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EPRO PR6423 Extension cord

Version with grounding transformer (see Fig. 3.5.10.5) This arrangement is widespread in North America. The maximum current for the grounding transformer is chosen to approximately equal the capacitive component of the E/F current Designing a scheme for connection to the generator terminals If the generator star-point is inaccessible, the stator E/F protection is connected to three v.t's at the generator terminals (see Fig. 3.5.10.6). In this case, the E/F current is supplied by the three v.t. primary windings. Assuming a permissible short-time current of the primary windings of 5 A, the E/F current must be limited to a maximum of 15 A. A secondary rated voltage must be chosen such that the secondary voltage does not exceed 300 V and, if possible, the secondary current does not exceed 250 A. Stator E/F protection with grounding transformer at the generator terminals For an E/F at a generator terminal, the voltage of the phase concerned becomes zero and the healthy phases are at phaseto-phase potential with respect to ground. The vectorial addition of the two phase-to-phase voltages produces three times the rated voltage across the broken delta connection of the v.t. secondary windings: U = 3 U2n where U2n is the rated secondary voltage. If U is greater than 100 V, the E/F protection must be connected via an interposing v.t. When designing the scheme, the maximum current flowing through the primaries of the v.t's during an E/F is determined first. Assuming that the permissible short-time primary current of the v.t's is 5 A, then IEmax = 15 A The corresponding minimum value for the grounding resistor is R U I U U e K GN n n min max
     3 3 2 1 2 where K is the influence of the v.t. reactance. A mean value of 0.7 may be assumed for v.t's.

Typical settings: 

V-Setting 0.05 UN Delay 0.5 s Note: If a generator circuit-breaker is installed between the generator and the step-up transformer, a second E/F protection scheme is required for the zone between the step-up transformer and the unit transformer. The second scheme is connected to the broken delta secondary windings of three v.t's. This scheme must also remain stable for E/F's on the HV system and during ferroresonance phenomena and for this reason there is a resistor across the broken delta as well. Frequently, the E/F protection is only required to protect the cables and bar conductors, because the transformers are protected by differential schemes and Buchholz relays. In this case, the E/F protection setting is determined by the voltage off-set for an E/F at the lowest load voltage. A typical setting for reliable E/F detection in an ungrounded system is 60 % UN, i.e. V-Setting = 0.6 UN The delay can remain the same at 0.5 s. The second E/F scheme usually gives only an alarm. Since the plant remains in operation for an E/F on the cables, the resistor across the broken delta must be continuously rated