ABB RET650 Applications Manual page 127

1MRK 504 169-UEN A
Transformer protection RET650 2.2 IEC
Application manual
• Earth-fault protection of feeders in effectively earthed distribution and
subtransmission systems. Normally these feeders have radial structure.
• Back-up earth-fault protection of transmission lines.
• Sensitive earth-fault protection of transmission lines. EF4PTOC can have
better sensitivity to detect resistive phase-to-earth-faults compared to distance
protection.
• Back-up earth-fault protection of power transformers.
• Earth-fault protection of different kinds of equipment connected to the power
system such as shunt capacitor banks, shunt reactors and others.
In many applications, several steps with different current operating levels and time
delays are needed. EF4PTOC can have up to four, individual settable steps. The
flexibility of each step of EF4PTOC is great. The following options are possible:
Non-directional/Directional function: In some applications the non-directional
functionality is used. This is mostly the case when no fault current can be fed from
the protected object itself. In order to achieve both selectivity and fast fault
clearance, the directional function can be necessary. This can be the case for earth-
fault protection in meshed and effectively earthed transmission systems. The
directional residual overcurrent protection is also well suited to operate in
teleprotection communication schemes, which enables fast clearance of earth faults
on transmission lines. The directional function uses the polarizing quantity as
decided by setting. Voltage polarizing is the most commonly used, but alternatively
current polarizing where currents in transformer neutrals providing the neutral
source (ZN) is used to polarize (IN · ZN) the function. Dual polarizing, where the
sum of both voltage and current components is allowed to polarize can also be
selected.
Choice of time characteristics: There are several types of time characteristics
available such as definite time delay and different types of inverse time
characteristics. The selectivity between different overcurrent protections is
normally enabled by co-ordination between the operate time of the different
protections. To enable optimal co-ordination all overcurrent protections, to be co-
ordinated against each other, should have the same time characteristic. Therefore a
wide range of standardized inverse time characteristics are available for IEC and
ANSI.
Table 17: Time characteristics
Curve name
ANSI Extremely Inverse
ANSI Very Inverse
ANSI Normal Inverse
ANSI Moderately Inverse
ANSI/IEEE Definite time
ANSI Long Time Extremely Inverse
ANSI Long Time Very Inverse
Table continues on next page
Section 7
Current protection
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