Date Code 20010731 Breaker Logic 3-29
SEL-352-1, -2 Instruction Manual
Cooling Time Constants (CRTC, TRTC)
Range: 1–1140 minutes in 1-minute steps
As soon as the difference voltage across the breaker drops below the 87TH setting, or the power
across the breaker drops below the 37OP setting, the relay models breaker resistor cooling. Use
SET <ENTER> to access the CRTC and TRTC settings and use these to define the close and
trip resistor cooling time constants, respectively.
Blown Potential Supervision (59L, 27D)
Range: 10.0–120.0 V in 0.1-V steps (59L)
1.0–120.0 V in 0.1-V steps (27D)
The 59L and 27D elements supervise the thermal logic. If one side of the closed breaker (on a
per-phase basis) is energized (59L) and the other side is dead (27D), the thermal logic is disabled
because a blown potential fuse condition exists. Without the voltage supervision, the thermal
logic may misoperate.
Refer to the Controlled Closing logic in Section 4: Close Logic for 59L and 27D setting
recommendations.
Setting Calculations
Thermal Model of Trip and Close Resistors
The output of the thermal models is a number in Joules calculated from either the heating or the
cooling equation. When power is applied greater than the 37OP setting from the OPpu pickup
time, the model is heating. Before the OPpu time expires, or if the power applied is less than the
37OP setting, the model is cooling.
When the model is heating, calculate the energy by integrating the power with respect to time. If
a constant power is applied, the energy is simply the power multiplied by the time.
×
¼¼D= dtIAVA)t(Energy
When the model is cooling, the energy is decaying exponentially based on the following
equation:
() ( )
RTC
t
0
etEnergytEnergy
-
¼=
where:
Energy(t
0
) is the initial energy
t = Seconds
RTC = Resistor time constant setting (CRTC, TRTC)
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