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SB510 3BSE000860R1 module

Giving due account to the available power, typical resistance values for most applications are REs = 1000  and RPs = 150 . Both RPs and REs must be able to conduct the maximum starpoint current for 10 s. The resistor RPs must also be continuously rated for the injection voltage (injected power < 100 VA). The maximum generator star-point current is determined by the resistors REs and RPs. Using the above resistors, this current would be, for example, 5.3 A for UGen = 10.5 kV or 13.5 A for UGen = 27 kV.

REs and MTR in the case of secondary injection at the starpoint An injection transformer block Type REX 011-1 is needed for this circuit. The displacement voltage can also be injected at the secondary of a grounding v.t. with the grounding resistor connected to its secondary winding (see Fig. 3.5.24.2). The two resistors R'Es and R'Ps limit the maximum current at the star-point which must not exceed 20 A. The total resistance is thus : Condition 1): R' R' U 3 I N N Es Ps Gen Emax 2 1 2
     where: UGen phase-to-phase voltage at the generator terminals IEmax max. star-point current = 20 A N1/N2 ratio of the grounding transformer

In most cases

 the calculated and determined values will not agree. Discrepancies of ±20 % are acceptable. Where the discrepancies - especially in the case of REs - are larger, check or measure the actual values of the grounding resistors and the grounding transformer. The v.t. must be designed such that for a solid ground fault at the generator terminals, the rated frequency component voltage signal Us = 100 ±20 %, i.e. the ratio MTR' = N'12/ N'11 must lie within the following range:The injection voltage Uis depends on the value of the parallel resistor R'Ps and can be either 0.85 V, 1.7 V or 3.4 V. The minimum value of the resistor R'Ps in relation to the corresponding injection voltage Uis can be seen from the following table. The maximum possible injection voltage should be chosen in each case.The two determination modes 'REs-Adjust' and 'MTR-Adjust' determine and display the values for REs and MTR, i.e. they present the secondary circuit reflected on the primary system side. Thus inaccuracies due to contact resistance, grounding resistor tolerances etc., are compensated. Determining the values for REs and MTR by means of the determination modes 'REs-Adjust' and 'MTR-Adjust' during commissioning is recommended in preference to calculating their values. As a check, calculate the values of R'Es and MTR' from the values given for RE and MTR in the measured value window as follows: