Some facts on protection

HRC fuse breaking capacity:- 80 KA

SF6 breaking capacity:- 30 KA

ACB breaking capacity :- 50 KA

hence fuses are used for S.C. protection and Relays for O.L. protection.

T1 and T2 reduce the fault current ( say from 400 A to 10 A ) which is the utility of these transformers.

Protections used in 132 KV/ 6.9 KV switchyard:-

• Lightening arrestor 120 KV, 10 KA

• IDMT O/C Relay + IDMT E/F Relay ( 51 + 51 N)

• IDMT O/C Relay + Instantaneous Relay + IDMT E/F Relay ( 51 + 50 + 51 N)
• Transformer Differential Relay(87)
• Restricted E/F Relay + Standby E/F Relay (64 R + 51 G)

Some notes on SubStation Relays

Trip Circuit Supervision Relay ( VAX ) :- 21 ( After closing only) 31 ( Preclosing and After Closing).
Application :- After Cl;osing or continuous supervision of the trip circuit or circuit breaker.
Detection:-
1.Failure of Trip Supply
2.Open Circuit of trip Coil or trip circuit wiring
3.Failure of mechanism to complete the tripping operation.

A CTZM Relay is used at ACB in PCC which feeds the MCC and the delay settins are provided there at.

When 10 MVA primary side SF6 primary side SF6 breaker operates, there should be an interlock to open the breaker on secondary side ( to prevent closing on load T/F) but this is not provided and is done manually.

CBCT :- Core Balance Current Transformer, Used for Unbalanced Current Protection.

Some questions answered.

Q)What is the difference between Screw Compressor and Reciprocating Compressor?
Ans) Screw Comp. is a constant torque load device. The output( i.e. air flow is also const.). unlike reciprocating comp, current of Screw Comp is much less fluctuating.

Q)Why three intermediate tap positions ( 11A,11B and 11C) are provided?
Ans) For Mechanical convenience.

Q) What re three MODES of CB/Feeder?
Ans) 1.Service :- Both Control and Power Supply will be there.
2.Test :- Only control Supply and no power supply.
3.Isolation:-Neither Control Now Power Supply.

Q)Where you find two diaphragms in transformer?
Ans) In the EXTRA VENT provided just below level og conservator.

A Lecture on Dielectrics

3A - Dielectric Materials

Some Rapid Fire Questions on Electrical Engineering

Q) Which machines have parabolic characteristics?

Ans) Fans and Pumps

Q) Which machine has linear load characteristics?

Ans) Screw Feeders

Q) What is difference between NEMA and IEC?

Ans) NEMA is American and IEC is Indian standard.

Power Management in Factories

There are three types of loads:-

• Load which cannot afford any power failure. ( on a small capacity DG set ;say 250 KVA)
• Load which can tolerate 2 min inturruption. ( on a small capacity DG set ;say 250 KVA)
• Load which can tolerate 30 min interruption. ( on a high capacity DG set ;say 4 MVA)

Motor Facts

Upto 30 KW DOL starters are used, above that, y-Delta starters are used.

For motors rated 55 KW+, motor heaters are used.

For motors rated /. 90 KW, PTC Thermisters are used.

For all motors rated 90 KW +, the vendor should furnish following data soon after submission of T-S curves and G D sqaure value of the driven equipment:- (This data is required for selection of proper motor protection relay).

• Thermal Withstand Curve: Hot and Cold.
• Stator Time Constant.
• Stalling Current.
• Stall Withstand:Hot and Cold
• Starting Current.
• Acceleration Time.