4.95
Date Code 20170927 Instruction Manual SEL-751 Relay
Protection and Logic Functions
Group Settings (SET Command)
The ambient temperature measurement (TAMB) setting specifies the remote
thermal device (RTD), such as the SEL-2600, internal RTD card, or remote
analogs input that are used to measure the ambient temperature surrounding
the device. The ambient temperature measured, TAMB, is used to calculate
the ambient temperature factor, FAMBn (n = 1–3), as defined by Equation 4.9.
If TAMB is set to OFF, FAMBn is then forced to 1. If TAMB is not set to OFF,
the FAMBn value is supervised by the THAMBH Relay Word bit. If this bit is
asserted, indicating the ambient temperature measurement is accurate, then
the relay computes the FAMBn values using Equation 4.9. If the THAMBH
bit is deasserted, then the FAMBn value is forced to 1.
If TAMB := RA, the THAMBH bit asserts when the relay receives five con-
secutive healthy data packets. A healthy packet is a valid data packet with the
received temperature measurement in the data packet within –50°C to
+250°C. The THAMBH bit has a dropout time of THAMBDO seconds and
deasserts if no healthy data packets are received for THAMBDO seconds.
If TAMB := RTD, the relay looks at the internal status bits associated with the
ambient temperature RTD (RTDnLOC := AMB, where n = 1–10 for
E49RTD := INT, and n = 1–12 for E49RTD := EXT). The THAMBH bit deas-
serts if any one of the internal status bits associated with the ambient RTD
assert, indicating open, short, diagnostic fail, or communication fail.
The TAMBLOC setting specifies the location of the ambient temperature
measurement. If the ambient temperature measurement is from an RTD, then
this setting should be the number of the ambient temperature measurement
RTD. If the ambient temperature measurement is from a remote analog, then
this setting should be the number of the remote analog receiving the ambient
temperature measurement.
The THAMBDO setting defines the dropout time of THAMBH, the ambient
temperature measurement health Relay Word bit.
The thermal model operating quantity (THROn) must use a current that
includes all of the additional heating effects of the current passing through the
protected equipment. For this reason, the operating current choices are the
three individual phase rms currents or the IMAX current, which is the maxi-
mum rms current seen among the three-phase currents.
The basic current value in per unit (IBASn) setting accounts for the specified
limiting value of the current for which the relay is required not to operate at
when considering steady state conditions. The product of the basic current
value, IBASn (n = 1–3), and the basic current correction factor, KCONSn
(described below), is the maximum continuous current, IMC, used by the
relay in computing the thermal level.
The basic current correction factor (KCONSn) setting dictates the maximum
continuous load current of the protected equipment. The product of the basic
current value, IBASn, and the basic current correction factor, KCONSn, is the
maximum continuous current, IMCn, used by the relay in computing the ther-
mal level.
The thermal element time constant state switch (THSTn) setting enables the
user to switch between two thermal element time constant states. The two
states correspondingly use two sets of time constants for thermal calculation
to cover the variety of heating and cooling conditions.
The heating thermal time constant (TCONH
nx) setting defines the heating
thermal time constant of thermal element n in state x of the equipment when
the equipment is energized (i.e., when the current is above the IEQPU value).
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