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PPD115A102 3BHE017628R0102 Control module

As can be seen from Fig. 3.5.10.2, the relay setting for a zone of protection of 95 % is 5 % of Umax. The scheme detects E/F's on the generator stator winding, the cables to the step-up transformer and the delta-connected windings of the step-up transformer. The capacitances between primary and secondary of the step-up transformer conduct currents emanating from E/F's on the HV side to the LV side and can cause false tripping of the stator E/F protection. The capacitive coupling of E/F currents on the HV side takes place regardless of whether the HV star-point is grounded or not. The capacitance C12 between HV and LV windings of the step-up transformer and the capacitance C of the generator circuit form a potential divider, which determines the potential of the generator star-point (see Fig. 3.5.10.3a). The value of the capacitance C is usually too low to definitely hold the star-point below the pick-up setting of the protection. For this reason, the generator star-point is grounded via a resistor RE (see Fig. 3.5.10.4), which ensures that the potential of the starpoint remains below the setting of the protection for an E/F on the HV power system. Correspondingly, the value of the resistor RE is chosen such that for a given C12 and an E/F at the HV terminals of the step-up transformer, the off-set of the generator star-point does not reach the pick-up setting of the 95 % E/F protection.

where:

 C12 capacitance between primary and secondary of the step-up transformer C capacitance to ground of the stator windings, the cables with protection capacitors and the LV step-up transformer winding U star-point off-set UOS rated voltage of the step-up transformer HV windings IE E/F current RE grounding resistor The value of the grounding resistor RE determines the E/F current. In view of the damage, which can be caused by an E/F current, especially to the laminations of the stator core, the maximum E/F current should be limited to 20 A for 10 s, i.e. the grounding resistor RE must not be too small. Tripping by the E/F protection is delayed by 0.5 s to avoid any risk of false tripping during transient phenomena. Designing a scheme for connection to the generator star-point 1. Version with grounding resistor and v.t. (see Fig. 3.5.10.4)

where: C12 capacitance between primary and secondary of the step-up transformer C capacitance to ground of the stator windings, the cables with protection capacitors and the LV step-up transformer winding U star-point off-set UOS rated voltage of the step-up transformer HV windings IE E/F current RE grounding resistor The value of the grounding resistor RE determines the E/F current. In view of the damage, which can be caused by an E/F current, especially to the laminations of the stator core, the maximum E/F current should be limited to 20 A for 10 s, i.e. the grounding resistor RE must not be too small. Tripping by the E/F protection is delayed by 0.5 s to avoid any risk of false tripping during transient phenomena. Designing a scheme for connection to the generator star-point 1. Version with grounding resistor and v.t. (see Fig. 3.5.10.4)