5-200 T35 TRANSFORMER PROTECTION SYSTEM – INSTRUCTION MANUAL
GROUPED ELEMENTS CHAPTER 5: SETTINGS
5
The calculation of differential (I
d
) and restraint (I
r
) currents for the purposes of the percent differential element is described
by the following block diagram, where “Σ” has as its output the vector sum of inputs, and “max” has as its output the input
of maximum magnitude; these calculations are performed for each phase.
The differential current is calculated as a vector sum of currents from all windings after magnitude and angle
compensation.
Eq. 5-30
The restraint current is calculated as a maximum of the same internally compensated currents.
Eq. 5-31
The element operates if I
d
> PKP and I
d
/ I
r
> K, where PKP represents a percent differential pickup setting and K is a
restraint factor defined by the relays settings Slope 1, Slope 2, and a transition area between breakpoint 1 and breakpoint
2 settings.
Figure 5-101: Percent differential calculations
OVEREXCITN INHIBIT
LEVEL: 10.0% fo
Range: 1.0 to 40.0% of f
0
in steps of 0.1
PERCENT DIFF BLOCK:
Off
Range: FlexLogic operand
PERCENT DIFFERENTIAL
TARGET: Self-reset
Range: Self-reset, Latched, Disabled
PERCENT DIFFERENTIAL
EVENTS: Disabled
Range: Disabled, Enabled
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