Correct connection of interposing(matching) CT 1

[linkect]

I am working on modification of protection system to suit a temporary transformer. Initially the system was designed for a 60MVA 132/33kv YNynd11 and A CDG66 O/C relay with a MCAG 14 REF relay are installed on the 132kV side. Main CTR = 500/1 connected in Y configuration. OC set at 0.75A i.e. 143% loading allowed. CDG 66 has a minimum setting of 0.5A.

Required to adapt the same system retaining the existing relays for a temporary 30MVA transformer same vector group. I decided to reduce loading to 135% due to transformer age. This will give me 0.35A on the main CT secondary. I can only use a Yy0 0.47/1 matching CT to get the current to the minimum trip level of the CDG 66 relay. Since there is an REF relay I should als use a 0.47/1 matching CT after the neutral 500/1 CT to maintain balance.

I have attached drawing (hand drawn) showing the current arrangement and the proposed arrangement. My worry is wether the proposed arrangement will work especially with the REF relay. Please advise if I am on the right track.

 

[linkect]

Somehow the current arrangement was missed out when I uploaded the proposed arrangement. I have added the current arrangement below.

 

[isquaredr]

Hello linkect
By carrying out your proposal you increase the risck of CT saturation and REF operation for through fault conditions, cheapest solution is to replace your OC relay.

 

[linkect]

I have pointed this out but the client is adamant that since its only a temporary measure we should not spent money on it. There is a diff circuit with a 4C21 relay which require similar step up interposing CT's on the HV and LV side, but the connections were straight forward.