ABB Relion 670 Series Applications Manual page 587

1MRK 511 401-UUS F
The requirements are a result of investigations performed in our network simulator. The current
transformer models are representative for current transformers of high remanence and low
remanence type. The results may not always be valid for non remanence type CTs (TPZ).
The performances of the protection functions have been checked in the range from symmetrical
to fully asymmetrical fault currents. Primary time constants of at least 120 ms have been
considered at the tests. The current requirements below are thus applicable both for symmetrical
and asymmetrical fault currents.
Depending on the protection function phase-to-ground, phase-to-phase and three-phase faults
have been tested for different relevant fault positions for example, close in forward and reverse
faults, zone 1 reach faults, internal and external faults. The dependability and security of the
protection was verified by checking for example, time delays, unwanted operations, directionality,
overreach and stability.
The remanence in the current transformer core can cause unwanted operations or minor
additional time delays for some protection functions. As unwanted operations are not acceptable
at all maximum remanence has been considered for fault cases critical for the security, for
example, faults in reverse direction and external faults. Because of the almost negligible risk of
additional time delays and the non-existent risk of failure to operate the remanence have not been
considered for the dependability cases. The requirements below are therefore fully valid for all
normal applications.
It is difficult to give general recommendations for additional margins for remanence to avoid the
minor risk of an additional time delay. They depend on the performance and economy
requirements. When current transformers of low remanence type (for example, TPY, PR) are used,
normally no additional margin is needed. For current transformers of high remanence type (for
example, P, PX, TPX) the small probability of fully asymmetrical faults, together with high
remanence in the same direction as the flux generated by the fault, has to be kept in mind at the
decision of an additional margin. Fully asymmetrical fault current will be achieved when the fault
occurs at approximately zero voltage (0°). Investigations have shown that 95% of the faults in the
network will occur when the voltage is between 40° and 90°. In addition fully asymmetrical fault
current will not exist in all phases at the same time.
24.1.3 Fault current
The current transformer requirements are based on the maximum fault current for faults in
different positions. Maximum fault current will occur for three-phase faults or single phase-to-
ground faults. The current for a single phase-to-ground fault will exceed the current for a three-
phase fault when the zero sequence impedance in the total fault loop is less than the positive
sequence impedance.
When calculating the current transformer requirements, maximum fault current for the relevant
fault position should be used and therefore both fault types have to be considered.
24.1.4 Secondary wire resistance and additional load
The voltage at the current transformer secondary terminals directly affects the current
transformer saturation. This voltage is developed in a loop containing the secondary wires and the
burden of all relays in the circuit. For ground faults the loop includes the phase and neutral wire,
normally twice the resistance of the single secondary wire. For three-phase faults the neutral
current is zero and it is just necessary to consider the resistance up to the point where the phase
wires are connected to the common neutral wire. The most common practice is to use four wires
Bay control REC670
Application manual
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Section 24
Requirements
M11610-4 v5
M11613-3 v1
M11613-4 v4
M11614-3 v1
M11614-4 v5
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