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
Thanks for any input






RE: High Resistant Ground Problem
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
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
RE: High Resistant Ground Problem
RE: High Resistant Ground Problem
Please, what is the transformer primary winding connection (the secondary is assumed Y because of the high resistance grounding)?