Surge Protection for Dry Type Transformers in Metro stations.

[prc]

Dry type transformers, (500 kVA to 3150 kVA 33/0.415 kV-Delta /star)) used in metro stations, are switched on by VCBs through a short length of cable. Power reach VCB through long cables. Usually when VCB switching is involved, surge arresters( between line to ground and line to line) and or snubbers( R-C in series with fuse) are provided at equipment terminals to mitigate the switching surges reaching the equipment. For the above application what is the practice adopted in various regions?

 

[cuky2000]

Vacuum circuit breakers (VCB) during certain network switching scenarios could produce current chop combined amplified by the cable capacitance and system inductance resulting in high frequency transient recovery voltage (TRV) and arc re-strikes that overstress the insulation of the dry type transformer winding inducing premature failure.

Surge arrester protects again overvoltage. Snubber with capacitor slows down rate-of-rise (dv/dt) and resistor helps to reduces DC offset and provides damping.

Modeling the transient behavior of VCB using EMTP, PLSCAD, PSSE or similar software could be and option but hard to justify the cost and effort. For small units, many engineers choose a practical approach having spare units, oversize the transformer BIL or add surge arrester and snubber rather than spend the capital cost in probabilistic study.

 

[prc]

Thank you cuky for the response. My views are same. The transient surges are not more than the BIL of transformer (except the case of breaking inrush current) but the generation of several surges during a single switching can weaken the insulation. What I find on ground is that for motors such protection is invariably used but rarely for dry type transformers. So I sought other's experience for this particular application.