A76 - New Standards for Transformers-Year 2022

Recently two important transformer standards were published, one by IEC on DGA interpretation (revision of Ed3.0) and the other a new standard by IEEE for establishing the short circuit withstand capability of transformers.

A)???IEC 60599 Ed4.0-2022 Mineral Oil-filled electrical equipment in service-Guidance on the analysis of dissolved and free gases.

IEC 60599 is one of the frequently referred IEC Standards whenever the user notices any of the dissolved gases as per the DGA test exceeds the typical gas levels due to some developing incipient fault. This standard was first issued in 1978 and then revised in 1999. This fourth edition published in May 2022 replaces the third edition issued in 2015.?

Major changes in this edition are

(a)??Clause A5 -Interpretation of DGA results of oil sampled from oil-impregnated paper bushings.

?This section has been thoroughly revised and expanded to include the contents of withdrawn IEC TR 61464-1998 -Insulated Bushings-Guide for interpretation of DGA in bushings and CIGRE Technical Brochure 771-2019 Advances in DGA Interpretation.

(b)??Clause A3 on DGA in Wind Turbine Transformers (WTT)

Revised to include the new findings available in CIGRE Technical Brochure 771(2019). Typical 90% values for gas concentrations for WTT are given in Table A.5

B) IEEE Std C57.164-2021-IEEE Guide for Establishing Short Circuit Withstand Capabilities of Liquid-Filled Power Transformers, Regulators, and Reactors

This new IEEE std, finalized in December 2021 gives guidance on checking and establishing short circuit withstand capabilities of power transformers, regulators, and reactors. The draft guide was prepared by the Short Circuit Guide Working Group, chaired by Sanjay Patel, and co-chaired by Raj-Ahuja with the secretary Joe Watson.

In a way, this is an equivalent IEEE standard to the widely referred IEC 60076-5 Ed3.0-2006 -Power Transformers-Part 5 Ability to withstand short circuits.?

For small transformers, included in Category I (5-500 kVA) and Category II (501-5000 kVA) of C57.12.00 Table 11, the preferred short circuit strength evaluation procedure is a short circuit withstand test since these transformers are purchased in bulk. To establish short circuit-withstand capability for higher-rated transformers, design review guidance is given in this standard.

Clauses 4.1 - 4.14 gives the short circuit current calculation method for various types of terminal faults on transformers. The subject is covered in detail in an easy-to-follow format. For short circuit withstand capability analysis, the maximum current condition is to be considered.

At present dynamic analysis of electromagnetic forces and mechanical stresses, are very complex to perform on every new design. Over the years and to date, these have been analyzed assuming static conditions (i.e., forces based on the amplitude of the peak current), which is the basis for this guide (for radial stress, the static analysis represents the worst-case). 2D and or 3D magnetic field calculation methods should be the basis for calculating the static forces.

Electromagnetic forces and mechanical stresses to be considered for core-type transformers during the overcurrent conditions are covered in clause 4.15. The forces and stresses to be considered in windings are (clause 4.15)

Forces

1)???The inward or outward radial forces on each winding

2)???The maximum axial compression and tilting forces on each winding

3)???The maximum axial end thrust forces (up/down) on each physical winding

4)???The maximum axial forces on pressure rings or plates, tie plates, and core clamp

5)???The thrust force acting on the lead exits of each winding.

Stress from Forces

1)???Maximum tensile hoop stress and compressive buckling stress on all windings

2)????Maximum axial and radial bending stresses on conductors in the span between axial sticks and between spacers

3) Maximum axial tilting stresses on the conductor

4) Maximum compressive stresses on radial spacers.

5) Maximum compressive stresses on conductor paper insulation

6) Maximum compressive stresses on insulation structures

7) Maximum compressive stress and deflection on pressure rings (or plates)

8) Maximum tensile stress on tie plates (flitch plates) or tie rods of the clamping structure. Note that the short-circuit stresses may not be the worst case; lifting and clamping stresses should also be evaluated.

9) Maximum stress on the upper and lower core frames, where applicable

Short circuit design details to be furnished by the manufacturer for design review (clause 6.5) are:

The calculated worst-case fault currents in each winding for various faults and design calculations including the following

1) ?Fault models with fault current sources, grounding details, tap positions, asymmetry factor and pre-fault voltages ii) Positive, negative, and zero sequence winding, system and source impedances used for the calculations

2) The electromagnetic forces and mechanical stresses resulting from the calculated worst-case fault currents for each type of fault on each winding

3) Winding descriptions, layout, and dimensions

4) Conductor type and dimensions

5) Rated 0.2% yield strength of all conductors

6) Type of epoxy used for all epoxy-bonded CTC

7) Any radial supports for the windings considered in the calculation of the winding's withstand strength, such as the core against the inner-most winding

8) The supports of the winding lead exits and leads or bars including internal bus work and leads connecting to all bushings and tap-changers

9) The mechanical short-circuit forces and mechanical withstand capabilities on the leads and bus work, the lead and bus work support structures, and the components connected to the leads and bus work such as bushings and tap changers

10) The assumptions used in the short-circuit force and stress calculations

The following additional details are also required for design review:

1) The number, sizes, and mechanical strength of axial and radial supports.

?2) Winding clamping pressure and description of the clamping system including pressure ring mechanical strength, material, and allowable deflection,

3) Tie plate (flitch plate) material, yield strength, dimensions and cross-sectional area.

4) Allowable maximum offset between the magnetic centers of any windings in any set of phase windings, after clamping.

5) The supports of the core against the tank, arrangement of the springs and clamping systems to hold the core, tank reinforcement structures, other support of the core laminations.

6) Documentation of quality assurance and quality control practices and procedures concerning both materials and manufacturing with specific reference to manufacturing activities, such as the winding of the conductors and control of the pulling force, sizing and elastic stabilizing of windings and coils, assembling within specified tolerances, drying and impregnation with oil, application of pre-load (clamping force), fastening/ securing of winding supports, leads and clamping devices, etc.

7) The winding drying/sizing and completed active part drying/clamping processes. This includes pressure applied to the winding during sizing and the final clamping pressure.

Evaluation of calculated stresses

Calculated stresses should be compared to the stresses in a similar transformer already short circuit tested. Qualifications for the referred similar transformer are given under clause 6.2.1.

If such a reference transformer is not available, a design review of the short circuit stresses and withstand capability of the proposed design shall be done based on the design criteria and rules of the manufacturer. The design rules shall be based on previously performed short-circuit tests.

If the manufacturer cannot provide validation of their design and/or factory-specific criteria with successful short-circuit test results, the purchaser should consider the calculated short-circuit design to be unproven and subject the same to additional scrutiny and/or margins on safety factors. Probably this open position in standard calls for mutual agreement between purchaser and manufacturer.?