Figure. 5.2.1 - 3. Nominal current calculation in differential protection relays.
Normally, the primary current ratings for phase current transformers are ten amperes to thousands of
amperes and their decimal multiples, while the secondary current ratings are 1 A and 5 A. Other, non-
standard ratings can be directly connected as the scaling settings are flexible and have large ranges.
For example, the ring core current transformer ratings may vary. Ring core current transformers are
commonly used for sensitive earth fault protection and their rated secondary may be as low as 0.2 A in
some cases.
The following chapter is an example on how to set the scaling of the relay measurements for the
selected current transformer and nominal load.
Example of CT scaling (application 1)
The following figure presents how CTs are connected to the relay's measurement inputs. It also shows
the CT ratings and the transformer nominal current. Note that S1 is always connected to an odd
connector regardless of the CT direction. The CT direction is selected in the settings of the transformer
differential protection function.
A
AQ
Q-M257
-M257
Instruction manual
Version: 2.07
© Arcteq Relays Ltd
IM00021
21
Summary of Contents for AQ-M257
Page 1: ...AQ M257 Motor protection IED Instruction manual...
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