S C Strength of Winding Exits and Leads : A critical area for Failure Prevention in Power Transformers by MANAN PANDYA SIEMENS LTD. manan.pandya@siemens.com 1
Introduction Short circuit withstand capability of power transformers is critical for its availability and overall power system reliability A vital concern for : Utilities - power equipment outage and loss of revenue Transformer manufacturers - repeat testing and project delays Practically implemented methodology of SC force evaluation on winding exit and leads is discussed in the paper 2
SC Failure Statistics alarming figures SC Test statistics of transformers 25 MVA Around 35% of 400kV class and 45% of transformers rated >200MVA failed during SC test 3
SC Failure Statistics - exploring deeper After windings, the major location of failure is leadsconnected components SC forces on leads and Winding exit location may lead to failure in windings, Tapchanger and Bushings SC Test statistics: 342 failures, V 100kV Motivation for the paper:- In comparison to windings, relatively less information is available on SC force calculation of winding exits and leads 4
Process of SC design of Leads in external magnetic field 3 step process: Focus of the paper Electromagnetic Fundamentals Not discussed Mechanics Specific to manufacturers 5
Force direction on current carrying conductors (Classical theory for Lorentz force) Fleming s left hand rule external magnetic field current flow in a lead Interaction of magnetic fields and resultant force 6
Force direction on current carrying conductors (AC magnetic field and current) Vectorial approach: = current and external field in phase Unidirectional pulsating force current and external field exactly out of phase Unidirectional pulsating force in opposite direction Force direction is linked to electrical as well as physical angle 7
Evaluation of force magnitude F = B I L sinθ [Classical Formula] F - force (N); B - flux density (T); I - current (A); L - conductor length (m) θ is physical angle, between the current and external magnetic field SC force for transformers : Design for the first SC peak Steady state rms-short circuit current and steady state rms-short circuit flux both shall be multiplied with the k 2 peak factor in above formula (k 2 is peak factor which depends on X/R ratio) non 8
Case Study successful application of methodology Largest-ever Made in India 1-ph. 315MVA, 23.5kV / 420 2 kv, generator transformer (GSU) - successfully tested in first attempt for dynamic short circuit withstand test April 2016 Transformer in Barge - KEMA Test Lab At Siemens Transformer HV Test Lab 9
Case Study Force Direction on LV winding radial exit Instantaneous direction of current in exit Instantaneous direction of stray flux Force direction Instantaneous direction of current in windings 10
Case Study Force magnitude on radial lead of LV wdg Plot the external magnetic field over the winding conductor Integrate perpendicular component of flux over the length Calculate force as per Formula 11
Case Study Force calculation on vertical LV lead crossing a phase direction of force at top direction of force at bottom Instantaneous stray flux direction at top Instantaneous direction of current in vertical lead Instantaneous stray flux direction at bottom SC force magnitude approach is similar to earlier case 12
Concluding Remarks It is critical to correctly calculate the SC forces for proper supports design and avoiding SC failures During SC test:- Very little reactance variation was observed and Integrity of support structure was maintained it was checked by detailed active part inspection after SC test
Concluding Remarks Steady state approach doesn t fully consider the dynamic effects like pulsating force effects, transient situations like flux saturation etc. BUT is good approximation of SC forces from design point of view Considerable expertise and computational efforts required No Short cuts!
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