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Electromechanical relays (not just earth fault relays) need to be calibrated to insure they are operating in the expected way. A simple overcurrent ground relay will have a tap setting, and a time dial setting. These are chosen by a protection engineer to coordinate with other ground protection devices upstream and downstream. If the relay is not calibrated correctly, the relay will not operate at the expected tap value, or may operate slower / faster due to the time dial calibration being different than expected. This can cause over tripping, removing more equipment from the power system than is necessary to clear a fault.
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Why ngt or ngr used in transformer?
Criteria-in order to protect the stator core from damage during internal earth fault it is necessary to limit the stator earth fault current al low as possible.may be upto 10amp for generator. Basis for selection of ngtr- if generator is connected to the system through generator transformer then generator can be treated as an isolated system which is not influenced by the distribution earthing system. In this case you can select earth fault current as low as possible, but if we use ngr then in order to limit the low earth fault current size of the ngr will be increased.due to big size of ngr we required more space & also its too costly. V = ir means if v constant i is inversely proportional to r exp:-to limit 10amp current , v =11kv/root 3 r = 11/root 3/10*1000=635.10 ohm ( very high value) for overcome on this, general practice to use a ngtr so that we can limit low earth current without any extra cost. Exp:- ngtr 11/240v to limit 10amp current current reflect on secondary side of ngtr will be = (11000/root 3/240)*10=264.6 amp. Resistor value on secondary side r =240/264.6 =0.9 ohm ( approximately) which is much lower than 635.10 ohm. Basis for selection of ngr- if generator is directly connected to the system then we can not limit the earth fault current up to 10 amp. We have to select the earth fault current value above the residual capacitive charging current of system.also we need to check the possibility of relay setting operation with ct ratio. For exmp- if ct ratio is 2000/1amp & earth fault limit current is 100amp relay selected with 10 % setting. Then that relay will see only 200amp earth fault current it will not see 100amp current. Also we can not keep generator solidly grounded because of high earth fault current stator can damage same will happen with ht motor if it is connected to this system. So for this case we need to select the ngr with proper earth fault current
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