ABB RELION RET670, Applications Manual

Page: 659 / 1016

op P
t TD
A
B
M C
= ×
æ ö
+
ç ÷
-
è ø
EQUATION1742-ANSI V1 EN-US
(Equation 488)
where:
t
op
is the operating time in seconds of the Inverse Time Overcurrent TOC/IDMT algorithm
TD is time multiplier (parameter setting)
M is ratio between measured current magnitude and set pickup current level
A, B, C and P are user settable coefficients which determine the curve used for Inverse Time Overcurrent
TOC/IDMT calculation
When the equation 485 is compared with the equation 487 for the inverse time
characteristic of the OC1 it is obvious that if the following rules are followed:
1. set TD equal to the generator negative sequence capability value
2. set
A_OC1 equal to the value 1/x2
3. set B_OC1 = 0.0, C_OC1 =0.0 and P_OC1 =2.0
4. set PickupCurr_OC1 equal to the value x
then the OC1 step of the CVGAPC function can be used for generator negative
sequence inverse overcurrent protection.
For this particular example the following settings shall be entered to insure proper
function operation:
1. select negative sequence current as measuring quantity for this CVGAPC function
2. make sure that the base current value for the CVGAPC function is equal to the
generator rated current
3. set TD_OC1 = 20
4. set
A_OC1 = 1/0.07
2
= 204.0816
5. set B_OC1 = 0.0, C_OC1 = 0.0 and P_OC1 = 2.0
6. set PickupCurr_OC1 = 7%
Proper timing of the CVGAPC function made in this way can easily be verified by
secondary injection. All other settings can be left at the default values. If required
delayed time reset for OC1 step can be set in order to ensure proper function operation
in case of repetitive unbalance conditions.
Furthermore the other built-in protection elements can be used for other protection and
alarming purposes (for example, use OC2 for negative sequence overcurrent alarm and
OV1 for negative sequence overvoltage alarm).
1MRK 504 163-UUS A
Section 12
Multipurpose protection
Transformer protection RET670 2.2 ANSI 653
Application manual