REG 316*4 1MRB520049-Uen / Rev. B ABB Switzerland Ltd
4-5
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
S
· 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 (see Fig. 4.1).
b) in addition to a) by the HV power system when the HV cir-
cuit-breaker is closed (see Fig. 4.2).
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 Fig. 4.2. Apart from the bur-
den of the cables, the high fault level results in a high c.t. flux
and a high probability of it saturating.
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