-line voltage. Minimum BIL rating for bushings should be as follows for ANSI standards: Nominal System Voltage (kV) BIL (kV) 1.2 kV and below----------------45 kV 1.3 kV to 2.5 kV-------------------60 kV 2.6 kV to 5.0 kV-------------------75 kV 5.1 kV to 8.7 kV-------------------95 kV 8.8 kV to 15 kV------------------110 kV

15.1 kV to 36 kV------------------145 kV b.

To conform with IEC standards, the Lightning Impulse Withstand Voltage should be selected from the reduced values of Table 2 of IEC 60076-3 (See Note* below), except where a transformer or reactor is to be directly connected to overhead lines which do not incorporate surge arrestor equipment. In this case the Rated Lightning Impulse Withstand Voltage should be selected from the increased impulse values. Note*: Table 2 in the 2000-03 (2nd edition) of IEC 60076-3 should be used with caution as the reduced impulse levels at Um 145 kV and 170 kV appear incorrect at 450 kV and 550 kV respectively. It is recommended that the minimum LI values to be used at these Um are 550 kV and 650 kV respectively. The amount of insulation applied to the winding conductors is usually influenced by the impulse voltage rating of the winding rather than by the power-frequency voltage rating. Impulse voltages due to lightning or switching activity appearing at the terminals of the

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transformer stress the winding insulation and this effect may be reduced by the application of surge arrestors. c.

Table 3 of IEC 60076-3 gives LI co-ordination for areas of the world where North American practice is applied to power systems, (see Note * below). On systems up to 15 kV the value should be selected from the column headed 'Distribution Transformers', unless the transformer or reactor is directly connected to an overhead line which has no surge arrestor equipment, in which case the value should be selected from the column headed 'Class II Transformers'. Note*: The North American Practice Tables given in the IEC Standard are no longer fully in accordance with the latest revisions of the ANSI/IEEE Transformer Standards and should be used with caution. It is preferred that current ANSI/IEEE standards are referred to when specifying transformers for use in these countries.

5.7.

Ambient temperature Table 3A gives the reference ambient temperatures for Both IEC and ANSI standards. The table continues with the standard temperature rises applicable to the particular standard. These rises should be reduced by the most dominant ambient temperature factor, i.e. that which exceeds the standard normal value by the greatest amount. Temperatures are given in degrees Celsius (oC) whereas temperature rises are in degrees Kelvin (K)

Table 3A Ambient temperature and permissible temperature rise (Outdoor oil-filled transformers) Rating factor

Standard IEC

ANSI

Minimum ambient temperature

o

C

-25

-20

Maximum ambient temperature

o

C

40

40

Monthly average of hottest month

o

C

30

N/a

Average of any 24 hour period

o

C

N/a

30

Yearly average

o

C

20

N/a

Max. cooling water temperature (water-cooled transformers)

o

C

25

30

Average cooling water temperature in any 24 hour period

o

C

N/a

25

Top-oil rise

K

60

65

Average winding rise (ON or OF cooling class)

K

65

65

Average winding rise (OD cooling class)

K

70

65

(1) (1) (1)

Note-1: These ANSI temperature rise values are based on thermally upgraded paper, which is the present industry standard in the USA. Older transformers using non-thermally upgraded paper are restricted to 55 K rise. Example: An IEC outdoor ONAN cooled transformer operating in a region with higher than normal ambient temperatures: • • •

Maximum ambient = 50oC. (This is 10oC in excess of IEC normal value.) Average ambient of hottest month = 35oC. (This is 5oC in excess of IEC normal value.) Annual average = 28oC. (This is 8oC in excess of IEC normal value.)

It is apparent that the dominant correction to be applied in this case is that arising from the maximum ambient temperature, i.e. 10oC. This requires an equivalent reduction of 10K in the allowable temperature rises: • •

Top-oil rise reduced from 60K to 50K. Average winding rise reduced from 65K to 55K

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5.8.


Winding hotspot Although not directly a factor in the rating of the transformer, it is increasingly common to specify the winding hotspot limit. This factor is particularly important for estimating the ageing of transformer insulation subjected to periodic operation above nameplate rating and/or ambient temperature. It should be remembered that with the indirect methods used in existing rating guides to estimate the hotspot temperature, the value is approximate and should be employed with caution. As with the average winding rise discussed previously (Table 3A), the allowable hotspot rise must be decreased accordingly when the transformer is intended for operation with abnormally high ambient temperatures. The hotspot values are different for IEC and ANSI standards and are given in Table 3B. The differences in hotspot rating philosophy between the two standards emphasise the need for caution when employing this factor. Table 3B Hotspot limits

Factor

Standard (3)

IEC

ANSI Existing

Average ambient temperature Hotspot rise

o

(2)

Previous

C

20

30

K

78

80

65

98

110

95

o

Hotspot temperature

(1)

C

30

Note-1:The existing ANSI C57.91-1995 standard is based on the use of thermally upgraded paper. Note-2:The previous ANSI standard was based on non-thermally upgraded paper. These figures must be used for transformers manufactured to the previous standard or for new transformers made with non-thermally upgraded paper for use in countries using the ANSI loading guide. Note-3:IEC 60354 does not recognise thermally upgraded paper. The higher hotspot value assigned by ANSI should not be used when this grade of paper is used in IEC countries. 5.9.

Altitude When transformers are to be operated at altitudes above 1000 m (3300 ft) an allowance must be made for a reduction for two key performance aspects because of the reduced air density. These factors are: Reduced dielectric performance Reduced power rating

5.9.1.

Dielectric correction

a.

With increasing altitude, the reduction in air density reduces the withstand strength of external insulation, such as the terminal bushings of the transformer. For operation at altitudes above 1000 m the insulation length will require increasing to maintain the power frequency (AC), lightning impulse (LI) and where applicable, switching impulse (SI) withstand voltages. No increase in creepage distance is required.

b.

IEC and ANSI offer different methods and the necessary correction appropriate to the standard employed should be selected. The corrections are summarised in Table-3C.

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Table 3C Dielectric withstand correction for altitudes > 1000 m Standard IEC

ANSI

1% per 100 m over 1000 m

See Table-1 of IEEE Standard C57.12.00-2000

The bushing manufacturer may have test data to demonstrate a higher than standard dielectric capability to meet this altitude requirement. Alternatively he may offer a prorata increase in insulation length. 5.9.2.

Power rating correction

It is necessary to demonstrate a power rating above nominal nameplate rating in the laboratory, to ensure that the transformer is capable of supplying rated power at altitudes above 1000 m. The correction is different for unforced and forced cooling and the methods are also different within IEC and ANSI standards. ANSI also provides data for maintaining rated power at altitude with a standard transformer if the ambient temperature is below certain values. The corrections are summarised in Table-3D. The reduced air density at high altitude reduces the cooling efficiency of the transformer such that a rating test at nominal kVA, carried out in a laboratory located between sea level and 1000 m, is invalid for a transformer that is to be operated at an altitude above 1000 m. Table 3D Rating correction for altitudes > 1000 m Liquid-immersed transformers Cooling

Standard IEC 60076-3 Average winding temp rise to be reduced by:

ANSI (IEEE Std C57.91) KVA rating to be reduced by:

Unforced cooling (AN)

1K per 400 m over 1000 m

0.4% per 100 m over 1000 m

Forced air cooling (AF)

1K per 250 m over 1000 m

0.5% per 100 m over 1000 m

Note Data is given here for only AN and AF cooling. For other methods, including watercooling and those involving heat-exchangers, see the appropriate standard. IEEE Std C57.91 Table E1 (Annex E) gives data for maintained power rating at altitude with reduced ambient temperature. Clearly the ANSI advice is a more practical guide for an application involving standard transformers that may be located in elevated altitudes. IEC specifications require manufacturers declaration concerning the relationship between temperature rise and rating.

6. 6.1.

Construction details Windings Copper windings are generally acceptable for power transformers. Where aluminium will be used special provision should be made for terminations. There are many instances of aluminium wound transformers offered by reputable manufacturers. Where these may offer cost advantages, their use should not be precluded. However the possible effects of corrosion or other deterioration should be taken into consideration. In general the manufacturer needs to demonstrate that these effects will not be an issue for the application and environment envisaged.

6.2.

Insulation medium, dry or liquid type a.

Transformers and reactors for use outdoors in onshore locations should be of mineral oilimmersed type. Mineral oil has good dielectric strength and thermal conductive properties.

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Its insulation level is however highly sensitive to the level of impurities. Therefore regular checks on oil quality are necessary in order to assure satisfactory performance. b.

Transformers and reactors for use indoors or for use offshore should be of the encapsulated winding dry type construction in accordance with IEC 60726, or of the synthetic liquid immersed type where the synthetic liquid is of the flame retardant type having a fire point in excess of 300ºC. Mineral oil immersed transformers present a potential fire hazard. In sensitive locations, such as those referred to; dry-type construction or non-flammable oilcooled transformers should then be used. Fully cast-resin encapsulated transformer units have the following advantages: • • •

c.

Unaffected by humidity, dust etc. Relatively simple assemblies, using few insulating materials and less prone to electromagnetic stress. High thermal time constant and superior short-circuit withstand giving good overload performance, often better than conventional air-cooled types.

Dry type transformers or reactors of the non-encapsulated winding type to IEC 60726 may be used for dry indoor locations requiring operation at ambient temperatures above those suitable for the manufacturer’s standard encapsulated products. This relates to conventional dry type transformers with Class H or Class C insulation temperature limits.

6.3.

Accessories a.

Transformers with external bushings, located in contaminated areas, such as coastal locations, may require bushing with extended creepage distances. Creepage distance is the surface distance of the bushing or insulator from energized connection to transformer tank or earth/ground. In contaminated areas. the contaminant (salt in coastal areas) builds up on the transformer bushings and this condition combined with condensation causes bushing/insulator flashover. This type of transformer bushing flashover can be minimized by extending the bushing's creepage distance. Bushing locations can be on cover or sidewall and in two of four transformer segments. High voltage bushings are located in one segment and the low voltage bushings in another segment. Segment 1 is front of transformer and other segments are numbered 2, 3 and 4 clockwise as viewed from top. For a transformer with star (wye) secondary and secondary bushings, when a neutral bushing is not specified, the neutral end of the winding is connected to the transformer tank.

b.

For certain conditions accessories additional to those implied by design considerations may be required as specified in GIS 12-051 and GIS 12-052. Accessories should be reviewed and justified before including on data sheet. An example of additional accessories is the requirement for contacts on temperature indicator for remote alarm indication. Another example is a nitrogen blanket to take the place of dry air in the space above the oil in an oil filled transformer.

c.

Winding temperature indicators should be provided on liquid-immersed transformers and reactors rated 10 MVA and above, and on dry type transformers and reactors rated 2 MVA and above. It is typical industry practice not to fit winding temperature indicators on transformers rated up to 5 MVA. At these low ratings, the economy of employing only oil-temperature indicators is usually adopted.

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Apart from the facility to monitor temperature, an important feature of the winding temperature indicator is to initiate automatic switch-on and switch-off of cooling fans and oil circulation pumps. In this way a dual rated transformer with a cooling classification of, for example, ONAN/ONAF will automatically switch from ONAN to ONAF (and back) according to the transformer loading conditions. d.

6.4.

Winding temperature indicators with alarm and switching contacts should be specified for ONAN transformers which have provision to be up-rated at some future date by conversion to ONAF, by addition of fans.

Type and location of termination facilities a.

Normally air terminated cable boxes should be specified in order that cable termination and future maintenance is eased. However particular attention needs to be paid to the possible ingress of water into cable boxes. Liquid immersed transformers rated 20 000 kVA and below are considered unit substations and can be equipped with enclosed bushings. Transformers rated above 20 MVA, are generally considered station-type and usually have open or exposed bushings. This generality reflects the manufacturers zone of reliable experience. Notwithstanding this, higher ratings can be supplied with cable terminations onto enclosed bushings. Voltage stress relief, multiple single core cable arrangements and the relative merits of bus ducting and multiple single core cable installations need to be considered for the larger ratings involving enclosed bushings. In the past some sites have found it advisable to have desiccant provisions for the cable box to prevent the effects of condensation.

b.

It is recommended that transformers, bus duct, switchgear and motor control centres be supplied by a single manufacturer to eliminate problems with bus duct fit.

c.

When someone other than the transformer manufacturer supplies bus duct, the bus duct supplier should also be provided a dimensional drawing of transformer throat to achieve lowest cost connection. The transformer manufacturer may require a dimensional drawing, if the transformer primary or secondary is connected to an existing bus duct or others supply a bus duct. Transformer manufacturers may have standard throats for bus duct connection.

d.

Air filled cable termination boxes should be of phase-insulated fault pressure relieving type. Phase-segregated or phase-separated air filled cable termination boxes or compoundfilled cable boxes should be used only under exceptional circumstances. Where these are used particular care should be taken to avoid the effects of circulating currents and to ensure that any filling is quality controlled.

e.

6.5.

Heat-shrink or factory-made fully-insulated cable terminations should be used in air filled cable termination boxes, and where applicable sufficient space should be provided by the manufacturer to allow for stress relieving terminations.

Audible noise Where the site installation has a need for sound limitation, the transformer noise should be considered as part of the overall plant layout design. In all cases sound data should be available for the transformer. There is concern about sound level because constant audible noise can be annoying and at high enough level can damage hearing. Transformers must have audible sound levels equal to or less than those described by IEC 60076-10 (NEMA TR1).

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6.6.


Earthing/Grounding requirements a.

Where an earthing/grounding transformer is required in a three wire, three phase power system for deriving a neutral, it should be continuously rated at 110% of the nominal neutral current rating of the system. In situations where the supply is derived from a distribution transformer having a delta connected secondary, the system neutral may be provided by an earthing/grounding transformer. It should be noted that neutral rating, and hence earthing/grounding transformer rating, will be influenced to a considerable extent by the nature of the system load. Where system loads comprise a considerable proportion of non-linear load e.g. switchedmode power supplies this can result in a considerable increase in neutral current. Such factors should therefore be taken into consideration when determining earthing transformer rating.

b.

Where an earthing/grounding transformer is required in a three wire, three phase power system for deriving a system earth/ground, the zero sequence impedance and rated short time current should be selected to match the earthing/grounding and protection practice adopted for the installation.

c.

Except where a system is to be solidly earthed/grounded via the earthing/grounding transformer, the zero sequence impedance and short time current rating should depend on an externally connected resistor. Various winding connection configurations are possible, each presenting different impedances to fault current. The selection of earthing/grounding transformer impedance, short time current rating and value of externally connected resistor are determined from the results of phase to earth/ground fault calculations. When considering resistance earthing/grounding, transformer manufacturers may need to provide mounting brackets etc., for earthing/grounding resistors and to assure their location does not interfere with other equipment mounted on transformer.

6.7.

Radiators a.

Offshore transformer radiators should be 316 SS. Radiator headers should be No. 12 gauge minimum, and radiator panels No. 11 gauge minimum. 316 SS. Radiator headers and panels should be painted with the same process and number of coats as the transformer body. Carbon steel or 304 SS have not been found to offer an acceptable performance in the past. The following radiator types are considered to offer an acceptable performance: • • •

b.

Plate Corrugated Fin-Fold Welded tubular

Where shipping could be an issue, radiator design should include detachable (removable) tubular types. All radiators should be removable and furnished with flanged shut off valves, seal welded to the tank, capable of sealing with radiator removed and transformer tank filled. Nothing (including fans) should be mounted on the radiators. Where radiators are removable from sealed tank designs, there should be provision and an identified procedure for tank refilling.

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6.8.


Tank Design a.

All liquid immersed transformers or reactors should either be of the sealed tank type or conservator type. The advantages of sealed tank designs over the general alternative of breathing type are their almost maintenance-free operation and with certain designs there is a space saving because of the absence of external pipe work.

b.

Below 2 MVA all liquid-immersed transformers and reactors should be of the sealed tank design, and at 2 MVA and above the use of sealed tank designs should be based on an economic assessment. The internal pressure characteristics of the sealed-type design limit the ratings available and eventually there is a need to employ conservator types. Generally, manufacturer’s standard products for sealed type designs have a maximum rating in the range 1.6 – 2 MVA. However the limitation relates to tank size and sealed tank designs of higher ratings have been used where the tank dimensions have been sufficiently small.

c.

All sealed tank transformers and reactors should be fitted with a resetting type stainless steel pressure relief device complete with alarm contacts. All transformers must have explosion protection either by Qualitrol pressure relief or some other means. The Qualitrol device is a sealed unit which fulfils the same function as the Buchholtz unit in a conservator type. It should have alarm and trip settings. The single pressure/vacuum unit equipped with the pressure settings and alarms/trips and which also has a pressure relief function is often faster than a bursting disc.

d.

Where there is a possible danger to personnel, the outlet of the sealed tank pressure relief device should be arranged to direct any liquid surge away from personnel access ways.

e.

All conservator type transformers or reactors should incorporate a Buchholz relay complete with alarm and trip contacts. Buchholz relays are considered mandatory for conservator type transformers. These are protective devices and designed to: • • •

detect a sudden surge movement of oil due to an internal transformer fault detect free gas being slowly produced in the main tank provide a chamber for collection and later analysis of evolved gas

The breathers and pressure relief devices fitted to conservator type transformers or reactors should be positioned where they will not constitute a danger to personnel should there be a sudden gas or liquid expulsion. f.

Conservator type transformers should be fitted with an oil-sampling valve suitable for condition monitoring applications. Sealed-type transformers used within their rating and temperature limits, are recognised as having potentially long service-life due to the oil remaining moisture-free during the life of the unit and are not usually specified with a sampling valve. If oil sampling is being considered for this type of transformer, the manufacturer should be consulted for advice, as sampling may disturb the internal pressure.

6.9.

Energy conservation a.

Consideration should be given to energy conservation features. Where efficiency is of concern, several cost-analysis techniques can be used to formalize procurement with the goal of maximizing efficiency or minimizing overall life-cycle cost.

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These techniques can be used to prepare an economical evaluation of several transformer design options. The cost of electric power varies between plant locations. Therefore, $/kW for a location should be obtained to determine operational costs. When $/kW is low, a more economical transformer design (higher transformer losses) may be considered versus what would be offered for a high $/kW number. b.

7.

The following information about the transformer should be supplied to the prospective vendors based on annualised operation projections: 1.

Cost in dollars/kW at which losses are valued

2.

Percentage of the transformer rating at which load losses will be evaluated during any comparison process.

Hazardous locations a.

Area classification must be determined before accessory classification can be defined. Transformers should be located in unclassified areas. Transformers may be inherently suitable for a Zone 2 (Class 1, Division 2) location, but related accessories could be a problem and very expensive. Special provisions may be required to ensure that surface temperatures and auxiliary devices are suitable for installation in hazardous locations. Distance to an unclassified area may be so great that cost of cable, conduit/tray/underground ducts may exceed cost of classified area equipment.

b.

Where the location for the transformer will be in a classified area there should be a recorded and verifiable analysis of the transformer and its accessories detailing the safety provisions. This record should be kept in a manner that is readily extracted by the maintenance management system. The requirements for electrical equipment for hazardous areas are detailed in GP12-60. These requirements should be followed in respect of the unusual application of power transformers being located in hazardous areas. In ANSI applications NFPA 70 can offer guidance.

8. 8.1.

Testing General A full range of transformer tests is listed in IEC 60076 (Section 8 of ANSI/IEEE C57.12.00). The objective of type testing is as follows: • • •

verify the basic design concept. demonstrate that the transformer meets the requirement of the specification. confirm the electrical and mechanical characteristics of the transformer.

For more information on the insulation power factor test and oil analysis refer to “A Guide to Transformer Maintenance” copyright 1981 by Transformer Maintenance Institute division of S. D. Myers, Library of Congress catalogue card number 81-50169 ISBN 0-939320-00-2. Gas in oil analysis should be completed after manufacturer’s testing is complete. 8.2.

Type tests a.

Type tests in accordance with the National or International standard should only be performed on a transformer or reactor when the manufacturer is unable to supply evidence of successful type tests on similar designs.

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8.3.


b.

Where more than one transformer or reactor of the same design is purchased, type tests should only be performed on one item. If however, the item tested fails, all remaining items should be type tested.

c.

The following tests should have previously been made on a sufficient number of transformers and ratings to demonstrate compliance. These tests should not need to be repeated unless the transformer design has substantially changed from the tested one. 1.

Temperature rise.

2.

Impulse.

3.

Pressure on tank.

Special tests a.

Special tests would constitute an unusual application. Such tests would only be undertaken following a special request to the manufacturer. Alternatively, a major programme involving a new supplier requires additional verification of performance and build quality and some special testing may be considered appropriate. It should be noted however that carrying out these may result in some overstressing of the transformer with possible reduction in plant life.

b.

Special tests may include the following: 1.

simulation of an incoming surge.

2.

zero sequence impedance measurement.

3.

short-circuit withstand testing.

4.

no-load harmonic measurement.

Short-circuit withstand testing of (in particular) large transformers is a very expensive undertaking and not common practice. c.

A noise test should be performed when there is an onerous plant noise specification and where this has not been already been the subject of a type test.

d.

Short circuit tests should not be requested because of the severe cost implications. If the design of a transformer or reactor involves a new construction concept a short circuit test may be justified if the equipment is to be used where there is a high system fault level. The short circuit rating of the transformer should be at least in accordance with IEC 60076-5. These values may be inadequate for some applications and it is always necessary to establish the local fault levels at the transformer service location: It is usual to appraise the calculations provided by the manufacturer to verify the theoretical mechanical and thermal margins in his design, with verification based on actual short-circuit tests on transformers of similar construction and rating. Nowadays manufacturer’s carry out detailed analysis of short circuit conditions generally based upon the use of finite element techniques. It is possible therefore assess the design by reviewing this analysis and requesting details of the validation of computer models used.

9.

Special transformer types

9.1.

Distribution (overhead) transformer

9.1.1.

General

a.

Distribution transformers are designed outdoor pole mounted installations.

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b. 9.1.2.

9.1.3.

9.1.4.


Distribution transformers are single-phase units but can be mounted in clusters of three to provide three-phase service.

Rating considerations

a.

Overhead (pole-mounted) transformers are limited in size to 167 kVA single-phase and 500 kVA three-phase.

b.

Overhead transformers should be sized at 125% of connected load to allow for future load growth.

c.

Economic evaluations and specifying special impedances are not recommended for distribution transformer selection because size limitations make these factors negligible. Also, cost to manufacture a special impedance overhead transformer probably warrants a different solution.

Fabrication and accessories

a.

One, fully-insulated, high voltage bushing is suitable for a solidly grounded distribution system. Two, fully-insulated, high-voltage bushings are required for application on either wye or delta distribution systems. Transformer manufacturers should be consulted to determine the bushing arrangement.

b.

Spade type secondary bushings should be specified to reduce connection resistance over installed life of transformer.

c.

An overhead transformer should be specified with aluminium windings.

d.

The Basic Impulse Level (BIL) is crest value of impulse voltage, which a transformer is required to withstand without failure. Standard BIL levels are a function of primary transformer voltage. To specify a BIL level that is “non-standard” will increase cost of a distribution transformer. However, the increased cost may be justified when distribution transformers are installed in areas of high lightning activity.

e.

Delta primary and wye secondary are the most common transformer connection, however delta/delta, wye/delta or wye/wye may be used to meet specific site requirements. Therefore, primary & secondary transformer connections need to be determined for specific location when specifying a three-phase transformer or making a three phase bank from single-phase overhead transformers.

f.

Overhead transformers information is available in manufacturer’s catalogue but approval drawings should be reviewed to assure manufacturing changes have not changed overall dimensions, accessory locations or mounting requirements. Modifications to mounting arrangements may be required when additional accessories have been specified.

g.

An additional accessory that may be specified is a secondary breaker for secondary fault and overload protection. Secondary protection schemes vary by location and it is important to duplicate these schemes for maintenance and training requirements.

h.

A ground pad is a steel pad attached to transformer tank to allow for a low resistance ground connection to transformer housing. There are situations where an additional ground pad may be necessary due to number or size of ground connections.

Testing

Since overhead transformers are such a standard item it would be unusual to require additional testing over the manufacturer’s standard tests.

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9.2.

Padmount (distribution) transformers

9.2.1.

General

9.2.2.

9.2.3.

a.

Pad mount transformers are designed for indoor or outdoor installation. There are no exposed electrical conductors, fan blades, screws, bolts or other fastening devices.

b.

It is recommended that dry-type transformers be used for indoor installations.

c.

Pad mount transformers can be either single-phase or three-phase units. However, plant use of pad mount transformer will be three-phase applications.

d.

Pad mount transformers provide a secure, tamper-proof installation without fencing. They are constructed for step down, underground systems. They can be used with an overhead distribution system by installing a pole with a conduit drop.

Ratings considerations

a.

Ratings of three-phase pad mount transformers fall between 500 kVA to 2 500 kVA. Although ratings higher than 2 500 kVA are available, IEC 60076 (ANSI/IEEE C57.12.22) limits rating to 2 500 kVA.

b.

Pad mount transformers are utilized in applications where load growth is stable. A 65°C rise is standard offering and is sufficient for most applications. Reviewing application and job requirements may determine that a 55/65°C rating is a better choice than moving up to next kVA size.

c.

Pad mount transformers meet tamper-resistant requirements for equipment used in public access areas.

d.

Forced-air cooling (ONAF) should not be used in public access areas due to the additional protective equipment required to meet public safety requirements.

e.

Single-phase pad mount transformers are not recommended due to unbalancing effect on distribution system.

Fabrication and accessories

a.

Pad mount transformers may be served by either radial or looped primary feeders. Bushing requirements double when a loop feeder is utilized. However, a looped feeder doubles the service reliability to the transformer and/or the second bushing on a radial fed transformer will permit plant expansion by extending the primary feeder at a later date.

b.

Dead front transformers have no exposed energized parts and thereby provide an extra margin of safety from accidental contact.

c.

Transformer costs are greater for a live-front unit because of space requirements between primary bushings for air insulation and an insulating barrier is required between phase bushings and ground to assure adequate electrical clearance.

d.

Pole mounted primary switches should be provided to isolate underground feeders serving radial and/or loop fed pad mount transformers. These switches are ordered separately from the transformer.

e.

Overcurrent protection should be considered for pad mount transformers.

f.

Six-hole spade connectors should be specified on secondary bushings. Four or two hole connectors can be provided, but may limit the addition of secondary conductor in the future.

g.

Copper windings are required for padmount transformers.

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BP Standard GP 12 05 Power Transformer & Reactor

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