ABB RELION 620 Series Technical Manual page 326

Section 4
Protection functions
320
Over/under frequency detection
The frequency detection module includes an overfrequency or underfrequency detection
based on the Operation mode setting.
In the "Freq>" mode, the measured frequency is compared to the set Pickup value Freq>.
If the measured value exceeds the set value of the Pickup value Freq> setting, the module
reports the exceeding of the value to the trip logic module.
In the "Freq<" mode, the measured frequency is compared to the set Pickup value Freq<.
If the measured value is lower than the set value of the Pickup value Freq< setting, the
module reports the value to the trip logic module.
df/dt detection
The frequency gradient detection module includes a detection for a positive or negative
rate-of-change (gradient) of frequency based on the set Pickup value df/dt value. The
negative rate-of-change protection is selected when the set value is negative. The positive
rate-of-change protection is selected when the set value is positive. When the frequency
gradient protection is selected and the gradient exceeds the set Pickup value df/dt value,
the module reports the exceeding of the value to the trip logic module.
The protection relay does not accept the set value "0.00" for the Pickup
value df/dt setting.
Operate logic
This module is used for combining different protection criteria based on the frequency and
the frequency gradient measurement to achieve a more sophisticated behavior of the
function. The criteria are selected with the Operation mode setting.
1MAC504801-IB E
620 series ANSI
Technical Manual