P54x/EN AP/La4 Application Notes
(AP) 6-62
MiCOM P543, P544, P545 & P546
AP
3.4 VT connections
3.4.1 Open delta (vee connected) VT's
The Mi
COM P54x relay can be used with vee connected VTs by connecting the VT
secondary’s to:
C19, C20 and C21 input terminals, with the C22 input left unconnected for P543 and P544
D19, D20 and D21 input terminals, with the D22 input left unconnected for P545 and P546
This type of VT arrangement cannot pass zero-sequence (residual) voltage to the relay, or
provide any phase to neutral voltage quantities. Therefore any protection that is dependent
upon phase to neutral voltage measurements should be disabled.
The ground directional comparison elements, ground distance elements, neutral voltage
displacement (residual overvoltage) and CT supervision all use phase-to-neutral voltage
signals for their operation and should be disabled. The DEF elements should be selected for
negative sequence polarization to avoid the use of phase-to-neutral voltages. Under and
over voltage protection can be set as phase-to-phase measuring elements, whereas all other
protection elements should remain operational.
The accuracy of the single phase voltage measurements can be impaired when using vee
connected VT’s. The relay attempts to derive the phase to neutral voltages from the phase
to phase voltage vectors. If the impedance of the voltage inputs were perfectly matched the
phase to neutral voltage measurements would be correct, provided the phase to phase
voltage vectors were balanced. However, in practice there are small differences in the
impedance of the voltage inputs, which can cause small errors in the phase to neutral
voltage measurements. This may give rise to an apparent residual voltage. This problem
also extends to single phase power measurements that are also dependent upon their
respective single phase voltages.
The phase to neutral voltage measurement accuracy can be improved by connecting 3, well
matched, load resistors between the phase voltage inputs (C19, C20, C21 for P543 and
P544 or D19, D20, D21 for P545 and P546) and neutral C22 for P543 and P544 or D22 for
P545 and P546 thus creating a ‘virtual’ neutral point. The load resistor values must be
chosen so that their power consumption is within the limits of the VT. It is recommended that
10 k 1% (6 W) resistors are used for the 110 V (Vn) rated relay, assuming the VT can
supply this burden.
3.4.2 VT single point earthing
The Mi
COM P54x will function correctly with conventional 3 phase VT’s earthed at any one
point on the VT secondary circuit. Typical earthing examples being neutral earthing, or
B-phase (UK: “yellow phase” earthing).
3.5 Trip circuit supervision (TCS)
The tri
p circuit, in most protective schemes, extends beyond the relay enclosure and passes
through components such as fuses, links, relay contacts, auxiliary switches and other
terminal boards. This complex arrangement, coupled with the importance of the trip circuit,
has led to dedicated schemes for its supervision.
Several trip circuit supervision scheme variants are offered. Although there are no dedicated
settings for TCS, in the MiCOM P54x the following schemes can be produced using the
programmable scheme logic (PSL). A user alarm is used in the PSL to issue an alarm
message on the relay front display. If necessary, the user alarm can be re-named using the
menu text editor to indicate that there is a fault with the trip circuit.
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