Nanhong Electric NRT-511 Digital Transformer Differential Protection Device

I. Overview
1.1 Scope of Application
The NRT-511 digital transformer differential protection device is suitable for transformer protection in substations with voltage levels of 110kV and below, serving as the main protection for three-winding transformers.
1.2 Device Function Configuration
1.2.1 Differential Instantaneous Trip Protection
Operates quickly to trip during severe internal faults in the transformer.
The protection operation criterion is: Id > Isd × Ie
Where: Id: differential current, Isd: differential instantaneous trip setting value,
Ie is the rated current on the high-voltage side.
If any phase of the three-phase differential current satisfies Id > Isd × Ie, the output action is triggered.
1.2.2 Percentage Restraint Differential Protection
Adopts the conventional percentage differential protection principle, reliably avoiding unbalanced differential current during external faults. Its operation equations are as follows:
Id > Iqd × Ie (before the breakpoint)
Id > Iqd × Ie + Kbl (Ir - Ieg × Ie) (after the breakpoint)
Where: Id: differential current; Ie: rated current on the high-voltage side; Ir: restraint current;
Kbl: percentage restraint coefficient; Iqd: differential starting setting value; Ieg: breakpoint current setting value.
For details, refer to Table 1 on page 2 of this manual.
1.2.3 Calculation Methods for Differential Current and Restraint Current
Id = | IH + IM + IL |
Ir = max{|IH|, |IM|, |IL|}
Where IH, IM, IL are the currents on the high-voltage, medium-voltage, and low-voltage sides of the transformer, respectively.