4.91
Date Code 20170927 Instruction Manual SEL-751 Relay
Protection and Logic Functions
Group Settings (SET Command)
IEC Thermal Elements
The SEL-751 implements three independent thermal elements that conform to
the IEC 60255-149 standard. Use these elements to activate a control action or
issue a warning or alarm when your power line overheats as a result of adverse
operating conditions. For simplicity, the equations used to represent the ther-
mal element calculations are presented in generic form. These equations are
applicable to all three elements (n = 1, 2, and 3) presented in Table 4.33.
The relay computes the thermal level, THRL, of the protected equipment. The
thermal level is a ratio between the estimated actual temperature of the equip-
ment and the steady state temperature of the equipment when the equipment is
operating at a maximum current value.
For each thermal element, you can select two sets of heating time constants
(TCONH[x], x = 1 or 2) and two sets of cooling time constants (TCONC[x],
x = 1 or 2) to cover a variety of heating and cooling conditions. The SEL
OGIC
setting THST allows the user to switch between the two time constants. When
THST = 0, the element uses the heating and cooling time constants
TCONH[1] and TCONC[1], respectively. When THST = 1, the corresponding
element uses the heating and cooling time constants TCONH[2] and
TCONC[2], respectively.
The relay computes the thermal level using the following equations:
Equation 4.5
194 90.00 134.70 191.64 154.90 12.51
212 100.00 138.50 200.64 161.80 12.90
230 110.00 142.29 209.85 168.80 13.28
248 120.00 146.06 219.29 176.00 13.67
266 130.00 149.83 228.96 183.30 14.06
284 140.00 153.58 238.85 190.90 14.44
302 150.00 157.32 248.95 198.70 14.83
320 160.00 161.05 259.30 206.60 15.22
338 170.00 164.77 269.91 214.80 15.61
356 180.00 168.47 280.77 223.20 16.00
374 190.00 172.17 291.96 231.80 16.39
392 200.00 175.85 303.46 240.70 16.78
410 210.00 179.53 315.31 249.80 17.17
428 220.00 183.17 327.54 259.20 17.56
446 230.00 186.82 340.14 268.90 17.95
464 240.00 190.45 353.14 278.90 18.34
482 250.00 194.08 366.53 289.10 18.73
Table 4.32 RTD Resistance Versus Temperature (Sheet 2 of 2)
Tem p ( ° F ) Tem p (°C) 100 Platinum 120 Nickel 100 Nickel 10 Copper
If THIEQ IEQPU
THRL
t
THRL
t 1–
TCONH x
TCONH x t+
-----------------------------------------
THIEQ
t
IMC
--------------------
2
t
TCONH x t+
-----------------------------------------
FAMB+
=
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