The protection element also provides a non-sensitive area to avoid instability due to small asymmetries and unbalances that can generally be present in the network system. This condition is represented by the characteristic reproduced below, where the hatched area is the tripping zone.
Under/Over frequency (81 U/O)
4.1.1 Description
Time delayed under and over frequency protection available on P127 provides the
fundamental form of frequency protection.
Six thresholds are available: Each one can be configured to detect an under or over
frequency within the range [fn – 4,9Hz, fn + 4,9Hz], where fn is the nominal frequency
selected (50Hz or 60Hz). A definite timer is assigned to each threshold.
When the frequency measured is crossing one of the 6 pre-defined thresholds, the relays
generates a start signal and after a user settable time delay, a trip signal.
NOTE: Under / Over frequency protection is available when voltage inputs are
connected.
4.2 Rate of frequency change protection (dF/dt) (81R)
4.2.1 Description
The calculation of the rate of change of frequency is an average measurement of the
instantaneous values over a programmable number of cycles (1 to 200); refer to the
‘CONFIGURATION / General options" menu. The instantaneous values of rate of change of
frequency are measured every cycle (20ms at 50Hz). The rate of frequency change
elements are very important to detect any power loss under severe disturbances and
eventually perform load shedding of secondary load.
These elements offer the possibility to detect the tendency of the variation of frequency, and
thus re establish the correct load/generation without waiting for big frequency reduction.
These elements could be combined to the frequency elements using the AND logic
equations to provide a very useful mechanism allowing a more secure trip decision to be
achieved during transient system disturbances.
According ‘CONFIGURATION / General options’ setting, this function will be inhibited in the
following case:
− if the voltage level for each phase is below the settable undervoltage blocking value
(see “Prot. Freq. Block U<” cell),
− if the frequency is out of range: fmeasured > (fn+20Hz) or fmeasured < (fn–20Hz)
− if dF/dt > ±20hz/s and “Inhib. dF/dt>20Hz/s” is set to “No”.
