Auto Transfer Scheme 2 I-C & B-C

SIEMENS

Operation Philosophy for 2: Incomer & 1: Bus-coupler Auto changeover Scheme

The scheme is based on an arrangement of two incoming feeders and one bus coupler and allows for automatic changeover in the case of an under-voltage on one of the incomers and also for intentional manual momentary paralleling of two incomers for restoration restoration of supply without interruption. interruption. Detailed operation Philosophy is explained explained below. Incomer

Each incomer can be closed in two modes 1. Dead Bus Closing Mode 2. Momentary Momentary paralleling Close Mode Dead Bus Close Mode

In the Dead bus close mode, the VCBs are so interlocked that only two VCBs shall be ‘ON’ at a time. If two VCBs are in closed condition the closing of the third VCB shall be prevented ensuring that the supplies are not paralleled at any point of time. This can be achieved through hard wire logic. The operation sequence sequence for closing the VCB in Independent mode is as follows:1. Auto-Independent-Manual switch (A-I-M switch) to be kept in Independent mode . 2. Then from Local or Remote we can give close command to breaker. 3. Breaker can be in Test or Service position. Momentary paralleling Close Mode

In Momentary paralleling close mode, paralleling of supplies is permitted, subject to certain conditions. After paralleling of two sources for some time, one breaker is tripped automatically. Let us say we want to close I/C-1 & trip B/C after paralleling. The operation sequence is as follows 1. Auto-Independent-Manual switch (A-I-M switch) to be kept in Manual. 2. The Trip Selector Switch (TSS switch) is to be kept in “Trip B/C” position. 3. When the close command is issued, Relay checks the synchronisation between Bus-A & Bus-B. 4. If sources are satisfying the synchronising condition, relay allows the closing of I/C1. 5. The other other condit conditio ions ns for closi closing ng are also checke checked d by relay relay before before giving giving the close close permissive command. 6. Now when when a close lose comm comman and d is give iven, the I/C-1 /C-1 will clos lose and suppl upplie ies s shall hall be paralleled. 7. The Closed status of all the three VCBs is sensed by the relay. So after a set time delay it issues a trip command command to B/C. Output relay-2 is used for tripping B/C. 8. Similar way if I/C-2 is to be tripped then user has to select the Trip Selector Switch in I/C-2 and Output of that particular relay is used to trip that feeder. 9. Similar procedure is followed for closing the I/C-1 and I/C-2 by keeping Trip Selector switch in respective position. In both the closing modes mentioned above following closing interlocks must be checked. 1. There must be no fault on bus onto which the incomer is closing (Fault contact of  I/C-1 & I/C-2 are not operated) 2. Trip circuit of the feeder to be closed must be healthy. 3. VCB must be in Serv Service ice Position.

The VCB can be tripped under following conditions - 1-

SIEMENS 1. Trip from T-N-C switch or remote. 2. Fault Trip command from protection relay of respective incomer 3. Under momentary paralleling condition if Breaker is selected for tripping through TSS. 4. Sustained under-voltage on line side. Bus-coupler

The Bus-coupler can be closed in three modes 1. Dead Bus closing mode 2. Momentary paralleling close Mode 3. Auto Closing mode Dead Bus Close Mode

In Dead bus close mode the VCBs are so i nterlocked that o nly two VCBs shall be ‘ON’ at a time. If two VCBs are in closed condition the closing of the third VCB shall be prevented ensuring that the supplies are not paralleled at any point of time. Momentary paralleling Close Mode

In Momentary paralleling close mode, paralleling of supplies is permitted, subject to certain conditions. After paralleling of two sources for some time, one breaker is tripped automatically. Let us say we want to close B/C & trip I/C-1 after paralleling. The operation sequence is as follows 1. The Trip Selector Switch is to be kept I/C-1 position. 2. Auto-Independent-Manual switch (A-I-M switch) is kept in Manual mode. 3. When the closing command is issued to relay, it checks synchronisation between Bus-A & Bus-B voltages. 4. If two voltages are synchronised, relay gives a close permissive command to the closing coil of B/C. 5. The other conditions for closing are also checked by relay before giving the close permissive command. 6. Now when a close command is given, the B/C will close and supplies shall be paralleled. 7. The Closed condition of all the three VCB`s is sensed by the relay. After a set time delay the I/C-1 relay issues a trip command to trip the I/C-1. 8. Similar way I/C-2 can be tripped by selecting it through TSS. Automatic close mode

The operation sequence shall be as follo ws. Under voltage settings mentioned are typical &  can be set by the user. 1. Incomer-1 & Incomer-2 are “ON” and Bus-coupler is in “OFF” condition and Automanual switch is to be kept in “Auto” position 2. In case the Bus-A voltage falls below set limit due to a line under-voltage, the Incomer-1 VCB trips on Under-voltage condition sensed by its protection relay. Status of line PT-1 secondary MCB is also monitored before issuing a trip command to ensure that there is a true under-voltage condition. 3. Following conditions are checked before an Auto-close command is generated. a. Voltage on the other bus (Bus-B) is healthy (Checked internally by relay) b. No fault is present on Bus-A. c. The residual voltage on the Bus-A has fallen below set limit (checked by relay). Also to bus PT secondary MCB status is monitored to ensure that there is a true under-voltage condition. 4. In case the above are met, an automatic close command to Bus-coupler is generated by the scheme. Now in other case if Bus-B voltage falls below set limit due to a line under-voltage, the Incomer-2 VCB trips on under-voltage condition sensed by its protection relay. Status of line PT-2 secondary MCB is also monitored before issuing trip command to ensure that there is a true under-voltage condition. 5. Following conditions are checked before an Auto close command is generated. a. Voltage on the other bus (Bus-A) i s healthy (Checked by relay). b. No fault is present on Bus-B. - 2-

SIEMENS c. The residual voltage on the Bus-B has fallen below set limit (checked internally by relay). Also the status of BUS-B PT secondary MCB is monitored. 6. In case the above are met, an automatic close command to Bus-coupler is generated by the scheme. 7. Incomer tripping & bus-coupler closing will be simultaneously. If there is no intentional time-delay. 8. We are not closing incomer first to restrict asynchronous paralleling. In all three closing modes following closing interlocks are checked 1. There is no fault on either buses (No fault condition of I/C-1, I/C-2.) 2. Trip circuit of breaker to be closed is healthy 3. VCB in Service The B/C can trip under following conditions 1. Trip from T-N-C switch or remote 2. Tripping after momentary paralleling if selected through TSS.  

While closing the incomers “No-Fault” condition of Bus-coupler is checked because if bus-coupler is opened due to fault & we cannot close the incomer. In make before break condition, we have to always check the synchronisation of  both the supply.

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