New Standards For Transformers-Year 2022
New Standards For Transformers-Year 2022
New Standards For Transformers-Year 2022
Year 2022
Published on November 11, 2022
P Ramachandran
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.
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This section has been thoroughly revised and expanded to include the contents of withdrawn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.
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
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.
Forces
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
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
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
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.
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.
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.