why Current transformer Star point is grounded

Mmm,

Star point current transformer?

let me tell you just regarding to current transformer's grounding.

According to IEEE standard C57.13.3, the gorunding of CTs is important to both safety and the correct operation of protective relays. To assure safe and reliable operation, the neutral of the CT secondary should have a single ground location for each circuit. The single ground is irrespective of the number of CTs or the chosen grond location.

see this too.

http://cr4.globalspec.com/thread/51469#comment535916

I am not familiar with a current transformer having a "star point," but I am open to explanations.

dear,

definitely, we have to assume 3 cts , one in each phase, common wire to be connected to ground. I hope, many a times, simple matters should be closed with open mind.

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,both the questions are same. Please clarify.

I seem to agree with the above mentioned points but in point 2 stated "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" ,For this isn't the star point of the transformer(system) should also be earthed, so earth and neutral are at the same potential, what if the system neutral is unearthed, I have this doubt, can you please throw some light on it....