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Generator Relay Data

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    A good starting point is to compile the data needed for setting the various generator protective functions.  Figures provide examples of organizing and massaging or converting the information to a more useful form for generators, step-up transformers, and their associated electrical systems. The information is for a 578.6 megavolt-amp (MVA), 24 kilovolt (kV) generator that is connected to the 765 kV bulk power transmission system. Organizing the data in this manner will save significant time in developing and documenting the basis for the relay settings.

     

     Generator Relay Data

     

     

    The top half of Fig shows the generator data needed for the relay settings, including generator nameplate data; the direct axis saturated subtransient, transient, and the synchronous per-unit reactances; and the various primary and secondary instrument transformer ratings. With the exception of converting primary ohms to secondary or relay ohms ,  and will not be discussed further here. The bottom half of  fig provides nominal currents, instrument transformer ratios, and various reactance ohms (not per unit) that will be used to develop the settings for the backup impedance, loss-of-excitation, and out-of-step relay functions.

     

    this figure presents the step-up transformer secondary or relay ohms for the backup impedance and out-of-step relay functions. The transformer per-unit base impedance at 555 MVA needs to be converted to the generator base of 578.6 and corrected for voltage differences on the generator side of the transformer and tap differences on the high voltage side of the transformer. With the exception of secondary or relay ohms, the per-unit MVA conversions and corrections have already been discussed.

     

     

    Fig develops the high voltage switchyard secondary or relay ohms for the generator out-of-step function. Normally, this calculation would not include the short circuit contribution from the generator. First, the system kV ohms and volt-amps (VA) are determined, then, the generator side amps and ohms are calculated, and finally, the secondary or relay ohms that represent the high voltage electrical system are developed. The system short circuit angle is not used in the calculation, but is needed for setting the out-of-step protective relay function.

     

     

     

     

     

     

    Reference: Electrical calculation by by Thomas E. Baker

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