A89 Understanding Power Transformer Purchases: Essential IEC Standards and Their Scope Part-1 – IEC 60076-1, Clauses 9-13 and other Details

Introduction

In the previous article A88, Clauses 1-8 of the IEC 60076-1:2011, Part 1 – General, were discussed from the perspective of the transformer purchaser. In this part of the article, we will delve into Clauses 9-13 and the Annexures, Figures, and Tables provided in the Standard.

Adhering to IEC standards is crucial in power transformer procurement, as they establish uniform specifications across design, construction, performance, and testing. This ensures consistency and compatibility across manufacturers, facilitating confident product comparison. Additionally, these standards offer guidelines for testing and certifying transformers, confirming compliance and assuring buyers of quality and reliability. Thus, they mitigate the risk of purchasing substandard products.

9 Safety, Environmental and other Requirements

These requirements are summarized briefly here; for detailed information, reference to the standard is recommended.

Liquid leaks: Transformer manufacturers must prioritize effectively containing the transformer's liquid and implementing measures to prevent leakage. This includes careful consideration of factors affecting long-term performance, such as joint design, choice of gasket materials, quality of welds, and corrosion prevention measures.

Transformers should be designed to ensure they are leak-free. Any instances of leakage discovered on-site at the conclusion of commissioning must be promptly addressed by the responsible supplier.

Safety considerations: The manufacturer must prioritize the safety of operators and maintenance staff when designing the transformer, with particular attention to the following aspects:

• Accessibility to parts with high temperatures
• Accessibility of live parts
• Accessibility of moving parts
• Provision for lifting and handling
• Access for maintenance, as needed
• Safety measures for working at height

In cases where installation may impact any of these factors, the transformer must come with appropriate installation instructions.

Additional requirements include:

Dimensioning of Neutral Connection: For transformers intended to carry a load between phase and neutral (e.g., distribution transformers), the neutral conductor and terminal must be rated for the appropriate load current and earth-fault current.

For transformers not intended to carry load between phase and neutral, the neutral conductor and terminal must be designed to carry earth-fault current as if the transformer were directly earthed.

Liquid Preservation System: The type of liquid preservation system may be specified in the inquiry and order by the purchaser. If not specified, the manufacturer should indicate the liquid preservation system in the tender."

DC Currents in Neutral Circuits: If a transformer may be exposed to DC currents, the purchaser should specify these current levels in the inquiry, along with any required limits on the consequences of these currents.?

Centre of Gravity Marking: For transformers weighing over 5 tonnes in transport configuration, the center of gravity should be permanently marked on at least two adjacent sides of the transformer.?

10 Tolerances ?

Tolerances are essential for specific transformer performance factors, either set by the customer or determined by the manufacturer. They account for manufacturing or material differences, measurement uncertainties, and the intricacies of design.

A transformer is deemed compliant with this standard when the quantities subject to tolerances fall within the limits provided in Table 1. If a tolerance in one direction is not specified, there is no restriction on the value in that direction.

This clause serves solely for acceptance or rejection purposes and does not supersede any guarantees prescribed by purchasers for economic evaluation, such as penalties for losses. It also does not override any limits specified in the inquiry.

Transformer users and manufacturers may choose to specify or adhere to tighter tolerances than those outlined in standards.

The tolerances on Transformer Parameters according to the IEC 60076 Standard are listed in Table 1:

11 Tests

This clause encompasses general requirements for routine tests, type tests, special tests, and those mandated by the IEC 60076 standard for compliance. Transformers must undergo these tests as outlined in the standard.

Tests should be conducted at the manufacturer's facility unless an alternative arrangement is agreed upon between the manufacturer and the purchaser. During testing, all external components and fittings that could impact the transformer's performance must be installed.

The standard also delineates guidelines for testing and assessing the performance of power transformers. These guidelines cover various types of tests, including routine tests (detailed in sub-clause 11.1.2), type tests (detailed in sub-clause 11.1.3), and special tests (detailed in sub-clause 11.1.4). These tests play a crucial role in ensuring the quality and reliability of power transformers.

12 Electromagnetic Compatibility (EMC)

Power transformers are considered passive elements regarding the emission of, and immunity to, electromagnetic disturbances. However, certain accessories may be susceptible to electromagnetic interference. Passive elements do not generate electromagnetic disturbances and are not affected by such disturbances.

13 High Frequency Switching Transients

When switching lightly loaded and/or low power factor (inductively loaded) transformers using vacuum and SF6 interrupters, the transformer may be exposed to potentially damaging voltage transients with frequencies reaching into the MHz range and voltages exceeding the transformer's impulse withstand capacity. While mitigation measures are not inherent to the transformer itself, they may include methods to increase damping, such as resistor-capacitor snubbers or pre-insertion resistors within the switches, or conducting switching operations under load. If requested by the purchaser, the manufacturer should provide detailed information on these mitigation measures.

Figure 1 illustrates potential snubber applications for safeguarding MV transformers from harmful switching transients. The snubber should be installed between the VCB and the transformer.

Annexures

The standard incorporates the following Annexures:

Annex A (informative): Check list of information to be provided with enquiry and order

Annex A serves as a valuable guideline for purchasers, providing necessary information and technical requirements when making inquiries and placing orders with transformer manufacturers. This helps ensure smooth execution of orders and avoids conflicts.

Annex B (informative): Examples of specifications for transformers with tappings

Annex C (informative): Specification of short-circuit impedance by boundaries

Annex D (informative): Examples of three-phase transformer connections

Annex E (normative): Temperature correction of load loss

Annex F (informative): Facilities for future fitting of condition monitoring systems to transformers

This annex suggests that for future installation of a monitoring system on a transformer, consider the sensors and facilities listed in Table F.1. The specific sensors and facilities should be agreed upon by the manufacturer and purchaser, depending on the transformer's size and criticality.

Annex G (informative): Environmental and safety considerations?

Figures given in the Standard

The standard includes the following figures:

Figure 1 – Different types of voltage variation

Figure 2 – Illustration of 'clock number' notation

Figure 3 – Illustration of 'clock number' notation for transformers with open windings

Figure 4 – Illustration of 'clock number' notation

Figure C.1 – Example of specification of short-circuit impedance by boundaries

Figure D.1 – Common connections

Figure D.2 – Additional connections

Figure D.3 – Designation of connections of three-phase auto-transformers by connection symbols (auto-transformer Ya0)

Figure D.4 – Example of three single-phase transformers connected to form a three-phase bank (connection symbol Yd5)

Tables given in the Standard

The standard comprises the following Tables each containing wealth of knowledge:

Table 1 – Tolerances

Table B.1 – Example of combined voltage variation

Table B.2 – Example of functional specification with HV voltage variation

Table B.3 – Example of functional specification with LV voltage variation

Table F.1 – Facilities for condition monitoring

REFERENCES

[1] https://webstore.iec.ch/

[2] IEC 60076-1:2011, Edition 3.0, Power transformers – Part 1: General.

[3] https://blog.se.com/datacenter/2020/04/24/how-to-protect-mv-transformers-from-damaging-switching-transients/

[4] https://www.dhirubhai.net/pulse/role-iec-standards-purchase-power-transformers-muhammad-hanif-2kprf/

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