The following diagram explains the reset characteristic. It will take tReset time for the thermal replica to reset
completely to zer
o after it has reached 100% of V/f>1 trip at stage 1. If the thermal replica has not reached 100%
of V/f>1 Trip, the reset time will be reduced proportionally. For example, if the reset time is set to 100 seconds, and
the thermal replica has only reached 50% of V/f> Trip when V/Hz resets, the reset time will be 50 seconds as
shown in stage 2. If another V/Hz excursion appears before the first reset reaches V/f Reset, the V/Hz time delay
takes the reset time left into consideration, as shown in stage 3.
V00868
V/f>1 Trip
V/f>1 Reset
V/f in p.u.
Stage1 Stage2 Stage3
tReset
Figure 117: Overfluxing reset characteristic
2.1.2 5TH HARMONIC BLOCKING
A differential current 5th harmonic blocking feature can be used to prevent possible maloperation under transient
ov
erfluxing conditions. The 5th harmonic signal is derived from the differential current waveform on each phase.
Blocking is on a per phase basis.
To ensure tripping for persistent overfluxing, caused by high system voltage or low system frequency, the device
provides time delayed overfluxing protection. Where there is any risk of persistent geomagnetic overfluxing, with
normal system voltage and frequency, the 5th harmonic differential current facility can be used to initiate tripping
after a long time delay. This time delay would need to be programmed in the PSL as shown:
Fault REC TRIG
V00863
Any Trip1
IA5H Diff Start
1000
0
Pick-up
Trip Initial
IC5H Diff Start
IB5H Diff Start 1
Other trip signals
Relay 3 Output
100
0
Dwell
Figure 118: 5th harmonic blocking time delay in PSL
2.1.3 OVERFLUXING PROTECTION LOGIC
The overfluxing protection logic is shown below. This diagram shows the case where the first stage timer is set to
IDMT
. You can select a DT timer for the first stage if required with the setting V/Hz>1 Trip Func. The diagram shown
is for the 3-phase VT element. The single-phase element follows the same principles.
Chapter 12 - Frequency Protection Functions P64x
256 P64x-TM-EN-1.3
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