110kv SF6 Breaker

To protect the switchgear, related equipment and the grid from people who randomly throw switches whose function they don't understand! The CTD is there to protect the breaker against loss of DC control power.

Just think of this scenario, today you remove all the control power from the switchgear by flipping the switch, and a few seconds later there's a fault in the system and the protective relaying calls for the now unpowered breaker to open... ka-boom..., one less engineer(?) who couldn't take the time to RTFM (Read the Fine Manual)...Darwinism at its finest.

CTD is normally used for tripping the CB, after it loss the power, & control supply, where a independent control supply source (eg. UPS) not availabe . CTD stores energy when the power supply is availabe.

Dear Jayachandran

As it was informed by fellow participants, it is to avoid keeping a breaker in service without auxiliary control circuit.

However consider a situation as indicated hereunder.

A grid substation having about 8 grid feeders. If all these feeder breakers trip by CTD due momentary switch off of DC supply, it will be more detrimental to the grid security

As the distance protection is provided the back 2nd and3rd zones, protection will taken care by them during the criticality of breaker not tripping because of the DC failure.

Therefore Tamil Nadu Electricity Board's Practice is to keep CTD in service only for the radial feeders. In grid feeders CTD is disabled.