CT star point has great importance in Protection & Metering applications of power system & power flow respectively. In protection application CT star has important role mainly in those application where the direction of current needs to be considered. Although in modern digital relays direction of fault current is decided by the phase angle instead of current Entering or leaving the neutral terminal for forward & reverse fault respectively, however, still conventional assumption of CT star point should be toward the protected object can’t be ignored completely in all protection application. For example if in a directional over current/ earth fault if the star point is not formed toward the protected object there is a chance of maloperation or if the coil of Neutral CT coil is connected with hard wire differential relay will assume forward fault as a reverse fault & there would be chance of compromise on overall protection. Similarly, in metering application star point has very crucial role. If the formation of start point is wrong the Power import will be considered as power export or vice versa, by the metering device, & it will ultimately affect the billing process.
Similarly, double earthing in CT secondary leads to multiple problems in protection applications. There would be only single earthing point according to the available Standard because double earthing can leads to maloperation of protection relay during fault condition, although, during normal condition it works fine.
In an unbalanced system double earthing provides a parallel path to fault current in the neutral wire & current remains circulating in neutral instead of reaching to relay coil in some cases, so, it will not operate the relay in faulty condition. In case of external faults in differential relay, double earthing can create potential difference in neutral & it will produce unnecessary more current that is not the projection of actual primary current & ultimately it will operate the differential relay in through faults. Therefore, always make single earthing point in CT secondary circuit according to the available standard.