STD Percentage Differential with Harmonic Restraint - Legacy

Overview

Manufacturing for STD relays has been discontinued. As alternatives, refer to 845, P64x, T35, or 345. The STD is a harmonic restrained percentage differential relay specifically designed for transformer fault protection. The STD15 through 21 relays differ from each other only in the number of restraint circuits that they include. The STD25 through 29 relays differ from STD15 through 21 in that they utilize only second harmonics for the harmonic restraint circuit while the others use all the harmonics. Harmonic restraint is employed to prevent undesired tripping as a result of exciting current inrush to the transformer being protected. Inrush to a transformer usually occurs when the transformer is energized or when a nearby fault is cleared thus suddenly restoring normal voltage to the bank. The second harmonic component is the predominant harmonic in transformer inrush current. Protection and control features: high-speed percentage differential and harmonic restraint. Resources: Brochures: https://www.gevernova.com/grid-solutions/resources?prod=std&type=1&node_id=4979; Manuals: https://www.gevernova.com/grid-solutions/resources?prod=std&type=3&node_id=4979; Support Documents: https://www.gevernova.com/grid-solutions/resources?prod=std&type=9&node_id=4979; Application Notes: https://www.gevernova.com/grid-solutions/resources?prod=std&type=2&node_id=4979. Applications explicitly listed for the STD relay are power and autotransformer protection. The harmonic restraint function is used to prevent undesired tripping caused by exciting current inrush that occurs when the transformer is energized or when a nearby fault is cleared and normal voltage is suddenly restored to the bank.