Protection System & Elements
Switch Gear : is a General Term Covering a Wide Range of Equipment connected with Switching and Interrupting the Currents, During Normal and Apnormal Conditions .
In Addition, The Equipment associated with Controlling Protecting and Measuring is also covered by Switch Gear.
Switch Gear includes Switches, Fuses, Circuit Breakers, Insulators, Relays, Control Panels, Lightening Arrestors, Current Transformers, Potential Transformers, and Other associated Items.
*Circuit Breakers :
During the Normal Operating Condition, The C.B can be Opened or closed Manually by the Station Operator for the Purpose of Maintenance.
During Apnormal or Faulty Conditions, The Relay senses the Fault and close The Trip Circuit of the C.B.
Therefore, The C.B opens.
The C.B has Two Working Positions, Namely Open and Closed.
The Apnormal Conditions of the C.B Operation are The Following:
- Faults : The Faults in an Electric Circuit may be Defined as Partial or Total Failure of the Insulation.
For The Purpose of Analysis, Fault can be Classified as :
- Single Line to Ground Fault.
- Double Line to Ground Fault.
- Line to Line Fault.
- Three Phase Fault.
- Winding Faults (Generators, Transformers, and Motors).
- Open Circuits (Arc due to Ionization of Air).
Beside Faults, There are Other Apnormal Conditions in Power Systems, This includes:
- Instability of the System.
- Over Voltage and Over Current.
- Under Voltage and Under Current.
- Over Frequency and Under Frequency.
- Power Reverse.
- Temperature Rise.
- Unbalanced Conditions.
The Standards defined The Circuit Breakers as a Device Capable of Making and Breaking of an Electric Circuit Under Normal Conditions and Upnormal Condituins such as, Short Circuits.
*The Process of Fault Clearing :
(1) As the Fault occurs, The Current increases and the Relay gets actuated or energized.
The Moving Part of the Relay moves because of the increasing in the Current.
The Relay tales some Time to close its Contacts.
- Relay Contacts close the Trip Circuit of the Circuit Breakers, and Trip is energized.
- The Operating Mechanism starts for the Opening of the Circuit Breaker, and The C.B Contacts separate.
- Arc is Drown between the Breaker Contacts, and That Arc is interrupted in the C.B BY Suitable Techniques. The Current reaches Final Zero as The Arc is interrupted and doesn’t restrike again.
*Maintenance of the Arc Extinction :
The Electrons so librated from the Cathode of The C.B make many Collisions with The Atoms of Gases, and produce more Electrons .
To interrupt The Arc, The Following Methods can be used:
- Lengthening of the Arc.
- Cooling the Arc.
- Splitting the Arc.
- Constraining the Arc.