Case study: Failure of a transformer & switchgear buses due to a ferroresonance event

Introduction to the problem

First of all, it’s good to know that the possibility of a transformer ferroresonance condition rapidly increases as the transformer is closer to generator zone. This condition may occur when the generator is disconnected, leaving the unit transformer delta winding energized and essentially ungrounded.

The grounded potential transformers are now connected to an ungrounded system with the possibility of a ferroresonance condition. As a result, a distorted 60-Hz or subharmonic voltage could overexcite the PT, forcing it into the saturated region.

The PT variable nonlinear inductance may now resonate with the ungrounded system capacitance, thus causing extreme high voltage that may initiate the failure of system equipment in the generator zone if damping is not present.

We describe the power system and associated protection, the failure mechanism, the ferroresonance phenomenon, corrective actions, and lessons learned.

Contents:

1. System single-line diagram of the system
2. Switching Sequence
3. Analysis of the digital fault recorder (DFR) record
4. Analysis of the station service transformer failure
5. Fault location using 3-phase fault calculation
6. Explanation of the ferroresonance phenomenon
7. Corrective actions
8. Conclusions

1. Description of the system single-line diagram

Figure 1 shows a portion of the system one-line diagram where the generating units of the hydro plant are connected to the 230 kV transmission systems. A bank of four units is also shown connected to the 230 kV system via a three-winding delta/delta/YG 13.8/13.8/230 kV transformer.

Dual units are bussed together and connected to each of the 13.8 kV delta windings. The transformer is protected by harmonic-restraint percentage differential relay device 87T. Other generating units, which are not shown, are connected to the 115 kV system.