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Correct connection of interposing(matching) CT

Correct connection of interposing(matching) CT

Correct connection of interposing(matching) CT

(OP)
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.

RE: Correct connection of interposing(matching) CT

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.

RE: Correct connection of interposing(matching) CT

(OP)
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.  

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