The operational logic consists of input magnitude processing, threshold comparator, two block signal
check, time delay characteristics and output processing.
Inputs for the function are the operating mode selections, setting parameters and measured and pre-
processed frequency magnitudes and binary input signals. Function outputs START, TRIP and
BLOCKED signals which can be used for direct IO controlling and also for user logic programming. The
function registers its operation into 12 last time-stamped registers and also generates general time
stamped ON/OFF events to the common event buffer from each of the three output signal. In instant
operating mode the function outputs START and TRIP events simultaneously with equivalent time
stamp. Time stamp resolution is 1ms. Function provides also cumulative counters for START, TRIP and
BLOCKED events.
In the following gure is presented the simpli ed function block diagram of the ROCOF function.
Figure. 5.4.16. - 87. Simpli ed function block diagram of the ROCOF function.
Measured input values
Rate of change of frequency protection function compares measured df/dt to pick-up setting given in
Hz/s. Source of measured frequency depends on the factory de ned tracking reference which can be
checked from frequency tab behind measurements-menu.
Table. 5.4.16. - 141. Measurement magnitudes used by the ROCOF function (depends on factory de nition)
Signals
Description
Time base
VT1 U1,U2,U3
L-N voltages of rst voltage transformer
5 ms
VT2 U1,U2,U3
L-N voltages of second voltage transformer
5 ms
Pick-up characteristics and time delay
Pick-up of each stage of ROCOF functions stage is controlled by
df/dt>/< (1) pick-up, df/dt>/< (2) pick-
up
etc.setting parameter, which de nes the maximum allowed change of frequency before action from
the function. The function constantly calculates the ratio in between of the pick-up setting and
measured df/dt. Reset ratio of 20mHz is inbuilt in the function and is always related to the pick-up
value. The f>/< limit value is used to block the operation of the function near the nominal frequency.
Table. 5.4.16. - 142. Pick-up characteristics setting
Name
Description
Range
Step
Default
df/dt>/<(1…8)pick-up
Pick-up setting
0.01…10.00Hz/s
0.01Hz/s
0.2 Hz/s
AQ-F255
Instruction manual
Version: 2.00
151
© Arcteq Relays Ltd
