Fault Current; Secondary Wire Resistance And Additional Load - ABB Relion REC670 Applications Manual

Section 22
Requirements
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-earth, 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.
22.1.3

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-
earth faults. The current for a single phase-to-earth 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.
22.1.4

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 earth 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 secondary cables so it normally is sufficient to consider just a single secondary
wire for the three-phase case.
The conclusion is that the loop resistance, twice the resistance of the single secondary wire,
must be used in the calculation for phase-to-earth faults and the phase resistance, the
resistance of a single secondary wire, may normally be used in the calculation for three-phase
faults.
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1MRK 511 358-UEN A
M11613-3 v1
M11613-4 v3
M11614-3 v1
M11614-4 v4
Application manual