CT Star Point

Why protection and metering CT's are always connected in star why not in delta? In star connection, line current and phase currents are equal. So, if you have a 5A CT secondary, when you connect the three phase CTs in STAR, the meters and Relays can be standardised at 5A. Whereas, if you connecet the three phase CTs in Delta, even though the individual CT seconadry current will still be 5A, the line current will be 2.89A, for which the meters and relays are not standardised. But, in Differential Protection of Delta/Star Transformers, the STAR side CTs are connected in Delta only. Connecting CT's for metering in star will allow star connection, in which star point may be connected to a grounding rod and eliminate the possibility of voltage rise in an open CT which may result to dangerous high voltages. insulation level is low and obviates the need of auxiliary transformer for earthing

1. I think it's a safety precaution against possible 'open circuit' on the CT secondary side, which can lead to generation of dangerously high voltage. 2. It is provided to flow the earth fault current. 3. During a secondary open circuit on one CT, a very high transient voltage would appear across all the three phase CTs as they are interconnected through the loads. If you earth one terminal of the secondary of all the three phase CTs, the insulation need to be done only for phase voltage and not for line voltage. This greatly reduces the size and insulation cost of the CT. There is a dedicated IEEE Standard on this topic. 4. CT Primary and secondary is linked though stray capacitance and resistance of insulator. If CT secondary is not earthed, the primary voltage may induce high potential in secondary winding. To keep the secondary voltage at designed rating (say 110 v) against the earth, secondary must be earthed. It appears,both the questions are same.

why Current transformer Star point is grounded 08/01/2010 7:04 AM

For CT's either you use for 3 phase or 2 phase or even if you use only 1 CT's for the Overcurrent Protection or for the Earth Faults Protection, their neutral point is always shorted to earth. This is NOT as what you explain as above but actually it is for the safety of the CT's when the current is passing thru the CT's. In generally, tripping of Earth faults and Overcurrent Protection has nothing to do with the earthing the neutral of the CT's. Even these CT's are not Grounded or Earthed, these Overcurrent and the Earth Faults Protection Relay still can operated. Operating of the Overcurrent Protection and the Earth Fauts Relays are by the Kichoff Law Principle where the total current flowing into the points is equal to the total of current flowing out from the point. Therefore, for the earth faults protection relays operating, it is that, if the total current

flowing in to the CT's is NOT equal total current flowing back out of the CT's then with the differeces of the leakage current, the Earth Faults Relays will operated. Here is the sample of the CT's connection

I refer to earlier a CR4 discussion,on "Why we must earth one terminal at secondary side of current transformer?" Several answers received are quoted for ready reference: 1. I think it's a safety precaution against possible 'open circuit' on the CT secondary side, which can lead to generation of dangerously high voltage. 2. It is provided to flow the earth fault current. 3. During a secondary open circuit on one CT, a very high transient voltage would appear across all the three phase CTs as they are interconnected through the loads. If you earth one terminal of the secondary of all the three phase CTs, the insulation need to be done only for phase voltage and not for line voltage. This greatly reduces the size and insulation cost of the CT. There is a dedicated IEEE Standard on this topic. 4. CT Primary and secondary is linked though stray capacitance and resistance of insulator. If CT secondary is not earthed, the primary voltage may induce high potential in secondary winding. To keep the secondary voltage at designed rating (say 110 v) against the earth, secondary must be earthed. It appears, the present topic of discussion / question is same. Please clarify.