Areva MiCom P120, Техническое руководство

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Application Guide P12x/EN AP/A96
MiCOM P120/P121/P122/P123
Page 17/80
6. RESTRICTED EARTH FAULT
MiCOM P120, P121, P122 and P123 provide Restricted Earth Fault protection. It should be
noted that:
The algorithms implemented in P120 and P121 for the first and second thresholds (I>, I0>
and I>>, I0>>) are similar to the ones implemented in the P122 and P123 for the same
thresholds. However the algorithm of the third threshold (I>>> and I0>>>) of P120 and P121
is different to the one of P122 and P123.
In fact, the algorithm of the third threshold of the P122 and P123 is based on a current
sample base in addition to the Fast Fourier Transformation base. This implementation allows
to trip faster on highly saturated current signals. The third threshold of P120 and P121- as it
is for the first and second threshold of P120, P121, P122 and P123 products - is based on
the Fourier transformation;
This explains the outstanding results obtained by the third threshold of P122 and P123
compared to the other thresholds regarding the high impedance restricted earth fault
application. So for:
•
P122 and P123: The user can use all the thresholds for REF application. Note that the
results of the third threshold will be greater due to the fact that a sample base
algorithm is used.
•
P120 and P121: The user can use all the threshold for REF application. The results of
the third threshold will be similar to one of the first and second threshold (since all the
thresholds are based on Fast Fourier Transformation).
NOTE: For P122 and P123, the maximum internal fault level for the third
threshold (for the 0.002 to 1In range) must not exceed 20In.
6.1 Introduction
The restricted earth fault relay is a high impedance differential scheme which balances zero
sequence current flowing in the transformer neutral against zero sequence current flowing in
the transformer phase windings. Any unbalance for in-zone fault will result in an increasing
voltage on the CT secondary and thus will activate the REF protection.
This scheme is very sensitive and can then protect against low levels of fault current in
resistance grounded systems where the earthing impedance and the fault voltage limit the
fault current.
In addition, this scheme can be used in a solidly grounded system. It provides a more
sensitive protection, even though the overall differential scheme provides a protection for
faults over most of the windings.
The high impedance differential technique ensures that the impedance of the circuit is of
sufficiently high impedance such that the differential voltage that may occur under external
fault conditions is lower than the voltage required to drive setting current through the relay.
This ensures stability against external fault conditions and then the relay will operate only for
faults occurring inside the protected zone.