Figure. 5.2.1. - 2. Current measurement terminology
PRI: Primary current, the current which flows in the primary circuit and through primary side of the
current transformer.
SEC: Secondary current, the current which the current transformer transforms according to its ratios.
This current is measured by the protection IED.
NOM: Nominal primary current of the protected transformer. Nominal current differs on the HV and LV
side according to the transformer voltage ratio. Nominal current is calculated based on transformers
MVA and nominal voltage on each winding.
For the measurements to be correct it needs to be made sure that the measurement signals are
connected to correct inputs, current direction is connected correctly and the scaling is set correctly.
For scaling, the relay calculates scaling factors based onto the set CT primary, secondary and nominal
current values. Relay measures secondary current which in this case mean the current output from the
current transformer which is installed into the primary circuit of the application. In order the relay to
“know” primary and per unit values it needs to be told the current transformer rated primary and
secondary currents. In case of a power transformer the protected unit nominal current in both windings
is calculated based on given nominal power (MVA) and nominal voltage (V), only when the nominal
current is known the settings can be per unitized to apparatus nominal. By knowing the transformer
nominal current it makes the unit protection a lot easier and straight forward. In modern protection
devices this scaling calculation is done internally after the current transformer primary, secondary and
machine nominal currents are known.
AQ-T257
Instruction manual
Version: 2.00
13
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