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High Resistant Ground Problem

High Resistant Ground Problem

High Resistant Ground Problem

(OP)
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

(OP)
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

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

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