Figure. 5.2.1 - 2. Current measurement terminology
PPRI:
RI: The primary current, i.e. the current which flows in the primary circuit and through the primary side
of the current transformer.
SEC:
SEC: The secondary current, i.e. the current which the current transformer transforms according to its
ratios. This current is measured by the protection relay.
NOM:
NOM: The nominal primary current of the protected transformer. The nominal current on the HV side
differs from that on the LV side according to the transformer voltage ratio. The nominal current is
calculated based on the transformer's MVA and the nominal voltage on each winding.
For the measurements to be correct the user needs to ensure that the measurement signals are
connected to the correct inputs, that the current direction is connected correctly, and that the scaling
is set correctly.
The relay calculates the scaling factors based on the set values of the CT primary, the CT secondary
and the nominal current. The relay measures the secondary current, the current output from the
current transformer installed into application's primary circuit. The rated primary and secondary
currents of the CT need to be set for the relay to "know" the primary and per-unit values. In power
transformers, the protected unit's nominal current in both windings is calculated based on the given
nominal power (MVA) and the nominal voltage. The settings can only give the apparatus nominal in p.u.
(per-unit) when the nominal current is known. Also, knowing what the transformer's nominal current is
makes the unit protection much easier and more straightforward to configure. In modern protection
devices this scaling calculation is done internally after the current transformer's primary current,
secondary current and machine nominal current are set.
A
AQ
Q-M257
-M257
Instruction manual
Version: 2.07
20
© Arcteq Relays Ltd
IM00021
Summary of Contents for AQ-M257
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