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Home / Technical Articles / How to prepare an enquiry for a power transformer: Design and tendering tips

Transformer  specification

Nothing is more crucial during the early phase of a transformer inquiry than a complete and explicit listing of all requirements that must be met from both the manufacturer’s and the user’s perspectives. The IEC standards include detailed requirements that affect the design and manufacture of transformers depending upon their rating, voltage and application.

How to prepare an enquiry for a power transformer: Design and tendering tips
How to prepare an enquiry for a power transformer: Design and tendering tips

Often, however, there are other additional local or regional technical requirements that need to be included in a specification, as well as requirements which arise from the purchaser’s previous experience. Therefore, the purchaser should state any technical requirements different from or not contained in the IEC standards or other listed standards.

Any additional technical information that will assist a manufacturer to optimize the design and manufacture of the transformer should also be provided by the purchaser.

A technical specification has three most important objectives:

1st OBJECTIVE – To give the manufacturer, or tenderer, all the technical details he needs to implement his design and which will differ from unit to unit, such as rating, voltage ratio, cooling type, etc.

2nd OBJECTIVE – To inform the manufacturer or tenderer of the strategic significance of the transformer and the importance that should be given to dependability, maintainability, and long service life.

3rd OBJECTIVE – To give the tenderer, or manufacturer, information that will guarantee the transformer will satisfactorily interface with its aforementioned plant and equipment and that installation and commissioning will go smoothly and without undue delays.

The first two objectives must be accomplished by the inquiry paper for the manufacturer to be able to create his tender, and it is obvious that they will have a considerable impact on the transformer’s price. The third will have a lot of elements that will have little to no impact on total cost and that may be resolved during contract engineering.

To minimize the use of engineers’ time during the contract stage and to ensure that there are no unneeded delays during the contract, it is discipline to identify in the technical specification all those aspects that should have been known at the time of initially drawing this up. It’s as simple as that! This also reduces the risk that these items might be overlooked during the detail engineering of the contract.

It is important to remember that the purpose of a specification is not solely to describe what is wanted but also, to state what is not wanted. The latter often result from the purchasers’ previous experience.

Equally, the manufacturers’ experience can also complement the purchaser’s specification. Therefore the opportunity exists during the tender stage for exchanges of further information between the purchaser and the manufacturer by means of formalized design reviews and consultations.

Table of Contents:

  1. IEC/ISO standards
  2. Normal and Abnormal Operating Conditions:
    1. Gas and Oil Actuated Relays
    2. Overloads
    3. Geomagnetic Induced Current (GIC) Effects
  3. Design Requirements:
    1. Flux Density
    2. Voltage Regulation
    3. Cooling
    4. Control Detail
    5. System Earthing
  4. Transformer Core
  5. Transformer Tank
    1. Handling Facilities
    2. Tank Cover
    3. Oil-Tight Joints
    4. Vacuum and Pressure Requirements
    5. Valves
    6. Circulating and Eddy-Currents
    7. Access Openings
    8. Conservator Tanks
    9. Tank Earthing
    10. Pressure Relief
  6. Insulating Fluid
  7. Bushings
  8. Secondary Wiring and Control Cabinets
  9. Fittings
    1. Fittings List
  10. Tap Changers
  11. Monitoring
  12. Interchangeability
  13. Standardization
  14. Exclusions

1. IEC/ISO standards

Transformers should conform to the standards listed in the specification. Please see Table 1 for recommended list of standards. Where the purchaser has a distinct preference for either a core type or shell form transformer this must be clearly stated in the specification.

Table 1 – Recommended list of IEC/ISO standards

IEC/ISO StandardDescription
IEC 61869Instrument transformers
IEC 61869-2Instrument transformers
IEC 60050International Electrotechnical Vocabulary
IEC 60050(421)International Electrotechnical Vocabulary – Chapter 421: Power transformers and reactors
IEC 60060-1General definitions and test requirements
IEC 60060-2Measuring systems
IEC 60071-1Insulation coordination – Part 1: Definitions, principles and rules
IEC 60071-2Insulation coordination – Part 2: Application guide
IEC 60076-1Power transformers – Part 1: General
1IEC 60076-2Power transformers – Part 2: Temperature Rise for liquid-immersed transformers
IEC 60076-3Power transformers – Part 3: Insulation levels, dielectric tests and external clearances in air
IEC 60076-4Power transformers – Part 4: Guide to the lightning impulse and switching impulse testing – Power transformers and reactors
IEC 60076-5Power transformers – Part 5: Ability to Withstand Short-circuits
IEC 60076-6Power transformers – Part 6: Reactors
IEC 60076-7Power transformers – Part 7: Loading guide for oil-immersed power transformers
IEC 60076-8Power transformers – Part 8: Application Guide
IEC 60076-10Power transformers – Part 10: Determination of sound levels
IEC 60076-18Power transformers – Part 18: Measurement of frequency response
IEC 60137Bushings for Alternating Voltages above 1000V
IEC 60214-1Tap-changers – Part 1: Performance requirements and test methods
IEC 60214-2Tap-changers – Part 2: Application Guide
IEC 60270High-voltage test techniques – Partial discharge measurements
IEC 60296Fluids for electrotechnical applications – Unused mineral insulating oils for transformers and switchgear
IEC 60422Mineral Insulating Oil in Electrical Equipment – Supervision and Maintenance Guide
IEC 60529Degrees of Protection provided by Enclosures (IP Code)
IEC 60567Oil-filled electrical equipment – Sampling of gases and analysis of free and dissolved gases (Guidance)
ISO 8501-1Preparation of steel substrates before application of paints and related products – visual assessment of surface cleanliness
ISO 9001Quality management systems – requirements
ISO 12944-2Paints and varnishes – corrosion protection of steel structure by protective paint systems – classification of environments
ISO 14001Environmental systems – requirements, with guidance for use
ISO 19011Guidelines for quality and/or environmental management systems auditing

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2. Normal and Abnormal Operating Conditions

The following should be specified:

2.1 Gas and Oil Actuated Relays

Gas and oil actuated relays, used to indicate presence of accumulated gas or sudden oil movements, should not operate inadvertently when any combination of pumps start up and run, or in the event of loss or restoration of the auxiliary supply.

Figure 1 – Buchholz Relay

Buchholz Relay
Figure 1 – Buchholz Relay

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2.2 Overloads

It is only necessary to specify overload requirements in detail where they are in excess of what is listed in IEC standard 60076-7. It would be as well to state this directly. Where more onerous requirements are specified, the following information should be included as a minimum:

  1. Preload (and duration)
  2. Overload (and duration)
  3. Ambient temperature
  4. Maximum allowable temperatures during overload
  5. Method of test or verification

In case of partial loss of cooling equipment, similar considerations will apply. Note that restrictions may apply to the use of tap changers during overloads.

Suggested Reading – Where and how to find the root cause of a power transformer failure

Where and how to find the root cause of a power transformer failure (troubleshooting guide)

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2.3 Geomagnetic Induced Current Effects

Solar activity can cause Geomagnetic Induced Currents (GIC) to flow in the earth and these currents can find their way onto the power system usually via the earthed neutral points of transformers. The occurrence of GIC’s in electrical grids is linked to position on the earths’ surface and to the orientation and length of overhead line circuits connected.

Higher latitudes are generally more affected, being closer to the magnetic poles. Purchasers should determine whether the transformer being specified will be located at a site which may be subjected to GIC events from time to time.

GIC’s are quasi-DC currents (they are not true DC but have a frequency of around 1 Hz) that will flow through the transformer neutral into the windings, creating an effective DC component on the transformer magnetizing flux. When a GIC flows in the transformer, the core may “half cycle saturate” and this can cause a significant increase in stray flux, increase in VAR consumption and generate harmonics. The stray flux can heat up windings, clamping, structural parts, flux shields and the transformer tank.

The temperature rise experienced in any object is depending on:

  1. Details of the design
  2. Constructional details
  3. Intensity of the GIC in duration and magnitude
  4. Loading condition of the transformer
  5. Heat transfer capacity of the affected structures

Purchasers should note that certain transformer types are more susceptible to GIC type events, including the use of five limb cores, single phase units and shell type transformers. Where GIC’s are a potential risk the purchaser may state this and any preference in transformer design for avoiding GIC effects.

Additionally the purchaser may specify the maximum magnitude of the GIC to be considered in the design and the time period that this current must be carried by the transformer.

Suggested Guide – Geomagnetic induced current as a severe threat to power systems

Geomagnetic induced current as a severe threat to power systems

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3. Design Requirements

3.1 Flux density

The flux density in any part of the magnetic circuit including shunts should not attain a value that causes saturation. This should apply under the specified voltage, frequency and tap positions, including transitory effects of combined system voltage and frequency fluctuations. An adequate safety margin should be included.

The purchaser should state the over-excitation capability of continuous operation above rated voltage and at frequencies above and below rated frequency. A minimum acceptable V/Hz ratio could be specified for unloaded and fully loaded conditions.

Important Note! For Generator Step-up Transformer’s (and unit auxiliary transformers) the purchaser should specify the short time over-excitation vs. time due to load-rejection.

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3.2 Voltage regulation

The purchaser shall state any requirements for De-Energized Tap Changer (DETC) or On-Load Tap Changer (OLTC), and specify the voltage and impedance variations for all tap positions.

Suggested Video – How On Load Tap Changer Works

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3.3 Cooling

The purchaser MUST specify the internal and external cooling mediums and the circulating mechanisms that are required, by referring to the cooling method identification symbols in an appropriate standard, such as IEC 60076. Any different cooling method should be clearly stated in the specification.

The purchaser should state the percentage of any spare cooling capacity if required. In the absence of such a requirement being specified, the manufacturer should state the minimum percentage cooling capacity that can be removed for maintenance or replacement.

If any forced internal or external cooling medium is specified, the specification should also state the minimum amount of inherent natural cooling required. If no amount of natural cooling is specified, the manufacturer should state the maximum natural cooling capability of the cooling equipment offered in the tender.

Figure 2 – Transformer cooling fans

Transformer cooling fans
Figure 2 – Transformer cooling fans

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3.4 Control detail

The cooling equipment and its control scheme MUST be designed to ensure that the desired transformer ratings can be obtained. Pumps and fans are usually initiated by thermostatic control derived from winding temperature hottest-spot indicators or other temperature monitors.

The appropriate sequence in which forced internal and external cooling mediums are required to operate should be specified, for instance ONAN / OFAN / OFAF or ONAN / ONAF / OFAF. It may be preferable to start or stop pumps sequentially, and/or with soft start capability to avoid sudden excessive oil velocity changes.

To accommodate sudden load increase, the cooler control system should incorporate means to initiate pumps and fans immediately upon sudden load increases above a threshold value agreed between purchaser and manufacturer.

Outdoor mounted cooling pump and fan control equipment should be housed in a weatherproof cabinet designed, for instance, for protection grade IP53.

Further Study – Specification For Erection, Testing and Commissioning Of 66/11 kV Grid Substation

Specification For Erection, Testing and Commissioning Of 66/11 kV Grid Substation

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3.5 System Earthing

The purchaser MUST state the method of earthing any transformer neutral terminals in the specification. In the absence of such information a manufacturer may design the transformers for use with solidly earthed neutral connections, and shall state on the name plate that the neutral shall be directly earthed.

The purchaser must state the type and ohmic impedance of the alternative earth connection if solidly earthed neutrals are not to be used.

Further Study – High voltage earthing system analysis and design for power substations

High voltage earthing system analysis and design for power substations and lines

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4. Transformer Core

The temperature of any part of the core or its support structure in contact with oil must not to exceed what is specified in IEC standard 60076-2. The purchaser or manufacturer may prefer to test these parts of the transformer to higher levels of voltage than specified in some standards.

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5. Transformer Tank

Transformer tanks are usually constructed from welded steel plate and reinforced to withstand transport, handling or excess pressures during fault conditions without distortion. The purchaser can specify whether or not a cover-type or a bell-type tank is required.

The design and positioning of lifting points, stiffeners and underbases on the tank should prevent distortion of the core during lifting and transport.
For personnel safety it is recommended to specify the maximum tank surface temperatures, according to local laws and regulations.

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5.1 Handling Facilities

Handling facilities may be required to permit movement, assembly and dismantling of the complete oil-filled transformer at site or elsewhere and should be agreed before a contract, unless otherwise specified.

Possible facilities include the following:

  1. Four jack pads near the corners of the tank, designed to take the weight of the complete transformer.
  2. Lugs for lifting the transformer during transport. The lifting lugs and attachments shall be designed to allow for possible unequal lifting forces,
    together with an adequate factor of safety allowance.
  3. Lifting eyes for main transformer tank cover, conservator tanks and on-load tap changer.
  4. If applicable, a suitable reinforced base frame to form a skid assembly for skidding the transformer in any direction using rollers.
  5. If applicable, permanently mounted or removable wheels, arranged to permit bidirectional movement.
  6. Hauling eyes on all sides of the tank.
  7. If applicable, riding lugs for transport on a side-beam road trailer or railway car. The riding lugs (removable if necessary) should be capable of taking the weight of the main unit, complete with oil filling if requested.

Interesting Video – Lifting the huge transformers into the Wind Farm

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5.2 Tank Cover

The tank cover may be bolted or welded to the tank. If the purchaser has a preference, this should be stated in the enquiry. In case of a welded cover, it is preferred to weld before final testing.

The tank cover should be designed with a sufficient slope to shed water. Fixings should be provided for attachments to ensure a safe working environment when personnel have to work on top of the transformer. All tubes, equipment, etc. on top of the transformer should be located in such a way as to minimize hindering movement of personnel.

Figure 3 – Transformer tank cover fabrication

Transformer tank cover fabrication
Figure 3 – Transformer tank cover fabrication

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5.3 Oil-tight Joints

Oil leaks from the main tank-cover joint or other joints are unacceptable under any static oil-head or forced oil conditions at any ambient or maximum operating temperatures. Only joints of proven design, capable of preventing deterioration of any seal or gasket materials should be specified or supplied.

All bolted flange joints should be provided with suitable gaskets and made from oil resistant, non-perishable material installed within smoothly machined grooves designed to stabilize the gasket position and to provide suitable compression stop.

The thermal performance of the material MUST exceed the maximum temperature attained by the metal parts in contact with the gaskets under all conditions.

For bolted pipe joints or similar, the “O-ring” type of flange seal may be preferred. If cork type gasket materials are used, the metal mating surface shall be thoroughly cleaned to prevent the gasket from sticking.

Figure 4 – Transformer tank oil leakage repair

Transformer tank oil leakage repair
Figure 4 – Transformer tank oil leakage repair

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5.4 Vacuum and Pressure Requirements

The assembled transformer, including the tank, coolers or radiators, conservator (if not equipped with a rubber diaphragm), oil pumps, all oil connections, valves, pressure relief devices and other fittings, MUST be capable of withstanding, with minimum permanent distortion:

  • When oil filled, an internal overpressure of 35 kPa.
  • Without oil, full internal vacuum.

The transformer conservator tank, if equipped with a rubber diaphragm, need not be designed for a full vacuum but a vacuum-tight valve should be provided in the connection between tank and conservator. The pressure relief diaphragm should be replaced by a steel plate.


It is necessary to ensure that the transformer tank is not accidentally sealed, as it might be the case by a valve between the tank and the conservator, unless a suitable bypass arrangement is specified.  It is usual for power transformers to be designed and equipped for vacuum filling and oil treatment in the field, whether or not the transformer is shipped with oil.

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5.5 Valves

The transformer tank should typically be equipped with the following valves and fittings, the positioning of which shall be subject to approval of the purchaser.

  1. At least an oil valve at the top and bottom of the tank for taking oil samples shall be provided, unless an alternative arrangement is proposed.
  2. A drain connection valve at each end of the tank at the bottom wall of the tank, complete with a blanking plate.
    The connection must vent the tank as close as possible to the junction of the tank wall and the base, so that no more than a few mm of oil will remain in the tank when empty.
  3. Two elbow valves, complete with a blanking plate for filling connections, should be provided on the tank cover and located at diagonally opposite corners.
  4. A valve fitted with a blanking plate and located on the tank cover in line with the bottom sampling valve should be provided for attaching a vacuum gauge, a pressure gauge or an oil level indicator when vacuum filling One or more valves for immediate or future connection of on-line monitors for dissolved gas.
  5. A siphon valve with no return valve for draining the OLTC tank (if applicable).
  6. Residual oil discharge valves for the expansion tank(s).

Figure 5 – Gas Density Monitor and fill valve

Gas Density Monitor and fill valve
Figure 5 – Gas Density Monitor and fill valve

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5.6 Circulating and Eddy-Currents

The tank should be designed or incorporate measures to minimize the losses caused by circulating and eddy-currents and avoid onerous temperatures at any part of the tank surface and at flanges between parts of the tank and its components especially at gasket sealed joints.

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5.7 Access Openings

Access openings should be provided as appropriate in the tank cover and walls to permit unhindered access to inspect, repair or remove current transformers, tap-changer components, winding connections and other devices that may require routine or emergency maintenance.

An opening that allows personnel access should not be less than 500mm diameter or 500mm × 500mm. Hand holes should be approximately 400mm diameter or 300mm × 600mm.

All openings on the cover should have a raised flange to prevent water from entering the openings when individual covers are removed. At least two openings should be provided on the tank cover for access to the interior without lowering the oil below the top of the core.

Figure 6 – Inspecting tap-changer components

Inspecting tap changer components
Figure 6 – Inspecting tap changer components

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5.8 Conservator Tanks

A conservator tank shall be provided of sufficient size to accommodate the change in oil volume that will occur between the specified ambient temperature limits in service with the transformer operating at full load or overload and the cold oil temperature with the transformer out of service.

The typical conservator oil volume is approximately 10% of the sum of the oil volumes of the main transformer tank and the coolers.

The main tank and on-load tap changer diverter compartment shall have separate conservator tanks. Oil proof rubber diaphragms (bladders) are typically used within the conservator tanks for the transformer main tank to minimize atmosphere contact with the insulating fluid.

Each conservator tank must have a suitable oil level gauge mounted on the conservator tank so as to be easily read from ground level. The gauge shall be graduated to indicate the oil level at temperatures of −10°C, +5°C, +15°C and +20°C or other values specified by the purchaser.

A float switch shall be provided having a set of low and high oil-level alarm contacts. A dehydrating breather shall be connected to each conservator tank. The inner diameter of the pipe connecting the breather and the conservator tank shall be sized large enough to not inhibit pressure equalization.

Oil transformer conservator
Figure 7 – Oil transformer conservator

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5.9 Tank Earthing

The following grounding and bonding facilities for earthing purposes should be provided on the tank and other separate free standing parts such as radiator banks:

  1. At least two suitable earthing terminals on the main tank.
  2. One earthing terminal should be located, for instance, towards the extreme right hand end of the low voltage side and the other diagonally opposite on the high voltage side.
  3. One suitable earthing terminal on each cooler bank support structure.
  4. Earthing straps, to bond the tank cover to the main tank.
  5. One earthing terminal on the main tank near each set of surge arresters to allow a “high frequency” earthing connection from the arresters.

Other internal and external metal parts of the transformer shall be earthed to the tank or separately and directly earthed. Whichever method is adopted, a uniform earth potential is required throughout the installation.

Closed circulating current loops within the earthing systems MUST be avoided.

Flanged joints should be electrically bridged. Internal earthing connections from the core and core clamping structure shall be brought out to bushings mounted in a secure, weatherproof terminal box, mounted on the tank surface and earthed externally in order to facilitate testing of the core earthing system.

Figure 8 – Earthing via copper brazing of earth tapes for 33kV power transformer

Earthing via copper brazing of earth tapes for 33kV power transformer
Figure 8 – Earthing via copper brazing of earth tapes for 33kV power transformer (photo credit:

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5.10 Pressure Relief

At least one suitable spring operated pressure relief vent should be located on the main tank (preferably on the cover) and on-load tap changer diverter compartment(s) complete with an approved oil deflection collar. In addition, piping can be connected to the oil deflection collar of each pressure relief device in order to direct the oil down near the base of the transformer.

The number of pressure relief devices required on a transformer is normally dependent on the total oil volume of the transformer. Suitably rated auxiliary contacts should be provided for these devices.

Figure 9 – Pressure Relief Device

Pressure Relief Device
Figure 9 – Pressure Relief Device

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6. Insulating Fluid

The purchaser should state which type of insulating fluid that should be supplied and the specification with which it must comply. The insulating fluid should be free of pcb, copper sulphide, or other chemical having a corrosive sulphur tendency.

The purchaser should state whether the fluid should be inhibited or non-inhibited, and if any special additives are required or conversely not permitted. The fluid should comply with the recognized IEC standard and any additional regional or purchaser requirements.

IMPORTANT! Under no circumstances shall any degree of forced-oil circulation create a static electrification hazard in any part of a transformer under any operating condition.

Figure 10 – Transformer oil is frequently used in electrical devices as a coolant, insulator, and arc-suppressor. Aromatic hydrocarbons, paraffins, and naphthenes are all components of transformer oil.

Transformer oil is frequently used in electrical devices as a coolant, insulator, and arc-suppressor
Figure 11 – Transformer oil is frequently used in electrical devices as a coolant, insulator, and arc-suppressor. Aromatic hydrocarbons, paraffins, and naphthenes are all components of transformer oil.

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7. Bushings

Bushings should comply with a recognized standard such as IEC 60137. The specification of oil/SF6 bushings should be agreed between purchaser and manufacturer before a contract. The interface between the transformer and gas insulated external connections requires special attention to dimensions, limits and tolerances.

These design aspects should be agreed between purchaser and manufacturer before a contract and should take into account any purchaser standardization policies.

Figure 12 – Maintenance of transformer bushings

Maintenance of transformer bushings
Figure 12 – Maintenance of transformer bushings

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8. Secondary Wiring and Control Cabinets

All conductors, connectors, terminal blocks, wire-ways, terminal markings, etc. MUST meet recognized standard and purchaser requirements. Control cabinets should be mounted in a manner to reduce vibration and should be designed to prevent moisture ingress and condensation.

Control cabinets should be mounted at a height that enables operational access from ground level.

Figure 13 – Checking power transformer control cabinet and secondary wiring

Checking power transformer control cabinet and secondary wiring
Figure 13 – Checking power transformer control cabinet and secondary wiring

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9. Fittings

The number and type of fittings required or provided on a transformer will depend upon several factors including its purpose, construction, rating and voltage. Some other considerations are, the amount of surveillance required or provided by the purchaser, the requirements for automatic control and protection, purchaser policies concerning on-line diagnostic and monitoring requirements.

Certain fittings may incorporate protection, control and remote indication facilities. Details of such facilities should be stated explicitly at the time of enquiry or tender, including any requirements for conformity with existing practices.

All labels, plates and markings MUST be manufactured from durable, non fading material.

Any instruments or indicators should be capable of being read from ground level. Where any equipment is to operate in parallel or perform in a similar manner with existing equipment, the purchaser should provide complete details of the key parameters of the existing equipment in the enquiry document.

Where alarm and trip contacts are required, the purchaser should state the range of operating settings required in the enquiry.

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9.1 Fittings List

The following list is representative of the fittings that may be required on each power transformer. In practice purchasers and manufacturers select fittings from such a list as this but may also adopt or recommend additional or alternative fittings for reasons of policy, improved safety, efficiency, security, and maintenance or lifetime costs.

  • Thermometer pockets in each top and bottom oil pipe adjacent to the transformer.
  • Gas and oil actuated relays for main conservator/tank oil pipes and externally mounted tap changer selector compartments, as appropriate. Sampling and test stopcocks may be required and mounted for operation at ground working height.
  • Conservators for the main tank and tap changer diverter compartments where required. Conservators should be provided with an oil gauge, drain valve, oil filling facility, lifting lugs, oil sumps and removable end covers.
  • Dehydrating breathers.
  • Air release and drain plugs or valves for pipe work, oil expansion bellows, pumps, etc.
  • Separate drain and filter valves at the top and bottom of the main tank, externally mounted tap changer selector compartments and cooler headers.
  • Isolating valves complete with an open/shut indicator and locking facility.
  • Valve location plate. The position of each valve in normal service should be shown, i.e. Normally Open (N.O.) or Normally Closed (N.C.) fans.
  • Pumps.
  • Oil-surge relays and pressure relief devices for tap changer diverter compartments.
  • Oil sampling devices easily accessed by personnel.
  • Anti-vibration pads.
  • Provision for blanking plate storage.
  • Earthing lugs on the main tank and each separate cooler structure.
  • Main tank jacking lugs.
  • Main tank transport lugs.
  • Name plate.
  • Diagram and rating plate.
  • Owner’s serial number plate.
  • Vacuum capabilities plate.
  • Main haulage points.
  • Main tank haulage rope guides.
  • Transport anchoring lugs.
  • Co-ordinating rod stands with co-ordinating gaps or fixings for mounting.
  • Surge arresters and supporting brackets.
  • Current transformer test loop(s) for HV and/or LV bushing turrets.
  • Terminal box for LV current transformer test loop.
  • Terminal box for HV current transformer test loop.
  • Terminal box for core and core structure earthing.
  • Current transformer terminal boxes.
  • Lifting lugs for tap-changer, cooler structures, main tank cover and other components as necessary.
  • Winding temperature indicator pockets with protective covers.
  • On-line combustible gas and moisture monitors.
  • Protective covers to protect projections, such as valves, from damage during transport.
  • Pressure relief devices and associated ducting.
  • Tank attached fixings for mounting external neutral current transformers.
  • Cover mounted safety lugs to permit fixing of toe boards / safety fences for safe working purposes.
  • Winding and oil temperature indicating instruments.

Suggested Guide – 25 High Voltage Transformer Tests and Commissioning Procedures

25 High Voltage Transformer Tests and Commissioning Procedures (Before Energizing)

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10. Tap Changers

If the transformer application requires that variation of the voltage ratio is required in order to make corrections for changes either in the supply-side or demand-side voltages, de-energized tap changers (DETC’s) and/or on-load tap changers (OLTC’s) should be specified.

The choice of which type of tap changer to be used will be dictated primarily by its function and economics, e.g.:

  1. The size of the tapping range needed to match the expected system voltage variation
  2. The step voltages required and number of steps
  3. Whether or not the transformer can be electrically disconnected from the network in order to change taps

De-energised tap changers (DETC’s) are only used when the need for voltage correction is infrequent because all tap changing by this means has to be undertaken when the transformer is off-line, which requires an outage.

In practice, DETC’s are mainly fitted to high voltage windings and when a low number of tap steps are required, e.g., 4, 6 or 8 steps. Usually, each tap step will vary the high voltage winding turns between 1% and 2.5%. The number of tap steps and percentage voltage variation per step should be specified by the purchaser and determined by the purpose of the tappings, i.e., to maintain the low voltage on load at rated value when the high voltage changes or alternatively to maintain the low voltage network voltage at some value as the load varies.

In most other respects the electrical, mechanical and thermal design requirements of DETC’s, e.g., current rating of contacts and voltage withstand
considerations, are similar to those of on-load tap changers.

On-load tap changers (OLTC’s) are designed for connection to line-end or neutral-end of high voltage or low voltage windings. The size of the tapping range is usually specified by the purchaser and is a compromise between the network high and low voltage ranges. Similarly, the number of tap steps will be determined by the range of voltage variation expected in service and the size of voltage change per step required.

Replacing an old OLTC with the new one
Figure 14 – Replacing an old OLTC with the new one (photo credit:

In practice the number of steps can vary between 10 and 40, depending on voltage, application and current rating of the tap changer. The range of voltage that can be accommodated is determined by the ac power frequency and impulse voltage withstand strength between adjacent taps.

OLTC’s are specialized precision electro-mechanical devices. They are invariably purchased by the transformer manufacturer under a sub-supplier contract from an original equipment manufacturer (OEM) and selected from a type tested and proven product range. The choice of OLTC suppliers is often specified by the transformer purchaser.

Several types of OLTC’s are commonly available and may be categorized as:

  1. Line-end
  2. Neutral-end
  3. In-tank
  4. Externally mounted

“Line-end” and “Neutral-end” describe the electrical position of the tap changer within the configuration of windings and connections. Line-end OLTC’s are usually positioned at the line end of lower voltage windings. This position is chosen for autotransformers, for instance, when the voltage ratio is low, e.g., of the order of 2:1.

Designs employing neutral-end tap changers are usually more economic when the voltage ratio is greater than 2:1. Where delta connected high voltage windings are required, line end tap changers may be specified on the high voltage winding.

“In-tank” and “Externally mounted” OLTC’s refer to the physical position of the tap changer in or on the transformer.

It should be noted that tap changers, particularly OLTC types, are still a frequent source of incipient or major transformer faults and a cause of unplanned transformer outages.

It is important therefore that proven equipment is used wherever possible and that a transformer design is adopted which best meets the needs of the purchaser’s operating regime, supply responsibilities and long-term maintenance requirements.

Suggested Video – How Off Load Tap Changer Works

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11. Monitoring

Purchasers of power transformers are routinely selecting equipment for on-line monitoring of the transformer operating status to minimize forced outages; planning maintenance activities; increased personnel safety; and as well for maximizing the performance of their transformers.

On-line monitoring equipment is available for DGA, moisture in oil, oil temperatures, oil pressure, load current and voltage measurements, pump/fan operation, conservator membrane condition, tank vibration, winding hot spot calculations and/or direct hot spot measurements, bushing condition, partial discharges and OLTC condition.

Purchasing monitoring equipment in the original specification is an efficient means to the option of adding monitors at a later date.

Online transformer condition control and monitoring
Figure 15 – Online transformer condition control and monitoring (on diagram: TraCoMo Transformer Control and Monitoring System with Automatic Voltage Regulation (AVR), transformer monitoring, tap changer monitoring, fibre optic winding temperature monitoring, online dissolved gas analysis (DGA), bushing monitoring, partial discharge, and breakdown voltage monitoring)

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12. Interchangeability

Interchangeability refers principally to transformers of similar rating, voltage and other operating characteristics that are or have been purchased under different contracts and sometimes from different suppliers, but are all designed to have common dimensions and layout, in order to allow them to be physically interchangeable with each other with a minimum of adaptation, if any.

It is possible for transformers purchased earlier to be replaced later by more modern designs, having larger ratings but designed to be installed and occupy the same “space”.

Utilities purchase transformers to meet requirements of this kind in order to increase the availability of electricity supply and reduce costs by minimizing the outage time in the event of a transformer having to be removed from service and replaced by a spare, stored strategically for that purpose.

Interchangeability is of special importance where transformers are required for installation and connection to gas insulated busbars. In these instances the concept of interchangeability extends beyond the transformers to include also the busbars, especially at the interface between the two systems.

When interchangeability is required, the purchaser should undertake to specify and detail the key features, dimensions and interfaces that are to be repeated on each transformer and provide all necessary reference drawings. The arrangement and physical dimensions of the high, low and possibly other voltage bushing connection points, (sometimes referred to as cover layout), is a vital part of this information.

When specified, the transformer including fittings and other major interfacing components shall be interchangeable with other transformers and related equipment defined by the purchaser.

Interesting reading – My worst experience in testing and commissioning power transformers

My worst experience in testing and commissioning power transformers (and how I fixed things up)

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13. Standardization

The concept of standardization is not to be confused with that of interchangeability. There is some similarity between the two requirements, especially when transformer components such as tap changers, valves and other interfacing fittings are required to be replaceable.

Standardization refers to a policy to limit the variation of transformer types, ratings, voltage ratios, impedances, tapping ranges and other principal electrical, mechanical and thermal characteristics of a purchaser’s transformers. The policy reduces the complexity of the purchaser’s stock of transformers, bushings, fittings, tap changer components and other items and tends to minimize maintenance practices and costs.

The aims of standardization are:

  1. Minimize system design, operating and capital costs
  2. Simplify maintenance procedures and requirements, and system planning
  3. Reduce stock held items
  4. Optimize spares
  5. Reduce purchasing and other front-end costs
Standardization need not be confined to utilities and other organizations that have large and varied stocks of transformers. Smaller purchasers can purchase transformers by reference to national and international standards that include recommended transformer ratings and other features, including in some cases, losses and dimensions, for which manufacturers have proven up-to-date designs offering economic savings that are not obtainable from custom made nonstandard alternatives.

This will also give the possibility to replace the transformer with one from another company in case of an emergency.

Further Study – A case study of maintenance and condition monitoring of the power transformers

A case study of maintenance and condition monitoring of the power transformers

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14. Exclusions

In most cases, every opportunity is taken by a manufacturer to comply with the requirements of the specification or to propose alternatives that permit improvements that best fulfil the purchaser’s needs or meet the manufacturer’s capabilities.

Deviations of this kind from a purchaser’s specification may be raised in pre-tender discussions between a purchaser and potential manufacturer but ultimately, any tender submission must either comply with the specification or, if this not possible, a manufacturer should categorically state the non-compliance and exclusions.

All exceptions should be discussed between the purchaser and tenderer and a resolution made prior to an award of order. The purpose is to avoid misunderstandings. If no exclusions are stated, the contract works have to be treated as fully compliant with the specification.

The tenderer should state any non-compliance with the specification in the tender submission and any alternative offers should be submitted in full and separately from the main offer.

Recommended Course – Electrical Transformer Theory, Design, Construction, Protection and Maintenance

Electrical Transformer Theory, Design, Construction, Protection and Maintenance

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  1. Manual for creating power transformer requirements by N. Buthelezi, A. Cancino, L. Cornelissen, E. de Groot, M. Figura, T. Fogelberg, T.Gradnik, AC Hall, J. Lackey, A. Manga, A. Mjelve, M. Oliva, V. Podobnik, S. Ryder, K. Ryen, C.Swinderman, J.Velek, and M. Zouiti – Cigre
  2. The J&P Transformer Book, Martin J. Heathcote, CEng, FIEE

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More Information

Edvard Csanyi

Electrical engineer, programmer and founder of EEP. Highly specialized for design of LV/MV switchgears and LV high power busbar trunking (<6300A) in power substations, commercial buildings and industry facilities. Professional in AutoCAD programming.


  1. hozefa morkas
    Aug 28, 2023

    Telawne india , good informative
    Can u pl share accesdories in pad mounted and sub station transformers suitable for polar region in canada -50deg.c as per CSA standurd, hermetically sealed with air caution

  2. Kok chee kheng
    Jun 27, 2023

    Great article, keep up the good work

  3. Saqlain
    Apr 13, 2023

    Nice article,plz brief how to measure GIC and how to avoid

  4. [email protected]
    Nov 27, 2022

    hello sir ,
    your topics are very great and informative according to electric field
    sir i m a student. belong to v.poor background so plz could anyone help me to take your perineum membership of your site
    I m thankful to you

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