ABB RET670 Applications Manual page 567

1MRK504116-UUS C
Application manual
When the equation 412 is compared with the equation
characteristic of the OC1 step in it is obvious that if the following rules are followed:
1. set TD equal to the IEC or ANSI standard generator capability value
2. set parameter A_OC1 equal to the value 1/x2
3. set parameter C_OC1 equal to the value 1/x2
4. set parameters B_OC1 = 0.0 and P_OC1=2.0
5. 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.
1. select positive sequence current as measuring quantity for this CVGAPC function
2. make sure that the base current value for CVGAPC function is equal to the
generator rated current
3. set TD = 37.5 for the IEC standard or TD = 41.4 for the ANSI standard
4. set A_OC1= 1/1.162 = 0.7432
5. set C_OC1= 1/1.162 = 0.7432
6. set B_OC1 = 0.0 and P_OC1 = 2.0
7. set PickupCurr_OC1 = 116%
Proper timing of 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 insure proper function operation
in case of repetitive overload conditions.
Furthermore the other built-in protection elements can be used for other protection and
alarming purposes.
In the similar way rotor overload protection in accordance with ANSI standard can be
achieved.
Open phase protection for transformer, lines or generators and circuit
breaker head flashover protection for generators
Example will be given how to use one CVGAPC function to provide open phase
protection. This can be achieved by using one CVGAPC function by comparing the
unbalance current with a pre-set level. In order to make such a function more secure it
is possible to restrain it by requiring that at the same time the measured unbalance
current must be bigger than 97% of the maximum phase current. By doing this it will
be insured that function can only pickup if one of the phases is open circuited. Such an
arrangement is easy to obtain in CVGAPC function by enabling the current restraint
feature. The following shall be done in order to insure proper operation of the function:
Section 3
IED application
413 for the inverse time
561