IEEE C37.91-2000 pdf free.IEEE Guide for Protective Relay Applications to Power Transformers.
IEEE C37.91 The most difficult transformer winding fault for which to provide protection is the fault that initially involvesone turn. A turn-to-turn fault will result in a terminal current of much less than rated full-load current.Forexample, as much as 10% of the winding may have to be shorted to cause full-load terminal current to flow.Therefore, a single turn-to-turn fault will result in an undetectable amount of current.
There is no limit to the maximum internal fault current that can flow, other than the system capability, whenthe fault is a terminal fault or a fault external to the transformer but in the relay zone. The relay systemshould be capable of withstanding the secondary current of the CT on a short-time basis. This may be afactor if the transformer is small relative to the system fault and if the CT ratio is chosen to match thetransformer rating.
Fault current through a transformer is limited by the transformer and source impedance. While currentthrough a transformer thus limited by its impedance can still cause incorrect relay operations or even trans-former failure,CT saturation is less likely to occur than with unlimited currents.
The above favorable aspect may disappear if the transformer protective zone includes a bus area with two ormore breakers on the same side of the transformer through which external fault current can flow with no relationship to the transformer rating.An example is a transformer connected to a section of a ring bus withthe transformer protection including the ring bus section.
During an internal fault, or a fault external to the transformer but in the protected zone of the relay system,the CTs may saturate, perhaps severely.Severe CT saturation can cause the harmonic restraint unit to pickup, thereby resulting in the failure of a transformer differential relay to operate or in a delay of its operation.The effect depends on the relay’s response to distorted current.On a transient basis, with a saturated CT,second and third harmonics predominate initially.Ultimately, the even harmonics disappear with the decayof the offset direct-current component of the short-circuit current. Whether or not the odd harmonicsdisappear depends on the CT’s steady-state saturation characteristic.CTs should be rated to avoid steady-state saturation.IEEE C37.91 pdf download.