kaspor
Electrical
- Aug 12, 2021
- 45
Hello
I am trying to understand whether there's inaccuracies with earth fault currents detected with a residual scheme (i.e. CT's connected in STAR to protection relay, protection relay measures imbalance current to determine earth fault current).
Can you please confirm if my understanding is correct for these 3 scenarios:
1. Assume DELTA/STAR transformer supplies an unloaded switchboard. During a supply cable earth fault on A phase, no current will flow through B and C phases. Residual current detected by protection relay is A phase current.
2. Assume DELTA/STAR transformer supplies a STAR connected motor (assume initially operating at 100A). During a supply cable earth fault on A phase, earth fault current (assume 1000A) will flow from phase A to ground. No current will flow through B & C phases (due to STAR connected motor). Residual current detected by protection relay is A phase current.
3. Assume DELTA/STAR transformer supplies a DELTA connected motor (assume initially operating at 100A). During a supply cable earth fault on A phase, earth fault current (assume 1000A) will flow from phase A to ground. Phase B and C current will circulate through the DELTA winding and return back to the transformer. Protection relay will not see 1000A, but instead the vector summation of Ia+Ib+Ic? Does this mean the earth fault current seen by the relay will be inaccurate?
I am trying to understand whether there's inaccuracies with earth fault currents detected with a residual scheme (i.e. CT's connected in STAR to protection relay, protection relay measures imbalance current to determine earth fault current).
Can you please confirm if my understanding is correct for these 3 scenarios:
1. Assume DELTA/STAR transformer supplies an unloaded switchboard. During a supply cable earth fault on A phase, no current will flow through B and C phases. Residual current detected by protection relay is A phase current.
2. Assume DELTA/STAR transformer supplies a STAR connected motor (assume initially operating at 100A). During a supply cable earth fault on A phase, earth fault current (assume 1000A) will flow from phase A to ground. No current will flow through B & C phases (due to STAR connected motor). Residual current detected by protection relay is A phase current.
3. Assume DELTA/STAR transformer supplies a DELTA connected motor (assume initially operating at 100A). During a supply cable earth fault on A phase, earth fault current (assume 1000A) will flow from phase A to ground. Phase B and C current will circulate through the DELTA winding and return back to the transformer. Protection relay will not see 1000A, but instead the vector summation of Ia+Ib+Ic? Does this mean the earth fault current seen by the relay will be inaccurate?