Consideration of Thermal Damage Curve for Protection Coordination Through Dyn Transformer (Solidly Grounded System)
The damage to transformers from through fault can be the result of both thermal and mechanical effects, which are a significant cause of transformer failure. Therefore, the overcurrent protective devices such as fuses and 50/51 relay shall have the well-defined operating characteristics related to the fault-current magnitude to operating time. The characteristic curves for these devices should be coordinated with the proper thermal damage curve of the transformer.
In addition, the through-fault protection curve values are based on winding-current relationships for a three-phase secondary fault, and it may be used directly for delta-delta- and wye-wye-connected transformers. However, for delta-wye-connected transformers (solidly grounded at secondary), the transformer damage curve should be shifted to the left by 58% as per recommendation by IEEE 242. The reason can be that the high-side line current is only 58% of the low-voltage per-unit fault current during the ground faults at secondary side, as shown in Figure 1.
This adjustment of the thermal damage curve provides the verification that the primary protective device (50/51 relay) can adequately protect the transformer for the single-phase to ground fault at its secondary side, as shown in Figure 2.
 IEEE 242 – IEEE Recommended Practice for Protection and Coordination of Industrial and Commercial Power Systems
 J. Lewis Blackburn Thomas J. Domin “Protective Relaying Principles and Application Third Edition”
 IEEE C37.91 IEEE Guide for protective relay applications to power transformers