Search

Premium Membership ♕

Limited Time Offer: Save 15% on PRO Plan with discount code: LRN15 and study specialized LV/MV/HV technical articles and studies.

Home / Technical Articles / 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.

Case study of transformer failure due to a ferroresonance occurrence
Case study of transformer failure due to a ferroresonance occurrence

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.

This case study illustrates an actual occurrence of a ferroresonance condition which resulted in the failure of a station service transformer and 13.8 kV switchgear buses. This case study provides a sequence of events with an actual DFR (digital fault recorder) record showing the three-phase fault that occurred when the station service transformer failed.

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.

Premium Membership Required

This technical article/guide requires a Premium Membership. You can choose an annually based Plus, Pro, or Enterprise membership plan. Subscribe and enjoy studying specialized technical articles, online video courses, electrical engineering guides, and papers. With EEP’s premium membership, you get additional essence that enhances your knowledge and experience in low- medium- and high-voltage engineering fields.

Check out each plan’s benefits and choose the membership plan that works best for you or your organization.

Limited time offer! – Save 15% on Pro Membership Plan with discount code LRN15

Log In »Purchase »

Premium Membership

Get access to premium HV/MV/LV technical articles, electrical engineering guides, research studies and much more! It helps you to shape up your technical skills in your everyday life as an electrical engineer.
More Information
author-pic

Edvard Csanyi

Electrical engineer, programmer and founder of EEP. Highly specialized for design of LV/MV switchgears and LV high power busbar trunking (<6300A) in power substations, commercial buildings and industry facilities. Professional in AutoCAD programming.

Leave a Comment

Tell us what you're thinking. We care about your opinion! Please keep in mind that comments are moderated and rel="nofollow" is in use. So, please do not use a spammy keyword or a domain as your name, or it will be deleted. Let's have a professional and meaningful conversation instead. Thanks for dropping by!

forty nine  ⁄  seven  =  

Learn How to Design Power Systems

Learn to design LV/MV/HV power systems through professional video courses. Lifetime access. Enjoy learning!

Subscribe to Weekly Newsletter

Subscribe to our Weekly Digest newsletter and receive free updates on new technical articles, video courses and guides (PDF).
EEP Academy Courses - A hand crafted cutting-edge electrical engineering knowledge