Auto Changeover Schemes

<...In 2 incomer (ON) & 1 buscoupler (Off ) scheme , during auto changeover there will be first a break in the system and then the buscoupler will be closed. I want to know if motor feeders get affected due to this break????...>

It depends. If the relays in each starter cubicle remain with their contacts closed during the changeover, then there's a good chance that the motors will see only a brief interruption to the 3Φ supply and continue running, depending on what they are turning. If the contacts open, then those affected will require re-starting.

If you are asking whether or not the motor control circuit will drop out, that depends upon where its control power comes from. If your control power comes from the 3 phase mains that are being switched, then yes, your control system will drop out and the motor will need to be restarted by what ever method was used to start it originally.

I have used a UPS in the past to power the control circuits of a motor that I want to keep "energized" during a swing of 3 phase sources. Works ok as long as the load on the motor can handle the drop in torque from the motor during the loss of 3 phase power, a fan comes to mind as a suitable load for such a setup, and in fact it was a couple of large ventilation units I did this on in a food plant.

Also... of paramount importance is verification of proper phasing to ensure correct motor shaft rotation!

Paralleling at the load end is prohibited to protect your equipment, not the utility's. A fault on the line would pass fault current through your service until both lines were deenergized. The fault current on the 11KV feeders can be well above the short circuit capacity of your service equipment. The paralleled incomers will also prevent the proper operation of any automated restoration. If a fault occurs, the supply breaker at the bus will trip. If the feeders are paralleled at the distribution end, the control system will see that the line is still energized, and assume the breaker failed to fully open. It will then automatically trip the next upstream protective device, deenergizing the entire bus.

For my customers who desire a closed transition for return to normal, I have a set of line switches installed between their incomers (in parallel with the buscoupler). When they want to transfer, I close the line switches. If a line fault occurs, most of the short circuit current passes through my switches instead of the customer's service equipment.

Once the transfer is complete, the line switches are opened to separate the incoming feeders.

Motor feeders shall require manual intervention to start again since bus voltage shall go dead momentarily during change over. You can consider providing reaccelaration feature facility on feeders which are critical and require start without manual intervention. Auto reaccleration feature requires hardware of timer or alternatively through numerical relay if provided in feeder. In case large number of feeders are to be provided with reacceleration feature then group starting of motors is to be considered.