Transformer core grounding 1

[Mr zezo]

How transformer core is being grounded although it consists of insulated laminations so how we get a conducting path through all the laminations to ground the core,and does this conducting path affected by edyy current?

 

[prc]

Normally the core stack is grounded at only one point. The laminations are individually insulated by the thin coating and hence the capacitance between laminations is very large. During surge conditions, laminations get automatically get grounded between each other through this capacitance.

 

[RRaghunath]

I have seen laminations divided in to number of packs, each pack having a short with the next pack.
prc could throw some light on this too please!

 

[prc]

Sometimes cooling ducts or insulation sheets (Nomex) are inserted in the stack. Then the laminations will not be touching each other and hence capacitance will be less there. There, sometimes, a copper shortiung piece is inserted to get continuity for the grounding path.

 

[waross]

I have seen small transformers such as lighting ballasts and small lighting transforms with a weld bead across the laminations to provide one point of contact to ground all of the laminations.

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Ohm's law
Not just a good idea;
It's the LAW!

 

[RRaghunath]

Thanks prc for that clarification.

 

[Mr zezo]

Thanks for your response prc , do you have any paper or book which clarify that point in details , and any practical pictures of that point?

 

[Mr zezo]

Prc , what do you mean by (During surge conditions, laminations get automatically get grounded between each other through this capacitance).how can it be,can you explain it ? And what's the main function of grounding transformer core ? I think it is to get rid of any induced voltage during normal operation and also to increase the current
During any fault related to the core to operate protection relays or protection devices, so since it has a function during normal operation how can grounding achieved without real conducting path.

 

[prc]

Voltage will be induced in the core when surge voltages enter the windings. Surge can be considered as a combination of several high-frequency sine waves. With high-frequency, capacitive reactance comes down (i/c omega ) drastically and surge currents find an easy path through the capacitances formed between laminations.