Operation P54x/EN OP/La4
MiCOM P543, P544, P545 & P546
(OP) 5-111
OP
NOTE : If communications breaks of up to 10 seconds during switching or fault
conditions on the power system cannot be tolerated by the P54x
application, a decision to implement a scheme using pilot wire circuits
should be reviewed.
2.1.12.1 Pilot isolation
Duri
ng primary earth faults, the strong magnetic field generated can induce a significant
voltage between the pilots and ground (longitudinal voltage). To prevent damage to any
equipment connected to the pilot circuit, it must be ensured that the modem can provide an
adequate isolation barrier between the pilot itself and all other electrically isolated circuits.
Although it may be difficult to accurately predict the induced pilot voltage during an earth
fault, the following equations can be used to give an approximation:
Induced voltage for un-screened pilots 0.3 x
F
x L
Induced voltage for screened pilots 0.1 x
F
x L
Where:
F
= Maximum prospective earth fault current in amperes
L = Length of pilot circuit in miles
In cases where the calculated voltage exceeds, typically 60% of the relay/modem isolation
level, additional isolation must be added. ALSTOM Grid offer the PCM-FLÜ 10 kV or 20 kV
isolating transformers for use in conjunction with such baseband modems. The choice of
10 kV or 20 kV will depend upon the predicted magnitude of the induced voltage.
Note: The PCM-FLÜ isolating transformer has “a”, “m” and “b” taps on both
primary and secondary windings. For all P54x applications,
connection must be made between taps ‘a’ and ‘m’, since the
frequency range of this winding extends to 2 MHz. Connection
between ‘a’ and ‘b’ may result in unreliable communications as the
maximum frequency for this tap configuration is 6 kHz. Connection to
‘a’ and ‘m’ taps must be adhered to on both primary and secondary so
as to maintain a 1:1 ratio.
2.1.12.2 Baseband modem and P590 specification
Depl
oyment of the Patton “Campus” 1092A baseband modem has been demonstrated with
the MiCOM relays and a scheme based on this is presented below.
The Patton “Campus” 1092A baseband modem offers a relatively short retrain time (by
baseband modem standards), but it should be noted that this can be as long as ten seconds
and the effect of this should be recognized as per the note in section 2.1.12.
On a
2-wire pilot connection a maximum link length of approximately 17 km can be achieved.
On a 4-wire pilot, approximately 18 km is possible. These figures are, however, dependent
upon the diameter and quality of the pilot wires. The effect of cable diameter on distance is
shown in the table below.
Wire guage Wire diameter Maximum distance
(2-wire connection)
Maximum distance
(4-wire connection)
19 AWG 0.9 mm 17.2 km 18.2 km
22 AWG 0.64 mm 11.5 km 12.1 km
24 AWG 0.5 mm 8 km 8.5 km
26 AWG 0.4 mm 5.5 km 5.7 km
For maximum security and performance it is strongly recommended that the pilots use
screened twisted pairs of conductors.
The Campus modem should be specified with a G.703 interface and should be used in
conjunction with a MiCOM P591.
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