RFL 9300 RFL Electronics Inc.
August 25, 2000 3 - 12 (973) 334-3100
The CT's secondary winding resistance and the relay burden of most modern low-burden relays are much
smaller than the secondary cable resistance. Because of this, these components can be ignored during the cal-
culations.
For SLG fault calculations, the two-way (or round-trip) cable resistance must be used. This is because the CT
for one phase must drive secondary current through the phase secondary cable resistance and
the neutral sec-
ondary cable resistance.
For three-phase faults, the following formula is used:
V
KPP
= I
3PH
x R
1W
x TF
P
where
V
KPP
= Kneepoint voltage of the CT's B-H curve for worst-case three-phase fault.
I
3PH
= Maximum expected three-phase fault current (symmetrical, in secondary amperes
rms).
R
1W
= "One-way" secondary cable resistance, in ohms.
TF
P
= "Transient factor" for phase faults. This is equal to (1+ X
L
/R) where (L/R) is the pri-
mary system time constant.
For phase faults, the CT for the faulted phase drives current through its secondary cable resistance only. The
voltage drop in the return cable (or cables) is driven by the other faulted phase CT (or CT's).
To select the proper CT class, take the higher of the calculated kneepoint voltages (Vkps or Vkpp), and divide it
by ten since Vk must be modified if residual flux is taken into consideration. The next-higher CT class will be
adequate for RFL 9300 operation on internal faults. This is because the RFL 9300's UHS circuit operated to
provide fast tripping even though one or more CT's are very severely
saturated.
Example: V
KPS
= 1700 volts
V
KPP
= 1550 volts
1700 / 10 = 170
CT's rated "C200" should be used.
3.9.2 EXTERNAL FAULT CALCULATION
The RFL 9300 tolerates less CT saturation on external faults than it does on internal faults. The UHS circuit
operates (blocks) correctly on the odd half-cycles (1st, 3rd, 5th, etc). However, the CCD (Charge Comparison
Data) on all half-cycles may trip incorrectly. The UHS may trip incorrectly on an even half-cycle (2nd, 4th, 6th,
etc) if the main CT is very severely saturated. (See Figure 3-9)
The large shifts in zero-crossings shown in Figure 3-9 only occur when there is a very large unbalance in CT
performance. The RFL 9300 bias characteristic (see Figure 2-4) has been designed to provide additional secu-
rity in the presence of moderate to large CT saturation. Also, because of the phase angle swings that are en-
countered in 3-terminal systems, extra security has been built into the 3-terminal bias characteristic (Figure 2-
4b). Dynamic time-tag compensation adds additional bias equal to 1/4A rms X the difference between the ad-
justed received time-tag of a validated CCD value and the time-tag representing the center of the “nested” half-
cycle stored at the local terminal. This is designed to be effective in the rare instance that a prefault load value is
not acquired prior to a fault. The 1:8 slope to the characteristic is designed to combat the effects of CT satura-
tion due to the higher scalar sums encountered in 3-terminal systems.
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