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The transformer is the main equipment that is very important at the substation.
However, sometimes there can be damage to the transformer used. Damage to the transformer
will be able to disrupt the distribution of electricity, so it must get the right security, determining
the right relay setting is an attempt to protect equipment, system security and consumer needs
from possible disruption in the distribution system. In some roles, efforts to protect the power
transformer if there is a short circuit between phase and phase to the ground, then this study will
study the coordination of the security system of power transformers in Padang Sambian main
substation. The results of the calculation of overcurrent relay (OCR) on the 150KV transformer
that has been done is obtained the results of the CT primary side current setting of 254.1 A and
for the secondary side of 3.178 A. For the actual working time of the relay is 0.91 seconds, if the
current exceeds value of this setting there will be a trip. The results of the calculation of
overcurrent relay (OCR) on the 20KV transformer that has been done is obtained the results of
the CT primary side current setting of 1905 A and for the secondary side is obtained for 4.76 A.
For the actual working time relay result is 3.69 seconds, if the current exceeds the value of this
setting there will be a trip.
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