POWER TRANSFORMER - TIPS TO ASCERTAIN SHORT CIRCUIT CAPABILITY

Comparison of proposed design drawings parameters with already successfully tested transformer

Use of already short circuit successfully tested components for example OLTC, Tap Switch, Bushings etc.?as far as possible

Tightness of coils

Use of winding copper of required hardness taking into account its wind ability

Critical check of winding resistance at different stages to ensure strong joints and no looseness

Flexibility in leads?so these do not break due to jerks experienced during short circuits.

Matching electrical centres of HV and LV coils

Best possible Ampere Turn?balancing of HV and LV windings at, and minimum tap positions,?with special attention for integral tab coils and course fine tap coils arrangements.

Use of Glued CTC in large Power Transformers especially for LV winding.

As for as possible use of fibreglass cylinder or SRBP cylinders on which coil to be found taking into account its electrical strength.

Use of additional spacers, locked?by intermediate blocks in LV coils. Also, use Diamond paper below the spacer strips to award paper insulation damage on S-band

Outermost coils shall have keyed blocks?

Proper processing and pressing of all the coils.

Use of wooden wedges if the gap is observed below the top yoke

Additional Perma wood segments of suitable thickness above Perma wood Rings on HV as well as LV side below pressing bolts to suit on assembly

Use of thick winding conductor, taking into account Eddy losses, gradient etc.

Use of Perma wood?blocks arrangement in outer Coil Centre gap over pressboard segment cylinder arrangement.

Tensile strength of vertical tie rods & flitch plates and dimensions shall be checked critically

?High tensile Steel shall be preferred.

Current density should be optimum for windings taking into account loss figures.

CTC epoxy-bonded conductor instead of strip conductor greatly enhances the resistance of the winding against buckling and tilting because of the bonding effect between strands due to epoxy coating.

High-grade proof stress copper for windings instead of?normal copper shall?be used.

A separately placed tapping?coil is always preferable from the point of view of short circuit forces.

A study on service conditions, installation and environmental conditions is required from the point of view of short circuit duty at the site.

Higher value of impedances is recommended in the Transformers to enhance short circuit capability.

Taps on more than one winding shall be avoided.

It is preferable to avoid split winding.?

Stabilization of windings and tight tolerances concerning winding dimensions with proper clamping forces and proper values of prestressing is absolutely necessary to obtain dimensional stability.

Ensure the tightness of conductors in radial & axial?dimensions.

Winding leads and connections to bushings and tap changers should be properly secured.

Use of Preshrunk and oil-impregnated?insulating components.

Placement of tight-fitting wooden blocks on the core in close contact with the insulating cylinders and radially in line with the axial spaces.

Vapour phase drying for better dimensional control.

Burr free edges rounding off strips and clacks for eliminating biting off the paper

The winding screws & nuts must be properly?tightened after oil impregnation.

Anchoring of leads, especially of Tap coils.

Special care of Conductors at Cross overs & Transpositions.