Ferroresonance

Ferroresonance Occ Occurrence urrence In Power Transf T ransf ormer  h ttp ://e l e c tr i c a l - en e n g i n e e r i n g - po p o r ta l .c o m/fe r r o r e s o n a n c e - oc o c c u r r e n c e - in in - po p o we r - tr tr a n s fo r me r

M a y 13, 2013

Edvard

Ferro reso nance Occurrence Occurrence In In Power T ransf ormer (pho to by SNC SNC Manuf Manuf acturing Co. Inc. Inc.))

Ferroresonance occurs when line wit h the magnetizing line capacitance capa citance resonates wit magnetizing reactance of  o f  while it goes go es in in and out of saturation. a core while Ferr oresonance oresonance is usually usually associat associated ed with wit h potential transformers transformers, which are instrument transformers t hat are used used to t o develop develop vo ltages used by rela relays; ys; however, itit can also also o ccur with power transformers under special circumstances. anot her occurrence occurrence that can cause cause equipment damage; fo rtunatel rtunat ely, y, itit is Ferroresonance is anot prevent preventabl able e by simpl simply y avoiding avoiding cert cert ain t ypes of t ransfo rmer rmer connections wit h the t he types t ypes of  circum circumst stances ances t hat enable itit t o o ccur. ccur. Because Because these t hese connections connect ions are rout inely inely avoided avoided in pract pract ice, ice, ferr f erroresonance oresonance is not encount encount ered ered very very of t en and there isn’t isn’t much much inf informa ormatt ion about it in t he litera literatt ure. ure. Ferroresonance erroresonance is worthy wo rthy o f mention, however, because it can utterly utterly destroy a transformer .

The necessary conditions f or  ferroresonance are established in the system shown in Figure 1. In t he example shown in Figure 1, the ∆-connected tertiary  winding of a large three-winding substat ion t ransfo rmer supplies a distribution t ype stat ionservice t ransformer with a Grd.-  Y primary winding . The supply lines to the station-service transformers are through a set of shielded cables. If the cable runs are f airly long, a significant amount of phase-to-ground capacitance may exist. When a transfo rmer core operates near saturation, t he B-  H curve is highly nonlinear, and the eff ective permeability of the core can take on a range of  values that vary with t he changes in f lux density. Each of the inductances shown as L1, L2, and L3, will have

Figure 1 - A situation where ferroresonance may occur. The cable capacitances f orm three parallel L-C elements that are in series with each ot her and the so urce voltages.

instantaneous inductance values that are proportional to the eff ective permeability of the core at any given instant

in time. These inductances form parallel L-C circuits that are in series with one another and in series with t he source voltage. Since L1, L2, and L3 are const antly varying along with t he effective permeability of the core, it is almost certain that a series resonant condition will exist at least part of the time during every cycle. When a series resonance exists, even for a brief period, this causes very large voltages across t he L-C elements. These voltages are capable of destroying the transformer and any  other equipment connected to it.

The nonlinear nature of this problem makes mathematical analysis virtually impossible, but the phenomenon has been observed bot h in the f ield and experimentally, and the vo ltages have been measured and recorded. In t he example above, t he conditions for f erroresonance can be disrupted by the simple expedient o f ∆-connected secondary winding to t he stat ion service transformer. The ∆-connected winding assures that the vector sum of the voltages of all three phases add to z ero, stabilizing the neutral point of the Y-connected primary winding and preventing excessive volt age across t he windings. The presence of a ∆-co nnected secondary winding will essentially ‘‘snuff out ’’ f erroresonance in this circuit.

Ferroresonance (VIDEO)

Dennis Merchant on ferro-resonance in distribution t ransformers. Cant see this video? Click here to watch it on Youtube.

*|YOUTUBE:[$vid=U7dNdQc1NL4]|* Reference: Power Transformers Principles and Applications – John J. Winders, Jr. ( Get it from Amazon )

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