Ground Fault Current Path for Multiple Sources


Thread Starter


I'm clear about single source ground fault current path, multiples sources or separate drive system still confusing me, please check the ground fault current path for multiple sources as per link, hopefully somebody can explain a bit for ground fault current path for multiple grounding involving multiple generators or transformers.
Jack… the Drawing you submitted is a generalized illustration of a 3-ph, 4-wire, LV (< 1,000V) distribution system.

Setting aside fault-duty concerns, both sources (Left & Right) are paralleled and each Xfmr’s secondary mid-point is normally Solidly-Grounded. Unfortunately you haven’t shown the Protective Ground-Fault devices so the following explanation will be similar to “Describing how a ratchet works without using your hands!”

A) Tie-CB Normally-Closed (as shown)
The fact that that both mid-points are grounded makes the GF-Fault protection system unreliable! The GF on either the Left or Right Bus will cause protective devices on both sources to operate resulting in loss of both sides! Thus, one or the other source is disconnectible via a link in the Grounding-Electrode-Conductor (EGC) between the Xfmr mid-pt and ground! This leads to the inconvenience of disconnecting and reconnecting those links!

B) Quasi-Differential Method
Residually-Derived GF Detectors are installed at both incoming-breakers and the tie-breaker! Selectivity is archived via settings but it is very difficult to achieve!

C) Current-Summation Method
Ring or Zero-Sequence CTs are placed around the 4-conductors at each of the 3-breakers. Please note the one placed at the Tie-CB can only be placed Upstream or Downstream (or Left-side or Right-side Bus) of the Tie-CB cubicle. Selectivity is achieved using both logic and time!

D) Cross-Link Method
Two sets of GF detectors are installed at the Tie-CB; one to the left, and one right. Fault location is instant! For example the Left-side GF-detector is paralleled with the Right-side bus' GF-detector! Thus, instantaneous differential protection is achieved!

E) Bus Encapsulation
The HPI Company I worked with never used a normally-closed Tie-CB. Furthermore, the Bus was fully encapsulated, and cubicles tightly sealed to greatly reduce risk of arcing-fault propagation throughout the switchgear.

Phil Corso