Substation Engineering - Interlockings in a Substation:

BB1(MAIN Bus) and BB2 ( SPARE bus )are the two buses. For each Feeder there are two bus bar isolators.

• Breaker is Q0
• Busbar Isolators are Q1 & Q2
• Line Isolator is Q9
• Line Earth switch is Q8
• Earth switches are Q51 , Q52 , Q53

I . To Close Breaker of a Feeder Q0 :

• Both Bus bar isolators should not be intermediate position (Q1 & Q2)
• Line Isolator should not be in intermediate position (Q9)

The Isolator can be either in OPEN position or in CLOSE position . But to Close the breaker Q0 of the feeder , they should not be in intermediate position.

II. To CLOSE Earth switches Q51 , Q52 , Q53 :

• Both Bus bar Isolators should be in OPEN position (Q1 & Q2)
• Line Isolator should be in OPEN position (Q9)

III. To CLOSE Bus bar isolator Q1 :

Case (i) – If the Other Bus bar Isolator Q2 is in OPEN position :

• Breaker should be in OPEN position (Q0)
• All Earthing switches should be in OPEN position (Q51 , Q52,Q53)
• The busbar earthing switch Q15 should be in OPEN position

Case (ii) – If the other Bus bar Isolator Q2 is in CLOSE position :

• Breaker should be in OPEN position (Q0)
• All Earthing switches should be in OPEN position (Q51 , Q52,Q53)
• The busbar earthing switch should be in OPEN position (Q15)
• Bus section should be in CLOSE position
• Bus coupler should be in CLOSE position

Vice versa for closing the busbar isolator Q2

While closing a busbar isolator say Q1 in this case , suppose if the busbar isolator is already closed , then the bus coupler should be in CLOSEd position . With the bus coupler in OPEN position we can not close both busbar isolator for a particular feeder . This is because , by closing both the isolators in the same bay , we are making that bay as a bus coupler with out a circuit breaker in between . So for any fault on the busbar , it will trip all bays in both busbars .This situation of course is not at all acceptable .

IV. To CLOSE Line isolator Q9 :

• Breaker should be in OPEN position (Q0)
• All Earthing switches should be in OPEN position (Q51 , Q52,Q53)
• Line Earthing switch should be in OPEN position (Q8)
• Opposite side Earthing must be OPEN

V. To CLOSE Line earth Q8 :

• Line isolator should be in OPEN position (Q9)
• There should be no voltage on Feeder
• Opposite side CB must be in OPEN position

I . To CLOSE Breaker in Bus section Q0:

• Bus section isolators should not be in intermediate position (Q12, Q11, Q21)

II . To CLOSE Bus section isolators Q12 & Q11 :

• Breaker should be in OPEN position (Q0)
• Earthing switches should be in OPEN position (Q52, Q51)
• Corresponding busbar earthing switch should be in OPEN pos (Q15 in case to close Q12 ; Q16 in case to close Q11 )

III . To CLOSE Earth switches Q52 & Q51 :

• Both the isolators should be in OPEN position (Q12 & Q11)

IV . To CLOSE Busbar earthing switch Q15 (BB1A – Q15) :

• Bus section isolator should be in OPEN position (Q12)
• All isolators of feeders connecting to that particular section of the bus bar should be in OPEN position ( eg : All Q1 s that are connecting to BB1A should be in OPEN pos )

V . To CLOSE Busbar earthing switch Q16 (BB1B – Q16) :

• Bus section isolator should be in OPEN position (Q11)
• All isolators of feeders connecting to that particular section of the bus bar should be in OPEN position ( eg : All Q1 s that are connecting to BB1B should be in OPEN position )

VI . To CLOSE Busbar earthing switch Q25 (BB2A – Q25) :

• Bus section isolator should be in OPEN position (Q21)
• All isolators of feeders connecting to that particular section of the bus bar should be in OPEN position ( eg : All Q2 s that are connecting to BB2A should be in OPEN pos )

VII. To CLOSE Busbar earthing switch Q26 (BB2B – Q26) :

• Bus section isolator should be in OPEN position (Q21)
• All isolators of feeders connecting to that particular section of the bus bar should be in OPEN position ( eg : All Q2 s that are connecting to BB2B should be in OPEN pos )

VIII. To CLOSE Bus section isolator Q21 :

• Earthing switches should be in OPEN position (Q25 & Q26)
• All isolators of feeders connecting to BB2 should be in OPEN position ( All Q2s of feeders connecting to BB2A and BB2B should be in OPEN position )

I . To CLOSE Bus coupler breaker Q0 :

• Busbar isolators should not be in intermediate position ( Q1 & Q2 )

II . To OPEN Bus coupler breaker Q0:

• Bus section should be in CLOSE position ( Q12, Q11, Q0, Q21 should be in CLOSE position )
• Both busbar isolators (Q1, Q2) should not be in closed position together per feeder on all bays

III. To CLOSE Busbar isolators Q1 , Q2 :

• Breaker should be in OPEN position (Q0)
• All earthing switches should be in OPEN position ( Q51, Q52 )
• Corresponding busbar earthing switch should be in OPEN position (Q16 in case to close Q1 & Q26 in case to close Q2)

IV. To CLOSE Earthing switches Q51 , Q52 :

• Both bus bar isolators should be in OPEN position

It is not possible to OPEN the bus coupler when both the busbar isolators are closed together on a single feeder. Because it will make that particular feeder a bus coupler without a circuit breaker in between.

* All these interlocking are applicable only when the Hand crank is not inserted and end position interlocking is not activated .