Transformer Protections

Fuses?

Surge protection my friend.

theres also the Buchols relay... I am not sure if the name of this engineer is correct dough!

A few corrections:

27 is AC Undervoltage Relay

50 is Instantaneous Overcurrent Relay

50N is Instantaneous Neutral Overcurrent Relay

51 is AC Time Delayed Overcurrent Relay

51N is AC Time Delayed Neutral Overcurrent Relay

64 is Earthfault Relay (64R is Restricted Earthfault Relay)

74 is Alarm Relay

86 is Lock-out Relay

94 is Tripping or Trip-free Relay

95 is Assigned for devices where none of the Device Function Numbers 1 to 94 are suitable.

Buch-Holz Relay shall be numbered 63;

Oil Temperature Relay shall be numbered 26T & Winding Temperature Relay shall be numbered 49T

This is as per ANSI C 37.2-1970.

And, as regards the protection of transformers, it depends upon the size & criticality of the transformer.

you forgot the most important one, percentage differential protection - which operates for internal fault, but will not operate for through fault. A simple differential protection does not take into account of CT's inaccuracy due to difference in magnetic saturation characteristic of field winding - which is more pronounsed during transient conditions.

sks

--------------------------------------

(serving humanity)

Acting too smart! huh! So, you think that differential protection is 'most important' for transformers! Would you provide it for any size of transformer, say 100kVA? What I have done is to correct the ANSI Code explanations, which were wrongly mentioned by the OP. Moreover, the OP has not specified the size of the transformer. Haven't you read what I have mentioned at the end of my posting that the extent of transformer protection depends upon the size and criticality of the transformer?

And, since you are trying to teach me 'percentage differential protection' - by the by, it should be "percentage biased differential protection" - let me ask you a question:

Generally, in transformer differential protection, one uses Class PS CTs, whose knee point voltage or the saturation level, is calculated duly taking into account all the worst case scenario parameters. Then why should one use a percentage biased differential relay?

To take care CTs mismatch during operation of protection.