ABB RELION REX640 - Page 595

and then compared with setting EF validity Min Curr . The current values used in
validity check are called as transient resistive and reactive components and noted as
Transient Ris Comp and Transient React Comp in Recorded data and TR_RIS_COMP
and TR_REACT_COMP in Monitored Data.
Validity of earth-fault current estimate is indicated by VALID_EF = TRUE.
In case EF validity Op mode = ‘Resistive’, then validity of earth-fault current estimate
is confirmed by evaluating the polarity of the resistive part of measured admittance
(real(Y
o
) = real(I
o
/U
o
)) during the fault ignition moment (during the transient
phase of fault). The measured “transient admittance” is converted into equivalent
current value by multiplying it with nominal phase-to-earth voltage value and then
compared with setting EF validity Min Curr . EF validity Op mode = ‘Resistive’ is
valid in compensated systems regardless of compensation degree and also in
high resistance earthed systems. Note that situation, when the compensation coil
becomes unintentionally disconnected, may result to condition where measured
resistive component is not sufficient for validity confirmation. In such case EF
validity Op mode should be selected as ‘Resistive, Reactive’.
In case EF validity Op mode = ‘Resistive, Reactive’, then validity of earth-fault
current estimate is confirmed by evaluating the polarity of the resistive (real(Y
o
)
= real(I
o
/U
o
)) and reactive (imaginary) part (imag(Y
o
) = imag(I
o
/U
o
)) of measured
admittance during the fault ignition moment (during the transient phase of fault).
The measured “transient admittance” is converted into equivalent current value
by multiplying it with nominal phase-to-earth voltage value and then compared
with setting EF validity Min Curr . Either one, resistive or reactive part of measured
admittance is used to confirm validity of earth-fault current estimate. EF validity
Op mode = ‘Resistive, Reactive’ is valid in compensated, unearthed and in high
resistance earthed systems. In compensated systems this criterion is applicable
unless the protected feeder is over-compensated i.e. earth-fault current produced
by protected feeder is inductive. In such case EF validity Op mode must be set to
‘Resistive‘.
In case EF validity Op mode = ‘No validity”, then validity of earth-fault current
estimate is not confirmed and protection operates based on estimated earth-fault
current or touch voltage magnitude. EF validity Op mode = ‘No validity’ can be
applied e.g. when current measurement is at incomer and when compensation coil
with parallel resistor is located at dedicated earthing transformer bay. In this case,
during an earth fault in the network, residual current at incomer is zero and thus
validity evaluation based on resistive or reactive component is not possible. Thus in
order to apply IFPTOC at the incomer, EF validity Op mode = ‘No validity‘, must be
selected.
Setting EF validity Min Curr is given in per unit format with residual current channel
primary current as base value. The value should be selected based on following
principles:
• In compensated networks EF validity Min Curr should be selected based on the
known system damping value, in practice e.g. based on the ampere value of the
parallel resistor of the coil. For example, if parallel resistor of the coil has rating
of 10A, then EF validity Min Curr should be smaller than this value with margin,
for example 5A. Based on practical experience, value of 3A is found to be suitable
for most applications. The value of EF validity Min Curr should also be set higher
than the transient resistive component generated by the protected feeder during
post-fault oscillation to avoid false activations of validity check module in healthy
feeders during an earth fault.
Transient resistive component can be approximated with following equation:
1MRS759142 F
Protection functions
REX640
Technical Manual
595