5Th Harmonic Blocking; Overfluxing Protection Logic; Figure 117: Overfluxing Reset Characteristic; Figure 118: 5Th Harmonic Blocking Time Delay In Psl - GE P642 Technical Manual

Chapter 12 - Frequency Protection Functions
The following diagram explains the reset characteristic. It will take tReset time for the thermal replica to reset
completely to zero 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.
V/f>1 Trip
V/f>1 Reset
V/f in p.u.
V00868

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
overfluxing 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:
IA5 H Diff Start
IB5 H Diff Start
IC5H Diff Start
Trip Initial
V00863

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.
256
Stage1
tReset
Other trip signals
1
1000
1
Pick-up
0
100
Stage2 Stage3
Dwell
0
P64x
Any Trip
Fault REC TRIG
Relay 3 Output
P64x-TM-EN-1.3