Premium Membership

Study specialized electrical engineering articles, papers & video courses in low/high voltage areas. Get 50% OFF on all courses at the EEP Academy with the Enterprise Plan.

Home / Technical Articles / The case of phase-to-phase fault in a 13.8 kV switchgear caused by heavy rain

Case study

In this case study we describe a 13.8 kV switchgear phase-to-phase fault due to by heavy rain evolving into a three-phase fault that was cleared by transformer numerical relays. The generator was out of service and the fault was fed only from the 138 kV system.

The case of phase-to-phase fault in a 13.8kV switchgear caused by heavy rain
The case of phase-to-phase fault in a 13.8kV switchgear caused by heavy rain

We illustrate how a relay oscillography record for the 138-kV transformer side can be used to develop the analysis for the classification of the fault and confirmation of the fault location.

Since this study uses phase transformer differential relay 87TP, so let’s first remind of the basics of this protection.

Phase transformer differential relays 87TP


The protective device provides restrained phase differential protection function with user-configurable multiple slope percentage restrained characteristic that allows to compensate both the static error and the dynamic error.

The static error accounts for transformer static magnetizing current and current measurement circuit calibration errors. The dynamic error may be caused by Tap Changing (OLTC) and by CT saturation caused by heavy fault currents.

ANSI 87TP – Transformer Differential Protection
Figure I0 – ANSI 87TP – Transformer Differential Protection

In addition, the static tripping characteristic can be modified temporarily at the User’s choice to prevent some nuisance tripping from the harmonic inrush during energization, over-excitation, or deep CT saturation. The harmonic inrush is evaluated through 2nd, 4th harmonics and 5th harmonics transient is monitored through the CT saturation detector.

The phase differential protection can be used for this application scenario:

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.

Note that an Enterprise subscription plan includes a 50% discount on all video courses and bundles at the EEP Academy.

Log In »Learn More »

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

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!

four  ×    =  36

Electrical Engineering Courses

Learn to design electrical power and industrial automation systems through professional video courses.
Protection testing