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.

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:

1^st 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.

2^nd 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.

3^rd 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.

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

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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

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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.

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.