dbrown20
05-02-2007, 11:15 AM
This is from an Ohio Brass bulletin.
Problems can be created with these transformer connections if they are normally operated with the wye grounded. Transformers can be destroyed due to a number of conditions that will exist when this bank is grounded. As noted the delta will attempt to balance loads. This will cause circulating currents to be generated if the delta sees an unbalanced load on the primary side. Also, grounding the wye produces a path in the delta for third harmonic currents. The bank will act as a grounding bank and supply fault current to faults on the circuit. If this fault is a single-phase fault, and is cleared by single-phase equipment, the transformer bank will be over loaded when one phase is opened since it will become an open delta bank. The combination of all these conditions will add to the loading of the bank causing conditions of overload to exist when they normally would not occur. Therefore, the best operation of this type of bank is to operate ungrounded. dbrown20
Problems can be created with these transformer connections if they are normally operated with the wye grounded. Transformers can be destroyed due to a number of conditions that will exist when this bank is grounded. As noted the delta will attempt to balance loads. This will cause circulating currents to be generated if the delta sees an unbalanced load on the primary side. Also, grounding the wye produces a path in the delta for third harmonic currents. The bank will act as a grounding bank and supply fault current to faults on the circuit. If this fault is a single-phase fault, and is cleared by single-phase equipment, the transformer bank will be over loaded when one phase is opened since it will become an open delta bank. The combination of all these conditions will add to the loading of the bank causing conditions of overload to exist when they normally would not occur. Therefore, the best operation of this type of bank is to operate ungrounded. dbrown20