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REG 316*4 1MRB520049-Uen / Rev. B
4.2.1.2.
Restricted E/F protection for a transformer
4.2.1.2.1.
Basic requirements
A restricted E/F scheme must be able to
detect E/F's in the protection zone
·
remain stable during both phase and earth through-faults.
·
The scheme is designed to remain stable
in the case of a solidly grounded star-point for an external
·
E/F current
in the case of an impedance grounded star-point for the
·
highest external phase and earth fault current.
When designing a scheme, it is assumed that one c.t. is fully
saturated and none of the others.
4.2.1.2.2.
Components of a restricted E/F scheme
A restricted E/F scheme comprises the following items:
linear stabilising resistor R
·
REG 316*4 overcurrent function
·
non-linear resistor
·
shorting contacts where necessary.
·
4.2.1.2.3.
Design
The E/F current is determined by
a)
the generator and step-up transformer reactances when the
HV circuit-breaker is open
b)
in addition to a) by the HV power system when the HV cir-
cuit-breaker is closed
As a result of the current distribution for a through-fault, the star-
point c.t. conducts the highest current in the case of a solidly
grounded transformer as shown in
den of the cables, the high fault level results in a high c.t. flux
and a high probability of it saturating.
S
(see Fig.
4.1).
(see Fig.
4.2).
Fig.
4.2. Apart from the bur-
ABB Switzerland Ltd
4-5
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