IEEE C57.109-1993 pdf free.IEEE Guide for Liquid-Immersed Transformer Through-Fault-Current Duration.
IEEE C57.109 The following definitions reflect usage as understood within the context of this document. The terminologyused in these definitions is in accordance with IEEE Std C57.12.80-1978(Reaff 1992).
normal base current:Rated current of a transformer corresponding to its rated voltage and ratedbase kilovoltamperes.
transformer short-circuit impedance:(1)For Category l and Category Il transformers, the transformerimpedance, expressed in percent on the transformer’s rated voltage and rated base kilovoltamperes. (2) ForCategory Ill and Category IV transformers,the sum of transformer impedance and system short-circuitimpedance at the transformer location, expressed in percent on the transformer’s rated voltage and ratedbase kilovoltamperes.
For purposes of coordinating overcurrent protective devices with transformer short-circuit withstand capa-bility, figures 1 through 4 are presented for Categories I-lV transformers as defined in IEEE Std C57.12.00-1993 and adopted in table 1.
For Categories I and lV transformers，a single curve represents both thermal and mechanical damageconsiderations.
For Categories ll and lll transformers, two curves are required. Depending upon the number of fault occur-rences in the transformer’s lifetime and fault current levels, mechanical damage considerations may be neg-ligible.On the curves that have both a solid and a dashed portion, the solid portion represents the total faultduration beyond which thermal damage to the transformer may occur. The dashed portion represents thetotal fault duration beyond which cumulative mechanical damage may occur. The increasing significance ofmechanical effects for higher-rated transformers is reflected in these curves. Transformers subjected to fre-quently occurring faults should be represented with the combination of mechanical and thermal portions ofthe curve, while transformers subjected to infrequently occurring faults are represented with the thermal por-tion only.The validity of these damage limit curves can not be demonstrated by tests, since the effects are cumulative over the transformer’s lifetime. They are based principally on informed engineering judgmentand favorable, historical field experience.
During the revision of C57.92 it became evident that the Times Rated Current capability of transformers asstated therein were thermal limits and did not recognize the mechanical withstand considerations of trans-formers. Consequently, the Protective Devices portion of the loading guide was removed. Later, this thermalcapability became part of this guide in an attempt to document the through-fault-current duration capabilityof transformers in sufficient detail to facilitate coordination of overcurrent protective devices with powertransformers.IEEE C57.109 pdf download.