Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

High Resistant Ground Problem

High Resistant Ground Problem

High Resistant Ground Problem

I had an application where I installed a breaker with wye-wye potential xfmrs for a relay on a high resistant ground system.  When a ground occured, the high voltage fried some of my PTs.  So, then I installed Delta-wye Pts.  This caused me to get a 30 degree shift to my relay.  Would zig-zag PTS be a better option or how can I correct the 30 degree shift?

Thanks for any input

RE: High Resistant Ground Problem

The PTs probably fried because they were rated for line to neutral voltage.  When a ground fault occurs on a high resistance grounded system, the voltage on the unfaulted phases changes to the full line to line value, which will burn out PTs that are rated for line-neutral.
Two clear choices -
1. Install wye-wye PTs rated for full line-line voltage
2. Correct the 30 degree phase shift on the delta-wye PTs by installing a set of auxiliary PTs connected delta-wye (if the main PTs are 30 degree lag, connect the auxiliary PTs 30 degrees lead)

RE: High Resistant Ground Problem

peterb's item 1 best describes the problem.  Assuming that the relay function is to sense ground faults, delta-primary (or open-delta) PTs will not allow the relay to see neutral shift [zero-sequence voltage].  With a ground on the system, usually phase-to-phase voltages do not necessarily change much, and that's the only quantities that delta-connected PTs are ’passing’ to the relay.  Wye-primary, wye-secondary PTs are the only practical way to do this.  (ZZ-connected PTs will work, but that’s a lot of unnecessary extra work.)   The PT primaries must be rated for full phase-to-phase voltage, even though connected phase-to-ground.  ‘ANSI C57.13’ PTs are meant to withstand continuous 15% overvoltage, and only intended for 173% for a very short interval, which is what you’re asking them to do long-term in a ground-fault situation.  

Example—take a hi-resistance-grounded 12kV system with 60:1-ratio PTs connected phase-to-ground.  Across each secondary winding is about 115 volts (12,000/3^0.5/60).  With balanced/symmetrical high-side voltage, each primary is seeing 6930V.  Now, fault a phase and then one PT has zero primary volts but the two others are at the full phase-to-phase potential, or 1.73x ‘normal.’  You need to go to 100:1 PTs that are meant for 12,000V.  Note that now each PT secondary runs about 69V under normal conditions, but a full 12kV across a primary won’t cook the winding.  Make sure your relay is intended to operate satisfactorily in this situation.

A grounded-wye primary, grounded-wye secondary bank with primary coils rated for continuous phase-to-phase voltage is the only sure fix.  The caution with this arrangement is that steady-state phase-to-neutral voltage is only 0.577 (1/3^0.5) per-unit of ‘normal.’

Follow the PT manufacturers recommendations for high- and low-side fusing for units mounted in switchgear.

In hi-resistance grounded systems, a grounded-wye/broken delta connection is often used with a sensitive overvoltage relay for ground sensing, (called a ‘59G function’) but this quantity can be calculated in numerical relays based on 4-wire wye PT inputs.

RE: High Resistant Ground Problem

Thanks for the help!  I never thought about wye-wye pts rated for line to line voltage.  my system voltage is 480V.  

RE: High Resistant Ground Problem

4:1 PTs will not be cooked in a 480V grounded-phase situation, but remember that the secondaries will be at ~69V phase-to-neutral [120V phase-to-phase] under normal conditions.  Verify that your relay package will work OK with that.

RE: High Resistant Ground Problem

Please, what is the transformer primary winding connection (the secondary is assumed Y because of the high resistance grounding)?

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members! Already a Member? Login


eBook - Integrating the Engineering Ecosystem
Aras Innovator provides multiple options for integrating data between systems, depending on the scenario. Utilizing the right approach to meet specific business requirements is vital. These needs range from authoring tools, federating data from various and dissimilar databases, and triggering processes and workflows. Download Now
Research Report - Simulation-Driven Design for SOLIDWORKS Users
In this engineering.com research report, we discuss the rising role of simulation and the paradigm shift commonly called the democratization of simulation. In particular, we focus on how SOLIDWORKS users can take advantage of simulation-driven design through two analysis tools: SOLIDWORKS Simulation and 3DEXPERIENCE WORKS. Download Now
White Paper - Industry 4.0 and the Future of Engineering Education
With industries becoming more automated, more tech-driven and more complex, engineers need to keep their skills and knowledge up to date in order to stay on top of this wave—and to be prepared for the Industry 4.0 future. The University of Cincinnati offers two online Master of Engineering degree programs designed specifically for practicing engineers. Download Now

Close Box

Join Eng-Tips® Today!

Join your peers on the Internet's largest technical engineering professional community.
It's easy to join and it's free.

Here's Why Members Love Eng-Tips Forums:

Register now while it's still free!

Already a member? Close this window and log in.

Join Us             Close